A10. Hypoxia - Biology

A10.  Hypoxia - Biology

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We have spend much time studying how the body deals with and utilizes the toxic byproducts of dioxygen reduction. What happens when the body doesn't get enough dioxygen - a condition call anoxia (no dioxygen) or hypoxia (too little dioxygen)? This might occur in muscles undergoing vigorous exercise, and in the brain and heart when clots occlude blood flow to these organs (as occurs in most strokes and heart attacks). Under low dioxygen concentrations, a family of protein transcription factors called hypoxia-inducible factor (HIF) become activated. This could be beneficial to cells since rapidly growing tissue, especially tumor cells, might be expected to experience low oxygen conditions.

Figure: Cell response to hypoxia

Effects of an Acute Hypoxic Event on Microplankton Community Structure in a Coastal Harbor of Southern California

Fish mortality and hypoxic events occur in many coastal and inland systems and may result from natural or anthropogenically mediated processes. The effects of consequent changes in water biogeochemistry have been investigated for communities of benthic invertebrates and pelagic metazoans. The responses of micro-plankton assemblages, however, have remained largely unstudied. The northern basin of King Harbor, a small embayment within Santa Monica Bay, CA, USA, suffered a massive fish kill in March 2011 as a consequence of acute hypoxia. Dissolved oxygen concentrations < 0.1 ml l −1 were measured in the northern basin of the harbor for several days following the mortality event, and a strong spatial gradient of oxygen was observed from the northern basin to waters outside the harbor. The microplankton community within King Harbor differed significantly from a diatom-dominated community present in neighboring Santa Monica Bay. The latter region appeared unaffected by physicochemical changes, induced by the fish kill, that were observed within the harbor. A trophic shift was observed throughout King Harbor from a photoautotrophic-dominated assemblage to one of heterotrophic forms, with relative abundances of bacterivorous ciliates increasing by more than 80 % in the most impacted part of the harbor. Significant changes in community structure were observed together with dramatically reduced photosynthetic yield of the remaining phytoplankton, indicating severe physiological stress during the extreme hypoxia.

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A-10 Rubble Identified in Colorado Pilot Still Missing

After a harrowing recovery operation Wednesday, the U.S. Air Force confirmed that wreckage high on a remote Colorado mountain is that of an A-10 attack bomber missing since April 2.

A recovery expert hanging 100 feet below a hovering helicopter retrieved two pieces of metal from an ice-encrusted cliff 13,000 feet up the mountainside. The two parts--a bent engine blade and a clump of wiring--carried manufacturers’ numbers matching those of the missing A-10 Thunderbolt, Maj. Gen. Nels Running said at a news conference.

“The aircraft debris at the site in the New York mountains near Gold Dust Peak is in fact our A-10 missing aircraft,” Running said. “But the search for the pilot continues. . . . We have no evidence he was with the plane when it met the mountain.”

The advanced mountaineer lowered from the belly of a MH-53 heavy-lift helicopter--the most powerful helicopter in the Air Force--spent only 15 minutes at the crash site in the Sawatch range about 15 miles southeast of the Vail ski resort.

A four-person team of rescuers was ferried to a natural bowl about 1,000 feet below the crash site to begin searching for the body of the pilot, Capt. Craig Button. The team planned to spend the night on the mountain and then start probing the snow, gingerly, with poles on Thursday.

Their effort is complicated by ground-hugging clouds and myriad potential hazards presumed buried under 10 feet of snow on an 80-degree slope: chunks of twisted metal, flares, hundreds of rounds of 30mm cannon ammunition and four 500-pound general-purpose bombs.

Given the additional danger of avalanches in this region of craggy peaks, deep valleys and strong wind currents, the searchers have been ordered to avoid any digging.

A strong weather front was expected to dump several inches of snow throughout the region over the next few days. The team is carrying enough supplies to last three days.

“We’re going to do everything we can to keep track of where they are,” said Col. Denver Peltcher. “If something comes up we are prepared to go get them.”

Running said a priority will be to remove the plane’s ordnance, but that may not be possible until the snow melts later this summer.

Any ordnance discovered over the next few weeks by searchers will be marked with a flag so that experts can defuse the explosives.

U.S. Forest Service authorities have suggested leaving the remainder of the estimated 30,000 pounds of wreckage on the mountain because removing it could cause damage to the wilderness ecology.

As a warning to hikers who might venture into the area over the weekend, Running said: “This is not a place for fun and games. If you are smart you’ll stay out.”

Now that the plane has been identified, Running said a safety investigation board will be appointed to investigate why Button mysteriously broke away from a three-plane formation over southern Arizona and then headed 800 miles northeast to Colorado.

His disappearance occurred about 15 minutes after he refueled while flying from Davis-Monthan Air Force Base in Tucson to the Barry M. Goldwater bombing range.

This much is known: Button was at the low-end of the range of his fuel by the time radar images and eyewitnesses saw his slow-moving tank destroyer making a tight circle over the Vail area about 12:40 p.m.

Some reported hearing an explosion and seeing smoke rising from the steep mountainside where bits of metal, wires and paper are now scattered across several hundred yards in all directions.

Was he looking for a place to land? Had he lost his bearings because of oxygen deprivation--a condition known as hypoxia--and slammed into the granite mountain face? Did he deliberately intend to end his life after a spectacular joy ride? Is he surviving somewhere in the snow-covered wilderness after bailing out?

Air Force officials had been reluctant to speculate on answers to those questions. But in an interview, Running downplayed the possibility that Button may have been suffering from the mind-altering influence of hypoxia, perhaps brought on during the aerial refueling.

“I can’t buy the idea that he gets a certain level of hypoxia and flies 800 miles to a different state through mountains and valleys and whatnot and then his fuel runs out and he’s gone,” he said.

“Right now, we can’t see anything that even suggests he was with the plane when it went down.”

Maj. Richard Dopierala, of the physiology training center at Peterson Air Force Base in Colorado Springs, disagreed.

“Hypoxia can do all kinds of things specific to an individual, and it is insidious--let it go and it’ll whack you,” he said. “Any number of things could have caused this. You can’t say it’s not one of them until they do an investigation.”

Louis Sahagún is a staff writer at the Los Angeles Times. He covers issues ranging from religion, culture and the environment to crime, politics and water. He was on the team of L.A. Times writers that earned the Pulitzer Prize in public service for a series on Latinos in Southern California and the team that was a finalist in 2015 for the Pulitzer Prize in breaking news. He is a CCNMA: Latino Journalists of California board member, and author of the book, “Master of the Mysteries: the Life of Manly Palmer Hall.”

With the highly contagious Delta variant of the coronavirus continuing to spread statewide, the Los Angeles County Department of Public Health is recommending that all residents wear masks in public indoor spaces — regardless of whether they’ve been vaccinated for COVID-19.

Hypoxia-inducible factor

The HIF transcriptional complex was discovered in 1995 by Gregg L. Semenza and postdoctoral fellow Guang Wang. [3] [4] [5] In 2016, William Kaelin Jr., Peter J. Ratcliffe and Gregg L. Semenza were presented the Lasker Award for their work in elucidating the role of HIF-1 in oxygen sensing and its role in surviving low oxygen conditions. [6] In 2019, the same three individuals were jointly awarded the Nobel Prize in Physiology or Medicine for their work in elucidating how HIF senses and adapts cellular response to oxygen availability. [7]

Most, if not all, oxygen-breathing species express the highly conserved transcriptional complex HIF-1, which is a heterodimer composed of an alpha and a beta subunit, the latter being a constitutively-expressed aryl hydrocarbon receptor nuclear translocator (ARNT). [4] [8] HIF-1 belongs to the PER-ARNT-SIM (PAS) subfamily of the basic helix-loop-helix (bHLH) family of transcription factors. The alpha and beta subunit are similar in structure and both contain the following domains: [9] [10] [11]

    – a bHLH domain for DNA binding
  • central region – Per-ARNT-Sim (PAS) domain, which facilitates heterodimerization – recruits transcriptional coregulatory proteins

The following are members of the human HIF family:

Member Gene Protein
HIF-1α HIF1A hypoxia-inducible factor 1, alpha subunit
HIF-1β ARNT aryl hydrocarbon receptor nuclear translocator
HIF-2α EPAS1 endothelial PAS domain protein 1
HIF-2β ARNT2 aryl-hydrocarbon receptor nuclear translocator 2
HIF-3α HIF3A hypoxia inducible factor 3, alpha subunit
HIF-3β ARNT3 aryl-hydrocarbon receptor nuclear translocator 3

HIF1α expression in haematopoietic stem cells explains the quiescence nature of stem cells [14] for being metabolically maintaining at a low rate so as to preserve the potency of stem cells for long periods in a life cycle of an organism.

The HIF signaling cascade mediates the effects of hypoxia, the state of low oxygen concentration, on the cell. Hypoxia often keeps cells from differentiating. However, hypoxia promotes the formation of blood vessels, and is important for the formation of a vascular system in embryos and tumors. The hypoxia in wounds also promotes the migration of keratinocytes and the restoration of the epithelium. [15] It is therefore not surprising that HIF-1 modulation was identified as a promising treatment paradigm in wound healing. [16]

In general, HIFs are vital to development. In mammals, deletion of the HIF-1 genes results in perinatal death. [17] HIF-1 has been shown to be vital to chondrocyte survival, allowing the cells to adapt to low-oxygen conditions within the growth plates of bones. HIF plays a central role in the regulation of human metabolism. [18]

The alpha subunits of HIF are hydroxylated at conserved proline residues by HIF prolyl-hydroxylases, allowing their recognition and ubiquitination by the VHL E3 ubiquitin ligase, which labels them for rapid degradation by the proteasome. [19] [20] This occurs only in normoxic conditions. In hypoxic conditions, HIF prolyl-hydroxylase is inhibited, since it utilizes oxygen as a cosubstrate. [21] [22]

Inhibition of electron transfer in the succinate dehydrogenase complex due to mutations in the SDHB or SDHD genes can cause a build-up of succinate that inhibits HIF prolyl-hydroxylase, stabilizing HIF-1α. This is termed pseudohypoxia.

HIF-1, when stabilized by hypoxic conditions, upregulates several genes to promote survival in low-oxygen conditions. These include glycolysis enzymes, which allow ATP synthesis in an oxygen-independent manner, and vascular endothelial growth factor (VEGF), which promotes angiogenesis. HIF-1 acts by binding to Hypoxia-responsive elements (HREs) in promoters that contain the sequence NCGTG (where N is either A or G). Recent work from the laboratories of Sónia Rocha and William Kaelin Jr. demonstrates that Hypoxia modulates histone methylation and reprograms chromatin [23] This paper was published back-to-back with that of 2019 Nobel Prize in Physiology or Medicine winner for Medicine William Kaelin Jr. [24] This work was highlighted in an independent editorial. [25]

It has been shown that muscle A kinase–anchoring protein (mAKAP) organized E3 ubiquitin ligases, affecting stability and positioning of HIF-1 inside its action site in the nucleus. Depletion of mAKAP or disruption of its targeting to the perinuclear (in cardiomyocytes) region altered the stability of HIF-1 and transcriptional activation of genes associated with hypoxia. Thus, "compartmentalization" of oxygen-sensitive signaling components may influence the hypoxic response. [26]

The advanced knowledge of the molecular regulatory mechanisms of HIF1 activity under hypoxic conditions contrast sharply with the paucity of information on the mechanistic and functional aspects governing NF-κB-mediated HIF1 regulation under normoxic conditions. However, HIF-1α stabilization is also found in non-hypoxic conditions through an, until recently, unknown mechanism. It was shown that NF-κB (nuclear factor κB) is a direct modulator of HIF-1α expression in the presence of normal oxygen pressure. siRNA (small interfering RNA) studies for individual NF-κB members revealed differential effects on HIF-1α mRNA levels, indicating that NF-κB can regulate basal HIF-1α expression. Finally, it was shown that, when endogenous NF-κB is induced by TNFα (tumour necrosis factor α) treatment, HIF-1α levels also change in an NF-κB-dependent manner. [27] HIF-1 and HIF-2 have different physiological roles. HIF-2 regulates erythropoietin production in adult life. [28]

Repair, regeneration and rejuvenation Edit

In normal circumstances after injury HIF-1a is degraded by prolyl hydroxylases (PHDs). In June 2015, scientists found that the continued up-regulation of HIF-1a via PHD inhibitors regenerates lost or damaged tissue in mammals that have a repair response and the continued down-regulation of Hif-1a results in healing with a scarring response in mammals with a previous regenerative response to the loss of tissue. The act of regulating HIF-1a can either turn off, or turn on the key process of mammalian regeneration. [29] [30] One such regenerative process in which HIF1A is involved is skin healing. [31] Researchers at the Stanford University School of Medicine demonstrated that HIF1A activation was able to prevent and treat chronic wounds in diabetic and aged mice. Not only did the wounds in the mice heal more quickly, but the quality of the new skin was even better than the original. [32] [33] [34] [35] Additionally the regenerative effect of HIF-1A modulation on aged skin cells was described [36] [37] and a rejuvenating effect on aged facial skin was demonstrated in patients. [38] HIF modulation has also been linked to a beneficial effect on hair loss. [39] The biotech company Tomorrowlabs GmbH, founded in Vienna in 2016 by the physician Dominik Duscher and pharmacologist Dominik Thor, makes use of this mechanism. [40] Based on the patent-pending HSF ("HIF strengthening factor") active ingredient, products have been developed that are supposed to promote skin and hair regeneration. [41] [42] [43] [44]

Anemia Edit

Recently, several drugs that act as selective HIF prolyl-hydroxylase inhibitors have been developed. [45] [46] The most notable compounds are: Roxadustat (FG-4592) [47] Vadadustat (AKB-6548), [48] Daprodustat (GSK1278863), [49] Desidustat (ZYAN-1), [50] and Molidustat (Bay 85-3934), [51] all of which are intended as orally acting drugs for the treatment of anemia. [52] Other significant compounds from this family, which are used in research but have not been developed for medical use in humans, include MK-8617, [53] YC-1, [54] IOX-2, [55] 2-methoxyestradiol, [56] GN-44028, [57] AKB-4924, [58] Bay 87-2243, [59] FG-2216 [60] and FG-4497. [61] By inhibiting prolyl-hydroxylase enzyme, the stability of HIF-2α in the kidney is increased, which results in an increase in endogenous production of erythropoietin. [62] Both FibroGen compounds made it through to Phase II clinical trials, but these were suspended temporarily in May 2007 following the death of a trial participant taking FG-2216 from fulminant hepatitis (liver failure), however it is unclear whether this death was actually caused by FG-2216. The hold on further testing of FG-4592 was lifted in early 2008, after the FDA reviewed and approved a thorough response from FibroGen. [63] Roxadustat, vadadustat, daprodustat and molidustat have now all progressed through to Phase III clinical trials for treatment of renal anemia. [47] [48] [49]

Inflammation and cancer Edit

In other scenarios and in contrast to the therapy outlined above, recent research suggests that HIF induction in normoxia is likely to have serious consequences in disease settings with a chronic inflammatory component. [64] [65] [66] It has also been shown that chronic inflammation is self-perpetuating and that it distorts the microenvironment as a result of aberrantly active transcription factors. As a consequence, alterations in growth factor, chemokine, cytokine, and ROS balance occur within the cellular milieu that in turn provide the axis of growth and survival needed for de novo development of cancer and metastasis. These results have numerous implications for a number of pathologies where NF-κB and HIF-1 are deregulated, including rheumatoid arthritis and cancer. [67] [68] [69] [70] Therefore, it is thought that understanding the cross-talk between these two key transcription factors, NF-κB and HIF, will greatly enhance the process of drug development. [27] [71]

HIF activity is involved in angiogenesis required for cancer tumor growth, so HIF inhibitors such as phenethyl isothiocyanate and Acriflavine [72] are (since 2006) under investigation for anti-cancer effects. [73] [74] [75]

Neurology Edit

Research conducted on mice suggests that stabilizing HIF using an HIF prolyl-hydroxylase inhibitor enhances hippocampal memory, likely by increasing erythropoietin expression. [76] HIF pathway activators such as ML-228 may have neuroprotective effects and are of interest as potential treatments for stroke and spinal cord injury. [77] [78]

Von Hippel–Lindau disease-associated renal cell carcinoma Edit

Belzutifan is an hypoxia-inducible factor-2α [79] under investigation for the treatment of von Hippel–Lindau disease-associated renal cell carcinoma. [80] [81] [82] [83]


Vascular diseases have been implicated as a major cause of global mortality [1]. Vascular reconstruction remains a foremost clinical challenge for patients undergoing cardiovascular surgery [2]. Although autografts can be used to replace obstructed vessels, they possess several limitations, such as limited availability, risk of donor site–associated infection, and mismatched length or diameter [3]. Consequently, there is a newfound interest for developing artificial vascular grafts. Artificial vascular grafts, including poly(ethylene terephthalate) (PET, Dacron), expanded poly(tetraflouroethylene) (ePTFE) and polyurethane (PU) have shown satisfactory performance for the reconstruction of large-diameter blood vessels (≥6 mm) [4]. However, these vascular grafts show several bottlenecks for the grafting of small diameter (<6 mm) owing to thrombosis and severe neointimal hyperplasia [5]. After implantation, functional vascular regeneration involving rapid endothelialization [6], regeneration of contractile SMCs [5], and modulation of the inflammatory reaction [5] is the ideal outcome to address the aforementioned problems and achieve long-term patency.

The cellular hypoxia response holds potential to improve blood vessel formation [7] and tissue regeneration [8], which are processes predominantly regulated by hypoxia inducible factor-1 (HIF-1) [9]. HIF-1 is a heterodimeric protein comprised of two subunits, HIF-1α and HIF-1β [9]. HIF-1β is constitutively expressed inside the nucleus, whereas HIF-1α activity is oxygen-dependent [10]. Under normoxia, HIF-1α protein undergoes proteasomal degradation by prolyl-hydroxylases (PHDs), which require Fe 2+ , 2-oxoglutarate, and oxygen to hydroxylate proline residues on HIF-1α [11]. During hypoxia, the activity of PHDs is impeded, leading to the stabilization and accumulation of HIF-1α, which translocates to the nucleus and binds to HIF-1β. The combined HIF-1 complex activates transcription of a multitude of genes involved in the cellular hypoxia response, including genes encoding the cytokines, vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF) [9], and stromal cell derived factor 1 (SDF-1) [12], which play prominent roles in the promotion of neovascularization and tissue regeneration.

Dimethyloxalylglycine (DMOG) is a competitive inhibitor of PHDs, which can effectively induce hypoxia-mimicking responses through the stabilization of HIF-1α expression under normoxic conditions [13]. Intraperitoneal or intravenous injection of DMOG provided neuroprotection after traumatic brain injury [14]. However, systemic administration of DMOG was suggested to confer potential side effects, such as elevating hematocrit, erythropoietin (EPO) level and red blood cell counts and increasing cardiac purinergic signaling [15]. Therefore, the DMOG functionalized scaffolds with the ability to locally release DMOG have been proposed to offer attractive options for the development of regenerative materials for tissue engineering applications. For example, Wu et al. demonstrated that DMOG incorporation into mesoporous bioactive glass (MBG) scaffolds improved the angiogenic capacity of human bone mesenchymal stem cells (hBMSCs), but also enhanced their osteogenic differentiation [16]. Ren et al. reported that aligned porous poly( l -lactic acid) (PLLA) electrospun fibrous membranes containing DMOG-loaded mesoporous silica nanospheres augmented capillary formation and wound healing in diabetic rats [17].

Arteries are at the interface with arterial blood, which possesses a much higher oxygen fraction compared to bone, brain, liver, skin, and other major organs [18]. The arterial wall is supplied with oxygen from two sources: the first is the process whereby oxygen is transported from arterial blood into the intima and avascular artery walls in close proximity to the intima the second is the process whereby oxygen is obtained from the vasa vasorum, which is located in the adventitia and the outer part of the media [19]. However, the oxygen access state of artificial vascular grafts implanted into arterial system and the effect of hypoxia-mimicking on vascular regeneration remains unclear. This unknown led to the rationale of the current investigation: we hypothesized that inducing a hypoxia-mimicking response, in terms of regulating HIF-1α stability, could enhance vascular regeneration of artificial vascular grafts during the critical early stage of implantation.

Electrospinning has been widely used to fabricate fibrous vascular grafts due to advantageous properties that support cell adhesion and proliferation [20]. The Walpoth group [21] confirmed that common electrospun PCL grafts provided better endothelial coverage, the induction of extracellular matrix (ECM) deposition, and neointima formation. Thus, common electrospun PCL grafts outperformed ePTFE grafts in these areas, after implantation into rat abdominal aorta for 6 months [21]. Subsequent research [22] showed that common electrospun PCL grafts maintained excellent structural integrity and patency after 18 months, in the same rat model. However, at 12 and 18 months, regression of cell number, capillary density and severe calcification were observed within the graft wall. These outcomes were likely due to the bio-inertness and dense fibrous structure of common PCL grafts. The dense fibrous structure of common electrospun grafts often result in small pore size, which negatively impacts cell migration into the graft walls [20]. Cellularization of vascular grafts is a key factor dictating the success or failure of tissue regeneration and remodeling [23]. Our previous study found that by increasing the fiber diameter of PCL fibers, the average pore size of electrospun PCL grafts could also be increased [23]. This in-turn improved cell infiltration and vascular regeneration [23]. We determined that macroporous electrospun PCL grafts (henceforth referred to PCL grafts) would serve as appropriate artificial grafts to use in the observation of hypoxia-mimicking effects on vascular regeneration, particularly the cellular and tissue regenerative outcomes within graft walls.

Recent advancements in electrospinning has enabled direct encapsulation of different types of drugs and bioactive factors to achieve local and sustained release to promote tissue regeneration [24]. Therefore, in this study, we first evaluated the oxygen access state of electrospun PCL grafts using Hypoxyprobe™-1 Kits in rat abdominal aorta replacement model. Then, we fabricated DMOG-loaded PCL vascular grafts by electrospinning. The DMOG release kinetics, blood compatibility, morphological and mechanical properties of DMOG-loaded PCL grafts were characterized. We assessed in vitro the effect of DMOG-loaded scaffolds on regulating the proliferation, migration, nitric oxide (NO) production, and vascular endothelial growth factor (VEGF) secretion of HUVECs and the effects on RAW264.7 macrophage polarization by stabilization of HIF-1α protein. The effects of mimicking hypoxic responses on improved vascular regeneration and regulation of the inflammatory response were then investigated by implanting DMOG-loaded PCL grafts into rat abdominal arteries.

A5. Oxidative Modification of Lipids:

The initial stages of cardiovascular disease appear to involve the development of fatty acid streaks under the artery walls. Macrophages, an immune cell, have receptors which appear to recognize oxidized lipoproteins in the blood, which they take-up. The cells then become fat-containing foam cells which form the streaks. Oxidation of fatty acids in lipoproteins (possibly by ozone) could produce lipid peroxides and to protein oxidation in lipoproteins. Cortical neurons from fetal Down's Syndrome patients show 3-4 times levels of intracellular reactive O2 species and increased levels of lipid peroxidation compared to control neurons. This damage is prevented by treatment of the neurons in culture with free radical scavengers or catalase.

A recent study of peroxiredoxins by Neumann et al showed the importance of these gene products in mice. Peroxidredoxins (which catalyze the conversion of a peroxides and thioredoxin into water and oxidized thioredoxin) are small proteins with an active site cysteine and are found in most organisms. Transcription of the mammalian peroxiredoxin 1 gene is activated by oxidative stress. They inactivated the gene which produced a mouse that could reproduce and appeared vital, but which had a shortened lifespan. These mice developed severe hemolytic anemia and several types of cancers. High levels of reactive oxygen species and resulting increased levels of oxidized proteins were found in red blood cells of the knockout mice with anemia. High levels of 8-oxoguanine, resulting from oxidative damage to DNA, were found in tumor cells.

Abstract A10: Correlating magnetic resonance and molecular imaging using three dimensional untreated virtual control

With mean survival from glioblastoma multiforme (GBM) barely edging over 15 months, it become imperative to develop novel tools informing on both progression and treatment efficacy. As the aggressive growth of GBM outstrips available resources, regions of hypoxia develop. Hypoxia can be clinically assessed through [18F]-flouromisonidazole (FMISO) PET. However, PET imaging suffers from low resolution and reconstruction artifact. We present the comparative and predictive results of a multi-stage model utilizing the mathematical biology of cancer and the physics of PET imaging.

Our mathematical model of GBM characterizes cancer cells into the interacting phenotypes of normoxic, hypoxic, and necrotic cells supported by vasculature. The pharmacokinetic activity of FMISO is induced in the simulated tissue then acquired and reconstructed by an analytic simulator of the PET process. This multi-stage process creates a patient-specific virtual PET images with characteristic of the clinical PET scan. Extending previous work, we model the full three-dimensional dynamics of tumor progression, pharmacokinetic activity, and FMISO PET for the tumor kinetics of six GBM patients. Virtual and clinical FMISO PET images are compared.

Overall hypoxic burden and spatial distribution show strong correspondence between virtual and clinical FMISO PET images for all six patients.

Simulated FMISO PET dynamics, with tumor kinetics derived from routine clinical MRI scans, provide a unique and evolving tool giving insight into the biological connections between magnetic resonance and molecular imaging.

Citation Format: Joshua Jacobs, Andrea Hawkins-Darrud, Robert Harrison, Sandra Johnston, Paul Kinahan, Kristin Swanson. Correlating magnetic resonance and molecular imaging using three dimensional untreated virtual control. [abstract]. In: Proceedings of the AACR Special Conference on Engineering and Physical Sciences in Oncology 2016 Jun 25-28 Boston, MA. Philadelphia (PA): AACR Cancer Res 201777(2 Suppl):Abstract nr A10.

PTHrP as a Therapeutic Target

Numerous studies have provided convincing evidence that PTHrP promotes tumor progression, and late recurrence by pushing tumor cells out of dormancy, resulting in poor patient survival. Thus, PTHrP would seem to be a promising therapeutic target for treating advanced human cancers. Several animal studies have demonstrated reduced distant metastasis to bone with PTHrP small molecule inhibitors (159) and neutralizing antibodies (68, 160, 161) however, human clinical data are lacking. Furthermore, there are several limitations in our current understanding of the biological activity of PTHrP that greatly complicate the development of safe and efficacious anti-PTHrP therapies at this time. PTHrP is an incredibly complex peptide with multiple distinct domains that can each influence its actions as an endocrine, paracrine, autocrine and intracrine signaling molecule. This coupled with the fact that its different isoforms and fragments can elicit diverse cellular responses could result in PTHrP targeting therapies that inadvertently promote tumor growth and recurrence if used in the wrong patient population or stage of disease progression. This is especially true in breast cancer, where preclinical and clinical data suggest that PTHrP inhibits early tumor progression, but promotes distant metastasis in advanced stages of disease (162). Studies fully defining PTHrP's role in different stages of cancer and in tumor dormancy are needed in order to identify the appropriate therapeutic window for targeting PTHrP.

In addition to direct PTHrP inhibition, alternative approaches including targeting upstream regulators of the peptide's expression have been explored. As discussed previously (20), Wnt signaling drives PTHrP expression in highly osteolytic cancer cells and thus presents a potential therapy to prevent tumor-induced bone destruction and metastatic outgrowth. However, there are challenges to targeting Wnt therapeutically due to deleterious off-target effects since signaling is critical during normal development and tissue homoestasis, especially bone formation (163�). However, the anti-tumor activity of Wnt inhibitors has been investigated and shown varying efficacy, primarily in preclinical gastrointestinal cancer models (166, 167). There are also numerous ongoing clinical trials investigating inhibitors of the Wnt pathway in multiple other solid tumor types [NCT01351103, NCT03901950, NCT02675946, NCT03447470, NCT03395080]. In recent years, more cancer cell-specific molecular targets such as vacuolar-ATPase (v-ATPase) have been explored in the development of Wnt signaling inhibitors (168, 169). Bafilomycin and concanamycin, which directly bind to and inhibit v-ATPase, markedly inhibit Wnt/β-catenin signaling in colorectal cancer cells in vitro and reduce tumor cell proliferation in vivo without significant toxicity (168). Selective inhibitors of Porcupine (PORCN), an acyltransferase that catalyzes post-translational modification and activation of WNT ligands, have also shown promising anti-tumor activity in vivo, while sparing WNT-dependent tissues (170, 171). While inhibiting the Wnt pathway may be an effective therapy to decrease PTHrP expression for the treatment of metastatic cancers, more extensive investigation is needed to identify the most selective inhibitors and safest therapeutic window.

Alternative upstream targets include TGF-β which upregulates expression of Gli2 and in turn increases tumor secretion of PTHrP (172, 173). Gli2 repression significantly reduces tumor-induced bone destruction mediated by TGF-β signaling in human breast cancer MDA-MB-231 cells (172). Inhibitors against TGF-β and GLI proteins have been evaluated in clinical trials as anti-cancer therapy (174) []. Another study demonstrated that the EGF receptor promotes PTHrP production, since treatment with erlotinib, an EGF receptor tyrosine kinase inhibitor, suppresses PTHrP expression in non-small cell lung cancer cells and reduces osteolysis (175). Other EGF receptor tyrosine kinase inhibitors including gefitinib also reduce PTHrP levels (176). Lastly, targeting downstream effectors of PTHrP may also provide an efficacious strategy. For instance, as mentioned previously, PTHrP (1�) overexpression in MCF7 cells also represses expression and downstream signaling of LIFR, a known breast tumor suppressor and dormancy factor in the bone (28). Consequently, LIFR downregulation promotes human MCF7 breast cancer cell emergence from dormancy in the bone. Treatment with the histone deacetylase inhibitor valproic acid subsequently increases LIFR expression in human MCF7 breast cancer cells in vitro, suggesting that targeting LIFR, a downstream factor in PTHrP signaling, may effectively maintain tumor cells in a dormant state to prevent metastatic outgrowth. Multiple strategies should therefore be considered in order to develop the most selective and effective PTHrP targeting therapies.

Hypoxia-inducible factor signaling in pulmonary hypertension

1 Department of Lung Development and Remodeling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), member of the Cardio-Pulmonary Institute (CPI), Bad Nauheim, Germany.

2 Department of Internal Medicine, Universities of Giessen and Marburg Lung Center, member of the DZL and CPI, Justus Liebig University, Giessen, Germany.

3 Institute for Lung Health (ILH), Justus Liebig University, Giessen, Germany.

4 Frankfurt Cancer Institute (FCI), Goethe University, Frankfurt am Main, Germany

Address correspondence to: Soni Savai Pullamsetti, Max Planck Institute for Heart and Lung Research, Parkstraße 1, 61231 Bad Nauheim, Germany. Phone: 49.6032.705.380 Email: [email protected]

Find articles by Pullamsetti, S. in: JCI | PubMed | Google Scholar | />

1 Department of Lung Development and Remodeling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), member of the Cardio-Pulmonary Institute (CPI), Bad Nauheim, Germany.

2 Department of Internal Medicine, Universities of Giessen and Marburg Lung Center, member of the DZL and CPI, Justus Liebig University, Giessen, Germany.

3 Institute for Lung Health (ILH), Justus Liebig University, Giessen, Germany.

4 Frankfurt Cancer Institute (FCI), Goethe University, Frankfurt am Main, Germany

Address correspondence to: Soni Savai Pullamsetti, Max Planck Institute for Heart and Lung Research, Parkstraße 1, 61231 Bad Nauheim, Germany. Phone: 49.6032.705.380 Email: [email protected]

Find articles by Mamazhakypov, A. in: JCI | PubMed | Google Scholar | />

1 Department of Lung Development and Remodeling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), member of the Cardio-Pulmonary Institute (CPI), Bad Nauheim, Germany.

2 Department of Internal Medicine, Universities of Giessen and Marburg Lung Center, member of the DZL and CPI, Justus Liebig University, Giessen, Germany.

3 Institute for Lung Health (ILH), Justus Liebig University, Giessen, Germany.

4 Frankfurt Cancer Institute (FCI), Goethe University, Frankfurt am Main, Germany

Address correspondence to: Soni Savai Pullamsetti, Max Planck Institute for Heart and Lung Research, Parkstraße 1, 61231 Bad Nauheim, Germany. Phone: 49.6032.705.380 Email: [email protected]

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1 Department of Lung Development and Remodeling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), member of the Cardio-Pulmonary Institute (CPI), Bad Nauheim, Germany.

2 Department of Internal Medicine, Universities of Giessen and Marburg Lung Center, member of the DZL and CPI, Justus Liebig University, Giessen, Germany.

3 Institute for Lung Health (ILH), Justus Liebig University, Giessen, Germany.

4 Frankfurt Cancer Institute (FCI), Goethe University, Frankfurt am Main, Germany

Address correspondence to: Soni Savai Pullamsetti, Max Planck Institute for Heart and Lung Research, Parkstraße 1, 61231 Bad Nauheim, Germany. Phone: 49.6032.705.380 Email: [email protected]

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1 Department of Lung Development and Remodeling, Max Planck Institute for Heart and Lung Research, member of the German Center for Lung Research (DZL), member of the Cardio-Pulmonary Institute (CPI), Bad Nauheim, Germany.

2 Department of Internal Medicine, Universities of Giessen and Marburg Lung Center, member of the DZL and CPI, Justus Liebig University, Giessen, Germany.

3 Institute for Lung Health (ILH), Justus Liebig University, Giessen, Germany.

4 Frankfurt Cancer Institute (FCI), Goethe University, Frankfurt am Main, Germany

Address correspondence to: Soni Savai Pullamsetti, Max Planck Institute for Heart and Lung Research, Parkstraße 1, 61231 Bad Nauheim, Germany. Phone: 49.6032.705.380 Email: [email protected]

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Published September 3, 2020 - More info

Pulmonary hypertension (PH) is characterized by pulmonary artery remodeling that can subsequently culminate in right heart failure and premature death. Emerging evidence suggests that hypoxia-inducible factor (HIF) signaling plays a fundamental and pivotal role in the pathogenesis of PH. This Review summarizes the regulation of HIF isoforms and their impact in various PH subtypes, as well as the elaborate conditional and cell-specific knockout mouse studies that brought the role of this pathway to light. We also discuss the current preclinical status of pan- and isoform-selective HIF inhibitors, and propose new research areas that may facilitate HIF isoform-specific inhibition as a novel therapeutic strategy for PH and right heart failure.

The transcription factor hypoxia-inducible factor (HIF) is a master regulator of oxygen homeostasis that acts as a heterodimeric complex composed of the oxygen-sensitive α subunit (HIF-α including HIF-1α, HIF-2α [EPAS1], and HIF-3α) and the oxygen-insensitive β subunit (HIF-β including HIF-1β [aryl hydrocarbon receptor nuclear translocator, ARNT1], ARNT2, and ARNT3) ( 1 ). In oxygenized cells, the HIF-α subunit is inactivated via hydroxylation by prolyl hydroxylase domain proteins (PHDs) and factor inhibiting HIF (FIH), which allows the binding of von Hippel–Lindau (VHL) tumor suppressor protein, a component of an E3 ubiquitin ligase complex that subsequently targets hydroxylated HIF-α for proteasomal degradation. Under hypoxic conditions, oxygen becomes limited, leading to the attenuation of HIF-α hydroxylation and resulting in stabilization of HIF-α subunits. This initiates nuclear translocation and binding of the HIF-α subunit with the HIF-β subunit, and this activated HIF initiates an adaptive response to hypoxia by inducing or repressing a broad range of genes involved in regulation of vascular tone, angiogenesis, erythropoiesis, cellular metabolism, proliferation, survival, and autophagic response ( 1 ). However, nonhypoxic conditions, i.e., growth factors, hormones, or cytokines, also modulate HIF-α subunits at various levels (gene transcription, mRNA processing, protein-protein interactions, and posttranslational modifications for a detailed description see refs. 1 , 2 ) and regulate a plethora of signaling pathways. In the lung, HIFs orchestrate a physiological response to hypoxia and contribute to the pathogenesis of numerous disorders, including lung cancer, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis (PF), and pulmonary hypertension.

Pulmonary hypertension (PH) is a severe pulmonary vascular disorder characterized by excessive proliferation of vascular cells, increased extracellular matrix deposition, and accumulation of inflammatory cells within the pulmonary vascular wall, collectively resulting in increased pulmonary vascular resistance ( 3 ). Despite extensive research in this field, the mechanisms underlying disease development and progression are incompletely understood ( 4 ). Among many dysregulated signaling pathways, HIF signaling has been identified as one underlying mechanism determining disease progression not only in pulmonary arterial hypertension (PAH group I PH), but also in PH due to lung diseases and/or hypoxia, including PH associated with chronic high altitude exposure ( 5 , 6 ), COPD ( 7 ), and PF ( 8 ) (group III PH). Notably, augmented expression of HIF-1α has been observed in lung tissues of patients with PAH ( 9 – 13 ), chronic thromboembolic PH ( 14 ), and idiopathic PF-associated PH ( 15 , 16 ), while HIF-2α has been associated with congenital diaphragmatic hernia–associated PH ( 17 ). In addition, neonatal patients with acute respiratory disease–associated PH also display increased circulating levels of HIF-1α ( 18 ). Moreover, HIF-1α and its target genes vascular endothelial growth factor (VEGF) and erythropoietin (EPO) are upregulated in peripheral blood cells of newborns with cyanosis and persistent PH, therefore representing early markers of generalized hypoxia ( 19 ). Similarly, increases in circulating bone marrow–derived progenitor cells observed in PAH patients are regulated by HIF-1α–driven C-X-C motif chemokine ligand 12 (CXCL12) expression in pulmonary artery endothelial cells (PAECs) ( 20 ). The cellular sources of increased HIF-1α expression in lung tissue of PH patients are PAECs ( 16 , 21 ) and pulmonary artery smooth muscle cells (PASMCs) ( 22 , 23 ). While some reports show HIF-1α upregulation in the pulmonary arteries of PAH patients ( 24 ), others provide evidence of decreased HIF-1α in PASMCs isolated from idiopathic PAH (IPAH) patients ( 25 , 26 ). PAH patients also display increased pulmonary expression of HIF-1β ( 11 ), and similarly, the expression of HIF-2α has been found to be increased in pulmonary arteries of patients with PAH ( 24 , 27 ) and IPF-associated PH ( 16 ). The cellular sources of increased HIF-2α expression in the lung tissue of PH patients are mainly PAECs ( 16 , 27 , 28 ), suggesting cell type– and context-specific regulation of HIF isoforms in PH.

Activation of HIFs in various subtypes of PH suggests that along with chronic hypoxia, other factors responsible for the initiation of PH (gene variants, vasoconstriction, endothelial dysfunction, mitochondrial abnormalities, dysregulated cell growth, and inflammation) can activate HIF signaling pathways to trigger alterations in pulmonary vascular cells, inflammatory cells, and cardiac cells that remodel lung vasculature and right ventricle (RV) (Figure 1).

Emerging concepts of HIF signaling in pulmonary hypertension. Emerging evidence shows that many pro-PH factors apart from hypoxia, such as inflammation, mechanical stretch, oxidative stress, and genetic predisposition, converge on HIF signaling pathways, causing alterations in vascular tone, angiogenesis, metabolism, and cell survival that subsequently lead to pulmonary vascular and right ventricular remodeling. VHL, von Hippel–Lindau tumor suppressor αAR, α1β-adrenergic receptor iNOS, inducible nitric oxide synthase HO-1, heme oxygenase-1 TRPC1, transient receptor potential canonical 1 KCNA5, potassium voltage-gated channel, shaker-related subfamily, member 5 KCNMB1, calcium-activated potassium channel subunit beta-1 PAI-1, plasminogen activator inhibitor-1 TF, transferrin TFR, transferrin receptor EPO, erythropoietin PDK1, pyruvate dehydrogenase kinase 1 IGF2BP1, insulin-like growth factor 2 mRNA-binding protein 1 p21, cyclin-dependent kinase inhibitor 1 CCNG2, cyclin-G2 DEC1, deleted in esophageal cancer 1.

Gene variants of the HIF pathway. Gene variants of HIF pathway molecules identified in high-altitude populations and in patients with Chuvash polycythemia illustrate the HIF pathway’s importance in pulmonary vascular adaptation and remodeling. Chuvash polycythemia is characterized by the presence of the R200W (598C>T) missense mutation in VHL, which reduces its binding to hydroxylated HIF-α subunits and thus increases HIF-1α and HIF-2α levels ( 29 ). This leads to expression of HIF target genes including EPO and VEGF and results in development of polycythemia. In Chuvash polycythemia, apart from erythrocytosis, several VHL loss-of-function mutations, including D126N (376G>A) ( 30 ), D126N (376G>A)/S183L (548C>T) ( 31 ), and M54I (162G>C), were found to be associated with higher resting pulmonary artery pressure (PAP), severe PH, and RV dysfunction ( 32 – 35 ). Moreover, mutation in HIF-2α G537R, which impairs HIF-2α hydroxylation, causing familial erythrocytosis, is also associated with PH ( 36 ).

Several genome-wide selection studies have been performed on high-altitude populations, including Tibetans, Ethiopians, and Andeans ( 5 , 6 ), and identified signals of positive selection for gene variants in and around the HIF pathway enabling these populations to adapt to life at high-altitude hypoxia. However, long-term high-altitude residency may lead to a sustained increase in PAP and development of PH. Among all high-altitude populations, Tibetans have the lowest PAP ( 37 ). A candidate gene study based on the results of genome-wide analyses that identified gene variants associated with high-altitude adaptation found that EPAS1 (HIF-2α) variants are associated with lower PAP in Tibetans ( 38 ). Furthermore, Tibetans who live at low altitudes but harbor gene variants in EPAS1 (encoding HIF-2α) and EGLN1 (encoding PHD2) display blunted hypoxic pulmonary vasoconstriction ( 39 ). Thus, future studies are required to delineate the role of HIF pathway gene variants in driving high-altitude PH susceptibility or resistance among indigenous high-altitude populations.

Vasomodulatory factors. Various vasomodulatory factors have been shown to regulate HIF isoform stability and transcriptional activity in PAECs (Figure 2 and Table 1). Nitric oxide (NO) maintains pulmonary vascular tone, and its downregulation is implicated in PH pathogenesis. Recent studies indicate that the NO plays a central role in hypoxia/HIF axis regulation. For example, in PAECs, hypoxia leads to post-transcriptional negative regulation of endothelial NO synthase (eNOS) expression by the cis–natural antisense RNA sONE ( 40 ), which results in lower levels of NO and increased HIF-1α stability. Accordingly, low levels of NO due to eNOS deletion also cause HIF-1α stabilization and migration of normoxic endothelial cells (ECs) ( 41 ). Moreover, endogenous regulators of NO production such as arginase-2 and asymmetric dimethyl arginine affect cell proliferation and inflammatory gene expression by stabilizing HIF-1α in PAECs ( 42 ). Endothelin 1 (ET-1), a potent vasoconstrictor, also stabilizes HIF-1α, which in turn promotes the HIF-1α–induced glycolytic switch via eNOS-mediated reactive oxygen species (ROS) production in PAECs ( 43 ). Likewise, stimulation of normoxic PASMCs with ET-1 increases stability of HIF-1α as a result of increased Ca 2+ and ROS and increases transcriptional activity of HIF-1α due to ERK1/2 pathway activation, which phosphorylates p300 to increase its binding to HIF-1α ( 44 ). Moreover, ET-1 promotes HIF-1α protein stabilization in normoxic PASMCs via calcineurin-dependent RACK1 dephosphorylation, which in turn inhibits PHD2 activity ( 45 ). Importantly, HIF has also been shown to regulate ET-1 synthesis. Mice with global deficiency for NEPAS, a transcript variant of HIF-3α, exhibit ET-1 overexpression, which leads to pulmonary vascular remodeling and dilated cardiomyopathy due to excessive tissue vascularization that is evident from birth and progresses in later stages of life ( 46 ). Thus, HIF and ET-1 form a bidirectional regulatory loop that plays an important role in driving pulmonary vascular remodeling.

HIF signaling in PH: upstream and downstream modulators in pulmonary artery endothelial cells. Vasomodulatory, mitochondrial, and inflammatory growth factors and epigenetic abnormalities associated with PH regulate HIF isoform stability and transcriptional activity in pulmonary artery endothelial cells (PAECs). Subsequently, HIF isoforms transcriptionally activate a series of genes that participate in vascular tone, angiogenesis, metabolism, and cell proliferation. Long black lines with arrows indicate an activating effect blocked red lines, an inhibiting effect ↑, activation or upregulation ↓, inactivation or downregulation. TFAM, mitochondrial transcription factor A PlGF, placental growth factor EPOR, erythropoietin receptor ETA/B, endothelin receptor type A and B sONE, antisense mRNA ADMA, asymmetric dimethylarginine Cul2, Cullin 2 ATOH8, atonal BHLH transcription factor 8 ISCU1/2, iron-sulfur (Fe-S) cluster assembly proteins 1 and 2 PGK1, phosphoglycerate kinase 1 PKM, pyruvate kinase M.

Summary of studies investigating upstream regulators of HIF signaling pathway in PH

Inflammation, growth factors, and microRNAs. Hypoxia induces upregulation of IL-33 and its receptor ST2 in PAECs, which activates downstream HIF-1α/VEGF signaling resulting in enhanced proliferation, adhesion, and angiogenesis in an ST2-dependent fashion ( 47 ). Similarly, hormones and growth factors such as bone morphogenetic protein (BMP), placental growth factor (PlGF), platelet-derived growth factor (PDGF), EPO, estradiol, and signaling molecules regulate HIF isoform transcriptional activity in PAECs. For example, in patients with sickle cell disease–associated (SCD-associated) PH, elevated levels of PlGF result in downregulation of microRNA-199a2 (miR-199a2), a negative regulator of HIF-1α. Furthermore, PPARα agonist–mediated transcription of miR-199a2 attenuates ET-1 expression and HIF-1α level, ameliorating PH in a mouse model of SCD ( 48 – 50 ). Estradiol negatively regulates ET-1 expression in PAECs by interfering with HIF activity, possibly through competition for limiting quantities of CBP/p300 ( 51 ). Estradiol also negatively regulates HIF-2α by promoting its degradation by estrogen receptor β–mediated (ERβ-mediated) PHD2 upregulation in hypoxic PAECs ( 52 ). By contrast, the BMP signaling molecules SMAD1 and SMAD5 transcriptionally activate atonal bHLH transcription factor 8 (ATOH8) expression, which interacts with HIF-2α to reduce its abundance and expression of its target genes delta-like protein 4 (DLL4) and angiopoietin-2 (ANGPT2) in hypoxia-exposed PAECs ( 53 ). ATOH8-KO mice spontaneously develop PH, suggesting an important role for BMP signaling in regulating the HIF pathway. Moreover, HIF-1α–driven expression of miR-322/424 in human ECs attenuates HIF-1α degradation by causing post-transcriptional repression of cullin-2, an E3 ubiquitin ligase scaffolding protein ( 54 ).

However, with regard to PASMCs, cross-regulation between the adhesion molecule CD146 and HIF-1α via the NF-κB pathway in PASMCs has been shown to trigger pulmonary vascular remodeling. Disruption of the CD146/HIF-1α axis in PASMCs blunts vascular remodeling and produces a marked attenuation of PH ( 55 ). Furthermore, exaggerated proliferation in PASMCs occurs after exposure to growth factors, such as epidermal growth factor (EGF), FGF2, PDGF, or is mediated by HIF-1α but not HIF-2α activation ( 56 ), suggesting that HIF-1α acts downstream of these growth factors, which are well established as disease-driving factors in PH. PPARγ agonists exert antiproliferative effects on PASMCs via PPARγ-mediated inhibition of HIF-1α and its downstream genes such as PDK-1, TRPC1, and TRPC6 ( 57 , 58 ). However, hypoxia induces PPARγ downregulation via HIF-1α in PASMCs ( 57 , 58 ), suggesting a negative feedback loop mechanism between PPARγ and HIF-1α. Among other regulators, hypoxia-induced downregulation of miR-206 ( 59 ) and miR-150 ( 60 ) promotes a pro-proliferative and promigratory phenotype of PASMCs by targeting HIF-1α. Furthermore, in rat PASMCs, upregulation of molecules related to both SUMOylation (SUMO-1) and deSUMOylation (SENP1) leads to increased HIF-1α stability and transcriptional activity, thus increasing proliferation ( 61 , 62 ).

Mitochondrial abnormalities. The interplay between mitochondrial abnormalities, NADPH oxidases (NOXs), and ROS has been established as an important activator of HIF-1α in pulmonary vascular cells in PH ( 63 – 65 ). For example, mitochondrial abnormalities that shift metabolism away from oxidative phosphorylation toward glycolysis (notably pyruvate dehydrogenase kinase [PDK] activation) lead to a normoxic impairment of electron flux and reduced mitochondrial ROS production ( 66 ). This pseudohypoxic signal is associated with nuclear translocation of HIF-1α. On the contrary, reports suggest that low levels of superoxide dismutase 2 (SOD2), as observed in PAECs from patients with IPAH, promote HIF-1α stabilization due to increased ROS levels ( 21 ). Mechanical stretch imposed on PASMCs due to pulmonary hemodynamic stress causes mitochondrial complex III–mediated ROS formation, which both induces the NF-κB pathway and inhibits Phd2, leading to HIF-1α activation ( 67 ), indicating that hemodynamic stress itself serves as an independent regulator of HIF-1α. Indeed, several molecules that are upstream regulators of ROS are implicated in HIF-1α activation. For instance, loss of sirtuin 3, a crucial regulator of mitochondrial function in PASMCs, causes mitochondrial dysfunction leading to ROS production and HIF-1α stabilization ( 68 ). As a consequence, sirtuin 3–KO mice develop spontaneous PH and RV hypertrophy ( 68 ). Recently, we identified a molecular mechanism in which a scaffold protein, Ras association domain family 1A (RASSF1A), acts as a crucial regulator of HIF-1α signaling in PASMCs. Upon hypoxia, HIF-1α upregulates RASSF1A expression, and RASSF1A is stabilized by ROS-driven and protein kinase C–mediated (PKC-mediated) phosphorylation. RASSF1A in turn stabilizes HIF-1α, leading to increased HIF-1 transcriptional activity ( 69 ). This crucial RASSF1A–HIF-1α feed-forward loop determines pro-proliferative and glycolytic switch of PASMCs and pulmonary artery adventitial fibroblasts (PAAFs) ( 69 ). Disruption of RASSF1A/HIF-1α crosstalk by genetic ablation of RASSF1A mitigates pulmonary vascular remodeling in mice exposed to chronic hypoxia.

Under extended exposure to reduced oxygen levels in pulmonary vascular cells, HIF isoforms transcriptionally activate a series of genes (Figures 2 and 3) that regulate vascular tone, angiogenesis, metabolism, and proliferation. Moreover, recent studies highlighted the potential role of HIFs and the underlying molecular mechanisms in the dysregulation of the innate and adaptive immune system in PH.

HIF signaling in PH: Upstream and downstream modulators in pulmonary artery smooth muscle cells. Signaling pathways associated with PH such as hypoxia, vasomodulation, growth factors, mechanical stress, and oxidative stress pathways regulate HIF isoform stability and transcriptional activity in PASMCs. This regulates genes related to cell proliferation and synthetic phenotypes, as well as genes related to Ca2+ modulation/ion channels, oxidative stress, mitochondrial fragmentation, and the renin-angiotensin-aldosterone system (RAAS) system. Long black lines with arrows indicate an activating effect blocked red lines, an inhibiting effect ↑, activation or upregulation ↓, inactivation or downregulation. C-III, mitochondrial complex III SIRT3, Sirtuin 3 TRPC6, transient receptor potential cation channel subfamily C member 1 or 6 FGFR, fibroblast growth factor receptor Ang-I, angiotensin I Ang-II, angiotensin II Ang-(1-7), angiotensin (1-7) Mas, Ang-(1-7) receptor ATR1/2, angiotensin receptor type 1 and 2 ACE, angiotensin converting enzyme PIP2, phosphatidylinositol 4,5-bisphosphate IP3, inositol trisphosphate DAG, diacylglycerol O2-, superoxide anion PKCα, protein kinase C alpha PAK1, P21 activated kinase 1 SENP-1, sentrin-specific protease 1.

PAECs. PAECs exhibit different phenotypes (proliferative, migratory, angiogenic, and/or endothelial-mesenchymal transition [EndoMT]) during PH pathogenesis, and HIF isoforms play a decisive role in defining these phenotypes. For example, HIF-1 induces cyclin-dependent kinase inhibitor 1B (p27 Kip1 ) upregulation and cyclin D1 downregulation, leading to decreased proliferation and migration of hypoxic PAECs ( 70 ). By contrast, HIF-2–driven octamer-binding transcription factor 4 (OCT4) expression via miR-130/131–mediated downregulation of PPARγ/apelin signaling results in increased proliferation of PAECs ( 71 ). In addition, both HIF-1 and HIF-2 in PAECs contribute to altered metabolic phenotype by modulating the expression of distinct mitochondrial enzymes such as pyruvate dehydrogenase kinase 1 (PDK1), hexokinase 1,2 (HK1,2), lactate dehydrogenase A (LDHA), and glucose transporter 1,3 (GLUT1,3) to regulate anaerobic glycolysis and the Warburg effect (aerobic glycolysis). The influence of HIF-1 on glycolytic metabolism is well established ( 72 ) the Warburg effect in IPAH could possibly be driven by HIF-1α stabilization, independently of the hypoxic environment. On the other hand, HIF-2, but not HIF-1, by upregulating SNAI1 transcription factors, triggers EndoMT, a mechanism potentially involved in the development of occlusive intimal/neointimal lesions and severe pulmonary vascular wall thickening in IPAH ( 28 ). Furthermore, EC HIF-2 influences the development of hypoxic PH via an arginase-1–dependent mechanism. The HIF-2/arginase-1 axis dysregulates vascular NO homeostasis ( 73 , 74 ), resulting in PH in hypoxia-exposed mice ( 73 ). Consequently, EC arginase-1 loss attenuates PH in hypoxia-exposed mice ( 73 ), and arginase inhibition prevents PH in monocrotaline-injected (MCT-injected) rats ( 75 ). However, the expression of HIF-2–mediated angiopoietin-1 and -2 in ECs is shown to be essential to maintaining proper pulmonary vascular homeostasis ( 76 , 77 ). These data propose that HIF-1 and HIF-2 exert pathogenic roles in PH by regulating distinct cellular processes in PAECs.

PASMCs. In PASMCs, HIF isoforms regulate not only genes related to cell proliferation and synthetic phenotypes but also genes related to vasoconstriction (Ca 2+ modulation/ion channels), oxidative stress, mitochondrial fragmentation, and the renin-angiotensin-aldosterone system (Figure 3). Increased PASMC proliferation and the prosynthetic phenotypic switch observed in hypoxia are mediated by HIF-1–driven expression of miR-9-1 and miR-9-3, which negatively regulate myocardin (MYOCD) expression ( 78 ). Augmented proliferation of rat PASMCs is associated with inhibition of the BMP pathway as a result of HIF-1–induced, but not HIF-2–induced, miR-322, which causes posttranslational repression of Bmpr1a and Smad5 genes ( 79 ). HIF-1–dependent upregulation of miR-210 causes apoptosis resistance in PASMCs by targeting transcription factor E2F3 ( 80 ). By contrast, HIF-2 promotes hypoxia-responsive PASMC migration and contractility by upregulating thrombospondin-1 ( 81 ). Thus, multiple mechanisms contribute to the pro-proliferative, promigratory, and apoptosis resistance phenotypes of PASMCs. Furthermore, hypoxia-induced muscularization of nonmuscularized pulmonary arterioles involves preexisting smooth muscle cell (SMC) progenitor cells that undergo dedifferentiation, migration to the distal vessel, proliferation, and redifferentiation ( 82 ). Elegant studies by Sheikh et al. demonstrated that activation of these progenitor cells starts with HIF-1α–mediated PDGF-β expression ( 83 ), and progresses with expansion of these progenitor-derived SMCs via HIF-1α–mediated Krüppel-like factor 4 (KLF4) expression ( 84 ). These studies demonstrate a central role of HIF-1 in the initiation as well as progression of pulmonary artery muscularization in hypoxia-induced PH.

Changes in intracellular K + and intracellular Ca 2+ concentration ([Ca 2+ ]i) play a pivotal role in the regulation of contraction, migration, and proliferation of PASMCs ( 2 ). Notably, HIF-1 plays an essential role in modulating [Ca 2+ ]i levels in PASMCs by regulating the expression of various ion channels. HIF-1 promotes overexpression of the transient receptor potential (TRPC) channel members TRPC1 and TRPC6 and subsequently increases [Ca 2+ ]i in hypoxic PASMCs ( 85 ). In addition, HIF-1 via ET-1 represses voltage-gated K + channels members that subsequently also increase [Ca 2+ ]i ( 86 ). On the other hand, HIF-1 activates expression of the β1 subunit (KCNMB1) of the calcium-sensitive K + channel BKCa, which prevents an excessive rise in [Ca 2+ ]i in PASMCs ( 87 ).

PAAFs. Although both HIF-1α and HIF-2α are activated in PAAFs in response to hypoxia, HIF-2α induction appears to play the dominant role in the proliferation response, whereas both HIF-1 and HIF-2 increase PAAF migration to a similar extent ( 88 ). Furthermore, studies suggest that HIF-1 via the regulation of ACE and ACE2 (a homolog of ACE that counterbalances the function of ACE) directly participates in the regulation of the renin-angiotensin-aldosterone system and consequently PAAF proliferation ( 89 – 91 ).

Inflammatory cells. Immune cells play an essential role in pulmonary vascular remodeling by regulating the functions of pulmonary vascular cells ( 92 ). For example, hypoxic PAAFs drive profibrotic macrophage phenotypes under the control of HIF-1, resulting in the release of various paracrine factors. Importantly, macrophage-produced VEGF and IL-6 are shown to promote pulmonary vascular remodeling ( 12 ).

Considering the multitude of cellular and mechanistic roles of the HIF pathway, it is not surprising that knockout mouse models of the HIF pathway (Hif1/2α, VHL, and Phd2) provided valuable insights on the HIF pathway in the hypoxic adaptation of the pulmonary vasculature and the development of PH (Table 2). Earlier studies revealed that the genes of the HIF pathway are crucial for embryonic development and that biallelic deletion of the majority of those genes is lethal. For example, homozygous deletion of Phd2 in mice leads to embryonic lethality, although Phd1/3 double knockout leads to viable and fertile mice ( 93 ). Complete deletion of Hif1a ( 94 ), Hif2a ( 95 ), or Hif1b ( 96 ) in mice results in embryonic lethality due to various developmental defects. In contrast, mice with global heterozygous deletion of either Hif1a or Hif2a reach adulthood and do not display phenotypes in homeostatic conditions, making them useful to study the role of HIFs in disease. For example, mice with heterozygous deletion of Hif1a exhibit attenuated PH and RV hypertrophy upon hypoxia exposure ( 97 ). By contrast, mice with heterozygous deletion of Hif2a are completely protected from hypoxia-induced PH ( 98 ). Further, to evaluate how global deletion of HIF isoforms during adult life affects hypoxia-induced PH, Hu et al. showed that global Hif1a deletion in mice did not prevent hypoxia-induced PH, whereas mice with global Hif2a deletion did not survive long-term hypoxia ( 99 ). Conversely, global partial Hif2a deletion diminishes development of hypoxia-induced PH at 5 weeks in adult mice ( 99 ).

Summary of studies evaluating the phenotypes of genetic manipulation of HIF and HIF regulators in animal models of PH

Furthermore, to tease out the cell- and postnatal-specific role of HIF pathway components in PH pathogenesis, various studies have used constitutive or inducible cell-specific knockout mouse models. For example, Ball et al. reported that SMC-specific postnatal (inducible) deletion of Hif1a attenuated PH but did not affect RV hypertrophy ( 100 ). Meanwhile, in another study, mice with constitutive SMC-specific Hif1a deletion showed exacerbated hypoxia-induced PH ( 101 ). More recent studies demonstrated that mice with constitutive EC-specific Hif1a deletion are not protected from hypoxia-induced PH and RV hypertrophy ( 28 , 73 ). However, in another study, mice with inducible EC Hif1a deletion were protected from PH and RV hypertrophy under hypoxia ( 84 ). Interestingly, inducible Hif1a deletion in either ECs and SMCs did not prevent hypoxia-induced PH and RV hypertrophy ( 102 ). In contrast, postnatal deletion of (PDGFR-β + )/SMC marker + progenitors completely prevents PH and RV hypertrophy in hypoxia-exposed mice ( 84 ). With regard to HIF-2α, Skuli et al. reported that mice with EC-specific Hif2a deletion develop PH and RV dilatation (but not RV hypertrophy) due to vascular leakage into the lung parenchyma ( 103 ). Similarly, Tang et al. demonstrated that EC-specific deletion of Hif2a, but not Hif1a, prevents mice from developing PH under hypoxic conditions ( 28 ). Interestingly, simultaneous deletion of Hif1a and Hif2a in ECs also provides protection against bleomycin-induced PH and RV hypertrophy despite lung fibrosis development ( 16 ). These data suggest that the prominent role of Hif2a in ECs is critical in the initiation and progression of PH. However, results of EC-specific gene-deletion mouse models should be interpreted cautiously, since, depending on whether a Cre/ERT2 or Cre23 system was used, gene deletion may be exclusively EC-specific or target other cell types, respectively ( 104 ).

Notably, multiple groups have shown that mice with Phd2 deletion in ECs spontaneously develop severe PH associated with massive pulmonary vascular lesions and adverse RV remodeling that is evident from the age of 1.5 months ( 10 , 105 , 106 ). Concomitant deletion of both Phd2 and Hif1a or Phd2 and Hif2a in ECs identified HIF-2α activation as a critical downstream modulator of PHD2 deficiency in PH development ( 10 , 105 ). Interestingly, these mice show increased mortality within 6–9 months of age, presumably due to progressive RV failure ( 10 , 105 ). Supporting this role of HIF-2α activity in the effect of PHD2 deficiency, mice with both heterozygous and homozygous Hif2a G536W gain-of-function mutations develop spontaneous PH and RV hypertrophy without RV dilatation ( 107 ). Furthermore, mice bearing homozygous knockin of a human R200W VHL mutation (as found in patients with Chuvash polycythemia) develop PH ( 108 ). Notably, development of PH in this model is attenuated in the setting of heterozygous deletion of Hif2a, but not of Hif1a ( 108 ), suggesting a prominent role of HIF-2α in PH induced by VHL loss of function.

Furthermore, in a study exploring the inflammatory- and immune-specific roles of HIF isoforms in the pathogenesis of PH, mice with EC-specific deletion of Phd2 developed spontaneous PH due to Hif2a stabilization, which was partially attenuated by transplantation with WT bone marrow–derived cells, suggesting that HIF-2 activation in bone marrow–derived cells contributes to pulmonary vascular remodeling ( 10 ). In addition, mice with Hif1a deletion in myeloid cells are partially protected from hypoxia-induced PH and RV hypertrophy, mainly as a result of attenuated macrophage activity ( 109 ). Similarly, mice with monocyte-specific Hif1a deletion display attenuated PH, PA remodeling, and RV hypertrophy in response to hypoxia or hypoxia plus Sugen 5416 (SuHx) exposure ( 110 ). In addition to myeloid cells, the HIF pathway is also involved in the regulation of lymphoid cells. For example, Hif2a activation induced by Phd2 deletion causes immunoregulatory dysfunction ( 111 ), which may partially explain PH development associated with reduced regulatory T cell function ( 112 ). Collectively, these studies suggest that both bone marrow–derived macrophages and thymus-derived T cells are involved in pulmonary vascular remodeling at least in part because of HIF pathway activation.

Taken together, global, inducible, and cell-specific deletion of HIF isoforms and HIF pathway molecules established cell type– and context-specific roles of HIF isoforms in early as well as late stages of PH development in adult mice.

RV failure is one of the most common causes of morbidity and mortality in PAH ( 113 ). Upon PH onset, the RV undergoes remodeling to maintain its contractility, characterized by increased RV wall thickness and mass and moderate dilatation, mediated by cardiomyocyte hypertrophy and extracellular matrix deposition. However, at some point during the course of persistent pressure overload, the compensatory mechanisms of the RV expire, and the RV fails ( 113 ).

In physiological conditions, HIF-1α expression is significantly higher in the right than in the left ventricle ( 114 ). RV HIF-1α expression is increased in a number of animal models of PH, including MCT rats ( 115 ), hypoxia-exposed (HOX) rats ( 116 ), pulmonary artery–banded (PAB) rats ( 117 ), SuHx rats ( 117 ), and pulmonary embolism (PE) rats ( 118 ). Rats with PE display increased RV HIF-1α expression, and its level is positively correlated with RV hypertrophy and PAP ( 118 ). Interestingly, mice with Hif2a gain-of-function mutations develop RV hypertrophy but do not show signs of RV dilatation despite a substantial increase in PAP, suggesting preserved RV function ( 107 ). In patients with repaired tetralogy of Fallot, the presence of gain-of-function mutations in HIF-1α is associated with preserved RV function and better outcome due to increased TGF-β1 (TGFB1) expression and myocardial fibrosis ( 119 ). Collectively, these reports suggest that presumably mild to moderate activation of both HIFs is associated with preserved RV function. By contrast, strong activation of HIFs adversely affects RV function. For example, mice with global inducible deletion of Phd2 display increased mortality due to severe polycythemia and dilated cardiomyopathy ( 120 ). Similarly, HIF-2 activation in mice with EC-specific Phd2 deletion leads to spontaneous PH with high mortality due to severe RV failure ( 10 ). However, the roles of HIFs and PHDs have not been studied in fixed-afterload models of RV hypertrophy and failure using cell-specific gene knockout or overexpression.

As we have discussed above, experiments in rodent models revealed that HIF-1/2α exerts a profound impact on pulmonary vascular remodeling. Moreover, antisense oligonucleotides to Hif2 (but not to Hif1) reduced vessel muscularization, rises in PAPs, and RV hypertrophy in mice exposed to hypoxia, suggesting that inhibition of HIF-2α can provide a therapeutic approach to prevent or reverse the development of PH ( 99 ). Thus, great interest has arisen in developing therapeutics targeting this pathway. Studies have specifically targeted components of the HIF pathway such as PHD2, HIF-1α, or HIF-2α with pharmacological agents in various rodent models of PH. Most tested agents that directly or indirectly inhibit HIFs have been able to prevent or reverse experimental PH (Table 3). Agents/compounds that inhibit HIF-1α at the level of mRNA (topotecan and camptothecin), protein synthesis (2-methoxyestradiol, digoxin, celastramycin, caffeic acid phenethyl ester), protein accumulation and transcriptional activity (YC-1), and targeting of the molecules regulating the HIF axis (anti-CD146, mAb AA98, apigenin), or that inhibit HIF-2α at the level of mRNA (C76) or at the level of heterodimerization and DNA binding (PT2567), have been evaluated. These inhibitors, given via different routes (intraperitoneal, intravenous, subcutaneous, oral), were shown to prevent as well as reverse PH in various animal models of PH (hypoxia, MCT, and SuHx). Notably, the HIF-2α inhibitor C76 showed strong anti-remodeling effects in three experimental models of PH ( 27 ), indicating that inhibition of HIF-2 may be a promising therapeutic approach for PH.

Summary of studies evaluating the effects of pharmacological agents targeting HIF signaling in animal models of PH

Data obtained from cell systems, animal models, and patient-derived materials have consistently confirmed that HIF isoforms are important components of PH pathogenesis. The hypoxic and pseudohypoxic states that occur in different groups of PH may vary in intensity and duration, thus allowing an intricate interplay between HIF-1 and HIF-2 in driving the pathological processes that underlie pulmonary vascular and RV remodeling. Animal models have helped elucidate the nonredundant and complementary roles of HIF-1 and HIF-2. For example, HIF-1 plays a major role in driving vasoconstriction, PASMC proliferation, angiogenesis, and RV contractility, whereas HIF-2 plays a major role in inflammatory cell recruitment and in EC phenotypic switch to a proinflammatory state and to EndoMT. These data suggest dynamic regulation of HIF isoforms as well as cell- and context-specific roles of HIF-1 and HIF-2 in the initiation, progression, and establishment phases of pulmonary vascular and RV remodeling. Although head-to-head comparisons of mice with cell-specific deletions of HIF-1α and HIF-2α at different time points of hypoxic exposure are still needed to determine their influence on pulmonary vasculature and RV, it is conceivable that HIF-2α may play a major role in the initiation of the disease, whereas HIF-1α may play a major role in the progression and perpetuation of the disease. However, in cancers, HIF-1α plays the dominant role in the response to acute hypoxia, whereas HIF-2α drives the response to chronic hypoxia, although both are involved in cancer progression ( 121 ). Intriguingly, this HIF switch is also observed in pulmonary vascular endothelial and smooth muscle cells upon exposure to hypoxia ( 122 ), which may allow HIF-1 and HIF-2 to play divergent and complementary roles during hypoxic and pseudohypoxic responses of pulmonary vascular and cardiac cells in PH.

These contradictory data in knockout mouse models versus in vitro cell models may be explained by the use of acute exposure to hypoxia (maximum exposure of 1–4 days) in the in vitro experiments versus the use of small-animal models (mice) for PH in the knockout studies. Thus, further studies with inducible deletion of HIF isoforms in severe animal models of PH (MCT, SuHx in mice and rats) and in vascular cells isolated from PH patients and large animals (cows), which exhibit and maintain their unique phenotypes in vitro ( 44 ), are needed to provide deeper insights into cell type– and context-specific roles of HIF isoforms in PH. Furthermore, understanding the molecular mechanisms that determine HIF-1/2 switches or activation of cell type– and context-specific HIF isoforms in PH will facilitate a better understanding of the pathophysiological roles exerted by HIF isoforms and the potential clinical implications of targeting them. For example, PHD2 has relatively more influence on HIF-1α, whereas PHD3 has relatively more influence on HIF-2α ( 123 ). In addition, molecules like sirtuin 1, hypoxia-associated factor (HAF), and heat shock proteins (HSP70, HSP90) differentially regulated the degradation and activities of different HIF isoforms in various cell types ( 121 , 124 ). Furthermore, translation of HIF-2α (but not HIF-1α) is linked to iron metabolism due to an iron-responsive element in the 5′-untranslated region of HIF-2α ( 125 , 126 ). However, no studies to date have explored the mechanisms regulating the HIF-1/2 switch in pulmonary vascular and cardiac cells upon exposure to hypoxia and other nonhypoxic PH stimuli. Considering the pathophysiological roles of iron metabolism, sirtuins, and heat shock proteins in PH ( 68 , 126 – 128 ), the possibility that these mechanisms are operative in the putative HIF-1/2 switch associated with PH warrants further investigation.

Despite striking similarity in protein sequence, dimerization partners, and binding sites among HIF-1α and HIF-2α proteins, it is well documented that HIF-1 and HIF-2 activate different subsets of hypoxia-inducible genes in various pathological conditions, including PH. Recent studies suggest that despite sharing an identical consensus recognition sequence, each HIF isoform has an inherent property that determines its binding distribution across the genome. For example, HIF-1 binds closer to promoters, while HIF-2 binds distal enhancers, and their inherent distributions are unaffected by the degree or duration of hypoxia or the cell type ( 129 ). In addition, differential recruitment of other transcription factors underlies HIF-mediated cell-specific hypoxia responses. Indeed, accumulating evidence suggests that HIF-1α and HIF-2α form separate multifactorial complexes with other transcription factors, cofactors, and RNA polymerase II to mediate the distinct functions of HIF-1 or HIF-2 ( 130 ). Hence, it is important to identify these complexes and their common and unique target genes, not only to understand the distinct pathological processes mediated by HIF isoforms, but also to selectively inhibit HIF isoform functions as a therapeutic approach.

In summary, rapidly advancing research has brought to light the isoform-specific, context-specific, and cell-specific roles of the HIF pathway in regulating pulmonary vascular remodeling. This has introduced a novel therapeutic approach for the treatment of PH. Inline, HIF-2–selective inhibitors reversed PH in various animal models of PH without any significant side effects. Notably, a HIF-2–specific small-molecule inhibitor developed to treat renal cancer has demonstrated a favorable safety profile in a recent phase I trial ( 131 ), entered into phase II clinical trials, and will be considered for clinical trials among PH patients in the future. Head-to-head comparisons and multicenter preclinical studies of pan-HIF inhibitors and HIF-1– and HIF-2–selective inhibitors in various animal models of PH and RV dysfunction are warranted before moving into clinical development ( 132 ). Notably, further studies are needed to develop personalized therapeutics, i.e., to determine under what conditions and in which PH patients HIF inhibition can provide an optimal therapeutic strategy. In addition, given the myriad roles of HIFs and their possible influence on extrapulmonary manifestations in patients with PH ( 15 , 133 ), it will be important to carefully assess the risk/benefit ratio of systemic versus pulmonary selective HIF inhibitors. Thus, more work needs to be done to identify novel, potent, and more specific inhibitors targeting clearly defined points in the HIF pathway, followed by lung-selective delivery of these inhibitors, which will be the key to developing potential therapeutic strategies for PH.

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Projektnummer 268555672, SFB 1213, project A01, A05, A06 and A10* grants and a European Research Council (ERC) Consolidator Grant (866051 to SSP).

Conflict of interest: The authors have declared that no conflict of interest exists.

RBR E3 ubiquitin ligases in tumorigenesis

RING-in-between-RING (RBR) E3 ligases are one class of E3 ligases that is characterized by the unique RING-HECT hybrid mechanism to function with E2s to transfer ubiquitin to target proteins for degradation. Emerging evidence has demonstrated that RBR E3 ligases play essential roles in neurodegenerative diseases, infection, inflammation and cancer. Accumulated evidence has revealed that RBR E3 ligases exert their biological functions in various types of cancers by modulating the degradation of tumor promoters or suppressors. Hence, we summarize the differential functions of RBR E3 ligases in a variety of human cancers. In general, ARIH1, RNF14, RNF31, RNF144B, RNF216, and RBCK1 exhibit primarily oncogenic roles, whereas ARIH2, PARC and PARK2 mainly have tumor suppressive functions. Moreover, the underlying mechanisms by which different RBR E3 ligases are involved in tumorigenesis and progression are also described. We discuss the further investigation is required to comprehensively understand the critical role of RBR E3 ligases in carcinogenesis. We hope our review can stimulate the researchers to deeper explore the mechanism of RBR E3 ligases-mediated carcinogenesis and to develop useful inhibitors of these oncogenic E3 ligases for cancer therapy.

Watch the video: HYPOXIA. Physiological and Pathological Effects of Hypoxia (July 2022).


  1. Leil

    Prompt reply)))

  2. Corybantes

    Wish it's no way me

  3. Newlyn

    Agree, this wonderful idea is just about

  4. Fateh

    This variant does not come close to me. Can the variants still exist?

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