Prolyl-hydroxylase inhibitor activating hypoxia-inducible transcription factors reduce levels of transplant arteriosclerosis in a murine aortic allograft model

Hypoxia inducible factor 2α promotes tolerogenic macrophage development during cardiac transplantation through transcriptional regulation of colony stimulating factor 1 receptor

Solid organ transplantation mobilizes myeloid cells, including monocytes and macrophages, which are central protagonists of allograft rejection. However, myeloid cells can also be functionally reprogrammed by perioperative costimulatory blockade to promote a state of transplantation tolerance. Transplantation tolerance holds promise to reduce complications from chronic immunosuppression and promote long-term survival in transplant recipients. We sought to identify different mediators of transplantation tolerance by performing single-cell RNA sequencing of acute rejecting or tolerized cardiac allografts. This led to the unbiased identification of the transcription factor, hypoxia inducible factor (HIF)-2α, in a subset of tolerogenic monocytes. Using flow cytometric analyses and mice with conditional loss or gain of function, we uncovered that myeloid cell expression of HIF-2α was required for costimulatory blockade-induced transplantation tolerance. While HIF-2α was dispensable for mobilization of tolerogenic monocytes, which were sourced in part from the spleen, it promoted the expression of colony stimulating factor 1 receptor (CSF1R). CSF1R mediates monocyte differentiation into tolerogenic macrophages and was found to be a direct transcriptional target of HIF-2α in splenic monocytes. Administration of the HIF stabilizer, roxadustat, within micelles to target myeloid cells, increased HIF-2α in splenic monocytes, which was associated with increased CSF1R expression and enhanced cardiac allograft survival. These data support further exploration of HIF-2α activation in myeloid cells as a therapeutic strategy for transplantation tolerance.

Hypoxia-inducible factors and essential hypertension: narrative review of experimental and clinical data

Hypoxia-inducible factor (HIFs) is a new class of drug developed for the management of anemia in chronic kidney disease (CKD) patients. HIFs increase the production of erythropoietin in the kidney and liver, enhance the absorption and utilization of iron, and stimulate the maturation and proliferation of erythroid progenitor cells. Besides, HIFs regulate many physiologic processes by orchestrating the transcription of hundreds of genes. Essential hypertension (HT) is an epidemic worldwide. HIFs play a role in many biological processes involved in the regulation of blood pressure (BP). In the current review, we summarize pre-clinical and clinical studies investigating the relationship between HIFs and BP regulation in patients with CKD, conflicting issues, and discuss future potential strategies.

Myeloid PHD2 deficiency accelerates neointima formation via Hif-1α

The key players of the hypoxic response are the hypoxia-inducible factors (Hif), whose α-subunits are tightly regulated by Prolyl-4-hydroxylases (PHD), predominantly by PHD2. Monocytes/Macrophages are involved in atherosclerosis but also restenosis and were found at hypoxic and sites of the lesion. Little is known about the role of the myeloid PHD2 in atherosclerosis and neointima formation. The study aimed to investigate the consequences of a myeloid deficiency of PHD2 in the process of neointima formation using an arterial denudation model. LysM-cre mice were crossed with PHD2^fl/fl, PHD2^fl/fl/Hif1α^fl/fl and PHD2^fl/fl/Hif2α^fl/fl to get myeloid specific knockout of PHD2 and the Hif-α subunits. Denudation of the femoral artery was performed and animals were fed a western type diet afterwards with analysis of neointima formation 5 and 35 days after denudation. Increased neointima formation in myeloid PHD2 knockouts was observed, which was blunted by double-knockout of PHD2 and Hif1α whereas double knockout of PHD2 and Hif-2α showed comparable lesions to the PHD2 knockouts. Macrophage infiltration was comparable to the neointima formation, suggesting a more inflammatory reaction, and was accompanied by increased intimal VEGF-A expression. Collagen-content inversely correlated to the extent of neointima formation suggesting a destabilization of the plaque. This effect might be triggered by macrophage polarization. Therefore, in vitro results showed a distinct expression pattern in differentially polarized macrophages with high expression of Hif-1α, VEGF and MMP-1 in proinflammatory M1 macrophages. In conclusion, the results show that myeloid Hif-1α is involved in neointima hyperplasia. Our in vivo and in vitro data reveal a central role for this transcription factor in driving plaque-vascularization accompanied by matrix-degradation leading to plaque destabilization.

Hypoxia-inducible factors not only regulate but also are myeloid-cell treatment targets

Hypoxia describes limited oxygen availability at the cellular level. Myeloid cells are exposed to hypoxia at various bodily sites and even contribute to hypoxia by consuming large amounts of oxygen during respiratory burst. Hypoxia-inducible factors (HIFs) are ubiquitously expressed heterodimeric transcription factors, composed of an oxygen-dependent α and a constitutive β subunit. The stability of HIF-1α and HIF-2α is regulated by oxygen-sensing prolyl-hydroxylases (PHD). HIF-1α and HIF-2α modify the innate immune response and are context dependent. We provide a historic perspective of HIF discovery, discuss the molecular components of the HIF pathway, and how HIF-dependent mechanisms modify myeloid cell functions. HIFs enable myeloid-cell adaptation to hypoxia by up-regulating anaerobic glycolysis. In addition to effects on metabolism, HIFs control chemotaxis, phagocytosis, degranulation, oxidative burst, and apoptosis. HIF-1α enables efficient infection defense by myeloid cells. HIF-2α delays inflammation resolution and decreases antitumor effects by promoting tumor-associated myeloid-cell hibernation. PHDs not only control HIF degradation, but also regulate the crosstalk between innate and adaptive immune cells thereby suppressing autoimmunity. HIF-modifying pharmacologic compounds are entering clinical practice. Current indications include renal anemia and certain cancers. Beneficial and adverse effects on myeloid cells should be considered and could possibly lead to drug repurposing for inflammatory disorders.

Turning the Oxygen Dial: Balancing the Highs and Lows

Oxygen is both vital and toxic to life. Molecular oxygen is the most used substrate in the human body and is required for several hundred diverse biochemical reactions. The discovery of the PHD-HIF-pVHL system revolutionized our fundamental understanding of oxygen sensing and cellular adaptations to hypoxia. It deepened our knowledge of the biochemical underpinnings of numerous diseases, ranging from anemia to cancer. Cellular dysfunction and tissue pathology can result from a mismatch of oxygen supply and demand. Recent work has shown that mitochondrial disease models display tissue hyperoxia and that disease pathology can be reversed by normalization of excess oxygen, suggesting that certain disease states can potentially be treated by modulating oxygen levels. In this review, we describe cellular and organismal mechanisms of oxygen sensing and adaptation. We provide a revitalized framework for understanding pathologies of too little or too much oxygen.

Oxygen homeostasis and cardiovascular disease: A role for HIF?

Hypoxia, the decline of tissue oxygen stress, plays a role in mediating cellular processes. Cardiovascular disease, relatively widespread with increased mortality, is closely correlated with oxygen homeostasis regulation. Besides, hypoxia-inducible factor-1(HIF-1) is reported to be a crucial component in regulating systemic hypoxia-induced physiological and pathological modifications like oxidative stress, damage, angiogenesis, vascular remodeling, inflammatory reaction, and metabolic remodeling. In addition, HIF1 controls the movement, proliferation, apoptosis, differentiation and activity of numerous core cells, such as cardiomyocytes, endothelial cells (ECs), smooth muscle cells (SMCs), and macrophages. Here we review the molecular regulation of HIF-1 in cardiovascular diseases, intended to improve therapeutic approaches for clinical diagnoses. Better knowledge of the oxygen balance control and the signal mechanisms involved is important to advance the development of hypoxia-related diseases.

Donor Treatment With a Hypoxia-Inducible Factor-1 Agonist Prevents Donation After Cardiac Death Liver Graft Injury in a Rat Isolated Perfusion Model

The protective role of hypoxia-inducible factor-1 (HIF-1) against liver ischemia-reperfusion injury has been well proved. However its role in liver donation and preservation from donation after cardiac death (DCD) is still unknown. The objective of this study was to test the hypothesis that pharmaceutical stabilization of HIF-1 in DCD donors would result in a better graft liver condition. Male SD rats (6 animals per group) were randomly given the synthetic prolyl hydroxylase domain inhibitor FG-4592 (Selleck, 6 mg/kg of body weight) or its vehicle (dimethylsulfoxide). Six hours later, cardiac arrest was induced by bilateral pneumothorax. Rat livers were retrieved 30 min after cardiac arrest, and subsequently cold stored in University of Wisconsin solution for 24 h. They were reperfused for 60 min with Krebs-Henseleit bicarbonate buffer in an isolated perfused liver model, after which the perfusate and liver tissues were investigated. Pretreatment with FG-4592 in DCD donors significantly improved graft function with increased bile production and synthesis of adenosine triphosphate, decreased perfusate liver enzyme release, histology injury scores and oxidative stress-induced cell injury and apoptosis after reperfusion with the isolated perfused liver model. The beneficial effects of FG-4592 is attributed in part to the accumulation of HIF-1 and ultimately increased PDK1 production. Pretreatment with FG-4592 in DCD donors resulted in activation of the HIF-1 pathway and subsequently protected liver grafts from warm ischemia and cold-stored injury. These data suggest that the pharmacological HIF-1 induction may provide a clinically applicable therapeutic intervention to prevent injury to DCD allografts.

Hypoxia-inducible factor prolyl 4-hydroxylase inhibition in cardiometabolic diseases

Hypoxia-inducible factor prolyl 4-hydroxylases (HIF-P4Hs, also called PHDs and EglNs) are enzymes that act as cellular oxygen sensors. They are the main downregulators of the hypoxia-inducible factor (HIF). HIF-P4Hs can be targeted with small molecule inhibitors, which stabilize HIF under normoxia and initiate the hypoxia response. Such inhibitors are in phase 2 and 3 clinical trials for the treatment of anemia due to their ability to induce erythropoietin and iron metabolism genes. Recent data suggest that HIF-P4H inhibition has a therapeutic role beyond anemia in cardiac ischemia, obesity and metabolic dysfunction, and atherosclerosis. The molecular level mechanisms involved are HIF stabilization driven changes in gene expression that improve perfusion and endothelial function, reprogram metabolism to promote glucose intake and glycolysis over oxidative metabolism, reduce inflammation and beneficially modify innate immune system. This review discusses the recent findings in detail.

Influence of antithymocyte globulin treatment of brain-dead organ donor on inflammatory response in cardiac grafts: an experimental study in mice

The expression of proinflammatory cytokines in donor hearts after antithymocyte globulin (ATG) treatment given prior to organ removal was evaluated to analyze changes in inflammatory response. Adult female OF-1 mice were randomized into brain death (BD) groups (BD Control, BD ATG) with or without treatment, and Controls (Control, ATG). BD induction was performed through gradual inflation of an intracranial positioned balloon catheter. At the end of a 6-h observation period, ATG (1 mg/kg BW) was given intravenously. After 45 min, the donor hearts were removed. Proinflammatory markers IL-2 and IL-6 were examined using ELISA and immunohistochemistry staining. After single administration of ATG, the inflammatory reaction in the myocardium showed a significant reduction in IL-2 expression (BD Control vs. BD ATG, P = 0.033). Our investigation showed expected increase in proinflammatory mediators after BD. This increase was abolished by single infusion of ATG, indicated by significant reduction in IL-2 levels in the myocardium. We observed a reduction of IL-6 deposition in media cells in ATG-treated specimens. Further research is necessary to evaluate the role of ATG in donor management considering a potentially positive effect of ATG on IL-2-directed inflammatory response and possible reduction of IL-6-mediated vascular changes.