Divergent Roles for Hypoxia-Inducible Factor-1 and -2 in Ischemic Retinal Disease

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SUMMARY. Diabetes accounts for 10% of healthcare spending in the United States. A growing portion of this cost is spent on treatments for diabetic eye disease, the most common ischemic retinopathy (IR) and the leading cause of blindness among working-age adults in the developed world. Hypoxia-inducible factors (HIFs) are transcriptional regulators that control the expression of the hyperpermeabilty and angiogenic mediators that promote the development of macular edema (ME) and retinal neovascularization (NV) in patients with IRs. Therapies targeting one HIF-regulated angiogenic mediator, vascular endothelial growth factor (VEGF), are the gold standard for treating ME in IR patients, and have more recently emerged as effective treatments for NV in diabetic patients. However, less than half of treated patients respond adequately to anti-VEGF therapies, supporting a role for other HIF-regulated angiogenic factors in the development of NV. HIFs are composed of an exquisitely oxygen-sensitive  subunit and a ubiquitously-expressed  subunit. HIF-1α, first HIF  subunit isoform to be identified, has been shown to play an important role in retinal vascular disease. Two other HIF isoforms, HIF-2α and HIF-3α, have subsequently been reported; while HIF-2α is closely related to HIF- 1α and also activates hypoxia-inducible gene transcription, HIF-3α is more distantly related and can either activate or repress expression of HIF target genes. The relative contribution of HIF-1 vs HIF-2 to the expression of genes that regulate the initiation, promotion, and progression of NV is not known. Similarly, the neuroprotective role of HIFs – and HIF-regulated gene products – following hypoxic injury to the vulnerable neurosensory retina remains poorly understood. We hypothesize that accumulation of specific HIFs in different cells at precise stages following hypoxic injury determines the HIF- dependent gene expression signature in hypoxic retinal cells; this cell-specific, time-dependent gene signature determines the “pathologic” vs “protective” role(s) of HIFs. To address this hypothesis, we propose to determine the contribution of HIFs to the initiation, promotion, and progression of NV by Müller cells in IRs (SA1); determine the contribution of HIFs to RGC survival following hypoxic injury in IRs (SA2); and determine the contribution of HIF-2 regulation of PAI-1 to the development of retinal NV in IRs (SA3). Successful completion of these studies will provide insight into the pathologic vs protective role(s) of HIF-1 and HIF-2 in different cells at different stages following hypoxic injury in patients with IRs, and will identify novel therapeutic approaches for preventing or treating patients with these vision threatening disease.