Oxygen Sensing in the Retina: Hypoxia Induces Contrasting Tissue Distributions of the Hypoxia Inducible Factors Hif-1 And Hif-2

Abstract

Purpose: : Hypoxia plays a key role in ischaemic and neovascular disorders of the retina. Cellular responses to oxygen are mediated by hypoxia-inducible transcription factors (HIFs), which are stabilized in hypoxia and induce the expression of a diverse range of genes through binding to their hypoxia response elements. The purpose of this study was to correlate the pattern of tissue hypoxia with apoptosis, the distributions of HIF-1α and HIF-2α subunits in the retina and the profiles of induced molecular mediators.

Methods: : We investigated the tissue distribution of hypoxia during oxygen-induced retinopathy (OIR) in mice using the bio-reductive drug pimonidazole. We measured the levels of HIF-1α and HIF-2α proteins by Western blotting and determined their cellular distribution by immunohistochemistry during the development of OIR. We measured the profiles of two downstream mediators, vascular endothelial growth factor (VEGF) and erythropoietin (EPO) by ELISA, and quantified ischaemia-associated apoptosis by TUNEL staining.

Results: : The degree of pimonidazole binding was maximal in the inner retina 2 hours after onset of hypoxia and gradually declined thereafter. Both HIF-1α and HIF-2α protein levels were significantly increased during retinal hypoxia but were found to have contrasting tissue distributions in the inner retina. Hypoxia was followed by a peak of apoptosis involving cells in the inner retina, and by a significantly increased presence of the downstream mediators VEGF and EPO.

Conclusions: : Hypoxia in areas of capillary loss in OIR is localized to the inner retina. Hypoxia results in stabilization of both HIF-1α and HIF-2α protein but these subunits have contrasting tissue distributions within the inner retina. These findings suggest the roles of HIF in the retina are isoform-specific. Therapeutic approaches to modulating the HIF system in retinal disorders should consider HIF isoform-specific distribution and function.

Keywords: hypoxia • transcription factors • retinal neovascularization