Purpose: Hypoxia inducible factor-1 (HIF-1) is a transcription factor composed of HIF-1alpha and HIF-1beta subunits. HIF-1 transactivates multiple genes whose products play key roles in oxygen homeostasis, including vascular endothelial growth factor (VEGF). This study was designed to determine whether HIF-1 levels are increased in ischemic retina and whether there is a correlation with increased expression of VEGF.
Methods: C57BL/6J mice were killed at time points that span retinal vascular development (PO to adult), or on postnatal day (P) 7 they were placed in a 75% oxygen environment for 5 days and then removed to room air and killed after 0, 2, or 6, or 24 hours and 5 or 14 days. Eyes were frozen, and retinas were isolated and used for immunoblot analysis, or eyes were sectioned for immunohisto chemical staining for HIF-1alpha or HIF-1beta, or for in situ hybridization for VEGF.
Results: Immunoblots of retinal lysates showed low levels of HIF-1alpha at PO that were markedly increased at P4, remained high throughout the period of retinal vascular development and then decreased to an intermediate level in adults. HIF-1beta levels were relatively constant at all time points. In mice with oxygen-induced ischemic retinopathy, HIF-1alpha levels were increased in the retina. The peak of increase occurred at 2 hours, and levels returned to baseline by 24 hours. Immunohistochemistry showed increased staining for HIF-1alpha throughout the hypoxic inner retina, but not in the normoxic outer retina. There was no modulation of HIF-1beta levels. There was constitutive expression of VEGF mRNA in the inner nuclear layer that was increased 6 hours after the onset of hypoxia and remained elevated for several days.
Conclusions: There are increased levels of HIF-1alpha in ischemic retina that show temporal and spatial correlation with increased expression of VEGF. These findings are consistent with the hypothesis that HIF-1 plays a role in upregulation of VEGF in ischemic retina.