Rapid, HIF-1α-independent, response of VEGF secretion to decreased O₂ in mouse retina

Vascular endothelial growth factor-A (VEGF) is a critical determinant of retinopathy due to vascular complications. Its expression is chronically regulated by transcription factors notably hypoxia-inducible factor-1α (HIF-1α) in response to decreased availability of oxygen [O₂]. We considered whether there is a distinct rapid response component of VEGF secretion in response to low O₂, faster than could be achieved by HIF-1α. To investigate this possibility, we measured real time responses of VEGF, lactate, and HIF-1α by retinas and other tissues to rapid and precise decreases in dissolved O_2.and showed that retinas, retinal pigment epithelium, and endothelial cells all can secrete VEGF within 5 min after a decrease in [O₂]. HIF-1α levels in retinas did not increase until 20 to 30 min after a decrease in dissolved [O₂]. Moreover, increased HIF-1α induced by the drug Roxadustat, did not stimulate VEGF secretion at atmospheric (21%) O₂ and a steady rate of VEGF secretion was sustained for over 120 min following elimination of HIF-1α and protein synthesis by exposure to the protein synthesis inhibitor cycloheximide. We then investigated the mechanism of VEGF release and found that VEGF was stored in the plasma membranes of retinas at 21% O₂. These stores are depleted within 30 min once [O₂] is decreased. Taken together, these findings indicate that VEGF is constitutively made (under the influence of HIF-1α and other transcription factors) and stored within the retina, and rapid release occurs after a substantial decrease in available O₂ independent of HIF-1α.