Decreased VEGF expression and microvascular density, but increased HIF-1 and 2α accumulation and EPO expression in chronic moderate hyperoxia in the mouse brain

… Arterial oxygen saturation (SaO 2 ), which is hemoglobin oxygen saturation in arterial blood, was
recorded in a subgroup of mice kept in normoxia or hyperoxia using a MouseOx™ Pulse Oximeter
(STARR Life Sciences Corp., Oakmount, PA), as described earlier (Benderro and …
Normal brain function is dependent on continuous and controlled oxygen delivery. Chronic moderate hypoxia leads to angiogenesis, suggesting a modulatory role for oxygen in determining capillary density. The objective of this study was to determine physiologic and brain angiogenic adaptational changes during chronic moderate normobaric hyperoxia in mice. Four-month old C56BL/6J mice were kept in a normobaric chamber at 50% O2 for up to 3 weeks. Normoxic littermates were kept in the same room outside the chamber. Freshly collected or fixed brain specimens were analyzed by RT-PCR, Western blot analysis and immunohistochemistry. Results show accumulation of hypoxia inducible factors 1 and 2α (HIF-1 and 2α), and increased expression of erythropoietin (EPO), cyclooxygenase-2 (COX-2) and angiopoietin-2 (Ang-2). Conversely, vascular endothelial growth factor (VEGF), and VEGF receptor-2 (KDR/Flk-1), Peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) and prolylhydroxylase-2 (PHD-2) expressions were decreased. VEGF mRNA level was diminished but there was no change in HIF-1α mRNA and von Hippel Lindau E3 ubiquitin ligase (VHL) protein expression. Microvascular density was significantly diminished by the end of the 3rd week of hyperoxia. Overall, our results are: (1) increased expression of the potent neuroprotective molecule, EPO; (2) diminished expression of the potent angiogenic factor, VEGF; and (3) decreased microvascular density. We can, therefore, conclude that brain microvascular density can be controlled by HIF-independent mechanisms, and that brain capillary density is a continuously adjusted variable with tissue oxygen availability as one of the controlling modulators.