Increased HIF-1α and HIF-2α accumulation, but decreased microvascular density, in chronic hyperoxia and hypercapnia in the mouse cerebral cortex.

The partial pressure of oxygen in the brain parenchyma is tightly controlled, and normal brain function is delicately sensitive to continuous and controlled oxygen delivery. The objective of this study was to determine brain angiogenic adaptive changes during chronic normobaric hyperoxia and hypercapnia in mice. Four-month-old C56BL/6 J mice were kept in a normobaric chamber at 50 % O2 and 2.5 % CO2 for up to 3 weeks. Normoxic littermates were kept adjacent to the chamber and maintained on the same schedule. Physiological variables were measured at time points throughout the 3 weeks or when the mice were sacrificed. Freshly collected or fixed brain specimens were analyzed by Western blot analysis and immunohistochemistry (IHC). We found significant accumulation of hypoxia-inducible factors 1α and 2α (HIF-1α and HIF-2α) and increased expression of erythropoietin (EPO), cyclooxygenase-2 (COX-2), and angiopoietin-2 (Ang-2) in hyperoxia and hypercapnia. Conversely, vascular endothelial growth factor (VEGF), and VEGF receptor-2 (KDR/Flk-1), peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), and prolyl hydroxylase-2 (PHD-2) expressions were decreased in hyperoxia and hypercapnia. Capillary density was significantly diminished by the end of the 3rd week of hyperoxia and hypercapnia as compared to control. We conclude that HIF-independent mechanisms contribute to brain capillary density modulation that is continuously adjusted in accordance with tissue oxygen tension.