Glucocorticoid-Dependent Mechanisms of Brain Tolerance to Hypoxia

Adaptation of organisms to stressors is coordinated by the hypothalamic-pituitary-adrenal axis (HPA), which involves glucocorticoids (GCs) and glucocorticoid receptors (GRs). Although the effects of GCs are well characterized, their impact on brain adaptation to hypoxia/ischemia is still understudied. The brain is not only the most susceptible to hypoxic injury, but also vulnerable to GC-induced damage, which makes studying the mechanisms of brain hypoxic tolerance and resistance to stress-related elevation of GCs of great importance. Cross-talk between the molecular mechanisms activated in neuronal cells by hypoxia and GCs provides a platform for developing the most effective and safe means for prevention and treatment of hypoxia-induced brain damage, including hypoxic pre- and post-conditioning. Taking into account that hypoxia- and GC-induced reprogramming significantly affects the development of organisms during embryogenesis, studies of the effects of prenatal and neonatal hypoxia on health in later life are of particular interest. This mini review discusses the accumulated data on the dynamics of the HPA activation in injurious and non-injurious hypoxia, the role of the brain GRs in these processes, interaction of GCs and hypoxia-inducible factor HIF-1, as well as cross-talk between GC and hypoxic signaling. It also identifies underdeveloped areas and suggests directions for further prospective studies.

Pharmacological HIF1 Inhibition Eliminates Downregulation of the Pentose Phosphate Pathway and Prevents Neuronal Apoptosis in Rat Hippocampus Caused by Severe Hypoxia

The pentose phosphate pathway (PPP) of glucose metabolism in the brain serves as a primary source of NADPH which in turn plays a crucial role in multiple cellular processes, including maintenance of redox homeostasis and antioxidant defense. In our model of protective mild hypobaric hypoxia in rats (3MHH), an inverse correlation between hypoxia-inducible factor-1 (HIF1) activity and mRNA levels of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of PPP, was observed. In the present study, it was demonstrated that severe hypobaric hypoxia (SH) induced short-term upregulation of HIF1 alpha-subunit (HIF1α) in the hippocampal CA1 subfield and decreased the activity of G6PD. The levels of NADPH were also reduced, promoting oxidative stress, triggering apoptosis, and neuronal loss. Injection of a HIF1 inhibitor (HIF1i), topotecan hydrochloride (5 mg/kg, i.p.), before SH prevented the upregulation of HIF1α and normalized G6PD activity. In addition, HIF1i injection caused an increase in NADPH levels, normalization of total glutathione levels and of the cellular redox status as well as suppression of free-radical and apoptotic processes. These results demonstrate a new molecular mechanism of post-hypoxic cerebral pathology development which involves HIF1-dependent PPP depletion and support a recently suggested injurious role of HIF1 activation in the acute phase of cerebral hypoxia/ischemia. Application of PPP stimulators in early post-hypoxic/ischemic period might represent a promising neuroprotective strategy. Graphical abstract HIF1-dependent down-regulation of the pentose phosphate pathway contributes to the hypoxia-induced oxidative stress and neuronal apoptosis in the rat hippocampus.