No abstract

Ischemia and chronic hypoxia (CH) trigger a variety of adverse effects arising from metabolic stress that injures cells. In response to reduced O 2, hypoxia-inducible factor 1␣ (HIF-1␣) activates erythropoietin (Epo) as well as many other target genes that counteract the effects of O 2 deficiency. Epo produced by the kidney stimulates erythrocyte production, leading to decreased HIF-1␣ production by improved tissue O 2 delivery. However, Epo is produced by many other tissues, and it is currently unclear to what extent, if any, locally produced Epo modulates HIF-1␣ expression. Derivatives of Epo that possess tissue-protective activities but do not stimulate erythropoiesis [e.g., carbamylated Epo (CEpo)] are useful tools with which to determine whether exogenous Epo modulates HIF-1␣ in the absence of changes in hemoglobin concentration. We compared the effects of CH (6.5% O 2 for 10 days) with or without CEpo administered by daily s.c. injection (10 g͞kg of body weight). CEpo administration did not alter the survival rate, weight loss, or increased hemoglobin concentration associated with CH. Therefore, CEpo does not directly suppress HIF-mediated erythropoiesis. CEpo does, however, prevent CH-induced neuronal increases of HIF-1␣ and Epo receptor-associated immunoreactivity (a measure of stress) while reducing the apoptotic index. In contrast, the myocardium did not exhibit increased HIF-1␣ expression during CH, although CEpo did reduce the apoptotic index. These observations therefore demonstrate that CEpo administration reduces the metabolic stress caused by severe CH, resulting in improved cellular survival independent of erythrocyte production.apoptosis ͉ brain ͉ hypoxia-inducible factor 1␣ ͉ heart ͉ Epo receptor

Exposure to hypoxia triggers a variety of adverse effects in the brain that arise from metabolic stress and induce neuron apoptosis. Overexpression of the hypoxia-inducible factor-1alpha (HIF-1alpha) is believed to be a major candidate in orchestrating the cell's defense against stress. To test the impact of HIF-1alpha on apoptosis during chronic hypoxia in vivo, we examined the protective effect of modulating the nitric oxide (NO)/cGMP pathway by sildenafil, a selective inhibitor of phosphodiesterase-5 (PDE-5). Male ICR/CD-1 mice were divided into 3 groups (n = 6/group): normoxic (21% O(2)), hypoxic (9.5% O(2)), and hypoxic with sildenafil (1.4-mg/kg intraperitoneal injections daily). At the end of the 8-day treatment period, the mice were euthanized and cerebral cortex biopsies were harvested for analyses. We found that sildenafil: (1) did not significantly alter the hypoxia-induced weight loss and hemoglobin increase, but did augment plasma nitrates+nitrites and the tissue content of cGMP and phosphorylated (P) NO synthase III; (2) reversed the hypoxia-induced neuron apoptosis (terminal deoxynucleotidyl transferase positivity and double-staining immunofluorescence, P = 0.009), presumably through increased bcl-2/Bax (P = 0.0005); and (3) did not affect HIF-1alpha, but rather blunted the hypoxia-induced increase in P-ERK1/2 (P = 0.0002) and P-p38 (P = 0.004). We conclude that upregulating the NO/cGMP pathway by PDE-5 inhibition during hypoxia reduces neuron apoptosis, regardless of HIF-1alpha, through an interaction involving ERK1/2 and p38.