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Phosphodiesterase-5 inhibition abolishes neuron apoptosis induced by chronic hypoxia independently of HIF-1 signaling

¹ University of Milan

^* To whom correspondence should be addressed. E-mail: michele.samaja{at}unimi.it.

	Abstract

Exposure to hypoxia triggers in brain a variety of adverse effects that arise from metabolic stress and induce neuron apoptosis. Over-expression of the hypoxia inducible factor-1 (HIF-1) is believed to be a major candidate in orchestrating the cell defense against stress. To test the impact of HIF-1 during chronic hypoxia in vivo on apoptosis, we examined the protective effect of modulating the NO/cyclic GMP pathway by sildenafil, a selective inhibitor of phosphodiesterase-5. Male ICR/CD-1 mice were divided into three groups (n=6/group): normoxic (21% O₂), hypoxic (9.5% O₂) and hypoxic with sildenafil (1.4 mg/kg ip daily injections). At the end of the 8-day treatment, mice were euthanized and cerebral cortex biopsies harvested for analyses. We found that sildenafil (1) did not alter the hypoxia-induced weight loss and hemoglobin increase, but augmented plasma nitrates+nitrites and the tissue content of cyclic GMP and phosphorylated NOS3; (2) reversed the hypoxia-induced neuron apoptosis (TdT-positivity and double-staining immunofluorescence, P=0.009), presumably through increased bcl-2/Bax (P=0.0005); and (3) did not affect HIF-1, but blunted the hypoxia-induced increase in phosphorylated-ERK1/2 (P=0.0002) and p38 (P=0.004). We conclude that up-regulating the NO/cyclic GMP pathway by phosphodiesterase-5 inhibition during hypoxia reduces neuron apoptosis irrespectively of HIF-1, through an interaction involving ERK1/2 and p38.

Key Words: Chronic hypoxia, HIF-1, NO, hypoxia signaling, bcl-2