The prolyl hydroxylase oxygen-sensing pathway is cytoprotective and allows maintenance of mitochondrial membrane potential during metabolic inhibition

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Abstract
The cellular oxygen sensor is a family of oxygen-dependent proline hydroxylase domain (PHD)-containing enzymes, whose reduction of activity initiate a hypoxic signal cascade. In these studies, prolyl hydroxylase inhibitors (PHIs) were used to activate the PHD-signaling pathway in cardiomyocytes. PHI-pretreatment led to the accumulation of glycogen and an increased maintenance of ATP levels in glucose-free medium containing cyanide. The addition of the glycolytic inhibitor 2-deoxy-d-glucose (2-DG) caused a decline of ATP levels that was indistinguishable between control and PHI-treated myocytes. Despite the comparable levels of ATP depletion, PHI-preconditioned myocytes remained significantly protected. As expected, mitochondrial membrane potential (ΔΨmito) collapses in control myocytes during cyanide and 2-DG treatment and it fails to completely recover upon washout. In contrast, ΔΨmitois partially maintained during metabolic inhibition and recovers completely on washout in PHI-preconditioned cells. Inclusion of rotenone, but not oligomycin, with cyanide and 2-DG was found to collapse ΔΨmitoin PHI-pretreated myocytes. Thus, continued complex I activity was implicated in the maintenance of ΔΨmitoin PHI-treated myocytes, whereas a role for the “reverse mode” operation of the F1F0-ATP synthase was ruled out. Further examination of mitochondrial function revealed that PHI treatment downregulated basal oxygen consumption to only ∼15% that of controls. Oxygen consumption rates, although initially lower in PHI-preconditioned myocytes, recovered completely upon removal of metabolic poisons, while reaching only 22% of preinsult levels in control myocytes. We conclude that PHD oxygen-sensing mechanism directs multiple compensatory changes in the cardiomyocyte, which include a low-respiring mitochondrial phenotype that is remarkably protected against metabolic insult.

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