Carbon Monoxide Potentiation of L-Type Ca2+ Channel Activity Increases HIF-1α-Independent VEGF Expression via an AMPKα/SIRT1-Mediated PGC-1α/ERRα Axis

Aims: The heme oxygenase-1 (HO-1)/carbon monoxide (CO) pathway induced in astrocytes after ischemic brain injury promotes vascular endothelial growth factor (VEGF) expression to maintain and repair neurovascular function. Although HO-1-derived CO has been shown to induce hypoxia-inducible factor-1 alpha (HIF-1 alpha)-dependent VEGF expression, the underlying mechanism independent of HIF-1 alpha remains to be elucidated. Results: HO-1 and VEGF were coexpressed in astrocytes of ischemic mouse brain tissues. Experiments with specific siRNAs and pharmacological activators/inhibitors of various target genes demonstrated that astrocytes pre-exposed to the CO-releasing compound, CORM-2, or transfected with HO-1 increased HIF-1 alpha-independent VEGF expression via sequential activation of the following signal cascades; Ca2+/calmodulin-dependent protein kinase kinase beta-mediated AMP-activated protein kinase (AMPK)alpha activation, AMPK alpha-induced increases in nicotinamide phosphoribosyltransferase (NAMPT) expression and cellular NAD(+) level, sirtuin 1 (SIRT1)-dependent peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha) stabilization and activation, and PGC-1 alpha/estrogen-related receptor (ERR)alpha-mediated VEGF expression. All of these sequential events were blocked by an L-type voltage-gated Ca2+ channel inhibitor and Ca2+ chelators, but not by other Ca2+ channel inhibitors. Innovation: HO-1-derived CO elicits Ca2+ influx by activating L-type Ca2+ channels, which is a key player in HIF-1 alpha-independent VEGF expression by activating the AMPK alpha-NAMPT-SIRT1-PGC-1 alpha/ERR alpha pathway. Conclusion: Our results provide new mechanistic insight into the possible role for L-type Ca2+ channels in HO-1/CO-induced angiogenesis.