The cardioprotective role of sirtuins is mediated in part by regulating K_ATP channel surface expression

Sirtuins are NAD⁺-dependent deacetylases with beneficial roles in conditions relevant to human health, including metabolic disease, type II diabetes, obesity, cancer, aging, neurodegenerative diseases, and cardiac ischemia. Since ATP-sensitive K⁺ (K_ATP) channels have cardioprotective roles, we investigated whether they are regulated by sirtuins. Nicotinamide mononucleotide (NMN) was used to increase cytosolic NAD⁺ levels and to activate sirtuins in cell lines, isolated rat and mouse cardiomyocytes or insulin-secreting INS-1 cells. K_ATP channels were studied with patch clamping, biochemistry techniques, and antibody uptake experiments. NMN led to an increase in intracellular NAD⁺ levels and an increase in the K_ATP channel current, without significant changes in the unitary current amplitude or open probability. An increased surface expression was confirmed using surface biotinylation approaches. The rate of K_ATP channel internalization was diminished by NMN, which may be a partial explanation for the increased surface expression. We show that NMN acts via sirtuins since the increased K_ATP channel surface expression was prevented by blockers of SIRT1 and SIRT2 (Ex527 and AGK2) and mimicked by SIRT1 activation (SRT1720). The pathophysiological relevance of this finding was studied using a cardioprotection assay with isolated ventricular myocytes, in which NMN protected against simulated ischemia or hypoxia in a K_ATP channel-dependent manner. Overall, our data draw a link between intracellular NAD⁺, sirtuin activation, K_ATP channel surface expression, and cardiac protection against ischemic damage.