Nicotinamide relaxes vascular smooth muscle by inhibiting myosin light chain kinase-dependent signaling pathways: implications for anticancer efficacy

Nicotinamide has been shown to be an effective tumor oxygenator in preclinical studies and is part of a promising clinical protocol for the treatment of cancer of the larynx. It has been known for some time that nicotinamide sensitizes tumors, at least in part, by modulating vascular smooth muscle contraction; however, the cellular target within the smooth muscle cell has yet to be identified. Our previous studies have eliminated targets within several agonist and antagonist signaling pathways in smooth muscle, suggesting that it must act at a point close to the contractile machinery of the cell. The present study investigated the effect of nicotinamide on four key steps responsible for force generation via actin/myosin interaction in the smooth muscle cell: calcium binding to calmodulin, calcium-calmodulin binding to smooth muscle myosin light chain kinase (MLCK) inhibitor peptide 480-501 (smMLCIP), modulation of MLCK-dependent signaling, and MLCK-induced phosphorylation of the regulatory myosin light chain, MLC20. Nicotinamide abolished the phosphorylation of MLC20, but had no significant effect on the other endpoints tested. We conclude that the vasorelaxant effects of nicotinamide are mediated mainly through inhibition of MLC20 phosphorylation, and that this could be a promising target for the development of novel tumor oxygenators to enhance radio- and chemotherapy.