Nicotinamide phosphoribosyltransferase (Nampt) is an important coenzyme involved in cellular redox reactions. Inside the cell, Nampt (iNampt) functions as a rate-limiting enzyme in the NAD salvage pathway, and outside the cell (eNampt), it acts as a proinflammatory cytokine. High-circulating levels of Nampt are reported in different pathological conditions. This study was designed to examine the role of Nampt in the development of cardiac hypertrophy and ventricular remodeling. We studied the hypertrophic response in Nampt heterozygous (+/-) knockout and cardiac-specific overexpressing Nampt transgenic mice. Whereas Nampt(+/-) mice were protected against agonist (isoproterenol and angiotensin II)-induced hypertrophy, Nampt transgenic mice spontaneously developed cardiac hypertrophy at 6 mo of age. Experiments conducted to gain insight into the mechanism revealed that treatment of cardiomyocytes with recombinant (eNampt) or overexpression with Nampt-synthesizing adenovirus vector (Ad.Nampt) induced cardiomyocyte hypertrophy. The prohypertrophic effects of eNampt and Ad.Nampt were blocked by the addition of a Nampt-blocking antibody into cultures, thus suggesting that Nampt was in fact invoking hypertrophic response of cardiomyocytes by acting on the cell surface receptors. We also found increased Nampt levels in the supernatant of cardiomyocyte cultures subjected to stress by either serum starvation or H(2)O(2) treatment. Exploration of signaling pathways in Nampt-induced cardiac hypertrophy and fibrosis revealed increased activation of mitogen-activated protein kinases, namely, JNK1, p38, and ERK. This was also associated with increased calcineurin levels and nuclear factor of activated T-cell localization into the nucleus. From these studies we conclude that cardiomyocytes are capable of secreting Nampt during stress, and exogenous Nampt is a positive regulator of cardiac hypertrophy and adverse ventricular remodeling.