Nicotinamide adenine dinucleotide (NAD+) is an essential molecule for living organisms. CD38 is a key NAD+-dependent enzyme which breaks down NAD+ to cyclic ADP-ribose (ADPR) and nicotinamide (NAM, vitamin B3), and NAM can be recycled to synthesize NAD+. CD38 expression is consistently silenced by methylation in prostate cancer and progressively downregulated in advanced castration-resistant prostate cancer, suggesting a connection between NAD+ and prostate carcinogenesis as well as prostate cancer progression...CD38 overexpression also dramatically reduced intracellular NAD+ levels and decreased mitochondrial respiration as measured by oxygen consumption rate. We further show that some but not all of these CD38-induced phenotypes could be rescued by exogenous NAM. Treatment of cells with NAM rescued CD38-induced apoptosis and mitochondrial stress but did not restore intracellular NAD+ levels. We also found that NAM demonstrated biphasic effect on mitochondria function, a finding that can be explained by the dual role of NAM as both a precursor of NAD+ and also as a suppressor of a number of NAD+-dependent enzymes...