Nicotinamide at mM concentration is a potent inhibitor of certain key molecules involved in cell survival, such as SIRT1 and PARP1, and affects cell survival in various conditions in vivo and in vitro. However, the effect of an acute treatment of nicotinamide on gene expression has rarely been closely examined. In our study, the treatment of 10 mM nicotinamide downregulated p21 WAF1 expression in various human cells including p53-negative or SIRT1-knockdown cells indicating gene regulation not mediated by p53 or SIRT1. Meanwhile, in the nicotinamide-treated cells, Sp1 activity and protein level was substantially reduced due to increased proteasome-mediated degradation. Our results indicate that nicotinamide treatment attenuates p21 WAF1 expression through Sp1 downregulation, and suggest a possible involvement of nicotinamide metabolism in cellular gene expression. (C) 2008 Elsevier Inc. All rights reserved.