The p21(WAFI/Cip1) gene plays a central role in cell cycle regulation. Here we show that topoisomerase II inhibitors, genistein and etoposide, induce p2(WAF1/Cip1) expression mainly in a p53-dependent manner in human lung cancer cell line A549. However, although p53 accumulated, p21(WAF1/Cip1) expression did not depend on the level of Ser15 phosphorylation of p53. Caffeine, an ataxia telangiectasia-mutated (ATM), and ATM- and Rad3-related kinase (ATR) inhibitor, abrogated genistein-induced p21(WAF1/Cip1) and largely blocked etoposide-induced p21(WAF1/Cip1) expression. Wortmannin, an ATM- and DNA-dependent protein kinase inhibitor, partially inhibited p21(WAF1/Cip1) expression induced by genistein and etoposide, whereas UCN-01, a Chk1 inhibitor, partially blocked etoposide, but not genistein-induced p21(WAF1/Cip1) expression. These data suggest that both genistein and etoposide induce p21(WAF1/Cip1) expression in a p53-dependent manner. Genistein appears to stimulate p21(WAF1/Cip1) expression through p53 via ATM, whereas etoposide may activate both ATM and ATR pathways. Our results suggest different mechanisms participate in genistein and etoposide induced p21(WAF1/Cip1) expression. (C) 2003 Elsevier Science (USA). All rights reserved.