CELL CYCLE-INDEPENDENT REGULATION OF P(21WAF1 CIP1) AND RETINOBLASTOMA PROTEIN DURING OKADAIC ACID-INDUCED APOPTOSIS IS COUPLED WITH INDUCTION OF BAX PROTEIN IN HUMAN BREAST-CARCINOMA CELLS/

Okadaic acid (OA) is a serine/threonine protein phosphatase inhibitor and has been shown to induce apoptosis in a number of different tumor cell lines, including human breast carcinoma (HBC) cells. The molecula r basis of OA-induced apoptosis remains to be investigated, Here, we d emonstrate that the OA concentration that inhibits only protein phosph atase 1 and 2A was sufficient to induce apoptosis in HBC cells, In MCF -7 cells, the OA-induced apoptosis was coupled with the overexpression of endogenous p53, p21(Waf1/Cip1), and Bar proteins, whereas the Rb p rotein levels were decreased, OA also induced apoptosis and concomitan tly enhanced the p21(Waf1/Clp1) and Bax levels in human papilloma viru s protein EG-transfected variants of MCF-7 cells, in which p53 functio n had been disrupted, OA, by contrast, had no effect on the levels or the subcellular localization of Gadd45 and Bcl2 proteins in either wil d-type or EG-transfected MCF-7 cells, Bcl-x(L), Bcl-x(s), and Bak leve ls were also unchanged after OA treatment in both cell types. OA-induc ed apoptosis and its effect on the expression of the above molecular m arkers occurred in the absence of any detectable changes in the cell c ycle phase distribution. On the basis of our findings, we conclude the following: (a) OA-induced apoptosis in HBC cells occurs independently of cell cycle arrest; (b) the wild-type p53 function is not an absolu te prerequisite for OA-induced cell death; and (c) OA-induced apoptosi s is associated with up-regulation of endogenous p21(Waf1/Clp1) and Ba r protein levels.