UP-REGULATION AND FUNCTIONAL-ROLE OF P21(WAF1 CIP1) DURING GROWTH ARREST OF HUMAN BREAST-CARCINOMA MCF-7 CELLS BY PHENYLACETATE/

Phenylacetate (PA) and related aromatic fatty acids constitute a novel class of relatively nontoxic antineoplastic agents. These compounds i nduce tumor cytostasis and growth inhibition and differentiation of ca ncer cells, but little is known regarding the molecular events mediati ng these biological effects. Using human breast carcinoma MCF-7 cells as a model, we show here that PA-induced growth arrest is associated w ith enhanced expression of the cyclin-dependent kinase inhibitor p21(W af1/Cip1) and dephosphorylation of the retinoblastoma protein (PRB). T he induction of p21(WAF1/CIP1) mRNA by PA was independent of the cellu lar p53 status. To directly assess the contribution of p21(Waf1/Cip1) to PA-mediated cytostasis, we compared the effects of PA in parental M CF-7 cells and cells expressing reduced levels of p21(Waf1/Cip1) prote in (clones AS.3 and AS.4), accomplished through constitutive expressio n of antisense p21(Waf1/Cip1) transcripts. In contrast to parental cel ls, AS.3 and AS.4 cells did not show reduced pRB phosphorylation follo wing PA treatment, indicating that p21(Waf1/Cip1) induction by PA is r equired for dephosphorylation (inactivation) of pRB, a known mediator of cell cycle control. A prominent role for p21(Waf1/Clp1) in mediatin g PA-induced growth arrest was further supported by the demonstration that embryonal fibroblasts derived from a p21(WAF1/CIP1) knockout mous e (p21(-/-) mouse embryonal fibroblasts) did not growth arrest followi ng PA treatment, whereas PA effectively induced p21(WAF1/CIP1) mRNA an d growth inhibition of the wild-type mouse embryonal fibroblasts. Take n together, our findings strongly support a role for p21(Waf1/Cip1) th e PA-mediated inhibition of cell growth.