Loss of the wild-type p53 activity and/or overexpression of the proto-oncogene bcl-2 are frequently detected in breast cancer and suggested to be related to chemotherapy and radiation therapy resistance. To identify the downstream signaling molecules for anti-proliferative and apoptotic activities of p53 and to investigate the interaction of bcl-2 with p53 in human breast epithelial cells, we have used the MCF10A cell line. We previously showed that overexpression of bcl-2 downregulates expression of p21(WAF1/CIP1) (a cyclin dependent kinase inhibitor which mediates p53 dependent G(1) arrest) and suppresses DNA damage-induced apoptosis in MCF10A cells. In the present study, we constitutively overexpressed p21(WAF1/CIP1) in bcl-2 overexpressing MCF10A cells to determine whether downregulation of p21(WAF1/CIP1) is necessary for the anti apoptotic activity of bcl-2, and to investigate the roles of p21(WAF1/CIP1) in p53-mediated cell death upon irradiation. Overexpression of p21(WAF1/CIP1) resulted in growth inhibition, but had no effect on bcl-2 inhibition of apoptosis following irradiation. Also, overexpression of p21(WAF1/CIP1) did not affect the dose-dependent radiation-induced cell lethality as determined by a clonogenic survival assay. These results suggest that bcl-2 downregulation of p21(WAF1/CIP1) is independent of the anti-apoptotic activity of bcl-2, and that p21(WAF/CIP1) is not involved in the p53-mediated cell death pathway.

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