Activation of the estrogen-signaling pathway by p21(WAF1/CIP1) in estrogenreceptor-negative breast cancer cells

Background: Estrogen stimulates the proliferation of cells in normal mammar y glands and most estrogen receptor (ER)positive mammary carcinomas by bind ing to the ER and promoting the transcription of ER-responsive genes. In ce lls with functional ERs, estrogen mediates the transition of cells from the G(1) to S phase of the cell cycle. Several cell cycle regulatory proteins have been implicated in the ER-signaling pathway involved in estrogen-media ted growth stimulation and antiestrogen-mediated growth arrest, We sought t o determine whether p21, a cyclin-dependent kinase inhibitor, is a componen t of this pathway and, if so, whether it can mediate estrogen's action in E R-negative breast cancer cells. Methods: We overexpressed p21 with a tetrac ycline-inducible system in ER-negative, p21-negative breast cancer cells. A ctivity of the ER-signaling pathway was monitored in transient transfection assays by using constructs in which the ER promoter or the estrogen-respon se element (ERE) controls Luciferase expression. The growth-modulating effe cts of estradiol and antiestrogens on p21-overexpressing clones were assess ed. All P values are from two-sided tests. Results: A strong positive assoc iation was found between the expression of p21 and ER in nine breast cancer cell lines and in tumor samples from 60 patients with breast cancer (P<.00 1). Overexpression of p21 in a p21-negative, ER-negative cell line induced both the ER and ERE promoters in an estrogen-responsive manner. Last, stabl e p21 clones that also lack the expression of wild-type ER were responsive to the growth-inhibitory effects of ICI 182,780, a potent antiestrogen, and the growth-stimulatory effects of 17 beta-estradiol, Conclusion: The abili ty of p21 to mediate the activation of the estrogen-signaling pathway in ER -negative tumor cells suggests that p21 plays a novel role in this pathway, a finding that also has important clinical implications.