Identification of an estrogen-inducible phosphatase (PP5) that converts MCF-7 human breast carcinoma cells into an estrogen-independent phenotype when expressed constitutively

The proliferation of many estrogen receptor (ER)-positive breast cancer cel ls depends on estradiol, and tumors arising from these cells are often resp onsive initially to treatment with Selective ER modulators, which produce a n antiestrogen effect. However, tumors that are refractory to the antiestro genic effects of selective ER modulators often reemerge, and the prognosis for these patients is poor because of the lack of additional effective ther apy, Accordingly, deciphering the cellular events associated with estrogen- dependent growth and the subsequent outgrowth of tumors with san estrogen-i ndependent phenotype is of considerable interest. Here me show that the exp ression of PP5, an evolutionarily conserved Ser/Thr phosphatase that functi ons as an inhibitor of glucocorticoid- and p53-induced signaling cascades l eading to growth suppression, is responsive to 17 beta -estradiol (E-2) in ER-positive human breast carcinoma cells (MCF-7). Northern analysis reveale d that E-2-induced PP5 expression is blocked by treatment with tamoxifen, a nd a consensus ER recognition element was identified in the PP5 promoter, T he PP5-ER recognition element associates with human ERs and confers E-2-ind uced transcriptional activation to reporter plasmids. The specific inhibiti on of PP5-expression ablates E-2-mediated proliferation in MCF-7 cells with out having an apparent effect on E-2-induced expression of c-myc or cyclin DI. Thus, although critical for cell growth, PP5 likely acts either downstr eam or independently of c-Myc and Cyclin D1. To further characterize the ro le of PP5 in E-2-regulated growth control, we constructed stable MCF-7 cell lines in which the expression of PP5 was placed under the control of tetra cycline-regulated transactivator and operator plasmids, Studies with these cells revealed that the constitutive overexpression of PP5 affords E-2-depe ndent MCF-7 cells with the ability to proliferate in E-2-depleted media. To gether, these studies indicate that E-2-induced PP5 expression functions to enhance E-2-initiated signaling cascades leading to cell division and that aberrant PP5 expression may contribute to the development of MCF-7 cells w ith an estrogen-independent phenotype.