INDUCTION OF P21 (CIP1 WAF1/SID1) BY ESTRADIOL IN A BREAST EPITHELIAL-CELL LINE TRANSFECTED WITH THE RECOMBINANT ESTROGEN-RECEPTOR GENE - APOSSIBLE MECHANISM FOR A NEGATIVE REGULATORY ROLE OF ESTRADIOL/

Estrogens stimulate the growth of a majority of estrogen receptor (ER) -positive breast cancer cells. In contrast, estradiol exerted a 75% in hibition of DNA synthesis in the MCF-1OAE(wt5) cell line, obtained by the transfection of the ER gene into a normal breast epithelial cell l ine, MCF-10A. The estradiol-mediated growth inhibitory effect was reve rsed by ICI 164384, a pure anti-estrogen. Analysis of cell cycle by fl ow cytometry showed a significant increase of G1 cells by estradiol tr eatment compared to controls. To understand the mechanism of action of estradiol on MCF-1OAE(wt5) cells, we examined the level of a cyclin d ependent kinase inhibitor (CKI), p21, by Western blot analysis. Our re sults showed a 5- to 10-fold increase in the level of p21 in estradiol -treated MCF-1OAE(wt5) cells compared to controls. ICI164354 reversed estradiol-mediated induction of p21. Northern blot analysis of p21 mRN A indicated that estradiol stimulated its message in MCF-1OAE(wt5) cel ls. Analysis of a panel of 6 breast cancer cell lines showed the absen ce of p21 protein, whereas it was present at a very low level in MCF-1 0A cells. Comparison of p21 in MCF-10A and MCF-10AE(wt5) cells showed an abundance of p21 in the ER-transfected cells. However, this p21 app ears to be inactive in the absence of estradiol. These results suggest a p21-mediated pathway as a possible mechanism for the growth inhibit ory effects of estradiol on at least a subset of ER-transfected cell l ines.