Estrogen-dependent and independent activation of the P1 promoter of the p53 gene in transiently transfected breast cancer cells

Loss of p53 function by mutational inactivation is the most common marker of the cancerous phenotype, Previous studies from our laboratory have demonstrated 17 beta estradiol (E-2) induction of p53 protein expression in breast cancer cells. Although direct effects of E-2 on the expression of p53 gene are not known, the steroid is a potent regulator of c-Myc transcription. In the present studies, we have examined the ability of E-2 and antiestrogens to regulate the P1 promoter of the p53 gene which contains a c-Myc responsive element, Estrogen receptor (ER)-positive T47D and MCF-7 cells were transiently transfected with the P1CAT reporter plasmid and levels of CAT activity in response to serum, E-2 and antiestrogens were monitored. Factors in serum were noted to be the dominant inducers of chloramphenicol acetyltransferase (CAT) expression in MCF-7 cells. The levels of CAT were drastically reduced when cells were maintained in serum free medium (SFM). However, a subtle ER-mediated induction of CAT expression was detectable when MCF-7 cells, cultured in SFM, were treated with E-2, In serum-stimulated T47D cells, the CAT expression was minimal. The full ER antagonist, ICI 182 780 (ICI) had no effect. Treatment with E-2 or 4-hydroxy tamoxifen (OHT) resulted in P1CAT induction; OHT was more effective than E-2. Consistent with c-Myc regulation of the P1 promoter, E-2 stimulated endogenous c-Myc in both cell lines. Two forms of c-Myc were expressed independent of E-2 stimuli. The expression of a third more rapidly migrating form was E-2-dependent and ER-mediated since it was blocked by the full ER antagonist, ICI, but not by the ER agonist/antagonist OHT, These data demonstrate both ER-mediated and ER-independent regulation of c-Myc and the P1 promoter of the p53 gene, and show differential effects of the two classes of antiestrogens in their ability to induce the P1 promoter of the p53 gene in breast cancer cells.