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 o f the cancerous phenotype, Previous studies from our laboratory have demons trated 17 beta estradiol (E-2) induction of p53 protein expression in breas t cancer cells. Although direct effects of E-2 on the expression of p53 gen e 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 antiestroge ns to regulate the P1 promoter of the p53 gene which contains a c-Myc respo nsive element, Estrogen receptor (ER)-positive T47D and MCF-7 cells were tr ansiently transfected with the P1CAT reporter plasmid and levels of CAT act ivity in response to serum, E-2 and antiestrogens were monitored. Factors i n serum were noted to be the dominant inducers of chloramphenicol acetyltra nsferase (CAT) expression in MCF-7 cells. The levels of CAT were drasticall y reduced when cells were maintained in serum free medium (SFM). However, a subtle ER-mediated induction of CAT expression was detectable when MCF-7 c ells, cultured in SFM, were treated with E-2, In serum-stimulated T47D cell s, 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 i n P1CAT induction; OHT was more effective than E-2. Consistent with c-Myc r egulation 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 ex pression of a third more rapidly migrating form was E-2-dependent and ER-me diated 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-i ndependent regulation of c-Myc and the P1 promoter of the p53 gene, and sho w differential effects of the two classes of antiestrogens in their ability to induce the P1 promoter of the p53 gene in breast cancer cells.