INHIBITION OF C-MYC IN BREAST AND OVARIAN-CARCINOMA CELLS BY 1,25-DIHYDROXYVITAMIN-D(3), RETINOIC ACID AND DEXAMETHASONE

The role and regulation of the c-myc protooncogene in breast and ovari an neoplasms is receiving increased attention. The downregulation of t he c-myc protooncogene by 1,25-dihydroxyvitamin D3 (calcitriol), retin oic acid (RA) and dexamethasone (Dex) is closely associated with growt h inhibition in leukemic cells. Calcitriol, RA and Dex have anti-proli ferative activity in breast and gynecologic carcinoma cells; however, the regulation of c-myc by these agents in breast and ovarian cancers is mostly unknown. We have addressed the regulation of c-myc in these cancers using an adaptation of a novel method which employs an immunoh istochemical procedure to detect c-myc protein followed by quantificat ion of c-myc staining with computerized image analysis. This system re presents an alternative to protein product assay by Western blotting a nd is straightforward, rapid (1 day), can be carried out on a small sc ale and provides a sample size that readily facilitates statistical an alysis of assay data. In MCF-7 human breast cancer cells, c-myc was su ppressed 29% by 0.5 nM Dex, 45% by 0.01 nM RA and 54% by 100 nM calcit riol after 24 h of drug treatment. At the same hormone concentrations, growth was inhibited 18% by Dex, 18% by RA and 39% by calcitriol afte r 3 days of treatment (p < 0.05 for all hormones). Similar patterns of growth and c-myc inhibition were seen in T47D human breast cancer cel ls and NIH:OVCAR3 human ovarian cancer cells, with the exception of De x in T47D cells, which caused no inhibition of c-myc or growth. Parall el control experiments in MCF-7, NIH:OVCAR3 and T47D cells showed that none of the three hormones suppressed epithelial membrane antigen, a non-growth-related protein. This suggested that the hormones specifica lly influenced c-myc expression. In conclusion, this study has shown t hat c-myc was repressed by calcitriol, RA and Dex in breast and ovaria n carcinoma lines where these same hormonal agents also inhibit growth , and suggests that these hormonal agents have commensurate effects on c-myc expression and growth in the breast and ovarian carcinoma lines examined.