Effects of tamoxifen and retinoic acid on cell growth and c-myc gene expression in human breast and cervical cancer cells

The effects of estradiol, tamoxifen, and retinoic acid on the proliferation of breast and cervical cancer cells were investigated. Estrogen stimulated only MCF-7 cell growth, whereas tamoxifen and retinoic acid inhibited the proliferation of all cells studied. Northern blot analysis indicated that estradiol up-regulates c-myc mRNA level in all cell lines studied regardless of the estrogen receptor status in the cells. On the contrary, tamoxifen inhibits c-myc gene expression in all cell lines studied except in MCF-7 cells where the c-myc transcript was not affected The inhibitory effect of tamoxifen on c-myc gene expression and cell proliferation in estrogen receptor-negative cells suggest an estrogen receptor-independent mechanism. The results also suggest that different mechanisms are involved in the regulation of cell growth and c-myc gene expression in different cancer cells by estrogen and tamoxifen.