Biological activity and binding of estradiol to SK-Mel 23 human melanoma cells

Patients expressing estradiol receptors in melanoma cells have been reported to have a better prognosis. We therefore decided to investigate the in vitro effects of beta-estradiol and tamoxifen on the growth and tyrosinase activity of SK-Mel 23 human melanoma cells. Twenty-four-hour treatment with 0.4 nM beta-estradiol inhibited cell proliferation in 30% (0.70 +/- 0.03 x 10(5) cells) and increased tyrosinase activity in 50% (7130.5 +/- 376.5 cpm/10(5) cells), as compared to untreated cells (1.0 +/- 0.05 x 10(5) cells and 4769 +/- 25.5 cpm/10(5) cells, respectively). Both responses were completely (100%) blocked by 1 microM tamoxifen. Higher concentrations (up to 1.6 nM) or longer treatments (up to 72 h) did not result in a larger effect of the hormone on proliferation or tyrosinase activity. Competition binding assays demonstrated the presence of binding sites to [2,4,6,7-3H]-beta-estradiol, and that the tritiated analogue was displaced by the unlabeled hormone (1 nM to 100 microM, Kd = 0.14 microM, maximal displacement of 93%) or by 10 microM tamoxifen (displacement of 60%). Beta-estradiol also increased the phosphorylated state of two proteins of 16 and 46 kDa, after 4-h treatment, as determined by Western blot. The absorbance of each band was 1.9- and 4-fold the controls, respectively, as determined with Image-Pro Plus software. Shorter incubation periods with beta-estradiol did not enhance phosphorylation; after 6-h treatment with the hormone, the two proteins returned to the control phosphorylation levels. The growth inhibition promoted by estradiol may explain the better prognosis of melanoma-bearing women as compared to men, and open new perspectives for drug therapy.