Cells by Epidermal Growth Factor and Insulin the Antiestrogen Hydroxyclomiphene on Human Breast Cancer Modulation of the Growth-inhibitory Effects of Progestins and Updated Version

The molecular basis of the growth-inhibitory effects of progestins or antiestrogens in human breast cancer has not been fully elucidated. Both direct actions and indirect actions, where the growth inhibition results from modulation of the production of, and/or the response to, growth factors, have been proposed. In this study the ability of some growth factors to modulate progestin-induced inhibition of cell proliferation was investigated in vitro, using 1-471) human breast cancer cells. When I -471) cells grown in insulin-containing medium were treated for 4 to 5 days with the synthetic progestin, ORG 2058, at a concentration of 10 UM, cell numbers were reduced to 10 to 20% of control. Simulta neous treatment with epidermal growth factor (EGF) and ORG 2058 led to a partial reversal of the growth-inhibitory effect of the progestin. The magnitude of the effect of EGF was concentration dependent, being halfmaximal at 0.48 ng/ml (0.08 MM)and maximal at concentrations > 5 ng/ ml (> 0.8 n\i ), where cell numbers were increased by 50% compared to those in the presence of ORG 2058 alone. ORG 2058 was no more potent in the absence of insulin, and, after several passages in insulin-free medium, addition of insulin failed to modulate the effect of ORG 2058. However, when maximal concentrations of insulin (5 Mg/ml) and EGF (10 ng/ml) were administered together with ORG 2058, insulin and EGF appeared to act synergistically to reduce the ORG 2058-induced inhibi tion of proliferation. In similar experiments in which cells were treated with hydroxyclomiphene, a potent antiestrogen, insulin was shown to partially reverse the growth-inhibitory effects of hydroxyclomiphene. Significant increases in cell number above hydroxyclomiphene-treated controls were apparent at insulin concentrations > 50 ng/ml, and at 5 Mg/ml the increase was approximately 2-fold. In contrast to the situation with progestins, simul taneous treatment with EGF and insulin had only an additive effect in reversing the growth-inhibitory effect of the antiestrogen. The results are compatible with the hypothesis that part of the growthinhibitory effects of progestin and antiestrogen on human breast cancer cell proliferation is mediated by inhibition of autocrine growth factor production. However, they do not exclude more direct mechanisms in volving modulation of progesterone and/or estrogen receptors by EGF and/or insulin. In addition the differential effects of insulin alone, and insulin in combination with EGF, on progestin and antiestrogen-treated cells suggest that, although progestin and antiestrogen-modulated path ways may converge, they are not identical in terms of their interactions with growth factor-mediated pathways.