In an attempt to understand the antiproliferative effects of progestins in endometrial cancer, we have examined the effects of the potent progestin, medroxyprogesterone acetate (MPA), on the cell proliferation and the expression of transforming growth factor (TGF) α and β genes in human endometrial adenocarcinoma cell lines. The two cell lines used were Ishikawa, var 1, and HEC-50. In addition, the effects of exogenous TGF-α and anti-epidermal growth factor (EGF) receptor monoclonal antibody on cell proliferation were determined. Incubation of both cell lines with MPA resulted in a time- and dose-dependent inhibition of cell proliferation. Half-maximal growth inhibition was observed at 0.6 nm. In Ishikawa cells, the relative abundance of TGF-α was significantly reduced by MPA. A significant decrease in TGF-α mRNA was apparent 6 h after exposure to MPA and a further decrease was seen 12–24 h after addition of the progestin. The concentration of TGF-α immunoreactivity in conditioned medium of MPA-treated cells was also significantly reduced compared to control cultures. MPA had no effect on TGF-α expression by HEC-50 cells. EGF mRNA was not detected by Northern blot analysis in either cell type. MPA had no significant effect on EGF receptor mRNA abundance but resulted in a small increase in EGF receptor number in Ishikawa cells. Anti-EGF receptor monoclonal antibody (0.6–6 nm) inhibited Ishikawa cell growth but had no effect on HEC-50 cell proliferation. Exogenous TGF-α stimulated proliferation of both cell lines, but Ishikawa cells were significantly more sensitive to exogenous TGF-α than HEC-50 cells. Furthermore, TGF-α could reverse the growth inhibitory effects of MPA on Ishikawa cells. A decrease in TGF-β mRNA abundance was also observed in MPA-treated Ishikawa and HEC-50 cells. This effect was of small magnitude, variable, and only observed after prolonged exposure to MPA. These observations are consistent with the hypothesis that the antiproliferative effects of progestins on Ishikawa cells are mediated by decreased expression and autocrine action of TGF-α. Since similar growth inhibition is also seen in the HEC-50 cells in which progestins have no effect on TGF-α expression, additional mechanisms are likely to be involved in the antiproliferative effects of progestins in human endometrial cancer.
This work was supported by grants from the National Institute of Cancer Canada and the Medical Research Council, Canada.