Alterations in Transforming Growth Factor-α and -β Production and Cell Responsiveness during the Progression of MCF-7 Human Breast Cancer Cells to Estrogen-Autonomous Growth

Abstract

Hormonal management of breast cancer is confounded by an almost inevitable progression of cell growth from a steroid-regulated to a steroid-autonomous state. We have experimentally induced this progression in the estrogen growth-responsive MCF-7 human breast cancer cell line by long-term culture in the absence of steroids. After an initial period (10–12 weeks) of slowed growth in response to steroid deprivation, rapid, steroid-independent growth rates were consistently established. In these cells, which contained 3-fold elevated, functional estrogen receptor levels (as determined by induction of PgR and transactivation of a transiently transfected estrogen-responsive gene construct), antiestrogens still effectively suppressed cell proliferation, although estrogens only minimally increased the proliferation rate. Depletion of steroids from the growth media also resulted in a marked (70–80%) transient decrease in transforming growth factor (TGF) α mRNA and TGF-α protein production at 2 weeks that was followed by a progressive, partial return to the initial parental TGF-α mRNA and protein levels. In contrast, the mRNAs for TGF-β1,-β2, and -β3 and bioactive TGF-β proteins transiently increased (3–10-fold) at 2 to 10 weeks of steroid deprivation and then returned by 24 weeks to the lower levels of the parental MCF-7 cells. These results suggest that the cells acquired steroid-independent means to regulate the production of these peptides. The long-term steroid-deprived sublines showed a loss of regulation of proliferation by TGF-α or anti-TGF-α antibodies and a 10-fold decrease in sensitivity to the growth-suppressive effects of TGF-β1, despite little change in receptor levels for these factors. The diminished contributions of TGF-α and TGF-βs to the regulation of cell proliferation in long-term steroid-deprived MCF-7 breast cancer cells suggest that the TGFs do not act as major growth regulators in these estrogen-autonomous sublines. However, the marked, transient alterations in the levels of these growth factors indicate that they may play a role in the events which accompany the progression from estrogen-responsive to estrogen-autonomous growth. In addition, continued exposure to estrogen may be needed for the long-term maintenance of cell responsiveness to these TGFs.