Progesterone induces focal adhesion in breast cancer cells MDA-MB-231 transfected with progesterone receptor complementary DNA

Since the effects of progesterone are mediated mainly via estrogen-dependen t progesterone receptor (PR), the expression of the effects of progesterone may be masked or overridden by the influence of estrogen under conditions in which priming with estrogens is required. We have established a PR-posit ive but estrogen receptor-alpha (ER-alpha) negative breast cancer cell mode l by transfecting PR cDNA into ER-alpha- and PR-negative MDA-MB-231 cells i n order that the functions of progesterone can be studied independently of estrogens. We have demonstrated using this model that progesterone markedly inhibited cell growth. We have also discovered that progesterone induced r emarkable changes in cell morphology and specific adhesion structures. Prog esterone-treated cells became considerably more flattened and well spread t han vehicle-treated control cells. This was associated with a striking incr ease of stress fibers, both in number and diameter, and increased focal con tacts as shown by the staining of focal adhesion proteins paxillin and tali n. There were also distinct increases in tyrosine phosphorylation of focal adhesion protein paxillin and focal adhesion kinase in association with inc reased focal adhesion. The staining of tyrosine-phosphorylated proteins was concentrated at focal adhesions in progesterone-treated cells. More intere stingly, monoclonal antibody (Ab) to beta 1 integrin was able to inhibit pr ogesterone-induced cell spreading and formation of actin cytoskeleton. To o ur knowledge, this is the first report describing a direct effect of proges terone in inducing spreading and adhesion of breast cancer cells, and beta 1-integrin appeared to play an essential role in the effect. It is known th at the initial step of tumor metastasis is the breakaway of tumor cells fro m primary tumor mass when they lose the ability to attach. Hence, progester one-induced cell spreading and adhesion may have significant implications i n tumor metastasis.