Transforming Growth Factor- Abrogates Glucocorticoid-stimulated Tight Junction Formation and Growth Suppression in Rat Mammary Epithelial Tumor Cells

The glucocorticoid and transforming growth factor-α (TGF-α) regulation of growth and cell-cell contact was investigated in the Con8 mammary epithelial tumor cell line derived from a 7,12-dimethylbenz(α)anthracene-induced rat mammary adenocarcinoma. In Con8 cell monolayers cultured on permeable filter supports, the synthetic glucocorticoid, dexamethasone, coordinately suppressed [ 3 H]thymidine incorporation, stimulated monolayer transepithelial electrical resistance (TER), and decreased the paracellular leakage of [ 3 H]inulin or [ 14 C]mannitol across the monolayer. These processes dose dependently correlated with glucocorticoid receptor occupancy and function. Constitutive production of TGF-α in transfected cells or exogenous treatment with TGF-α prevented the glucocorticoid growth suppression response and disrupted tight junction formation without affecting glucocorticoid responsiveness. Treatment with hydroxyurea or araC demonstrated that de novo DNA synthesis is not a requirement for the growth factor disruption of tight junctions. Immunofluorescence analysis revealed that the ZO-1 tight junction protein is localized exclusively at the cell periphery in dexamethasone-treated cells and that TGF-α caused ZO-1 to relocalize from the cell periphery back to a cytoplasmic compartment. Taken together, our results demonstrate that glucocorticoids can coordinately regulate growth inhibition and cell-cell contact of mammary tumor cells and that TGF-α, can override both effects of glucocorticoids. These results have uncovered a novel functional “cross-talk” between glucocorticoids and TGF-α which potentially regulates the proliferation and differentiation of mammary epithelial cells.