TRANSFORMING GROWTH-FACTOR-ALPHA ABROGATES GLUCOCORTICOID-STIMULATED TIGHT JUNCTION FORMATION AND GROWTH SUPPRESSION IN RAT MAMMARY EPITHELIAL TUMOR-CELLS

The glucocorticoid and transforming growth factor-alpha (TGF-alpha) re gulation of growth and cell-cell contact was investigated in the Con8 mammary epithelial tumor cell line derived from a 7,12-dimethylbenz(al pha)anthracene-induced rat mammary adenocarcinoma. In Con8 cell monola yers cultured on permeable filter supports, the synthetic glucocortico id, dexamethasone, coordinately suppressed [H-3]thymidine incorporatio n, stimulated monolayer transepithelial electrical resistance (TER), a nd decreased the paracellular leakage of [H-3]inulin or [C-14]mannitol across the monolayer. These processes dose dependently correlated wit h glucocorticoid receptor occupancy and function. Constitutive product ion of TGF-alpha in transfected cells or exogenous treatment with TGF- alpha prevented the glucocorticoid growth suppression response and dis rupted tight junction formation without affecting glucocorticoid respo nsiveness. Treatment with hydroxyurea or araC demonstrated that de nov o 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-alpha caused ZO-1 to relo calize from the cell periphery back to a cytoplasmic compartment. Take n together, our results demonstrate that glucocorticoids can coordinat ely regulate growth inhibition and cell-cell contact of mammary tumor cells and that TGF-alpha, can override both effects of glucocorticoids . These results have uncovered a novel functional ''cross-talk'' betwe en glucocorticoids and TGF-alpha which potentially regulates the proli feration and differentiation of mammary epithelial cells.