ANTAGONISTIC REGULATION OF TIGHT JUNCTION DYNAMICS BY GLUCOCORTICOIDSAND TRANSFORMING GROWTH-FACTOR-BETA IN MOUSE MAMMARY EPITHELIAL-CELLS

The synthetic glucocorticoid, dexamethasone, stimulated the transepith elial electrical resistance and suppressed the DNA synthesis of 31EG4 nontransformed mouse mammary epithelial cells, The addition of transfo rming growth factor-beta 1 (TGF-beta) to mammary cells simultaneously with or up to 24 h after dexamethasone treatment prevented the steroid induction of transepithelial electrical resistance and stimulated the incorporation of [H-3]thymidine. However, the TGF-beta inhibition of tight junction formation did not require de novo DNA synthesis. Confoc al microscopy revealed that the organized immunostaining pattern of th e tight junction protein, ZO-1, and F-actin at the cell periphery was disrupted by TGF-beta, resulting in disorganized and diffuse staining patterns throughout the cell. Western blot analysis demonstrated that TGF-beta did not alter the protein levels of ZO-1, In contrast to cell s not treated or pretreated with steroid for up to 24 h, TGF-beta had no effect on cells pretreated with dexamethasone for 48 h, Transfectio n of chimeric reporter genes containing promoters responsive to either glucocorticoid or TGF-beta demonstrated that the mutual antagonism of tight junction dynamics by dexamethasone and TGF-beta occurs in the p resence of intact signaling pathways. Taken together, our results esta blish for the first time that glucocorticoids and TGF-beta can antagon istically regulate tight junction formation in a nontransformed mammar y cell line.