Transforming Growth Factor-α Abrogates the Glucocorticoid Stimulation of Tight Junction Formation and Reverses the Steroid-Induced Down-regulation of Fascin in Rat Mammary Epithelial Tumor Cells by a Ras-Dependent Pathway

In Con8 rat mammary epithelial tumor cells, the synthetic glucocorticoid dexamethasone stimulates transepithelial electrical resistance (TER), promotes the remodeling of apical junctions, and down-regulates the level of fascin, an actin-bundling protein that can bind to β-catenin. We have previously shown that ectopic expression of fascin prevented the glucocorticoid-mediated recruitment of tight junction and adherens junction proteins to the site of cell–cell contact. Here we demonstrate that exogenous treatment or constitutive production of transforming growth factor-α (TGF-α) ablated the dexamethasone down-regulation of the fascin protein level and disrupted the dexamethasone-induced remodeling of the apical junction and stimulation of the monolayer TER. The response to TGF-α was polarized in that basolateral, but not apical, exposure to this growth factor coordinately reversed the steroid control of fascin production and tight junction formation. Expression of dominant negative RasN17 or treatment with the PD098059 MEK inhibitor abolished or attenuated the TGF-α disruptive effects on TER, junction remodeling, and fascin protein levels. Our results implicate the regulation of fascin protein levels as a target of cross-talk between the Ras-dependent growth factor signaling and glucocorticoid signaling pathways that controls tight junction dynamics in mammary epithelial tumor cells. We propose that reversing the down-regulation of fascin is critical for the ability of TGF-α to disrupt the glucocorticoid-induced remodeling of the apical junction that leads to tight junction formation.