RhoA activity is required for fibronectin assembly and counteracts beta 1Bintegrin inhibitory effect in FRT epithelial cells

FRT thyroid epithelial cells synthesize fibronectin and organize a network of fibronectin fibrils at the basal surface of the cells. Fibronectin fibri l formation is enhanced by the overexpression of the ubiquitous beta 1A int egrin and is inhibited by the expression of the dominant-negative beta 1B s ubunit. We tested the hypotheses that RhoA activity might mediate the integ rin-dependent fibronectin fibrillogenesis and might counteract beta 1B inte grin inhibitory effect, FRT-beta 1A cells were transfected with a vector ca rrying a dominant negative form of RhoA (RhoAN19) or treated with the C3 tr ansferase exoenzyme, Both treatments inhibited fibronectin assembly and cau sed loss of actin microfilaments and adhesion plaques. On the other hand, F RT-beta 1B cells were transfected with the constitutively activated form of RhoA (RhoAV14) or treated with the E, coli cytotoxic necrotizing factor 1, which directly activates RhoA, Either treatment restored microfilament and adhesion plaque assembly and promoted fibronectin fibril organization. A g reat increase in fibronectin fibril assembly was also obtained by treatment of FRT-beta 1B cells with TGF-beta, Our data indicate that RhoA is require d to promote fibronectin matrix assembly in FRT cells and that the activati on of the signal transduction pathway downstream of RhoA can overcome the i nhibitory effect of beta 1B integrin.