ADP-RIBOSYLATION OF RHO ENHANCES ADHESION OF U937 CELLS TO FIBRONECTIN VIA THE ALPHA-5-BETA-1-INTEGRIN RECEPTOR

To examine the role of Rho GTP binding proteins in the adhesion of mon ocytic cells to fibronectin we used the C3 exoenzyme of Clostridium bo tulinum which ADP-ribosylates and inactivates Rho proteins in situ. Tr eatment of human monocytic U937 cells with C3 exoenzyme (10 mu g/ml, 2 4 h) increased adhesion to fibronectin 2-fold but had no effect on adh esion to collagen or human serum albumin. The increase in fibronectin adhesion was prevented by antibodies against the alpha 5 and beta 1 in tegrin subunits, but surface expression of beta 1 and alpha 5 was not altered. These results suggest that Rho proteins regulate the interact ion of the monocyte alpha 5 beta 1 integrin receptor with fibronectin by post receptor mechanisms.