ACTIVATION OF THE SMALL GTP-BINDING PROTEINS RHO AND RAC BY GROWTH-FACTOR RECEPTORS

The small GTP-binding proteins, rho and rac, control signal transducti on pathways that link growth factor receptors to the activation of act in polymerization. In Swiss 3T3 cells, rho proteins mediate the lysoph osphatidic acid and bombesin-induced formation of focal adhesions and actin stress fibres, whilst rac proteins are required for the platelet -derived growth factor-, insulin-, bombesin- and phorbol ester (phorbo l 12-myristate 13-acetate)-stimulated actin polymerization at the plas ma membrane that results in membrane ruffling. To investigate the role of p85/p110 phosphatidylinositol 3-kinase in the rho and rac signalli ng pathways, we have used a potent inhibitor of this activity, wortman nin. Wortmannin has no effect on focal adhesion or actin stress fibre formation induced by lysophosphatidic acid, bombesin or microinjected recombinant rho protein. In contrast, it totally inhibits plasma membr ane edge-ruffling induced by platelet-derived growth factor and insuli n though not by bombesin, phorbol ester or microinjected recombinant r ac protein. We conclude that phosphatidylinositol 3,4,5 trisphosphate mediates activation of rac by the platelet-derived growth factor and i nsulin receptors. The effects of lysophosphatidic acid on the Swiss 3T 3 actin cytoskeleton can be blocked by the tyrosine kinase inhibitor, tyrphostin. Since tyrphostin does not inhibit the effects of microinje cted rho protein, we conclude that lysophosphatidic acid activation of rho is mediated by a tyrosine kinase.