LYSOPHOSPHATIDIC ACID INDUCES THREONINE PHOSPHORYLATION OF TIAM1 IN SWISS 3T3 FIBROBLASTS VIA ACTIVATION OF PROTEIN-KINASE-C

The Rho family of GTPases plays an important role in the control of ce ll shape, adhesion, movement, and growth. Several guanine nucleotide e xchange factors have been identified that activate Rho family GTPases by promoting the binding of GTP to these proteins. However, little is known concerning the regulation of these GDP/GTP exchange factors. In this study, we demonstrate that lysophosphatidic acid (LPA) induces a rapid, sustainable phosphorylation of the Rac1-specific nucleotide exc hange factor Tiam1 in Swiss 3T3 fibroblasts. LPA stimulated Tiam1 phos phorylation in a dose-dependent manner, and the protein was phosphoryl ated on threonine, but not tyrosine or serine, Tiam1 phosphorylation w as also induced by platelet-derived growth factor, endothelin-1, bombe sin, and bradykinin but not by epidermal growth factor. Significantly, pretreatment of Swiss 3T3 fibroblasts with 1 mu M phorbol 12-myristat e 13-acetate for 24 h, or with the selective protein kinase C inhibito r Ro-31-8220, reduced LPA-stimulated phosphorylation of Tiam1 by appro ximately 75%. Moreover, acute stimulation with 100 nM phorbol 12-myris tate 13-acetate was sufficient to induce Tiam1 phosphorylation in vivo , and protein kinase C could phosphorylate purified Tiam1 on threonine residues in vitro. These data indicate that agonist-induced phosphory lation of Tiam1 is a general mechanism and suggest that it is likely t o be important in its regulation. Protein kinase C appears to play a k ey role in phosphorylation of Tiam1.