βPix-enhanced p38 activation by Cdc42/Rac/PAK/MKK3/6-mediated pathway -: Implication in the regulation of membrane ruffling

beta Pix (PAK-interacting exchange factor) is a recently identified guanine nucleotide exchange factor for Rho family small G protein Cdc42/Rac, The protein interacts with p21-activated protein kinase (PAK) through its SH3 domain. We examined the effect of beta Pix on MAP kinase signaling and cytoskeletal rearrangement in NIH3T3 fibroblast cells. Overexpression of beta Pix enhanced the activation of p38 in the absence of other stimuli and also induced translocation of p38 to the nucleus. This beta Pix-induced p38 activation was blocked by coexpression of dominant-negative Cdc42/Rac or kinase-inactive PAK, indicating that the effect of beta Pix on p38 is exerted through the Cdc42/Rac-PAK pathway and requires PAK kinase activity. The essential role of beta Pix in growth factor-stimulated p38 activation was evidenced by the blocking of platelet-derived growth factor-induced p38 activation in the cells expressing beta Pix SH3m (W43K) and beta Pix DHm (L238R,L239R). In addition, SB203580, a p38 inhibitor, and kinase-inactive p38 (T180A;Y182F) blocked membrane ruffling induced by beta Pix, suggesting that p38 might be involved in mediating beta Pix-induced membrane ruffling. The results in this study suggest that beta Pix might have a role in nuclear signaling, as well as in actin cytoskeleton regulation, and that some part of these cellular functions is possibly mediated by p38 MAP kinase.