Vimentin intermediate filament reorganization by Cdc42: involvement of PAK and p70 S6 kinase

Rho family GTPases play a major role in actin cytoskeleton reorganization. Recent studies have shown that the activation of Rho family GTPases also induces collapse of the vimentin intermediate filament (IF) network in fibroblasts. Here, we report that Cdc42V12 induces the reorganization of vimentin IFs in Hela cells, and such reorganization is independent of actin and microtubule status. We analyzed the involvement of three serine/threonine kinase effectors, MRCK, PAK and p70 S6K in the Cdc42-induced vimentin reorganization. Surprisingly, the ROK-related MRCK is not involved in this IF reorganization. We detected phosphorylation of vimentin Ser72, a site phosphorylated by PAK, after Cdc42 activation. PAK inhibition partially blocked Cdc42-induced vimentin IF collapse suggesting the involvement of other effectors. We report that p70 S6 kinase (S6K)1 participates in this IF rearrangement since the inhibitor rapamycin or a dominant inhibitory S6K could reduce the Cdc42V12 or bradykinin-induced vimentin collapse. Further, inhibition of PAK and S6K in combination very effectively prevents Cdc42-induced vimentin IF collapse. Conversely, only in combination active PAK and S6K could induce a vimentin IF rearrangement that mimics the Cdc42 effect. Thus, Cdc42-induced vimentin reorganization involves PAK and, in a novel cytoskeletal role, p70 S6K.