Rac1 mediates phorbol 12-myristate 13-acetate-induced migration of glioblastoma cells via paxillin

Previously, we reported that phorbol 12-myristate 13-acetate (PMA)-activated protein kinase C (PKC) induced Rac1 activation in A172 glioblastoma cells. In this study, we investigated the mechanism of PMA-activated PKC-induced migration of glioblastoma cells by focusing on Rac1. PMA-induced formation of lamellipodia and focal complexes following migration were blocked by inhibiting Rac1 with small interfering RNA (siRNA), implicating Rac1 in PMA-induced glioblastoma cell migration. PMA-activated PKC induced phosphorylation of c-jun N-terminal kinase (JNK), one of the downstream effectors of Rac1. Immunohistochemical analysis showed that phosphorylated JNK was translocated to paxillin-containing focal complexes upon PMA stimulation and that Rac1 siRNA blocked these phenomena. These results suggest that phosphorylated JNK functions in cell migration and that JNK phosphorylation and translocation are mediated by Rac1. Furthermore, inhibition of Rac1 reduced phosphorylation of paxillin, a focal adhesion component and a downstream effector of JNK, at serine 178 (Ser178). Paxillin phosphorylation at this site has been shown to be involved in cell migration. Immunohistochemical analysis detected phosphorylation of paxillin (Ser178) in focal complexes upon PMA stimulation that was blocked by Rac1 siRNA. SP600125, a JNK inhibitor, also blocked PMA-induced phosphorylation of paxillin and aggregation of phosphorylated paxillin (Ser178) in focal complexes. In conclusion, paxillin is a critical downstream effector of Rac1 that may be involved in PMA-stimulated migration presumably by modulating the integrity of focal complex formation.