Role of group A p21-activated kinases in activation of extracellular- regulated kinase by growth factors

The canonical extracellular-regulated kinase (ERK) signaling cascade, consisting of the Ras-Raf-Mek-ERK module, is critically important to many cellular functions. Although the general mechanism of activation of the ERK cascade is well established, additional noncanonical components greatly influence the activity of this pathway. Here, we focus on the group A p21-activated kinases (Paks), which have previously been implicated in regulating both c-Raf and Mek1 activity, by phosphorylating these proteins at Ser³³⁸ and Ser²⁹⁸, respectively. In NIH-3T3 cells, expression of an inhibitor of all three group A Paks reduced activation of ERK in response to platelet-derived growth factor (PDGF) but not to epidermal growth factor (EGF). Similar results were obtained in HeLa cells using small interference RNA-mediated simultaneous knockdown of both Pak1 and Pak2 to reduce group A Pak function. Inhibition of Pak kinase activity dramatically decreased phosphorylation of Mek1 at Ser²⁹⁸ in response to either PDGF or EGF, but this inhibition did not prevent Mek1 activation by EGF, suggesting that although Pak can phosphorylate Mek1 at Ser²⁹⁸, this event is not required for Mek1 activation by growth factors. Inhibition of Pak reduced the Ser³³⁸ phosphorylation of c-Raf in response to both PDGF and EGF; however, in the case of EGF, the reduction in Ser³³⁸ phosphorylation was not accompanied by a significant decrease in c-Raf activity. These findings suggest that Paks are required for the phosphorylation of c-Raf at Ser ³³⁸ in response to either growth factor, but that the mechanisms by which EGF and PDGF activate c-Raf are fundamentally different.