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 Ser(338) and Ser(298), 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 Ser(298) 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 Ser(298), this event is not required for Mek1 activation by growth factors. Inhibition of Pak reduced the Ser(338) phosphorylation of c-Raf in response to both PDGF and EGF; however, in the case of EGF, the reduction in Ser(338) 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.
Beeser
Alexander
A
Tumor Cell Biology Program, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111, USA.
Jaffer
Zahara M
ZM
Hofmann
Clemens
C
Chernoff
Jonathan
J
D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D004789:Enzyme Activation; D004815:Epidermal Growth Factor / Q000494:pharmacology*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D048369:MAP Kinase Kinase 1 / Q000378:metabolism; D051379:Mice; D041681:NIH 3T3 Cells; D010766:Phosphorylation; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D012694:Serine; D015398:Signal Transduction; D054462:p21-Activated Kinases
D010982:Platelet-Derived Growth Factor; D034741:RNA, Small Interfering; D012694:Serine; D004815:Epidermal Growth Factor; C515903:PAK1 protein, human; C515907:PAK2 protein, human; C515905:Pak1 protein, mouse; C515909:Pak2 protein, mouse; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D054462:p21-Activated Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D048369:MAP Kinase Kinase 1