PDGF-BB-mediated activation of p42(MAPK) is independent of PDGF beta-receptor tyrosine phosphorylation

Herein, we investigated the activity of mitogen-activated,protein kinase (M APK), a key component of downstream signaling events, which is activated su bsequent to platelet-derived growth factor (PDGF)-BB stimulation. Specifica lly, p42(MAPK) activity peaked 60 min after addition of PD,GF-BB, declined thereafter, and was determined not to be a direct or necessary component of glycosaminoglycan (GAG) synthesis. PDGF-BB also activated MAPK kinase 2 (M APKK2) but had no effect on MAPKK1 and Raf-1 activity, Chemical inhibition of Janus kinase, phosphatidylinositol 3-kinase, Src kinase, or tyrosine pho sphorylation inhibition of the PDGF beta -receptor (PDGFR-beta) did not abr ogate PDGF-BB-induced p42(MAPK) activation or its threonine or tyrosine pho sphorylation. A dominant negative cytoplasmic receptor for hyaluronan-media ted motility variant 4 (RHAMMv4), a regulator of MAPKK-MAPK interaction and activation, did not inhibit PDGF-BB-induced p42(MAPK) activation nor did a construct expressing PDGFR-beta with cytoplasmic tyrosines mutated to phen ylalanine. However, overexpression of a dominant negative PDGFR-beta lackin g the cytoplasmic signaling domain abrogated p42(MAPK) activity. These resu lts suggest that PDGF-BB-mediated activation of p42(MAPK) requires the PDGF R-beta but is independent of its tyrosine phosphorylation.