We have shown previously that GH stimulates the mitogen-activated protein (MAP) kinases designated ERKs (extracellular signal-regulated kinases) 1 and 2. To examine pathways coupling GH receptor (GHR) to MAP kinase activation, we have determined the effects of GH on SHC-growth factor receptor bound 2-son of Sevenless (SHC-Grb2-SOS) association and activation of Ras, Raf, and MAPERK kinase (MEK). GH promoted the rapid, transient association of SHC with the Grb2-SOS complex, which correlated with the time course of Ras, Raf, and MEK activation. Despite the continuous presence of GH, these activation events were transient with Ras, Raf, and MEK returning to near basal activity by 15 or 30 min. The inactivation of Ras, Raf, and MEK directly correlated with the serine/threonine phosphorylation of SOS and dissociation of SOS from Grb2 but not Grb2 from tyrosine-phosphorylated SHC. Phosphorylation was blocked by the MEK inhibitor, PD98059. Based upon the established functions of the MAP kinase pathway, these data indicate that GH stimulation results in the assembly of a SHC-Grb2-SOS complex that serves to activate Ras and thereby engage the Raf-MEK-ERK pathway. Activation of this pathway generates a feedback kinase cascade that phosphorylates SOS resulting in the dissociation of SHC-Grb2 complexes from SOS, thereby causing a more rapid termination of the signaling pathway than would result from SHC dephosphorylation.
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