Insulin, Insulin-like Growth Factor-I, and Platelet-Derived Growth Factor Activate Extracellular Signal-Regulated Kinase by Distinct Pathways in Muscle Cells

We have investigated the signaling pathways initiated by insulin, insulin-like growth factor-1 (IGF-I), and platelet-derived growth factor (PDGF) leading to activation of the extracellular signal-regulated kinase (ERK) in L6 myotubes. Insulin but not IGF-I or PDGF-induced ERK activation was abrogated by Ras inhibition, either by treatment with the farnesyl transferase inhibitor FTP III, or by actin disassembly by cytochalasin D, previously shown to inhibit Ras activation. The protein kinase C (PKC) inhibitor bisindolylmaleimide abolished PDGF but not IGF-I or insulin-induced ERK activation. ERK activation by insulin, IGF-I, or PDGF was unaffected by the phosphatidylinositol 3-kinase inhibitor wortmannin but was abolished by the MEK inhibitor PD98059. In contrast, activation of the pathway involving phosphatidylinositol 3-kinase (PI3k), protein kinase B, and glycogen synthase kinase 3 (GSK3) was mediated similarly by all three receptors, through a PI 3-kinase-dependent but Ras- and actin-independent pathway. We conclude that ERK activation is mediated by distinct pathways including: (i) a cytoskeleton- and Ras-dependent, PKC-independent, pathway utilized by insulin, (ii) a PKC-dependent, cytoskeleton- and Ras-independent pathway used by PDGF, and (iii) a cytoskeleton-, Ras-, and PKC-independent pathway utilized by IGF-I.