Tyrosine phosphorylation-dependent and -independent role of Shc in the regulation of IGF-1-induced mitogenesis and glycogen synthesis

To examine the functional role of She tyrosine phosphorylation in IGF-1 sig naling, wild-type (WT)-Shc and Y239,240,317F (3F)-Shc were transiently tran sfected into L6 myoblasts. IGF-l signaling was compared among the transfect ed cells. IGF-1-induced tyrosine phosphorylation of She and its subsequent association with Grb2 were increased in WT-Shc cells, whereas they were dec reased in 3F-Shc cells compared with those in parental L6 cells. Consistent with their changes, IGF-1-induced MAP activation and thymidine incorporati on were enhanced in WT-Shc cells, whereas they were again decreased in 3F-S hc cells. It is possible that She and insulin receptor substrate (IRS)-1 ca n interact competitively, via their phosphotyrosine binding (PTB) domains, with the activated IGF-l receptor. In this regard, IGF-1-induced tyrosine p hosphorylation of IRS-1 was decreased by overexpressing both WT-Shc and 3F- Shc cells. Consistent with the decrease, IGF-1-induced IRS-1 association wi th the p85 subunit of PI3K and activation of PI3K and Akt were reduced in b oth WT-Shc and 3F-Shc cells. As a result, IGF-1-induced glycogen synthesis was also decreased in both cells. Furthermore, expression of She PTB domain alone inhibited IGF-l stimulation of Akt and glycogen synthesis. These res ults indicate that tyrosine phosphorylation of She is important for IGF-1 s timulation of MAPK leading to mitogenesis and that She, via its PTB domain, negatively regulates IGF-1-induced glycogen synthesis by competing with IR S-1, which is not relevant to She tyrosine phosphorylation.