C-peptide attenuates protein tyrosine phosphatase activity and enhances glycogen synthesis in L6 myoblasts

Recent studies suggest that C-peptide might play a role in a broad range of biological activities. We have provided evidence that C-peptide stimulates glycogen synthesis in insulin-responsive rat skeletal muscle cells in a dose-related manner. To explore the mechanism by which C-peptide exerts this insulinomimetic effect, here we report the effect of C-peptide on protein tyrosine phosphatase (PTP) activity and phosphorylation of the insulin receptor and insulin receptor substrate-1 (IRS-1). C-peptide inhibited PTP activity in a dose-dependent manner. A reverse bell-shaped dose-response curve was shown with the maximum inhibition of PTP activity at a concentration of 3 nM of C-peptide, which is the same concentration achieving the maximum stimulatory effect on glycogen synthesis. In association with the PTP inhibition by C-peptide, autophosphorylation of the insulin receptor and activation of IRS-1 were enhanced. These results suggest that C-peptide signal transduction may crosstalk with the insulin signaling pathway at the level of the insulin receptor.