Molecular basis for the insulinomimetic effects of C-peptide

Aims/hypothesis. C-peptide, released by the beta-cells of pancreatic islets , elicits salutary responses in Type I (insulin-dependent) diabetes mellitu s but the molecular mechanisms behind these effects are not known. We asses sed whether synthetic rat C-peptide stimulates insulin-like cellular effect s in a classic insulin target tissue. Methods. To clarify the molecular mechanisms involved in several insulinomi metic actions, we investigated the effect of C-peptide on the insulin signa lling pathway in rat skeletal muscle cells. We used L6 myoblasts and myocyt es to measure the effects of C-peptide or insulin or both on glycogen synth esis and amino acid uptake. We also studied the effects of C-peptide on ins ulin receptor autophosphorylation, its tyrosine kinase activity, phosphoryl ation of IRS-1, PI 3-kinase, Akt, p90Rsk, MAPK, and GSK3 in these cells. Results. In L6 cells, physiological concentrations of C-peptide (0.3-3 nmol /l) significantly activated insulin receptor tyrosine kinase, IRS-1 tyrosin e phosphorylation, PI 3-kinase activity, MAPK phosphorylation, p90Rsk, and GSK3 phosphorylation. A scrambled C-peptide sequence - the control - showed no effects. Wortmannin blocked C-peptide-induced glycogen synthesis while pertussis toxin had no effect. Only submaximal insulin concentrations (up t o 10 nmol/l) combined with submaximal C-peptide concentrations led to addit ive effects. Conclusion/interpretation. C-peptide added to the maximal insulin dose (100 nmol/l) did not increase the effect of insulin alone. We thus conclude tha t the same signalling elements are used by both ligands. However, the lack of Akt activation by C-peptide and the bell-shaped dose response induced by C-peptide indicate that C-peptide has some effects by another distinct mec hanism. We speculate that C-peptide could modulate the metabolic effects of insulin by enhancing them at low hormone concentrations and dampening them at high hormone concentrations.