Molecular basis for the insulinomimetic effects of C-peptide

Diabetologia. 2001 Oct;44(10):1247-57. doi: 10.1007/s001250100632.

Abstract

Aims/hypothesis: C-peptide, released by the beta-cells of pancreatic islets, elicits salutary responses in Type I (insulin-dependent) diabetes mellitus but the molecular mechanisms behind these effects are not known. We assessed whether synthetic rat C-peptide stimulates insulin-like cellular effects in a classic insulin target tissue.

Methods: To clarify the molecular mechanisms involved in several insulinomimetic actions, we investigated the effect of C-peptide on the insulin signalling pathway in rat skeletal muscle cells. We used L6 myoblasts and myocytes to measure the effects of C-peptide or insulin or both on glycogen synthesis and amino acid uptake. We also studied the effects of C-peptide on insulin receptor autophosphorylation, its tyrosine kinase activity, phosphorylation 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 tyrosine 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 to 10 nmol/l) combined with submaximal C-peptide concentrations led to additive 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 that 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 mechanism. 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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • 3T3 Cells
  • Amino Acids / metabolism
  • Animals
  • C-Peptide / pharmacology*
  • Calcium-Calmodulin-Dependent Protein Kinases / metabolism
  • Cell Line
  • Enzyme Activation / drug effects
  • Glycogen / biosynthesis
  • Glycogen Synthase Kinase 3
  • Insulin / pharmacology*
  • Insulin Receptor Substrate Proteins
  • Mice
  • Mitogen-Activated Protein Kinases / metabolism
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / metabolism
  • Pertussis Toxin
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Phosphotyrosine / metabolism
  • Protein-Serine-Threonine Kinases*
  • Proto-Oncogene Proteins / metabolism
  • Proto-Oncogene Proteins c-akt
  • Rats
  • Receptor, Insulin / metabolism
  • Ribosomal Protein S6 Kinases / metabolism
  • Signal Transduction
  • Virulence Factors, Bordetella / pharmacology

Substances

  • Amino Acids
  • C-Peptide
  • Insulin
  • Insulin Receptor Substrate Proteins
  • Irs1 protein, mouse
  • Irs1 protein, rat
  • Phosphoproteins
  • Proto-Oncogene Proteins
  • Virulence Factors, Bordetella
  • Phosphotyrosine
  • Glycogen
  • Pertussis Toxin
  • Phosphatidylinositol 3-Kinases
  • Receptor, Insulin
  • Akt1 protein, rat
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases
  • Calcium-Calmodulin-Dependent Protein Kinases
  • Mitogen-Activated Protein Kinases
  • Glycogen Synthase Kinase 3