Molecular basis for the insulinomimetic effects of C-peptide.
OBJECTIVE
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.


CONCLUSIONS
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.
Grunberger
G
G
Centre for Molecular Medicine and Genetics, Wayne State University, Detroit, MI 48201, USA. g.grunberger@wayne.edu
Qiang
X
X
Li
Z
Z
Mathews
S T
ST
Sbrissa
D
D
Shisheva
A
A
Sima
A A
AA
D016475:3T3 Cells; D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D002096:C-Peptide / Q000494:pharmacology*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects; D006003:Glycogen / Q000096:biosynthesis; D038362:Glycogen Synthase Kinase 3; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism; D037342:Pertussis Toxin; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D019000:Phosphotyrosine / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D000596:Amino Acids; D002096:C-Peptide; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D006003:Glycogen; D038362:Glycogen Synthase Kinase 3; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D020928:Mitogen-Activated Protein Kinases; D037342:Pertussis Toxin; D019869:Phosphatidylinositol 3-Kinases; D010750:Phosphoproteins; D019000:Phosphotyrosine; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D011972:Receptor, Insulin; D019893:Ribosomal Protein S6 Kinases; D010566:Virulence Factors, Bordetella; C526295:Irs1 protein, mouse; C526296:Irs1 protein, rat; C494920:Akt1 protein, rat