Proinsulin C-peptide rapidly stimulates mitogen-activated protein kinases in Swiss 3T3 fibroblasts: requirement of protein kinase C, phosphoinositide3-kinase and pertussis toxin-sensitive G-protein

It has been demonstrated that proinsulin C-peptide possesses several biolog ical activities and that its specific binding sites are present on the surf ace of cell membranes. However, the molecular and cellular mechanisms of C- peptide actions are poorly known. In the present study we examined the poss ible involvement of the mitogen-activated protein kinase (MAPK) pathway in C-peptide effects. C-peptide induced the phosphorylation of MAPK [p44 extra cellular signal-regulated kinase 1 (ERK1) and p42 ERK2] in Swiss 3T3 and 3T 3-F442A fibroblasts but not in 3T3-L1 fibroblasts and some other cell lines such as L6E9 muscle cells. In Swiss 3T3 cells, C-peptide-induced phosphory lation of MAPK was dependent on time and concentration, being maximal at 1 min and at 1 nM C-peptide and was accompanied by an increase in MAPK activi ty and MAPK kinase (MEK) phosphorylation. The MAPK phosphorylation by C-pep tide was abolished by treatment with pertussis toxin (PTX) and also with a MEK inhibitor, PD 98059. In addition, MAPK phosphorylation was attenuated b y treatment with a phosphoinositide 3-kinase (PI-3K) inhibitor. wortmannin, and with a protein kinase C (PKC) inhibitor, GF109203X, and by down-regula tion of PKC by prolonged treatment with PMA. Similar effects of the inhibit ors and PTX were found on the MAPK phosphorylation induced by neuropeptide Y. These results suggest that C-peptide activates MAPK through a putative G (1)/G(0)-linked receptor for C-peptide and through PI-3K-dependent and PKC- dependent pathways.