High insulin levels do not influence PC-1 gene expression and protein content in human muscle tissue and hepatoma cells

Background To verify whether insulin levels influence PC-1 tissue content, we studied PC-1 gene expression and protein content in skeletal muscle of patients with insulinoma, a model of primary hyperinsulinemia. Data were compared with those obtained in matched insulin sensitive or resistant healthy subjects. In addition, the effect of high insulin concentration on PC-1 protein content was studied in HepG2 cells. Methods The following measurements were performed: insulin sensitivity by euglycemic clamp; PC-1 protein content and insulin receptor autophosphorylation by specific ELISAs; PC-1 gene expression by competitive polymerase chain reaction (PCR); phosphatidyl-inositol-3 kinase by immunoprecipitation and thin layer chromatography; glycogen synthesis by 14C-glucose incorporation. Results Muscle PC-1 content was similar in the insulinoma patients and in insulin sensitive controls but higher (p<0.01) in insulin resistant controls (21.9±4.6 ng/mg protein, 23.8±3.9, 48.0±8.7, respectively). PC-1 protein content was inversely correlated with insulin sensitivity (r=−0.5, p<0.015) but with neither plasma insulin nor glucose levels. PC-1 protein content was correlated with PC-1 gene expression (r=0.53, p<0.05, n=14). Exposure to high insulin (100 nmol/l for 16 h) caused a significant (p<0.05–0.01) impairment of insulin receptor autophosphorylation, phosphatidyl-inositol-3 kinase activity and glycogen synthesis, but not of PC-1 protein content (114±3 vs 102±14 ng/mg protein) in HepG2 cells. Conclusion These findings suggest that chronic high insulin levels do not influence PC-1 expression. Copyright © 2000 John Wiley & Sons, Ltd.