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-I 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 C-14-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-I protein content was correlated with PC-1 gene expression (r = 0.53, p<0.05, n = 14). Exposure to high insulin (10 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