Increased adipose tissue PC-1 protein content, but not tumour necrosis factor-alpha gene expression, is associated with a reduction of both whole body insulin sensitivity and insulin receptor tyrosine-kinase activity

In the present study we measured PC-1 content, tumour necrosis factor (TNF)-alpha gene expression, and insulin stimulation of insulin receptor tyrosine-kinase activity in adipose tissue from nonobese, non-diabetic subjects. These parameters were correlated with in vivo insulin action as measured by the intravenous insulin tolerance test (K-itt values). PC-1 content was negatively correlated with K-itt values (r = -0.5, p = 0.04) and positively with plasma insulin levels both fasting (r = 0.58, p = 0.009) and after 120 min during oral glucose tolerance test (OGTT) (r = 0.67, p = 0.002). Moreover, adipose tissue PC-1 content was higher in relatively insulin-resistant subjects (K-itt values lower than 6) than in relatively insulin-sensitive subjects (K-itt values higher than 6) (525+/-49 ng/mg protein vs 336+/-45, respectively, p = 0.012). Adipose tissue insulin receptor tyrosine-kinase activity in response to insulin was significantly lower at all insulin concentrations tested (p = 0.017, by two-way analysis of variance test) in insulin-resistant than in insulin-sensitive subjects (K-itt values lower or higher than 6, respectively). In contrast to PC-1, no significant correlation was observed between adipose tissue TNF-alpha mRNA content and K-itt values, and plasma insulin levels, both fasting and at after 120 min during OGTT. Also, no difference was observed in TNF-alpha mRNA content between subjects with K-itt values higher or lower than 6. These studies in adipose tissue, together with our previous studies in skeletal muscle raise the possibility that PC-1, by regulating insulin receptor function, may play a role in the degree of insulin sensitivity in non-obese, non-diabetic subjects.