Insulin receptor substrate (IRS) 1 is reduced and IRS-2 is the main docking protein for phosphatidylinositol 3-kinase in adipocytes from subjects with non-insulin-dependent diabetes mellitus

Abstract

The large docking protein IRS-1 is a major substrate for the insulin receptor and other tyrosine kinases. It plays a key role in eliciting many of insulin's actions, including binding and activation of phosphatidylinositol (PI) 3-kinase and the subsequent increase in glucose transport. Gene disruption of IRS-1 in mice is associated with an impaired insulin-stimulated glucose disposal in vivo and glucose transport in vitro, but the survival of the animals and residual insulin sensitivity is dependent on the presence of the alternative docking protein IRS-2. We examined the expression and function of IRS-1 and IRS-2 in adipocytes from healthy and diabetic individuals. Cells from subjects with non-insulin-dependent diabetes mellitus (NIDDM), but not with insulin-dependent diabetes mellitus, had an impaired insulin effect and a marked reduction (70 ؎ 6%) in the expression of IRS-1 protein, whereas IRS-2 was unchanged. In normal cells, IRS-1 was the main docking protein for the binding and activation of insulin-stimulated PI 3-kinase; IRS-2 was also functional but required a higher insulin concentration for a similar binding and activation of PI 3-kinase. In contrast in NIDDM cells with a low IRS-1 content, IRS-2 became the main docking protein. These findings may provide important reasons for the insulin resistance in NIDDM.

Improved insulin sensitivity and adipose tissue dysregulation after short-term treatment with pioglitazone in non-diabetic, insulin-resistant subjects

“…Previous studies from this laboratory have shown an early association between adipose tissue dysfunction and insulin resistance [17][18][19][20][21]31].…”

“…Ten non-diabetic subjects, previously identified as having low expression of IRS-1 and GLUT4 protein in the adipose cells as markers of insulin resistance [17,18], volunteered to undergo 3 weeks of treatment with pioglitazone (Actos, 30 mg/day). Body weight and height were recorded with standard techniques.…”

“…Detergent-insoluble material was sedimented through centrifugation at 12,000×g; this was done for 10 min at 4°C. Supernatants were collected and stored at −80°C prior to use [17]. Protein concentration was measured using the bicinchonic acid method (Pierce, Rockford, IL, USA).…”

“…The insulin-stimulated activation of phosphoinositide-3-kinase (PI3-kinase), phosphoinositide-dependent protein kinase B (PKB)/Akt and glucose transport in the adipose cells are also impaired in insulin-resistant states [17,18]. In addition to these perturbations, we have identified other markers for dysregulated adipose tissue in insulin resistance suggesting impaired adipose cell differentiation, i.e.…”

“…For instance, several key molecules for insulin signalling and action, such as IRS-1 and GLUT4, are reduced in adipose cells from insulinresistant subjects with or without type 2 diabetes [17,18].…”