Tumor Necrosis Factor α-induced Pancreatic β-Cell Insulin Resistance Is Mediated by Nitric Oxide and Prevented by 15-Deoxy-Δ12,14-prostaglandin J2 and Aminoguanidine

Abstract

Recent studies have identified a-cell insulin receptor that functions in the regulation of protein translation and mitogenic signaling similar to that described for insulin-sensitive cells. These findings have raised the novel possibility that-cells may exhibit insulin resistance similar to skeletal muscle, liver, and fat. To test this hypothesis, the effects of tumor necrosis factor-(TNF), a cytokine proposed to mediate insulin resistance by interfering with insulin signaling at the level of the insulin receptor and its substrates, was evaluated. TNF inhibited p70 s6k activation by glucose-stimulated-cells of the islets of Langerhans in a dose-and time-dependent manner, with maximal inhibition observed at 20-50 ng/ml, detected after 24 and 48 h of exposure. Exogenous insulin failed to prevent TNF-induced inhibition of p70 s6k , suggesting a defect in the insulin signal-ing pathway. To further define mechanisms responsible for this inhibition and also to exclude cytokine-induced nitric oxide (NO) as a mediator, the ability of exogenous or endogenous insulin inhibitors of nitric-oxide syn-thase (NOS) activity, aminoguanidine or N-monomethyl-L-arginine, was evaluated. Unexpectedly, TNF and also interleukin 1 (IL-1)-induced inhibition of p70 s6k was completely prevented by inhibitors that block NO production. Western blot analysis verified inducible NOS (iNOS) expression after TNF exposure. Furthermore, the ability of IL-1 receptor antagonist protein, IRAP, to block TNF-induced inhibition of p70 s6k indicated that activation of intra-islet macrophages and the release of IL-1 that induces iNOS expression in-cells was responsible for the inhibitory effects of TNF. This mechanism was confirmed by the ability of the peroxisome prolif-erator-activated receptor-agonist 15-deoxy-12,14-pros-taglandin J 2 to attenuate TNF-induced insulin resistance by down-regulating iNOS expression and/or blocking IL-1 release from activated macrophages. Overall, TNF-mediated insulin resistance in-cells is characterized by a global inhibition of metabolism mediated by NO differing from that proposed for this proin-flammatory cytokine in insulin-sensitive cells. A hallmark of human type 2 diabetes is an initial attenuated responsiveness of cells to insulin (a term designated as insulin resistance) that is countered or compensated by the ability of pancreatic-cells to secrete more insulin than normally required to achieve euglycemia (1-3). Sustained hyperglycemia, however, occurs when the ability of-cells to over-secrete insulin fails, resulting in overt diabetes. This sequence of events has generated the concept that a specific defect in the insulin-signaling pathway of insulin-sensitive cells such as skeletal muscle, fat, and liver is a primary cause for insulin resistance. As a secondary effect of insulin resistance,-cell failure or the loss of the ability of-cells to compensate has been proposed to be mediated by glucose toxicity, cellular exhaustion, and/or other undefined cellular mechanisms (4-6). More recent findings using gene knockout approaches have shown that disruption of insulin receptor substrate protein-2 (IRS-2) 1 in mice results in defects in both insulin action and insulin secretion by-cells, which closely mimics the development of human type 2 diabetes (7). Of particular significance, IRS-2 knockout mice exhibit a reduced mass of-cells compared with wild-type mice, suggesting that this defect in the insulin signaling pathway also produces a functional defect in mitogenic signaling, resulting in decreased-cell replication and growth. These novel results have altered this previous concept for the development of type 2 diabetes by demonstrating that disruption of a single gene product, i.e. IRS-2, involved in insulin signaling may be responsible for insulin resistance in insulin-sensitive tissues and also in pancreatic-cells. Thus,-cell failure may be because of insulin resistance rather than-cell toxicity or exhaustion that occurs in parallel with the development of peripheral insulin resistance in type 2 diabetes. Evidence in support of a functional-cell insulin receptor has been reported recently by several groups. Studies by Rothenberg et al. (8) characterize an insulin-activated cell surface receptor tyrosine kinase and IRS-1 that associates with the 85-kDa subunit of phosphoinositide 3-kinase in the-cell line TC3. Overexpression of the human insulin receptor in the-cell line TC6-F7 also regulates insulin gene expression and insulin content (9). Harbeck et al. (10) also describe expression of insulin receptor mRNA and IRS-1 in single primary rat-cells. Furthermore, Leibiger et al. (11) show that endogenous insulin secreted from the-cell line HIT promotes insulin bio