An in Vivo Model for Elucidation of the Mechanism of Tumor Necrosis Factor-  (TNF- )-Induced Insulin Resistance: Evidence for Differential Regulation of Insulin Signaling by TNF- 

Abstract

Tumor necrosis factor-alpha (TNF-alpha) has been shown to induce insulin resistance in cultured cells as well as in animal models. The aim of this study was to map the in vivo mechanism whereby TNF-alpha contributes to the pathogenesis of impaired insulin signaling, using obese and lean Zucker rats in which TNF-alpha activity was inhibited through adenovirus-mediated gene transfer. We employed a replication-incompetent adenovirus-5 (Ad5) vector to endogenously express a TNF inhibitor (TNFi) gene, which encodes a chimeric protein consisting of the extracellular domain of the human 55-kDa TNF receptor joined to a mouse IgG heavy chain. Control animals consisted of rats infected with the same titer of adenovirus carrying the lac-z complementary DNA, encoding for beta-galactosidase. There was a significant reduction in plasma insulin and free fatty acid levels in TNFi obese rats 2 days following Ad5 administration. The peripheral insulin sensitivity index was 50% greater, whereas hepatic glucose output was completely suppressed during hyperinsulinemic glucose clamps in TNFi obese animals, with no differences observed between the two lean groups. The improvement in peripheral and hepatic sensitivity to insulin seen in the obese animals was independent of insulin receptor (IR) number and insulin binding affinity for IR. However, TNF-alpha neutralization led to a 2.5-fold increase in tyrosine phosphorylation of IR in skeletal muscle, whereas this was unchanged in liver. There was also a 4-fold increase in particulate protein tyrosine phosphatase activity of skeletal muscle in TNFi obese animals vs. beta-galactosidase controls, whereas protein tyrosine phosphatase activity in liver was unchanged. These results suggest that TNF-alpha is a mediator of insulin resistance in obesity and may modulate IR signaling in skeletal muscle and liver through different pathways. TNF-alpha may affect insulin action in the liver either at sites distal to the IR or indirectly, possibly because of increased provision of gluconeogenic substrates or altered counterregulation. In addition, the Ad5-mediated gene delivery system employed here provides an in vivo model that is efficient and economical for exploring mechanisms involved in TNF-alpha-induced insulin resistance in various genetic models of obesity-linked diabetes.

Hypoadiponectinemia in Obesity: Association with Insulin Resistance

“…Tumor necrosis factora is a cytokine which has direct effects on the insulin signaling cascade by improved release of free fatty acids by adipocytes and decrease of adiponectin synthesis [25]. In numerous obesity-diabetes models, tumor necrosis factor-a over expressed in adipose and muscle tissues and tumor necrosis factor-a blocks the action of insulin in cultured cells [26]. In humans, tumor necrosis factor-a also over expressed in the adipose and muscle tissues of obese insulin resistance subjects [27].…”

Regulation of Muscle Protein Degradation

“…A possible mechanism is insulin resistance due to phosphorylation of serine 307 on IRS-1; this phosphorylation impairs IRS-1 docking with PI3K to decrease PI3K/Akt activity (25,26). Because such changes have been linked to TNF␣ responses, the mechanism we describe could apply to many conditions associated with high TNF␣ levels, including chronic kidney disease (24,27).…”

“…Besides suppressing protein synthesis, our findings show that a reduced PI3K activity contributes to muscle atrophy via coordinated regulation of apoptotic and Ub-P'some systems, ultimately leading to protein degradation. It is tempting to speculate that the identified pathways account for the excessive muscle protein loss that occurs in conditions associated with insulin resistance, including metabolic acidosis, uremia, and conditions that increase glucocorticoids (11,16,27,(35)(36)(37)(38)(39).…”

Fat poetry: a kingdom for PPARγ

“…Besides its role in WAT, TNF-α increases the expression of genes involved in the de novo synthesis of FA, and decreases that of genes involved in FA oxidation in liver. The autocrine and paracrine effects of TNF-α are responsible for the insulin resistance observed in humans and rats to whom this cytokine has been administrated [90][91][92]. TNF-α also regulates the expression of other adipokines in WAT.…”