Analysis of Thermal Injury-induced Insulin Resistance in Rodents

Abstract

Burn injury is associated with insulin resistance. The molecular basis of this resistance was investigated by examining insulin receptor signaling in rats after thermal injury. The impaired insulin-stimulated transport of [ 3 H]2-deoxyglucose into soleus muscle strips confirmed the insulin resistance following burns. In vivo insulin-stimulated phosphoinositide 3-kinase activity, pivotal in translocation of GLUT4, was decreased in burns when assessed by its insulin receptor substrate-1 (IRS-1)-associated activity. Insulin-induced tyrosine kinase activity of insulin receptor (IR) and tyrosine phosphorylation of IRS-1 were also attenuated. Immunoprecipitated IR, however, appeared to have normal insulinresponsive kinase activity. Finally, immunoprecipitated IRS-1 was tested for its effect on partially purified recombinant IR and was found to inhibit its kinase activity. This inhibitory effect of IRS-1 was abolished by prior treatment of IRS-1 with alkaline phosphatase, indicating that burn injury-related hyperphosphorylation of IRS-1 is similar to that observed in TNF␣-induced inhibition of IR signaling. All of these changes were observed in the absence of quantitative changes in IR, IRS-1, and phosphoinositide 3-kinase. Alterations in postreceptor insulin signaling, therefore, may be responsible for the insulin resistance after thermal injury.Insulin, the principle hormone maintaining glucose homeostasis, stimulates glucose influx into muscle, glycogen synthesis in the liver, and fat deposition in adipocytes (1, 2). Insulin resistance is a major metabolic abnormality following burn injury (3-6). Insulin resistance of burn injury is indicated by elevated glucose production (gluconeogenesis) and blood glucose levels in association with normal or increased insulin levels. Muscle constitutes approximately 40% of the body mass, and insulin resistance results in muscle wasting due to the lack of anabolic effects. Indeed, insulin resistance as documented in skeletal muscle after burn injury results in enhanced gluconeogenesis and protein catabolism (3, 5, 7). Vigorous nutritional support to optimize the hypermetabolic state and promote anabolism has not been effective. Attempts have been made to correct this burn stress-induced pseudodiabetes, hyperglycemia, and muscle wasting with pharmacological doses of insulin, but this treatment resulted in fatty liver and increased wasted substrate cycling, enhanced CO 2 production, and attendant complications of hepatic and respiratory failure (8 -10). The molecular mechanism of this abnormal insulin function has not been elucidated.The binding of insulin to the ␣-subunit of the IR 1 induces rapid autophosphorylation of its ␤-subunits, activating its tyrosine kinase activity (1, 2). This leads to tyrosine phosphorylation of cytosolic molecules, including IRS-1, whose phosphorylation is pivotal in the biological actions of insulin (11). The tyrosine phosphorylation of IRS-1 leads to the second intracellular step in insulin action, the association of phosphorylated IRS-1 with the enz...

Burn injury causes mitochondrial dysfunction in skeletal muscle

“…The absence of the anabolic effects of insulin after burn trauma might underlie increased protein catabolism and muscle wasting (31,32), and alterations in postreceptor insulin signaling could promote such insulin resistance (31). Here, we demonstrate that reduced glucose utilization in the skeletal muscle of burned mice is at least in part due to coordinated changes in the expression of enzymes and transporters of glucose uptake and metabolism.…”

“…Here, by analyzing coordinated changes in gene expression and measuring ATP synthesis reactions with in vivo 31 P NMR, we show that perturbed mitochondrial function also plays a role in skeletal muscle burn trauma. Furthermore, we identify metabolic and mitochondrial pathways that may mediate medically significant alterations in muscle metabolism after thermal injury, and thus might serve as novel targets for burn therapy.…”

“…The mice were transiently anesthetized by using a nose cone and vaporizer with a halothane͞oxygen mixture (5% for induction, 1% for maintenance), and placed in a customized restraining tube. Their left hind limb was placed into a solenoid coil (four turns; length, 2 cm; diameter, 1 cm) tuned to 31 P frequency (162.1 MHz). The rectal body temperature was maintained at 37 Ϯ 1°C by using heated water blankets.…”

“…The rectal body temperature was maintained at 37 Ϯ 1°C by using heated water blankets. All in vivo 31 P NMR experiments were performed in a horizontal bore magnet (proton frequency at 400 MHz, 21 cm diameter, Magnex Scientific) using a Bruker Advance console. Field homogeneity was adjusted by using the 1 H signal of tissue water.…”

The c-Jun NH2-terminal Kinase Promotes Insulin Resistance during Association with Insulin Receptor Substrate-1 and Phosphorylation of Ser307

“…Moreover, TNF␣ treatment of adipocytes increases serine phosphorylation of IRS-proteins, which inhibits insulin-stimulated tyrosine phosphorylation and impairs insulin signaling (28,29,34). These results suggest that serine phosphorylation of IRS-1 promotes an inhibitory effect of proinflammatory cytokines on insulin receptor signaling (30).…”