Diacylglycerols modulate phosphorylation of the insulin receptor from human mononuclear cells

Insulin inhibits intranuclear nuclear factor kappaB and stimulates IkappaB in mononuclear cells in obese subjects: evidence for an anti-inflammatory effect?

In view of the fact that insulin resistance is associated with atherogenesis and that troglitazone, an insulin sensitizer, has anti-inflammatory effects, which may be potentially antiatherogenic in the long term, we have now investigated whether insulin has potential anti-inflammatory effects. We infused 2.0 to 2.5 IU/h in 5% dextrose (100 mL/h) iv into 10 obese subjects for 4 h followed by 5% dextrose alone for 2 h. The rate of insulin infusion was varied to maintain glucose concentrations as close to the baseline as possible. Blood samples were obtained before and at 2, 4, and 6 h. Subjects were also infused with 5% dextrose without insulin and with saline on separate occasions. Intranuclear nuclear factor kappaB (NFkappaB) in mononuclear cells fell at 2 and further at 4 h, reverting toward the baseline at 6 h (P < 0.05). IkappaB increased significantly at 2 h, increasing further at 4 h and remaining elevated at 6 h (P < 0.001). Reactive oxygen species (ROS) generation by mononuclear cells fell significantly at 2 h and fell further at 4 h; it partially reverted to baseline at 6 h (P < 0.005). p47(phox) subunit, the key protein of nicotinamide adenine dinucleotide phosphate oxidase also fell at 2 h and 4 h, reverting toward the baseline at 6 h (P < 0.05). In addition, soluble intercellular adhesion molecule-1 (sICAM-1), monocyte chemoattractant protein-1 (MCP-1), and plasminogen activator inhibitor-1 (PAI-1) fell significantly following insulin infusion. Glucose or saline infusions without insulin caused no alteration in NFkappaB, IkappaB, ROS generation, p47(phox) subunit, sICAM-1, MCP-1, or PAI-1. We conclude that insulin has a potent acute anti-inflammatory effect including a reduction in intranuclear NFkappaB, an increase in IkappaB, and decreases in ROS generation, p47(phox) subunit, plasma soluble intercellular adhesion molecule-1 (sICAM-1), monocyte chemoattractant protein-1 (MCP-1), and plasminogen activator inhibitor-1 (PAI-1. This acute anti-inflammatory effect, if demonstrated in the long term, may have implications for atherosclerosis and its complications.

Diacylglycerol directly stimulates the insulin receptor tyrosine kinase

Studies with detergent:lipid mixed micelles reveal that diacylglycerol directly stimulates the intrinsic tyrosine kinase activity of the insulin receptor. Kinetic analyses indicate that diacylglycerol activates the kinase by causing a marked increase in the affinity of the receptor for insulin. In contrast, diacylglycerol has no effect on the insulin receptor's catalytic activity or its affinity for ATP. Stimulation of the insulin receptor is not a result of protein kinase C activation. First, phorbol myristate acetate, a potent activator of protein kinase C, has no effect on insulin receptor activity. Second, the activation by diacylglycerol is not stereospecific, in marked contrast to the specificity for 1,2-diacyl-sn-glycerol in the activation of protein kinase C. Because circulating levels of insulin are below the Kd of the insulin receptor for insulin, the ability of diacylglycerol to modulate the affinity of the receptor for ligand suggests that increases in cellular levels of diacylglycerol directly sensitize the receptor to insulin.

Inhibitors such as staurosporine, H-7 or polymyxin B cannot be used in skeletal muscle to prove the role of protein kinase C on insulin action

The precise role of protein kinase C in insulin action in skeletal muscle is not well defined. Based on the fact that inhibitors of protein kinase C block some insulin effects, it has been concluded that some of the biological actions of insulin are mediated via protein kinase C. In this study, we present evidence that inhibitors of protein kinase C such as staurosporine, H-7 or polymyxin B cannot be used to ascertain the role of protein kinase C in skeletal muscle. This is based on the following experimental evidences: a) staurosporine, H-7 and polymyxin B markedly block in muscle the effect of insulin on System A transport activity; however, this effect of insulin is not mimicked in muscle by TPA-induced stimulation of protein kinase C, b) H-7 and polymyxin B block insulin action on System A transport activity in an additive manner to the inhibitory effect of phorbol esters, c) staurosporine, H-7 and polymyxin B block the effect of insulin on lactate production, a process that is activated by insulin and TPA in an additive fashion, and d) staurosporine completely blocks the tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle.

Abnormal insulin receptor binding in cultured monocytes in myotonic muscular dystrophy

Even though one of the characteristic features of myotonic dystrophy (MyD) is the high level of circulating insulin, 125I-insulin-binding data in MyD have been controversial. In the present study we utilized cultured monocytes to avoid problems of reproducibility and variability in age, and examined the affinity and binding characteristics of 125I-insulin binding in MyD patients and controls. The Bmax and mean affinity constant, Ka, were significantly lower, while the number of receptors per cell had increased in the patient group as compared to the controls. The data confirm our earlier findings that there is no primary defect in insulin receptors in MyD, and the disturbed insulin response may be due to an abnormality in the membrane environment. Since the insulin receptor is an integral membrane protein, abnormal plasma membrane lipid composition may lead to impaired lipid-protein interactions, and hence affect the binding characteristics in MyD.