Nonenzymatic glycation is increased in diabetes and leads to elevated levels of advanced glycation end products (AGEs), which link hyperglycemia to the induction of insulin resistance. In hyperglycemic conditions, intracellularly formed ␣-ketoaldehydes, such as methylglyoxal, are an essential source of intracellular AGEs, and the abnormal accumulation of methylglyoxal is related to the development of diabetes complications in various tissues and organs. We have previously shown in skeletal muscle that AGEs induce insulin resistance at the level of metabolic responses. Therefore, it was important to extend our work to intermediates of the biosynthetic pathway leading to AGEs. Hence, we asked the question whether the reactive ␣-ketoaldehyde methylglyoxal has deleterious effects on insulin action similar to AGEs. We analyzed the impact of methylglyoxal on insulin-induced signaling in L6 muscle cells. We demonstrate that a short exposure to methylglyoxal induces an inhibition of insulin-stimulated phosphorylation of protein kinase B and extracellular-regulated kinase 1/2, without affecting insulin receptor tyrosine phosphorylation. Importantly, these deleterious effects of methylglyoxal are independent of reactive oxygen species produced by methylglyoxal but appear to be the direct consequence of an impairment of insulin-induced insulin receptor substrate-1 tyrosine phosphorylation subsequent to the binding of methylglyoxal to these proteins. Our data suggest that an increase in intracellular methylglyoxal content hampers a key molecule, thereby leading to inhibition of insulin-induced signaling. By such a mechanism, methylglyoxal may not only induce the debilitating complications of diabetes but may also contribute to the pathophysiology of diabetes in general.

Hepatic ischemia occurs in liver transplantation, hemodynamic or cardiogenic shock, and liver resection associated with trauma or tumor. Ischemia/reperfusion (I/R) injury results in microcirculation failure followed by apoptosis and necrosis. Matrix metalloproteinases (MMPs) are involved in many physiological and pathological processes, but their expression and function during liver I/R remains poorly documented. In this study, we evaluated the expression of nine MMPs and their natural inhibitors, tissue inhibitors of MMPs (TIMPs), in a rat model of liver I/R. Analysis of MMP and TIMP expression show that although most of these genes are not constitutively expressed in the normal liver, they are induced in a specific time-dependent manner following I/R. Stromelysin-1, gelatinase B, and collagenase-3 are induced during the early phase of acute liver injury associated with inflammation and increased necrosis/apoptosis, whereas gelatinase A, membrane type-MMP, stromelysin-3, metalloelastase, TIMP-1, and TIMP-2 are essentially detectable during the recovery phase of liver injury corresponding to hepatocyte regeneration. This observation suggested that MMPs and TIMPs could play both deleterious and beneficial roles following I/R. We thus tested the effect of a specific phosphinic MMP inhibitor on acute liver I/R injury. Inhibition of MMP activity was shown to significantly decrease liver injury in ischemic/reperfused liver tissue as assessed by histological studies and serum hepatic enzyme levels. We therefore propose that MMP inhibitors may be of clinical relevance in liver-associated ischemic diseases or after liver transplantation.

Nonenzymatic glycation is increased in diabetes and leads to increased levels of glycated proteins. Most studies have focused on the role of glycation products in vascular complications. Here, we have investigated the action of human glycated albumin (HGA) on insulin signaling in L6 skeletal muscle cells. Exposure of these cells to HGA inhibited insulin-stimulated glucose uptake and glycogen synthase activity by 95 and 80%, respectively. These effects were time-and dose-dependent, reaching a maximum after 12 h incubation with 0.1 mg/ml HGA. In contrast, exposure of the cells to HGA had no effect on thymidine incorporation. Further, HGA reduced insulin-stimulated serine phosphorylation of PKB and GSK3, but did not alter ERK1/2 activation. HGA did not affect either insulin receptor kinase activity or insulin-induced Shc phosphorylation on tyrosine. In contrast, insulin-dependent IRS-1 and IRS-2 tyrosine phosphorylation was severely reduced in cells preincubated with HGA for 24 h. Insulin-stimulated association of PI3K with IRS-1 and IRS-2, and PI3K activity were reduced by HGA in parallel with the changes in IRS tyrosine phosphorylation, while Grb2-IRS association was unchanged. In L6 myotubes, exposure to HGA increased PKC activity by 2-fold resulting in a similar increase in Ser/Thr phosphorylation of IRS-1 and IRS-2. These phosphorylations were blocked by the PKC inhibitor bisindolylmaleimide (BDM). BDM also blocked the action of HGA on insulin-stimulated PKB and GSK3␣. Simultaneously, BDM rescued insulin-stimulation of glucose uptake and glycogen synthase activity in cells exposed to HGA. The use of antibodies specific to PKC isoforms shows that this effect appears to be mediated by activated PKC␣, independent of reactive oxygen species production. In summary, in L6 skeletal muscle cells, exposure to HGA leads to insulin resistance selectively in glucose metabolism with no effect on growth-related pathways regulated by the hormone.

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Aims/hypothesis Chronic hyperglycaemia aggravates insulin resistance, at least in part, by increasing the formation of advanced glycation end-products (AGEs). Methylglyoxal (MGO) is the most reactive AGE precursor and its abnormal accumulation participates in damage in various tissues and organs. Here we investigated the ability of MGO to interfere with insulin signalling and to affect beta cell functions in the INS-1E beta cell line. Methods INS-1E cells were incubated with MGO and then exposed to insulin or to glucose. Western blotting was used to study signalling pathways, and real-time PCR to analyse gene expression; insulin levels were determined by radioimmunoassay.Results Non-cytotoxic MGO concentrations inhibited insulin-induced IRS tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) pathway activation independently from reactive oxygen species (ROS) production. Concomitantly, formation of AGE adducts on immunoprecipitated IRS was observed. Aminoguanidine reversed MGO inhibitory effects and the formation of AGE adducts on IRS. Further, the insulin-and glucose-induced expression of Ins1, Gck and Pdx1 mRNA was abolished by MGO. Finally, MGO blocked glucoseinduced insulin secretion and PI3K/PKB pathway activation. These MGO effects were abolished by LiCl, which inhibits glycogen synthase kinase-3 (GSK-3). Conclusions/interpretation MGO exerted major damaging effects on INS-1E cells impairing both insulin action and secretion. An important actor in these noxious MGO effects appears to be GSK-3. In conclusion, MGO participates not only in the pathogenesis of the debilitating complications of type 2 diabetes, but also in worsening of the diabetic state by favouring beta cell failure.

Background The role of vascular endothelium in the control of epicardial coronary artery vasomotion during treadmill exercise remains unclear. Therefore, we examined the consequences of in vivo balloon endothelial denudation on external coronary diameter of the left circumflex artery during exercise in conscious dogs.Methods and Results Seven dogs instrumented for the measurement of arterial blood pressure, external coronary artery diameter, and coronary blood flow were studied during exercise before and up to 21 days after balloon endothelial denudation of the proximal left circumflex artery. Endothelial denudation was confirmed by abolition of the epicardial cor-