Hyperglycemia-induced oxidative stress is detrimental for endothelial cells, contributing to the vascular complications of diabetes. The mitochondrial permeability transition pore (PTP) is an oxidative stress-sensitive channel involved in cell death; therefore, we have examined its potential role in endothelial cells exposed to oxidative stress or high glucose level. Metformin, an antihyperglycemic agent used in type 2 diabetes, was also investigated because it inhibits PTP opening in transformed cell lines. Cyclosporin A (CsA), the reference PTP inhibitor, and a therapeutic dose of metformin (100 mol/l) led to PTP inhibition in permeabilized human microvascular endothelial cells (HMEC-1). Furthermore, exposure of intact HMEC-1 or primary endothelial cells from either human umbilical vein or bovine aorta to the oxidizing agent tert-butylhydroperoxide or to 30 mmol/l glucose triggered PTP opening, cytochrome c decompartmentalization, and cell death. CsA or metformin prevented all of these effects. The antioxidant N-acetyl-L-cysteine also prevented hyperglycemia-induced apoptosis. We conclude that 1) elevated glucose concentration leads to an oxidative stress that favors PTP opening and subsequent cell death in several endothelial cell types and 2) metformin prevents this PTP opening-related cell death. We propose that metformin improves diabetes-associated vascular disease both by lowering blood glucose and by its effect on PTP regulation. Diabetes 54

AMP-activated protein kinase (AMPK) controls glucose uptake and glycolysis in muscle. Little is known about its role in liver glucose uptake, which is controlled by glucokinase. We report here that 5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside (AICAR), metformin, and oligomycin activated AMPK and inhibited glucose phosphorylation and glycolysis in rat hepatocytes. In vitro experiments demonstrated that this inhibition was not due to direct phosphorylation of glucokinase or its regulatory protein by AMPK. By contrast, AMPK phosphorylated liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase without affecting activity. Inhibitors of the endothelial nitric oxide synthase, stress kinases, and phosphatidylinositol 3-kinase pathways did not counteract the effects of AICAR, metformin, or oligomycin, suggesting that these signaling pathways were not involved. Interestingly, the inhibitory effect on glucose phosphorylation of these well-known AMPK activators persisted in primary cultured hepatocytes from newly engineered mice lacking both liver ␣ 1 and ␣ 2 AMPK catalytic subunits, demonstrating that this effect was clearly not mediated by AMPK. Finally, AICAR, metformin, and oligomycin were found to inhibit the glucose-induced translocation of glucokinase from the nucleus to the cytosol by a mechanism that could be related to the decrease in intracellular ATP concentrations observed in these conditions. Diabetes 55: [865][866][867][868][869][870][871][872][873][874] 2006

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Although metformin has been used for over 60 years, the balance between the drug's beneficial and adverse effects is still subject to debate. Following an analysis of how cases of so-called "metformin-associated lactic acidosis" (MALA) are reported in the literature, the present article reviews the pitfalls to be avoided when assessing the purported association between metformin and lactic acidosis. By starting from pathophysiological considerations, we propose a new paradigm for lactic acidosis in metformin-treated patients. Metformin therapy does not necessarily induce metformin accumulation, just as metformin accumulation does not necessarily induce hyperlactatemia, and hyperlactatemia does not necessarily induce lactic acidosis. In contrast to the conventional view, MALA probably accounts for a smaller proportion of cases than either metformin-unrelated lactic acidosis or metformin-induced lactic acidosis. Lastly, this review highlights the need for substantial improvements in the reporting of cases of lactic acidosis in metformin-treated patients. Accordingly, we propose a check-list as a guide to clinical practice.

Accumulating evidence suggests that high concentrations of leptin observed in obesity and diabetes may contribute to their adverse effects on cardiovascular health. Metformin monotherapy is associated with reduced macrovascular complications in overweight patients with type 2 diabetes. It is uncertain whether such improvement in the cardiovascular outcome is related to specific vasculoprotective effects of this drug. In the present study, we determined the effect of leptin on human aortic smooth muscle cell (HASMC) proliferation and matrix metalloproteinase (MMP)-2 expression, the signaling pathways mediating these effects, and the modulatory effect of metformin on these parameters. Incubation of HASMCs with leptin enhanced the proliferation and MMP-2 expression in these cells and increased the generation of intracellular reactive oxygen species (ROS). These effects were abolished by vitamin E. Inhibition of NAD(P)H oxidase and protein kinase C (PKC) suppressed the effect of leptin on ROS production. In HASMCs, leptin induced PKC, extracellular signal-regulated kinase (ERK)1/2, and nuclear factor-B (NF-B) activation and inhibition of these signaling pathways abrogated HASMC proliferation and MMP-2 expression induced by this hormone. Treatment of HASMCs with metformin decreased leptin-induced ROS production and activation of PKC, ERK1/2, and NF-B. Metformin also inhibited the effect of leptin on HASMC proliferation and MMP-2 expression. Overall, these results demonstrate that leptin induced HASMC proliferation and MMP-2 expression through a PKCdependent activation of NAD(P)H oxidase with subsequent activation of the ERK1/2/NF-B pathways and that therapeutic metformin concentrations effectively inhibit these biological effects. These results suggest a new mechanism by which metformin may improve cardiovascular outcome in patients with diabetes. Diabetes