Identification and characterization of enzymes responsible for methylglyoxal overproduction in the metabolic syndrome

Methylglyoxal (MG) is a highly reactive metabolite produced in the cells. Insulin-insensitive vascular cells are the major sites for endogenous MG formation. Elevated levels of MG in vascular tissues were reported in metabolic syndrome with hyperglycemia (such as type 2 diabetes) or without hyperglycemia (such as hypertension), but the underlying mechanism is largely unknown. We observed that in cultured vascular smooth muscle cells (VSMCs), fructose (25 mM) treatment up-regulated gene expression of aldolase B and enhanced MG formation. Glucose (25 mM) treatment of VSMCs activated the polyol pathway and increased fructose formation, leading to aldolase B up-regulation and MG overproduction. In our tested rat models with obesity, hypertension or diabetes, serum and aortic MG and fructose levels were increased, and the expression of aldolase B in the aorta was up-regulated. Our study indicates that aldolase B up-regulation by elevated fructose is a common mechanism for vascular MG overproduction in the metabolic syndrome. Increased MG accumulation is considered an important molecular mechanism for endothelial cell damage in diabetes. Whether knockdown of aldolase B prevents high glucose-induced MG overproduction and cellular dysfunction was investigated in the cultured endothelial cells. High glucose (25 mM) incubation increased aldolase B mRNA expression and MG formation in endothelial EA. hy926 cells. We found that siRNA knockdown of aldolase B prevented high glucose-elevated MG levels and the activation of multiple metabolic and signaling pathways (i.e. increase in advanced glycation endproducts accumulation, oxidative stress, O-linked N-acetyl glucosamine modification of proteins, membrane protein kinase C activity and nuclear translocation of nuclear factor κB). Our study further suggests that aldolase B is likely a key target for prevention of MG overproduction and related cellular dysfunction not only in VSMCs but also in vascular endothelial cells in diabetes and its vascular complications. Recently, MG formation in insulin-sensitive cells has received much attention since high levels of MG have been found to disturb insulin signaling in 3T3-L1 adipocytes and skeletal muscle L6 cells. We, therefore, investigated the mechanisms for MG formation in insulin-sensitive 3T3-L1 adipocytes under physiological and pathological conditions. We found that insulin (100 nM), glucose (25 mM), or their combination has no effect on cellular levels of sorbitol and fructose, in comparison with the control group (5 mM glucose alone). Insulin, glucose (25 mM), or their combination decreased aldolase B mRNA to a similar level. Glucose (25 mM) had no effect on aldolase A gene expression, but insulin (100 nM) markedly increased aldolase A mRNA and protein levels in the absence or presence of 25 mM glucose. Application of insulin (100 nM) increased the levels of basal or glucose (25 mM)-induced MG and glucose 6-phosphate. Knockdown of aldolase A prevented the increased MG levels induced by insulin (100 nM), glucose (25 mM), or their combination. Our data suggest that aldolase A and glycolysis are responsible for the basal and excess MG generation in insulin-sensitive adipose cells, especially under the stimulus of insulin. In summary, all experiments taken together, show for the first time, that aldolase B is responsible for vascular MG overproduction and aldolase A is responsible for adipose MG overproduction in metabolic syndrome. Increased MG levels in these cells may contribute to endothelial dysfunction, pathogenesis of hypertension and hypertriglyceridemia that characterize the metabolic syndrome