The pathobiology of diabetic complications: a unifying mechanism

Hexosamines, insulin resistance, and the complications of diabetes: current status

The hexosamine biosynthesis pathway (HBP) is a relatively minor branch of glycolysis. Fructose 6-phosphate is converted to glucosamine 6-phosphate, catalyzed by the first and rate-limiting enzyme glutamine:fructose-6-phosphate amidotransferase (GFAT). The major end product is UDP-N-acetylglucosamine (UDP-GlcNAc). Along with other amino sugars generated by HBP, it provides essential building blocks for glycosyl side chains, of proteins and lipids. UDP-GlcNAc regulates flux through HBP by regulating GFAT activity and is the obligatory substrate of O-GlcNAc transferase. The latter is a cytosolic and nuclear enzyme that catalyzes a reversible, posttranslational protein modification, transferring GlcNAc in O-linkage (O-GlcNAc) to specific serine/threonine residues of proteins. The metabolic effects of increased flux through HBP are thought to be mediated by increasing O-GlcNAcylation. Several investigators proposed that HBP functions as a cellular nutrient sensor and plays a role in the development of insulin resistance and the vascular complications of diabetes. Increased flux through HBP is required and sufficient for some of the metabolic effects of sustained, increased glucose flux, which promotes the complications of diabetes, e.g., diminished expression of sarcoplasmic reticulum Ca(2+)-ATPase in cardiomyocytes and induction of TGF-beta and plasminogen activator inhibitor-1 in vascular smooth muscle cells, mesangial cells, and aortic endothelial cells. The mechanism was consistent with enhanced O-GlcNAcylation of certain transcription factors. The role of HBP in the development of insulin resistance has been controversial. There are numerous papers showing a correlation between increased flux through HBP and insulin resistance; however, the causal relationship has not been established. More recent experiments in mice overexpressing GFAT in muscle and adipose tissue or exclusively in fat cells suggest that the latter develop in vivo insulin resistance via cross talk between fat cells and muscle. Although the relationship between HBP and insulin resistance may be quite complex, it clearly deserves further study in concert with its role in the complications of diabetes.

Metabolic effects of the incretin mimetic exenatide in the treatment of type 2 diabetes

Interventional studies have demonstrated the impact of hyperglycemia on the development of vascular complications associated with type 2 diabetes, which underscores the importance of safely lowering glucose to as near-normal as possible. Among the current challenges to reducing the risk of vascular disease associated with diabetes is the management of body weight in a predominantly overweight patient population, and in which weight gain is likely with many current therapies. Exenatide is the first in a new class of agents termed incretin mimetics, which replicate several glucoregulatory effects of the endogenous incretin hormone, glucagon-like peptide-1 (GLP-1). Currently approved in the US as an injectable adjunct to metformin and/or sulfonylurea therapy, exenatide improves glycemic control through multiple mechanisms of action including: glucose-dependent enhancement of insulin secretion that potentially reduces the risk of hypoglycemia compared with insulin secretagogues; restoration of first-phase insulin secretion typically deficient in patients with type 2 diabetes; suppression of inappropriately elevated glucagon secretion to reduce postprandial hepatic output; and slowing the rate of gastric emptying to regulate glucose appearance into the circulation. Clinical trials in patients with type 2 diabetes treated with subcutaneous exenatide twice daily demonstrated sustained improvements in glycemic control, evidenced by reductions in postprandial and fasting glycemia and glycosylated hemoglobin (HbA(1c)) levels. Notably, improvements in glycemic control with exenatide were coupled with progressive reductions in body weight, which represents a distinct therapeutic benefit for patients with type 2 diabetes. Acute effects of exenatide on beta-cell responsiveness along with significant reductions in body weight in patients with type 2 diabetes may have a positive impact on disease progression and potentially decrease the risk of associated long-term complications.

Redox dependence of glomerular epithelial cell hypertrophy in response to glucose

Podocytes or glomerular epithelial cells (GECs) are important targets of the diabetic microenvironment. Podocyte foot process effacement and widening, loss of GECs and hypertrophy are pathological features of this disease. ANG II and oxidative stress are key mediators of renal hypertrophy in diabetes. The cellular mechanisms responsible for GEC hypertrophy in diabetes are incompletely characterized. We investigated the effect of high glucose on protein synthesis and GEC hypertrophy. Exposure of GECs to high glucose dose dependently stimulated [(3)H]leucine incorporation, but not [(3)H]thymidine incorporation. High glucose resulted in the activation of ERK1/2 and Akt/PKB. ERK1/2 pathway inhibitor or the dominant negative mutant of Akt/PKB inhibited high glucose-induced protein synthesis. High glucose elicited a rapid generation of reactive oxygen species (ROS). The stimulatory effect of high glucose on ROS production, ERK1/2, and Akt/PKB activation was prevented by the antioxidants catalase, diphenylene iodonium, and N-acetylcysteine. Exposure of the cells to hydrogen peroxide mimicked the effects of high glucose. In addition, ANG II resulted in the activation of ERK1/2 and Akt/PKB and GEC hypertrophy. Moreover, high glucose and ANG II exhibited additive effects on ERK1/2 and Akt/PKB activation as well as protein synthesis. These additive responses were abolished by treatment of the cells with the antioxidants. These data demonstrate that high glucose stimulates GEC hypertrophy through a ROS-dependent activation of ERK1/2 and Akt/PKB. Enhanced ROS generation accounts for the additive effects of high glucose and ANG II, suggesting that this signaling cascade contributes to GEC injury in diabetes.

Myocardial infarction and heart failure in the db/db diabetic mouse

Clinical studies have reported that the incidence and severity of myocardial infarction is significantly greater in diabetics compared with nondiabetics after correction for all other risk factors. The majority of studies investigating the pathophysiology of myocardial ischemia-reperfusion injury have focused on otherwise healthy animals. At present, there is a paucity of experimental investigations on the pathophysiology of heart failure in diabetic animals. We hypothesized that the severity of myocardial reperfusion injury and the development of congestive heart failure would be markedly enhanced in the db/db diabetic mouse. Accordingly, we studied the effects of varying durations of in vivo myocardial ischemia and reperfusion on the incidence of heart failure in db/db diabetic mice. Nondiabetic and db/db diabetic mice (10 wk of age) were subjected to 30, 45, or 60 min of left coronary artery occlusion and 28 days of reperfusion. Survival at 24 h of reperfusion was 100% in nondiabetic mice subjected to 30 min of myocardial ischemia and 88% in nondiabetic mice subjected to 45 min of myocardial ischemia. In contrast, survival was 53% in db/db diabetic mice subjected to 30 min of myocardial ischemia and 44% in db/db mice after 45 min of myocardial ischemia. Prolonged survival in nondiabetic mice was not significantly attenuated when compared during the 28-day follow-up period with all groups experiencing >90% survival. Prolonged survival was significantly decreased in db/db mice after both 30 and 45 min of myocardial ischemia compared with sham controls. Furthermore, we observed a significant degree or left ventricular dilatation, cardiac hypertrophy, and cardiac contractile dysfunction in db/db mice subjected to 45 min of myocardial ischemia and 28 days reperfusion. In nondiabetic mice subjected to 45 min of myocardial ischemia, we failed to observe any changes in left ventricular dimensions or fractional shortening. These studies provide a feasible experimental model system for the investigation of heart failure secondary to acute myocardial infarction in the db/db diabetic mouse.