Histological manifestations of AGE accumulation include basement membrane thickening, reduced protein degradation that results in an increase in mesangial matrix, and an increase in interstitial extracellular volume. Damage Induced by Products of Glucose Metabolism Glucose can induce damage in cells independent of glycation such as by the activation of the polyol pathway, hexosamine pathway, or protein kinase C (PKC) pathway or through the generation of ROS. Polyol Pathway The polyol pathway involves the activation of the enzyme aldose reductase within cells when intracellular concentrations of glucose rise to hyperglycemic levels (11). This depletes the cellular nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) concentration and alters the redox ratio, which can reduce NO bioavailability and alter enzyme function. Although aldose reductase inhibitors were found to be effective in rodent models of diabetes, human trials have failed to reveal protection from an important microvascular complication of diabetes—eye disease—in a randomized trial (13). Hexosamine Pathway The hexosamine pathway is important for the synthesis of proteoglycans, glycolipids, and glycoproteins (14). The synthesis of these molecules requires an amino sugar substrate called UDP-N-acetylglucosamine, which is the final product of the hexosamine pathway. The rate-limiting enzyme of the hexosamine pathway is glutamine:fructose-6-phosphate-amidotransferase (GFAT), which catalyzes the reaction between fructose-6-phosphate and the amine-donor glutamine to produce glucosamine-6-phosphate (14). In cultured mesangial cells, high glucose levels provoke production of transforming growth factor β1 (TGF-β1); this effect is eliminated by inhibition of GFAT. In contrast, stable overexpression of GFAT increases TGF-β1 production. Furthermore, the effects appear to be transduced by PKC. In humans, GFAT is absent in glomerular cells. However, in patients with diabetic nephropathy, GFAT is expressed in the glomerulus, suggesting that it may play a pathophysiological role (14). PKC Pathway PKC is a family of enzymes that are critical intracellular signaling molecules and are important for vascular function. In the physiological state, receptor-mediated activation of PKC releases intracellular calcium ions and diacylglycerol (DAG) and activates these enzymes. In pathological states such as in diabetes, DAG production can be abnormally increased and can lead to activation of PKC. In diabetes, DAG production is increased by increased glycolysis and an elevated level of intracellular glyceraldehyde-3-phosphate and glycerol-3-phosphate. PKC FIGURE 2 Metabolic pathways of diabetic nephropathy. Hyperglycemia provokes the accumulation of AGEs and other products of glucose metabolism. Activation of each of these pathways can injure the kidney. AGEs can produce cell injury by receptor and non-receptor pathways. Outside the cells, they can cause tissue damage by glycating molecules such as collagen that can reduce tissue compliance through crosslinking. Increased glucose flux can result in activation of pathways such as polyol, hexosamine, and PKC that can result in cellular injury and organ dysfunction. AGEs Receptor-mediated RAGE Polyol pathway ↑ Aldose reductase ↓ NADPH NFκB ↑ Oxidative stress Collagen crosslinking Cell growth, inflammation, angiogenesis, endothelial dysfunction, ECM production Non–receptor-mediated Within cells Outside cells Hexosamine pathway ↑ GFAT ↑ TGFβ1 Tissue fibrosis Vascular dysfunction ROS ECM ↓ NO bioavailability ↑ CTGF PKC pathway ↑ DAG ↑ PKC ↓ MMPs Glucose metabolism byproducts 4 CHRONIC KIDNEY DISEASE AND TYPE 2 DIABETES can also be activated by ROS and AGEs. An inhibitor of PKC-β— ruboxistaurin—has been tested in a phase 2 randomized clinical trial in patients with type 2 diabetes and persistent albuminuria (albumin-to-creatinine ratio [ACR] 200–2,000 mg/g creatinine) despite therapy with renin-angiotensin system inhibitors (15). Compared to placebo, the reduction in ACR at 1 year—the primary endpoint of the study—was not significant. Hemodynamic Factors The increases in glomerular capillary pressure increase the single nephron glomerular filtration rate—hyperfiltration—and this occurs early in the course of diabetes. An increase in intraglomerular pressure is the result of an increase in efferent arteriolar tone and a reduction in afferent arteriolar tone (Figure 3) (16). How this process occurs is not settled, but two theories have emerged. One group believes that hyperfiltration is mediated by circulating molecules that primarily operate within the glomerulus (17). Several mediators have been proposed to increase intraglomerular pressure via increasing efferent arteriolar tone and reducing afferent arteriolar tone. Increase in efferent arteriolar resistance can result from an increase in the concentration of angiotensin II, thromboxane A2 (TxA2), endothelin 1 (ET-1), and ROS (16). Reduction in afferent arteriolar resistance can be provoked by reduction in NO oxide bioavailability; increased cyclooxygenase-2 (COX-2) prostanoids; activation of the kallikrein-kinin system, atrial natriuretic peptide, and angiotensin 1-7; and an increase in insulin (16). However, another group proposes that tubular mechanisms remain the primary driver of the intraglomerular hypertension (12). The