Diabetes-specific microvascular disease is a leading cause of blindness, renal failure and nerve damage, and diabetes-accelerated atherosclerosis leads to increased risk of myocardial infarction, stroke and limb amputation. Four main molecular mechanisms have been implicated in glucose-mediated vascular damage. All seem to reflect a single hyperglycaemia-induced process of overproduction of superoxide by the mitochondrial electron-transport chain. This integrating paradigm provides a new conceptual framework for future research and drug discovery.

http://www.chem.uwec.edu/Chem454/BiochemDiabetes.pdf

Biochemistry and molecular cell biology of diabetic complications

It’s a great honor to join the exceptional club of Banting Award winners, many of whom were my role models and mentors. In addition, giving the Banting Lecture also has a very personal meaning to me, because without Frederick Banting, I would have died from type 1 diabetes when I was 8 years old. However, it was already apparent at the time I was diagnosed that for too many people like me, Banting’s discovery of insulin only allowed them to live just long enough to develop blindness, renal failure, and coronary disease. For example, when I started college, the American Diabetes Association’s Diabetes Textbook had this to say to my parents: “The person with type 1 diabetes can be reassured that it is highly likely that he will live at least into his 30s.” Not surprisingly, my parents did not find this particularly reassuring.

At the same time we were reading this in 1967, however, the first basic research discovery about the pathobiology of diabetic complications had just been published in Science the previous year. In my Banting Lecture today, I am thus going to tell you a scientific story that is also profoundly personal.

I’ve divided my talk into three parts. The first part is called “pieces of the puzzle,” and in it I describe what was learned about the pathobiology of diabetic complications starting with that 1966 Science paper and continuing through the end of the 1990s. In the second part, I present a unified mechanism that links together all of the seemingly unconnected pieces of the puzzle. Finally, in the third part, I focus on three examples of novel therapeutic approaches for the prevention and treatment of diabetic complications, which are all based on the new paradigm of a unifying mechanism for the pathogenesis of diabetic complications. …

/pdf/the-pathobiology-of-diabetic-complications-a-unifying-10rf12tje8.pdf

The pathobiology of diabetic complications: a unifying mechanism.

In both type 1 and type 2 diabetes, the late diabetic complications in nerve, vascular endothelium, and kidney arise from chronic elevations of glucose and possibly other metabolites including free fatty acids (FFA). Recent evidence suggests that common stress-activated signaling pathways such as nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases underlie the development of these late diabetic complications. In addition, in type 2 diabetes, there is evidence that the activation of these same stress pathways by glucose and possibly FFA leads to both insulin resistance and impaired insulin secretion. Thus, we propose a unifying hypothesis whereby hyperglycemia and FFA-induced activation of the nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases stress pathways, along with the activation of the advanced glycosylation end-products/receptor for advanced glycosylation end-products, protein kinase C, and sorbitol stress pathways, plays a key role in causing late complications in type 1 and type 2 diabetes, along with insulin resistance and impaired insulin secretion in type 2 diabetes. Studies with antioxidants such as vitamin E, alpha-lipoic acid, and N-acetylcysteine suggest that new strategies may become available to treat these conditions.

/pdf/oxidative-stress-and-stress-activated-signaling-pathways-a-3134ynljka.pdf

Oxidative Stress and Stress-Activated Signaling Pathways: A Unifying Hypothesis of Type 2 Diabetes

The major focus of this Review is on the mechanisms of islet beta cell failure in the pathogenesis of obesity-associated type 2 diabetes (T2D). As this demise occurs within the context of beta cell compensation for insulin resistance, consideration is also given to the mechanisms involved in the compensation process, including mechanisms for expansion of beta cell mass and for enhanced beta cell performance. The importance of genetic, intrauterine, and environmental factors in the determination of "susceptible" islets and overall risk for T2D is reviewed. The likely mechanisms of beta cell failure are discussed within the two broad categories: those with initiation and those with progression roles.

/pdf/islet-b-cell-failure-in-type-2-diabetes-3tcwflat73.pdf

Islet β cell failure in type 2 diabetes

The vitamin D endocrine system is essential for calcium and bone homeostasis. The precise mode of action and the full spectrum of activities of the vitamin D hormone, 1,25-dihydroxyvitamin D [1,25-(OH)2D], can now be better evaluated by critical analysis of mice with engineered deletion of the vitamin D receptor (VDR). Absence of a functional VDR or the key activating enzyme, 25-OHD-1α-hydroxylase (CYP27B1), in mice creates a bone and growth plate phenotype that mimics humans with the same congenital disease or severe vitamin D deficiency. The intestine is the key target for the VDR because high calcium intake, or selective VDR rescue in the intestine, restores a normal bone and growth plate phenotype.

The VDR is nearly ubiquitously expressed, and almost all cells respond to 1,25-(OH)2D exposure; about 3% of the mouse or human genome is regulated, directly and/or indirectly, by the vitamin D endocrine system, suggesting a more widespread function. VDR-deficient mice, but not vitamin D- or 1α-hydroxylase-deficient mice, and man develop total alopecia, indicating that the function of the VDR and its ligand is not fully overlapping. The immune system of VDR- or vitamin D-deficient mice is grossly normal but shows increased sensitivity to autoimmune diseases such as inflammatory bowel disease or type 1 diabetes after exposure to predisposing factors. VDR-deficient mice do not have a spontaneous increase in cancer but are more prone to oncogene- or chemocarcinogen-induced tumors. They also develop high renin hypertension, cardiac hypertrophy, and increased thrombogenicity. Vitamin D deficiency in humans is associated with increased prevalence of diseases, as predicted by the VDR null phenotype. Prospective vitamin D supplementation studies with multiple noncalcemic endpoints are needed to define the benefits of an optimal vitamin D status.

Vitamin D and Human Health: Lessons from Vitamin D Receptor Null Mice

https://www.kidney-international.org/article/S0085-2538(15)47416-2/pdf

Aldose reductase and the role of the polyol pathway in diabetic nephropathy

A large number of experimental studies in animals and retrospective or non-randomised prospective studies in humans provide support for the concept that the microvascular complications of diabetes mellitus are dependent on hyperglycaemia. This review focuses on four potential bio-chemical pathways linking hyperglycaemia to changes within the kidney which can plausibly be linked to the functional and structural changes characterising diabetic nephropathy. These four pathways are the polyol pathway, non-enzymatic glycation, glucose autoxidation and de novo synthesis of diacylglycerol leading to protein kinase C and phospholipase A2 activation. Rather than being independent, there are several potential interactions between these four pathways which may explain confusing and overlapping effects observed in studies examining inhibitors of individual pathways. As many of the steps which follow on glucose metabolism are subject to modification by dietary and pharmacological means, the further delineation of the pathogenetic sequence leading to tissue damage in diabetes should allow a logical and effective approach to the prevention or treatment of the complications of diabetes.

/pdf/the-link-between-hyperglycaemia-and-diabetic-nephropathy-414ura5uxw.pdf

The link between hyperglycaemia and diabetic nephropathy.

Advanced glycation is an important pathogenic mechanism in the development of diabetic complications. However, other biochemical processes, such as the polyol pathway or lipid and protein oxidation which can interact with advanced glycation can also yield tissue fluorescence and may also be implicated in the genesis of diabetic microangiopathy. Aminoguanidine is an inhibitor of advanced glycation, but it is not known if all of its effects are mediated by this mechanism. The present study explores the relative contributions of aldose reductase, oxidative stress and advanced glycation on the development of aortic and renal fluorescence and urinary albumin excretion in streptozotocin diabetic rats. The study groups included non-diabetic (control), streptozotocin diabetic rats and diabetic rats receiving aminoguanidine, the anti-oxidants butylated hydroxytoluene and probucol and the aldose reductase inhibitor, ponalrestat. Serial measurements of glycaemic control and urinary albumin excretion were performed every 8 weeks. At 32 weeks, animals were killed, tissues removed and collagen extracted for measurement of fluorescence. Diabetic rats had increased fluorescence in aorta, glomeruli and renal tubules. Aminoguanidine prevented an increase in fluorescence at all three sites suggesting that diabetes-related tissue fluorescence is predominantly due to advanced glycation. Ponalrestat retarded fluorescence in aorta only and butylated hydroxytoluene attenuated fluorescence at the renal sites but not in the aorta. Diabetic rats had increased renal cortical sorbitol levels. Ponalrestat normalized renal cortical sorbitol levels but aminoguanidine did not affect this parameter. The only agent to decrease plasma thiobarbituric acid reactive substances was butylated hydroxytoluene. Diabetic rats developed albuminuria over the 32-week period.(ABSTRACT TRUNCATED AT 250 WORDS)

/pdf/the-relative-roles-of-advanced-glycation-oxidation-and-2cllu6o8b3.pdf

The relative roles of advanced glycation, oxidation and aldose reductase inhibition in the development of experimental diabetic nephropathy in the Sprague-Dawley rat

Pancreatic islets synthesize phospholipids de novo from glucose via acyl-dihydroxyacetone phosphate

Activation of the polyol pathway under hyperglycemic conditions is proposed to contribute to the development of diabetic nephropathy. The mechanisms by which this activation may lead to functional and structural changes within the kidney are yet to be definitively established. We have examined in vitro the steps linking increased polyol pathway activity resulting from hyperglycemia to prostaglandin production. Following the demonstration of increased prostaglandin E (PGE) levels in glomeruli from diabetic rats (14.9 -+ 2.5 v 59.1 -+ 19.4 ng PGE/mg protein), a specific inhibitor of aldose reductase, HOE-843, was used in vitro to analyze the response to hyperglycemia of the steps preceding prostaglandin production. In explants of glomeruli from control animals, increasing the glucose concentration in vitro from 5.6 mmol/L to 25 mmol/L resulted in a significant increase in the flux of glucose through the pentose phosphate pathway ([PPP] 1.29 _ 0.08 v2.00 - 0.11 nmol/h), de novo diacylglycerol synthesis (2.2 _ 0.1 v3.1 _-_ 0.2 i~mol/mg protein), membrane protein kinase C (PKC) activity (18.7 -- 0.5 v 24.3 --- 0.75 pmol/l~g protein), and in vitro phospholipase A2 (PLA2) activity (2.18 -+ 0.46 v 3.83 + 1.07 nmol arachidonic acid hydrolyzed/min/mg cytosolic protein). For all parameters measured, the increase resulting from the increased glucose concentration could be prevented by in vitro addition of HOE-843 for 24 hours before measurement. These findings provide evidence to suggest a mechanism linking increased polyol pathway activity and an increase in PLA2 activity to increased prostaglandin production, which is observed in diabetes of recent onset and may ultimately lead to changes associated with the development of diabetic nephropathy. Copyright© 1997by W.B. Saunders Company S EVERAL BIOCHEMICAL mechanisms have been implicated as contributing to the development of diabetic nephropathy with its characteristic structural changes of enlargement of the mesangial matrix ~,2 and thickened glomerular capillary basement membrane. 3 These mechanisms include increased nonenzymatic glycation of proteins, 4 decreased mesangium degradation, 5 increased oxidative stress resulting from excess free-radical production 6 and impairment of free-radical scavenging, 7-9 increased aldose reductase pathway activity] ° and increased diacylglycerol synthesis with protein kinase C (PKC) activation and increased prostaglandin production. 11 Although there is abundant evidence from experimental diabetes that the aldose reductase pathway is important in the development of diabetic nephropathy, the mechanisms by which it might cause the functional and structural changes have not been elucidated. Initial changes in kidney function in diabetes that precede excess matrix synthesis include glomerular hyperfiltration due to an increase in the renal blood flow and an associated increase in glomerular filtration rate. Glomerular hyperfiltration has been demonstrated both in experimental animal models of diabetes ~2,~3 and in humans3 4,15 The vascular reactivity of the renal glomemlar efferent arterioles has been shown to be controlled at least in part by the release of endogenously synthesized prostaglandins, allowing autoregulation of the glomerular capillary pressure and thus the ultrafiltration rate within the glomerulus. 16 Due to the role prostaglandins play in controlling renal function, it is proposed that changes in prostaglandin production may contribute to the bemodynamic changes seen in diabetes. Several studies have shown that renal prostaglandin production is increased at the onset of diabetes. 17-2° However, the cellular mechanisms underlying this change are yet to be definitively established. It has been demonstrated that glomerular byperfiltration of diabetes can be reduced in both rats 2j and humans 22 by inhibition of the polyol pathway enzyme aldose reductase. Furthermore, glomerular production of the vasodilatory prostaglandin g 2 (PGE2) and prostacyclin can be suppressed in diabetic rats treated with an aldose reductase inhibitor in vivo. 2>25 This suggests a potential connection between the polyol pathway and increased prostaglandin production in diabetes. A hypothesis that involves an enhancement of polyol pathway activity may be proposed to link the increased glucose levels seen in diabetes with the observed alterations in renal prostaglandin production. At elevated glucose concentrations, there is a concomitant increase in the activity of the polyol pathway. This has been shown as increased sorbitol production not only in diabetic kidney 24,26 but also in peripheral nerve and retina 27 and lens. 2s As a consequence of the acceleration of the polyol pathway, there are changes in the ratios NADPH! NADP + (following glucose conversion to sorbitol via aldose reductase) and NAD+/NADH (following sorbitol conversion to fructose via sorbitol dehydrogenase) that have been shown to be associated with increased pentose phosphate pathway (PPP) activity. 29 The PPP is one source of glucose-derived triose phosphate precursors used in the de novo formation of diacylglycerol, a physiologic activator of PKC, and increasing glucose levels have been shown to increase de novo diacylglycerol formation. 3° PKC has been demonstrated to be activated in glomeruli from diabetic rats 31 and in glomerular mesangial cells exposed to high glucose concentrations in vitro. 32 PKC is able to phosphorylate and activate a cytosolic form of phospholipase

Marjorie Dunlop

Papers

Aldose reductase and the role of the polyol pathway in diabetic nephropathy

The link between hyperglycaemia and diabetic nephropathy.

The relative roles of advanced glycation, oxidation and aldose reductase inhibition in the development of experimental diabetic nephropathy in the Sprague-Dawley rat

Pancreatic islets synthesize phospholipids de novo from glucose via acyl-dihydroxyacetone phosphate

Effect of inhibition of aldose reductase on glucose flux, diacylglycerol formation, protein kinase C, and phospholipase A2 activation.