Introduction to the Pathogenesis of Microangiopathy
Hyperglycemia has three important effects that are to some extent interdependent: nonenzymatic glycation, activation of the polyol pathway, and generation of reactive oxygen species (ROS).
The formation of early glycation endproducts (Schiff base and Amadori products) is reversible, but glycated long-lived molecules like DNA and matrix/basement membrane proteins may undergo irreversible cross-linking and other not yet fully understood complex reactions to form advanced glycation endproducts (AGE).
This process leads to considerable changes in their three-dimensional structure and function. Glycated LDL (low-density lipoproteins) are more atherogenic than nonglycated LDL The charge of matrix proteins may be altered, leading to increased permeability, and basement membrane proteins may become resistant to degradation andincrease in thickness and stiffness. AGE may impair the relaxation of vessels by quenching nitric oxide [150]. Moreover, they favor coagulation and thrombosis and initiate atherosclerosis by stimulating macrophages to express AGE receptors (RAGE) and to release cytokines such as TNF-α. IL-1. and IGF.
The activation of the polyol pathway leads to the accumulation of sorbitol, which is followed by myoinositol depletion and inactivation of Na+-K+-ATPase channels. In addition, the production of fructose and sorbitol, which contribute to the formation of AGE, is increased.
There is much evidence that both advanced glycation endproducts (AGE) and high glucose are able to induce the generation of ROS in various types of vascular cells and that this process plays an important role in the initiation of vascular complications in diabetes. In addition to the autoxidation of glucose, three different mechanisms of ROS production are presently under debate:
1. Activation of a membrane-bound, macrophage-like NADH-oxidase. Activation of NADH-oxidase in endothelial cells and smooth muscle cells has been reported in subjects with hypertension and hypercholesterolemia. AGE and angiotensin II are strong activators of this enzyme in vascular cells.
2. Alternatively, it has been suggested that the electron flux in endothelial nitric oxide synthase (NOS III) becomes uncoupled in diabetes and hyperglycemia. In this uncoupled state the electrons flowing from the reductase domain to the oxygenase domain of the NOS complex are diverted to molecular oxygen rather than to L-arginine. In line with this assumption, production of ROS was prevented in human and rat endothelial cells in the presence of inhibitors of NOS.
3. Recently, Nishigawa et al. [146] demonstrated in cultured bovine aortic endothelial cells that in hyperglycemic conditions the mitochondrial electron flux becomes uncoupled from ATP synthesis, resulting in increased ROS production. ROS production was prevented by various uncouplers of the mitochondrial electron chain and overexpression of the uncoupling protein (UCP-2). The activation of protein kinase C, the polyol pathway, the transcription factor NFkB, and the increased formation of AGE and glucosamine were clearly dependent on the formation of ROS, suggesting that at least in these cultured endothelial cells the formation of ROS is the central, initiating step for the transformation of endothelial cells into an active, prothrombotic state. According to these observations, an accelerated substrate flow from either glucose or fatty acids seems to be the final cause for the generation of ROS and oxidative stress.
Against this background, the great importance of accelerated conversion of glucose to fructose by the so-called sorbitol pathway and the changes in the cellular redox state by these processes is obvious. The conversion of glucose consumes NADPH and leads to increased flow of NADH to mitochondria. Since an important cofactor of glutathione peroxidase is diminished, the regeneration of glutathione is impaired, which may limit the antioxidative capacity of the cells and contribute to the occurrence of oxidative stress in diabetes.
The generation of ROS seems to be the initiating factor for a number of processes known to be relevant to the development of vascular complication:
1. Activation of protein kinase C(PKC). The activation of PKC in cells and tissues that take up glucose independently of insulin is mediated not only by a hyperglycemia-dependent increase in diacylglycerol (DAG), but also by the enhanced formation of ROS. Activation of PKC seems to be a common downstream mechanism to which multiple cellular and functional abnormalities in the diabetic vascular tissue can be attributed, including changes in vascular blood flow, vascular permeability, extracellular matrix components, and cell growth.
2. Activation of redox-sensitive transcription factors by AGE and hyperglycemia. AGE formation has so far mostly been in discussion as a process of protein modification. From recent studies it follows, however, that interactions of AGE-modified proteins with specific AGE receptors serve not only to eliminate AGE proteins, but also to induce signal transaction pathways which lead to the generation of ROS, depletion of cellular antioxidant defense mechanisms (e. g., glutathione, ascorbate) and the activation of redox-sensitive transcription factors such as NFkB [144,151]. The activation of NFkB and presumably also other redox-sensitive transcription factors promotes the expression of a variety of kinins, such as the procoagulant tissue factor, endothelin-1, and the adhesion molecules VCAM-1 (vascular cellular adhesion molecule 1), ICAM-1 (intercellular adhesion molecule 1), and MCP-1 (monocyte chemoattracting protein 1), all of which have been found to be increased in the diabetic state. The concept of AGE-induced oxidative stress which activates transcription factors could explain the concomitant occurrence of oxidative stress and changes in the dynamic endothelial balance from an anticoagulant to a procoagulant state, from vasodilatation to vasoconstriction and impaired microcirculation.
3. Activation of the hexosamine pathway and activation of the transcription factor SP-1. ROS have been shown to inhibit glyceraldehyde 3-P-dehydrogenase. In consequence, more glucose will be metabolized to glutamine 6-phosphate. This molecule has been shown to play a role in the induction of insulin resistance. It also enhances glycosylation and activation of the transcription factor SP-1, which accelerates synthesis of plasminogen activator inhibitor 1 (PAI-1) and transforming growth factor β1 (TGF-β1), both of which contribute to the pathogenesis of vascular complications by changes in the hemostatic balance and remodeling processes of the vessel wall.
4. Quenching of nitric oxide. Oxidative stress seems thereby to initiate a vicious cycle reinforcing the imbalance in the redox state of cells and the generation of ROS. These therefore counterbalance the cytoprotective effects of nitric oxide on microcirculation, on the permeability and adhesiveness of the vessel wall, and growth inhibition of smooth muscle cells.
5. Taken together, ROS activate interrelated processes and mechanisms which play a major role in the development of vascular complications in diabetes. However, it must be borne in mind that the outcome of the processes may vary depending the site affected (large vessels, resistance vessels, capillaries).
Following the metabolic concept of pathogenesis, one would expect that the extracellular changes are ubiquitous systemic disorders and that all insulinindependentcells (for only these are exposed to intracellular hyperglycemia) are affected. However, microangiopathy is clinically evident only in the kidney and the eye, and is thought to play a role in neuropathy. Microangiopathy usually begins with reversible functional disorders and may end with irreversible loss of organ function. For this reason, early detection (screening) and monitoring are of paramount importance.
Retinopathy
Epidemiology
The prevalence of diabetic retinopathy is highest in early-onset insulin-treated diabetic subjects and lowest in late-onset non-insulin-treated diabetic
