Darlene McCord – Pathogenesis

Pathogenesis
The macrovascular and microvascular complications of diabetes are closely related to hyperglycemia and oxidative stress, which is when cells fail to detoxify the reactive oxygen species (ROS) produced during metabolism. Four hypotheses have been proposed to explain how hyperglycemia causes complications: 1) increased polyol pathway flux, 2) increased intracellular formation of advanced glycation end-products (AGE), 3) activation of protein kinase C (PKC) isoforms, and 4) increased flux through the hexosamine pathway.

A unifying concept is that hyperglycemia-induced mitochondrial superoxide overproduction activates these 4 pathways. Excess superoxide partially inhibits the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) thereby diverting upstream metabolites from glycolysis to pathways of glucose over-utilization. Superoxide anion achieves this by causing DNA strand breaks that result in activation of poly ADP ribose polymerase (PARP) which in turn ribosylates and deactivates GAPDH. By preventing their metabolism, this process increases energy substrates resulting in increased flux of dihydroxyacetone phosphate (DHAP) to diacylglycerol (DAG), an activator of PKC, and of triose phosphates to methylglyoxol, which is the main intracellular AGE precursor. Increased flux of fructose-6-phosphate to UDP-N-acetylglucosamine in the hexosamine pathway increases modification of proteins by O-linked N-acetylglucosamine and increased glucose flux through the polyol pathway consumes the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) and depletes GSH (reduced glutathione, a natural potent anti-oxidant).

Several mechanisms have been postulated to explain why increasing the polyol pathway flux is detrimental. These are sorbitol-induced osmotic stress, decreased (Na++K+) ATPase activity, increased cytosolic NADH/NAD+ and decreased cytosolic NADPH.

Activation of the hexosamine pathway results in intracellular glycosylation and donation of N-acetyl glucosamine to serine and threonine residues of transcription factors such as Sp1 resulting in increased production of factors such as plasminogen activator inhibitor-1 (PAI-1) and transforming growth factor beta 1 (TGF-beta 1).5

Production of intracellular AGEs damages target cells by three mechanisms. Intracellular proteins modified by AGEs have altered function (like neurotropism, axonal transport, and gene expression). Secondly, extra-cellular matrix components modified by AGE precursors interact abnormally with other matrix components and with the receptors for matrix proteins (integrins) on cells. Thirdly, plasma proteins modified by AGE precursors bind to AGE receptors (RAGE) on endothelial cells, mesangial cells, and macrophages inducing receptor-mediated production of ROS as a second messenger to activate the nuclear factor kappa B (NF-kappa B), a transcription factor causing pathological changes in gene expression.

Hyperglycemia-induced activation of PKC has a number of pathogenic consequences by affecting expression of endothelial nitric oxide synthetase (eNOS), endothelin-1 (ET-1), vascular endothelial growth factor (VEGF), TGF-beta 1, and PAI-1, and by activating NF-kappa B and NAD(P)H oxidases. Increased eNOS and decreased ET-1 decrease blood flow causing hypoxia. Increased VEGF causes increased vascular permeability and angiogenesis. Increased TGF-beta leads to increased collagen, fibronectin, extra-cellular matrix, and basement membrane resulting in capillary occlusion. Increased PAI-1 decreases fibrinolysis leading to vascular occlusion. Increased NF-kappa B causes an increase in pro-inflammatory gene expression. Increased NAD(P)H oxidase causes increased ROS (resulting in DNA damage, oxidation of polydesaturated fatty acids in lipids, and oxidation of amino acids in proteins). These pathogenic mechanisms can all be characterized as a result of ROS effects on genes and proteins.5

Nishikawa T, et al. Normalizing mitochondrial superoxide production blocks three pathways of hyperglycaemic damage. Nature 2000; 404: 787-90.

Du X, et al. Inhibition of GAPDH activity by poly (ADP-ribose) polymerase activates three major pathways of hyperglycemic damage in endothelial cells. J Clin Invest 2000; 112: 1049-57.

Brownlee M. Biochemistry and molecular cell biology of diabetic complications. Nature 2001; 414: 813-20.

Tags: Darlene McCord, Dr. Darlene E McCord, GAPDH, Health, mccord research, Pathogenesis, Science, skin care