Thus, hyperglycemia-mediated generation of O₂ ^•− inhibits glyceraldehyde 3-phosphate dehydrogenase (GAPDH) activity in vivo by modifying the enzyme with polymers of ADP-ribose (Fig. 8.2) (Du et al. 2003). Specific inhibitors of poly(ADP-ribose) polymerase (PARP) prevent the inhibition of GAPDH. Under normal conditions, PARP resides in the nucleus in an inactive form. Under hyperglycemic conditions DNA damage due to increase in ROS production results in activation of PARP in the nucleus. PARP splits the NAD⁺molecule into nicotinic acid and ADP-ribose. PARP then proceeds to make polymers of ADP-ribose, which accumulate on GAPDH and on other nuclear proteins. GAPDH is commonly thought to reside exclusively in the cytosol. However, it normally shuttles in and out of the nucleus, where it plays a critical role in DNA repair (Sawa et al. 1997; Du et al. 2003). Under physiological conditions, glucose is metabolized to glyceraldehyde-3-phosphate (G3P) via glycolysis. G3P is then converted to 1,3-diphosphoglycerate by GAPDH, which is upregulated and G3P is further metabolized to form pyruvate. Under hyperglycemic conditions, insulin resistance produces down regulation of GAPDH not only slowing glucose metabolism but increasing glucose metabolism via the polyol pathway (Phillips and Thornalley 1993; Beisswenger et al. 2003; Alexander et al. 1988) involving the enzyme aldose reductase. This enzyme utilizes a wide variety of carbonyl compounds as substrates and reduces them into sugar alcohols (polyols). It uses the nicotinic acid adenine dinucleotide phosphate (NADPH) as a cofactor. NADPH is the essential cofactor for regenerating a critical intracellular antioxidant (reduced glutathione). By reducing the amount of reduced glutathione, the polyol pathway increases susceptibility to intracellular oxidative stress. Accumulating evidence suggests that in polyol pathway, glucose is transformed into sorbitol by the enzyme aldose reductase and sorbitol, which is then oxidized to fructose by the enzyme sorbitol dehydrogenase (SDH) with NAD⁺as a cofactor (Giacco and Brownlee 2010). The production of sorbitol from glucose increases osmotic stress, which is associated with the pathophysiology of diabetic complications (Fig. 8.2). Accumulating evidence suggests that there are three potential mechanisms by which the polyol pathway contributes to oxidative stress. First, under hyperglycemic conditions, about 30 % of the glucose is channeled into aldolase reductase-dependent polyol pathway, which depletes NADPH and consequently reduces levels of GSH (Cheng and Gonzalez 1986). Second, oxidative stress is generated during the conversion of sorbitol into fructose by sorbitol dehydrogenase. In this step, the co-factor NAD⁺is transformed into NADH by sorbitol dehydrogenase. NADH is a substrate for NADH oxidase, which facilitates the formation of superoxide anions (Morre et al. 2000). Third, the polyol pathway converts glucose into fructose, and fructose can be further metabolized into fructose-3-phosphate and 3-deoxyglucosone, which are much more potent non-enzymic glycation agent than glucose (Hamada et al. 1996a, 1996b). Thus, the utilization of glucose through the polyol pathway increases the formation of advanced glycation end-products (AGEs) , ultimately leading to ROS generation. Furthermore, aldose reductase has broad substrate specificity. It recognizes short chain sugar derived carbonyls (glycoaldehyde, methylglyoxal) , medium chain hydrophobic aldehydes (4-hydroxynonenal, hexenal) and phospholipid aldehydes (1-palmitoyl-2-oxo valeroyl phosphatidylcholine, POVPC). Aldose reductase also catalyzes the reduction of the glutathione conjugates of 4-HNE and acrolein. Based on this substrate specificity, it is proposed that the enzyme is involved in the metabolism and detoxification of AGEs and advanced lipoxidation end products (ALEs) precursors and therefore it retards tissue injury and inflammation associated with ALEs and AGEs accumulation. Pharmacological inhibition of the aldose reductase exerts significant benefit against the development of various diabetic complications in animal models (Altan 2003; Drel et al. 2008). Downregulation of the polyol pathway activity by inhibiting aldose reductase prevents high-glucose-induced diacylglycerol accumulation and PKC activation in smooth muscle cells (Ramana et al. 2005, 2006). Inhibition of aldose reductase also retards high-glucose-induced stimulation of the extracellular signal-related kinase/mitogen-activated protein kinase and phosphatidylinositol 3-kinase (Campbell and Trimble 2005) and activation of NF-κB (Ramana et al. 2004), thereby decreasing smooth muscle cells chemotaxis, vascular inflammation, and adhesion. Similar processes may occur in neurons and glial cells under hyperglycemic conditions.

As stated above, hyperglycemia also results in production of AGEs, which are formed by the nonenzymatic reaction of glucose and glucose-derived compounds with proteins. The AGEs are formed when α-dicarbonyl and oxoaldehydes (4-HNE and acrolein) react with free amines or amino groups of proteins to form a labile Schiff base (Maillard reaction). A cascade of reactions results thereafter in the formation of AGEs, which are composed of irreversibly cross-linked heterogeneous protein aggregates.

Methylglyoxal is generated nonenzymatically from the oxidation and spontaneous dismutation of intermediates in the glycolysis pathway or enzymic oxidation reaction catalyzed by peroxidases, whereas other AGE precursors such as deoxyglucosone are generated from fructose or from nonenzymatic degradation of Amadori rearrangement compounds (Fig. 8.3) (Conklin et al. 2007). Increase in plasma levels of methylglyoxal significantly correlates with the degree of hypertension in spontaneously hypertensive rats (Wang et al. 2005) as well as in patients with type 2 diabetes (Wang et al. 2007). This metabolite reduces activity of antioxidant enzymes like glutathione reductase and glutathione peroxidase (Wu and Juurlink 2002), leading to increased oxidative stress, which in turn mediates the pathophysiological changes in diabetes, hypertension, and aging (Ceriello and Motz 2004). Hypertension is a multifactorial condition cause by an increase in renin angiotensin aldosterone system (RAAS) activity, increase in uric acid, insulin resistance, renal disease and oxidative stress (Ceriello 2008; Manrique et al. 2009; Dhar et al. 2013). The RAAS plays an important role in maintaining fluid balance, vascular tone and blood pressure (Fyhrquist and Saijonmaa 2008; Zaman et al. 2002). Elevation in angiotensin II levels increases oxidative stress (Manrique et al. 2009; Hitomi et al. 2007) due to interactions of Angiotensin II with type I receptor (AT₁ receptor). These interactions results in potent vasoconstriction, proinflammatory, pro-oxidative, proliferative and hypertrophic effects.

Methylglyoxal is a major precursor for the formation of advanced glycation end products (AGEs) (Desai and Wu 2007). AGEs are generated normally as tissues age, but they are formed at an accelerated rate during diabetes (Wautier and Schmidt 2004). Production of AGE precursors damages cells by three general mechanisms including (a) modification of intracellular proteins by AGEs results in alterations in function, such as resistance of glycated proteins to lysosomal enzyme degradation and poor recognition of glycated haemoglobin by lipoprotein receptors and scavenger receptors (Zimmermann et al. 2001), (b) modification of extracellular matrix components by AGE precursors produces abnormalities in between other matrix components and matrix integrins receptors, which are located on the surface of cells, and (c) modification of plasma proteins by AGEs. AGEs interact with AGE receptors (RAGE), which are found on macrophages, vascular endothelial cells, vascular smooth muscle cells, neurons, astrocytes and microglial cells to induce the production of ROS, which in turn activates the pleiotropic transcription factor NF-κB, triggering multiple pathological changes in gene expression involved in proinflammatory events (Fig. 8.4) (Lue et al. 2001; Sasaki et al. 2001; Goldin et al. 2006). RAGEs modulate many signal transductions pathways associated not only with generation of more oxidative stress, but also inflammatory events (Fig. 8.5). In addition, hyperglycemia-mediated insulin resistance activates protein kinase C isoforms (due to increase in intracellular levels of diacylglycerol) and downregulates endothelial NO synthesis, as well as a induces a variety of other deleterious vascular effects, such as coagulation abnormalities, increase in production of vasoconstrictors (e.g. endothelin) and enhancement in vascular permeability along with pathological alterations in angiogenetic pathways (Sheetz and King 2002; Coppey et al. 2003). Finally, another pathway for hyperglycemia-mediated effects involves fructose-6 phosphate. This metabolite is converted by the enzyme glutamine:fructose-6 phosphate amidotransferase into UDP–N-acetylglucosamine (UDP-GlcNAc) leading to over-modification of various proteins with N-acetyl-hexosamine (Brownlee 2005) (Fig. 8.2). Collective evidence suggests that hyperglycemia-mediated increase in ROS production causes a cascade of cellular metabolic alterations, such as increase in polyol pathway influx, generation of AGEs, activation of NF-κB along with increase in hexosamine pathway (Brownlee 2005). These downstream effects may lead to blood-flow abnormalities, increased vascular permeability, angiogenesis, capillary occlusion, and pro-inflammatory gene expression (Koya et al. 1997). In addition, lipotoxicity, sympathetic nervous system activation, and extracellular matrix deposition also contribute to tissue damage caused by hyperglycemia (Dungan et al. 2009). Thus, hyperglycemia-mediated insulin resistance (Hoehn et al. 2009) is a key component of various health problems caused by the prolonged consumption of high carbohydrate diet resulting in positive energy imbalance (overeating) along with lack of physical activity. These processes result in the development of oxidative stress and low grade inflammation. The major health problems caused by hyperglycemia and insulin resistance include metabolic syndrome (MetS) , heart disease, cerebrovascular diseases, Alzheimer disease, and aging (Fig. 8.3) (Moreira et al. 2007; Ning et al. 2011) .