Advanced protein glycosylation in diabetes and aging

Differential expression of fructosyllysine-specific receptors on monocytes and macrophages and possible pathophysiological significance

A differing individual expression of fructosyllysine-specific receptors has been found on the monocytes of 90 insulin-dependent diabetic patients and 101 healthy control subjects. The degree of receptor expression is neither age- nor sex-dependent; however, in the diabetic group it correlates significantly with the severity and age of onset of diabetic microangiopathy. To interpret the results of the human study, spontaneously diabetic and non-diabetic BB/OK rats were used to estimate tissue content of glucose-modified proteins and capillary basement membrane thickness in relation to the receptor expression on macrophages. In non-diabetic and diabetic rats no correlation was found between receptor expression and tissue content (i.e. artery, nerve) of fructosyllsine and fluorescent advanced glycation end products. However, animals which express the fructosyllysine receptor showed a greater increase in muscle capillary basement membrane thickness. There are indications that fructosyllysine receptor expression is positively associated with indices of diabetic complications such as microangiopathy and/or capillary basement membrane thickening.

The cellular and molecular mechanisms of diabetic complications

In this article, the cellular and molecular mechanisms of diabetic complications have been reviewed. Hyperglycemia-induced mechanisms that may induce vascular dysfunction in specific sites of diabetic microvascular damage include increased polyol pathway flux, altered cellular redox state, increased formation of diacylglycerol and the subsequent activation of specific PKC isoforms, and accelerated nonenzymatic formation of advanced glycation endproducts. Several of these mechanisms may be responsible for the potentially damaging overproduction of reactive oxygen species observed with hyperglycemia. Each of these mechanisms may contribute to the known pathophysiologic features of diabetic complications by a number of mechanisms, including the upregulation of cytokines and growth factors. Diabetic macrovascular disease may arise more from insulin resistance than from hyperglycemia, and the authors speculate that this may reflect a selective loss of insulin-dependent vascular homeostasis.

Quantitation of the reactive oxygen species generated by the UVA irradiation of ascorbic acid-glycated lens proteins

The oxidation products of ascorbic acid rapidly glycate proteins and produce protein-bound, advanced glycation endproducts. These endproducts can absorb UVA light and cause the photolytic oxidation of proteins (Ortwerth, Linetsky and Olesen, Photochem. Photobiol. 62, 454-463, 1995), which is mediated by the formation of reactive oxygen species. A dialyzed preparation of calf lens proteins, which had been incubated for 4 weeks with 20 mM ascorbic acid in air, was irradiated for 1 h with 200 mW/cm2 of absorbed UVA light (gamma > 338 nm), and the concentration of individual oxygen free radicals was measured. Superoxide anion attained a level of 76 microM as determined by the superoxide dismutase (SOD)-dependent increase in hydrogen peroxide formation and of 52 microM by the SOD-inhibitable reduction of cytochrome c. Hydrogen peroxide formation increased linearly to 81 microM after 1 h. Neither superoxide anion nor hydrogen peroxide, however, could account for the UVA photolysis of Trp and His seen in this system. Singlet oxygen levels approached 1.0 mM as measured by the oxidation of histidine, which was consistent with singlet oxygen measurements by the bleaching of N,N-dimethyl-4-nitrosoaniline. High concentrations of sodium azide, a known singlet oxygen quencher, inhibited the photolytic destruction of both His and Trp. Little or no protein damage could be ascribed to hydroxyl radical based upon quenching experiments with added mannitol. Therefore, superoxide anion and H2O2 were generated by the UVA irradiation of ascorbate advanced glycation endproducts, however, the major reactive oxygen species formed was singlet oxygen.

BCL-2 expression or antioxidants prevent hyperglycemia-induced formation of intracellular advanced glycation endproducts in bovine endothelial cells

Hyperglycemia rapidly induces an increase in intracellular advanced glycation end products (AGEs) in bovine endothelial cells, causing an alteration in bFGF activity (Giardino, I., D. Edelstein, and M. Brownlee. 1994. J. Clin. Invest. 94:110-117). Because sugar or sugar-adduct autoxidation is critical for AGE formation in vitro, we evaluated the role of reactive oxygen species (ROS) in intracellular AGE formation, using bovine aortic endothelial cells. 30 mM glucose increased intracellular ROS formation by 250% and lipid peroxidation by 330%, while not affecting ROS in the media. In cells depleted of glutathione, intracellular AGE accumulation increased linearly with ROS generation as measured by immunoblotting and the fluorescent probe DCFH (AGE 0.258-3.531 AU* mm/5x10(4) cells, DCF 57-149 mean AU, r = .998, P < .002). Deferoxamine, alpha-tocopherol, and dimethylsulfoxide each inhibited hyperglycemia-induced formation of both ROS and AGE. To differentiate an effect of ROS generation on AGE formation from an effect of more distal oxidative processes, GM7373 endothelial cell lines were generated that stably expressed the peroxidation-suppressing proto-oncogene bcl-2. bcl-2 had no effect on hyperglycemia-induced intracellular ROS formation. In contrast, bcl-2 expression decreased both lipid peroxidation (100% at 3 h and 29% at 168 h) and AGE formation (55% at 168 h). These data show that a ROS-dependent process plays a central role in the generation of intracellular AGEs, and that inhibition of oxidant pathways prevents intracellular AGE formation.

Aging and oxidative stress: modulation by dietary restriction

Aging is an inevitable biological process that affects most living organisms. Despite the enormous consequences associated with the aging process, until recently, relatively little systematic effort has been expended on the scientific understanding of this important life process. Society, however, urged by an ever increasing older population, is challenging scientists from many disciplines to explore one of nature's most complex phenomena-biological aging. For the past two decades, research directed toward the basic understanding of biological aging mechanisms and possible aging interventions have given us new insights into the molecular bases and the biological events that contribute to age-related deterioration. To further investigate the aging processes, one probe uniquely suited to exploring the progression of aging in animal models is dietary restriction, currently the only antiaging intervention accepted by gerontologists and nutritionists. Recent research renders a better understanding of how reduced dietary intake extends the life span, supplying evidence that dietary restriction is a diverse and effective modulator of oxidative stress. It has been proposed that this antioxidative mechanism is the underlying anti-aging action of dietary restriction.

Glutamine transaminase K and omega-amidase activities in primary cultures of astrocytes and neurons and in embryonic chick forebrain: marked induction of brain glutamine transaminase K at time of hatching

Glutamine transaminase K and omega-amidase activities are present in the chick brain and in the brains of adult mice, rats, and humans. However, the activity of glutamine transaminase K in adult mouse brain is relatively low. In the chick embryo, cerebral glutamine transaminase K activity is low between embryonic days 5 and 17, but by day 23 (day of hatching) activity rises dramatically (> 15-fold). Cerebral omega-amidase activity is relatively high at embryonic day 5 but lower between days 5 and 17; at embryonic day 23 the activity rises to a maximum. Both glutamine transaminase K and omega-amidase are present in cultured chick, rat, and mouse astrocytes and neurons. For each species, the activity of glutamine transaminase K is higher in the astrocytes than in the neurons. The activity of omega-amidase is about the same in the cultured chick astrocytes and neurons but significantly higher in rat astrocytes than in rat neurons. The data suggest that the rise in brain glutamine transaminase K activity in the chick embryo at hatching correlates with maturation of astrocytes. Glutamine transaminase K may be involved in glutamine cycling in astrocytes. Glutamine transaminase K appears to be a major cysteine S-conjugate beta-lyase of the brain and may play a role in the neurotoxicity associated with exposure to dichloroacetylene and perhaps to other toxins.