O-GlcNAc and aging: C. elegans as a genetic model to test O-GlcNAc roles in type II diabetic insulin resistance

High-fat diet increases O-GlcNAc levels in cerebral arteries: a link to vascular dysfunction associated with hyperlipidaemia/obesity?

Obesity and high fat intake induce alterations in vascular function and structure. Aberrant O-GlcNAcylation (O-GlcNAc) of vascular proteins has been implicated in vascular dysfunction associated with cardiovascular and metabolic diseases. In the present study, we tested the hypothesis that high-fat diet (HFD)-mediated increases in O-GlcNAc-modified proteins contribute to cerebrovascular dysfunction. O-GlcNAc-protein content was increased in arteries from male Wistar rats treated with a HFD (45% fat) for 12 weeks compared with arteries from rats on control diet (CD). HFD augmented body weight [(g) 550±10 compared with 502±10 CD], increased plasma triacylglycerols [(mg/dl) 160±20 compared with 95±15 CD] and increased contractile responses of basilar arteries to serotonin [5-hydroxytryptamine (5-HT)] [(pD2) 7.0±0.1 compared with 6.7±0.09 CD] and the thromboxane analogue 9,11-dideoxy-9α,11α-methanoepoxy prostaglandin F2α (U-46619) [(pD2) 7.2±0.1 compared with 6.8±0.09 CD]. Of importance, increased levels of O-GlcNAc [induced by 24 h-incubation of vessels with a potent inhibitor of O-GlcNAcase (OGA), O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate (PugNAc)] increased basilar artery contractions in response to U-46619 [(pD2) 7.4±0.07 compared with 6.8±0.08 CD] and 5-HT [(pD2) 7.5±0.06 compared with 7.1±0.1 CD]. Vessels from rats on the HFD for 12 weeks and vessels treated with PugNAc displayed increased phosphorylation of p38 (Thr(180/182)) and extracellular signal-regulated kinase 1/2 (Erk1/2) (Ser(180/221)). Increased 5HT-induced contractions in arteries from rats on the HFD or in arteries incubated with PugNAc were abrogated by mitogen-activated protein kinase (MAPK) inhibitors. Our data show that HFD augments cerebrovascular O-GlcNAc and this modification contributes to increased contractile responses and to the activation of the MAPK pathway in the rat basilar artery.

Answering the ultimate question "what is the proximal cause of aging?"

Recent discoveries suggest that aging is neither driven by accumulation of molecular damage of any cause, nor by random damage of any kind. Some predictions of a new theory, quasi-programmed hyperfunction, have already been confirmed and a clinically-available drug slows aging and delays diseases in animals. The relationship between diseases and aging becomes easily apparent. Yet, the essence of aging turns out to be so startling that the theory cannot be instantly accepted and any possible arguments are raised for its disposal. I discuss that these arguments actually support a new theory. Are any questions remaining? And might accumulation of molecular damage still play a peculiar role in aging?

Enhanced cardiac protein glycosylation (O-GlcNAc) of selected mitochondrial proteins in rats artificially selected for low running capacity

O-linked β-N-acetyl glucosamine (O-GlcNAc) is a posttranslational modification consisting of a single N-acetylglucosamine moiety attached by an O-β-glycosidic linkage to serine and threonine residues of both nuclear and cytosolic proteins. Analogous to phosphorylation, the modification is reversible and dynamic, changing in response to stress, nutrients, hormones, and exercise. Aims of this study were to examine differences in O-GlcNAc protein modification in the cardiac tissue of rats artificially selected for low (LCR) or high (HCR) running capacity. Hyperinsulinemic-euglycemic clamps in conscious animals assessed insulin sensitivity while 2-[(14)C] deoxyglucose tracked both whole body and tissue-specific glucose disposal. Immunoblots of cardiac muscle examined global O-GlcNAc modification, enzymes that control its regulation (OGT, OGA), and specific proteins involved in mitochondrial oxidative phosphorylation. LCR rats were insulin resistant disposing of 65% less glucose than HCR. Global tissue O-GlcNAc, OGT, OGA, and citrate synthase were similar between groups. Analysis of cardiac proteins revealed enhanced O-GlcNAcylation of mitochondrial Complex I, Complex IV, VDAC, and SERCA in LCR compared with HCR. These results are the first to establish an increase in specific protein O-GlcNAcylation in LCR animals that may contribute to progressive mitochondrial dysfunction and the pathogenesis of insulin resistance observed in the LCR phenotype.

Pleiotropic and age-dependent effects of decreased protein modification by O-linked N-acetylglucosamine on pancreatic β-cell function and vascularization

The hexosamine biosynthesis pathway (HBP) regulates the post-translational modification of nuclear and cytoplasmic protein by O-linked N-acetylglucosamine (O-GlcNAc). Numerous studies have demonstrated increased flux through this pathway contributes to the development of β-cell dysfunction. The effect of decreased O-GlcNAc on the maintenance of normal β-cell function, however, is not well understood. We studied transgenic mice that over express β-N-acetylglucosaminidase (O-GlcNAcase), an enzyme that catalyzes the removal of O-GlcNAc from proteins, in the pancreatic β-cell under control of the rat insulin promoter. 3-4-Month-old O-GlcNAcase transgenic mice have higher glucose excursions with a concomitant decrease in circulating insulin levels, insulin mRNA levels, and total islet insulin content. In older (8-9-month-old) O-GlcNAcase transgenic mice glucose tolerance is no longer impaired. This is associated with increased serum insulin, islet insulin content, and insulin mRNA in the O-GlcNAcase transgenic mice. These improvements in β-cell function with aging are associated with increased angiogenesis and increased VEGF expression, with parallel increases in activation of Akt and expression of PGC1α. The biphasic effects as a function of age are consistent with published observations of mice with increased O-GlcNAc in islets and demonstrate that O-GlcNAc signaling exerts multiple effects on both insulin secretion and islet survival.