Involvement of SIRT1 in Zn2+, Streptozotocin, Non-Obese Diabetic, and Cytokine-Mediated Toxicities of β-cells

Sirtuins as molecular targets, mediators, and protective agents in metal-induced toxicity

Metal dyshomeostasis, and especially overexposure, is known to cause adverse health effects due to modulation of a variety of metabolic pathways. An increasing body of literature has demonstrated that metal exposure may affect SIRT signaling, although the existing data are insufficient. Therefore, in this review we discuss the available data (PubMed-Medline, Google Scholar) on the influence of metal overload on sirtuin (SIRT) signaling and its association with other mechanisms involved in metal-induced toxicity. The existing data demonstrate that cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), aluminium (Al), hexavalent chromium (Cr^VI), manganese (Mn), iron (Fe), and copper (Cu) can inhibit SIRT1 activity. In addition, an inhibitory effect of Cd, Pb, As, and Fe on SIRT3 has been demonstrated. In turn, metal-induced inhibition of SIRT was shown to affect deacetylation of target proteins including FOXO, PGC1α, p53 and NF-kB. Increased acetylation downregulates PGC1α signaling pathway, resulting in cellular altered redox status and increased susceptibility to oxidative stress, as well as decreased mitochondrial biogenesis. Lower rates of LKB1 deacetylation may be responsible for metal-induced decreases in AMPK activity and subsequent metabolic disturbances. A shift to the acetylated FOXO results in increased expression of pro-apoptotic genes which upregulates apoptosis together with increased p53 signaling. Correspondingly, decreased NF-kB deacetylation results in upregulation of target genes of proinflammatory cytokines, enzymes, and cellular adhesion molecules thus promoting inflammation. Therefore, alterations in sirtuin activity may at least partially mediate metal-induced metabolic disturbances that have been implicated in neurotoxicity, nephrotoxicity, cardiotoxicity, and other toxic effects of heavy metals.

NAD+ Deficiency Is a Common Central Pathological Factor of a Number of Diseases and Aging: Mechanisms and Therapeutic Implications

Increasing evidence has indicated critical roles of nicotinamide adenine dinucleotide, oxidized form (NAD⁺) in various biological functions. NAD⁺ deficiency has been found in models of a number of diseases such as cerebral ischemia, myocardial ischemia, and diabetes, and in models of aging. Applications of NAD⁺ or other approaches that can restore NAD⁺ levels are highly protective in these models of diseases and aging. NAD⁺ produces its beneficial effects by targeting at multiple pathological pathways, including attenuating mitochondrial alterations, DNA damage, and oxidative stress, by modulating such enzymes as sirtuins, glyceraldehyde-3-phosphate dehydrogenase, and AP endonuclease. These findings have suggested great therapeutic and nutritional potential of NAD⁺ for diseases and senescence. Recent Advances: Approaches that can restore NAD⁺ levels are highly protective in the models of such diseases as glaucoma. The NAD⁺ deficiency in the diseases and aging results from not only poly(ADP-ribose) polymerase-1 (PARP-1) activation but also decreased nicotinamide phosphoribosyltransferase (Nampt) activity and increased CD38 activity. Significant biological effects of extracellular NAD⁺ have been found. Increasing evidence has suggested that NAD⁺ deficiency is a common central pathological factor in a number of diseases and aging. Critical Issues and Future Directions: Future studies are required for solidly establishing the concept that "NAD⁺ deficiency is a common central pathological factor in a number of disease and aging." It is also necessary to further investigate the mechanisms underlying the NAD⁺ deficiency in the diseases and aging. Preclinical and clinical studies should be conducted to determine the therapeutic potential of NAD⁺ for the diseases and aging.

SIRT1 rs10823108 and FOXO1 rs17446614 responsible for genetic susceptibility to diabetic nephropathy

SIRT1 and FOXO1 play an important role in the pathogenesis of diabetic nephropathy (DN). However, the association between genetic polymorphisms and susceptibility to type 2 DN (T2DN) has not been explored. In this study, a total of 1066 patients with type 2 diabetes mellitus (T2DM) (413 without and 653 with DN) were enrolled. The genotypes of three htSNPs (rs3818292, rs4746720, rs10823108) within SIRT1 and two htSNPs (rs2721068, rs17446614) in FOXO1 were determined by PCR-RFLP. HbA1C, LDL, HDL, TC, and TG levels were also examined. SIRT1 rs10823108 AA genotype was significantly associated with a decreased risk of DN (OR = 0.60, 95%CI: 0.38–0.97), while GA genotype (OR = 1.77, 95%CI: 1.33–2.35) and AA genotype (OR = 2.32, 95%CI: 1.25–4.34) of FOXO1 rs17446614 was associated with an increased T2DN risk. The interactions among rs1744 6614, BMI and duration of diabetes (OR: 2.63, 95%CI: 1.23–4.31) were also observed. Subsequent haplotype analysis revealed that two haplotype defined by AC (OR: 1.50, 95%CI: 1.15–1.94) and AT (OR: 1.79, 95%CI: 1.06–2.80) within FOXO1 gene may increase the risk of T2DN. In conclusion, genetic variant rs10823108 in SIRT1 and variant rs17446614 in FoxO1 may contribute to the risk of DN in T2DM patients.

High Glucose Induces Reactivation of Latent Kaposi's Sarcoma-Associated Herpesvirus

A high prevalence of Kaposi's sarcoma (KS) is seen in diabetic patients. It is unknown if the physiological conditions of diabetes contribute to KS development. We found elevated levels of viral lytic gene expression when Kaposi's sarcoma-associated herpesvirus (KSHV)-infected cells were cultured in high-glucose medium. To demonstrate the association between high glucose levels and KSHV replication, we xenografted telomerase-immortalized human umbilical vein endothelial cells that are infected with KSHV (TIVE-KSHV cells) into hyperglycemic and normal nude mice. The injected cells expressed significantly higher levels of KSHV lytic genes in hyperglycemic mice than in normal mice. We further demonstrated that high glucose levels induced the production of hydrogen peroxide (H₂O₂), which downregulated silent information regulator 1 (SIRT1), a class III histone deacetylase (HDAC), resulting in the epigenetic transactivation of KSHV lytic genes. These results suggest that high blood glucose levels in diabetic patients contribute to the development of KS by promoting KSHV lytic replication and infection. IMPORTANCE Multiple epidemiological studies have reported a higher prevalence of classic KS in diabetic patients. By using both in vitro and in vivo models, we demonstrated an association between high glucose levels and KSHV lytic replication. High glucose levels induce oxidative stress and the production of H₂O₂, which mediates the reactivation of latent KSHV through multiple mechanisms. Our results provide the first experimental evidence and mechanistic support for the association of classic KS with diabetes.

Dietary zinc reduction, pyruvate supplementation, or zinc transporter 5 knockout attenuates β-cell death in nonobese diabetic mice, islets, and insulinoma cells

Pancreatic zinc (Zn(2+)) concentrations are linked to diabetes and pancreatic dysfunction, but Zn(2+) is also required for insulin processing and packaging. Zn(2+) released with insulin increases β-cell pancreatic death after streptozotocin toxin exposure in vitro and in vivo. Triosephosphate accumulation, caused by NAD(+) loss and glycolytic enzyme dysfunction, occur in type-1 diabetics (T1DM) and animal models. We previously showed these mechanisms are also involved in Zn(2+) neurotoxicity and are attenuated by nicotinamide- or pyruvate-induced restoration of NAD(+) concentrations, Zn(2+) restriction, or inhibition of Sir2 proteins. We tested the hypothesis that similar Zn(2+)- and NAD(+)-mediated mechanisms are involved in β-cell toxicity in models of ongoing T1DM using mouse insulinoma cells, islets, and nonobese diabetic (NOD) mice. Zn(2+), streptozotocin, and cytokines caused NAD(+) loss and death in insulinoma cells and islets, which were attenuated by Zn(2+) restriction, pyruvate, nicotinamide, NAD(+), and inhibitors of Sir2 proteins. We measured diabetes incidence and mortality in NOD mice and demonstrated that pyruvate supplementation, or genetic or dietary Zn(2+) reduction, attenuated these measures. T-lymphocyte infiltration, punctate Zn(2+) staining, and β-cell loss increased with time in islets of NOD mice. Dietary Zn(2+) restriction or Zn(2+) transporter 5 knockout reduced pancreatic Zn(2+) staining and increased β-cell mass, glucose homeostasis, and survival in NOD mice, whereas Zn(2+) supplementation had the opposite effects. Pancreatic Zn(2+) reduction or NAD(+) restoration (pyruvate or nicotinamide supplementation) are suggested as novel targets for attenuating T1DM.