Telomere shortening: a marker of atherosclerosis?

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Differential gene expression of NADPH oxidase (p22phox) and hemoxygenase-1 in patients with Type 2 diabetes and microangiopathy

Aims: While the downstream effects of increased reactive oxygen species (ROS) in the pathogenesis of diabetes were well studied, only a few studies have explored the cellular sources of ROS. We examined whether protection against oxidative stress is altered in patients with diabetes and microangiopathy by examining changes in NADPH oxidase (p22phox) and hemoxygenase‐1 (HO‐1) levels. Methods: NADPH oxidase (p22phox) and HO‐1 gene expression were probed by RT‐PCR using leucocytes from patients with Type 2 diabetes without (n = 19) and with microangiopathy (n = 20) and non‐diabetic subjects (n = 17). Levels of lipid peroxidation as measured by thiobarbituric reactive substances (TBARS) and protein carbonyl content (PCO) were determined by fluorimetric and spectrophotometric methods, respectively. Results: p22phox gene expression (mean ± se) was significantly (P < 0.05) higher in diabetic patients with (0.99 ± 0.04) and without microangiopathy (0.86 ± 0.05) compared with control subjects (0.66 ± 0.05). Consistent with the mRNA data, the p22phox protein expression and NADPH oxidase activity was also increased in cells from diabetic patients compared with control subjects. However, HO‐1 gene expression was significantly (P < 0.05) lower in patients with (0.73 ± 0.03) and without microangiopathy (0.85 ± 0.02) compared with control subjects (1.06 ± 0.03). The mean (± se) levels of TBARS were significantly (P < 0.05) higher in diabetic patients with (14.36 ± 1.3 nm/ml) and without microangiopathy (12.20 ± 1.3 nm/ml) compared with control subjects (8.58 ± 0.7 nm/ml). The protein carbonyl content was also significantly (P < 0.05) higher in diabetic patients with (1.02 ± 0.04 nmol/mg protein) and without microangiopathy (0.84 ± 0.06 nmol/mg protein) compared with control subjects (0.48 ± 0.02 nmol/mg protein). In diabetic subjects, increased p22phox gene expression was negatively correlated with HO‐1 and positively correlated with TBARS, PCO, HbA1c and diabetes duration. In contrast, HO‐1 gene expression was correlated negatively with p22phox, TBARS, PCO, HbA1c and diabetes duration. Conclusion: Our results indicate that increased oxidative damage is seen in Asian Indians with Type 2 diabetes and microangiopathy and is associated with increased NADPH oxidase (p22phox) and decreased HO‐1 gene expression

Biomarkers of oxidative stress: methods and measures of oxidative DNA damage (COMET assay) and telomere shortening

Oxidative stress is fast becoming the nutritional and medical buzzword for the twenty-first century. The theoretical importance of oxidative stress in diabetes is highlighted by its potential double impact on metabolic dysfunction on one hand and the vascular system on the other hand. The new concept of oxidative stress, being an important trigger in the onset and progression of diabetes and its complications, emphasizes the need for measurement of markers of oxidation to assess the degree of oxidative stress. While we have been routinely measuring biomarkers in our molecular epidemiology projects, here we discuss the utility of two assays, (a) DNA damage assessment by COMET measurement and (b) telomere length measurement. As DNA damage is efficiently repaired by cellular enzymes, its measurement gives a snapshot view of the level of oxidative stress. The protocol allows for measurement of oxidative DNA damage (FPG-sensitive DNA strand breaks). Telomere length measured by Southern blotting technique allows one to estimate the chronic burden of oxidative stress at the molecular level and is now considered as biomarker of biological aging

Association of telomere shortening with impaired glucose tolerance and diabetic macroangiopathy

Objective: Shortening of telomere length has been reported in several conditions including Type 2 diabetes and atherosclerosis. The aims of this study were (1) to assess whether telomere shortening occurs at the stage of pre-diabetes, i.e., impaired glucose tolerance (IGT) and (2) whether telomere shortening was greater in Type 2 diabetic subjects with atherosclerotic plaques. Methods: Subjects with impaired glucose tolerance (IGT) (n = 30), non-diabetic control subjects (n = 30), Type 2 diabetic patients without (n = 30) and with atherosclerotic plaques (n = 30) were selected from the Chennai Urban Rural Epidemiology Study (CURES), an ongoing epidemiological population-based study. Southern-blot analysis was used to determine mean terminal restriction fragment (TRF) length, a measure of average telomere size, in leukocyte DNA. Levels of thiobarbituric acid reactive substances (TBARS), protein carbonyl content (PCO) and high sensitive C-reactive protein (hs-CRP) were measured by standard methodologies. Carotid intima-media thickness (IMT) was assessed by high resolution B-mode ultrasonography. Results: The mean (±S.E.) TRF lengths were significantly lower in IGT subjects (6.97 ± 0.3 kb; p = 0.002) and lower still in Type 2 diabetic subjects without plaques (6.21 ± 0.2; p = 0.0001) and lowest in Type 2 diabetic subjects with atherosclerotic plaques (5.39 ± 0.2; p = 0.0001) when compared to control subjects (8.7 ± 0.5). In IGT subjects, TRF length was positively correlated to HDL cholesterol and negatively correlated to glycated hemoglobin (HbA1c), TBARS, PCO, HOMA-IR and IMT. In multiple linear regression analysis, presence of diabetes, HDL cholesterol and increased TBARS levels appear as significant determinants of telomere shortening. Conclusion: Telomere shortening is seen even at the stage of IGT. Among subjects with Type 2 diabetes, those with atherosclerotic plaques had greater shortening of telomere length compared to those without plaques

Telomere shortening & metabolic/vascular diseases

Telomeres are specialized DNA-protein structures located at the ends of eukaryotic chromosomes whose length is progressively reduced in most somatic cells during ageing. Over the past decade, emerging evidence has shown that the telomeres are essential regulators of cellular life span and chromosome integrity in a dynamic fashion. By inducing genomic instability, replicative senescence and apoptosis, shortening of telomeres is thought to contribute to organismal ageing. While the aetiology of cardiovascular diseases and diabetes represent a complex interaction between various risk factors overlaid on different genetic backgrounds, the conventional risk factors often did not explain the inter-individual variability related to predisposition of disease states. This underscores the need for biological indicators of ageing in evaluating the aetiology of several age-related disorders, and recent studies indicate that telomere length could qualify as an ideal marker of biological ageing. Short telomeres have been detected in senescent endothelial cells and vascular smooth muscle cells from human atherosclerotic plaque as well as in myocardial tissue from patients with end-stage heart failure and cardiac hypertrophy. In addition, telomere shortening has been demonstrated in WBCs from patients with coronary heart disease, premature myocardial infarction, hypertension and diabetes mellitus. In this review, we discuss the telomere hypothesis of ageing as well as human studies that address the role of telomeres in cardiovascular, diabetes and other cardio-metabolic pathologies

Oxidative DNA damage and augmentation of poly(ADP-ribose) polymerase/nuclear factor-kappa B signaling in patients with Type 2 diabetes and microangiopathy

Although oxidative stress and the subsequent DNA damage is one of the obligatory signals for poly(ADP-ribose) polymerase (PARP) activation and nuclear factor-kappa B (NFκB) alterations, these molecular aspects have not been collectively examined in epidemiological and clinical settings. Therefore, this study attempts to assess the oxidative DNA damage and its downstream effector signals in peripheral blood lymphocytes from Type 2 diabetes subjects without and with microangiopathy along with age-matched non-diabetic subjects. The basal DNA damage, lipid peroxidation and protein carbonyl content were significantly (p < 0.05) higher in patients with and without microangiopathy compared to control subjects. Formamido Pyrimidine Glycosylase (FPG)-sensitive DNA strand breaks which represents reliable indicator of oxidative DNA damage were also significantly (p < 0.001) higher in diabetic patients with (19.41 ± 2.5) and without microangiopathy (16.53 ± 2.0) compared to control subjects (1.38 ± 0.85). Oxidative DNA damage was significantly correlated to poor glycemic control. PARP mRNA expression and PARP activity were significantly (p < 0.05) increased in cells from diabetic patients with (0.31 ± 0.03 densitometry units; 0.22 ± 0.02 PARP units/mg protein, respectively) and without (0.35 ± 0.02; 0.42 ± 0.05) microangiopathy compared to control (0.19 ± 0.02; 0.11 ± 0.02) subjects. Diabetic subjects with and without microangiopathy exhibited a significantly (p < 0.05) higher (80%) NFκB binding activity compared to control subjects. In diabetic patients, FPG-sensitive DNA strand breaks correlated positively with PARP gene expression, PARP activity and NFκB binding activity. This study provides a comprehensive molecular evidence for increased oxidative stress and genomic instability in Type 2 diabetic subjects even prior to vascular pathology and hence reveals a window of opportunity for early therapeutic intervention

Glucose-induced down regulation of thiamine transporters in the kidney proximal tubular epithelium produces thiamine insufficiency in diabetes

Increased renal clearance of thiamine (vitamin B1) occurs in experimental and clinical diabetes producing thiamine insufficiency mediated by impaired tubular re-uptake and linked to the development of diabetic nephropathy. We studied the mechanism of impaired renal re-uptake of thiamine in diabetes. Expression of thiamine transporter proteins THTR-1 and THTR-2 in normal human kidney sections examined by immunohistochemistry showed intense polarised staining of the apical, luminal membranes in proximal tubules for THTR-1 and THTR-2 of the cortex and uniform, diffuse staining throughout cells of the collecting duct for THTR-1 and THTR-2 of the medulla. Human primary proximal tubule epithelial cells were incubated with low and high glucose concentration, 5 and 26 mmol/l, respectively. In high glucose concentration there was decreased expression of THTR-1 and THTR-2 (transporter mRNA: −76% and −53% respectively, p<0.001; transporter protein −77% and −83% respectively, p<0.05), concomitant with decreased expression of transcription factor specificity protein-1. High glucose concentration also produced a 37% decrease in apical to basolateral transport of thiamine transport across cell monolayers. Intensification of glycemic control corrected increased fractional excretion of thiamine in experimental diabetes. We conclude that glucose-induced decreased expression of thiamine transporters in the tubular epithelium may mediate renal mishandling of thiamine in diabetes. This is a novel mechanism of thiamine insufficiency linked to diabetic nephropathy

Low vitamin B12 and lipid metabolism: evidence from pre-clinical and clinical studies

Obesity is a worldwide epidemic responsible for 5% of global mortality. The risks of developing other key metabolic disorders like diabetes, hypertension and cardiovascular diseases (CVDs) are increased by obesity, causing a great public health concern. A series of epidemiological studies and animal models have demonstrated a relationship between the importance of vitamin B12 (B12) and various components of metabolic syndrome. High prevalence of low B12 levels has been shown in European (27%) and South Indian (32%) patients with type 2 diabetes (T2D). A longitudinal prospective study in pregnant women has shown that low B12 status could independently predict the development of T2D five years after delivery. Likewise, children born to mothers with low B12 levels may have excess fat accumulation which in turn can result in higher insulin resistance and risk of T2D and/or CVD in adulthood. However, the independent role of B12 on lipid metabolism, a key risk factor for cardiometabolic disorders, has not been explored to a larger extent. In this review, we provide evidence from pre-clinical and clinical studies on the role of low B12 status on lipid metabolism and insights on the possible epigenetic mechanisms including DNA methylation, micro-RNA and histone modifications. Although, there are only a few association studies of B12 on epigenetic mechanisms, novel approaches to understand the functional changes caused by these epigenetic markers are warranted

An update on vitamin B12-related gene polymorphisms and B12 status.

Vitamin B12 is an essential micronutrient in humans needed for health maintenance. Deficiency of vitamin B12 has been linked to dietary, environmental and genetic factors. Evidence for the genetic basis of vitamin B12 status is poorly understood. However, advancements in genomic techniques have increased the knowledge-base of the genetics of vitamin B12 status. Based on the candidate gene and genome-wide association (GWA) studies, associations between genetic loci in several genes involved in vitamin B12 metabolism have been identified. The objective of this literature review was to identify and discuss reports of associations between single-nucleotide polymorphisms (SNPs) in vitamin B12 pathway genes and their influence on the circulating levels of vitamin B12. Relevant articles were obtained through a literature search on PubMed through to May 2017. An article was included if it examined an association of a SNP with serum or plasma vitamin B12 concentration. Beta coefficients and odds ratios were used to describe the strength of an association, and a  < 0.05 was considered as statistically significant. Two reviewers independently evaluated the eligibility for the inclusion criteria and extracted the data. From 23 studies which fulfilled the selection criteria, 16 studies identified SNPs that showed statistically significant associations with vitamin B12 concentrations. Fifty-nine vitamin B12-related gene polymorphisms associated with vitamin B12 status were identified in total, from the following populations: African American, Brazilian, Canadian, Chinese, Danish, English, European ancestry, Icelandic, Indian, Italian, Latino, Northern Irish, Portuguese and residents of the USA. Overall, the data analyzed suggests that ethnic-specific associations are involved in the genetic determination of vitamin B12 concentrations. However, despite recent success in genetic studies, the majority of identified genes that could explain variation in vitamin B12 concentrations were from Caucasian populations. Further research utilizing larger sample sizes of non-Caucasian populations is necessary in order to better understand these ethnic-specific associations

Vitamin B12 induces hepatic fatty infiltration through altered fatty acid metabolism

Background/Aims: Rise in global incidence of obesity impacts metabolic health. Evidence from human and animal models show association of vitamin B12 (B12) deficiency with elevated BMI and lipids. Human adipocytes demonstrated dysregulation of lipogenesis by low B12 via hypomethylation and altered microRNAs. It is known de novo hepatic lipogenesis plays a key role towards dyslipidaemia, however, whether low B12 affects hepatic metabolism of lipids is not explored. Methods: HepG2 was cultured in B12-deficient EMEM medium and seeded in different B12 media: 500nM(control), 1000pM(1nM), 100pM and 25pM(low) B12. Lipid droplets were examined by Oil Red O (ORO) staining using microscopy and then quantified by elution assay. Gene expression were assessed with real-time quantitative polymerase chain reaction (qRT-PCR) and intracellular triglycerides were quantified using commercial kit (Abcam, UK) and radiochemical assay. Fatty acid composition was measured by gas chromatography and mitochondrial function by seahorse XF24 flux assay. Results: HepG2 cells in low B12 had more lipid droplets that were intensely stained with ORO compared with control. The total intracellular triglyceride and incorporation of radio-labelled-fatty acid in triglyceride synthesis were increased. Expression of genes regulating fatty acid, triglyceride and cholesterol biosynthesis were upregulated. Absolute concentrations of total fatty acids, saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), trans-fatty acids and individual even-chain and odd-chain fatty acids were significantly increased. Also, low B12 impaired fatty acid oxidation and mitochondrial functional integrity in HepG2 compared with control. Conclusion: Our data provide novel evidence that low B12 increases fatty acid synthesis and levels of individual fatty acids, and decreases fatty acid oxidation and mitochondrial respiration, thus resulting in dysregulation of lipid metabolism in HepG2. This highlights the potential significance of de novo lipogenesis and warrants possible epigenetic mechanisms of low B12