Hyperhomocysteinemia is associated with visceral adiposity in Japanese patients with type 2 diabetes mellitus

Nuclear factor erythroid 2-related factor 2 activation mediates hyperhomocysteinemia-associated lipolysis suppression in adipocytes

Hyperhomocysteinemia (HHcy) is associated with suppressed lipolytic response in adipocytes/adipose tissue, however, the underlying mechanism remains to be extensively studied. Nuclear factor erythroid 2-related factor 2 (Nrf2), a master transcriptional factor regulating antioxidant generation, has been recently reported to mediate lipid metabolism. Employing both fully differentiated 3T3-L1 adipocytes and male C57BL/6 mice, in the present study, we investigated the potential involvement of Nrf2 activation in HHcy-mediated lipolytic suppression. Our results showed that homocysteine (Hcy) treatment resulted in suppressed lipolysis, evidenced by increased intracellular triglyceride (TG) accumulation, decreased glycerol and free fatty acid (FFA) in fully differentiated 3T3-L1 adipocytes. Interestingly, Hcy exposure was associated with Nrf2 activation in adipocytes. Further studies showed that Nrf2 knockdown via siRNA transfection ameliorated Hcy-induced glycerol release in adipocytes. On the contrary, Nrf2 activators, epigallocatechin gallate (EGCG) and tert-butylhydroquinone (t-BHQ), increased intracellular TG content and decreased glycerol release in adipocytes. Importantly, our in vitro observations were corroborated by our in vivo findings, in which Hcy feeding (0.1% wt/vol) for four weeks induced Nrf2 expression in adipose tissue and lowered circulating FFA and glycerol levels in mice. Furthermore, EGCG injection (5 mg/kg/d) decreased circulating glycerol levels in comparison to the control group in mice. In conclusion, these results indicated that Nrf2 activation in response to HHcy plays an important role in mediating Hcy-suppressed lipolysis in adipocytes.

Association of epicardial adipose tissue with cardiometabolic risk and metabolic syndrome in patients with rheumatoid arthritis

OBJECTIVE

Patients with rheumatoid arthritis (RA) have increased coronary atherosclerosis possibly related to increased prevalence of visceral adiposity, insulin resistance, and metabolic syndrome. Epicardial adipose tissue (EAT), a type of visceral fat, may contribute to cardiometabolic risk. The aim of this study was to measure EAT volume in patients with RA and determine its relationship with cardiometabolic risk markers and coronary artery calcium.

METHODS

EAT volume and coronary artery calcium score were measured by noncontrast cardiac computed tomography and compared in RA patients (n = 162) and controls (n = 89). The relationships between EAT volume and markers of cardiometabolic risk in RA were examined with adjustment for age, race, and sex.

RESULTS

Among RA patients, EAT volume was positively associated with interleukin-6 (P = 0.03), triglycerides (P = 0.004), hypertension (P = 0.01), homeostatic model of insulin resistance (HOMA) (P < 0.001), smoking history (P = 0.04), and homocysteine level (P = 0.001), and negatively associated with high-density lipoprotein (P = 0.005). With further adjustment for waist circumference (a measure of visceral obesity), EAT volume remained independently associated with triglycerides, HOMA, current smoking, and homocysteine level (all P < 0.05). EAT volume was not associated with corticosteroid use or coronary artery calcium score. Patients with metabolic syndrome had significantly greater EAT volume (P < 0.001) and each increase in metabolic syndrome criteria was associated, on average, with a 20% increase (95% confidence interval 14-26%) in EAT volume (P < 0.001).

CONCLUSION

EAT volume is associated with metabolic syndrome and cardiometabolic risk factors, including insulin resistance, triglycerides, current smoking, and homocysteine levels, but not with coronary artery calcium in RA patients.

Methylenetetrahydrofolate reductase gene polymorphism and risk of type 2 diabetes mellitus

OBJECTIVE

This review aimed to comprehensively assess the literature examining a possible link between the rs1801133 polymorphism (677C → T) in the gene encoding the methylenetetrahydrofolate reductase (MTHFR) gene and risk of type 2 diabetes mellitus (DM).

RESEARCH DESIGN AND METHODS

Several research databases were systematically searched for studies examining the genotype at the rs1801133 polymorphism in healthy control individuals and individuals with type 2 DM. Genotype frequency data were examined across all studies and across subsets of studies according to ethnicity and presence of serious DM-related complications. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated.

RESULTS

A total of 4855 individuals with type 2 DM and 5242 healthy controls from 15 countries comprising Asian, Caucasian and African ethnicities were found to satisfy the inclusion criteria and included in the review. Genotype at the rs1801133 polymorphism was not consistently associated with either increased or reduced risk of type 2 DM; the OR across all studies was 0.91 (95%CI 0.82 to 1.00) for the C- vs. T-allele, 0.88 (0.75 to 1.03) for CC vs. CT+TT, 0.82 (0.71 to 0.95) for CC vs. TT, and 1.15 (1.03 to 1.29) for TT vs. CC+CT. Similar results were found when the meta-analysis was repeated separately for each ethnic subgroup, and for subgroups with or without serious DM-related complications.

CONCLUSIONS

There does not appear to be compelling evidence of an association between the genotype at the rs1801133 polymorphism of the MTHFR gene and risk of type 2 DM.

Adipose tissue as a source of nicotinamide N-methyltransferase and homocysteine

Nicotinamide N-methyltransferase (NNMT) catalyses the conversion of nicotinamide to 1-methylnicotinamide and plays an important role in hepatic detoxification reactions. Here we show that, in addition to the liver, 3T3-L1 adipocytes as well as human and murine adipose tissue explants express high amounts of enzymatically active NNMT. NNMT mRNA levels and enzyme activity increased in 3T3-L1 cells in a differentiation-dependent manner. Homocysteine, the atherogenic product of the NNMT-catalyzed reaction, was secreted from 3T3-L1 cells or adipose tissue cultures. Homocysteine release increased during 3T3-L1 differentiation and was reduced when adipose tissue was treated with the NNMT inhibitor 1-methylnicotinamide. Nicotinic acid (NA), a widely used drug to lower elevated plasma lipid levels, induced NNMT enzyme activity in white adipose tissue of mice. In tissue culture nicotinamide treatment led to an increase in adipose tissue homocysteine secretion. These data support the concept that adipose tissue NNMT contributes to the increased plasma homocysteine levels in patients treated with NA.

Homocysteine upregulates resistin production from adipocytes in vivo and in vitro

OBJECTIVE

Homocysteine (Hcy) is epidemiologically related to insulin resistance, which has been speculated to be a low-grade systemic inflammatory condition. Resistin acts as a critical mediator of insulin resistance associated with inflammatory conditions. We aimed to determine whether Hcy can induce insulin resistance by directly regulating the expression and secretion of resistin from adipose tissue.

RESEARCH DESIGN AND METHODS

The effect of Hcy on the expression and secretion of resistin and insulin resistance was investigated using primary rat adipocytes and mice with hyperhomocysteinemia (HHcy).

RESULTS

Hcy impaired glucose transport and, particularly, the insulin signaling pathway as shown by decreased insulin-stimulated tyrosine phosphorylation of insulin receptor and insulin receptor substrate (IRS)-1, increased serine phosphorylation of IRS-1, and inhibited Akt phosphorylation both in vitro and in vivo, and these impairments were accompanied by an increase in resistin expression. Compared with normal mice, HHcy mice with a clinically relevant level of plasma Hcy (19 micromol/l) showed significantly increased resistin production from adipose tissue (33.38 +/- 3.08 vs. 19.27 +/- 1.71 ng/ml, P < 0.01). Hcy (300-1000 micromol/l) also increased mRNA expression of resistin in primary rat adipocytes in a time- and concentration-dependent manner, with maximal induction at 24 h of approximately fourfold with 1,000 micromol/l. In addition, Hcy-induced resistin expression attenuated by treatment with reactive oxygen species (ROS) scavengers, protein kinase C (PKC), and nuclear factor (NF)-kappaB inhibitors implies a role in the process for ROS, PKC, and NF-kappaB.

CONCLUSIONS

HHcy may promote insulin resistance through the induction of resistin expression and secretion from adipocytes via the activation of the ROS-PKC-NF-kappaB pathway.

Factors associated with serum total homocysteine level in type 2 diabetes

OBJECTIVES

The aim of this study was to investigate the determinants of serum total homocysteine level (tHcy) in patients with type 2 diabetes mellitus (DM) according to sex.

METHODS

A total of 1,276 Japanese, diabetics (n = 280) with a control group of non-diabetics (n = 996), were enrolled into the study from 2003 to 2005. This cross-sectional study was conducted for all the subjects, using personal data regarding clinical characteristics and lifestyle. Multiple regression analysis was performed to analyze the association of tHcy with selected factors.

RESULTS

In diabetic subjects, estimated glomerular filtration rate (eGFR) and serum creatinine levels (Cre), even those within the normal range, were strongly associated with tHcy after adjustment in both sexes; the standardized partial regression coefficient of eGFR for tHcy was -0.251, (p = 0.001) in diabetic men and -0.523, (p < 0.001) in diabetic women. Furthermore, the eGFR of the diabetics, except patients with nephropathy, also had significant association with tHcy in both sexes. Fasting plasma glucose levels and serum triglyceride levels were strongly associated with tHcy in diabetic men only. HbA1c was also associated with tHcy in diabetic men only, though not as significantly. Age and presence of hypertension were significantly associated with tHcy in women.

CONCLUSIONS

This study suggests that there are some differences in the factors associated with tHcy between diabetics and non-diabetics, and between the sexes. There is, therefore, circumstantial evidence that elevated tHcy should be evaluated clinically. Because tHcy was strongly associated with eGFR and Cre, even within the normal ranges, tHcy may have important implications regarding the microangiopathy of the kidney and atherosclerosis.