Enhancement of homocysteine toxicity to insulin-secreting BRIN-BD11 cells in combination with alloxan

MTHFR gene polymorphisms in diabetes mellitus

The methylenetetrahydrofolate reductase (MTHFR), methionine synthase (MTR), and methionine synthase reductase (MTRR) are three regulatory enzymes in the folic acid (FA) cycle play a critical role in the balance of methionine and homocysteine. MTHFR and MTRR gene polymorphisms affect the biochemical activities of enzymes, impairing the remethylation of homocysteine to methionine. In 1972, severe MTHFR deficiency resulting in homocystinuria was first reported, suggesting MTHFR involvement in the disease. MTHFR C677T polymorphism can independently increase the risk of high homocysteine (HHcy) in plasma. Elevation of homocysteine levels could increase the risk of microvascular damage, thrombosis, heart disease, etc. Vascular complications were regarded as a leading major cause of diabetes mortality, and disability increases individual health and economic burden. Diabetes mellitus (DM) is a chronic inflammatory disease, and conventional medications do not provide a complete cure for diabetes. It was essential to identify other risk factors for the intervention and prevention of diabetes. MTHFR gene polymorphism is an emerging risk factor in diabetes. Recent studies have shown that polymorphisms of the MTHFR gene play a significant role in the pathophysiology of diabetes, including inflammation and insulin resistance. This review summarizes the association between MTHER gene polymorphism and diabetes.

Circulating homocysteine and folate concentrations and risk of type 2 diabetes: A retrospective observational study in Chinese adults and a Mendelian randomization analysis

Background

The relation between circulating homocysteine (hcy) and folate concentrations and risk of type 2 diabetes mellitus (T2DM) has been evaluated in several observational studies with inconsistent results; and it is unclear about their causal relationships. Our aim was to assess the causality association between circulating hcy or folate concentrations and the development of T2DM using Mendelian randomization (MR) analysis, based on results of an observational study in Chinese adults.

Methods

We conducted an observational study of 370 patients with T2DM and 402 controls after routine physical examination who consulted at the Zhongnan Hospital of Wuhan University between March 2021 and December 2021. Correlations between hcy and folate and the incidence of T2DM were quantified using logistic regression models. Two-sample MR analysis was conducted using summary statistics of genetic variants gained from 2 genome-wide association studies (GWAS) on circulating hcy and folate concentrations in individuals of European ancestry and from an independent GWAS study based on DIAMANTE meta-analysis.

Results

In the observational study, after logistic regression with multiple adjustment, lower hcy and higher folate levels were identified to be associated with the risk of T2DM, with OR (95% CI) for hcy of 1.032 (1.003–1.060); while 0.909 (0.840–0.983) for folate. In the MR analysis, the OR for T2DM was 1.08 (95% CI: 0.95, 1.21; P = 0.249) for each SD unit increase in genetically predicted homocysteinemia and the OR for T2DM per SD increase in genetically predicted folate elevation was 0.80 (95% CI: 0.60, 1.00, P = 0.026).

Conclusions

We discovered that high circulating hcy and low folate concentrations were related with an increased risk of developing T2DM in Chinese adults. Moreover, MR analysis provided genetic evidence for a possible causal relationship between serum folate and the risk of T2DM.

LDL Promotes Disorders in β-Cell Cholesterol Metabolism, Implications on Insulin Cellular Communication Mediated by EVs

Dyslipidemia is described as a hallmark of metabolic syndrome, promoting a stage of metabolic inflammation (metainflammation) that could lead to misbalances in energetic metabolism, contributing to insulin resistance, and modifying intracellular cholesterol pathways and the renin–angiotensin system (RAS) in pancreatic islets. Low-density lipoprotein (LDL) hypercholesterolemia could disrupt the tissue communication between Langerhans β-cells and hepatocytes, wherein extracellular vesicles (EVs) are secreted by β-cells, and exposition to LDL can impair these phenomena. β-cells activate compensatory mechanisms to maintain insulin and metabolic homeostasis; therefore, the work aimed to characterize the impact of LDL on β-cell cholesterol metabolism and the implication on insulin secretion, connected with the regulation of cellular communication mediated by EVs on hepatocytes. Our results suggest that β-cells can endocytose LDL, promoting an increase in de novo cholesterol synthesis targets. Notably, LDL treatment increased mRNA levels and insulin secretion; this hyperinsulinism condition was associated with the transcription factor PDX-1. However, a compensatory response that maintains basal levels of intracellular calcium was described, mediated by the overexpression of calcium targets PMCA1/4, SERCA2, and NCX1, together with the upregulation of the unfolded protein response (UPR) through the activation of IRE1 and PERK arms to maintain protein homeostasis. The LDL treatment induced metainflammation by IL-6, NF-κB, and COX-2 overexpression. Furthermore, LDL endocytosis triggered an imbalance of the RAS components. LDL treatment increased the intracellular levels of cholesterol on lipid droplets; the adaptive β-cell response was portrayed by the overexpression of cholesterol transporters ABCA1 and ABCG1. Therefore, lipotoxicity and hyperinsulinism induced by LDL were regulated by the natural compound auraptene, a geranyloxyn coumarin modulator of cholesterol-esterification by ACAT1 enzyme inhibition. EVs isolated from β-cells impaired insulin signaling via mTOR/p70S6Kα in hepatocytes, a phenomenon regulated by auraptene. Our results show that LDL overload plays a novel role in hyperinsulinism, mechanisms associated with a dysregulation of intracellular cholesterol, lipotoxicity, and the adaptive UPR, which may be regulated by coumarin-auraptene; these conditions explain the affectations that occur during the initial stages of insulin resistance.

Homocysteine Metabolism Pathway Is Involved in the Control of Glucose Homeostasis: A Cystathionine Beta Synthase Deficiency Study in Mouse

Cystathionine beta synthase (CBS) catalyzes the first step of the transsulfuration pathway from homocysteine to cystathionine, and its deficiency leads to hyperhomocysteinemia (HHcy) in humans and rodents. To date, scarce information is available about the HHcy effect on insulin secretion, and the link between CBS activity and the setting of type 2 diabetes is still unknown. We aimed to decipher the consequences of an inborn defect in CBS on glucose homeostasis in mice. We used a mouse model heterozygous for CBS (CBS+/−) that presented a mild HHcy. Other groups were supplemented with methionine in drinking water to increase the mild to intermediate HHcy, and were submitted to a high-fat diet (HFD). We measured the food intake, body weight gain, body composition, glucose homeostasis, plasma homocysteine level, and CBS activity. We evidenced a defect in the stimulated insulin secretion in CBS+/− mice with mild and intermediate HHcy, while mice with intermediate HHcy under HFD presented an improvement in insulin sensitivity that compensated for the decreased insulin secretion and permitted them to maintain a glucose tolerance similar to the CBS+/+ mice. Islets isolated from CBS+/− mice maintained their ability to respond to the elevated glucose levels, and we showed that a lower parasympathetic tone could, at least in part, be responsible for the insulin secretion defect. Our results emphasize the important role of Hcy metabolic enzymes in insulin secretion and overall glucose homeostasis.

Redox Regulation of Hexokinases

SIGNIFICANCE

Hexokinases are key enzymes that are responsible for the first reaction of glycolysis, but they also moonlight other cellular processes, including mitochondrial redox signaling regulation. Modulation of hexokinase activity and spatiotemporal location by reactive oxygen and nitrogen species as well as other gasotransmitters serves as the basis for a unique, underexplored method of tight and flexible regulation of these fundamental enzymes. Recent Advances: Redox modifications of thiols serve as a molecular code that enables the precise and complex regulation of hexokinases. Redox regulation of hexokinases is also used by multiple parasites to cause widespread and severe diseases, including malaria, Chagas disease, and sleeping sickness. Redox-active molecules affect each other, and the moonlighting activity of hexokinases provides another feedback loop that affects the cellular redox status and is hijacked in malignantly transformed cells.

CRITICAL ISSUES

Several compounds affect the redox status of hexokinases in vivo. These include the dehydroascorbic acid (oxidized form of vitamin C), pyrrolidinium porrolidine-1-carbodithioate (contraceptive), peroxynitrite (product of ethanol metabolism), alloxan (a glucose analog), and isobenzothiazolinone ebselen. However, very limited information is available regarding which amino acid residues in hexokinases are affected by redox signaling. Except in cases of monogenic diabetes, direct evidence is absent for disease phenotypes that are associated with variations within motifs that are susceptible to redox signaling.

FUTURE DIRECTIONS

Further studies should address the propensity of hexokinases and their disease-associated variants to participate in redox regulation. Robust and straightforward proteomic methods are needed to understand the context and consequences of hexokinase-mediated redox regulation in health and disease.

Type II diabetes mellitus and hyperhomocysteinemia: a complex interaction

BACKGROUND

Elevated homocysteine (Hc) levels have a well-established and clear causal relationship to epithelial damage leading to coronary artery disease. Furthermore, it is strongly associated with other metabolic syndrome variables, such as hypertension, which is correlated with type II diabetes mellitus (T2DM). Studies on T2DM in relation to Hc levels have shown both positive and negative associations. The aim of the present study is to examine the relationship between Hc levels and risk of T2DM in the Lebanese population.

METHODS

We sought to identify whether Hc associates positively or negatively with diabetes in a case-control study, where 2755 subjects enrolled from patients who had been catheterized for coronary artery diagnosis and treatment. We further sought to identify whether the gene variant MTHFR 667C>T is associated with T2DM, and how Hc and MTHFR 667C>T also impact other correlates of T2DM, including the widely used diuretics in this study population.

RESULTS

We found that Hc levels were significantly reduced among subjects with diabetes compared to those without diabetes when adjusted for all potential confounders (OR 0.640; 95% CI [0.44-0.92]; p = 0.0200). The associations between Hc levels and other variates contradicted the result: hypertension associates positively with high Hc levels, and with T2DM. The MTHFR 667C>T only associated significantly with high Hc levels.

CONCLUSION

These results suggest population-specific variations among a range of mechanisms that modulate the association of Hc and T2DM, providing a probe for future studies.

Relationship between MTHFR C677T and A1298C gene polymorphisms and complications of type 2 diabetes mellitus in an Emirati population

Purpose

Type 2 diabetes mellitus (T2DM) is the most common form of diabetes with clinical consequences giving rise to chronic multiple organ complications. Methylenetetrahydrofolate reductase (MTHFR) polymorphisms are genetic variations that have been linked to T2DM, and micro/macrovascular complications. The link between MTHFR and T2DM however is strongly dependent on the ethnic group studied. The objective of this study was to investigate the possible association between two MTHFR polymorphisms (C677T and A1298C) and T2DM and specifically examine if there are any associations with clinical and demographic characteristics among patients in the United Arab Emirates (UAE).

Methods

The study included 169 T2DM patients and 209 healthy controls. Genomic DNA was isolated and genotyped using TaqMan real-Time PCR assays for the MTHFR C677T and A1298C polymorphisms.

Results

There were no significant differences in genotype and haplotype distributions observed between groups. A significant association was observed between the C677T polymorphism and history of cerebrovascular accident (CVA) (p = 0.0330), history of nephropathy (p = 0.0280) and levels of LDL cholesterol (p = 0.0409). Also, the A1298C polymorphism was associated with hypertriglyceridemia (p = 0.0305) in T2DM patients.

Conclusion

These findings demonstrate that the MTHFR gene polymorphisms are not related to T2DM in the Emirati population. However, these polymorphisms can be used as risk markers for CVA, nephropathy, high LDL cholesterol and triglycerides in T2DM patients and allow timely treatment.

Associations of MTHFR C677T and MTRR A66G gene polymorphisms with metabolic syndrome: a case-control study in Northern China

Prior evidence indicates that homocysteine plays a role in the development of metabolic syndrome (MetS). Methylenetetrahydrofolate reductase (MTHFR) C677T and methionine synthase reductase (MTRR) A66G polymorphisms are common genetic determinants of homocysteine levels. To investigate the associations of the MTHFR C677T and MTRR A66G polymorphisms with MetS, 692 Chinese Han subjects with MetS and 878 controls were recruited. The component traits of MetS and the MTHFR C677T and MTRR A66G genotypes were determined. A significant association was observed between the MTHFR 677T allele and increased risk of MetS, high fasting blood glucose, high waist circumference, and increasing number of MetS components. The MTRR A66G polymorphism was associated with an increased risk of MetS when combined with the MTHFR 677TT genotype, although there was no association found between MetS and MTRR A66G alone. Furthermore, the MTRR 66GG genotype was associated with high fasting blood glucose and triglycerides. Our data suggest that the MTHFR 677T allele may contribute to an increased risk of MetS in the northern Chinese Han population. The MTRR A66G polymorphism is not associated with MetS. However, it may exacerbate the effect of the MTHFR C677T variant alone. Further large prospective population-based studies are required to confirm our findings.

Methylenetetrahydrofolate reductase C677T polymorphism and type 2 diabetes mellitus in Chinese population: a meta-analysis of 29 case-control studies

BACKGROUND

Methylenetetrahydrofolate reductase (MTHFR), a key enzyme in folate metabolism, had significant effects on the homocysteine levels. The common functional MTHFR C677T polymorphism had been extensively researched. Several studies had evaluated the relationship between MTHFR C677T polymorphism and type 2 diabetes mellitus (T2DM), but the results were still controversial in the Chinese Han population. This meta-analysis was conducted to evaluate the relationship between MTHFR C677T polymorphism and T2DM in the Chinese Han population.

METHODS

We searched the relevant studies in multiple electronic databases, which published up to December 2013. We reviewed and extracted data from all the included studies on the relationship between MTHFR C677T polymorphism and T2DM in the Chinese Han population. The odds ratios (ORs) and their 95% confidence intervals (95%CIs) were used to evaluate the relationship. Fixed-effects and random-effects meta-analysis were used to pool ORs by the heterogeneity. Publication bias and sensitivity analysis were also examined.

RESULTS

29 studies were finally included in our meta-analysis, which contained 4656 individuals with T2DM and 2127 healthy controls. There was a significant relationship between MTHFR C677T polymorphism and T2DM under dominant (OR: 1.70, 95% CI: 1.42-2.02), recessive (OR: 1.48, 95% CI: 1.21-1.80), homozygous (OR: 1.89, 95% CI: 1.47-2.42), heterozygous (OR: 1.58, 95% CI: 1.33-1.87), and additive (OR: 1.46, 95% CI: 1.28-1.68) genetic model in a random-effects model. Subgroup analysis also reached similar results. Sensitivity analysis indicated that the overall result were dependable.

CONCLUSIONS

There was a significant relationship between MTHFR C677T polymorphism and T2DM in the Chinese Han population. The results of our meta-analysis suggested that MTHFR 677T allele might be a risk genetic factor of T2DM in the Chinese Han population.

Association of homocysteine with type 2 diabetes: a meta-analysis implementing Mendelian randomization approach

BACKGROUND

We tested the hypothesis that elevated homocysteine (Hcy) level is causally associated with increased risk of type 2 diabetes mellitus (T2DM).

RESULTS

The meta-analysis and Mendelian randomization analysis were performed among 4011 cases and 4303 controls. The absolute pooled mean Hcy concentration in subjects with MTHFR 677TT was 5.55 μmol/L (95% CI, 1.33 to 9.77) greater than that in subjects with MTHFR 677CC in T2DM. Overall, the T allele of the MTHFR 677 C > T conferred a greater risk for T2DM [Random effect (RE) OR = 1.31(1.17-1.64), I² = 41.0%, p = 0.055]. The random effect (RE) pooled OR associated with T2DM for MTHFR 677TT relative to the 677CC was [RE OR = 1.38(1.18-1.62)]. The fixed-effect pooled OR of the association for the MTHFR 677 TT vs CT was 1.29 (95% CI, 1.09-1.51). MTHFR 677 TT showed a significantly higher risk for T2DM compared with MTHFR 677 CC + CT [Fixed effect (FE) OR = 1.32(1.14-1.54), I² = 0.0%, p = 0.686]. The absolute pooled mean Hcy concentration in individuals with T2DM was 0.94 μmol/L (95% CI, 0.40-1.48) greater than that in control subjects. The estimated causal OR associated with T2DM was 1.29 for 5 μmol/L increment in Hcy.

CONCLUSIONS

Our findings provided strong evidence on the causal association of Hcy level with the development of T2DM.

Folate deficiency triggers an oxidative-nitrosative stress-mediated apoptotic cell death and impedes insulin biosynthesis in RINm5F pancreatic islet β-cells: relevant to the pathogenesis of diabetes

It has been postulated that folic acid (folate) deficiency (FD) may be a risk factor for the pathogenesis of a variety of oxidative stress-triggered chronic degenerative diseases including diabetes, however, the direct evidence to lend support to this hypothesis is scanty. For this reason, we set out to study if FD can trigger the apoptotic events in an insulin-producing pancreatic RINm5F islet β cells. When these cells were cultivated under FD condition, a time-dependent growth impediment was observed and the demise of these cells was demonstrated to be apoptotic in nature proceeding through a mitochondria-dependent pathway. In addition to evoke oxidative stress, FD condition could also trigger nitrosative stress through a NF-κB-dependent iNOS-mediated overproduction of nitric oxide (NO). The latter compound could then trigger depletion of endoplasmic reticulum (ER) calcium (Ca²⁺) store leading to cytosolic Ca²⁺ overload and caused ER stress as evidence by the activation of CHOP expression. Furthermore, FD-induced apoptosis of RINm5F cells was found to be correlated with a time-dependent depletion of intracellular gluthathione (GSH) and a severe down-regulation of Bcl-2 expression. Along the same vein, we also demonstrated that FD could severely impede RINm5F cells to synthesize insulin and their abilities to secret insulin in response to glucose stimulation were appreciably hampered. Even more importantly, we found that folate replenishment could not restore the ability of RINm5F cells to resynthesize insulin. Taken together, our data provide strong evidence to support the hypothesis that FD is a legitimate risk factor for the pathogenesis of diabetes.