Insulin in methionine and homocysteine kinetics in healthy humans: plasma vs. intracellular models

Prevotella copri alleviates hyperglycemia and regulates gut microbiota and metabolic profiles in mice

Prevotella copri is the dominant species of the Prevotella genus in the gut, which is genomically heterogeneous and difficult to isolate; hence, scarce research was carried out for this species. This study aimed to investigate the effect of P. copri on hyperglycemia. Thirty-nine strains were isolated from healthy individuals, and three strains (HF2123, HF1478, and HF2130) that had the highest glucose consumption were selected to evaluate the effects of P. copri supplementation on hyperglycemia. Microbiomics and non-target metabolomics were used to uncover the underlying mechanisms. Oral administration of P. copri in diabetic db/db mice increased the expression and secretion of glucagon-like peptide-1 (GLP-1), significantly improved hyperglycemia, insulin resistance, and lipid accumulation, and alleviated the pathological morphology in the pancreas, liver, and colon. P. copri changed the composition of the gut microbiota of diabetic db/db mice, which was characterized by increasing the ratio of Bacteroidetes to Firmicutes and increasing the relative abundance of genera Bacteroides, Akkermansia, and Faecalibacterium. After intervention with P. copri, fecal metabolic profiling showed that fumaric acid and homocysteine contents decreased, and glutamine contents increased. Furthermore, amino acid metabolism and cAMP/PKA signaling pathways were enriched. Our findings indicate that P. copri improved glucose metabolism abnormalities in diabetic db/db mice. Especially, one of the P. copri strains, HF2130, has shown superior performance in improving hyperglycemia, which may have the potential as a probiotic against hyperglycemia.

IMPORTANCE

As a core member of the human intestinal ecosystem, Prevotelal copri has been associated with glucose metabolic homeostasis in previous studies. However, these results have often been derived from metagenomic studies, and the experimental studies have been based solely on the type of strain DSM 18205T. Therefore, more experimental evidence from additional isolates is needed to validate the results according to their high genomic heterogeneity. In this study, we isolated different branches of strains and demonstrated that P. copri could improve the metabolic profile of hyperglycemic mice by modulating microbial activity. This finding supports the causal contribution of P. copri in host glucose metabolism.

Acute Antiplatelet Effects of an Oleocanthal-Rich Olive Oil in Type II Diabetic Patients: A Postprandial Study

Postprandial dysmetabolism is a common entity of type 2 diabetes mellitus (T2DM) and may act as a daily stressor of the already dysfunctional diabetic platelets. This study aims to investigate whether oleocanthal-rich olive oils (OO), incorporated into a carbohydrate-rich meal, can affect postprandial dysmetabolism and platelet aggregation. Oleocanthal is a cyclooxygenase inhibitor with putative antiplatelet properties. In this randomized, single-blinded, crossover study, ten T2DM patients consumed five isocaloric meals containing 120 g white bread combined with: (i) 39 g butter, (ii) 39 g butter and 400 mg ibuprofen, (iii) 40 mL OO (phenolic content < 10 mg/Kg), (iv) 40 mL OO with 250 mg/Kg oleocanthal and (v) 40 mL OO with 500 mg/Kg oleocanthal. Metabolic markers along with ex vivo ADP- and thrombin receptor-activating peptide (TRAP)-induced platelet aggregation were measured before and for 4 h after the meals. The glycemic and lipidemic response was similar between meals. However, a sustained (90-240 min) dose-dependent reduction in platelets' sensitivity to both ADP (50-100%) and TRAP (20-50%) was observed after the oleocanthal meals in comparison to OO or butter meals. The antiplatelet effect of the OO containing 500 mg/Kg oleocanthal was comparable to that of the ibuprofen meal. In conclusion, the consumption of meals containing oleocanthal-rich OO can reduce platelet activity during the postprandial period, irrespective of postprandial hyperglycemia and lipidemia.

Stepwise Discovery of Insulin Effects on Amino Acid and Protein Metabolism

A clear effect of insulin deficiency and replacement on body/muscle mass was a landmark observation at the start of the insulin age. Since then, an enormous body of investigations has been produced on the pathophysiology of diabetes mellitus from a hormonal/metabolic point of view. Among them, the study of the effects of insulin on body growth and protein accretion occupies a central place and shows a stepwise, continuous, logical, and creative development. Using a metaphor, insulin may be viewed as a director orchestrating the music (i.e., the metabolic effects) played by the amino acids and proteins. As a hormone, insulin obviously does not provide either energy or substrates by itself. Rather, it tells cells how to produce and utilize them. Although the amino acids can be released and taken up by cells independently of insulin, the latter can powerfully modulate these movements. Insulin regulates (inhibits) protein degradation and, in some instances, stimulates protein synthesis. This review aims to provide a synthetic and historical view of the key steps taken from the discovery of insulin as an "anabolic hormone", to the in-depth analysis of its effects on amino acid metabolism and protein accretions, as well as of its interaction with nutrients.

Metabolic Changes Induced by Cerebral Ischemia, the Effect of Ischemic Preconditioning, and Hyperhomocysteinemia

1H Nuclear Magnetic Resonance (NMR) metabolomics is one of the fundamental tools in the fast-developing metabolomics field. It identifies and quantifies the most abundant metabolites, alterations of which can describe energy metabolism, activated immune response, protein synthesis and catabolism, neurotransmission, and many other factors. This paper summarizes our results of the 1H NMR metabolomics approach to characterize the distribution of relevant metabolites and their alterations induced by cerebral ischemic injury or its combination with hyperhomocysteinemia in the affected tissue and blood plasma in rodents. A decrease in the neurotransmitter pool in the brain tissue likely follows the disordered feasibility of post-ischemic neurotransmission. This decline is balanced by the increased tissue glutamine level with the detected impact on neuronal health. The ischemic injury was also manifested in the metabolomic alterations in blood plasma with the decreased levels of glycolytic intermediates, as well as a post-ischemically induced ketosis-like state with increased plasma ketone bodies. As the 3-hydroxybutyrate can act as a likely neuroprotectant, its post-ischemic increase can suggest its supporting role in balancing ischemic metabolic dysregulation. Furthermore, the 1H NMR approach revealed post-ischemically increased 3-hydroxybutyrate in the remote organs, such as the liver and heart, as well as decreased myocardial glutamate. Ischemic preconditioning, as a proposed protective strategy, was manifested in a lower extent of metabolomic changes and/or their faster recovery in a longitudinal study. The paper also summarizes the pre- and post-ischemic metabolomic changes in the rat hyperhomocysteinemic models. Animals are challenged with hyperglycemia and ketosis-like state. A decrease in several amino acids in plasma follows the onset and progression of hippocampal neuropathology when combined with ischemic injury. The 1H NMR metabolomics approach also offers a high potential for metabolites in discriminatory analysis in the search for potential biomarkers of ischemic injury. Based on our results and the literature data, this paper presents valuable findings applicable in clinical studies and suggests the precaution of a high protein diet, especially foods which are high in Met content and low in B vitamins, in the possible risk of human cerebrovascular neuropathology.

Taurine Supplementation as a Neuroprotective Strategy upon Brain Dysfunction in Metabolic Syndrome and Diabetes

Obesity, type 2 diabetes, and their associated comorbidities impact brain metabolism and function and constitute risk factors for cognitive impairment. Alterations to taurine homeostasis can impact a number of biological processes, such as osmolarity control, calcium homeostasis, and inhibitory neurotransmission, and have been reported in both metabolic and neurodegenerative disorders. Models of neurodegenerative disorders show reduced brain taurine concentrations. On the other hand, models of insulin-dependent diabetes, insulin resistance, and diet-induced obesity display taurine accumulation in the hippocampus. Given the possible cytoprotective actions of taurine, such cerebral accumulation of taurine might constitute a compensatory mechanism that attempts to prevent neurodegeneration. The present article provides an overview of brain taurine homeostasis and reviews the mechanisms by which taurine can afford neuroprotection in individuals with obesity and diabetes. We conclude that further research is needed for understanding taurine homeostasis in metabolic disorders with an impact on brain function.

Exhaustive Exercise and Post-exercise Protein Plus Carbohydrate Supplementation Affect Plasma and Urine Concentrations of Sulfur Amino Acids, the Ratio of Methionine to Homocysteine and Glutathione in Elite Male Cyclists

Plasma and tissue sulfur amino acid (SAA) availability are crucial for intracellular methylation reactions and cellular antioxidant defense, which are important processes during exercise and in recovery. In this randomized, controlled crossover trial among eight elite male cyclists, we explored the effect of exhaustive exercise and post-exercise supplementation with carbohydrates and protein (CHO+PROT) vs. carbohydrates (CHO) on plasma and urine SAAs, a potential new marker of methylation capacity (methionine/total homocysteine ratio [Met/tHcy]) and related metabolites. The purpose of the study was to further explore the role of SAAs in exercise and recovery. Athletes cycled to exhaustion and consumed supplements immediately after and in 30 min intervals for 120 min post-exercise. After ~18 h recovery, performance was tested in a time trial in which the CHO+PROT group cycled 8.5% faster compared to the CHO group (41:53 ± 1:51 vs. 45:26 ± 1:32 min, p < 0.05). Plasma methionine decreased by ~23% during exhaustive exercise. Two h post-exercise, further decline in methionine had occured by ~55% in the CHO group vs. ~33% in the CHO+PROT group (p_group × _time < 0.001). The Met/tHcy ratio decreased by ~33% during exhaustive exercise, and by ~54% in the CHO group vs. ~27% in the CHO+PROT group (p_group × _time < 0.001) post-exercise. Plasma cystathionine increased by ~72% in the CHO group and ~282% in the CHO+PROT group post-exercise (p_group × _time < 0.001). Plasma total cysteine, taurine and total glutathione increased by 12% (p = 0.03), 85% (p < 0.001) and 17% (p = 0.02), respectively during exhaustive exercise. Using publicly available transcriptomic data, we report upregulated transcript levels of skeletal muscle SLC7A5 (log₂ fold-change: 0.45, FDR:1.8^e−07) and MAT2A (log₂ fold-change: 0.38, FDR: 3.4^e−0.7) after acute exercise. Our results show that exercise acutely lowers plasma methionine and the Met/tHcy ratio. This response was attenuated in the CHO+PROT compared to the CHO group in the early recovery phase potentially affecting methylation capacity and contributing to improved recovery.

Effects of dietary restriction on metabolic and cognitive health

Life expectancy in most developed countries has been rising over the past century. In the UK alone, there are about 12 million people over 65 years old and centenarians have increased by 85% in the past 15 years. As a result of the ageing population, which is due mainly to improvements in medical treatments, public health, improved housing and lifestyle choices, there is an associated increase in the prevalence of pathological conditions, such as metabolic disorders, type 2 diabetes, cardiovascular and neurodegenerative diseases, many types of cancer and others. Statistics suggest that nearly 54% of elderly people in the UK live with at least two chronic conditions, revealing the urgency for identifying interventions that can prevent and/or treat such disorders. Non-pharmacological, dietary interventions such as energetic restriction (ER) and methionine restriction (MR) have revealed promising outcomes in increasing longevity and preventing and/or reversing the development of ageing-associated disorders. In this review, we discuss the evidence and mechanisms that are involved in these processes. Fibroblast growth factor 1 and hydrogen sulphide are important molecules involved in the effects of ER and MR in the extension of life span. Their role is also associated with the prevention of metabolic and cognitive disorders, highlighting these interventions as promising modulators for improvement of health span.

Substitution of Dietary Sulfur Amino Acids by DL-2-hydroxy-4-Methylthiobutyric Acid Increases Remethylation and Decreases Transsulfuration in Weaned Piglets

BACKGROUND

DL-2-hydroxy-4-methylthiobutyric acid (DL-HMTBA), an L-methionine (L-Met) hydroxyl analogue, has been suggested to be a dietary L-Met source. How dietary DL-HMTBA compared with L-Met affects whole-body L-Met kinetics in growing individuals is unknown.

OBJECTIVES

We determined to what extent DL-HMTBA supplementation of an L-Met-deficient diet affects whole-body L-Met and L-cysteine (L-Cys) kinetics, protein synthesis (PS), and the L-Met incorporation rate in liver protein (L-MetInc) compared with L-Met and DL-Met supplementation in a piglet model.

METHODS

Forty-five, 28-d-old weaned piglets (male, German Landrace) were allocated to 4 dietary groups: L-Met-deficient diet [Control: 69% of recommended L-Met plus L-Cys supply; 0.22% standardized ileal digestible (SID) L-Met; 0.27% SID L-Cys; n = 12] and Control diet supplemented equimolarly to 100% of recommended intake with either L-Met (n = 12; LMET), DL-Met (n = 11; DLMET), or DL-HMTBA (n = 10; DLHMTBA). At 47 d of age, the piglets were infused with L-[1-13C; methyl-2H3]-Met and [3,3-2H2]-Cys to determine the kinetics and PS rates. Plasma amino acid (AA) concentrations, hepatic mRNA abundances of L-Met cycle and transsulfuration (TS) enzymes, and L-MetInc were measured.

RESULTS

During feed deprivation, L-Met kinetics did not differ between groups, and were ≤3 times higher in the fed state (P < 0.01). Remethylation (RM) was 31% and 45% higher in DLHMTBA than in DLMET and Control pigs, respectively, and the RM:transmethylation (TM) ratio was 50% higher in DLHMTBA than in LMET (P < 0.05). Furthermore, TS and the TS:TM ratio were 32% lower in DLHMTBA than in LMET (P < 0.05). L-MetInc was 42% lower in DLMET and DLHMTBA than in L-Met-deficient Control pigs, whereas plasma AA and hepatic mRNA abundances were similar among DL-HMTBA-, L-Met-, and DL-Met-supplemented pigs.

CONCLUSIONS

In piglets, DL-HMTBA compared with L-Met and DL-Met supplementation increases RM and reduces the TS rate to conserve L-Met, but all 3 Met isomers support growth at a comparable rate.

Decreased Homocysteine Trans-Sulfuration in Hypertension With Hyperhomocysteinemia: Relationship With Insulin Resistance

CONTEXT

Homocysteine is an independent cardiovascular risk factor and is elevated in essential hypertension. Insulin stimulates homocysteine catabolism in healthy individuals. However, the mechanisms of hyperhomocysteinemia and its relationship with insulin resistance in essential hypertension are unknown.

OBJECTIVE

To investigate whole body methionine and homocysteine kinetics and the effects of insulin in essential hypertension.

DESIGN AND SETTING

Eight hypertensive male subjects and six male normotensive controls were infused with l-[methyl-2H3,1-13C]methionine for 6 hours. In the last 3 hours a euglycemic, hyperinsulinemic clamp was performed. Steady-state methionine and homocysteine kinetics were determined in postabsorptive and hyperinsulinemic conditions.

RESULTS

Postabsorptive hypertensive subjects had elevated homocysteine concentrations (+30%, P = 0.035) and slightly (by 15% to 20%) but insignificantly lower methionine rates of appearance (Ras) (P = 0.07 to P = 0.05) and utilization for protein synthesis (P = 0.06) than postabsorptive normotensive controls. Hyperinsulinemia suppressed methionine Ra and protein synthesis, whereas it increased homocysteine trans-sulfuration, clearance, and methionine transmethylation (the latter only in the normotensive subjects). However, in the hypertensive subjects trans-sulfuration was significantly lower (P < 0.05) and increased ~50% less [by +1.59 ± 0.34 vs +3.45 ± 0.52 µmol/kg lean body mass (LBM) per hour, P < 0.005] than in normotensive controls. Homocysteine clearance through trans-sulfuration was ~50% lower in hypertensive than in normotensive subjects (P < 0.005). In the hypertensive subjects, insulin-mediated glucose disposal was ~45% lower (460 ± 44 vs 792 ± 67 mg/kg LBM per hour, P < 0.0005) than in normotensive controls and was positively correlated with the increase of trans-sulfuration (P < 0.0015).

CONCLUSIONS

In subjects with essential hypertension, hyperhomocysteinemia is associated with decreased homocysteine trans-sulfuration and probably represents a feature of insulin resistance.

Serum Homocysteine Levels in Men with and without Erectile Dysfunction: A Systematic Review and Meta-Analysis

OBJECTIVES

Elevated levels of serum homocysteine (Hcy) have been associated with cardiovascular diseases and endothelial dysfunction, conditions closely associated with erectile dysfunction (ED). This meta-analysis was aimed to assess serum Hcy levels in subjects with ED compared to controls in order to clarify the role of Hcy in the pathogenesis of ED.

METHODS

Medline, Embase, and the Cochrane Library were searched for publications investigating the possible association between ED and Hcy. Results were restricted by language, but no time restriction was applied. Standardized mean difference (SMD) was obtained by random effect models.

RESULTS

A total of 9 studies were included in the analysis with a total of 1320 subjects (489 subjects with ED; 831 subjects without ED). Pooled estimate was in favor of increased Hcy in subjects with ED with a SMD of 1.00, 95% CI 0.65-1.35, p < 0.0001. Subgroup analysis based on prevalence of diabetes showed significantly higher SMD in subjects without diabetes (1.34 (95% CI 1.08-1.60)) compared to subjects with diabetes (0.68 (95% CI 0.39-0.97), p < 0.0025 versus subgroup w/o diabetes).

CONCLUSIONS

Results from our meta-analysis suggest that increased levels of serum Hcy are more often observed in subjects with ED; however, increase in Hcy is less evident in diabetic compared to nondiabetic subjects. This study is registered with Prospero registration number CRD42018087558.

Differential control of muscle mass in type 1 and type 2 diabetes mellitus

Diabetes mellitus--whether driven by insulin deficiency or insulin resistance--causes major alterations in muscle metabolism. These alterations have an impact on nutrient handling, including the metabolism of glucose, lipids, and amino acids, and also on muscle mass and strength. However, the ways in which the distinct forms of diabetes affect muscle mass differ greatly. The most common forms of diabetes mellitus are type 1 and type 2. Thus, whereas type 1 diabetic subjects without insulin treatment display a dramatic loss of muscle, most type 2 diabetic subjects show no changes or even an increase in muscle mass. However, the most commonly used rodent models of type 2 diabetes are characterized by muscle atrophy and do not mimic the features of the disease in humans in terms of muscle mass. In this review, we analyze the processes that are differentially regulated under these forms of diabetes and propose regulatory mechanisms to explain them.

Hypomethylation of the promoter of the catalytic subunit of protein phosphatase 2A in response to hyperglycemia

In order to identify epigenetic mechanisms through which hyperglycemia can affect gene expression durably in β cells, we screened DNA methylation changes induced by high glucose concentrations (25 mmol/L) in the BTC3 murine cell line, using an epigenome‐wide approach. Exposure of BTC3 cells to high glucose modified the expression of 1612 transcripts while inducing significant methylation changes in 173 regions. Among these 173 glucose‐sensitive differentially methylated regions (DMRs), 14 were associated with changes in gene expression, suggesting an epigenetic effect of high glucose on gene transcription at these loci. Among these 14 DMRs, we selected for further study Pp2ac, a gene previously suspected to play a role in β‐cell physiology and type 2 diabetes. Using RT‐qPCR and bisulfite pyrosequencing, we confirmed our previous observations in BTC3 cells and found that this gene was significantly demethylated in the whole blood cells (WBCs) of type 2 diabetic patients compared to controls.

In order to identify epigenetic mechanisms through which hyperglycemia can affect gene expression durably in β cells, we screened DNA methylation changes induced by high glucose concentration in the BTC3 murine cell line. We identified one interesting gene, PP2AC, and confirmed it in type 2 diabetic patients.

Tissue-specific alterations of methyl group metabolism with DNA hypermethylation in the Zucker (type 2) diabetic fatty rat

BACKGROUND

Altered methyl group and homocysteine metabolism were tissue-specific, persistent, and preceded hepatic DNA hypomethylation in type 1 diabetic rats. Similar metabolic perturbations have been shown in the Zucker (type 2) diabetic fatty (ZDF) rat in the pre-diabetic and early diabetic stages, but tissue specificity and potential impact on epigenetic marks are unknown, particularly during pathogenesis.

METHODS

ZDF (fa/fa) and lean (+/?) control rats were killed at 12 and 21 weeks of age, representing early and advanced diabetic conditions. Blood and tissues were analysed with respect to methyl group and homocysteine metabolism, including DNA methylation.

RESULTS

At 12 weeks, hepatic glycine N-methyltransferase (GNMT), methionine synthase, and cystathionine β-synthase (CBS) activity and/or abundance were increased in ZDF rats. At 21 weeks, GNMT activity was increased in liver and kidney; however, only hepatic CBS protein abundance (12 weeks) and betaine-homocysteine S-methyltransferase mRNA expression (21 weeks) were significantly elevated (78 and 100%, respectively). Hepatic phosphatidylethanolamine N-methyltransferase expression was also elevated in the ZDF rat. Homocysteine concentrations were decreased in plasma and kidney, but not in liver, at 12 and 21 weeks. In contrast to hepatic DNA hypomethylation in the type 1 diabetic rat, genomic DNA was hypermethylated at 12 and 21 weeks in the liver of ZDF rats, concomitant with an increase in DNA methyltransferase 1 expression at 21 weeks.

CONCLUSIONS

The pathogenesis of type 2 diabetes in the ZDF rat was associated with tissue and disease stage-specific aberrations of methyl group and homocysteine metabolism, with persistent hepatic global DNA hypermethylation.

T2DM: Why Epigenetics?

Type 2 Diabetes Mellitus (T2DM) is a metabolic disorder influenced by interactions between genetic and environmental factors. Epigenetics conveys specific environmental influences into phenotypic traits through a variety of mechanisms that are often installed in early life, then persist in differentiated tissues with the power to modulate the expression of many genes, although undergoing time-dependent alterations. There is still no evidence that epigenetics contributes significantly to the causes or transmission of T2DM from one generation to another, thus, to the current environment-driven epidemics, but it has become so likely, as pointed out in this paper, that one can expect an efflorescence of epigenetic knowledge about T2DM in times to come.

Insulin resistance of amino acid and protein metabolism in type 2 diabetes

Although insulin resistance in T2DM (type 2 diabetes mellitus) is usually referred to glucose and lipid metabolism, the question whether such a resistance affects also amino acid and protein metabolism is both relevant and not easy to be answered. Available data indicate a reduced response to insulin in the inhibition of proteolysis at low, near basal hormone levels, whereas such a response appears to be normal at high physiological doses. In most studies in T2DM subjects the stimulation of whole-body protein synthesis in the presence of hyperinsulinemia and euaminoacidemia appears to be normal, although one single study reported lower rates in male T2DM subjects with obesity. The response to insulin of plasma protein synthesis (albumin and fibrinogen) is also normal. However, some metabolic steps of amino acids related to vascular complications (methionine and arginine) exhibit a defective response to insulin in T2DM subjects with nephropathy. In summary, although gross alterations in the response of whole-body protein turnover are not evident in T2DM, specific investigations reveal subtle abnormalities in metabolic steps of selected amino acids. Furthermore, the effects of interaction between diabetes (with the associated insulin resistance) and older age in the pathogenesis of sarcopenia in the elderly deserve more specific studies.

Intestinal metabolism of sulfur amino acids

The gastrointestinal tract (GIT) is a metabolically significant site of sulfur amino acid (SAA) metabolism in the body and metabolises about 20 % of the dietary methionine intake which is mainly transmethylated to homocysteine and trans-sulfurated to cysteine. The GIT accounts for about 25 % of the whole-body transmethylation and trans-sulfuration. In addition, in vivo studies in young pigs indicate that the GIT is a site of net homocysteine release and thus may contribute to the homocysteinaemia. The gut also utilises 25 % of the dietary cysteine intake and the cysteine uptake by the gut represents about 65 % of the splanchnic first-pass uptake. Moreover, we recently showed that SAA deficiency significantly suppresses intestinal mucosal growth and reduces intestinal epithelial cell proliferation, and increases intestinal oxidant stress in piglets. These recent findings indicate that intestinal metabolism of dietary methionine and cysteine is nutritionally important for intestinal mucosal growth. Besides their role in protein synthesis, methionine and cysteine are precursors of important molecules. S-adenosylmethionine, a metabolite of methionine, is the principal biological methyl donor in mammalian cells and a precursor for polyamine synthesis. Cysteine is the rate-limiting amino acid for glutathione synthesis, the major cellular antioxidant in mammals. Further studies are warranted to establish how SAA metabolism regulates gut growth and intestinal function, and contributes to the development of gastrointestinal diseases. The present review discusses the evidence of SAA metabolism in the GIT and its functional and nutritional importance in gut function and diseases.

Tissue methionine cycle activity and homocysteine metabolism in female rats: impact of dietary methionine and folate plus choline

Impaired transfer of methyl groups via the methionine cycle leads to plasma hyperhomocysteinemia. The tissue sources of plasma homocysteine in vivo have not been quantified nor whether hyperhomocysteinemia is due to increased entry or decreased removal. These issues were addressed in female rats offered diets with either adequate or excess methionine (additional methyl groups) with or without folate and choline (impaired methyl group transfer) for 5 wk. Whole body and tissue metabolism was measured based on isotopomer analysis following infusion with either [1-(13)C,methyl-(2)H3]methionine or [U-(13)C]methionine plus [1-(13)C]homocysteine. Although the fraction of intracellular methionine derived from methylation of homocysteine was highest in liver (0.18-0.21), most was retained. In contrast, the pancreas exported to plasma more of methionine synthesized de novo. The pancreas also exported homocysteine to plasma, and this matched the contribution from liver. Synthesis of methionine from homocysteine was reduced in most tissues with excess methionine supply and was also lowered in liver (P<0.01) with diets devoid of folate and choline. Plasma homocysteine concentration (P<0.001) and flux (P=0.001) increased with folate plus choline deficiency, although the latter still represented <12% of estimated tissue production. Hyperhomocysteinemia also increased (P<0.01) the inflow of homocysteine into most tissues, including heart. These findings indicate that a full understanding of hyperhomocysteinemia needs to include metabolism in a variety of organs, rather than an exclusive focus on the liver. Furthermore, the high influx of homocysteine into cardiac tissue may relate to the known association between homocysteinemia and hypertension.

Effects of insulin and glucose on cellular metabolic fluxes in homocysteine transsulfuration, remethylation, S-adenosylmethionine synthesis, and global deoxyribonucleic acid methylation

BACKGROUND

The mechanisms underlying the impact of pathophysiological elevations in insulin or glucose on hepatic cellular homocysteine kinetics is not fully understood.

OBJECTIVE

The objective of the study was to investigate the impact of elevated insulin/glucose on hepatic homocysteine kinetics at the cellular level.

DESIGN AND METHODS

Effects of insulin and glucose on homocysteine remethylation and transsulfuration metabolic fluxes were investigated in a cell model using stable isotopic tracers and gas chromatography/mass spectrometry. The methylation status was assessed by S-adenosylmethionine (adoMet), the adoMet to S-adenosylhomocysteine ratio, DNA methyltransferase activity, and methylated cytidine content of DNA. The expression profile of homocysteine remethylation, transmethylation, and transsulfuration-associated genes was determined.

RESULTS

Insulin increased cellular homocysteine production primarily by its inhibition of transsulfuration. When cells were exposed to elevated insulin and glucose, homocysteine remethylation was enhanced, which consequently increased intracellular adoMet concentrations by inducing adoMet synthase activity. Elevated glucose further enhanced DNA methyltransferase activity that subsequently led to increased global DNA methylation.

CONCLUSIONS

We demonstrated the novel finding of a direct promoting effect of high cellular insulin or glucose exposure on homocysteine remethylation, adoMet synthase activity, and adoMet synthesis. We also provided new evidence indicating that when hepatic tissue is exposed to elevated insulin or glucose, the cellular methylation balance can be altered, which may have potential epigenetic impacts gene regulation in diabetic individuals. These findings in a cell line may or may not reflect what happens in humans. In vivo studies on the homocysteine transmethylation fluxes and DNA methylation in diabetic state are underway.

Acute effect of insulin on nitric oxide synthesis in humans: a precursor-product isotopic study

Nitric oxide (NO) is a key regulatory molecule with wide vascular, cellular, and metabolic effects. Insulin affects NO synthesis in vitro. No data exist on the acute effect of insulin on NO kinetics in vivo. By employing a precursor-product tracer method in humans, we have directly estimated the acute effect of insulin on intravascular NO(x) (i.e., the NO oxidation products) fractional (FSR) and absolute (ASR) synthesis rates in vivo. Nine healthy male volunteers were infused iv with L-[(15)N(2)-guanidino]arginine ([(15)N(2)]arginine) for 6 h. Timed measurements of (15)NO(x) and [(15)N(2)]arginine enrichments in whole blood were performed in the first 3 h in the fasting state and then following a 3-h euglycemic-hyperinsulinemic clamp (with plasma insulin raised to approximately 1,000 pmol/l). In the last 60 min of each experimental period, at approximately steady-state arginine enrichment, a linear increase of (15)NO(x) enrichment (mean r = 0.9) was detected in both experimental periods. In the fasting state, NO(x) FSR was 27.4 +/- 4.3%/day, whereas ASR was 0.97 +/- 0.36 mmol/day, accounting for 0.69 +/- 0.27% of arginine flux. Following hyperinsulinemia, both FSR and ASR of NO(x) increased (FSR by approximately 50%, to 42.4 +/- 6.7%/day, P < 0.005; ASR by approximately 25%, to 1.22 +/- 0.41 mmol/day, P = 0.002), despite a approximately 20-30% decrease of arginine flux and concentration. The fraction of arginine flux used for NO(x) synthesis was doubled, to 1.13 +/- 0.35% (P < 0.003). In conclusion, whole body NO(x) synthesis can be directly measured over a short observation time with stable isotope methods in humans. Insulin acutely stimulates NO(x) synthesis from arginine.

Methionine transmethylation and transsulfuration in the piglet gastrointestinal tract

Methionine is an indispensable sulfur amino acid that functions as a key precursor for the synthesis of homocysteine and cysteine. Studies in adult humans suggest that splanchnic tissues convert dietary methionine to homocysteine and cysteine by means of transmethylation and transsulfuration, respectively. Studies in piglets show that significant metabolism of dietary indispensable amino acids occurs in the gastrointestinal tissues (GIT), yet the metabolic fate of methionine in GIT is unknown. We show here that 20% of the dietary methionine intake is metabolized by the GIT in piglets implanted with portal and arterial catheters and fed milk formula. Based on analyses from intraduodenal and intravenous infusions of [1-(13)C]methionine and [(2)H(3)]methionine, we found that the whole-body methionine transmethylation and remethylation rates were significantly higher during duodenal than intravenous tracer infusion. First-pass splanchnic metabolism accounted for 18% and 43% of the whole-body transmethylation and remethylation, respectively. Significant transmethylation and transsulfuration was demonstrated in the GIT, representing approximately 27% and approximately 23% of whole-body fluxes, respectively. The methionine used by the GIT was metabolized into homocysteine (31%), CO(2) (40%), or tissue protein (29%). Cystathionine beta-synthase mRNA and activity was present in multiple GITs, including intestinal epithelial cells, but was significantly lower than liver. We conclude that the GIT consumes 20% of the dietary methionine and is a significant site of net homocysteine production. Moreover, the GITs represent a significant site of whole-body transmethylation and transsulfuration, and these two pathways account for a majority of methionine used by the GITs.

Effects of insulin on methionine and homocysteine kinetics in type 2 diabetes with nephropathy

Although hyperhomocysteinemia, an independent cardiovascular risk factor, is common in type 2 diabetes with nephropathy, the mechanism(s) of this alteration is not known. In healthy humans, hyperinsulinemia increases methionine transmethylation, homocysteine transsulfuration, and clearance. No such data exist in type 2 diabetes either in the fasting state or in response to hyperinsulinemia. To this purpose, seven male type 2 diabetic patients with albuminuria (1.2 +/- 0.4 g/day, three with mild to moderate renal insufficiency) and seven matched control subjects were infused for 6 h with L-[methyl-(2)H(3), 1-(13)C]methionine. Methionine flux, transmethylation, and disposal into proteins as well as homocysteine remethylation, transsulfuration, and clearance were determined before and after euglycemic hyperinsulinemia (approximately 1,000 pmol/l). In type 2 diabetic subjects, homocysteine concentration was twofold greater (P < 0.01) and methionine transmethylation and homocysteine clearance lower (from approximately 15 to >50% and from approximately 40 to >100%, respectively; P < 0.05) than in control subjects. The insulin-induced increments of methionine transmethylation, homocysteine transsulfuration, and clearance were markedly reduced in type 2 diabetic subjects (by more than threefold, P < 0.05 or less vs. control subjects). In contrast, methionine methyl and carbon flux were not increased in the patients. In conclusion, pathways of homocysteine disposal are impaired in type 2 diabetes with nephropathy, both in postabsorptive and insulin-stimulated states, possibly accounting for the hyperhomocysteinemia of this condition.

Homocysteine, vitamin B 12 and folate in Alzheimer's and vascular dementias: The paradoxical effect of the superimposed type II diabetes mellitus condition

BACKGROUND

Increased concentration of plasmatic homocysteine (tHcy) and decreased vitamin B 12 (B12) and folate (FOL) are associated with Alzheimer's (AD) and vascular (VaD) dementias, with type II diabetes mellitus (DM), and reported as risk factors of these diseases.

METHODS

The sample (n=122; males=60; mean age=73+/-7 years) comprised AD and VaD patients without DM, with a concomitant DM (AD+DM, VaD+DM), DM alone and controls (CTR), resulting in 6 groups. tHcy, B12 and FOL were determined in duplicate.

RESULTS

The one-way ANOVA yielded significant differences between groups for all variables: tHcy p<10(-12); B12 p<10(-3); FOL p<10(-4). Significance for comparisons between groups was set at alpha=0.05, using the Bonferroni's statistic. The comparisons: DM vs. CTR, AD+DM vs. AD, VaD+DM vs. VaD, and DM demented vs. DM non-demented resulted significant for all variables, except for B12 in 2 comparisons.

CONCLUSIONS

In demented and control subjects, tHcy and FOL exhibit extreme differences, not so marked between DM and controls. Demented patients with concomitant diabetes are closer to controls than their non-diabetic counterparts. Diabetes affects tHcy and FOL values, which are changed with opposite sign to non-demented. These results suggests a paradoxical phenomenon when diabetes is superimposed to dementias.