Effects of streptozotocin-induced diabetes and of insulin treatment on homocysteine metabolism in the rat

Significance of hydrogen sulfide production in the pancreatic β-cell

Hydrogen sulfide (H(2)S) is an important signaling molecule in various mammalian cells and tissues. H(2)S is synthesized from L-cysteine and regulates several cellular and physiological phenomena (vasorelaxation, hormone secretion, and apoptosis) and multicellular events (neuromodulation and inflammatory responses). H(2)S can be produced in pancreatic β-cells by cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE). H(2)S inhibits insulin release and regulates β-cell survival. We found that glucose stimulation increased CSE expression at transcript and protein levels in mouse pancreatic islets. We also found that H(2)S protects β-cells that were chronically exposed to high glucose from apoptotic cell death. Loss of β-cell mass and failures of β-cell function are important in the pathogenesis and/or progression of diabetes mellitus; therefore, molecular analyses of the mechanisms of H(2)S production and its protective effects on β-cells may lead to new insights into diabetes mellitus.

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.

Methylcobalamin effects on diabetic neuropathy and nerve protein kinase C in rats

BACKGROUND

Methyl-base-attached cobalamin (Methycobalamin) (MC) has a special affinity for nerve tissues to promote myelination and transport of axonal cytoskeleton. It is not known, however, how MC influences on peripheral nerve in experimental diabetic neuropathy.

MATERIALS AND METHODS

We studied the effects of MC on expressions and activities of protein kinase C (PKC) in peripheral nerve of streptozotocin-induced diabetic rats. Wistar rats, 8 weeks of age, were rendered diabetic by streptozotocin (40 mg kg(-1), iv) and followed for 16 weeks. A half of diabetic animals were treated with MC (10 mg kg(-1) per every other day, im) after the induction of diabetes. Normal Wistar rats were served as control.

RESULTS

At the end, untreated diabetic animals developed significant delay of nerve conduction velocity (NCV), and MC treatment normalized the NCV. Nerve PKC activity was significantly suppressed in untreated diabetic rats, while the activity was normalized in treated animals. While PKCα located in Schwann cells, PKCβΙα and βII distributed in axoplasm, vascular walls and macrophages. The decreased PKC activity in diabetic nerve was associated with reduced expression of membrane PKCα and increased membrane expression of PKCβII, and MC treatment corrected these changes. Diabetic nerve contained an increased number of macrophages and 8-hydroxydeoxyguanosine-positive cells in the endoneurium, the latter of which was significantly suppressed by MC treatment. Elevated nerve polyol levels in diabetic nerve were partially corrected by MC treatment.

CONCLUSIONS

This study suggested that correction of impaired neural signalling of PKC and oxidative stress-induced damage may be a major attribute to the beneficial effects of MC on diabetic nerve.

Epigenetics: deciphering how environmental factors may modify autoimmune type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease that has increased two- to threefold over the past half century by as yet unknown means. It is generally accepted that T1D is the result of gene-environment interactions, but such rapid increases in incidence are not explained by Mendelian inheritance. There have been numerous advances in our knowledge of the pathogenesis of T1D. Indeed, there has been a large number of genes identified that contribute to risk for this disease and several environmental factors have been proposed. The complexity of such interactions is yet to be understood for any major chronic disease. Epigenetic regulation is one way to explain the rapid increase in incidence and could be a central mechanism by which environmental factors influence development of diabetes. However, there is remarkably little known about the contribution of epigenetics to T1D pathogenesis. Here we speculate on various candidate processes and molecules of the immune and endocrine systems that could modify risk for T1D through epigenetic regulation.

Plasma Homocysteine and Insulin in Diabetic Nephropathy: Relationship to Body Mass Index

The data on plasma homocysteine and endogenous insulin in type 2 diabetes mellitus with nephropathy and relationship to body mass index (BMI) is particularly from the Indian subcontinent. A prospective study was carried out in 50 patients of type 2 diabetes mellitus with overt nephropathy (Group A). The results were compared with 25 diabetics without nephropathy (Group B), and 25 age and sex matched healthy controls (Group C). Microenzyme immunoassay and ELISA estimated the plasma homocysteine and insulin, respectively. The mean values of plasma homocysteine were significantly elevated in diabetic nephropathy (21.3+/-7.2 micromol/L) and diabetics without nephropathy (19.4+/-7.1) when compared to healthy control (11.5+/-2.3). The insulin levels and BMI were significantly higher in diabetics as compared to controls. There was no correlation between homocysteine and insulin, homocysteine and BMI, and homocysteine with the degree of renal failure.

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.

Regulation of homocysteine homeostasis through the transcriptional coactivator PGC-1alpha

Plasma homocysteine (Hcy) is an independent risk factor for cardiovascular disease. Hcy is a nonprotein amino acid derivative that is generated from the methionine cycle, which provides the methyl group for essentially all biological methylation reactions. Although plasma Hcy levels are elevated in patients with cardiovascular disease, the mechanisms that regulate Hcy homeostasis remain poorly defined. In this study, we found that the expression of key enzymes involved in Hcy metabolism is induced in the liver in response to fasting. This induction coincides with increased expression of peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha, a transcriptional coactivator that regulates hepatic gluconeogenesis and mitochondrial function. PGC-1alpha stimulates the expression of genes involved in Hcy metabolism in cultured primary hepatocytes as well as in the liver. Adenoviral-mediated expression of PGC-1alpha in vivo leads to elevated plasma Hcy levels. In contrast, mice deficient in PGC-1alpha have lower plasma Hcy concentrations. These results define a novel role for the PGC-1alpha coactivator pathway in the regulation of Hcy homeostasis and suggest a potential pathogenic mechanism that contributes to hyperhomocysteinemia.

Type I diabetes leads to tissue-specific DNA hypomethylation in male rats

Numerous perturbations of methyl group and homocysteine metabolism have been documented as an outcome of diabetes. It has also been observed that there is a transition from hypo- to hyperhomocysteinemia in diabetes, often concurrent with the development of nephropathy. The objective of this study was to characterize the temporal changes in methyl group and homocysteine metabolism in the liver and kidney and to determine the impact these alterations have on DNA methylation in type 1 diabetic rats. Male Sprague-Dawley rats were injected with streptozotocin (60 mg/kg body weight) to induce diabetes and samples were collected at 2, 4, and 8 wk. At 8 wk, hepatic and renal betaine-homocysteine S-methyltransferase activities were greater in diabetic rats, whereas methionine synthase activity was lower in diabetic rat liver and kidney did not differ. Cystathionine beta-synthase abundance was greater in the liver but less in the kidney of diabetic rats. Both hepatic and renal glycine N-methyltransferase (GNMT) activity and abundance were greater in diabetic rats; however, changes in renal activity and/or abundance were present only at 2 and 4 wk, whereas hepatic GNMT was induced at all time points. Most importantly, we have shown that genomic DNA was hypomethylated in the liver, but not the kidney, in diabetic rats. These results suggest that diabetes-induced perturbations of methyl group and homocysteine metabolism lead to functional methyl deficiency, resulting in the hypomethylation of DNA in a tissue-specific fashion.

Protein and amino acid metabolism in the human newborn

Birth and adaptation to extrauterine life involve major shifts in the protein and energy metabolism of the human newborn. These include a shift from a state of continuous supply of nutrients including amino acids from the mother to cyclic periodic oral intake, a change in the redox state of organs, thermogenesis, and a significant change in the mobilization and use of oxidative substrates. The development of safe, stable isotopic tracer methods has allowed the study of protein and amino acid metabolism not only in the healthy newborn but also in those born prematurely and of low birth weight. These studies have identified the unique and quantitative aspects of amino acid/protein metabolism in the neonate, thus contributing to rational nutritional care of these babies. The present review summarizes the contemporary data on some of the significant developments in essential and dispensable amino acids and their relationship to overall protein metabolism. Specifically, the recent data of kinetics of leucine, phenylalanine, glutamine, sulfur amino acid, and threonine and their relation to whole-body protein turnover are presented. Finally, the physiological rationale and the impact of nutrient (amino acids) interventions on the dynamics of protein metabolism are discussed.

Metabolism of methionine in the newborn infant: response to the parenteral and enteral administration of nutrients

The rates of transmethylation and transsulfuration of methionine were quantified using [1-(13)C]methionine and [C2H3]methionine tracers in newborn infants born at term gestation and in prematurely born low birth weight infants. Whole body rate of protein breakdown was also measured using [2H5]phenylalanine. The response to enteral formula feeding and parenteral nutrition was examined in full term and prematurely born babies, respectively. The relative rates of appearance of methionine and phenylalanine were comparable to the amino acid composition of mixed body proteins. Rates of transmethylation were high, both in full term infants (fast 32 +/- 14 micromol kg(-1) x h(-1); fed 21.7 +/- 3.2) and in preterm infants (57.2 +/- 14.8). Significant flux through the transsulfuration pathway was evident (full term: fast 6.0 +/- 4.4, fed 4.1 +/- 2.1; preterm: 24.9 +/- 9.9 micromol kg(-1) x h(-1)). Transsulfuration of methionine is evident in the human newborn in the immediate neonatal period, suggesting that cysteine may not be considered a "conditionally" essential amino acid for the neonate. The high rate of transmethylation may reflect the high methylation demand, whereas high rates of transsulfuration in premature babies may be related to high demands for glutathione and to the amounts of methionine in parenteral amino acid mixtures.

Comparative effects of atorvastatin and simvastatin on the plasma total homocysteine levels in women with polycystic ovary syndrome: a prospective randomized study

OBJECTIVE

To test the hypothesis that statins improve hyperhomocysteinemia in women with polycystic ovary syndrome (PCOS).

DESIGN

A prospective randomized study.

SETTING

University Hospital.

PATIENT(S)

Fifty-two women with PCOS and 52 women matched for age and body mass index as controls.

INTERVENTION(S)

Patients were randomly divided into two groups for treatment: group 1, atorvastatin, 20 mg daily (n = 26), and group 2, simvastatin, 20 mg daily (n = 26). Blood samples were obtained before and after treatment.

MAIN OUTCOME MEASURE(S)

Serum homocysteine levels.

RESULT(S)

After 12 weeks of treatment, serum homocysteine levels in group 1 had decreased from 14.3 +/- 2.9 to 10.6 +/- 1.7 micromol/L; in group 2, the levels decreased from 13.6 +/- 2.1 to 11.1 +/- 1.9 micromol/L. Both two groups, free testosterone and total testosterone declined statistically significantly (38.3% and 36.5%; and 40.6% and 46.0%, respectively). In group 1, vitamin B(12) increased from 362.1 +/- 107 to 478.7 +/- 267 pg/mL; in group 2, it increased from 391.3 +/- 107 to 466 +/- 211 pg/mL, but the change did not reach statistical significance. There was a considerable decline in the homeostatic model assessment index in group 1 (40.0% to 32.1%).

CONCLUSION(S)

Treatment with statins in women with PCOS leads to decreases in serum homocysteine levels.

Homocysteine metabolism in diabetes

An increase in the plasma level of Hcy (homocysteine), an intermediate in the catabolism of methionine, has been identified as a risk factor for many diseases including CVD (cardiovascular disease). CVD is the major cause of death in patients with diabetes mellitus. Therefore the study of Hcy metabolism in diabetes mellitus has been a major focus of current research. Studies conducted in our laboratory were able to show that in both Type 1 and Type 2 diabetes with no renal complications, the plasma Hcy levels were lower than in controls. In Type 1 diabetes, increased activities of the trans-sulfuration enzymes were the major cause for the reduction in plasma Hcy. In Type 2 diabetes, BHMT (betaine:homocysteine methyltransferase) was also observed to play a major role in the increased catabolism of Hcy in addition to the trans-sulfuration enzymes. We were also able to demonstrate the direct effect of insulin and the counter-regulatory hormones on the regulation of cystathionine beta-synthase and BHMT, which accounts for the changes in the activities of these two enzymes seen in diabetes mellitus.

Circulating homocysteine levels in patients with type 2 diabetes mellitus

BACKGROUND AND AIM

Previous studies have shown conflicting results regarding circulating homocysteine levels in patients with type 2 diabetes.

METHODS AND RESULTS

This observational study included 2121 patients with angiographically proven coronary artery disease (507 patients with type 2 diabetes and 1614 patients without diabetes). Circulating homocysteine levels, methylenetetrahydrofolate reductase (MTHFR) C677T gene polymorphism, renal function, presence of coronary artery disease (CAD) diagnosed by coronary angiography, and circulating folate and vitamin B12 status were assessed. Plasma homocysteine levels [median (25th; 75th percentile)] were significantly higher in patients with diabetes than in those without [12.4 micromol/L (9.9 micromol/L; 15.9 micromol/L) versus 11.7 micromol/L (9.6 micromol/L; 14.5 micromol/L), P=0.011]. Diabetes affected homocysteine levels only in patients with a glomerular filtration rate <90 mL/min [13.0 micromol/L (10.5 micromol/L; 16.7 micromol/L) in patients with diabetes versus 12.2 micromol/L (10.1 micromol/L; 15.2 micromol/L) in patients without diabetes, P=0.006] but not in those with a glomerular filtration rate > or = 90 mL/min [10.1 micromol/L (8.1 micromol/L; 12.4 micromol/L) versus 10.2 micromol/L (8.8 micromol/L; 12.3 micromol/L), P=0.267]. Multivariable analysis did not show an independent association between diabetes and homocysteine level (P=0.342).

CONCLUSION

Circulating homocysteine levels are increased in patients with type 2 diabetes compared with non-diabetic patients due to a more diabetes-associated adverse risk profile rather than to diabetes itself.

Relationship between metabolic syndrome categorized by newly recommended by International Diabetes Federation criteria with plasma homocysteine concentration

Plasma total homocysteine (tHcy) is an independent risk factor for cardiovascular disease and increased tHcy levels have been reported to be a novel risk factor of atherosclerotic disease. The aim of this study was to assess the association of the metabolic syndrome components with plasma (tHcy) level. Total 722 participants (284 men, 438 women) from the medical checkup program were enrolled in this study. The clinical characteristics and biochemical parameters of the subjects were assessed and the tHcy levels were compared according to the components of metabolic syndrome diagnosed by Adult Treatment Panel (ATP) III guideline and International Diabetes Federation (IDF) criteria. Among the components, groups with larger waist circumference and higher fasting blood glucose levels showed significantly higher tHcy level than the counterparts. Although statistically insignificant, mean concentrations of tHcy was higher in subjects with metabolic syndrome defined by both criteria. In multiple regression analysis, age, sex and systolic blood pressure were the independent determinants of tHcy level. In conclusion, tHcy level was not associated with metabolic syndrome defined by either criteria in Korean subjects.

Effect of pre-germinated brown rice intake on diabetic neuropathy in streptozotocin-induced diabetic rats

BACKGROUND

To study the effects of a pre-germinated brown rice diet (PR) on diabetic neuropathy in streptozotocin (STZ)-induced diabetic rats.

METHODS

The effects of a PR diet on diabetic neuropathy in STZ-induced diabetic rats were evaluated and compared with those fed brown rice (BR) or white rice (WR) diets with respect to the following parameters: blood-glucose level, motor-nerve conduction velocity (NCV), sciatic-nerve Na+/K+-ATPase activity, and serum homocysteine-thiolactonase (HTase) activity.

RESULTS

Compared with diabetic rats fed BR or WR diets, those fed a PR diet demonstrated significantly lower blood-glucose levels (p < 0.001), improved NCV (1.2- and 1.3-fold higher, respectively), and increased Na+/K+-ATPase activity (1.6- and 1.7-fold higher, respectively). The PR diet was also able to normalize decreased serum homocysteine levels normally seen in diabetic rats. The increased Na+/K+-ATPase activity observed in rats fed PR diets was associated with elevations in HTase activity (r = 0.913, p < 0.001). The in vitro effect of the total lipid extract from PR bran (TLp) on the Na+/K+-ATPase and HTase activity was also examined. Incubation of homocysteine thiolactone (HT) with low-density lipoprotein (LDL) in vitro resulted in generation of HT-modified LDL, which possessed high potency to inhibit Na+/K+-ATPase activity in the sciatic nerve membrane. The inhibitory effect of HT-modified LDL on Na+/K+-ATPase activity disappeared when TLp was added to the incubation mixture. Furthermore, TLp directly activated the HTase associated with high-density lipoprotein (HDL).

CONCLUSION

PR treatment shows efficacy for protecting diabetic deterioration and for improving physiological parameters of diabetic neuropathy in rats, as compared with a BR or WR diet. This effect may be induced by a mechanism whereby PR intake mitigates diabetic neuropathy by one or more factors in the total lipid fraction. The active lipid fraction is able to protect the Na+/K+-ATPase of the sciatic-nerve membrane from the toxicity of HT-modified LDL and to directly activate the HTase of HDL.

Beer ethanol consumption and plasma homocysteine among patients with type 2 diabetes

We analyzed the association between beer and other type of ethanol consumption and tHcy levels among type 2 diabetic patients. Male type 2 diabetic patients without overt nephropathy were studied (n=242). Ethanol consumptions of the patients were 35.1+/-37.8mL/day for total ethanol, 13.9+/-15.2mL/day for beer ethanol and 21.2+/-32.1mL/day for non-beer ethanol. Both, total and non-beer ethanol consumption correlated with tHcy, whereas beer ethanol consumption showed a trend to inverse association with tHcy (standard regression coefficient, 0.184, 0.283 and -0.110, respectively). Each intake of 30mL/day ethanol consumption was associated with an increase of tHcy of 0.6micromol/L for total ethanol and 1.1micromol/L for non-beer ethanol and a decrease of tHcy of 0.7micromol/L for beer ethanol. Similar trend was observed in the analysis model which included only drinkers, and also in an adjusted analysis model. Plasma tHcy of beer only drinkers was lower than that of non-beer alcohol only drinkers (8.9+/-1.9micromol/L versus 11.5+/-5.5micromol/L, P=0.003). Non-beer ethanol consumption might be less healthy compared with beer ethanol consumption among type 2 diabetic patients in terms of the effects on tHcy.

Correlations between homocysteine levels and atherosclerosis in Japanese type 2 diabetic patients

Elevated total plasma homocysteine (tHcy) level and aortic stiffness are associated with high mortality in type 2 diabetic patients. We tested the hypothesis that tHcy correlates with aortic stiffness and insulin resistance in type 2 diabetic patients. The study consisted of 40 Japanese patients with type 2 diabetes mellitus and high tHcy levels (mean age +/- SD, 57 +/- 7 years) and a control group of 45 age-matched patients with normal tHcy levels (mean age +/- SD, 57 +/- 6 years). Brachial-ankle pulse wave velocity (BaPWV) was measured by an automatic oscillometric method. Brachial-ankle pulse wave velocity was used as an index of atherosclerosis. Body mass index values (P < .05), waist circumferences (P < .05), and the waist-to-hip ratios (P < .05) were larger in the high-tHcy group than in the normal-tHcy group. The BaPWV was higher in the high-tHcy group than in the normal-tHcy group (P < .0001). Fasting plasma glucose (P < .005) and insulin concentrations (P < .0001), and the homeostasis model assessment (HOMA) index (P < .0001) were higher in the high-tHcy group than in the normal-tHcy group. Multiple regression analysis showed that tHcy levels were independently predicted by BaPWV and the HOMA index. In conclusion, our results indicate that the elevated level of tHcy in Japanese patients with type 2 diabetes mellitus is characterized by increased aortic stiffness and insulin resistance, and that the BaPWV and the HOMA index are independent predictors of tHcy.

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

This study aims to investigate the relationship between the circulating level of homocysteine and body adiposity in Japanese patients with type 2 diabetes mellitus. We measured the body mass index (BMI), waist and hip circumferences, visceral and subcutaneous adiposities, visceral/subcutaneous (V/S) adiposity ratio, and insulin resistance as assessed by the Homeostasis Model Assessment (HOMA) index in patients with hyperhomocysteinemia. The study group consisted of 17 Japanese patients with type 2 diabetes and hyperhomocysteinemia (age: 62+/-10 years, mean+/-S.D.), and the control group consisted of 24 age-matched type 2 diabetes patients with normohomocysteinemia (60+/-11 years). The visceral adiposity, HOMA index, and V/S ratio were significantly higher in the hyperhomocysteinemia group than in the normohomocysteinemia group (P<0.05). In contrast, the BMI, hip circumference, and subcutaneous adiposity were similar between the two groups (P>0.1). Furthermore, multiple regression analysis showed that hyperhomocysteinemia was closely related to insulin resistance and visceral adiposity. Our results indicate that the presence of hyperhomocysteinemia in our population of Japanese patients with type 2 diabetes-associated insulin resistance was associated with increased visceral but not subcutaneous adiposity.

Hyperhomocysteinemia induces insulin resistance in male Sprague-Dawley rats

Association between elevated plasma homocysteine levels and insulin resistance has been reported, however, whether hyperhomocysteinemia induces insulin resistance or it is actually hyperinsulinemia that causes elevated plasma homocysteine levels, the direction of causality in this association is not still clear. In this study, we examined the hypothesis that hyperhomocysteinemia may cause hyperinsulinemia leading to insulin resistance in rats. Plasma glucose, insulin and total homocysteine concentrations were determined in two groups of male Sprague-Dawley rats, a test group that administered with homocysteine and a control group with no homocysteine in daily drinking water before and after 50 days. Oral glucose tolerance tests were also performed in control and test groups before and after 50 days. Mean fasting plasma insulin level was significantly higher (42.5+/-20.4 mU/L versus 23.2+/-5.9 mU/L, p=0.01), whereas mean glucose: insulin ratio was significantly lower in test rats than in control rats (0.12+/-0.07 versus 0.17+/-0.05, p=0.04) after 50 days. In addition, mean homeostasis assessment insulin resistance index was significantly higher in test rats than in control rats (7.5+/-3.5 versus 4.0+/-1.6, p=0.02) after 50 days. The mean plasma glucose level was not significantly different (4.1+/-1.1 mmol/L versus 3.9+/-0.8 mmol/L, p=0.57) between controls and test rats, however, the results from oral glucose tolerance tests showed the development of insulin resistance in test rats after 50 days administration of homocysteine. Results from this in vivo study suggest that homocysteine can cause insulin resistance and this relationship may need to be considered when evaluating the role of plasma homocysteine as a risk factor in patients with obesity and type II diabetes.

New insights into the regulation of methyl group and homocysteine metabolism

Hepatic folate, methyl group, and homocysteine metabolism are interrelated pathways that when disrupted are associated with numerous pathologies. Maintenance of normal methyl group and homocysteine homeostasis is dependent on the balance between: S-adenosylmethionine (SAM)-dependent transmethylation, which utilizes methyl groups and produces homocysteine; remethylation of homocysteine back to methionine by folate-dependent and -independent mechanisms; and homocysteine catabolism via the transsulfuration pathway. Recent studies have demonstrated that hormonal imbalance is a factor in the control of key proteins that regulate these pathways. A diabetic state is characterized by increased expression of specific methyltransferases that utilize SAM-derived methyl groups and produce homocysteine. Although the supply of methyl groups from the folate-dependent 1-carbon pool appears to be diminished under diabetic conditions, the increased production of homocysteine is compensated for by stimulation of folate-independent remethylation and catabolism by transsulfuration, resulting in hypohomocysteinemia. Similar changes have been observed with glucocorticoid administration and in a growth hormone-deficient model, which can be prevented by insulin and growth hormone treatment, respectively. Taken together, these reports clearly indicate that hormonal regulation is a major factor in the metabolic control of folate, methyl groups, and homocysteine, thereby providing a potential link between the pathologies associated with these pathways and hormonal imbalance.

Folate status modulates the induction of hepatic glycine N-methyltransferase and homocysteine metabolism in diabetic rats

A diabetic state induces the activity and abundance of glycine N-methyltransferase (GNMT), a key protein in the regulation of folate, methyl group, and homocysteine metabolism. Because the folate-dependent one-carbon pool is a source of methyl groups and 5-methyltetrahydrofolate allosterically inhibits GNMT, the aim of this study was to determine whether folate status has an impact on the interaction between diabetes and methyl group metabolism. Rats were fed a diet containing deficient (0 ppm), adequate (2 ppm), or supplemental (8 ppm) folate for 30 days, after which diabetes was initiated in one-half of the rats by streptozotocin treatment. The activities of GNMT, phosphatidylethanolamine N-methyltransferase (PEMT), and betaine-homocysteine S-methyltransferase (BHMT) were increased about twofold in diabetic rat liver; folate deficiency resulted in the greatest elevation in GNMT activity. The abundance of GNMT protein and mRNA, as well as BHMT mRNA, was also elevated in diabetic rats. The marked hyperhomocysteinemia in folate-deficient rats was attenuated by streptozotocin, likely due in part to increased BHMT expression. These results indicate that a diabetic state profoundly modulates methyl group, choline, and homocysteine metabolism, and folate status may play a role in the extent of these alterations. Moreover, the upregulation of BHMT and PEMT may indicate an increased choline requirement in the diabetic rat.

Hepatic phosphatidylethanolamine N-methyltransferase expression is increased in diabetic rats

Phosphatidylcholine is an essential phospholipid that is synthesized by 2 different pathways, the CDP-choline pathway and the methylation of phosphatidylethanolamine by phosphatidylethanolamine N-methyltransferase (PEMT). Recent studies have suggested that PEMT is an important consumer of methyl groups from S-adenosylmethionine (SAM) and is a major determinant of homocysteine pools. Diabetes and all-trans-retinoic acid (ATRA) have been shown to alter the activities of several enzymes involved in methyl group metabolism. Thus, we investigated how diabetes and ATRA, individually and together, affect SAM-dependent phospholipid methylation. Rats received a single injection of streptozotocin (60 mg/kg body wt) or vehicle followed by administration of ATRA (30 mumol/kg body wt) or vehicle for 5 d. The hepatic activity of PEMT increased 50% in both diabetic rat groups, whereas administration of ATRA was without effect. In diabetic rats, plasma total homocysteine decreased 30-35% in all treatment groups as compared with the control group. Thus, alterations in the activity of PEMT were not directly correlated to changes in homocysteine concentrations. Moreover, treatment of diabetic rats with insulin prevented the increase in PEMT activity and abundance. Because these observations support an increased need for SAM-dependent phosphatidylcholine synthesis, this may also indicate an increased choline requirement in diabetes.

Effects of insulin deprivation and treatment on homocysteine metabolism in people with type 1 diabetes

CONTEXT

Abnormal homocysteine metabolism may contribute to increased cardiovascular death in type 1 diabetes (T1DM). Amino acid metabolism is altered in T1DM. In vitro, insulin reduces hepatic catabolism of homocysteine by inhibiting liver transsulfuration. It remains to be determined whether methionine-homocysteine metabolism is altered in T1DM.

OBJECTIVE

We sought to determine whether insulin deficiency during insulin deprivation or high plasma insulin concentration after insulin treatment alters homocysteine metabolism in T1DM.

DESIGN

This was an acute interventional study with paired and comparative controls.

SETTING

The study was conducted at a general clinical research center.

PATIENTS AND INTERVENTION

We used stable isotope tracers to measure methionine-homocysteine kinetics in six patients with T1DM during insulin deprivation (I-) and also during insulin treatment (I+) and compared them with nondiabetic controls (n = 6).

MAIN OUTCOME MEASURES

Homocysteine kinetics (transmethylation, transsulfuration, and remethylation) were from plasma isotopic enrichment of methionine and homocysteine and (13)CO(2).

RESULTS

T1DM (I-) had lower rates of homocysteine-methionine remethylation (P < 0.05 vs. control and I+). In contrast, transsulfuration rates were higher in I- than controls and I+ (P < 0.05). Insulin treatment normalized transsulfuration and remethylation (P < 0.05 vs. I- and P > 0.8 vs. control). Plasma homocysteine concentrations were lower in T1DM (P < 0.05 vs. control during both I- and I+), which may be explained by increased homocysteine transsulfuration. Thus, significant alterations of methionine-homocysteine metabolism occur during insulin deprivation in humans with T1DM.

CONCLUSIONS

Insulin plays a key role in the regulation of methionine-homocysteine metabolism in humans, and altered homocysteine may occur during insulin deficiency in type 1 diabetic patients.

Homocysteine levels are associated with the results of 123I-metaiodobenzylguanidine myocardial scintigraphy in type 2 diabetic patients

PURPOSE

Elevated total plasma homocysteine (tHcy) levels and cardiovascular autonomic dysfunction are associated with a high mortality in type 2 diabetic patients. We tested the hypothesis that hyperhomocysteinemia is associated with insulin resistance and cardiovascular autonomic dysfunction in type 2 diabetic patients not receiving insulin treatment.

METHODS

The study group consisted of 17 type 2 diabetic patients with high tHcy levels (>15 mmol/l, age 58+/-5 years, high tHcy group). The control group consisted of 23 age-matched type 2 diabetic patients with normal tHcy levels (<or=15 mmol/l, age 58+/-9 years, normal tHcy group). Cardiovascular autonomic function was assessed by baroreflex sensitivity, heart rate variability, plasma norepinephrine concentrations, and cardiac (123)I-metaiodobenzylguanidine (MIBG) scintigraphy.

RESULTS

Early and delayed (123)I-MIBG myocardial uptake values were lower (p<0.005 and p<0.01, respectively) and the percent washout rate of (123)I-MIBG was higher (p<0.001) in the high tHcy group than in the normal tHcy group. The fasting plasma insulin concentrations (p<0.0001) and the homeostasis model assessment (HOMA) index values (p<0.0001) were higher in the high tHcy group than in the normal tHcy group. Multiple regression analysis revealed that the level of tHcy was independently predicted by the HOMA index values and the myocardial uptake of (123)I-MIBG at the delayed phase.

CONCLUSION

Our results demonstrate that high levels of tHcy are associated with depressed cardiovascular autonomic function and insulin resistance in patients with type 2 diabetes mellitus.

Associations between serum insulin and homocysteine in a Swedish population-a potential link between the metabolic syndrome and hyperhomocysteinemia: the Skaraborg project

The objective of this study is to examine the association between serum levels of insulin and homocysteine (Hcy) in a population-based sample of Swedish men and women. Men and women (537 and 571, respectively) 40 years or older, who were randomly selected from the population in Skara, southwestern Sweden, with valid information on serum levels of Hcy and insulin, were subject to a physical examination, including anthropometric measurement. Lifestyle factors were assessed by a questionnaire, and venous blood samples were drawn after an overnight fast. Insulin resistance was estimated by the homeostasis model assessment index. Homocysteine was higher in men (11.0 micromol/L) than in women (9.7 micromol/L) (P < .001) and was positively associated with age (P < .001 in both sexes) and serum creatinine (P = .009 in men, P < .001 in women), but inversely associated with leisure time physical activity (P = .012 in men, P = .001 in women). There was a positive association between serum insulin and serum Hcy independent of age and sex (P = .004). Upon exclusion of patients with diabetes and individuals with serum creatinine level greater than 130 microcat/L, this association was significant in the remaining 999 individuals also after adjustment for age, sex, serum creatinine, leisure time physical activity, body mass index, and smoking status (P = .003). A 1 SD difference in serum insulin corresponded to a difference of 0.5 micromol/L in serum Hcy. A similar association was found between insulin resistance and serum Hcy. In conclusion, there is an association between serum insulin and Hcy that may constitute a link between the metabolic syndrome and Hcy, either unilaterally or as part of a vicious circle.

Effects of diabetes and insulin on betaine-homocysteine S-methyltransferase expression in rat liver

Elevation of plasma homocysteine levels has been recognized as an independent risk factor for the development of cardiovascular disease, a major complication of diabetes. Plasma homocysteine reflects a balance between its synthesis via S-adenosyl-L-methionine-dependent methylation reactions and its removal through the transmethylation and the transsulfuration pathways. Betaine-homocysteine methyltransferase (BHMT, EC 2.1.1.5) is one of the enzymes involved in the remethylation pathway. BHMT, a major zinc metalloenzyme in the liver, catalyzes the transfer of methyl groups from betaine to homocysteine to form dimethylglycine and methionine. We have previously shown that plasma homocysteine levels and the transsulfuration pathway are affected by diabetes. In the present study, we found increased BHMT activity and mRNA levels in livers from streptozotocin-diabetic rats. In the rat hepatoma cell line (H4IIE cells), glucocorticoids (triamcinolone) increased the level and rate of BHMT mRNA synthesis. In the same cell line, insulin decreased the abundance of BHMT mRNA and the rate of de novo mRNA transcription of the gene. Thus the decreased plasma homocysteine in various models of diabetes could be due to enhanced homocysteine removal brought about by a combination of increased transsulfuration of homocysteine to cysteine and increased remethylation of homocysteine to methionine by BHMT.

L-cysteine inhibits insulin release from the pancreatic beta-cell: possible involvement of metabolic production of hydrogen sulfide, a novel gasotransmitter

Hydrogen sulfide (H(2)S) was historically recognized as a toxic gas generated by natural resources. However, its enzymatic production from L-cysteine has recently been demonstrated in mammals. Cystathionine beta-synthase and cystathionine gamma-lyase, both of which can produce H(2)S, were expressed in mouse pancreatic islet cells and the beta-cell line, MIN6. L-cysteine and the H(2)S donor NaHS inhibited glucose-induced insulin release from islets and MIN6 cells. These inhibitory effects were reproduced when insulin release was stimulated by alpha-ketoisocaproate, tolbutamide, or high K+. L-cysteine and NaHS inhibited glucose-potentiated insulin release in the copresence of diazoxide and high K+. Real-time imaging of intracellular Ca2+ concentration ([Ca2+](i)) demonstrated that both L-cysteine and NaHS reversibly suppressed glucose-induced [Ca2+](i) oscillation in a single beta-cell without obvious changes in the mean value. These substances inhibited Ca2+ - or guanosine 5'-0-3-thiotriphosphate-induced insulin release from islets permeabilized with streptolysin-O. L-cysteine and NaHS reduced ATP production and attenuated glucose-induced hyperpolarization of the mitochondrial membrane potential. Finally, L-cysteine increased H(2)S content in MIN6 cells. We suggest here that L-cysteine inhibits insulin release via multiple actions on the insulin secretory process through H(2)S production. Because the activities of H(2)S-producing enzymes and the tissue H(2)S contents are known to increase under diabetic conditions, the inhibition may participate in the deterioration of insulin release in this disease.

Plasma glutathione and cystathionine concentrations are elevated but cysteine flux is unchanged by dietary vitamin B-6 restriction in young men and women

Cysteine synthesis from homocysteine is catalyzed by two pyridoxal 5'-phosphate (PLP)-dependent enzymes. This suggests that vitamin B-6 status might affect cysteine and glutathione homeostasis, but it is unclear whether this occurs in humans. We assessed the effects of vitamin B-6 status on static and kinetic parameters of cysteine and glutathione metabolism in healthy female (n=5) and male (n=4) volunteers (20-30 y) before and after 4 wk of dietary vitamin B-6 restriction (<0.5 mg vitamin B-6/d). Rates of reactions related to cysteine metabolism were measured from blood sampled during primed, constant infusions of [(13)C(5)]methionine, [3-(13)C]serine, and [(2)H(2)]cysteine that were conducted after an overnight fast at baseline and after the dietary protocol. Vitamin B-6 restriction reduced the concentration of PLP (55.1+/- 8.3 vs. 22.6+/-1.3 nmol/L; P=0.004) and increased concentrations of cystathionine (124%; P<0.001) and total glutathione (38%; P<0.008) in plasma. Concentrations of plasma homocysteine, cysteine, cysteinylglycine, and C-reactive protein (an indicator of systemic inflammation) were not affected by dietary vitamin B-6 restriction. The rate of cysteine synthesis via transsulfuration was below detection limits in this protocol. Neither the fractional synthesis rate of cystathionine nor whole-body cysteine flux was affected by vitamin B-6 restriction. These data indicate that glutathione homeostasis is altered by dietary vitamin B-6 deficiency and appears to be unrelated to cysteine flux under conditions of minimal amino acid intake as evaluated in this study.

Differences in plasma homocysteine levels between Zucker fatty and Zucker diabetic fatty rats following 3 weeks oral administration of organic vanadium compounds

PURPOSE

Recently, our laboratory group has reported that rats with Type 1 diabetes have decreased plasma homocysteine and cysteine levels compared to non-diabetic controls and that organic vanadium treatment increased plasma homocysteine concentrations to non-diabetic concentrations. However, to date, no studies have been done investigating the effects of organic vanadium compounds on plasma homocysteine and its metabolites in Type 2 diabetic animal model. These studies examined the effect of organic vanadium compounds [bis(maltolato)oxovanadium(IV) and bis(ethylmaltolato)oxovanadium(IV); BMOV and BEOV] administered orally on plasma concentrations of homocysteine and its metabolites (cysteine and cysteinylglycine) in lean, Zucker fatty (ZF) and Zucker diabetic fatty (ZDF) rats. ZF rats are a model of pre-diabetic Type 2 diabetes characterized by hyperinsulinemia and normoglycemia. The ZDF rat is a model of Type 2 diabetes characterized by relative hypoinsulinemia and hyperglycemia.

METHODS

Zucker lean and ZF rats received BMOV in the drinking water at a dose of 0.19 +/- 0.02 mmol/kg/day. Lean and ZDF rats received BEOV by oral gavage daily at dose of 0.1 mmol/kg. The treatment period for both studies was 21 days. At termination, animals were fasted overnight (approximately 16 h) and blood samples were collected by cardiac puncture for determination of plasma glucose, insulin and homocysteine levels. Plasma homocysteine and its metabolites levels were determined using high-pressure liquid chromatography. Plasma glucose was determined using a Glucose Analyzer 2. Plasma insulin levels were determined by radioimmunoassay. Plasma triglycerides were determined by an enzymatic assay methodology.

RESULTS

ZF (n = 4) and ZDF (n = 10) rats had significantly lower plasma homocysteine as compared to their respective lean groups (ZF 0.78 +/- 0.1 micromol/L vs. Zucker lean 2.19 +/- 0.7 micromol/L; ZDF 1.71 +/- 0.2 micromol/L vs. Zucker lean 3.02 +/- 0.3 micromol/L; p < 0.05). BMOV treatment in ZF rats restored plasma homocysteine levels to those observed in lean untreated rats (ZF treated: 2.04 +/- 0.2 micromol/L; lean 2.19 +/- 0.7 micromol/L). There was a modest effect of BMOV treatment on plasma glucose levels in ZF rats. BEOV treatment significantly decreased the elevated plasma glucose levels in the ZDF rats (lean 7.9 +/- 0.1 mmol/L; lean + vanadium 7.7 +/- 0.2 mmol/L; ZDF 29.9 +/- 0.4 mmol/L; ZDF + vanadium 17.4 +/- 0.3 mmol/L, p < 0.05). Organic vanadium treatment reduced cysteine levels in both ZF and ZDF rats. No differences in total plasma cysteinylglycine concentrations were observed.

CONCLUSION

Plasma homocysteine levels are significantly reduced in a pre-diabetic model of Type 2 diabetes, which was restored to lean levels upon vanadium treatment; however, this restoration of plasma homocysteine levels was not seen in ZDF Type 2 diabetic rats following vanadium treatment. In the latter case vanadium treatment may not have totally overcome the insulin resistance seen in these animals.

Homocysteine and vascular disease in diabetes: a double hit?

Cardiovascular disease is a major problem in diabetes, and risk factors presumably unrelated to diabetes, such as hyperhomocysteinaemia, may be related to the development of cardiovascular complications in diabetic individuals. Plasma homocysteine levels are usually normal in diabetes, although both lower and higher levels have been reported. Homocysteine levels in diabetes are modulated by hyperfiltration and renal dysfunction, as well as low folate status. Insulin resistance does not appear to be a major determinant of plasma homocysteine level. Hyperhomocysteinaemia has been associated with microalbuminuria and retinopathy in type 1 and type 2 diabetes. In patients with type 2 diabetes, plasma homocysteine concentration is a significant predictor of cardiovascular events and death. This relation seems to be stronger in subjects with diabetes than without. The underlying pathophysiological mechanism of this increased vascular risk remains unexplained, but may be related to worsening of endothelial dysfunction and/or structural vessel properties induced by oxidative stress. Because homocysteine and diabetes have apparent synergistic detrimental vascular effects, patients with diabetes are candidates for screening and treatment with folic acid until the results of ongoing clinical trials are available.

Elevated homocysteine as a risk factor for the development of diabetes in women with a previous history of gestational diabetes mellitus: a 4-year prospective study

OBJECTIVE

To investigate the potential use of the plasma homocysteine level as a predictor of diabetes in women with a previous history of gestational diabetes mellitus (GDM).

RESEARCH DESIGN AND METHODS

At 6 weeks' postpartum, baseline examination was performed in 177 GAD-negative subjects. Of these subjects, 7 who were diagnosed with diabetes at baseline were excluded from further evaluation, and 170 with normal or impaired glucose tolerance (IGT) at baseline were followed annually over 4 years. The follow-up examinations included 2-h 75-g oral glucose tolerance tests (OGTTs), lipid profiles, homocysteine levels, anthropometric measurements, history taking, diet, and lifestyle. During the OGTTs, insulin and glucose levels were assayed every 30 min. Plasma homocysteine levels were determined by ion-exchange chromatography.

RESULTS

Of the 170 women, 18 (10.6%) converted to diabetes during the 4-year follow-up period. Mean age, BMI, fasting insulin, and total cholesterol at baseline (6 weeks' postpartum test) were similar in the three study groups (i.e., normal, IGT, and diabetes). Fasting glucose levels, insulin-to-glucose ratios, and homocysteine levels were significantly higher in the diabetic group (P < 0.05). Higher glucose at the time of the diagnosis of GDM and higher homocysteine levels at baseline were independently associated with the onset of postpartum diabetes. These relationships were independent of age, BMI, and family history of diabetes.

CONCLUSIONS

This prospective study identified homocysteine level as a significant risk factor for development of diabetes in women with previous GDM.

Homocysteine metabolism in ZDF (type 2) diabetic rats

Mild hyperhomocysteinemia is a risk factor for many diseases, including cardiovascular disease. We determined the effects of insulin resistance and of type 2 diabetes on homocysteine (Hcy) metabolism using Zucker diabetic fatty rats (ZDF/Gmi fa/fa and ZDF/Gmi fa/?). Plasma total Hcy was reduced in ZDF fa/fa rats by 24% in the pre-diabetic insulin-resistant stage, while in the frank diabetic stage there was a 59% reduction. Hepatic activities of several enzymes that play a role in the removal of Hcy:cystathionine beta-synthase (CBS), cystathionine gamma-lyase, and betaine:Hcy methyltransferase (BHMT) were increased as was methionine adenosyltransferase. CBS and BHMT mRNA levels and the hepatic level of S-adenosylmethionine were also increased in the ZDF fa/fa rats. Studies with primary hepatocytes showed that Hcy export and the transsulfuration flux in cells from ZDF fa/fa rats were particularly sensitive to betaine. Interestingly, liver betaine concentration was found to be significantly lower in the ZDf fa/fa rats at both 5 and 11 weeks. These results emphasize the importance of betaine metabolism in determining plasma Hcy levels in type 2 diabetes.

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.

Elevated plasma homocysteine concentrations as a predictor of steatohepatitis in patients with non-alcoholic fatty liver disease

BACKGROUND

Although steatosis is common in patients with severe hyperhomocysteinemia due to deficiency of cystathionine beta-synthase, there are no satisfactory data on homocysteine concentrations in patients with non-alcoholic fatty liver disease. The main aim of the present study was to evaluate the clinical significance of plasma homocysteine concentrations in patients with non-alcoholic fatty liver disease.

METHODS

Seventy-one non-alcoholic fatty liver disease patients, 36 patients with chronic viral hepatitis and 30 healthy persons were enrolled in the study. Homocysteine levels were measured by high-performance liquid chromatography. Insulin, folate, vitamin B(12) and lipoprotein levels were also determined in all groups.

RESULTS

Homocysteine in the non-alcoholic fatty liver disease group was found to be significantly higher than other groups. Homocysteine was found to be significantly higher in the non-alcoholic steatohepatitis group when compared with simple steatosis group. A positive correlation was found between homocysteine and triglyceride, very-low-density-lipoprotein (VLDL) cholesterol, insulin, and index of insulin resistance in the non-alcoholic fatty liver disease group, and a negative correlation was found between homocysteine and folate, or vitamin B(12) in all groups. The homocysteine threshold for the prediction of steatohepatitis was 11.935 ng/mL. Furthermore; plasma homocysteine was a statistically significant predictor for severity of necroinflammatory activity in non-alcoholic steatohepatitis.

CONCLUSIONS

The plasma homocysteine concentrations were significantly higher in patients with non-alcoholic fatty liver disease, while the concentrations were not affected by chronic viral hepatitis. Plasma homocysteine is a parameter for discriminating steatohepatitis from simple steatosis. Determining the plasma homocysteine concentrations may facilitate selection of steatosis patients in whom a liver biopsy should be performed.

Streptozotocin-induced diabetes in the rat is associated with enhanced tissue hydrogen sulfide biosynthesis

This investigation is aimed to determine whether the biosynthesis of H(2)S, an endogenous vasodilator gas, is altered in the streptozotocin-diabetic rat. Plasma H(2)S concentration as well as the activity, and expression, of H(2)S synthesizing enzymes (namely cystathionine-gamma-lyase (CSE) and cystathionine-beta-synthetase (CBS)) were measured in various tissues of non-diabetic, streptozotocin-diabetic and insulin-treated diabetic rats. H(2)S formation in pancreas and liver was increased in diabetic rats. Both CSE and CBS mRNAs were increased in liver of diabetic animals. Similarly, CBS mRNA was increased in pancreas. Insulin treatment restored the changes in H(2)S metabolism seen. The findings of this study suggest that the metabolism of H(2)S in pancreas and liver is altered in the streptozotocin-diabetic rat. This is the first study in which a derangement in H(2)S biosynthesis in diabetes has been demonstrated. H(2)S may play a part in the aetiology or development of diabetes in this animal model.

The impact of metabolism on DNA methylation

Methylation of genomic cytosines is one of the best characterized epigenetic mechanisms, and investigation of its relationship with other biochemical pathways represents a critical stage in the elucidation of biological information processing. The field also has immense potential for the development of medical treatments for any number of conditions ranging from aging to neurological disorders. The DNA methylation status of genes is responsible for many heritable traits and varies more or less independently of the genetic code. This variation is often a result of cellular environmental factors including metabolism. A key metabolite in this regard is homocysteine. Knowledge of homocysteine metabolism continues to be amassed, yet the part played by aberrant DNA methylation in homocysteine-related pathologies is often, at best, conjectural. In this analysis, we will review recent insights and attempt to speculate meaningfully concerning the dynamics of the methionine cycle in relation to DNA methylation and disease.

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

Methionine is a sulfur-containing amino acid that is reversibly converted into homocysteine. Homocysteine is an independent cardiovascular risk factor frequently associated with the insulin resistance syndrome. The effects of insulin on methionine and homocysteine kinetics in vivo are not known. Six middle-aged male volunteers were infused with L-[methyl-2H3,1-13C]methionine before (for 3 h) and after (for 3 additional hours) an euglycemic hyperinsulinemic (150 mU/l) clamp. Steady-state methionine and homocysteine kinetics were determined using either plasma (i.e., those of methionine) or intracellular (i.e., those of plasma homocysteine) enrichments. By use of plasma enrichments, insulin decreased methionine rate of appearance (Ra; both methyl- and carbon Ra) by 25% (P < 0.003 vs. basal) and methionine disposal into proteins by 50% (P < 0.0005), whereas it increased homocysteine clearance by approximately 70% (P < 0.025). With intracellular enrichments, insulin increased all kinetic rates, mainly because homocysteine enrichment decreased by approximately 40% (P < 0.001). In particular, transmethylation increased sixfold (P < 0.02), transsulfuration fourfold (P = 0.01), remethylation eightfold (P < 0.025), and clearance eightfold (P < 0.004). In summary, 1) physiological hyperinsulinemia stimulated homocysteine metabolic clearance irrespective of the model used; and 2) divergent changes in plasma methionine and homocysteine enrichments were observed after hyperinsulinemia, resulting in different changes in methionine and homocysteine kinetics. In conclusion, insulin increases homocysteine clearance in vivo and may thus prevent homocysteine accumulation in body fluids. Use of plasma homocysteine as a surrogate of intracellular methionine enrichment, after acute perturbations such as insulin infusion, needs to be critically reassessed.

Hydrogen sulfide as an endogenous modulator of biliary bicarbonate excretion in the rat liver

Cystathionine gamma-lyase (CSE) is an enzyme catalyzing cystathionine and cysteine to yield cysteine and hydrogen sulfide (H(2)S), respectively. This study aimed to examine if H(2)S generated from the enzyme could serve as an endogenous regulator of hepatobiliary function. Gas chromatographic analyses indicated that, among rat organs herein examined, liver constituted one of the greatest components of H(2)S generation in the body, at 100 mumol/g of tissue, comparable to that in kidney and 1.5-fold greater than that in brain, where roles of the gas in the regulation of neurotransmission were reported previously. At least half of the gas amount in the liver appeared to be derived from CSE, because blockade of the enzyme by propargylglycine suppressed it by 50%. Immunohistochemistry revealed that CSE occurs not only in hepatocytes, but also in bile duct. In livers in vivo, as well as in those perfused ex vivo, treatment with the CSE inhibitor induced choleresis by stimulating the basal excretion of bicarbonate in bile samples. Transportal supplementation of NaHS at 30 mumol/L, but not that of N-acetylcysteine as a cysteine donor, abolished these changes elicited by the CSE inhibitor in the perfused liver. The changes elicited by the CSE blockade did not coincide with alterations in hepatic vascular resistance, showing little involvement of vasodilatory effects of the gas in these events, if any. These results first provided evidence that H(2)S generated through CSE modulates biliary bicarbonate excretion and is thus a determinant of bile salt-independent bile formation in the rat liver.

Reduced serum homocysteine levels in type 2 diabetes

OBJECTIVE

To assess the contribution of fasting blood glucose and methylene-tetrahydrofolate reductase (MTHFR) gene polymorphism on fasting serum homocysteine (tHcy) levels in patients with uncomplicated type 2 diabetes compared with healthy subjects.

METHODS AND RESULTS

We studied 105 type 2 diabetic patients without cardiovascular complications or diabetic nephropathy (55 males, 50 females, mean age 53+/-10 years, mean duration of diabetes 11.4+/-8 years) and 120 age- and sex-matched control subjects (65 males, 55 females, mean age 52+/-8 years). tHcy and other biochemical variables were measured. The C677T MTHFR gene polymorphism was determined by analysis of HinfI restriction fragment length polymorphism tHcy levels were significantly lower in diabetic patients compared with control subjects (7.7 +/- 2.2 vs. 11.8 +/- 4.5 micromol/l, P < 0.0001). In both patients and control subjects, homocysteinemia was higher in men than in women (8.4+/-2.6 vs. 7.3+/-2.0 micromol/l, P < 0.03, and 13.0+/-5.3 vs. 10.4+/-2.6 micromol/l, P < 0.0001, respectively). Levels were slightly higher in subjects with the mutated Val/Val genotype compared with the Ala/Val plus Ala/Ala genotypes in both diabetic patients (P < 0.02) and control subjects (P < 0.003). On simple regression analysis, tHcy was inversely related with blood glucose levels (P < 0.02) and directly with sex (P < 0.04) in diabetic patients, and with sex (P < 0.0001), age (P < 0.02), BMI (P < 0.03), systolic and diastolic blood pressure (P < 0.0004 and P < 0.0002), uric acid and creatinine (P < 0.0001 and P < 0.0003) in control subjects. On multiple regression, tHcy levels were associated with sex (P < 0.03) and glucose levels (P < 0.04) in diabetic patients, and with uric acid (P < 0.002) and MTHFR genotype (P < 0.03) in control subjects.

CONCLUSION

In type 2 diabetic patients without nephropathy, basal levels of tHcy were 35% lower compared with healthy controls. Chronic hyperglycemia may control tHcy by affecting its renal excretion, or accelerate hepatic trans-sulfuration secondary to insulin disorders.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

Triiodothyronine treatment attenuates the induction of hepatic glycine N-methyltransferase by retinoic acid and elevates plasma homocysteine concentrations in rats

Recent studies indicated that hormonal imbalances have a role in modulating the metabolism of methyl groups and homocysteine, interrelated pathways that when disrupted, are associated with a number of pathologies. Retinoic acid (RA) was shown to induce hepatic glycine N-methyltransferase (GNMT), a key regulatory protein in methyl group metabolism, and to reduce circulating homocysteine levels. Because thyroid status influences the hepatic folate-dependent one-carbon pool and retinoids can alter thyroid hormone levels, the aim of this study was to examine the interaction between retinoids and thyroid function. For hypothyroid studies, rats were administered 0.5 g/L propylthiouracil in the drinking water for 15 d, and RA [30 micromol/(kg . d)] for the final 5 d. For hyperthyroid studies, rats were treated with RA [30 micromol/(kg . d)] for 8 d and triiodothyronine [T(3); 50 microg/(100 g . d)] the last 4 d. T(3) treatment prevented the RA-mediated increase in GNMT activity. However, GNMT abundance remained elevated, indicating that GNMT regulation by T(3) in RA-treated rats may be, at least in part, at the post-translational level. In addition, T(3) treatment elevated plasma levels of homocysteine 177%, an elevation that was prevented by RA. T(3)-mediated hyperhomocysteinemia may be due to a 70% decrease in hepatic betaine-homocysteine S-methyltransferase, the enzyme that catalyzes folate-independent remethylation of homocysteine, whereas the RA-mediated stimulation of hepatic homocysteine remethylation by folate-dependent methionine synthase may contribute to lowering plasma homocysteine levels. These findings indicate that thyroid hormones, alone and in conjunction with RA, play an important role in the regulation of methyl group and homocysteine metabolism.

Effects of paradoxical sleep deprivation on blood parameters associated with cardiovascular risk in aged rats

The effects of 96 h of paradoxical sleep deprivation (PSD) on blood parameters associated with cardiovascular risk were studied in young (3-month old) and aged (22-month old) rats. In general, aging was associated with an overall increase in most measures, irrespective of sleep deprivation condition. The latter manipulation also had significant effects on blood variables, but not in a consistent pattern. Thus, PSD significantly reduced triglyceride levels in both young and aged rats; it reduced blood viscosity in aged but not in young rats, and had no effect on the increased cholesterol levels observed in aged controls. Examinations of cholesterol fractions revealed significant increases in low density lipoprotein and high density lipoprotein in aged PSD rats compared to respective controls, whereas very low density lipoprotein was significant decreased after PSD in both young and aged animals. PSD increased vitamin B(12) levels in aged rats, and significantly decreased homocysteine levels in young but not in aged rats which in turn were already reduced. Folate levels were the only variable that was unaffected by aging and/or PSD. These results indicate that PSD has significant but heterogeneous physiological effects in aged rats and may intensify certain aging-related effects which contribute to cardiovascular disease risk while attenuating others.

Modulation of methyl group metabolism by streptozotocin-induced diabetes and all-trans-retinoic acid

The hepatic enzyme glycine N-methyltransferase (GNMT) plays a major role in the control of methyl group and homocysteine metabolism. Because disruption of these vital pathways is associated with numerous pathologies, understanding GNMT control is important for evaluating methyl group regulation. Recently, gluconeogenic conditions have been shown to modulate homocysteine metabolism and treatment with glucocorticoids and/or all-trans-retinoic acid (RA)-induced active GNMT protein, thereby leading to methyl group loss. This study was conducted to determine the effect of diabetes, alone and in combination with RA, on GNMT regulation. Diabetes and RA increased GNMT activity 87 and 148%, respectively. Moreover, the induction of GNMT activity by diabetes and RA was reflected in its abundance. Cell culture studies demonstrated that pretreatment with insulin prevented GNMT induction by both RA and dexamethasone. There was a significant decline in homocysteine concentrations in diabetic rats, owing in part to a 38% increase in the abundance of the transsulfuration enzyme cystathionine beta-synthase; treatment of diabetic rats with RA prevented cystathionine beta-synthase induction. A diabetic state also increased the activity of the folate-independent homocysteine remethylation enzyme betaine-homocysteine S-methyltransferase, whereas the activity of the folate-dependent enzyme methionine synthase was diminished 52%. In contrast, RA treatment attenuated the streptozotocin-mediated increase in betaine-homocysteine S-methyltransferase, whereas methionine synthase activity remained diminished. These results indicate that both a diabetic condition and RA treatment have marked effects on the metabolism of methyl groups and homocysteine, a finding that may have significant implications for diabetics and their potential sensitivity to retinoids.

Retinoic acid and glucocorticoid treatment induce hepatic glycine N-methyltransferase and lower plasma homocysteine concentrations in rats and rat hepatoma cells

Perturbation of folate and methyl group metabolism is associated with a number of pathological conditions, including cardiovascular disease and neoplastic development. Glycine N-methyltransferase (GNMT) is a key protein that functions to regulate the supply and utilization of methyl groups for S-adenosylmethionine (SAM)-dependent transmethylation reactions. Factors or conditions that have the ability to regulate GNMT and the generation of homocysteine, a product of transmethylation, have important implications in the potential perturbation of methyl group metabolism. We showed that retinoid compounds induce active hepatic GNMT, resulting in compromised transmethylation processes. Because retinoids can stimulate gluconeogenesis, a condition known to alter methyl group and homocysteine metabolism, the current study was undertaken to determine the relationship between all-trans-retinoic acid (RA) and gluconeogenic hormones on these metabolic pathways. Intact adrenal function was not required for RA to induce and activate hepatic GNMT; however, treatment of rats with dexamethasone (DEX) was as effective as RA in inducing GNMT in rat liver. The marked increase in plasma total homocysteine levels observed in adrenalectomized rats was reduced to normal levels by treatment with either RA or DEX, indicating that the transsulfuration and/or remethylation pathways may be enhanced. Moreover, coadministration of RA and DEX had an additive effect on GNMT induction. Similar findings were also observed in a rat hepatoma cell culture model using H4IIE cells. Taken together, these results demonstrate that both RA and DEX independently induce GNMT, thereby having substantial implications for the potential interaction of retinoid administration with diabetes.