Cystathionine beta-synthase deficiency causes fat loss in mice

Mechanism of action and impact of thiol homeostasis on efficacy of an enzyme replacement therapy for classical homocystinuria

Homocystinuria (HCU) due to cystathionine beta-synthase (CBS) deficiency is characterized by elevated plasma and tissue homocysteine levels. There is no cure, but HCU is typically managed by methionine/protein restriction and vitamin B6 supplementation. Enzyme replacement therapy (ERT) based on human CBS has been developed and has shown significant efficacy correcting HCU phenotype in several mouse models by bringing plasma total homocysteine below the clinically relevant 100 μM threshold. As the reactive nature of homocysteine promotes disulfide formation and protein binding, and ERT is unable to normalize plasma total homocysteine levels, the mechanism of action of ERT in HCU remains to be further characterized. Here we showed that only a reduced homocysteine serves as a substrate for CBS and its availability restricts the homocysteine-degrading capacity of CBS. We also demonstrated that cells export homocysteine in its reduced form, which is efficiently metabolized by CBS in the culture medium. Availability of serine, a CBS co-substrate, was not a limiting factor in our cell-based model. Biological reductants, such as N-acetylcysteine, MESNA or cysteamine, increased the availability of the reduced homocysteine and thus promoted its subsequent CBS-based elimination. In a transgenic I278T mouse model of HCU, administration of biological reductants significantly increased the proportion of protein-unbound homocysteine in plasma, which improved the efficacy of the co-administered CBS-based ERT, as evidenced by significantly lower plasma total homocysteine levels. These results clarify the mechanism of action of CBS-based ERT and unveil novel pharmacological approaches to further increase its efficacy.

Pharmacology of Hydrogen Sulfide and Its Donors in Cardiometabolic Diseases

Cardiometabolic diseases (CMDs) are major contributors to global mortality, emphasizing the critical need for novel therapeutic interventions. Hydrogen sulfide (H2S) has garnered enormous attention as a significant gasotransmitter with various physiological, pathophysiological, and pharmacological impacts within mammalian cardiometabolic systems. In addition to its roles in attenuating oxidative stress and inflammatory response, burgeoning research emphasizes the significance of H2S in regulating proteins via persulfidation, a well known modification intricately associated with the pathogenesis of CMDs. This review seeks to investigate recent updates on the physiological actions of endogenous H2S and the pharmacological roles of various H2S donors in addressing diverse aspects of CMDs across cellular, animal, and clinical studies. Of note, advanced methodologies, including multiomics, intestinal microflora analysis, organoid, and single-cell sequencing techniques, are gaining traction due to their ability to offer comprehensive insights into biomedical research. These emerging approaches hold promise in characterizing the pharmacological roles of H2S in health and diseases. We will critically assess the current literature to clarify the roles of H2S in diseases while also delineating the opportunities and challenges they present in H2S-based pharmacotherapy for CMDs. SIGNIFICANCE STATEMENT: This comprehensive review covers recent developments in H2S biology and pharmacology in cardiometabolic diseases CMDs. Endogenous H2S and its donors show great promise for the management of CMDs by regulating numerous proteins and signaling pathways. The emergence of new technologies will considerably advance the pharmacological research and clinical translation of H2S.

Deciphering pathophysiological mechanisms underlying cystathionine beta-synthase-deficient homocystinuria using targeted metabolomics, liver proteomics, sphingolipidomics and analysis of mitochondrial function

BACKGROUND

Cystathionine β-synthase (CBS)-deficient homocystinuria (HCU) is an inherited disorder of sulfur amino acid metabolism with varying severity and organ complications, and a limited knowledge about underlying pathophysiological processes. Here we aimed at getting an in-depth insight into disease mechanisms using a transgenic mouse model of HCU (I278T).

METHODS

We assessed metabolic, proteomic and sphingolipidomic changes, and mitochondrial function in tissues and body fluids of I278T mice and WT controls. Furthermore, we evaluated the efficacy of methionine-restricted diet (MRD) in I278T mice.

RESULTS

In WT mice, we observed a distinct tissue/body fluid compartmentalization of metabolites with up to six-orders of magnitude differences in concentrations among various organs. The I278T mice exhibited the anticipated metabolic imbalance with signs of an increased production of hydrogen sulfide and disturbed persulfidation of free aminothiols. HCU resulted in a significant dysregulation of liver proteome affecting biological oxidations, conjugation of compounds, and metabolism of amino acids, vitamins, cofactors and lipids. Liver sphingolipidomics indicated upregulation of the pro-proliferative sphingosine-1-phosphate signaling pathway. Liver mitochondrial function of HCU mice did not seem to be impaired compared to controls. MRD in I278T mice improved metabolic balance in all tissues and substantially reduced dysregulation of liver proteome.

CONCLUSION

The study highlights distinct tissue compartmentalization of sulfur-related metabolites in normal mice, extensive metabolome, proteome and sphingolipidome disruptions in I278T mice, and the efficacy of MRD to alleviate some of the HCU-related biochemical abnormalities.

Cysteine-lowering treatment with mesna against obesity: Proof of concept and results from a human phase I, dose-finding study

AIM

To investigate whether mesna-sodium-2-mercaptoethane sulfonate) can reduce diet-induced fat gain in mice, and to assess the safety of single ascending mesna doses in humans to find the dose associated with lowering of plasma tCys by at least 30%.

METHODS

C3H/HeH mice were shifted to a high-fat diet ± mesna in drinking water; body composition was measured at weeks 0, 2 and 4. In an open, phase I, single ascending dose study, oral mesna (400, 800, 1200, 1600 mg) was administered to 17 men with overweight or obesity. Mesna and tCys concentrations were measured repeatedly for a duration of 48 hours postdosing in plasma, as well as in 24-hour urine.

RESULTS

Compared with controls, mesna-treated mice had lower tCys and lower estimated mean fat mass gain from baseline (week 2: 4.54 ± 0.40 vs. 6.52 ± 0.36 g; week 4: 6.95 ± 0.35 vs. 8.19 ± 0.34 g; Poverall  = .002), but similar lean mass gain. In men with overweight, mesna doses of 400-1600 mg showed dose linearity and were well tolerated. Mesna doses of 800 mg or higher decreased plasma tCys by 30% or more at nadir (4h post-dosing). With increasing mesna dose, tCys AUC0-12h decreased (Ptrend  < .001), and urine tCys excretion increased (Ptrend  = .004).

CONCLUSIONS

Mesna reduces diet-induced fat gain in mice. In men with overweight, single oral doses of mesna (800-1600 mg) were well tolerated and lowered plasma tCys efficiently. The effect of sustained tCys-lowering by repeated mesna administration on weight loss in humans deserves investigation.

Paracetamol toxicity in classic homocystinuria: Effect of N-acetylcysteine on total homocysteine

Classical homocystinuria (HCU) is caused by cystathionine β-synthase deficiency leading to impaired homocysteine transsulfuration and accumulation of homocysteine and methionine. Patients present with a wide spectrum of manifestations including ocular, skeletal, neuropsychiatric, and vascular manifestations. We report a 48-year-old female with pyridoxine-unresponsive HCU treated with betaine, cyanocobalamin, and folate. Her diet was non-restricted due to intolerance of low-methionine diet. She was admitted to hospital following a fall, with multiple fractures and subsequently developed acute liver failure with encephalopathy. Shock, sepsis, and liver ischaemia/thrombosis were excluded. In the context of glutathione depletion expected in HCU, hepatic dysfunction was presumed to be due to iatrogenic paracetamol toxicity, despite paracetamol intake at conventional therapeutic dose, with role of hypermethioninemia as a contributing factor being uncertain. Betaine was discontinued on hospital admission. N-Acetylcysteine (NAC) infusion was initiated. Plasma total homocysteine (tHcy) was 3.4 μmol/L 9 days following initiation of NAC treatment with a markedly elevated plasma methionine of 1278 μmol/L. tHcy concentration returned to pre-admission baseline after NAC was discontinued. Recovery following this episode was slow with a prolonged cholestatic phase and gradual improvement in jaundice and coagulopathy. We recommend that paracetamol should be administered cautiously in HCU patients due to underlying glutathione depletion and risk of toxicity even at therapeutic doses. NAC is clearly effective in lowering tHcy in classical HCU in the short-term however further research is required to assess clinical efficacy and use as a potential therapy in classical HCU.

Selective Hepatic Cbs Knockout Aggravates Liver Damage, Endothelial Dysfunction and ROS Stress in Mice Fed a Western Diet

Cystathionine-β-synthase (CBS) is highly expressed in the liver, and deficiencies in Cbs lead to hyperhomocysteinemia (HHCy) and disturbed production of antioxidants such as hydrogen sulfide. We therefore hypothesized that liver-specific Cbs deficient (LiCKO) mice would be particularly susceptible to the development of non-alcoholic fatty liver disease (NAFLD). NAFLD was induced by a high-fat high-cholesterol (HFC) diet; LiCKO and controls were split into eight groups based on genotype (con, LiCKO), diet (normal diet, HFC), and diet duration (12 weeks, 20 weeks). LiCKO mice displayed intermediate to severe HHCy. Plasma H₂O₂ was increased by HFC, and further aggravated in LiCKO. LiCKO mice fed an HFC diet had heavier livers, increased lipid peroxidation, elevated ALAT, aggravated hepatic steatosis, and inflammation. LiCKO mice showed decreased L-carnitine in the liver, but this did not result in impaired fatty acid oxidation. Moreover, HFC-fed LiCKO mice demonstrated vascular and renal endothelial dysfunction. Liver and endothelial damage correlated significantly with systemic ROS status. In conclusion, this study demonstrates an important role for CBS in the liver in the development of NAFLD, which is most probably mediated through impaired defense against oxidative stress.

Proximal tubular Bmal1 protects against chronic kidney injury and renal fibrosis by maintaining of cellular metabolic homeostasis

Recent studies suggest that deletion of the core clock gene Bmal1 in the kidney has a significant influence on renal physiological functions. However, the role of renal Bmal1 in chronic kidney disease (CKD) remains poorly understood. Here by generating mice lacking Bmal1 in proximal tubule (Bmal1^flox/flox-KAP-Cre⁺, ptKO) and inducing CKD with the adenine diet model, we found that lack of Bmal1 in proximal tubule did not alter renal water and electrolyte homeostasis. However, adenine-induced renal injury indexes, including blood urea nitrogen, serum creatinine, and proteinuria, were markedly augmented in the ptKO mice. The ptKO kidneys also developed aggravated tubulointerstitial fibrosis and epithelial-mesenchymal transformation. Mechanistically, RNAseq analysis revealed significant downregulation of the expression of genes related to energy and substance metabolism, in particular fatty acid oxidation and glutathione/homocysteine metabolism, in the ptKO kidneys. Consistently, the renal contents of ATP and glutathione were markedly reduced in the ptKO mice, suggesting the disruption of cellular metabolic homeostasis. Moreover, we demonstrated that Bmal1 can activate the transcription of cystathionine β-synthase (CBS), a key enzyme for homocysteine metabolism and glutathione biosynthesis, through direct recruitment to the E-box motifs of its promoter. Supporting the in vivo findings, knockdown of Bmal1 in cultured proximal tubular cells inhibited CBS expression and amplified albumin-induced cell injury and fibrogenesis, while glutathione supplementation remarkably reversed these changes. Taken together, we concluded that deletion of Bmal1 in proximal tubule may aggravate chronic kidney injury and exacerbate renal fibrosis, the mechanism is related to suppressing CBS transcription and disturbing glutathione related metabolic homeostasis. These findings suggest a protective role of Bmal1 in chronic tubular injury and offer a novel target for treating CKD.

Differential expression of cystathionine beta synthase in adolescents with Down syndrome: impact on adiposity

Purpose

Obesity is more prevalent among people with Down Syndrome (DS) compared to general population. In this pilot study, we investigated the effect of cystathionine beta-synthase (CBS) overdosage on the regulation of transsulfuration pathway and the obesity phenotype in fifty adolescents (25 obese/overweight and 25 lean) with trisomy 21.

Methods

The transcriptional levels of CBS in leukocytes and its translational levels in plasma were quantified using real time polymerase chain reaction and enzyme-linked immunosorbent assay respectively. Meanwhile, ultra performance liquid chromatography tandem mass spectrometry was used to determine the plasma concentrations of methionine, homocysteine, cystathionine and cysteine. Fasting plasma lipid profiles were assessed by colorimetric assays. The anthropometric measurements and indices of all subjects were recorded.

Results

Both DS groups had comparable levels of CBS transcripts (p = 0.2734). The plasma levels of the enzyme were significantly higher in the lean DS cases (p = 0.0174) compared to the obese/overweight participants. Total cholesterol, triglycerides, high-density lipoprotein, low-density lipoprotein, methionine, homocysteine, cystathionine and cysteine showed similar plasma levels in both groups. However, the plasma cysteine levels exceeded the normal range in all DS cases. We reported a statistically significant inverse association between CBS enzyme levels and weight (r= - 0.3498, p = 0.0128), hip circumference (r= - 0.3584, p = 0.0106), body mass index (r= - 0.3719, p = 0.0078) and body adiposity index (r= - 0.3183, p = 0.0243).

Conclusions

Our data suggests that the high concentrations of CBS enzyme together with cysteine modulate the DS obesity presumably through increased hydrogen sulfide production which has recently showed anti-adiposity effects.

Inhibition of the 3-mercaptopyruvate sulfurtransferase-hydrogen sulfide system promotes cellular lipid accumulation

H₂S is generated in the adipose tissue by cystathionine γ-lyase, cystathionine β-synthase, and 3-mercaptopyruvate sulfurtransferase (3-MST). H₂S plays multiple roles in the regulation of various metabolic processes, including insulin resistance. H₂S biosynthesis also occurs in adipocytes. Aging is known to be associated with a decline in H₂S. Therefore, the question arises whether endogenous H₂S deficiency may affect the process of adipocyte maturation and lipid accumulation. Among the three H₂S-generating enzymes, the role of 3-MST is the least understood in adipocytes. Here we tested the effect of the 3-MST inhibitor 2-[(4-hydroxy-6-methylpyrimidin-2-yl)sulfanyl]-1-(naphthalen-1-yl)ethan-1-one (HMPSNE) and the H₂S donor (GYY4137) on the differentiation and adipogenesis of the adipocyte-like cells 3T3-L1 in vitro. 3T3-L1 cells were differentiated into mature adipocytes in the presence of GYY4137 or HMPSNE. HMPSNE significantly enhanced lipid accumulation into the maturing adipocytes. On the other hand, suppressed lipid accumulation was observed in cells treated with the H₂S donor. 3-MST inhibition increased, while H₂S donation suppressed the expression of various H₂S-producing enzymes during adipocyte differentiation. 3-MST knockdown also facilitated adipocytic differentiation and lipid uptake. The underlying mechanisms may involve impairment of oxidative phosphorylation and fatty acid oxidation as well as the activation of various differentiation-associated transcription factors. Thus, the 3-MST/H₂S system plays a tonic role in suppressing lipid accumulation and limiting the differentiation of adipocytes. Stimulation of 3-MST activity or supplementation of H₂S—which has been recently linked to various experimental therapeutic approaches during aging—may be a potential experimental approach to counteract adipogenesis.

Long-term functional correction of cystathionine β-synthase deficiency in mice by adeno-associated viral gene therapy

Cystathionine β-synthase (CBS) deficiency is a recessive inborn error of sulfur metabolism characterized by elevated blood levels of total homocysteine (tHcy). Patients diagnosed with CBS deficiency are currently treated by a combination of vitamin supplementation and restriction of foods containing the homocysteine precursor methionine, but the effectiveness of this therapy is limited due to poor compliance. A mouse model for CBS deficiency (Tg-I278T Cbs^-/- ) was used to evaluate a potential gene therapy approach to treat CBS deficiency utilizing an AAVrh.10-based vector containing the human CBS cDNA downstream of the constitutive, strong CAG promoter (AAVrh.10hCBS). Mice were administered a single dose of virus and followed for up to 1 year. The data demonstrated a dose-dependent increase in liver CBS activity and a dose-dependent decrease in serum tHcy. Liver CBS enzyme activity at 1 year was similar to Cbs^+/- control mice. Mice given the highest dose (5.6 × 10¹¹ genomes/mouse) had mean serum tHcy decrease of 97% 1 week after injection and an 81% reduction 1 year after injection. Treated mice had either full- or substantial correction of alopecia, bone loss, and fat mass phenotypes associated with Cbs deficiency in mice. Our findings show that AAVrh.10-based gene therapy is highly effective in treating CBS deficiency in mice and supports additional pre-clinical testing for eventual use human trials.

Hyperhomocysteinemia: Metabolic Role and Animal Studies with a Focus on Cognitive Performance and Decline-A Review

Disturbances in the one-carbon metabolism are often indicated by altered levels of the endogenous amino acid homocysteine (HCys), which is additionally discussed to causally contribute to diverse pathologies. In the first part of the present review, we profoundly and critically discuss the metabolic role and pathomechanisms of HCys, as well as its potential impact on different human disorders. The use of adequate animal models can aid in unravelling the complex pathological processes underlying the role of hyperhomocysteinemia (HHCys). Therefore, in the second part, we systematically searched PubMed/Medline for animal studies regarding HHCys and focused on the potential impact on cognitive performance and decline. The majority of reviewed studies reported a significant effect of HHCys on the investigated behavioral outcomes. Despite of persistent controversial discussions about equivocal findings, especially in clinical studies, the present evaluation of preclinical evidence indicates a causal link between HHCys and cognition-related- especially dementia-like disorders, and points out the further urge for large-scale, well-designed clinical studies in order to elucidate the normalization of HCys levels as a potential preventative or therapeutic approach in human pathologies.

Activation of Endogenous H2S Biosynthesis or Supplementation with Exogenous H2S Enhances Adipose Tissue Adipogenesis and Preserves Adipocyte Physiology in Humans

Aims: To investigate the impact of exogenous hydrogen sulfide (H₂S) and its endogenous biosynthesis on human adipocytes and adipose tissue in the context of obesity and insulin resistance. Results: Experiments in human adipose tissue explants and in isolated preadipocytes demonstrated that exogenous H₂S or the activation of endogenous H₂S biosynthesis resulted in increased adipogenesis, insulin action, sirtuin deacetylase, and PPARγ transcriptional activity, whereas chemical inhibition and gene knockdown of each enzyme generating H₂S (CTH, CBS, MPST) led to altered adipocyte differentiation, cellular senescence, and increased inflammation. In agreement with these experimental data, visceral and subcutaneous adipose tissue expression of H₂S-synthesising enzymes was significantly reduced in morbidly obese subjects in association with attenuated adipogenesis and increased markers of adipose tissue inflammation and senescence. Interestingly, weight-loss interventions (including bariatric surgery or diet/exercise) improved the expression of H₂S biosynthesis-related genes. In human preadipocytes, the expression of CTH, CBS, and MPST genes and H₂S production were dramatically increased during adipocyte differentiation. More importantly, the adipocyte proteome exhibiting persulfidation was characterized, disclosing that different proteins involved in fatty acid and lipid metabolism, the citrate cycle, insulin signaling, several adipokines, and PPAR, experienced the most dramatic persulfidation (85-98%). Innovation: No previous studies investigated the impact of H₂S on human adipose tissue. This study suggests that the potentiation of adipose tissue H₂S biosynthesis is a possible therapeutic approach to improve adipose tissue dysfunction in patients with obesity and insulin resistance. Conclusion: Altogether, these data supported the relevance of H₂S biosynthesis in the modulation of human adipocyte physiology. Antioxid. Redox Signal. 35, 319-340.

Cystathionine β-synthase deficiency in the E-HOD registry-part I: pyridoxine responsiveness as a determinant of biochemical and clinical phenotype at diagnosis

Cystathionine β-synthase (CBS) deficiency has a wide clinical spectrum, ranging from neurodevelopmental problems, lens dislocation and marfanoid features in early childhood to adult onset disease with predominantly thromboembolic complications. We have analysed clinical and laboratory data at the time of diagnosis in 328 patients with CBS deficiency from the E-HOD (European network and registry for Homocystinurias and methylation Defects) registry. We developed comprehensive criteria to classify patients into four groups of pyridoxine responsivity: non-responders (NR), partial, full and extreme responders (PR, FR and ER, respectively). All groups showed overlapping concentrations of plasma total homocysteine while pyridoxine responsiveness inversely correlated with plasma/serum methionine concentrations. The FR and ER groups had a later age of onset and diagnosis and a longer diagnostic delay than NR and PR patients. Lens dislocation was common in all groups except ER but the age of dislocation increased with increasing responsiveness. Developmental delay was commonest in the NR group while no ER patient had cognitive impairment. Thromboembolism was the commonest presenting feature in ER patients, whereas it was least likely at presentation in the NR group. This probably is due to the differences in ages at presentation: all groups had a similar number of thromboembolic events per 1000 patient-years. Clinical severity of CBS deficiency depends on the degree of pyridoxine responsiveness. Therefore, a standardised pyridoxine-responsiveness test in newly diagnosed patients and a critical review of previous assessments is indispensable to ensure adequate therapy and to prevent or reduce long-term complications.

Host cystathionine-γ lyase derived hydrogen sulfide protects against Pseudomonas aeruginosa sepsis

Hydrogen sulfide (H2S) has recently been recognized as a novel gaseous transmitter with several anti-inflammatory properties. The role of host- derived H2S in infections by Pseudomonas aeruginosa was investigated in clinical and mouse models. H2S concentrations and survival was assessed in septic patients with lung infection. Animal experiments using a model of severe systemic multidrug-resistant P. aeruginosa infection were performed using mice with a constitutive knock-out of cystathionine-γ lyase (Cse) gene (Cse-/-) and wild-type mice with a physiological expression (Cse+/+). Experiments were repeated in mice after a) treatment with cyclophosphamide; b) bone marrow transplantation (BMT) from a Cse+/+ donor; c) treatment with H2S synthesis inhibitor aminooxyacetic acid (ΑΟΑΑ) or propargylglycine (PAG) and d) H2S donor sodium thiosulfate (STS) or GYY3147. Bacterial loads and myeloperoxidase activity were measured in tissue samples. The expression of quorum sensing genes (QS) was determined in vivo and in vitro. Cytokine concentration was measured in serum and incubated splenocytes. Patients survivors at day 28 had significantly higher serum H2S compared to non-survivors. A cut- off point of 5.3 μΜ discriminated survivors with sensitivity 92.3%. Mortality after 28 days was 30.9% and 93.7% in patients with H2S higher and less than 5.3 μΜ (p = 7 x 10-6). In mice expression of Cse and application of STS afforded protection against infection with multidrug-resistant P. aeruginosa. Cyclophosphamide pretreatment eliminated the survival benefit of Cse+/+ mice, whereas BMT increased the survival of Cse-/- mice. Cse-/- mice had increased pathogen loads compared to Cse+/+ mice. Phagocytic activity of leukocytes from Cse-/- mice was reduced but was restored after H2S supplementation. An H2S dependent down- regulation of quorum sensing genes of P.aeruginosa could be demonstrated in vivo and in vitro. Endogenous H2S is a potential independent parameter correlating with the outcome of P. aeruginosa. H2S provides resistance to infection by MDR bacterial pathogens.

The Role of H2S in the Metabolism of Glucose and Lipids

Glucose and lipids are essential elements for maintaining the body's homeostasis, and their dysfunction may participate in the pathologies of various diseases, particularly diabetes, obesity, metabolic syndrome, cardiovascular ailments, and cancers. Among numerous endogenous mediators, the gasotransmitter hydrogen sulfide (H₂S) plays a central role in the maintenance of glucose and lipid homeostasis. Current evidence from both pharmacological studies and transgenic animal models suggest a complex relationship between H₂S and metabolic dysregulation, especially in diabetes and obesity. This notion is achieved through tissue-specific expressions and actions of H₂S on target metabolic and hormone organs including the pancreas, skeletal muscle, livers, and adipose. In this chapter, we will summarize the roles and mechanisms of H₂S in several metabolic organs/tissues that are necessary for glucose and lipid metabolic homeostasis. In addition, future research directions and valuable therapeutic avenues around the pharmacological regulation of H₂S in glycolipid metabolism disorder will be also discussed.

Regulation of the parental gene GRM4 by circGrm4 RNA transcript and glutamate-mediated neurovascular toxicity in eyes

Epigenetic memory plays crucial roles in gene regulation. It not only modulates the expression of specific genes but also has ripple effects on transcription as well as translation of other genes. Very often an alteration in expression occurs either via methylation or demethylation. In this context, "1-carbon metabolism" assumes a special significance since its dysregulation by higher levels of homocysteine; Hcy (known as hyperhomocysteinemia; HHcy), a byproduct of "1-Carbon Metabolism" during methionine biosynthesis leads to serious implications in cardiovascular, renal, cerebrovascular systems, and a host of other conditions. Currently, the circular RNAs (circRNAs) generated via non-canonical back-splicing events from the pre-mRNA molecules are at the center stage for their essential roles in diseases via their epigenetic manifestations. We recently identified a circular RNA transcript (circGRM4) that is significantly upregulated in the eye of cystathionine β-synthase-deficient mice. We also discovered a concurrent over-expression of the mGLUR4 receptor in the eyes of these mice. In brief, circGRM4 is selectively transcribed from its parental mGLUR4 receptor gene (GRM4) functions as a "molecular-sponge" for the miRNAs and results into excessive turnover of the mGLUR4 receptor in the eye in response to extremely high circulating glutamate concentration. We opine that this epigenetic manifestation potentially predisposes HHcy people to retinovascular malfunctioning.

Interplay of Enzyme Therapy and Dietary Management of Murine Homocystinuria

Albeit effective, methionine/protein restriction in the management of classical homocystinuria (HCU) is suboptimal and hard to follow. To address unmet need, we developed an enzyme therapy (OT-58), which effectively corrected disease symptoms in various mouse models of HCU in the absence of methionine restriction. Here we evaluated short- and long-term efficacy of OT-58 on the background of current dietary management of HCU. Methionine restriction resulted in the lowering of total homocysteine (tHcy) by 38–63% directly proportional to a decreased methionine intake (50–12.5% of normal). Supplemental betaine resulted in additional lowering of tHcy. OT-58 successfully competed with betaine and normalized tHcy on the background of reduced methionine intake, while substantially lowering tHcy in mice on normal methionine intake. Betaine was less effective in lowering tHcy on the background of normal or increased methionine intake, while exacerbating hypermethioninemia. OT-58 markedly reduced both hyperhomocysteinemia and hypermethioninemia caused by the diets and betaine in HCU mice. Withdrawal of betaine did not affect improved metabolic balance, which was established and solely maintained by OT-58 during periods of fluctuating dietary methionine intake. Taken together, OT-58 may represent novel, highly effective enzyme therapy for HCU performing optimally in the presence or absence of dietary management of HCU.

Cystathionine-β-Synthase: Molecular Regulation and Pharmacological Inhibition

Cystathionine-β-synthase (CBS), the first (and rate-limiting) enzyme in the transsulfuration pathway, is an important mammalian enzyme in health and disease. Its biochemical functions under physiological conditions include the metabolism of homocysteine (a cytotoxic molecule and cardiovascular risk factor) and the generation of hydrogen sulfide (H2S), a gaseous biological mediator with multiple regulatory roles in the vascular, nervous, and immune system. CBS is up-regulated in several diseases, including Down syndrome and many forms of cancer; in these conditions, the preclinical data indicate that inhibition or inactivation of CBS exerts beneficial effects. This article overviews the current information on the expression, tissue distribution, physiological roles, and biochemistry of CBS, followed by a comprehensive overview of direct and indirect approaches to inhibit the enzyme. Among the small-molecule CBS inhibitors, the review highlights the specificity and selectivity problems related to many of the commonly used "CBS inhibitors" (e.g., aminooxyacetic acid) and provides a comprehensive review of their pharmacological actions under physiological conditions and in various disease models.

Hydrogen sulfide regulates insulin secretion and insulin resistance in diabetes mellitus, a new promising target for diabetes mellitus treatment? A review

Background

Insulin resistance and impaired insulin secretion lead to disorders of glucose metabolism, which contributes to the development of diabetes. Hydrogen sulfide (H2S), a novel gasotransmitter, is found to play important roles in regulation of glucose metabolism homeostasis.

Aim of Review

This study aimed to summarize and discuss current data about the function of H2S in insulin secretion and insulin resistance regulation as well as the underlying mechanisms.

Key Scientific Concepts of Review

H₂S could be endogenously produced in islet β cells, liver, adipose, skeletal muscles, and the hypothalamus, and regulates local and systemic glucose metabolism. It is reported that H₂S suppresses insulin secretion, promotes or reduces the apoptosis of islet β cells. It plays important roles in the regulation of insulin sensitivity in insulin responsive tissues. H₂S inhibits glucose uptake and glycogen storage, and promotes or inhibits gluconeogenesis, mitochondrial biogenesis and mitochondrial bioenergetics in the liver. In adipose tissue, several investigators indicated that H2S promoted glucose uptake in adipocytes, while other studies reported that H₂S inhibits this process. H₂S has also been shown to promote adipogenesis, inhibit lipolysis, and regulate adiponectin and MCP-1 secretion from adipocytes. In skeletal muscle, H₂S increases glucose uptake and improves insulin sensitivity. It is also observed that H₂S modulates circadian-clock genes in muscle. Hypothalamic CBS/H₂S pathway reduces obesity and improves insulin sensitivity via the brain-adipose interaction. Most studies indicated plasma H₂S levels decreased in diabetic patients. However, the mechanisms by which H₂S regulates systemic glucose metabolism remain unclear. Whether H₂S acts as a new promising target for diabetes mellitus treatment merits further studies.

Treatment of Cystathionine β-Synthase Deficiency in Mice Using a Minicircle-Based Naked DNA Vector

Cystathionine β-synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by extremely elevated total homocysteine (tHcy) in the blood. Patients diagnosed with CBS deficiency have a variety of clinical problems, including dislocated lenses, osteoporosis, cognitive and behavioral issues, and a significantly increased risk of thrombosis. Current treatment strategies involve a combination of vitamin supplementation and restriction of foods containing the homocysteine precursor methionine. Here, a mouse model for CBS deficiency (Tg-I278T Cbs-/-) was used to evaluate the potential of minicircle-based naked DNA gene therapy to treat CBS deficiency. A 2.3 kb DNA-minicircle containing the liver-specific P3 promoter driving the human CBS cDNA (MC.P3-hCBS) was delivered into Tg-I278T Cbs-/- mice via a single hydrodynamic tail vein injection. Mean serum tHcy decreased from 351 μM before injection to 176 μM 7 days after injection (p = 0.0005), and remained decreased for at least 42 days. Western blot analysis reveals significant minicircle-directed CBS expression in the liver tissue. Liver CBS activity increased 34-fold (12.8 vs. 432 units; p = 0.0004) in MC.P3-hCBS-injected animals. Injection of MC.P3-hCBS in young mice, subsequently followed for 202 days, showed that the vector can ameliorate the mouse homocystinuria alopecia phenotype. The present findings show that minicircle-based gene therapy can lower tHcy in a mouse model of CBS deficiency.

Regulators of the transsulfuration pathway

The transsulfuration pathway is a metabolic pathway where transfer of sulfur from homocysteine to cysteine occurs. The pathway leads to the generation of several sulfur metabolites, which include cysteine, GSH and the gaseous signalling molecule hydrogen sulfide (H₂ S). Precise control of this pathway is critical for maintenance of optimal cellular function and, therefore, the key enzymes of the pathway, cystathionine β-synthase and cystathionine γ-lyase, are regulated at multiple levels. Disruption of the transsulfuration pathway contributes to the pathology of several conditions such as vascular dysfunction, Huntington's disease and during ageing. Treatment with donors of hydrogen sulfide and/or stimulation of this pathway have proved beneficial in several of these disorders. In this review, we focus on the regulation of the transsulfuration pathway pertaining to cysteine and H₂ S, which could be targeted to develop novel therapeutics. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Attenuation of neuro-inflammation improves survival and neurodegeneration in a mouse model of severe neonatal hyperbilirubinemia

All pre-term newborns and a high proportion of term newborns develop neonatal jaundice. Neonatal jaundice is usually a benign condition and self-resolves within few days after birth. However, a combination of unfavorable complications may lead to acute hyperbilirubinemia. Excessive hyperbilirubinemia may be toxic for the developing nervous system leading to severe neurological damage and death by kernicterus. Survivors show irreversible neurological deficits such as motor, sensitive and cognitive abnormalities. Current therapies rely on the use of phototherapy and, in unresponsive cases, exchange transfusion, which is performed only in specialized centers. During bilirubin-induced neurotoxicity different molecular pathways are activated, ranging from oxidative stress to endoplasmic reticulum (ER) stress response and inflammation, but the contribution of each pathway in the development of the disease still requires further investigation. Thus, to increase our understanding of the pathophysiology of bilirubin neurotoxicity, encephalopathy and kernicterus, we pharmacologically modulated neurodegeneration and neuroinflammation in a lethal mouse model of neonatal hyperbilirubinemia. Treatment of mutant mice with minocycline, a second-generation tetracycline with anti-inflammatory and neuroprotective properties, resulted in a dose-dependent rescue of lethality, due to reduction of neurodegeneration and neuroinflammation, without affecting plasma bilirubin levels. In particular, rescued mice showed normal motor-coordination capabilities and behavior, as determined by the accelerating rotarod and open field tests, respectively. From the molecular point of view, rescued mice showed a dose-dependent reduction in apoptosis of cerebellar neurons and improvement of dendritic arborization of Purkinje cells. Moreover, we observed a decrease of bilirubin-induced M1 microglia activation at the sites of damage with a reduction in oxidative and ER stress markers in these cells. Collectively, these data indicate that neurodegeneration and neuro-inflammation are key factors of bilirubin-induced neonatal lethality and neuro-behavioral abnormalities. We propose that the application of pharmacological treatments having anti-inflammatory and neuroprotective effects, to be used in combination with the current treatments, may significantly improve the management of acute neonatal hyperbilirubinemia, protecting from bilirubin-induced neurological damage and death.

Homocysteine regulates fatty acid and lipid metabolism in yeast

S-Adenosyl-l-homocysteine hydrolase (AdoHcy hydrolase; Sah1 in yeast/AHCY in mammals) degrades AdoHcy, a by-product and strong product inhibitor of S-adenosyl-l-methionine (AdoMet)-dependent methylation reactions, to adenosine and homocysteine (Hcy). This reaction is reversible, so any elevation of Hcy levels, such as in hyperhomocysteinemia (HHcy), drives the formation of AdoHcy, with detrimental consequences for cellular methylation reactions. HHcy, a pathological condition linked to cardiovascular and neurological disorders, as well as fatty liver among others, is associated with a deregulation of lipid metabolism. Here, we developed a yeast model of HHcy to identify mechanisms that dysregulate lipid metabolism. Hcy supplementation to wildtype cells up-regulated cellular fatty acid and triacylglycerol content and induced a shift in fatty acid composition, similar to changes observed in mutants lacking Sah1. Expression of the irreversible bacterial pathway for AdoHcy degradation in yeast allowed us to dissect the impact of AdoHcy accumulation on lipid metabolism from the impact of elevated Hcy. Expression of this pathway fully suppressed the growth deficit of sah1 mutants as well as the deregulation of lipid metabolism in both the sah1 mutant and Hcy-exposed wildtype, showing that AdoHcy accumulation mediates the deregulation of lipid metabolism in response to elevated Hcy in yeast. Furthermore, Hcy supplementation in yeast led to increased resistance to cerulenin, an inhibitor of fatty acid synthase, as well as to a concomitant decline of condensing enzymes involved in very long-chain fatty acid synthesis, in line with the observed shift in fatty acid content and composition.

Enzyme Replacement Therapy Ameliorates Multiple Symptoms of Murine Homocystinuria

Classical homocystinuria (HCU) is the most common inherited disorder of sulfur amino acid metabolism caused by deficiency in cystathionine beta-synthase (CBS) activity and characterized by severe elevation of homocysteine in blood and tissues. Treatment with dietary methionine restriction is not optimal, and poor compliance leads to serious complications. We developed an enzyme replacement therapy (ERT) and studied its efficacy in a severe form of HCU in mouse (the I278T model). Treatment was initiated before or after the onset of clinical symptoms in an effort to prevent or reverse the phenotype. ERT substantially reduced and sustained plasma homocysteine concentration at around 100 μM and normalized plasma cysteine for up to 9 months of treatment. Biochemical balance was also restored in the liver, kidney, and brain. Furthermore, ERT corrected liver glucose and lipid metabolism. The treatment prevented or reversed facial alopecia, fragile and lean phenotype, and low bone mass. In addition, structurally defective ciliary zonules in the eyes of I278T mice contained low density and/or broken fibers, while administration of ERT from birth partially rescued the ocular phenotype. In conclusion, ERT maintained an improved metabolic pattern and ameliorated many of the clinical complications in the I278T mouse model of HCU.

Enzyme replacement therapy prevents loss of bone and fat mass in murine homocystinuria

Skeletal and connective tissue defects are the most striking symptoms in patients suffering from classical homocystinuria (HCU). Here, we determined body composition and bone mass in three mouse models of HCU and assessed whether a long-term administration of enzyme replacement therapy (ERT) corrected the phenotype. The mouse models of HCU were analyzed using dual-energy X-ray absorptiometry and the data were complemented by plasma biochemical profiles. Both the mouse model lacking CBS (KO) and the one expressing human CBS mutant transgene on a mouse CBS null background (I278T) showed marked bone loss and decreased weight mostly due to a lower fat content compared with negative controls. In contrast, the HO mouse expressing the human CBS WT transgene on a mouse CBS null background showed no such phenotype despite similar plasma biochemical profile to the KO and I278T mice. More importantly, administration of ERT rescued bone mass and changes in body composition in the KO mice treated since birth and reversed bone loss and improved fat content in the I278T mice injected after the development of clinical symptoms. Our study suggests that ERT for HCU may represent an effective way of preventing the skeletal problems in patients without a restricted dietary regime.

Leptin concentrations and SCD-1 indices in classical homocystinuria: Evidence for the role of sulfur amino acids in the regulation of lipid metabolism

BACKGROUND

We describe body composition, lipid metabolism and Stearoyl-CoA desaturase-1 (SCD-1) indices in patients with classical homocystinuria (HCU).

METHODS

Eleven treated HCU patients and 16 healthy controls were included. Body composition and bone mineral density were assessed by dual X-ray absorptiometry. Sulfur amino acids (SAA) and their derivatives (total homocysteine, cysteine, methionine, S-adenosylmethionine, S-adenosylhomocysteine, and glutathione), lipids (free fatty acids, acylcarnitines, triglycerides and lipoproteins), glucose, insulin, leptin, adiponectin, and isoprostanes were measured in plasma. Insulin resistance was evaluated by HOMA-IR. To estimate liver SCD-1 activity, SCD-16 [16:1(n-7)/16:0] and SCD-18 [18:1(n-9)/18:0] desaturation indices were determined.

RESULTS

In HCU patients, SCD-16 index was significantly reduced (p=0.03). A trend of an association of SCD-16 index with cysteine was observed (r=0.624, p=0.054). HCU patients displayed lower lean mass (p<0.05), with no differences in fat mass percentage. Leptin and low-density lipoprotein concentrations were lower in HCU patients (p<0.05). Femur bone mineral density Z-scores were correlated with plasma cysteine (r=0.829; p=0.04) and total homocysteine (r=-0.829; p=0.04) in HCU patients.

CONCLUSIONS

We report alterations in leptin and SCD-1 in HCU patients. These results agree with previous findings from epidemiologic and animal studies, and support a role for SAA on lipid homeostasis.

Cystathionine β-synthase deficiency: Of mice and men

Cystathionine β-synthase (CBS) deficiency (Online Mendelian Inheritance in Man [OMIM] 236,200) is an autosomal recessive disorder that is caused by mutations in the CBS gene. It is the most common inborn error of sulfur metabolism and is the cause of classical homocystinuria, a condition characterized by very high levels of plasma total homocysteine and methionine. Although recognized as an inborn error of metabolism over 60years ago, these is still much we do not understand related to how this specific metabolic defect gives rise to its distinct phenotypes. To try and answer these questions, several groups have developed mouse models on CBS deficiency. In this article, we will review various mouse models of CBS deficiency and discuss how these mouse models compare to human CBS deficient patients.

Lack of global epigenetic methylation defects in CBS deficient mice

Cystathionine β-synthase (CBS) deficiency is a recessive inborn error of metabolism in which patients have extremely elevated plasma total homocysteine and have clinical manifestations in the vascular, visual, skeletal, and nervous systems. Homocysteine is an intermediary metabolite produced from the hydrolysis of S-adenosylhomocysteine (SAH), which is a by-product of methylation reactions involving the methyl-donor S-adenosylmethionine (SAM). Here, we have measured SAM, SAH, DNA and histone methylation status in an inducible mouse model of CBS deficiency to test the hypothesis that homocysteine-related phenotypes are caused by inhibition of methylation due to elevated SAH and reduced SAM/SAH ratio. We found that mice lacking CBS have elevated cellular SAH and reduced SAM/SAH ratios in both liver and kidney, but this was not associated with alterations in the level of 5-methylcytosine or various histone modifications. Using methylated DNA immunoprecipitation in combination with microarray, we found that of the 241 most differentially methylated promoter probes, 89 % were actually hypermethylated in CBS deficient mice. In addition, we did not find that changes in DNA methylation correlated well with changes in RNA expression in the livers of induced and uninduced CBS mice. Our data indicates that reduction in the SAM/SAH ratio, due to loss of CBS activity, does not result in overall hypomethylation of either DNA or histones.

Hydrogen Sulfide in the Adipose Tissue-Physiology, Pathology and a Target for Pharmacotherapy

Hydrogen sulfide (H₂S) is synthesized in the adipose tissue mainly by cystathionine γ-lyase (CSE). Several studies have demonstrated that H₂S is involved in adipogenesis, that is the differentiation of preadipocytes to adipocytes, most likely by inhibiting phosphodiesterases and increasing cyclic AMP concentration. The effect of H₂S on adipose tissue insulin sensitivity and glucose uptake is controversial. Some studies suggest that H₂S inhibits insulin-induced glucose uptake and that excess of H₂S contributes to adipose tissue insulin resistance in metabolic syndrome. In contrast, other studies have demonstrated that H₂S stimulates glucose uptake and its deficiency contributes to insulin resistance. Similarly, the effect of H₂S on adipose tissue lipolysis is controversial. H₂S produced by perivascular adipose tissue decreases vascular tone by activating ATP-sensitive and/or voltage-gated potassium channels in smooth muscle cells. Experimental obesity induced by high calorie diet has a time dependent effect on H₂S in perivascular adipose tissue; short and long-term obesity increase and decrease H₂S production, respectively. Hyperglycemia has been consistently demonstrated to suppress CSE-H₂S pathway in various adipose tissue depots. Finally, H₂S deficiency may contribute to adipose tissue inflammation associated with obesity/metabolic syndrome.

Homocysteine and disease: Causal associations or epiphenomenons?

Nutritional and genetic deficiencies of folate and vitamin B₁₂ lead to elevation of cellular homocysteine (Hcy), which translates in increased plasma Hcy. The sources and role of elevated plasma Hcy in pathology continues to be a subject of intense scientific debate. Whether a cause, mediator or marker, little is known about the molecular mechanisms and interactions of Hcy with cellular processes that lead to disease. The use of folic acid reduces the incidence of neural tube defects, but the effect of Hcy-lowering interventions with folic acid in cardiovascular disease and cognitive impairment remains controversial. The fact that levels of Hcy in plasma do not always reflect cellular status of this amino acid may account for the substantial gaps that exist between epidemiological, intervention and basic research studies. Understanding whether plasma Hcy is a mechanistic player or an epiphenomenon in pathogenesis requires further investigation, and this research is essential to improve the assessment and potential treatment of hyperhomocysteinemias.

H2S biosynthesis and catabolism: new insights from molecular studies

Hydrogen sulfide (H₂S) has profound biological effects within living organisms and is now increasingly being considered alongside other gaseous signalling molecules, such as nitric oxide (NO) and carbon monoxide (CO). Conventional use of pharmacological and molecular approaches has spawned a rapidly growing research field that has identified H₂S as playing a functional role in cell-signalling and post-translational modifications. Recently, a number of laboratories have reported the use of siRNA methodologies and genetic mouse models to mimic the loss of function of genes involved in the biosynthesis and degradation of H₂S within tissues. Studies utilising these systems are revealing new insights into the biology of H₂S within the cardiovascular system, inflammatory disease, and in cell signalling. In light of this work, the current review will describe recent advances in H₂S research made possible by the use of molecular approaches and genetic mouse models with perturbed capacities to generate or detoxify physiological levels of H₂S gas within tissues.

Exploring the Lean Phenotype of Glutathione-Depleted Mice: Thiol, Amino Acid and Fatty Acid Profiles

BACKGROUND

Although reduced glutathione (rGSH) is decreased in obese mice and humans, block of GSH synthesis by buthionine sulfoximine (BSO) results in a lean, insulin-sensitive phenotype. Data is lacking about the effect of BSO on GSH precursors, cysteine and glutamate. Plasma total cysteine (tCys) is positively associated with stearoyl-coenzyme A desaturase (SCD) activity and adiposity in humans and animal models.

OBJECTIVE

To explore the phenotype, amino acid and fatty acid profiles in BSO-treated mice.

DESIGN

Male C3H/HeH mice aged 11 weeks were fed a high-fat diet with or without BSO in drinking water (30 mmol/L) for 8 weeks. Amino acid and fatty acid changes were assessed, as well as food consumption, energy expenditure, locomotor activity, body composition and liver vacuolation (steatosis).

RESULTS

Despite higher food intake, BSO decreased particularly fat mass but also lean mass (both P<0.001), and prevented fatty liver vacuolation. Physical activity increased during the dark phase. BSO decreased plasma free fatty acids and enhanced insulin sensitivity. BSO did not alter liver rGSH, but decreased plasma total GSH (tGSH) and rGSH (by ~70%), and liver tGSH (by 82%). Glutamate accumulated in plasma and liver. Urine excretion of cysteine and its precursors was increased by BSO. tCys, rCys and cystine decreased in plasma (by 23-45%, P<0.001 for all), but were maintained in liver, at the expense of decreased taurine. Free and total plasma concentrations of the SCD products, oleic and palmitoleic acids were decreased (by 27-38%, P <0.001 for all).

CONCLUSION

Counterintuitively, block of GSH synthesis decreases circulating tCys, raising the question of whether the BSO-induced obesity-resistance is linked to cysteine depletion. Cysteine-supplementation of BSO-treated mice is warranted to dissect the effects of cysteine and GSH depletion on energy metabolism.

Adipose tissue-liver axis in alcoholic liver disease

Alcoholic liver disease (ALD) remains an important health problem worldwide. The disease spectrum is featured by early steatosis, steatohepatitis (steatosis with inflammatory cells infiltration and necrosis), with some individuals ultimately progressing to fibrosis/cirrhosis. Although the disease progression is well characterized, no effective therapies are currently available for the treatment in humans. The mechanisms underlying the initiation and progression of ALD are multifactorial and complex. Emerging evidence supports that adipose tissue dysfunction contributes to the pathogenesis of ALD. In the first part of this review, we discuss the mechanisms whereby chronic alcohol exposure contributed to adipose tissue dysfunction, including cell death, inflammation and insulin resistance. It has been long known that aberrant hepatic methionine metabolism is a major metabolic abnormality induced by chronic alcohol exposure and plays an etiological role in the pathogenesis of ALD. The recent studies in our group documented the similar metabolic effect of chronic alcohol drinking on methionine in adipose tissue. In the second part of this review, we also briefly discuss the recent research progress in the field with a focus on how abnormal methionine metabolism in adipose tissue contributes to adipose tissue dysfunction and liver damage.

Betaine supplementation is less effective than methionine restriction in correcting phenotypes of CBS deficient mice

Cystathionine beta synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by elevated serum total homocysteine (tHcy). Betaine supplementation, which can lower tHcy by stimulating homocysteine remethylation to methionine, is often given to CBS deficient patients in combination with other treatments such as methionine restriction and supplemental B-vitamins. However, the effectiveness of betaine supplementation by itself in the treatment of CBS deficiency has not been well explored. Here, we have examined the effect of a betaine supplemented diet on the Tg-I278T Cbs (-/-) mouse model of CBS deficiency and compared its effectiveness to our previously published data using a methionine restricted diet. Tg-I278T Cbs (-/-) mice on betaine, from the time of weaning until for 240 days of age, had a 40 % decrease in mean tHcy level and a 137 % increase in serum methionine levels. Betaine-treated Tg-I278T Cbs (-/-) mice also exhibited increased levels of betaine-dependent homocysteine methyl transferase (BHMT), increased levels of the lipogenic enzyme stearoyl-coenzyme A desaturase (SCD-1), and increased lipid droplet accumulation in the liver. Betaine supplementation largely reversed the hair loss phenotype in Tg-I278T Cbs (-/-) animals, but was far less effective than methionine restriction in reversing the weight-loss, fat-loss, and osteoporosis phenotypes. Surprisingly, betaine supplementation had several negative effects in control Tg-I278T Cbs (+/-) mice including decreased weight gain, lean mass, and bone mineral density. Our findings indicate that while betaine supplementation does have some beneficial effects, it is not as effective as methionine restriction for reversing the phenotypes associated with severe CBS deficiency in mice.

The effect of dietary modulation of sulfur amino acids on cystathionine β synthase-deficient mice

Cystathionine β synthase (CBS) is a key enzyme in the methionine and cysteine metabolic pathway, acting as a metabolic gatekeeper to regulate the flow of fixed sulfur from methionine to cysteine. Mutations in the CBS gene cause clinical CBS deficiency, a disease characterized by elevated plasma total homocysteine (tHcy) and methionine and decreased plasma cysteine. The treatment goal for CBS-deficient patients is to normalize the metabolic values of these three metabolites using a combination of vitamin therapy and dietary manipulation. To better understand the effectiveness of nutritional treatment strategies, we have performed a series of long-term dietary manipulation studies using our previously developed Tg-I278T Cbs(-/-) mouse model of CBS deficiency and sibling Tg-I278T Cbs(+/-) controls. Tg-I278T Cbs(-/-) mice have undetectable levels of CBS activity, extremely elevated plasma tHcy, modestly elevated plasma methionine, and low plasma cysteine. They exhibit several easily assayable phenotypes, including osteoporosis, loss of fat mass, reduced life span, and facial alopecia. The diets used in these studies differed in the amounts of sulfur amino acids or sulfur amino acid precursors. In this review, we will discuss our findings and their relevance to CBS deficiency and the concept of gene-diet interaction.

Cysteine and hydrogen sulphide in the regulation of metabolism: insights from genetics and pharmacology

Obesity and diabetes represent a significant and escalating worldwide health burden. These conditions are characterized by abnormal nutrient homeostasis. One such perturbation is altered metabolism of the sulphur‐containing amino acid cysteine. Obesity is associated with elevated plasma cysteine, whereas diabetes is associated with reduced cysteine levels. One mechanism by which cysteine may act is through its enzymatic breakdown to produce hydrogen sulphide (H₂S), a gasotransmitter that regulates glucose and lipid homeostasis. Here we review evidence from both pharmacological studies and transgenic models suggesting that cysteine and hydrogen sulphide play a role in the metabolic dysregulation underpinning obesity and diabetes. We then outline the growing evidence that regulation of hydrogen sulphide levels through its catabolism can impact metabolic health. By integrating hydrogen sulphide production and breakdown pathways, we re‐assess current hypothetical models of cysteine and hydrogen sulphide metabolism, offering new insight into their roles in the pathogenesis of obesity and diabetes. © 2015 The Authors. Pathological Society of Great Britain and Ireland.

Stearoyl-CoA Desaturase-1: Is It the Link between Sulfur Amino Acids and Lipid Metabolism?

An association between sulfur amino acids (methionine, cysteine, homocysteine and taurine) and lipid metabolism has been described in several experimental and population-based studies. Changes in the metabolism of these amino acids influence serum lipoprotein concentrations, although the underlying mechanisms are still poorly understood. However, recent evidence has suggested that the enzyme stearoyl-CoA desaturase-1 (SCD-1) may be the link between these two metabolic pathways. SCD-1 is a key enzyme for the synthesis of monounsaturated fatty acids. Its main substrates C16:0 and C18:0 and products palmitoleic acid (C16:1) and oleic acid (C18:1) are the most abundant fatty acids in triglycerides, cholesterol esters and membrane phospholipids. A significant suppression of SCD-1 has been observed in several animal models with disrupted sulfur amino acid metabolism, and the activity of SCD-1 is also associated with the levels of these amino acids in humans. This enzyme also appears to be involved in the etiology of metabolic syndromes because its suppression results in decreased fat deposits (regardless of food intake), improved insulin sensitivity and higher basal energy expenditure. Interestingly, this anti-obesogenic phenotype has also been described in humans and animals with sulfur amino acid disorders, which is consistent with the hypothesis that SCD-1 activity is influenced by these amino acids, in particularly cysteine, which is a strong and independent predictor of SCD-1 activity and fat storage. In this narrative review, we discuss the evidence linking sulfur amino acids, SCD-1 and lipid metabolism.

Body composition in gene knockouts of sulfur amino acid-metabolizing enzymes

Plasma concentrations of several amino acids are elevated in human obesity and insulin resistance, but there is no conclusive evidence on whether the amino acid alterations are causal. Dietary restriction of the essential SAA methionine (MR) in rats produces a hypermetabolic phenotype, with an integrated set of transcriptional changes in lipid enzymes in liver and adipose tissue. MR also induces an array of changes in methionine metabolites, including elevated plasma homocysteine and decreased cystathionine, cysteine, glutathione, and taurine. Several knockouts of enzymes acting downstream of methionine recapitulate the phenotypic results of MR, suggesting that the MR phenotype may be driven by changes distal to methionine. Here we review the changes in SAA and body composition in seven relevant knockout mouse models. All seven models feature decreased body weight, which in five of these have been further explored and shown to result from predominantly decreased fat mass. Common to several models is increased energy expenditure, enhanced insulin sensitivity, and protection against dietary obesity, as occurs in MR. A decrease in plasma total cysteine concentrations is also seen in most models. The lean phenotype could often be reversed by dietary supplementation of cysteine or choline, but not taurine, betaine or a H2S donor. Importantly, the plasma concentrations of both cysteine and choline are positively associated with fat mass in large populations studies, while taurine, betaine, and H2S are not. Collectively, the emerging data from dietary and knockout models are in harmony with human epidemiologic data, suggesting that the availability of key nutrients in the SAA pathway regulates fat storage pathways.

Body composition in patients with classical homocystinuria: body mass relates to homocysteine and choline metabolism

INTRODUCTION

Classical homocystinuria is a rare genetic disease caused by cystathionine β-synthase deficiency, resulting in homocysteine accumulation. Growing evidence suggests that reduced fat mass in patients with classical homocystinuria may be associated with alterations in choline and homocysteine pathways. This study aimed to evaluate the body composition of patients with classical homocystinuria, identifying changes in body fat percentage and correlating findings with biochemical markers of homocysteine and choline pathways, lipoprotein levels and bone mineral density (BMD) T-scores.

METHODS

Nine patients with classical homocystinuria were included in the study. Levels of homocysteine, methionine, cysteine, choline, betaine, dimethylglycine and ethanolamine were determined. Body composition was assessed by bioelectrical impedance analysis (BIA) in patients and in 18 controls. Data on the last BMD measurement and lipoprotein profile were obtained from medical records.

RESULTS

Of 9 patients, 4 (44%) had a low body fat percentage, but no statistically significant differences were found between patients and controls. Homocysteine and methionine levels were negatively correlated with body mass index (BMI), while cysteine showed a positive correlation with BMI (p<0.05). There was a trend between total choline levels and body fat percentage (r=0.439, p=0.07). HDL cholesterol correlated with choline and ethanolamine levels (r=0.757, p=0.049; r=0.847, p=0.016, respectively), and total cholesterol also correlated with choline levels (r=0.775, p=0.041). There was no association between BMD T-scores and body composition.

CONCLUSIONS

These results suggest that reduced fat mass is common in patients with classical homocystinuria, and that alterations in homocysteine and choline pathways affect body mass and lipid metabolism.

Epigenetic regulation of aortic remodeling in hyperhomocysteinemia

Hyperhomocysteinemia (HHcy) is prevalent in patients with hypertension and is an independent risk factor for aortic pathologies. HHcy is known to cause an imbalance between matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), leading to the accumulation of collagen in the aorta and resulting in stiffness and development of hypertension. Although the exact mechanism of extracellular matrix (ECM) remodeling is unclear, emerging evidence implicates epigenetic regulation involving DNA methylation. Our purpose was to investigate whether 5-aza-2'-deoxycytidine (Aza), a DNA methyltransferase (DNMT1) inhibitor, reduces high blood pressure (BP) by regulating aortic ECM remodeling in HHcy. Wild-type and cystathionine β-synthase (CBS)(+/-) HHcy mice were treated with Aza (0.5 mg/kg body weight). In HHcy mice, Aza treatment normalized the plasma homocysteine (Hcy) level and BP. Thoracic and abdominal aorta ultrasound revealed a reduction in the resistive index and wall-to-lumen ratio. Vascular response to phenylephrine, acetylcholine, and sodium nitroprusside improved after Aza in HHcy mice. Histology showed a marked reduction in collagen deposition in the aorta. Aza treatment decreased the expression of DNMT1, MMP9, TIMP1, and S-adenosyl homocysteine hydrolase (SAHH) and upregulated methylene tetrahydrofolate reductase (MTHFR). We conclude that reduction of DNA methylation by Aza in HHcy reduces adverse aortic remodeling to mitigate hypertension.

Protein arginine hypomethylation in a mouse model of cystathionine β-synthase deficiency

Accumulation of the homocysteine (Hcy) precursor S-adenosylhomocysteine (AdoHcy) may cause cellular hypomethylation in the setting of hyperhomocysteinemia because of cystathionine β-synthase (CBS) deficiency, an inborn error of metabolism. To test this hypothesis, DNA and protein arginine methylation status were assessed in liver, brain, heart, and kidney obtained from a previously described mouse model of CBS deficiency. Metabolite levels in tissues and serum were determined by high-performance liquid chromatography or liquid chromatography-electrospray ionization-tandem mass spectrometry. Global DNA and protein arginine methylation status were evaluated as the contents of 5-methyldeoxycytidine in DNA and of methylarginines in proteins, respectively. In addition, histone arginine methylation was assessed by Western blotting. CBS-deficient mice exhibited increased (>6-fold) Hcy and AdoHcy levels in all tissues examined compared with control levels. In addition, global DNA methylation status was not affected, but global protein arginine methylation status was decreased (10-35%) in liver and brain. Moreover, asymmetric dimethylation of arginine 3 on histone H4 (H4R3me2a) content was markedly decreased in liver, and no differences were observed for the other histone arginine methylation marks examined. Our results show that CBS-deficient mice present severe accumulation of tissue Hcy and AdoHcy, protein arginine hypomethylation in liver and brain, and decreased H4R3me2a content in liver. Therefore, protein arginine hypomethylation arises as a potential player in the pathophysiology of CBS deficiency.

S-adenosylmethionine is associated with fat mass and truncal adiposity in older adults

S-adenosylmethionine (SAM) is synthesized from methionine, which is abundant in animal-derived protein, in an energy-consuming reaction. SAM and S-adenosylhomocysteine (SAH) correlate with body mass index (BMI). Plasma total concentration of the SAM-associated product cysteine (tCys) correlates with fat mass in humans and cysteine promotes adiposity in animals. In a cross-sectional study of 610 participants, we investigated whether SAM and SAH are associated with BMI via lean mass or fat mass and dietary protein sources as determinants of SAM and tCys concentrations. Plasma SAM was not associated with lean mass, but mean adjusted fat mass increased from 24 kg (95% CI: 22.6, 25.1) to 30 kg (95% CI: 28.7, 31.3) across SAM quartiles (P < 0.001) and trunk fat:total fat ratio increased from 0.48 to 0.52 (P < 0.001). Erythrocyte SAM was also positively associated with fat mass and trunk fat:total fat ratio. The association of SAM with fat mass was not weakened by adjustment for serum tCys, lipids, creatinine, or dietary or lifestyle confounders. Concentrations of the SAM precursor, methionine, and the SAM product, SAH, were not independently associated with adiposity. Intake of animal-derived protein was not related to serum methionine but was positively associated with plasma SAM (partial r = 0.11) and serum tCys (partial r = 0.13; P < 0.05 for both after adjustment for age, gender, and total energy intake). In conclusion, plasma SAM, but not methionine, is independently associated with fat mass and truncal adiposity, suggesting increased conversion of methionine to SAM in obese individuals. Prospective studies are needed to investigate the interactions among dietary energy and animal protein content, SAM concentrations, and change in body weight and cardiometabolic risk.

Cystathionine β-synthase-deficient mice thrive on a low-methionine diet

Cystathionine β-synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by elevated serum total homocysteine (tHcy). Previously, our laboratory developed a mouse model of CBS deficiency, TgI278T Cbs(-)/(-) (abbreviated as Cbs(-/-)), characterized by low weight, low adiposity, decreased Scd-1 expression, facial alopecia, and osteoporosis. To determine the potential benefit of a methionine-restricted diet (MRD), we fed Cbs(-/-) and Cbs(+/-) control mice either an MRD or a regular diet (RD) from weaning till 240 d of age. Cbs(-/-) mice fed the MRD had a 77% decrease in tHcy, 28% increase in weight, 130% increase in fat mass, 82% increase in Scd-1 expression, and 10.6% increase in bone density and entirely lacked the alopecia phenotype observed in age-matched Cbs(-/-) mice fed the RD. At the end of the study, Cbs(-/-) mice fed the MRD were phenotypically indistinguishable from Cbs(+/-) mice fed the RD. Notably, whereas the MRD diet was highly beneficial to Cbs(-/-) mice, it had nearly opposite effect on Cbs(+/-) mice. These studies show that a low-methionine diet can correct the phenotypic consequences of loss of CBS and provide a striking example of how genotype and diet can interact to influence phenotype in mammals.

Plasma sulfur amino acids and stearoyl-CoA desaturase activity in two Caucasian populations

In rats, dietary restriction of the cysteine precursor methionine suppresses hepatic stearoyl-CoA desaturase (SCD)-1 expression and activity, whereas cysteine supplementation reverses these effects. In 2 independent cohorts: Hordaland Health Study (HUSK; N=2021, aged 71-74y), Norway, and Hoorn study (N=686, aged 50-87y), Netherlands, we examined the cross-sectional associations of plasma sulfur-containing compounds (SCC; methionine, S-adenosylmethionine, S-adenosylhomocysteine, homocysteine, cystathionine, total cysteine (tCys), glutathione and cysteinylglycine) with SCD-16 index (16:1n-7/16:0), estimated from fatty acid profiles of total plasma or serum lipids. Only tCys was consistently associated with SCD-16 index after adjustments for sex and age (HUSK: partial r=0.14; Hoorn: partial r=0.11, P<0.001 for both), and after further adjustments for other SCC, body fat, diet, exercise and plasma lipids (HUSK: partial r=0.07, P=0.004; Hoorn: partial r=0.12, P=0.013). Together with animal data showing an effect of dietary cysteine on SCD1, our results suggest a role for cysteine in SCD1 regulation in humans.

Identification of p130Cas/ErbB2-dependent invasive signatures in transformed mammary epithelial cells

Understanding transcriptional changes during cancer progression is of crucial importance to develop new and more efficacious diagnostic and therapeutic approaches. It is well known that ErbB2 is overexpressed in about 25% of human invasive breast cancers. We have previously demonstrated that p130Cas overexpression synergizes with ErbB2 in mammary cell transformation and promotes ErbB2-dependent invasion in three-dimensional (3D) cultures of human mammary epithelial cells. Here, by comparing coding and non-coding gene expression profiles, we define the invasive signatures associated with concomitant p130Cas overexpression and ErbB2 activation in 3D cultures of mammary epithelial cells. Specifically, we have found that genes involved in amino acids synthesis (CBS, PHGDH), cell motility, migration (ITPKA, PRDM1), and angiogenesis (HEY1) are upregulated, while genes involved in inflammatory response (SAA1, S100A7) are downregulated. In parallel, we have shown that the expression of specific miRNAs is altered. Among these, miR-200b, miR-222, miR-221, miR-R210, and miR-424 are upregulated, while miR-27a, miR-27b, and miR-23b are downregulated. Overall, this study presents, for the first time, the gene expression changes underlying the invasive behavior following p130Cas overexpression in an ErbB2 transformed mammary cell model.

Effect of taurine and N-acetylcysteine on methionine restriction-mediated adiposity resistance

OBJECTIVES

Methionine-restricted (MR) rats, which are lean and insulin sensitive, have low serum total cysteine (tCys) and taurine and decreased hepatic expression and activity indices of stearoyl-coenzyme A desaturase-1 (SCD1). These effects are partly or completely reversed by cysteine supplementation. We investigated whether reversal of MR phenotypes can be achieved by other sulfur compounds, namely taurine or N-acetylcysteine (NAC).

METHODS

MR and control-fed (CF) rats were supplemented with taurine (0.5%) or NAC (0.5%) for 12weeks. Adiposity, serum sulfur amino acids (SAA), Scd1 gene expression in liver and white adipose tissue, and SCD1 activity indices (calculated from serum fatty acid profile) were monitored.

RESULTS

Taurine supplementation of MR rats did not restore weight gain or hepatic Scd1 expression or indices to CF levels, but further decreased adiposity. Taurine supplementation of CF rats did not affect adiposity, but lowered triglyceridemia. NAC supplementation in MR rats raised tCys and partly or completely reversed MR effects on weight, fat %, Scd1 expression in liver and white adipose tissue, and estimated SCD1 activity. In CF rats, NAC decreased body fat % and lowered SCD1-18 activity index (P<0.001). Serum triglycerides and leptin were over 40% lower in CF+NAC relative to CF rats (P≤0.003 for both). In all groups, change in tCys correlated with change in SCD1-16 index (partial r=0.60, P<0.001) independent of other SAA.

CONCLUSION

The results rule out taurine as a mediator of increased adiposity produced by cysteine in MR, and show that NAC, similar to L-cysteine, blocks anti-obesity effects of MR. Our data show that dietary SAA can influence adiposity in part through mechanisms that converge on SCD1 function. This may have implications for understanding and preventing human obesity.