Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:4.2.1.22 (cystathionine beta-synthase)
 965 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Homocysteine is a key junction metabolite in methionine metabolism. It suffers two major metabolic fates: transmethylation catalyzed by methionine synthase or betaine homocysteine methyl transferase and transsulfuration catalyzed by cystathionine beta-synthase leading to cystathionine. The latter is subsequently converted to cysteine, a precursor of glutathione. Studies with purified mammalian methionine synthase and cystathionine beta-synthase have revealed the oxidative sensitivity of both junction enzymes, suggesting the hypothesis that redox regulation of this pathway may be physiologically significant. This hypothesis has been tested in a human hepatoma cell line in culture in which the flux of homocysteine through transsulfuration under normoxic and oxidative conditions has been examined. Addition of 100 microM H(2)O(2) or tertiary butyl hydroperoxide increased cystathionine production 1.6- and 2.1-fold from 82 +/- 7 micromol h(-)(1) (L of cells)(-)(1) to 136 +/- 15 and 172 +/- 23 micromol h(-)(1) (L of cells)(-)(1), respectively. The increase in homocysteine flux through the transsulfuration pathway exhibited a linear dose dependence on the concentrations of both oxidants (50-200 microM H(2)O(2) and 10-200 microM tertiary butyl hydroperoxide). Furthermore, our results reveal that approximately half of the intracellular glutathione pool in human liver cells is derived from homocysteine via the transsulfuration pathway. The redox sensitivity of the transsulfuration pathway can be rationalized as an autocorrective response that leads to an increased level of glutathione synthesis in cells challenged by oxidative stress. In summary, this study demonstrates the importance of the homocysteine-dependent transsulfuration pathway in the maintenance of the intracellular glutathione pool, and the regulation of this pathway under oxidative stress conditions. Aberrations in this pathway could compromise the redox buffering capacity of cells, which may in turn be related to the pathophysiology of the different homocysteine-related diseases.
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PMID:The quantitatively important relationship between homocysteine metabolism and glutathione synthesis by the transsulfuration pathway and its regulation by redox changes. 1104 66

Homocysteine metabolism is altered in diabetic patients. Cystathionine beta-synthase (CBS), a key enzyme involved in the transsulfuration pathway, which irreversibly converts homocysteine to cysteine, catalyzes the condensation of serine and homocysteine to cystathionine. Studies in streptozotocin-induced diabetic rats have shown that CBS enzyme activity is elevated in the liver but not in the kidney, and this effect is reversed by insulin treatment. To determine whether these effects resulted from alterations at the level of gene transcription, CBS mRNA was measured in diabetic and insulin-treated diabetic rats. CBS mRNA levels were found to be markedly higher in streptozotocin-induced diabetic rat livers; these were reduced by insulin administration. In H4IIE cells, a rat hepatoma cell culture model, glucocorticoids increased the cellular levels of CBS enzyme protein and CBS mRNA; insulin inhibited this stimulatory effect. Treatment with insulin also decreased CBS levels in HepG2 cells, a human hepatoma cell line. Nuclear run-on experiments in the rat cells confirmed that stimulation of CBS gene expression by glucocorticoids and the inhibition by insulin occurred at the transcriptional level. Transient transfections of HepG2 cells with a CBS-1b promoter luciferase reporter construct showed that the promoter activity was decreased by 70% after insulin treatment. These results show that insulin has a direct role in regulating homocysteine metabolism. Altered insulin levels in diseases such as diabetes may influence homocysteine metabolism by regulating the hepatic transsulfuration pathway.
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PMID:Hormonal regulation of cystathionine beta-synthase expression in liver. 1219 28

Metallothionein (MT) promoter was methylated in rat hepatoma and in mouse lymphosarcoma cells by methylation of cytosine within the CpG dinucleotide region. After demethylation of MT-I promoter in mouse lymphosarcoma cells or in the transplanted rat hepatoma with 5-azacytidine, a potent inhibitor of DNA methyltransferase, the promoter was activated in response to heavy metal treatment. MT-I promoter was also suppressed in human prostate cancer lines PC3 and DU145, probably by promoter methylation, whereas cadmium induced MT-I in the human prostate cancer line LNCaP. In the prostate cancer lines where MT-I was suppressed, glutathione-S-transferase-pi (GST-pi) was expressed. On the contrary, GST-pi gene was repressed in the cell line where MT-I was induced, which suggests an inverse relationship between MT-I induction and GST-pi expression in some prostate cancer lines. The expressions of GST-pi and gamma-glutamyl cysteine synthase were also significantly higher (5- to 12-fold) in the lymphosarcoma cells and the hepatoma relative to the parental tissues. The higher expressions of these two genes suggest a compensatory mechanism in the cells where the gene for the antioxidant MT-I/II is not induced. MT-I/II may function as a growth suppressor either alone or in concert with other factor(s), and consequently their lack of expression could facilitate the tumor growth. In addition to suppression of MT-I/II expression by promoter methylation, the lack of MT induction could also be brought about by nuclear factor I (NFI), probably by interaction with the metal transcription factor MTF-1. An inverse relationship was observed between the level of NFI and MT-I expression in some cells, which suggests a role for NFI in the relatively low constitutive levels of MT-I expression in these cells.
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PMID:Suppression of metallothionein-I/II expression and its probable molecular mechanisms. 1242 40

In certain tissues, glutathione biosynthesis is connected to methionine metabolism via the trans-sulfuration pathway. The latter condenses homocysteine and serine to cystathionine in a reaction catalyzed by cystathionine beta-synthase followed by cleavage of cystathionine to cysteine and alpha-ketoglutarate by gamma-cystathionase. Cysteine is the limiting amino acid in glutathione biosynthesis, and studies in our laboratory have shown that approximately 50% of the cysteine in glutathione is derived from homocysteine in human liver cells. In this study, we have examined the effect of pro- and antioxidants on the flux of homocysteine through the trans-sulfuration pathway in the human hepatoma cell line, HepG2. Our studies reveal that pyrrolidine dithiocarbamate and butylated hydroxyanisole enhance the flux of homocysteine through the trans-sulfuration pathway as has been observed previously with the pro-oxidants, H(2)O(2) and tertiary butyl hydroperoxide. In contrast, antioxidants such as catalase, superoxide dismutase and a water-soluble derivative of vitamin E elicit the opposite effect and result in diminished flux of homocysteine through the trans-sulfuration pathway. These studies provide the first evidence for the reciprocal sensitivity of the trans-sulfuration pathway to pro- and antioxidants, and demonstrate that the upstream half of the glutathione biosynthetic pathway (i.e. leading to cysteine biosynthesis) is redox sensitive as is the regulation of the well-studied enzymes in the downstream half (leading from cysteine to glutathione), namely, gamma-glutamyl-cysteine ligase and glutathione synthetase.
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PMID:Redox regulation of homocysteine-dependent glutathione synthesis. 1263 46

Derangements in methionine metabolism are a hallmark of cancers and homocystinuria, an inborn error of metabolism. In this study, the metabolic consequences of the pathological changes associated with the key pathway enzymes, methionine adenosyl transferase (MAT), glycine N-methyl transferase (GNMT) and cystathionine beta-synthase (CBS) as well as an activation of polyamine metabolism, were analyzed using a simple mathematical model describing methionine metabolism in liver. The model predicts that the mere loss of allosteric regulation of CBS by adenosylmethionine (AdoMet) leads to an increase in homocysteine concentration. This is consistent with the experimental data on the corresponding genetic defects, which specifically impair allosteric activation but not basal enzyme activity. Application of the characteristics of transformed hepatocytes to our model, i.e., substitution of the MAT I/III isozyme by MAT II, loss of GNMT activity and activation of polyamine biosynthesis, leads to the prediction of a significantly different dependence of methionine metabolism on methionine concentrations. The theoretical predictions were found to be in good agreement with experimental data obtained with the human hepatoma cell line, HepG2.
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PMID:Analysis of pathological defects in methionine metabolism using a simple mathematical model. 1596 1

The transsulfuration pathway converts homocysteine to cysteine and represents the metabolic link between antioxidant and methylation metabolism. The first and committing step in this pathway is catalyzed by cystathionine beta-synthase (CBS), which is subject to complex regulation, including allosteric activation by the methyl donor, S-adenosylmethionine (AdoMet). In this study, we demonstrate that methionine restriction leads to a >10-fold decrease in CBS protein levels, and pulse proteolysis studies reveal that binding of AdoMet stabilizes the protein against degradation by approximately 12 kcal/mol. These observations predict that under pathological conditions where AdoMet levels are diminished, CBS, and therefore glutathione levels, will be reduced. Indeed, we demonstrate this to be the case in a mouse model for spontaneous steatohepatitis in which the gene for the MAT1A isoenzyme encoding AdoMet synthetase has been disrupted, and in human hepatocellular carcinoma, where MAT1A is silenced. Furthermore, diminished CBS levels are associated with reduced cell viability in hepatoma cells challenged with tert-butyl hydroperoxide. This study uncovers a mechanism by which CBS is allosterically activated by AdoMet under normal conditions but is destabilized under pathological conditions, for redirecting the metabolic flux toward methionine conservation. A mechanistic basis for the coordinate changes in redox and methylation metabolism that are a hallmark of several complex diseases is explained by these observations.
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PMID:S-adenosylmethionine stabilizes cystathionine beta-synthase and modulates redox capacity. 1661 71

Chronic exposure to elevated levels of free fatty acids (FFAs) has been shown to cause cell death (lipotoxicity), but the underlying mechanisms of lipotoxicity in hepatocytes remain unclear. We have previously shown that the saturated FFAs cause much greater toxicity to human hepatoma cells (HepG2) than the unsaturated ones (Srivastava and Chan, 2007). In this study, metabolic flux analysis (MFA) was applied to identify the metabolic changes associated with the cytotoxicity of saturated FFA. Measurements of the fluxes revealed that the saturated FFA, palmitate, was oxidized to a greater extent than the non-toxic oleate and had comparatively less triglyceride synthesis and reduced cystine uptake. Although fatty acid oxidation had a high positive correlation to the cytotoxicity, inhibitor experiments indicated that the cytotoxicity was not due to the higher fatty acid oxidation. Application of MFA revealed that cells exposed to palmitate also had a consistently reduced flux of glutathione (GSH) synthesis but greater de novo ceramide synthesis. These predictions were experimentally confirmed. In silico sensitivity analyses identified that the GSH synthesis was limited by the uptake of cysteine. Western blot analyses revealed that the levels of the cystine transporter xCT, but not that of the GSH-synthesis enzyme glutamyl-cysteine synthase (GCS), were reduced in the palmitate cultures, suggesting the limitation of cysteine import as the cause of the reduced GSH synthesis. Finally, supplementing with N-acetyl L-cysteine (NAC), a cysteine-provider whose uptake does not depend on xCT levels, reduced the FFA-toxicity significantly. Thus, the metabolic alterations that contributed to the toxicity and suggested treatments to reduce the toxicity were identified, which were experimentally validated.
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PMID:Application of metabolic flux analysis to identify the mechanisms of free fatty acid toxicity to human hepatoma cell line. 1761 59

The cystathionine beta-synthase (CBS) gene encodes an enzyme that catalyzes the synthesis of cystathionine in the trans-sulfuration pathway and is subject to tight regulation because of its critical role in antioxidant and methylation metabolism. The expression level of CBS in 120 hepatocellular carcinoma (HCC) specimens evaluated by real-time reverse transcriptase PCR (RT-PCR) is markedly lower than in surrounding non-cancerous liver (P<0.0001). The correlation between CBS gene expression in HCC and clinicopathological parameters or survival of HCC patients was statistically analyzed in the present study. Our study demonstrated that reduced CBS expression is significantly correlated with high tumor stage (P=0.0019), high Edmondson grade (P=0.00084), and high AFP level (P=6.2x10(-5)). Interestingly, a survival analysis showed that a significantly shorter overall survival (OS) time is observed in patients with reduced CBS expression (P=0.0022), although CBS expression was determined not to be an independent prognostic factor for OS (P=0.071) after considering tumor stage, tumor size, and AFP level. However, for the 62 patients with low AFP levels (<100 ng/ml), reduced CBS expression was found to be an independent prognostic factor for OS (P=0.0042) after considering tumor stage and tumor size. Thus, the expression level of CBS mRNA could be useful to predict clinical outcome of HCC, especially for patients with low AFP levels.
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PMID:Expression of cystathionine beta-synthase is downregulated in hepatocellular carcinoma and associated with poor prognosis. 1942 22