Hormonal Regulation of Cystathionine β-Synthase Expression in Liver

Abstract

Homocysteine metabolism is altered in diabetic patients. Cystathionine ␤-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.Cystathionine ␤-synthase (CBS) 1 (EC 4.2.1.22) catalyzes the first committed step in cysteine biosynthesis, the irreversible condensation of homocysteine with serine to form cystathionine (1). Homocysteine, a sulfur-containing nonprotein amino acid, is an intermediate in the metabolism of methionine. It is at a metabolic crossroads between its synthesis from methionine and its removal through the transsulfuration or remethylation pathways (2). Two pyridoxal 5Ј-phosphate-dependent enzymes comprise the transsulfuration pathway: CBS, which catalyzes the condensation of serine and homocysteine to cystathionine, and cystathionine ␥-lyase, which catalyzes the formation of cysteine, ␣-ketobutyrate, and ammonia (3, 4). The regulation of CBS gene expression is important in a number of physiological situations. Feeding rats a high protein diet or a high methionine diet increases CBS activity (5). It is also known that flux through the transsulfuration pathway provides cysteine for glutathione synthesis, so that altered CBS levels may be of importance in oxidative stress (6). The well known sparing effect of cysteine on methionine requirements is mediated, at least in part, via alterations in CBS activity. Our own recent work shows that glucagon administration to rats increases hepatic CBS enzyme activity and mRNA levels (7). CBS enzyme deficiency, an autosomal recessively inherited disorder, is the leading cause of homocystinuria. Partial d...

Elevated Homocysteine as a Risk Factor for the Development of Diabetes in Women With a Previous History of Gestational Diabetes Mellitus

“…The level of hepatic cystathionine ␤-synthase, a transmethylation enzyme that metabolizes conversion of homocysteine to cystathione (36), was elevated and plasma homocysteine reduced in untreated, insulin-deficient diabetic rats, and the defects were normalized by treatment with insulin. Furthermore, cystathionine ␤-synthase mRNA levels were markedly elevated in streptozotocininduced diabetic rat livers and reduced by insulin administration (37). Considering that insulin resistance may be an expression of diffuse arterial endothelial dysfunction, which contributes to atherosclerosis directly or indirectly (38), it is not surprising that these two risk factors are associated with cardiovascular disease.…”

Highly sensitive sulphide mapping in live cells by kinetic spectral analysis of single Au-Ag core-shell nanoparticles

“…Previous studies have demonstrated that the cystathionine b-synthase (CBS) enzyme, which is involved in H 2 S generation using cysteine and homocysteine as sulphide sources, is strongly expressed in nonconfluent HepG2 cells, but the activity of CBS is significantly decreased after the cells are treated with human insulin [49][50][51][52] . Thus, we adopted normal and insulin-pretreated HepG2 cells as model cells, incubated them with the PNPs, and examined whether variations in intracellular H 2 S production could be detected.…”

Exercise prevents hyperhomocysteinemia in a dietary folate-restricted mouse model

“…This shift is also reflected in other 270 gluconeogenic states, such as diabetes. We and others have demonstrated that a diabetic state or 271 administration of synthetic glucocorticoid compounds has a major impact on methyl group and 272 homocysteine metabolism [23][24][25][26][49][50][51][52]. A consistent finding from these reports is a reduction 273 in circulating homocysteine concentrations owing to an increase in folate-independent 274 remethylation (i.e., BHMT) and catabolism of homocysteine through the transsulfuration 275 pathway.…”