Changes in the expression of methionine adenosyltransferase genes and S-adenosylmethionine homeostasis during hepatic stellate cell activation.

UNLABELLED: Hepatic stellate cell (HSC) activation is an essential event during liver fibrogenesis. Methionine adenosyltransferase (MAT) catalyzes biosynthesis of S-adenosylmethionine (SAMe), the principle methyl donor. SAMe metabolism generates two methylation inhibitors, methylthioadenosine (MTA) and S-adenosylhomocysteine (SAH). Liver cell proliferation is associated with induction of two nonliver-specific MATs: MAT2A, which encodes the catalytic subunit alpha2, and MAT2beta, which encodes a regulatory subunit beta that modulates the activity of the MAT2A-encoded isoenzyme MATII. We reported that MAT2A and MAT2beta genes are required for liver cancer cell growth that is induced by the profibrogenic factor leptin. Also, MAT2beta regulates leptin signaling. The strong association of MAT genes with proliferation and leptin signaling in liver cells led us to examine the role of these genes during HSC activation. MAT2A and MAT2beta are induced in culture-activated primary rat HSCs and HSCs from 10-day bile duct ligated (BDL) rat livers. HSC activation led to a decline in intracellular SAMe and MTA levels, a drop in the SAMe/SAH ratio, and global DNA hypomethylation. The decrease in SAMe levels was associated with lower MATII activity during activation. MAT2A silencing in primary HSCs and MAT2A or MAT2beta silencing in the human stellate cell line LX-2 resulted in decreased collagen and alpha-smooth muscle actin (alpha-SMA) expression and cell growth and increased apoptosis. MAT2A knockdown decreased intracellular SAMe levels in LX-2 cells. Activation of extracellular signal-regulated kinase and phosphatidylinositol-3-kinase signaling in LX-2 cells required the expression of MAT2beta but not that of MAT2A. CONCLUSION: MAT2A and MAT2beta genes are induced during HSC activation and are essential for this process. The SAMe level falls, resulting in global DNA hypomethylation.

Mechanisms of protection by the betaine-homocysteine methyltransferase/betaine system in HepG2 cells and primary mouse hepatocytes.

UNLABELLED: Betaine-homocysteine methyltransferase (BHMT) regulates homocysteine levels in the liver. We previously reported that the alteration of BHMT is associated with alcoholic liver steatosis and injury. In this study, we tested whether BHMT protects hepatocytes from homocysteine-induced injury and lipid accumulation. Both BHMT transfectants of HepG2 cells and primary mouse hepatocytes with suppressed BHMT were generated. Comparisons were made between the cell models with respect to their response to homocysteine treatments. Homocysteine metabolism was impaired in HepG2 cells, and the expression of BHMT in HepG2 cells ameliorated the impairment and stabilized the levels of intracellular homocysteine after the addition of exogenous homocysteine. BHMT expression inhibited homocysteine-induced glucose-regulated protein 78 (GRP78) and C/EBP-homologous protein (CHOP) and homocysteine-induced cell death. A betaine treatment protected primary mouse hepatocytes from a homocysteine-induced increase in GRP78 and cell death but not a tunicamycin-induced increase. Homocysteine induced greater CHOP expression (2.7-fold) in BHMT small interfering RNA (siRNA)-transfected cells than in a control (1.9-fold). Homocysteine-induced cell death was increased by 40% in the siRNA-treated cells in comparison with the control. Apolipoprotein B (apoB) expression was higher and triglycerides and cholesterol were lower in HepG2 expressing BHMT. In primary mouse hepatocytes, homocysteine induced the accumulation of triglycerides and cholesterol, which was reduced in the presence of betaine. Betaine partially reduced homocysteine-induced sterol regulatory element binding protein 1 expression in HepG2 cells and increased S-adenosylmethionine in primary mouse hepatocytes. CONCLUSION: The BHMT/betaine system directly protects hepatocytes from homocysteine-induced injury but not tunicamycin-induced injury, including an endoplasmic reticulum stress response, lipid accumulation, and cell death. This system also exhibits a more generalized effect on liver lipids by inducing ApoB expression and increasing S-adenosylmethionine/S-adenosylhomocysteine.

Role of methionine adenosyltransferase 2A and S-adenosylmethionine in mitogen-induced growth of human colon cancer cells.

BACKGROUND & AIMS: Two genes (MAT1A and MAT2A) encode for methionine adenosyltransferase, an essential enzyme responsible for S-adenosylmethionine (SAMe) biosynthesis. MAT1A is expressed in liver, whereas MAT2A is widely distributed. In liver, increased MAT2A expression is associated with growth, while SAMe inhibits MAT2A expression and growth. The role of MAT2A in colon cancer in unknown. The aims of this study were to examine whether MAT2A expression and SAMe and its metabolite methylthioadenosine (MTA) can modulate growth of colon cancer cells. METHODS: Studies were conducted using resected colon cancer specimens, polyps from Min mice, and human colon cancer cell lines RKO and HT-29. MAT2A expression was measured by real-time polymerase chain reaction and cell growth by the 3-(4,5-dimethylthiazolyl-2-yl)-2,5-diphenyltetrazolium bromide assay. RESULTS: In 12 of 13 patients and all 9 polyps from Min mice, the MAT2A messenger RNA levels were 200%-340% of levels in adjacent normal tissues, respectively. Epidermal growth factor, insulin-like growth factor 1, and leptin increased growth and up-regulated MAT2A expression and MAT2A promoter activity in RKO and HT-29 cells. SAMe and MTA lowered the baseline expression of MAT2A and blocked the growth factor-mediated increase in MAT2A expression and growth in colon cancer cell lines. Importantly, the mitogenic effect of these growth factors was inhibited if MAT2A induction was prevented by RNA interference. SAMe and MTA supplementation in drinking water increased intestinal SAMe levels and lowered MAT2A expression. CONCLUSIONS: Similar to the liver, up-regulation of MAT2A also provides a growth advantage and SAMe and MTA can block mitogenic signaling in colon cancer cells.

In vitro hepatotoxicity of the cyanobacterial alkaloid cylindrospermopsin and related synthetic analogues.

Cylindrospermopsin (CY), a sulfate ester of a tricyclic guanidine substituted with a hydroxymethyluracil, is a cyanobacterial toxin of increasing environmental import as it frequently occurs in drinking water reservoirs. As a toxin, CY mainly targets the liver but also involves other organs. In hepatocytes CY inhibits the synthesis of protein and of glutathione, leading to cell death. The total chemical synthesis of CY has recently been reported (Xie et al., 2000, J. Am. Chem. Soc. 22, 5017-5024). The synthesis has provided analogues of CY to study aspects of the relationship between chemical structure and activity that contribute to toxicity. Protein synthesis inhibition was measured in vitro using a rabbit reticulocyte system. Primary cultures of rat hepatocytes were used to determine the biological activity of CY and analogues in intact cells. Protein synthesis and cell glutathione levels were measured. We could distinguish between CY transport and biological activity by comparing the results in vitro to those in intact cells. The role of the sulfate group in CY toxicity was examined by comparing biological effects of CY with that of CY-DIOL (synthetic CY lacking the sulfate group). The sulfate group was found not to play a role in CY activity or in its uptake into cells, since there was no significant difference in biological activity in vitro or in cells between natural CY and CY-DIOL. The orientation of the hydroxyl group at C7 also had no impact on biological activity or transport of CY, since the C7 epimer of CY (EPI-CY) and the corresponding diol (EPI-DIOL) had activity similar to RAC-CY in vitro and in intact cells. AB-MODEL, the analogue lacking an intact C ring, and the methyl and hydroxyl groups of ring A could inhibit protein synthesis (but at concentrations 500-1000-fold higher than natural CY). Other structurally simpler synthetic analogues lacked biological activity.