12 SAMe also donates propylamine moiety for polyamine biosynthesis and in the process generates methylthioadenosine (MTA), which is an inhibitor of methylation.13 Transmethylation reactions of SAMe result in its conversion to another potent methylation inhibitor, S-adenosylhomocysteine (SAH).14 Mammalian cells express two genes, MAT1A and MAT2A, that encode the two MAT catalytic subunits, α1 and α2, and a third gene MAT2β, which encodes the regulatory subunit β that regulates the activity of MAT2A-encoded

isoenzyme MAT II by lowering the inhibition constant (Ki) for SAMe and Michaeli’s constant (Km) for methionine.15, 16 MAT1A is expressed mainly in hepatocytes and maintains the differentiated state of these cells.12 MAT2A is expressed in all extrahepatic tissues and is induced in liver during active growth and dedifferentiation.12, 17, 18 The MAT2β gene is induced during Selleckchem Epacadostat liver cirrhosis and hepatocellular carcinoma (HCC).19 Hepatic stellate cells do not express MAT1A.20 MAT2A is the only enzyme responsible for SAMe biosynthesis in these selleck inhibitor cells. Our recent work in liver cancer cells showed that induction of MAT2A and MAT2β genes is required for cell

growth that is induced by leptin,21 an adipokine that plays a pivotal role in liver fibrogenesis and carcinogenesis.4, 22 Furthermore, leptin signaling in the liver cancer cell line HepG2 requires the expression of the MAT2β gene but not that of MAT2A. Tau-protein kinase Knockdown of MAT2β inhibits upstream events like leptin-mediated signal transducers and activators of transcription 3 (STAT3) activation as well as downstream events like extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3-K) activation.21 Because leptin is a potent profibrogenic growth factor regulated by MAT gene expression and MAT genes are associated with cellular proliferation,

we investigated the hypothesis that MAT2A and MAT2β genes may play important roles in the activation of HSCs. Our results indicate dramatic changes in MAT genes and SAMe homeostasis during activation of HSCs and provide evidence that activation of the MAT genes is an essential event during fibrogenesis. α-SMA, alpha-smooth muscle actin; AKT, AK strain transforming; BDL, bile duct ligation; BrDU, bromodeoxyuridine; Col1A2, alpha2(1) collagen mRNA; ECM, extracellular matrix; ERK, extracellular signal-regulated kinase; FBS, fetal bovine serum; HPLC, high-performance liquid chromatography; HPRT1, hypoxanthine phosphoribosyl-transferase 1; HSC, hepatic stellate cell; MAT, methionine adenosyltransferase; MTA, methylthioadenosine; RT, reverse transcription; SAH, S-adenosylhomocysteine; SAMe, S-adenosylmethionine; siRNA, short interfering RNA; RNAi, RNA interference, STAT, signal transducers and activators of transcription. All reagents used in this study were of analytical grade and obtained from commercial sources.