Asunto(s)
Factor de Crecimiento de Hepatocito/farmacología , Hepatocitos/metabolismo , Regeneración Hepática , Metionina Adenosiltransferasa/metabolismo , S-Adenosilmetionina/farmacología , Acetilación , Animales , Células Cultivadas , ADN/metabolismo , Hepatectomía , Histonas/metabolismo , Metionina Adenosiltransferasa/genética , ARN Mensajero/metabolismo , RatasRESUMEN
Methionine adenosyltransferase (MAT) catalyzes the biosynthesis of S-adenosylmethionine (AdoMet). In mammals MAT activity derives from two separate genes which display a tissue-specific pattern of expression. While MAT1A is expressed only in the adult liver, MAT2A is expressed in non-hepatic tissues. The mechanisms behind the selective expression of these two genes are not fully understood. In the present report we have evaluated MAT1A and MAT2A methylation in liver and in other tissues, such as kidney, by methylation-sensitive restriction enzyme digestion of genomic DNA. Our data indicate that MAT1A is hypomethylated in liver and hypermethylated in non-expressing tissues. The opposite situation is found for MAT2A. Additionally, histones associated to MAT1A and MAT2A genes showed enhanced levels of acetylation in expressing tissues (two-fold for MAT1A and 3.5-fold for MAT2A liver and kidney respectively). These observations support a role for chromatin structure and its modification in the tissue-specific expression of both MAT genes.
Asunto(s)
Metilación de ADN , Histonas/metabolismo , Metionina Adenosiltransferasa/genética , Acetilación , Animales , Southern Blotting , Western Blotting , Histonas/química , Técnicas In Vitro , Riñón/metabolismo , Hígado/metabolismo , Metionina Adenosiltransferasa/química , Metionina Adenosiltransferasa/metabolismo , Miocardio/metabolismo , Especificidad de Órganos , Ratas , Ratas Wistar , Bazo/metabolismoRESUMEN
Methionine adenosyltransferase (MAT) is the enzyme that catalyzes the synthesis of S-adenosylmethionine (AdoMet), the main donor of methyl groups in the cell. In mammals MAT is the product of two genes, MAT1A and MAT2A. MAT1A is expressed only in the mature liver whereas fetal hepatocytes, extrahepatic tissues and liver cancer cells express MAT2A. The mechanisms behind the tissue and differentiation state specific MAT1A expression are not known. In the present work we examined MAT1A promoter methylation status by means of methylation sensitive restriction enzyme analysis. Our data indicate that MAT1A promoter is hypomethylated in liver and hypermethylated in kidney and fetal rat hepatocytes, indicating that this modification is tissue specific and developmentally regulated. Immunoprecipitation of mononucleosomes from liver and kidney tissues with antibodies mainly specific to acetylated histone H4 and subsequent Southern blot analysis with a MAT1A promoter probe demonstrated that MAT1A expression is linked to elevated levels of chromatin acetylation. Early changes in MAT1A methylation are already observed in the precancerous cirrhotic livers from rats, which show reduced MAT1A expression. Human hepatoma cell lines in which MAT1A is not expressed were also hypermethylated at this locus. Finally we demonstrate that MAT1A expression is reactivated in the human hepatoma cell line HepG2 treated with 5-aza-2'-deoxycytidine or the histone deacetylase inhibitor trichostatin, suggesting a role for DNA hypermethylation and histone deacetylation in MAT1A silencing.