Expression Pattern, Regulation, and Functions of Methionine Adenosyltransferase 2β Splicing Variants in Hepatoma Cells

Abstract

Methionine adenosyltransferase (MAT) catalyzes S-adenosylmethionine biosynthesis. Two genes (MAT1A and MAT2A) encode for the catalytic subunit of MAT, while a third gene (MAT2beta) encodes for a regulatory subunit that modulates the activity of MAT2A-encoded isoenzyme. We uncovered multiple splicing variants while characterizing its 5'-flanking region. The aims of our current study are to examine the expression pattern, regulation, and functions of the 2 major variants: V1 and V2. Studies were conducted using RNA from normal human tissues, resected hepatocellular carcinoma specimens, and cell lines. Gene expression, promoter and nuclear binding activities, growth, and apoptosis were measured by routine assays. MAT2beta is expressed in most but not all tissues, and the 2 variants are differentially expressed. The messenger RNA levels of both variants are markedly increased in hepatocellular carcinoma. Tumor necrosis factor (TNF)-alpha, which induces MAT2A in HepG2 cells, also induced V1 (but not V2) expression. TNF-alpha induced the promoter activity of MAT2beta V1, likely via nuclear factor kappaB and activator protein 1. Both variants regulate growth, but only V1 regulates apoptosis. Reduced expression of V1 led to c-Jun-N-terminal kinase (JNK) activation, apoptosis, and sensitized HepG2 cells to TNF-alpha-induced apoptosis, while overexpression of V1 was protective. However, blocking JNK1 or JNK2 activation did not prevent apoptosis induced by V1 knockdown. V1 (but not V2) knockdown also leads to apoptosis in a colon cancer cell line, suggesting these variants play similar roles in many cell types. Different variants of MAT2beta regulate growth and death, which broadens their importance in biology.