Induction of human methionine adenosyltransferase 2A expression by tumor necrosis factor α: Role of NF-κB and AP-1

Abstract

Two genes (MAT1A and MAT2A) encode for methionine adenosyltransferase (MAT), an essential cellular enzyme responsible for S-adenosylmethionine biosynthesis. MAT1A is expressed mostly in the liver, whereas MAT2A is widely distributed. We showed a switch from MAT1A to MAT2A expression in human hepatocellular carcinoma (HCC), which facilitates cancer cell growth. Using DNase I footprinting analysis, we previously identified a region in the MAT2A promoter protected from DNase I digestion in HCC. This region contains NF-κB and AP-1 elements, and the present study examined whether they regulate MAT2A promoter activity. We found nuclear binding of NF-κB and AP-1 to the MAT2A promoter increased in HCC. Tumor necrosis factor α (TNFα), which activates both NF-κB and AP-1, increased MAT2A expression in a dose- and time-dependent manner, binding of both NF-κB and AP-1 to the MAT2A promoter and MAT2A promoter activity, with the latter effect blocked by site-directed mutagenesis of the NF-κB and AP-1 binding sites. Blocking NF-κB with IκB super-repressor or AP-1 with dominant-negative c-Jun led to decreased basal MAT2A expression and prevented the TNFα-induced increase in MAT2A expression. Although blocking NF-κB had no influence on the ability of TNFα to increase AP-1 nuclear binding, blocking AP-1 with dominant-negative c-Jun prevented the TNFα-mediated increase in NF-κB binding. In conclusion, both NF-κB and AP-1 are required for basal MAT2A expression in HepG2 cells and mediate the increase in MAT2A expression in response to TNFα treatment. Increased transactivation of these two sites also contributes to MAT2A up-regulation in HCC.