The role of c-Myb in the up-regulation of methionine adenosyltransferase 2A expression in activated Jurkat cells.

Methionine adenosyltransferase (MAT) is a critical cellular enzyme which catalyses the formation of S-adenosylmethionine (SAM), the principal methyl donor. In mammals, two different genes, MAT1A and MAT2A, encode liver-specific and non-liver-specific MATs, respectively. SAM level increases during T-lymphocyte activation and is required for proliferation. A major mechanism for the increase in SAM level is increased MAT2A transcription. In the current work we examined the molecular mechanism of increased MAT2A expression in activated Jurkat cells. Treatment of Jurkat cells with interleukin-2 (IL-2), PMA or PMA plus phytohaemagglutinin (PHA) resulted in a 2-fold increase in MAT2A mRNA levels and a 2-fold increase in luciferase activity driven by the transfected human MAT2A promoter construct -571/+60 but not -270/+60. The region -571 to -270 of the human MAT2A contains a c-Myb consensus binding site. c-Myb is known to be induced during T-lymphocyte activation and its mRNA level was increased after treatment of Jurkat cells with IL-2, PMA or PMA plus PHA. Increased nuclear binding to the MAT2A c-Myb site was confirmed on electrophoretic mobility-shift and supershift analyses. Mutation of the MAT2A c-Myb site abolished the stimulatory effect of these agents on c-Myb nuclear binding and MAT2A promoter activities. Overexpression of c-Myb increased MAT2A promoter activity by 2-fold. Dexamethasone, a known inhibitor of lymphocyte activation, blocked the effect of these agents on MAT2A expression by preventing the increase in c-Myb expression.