Cloning and functional characterization of the 5 '-flanking region of human methionine adenosyltransferase 1A gene

Methionine adenosyltransferase (MAT) is an essential cellular enzyme which catalyses the formation of S-adenosylmethionine, the principal methyl donor and precursor for polyamines. In mammals, two different genes, MAT1A and M AT2A, encode for liver-specific and non-liver-specific MAT respectively. We previously described a switch in the MAT expression from MAT1A to MAT2A in human liver cancer, which offered the cancerous cell a growth advantage. L oss of MAT1A expression was due to lack of gene transcription. To study reg ulation of the MAT1A gene, we have cloned and characterized a 1.9 kb 5'-fla nking region of the human MA TIA gene. One transcriptional start site, loca ted 25 nt downstream from a consensus TATA box, was identified by primer ex tension and RNase protection assays. The promoter contains several consensu s binding sites for CAAT enhancer binding protein (C/EBP) and hepatocyte-en riched nuclear factor (HNF), transcriptional factors important in liver-spe cific gene expression. The human MA TIA promoter was able to efficiently dr ive luciferase expression in Chang cells, a human liver cell line, but not in HeLa cells. Sequential deletion analysis of the promoter revealed two DN A regions upstream of the translational start site, -705 to -839 bp and -11 11 to -1483 bp, which are involved in positive and negative gene regulation , respectively. Specific protein binding to these regions was confirmed by electrophoretic-mobility-shift and DNase I footprinting assays. Similar to the situation with the rat MA TIA, glucocorticoid treatment also increased human MAT1A expression and promoter activity in a dose- and time-dependent manner.