Downregulation of liver-specific MAT1Agene, encoding S-adenosylmethionine (SAM) synthesizing isozymes MATI/III, and upregulation of widely expressedMAT2A, encoding MATII isozyme, known as MAT1A:MAT2A switch, occurs in hepatocellular carcinoma (HCC). Here, we found Mat1A:Mat2A switch and low SAM levels, associated with CpG hypermethylation and histone H4 deacetylation of Mat1A promoter, and prevalent CpG hypomethylation and histone H4 acetylation in Mat2A promoter of fast growing HCC of F344 rats, genetically susceptible to hepatocarcinogenesis. In HCC of genetically resistant BN rats, very low changes in Mat1A:Mat2A ratio, CpG methylation, and histone H4 acetylation occurred. Highest MAT1A promoter hypermethylation and MAT2A promoter hypomethylation occurred in human HCC with poorer prognosis. Furthermore, levels of AUF1 protein, which destabilizes MAT1A mRNA, MAT1A-AUF1 ribonucleoprotein, HuR protein, which stabilizes MAT2AmRNA, and MAT2A-HuR ribonucleoprotein, sharply increased in F344 and human HCC, and underwent low/no increase in BN HCC. In human HCC, MAT1A:MAT2Aexpression and MATI/III:MATII activity ratios correlated negatively with cell proliferation and genomic instability, and positively with apoptosis and DNA methylation. Noticeably, MATI/III:MATII ratio strongly predicted patients' survival length. Forced MAT1A overexpression in HepG2 and HuH7 cells led to rise in SAM level, decreased cell proliferation, increased apoptosis, downregulation of Cyclin D1, E2F1, IKK, NF-kB,and antiapoptotic BCL2and XIAP genes, and upregulation of BAX and BAK proapoptotic genes. In conclusion, we found for the first time a post-transcriptional regulation of MAT1A and MAT2A by AUF1 and HuR in HCC. Low MATI/III:MATII ratio is a prognostic marker that contributes to determine a phenotype susceptible to HCC and patients' survival. Interference with cell cycle progression and IKK/NF-kB signaling contributes to the anti-proliferative and pro-apoptotic effect of high SAM levels in HCC. (HEPATOLOGY 2012.)