S-adenosylmethionine and its metabolite induce apoptosis in HepG2 cells: Role of protein phosphatase 1 and Bcl-xS

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Abstract
S-adenosylmethionine (SAMe) and its metabolite 5′-methylthioadenosine (MTA) are proapoptotic in HepG2 cells. In microarray studies, we found SAMe treatment induced Bcl-x expression. Bcl-x is alternatively spliced to produce two distinct mRNAs and proteins, Bcl-xL and Bcl-xS. Bcl-xL is antiapoptotic, while Bcl-xS is proapoptotic. In this study we showed that SAMe and MTA selectively induced Bcl-xS in a time- and dose-dependent manner in HepG2 cells. There are three transcription start sites in the human Bcl-x gene which yield only Bcl-xL in control HepG2 cells. SAMe and MTA treatment did not affect promoter usage, but while one promoter yielded only Bcl-xL, the other two yielded both Bcl-xL and Bcl-xS, with Bcl-xS as the predominant messenger RNA (mRNA) species. Trichostatin A, 3-deaza-adenosine, cycloleucine, and okadaic acid had no effect on Bcl-xS induction by SAMe or MTA. Calyculin A and tautomycin, on the other hand, blocked SAMe and MTA-mediated Bcl-xS induction and apoptosis in a dose-dependent manner. SAMe and MTA increased protein phosphatase 1 (PP1) catalytic subunit mRNA and protein levels and dephosphorylation of serine-arginine proteins, with the latter blocked by calyculin A. The effects of SAMe and MTA on Bcl-xS, PP1 expression, and apoptosis were also seen in 293 cells, but not in primary hepatocytes. Induction of Bcl-xS by ceramide in HepG2 cells also resulted in apoptosis. In conclusion, we have uncovered a highly novel action of SAMe and MTA, namely the ability to affect the cellular phosphorylation state and alternative splicing of genes, in this case resulting in the induction of Bcl-xS leading to apoptosis. Supplementary material for this article can be found on the Hepatology website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html). (Hepatology 2004;40:221-231.)