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

✍ Scribed by Heping Yang; Mamatha R. Sadda; Mei Li; Ying Zeng; Lixin Chen; Wanjun Bae; Xiaopeng Ou; Maria T. Runnegar; José M. Mato; Shelly C. Lu

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 870 KB
• Volume: 40
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.20274

✦ Synopsis

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-x L and Bcl-x S . Bcl-x L is antiapoptotic, while Bcl-x S is proapoptotic. In this study we showed that SAMe and MTA selectively induced Bcl-x S 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-x L in control HepG2 cells. SAMe and MTA treatment did not affect promoter usage, but while one promoter yielded only Bcl-x L , the other two yielded both Bcl-x L and Bcl-x S , with Bcl-x S as the predominant messenger RNA (mRNA) species. Trichostatin A, 3-deaza-adenosine, cycloleucine, and okadaic acid had no effect on Bcl-x S induction by SAMe or MTA. Calyculin A and tautomycin, on the other hand, blocked SAMe and MTA-mediated Bcl-x S 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-x S , PP1 expression, and apoptosis were also seen in 293 cells, but not in primary hepatocytes. Induction of Bcl-x S 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-x S 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.