title: S-Adenosylhomocysteine hydrolase overexpression in HEK-293 cells: effect on intracellular adenosine levels, cell viability, and DNA methylation.
abstract: OBJECTIVE
  S-Adenosylhomocysteine hydrolase (AdoHcyase) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (AdoHcy), which is a potent product inhibitor of S-adenosylmethionine (AdoMet)-dependent methyltransferases. While previous studies have shown that AdoHcyase inhibition or deficiency lead to a decreased AdoMet/AdoHcy ratio resulting in impaired transmethylation, the effect of enhanced AdoHcyase activity on AdoMet/AdoHcy metabolism and methylation reactions has not been studied in detail.
  
  
  METHODS
  To investigate the effect of enhanced AdoHcyase activity, we generated HEK-293 cell lines stably overexpressing AdoHcyase.
  
  
  RESULTS
  Initial studies revealed that 2-10-fold AdoHcyase overexpression resulted in decreased intracellular AdoHcy and elevated adenosine levels, whereas 16-fold AdoHcyase overexpression increased adenosine and AdoHcy levels, lowered energy charge, and altered cell morphology. Furthermore, we found a correlation between AdoHcyase activity and cell viability. Caspase-activity assays and DNA fragmentation analysis revealed that the cell death in AdoHcyase overexpressing cells was due to apoptosis. Global DNA methylation was not altered in the different AdoHcyase overexpressing cell lines.
  
  
  CONCLUSIONS
  Taken together, these data show that 2-5-fold enhanced AdoHcyase activity is well tolerated by the cell, while greatly enhanced AdoHcyase activity results in adenosine-induced apoptosis. The fact that enhanced AdoHcyase activity does not increase transmethylation activity suggests that AdoHcyase activity under physiological conditions is not rate limiting for efficient transmethylation.
authors.affiliation: Department of Pharmacology and Toxicology, Faculty of Medicine, University of Tuebingen, Germany.
authors.forename: Marina
authors.initials: M
authors.lastname: Hermes
authors.forename: Hartmut
authors.initials: H
authors.lastname: Osswald
authors.forename: Renate
authors.initials: R
authors.lastname: Riehle
authors.forename: Christine
authors.initials: C
authors.lastname: Piesch
authors.forename: Doris
authors.initials: D
authors.lastname: Kloor