ADENOSINE AND HOMOCYSTEINE TOGETHER ENHANCE TNF-MEDIATED CYTOTOXICITYBUT DO NOT ALTER ACTIVATION OF NUCLEAR FACTOR-KAPPA-B IN L929

This paper shows that a combination of adenosine and homocysteine pote ntiates TNF-alpha-mediated cytotoxicity, but does not modulate activat ion of NF-kappa B transcription factor controlling the expression of v arious TNF-alpha-inducible genes. Adenosine and homocysteine at concen trations (1 mM each) that enhance TNF-alpha-induced cytotoxicity accum ulate S-adenosyl-L-homocysteine (AdoHcy), a potent inhibitor of S-aden osyl-L-methionine-dependent methylation reactions. In addition, preloa ding L999 cells with AdoHcy resulted in enhanced responses to TNF-alph a, suggesting that AdoHcy potentiates TNF-alpha-induced cytotoxicity. Moreover, the combination of adenosine and homocysteine changed the de pendency of the TNF-alpha-mediated cytolysis on reactive oxygen interm ediates. In the absence of adenosine and homocysteine TNF-alpha-mediat ed cytotoxicity was inhibited by antioxidants such as butylated hydrox yanisole and pyrrolidine dithiocarbamate. In the presence of adenosine and homocysteine, however, TNF-alpha-mediated cytotoxicity is not inh ibited by these antioxidants. A L929 subclone, defective in the respir atory chain, resisted the cytotoxic action of TNF-alpha, but was rende red TNF-alpha sensitive in the presence of adenosine and homocysteine. Unlike TNF-alpha-mediated cytotoxicty, the TNF-alpha-induced activati on of NF-kappa B was inhibited by antioxidants regardless whether aden osine and homocysteine were present or absent in the culture medium. I n conclusion, a combination of adenosine and homocysteine selectively modulates TNF-alpha-mediated cytotoxicity without changing the TNF-alp ha-induced activation of NF-kappa B. Our results could facilitate the development of strategies that permit dissection of cytotoxic and gene -activating pathways.