Modulation of tumor necrosis factor-α-mediated cytotoxicity by changes of the cellular methylation state: mechanism and in vivo relevance

Abstract

A combination of adenosine (Ado) and homocysteine (Homo) enhances tumor necrosis factor (TNF)-α cytotoxicity in vitro and in vivo in several tumor cells. Ado and Homo at concentrations that enhanced TNF-α-mediated cytotoxicity accumulated S-adenosylhomocysteine (AdoHcy) and as consequence decreased the cellular methylation state, i.e. the ratio of S-adenosylmethionine to AdoHcy. This decrease led to inhibition of the isoprenylcysteine carboxyl methyltransferase (MTase), an enzyme that catalyzes carboxyl methylation of C-terminal cysteine residues on isoprenylated proteins. The effect of Ado and Homo on TNF-α cytotoxicity was at least partly mimicked by S-farnesylthioacetic acid, a selective inhibitor of the isoprenylcysteine carboxyl MTase, suggesting involvement of methylations of prenylated proteins in TNF-α-mediated cytotoxicity. Blockage of methylation reactions was associated with an enhancement of the TNF-α-induced disruption of the mitochondrial membrane potential (ΔΨ_m). In nude mice, a combination of Ado, Homo and TNF-α led to TNF-α-induced hemorrhagic necrosis and growth inhibition of TNF-sensitive L929 tumors, whereas little effect was observed with TNF-α alone. Even more important, the TNF-resistant L929 M1 tumors were rendered TNF-sensitive by the combined action of Ado and Homo. We conclude that Ado and Homo together enhance the effectiveness of TNF-α in vitro and in vivo, results that may have therapeutic implications.