Methyltransferase inhibitor S-adenosyl-L-homocysteine sensitizes human breast carcinoma MCF7 cells and related TNF-resistant derivatives to TNF-mediated cytotoxicity via the ceramide-independent pathway.

In this study we investigated the signalling requirements for TNF-induced cytotoxicity modulated by the methyltransferase inhibitor S-adenosyl-L-homocysteine (AdoHcy) using the TNF-sensitive human breast carcinoma MCF7 cells and its established TNF-resistant clones (R-A1 and clone 1001). Our data indicate that inhibition of methylation reactions by adenosine plus homocysteine, which are known to condense within cells to AdoHcy, markedly potentiated TNF-induced cytotoxicity in MCF7 cells and rendered related TNF-resistant variants, TNF-sensitive by a mechanism independent from the ceramide pathway. We demonstrated that the dominant-negative derivative of FADD (FADD-DN) blocked methylation inhibition/TNF-induced cell death. Moreover, TNF-mediated cytotoxicity modulated by AdoHcy was blocked by the ICE-inhibiting peptide z-VAD-fmk, suggesting that an ICE-like protease is required for the methylation inhibition/TNF-inducible death pathway. In conclusion, these results suggest that the methyltransferase inhibitor AdoHcy potentiates TNF-induced cytotoxicity in MCF7 cells and renders TNF-resistant MCF7 clones, TNF-sensitive via the ceramide independent pathway and that FADD and the ICE-like protease are likely necessary components in transducing methylation inhibition/TNF signals for cell death.