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 c ytotoxicity modulated by the methyltransferase inhibitor S-adenosyl-L-homoc ysteine (AdoHcy) using the TNF-sensitive human breast carcinoma MCF7 cells and its established TNF-resistant clones (R-A1 and clone 1001), Our data in dicate that inhibition of methylation reactions by adenosine plus homocyste ine, which are known to condense within cells to AdoHcy, markedly potentiat ed TNF-induced cytotoxicity in MCF7 cells and rendered related TNF-resistan t variants, TNF-sensitive by a mechanism independent from the ceramide path way. We demonstrated that the dominant-negative derivative of FADD (FADD-DN ) blocked methylation inhibition/TNF-induced cell death. Moreover, TNF-medi ated cytotoxicity modulated by AdoHcy was blocked by the ICE-inhibiting pep tide z-VAD-fmk, suggesting that an ICE-like protease is required for the me thylation inhibition/TNF-inducible death pathway. In conclusion, these resu lts suggest that the methyltransferase inhibitor AdoHcy potentiates TNF-ind uced cytotoxicity in MCF7 cells and renders TNF-resistant MCF7 clones, TNF- sensitive via the ceramide independent pathway and that FADD and the ICE-li ke protease are likely necessary components in transducing methylation inhi bition/TNF signals for cell death.