The apoptotic signaling of TNF-alpha in multidrug resistant Friend leukemia cells.

Drug resistance, especially in its multiple forms (multidrug resistance, MDR), is a major and difficult problem to resolve in cancer therapy. Certain cytokines might be capable of bypassing this process and here we report on the in vitro effects of Tumor Necrosis Factor alpha, (TNF) on a MDR variant (FLC/DOX) of Friend leukemia. Drug resistance of FLC/DOX is associated with at least two mechanisms, i.e. overexpression of P-glycoprotein and increase in glutathione-related detoxifying activities. Nevertheless, TNF exerts more cytotoxicity in FLC/DOX than in its parental, drug-sensitive, counterpart and this effect is related to the induction of apoptosis. In contrast, Doxorubicin (DOX) never induces apoptosis in FLC/DOX, even when applied at high, fully cytotoxic, concentrations. We have tried to elucidate TNF signaling in FLC/DOX. The results have indicated that in this cell line TNF-triggered apoptosis exhibits some distinct features. It occurs mostly through type I (p55) TNF receptors, probably involves a calphostin-C sensitive protein kinase C activity and requires synthesis of proteins (it is inhibited by actinomycin D or cycloheximide) and of inducible nitric oxide (NO) synthase (it is inhibited by NG-methyl-L-arginine or aminoguanidine). Further, it is not influenced by agents which increase or decrease cell sulfhydryl groups, such as N-acetylcysteine or buthionine sulfoximine, respectively. These steps appeared to be either not or dissimilarly involved in the resistance to DOX of the same cells. In particular, DOX activity was stimulated by calphostin C and buthionine sulfoximine, and reduced by N-acetyl-cysteine. These findings illustrate that TNF may activate fresh cytotoxic pathways in tumor cells which are multidrug resistant, also owing to multifactorial causes.