Els Decoster

Resistance to tumor necrosis factor (TNF) cytotoxicity by autocrine TNF production is independent of intracellular signaling pathways

We previously showed that autocrine tumor necrosis factor (TNF) production in the TNF‐sensitive L929sA fibrosarcoma cell line induced TNF resistance, which is correlated with downmodulation of both TNF receptors on the cell surface. We now analyzed whether autocrine TNF production also interfered with intracellular TNF signaling pathways. The L929sA‐CAT‐R55i cell line, in which cell death can be induced by controlled cytoplasmic expression of a trimeric fusion protein between chloramphenicol acetyltransferase and the intracellular domain of TNF‐R55 (CAT‐R55i), was supertransfected with the murine TNF gene. Expression of the latter conferred resistance to cell death induced by exogenous TNF, while cytotoxicity induced by CAT‐R55i was not impaired. This demonstrates that autocrine TNF did not induce intracellular mechanisms that block TNF signaling leading to cell death. Thus the induction of TNF resistance via autocrine TNF production in L929sA cells is solely due to downmodulation of TNF receptors on the cell surface.

Tumor Necrosis Factor: Mechanism of Action and its Potential for Anticancer Therapy

Tumor Necrosis Factor (TNF) is secreted by appropriately induced monocytes and also by some T-cells. The subunit of TNF is a 17 kDa polypeptide, while the native molecule corresponds to a trimer. The three-dimensional structure has been solved at 2.6 A resolution. Studies on mutants provided evidence for localization of receptor binding sites (three per trimer) in clefts between subunits (one site for each cleft). TNF (in combination with IFN-γ) is a promising anticancer agent, provided the therapeutic index can be improved. The antitumor activity can be enhanced by the presence of LiC1, but this combination is not without effects on normal cells; when TNF + LiCI was injected in the skin, a rapid, local inflammation was observed.