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

Abstract

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 Å 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.

TNF has two pathways of action, a nucleus-independent cytotoxic effect and transcriptional activation of a defined set of genes leading e.g. to IL6 synthesis. This transcriptional activation of the IL6 gene is also very strongly enhanced by Li⁺, suggesting that Li⁺ acts early in the common signal transduction pathway. There is an overlap with the signal transduction pathway induced by IL6, as also some ILl effects are dramatically increased by Li⁺.

Expression of the endogenous or an exogenous TNF gene in a sensitive cell renders it resistant to TNF. Are TNF-producing tumor cells more tumorigenic? We compared the tumorigenicity in nude mice of tumor cells varying in their TNF expression level. No cachexia was observed. However, with these subcutaneous tumors, the expression level of TNF correlated with a much reduced tumor growth. In fact, the (locally produced) TNF seems to induce a host response leading to encapsulation of the tumor. This may possibly be due to the mitogenic effect of TNF on fibroblasts.

TNF and IL 1 are strong inducers of IL6. The latter peaks at about 2–3 h and then disappears from circulation. However, in the case of highly toxic treatments (high mTNF dose, TNF together with ILI or TNF together with RU486), the 1L6 level in circulation at later times even increases and this correlates with subsequent death. Possibly small amounts of IL6 are protective, but high concentrations contribute to lethality.

One can tolerize mice towards moderate doses of TNF by daily administration of small concentrations of TNF for 5–6 days. This tolerization also works in mice bearing a B16 melanoma tumor. After tolerization, the anticancer treatment can be started by paralesional administration of moderate doses of TNF + IFN-γ. This leads to complete tumor regression with a stirvival of 80%. These tumor model systems offer promising indications for therapeutic use of TNF in cancer patients.

Research supported by the Belgian Fund for Medical Scientific Research (FGWO), the “Algemene Spaar-en Lijfrentekas” (ASLK), the National Incentive Program in Life Sciences, the Interuniversity Attraction Poles (IUAP), “Levenslijn”, the “Vlaams Actieprogramma Biotechnologie” and the National Lottery. RB, CL, SVB and BV hold fellowships from the NFWO, and DDV, AL and CVD from the IWONL. NT is a recipient of a grant from the Canon Foundation. FVR is a Senior Research Associate with the NFWO.