Tumour Necrosis Factor and Interleukin-6: Structure and Mechanism of Action of the Molecular, Cellular and in Vivo Level

Abstract

Tumour necrosis factor (TNF) can be induced in experimental animals by injection of Bacillus Calmette-Guérin followed, after one or two weeks, by treatment with lipopolysaccharide (LPS); serum taken a few hours later contains a high concentration of TNF (Carswell et al., 1975). Isolated macrophages, e.g. obtained from placenta, can be activated with interferon-γ (LFN-γ) and 24 h later induced to produce TNF by treatment with LPS. Also monocytic cell lines, such as the human U-937 line or the murine PU-518 line, can be induced under proper conditions to produce TNF (Männel et al., 1980; Fransen et al., 1985; Marmenout et al., 1985). We have cloned and expressed to a high specific activity in E. coli both the human TNF (hTNF) gene (Marmenout et al., 1985) and the murine TNF (mTNF) gene (Fransen et al., 1985). Also the sequence of the rabbit TNF gene has been reported (Ito et al., 1986). TNF obtained from various species is highly homologous (about 80%). The subunit of the mature hTNF is a 157 amino acids long polypeptide (156 amino acids for mTNF). The native protein is a nearly spherical, trimeric molecule, containing 45% β-structure and little or no α-helix (Wingfield et al., 1987). TNF, as its name implies, was originally recognized as a substance causing necrosis of tumours in experimental animals; this was usually demonstrated by means of a methyl-cholanthrene-induced sarcoma, and it may be noted that obtaining effective tumour regression requires a rather strict adherence to a defined treatment protocol. Remarkably (and almost by coincidence as it later turned out), TNF is also selectively toxic to some transformed cell lines. But in the presence of concomitant treatment with interferon (IFN), many more transformed and malignant cell lines become sensitive to the cytotoxic action of TNF (Williamson et al., 1983; Fransen et al., 1986b).