Cellular receptor for 125I-labeled tumor necrosis factor: specific binding, affinity labeling, and relationship to sensitivity.

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Abstract
Tumor necrosis factor (TNF) is a proteinaceous toxin shed by stimulated myeloid cells. Murine TNF was radioiodinated to a specific activity of 1 mCi/nmol (1 Ci = 37 GBq) of monomer. 125I-labeled TNF (125I-TNF) retained complete cytotoxic activity and it was immunochemically identical to the native toxin in a quantitative immunoprecipitation assay. It could be shown by competition binding that 125I-TNF bound to intact [mouse] L929 cells with a specificity equal to that of native toxin. The conditions of time, temperature, and concentration involved in equilibrium specific binding to intact cells were studied in detail. When binding was carried out at 4.degree. C for 18 hr, four cell lines sensitive to 125I-TNF cytotoxicity demonstrated high-affinity binding. The binding reached half-maximal level at 3 pM and saturated at 30 pM. These concentrations approximated those required for cell death. Scatchard analysis gave .apprxeq. 1000 sites per cell. J774.1 cells, the source of the toxin, demonstrated similar binding but were not sensitive to 125I-TNF cytotoxicity. Other sensitive cell lines and freshly extracted tumor cells showed specific binding at 3 pM. Normal lymphoid organ cell suspensions and two human tumorigenic cell lines were not sensitive and failed to demonstrate specific binding. 125I-TNF, covalently cross-linked to its receptor on sensitive L-M cells with disuccinimidyl suberate, was isolated and analyzed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and autoradiography. Two specific bands were identified. The most prominent band had a mobility corresponding to a molecular mass of 95 kDa and the second band had a molecular mass of 75 kDa. The presence of the binding site appears to be necessary but not sufficient to explain the sensitivity of cells to the cytotoxic action of TNF.