Autocrine Tumor Necrosis Factor (TNF) and Lymphotoxin (LT) α Differentially Modulate Cellular Sensitivity to TNF/LT-α Cytotoxicity in L929 Cells

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Abstract
Tumor necrosis factor (TNF) and lymphotoxin (LT) α are structurally and functionally related cytokines. We expressed the TNF and LT-α genes in murine fibrosarcoma L929r2 cells, which can be sensitized to TNF/LT-α–dependent necrosis by inhibitors of transcription or translation. Autocrine production of murine TNF in L929r2 cells completely downmodulated the expression of the 55- and 75-kD TNF receptors, resulting in resistance to TNF/LT-α cytotoxicity. Partial downmodulation of the 55-kD receptor was observed in human TNF-producing L929r2 cells. In contrast, an unaltered TNF receptor expression was found on LT-α L929r2 transfectants. Hence, although similar cytotoxic effects are induced by extracellularly administered TNF and LT-α, endogenous expression of these cytokines fundamentally differs in the way they modulate TNF receptor expression. Unlike LT-α, secreted by the classical pathway, TNF is first formed as a membrane-bound protein, which is responsible for receptor downmodulation. To explore whether the different pathways for secretion of TNF and LT-α explain this difference, we examined the effect of membrane-bound LT-α expression. This was obtained by exchange of the classical signal sequence of LT-α for the membrane anchor of chicken hepatic lectin. Membrane retention of LT-α resulted indeed in receptor downmodulation and TNF/LT-α resistance. We conclude that membrane retention of newly synthesized TNF or LT-α is absolutely required for receptor downmodulation and TNF/LT-α resistance.