Autocrine tumor necrosis factor (TNF) and lymphotoxin (LT) alpha differentially modulate cellular sensitivity to...

Tumor necrosis factor (TNF) and lymphotoxin (LT) alpha are structurally and functionally related cytokines. We expressed the TNF and LT-alpha genes in murine fibrosarcoma L929r2 cells, which can be sensitized to TNF/LT-alpha-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-alpha 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-alpha L929r2 transfectants. Hence, although similar cytotoxic effects are induced by extracellularly administered TNF and LT-alpha, endogenous expression of these cytokines fundamentally differs in the way they modulate TNF receptor expression. Unlike LT-alpha, 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-alpha explain this difference, we examined the effect of membrane-bound LT-alpha expression. This was obtained by exchange of the classical signal sequence of LT-alpha for the membrane anchor of chicken hepatic lectin. Membrane retention of LT-alpha resulted indeed in receptor downmodulation and TNF/LT-alpha resistance. We conclude that membrane retention of newly synthesized TNF or LT-alpha is absolutely required for receptor downmodulation and TNF/LT-alpha resistance.