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The expression and cytokine-mediated regulation of the two different receptors for tumor necrosis factor, TNF-R-75 and TNF-R-55, was investigated in human malignant epithelial cell lines. Here we show that cells treated with TNF-alpha up-regulate the TNF-R-75 mRNA and protein levels. No changes were seen regarding the level of TNF-R-55 transcripts. Phospholipase and protein kinase C inhibitors abrogated the signal transduction pathway of TNF-mediated TNF-R-75 mRNA up-regulation which proceeded in the absence of transcriptional activation. This process was also elicited by an agonistic antibody binding specifically to TNF-R-55. Ligand binding assays using specific inhibitory antibodies showed a marked shift in active binding sites from p55 to p75 without significant changes in the total binding for TNF-alpha after up-regulation of p75 TNF-R. This ligand-induced regulation of one of the corresponding receptors has so far only been detected in malignant epithelial cells and not in hematopoietic cell lines. In our search for a specific function we were able to show that p75 is the specific receptor for TNF-mediated up-regulation of transforming growth factor alpha mRNA, whereas p55 is the signal transducer for TNF-induced up-regulation of epidermal growth factor receptor mRNA. This is the first demonstration of an exclusive function of TNF-R-75 in cells of epithelial origin.
Kalthoff et al. (Mon,) studied this question.