Role of sulfhydryl groups in induction of cell surface down-modulation and shedding of extracellular domain of human TNF receptors in human histiocytic lymphoma U937 cells.

The interaction of different polypeptide growth factors with their cell surface receptors, which are typically rich in cysteine, is modulated by sulfhydryl (SH) reagents. Because the extracellular domain of both human TNFRs are likewise rich in cysteine residues, we examined the effect of SH reagents on these receptors in human histiocytic lymphoma U937 cells, which express both p60 and p80 forms. Iodoacetamide induced down-regulation of cell surface TNFRs in a dose- and time-dependent manner. Down-regulation was complete at 10 mM IAA for 1 h at 37 degrees C. The decrease was minimal at 4 degrees C. The down-modulation was also observed with other cell-permeable and -impermeable thiol reagents. The expression of other cell surface molecules such as CD3, CD11b, CD23, and CD25 were not affected. IAA induced the down-regulation of TNFR on cells of both epithelial and myeloid origin. With the use of receptor-specific Abs, we found that the kinetics of down-modulation of the p60 and p80 receptors by IAA were very similar. We also found that down-modulation was not a result of internalization but rather of the shedding of the receptors, as ascertained by receptor-ligand cross-linking analysis, immunoassay, Western blot analysis, and ligand-blot analysis. When TNFRs with a molecular mass of 80 kDa disappeared from the cell surface, a 42-kDa polypeptide appeared in the medium. Thus, we demonstrate that SH reagents down-regulate TNFRs by inducing shedding of both receptors from the cell surface, most likely by activating an endopeptidase that cleaves the receptor.