Effect of phorbol esters on down-regulation and redistribution of cell surface receptors for tumor necrosis...

The effects of various phorbol esters on the interaction of human cells with recombinant human tumor necrosis factor-alpha (rTNF-alpha) was investigated. Preexposure of several different types of cells with only biologically active tumor promoter, i.e. 4 beta-phorbol 12-myristate 13-acetate (PMA), inhibited the specific binding of rTNF-alpha to its receptor. The reduction in specific binding of TNF-alpha was observed only by PMA but not with several other phorbol esters tested. 1-oleoyl-2-acetylglycerol, which is an analogue of the natural protein kinase C activator, diacylglycerol, was active in down-regulating TNF-alpha receptors but only at 1000 times concentration than PMA. Scatchard analysis of the binding data on U-937 cells revealed that PMA caused a decrease in high affinity cell surface receptor number (approximately 8300 versus approximately 2500 binding sites/cell) without any significant change in the dissociation constant (0.38 nM versus 0.32 nM). This decrease in receptor number is dependent on temperature, the time of exposure, and dose of PMA. Greater than 95% of the specific binding of 125I-TNF-alpha could be abolished within 10 min by preexposure of cells to 10 nM PMA at 37 degrees C. The down-regulation of receptors by PMA occurred only at 37 degrees C but not at 4 degrees C, suggesting a probable internalization of the receptors. The specific binding of TNF-alpha to detergent-solubilized cell extracts remained unchanged after exposure of cells to PMA. The rates of dissociation of TNF-alpha from the cell surface and the rate of internalization was not significantly affected by PMA, but the rate of disappearance from cell interior and its appearance into the medium was slightly enhanced by PMA. PMA did not alter the rate of degradation of the TNF-alpha nor cause the shedding of receptors into the medium. Approximately 70% of TNF-alpha cell surface receptors could be regenerated within 16 h after PMA removal. These results suggest the involvement of PMA-activated protein kinase C in down-regulation and redistribution of TNF-alpha receptors.