Downregulation of tumor necrosis factor receptors on macrophages and endothelial cells by microtubule depolymerizing agents.

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Abstract
Exposure of murine and human macrophages and human umbilical vein endothelial cells to micromolar concentrations of five microtubule (MT)-depolymerizing agents (colchicine, nocodazole, podophyllotoxin, vincristine, and vinblastine) resulted in a loss of binding sites for iodinated TNF-alpha. The reduction amounted to 40-60% by 1 h and approximately 75% by 2-4 h. In 1 h, specific binding was reduced 50% by 0.1-5 microM of these drugs at 37 degrees C, but not at 4 degrees C. Inactive isomers of colchicine were ineffective, as were microfilament-destabilizing cytochalasins. The active agents did not compete with TNF-alpha R for binding. Antiserum against TNF-alpha did not neutralize the effect of colchicine and nocodazole. PGE1 and dibutyryl-cAMP could not mimic, and cyclooxygenase inhibitors could not prevent the drug effects. All the binding sites were regenerated within 3 h after removal of nocodazole, which binds tubulin reversibly, whereas little recovery was found even 18 h after the removal of colchicine, which binds tubulin irreversibly. These findings suggested that MT disassembly was responsible for the observed downregulation of TNF-alpha R. The protein synthesis inhibitor cycloheximide inhibited binding of TNF-alpha to a similar extent and with a similar time course as colchicine in the absence of added ligand. Neither drug affected binding of IFN-gamma to macrophages, nor binding of TNF-alpha to human polymorphonuclear leukocytes. Thus, an intact MT network appears to be important in maintenance of the steady state of TNF-alpha R on those cells in which TNF-alpha R turns over rapidly in the absence of ligand. The antiinflammatory actions of MT-depolymerizing agents may result in part from their interference with the ability of such cells to respond to TNF-alpha.