MECHANISMS INVOLVED IN THE PROCESSING OF THE P55 AND THE P75 TUMOR-NECROSIS-FACTOR (TNF) RECEPTORS TO SOLUBLE RECEPTOR FORMS

The two tumor necrosis factor (TNF) receptors (TNF-R55 and TNF-R75) ca n release soluble TNF-binding proteins (TNF-R55-BP and TNF-R75-BP) by proteolytic cleavage. The proteolytic processing of the TNF receptors was investigated in monoblastic THP-1 and promyelocytic HL-60-10 leuke mic cell lines. The release of soluble forms of both receptors was rap idly stimulated by staurosporine-sensitive protein kinase C activation by phorbol myristate acetate (PMA) and more slowly stimulated by TNF. No receptor release was seen below a temperature of 16 degrees C. NH4 Cl (10 mmol/liter) and monensin (1 mu mol/liter), known to increase in tracellular pH, inhibited to some extent PMA- and TNF-induced release of both TNF-R55-BP and TNF-R55-BP. The inhibitory effect of monensin m ight be explained by a diminished translocation of newly synthesized r eceptor to the plasma membrane. The weak inhibitory effect of NH4Cl on PMA-induced release of soluble receptor forms could be due to effects on a pH-sensitive compartment. PMA-induced down-regulation of recepto rs was not dependent on acidity as it occurred also in the presence of monensin and NH4Cl when the release of TNF-BPs is partially blocked. Dibutyryl cAMP inhibited the PMA-induced release of TNF-R55-BP but not of TNF-R75-BP in both cell lines investigated. In addition, dibutyryl cAMP alone stimulated the release of both receptors but only in THP-1 cells. Our data show that the generation of soluble forms of both TNF receptors can be regulated by both PKC and PKA.