METALLOPROTEASES AND SERINE PROTEASES ARE INVOLVED IN THE CLEAVAGE OFTHE 2 TUMOR-NECROSIS-FACTOR (TNF) RECEPTORS TO SOLUBLE FORMS IN THE MYELOID CELL-LINES U-937 AND THP-1

The proteolytic processing of the two TNF receptors (TNF-R55 and TNF-R 75) into soluble forms was investigated in the myeloid cell lines U-93 7 and THP-1. Phorbol myristate acetate (PMA) rapidly stimulated releas e of soluble forms of both TNF-receptors. Incubations were made with P MA and protease inhibitors directed against different target protease groups. The serineprotease inhibitors TPCK and dichloroisocoumarin and the metalloprotease inhibitor 1,10-phenanthroline reduced PMA-induced release of both soluble receptor forms with about 60-70%. Furthermore , 1,10-phenanthroline also reduced PMA-induced down-regulation of TNF- receptors in both cell lines as judged by TNF-binding to cells. Reduce d down-regulation and TNF-receptor shedding by 1,10-phenanthroline was reversed by Zn2+, indicating involvement of a Zn2+-dependent metallop rotease. Thus, both serine proteases and metalloproteases are involved in the processing of TNF-receptors.