MECHANISM OF RELEASE OF SOLUBLE FORMS OF TUMOR-NECROSIS-FACTOR LYMPHOTOXIN RECEPTORS BY PHORBOL-MYRISTATE ACETATE-STIMULATED HUMAN THP-1 CELLS IN-VITRO

The mechanism involved in the release of the soluble forms of 55 and 7 5 kDa TNF and lymphotoxin (LT) membrane receptors was studied in a con tinuous human monocytic cell line, THP-1, in vitro. THP-1 cells were f ound to spontaneously release soluble forms of both 55 and 75 kDa TNF/ LT receptors. Release was up-regulated by PMA, and optimal release was achieved at 10(-8) M PMA. Serine protease inhibitors such as PMSF,3,4 dichloroisocoumarin, Nalpha-p-tosyl-L-lysine chloromethyl ketone (TLC K), and N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) were found to inhibit the production of both soluble TNF/LT receptors. PMSF (2 mM ) also blocked receptors shedding from paraformaldehyde-fixed THP-1 ce lls coincubated with conditioned media from PMA-stimulated THP-1 cells . Colchicine at 1 and 10 muM stimulated the production of both soluble TNF/LT receptors, but the PMA-induced release of both soluble TNF/LT receptors was inhibited. It appears that the PMA-induced release of so luble TNF/LT receptors involves serine proteases in the extracellular space where the soluble parts of the TNF/LT receptors are cleaved dire ctly off the cell membrane.