A metalloprotease inhibitor blocks shedding of the IL-6 receptor and the p60 TNF receptor

Many cytokines and soluble cytokine receptors are generated by limited proteolysis of membrane-bound precursors. We have examined the ability of the recently described metalloprotease inhibitor, TNF-alpha protease inhibitor (TAPI), and other protease inhibitors to modulate shedding. The membrane-bound forms of the ligands TNF-alpha and CSF-1, the p60 TNFR and the IL-6R, were expressed in COS-7 cells. As expected, TAPI blocked the spontaneous and PMA-induced release of TNF-alpha from transfected cells. Interestingly, TAPI also inhibited the release of soluble forms of p60 TNFR and IL-6R in COS-7 cells. However, the processing of CSF-1, which also requires proteolytic cleavage of a membrane protein, was not affected. The ability of TAPI to inhibit shedding was unique, since several other classes of protease inhibitors, including three other metalloprotease inhibitors, did not inhibit shedding of IL-6R. To determine whether TAPI would prevent shedding under more physiologic conditions, we demonstrated that TAPI was able to prevent unstimulated and PMA-induced release of the soluble forms of TNF-alpha, p60 TNFR, and IL-6R from the monocytic cell line, THP-1, and from human peripheral blood monocytes. In addition, TAPI was able to inhibit LPS-induced shedding of the p60 TNFR and TNF-alpha from monocytes. In summary, our results indicate that a metalloprotease or group of related metalloproteases is responsible for the proteolytic cleavage of several cell surface proteins.