Shedding of CD163, a novel regulatory mechanism for a member of the scavenger receptor cysteine-rich family

The glucocorticoid-inducible transmembrane protein CD163 is a member of the scavenger receptor cysteine-rich (SRCR) family which is expressed exclusively on human monocytes and macrophages. The expression of the protein is significantly downregulated in response to phorbol 12-myristate 13-acetate (PMA) by a yet unknown mechanism. We now demonstrate that PMA induces shedding of a soluble form of CD163 rather than internalization, revealing a novel regulatory mechanism for a member of the SRCR family. Bisindolylmaleimide I was shown to inhibit phorbol ester-induced shedding, thus implying an involvement of protein kinase C (PKC). Furthermore, cleavage could be prevented by protease inhibitors. Therefore, we suggest that PMA-induced activation of PKC leads to protease-mediated shedding of CD163. These results indicate a specific release mechanism of soluble CD163 by human monocytes which could play an important role in modulating inflammatory processes. (C) 1999 Academic Press.