Phorbol 12-Myristate 13-Acetate-induced Ectodomain Shedding and Phosphorylation of the Human Meprinβ Metalloprotease

Abstract

Shedding of proteins localized at the cell surface is an important regulatory step in the function of many of these proteins. Human meprin (N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase, PPH, EC 3.4.24.18) a zincmetalloendopeptidase of the astacin family is an oligomeric protein complex of α- and β-subunits and is expressed abundantly in the intestine and kidney as well as in leukocytes of the lamina propria and in cancer cells. In transfected cells intracellular proteolytic removal of the membrane anchor results in the secretion of the meprin α-subunit. In rats and mice, the β-subunit exists in a membrane-anchored form. In contrast, human meprinβ is constitutively converted into a secretable form. We now show that phorbol 12-myristate 13-acetate (PMA) stimulates an increased release of hmeprinβ from transfected COS-1 cells, whereas hmeprinβ secretion is not influenced. This stimulatory effect is inhibited by the protein kinase C (PKC) inhibitor staurosporine, suggesting that activation of PKC mediates PMA-induced hmepriniβ shedding. The use of different protease inhibitors shows that two different metalloprotease activities are responsible for the constitutive and the PMA-stimulated hmepriniβ shedding. We identified tumor necrosis factor α-converting enzyme (TACE or ADAM17) as the protease that mediates the PMA-induced release. We also demonstrate that hmepriniβ is phosphorylated by PMA treatment on Ser687 within a PKC consensus sequence in the cytosolic domain of the protein. This phosphorylation of hmeprini3 is not, however, implicated in the enhanced secretion by PMA treatment.