Human bradykinin B-2 receptor is activated by kallikrein and other serine proteases

Bradykinin (BK) and kallidin (Lys-BK), liberated from kininogens by kallikr eins, are ligands of the BK B-2 receptor. We investigated whether kallikrei ns, besides releasing peptide agonist, could also activate the receptor dir ectly. We studied the effect of porcine and human recombinant tissue kallik rein and plasma kallikrein on [Ca2+](i) mobilization and [H-3]arachidonic a cid release from cultured cells stably transfected to express human BK B-2 receptor (CHO/B-2, MDCK/B-2, HEK/B-2), and endothelial cells were used as c ontrol cells. As with BK, the actions of kallikrein were blocked by the B-2 antagonist, HOE 140. Kallikrein was inactive on cells lacking B-2 receptor . Kallikrein and BK desensitized the receptor homologously but there was no cross-desensitization. Furthermore, 50 nM human cathepsin G and 50 nM tryp sin also activated the receptor; this also was blocked by HOE 140. Experime nts excluded a putative kinin release by proteases. [H-3]AA release by BK w as reduced by 40% by added kininase I (carboxypeptidase M); however, recept or activation by tissue kallikrein, trypsin, or cathepsin G was not affecte d. Prokallikrein and inhibited kallikrein were inactive, suggesting cleavag e of a peptide bond in the receptor. Kallikreins were active on mutated B-2 receptor missing the 19 N-terminal amino acids, suggesting a type of activ ation different from that of thrombin receptor. Paradoxically, tissue kalli kreins decreased the [H-3]BK binding to the receptor with a low K-D (3 nM) and inhibited it 78%. Thus, kallikreins and some other proteases activate h uman BK B-2 receptor directly, independent of BK release. The BK B-2 recept or may belong to a new group of serine protease-activated receptors.