Human Bradykinin B2Receptor Is Activated by Kallikrein and Other Serine Proteases

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Abstract
Bradykinin (BK) and kallidin (Lys-BK), liberated from kininogens by kallikreins, are ligands of the BK B2 receptor. We investigated whether kallikreins, besides releasing peptide agonist, could also activate the receptor directly. We studied the effect of porcine and human recombinant tissue kallikrein and plasma kallikrein on [Ca2+]i mobilization and [3H]arachidonic acid release from cultured cells stably transfected to express human BK B2 receptor (CHO/B2, MDCK/B2, HEK/B2), and endothelial cells were used as control cells. As with BK, the actions of kallikrein were blocked by the B2 antagonist, HOE 140. Kallikrein was inactive on cells lacking B2receptor. Kallikrein and BK desensitized the receptor homologously but there was no cross-desensitization. Furthermore, 50 nM human cathepsin G and 50 nM trypsin also activated the receptor; this also was blocked by HOE 140. Experiments excluded a putative kinin release by proteases. [3H]AA release by BK was reduced by 40% by added kininase I (carboxypeptidase M); however, receptor activation by tissue kallikrein, trypsin, or cathepsin G was not affected. Prokallikrein and inhibited kallikrein were inactive, suggesting cleavage of a peptide bond in the receptor. Kallikreins were active on mutated B2receptor missing the 19 N-terminal amino acids, suggesting a type of activation different from that of thrombin receptor. Paradoxically, tissue kallikreins decreased the [3H]BK binding to the receptor with a low KD (3 nM) and inhibited it 78%. Thus, kallikreins and some other proteases activate human BK B2 receptor directly, independent of BK release. The BK B2 receptor may belong to a new group of serine protease-activated receptors.