Kinin B2 Receptor-Mediated Bradykinin Internalization and Metalloendopeptidase EP24.15-Dependent Intracellular Bradykinin Degradation

Kinins are potent proinflammatory peptides that are produced extracellularly and rapidly degraded by extracellular peptidases and by intracellular peptidases accessed by kinins via receptor-mediated endocytosis. Here, we developed model cell systems expressing the kinin B2 receptor (B2R) and the metalloendopeptidase thimet oligopeptidase (EC 3.4.24.15; EP24.15) either individually or together to address 1) the cellular and functional relationship between these proteins and 2) the participation of EP24.15 in the metabolism of bradykinin (BK) following BK internalization via B2R. B2R was localized almost exclusively in the plasma membrane, whereas EP24.15 was localized both intracellularly and on the cell surface, and secreted in the media. Intracellular EP24.15 was present throughout the cell, both cytosolic and particulate, with less nuclear localization and no co-localization with either the endoplasmatic reticulum marker calnexin or Golgi marker GM130. No direct co-localization of B2R and EP24.15 was observed using immunofluorescence microscopy. However, the two proteins co-immunoprecipitated specifically, and EP24.15 attenuated maximal B2R responsiveness without influencing the potency of BK to stimulate phosphoinositide hydrolysis and intracellular Ca(2+) mobilization. Cell surface-bound BK remained intact in cells overexpressing EP24.15 but was degraded intracellularly in an EP24.15-dependent manner upon B2R-mediated endocytosis. These results show that EP24.15 acts to negatively regulate B2R responsiveness and by serving as an intracellular peptidase in the degradation of BK specifically internalized via this receptor.