Abstract

Bradykinin (BK) has been implicated in the regulation of renal function. Activation of extracellular signal-regulated protein kinase (ERK1/2) has been demonstrated in several models of toxic or proliferative renal injury. We studied activation of ERK1/2 by BK in a cell model of the most distal part of the nephron, inner medullary collecting duct (mIMCD-3) cells. Exposure of mIMCD-3 cells to BK (10⁻¹⁰–10⁻⁵ M) resulted in a concentration-dependent increase in tyrosine phosphorylation of ERK1/2, with maximal effect at 10⁻⁸ M BK. ERK1/2 activation by BK was observed as early as 1 min, peaked at 5 min, and was sustained at least for 1 h. The effect of BK was mediated by the B₂receptor and was pertussis toxin-independent. Inhibition of phospholipase C, protein kinase C, or phosphatidylinositol 3-kinase did not alter ERK1/2 activation by BK. BK-induced ERK1/2 activation was Ca²⁺-calmodulin-independent but was sensitive to genistein, an inhibitor of tyrosine kinase(s). AG1478, a specific inhibitor of epidermal growth factor receptor (EGFR) kinase, completely blocked the effect of BK, suggesting an essential role of EGFR in ERK1/2 activation by BK. Immunoprecipitation/Western blot studies revealed that BK stimulated tyrosine phosphorylation of EGFR, its association with an adapter molecule Grb2, and complex formation between Grb2 and the adapter protein Shc. Activation studies of monomeric G protein Ras showed that BK-induced stimulation of Ras was dependent on EGFR tyrosine kinase activity. These studies demonstrate that BK stimulates Ras-dependent activation of ERK1/2 in mIMCD-3 cells via transactivation of EGFR through a novel mechanism.