Abstract
The kallikrein-kinin system, activated during inflammatory conditions and the regulation of specific cardiovascular and renal functions, includes two G protein-coupled receptors for bradykinin (BK)-related peptides. The B1 receptor (B1R) subtype is not believed to undergo agonist-induced phosphorylation and endocytosis. A conjugate made of the rabbit B1R fused with the yellow variant of green fluorescent protein (YFP) was expressed in mammalian cells. In COS-1 or human embryonic kidney (HEK) 293 cells, the construction exhibited a nanomolar affinity for the agonist radioligand [3H]Lys-des-Arg9-BK or the antagonist ligand [3H]Lys-[Leu8]des-Arg9-BK and a pharmacological profile virtually identical to that of wild-type B1R. Lys-des-Arg9-BK stimulation of HEK 293 cells stably expressing B1R-YFP but not stimulation of untransfected cells released [3H]arachidonate in a phospholipase A2 assay. B1R-YFP was visualized as a continuous labeling of the plasma membranes in stably transfected HEK 293 cells (confocal microscopy). Addition of Lys-des-Arg9-BK (1–100 nM) rapidly concentrated the receptor-associated fluorescence into multiple aggregates that remained associated with the plasma membrane (no significant internalization) and colocalized with caveolin-1. This reaction was slowly reversible upon agonist washing at 37°C and prevented pretreatment with a B1R antagonist. β-Cyclodextrin treatment, which extracts cholesterol from membranes and disrupts caveolae-related rafts, prevented agonist-induced redistribution of B1R-YFP but not the PLA2 activation mediated by this receptor. The agonist radioligand copurified with caveolin-1 to a greater extent than the tritiated antagonist in buoyant fractions of HEK 293 cells treated with the ligands. Agonist-induced cellular translocation of the kinin B1R to caveolae-related rafts without endocytosis is a novel variation on the theme of G protein-coupled receptor adaptation.
- The American Society for Pharmacology and Experimental Therapeutics
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