The endogenous opioid spinorphin blocks fMet-leu-phe-induced neutrophil chemotaxis by acting as a specific antagonist at the N-formylpeptide receptorsubtype FPR

Spinorphin is an endogenous heptapeptide (leucylvalylvalyltyrosylprolyltryp tophylthreonine), first isolated from bovine spinal cord, whose sequence ma tches a conserved region of beta -hemoglobin. Also referred to as LVV-hemor phin-4 and a member of the nonclassical opioid hemorphin family, spinorphin inhibits enkephalin-degrading enzymes and is analgesic. Recently, spinorph in was reported to block neutrophil activation induced by the chemotactic N -formylpeptide N-formylmethionylleucylphenylalanine (fMLF), suggesting a po tential role as an endogenous negative regulator of inflammation. Here we u se both gain- and loss-of-function genetic tests to identify the specific m echanism of spinorphin action on neutrophils. Spinorphin induced calcium fl ux in normal mouse neutrophils, but was inactive in neutrophils from mice g enetically deficient in the fMLF receptor subtype FPR (N-formylpeptide rece ptor). Consistent with this, spinorphin induced calcium flux in human embry onic kidney 293 cells transfected with mouse FPR, but had no effect on cell s expressing the closely related fMLF receptor subtype FPR2. Despite acting as a calcium-mobilizing agonist at FPR, spinorphin was a weak chemotactic agonist and effectively blocked neutrophil chemotaxis induced by fMLF at co ncentrations selective for FPR. Spinorphin did not affect mouse neutrophil chemotaxis induced by concentrations of fMLF that selectively activate FPR2 . Thus, spinorphin blocks fMLF-induced neutrophil chemotaxis by acting as a specific antagonist at the fMLF receptor subtype FPR.