HIGH CONSTITUTIVE ACTIVITY OF THE HUMAN FORMYL PEPTIDE RECEPTOR

The formyl peptide receptor (FPR) couples to pertussis toxin (PTX)-sen sitive Gi-proteins to activate chemotaxis and exocytosis in neutrophil s. PTX reduces not only formyl peptide-stimulated but also agonist-ind ependent (''basal'') G(i)-protein activity, suggesting that the FPR is constitutively active. We aimed at identifying an inverse FPR agonist , i.e. a compound that suppresses constitutive FPR activity. In Sf9 in sect cell membranes, the G-protein heterotrimer G(i)alpha(2)beta(1)gam ma(2) reconstituted N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FML P)stimulated guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) binding and GTP gamma S-sensitive high affinity [H-3]FMLP binding. The FPR '' antagonist'' cyclosporin H (CsH) potently and efficiently reduced basa l GTP gamma S binding in Sf9 membranes. Another FPR antagonist, l-L-le ucyl-L-phenylalanyl-L-leucyl-L-phenylalanine did not inhibit basal GTP gamma S binding but blocked the inhibitory effect of CsH on GTP gamma S binding. Na+ reduced basal GTP gamma S binding and eliminated the i nhibitory effect of CsH. Similar effects of FMLP, CsH, and Na+ as in S f9 membranes were observed with FPR expressed in the mammalian cell li ne HEK293. Our data show that the human FPR possesses high constitutiv e activity. CsH is an inverse FPR agonist and stabilizes the FPR in an inactive state. Na+ also stabilizes the FPR in an inactive state and, thereby, diminishes inverse agonist efficacy.