cDNA cloning and characterization of guinea-pig leukotriene B4 receptor

The cDNA for leukotriene B(4) (LTB(4)) receptor (BLT) was cloned from a guinea-pig leucocyte cDNA library. The cloned receptor cDNA encodes 348 amino acid residues and shares 73% identity with the amino acid sequence of human BLT. Northern blot analysis showed the highest expression of the receptor mRNA in leucocytes, followed by lung and spleen. The membrane fractions of HEK-293 and Cos-7 cells transfected with the cDNA showed specific LTB(4)-binding activities, with K(d) values of 0.27 and 0.17 nM respectively. Xenopus laevis oocytes injected with the cRNA of guinea-pig BLT showed LTB(4)-induced Cl(-) currents, indicating that the cloned receptor is functional. LTB(4) is metabolized to 20-hydroxy-LTB(4) and then to 20-carboxy-LTB(4), a transformation considered as a major inactivation pathway of the compound. Using the cloned receptor, we analysed the agonistic effects of LTB(4) and these two metabolites. 20-Carboxy-LTB(4) is a much weaker agonist, with a K(d) value higher than that of LTB(4) by three orders of magnitude, corresponding to a much weaker chemotactic activity. Although 20-hydroxy-LTB(4) is as potent as LTB(4) in inhibiting [(3)H]LTB(4) binding and cAMP formation, it is less potent than LTB(4) in the mobilization of intracellular Ca(2+) and the chemotaxis of Chinese hamster ovary cells expressing the guinea-pig BLT. The present study demonstrated that although LTB(4) and 20-hydroxy-LTB(4) bind to the receptor with similar affinities, they do differ in activating intracellular signalling.