cDNA cloning and characterization of guinea-pig leukotriene B-4 receptor

The cDNA for leukotriene B-4 (LTB4) receptor (BLT) was cloned from a guinea -pig leucocyte cDNA library. The cloned receptor cDNA encodes 348 amino aci d residues and shares 73% identity with the amino acid sequence of human BL T. Northern blot analysis showed the highest expression of the receptor mRN A in leucocytes, followed by lung and spleen. The membrane fractions of HEK -293 and Cos-7 cells transfected with the cDNA showed specific LTB4-binding activities, with K-d values of 0.27 and 0.17 nM respectively. Xenopus laev is oocytes injected with the cRNA of guinea-pig BLT showed LTB4-induced Cl- currents, indicating that the cloned receptor is functional. LTB4 is metab olized to 20-hydroxy-LTB4 and then to 20-carboxy-LTB4, a transformation con sidered as a major inactivation pathway of the compound. Using the cloned r eceptor, we analysed the agonistic effects of LTB4 and these two metabolite s. 20-Carboxy-LTB4 is a much weaker agonist, with a K-d value higher than t hat of LTB4 by three orders of magnitude, corresponding to a much weaker ch emotactic activity. Although 20-hydroxy-LTB4 is as potent as LTB4 in inhibi ting [H-3]LTB4 binding and cAMP formation, it is less potent than LTB4 in t he mobilization of intracellular Ca2+ and the chemotaxis of Chinese hamster ovary cells expressing the guinea-pig BLT. The present study demonstrated that although LTB4 and 20-hydroxy-LTB4 bind to the receptor with Similar af finities, they do differ in activating intracellular signalling.