Cloning and characterization of a novel human histamine receptor

Histamine exerts its numerous physiological functions through interaction w ith G protein-coupled receptors. Three such receptors have been defined at both the pharmacological and molecular level, while pharmacological evidenc e hints at the existence of further subtypes. We report here the cloning an d characterization of a fourth histamine receptor subtype. Initially discov ered in an expressed-sequence tag database, the full coding sequence (SP914 4) was subsequently identified in chromosome 18 genomic sequence. This virt ual coding sequence exhibited highest homology to the H-3 histamine recepto r and was used to generate a full-length clone by polymerase chain reaction (PCR). The distribution of mRNA encoding SP9144 was restricted to cells of the immune system as determined by quantitative PCR. HEK-293 cells transie ntly transfected with SP9144 and a chimeric G protein alpha -subunit (G(alp haq/i1,2)) exhibited increases in intracellular [Ca2+] in response to hista mine but not other biogenic amines. SP9144-transfected cells exhibited satu rable, specific, high-affinity binding of [H-3] histamine, which was potent ly inhibited by H-3 receptor-selective compounds. The rank order and potenc y of these compounds at SP9144 differed from the rank order at the H-3 rece ptor. Although SP9144 apparently coupled to G alpha (i), HEK-293 cells stab ly transfected with SP9144 did not exhibit histamine-mediated inhibition of forskolin-stimulated cAMP levels. However, both [S-35] GTP gammaS binding and phosphorylation of mitogen-activated protein kinase were stimulated by histamine via SP9144 activation. In both of these assays, SP9144 exhibited evidence of constitutive activation. Taken together, these data demonstrate that SP9144 is a unique, fourth histamine receptor subtype.