Pharmacological characterization of cloned human NK-2 (neurokinin A) receptor expressed in a baculovirus/Sf-21 insect cell system

Abstract

Using the novel ligand [4,5-3H-Leu9]neurokinin A ([4,5-3H-Leu9] NKA) in a receptor binding assay, we characterized the pharmacology of a cloned neurokinin NK-2 receptor from human lung (hNK-2R), expressed in baculovirus-infected Sf-21 insect cells. Functional hNK-2R cDNA clones were isolated from human lung using a polymerase chain reaction-based methodology. hNK-2R was cloned into pAcYM1, a vector designed to couple expression to the polyhedrin promoter, and the recombinant baculovirus was isolated and used to infect Sf-21 insect cells. hNK-2R expression levels were monitored by Northern blots and 125-I-NKA binding assays. Isolates demonstrating the highest specific binding of 125-I-NKA were grown and membrane preparations from high-speed centrifugations were prepared from both hNK-2R-expressing and wild-type virus-infected cells. [3H]NKA bound in a protein-dependent, saturable (Bmax = 820 +/- 167 fmol/mg of protein), and highly specific (88 +/- 5%) manner to hNK-2R, but not to membranes from cells infected with wild-type virus (14 +/- 8%, 7 +/- 10 fmol/mg of protein). [3H]NKA binding was rapid (k1 = 0.085 nM-1 x min-1) and reversible (t1/2 = 4-5 min). Equilibrium binding experiments demonstrated binding to a mixture of receptors in high and low affinity states (Kd1 = 2.28 +/- 0.26 nM and Kd2 = 266 +/- 91 nM). Binding to hNK-2R was greatly enhanced (400%-600%) by Ca2+ and Mg2+ (EC50 values of 30 microM and 140 microM, respectively), whereas guanosine-5'-O-(3'-thio)triphosphate and guanosine-5'-(beta, gamma-imido)diphosphate were inhibitory. Competition experiments with agonists also demonstrated binding to high and low affinity states, with the following order of potency: NKA > [Nle10]NKA(4-10) > [beta-Ala8]NKA(4-10) >> substance P; Senktide and the NK-1 antagonist CP96,345 (10 microM) did not inhibit binding. Inhibition of binding by selective NK-2 antagonists was consistent with a single affinity state and demonstrated the following order of affinity: SR48,968 >> MEN10,376 > L659,877 > R396. These data suggest that infection of Sf-21 cells with baculovirus expression vector harboring the cDNA of hNK-2R resulted in expression of high affinity, G protein-coupled hNK-2R, with pharmacological selectivity compatible with the NK-2A receptor subtype.