Kappa opioid receptors expressed on three related thymoma cell lines: Differences in receptor-effector coupling

Abstract

The mouse thymoma R1.1 cell line was shown previously to express a single high-affinity K 1 opioid receptor that is negatively coupled through a pertussis toxin-sensitive G-protein to adenylyl cyclase. This study compared opioid receptor binding and inhibition of adenylyl cyclase activity in three unique derivatives of the R1.1 cell line. Membranes from the R1.G1 and R1E/TL8x.1.G1.OUA r.1 (R1EGO) cell lines bound both [ 3H]U69,593 and [ 3H](−)-bremazocine with similar affinities compared with R1.1 membranes, whereas membranes from the R1E/TL8x.1 (R1E) cell line did not posses any opioid binding sites, detected by radioreceptor binding. The B max values for [ 3H]U69,593 and [ 3H](−)-bremazocine binding to R1.G1 and R1EGO cell membranes were, respectively, 3- and 6-fold greater than those obtained with the parent R1.1 cell line. GTP and its nonhydrolyzable analog, Gpp(NH)p, inhibited [ 3H]U69,593 binding to all three cell lines. Stimulation of low- K m GTPase activity by the k-selective agonist (−)U50,488 was greatest in R1.G1 membranes, followed by R1EGO and R1.1. The maximal inhibition of forskolin-stimulated adenylyl cyclase activity by (−)U50,488 was 66 ± 2% in R1.G1 and 49 ± 2% in R1EGO, compared with 37 ± 1% in R1.1 membranes. Whereas maximal inhibition of adenylyl cyclase activity did not correlate with receptor number among cell lines, the inhibition of cyclic AMP production did correlate with stimulation of low- K m GTPase activity. The R1.1 cell line and its derivatives, R1.G1 and R1EGO, express a similar type of k opioid receptor, which exhibits differences in coupling to G-proteins and to adenylyl cyclase among cell lines. These cell lines provide an excellent model system for studying the regulation of opioid receptor-adenylyl cyclase coupling efficiency.