Ligands regulate cell surface level of the human κ opioid receptor by activation-induced down-regulation and pharmacological chaperone-mediated enhancement:: Differential effects of nonpeptide and peptide agonists

Two peptide agonists, eight nonpeptide agonists, and five nonpeptide antagonists were evaluated for their capacity to regulate FLAG (DYKDDDDK)-tagged human kappa opioid receptors (hKORs) stably expressed in Chinese hamster ovary cells after incubation for 4 h with a ligand at a concentration similar to 1000-fold of its EC50 (agonist) or K-i (antagonist) value. Dynorphins A and B decreased the fully glycosylated mature form (55-kDa) of FLAG-hKOR by 70%, whereas nonpeptide full agonists [2-(3,4-dichlorophenyl)-N-methyl-N-[(2R)-2-pyrrolidin-1-ylcyclohexyl] acetamide (U50,488H), 17-cyclopropylmethyl-3, 14-dihydroxy-4,5-epoxy-6-[N-methyl-trans-3-(3-furyl) acrylamido]morphinan hydrochloride (TRK-820), ethylketocyclazocine, bremazocine, asimadoline, and (RS)-[3-[1-[[(3,4-dichlorophenyl) acetyl]methylamino]-2-(1-pyrrolidinyl)ethyl]phenoxy] acetic acid hydrochloride (ICI 204,448) caused 10-30% decreases. In contrast, pentazocine (partial agonist) and etorphine (full agonist) up-regulated by similar to 15 and 25%, respectively. The antagonists naloxone and norbinaltorphimine also significantly increased the 55-kDa receptor, whereas selective mu, delta, and D-1 receptor antagonists had no effect. Naloxone up-regulated the receptor concentration- and time-dependently and enhanced the receptor maturation extent, without affecting its turnover. Treatment with brefeldin A (BFA), which disrupts Golgi, resulted in generation of a 51-kDa form that resided intracellularly. Naloxone up-regulated the new species, indicating that its action site is in the endoplasmic reticulum as a pharmacological chaperone. After treatment with BFA, all nonpeptide agonists up-regulated the 51-kDa form, whereas dynorphins A and B did not, indicating that nonpeptide agonists act as pharmacological chaperones, but peptide agonists do not. BFA treatment enhanced down-regulation of the cell surface receptor induced by nonpeptide agonists, but not that by peptide agonists, and unmasked etorphine- and pentazocine-mediated receptor down-regulation. These results demonstrate that ligands have dual effects on receptor levels: enhancement by chaperone-like effects and agonist-promoted down-regulation, and the net effect reflects the algebraic sum of the two.