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

Publisher Website
Google Scholar
Abstract
Two peptide agonists, eight nonpeptide agonists, and five nonpeptide antagonists were evaluated for their capacity to regulate FLAG (DYKDDDDK)-tagged human κ opioid receptors (hKORs) stably expressed in Chinese hamster ovary cells after incubation for 4 h with a ligand at a concentration ∼1000-fold of its EC50 (agonist) or Ki (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 ∼15 and 25%, respectively. The antagonists naloxone and norbinaltorphimine also significantly increased the 55-kDa receptor, whereas selective μ, δ, and D1 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.