Abstract
Internalization, recycling, and resensitization of the human δ-opioid receptor (hDOR) were studied in the neuroblastoma cell line SK-N-BE, endogenously expressing this receptor. Conventional and confocal fluorescence microscopy observations, corroborated by Scatchard analysis, indicated that after a 100 nM Eto treatment, 60 to 70% of hDOR were rapidly internalized (t1/2 < 15 min). This agonist-triggered internalization was reversible for a treatment not exceeding 1 h and became irreversible for prolonged treatment (4 h), leading probably to the degradation and/or down-regulation of the receptor. The rapid internalization of hDOR was totally blocked in the presence of heparin, known as an inhibitor of G protein-coupled receptor kinases (Benovic et al., 1989), a result indicating that phosphorylation by these kinases is a critical step in desensitization (Hasbi et al., 1998) and internalization of hDOR (present study) in SK-N-BE cell line. Blockade of internalization by agents not interferring with phosphorylation, as hypertonic sucrose or concanavalin A, also blocked the resensitization (receptor functional recovering) process. Furthermore, blockade of dephosphorylation of the internalized hDOR by okadaic acid totally suppressed its recycling to the plasma membrane and its subsequent resensitization. These results indicate that regulatory events leading to desensitization, internalization, and recycling in a functional state of hDOR involve phosphorylation by a G protein-coupled receptor kinase, internalization via clathrin-coated vesicles, and dephosphorylation by acid phosphatases.
Footnotes
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Send reprint requests to: Dr. Ahmed Hasbi, Universite de Caen BP 5229, Centre National de la Recherche Scientifique UnitéMixte de Recherche 6551, Laboratoire des Neurosciences, 14074 Caen Cedex, France. E-mail: jauzac-p{at}chu-caen.fr
- Abbreviations:
- GPCR
- G protein-coupled receptor
- DOR
- δ-opioid receptor
- GRK
- G protein-coupled receptor kinase
- MOR
- μ-opioid receptor
- hDOR
- human δ-opioid receptor
- OA
- okadaic acid
- Eto
- etorphine
- Received June 29, 1999.
- Accepted January 11, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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