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CELLULAR AND MOLECULAR

Localization of the Opioid Receptor in Lipid Rafts

Department of Pharmacology, Center for Substance Abuse Research (W.X., P.H., Y.W., C.C., L.-Y.L.-C.) and Department of Biochemistry (S.-I.Y., P.L.-G.C.), Temple University School of Medicine, Philadelphia, Pennsylvania

Lipid rafts are microdomains of plasma membranes enriched in cholesterol and sphingolipids in the outer layer. We determined whether opioid receptors (KOR) in human placenta and FLAG (DYKDDDDK)-tagged human KOR (FLAG-hKOR) expressed in Chinese hamster ovary (CHO) cells are localized in lipid rafts and whether changes in cholesterol contents affect hKOR properties and signaling. Lipid rafts were prepared from placenta membranes and CHO cells expressing FLAG-hKOR using the Na₂CO₃ method and fractionation through a sucrose density gradient. The majority of the KOR in the placenta and FLAG-hKOR in CHO cells, determined by [³H]diprenorphine binding and/or immunoblotting with an anti-FLAG antibody, was present in low-density fractions, coinciding with high levels of caveolin-1 and cholesterol, markers of lipid rafts, which indicated that the KOR is localized in lipid rafts. Pretreatment with 2% methyl -cyclodextrin (MCD) reduced cholesterol content by 48% and changed the cells from spindle-shaped to spherical. MCD treatment disrupted lipid rafts, shifted caveolin-1 and FLAG-hKOR to higher density fractions, increased the affinity of (–)-(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide (U50,488H) for the hKOR, and greatly increased U50,488H-induced [³⁵S]guanosine 5'-O-(3-thio)triphosphate binding and p42/44 mitogen-activated protein kinase phosphorylation. Cholesterol replenishment reversed all the MCD effects. Caveolin-1 immunoprecipitated with G_i proteins and MCD treatment reduced caveolin-1 associated with G_i proteins, which may contribute to the enhanced agonist-induced G protein activation. Caveolin-1 also immunoprecipitated with FLAG-hKOR, but MCD treatment had no effect on the association. Thus, the KOR is located in lipid rafts and its localization in the microdomains greatly affects coupling to G proteins.

Address correspondence to: Dr. Lee-Yuan Liu-Chen, Department of Pharmacology, Temple University School of Medicine, 3420 N. Broad St., Philadelphia, PA 19140. E-mail: lliuche{at}temple.edu

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