Agonist-Induced Desensitization and Down-Regulation of δ Opioid Receptors Alter the Levels of Their 125I-β-Endorphin Cross-Linked Products in Subcellular Fractions from NG108-15 Cells

Publisher Website
Google Scholar
Abstract
The δ opioid binding sites in subcellular fractions from NG108-15 cells were characterized with respect to their relative molecular size and levels under conditions of receptor adaptation. 125I-β-Endorphin was cross-linked to preparations enriched in plasma membranes (P20), nuclear membranes or nuclear matrices. Five cross-linked bands appear in all subcellular fractions. The largest molecular size reaction product in nuclear matrix preparations (∼72 kDa) differed from that in the other two fractions(∼83 kDa). Immunoblot analyses with an antibody to the δ opioid receptor gave a P20 band pattern similar to that for the corresponding cross-linked products. To determine which cross-linked products in P20 are glycoproteins, labeled membranes were solubilized and purified by wheat germ agglutinin chromatography. The absence of a ∼36 kDa band after purification suggests that this product is not a glycoprotein. The remaining four bands were present in N-acetyl-d-glucosamine eluates, although their % distribution changes in favor of the largest molecular size band (∼83 kDa). Immunoblotting of the eluate gave a single diffuse band at ∼73 kDa, suggesting the native glycoprotein has a molecular size in the 70−80 kDa range. Etorphine-induced desensitization of cell surface receptors increased the amount of some cross-linked products associated with nuclear membranes. The same treatment did not affect the relative density of the four larger molecular size bands in P20, but increased the density of the ∼26 kDa product two fold. Etorphine-induced down-regulation evoked an elevation of cross-linked products in nuclear matrix preparations, while all band densities of P20 were diminished. These results suggest that nuclear matrix associated opioid binding sites represent internalized, truncated forms of the glycosylated δ opioid receptor found in P20.