Glycosylation‐Dependent Regulation of Opiate (Enkephalin) Receptors in Neurotumor Cells

Abstract

Electron inactivation analysis revealed that the opiate (enkephalin) binding site in neurotumor [hybrid] cell lines [mouse neuroblastoma .times. rat glioma] NG108-15 and [mouse neuroblastoma .times. Chinese hamster brain] NCB-20 had an apparent target size of 200,000 daltons. Expression of functional opiate receptors in neurotumor cells appeared to require glycosylation, as treatment of such cells with tunicamycin (TM; under conditions where de novo glycosylation of asparagine residues in protein was reduced by 80%, but overall protein and DNA synthesis were inhibited by < 10%) resulted in the loss of 50% of the opiate binding sites. The loss of binding sites could not be prevented by addition of protease inhibitors to cell cultures, but binding sites were partially restored 48-60 h after removal of the TM. The number of enkephalin binding sites in TM-treated cells was also restored to near-normal levels by addition of physiological concentrations (1-10 mM) of Mn to the in vitro receptor binding incubation mixture. TM treatment resulted in receptor supersensitivity to Mn for both opiate agonists and antagonists, no change in the Na effect for either agonists or antagonists, and subsensitivity to GTP for both agonists and antagonists. Opiate binding to cell membranes was not substantially inhibited by either neuraminidase treatment or short-term incubation with lectins such as wheat germ agglutinin, ricin or concanavalin A. Evidently, oligosaccharide units are not directly involved in opiate receptor-ligand interactions, but protein glycosylation is required for functional expression of receptors.