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The class A scavenger receptor (SR-A) is a multifunctional trimeric membrane glycoprotein involved in atherogenesis. The mature receptor can mediate the binding and internalization of a number of specific ligands, including modified low-density lipoprotein. We have investigated the effects of inhibiting N-glycan processing on SR-A expression, distribution, and activity in the murine macrophage cell line RAW264.7. We have found that SR-A normally interacts with calnexin in the endoplasmic reticulum and in its mature form carries complex N-glycans. The imino sugar, N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of the N-glycan processing enzymes alpha-glucosidases I and II. Following NB-DNJ treatment SR-A became Endo H-sensitive, consistent with inhibition of N-glycan processing. A dose-dependent increase in cell surface expression of SR-A was observed in response to NB-DNJ treatment. The receptor on inhibitor-treated cells was still functional because the increased surface expression resulted in a proportional enhancement in the endocytosis of the ligand, acetylated low-density lipoprotein. The expression of SR-A on NB-DNJ cultured cells was further enhanced by co-treatment with interferon-gamma. Quantitative reverse transcriptase-PCR analysis did not show a significant difference in the amount of SR-A mRNA in NB-DNJ-treated RAW264.7 cells. However, the half-life of SR-A protein was significantly increased. These data indicate the retention of glucosylated N-glycans does not result in gross misfolding and degradation of this receptor or prevent its transport to the cell surface. SR-A interacts with calnexin and when the association is prevented changes in the recycling kinetics and rate of turnover of the receptor result, leading to enhanced cell surface expression.

Original publication

DOI

10.1074/jbc.M405219200

Type

Journal article

Journal

J Biol Chem

Publication Date

17/09/2004

Volume

279

Pages

39303 - 39309

Keywords

1-Deoxynojirimycin, Animals, Antineoplastic Agents, Calnexin, Carbocyanines, Drug Synergism, Electrophoretic Mobility Shift Assay, Endocytosis, Enzyme Inhibitors, Glycoside Hydrolase Inhibitors, Glycosylation, Interferon-gamma, Lipoproteins, LDL, Macrophages, Mice, Protein Transport, RNA, Messenger, Receptors, Cell Surface, Receptors, Immunologic, Receptors, Scavenger, Reverse Transcriptase Polymerase Chain Reaction, Scavenger Receptors, Class A, alpha-Glucosidases