title: ORMDL orosomucoid-like proteins are degraded by free-cholesterol-loading-induced autophagy.
abstract: Eukaryotic cells have evolved robust mechanisms to counter excess cholesterol including redistribution of lipids into different compartments and compensatory up-regulation of phospholipid biosynthesis. We demonstrate here that excess cellular cholesterol increased the activity of the endoplasmic reticulum (ER) enzyme serine palmitoyl-CoA transferase (SPT), the rate-limiting enzyme in sphingomyelin synthesis. This increased SPT activity was not due to altered levels of SPTLC1 or SPTLC2, the major subunits of SPT. Instead, cholesterol loading decreased the levels of ORMDL1, a negative regulator of SPT activity, due to its increased turnover. Several lines of evidence demonstrated that free-cholesterol-induced autophagy, which led to increased turnover of ORMDL1. Cholesterol loading induced ORMDL1 redistribution from the ER to cytoplasmic p62 positive autophagosomes. Coimmunoprecipitation analysis of cholesterol-loaded cells showed increased association between ORMDL1 and p62. The lysosomal inhibitor chloroquine or siRNA knockdown of Atg7 inhibited ORMDL1 degradation by cholesterol, whereas proteasome inhibitors showed no effect. ORMDL1 degradation was specific to free-cholesterol loading as autophagy induced by serum starvation or general ER stress did not lead to ORMDL1 degradation. ORMDL proteins are thus previously unidentified responders to excess cholesterol, exiting the ER to activate SPT and increase sphingomyelin biosynthesis, which may buffer excess cellular cholesterol.
authors.affiliation: Departments of Cellular and Molecular Medicine and.
authors.forename: Shuhui
authors.initials: S
authors.lastname: Wang
authors.affiliation: Departments of Cellular and Molecular Medicine and.
authors.forename: Peggy
authors.initials: P
authors.lastname: Robinet
authors.affiliation: Departments of Cellular and Molecular Medicine and Cardiovascular Medicine, Cleveland Clinic, Cleveland, OH 44195 gulshak@ccf.org smithj4@ccf.org.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Smith
authors.affiliation: Departments of Cellular and Molecular Medicine and gulshak@ccf.org smithj4@ccf.org.
authors.forename: Kailash
authors.initials: K
authors.lastname: Gulshan
mesh_terms: D000818:Animals; D001249:Asthma / Q000378:metabolism; D050197:Atherosclerosis / Q000378:metabolism; D001343:Autophagy; D001692:Biological Transport; D002460:Cell Line; D002784:Cholesterol / Q000378:metabolism; D003513:Cycloheximide / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism; D059865:Endoplasmic Reticulum Stress; D004804:Eosinophils / Q000378:metabolism; D006706:Homeostasis; D008055:Lipids / Q000737:chemistry; D008264:Macrophages / Q000378:metabolism; D021962:Membrane Microdomains / Q000737:chemistry; D008565:Membrane Proteins; D051379:Mice; D009961:Orosomucoid / Q000378:metabolism*; D021381:Protein Transport; D051102:Serine C-Palmitoyltransferase / Q000737:chemistry; D013107:Sphingolipids / Q000737:chemistry; D013109:Sphingomyelins / Q000737:chemistry
chemical_list: D008055:Lipids; D008565:Membrane Proteins; C000598563:ORMDL1 protein, mouse; D009961:Orosomucoid; D013107:Sphingolipids; D013109:Sphingomyelins; D002784:Cholesterol; D003513:Cycloheximide; D051102:Serine C-Palmitoyltransferase
keywords: ORMDL1; autophagy; free cholesterol; serine palmitoyl-CoA transferase; sphingomyelin