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标题：ORMDL orosomucoid-like proteins are degraded by free-cholesterol-loading–induced autophagy
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作者：Shuhui Wang ; Peggy Robinet ; Jonathan D. Smith 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：12
页码：3728-3733
DOI：10.1073/pnas.1422455112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceAccumulation of toxic lipids in macrophages or human plaques leads to endoplasmic reticulum (ER) stress and induction of autophagy. However, the mechanism by which ER stress and autophagy help in fine tuning cellular lipid homeostasis to counter free-cholesterol toxicity is not clear. Our studies demonstrate that cholesterol induces the translocation of ORMDL orosomucoid-like proteins out of the ER and targets them to autophagosomes, thereby relieving their negative regulation on cellular de novo sphingolipid synthesis. In addition, ORMDL3 has been recently implicated in childhood asthma and in eosinophil trafficking and activation. Thus, our finding of increased turnover of ORMDL proteins by free cholesterol is relevant not only in context to atherosclerotic disease progression but may also shed new insights on lipid homeostasis in other human diseases. 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.
关键词：ORMDL1 ; autophagy ; free cholesterol ; sphingomyelin ; serine palmitoyl-CoA transferase