The endoplasmic reticulum (ER) engages mitochondria at specialized ER domains known as mitochondria-associated membranes (MAMs). Here, we used three-dimensional high-resolution imaging to investigate the formation of pleomorphic “megamitochondria” with altered MAMs in brown adipocytes lacking the Sel1L-Hrd1 protein complex of ER-associated protein degradation (ERAD). Mice with ERAD deficiency in brown adipocytes were cold sensitive and exhibited mitochondrial dysfunction. ERAD deficiency affected ER-mitochondria contacts and mitochondrial dynamics, at least in part, by regulating the turnover of the MAM protein, sigma receptor 1 (SigmaR1). Thus, our study provides molecular insights into ER-mitochondrial cross-talk and expands our understanding of the physiological importance of Sel1L-Hrd1 ERAD.
Inhibition of de novo lipid biosynthesis by suppressing the SREBP pathway prevents HCC. This study identifies a previously underappreciated role of the SREBP pathway in HCC and suggests a novel metabolic strategy to control liver cancer. (Hepatology 2017;65:1936-1947).
This article is available online at http://www.jlr.org diffuses to the brush border membrane of enterocytes ( 1 ). Then cholesterol moves to the endoplasmic reticulum (ER) and is esterifi ed by ACAT2 (1-5). Finally, the newly formed cholesterol ester is packed into chylomicrons and secreted to lymph ( 1 ).The Niemann-Pick C1 like1 (NPC1L1) protein is the gatekeeper of dietary cholesterol absorption. Genetic deletion of NPC1L1 decreases cholesterol absorption by more than 70% in mice ( 6 ). In rodents, NPC1L1 is selectively expressed in the small intestine and localizes on the brush border membrane ( 6-8 ). It has been proposed that NPC1L1 mediates cholesterol movement into enterocytes in vivo. However, direct evidence is lacking.Studies in cultured cells indicate that NPC1L1 facilitates cholesterol entering cytoplasm by vesicular endocytosis ( 9, 10 ). NPC1L1 forms cholesterol-enriched membrane microdomains on plasma membrane with lipid raft proteins Flotillin-1/-2 ( 11 ). The NPC1L1-Flotillin-cholesterol microdomains are internalized via clathrin/AP2 pathway and transported to the endocytic recycling compartment (ERC) ( 9, 11 ). The ERC is a cellular cholesterol pool and a Rab11a-positive compartment ( 9, 12-14 ). When the ERC cholesterol level drops, NPC1L1 moves to plasma membrane to mediate another round of cholesterol transport ( 9,13,(15)(16)(17). Ezetimibe, a cholesterol absorption inhibitor, blocks the internalization of NPC1L1-Flotillin-cholesterol microdomains and therefore decreases cholesterol uptake in cultured cells ( 9,11,17,18 ). How NPC1L1 and ezetimibe work in vivo is unknown.There are two isoforms of ACAT enzymes in mammals, the ACAT1 and ACAT2. ACAT1 is ubiquitously expressed in all tissues, whereas ACAT2 is specifi cally expressed in liver and small intestine (19)(20)(21)(22)(23)(24). Previous studies have indicated Dietary cholesterol absorption in small intestine is a major way for mammals to obtain cholesterol. In intestinal lumen, cholesterol incorporates into bile salt micelles and
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