Feedback inhibition of SREBP proteolysis is mediated by Insig proteins, membrane proteins that retain the Scap/ SREBP complex in the ER when the cholesterol content of Abstract Enterocyte cholesterol homeostasis refl

Journal of Lipid Research Volume 53, 2012 1359 Copyright © 2012 by the American Society for Biochemistry and Molecular Biology, Inc. unusual in that they have three, rather than two, sources of cholesterol. Enterocytes uniquely absorb free cholesterol from the gut lumen, and they also share two ubiquitous sources with other cells: endogenous synthesis of sterols de novo and uptake of LDL-derived cholesterol from plasma. The small intestine is one of the major sites of cholesterol synthesis in mammals. It is second only to liver in mice, and it may be quantitatively the single most important organ in other species, such as rabbit and guinea pig ( 1, 2 ). Likewise, the small intestine is second only to the liver in terms of overall LDL clearance in multiple species ( 1, 3, 4 ). The transcriptional regulation of de novo sterol synthesis and LDL receptor-mediated sterol uptake in cultured cells and in liver has been well characterized ( 5 ). Transcription of the LDL receptor (LDLR) and of every gene of the sterol biosynthetic pathway is regulated by sterol regulatory element-binding protein-2 (SREBP-2), one of three SREBP family members ( 6 ). SREBPs are synthesized as transcriptionally inactive precursor molecules that are integral membrane proteins of the endoplasmic reticulum (ER). To become transcriptionally active, they must move to the Golgi complex, where they are proteolyzed to liberate the soluble N-terminal domain, which can enter the nucleus to activate transcription ( 7 ). ER-to-Golgi transport is accomplished by the action of Scap, a polytopic ER membrane protein that interacts with SREBPs and mediates their incorporation into COPII-coated vesicles ( 8, 9 ). Feedback inhibition of SREBP proteolysis is mediated by Insig proteins, membrane proteins that retain the Scap/ SREBP complex in the ER when the cholesterol content of Abstract Enterocyte cholesterol homeostasis refl ects aggregated rates of sterol synthesis, effl ux, and uptake from plasma and gut lumen. Cholesterol synthesis and LDL uptake are coordinately regulated by sterol regulatory elementbinding proteins (SREBP), whereas sterol effl ux is regulated by liver X receptors (LXR). How these processes are coordinately regulated in enterocytes, the site of cholesterol absorption, is not well understood. Here, we treat mice with ezetimibe to investigate the effect of blocking cholesterol absorption on intestinal SREBPs, LXRs, and their effectors. Ezetimibe increased nuclear SREBP-2 8-fold. HMG-CoA reductase (HMGR) and LDL receptor (LDLR) mRNA levels increased less than 3-fold, whereas their protein levels increased 30and 10-fold, respectively. Expression of inducible degrader of LDLR (IDOL), an LXR-regulated gene that degrades LDLRs, was reduced 50% by ezetimibe. Coadministration of ezetimibe with the LXR agonist T0901317 abolished the reduction in IDOL and prevented the increase in LDLR protein. Ezetimibe-stimulated LDLR expression was independent of proprotein convertase subtilisin/kexin type 9 (PSCK9), a protein that degrades LDLRs. To maintain cholesterol homeostasis in the face of ezetimibe, enterocytes boost LDL uptake by increasing LDLR number, and they boost sterol synthesis by increasing HMGR and other cholesterologenic genes. These studies reveal a hitherto undescribed homeostatic network in enterocytes triggered by blockade of cholesterol absorption. —Engelking, L. J., M. R. McFarlane, C. K. Li, and G. Liang. Blockade of cholesterol absorption by ezetimibe reveals a complex homeostatic network in enterocytes. J. Lipid Res. 2012. 53: 1359–1368.