Insig required for sterol-mediated inhibition of Scap/SREBP binding to COPII proteins in vitro

When added to living cells, sterols such as cholesterol and 25-hydroxycholesterol block the lateral movement of sterol regulatory element-binding proteins (SREBPs) into COPII-coated vesicles on endoplasmic reticulum (ER) membranes and thereby prevent the SREBPs from reaching the Golgi complex for processing to the mature forms that activate cholesterol synthesis. Sorting of SREBPs into COPII vesicles is mediated by Sar1 and the coat proteins Sec23 and Sec24. Here, we explore the mechanism of sterol inhibition in vitro through use of protein pull-down assays. We show that addition of cholesterol or 25-hydroxycholesterol to microsomal membranes in vitro blocks Sar1-dependent binding of the Sec23/24 complex to Scap, the SREBP escort protein. This in vitro inhibition is dependent on the presence of Insig-1, an ER resident protein that is necessary for sterol-mediated inhibition of Scap/SREBP transport in intact cells. Sec23/24 binding to Scap requires the hexapeptide sequence MELADL located in a cytoplasmic loop of Scap. This hexapeptide acts as a sterol-regulated ER sorting signal. These studies define the biochemical parameters responsible for regulated sorting of an ER membrane protein into COPII-coated vesicles.     

Identification of luminal Loop 1 of Scap protein as the sterol sensor that maintains cholesterol homeostasis

Cellular cholesterol homeostasis is maintained by Scap, an endoplasmic reticulum (ER) protein with eight transmembrane helices. In cholesterol-depleted cells, Scap transports sterol regulatory element-binding proteins (SREBPs) to the Golgi, where the active fragment of SREBP is liberated by proteases so that it can activate genes for cholesterol synthesis. When ER cholesterol increases, Scap binds cholesterol, and this changes the conformation of cytosolic Loop 6, which contains the binding site for COPII proteins. The altered conformation precludes COPII binding, abrogating movement to the Golgi. Consequently, cholesterol synthesis declines. Here, we identify the cholesterol-binding site on Scap as Loop 1, a 245-amino acid sequence that projects into the ER lumen. Recombinant Loop 1 binds sterols with a specificity identical to that of the entire Scap membrane domain. When tyrosine 234 in Loop 1 is mutated to alanine, Loop 6 assumes the cholesterol-bound conformation, even in sterol-depleted cells. As a result, full-length Scap(Y234A) cannot mediate SREBP processing in transfected cells. These results indicate that luminal Loop 1 of Scap controls the conformation of cytosolic Loop 6, thereby determining whether cells produce cholesterol.     

Insig Proteins Mediate Feedback Inhibition of Cholesterol Synthesis in the Intestine

Enterocytes are the only cell type that must balance the de novo synthesis and absorption of cholesterol, although the coordinate regulation of these processes is not well understood. Our previous studies demonstrated that enterocytes respond to the pharmacological blockade of cholesterol absorption by ramping up de novo sterol synthesis through activation of sterol regulatory element-binding protein-2 (SREBP-2). Here, we genetically disrupt both Insig1 and Insig2 in the intestine, two closely related proteins that are required for the feedback inhibition of SREBP and HMG-CoA reductase (HMGR). This double knock-out was achieved by generating mice with an intestine-specific deletion of Insig1 using Villin-Cre in combination with a germ line deletion of Insig2. Deficiency of both Insigs in enterocytes resulted in constitutive activation of SREBP and HMGR, leading to an 11-fold increase in sterol synthesis in the small intestine and producing lipidosis of the intestinal crypts. The intestine-derived cholesterol accumulated in plasma and liver, leading to secondary feedback inhibition of hepatic SREBP2 activity. Pharmacological blockade of cholesterol absorption was unable to further induce the already elevated activities of SREBP-2 or HMGR in Insig-deficient enterocytes. These studies confirm the essential role of Insig proteins in the sterol homeostasis of enterocytes.     

Piperine Induces Hepatic Low-Density Lipoprotein Receptor Expression through Proteolytic Activation of Sterol Regulatory Element-Binding Proteins

Elevated plasma low-density lipoprotein (LDL) cholesterol is considered as a risk factor for atherosclerosis. Because the hepatic LDL receptor (LDLR) uptakes plasma lipoproteins and lowers plasma LDL cholesterol, the activation of LDLR is a promising drug target for atherosclerosis. In the present study, we identified the naturally occurring alkaloid piperine, as an inducer of LDLR gene expression by screening the effectors of human LDLR promoter. The treatment of HepG2 cells with piperine increased LDLR expression at mRNA and protein levels and stimulated LDL uptake. Subsequent luciferase reporter gene assays revealed that the mutation of sterol regulatory element-binding protein (SREBP)-binding element abolished the piperine-mediated induction of LDLR promoter activity. Further, piperine treatments increased mRNA levels of several SREBP targets and mature forms of SREBPs. However, the piperine-mediated induction of the mature forms of SREBPs was not observed in SRD–15 cells, which lack insulin-induced gene–1 (Insig–1) and Insig–2. Finally, the knockdown of SREBPs completely abolished the piperine-meditated induction of LDLR gene expression in HepG2 cells, indicating that piperine stimulates the proteolytic activation of SREBP and subsequent induction of LDLR expression and activity.