Disease relevance of SCAP

• Using human hepatoma cells, we show that these compounds act through the sterol-responsive element of the LDLr promoter and activate the SCAP/SREBP pathway, leading to increased LDLr expression and activity, even in presence of excess of sterols [1].
• When insect cells were infected with SREBP-2 or SCAP recombinant viruses, it was found that these ER membrane proteins appeared on the budded baculovirus in addition to the host cell membrane fraction [2].
• Through immunoisolation, we show that SCAP-containing vesicles, formed in vitro, also contain vesicular stomatitis virus glycoprotein (VSVG) protein, a classic marker of COPII-coated vesicles [3].
• Chloramphenicol sodium succinate (SCAP) kinetics were studied in 10 critically ill patients [4].
• Effects of SREBF-1a and SCAP polymorphisms on plasma levels of lipids, severity, progression and regression of coronary atherosclerosis and response to therapy with fluvastatin [5].

High impact information on SCAP

• Crucial step in cholesterol homeostasis: sterols promote binding of SCAP to INSIG-1, a membrane protein that facilitates retention of SREBPs in ER [6].
• In sterol-depleted cells, SCAP escorts SREBPs from ER to Golgi for proteolytic processing, thereby allowing SREBPs to stimulate cholesterol synthesis [6].
• In sterol-depleted cells, SCAP escorts SREBPs from ER to Golgi, where SREBPs are cleaved by Site-1 protease (S1P) [7].
• Proteolytic release of the active domains of SREBPs from membranes requires a sterol-sensing protein, SCAP, which forms a complex with SREBPs [7].
• SCAP has multiple membrane-spanning regions, five of which resemble the sterol-sensing domain of HMG CoA reductase, an endoplasmic reticulum enzyme whose degradation is accelerated by sterols [8].

Biological context of SCAP

• By contrast, NPC1 proteins carrying a L657F or D787N point mutation, which correspond to the activating SCAP L315F and D443N mutations, respectively, exhibit a gain of function phenotype [9].
• Here, we use transfection techniques to overexpress a segment of SCAP containing transmembrane helices 1-6 in hamster and human cells [10].
• Silencing of the mutant SCAP allele accounts for restoration of a normal phenotype in CT60 cells selected for NPC1 expression [11].
• Downregulation of cholesterol synthesis via the SREBP/SCAP regulatory pathway is common to the initial hydroxycholesterols, but more variations exist with respect to these intermediates functioning as ligands for the nuclear receptor LXRalpha [12].
• This suggests that SCAP gene expression is under the control of SREBP-1, a key regulator of the expression of genes essential for intracellular lipid metabolism [13].

Anatomical context of SCAP

• The COOH-terminal domain of SCAP, like that of the SREBPs, is located on the cytosolic face of the membranes [14].
• SREBP cleavage-activating protein (SCAP) stimulates the proteolytic cleavage of membrane-bound SREBPs, thereby initiating the release of NH2-terminal fragments from cell membranes [14].
• Unexpectedly, the YAC-containing cell lines have overall cellular cholesterol concentrations that are comparable to wild-type levels, despite the assumed presence of the SCAP mutation [11].
• This cleavage step is regulated by a putative sterol-sensing molecule, SREBP-activating protein (SCAP), that forms a complex with SREBPs and traffics between the rough endoplasmic reticulum and Golgi [15].
• SCAP RNAi or 25-HC inhibited VEGF-induced pseudopodia extension and migration of ECs [16].

Associations of SCAP with chemical compounds

• Sterol resistance was traced to a G-->A transition at codon 443 of SCAP, changing aspartic acid to asparagine [8].
• SCAP appears to be a central regulator of cholesterol metabolism in animal cells [8].
• We identified a new common amino acid polymorphism of isoleucine/valine at codon 796 in exon 16 of the gene for human sterol regulatory element binding protein (SREBP) cleavage-activating protein (SCAP), a central regulator of lipid synthesis and metabolism in animal cells [17].
• METHODS AND RESULTS: Our studies uncovered several new features that distinguish SCAP ligand from lovastatin, a classic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, and from OM, which utilize an SREBP-independent regulatory pathway [18].
• In transfected hamster cells, mutant SCAP in which Asp-428 is replaced by alanine (D428A) remained in an Insig-binding conformation when cells were depleted of sterols [19].

Physical interactions of SCAP

• The membranous nature of Insig-1 is consistent with its sterol-dependent binding to hydrophobic sterol-sensing domains in SCAP and HMG CoA reductase [20].
• Truncation of SREBP-2 at its COOH terminus prevents the formation of complexes with SCAP and simultaneously reduces proteolytic cleavage [14].
• The C-terminal domain of SREBP interacts with the C-terminal domain of SREBP-cleavage-activating protein (SCAP) [21].
• These studies suggest that sterols regulate the cleavage of SREBPs by modulating the ability of SCAP to transport SREBPs to a post-ER compartment that houses active Site-1 protease [22].

Regulatory relationships of SCAP

• Cholesterol-induced conformational change in SCAP enhanced by Insig proteins and mimicked by cationic amphiphiles [23].
• Insig-1 and Insig-2 also enhance the conformational change in SCAP that occurs upon addition of certain cationic amphiphiles, such as chlorpromazine, trifluoperazine, and imipramine, which mimic the effect of cholesterol [23].

Other interactions of SCAP

• These findings suggest that the INSIG-1 protein alters sterol balance by modulating SREBP processing jointly with SCAP [24].
• Furthermore, our data show that the putative SSD of NPC1 is oriented in the same manner as those of HMG-R and SCAP, providing strong evidence that this domain is functionally important [25].
• Insig-1 appears to play an essential role in the sterol-mediated trafficking of two proteins with sterol-sensing domains, HMG CoA reductase and SCAP [26].
• Complex formation occurs when cells express only the COOH-terminal domain of either SREBP-2 or SCAP, indicating that the complex forms between the two COOH-terminal domains [14].
• Subgroup analysis considering the most frequent (N>/=24) LDL receptor mutations (del191-2, ins313+1-2, C660X, E207K, S285L) revealed further gene-dosage- and gender-dependent effects of the SCAP mutations on LDL-cholesterol concentrations (p=0.0345) [27].

Analytical, diagnostic and therapeutic context of SCAP

• Co-immunoprecipitation experiments show that SCAP and SREBP-2 form a complex that can be precipitated with antibodies to either component [14].
• The SCAP gene was localized to chromosome 3p21.3 by fluorescence in situ hybridization [13].
• High-performance liquid chromatography was used to assay SCAP and chloramphenicol (CAP) in serum and urine [4].
• 5. The calibration studies were performed by using SIAP and SCAP polarography and satisfactory results were observed for all techniques [28].
• Isolation and radiation hybrid mapping of a highly polymorphic CA repeat sequence at the SREBP cleavage-activating protein (SCAP) locus [29].

References