Search Article Keyword  
PubMed Submission Abstract PDF Cited  Click Count: 8842 DownLoad Count: 4188 

ISSN 1007-9327 CN 14-1219/R  World J Gastroenterol  2004 November 1;10(21):3081-3087

Maintaining cholesterol homeostasis: Sterol regulatory element-binding proteins 

Lutz W. Weber, Meinrad Boll, Andreas Stampfl

Lutz W. Weber, 554 Mariner Point Drive, Clinton, TN 37716, USA
Meinrad Boll, Andreas Stampfl, Institute for Toxicology, GSF - National Research Center for Environment and Health, Munich, D-85758 Neuherberg, Germany
Correspondence to: Lutz W. Weber, Institute of Toxicology, GSF - National Research Center for Environment and Health, Munich, D-85758 Neuherberg, Germany.
Telephone: +49-89-3187-2625    Fax: +49-89-3187-3449
Received: 2003-10-15    Accepted: 2004-04-13

The molecular mechanism of how hepatocytes maintain cholesterol homeostasis has become much more transparent with the discovery of sterol regulatory element binding proteins (SREBPs) in recent years. These membrane proteins are members of the basic helix-loop-helix-leucine zipper (bHLH-Zip) family of transcription factors. They activate the expression of at least 30 genes involved in the synthesis of cholesterol and lipids. SREBPs are synthesized as precursor proteins in the endoplasmic reticulum (ER), where they form a complex with another protein, SREBP cleavage activating protein (SCAP).The SCAP molecule contains a sterol sensory domain. In the presence of high cellular sterol concentrations SCAP confines SREBP to the ER. With low cellular concentrations, SCAP escorts SREBP to activation in the Golgi. There, SREBP undergoes two proteolytic cleavage steps to release the mature, biologically active transcription factor, nuclear SREBP (nSREBP). nSREBP translocates to the nucleus and binds to sterol response elements (SRE) in the promoter/enhancer regions of target genes. Additional transcription factors are required to activate transcription of these genes. Three different SREBPs are known, SREBPs-1a, -1c and -2. SREBP-1a and -1c are isoforms produced from a single gene by alternate splicing. SREBP-2 is encoded by a different gene and does not display any isoforms. It appears that SREBPs alone, in the sequence described above, can exert complete control over cholesterol synthesis, whereas many additional factors (hormones, cytokines, etc.) are required for complete control of lipid metabolism. Medicinal manipulation of the SREBP/SCAP system is expected to prove highly beneficial in the management of cholesterol-related disease.

Weber LW, Boll M, Stampfl A. Maintaining cholesterol homeostasis: Sterol regulatory element-binding proteins. World J Gastroenterol  2004; 10(21): 3081-3087

The view of cholesterol as a nasty substance which clogs arteries and causes heart disease is wide-spread, but it does not do the molecule justice. Not only is it a vital component of cell membranes without which the cell cannot function, but it is also the precursor to all steroid hormones, bile acids, and oxysterols, which by themselves are important regulatory molecules in many metabolic pathways.
     Cholesterol and fatty acids as building blocks of cell membranes are synthesized via regulated pathways. All cells must control  these pathways in order to maintain levels within physiological boundaries. Excessive amounts of cholesterol in cells can destroy membrane function, precipitate as crystals which will kill the cell or result in atherosclerotic damage if spread to blood[1]. However, the original view of random distribution of cholesterol and lipids in the cell membrane no longer holds: not only differs the lipid composition of the outer leaflet of the plasma membrane from the inner one, but the distribution of lipids and cholesterol in the outer leaflet is organized into domains, with so-called rafts[2] and caveolae[3] being rich in cholesterol and sphingomyelin. These structures play intricate roles in cholesterol trafficking to maintain cellular homeostasis, and they are also components of the cellular signalling system.  The membranes of endoplasmic reticulum (ER) and Golgi, on the other hand, contain comparatively little cholesterol, a factor important in its own homeostasis, and one objective of this overview.
      The understanding of cholesterol regulation has come a long way from the initial recognition of cholesterol feedback inhibition of its rate-limiting synthetic enzyme, 3-hydroxy-3- methylglutaryl coenzyme A (HMGCoA) reductase, through the role of lipoproteins in maintaining plasma cholesterol levels, to the recent discoveries of regulation of cholesterol synthesis via sterol-sensitive response elements (SREs), and degradation via liver X receptor (LXR) - or bile acid receptor (BAR) -regulated pathways.
      Lipid homeostasis via SREs in animal cells is achieved by a family of transcription factors called SRE-binding proteins (SREBPs). SREBPs activate directly the expression of some 30-plus genes participating in the metabolism mostly of lipids, but also glucose. Activation of these originally membrane-bound transcription factors involves a proteolytic cascade through which the SREBP molecule is released from the membrane and obtains its mature form as a transcription factor. The active SREBP enters the nucleus and binds to those special DNA sequences, the SREs, in the promoter regions of many different genes.
      In the health arena, SREBPs stand at a crucial point: they regulate expression of the LDL receptor, the molecule which enables the hepatocytes to remove cholesterol contained in LDL particles from the bloodstream. High (dietary) cholesterol prevents maturation of SREBPs and not only cuts off cholesterol synthesis, but also LDL receptor synthesis, resulting in high blood cholesterol and the imminent danger of atherosclerotic plaque formation. At this point in time the so-called statins, drugs which block HMGCoA reductase, another target of SREBP-mediated gene expression, are the most effective way to interrupt this vicious circle[4].
     This brief overview is concerned with the SREBP-mediated control of cholesterol and lipid synthesis. Lipid synthesis is subject to many other regulatory influences which cannot be addressed here, nor can the degradation of cholesterol via hormone or bile acid synthesis be covered, which again commands its own set of regulatory substances. The focus of this minireview is to present the latest data on the SREBP-induced mechanism. Recent in-depth reviews have been available[5-9].

Figure 1(PDF) Feedforward and feedback effects of oxysterols FXR = farnesoid×receptor; LRH-1 = liver receptor homologue-1; LXR = liver×receptor; SF-1 = steroidogenic factor-1; SREBP = sterol responsive element binding protein.


Cholesterol is not the only sterol inside the cell to act as a regulatory substance, many of its hydroxylated derivatives, the oxysterols, share these important functions. Oxysterols exert a feed-back effect and down-regulate cholesterol synthesis, to be detailed in the following sections of this overview, but they also up-regulate their own metabolism and elimination via a feed-forward effect which will be briefly described in this section. 
      Cholesterol directly affects two enzymes which are vital in its own removal. It activates acyl-CoA: cholesterol acyl transferase (ACAT), the enzyme required to synthesize cholesteryl esters, its major storage form, and it also activates cholesterol 7a-hydroxylase (CYP7A1), the initial and rate-limiting enzyme of bile acid synthesis, the leading pathway of cholesterol elimination[10].
     The side chain-hydroxylated 27-hydroxycholesterol is presumed to function in cholesterol homeostasis by sustaining HDL-mediated reverse cholesterol transport, the process by which cholesterol is brought back to the liver from peripheral tissues[11]. The most important action of oxysterols, however, is to serve as ligands for nuclear orphan receptors. This family of proteins has been recognized as nuclear hormone receptors based on their molecular properties, but since ligands were initially not known, they were named orphan receptors. Most important among these is liver X receptor (LXR), which controls expression of CYP7A1[12] as the rate-limiting step of bile acid synthesis[13,14], reverse cholesterol transport[15], trafficking of cholesteryl esters, and aids in lipid metabolism[16] and the intestinal absorption of cholesterol. This receptor displays specific requirements for the position of additional hydroxy group (s) on the cholesterol molecule, the most potent ligands being 20 (S) -OH-, 22 (R) -OH-, 20, 22-di-OH-, and 24-OH-cholesterol[17,18]. An overview of oxysterol actions is shown in Figure 1.
       Furthermore, oxysterols are ligands for steroidogenic factor-1 (SF-1) and liver receptor-homologue 1 (LRH-1), in which they regulate steroid hormone synthesis and sexual differentiation during prenatal development. SF-1 is limited to steroidogenic tissues, whereas LRH-1 occurs in liver and other tissues derived from the gut endoderm[7]. Oxysterols also play a role in meiosis[19]. Finally, these bile acids activate the farnesol X receptor (FXR) /retinoid X receptor (RXR)/receptor-interacting protein 14 (Rip14) system, FXR is also known as bile acid receptor (BAR).  Binding of bile acids to FXR inhibits bile acids synthesis.

SREBPs belong to the large family of basic helix-loop-helix-leucine zipper (bHLH-Zip) transcription factors. Three members of the SREBP family, SREBPs-1a, -1c, and -2, have been identified[20,21]. They are synthesized as inactive precursors.  Isoforms -1a and -1c are produced from a single gene on human chromosome 17p11.2 by use of alternate promoters and splicing, resulting in different forms of exon-1[21-23]. SREBP-2 is encoded by a separate gene on human chromosome 22q13[20,24]. It has about 50% sequence identity with SREBP-1. Adipocyte determination differentiation factor (ADD)-1, a transcription factor which binds to E-boxes and promotes adipocyte differentiation in rats[25] is the homologue of  human SREBP-1c[23].
      Each nascent SREBP protein has a molecular size of about 125 ku and consists of about 1 150 amino acids (aa) in 3 functional domains. The first, NH2-terminal domain of SREBP contains the bHLH-Zip and an acidic domain, located at the very NH2 terminus. The acidic domain has to bind a transcription coactivator for function, it is shorter in SREBP-1c than in SREBP-1a, making SREBP-1c a weaker transcription activator[26]. The acidic domain is essential for function. When removed, the bHLH-Zip portion of SREBP still binds to DNA, but no longer activates transcription, thus acting as an inhibitor[27].
     A djacent to the acidic DNA-binding domain is a variable area rich in serine, proline, glutamine, and glycine. Next follows the bHLH-Zip sequence, whose basic region mediates DNA binding. The rest of the bHLH-Zip motif imparts the ability to dimerize. Other bHLH-Zips tend to homo-dimerize, whereas SREBP needs other transcription factors such as SP-1 or NF-Y for full function. The remainder of the SREBP molecule has no analogy with other bHLH-Zip transcription factors[5].
      SREBPs are embedded in membranes of the endoplasmic reticulum (ER) and the nuclear envelope, forming what has been called a hairpin shape[28,29]. The NH2-terminal portion of the SREBP molecule through the bHLH-Zip region protrudes into the cytosol. The following central portion of SREBP, the membrane anchoring region, is about 90 aa in length. It consists of two hydrophobic, membrane-spanning segments separated by a hydrophilic loop which extends into the lumen of the ER.  The COOH terminal segment of about 590 aa again extends into the cytosol and serves as the regulatory domain for transformation into the mature, a transcriptionally active form also known as nuclear SREBP (nSREBP). Neither the membrane-spanning, nor the regulatory regions, are found in other bHLH-Zip transcription factors.
      The NH2-terminal domain of SREBP binds to a sterol binding element (SRE) which must contain a direct, or tandem, repeat of the recognition sequence[30]. This is another evident difference to bHLH-Zip transcription factors, which will bind only to palindromic (i.e., tail-to-tail connected) repeats. SREBPs display no in vivo activity with palindromic sequences[5,19].

SREBPs attain biological activity only after being transferred to the membrane of the Golgi complex, where they are cleaved at the lumenal loop and the NH2-end is released from the membrane. They are confined to the membranes of the ER unless they are paired up with another protein which serves two functions: to sense the levels of sterols in the cell, and in response to low sterol levels, to escort SREBP to its place of activation.  This is achieved by SREBP cleavage activating protein (SCAP), a protein originally cloned from a mutant cell line of Chinese hamster ovary cells which will not suppress SREBP cleavage even with very high sterol levels[31]. The mutation affects one specific codon, 443, where a C (ytosine) to G (uanine) transition on the DNA side replaces an aspartic acid by an asparagine in the amino acid sequence, rendering SCAP unresponsive to sterols[32].
     SCAP has 1 276 amino acids[31] in two domains[33]: a membrane-spanning NH2 terminal domain of 730 aa, and a COOH terminal domain of 546 aa, which extends into the cytosol.  The NH2-terminus contains eight hydrophobic sequences separated by short hydrophilic loops[31], the hydrophobic sequences are thought to span the membrane with the hydrophilic loops protruding at either side. SCAP shares this feature with HMGCoA reductase[34,35]. The membrane-spanning stretches comprise the sterol-sensing area, whereas the COOH-terminus contains all of the remaining biological activity[31]. The COOH-terminus is organized in five repeat sequences characteristic of the WD family. Wherever this sequence occurs, it mediates protein-protein interactions[36].
     Newly synthesized SREBP forms a tight complex with the WD repeat domains of SCAP[37], but in the presence of oxysterols this complex is confined to the ER, establishing a feedback loop to control cholesterol synthesis. With oxysterol depletion, the SCAP/SREBP complex appears in vesicles budding from ER membranes[38] and translocates to another subcellular compartment, the Golgi[39-43]. Transfer of SREBP and proteolytic cleavage to release nSREBP cannot occur unless this complex has formed[42]. Oxysterols induce a change of the conformation of SCAP, and an additional protein has been postulated to retain SCAP in the ER once in its sterol-induced conformation[44].  Not only cholesterol, but also several other oxysterols display full activity in suppressing SCAP translocation into vesicles, but it has been suggested that cholesterol alone exerts the effect, while high levels of oxysterols simply force translocation of cholesterol from the plasma membrane to the ER[44]. Complete understanding could be of immense benefit in the management of hyperlipidemias.

The SREBP/SCAP-containing vesicles from the ER also contain a membrane-anchored serine protease of the subtilisin family, Site-1 protease (S1P), in an inactive form which becomes activated only during its transport to the Golgi[45]. The SREBP/SCAP complex and S1P now incorporate into the Golgi membrane. As the next step, the activated S1P attaches to the SCAP/SREBP complex and cuts the SREBP molecule right in the middle of its lumenal hydrophilic loop[29]. When active S1P is inserted into the ER, it will cleave SREBP without a need for SCAP[43].
      To release active SREBP, another enzyme is required, Site-2 protease (S2P, a trans-membrane zinc metalloprotease). The cellular location of this enzyme is as yet unclear, but likely resides in the Golgi. S2P cuts the still membrane-anchored SREBP in a rather unusual place, viz., three amino acids into the membrane-spanning portion on the cytoplasmic side[46,47]. This process is known to regulate intra-membrane proteolysis (Rip)[48], and typically produces proteins which are transcriptionally active and participate in the control of various cellular processes[6]. S2P action is not directly affected by cellular oxysterol levels, since this enzyme cannot act unless S1P has separated the bHLH-Zip portion of SREBP from the regulatory COOH terminus[49]. S2P action results in the release of a mature, 68 kDa nSREBP, consisting of the bHLH-zipper domain with the first three membrane-spanning amino acids attached, which now can migrate to the nucleus and bind to a sterol-responsive element (SRE). SCAP is recycled back to the ER to chaperone another 125 kDa SREBP molecule to the Golgi[50], whereas the 68 kDa SREBP is degraded. Interestingly, the nuclear action of nSREBP induces new SREBP mRNA synthesis by way of SREs located in the promoter regions of their own genes[51]. A synopsis is shown in Figure 2.
     The process of SREBP cleavage and activation thus comprises 4 components which, upon mutation, can result in loss of oxysterol-sensitive regulation of cholesterol homeostasis. As a matter of fact, much of the knowledge of the SREBP regulatory system stems from experimentally mutated cell lines and subsequent selection for oxysterol resistance or cholesterol auxotrophy[8].  Oxysterol resistance has aided in cloning the genes for SREBP and SCAP, respectively. One type, class 1 mutation, produces an NH2-terminal bHLH-Zip portion of SREBP which is truncated even before the S2P cleavage site[52,53]. This molecule is therefore not membrane bound and can access the nucleus immediately after its synthesis, turning on the complete SREBP gene battery regardless of cellular oxysterol levels. Class 2 mutants contain a mutated SCAP molecule[54] which holds the SREBP/SCAP/S1P complex in a permanently active configuration, no matter how high the oxysterol level in the cell, resulting in permanent release of nSREBP with concomitant overproduction of cholesterol[31].
      Cholesterol auxotrophy has been used as a selection criterion to clone the genes for S1P and S2P. With S1P[55], S2P[41,46,56] or SCAP[57] rendered non-functional, cells become dependent on exogenous cholesterol because they cannot produce the enzymes necessary for its biosynthesis.
     It is interesting to know that the proteolytic activation of SREBP-1 and SREBP-2, respectively, can be regulated individually. In rodents, treatment with cholesterol suppressing drugs (statins) or with cholesterol-sequestering agents results in up-regulation and increased activation of SREBP-2 while reducing activation of SREBP-1[58]. Another interesting feature is that polyunsaturated fatty acids inhibit the proteolytic activation of SREBP-1, but contrary to the action of oxysterols, they have no effect on SREBP-2 maturation[59]. In addition, glucose metabolites such as glucose-6-phosphate may serve to accelerate maturation of SREBP-1c only[60], but mechanistic details have not been elucidated.

Figure 2
(PDF) Maturation of SREBPs -NH2 = amino-terminal ends of SREBP or SCAP. S1P = Site-1 protease (crossed-out = inactive); S2P = Site-2 protease. SRE = sterol-responsive element; nSREBP = nuclear SREBP. Arrangement of the additional transcription factors NF-Y, Sp1, CREB, ARC, CBP, TBP, and DRIC (see text) is tentative as their requirements are not exactly known.

Immediately after the second cleavage of the SREBPs the now mature protein enters the nucleus where it binds to SREs in the promoters of the target genes and activates transcription. The SRE nucleotide sequence displays considerable variation among the promoters of the many genes activated by an SRE, a commonly found sequence is represented by 5'-nTCACnCCA C n-3' (cf.[6]). However, SREBP alone cannot activate transcription, but must act co-ordinately with additional transcription factors to obtain full activation of target genes.
      Transcription factors known to activate SREs in conjunction with SREBP are Sp1, nuclear factor (NF)-Y, and cAMP response element binding protein (CREB)[10,61-64]. NF-Y interacts directly with SREBP[62], and Sp1 can stabilize the complex[61]. It appears that the promoters of different SRE-activated genes respond to different combinations of these transcription factors. Once a stable complex has formed in a promoter region, additional factors such as CREB binding protein (CBP)[65,66], activated recruited cofactor (ARC)[67], vitamin D receptor interacting protein (DRIC)[67], or TATA box-binding protein (TBP)-associated factors may be recruited to initiate transcription.  Histone acetylation is also required for full transcriptional activity[68]. Such complexes have been shown to exist with SREBP-1a and -2, but not with SREBP-1c. This may explain its rather weak potency to activate the SRE gene battery.
      SREBP-activated genes predominantly belong to lipid metabolism pathways, viz, cholesterogenesis, fatty acid synthesis, lipogenesis, triglyceride and phospholipid synthesis, but also glucose metabolism. Yet, the three SREBPs do not activate identical gene batteries. Using transgenic mice over-expressing just one type of nSREBP it has been discovered that SREBPs-1a and -1c actions favour fatty acid synthesis, whereas SREBP-2 action favours cholesterol synthesis[5,69-71], but in vitro both SREBPs-1a and -2 can trigger expression of the complete set of enzymes required for cholesterol synthesis with similar potency[72]. While SREBPs-1a and -2 predominate in cultured cells, intact liver and most other tissues primarily express SREBP-1c and -2[9]. An overview of SREBP pathways is given in Figure 3.

Figure 3(PDF) Metabolic pathways regulated by SREBPs G-6-P = glucose-6-phosphate; 6-PG = 6-phosphogluconate; Rib-5-P = ribulose-5-phosphate; PEP = phosphoenol pyruvate; CoA = coenzyme A; HMGCoA = 3-hydroxy-3-methylglutaryl coenzyme A. SREBP-1c and SREBP-2 activate genes for the generation of NADPH (ATP citrate lyase, malic enzyme, G-6-P dehydrogenase, 6-PG dehydrogenase) required in various steps of lipid synthesis.

      SREBP-2 over-expression induces all 12 enzymes of the cholesterol biosynthetic pathway[72], most notably the mRNA for HMGCoA reductase, which may increase as many as 75-fold[73]. Overall cholesterol synthesis in such animals is up 28-fold, whereas fatty acid synthesis is increased only 4-fold. In contrast, expression of enzymes does not involve cholesterol synthesis but is related to cholesterol metabolism. Cholesterol 7a-hydroxylase (rate-limiting enzyme for bile acid synthesis) and acyl-CoA:cholesterol acyltransferase (ACAT, catalyses cholesterol ester formation), are not activated[72].
      SREBP-1a appears to be constitutively expressed in most tissues, with as yet no known factor to stimulate its low expression[26]. Over-expression in adult rats also resulted in over-stimulation of lipid synthesis, but in this case fatty acid synthesis was increased 26-fold, and cholesterol synthesis 5-fold. Since SREBP-1a and -1c (see below) also induce enzymes for fatty acid elongation and desaturation[73,74], SREBP-1a over-expression resulted in elevated hepatic levels of oleate[75].
      SREBP-1c is predominantly involved in the regulation of adipogenesis and also in the regulation of insulin-responsive genes which control lipogenesis and glucose metabolism[76,77], but in vitro it does not stimulate cholesterol synthesis[75]. SREBP-1c mRNA synthesis responds to nutritional changes in parallel to insulin levels[78]. Insulin heightens the expression of SREBP-1c and its battery of genes involved in the synthesis of saturated and unsaturated fatty acids[75,79] as well as glucose metabolism genes[77,78]. The effect is opposed by glucagon and cyclic AMP. In short, available experimental data indicate that SREBP-1c mediates all effects of insulin on lipogenesis. In response to nutritional stimuli SREBP-1c also triggers expression of genes of enzymes required for fatty acid elongation[74], and of glycerol 3-phosphate acyltransferase required for triglyceride and phospholipid synthesis[6]. Last but not least SREBPs activate 3 genes necessary for the generation of NADPH, which is needed in fatty acid and cholesterol synthesis.
      The promoter of the SREBP-1c gene contains response elements for insulin, glucagon, as well as liver X-activated receptors (LXR)a
and LXRb, the latter is activated by sterols[17,43]. This regulatory pathway, among others, results in increased synthesis of oleate[75], the major fatty acid used for cholesterol esterification, to ensure its removal from the liver.

Knock-out mice which lack all SREBPs, or S1P to activate them, die at an early stage of embryonic development[80,81]. Specific knock-out of SREBP-2 also results in embryonic lethality. Deletion of SREBP-1a allows some foetuses to survive, whereas lack of SREBP-1c appears to be of no consequence. The survivors are found to compensate by higher SREBP-2 levels, and consequently, these animals have elevated hepatic cholesterol, but lower fatty acid levels[9].
      In order to study SREBP knock-outs in adulthood, a gene manipulation was used in Brown and Goldstein's laboratory which allows to turn specific genes off at will by stimulating interferon production. Disruption of the SCAP or S1P gene, respectively, almost abolished nSREBPs-1 and -2 in liver and diminished expression of all target genes of cholesterol and fatty acid synthesis. The result was a reduction in cholesterol and fatty acid levels in livers by 70-80%[9].
     The SREBP-related links between fatty acid and glucose metabolism draw immediate attention to the wide-spread disease, diabetes. It would appear that the fatty liver frequently observed in insulin-resistant diabetics is a result of high SREBP-1c levels caused by high insulin levels. Leptin, the organism's response to fat accumulation in adipocytes, opposes SREBP-1c action[82], and consequently can heal the derailment of fat metabolism in diabetic animals[83].
     One of the genes activated by nSREBP-1c is that for the hepatic LDL receptor, an effect clearly targeting at maintaining plasma lipid homeostasis. At first sight this would appear beneficial for arteriosclerosis since it should reduce elevated LDL-cholesterol levels in blood. However, since nSREBP-1c also induces lipogenesis, its effect is ambiguous, and other factors will decide whether the net result fights arteriosclerosis, or rather exacerbates it[7]. It is at this point where the HMGCoA reductase inhibitors or statins work, but they are effective only in one third of cases[84].
     It must be understood that fatty acid synthesis is not only subject to SREBP regulation, but a host of other factors, whereas cholesterol synthesis appears to be exclusively controlled by SREPBs. Additional unknowns are the exact regulation of system by which HDL via the scavenger receptor B-1 mediates efflux of cholesterol from the liver, and the ATP-binding cassette-1 (ABC-1) system which mediates transfer of cholesterol to HDL[84]. Profound knowledge of these pathways will open a perspective beyond the statin drugs to specifically lower cholesterol levels in disease conditions.

1    Small DM, Shipley GG. Physical-chemical basis of lipid deposition in atherosclerosis. Science 1974; 185: 222-229
2    Simons K, Ikonen E. Functional rafts in cell membranes. Nature 1997; 387: 569-572
3    Anderson RG. The caveolae membrane system. Annu Rev Biochem 1998; 67: 199-225
4    Goldstein JL, Brown MS. Regulation of the mevalonate pathway. Nature 1990; 343: 425-430
5    Brown MS, Goldstein JL. The SREBP pathway: Regulation of cholesterol metabolism by proteolysis of a membrane-bound 
      transcription factor. Cell 1997; 89: 331-340
6    Edwards PA, Tabor D, Kast HR, Venkateswaran A. Regulation of gene expression by SREBP and SCAP. Biochim Biophys 
      Acta 2000; 1529: 103-113
7    Schoonjans K, Brendel C, Mangelsdorf D, Auwerx J. Sterols and gene expression: control of affluence. Biochim Biophys 
      Acta 2000; 1529: 114-125
8    Goldstein JL, Rawson RB, Brown MS. Mutant mammalian cells as tools to delineate the sterol regulatory element-binding 
      protein pathway for feedback regulation of lipid synthesis.  Arch Biochem Biophys 2002; 329: 139-148
9    Horton JD, Goldstein JL, Brown MS.  SREBPs: activators of the complete program of cholesterol and fatty acid synthesis 
      in the liver. J Clin Invest 2002; 109: 1125-1131
10  Jackson MS, Ericsson J, Edwards PA. Signaling molecules derived from the cholesterol biosynthetic pathway. Subcell 
      Biochem 1997; 28: 1-21
11  Björkhem I, Andersson O, Diczfalusy U, Sevastik B, Xin RJ, Duan C, Lund E. Atherosclerosis and sterol 27-hydroxylase: 
      Evidence for a role of this enzyme in elimination of cholesterol from human macrophages. Proc Natl Acad Sci U S A 
      1994; 91: 8592-8596
12  Lehmann JM, Kliewer ST, Moore LB, Smith-Oliver TA, Oliver BB, Su JL, Sundseth SS, Winegar DA, Blanchard DE, 
      Spencer TA, Willson TM. Activation of nuclear receptor LXR by oxysterols defines a new hormone pathway. J Biol 
      Chem 1997; 272: 3137-3140
13  Schwarz M, Lund EG, Russell DW. Two 7 alpha-hydroxylase enzymes in bile acid biosynthesis. Curr Opin Lipidol 
      1998; 9: 113-118
14  Russell DW. The enzymes, regulation, and genetics of bile acid synthesis. Annu Rev Biochem 2003; 73: 137-174
15  Babiker A, Anderson O, Lund E, Xiu RJ, Deeb S, Reshef A, Leitersdorf E, Diczfalusy U, Björkhem I. Elimination of 
      cholesterol in macrophage and endothelial cells by the sterol 27-hydroxylase mechanism. Comparison with high 
      density lipoprotein-mediated reverse cholesterol transport. J Biol Chem 1997; 272: 26253-26261
16  Ishibashi S, Schwarz M, Frykman PK, Herz J, Russell DW.  Disruption of cholesterol 7a-hydroxylase gene in mice. I. 
      Postnatal lethality reversed by bile acids and vitamin supplementation. J Biol Chem 1996; 271: 18017-18023
17  Janowski BA, Willy PJ, Devi TR, Falck JR, Mangelsdorf DJ. An oxysterol signalling pathway mediated by the nuclear 
      receptor LXRa. Nature 1996; 383: 728-731
18  Ventakeswaran A, Lefitte BA, Josyl SB, Mak PA, Wilpitz DC, Edwards PA, Tontonoz P. Control of cellular cholesterol 
      efflux by the nuclear oxysterol receptor LXRa. Proc Natl Acad Sci U S A 2000; 97: 12097-12102
19  Edwards PA, Ericsson J. Sterols and isoprenoids: Signaling molecules derived from the cholesterol biosynthetic pathway.  
      Annu Rev Biochem 1999; 68: 157-185
20  Hua X, Yokoyama C, Wu J, Briggs MR, Brown MS, Goldstein JL, Wang X. SREBP-2, a second basic-helix-loop-helix-
      leucine zipper protein that stimulates transcription by binding to sterol regulatory elements. Proc Natl Acad Sci U S A 
      1993; 90: 11603-11607
21  Yokoyama C, Wang X, Briggs MR, Admon A, Wu J, Hua X, Goldstein JL, Brown MS. SREBP-1, a basic 
      helix-loop-helix-leucine zipper protein that controls transcription of the LDL receptor gene. Cell 
      1993; 75: 187-197
22  Hua X, Wu J, Goldstein JL, Brown MS, Hobbs HH. Structure of human gene encoding sterol regulatory element binding 
      protein-1 (SREBF1) and localization of SREBF1 and SREBF2 to chromosomes 17p11.2 and 22q13. Genomics 
      1995; 25: 667-673
23  Shimomura I, Shimano H, Horton JD, Goldstein JL, Brown MS. Differential expression of exons 1a and 1c in the mRNAs 
      of sterol regulatory element binding protein-1 in human and mouse organs and cultured cells. J Clin Invest 
      1997; 99: 838-845
24  Miserez, AR, Cao G, Probst L, Hobbs HH. Structure of the human gene encoding sterol regulatory element binding 
      protein-2 (SREBP-2). Genomics 1997; 40: 31-40
25  Tontonoz P, Kim JB, Graves RA, Spiegelman BM. ADD 1: a novel helix-loop- helix transcription factor associated with 
      adipocyte determination and differentiation. Mol Cell Biol 1993; 13: 4752-4759
26  Shimano H, Horton JD, Shimomura I, Hammer RE, Brown MS, Goldstein JL. Isoform-1c of sterol regulatory 
      element-binding protein is less active than isoform 1a in livers of transgenic mice and in cultured cells. J Clin 
      Invest 1997; 99: 846-854
27  Sato R, Yang J, Wang X, Evans MJ, Ho YK, Goldstein JL, Brown MS. Assignment of the membrane attachment, DNA 
      binding, and transcriptional activation domains of sterol regulatory element-binding protein-1 (SREBP-1). J Biol Chem 
      1994; 269: 17267-17273
28  Hua X, Sakai J, Ho YK, Brown JL, Goldstein MS. Hairpin orientation of sterol regulatory element binding protein 2 in cell 
      membranes as determined by protease protection. J Biol Chem 1995; 270: 29422-29427
29  Duncan EA, Brown MS, Goldstein JL, Sakai J. Cleavage site for sterol regulatory protease localized to a Leu-Ser bond in 
      lumenal loop of sterol regulatory element binding protein-2. J Biol Chem 1997; 272: 12778-12785
30  Magaña MM, Osborne TF. Two tandem binding sites for sterol regulatory element binding proteins are required for 
      sterol regulation of fatty acid synthase promoter. J Biol Chem 1996; 271: 32689-32694
31  Hua X, Nohturfft A, Goldstein JL, Brown MS. Sterol resistance in CHO cells traced to point mutations in SREBP cleavage 
      activating protein (SCAP). Cell 1996; 87: 415-426
32  Nohturfft A, Hua X, Brown MS, Goldstein JL. Recurrent G-to-A substitution in a single codon of SREBP cleavage-activating 
      protein causes sterol resistance in three mutant CHO cell lines.  Proc Natl Acad Sci U S A 1996; 93: 13709-13714
33  Nohturfft A, Brown MS, Goldstein JL. Topology of SREBP cleavage activating protein, a polytopic membrane protein with 
      a sterol sensing domain. J Biol Chem 1998; 273: 17243-17250
34  Liscum L, Finer-Moore J, Stroud RM, Luskey KL, Brown MS, Goldstein JL. Domain structure of 3-hydroxy-3-methylglutaryl 
      coenzyme A reductase, a glycoprotein of the endoplasmic reticulum. J Biol Chem 1985; 260: 522-530
35  Olender EH, Simoni RD. The intracellular targeting and membrane topology of 3-hydroxy-3-methylglutaryl coenzyme A 
      reductase. J Biol Chem 1992; 267: 4223-4235
36  Neer EJ, Schmidt CJ, Nambudripad R, Smith TF. The ancient regulatory protein family of WD-repeat proteins. Nature 
      1994; 371: 297-300
37  Sakai J, Nohturfft A, Cheng D, Ho YK, Brown MS, Goldstein JL. Identification of complexes between the COOH-terminal 
      domain of sterol regulatory element binding protein (SREBPs) and SREBP cleavage-activating protein (SCAP). J Biol 
      Chem 1997; 272: 20213-20221
38  Nohturfft A, Jabe D, Goldstein JL, Brown MS, Espenshade PJ.  Regulated step in cholesterol feed back localized to 
      budding of SCAP from ER membranes. Cell 2000; 102: 315-323
39  Wang X, Sato R, Brown MS, Hua X, Goldstein JL. SREBP-1, a membrane-bound transcription factor released by 
      sterol-regulated proteolysis. Cell 1994; 77: 53-62
40  Hua X, Sakai J, Brown MS, Goldstein JL. Regulated cleavage of sterol regulatory element binding proteins (SREBPs) 
      requires sequences on both sides of the endoplasmic reticulum membrane. J Biol Chem 1996; 271: 10379-10384
41  Sakai J, Duncan EA, Rawson RB, Hua X, Brown MS, Goldstein JL. Sterol regulated release of SREBP-2 from cell 
      membranes require two sequential cleavages, one within a transmembrane segment. Cell 1996; 85: 1037-1046
42  Sakai J, Nohturfft A, Goldstein JL, Brown MS. Cleavage of sterol regulatory element-binding proteins (SREBPs) at 
      site-1  requires interaction with SREBP cleavage-activating protein. Evidence from in vivo competition studies. J Biol 
      Chem 1998; 273: 5785-5793
43  DeBose-Boyd RA, Brown MS, Li WP, Nohturfft A, Goldstein JL, Espenshade PJ. Transport dependent proteolysis of 
      SREBP: relocation of site-1 protease protein from Golgi to ER. Cell 1999; 99: 703-712
44  Brown AJ, Sun L, Feramisco JD, Brown MS, Goldstein JL.  Cholesterol addition to ER membranes alters conformation 
      of SCAP, the SREBP escort protein that regulates cholesterol metabolism. Mol Cell 2002; 10: 237-245
45  Espenshade PJ, Cheng D, Goldstein JL, Brown MS. Autocatalytic processing of site-1 protease removes propeptide and 
      permits cleavage of sterol regulatory element-binding proteins.  J Biol Chem 1999; 274: 22795-22804
46  Rawson RB, Zelenski NG, Nijhawan D, Ye J, Sakai J, Hasan MT, Chang TY, Brown MS, Goldstein JL. Complementation 
      cloning of S2P, a gene encoding a putative metalloprotease required for intramembrane cleavage of SREBPs. Mol Cell 
      1997; 1: 47-57
47  Duncan EA, Dave UP, Sakai J, Goldstein JL, Brown MS. Second-site cleavage in sterol regulatory element-binding protein 
      occurs at transmembrane junction as determined by cysteine panning. J Biol Chem 1998; 273: 17801-17809
48  Brown MS, Ye J, Rawson RB, Goldstein JL. Regulated intramembrane proteo-lysis: a control mechanism conserved 
      from bacteria to humans. Cell 2000; 100: 391-398
49  Brown MS, Goldstein JL. A proteolytic pathway that controls the cholesterol content of membranes, cells and blood. 
      Proc Natl Acad Sci U S A 1999; 96: 11041-11048
50  Nohturfft A, DeBose-Boyd RA, Scheek S, Goldstein JL, Brown MS. Sterols regulate cycling of SREBP cleavage activating 
      protein (SCAP) between endoplasmic reticulum and Golgi. Proc Natl Acad Sci U S A 1999; 96: 11235-11240
51  Sato R, Inoue J, Kawabe Y, Kodama T, Takano T, Maeda M.  Sterol-dependent transcriptional regulation of sterol 
      regulatory element-binding protein-2. J Biol Chem 1996; 271: 26461-26464
52  Metherall JE, Ridgway ND, Dawson PA, Goldstein JL, Brown MS. A 25-hydroxycholesterol-resistant cell line deficient in 
      acyl-CoA: cholesterol acyltransferase. J Biol Chem 1991; 266: 12734-12740
53  Yang J, Sato R, Goldstein JL, Brown MS. Sterol-resistant transcription in CHO cells caused by gene rearrangement that 
      truncates SREBP-2. Genes Dev 1994; 8: 1910-1919
54  Korn BS, Shimomura I, Bashmakov Y, Hammer RE, Horton JD, Goldstein JL, Brown MS. Blunted feedback suppression of 
      SREBP processing by dietary cholesterol in transgenic mice expressing sterol-resistant SCAP/D443N. J Clin Invest 
      1998; 102: 2050-2060
55  Rawson RB, Cheng D, Brown MS, Goldstein JL. Isolation of cholesterol-requiring mutant Chinese hamster ovary cells 
      with defects in cleavage of sterol regulatory element-binding proteins at site 1. J Biol Chem 1998; 273: 28261-28269
56  Hasan MT, Chang CC, Chang TY. Somatic cell genetic and biochemical characterization of cell lines resulting from 
      human genomic DNA transfections of Chinese hamster ovary cell mutants defective in sterol-dependent activation of 
      sterol synthesis and LDL receptor expression. Somat Cell Mol Genet 1994; 20: 183-194
57  Rawson RB, DeBose-Boyd R, Goldstein JL, Brown MS. Failure to cleave sterol regulatory element-binding proteins 
      (SREBPs) causes cholesterol auxotrophy in Chinese hamster ovary cells with genetic absence of SREBP 
      cleavage-activating protein. J Biol Chem 1999; 274: 28549-28556
58  Sheng Z, Otani H, Brown MS, Goldstein JL. Independent regulation of sterol regulatory element-binding proteins 1 and 2 
      in hamster liver. Proc Natl Acad Sci U S A 1995; 92: 935-938
59  Yahagi N, Shimano H, Hasty AH, Amemiya-Kudo M, Okazaki H, Tamura Y, Iizuka Y, Shionoiri F, Ohashi K, Osuga J, 
      Harada K, Gotoda T, Nagai R, Ishibashi S, Yamada N. A crucial role of sterol regulatory element-binding protein-1 in 
      the regulation of lipogenic gene expression by polyunsaturated fatty acids. J Biol Chem 1999; 274: 35840-35844
60  Mourrieras F, Foufelle F, Foretz M, Morin J, Bouche S, Ferre P.  Induction of fatty acid synthase and S14 gene 
      expression by glucose, xylitol and dihydroxyacetone in cultured rat hepatocytes is closely correlated with glucose 
      6-phosphate concentrations. Biochem J 1997; 326: 345-349
61  Sanchez HB, Yieh L, Osborne TF. Cooperation by sterol regulatory element-binding protein and Sp1 in sterol regulation 
      of low density lipoprotein receptor gene. J Biol Chem 1995; 270: 1161-1169
62  Dooley KA, Millinder S, Osborne TF. Sterol regulation of 3-hydroxy-3-methylglutaryl-coenzyme A synthase gene through 
      a direct interaction between sterol regulatory element binding protein and the trimeric CCAAT-binding factor/nuclear 
      factor Y. J Biol Chem 1998; 273: 1349-1356
63  Dooley KA, Benett MK, Osborne TF. A critical role for cAMP response element-binding protein (CREB) as a coactivator in 
      sterol-regulated transcription of 3-hydroxy-3-methylglutaryl coenzymne A synthase promoter. J Biol Chem 
      1999; 274: 5285-5291
64  Magaña MM, Koo SH, Towle HC, Osborne TF. Different sterol regulatory element-binding protein-1 isoforms utilize 
      distinct co-regulatory factors to activate the promoter of fatty acid synthetase. J Biol Chem 2000; 275: 4762-4733
65  Oliner JD, Andresen JM, Hansen SK, Zhou S, Tjian R. SREBP transcriptional activity is mediated through an interaction 
      with the CREB-binding protein. Genes Dev 1996; 10: 2903-2911
66  Ericsson J, Edwards PA. CBP is required for sterol-regulated and sterol regulatory element-binding protein-regulated 
      transcription. J Biol Chem 1998; 273: 17865-17870
67  Näär AM, Beaurang PA, Zhou S, Abraham S, Solomon W, Tjian R. Composite co-activator ARC mediates 
      chromatin-directed transcriptional activation. Nature 1999; 389: 828-832
68  Näär AM, Beaurang PA, Robinson KM, Oliner JD, Avizonis D, Scheek S, Zwicker J, Kadonaga JT, Tjian R. Chromatin, 
      TAFs, and a novel multiprotein coactivator are required for synergistic activation by Sp1 and SREBP-1a in vitro. Genes 
      Dev 1998; 12: 3020-3031
69  Ericsson J, Jackson SM, Kim JB, Spiegelman BM, Edwards PA.  Identification of glycerol-3-phosphate acyltransferase as 
      an adipocyte determination and differentiation factor 1- and sterol regulatory element-binding protein-responsive gene. 
      J Biol Chem 1997; 272: 7298-7305
70  Guan G, Dai PH, Osborne TF, Kim JB, Shechter I. Multiple sequence elements are involved in the transcriptional
      regulation of the human squalene synthase gene. J Biol Chem 1997; 272: 10295-10302
71  Guan G, Dai PH, Shechter I. Differential transcriptional regulation of the human squalene synthase gene by sterol 
      regulatory element-binding proteins (SREBP) 1a and 2 and involvement of 5' DNA sequence elements in the regulation. 
      J Biol Chem 1998; 273: 12526-12535
72  Sakakura Y, Shimano H, Sone H, Takahashi A, Inoue K, Toyoshima H, Suzuki S, Yamada N. Sterol regulatory element 
      -binding proteins induce an entire pathway of cholesterol  synthesis. Biochem Biophys Res Commun 
      2001; 286: 176-183
73  Horton JD, Shimomura I, Brown MS, Hammer RE, Goldstein JL, Shimano H. Acivation of cholesterol synthesis in 
      preference to fatty acid synthesis in liver and adipose tissue of transgenic mice overproducing sterol regulatory 
      element binding protein-2. J Clin Invest 1998; 101: 2331-2339
74  Moon YA, Shah NA, Mohapatra S, Warrington JA, Horton JD.  Identification of a mammalian long chain fatty acyl 
      elongase regulated by sterol regulatory element-binding proteins. J Biol Chem 2001; 276: 45358-45366
75  Shimomura I, Shimano H, Korn BS, Bashmakov Y, Horton JD.  Nuclear sterol regulatory element-binding proteins 
      activate genes responsible for the entire program of unsaturated fatty acid biosynthesis in transgenic mouse livers. 
      J Biol Chem 1998; 273: 35299-35306
76  Flier JS, Hollenberg AN. ADD-1 provides major new insight into the mechanism of insulin action. Proc Natl Acad Sci 
      U S A 1999; 96: 14191-14192
77  Foretz M, Guichard C, Ferre P, Foufelle F. Sterol regulatory element binding protein-1c is a major mediator of insulin 
      action on the hepatic expression of gluco-kinase and lipogenesis-related genes. Proc Natl Acad Sci U S A 
      1999; 96: 12737-12742
78  Kim JB, Sarraf P, Wright M, Yao KM, Mueller E, Solanes G, Lowell BB, Spiegelman BM.  Nutritional and insulin regulation 
      of fatty acid synthetase and leptin gene expression through ADD1/SREBP1. J Clin Invest 1998; 101: 1-9
79  Shimano H, Yahagi N, Amemiya-Kudo M, Hasty AH, Osuga J, Tamura Y, Shionoiri F, Iizuka Y, Ohashi K, Harada K, 
      Gotoda T, Ishibashi S, Yamada N. Sterol regulatory element-binding protein-1 as a key transcription factor for 
      nutritional induction of lipogenic enzyme genes. J Biol Chem 1999; 274: 35832-35839
80  Mitchell KJ, Pinson KI, Kelly OG, Brennan J, Zupicich J, Scherz P, Leighton PA, Goodrich LV, Lu X, Avery BJ, Tate P, Dill K, 
      Pangilinan E, Wakenight P, Tessier-Lavigne M, Skarnes WC.  Functional analysis of secreted and transmembrane 
      proteins critical to mouse development. Nat Genet 2001; 28: 241-249
81  Yang J, Goldstein JL, Hammer RE, Moon YA, Brown MS, Horton JD. Decreased lipid synthesis in livers of mice with 
      disrupted site-1 protease gene. Proc Natl Acad Sci U S A 2001; 98: 13607-13612
82  Soukas A, Cohen P, Socci ND, Friedman JM. Leptin-specific patterns of gene expression in white adipose tissue. Genes 
      Dev 2000; 14: 963-980
83  Shimomura I, Hammer RE, Ikemoto S, Brown MS, Goldstein JL. Leptin reverses insulin resistance and diabetes mellitus 
      in mice with congenital lipodystrophy. Nature 1999; 401: 73-76
84  Libby P, Aikawa M, Schönbeck U. Cholesterol and atherosclerosis. Biochim Biophys Acta 2000; 1529: 299-309

   Edited by Wang XL  Proofread by Xu FM 


Reviews Add

Related Articles:
Maintaining cholesterol homeostasis: Sterol regulatory element-binding proteins
Establishment of transgenic mice carrying gene encoding human zinc finger protein 191