Sterol resistance in CHO cells traced to point mutation in SREBP cleavage-activating protein

Retinol binding protein 4 stimulates hepatic sterol regulatory element-binding protein 1 and increases lipogenesis through the peroxisome proliferator-activated receptor-γ coactivator 1β-dependent pathway

Recent studies have revealed the essential role of retinol binding protein 4 (RBP4) in insulin resistance. However, the impact of RBP4 on aberrant lipogenesis, the common hepatic manifestation in insulin resistance states, and the underlying mechanism remain elusive. The present study was designed to examine the effect of RBP4 on sterol regulatory element-binding protein (SREBP-1) and hepatic lipogenesis. Treatment with human retinol-bound RBP4 (holo-RBP4) significantly induced intracellular triglyceride (TAG) synthesis in HepG2 cells and this effect is retinol-independent. Furthermore, RBP4 treatment enhanced the levels of mature SREBP-1 and its nuclear translocation, thereby increasing the expression of lipogenic genes, including fatty acid synthase (FAS), acetyl coenzyme A carboxylase-1 (ACC-1), and diacylglycerol O-acyltransferase 2 (DGAT-2). Stimulation of HepG2 cells with RBP4 strongly up-regulated the expression of transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator 1β (PGC-1β) at both the messenger RNA (mRNA) and protein levels. The transcriptional activation of PGC-1β is necessary and sufficient for the transcriptional activation of SREBP-1 in response to RBP4. The cyclic adenosine monophosphate (cAMP)-response element binding protein (CREB) was identified as the target transcription factor involved in the RBP4-mediated up-regulation of PGC-1β transcription as a result of phosphorylation on Ser133. Furthermore, in vivo RBP4 infusion induced SREBP-1c activation and consequently accelerated hepatic lipogenesis and plasma TAG in C57BL/6J mice, a phenomenon not observed in Ppargc1b knockout mice.

CONCLUSION

These findings reveal a novel mechanism by which RBP4 achieves its effects on hepatic lipid metabolism.

Protein trafficking and maturation regulate intramembrane proteolysis

Intramembrane-cleaving proteases (I-CLiPs) are membrane embedded proteolytic enzymes. All substrates identified so far are also membrane proteins, involving a number of critical cellular signaling as well as human diseases. After synthesis and assembly at the endoplasmic reticulum, membrane proteins are exported to the Golgi apparatus and transported to their sites of action. A number of studies have revealed the importance of the intracellular membrane trafficking in i-CLiP-mediated intramembrane proteolysis, not only for limiting the unnecessary encounter between i-CLiPs and their substrate but also for their cleavage site preference. In this review, we will discuss recent advances in our understanding of how each i-CLiP proteolysis is regulated by intracellular vesicle trafficking. This article is part of a Special Issue entitled: Intramembrane Proteases.

Point mutation in luminal loop 7 of Scap protein blocks interaction with loop 1 and abolishes movement to Golgi

Scap is a polytopic protein of the endoplasmic reticulum (ER) that controls cholesterol homeostasis by transporting sterol regulatory element-binding proteins (SREBPs) from the ER to the Golgi complex. Scap has eight transmembrane helices (TM) joined by four small hydrophilic loops and three large loops. Two of the large loops (Loops 1 and 7) are in the ER lumen, and the other large loop (Loop 6) faces the cytosol where it binds COPII proteins that initiate transport to Golgi. Cholesterol binding to Loop 1 alters the configuration of Loop 6, precluding COPII binding and preventing the exit of Scap from the ER. Here, we create a point mutation (Y640S) in luminal Loop 7 that prevents Scap movement to Golgi. Trypsin cleavage assays show that Loop 6 of Scap(Y640S) is always in the configuration that precludes COPII binding, even in the absence of cholesterol. When expressed separately by co-transfection, the NH2-terminal portion of Scap (containing TM helices 1-6, including Loop 1) binds to the COOH-terminal portion (containing TM helices 7-8 and Loop 7) as determined by co-immunoprecipitation. This binding does not occur when Loop 7 contains the Y640S mutation. Co-immunoprecipitation is also abolished by a point mutation in Loop 1 (Y234A) that also prevents Scap movement. These data suggest that Scap Loop 1 must interact with Loop 7 to maintain Loop 6 in the configuration that permits COPII binding. These results help explain the operation of Scap as a sterol sensor.

Sterol regulatory element-binding proteins are regulators of the NIS gene in thyroid cells

The uptake of iodide into the thyroid, an essential step in thyroid hormone synthesis, is an active process mediated by the sodium-iodide symporter (NIS). Despite its strong dependence on TSH, the master regulator of the thyroid, the NIS gene was also reported to be regulated by non-TSH signaling pathways. In the present study we provide evidence that the rat NIS gene is subject to regulation by sterol regulatory element-binding proteins (SREBPs), which were initially identified as master transcriptional regulators of lipid biosynthesis and uptake. Studies in FRTL-5 thyrocytes revealed that TSH stimulates expression and maturation of SREBPs and expression of classical SREBP target genes involved in lipid biosynthesis and uptake. Almost identical effects were observed when the cAMP agonist forskolin was used instead of TSH. In TSH receptor-deficient mice, in which TSH/cAMP-dependent gene regulation is blocked, the expression of SREBP isoforms in the thyroid was markedly reduced when compared with wild-type mice. Sterol-mediated inhibition of SREBP maturation and/or RNA interference-mediated knockdown of SREBPs reduced expression of NIS and NIS-specific iodide uptake in FRTL-5 cells. Conversely, overexpression of active SREBPs caused a strong activation of the 5'-flanking region of the rat NIS gene mediated by binding to a functional SREBP binding site located in the 5'-untranslated region of the rat NIS gene. These findings show that TSH acts as a regulator of SREBP expression and maturation in thyroid epithelial cells and that SREBPs are novel transcriptional regulators of NIS.

A pathway approach to investigate the function and regulation of SREBPs

The essential function of sterol regulatory element-binding proteins (SREBPs) in cellular lipid metabolism and homeostasis has been recognized for a long time, and the basic biological pathway involving SREBPs has been well described; however, a rapidly growing number of studies reveal the complex regulation of these SREBP transcription factors at multiple levels. This regulation allows the integration of signals of diverse pathways involving nutrients, contributing to cellular lipid and energy homeostasis. This review attempts to integrate this knowledge. The description of the SREBP pathway is Web-linked as it refers to the online version of the pathway on wikipathways.org , which is interactively linked to genomics databases and literature. This allows a more extensive study of the pathway through reviewing these links.

Dual functions of Insig proteins in cholesterol homeostasis

The molecular mechanism of how cells maintain cholesterol homeostasis has become clearer for the understanding of complicated association between sterol regulatory element-binding proteins (SREBPs), SREBP cleavage-activating protein (SCAP), 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) and Insuin induced-genes (Insigs). The pioneering researches suggested that SREBP activated the transcription of genes encoding HMG-CoA reductase and all of the other enzymes involved in the synthesis of cholesterol and lipids. However, SREBPs can not exert their activities alone, they must form a complex with another protein, SCAP in the endoplasmic reticulum (ER) and translocate to Golgi. Insigs are sensors and mediators that regulate cholesterol homeostasis through binding to SCAP and HMG-CoA reductase in diverse tissues such as adipose tissue and liver, as well as the cultured cells. In this article, we aim to review on the dual functions of Insig protein family in cholesterol homeostasis.

The lifetime of UDP-galactose:ceramide galactosyltransferase is controlled by a distinct endoplasmic reticulum-associated degradation (ERAD) regulated by sigma-1 receptor chaperones

The glycosphingolipid biosynthesis is initiated by monoglycosylation of ceramides, the action of which is catalyzed either by UDP-glucose:ceramide glucosyltransferase or by UDP-galactose:ceramide galactosyltransferase (CGalT). CGalT is expressed predominantly at the endoplasmic reticulum (ER) of oligodendrocytes and is responsible for synthesizing galactosylceramides (GalCer) that play an important role in regulation of axon conductance. However, despite the importance of ceramide monoglycosylation enzymes in a spectrum of cellular functions, the mechanism that fine tunes activities of those enzymes is largely unknown. In the present study, we demonstrated that the sigma-1 receptor (Sig-1R) chaperone, the mammalian homologue of a yeast C8-C7 sterol isomerase, controls the protein level and activity of the CGalT enzyme via a distinct ER-associated degradation system involving Insig. The Sig-1R forms a complex with Insig via its transmembrane domain partly in a sterol-dependent manner and associates with CGalT at the ER. The knockdown of Sig-1Rs dramatically prolonged the lifetime of CGalT without affecting the trimming of N-linked oligosaccharides at CGalT. The increased lifetime leads to the up-regulation of CGalT protein as well as elevated enzymatic activity in CHO cells stably expressing CGalT. Knockdown of Sig-1Rs also decreased CGalT degradation endogenously expressed in D6P2T-schwannoma cells. Our data suggest that Sig-1Rs negatively regulate the activity of GalCer synthesis under physiological conditions by enhancing the degradation of CGalT through regulation of the dynamics of Insig in the lipid-activated ER-associated degradation system. The GalCer synthesis may thus be influenced by sterols at the ER.

Estradiol regulates insulin dependent stimulation of LDL-receptor expression in HepG(2) cells

The low density lipoprotein, one of the major vehicle for extraneous cholesterol, internalizes into the cells through the process of LDL-receptor mediated endocytosis. The expression of LDL-receptor on the cell surface is a function of various hormone regulated transcription of the receptor gene. The present study elucidates the differential expression pattern of LDL-receptor protein in human hepatoma HepG(2) cells by the influence of two hormones, insulin and estrogen (β-estradiol), as compared to the basal level expression of the receptor protein. The combined effect of insulin and β-estradiol reveals that β-estradiol is the ultimate regulator between these two hormones and supershades the message of insulin on LDL-receptor expression. The receptor protein level immunobloted by anti LDL-receptor antibody after treating cells with insulin, β-estradiol and mixture of both also reflects the same phenomena. This comparative study makes it consistent that cell saturation with sterol (β-estradiol) is the prime regulator of LDL-receptor expression between the two hormones, insulin and estrogen.

Scavenger receptor class B type I is a plasma membrane cholesterol sensor

RATIONALE

Signal initiation by the high-density lipoprotein (HDL) receptor scavenger receptor class B, type I (SR-BI), which is important to actions of HDL on endothelium and other processes, requires cholesterol efflux and the C-terminal transmembrane domain. The C-terminal transmembrane domain uniquely interacts with plasma membrane (PM) cholesterol.

OBJECTIVE

The molecular basis and functional significance of SR-BI interaction with PM cholesterol are unknown. We tested the hypotheses that the interaction is required for SR-BI signaling, and that it enables SR-BI to serve as a PM cholesterol sensor.

METHODS AND RESULTS

In studies performed in COS-M6 cells, mutation of a highly conserved C-terminal transmembrane domain glutamine to alanine (SR-BI-Q445A) decreased PM cholesterol interaction with the receptor by 71% without altering HDL binding or cholesterol uptake or efflux, and it yielded a receptor incapable of HDL-induced signaling. Signaling prompted by cholesterol efflux to methyl-β-cyclodextrin also was prevented, indicating that PM cholesterol interaction with the receptor enables it to serve as a PM cholesterol sensor. Using SR-BI-Q445A, we further demonstrated that PM cholesterol sensing by SR-BI does not influence SR-BI-mediated reverse cholesterol transport to the liver in mice. However, the PM cholesterol sensing does underlie apolipoprotein B intracellular trafficking in response to postprandial micelles or methyl-β-cyclodextrin in cultured enterocytes, and it is required for HDL activation of endothelial NO synthase and migration in cultured endothelial cells and HDL-induced angiogenesis in vivo.

CONCLUSIONS

Through interaction with PM cholesterol, SR-BI serves as a PM cholesterol sensor, and the resulting intracellular signaling governs processes in both enterocytes and endothelial cells.

[Control and analysis of cells by synthetic small molecules]

In human history, small organic molecules have been utilized for improving human health and for revealing secrets of life. Discovery or design of small organic molecules with unique biological activity permits small-molecule-initiated exploration of biology. Our laboratory has been discovering and designing bioactive small synthetic molecules to use them as tools to analyze or modulate biological processes. This personal perspective summarizes our contributions to chemical biology, particularly in the fields of gene transcription, cell therapy, growth factor signaling, and target identification.

Haplotype combination of the bovine INSIG1 gene sequence variants and association with growth traits in Nanyang cattle

The insulin-induced gene 1 (INSIG1) gene encodes a protein that blocks proteolytic activation of sterol regulatory element binding proteins, which are transcription factors that activate genes that regulate cholesterol, fatty acid, and glucose metabolism. However, similar research for the bovine INSIG1 gene is lacking. Therefore, in this study, polymorphisms of the bovine INSIG1 gene were detected in 643 individuals from four cattle breeds by DNA pooling, forced PCR-RFLP, PCR-SSCP, and DNA sequencing methods. Only 10 novel SNPs were identified, which included four mutations in the coding region and the others in the introns. In Nanyang individuals, seven common haplotypes were identified based on four coding region SNPs. The haplotype GACT, with a frequency of 75.4%, was the most prevalent haplotypes and SNPs formed two linkage disequilibrium blocks with strong multi-allelic D' (D' = 1). Additionally, association analysis between mutations of the bovine INSIG1 gene and growth traits in Nanyang cattle at 6, 12, 18, and 24 months old was performed, and the results indicated that the polymorphisms were not significantly associated with body mass.

ER stress contributes to renal proximal tubule injury by increasing SREBP-2-mediated lipid accumulation and apoptotic cell death

Renal proximal tubule injury is induced by agents/conditions known to cause endoplasmic reticulum (ER) stress, including cyclosporine A (CsA), an immunosuppressant drug with nephrotoxic effects. However, the underlying mechanism by which ER stress contributes to proximal tubule cell injury is not well understood. In this study, we report lipid accumulation, sterol regulatory element-binding protein-2 (SREBP-2) expression, and ER stress in proximal tubules of kidneys from mice treated with the classic ER stressor tunicamycin (Tm) or in human renal biopsy specimens showing CsA-induced nephrotoxicity. Colocalization of ER stress markers [78-kDa glucose regulated protein (GRP78), CHOP] with SREBP-2 expression and lipid accumulation was prominent within the proximal tubule cells exposed to Tm or CsA. Prolonged ER stress resulted in increased apoptotic cell death of lipid-enriched proximal tubule cells with colocalization of GRP78, SREBP-2, and Ca(2+)-independent phospholipase A(2) (iPLA(2)β), an SREBP-2 inducible gene with proapoptotic characteristics. In cultured HK-2 human proximal tubule cells, CsA- and Tm-induced ER stress caused lipid accumulation and SREBP-2 activation. Furthermore, overexpression of SREBP-2 or activation of endogenous SREBP-2 in HK-2 cells stimulated apoptosis. Inhibition of SREBP-2 activation with the site-1-serine protease inhibitor AEBSF prevented ER stress-induced lipid accumulation and apoptosis. Overexpression of the ER-resident chaperone GRP78 attenuated ER stress and inhibited CsA-induced SREBP-2 expression and lipid accumulation. In summary, our findings suggest that ER stress-induced SREBP-2 activation contributes to renal proximal tubule cell injury by dysregulating lipid homeostasis.

Experimental strategies for cloning or identifying genes encoding DNA-binding proteins

This article describes experimental strategies for cloning or identifying genes encoding DNA-binding proteins. DNA-binding proteins are most commonly identified by electrophoretic mobility-shift assay (EMSA) or DNase I footprinting. To identify the gene encoding a protein detected by EMSA or DNase footprinting, the protein often needs to be purified and its sequence analyzed, as described here. Other methods are also available which do not resort to protein purification, including the one-hybrid screen, in vitro expression library screen, and mammalian expression cloning. These methods are outlined, and their advantages and disadvantages are discussed.

Human cytomegalovirus infection induces adipocyte-like lipogenesis through activation of sterol regulatory element binding protein 1

Sterol regulatory element binding proteins (SREBPs) are essential transcriptional factors that control expression of lipogenic genes and adipocyte differentiation. Human cytomegalovirus (HCMV) infection has been shown to require the induction of lipogenesis. Here we show that the induction of lipogenesis and expression of key lipogenic enzymes in human fibroblasts occurs by 24 h post-HCMV infection. This activation correlates with increased cleavage of the SREBP1 precursors to form the mature active transcription factors that enter the nucleus to transcriptionally activate lipogenic genes. SREBP1 cleavage is normally inhibited by increased sterol levels; however, our data show that this level of control is overridden in infected cells to allow constitutive activation of lipogenesis. This process requires viral protein synthesis, since UV-irradiated HCMV cannot activate SREBP cleavage. The cleavage of SREBP1 requires it to be in complex with SREBP cleavage activation protein (SCAP). Depleting SCAP using short hairpin RNA (shRNA) showed that SREBP1 cleavage and the induction of lipogenic genes and lipid synthesis are all inhibited in HCMV-infected cells. As a result, production of infectious virions is reduced in SCAP-depleted cells. Thus, the SCAP-mediated mechanism for SREBP cleavage is utilized by HCMV during infection. Our studies suggest that HCMV induces adipocyte-like lipogenesis and overrides normal sterol feedback controls in order to maintain high levels of constitutive lipid synthesis during infection.

A conserved SREBP-1/phosphatidylcholine feedback circuit regulates lipogenesis in metazoans

Sterol regulatory element-binding proteins (SREBPs) activate genes involved in the synthesis and trafficking of cholesterol and other lipids and are critical for maintaining lipid homeostasis. Aberrant SREBP activity, however, can contribute to obesity, fatty liver disease, and insulin resistance, hallmarks of metabolic syndrome. Our studies identify a conserved regulatory circuit in which SREBP-1 controls genes in the one-carbon cycle, which produces the methyl donor S-adenosylmethionine (SAMe). Methylation is critical for the synthesis of phosphatidylcholine (PC), a major membrane component, and we find that blocking SAMe or PC synthesis in C. elegans, mouse liver, and human cells causes elevated SREBP-1-dependent transcription and lipid droplet accumulation. Distinct from negative regulation of SREBP-2 by cholesterol, our data suggest a feedback mechanism whereby maturation of nuclear, transcriptionally active SREBP-1 is controlled by levels of PC. Thus, nutritional or genetic conditions limiting SAMe or PC production may activate SREBP-1, contributing to human metabolic disorders.

Oxysterols in bile acid metabolism

Increasing body of evidence is available indicating that oxysterols are more much than intermediates of metabolic pathways. Oxysterols play a role in the regulation of cholesterol synthesis, transport and efflux. A scavenger effect of cholesterol 27-hydroxylase on elevated serum cholesterol levels is well demonstrated. Bile acid synthesis occurs through two main pathways, the classic and the alternative ones. Since plasma concentrations of 27-hydroxycholesterol were clearly shown to reflect its production rate the alternative pathway of bile acid synthesis can be easily explored. Conversely this was not true for 7α-hydroxycholesterol and also the direct evaluation of the classic pathway by kinetic studies is more difficult since the rate of plasma appearance during continuous infusion of deuterated isotopomers may not exactly measure its production rate. Hepatic cholesterol 7alpha-hydroxylase activity is absent during fetal life in humans and upregulates after birth. Both the classic and alternative pathways become mature after the age of 4 years. It has been clearly demonstrated that in patients with liver disease the classic pathway is impaired while the alternative one is preserved. Conversely, in obese patients, preliminary data suggest an increase of the production rate of 27-hydroxycholesterol, a possible mechanism to counteract the increase of atherosclerotic risk.

Regulation of cholesterol and fatty acid synthesis

In mammals, intracellular levels of cholesterol and fatty acids are controlled through a feedback regulatory system mediated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs). SREBPs are synthesized as inactive precursors bound to membranes of the endoplasmic reticulum. When cells are deprived of cholesterol and fatty acids, NH(2)-terminal fragments of SREBPs become proteolytically released from membranes and migrate to the nucleus to activate transcription of genes required for lipid synthesis and uptake. Conversely, lipid repletion inhibits proteolytic processing of SREBPs and thereby suppresses lipid accumulation. We review here studies in cultured cells that reveal the mechanism for regulation of SREBP proteolytic activation, and those in animal models in which SREBP proteolysis has been either activated or inhibited to show the essential role of SREBPs in regulating hepatic lipid homeostasis.

Intramembrane proteolysis in regulated protein trafficking

Regulated intramembrane proteolysis is an evolutionarily conserved mechanism by which membrane-anchored bioactive molecules are released from cellular membranes. In eukaryotic cells, intramembrane proteases are found in different cellular organelles ranging from the endosomal system to mitochondria and chloroplasts. These proteases function in diverse processes such as transcription control, regulated growth factor secretion and recently even a role in the control of mitophagy has been suggested. Genomic annotation has predicted 13 different intramembrane proteases in humans. Apart from few studied examples, very little is known about their function. This review describes emerging principles of how intramembrane proteases contribute to the regulation of cellular protein trafficking in eukaryotic cells and raises the important question of how their activity is controlled.

Synthesis and evaluation of diarylthiazole derivatives that inhibit activation of sterol regulatory element-binding proteins

Fatostatin, a recently discovered small molecule that inhibits activation of sterol regulatory element-binding protein (SREBP), blocks biosynthesis and accumulation of fat in obese mice. We synthesized and evaluated a series of fatostatin derivatives. Our structure-activity relationships led to the identification of N-(4-(2-(2-propylpyridin-4-yl)thiazol-4-yl)phenyl)methanesulfonamide (24, FGH10019) as the most potent druglike molecule among the analogues tested. Compound 24 has high aqueous solubility and membrane permeability and may serve as a seed molecule for further development.

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.

C. elegans patched-3 is an essential gene implicated in osmoregulation and requiring an intact permease transporter domain

The nematode Caenorhabditis elegans has retained a rudimentary Hedgehog (Hh) signalling pathway; Hh and Smoothened (Smo) homologs are absent, but two highly related Patched gene homologs, ptc-1 and ptc-3, and 24 ptc-related (ptr) genes are present. We previously showed that ptc-1 is essential for germ line cytokinesis. Here, we report that ptc-3 is also an essential gene; the absence of ptc-3 results in a late embryonic lethality due to an apparent defect in osmoregulation. Rescue of a ptc-3 mutant with a ptc-3::gfp translational reporter reveals that ptc-3 is dynamically expressed in multiple tissues across development. Consistent with this pattern of expression, ptc-3(RNAi) reveals an additional postembryonic requirement for ptc-3 activity. Tissue-specific promoter studies indicate that hypodermal expression of ptc-3 is required for normal development. Missense changes in key residues of the sterol sensing domain (SSD) and the permease transporter domain GxxxD/E motif reveal that the transporter domain is essential for PTC-3 activity, whereas an intact SSD is dispensable. Taken together, our studies indicate that PTC proteins have retained essential roles in C. elegans that are independent of Smoothened (Smo). These observations reveal novel, and perhaps ancestral, roles for PTC proteins.

Nelfinavir induces liposarcoma apoptosis through inhibition of regulated intramembrane proteolysis of SREBP-1 and ATF6

PURPOSE

We previously reported that nelfinavir (NFV) induces G(1) cell-cycle block and apoptosis selectively in liposarcoma cell lines due to increased SREBP-1 (sterol regulatory element binding protein-1) expression in the absence of increased transcription. We postulate that NFV interferes with regulated intramembrane proteolysis of SREBP-1 and ATF6 (activating transcription factor 6).

EXPERIMENTAL DESIGN

Time-lapse, confocal microscopic studies show that NFV inhibits the nuclear translocation of full-length SREBP-1-EGFP and ATF6-EGFP fusion proteins. siRNA-mediated knockdown of site-1 protease (S1P) and/or site-2 protease (S2P) leads to inhibition of SREBP-1 intracellular trafficking to the nucleus and reduces liposarcoma cell proliferation. Treatment of LiSa-2 liposarcoma cells with 3,4-dichloroisocoumarin, a serine protease inhibitor of S1P, did not affect SREBP-1 processing. In contrast, 1,10-phenanthroline, an S2P-specific inhibitor, reproduces the molecular and biological phenotypes observed in NFV-treated cells, which implicates S2P as a target of NFV. In vivo evaluation of NFV in a murine liposarcoma xenograft model leads to inhibition of tumor growth without significant toxicity.

RESULTS

NFV-induced upregulation of SREBP-1 and ATF6 results from inhibition of S2P, which together with S1P mediates regulated intramembrane proteolysis from their precursor to their transcriptionally active forms. The resulting endoplasmic reticulum (ER) stress and concurrent inhibition of the unfolded protein response induce caspase-mediated apoptosis.

CONCLUSIONS

These results provide new insight into the mechanism of NFV-mediated induction of ER stress and cell death in liposarcomas and are the first to report targeting S2P for cancer therapy.

CREB3 subfamily transcription factors are not created equal: Recent insights from global analyses and animal models

The CREB3 subfamily of membrane-bound bZIP transcription factors has five members in mammals known as CREB3 and CREB3L1-L4. One current model suggests that CREB3 subfamily transcription factors are similar to ATF6 in regulated intramembrane proteolysis and transcriptional activation. Particularly, they were all thought to be proteolytically activated in response to endoplasmic reticulum (ER) stress to stimulate genes that are involved in unfolded protein response (UPR). Although the physiological inducers of their proteolytic activation remain to be identified, recent findings from microarray analyses, RNAi screens and gene knockouts not only demonstrated their critical roles in regulating development, metabolism, secretion, survival and tumorigenesis, but also revealed cell type-specific patterns in the activation of their target genes. Members of the CREB3 subfamily show differential activity despite their structural similarity. The spectrum of their biological function expands beyond ER stress and UPR. Further analyses are required to elucidate the mechanism of their proteolytic activation and the molecular basis of their target recognition.

Endoplasmic reticulum stress and lipid dysregulation

Cellular cholesterol homeostasis is a fundamental and highly regulated process. Transcription factors known as sterol regulatory element binding proteins (SREBPs) coordinate the expression of many genes involved in the biosynthesis and uptake of cholesterol. Dysregulation of SREBP activation and cellular lipid accumulation has been associated with endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). This review will provide an overview of ER stress and the UPR as well as cholesterol homeostasis and SREBP regulation, with an emphasis on their interaction and biological relevance.

Insulin-induced gene: a new regulator in lipid metabolism

Insulin-induced genes (Insigs) including Insig-1 and Insig-2, are proteins that mediate sterol regulation of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase). Insigs perform distinct tasks in the regulation of these effectors: they promote the endoplasmic reticulum (ER) retention of SCAP, but ubiquitin-mediated degradation of HMG-CoA reductase. Through these activities, Insig-1 and Insig-2 influence cholesterol metabolism, lipogenesis, and glucose homeostasis in diverse tissues such as adipose tissue and liver. In this article, we focus on the functions, expression and regulation, gene polymorphisms of Insigs, and their deficiency with diseases.

Interleukin-1β enhances the intracellular accumulation of cholesterol by up-regulating the expression of low-density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase in podocytes

The aim of this article is to investigate whether interleukin-1β (IL-1β) could regulate the intracellular accumulation of cholesterol and the expression of lipid-metabolism-related regulators in podocytes in vitro and the potential mechanisms. Podocytes were treated with 200 μg/ml of low-density protein (LDL), 20 ng/ml of IL-1β, or 200 μg/ml of LDL plus 5-20 ng/ml of IL-1β for 24 h in vitro. The contents of intracellular cholesterol were determined by enzymatic assays and Oil Red O staining. The levels of LDL receptor (LDLr), 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, sterol regulatory element binding protein 2 (SREBP-2), SREBP cleavage activating protein (SCAP), and insulin-induced gene-1 (Insig-1) expression were characterized by real-time polymerase chain reaction (RT-PCR) and Western blot assays. Treatment with IL-1β or LDL alone increased the contents of intracellular cholesterol (P < 0.05 for both) and lipid droplets, and treatment with both IL-1β and LDL further increased the accumulation of intracellular cholesterol in podocytes (P < 0.05 vs. LDL alone). While loading with LDL significantly inhibited the expression of LDLr, HMG-CoA reductase, nuclear SREBP-2 (nSREBP-2), SCAP, and Insig-1 by 40-62% treatment with IL-1β enhanced the expression of LDLr, HMG-CoA reductase and nSREBP-2, but not Insig-1, in podocytes (P < 0.05 vs. control). Treatment with both LDL and IL-1β significantly increased the levels of LDLr and HMG-CoA reductase expression and the ratio of SCAP to Insig-1, as compared with that in the LDL-treated podocytes (P < 0.05 vs. LDL alone). Our data indicated that IL-1β mitigated the LDL-triggered SCAP-SREBP-2-mediated feedback inhibition on the expression of LDLr and HMG-CoA reductase, leading to the intracellular accumulation of LDL-cholesterol in podocytes in vitro.

Chemical combinations elucidate pathway interactions and regulation relevant to Hepatitis C replication

SREBP-2, oxidosqualene cyclase (OSC) or lanosterol demethylase were identified as novel sterol pathway-associated targets that, when probed with chemical agents, can inhibit hepatitis C virus (HCV) replication.

Using a combination chemical genetics approach, combinations of chemicals targeting sterol pathway enzymes downstream of and including OSC or protein geranylgeranyl transferase I (PGGT) produce robust and selective synergistic inhibition of HCV replication. Inhibition of enzymes upstream of OSC elicit proviral responses that are dominant to the effects of inhibiting all downstream targets.

Inhibition of the sterol pathway without inhibition of regulatory feedback mechanisms ultimately results in an increase in HCV replication because of a compensatory upregulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) expression. Increases in HMGCR expression without inhibition of HMGCR enzymatic activity ultimately stimulate HCV replication through increasing the cellular pool of geranylgeranyl pyrophosphate (GGPP).

Chemical inhibitors that ultimately prevent SREBP-2 activation, inhibit PGGT or encourage the production of polar sterols have great potential as HCV therapeutics if associated toxicities can be reduced.

Chemical inhibition of enzymes in either the cholesterol or the fatty acid biosynthetic pathways has been shown to impact viral replication, both positively and negatively (Su et al, 2002; Ye et al, 2003; Kapadia and Chisari, 2005; Sagan et al, 2006; Amemiya et al, 2008). FBL2 has been identified as a 50 kDa geranylgeranylated host protein that is necessary for localization of the hepatitis C virus (HCV) replication complex to the membranous web through its close association with the HCV protein NS5A and is critical for HCV replication (Wang et al, 2005). Inhibition of the protein geranylgeranyl transferase I (PGGT), an enzyme that transfers geranylgeranyl pyrophosphate (GGPP) to cellular proteins such as FBL2 for the purpose of membrane anchoring, negatively impacts HCV replication (Ye et al, 2003). Conversely, chemical agents that increase intracellular GGPP concentrations promote viral replication (Kapadia and Chisari, 2005). Statin compounds that inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme in the sterol pathway (Goldstein and Brown, 1990), have been suggested to inhibit HCV replication through ultimately reducing the cellular pool of GGPP (Ye et al, 2003; Kapadia and Chisari, 2005; Ikeda et al, 2006). However, inhibition of the sterol pathway with statin drugs has not yielded consistent results in patients. The use of statins for the treatment of HCV is likely to be complicated by the reported compensatory increase in HMGCR expression in vitro and in vivo (Stone et al, 1989; Cohen et al, 1993) in response to treatment. Enzymes in the sterol pathway are regulated on a transcriptional level by sterol regulatory element-binding proteins (SREBPs), specifically SREBP-2 (Hua et al, 1993; Brown and Goldstein, 1997). When cholesterol stores in cells are depleted, SREBP-2 activates transcription of genes in the sterol pathway such as HMGCR, HMG-CoA synthase, farnesyl pyrophosphate (FPP) synthase, squalene synthase (SQLS) and the LDL receptor (Smith et al, 1988, 1990; Sakai et al, 1996; Brown and Goldstein, 1999; Horton et al, 2002). The requirement of additional downstream sterol pathway metabolites for HCV replication has not been completely elucidated.

To further understand the impact of the sterol pathway and its regulation on HCV replication, we conducted a high-throughput combination chemical genetic screen using 16 chemical probes that are known to modulate the activity of target enzymes relating to the sterol biosynthesis pathway (Figure 1). Using this approach, we identified several novel antiviral targets including SREBP-2 as well as targets downstream of HMGCR in the sterol pathway such as oxidosqualene cyclase (OSC) and lanosterol demethylase. Many of our chemical probes, specifically SR-12813, farnesol and squalestatin, strongly promoted replicon replication. The actions of both farnesol and squalestatin ultimately result in an increase in the cellular pool of GGPP, which is known to increase HCV replication (Ye et al, 2003; Kapadia and Chisari, 2005; Wang et al, 2005).

Chemical combinations targeting enzymes upstream of squalene epoxidase (SQLE) at the top of the sterol pathway (Figure 4A) elicited Bateson-type epistatic responses (Boone et al, 2007), where the upstream agent's response predominates over the effects of inhibiting all downstream targets. This was especially notable for combinations including simvastatin and either U18666A or squalestatin, and for squalestatin in combination with Ro48-8071. Treatment with squalestatin prevents the SQLS substrate, farnesyl pyrophosphate (FPP) from being further metabolized by the sterol pathway. As FPP concentrations increase, the metabolite can be shunted away from the sterol pathway toward farnesylation and GGPP synthetic pathways, resulting in an increase in host protein geranylgeranylation, including FBL2, and consequently replicon replication. This increase in replicon replication explains the source of the observed epistasis over Ro48-8071 treatment.

Combinations between probes targeting enzymes downstream of and including OSC produced robust synergies with each other or with a PGGT inhibitor. Figure 4B highlights examples of antiviral synergy resulting from treatment of cells with an OSC inhibitor in combination with an inhibitor of either an enzyme upstream or downstream of OSC. A combination of terconazole and U18666A is synergistic without similar combination effects in the host proliferation screen. Likewise, clomiphene was also synergistic when added to replicon cells in combination with U18666A. One of the greatest synergies observed downstream in the sterol pathway is a combination of amorolfine and AY 9944, suggesting that there is value in developing combinations of drugs that target enzymes in the sterol pathway, which are downstream of HMGCR.

Interactions with the protein prenylation pathway also showed strong mechanistic patterns (Figure 4C). GGTI-286 is a peptidomimetic compound resembling the CAAX domain of a protein to be geranylgeranylated and is a competitive inhibitor of protein geranylgeranylation. Simvastatin impedes the antiviral effect of GGTI-286 at low concentrations but that antagonism is balanced by comparable synergy at higher concentrations. At the low simvastatin concentrations, a compensatory increase in HMGCR expression leads to increased cellular levels of GGPP, which are likely to result in an increase in PGGT enzymatic turnover and decreased GGTI-286 efficacy. The antiviral synergy observed at the higher inhibitor concentrations is likely nonspecific as synergy was also observed in a host viability assay. Further downstream, however, a competitive interaction was observed between GGTI-286 and squalestatin, where the opposing effect of one compound obscures the other compound's effect. This competitive relationship between GGTI and SQLE explains the epistatic response observed between those two agents. For inhibitors of targets downstream of OSC, such as amorolfine, there are strong antiviral synergies with GGTI-286. Notably, combinations with OSC inhibitors and GGTI-286 were selective, in that comparable synergy was not found in a host viability assay. This selectivity suggests that jointly targeting OSC and PGGT is a promising avenue for future HCV therapy development.

This study provides a comprehensive and unique perspective into the impact of sterol pathway regulation on HCV replication and provides compelling insight into the use of chemical combinations to maximize antiviral effects while minimizing proviral consequences. Our results suggest that HCV therapeutics developed against sterol pathway targets must consider the impact on underlying sterol pathway regulation. We found combinations of inhibitors of the lower part of the sterol pathway that are effective and synergistic with each other when tested in combination. Furthermore, the combination effects observed with simvastatin suggest that, though statins inhibit HMGCR activity, the resulting regulatory consequences of such inhibition ultimately lead to undesirable epistatic effects. Inhibitors that prevent SREBP-2 activation, inhibit PGGT or encourage the production of polar sterols have great potential as HCV therapeutics if associated toxicities can be reduced.

The search for effective Hepatitis C antiviral therapies has recently focused on host sterol metabolism and protein prenylation pathways that indirectly affect viral replication. However, inhibition of the sterol pathway with statin drugs has not yielded consistent results in patients. Here, we present a combination chemical genetic study to explore how the sterol and protein prenylation pathways work together to affect hepatitis C viral replication in a replicon assay. In addition to finding novel targets affecting viral replication, our data suggest that the viral replication is strongly affected by sterol pathway regulation. There is a marked transition from antagonistic to synergistic antiviral effects as the combination targets shift downstream along the sterol pathway. We also show how pathway regulation frustrates potential hepatitis C therapies based on the sterol pathway, and reveal novel synergies that selectively inhibit hepatitis C replication over host toxicity. In particular, combinations targeting the downstream sterol pathway enzymes produced robust and selective synergistic inhibition of hepatitis C replication. Our findings show how combination chemical genetics can reveal critical pathway connections relevant to viral replication, and can identify potential treatments with an increased therapeutic window.

Control of cholesterol synthesis through regulated ER-associated degradation of HMG CoA reductase

Multiple mechanisms for feedback control of cholesterol synthesis converge on the rate-limiting enzyme in the pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase. This complex feedback regulatory system is mediated by sterol and nonsterol metabolites of mevalonate, the immediate product of reductase activity. One mechanism for feedback control of reductase involves rapid degradation of the enzyme from membranes of the endoplasmic reticulum (ER). This degradation results from the accumulation of sterols in ER membranes, which triggers binding of reductase to ER membrane proteins called Insig-1 and Insig-2. Insig binding leads to the recruitment of a membrane-associated ubiquitin ligase called gp78 that initiates ubiquitination of reductase. Ubiquitinated reductase then becomes extracted from ER membranes and is delivered to cytosolic 26S proteasomes through an unknown mechanism that is mediated by the gp78-associated ATPase Valosin-containing protein/p97 and appears to be augmented by nonsterol isoprenoids. Here, we will highlight several advances that have led to the current view of mechanisms for sterol-accelerated, ER-associated degradation of reductase. In addition, we will discuss potential mechanisms for other aspects of the pathway such as selection of reductase for gp78-mediated ubiquitination, extraction of the ubiquitinated enzyme from ER membranes, and the contribution of Insig-mediated degradation to overall regulation of reductase in whole animals.

Exosome secretion ameliorates lysosomal storage of cholesterol in Niemann-Pick type C disease

Niemann-Pick type C1 disease is an autosomal-recessive lysosomal storage disorder. Loss of function of the npc1 gene leads to abnormal accumulation of free cholesterol and sphingolipids within the late endosomal and lysosomal compartments resulting in progressive neurodegeneration and dysmyelination. Here, we show that oligodendroglial cells secrete cholesterol by exosomes when challenged with cholesterol or U18666A, which induces late endosomal cholesterol accumulation. Up-regulation of exosomal cholesterol release was also observed after siRNA-mediated knockdown of NPC1 and in fibroblasts derived from NPC1 patients and could be reversed by expression of wild-type NPC1. We provide evidence that exosomal cholesterol secretion depends on the presence of flotillin. Our findings indicate that exosomal release of cholesterol may serve as a cellular mechanism to partially bypass the traffic block that results in the toxic lysosomal cholesterol accumulation in Niemann-Pick type C1 disease. Furthermore, we suggest that secretion of cholesterol by exosomes contributes to maintain cellular cholesterol homeostasis.

Sterol metabolism and SREBP activation

Cholesterol is one of the major components of membrane lipids. Adjustment of the membrane cholesterol balance is, therefore, pivotal in the maintenance of cellular homeostasis. Cholesterol biosynthesis and uptake are tightly regulated at the transcriptional level through a negative feedback control. The transcription factor family known as sterol regulatory element-binding proteins (SREBPs) is in charge of this feedback control. In particular, SREBP-2 is activated in a cholesterol-dependent manner, and hence is deeply involved in regulation of the expression of genes closely related to cholesterol metabolism. Sterol metabolites, including cholesterol, 24,25-dihydrolanosterol and oxysterols, strongly participate in the regulation of sterol metabolism via binding to its particular proteins.

Obesity, cholesterol, and clear-cell renal cell carcinoma (RCC)

Multiple epidemiologic studies have linked the development of renal cancer to obesity. In this chapter, we begin with a review of selected population studies, followed by recent mechanistic discoveries that further link lipid deregulation to the RCC development. The upregulation of leptin and downregulation of adiponectin pathways in obesity fit well with our molecular understanding of RCC pathogenesis. In addition, two forms of hereditary RCC involve proteins, Folliculin and TRC8, that are positioned to coordinately regulate lipid and protein biosynthesis. Both of these biosynthetic pathways have important downstream consequences on HIF-1/2alpha levels and angiogenesis, key aspects in the disease pathogenesis. The role of lipid biology and its interface with protein translation regulation represents a new dimension in RCC research with potential therapeutic implications.

A feed-forward loop amplifies nutritional regulation of PNPLA3

The upsurge in prevalence of obesity has spawned an epidemic of nonalcoholic fatty liver disease (NAFLD). Previously, we identified a sequence variant (I148M) in patatin-like phospholipase domain-containing protein 3 (PNPLA3) that confers susceptibility to both hepatic triglyceride (TG) deposition and liver injury. To glean insights into the biological role of PNPLA3, we examined the molecular mechanisms by which nutrient status controls hepatic expression of PNPLA3. PNPLA3 mRNA levels, which were low in fasting animals, increased approximately 90-fold with carbohydrate feeding. The increase was mimicked by treatment with a liver X receptor (LXR) agonist and required the transcription factor SREBP-1c. The site of SREBP-1c binding was mapped to intron 1 of Pnpla3 using chromatin immunoprecipitation and electrophoretic mobility shift assays. SREBP-1c also promotes fatty acid synthesis by activating several genes encoding enzymes in the biosynthetic pathway. Addition of fatty acids (C16:0, C18:1, and C18:2) to the medium of cultured hepatocytes (HuH-7) increased PNPLA3 protein mass without altering mRNA levels. The posttranslational increase in PNPLA3 levels persisted after blocking TG synthesis with triascin C. Oleate (400 muM) treatment prolonged the half-life of PNPLA3 from 2.4 to 6.7 h. These findings are consistent with nutritional control of PNPLA3 being effected by a feed-forward loop; SREBP-1c promotes accumulation of PNPLA3 directly by activating Pnpla3 transcription and indirectly by inhibiting PNPLA3 degradation through the stimulation of fatty acid synthesis.

High density lipoprotein inhibits the activation of sterol regulatory element-binding protein-1 in cultured cells

A link between cellular uptake of high density lipoprotein (HDL) and regulation of sterol regulatory element-binding protein-1 (SREBP-1) was investigated in vitro. HDL decreased nuclear SREBP-1 levels as well as SREBP-1 target gene expression in HepG2 and HEK293 cells. However, HDL did not repress an exogenously expressed, constitutively active form of SREBP-1. HDL increased cellular cholesterol levels, and cellular cholesterol depletion by methyl-beta-cyclodextrin abolished the effects of HDL. These results suggest that HDL inhibits the activation of SREBP-1 through a cholesterol-dependent mechanism, which may play an important role in regulating lipid synthetic pathways mediated by SREBP-1.

Intramembrane glycine mediates multimerization of Insig-2, a requirement for sterol regulation in Chinese hamster ovary cells

Sterol-induced binding of endoplasmic reticulum (ER) membrane proteins Insig-1 and Insig-2 to SREBP cleavage-activating protein (Scap) and HMG-CoA reductase triggers regulatory events that limit cholesterol synthesis in animal cells. Binding of Insigs to Scap prevents proteolytic activation of sterol-regulatory element binding proteins (SREBPs), membrane-bound transcription factors that enhance cholesterol synthesis, by trapping Scap-SREBP complexes in the ER. Insig binding to reductase causes ubiquitination and subsequent proteasome-mediated degradation of the enzyme from ER membranes, slowing a rate-limiting step in cholesterol synthesis. Here, we report the characterization of mutant Chinese hamster ovary cells, designated SRD-20, that are resistant to 25-hydroxycholesterol, which potently inhibits SREBP activation and stimulates degradation of reductase. SRD-20 cells were produced by mutagenesis of Insig-1-deficient SRD-14 cells, followed by selection in 25-hydroxycholesterol. DNA sequencing reveals that SRD-20 cells harbor a point mutation in one Insig-2 allele that results in production of a truncated, nonfunctional protein, whereas the other allele contains a point mutation that results in substitution of glutamic acid for glycine-39. This glycine residue localizes to the first membrane-spanning segment of Insig-2 and is also present in the corresponding region of Insig-1. Mutant forms of Insig-1 and Insig-2 containing the Glu-to-Gly substitution fail to confer sterol regulation upon overexpressed Scap and reductase. These studies identify the intramembrane glycine as a key residue for normal sterol regulation in animal cells.

Activation of sterol regulatory element binding proteins in the absence of Scap in Drosophila melanogaster

The escort factor Scap is essential in mammalian cells for regulated activation of sterol regulatory element binding proteins (SREBPs). SREBPs are membrane-bound transcription factors. Cells lacking Scap cannot activate SREBP. They are therefore deficient in the transcription of numerous genes involved in lipid synthesis and uptake; they cannot survive in the absence of exogenous lipid. Here we report that, in contrast to mammalian cells, Drosophila completely lacking dscap are viable. Flies lacking dscap emerge at approximately 70% of the expected rate and readily survive as homozygous stocks. These animals continue to cleave dSREBP in some tissues. Transcription of dSREBP target genes in dscap mutant larvae is reduced compared to wild type. It is greater than in mutants lacking dSREBP and remains responsive to dietary lipids in dscap mutants. Flies lacking dscap do not require the caspase Drice to activate dSREBP. This contrasts with ds2p mutants. ds2p encodes a protease that releases the transcription factor domain of dSREBP from the membrane. Larvae doubly mutant for dscap and ds2p exhibit phenotypes similar to those of ds2p single mutants. Thus, dScap and dS2P, essential components of the SREBP activation machinery in mammalian cells, are dispensable in Drosophila owing to different compensatory mechanisms.

PPARgamma regulates the expression of cholesterol metabolism genes in alveolar macrophages

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a nuclear transcription factor involved in lipid metabolism that is constitutively expressed in the alveolar macrophages of healthy individuals. PPARgamma has recently been implicated in the catabolism of surfactant by alveolar macrophages, specifically the cholesterol component of surfactant while the mechanism remains unclear. Studies from other tissue macrophages have shown that PPARgamma regulates cholesterol influx, efflux, and metabolism. PPARgamma promotes cholesterol efflux through the liver X receptor-alpha (LXRalpha) and ATP-binding cassette G1 (ABCG1). We have recently shown that macrophage-specific PPARgamma knockout (PPARgamma KO) mice accumulate cholesterol-laden alveolar macrophages that exhibit decreased expression of LXRalpha and ABCG1 and reduced cholesterol efflux. We hypothesized that in addition to the dysregulation of these cholesterol efflux genes, the expression of genes involved in cholesterol synthesis and influx was also dysregulated and that replacement of PPARgamma would restore regulation of these genes. To investigate this hypothesis, we have utilized a Lentivirus expression system (Lenti-PPARgamma) to restore PPARgamma expression in the alveolar macrophages of PPARgamma KO mice. Our results show that the alveolar macrophages of PPARgamma KO mice have decreased expression of key cholesterol synthesis genes and increased expression of cholesterol receptors CD36 and scavenger receptor A-I (SRA-I). The replacement of PPARgamma (1) induced transcription of LXRalpha and ABCG1; (2) corrected suppressed expression of cholesterol synthesis genes; and (3) enhanced the expression of scavenger receptors CD36. These results suggest that PPARgamma regulates cholesterol metabolism in alveolar macrophages.

Signaling by the high-affinity HDL receptor scavenger receptor B type I

Scavenger receptor B type I (SR-BI) plays an important role in mediating cholesterol exchange between cells, high-density lipoprotein (HDL) cholesterol, and other lipoproteins. SR-BI in hepatocytes is essential for reverse cholesterol transport and biliary secretion of HDL cholesterol; thus, it is atheroprotective. More recently, it has been discovered that the HDL-SR-BI tandem serves other functions that also likely contribute to HDL-related cardiovascular protection. A number of the latter mechanisms, particularly in endothelial cells, involve unique direct signal initiation by SR-BI that leads to the activation of diverse kinase cascades. SR-BI signaling occurs in response to plasma membrane cholesterol flux. It requires the C-terminal PDZ-interacting domain of the receptor, which mediates direct interaction with the adaptor molecule PDZK1; and the C-terminal transmembrane domain, which directly binds membrane cholesterol. In endothelium, direct SR-BI signaling in response to HDL results in enhanced production of the antiatherogenic molecule nitric oxide; in a nitric oxide-independent manner, it serves to maintain endothelial monolayer integrity. The role of SR-BI signaling in the numerous other cellular targets of HDL, including hepatocytes, macrophages, and platelets, and the basis by which SR-BI senses plasma membrane cholesterol movement to modify cell behavior are unknown. Further understanding of signaling by SR-BI will optimize the capacity to harness the mechanisms of action of HDL-SR-BI for cardiovascular benefit.

Sterol regulatory element binding proteins in fungi: hypoxic transcription factors linked to pathogenesis

Sterol regulatory element binding proteins (SREBPs) are membrane-bound transcription factors whose proteolytic activation is controlled by the cellular sterol concentration. Mammalian SREBPs are activated in cholesterol-depleted cells and serve to regulate cellular lipid homeostasis. Recent work demonstrates that SREBP is functionally conserved in fungi. While the ability to respond to sterols is conserved, fungal SREBPs are hypoxic transcription factors required for adaptation to a low-oxygen environment. In the fission yeast Schizosaccharomyces pombe, oxygen regulates the SREBP homolog Sre1 by independently controlling both its proteolytic activation and its degradation. SREBP is also required for adaptation to hypoxia in the human pathogens Cryptococcus neoformans and Aspergillus fumigatus. In these organisms, SREBP is required for virulence and resistance to antifungal drugs, making the SREBP pathway a potential target for antifungal therapy.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Sterol-protein interactions in cholesterol and bile acid synthesis

Cholesterol and other cholesterol related metabolites, oxysterols, and bile acids, establish specific interactions with enzymes and other proteins involved in cholesterol and bile acid homeostasis, triggering a variety of biological responses. The substrate-enzyme binding represents the best-characterized type of complementary interaction between proteins and small molecules. Key enzymes in the pathway that converts cholesterol to bile acids belong to the cytochrome P450 superfamily. In contrast to the majority of P450 enzymes, those acting on cholesterol and related metabolites exhibit higher stringency with respect to substrate molecules. This stringency, coupled with the specificity of the reactions, dictates the chemical features of intermediate metabolites (oxysterols) and end products (bile acids). Both oxysterols and bile acids have emerged in recent years as new signalling molecules due to their ability to interact and activate nuclear receptors, and consequently to regulate the transcription of genes involved in cholesterol and bile acid homeostasis and metabolism, but also in glucose and fatty acid metabolism. Interestingly, other proteins function as bile acid or sterol receptors. New findings indicate that bile acids also interact with a membrane G protein-coupled receptor, triggering a signalling cascade that ultimately promote energy expenditure. On the other end, cholesterol and side chain oxysterols establish specific interactions with different proteins residing in the endoplasmic reticulum that result in controlled protein degradation and/or trafficking to the Golgi and the nucleus. These regulatory pathways converge and contribute to adapt cholesterol uptake and synthesis to the cellular needs.

A small molecule that blocks fat synthesis by inhibiting the activation of SREBP

Sterol regulatory element binding proteins (SREBPs) are transcription factors that activate transcription of the genes involved in cholesterol and fatty acid biosynthesis. In the present study, we show that a small synthetic molecule we previously discovered to block adipogenesis is an inhibitor of the SREBP activation. The diarylthiazole derivative, now called fatostatin, impairs the activation process of SREBPs, thereby decreasing the transcription of lipogenic genes in cells. Our analysis suggests that fatostatin inhibits the ER-Golgi translocation of SREBPs through binding to their escort protein, the SREBP cleavage-activating protein (SCAP), at a distinct site from the sterol-binding domain. Fatostatin blocked increases in body weight, blood glucose, and hepatic fat accumulation in obese ob/ob mice, even under uncontrolled food intake. Fatostatin may serve as a tool for gaining further insights into the regulation of SREBP.

Regulated endoplasmic reticulum-associated degradation of a polytopic protein: p97 recruits proteasomes to Insig-1 before extraction from membranes

Polytopic membrane proteins subjected to endoplasmic reticulum (ER)-associated degradation are extracted from membranes and targeted to proteasomes for destruction. The extraction mechanism is poorly understood. One polytopic ER protein subjected to ER-associated degradation is Insig-1, a negative regulator of cholesterol synthesis. Insig-1 is rapidly degraded by proteasomes when cells are depleted of cholesterol, and its degradation is inhibited when sterols accumulate in cells. Insig-2, a functional homologue of Insig-1, is degraded slowly, and its degradation is not regulated by sterols. Here, we report that a single amino acid substitution in Insig-2, Insig-2(L210A), causes Insig-2 to be degraded in an accelerated and sterol-regulated manner similar to Insig-1. In seeking an explanation for the accelerated degradation, we found that proteasomes bind to wild type Insig-1 and mutant Insig-2(L210A) but not to wild type Insig-2, whereas the proteins are still embedded in cell membranes. This binding depends on at least two factors, ubiquitination of Insig and association with the ATPase p97/VCP complex. These data suggest that p97 recruits proteasomes to polytopic ER proteins even before they are extracted from membranes.

Direct regulation of prokaryotic Kir channel by cholesterol

Our earlier studies have shown that channel activity of Kir2 subfamily of inward rectifiers is strongly suppressed by the elevation of cellular cholesterol. The goal of this study is to determine whether cholesterol suppresses Kir channels directly. To achieve this goal, purified prokaryotic Kir (KirBac1.1) channels were incorporated into liposomes of defined lipid composition, and channel activity was assayed by (86)Rb(+) uptake. Our results show that (86)Rb(+) flux through KirBac1.1 is strongly inhibited by cholesterol. Incorporation of 5% (mass cholesterol/phospholipid) cholesterol into the liposome suppresses (86)Rb(+) flux by >50%, and activity is completely inhibited at 12-15%. However, epicholesterol, a stereoisomer of cholesterol with similar physical properties, has significantly less effect on KirBac-mediated (86)Rb(+) uptake than cholesterol. Furthermore, analysis of multiple sterols suggests that cholesterol-induced inhibition of KirBac1.1 channels is mediated by specific interactions rather than by changes in the physical properties of the lipid bilayer. In contrast to the inhibition of KirBac1.1 activity, cholesterol had no effect on the activity of reconstituted KscA channels (at up to 250 microg/mg of phospholipid). Taken together, these observations demonstrate that cholesterol suppresses Kir channels in a pure protein-lipid environment and suggest that the interaction is direct and specific.

Interactions among genetic variants from SREBP2 activating-related pathway on risk of coronary heart disease in Chinese Han population

Sterol regulatory element binding proteins (SREBPs), as a family of membrane-bound transcription factors, control the metabolism of cholesterol and fatty acids. We conducted a case-control study to investigate whether the common variants of genes from the SREBP2 activating-related pathway, involving SREBP2, SCAP, INSIG1 and INSIG2 genes, were associated with coronary heart disease (CHD) of Chinese Han population individually or interactively. Three, four and two single nucleotide polymorphisms (SNPs) from the INSIG1, INSIG2 and SCAP genes were chosen as haplotype-tagging SNPs (htSNPs), respectively, and one nonsynonymous coding SNP was selected from SREBP2. All of the SNPs were genotyped in 853 CHD cases and 948 unrelated control subjects. The interactions among SNPs of the four genes were evaluated with multifactor-dimensionality reduction (MDR) and logistic regression models (LRM). The results from MDR indicated that there existed the SNP-SNP interactive effect of INSIG1 gene on CHD (best prediction accuracy=56.09%, p=0.002 on 1000 permutations). The results from LRM also identified the 2-locus interaction model (adjusted p< or =0.001 for interaction) as well as the 3-locus gene-gene interaction (adjusted p=0.026 for interaction). Single polymorphism analysis showed that the rs4822063 of SREBP2 was associated with LDL-C in the controls. The genotype CC carriers had higher LDL-C than the major allele G carriers (3.44+/-0.90 mmol/L versus 3.17+/-0.84 mmol/L, adjusted p=0.038). Our results suggested that the INSIG1 gene was associated with CHD; there might be potential interactive effect on CHD among genes from SREBP2 activating-related pathway; and the SREBP2 gene might be associated with plasma lipid level.