The requirements for sterol regulatory element-binding protein (Srebp) and stimulatory protein 1 (Sp1)-binding elements in the transcriptional activation of two freshwater fish Channa striata and Danio rerio elovl5 elongase

Fish are a major source of beneficial n-3 LC-PUFA in human diet, and there is considerable interest to elucidate the mechanism and regulatory aspects of LC-PUFA biosynthesis in farmed species. Long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis involves the activities of two groups of enzymes, the fatty acyl desaturase (Fads) and elongase of very long-chain fatty acid (Elovl). The promoters of elovl5 elongase, which catalyses the rate-limiting reaction of elongating polyunsaturated fatty acid (PUFA), have been previously described and characterized from several marine and diadromous teleost species. We report here the cloning and characterization of elovl5 promoter from two freshwater fish species, the carnivorous snakehead fish (Channa striata) and zebrafish. Results show the presence of sterol-responsive elements (SRE) in the core regulatory region of both promoters, suggesting the importance of sterol regulatory element-binding protein (Srebp) in the regulation of elovl5 for both species. Mutagenesis luciferase and electrophoretic mobility shift assays further validate the role of SRE for basal transcriptional activation. In addition, several Sp1-binding sites located in close proximity with SRE were present in the snakehead promoter, with one having a potential synergy with SRE in the regulation of elovl5 expression. The core zebrafish elovl5 promoter fragment also directed in vivo expression in the yolk syncytial layer of developing zebrafish embryos.

AMPKα Subunit Ssp2 and Glycogen Synthase Kinases Gsk3/Gsk31 are involved in regulation of sterol regulatory element-binding protein (SREBP) activity in fission yeast

Sterol regulatory element-binding protein (SREBP), a highly conserved family of membrane-bound transcription factors, is an essential regulator for cellular cholesterol and lipid homeostasis in mammalian cells. Sre1, the homolog of SREBP in the fission yeast Schizosaccharomyces pombe (S. pombe), regulates genes involved in the transcriptional responses to low sterol as well as low oxygen. Previous study reported that casein kinase 1 family member Hhp2 phosphorylated the Sre1 N-terminal transcriptional factor domain (Sre1N) and accelerated Sre1N degradation, and other kinases might exist for regulating the Sre1 function. To gain insight into the mechanisms underlying the Sre1 activity and to identify additional kinases involved in regulation of Sre1 function, we developed a luciferase reporter system to monitor the Sre1 activity through its binding site called SRE2 in living yeast cells. Here we showed that both ergosterol biosynthesis inhibitors and hypoxia-mimic CoCl₂ caused a dose-dependent increase in the Sre1 transcription activity, concurrently, these induced transcription activities were almost abolished in Δsre1 cells. Surprisingly, either AMPKα Subunit Ssp2 deletion or Glycogen Synthase Kinases Gsk3/Gsk31 double deletion significantly suppressed ergosterol biosynthesis inhibitors- or CoCl₂-induced Sre1 activity. Notably, the Δssp2Δgsk3Δgsk31 mutant showed further decreased Sre1 activity when compared with their single or double deletion. Consistently, the Δssp2Δgsk3Δgsk31 mutant showed more marked temperature sensitivity than any of their single or double deletion. Moreover, the fluorescence of GFP-Sre1N localized at the nucleus in wild-type cells, but significantly weaker nuclear fluorescence of GFP-Sre1N was observed in Δssp2, Δgsk3Δgsk31, Δssp2Δgsk3, Δssp2Δgsk31 or Δssp2Δgsk3Δgsk31 cells. On the other hand, the immunoblot showed a dramatic decrease in GST-Sre1N levels in the Δgsk3Δgsk31 or the Δssp2Δgsk3Δgsk31 cells but not in the Δssp2 cells. Altogether, our findings suggest that Gsk3/Gsk31 may regulate Sre1N degradation, while Ssp2 may regulate not only the degradation of Sre1N but also its translocation to the nucleus.

Cyp1b1 directs Srebp-mediated cholesterol and retinoid synthesis in perinatal liver; Association with retinoic acid activity during fetal development

Background

Cytochrome P450 1b1 (Cyp1b1) deletion and dietary retinol deficiency during pregnancy (GVAD) affect perinatal liver functions regulated by Srebp. Cyp1b1 is not expressed in perinatal liver but appears in the E9.5 embryo, close to sites of retinoic acid (RA) signaling.

Hypothesis

Parallel effects of Cyp1b1 and retinol on postnatal Srebp derive from effects in the developing liver or systemic signaling.

Approach

Cluster postnatal increases in hepatic genes in relation to effects of GVAD or Cyp1b1 deletion. Sort expression changes in relation to genes regulated by Srebp1 and Srebp2.Test these treatments on embryos at E9.5, examining changes at the site of liver initiation. Use in situ hybridization to resolve effects on mRNA distributions of Aldh1a2 and Cyp26a1 (RA homeostasis); Hoxb1 and Pax6 (RA targets). Assess mice lacking Lrat and Rbp4 (DKO mice) that severely limits retinol supply to embryos.

Results

At birth, GVAD and Cyp1b1 deletion stimulate gene markers of hepatic stellate cell (HSC) activation but also suppress Hamp. These treatments then selectively prevent the postnatal onset of genes that synthesize cholesterol (Hmgcr, Sqle) and fatty acids (Fasn, Scd1), but also direct cholesterol transport (Ldlr, Pcsk9, Stard4) and retinoid synthesis (Aldh1a1, Rdh11). Extensive support by Cyp1b1 is implicated, but with distinct GVAD interventions for Srebp1 and Srebp2. At E9.5, Cyp1b1 is expressed in the septum transversum mesenchyme (STM) with β-carotene oxygenase (Bco1) that generates retinaldehyde. STM provides progenitors for the HSC and supports liver expansion. GVAD and Cyp1b1-/- do not affect RA-dependent Hoxb1 and Pax6. In DKO embryos, RA-dependent Cyp26a1 is lost but Hoxb1 is sustained with Cyp1b1 at multiple sites.

Conclusion

Cyp1b1-/- suppresses genes supported by Srebp. GVAD effects distinguish Srebp1 and Srebp2 mediation. Srebp regulation overlaps appreciably in cholesterol and retinoid homeostasis. Bco1/Cyp1b1 partnership in the STM may contribute to this later liver regulation.

SREBP Plays a Regulatory Role in LH/hCG Receptor mRNA Expression in Human Granulosa-Lutein Cells

CONTEXT

LH receptor (LHR) expression has been shown to be regulated posttranscriptionally by LHR mRNA binding protein (LRBP) in rodent and human ovaries. LRBP was characterized as mevalonate kinase. The gene that encodes mevalonate kinase is a member of a family of genes that encode enzymes involved in lipid synthesis and are regulated by the transcription factor sterol regulatory element binding proteins (SREBPs).

OBJECTIVE

The current study examined the regulation of LHR mRNA expression in human granulosa-lutein cells in response to alterations in cholesterol metabolism.

DESIGN

Using atorvastatin, an inhibitor of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase to inhibit cholesterol biosynthesis, we examined its effect on LHR mRNA expression. The effect of atorvastatin on SREBP and mRNA expression as well as LHR mRNA binding protein expression was examined. Finally, the effect of atorvastatin on human chorionic gonadotropin (hCG)-stimulated progesterone production and the expression of key steroidogenic enzymes was also examined.

RESULTS

Statin treatment reduced LHR mRNA expression by increasing the levels of SREBP1a and SREBP2, leading to an increase in LRBP. RNA gel shift assay showed that increased binding of LHR mRNA to LRBP occurred in response to atorvastatin, leading to LHR mRNA degradation. The granulosa-lutein cells pretreated with atorvastatin also showed decreased responsiveness to hCG by decreasing the mRNA and protein expression of steroidogenic enzymes. Atorvastatin also attenuated LH/hCG-induced progesterone production.

CONCLUSION

These results imply that LHR mRNA expression by the human granulosa-lutein cells is regulated by cholesterol, through a mechanism involving SREBP and SREBP cleavage activating protein serving as the cholesterol sensor.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

Sulfation of 25-hydroxycholesterol regulates lipid metabolism, inflammatory responses, and cell proliferation

Intracellular lipid accumulation, inflammatory responses, and subsequent apoptosis are the major pathogenic events of metabolic disorders, including atherosclerosis and nonalcoholic fatty liver diseases. Recently, a novel regulatory oxysterol, 5-cholesten-3b, 25-diol 3-sulfate (25HC3S), has been identified, and hydroxysterol sulfotransferase 2B1b (SULT2B1b) has been elucidated as the key enzyme for its biosynthesis from 25-hydroxycholesterol (25HC) via oxysterol sulfation. The product 25HC3S and the substrate 25HC have been shown to coordinately regulate lipid metabolism, inflammatory responses, and cell proliferation in vitro and in vivo. 25HC3S decreases levels of the nuclear liver oxysterol receptor (LXR) and sterol regulatory element-binding proteins (SREBPs), inhibits SREBP processing, subsequently downregulates key enzymes in lipid biosynthesis, decreases intracellular lipid levels in hepatocytes and THP-1-derived macrophages, prevents apoptosis, and promotes cell proliferation in liver tissues. Furthermore, 25HC3S increases nuclear PPARγ and cytosolic IκBα and decreases nuclear NF-κB levels and proinflammatory cytokine expression and secretion when cells are challenged with LPS and TNFα. In contrast to 25HC3S, 25HC, a known LXR ligand, increases nuclear LXR and decreases nuclear PPARs and cytosol IκBα levels. In this review, we summarize our recent findings, including the discovery of the regulatory oxysterol sulfate, its biosynthetic pathway, and its functional mechanism. We also propose that oxysterol sulfation functions as a regulatory signaling pathway.

Hepatitis C virus infection activates an innate pathway involving IKK-α in lipogenesis and viral assembly

Hepatitis C virus (HCV) interacts extensively with host factors to not only establish productive infection but also trigger unique pathological processes. Our recent genome-wide siRNA screen demonstrated that IκB kinase-α (IKK-α) is a crucial host factor for HCV. Here we describe a new nuclear factor κB (NF-κB)-independent and kinase-mediated nuclear function of IKK-α in HCV assembly. HCV, through its 3' untranslated region, interacts with DEAD box polypeptide 3, X-linked (DDX3X) to activate IKK-α, which translocates to the nucleus and induces a CBP/p300-mediated transcriptional program involving sterol regulatory element-binding proteins (SREBPs). This innate pathway induces lipogenic genes and enhances core-associated lipid droplet formation to facilitate viral assembly. Chemical inhibitors of IKK-α suppress HCV infection and IKK-α-induced lipogenesis, offering a proof-of-concept approach for new HCV therapeutic development. Our results show that HCV uses a novel mechanism to exploit intrinsic innate responses and hijack lipid metabolism, which may contribute to high chronicity rates and the pathological hallmark of steatosis in HCV infection.

SREBP: a novel therapeutic target

Sterol regulatory element-binding proteins (SREBPs) are major transcription factors regulating the biosynthesis of cholesterol, fatty acid, and triglyceride. They control the expression of crucial genes involved in lipogenesis and uptake. In this review, we summarize the processing of SREBPs and their regulation by insulin, cAMP, and vitamin A, and the relationship between miRNA and lipid metabolism. We also discuss the recent functional studies on SREBPs. These discoveries suggest that inhibition of SREBP can be a novel strategy to treat metabolic diseases, such as type II diabetes, insulin resistance, fatty liver, and atherosclerosis.

SREBPs: metabolic integrators in physiology and metabolism

Recent advances have significantly increased our understanding of how sterol regulatory element binding proteins (SREBPs) are regulated at the transcriptional and post-transcriptional levels in response to cellular signaling. The phosphatidyl inositol-3-kinase (PI3K) and SREBP pathways intersect at multiple points, and recent insights demonstrate the importance of tight regulation of the PI3K pathway for regulating SREBPs in the adaptation to fluctuating dietary calorie load in the mammalian liver. In addition, genetic and genome-wide approaches highlight new functions for SREBPs in connecting lipid metabolism with other cellular processes where lipid pathway flux affects physiologic or pathophysiologic adaptation, such as cancer, steatosis, and innate immunity. This review focuses on recent advances and new roles for mammalian SREBPs in physiology and metabolism.

[Disorder of cholesterol metabolism: regulation of intracellular cholesterol and membrane trafficking]

It has been clarified that several transcription factors and functioning proteins play important roles regulating intracellular cholesterol levels. They bind to the ER membrane and sense changes in cholesterol levels in the membrane through SSD. An important membrane-binding transcription factor, SREBP, is retained in the ER membrane, forming an SREBP/SCAP/INSIG trimer when cellular cholesterol levels are abundant. This complex blocks the transport of SREBPs to the Golgi apparatus, thus preventing subsequent transcriptional activation. When cellular cholesterol levels are low, the ER cholesterol concentration is below a threshold value ( <5 mol %). Under these conditions, SCAP escorts SREBPs from the ER to Golgi apparatus by binding to a component of the CopII protein coat. Once in the Golgi apparatus, the SREBPs are proteolytically processed to generate their nuclear form, the bHLH leucine zipper, that activates genes for cholesterol synthesis and uptake. HMG-CoA reductase is also post-transcriptionally regulated by sterol, with INSIG binding of the protein leading to its proteosomal degradation. We demonstrated that Tangier disease and Niemann-Pick disease type B and type C are metabolic disorders of membrane cholesterol. These diseases are not so common in clinical medicine; however, it is very important to understand membrane lipid metabolism, especially in the ER. It will be clarified in the near future disorders of membrane cholesterol trafficking contribute to the pathogeneses of many kinds of disease affecting through ER functioning.

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.

The Akt-SREBP nexus: cell signaling meets lipid metabolism

Phosphatidylinositol 3'-kinase (PI3K) and Akt are signaling kinases involved in cell survival and proliferation. Recent evidence suggests that PI3K/Akt activates the sterol-regulatory element-binding proteins (SREBPs), master transcriptional regulators of lipid metabolism. The precise molecular mechanisms are controversial and differ between SREBP isoforms; proposed mechanisms include increased trafficking and processing of SREBP, reduced degradation, and involvement of the downstream signaling hub, mammalian target of rapamycin complex 1 (mTORC1). In this report, we explore the various mechanistic links between Akt and SREBP. We consider this relationship in diseases where Akt and lipids play crucial roles, including diabetes, viral infections and cancer, suggesting that this Akt-SREBP link provides fresh insights into human health and disease.

Identification of novel genes and pathways regulating SREBP transcriptional activity

BACKGROUND

Lipid metabolism in mammals is orchestrated by a family of transcription factors called sterol regulatory element-binding proteins (SREBPs) that control the expression of genes required for the uptake and synthesis of cholesterol, fatty acids, and triglycerides. SREBPs are thus essential for insulin-induced lipogenesis and for cellular membrane homeostasis and biogenesis. Although multiple players have been identified that control the expression and activation of SREBPs, gaps remain in our understanding of how SREBPs are coordinated with other physiological pathways.

METHODOLOGY

To identify novel regulators of SREBPs, we performed a genome-wide cDNA over-expression screen to identify proteins that might modulate the transcription of a luciferase gene driven from an SREBP-specific promoter. The results were verified through secondary biological assays and expression data were analyzed by a novel application of the Gene Set Enrichment Analysis (GSEA) method.

CONCLUSIONS/SIGNIFICANCE

We screened 10,000 different cDNAs and identified a number of genes and pathways that have previously not been implicated in SREBP control and cellular cholesterol homeostasis. These findings further our understanding of lipid biology and should lead to new insights into lipid associated disorders.

Hepatic steatosis in response to acute alcohol exposure in zebrafish requires sterol regulatory element binding protein activation

Steatosis is the most common consequence of acute alcohol abuse and may predispose to more severe hepatic disease. Increased lipogenesis driven by the sterol response element binding protein (SREBP) transcription factors is essential for steatosis associated with chronic alcohol ingestion, but the mechanisms underlying steatosis following acute alcohol exposure are unknown. Zebrafish larvae represent an attractive vertebrate model for studying alcoholic liver disease (ALD), because they possess the pathways to metabolize alcohol, the liver is mature by 4 days post-fertilization (dpf), and alcohol can be simply added to their water. Exposing 4 dpf zebrafish larvae to 2% ethanol (EtOH) for 32 hours achieves approximately 80 mM intracellular EtOH and up-regulation of hepatic cyp2e1, sod, and bip, indicating that EtOH is metabolized and provokes oxidant stress. EtOH-treated larvae develop hepatomegaly and steatosis accompanied by changes in the expression of genes required for hepatic lipid metabolism. Based on the importance of SREBPs in chronic ALD, we explored the role of Srebps in this model of acute ALD. Srebp activation was prevented in gonzo larvae, which harbor a mutation in the membrane-bound transcription factor protease 1 (mbtps1) gene, and in embryos injected with a morpholino to knock down Srebp cleavage activating protein (scap). Both gonzo mutants and scap morphants were resistant to steatosis in response to 2% EtOH, and the expression of many Srebp target genes are down-regulated in gonzo mutant livers.

CONCLUSION

Zebrafish larvae develop signs of acute ALD, including steatosis. Srebp activation is required for steatosis in this model. The tractability of zebrafish genetics provides a valuable tool for dissecting the molecular pathogenesis of acute ALD.

[Activation of sterol regulatory element binding protein and its involvement in endothelial cell migration]

OBJECTIVE

To study the activation of sterol regulatory element binding protein (SREBP) and its critical role in endothelial cell migration.

METHODS

Bovine aortic endothelial cells (ECs) were cultured. The expression of SREBP and Cdc42 were determined by Western blot and quantitative real-time PCR. Moreover, outward growth migration model and transwell chamber assay were used to detect ECs migration.

RESULTS

(1) SREBP was activated during ECs migration. Western blot analysis demonstrated increased active form SREBP in migrating as compared to non-migrating ECs population. SREBP activation decreased as ECs migration slowed;(2) Coincidental with SREBP activation, mRNA expression of its target genes such as low density lipoprotein receptor, HMG-CoA reductase, and fatty acid synthase also increased in migrating ECs population as detected by real-time PCR; (3) Migration induced SREBP activation in ECs was inhibited by SREBP-acting protein RNAi and pharmacologically by 25-hydroxycholesterol; (4) Inhibition of SREBP led to decreased ECs migration in various models; (5) Cells genetically deficient in SREBP-acting protein, S1P, or S2P, phenotypically exhibited impaired migration; (6) SREBP inhibition in ECs suppressed the activity of small GTPase Cdc42, a key molecule for ECs motility.

CONCLUSIONS

SREBP is activated during and plays a critical role in ECs migration. Targeting SREBP could become a novel approach in fighting diseases involving abnormal ECs migration.

Sterol regulators of cholesterol homeostasis and beyond: the oxysterol hypothesis revisited and revised

This review traces the evolution of the 'Oxysterol Hypothesis', which was first formulated by Kandutsch and colleagues in 1978. The original hypothesis asserted that the suppressive effect of cholesterol on its own synthesis is mediated not by cholesterol itself, but by oxygenated forms of cholesterol, so called oxysterols. Subsequently, it has become clear that cholesterol plays a pivotal role in its own feedback regulation. However, recent findings have rekindled interest in oxysterols as potential physiological regulators of cholesterol homeostasis, in addition to drawing attention to other sterol regulators. Thus, certain oxysterols can suppress the activation of the master transcriptional regulators of lipid homeostasis (SREBPs) by binding to an oxysterol sensing protein in the Endoplasmic Reticulum (Insig). Some (oxy)sterols can accelerate the degradation of the key cholesterol biosynthetic enzyme, HMG-CoA reductase, and/or serve as natural ligand activators of a nuclear receptor (LXR) involved in coordinating many aspects of reverse cholesterol transport. Recent studies on endogenously produced oxysterols indicate that they may play a more subtle and acute role than originally envisaged, smoothing cholesterol responses in the short term. We also review the metabolism of oxysterols and other recent findings about oxysterols beyond a purely cholesterol homeostatic context, such as their proposed role in the Hedgehog development pathway.

SREBPs: the crossroads of physiological and pathological lipid homeostasis

The uptake, biosynthesis and metabolism of cholesterol and other lipids are exquisitely regulated by feedback and feed-forward pathways in organisms ranging from Caenorhabditis elegans to humans. As endoplasmic reticulum (ER) membrane-embedded transcription factors that are activated in the Golgi apparatus, sterol regulatory element-binding proteins (SREBPs) are central to the intracellular surveillance of lipid catabolism and de novo biogenesis. The biosynthesis of SREBP proteins, their migration from the ER to the Golgi compartment, intra-membrane proteolysis, nuclear translocation and trans-activation potential are tightly controlled in vivo. Here we summarize recent studies elucidating the transcriptional and post-transcriptional regulation of SREBP-1c through nutrition and the action of hormones, particularly insulin, and the resulting implications for dyslipidemia of obesity, metabolic syndrome and type 2 diabetes.

Modulation of sterol regulatory element binding proteins (SREBPs) as potential treatments for non-alcoholic fatty liver disease (NAFLD)

Non-alcoholic fatty liver disease (NAFLD) is associated with diabetes, obesity and insulin resistance. The pathogenesis of NAFLD is complex, but modulation of the activities of transcription factors that regulate hepatic lipid and glucose homeostasis may be a key to treating NAFLD. An example of a key transcription factor regulating hepatic lipid metabolism is sterol regulatory element binding proteins (SREBPs), and in this review we present evidence supporting a key role for SREBPs in NAFLD. Currently, the only effective treatment for NAFLD is caloric restriction and peroxisome proliferator activated receptor (PPAR-gamma) agonists. We suggest that further studies are urgently needed to evaluate modulation of SREBP activity as a potential new treatment for NAFLD.

Inhibition of fatty acid synthase activity in prostate cancer cells by dutasteride

PURPOSE

With malignant progression to androgen independence, prostate cancer cells develop resistance to apoptosis and exhibit a variety of gene expression changes, including increased fatty acid synthase (FASN) expression. Increased FASN expression has been shown to correlate with poor prognosis, and correspondingly, the FASN gene has been proposed as a therapeutic target. Because FASN is an androgen regulated gene in the prostate, we have examined the effects of dutasteride on FASN in prostate cancer cells in vitro. Dutasteride is a novel dual inhibitor of the 5 alpha-reductase enzymes and is currently in use both for treatment of benign prostate hyperplasia (BPH) and in the reduction by dutasteride of prostate cancer events (REDUCE) prostate cancer prevention trial.

METHODS

Microarray analysis was used to identify genes affected by treatment with dutasteride, followed by real time PCR confirmation. FASN expression at the protein level was examined using Western blotting and immunocytochemistry. Enzymatic activity of FASN was assayed by (14)C-labeled malonyl-CoA incorporation. Viability after dutasteride treatment was assayed by MTS (Promega) and apoptosis via caspase 3/7 by DEVD cleavage assay.

RESULTS

We have demonstrated that the 5 alpha-reductase inhibitor dutasteride, at clinically relevant levels, inhibits FASN mRNA, protein expression and enzymatic activity in prostate cancer cells.

CONCLUSIONS

This is the first study to examine the effects of clinically relevant levels of dutasteride on prostate cancer cells at the molecular level and specifically, demonstrating the inhibition of FASN in these cells.

SREBP controls oxygen-dependent mobilization of retrotransposons in fission yeast

Retrotransposons are mobile genetic elements that proliferate through an RNA intermediate. Transposons do not encode transcription factors and thus rely on host factors for mRNA expression and survival. Despite information regarding conditions under which elements are upregulated, much remains to be learned about the regulatory mechanisms or factors controlling retrotransposon expression. Here, we report that low oxygen activates the fission yeast Tf2 family of retrotransposons. Sre1, the yeast ortholog of the mammalian membrane-bound transcription factor sterol regulatory element binding protein (SREBP), directly induces the expression and mobilization of Tf2 retrotransposons under low oxygen. Sre1 binds to DNA sequences in the Tf2 long terminal repeat that functions as an oxygen-dependent promoter. We find that Tf2 solo long terminal repeats throughout the genome direct oxygen-dependent expression of adjacent coding and noncoding sequences, providing a potential mechanism for the generation of oxygen-dependent gene expression.

A link between virulence and homeostatic responses to hypoxia during infection by the human fungal pathogen Cryptococcus neoformans

Fungal pathogens of humans require molecular oxygen for several essential biochemical reactions, yet virtually nothing is known about how they adapt to the relatively hypoxic environment of infected tissues. We isolated mutants defective in growth under hypoxic conditions, but normal for growth in normoxic conditions, in Cryptococcus neoformans, the most common cause of fungal meningitis. Two regulatory pathways were identified: one homologous to the mammalian sterol-response element binding protein (SREBP) cholesterol biosynthesis regulatory pathway, and the other a two-component-like pathway involving a fungal-specific hybrid histidine kinase family member, Tco1. We show that cleavage of the SREBP precursor homolog Sre1—which is predicted to release its DNA-binding domain from the membrane—occurs in response to hypoxia, and that Sre1 is required for hypoxic induction of genes encoding for oxygen-dependent enzymes involved in ergosterol synthesis. Importantly, mutants in either the SREBP pathway or the Tco1 pathway display defects in their ability to proliferate in host tissues and to cause disease in infected mice, linking for the first time to our knowledge hypoxic adaptation and pathogenesis by a eukaryotic aerobe. SREBP pathway mutants were found to be a hundred times more sensitive than wild-type to fluconazole, a widely used antifungal agent that inhibits ergosterol synthesis, suggesting that inhibitors of SREBP processing could substantially enhance the potency of current therapies.

Opportunistic environmental pathogens adapt to hostile conditions within the host to cause disease. We describe two pathways in the pathogenic fungus Cryptococcus neoformans that are both necessary for adaptation to hypoxia and required for its virulence. One pathway uses a pathway homologous to the mammalian sterol-response element binding protein (SREBP) pathway to activate genes involved in sterol biosynthesis in response to low oxygen levels, while the other pathway involves the two-component hybrid histidine kinase protein Tco1. Mutant strains containing deletions of genes encoding components in either of these pathways were found to be less virulent in experimental mouse models. This study suggests that this pathogenic fungus experiences low levels of oxygen in the mammalian host, and that adaptation to these conditions is important for infection. Targeting components of the hypoxia response could yield more effective treatments for C. neoformans infections, which cause a large fraction of HIV/AIDS-related deaths worldwide. Notably, we find that mutants in the SREBP-like pathway are a hundred times more sensitive than wild-type cells to the widely used antifungal drug fluconazole.

Cholesterol, statins and cancer

1. The link between cholesterol and cardiovascular disease is well-established. Emerging evidence is now forging a tantalizing link between cholesterol and cancer. 2. Results from a number of case-control studies have indicated that the commonly prescribed cholesterol-lowering drugs, the statins, may reduce the risk of certain cancers, although this area certainly remains controversial. 3. Herein, the recent literature examining statins and cancer is reviewed briefly and the relationship between a key cholesterol homeostatic pathway and signalling pathways that are involved in carcinogenesis is discussed. In particular, how the sterol-regulatory element binding protein, Akt and Hedgehog pathways may converge in cancer is reviewed.

Vascular endothelial growth factor receptor 2 plays a role in the activation of aortic endothelial cells by oxidized phospholipids

OBJECTIVE

Previous studies have shown that oxidized products of PAPC (Ox-PAPC) regulate cell transcription of interleukin-8, LDL receptor, and tissue factor. This upregulation takes place in part through the activation of sterol regulatory element-binding protein (SREBP) and Erk 1/2. The present studies identify vascular endothelial growth factor receptor 2 (VEGFR2) as a major regulator in the activation of SREBP and Erk 1/2 in endothelial cells activated by Ox-PAPC.

METHODS AND RESULTS

Ox-PAPC induced the phosphorylation of VEGFR2 at Tyr1175 in human aortic endothelial cells. Inhibitors and siRNA for VEGFR2 decreased the transcription of interleukin-8, LDL receptor, and tissue factor in response to Ox-PAPC and the activation of SREBP and Erk 1/2, which mediate this transcription. We provide evidence that the activation of VEGFR2 is rapid, sustained, and c-Src-dependent.

CONCLUSIONS

These data point to a major role of VEGFR2 in endothelial regulation by oxidized phospholipids which accumulate in atherosclerotic lesions and apoptotic cells.

Hepatocyte Growth Factor Regulates E Box–Dependent Plasminogen Activator Inhibitor Type 1 Gene Expression in HepG2 Liver Cells

OBJECTIVE

We sought to determine the etiologic mechanism of pleiotropic growth factor, hepatocyte growth factor (HGF), as a regulator of hepatic synthesis of plasminogen activator inhibitor (PAI)-1, the physiological inhibitor of fibrinolysis and a potential inducer of atherothrombosis.

METHODS AND RESULTS

HGF increased PAI-1 mRNA expression and PAI-1 protein accumulation in the conditioned media of human liver-derived HepG2 cells, and increased hepatic PAI-1 mRNA expression in vivo in mice. HGF-inducible PAI-1 mRNA was attenuated by U0126, a specific inhibitor of mitogen-activated protein kinase (MAPK) kinase, and genistein, an inhibitor of tyrosine kinase. HGF increased the human PAI-1 promoter (-829 to +36 bp) activity, and deletion and mutation analysis uncovered a functional E box (5'-CACATG-3') at positions -158 to -153 bp. Electrophoretic mobility shift assays demonstrated that this E box binds upstream stimulatory factors (USFs). HGF phosphorylated USFs through MAPK and tyrosine kinase pathways. Co-transfection of USF1 expression vector increased PAI-1 promoter activity. Sterol regulatory element-binding protein-1 attenuated HGF-inducible PAI-1 promoter activity.

CONCLUSIONS

Because USFs are involved in the regulation of carbohydrates and lipid metabolism, HGF-mediated PAI-1 production may provide a novel link between atherothrombosis and metabolic derangements. Targeting HGF signaling pathway may modulate the thrombotic risk in high-risk patients.

Nutritional control, gene regulation, and transformation of vascular smooth muscle cells in atherosclerosis

Contractile-state smooth muscle cells (SMC), the only cell type in the arterial media, undergoes migration to the intima, proliferation, and abundant extracellular matrix production during the early stages of atherosclerosis. This involves the ingestion of low-density lipoprotein (LDL) and modified or oxidised LDL by macrophages together with SMC by several pathways including a scavenger pathway leading to accumulation of cholesterol esters and formation of foam cells. High-plasma cholesterol levels constitute a major causative risk for atherosclerosis. The membrane-bound transcription factor called sterol regulatory element binding protein (SREBP) activates gene-encoding enzymes of cholesterol and fatty acid biosynthesis. The SREBP expression, in response to diet, shows that are involved in both lipogenesis and cholesterol homeostasis, moreover SREBPs are regulated directly by cholesterol. Animal models were used in trials of atherosclerosis, and cholesterol feeding has been described elsewhere as producing atherosclerotic lesions. We have examined the morphological, molecular and proliferative change in arterial SMC mimicking such a cholesterol diet, this transformed SMC is a good model to study the alterations of the differentiated state of SMC, and the transformation into foam cell, caused by cholesterol-rich diet. Despite the complexity of the interactions in atherosclerosis, there are many opportunities to affect the homeostatic balance of the artery wall at SMC levels. We have considered here some of the possible targets for intervention with promising strategies for the nutritional control of the genes, and, in a general way, the possibilities for modulating the expression of genes influencing atherosclerosis.

Androgen regulation of prostasin gene expression is mediated by sterol-regulatory element-binding proteins and SLUG

BACKGROUND

Prostasin is downregulated in hormone-refractory prostate cancers (HRPC). The mechanisms by which androgens regulate prostasin expression are unclear.

METHODS

LNCaP cells were treated with dihydrotestosterone (DHT), and mRNA expression of prostasin, SREBPs, SNAIL, and SLUG was examined by real-time PCR following reverse transcription. A human prostasin promoter was evaluated in HEK-293 cells co-transfected with transcription factor cDNAs. Regulation of endogenous prostasin expression by transfected SREBP-2 or SLUG was evaluated. Expression of SNAIL and SLUG mRNA in DU-145 cells treated with epidermal growth factor (EGF) was examined.

RESULTS

Prostasin mRNA expression in LNCaP cells was not responsive to DHT treatment. DHT marginally upregulated mRNA expression of SREBP-1c, SREBP-2, and SNAIL, but not SREBP-1a, while dramatically increased SLUG mRNA expression, in a dose-dependent manner. Co-transfection of prostasin promoter and SREBP cDNA in HEK-293 cells resulted in stimulation of promoter activity at approximately twofold by SREBP-1c, and up to sixfold by SREBP-2; while co-transfection with SNAIL or SLUG cDNA resulted in repression of promoter activity to 43% or 59%, respectively. Co-transfection of the SLUG cDNA negated SREBP-2's stimulation of prostasin promoter in a dose-dependent manner. Transfection of an SREBP-2 cDNA in HEK-293 and DU-145 resulted in upregulation of prostasin while transfection of a SLUG cDNA in LNCaP repressed prostasin expression. EGF upregulated SNAIL and SLUG mRNA in DU-145.

CONCLUSIONS

DHT regulates prostasin expression in prostate cells via SREBP stimulation and SLUG repression of prostasin promoter. SLUG is upregulated by DHT and EGF, providing a molecular mechanism by which epithelial cell-specific genes are silenced during prostate cancer development and progression.

Nutrition-/diet-induced changes in gene expression in white adipose tissue

Nutrients regulate metabolic fluxes and homeostasis through transcriptional and translational control of enzyme concentrations and allosteric modulation of enzyme activity. Dietary omega-3 polyunsaturated fatty acids (PUFAs) have been shown to exert a variety of beneficial health effects such as reducing adiposity and increasing insulin sensitivity in rodents. It is now clear that PUFAs regulate fundamental adipose cell and liver functions through modulation of activity and abundance of key transcription factors that act as nutrient sensors, including peroxisome proliferator-activated receptors (PPARalpha/delta/gamma), sterol regulatory element binding proteins (SREBP-1/2), and liver X receptors (LXRalpha/beta). However, in the state of obesity, where adipose tissue shows elevated storage of triglycerides, many lipogenic genes that are essential for adipose cell function including PPARgamma, SREBP-1c, CCAAT-enhancer binding protein alpha and stearoyl-CoA desaturase-1 are downregulated, apparently due to desensitization of the very same crucial nutrient sensors. This chapter will summarize recent studies of PUFA- and obesity-induced changes in gene expression in white adipose tissue.

[Metabolism of cholesterol and fatty acids in nephrotic syndrome and its regulation by sterol regulatory element binding proteins (SREBP's). Effect of soy protein consumption]

Hyperlipidemia occurs during nephrotic syndrome (NS). It is known that cholesterol and fatty acid biosynthesis is controlled by the transcription factors sterol regulatory element binding proteins (SREBPs). Soy protein consumption reduces the concentration of these lipids, although its mechanism of action is not well known. The aim of the present study was to establish whether soy protein consumption reduces cholesterol and triglycerides levels by regulating of SREBPs. Male Wistar rats with experimental NS were studied for 64 days. The results showed that rats fed with soy protein had significantly lower plasma cholesterol and triglyceride concentrations as well as proteinuria than rats fed with casein diet. These decrements were associated with a decrease in the expression of SREBP-1 and fatty acid biosynthetic enzymes. In addition, Western blot analysis revealed that in nuclear extracts from hepatocytes of rats fed with soy protein, there was a lower concentration of SREBP-1 than in rats fed with casein. The results of this study indicate that consumption of a soy protein diet has beneficial effects on nephrotic syndrome.

Sterol regulatory element-binding proteins activate insulin gene promoter directly and indirectly through synergy with BETA2/E47

Insulin gene expression is regulated by pancreatic beta cell-specific factors, PDX-1 and BETA2/E47. Here we have demonstrated that the insulin promoter is a novel target for SREBPs established as lipid-synthetic transcription factors. Promoter analyses of rat insulin I gene in non-beta cells revealed that nuclear SREBP-1c activates the insulin promoter through three novel SREBP-binding sites (SREs), two of which overlap with E-boxes, binding sites for BETA2/E47. SREBP-1c activation of the insulin promoter was markedly enhanced by co-expression of BETA2/E47. This synergistic activation by SREBP-1c/BETA2/E47 was not mediated through SREs but through the E-boxes on which BETA2/E47 physically interacts with SREBP-1c, suggesting a novel function of SREBP as a co-activator. These two cis-DNA regions, E1 and E2, with an appropriate distance separating them, were mandatory for the synergism, which implicates formation of SREBP-1c.BETA2.E47 complex in a DNA looping structure for efficient recruitment of CREB-binding protein/p300. However, in the presence of PDX1, the synergistic action of SREBP-1c with BETA2/E47 was canceled. SREBP-1c-mediated activation of the insulin promoter and expression became overt in beta cell lines and isolated islets when endogenous PDX-1 expression was low. This cryptic SREBP-1c action might play a compensatory role in insulin expression in diabetes with beta cell lipotoxicity.

Oxygen sensing: getting pumped by sterols

Oxygen plays a pivotal role in the maintenance of life for all eukaryotes, with the exception of strict anaerobes. Eukaryotes have developed mechanisms to sense and respond to decreased oxygen levels. How eukaryotes sense oxygen is still not fully understood. What is (or are) the oxygen sensor(s)? This question has vital physiological and pathophysiological implications, because all living aerobic organisms have adaptive mechanisms to maintain oxygen homeostasis. A recent report describes a novel eukaryotic oxygen-sensing mechanism in the fission yeast Schizosaccharomyces pombe, involving the depletion of sterols as a trigger to induce gene expression in response to decreased oxygen levels. It is not yet clear whether this mechanism is involved in the mammalian response to hypoxia, possibly in conjunction with activation of one or both of the hypoxia-inducible factor (HIF-1 or HIF-2) transcription factors.