Multiple sequence elements are involved in the transcriptional regulation of the human squalene synthase gene

Unveiling the Therapeutic Potential of Squalene Synthase: Deciphering Its Biochemical Mechanism, Disease Implications, and Intriguing Ties to Ferroptosis

Squalene synthase (SQS) has emerged as a promising therapeutic target for various diseases, including cancers, owing to its pivotal role in the mevalonate pathway and the antioxidant properties of squalene. Primarily, SQS orchestrates the head-to-head condensation reaction, catalyzing the fusion of two farnesyl pyrophosphate molecules, leading to the formation of squalene, which has been depicted as a highly effective oxygen-scavenging agent in in vitro studies. Recent studies have depicted this isoprenoid as a protective layer against ferroptosis due to its potential regulation of lipid peroxidation, as well as its protection against oxidative damage. Therefore, beyond its fundamental function, recent investigations have unveiled additional roles for SQS as a regulator of lipid peroxidation and programmed cell death pathways, such as ferroptosis-a type of cell death characterized by elevated levels of lipid peroxide, one of the forms of reactive oxygen species (ROS), and intracellular iron concentration. Notably, thorough explorations have shed light on the distinctive features that set SQS apart from other members within the isoprenoid synthase superfamily. Its unique biochemical structure, intricately intertwined with its reaction mechanism, has garnered significant attention. Moreover, considerable evidence substantiates the significance of SQS in various disease contexts, and its intriguing association with ferroptosis and lipid peroxidation. The objective of this report is to analyze the existing literature comprehensively, corroborating these findings, and provide an up-to-date perspective on the current understanding of SQS as a prospective therapeutic target, as well as its intricate relationship with ferroptosis. This review aims to consolidate the knowledge surrounding SQS, thereby contributing to the broader comprehension of its potential implications in disease management and therapeutic interventions.

A Novel Module Promotes Horizontal Gene Transfer in Azorhizobium caulinodans ORS571

Azorhizobium caulinodans ORS571 contains an 87.6 kb integrative and conjugative element (ICE^Ac) that conjugatively transfers symbiosis genes to other rhizobia. Many hypothetical redundant gene fragments (rgfs) are abundant in ICE^Ac, but their potential function in horizontal gene transfer (HGT) is unknown. Molecular biological methods were employed to delete hypothetical rgfs, expecting to acquire a minimal ICE^Ac and consider non-functional rgfs as editable regions for inserting genes related to new symbiotic functions. We determined the significance of rgf4 in HGT and identified the physiological function of genes designated rihF1a (AZC_3879), rihF1b (AZC_RS26200), and rihR (AZC_3881). In-frame deletion and complementation assays revealed that rihF1a and rihF1b work as a unit (rihF1) that positively affects HGT frequency. The EMSA assay and lacZ-based reporter system showed that the XRE-family protein RihR is not a regulator of rihF1 but promotes the expression of the integrase (intC) that has been reported to be upregulated by the LysR-family protein, AhaR, through sensing host’s flavonoid. Overall, a conservative module containing rihF1 and rihR was characterized, eliminating the size of ICE^Ac by 18.5%. We propose the feasibility of constructing a minimal ICE^Ac element to facilitate the exchange of new genetic components essential for symbiosis or other metabolic functions between soil bacteria.

Hepatic transcriptional responses to fasting and feeding

Mammals undergo regular cycles of fasting and feeding that engage dynamic transcriptional responses in metabolic tissues. Here we review advances in our understanding of the gene regulatory networks that contribute to hepatic responses to fasting and feeding. The advent of sequencing and -omics techniques have begun to facilitate a holistic understanding of the transcriptional landscape and its plasticity. We highlight transcription factors, their cofactors, and the pathways that they impact. We also discuss physiological factors that impinge on these responses, including circadian rhythms and sex differences. Finally, we review how dietary modifications modulate hepatic gene expression programs.

Metabolic coessentiality mapping identifies C12orf49 as a regulator of SREBP processing and cholesterol metabolism

Coessentiality mapping has been useful to systematically cluster genes into biological pathways and identify gene functions1-3. Here, using the debiased sparse partial correlation (DSPC) method3, we construct a functional coessentiality map for cellular metabolic processes across human cancer cell lines. This analysis reveals 35 modules associated with known metabolic pathways and further assigns metabolic functions to unknown genes. In particular, we identify C12orf49 as an essential regulator of cholesterol and fatty acid metabolism in mammalian cells. Mechanistically, C12orf49 localizes to the Golgi, binds membrane-bound transcription factor peptidase, site 1 (MBTPS1, site 1 protease) and is necessary for the cleavage of its substrates, including sterol regulatory element binding protein (SREBP) transcription factors. This function depends on the evolutionarily conserved uncharacterized domain (DUF2054) and promotes cell proliferation under cholesterol depletion. Notably, c12orf49 depletion in zebrafish blocks dietary lipid clearance in vivo, mimicking the phenotype of mbtps1 mutants. Finally, in an electronic health record (EHR)-linked DNA biobank, C12orf49 is associated with hyperlipidaemia through phenome analysis. Altogether, our findings reveal a conserved role for C12orf49 in cholesterol and lipid homeostasis and provide a platform to identify unknown components of other metabolic pathways.

Identification of foam cell biomarkers by microarray analysis

Background

Lipid infiltration and inflammatory response run through the occurrence of atherosclerosis. Differentiation into macrophages and foam cell formation are the key steps of AS. Aim of this study was that the differential gene expression between foam cells and macrophages was analyzed to search the key links of foam cell generation, so as to explore the pathogenesis of atherosclerosis and provide targets for the early screening and prevention of coronary artery disease (CAD).

Methods

The gene expression profiles of GSE9874 were downloaded from Gene Expression Omnibus (https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE9874) on GPL96 [HG-U133A] Affymetrix Human Genome U133. A total of 22,383 genes were analyzed for differentially expression genes (DEGs) by Bayes package. GO enrichment analysis and KEGG pathway analysis for DEGs were performed using KOBAS 3.0 software (Peking University, Beijing, China). STRING software (STRING 10.0; European Molecular Biology Laboratory, Heidelberg, Germany) was used to analyze the protein-protein interaction (PPI) of DEGs.

Results

A total of 167 DEGs between macrophages and foam cells were identified. Compared with macrophages, 102 genes were significantly upregulated and 65 genes were significantly downregulated (P < 0.01, fold-change > 1) in foam cells. DEGs were mainly enrich in ‘sterol biosynthetic and metabolic process’, ‘cholesterol metabolic and biosynthetic process’ by GO enrichment analysis. The results of KEGG pathway analysis showed all differential genes are involved in biological processes through 143 KEGG pathways. A PPI network of the DEGs was constructed and 10 outstanding genes of the PPI network was identified by using Cytoscape, which include HMGCR, SREBF2, LDLR, HMGCS1, FDFT1, LPL, DHCR24, SQLE, ABCA1 and FDPS. Conclusion: Lipid metabolism related genes and molecular pathways were the key to the transformation of macrophages into foam cells. Therefore, lipid metabolism disorder is the key to turn macrophages into foam cells, which plays a major role in CAD.

Side-chain oxysterols suppress the transcription of CTP: Phosphoethanolamine cytidylyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase by inhibiting the interaction of p300 and NF-Y, and H3K27 acetylation

CTP: phosphoethanolamine cytidylyltransferase (Pcyt2) is the rate-limiting enzyme in mammalian phosphatidylethanolamine (PE) biosynthesis. Previously, we reported that increasedPcyt2 mRNA levels after serum starvation are suppressed by 25-hydroxycholesterol (HC) (25-HC), and that nuclear factor-Y (NF-Y) is involved in the inhibitory effects. Transcription of Hmgcr, which encodes 3-hydroxy-3-methylglutaryl-CoA reductase, is suppressed in the same manner. However, no typical sterol regulatory element (SRE) was detected in the Pcyt2 promoter. We were therefore interested in the effect of 25-HC on the modification of histones and thus treated cells with histone acetyltransferase inhibitor (anacardic acid) or histone deacetylase inhibitor (trichostatin A). The suppressive effect of 25-HC on Pcyt2 and Hmgcr mRNA transcription was ameliorated by trichostatin A. Anacardic acid, 25-HC and 24(S)-HC suppressed their transcription by inhibiting H3K27 acetylation in their promoters as evaluated by chromatin immunoprecipitation (ChIP) assays. 27-HC, 22(S)-HC and 22(R)-HC also suppressed their transcription, but 7α-HC, 7β-HC, the synthetic LXR agonist T0901317 and cholesterol did not. Furthermore, 25-HC inhibited p300 recruitment to the Pcyt2 and Hmgcr promoters, and suppressed H3K27 acetylation. 25-HC in the medium was easily conducted into cells. Based on these results, we concluded that 25-HC (and other side-chain oxysterols) in the medium was easily transferred into cells, suppressed H3K27 acetylation via p300 recruitment on the NF-Y complex in the Pcyt2 and Hmgcr promoters, and then suppressed transcription of these genes although LXR is not involved.

DHCR7: A vital enzyme switch between cholesterol and vitamin D production

The conversion of 7-dehydrocholesterol to cholesterol, the final step of cholesterol synthesis in the Kandutsch-Russell pathway, is catalyzed by the enzyme 7-dehydrocholesterol reductase (DHCR7). Homozygous or compound heterozygous mutations in DHCR7 lead to the developmental disease Smith-Lemli-Opitz syndrome, which can also result in fetal mortality, highlighting the importance of this enzyme in human development and survival. Besides serving as a substrate for DHCR7, 7-dehydrocholesterol is also a precursor of vitamin D via the action of ultraviolet light on the skin. Thus, DHCR7 exerts complex biological effects, involved in both cholesterol and vitamin D production. Indeed, we argue that DHCR7 can act as a switch between cholesterol and vitamin D synthesis. This review summarizes current knowledge about the critical enzyme DHCR7, highlighting recent findings regarding its structure, transcriptional and post-transcriptional regulation, and its links to vitamin D synthesis. Greater understanding about DHCR7 function, regulation and its place within cellular metabolism will provide important insights into its biological roles.

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

Sterol regulatory element-binding proteins (SREBPs)-1c and -2, which were initially discovered as master transcriptional regulators of lipid biosynthesis and uptake, were recently identified as novel transcriptional regulators of the sodium-iodide symporter gene in the thyroid, which is essential for thyroid hormone synthesis. Based on this observation that SREBPs play a role for thyroid hormone synthesis, we hypothesized that another gene involved in thyroid hormone synthesis, the thyroid peroxidase (TPO) gene, is also a target of SREBP-1c and -2. Thyroid epithelial cells treated with 25-hydroxycholesterol, which is known to inhibit SREBP activation, had about 50% decreased mRNA levels of TPO. Similarly, the mRNA level of TPO was reduced by about 50% in response to siRNA mediated knockdown of both, SREBP-1 and SREBP-2. Reporter gene assays revealed that overexpression of active SREBP-1c and -2 causes a strong transcriptional activation of the rat TPO gene, which was localized to an approximately 80 bp region in the intron 1 of the rat TPO gene. In vitro- and in vivo-binding of both, SREBP-1c and SREBP-2, to this region in the rat TPO gene could be demonstrated using gel-shift assays and chromatin immunoprecipitation. Mutation analysis of the 80 bp region of rat TPO intron 1 revealed two isolated and two overlapping SREBP-binding elements from which one, the overlapping SRE+609/InvSRE+614, was shown to be functional in reporter gene assays. In connection with recent findings that the rat NIS gene is also a SREBP target gene in the thyroid, the present findings suggest that SREBPs may be possible novel targets for pharmacological modulation of thyroid hormone synthesis.

The yeast anaerobic response element AR1b regulates aerobic antifungal drug-dependent sterol gene expression

Saccharomyces cerevisiae ergosterol biosynthesis, like cholesterol biosynthesis in mammals, is regulated at the transcriptional level by a sterol feedback mechanism. Yeast studies defined a 7-bp consensus sterol-response element (SRE) common to genes involved in sterol biosynthesis and two transcription factors, Upc2 and Ecm22, which direct transcription of sterol biosynthetic genes. The 7-bp consensus SRE is identical to the anaerobic response element, AR1c. Data indicate that Upc2 and Ecm22 function through binding to this SRE site. We now show that it is two novel anaerobic AR1b elements in the UPC2 promoter that direct global ERG gene expression in response to a block in de novo ergosterol biosynthesis, brought about by antifungal drug treatment. The AR1b elements are absolutely required for auto-induction of UPC2 gene expression and protein and require Upc2 and Ecm22 for function. We further demonstrate the direct binding of recombinant expressed S. cerevisiae ScUpc2 and pathogenic Candida albicans CaUpc2 and Candida glabrata CgUpc2 to AR1b and SRE/AR1c elements. Recombinant endogenous promoter studies show that the UPC2 anaerobic AR1b elements act in trans to regulate ergosterol gene expression. Our results indicate that Upc2 must occupy UPC2 AR1b elements in order for ERG gene expression induction to take place. Thus, the two UPC2-AR1b elements drive expression of all ERG genes necessary for maintaining normal antifungal susceptibility, as wild type cells lacking these elements have increased susceptibility to azole antifungal drugs. Therefore, targeting these specific sites for antifungal therapy represents a novel approach to treat systemic fungal infections.

Controlling cholesterol synthesis beyond 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR)

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGCR) is the target of the statins, important drugs that lower blood cholesterol levels and treat cardiovascular disease. Consequently, the regulation of HMGCR has been investigated in detail. However, this enzyme acts very early in the cholesterol synthesis pathway, with ∼20 subsequent enzymes needed to produce cholesterol. How they are regulated is largely unexplored territory, but there is growing evidence that enzymes beyond HMGCR serve as flux-controlling points. Here, we introduce some of the known regulatory mechanisms affecting enzymes beyond HMGCR and highlight the need to further investigate their control.

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.

Regulation of the human prostacyclin receptor gene by the cholesterol-responsive SREBP1

Prostacyclin and its prostacyclin receptor, the I Prostanoid (IP), play essential roles in regulating hemostasis and vascular tone and have been implicated in a range cardio-protective effects but through largely unknown mechanisms. In this study, the influence of cholesterol on human IP [(h)IP] gene expression was investigated in cultured vascular endothelial and platelet-progenitor megakaryocytic cells. Cholesterol depletion increased human prostacyclin receptor (hIP) mRNA, hIP promoter-directed reporter gene expression, and hIP-induced cAMP generation in all cell types. Furthermore, the constitutively active sterol-response element binding protein (SREBP)1a, but not SREBP2, increased hIP mRNA and promoter-directed gene expression, and deletional and mutational analysis uncovered an evolutionary conserved sterol-response element (SRE), adjacent to a known functional Sp1 element, within the core hIP promoter. Moreover, chromatin immunoprecipitation assays confirmed direct cholesterol-regulated binding of SREBP1a to this hIP promoter region in vivo, and immunofluorescence microscopy corroborated that cholesterol depletion significantly increases hIP expression levels. In conclusion, the hIP gene is directly regulated by cholesterol depletion, which occurs through binding of SREBP1a to a functional SRE within its core promoter. Mechanistically, these data establish that cholesterol can regulate hIP expression, which may, at least in part, account for the combined cardio-protective actions of low serum cholesterol through its regulation of IP expression within the human vasculature.

Sterols regulate 3β-hydroxysterol Δ24-reductase (DHCR24) via dual sterol regulatory elements: cooperative induction of key enzymes in lipid synthesis by Sterol Regulatory Element Binding Proteins

3β-Hydroxysterol Δ24-reductase (DHCR24) catalyzes a final step in cholesterol synthesis, and has been ascribed diverse functions, such as being anti-apoptotic and anti-inflammatory. How this enzyme is regulated transcriptionally by sterols is currently unclear. Some studies have suggested that its expression is regulated by Sterol Regulatory Element Binding Proteins (SREBPs) while another suggests it is through the Liver X Receptor (LXR). However, these transcription factors have opposing effects on cellular sterol levels, so it is likely that one predominates. Here we establish that sterol regulation of DHCR24 occurs predominantly through SREBP-2, and identify the particular region of the DHCR24 promoter to which SREBP-2 binds. We demonstrate that sterol regulation is mediated by two sterol regulatory elements (SREs) in the promoter of the gene, assisted by two nearby NF-Y binding sites. Moreover, we present evidence that the dual SREs work cooperatively to regulate DHCR24 expression by comparison to two known SREBP target genes, the LDL receptor with one SRE, and farnesyl-diphosphate farnesyltransferase 1, with two SREs.

Role of squalene synthase in prostate cancer risk and the biological aggressiveness of human prostate cancer

BACKGROUND

We previously conducted a genome-wide linkage analysis of Japanese nuclear families affected with prostate cancer and showed that the susceptibility to prostate cancer was closely linked to D8S550 at 8p23. The role of farnesyl diphosphate farnesyltransferase (FDFT1), which is located under the peak marker D8S550 at 8p23, and squalene synthase, the enzyme encoded by FDFT1, in prostate cancer was studied.

METHODS

The association among common variants of FDFT1 with prostate cancer risk, the promoter activities of FDFT1 with different genotypes and the effects of inhibition of squalene synthase were studied, and the FDFT1 transcript levels of human prostate samples were quantified.

RESULTS

The A allele of rs2645429 was significantly associated with prostate cancer risk in a Japanese familial prostate cancer population. Rs2645429 was located in the promoter region of FDFT1, and the AA genotype showed significantly increased promoter activity. The knockdown of FDFT1 mRNA expression or squalene synthase inhibition led to a significant decrease in prostate cancer cell proliferation. Additionally, human prostate cancer specimens expressed significantly higher levels of FDFT1 mRNA compared with noncancerous specimens. Finally, aggressive cancers showed higher transcript levels.

CONCLUSIONS

FDFT1 and its encoded enzyme, squalene synthase, may play an important role in prostate cancer development and its aggressive phenotypes.

Biomineralization of bone: a fresh view of the roles of non-collagenous proteins

The impact of genetics has dramatically affected our understanding of the functions of non-collagenous proteins. Specifically, mutations and knockouts have defined their cellular spectrum of actions. However, the biochemical mechanisms mediated by non-collagenous proteins in biomineralization remain elusive. It is likely that this understanding will require more focused functional testing at the protein, cell, and tissue level. Although initially viewed as rather redundant and static acidic calcium binding proteins, it is now clear that non-collagenous proteins in mineralizing tissues represent diverse entities capable of forming multiple protein-protein interactions which act in positive and negative ways to regulate the process of bone mineralization. Several new examples from the author's laboratory are provided which illustrate this theme including an apparent activating effect of hydroxyapatite crystals on metalloproteinases. This review emphasizes the view that secreted non-collagenous proteins in mineralizing bone actively participate in the mineralization process and ultimately control where and how much mineral crystal is deposited, as well as determining the quality and biomechanical properties of the mineralized matrix produced.

Inhibition of proprotein convertase SKI-1 blocks transcription of key extracellular matrix genes regulating osteoblastic mineralization

Mineralization, a characteristic phenotypic property of osteoblastic lineage cells, was blocked by 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF) and decanoyl-Arg-Arg-Leu-Leu-chloromethyl ketone (dec-RRLL-cmk), inhibitors of SKI-1 (site 1; subtilisin kexin like-1) protease. Because SKI-1 is required for activation of SREBP and CREB (cAMP-response element-binding protein)/ATF family transcription factors, we tested the effect of these inhibitors on gene expression. AEBSF decreased expression of 140 genes by 1.5-3.0-fold including Phex, Dmp1, COL1A1, COL11A1, and fibronectin. Direct comparison of AEBSF and dec-RRLL-cmk, a more specific SKI-1 inhibitor, demonstrated that expression of Phex, Dmp1, COL11A1, and fibronectin was reduced by both, whereas COL1A2 and HMGCS1 were reduced only by AEBSF. AEBSF and dec-RRLL-cmk decreased the nuclear content of SKI-1-activated forms of transcription factors SREBP-1, SREBP-2, and OASIS. In contrast to AEBSF, the actions of dec-RRLL-cmk represent the sum of its direct actions on SKI-1 and indirect actions on caspase-3. Specifically, dec-RRLL-cmk reduced intracellular caspase-3 activity by blocking the formation of activated 19-kDa caspase-3. Conversely, overexpression of SKI-1-activated SREBP-1a and CREB-H in UMR106-01 osteoblastic cells increased the number of mineralized foci and altered their morphology to yield mineralization nodules, respectively. In summary, SKI-1 regulates the activation of transmembrane transcription factor precursors required for expression of key genes required for mineralization of osteoblastic cultures in vitro and bone formation in vivo. Our results indicate that the differentiated phenotype of osteoblastic cells and possibly osteocytes depends upon the non-apoptotic actions of SKI-1.

Sterol-regulatory-element-binding protein 2 and nuclear factor Y control human farnesyl diphosphate synthase expression and affect cell proliferation in hepatoblastoma cells

FDPS (farnesyl diphosphate synthase) catalyses the formation of farnesyl diphosphate, a key intermediate in the synthesis of cholesterol and isoprenylated cellular metabolites. FDPS is also the molecular target of nitrogen-containing bisphosphonates, which are used as bone-antiresorptive drugs in various disorders. In the present study, we characterized the sterol-response element and NF-Y (nuclear factor Y)-binding site in the human FDPS promoter. Using a luciferase assay, electrophoretic mobility-shift assay and chromatin immunoprecipitation assay, we demonstrated that these elements are responsible for the transcription of the FDPS gene, and that its transcriptional activation is mediated by SREBP-2 (sterol-regulatory-element-binding protein 2) and NF-Y. We also investigated whether sterol-mediated FDPS expression is involved in the cell proliferation induced by zoledronic acid, an FDPS inhibitor. We show that the SREBP-2- and NF-Y-mediated regulation of FDPS gene transcription modulates cell proliferation. These results suggest that SREBP-2 and NF-Y are required to trigger cell proliferation through the induction of FDPS expression and that the pharmacological action of zoledronic acid is involved in this pathway.

Evolutionary conservation and adaptation in the mechanism that regulates SREBP action: what a long, strange tRIP it's been

Sterol regulatory element-binding proteins (SREBPs) are a subfamily of basic helix-loop-helix leucine zipper (bHLH-LZ) transcription factors that are conserved from fungi to humans and are defined by two key features: a signature tyrosine residue in the DNA-binding domain, and a membrane-tethering domain that is a target for regulated proteolysis. Recent studies including genome-wide and model organism approaches indicate SREBPs coordinate cellular lipid metabolism with other cellular physiologic processes. These functions are broadly related as cellular adaptation to environmental changes ranging from nutrient fluctuations to toxin exposure. This review integrates classic features of the SREBP pathway with newer information regarding the regulation and sensing mechanisms that serve to assimilate different cellular physiologic processes for optimal function and growth.

In vivo and in vitro effects of SREBP-1 on diabetic renal tubular lipid accumulation and RNAi-mediated gene silencing study

Lipid deposits can injury the kidney of diabetic patients and models. Sterol regulatory element binding protein-1 (SREBP-1) is transcription factor regulating the synthesis of fatty acid and triglyceride. At present whether the expression of SREBP-1 makes some effects on the lipid accumulation in diabetic kidney is not still clear completely. The purpose of our in vivo and in vitro study is to investigate the relationship between the expression of SREBP-1 and lipid abnormal metabolism in the type 1 diabetic rats and explore to inhibit SREBP-1 gene expression by RNA interfere in human renal proximal tubular epithelial cells line (HKC cells). The animal experiment showed that triglyceride and SREBP-1 were up-regulated in proximal tubule of diabetic rats' kidney, which may result in increase of transforming growth factor-beta1 (TGF-beta1) and accumulation of extracellular matrix (ECM). The further HKC cells experiment confirmed SREBP-1 increasing resulted into lipid droplet formation. The expression of fatty acid synthase (FAS) in HKC cells transfected with specific plasmid for SREBP-1 gene was significantly more than that of the cells transfected with the control plasmid pcDNA3.1 and that of the untransfected cells. Simultaneously, up-regulation of TGF-beta1 and fibronectin, an ECM glycoprotein, was evident in HKC cells transfected by specific SREBP-1 plasmid. Furthermore, we found that high glucose was a positive factor on the expression of SREBP-1 at protein and mRNA levels in HKC cells. High glucose makes effects on SREBP-1 in time-dependent manner, and the greatest effect was at 48 h. In addition, two effective eukaryotic expression plasmid vectors of shRNA aimed at SREBP-1 were designed and constructed successfully. Compared with the negative control plasmid group, the levels of the expression of SREBP-1 were inhibited by 24.11 and 36.15%, respectively, at mRNA level, 20.80 and 37.59%, respectively, at precursor segment of protein level, and 38.12 and 52.24%, respectively, at mature segment of protein level at 48 h after transfection. In vivo and in vitro study suggested that high glucose caused increasing SREBP-1 mRNA and protein in renal proximal tubule epithelial cells of type 1 diabetic rats. Increasing SREBP-1 plays an important role in the pathogenesis of renal lipid accumulation by up-regulation of FAS and ECM accumulation by inducing TGF-beta1 expression. The application of vector-mediated RNAi could markedly inhibit the expression of SREBP-1 in HKC cells, which is a promising tool for future research into the mechanisms of renal lipid accumulation in vivo.

Selective binding of sterol regulatory element-binding protein isoforms and co-regulatory proteins to promoters for lipid metabolic genes in liver

Mice were subjected to different dietary manipulations to selectively alter expression of hepatic sterol regulatory element-binding protein 1 (SREBP-1) or SREBP-2. mRNA levels for key target genes were measured and compared with the direct binding of SREBP-1 and -2 to the associated promoters using isoform specific antibodies in chromatin immunoprecipitation studies. A diet supplemented with Zetia (ezetimibe) and lovastatin increased and decreased nuclear SREBP-2 and SREBP-1, respectively, whereas a fasting/refeeding protocol dramatically altered SREBP-1 but had modest effects on SREBP-2 levels. Binding of both SREBP-1 and -2 increased on promoters for 3-hydroxy-3-methylglutaryl-CoA reductase, fatty-acid synthase, and squalene synthase in livers of Zetia/lovastatin-treated mice despite the decline in total SREBP-1 protein. In contrast, only SREBP-2 binding was increased for the low density lipoprotein receptor promoter. Decreased SREBP-1 binding during fasting and a dramatic increase upon refeeding indicates that the lipogenic "overshoot" for fatty-acid synthase gene expression known to occur during high carbohydrate refeeding can be attributed to a similar overshoot in SREBP-1 binding. SREBP co-regulatory protein recruitment was also increased/decreased in parallel with associated changes in SREBP binding, and there were clear distinctions for different promoters in response to the dietary manipulations. Taken together, these studies reveal that there are alternative molecular mechanisms for activating SREBP target genes in response to the different dietary challenges of Zetia/lovastatin versus fasting/refeeding. This underscores the mechanistic flexibility that has evolved at the individual gene/promoter level to maintain metabolic homeostasis in response to shifting nutritional states and environmental fluctuations.

Squalene synthase, a determinant of Raft-associated cholesterol and modulator of cancer cell proliferation

Several cues for cell proliferation, migration, and survival are transmitted through lipid rafts, membrane microdomains enriched in sphingolipids and cholesterol. Cells obtain cholesterol from the circulation but can also synthesize cholesterol de novo through the mevalonate/isoprenoid pathway. This pathway, however, has several branches and also produces non-sterol isoprenoids. Squalene synthase (SQS) is the enzyme that determines the switch toward sterol biosynthesis. Here we demonstrate that in prostate cancer cells SQS expression is enhanced by androgens, channeling intermediates of the mevalonate/isoprenoid pathway toward cholesterol synthesis. Interestingly, the resulting increase in de novo synthesis of cholesterol mainly affects the cholesterol content of lipid rafts, while leaving non-raft cholesterol levels unaffected. Conversely, RNA interference-mediated SQS inhibition results in a decrease of raft-associated cholesterol. These data show that SQS activity and de novo cholesterol synthesis are determinants of membrane microdomain-associated cholesterol in cancer cells. Remarkably, SQS knock down also attenuates proliferation and induces death of prostate cancer cells. Similar effects are observed when cancer cells are treated with the chemical SQS inhibitor zaragozic acid A. Importantly, although the anti-tumor effect of statins has previously been attributed to inhibition of protein isoprenylation, the present study shows that specific inhibition of the cholesterol biosynthesis branch of the mevalonate/isoprenoid pathway also induces cancer cell death. These findings significantly underscore the importance of de novo cholesterol synthesis for cancer cell biology and suggest that SQS is a potential novel target for antineoplastic intervention.

Direct interaction between USF and SREBP-1c mediates synergistic activation of the fatty-acid synthase promoter

To understand the molecular mechanisms underlying transcriptional activation of fatty-acid synthase (FAS), we examined the relationship between upstream stimulatory factor (USF) and SREBP-1c, two transcription factors that we have shown previously to be critical for FAS induction by feeding/insulin. Here, by using a combination of tandem affinity purification and coimmunoprecipitation, we demonstrate, for the first time, that USF and SREBP-1 interact in vitro and in vivo. Glutathione S-transferase pulldown experiments with various USF and sterol regulatory element-binding protein (SREBP) deletion constructs indicate that the basic helix-loop-helix domain of USF interacts directly with the basic helix-loop-helix and an N-terminal region of SREBP-1c. Furthermore, cotransfection of USF and SREBP-1c with an FAS promoter-luciferase reporter construct in Drosophila SL2 cells results in highly synergistic activation of the FAS promoter. We also show similar cooperative activation of the mitochondrial glycerol-3-phosphate acyltransferase promoter by USF and SREBP-1c. Chromatin immunoprecipitation analysis of mouse liver demonstrates that USF binds constitutively to the mitochondrial glycerol 3-phosphate acyltransferase promoter during fasting/refeeding in vivo, whereas binding of SREBP-1 is observed only during refeeding, in a manner identical to that of the FAS promoter. In addition, we show that the synergy we have observed depends on the activation domains of both proteins and that mutated USF or SREBP lacking the N-terminal activation domain could inhibit the transactivation of the other. Closely positioned E-boxes and sterol regulatory elements found in the promoters of several lipogenic genes suggest a common mechanism of induction by feeding/insulin.

Visceral obesity is associated with high levels of serum squalene

OBJECTIVE

To investigate the impact of visceral obesity on cholesterol metabolism in normoglycemic offspring of patients with type 2 diabetes.

RESEARCH METHODS AND PROCEDURES

The proportion of intra-abdominal fat (IAF) was measured by abdominal computer tomography, and serum cholesterol synthesis and absorption markers were determined by gas-liquid chromatography in 109 normoglycemic offspring of patients with type 2 diabetes. Insulin action was measured with the hyperinsulinemic euglycemic clamp. The gene encoding squalene synthase (farnesyl-diphosphate farnesyltransferase 1) was screened with the single-strand conformation polymorphism analysis and direct sequencing.

RESULTS

Cholesterol synthesis markers correlated positively with IAF (r = 0.213 to 0.309, p < or = 0.027) and negatively with the rates of insulin-stimulated whole-body glucose uptake (r = -0.372 to -0.248, p < or = 0.010). However, serum squalene, the first measured precursor of cholesterol synthesis, showed a positive correlation with IAF (r = 0.309, p = 0.001) without any association with subcutaneous fat or insulin sensitivity. Variation in the farnesyl-diphosphate farnesyltransferase 1 gene did not explain elevated serum squalene levels in viscerally obese subjects. From the cholesterol absorption markers, cholestanol was associated negatively with IAF and positively with whole-body glucose uptake (p < 0.05).

DISCUSSION

High serum squalene levels are associated with visceral obesity but not with subcutaneous obesity. Whether this finding is causally connected to visceral obesity remains to be established.

Sterol dependent regulation of human TM7SF2 gene expression: role of the encoded 3beta-hydroxysterol Delta14-reductase in human cholesterol biosynthesis

3Beta-hydroxysterol Delta(14)-reductase operates during the conversion of lanosterol to cholesterol in mammalian cells. Besides the endoplasmic reticulum 3beta-hydroxysterol Delta(14)-reductase (C14SR) encoded by TM7SF2 gene, the lamin B receptor (LBR) of the inner nuclear membrane possesses 3beta-hydroxysterol Delta(14)-reductase activity, based on its ability to complement C14SR-defective yeast strains. LBR was indicated as the primary 3beta-hydroxysterol Delta(14)-reductase in human cholesterol biosynthesis, since mutations in LBR gene were found in Greenberg skeletal dysplasia, characterized by accumulation of Delta(14)-unsaturated sterols. This study addresses the issue of C14SR and LBR role in cholesterol biosynthesis. Both human C14SR and LBR expressed in COS-1 cells exhibit 3beta-hydroxysterol Delta(14)-reductase activity in vitro. TM7SF2 mRNA and C14SR protein expression in HepG2 cells grown in delipidated serum (LPDS) plus lovastatin (sterol starvation) were 4- and 8-fold higher, respectively, than in LPDS plus 25-hydroxycholesterol (sterol feeding), resulting in 4-fold higher 3beta-hydroxysterol Delta(14)-reductase activity. No variations in LBR mRNA and protein levels were detected in the same conditions. The induction of TM7SF2 gene expression is turned-on by promoter activation in response to low cell sterol levels and is mediated by SREBP-2. The results suggest a primary role of C14SR in human cholesterol biosynthesis, whereas LBR role in the pathway remains unclear.

Increased cholesterol biosynthesis and hypercholesterolemia in mice overexpressing squalene synthase in the liver

Squalene synthase (SS) is the first committed enzyme for cholesterol biosynthesis, located at a branch point in the mevalonate pathway. To examine the role of SS in the overall cholesterol metabolism, we transiently overexpressed mouse SS in the livers of mice using adenovirus-mediated gene transfer. Overexpression of SS increased de novo cholesterol biosynthesis with increased 3-hydroxy-3-methyglutaryl-CoA (HMG-CoA) reductase activity, in spite of the downregulation of its own mRNA expression. Furthermore, overexpression of SS increased plasma concentrations of LDL, irrespective of the presence of functional LDL receptor (LDLR). Thus, the hypercholesterolemia is primarily caused by increased hepatic production of cholesterol-rich VLDL, as demonstrated by the increases in plasma cholesterol levels after intravenous injection of Triton WR1339. mRNA expression of LDLR was decreased, suggesting that defective LDL clearance contributed to the development of hypercholesterolemia. Curiously, the liver was enlarged, with a larger number of Ki-67-positive cells. These results demonstrate that transient upregulation of SS stimulates cholesterol biosynthesis as well as lipoprotein production, providing the first in vivo evidence that SS plays a regulatory role in cholesterol metabolism through modulation of HMG-CoA reductase activity and cholesterol biosynthesis.

The sterol response element binding protein regulates cyclooxygenase-2 gene expression in endothelial cells

We previously demonstrated that cholesterol deprivation increases endothelial cyclooxygenase-2 (COX-2)-dependent prostacyclin [prostaglandin I2 (PGI2)] production in vitro. Cholesterol directly regulates gene transcription through the sterol response element binding protein (SREBP). In this work, we demonstrate that SREBP directly regulates COX-2 expression. Cholesterol reduces human COX-2 promoter-luciferase reporter construct activity in transiently transfected endothelial cells. Conversely, cotransfection with a constitutively active mutant SREBP increases COX-2 promoter activity. SREBP-1a and -2 specifically bind a putative sterol response element (SRE) sequence in the COX-2 promoter. This sequence competes for SREBP binding to a low density lipoprotein receptor consensus sequence in an electromobility-shift assay. These data indicate that endothelial COX-2 is regulated by cholesterol via the SREBP pathway. The present study identifies COX-2 as the first vascular gene without a clear role in lipid metabolism transactivated by SREBP, and suggests that enhanced production of PGI2 through this pathway may be an additional benefit of cholesterol-lowering therapies.

Novel role for a sterol response element binding protein in directing spermatogenic cell-specific gene expression

Sperm are highly specialized cells, and their formation requires the synthesis of a large number of unique mRNAs. However, little is known about the transcriptional mechanisms that direct male germ cell differentiation. Sterol response element binding protein 2gc (SREBP2gc) is a spermatogenic cell-enriched isoform of the ubiquitous transcription factor SREBP2, which in somatic cells is required for homeostatic regulation of cholesterol. SREBP2gc is selectively enriched in spermatocytes and spermatids, and, due to its novel structure, its synthesis is not subject to cholesterol feedback control. This suggested that SREBP2gc has unique cell- and stage-specific functions during spermatogenesis. Here, we demonstrate that this factor activates the promoter for the spermatogenesis-related gene proacrosin in a cell-specific manner. Multiple SREBP2gc response elements were identified within the 5'-flanking and proximal promoter regions of the proacrosin promoter. Mutating these elements greatly diminished in vivo expression of this promoter in spermatogenic cells of transgenic mice. These studies define a totally new function for an SREBP as a transactivator of male germ cell-specific gene expression. We propose that SREBP2gc is part of a cadre of spermatogenic cell-enriched isoforms of ubiquitously expressed transcriptional coregulators that were specifically adapted in concert to direct differentiation of the male germ cell lineage.

Maintaining cholesterol homeostasis: Sterol regulatory element-binding proteins

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

Transcription factor sterol regulatory element binding protein 2 regulates scavenger receptor Cla-1 gene expression

OBJECTIVE

The human scavenger receptor class B type I (Cla-1) plays a key role in cellular cholesterol movement in facilitating transport of cholesterol between cells and lipoproteins. Indirect evidence has suggested that Cla-1 gene expression is under the feedback control of cellular cholesterol content. To define the molecular mechanisms underlying such putative regulation, we evaluated whether Cla-1 is a target gene of the sterol regulatory element binding protein (SREBP) transcription factor family.

METHODS AND RESULTS

Transient transfections demonstrated that SREBP factors induce Cla-1 promoter activity and that SREBP-2 is a more potent inducer than the SREBP-1a isoform. The 5'-deletion analysis of 3 kb of the 5'-flanking sequence of the Cla-1 gene, combined with site-directed mutagenesis and electrophoretic mobility shift assay, allowed identification of a unique sterol responsive element. SREBP-mediated Cla-1 regulation was confirmed in stably transfected human embryonic kidney 293 cells expressing the active form of SREBP-2 at incremental levels. In these cell lines, Cla-1 mRNA and protein levels were increased in direct proportion to the level of SREBP-2 expression.

CONCLUSIONS

These findings provide evidence that SREBP-2, a key regulator of cellular cholesterol uptake through modulation of the expression of the low-density lipoprotein receptor gene, may influence cellular cholesterol homeostasis via regulation of Cla-1 gene expression.

Selective coactivator interactions in gene activation by SREBP-1a and -1c

Requisite levels of intracellular cholesterol and fatty acids are maintained in part by the sterol regulatory element binding proteins (SREBPs). Three major SREBP isoforms exist; SREBP-1a and SREBP-1c are expressed from overlapping mRNAs, whereas SREBP-2 is encoded by a separate gene. The active forms of SREBP-1a and SREBP-1c differ only at their extreme N termini; SREBP-1c lacks 28 aa present in SREBP-1a and instead contains 4 unique aa of its own. While the SREBP-1a and -1c isoforms differentially activate transcription, the molecular basis of this difference is unknown. Here we define the differences between these proteins that confer the enhanced activity of SREBP-1a and demonstrate that this enhancement is a direct result of its avid binding to the coactivator CREB binding protein (CBP) and the mammalian mediator complex. While previous work determined that the C/H1 zinc finger and KIX domains of CBP bind to SREBP-1a, we provide evidence that the interaction with C/H1 is important for gene activation. We further show that the association between the activation domain of SREBP-1 and mediator is through aa 500 to 824 of DRIP150. Finally, we demonstrate the recruitment of mediator to an SREBP-responsive promoter in a sterol-dependent manner.

Differential Regulation of Human and Mouse Orphan Nuclear Receptor Small Heterodimer Partner Promoter by Sterol Regulatory Element Binding Protein-1

Small heterodimer partner (SHP; NR0B2) is an unusual orphan nuclear receptor that lacks a conventional DNA-binding domain and acts as a modulator of transcriptional activities of a number of nuclear receptors. Herein, we report that the human SHP promoter (hSHP) is activated by sterol regulatory element-binding protein-1 (SREBP-1), which regulates the expression of various genes involved in cholesterol and fatty acid synthesis. Overexpression of SREBP-1 activated the human but not mouse SHP promoter, although SREBP-2 had little effect on the SHP promoter in CV-1 cells. Serial deletion reporter assays revealed that SREBP-1-responsive region is located within the sequences from -243 to -120 bp in the hSHP promoter. DNase I footprinting, gel shift assays, and chromatin immunoprecipitation assays demonstrated that SREBP-1 binds directly to the hSHP promoter. Site-directed mutagenesis made it clear that the hSHP promoter activation by SREBP-1 is mostly mediated by the SRE1 (-186 to -195 bp) in the hSHP promoter, which is not conserved in the mouse SHP promoter. Moreover, adenovirus-mediated overexpression of SREBP-1c/ADD-1 induced SHP mRNA expression and repressed CYP7A1 expression in HepG2 cells. Finally, we found that a four-nucleotide deletion (-195CT-GAdel) in the hSHP promoter, which is reported to be associated with altered body weight and insulin secretion in human, coincides with the SRE1. This mutation strongly decreased both basal and SREBP-1 dependent activities of the hSHP promoter, because of the reduced binding of SREBP-1 to the mutated SRE1. Overall, our results demonstrate a differential regulation of human and mouse SHP promoters by SREBP-1. We propose a possible role of SREBP-1 in the species differential regulation of cholesterol and bile acid homeostasis via a novel mechanism of up-regulation of the hSHP gene expression.

IDH1 gene transcription is sterol regulated and activated by SREBP-1a and SREBP-2 in human hepatoma HepG2 cells: evidence that IDH1 may regulate lipogenesis in hepatic cells

The mRNA level for cytosolic NADP-dependent isocitrate dehydrogenase (IDH1) increases 2.3-fold, and enzyme activity of NADP-isocitrate dehydrogenase (IDH) 63%, in sterol-deprived HepG2 cells. The mRNA levels of the NADP- and NAD-dependent mitochondrial enzymes show limited or lack of regulation under the same conditions. Nucleotide sequences that are required, and sufficient, for the sterol regulation of transcription are located within a 67 bp region of an IDH1-secreted alkaline phosphatase promoter-reporter gene. The IDH1 promoter is fully activated by the expression of SREBP-1a in the cells and, to a lesser degree, by that of SREBP-2. A 5'-end truncation of 23 bp containing a CAAT and a GC-Box results in 6.5% residual activity. The promoter region involved in the activation by the sterol regulatory element binding proteins (SREBPs) is located at nucleotides -44 to -25. Mutagenesis analysis identified within this region the IDH1-SRE sequence element GTGGGCTGAG, which binds the SREBPs. Similar to the promoter activation, electrophoretic mobility shifts of probes containing the IDH1-SRE element exhibit preferential binding to SREBP-1a, as compared with SREBP-2. These results indicate that IDH1 activity is coordinately regulated with the cholesterol and fatty acid biosynthetic pathways and suggest that it is the source for the cytosolic NADPH required by these pathways.

Sterol regulatory element-binding protein-2 interacts with hepatocyte nuclear factor-4 to enhance sterol isomerase gene expression in hepatocytes

In the course of an effort to identify unknown targets genes for sterol regulatory element-binding proteins (SREBPs) by PCR, the gene for ATP citrate-lyase was determined to be one such gene. (Sato, R., Okamoto, A., Inoue, J., Miyamoto, W., Sakai, Y., Emoto, N., Shimano, H., and Maeda, M. (2000) J. Biol. Chem. 275, 12497-12502). We here report that gene expression of sterol Delta8-isomerase (SI), which catalyzes the conversion of the 8-ene isomer into the 7-ene isomer in the last steps of the cholesterol biosynthetic pathway, is regulated by SREBPs, mainly by SREBP-2. Luciferase assays using the promoter of the human SI gene revealed that a 200-base pair segment upstream region from the transcription start site contains functional elements required for the activity of the SREBPs, Sp1 and NF-Y. Interestingly, SI gene expression was well regulated by sterols in Caco-2 and HepG2 cells, in contrast with HEK293 and HeLa cells. Overexpression of hepatocyte nuclear factor (HNF)-4 in HEK293 cells augmented expression of SREBP-responsive genes including the SI gene, whereas inactivation of HNF-4 by small interfering RNAs in HepG2 cells reduced the SI gene promoter activity. The in vitro pull-down and in vivo co-immunoprecipitation experiments showed the direct interaction between SREBP-2 and HNF-4. These data provide a novel pathway by which HNF-4 potentiates the SREBP functions and stimulates expression of SREBP-responsive genes in enterohepatic cells.

Occupancy and function of the -150 sterol regulatory element and -65 E-box in nutritional regulation of the fatty acid synthase gene in living animals

Upstream regulatory factor (USF) and sterol regulatory element binding protein (SREBP) play key roles in the transcriptional regulation of the fatty acid synthase (FAS) gene by feeding and insulin. Due to the dual binding specificity of SREBP, as well as the presence of multiple consensus sites for these transcription factors in the FAS promoter, their physiologically relevant functional binding sites have been controversial. Here, in order to determine the occupancy of the putative USF and SREBP binding sites, we examined their protein-DNA interactions in living animals by using formaldehyde cross-linking and immunoprecipitation of chromatin and tested the function of these elements by employing mice transgenic for a reporter gene driven by various 5' deletions as well as site-specific mutations of the FAS promoter. We show that the -332 and -65 E-boxes are bound by USF in both fasted and refed mice, while the -150 SRE is bound by SREBP-1 only in refed mice. We also found that mutation of either the -150 SRE or the -65 E-box abolishes the feeding-induced activation of the FAS promoter in transgenic mice. Furthermore, in vivo occupancy of the FAS promoter by SREBP in the fed state can be prevented by mutation not only of the -150 SRE but, unexpectedly, of the -65 E-box as well. We conclude that the FAS promoter is activated during refeeding via the induced binding of SREBP to the -150 SRE and that USF binding to the -65 E-box is also required for SREBP binding and activation of the FAS promoter.

Low-temperature effect on the sterol-dependent processing of SREBPs and transcription of related genes in HepG2 cells

Lowering the growth temperature of HepG2 cells from 37 degrees C to 20 degrees C results in a 73% reduction in human squalene synthase (HSS) protein, a 76% reduction in HSS mRNA, and a 96% reduction in promoter activity of a secreted alkaline phosphatase-HSS reporter gene. A similar decrease in either mRNA or protein levels is observed for 3-hydroxy-3-methylglutaryl CoA reductase, farnesyl diphosphate synthase, the LDL receptor, and fatty acid synthase. All these proteins and mRNAs show either a decrease or a complete loss of sterol-dependent regulation in cells grown at 20 degrees C. In contrast, sterol regulatory element binding proteins (SREBPs)-1 and -2 exhibit a 2- to 3-fold increase in mRNA levels at 20 degrees C. The membrane-bound form of the SREBPs is dramatically increased, but the proteolytic processing to the nuclear (N-SREBP) form is inhibited under these conditions. Overexpression of the N-SREBP or SREBP cleavage-activating protein (SCAP), but not site-1 or site-2 proteases, restores the activation of the HSS promoter at 20 degrees C, most likely by liberating the SCAP-SREBP complex so that it can move to the Golgi for processing. These results indicate that the cholesterol synthesizing machinery is down-regulated at low temperatures, and points to the transport of the SCAP-SREBP complex to the Golgi as the specific down-regulated step.

Sterol regulatory element-binding protein family as global regulators of lipid synthetic genes in energy metabolism

Sterol regulatory element-binding proteins (SREBPs) have been established as lipid synthetic transcription factors for cholesterol and fatty acid synthesis. SREBPs are synthesized as membrane-bound precursors with their N-terminal active portions entering the nucleus to activate target genes after proteolytic cleavage in a sterol-regulated manner. This cleavage step is regulated by a putative sterol-sensing molecule, SREBP-activating protein (SCAP), that forms a complex with SREBPs and traffics between the rough endoplasmic reticulum and Golgi. DNA cis-elements that SREBPs bind, originally identified as sterol-regulatory elements (SREs), now expands to a variety of SRE-like sequences and some of E-boxes, which makes SREBPs eligible to regulate a wide range of lipid genes. Animal experiments including transgenic and knockout mice suggest that three isoforms, SREBP-1a, -1c, and -2, have different roles in lipid synthesis. In differentiated tissues and organs, SREBP-1c is involved in fatty acid, whereas SREBP-2 plays a major role in regulation of cholesterol synthesis. SREBP-1a is expressed in growing cells, providing both cholesterol and fatty acids that are required for membrane synthesis. SREBP-1c seems to be a mediator for insulin/glucose signaling to lipogenesis, and could be involved in insulin resistance, remnant lipoproteins, and fatty livers. Future studies in this field will certainly focus on understanding the molecular mechanisms sensing cellular sterol and energy states leading to the activation of SREBP-mediated gene transcription.

Regulation of squalene synthase, a key enzyme of sterol biosynthesis, in tobacco

Squalene synthase (SS) represents a putative branch point in the isoprenoid biosynthetic pathway capable of diverting carbon flow specifically to the biosynthesis of sterols and, hence, is considered a potential regulatory point for sterol metabolism. For example, when plant cells grown in suspension culture are challenged with fungal elicitors, suppression of sterol biosynthesis has been correlated with a reduction in SS enzyme activity. The current study sought to correlate changes in SS enzyme activity with changes in the level of the corresponding protein and mRNA. Using an SS-specific antibody, the initial suppression of SS enzyme activity in elicitor-challenged cells was not reflected by changes in the absolute level of the corresponding polypeptide, implicating a post-translational control mechanism for this enzyme activity. In comparison, the absolute level of the SS mRNA did decrease approximately 5-fold in the elicitor-treated cells, which is suggestive of decreased transcription of the SS gene. Study of SS in intact plants was also initiated by measuring the level of SS enzyme activity, the level of the corresponding protein, and the expression of SS gene promoter-reporter gene constructs in transgenic plants. SS enzyme activity, polypeptide level, and gene expression were all localized predominately to the shoot apical meristem, with much lower levels observed in leaves and roots. These later results suggest that sterol biosynthesis is localized to the apical meristems and that apical meristems may be a source of sterols for other plant tissues.

Comparative squalene synthase gene expression in mouse liver and testis

RNA from various mouse organs was analyzed by Northern hybridization to determine the response of squalene synthase (SQS) mRNA to dietary cholesterol, or lovastatin and cholestyramine, administration. Two size-classes of highly abundant mouse SQS (mSQS) mRNAs of approximately 1.9 and 2.0 kb were found in testis. These transcripts were unresponsive to sterol regulation. A single size-class of liver mSQS mRNA of approximately 1.9 kb was sterol-regulated. Studies using primer extension and 5' rapid amplification of cDNA ends (RACE) indicated that the size differences in liver and testis mSQS transcripts were due to variations in the lengths of the 5' untranslated regions (UTRs). The longest testis 5' UTR extended approximately 106 nt 5' of the primary transcription initiation site in liver of mice fed lovastatin and cholestyramine. These results suggest that tissue-specific promoter elements control the transcriptional regulation of the promoters for the mSQS gene in liver and testis.

Transcriptional regulation of the murine acetyl-CoA synthetase 1 gene through multiple clustered binding sites for sterol regulatory element-binding proteins and a single neighboring site for Sp1

Cytosolic acetyl-CoA synthetase (AceCS1) activates acetate to supply the cells with acetyl-CoA for lipid synthesis. The cDNA for the mammalian AceCS1 has been isolated recently, and the mRNA was shown to be negatively regulated by sterols in cultured cells. In the current study, we describe the molecular mechanisms directing the sterol-regulated expression of murine AceCS1 by cloning and functional studies of the 5'-flanking region of the AceCS1 gene. An AceCS1 promoter-reporter gene (approximately 2.1 kilobase pairs) was negatively regulated when sterols were added to the medium of cultured cells, and the promoter was markedly induced by co-transfection of a plasmid that expresses the transcriptionally active nuclear form of either sterol regulatory element-binding protein (SREBP)-1a or -2 in HepG2 cells. Sequence analysis suggested that the AceCS1 promoter contains an E-box, two putative CCAAT-boxes, eight sterol regulatory element (SRE) motifs, and six GC-boxes. Gel shift assays demonstrated that all eight SRE motifs bound purified SREBP-1a in vitro with similar affinity. Luciferase reporter gene assays revealed that sterol regulation was critically dependent on three closely spaced SRE motifs and an adjacent GC-box. However, mutation of two putative upstream CCAAT-boxes did not affect SREBP dependent activation. Electrophoretic mobility "supershift" analyses confirmed that both Sp1 and Sp3 bound to the critical GC-box. In addition, transfection studies in Drosophila SL2 cells demonstrated that SREBP synergistically activated the AceCS1 promoter along with Sp1 or Sp3 but not with nuclear factor-Y.

The FIRE3-mediated sterol response of the FAS promoter requires NF-Y/CBF as a coactivator

The transcription of the fatty acid synthase (FAS) gene is regulated by the sterol status of the cell via cleavage of the sterol regulatory element-binding protein (SREBP). When human HepG2 hepatoma cells were cotransfected with an expression plasmid for mature SREBP-1a together with FAS promoter/reporter constructs significant increases in reporter activity were observed. Deletion analysis of the FAS promoter between -151 and -52 relative to the transcription start site pinpoint two cis-elements important in sterol regulation of the FAS gene. One element, FIRE3, between -71 and -52 can bind in vitro translated and transcribed SREBP-1a whereas the other element, the inverted CCAAT element ICE(-97/-92), binds the trimeric transcription factor NF-Y/CBF as shown with rat liver extract and reconstituted, recombinant NF-Y. The results clearly show that the coactivator for SREBP-1a in this cell line is NF-Y. This finding was confirmed by using a dominant negative form of NF-YA, NF-YAm29, which interferes with the effect of ectopically expressed SREBP-1a on FAS reporter activity.

Sterol regulatory element binding protein-1a regulation of the steroidogenic acute regulatory protein gene

The binding of tropic hormones to their specific receptors in steroidogenic cells stimulates the cAMP second-messenger system in the presence of steroidogenic factor-1 (SF-1) to increase expression of steroidogenic acute regulatory (StAR) protein, facilitating the transfer of cholesterol to the inner mitochondrial membrane. The increased use of cholesterol in steroidogenesis triggers activation of sterol- sensitive genes through a second regulatory pathway involving the binding of sterol regulatory element (SRE)-binding proteins (SREBP) to SREs located in the promoter regions of these genes. A search of the rat StAR promoter revealed five potential SRE sites, which demonstrated specific binding with recombinant SREBP-1a. Overexpression of SREBP-1a, -1c or -2 in HTB-9 cells cotransfected with the rat StAR promoter resulted in an increase in promoter-driven luciferase activity. In addition, SREBP-1a was able to activate the StAR promoter through an E-box but only in a promoter construct lacking SREs. SREBPs are known to be weak transcriptional activators and require the presence of additional coactivators like Sp1 and nuclear factor-Y (NF-Y) to elicit maximum activation. Electrophoretic mobility shift assays demonstrated that Sp1, SF-1, and NF-Y enhanced SREBP-1a binding to SREs in the StAR promoter. There was a 4-fold increase in StAR promoter luciferase reporter gene expression when HTB-9 cells were cotransfected with expression vectors for SREBP-1a and NF-Y. In addition, the combined action of SREBP-1a and SF-1 increased both basal (1.6-fold) and cAMP-induced (3.5-fold) activation of the rat StAR promoter. Although Sp1 enhanced SREBP-1a binding to an SRE, Sp1 was not able to increase StAR promoter activity in the presence of SREBP-1a. These results suggest that SREBP-induced regulation of the rat StAR gene is responsive to selective combinations of transcriptional cofactors that could necessitate the convergence of multiple regulatory pathways to enhance gene transcription.

A novel sequence element is involved in the transcriptional regulation of expression of the ERG1 (squalene epoxidase) gene in Saccharomyces cerevisiae

Squalene epoxidase is an essential enzyme in the ergosterol-biosynthesis pathway. It catalyzes the epoxidation of squalene to 2,3-oxidosqualene and is the specific target of the antifungal drug terbinafine. Treatment of yeast cells with this inhibitor leads to squalene accumulation and sterol depletion. As ergosterol fulfils several essential functions, each requiring optimal sterol concentrations, synthesis of sterols in yeast must be tightly regulated. This study focuses on the sterol-mediated regulation of expression of the ERG1 gene, which codes for squalene epoxidase in Saccharomyces cerevisiae. Inhibition of ergosterol biosynthesis with terbinafine increases the expression of ERG1 in a concentration-dependent manner to a maximum of sevenfold. Inhibition of later steps in the ergosterol-biosynthetic pathway by ketoconazole, an inhibitor of the lanosterol-14alpha-demethylase, and U18666A, an inhibitor of the squalene-2,3-epoxide-lanosterol cyclase, also induce expression of ERG1, suggesting that ERG1 expression is positively regulated by diminished intracellular ergosterol levels. The regulatory effect of sterols is manifested at the level of transcription. Deletion analysis of the ERG1 promoter identified a novel regulatory DNA sequence element. Two 6-bp direct repeats, separated by 4 bp, AGCTCGGCCGAGCTCG, are unique to the ERG1 promoter. A DNA fragment containing this region confers ergosterol-regulated expression on an otherwise unregulated CYC1 promoter construction. One copy of the 6-bp element, AGCTCG, is sufficient to confer regulation, albeit less effectively than when both elements are present, whereas the removal of both elements from the ERG1 promoter leads to the loss of sterol-dependent ERG1 regulation.

Conditional response of the human steroidogenic acute regulatory protein gene promoter to sterol regulatory element binding protein-1a

The steroidogenic acute regulatory protein (StAR) gene controls the rate-limiting step in the biogenesis of steroid hormones, delivery of cholesterol to the cholesterol side-chain cleavage enzyme on the inner mitochondrial membrane. We determined whether the human StAR promoter is responsive to sterol regulatory element-binding proteins (SREBPs). Expression of SREBP-1a stimulated StAR promoter activity in the context of COS-1 cells and human granulosa-lutein cells. In contrast, expression of SREBP-2 produced only a modest stimulation of StAR promoter activity. One of the SREBP-1a response elements in the StAR promoter was mapped in deletion constructs and by site-directed mutagenesis between nucleotides -81 to -70 from the transcription start site. This motif bound recombinant SREBPs in electrophoretic mobility shift assays, but with lesser affinity than a low density lipoprotein receptor SREBP-binding site. An additional binding site for the transcriptional modulator, yin yang 1 (YY1), was observed within the SREBP-binding site (nucleotides -73 to -70). Mutation of the YY1-binding site increased the responsiveness of the StAR promoter to exogenous SREBP-1a, but did not alter the affinity for SREBP-1a binding in electrophoretic mobility gel shift assays. Manipulations that altered endogenous mature SREBP-1a levels (e.g. culture in lipoprotein-deficient medium and addition of 27-hydroxycholesterol) did not affect StAR promoter function, but influenced low density lipoprotein receptor promoter activity. We conclude that 1) the human StAR promoter is conditionally responsive to SREBP-1a such that promoter activity is up-regulated in the presence of high levels of SREBP-1a, but is unaffected when mature SREBP levels are suppressed; and 2) the human StAR promoter is selectively responsive to SREBP-1a.