YY1 is a positive regulator of transcription of the Col1a1 gene

Both cell-specific and ubiquitous transcription factors in fibroblasts have been identified as critical for expression of the Col1a1 gene, which encodes the alpha1 chain of type I collagen. Here, we report that Yin Yang 1 (YY1) binds to the Col1a1 promoter immediately upstream of the TATA box, and we examine the functional implications of YY1 binding for regulation of Col1a1 gene expression in BALBc/3T3 fibroblasts. The Col1a1 promoter region spanning base pairs (bp) -56 to -9 bound purified recombinant YY1 and the corresponding binding activity in nuclear extracts was supershifted using a YY1-specific antibody. Mutation of the TATA box to TgTA enhanced YY1 complex formation. Mutation analysis revealed two YY1 core binding sites at -40/-37 bp (YY1A) and, on the reverse strand, at -32/-29 bp (YY1B) immediately adjacent to the TATA box. In transfections using Col1a1-luciferase constructs, mutation of YY1A decreased activity completely (wild-type p350 (p350wt), -222/+113 bp) or partially (p130wt, -84 bp/+13 bp), whereas mutation of YY1B blocked the expression of both promoter constructs. Cotransfection with pCMV-YY1 increased p350wt and p130wt activities by as much as 10-fold, whereas antisense YY1 decreased constitutive expression and blocked the increased activity due to pCMV-YY1 overexpression. The mTgTA constructs were devoid of activity, arguing for a requirement for cognate binding of the TATA box-binding protein (TBP). Electrophoretic mobility shift assays performed under conditions permitting TBP binding showed that recombinant TBP/TFIID and YY1 could bind to the -56/-9 bp fragment and that YY1B was the preferred site for YY1 binding. Our results indicate that YY1 binds to the Col1a1 proximal promoter and functions as a positive regulator of constitutive activity in fibroblasts. Although YY1 is not sufficient for transcriptional initiation, it is a required component of the transcription machinery in this promoter.

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.

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.

Nutrient regulation of gene expression by the sterol regulatory element binding proteins: increased recruitment of gene-specific coregulatory factors and selective hyperacetylation of histone H3 in vivo

We have evaluated the mechanism for sterol-regulated gene expression by the sterol regulatory element binding proteins (SREBPs) in intact cells. We show that activation of SREBPs by sterol depletion results in the increased binding of Sp1 to a site adjacent to SREBP in the promoter for the low density lipoprotein (LDL) receptor gene in vivo. Similarly, sterol depletion resulted in the increased recruitment of two distinct SREBP coregulatory factors, NF-Y and CREB, to the promoter for hydroxymethyl glutaryl CoA reductase, another key gene of intracellular cholesterol homeostasis. Furthermore, increased acetylation of histone H3 but not H4 was also detected in chromatin from both promoters on SREBP activation. Thus, SREBP activation results in the similar selective recruitment of different coregulatory generic transcription factors to two separate cholesterol-regulated promoters. These studies demonstrate the utility of the chromatin immunoprecipitation technique for analyzing the differential action of low-abundance transcription factors in fundamental regulatory events in intact cells. Our results also provide key in vivo support for the mechanism proposed from cell-free experiments, where SREBP increased the binding of Sp1 to the LDL receptor promoter. Finally, our findings also indicate that subtle differences in the pattern of core histone acetylation play a role in selective gene activation.

Different sterol regulatory element-binding protein-1 isoforms utilize distinct co-regulatory factors to activate the promoter for fatty acid synthase

Sterol regulatory element-binding proteins (SREBPs) activate genes of cholesterol and fatty acid metabolism. In each case, a ubiquitous co-regulatory factor that binds to a neighboring recognition site is also required for efficient promoter activation. It is likely that gene- and pathway-specific regulation by the separate SREBP isoforms is dependent on subtle differences in how the individual proteins function with specific co-regulators to activate gene expression. In the studies reported here we extend these observations significantly by demonstrating that SREBPs are involved in both sterol regulation and carbohydrate activation of the FAS promoter. We also demonstrate that the previously implicated Sp1 site is largely dispensable for sterol regulation in established cultured cells, whereas a CCAAT-binding factor/nuclear factor Y is critically important. In contrast, carbohydrate activation of the FAS promoter in primary hepatocytes is dependent upon SREBP and both the Sp1 and CCAAT-binding factor/nuclear factor Y sites. Because 1c is the predominant SREBP isoform expressed in hepatocytes and 1a is more abundant in sterol depleted established cell lines, this suggests that the different SREBP isoforms utilize distinct co-regulatory factors to activate target gene expression.

Co-stimulation of promoter for low density lipoprotein receptor gene by sterol regulatory element-binding protein and Sp1 is specifically disrupted by the yin yang 1 protein

Sterol regulation of gene expression in mammalian cells is mediated by an interaction between the cholesterol-sensitive sterol regulatory element-binding proteins (SREBPs) and promoter-specific but generic co-regulatory transcription factors such as Sp1 and NF-Y/CBF. Thus, sterol-regulated promoters that require different co-regulatory factors could be regulated independently through targeting the specific interaction between the SREBPs and the individual co-regulatory proteins. In the present studies we demonstrate that transiently expressed yin yang 1 protein (YY1) inhibits the SREBP-mediated activation of the low density lipoprotein (LDL) receptor in a sensitive and dose-dependent manner. The inhibition is independent of YY1 binding directly to the LDL receptor promoter, and we show that the same region of YY1 that interacts in solution with Sp1 also interacts with SREBP. Furthermore, other SREBP-regulated genes that are not co-regulated by Sp1 are either not affected at all or are not as sensitive to the repression. Thus, the specific interaction that occurs between SREBPs and Sp1 to stimulate the LDL receptor promoter is a specific target for inhibition by the YY1 protein, and we provide evidence that the mechanism can be at least partially explained by the ability of YY1 to inhibit the interaction between SREBP and Sp1 in solution in vitro. The LDL receptor is the key gene of cholesterol uptake, and the rate-controlling genes of cholesterol synthesis are stimulated by the concerted action of SREBPs along with coregulators that are distinct from Sp1. Therefore, repression of gene expression through specifically targeting the interaction between SREBP and Sp1 would provide a molecular mechanism to explain how cholesterol uptake can be regulated independently from cholesterol biosynthesis in mammalian cells.

A critical role for cAMP response element-binding protein (CREB) as a Co-activator in sterol-regulated transcription of 3-hydroxy-3-methylglutaryl coenzyme A synthase promoter

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase, a key regulatory enzyme in the pathway for endogenous cholesterol synthesis, is a target for negative feedback regulation by cholesterol. When cellular sterol levels are low, the sterol regulatory element-binding proteins (SREBPs) are released from the endoplasmic reticulum membrane, allowing them to translocate to the nucleus and activate SREBP target genes. However, in all SREBP-regulated promoters studied to date, additional co-regulatory transcription factors are required for sterol-regulated activation of transcription. We have previously shown that, in addition to SREBPs, NF-Y/CBF is required for sterol-regulated transcription of HMG-CoA synthase. This heterotrimeric transcription factor has recently been shown to function as a co-regulator in several other SREBP-regulated promoters, as well. In addition to cis-acting sites for both SREBP and NF-Y/CBF, the sterol regulatory region of the synthase promoter also contains a consensus cAMP response element (CRE), an element that binds members of the CREB/ATF family of transcription factors. Here, we show that this consensus CRE is essential for sterol-regulated transcription of the synthase promoter. Using in vitro binding assays, we also demonstrate that CREB binds to this CRE, and mutations within the CRE that result in a loss of CREB binding also result in a loss of sterol-regulated transcription. We further show that efficient activation of the synthase promoter in Drosophila SL2 cells requires the simultaneous expression of all three factors: SREBPs, NF-Y/CBF, and CREB. To date this is the first promoter shown to require CREB for efficient sterol-regulated transcription, and to require two different co-regulatory factors in addition to SREBPs for maximal activation.

Oxysterol regulation of steroidogenic acute regulatory protein gene expression. Structural specificity and transcriptional and posttranscriptional actions

Oxysterols exert a major influence over cellular cholesterol homeostasis. We examined the effects of oxysterols on the expression of steroidogenic acute regulatory protein (StAR), which increases the delivery of cholesterol to sterol-metabolizing P450s in the mitochondria. 22(R)-hydroxycholesterol (22(R)-OHC), 25-OHC, and 27-OHC each increased steroidogenic factor-1 (SF-1)-mediated StAR gene transactivation by approximately 2-fold in CV-1 cells. In contrast, cholesterol, progesterone, and the 27-OHC metabolites, 27-OHC-5beta-3-one and 7alpha,27-OHC, had no effect. Unlike our findings in CV-1 cells, SF-1-dependent StAR promoter activity was not augmented by 27-OHC in COS-1 cells, Y-1 cells, BeWo choriocarcinoma cells, Chinese hamster ovary (CHO) cells, and human granulosa cells. Studies examining the metabolism of 27-OHC indicated that CV-1 cells formed a single polar metabolite, 3beta-OH-5-cholestenoic acid from radiolabeled 27-OHC. However, this metabolite inhibited StAR promoter activity in CV-1, COS-1 and CHO cells. Because 7alpha,27-OHC was unable to increase SF-1-dependent StAR promoter activity, we examined 27-OHC 7alpha-hydroxylase in COS-1 and CHO cells. COS-1 cells contained high 7alpha-hydroxylase activity, whereas the enzyme was undetectable in CHO cells. The hypothesis that oxysterols act in CV-1 cells to increase StAR promoter activity by reducing nuclear levels of sterol regulatory element binding protein was tested. This notion was refuted when it was discovered that sterol regulatory element binding protein-1a is a potent activator of the StAR promoter in CV-1, COS-1, and human granulosa cells. Human granulosa and theca cells, which express endogenous SF-1, contained more than 5-fold more StAR protein following addition of 27-OHC, whereas StAR mRNA levels remained unchanged. We conclude that 1) there are cell-specific effects of oxysterols on SF-1-dependent transactivation; 2) the ability to increase transactivation is limited to certain oxysterols; 3) there are cell-specific pathways of oxysterol metabolism; and 4) oxysterols elevate StAR protein levels through posttranscriptional actions.

Polyunsaturated fatty acids decrease expression of promoters with sterol regulatory elements by decreasing levels of mature sterol regulatory element-binding protein

Membrane physiology, plasma lipid levels, and intracellular sterol homeostasis are regulated by both fatty acids and cholesterol. Sterols regulate gene expression of key enzymes of cholesterol and fatty acid metabolism through proteolysis of the sterol regulatory element-binding protein (SREBP), which binds to sterol regulatory elements (SRE) contained in promoters of these genes. We investigated the effect of fatty acids on SRE-dependent gene expression and SREBP. Consistent results were obtained in three different cell lines (HepG2, Chinese hamster ovary, and CV-1) transfected with SRE-containing promoters linked to the luciferase expression vector. We show that micromolar concentrations of oleate and other polyunsaturated fatty acids (C18:2-C22:6) dose-dependently (0.075-0.6 mmol) decreased transcription of SRE-regulated genes by 20-75%. Few or no effects were seen with saturated free fatty acids. Fatty acid effects on SRE-dependent gene expression were independent and additive to those of exogenous sterols. Oleate decreased levels of the mature sterol regulatory element-binding proteins SREBP-1 and -2 and HMG-CoA synthase mRNA. Oleate had no effect in sterol regulation defective Chinese hamster ovary cells or in cells transfected with mutant SRE-containing promoters. We hypothesize that unsaturated fatty acids increase intracellular regulatory pools of cholesterol and thus affect mature SREBP levels and expression of SRE-dependent genes.

Molecular aspects in feedback regulation of gene expression by cholesterol in mammalian cells

Intracellular cholesterol balance is maintained by a tight feedback mechanism that prevents the overaccumulation of cholesterol to cytotoxic levels. This is achieved through the coordinate regulation of genes of cholesterol uptake and biosynthesis by the sterol regulatory element binding proteins (SREBPs). The SREBPs are synthesized as membrane bound precursors that are released from their membrane tether when the cell needs new cholesterol. In the present article we present a model for how the cholesterol uptake pathway may be activated before the biosynthetic pathway to prevent wasting cellular energy and carbon on unneeded synthesis. Then we introduce a system for analyzing the differential localization and cellular trafficking of the different SREBP isoforms that can be performed over time in living cells.

Sterol regulatory element binding protein-1 activates the cholesteryl ester transfer protein gene in vivo but is not required for sterol up-regulation of gene expression

The plasma cholesteryl ester transfer protein (CETP) plays a central role in high density lipoprotein metabolism and reverse cholesterol transport. Plasma CETP levels are increased in response to dietary or endogenous hypercholesterolemia as a result of increased gene transcription in liver and periphery. Deletional analysis in human CETP transgenic mice localized this response to a region of the proximal promoter which contains a tandem repeat of the sterol regulatory element (SRE) of the 3-hydroxy-3-methylglutaryl-CoA reductase gene. The purpose of the present study was to evaluate the role of the SRE-like element in CETP promoter activity. Gel shift assays using CETP promoter fragments containing these elements showed binding of the transcription factors, sterol regulatory element-binding protein-1 (SREBP-1) and Yin Yang-1 (YY-1). Point mutations in the SRE-like element, designated MUT1 and MUT2, resulted in decreased binding of SREBP-1 (MUT1) or SREBP-1 and YY-1 (MUT2). To determine the in vivo significance of this binding activity, CETP transgenic mice were prepared containing these promoter point mutations. MUT1 and MUT2 transgenic mice expressed CETP activity and mass in plasma. In response to high fat, high cholesterol diets, both MUT1-CETP and MUT2-CETP transgenic mice displayed induction of plasma CETP activity similar to that observed in natural flanking region (NFR) CETP transgenic mice. Moreover, in stably transfected adipocyte cell lines, MUT1 and MUT2 CETP promoter-reporter genes showed significant induction of reporter activity in response to sterols. To evaluate transactivation by SREBP-1, NFR- and MUT1-CETP transgenic mice were crossed with SREBP-1 transgenic mice. Induction of the SREBP transgene in the liver with a low carbohydrate diet resulted in a 3-fold increase in plasma CETP activity in NFR-CETP/SREBP transgenic mice, but there was no significant change in activity in MUT1-CETP/SREBP transgenic mice. Thus, SREBP-1 transactivates the NFR-CETP transgene in vivo, as a result of interaction with the CETP promoter SREs. However, this interaction is not required for positive sterol induction of CETP gene transcription. The results suggest independent regulation of the CETP gene by SREBP-1 and a distinct positive sterol response factor.

HLH106, a Drosophila sterol regulatory element-binding protein in a natural cholesterol auxotroph

In mammalian cells, sterol regulatory element-binding proteins (SREBPs) coordinate metabolic flux through the cholesterol and fatty acid biosynthetic pathways in response to intracellular cholesterol levels. We describe experiments that evaluate the functional equivalence of mammalian SREBPs and the insect homologue of SREBP-1a, HLH106, in both mammalian and insect cell culture systems. HLH106 binds to both palindromic E-boxes and direct repeat sterol regulatory elements (SREs) efficiently, suggesting that it has a dual DNA binding specificity similar to the mammalian proteins. The amino-terminal "mature" protein activates transcription from mammalian SREs in both mammalian and Drosophila tissue culture cells. Additionally, HLH106 also requires a ubiquitous regulatory co-activator to efficiently activate transcription from mammalian SREs. These properties are shared with its mammalian counterparts. When expressed in mammalian cells, the carboxyl-terminal portion also localizes to perinuclear membranes similar to mammalian SREBPs. Furthermore, membrane-bound HLH106 is proteolytically processed in response to intracellular sterol levels in mammalian cells in an SREBP cleavage-activating protein-stimulated fashion. The presence of an SREBP homologue in Drosophila whose processing is regulated by intracellular sterol levels when expressed in mammalian cells suggests that related processing machinery exists in insect cells. This is notable, since insects are reportedly incapable of de novo sterol biosynthesis.

Specificity in cholesterol regulation of gene expression by coevolution of sterol regulatory DNA element and its binding protein

When demand for cholesterol rises in mammalian cells, the sterol regulatory element (SRE) binding proteins (SREBPs) are released from their membrane anchor through proteolysis. Then, the N-terminal region enters the nucleus and activates genes of cholesterol uptake and biosynthesis. Basic helix-loop-helix (bHLH) proteins such as SREBPs bind to a palindromic DNA sequence called the E-box (5'-CANNTG-3'). However, SREBPs are special because they also bind direct repeat elements called SREs. Importantly, sterol regulation of all promoters studied thus far is mediated by SREBP binding only to SREs. To study the reason for this we converted the direct repeat SRE from the sterol-regulated low-density lipoprotein receptor promoter into an E-box. In this report we show that SREBPs are still able to bind and activate this promoter however, sterol regulation is lost. The results are consistent with the mutant promoter being a target for promiscuous activation by constitutively expressed E-box binding bHLH proteins that are not regulated by cholesterol. Kim and coworkers [Kim, J. B., Spotts, G. D., Halvorsen, Y.-D., Shih, H.-M., Ellenberger, T., Towle, H. C. & Spiegelman, B. M. (1995) Mol. Cell. Biol. 15, 2582-2588] demonstrated that the dual DNA binding specificity of SREBPs is caused by a specific tyrosine in the conserved basic region of the DNA binding domain that corresponds to an arginine in all other bHLH proteins that recognize only E-boxes. Taken together the data suggest an evolutionary mechanism where a DNA binding protein along with its recognition site have coevolved to ensure maximal specificity and sensitivity in a crucial nutritional regulatory response.

Related membrane domains in proteins of sterol sensing and cell signaling provide a glimpse of treasures still buried within the dynamic realm of intracellular metabolic regulation

Recent discoveries in the regulation of cholesterol metabolism have documented a two step proteolytic pathway that regulates nuclear targeting of the sterol regulatory element binding proteins. Sterol regulatory element binding protein cleavage activating protein is a newly identified protein that modulates the proteolytic maturation of the sterol regulatory element binding proteins. It contains a domain that is quite similar in sequence to the membrane spanning region of the rate controlling enzyme of cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase. The membrane domain of the reductase is involved in its post-translational regulation by cholesterol. The molecular defect in the intracellular cholesterol storage disease, Niemann-Pick type C, has also recently been identified. Surprisingly, the affected gene encodes a protein with similarity to the membrane domains that are conserved in 3-hydroxy-3-methylglutaryl reductase and sterol regulatory element binding protein cleavage activating protein. Furthermore, the cell surface receptor for the sterol modified hedgehog morphogen, Patched, also contains a membrane domain with significant similarity to this putative sterol monitoring domain. These recent developments suggest a common mechanism for sensing intracellular sterol levels and cell signaling, which is based on the function of related membrane domains that are contained in key regulatory proteins.

Sterol regulation of 3-hydroxy-3-methylglutaryl-coenzyme A synthase gene through a direct interaction between sterol regulatory element binding protein and the trimeric CCAAT-binding factor/nuclear factor Y

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, a key regulatory enzyme in the pathway for endogenous cholesterol synthesis, is a target for negative feedback regulation by cholesterol. The promoter for HMG-CoA synthase contains two binding sites for the sterol regulatory element-binding proteins (SREBPs). When cellular sterol levels are low, the SREBPs are released from the endoplasmic reticulum membrane, allowing them to translocate to the nucleus and activate SREBP target genes. In all SREBP-regulated promoters studied to date, additional co-regulatory transcription factors are required. In the HMG-CoA synthase promoter there are several potential co-regulatory transcription factor binding sites, including an inverted CCAAT box. A similar element has been shown to function with SREBP to mediate sterol regulation of another gene involved in cholesterol metabolism, farnesyl diphosphate synthase. Here, we show that CCAAT binding factor/nuclear factor Y (CBF/NF-Y) binding to the CCAAT box is required for sterol-regulated transcription of HMG-CoA synthase. The SREBP sites and the inverted CCAAT box are normally separated by 17 base pairs, and we show that increasing this distance results in a decrease in the level of transcriptional regulation by sterols. Furthermore, we provide evidence that there is a direct interaction between CBF/NF-Y and the basic helix-loop-helix-zipper region of SREBP. Interestingly, this interaction does not occur efficiently with any of the isolated subunits and appears to require all three nonidentical CBF/NF-Y subunits in a preassembled complex. Since CBF/NF-Y only binds to DNA when all three subunits are in a complex, this would prevent SREBP from forming nonproductive associations with the individual subunits.

Promoter selective transcriptional synergy mediated by sterol regulatory element binding protein and Sp1: a critical role for the Btd domain of Sp1

Cellular cholesterol and fatty acid levels are coordinately regulated by a family of transcriptional regulatory proteins designated sterol regulatory element binding proteins (SREBPs). SREBP-dependent transcriptional activation from all promoters examined thus far is dependent on the presence of an additional binding site for a ubiquitous coactivator. In the low-density lipoprotein (LDL) receptor, acetyl coenzyme A carboxylase (ACC), and fatty acid synthase (FAS) promoters, which are all regulated by SREBP, the coactivator is the transcription factor Sp1. In this report, we demonstrate that Sp3, another member of the Sp1 family, is capable of substituting for Sp1 in coactivating transcription from all three of these promoters. Results of an earlier study showed that efficient activation of transcription from the LDL receptor promoter required domain C of Sp1; however, this domain is not crucial for activation of the simian virus 40 promoter, where synergistic activation occurs through multiple Sp1 binding sites and does not require SREBP. Also in the present report, we further localize the critical determinant of the C domain required for activation of the LDL receptor to a small region that is highly conserved between Sp1 and Sp3. This crucial domain encompasses the buttonhead box, which is a 10-amino-acid stretch that is present in several Sp1 family members, including the Drosophila buttonhead gene product. Interestingly, neither the buttonhead box nor the entire C domain is required for the activation of the FAS and ACC promoters even though both SREBP and Sp1 are critical players. ACC and FAS each contain two critical SREBP sites, whereas there is only one in the LDL receptor promoter. This finding suggested that buttonhead-dependent activation by SREBP and Sp1 may be limited to promoters that naturally contain a single SREBP recognition site. Consistent with this model, a synthetic construct containing three tandem copies of the native LDL receptor SREBP site linked to a single Sp1 site was also significantly activated in a buttonhead-independent fashion. Taken together, these studies indicate that transcriptional activation through the concerted action of SREBP and Sp1 can occur by at least two different mechanisms, and promoters that are activated by each one can potentially be identified by the number of critical SREBP binding sites that they contain.

Sterol regulation of acetyl coenzyme A carboxylase promoter requires two interdependent binding sites for sterol regulatory element binding proteins

The sterol regulatory element binding proteins (SREBPs) are central regulators of lipid homeostasis in mammalian cells. Their activity is controlled by a sterol-regulated two-step proteolytic process that releases the nuclear targeted amino-terminal domain from the membrane anchored carboxyl-terminal remnant. This ensures that transcriptional stimulation of the appropriate genes occurs only when increased intracellular sterol accumulation is required. Gene targets for SREBP encode key proteins of cholesterol metabolism as well as essential proteins of fatty acid biosynthesis, providing a mechanism for coordinate control of these two major lipid pathways when sterols and fatty acids need to accumulate together. However, the regulatory mechanism must provide a way to uncouple these two pathways to allow separate regulation when sterol or fat levels need to increase independently of each other. We compared the similarities and differences for how SREBP activates the promoter for the low density lipoprotein (LDL) receptor, which is the key protein involved in cholesterol uptake, relative to how it activates promoters for acetyl coenzyme A carboxylase (ACC) and fatty acid synthase (FAS), which are both key enzymes of fatty acid biosynthesis. In the current studies we show there are two distinct sites for SREBP binding that control activation of the ACC PII promoter whereas previous work has shown there is only a single SREBP site in the LDL receptor. Additionally, disruption of either ACC site results in a total loss in promoter function and a severe decrease in SREBP binding even to the neighboring unaltered site. Thus, the two sites are equally important and dependent on one another for optimal function. This is in contrast to the FAS promoter where SREBP binds to two adjacent sites independently and the one located closer to the binding site for the Sp1 co-regulator is more critical for sterol regulation and activation by SREBP over-expression.

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

The expression of human squalene synthase (HSS) gene is transcriptionally regulated in HepG-2 cells, up to 10-fold, by variations in cellular cholesterol homeostasis. An earlier deletion analysis of the 5'-flanking region of the HSS gene demonstrated that most of the HSS promoter activity is detected within a 69-base pair sequence located between nucleotides -131 and -200. ADD1/SREBP-1c, a rat homologue of sterol regulatory element-binding protein (SREBP)-1c binds to sterol regulatory element (SRE)-1-like sequence (HSS-SRE-1) present in this region (Guan, G., Jiang, G., Koch, R. L. and Shechter, I. (1995) J. Biol. Chem. 270, 21958-21965). In our present study, we demonstrate that mutation of this HSS-SRE-1 element significantly reduced, but did not abolish, the response of HSS promoter to change in sterol concentration. Mutation scanning indicates that two additional DNA promoter sequences are involved in sterol-mediated regulation. The first sequence contains an inverted SRE-3 element (Inv-SRE-3) and the second contains an inverted Y-box (Inv-Y-box) sequence. A single mutation in any of these sequences reduced, but did not completely remove, the response to sterols. Combination mutation studies showed that the HSS promoter activity was abolished only when all three elements were mutated simultaneously. Co-expression of SRE-1- or SRE-2-binding proteins (SREBP-1 or SREBP-2) with HSS promoter-luciferase reporter resulted in a dramatic increase of HSS promoter activity. Gel mobility shift studies indicate differential binding of the SREBPs to regulatory sequences in the HSS promoter. These results indicate that the transcription of the HSS gene is regulated by multiple regulatory elements in the promoter.

Cholesterol homeostasis: clipping out a slippery regulator

SREBPs are transcriptional activators central to cholesterol homeostasis. Recent work has shown that a two-step cleavage of membrane-bound SREBPs frees them to enter the nucleus. An activator of the first, sterol-regulated proteolysis step has also been identified.

Two tandem binding sites for sterol regulatory element binding proteins are required for sterol regulation of fatty-acid synthase promoter

We previously reported that sterol regulation of the rat fatty-acid synthase was lost when the DNA sequence between -73 and -43 of the promoter was deleted from a luciferase reporter construct (Bennett, M. K., Lopez, J. M., Sanchez, H. B., and Osborne, T. F. (1995) J. Biol. Chem. 270, 25578-25583). We also showed that there was a binding site for sterol regulatory element binding protein-1 (SREBP-1) in this region that contains a palindromic E-box motif (5'-CANNTG-3'). This is the consensus recognition element for basic-helix-loop-helix leucine zipper containing proteins such as the SREBPs. However, the SREBPs are unique basic-helix-loop-helix leucine zipper proteins that not only bind to a subset of E-boxes but also to the direct repeat SRE-1 element of the low density lipoprotein receptor promoter as well as to variant sites present in the promoters for key enzymes of both cholesterol and fatty acid biosynthesis. Based on the sequence of the variant SREBP recognition sites in these other promoters, we noted there was more than one potential recognition site for SREBP within the -73 to -43 interval of the fatty-acid synthase promoter. In the present studies we have systematically mutated these potential SREBP sites and have analyzed the consequences on sterol regulation, activation by exogenously supplied SREBPs, and binding by SREBPs in vitro. The results clearly show that the E-box element is not the SREBP recognition site in this region. Rather, there are two independent SREBP binding sites that flank the E-box, and both are required for maximal sterol regulation and activation by transfected SREBP protein.

A direct role for sterol regulatory element binding protein in activation of 3-hydroxy-3-methylglutaryl coenzyme A reductase gene

In earlier studies the DNA site required for sterol regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase was shown to be distinct from the classic sterol regulatory element (SRE-1) of the low density lipoprotein receptor gene (Osborne, T. F. (1991) J. Biol. Chem 266, 13947-13951). However, oxysterol-resistant cells that continuously overproduce one of the sterol regulatory element binding proteins in the nucleus result in high unregulated expression of both genes (Yang, J., Brown, M. S., Ho, Y. K., and Goldstein, J. L. (1995) J. Biol. Chem. 270, 12152-12161) suggesting a direct role for the SREBPs in the activation of the reductase gene. In the present studies we demonstrate that SREBP-1 binds to two adjacent sites within the previously identified sterol regulatory element of the reductase gene even though there is only limited homology with the SRE-1 of the receptor. We also show that SREBP-1 specifically activates the reductase promoter in transient DNA transfection studies in HepG2 cells and that mutations which eliminate sterol regulation and SREBP-1 binding also abolish transient activation by SREBP-1. Although specific, the magnitude of the activation observed is considerably lower than for the low density lipoprotein (LDL) receptor analyzed in parallel, suggesting there is an additional protein required for activation of the reductase promoter that is limiting in the transient assay. SREBP also binds to two additional sites in the reductase promoter which probably plan an auxiliary role in expression. When the DNA sequence within the sites are aligned with each other and with the LDL receptor SRE-1, a consensus half-site is revealed 5'-PyCAPy-3'. The LDL receptor element contains two half-sites oriented as a direct repeat spaced by one nucleotide. The SREBP proteins are special members of the basic-helix-loop-helix-zipper (bHLHZip) family of DNA binding proteins since they bind the classic palindromic E-box site as well as the direct repeat SRE-1 element. The SREBP binding sites in both the reductase and those recently identified in other sterol regulated promoters appear to contain a half-site with considerable divergence in the flanking residues. Here we also show that a 22-amino acid domain located immediately adjacent to the basic domain of the bHLHZip region is required for SREBP to efficiently recognize divergent sites in the reductase and 3-hydroxy-3-methylglutaryl-CoA synthase promoters but, interestingly, this domain is not required for efficient binding to the LDL direct repeat SRE-1 or to a palindromic high-affinity E-box element.

Sterol regulation of acetyl coenzyme A carboxylase: a mechanism for coordinate control of cellular lipid

Transcription from the housekeeping promoter for the acetyl coenzyme A carboxylase (ACC) gene, which encodes the rate-controlling enzyme of fatty acid biosynthesis, is shown to be regulated by cellular sterol levels through novel binding sites for the sterol-sensitive sterol regulatory element binding protein (SREBP)-1 transcription factor. The position of the SREBP sites relative to those for the ubiquitous auxiliary transcription factor Sp1 is reminiscent of that previously described for the sterol-regulated low density lipoprotein receptor promoter. The experiments provide molecular evidence that the metabolism of fatty acids and cholesterol, two different classes of essential cellular lipids, are coordinately regulated by cellular lipid levels.

Sterol regulation of fatty acid synthase promoter. Coordinate feedback regulation of two major lipid pathways

The gene encoding fatty acid synthase, the essential multi-functional enzyme of fatty acid biosynthesis, is shown to be regulated by cellular sterol levels similar to genes that encode important proteins of cholesterol metabolism. We show that expression of the endogenous FAS gene is repressed when regulatory sterols are included in the culture medium of HepG2 cells and that the FAS promoter is subject to similar regulation when fused to the luciferase reporter gene. Mutational studies demonstrate that sterol regulation is mediated by binding sites for the sterol regulatory element-binding protein (SREBP) and transcription factor Sp1, making it mechanistically similar to sterol regulation of the low density lipoprotein receptor gene. It is also demonstrated that SREBP and Sp1 synergistically activate the FAS promoter in Drosophila tissue culture cells, which lack endogenous Sp1. These experiments provide key molecular evidence that directly links the metabolism of fatty acids and cholesterol together.

NF-Y has a novel role in sterol-dependent transcription of two cholesterogenic genes

The transcription of farnesyl diphosphate (FPP) synthase is regulated up to 30-fold by the sterol status of the cell. Point mutations in a 6-base pair ATTGGC sequence in the promoter disrupt both sterol-dependent transcription in vivo as well as binding of the transcription factor NF-Y in vitro. Co-transfection of cells with NF-YA29, a dominant negative form of NF-Y, and various promoter-reporter genes specifically inhibits the sterol-dependent regulation of FPP synthase and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase. In contrast, NF-YA29 does not affect the regulation of reporter genes under the control of promoters derived from either the HMG-CoA reductase or the low density lipoprotein receptor gene. Transient expression of the 68-kDa transcriptionally active fragment of sterol regulatory element-binding protein in cells stimulates an HMG-CoA synthase-reporter gene over 90-fold. This induction is blocked in cells co-expressing NF-YA29. We hypothesize that NF-Y plays a novel role in sterol-dependent regulation of two key genes in the cholesterol biosynthetic pathway and that this role requires a specific interaction with the sterol regulatory element-binding protein or related transcription factors.

Domains of transcription factor Sp1 required for synergistic activation with sterol regulatory element binding protein 1 of low density lipoprotein receptor promoter

Feedback regulation of transcription from the low density lipoprotein (LDL) receptor gene is fundamentally important in the maintenance of intracellular sterol balance. The region of the LDL receptor promoter responsible for normal sterol regulation contains adjacent binding sites for the ubiquitous transcription factor Sp1 and the cholesterol-sensitive sterol regulatory element-binding proteins (SREBPs). Interestingly, both are essential for normal sterolmediated regulation of the promoter. The cooperation by Sp1 and SREBP-1 occurs at two steps in the activation process. SREBP-1 stimulates the binding of Sp1 to its adjacent recognition site in the promoter followed by enhanced stimulation of transcription after both proteins are bound to DNA. In the present report, we have defined the protein domains of Sp1 that are required for both synergistic DNA binding and transcriptional activation. The major activation domains of Sp1 that have previously been shown to be essential to activation of promoters containing multiple Sp1 sites are required for activation of the LDL receptor promoter. Additionally, the C domain is also crucial. This slightly acidic approximately 120-amino acid region is not required for efficient synergistic activation by multiple Sp1 sites or in combination with other recently characterized transcriptional regulators. We also show that Sp1 domain C is essential for full, enhanced DNA binding by SREBP-1. Taken together with other recent studies on the role of Sp1 in promoter activation, the current experiments suggest a unique combinatorial mechanism for promoter activation by two distinct transcription factors that are both essential to intracellular cholesterol homeostasis.

Cooperation by sterol regulatory element-binding protein and Sp1 in sterol regulation of low density lipoprotein receptor gene

Regulation of the low density lipoprotein (LDL) receptor promoter by cholesterol requires a well defined sterol regulatory site and an adjacent binding site for the universal transcription factor Sp1. These elements are located in repeats 2 and 3 of the wild type promoter, respectively. The experiments reported here demonstrate that Sp1 participates in sterol regulation of the LDL receptor in an orientation-specific fashion. We present data which suggest that sterol regulatory element-binding protein (SREBP) increases the binding of Sp1 to the adjacent repeat 3 sequence. We also demonstrate that SREBP and Sp1 synergistically activate expression from the LDL receptor promoter inside the cell by cotransfecting expression vectors encoding each protein into Drosophila tissue culture cells that are devoid of endogenous Sp1. In addition, other transcription factor sites were unable to substitute for Sp1 in sterol regulation when placed next to the SREBP-binding site. These studies together with recent data from others provide the basis of a working model for sterol regulation of the LDL receptor promoter. The presence of Sp1 sites in several other regulated promoters suggests that this universal transcription factor has been recruited to participate in many regulatory responses possibly by a similar mechanism.

Transcriptional control mechanisms in the regulation of cholesterol balance

The mechanisms that govern regulation of cholesterol metabolism in higher eukaryotic cells provide an example of how metabolic regulation has evolved to establish growth and nutritional control in a multicellular environment. Two sources of cholesterol must be balanced to ensure optimum growth and viability. Much of the control is established by regulating the levels of key proteins involved in cholesterol uptake and biosynthesis and this occurs by alterations in promoter activity. The studies discussed here track the progression in understanding the mechanism for transcriptional regulation by cholesterol from the isolation of the key genes involved, to the careful dissection of the cis-acting sequences that control expression, and on to what is currently known about the trans-acting proteins that mediate the regulatory response.

Two separate sites contribute to AP-1 activation of the promoter for 3-hydroxy-3-methylglutaryl coenzyme A synthase

Metabolic flux into the mevalonate pathway is regulated by end product repression and cell growth. In the experiments reported here the transcriptional promoter for an early enzyme of the pathway, 3-hydroxy-3-methylglutaryl coenzyme A synthase, is shown to be activated by the growth stimulatory agent tetraphorbol acetate (TPA). We show that TPA has a direct stimulatory action on the promoter and further that this is mediated by the AP-1 transcription factor. In addition, we show that there are two separate cis-acting sites that bind AP-1 and both are required for maximal stimulation. We further show that in AP-1-deficient cells ectopic expression of AP-1 stimulates synthetic promoters containing two copies of each synthase element upstream of a minimal promoter. The physiological rationale of having both end product repression and direct activation by growth stimulatory cues is discussed.

Red 25, a protein that binds specifically to the sterol regulatory region in the promoter for 3-hydroxy-3-methylglutaryl-coenzyme A reductase

A protein that binds to the sterol regulatory region of the hamster promoter for 3-hydroxy-3-methylglutaryl-coenzyme A reductase has been identified. All of the DNA bases crucial to the binding of this protein were previously shown to be essential for sterol regulation of the intact promoter in cultured cells. This low abundance protein, called Red 25, has been purified from nuclear extracts of hamster liver by a series of standard chromatographic techniques coupled with a DNA affinity step. Its size has been estimated as approximately 42 kDa by gel electrophoresis, size exclusion chromatography, and protein-DNA cross-linking studies. Furthermore, it binds to its target site with a Kd = 6 x 10(-11) M. Red 25 does not bind to the sterol regulatory regions of the LDL receptor or 3-hydroxy-3-methylglutaryl-coenzyme A synthase. This is consistent with recent studies that show there is a unique site for sterol regulation in the reductase promoter. The identification and purification of this protein represents a significant step in the study of feedback regulation by cholesterol.

Single nucleotide resolution of sterol regulatory region in promoter for 3-hydroxy-3-methylglutaryl coenzyme A reductase

Sterol-dependent regulation of the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase promoter was previously localized to a 42-base pair region containing an octamer sequence, referred to as the sterol regulatory element (SRE-1). A similar motif is found in the region of DNA that is required for sterol-dependent regulation of the HMG-CoA synthase and low density lipoprotein receptor genes. Single nucleotide substitution analyses of the low density lipoprotein receptor and HMG-CoA synthase promoters confirmed that the SRE-1 is an important sterol regulatory motif. In the current studies, a series of single nucleotide mutations were introduced into the HMG-CoA reductase regulatory region and transfected into Chinese hamster ovary cells. RNA produced by each mutant promoter was then measured in the presence or absence of sterols. Thirty-seven independent mutations were analyzed, and two separate domains were identified as being critical. One essential region was spread over 10 bases and contained half of the SRE-1; however, the other half of the SRE-1 was not important for sterol regulation. The second essential region spanned four contiguous bases. These two critical elements are separated from each other by three nonessential bases. The results are interpreted to suggest that regulation of HMG-CoA reductase gene transcription by sterols requires additional or possibly separate factors from those required for sterol regulation of the low density lipoprotein receptor and HMG-CoA synthase promoters.

NF-I proteins from brain interact with the proenkephalin cAMP inducible enhancer

A short region of the human proenkephalin promoter has been shown previously to mediate transcriptional regulation in response to activation of the cAMP, TPA, and Ca+ + dependent intracellular signalling pathways. Two adjacent DNA elements, CRE-1 and CRE-2, are essential for this regulation although neither element alone is sufficient for inducible expression. The CRE-2 element consists of overlapping binding sites for the transcription factors AP-1 and AP-4. The CRE-1 element has been shown to interact with a DNA binding factor called ENKTF-1. Here we characterize proteins from bovine brain which bind the CRE-1 element of the human proenkephalin gene. Interactions between proteins binding the CRE-1 and CRE-2 elements are characterized in vitro using affinity purified DNA binding proteins. We demonstrate that CRE-1 binding proteins from bovine brain consist of three different polypeptides each belonging to the NF-I family of transcription factors. Point mutation analysis of the contacts of these proteins with the CRE-1 element indicate that NF-I proteins contact the inducible enhancer at the sequence CTGGCxxxxxxCCT which overlaps the CRE-1 element (underlined) defined by in vivo point mutation analysis. Cotransfection of one of the three NF-I proteins purified from bovine brain, NF-I/Red1, together with a proenkephalin/bacterial chloramphenicol acetyl transferase (CAT) fusion gene repressed protein kinase A or forskolin stimulated CAT expression.

Identification of nucleotides responsible for enhancer activity of sterol regulatory element in low density lipoprotein receptor gene

Sterol-dependent regulation of the low density lipoprotein (LDL) receptor promoter has been localized previously to a 16-base pair sequence, designated repeat 2, in the 5'-flanking region of the gene. In the current study, we show that the central 10 nucleotides of repeat 2 are crucial for the sterol regulatory activity. This sequence includes an octamer, designated sterol regulatory element 1 (SRE-1), which was identified previously in the promoter of the gene for 3-hydroxy-3-methylglutaryl coenzyme A synthase, a sterol-regulated enzyme of cholesterol biosynthesis. We made a series of single-base substitutions within a 1471-base pair fragment of the intact LDL receptor promoter, introduced the mutant plasmids into hamster cells by transfection, and measured mRNA levels in the absence and presence of sterols. Substitutions within the 10-base pair sequence in repeat 2 largely prevented the induction of transcription which occurs in the absence of sterols. None of these point mutations affected transcription in the presence of sterols. Like an enhancer, the SRE-1 in repeat 2 functioned in an orientation-independent manner. We interpret these findings to indicate that the SRE-1 of the LDL receptor promoter is a conditional positive element that cooperates with other elements to enhance transcription in the absence of sterols and loses its function in the presence of sterols.

Purification of a protein doublet that binds to six TGG-containing sequences in the promoter for hamster 3-hydroxy-3-methylglutaryl-coenzyme A reductase

The gene for 3-hydroxy-3-methylglutaryl-coenzyme A reductase, the rate-controlling enzyme of cholesterol biosynthesis, is transcribed at a relatively high level when cellular sterols are depleted and is repressed when sterols accumulate. We have previously reported that the regulatory region of the hamster reductase gene contains eight different sequences that bind nuclear proteins as determined by DNase I footprinting assays. We here report the purification of a single activity that accounts for six of these footprints. This activity was found in a doublet of proteins (designated reductase promoter factor 1, RPF-1) that have apparent molecular weights of 33,000 and 35,000. They were isolated by DNA affinity chromatography using oligonucleotides corresponding to either of two footprinted sequences. The 33- and 35-kDa species were present as monomers, as indicated by gel filtration and gradient ultracentrifugation. Oligonucleotides corresponding to any one of the six footprinted sequences prevented the binding of RPF-1 to all of the other sequences, indicating that all six bind to a single site in RPF-1. The only sequence shared by all six footprinted sequences is the trinucleotide, TGG, both of whose guanosines made contact with RPF-1, as determined by methylation interference assays. The footprinted sequence that binds RPF-1 with highest affinity contains the palindrome, TGG(N7)CCA, which conforms to the consensus sequence for binding NF-1, a nuclear protein that stimulates replication of adeno-virus-2. Purified RPF-1 was shown to bind to the adenovirus NF-1 binding site with high affinity. Although the apparent molecular weight of the RPF-1 doublet was lower than the molecular weight range for NF-1 proteins (52,000-66,000), it is likely that the 33-35-kDa doublet is derived from a larger NF-1-like protein as a result of proteolysis. We conclude that RPF-1 belongs to a group of TGG-binding proteins that includes NF-1 and other proteins previously described as CCAAT binding proteins. This protein binds to six sites in the promoter region for hamster 3-hydroxy-3-methylglutaryl CoA reductase, where its function remains to be determined.

Multiple sterol regulatory elements in promoter for hamster 3-hydroxy-3-methylglutaryl-coenzyme A synthase

Through substitution mutagenesis and gene transfer experiments in cultured cells, we have identified three sequences in the 5' flanking region of the gene for hamster 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase that are required for sterol-mediated regulation of transcription. Point mutations in any one of these sequences largely prevented the increase in transcription that normally follows cellular sterol depletion. These mutations did not alter the low level of transcription that occurs in the presence of sterols. Two of the three sterol regulatory sequences contain an octanucleotide that shows a 7/8-base pair match with a sequence that was previously identified as a sterol regulatory element in the genes for HMG-CoA reductase and the low density lipoprotein receptor, both of which are induced by sterol deprivation. The third sterol regulatory region in the HMG-CoA synthase promoter shows only a low-level match with the other sterol regulatory elements. The current data suggest that the sterol regulatory elements in the HMG-CoA synthase promoter operate by a conditional positive mechanism: in the absence of sterols, regulatory proteins bind to these elements and stimulate transcription; in the presence of sterols, the regulatory proteins are inactivated and transcription decreases to the basal rate.

Multiple genes encode nuclear factor 1-like proteins that bind to the promoter for 3-hydroxy-3-methylglutaryl-coenzyme A reductase

DNA-binding proteins of the nuclear factor 1 (NF1) family recognize sequences containing TGG. Two of these proteins, termed reductase promoter factor (RPF) proteins A and B, bind to the promoter for hamster 3-hydroxy-3-methylglutaryl-coenzyme A reductase, a negatively regulated enzyme in cholesterol biosynthesis. In the current study, we determined the sequences of peptides derived from hamster RPF proteins A and B and used this information to isolate a cDNA, designated pNF1/Red1, that encodes RPF protein B. The peptide sequence of RPF protein A, the other reductase-related protein, suggests that it is the hamster equivalent of NF1/L, which was previously cloned from rat liver. We also isolated a hamster cDNA for an additional member of the NF1 family, designated NF1/X. Thus, the hamster genome contains at least three genes for NF1-like proteins. It is likely to contain a fourth gene, corresponding to NF1/CTF, which was previously cloned from the human. The NH2-terminal sequences of all four NF1-like proteins (NF1/Red1, NF1/L, NF1/X, and NF1/CTF), which are virtually identical, contain the DNA-binding domain that recognizes TGG. Functional diversity may arise from differences in the COOH-terminal sequences. We hypothesize that the COOH-terminal domain interacts with adjacent DNA-binding proteins, thereby stabilizing the binding of a particular NF1-like protein to a particular promoter. This protein-protein interaction confers specificity to a class of proteins whose DNA-recognition sequence is widespread in the genome. Sterols may repress transcription of the reductase gene by disrupting this protein-protein interaction.

Operator constitutive mutation of 3-hydroxy-3-methylglutaryl coenzyme A reductase promoter abolishes protein binding to sterol regulatory element

Through substitution mutagenesis we identified the promoter elements responsible for basal expression and sterol-mediated repression of transcription of the gene for 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-controlling enzyme of cholesterol biosynthesis. Mutant promoters containing 277 base pairs (bp) of reductase 5' flanking sequence were inserted into recombinant plasmids upstream of the coding region for bacterial chloramphenicol acetyltransferase. The plasmids were transfected into hamster fibroblasts, and transcription was measured in the presence and absence of sterols. Mutations in three regions that are known to bind nuclear proteins markedly reduced transcription. Mutation of another protein-binding region of 20 bp in length did not reduce transcription, but it did abolish sterol-mediated repression, producing an operator constitutive phenotype. This mutation also abolished protein binding to the corresponding 20-bp region of DNA as determined by footprinting assays. When a DNA fragment containing these 20 bp was inserted into the herpes simplex virus thymidine kinase promoter, sterol-mediated repression was observed. This sequence contains an octanucleotide that shows a 7/8-bp match with a previously identified regulatory sequence in repeat 2 of the low density lipoprotein receptor promoter, another sterol-repressible gene. We hypothesize that this octanucleotide, GTGGCGGTG, is the core binding site for a sterol-dependent protein that represses transcription.

Identification of promoter elements required for in vitro transcription of hamster 3-hydroxy-3-methylglutaryl coenzyme A reductase gene

The 5'-flanking region of the gene for hamster 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) is shown to contain promoter sequences that drive transcription in vitro in the presence of a HeLa whole-cell extract. DNase I protection studies revealed at least six different regions within the 277-base-pair (bp) promoter that bind nuclear proteins and produce "footprints." The functional significance of these sequences was determined through transcriptional analysis of a series of substitution mutations that scrambled short sequences throughout this region. Two of the footprint sequences were crucial for transcription in vitro; one of these contains a match in 6 of 6 bp, with a sequence in the adenovirus type 2 major late promoter that is known to be required for transcription. Scrambling a 26-bp sequence in a third footprint led to a consistent 2-fold increase in transcription, suggesting that this sequence might be a site for negative regulation. These studies define three regions that play a role in regulating transcription of the gene for HMG-CoA reductase, a negatively regulated enzyme in the cholesterol biosynthetic pathway.

5' end of HMG CoA reductase gene contains sequences responsible for cholesterol-mediated inhibition of transcription

Cholesterol homeostasis is maintained by feedback inhibition of transcription of the gene encoding HMG CoA reductase. To study this mechanism, we joined the 5' end of the hamster reductase gene to the coding region for chloramphenicol acetyltransferase (CAT). The chimeric gene produced high levels of CAT activity in mouse L cells; sterols suppressed expression by 70% to 90%. Sequences responsible for both promotion and inhibition of transcription were distributed over 500 bp extending 300 bp upstream of the reductase transcription initiation sites. Any sizable deletion within this region decreased CAT expression in vivo and CAT mRNA transcription in vitro. This region contains five hexanucleotide repeats (CCGCCC or GGGCGG) that occur in promoters of viral and cellular housekeeping genes. Every reductase-CAT plasmid that showed transcriptional activity also showed inhibition by sterols, indicating that the sites for promotion and inhibition of transcription are closely associated.

HMG CoA reductase: a negatively regulated gene with unusual promoter and 5' untranslated regions

The rate-limiting enzyme of cholesterol biosynthesis, HMG CoA reductase, is controlled by negative feedback regulation of transcription. We have isolated the reductase gene from a bacteriophage lambda genomic library prepared from hamster UT-1 cells. The 25 kilobase gene is split into 20 exons. The 5' untranslated and promoter regions differ from those of previously characterized genes. The 5' untranslated region encompasses as many as 670 nucleotides; contains up to eight AUG codons upstream of the codon used to initiate translation; and has multiple transcription initiation sites as determined by S1 nuclease mapping and primer extension analysis. The promoter region lacks a characteristic TATA box and CCAAT box; is rich in G + C residues (65%); and contains repeat sequences homologous to the 21 base pair repeats of the SV40 promoter. These unusual features may be relevant to the mechanism of expression of "housekeeping" genes, particularly those that are subject to negative feedback regulation.

Transcription control region within the protein-coding portion of adenovirus E1A genes

A single-base deletion within the protein-coding region of the adenovirus type 5 early region 1A (E1A) genes, 399 bases downstream from the transcription start site, depresses transcription to 2% of the wild-type rate. Complementation studies demonstrated that this was due to two effects of the mutation: first, inactivation of an E1A protein, causing a reduction by a factor of 5; second, a defect which acts in cis to depress E1A mRNA and nuclear RNA concentrations by a factor of 10. A larger deletion within the protein-coding region of E1A which overlaps the single-base deletion produces the same phenotype. In contrast, a linker insertion which results in a similar truncated E1A protein does not produce the cis-acting defect in E1A transcription. These results demonstrate that a critical cis-acting transcription control region occurs within the protein coding sequence in adenovirus type 5 E1A. The single-base deletion occurs in a sequence which shows extensive homology with a sequence from the enhancer regions of simian virus 40 and polyomavirus. This region is not required for E1A transcription during the late phase of infection.

Far upstream initiation sites for adenovirus early region 1A transcription are utilized after the onset of viral DNA replication

Adenovirus early region 1A (E1A) is the first transcription unit expressed after infection. It encodes a protein which controls the expression of all other early viral genes. The E1A mRNAs have one major capped 5' terminus which maps 31 nucleotides downstream from a T-A-T-A sequence (C. Baker and E. Ziff. J. Mol. Biol. 149:189-221, 1981). In addition, a minor set of E1A mRNAs are observed during the early phase of infection which have 5' termini mapping at approximately -160, -185, and -230 relative to the major cap site (Osborne et al., Cell 29:139-148, 1982). Here we report the occurrence of another set of minor E1A mRNAs which were observed exclusively after the initiation of viral DNA replication. These late specific E1A mRNAs had cap sites which mapped at approximately -300, -325, -360, and -375 relative to the major cap site. The appearance of these minor late E1A mRNAs was blocked by the DNA synthesis inhibitor cytosine arabinoside. These same late specific E1A mRNAs were synthesized from E1A-containing plasmids which replicate in monkey cells. This demonstrated that neither late specific adenovirus proteins nor adenovirus-specific chromatin structure was required for the production of the late specific E1A mRNAs. Adenovirus mutants in which the E1A T-A-T-A box region had been deleted also synthesized the corresponding deleted forms of the late specific mRNAs after initiation of DNA replication. These results indicate that the process of DNA replication alters the specificity of E1A transcription initiation in a promoter region which is at least 375 nucleotides in length.

The TATA homology and the mRNA 5' untranslated sequence are not required for expression of essential adenovirus E1A functions

Adenovirus E1A encodes a protein that facilitates the transcription of other early viral transcriptional units and is required for virus-induced transformation. To study the function of non-protein-coding DNA sequence at the 5' end of this transcriptional unit, we constructed mutant viruses with deletions in this region. Deletion of sequence just upstream from the TATA homology does not affect the level or sequence of E1A mRNAs. Deletion of the TATA homology decreases the level of E1A mRNAs by a factor of 5-10 and shifts the mRNA 5' ends from the major 5' end found in wild-type transcripts to a set of minor ends found in wild-type E1A mRNAs. This suggests that the TATA homology is required for an efficient transcription initiation mechanism, and that in its absence a less efficient, less precise mechanism is unmasked. Analysis of mRNAs from other early regions establishes that E2 and E3 regions are most dependent on E1A functions for expression of maximal mRNA levels, E4 is less dependent and E1B is the least dependent. Deletion of the TATA homology, a sequence highly conserved among human adenoviruses, and the entire 5' untranslated sequence of the E1A mRNAs decreases neither the rate of virus replication in a host in which E1A expression is required, nor the efficiency of transformation of rat embryo cells.

Mapping a eukaryotic promoter: a DNA sequence required for in vivo expression of adenovirus pre-early functions

This study defines a DNA sequence upstream from the mRNA cap site required for in vivo expression of the adenovirus 2 pre-early region. The adenovirus 2 pre-early region and flanking sequences were cloned in Escherichia coli plasmid pBR322. Derivatives of the plasmid lacking portions of the upstream viral DNA sequence were constructed. An assay was devised to test the ability of these plasmid DNAs to complement an adenovirus 5 mutant with a deletion in the pre-early region. Plasmids that retained at least 38 base pairs upstream from the mRNA cap site had complementing activity similar to that of the original plasmid, which contains 229 base pairs of upstream viral sequence. However, plasmids retaining 23 or fewer base pairs of viral sequence upstream from the cap site had significantly reduced complementing activity. These results indicate that a portion of the adenovirus 2 sequence between 23 and 38 base pairs upstream from the mRNA cap site is required for expression of the pre-early region. This interval includes the Goldberg-Hogness box-like sequence T-A-T-T-T-A-T-A.