SREBP-1, a basic-helix-loop-helix-leucine zipper protein that controls transcription of the low density lipoprotein receptor gene

DFF, a heterodimeric protein that functions downstream of caspase-3 to trigger DNA fragmentation during apoptosis

We have identified and purified from HeLa cytosol a protein that induces DNA fragmentation in coincubated nuclei after it is activated by caspase-3. This protein, designated DNA Fragmentation Factor (DFF), is a heterodimer of 40 kDa and 45 kDa subunits. The amino acid sequence of the 45 kDa subunit, determined from its cDNA sequence, reveals it to be a novel protein. Caspase-3 cleaves the 45 kDa subunit at two sites to generate an active factor that produces DNA fragmentation without further requirement for caspase-3 or other cytosolic proteins. In cells undergoing apoptosis, the 45 kDa subunit is cleaved in the same pattern as it is cleaved by caspase-3 in vitro. These data delineate a direct signal transduction pathway during apoptosis: caspase-3 to DFF to DNA fragmentation.

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.

Identification of glycerol-3-phosphate acyltransferase as an adipocyte determination and differentiation factor 1- and sterol regulatory element-binding protein-responsive gene

We demonstrate that the mRNA levels of glycerol-3-phosphate acyltransferase (GPAT), a mitochondrial enzyme catalyzing the initial step in glycerolipid synthesis, are induced during the differentiation of 3T3-L1 preadipocytes to adipocytes and following ectopic expression of rat adipocyte determination and differentiation factor 1 (ADD1), a protein with high homology to the human sterol regulatory element-binding protein-1 (SREBP-1). The increase in GPAT mRNA levels that occurs during differentiation is partially prevented by ectopic expression of a dominant negative form of ADD1. Nucleotide sequences corresponding to the proximal promoter of the murine mitochondrial GPAT gene (Jerkins, A. A., Liu, W. R., Lee, S., and Sul, H. S. (1995) J. Biol. Chem. 270, 1416-1421) bound SREBP-1a and NF-Y in electromobility shift assays. In addition, GPAT promoter-luciferase reporter genes were stimulated by co-expression of SREBP-1a. This increase was attenuated when either a dominant negative form of NF-Y was co-transfected into the cells or when the GPAT promoter contained mutations in the putative binding sites for SREBP-1a or NF-Y. These studies demonstrate that the regulated expression of the mitochondrial GPAT gene requires both NF-Y and ADD1/SREBPs. Thus, SREBPs/ADD1 regulate not only genes involved in cholesterol homeostasis and fatty acid synthesis but also a key enzyme in glycerolipid synthesis.

Carbohydrate regulation of hepatic gene expression. Evidence against a role for the upstream stimulatory factor

Hepatic expression of the genes encoding L-type pyruvate kinase (L-PK) and S14 is induced in rats upon feeding them a high carbohydrate, low fat diet. A carbohydrate response element (ChoRE) containing two CACGTG-type E boxes has been mapped in the 5'-flanking region of both of these genes. The nature of the ChoRE suggests that a member of the basic/helix-loop-helix/leucine zipper family of proteins may be responsible for mediating the response to carbohydrate. Indeed, the upstream stimulatory factor (USF), a ubiquitous basic/helix-loop-helix/leucine zipper protein, is present in hepatic nuclear extracts and binds to the ChoREs of L-PK and S14 in vitro. We have conducted experiments to determine whether USF is involved in the carbohydrate-mediated regulation of L-PK and S14. For this purpose, dominant negative forms of USF that are capable of heterodimerizing with endogenous USF but not of binding to DNA were expressed in primary hepatocytes. Expression of these forms did not block either S14 or L-PK induction by glucose. In addition, we have constructed mutant ChoREs that retain their carbohydrate responsiveness but have lost the ability to bind USF. Together, these data suggest that USF is not the carbohydrate-responsive factor that stimulates S14 and L-PK expression and that a distinct hepatic factor is likely to be responsible for the transcriptional response.

Structure of the human gene encoding sterol regulatory element binding protein 2 (SREBF2)

Sterol-regulatory element binding protein (SREBP) 1 and SREBP2 are ubiquitously expressed transcription factors that play key roles in the regulation of cholesterol and fatty acid metabolism. SREBP1 and SREBP2 share approximately 47% sequence identity and map to chromosomes 17 and 22, respectively. The gene encoding SREBP1 (SREBF1) has been cloned and characterized. In this paper we describe the gene structure and 5'-flanking sequence of SREBF2. SREBF2 spans 72 kb and is composed of 19 exons and 18 introns. The locations of the exon/intron boundaries of SREBF2 are remarkably similar to those of SREBF1, but SREBF2 is approximately 2.8 times larger in size. The 5'-flanking regions of SREBF2 and of two alternatively spliced forms of SREBF1, SREBF1a and SREBF1c, were sequenced, and the SREBF2 transcription start site was determined. A perfect 10-bp sterol regulatory element (SRE)-1 sequence was present in the promoter region of SREBF2. No SRE-1 was identified in the 5'-flanking sequences of either SREBF1a or SREBF1c, but several E-box sequences were present in SREBP1c. Thus, analysis of the 5'-flanking regions provides support that these two transcription factors, though similar in their coding sequence and overall gene structure, have different physiological roles. Finally, evidence is presented for the presence of another expressed gene of unknown function located 500 bp upstream of SREBF2.

Identification of a novel cis-acting element participating in maximal induction of the human low density lipoprotein receptor gene transcription in response to low cellular cholesterol levels

In this paper, we present both in vivo and in vitro evidence for the presence of a novel cis-acting regulatory element that is required for maximal induction of the human low density lipoprotein (LDL) receptor gene following depletion of cellular sterols in HepG2 cells. First, in vivo dimethyl sulfate footprinting of the human LDL receptor promoter before and after transcriptional induction in HepG2 cells revealed protection from -145 to -126, 5'-GAGCTTCACGGGTTAAAAAG-3' (referred to as FP1 site). Second, transient transfections of HepG2 cells with promoter luciferase reporter constructs containing the FP1 site resulted in significant enhancement (approximately 375%) of reporter gene expression in response to low levels of sterols compared with parallel plasmid without the FP1 site. In addition, this response was markedly attenuated on nucleotide substitutions within the FP1 site. Third, by electrophoretic mobility shift assays, the FP1 sequence was found to bind protein(s) from HepG2 nuclear extracts in a sequence-specific manner. In vitro binding of the FP1 mutants paralleled the results obtained for their in vivo transcription. On the basis of competition profiles, the FP1-binding factor is different from the known transcription factors binding to the AT-rich CArG and GArC motifs. Furthermore, the FP1-binding protein is not specific to HepG2 cells because nuclear factor(s) with the same specificity was observed in nuclear extracts of non-hepatic HeLa cells. We conclude that transcriptional induction of the LDL receptor gene in response to sterol depletion is mediated, in part, by an highly conserved novel cis-acting element through the binding of specific nuclear protein(s).

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.

Human cholesteryl ester transfer protein gene proximal promoter contains dietary cholesterol positive responsive elements and mediates expression in small intestine and periphery while predominant liver and spleen expression is controlled by 5'-distal sequences. Cis-acting sequences mapped in transgenic mice

The plasma cholesteryl ester transfer protein (CETP) facilitates the transfer of high density lipoprotein cholesteryl esters to other lipoproteins and appears to be a key regulated component of reverse cholesterol transport. Earlier studies showed that a CETP transgene containing natural flanking sequences (-3.4 kilobase pairs (kbp) upstream, +2.2 kbp downstream) was expressed in an authentic tissue distribution and induced in liver and other tissues in response to dietary or endogenous hypercholesterolemia. In order to localize the DNA elements responsible for these effects, we prepared transgenic mice expressing six new DNA constructs containing different amounts of natural flanking sequence of the CETP gene. Tissue-specific expression and dietary cholesterol response of CETP mRNA were determined. The native pattern of predominant expression in liver and spleen with cholesterol induction was shown by a -3.4 (5'), +0.2 (3') kbp transgene, indicating no major contribution of distal 3'-sequences. Serial 5'-deletions showed that a -570 base pairs (bp) transgene gave predominant expression in small intestine with cholesterol induction of CETP mRNA in that organ, and a -370 bp transgene gave highest expression in adrenal gland with partial dietary cholesterol induction of CETP mRNA and plasma activity. Further deletion to -138 bp 5'-flanking sequence resulted in a transgene that was not expressed in vivo. Both the -3.4 kbp and -138 bp transgenes were expressed when transfected into a cultured murine hepatocyte cell line, but only the former was induced by treating the cells with LDL. When linked to a human apoA-I transgene, the -570 to -138 segment of the CETP gene promoter gave rise to a relative positive response of hepatic apoA-I mRNA to the high cholesterol diet in two out of three transgenic lines. Thus, 5'-elements between -3,400 and -570 bp in the CETP promoter endow predominant expression in liver and spleen. Elements between -570 and -370 are required for expression in small intestine and some other tissues, and elements between -370 and -138 contribute to adrenal expression. The minimal CETP promoter element associated with a positive sterol response in vivo was found in the proximal CETP gene promoter between -370 and -138 bp. This region contains a tandem repeat of a sequence known to mediate sterol down-regulation of the HMG-CoA reductase gene, suggesting either the presence of separate positive and negative sterol response elements in this region or the use of a common DNA element for both positive and negative sterol responses.

Recurrent G-to-A substitution in a single codon of SREBP cleavage-activating protein causes sterol resistance in three mutant Chinese hamster ovary cell lines

Oxygenated sterols such as 25-hydroxycholesterol kill Chinese hamster ovary cells because they inhibit the proteolytic processing of sterol regulatory element binding proteins (SREBPs), a pair of membrane-bound transcription factors that activate genes controlling cholesterol synthesis and uptake from lipoproteins. The unprocessed SREBPs remain membrane-bound, they cannot activate the cholesterol biosynthetic pathway, and the cells die of cholesterol deprivation. Several sterol-resistant hamster cell lines have been isolated previously by chemical mutagenesis and selection for resistance to killing by 25-hydroxycholesterol. We recently identified the defect in one such cell line (25-RA cells) as a point mutation in a newly discovered membrane protein of 1276 amino acids, designated SREBP cleavage-activating protein (SCAP). The mutation in the 25-RA cells resulted from a G-to-A transition in codon 443 of the SCAP gene, changing aspartic acid to asparagine. Wild-type SCAP, when overexpressed by transfection, stimulates the proteolytic processing of both SREBPs. The D443N substitution is an activating mutation that increases the activity of SCAP and renders it resistant to inhibition by 25-hydroxycholesterol. We here report the identical G-to-A transition in two additional lines of Chinese hamster ovary cells that were mutagenized and isolated by a similar protocol. The three mutations occurred independently as indicated by haplotype analysis of the mutant genes using two intragenic sequence polymorphisms. All three cell lines were mutagenized with alkylating agents (nitrosoethylurea or ethylmethane sulfonate) that favor G-to-A transitions. Nevertheless, the finding of the same nucleotide substitution at the same location in all three cell lines indicates that SCAP may be unique in its ability to stimulate SREBP cleavage, and residue 443 is a crucial determinant of the protein's ability to be inhibited by 25-hydroxycholesterol.

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

Through expression cloning we have isolated a cDNA-encoding SREBP cleavage-activating protein (SCAP), which regulates cholesterol metabolism by stimulating cleavage of transcription factors SREBP-1 and -2, thereby releasing them from membranes. The cDNA was isolated from Chinese hamster ovary cells with a dominant mutation that renders them resistant to sterol-mediated suppression of cholesterol synthesis and uptake. Sterol resistance was traced to a G-->A transition at codon 443 of SCAP, changing aspartic acid to asparagine. The D443N mutation enhances the cleavage-stimulating ability of SCAP and renders it resistant to inhibition by sterols. SCAP has multiple membrane-spanning regions, five of which resemble the sterol-sensing domain of HMG CoA reductase, an endoplasmic reticulum enzyme whose degradation is accelerated by sterols. SCAP appears to be a central regulator of cholesterol metabolism in animal cells.

Sterol-dependent transcriptional regulation of sterol regulatory element-binding protein-2

We show in this manuscript that expression of the mRNA for sterol regulatory element-binding protein-2 (SREBP-2) is regulated by the cellular sterol level in cultured HeLa cells. We have cloned the 5'-flanking region of the gene encoding human SREBP-2. Characterization of this region shows the minimum 50-base pair segment, which contains a 10-base pair sterol regulatory element 1 (SRE-1) identical to the one in the human LDL receptor promoter, confers sterol responsiveness when fused to the luciferase reporter gene. Enforced expression of the truncated SREBP-2 protein (amino acid residues 1-481) also shows that this upstream segment contains the information required for transcriptional activation. The luciferase assays using mutant versions of the reporter genes reveal that the sterol-dependent transcriptional regulation is mediated by two nearby motifs, the SRE-1 and the NF-Y binding site (the inverted CCAAT box, ATTGGC); the latter is reported to play a critical role in sterol-dependent regulation of 3-hydroxy-3-methylglutaryl-coenzyme A synthase and farnesyl diphosphate synthase genes (Jackson, S. M., Ericsson, J., Osborne, T. F., and Edwards, P. A. (1995) J. Biol. Chem. 270, 21445-21448). Gel mobility shift assays demonstrate that the transcription factor NF-Y truly binds to the ATTGGC sequence. These findings suggest that the activity of SREBP-2 is controlled not only post-translationally by proteolytic activation of the precursor protein but also transcriptionally by itself together with NF-Y.

Synergistic binding of sterol regulatory element-binding protein and NF-Y to the farnesyl diphosphate synthase promoter is critical for sterol-regulated expression of the gene

Sterol-regulated transcription of the farnesyl diphosphate (FPP) synthase gene is dependent on two cis elements in the proximal promoter. These elements, an inverted CCAAT box and sterol regulatory element 3 (SRE-3), bind NF-Y and sterol regulatory element-binding protein 1 (SREBP-1), respectively. We now demonstrate that the binding of recombinant SREBP-1 to its cognate site (SRE-3) within the FPP synthase promoter in vitro is enhanced by binding of NF-Y to the upstream inverted CCAAT box. Using an FPP synthase promoter fragment containing the binding sites for both NF-Y and SREBP-1 in gel mobility shift assays, we demonstrate that the addition of NF-Y increases the binding of SREBP-1 to SRE-3 over 20-fold. In contrast, NF-Y does not stimulate the binding of SREBP-1 to SRE-3 when the inverted CCAAT box is either mutated or 4 base pairs (bp) are inserted between the inverted CCAAT box and SRE-3. Promoter-reporter genes, containing either the wild-type FPP synthase promoter sequence or containing the 4-bp insertion between the inverted CCAAT box and SRE-3, were transiently transfected into cells. The activity of the wild-type promoter-reporter gene increased when the cells were either incubated in sterol-depleted medium or were co-transfected with an expression vector encoding transcriptionally active SREBP-1. This increase in activity was attenuated when the promoter contained the 4-bp insert, consistent with defective binding of SREBP to the promoter in vivo. These studies suggest that the binding of SREBP-1 to SRE-3 in the FPP synthase promoter, and subsequent stimulation of transcription, is dependent on synergistic binding and a functional interaction between SREBP-1 and NF-Y.

Site-directed mutagenesis of human prostacyclin synthase: Alteration of Cys441 of the Cys-pocket, and Glu347 and Arg350 of the EXXR motif

The possible active site Cys441 in the Cys-pocket and Glu347 and Arg350 of the EXXR motif of the human prostacyclin synthase, which catalyzes the conversion of prostaglandin H2 to prostacyclin, were subjected to site-directed mutagenesis in order to understand the role of these residues in expressing the enzymatic activity. Five expression vectors encoding the mutant enzymes with a single replacement, Cys441 Ala, Cys441 Ser, Cys441 His, Glu347 Ala and Arg350 Ala, as well as the wild-type enzyme were expressed in 293 cells. The microsomal fraction of the cells expressing the wild-type enzyme showed a specific activity of 96 nmol 6-keto-PGF1alpha/min per mg protein. All of the mutant enzymes examined showed no detectable enzyme activity, although immunoblot analysis demonstrated that levels of all the expressed mutant enzymes were similar to that of the wild-type enzyme. These results indicated that the Cys441 in the Cys-pocket, and Glu347 and Arg350 of the EXXR motif of human prostacyclin synthase are important for expressing the enzymatic activity.

Sterol-regulated release of SREBP-2 from cell membranes requires two sequential cleavages, one within a transmembrane segment

Sterol regulatory element binding proteins (SREBPs) are transcription factors attached to the endoplasmic reticulum. The NH2-segment, which activates transcription, is connected to membranes by a hairpin anchor formed by two transmembrane sequences and a short lumenal loop. Using H-Ras-SREBP-2 fusion proteins, we show that the NH2-segment is released from membranes by two sequential cleavages. The first, regulated by sterols, occurs in the lumenal loop. The second, not regulated by sterols, occurs within the first transmembrane domain. The liberated NH2-segment enters the nucleus and activates genes controlling cholesterol synthesis and uptake. Certain mutant Chinese hamster ovary cells are auxotrophic for cholesterol because they fail to carry out the second cleavage; the NH2-segment remains membrane-bound and transcription is not activated.

Complementation of mutation in acyl-CoA:cholesterol acyltransferase (ACAT) fails to restore sterol regulation in ACAT-defective sterol-resistant hamster cells

A previously described mutant line of Chinese hamster ovary cells, designated SRD-4, fails to synthesize cholesteryl esters, owing to a deficiency in the activity of acyl-CoA:cholesterol acyltransferase (ACAT). These cells also fail to suppress low density lipoprotein receptors or cholesterol synthesizing enzymes in the presence of 25-hydroxycholesterol. In the current studies we show that SRD-4 cells have three defects: 1) a point mutation in one allele at the ACAT locus that changes codon 265 from Ser to Leu, resulting in an inactive enzyme; 2) a silent allele at the other ACAT locus that does not produce detectable mRNA; and 3) a mutation, as yet undefined, that abolishes the ability of 25-hydroxycholesterol to inhibit the cleavage of both sterol regulatory element binding proteins (SREBP-1 and SREBP-2). Correction of the ACAT deficiency by transfection of a wild-type cDNA failed to restore inhibition of SREBP cleavage by 25-hydroxycholesterol, indicating that the ACAT deficiency and the sterol regulatory defect are caused by independent mutations. These data provide further insight into the interplay between ACAT activation and inhibition of SREBP cleavage by 25-hydroxycholesterol, and they indicate that these two processes can be disrupted independently by mutation.

Activation of acyl-coenzyme A:cholesterol acyltransferase activity by cholesterol is not due to altered mRNA levels in HepG2 cells

Many studies have shown that sterols can stimulate acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in cells. To elucidate this mechanism, effects of sterol-mediated induction on both the enzyme activity of ACAT and its mRNA levels were studied in human hepatoblastoma cell line, HepG2 cells. When HepG2 cells were loaded with cholesterol and 25-hydroxycholesterol, both the whole-cell ACAT activity and the microsomal ACAT activity were increased by 85.1% and 41.3%. In contrast, cholesterol depletion of HepG2 cells with compactin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, resulted in a decrease in both the whole-cell and the microsomal ACAT activity by 46.4% and 58.3%. Under identical conditions, RT-PCR and Northern blotting analyses revealed that neither cholesterol loading nor cholesterol depletion of HepG2 cells altered the amounts of ACAT mRNA. Moreover, these treatments had no effect on the enzymatic ACAT activity determined by the reconstituted assay in which HepG2 cell homogenate had been supplemented in vitro with a saturating level of exogenous cholesterol. These results indicate that cholesterol-induced up-regulation of ACAT activity in HepG2 cells does not occur at the level of transcription, but rather at a posttranscriptional level.

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.

Regulated cleavage of sterol regulatory element binding proteins requires sequences on both sides of the endoplasmic reticulum membrane

Sterol regulatory element binding proteins (SREBP-1 and SREBP-2) are attached to the endoplasmic reticulum (ER) and nuclear envelope by a hairpin domain consisting of two transmembrane regions connected by a short lumenal loop of approximately 30 hydrophilic amino acids. In sterol-depleted cells, a protease cleaves the protein in the region of the first transmembrane domain, releasing an NH2-terminal fragment of approximately 500 amino acids that activates transcription of genes encoding the low density lipoprotein receptor and enzymes of cholesterol synthesis. In sterol-overloaded cells, proteolysis does not occur, and transcription is repressed. Through mutational analysis in transfected cells, we identify two segments of SREBPs that are required for proteolysis, one on either side of the ER membrane. An arginine in the lumenal loop is essential. A tetrapeptide sequence (DRSR) on the cytosolic face adjacent to the first transmembrane domain is also required for maximal cleavage. Both of these elements are conserved in the human and hamster versions of SREBP-1 and SREBP-2. Sterol-mediated suppression of cleavage of SREBP-1 was found to be dependent on the extreme COOH-terminal region (residue 1034 to the COOH terminus), which exists in two forms as a result of alternative splicing. The form encoded by the "a" class exons (exons 18a and 19a) undergoes sterol-regulated cleavage. The form encoded by the "c" class exons (18c and 19c) is cleaved less efficiently and is not suppressed by sterols. These studies were made possible through use of a vector that achieves low level expression of epitope-tagged SREBPs under control of the relatively weak thymidine kinase promoter from herpes simplex virus. In contrast to SREBPs overproduced by high level expression vectors, the SREBPs produced at low levels were subject to the same regulated cleavage pattern as the endogenous SREBPs. These results indicate that sterol-regulated proteolysis of SREBPs is a complex process, requiring sequences on both sides of the ER membrane.

Transcriptional regulation of squalene epoxidase by sterols and inhibitors in HeLa cells

Regulation of squalene epoxidase (SE) gene expression was studied in comparison with those of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase and low density lipoprotein (LDL) receptor. An increased expression of SE mRNA and protein content in mouse L929 cells grown in 10% lipoprotein-deficient fetal bovine serum (LPDS) for 48 h was found by performing immunoblot and Northern blot analyses when compared with the culture in the presence of fetal bovine serum (FBS). The same results in mRNA levels were seen using human cell lines HepG2, HeLa, and Chang liver cells. The increase of SE mRNA in HeLa cells grown in LPDS was preventable in a dose-dependent manner by feeding cells with 25-hydroxycholesterol or cholesterol. When an SE inhibitor, NB-598, was fed to HeLa cells grown in LPDS, it caused further increases in mRNA levels of SE, HMG-CoA reductase, and LDL receptor. In contrast, NB-598 had no effect on the message levels of these genes when fed to HeLa cells grown in FBS. These results suggest that sterol produced endogenously can also regulate SE expression at the level of transcription.

SREBP-1 mediates activation of the low density lipoprotein receptor promoter by insulin and insulin-like growth factor-I

Transcription of the low density lipoprotein (LDL) receptor gene is regulated by intracellular cholesterol concentration, hormones, and growth factors. We studied the mechanisms by which insulin and estradiol stimulate promoter activity of the LDL receptor gene. Hormonal effects were analyzed in HepG2 cells after transient transfection with promotor reporter gene constructs. Successive 5' deletions of the LDL receptor promoter fragment from -537 to +88 revealed the sterol regulatory element 1 (SRE-1) between -65 and -56 as an insulin- and estradiol-sensitive cis-element. If the SRE-1 is point mutated at position -59 (C to G), which abolishes the binding of the SRE binding proteins (SREBP-1 and SREBP-2), no insulin or estradiol stimulatory effect on reporter gene expression was observed, indicating a role of SRE binding proteins in this regulatory mechanism. The concentration of the 125-kDa membrane-integrated SREBP-1 precursor protein in LDL repressed HepG2 cells is not altered by hormone treatment. Concentrations of SREBP-1 mRNA and precursor protein are reduced significantly by high and stable expression of an SREBP-1 antisense cDNA fragment in HepG2 cells (SREBP1(-) cells). Transfection of SREBP1(-) cells with promoter construct phLDL4 (-105 to +88) reduces induction of reporter gene activity by insulin and insulin-like growth factor-I to 35 and 17%, respectively, compared with HepG2 cells. The stimulatory effect of estradiol remains unchanged, and the inductions by pravastatin are enlarged. We conclude that different regulatory effects converge at SRE-1, but that SREBP-1 is selectively involved in the signal transduction pathway of insulin and insulin-like growth factor-I leading to LDL receptor gene activation.

Transactivation by Rtg1p, a basic helix-loop-helix protein that functions in communication between mitochondria and the nucleus in yeast

Rtg1p is a basic helix-loop-helix transcription factor in the yeast Saccharomyces cerevisiae that is required for basal and regulated expression of CIT2, the gene encoding a peroxisomal isoform of citrate synthase. In respiratory incompetent rho degree petite cells, CIT2 transcription is elevated as much as 30-fold compared with respiratory competent rho + cells. Here we provide evidence that Rtg1p interacts directly with a CIT2 upstream activation site (UASr) and that the rho degree/rho + regulation is not due to a change in the levels of Rtg1p. A fusion protein consisting of the DNA binding domain of Gal4p fused to the NH2 terminus of the full-length wild-type Rtg1p was able to transactivate an integrated LacZ reporter under control of the Gal4p-responsive GAL1 UASG in a rho degree/rho(+)-dependent manner. Other Gal4p fusions to deletions or mutations of Rtg1p indicate that the helix-loop-helix domain is essential for transactivation. Regulated expression of CIT2 also requires the RTG2 gene product. The Gal4-Rtg1p fusion was unable to transactivate the LacZ reporter gene in a strain deleted for RTG2, suggesting that the RTG2 product does not act independently of Rtg1p in the rho degree/rho + transcriptional response.

Hairpin orientation of sterol regulatory element-binding protein-2 in cell membranes as determined by protease protection

Sterol regulatory element-binding proteins (SREBP-1 and SREBP-2) are proteins of approximately 1150 amino acids each that are attached to membranes of the endoplasmic reticulum (ER). In sterol-depleted cells, a protease releases an NH2-terminal fragment of approximately 500 amino acids that contains a basic helix-loop-helix leucine zipper motif. This fragment enters the nucleus and stimulates transcription of genes encoding the low density lipoprotein receptor and enzymes of cholesterol biosynthesis. Prior evidence indicates that the SREBPs are attached to membranes by virtue of an 80-residue segment located approximately 80 amino acids to the COOH-terminal side of the leucine zipper. This segment contains two long hydrophobic sequences separated by a short hydrophilic sequence of approximately 30 amino acids. We have proposed a hairpin model in which the two hydrophobic sequences span the membrane, separated by the short hydrophilic sequence which projects into the lumen of the ER (the "lumenal loop"). The model predicts that the NH2- and COOH-terminal segments face the cytosol. To test this model, we constructed a cDNA encoding human SREBP-2 with epitope tags at the NH2 terminus and in the lumenal loop. The COOH-terminal region was visualized with a newly developed monoclonal antibody against this region. Sealed membrane vesicles were isolated from cells expressing the epitope-tagged version of SREBP-2. Trypsin treatment of these vesicles destroyed the NH2- and COOH-terminal segments and reduced the lumenal epitope to a size consistent with protection of the lumenal sequence plus the two membrane-spanning segments. The lumenal epitope tag contained two potential sites for N-linked glycosylation. The size of the trypsin-protected fragment was reduced by treatment with N-Glycanase and endoglycosidase H, indicating that this segment was located in the lumen of the ER where it was glycosylated. These data provide strong support for the hairpin model.

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.

Transcriptional modulators affect in vivo protein binding to the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase promoters

Treatment of HepG2 cells with known effectors of low density lipoprotein receptor (LDLR) gene expression altered the in vivo pattern of protein-DNA interactions in the promoter. The observed changes are consistent with proteins binding in vivo to the sterol regulatory element (SRE), to Sp1-like sites, as well as to other regions. Protein bound to the SRE in all conditions, but the nature of the dimethyl sulfate reactivity changed depending on the physiological state of the cell. Hypermethylation within the SRE of the low density lipoprotein receptor promoter was observed when cells were treated with cholesterol synthesis inhibitors, insulin, or phorbol 12-myristate 13-acetate, suggesting that the SRE regulates this promoter through sterol-independent as well as sterol-dependent mechanisms. No significant changes were observed in binding to the Sp1-like sites, suggesting that differential binding to these sites does not play a role in altered transcription levels. Analysis of the 3-hydroxy-3-methylglutaryl coenzyme A reductase promoter also revealed protections that varied in a cell type-specific manner. Binding to the 3-hydroxy-3-methylglutaryl coenzyme A reductase SRE and putative nuclear factor 1 sites could be observed but varied little in different physiological conditions.

Tyrosine kinase inhibitors potentiate the induction of low density lipoprotein receptor gene expression by hepatocyte growth factor

The role of tyrosine kinase, protein kinase C, cyclic nucleotide- and Ca(2+)-calmodulin-dependent protein kinase second messenger pathways in the induction of LDL receptor gene expression by hepatocyte growth factor (HGF) was studied in the human hepatoma cell line Hep-G2. Incubation with media containing HGF increased the level of LDL receptor mRNA by 6.5-fold. Co-incubation with HGF and either of two tyrosine kinase inhibitors genistein (2.0-20.0 micrograms/ml) and herbimycin A (0.5-500.0 ng/ml) increased the level of LDL receptor mRNA above that observed with HGF alone by 40-60%. Incubation with HGF in the presence of the calmodulin antagonist W7 (10-30 microM) also super-induced the level of LDL receptor mRNA by nearly 230%. The protein kinase C and A inhibitors chelerythrine (0.1-10.0 microM) and H8 (0.5-5.0 microM), respectively, had no significant effects on the induction of LDL receptor mRNA by HGF. Taken together, these data suggest that tyrosine kinase, protein kinases C and A, and Ca(2+)-calmodulin dependent protein kinase activities are not essential for activation of LDL receptor gene expression in Hep-G2 cells by HGF.

Molecular cloning and functional analysis of the promoter of the human squalene synthase gene

We have cloned and characterized the 5'-flanking region of the gene encoding human squalene synthase. We report here the promoter activity of successively 5'-truncated sections of a 1 kilobase of this region by fusing it to the coding region of a luciferase reporter gene. DNA segments of 200 base pairs (bp) 5' to the transcription start site, as determined by primer extension analysis, show a strong promoter effect on the expression of the luciferase chimeric gene and a high response to the presence of sterols when transiently transfected into the human hepatoma cell line HepG2 or to the hamster-derived CHO-K1 cells. An approximately 50-fold induction of luciferase activity, in the absence of sterols, was observed in transiently transfected HepG2 cells for fusion constructs containing sections of 200, 459, and 934 bp of the putative human squalene synthase promoter. Loss of promoter activity and response to sterols was localized to a 69-bp section located 131 nucleotides 5' to the transcription start site. Sequence analysis of this region showed that it contained a sterol regulatory element 1 (SRE-1) previously identified in other sterol regulated genes (Smith, J. R., Osborne, T. F., Brown, M. S., Goldstein, J. L., and Gil, G. (1988). J. Biol. Chem. 263, 18480-18487) and two potential NF-1 binding sites. Additional CCAAT box, SRE-1 element, and two Sp1 sites were identified 3' to this section. Sequences within this 69-bp DNA, including the SRE-1 cis-acting element, show strong binding to the purified nuclear transcription factor ADD1 (Tonzonoz, P., Kim, J. B., Graves, R. A., and Spiegelman B. M. (1993) Mol. Cell Biol. 13, 4753-4759) by mobility shift assay and footprinting analyses.

Platelet-derived growth factor enhances Sp1 binding to the LDL receptor gene

We have previously demonstrated that growth activation of quiescent cells enhances LDL receptor gene transcription and that the proximal 5' flanking region of the LDL receptor gene could transduce a platelet-derived growth factor (PDGF) response. This portion of the LDL receptor gene encompasses a previously characterized sterol response element and an adjacent Sp1 binding site. By use of mobility shift analyses we show that PDGF activation of quiescent cells enhances binding of Sp1 to the LDL receptor gene. Transfection analyses indicated that the Sp1 site, but not the sterol response element binding protein site, could confer PDGF responsiveness to a heterologous promoter in quiescent cells. Furthermore, cotransfection of an LDL receptor reporter gene (containing -141 to +35 bp of the LDL receptor gene promoter) along with an expression construct coding for high-level constitutive expression of an Sp1 cDNA led to marked enhancement in expression of the LDL receptor reporter gene in quiescent cells. Increased Sp1 binding due to PDGF could be due to enhanced production of Sp1; alternatively, posttranslational activation of binding could be involved. Western blot analysis showed no difference in Sp1 abundance in quiescent cells versus PDGF-stimulated cells, suggesting a posttranslational mechanism for activation of Sp1 binding by growth induction. Our data demonstrate that PDGF stimulation of quiescent cells leads to enhanced Sp1 binding to the LDL receptor gene. This enhanced binding could participate in PDGF induction of LDL receptor gene transcription.

Purification of an interleukin-1 beta converting enzyme-related cysteine protease that cleaves sterol regulatory element-binding proteins between the leucine zipper and transmembrane domains

We describe the characterization and purification of a protease that cleaves sterol regulatory element-binding protein-1 (SREBP-1) and SREBP-2 in vitro. Cleavage occurs between the basic helix-loop-helix-leucine zipper and the first transmembrane domain of each SREBP. This is the region in which the SREBPs are cleaved physiologically by a sterol-regulated protease that releases an NH2-terminal fragment that activates transcription of the genes for the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl CoA synthase. The cleavage enzyme, designated SREBP cleavage activity (SCA), belongs to a new class of cysteine proteases of the interleukin-1 beta-converting enzyme (ICE) family, all of which cleave at aspartic acid residues. Like ICE, SCA was inactive in cytosol, and it was activated in vitro by incubation at 30 degrees C. SCA was resistant to inhibitors of serine, aspartyl, and metalloproteases, but it was sensitive to N-ethylmaleimide. The enzyme cleaved SREBP-1 and SREBP-2 between the Asp and Ser of a conserved sequence (S/DEPDSP). The activity was blocked by a tetrapeptide aldehyde, Ac-Asp-Glu-Ala-Asp-aldehyde (Ac-DEAD-CHO). A purified preparation of SCA from hamster liver contained a prominent 20-kDa polypeptide that could be labeled with [14C]iodoacetic acid. Labeling was blocked by Ac-DEAD-CHO. Partial amino acid sequence of this polypeptide revealed that it was the hamster equivalent of human CPP32, a putative protease whose cDNA was recently identified by virtue of sequence homology to ICE. CPP32 and ICE have been implicated in apoptosis in animal cells. Whether SCA/CPP32 participates in vivo in the sterol-regulated activation of SREBP, or whether it activates SREBPs during apoptosis, remains to be determined.

Evidence for a cholesterol-lowering gene in a French-Canadian kindred with familial hypercholesterolemia

We describe a four-generation kindred with familial hypercholesterolemia (FH) in which two of the eight heterozygotes for a 5-kb deletion (exons 2 and 3) in the low density lipoprotein (LDL) receptor gene were found to have normal LDL-cholesterol levels. In our search for a gene responsible for the cholesterol-lowering effect in this family, we have studied variation in the genes encoding the LDL receptor, apolipoprotein (apo) B, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, apoAI-CIII-AIV, and lipoprotein lipase. The analysis showed that it was unlikely that variation in any of these genes was responsible for the cholesterol-lowering effect. Expression of the LDL receptor, as assessed in vitro with measurements of activity and mRNA levels, was similar in normo and hyperlipidemic subjects carrying the deletion. Analysis of the apo E isoforms revealed that most of the e2 allele carriers in this family, including the two normolipidemic 5-kb deletion carriers, were found to have LDL-cholesterol levels substantially lower than subjects with the other apo E isoforms. Thus, this kindred provides evidence for the existence of a gene or genes, including the apo e2 allele, with profound effects on LDL-cholesterol levels.

Nup358, a cytoplasmically exposed nucleoporin with peptide repeats, Ran-GTP binding sites, zinc fingers, a cyclophilin A homologous domain, and a leucine-rich region

The Ras-related nuclear protein, Ran, has been implicated in nuclear transport. By screening a HeLa cell lambda expression library with Ran-GTP and sequencing overlapping cDNA clones, we have obtained the derived primary structure of a protein with a calculated molecular mass of 358 kDa. Using antibodies raised against an expressed segment of this protein, we obtained punctate nuclear surface staining by immunofluorescence microscopy that is characteristic for nucleoporins. Electron microscopy of immunogold-decorated rat liver nuclear envelopes sublocalized the 358-kDa protein at (or near) the tip of the cytoplasmic fibers of the nuclear pore complex (NPC). In agreement with current convention, this protein was therefore termed Nup358 (for nucleoporin of 358 kDa). Nup358 contains a leucine-rich region, four potential Ran binding sites (i.e. Ran binding protein 1 homologous domains) flanked by nucleoporin-characteristic FXFG or FG repeats, eight zinc finger motifs, and a C-terminal cyclophilin A homologous domain. Consistent with the location of Nup358 at the cytoplasmic fibers of the NPC, we found decoration with Ran-gold at only the cytoplasmic side of the NPC. Thus, Nup358 is the first nucleoporin shown to contain binding sites for two of three soluble nuclear transport factors so far isolated, namely karyopherin and Ran-GTP.

Three different rearrangements in a single intron truncate sterol regulatory element binding protein-2 and produce sterol-resistant phenotype in three cell lines. Role of introns in protein evolution

The cholesterol analogue 25-hydroxycholesterol kills animal cells by blocking the proteolytic activation of two sterol-regulated transcription factors designated sterol regulatory element binding protein-1 and -2 (SREBP-1 and SREBP-2). These proteins, each approximately 1150 amino acids in length, are embedded in the membranes of the nucleus and endoplasmic reticulum by virtue of hydrophobic COOH-terminal segments. In cholesterol-depleted cells the proteins are cleaved to release soluble NH2-terminal fragments of approximately 480 amino acids that enter the nucleus and activate genes encoding the low density lipoprotein receptor and enzymes of cholesterol synthesis. 25-Hydroxycholesterol blocks this cleavage, and cells die of cholesterol deprivation. We previously described a mutant 25-hydroxycholesterol-resistant hamster cell line (SRD-1 cells) in which the SREBP-2 gene had undergone a recombination between the intron following codon 460 and an intron in an unrelated gene. The SREBP-2 sequence terminated at residue 460, eliminating the membrane attachment domain and producing a constitutively active factor that no longer required proteolysis and thus was not inhibited by 25-hydroxycholesterol. Here, we report that two additional sterol-resistant cell lines (SRD-2 and SRD-3) have also undergone genomic rearrangements in the intron following codon 460 of the SREBP-2 gene. Although the molecular rearrangements differ in the three mutant lines, each leads to the production of a constitutively active transcription factor whose SREBP-2 sequence terminates at residue 460. These findings provide a dramatic illustration of the advantage that introns provide in allowing proteins to gain new functions in response to new environmental challenges.

Cellular nucleic acid binding protein regulates the CT element of the human c-myc protooncogene

The CT element of the c-myc gene is required for promoter P1 usage and can drive expression of a heterologous promoter. Both double strand (Sp1) and single strand (hnRNP K) CT-binding proteins have been implicated as mediators of CT action. Although significant levels of CT activity persisted following Sp1 immunodepletion, EGTA totally abolished transactivation, thus implicating another metal requiring factor in CT element activity. As hnRNP K binds to one strand of the CT element, but has no metal requirement, the opposite (purine-rich strand) was examined as a target for a metal-dependent protein. A zinc-requiring purine strand binding activity was identified as cellular nucleic acid binding protein (CNBP), a protein previously implicated in the regulation of sterol responsive genes. Two forms of CNBP differed in their relative binding to the CT- or sterol-response elements. CNBP was shown to be a bona fide regulator of the CT element by cotransfection of a CNBP expression vector that stimulated expression of a CT-driven but not an AP1-dependent reporter. These data suggest that hnRNP K and CNBP bind to opposite strands and co-regulate the CT element.

Brefeldin A renders Chinese hamster ovary cells insensitive to transcriptional suppression by 25-hydroxycholesterol

The effect of disruption of the Golgi apparatus on 25-hydroxycholesterol-mediated transcriptional suppression and activation of acyl-CoA:cholesterol acyltransferase was examined. In Chinese hamster ovary (CHO) cells, brefeldin A (BFA) caused dose-dependent inhibition of 25-hydroxycholesterol-mediated suppression of mRNAs for four sterol-regulated genes: 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, HMG-CoA synthase, farnesyl-diphosphate synthase, and the low density lipoprotein receptor. BFA prevented suppression whether added prior to or following a 4-h pretreatment with 25-hydroxycholesterol. In the presence of BFA (1 microgram/ml), 25-hydroxycholesterol-mediated suppression of mRNAs for HMG-CoA reductase, the low density lipoprotein receptor, and farnesyl-diphosphate synthase was almost completely blocked. HMG-CoA synthase mRNA was 80-90% suppressed by 25-hydroxycholesterol compared with 50-60% suppression in the presence of BFA. These effects of BFA were not due to alterations in mRNA stability. Disruption of the Golgi apparatus, as assessed by staining with a fluorescent lectin, correlated with concentrations of BFA that reversed mRNA suppression. Monensin was also found to block the effects of 25-hydroxycholesterol on suppression of HMG-CoA reductase. However, this ionophore decreased the other three sterol-regulated mRNAs to a similar degree as 25-hydroxycholesterol. In contrast to CHO cells, BFA-resistant PtK1 cells displayed normal down-regulation of HMG-CoA reductase and an intact Golgi apparatus in the presence of BFA and 25-hydroxycholesterol. Cholesterol esterification in CHO cells was stimulated to a similar extent by BFA (1 microgram/ml) and 25-hydroxycholesterol, and simultaneous treatment of CHO cells with both compounds was 60-70% additive. These results suggest that an intact Golgi apparatus is required for 25-hydroxycholesterol-mediated suppression of mRNA.

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.