Identification of a 6-base pair element involved in the sterol-mediated transcriptional regulation of farnesyl diphosphate synthase

Characterization and functional analysis of cis-acting elements of the human farnesyl diphosphate synthetase (FDPS) gene 5' flanking region

Farnesyl diphosphate synthetase (FDPS) is a key enzyme in the isoprenoid pathway responsible for cholesterol biosynthesis, post-translational protein modifications and synthesis of steroid hormones, whose expression is regulated by phorbol esters and polyunsaturated fatty acids. Genomic comparison of the 5' upstream sequence of the FDPS genes identifies conserved binding sites for NF-Y, SP1, SRE3, and YY1 regulatory elements in rat, mouse, dog and chimpanzee. Two additional specific consensus sequences, upstream of the core promoter that had not been analysed previously, are shared only by human and chimpanzee genomes. The work presented here aimed at characterizing these genomic sequence elements in the human FDPS promoter region and their contribution to gene expression. We have characterized functionally the minimal basal promoter of the human FDPS gene by means of deletion mutants and we have identified two cis-acting elements which modulate the FDPS gene expression and are recognized by Pax5 and OCT-1 transcription factors.

Transcriptional regulation of farnesyl pyrophosphate synthase by liver X receptors

Liver X receptors (LXRs) are members of the nuclear receptor superfamily that are involved in cholesterol and lipid metabolism. In addition to liver, the brain is another site where LXRs may control cholesterol homeostasis. In the brain, the regulation of cholesterol homeostasis is independent from other parts of the body, and its disturbance is associated with neurodegenerative disorders, such as Alzheimer's disease. We have used PCR-based suppressive subtractive cloning to identify new LXR target genes in brain cells. In this report, we show that farnesyl pyrophosphate synthase (FPPS) is a new target gene for LXR in astrocytes and neurons. Farnesyl pyrophosphate is an obligate intermediate for de novo cholesterol synthesis and a substrate for protein farnesylation. Stimulation of FPPS mRNA synthesis by an LXR agonist, Hypocholamide, was observed in several cell lines from the central nervous system. We identified a single putative direct repeat 4 (DR4) LXR response element in the FPPS promoter. In a reporter gene assay, LXR transactivated a reporter gene bearing a truncated FPPS promoter containing this DR4 cis-element but not if the DR4 element was mutated. Using gel-mobility shift assay, we further demonstrated the direct interaction between the LXR/retinoid X receptor (RXR) heterodimer and the response element. Taken together, our results indicate that LXRs directly regulate FPPS gene expression, and thus may play a role in modulating cholesterol synthesis in the brain.

Atherogenic diet and minimally oxidized low density lipoprotein inhibit osteogenic and promote adipogenic differentiation of marrow stromal cells

In osteoporosis, the bone marrow stroma osteogenic cell population declines and adipocyte numbers increase. We recently showed that oxidized lipids inhibit differentiation of preosteoblasts. In this report, we assess the effect of minimally oxidized low density lipoprotein (MM-LDL) on osteoblastic differentiation of murine marrow stromal cells, M2-10B4. MM-LDL, but not native LDL, inhibited stromal cell osteoblastic differentiation as demonstrated by inhibition of alkaline phosphatase activity, collagen I processing, and mineralization, through a mitogen-activated protein kinase-dependent pathway. In addition, marrow stromal cells from C57BL/6 mice fed a high fat, atherogenic diet failed to undergo osteogenic differentiation in vitro. The ability of MM-LDL to regulate adipogenesis was also assessed. Treatment of M2-10B4 as well as 3T3-L1 preadipocytes with MM-LDL, but not native LDL, promoted adipogenic differentiation in the presence of peroxisome proliferator-activated receptor (PPAR) gamma agonist thiazolidinediones, BRL49653 and ciglitizone. Based on promoter-reporter construct experiments, MM-LDL may be acting in part through activating PPARalpha. These observations suggest that LDL oxidation products promote osteoporotic loss of bone by directing progenitor marrow stromal cells to undergo adipogenic instead of osteogenic differentiation. These data lend support to the "lipid hypothesis of osteoporosis."

NF-Y organizes the gamma-globin CCAAT boxes region

The CCAAT-binding activator NF-Y is formed by three evolutionary conserved subunits, two of which contain putative histone-like domains. We investigated NF-Y binding to all CCAAT boxes of globin promoters in direct binding, competition, and supershift electrophoretic mobility shift assay; we found that the alpha, zeta, and proximal gamma CCAAT boxes of human and the prosimian Galago bind avidly, and distal gamma CCAAT boxes have intermediate affinity, whereas the epsilon and beta sequences bind NF-Y very poorly. We developed an efficient in vitro transcription system from erythroid K562 cells and established that both the distal and the proximal CCAAT boxes are important for optimal gamma-globin promoter activity. Surprisingly, NF-Y binding to a mutated distal CCAAT box (a C to T at position -114) is remarkably increased upon occupancy of the high affinity proximal element, located 27 base pairs away. Shortening the distance between the two CCAAT boxes progressively prevents simultaneous CCAAT binding, indicating that NF-Y interacts in a mutually exclusive way with CCAAT boxes closer than 24 base pairs apart. A combination of circular permutation and phasing analysis proved that (i) NF-Y-induced angles of the two gamma-globin CCAAT boxes have similar amplitudes; (ii) occupancy of the two CCAAT boxes leads to compensatory distortions; (iii) the two NF-Y bends are spatially oriented with combined twisting angles of about 100 degrees. Interestingly, such distortions are reminiscent of core histone-DNA interactions. We conclude that NF-Y binding imposes a high level of functionally important coordinate organization to the gamma-globin promoter.

Characterization of the mevalonate kinase 5'-untranslated region provides evidence for coordinate regulation of cholesterol biosynthesis

Using a probe derived from the 5'-untranslated region of the human mevalonate kinase (MK) cDNA, we screened a lambda gt 11 genomic library and obtained a single clone containing the 5' untranslated region of the gene. Nucleotide sequencing identified several putative regulatory elements, including two Sp1 (GC box) elements and a CCAAT box. A canonical TATA box was not detected. Directly adjacent to one Sp1 element was a sterol regulatory element (SRE), 5'-CACCCCAG-3', which was a 7/8 base pair match to the consensus sequences identified in the genes encoding 3-hydroxy-3-methyl-glutaryl-coenzyme A synthase and reductase, and the LDL receptor. There was no Sp1 element upstream of the SRE. Northern blot analysis in human CRL1508T cells revealed that quantities of MK poly A+ RNA increased for cells grown in the presence of lipid-deficient calf serum, and further increased upon addition of 1 microM lovastatin. Primer extension analysis with human poly A+ RNA suggested at least 4 transcription initiation sites downstream from the CCAAT box. To assess sterol responsiveness of transcription initiation, a 1.4 kb genomic fragment upstream of the translational start site was fused to the pSV2cat vector for transient expression in COS-7 cells, with chloramphenicol acetyltransferase (CAT) as the reporter gene. This construct demonstrated modest levels of CAT expression which was induced > 2-fold when cells were grown in lipoprotein-deficient calf serum. Our data provide further evidence for coordinate regulation of cholesterol biosynthesis in response to sterol.

Two sterol regulatory element-like sequences mediate up-regulation of caveolin gene transcription in response to low density lipoprotein free cholesterol

Caveolae form the terminus for a major pathway of intracellular free cholesterol (FC) transport. Caveolin mRNA levels in confluent human skin fibroblasts were up-regulated following increased uptake of low density lipoprotein (LDL) FC. The increase induced by FC was not associated with detectable change in mRNA stability, indicating that caveolin mRNA levels were mediated at the level of gene transcription. A total of 924 bp of 5' flanking region of the caveolin gene were cloned and sequenced. The promoter sequence included three G+C-rich potential sterol regulatory elements (SREs), a CAAT sequence and a Sp1 consensus sequence. Deletional mutagenesis of individual SRE-like sequences indicated that of these two (at -646 and -395 bp) were essential for the increased transcription rates mediated by LDL-FC, whereas the third was inconsequential. Gel shift analysis of protein binding from nuclear extracts to these caveolin promoter DNA sequences, together with DNase I footprinting, confirmed nucleoprotein binding to the SRE-like elements as part of the transcriptional response to LDL-FC. A supershift obtained with antibody to SRE-binding protein 1 (SPEBP-1) indicated that this protein binds at -395 bp. There was no reaction at -395 bp with anti-Sp1 antibody nor with either antibody at -646 bp. The cysteine protease inhibitor N-acetyl-leu-leu-norleucinal (ALLN), which inhibits SREBP catabolism, superinhibited caveolin mRNA levels regardless of LDL-FC. This finding suggests that SREBP inhibits caveolin gene transcription in contrast to its stimulating effect on other promoters. The findings of this study are consistent with the postulated role for caveolin as a regulator of cellular FC homeostasis in quiescent peripheral cells, and the coordinate regulation by SREBP of FC influx and efflux.

Mechanisms of Giardia lamblia differentiation into cysts

Microbiologists have long been intrigued by the ability of parasitic organisms to adapt to changes in the environment. Since most parasites occupy several niches during their journey between vectors and hosts, they have developed adaptive responses which allow them to survive under adverse conditions. Therefore, the life cycles of protozoan and helminthic parasites are excellent models with which to study numerous mechanisms involved in cell differentiation, such as the regulation of gene expression, signal transduction pathways, and organelle biogenesis. Unfortunately, many of these studies are very difficult because the conditions needed to elicit developmental changes in parasites remain undetermined in most cases. Recently, several interesting findings were reported on the process of differentiation of Giardia lamblia trophozoites into cysts. G. lamblia is a flagellated protozoan that inhabits the upper small intestine of its vertebrate host and is a major cause of enteric disease worldwide. It belongs to the earliest identified lineage among eukaryotes and therefore offers a unique insight into the progression from primitive to more complex eukaryotic cells. The discovery of a specific stimulus that induces trophozoites to differentiate into cysts, the identification and characterization of encystation-specific molecules, the elucidation of novel biochemical pathways, and the development of useful reagents and techniques have made this parasite an excellent model with which to study differentiation in eukaryotic cells. In this review, we summarize the most recent fundings on several aspects of Giardia differentiation and discuss the significance of these findings within the context of current knowledge in the field.

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.

Polyprenyl diphosphate synthases

It is noteworthy that in spite of the similarity of the reactions catalyzed by these prenyltransferases, the modes of expression of catalytic function are surprisingly different, varying according to the chain length and stereochemistry of reaction products. These enzymes are summarized and classified into four groups, as shown in Figure 13. Short-chain prenyl diphosphates synthases such as FPP and GGPP synthases require no cofactor except divalent metal ions, Mg2+ or Mn2+, which are commonly required by all prenyl diphosphate synthases. Medium-chain prenyl diphosphate synthases, including the enzymes for the synthesis of all-E-HexPP and all-E-HepPP, are unusual because they each consist of two dissociable dissimilar protein components, neither of which has catalytic activity. The enzymes for the synthesis of long-chain all-E-prenyl diphosphates, including octaprenyl (C40), nonaprenyl-(C45), and decaprenyl (C50) diphosphates, require polyprenyl carrier proteins that remove polyprenyl products from the active sites of the enzymes to maintain efficient turnovers of catalysis. The enzymes responsible for Z-chain elongation include Z,E-nonaprenyl-(C45) and Z,E-undecaprenyl (C55) diphosphate synthases, which require a phospholipid. The classification of mammalian synthases seems to be fundamentally similar to that of bacterial synthases except that no medium-chain prenyl diphosphate synthases are included. The Z-prenyl diphosphate synthase in mammalian cells is dehydrodolichyl PP synthase, which catalyzes much longer chain elongations than do bacterial enzymes. Dehydrodolichyl PP synthase will be a major target of future studies in this field in view of its involvement in glycoprotein biosynthesis.

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.

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.

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.

Somatic cell genetic analysis of two classes of CHO cell mutants expressing opposite phenotypes in sterol-dependent regulation of cholesterol metabolism

Two different classes of hamster cell mutants (25RA cells and M1 cells) express opposite phenotypes in sterol dependent regulation. In 25RA cells, sterols added in growth medium fail to cause down-regulation of sterol synthesis rate and low density lipoprotein (LDL) receptor activity, while in M1 cells, removal of lipids from growth medium fail to cause up-regulation of sterol synthesis rate and LDL receptor activity. Cell hybridization analysis showed that the 25RA phenotype is semidominant, while the M1 phenotype is recessive. Using 25RA as the parental cells, we isolated eight independent mutant cells (DM cells) and showed that all of them belong to the same genetic complementation group as the M1 mutant, indicating that the normal (unmutated) M1 gene product(s) is required to express the 25RA phenotype. We next performed gene transfer experiments using hamster cell genomic DNAs containing the functional human M1 gene as the donor, and the double mutant cell DM7 as the recipient. The resultant transfectant cells express the 25RA cell phenotype (instead of the wild-type cell phenotype). This result, along with the results obtained from cell hybridization analysis, shows that the 25RA and M1 cell phenotypes are caused by mutations at two different genes.