Archaic structure of the gene encoding transcription factor USF

Cyclin-Dependent Kinase 5 (CDK5)-Mediated Phosphorylation of Upstream Stimulatory Factor 2 (USF2) Contributes to Carcinogenesis

The transcription factor USF2 is supposed to have an important role in tumor development. However, the regulatory mechanisms contributing to the function of USF2 are largely unknown. Cyclin-dependent kinase 5 (CDK5) seems to be of importance since high levels of CDK5 were found in different cancers associated with high USF2 expression. Here, we identified USF2 as a phosphorylation target of CDK5. USF2 is phosphorylated by CDK5 at two serine residues, serine 155 and serine 222. Further, phosphorylation of USF2 at these residues was shown to stabilize the protein and to regulate cellular growth and migration. Altogether, these results delineate the importance of the CDK5-USF2 interplay in cancer cells.

Protein kinases as switches for the function of upstream stimulatory factors: implications for tissue injury and cancer

The upstream stimulatory factors (USFs) are regulators of important cellular processes. Both USF1 and USF2 are supposed to have major roles in metabolism, tissue protection and tumor development. However, the knowledge about the mechanisms that control the function of USFs, in particular in tissue protection and cancer, is limited. Phosphorylation is a versatile tool to regulate protein functions. Thereby, phosphorylation can positively or negatively affect different aspects of transcription factor function including protein stability, protein-protein interaction, cellular localization, or DNA binding. The present review aims to summarize the current knowledge about the regulation of USFs by direct phosphorylation and the consequences for USF functions in tissue protection and cancer.

Gene trapping uncovers sex-specific mechanisms for upstream stimulatory factors 1 and 2 in angiotensinogen expression

A single-nucleotide polymorphism (C/A) located within an E-box at the -20 position of the human angiotensinogen (AGT) promoter may regulate transcriptional activation through differential recruitment of the transcription factors upstream stimulatory factor (USF) 1 and 2. To study the contribution of USF1 on AGT gene expression, mice carrying a (-20C) human AGT (hAGT) transgene were bred with mice harboring a USF1 gene trap allele designed to knock down USF1 expression. USF1 mRNA was reduced relative to controls in liver (9 ± 1%), perigenital adipose (16 ± 3%), kidney (17 ± 1%), and brain (34 ± 2%) in double-transgenic mice. This decrease was confirmed by electrophoretic mobility shift assay. Chromatin immunoprecipitation analyses revealed a decrease in USF1, with retention of USF2 binding at the hAGT promoter in the liver of male mice. hAGT expression was reduced in the liver and other tissues of female but not male mice. The decrease in endogenous AGT expression was insufficient to alter systolic blood pressure at baseline but caused reduced systolic blood pressure in female USF1 gene trap mice fed a high-fat diet. Treatment of USF1 knockdown males with intravenous adenoviral short hairpin RNA targeting USF2 resulted in reduced expression of USF1, USF2, and hAGT protein. Our data from chromatin immunoprecipitation assays suggests that this decrease in hAGT is attributed to decreased USF2 binding to the hAGT promoter. In conclusion, both USF1 and USF2 are essential for AGT transcriptional regulation, and distinct sex-specific and tissue-specific mechanisms are involved in the activities of these transcription factors in vivo.

Opposite expression of the antioxidant heme oxygenase-1 in primary cells and tumor cells: regulation by interaction of USF-2 and Fra-1

Heme oxygenase-1 is the rate-limiting enzyme for the degradation of the prooxidant heme. Previously, we showed that an E-box within the HO-1 promoter is crucial for the regulation of HO-1 expression in primary hepatocytes. Further to investigate the importance of this E-box, we determined the regulatory capacity of the E-box-binding factor USF-2 in primary cells in comparison with transformed cell lines. We found that HO-1 expression was inhibited by USF-2 in primary cells, whereas it was induced in tumor cell lines. Mutation of either the E-box or the AP-1 site within the HO-1 promoter only partially affected the USF-dependent regulation. However, this regulation was dramatically reduced in tumor cells and completely abolished in primary cells transfected with an HO-1 promoter construct containing mutations in both the E-box and the AP-1 site, suggesting that AP-1 factors and USF-2 may act in a cooperative manner. Indeed, protein-protein interaction studies revealed that USF proteins interacted with Fra-1. Further, the USF-dependent HO-1 promoter activity was not detectable with an USF-2 mutant lacking residues of the USF-specific region (USR) or the transactivation domain encoded by exon 4. Together, these data suggest that USF-2 has opposite regulatory roles for HO-1 gene expression in primary cells and tumor cell lines.

USF : un régulateur essentiel de la transcription

Upstream stimulating factors, USF-1 and -2, are members of the evolutionary conserved basic-Helix-Loop-Helix-Leucine Zipper transcription factor family. The ubiquitously expressed USF-1 and -2 proteins of respectively 43 kDa and 44 kDa interact with high affinity to cognate E-box regulatory elements (CANNTG) which are particularly represented over the genome. The USF transcription factors are key regulatory elements of the transcriptional machinery mediating recruitment of chromatin remodelling enzymes, interacting with co-activators and members of the pre-initiation complex (PIC). Furthermore, transcriptionnal activity and DNA-binding of the USF proteins can be modulated by multiple ways including phosphorylation by distinct kinases (p38, protein kinase A and C, cdk1 and PI3Kinase), homo or heterodimerization formation and DNA modification of the E-box binding motif (methylation, SNP). Taken together, these parameters render very complex the understanding of the USF-dependent gene expression regulation. USF transcription factors have thus been involved as key regulators of a wide number of gene regulation network including stress and immune response, cell cycle and proliferation. This review will thus focus on general aspect of the USF transcription factors and their implications in some regulatory networks.

Upstream stimulating factors: highly versatile stress-responsive transcription factors

Upstream stimulating factors (USF), USF-1 and USF-2, are members of the eucaryotic evolutionary conserved basic-Helix-Loop-Helix-Leucine Zipper transcription factor family. They interact with high affinity to cognate E-box regulatory elements (CANNTG), which are largely represented across the whole genome in eucaryotes. The ubiquitously expressed USF-transcription factors participate in distinct transcriptional processes, mediating recruitment of chromatin remodelling enzymes and interacting with co-activators and members of the transcription pre-initiation complex. Results obtained from both cell lines and knock-out mice indicates that USF factors are key regulators of a wide number of gene regulation networks, including the stress and immune responses, cell cycle and proliferation, lipid and glucid metabolism, and in melanocytes USF-1 has been implicated as a key UV-activated regulator of genes associated with pigmentation. This review will focus on general characteristics of the USF-transcription factors and their place in some regulatory networks.

Decreased tumorigenicity of c-Myc-transformed fibroblasts expressing active USF2

USF is a small family of basic helix-loop-helix leucine zipper (bHLH-zip) transcription factors with DNA binding specificities similar to that of the c-Myc oncoprotein. Evidence for a role of USF in growth control includes inhibition of c-Myc-dependent cellular transformation in vitro and loss of USF transcriptional activity in many cancer cell lines. However, a direct effect of USF on the tumorigenicity of an established cell line has never been demonstrated. Here, cell lines derived from rat embryo fibroblasts transformed by c-Ha-Ras and either c-Myc or E1A were used as model system to investigate the tumor suppression ability of USF. Overexpression of USF2 stimulated transcription and inhibited colony formation in c-Myc-transformed, but not E1A-transformed, fibroblasts. Stable clones expressing high USF2 levels were constructed from c-Myc-transformed fibroblasts. In two of these clones, overexpressed USF2 did not activate transcription, and there was no significant change in the transformed phenotype. In contrast, a clone that expressed transcriptionally active USF2 exhibited altered morphology and a strongly decreased ability to proliferate in semisolid medium. The ability of these cells to form tumors in nude mice was also decreased by a factor of more than 30 as compared to the parental cell line or cells overexpressing transcriptionally inactive USF2. Cotransfection assays with USF- or Myc-specific dominant-negative mutants indicated that active USF2 inhibited cellular transformation by preventing transcriptional repression by c-Myc.

Identification of basal promoter and enhancer elements in an untranslated region of the TT virus genome

The regulation of TT virus (TTV) gene expression was characterized. Transient-transfection assays using reporter constructs revealed that a 113-nucleotide (nt) sequence within the untranslated region, proximal to the transcription initiation site and containing a TATA box motif, has a basal promoter activity. This sequence is well conserved among different TTV genotypes. Upstream stimulating factor bound to a consensus binding motif within this region and positively regulates TTV transcription. Furthermore, a 488-nt region upstream of the basal promoter exhibited enhancer activity, presumably in a cell type-specific manner. This study illustrates some of the mechanisms involved in the transcriptional regulation of TTV.

Glucose up-regulates thrombospondin 1 gene transcription and transforming growth factor-beta activity through antagonism of cGMP-dependent protein kinase repression via upstream stimulatory factor 2

Thrombospondin 1 (TSP1) transcription is stimulated by glucose, resulting in increased TGF-beta activation and matrix protein synthesis. We previously showed that inducible expression of the catalytic domain of cGMP-dependent protein kinase (PKG) inhibits glucose-regulated TSP1 transcription and transforming growth factor (TGF)-beta activity in stably transfected rat mesangial cells (RMCs(tr/cd)). However, the molecular mechanisms by which PKG represses glucose-regulated TSP1 transcription are unknown. Using a luciferase-promoter deletion assay, we now identify a single region of the human TSP1 promoter (-1172 to -878, relative to the transcription start site) that is responsive to glucose. Further characterization of this region identified an 18-bp sequence that specifically binds nuclear proteins from mesangial cells. Moreover, binding is significantly enhanced by high glucose treatment and is reduced by increased PKG activity. Gel mobility shift and supershift assays show that the nuclear proteins binding to the 18-bp sequence are USF1 and -2. USF1 and USF2 bound to the endogenous TSP1 promoter using a chromatin immunoprecipitation assay. Glucose stimulates nuclear USF2 protein accumulation through protein kinase C, p38 MAPK, and extracellular signal-regulated kinase pathways. Increased PKG activity down-regulates USF2 protein levels and its DNA binding activity under high glucose conditions, resulting in inhibition of glucose-induced TSP1 transcription and TGF-beta activity. Overexpression of USF2 reversed the inhibitory effect of PKG on glucose-induced TSP1 gene transcription and TGF-beta activity. Taken together these data present the first evidence that USF2 mediates glucose-induced TSP1 expression and TSP1-dependent TGF-beta bioactivity in mesangial cells, suggesting that USF2 is an important transcriptional regulator of diabetic complications.

Comparative analysis of mouse hepcidin 1 and 2 genes: evidence for different patterns of expression and co-inducibility during iron overload

In contrast to the human genome, the mouse genome contains two HEPC genes encoding hepcidin, a key regulator of iron homeostasis. Here we report a comparative analysis of sequence, genomic structure, expression and iron regulation of mouse HEPC genes. The predicted processed 25 amino acid hepcidin 2 peptide share 68% identity with hepcidin 1 with perfect conservation of eight cysteine residues. Both HEPC1 and HEPC2 genes have similar genomic organization and have probably arisen from a recent duplication of chromosome 7 region, including the HEPC ancestral gene and a part of the adjacent USF2 gene. Insertion of a retroviral intracisternal A-particle element was found upstream of the HEPC1 gene. Both genes are highly expressed in the liver and to a much lesser extent in the heart. In contrast to HEPC1, a high amount of HEPC2 transcripts was detected in the pancreas. Expression of both genes was increased in the liver during carbonyl-iron and iron-dextran overload. Overall our data suggest that both HEPC1 and HEPC2 genes are involved in iron metabolism regulation but could exhibit different activities and/or play distinct roles.

Genomic structure and transcriptional regulation of human Galbeta1,3GalNAc alpha2,3-sialyltransferase (hST3Gal I) gene

Previous studies have shown that hST3Gal I mRNA is overexpressed in colorectal cancer tissues and primary breast carcinoma compared with nonmalignant or benign tissue, suggesting that the transcriptional regulation of hST3Gal I gene is altered during malignant transformation. We report transcriptional regulation of the hST3Gal I gene in colon adenocarcinoma and leukemia cell lines. To determine the genomic structure of the 5'-untranslated region, we cloned and identified the 5'-untranslated region of hST3Gal I from a human genome library. The 5'-untranslated region was found to be divided into three exons, namely, exons Y, X, and C1. The transcription initiation sites map at -1035 bp from the translation initiation site. Our results indicate that the transcriptional regulation of hST3Gal I depends on the pI promoter that exists 5'-upstream of exon Y in these cell lines. The results of luciferase assay suggest that the nt -304 to -145 region is important for transcriptional activity of hST3Gal I gene in both cell lines. The nt -304 to -145 region contains two sequences similar to the Sp1 recognition elements (GC-box) and one USF binding site. The results of site-directed mutagenesis indicated that the Sp1 binding sites and USF binding site of the pI promoter are involved in the transcription of hST3Gal I mRNA. However, the triple mutant of these sites still exhibits about 50% transcriptional activity, suggesting that there are other transcription factors involved in the transcription of hST3Gal I mRNA. These results suggest that these factors may play a critical role in the up-regulation of the hST3Gal I gene during malignant transformation.

Molecular cloning of rat USF2 cDNA and characterization of splicing variants

The complete nucleotide sequence of rat USF2 cDNA was determined. In addition to the full length clone (USF2FL), four isoforms (delta1, delta2, delta3, and delta4) suggested to be generated by alternative splicing were isolated. USF2delta1 and delta2 lacked 27 and 67 internal amino acid residues, respectively. USF2delta3 and delta4 lacked most of the entire sequence but encoded short peptides of an N-terminal portion of USF2FL. Overexpression of USF2FL increased the transcription of the human high affinity IgE receptor (FcepsilonRI) alpha chain gene through specific binding to the CAGCTG motif in the first intron. On the other hand, overexpression of USF2delta1 or delta2 reduced the transcription of the human FcepsilonRI alpha chain gene. Both USF2FL and USF2delta1 bound to CACGTG as well as CAGCTG, while USF2delta2 bound to CACGTG but not to CAGCTG. These results suggested the presence of a different and definitive role of each variant in the expression of the alpha chain gene.

Upstream stimulatory factor 2 stimulates transcription through an initiator element in the mouse cytochrome c oxidase subunit Vb promoter

Upstream stimulatory factor (USF) is a basic helix-loop-helix-leucine zipper transcription factor that plays an important role in transcriptional activation and cell proliferation. In this article, we demonstrate that the mouse cytochrome c oxidase subunit Vb gene (Cox5b) can be transactivated by ectopic expression of USF2 through an initiator (Inr) element in the core promoter. Importantly, using a dominant-negative mutant of USF2, we demonstrate the role of endogenous USF2 proteins in the transcriptional activation of the Cox5b Inr. Domains of USF2 encoded by exon 4, exon 5 and the USF-specific region are important for maximum activation of the Cox5b Inr. Using the adenovirus E1A oncoprotein, we show that p300/CBP acts as a coactivator in the USF2-dependent activation of the Cox5b Inr. We also demonstrate that although expression of multifunctional regulatory factor, Yin Yang 1 (YY1), can stimulate transcription of the Cox5b Inr to a modest extent, expression of YY1 together with USF2 greatly reduces the level of activation of the Cox5b Inr. Furthermore, we show that the transcription factor, Sp1, represses both the YY1- and the USF2-dependent activation of the Cox5b Inr, indicating competition among Sp1, YY1, and USF2.

The USF proteins regulate transcription of the follicle-stimulating hormone receptor but are insufficient for cell-specific expression

Expression of the FSH receptor (FSHR) is limited to granulosa cells of the ovary and Sertoli cells of the testis. Previous studies showed that an E box in the proximal promoter of the FSHR gene is required for transcription and that the predominant E box binding proteins are the ubiquitous transcription factors, upstream stimulatory factor 1 (USF1) and USF2. Through cotransfection analysis, we have shown that both wild-type and dominant negative forms of the USF proteins regulate the rat FSHR promoter and that transcriptional activation of FSHR required several domains within the amino-terminal portion of the USF proteins. Analysis of the FSHR promoter region using in vivo genomic footprinting indicated that the E box is occupied by proteins in Sertoli cells but not in cells that fail to express the receptor, despite the presence of the USF proteins. To help delineate the regions of the rat FSHR gene required for correct spatial and temporal expression, transgenic mice harboring two constructs containing variable amounts of 5'-flanking sequence (5,000 bp and 100 bp) were generated. Examination of 16 different transgenic lines revealed varied transgene expression profiles with multiple lines having different amounts of ectopic expression and two lines failing to express the transgene. In addition, little or no expression was observed in Sertoli cells. These studies indicate that additional regulatory sequences outside the region from -5,000 to +123 bp are needed for proper expression in Sertoli cells.

Growth state-dependent binding of USF-1 to a proximal promoter E box element in the rat plasminogen activator inhibitor type 1 gene

Induced PAI-1 gene expression in renal epithelial (NRK-52E, clone EC-1) cells occurs as part of the immediate-early response to serum. PAI-1 transcripts are maximally expressed early in G(1) (within 4 h of serum addition to quiescent EC-1 cells) and then subsequently decline to basal levels prior to entry into DNA synthetic phase. Comparative analysis of PAI-1 mRNA abundance and de novo-synthesized thiolated RNA in quiescent cells, as well as at 4 h (early G(1)) and 20 h (late G(2)) postserum addition, in conjunction with RNA decay measurements indicated that PAI-1 gene regulation upon growth activation was predominantly transcriptional. An E box motif (CACGTG), important in the induced expression of some growth state-dependent genes, mapped to nucleotides -160 to -165 upstream of the transcription start site in the PAI-1 proximal promoter. Mobility-shift assessments, using a 18-bp deoxyoligonucleotide construct containing the E box within the context of PAI-1-specific flanking sequences, confirmed binding of EC-1 nuclear protein(s) to this probe and, specifically, to the E box hexanucleotide site. The specificity of this protein-probe interaction was verified by competition analyses with double-stranded DNA constructs that included E box deoxyoligonucleotides with non-PAI-1 flanking bases, mutant E box sequences incapable of binding NRK nuclear proteins, and unrelated (i.e., AP-1) target motifs. Extract immunodepletion and supershift/complex-blocking experiments identified one PAI-1 E box-binding protein to be upstream stimulatory factor-1 (USF-1), a member of the HLH family of transcription factors. Mutation of the CACGTG site to TCCGTG in an 18-bp PAI-1 probe inhibited the formation of USF-1-containing complexes confirming that an intact E box motif at -160 to -165 bp in the PAI-1 promoter and, in particular, the CA residues at -165 and -164 are essential for USF-1 binding. Incorporation of this 2 bp change into a reporter construct containing 764 bp of the proximal PAI-1 "promoter" ligated to a CAT gene effectively reduced (by 74%) CAT activity in cycling cells. An intact E box motif at nucleotides -160 to -165 in the PAI-1 promoter, thus, is an important functional element in the regulation of PAI-1 transcriptional activity in renal cells.

Upstream stimulatory factor regulates major histocompatibility complex class I gene expression: the U2DeltaE4 splice variant abrogates E-box activity

The tissue-specific expression of major histocompatibility complex class I genes is determined by a series of upstream regulatory elements, many of which remain ill defined. We now report that a distal E-box element, located between bp -309 and -314 upstream of transcription initiation, acts as a cell type-specific enhancer of class I promoter activity. The class I E box is very active in a neuroblastoma cell line, CHP-126, but is relatively inactive in the HeLa epithelial cell line. The basic helix-loop-helix leucine zipper proteins upstream stimulatory factor 1 (USF1) and USF2 were shown to specifically recognize the class I E box, resulting in the activation of the downstream promoter. Fine mapping of USF1 and USF2 amino-terminal functional domains revealed differences in their abilities to activate the class I E box. Whereas USF1 contained only an extended activation domain, USF2 contained both an activation domain and a negative regulatory region. Surprisingly, the naturally occurring splice variant of USF2 lacking the exon 4 domain, U2DeltaE4, acted as a dominant-negative regulator of USF-mediated activation of the class I promoter. This latter activity is in sharp contrast to the known ability of U2DeltaE4 to activate the adenovirus major late promoter. Class I E-box function is correlated with the relative amount of U2DeltaE4 in a cell, leading to the proposal that U2DeltaE4 modulates class I E-box activity and may represent one mechanism to fine-tune class I expression in various tissues.

SREBP-1 binds to multiple sites and transactivates the human ApoA-II promoter in vitro : SREBP-1 mutants defective in DNA binding or transcriptional activation repress ApoA-II promoter activity

-Screening of an expression human liver cDNA library resulted in the isolation of several cDNA clones homologous to sterol regulatory element-binding protein-1 (SREBP-1) that recognize the regulatory element AIIAB and AIIK of the human apoA-II promoter. DNaseI footprinting of the apoA-II promoter using SREBP-1 (1 to 460) expressed in bacteria identified 5 overall protected regions designated AIIAB (-64 to -48), AIICD (-178 to -154), AIIDE (-352 to -332), AIIHI (-594 to -574), and AIIK (-760 to -743). These regions contain inverted E-box palindromic or direct repeat motifs and bind SREBP-1 with different affinities. Transient cotransfection experiments in HepG2 cells showed that SREBP-1 transactivated the -911/29 apoA-II promoter 3.5-fold as well as truncated apoA-II promoter segments that contain 1, 2, 3, or 4 SREBP binding sites. Mutagenesis analysis showed that transactivation by SREBP was mainly affected by mutations in element AIIAB. Despite the strong transactivation of the apoA-II promoter by SREBP-1 we could not demonstrate significant changes on the endogenous apoA-II mRNA levels of HepG2 cells after cotransfection with SREBP-1 or in the presence or absence of cholesterol and 25-OH-cholesterol. An SREBP-1 mutant lacking the amino-terminal activation domain bound normally to its cognate sites and repressed the apoA-II promoter activity. Repression was also caused by specific amino acid substitutions of Leu, Val, or Gly for Lys359, which affected DNA binding. Repression by the DNA binding-deficient mutants was abolished by deletion of the amino-terminal activation domain (1 to 90) of SREBP-1. Overall, the findings suggest that the wild-type SREBP-1 can bind and transactivate efficiently the apoA-II promoter in cell culture. SREBP-1 mutants lacking the activation domain bind to their cognate sites and directly repress the apoA-II promoter whereas mutants defective in DNA binding indirectly repress the apoA-II promoter activity, possibly by a squelching mechanism.

Cell-type-dependent activity of the ubiquitous transcription factor USF in cellular proliferation and transcriptional activation

USF1 and USF2 are basic helix-loop-helix transcription factors implicated in the control of cellular proliferation. In HeLa cells, the USF proteins are transcriptionally active and their overexpression causes marked growth inhibition. In contrast, USF overexpression had essentially no effect on the proliferation of the Saos-2 osteosarcoma cell line. USF1 and USF2 also lacked transcriptional activity in Saos-2 cells when assayed by transient cotransfection with USF-dependent reporter genes. Yet, there was no difference in the expression, subcellular localization, or DNA-binding activity of the USF proteins in HeLa and Saos-2 cells. Furthermore, Gal4-USF1 and Gal4-USF2 fusion proteins activated transcription similarly in both cell lines. Mutational analysis and domain swapping experiments revealed that the small, highly conserved USF-specific region (USR) was responsible for the inactivity of USF in Saos-2 cells. In HeLa, the USR serves a dual function. It acts as an autonomous transcriptional activation domain at promoters containing an initiator element and also induces a conformational change that is required for USF activity at promoters lacking an initiator. Taken together, these results suggest a model in which the transcriptional activity of the USF proteins, and consequently their antiproliferative activity, is tightly controlled by interaction with a specialized coactivator that recognizes the conserved USR domain and, in contrast to USF, is not ubiquitous. The activity of USF is therefore context dependent, and evidence for USF DNA-binding activity in particular cells is insufficient to indicate USF function in transcriptional activation and growth control.

Differential roles of upstream stimulatory factors 1 and 2 in the transcriptional response of liver genes to glucose

USF1 and USF2 are ubiquitous transcription factors of the basic helix-loop-helix leucine zipper family. They form homo- and heterodimers and recognize a CACGTG motif termed E box. In the liver, USF binding activity is mainly accounted for by the USF1/USF2 heterodimer, which binds in vitro the glucose/carbohydrate response elements (GlRE/ChoRE) of glucose-responsive genes. To assign a physiological role of USFs in vivo, we have undertaken the disruption of USF1 and USF2 genes in mice. We present here the generation of USF1-deficient mice. In the liver of these mice, we demonstrate that USF2 remaining dimers can compensate for glucose responsiveness, even though the level of total USF binding activity is reduced by half as compared with wild type mice. The residual USF1 binding activity was similarly reduced in the previously reported USF2 -/- mice in which an impaired glucose responsiveness was observed (Vallet, V. S., Henrion, A. A., Bucchini, D., Casado, M. , Raymondjean, M., Kahn, A., and Vaulont, S. (1997) J. Biol. Chem. 272, 21944-21949). Taken together, these results clearly suggest differential transactivating efficiencies of USF1 and USF2 in promoting the glucose response. Furthermore, they support the view that USF2 is the functional transactivator of the glucose-responsive complex.

Overlapping roles and asymmetrical cross-regulation of the USF proteins in mice

USF1 and USF2 are ubiquitously expressed transcription factors implicated as antagonists of the c-Myc protooncoprotein in the control of cellular proliferation. To determine the biological role of the USF proteins, mutant mice were generated by homologous recombination in embryonic stem cells. USF1-null mice were viable and fertile, with only slight behavioral abnormalities. However, these mice contained elevated levels of USF2, which may compensate for the absence of USF1. In contrast, USF2-null mice contained reduced levels of USF1 and displayed an obvious growth defect: they were 20-40% smaller at birth than their wild-type or heterozygous littermates and maintained a smaller size with proportionate features throughout postnatal development. Some of the USF-deficient mice, especially among the females, were prone to spontaneous epileptic seizures, suggesting that USF is important in normal brain function. Among the double mutants, an embryonic lethal phenotype was observed for mice that were homozygous for the Usf2 mutation and either heterozygous or homozygous for the Usf1 mutation, demonstrating that the USF proteins are essential in embryonic development.

The keratinocyte-specific Epstein-Barr virus ED-L2 promoter is regulated by phorbol 12-myristate 13-acetate through two cis-regulatory elements containing E-box and Krüppel-like factor motifs

We previously employed 782 base pairs of the Epstein-Barr virus ED-L2 early lytic cycle promoter in a transgenic mouse model to target cyclin D1 to the stratified squamous epithelium of the tongue and esophagus. This promoter is located 5' to the transcriptional start site of a short open reading frame BNLF-2A and is immediately 3' to the BNLF-1 (LMP-1 oncogene) open reading frame. We studied transcriptional regulation of the ED-L2 promoter by phorbol 12-myristate 13-acetate (PMA) as a means of understanding the tissue specificity of this promoter. The transcriptional activity of the ED-L2 promoter was stimulated 40-fold by PMA and could be blocked with the compound H7 through antagonism of protein kinase C. 5' deletion analysis of the 782-base pair promoter demonstrated that the sequences necessary for PMA-stimulated trans-activation were located in two separate cis-regulatory regions of the promoter: -187 to -164 and -144 to -114 base pairs from the transcription start site of BNLF-2A. Importantly, mutation of critical base pairs in each region was sufficient to abolish PMA-stimulated trans-activation in the native ED-L2 promoter. Region -187 to -164 contains a CACCTG (E-box) motif, and region -144 to -114 contains a CACACCC motif. Both of these motifs are necessary for trans-activation by PMA. These regions do not, however, demonstrate enhancer characteristics when tested in a heterologous minimal promoter system. Variations of the CACACCC motif are found in other keratinocyte-specific promoters, as well as in the DNA binding motifs of the Krüppel-like family of transcription factors. Electrophoretic mobility shift assays with specific competitors and factor-specific antibody supershift assays demonstrated that one complex binding the -187 to -164 region containing the CACCTG nucleotides has characteristics of the helix-loop-helix protein upstream stimulatory factor, whereas a factor binding the CACACCC motif may be a member of the Krüppel-like family. These experiments show how ubiquitous and tissue-specific transcription factors induced by PMA regulate the ED-L2 promoter in squamous epithelial cells.

Glucose-dependent liver gene expression in upstream stimulatory factor 2 -/- mice

Upstream stimulatory factors (USF) 1 and 2 belong to the Myc family of transcription factors characterized by a basic/helix loop helix/leucine zipper domain responsible for dimerization and DNA binding. These ubiquitous factors form homo- and heterodimers and recognize in vitro a CACGTG core sequence termed E box. Through binding to E boxes of target genes, USF factors have been demonstrated to activate gene transcription and to enhance expression of some genes in response to various stimuli. In particular, in the liver USF1 and USF2 have been shown to bind in vitro glucose/carbohydrate response elements of glycolytic and lipogenic genes and have been proposed, from ex vivo experiments, to be involved in their transcriptional activation by glucose. However, the direct involvement of these factors in gene expression and nutrient gene regulation in vivo has not yet been demonstrated. Therefore, to gain insight into the specific role of USF1 and USF2 in vivo, and in particular to determine whether the USF products are required for the response of genes to glucose, we have created, by homologous recombination, USF2 -/- mice. In this paper, we provide the first evidence that USF2 proteins are required in vivo for a normal transcriptional response of L-type pyruvate kinase and Spot 14 genes to glucose in the liver.

Multiple regulatory elements and binding proteins of the 5'-flanking region of the human estrogen-responsive finger protein (efp) gene

The efp, a member of the RING finger family, was previously identified as an estrogen responsive gene. Here, we characterized basal promoter of the human efp gene. Transcription initiation site was found at position -60 G relative to the site for translation initiation, and TATA motif was absent. Deletion and mutation analyses of the 5'-flanking region using MCF-7 breast cancer cells indicated that the sequences located at -137 to -72 had the promoter activity for which an E-box (CACGTG) element at -110 to -105 was essential. EMSA showed that USF-1 bound to the E-box and that a protein-DNA complex was formed at the positive regulatory region (-137 to -110). Moreover, a strong negative regulatory region was present in -235 to -174. These findings suggest that the human efp promoter is regulated by multiple elements and their interacting factors, and the E-box near the transcription initiation site is essential for the basal promoter activity.

Regulation of the nuclear gene that encodes the alpha-subunit of the mitochondrial F0F1-ATP synthase complex. Activation by upstream stimulatory factor 2

We have previously identified several positive cis-acting regulatory regions in the promoters of the bovine and human nuclear-encoded mitochondrial F0F1-ATP synthase alpha-subunit genes (ATPA). One of these cis-acting regions contains the sequence 5'-CACGTG-3' (an E-box), to which a number of transcription factors containing a basic helix-loop-helix motif can bind. This E-box element is required for maximum activity of the ATPA promoter in HeLa cells. The present study identifies the human transcription factor, upstream stimulatory factor 2 (USF2), as a nuclear factor that binds to the ATPA E-box and demonstrates that USF2 plays a critical role in the activation of the ATPA gene in vivo. Evidence includes the following. Antiserum directed against USF2 recognized factors present in HeLa nuclear extracts that interact with the ATPA promoter in mobility shift assays. Wild-type USF2 proteins synthesized from expression vectors trans-activated the ATPA promoter through the E-box, whereas truncated USF2 proteins devoid of the amino-terminal activation domains did not. Importantly, expression of a dominant-negative mutant of USF2 lacking the basic DNA binding domain but able to dimerize with endogenous USF proteins significantly reduced the level of activation of the ATPA promoter caused by ectopically coexpressed USF2, demonstrating the importance of endogenous USF2 in activation of the ATPA gene.

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.

Mouse USF1 gene cloning: comparative organization within the c-myc gene family

Upstream stimulatory factors (USF/MLTF) belong to the c-myc family of transcription factors. Through binding to target DNA as dimers, the ubiquitous USF proteins regulate a variety of genes. USF proteins are encoded by two genes, USF1 and USF2. Protein sequences of USF1 and 2 are highly homologous across species, suggesting functional conservation. To determine whether the genomic organization was conserved between USF1 and USF2, we isolated the murine USF1 gene and characterized its genomic structure. Both genes are similarly organized in 10 exons spanning over 10 kbp. By the 5'-rapid amplification of cDNA ends and S1 nuclease mapping methods, exon 1 was defined and the transcription initiation sites were mapped. The sequence of 8 kb of the gene, including 1.75 kb of 5'-flanking DNA, was determined. The promoter region is GC rich and lacks a typical TATA or CCAAT element. Strikingly, a comparison of the murine and human untranslated sequences reveals regions that exhibit greater than 73% sequence identity. A genomic alignment of the dimerization and DNA binding domains is presented for five genes of the c-myc family, suggesting a hypothetical common ancestor gene.

Retinoic acid-induced transcriptional modulation of the human interferon-gamma promoter

Disregulation of vitamin A metabolism is able to generate different immunological effects, including altered response to infection, reduced IgG production, and differential regulation of cytokine gene expression (including interleukin-2 and -4 and interferon-gamma (IFN-gamma)). In particular, IFN-gamma gene expression is significantly affected by vitamin A and/or its derivatives (e.g. retinoic acid (RA)). Here, we analyze the effect of retinoic acid on IFN-gamma transcription. Transient transfection assays in the human T lymphoblastoid cell line Jurkat demonstrated that the activation of the IFN-gamma promoter was significantly down-regulated in the presence of RA. Surprisingly, two different AP-1/CREB-ATF-binding elements situated in the initial 108 base pairs of the IFN-gamma promoter and previously shown to be critical for transcriptional activity were unaffected by RA. Utilizing promoter deletions and electrophoretic mobility shift analysis, we identified a USF/EGR-1-binding element cooperating in the modulation of IFN-gamma promoter activity by RA. This element was found to be situated in a position of the IFN-gamma promoter close to a silencer element previously identified in our laboratory. These results suggest that direct modulation of IFN-gamma promoter activity is one of the possible mechanisms involved in the inhibitory effect of retinoids on IFN-gamma gene expression.

The basic region/helix-loop-helix/leucine repeat transcription factor USF interferes with Ras transformation

Upstream stimulatory factor (USF) is a transcription factor of the basic region/helix-loop-helix/leucine repeat family. It shares the same DNA-binding sequence as the myc oncogene. Based on the three-dimensional structures, its DNA-binding domain is structurally related to that of Max, the partner of Myc. In addition, USF can form heterodimers with a related factor, Fos-interacting protein/upstream stimulatory factor 2 (FIP/USF2), which has been shown to directly interact with Fos. In view of the provocative relationship of USF with other factors involved in cell proliferation, we investigated whether USF could also play a role in cellular growth control. In this study, we report that USF is not an oncogene, but interferes with Ras-driven transformation. This inhibitory effect is independent of USF transactivating domains, but requires its DNA-binding activity. However, the minimal USF DNA-binding domain does not display this inhibitory effect, and even slightly enhances Ras transformation. On the basis of these data, we propose that USF may play an important role in the control of cell growth and proliferation, through both binding to promoter sequences and specific protein/protein interactions.

Immunochemical characterization and transacting properties of upstream stimulatory factor isoforms

The ubiquitous upstream stimulatory factor (USF) transcription factors encoded by two distinct genes (USF1 and USF2) exist under the form of various dimers able to bind E-boxes. We report the molecular cloning and functional characterization of USF2 isoforms, corresponding to a 44-kDa subunit, USF2a, and a new 38-kDa subunit, USF2b, generated by differential splicing. Using specific anti-USF antibodies, we define the different binding complexes in various nuclear extracts. In vivo, the USF1/USF2a heterodimer represents over 66% of the USF binding activity whereas the USF1 and USF2a homodimers represent less than 10%, which strongly suggests an in vivo preferential association in heterodimers. In particular, an USF1/USF2b heterodimer accounted for almost 15% of the USF species in some cells. The preferential heterodimerization of USF subunits was reproduced ex vivo, while the in vitro association of cotranslated subunits, or recombinant USF proteins, appeared to be random. In transiently transfected HeLa or hepatoma cells, USF2a and USF1 homodimers transactivated a minimal promoter with similar efficiency, whereas USF2b, which lacks an internal 67-amino acid domain, was a poor transactivator. Additionally, USF2b was an efficient as USF1 and USF2a homodimers in transactivating the liver-specific pyruvate kinase gene promoter.