Identification of an upstream region of the mouse p53 promoter critical for transcriptional expression

Context is key: Understanding the regulation, functional control, and activities of the p53 tumour suppressor

The p53 tumour suppressor is considered one of the most critical genes in cancer biology. By upregulating apoptosis, cell cycle arrest, and DNA damage repair in normal cells, p53 prevents the propagation of cells with tumorigenic potential; therefore, mutations in p53 are associated with carcinogenic transformation and can be accompanied by the accumulation of a novel gain-of-function oncogenic protein, mutant p53. Although p53 is most often understood to utilize context-dependent post-translational modifications to achieve regulation of its many target genes, recent research has also sought to define other mechanisms of regulating p53 gene expression prior to translation and to understand how this alternative regulation of p53 may influence target gene expression and cellular outcome. This review attempts to summarize what is known about p53 regulation at the transcriptional, post-transcriptional, and post-translational levels while paying special attention to the ways in which context may influence p53 regulation and subsequent regulation of its target genes.

Ectopic Methylation of a Single Persistently Unmethylated CpG in the Promoter of the Vitellogenin Gene Abolishes Its Inducibility by Estrogen through Attenuation of Upstream Stimulating Factor Binding

The enhancer/promoter of the vitellogenin II gene (VTG) has been extensively studied as a model system of vertebrate transcriptional control. While deletion mutagenesis and in vivo footprinting identified the transcription factor (TF) binding sites governing its tissue specificity, DNase hypersensitivity and DNA methylation studies revealed the epigenetic changes accompanying its hormone-dependent activation. Moreover, upon induction with estrogen (E₂), the region flanking the estrogen-responsive element (ERE) was reported to undergo active DNA demethylation. We now show that although the VTG ERE is methylated in embryonic chicken liver and in LMH/2A hepatocytes, its induction by E₂ was not accompanied by extensive demethylation. In contrast, E₂ failed to activate a VTG enhancer/promoter-controlled luciferase reporter gene methylated by SssI. Surprisingly, this inducibility difference could be traced not to the ERE but rather to a single CpG in an E-box (CACGTG) sequence upstream of the VTG TATA box, which is unmethylated in vivo but methylated by SssI. We demonstrate that this E-box binds the upstream stimulating factor USF1/2. Selective methylation of the CpG within this binding site with an E-box-specific DNA methyltransferase, Eco72IM, was sufficient to attenuate USF1/2 binding in vitro and abolish the hormone-induced transcription of the VTG gene in the reporter system.

Transcription Factor USF1 Is Required for Maintenance of Germline Stem Cells in Male Mice

A prerequisite for lifelong sperm production is that spermatogonial stem cells (SSCs) balance self-renewal and differentiation, yet factors required for this balance remain largely undefined. Using mouse genetics, we now demonstrate that the ubiquitously expressed transcription factor upstream stimulatory factor (USF)1 is critical for the maintenance of SSCs. We show that USF1 is not only detected in Sertoli cells as previously reported, but also in SSCs. Usf1-deficient mice display progressive spermatogenic decline as a result of age-dependent loss of SSCs. According to our data, the germ cell defect in Usf1-/- mice cannot be attributed to impairment of Sertoli cell development, maturation, or function, but instead is likely due to an inability of SSCs to maintain a quiescent state. SSCs of Usf1-/- mice undergo continuous proliferation, which provides an explanation for their age-dependent depletion. The proliferation-coupled exhaustion of SSCs in turn results in progressive degeneration of the seminiferous epithelium, gradual decrease in sperm production, and testicular atrophy. We conclude that the general transcription factor USF1 is indispensable for the proper maintenance of mammalian spermatogenesis.

p53 requires the stress sensor USF1 to direct appropriate cell fate decision

Genomic instability is a major hallmark of cancer. To maintain genomic integrity, cells are equipped with dedicated sensors to monitor DNA repair or to force damaged cells into death programs. The tumor suppressor p53 is central in this process. Here, we report that the ubiquitous transcription factor Upstream Stimulatory factor 1 (USF1) coordinates p53 function in making proper cell fate decisions. USF1 stabilizes the p53 protein and promotes a transient cell cycle arrest, in the presence of DNA damage. Thus, cell proliferation is maintained inappropriately in Usf1 KO mice and in USF1-deficient melanoma cells challenged by genotoxic stress. We further demonstrate that the loss of USF1 compromises p53 stability by enhancing p53-MDM2 complex formation and MDM2-mediated degradation of p53. In USF1-deficient cells, the level of p53 can be restored by the re-expression of full-length USF1 protein similarly to what is observed using Nutlin-3, a specific inhibitor that prevents p53-MDM2 interaction. Consistent with a new function for USF1, a USF1 truncated protein lacking its DNA-binding and transactivation domains can also restore the induction and activity of p53. These findings establish that p53 function requires the ubiquitous stress sensor USF1 for appropriate cell fate decisions in response to DNA-damage. They underscore the new role of USF1 and give new clues of how p53 loss of function can occur in any cell type. Finally, these findings are of clinical relevance because they provide new therapeutic prospects in stabilizing and reactivating the p53 pathway.

Cancer is a complex disease that is characterized by the sequential accumulation of genetic mutations. Exposure to environmental agents, such as solar ultraviolet, induces such alterations and thus contributes to the development of genomic instability. The tumor suppressor p53 has a central role in orchestrating cellular responses to genotoxic stress. In response to DNA-damage, p53 is stabilized and activated to direct cell fate decisions. Cells in which p53 stabilization is compromised become more vulnerable to mutagenic agents and hence the mutation rate increases, which promotes tumor development. Stabilization of p53 is thus a critical step towards cancer prevention. Using a genetic approach, we demonstrate that the ubiquitous transcription factor Upstream Stimulatory factor 1 (USF1) is required for immediate p53 stabilization and appropriate cell fate decisions following genotoxic stress. Furthermore, we show that this involves a novel function of USF1 that underscores its critical role as a stress sensor. The loss of USF1 expression should thus be considered as a potential initiator of tumorigenesis in the context of environmental insults.

Upstream stimulatory factor 1 activates GATA5 expression through an E-box motif

Silencing of GATA5 gene expression as a result of promoter hypermethylation has been observed in lung, gastrointestinal and ovarian cancers. However, the regulation of GATA5 gene expression has been poorly understood. In the present study, we have demonstrated that an E (enhancer)-box in the GATA5 promoter (bp -118 to -113 in mice; bp -164 to -159 in humans) positively regulates GATA5 transcription by binding USF1 (upstream stimulatory factor 1). Using site-directed mutagenesis, EMSA (electrophoretic mobility-shift analysis) and affinity chromatography, we found that USF1 specifically binds to the E-box sequence (5'-CACGTG-3'), but not to a mutated E-box. CpG methylation of this E-box significantly diminished its binding of transcription factors. Mutation of the E-box within a GATA5 promoter fragment significantly decreased promoter activity in a luciferase reporter assay. Chromatin immunoprecipitation identified that USF1 physiologically interacts with the GATA5 promoter E-box in mouse intestinal mucosa, which has the highest GATA5 gene expression in mouse. Co-transfection with a USF1 expression plasmid significantly increased GATA5 promoter-driven luciferase transcription. Furthermore, real-time and RT (reverse transcription)-PCR analyses confirmed that overexpression of USF1 activates endogenous GATA5 gene expression in human bronchial epithelial cells. The present study provides the first evidence that USF1 activates GATA5 gene expression through the E-box motif and suggests a potential mechanism (disruption of the E-box) by which GATA5 promoter methylation reduces GATA5 expression in cancer.

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.

Increased p53 transcription prior to DNA synthesis is regulated through a novel regulatory element within the p53 promoter

p53 mRNA levels are tightly regulated during the cell cycle with its transcription being induced prior to DNA synthesis. However, the mechanism controlling this regulation is not well defined. Through characterizing an additional 1000 bp of upstream DNA sequences of the murine p53 gene, we identified new positive and negative regulatory elements. Furthermore, we found a trans-acting factor(s) that binds within a positive cis-acting element (-972/-953) in a manner indicative of regulation during the cell cycle. When Swiss3T3 cells are arrested by serum depletion p53 mRNA levels decrease and binding of this regulatory factor(s) to the promoter is reduced. Upon serum stimulation, the regulatory factor(s) binds the promoter and p53 mRNA levels increase prior to the cells entering S phase. When the factors are experimentally sequestered from the promoter or when the regulatory element is deleted from the promoter, p53 promoter activity is reduced. There is no further reduction in p53 promoter activity upon serum depletion and the kinetics of induction upon serum stimulation is delayed by approximately 5 h. These findings indicate that a factor(s) binding within the -972/-953 regulatory element on the p53 promoter is important for the proper regulation of p53 mRNA expression in response to mitogen stimulation. Our initial findings indicate that a member of the C/EBP family of transcription factors may play a role in this regulation.

The Y-box-binding protein, YB1, is a potential negative regulator of the p53 tumor suppressor

The p53 tumor suppressor plays a major role in preventing tumor development by transactivating genes to remove or repair potentially tumorigenic cells. Here we show that the Y-box-binding protein, YB1, acts as a negative regulator of p53. Using reporter assays we show that YB1 represses transcription of the p53 promoter in a sequence-specific manner. We also show that YB1 reduces endogenous levels of p53, which in turn reduces p53 activity. Conversely, inhibiting YB1 in a variety of tumor cell lines induces p53 activity, resulting in significant apoptosis via a p53-dependent pathway. These data suggest that YB1 may, in some situations, protect cells from p53-mediated apoptosis, indicating that YB1 may be a good target for the development of new therapeutics.

Partial purification and characterization of an 80-kDa transcription factor binding to bHLH motif in the rat p53 promoter

E-box is one of potential cis-regulatory elements for the p53 gene. It was previously reported that USF bound to the E-box of the p53 gene. Recently, we demonstrated that an 80-kDa protein other than USF bound to the E-box and activated the transcription of the p53 gene. In the present study, the 80-kDa protein was partially purified and characterized. First, we confirmed that nuclear factors bound to the E-box in sequence-specific manner by the oligonucleotide competition assay. The binding protein to the E-box was partially purified by a sequence-specific DNA affinity chromatography. The active fraction was analyzed by SDS-PAGE and southwestern blotting assay, which showed that the 80-kDa protein was enriched. The binding activity of the 80-kDa protein was not decreased in the presence of 1.4 M urea. In addition, the binding activity was stable up to 50 degrees C. Treatment of EDTA showed that the 80-kDa protein did not require divalent cation such as Mg2+ for the maximum DNA binding activity. The competition assay with non-specific competitor, poly (dI-dC) showed that the 80-kDa protein had high affinity to its binding site. These biochemical properties provide useful insights into the 80-kDa nuclear factor binding to the p53 promoter.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Identification of a rare polymorphism in the human TP53 promoter

The majority of families with classic Li-Fraumeni Syndrome (LFS) and a significant proportion of Li-Fraumeni-like (LFL) families have a germline mutation in the TP53 tumor suppressor gene. However around 20% of LFS and 60% of LFL families have no identifiable genetic defect in the coding region or splice junctions of TP53, and the genetic basis for cancer susceptibility in these families remains largely uncharacterized. To determine whether promoter mutations could be responsible for the Li-Fraumeni phenotype, we sequenced the TP53 promoter in index cases from members of classic LFS and LFL families without detectable TP53 mutations. We identified an identical single nucleotide deletion within the C/EBP- like site of the promoter in two out of eighteen such families (11%), compared to only one of a total of 366 control samples (0.3%). Although this result is highly significant (P=0.006, Fischer's exact test), the mutation did not affect the expression of TP53 in our hands. We provide evidence that this site is not utilized in the wild type TP53 promoter and further, that mutation of this site in LFS/LFL does not have a functional effect. We conclude that the sequence variant is a rare polymorphism arising within the TP53 promoter. However, the significantly increased frequency of this variant in LFS/LFL remains intriguing.

Cloning and characterization of murine p53 upstream sequences reveals additional positive transcriptional regulatory elements

Transcriptional regulation of the p53 gene plays an important role leading to elevated expression of mutant p53 alleles in tumor cells. In addition, alterations in p53 transcription levels occur in response to changes in the cell cycle. Previous work had identified a number of regulatory sites at the 5'-end of the murine p53 promoter. During the characterization of the 5'-end of the cloned murine p53 promoter, we identified a 28 bp positive regulatory element that participates in three distinct DNA-protein complexes. The binding by nuclear factors to each one of these sites contributes to the overall activity of the p53 promoter. One site is a potential recognition sequence for members of the ETS family of transcription factors, which are known regulators of the human p53 promoter. Since six nucleotides in the middle of this required element were not present in the previously published sequence of the murine promoter, we recloned this region from C57/BL6 cells and confirmed their presence in the genome. The removal of this regulatory element completely abolishes p53 promoter activity.

A p53 dose-response relationship for sensitivity to DNA damage in isogenic teratocarcinoma cells

Teratocarcinomas are tumors that arise from primordial germ cells and are readily curable with DNA-damaging chemotherapeutic drugs. Teratocarcinoma cells ex vivo in tissue culture are also relatively chemosensitive and undergo apoptotic death in response to DNA damage. We have previously hypothesized that the observed sensitivity of this tumor type to DNA damage is related to high basal expression of wild-type p53 protein. We have now addressed this issue by characterizing the DNA damage response of isogenic teratocarcinoma cells that differ only in their level of expression of wild-type p53 protein. We find a clear p53 dose-response relationship in these cells for rapid apoptosis following DNA damage that correlates with diminished colony formation in clonogenic survival assays. These results suggest that strategies to increase basal wild-type p53 protein expression prior to treatment with DNA-damaging drugs may improve curability in other tumor types.

Characterization of nuclear factors binding to AT-rich element in the rat p53 promoter

In this study, we identified AT-rich element located at positions -504 to -516 in the rat p53 promoter by DNase I foot printing assay. This region was previously identified as a positive regulatory element in the murine p53 promoter and designated as PBF1 (p53 binding factor 1) binding site. However, the proteins binding to this AT-rich element have not been identified yet. Therefore, we characterized the binding protein by various biochemical methods. First, we confirmed that by the oligonucleotide competition assay, nuclear factors bound to the AT-rich element in a sequence-specific manner. Two binding proteins were identified in southwestern blotting analysis and the molecular masses of the proteins were 60 and 40 kDa, respectively. The proteins were stable to denaturants or ionic strength. Treatment of chelators showed that the binding proteins did not require divalent cation for DNA-binding activity. In addition, the binding proteins were labile to protease treatment. This study showed that 60 and 40 kDa proteins bound to AT-rich element and the physico-chemical properties provided new insights into the binding proteins.

Identification of a transcription factor, an 80-kDa protein that interacts with the HLH recognition motif of the rat p53 promoter

The p53 promoter has been shown to contain a number of potential regulatory motifs. It was previously reported that the upstream stimulating factor (USF) played a central role in regulating the p53 expression. The USF binding site, E-box, is located around 40 bp upstream of the major transcription start site. In this study, it was confirmed that the E-box binds to proteins by DNase I footprinting assay. In the electrophoretic mobility shift assay (EMSA), two retarded bands were detected. One band was abolished by the competition of USF consensus oligonucleotide, but the other band was not. This result indicated that a factor, other than USF, was bound to the E-box. The molecular masses of the binding proteins were determined by a Southwestern-blotting assay. As a result, 46- and 80-kDa proteins were detected. The 46-kDa protein was eliminated by the competition of USF consensus oligonucleotide. Also, the Southwestern-blotting assay with 32P-labeled USF consensus oligonucleotide showed only a 46-kDa protein. Therefore, the 46-kDa protein was USF. These results showed that USF and the 80-kDa protein were bound to the E-box. In addition, it was proved by in vitro transcription assay that this 80-kDa protein had a basal transcriptional activity.Key words: E-box, HLH, rat p53 promoter, transcription factor, upstream stimulating factor (USF).

Characterization of a nuclear factor that binds to AP1-like element in the rat p53 promoter during liver regeneration

The transcription level of the rat p53 gene increases at 5-12 h in the regenerating liver after partial hepatectomy. It was previously reported that an activator protein 1 (AP1)-like element (-264--284) mediated the induced transcription of the rat p53 gene during liver regeneration. In this study, we characterize the protein binding to the AP1-like element by various methods. Oligonucleotide competition assays showed that the binding protein did not require AP1 consensus sequence. Therefore, the binding protein is not an AP1 family protein. Zn(2+) was required for maximum DNA-binding activity of the protein, suggesting that the binding protein contains zinc fingers. The binding protein was highly resistant to denaturant. Even 1.8 M urea did not eliminate the protein-DNA complexes. In addition, the binding protein was stable up to 55 degrees C. The protein-DNA complexes were abolished in the presence of 0.6 M NaCl and higher. Protease clipping assay showed that the protein had a protease-resistant core DNA binding domain. These results provided new insights into the structure of the protein that binds to the AP1-like element of the p53 promoter during liver regeneration.

Maf transcriptionally activates the mouse p53 promoter and causes a p53-dependent cell death

An increase in the level of the tumor suppressor protein p53 can induce cell cycle arrest or cell death. Although mechanisms for regulating the life span of p53 have been described, there is growing evidence that transcriptional regulation of the p53 gene contributes significantly to controlling p53 protein levels and therefore the fate of a cell. However, the signal transduction pathways that lead to transcriptional activation of the p53 gene are poorly understood. The oncoprotein v-Maf and its cellular counterparts belong to the large combinatorially complex basic leucine zipper family of transcription factors, which include the AP1 family. To date few cellular targets of c-Maf have been identified. It is demonstrated here that v-Maf can bind as a homodimer to a variant Maf recognition element located between -66 and -54 upstream in the mouse p53 promoter. V-Maf and its cellular counterparts are shown to activate p53 expression through this site. The ability of v-Maf to activate p53 expression is modulated by AP1 family members. In addition, overexpression of v-Maf in primary cells leads to a p53-dependent cell death. Thus, Maf and members of the AP1 family are able to regulate p53 expression through this site in the p53 promoter.

Biochemical characterization of a nuclear factor that binds to NF1-like elements in the rat p53 promoter

We previously reported that two nuclear factor 1-like elements mediated the transcription of the rat p53 gene. A 40-kDa protein was shown to bind to these elements, which was different from common NF1 family proteins. In this study, the biochemical properties of the 40-kDa binding protein were investigated. The metal ion dependency of the protein was examined with various chelators; the protein was proved to require Mg(2+) for maximum DNA-binding activity. The binding protein was highly resistant to ionic strength and denaturant. The protein-DNA complex was reduced at high NaCl concentration, but residual DNA-binding activity remained. Even 2 M urea did not completely eliminate the formation of protein-DNA complex. DNA-binding activity of the protein was also stable at high temperature. Treatment of the protein-DNA complex with increasing concentrations of proteinase K or trypsin demonstrated the existence of a protease-resistant DNA-bound core. These biochemical properties provide new insight into the 40-kDa NF1-like nuclear factor.

The adenovirus oncoprotein E1a stimulates binding of transcription factor ETF to transcriptionally activate the p53 gene

Expression of the tumor suppressor protein p53 plays an important role in regulating the cellular response to DNA damage. During adenovirus infection, levels of p53 protein also increase. It has been shown that this increase is due not only to increased stability of the p53 protein but to the transcriptional activation of the p53 gene during infection. We demonstrate here that the E1a proteins of adenovirus are responsible for activating the mouse p53 gene and that both major E1a proteins, 243R and 289R, are required for complete activation. E1a brings about the binding of two cellular transcription factors to the mouse p53 promoter. One of these, ETF, binds to three upstream sites in the p53 promoter and one downstream site, whereas E2F binds to one upstream site in the presence of E1a. Our studies indicate that E2F binding is not essential for activation of the p53 promoter but that ETF is. Our data indicate the ETF site located downstream of the start site of transcription is the key site in conferring E1a responsiveness on the p53 promoter.

A 40-kDa NF1-like protein, not YY1, binds to the rat p53 promoter for transactivation in various rat organs

Two recognition motifs of a 40-kDa NF1-like protein were previously identified in the rat p53 promoter. One is located between -296 and -312 (NF1-like element 1) and the other between -195 and -219 (NF1-like element 2). The latter one was also identified as a NF1/YY1 recognition motif in the human p53 promoter. NF1 or YY1 binds to the motif and regulates the expression of the human p53 gene in a tissue-specific manner. In this study, we investigated the binding protein for NF1-like element 2 in various rat tissues. Unlike the human p53 transcription, an NF1-like protein, not YY1, bound to the motif in every tested tissue: thymus, kidney, and spleen. In vitro transcription assay also confirmed that the NF1-like protein regulated the p53 transcription in rat spleen, although the human p53 transcription was regulated by YY1 in that organ. The molecular mass of the binding protein was determined to be 40 kDa, which was the same as that of the NF1-like protein identified in liver. Therefore, the 40-kDa NF1-like protein may be a universal transcription regulator for the rat p53 gene.Key words: NF1-like protein, p53, promoter, transcription regulation, YY1.

Transcription of the rat p53 gene is mediated by factor binding to two recognition motifs of NF1-like protein

In this study we analyzed the ratp53 promoter by electrophoretic mobility shift assay (EMSA) and DNase I footprinting analysis. As a result we identified two protein binding elements (element 1: -296 to -312, element 2: -195 to -219) with sequence homology to each other. The two identified elements bind to the same kind of protein. To identify the protein binding to these elements, competition assays were carried out with double stranded oligonucleotides containing NF1, YY1, and CRE consensus motifs. Only the NF1 consensus motif competed with element 1 and 2. Element 2 is conserved between the rat, human, and mouse p53 promoters, and has an NF1 consensus motif. However, the sequences of element 1 are comparatively variable between the species. Only the element 1 region of the rat p53 promoter has partial homology to the NF1 consensus motif. This suggests that the element 1 is specific for the rat p53 gene. The molecular mass of the binding protein, determined by Southwestern blotting analysis, was 40 kDa, which is different from that of NF1. In EMSA with an anti-NF1 antibody, DNA-protein complexes were neither supershifted nor decreased. The 40 kDa protein was also detected in rat spleen and lung, but not in kidney. The binding protein was purified by sequence-specific DNA affinity chromatography and it was confirmed that the purified protein binds to the two regions. It was also proved that the identified two elements are required for basal level transcription of the rat p53 gene by in vitro transcription assay.

Autoregulation of eukaryotic transcription factors

The structures of several promoters regulating the expression of eukaryotic transcription factors have in recent years been examined. In many cases there is good evidence for autoregulation, in which a given factor binds to its own promoter and either activates or represses transcription. Autoregulation occurs in all eukaryotes and is an important component in controlling expression of basal, cell cycle specific, inducible response and cell type-specific factors. The basal factors are autoregulatory, being strictly necessary for their own expression, and as such must be epigenetically inherited. Autoregulation of stimulus response factors typically serves to amplify cellular signals transiently and also to attenuate the response whether or not a given inducer remains. Cell cycle-specific transcription factors are positively and negatively autoregulatory, but this frequently depends on interlocking circuits among family members. Autoregulation of cell type-specific factors results in a form of cellular memory that can contribute, or define, a determined state. Autoregulation of transcription factors provides a simple circuitry, useful in many cellular circumstances, that does not require the involvement of additional factors, which, in turn, would need to be subject to another hierarchy of regulation. Autoregulation additionally can provide a direct means to sense and control the cellular conce]ntration of a given factor. However, autoregulatory loops are often dependent on cellular pathways that create the circumstances under which autoregulation occurs.

Binding of upstream stimulatory factor to an E-box in the 3'-flanking region stimulates alpha1(I) collagen gene transcription

Since several lines of evidence implicate the 3'-flanking region in regulating alpha1(I) collagen gene transcription, we analyzed 12. 4-kilobase pairs of 3'-flanking sequence of the murine alpha1(I) collagen gene for transcriptional elements. A region of the 3'-flanking region stimulated expression of the heterologous beta-globin gene promoter in an enhancer trap plasmid and of the alpha1(I) collagen gene promoter in a collagen-luciferase reporter gene construct when located 3' to the luciferase reporter gene. DNase I footprinting analysis demonstrated the presence of three regions where DNA binding proteins specifically interact within this 3'-stimulatory region. Inspection of the DNA sequence revealed a consensus E-box, a binding site for basic helix-loop-helix proteins, in one of the protein binding sites. Mobility shift assays demonstrated that upstream stimulatory factors (USF) USF-1 and USF-2 bind to this E-box. Mutating the E-box in the context of the 3'-flanking region confirmed that it contributes to the enhancement of transcriptional activity of the alpha1(I) collagen gene promoter. Mutations in all three protein binding sites abolished transcriptional activation by the 3'-flanking region, suggesting a complex interaction among the trans-acting factors in enhancing transcriptional activity. Thus, a region of the 3'-flanking region of the alpha1(I) collagen gene stimulates transcription of the alpha1(I) collagen gene promoter, and USF-1 and USF-2 contribute to this transcriptional stimulation.