Evidence for three major transcription activation elements in the proximal mouse proalpha2(I) collagen promoter

High doses of TGF-β potently suppress type I collagen via the transcription factor CUX1

Uncontrolled expression of collagen often leads to tissue scarring and loss of organ function. In this study, we identify a molecular mechanism that may enable us to switch off collagen production when unnecessary (i.e., fibrosis). We conclude that CUX1, which is a CCAAT binding factor displacement protein, may serve as a therapeutic target in treating fibrosis.

Transforming growth factor-β (TGF-β) is an inducer of type I collagen, and uncontrolled collagen production leads to tissue scarring and organ failure. Here we hypothesize that uncovering a molecular mechanism that enables us to switch off type I collagen may prove beneficial in treating fibrosis. For the first time, to our knowledge, we provide evidence that CUX1 acts as a negative regulator of TGF-β and potent inhibitor of type I collagen transcription. We show that CUX1, a CCAAT displacement protein, is associated with reduced expression of type I collagen both in vivo and in vitro. We show that enhancing the expression of CUX1 results in effective suppression of type I collagen. We demonstrate that the mechanism by which CUX1 suppresses type I collagen is through interfering with gene transcription. In addition, using an in vivo murine model of aristolochic acid (AA)-induced interstitial fibrosis and human AA nephropathy, we observe that CUX1 expression was significantly reduced in fibrotic tissue when compared to control samples. Moreover, silencing of CUX1 in fibroblasts from kidneys of patients with renal fibrosis resulted in increased type I collagen expression. Furthermore, the abnormal CUX1 expression was restored by addition of TGF-β via the p38 mitogen-activated protein kinase pathway. Collectively, our study demonstrates that modifications of CUX1 expression lead to aberrant expression of type I collagen, which may provide a molecular basis for fibrogenesis.

Transcriptional regulation of T helper 17 cell differentiation

The third lineage of T helper subsets, Th17, has recently been identified as an IL- 17-producing CD4+ Th cell, and its functions and regulatory mechanisms have been extensively characterized in immune responses. Functional studies have provided evidence that Th17 cells are important for the modulation of autoimmune responses, such as chronic asthma, rheumatoid arthritis, inflammatory bowel diseases, and multiple sclerosis. Murine Th17 cell differentiation is enhanced by the coordinated functions of distinct cytokines including TGFbeta, IL-6, IL-21, and IL-23, whereas IL-2, IL-4, IFNgamma, and IL-27 inhibit its differentiation. In addition, Th17 cells are controlled by several transcription factors such as RORgammat, IRF4, BATF, FoxP3, T-bet, PPARgamma, E-FABP, and SOCSs. This review focuses on the functions and regulatory mechanisms of several transcription factors in the control of Th17 cell differentiation.

Regulatory effect of extracellular signal-regulated kinases (ERK) on type I collagen synthesis in human dermal fibroblasts stimulated by IL-4 and IL-13

Synthesis of collagen is up-regulated by pro-fibrogenic growth factors and cytokines such as TGF-beta 1, IL-4, and IL-13 binding to their corresponding cell membrane receptors of fibroblasts. The ERK pathway is an important MAPK signaling pathway that is involved in regulating cell function. The aim of our studies was to examine effects of IL-4 and IL-13 on the ERK signaling pathway and its function in regulating type I collagen gene expression in human fibroblasts. We found that human dermal fibroblasts treated with IL-4 and IL-13 exhibited an increase in the activated ERK1/2 pathway. As well, pro-fibrogenic cytokines increased the promoter activity of type I collagen, and this activity decreased with cells that were co-transfected with dominant negative plasmids of ERK1 and 2. RT-PCR confirmed that collagen transcript levels decreased when cells were transfected with dn ERK1 and 2 and then further stimulated with IL-4 and IL-13. These results were also mirrored with collagen secretion assays. In addition, we studied the role for transcription factor Elk-1 known to be activated via the ERK pathway. Dominant negative Elk-1 showed inhibition of collagen promoter activity in fibroblasts transfected with full collagen type I promoter or two fragments which contain the Elk-1 binding site. Our results suggest that the modulation of collagen gene expression may occur via the ERK pathway and is mediated by Elk-1.

IGF-I-induced VEGF expression in HUVEC involves phosphorylation and inhibition of poly(ADP-ribose)polymerase

Insulin-like growth factor-I (IGF-I) has been shown to promote angiogenesis by enhancing vascular endothelial growth factor (VEGF) expression. However, how IGF-I-induces VEGF expression is not yet fully understood. With this investigation, we propose a new possible mechanism involving downregulation of poly(ADP-ribosyl)ation (pADPR). We first demonstrated that IGF-I increased VEGF protein expression in endothelial cells. Inhibitors of mitogen activated kinase (PD 98059), phosphatidyl-3-inositol-kinase (LY 294002), and protein kinase C (staurosporine) diminished the IGF-I effect suggesting the involvement of signal transduction. Since there is an established link between pADPR and transcriptional activity, we focused on a possible role of poly(ADP-ribose)polymerase (PARP). The inhibition of PARP by 3-aminobenzamide or nicotinamide enhanced VEGF expression. Additionally, IGF-I markedly decreased PARP activity. Furthermore, the IGF-I-mediated inhibition of PARP could be demonstrated as a result of protein phosphorylation since phosphorylation of PARP decreased its activity in vitro and IGF-I treatment of endothelial cells induced PARP phosphorylation. The IGF-I-mediated phosphorylation and inhibition of PARP represent a novel mechanism of VEGF protein expression.

Fibrotic lung fibroblasts show blunted inhibition by cAMP due to deficient cAMP response element-binding protein phosphorylation

Pulmonary fibroblasts regulate extracellular matrix production and degradation; thus, they are critical for maintenance of lung structure, function, and repair. In pulmonary fibrosis, fibroblasts produce excess collagen and form fibrotic foci that eventually impair lung function, but the mechanisms responsible for these alterations are not known. Receptors coupled to the stimulation of cAMP production can inhibit activation of fibroblasts and thereby are antifibrotic. To test whether this signaling pathway is altered in pulmonary fibrosis, we compared the ability of normal adult human pulmonary fibroblasts to generate and respond to cAMP with that of cells isolated from lungs with idiopathic pulmonary fibrosis. Serum- and transforming growth factor (TGF)-beta-stimulated cell proliferation was inhibited approximately 50% by forskolin and approximately 100% by prostaglandin (PG) E(2) in the normal cells but substantially less in the diseased cells. Collagen synthesis was also inhibited >50% by the same drugs in the normal cells but significantly less so in the diseased cells, despite responding with similar increases in cAMP production. Although expression of protein kinase A (PKA) and cAMP-stimulated PKA activity were similar in both the normal and diseased cell types, forskolin- and PGE(2)-stimulated cAMP response element-binding protein (CREB) phosphorylation was decreased in the diseased cell lines compared with the normal cells. cAMP-mediated activation and TGF-beta-mediated inhibition of CREB DNA binding was also diminished in the diseased cells. Thus, pulmonary fibroblasts derived from patients with pulmonary fibrosis are refractory to the inhibition by cAMP due to altered activity of components distal to the activity of PKA, in particular the phosphorylation of CREB.

Regulation of human methylthioadenosine phosphorylase gene by the CBF (CCAAT binding factor)/NF-Y (nuclear factor-Y)

hMTAP (human 5'-deoxy-5'-methylthioadenosine phosphorylase) is a key enzyme in the methionine salvage pathway and is frequently inactivated in human tumour cells. To understand the mechanism of the transcriptional regulation of the MTAP gene, we have cloned the 1.29 kb fragment of the hMTAP promoter and identified cis-acting regulatory sequences using a luciferase reporter gene assay. Maximal promoter activity was associated with sequences between -446 and -152, where two CCAAT elements were located. Electrophoretic mobility-shift assay reveals binding of specific complexes at both CCAAT motifs within the MTAP promoter, although more prominent bands were associated with the distal motif (-372 to -368). Supershift experiments and chromatin immunoprecipitation assays indicate that both the proximal and distal complexes bind CBF (CCAAT-binding factor; also known as nuclear factor-Y), and that the distal CCAAT motif has increased levels of CBF binding. We have mapped seven different transcriptional start sites between -135 and -58. Our results show that the hMTAP expression is regulated by a CBF and that the distal one of two CCAAT motifs plays a major role in the transcriptional activation of hMTAP gene.

Characterization of the c-Myb-responsive region and regulation of the human type I collagen alpha 2 chain gene by c-Myb

We have characterized the role of c-Myb and B-Myb in the regulation of human type I collagen alpha2 chain gene expression in fibroblastic cells. We have identified four Myb-binding sites (MBSs) in the promoter. Transactivation assays on wild type and mutant promoter-reporter constructs demonstrated that c-Myb, but not B-Myb, can transactivate the human type I collagen alpha 2 chain gene promoter via the MBS-containing region. Electrophoretic mobility shift assay experiments showed that c-Myb specifically binds to each of the four MBS; however, the mutagenesis of site MBS-4 completely inhibited transactivation by c-Myb, at least in the full-length promoter. In agreement with these results, c-myb(-/-) mouse embryo fibroblasts (MEFs) showed a selective lack of expression of type I collagen alpha 2 chain gene but maintained the expression of fibronectin and type III collagen. Furthermore, transforming growth factor-beta induced type I collagen alpha 2 chain gene expression in c-myb(-/-) MEFs, implying that the transforming growth factor-beta signaling pathway is maintained and that the absence of COL1A2 gene expression in c-myb(-/-) MEFs is a direct consequence of the lack of c-Myb. The demonstration of the importance of c-Myb in the regulation of the type I collagen alpha 2 chain gene suggests that uncontrolled expression of c-Myb could be an underlying mechanism in the pathogenesis of several fibrotic disorders.

A repressor with similarities to prokaryotic and eukaryotic DNA helicases controls the assembly of the CAAT box binding complex at a photosynthesis gene promoter

A single nucleotide exchange in a promoter region located immediately upstream of the CAAT box of the spinach photosynthesis gene AtpC (gene product is subunit gamma of the chloroplast ATP synthase) prevents the formation of a secondary structure and causes an unregulated, constitutive high level of expression (Kusnetsov, V., Landsberger, M., Oelmüller, R. (1999) J. Biol. Chem. 274, 36009-36014). We have isolated cDNAs for ATPC-2, a new polypeptide with homologies to pro- and eukaryotic helicases, which specifically binds to this promoter region. Binding of ATPC-2 competes strongly with that of a CAAT box binding factor (CBF), consistent with the idea that both complexes cannot be formed simultaneously because of sterical reasons. In gel mobility shift assays, high binding activities of ATPC-2 and low binding activities of CBF were observed with nuclear extracts from tissue with low AtpC expression levels, and the opposite was observed with extracts from tissues with high AtpC expression levels. Binding of ATPC-2 to the mutant sequence, which directs a constitutively high level expression in vivo and prevents the formation of a secondary structure in vitro, is significantly weaker than binding to the wild-type sequence. Again, the opposite results were obtained for the CBF. Thus, we conclude that the assembly of the CBF.DNA complex stimulates transcription of AtpC and that CBF binding is prevented if ATPC-2 is bound to the promoter region. The novel mechanism of gene regulation and the role of the helicase-like protein ATPC-2 as a potential transcriptional repressor is discussed in relation to its modular structure.

Interaction between GC box binding factors and Smad proteins modulates cell lineage-specific alpha 2(I) collagen gene transcription

Type I collagen is produced predominantly in mesenchymal cells, but molecular mechanisms responsible for cell type-specific expression are virtually unknown. During fibrogenic process in the liver, activated hepatic stellate cells (HSC) are the main producers of type I collagen, whereas parenchymal hepatocytes produce little, if any, of this protein. We have previously reported that Sp1 and an interacting unknown factor(s) bind to the -313 to -255 sequence of the alpha2(I) collagen gene (COL1A2) and play essential roles for basal and TGF-beta-stimulated transcription in skin fibroblasts and HSC. Recently, Smad3 has been shown to bind to this region, and its interaction with Sp1 has been implicated in TGF-beta-elicited COL1A2 stimulation. The present study demonstrates predominant binding of Sp3 rather than Sp1 to this regulatory element in parenchymal hepatocytes. In these cells, this region did not exhibit strong enhancer activity or mediate the effect of TGF-beta. Transfection of HSC with an Sp3 expression plasmid abolished the COL1A2 response to TGF-beta, whereas overexpression of Sp1 in hepatocytes increased basal COL1A2 transcription and conferred TGF-beta responsiveness. Functional and physical interactions between Sp1 and Smad3, but not between Sp3 and Smad3, were demonstrated using the bacterial GAL4 system and immunoprecipitation-Western blot analyses. These results indicate that cell lineage-specific interactions between GC box binding factors and Smad protein(s) may account, at least in part, for differential COL1A2 transcription and TGF-beta responsiveness in HSC and parenchymal hepatocytes.

PTRF (polymerase I and transcript-release factor) is tissue-specific and interacts with the BFCOL1 (binding factor of a type-I collagen promoter) zinc-finger transcription factor which binds to the two mouse type-I collagen gene promoters

We have used the yeast two-hybrid system to clone the protein that interacts with the BFCOL1 (binding factor of a type-I collagen promoter) zinc-finger transcription factor that was cloned previously as the factor that binds to the two mouse proximal promoters of the type-I collagen genes. We utilized as bait the N-terminal domain of BFCOL1 that includes the zinc-finger DNA-binding domain. One cDNA contained a potential open reading frame for a polypeptide of 392 amino acids and was identical to PTRF (polymerase I and transcript-release factor), which is involved in transcription termination of the RNA polymerase I reaction. Northern-blot analysis revealed that the pattern of mRNA expression was similar to that of the type-I collagen gene. In addition, we detected the mRNA expression only in a fibroblast cell line and two bone cell lines, but not in other blood and neuronal cell lines. Recombinant protein was shown to enhance the binding of BFCOL1 to its binding site in the mouse proalpha2(I) collagen proximal promoter in vitro. The transient-transfection experiment showed that PTRF had a suppressive effect on the mouse proalpha2(I) collagen proximal promoter activity. We speculate that PTRF might play a role in the RNA polymerase II reaction as well as that of RNA polymerase I.

PTRF (polymerase I and transcript-release factor) is tissue-specific and interacts with the BFCOL1 (binding factor of a type-I collagen promoter) zinc-finger transcription factor which binds to the two mouse type-I collagen gene promoters

We have used the yeast two-hybrid system to clone the protein that interacts with the BFCOL1 (binding factor of a type-I collagen promoter) zinc-finger transcription factor that was cloned previously as the factor that binds to the two mouse proximal promoters of the type-I collagen genes. We utilized as bait the N-terminal domain of BFCOL1 that includes the zinc-finger DNA-binding domain. One cDNA contained a potential open reading frame for a polypeptide of 392 amino acids and was identical to PTRF (polymerase I and transcript-release factor), which is involved in transcription termination of the RNA polymerase I reaction. Northern-blot analysis revealed that the pattern of mRNA expression was similar to that of the type-I collagen gene. In addition, we detected the mRNA expression only in a fibroblast cell line and two bone cell lines, but not in other blood and neuronal cell lines. Recombinant protein was shown to enhance the binding of BFCOL1 to its binding site in the mouse proalpha2(I) collagen proximal promoter in vitro. The transient-transfection experiment showed that PTRF had a suppressive effect on the mouse proalpha2(I) collagen proximal promoter activity. We speculate that PTRF might play a role in the RNA polymerase II reaction as well as that of RNA polymerase I.

Stable expression of a dominant negative mutant of CCAAT binding factor/NF-Y in mouse fibroblast cells resulting in retardation of cell growth and inhibition of transcription of various cellular genes

The heterotrimeric CCAAT-binding factor CBF specifically interacts with the CCAAT motif present in the proximal promoters of numerous mammalian genes. To understand the in vivo function of CBF, a dominant negative mutant of CBF-B subunit that inhibits DNA binding of wild type CBF was stably expressed in mouse fibroblast cells under control of tetracycline-responsive promoter. Expression of the mutant CBF-B but not the wild-type CBF-B resulted in retardation of fibroblast cell growth. The analysis of cell growth using bromodeoxyuridine labeling showed that expression of the mutant CBF-B decreased the number of cells entering into S phase, and also delayed induction of S phase in the quiescent cells after serum stimulation, thus indicating that the inhibition of CBF binding prolonged the progression of S phase in fibroblasts. These results provide direct evidence for the first time that CBF is an important regulator of fibroblast growth. The inhibition of CBF binding reduced expression of various cellular genes including the alpha2(1) collagen, E2F1, and topoisomerase IIalpha genes which promoters contain the CBF-binding site. This result implied that expression of many other genes which promoters contain CBF-binding site was also decreased by the inhibition of CBF binding, and that the decreased expression of multiple cellular genes possibly caused the retardation of fibroblast cell growth.

The assembly of the CAAT-box binding complex at a photosynthesis gene promoter is regulated by light, cytokinin, and the stage of the plastids

A functionally important region in the promoter of the spinach photosynthesis gene AtpC, which encodes the subunit gamma of the chloroplast ATP synthase, is located immediately upstream of the CAAT-box. A single nucleotide exchange in this region (AAAATTCAAT --> AAGATCAAT) uncouples the expression of an AtpC promoter::uidA gene fusion from the regulation by light, cytokinin, and functional plastids and results in a high constitutive expression of the reporter gene. By screening an Arabidopsis thaliana expression library with a double-stranded wild-type oligonucleotide from this promoter region, we have isolated cDNAs from Arabidopsis libraries that code for plant homologs of the CAAT-box binding factor (CBF)-C. Binding occurs only in the presence of nuclear extracts, consistent with reports from metazoa CBFs that the subunits A and B in addition to C are required for the formation of the CBF-DNA complex. At least eight genes with homologies to CBF-C are present in the Arabidopsis genome; one of them exhibits striking similarities to the gene for the human global transcriptional repressor Drap1. In gel mobility shift assays, low binding activity of CBF to the wild-type AtpC promoter sequence was observed with nuclear extracts from tissue with low AtpC expression levels, i.e. extracts from etiolated and photobleached seedlings, whereas high binding activity was detectable with extracts from tissues with high AtpC expression levels, i.e. extracts from light-grown seedlings and etiolated seedlings treated with cytokinin. Binding to the mutant sequence, which directs constitutive high level uidA expression in vivo, is significantly stronger than to the wild-type sequence. The data are consistent with the idea that the assembly of CBF at the AtpC promoter is regulated in response to light and cytokinin and that the low level of expression in etiolated and photobleached material is caused by an inhibitory effect. The structure/function relationships of the Arabidopsis CBFs are discussed in relation to their regulatory function in AtpC gene expression.

Human ZHX1: cloning, chromosomal location, and interaction with transcription factor NF-Y

NF-YA, B, and C comprise the heterotrimeric transcription factor known as nuclear factor Y (NF-Y) or CCAAT-binding protein (CBF). NF-Y binds many CCAAT and Y box (an inverted CCAAT box, ATTGG) elements. Mutations of these elements that disrupt the binding of NF-Y result in decreased transcription from various tissue-specific and inducible promoters. We employed a yeast two-hybrid system to screen a human liver cDNA library in an effort to isolate proteins that interact with NF-Y and that may play a role in tissue-specific or hormone-inducible promoter activity. Using a fragment of the NF-YA subunit as bait we isolated a cDNA that encodes most of the open reading frame of the human zinc fingers and homeobox 1 (ZHX1) protein. The complete open reading frame was subsequently isolated and found to encode a protein of 873 amino acids that contains two zinc fingers and five homeodomain motifs. Northern blot analysis of poly(A)(+) RNA isolated from various tissues revealed two major ZHX1 transcripts of about 4.5 and 5 kilobases. Both transcripts were expressed ubiquitously, although the 5-kilobase transcript is of greater abundance in most tissues examined. The human ZHX1 gene is located on chromosome 8q, between markers CHCL.GATA50B06 and CHLC. GATA7G07.

C-myb, but not B-myb, upregulates type I collagen gene expression in human fibroblasts

C-myb and B-myb belong to the myb family of transcription factors. We have shown previously that c-myb is deregulated in fibroblasts from systemic sclerosis (scleroderma) patients relative to normal fibroblasts. Scleroderma fibroblasts are known to express elevated levels of collagen genes and transforming growth factor beta is known to be a pro-fibrotic cytokine and to induce transcription of type I collagen genes. We have therefore investigated the role of c-myb and B-myb in the regulation of type I collagen genes in response to transforming growth factor beta in normal human fibroblasts. We show that, in these cells, transforming growth factor beta treatment induces c-myb as well as collagen alpha1(I) and alpha2(I) gene expression, but not B-myb. Furthermore we demonstrate by cotransfection assays that c-myb can upregulate alpha1(I) and alpha2(I) collagen promoters by 6-10-fold whereas B-myb is inactive. The activity of c-myb on both type I collagen promoters requires a functional c-myb DNA binding domain suggesting a direct interaction between c-myb and these promoters. Indeed c-myb is active also on a 500 bp fragment of the alpha2(I) collagen promoter and can bind to this fragment in electrophoretic mobility shift assays. Finally, we show that anti-c-myb anti-sense treatment reduces alpha1(I) and to a lesser extent alpha2(I) collagen gene expression. These data strongly suggest that c-myb, but not B-myb, plays a direct role in the upregulation of type I collagen gene expression in response to transforming growth factor beta.

Methylation in the initiation region of the first exon suppresses collagen pro-alpha2(I) gene transcription

Our previous studies demonstrated that the collagen alpha2(I) gene is hypermethylated in the promoter/first exon region after chemical transformation and the alpha2(I) promoter/first exon is sensitive to methylation in transfection studies. In this paper, we demonstrate that a minimum collagen promoter containing the preinitiation region (-41 to +54) driving luciferase reporter gene was inactivated by DNA methylation as judged by transfection assays. All the methylation sites within the preinitiation region were located in the first exon, not in the promoter. Methylation of the promoter construct inhibited transcription as determined by an in vitro assay, only if proteins were extracted from nuclei using 500 mM NaCl. Gel mobility shift analysis suggested that methylation within the first exon decreased the formation of the largest preinitiation complex while increasing the formation of faster migrating protein-DNA complexes. Competition gel mobility shift analysis indicated that the faster migrating protein-DNA complex could be competed by a smaller initiator probe which did not contain TATA binding region. A protein-DNA complex with increased affinity to methylated sequences was detected using the initiator probe, which contained two methylation sites and no TATA sequence (-25 to 30) suggesting that a separate repressor complex binds to the methylated sequences. Mutations at the methylation sites (+7, +23) in the first exon also increased the protein-DNA complex formation in gel shift analysis and inhibited collagen alpha2(I) transcription as judged by transient transfection and in vitro transcription assays. Therefore, these methylation sites in the preinitiation region are important for transcription of alpha2(I) gene and the protein responsible for the repression of transcription is extractable using high salt nuclear extracts.

Role of the CCAAT-binding protein CBF/NF-Y in transcription

The CCAAT motif is one of the common promoter elements present in the proximal promoter of numerous mammalian genes transcribed by RNA polymerase II. CBF (also called NF-Y and CP1) consists of three different subunits and interacts specifically with the CCAAT motif. In each CBF subunit, the segment needed for formation of the CBF-DNA complex is conserved from yeast to human and, interestingly, the conserved segment of two CBF subunits, CBF-A and CBF-C, are homologous to the histone-fold motif of eukaryotic histones and archaebacterial histone-like protein HMf-2. The histone fold motifs of CBF-A and CBF-C interact with each other to form a heterodimer that associates with CBF-B to form a heterotrimeric CBF molecule, which then binds to DNA.

Cloning and characterization of a novel sequence-specific single-stranded-DNA-binding protein

The promoter region of the chicken alpha2(I) collagen gene contains a pyrimidine-rich element that is well conserved in different mammalian species. This sequence can also form an unusual DNA structure as shown by its sensitivity to SI nuclease in vitro and it lies in a region that is DNase I-hypersensitive only when this promoter is active. We have recently reported that fibroblast nuclear proteins, including chicken Y-box-binding protein 1, bind to this single-stranded pyrimidine-rich sequence. Here we report the isolation, from a chick embryo fibroblast cDNA expression library, of a partial cDNA clone encoding a previously unknown protein, designated SSDP (sequence-specific single-stranded DNA-binding protein), that binds this single-stranded sequence. This clone contains 1199 bp of chicken sequence and has a single long open reading frame that encodes 284 amino acid residues. The affinity-purified recombinant protein encoded by this cDNA binds sequence-specifically to the single-stranded pyrimidine sequence. This cDNA sequence lacks significant similarity to any known gene in the data banks, but it is highly conserved in expressed sequence tags derived from both mouse and human. The corresponding amino acid sequence is remarkably conserved, having 97% identity with mouse and human expressed sequences. The corresponding mRNA is approx. 1800 nt in length and is expressed in both fibroblasts and chondrocytes. The high affinity of this protein for this conserved pyrimidine-rich region suggests that it might be involved in the transcriptional regulation of the alpha2(I) collagen gene.

DNA binding specificity of the CCAAT-binding factor CBF/NF-Y

CBF is a heterotrimeric protein that binds to DNA containing CCAAT motifs. Here we have analyzed interactions of recombinant CBF with DNA using hydroxyl radical footprinting and methylation interference assays. In the CBF-DNA complex, three separate DNA regions are protected from hydroxyl radical cleavage, one located over and immediately adjacent to the CCAAT motif itself and the other two located on both sides of the CCAAT motif. The methylation interference assay showed, however, that only in the CCAAT motif region methylation of bases was able to interfere with the formation of a CBF-DNA complex, suggesting that CBF makes sequence-specific contacts only in the CCAAT motif region. To further determine the specific DNA sequences necessary for CBF binding, we employed a polymerase chain reaction-mediated random binding site selection method. This analysis showed that CBF binding to DNA requires the CCAAT sequence and other specific sequences immediately flanking both ends of the CCAAT motif. We also showed that the nature of the flanking nucleotide sequences affects the affinity of CBF for DNA. Interestingly, most of the CCAAT motifs present in various higher eukaryotic promoters correspond to the CBF binding sites that were selected, consistent with the hypothesis that these motifs are binding sites for CBF and, hence, that CBF could regulate transcription of numerous eukaryotic genes.

Cloning and characterization of the 5'-flanking region of the mouse diastrophic dysplasia sulfate transporter gene

Dyastrophic dysplasia sulfate transporter (DTDST) plays an important role in proteoglycan synthesis in the extracellular matrix of bone and cartilage. Recently, we found that the mouse DTDST gene was induced in pluripotent C3H10T1/2 cells during differentiation by bone morphogenetic protein-2 (BMP-2). To clarify the transcriptional regulation of the DTDST gene, we have cloned the 5'-flanking region of the mouse DTDST gene by the PCR based gene walking method. Sequence analysis revealed the presence of the TATA box followed by GC rich sequences containing two Sp-1 binding sites and a CBFA1 binding site. Transient transfection assays demonstrated that the basal transcriptional activity in osteoblastic MC3T3-E1 cells was mainly present between -309 and -275 bp upstream of the transcription start site (Segment -309/-275) which contained the consensus sequence for the xenobiotic-responsible element (XRE). Nuclear proteins from MC3T3-E1 cells and C3H10T1/2 cells could bind to this short segment in vitro. BMP-2 increased the promoter activity as well as the nuclear protein binding to the sequence in C3H10T1/2 cells. The present data suggest that the DTDST gene expression in osteoblasts and differentiating precursor cells to osteoblast/chondrocyte lineage would be mainly regulated by undetermined XRE binding transcription factors.

Sp3 is a transcriptional activator of the human alpha2(I) collagen gene

Sp3 is an ubiquitously expressed transcription factor, closely related to Sp1 but, unlike Sp1, it often functions as a transcriptional repressor. In this study we investigated the role of Sp3 in regulating the transcription of the human alpha2(I) collagen gene. We show that Sp1 and Sp3 specifically bind to three of the previously characterized cis -elements in this promoter, including two positive cis-elements between -303 and -271 and -128 and -123, and a repressor site between -164 and -159, but do not bind to the fourth cis-element bound by CBF. Functional analyses of Sp3 and Sp1 in Drosophila cells indicate that each protein transactivates the human alpha2(I) collagen promoter with equal potency and, when tested together, have an additive effect on the promoter activity. Furthermore, in vitro transcription assays demonstrate that both Sp1 and Sp3 are capable of supporting transcription from the collagen promoter independently of each other. However, when activities of both Sp1 and Sp3 are blocked with specific antibodies, in vitro transcription from this promoter is almost completely abolished. The results of this study demonstrate that Sp3 is as potent an activator of the human alpha2(I) collagen promoter as is Sp1 and that a transcriptional activity of the human alpha2(I) promoter is dependent on both proteins.

Cell-specific in vivo DNA-protein interactions at the proximal promoters of the pro alpha 1(I) and the pro alpha2(I) collagen genes

We performed in vivo dimethylsulfate footprinting of the 220 bp mouse proximal proalpha1(I) collagen promoter and the 350 bp mouse proximal proalpha2(I) collagen promoter in BALB/3T3 fibroblasts, primary mouse skin fibroblasts, S-194 B cells, NMuLi liver epithelial cells and RAG renal adenocarcinoma cells and in vitro DNase I footprinting of these promoters using nuclear extracts of these different cell types. Whereas proalpha1(I) and proalpha2(I) collagen RNAs were present in BALB/3T3 fibroblasts and primary fibroblasts, these RNAs could not be detected in the three other cell lines. Comparison of in vitro DNase I footprints for each of the two proximal collagen promoters indicated that the patterns of protection were very similar with the different nuclear extracts, suggesting that the DNA binding proteins binding to these promoters were present in all cell types tested. In contrast, in vivo footprints over these proximal promoters were cell-specific, occurring only in fibroblast cells and not in the other three cell types. The in vivo footprints were generally located within the in vitro footprinted regions. Our results suggest that although all cell types tested contained nuclear proteins that can bind to the proximal proalpha1(I) and proalpha2(I) collagen promoters in vitro , it is only in fibroblasts that these proteins bind to their cognate sites in vivo . We discuss possible regulatory mechanisms in type I collagen genes that can contribute to the cell-specific in vivo protein-DNA interactions at the proximal promoters.

Sp1 is required for the early response of alpha2(I) collagen to transforming growth factor-beta1

It is currently debated whether AP1 or Sp1 is the factor that mediates transforming growth factor beta1 (TGF-beta) stimulation of the human alpha2(I) collagen (COL1A2) gene by binding to an upstream promoter element (TbRE). The present study was designed to resolve this controversy by correlating expression of COL1A2, AP1, and Sp1 in the same cell line and under different experimental conditions. The results strongly indicate that Sp1 is required for the immediate early response of COL1A2 to TGF-beta and AP1 is not. The Sp1 inhibitor mithramycin blocked stimulation of alpha2(I) collagen mRNA accumulation by TGF-beta, whereas the AP1 inhibitor curcumin had no effect. Furthermore, antibodies against Jun-B and c-Jun failed to identify immunologically related proteins in the TbRE-bound complex, irrespective of whether they were purified from untreated or TGF-beta-treated cells. AP1 did bind to the TbRE probe in vitro, but only in the absence of the upstream Sp1 recognition sequence. Based on this finding and DNA transfection results, we conclude that the AP1 sequence of the TbRE represents a cryptic site used under experimental conditions that either eliminate the more favorable Sp1 binding site or force the balance toward the less probable. Finally, a combination of cell transfections and DNA-binding assays excluded that COL1A2 transactivation involves the retinoblastoma gene product (pRb), an activator of Sp1, the pRb-related protein p107, an inhibitor of Sp1, or the Sp1-related repressor, Sp3.

Cloning and characterization of a transcription factor that binds to the proximal promoters of the two mouse type I collagen genes

We have used the yeast one-hybrid system to clone transcription factors that bind to specific sequences in the proximal promoters of the type I collagen genes. We utilized as bait the sequence between -180 and -136 in the pro-alpha2(I) collagen promoter because it acts as a functional promoter element and binds several DNA-binding proteins. Three cDNA clones were isolated that encoded portions of the mouse SPR2 transcription factor, whereas a fourth cDNA contained a potential open reading frame for a polypeptide of 775 amino acids and was designated BFCOL1. Recombinant BFCOL1 was shown to bind to the -180 to -152 segment of the mouse pro-alpha2(I) collagen proximal promoter and to two discrete sites in the proximal promoter of the mouse pro-alpha1(I) gene. The N-terminal portion of BFCOL1 contains its DNA-binding domain. DNA transfection experiments using fusion polypeptides with the yeast GAL4 DNA-binding segment indicated that the C-terminal part of BFCOL1 contained a potential transcriptional activation domain. We speculate that BFCOL1 participates in the transcriptional control of the two type I collagen genes.

A gene for a novel zinc-finger protein expressed in differentiated epithelial cells and transiently in certain mesenchymal cells

We have identified a novel zinc-finger protein whose mRNA is expressed at high levels in the epidermal layer of the skin and in epithelial cells in the tongue, palate, esophagus, stomach, and colon of newborn mice. Expression in epithelial cells is first detected at the time of their differentiation during embryonic development. In addition, during early embryonic development there is expression in mesenchymal cells of the skeletal primordia and the metanephric kidney which is later down-regulated. The expression pattern suggests that the protein could be involved in terminal differentiation of several epithelial cell types and could also be involved in early differentiation of the skeleton and kidney. The carboxyl terminus of the protein contains three zinc fingers with a high degree of homology to erythroid krüppel-like factor and binds to DNA fragments containing CACCC motifs. The amino-terminal portion of the protein is proline and serine-rich and can function as a transcriptional activator. The chromosomal location of the gene was mapped using mouse interspecific backcrosses and was shown to localize to mouse chromosome 4 and to cosegregate with the thioredoxin gene.