Analysis of the collagen alpha 1(I) promoter

Origin of myofibroblasts in the fibrotic liver in mice

Hepatic myofibroblasts are activated in response to chronic liver injury of any etiology to produce a fibrous scar. Despite extensive studies, the origin of myofibroblasts in different types of fibrotic liver diseases is unresolved. To identify distinct populations of myofibroblasts and quantify their contribution to hepatic fibrosis of two different etiologies, collagen-α1(I)-GFP mice were subjected to hepatotoxic (carbon tetrachloride; CCl4) or cholestatic (bile duct ligation; BDL) liver injury. All myofibroblasts were purified by flow cytometry of GFP(+) cells and then different subsets identified by phenotyping. Liver resident activated hepatic stellate cells (aHSCs) and activated portal fibroblasts (aPFs) are the major source (>95%) of fibrogenic myofibroblasts in these models of liver fibrosis in mice. As previously reported using other methodologies, hepatic stellate cells (HSCs) are the major source of myofibroblasts (>87%) in CCl4 liver injury. However, aPFs are a major source of myofibroblasts in cholestatic liver injury, contributing >70% of myofibroblasts at the onset of injury (5 d BDL). The relative contribution of aPFs decreases with progressive injury, as HSCs become activated and contribute to the myofibroblast population (14 and 20 d BDL). Unlike aHSCs, aPFs respond to stimulation with taurocholic acid and IL-25 by induction of collagen-α1(I) and IL-13, respectively. Furthermore, BDL-activated PFs express high levels of collagen type I and provide stimulatory signals to HSCs. Gene expression analysis identified several novel markers of aPFs, including a mesothelial-specific marker mesothelin. PFs may play a critical role in the pathogenesis of cholestatic liver fibrosis and, therefore, serve as an attractive target for antifibrotic therapy.

The types of hepatic myofibroblasts contributing to liver fibrosis of different etiologies

Liver fibrosis results from dysregulation of normal wound healing, inflammation, activation of myofibroblasts, and deposition of extracellular matrix (ECM). Chronic liver injury causes death of hepatocytes and formation of apoptotic bodies, which in turn, release factors that recruit inflammatory cells (neutrophils, monocytes, macrophages, and lymphocytes) to the injured liver. Hepatic macrophages (Kupffer cells) produce TGFβ1 and other inflammatory cytokines that activate Collagen Type I producing myofibroblasts, which are not present in the normal liver. Secretion of TGFβ1 and activation of myofibroblasts play a critical role in the pathogenesis of liver fibrosis of different etiologies. Although the composition of fibrogenic myofibroblasts varies dependent on etiology of liver injury, liver resident hepatic stellate cells and portal fibroblasts are the major source of myofibroblasts in fibrotic liver in both experimental models of liver fibrosis and in patients with liver disease. Several studies have demonstrated that hepatic fibrosis can reverse upon cessation of liver injury. Regression of liver fibrosis is accompanied by the disappearance of fibrogenic myofibroblasts followed by resorption of the fibrous scar. Myofibroblasts either apoptose or inactivate into a quiescent-like state (e.g., stop collagen production and partially restore expression of lipogenic genes). Resolution of liver fibrosis is associated with recruitment of macrophages that secrete matrix-degrading enzymes (matrix metalloproteinase, collagenases) and are responsible for fibrosis resolution. However, prolonged/repeated liver injury may cause irreversible crosslinking of ECM and formation of uncleavable collagen fibers. Advanced fibrosis progresses to cirrhosis and hepatocellular carcinoma. The current review will summarize the role and contribution of different cell types to populations of fibrogenic myofibroblasts in fibrotic liver.

Sp1 and Sp3 transcription factors mediate leptin-induced collagen α1(I) gene expression in primary culture of male rat hepatic stellate cells

Mechanisms by which leptin stimulates collagen α(1)(I) [Col1a(I)] gene expression are unclear. The purposes of this study were to identify the trans-acting factors and cis-acting elements in Col1a(I) promoter involved in this effect as well as the pathways that are implicated. In primary cultures of rat hepatic stellate cells (HSCs), we measured the effects of leptin on Col1a(I) gene and protein expression and on the binding of nuclear proteins to the Col1a(I) promoter. We found that leptin increased Col1a(I) gene and protein expression in activated HSCs. Transient transfections showed that leptin exerted its effects through elements located between -220 and -112 bp of the Col1a(I) promoter. Gel retardation assays demonstrated that leptin induced the binding of transcription factors specific protein (Sp)-1 and Sp3 to two elements located between -161 and -110 bp of the Col1a(I) promoter. Leptin-induced Sp1/Sp3 phosphorylation, but this effect was suppressed by inhibiting or silencing Janus kinase-2, phosphatidylinositol-3-kinase, nonphagocytic adenine dinucleotide phosphate (NADPH) oxidase, or ERK1/2, by the use of antioxidants or catalase, or by preventing protein-aldehyde adduct formation. Leptin provoked oxidative stress, aldehyde-protein adduct formation, and increased gene expression of some components of the NADPH oxidase complex. In conclusion, in HSCs, leptin up-regulates Col1a(I) gene expression after activating NADPH oxidase, inducing oxidative stress, aldehyde-protein adduct formation, and ERK1/2 phosphorylation, which in turn activates Sp1/Sp3 and provokes the binding of these two factors to regulatory elements located between -161 and -110 bp of the Col1a(I) promoter. These findings may contribute to a better understanding of mechanisms involved in the leptin-induced liver fibrosis.

Tryptase/Protease-activated receptor 2 interactions induce selective mitogen-activated protein kinase signaling and collagen synthesis by cardiac fibroblasts

The mast cell product, tryptase, has recently been implicated to mediate fibrosis in the hypertensive heart. Tryptase has been shown to mediate noncardiac fibroblast function via activation of protease-activated receptor 2 and subsequent activation of the mitogen-activated protein kinase pathway, including extracellular signal-regulated kinase 1/2. Therefore, we hypothesized that this pathway may be a mechanism leading to fibrosis in the hypertensive heart. Isolated adult cardiac fibroblasts were treated with tryptase, which induced activation of extracellular signal-regulated kinase 1/2 via protease-activated receptor 2. Blockade of protease activated receptor 2 with FSLLRY (10 μmol/L) and inhibition of the extracellular signal-regulated kinase pathway with PD98059 (10 μmol/L) prevented collagen synthesis in isolated cardiac fibroblasts stimulated with tryptase. In contrast, p38 mitogen-activated protein kinase and stress-activated protein/c-Jun N-terminal kinase were not activated by tryptase. Cardiac fibroblasts isolated from spontaneously hypertensive rats showed this same pattern of activation. Treatment of spontaneously hypertensive rats with FSLLRY prevented fibrosis in these animals, indicating the in vivo applicability of the cultured fibroblast findings. Also, tryptase induced a myofibroblastic phenotype indicated by elevations in α-smooth muscle actin and extra type III domain A (ED-A) fibronectin. Thus, the results from this study demonstrate the importance of tryptase for inducing a cardiac myofibroblastic phenotype, ultimately leading to the development of cardiac fibrosis. Specifically, tryptase causes cardiac fibroblasts to increase collagen synthesis via a mechanism involving activation of protease-activated receptor 2 and subsequent induction of extracellular signal-regulated kinase signaling.

Homeobox Gene Prx1 Is Expressed in Activated Hepatic Stellate Cells and Transactivates Collagen α1(I) Promoter

Hepatic stellate cells (HSCs) are mesenchymal cells of the liver, which are normally in quiescent state and synthesize tracing amounts of extracellular matrix proteins. Upon fibrogenic stimulus, HSCs become activated and increase synthesis of type I collagen 50–100 fold. Prx1 and Prx2 are two homeobox transcription factors which are required for mesenchymal tissue formation during embryogenesis. The present study shows that Prx1 mRNA is expressed in in vivo and in vitro activated HSCs, but not in quiescent HSCs. Prx1 is also expressed in fibrotic livers, while it is undetectable in normal livers. Overexpression of Prx1a in quiescent HSCs cultured in vitro induced collagen α1(I) mRNA and TGFβ3 mRNA expression. Prx1 transactivated TGFβ3 promoter 3 fold in transient transfection experiments. In the whole liver, Prx1a induced expression of collagen α1(I), α2(I), α1(III) and α-smooth muscle mRNAs, which are the markers of activation of HSCs. Prx1 also increased expression of collagen α1(I) mRNA after acute liver injury. This suggests that Prx1a promotes activation of HSCs and expression of type I collagen. Several regions in the collagen α1(I) promoter were identified which mediate transcriptional induction by Prx1. The regions are scattered throughout the promoter and individually have modest effects; however, the cumulative effect of all sequences is >50 fold. This is the first description of the effects of Prx1 in HSCs and in the liver, and identification of the two Prx1 target genes, which play a pivotal role in development of liver fibrosis, is a novel finding for liver pathophysiology.

Gene modulation for treating liver fibrosis

Despite tremendous progress in our understanding of fibrogenesis, injury stimuli process, inflammation, and hepatic stellate cell (HSC) activation, there is still no standard treatment for liver fibrosis. Delivery of small molecular weight drugs, proteins, and nucleic acids to specific liver cell types remains a challenge due to the overexpression of extracellular matrix (ECM) and consequent closure of sinusoidal gaps. In addition, activation of HSCs and subsequent release of inflammatory cytokines and infiltration of immune cells are other major obstacles to the treatment of liver fibrosis. To overcome these barriers, different therapeutic approaches are being investigated. Among them, the modulation of certain aberrant protein production is quite promising for treating liver fibrosis. In this review, we describe the mechanism of antisense, antigene, and RNA interference (RNAi) therapies and discuss how the backbone modification of oligonucleotides affects their in vivo stability, biodistribution, and bioactivity. Strategies for delivering these nucleic acids to specific cell types are discussed. This review critically addresses various insights developed with each individual strategy and for multipronged approaches, which will be helpful in achieving more effective outcomes.

Sequence-specific triple helix formation with genomic DNA

We have previously demonstrated site-specific delivery of antiparallel phosphorothioate triplex forming oligonucleotide (TFO) specific to -165 to -141 promoter region of alpha1(I) collagen (abbreviated as APS165) to hepatic stellate cells (HSCs) of fibrotic rats after conjugation with mannose 6-phosphate-bovine serum albumin. However, we still need to determine whether there is correlation between transcription inhibition and triplex formation with genomic DNA. In this study, APS165 was modified with psoralen and the extent of triplex formation with alpha1(I) collagen DNA was determined in naked genomic DNA, isolated nuclei of HSC-T6 cells and whole cells by using a simple real-time PCR based method. In this method, a purification step was added to remove unbound APS165, which eliminated the possible artifacts during real-time PCR. Psoralen photoadduct formation was shown to be essential to retain triplex structure under denaturing conditions. On naked genomic DNA, 82.2% of DNA formed triplex and 36.7% of genomic DNA in isolated nuclei at 90 min contained triplex structure. As quantified by real-time PCR, 50% of genomic DNA in living cells at 12 h postincubation contained triplex structures. Furthermore, the triplex formation was dose-dependent with 26.5% and 50% of DNA having triplex structure at concentrations of 1 microM and 5 microM, respectively. Moreover, on a plasmid pCol-CAT220 containing rat alpha1(I) gene promoter (-225 to +113), 75.3% of triplex formation was observed, which was correlated with a 73.6% of transcription inhibition. These findings will further strengthen the therapeutic applications of APS165.

Modulation of gene expression by antisense and antigene oligodeoxynucleotides and small interfering RNA

Antisense oligodeoxynucleotides, triplex-forming oligodeoxynucleotides and double-stranded small interfering RNAs have great potential for the treatment of many severe and debilitating diseases. Concerted efforts from both industry and academia have made significant progress in turning these nucleic acid drugs into therapeutics, and there is already one FDA-approved antisense drug in the clinic. Despite the success of one product and several other ongoing clinical trials, challenges still exist in their stability, cellular uptake, disposition, site-specific delivery and therapeutic efficacy. The principles, strategies and delivery consideration of these nucleic acids are reviewed. Furthermore, the ways to overcome the biological barriers are also discussed so that therapeutic concentrations at their target sites can be maintained for a desired period.

Regulation of collagen type I in vascular smooth muscle cells by competition between Nkx2.5 and deltaEF1/ZEB1

A major component of the vessel wall of large arteries and veins is the extracellular matrix (ECM), which consists of collagens, elastin, and proteoglycans. Collagen type I is one of the most abundant of the ECM proteins. We have previously shown that the pro-collagen type I alpha 2 gene contains an enhancer which confers tissue-specific expression in the majority of collagen-producing cells, including blood vessels. In this paper, we delineate a specific vascular smooth muscle cell (vSMC) element: a 100-bp sequence around -16.6 kb upstream of the transcription start site that regulates collagen expression exclusively in vSMCs. Furthermore, we show that the expression is activated through the binding of the homeodomain protein Nkx2.5, which is further potentiated in the presence of GATA6. In contrast, this element was repressed by the binding of the zinc-finger protein deltaEF1/ZEB1. We propose a model of regulation where the activating transcription factor Nkx2.5 and the repressor deltaEF1/ZEB1 compete for an overlapping DNA binding site. This element is important in understanding the molecular mechanisms of vessel remodeling and is a potential target for intervention in vascular diseases where there is excessive deposition of collagen in the vessel wall.

Collagen alpha1(I) gene contains an element responsive to tumor necrosis factor-alpha located in the 5' untranslated region of its first exon

The aims of the present study were to identify the cis-acting element through which tumor necrosis factor-alpha (TNFalpha) inhibits collagen alpha1(I) gene transcription and the trans-acting factors involved in this effect in cultured hepatic stellate cells. Deletion analysis of the collagen alpha1(I) promoter demonstrated that TNFalpha inhibited gene expression through an element located between -59 and + 116 bp relative to the transcription start site. DNase I protection assays revealed a footprint between +68 and +86 bp of the collagen first exon, the intensity of which decreased when the DNA probe was incubated with nuclear protein from TNFalpha-treated hepatic stellate cells. This footprint contained a G+C-rich box. Transfection experiments demonstrated that mutations in this G+C-rich element abrogated the inhibitory effect of TNFalpha on the collagen alpha1(I) promoter. Gel retardation experiments using a radiolabeled oligonucleotide containing sequences of this region confirmed that TNFalpha treatment decreased the formation of two complexes between nuclear proteins and DNA. These complexes were efficiently blocked with an oligonucleotide containing an Spl-binding site and were supershifted with specific Spl and Sp3 antibodies. These results suggest that TNFalpha inhibits collagen alpha1(I) gene expression by decreasing the binding of Spl to a G+C-rich box in the 5' untranslated region of its first exon.

Interaction of Ap1, Ap2, and Sp1 with the regulatory regions of the human pro-alpha1(I) collagen gene

In the pro-alpha1(I) collagen gene a number of cis-regulatory elements, which interact with a variety of trans-acting factors, are present in the promoter and first intron. We have undertaken a comprehensive study of Sp1, Ap1, and Ap2 binding in the region spanning -442 to +1697 nt. DNase I footprinting analysis revealed these factors bind with varying affinities to some of the potential sites: Sp1 binds to 16 of 34 potential sites, Ap2 binds to 22 of 40 potential binding sites, and Ap1 binds to its only potential site. The Sp1 sites were mostly clustered in the intron region, while the Ap2 sites were clustered in the promoter region. Transmission electron microscopic analysis of DNA-protein complexes not only confirmed these results, but also clearly showed that heterologous and/or homologous protein-protein interactions between Sp1 and/or Ap2 bring the promoter and intron in contact with each other, with the resulting looping out of the intervening DNA. This strongly suggests that the DNA-looping model is an explanation for the orientation preference of the enhancing element in the first intron as these interactions possibly create an optimum environment for the binding of the rest of the transcriptional machinery.

Characterization of a novel transcription factor binding to the regulatory regions of the human pro-alpha1(I) collagen gene

We have identified a novel transcription factor TGP (TG Binding Protein) that binds to the consensus sequence 5'-TGTGGGGTGG-3' in the promoter and intron of the human pro-alpha1(I) collagen gene. This recognition sequence, or sequences closely resembling these sequences, was also identified in the pro-alpha1(I) and pro-alpha2(I) collagen genes of other species. Competition experiments revealed that TGP is related to but distinguishable from the Ap4/5 family of transcription factors and that it can be separated from Ap4/5 according to size.

The DNA binding protein BTEB mediates acetaldehyde-induced, jun N-terminal kinase-dependent alphaI(I) collagen gene expression in rat hepatic stellate cells

Alcohol-induced cirrhosis results partially from the excessive production of collagen matrix proteins, which, predominantly alphaI(I) collagen, are produced and secreted by activated hepatic stellate cells (HSC). The accumulation of alphaI(I) collagen in HSC during cirrhosis is largely due to an increase in alphaI(I) collagen gene expression. Acetaldehyde, the major active metabolite of alcohol, is known to stimulate alphaI(I) collagen production in HSC. However, the mechanisms responsible for it remain unknown. The aim of this study was to elucidate the mechanisms by which alphaI(I) collagen gene expression is induced by acetaldehyde in rat HSC. In the present study, the acetaldehyde response element was located in a distal GC box, previously described as the UV response element, in the promoter of the alphaI(I) collagen gene (-1484 to -1476). The GC box was predominantly bound by the DNA binding transcription factor BTEB (basic transcription element binding protein), expression of which was acetaldehyde and UV inducible. Blocking BTEB protein expression significantly reduced the steady-state levels of the acetaldehyde-induced alphaI(I) collagen mRNA, suggesting that BTEB is required for this gene expression. Further studies found that acetaldehyde activated Jun N-terminal kinase (JNK) 1 and 2 and activator protein 1 (AP-1) transactivating activity. Inhibition of JNK activation resulted in the reduction of the acetaldehyde-induced BTEB protein abundance and alphaI(I) collagen mRNA levels, indicating that the expression of both genes is JNK dependent in HSC. Taken together, these studies demonstrate that BTEB mediates acetaldehyde-induced, JNK-dependent alphaI(I) collagen gene expression in HSC.

Platelet-derived growth factor is a principal inductive factormodulating mannose 6-phosphate/insulin-like growth factor-II receptorgene expression via a distal E-box in activated hepatic stellate cells

Hepatic stellate cells (HSCs) become activated during the earlystages of hepatic injury associated with fibrogenesis. The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGFIIR) plays animportant role in early fibrogenesis by participating in the activationof latent transforming growth factor-beta, a potent inducer of thematrix proteins in activated stellate cells that produce the hepaticnodule. Platelet-derived growth factor (PDGF), a potent HSC mitogen, isreleased early in hepatic injury and activates several signallingpathways in HSCs. In this study we examined the role of PDGF-BB in HSCregulation of M6P/IGFIIR gene expression. Several promoter elementswere found and characterized that modulate M6P/IGFIIR expression inactivated stellate cells. The presence of a distal CACGTG E-box at-2695 was required for M6P/IGFIIR expression in transfectedstellate cells. When the distal E-box was removed there was no significant M6P/IGFIIR promoter activity. The distal E-box-binding protein responded specifically to PDGF-BB with increased binding. This coincided with PDGF-BB up-regulation of M6P/IGFIIR mRNA transcript levels. Downstream elements include two proximal (-2 to-48) CACGTG E-boxes that bind a different protein to the distal(-2695) E-box. The proximal E-boxes respond moderately to PDGF-BB. The promoter segment encompassing -144 to +109 is able to respond dramatically to serum but is refractory to PDGF-BB. However, a constitutively bound protein binding to the -611/-716 fragment appears to be a repressor that suppresses inductive changes in protein binding occurring downstream of -611. These results indicate that the M6P/IGFIIR promoter responds primarily and specifically to PDGF-BB through a distal E-box element and possibly through two proximal E-box elements.

Sp family of transcription factors is involved in iron-induced collagen alpha1(I) gene expression

The purpose of this study was to identify the cis-acting elements and the trans-acting factors involved in the iron-induced expression of the collagen alpha1(I) (COL1aI) gene. Rat hepatic stellate cells were cultured in the presence of 50 microM ferric chloride, 50 microM ascorbic acid, and 250 microM citric acid (Fe/AA/CA), and the effects on collagen gene expression and the binding of nuclear proteins to the COL1aI promoter were measured. The Fe/AA/CA treatment induced a time- and dose-dependent increase in the cellular levels of COL1aI mRNA that was abrogate by pretreating cells with cycloheximide, antioxidants, and inhibitors of aldehyde-protein adduct formation. Transient transfection experiments showed that Fe/AA/CA exerted its effect through regulatory elements located between -220 and -110 bp of the COL1aI promoter. Gel retardation assays showed that Fe/AA/CA increased the binding of nuclear proteins to two elements located between -161 and -110 bp of the COL1aI promoter. These bindings were blocked by unlabeled consensus Sp1 oligonucleotide and supershifted with Sp1 and Sp3 antibodies. Finally, Fe/AA/CA increased cellular levels of the Sp1 and Sp3 proteins and Sp1 mRNA. Treatment with Fe/AA/CA stimulates COL1aI gene expression by inducing the synthesis of Sp1 and Sp3 and their binding to two regulatory elements located between -161 and -110 bp of the COL1aI promoter.

Platelet-derived growth factor is a principal inductive factormodulating mannose 6-phosphate/insulin-like growth factor-II receptorgene expression via a distal E-box in activated hepatic stellate cells

Hepatic stellate cells (HSCs) become activated during the earlystages of hepatic injury associated with fibrogenesis. The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGFIIR) plays animportant role in early fibrogenesis by participating in the activationof latent transforming growth factor-beta, a potent inducer of thematrix proteins in activated stellate cells that produce the hepaticnodule. Platelet-derived growth factor (PDGF), a potent HSC mitogen, isreleased early in hepatic injury and activates several signallingpathways in HSCs. In this study we examined the role of PDGF-BB in HSCregulation of M6P/IGFIIR gene expression. Several promoter elementswere found and characterized that modulate M6P/IGFIIR expression inactivated stellate cells. The presence of a distal CACGTG E-box at-2695 was required for M6P/IGFIIR expression in transfectedstellate cells. When the distal E-box was removed there was no significant M6P/IGFIIR promoter activity. The distal E-box-binding protein responded specifically to PDGF-BB with increased binding. This coincided with PDGF-BB up-regulation of M6P/IGFIIR mRNA transcript levels. Downstream elements include two proximal (-2 to-48) CACGTG E-boxes that bind a different protein to the distal(-2695) E-box. The proximal E-boxes respond moderately to PDGF-BB. The promoter segment encompassing -144 to +109 is able to respond dramatically to serum but is refractory to PDGF-BB. However, a constitutively bound protein binding to the -611/-716 fragment appears to be a repressor that suppresses inductive changes in protein binding occurring downstream of -611. These results indicate that the M6P/IGFIIR promoter responds primarily and specifically to PDGF-BB through a distal E-box element and possibly through two proximal E-box elements.

NF-kappaB inhibits expression of the alpha1(I) collagen gene

Fibrosis results from an increase in the synthesis and deposition of type I collagen. Fibrosis is frequently associated with inflammation, which is accompanied by increased levels of tumor necrosis factor-alpha (TNFalpha) and activation of the transcription factor NF-kappaB. However, several agents known to activate NF-kappaB, such as phorbol 12-myristate 13-acetate (PMA) and TNFalpha, result in decreased expression of type I collagen. Therefore, we directly examined the effects of NF-kappaB on alpha1(I) collagen gene expression in two collagen-producing cells, NIH 3T3 fibroblasts and hepatic stellate cells (HSCs). Transient transfections of NIH 3T3 cells or HSCs using NF-kappaB p50, p65, and c-Rel expression plasmids with collagen reporter gene plasmids demonstrated a strong inhibitory effect on transcription of the collagen gene promoter. Dose-response curves showed that p65 was a stronger inhibitor of collagen gene expression than was NF-kappaB p50 or c-Rel (maximum inhibition 90%). Transient transfections with reporter gene plasmids containing one or two Spl binding sites demonstrated similar inhibitory effects of NF-kappaB p65 on the activity of these reporter genes, suggesting that the inhibitory effects of NF-kappaB p65 are mediated through the critical Spl binding sites in the alpha1(I) collagen gene promoter. Cotransfection experiments using either a super-repressor I[ke]B or Spl partially blocked the inhibitory effects of p65 on collagen reporter gene activity. Coimmunoprecipitation experiments demonstrated that NF-kappaB and Spl do interact in vivo. Nuclear run-on assays showed that NF-kappaB p65 inhibited transcription of the endogenous alpha1(I) collagen gene. Together, these results demonstrate that NF-kappaB decreases transcription of the alpha1(I) collagen gene.

UV irradiation activates JNK and increases alphaI(I) collagen gene expression in rat hepatic stellate cells

Hepatic stellate cells (HSCs) become activated into myofibroblast-like cells during the early stages of hepatic injury associated with fibrogenesis. The subsequent dysregulation of alphaI(I) collagen gene expression is a central pathogenetic step during the development of cirrhosis. Our recent study in rat HSCs (Davis, B. H., Chen, A., and Beno, D. (1996) J. Biol. Chem. 271, 11039-11042) found that ERK1,2 activation might be required for maximal alphaI(I) collagen gene expression. However, the role of the parallel JNK cascade in regulating alphaI(I) collagen gene expression was unknown. In this study, we initially found that UV irradiation of HSCs activated JNK but not ERK1,2. Furthermore, UV irradiation increased endogenous alpha I(I) collagen mRNA abundance and stimulated alpha I(I) collagen gene transcription in HSCs. The effect of the activation of JNK and Jun on alpha I(I) collagen gene expression was further evaluated via transfection of chloramphenicol acetyltransferase reporter plasmids with various sizes of truncated 5' upstream promoter sequence (UPS) of the alphaI(I) collagen gene. This revealed that dominant negative transcription factor JUN suppressed alpha I(I) collagen gene transcription in HSCs maintained in media with 20% serum and constitutively activated JUN increased alphaI(I) collagen gene transcription in HSCs cultured in media with 0.4% serum. UV activated JNK utilized a distal GC box in the 5'-UPS of the collagen gene to regulate gene transcription. This observation was confirmed by site-directed mutagenesis. In co-transfection experiments, the col-chloramphenicol acetyltransferase reporter with a mutagenized GC box was not suppressed by dn-JUN and was not stimulated by activated JUN or by UV irradiation. Southwestern blotting analyses and gel shift assays with basic transcription element-binding protein antiserum suggested that the GC box was bound by basic transcription element-binding protein, a recently described DNA-binding protein. In conclusion, the current study combined with our previous report suggests that ERK1,2 and JNK cascades regulate alphaI(I) collagen expression in HSCs through different regions of the 5'-UPS of the gene. The distal GC box in the 5'-UPS of the alphaI(I) collagen gene may play a central role in receiving extracellular signals through the JNK pathway.

Transcriptional and posttranscriptional silencing of rodent alpha1(I) collagen by a homologous transcriptionally self-silenced transgene

Transient transfection of rodent fibroblasts with plasmids carrying a full-size pro-alpha1(I) collagen gene (pWTC1) results in rapid reduction of the endogenous transcripts by >90%, while the transgene mRNA is undetectable. Using deletion constructs, two adjacent 5' noncoding regions of the gene are identified as causing transcriptional silencing of the endogene in normal and v-fos-transformed cells but not in nontumorigenic revertants, which show partial relief from v-fos transformation-induced alpha1(I) gene suppression. The 3' end of the transgene carries an additional element(s), causing posttranscriptional silencing of the endogene in all cells including the revertant. Data indicate that the transgenes are transcriptionally self-silenced. Genome-integrated transgenes that are transcriptionally active also allow expression of the endogene, suggesting gene activation by chromosomal factors missing in pWTC1. Silencing is not regulated by antisense RNA. Silencing of the endogenous pro-alpha1(I) collagen gene is not linked to the level of transgene expression.

chk-YB-1b, a Y-box binding protein activates transcription from rat alpha1(I) procollagen gene promoter

Type-I collagen, the predominant component of extracellular matrix, is a triple-helical protein consisting of two alpha1 polypeptides and one alpha2 polypeptide. Expression of alpha1 and alpha2 procollagen genes is co-ordinately regulated under both normal and various pathological conditions. However, the basis of this co-ordinate regulation is not well known. YB-1b, a Y-box protein, has been shown to bind to the polypyrimidine tract present in the alpha2 procollagen gene. Here, we show that chk-YB-1b, a YB-1 homologue, binds in a single-strand-sequence-specific manner to the highly conserved pyrimidine-rich sequences in both alpha1(I) and alpha2(I) procollagen promoters from different species, as demonstrated by electrophoretic-mobility-shift assays and by DNaseI footprinting experiments. Transiently transfected and retrovirally expressed antisense oligonucleotides directed against chk-YB-1b specifically inhibited the alpha1(I) procollagen promoter-driven transcription in cultured fibroblasts. Considering these data and the fact that the chk-YB-1b binding site is one of the few sites between alpha1(I) and alpha2(I) procollagen promoters that is conserved from chicken to human, it is proposed that chk-YB-1b may be involved in co-ordinate expression of these two collagen genes.

Triple helix formation with the promoter of human alpha1(I) procollagen gene by an antiparallel triplex-forming oligodeoxyribonucleotide

The promoters of alpha1(I) procollagen genes of vertebrates contain two contiguous stretches of polypyrimidine/polypurine sequences, referred to as C1 (-140 to -170) and C2 (-171 to -200). Antiparallel triplex-forming upstream oligonucleotides form efficient triplexes with C1. The C1 tract of human differs from rodent alpha1(I) promoters by 7 nt which are mainly A-->G transitions. Human triplex-forming oligodeoxyribonucleotide (TFO) formed stable triplexes efficiently with a K d of approximately 10-20 nM compared with a K d of approximately 100 nM for rodent TFO. Mutational analysis indicated that 3 or 4 nt (-153 to -155) are sufficient for this higher affinity. TFOs specific for human C1 inhibited transcription from human promoter both in vitro in HeLa cell nuclear extracts and in vivo in cultured chick embryo fibroblasts.

Identification of elements in the promoter region of the alpha1(I) procollagen gene involved in its up-regulated expression in systemic sclerosis

OBJECTIVE

To identify regulatory elements in the promoter region of the alpha1(I) procollagen gene (COL1A1) involved in the transcriptional activation of this gene in systemic sclerosis (SSc), and to identify the transcription factors interacting with these regulatory elements.

METHODS

Dermal fibroblasts from 6 patients with diffuse SSc of recent onset and from 6 healthy individuals were studied. The transcriptional regulation of COL1A1 was examined by transient transfections with deletion constructs containing portions of the COL1A1 promoter. The DNA binding activity of nuclear proteins recognizing the regulatory regions in the COL1A1 promoter was examined by gel mobility shift assays. A procedure was established to allow the quantitative determination of the amount of DNA binding proteins interacting with the COL1A1 promoter, employing DNA binding protein and DNA titration experiments analyzed by gel mobility shift assays.

RESULTS

Maximal chloramphenicol acetyltransferase activity was observed with a -174-bp to +42-bp COL1A1 promoter construct in both normal and SSc cells; however, the activity driven by this construct was 70-260% higher in SSc fibroblasts. Most of the transcriptional activity of the COL1A1 promoter was contained in a minimal promoter region encompassing -174 bp to -84 bp. Electrophoretic mobility shift assays performed with oligonucleotides corresponding to the regions spanning -129/-107 bp and -104/-78 bp of the COL1A1 promoter revealed marked increases in the intensities of DNA-protein complexes formed with both oligonucleotides in nuclear extracts prepared from each of the SSc cell lines in comparison with normal fibroblasts. Competition experiments showed that each of these regions contained elements recognized by Sp1 and nuclear factor 1 (NF-1) binding proteins. A quantitative determination of DNA binding activity recognizing the Sp1 binding element within the -129/-107-bp region showed that it was 23.6 nM in SSc fibroblasts compared with 6.9 nM in normal fibroblasts.

CONCLUSION

The results demonstrate that a short region in the proximal promoter of COL1A1 containing 2 tandem NF-1/Sp1 elements displays up-regulated transcriptional activity in SSc fibroblasts, and that SSc fibroblasts contain 3.4-fold greater DNA binding activity recognizing these elements than normal cells.

An RNA oligonucleotide corresponding to the polypyrimidine region of the rat alpha 1(I) procollagen promoter forms a stable triplex and inhibits transcription

In this report we demonstrate formation of a triplex structure by an antiparallel RNA oligonucleotide corresponding to the 21 bp polypurine-pyrimidine stretch from -141 to -162 of the rat alpha 1(I) procollagen promoter with a Kd of 0.1-0.2 microM. The formation of triplexes by the triplex forming oligoribonucleotide (ORN) was also observed under physiological conditions. In vitro transcription run-off experiments showed that triplex formation results in inhibited transcription from the rat alpha 1(I) procollagen gene. Our results demonstrate a novel approach for down-regulation of procollagen gene transcription in vivo.

E-box-binding repressor is down-regulated in hepatic stellate cells during up-regulation of mannose 6-phosphate/insulin-like growth factor-II receptor expression in early hepatic fibrogenesis

Hepatic stellate cells become activated during the early stages of hepatic injury associated with fibrogenesis. The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGFIIR) plays an important role in early fibrogenesis by participating in the activation of latent transforming growth factor-beta, a potent inducer of the matrix proteins in activated stellate cells that define the fibrotic phenotype. In this study we examined hepatic stellate cell regulation of M6P/IGFIIR expression and found that M6P/IGFIIR mRNA transcript levels increased in stellate cells from rats exposed to carbon tetrachloride (CCl4), a potent fibrogenic stimulant. Two E-boxes residing in the proximal promoter of M6P/IGFIIR were found to each bind a novel 75-kDa transcription factor (P75) in quiescent stellate cells of normal livers. This E-box binding was down-regulated as an early response in stellate cells exposed to CCl4, coinciding with increased M6P/IGFIIR transcript levels. Mutagenized E-boxes in M6P/IGFIIR promoter-chloramphenicol acetyltransferase (CAT) reporter constructs produced a substantial increase in reporter expression when compared with the corresponding native promoter-CAT construct when transfected in culture-activated stellate cells, suggesting P75's role as a repressor. The results indicate P75's participation in the regulation of M6P/IGFIIR transcription in hepatic stellate cells during fibrogenesis.

Posttranscriptional regulation of collagen alpha1(I) mRNA in hepatic stellate cells

The hepatic stellate cell (HSC) is the primary cell responsible for the dramatic increase in the synthesis of type I collagen in the cirrhotic liver. Quiescent HSCs contain a low level of collagen alpha1(I) mRNA, while activated HSCs contain about 60- to 70-fold more of this mRNA. The transcription rate of the collagen alpha1(I) gene is only two fold higher in activated HSCs than in quiescent HSCs. In assays using actinomycin D or 5,6-dichlorobenzimidazole riboside collagen alpha1(I) mRNA has estimated half-lives of 1.5 h in quiescent HSCs and 24 h in activated HSCs. Thus, this 16-fold change in mRNA stability is primarily responsible for the increase in collagen alpha1(I) mRNA steady-state level in activated HSCs. We have identified a novel RNA-protein interaction targeted to the C-rich sequence in the collagen alpha1(I) mRNA 3' untranslated region (UTR). This sequence is localized 24 nucleotides 3' to the stop codon. In transient transfection experiments, mutation of this sequence diminished accumulation of an mRNA transcribed from a collagen alpha1(I) minigene and in stable transfections decreased the half-life of collagen alpha1(I) minigene mRNA. Binding to the collagen alpha1(I) 3' UTR is present in cytoplasmic extracts of activated but not quiescent HSCs. It contains as a subunit alphaCP, which is also found in the complex involved in stabilization of alpha-globin mRNA. The auxiliary factors necessary to promote binding of alphaCP to the collagen 3' UTR are distinct from the factors necessary for binding to the alpha-globin sequence. Since alphaCP is expressed in both quiescent and activated HSCs, these auxiliary factors are responsible for the differentially expressed RNA-protein interaction at the collagen alpha1(I) mRNA 3' UTR.

Antiparallel polypurine phosphorothioate oligonucleotides form stable triplexes with the rat alpha1(I) collagen gene promoter and inhibit transcription in cultured rat fibroblasts

The rat alpha1(I) collagen promoter contains a unique polypurine-polypyrimidine sequence between -141 and -200 upstream of the transcription start site. The polypurine sequence from -171 to -200 (C2) is on the coding strand and the adjacent polypurine sequence from -141 to -170 (C1) is on the non-coding strand. Earlier we demonstrated triplex formation with a polypurine 30 nt parallel triplex-forming oligonucleotide (TFO) corresponding to C1 and inhibition of transcriptional activity of the rat alpha1(I) collagen promoter. In the present work we have tested triplex-forming abilities of shorter (18 nt) purine and pyrimidine TFOs in parallel and antiparallel orientation to the C1 purine sequence. Our results show that purine antiparallel TFOs formed triplexes with the highest binding affinities, while pyrimidine oligodeoxyribonucleotides (ODNs) did not show appreciable binding. Phosphorothioate modification of purine TFOs did not significantly reduce binding affinity. We also demonstrate that preformed triplexes are quite stable when precipitated with ethanol and resuspended in water. Further analysis was carried out using two purine phosphorothioate antiparallel TFOs, 158 APS and 164 APS, designed to bind to the promoter region from -141 to -158 and -147 to -164, respectively, which were found to form triplexes even under physiological conditions. DNase I footprinting experiments showed the ability of these TFOs to protect target sequences in the promoter region; both purine sequences (C1 and C2) were protected in the case of 158 APS. Transfection experiments using preformed triplexes with a reporter plasmid containing the collagen promoter sequence showed significant inhibition of transcription when compared with a control phosphorothioate ODN. The effect of 164 APS was greater than that of 158 APS. These results indicate that this triplex strategy could be used in the down-regulation of collagen synthesis in cultured cells and offer the potential to control fibrosis in vivo.

Identification of negative and positive regulatory elements in the rat alpha 1(I) collagen gene promoter

Type I collagen is the main constituent of extracellular matrix found in various organs including the heart. Under some pathological conditions accumulation of excess type I collagen in the interstitium leads to organ dysfunction. In order to identify the regulatory elements in the rat alpha 1(I) collagen gene promoter, deletions were made in the promoter region. Various plasmid constructs were transfected into different fibroblasts using LipofectAMINE. The results indicated a negative cis-element between nucleotides -310 to -440 in the rat alpha 1(I) collagen gene promoter. Presence of this sequence significantly diminished the reporter gene activity. In addition we have observed that the sequence between -220 to -330 contained a positively acting cis-element, which is highly active in rat fibroblasts. Analysis of the nuclear factors binding to the negative element by electrophoretic mobility shift assays indicated that similar or identical factors are present in different fibroblasts as well as human HeLa cells and that these factors appear to bind to a composite sequence within -325 to -400. Competition with different oligonucleotides suggested that two distinct but contiguous sequence motifs may constitute the negative regulatory element. Our results with the rat alpha 1(I) collagen promoter confirm the presence of a negative cis-element previously described for the mouse promoter and provided additional information on the bipartite nature of this element.

Suppression of stellate cell type I collagen gene expression involves AP-2 transmodulation of nuclear factor-1-dependent gene transcription

The regulation of collagen gene expression was studied in culture-activated rat hepatic stellate cells, the fibrogenic effector cell involved in hepatic fibrogenesis. Treatment of cells with a 5-lipoxygenase-specific inhibitor caused a reduction in alphaI(I) collagen mRNA transcript abundance, which suggested that leukotriene production was involved in maintaining the activated cell's high level of collagen mRNA production. The underlying mechanism involved a decrease in collagen gene transcription. Suppression of gene transcription was localized to an nuclear factor-1 (NF-1) binding domain in the proximal promoter and an AP-2 binding domain adjacent to it. Gel retardation assays demonstrated that an increase in AP-2 binding adjacent to the NF-1 site was likely to be the transmodulator responsible for the suppression of the NF-1-dependent gene expression. The data suggest that post-translational alterations in AP-2 activity are responsible for this unappreciated mechanism of regulating the collagen gene.

c-Krox binds to several sites in the promoter of both mouse type I collagen genes. Structure/function study and developmental expression analysis

We have previously shown that c-Krox is a zinc finger protein that can increases the transcriptional activity of the mouse alpha1(I) collagen promoter through its binding to two GC-rich sequences (Galéra, P., Musso, M., Ducy, P., and Karsenty, G. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 9372-9376). In this report we show that c-Krox can bind to an additional site in the promoter of the alpha1(I) collagen gene and to three sites in the promoter of the alpha2(I) collagen gene, the other gene coding for type I collagen. One of the binding sites present in both promoters is adjacent to the CCAAT box. We have performed a structure/function analysis of c-Krox locating the transactivation domain in the zinc finger and C-terminal domains and the dimerization domain in the C-terminal end of the protein. We also demonstrate that c-Krox is an early response gene, whose expression is detectable as early as 9.5-day postcoitum in mouse embryos. Whole-mount in situ hybridization shows that c-Krox is expressed in dermatomes, the somite derivatives that generate dermis, and section in situ hybridization shows that c-Krox and alpha1(I) collagen mRNAs colocalized in skin but not in bone during development. This result is consistent with the predominant expression of c-Krox in skin in postnatal life. Thus, our findings suggest that c-Krox is one transcription factor controlling the coordinated expression of the two type I collagen genes in skin.

Raf and mitogen-activated protein kinase regulate stellate cell collagen gene expression

Hepatic stellate cells become activated into myofibroblast-like cells during the early stages of hepatic injury associated with fibrogenesis. The subsequent dysregulation of hepatic stellate cell collagen gene expression is a central pathogenetic step during the development of cirrhosis. The cytoplasmic Raf and mitogen-activated protein (MAPK) kinases were found to differentially regulate alpha I(I) collagen gene expression in activated stellate cells. This suggests an unappreciated branch point exists between Raf and MAPK. A MAPK-stimulatory signal was mapped to the most proximal NF-1 and Sp-1 binding domains of the 5'-untranslated region of the collagen gene. A Raf-inhibitory signal was mapped to a further upstream binding domain involving a novel 60-kDa DNA-binding protein (p60). The cell-specific expression and induction of p60 in stellate cells during the early stages of hepatic fibrogenesis in vivo suggest a central role for this pathway during liver injury and stellate cell activation.

Triple helix-forming oligonucleotide corresponding to the polypyrimidine sequence in the rat alpha 1(I) collagen promoter specifically inhibits factor binding and transcription

Type I and III fibrillar collagens are the major structural proteins of the extracellular matrix found in various organs including the myocardium. Abnormal and progressive accumulation of fibrillar type I collagen in the interstitial spaces compromises organ function and therefore, the study of transcriptional regulation of this gene and specific targeting of its expression is of major interest. Transient transfection of adult cardiac fibroblasts indicate that the polypurine-polypyrimidine sequence of alpha 1(I) collagen promoter between nucleotides - 200 and -140 represents an overall positive regulatory element. DNase I footprinting and electrophoretic mobility shift assays suggest that multiple factors bind to different elements of this promoter region. We further demonstrate that the unique polypyrimidine sequence between -172 and -138 of the promoter represents a suitable target for a single-stranded polypurine oligonucleotide (TFO) to form a triple helix DNA structure. Modified electrophoretic mobility shift assays show that this TFO specifically inhibits the protein-DNA interaction within the target region. In vitro transcription assays and transient transfection experiments demonstrate that the transcriptional activity of the promoter is inhibited by this oligonucleotide. We propose that TFOs represent a therapeutic potential to specifically influence the expression of alpha 1(I) collagen gene in various disease states where abnormal type I collagen accumulation is known to occur.

G/C element contributes to the cell line-specific expression of the proximal osteocalcin promoter

Sequential activation of cell type-specific genes occurs during osteoblast development. The promoter of one such gene, osteocalcin, has been widely studied, but the DNA sequences that govern osteoblast-specific expression have not been defined. The proximal osteocalcin promoter linked to pTKCAT directs strong promoter activity in osteoblast-like ROS17/2.8 cells and comparatively weak promoter activity in nonosteoblastic NIH3T3 cells. To identify sequences important in conferring cell-specific expression of the osteocalcin gene, a deletion series of the human proximal promoter was constructed and the activities assessed in ROS17/2.8 and NIH3T3 cells. These studies identified a 30 bp sequence within the proximal promoter (osteocalcin repressor element-1 [ORE-1]) which is responsible for repressing the transcriptional activity in NIH3T3 cells. In electrophoretic mobility shift assays from both NIH3T3 and ROS17/2.8 cells, a protein complex bound to the ORE-1 that was related to a complex which binds the G/C-rich repressor element in the collagen type I (alpha 1) promoter. In addition, there was a second complex from NIH3T3 cells but not ROS17/2.8 cells that bound the ORE-1 fragment. The presence of this additional factor in NIH3T3 cells parallels the observation that constructs carrying the ORE-1 sequence have repressed promoter activity relative to the analogous constructs lacking the ORE-1 when transfected into NIH3T3 and suggests that the NIH3T3-specific factor is a repressor. These data indicate that the G/C element in the ORE-1 contributes to the repression of osteocalcin gene transcription in a nonosteoblast cell line. The high homology between the ORE-1 sequence and a related sequence in the collagen type I (alpha 2) proximal promoter suggests that homologous regions in other osteoblast-expressed genes may function similarly.

DNA methylation represses the murine alpha 1(I) collagen promoter by an indirect mechanism

Several lines of evidence indicate that DNA methylation plays a role in the transcriptional regulation of the murine alpha 1(I) collagen gene. To study the molecular mechanisms involved, a reporter gene construct containing the alpha 1(I) promoter and part of the first exon linked to the luciferase gene (Col3luc) was methylated in vitro and transfected into murine fibroblasts and embryonal carcinoma cells. Methylation resulted in repression of the alpha 1(I) promoter in both cell types, although it was less pronounced in embryonal carcinoma cells than in fibroblasts. The extent of repression depended on the density of methylation. DNase footprint and mobility shift assays indicated that the trans-acting factors binding to the alpha 1(I) promoter and first exon are ubiquitous factors and that their DNA binding is not inhibited by methylation. Transfection of Col3luc into Drosophila SL2 cells together with expression vectors for the transcription factors Sp1 and NF-1 showed that DNA methylation also inhibits the alpha 1(I) promoter in nonvertebrate cells, although to a much lesser extent than in murine cells. However, Sp1 and NF-1 transactivated the unmethylated and methylated reporter gene in SL2 cells equally well, confirming that these factors can bind and transactivate methylated DNA and indicating that DNA methylation represses the alpha 1(I) promoter by an indirect mechanism. This was further confirmed by cotransfection experiments with unspecific methylated competitor DNA which partially restored the activity of the methylated alpha 1(I) promoter. Our results suggest that DNA methylation can inhibit promoter activity by an indirect mechanism independent of methyl-C-binding proteins and that in vertebrate cells, chromatin structure and methyl-C-binding proteins cooperatively mediate the transcriptional inhibitory effect of DNA methylation.

DNA methylation represses the murine alpha 1(I) collagen promoter by an indirect mechanism

Several lines of evidence indicate that DNA methylation plays a role in the transcriptional regulation of the murine alpha 1(I) collagen gene. To study the molecular mechanisms involved, a reporter gene construct containing the alpha 1(I) promoter and part of the first exon linked to the luciferase gene (Col3luc) was methylated in vitro and transfected into murine fibroblasts and embryonal carcinoma cells. Methylation resulted in repression of the alpha 1(I) promoter in both cell types, although it was less pronounced in embryonal carcinoma cells than in fibroblasts. The extent of repression depended on the density of methylation. DNase footprint and mobility shift assays indicated that the trans-acting factors binding to the alpha 1(I) promoter and first exon are ubiquitous factors and that their DNA binding is not inhibited by methylation. Transfection of Col3luc into Drosophila SL2 cells together with expression vectors for the transcription factors Sp1 and NF-1 showed that DNA methylation also inhibits the alpha 1(I) promoter in nonvertebrate cells, although to a much lesser extent than in murine cells. However, Sp1 and NF-1 transactivated the unmethylated and methylated reporter gene in SL2 cells equally well, confirming that these factors can bind and transactivate methylated DNA and indicating that DNA methylation represses the alpha 1(I) promoter by an indirect mechanism. This was further confirmed by cotransfection experiments with unspecific methylated competitor DNA which partially restored the activity of the methylated alpha 1(I) promoter. Our results suggest that DNA methylation can inhibit promoter activity by an indirect mechanism independent of methyl-C-binding proteins and that in vertebrate cells, chromatin structure and methyl-C-binding proteins cooperatively mediate the transcriptional inhibitory effect of DNA methylation.

Cell-specific expression of alpha 1(I) collagen-hGH minigenes in transgenic mice

Sequences within the first intron of the alpha 1(I) collagen gene have been implicated in the regulation of expression of alpha 1(I) collagen-reporter gene constructs in cultured cells. However, the physiological significance of these intronic elements has not been established. We have used in situ hybridization to examine whether a cell-specific pattern of expression of human alpha 1(I) collagen-human growth hormone minigenes exists in transgenic mice. Our results indicate that transgenes which contained 2,300 bp of promoter/5' flanking sequence and an intact first intron were well expressed by fibroblasts in dermis and fascia, whereas transgenes lacking the intronic sequence, +292 to +1440, were not expressed in dermis and poorly expressed in fascia. Analysis of transgene expression in cultured fibroblasts obtained from dermal explants of transgenic animals confirmed the requirement for these intronic sequences in the regulation of the alpha 1(I) collagen gene. In contrast, transgenes with or without the intronic deletion were expressed equally well in tendon and bone, in a manner comparable to the endogenous mouse alpha 1(I) collagen gene, and expression of neither transgene was detected in skeletal muscle or perichondrium. These data support a model in which cis-acting elements in the first intron, and their cognate DNA-binding proteins, mediate transcription of the alpha 1(I) collagen gene in some cells, such as dermal fibroblasts, but not in tendon cells or osteoblasts. Moreover, regions of the gene not included in the sequence, -2300 to +1440, appear to be required for transcription in tissues such as skeletal muscle and perichondrium.

Mithramycin selectively inhibits collagen-alpha 1(I) gene expression in human fibroblast

The products of the collagen-alpha 1(I) and -alpha 2(I) genes form the triple helical molecule collagen type I, which constitutes the major ECM protein in tissue fibrosis. The collagen-alpha 1(I) gene is mainly transcriptionally regulated, and its promoter activity depends on the interaction of the transcription factors NF-I and Sp1 with a tandem repeat of evolutionary conserved NF-I/Sp1 switch elements. An increased affinity of Sp1 to these elements has been observed in experimental liver fibrosis. Here, we demonstrate that the DNA binding drug mithramycin displays a high affinity binding to the GC-rich elements in the collagen-alpha 1(I) promoter as measured by DNAse I protection and gel retardation assays. Mithramycin interferes with Sp1 but not with NF-I binding to these sites. At a concentration of 100 nM, mithramycin efficiently reduces basal and TGF-beta-stimulated alpha 1(I) gene expression in human primary fibroblasts. The transcriptional activity and mRNA steady state levels of other genes, including the collagenase gene, as well as the growth rate of fibroblasts remained unchanged on exposure to this drug. Taken together, our results indicate that the transcriptional activity of the type I collagen gene highly depends on its GC-rich regulatory elements, and further, that these elements can be differentially blocked, thereby changing the balance between ECM structural and degrading gene activities in human fibroblasts.

Purification of a novel factor which binds to the mouse alpha 2 (I) collagen promoter

We have identified and purified a DNA binding protein which specifically binds to a segment of the mouse alpha 2 (I) collagen promoter between -420 and -399 bp upstream of the start of transcription. Purification included heparin-agarose and sequence-specific DNA-affinity chromatography, followed by SDS-PAGE and renaturation of the DNA binding activity after elution from SDS-polyacrylamide gel. The DNA binding activity resides in two species of 42 kDa and 40 kDa, respectively. The levels of DNA binding activity of this factor, which has been tentatively designated as ColF1, are considerably higher in nuclear extracts of NIH-3T3 fibroblasts than in nuclear extracts from epidermal cells, lymphoid cells and transformed NIH-3T3 fibroblasts.

Regulation of expression of the type I collagen genes

The identification and functional analysis of DNA-protein interactions in the intronic and 5' flanking regions of the type I collagen genes has begun to define a series of cis-elements and trans-acting factors which regulate transcription of these genes. Studies such as these will eventually be expected to elucidate the mechanisms responsible for coordinate transcription of the alpha 1 and alpha 2 genes, a question which remains central to the field of collagen research. Although it is relatively straightforward to define sites of DNA-protein binding, interpretation of the functional importance of such interactions can be extremely complex. Furthermore, while mutation or deletion of a particular binding site may alter the functional activity of a construct transfected into cultured cells, there is no guarantee that a similar change will have the same effect in vivo, where the entire gene locus is present in its native chromosomal context. Nevertheless, these kinds of in vitro studies offer the best current approach to defining and isolating transcription factors that control expression of the alpha 1 and alpha 2 genes. Ultimately, it will be necessary to test the activity of such factors (and their respective cis-elements) in defined systems in vivo.

The transcriptional tissue specificity of the human pro alpha 1 (I) collagen gene is determined by a negative cis-regulatory element in the promoter

The transcriptional activity of plasmid pCOL-KT, in which human pro alpha 1 (I) collagen gene upstream sequences up to -804 and most of the first intron (+474 to +1440) drive expression of the chloramphenicol acetyltransferase (CAT) gene [Thompson, Simkevich, Holness, Kang & Raghow (1991) J. Biol. Chem. 266, 2549-2556], was tested in a number of mesenchymal and non-mesenchymal cells. We observed that pCOL-KT was readily expressed in fibroblasts of human (IMR-90 and HFL-1), murine (NIH 3T3) and avian (SL-29) origin and in a human rhabdomyosarcoma cell line (A204), but failed to be expressed in human erythroleukaemia (K562) and rat pheochromocytoma (PC12) cells, indicating that the regulatory elements required for appropriate tissue-specific expression of the human pro alpha 1 (I) collagen gene were present in pCOL-KT. To delineate the nature of cis-acting sequences which determine the tissue specificity of pro alpha 1 (I) collagen gene expression, functional consequences of deletions in the promoter and first intron of pCOL-KT were tested in various cell types by transient expression assays. Cis elements in the promoter-proximal and intronic sequences displayed either a positive or a negative influence depending on the cell type. Thus deletion of fragments using EcoRV (nt -625 to -442 deleted), XbaI (-804 to -331) or SstII (+670 to +1440) resulted in 2-10-fold decreased expression in A204 and HFL-1 cells. The negative influences of deletions in the promoter-proximal sequences was apparently considerably relieved by deleting sequences in the first intron, and the constructs containing the EcoRV/SstII or XbaI/SstII double deletions were expressed to a much greater extent than either of the single deletion constructs. In contrast, the XbaI* deletion (nt -804 to -609), either alone or in combination with the intronic deletion, resulted in very high expression in all cells regardless of their collagen phenotype; the XbaI*/(-SstII) construct, which contained the intronic SstII fragment (+670 to +1440) in the reverse orientation, was not expressed in either mesenchymal or nonmesenchymal cells. Based on these results, we conclude that orientation-dependent interactions between negatively acting 5'-upstream sequences and the first intron determine the mesenchymal cell specificity of human pro alpha 1 (I) collagen gene transcription.

NF-I/Sp1 switch elements regulate collagen alpha 1(I) gene expression

The expression of type I collagen is regulated developmentally and tissue specifically. Two sets of binding sites for nuclear factor I (NF-I) and Sp1 transcription factors arrayed as an imperfect tandem repeat are critical for high activity of the murine alpha 1(I) collagen gene in NIH-3T3 fibroblasts and are conserved in evolution. Gel retardation analysis combined with methylation interference studies show that NF-I and Sp1 bind to overlapping sites in a mutually exclusive manner. Cotransfection studies using Drosophila Schneider L2 cells, which lack both transcription factors, demonstrate that each factor alone trans-activates the gene, while cotransfection of both factors results in the inhibition of the strong Sp1 trans-activation. In contrast, the herpes simplex virus thymidine kinase promoter, which contains functionally independent NF-I and Sp1 binding sites, is maximally transactivated by the cotransfection of both factors. Because the two NF-I/Sp1 binding sites overlap, the ratio of the activities of the two factors rather than their absolute concentrations determine alpha 1(I) gene expression, characterizing these promoter sequences as transcription factor switch elements.

Developmental changes in the methylation status of regulatory elements in the murine alpha 1(I) collagen gene

Regulatory elements contributing to the tissue-specific regulation of the murine alpha 1(I) collagen (Colla1) gene have previously been identified in its promoter region and first intron. Because several lines of evidence indicate that DNA methylation may be involved in the tissue-specific regulation of Colla1 gene expression, we have analyzed the methylation status of the 5' region of the gene by restriction analysis and a methylation-dependent PCR assay. All sites tested were unmethylated in sperm DNA. The region surrounding the start site of transcription was partially or completely methylated in undifferentiated embryonal cell lines, suggesting that it may be marked by de novo methylation during early embryonic development. In differentiated cells and adult tissues, the Colla1 promoter was completely demethylated in collagen-producing and some nonproducing cells, and partially methylated in other nonproducing cells. The first intron was unmethylated in collagen-producing as well as nonproducing cells. Only sites in the first exon showed an inverse correlation with transcriptional activity, i.e., they were unmethylated in cells that express the gene, but predominantly methylated in cells that do not. Our results indicate that the methylation status of a small area (less than 1 kb) downstream of the Colla1 promoter, but not of the promoter itself or the first intron, may be critical for transcriptional activity of the promoter, presumably through an indirect mechanism.

Transcription factors nuclear factor I and Sp1 interact with the murine collagen alpha 1 (I) promoter

The collagen alpha 1(I) promoter, which is efficiently transcribed in NIH 3T3 fibroblasts, contains four binding sites for trans-acting factors, as demonstrated by DNase I protection assays (D. A. Brenner, R. A. Rippe, and L. Veloz, Nucleic Acids Res. 17:6055-6064, 1989). This study characterizes the DNA-binding proteins that interact with the two proximal footprinted regions, both of which contain a reverse CCAAT box and a G + C-rich 12-bp direct repeat. Analysis by DNase I protection assays, mobility shift assays, competition with specific oligonucleotides, binding with recombinant proteins, and reactions with specific antisera showed that the transcriptional factors nuclear factor I (NF-I) and Sp1 bind to these two footprinted regions. Because of overlapping binding sites, NF-I binding and Sp1 binding appear to be mutually exclusive. Overexpression of NF-I in cotransfection experiments with the alpha 1(I) promoter in NIH 3T3 fibroblasts increased alpha 1(I) expression, while Sp1 overexpression reduced this effect, as well as basal promoter activity. The herpes simplex virus thymidine kinase promoter, which contains independent NF-I- and Sp1-binding sites, was stimulated by both factors. Therefore, expression of the collagen alpha 1(I) gene may depend on the relative activities of NF-I and Sp1.

Transcription factors nuclear factor I and Sp1 interact with the murine collagen alpha 1 (I) promoter

The collagen alpha 1(I) promoter, which is efficiently transcribed in NIH 3T3 fibroblasts, contains four binding sites for trans-acting factors, as demonstrated by DNase I protection assays (D. A. Brenner, R. A. Rippe, and L. Veloz, Nucleic Acids Res. 17:6055-6064, 1989). This study characterizes the DNA-binding proteins that interact with the two proximal footprinted regions, both of which contain a reverse CCAAT box and a G + C-rich 12-bp direct repeat. Analysis by DNase I protection assays, mobility shift assays, competition with specific oligonucleotides, binding with recombinant proteins, and reactions with specific antisera showed that the transcriptional factors nuclear factor I (NF-I) and Sp1 bind to these two footprinted regions. Because of overlapping binding sites, NF-I binding and Sp1 binding appear to be mutually exclusive. Overexpression of NF-I in cotransfection experiments with the alpha 1(I) promoter in NIH 3T3 fibroblasts increased alpha 1(I) expression, while Sp1 overexpression reduced this effect, as well as basal promoter activity. The herpes simplex virus thymidine kinase promoter, which contains independent NF-I- and Sp1-binding sites, was stimulated by both factors. Therefore, expression of the collagen alpha 1(I) gene may depend on the relative activities of NF-I and Sp1.

An upstream regulatory region mediates high-level, tissue-specific expression of the human alpha 1(I) collagen gene in transgenic mice

Studies in vitro have not adequately resolved the role of intronic and upstream elements in regulating expression of the alpha 1(I) collagen gene. To address this issue, we generated 12 separate lines of transgenic mice with alpha 1(I) collagen-human growth hormone (hGH) constructs containing different amounts of 5'-flanking sequence, with or without most of the first intron. Transgenes driven by 2.3 kb of alpha 1(I) 5'-flanking sequence, whether or not they contained the first intron, were expressed at a high level and in a tissue-specific manner in seven out of seven independent lines of transgenic mice. In most tissues, the transgene was expressed at levels approaching that of the endogenous alpha 1(I) gene and was regulated identically with the endogenous gene as animals aged. However, in lung, expression of the transgene was anomalously high, and in muscle, expression was lower than that of the endogenous gene, suggesting that in these tissues other regions of the gene may participate in directing appropriate expression. Five lines of mice were generated containing transgenes driven by 0.44 kb of alpha 1(I) 5'-flanking sequence (with or without the first intron), and expression was detected in four out of five of these lines. The level of expression of the 0.44-kb constructs in the major collagen-producing tissues was 15- to 500-fold lower than that observed with the longer 2.3-kb promoter. While transgenes containing the 0.44-kb promoter and the first intron retained a modest degree of tissue-specific expression, those without the first intron lacked tissue specificity and were poorly expressed in all tissues except lung. These results contribute to our understanding of the role of the first intron in regulating alpha1(I) gene expression and identify a region, upstream of the basal alpha1(I) promotor, which is necessary for full tissue-specific, developmentally regulated expression of the alpha1(I) collagen gene.

An upstream regulatory region mediates high-level, tissue-specific expression of the human alpha 1(I) collagen gene in transgenic mice

Studies in vitro have not adequately resolved the role of intronic and upstream elements in regulating expression of the alpha 1(I) collagen gene. To address this issue, we generated 12 separate lines of transgenic mice with alpha 1(I) collagen-human growth hormone (hGH) constructs containing different amounts of 5'-flanking sequence, with or without most of the first intron. Transgenes driven by 2.3 kb of alpha 1(I) 5'-flanking sequence, whether or not they contained the first intron, were expressed at a high level and in a tissue-specific manner in seven out of seven independent lines of transgenic mice. In most tissues, the transgene was expressed at levels approaching that of the endogenous alpha 1(I) gene and was regulated identically with the endogenous gene as animals aged. However, in lung, expression of the transgene was anomalously high, and in muscle, expression was lower than that of the endogenous gene, suggesting that in these tissues other regions of the gene may participate in directing appropriate expression. Five lines of mice were generated containing transgenes driven by 0.44 kb of alpha 1(I) 5'-flanking sequence (with or without the first intron), and expression was detected in four out of five of these lines. The level of expression of the 0.44-kb constructs in the major collagen-producing tissues was 15- to 500-fold lower than that observed with the longer 2.3-kb promoter. While transgenes containing the 0.44-kb promoter and the first intron retained a modest degree of tissue-specific expression, those without the first intron lacked tissue specificity and were poorly expressed in all tissues except lung. These results contribute to our understanding of the role of the first intron in regulating alpha1(I) gene expression and identify a region, upstream of the basal alpha1(I) promotor, which is necessary for full tissue-specific, developmentally regulated expression of the alpha1(I) collagen gene.