Regulation of collagen gene expression

Regulation of the human alpha 2(1) procollagen gene by sequences adjacent to the CCAAT box

The human, rat, mouse and chicken alpha 2(1) procollagen promoters analysed to date all contain an inverted CCAAT box at -80. In this study we have examined the binding of nuclear proteins to the proximal promotor of the human alpha 2(1) procollagen gene, where an inverted CCAAT box is flanked by a downstream GGAGG sequence and its inverted counterpart (CCTCC) on the upstream end. Each of the GGAGG sequences is separated from the inverted CCAAT box by a single pyrimidine nucleotide (5'-CCTCCCATTGGTGGAGGCCCTTTT-3'). Electrophoretic mobility-shift assays (EMSAs) revealed that two distinct DNA-protein complexes formed on this DNA sequence. Methylation interference analysis and in vitro mutagenesis studies revealed that the integrity of the sequence 5'-CCTCCCATTGG-3' (the GGAGG/CCAAT-binding element or G/CBE) was important for the binding of the CCAAT-binding factor (CBF) (complex I). Competition studies showed that complex formation on the human G/CBE could be competed by mouse CBE and nuclear factor-Y (NF-Y) oligonucleotides, suggesting that mouse CBE and human G/CBE-binding proteins belong to the same family of CCAAT box binding proteins. Furthermore, antibodies to mouse CBF specifically supershifted the G/CBE complex (complex I) in EMSAs. The downstream GGAGG and 3'-flanking sequences (5'-GGAGGCCCTTTT-3') or collagen modulating element (CME), however, were important for the formation of a novel DNA protein complex (complex III). The formation of this complex was not competed out by CBE or NF-Y oligonucleotides, nor was DNA-protein complex formation affected by the anti-CBF antibody. Functional analysis of G/CBE and CME elements subjected to mutagenesis, using promoter-chloroamphenicol acetyl transferase constructs in transient transfection assays, showed that both these elements were essential for activity of the human promoter. These experiments identified a novel regulatory element in the human alpha 2(1) procollagen gene which is not present in the rodent gene.

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

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

Cell-specific expression of the alpha 1 (I) collagen promoter-CAT transgene in skin and lung: a response to TGF-beta subcutaneous injection and bleomycin endotracheal instillation

Transgenic mice containing a rat collagen alpha 1 (I) promoter (3.6 kilobases) fused to the reporter gene chloramphenicol acetyl transferase (CAT) express the reporter gene parallel to endogenous gene in most connective tissues other than vascular tissue [Pavlin et al. (1992): J Cell Biol 116:227-236; Bedalov et al. (1994): J Biol Chem 269:4903-4909]. We have challenged transgenic mice with subcutaneous injections of transforming growth factor-beta (TGF-beta) or intratracheal instillation of bleomycin. In situ hybridization studies of skin revealed increased CAT expression in the papillary dermis of TGF-beta treated animals. In contrast, alpha 1 (I) collagen mRNA was expressed throughout the dermis including granulation tissue and reticular dermis. Therefore, the transgenic promoter responds to TGF-beta in a subset of dermal fibroblasts. Endotracheal instillation of bleomycin induces lung fibrosis which is thought to be mediated in part by TGF-beta. CAT gene expression in lungs was increased 6-8-fold at 2 weeks post bleomycin treatment. In situ hybridization studies revealed focal areas of cells expressing both CAT and collagen genes in the interstitium. However, most regions, especially around airways, contained a subset of cells expressing the endogenous gene with little or no CAT expression as judged by in situ hybridization. These cells could be myofibroblasts that require additional cis-acting elements to activate alpha 1 (I) collagen gene expression similar to smooth muscle cells.

Pathogenesis of scleroderma. Collagen

It is now evident that persistent overproduction of collagen and other connective tissue macromolecules results in excessive tissue deposition, and is responsible for the progressive nature of fibrosis in SSc. Up-regulation of collagen gene expression in SSc fibroblasts appears to be a critical event in the development of tissue fibrosis. The coordinate transcriptional activation of a number of extracellular matrix genes suggests a fundamental alteration in the regulatory control of gene expression in SSc fibroblasts. Trans-acting nuclear factors that bind to cis-acting elements in enhancer and promoter regions of the genes modulate the basal and inducible transcriptional activity of the collagen genes. The identity of the nuclear transcriptional factors that regulate normal collagen gene expression remains to be firmly established, and to date, no alterations in the level or in the activity of such DNA binding factors has been demonstrated in SSc fibroblasts. In addition to important interactions between fibroblasts and the extracellular matrix, cytokines and other cellular mediators can positively and negatively influence fibroblast collagen synthesis. Some of these signaling molecules may have physiologic roles, and their aberrant expression, or altered responsiveness of SSc fibroblasts to them, may result in the acquisition of the activated phenotype. The rapid expansion of knowledge regarding the effects of cytokines on extracellular matrix synthesis has led to an appreciation of the enormous complexity of regulatory networks that operate in the physiologic maintenance of connective tissue and which may be responsible for the occurrence of pathologic fibrosis. The ubiquitous growth factor TGF beta is the most potent inducer of collagen gene expression and connective tissue accumulation yet discovered. The expression of TGF beta in activated infiltrating mononuclear cells suggests a role for this cytokine as a mediator of fibroblast activation in SSc. Furthermore, the recognition that TGF beta is capable of inducing its own expression in a variety of cell types, coupled with the demonstration that a subpopulation of SSc dermal fibroblasts produces TGF beta, indicates the existence of a possible autocrine loop whereby lymphocyte-derived TGF beta in early SSc not only signals biosynthetic activation of fibroblasts in a paracrine manner, but autoinduces endogenous TGF beta production by the target fibroblasts themselves. Such an autocrine loop involving TGF beta may explain the persistent activation of collagen gene expression in SSc fibroblasts, and could be responsible for the progressive nature of fibrosis in SSc. Numerous other cytokines, as well as cell-matrix interactions, also modify collagen gene expression and can significantly influence the effects of TGF beta. Although their physiologic function in tissue remodeling or their involvement in abnormal fibrogenesis has not yet been conclusively demonstrated, the study of the biologic effects of these cytokines may provide important clues to understanding the pathogenesis of SSc, and to the development of rational drug therapy aimed at interrupting the abnormal fibrogenic process in this disease.

Y-box proteins interact with the S1 nuclease-sensitive site in the chicken alpha 2(I) collagen gene promoter

The sequence of the chicken alpha 2(I) collagen promoter from -712 to -85, relative to exon 1, has been shown to be important for transcriptional activity. Within this region a pyrimidine/purine asymmetrical element at -200 bp forms an in vitro S1 nuclease-sensitive site. The pyrimidine-rich strand of this element interacts specifically with single-stranded DNA-binding proteins present in fibroblast nuclear extracts [Bayarsaihan and Lukens (1996) Biochem. J. 314, 293-296]. To identify these proteins we performed expression screening of a chick embryo fibroblast cDNA library using a single-stranded polypyrimidine sequence derived from this element. One of the isolated clones was found to encode a member of the cold-shock gene family, either chicken YB-1 or a highly homologous protein. This protein and a known chicken Y-box protein were both found to bind sequence-specifically to the pyrimidine-rich strand of the pyrimidine/purine asymmetrical element in the chicken alpha 2(I) collagen promoter. The binding mechanism of these proteins could be based on the formation of a non-canonical triplex DNA structure (H-DNA). Although members of this widespread and conserved protein family have been reported to modulate the expression of a number of genes, the findings reported here provide the first evidence for a possible role of cold-shock proteins in the regulation of type I collagen genes.

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

In vivo transient expression and in vitro transcription experiments indicated that a segment between -170 and -40 bp upstream of the start of transcription of the mouse proalpha2(I) collagen gene was essential to activate transcription. DNase I protection experiments identified three strong footprints in this segment. Experiments with deletion mutants encompassing the sequences defined by these three footprints indicated that each of the three elements contributed to the transcriptional activity of the promoter. All three elements are GC-rich, redundant sites for a complex set of DNA binding proteins that includes SP1, other proteins that bind to an SP1 consensus site and proteins that bind to a Krox consensus site. In addition, the segment corresponding to the most proximal footprint also binds the multimeric CCAAT binding protein CBF. Addition of an excess amount of oligo- nucleotides corresponding to either of the two distal footprints significantly inhibited in vitro transcription of the -350 bp proalpha2(I) collagen promoter. Anti-SP1 antibodies that completely inhibited transcription of the early SV40 promoter had little effect on transcription of the wild-type -350 bp promoter, suggesting that SP1 has only a minor role in activity of this promoter. Our results show that the segment between base pairs -170 and -40 of the proalpha2(I) collagen promoter, which contains redundant binding sites for a complex set of nuclear proteins, is essential in the transcriptional activity of this promoter in fibroblasts.

Transcriptional activation of the alpha 1(I) procollagen gene in systemic sclerosis dermal fibroblasts. Role of intronic sequences

OBJECTIVE

To investigate the transcriptional regulation of the alpha 1(I) procollagen gene (COL1A1) in cultured dermal fibroblasts from patients with diffuse systemic sclerosis (SSc) of recent onset and to evaluate the role that intronic sequences may play in the upregulated expression of COL1A1 in SSc dermal fibroblasts.

METHODS

Dermal fibroblasts from 6 patients with diffuse SSc of recent onset and from 3 healthy individuals were studied. The steady-state levels of alpha 1(I) procollagen messenger RNA were evaluated by Northern hybridization analysis, and the transcriptional regulation of COL1A1 was examined by transient transfection experiments with deletion constructs containing portions of COL1A1 promoter (with 5' end points at -5.3 kb, -2.3 kb, and -804 bp and 3' end point at +42 bp) ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. To examine the role of intronic sequences, constructs containing, in addition to the COL1A1 promoter, a portion of the first intron (+380 bp to +1,440 bp) cloned in front of the CAT gene were transfected. The efficiency of transfections was normalized relative to the net amount of CAT plasmid actually transfected into recipient cells, determined by a modified Southern hybridization procedure.

RESULTS

Maximal CAT activity was observed with constructs extending from -804 bp to +42 bp in both normal and SSc fibroblasts. However, the activity driven by this construct was 80-110% higher in SSc fibroblasts. The CAT activity driven by a construct with a 5' end point at -5.3 kb was only 15-20% higher in SSc cells, and the CAT activity driven by a construct with a 5' end point -2.3 kb was 35-45% higher in SSc fibroblasts. The CAT activity driven by the -804-bp promoter construct was increased up to 4-fold in SSc fibroblasts in comparison with normal cells when the intronic segment spanning +380 bp to +1,440 bp was included in the transfected construct.

CONCLUSION

The results directly demonstrate the transcriptional activation of COL1A1 in dermal fibroblasts from SSc patients. The data also indicate that first-intron sequences of COL1A1 are required for maximal transcriptional activity of the collagen gene and may play an important role in the up-regulation of its expression in SSc fibroblasts.

Surface adhesion-mediated regulation of chondrocyte-specific gene expression in the nontransformed RCJ 3.1C5.18 rat chondrocyte cell line

Recent evidence suggests that decreased chondrocyte function in osteoarthritis and other articular disorders may be due to chondrocyte dedifferentiation produced by altered regulatory signals from the cartilage extracellular matrix (ECM). However, there are currently no mammalian chondrocytic cell line systems adapted to the study of this process. We therefore examined the effects of ECM growth conditions on markers of differentiated chondrocytic phenotype expression in the nontransformed rat RCJ 3.1C5.18 (RCJ) chondrocyte cell line, including type II collagen expression, aggrecan production, link protein gene expression, and parathyroid hormone (PTH) receptor number. RCJ cells grown in monolayer on plastic exhibited a dedifferentiated phenotype characterized by flattened cell morphology, with > 80% type I collagen and < 5% type II collagen production, as determined by two-dimensional gel mapping electrophoresis of collagen cyanogen bromide peptides. In addition, aggrecan production was low, and link protein mRNA was not expressed at detectable levels. After transfer to growth under minimal attachment conditions on the surface of a composite type I collagen/agarose (0.15%-0.8%) gel (CAG) for 7 days, RCJ cells developed a rounded, chondrocytic morphology and a pattern of differentiated, chondrocytic gene expression, with 79% type II and 8% type I collagen production. Steady-state type I and type II procollagen mRNA levels were altered in parallel with collagen protein expression. In cells grown on CAG, aggrecan production increased 6-fold, and there was a marked increase in both aggrecan core protein and link protein mRNA levels. In addition, maximal PTH-stimulated cAMP generation increased 15-fold in association with an increased PTH receptor number. Therefore, the RCJ chondrocyte cell line is highly sensitive to ECM regulation of chondrocyte-specific gene expression.

Two different cis-acting regulatory regions direct cell-specific transcription of the collagen alpha 1(I) gene in hepatic stellate cells and in skin and tendon fibroblasts

The expression of the collagen alpha 1(I) gene in activated stellate cells plays an important role during liver fibrogenesis. To identify the critical cis-elements of the collagen alpha 1(I) gene in stellate cells, we used transgenic animals bearing various collagen alpha 1(I) regulatory regions directing the expression of either a human growth hormone minigene or the bacterial beta-galactosidase gene. We found that collagen alpha 1(I)-human growth hormone transgene expression was constitutively high in tendon and skin, provided the transgene contained the -2.3 to -0.44 kb collagen regulatory region. However in the liver, expression was stimulated several-fold, as was the endogeneous gene, by the fibrogenic hepatotoxin carbon tetrachloride. This stimulation occurred whether the collagen 5' regulatory region extended -2.3, -1.6 or -0.44 kb, and in the presence or absence of much of the first intron (+292 to +1607 bp). In addition, the -0.44 kb 5' region was sufficient for high-level transgene expression in stellate cells, following their activation by culture on plastic. In contrast, in skin and tendon, high-level transcription of the collagen alpha 1(I) gene required the -2.3 to -0.44 kb 5' flanking region. Thus, two different cis-regulatory regions direct cell-specific transcription of the collagen alpha 1(I) gene in stellate cells and in skin and tendon.

Positive regulation of human alpha 1 (I) collagen promoter activity by transcription factor Sp1

Analysis of the regulatory promoter region of the human alpha 1 (I) collagen-encoding gene (COL1A1) gene indicated the presence of G+C-rich sequence elements that are potential binding sites for the transcription factor Sp1. As a step toward understanding transcriptional regulation of the human COL1A1, we examined Sp1 binding in the promoter region using DNase I footprinting, and analyzed the effect of Sp1 expression on COL1A1 promoter activity in transiently transfected Drosophila melanogaster cells in vivo. The results indicated that recombinant human Sp1 interacted specifically with two G+C-rich sequences within the COL1A1 promoter. Binding of factors in nuclear extracts prepared from human dermal fibroblasts to a 22-nucleotide deoxyribonucleotide (oligo) spanning the 5' G+C-rich sequence required Zn2+, and was abolished by excess Sp1 consensus binding site oligos, or by anti-Sp1 antibodies. Studies in which a series of progressively 5'-deleted COL1A1 promoter::cat constructs were co-expressed with an Sp1 expression plasmid in a cellular background devoid of Sp1 homology demonstrated that Sp1 markedly enhanced the COL1A1 promoter activity. These results suggest that the transcriptional activity of the human COL1A1 can be positively regulated by Sp1.

Controlled conversion of an immortalized mesodermal progenitor cell towards osteogenic, chondrogenic, or adipogenic pathways

The teratocarcinoma-derived C1 clone behaves as a mesodermal tripotential progenitor cell whose choice of fate, either osteoblast, chondroblast, or adipoblast, is strictly dependent on the spatial organization of the cells and the nature of the induction. In the absence of cell contact before the addition of inducers, the C1 cells maintain a stable undifferentiated phenotype while expressing potential regulators of embryonic mesodermal stem cell fate such a M-twist and Id1. Upon establishment of cell contacts before the induction of differentiation, the early genes characteristic of the three fates become expressed. In the presence of beta glycerophosphate and ascorbate, provided the cells have formed aggregates, 95% of the C1 cells mineralize with a kinetics of gene expression close to that of osteoblasts (Poliard, A., D. Lamblin, P. J. Marie, M. H. Buc, and O. Kellerman. 1993. J. Cell Sci. 106:503-512). With 10(-6)M dexamethasone, 80% of the same aggregates differentiate into foci of chondroblast-like cells. The kinetics of expression of the genes encoding type II, IX, X, and XI collagens, aggrecan and link protein during the conversion toward cartilage hypertrophy resembles that accompanying in vivo chondrogenesis. The synergistic action of dexamethasone and insulin convert most confluent C1 cells into functional adipocytes and induce a pattern of gene expression close to that reported for adipoblast cell lines. The C1 clone with its capacity to differentiate along three alternative pathways with high frequency, therefore appears as a valid in vitro model for deciphering the molecular basis of mesoblast ontogeny.

Age-related alterations in collagen and total protein metabolism determined in cultured rat dermal fibroblasts: age-related trends parallel those observed in rat skin in vivo

The cultured fibroblast has been extensively used as a model system to study aging. However, few studies have examined the veracity of observations obtained in cultured fibroblasts aged in vitro to those made in animal tissues in vivo. This paper compares age-related alterations in collagen metabolism measured in cultured cells with previously reported results in the aging rat (Mays et al. (1991) Biochem. J. 276, 307-313). Age-related changes in collagen synthesis in rat skin fibroblasts in vitro over 30 population doublings were determined based on the production of hydroxy-[14C]proline. Degradation of newly synthesized collagen was based on the appearance of free hydroxy-[14C]proline in the culture system. Total protein synthesis rates were based on the incorporation of [14C]proline into proteins. In vitro rates of collagen synthesis decreased 5-fold over 30 population doublings (P < 0.05). Degradation of newly synthesized collagen increased from 33.0 +/- 0.8% (n = 4, SEM) to 45.2 +/- 1.1% (n = 4; P < 0.05) over the same period, with a maximum after 25 population doublings of 55.8 +/- 1.1% (n = 4). Total protein synthesis rates decreased by one-half over 30 population doublings (P < 0.05). The results indicated that collagen production decreased as cells aged in vitro and that this was due to both changes in synthesis and degradation. The results demonstrate that age-related alterations in collagen and total protein metabolism of skin fibroblasts in culture were similar to those reported previously for skin in vivo, suggesting that for studies of these processes, fibroblasts in culture provide an appropriate model.

Identification and characterization of a fibroblast marker: FSP1

We performed subtractive and differential hybridization for transcript comparison between murine fibroblasts and isogenic epithelium, and observed only a few novel intracellular genes which were relatively specific for fibroblasts. One such gene encodes a filament-associated, calcium-binding protein, fibroblast-specific protein 1 (FSP1). The promoter/enhancer region driving this gene is active in fibroblasts but not in epithelium, mesangial cells or embryonic endoderm. During development, FSP1 is first detected by in situ hybridization after day 8.5 as a postgastrulation event, and is associated with cells of mesenchymal origin or of fibroblastic phenotype. Polyclonal antiserum raised to recombinant FSP1 protein stained the cytoplasm of fibroblasts, but not epithelium. Only occasional cells stain with specific anti-FSP1 antibodies in normal parenchymal tissue. However, in kidneys fibrosing from persistent inflammation, many fibroblasts could be identified in interstitial sites of collagen deposition and also in tubular epithelium adjacent to the inflammatory process. This pattern of anti-FSP1 staining during tissue fibrosis suggests, as a hypothesis, that fibroblasts in some cases arise, as needed, from the local conversion of epithelium. Consistent with this notion that FSP1 may be involved in the transition from epithelium to fibroblasts are experiments in which the in vitro overexpression of FSP1 cDNA in tubular epithelium is accompanied by conversion to a mesenchymal phenotype, as characterized by a more stellate and elongated fibroblast-like appearance, a reduction in cytokeratin, and new expression of vimentin. Similarly, tubular epithelium submerged in type I collagen gels exhibited the conversion to a fibroblast phenotype which includes de novo expression of FSP1 and vimentin. Use of the FSP1 marker, therefore, should further facilitate both the in vivo studies of fibrogenesis and the mapping of cell fate among fibroblasts.

Separate cis-acting DNA elements of the mouse pro-alpha 1(I) collagen promoter direct expression of reporter genes to different type I collagen-producing cells in transgenic mice

The genes coding for the two type I collagen chains, which are active selectively in osteoblasts, odontoblasts, fibroblasts, and some mesenchymal cells, constitute good models for studying the mechanisms responsible for the cell-specific activity of genes which are expressed in a small number of discrete cell types. To test whether separate genetic elements could direct the activity of the mouse pro-alpha 1(I) collagen gene to different cell types in which it is expressed, transgenic mice were generated harboring various fragments of the proximal promoter of this gene cloned upstream of the Escherichia coli beta-galactosidase gene. During embryonic development, X-gal staining allows for the precise identification of the different cell types in which the beta-galactosidase gene is active. Transgenic mice harboring 900 bp of the pro-alpha 1(I) proximal promoter expressed the transgene at relatively low levels almost exclusively in skin. In mice containing 2.3 kb of this proximal promoter, the transgene was also expressed at high levels in osteoblasts and odontoblasts, but not in other type I collagen-producing cells. Transgenic mice harboring 3.2 kb of the proximal promoter showed an additional high level expression of the transgene in tendon and fascia fibroblasts. The pattern of expression of the lacZ transgene directed by the 0.9- and 2.3-kb pro-alpha 1(I) proximal promoters was confirmed by using the firefly luciferase gene as a reporter gene. The pattern of expression of this transgene, which can be detected even when it is active at very low levels, paralleled that of the beta-galactosidase gene. These data strongly suggest a modular arrangement of separate cell-specific cis-acting elements that can activate the mouse pro-alpha(I) collagen gene in different type I collagen-producing cells. At least three different types of cell-specific elements would be located in the first 3.2 kb of the promoter: (a) an element that confers low level expression in dermal fibroblasts; (b) a second that mediates high level expression in osteoblasts and odontoblasts; and (c) one responsible for high level expression in tendon and fascia fibroblasts. Our data also imply that other cis-acting cell-specific elements which direct activity of the gene to still other type I collagen-producing cells remain to be identified.

Tissue-specific expression of the gene for type I procollagen (COL1A1) in transgenic mice. Only 476 base pairs of the promoter are required if collagen genes are used as reporters

Inconsistent data have been reported on the size of the promoter that is necessary for high levels of tissue-specific expression of the COL1A1 gene for type I procollagen. Some of the inconsistencies may be traced to the use of reporter gene constructs. Therefore, we prepared transgenic mice with modifications of the intact gene engineered so that the level of expression of the transgene could be assayed both as mRNA and protein that were similar to the products from the endogenous COL1A1 gene. The results with a mini-COL1A1 gene lacking 41 internal exons and introns indicated that the first intron and 90% of the 3'-untranslated region were not essential for tissue-specific expression. In a hybrid COL1A1/COL2A1 construct, a 1.9-kilobase 5'-fragment from the COL1A1 gene that contained only 476 of the promoter was linked to a promoterless 29.5-kilobase fragment of the human COL2A1 gene for type II procollagen. The hybrid COL1A1/COL2A1 construct was expressed as both mRNA and protein in tissues that normally synthesize type I procollagen but not type II procollagen. Apparently, 476 base pairs of the promoter are sufficient to drive tissue-specific expression of the COL1A1 gene and totally inappropriate expression of the COL2A1 gene.

Effect of the 3'-untranslated region on the expression levels and mRNA stability of alpha 1(I) collagen gene

Changes in the synthesis of type I collagen, a major extracellular matrix component in skin and bones, are associated with both normal growth or repair processes and with several pathological conditions such as lung fibrosis and liver cirrhosis. The expression of the alpha 1(I) collagen gene is regulated by transcriptional and post-transcriptional mechanisms. Regulation at both these levels are usually utilised when extensive changes occur in collagen synthesis. We constructed plasmids carrying the whole or partially deleted 3'-UTR sequences of the alpha 1(I) collagen gene, fused to two hGH exons and to the promoter of the alpha 1(I) collagen gene. A control plasmid contained the 3'-UTR of the hGH gene. In transient transfections into Rat-1 fibroblasts, no significant differences between plasmids were found, which suggests that although 3'-end of the gene has been shown in previous studies to contain DNaseI hypersensitive sites and to bind sequence-specific nuclear proteins it does not seem to function as a transcriptional regulator. This was further supported by the finding that TGF-beta treatment induced a 2.5-fold expression of hGH mRNA from plasmids containing collagen promoter and either hGH or alpha 1(I) collagen 3'-UTR. In stable transfections, mRNAs using the first polyadenylation site were not as stable as those transcribed from the endogenous alpha 1(I) collagen gene. We suggest that the 3'-UTR alone may not be sufficient to determine the stability of the shorter alpha 1(I) collagen mRNA species.

Down-regulation of collagen XII in transformed mesenchymal cells

Collagen XII is a complex multidomain protein associated with the surface of interstitial collagen fibrils. This protein is produced in large amounts by fibroblasts cultivated in vitro. However, it is completely absent from cells transformed by the oncogene v-myc or v-src and from cells derived from a methylcholanthrene-induced fibrosarcoma. Since all these cells lack any mRNA for collagen, XII, it seems likely that the synthesis is blocked at the transcriptional level. Experiments with a temperature-sensitive mutant of Rous sarcoma virus demonstrated that a single oncogene product is sufficient to inhibit the synthesis. A reduction in the expression of collagen XII might have profound effects on the stability of the extracellular matrix of transformed cells.

Human COL2A1-directed SV40 T antigen expression in transgenic and chimeric mice results in abnormal skeletal development

The ability of SV40 T antigen to cause abnormalities in cartilage development in transgenic mice and chimeras has been tested. The cis-regulatory elements of the COL2A1 gene were used to target expression of SV40 T antigen to differentiating chondrocytes in transgenic mice and chimeras derived from embryonal stem (ES) cells bearing the same transgene. The major phenotypic consequences of transgenic (pAL21) expression are malformed skeleton, disproportionate dwarfism, and perinatal/neonatal death. Expression of T antigen was tissue specific and in the main characteristic of the mouse alpha 1(II) collagen gene. Chondrocyte densities and levels of alpha 1(II) collagen mRNAs were reduced in the transgenic mice. Islands of cells which express cartilage characteristic genes such as type IIB procollagen, long form alpha 1(IX) collagen, alpha 2(XI) collagen, and aggrecan were found in the articular and growth cartilages of pAL21 chimeric fetuses and neonates. But these cells, which were expressing T antigen, were not properly organized into columns of proliferating chondrocytes. Levels of alpha 1(II) collagen mRNA were reduced in these chondrocytes. In addition, these cells did not express type X collagen, a marker for hypertrophic chondrocytes. The skeletal abnormality in pAL21 mice may therefore be due to a retardation of chondrocyte maturation or an impaired ability of chondrocytes to complete terminal differentiation and an associated paucity of some cartilage matrix components.

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.

LAP (NF-IL6) transactivates the collagen alpha 1(I) gene from a 5' regulatory region

Although collagen is known to enhance hepatocyte differentiation and hepatocytes produce collagen in vivo, the transcriptional factors responsible for collagen type I gene expression in hepatic cells are not known. LAP (Liver Activator Protein) is a member of the C/EBP family, which in differentiated hepatocytes contributes to the high levels of liver-specific gene expression. In this study we show that LAP binds to the collagen alpha 1(I) promoter at both reverse CCAAT motifs and activates transcription. Furthermore, an upstream element, collagen element I (-370/-344), which shares homology with the LAP binding cis-element of the albumin promoter (9 of 13 bp) is described. This collagen element I stimulates transcription in both orientations and when placed in front of either a homologous or a heterologous chimeric report construct. These experiments suggest that LAP may be important in the expression of collagen in differentiated hepatocytes through both the promoter and a newly described upstream element.

Enalapril reduces collagen type IV synthesis and expansion of the interstitium in the obstructed rat kidney

Chronic unilateral ureteral obstruction (UUO) results in interstitial fibrosis of the affected kidney. In this study we determined that enalapril ameliorates the increased production of extracellular matrix (ECM) protein in the tubulointerstitium during UUO. The relative volume (Vv) of the tubulointerstitium measured by a point-counting method increased significantly at three or five days of UUO as compared to the contralateral kidney. Enalapril significantly blunted this increase at either three or five days. Immunofluorescence studies revealed that collagen type IV increased remarkably in both the tubular basement membrane (TBM) and the interstitial space at three or five days of UUO. Glomeruli did not show any change. Collagen types I and III were faintly stained in the control kidneys while they were obviously increased in the interstitial space of the obstructed kidney. We examined the expression of collagen type IV (COL IV) because this basement membrane matrix protein appeared to be a major ECM protein deposited in the tubulointerstitium of the obstructed kidney. Semiquantitative analysis of COL IV by immunofluorescence microscopy revealed that enalapril reduced slightly (21%) but significantly (P < 0.01) the deposition of COL IV in the obstructed kidney. Measurement of cyanogen bromide peptides from the obstructed kidney by Western blotting showed an increase of COL IV. This increase was reduced slightly (20%) by enalapril. The level of COL IV mRNA measured by reverse transcription-PCR was very low or undetectable in the control and contralateral kidneys, while it was significantly increased in the obstructed kidney at three or five days of UUO. COL IV mRNA was abundant in glomeruli while it was almost undetectable in renal tubules in the control and contralateral kidneys. However, COL IV mRNA was increased in renal tubules but not in the glomeruli of the obstructed kidney. Enalapril treatment resulted in a 42% decrease (P < 0.01) in COL IV mRNA in the cortex and a remarkable decrease in the renal tubules of the obstructed kidney at five days. Enalapril treatment resulted in an 89% decrease in the number of infiltrating ED-1 positive monocytes/macrophages. These results indicate that enalapril treatment ameliorates the tubulointerstitial fibrosis of the affected kidney in UUO. This effect of enalapril on fibrosis may be due to the severe reduction in monocytes/macrophages capable of secreting the profibrotic factor TGF-beta 1.

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.

Nuclear and cytoplasmic alpha 1 (I) collagen mRNA-binding proteins

We have recently identified a cytoplasmic protein, alpha 1-RBF67, that specifically interacts with the conserved 3'-untranslated region of the alpha 1 (I) collagen gene. The binding activity was decreased in extracts from dexamethasone treated cells, which correlates with the known accelerated turnover of the COL1A1 RNA [Määttä, A. and Penttinen, R.P.K. (1993) Biochem. J. 295, 691-698]. Now we report that a very similar protein is present in nuclear extracts of NIH 3T3, human fibroblast and HeLa cells, which suggests that determination of cytoplasmic mRNA stability is not the sole function of the alpha 1-RBF67 activity. The binding to the RNA probe can be inhibited by annealing a DNA oligonucleotide or using excess of cold specific competitors. In UV-cross linking assays the nuclear protein has the same molecular weight (67 kDa) as the cytoplasmic one and the RNA-bound peptides generated by CNBr or V8 protease cleavage from both the cytoplasmic and the nuclear protein were identical. This protein was the only one of several nuclear collagen mRNA 3'-UTR binding proteins that was present in both nuclear and cytoplasmic extracts. In fibroblasts heparin-resistant nuclear RNA binding proteins had molecular weights of 45, 67 (alpha 1-RBF67), and 71 kDa. HeLa-cells contained an additional protein of 51 kDa and several non-specific RNA-binding proteins. The binding activity is modified by changes in the redox state, which implicates that in the nucleus the binding affinities of alpha 1(I) collagen RNA-binding protein and AP-1, a redox sensitive nuclear factor, that is important in the transcription of alpha 1(I) collagen gene, can be regulated simultaneously to the same direction.

The pathophysiology of obstructive nephropathy: the role of vasoactive compounds in the hemodynamic and structural abnormalities of the obstructed kidney

Vasoactive compounds such as angiotensin II, thromboxane A2, nitric oxide, and eicosanoids have a role in the hemodynamic and structural abnormalities that occur following obstruction of the urinary tract. Inhibition of angiotensin II and thromboxane synthesis increases glomerular filtration rate and renal plasma flow significantly in the postobstructed kidney. Angiotensin II also appears to mediate tubular interstitial fibrosis in the obstructed kidney. Indeed, use of angiotensin-converting enzyme inhibitors decreases the degree of interstitial fibrosis and the increased deposition of collagen seen in rats with prolonged ureteral obstruction.

Thyroid hormones, procollagen III peptide, body composition and basal metabolic rate in euthyroid individuals

We examined 103 euthyroid men and women within a wide range of body weights and ages. Fat free mass (FFM) and body fat (BF) were determined with the total body potassium technique, basal metabolic rate (BMR) by indirect calorimetry and serum concentrations of thyroid hormones (free and total T3 and T4) and the aminoterminal propeptide of collagen III (pIIIp) by immunoassays. BMR was positively related to FFM, BF, total T3, the free T3/free T4 ratio and pIIIp, and negatively to free T4 (men) and to the ratios free T4/total T4 and free T3/total T3. pIIIp was as strongly related to BMR as to total T3. It is suggested that pIIIp may serve as an indicator of peripheral energy expenditure. The negative relationship between BMR and free T4 was unexpected and different to the situation in hypo- and hyperthyreosis where BMR and thyroid hormone are positively related. Our hypothesis is that euthyroid subjects with low serum free thyroid hormone concentrations and comparatively high BMR may have high intracellular thyroid hormone concentrations.

A fibroblast protein binds the 3'-untranslated region of pro-alpha 1(I) collagen mRNA

Post-transcriptional regulation of the expression of the pro alpha 1(I) chain of type I collagen (COL1A1) was studied by analysing cytoplasmic RNA-binding proteins and by transient transfections with collagen minigene plasmids. In this paper we present evidence for a factor from NIH 3T3 cells and human skin fibroblasts that interacts with the conserved 3'-untranslated region (UTR) of the shorter 4.8 kb mRNA species of the COL1A1 gene. The specificity of the interaction was confirmed by using (i) unlabelled specific and non-specific competitor RNAs and (ii) oligodeoxyribonucleotides annealed to the probe or used as single-stranded competitors. An antisense oligonucleotide annealed to the RNA probe near its 3'-terminus [20-42 nucleotides upstream of the first polyadenylation signal of the alpha 1(I) collagen mRNA] inhibited the binding, whereas other sense or antisense oligonucleotides had no effect on the interaction. The binding was sensitive to alkylation of free SH groups but not to phosphatase treatment of the extracts. In u.v. cross-linking analysis this factor migrated as a single polypeptide chain of about 67 kDa, and was named alpha 1-RBF67 (type I collagen alpha 1 chain RNA-binding factor). Dexamethasone treatment of fibroblasts, which is known to accelerate the turnover of COL1A1 mRNA, decreased the alpha 1-RBF67 activity markedly as evaluated by gel-retardation and u.v. cross-linking assays. Transient transfections with plasmids carrying the alpha 1(I) collagen promoter and 3'-UTR sequences demonstrated that the 3'-UTR participates in the response to dexamethasone. Thus the loss of alpha 1-RBF67 activity might be associated with decreased alpha 1(I) collagen mRNA levels after dexamethasone treatment.

Transforming growth factor beta and the cell surface in tumor progression

Type 1 transforming growth beta (TGF-beta 1) is a multifunctional regulator of cellular differentiation, motility and growth. It is capable of inhibiting or stimulating these processes depending on cell type, cell density, culture conditions and TGF-beta 1 concentration. TGF-beta 1 regulates growth, in part, by inducing the expression and secretion of various types of collagen, which participate in the control of cell adhesion and migration, as well as growth. TGF-beta 1 also regulates cell growth by controlling the response to epidermal growth factor (EGF) and other growth factors, in ways that can either decrease or increase their growth-promoting effects. Alterations in both negative and positive growth responses to TGF-beta 1 play important roles in tumor progression. Loss of sensitivity to growth inhibition by TGF-beta 1 can occur as a result of decreased expression of collagen. Acquisition of sensitivity to growth stimulation, and autocrine transformation by TGF-beta 1, are associated with aberrant EGF receptor regulation. Aberrant growth factor receptor regulation by TGF-beta 1 may be mediated by a protein kinase C (PKC)-dependent pathway which inhibits degradation of growth factor receptor/ligand complexes. The evidence reviewed is consistent with a minimal two-step mechanism for autocrine transformation, which involves production of growth factor and enhanced cellular response as a result of aberrant membrane traffic. Defects in membrane traffic regulation may provide an explanation for common alterations in tumor cell response to both multiple growth inhibitors and growth stimulators, and may also suggest novel approaches to cancer chemotherapy.

Nuclear factor binding to an AP-1 site is associated with the activation of pro-alpha 1(I)-collagen gene in dedifferentiating chondrocytes

Isolated chondrocytes grown on plastic gradually lose their differentiated phenotype upon subculturing. This dedifferentiation is manifested by an altered production of extracellular-matrix molecules (ECM): e.g., the cartilage specific type II collagen is replaced by types I and III. We have studied the regulation of ECM gene expression in dedifferentiating human and murine fetal chondrocytes. Nuclear extracts from dedifferentiated cells, human fetal fibroblasts and 3T3 cells contained a protein that bound in an electrophoretic mobility shift assay to an AP-1 site in the first intron of the human alpha 1(I) collagen gene. This binding activity was not present in freshly isolated human or murine chondrocytes, which produced type II, but not type I, collagen mRNA in culture. Thus the binding activity was induced simultaneously with alpha 1(I)-collagen-gene expression during dedifferentiation. The specific interaction was sensitive to dephosphorylation of the nuclear extract and to chemical modification of reduced cysteine residues. The AP-1 site we studied had previously been shown to be a positive transcriptional contributor in the first intron to the expression of the alpha 1(I) collagen gene. In transient transfections into dedifferentiating chondrocytes, an alpha 1(I) collagen expression plasmid carrying a mutated AP-1 site in the first intron resulted in three-times-lower reporter gene RNA levels than a plasmid carrying the respective functional AP-1 site. These data suggest that the AP-1 sequence and its respective trans-acting factors may play a role in the transcriptional regulation of the alpha 1(I) collagen gene during dedifferentiation of chondrocytes.

Characterization of an intronless collagen gene family in the marine sponge Microciona prolifera

Two independent clones from the genomic DNA of a marine sponge Microciona prolifera were isolated by hybridization to the Caenorhabditis elegans Col-1 gene and one clone was obtained from genomic DNA by PCR. They contain open reading frames (MpCol1, MpCol2, MpCol3, MpCol4) capable of coding for a family of collagens different from those previously found in sponges. Southern blotting of genomic DNA suggested the presence of several other homologous genes. cDNA clones covering most of the triple-helical coding domain and the 3' untranslated region of MpCol1 were isolated by specific primers and reverse PCR. Two cDNA clones end in the middle of an AATAAA sequence 170 bp downstream from the translation stop codon of MpCol1. The putative NH2-terminal noncollagenous peptide is composed of only seven amino acid residues. The 1074-bp triple-helical coding region is not interrupted by intervening sequences. It codes for a polypeptide of 120 Gly-Xaa-Yaa triplets with only one short interruption near the COOH terminus. A putative N-glycosylation sequence (Asn-Gly-Ser), three Arg-Gly-Asp triplets known as cell recognition peptides, frequent Lys residues in the Yaa position (which are templates for hydroxylation), several Lys-Gly-Asn/Xaa-Arg peptides known as the lysyl oxidase recognition site, and long stretches without imino acids could be found within the triple-helical domain. The short COOH-terminal noncollagenous domain closely resembles that of nematode cuticular collagens and vertebrate nonfibrillar collagens. Our results strongly support the idea that the diversity of collagen genes and gene families found in higher organisms already existed in sponge.

Increased expression of TGF-beta 1 mRNA in the obstructed kidney of rats with unilateral ureteral ligation

Renal interstitial fibrosis is a common consequence of chronic ureteral obstruction. While several cytokines may initiate fibrogenesis, TGF-beta is considered to be a major stimulating factor. It has been reported that TGF-beta 1 regulates extracellular matrix (ECM) synthesis, that thromboxane (Tx) stimulates ECM protein synthesis, and that angiotensin II (Ang II) increases expression of TGF-beta 1 mRNA in rat aortic smooth muscle cells. Therefore, we measured TGF-beta 1 mRNA expression by reverse transcription coupled with polymerase chain reaction in renal cortex of rats with unilateral ureteral obstruction (UUO) to determine whether Ang II and/or Tx stimulates increases in TGF-beta 1 mRNA. TGF-beta 1 mRNA levels in contralateral kidneys of rats with UUO did not change significantly during 14 days of obstruction, while in the obstructed kidney TGF-beta 1 mRNA levels were increased significantly after three days as compared to the control (unoperated rats) kidneys. The increase in TGF-beta 1 mRNA expression in the obstructed kidney cortex was found in tubular cells rather than glomeruli. OKY-046, an inhibitor of thromboxane synthase, did not affect the changes in TGF-beta 1 mRNA in the obstructed kidney. Enalapril, an angiotensin I converting enzyme inhibitor, significantly blunted but did not completely abrogate the increase in TGF-beta 1 mRNA. These data suggest that in obstruction TGF-beta 1 is increased at the transcriptional level and thus may play a role in initiating fibrogenesis in obstructive nephropathy. The effect of thromboxane on extracellular matrix synthesis does not appear to be mediated by TGF-beta 1.(ABSTRACT TRUNCATED AT 250 WORDS)

A new form of tumor and fetal collagen that binds laminin

Human breast and colon carcinoma tissues contain a form of collagen, not described before, composed of alpha 1 chains of similar size (approximately 100 kDa) but different charge. The three constitutive chains, separated by two-dimensional electrophoresis, are a unique acidic component, undetectable in other collagen types, with an apparent isoelectric point of 4-5, and two more basic components displaying the same electrophoretic behavior as alpha 1(III) and alpha 1(I), respectively. The acidic chain is structurally distinct from alpha 1(I) and displays a cyanogen bromide-derived fragment of similar size to CB5(III). This collagen in its native state is resistant to trypsin and metalloproteinase 3, while it is fully degraded by metalloproteinases 1 and 9. Moreover, this collagen appears able to bind to laminin, as tested by affinity chromatography. The biological significance of our data is related to the finding of this collagen form not only in the tumor tissue tested but also in embryonic-fetal tissues (bovine skin and intestine and human umbilical cord). For its peculiar laminin-binding ability and occurrence in tumoral and embryonic-fetal tissues, we propose to temporarily term this new collagen form OF/LB collagen (onco-fetal, laminin-binding collagen). The presence of OF/LB collagen during development and cancer, and its absence in normal adult tissues, make this protein a potential stromal marker of malignancy.

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.

Suppression of ras-mediated transformation. Differential expression of genes encoding extracellular matrix proteins in normal, transformed and revertant cells

A subtraction hybridization technique was used to identify genes specifically expressed in phenotypic revertants derived from cells transformed by the H-ras oncogene. The expression of genes coding for components of the extracellular matrix appears to be frequently down-regulated in transformed cells. Partial restoration is associated with the reexpression of the normal phenotype in revertants.

An AP-1-like motif in the first intron of human Pro alpha 1(I) collagen gene is a critical determinant of its transcriptional activity

The first intron of the human Pro alpha 1(I) collagen gene contains an orientation-dependent enhancer composed of both positive and negative cis-acting elements involved in the transcriptional regulation of this gene. Deletion of a 360 bp Sau 3A intronic fragment spanning nucleotide +494 to +854 (S360) resulted in dramatic down-regulation of pCOL-KT (Thompson et al., J Biol Chem 266: 2549-2556, 1991). Using a DNaseI protection assay, we demonstrate a single footprint located at +590 to +615 in the S360 fragment; nuclear extracts prepared from mesenchymal and nonmesenchymal cells exhibited similar binding characteristics. A double stranded oligonucleotide representing a consensus Ap-1 binding sequence competed with S360 for binding. In contrast to what occurred in response to S360 deletion which was always accompanied by reduced expression, the deletion of the Ap-1 binding site (+598 to +off) caused either increased or decreased expression of the reporter gene depending on the target cell. Site-directed mutations in the Ap-1-like cis-element of Pro alpha 1(I) were also tested in transient expression assays. Consistent with the paradoxical results of Ap-1 deletion, we observed that the functional consequences of mutations in the Ap-1 site also varied in different cells. In A204 cells, one point mutation, which resulted in the loss of protein binding to S360, led to increased CAT activity while another point mutant, which retained binding of the Ap-1 like trans-acting factor(s), showed decreased CAT expression. The effects of these two mutations in the HFL-1 cells were exactly opposite of what was seen for A204 cells. Based on these observations, we postulate that the Ap-1 site plays a critical role in the transcriptional activity of the human Pro alpha 1(I) gene. The implications of an apparently dual mode of regulation through a single cis-regulatory element are discussed.

The abolition of collagen gene expression in SV40-transformed fibroblasts is associated with trans-acting factor switching

The goal of this study was to determine whether alpha 2(1) procollagen gene expression is modulated by positive or negative trans-acting DNA-binding proteins. Previous studies have shown that a clone of SV40-transformed WI-38 fibroblasts (SVWI-38) does not produce any alpha 2(1) procollagen mRNA (Parker et al (1989), J. Biol Chem. 264, 7147-7152). In order to elucidate the mechanism(s) responsible for such inactivation, we have examined the activity of a transfected wild type COL1A2 promoter in SVWI-38 cells. A set of 5' promoter deletions was linked to the chloramphenicol acetyltransferase (CAT) gene and transfected into SVWI-38 and other cell lines expression type I collagen. The resulting CAT assays confirmed the importance of several upstream regions for promoter activity and documented the decreased transcriptional activity from an exogenous COL1A2 promoter in the SVWI-38 cell line. Competition experiments with an excess of COL1A2 promoter DNA fragment and a constant amount of COL1A2/CAT construct displayed a linear relationship between excess COL1A2 fragment and CAT activity in SVWI-38 cells, suggesting the involvement of a titratable negative effector. Electrophoretic mobility shift assays revealed the presence of a specific DNA-protein complex which was present in SVWI-38 cells and almost absent in control fibroblasts. Methylation interference analysis mapped the region of binding of this factor between nucleotides -80 and -72, relative to the transcription start site. Thus the data presented provide strong evidence for the existence of a negative trans-acting factor that may play a role in the repression of COL1A2 expression in SVWI-38 fibroblasts.

Expression of human collagen type IV genes is regulated by transcriptional and post-transcriptional mechanisms

The molecules of the basement membrane specific collagen type IV are heterotrimers consisting of two alpha 1(IV) and one alpha 2(IV) polypeptide chains. Comparison of the ratios of transcription by nuclear run-on analysis and mRNAs by RNAse protection assay indicates the involvement of transcriptional as well as post-transcriptional events in the control of overall collagen type IV expression. The relative ratios of transcription of the respective genes COL4A1 and COL4A2 remained near 2:1 in most cells, whereas the ratio of mRNA steady-state levels alpha 1(IV)/alpha 2(IV) varied from 0.3:1 to 1:1 and did not parallel the subunit structure of the protein. Nevertheless, secreted protein shows a 2:1 ratio of the subunit polypeptides. This indicates that post-translational processes during chain selection, aggregation and secretion finally determine the amount of secreted protein.

Post-transcriptional regulation of the pro alpha 1(I) collagen gene in pro alpha 1(I)-deficient, chemically transformed Syrian hamster embryo fibroblasts

4-Nitroquinoline-1-oxide-transformed Syrian hamster embryo fibroblasts (NQT-SHE) synthesize the pro alpha 2 chain but not the pro alpha 1 subunit of type I procollagen, and they contain little pro alpha 1(I)mRNA. This study shows that there was no accumulation of pro alpha 1(I) poly(A)+ mRNA in NQT-SHE fibroblasts. BHK cells, a normal established line of hamster fibroblasts that synthesized collagen at approximately the same rate as NQT-SHE fibroblasts, nevertheless produced both subunits of type I collagen and contained pro alpha 1(I)mRNA. Run-off transcription assays with isolated nuclei showed that both the pro alpha 1(I) and pro alpha 2(I) genes were transcribed at about the same rate in NQT-SHE cells as well as in the normal BHK cells. These results suggest that a post-transcriptional defect, probably resulting from transformation, prevents the accumulation of pro alpha 1(I)mRNA in NQT-SHE cells.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.