Tissue-specific expression of the mouse alpha 2(I) collagen promoter. Studies in transgenic mice and in tissue culture cells

Novel method to repair articular cartilage by direct reprograming of prechondrogenic mesenchymal stem cells

Age-related cartilage loss is worsened by the limited regenerative capacity of chondrocytes. The role of cell-based therapies using mesenchymal stem cells is gaining interest. Adipose tissue-derived mesenchymal stem cells (ADSCs) are an attractive source to generate the optimal number of chondrocytes required to repair a cartilage defect and regenerate hyaline articular cartilage. Here, we report an outstanding technique to prepare chondrocytes for cartilage repair using canine ADSCs. We hypothesized that external electrical fields promote prechondrogenic condensation without requiring genetic modifications or exogenous factors. We analyzed the effect of electrical stimulation (ES) on the differentiation of ADSC micromass into chondrocytes. Highly compact structures were formed within 3 days of ES of canine ADSC micromass. The expression of type I collagen gene was abolished in these cells compared with that in control micromass cultures and monolayer cultures. We further found that ES enhanced the production of proteoglycan, a highly produced extracellular matrix component in chondrocytes. Additionally, single-cell RNA sequencing analysis showed that canine ADSC micromass undergoing ES developed a prechondrogenic cell aggregation, suggesting their metabolic conversion, biogenesis, and calcium ion change. Collectively, our findings demonstrate the capacity of ES to drive the chondrogenesis of ADSCs in the absence of exogenous factors and confirm its commercial potential as a budget-friendly therapy for the repair of cartilage defects.

Proteome analysis of human adipocytes identifies depot-specific heterogeneity at metabolic control points

Adipose tissue is a primary regulator of energy balance and metabolism. The distribution of adipose tissue depots is of clinical interest because the accumulation of upper-body subcutaneous (ASAT) and visceral adipose tissue (VAT) is associated with cardiometabolic diseases, whereas lower-body glutealfemoral adipose tissue (GFAT) appears to be protective. There is heterogeneity in morphology and metabolism of adipocytes obtained from different regions of the body, but detailed knowledge of the constituent proteins in each depot is lacking. Here, we determined the human adipocyte proteome from ASAT, VAT, and GFAT using high-resolution Sequential Window Acquisition of all Theoretical (SWATH) mass spectrometry proteomics. We quantified 4,220 proteins in adipocytes, and 2,329 proteins were expressed in all three adipose depots. Comparative analysis revealed significant differences between adipocytes from different regions (6% and 8% when comparing VAT vs. ASAT and GFAT, 3% when comparing the subcutaneous adipose tissue depots, ASAT and GFAT), with marked differences in proteins that regulate metabolic functions. The VAT adipocyte proteome was overrepresented with proteins of glycolysis, lipogenesis, oxidative stress, and mitochondrial dysfunction. The GFAT adipocyte proteome predicted the activation of peroxisome proliferator-activated receptor α (PPARα), fatty acid, and branched-chain amino acid (BCAA) oxidation, enhanced tricarboxylic acid (TCA) cycle flux, and oxidative phosphorylation, which was supported by metabolomic data obtained from adipocytes. Together, this proteomic analysis provides an important resource and novel insights that enhance the understanding of metabolic heterogeneity in the regional adipocytes of humans.NEW & NOTEWORTHY Adipocyte metabolism varies depending on anatomical location and the adipocyte protein composition may orchestrate this heterogeneity. We used SWATH proteomics in patient-matched human upper- (visceral and subcutaneous) and lower-body (glutealfemoral) adipocytes and detected 4,220 proteins and distinguishable regional proteomes. Upper-body adipocyte proteins were associated with glycolysis, de novo lipogenesis, mitochondrial dysfunction, and oxidative stress, whereas lower-body adipocyte proteins were associated with enhanced PPARα activation, fatty acid, and BCAA oxidation, TCA cycle flux, and oxidative phosphorylation.

New transgenic NIS reporter rats for longitudinal tracking of fibrogenesis by high-resolution imaging

Fibrogenesis is the underlying mechanism of wound healing and repair. Animal models that enable longitudinal monitoring of fibrogenesis are needed to improve traditional tissue analysis post-mortem. Here, we generated transgenic reporter rats expressing the sodium iodide symporter (NIS) driven by the rat collagen type-1 alpha-1 (Col1α1) promoter and demonstrated that fibrogenesis can be visualized over time using SPECT or PET imaging following activation of NIS expression by rotator cuff (RC) injury. Radiotracer uptake was first detected in and around the injury site day 3 following surgery, increasing through day 7–14, and declining by day 21, revealing for the first time, the kinetics of Col1α1 promoter activity in situ. Differences in the intensity and duration of NIS expression/collagen promoter activation between individual RC injured Col1α1-hNIS rats were evident. Dexamethasone treatment delayed time to peak NIS signals, showing that modulation of fibrogenesis by a steroid can be imaged with exquisite sensitivity and resolution in living animals. NIS reporter rats would facilitate studies in physiological wound repair and pathological processes such as fibrosis and the development of anti-fibrotic drugs.

Fra-2/AP-1 controls bone formation by regulating osteoblast differentiation and collagen production

The activator protein-1 (AP-1) transcription factor complex, in particular the Fos proteins, is an important regulator of bone homeostasis. Fra-2 (Fosl2), a Fos-related protein of the AP-1 family, is expressed in bone cells, and newborn mice lacking Fra-2 exhibit defects in chondrocytes and osteoclasts. Here we show that Fra-2-deficient osteoblasts display a differentiation defect both in vivo and in vitro. Moreover, Fra-2-overexpressing mice are osteosclerotic because of increased differentiation of osteoblasts, which appears to be cell autonomous. Importantly, the osteoblast-specific osteocalcin (Oc) gene and collagen1α2 (col1α2) are transcriptional targets of Fra-2 in both murine and human bone cells. In addition, Fra-2, Oc, and col1 are expressed in stromal cells of human chondroblastic and osteoblastic osteosarcomas (Os's) as well as during osteoblast differentiation of human Os cell lines. These findings reveal a novel function of Fra-2/AP-1 as a positive regulator of bone and matrix formation in mice and humans.

The effect of class II transactivator mutations on bleomycin-induced lung inflammation and fibrosis

IFN-γ expression increases during the inflammatory response after bleomycin injury in mice. IFN-γ deficiency attenuates lung inflammation and fibrosis. Because IFN-γ stimulates class II transactivator (CIITA) expression, which activates major histocompatibility class (MHC) II and represses collagen expression, it was hypothesized that CIITA mediates IFN-γ action after bleomycin injury. To test this hypothesis, two CIITA mouse lines, one carrying a mutation of the leucine-rich region of CIITA (CIITA C-/-) and one with a deletion extending into the GTP-binding domain (CIITA G-/-), were used. IFN-γ treatment of lung cells isolated from both strains of mice induced mutant CIITA expression, which did not activate MHC II transcription. Collagen expression was similar in both mutant mouse strains and comparable to C57BL/6 (wild-type) mice. When mice were exposed to intratracheal bleomycin, both strains of CIITA mutant mice retained body weight and altered inflammation at 14 days after bleomycin injury compared with bleomycin-treated wild-type mice. However, there was no difference in fibrosis as judged by histology, mRNA, and protein expression of lungs. Bronchoalveolar lavage cells from CIITA C-/- and C57BL/6 lungs were examined at 3, 7, and 14 days after bleomycin injury. CD4 mRNA expression in bronchoalveolar lavage cells was down-regulated, whereas IL-4 and IL-10 expression was up-regulated, in CIITA C-/- mice, indicating a diminished, skewed Th2 response. The expression of IFN-γ was the same in all mice tested. Combined, our data suggest that CIITA mutations altered the immune response without affecting fibrosis.

RFXB and its splice variant RFXBSV mediate the antagonism between IFNgamma and TGFbeta on COL1A2 transcription in vascular smooth muscle cells

Cytokines secreted by infiltrating immune cells during atherogenesis modulate vascular remodeling. One exemplary event is the antagonism between transformed growth factor (TGF-β) and interferon gamma (IFN-γ) on the transcriptional control of type I collagen gene (COL1A2). Previously we have reported that IFN-γ up-regulates regulatory factor for X-box B (RFXB) to repress collagen transcription while down-regulates the expression of RFXBSV, a splice variant of RFXB that blocks collagen repression in fibroblasts. Here we demonstrate that TGF-β abrogated COL1A2 repression by IFN-γ through altering the relative expression of RFXB and RFXBSV. Unlike RFXB, RFXBSV did not bind to the collagen promoter and competed with RFXB for the co-repressor histone deacetylase 2 (HDAC2), limiting HDAC2 recruitment to the collagen transcription start site as evidenced by chromatin immunoprecipitation assays. Over-expression of RFXB by lentiviral infection in HASMCs enhanced HDAC2 enlistment, promoted histone deacetylation surrounding the collagen site by IFN-γ, and blocked the TGF-β antagonism, a pattern reversed by RFXBSV infection. On the contrary, silencing of RFXB, but not both RFXB and RFXBSV, expression promoted the TGF-β antagonism. Thus, we have identified a novel mechanism whereby TGF-β antagonizes the IFN-γ repression of collagen transcription in HASMCs and as such provided new insights into antiatherogenic strategies.

Regulation of collagen gene expression in the Tsk2 mouse

The tight skin 2 (Tsk2) mutation is an ENU induced dominant mutation localized on mouse chromosome 1. While the molecular defect is unknown, Tsk2/+ mice display cutaneous thickening associated with excessive matrix production and are used as a model of scleroderma. The purpose of this study was to examine the cellular mechanisms associated with the excessive synthesis of matrix macromolecules using a collagen promoter GFP reporter transgene (pOBCol3.6GFP) as a marker of Col1a1 expression. This analysis of pOBCol3.6GFP expression in Tsk2/+ skin showed an increase in transgene activity compared to wild-type (+/+) samples. In addition, an increased area of "high" GFP fluorescence in Tsk2/+ dermis in both 1- and 4-month-old mice was observed that was also associated with an increased number of dermal fibroblasts per unit area of dermis. These data collectively suggest an important mechanism of Tsk2/+ skin fibrosis; an increased number of collagen expressing cells as well as elevated collagen expression on a per cell basis. During this study it was noted that Tsk2/+ mice appeared consistently smaller than wild-type (+/+) siblings and measurements of body length revealed a decrease (5-10%) in 1- and 2-month-old Tsk2/+ mice as well as a decrease in body weight in both age groups as compared to wild-type (+/+) control mice. Femur length was also decreased (2-9%) in Tsk2/+ mice. Finally, in contrast to Tsk/+ mice that display an emphysema-like lung pathology, histological sections of lungs from Tsk2/+ mice were normal and indistinguishable from wild-type (+/+) controls.

CIITA mediates interferon-gamma repression of collagen transcription through phosphorylation-dependent interactions with co-repressor molecules

Previously, we have demonstrated that major histocompatibility class II trans-activator (CIITA) is crucial in mediating interferon-gamma (IFN-gamma)-induced repression of collagen type I gene transcription. Here we report that CIITA represses collagen transcription through a phosphorylation-dependent interaction between its proline/serine/threonine domain and co-repressor molecules such as histone deacetylase (HDAC2) and Sin3B. Mutation of a serine (S373A) in CIITA, within a glycogen synthase kinase 3 (GSK3) consensus site, decreases repression of collagen transcription by blocking interaction with Sin3B. In vitro phosphorylation of CIITA by GSK3 relies on a casein kinase I site three amino acids C-terminal to the GSK3 site in CIITA. Both GSK3 and casein kinase I inhibitors alleviate collagen repression and disrupt IFN-gamma-mediated recruitment of Sin3B and HDAC2 to the collagen start site. Therefore, we have identified the region within CIITA responsible for mediating IFN-gamma-induced inhibition of collagen synthesis.

Peroxisome proliferator-activated receptor gamma interacts with CIITA x RFX5 complex to repress type I collagen gene expression

Recent reports demonstrate that peroxisome proliferator-activated receptor gamma (PPARgamma), a member of the nuclear receptor superfamily, acts as a repressor of type I collagen synthesis. Our data demonstrate that exogenously expressed PPARgamma down-regulates collagen expression in a dose-responsive manner in human lung fibroblast cells. Silencing PPARgamma using lentiviruses expressing short hairpin RNAs partially reverses interferon-gamma (IFN-gamma)-induced repression and activates collagen mRNA levels. Previous studies indicate that IFN-gamma represses collagen gene expression and induces major histocompatibility complex II (MHC II) expression by activating the formation of a regulatory factor for X-box 5 (RFX5) complex with class II transactivator (CIITA). This report demonstrates that PPARgamma is within the RFX5.CIITA complex as judged by co-immunoprecipitation and DNA affinity precipitation studies. Most importantly, occupancy of PPARgamma on the collagen transcription start site and MHC II promoter increases with IFN-gamma treatment. The PPARgamma agonist, troglitazone, sensitizes the cells to IFN-gamma treatment by increasing recruitment of PPARgamma to collagen gene while repressing collagen expression, and these effects are blocked by the PPARgamma antagonist T0070907. PPARgamma may mediate IFN-gamma-stimulated collagen transcription down-regulation and MHC II up-regulation by interacting with CIITA as well as regulating CIITA expression. Therefore, PPARgamma is a critical target for investigations into therapeutics of diseases involving extracellular matrix remodeling and the immune response.

Collagen and major histocompatibility class II expression in mesenchymal cells from CIITA hypomorphic mice

Major histocompatibility class II (MHC II) transactivator (CIITA) is critical for interferon-gamma (IFN-gamma)-induced repression of collagen [Xu, Y., Wang, L., Buttice, G., Sengupta, P.K., Smith, B.D., 2004. Major histocompatibility class II transactivator (CIITA) mediates repression of collagen (COL1A2) transcription by interferon gamma (IFN-gamma). J. Biol. Chem. 279, 41319-41332] and activation of MHC II transcription. To better understand the role of CIITA and IFN-gamma induced repression of collagen, mesenchymal cells (lung fibroblasts, adventitial fibroblasts, and smooth muscle cells) were isolated from a CIITA deficient mouse (C2ta(tm1Ccum)). IFN-gamma induced MHC II expression and repressed collagen type I expression in all three cell types isolated from the wild type background. As expected, IFN-gamma treatment of cells isolated from CIITA deficient mice did not induce MHC II production or activate the MHC II promoter. Interestingly, collagen gene expression and promoter activity was similar to that of wild type. Moreover, IFN-gamma induced CIITA mRNA and a truncated form of CIITA protein in all cells isolated from CIITA deficient mice. Most importantly, truncated CIITA occupied the collagen alpha 2(I) gene (col1a2) transcription start site during IFN-gamma treatment, but it did not occupy the MHC II promoter as judged by chromatin immunoprecipitation assays. Exogenous expression of a similar truncated form of CIITA maintained its ability to repress col1a2 transcription, but lost its ability to activate MHC II gene transcription suggesting a role for the CIITA C-terminal domain in activation, but not repression. IFN-gamma induced primarily types I and IV CIITA isoforms in the mouse cells. All three isoforms of CIITA were capable of repressing col1a2 and activating MHC II gene transcription. These data suggest that the previously described CIITA knockout mouse carries a hypomorphic mutation, rather than a null mutation. The removal of the leucine rich region in CIITA blocks activation of MHC II without altering repression of collagen transcription.

The phylogenetically conserved molluscan chitinase-like protein 1 (Cg-Clp1), homologue of human HC-gp39, stimulates proliferation and regulates synthesis of extracellular matrix components of mammalian chondrocytes

Members of chitinase-like proteins (CLPs) have attracted much attention because of their ability to promote cell proliferation in insects (imaginal disc growth factors) and mammals (YKL-40). To gain insights into the molecular processes underlying the physiological control of growth and development in Lophotrochozoa, we report here the cloning and biochemical characterization of the first Lophotrochozoan CLP from the oyster Crassostrea gigas (Cg-Clp1). Gene expression profiles monitored by real time quantitative reverse transcription-PCR in different adult tissues and during development support the involvement of this protein in the control of growth and development in C. gigas. Recombinant Cg-Clp1 demonstrates a strong affinity for chitin but no chitinolytic activity, as was described for the HC-gp39 mammalian homolog. Furthermore, transient expression of Cg-Clp1 in primary cultures of rabbit articular chondrocytes as well as the use of both purified recombinant protein and conditioned medium from Cg-Clp1-expressing rabbit articular chondrocytes established that Cg-Clp1 stimulates cell proliferation and regulates extracellular matrix component synthesis, showing for the first time a possible involvement of a CLP on type II collagen synthesis regulation. These observations together with the fact that Cg-Clp1 gene organization strongly resembles that of its mammalian homologues argue for an early evolutionary origin and a high conservation of this class of proteins at both the structural and functional levels.

TEF-1 and C/EBPbeta are major p38alpha MAPK-regulated transcription factors in proliferating cardiomyocytes

p38 MAPKs (mitogen-activated protein kinases) play important roles in the regulation of cellular responses to environmental stress. Recently, this signalling pathway has also been implicated in the regulation of processes unrelated to stress, for example, in T lymphocytes and cardiomyocytes. In order to identify molecular targets responsible for the housekeeping functions of p38 MAPKs, we have analysed the differences in the transcriptomes of normally proliferating wild-type and p38alpha knockout immortalized embryonic cardiomyocytes. Interestingly, many potential components of the myocardium extracellular matrix were found to be upregulated in the absence of p38alpha. Further analysis of the microarray data identified TEF-1 (transcriptional enhancer factor-1), a known regulator of heart-specific gene expression, and C/EBPbeta (CCAAT/enhancer-binding protein beta), as the two transcription factors the binding sites of which were most enriched in the promoters of p38alpha-regulated genes. We have focused on the study of the extracellular matrix component COL1A1 (alpha1 chain of type I collagen) and found evidence for the involvement of both TEF-1 and C/EBPbeta in the p38alpha-dependent inhibition of COL1A1 transcription. Our data therefore show that p38 MAPKs regulate TEF-1 and C/EBPbeta transcriptional activity in the absence of environmental stress and suggests a role for p38alpha in the expression of extracellular matrix components that maintain organ architecture.

Regulatory factor for X-box family proteins differentially interact with histone deacetylases to repress collagen alpha2(I) gene (COL1A2) expression

Our studies indicate that the regulatory factor for X-box (RFX) family proteins repress collagen alpha2(I) gene (COL1A2) expression (Xu, Y., Wang, L., Buttice, G., Sengupta, P. K., and Smith, B. D. (2003) J. Biol. Chem. 278, 49134-49144; Xu, Y., Wang, L., Buttice, G., Sengupta, P. K., and Smith, B. D. (2004) J. Biol. Chem. 279, 41319-41332). In this study, we examined the mechanism(s) underlying the repression of collagen gene by RFX proteins. Two members of the RFX family, RFX1 and RFX5, associate with distinct sets of co-repressors on the collagen transcription start site in vitro. RFX5 specifically interacts with histone deacetylase 2 (HDAC2) and the mammalian transcriptional repressor (mSin3B), whereas RFX1 preferably interacts with HDAC1 and mSin3A. HDAC2 cooperates with RFX5 to down-regulate collagen promoter activity, whereas HDAC1 enhances inhibition of collagen promoter activity by RFX1. Interferon-gamma promotes the recruitment of RFX5/HDAC2/mSin3B to the collagen transcription start site but decreases the occupancy by RFX1/mSin3A as manifested by chromatin immunoprecipitation assay. RFX1 binds to the methylated collagen sequence with much higher affinity than unmethylated sequence, recruiting more HDAC1 and mSin3A. The DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine, which inhibits DNA methylation, reduces RFX1/HDAC1 binding to the collagen transcription start site in chromatin immunoprecipitation assays. Finally, both RFX1 and RFX5 are acetylated in vivo. Trichostatin A stimulates the acetylation of RFX proteins and activates the collagen promoter activity. Collectively, our data strongly indicate two separate pathways for RFX proteins to repress collagen gene expression as follows: one for RFX5/HDAC2 in interferon-gamma-mediated repression, and the other for RFX1/HDAC1 in methylation-mediated collagen silencing.

c-Krox down-regulates the expression of UDP–glucose dehydrogenase in chondrocytes

Chondrocyte glycosaminoglycan (GAG) synthesis is regulated by the availability of UDP-glucuronate, the substrate of glucuronosyl transferases which form the GAG chains in proteoglycans and hyaluronan. UDP-glucose dehydrogenase (UDPGD) is therefore a key enzyme in the synthesis of UDP-glucuronate from glucose. However, the mechanisms regulating its expression in chondrocytes are not fully understood. We investigated the effect of c-Krox, a zinc-finger transcription factor previously shown to modulate several matrix genes, on the synthesis of GAG and transcriptional activity of several UDPGD gene promoter constructs, using transient transfection and decoy experiments in rabbit articular chondrocytes (RACs). We show that overexpression of c-Krox inhibits radiosulfate incorporation into neosynthesized GAG and that the effect was mediated by a cis-sequence located between +18 and +39bp of the UDPGD gene. Since that sequence can also bind Sp1/Sp3 factors, it is likely that c-Krox acts in concert with these proteins to modulate the UDPGD gene expression in articular chondrocytes.

Species-specific regulation of the alpha-2(I) procollagen gene by proximal promoter elements

Transcriptional regulation of the human alpha 2(I) procollagen proximal promoter involves the interaction of trans-acting factors at the inverted CCAAT box (G/CBE) located at position -80 and an adjacent GGAGGCCC-box at -70. Both these elements have previously been shown to be essential for activity of the human promoter. This study investigated nucleotide differences at three sites (-74, -72 and -71) between the human and mouse promoters that were sufficient to abolish trans-acting factor binding with the mouse sequence (GGAGACGT). Two distinct DNA-protein interactions were detected on the human -107/+54 promoter fragment while a single interaction was observed at the equivalent mouse promoter. One of these factors is the CCAAT-binding factor (CBF) and it's binding was observed on both the human and mouse promoters. Although the GGAGGCCC DNA-binding element was not detected on the mouse promoter, GGAGGCC-binding proteins were present in mouse nuclear extracts as observed by their interaction with the human promoter. Functional analysis of the human and mouse -343/+54 and -107/+54 promoter regions revealed significant differences between species; the human constructs having higher activity than the mouse. The differences in promoter activity between species may in part be a result of the nucleotide differences in the GGAGGCCC-box. Mutations in this region of the human -107/+54 promoter prevented DNA-protein interaction and lowered promoter activity. These results support the hypothesis that the GGAGGCCC-box in the human alpha 2(1) procollagen promoter has a regulatory function and that there exists a species-specific difference in transcription factor binding and regulation of the gene.

Major histocompatibility class II transactivator (CIITA) mediates repression of collagen (COL1A2) transcription by interferon gamma (IFN-gamma)

Interferon gamma (IFN-gamma) plays an important role during inflammation by repressing collagen and activating major histocompatibility class II (MHC-II) expression. Activation of MHC-II by IFN-gamma requires regulatory factor for X-box 5 (RFX5) complex as well as class II transactivator (CIITA). We have shown that the RFX family binds to the COL1A2 transcription start site, and the RFX5 complex represses COL1A2 gene expression during IFN-gamma response. In this report, we demonstrate that CIITA is a key mediator of COL1A2 repression by IFN-gamma. IFN-gamma up-regulates the expression of CIITA in a time-dependent manner in lung fibroblasts and promotes CIITA protein occupancy on COL1A2 transcription start site in vivo as judged by chromatin immunoprecipitation (ChIP) assays. There are coordinate decreases in the occupancy of RNA polymerase II on the collagen transcription start site with increasing CIITA occupancy during IFN-gamma treatment. In addition, we are able to specifically knockdown the IFN-gamma-stimulated expression of CIITA utilizing short hairpin interference RNA (shRNA) against CIITA. This leads to the alleviation of COL1A2 repression and MHC-II activation by IFN-gamma. RFX5 recruits CIITA to the collagen site as evidenced by DNA affinity chromatography. The presence of RFX5 complex proteins enhances the collagen repression by CIITA reaching levels occurring during IFN-gamma treatment. Co-expression of CIITA with deletion mutations and collagen promoter constructs demonstrates that CIITA represses collagen promoter mainly through its N-terminal region including the acidic domain and the proline/serine/threonine domain. Our data suggest that CIITA is a crucial member of a repressor complex responsible for mediating COL1A2 transcription repression by IFN-gamma.

Col1a2 enhancer regulates collagen activity during development and in adult tissue repair

An enhancer region in the type I collagen alpha 2 chain (pro-Col1a2) promoter has been previously identified approximately -17 kb away from the transcription start site. This upstream region termed the far-upstream-enhancer contains three DNAse I hypersensitive sites and has been shown to be conserved between mouse and human genes. In this study, we used transgenic mice harbouring the complete promotor sequence of the pro-Col1a2 gene up to -17 kb to examine the role of this enhancer in the expression and regulation of the collagen gene during development and in adult tissues pre and post injury. By careful histological mapping of the collagen type I endogenous gene distribution with that of the transgene driven by the mouse far upstream enhancer, we are able to show that in early days of collagen expression, E8.5-9.5, the endogenous gene preceded transgene expression. However, by E11.5 the overall pattern becomes synchronous with a few exceptions. In adult tissue, both endogenous and transgene expression are attenuated and both are reactivated in parallel in various organs by physical injury or fibrogenic cytokine injection. These findings suggest that the enhancer is central to the activation of the collagen type I and that mice harbouring this enhancer/reporter provide a useful model to follow collagen gene transcription activity and for investigating cellular activity in tissue fibrosis.

Interferon gamma repression of collagen (COL1A2) transcription is mediated by the RFX5 complex

Interferon gamma (IFN-gamma) plays an important physiological role during inflammation by down-regulating collagen gene expression and activating major histocompatibility II (MHC-II) complex. The activation of MHC-II by IFN-gamma requires activation of a trimeric DNA binding transcriptional complex, RFX5 complex, containing RFXB (also called RFXANK or Tvl-1), RFXAP, as well as RFX5 protein. Previously, we demonstrated that RFX5 binds to the collagen transcription start site and represses collagen gene expression (Sengupta, P. K., Fargo, J., Smith, B. D. (2002) J. Biol. Chem. 277, 24926-24937). In this report, we have examined the role of RFXB and RFXAP proteins within the RFX5 complex to regulate collagen gene expression. The data show that all three RFX5 complex proteins are required for maximum repression. Expression of proteins with mutations known to be important for RFX5 complex formation does not repress collagen promoter activity. Two mutated forms of RFX5 act as dominant negative proteins activating collagen expression and reversing IFN-gamma down-regulation of collagen expression in human lung fibroblasts. IFN-gamma increases expression and nuclear translocation of RFX5. RFXB has a naturally occurring splice variant isoform (RFX SV). Interferon increases expression of the long form of RFXB and decreases expression of RFX SV with the same kinetics as collagen gene expression. Overexpression of the splice variant form reverses the IFN-gamma induced collagen repression in human lung fibroblasts. Finally, all three RFX5 complex proteins increase at the collagen transcription start site with IFN-gamma treatment using chromatin immunoprecipitation analysis. Thus, these studies suggest an important role for RFX5 complex in collagen repression.

Down-regulation of human type II collagen gene expression by transforming growth factor-beta 1 (TGF-beta 1) in articular chondrocytes involves SP3/SP1 ratio

Although transforming growth factor beta1 (TGF-beta1) is generally considered as a stimulator of type I collagen production in smooth organs, we found that it can inhibit type II collagen biosynthesis in primary rabbit articular chondrocytes (RAC) at transcriptional levels. Constructs of promoter and first intron sequences associated with the luciferase reporter gene were used to delineate the gene sequences involved in TGF-beta1 control of human COL2A1 gene transcription. Cotransfection of these DNA fragments with a TbetaRII/I cDNA hybrid receptor, capable of inducing a TGF-beta1 dominant negative effect, showed that TGF-beta1 inhibits specifically COL2A1 gene transcription in RAC by a 63-bp proximal promoter. Footprint and gel retardation analyses revealed that the TGF-beta1-induced inhibition effect exerted through the 63-bp promoter sequence implies a multimeric complex that binds to the -41/-33 sequence and involves Sp1 and Sp3 transcription factors. Transfection of decoy Sp-binding oligonucleotides corroborated the implication of the proximal promoter in the TGF-beta1-induced inhibition of COL2A1 gene transcription. In addition, TGF-beta1 was found to increase the expression of Sp3 without significant changes to its binding level, but repressed both the biosynthesis and binding activity of Sp1. In functional assays, Sp3 inhibited the 63-bp promoter activity and prevented Sp1 induction of transcription. These findings suggest that TGF-beta1 inhibition of COL2A1 gene transcription in RAC is mediated by an increase of the Sp3/Sp1 ratio and by the repression of Sp1 transactivating effects on that gene.

The RFX family interacts at the collagen (COL1A2) start site and represses transcription

The transcription start site of the collagen alpha2(1) gene (COL1A2) has a sequence-specific binding site for a DNA methylation-responsive binding protein called regulatory factor for X-box 1 (RFX1) (Sengupta, P. K., Erhlich, M., and Smith, B. D. (1999) J. Biol. Chem. 274, 36649-36655). In this report, we demonstrate that RFX1 forms homodimers as well as heterodimers with RFX2 spanning the collagen transcription start site. Methylation at +7 on the coding strand increases RFX1 complex formation in gel shift assays. Methylation on the template strand, however, does not increase RFX1 complex formation. DNA from human fibroblasts contains minimal methylation on the coding strand (<4%) with variable methylation on the template strand. RFX1 acts as a repressor of collagen transcription as judged by in vitro transcription and co-transfection assays with an unmethylated collagen promoter-reporter construct. In addition, an RFX5 complex present in human fibroblasts interacts with the collagen RFX site, which is not sensitive to methylation. This is the first demonstration of RFX5 complex formation on a gene other than major histocompatibility complex (MHC) promoters. Also, RFX5 represses transcription of a collagen promoter-reporter construct in rat fibroblasts that have no detectable RFX5 complex formation or protein. RFX5 complex activates MHC II transcription by interacting with an interferon-gamma (IFN-gamma)-inducible protein, major histocompatibility class II trans-activator (CIITA). Collagen transcription is repressed by IFN-gamma in a dose-dependent manner in human but not in rat fibroblasts. IFN-gamma enhances RFX5 binding activity, and CIITA is present in the RFX5 complex of IFN-gamma-treated human fibroblasts. CIITA repressed collagen gene transcription more effectively in human fibroblasts than in rat fibroblasts, suggesting that the RFX5 complex may, in part, recruit CIITA protein to the collagen transcription start site. Thus the RFX family may be important repressors of collagen gene transcription through a RFX binding site spanning the transcription start site.

Factors involved in the regulation of type I collagen gene expression: implication in fibrosis

Type I collagen, the major component of extracellular matrix in skin and other tissues, is a heterotrimer of two alpha1 and one alpha2 collagen polypeptides. The synthesis of both chains is highly regulated by different cytokines at the transcriptional level. Excessive synthesis and deposition of collagen in the dermal region causes thick and hard skin, a clinical manifestation of scleroderma. To better understand the causes of scleroderma or other tissue fibrosis, it is very important to investigate the molecular mechanisms that cause upregulation of the Type I collagen synthesis in these tissues. Several cis-acting regulatory elements and trans-acting protein factors, which are involved in basal as well as cytokine-modulated Type I collagen gene expression, have been identified and characterized. Hypertranscription of Type I collagen in scleroderma skin fibroblasts may be due to abnormal activities of different positive or negative transcription factors in response to different abnormally induced signaling pathways. In this review, I discuss the present day understanding about the involvement of different factors in the regulation of basal as well as cytokine-modulated Type I collagen gene expression and its implication in scleroderma research.

Functional analysis of a minimal mouse serum amyloid A3 promoter in transgenic mice

We report the generation of transgenic mice harboring the SAA3/LacZ transgene and analysis of its expression patterns in vivo following LPS-induced inflammation. Our results show that a 210-bp fragment of the mouse SAA3 promoter when placed in front of the LacZ gene was sufficient to confer basal and inflammation-induced reporter gene expression. Consistent with endogenous SAA3 expression, the basal level of LacZ expression was high in the lung and liver of newborn and 1-week-old transgenic mice. Its expression however decreased with increasing age and at 3-weeks ofage, LacZ expression was very low in the lung and was essentially undetectable in the liver. When SAA3/LacZ transgenic mice were injected with lipopolisaccharide to induce inflammation, beta-gal activities were increased approximately 6- and 16-fold in the lung and liver, respectively. Histological examination of lung and liver tissues stained with X-gal revealed that the LacZ transgene was expressed primarily in the macrophages. Thus, this minimal SAA3 promoter fragment contains the necessary regulatory sequences for its expression and cytokine responsiveness in macrophages albeit is insufficient to confer expression in hepatocytes.

CBF/NF-Y functions both in nucleosomal disruption and transcription activation of the chromatin-assembled topoisomerase IIalpha promoter. Transcription activation by CBF/NF-Y in chromatin is dependent on the promoter structure

To understand the role of CCAAT-binding factor (CBF) in transcription in the context of chromatin-assembled DNA, we used regularly spaced nucleosomal DNA using topoisomerase IIalpha (topo IIalpha) and alpha2(1) collagen promoter templates, which were subsequently reconstituted in an in vitro transcription reaction. Binding of CBF to the nucleosomal wild-type topo IIalpha promoter containing four CBF-binding sites disrupted the regular nucleosomal structure not only in the promoter region containing the CBF-binding sites but also in the downstream region over the transcription start site. In contrast, no nucleosome disruption was observed in a mutant topo IIalpha promoter containing mutations in all CBF-binding sites. Interestingly, CBF also activated transcription from nucleosomal wild-type topo IIalpha promoter. In this experiment, a promoter containing one wild-type CBF-binding site was activated very weakly, whereas the promoter containing mutations in all sites was not activated by CBF. A truncated CBF that lacked the glutamine-rich domains did not activate transcription from nucleosomal wild-type topo IIalpha promoter but disrupted the nucleosomal structure about as much as did the binding of full-length CBF. Two nucleosomal mouse alpha2(1) collagen promoter DNAs, one containing a single and the other containing four CBF- binding sites, were also reconstituted in an in vitro transcription reaction. None of the nucleosomal collagen promoters was activated by CBF. However, both of these collagen promoters were activated by CBF when the transcription reaction was performed using naked DNA templates. Binding of CBF to the nucleosomal collagen promoter containing four binding sites disrupted the nucleosomal structure, similarly as observed in the topo IIalpha promoter. Altogether this study indicates that CBF-mediated nucleosomal disruption occurred independently of transcription activation. It also suggests that specific promoter structure may play a role in the CBF-mediated transcription activation of nucleosomal topo IIalpha promoter template.

Haploinsufficiency of Sox9 results in defective cartilage primordia and premature skeletal mineralization

In humans, SOX9 heterozygous mutations cause the severe skeletal dysmorphology syndrome campomelic dysplasia. Except for clinical descriptions, little is known about the pathogenesis of this disease. We have generated heterozygous Sox9 mutant mice that phenocopy most of the skeletal abnormalities of this syndrome. The Sox9(+/-) mice died perinatally with cleft palate, as well as hypoplasia and bending of many skeletal structures derived from cartilage precursors. In embryonic day (E)14.5 heterozygous embryos, bending of radius, ulna, and tibia cartilages was already prominent. In E12.5 heterozygotes, all skeletal elements visualized by using Alcian blue were smaller. In addition, the overall levels of Col2a1 RNA at E10.5 and E12.5 were lower than in wild-type embryos. We propose that the skeletal abnormalities observed at later embryonic stages were caused by delayed or defective precartilaginous condensations. Furthermore, in E18.5 embryos and in newborn heterozygotes, premature mineralization occurred in many bones, including vertebrae and some craniofacial bones. Because Sox9 is not expressed in the mineralized portion of the growth plate, this premature mineralization is very likely the consequence of allele insufficiency existing in cells of the growth plate that express Sox9. Because the hypertrophic zone of the heterozygous Sox9 mutants was larger than that of wild-type mice, we propose that Sox9 also has a role in regulating the transition to hypertrophic chondrocytes in the growth plate. Despite the severe hypoplasia of cartilages, the overall organization and cellular composition of the growth plate were otherwise normal. Our results suggest the hypothesis that two critical steps of the chondrocyte differentiation pathway are sensitive to Sox9 dosage. First, an early step presumably at the stage of mesenchymal condensation of cartilage primordia, and second, a later step preceding the transition of chondrocytes into hypertrophic chondrocytes.

Activation of a fibroblast-specific enhancer of the proalpha2(I) collagen gene in tight-skin mice

OBJECTIVE

Reporter transgenes were introduced into the type 1 tight-skin (Tsk1/+) mouse model of scleroderma to test the hypothesis that fibroblast-specific genetic programs are activated in fibrosis.

METHODS

Transgenes harboring upstream fragments of the 5' flanking region of the mouse proalpha2(I) collagen gene (Col1a2), linked to a 400-bp minimal Col1a2 promoter driving an Escherichia coli beta-galactosidase (LacZ) reporter gene, were introduced into Tsk1/+ mice by breeding. Expression of these transgenes, which function as lineage-specific markers of fibroblast differentiation, was compared between the Tsk-LacZ mice and non-Tsk littermates. Responsiveness of these constructs to the profibrotic cytokine, transforming growth factor beta1 (TGFbeta1), was investigated by transient transfection of reporter constructs in tissue-culture cells.

RESULTS

There was significant activation of reporter genes harboring the upstream enhancer in Tsk1/+ mice starting from 1 week of age. This was maximal at 6 weeks old (mean +/- SD 237 +/- 24% of non-Tsk controls; P= 0.001). Recombinant TGFbeta1 significantly activated reporter genes regulated by the upstream enhancer in transient transfection, and Tsk-LacZ fibroblasts showed elevated LacZ expression in tissue culture.

CONCLUSION

These data suggest that activating signals in Tsk1/+ mice may act via fibroblast-specific regulatory elements within the murine Col1a2 gene. Although TGFbeta has been implicated in the pathogenesis of fibrosis, and reporter genes regulated by the upstream enhancer appear to be TGFbeta responsive in vitro, our results suggest that fibroblast-specific pathways may also be involved.

Collagen XII mutation disrupts matrix structure of periodontal ligament and skin

Collagen XII has been postulated to organize the extracellular matrix (ECM) architecture of dense connective tissues such as the periodontal ligament (PDL) and skin. The objective of this study was to test this hypothesis in transgenic mice carrying a dominant interference mutation of collagen XII. The truncated alpha1(XII) collagen minigene construct MXIINC3(-), driven by the mouse alpha2(I) collagen promoter, was prepared and used to generate transgenic mouse lines. The PDL matrix fibers of molar teeth lost the ordered architecture characteristic of ligament tissue without noticeable inflammation. Cellular cement appeared to be disrupted at the PDL insertion. By confocal laser scanning microscopy, the PDL of transgenic mice demonstrated swollen and irregularly arranged collagen fibers associated with internal porosity. The skin of transgenic mice revealed the lack of matrix fiber structure in the papillary dermis. These results indicated that the dominant interference mutation of collagen XII disorganized the ECM architecture of PDL and skin.

Identification of a short cis-acting element in the human vasopressin type 2 receptor gene which confers high-level expression of a reporter gene specifically in collecting duct cells

In the kidney, water reabsorption is mainly regulated by the binding of arginine vasopressin to vasopressin type 2 (V2) receptors. These receptors are expressed selectively in principal cells of the collecting ducts. To identify molecular mechanisms responsible for the cell-specific expression of the V2 receptor, we have analyzed the proximal promoter of the corresponding gene. We report the identification of a 33-bp enhancer [collecting duct tissue-specific element 1 (CSE1)] that induced high levels of expression of the luciferase reporter gene in three collecting duct cell lines, but not in other renal cell lines. In gel shift assays, CSE1 bound a DNA-binding protein expressed selectively in collecting duct cell lines, and a 7-bp mutation, which abolished the activity of CSE1 in transient transfection experiments, also abolished the binding of this protein. Furthermore, decoy experiments performed using CSE1 showed that this sequence was involved not only in the expression of a construct containing 4.2 kb of the V2 receptor proximal promoter, but also in the expression of the endogenous V2 receptor gene. CSE1 appears to act mostly by counteracting the inhibitory effects of a strong ubiquitous repressor element that we called CIE1. Collectively, these results identify the first functional collecting duct-specific cis-acting element.

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

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

Chondrocyte-specific enhancer elements in the Col11a2 gene resemble the Col2a1 tissue-specific enhancer

Type XI collagen and type II collagen are coexpressed in all cartilage, and both are essential for normal cartilage differentiation and skeletal morphogenesis. This laboratory has recently identified a 48-base pair (bp) enhancer element in the type II collagen gene Col2a1 that contains several HMG-type protein-binding sites and that can direct chondrocyte-specific expression in transient transfection and in transgenic mice. The present study has identified two short chondrocyte-specific enhancer elements within a region in the 5' portion of the type XI collagen gene Col11a2 that has previously been shown to influence chondrocyte-specific expression in transgenic mice. These Col11a2 enhancer elements, like the Col2a1 enhancer, contain several sites with homology to the high mobility group (HMG) protein-binding consensus sequence. In electrophoretic mobility shift assays, the Col11a2 elements formed a DNA-protein complex that was dependent on the presence of the HMG-like sites. It had the same mobility as the complex formed with the Col2a1 48-bp enhancer and appeared to contain the same or similar proteins, including SOX9. The Col11a2 elements directed gene expression in transient transfections of chondrocytes but not fibroblasts, and their activity was abolished by mutation of the HMG-like sites. Ectopically expressed SOX9 activated these enhancers in non-chondrocytic cells, as it also activates the Col2a1 enhancer. Finally, the Col11a2 enhancer elements both directed transgene expression to cartilage in developing mouse embryos. Overall, our results indicate that the two Col11a2 chondrocyte-specific enhancer elements share many similarities with the Col2a1 48-bp enhancer. These similarities suggest the existence of a genetic program designed to coordinately regulate the expression of these and perhaps other genes involved in the chondrocyte differentiation pathway.

The upstream regulatory regions of the hepatocyte growth factor gene promoter are essential for its expression in transgenic mice

To understand the molecular mechanisms of hepatocyte growth factor (HGF) gene transcription in vivo, we report the generation and characterization of transgenic mice harboring various lengths of the mouse HGF promoter linked to the chloramphenicol acetyltransferase reporter gene. Analysis of different tissues of the transgenic mouse lines having the 2.7-kilobase (kb) promoter construct revealed a pattern of reporter gene expression in embryonic and adult tissues that paralleled that of endogenous HGF gene expression. A similar expression pattern was observed in the 0.7-kb transgenic lines. However, in contrast to in vitro data, no promoter activity was detected in four independent transgenic lines harboring the 0.1-kb construct. Akin to the activity of the endogenous HGF gene, which is induced in the liver, lung, and spleen in response to 70% partial hepatectomy, the reporter gene driven by the 2.7-kb promoter construct was strongly induced, whereas that driven by the 0.7-kb promoter construct was modestly induced in these organs after partial hepatectomy. Together, these data suggest that the region between -0.1 and -0.7 kb of the HGF gene promoter is essential to drive its expression in vivo and that additional upstream sequences located between -0.7 and -2.7 kb are also necessary for its maximum inducibility in response to cues that stimulate tissue growth and regeneration.

A transcription activator with restricted tissue distribution regulates cell-specific expression of alpha1(XI) collagen

Different regulatory programs are likely to control expression of the alpha1(XI) collagen (COL11A1) gene in cartilaginous and non-cartilaginous tissues and in coordination with different collagen genes. Here, we report the identification of a cis-acting element that is required for constitutive and tissue-specific activity of the proximal COL11A1 promoter. The element binds an apparently novel activator whose expression is restricted mostly, but not exclusively, to cells of mesenchymal origin. Transient transfection experiments using wild-type and mutant constructs demonstrated the critical contribution of a 45-base pair upstream element (FP9) to promoter activity. The same functional tests and DNA binding assays narrowed down the critical portion of FP9 to a 20-base pair sequence, which consists of an imperfect palindrome with strong homology to the GATA consensus motif. Despite being able to bind GATA proteins in vitro, FP9 is actually recognized by a distinct approximately 100-kDa polypeptide (FP9C) probably belonging to the zinc-finger family of transcription factors. FP9C binding was mostly identified in nuclei from cells of mesenchymal origin, including those actively engaged in COL11A1 transcription. A positive correlation was also established between the level of FP9C binding and the degree of cell differentiation in vitro. Thus, FP9C represents an unusual example of tissue-specific and differentiation-related transcription factor with overlapping expression in hard and soft connective tissues.

Functional interaction between the DNA binding subunit trimerization domain of NF-Y and the high mobility group protein HMG-I(Y)

The mammalian transcription factor, NF-Y(CBF), contains three known subunit components, NF-YA (CBF-B), NF-YB(CBF-A), and NF-YC(CBF-C), which are all required to reconstitute specific CCAAT box DNA binding activity. In this study, the high mobility group chromosomal protein, HMG-I(Y), has been shown to activate NF-Y in transient transfections in vivo using the natural murine alpha2(I) collagen promoter and a multimerized version of the proximal NF-Y(CBF) CCAAT box element. In vitro analysis of the alpha2(I) collagen promoter region inclusive of the NF-Y(CBF) binding site (-106 to -65 base pairs) failed to identify any high affinity HMG-I(Y) DNA-binding sites. However, the heterotrimeric NF-Y complex, as well as the NF-YA subunit alone, was shown to stably interact in vitro with both HMG-I(Y) and phosphorylated HMG-I, as modified by casein kinase II, using far Western and protein-protein interaction solution assays in the absence of CCAAT box DNA. Furthermore, the interaction between HMG-I(Y) and NF-Y was mapped to the highly conserved DNA binding-subunit interaction domain (DBD) of the NF-YA subunit and to a single AT-hook motif in HMG-I(Y). Recombinant HMG-I was also found to stabilize the CCAAT box DNA binding activity of recombinant NF-Y, as well as the native NF-Y complex, in vitro. Together, these results suggest a functional HMG-I(Y) protein binding site has been identified in the NF-Y complex and mapped to the conserved DBD and AT-hook regions of NF-YA and HMG-I(Y), respectively. This protein-protein interaction site may function to modulate NF-Y activity through stabilization of NF-Y binding to its CCAAT box DNA-binding site.

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

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

Activity of the osteocalcin promoter in skeletal sites of transgenic mice and during osteoblast differentiation in bone marrow-derived stromal cell cultures: effects of age and sex

The bone-specific osteocalcin gene is a well established marker of osteoblast activity. We have studied osteocalcin transcription in transgenic mice carrying rat osteocalcin promoter-chloramphenicol acetyltransferase (CAT) reporter constructs. Transgenic lines carrying each of the 1.7-, 1.1-, 0.72-, or 0.35-kilobase promoter constructs expressed the reporter gene in a tissue-specific manner. However, each of these constructs was sensitive to site of integration effects, reflected by a high frequency of nonexpressing transgenic lines. High expression of the 1.7-kilobase promoter in osseous tissues was accompanied by low ectopic expression in the brain. Analysis of CAT expression in femurs, calvariae, and lumbar vertebrae of this line indicated considerable variability in promoter activity among individual transgenic animals. Analysis of the variance in CAT activity demonstrated a linkage between promoter activities in these distant skeletal sites. Promoter activity was inversely correlated with age, and females exhibited severalfold higher activity than age-matched males. Bone marrow stromal cells from these animals, cultured under conditions that support osteoblast differentiation, exhibited the expected postproliferative onset of osteocalcin promoter activity, as assessed by CAT assay. The ex vivo CAT activity was not dependent on the sex or the age of the donor transgenic mouse. Taken together, our results are consistent with the hypothesis that a common, probably humoral, factor(s) regulates osteocalcin transcription in distant skeletal sites. We suggest that the abundance of this factor(s) is different between males and females and among individual mice at a given time point, and that ex vivo culturing of osteoblasts reduces the variation in osteocalcin promoter activity by eliminating the physiological contribution of this factor.

Extracellular signal-regulated kinase and the small GTP-binding protein, Rac, contribute to the effects of transforming growth factor-beta1 on gene expression

The kinases and regulatory proteins that convey signals initiated by transforming growth factor-beta (TGF-beta) to the nucleus are poorly characterized. To study the role of the extracellular signal-regulated kinase (ERK) pathway in this process, we transiently transfected NIH 3T3 fibroblasts with TGF-beta-responsive luciferase reporter genes and expression vectors designed to interrupt this kinase cascade. Mitogen-activated protein (MAP) kinase phosphatase-1 and a dominant negative MAP/ERK kinase 1 mutant reduced stimulation of plasminogen activator inhibitor-1 (PAI-1) promoter activity by TGF-beta1 from 11.5- to 4-fold and 4.9-fold, respectively. Similar results were observed with the type I collagen promoters. TGF-beta1 increased ERK1 activity 4.5-fold at 5 min and 3. 1-fold at 3 h, while Jun kinase and p38 activity were not affected. Cotransfection of a dominant negative mutant of the small G protein, Rac, but not dominant negative Ras, Cdc42, or Rho mutants, reduced the effects of TGF-beta1 on the PAI-1 promoter by approximately half. In support of a role for Rac in signaling by TGF-beta, GTP binding to Rac was increased 3.7-fold following exposure of NIH 3T3 cells to TGF-beta1 for 3 min. These findings indicate that TGF-beta1 modulates gene expression partly through ERK and Rac in NIH 3T3 cells.

The transcriptional activity of the CCAAT-binding factor CBF is mediated by two distinct activation domains, one in the CBF-B subunit and the other in the CBF-C subunit

CBF-A, CBF-B, and CBF-C together form the heterotrimeric mammalian CCAAT-binding factor, CBF, which binds to DNA to form a CBF-DNA complex. Here we examined the transcription activation function of CBF in an in vitro reconstituted system using the three purified recombinant CBF subunits expressed in Escherichia coli. Two of the subunits, CBF-A and CBF-C, were coexpressed and purified as a CBF-A/CBF-C complex. Addition of the three wild-type recombinant CBF subunits to EL4 cell nuclear extracts depleted of CBF stimulated transcription 5-20-fold from proalpha2(1) collagen promoters and 10-fold from the Rous sarcoma virus long terminal repeat. Two CBF deletion mutants, one containing full-length CBF-A and CBF-C and a CBF-B lacking the NH2-terminal residues 1-224, and the other containing full- length CBF-A and CBF-B and a CBF-C lacking the COOH-terminal residues 114-309, also stimulated transcription from these promoters, but the level of activation was reduced to half that obtained with the full-length CBF subunits. In contrast, a CBF deletion mutant protein containing full-length CBF-A and deleted forms of both CBF-B and CBF-C showed very little transcription activation from these promoters. Hence, this study demonstrates that the heterotrimeric CBF protein consists of two transcription activation domains, one present in CBF-B and the other in CBF-C, and that the two domains act additively in the in vitro assay. The activation domains of both CBF-B and CBF-C, which are rich in glutamine and hydrophobic residues, showed amino acid sequence similarities with each other and with the glutamine-rich activation domain of transcription factor Sp1.

Expression of rat bone sialoprotein promoter in transgenic mice

Bone sialoprotein (BSP) is a major protein of the mineralized bone extracellular matrix that has been implicated in the nucleation of hydroxyapatite crystals. Our previous studies have demonstrated that BSP mRNA is expressed by differentiated osteoblasts, odontoblasts, and cementoblasts involved in de novo mineralized tissue formation in a tissue-specific and developmentally regulated manner. To determine the basis of the selective expression of the BSP gene, we have generated four transgenic mouse lines in which 2.7 kb of the rat BSP promoter ligated to a luciferase reporter gene has been stably integrated into the mouse genome. Assays of luciferase activities in 5-day-old animals has revealed consistently high levels in bone tissues with negligible activities in various other organs including kidney, liver, stomach, intestine, and spleen. In some animals, variable expression was observed in brain and skin. Temporal analyses revealed the highest luciferase expression in neonatal bones, with expression decreasing markedly with subsequent growth and development, as observed previously for the endogenous gene in rats. Immunohistochemical analysis of luciferase activity and in situ hybridization of luciferase mRNA in bone tissues show that differentiated osteoblasts express the highest levels of luciferase, consistent with the induction of endogenous gene expression. These studies demonstrate that the regulation of the BSP gene during osteoblastic differentiation, together with its tissue-specific, developmentally regulated expression, is primarily mediated within the 2.7 kb region of the promoter.

Inhibition of collagen synthesis and changes in skin morphology in murine graft-versus-host disease and tight skin mice: effect of halofuginone

The effect of halofuginone, a plant alkaloid known to inhibit collagen type I synthesis, on skin collagen content and skin morphology was evaluated in two in vivo models of scleroderma: the murine chronic graft-versus-host disease (cGvHD) and the tight skin mouse. Skin collagen was assessed by hydroxyproline levels in skin biopsies and by immunohistochemistry using anti-collagen type I antibodies. Daily intraperitoneal injections of halofuginone (1 microgram/mouse) for 52 d starting 3 d before spleen cell transplantation, abrogated the increase in skin collagen and prevented the thickening of the dermis and the loss of the subdermal fat, all of which are characteristic of the cGvHD mice. Halofuginone had a minimal effect on collagen content of the control mice. The halofuginone-dependent decrease in skin collagen content was concentration-dependent and was not accompanied by changes in body weight in either the cGvHD or the control mice. Injections of halofuginone (1 microgram/mouse) for 45 d caused a decrease in the collagen content and dermis width in tight skin mice, but did not affect the dermis width of control mice. Collagen content determination from skin biopsies confirmed the immunohistochemical results in the same mice. The low concentration of halofuginone needed to prevent collagen deposition in fibrotic skin without affecting body weight suggests that halofuginone may serve as a novel and promising anti-fibrotic therapy.

Coordinate patterns of expression of type I and III collagens during mouse development

The extracellular proteins types I and III collagen are abundantly expressed during development. Here, the patterns of the pro alpha 1(I), pro alpha 2(I), and pro alpha 1(III) collagen mRNAs are systematically examined from 7.5 to 17.5 days of development (E7.5 to E17.5) in the mouse using in situ hybridization with specific riboprobes. Coordinated expression of pro alpha 1(I) and pro alpha 2(I) collagen mRNA was found throughout development in all regions examined. Widespread type I collagen expression starting at E8.5 occurred in embryonic mesoderm, sclerotomes, dermatomes, and in the forming connective tissues. After E14.5, regions of ossification showed highest levels of type I collagen expression. Pro alpha 1(III) collagen expression was specific to and coordinated with patterns of type I collagen expression in many fibroblast-containing tissues. No expression of type III collagen occurred in osteoblasts. This comprehensive study of the transcripts of abundantly expressed structural proteins should provide a basis for comparison of other key extracellular matrix molecules and serve as a reference for studies on the patterns of activities of various promoter/enhancer-reporter gene constructions of type I and III collagen genes in transgenic mice.

Use of a new rat chondrosarcoma cell line to delineate a 119-base pair chondrocyte-specific enhancer element and to define active promoter segments in the mouse pro-alpha 1(II) collagen gene

We show that a new rat chondrosarcoma (RCS) cell line established in long-term culture from the Swarm tumor displayed a stable differentiated chondrocyte-like phenotype. Indeed, these cells produced the collagen types II, IX, and XI and alcian blue-stainable cartilage-specific proteoglycans, but no type I or type III collagen. To functionally characterize their chondrocytic nature, the cells were stably transfected with a type II collagen/beta geo chimeric gene which confers essentially perfect chondrocyte-specific expression in transgenic mice. RCS cells expressed both beta-galactosidase and G418 resistance, in comparison with similarly transfected 10T1/2 and NIH/3T3 fibroblasts which did not. These cells were then used to perform a systematic deletion analysis of the first intron of the mouse type II collagen gene (Col2a1) using transient expression experiments to determine which segments stimulated expression of a luciferase reporter gene in RCS cells but not in 10T1/2 fibroblasts. Cloning of two tandem copies of a 156-base pair (bp) intron 1 fragment (+2188 to +2343) in a construction containing a 314-bp Col2a1 promoter caused an almost 200-fold increase in promoter activity in RCS cells but no increase in 10T1/2 cells. DNase I footprint analysis over this 156-bp fragment revealed two adjacent protected regions, FP1 and FP2, located in the 3'-half of this segment, but no differences were seen with nuclear extracts of RCS cells and 10T1/2 fibroblasts. Deletion of FP2 to leave a 119-bp segment decreased enhancer activity by severalfold, but RCS cell specificity was maintained. Further deletions indicated that sequences both in the 5' part of the 119-bp fragment and in FP1 were needed simultaneously for RCS cell-specific enhancer activity. A series of deletions in the promoter region of the mouse Col2a1 gene progressively reduced activity when these promoters were tested by themselves in transient expression experiments. However, these promoter deletions were all activated to a similar level in RCS cells by a 231-bp intron 1 fragment that included the 156-bp enhancer. The RCS cell-specific activity persisted even if the Col2a1 promoter was replaced by a minimal adenovirus major late promoter. This 231-bp intron 1 fragment also had strong enhancing activity in transiently transfected mouse primary chondrocytes. Our experiments establish the usefulness of RCS cells as an experimental system for studies of the control of chondrocyte-specific genes, provide an extensive delineation of segments in the Col2a1 first intron involved in chondrocyte-specific activity, and show that promoter sequences are dispensable for chondrocyte specificity.

Requirements for transforming growth factor-beta regulation of the pro-alpha 2(I) collagen and plasminogen activator inhibitor-1 promoters

Experiments were designed to clarify the role of several proteins, junB, retinoblastoma protein (RB), and the transforming growth factor-beta (TGF-beta) receptors that are potential intermediates in TGF-beta activation of the alpha 2(I) collagen promoter. Treatment of NIH-3T3 cells with TGF-beta increased the activity of a transiently transfected murine alpha 2(I) collagen promoter (nucleotides -350 to +54) fused to a luciferase reporter gene 9-fold. Cotransfection of a junB stimulated the basal activity of the alpha 2(I) collagen promoter 93-fold, respectively. Expression of antisense junB RNA attenuated the effect of TGF-beta. Simian virus 40 large T antigen, an inhibitor RB function, did not prevent TGF-beta effects on the alpha 2(I) collagen promoter. A chimeric receptor containing the extracellular domain of the colony-stimulating factor-1 receptor and the intracellular domain of the type I TGF-beta receptor enhanced alpha 2(I) collagen promoter activity 4.8-fold, whereas a similar chimera containing the type II receptor intracellular domain had much weaker effects. Similar results were obtained with a plasminogen activator inhibitor-1 promoter, previously shown to be activated by TGF-beta through AP-1 elements. We conclude that TGF-beta activates the alpha 2(I) collagen and plasminogen activator inhibitor-1 promoters in NIH-3T3 cells through junB and the type I TGF-beta receptor kinase domain.

Characterization of a GC-rich region containing Sp1 binding site(s) as a constitutive responsive element of the alpha 2(I) collagen gene in human fibroblasts

To analyze regulatory elements in the human alpha 2(I) collagen gene (COL1A2) promoter, a series of deletion mutants from -323 to -186 base pairs was tested in transient transfection assays in human fibroblasts. A strong positive responsive element was mapped to a GC-rich region located between base pairs -303 and -271. This region contains three binding sites (GC-boxes) resembling recognition sites for the transcription factor Sp1. Substitution mutations in the GC-boxes abolished binding to the GC-rich region in gel shift analyses and resulted in 90% reduction of promoter activity in transient transfection assays. We demonstrated that transcription factor Sp1 is essential for binding based on the following observations. 1) Sp1 consensus binding site alone competes by binding to the GC-rich region in the DNase I protection assay; 2) both Sp1 consensus binding site and Sp1 antibodies prevent the formation of a DNA-protein complex in the mobility shift assay; 3) anti-Sp1 antibodies recognize a component of the complex competed for by Sp1 consensus binding site.

Overlapping pathways mediate the opposing actions of tumor necrosis factor-alpha and transforming growth factor-beta on alpha 2(I) collagen gene transcription

Transforming growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) are multifunctional peptides intimately involved in the process of extracellular matrix remodeling. We recently showed that TGF-beta stimulates the human alpha 2(I) collagen gene by increasing the affinity of an Sp1-containing transcriptional complex bound to an upstream sequence termed the TbRE (Inagaki, Y., Truter, S. and Ramirez, F. (1994) J. Biol. Chem. 269, 14828-14834). Here, we report that the TbRE-bound complex also mediates the inhibitory signal of TNF-alpha. Nuclear proteins from cells treated with TNF-alpha bind to the TbRE sequence substantially more strongly than those from untreated cells. Additionally, TNF-alpha increases binding of a second protein complex that recognizes the negatively cis-acting element located immediately next to the TbRE. Thus, we postulate that TNF-alpha counteracts the TGF-beta-elicited stimulation of collagen gene expression through overlapping nuclear signaling pathways. One modifies the TGF-beta-targeted transcriptional complex, probably by reducing its stimulatory effect on collagen transcription. The other acts on the binding of the adjacent factor, presumably by increasing its effectiveness in repressing the activity of the collagen promoter. The convergence of the TGF-beta and TNF-alpha pathways on the same sequence of the alpha 2(I) collagen promoter is yet another example of combinatorial gene regulation achieved through composite response elements.

Studies on transcription activation by the multimeric CCAAT-binding factor CBF

The CCAAT-binding factor CBF is a heteromeric transcription factor that specifically binds to CCAAT sequences in many eukaryotic genes. CBF consists of three subunits, CBF-A, CBF-B, and CBF-C, all three of which are necessary for DNA binding. In this study we examined the transcription activation function of CBF by two different approaches. We first used a heterologous system in which a series of deletion mutations of CBF-B, fused to the bacterial LexA DNA binding domain, were transfected into HeLa cells together with a reporter gene driven by a minimal promoter containing LexA binding sites. These experiments showed that CBF-B needed both a glutamine-rich domain and an adjacent serine/threonine-rich domain to activate the reporter gene optimally. The glutamine-rich domain by itself activated transcription only modestly. We also set up an in vitro transcription reconstituted system in which trans-activation by CBF occurred through a physiological CCAAT motif. Nuclear extracts from NIH 3T3 cells were first depleted of CBF and then complemented with recombinant CBF-B and a highly purified fraction containing native CBF-A and CBF-C. Recombinant full-length CBF-B together with CBF-A and CBF-C activated transcription of several alpha 2(I) collagen gene promoter constructs. We then tested whether in this system the glutamine- and serine/threonine-rich domains of CBF-B were needed for trans-activation by CBF. We generated a truncated form of CBF-B that was still able to bind DNA in the presence of CBF-A and CBF-C. Even in the absence of the glutamine- and serine/threonine-rich domains of CBF-B, reconstituted CBF did activate transcription, suggesting that CBF transcriptional activation can also be mediated by the other subunits of CBF or by another transcription factor present in the nuclear extracts that interacts with CBF. Taken together our results suggest a model in which CBF has the potential to activate transcription either through the glutamine- and serine/threonine-rich domains of CBF-B or through the other subunits of CBF or through another component recruited by CBF.

Regulation of type I collagen gene expression in bone

The regulation of COL1A1 gene expression in bone was studied by measuring the activity of type I collagen promoter fusion genes (ColCAT) in permanently transfected osteoblastic cells and calvariae from transgenic animals. The basal activity of ColCAT fusion genes in transfected cells is mediated by DNA sequences between -3.5 to -2.3 kb while expression in vivo requires sequences between -2.3 and -1.7 kb. Parathyroid hormone, 1,25-dihydroxyvitamin D3 and interleukin-1 decrease the activity of ColCAT fusion genes in osteoblastic cells and transgenic calvariae. Because there may be differences between the expression of ColCAT fusion genes in cultured cells and intact bone, it will be important to compare data obtained from transfected cells with an in vivo model such as calvariae from transgenic mice.

Cell-type specific promoters of the chick alpha 2(I) collagen gene in chondrocytes and fibroblasts

Two different promoter regions responsible for the cell-type specific transcripts of the collagen alpha 2(I) gene in chick embryonic fibroblasts and chondrocytes are described. A region including part of intron 1 of the fibroblast alpha 2(I) gene is able to promote transcription in chondrocytes. This promoter region is located 5' to the chondrocyte-specific transcript, from -2205 to +42, relative to the chondrocyte transcription start site. When it is placed upstream of a luciferase reporter gene in transiently transfected chondrocytes, luciferase activity is stimulated 78-fold. The region between -2205 and -1298 is most important for the activity of the chondrocyte promoter, since a truncated promoter from -1298 through +42 gives only 15% as much activity. A similar low level of activity is given by the sequence -96 through +42. In transfected fibroblasts, a region from -1600 through +46, relative to the fibroblast transcription start site, stimulates transcription of a luciferase reporter gene by more than 1,000-fold. Deletions from -1185 through -807 and from -707 through -90 reduce promoter activity to 22% and 32%, respectively, of that given by the intact promoter. The chondrocyte and fibroblast promoters are each cell-type specific and are relatively inactive in the other cell type. Subregions within the fibroblast promoter, from -1154 through -1055, from -712 through -85, and the sequence containing the inverted CCAAT motif from -93 through -67, differentially bind factors in fibroblast but not chondrocyte nuclear extracts.