Stable murine chondrogenic cell lines derived from c-fos-induced cartilage tumors

Role of the Transcription Factor FOSL1 in Organ Development and Tumorigenesis

The transcription factor FOSL1 plays an important role in cell differentiation and tumorigenesis. Primarily, FOSL1 is crucial for the differentiation of several cell lineages, namely adipocytes, chondrocytes, and osteoblasts. In solid tumors, FOSL1 controls the progression of tumor cells through the epithelial-mesenchymal transformation. In this review, we summarize the available data on FOSL1 expression, stabilization, and degradation in the cell. We discuss how FOSL1 is integrated into the intracellular signaling mechanisms and provide a comprehensive analysis of FOSL1 influence on gene expression. We also analyze the pathological changes caused by altered Fosl1 expression in genetically modified mice. In addition, we dedicated a separate section of the review to the role of FOSL1 in human cancer. Primarily, we focus on the FOSL1 expression pattern in solid tumors, FOSL1 importance as a prognostic factor, and FOSL1 perspectives as a molecular target for anticancer therapy.

Wnt signaling and Loxl2 promote aggressive osteosarcoma

Osteosarcoma (OS) is the most frequent primary malignant bone tumor in urgent need of better therapies. Using genetically modified mouse models (GEMMs), we demonstrate that Wnt signaling promotes c-Fos-induced OS formation via the actions of the collagen-modifying enzyme Loxl2. c-Fos/AP-1 directly regulates the expression of the Wnt ligands Wnt7b and Wnt9a in OS cells through promoter binding, and Wnt7b and Wnt9a in turn promote Loxl2 expression in murine and human OS cells through the transcription factors Zeb1 and Zeb2. Concordantly, inhibition of Wnt ligand secretion by inactivating the Wnt-less (Wls) gene in osteoblasts in c-Fos GEMMs either early or in a therapeutic setting reduces Loxl2 expression and progression of OS. Wls-deficient osteosarcomas proliferate less, are less mineralized and are enriched in fibroblastic cells surrounded by collagen fibers. Importantly, Loxl2 inhibition using either the pan-Lox inhibitor BAPN or a specific inducible shRNA reduces OS cell proliferation in vitro and decreases tumor growth and lung colonization in murine and human orthotopic OS transplantation models. Finally, OS development is delayed in c-Fos GEMMs treated with BAPN or with specific Loxl2 blocking antibodies. Congruently, a strong correlation between c-FOS, LOXL2 and WNT7B/WNT9A expression is observed in human OS samples, and c-FOS/LOXL2 co-expression correlates with OS aggressiveness and decreased patient survival. Therefore, therapeutic targeting of Wnt and/or Loxl2 should be considered to potentiate the inadequate current treatments for pediatric, recurrent, and metastatic OS.

Fos metamorphoses: Lessons from mutants in model organisms

The Fos oncogene gene family is evolutionarily conserved throughout Eukarya. Fos proteins characteristically have a leucine zipper and a basic region with a helix-turn-helix motif that binds DNA. In vertebrates, there are several Fos homologs. They can homo- or hetero-dimerize via the leucine zipper domain. Fos homologs coupled with other transcription factors, like Jun oncoproteins, constitute the Activator Protein 1 (AP-1) complex. From its original inception as an oncogene, the subsequent finding that they act as transcription factors binding DNA sequences known as TRE, to the realization that they are activated in many different scenarios, and to loss-of-function analysis, the Fos proteins have traversed a multifarious path in development and physiology. They are instrumental in 'immediate early genes' responses, and activated by a seemingly myriad assemblage of different stimuli. Yet, the majority of these studies were basically gain-of-function studies, since it was thought that Fos genes would be cell lethal. Loss-of-function mutations in vertebrates were recovered later, and were not cell lethal. In fact, c-fos null mutations are viable with developmental defects (osteopetrosis and myeloid lineage abnormalities). It was then hypothesized that vertebrate genomes exhibit partial redundancy, explaining the 'mild' phenotypes, and complicating assessment of complete loss-of-function phenotypes. Due to its promiscuous activation, fos genes (especially c-fos) are now commonly used as markers for cellular responses to stimuli. fos homologs high sequence conservation (including Drosophila) is advantageous as it allows critical assessment of fos genes functions in this genetic model. Drosophila melanogaster contains only one fos homolog, the gene kayak. kayak mutations are lethal, and allow study of all the processes where fos is required. The kayak locus encodes several different isoforms, and is a pleiotropic gene variously required for development involving cell shape changes. In general, fos genes seem to primarily activate programs involved in cellular architectural rearrangements and cell shape changes.

β2-adrenergic receptors inhibit the expression of collagen type II in growth plate chondrocytes by stimulating the AP-1 factor Jun-B

The sympathetic nervous system can regulate both osteoblast and chondrocyte growth and activity through β(2)-adrenergic receptors (β(2)-AR). We have shown previously that β(2)-AR activate both adenylyl cyclase and mitogen-activated protein kinases ERK1/2 in growth plate chondrocytes prepared from ribs of embryonic E18.5 mice. Here we examined β(2)-AR inhibition of collagen type II (Col II) expression in growth plate chondrocytes and the molecular pathways involved. Stimulation of β(2)-AR by isoproterenol inhibited Col II mRNA and protein levels by ∼50% beginning at 2 h, with both remaining suppressed over 24 h. This inhibition was blocked by propranolol and inhibitors of either MEK1 or PKA. Isoproterenol stimulated an AP-1-luciferase reporter and increased the expression of AP-1 factors c-Fos, Fra-1, Fra-2, c-Jun, and Jun-B but had no effect on Jun-D. Stimulation of AP-1 activity was blocked by inhibitors of MEK1 or PKA. siRNA inhibition of AP-1 factors showed that depletion of only Jun-B attenuated isoproterenol-mediated inhibition of Col II. Transfection with jun-B or c-fos showed selective inhibition of Col II mRNA and a Col II luciferase reporter construct by jun-B. Isoproterenol as well as jun-B overexpression in the chondrocytes also inhibited the expression of Sox-6 mRNA and protein, and depletion of Jun-B abrogated β(2)-AR inhibition of Sox-6. Collectively, these findings demonstrate regulation of chondrocyte differentiation through β(2)-AR mediated by ERK1/2 and PKA stimulation of the AP-1 factor Jun-B that inhibits the expression of Sox-6 and Col II.

Differentiation-dependent activation of the extracellular fatty acid binding protein (Ex-FABP) gene during chondrogenesis

Chicken hypertrophic chondrocytes secrete the extracellular fatty acid binding protein (Ex-FABP), a lipocalin not expressed by their undifferentiated precursors. Genomic clones coding for the full protein are here structurally and functionally analyzed. We first determined that the promoter sequence markedly differs from that reported for the homologous p20K, and we confirmed by genomic DNA Southern analysis the exactness of our sequence. This is of relevance since we have identified another lipocalin gene within the region of discrepancy, indicating thereby the existence of a lipocalin cluster within the same chromosomal locus. By transient transfections with 5'-deletions and the chloramphenicol acetyl transferase (CAT) reporter gene, the region between nt -926 and nt -629 was shown to be strongly active, specifically in hypertrophic chondrocytes and not in dedifferentiated cells. Responsive elements for several potential transcription factors lay within this sequence. Among those, activating protein-1 (AP-1) was shown to be involved in the regulation of the Ex-FABP gene during chondrocyte differentiation, as indicated by electrophoretic mobility shift assay, AP-1 site mutagenesis and functional interference assays.

Establishment of a novel chondrocytic cell line N1511 derived from p53-null mice

We established a clonal chondrocytic cell line N1511 derived from rib cartilage of a p53-null mouse. N1511 cells proliferated in polygonal shape and elicited differentiation at confluence when treated with combination of bone morphogenetic protein (BMP) 2 and insulin or parathyroid hormone (PTH) and dexamethasone. BMP-2/insulin-treated cells became refractile without forming cartilaginous nodules and reached terminal differentiation, became positive for alizarin red staining, and developed considerable ALP activity. In contrast, PTH/dexamethasone-treated cells formed Alcian blue-positive nodules but remained negative for alizarin red staining and ALP activity. Northern blot analysis revealed that BMP-2/insulin-treated cells sequentially expressed type II, IX, and X collagens, whereas PTH/dexamethasone-treated cells slowly expressed type II collagen and then type IX, and they did not exhibit type X collagen expression. These results show that BMP-2/insulin treatment induces full differentiation toward hypertrophy, whereas treatment with PTH/dexamethasone slows and limits differentiation. Recovery of p53 expression in N1511 cells by transient transfection inhibited cell proliferation, suggesting that cell proliferation could be regulated with p53 in this cell line. These results indicate that N1511 is the only cell line with known genetic mutation, which undergoes multiple steps of chondrocyte differentiation toward hypertrophy, and because proliferation could be regulated by expression of p53, N1511 could be an excellent model for studies of chondrogenesis, the function of p53, and genetic engineering of cartilage tissue.

Establishment and characterization of chondrocyte cell lines from the costal cartilage of SV40 large T antigen transgenic mice

Complete understanding of the physiology and pathology of the cartilage is essential to establish treatments for a variety of cartilage disorders and defects such as rheumatoid arthritis, congenital malformations, and tumors of cartilage. Although synthetic materials have been used in many cases, they possess inherent problems including wear of the materials and low mechanical strength. Autograft has been considered very effective to overcome these problems. However, the limitation of the transplant volume is a major problem in autograft to be overcome. The costal cartilage is the most serious candidate for donor site transplantation, since it is the largest permanent hyaline cartilage in the body. To investigate the possibility using the costal cartilage as a transplant source, we have established and characterized three mouse chondrocyte cell lines (MCC-2, MCC-5, and MCC-35) derived from the costal cartilage of 8-week-old male SV40 large T-antigen transgenic mice. At confluence, all the cell lines formed nodules that could be positively stained with alcian blue (pH 2.5). The size of nodules gradually increased during culturing time. After 2 and 6 weeks of culture, RT-PCR analysis demonstrated that all three cell lines expressed mRNA from the cartilage-specific genes for type II collagen, type XI collagen, aggrecan, and link protein. Furthermore, type X collagen expression was detected in MCC-5 and MCC-35 but not in MCC-2. Any phenotypic changes were not observed over 31 cell divisions. Immunocytochemistry showed further that MCC-2, MCC-5, and MCC-35 produced cartilage-specific proteins type II collagen and type XI collagen, while in addition MCC-5 and MCC-35 produced type X collagen. Treatment with 1alpha, 25-dihydroxyvitamin D(3) inhibited cell proliferation and differentiation of the three cell lines in a dose-dependent manner. These phenotypic characteristics have been found consistent with chondrocyte cell lines established from cartilage tissues other than costal cartilage. In conclusion, costal cartilage shows phenotypic similarities to other cartilages, i.e., articular cartilage and embryonic limbs, suggesting that costal cartilage may be very useful as the donor transplantation site for the treatment of cartilage disorders. Furthermore, the cell lines established in this study are also beneficial in basic research of cartilage physiology and pathology.

Detection of c-fos expression in benign and malignant musculoskeletal lesions

The proto-oncogene c-fos has been implicated in the development of both benign and malignant lesions of bone. Although c-fos expression in such lesions has been well studied in transgenic mouse models, less is known about its role in human musculoskeletal pathology. To clarify this relationship, we used in situ hybridization to localize c-fos m-RNA transcripts in 26 fibrous lesions (eight cases of extra-abdominal fibromatosis and six cases each of fibrous dysplasia, fibrosarcoma, and malignant fibrous histiocytoma of bone) as well as six chondrosarcomas and eight conventional high grade osteosarcomas. We found detectable levels of c-fos expression in tissues from each type of lesion tested. Moreover, all fibrous lesions consistently demonstrated high levels of expression in a majority of cells in each lesion. Chondrosarcomas and osteosarcomas exhibited more heterogeneity in c-fos expression than fibrous tissues. Three of six chondrosarcomas showed moderate expression of c-fos while only one of six was considered high. Similarly, only three of eight osteosarcomas had high expression of c-fos. These findings indicate that the expression of c-fos may be important in the development of a broad range of fibrous lesions as well as in bone and cartilaginous tumors. Additionally, this is the first report, to our knowledge, of detectable c-fos m-RNA in human chondrosarcoma.

A novel in vitro culture system for analysis of functional role of phosphate transport in endochondral ossification

In vivo expression of the type III sodium-dependent phosphate transporter (NaPiT) Glvr-1 during endochondral ossification, suggests a functional role for inorganic phosphate (Pi) transport in cartilage calcification. For further analysis of this relationship, an in vitro model of endochondral ossification is required. In this context, we investigated the characteristics of Pi transport in the new chondrogenic cell line ATDC5 in relation to extracellular matrix (ECM) formation and mineralization. Pi uptake in ATDC-5 cells and in isolated matrix vesicles (MVs) is mediated by an Na-dependent Pi transporter with a pH dependency characteristic of a type III Pi carrier (lower activity at alkaline pH). Northern blot analysis indicated that ATDC-5 cells express Glvr-1 transcripts during the various stages of their maturation with a maximal level during the proliferating stage. In isolated MVs, Pi transport activity was maximal at day 21, concomitant with the beginning of type X collagen messenger RNA expression. These events preceded the initiation of matrix mineralization, which was apparent at day 25, and then gradually increased until day 47. This temporal relationship between maximal Pi transport activity in MVs and the expression of a marker of mineralizing chondrocytes is compatible with the possible involvement of Pi transport in the ECM calcification observed in ATDC-5 cell cultures. In conclusion, these observations suggest that ATCD-5 cells in culture represent a promising model for the analysis of a functional role of Pi transport in the initial events of endochondral ossification.

Presence and expression of the simian virus-40 genome in human giant cell tumors of bone

SV40 DNA sequences have been found in human tumors, such as mesotheliomas, ependymomas, and bone tumors, suggesting that SV40 may be involved in their etiology. The FOS oncogene could play an important role in bone development because SV40 is able to induce FOS in cell culture. In this study, the presence of SV40 sequences, large T antigen (Tag), and FOS protein expression were investigated in 120 giant cell tumors (GCTs), moderately benign bone tumors that in some cases can progress to a malignant phenotype. Polymerase chain reaction (PCR), using primers that amplify the RB1 pocket binding domain and the intron of Tag, was used to analyze GCT for the presence of SV40 DNA. Tag and FOS protein expression was evaluated by immunohistochemistry. SV40 sequences were found in 30/107 GCTs, and of these, 22/30 samples expressed Tag protein (73%) and 15/30 overexpressed the FOS oncogene (50%). FOS was undetectable in 77 SV40-negative GCTs. Sequence analysis of the amplified DNAs confirmed that the amplified sequences corresponded to SV40 DNA. The correlation between FOS overexpression and SV40-positive GCTs was highly statistically significant (P < 0.001). These results show that SV40 DNA sequences and SV40 Tag are present in GCTs and might induce FOS activity. These data suggest that SV40 might play a role in the development and progression of some GCTs.

Inhibition of chondrocyte differentiation in vitro by constitutive and inducible overexpression of the c-fos proto-oncogene

We have investigated the role of c-Fos in chondrocyte differentiation in vitro using both constitutive and inducible overexpression approaches in ATDC5 chondrogenic cells, which undergo a well-defined sequence of differentiation from chondroprogenitors to fully differentiated hypertrophic chondrocytes. Initially, we constitutively overexpressed exogenous c-fos in ATDC5 cells. Several stable clones expressing high levels of exogenous c-fos were isolated and those also expressing the cartilage marker type II collagen showed a marked decrease in cartilage nodule formation. To investigate further whether c-Fos directly regulates cartilage differentiation independently of potential clonal variation, we generated additional clones in which exogenous c-fos expression was tightly controlled by a tetracycline-regulatable promoter. Two clones, DT7.1 and DT12.4 were capable of nodule formation in the absence of c-fos. However, upon induction of exogenous c-fos, differentiation was markedly reduced in DT7.1 cells and was virtually abolished in clone DT12.4. Pulse experiments indicated that induction of c-fos only at early stages of proliferation/differentiation inhibited nodule formation, and limiting dilution studies suggested that overexpression of c-fos decreased the frequency of chondroprogenitor cells within the clonal population. Interestingly, rates of proliferation and apoptosis were unaffected by c-fos overexpression under standard conditions, suggesting that these processes do not contribute to the observed inhibition of differentiation. Finally, gene expression analyses demonstrated that the expression of the cartilage markers type II collagen and PTH/PTHrP receptor were down-regulated in the presence of exogenous c-Fos and correlated well with the differentiation status. Moreover, induction of c-fos resulted in the concomitant increase in the expression of fra-1 and c-jun, further highlighting the importance of AP-1 transcription factors in chondrocyte differentiation. These data demonstrate that c-fos overexpression directly inhibits chondrocyte differentiation in vitro, and therefore these cell lines provide very useful tools for identifying novel c-Fos-responsive genes that regulate the differentiation and activity of chondrocytes.

Prognostic relevance of C-myc gene expression in giant-cell tumor of bone

Giant-cell tumor is a primary bone tumor, of uncertain origin, with the potential capacity to metastasize. To study the role of c-myc and c-fos oncogene overexpression in the tumorigenesis and metastatic spread of giant-cell tumors, 32 primary tumors were collected; of these, 19 remained disease-free and 13 metastasized to the lung. Samples of lung metastasis from these 13 patients were also available for study. The expression of c-myc and c-fos mRNA was studied by reverse transcription-polymerase chain reaction and by in situ hybridization. The expression of protein was studied by Western blot analysis and by immunohistochemistry. C-myc mRNA was overexpressed in 12 (38%) of the 32 primary tumors. Thirteen primary tumors metastasized to the lung; in nine (69%) of these, c-myc mRNA was overexpressed. The c-myc protein was overexpressed in seven (54%) of the 13 tumors that metastasized to the lung. C-fos was overexpressed in only one lung metastasis. A strong correlation between the overexpression of c-myc, and the occurrence of metastases was found: thus, c-myc seems a powerful prognosticator in giant-cell tumor. C-myc was overexpressed both in giant cells and in mononuclear cells, suggesting that both cell types are involved in the progression of this tumor.

Localization of silencer and enhancer elements in the human type X collagen gene

Collagen type X is a short, network-forming collagen expressed temporally and spatially tightly controlled in hypertrophic chondrocytes during endochondral ossification. Studies on chicken chondrocytes indicate that the regulation of type X collagen gene expression is regulated at the transcriptional level. In this study, we have analyzed the regulatory elements of the human type X collagen (Col10a1) by reporter gene constructs and transient transfections in chondrogenic and nonchondrogenic cells. Four different promoter fragments covering up to 2,864 bp of 5'-flanking sequences, either including or lacking the first intron, were linked to luciferase reporter gene and transfected into 3T3 fibroblasts, HT1080 fibrosarcoma cells, prehypertrophic chondrocytes from the resting zone, hypertrophic chondrocytes, and chondrogenic cell lines. The results indicated the presence of three regulatory elements in the human Col10a1 gene besides the proximal promoter. First, a negative regulatory element located between 2.4 and 2.8 kb upstream of the transcription initiation site was active in all nonchondrogenic cells and in prehypertrophic chondrocytes. Second, a positive, but also non-tissue-specific positive regulatory element was present in the first intron. Third, a cell-type-specific enhancer element active only in hypertrophic chondrocytes was located between -2.4 and -0.9 kb confirming a previous report by Thomas et al. [(1995): Gene 160:291-296]. The enhancing effect, however, was observed only when calcium phosphate was either used for transfection or included in the culture medium after lipofection. These findings demonstrate that the rigid control of human Col10a1 gene expression is achieved by both positive and negative regulatory elements in the gene and provide the basis for the identification of factors binding to those elements.

Establishment of a novel chondrocyte-like cell line derived from transgenic mice harboring the temperature-sensitive simian virus 40 large T-antigen gene

We established a clonal chondrocyte-like cell line (TC6, TC stands for large T immortalized chondrocyte-like cell line) derived from articular cartilage of transgenic mice harboring a temperature-sensitive simian virus 40 large T-antigen gene. TC6 cells exhibited spindle-like or polygonal morphology and grew well at 33 degrees C in alpha-minimal essential medium supplemented with 0.5% fetal bovine serum. After confluence, these cells formed nodules that were positive for staining with alcian blue. Northern blot analysis demonstrated that these cells expressed messenger RNAs (mRNA) of the genes encoding cartilage-specific proteins such as type II procollagen, link protein, and aggrecan. Furthermore, the expression of type II procollagen and link protein genes in TC6 cells was regulated by parathyroid hormone and basic fibroblast growth factor, suggesting the presence of the receptors for the hormone and cytokine. The expression of link protein mRNA in TC6 cells was regulated in a time-dependent manner and was enhanced in culture within a week and increased continuously up to 10-fold by the end of 4 weeks. Expression of mRNAs encoding type II procollagen and versican/PG-M also increased moderately during the culture period. TC6 cells expressed type I procollagen mRNA, however, its level declined along with time in culture in contrast to the enhancement of the genes encoding cartilage-specific molecules in these cells. Interestingly, alkaline phosphatase mRNA expression was barely detectable in the TC6 cells in their growing phase while it was enhanced dramatically more than 7-fold by day 14 in culture. These results indicate that the TC6 cells could serve as an excellent model for the studies on chondrocyte physiology.

Analysis of chondroprogenitor frequency and cartilage differentiation in a novel family of clonal chondrogenic rat cell lines

We have isolated through sequential steps of subcloning a series of normal clonal cell lines enriched for chondroprogenitors that undergo differentiation in vitro from progenitors to mature chondroblasts and chondrocytes forming three-dimensional cartilage nodules. In the parental chondroblast clone RCJ 3.1C5 (C5), differentiation and cartilage formation occurred without added hormones or growth factors, but chondrogenesis could be stimulated markedly in the presence of the glucocorticoid steroid Dexamethasone (Dex). Limiting dilution analysis indicated that greater than one in ten C5 cells plated was a chondroprogenitor capable of differentiating and forming a cartilage nodule in low density cultures, but chondrogenesis was down-regulated in higher density cultures. Dex elicited a greater stimulatory effect on cartilage nodule formation when C5 cells were plated at higher rather than lower densities. Since Dex also maintained the chondrogenic potential of C5 cells passaged repeatedly, we subcloned C5 in the presence of Dex. Eight of eleven subclones were chondrogenic and the frequency of chondroprogenitors capable of cartilage formation in isolated subclones ranged from lower to much higher than in the parental C5 clone. Both Dex-independent as well as Dex-dependent clones were identified, although long-term maintenance of the chondrocyte phenotype in all subclones required Dex. These data suggest that there are Dex-dependent and Dex-independent chondroprogenitor cells, that cell-cell interactions and/or local factors can modulate cartilage nodule formation and that Dex-responsive steps are involved in long-term maintenance of chondroprogenitors in vitro. Thus, this unique family of non-transformed, clonal chondrogenic cell lines provides a quantifiable, readily manipulatable system in which cartilage differentiation and metabolism can be assessed.

Bone morphogenetic proteins and C-FOS: early signals in endochondral bone formation

The earliest events in endochondral ossification, whether in the formation of the skeleton during embryogenesis or in the regeneration of a bone during fracture healing, involve the establishment of a tissue-specific plan of gene expression arising from pluripotent mesenchymal cells. Recent data from studies of vertebrate limb bud development implicate gradients of diffusable morphogens that specify complex patterns of position-specific gene expression. Diffusable signaling molecules involved at the earliest stages of tissue development include fibroblast growth factors, platelet derived growth factors, bone morphogenetic proteins, transforming growth factor beta, and sonic hedgehog; there are undoubtedly others. An equally elaborate array of trans-membrane receptors in the tyrosine kinase and serine-threonine kinase signaling pathways are responsible for transducing these morphogenetic signals and for establishing boundaries of gene expression. Nuclear signals for transcriptional activation, such as the fos-jun proto-oncogene family and homeobox genes, are also involved in the specification and modification of morphogenetic plans. While much has been learned recently about the presence and interaction of these growth factors and transcription factors in the development of the vertebrate limb bud, very little is known about their presence and interaction in the earliest stages of post-natal bone formation at a fracture site or in heterotopic osteogenesis. Fibrodysplasia ossificans progressiva (FOP) a rare genetic disorder of heterotopic ossification, provides a unique opportunity to study the role of these morphogens and proto-oncogenes in the earliest events of endochondral bone formation.

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.

Fos and bone cell development: lessons from a nuclear oncogene

Vertebrate embryologists are beginning to understand the early developmental decisions that control the origin and patterning of skeletal elements. However, the regulators governing the development of the cells that form the skeleton, namely, bone and cartilage cells, are poorly understood. Recent studies using transgenic and knockout mice have established a unique role for the proto-oncogene and nuclear transcription factor, Fos, in regulating the differentiation and activity of specific bone cell populations, both during normal development and in bone disease.

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.

Transcriptional activation of the fra-1 gene by AP-1 is mediated by regulatory sequences in the first intron

Constitutive expression of c-Fos, FosB, Fra-1, or c-Jun in rat fibroblasts leads to up-regulation of the immediate-early gene fra-1. Using the posttranslational FosER induction system, we demonstrate that this AP-1-dependent stimulation of fra-1 expression is rapid, depends on a functional DNA-binding domain of FosER, and is a general phenomenon observed in different cell types. In vitro mutagenesis and functional analysis of the rat fra-1 gene in stably transfected Rat-1A-FosER fibroblasts indicated that basal and AP-1-regulated expression of the fra-1 gene depends on regulatory sequences in the first intron which comprise a consensus AP-1 site and two AP-1-like elements. We have also investigated the transactivating and transforming properties of the Fra-1 protein to address the significance of fra-1 up-regulation. The entire Fra-1 protein fused to the DNA-binding domain of Ga14 is shown to lack any transactivation function, and yet it possesses oncogenic potential, as overexpression of Fra-1 in established rat fibroblasts results in anchorage-independent growth in vitro and tumor development in athymic mice, fra-1 is therefore not only induced by members of the Fos family, but its gene product may also contribute to cellular transformation by these proteins. Together, these data identify fra-1 as a unique member of the fos gene family which is under positive control by AP-1 activity.

Fos-related antigen (Fra-1), junB, and junD activate human involucrin promoter transcription by binding to proximal and distal AP1 sites to mediate phorbol ester effects on promoter activity

Human involucrin (hINV) is a cornified envelope precursor that is specifically expressed in the suprabasal epidermal layers. We previously demonstrated that 2500 base pairs of the hINV gene upstream regulatory region confers differentiation appropriate regulation in transgenic mice. An analysis of the hINV gene sequence upstream of the transcription start site reveals five potential AP1 binding sites (AP1-1 to 5). Using reporter gene constructs in human keratinocytes, we show that the most distal (AP1-5) and most proximal (AP1-1) AP1 sites are essential for high level transcriptional activity. Simultaneous mutation of these sites reduces transcription by 80%. Gel supershift experiments indicate the interaction of these sites with Fra-1, junB, and junD. Involucrin mRNA levels increase 10-fold and promoter activity 5-11-fold when differentiation is induced by phorbol ester. Functional studies implicate AP1-1 and AP1-5 in mediating the phorbol ester-dependent increase in promoter activity. No involucrin promoter activity or involucrin mRNA was detected in 3T3 fibroblasts. We conclude that (i) two AP1 sites in the hINV promoter are important elements required for keratinocyte-specific expression, (ii) these AP1-1 sites mediate the phorbol ester-dependent increase in promoter activity, and (iii) Fra-1, junB, and junD may be important regulators of hINV expression in epidermis.

Inhibitory effects of basic fibroblast growth factor on chondrocyte differentiation

The role of basic fibroblast growth factor (bFGF) and insulin-like growth factor I (IGF-I) in cartilage growth was studied in primary cultures of rat rib growth plate chondrocytes. Growth factors effects on expression of the proto-oncogene c-fos, DNA synthesis, differentiation, and morphological changes were analyzed by in situ hybridization, 3H-thymidine incorporation, and light and fluorescence microscopy. In serum-deprived cells, bFGF induced a transient expression of c-fos with a maximal effect 15-30 minutes after stimulation. After 24 h of culture it had a slightly lower stimulatory effect on DNA synthesis than IGF-I, but became a significantly more potent mitogen than IGF-I after 48 and 72 h. The stimulatory effect of bFGF on DNA synthesis coincided with a decrease in collagen type II and IGF-II expression. In contrast, IGF-I alone stimulated expression of these genes. In bFGF-treated cultures, cell morphology and the appearance of actin filaments was changed. Polygonal chondrocytes became elongated, fibroblast-like, and the smooth actin filaments were brush-like and disrupted. Addition of IGF-I reduced these changes without affecting c-fos expression induced by bFGF. Our results suggest that bFGF stimulates cell proliferation by preventing terminal differentiation of chondrocytes. This effect is mediated by induction of c-fos expression and a decrease in the steady-state levels of transcripts for collagen II and IGF-II.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Type X collagen gene expression in mouse chondrocytes immortalized by a temperature-sensitive simian virus 40 large tumor antigen

Mouse endochondral chondrocytes were immortalized with a temperature-sensitive simian virus 40 large tumor antigen. Several clonal isolates as well as pools of immortalized cells were characterized. In monolayer cultures at the temperature permissive for the activity of the large tumor antigen (32 degrees C), the cells grew continuously with a doubling time of approximately 2 d, whereas they stopped growing at nonpermissive temperatures (37 degrees C-39 degrees C). The cells from all pools and from most clones expressed the genes for several markers of hypertrophic chondrocytes, such as type X collagen, matrix Gla protein, and osteopontin, but had lost expression of type II collagen mRNA and failed to be stained by alcian blue which detects cartilage-specific proteoglycans. The cells also contained mRNAs for type I collagen and bone Gla protein, consistent with acquisition of osteoblastic-like properties. Higher levels of mRNAs for type X collagen, bone Gla protein, and osteopontin were found at nonpermissive temperatures, suggesting that the expression of these genes was upregulated upon growth arrest, as is the case in vivo during chondrocyte hypertrophy. Cells also retained their ability to respond to retinoic acid, as indicated by retinoic acid dose-dependent and time-dependent increases in type X collagen mRNA levels. These cell lines, the first to express characteristic features of hypertrophic chondrocytes, should be very useful to study the regulation of the type X collagen gene and other genes activated during the last stages of chondrocyte differentiation.

Osteoblasts are target cells for transformation in c-fos transgenic mice

We have generated transgenic mice expressing the proto-oncogene c-fos from an H-2Kb class I MHC promoter as a tool to identify and isolate cell populations which are sensitive to altered levels of Fos protein. All homozygous H2-c-fosLTR mice develop osteosarcomas with a short latency period. This phenotype is specific for c-fos as transgenic mice expressing the fos- and jun-related genes, fosB and c-jun, from the same regulatory elements do not develop any pathology despite high expression in bone tissues. The c-fos transgene is not expressed during embryogenesis but is expressed after birth in bone tissues before the onset of tumor formation, specifically in putative preosteoblasts, bone-forming osteoblasts, osteocytes, as well as in osteoblastic cells present within the tumors. Primary and clonal cell lines established from c-fos-induced tumors expressed high levels of exogenous c-fos as well as the bone cell marker genes, type I collagen, alkaline phosphatase, and osteopontin/2ar. In contrast, osteocalcin/BGP expression was either low or absent. All cell lines were tumorigenic in vivo, some of which gave rise to osteosarcomas, expressing exogenous c-fos mRNA, and Fos protein in osteoblastic cells. Detailed analysis of one osteogenic cell line, P1, and several P1-derived clonal cell lines indicated that bone-forming osteoblastic cells were transformed by Fos. The regulation of osteocalcin/BGP and alkaline phosphatase gene expression by 1,25-dihydroxyvitamin D3 was abrogated in P1-derived clonal cells, whereas glucocorticoid responsiveness was unaltered. These results suggest that high levels of Fos perturb the normal growth control of osteoblastic cells and exert specific effects on the expression of the osteoblast phenotype.