Regulation of osteosarcoma cell lung metastasis by the c-Fos/AP-1 target FGFR1.

Osteosarcoma is the most common primary malignancy of the skeleton and is prevalent in children and adolescents. Survival rates are poor and have remained stagnant owing to chemoresistance and the high propensity to form lung metastases. In this study, we used in vivo transgenic models of c-fos oncogene-induced osteosarcoma and chondrosarcoma in addition to c-Fos-inducible systems in vitro to investigate downstream signalling pathways that regulate osteosarcoma growth and metastasis. Fgfr1 (fibroblast growth factor receptor 1) was identified as a novel c-Fos/activator protein-1(AP-1)-regulated gene. Induction of c-Fos in vitro in osteoblasts and chondroblasts caused an increase in Fgfr1 RNA and FGFR1 protein expression levels that resulted in increased and sustained activation of mitogen-activated protein kinases (MAPKs), morphological transformation and increased anchorage-independent growth in response to FGF2 ligand treatment. High levels of FGFR1 protein and activated pFRS2α signalling were observed in murine and human osteosarcomas. Pharmacological inhibition of FGFR1 signalling blocked MAPK activation and colony growth of osteosarcoma cells in vitro. Orthotopic injection in vivo of FGFR1-silenced osteosarcoma cells caused a marked twofold to fivefold decrease in spontaneous lung metastases. Similarly, inhibition of FGFR signalling in vivo with the small-molecule inhibitor AZD4547 markedly reduced the number and size of metastatic nodules. Thus deregulated FGFR signalling has an important role in osteoblast transformation and osteosarcoma formation and regulates the development of lung metastases. Our findings support the development of anti-FGFR inhibitors as potential antimetastatic therapy.

Root and Eruption Defects in c-Fos Mice Are Driven by Loss of Osteoclasts.

c-Fos homozygous mice lack osteoclasts with a failure of the teeth to erupt and with an arrest of root development. Here, we characterize the defects associated with the failure in root development and the loss of the tooth-bone interface, and we investigate the underlying causes. We show that, while homozygous c-Fos mice have no multinucleated osteoclasts, heterozygous mice have a reduction in the number of osteoclasts with a reduction in the tooth-bone interface during development and subtle skeletal defects postnatally. In the homozygous mutants bone is found to penetrate the tooth, particularly at the apical end, physically disrupting the root forming HERS (Hertwig's epithelial root sheath) cells. The cells of the HERS continue to proliferate but cannot extend downward due to the presence of bone, leading to a loss of root formation. Tooth germ culture showed that the developing tooth invaded the static bone in mutant tissue, rather than the bone encroaching on the tooth. Although c-Fos has been shown to be expressed in developing teeth, the defect in maintenance of the tooth-bone interface appears to be driven solely by the lack of osteoclasts, as this defect can be rescued in the presence of donor osteoclasts. The rescue suggests that signals from the tooth recruit osteoclasts to clear the bone from around the tooth, allowing the tooth to grow, form roots, and later erupt.

Mesenchymal differentiation propensity of a human embryonic stem cell line.

OBJECTIVES: To characterize basal differentiation tendencies of a human embryonic stem (hES) cell line, KCL-002. MATERIALS AND METHODS: In vitro specification and differentiation of hES cells were carried out using embryoid body (EB) cultures and tests of pluripotency and in vivo differentiation were performed by teratoma assays in SCID mice. Real-time PCR, immunohistochemistry, flow cytometry and histological analyses were used to identify expression of genes and proteins associated with the ectodermal, endodermal and mesodermal germ layers. RESULTS: Undifferentiated KCL-002 cells expressed characteristic markers of pluripotent stem cells such as Nanog, Sox-2, Oct-4 and TRA 1-60. When differentiated in vitro as EB cultures, expression of pluripotency, endodermal and ectodermal markers decreased rapidly. In contrast, mesodermal and mesenchymal markers such as VEGFR-2, α-actin and vimentin increased during EB differentiation as shown by qPCR, immunostaining and flow cytometric analyses. Teratoma formation in SCID mice demonstrated the potential to form all germ layers in vivo with a greater proportion of the tumours containing mesenchymal derivatives. CONCLUSIONS: The data presented suggest that the KCL-002 hES cell line is pluripotent and harbours a bias in basal differentiation tendencies towards mesodermal and mesenchymal lineage cells. Characterizing innate differentiation propensities of hES cell lines is important for understanding heterogeneity between different cell lines and for further studies aimed at deriving specific lineages from hES cells.

Dlx homeobox gene family expression in osteoclasts.

Skeletal growth and homeostasis require the finely orchestrated secretion of mineralized tissue matrices by highly specialized cells, balanced with their degradation by osteoclasts. Time- and site-specific expression of Dlx and Msx homeobox genes in the cells secreting these matrices have been identified as important elements in the regulation of skeletal morphology. Such specific expression patterns have also been reported in osteoclasts for Msx genes. The aim of the present study was to establish the expression patterns of Dlx genes in osteoclasts and identify their function in regulating skeletal morphology. The expression patterns of all Dlx genes were examined during the whole osteoclastogenesis using different in vitro models. The results revealed that Dlx1 and Dlx2 are the only Dlx family members with a possible function in osteoclastogenesis as well as in mature osteoclasts. Dlx5 and Dlx6 were detected in the cultures but appear to be markers of monocytes and their derivatives. In vivo, Dlx2 expression in osteoclasts was examined using a Dlx2/LacZ transgenic mouse. Dlx2 is expressed in a subpopulation of osteoclasts in association with tooth, brain, nerve, and bone marrow volumetric growths. Altogether the present data suggest a role for Dlx2 in regulation of skeletal morphogenesis via functions within osteoclasts.

Changes in RANKL/OPG/RANK gene expression in peripheral mononuclear cells following treatment with estrogen or raloxifene.

The RANKL/OPG/RANK pathway is the key mediator of osteoclastogenesis. Mononuclear cells may be implicated in post-menopausal osteoporosis. The effect of estrogen or raloxifene on bone resorption and the expression of RANKL/OPG/RANK in peripheral blood mononuclear cells (PBMCs) was examined. Twenty-nine women with post-menopausal osteoporosis were treated with estrogen (HRT) or raloxifene for 12 months. Bone mineral density (BMD) was measured at baseline and at 12 months at the spine and hip. Serum C-terminal telopeptide (CTX) and OPG were measured at baseline and at 1, 3, 6 and 12 months. PBMCs were isolated from 17 women and changes in RANKL, OPG and RANK mRNA were determined. The effects of estrogen or raloxifene in PBMCs in vitro were also assessed. BMD increased following treatment (lumbar spine % change mean [S.E.M.]: 4.3% [0.9], p<0.001). Serum CTX decreased (6 months: -43.7% [6.0], p<0.0001). Serum OPG declined gradually (12 months: -26.4% [4.4], p<0.001). RANKL, OPG and RANK gene expression decreased (6 months: RANKL 50.0% [24.8] p<0.001, OPG: 21.7% [28] p<0.001, RANK: 76.6% [10.2] p=0.015). Changes in OPG mRNA correlated with changes in BMD (r=-0.53, p=0.027) and CTX (r=0.7, p=0.0044). Down-regulation in RANKL, OPG, RANK mRNA and reduction in bone resorption was also seen in vitro. These results suggest that the expression of RANKL/OPG/RANK in PBMCs are responsive to the slowing in bone turnover/remodeling associated with treatment with estrogen or raloxifene. Further confirmatory studies are needed.

Pasteurella multocida toxin: the mitogenic toxin that stimulates signalling cascades to regulate growth and differentiation.

Pasteurella multocida toxin (PMT) is an unusual toxin that acts as a mitogen by stimulating various intracellular signalling cascades. Pathways downstream of the G-protein Gq and also downstream of the Rho proteins are activated. Thus PMT action stimulates phospholipase C leading to activation of protein kinase C, an increase in inositol phosphates, and a rise in intracellular calcium. Rho activation of the Rho kinase leads to cytoskeletal reorganisation, tyrosine phosphorylation of the focal adhesion kinase, and activation of the Src proto-oncogene. In addition, signalling through the Ras-MAP kinase signalling pathway is also initiated. PMT is an intracellularly acting toxin, and functional domains that carry out different aspects of its function have been described. The intracellular target of the toxin is currently not known. PMT also acts to inhibit differentiation, in particular of bone cells, where it prevents the formation of mineralised bone nodules in vitro. The toxin is the causative agent of a porcine disease that is characterised by bone resorption. Injection of very low doses of toxin leads to proliferative effects, but at higher doses is lethal. The possible effect of PMT-induced perturbation of signal transduction pathways is discussed.

Expression of collagenase-3 (MMP-13) in c-fos-induced osteosarcomas and chondrosarcomas is restricted to a subset of cells of the osteo-/chondrogenic lineage.

The interstitial collagenases have been suggested to play a critical role in bone formation, remodeling, and cancerogenesis. We have previously shown that during mouse development expression of collagenase-3 (MMP-13) is restricted to bone and cartilage (Gack et al., 1995; Tuckermann et al., 2000) and is affected in mice with altered c-Fos and Cbfa-1 expression (Gack et al., 1994; Porte et al., 1999). In this study, using immunohistochemistry (IHC) and in situ hybridization (ISH) techniques, we have identified cells of the osteoblastic lineage to be the origin of strongly enhanced levels of MMP-13 transcripts in c-fos-induced osteosarcomas. Expression in these cells is further increased in c-fos/c-jun double transgenic mice and paralleled by Cbfa-1 expression. Similarly, in spontaneous and radiation-induced osteosarcomas, both c-Fos and MMP-13 proteins are detectable, suggesting that overexpression of both genes is a characteristic feature of osteosarcomas of different origin. We also observed high levels of MMP-13 in c-Fos-induced chondrosarcomas. In osteoblast-like cells and in cells of late chondrocyte differentiation such as hypertrophic chondrocytes, high levels of MMP-13 transcripts were found. In contrast, in anaplastic areas of the tumors representing highly proliferating chondrocytes, no MMP-13 expression is detectable, suggesting that in addition to Fos/AP-1, bone-specific transcription factors are responsible for restricted expression of collagenase-3/MMP-13 in a specific subset of cells of bone and cartilage in physiology and pathology.

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.

A putative role for c-Fos in the pathophysiology of Paget's disease.

The molecular mechanisms underlying Paget's disease and subsequent osteosarcoma formation are not well understood. In this study, we aim to delineate the function of the c-Fos oncogene in Paget's disease using transgenic mice, based on previous findings that c-Fos is highly expressed in Pagetic osteoclasts and that c-Fos is an essential gene for osteoclast differentiation and skeletal neoplasia. We have generated transgenic mice in which c-Fos is overexpressed specifically in osteoclasts using the tartrate-resistant acid phosphatase (TRAP) promoter, and five founder mice have been identified. All transgene-expressing animals developed severe bone remodeling lesions, some of which progressed to large bone tumors. Histopathologic analysis indicated that the lesions contained a marked increase in the number of osteoclasts that contained a large number of nuclei. Osteoclasts were identified by histochemical staining for TRAP and by in situ hybridization for matrix metalloproteinase-9 (MMP-9) expression. Moreover, transgenic osteoclasts, and in some cases, osteoblasts and chondrocytes, expressed high levels of c-Fos protein as judged by immunocytochemistry. This phenotype of increased osteoclast number and activity, together with an apparently high rate of bone turnover, resembles some characteristics of Paget's disease. These data therefore support an important function for c-Fos in the Pagetic phenotype, and further support the notion that this gene is important in osteoclastogenesis and in bone remodeling disorders.

Control of cell cycle gene expression in bone development and during c-Fos-induced osteosarcoma formation.

We have used c-Fos transgenic mice which develop osteosarcomas to determine the expression patterns of cyclins, cyclin-dependent kinases (CDKs), and cyclin-dependent kinase inhibitors (CKIs) in different bone cell populations in order to define the potential mechanisms of c-Fos transformation. Immunohistochemical analysis in embryonic and early postnatal bone demonstrated that cyclin E and its kinase partner CDK2 were expressed specifically in bone-forming osteoblasts. Cyclin D1 expression was absent despite high levels of CDK4 and CDK6, and the CKI p27 was expressed in chondrocytes, osteoclasts, and at lower levels in osteoblasts. Following activation of the c-fos transgene in vivo and before overt tumor formation, cyclin D1 expression increased dramatically and was colocalized with exogenous c-Fos protein specifically in osteoblasts and chondrocytes, but not in osteoclasts. Prolonged activation of c-Fos resulted in osteosarcoma formation wherein the levels of cyclin D1, cyclin E, and CDKs 2, 4, and 6 were high in a wide spectrum of malignant cell types, especially in transformed osteoblasts. The CKI p27 was expressed at very high levels in bone-resorbing osteoclasts, and to a lesser extent in chondrocytes and osteoblasts. These in vivo observations suggest that cyclin D1 may be a target for c-Fos action and that elevation of cyclin D1 in osteoblasts which already express cyclin E/CDK2 and the cyclin D1 partners CDKs-4 and 6, may predispose cells to uncontrolled cell growth leading to osteosarcoma development. This study implicates altered cell cycle control as a potential mechanism through which c-Fos causes osteoblast transformation and bone tumor formation.

Embryonic overexpression of the c-Fos protooncogene. A murine stem cell chimera applicable to the study of fibrodysplasia ossificans progressiva in humans.

Murine embryonic overexpression of the c-fos protooncogene leads to early postnatal heterotopic chondrogenesis and osteogenesis with phenotypic features similar to those seen in children who have the disabling heritable disease fibrodysplasia ossificans progressiva. The overexpression of Fos in embryonic stem cell chimeras leads to heterotopic endochondral osteogenesis at least in part through a bone morphogenetic protein 4 mediated signal transduction pathway. In contrast, early fibrodysplasia ossificans progressiva lesions express abundant bone morphogenetic protein 4, without abundant expression of c-Fos, suggesting that the primary molecular defect in fibrodysplasia ossificans progressiva may be independent of the sustained Fos effects on chondrogenesis and osteogenesis. Comparisons of the clinical, molecular, and pathogenetic features of the c-Fos embryonic stem cell chimeras with those of fibrodysplasia ossificans progressiva provide insight into the earliest events in the molecular pathogenesis of genetically induced heterotopic chondrogenesis and osteogenesis. The relevance of the c-Fos embryonic stem cell chimera to the study of the currently untreatable human disease fibrodysplasia ossificans progressiva demonstrates the power of using embryonic stem cell technology for generating gain of function mutations in the study of human bone disease.

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.

c-fos-induced osteosarcoma formation in transgenic mice: cooperativity with c-jun and the role of endogenous c-fos.

Transgenic mice overexpressing the c-fos proto-oncogene in bone develop osteosarcomas, whereas mice overexpressing c-Jun are normal. In this study, we investigated whether Fos and Jun would cooperate in vivo and whether the threshold levels of Fos are important in osteosarcoma formation. Fos-Jun double-transgenic mice develop osteosarcomas at a higher frequency than single-Fos transgenic mice with no differences in the time of onset of tumor formation. Histological and histochemical analyses indicated that Fos-Jun tumors contained greater quantities of neoplastic bone, were more remodeled, and contained a greater number of multinucleated osteoclast-like cells than tumors isolated from age-matched, single transgenic littermates. In contrast, overexpression of Fos in knockout mice that lack endogenous Fos resulted in a decrease in the number of tumor-bearing mice; osteosarcomas were almost absent in c-fos -/- mice, whereas tumor incidence was reduced to approximately 50% in c-fos +/- mice. Cell lines isolated from Fos-Jun transgenic tumors expressed high levels of both transgenes but significantly lower levels of the jun-related gene junB compared with cells expressing only a c-fos transgene. Osteoblastic marker genes were expressed at varying levels in different cell lines, but expression of interstitial collagenase (matrix metalloproteinase-1) was enhanced in cells derived from Fos-Jun tumors. These studies demonstrate that coexpression of a c-jun transgene can enhance Fos-induced oncogenesis in vivo and suggest that a critical level of Fos is necessary for osteosarcoma development.

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.

c-Fos: a key regulator of osteoclast-macrophage lineage determination and bone remodeling.

Mice lacking the proto-oncogene c-fos develop the bone disease osteopetrosis. Fos mutant mice were found to have a block in the differentiation of bone-resorbing osteoclasts that was intrinsic to hematopoietic cells. Bone marrow transplantation rescued the osteopetrosis, and ectopic c-fos expression overcame this differentiation block. The lack of Fos also caused a lineage shift between osteoclasts and macrophages that resulted in increased numbers of bone marrow macrophages. These results identify Fos as a key regulator of osteoclast-macrophage lineage determination in vivo and provide insights into the molecular mechanisms underlying metabolic bone diseases.

Mice lacking c-fos have normal hematopoietic stem cells but exhibit altered B-cell differentiation due to an impaired bone marrow environment.

Mice lacking c-fos develop severe osteopetrosis with deficiencies in bone remodeling and exhibit extramedullary hematopoiesis, thymic atrophy, and altered B-cell development. In this study, we have used these mice to characterize in detail the developmental potential of hematopoietic stem cells lacking c-fos and to analyze how the lymphoid differentiation is altered. In c-fos -/- mice, B-cell numbers are reduced in the spleen, lymph nodes, and the peripheral blood as a result of a marked reduction (> 90%) in the number of clonogenic B-cell precursors. In contrast, the number and lineage distribution of myeloid progenitor cells are not affected. The thymic defects observed in a large number of these mice correlate with their health status, suggesting that this may be an indirect effect of the c-fos mutation. In vitro differentiation and bone marrow reconstitution experiments demonstrated that hematopoietic stem cells lacking c-fos can give rise to all mature myeloid as well as lymphoid cells, suggesting that the observed B lymphopenia in the mutant mice is due to an altered environment. Transplantation of wild-type bone marrow cells into newborn mutant mice resulted in the establishment of a bone marrow space and subsequent correction of the B-cell defect. These results demonstrate that hematopoietic stem cells lacking Fos have full developmental potential and that the observed defect in B-cell development is most likely due to the impaired bone marrow environment as a consequence of osteopetrosis.

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.

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

This study describes the detailed characterization of four murine chondrogenic cell lines (wT2-1, wT2-7, wT2-8, and wT2-9) that were isolated from a cartilage tumor induced by the protooncogene c-fos in chimeric mice. All cell lines are clonal and display a fibroblastic morphology with a doubling time of 1-2 days. Northern blot analysis demonstrated that in addition to expressing high levels of exogenous c-fos, all clones express varying levels of the cartilage marker gene type II collagen in addition to type I collagen. The clones also expressed high levels of the AP-1 genes c-jun and fra-1. The doubling times of these clones did not change over a period of 14 months in culture. Most importantly, however, expression of type II collagen was maintained in all cell lines for 8 months in culture, and two cell lines maintained type II collagen expression when analyzed after 14 months. Interestingly, type I collagen expression was lost after long-term culture. Following injection into syngeneic and nude mice, all cell lines formed tumors containing areas with the morphologic appearance of hyaline cartilage, indicating that these cell lines are chondrogenic. Thus, these stable murine chondrogenic cell lines provide a useful tool for studying the transcriptional control of cartilage-specific gene expression, as well as the growth control of chondrogenic cells.

Bone and haematopoietic defects in mice lacking c-fos.

The proto-oncogene c-fos is the cellular homologue of v-fos originally isolated from murine osteosarcoma. Fos protein is a major component of the AP-1 transcription factor complex, which includes members of the jun family. Stable expression of c-fos in mice has been demonstrated in developing bones and teeth, haematopoietic cells, germ cells and in the central nervous system. It has been proposed that c-fos has an important role in signal transduction, cell proliferation and differentiation. We have previously demonstrated that overexpression of c-fos in transgenic and chimaeric mice specifically affects bone, cartilage and haematopoietic cell development. To understand better the function of c-fos in vivo, we used gene targeting in embryonic stem cells to generate cells and mice lacking c-fos. Here we report that heterozygous fos +/- mice appear normal, although females exhibit a distorted transmission frequency. All homozygous fos -/- mice are growth-retarded, develop osteopetrosis with deficiencies in bone remodelling and tooth eruption, and have altered haematopoiesis. These data define the c-Fos protein as an essential molecule for the development of specific cellular compartments.