Fra-1 potentiates osteoclastic differentiation in osteoclast-macrophage precursor cell lines

The Fra-1: Novel role in regulating extensive immune cell states and affecting inflammatory diseases

Fra-1(Fos-related antigen1), a member of transcription factor activator protein (AP-1), plays an important role in cell proliferation, apoptosis, differentiation, inflammation, oncogenesis and tumor metastasis. Accumulating evidence suggest that the malignancy and invasive ability of tumors can be significantly changed by directly targeting Fra-1. Besides, the effects of Fra-1 are gradually revealed in immune and inflammatory settings, such as arthritis, pneumonia, psoriasis and cardiovascular disease. These regulatory mechanisms that orchestrate immune and non-immune cells underlie Fra-1 as a potential therapeutic target for a variety of human diseases. In this review, we focus on the current knowledge of Fra-1 in immune system, highlighting its unique importance in regulating tissue homeostasis. In addition, we also discuss the possible critical intervention strategy in diseases, which also outline future research and development avenues.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

E proteins regulate osteoclast maturation and survival

Osteoclasts are bone-specific polykaryons derived from myeloid precursors under the stimulation of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). E proteins are basic helix-loop-helix (bHLH) transcription factors that modulate lymphoid versus myeloid cell fate decisions. To study the role of E proteins in osteoclasts, myeloid-specific E protein gain-of-function transgenic mice were generated. These mice have high bone mass due to decreased osteoclast numbers and increased osteoclast apoptosis leading to overall reductions in resorptive capacity. The molecular mechanism of decreased osteoclast numbers and resorption is in part a result of elevated expression of CD38, a regulator of intracellular calcium pools with known antiosteoclastogenic properties, which increases sensitivity to apoptosis. In vivo, exogenous RANKL stimulation can overcome this inhibition to drive osteoclastogenesis and bone loss. In vitro-derived ET2 osteoclasts are more spread and more numerous with increases in RANK, triggering receptor expressed on myeloid cells 2 (TREM2), and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) compared to wild type. However, their resorptive capacity does not increase accordingly. Thus, E proteins participate in osteoclast maturation and survival in homeostatic bone remodeling.

M-CSF potently augments RANKL-induced resorption activation in mature human osteoclasts

Macrophage-CSF (M-CSF) is critical for osteoclast (OC) differentiation and is reported to enhance mature OC survival and motility. However, its role in the regulation of bone resorption, the main function of OCs, has not been well characterised. To address this we analysed short-term cultures of fully differentiated OCs derived from human colony forming unit-granulocyte macrophages (CFU-GM). When cultured on dentine, OC survival was enhanced by M-CSF but more effectively by receptor activator of NFκB ligand (RANKL). Resorption was entirely dependent on the presence of RANKL. Co-treatment with M-CSF augmented RANKL-induced resorption in a concentration-dependent manner with a (200-300%) stimulation at 25 ng/mL, an effect observed within 4-6 h. M-CSF co-treatment also increased number of resorption pits and F-actin sealing zones, but not the number of OCs or pit size, indicating stimulation of the proportion of OCs activated. M-CSF facilitated RANKL-induced activation of c-fos and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, but not NFκB nor nuclear factor of activated T-cells, cytoplasmic-1 (NFATc1). The mitogen-activated protein kinase kinase (MEK) 1 inhibitor PD98059 partially blocked augmentation of resorption by M-CSF. Our results reveal a previously unidentified role of M-CSF as a potent stimulator of mature OC resorbing activity, possibly mediated via ERK upstream of c-fos.

Triiodothyronine (T3) does not induce Rankl expression in rat Ros 17/2.8 cells

Osteoclastogenesis may be regulated via activation of the RANK/RANKL (receptor activator of nuclear factor-kappa B/receptor activator of nuclear factor-kappa B ligand) system, which is mediated by osteoblasts. However, the bone loss mechanism induced by T3 (triiodothyronine) is still controversial. In this study, osteoblastic lineage rat cells (ROS 17/2.8) were treated with T3 (10(-8) M, 10(-9) M, and 10(-10) M), and RANKL mRNA (messenger RNA) expression was measured by semiquantitative RT-PCR. Our results show that T3 concentrations used did not significantly enhance RANKL expression compared to controls without hormone treatment. This data suggests that other mechanisms, unrelated to the RANK/RANKL system, might be to activate osteoclast differentiation in these cells.

Cartilage-selective genes identified in genome-scale analysis of non-cartilage and cartilage gene expression

Background

Cartilage plays a fundamental role in the development of the human skeleton. Early in embryogenesis, mesenchymal cells condense and differentiate into chondrocytes to shape the early skeleton. Subsequently, the cartilage anlagen differentiate to form the growth plates, which are responsible for linear bone growth, and the articular chondrocytes, which facilitate joint function. However, despite the multiplicity of roles of cartilage during human fetal life, surprisingly little is known about its transcriptome. To address this, a whole genome microarray expression profile was generated using RNA isolated from 18–22 week human distal femur fetal cartilage and compared with a database of control normal human tissues aggregated at UCLA, termed Celsius.

Results

161 cartilage-selective genes were identified, defined as genes significantly expressed in cartilage with low expression and little variation across a panel of 34 non-cartilage tissues. Among these 161 genes were cartilage-specific genes such as cartilage collagen genes and 25 genes which have been associated with skeletal phenotypes in humans and/or mice. Many of the other cartilage-selective genes do not have established roles in cartilage or are novel, unannotated genes. Quantitative RT-PCR confirmed the unique pattern of gene expression observed by microarray analysis.

Conclusion

Defining the gene expression pattern for cartilage has identified new genes that may contribute to human skeletogenesis as well as provided further candidate genes for skeletal dysplasias. The data suggest that fetal cartilage is a complex and transcriptionally active tissue and demonstrate that the set of genes selectively expressed in the tissue has been greatly underestimated.

Discriminate gene lists derived from cDNA microarray profiles of limited samples permit distinguishing mesenchymal neoplasia ex vivo

BACKGROUND

Mesenchymal neoplasia comprises a heterogeneous group of tumors with over 200 benign neoplasms and 100 sarcomas. Currently, tumors are classified using histologic and immunocytologic characteristics, with diagnostic error rates reported as high as 40% of cases. As a feasibility study, our goal was to generate a preliminary discriminatory gene list for selected mesenchymal tumors, including sarcomas. This technique may enable an eventual molecular classification schema based on expression profiles that can complement current clinical and pathologic diagnostic procedures in mesenchymal tumors.

METHODS

cDNA microarray analyses were preformed on connective tissue tumors obtained at time of surgical resection or biopsy. Messenger RNA (mRNA) from four general tumor classes was competitively hybridized against a human dermal fibroblast cell line comparator and the resulting gene expression profiles processed by ANOVA and linear discriminate analysis.

RESULTS

The tissue classification involved 18 patients with malignant peripheral nerve sheath tumors, giant cell containing tumors, benign spindle cell lesions, or Ewing's family of tumors. Lymph nodes from two patients served comparative purposes. Twenty-five differentially regulated genes considered most variable among the five tissue classes were identified. The tissues were segregated into five classes by linear discriminate analysis.

CONCLUSIONS

Linear discriminate analysis of cDNA gene expression profiles partitioned mesenchymal tumor classes, even when constrained by limited sample sizes.

Serotonin regulates osteoclast differentiation through its transporter

5-HTT mediates antidepressant-sensitive clearance of 5-HT after its release into neural synapses. We found increased expression of 5-HTT in RANKL-induced osteoclast-like cells. Fluoxetine, an inhibitor of 5-HTT, reduced osteoclast differentiation but not activation. Reserpine, an inhibitor of 5-HT intracellular transport, potentiated differentiation. These results indicate a role for 5-HTT in osteoclast function and suggest that commonly used antidepressive agents may affect bone mass.

INTRODUCTION

Interactions between the serotonergic and skeletal systems are suggested by various clinical observations but are poorly understood.

MATERIALS AND METHODS

Using gene microarrays, we found that the serotonin transporter (5-HTT) was strongly expressed in RANKL-induced osteoclasts. Using RANKL stimulation of RAW264.7 cells and mouse bone marrow cells as a model system for osteoclast differentiation, we studied the possible role/s of the different components of the serotonin (5-HT) system on the differentiation process.

RESULTS

Osteoclast 5-HTT exhibited typical 5-HT uptake activity that was inhibitable by fluoxetine (Prozac). Fluoxetine reduced osteoclast differentiation but did not inhibit the activation of preformed osteoclasts, whereas the addition of 5-HT itself enhanced differentiation. Fluoxetine-treated osteoclast precursors had reduced NF-kappa B activation and elevated inhibitory protein kappa B alpha (I kappa B alpha) levels compared with untreated cells. 5-HT, on the other hand, resulted in activation of NF-kappa B. Reserpine inhibition of intracellular transport of 5-HT into cytoplasmic vesicles potentiated RANKL-induced osteoclast formation, suggesting the importance of intracellular 5-HT in regulating osteoclast differentiation. Reserpine also modestly enhanced the expression of the osteoclast marker TRACP in the absence of RANKL.

CONCLUSIONS

Taken together, these data suggest that the 5-HT system plays an important role in bone homeostasis through effects on osteoclast differentiation and implies that commonly used antidepressive agents may affect bone mass.

The Fos-related antigen Fra-1 is an activator of bone matrix formation

Ectopic expression of the transcription factor Fra-1 in transgenic mice leads to osteosclerosis, a bone disorder characterized by increased bone mass. The molecular basis for this phenotype is unknown and Fra-1 functions cannot be studied by a conventional loss-of-function approach, since fra-1-knockout mice die in utero likely due to placental defects. Here we show that the lethality of fra-1-knockout mice can be rescued by specific deletion of Fra-1 only in the mouse embryo and not in the placenta. Mice lacking Fra-1 (fra-1(delta/delta)) are viable and develop osteopenia, a low bone mass disease. Long bones of fra-1(delta/delta) mice appear to have normal osteoclasts but express reduced amounts of bone matrix components produced by osteoblasts and chondrocytes such as osteocalcin, collagen1a2 and matrix Gla protein. The gene for matrix Gla protein seems to be a specific target of Fra-1 since its expression was markedly increased in the long bones of fra-1-transgenic mice. These results uncover a novel function of Fra-1 in regulating bone mass through bone matrix production by osteoblasts and chondrocytes.

The calcineurin/nuclear factor of activated T cells signaling pathway regulates osteoclastogenesis in RAW264.7 cells

Although best known for its role in T lymphocyte activation, the calcineurin/nuclear factor of activated T cells (NFAT) signaling pathway is also known to be involved in a wide range of other biological responses in a variety of different cell types. Here we have investigated the role of the calcineurin/NFAT signaling pathway in the regulation of osteoclast differentiation. Osteoclasts are bone-resorbing multinucleated cells that are derived from the monocyte/macrophage cell lineage after stimulation with a member of the tumor necrosis factor family of ligands known as receptor activator of nuclear factor-kappaB ligand (RANKL). We now report that inhibition of calcineurin with either the immunosuppressant drugs cyclosporin A and FK506, or the retrovirally mediated ectopic expression of a specific calcineurin inhibitory peptide, all potently inhibit the RANKL-induced differentiation of the RAW264.7 monocyte/macrophage cell line into mature multinucleated osteoclasts. In addition, we find that NFAT family members are expressed in RAW264.7 cells and that their expression is up-regulated in response to RANKL stimulation. Most importantly, we find that ectopic expression of a constitutively active, calcineurin-independent NFATc1 mutant in RAW264.7 cells is sufficient to induce these cells to express an osteoclast-specific pattern of gene expression and differentiate into morphologically distinct, multinucleated osteoclasts capable of inducing the resorption of a physiological mineralized matrix substrate. Taken together, these data define calcineurin as an essential downstream effector of the RANKL-induced signal transduction pathway leading toward the induction of osteoclast differentiation and furthermore, indicate that the activation of the NFATc1 transcription factor is sufficient to initiate a genetic program that results in the specification of the mature functional osteoclast cell phenotype.

Thyroid hormone stimulates osteoclast differentiation by a mechanism independent of RANKL-RANK interaction

It is well known that thyroid hormone excess causes bone loss. However, the precise mechanism of bone loss by thyroid hormone still remains unclear. When T(3) was added to unfractionated bone cells after degeneration of pre-existent osteoclasts, T(3) (1 pM-100 nM) dose-dependently stimulated osteoclast-like cell formation, irrespective of the presence of indomethacin and IL-6 Ab. T(3) increased the expression of osteoprotegerin (OPG) messenger RNA (mRNA), but not of receptor activator of nuclear factor kappaB ligand (RANKL) in unfractionated bone cells, suggesting that the stimulatory effect of T(3) on osteoclast formation was not mediated by the RANKL/OPG system. We next examined the direct effect of T(3) on osteoclast precursors in the absence of osteoblasts, using hemopoietic blast cells derived from spleen cells. T(3) (1 pM-100 nM) dose-dependently stimulated osteoclast-like cell formation from osteoclast precursors. OPG did not inhibit T(3)-induced osteoclast formation from osteoclast precursor cells. The polymerase chain reaction (PCR) product corresponding in size to the mouse T(3) receptor alpha1 cDNA was detected in osteoclast precursors from mouse hemopoietic blast cells as well as mouse heart and mouse osteoblastic cell line MC3T3-E1 cells, suggesting that T(3) directly stimulated osteoclast-like cell formation from osteoclast precursors in the absence of osteoblasts. Further, T(3) increased the expression of c-Fos mRNA at 15 min and 24 h and Fra-1 mRNA at 2 and 6 h in osteoclast precursors. Consistent with the increased expression of c-Fos mRNA observed by RT-PCR, the activation of c-Fos occurred in osteoclast precursor cells stimulated by T(3), while the activation of neither NF-kappaB nor MAPKs was observed by immunoblot analysis. Antisense oligodeoxynucleotides (as-ODN) complementary to c-Fos mRNA at 1 microM significantly inhibited T(3)-induced osteoclast-like cell formation from osteoclast precursors in the absence of stromal cells while sense-ODN did not affect T(3)-induced osteoclast-like cell formation. These results indicate that T(3) directly stimulates osteoclast differentiation at least in part by up-regulation of c-fos protein in osteoclast precursor cells.

Functions of AP1 (Fos/Jun) in bone development

Genetically modified mice and cells have provided important insights into the biological functions of the dimeric transcription factor complex AP1, in particular into its role in skeletal development. Data obtained from knockout mice revealed that some components, such as c-Fos are key regulators of bone cell differentiation, whereas others, like c-Jun, JunB and Fra-1 are essential in embryonic and/or postnatal development. Apart from identifying the specific roles of AP1 proteins in developmental processes, researchers are beginning to obtain a better molecular understanding of their cell-context dependent functions, their downstream target genes and how they regulate bone cell proliferation, differentiation, and apoptosis.

Reaching a genetic and molecular understanding of skeletal development

In the last ten years, we have made considerable progress in our genetic and molecular understanding of all aspects of skeletal development, chondrogenesis, joint formation, and osteogenesis. This review addresses the role of the principal growth factors and transcription factors affecting these different processes and presents, in several cases, the genetic cascade leading to cell differentiation.

The reprogrammed host: Chlamydia trachomatis-induced up-regulation of glycoprotein 130 cytokines, transcription factors, and antiapoptotic genes

OBJECTIVE

Infection with Chlamydia trachomatis is a known cause of sexually transmitted diseases, eye infections (including trachoma), and reactive arthritis (ReA). Because the mechanisms of Chlamydia-induced changes leading to ReA are poorly defined, this study sought to identify the target genes involved at the molecular level.

METHODS

Chlamydia-induced changes in host cells were investigated by combining a screening technique, which utilized complementary DNA arrays on C trachomatis-infected and mock-infected epithelial HeLa cells, with real-time reverse transcription-polymerase chain reaction or enzyme-linked immunosorbent assay of gene products. Some responses were additionally demonstrated on human primary chondrocytes and a human synovial fibroblast cell line, both of which served as model cells for ReA.

RESULTS

Eighteen genes (of 1,176) were found to be up-regulated after 24 hours of infection with this obligate intracellular bacterium, among them the glycoprotein 130 family members IL-11 and LIF, the chemokine gene MIP2-alpha, the transcription factor genes EGR1, ETR101, FRA1, and c-jun, the apoptosis-related genes IEX-1L and MCL-1, adhesion molecule genes such as ICAM1, and various other functionally important genes. In the context of this rheumatic disease, the cytokines and transcription factors seem to be especially involved, since various connections to chondrocytes, synoviocytes, bone remodeling, joint pathology, and other rheumatic diseases have been demonstrated.

CONCLUSION

Infection with C trachomatis seems to reprogram the host cells (independent of activation by lipopolysaccharide or other ultraviolet-resistant bacterial components) at various key positions that act as intra- or intercellular switches, suggesting that these changes and similar Chlamydia-induced functional alterations constitute an important basis of the pathogenic inflammatory potential of these cells in ReA. Our results suggest that this approach is generally useful for the broad analysis of host-pathogen interactions involving obligate intracellular bacteria, and for the identification of target genes for therapeutic intervention in this rheumatic disease.

Carbonic anhydrase II is an AP-1 target gene in osteoclasts

c-Fos, a member of the AP-1 family of transcription factors, is necessary for osteoclast differentiation but to date, none of the osteoclast-phenotypic markers have been identified as AP-1 target genes. Here, we demonstrate that carbonic anhydrase II (CA II), an enzyme necessary for osteoclast activity, is transcriptionally upregulated by c-Fos/AP-1. A functional AP-1 binding site is present in the CA II promoter and is necessary for this regulation. Furthermore, we show that AP-1 binding activity, mainly composed of Fra-2 and JunD, is induced by treatment of bone marrow cultures with the osteoclastogenic hormone 1,25 dihydroxyvitamin D(3). Fra-2 and JunD are found in mature osteoclasts as well. Thus, our data demonstrate that cFos/AP-1 can directly regulate the expression of this osteoclast marker and that AP-1 activity is upregulated in osteoclast progenitors in response to osteoclastogenic signals.

AP-1 in mouse development and tumorigenesis

Genetically modified mice have provided important insights into the biological functions of the dimeric transcription factor complex AP-1. Extensive analyses of mice and cells with genetically modified Fos or Jun proteins provide novel insights into the physiological functions of AP-1 proteins. Using knock-out strategies it was found that some components, such as c-Fos, FosB and JunD are dispensable, whereas others, like c-Jun, JunB and Fra-1 are essential in embryonic development and/or in the adult organism. Besides the specific roles of AP-1 proteins in developmental processes, we are beginning to obtain a better molecular understanding of the cell-context dependent function of AP-1 in cell proliferation and apoptosis, in bone biology as well as in multistep tumorigenesis.

Placental vascularisation requires the AP-1 component fra1

Fra1 is an immediate-early gene encoding a member of the AP-1 transcription factor family, which has diverse roles in development and oncogenesis. To determine the function of Fra1 in mouse development, the gene was inactivated by gene targeting. Foetuses lacking Fra1 were severely growth retarded and died between E10.0 and E10.5, owing to defects in extra-embryonic tissues. The placental labyrinth layer, where X-gal staining revealed expression of Fra1, was reduced in size and largely avascular, owing to a marked decrease in the number of vascular endothelial cells, as shown by the lack of Flk1 expression. In contrast, the spongiotrophoblast layer was unaffected and expressed the marker genes 4311 (Tpbp) and Flt1. Furthermore, mutant foetuses exhibited yolk-sac defects that may contribute to their growth retardation and lethality. Importantly, when the placental defect was rescued by injection of Fra1(−)(/)(−) ES cells into tetraploid wild-type blastocysts, Fra1(−)(/)(−) pups were obtained that were no longer growth retarded and survived up to 2 days after birth without apparent phenotypic defects. These data indicate that a defect in the extra-embryonic compartment is causal to the observed lethality, and suggest that Fra1 plays a crucial role in establishing normal vascularisation of the placenta.

Increased bone formation and osteosclerosis in mice overexpressing the transcription factor Fra-1

Bone formation by osteoblasts is essential for skeletal growth and remodeling. Fra-1 is a c-Fos-related protein belonging to the AP-1 family of transcription factors. Here we show that transgenic mice overexpressing Fra-1 in various organs develop a progressive increase in bone mass leading to osteosclerosis of the entire skeleton, which is due to a cell-autonomous increase in the number of mature osteoblasts. Moreover, osteoblast differentiation, but not proliferation, was enhanced and osteoclastogenesis was also elevated in vitro. These data indicate that, unlike c-Fos, which causes osteosarcomas, Fra-1 specifically enhances bone formation, which may be exploited to stimulate bone formation in pathological conditions.

Osteoclast lineage commitment of bone marrow precursors through expression of membrane-bound TRANCE

Osteoclast formation from hemopoietic precursors is induced by TRANCE (also called RANKL, ODF, and OPGL), a membrane-bound ligand expressed by bone marrow stromal cells. Because soluble recombinant TRANCE is a suboptimal osteoclastogenic stimulus, and to eliminate the need for such dependence on stromal cells, membrane-bound TRANCE was expressed in hematopoietic precursors using retroviral gene transfer. Four TRANCE-expressing osteoclast cell lines were established that continuously generate large numbers of multinucleated cells and express tartrate-resistant acid phosphatase and calcitonin receptors. The multinuclear cells are long-lived and either fuse continuously with each other and with mononuclear cells to form enormous syncytia, or separate to form daughter multinuclear cells. When formed on bone, but not on plastic, the majority of multinuclear cells develop actin rings on bone, and resorb bone, suggesting that bone matrix may provide additional signals that facilitate osteoclastic functional maturation. Surprisingly, multinuclear cells originate from fusion of proliferating mononuclear cells that strongly express the mature macrophage markers F4/80 and Fc receptor, which are not expressed by osteoclasts. These results indicate that osteoclasts can be derived from F4/80-positive and Fc receptor-positive cells, and that TRANCE induces osteoclastic differentiation partly by suppressing the macrophage phenotype.

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.

Fosl1 is a transcriptional target of c-Fos during osteoclast differentiation

Osteoclasts are bone-resorbing cells derived from haematopoietic precursors of the monocyte-macrophage lineage. Mice lacking Fos (encoding c-Fos) develop osteopetrosis due to an early differentiation block in the osteoclast lineage. c-Fos is a component of the dimeric transcription factor activator protein-1 (Ap-1), which is composed mainly of Fos (c-Fos, FosB, Fra-1 and Fra-2) and Jun proteins (c-Jun, JunB and JunD). Unlike Fra-1 (encoded by Fosl1), c-Fos contains transactivation domains required for oncogenesis and cellular transformation. The mechanism by which c-Fos exerts its specific function in osteoclast differentiation is not understood. Here we show by retroviral-gene transfer that all four Fos proteins, but not the Jun proteins, rescue the differentiation block in vitro. Structure-function analysis demonstrated that the major carboxy-terminal transactivation domains of c-Fos and FosB are dispensable and that Fra-1 (which lacks transactivation domains) has the highest rescue activity. Moreover, a transgene expressing Fra-1 rescues the osteopetrosis of c-Fos-mutant mice in vivo. The osteoclast differentiation factor Rankl (also known as TRANCE, ODF and OPGL; refs 8-11) induces transcription of Fosl1 in a c-Fos-dependent manner, thereby establishing a link between Rank signalling and the expression of Ap-1 proteins in osteoclast differentiation.