WHIM Syndrome-linked CXCR4 mutations drive osteoporosis

WHIM Syndrome is a rare immunodeficiency caused by gain-of-function CXCR4 mutations. Here we report a decrease in bone mineral density in 25% of WHIM patients and bone defects leading to osteoporosis in a WHIM mouse model. Imbalanced bone tissue is observed in mutant mice combining reduced osteoprogenitor cells and increased osteoclast numbers. Mechanistically, impaired CXCR4 desensitization disrupts cell cycle progression and osteogenic commitment of skeletal stromal/stem cells, while increasing their pro-osteoclastogenic capacities. Impaired osteogenic differentiation is evidenced in primary bone marrow stromal cells from WHIM patients. In mice, chronic treatment with the CXCR4 antagonist AMD3100 normalizes in vitro osteogenic fate of mutant skeletal stromal/stem cells and reverses in vivo the loss of skeletal cells, demonstrating that proper CXCR4 desensitization is required for the osteogenic specification of skeletal stromal/stem cells. Our study provides mechanistic insights into how CXCR4 signaling regulates the osteogenic fate of skeletal cells and the balance between bone formation and resorption.

Chronic circadian disruption modulates breast cancer stemness and immune microenvironment to drive metastasis in mice

Breast cancer is the most common type of cancer worldwide and one of the major causes of cancer death in women. Epidemiological studies have established a link between night-shift work and increased cancer risk, suggesting that circadian disruption may play a role in carcinogenesis. Here, we aim to shed light on the effect of chronic jetlag (JL) on mammary tumour development. To do this, we use a mouse model of spontaneous mammary tumourigenesis and subject it to chronic circadian disruption. We observe that circadian disruption significantly increases cancer-cell dissemination and lung metastasis. It also enhances the stemness and tumour-initiating potential of tumour cells and creates an immunosuppressive shift in the tumour microenvironment. Finally, our results suggest that the use of a CXCR2 inhibitor could correct the effect of JL on cancer-cell dissemination and metastasis. Altogether, our data provide a conceptual framework to better understand and manage the effects of chronic circadian disruption on breast cancer progression.

Circadian disruption is implicated in the development of different human cancers. Here the authors show that chronic circadian disruption, through continuous jet lag, only moderately affects primary tumour growth but promotes cancer-cell dissemination and metastasis in a mouse model of spontaneous mammary tumorigenesis.

PiT1/Slc20a1 Is Required for Endoplasmic Reticulum Homeostasis, Chondrocyte Survival, and Skeletal Development

During skeletal mineralization, the sodium-phosphate co-transporter PiT1Slc20a1 is assumed to meet the phosphate requirements of bone-forming cells, although evidence is missing. Here, we used a conditional gene deletion approach to determine the role of PiT1 in growth plate chondrocytes. We show that PiT1 ablation shortly after birth generates a rapid and massive cell death in the center of the growth plate, together with an uncompensated endoplasmic reticulum (ER) stress, characterized by morphological changes and increased Chop, Atf4, and Bip expression. PiT1 expression in chondrocytes was not found at the cell membrane but co-localized with the ER marker ERp46, and was upregulated by the unfolded protein response cascade. In addition, we identified the protein disulfide isomerase (Pdi) ER chaperone as a PiT1 binding partner and showed that PiT1 ablation impaired Pdi reductase activity. The ER stress induced by PiT1 deficiency in chondrocytes was associated with intracellular retention of aggrecan and vascular endothelial growth factor A (Vegf-A), which was rescued by overexpressing a phosphate transport-deficient mutant of PiT1. Our data thus reveal a novel, Pi-transport independent function of PiT1, as a critical modulator of ER homeostasis and chondrocyte survival during endochondral ossification. © 2018 American Society for Bone and Mineral Research.

Calpain-6 controls the fate of sarcoma stem cells by promoting autophagy and preventing senescence

Sarcomas are still unsolved therapeutic challenges. Cancer stem cells are believed to contribute to sarcoma development, but lack of specific markers prevents their characterization and targeting. Here, we show that calpain-6 expression is associated with cancer stem cell features. In mouse models of bone sarcoma, calpain-6-expressing cells have unique tumor-initiating and metastatic capacities. Calpain-6 levels are especially high in tumors that have been successfully propagated in mouse to establish patient-derived xenografts. We found that calpain-6 levels are increased by hypoxia in vitro and calpain-6 is detected within hypoxic areas in tumors. Furthermore, calpain-6 expression depends on the stem cell transcription network that involves Oct4, Nanog, and Sox2 and is activated by hypoxia. Calpain-6 knockdown blocks tumor development in mouse and induces depletion of the cancer stem cell population. Data from transcriptomic analyses reveal that calpain-6 expression in sarcomas inversely correlates with senescence markers. Calpain-6 knockdown suppresses hypoxia-dependent prevention of senescence entry and also promotion of autophagic flux. Together, our results demonstrate that calpain-6 identifies sarcoma cells with stem-like properties and is a mediator of hypoxia to prevent senescence, promote autophagy, and maintain the tumor-initiating cell population. These findings open what we believe is a novel therapeutic avenue for targeting sarcoma stem cells.

Abstract IA2: Microenvironmental control of bone metastasis

Breast cancer is often associated with a poor prognosis largely due to its tendency to metastasize. Breast cancer preferentially metastasizes to the skeleton. Since Paget's seed and soil hypothesis from more than a century ago, it is known that bones constitute a very hospitable soil that attracts and allows breast cancer cells to thrive. Surprisingly, however, only a few studies have addressed the role of the bone microenvironment in bone metastasis, and most of them emphasized the role of osteoclasts (bone-destructing cells). Osteoclasts are known to stimulate bone metastasis because of their bone resorbing activity, which releases into the bone marrow space growth factors normally embedded in the bone matrix. On the other hand, the role of osteoblasts (bone-making cells) in this process has not been addressed directly. There is growing evidence that osteoblasts' function is not limited to making bone. In particular, these cells regulate various aspects of hematopoiesis, insulin secretion in the pancreas, or even male fertility in mice. We hypothesized that osteoblasts play an important role in breast cancer metastasis to bone. We tested this possibility by inoculating a breast cancer cell line (PyMT-FVB-Luc+ cells) into either the mammary fat pad, or directly into the blood stream of syngeneic recipient mice, which presented either an increased or a decreased osteoblast number. Hypoxia and the hypoxia inducible factor 1alpha (Hif-1alpha) play a fundamental role in bone by stimulating the differentiation of the osteoblasts in hypoxic niches where metastatic cells coming from distant tumors are often found. However, the role of the hypoxic microenvironment in bone metastasis is unknown. We used a conditional knockout approach to suppress either Hif-1alpha or von Hippel Lindau (VHL, an E3 ubiquitin-ligase that targets Hif-1alpha for degradation) specifically in osteoblasts (ΔHif-1alphaOB and ΔVHLOB mice). VHL suppression leads to ectopic expression of Hif-1alpha, and thus functions as a gain-of-function model for Hif-1alpha. Tissue specific ablation of Hif-1alpha and VHL in osteoblasts was performed by mating floxed Hif-1alpha or VHL mice, with mice expressing the Cre recombinase under the control of the osterix promoter, which is solely activated in immature osteoblasts. ΔHif-1alphaOB mice presented a decreased number of osteoblasts and reduced bone mass. Conversely, ΔVHLOB mice exhibited a dramatic increase in osteoblast number and bone mass. Intracardiac injection of PyMT-FVB-Luc+ cells into these loss- or gain-of-function models revealed that osteoblasts directly or indirectly stimulate breast cancer metastasis to bone. Strikingly, our results also show that osteoblasts produce systemic changes, controlling breast cancer metastasis to the lungs, as well as the growth of primary mammary tumors. These results were obtained by either transplanting PyMT-FVB-Luc+ cells into the mammary gland, or by tail vein (intravenous) injections. Thus, osteoblasts can affect distant tissues, well beyond the bone microenvironment. By providing the first evidence that the skeleton exerts a systemic control of breast cancer growth and dissemination, our study expands the biological importance of this organ.

Citation Format: Sylvain Provot, Audrey Brenot, Amy-Jo Casbon, Ying Yu, Zena Werb. Microenvironmental control of bone metastasis. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr IA2.

GATA3 suppresses metastasis and modulates the tumour microenvironment by regulating microRNA-29b expression

Despite advances in our understanding of breast cancer, patients with metastatic disease have poor prognoses. GATA3 is a transcription factor that specifies and maintains mammary luminal epithelial cell fate, and its expression is lost in breast cancer, correlating with a worse prognosis in human patients. Here, we show that GATA3 promotes differentiation, suppresses metastasis and alters the tumour microenvironment in breast cancer by inducing microRNA-29b (miR-29b) expression. Accordingly, miR-29b is enriched in luminal breast cancers and loss of miR-29b, even in GATA3-expressing cells, increases metastasis and promotes a mesenchymal phenotype. Mechanistically, miR-29b inhibits metastasis by targeting a network of pro-metastatic regulators involved in angiogenesis, collagen remodelling and proteolysis, including VEGFA, ANGPTL4, PDGF, LOX and MMP9, and targeting ITGA6, ITGB1 and TGFB, thereby indirectly affecting differentiation and epithelial plasticity. The discovery that a GATA3-miR-29b axis regulates the tumour microenvironment and inhibits metastasis opens up possibilities for therapeutic intervention in breast cancer.

Wdr5 is required for chick skeletal development

Wdr5, a bone morphogenetic protein 2 (BMP-2)-induced protein belonging to the family of the WD repeat proteins, is expressed in proliferating and hypertrophic chondrocytes of the growth plate and in osteoblasts. Although previous studies have provided insight into the mechanisms by which Wdr5 affects chondrocyte and osteoblast differentiation, whether Wdr5 is required in vivo for endochondral bone development has not been addressed. In this study, using an avian replication competent retrovirus (RCAS) system delivering Wdr5 short hairpin (sh) RNA to silence Wdr5 in the developing limb, we report that reduction of Wdr5 levels delays endochondral bone development and consequently results in shortening of the skeletal elements. Shortening of the skeletal elements was due to impaired chondrocyte maturation, evidenced by a significant reduction of Runx2, type X collagen, and osteopontin expression. A decrease in Runx2, type collagen I, and ostepontin expression in osteoblasts and a subsequent defect in mineralized bone was observed as well when Wdr5 levels were reduced. Most important, retroviral misexpression of Runx2 rescued the phenotype induced by Wdr5 shRNA. These findings suggest that during limb development, Wdr5 is required for endochondral bone formation and that Wdr5 influences this process, at least in part, by regulating Runx2 expression.

GATA3 in development and cancer differentiation: cells GATA have it!

There is increasing evidence that the numerous mechanisms that regulate cell differentiation during normal development are also involved in tumorigenesis. In breast cancer, differentiation markers expressed by the primary tumor are routinely profiled to guide clinical decisions. Indeed, numerous studies have shown that the differentiation profile correlates with the metastatic potential of tumors. The transcription factor GATA3 has emerged recently as a strong predictor of clinical outcome in human luminal breast cancer. In the mammary gland, GATA3 is required for luminal epithelial cell differentiation and commitment, and its expression is progressively lost during luminal breast cancer progression as cancer cells acquire a stem cell-like phenotype. Importantly, expression of GATA3 in GATA3-negative, undifferentiated breast carcinoma cells is sufficient to induce tumor differentiation and inhibits tumor dissemination in a mouse model. These findings demonstrate the exquisite ability of a differentiation factor to affect malignant properties, and raise the possibility that GATA3 or its downstream genes could be used in treating luminal breast cancer. This review highlights our recent understanding of GATA3 in both normal mammary development and tumor differentiation.

The transcriptional cofactor Lbh regulates angiogenesis and endochondral bone formation during fetal bone development

Lbh is thought to act as a transcriptional cofactor and is highly conserved among species. Here we show that Lbh is expressed in chondrocytes, cells of the perichondrium, and the primary spongiosa in fetal growth plates of mice and chickens. Lbh overexpression in chick wings, using the RCAS-retroviral vector strategy, results in shortened skeletal elements and delayed hypertrophic chondrocyte maturation and bone formation. Additionally, osteoclast and endothelial cell invasion are delayed in the Lbh-overexpressing bones. Finally, we find a dramatic suppression of Runx2 and VEGF mRNAs in chondrocytes and osteoblasts that overexpress Lbh. Strikingly, this abnormal bone development in infected limbs can be rescued by concurrent overexpression of Runx2. These results suggest that during endochondral bone formation, Lbh may negatively regulate vascular invasion and formation of the early ossification center at least in part by interfering with Runx2 and/or VEGF expression.

Hif-1alpha regulates differentiation of limb bud mesenchyme and joint development

Recent evidence suggests that low oxygen tension (hypoxia) may control fetal development and differentiation. A crucial mediator of the adaptive response of cells to hypoxia is the transcription factor Hif-1α. In this study, we provide evidence that mesenchymal condensations that give origin to endochondral bones are hypoxic during fetal development, and we demonstrate that Hif-1α is expressed and transcriptionally active in limb bud mesenchyme and in mesenchymal condensations. To investigate the role of Hif-1α in mesenchymal condensations and in early chondrogenesis, we conditionally inactivated Hif-1α in limb bud mesenchyme using a Prx1 promoter-driven Cre transgenic mouse. Conditional knockout of Hif-1α in limb bud mesenchyme does not impair mesenchyme condensation, but alters the formation of the cartilaginous primordia. Late hypertrophic differentiation is also affected as a result of the delay in early chondrogenesis. In addition, mutant mice show a striking impairment of joint development. Our study demonstrates a crucial, and previously unrecognized, role of Hif-1α in early chondrogenesis and joint formation.

Nkx3.2/Bapx1 acts as a negative regulator of chondrocyte maturation

Parathyroid hormone-related protein (PTHrP) is essential to maintain a pool of dividing, immature chondrocytes in the growth plate of long bones. In chick and mouse, expression of Nkx3.2/Bapx1 in the growth plate is restricted to the proliferative zone and is down regulated as chondrocyte maturation begins. Nkx3.2/Bapx1 expression is lost in the growth plates of mice engineered to lack PTHrP signaling and, conversely, is maintained by ectopic expression of PTHrP in developing bones. Artificially preventing Nkx3.2/Bapx1 downregulation, by forced expression of either retroviral-encoded PTHrP or Nkx3.2 inhibits chondrocyte maturation. Although wild-type Nkx3.2 blocks chondrocyte maturation by acting as a transcriptional repressor, a ;reverse function' mutant of Nkx3.2 that has been converted into a transcriptional activator conversely accelerates chondrocyte maturation. Nkx3.2 represses expression of the chondrocyte maturation factor Runx2, and Runx2 mis-expression can rescue the Nkx3.2-induced blockade of chondrocyte maturation. Taken together, these results suggest that PTHrP signals block chondrocyte hypertrophy by, in part, maintaining the expression of Nkx3.2/Bapx1, which in turn represses the expression of genes required for chondrocyte maturation.

Molecular mechanisms of endochondral bone development

Endochondral bone development is a complex process in which undifferentiated mesenchymal cells differentiate into chondrocytes, which then undergo well-ordered and controlled phases of proliferation, hypertrophic differentiation, death, blood vessel invasion, and finally replacement of cartilage with bone. The process recapitulates basic and fundamental mechanisms of cell biology with a highly specific spatial and temporal pattern, and it thus constitutes an excellent model for the analysis of such mechanisms. In recent years, the tools provided by modern genetic both in mice and men have been instrumental in the process of identifying and dissecting basic molecular mechanisms of endochondral bone formation. This review is a brief summary of the current knowledge about some of the crucial factors involved in growth plate development.

PTHrP, PTH, and the PTH/PTHrP receptor in endochondral bone development

Endochondral bone development is a fascinating story of proliferation, maturation, and death. An understanding of this process at the molecular level is emerging. In particular, significant advances have been made in understanding the role of parathyroid-hormone-related peptide (PTHrP), parathyroid hormone (PTH), and the PTH/PTHrP receptor in endochondral bone development. Mutations of the PTH/PTHrP receptor have been identified in Jansen metaphyseal chondrodysplasia, Blomstrand's lethal chondrodysplasia, and enchondromatosis. Furthermore, genetic manipulations of the PTHrP, PTH, and the PTH/PTHrP receptor genes, respectively, have demonstrated the critical role of these proteins in regulating both the switch between proliferation and differentiation of chondrocytes, and their replacement by bone cells. A future area of investigation will be the identification of downstream effectors of PTH, PTHrP, and PTH/PTHrP receptor activities. Furthermore, it will be of critical importance to study how these proteins cooperate and integrate with other molecules that are essential for growth plate development.

The implementation of a quality assurance procedure for the Veterinary Services of France

Due to the increasing complexity of food production systems and the concerns that these systems raise, there has been increasing demand from the general public for more State control of these processes. In France, it is the official Veterinary Services who are responsible for food safety and who must respond to these demands. The Veterinary Service is formulating a quality assurance procedure in accordance with standard EN 45004-ISO 17020, which determines the requirements that inspection bodies must follow to be recognised, at national, European and international level, as competent and reliable. As part of this procedure, the Veterinary Service will review requirements in terms of organisation, functions, qualifications and resources. The progress of inspection service orders, from their conception by the Central Administration, to their implementation by decentralised services, must be carefully managed. It is essential that service orders be implemented effectively and systematically by using recognised methods and issuing adequate inspection reports. The training and qualifications of inspectors are very important: their skills must remain up-to-date so that there is always a network of qualified staff, that is, staff who have an understanding of production processes and who have recognised competences in terms of initial training, continuous professional development and adequate experience. The quality systems implemented will only meet expectations if they are continuously monitored by means of regular evaluations. For this reason, both internal and external audits are performed. These new practices contribute to establishing a basis for the improvement of internal evaluation. In order to facilitate the implementation of a quality assurance procedure for inspection services, several tools, that are linked with the information system of the government department responsible for food, are, or will be, at the disposal of the decentralised Veterinary Services, i.e. a national database, mail and service order processing software, and inspection procedures.

Phosphorylation of MafA is essential for its transcriptional and biological properties

We previously described the identification of quail MafA, a novel transcription factor of the Maf bZIP (basic region leucine zipper) family, expressed in the differentiating neuroretina (NR). In the present study, we provide the first evidence that MafA is phosphorylated and that its biological properties strongly rely upon phosphorylation of serines 14 and 65, two residues located in the transcriptional activating domain within a consensus for phosphorylation by mitogen-activated protein kinases and which are conserved among Maf proteins. These residues are phosphorylated by ERK2 but not by p38, JNK, and ERK5 in vitro. However, the contribution of the MEK/ERK pathway to MafA phosphorylation in vivo appears to be moderate, implicating another kinase. The integrity of serine 14 and serine 65 residues is required for transcriptional activity, since their mutation into alanine severely impairs MafA capacity to activate transcription. Furthermore, we show that the MafA S14A/S65A mutant displays reduced capacity to induce expression of QR1, an NR-specific target of Maf proteins. Likewise, the integrity of serines 14 and 65 is essential for the MafA ability to stimulate expression of crystallin genes in NR cells and to induce NR-to-lens transdifferentiation. Thus, the MafA capacity to induce differentiation programs is dependent on its phosphorylation.

Induction of postmitotic neuroretina cell proliferation by distinct Ras downstream signaling pathways

Ras-induced cell transformation is mediated through distinct downstream signaling pathways, including Raf, Ral-GEFs-, and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathways. In some cell types, strong activation of the Ras-Raf-MEK-extracellular signal-regulated kinase (ERK) cascade leads to cell cycle arrest rather than cell division. We previously reported that constitutive activation of this pathway induces sustained proliferation of primary cultures of postmitotic chicken neuroretina (NR) cells. We used this model system to investigate the respective contributions of Ras downstream signaling pathways in Ras-induced cell proliferation. Three RasV12 mutants (S35, G37, and C40) which differ by their ability to bind to Ras effectors (Raf, Ral-GEFs, and the p110 subunit of PI 3-kinase, respectively) were able to induce sustained NR cell proliferation, although none of these mutants was reported to transform NIH 3T3 cells. Furthermore, they all repressed the promoter of QR1, a neuroretina growth arrest-specific gene. Overexpression of B-Raf or activated versions of Ras effectors Rlf-CAAX and p110-CAAX also induced NR cell division. The mitogenic effect of the RasC40-PI 3-kinase pathway appears to involve Rac and RhoA GTPases but not the antiapoptotic Akt (protein kinase B) signaling. Division induced by RasG37-Rlf appears to be independent of Ral GTPase activation and presumably requires an unidentified mechanism. Activation of either Ras downstream pathway resulted in ERK activation, and coexpression of a dominant negative MEK mutant or mKsr-1 kinase domain strongly inhibited proliferation induced by the three Ras mutants or by their effectors. Similar effects were observed with dominant negative mutants of Rac and Rho. Thus, both the Raf-MEK-ERK and Rac-Rho pathways are absolutely required for Ras-induced NR cell division. Activation of these two pathways by the three distinct Ras downstream effectors possibly relies on an autocrine or paracrine loop, implicating endogenous Ras, since the mitogenic effect of each Ras effector mutant was inhibited by RasN17.

Characterization of a novel quiescence responsive element downregulated by v-Src in the promoter of the neuroretina specific QR1 gene

The neuroretina is a functional unit of the central nervous system which arises through successive steps of division, growth arrest and differentiation of neuroectodermal precursors. Postmitotic quail neuroretina (QNR) cells are conditionally induced to divide upon infection with temperature sensitive mutants of Rous sarcoma virus (RSV), since QNR cell division can be arrested by either inactivating p60v-Src at the nonpermissive temperature (41 degrees C) or by serum deprivation at 37 degrees C. We are studying the transcriptional control of QR1, a neuroretina specific gene, whose expression is down-regulated in proliferating cells at 37 degrees C and is fully restored when these cells are made quiescent. We previously showed that this quiescence specific upregulation implicates a promoter region named A box, which binds Maf transcription factors. We report the identification of the C box, a second promoter sequence that activates QR1 transcription in non dividing cells. This sequence is able to form two DNA-protein complexes, one of which (C4) is predominantly detected in growth arrested NR cells. We identified the DNA binding site for C4 and described mutations that abolish both C4 binding and promoter activity in quiescent cells. Moreover, we show that a multimerized C box is able to stimulate a heterologous promoter in non dividing cells and constitutes, therefore, a novel quiescence responsive enhancer. Finally, we report that QR1 transcriptional response to cell quiescence requires cooperation between the C box and A box.

mafA, a novel member of the maf proto-oncogene family, displays developmental regulation and mitogenic capacity in avian neuroretina cells

Transcription factors of the Maf proto-oncogene family have been shown to participate in the regulation of several differentiation specific genes. We previously reported that a member(s) of this family is involved in the regulation of the neuroretina specific gene, QR1, through a promoter region, designated the A box, that is closely related to the Maf recognition element (MARE). We undertook an identification of Maf family genes expressed in the quail neuroretina (QNR) and we report the isolation of mafA, a gene encoding a novel member of the large Maf proteins subgroup. Expression of this gene is developmentally regulated in the neuroretina. MafA is able to bind to MARE sequence and to heterodimerize with v-Maf, MafB, Jun and Fos, but not with the small MafF and MafK proteins. Accordingly, it is able to transactivate the QR1 promoter A box. We also show that increased expression of mafA induces sustained proliferation of postmitotic QNR cells.

Characterization of a leucine zipper-containing protein identified by retroviral insertion in avian neuroretina cells

We reported previously that post-mitotic chicken embryonic neuroretina (NR) cells are induced to proliferate following in vitro infection with RAV-1, a retrovirus that does not carry an oncogene. NR cell multiplication results from the frequent activation and subsequent retroviral transduction of two related serine/threonine protein kinases, the c-mil/c-raf or c-Rmil/B-raf genes. We also showed that a very early event in the activation of these proto-oncogenes is the synthesis of chimeric mRNAs containing viral and cellular sequences joined by a splicing mechanism. In the current study, we have examined the ability of RAV-1 to induce proliferation of quail NR cells. By using the reverse transcription-polymerase chain reaction technique, we identified, in several proliferating quail NR cultures infected with RAV-1, a chimeric mRNA containing cellular sequences joined to the RAV-1 splice donor site. These cellular sequences are derived from a gene designated R10, which is expressed through a 1.9-kilobase (kb) mRNA detected in several embryonic tissues. A second transcript of 2.3 kb is specifically expressed in the NR, where both transcripts are developmentally regulated. The R10 cDNA encodes a 251-amino acid polypeptide that contains a leucine zipper motif. It exhibits significant similarity with the putative D52/N8L protein, encoded by an mRNA reported previously to be overexpressed in human breast and lung carcinomas. By using polyclonal antibodies specific for its amino-terminal and leucine zipper-containing regions, we identified the R10 gene product as a cytoplasmic protein of 23 kDa in cultured avian fibroblasts. A second protein of 30 kDa is detected in post-mitotic NR cells that express the 2.3-kb transcript. We also show, by in vitro transcription/translation and immunoprecipitation, that the R10 protein can readily form homodimers, presumably through its leucine zipper motif.