The retinoblastoma protein acts as a transcriptional coactivator required for osteogenic differentiation

Osteosarcoma: prognosis plateau warrants retinoblastoma pathway targeted therapy

Osteosarcoma (OS) is the most common primary bone cancer in children and adolescents, affecting ~560 young patients in the United States annually. The term OS describes a diverse array of subtypes with varying prognoses, but the majority of tumors are high grade and aggressive. Perhaps because the true etiology of these aggressive tumors remains unknown, advances in OS treatment have reached a discouraging plateau, with only incremental improvements over the past 40 years. Thus, research surrounding the pathogenesis of OS is essential, as it promises to unveil novel therapeutic targets that can attack tumor cells with greater specificity and lower toxicity. Among the candidate molecular targets in OS, the retinoblastoma (RB) pathway demonstrates the highest frequency of inactivation and thus represents a particularly promising avenue for molecular targeted therapy. This review examines the present thinking and practices in OS treatment and specifically highlights the relevance of the RB pathway in osteosarcomagenesis. Through further investigation into RB pathway-related novel therapeutic targets, we believe that a near-term breakthrough in improved OS prognosis is possible.

The Retinoblastoma Tumor Suppressor Transcriptionally Represses Pak1 in Osteoblasts

We previously characterized the retinoblastoma tumor suppressor protein (Rb) as a regulator of adherens junction assembly and cell-to-cell adhesion in osteoblasts. This is a novel function since Rb is predominantly known as a cell cycle repressor. Herein, we characterized the molecular mechanisms by which Rb performs this function, hypothesizing that Rb controls the activity of known regulators of adherens junction assembly. We found that Rb represses the expression of the p21-activated protein kinase (Pak1), an effector of the small Rho GTPase Rac1. Rac1 is a well-known regulator of adherens junction assembly whose increased activity in cancer is linked to perturbations of intercellular adhesion. Using nuclear run-on and luciferase reporter transcription assays, we found that Pak1 repression by Rb is transcriptional, without affecting Pak1 mRNA and protein stability. Pak1 promoter bioinformatics showed multiple E2F1 binding sites within 155 base pairs of the transcriptional start site, and a Pak1-promoter region containing these E2F sites is susceptible to transcriptional inhibition by Rb. Chromatin immunoprecipitations showed that an Rb-E2F complex binds to the region of the Pak1 promoter containing the E2F1 binding sites, suggesting that Pak1 is an E2F target and that the repressive effect of Rb on Pak1 involves blocking the trans-activating capacity of E2F. A bioinformatics analysis showed elevated Pak1 expression in several solid tumors relative to adjacent normal tissue, with both Pak1 and E2F increased relative to normal tissue in breast cancer, supporting a cancer etiology for Pak1 up-regulation. Therefore, we propose that by repressing Pak1 expression, Rb prevents Rac1 hyperactivity usually associated with cancer and related to cytoskeletal derangements that disrupt cell adhesion, consequently enhancing cancer cell migratory capacity. This de-regulation of cell adhesion due to Rb loss could be part of the molecular events associated with cancer progression and metastasis.

Estrogen-Related Receptor γ Plays a Key Role in Vascular Calcification Through the Upregulation of BMP2 Expression

OBJECTIVE

Vascular calcification which refers to ectopic mineralization in vascular cells is associated with several conditions, such as chronic kidney disease, atherosclerosis, and diabetes mellitus. Estrogen-related receptor (ERR)γ is a member of the orphan nuclear receptor superfamily, which plays diverse roles in regulating homeostatic and metabolic processes. However, the role of ERRγ in vascular calcification has not been investigated to date. The aim of the present study was to examine the role of ERRγ in vascular calcification.

APPROACH AND RESULTS

Vascular calcification was induced by treating rat aortic vascular smooth muscle cells with calcification medium. ERRγ expression in vascular smooth muscle cells was induced during calcification medium-induced vascular calcification. Adenovirus-mediated overexpression of ERRγ in vascular smooth muscle cells resulted in the upregulation of the expression of osteogenic genes, including runt-related transcription factor 2, osteopontin, and Msx2, and the downregulation of α-smooth muscle actin. Adenovirus-mediated overexpression of ERRγ induced bone morphogenetic protein 2 (BMP2) expression, leading to increased phosphorylation of the intracellular BMP2 effector proteins SMAD1/5/8. Collectively, these data suggested that ERRγ promotes dedifferentiation of vascular smooth muscle cells to an osteogenic phenotype during the vascular calcification process. Inhibition of endogenous ERRγ expression or activity using a specific siRNA or the selective inverse agonist GSK5182 attenuated vascular calcification and osteogenic gene expression in vitro and in vivo.

CONCLUSIONS

The present results indicate that ERRγ plays a key role in vascular calcification by upregulating the BMP2 signaling pathway, suggesting that inhibition of ERRγ is a potential therapeutic strategy for the prevention of vascular calcification.

Rb1 and Pten Co-Deletion in Osteoblast Precursor Cells Causes Rapid Lipoma Formation in Mice

The Rb and Pten tumor suppressor genes are important regulators of bone development and both are frequently mutated in the bone cancer osteosarcoma (OS). To determine if Rb1 and Pten synergize as tumor suppressor genes for osteosarcoma, we co-deleted them in osteoprogenitor cells. Surprisingly, we observed rapid development of adipogenic but not osteosarcoma tumors in the ΔRb1/Pten mice. ΔPten solo deleted mice also developed lipoma tumors but at a much reduced frequency and later onset than those co-deleted for Rb1. Pten deletion also led to a marked increase in adipocytes in the bone marrow. To better understand the function of Pten in bone development in vivo, we conditionally deleted Pten in OSX⁺ osteoprogenitor cells using OSX-Cre mice. μCT analysis revealed a significant thickening of the calvaria and an increase in trabeculae volume and number in the femur, consistent with increased bone formation in these mice. To determine if Pten and Rb1 deletion actively promotes adipogenic differentiation, we isolated calvarial cells from Pten^fl/fl and Pten^fl/fl; Rb1^fl/fl mice, infected them with CRE or GFP expressing adenovirus, treated with differentiation media. We observed slightly increased adipogenic, and osteogenic differentiation in the ΔPten cells. Both phenotypes were greatly increased upon Rb1/Pten co-deletion. This was accompanied by an increase in expression of genes required for adipogenesis. These data indicate that Pten deletion in osteoblast precursors is sufficient to promote frequent adipogenic, but only rare osteogenic tumors. Rb1 hetero- or homo-zygous co-deletion greatly increases the incidence and the rapidity of onset of adipogenic tumors, again, with only rare osteosarcoma tumors.

Increased mitochondrial function downstream from KDM5A histone demethylase rescues differentiation in pRB-deficient cells

The retinoblastoma tumor suppressor protein pRb restricts cell growth through inhibition of cell cycle progression. Increasing evidence suggests that pRb also promotes differentiation, but the mechanisms are poorly understood, and the key question remains as to how differentiation in tumor cells can be enhanced in order to diminish their aggressive potential. Previously, we identified the histone demethylase KDM5A (lysine [K]-specific demethylase 5A), which demethylates histone H3 on Lys4 (H3K4), as a pRB-interacting protein counteracting pRB's role in promoting differentiation. Here we show that loss of Kdm5a restores differentiation through increasing mitochondrial respiration. This metabolic effect is both necessary and sufficient to induce the expression of a network of cell type-specific signaling and structural genes. Importantly, the regulatory functions of pRB in the cell cycle and differentiation are distinct because although restoring differentiation requires intact mitochondrial function, it does not necessitate cell cycle exit. Cells lacking Rb1 exhibit defective mitochondria and decreased oxygen consumption. Kdm5a is a direct repressor of metabolic regulatory genes, thus explaining the compensatory role of Kdm5a deletion in restoring mitochondrial function and differentiation. Significantly, activation of mitochondrial function by the mitochondrial biogenesis regulator Pgc-1α (peroxisome proliferator-activated receptor γ-coactivator 1α; also called PPARGC1A) a coactivator of the Kdm5a target genes, is sufficient to override the differentiation block. Overexpression of Pgc-1α, like KDM5A deletion, inhibits cell growth in RB-negative human cancer cell lines. The rescue of differentiation by loss of KDM5A or by activation of mitochondrial biogenesis reveals the switch to oxidative phosphorylation as an essential step in restoring differentiation and a less aggressive cancer phenotype.

Transcriptional control of stem cell fate by E2Fs and pocket proteins

E2F transcription factors and their regulatory partners, the pocket proteins (PPs), have emerged as essential regulators of stem cell fate control in a number of lineages. In mammals, this role extends from both pluripotent stem cells to those encompassing all embryonic germ layers, as well as extra-embryonic lineages. E2F/PP-mediated regulation of stem cell decisions is highly evolutionarily conserved, and is likely a pivotal biological mechanism underlying stem cell homeostasis. This has immense implications for organismal development, tissue maintenance, and regeneration. In this article, we discuss the roles of E2F factors and PPs in stem cell populations, focusing on mammalian systems. We discuss emerging findings that position the E2F and PP families as widespread and dynamic epigenetic regulators of cell fate decisions. Additionally, we focus on the ever expanding landscape of E2F/PP target genes, and explore the possibility that E2Fs are not simply regulators of general ‘multi-purpose’ cell fate genes but can execute tissue- and cell type-specific gene regulatory programs.

Osteoblast dysfunctions in bone diseases: from cellular and molecular mechanisms to therapeutic strategies

Several metabolic, genetic and oncogenic bone diseases are characterized by defective or excessive bone formation. These abnormalities are caused by dysfunctions in the commitment, differentiation or survival of cells of the osteoblast lineage. During the recent years, significant advances have been made in our understanding of the cellular and molecular mechanisms underlying the osteoblast dysfunctions in osteoporosis, skeletal dysplasias and primary bone tumors. This led to suggest novel therapeutic approaches to correct these abnormalities such as the modulation of WNT signaling, the pharmacological modulation of proteasome-mediated protein degradation, the induction of osteoprogenitor cell differentiation, the repression of cancer cell proliferation and the manipulation of epigenetic mechanisms. This article reviews our current understanding of the major cellular and molecular mechanisms inducing osteoblastic cell abnormalities in age-related bone loss, genetic skeletal dysplasias and primary bone tumors, and discusses emerging therapeutic strategies to counteract the osteoblast abnormalities in these disorders of bone formation.

Molecular cloning, expression pattern analysis of porcine Rb1 gene and its regulatory roles during primary dedifferentiated fat cells adipogenic differentiation

Adipocytes are the main constituent of adipose tissue and are considered to be a corner stone in the homeostatic control of whole body metabolism. Recent reports evidenced that retinoblastoma 1 (Rb1) gene plays an important role in fat development and adipogenesis in mice. Here, we cloned the partial cDNA sequences of the porcine Rb1 gene which contains the complete coding sequences (CDS) of 2820bp encoding a protein of 939 amino acids. Bioinformatic analysis revealed that the CDS of porcine Rb1 was highly identical with those of cattle, human and mice. The porcine Rb1 has three typical conserved structural domains, including Rb-A pocket domain, CYCLIN domain and C-terminus domain, and the phylogenetic tree indicates a closer genetic relationship with cattle and human. Tissue distribution analysis showed that Rb1 expression appeared to be ubiquitously in various tissues, being higher in heart, liver, muscle, and stomach. Furthermore, significant downregulation of Rb1 was found at the initial stage of dedifferentiated fat (DFAT) cells adipogenic differentiation. With the knockdown of the Rb1 expression by siRNA, the number of DFAT cells recruited to white rather than brown adipogenesis was promoted, and mRNA levels of adipogenic markers, such as PPARγ, aP2, LPL and adiponectin and protein expression of PPARγ and adiponectin were increased after hormone stimulation. The underlying mechanisms may be that knockdown of Rb1 promotes the mitotic clonal expansion and PPARγ expression by derepressing the transcriptional activity of E2F so as to facilitate the first steps of adipogenesis. In summary, we cloned and characterized an important negative regulator in adipogenic commitment of porcine DFAT cells.

Generation of a Retinoblastoma (Rb)1-inducible dominant-negative (DN) mouse model

Retinoblastoma 1 (Rb1) is an essential gene regulating cellular proliferation, differentiation, and homeostasis. To exert these functions, Rb1 is recruited and physically interacts with a growing variety of signaling pathways. While Rb1 does not appear to be ubiquitously expressed, its expression has been confirmed in a variety of hematopoietic and neuronal-derived cells, including the inner ear hair cells (HCs). Studies in transgenic mice demonstrate that complete germline or conditional Rb1 deletion leads to abnormal cell proliferation, followed by massive apoptosis; making it difficult to fully address Rb1's biochemical activities. To overcome these limitations, we developed a tetracycline-inducible TetO-CB-myc6-Rb1 (CBRb) mouse model to achieve transient and inducible dominant-negative (DN) inhibition of the endogenous RB1 protein. Our strategy involved fusing the Rb1 gene to the lysosomal protease pre-procathepsin B (CB), thus allowing for further routing of the DN-CBRb fusion protein and its interacting complexes for proteolytic degradation. Moreover, reversibility of the system is achieved upon suppression of doxycycline (Dox) administration. Preliminary characterization of DN-CBRb mice bred to a ubiquitous rtTA mouse line demonstrated a significant inhibition of the endogenous RB1 protein in the inner ear and in a number of other organs where RB1 is expressed. Examination of the postnatal (P) DN-CBRb mice inner ear at P10 and P28 showed the presence of supernumerary inner HCs (IHCs) in the lower turns of the cochleae, which corresponds to the described expression domain of the endogenous Rb1 gene. Selective and reversible suppression of gene expression is both an experimental tool for defining function and a potential means to medical therapy. Given the limitations associated with Rb1-null mice lethality, this model provides a valuable resource for understanding RB1 activity, relative contribution to HC regeneration and its potential therapeutic application.

The RUNX family: developmental regulators in cancer

RUNX proteins belong to a family of metazoan transcription factors that serve as master regulators of development. They are frequently deregulated in human cancers, indicating a prominent and, at times, paradoxical role in cancer pathogenesis. The contextual cues that direct RUNX function represent a fast-growing field in cancer research and could provide insights that are applicable to early cancer detection and treatment. This Review describes how RUNX proteins communicate with key signalling pathways during the multistep progression to malignancy; in particular, we highlight the emerging partnership of RUNX with p53 in cancer suppression.

Protein kinase inhibitor γ reciprocally regulates osteoblast and adipocyte differentiation by downregulating leukemia inhibitory factor

The protein kinase inhibitor (Pki) gene family inactivates nuclear protein kinase A (PKA) and terminates PKA-induced gene expression. We previously showed that Pkig is the primary family member expressed in osteoblasts and that Pkig knockdown increases the effects of parathyroid hormone and isoproterenol on PKA activation, gene expression, and inhibition of apoptosis. Here, we determined whether endogenous levels of Pkig regulate osteoblast differentiation. Pkig is the primary family member in murine embryonic fibroblasts (MEFs), murine marrow-derived mesenchymal stem cells, and human mesenchymal stem cells. Pkig deletion increased forskolin-dependent nuclear PKA activation and gene expression and Pkig deletion or knockdown increased osteoblast differentiation. PKA signaling is known to stimulate adipogenesis; however, adipogenesis and osteogenesis are often reciprocally regulated. We found that the reciprocal regulation predominates over the direct effects of PKA since adipogenesis was decreased by Pkig deletion or knockdown. Pkig deletion or knockdown also simultaneously increased osteogenesis and decreased adipogenesis in mixed osteogenic/adipogenic medium. Pkig deletion increased PKA-induced expression of leukemia inhibitory factor (Lif) mRNA and LIF protein. LIF neutralizing antibodies inhibited the effects on osteogenesis and adipogenesis of either Pkig deletion in MEFs or PKIγ knockdown in both murine and human mesenchymal stem cells. Collectively, our results show that endogenous levels of Pkig reciprocally regulate osteoblast and adipocyte differentiation and that this reciprocal regulation is mediated in part by LIF. Stem Cells 2013;31:2789-2799.

Skeletal (stromal) stem cells: an update on intracellular signaling pathways controlling osteoblast differentiation

Skeletal (marrow stromal) stem cells (BMSCs) are a group of multipotent cells that reside in the bone marrow stroma and can differentiate into osteoblasts, chondrocytes and adipocytes. Studying signaling pathways that regulate BMSC differentiation into osteoblastic cells is a strategy for identifying druggable targets for enhancing bone formation. This review will discuss the functions and the molecular mechanisms of action on osteoblast differentiation and bone formation; of a number of recently identified regulatory molecules: the non-canonical Notch signaling molecule Delta-like 1/preadipocyte factor 1 (Dlk1/Pref-1), the Wnt co-receptor Lrp5 and intracellular kinases. This article is part of a Special Issue entitled: Stem Cells and Bone.

THE RETINOBLASTOMA PROTEIN: A MASTER TUMOR SUPPRESSOR ACTS AS A LINK BETWEEN CELL CYCLE AND CELL ADHESION

RB1 was the first tumor suppressor gene discovered. Over four decades of work have revealed that the Rb protein (pRb) is a master regulator of biological pathways influencing virtually every aspect of intrinsic cell fate including cell growth, cell-cycle checkpoints, differentiation, senescence, self-renewal, replication, genomic stability and apoptosis. While these many processes may account for a significant portion of RB1's potency as a tumor suppressor, a small, but growing stream of evidence suggests that RB1 also significantly influences how a cell interacts with its environment, including cell-to-cell and cell-to-extracellular matrix interactions. This review will highlight pRb's role in the control of cell adhesion and how alterations in the adhesive properties of tumor cells may drive the deadly process of metastasis.

Bone environment is essential for osteosarcoma development from transformed mesenchymal stem cells

The cellular microenvironment plays a relevant role in cancer development. We have reported that mesenchymal stromal/stem cells (MSCs) deficient for p53 alone or together with RB (p53(-/-)RB(-/-)) originate leiomyosarcoma after subcutaneous (s.c.) inoculation. Here, we show that intrabone or periosteal inoculation of p53(-/-) or p53(-/-)RB(-/-) bone marrow- or adipose tissue-derived MSCs originated metastatic osteoblastic osteosarcoma (OS). To assess the contribution of bone environment factors to OS development, we analyzed the effect of the osteoinductive factor bone morphogenetic protein-2 (BMP-2) and calcified substrates on p53(-/-)RB(-/-) MSCs. We show that BMP-2 upregulates the expression of osteogenic markers in a WNT signaling-dependent manner. In addition, the s.c. coinfusion of p53(-/-)RB(-/-) MSCs together with BMP-2 resulted in appearance of tumoral osteoid areas. Likewise, when p53(-/-)RB(-/-) MSCs were inoculated embedded in a calcified ceramic scaffold composed of hydroxyapatite and tricalciumphosphate (HA/TCP), tumoral bone formation was observed in the surroundings of the HA/TCP scaffold. Moreover, the addition of BMP-2 to the ceramic/MSC implants further increased the tumoral osteoid matrix. Together, these data indicate that bone microenvironment signals are essential to drive OS development.

p107-Dependent recruitment of SWI/SNF to the alkaline phosphatase promoter during osteoblast differentiation

The retinoblastoma protein family is intimately involved in the regulation of tissue specific gene expression during mesenchymal stem cell differentiation. The role of the following proteins, pRB, p107 and p130, is particularly significant in differentiation to the osteoblast lineage, as human germ-line mutations of RB1 greatly increase susceptibility to osteosarcoma. During differentiation, pRB directly targets certain osteogenic genes for activation, including the alkaline phosphatase-encoding gene Alpl. Chromatin immunoprecipitation (ChIP) assays indicate that Alpl is targeted by p107 in differentiating osteoblasts selectively during activation with the same dynamics as pRB, which suggests that p107 helps promote Alpl activation. Mouse models indicate overlapping roles for pRB and p107 in bone and cartilage formation, but very little is known about direct tissue-specific gene targets of p107, or the consequences of targeting by p107. Here, the roles of p107 and pRB were compared using shRNA-mediated knockdown genetics in an osteoblast progenitor model, MC3T3-E1 cells. The results show that p107 has a distinct role along with pRB in induction of Alpl. Deficiency of p107 does not impede recruitment of transcription factors recognized as pRB co-activation partners at the promoter; however, p107 is required for the efficient recruitment of an activating SWI/SNF chromatin-remodeling complex, an essential event in Alpl induction.

pRb-E2F signaling in life of mesenchymal stem cells: Cell cycle, cell fate, and cell differentiation

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into various mesodermal lines forming fat, muscle, bone, and other lineages of connective tissue. MSCs possess plasticity and under special metabolic conditions may transform into cells of unusual phenotypes originating from ecto- and endoderm. After transplantation, MSCs release the humoral factors promoting regeneration of the damaged tissue. During last five years, the numbers of registered clinical trials of MSCs have increased about 10 folds. This gives evidence that MSCs present a new promising resource for cell therapy of the most dangerous diseases. The efficacy of the MSCs therapy is limited by low possibilities to regulate their conversion into cells of damaged tissues that is implemented by the pRb-E2F signaling. The widely accepted viewpoint addresses pRb as ubiquitous regulator of cell cycle and tumor suppressor. However, current publications suggest that basic function of the pRb-E2F signaling in development is to regulate cell fate and differentiation. Through facultative and constitutive chromatin modifications, pRb-E2F signaling promotes transient and stable cells quiescence, cell fate choice to differentiate, to senesce, or to die. Loss of pRb is associated with cancer cell fate. pRb regulates cell fate by retaining quiescence of one cell population in favor of commitment of another or by suppression of genes of different cell phenotype. pRb is the founder member of the "pocket protein" family possessing functional redundancy. Critical increase in the efficacy of the MSCs based cell therapy will depend on precise understanding of various aspects of the pRb-E2F signaling.

The impact of osteoblastic differentiation on osteosarcomagenesis in the mouse

Osteosarcomas remain an enigmatic group of malignancies that share in common the presence of transformed cells producing osteoid matrix, even if these cells comprise a minority of the tumor volume. The differentiation state of osteosarcomas has therefore become a topic of interest and challenge to those who study this disease. In order to test how the cell of origin contributes to the final state of differentiation in the transformed cells, we compared the relative tumorigenicity of Cre-LoxP conditional disruption of the cell cycle checkpoint tumor-suppressor genes Trp53 and Rb1 using Prx1-Cre, Collagen-1α1-Cre and Osteocalcin-Cre to transform undifferentiated mesenchyme, preosteoblasts and mature osteoblasts, respectively. The Prx1 and Col1α1 lineages developed tumors with nearly complete penetrance, as anticipated. Osteosarcomas also developed in 44% of Oc-Cre;Rb1(fl/fl);Trp53(fl/fl) mice. We confirmed using 5-ethynyl-2'-deoxyuridine click chemistry that the Oc-Cre lineage includes very few actively cycling cells. By assessing radiographic mineralization and histological osteoid production, the differentiation state of tumors did not correlate with the differentiation state of the lineage of origin. Some of the osteocalcin-lineage-derived osteosarcomas were among the least osteoblastic. Osteocalcin immunohistochemistry in tumors correlated well with the expression of DNA methyl transferases, suggesting that silencing of these epigenetic regulators may influence the final differentiation state of an osteosarcoma. Transformation of differentiated, minimally proliferative osteoblasts is possible but may require such an epigenetic reprogramming that the tumors no longer resemble their differentiated origins.

Screening of diagnostic markers for osteosarcoma

Osteosarcoma, which is the most common type of highly malignant bone tumor in children and adolescents, has poor diagnosis and 2-year survival rates of 15-20% following surgery or radiotherapy, and has therefore generated marked attention. In order to investigate the potential biomarkers for diagnosing osteosarcoma, the expression profiling data from normal and disease tissues were compared, respectively, and the differentially‑expressed genes were analyzed by three different statistical tests. Interacting proteins were determined and an interaction network was constructed by Search Tool for the Retrieval of Interacting Genes database. Subsequently, the protein interaction network was decomposed and Gene Otology annotation using Cytoscape, Mcode and Bingo, was conducted on the function modules. Finally, three differentially‑expressed genes GJA1, COL1A2 and COL5A2 were identified, and an interaction network was successfully generated with COL1A2 and COL5A2 at the core. From the results, it was observed that COL1A2 and COL5A2 interact with a number of genes of the matrix metalloprotease (MMP) family, including MMP1, MMP2, MMP3 and MMP14, TGFβ and RUNX2. Furthermore, these genes have been confirmed to be important in the tumorigenesis of osteosarcoma. It was hypothesized that the upregulation of the COL gene family may be considered as a diagnostic marker for osteosarcoma and collagen may be administered as a therapy.

GATA2 regulates differentiation of bone marrow-derived mesenchymal stem cells

The bone marrow microenvironment comprises multiple cell niches derived from bone marrow mesenchymal stem cells. However, the molecular mechanism of bone marrow mesenchymal stem cell differentiation is poorly understood. The transcription factor GATA2 is indispensable for hematopoietic stem cell function as well as other hematopoietic lineages, suggesting that it may maintain bone marrow mesenchymal stem cells in an immature state and also contribute to their differentiation. To explore this possibility, we established bone marrow mesenchymal stem cells from GATA2 conditional knockout mice. Differentiation of GATA2-deficient bone marrow mesenchymal stem cells into adipocytes induced accelerated oil-drop formation. Further, GATA2 loss- and gain-of-function analyses based on human bone marrow mesenchymal stem cells confirmed that decreased and increased GATA2 expression accelerated and suppressed bone marrow mesenchymal stem cell differentiation to adipocytes, respectively. Microarray analysis of GATA2 knockdowned human bone marrow mesenchymal stem cells revealed that 90 and 189 genes were upregulated or downregulated by a factor of 2, respectively. Moreover, gene ontology analysis revealed significant enrichment of genes involved in cell cycle regulation, and the number of G1/G0 cells increased after GATA2 knockdown. Concomitantly, cell proliferation was decreased by GATA2 knockdown. When GATA2 knockdowned bone marrow mesenchymal stem cells as well as adipocytes were cocultured with CD34-positive cells, hematopoietic stem cell frequency and colony formation decreased. We confirmed the existence of pathological signals that decrease and increase hematopoietic cell and adipocyte numbers, respectively, characteristic of aplastic anemia, and that suppress GATA2 expression in hematopoietic stem cells and bone marrow mesenchymal stem cells.

Animal models in osteosarcoma

Osteosarcoma (OS) is the most common non-hematologic primary tumor of bone in children and adults. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for non-metastatic disease approaching 70%. However, most OS tumors are high grade and tend to rapidly develop pulmonary metastases. Despite clinical advances, patients with metastatic disease or relapse have a poor prognosis. Toward a better understanding of the molecular pathogenesis of human OS, several genetically modified OS mouse models have been developed and will be reviewed here. However, better animal models that more accurately recapitulate the natural progression of the disease are needed for the development of improved prognostic and diagnostic markers as well as targeted therapies for both primary and metastatic OS.

Molecular ties between the cell cycle and differentiation in embryonic stem cells

Attainment of the differentiated state during the final stages of somatic cell differentiation is closely tied to cell cycle progression. Much less is known about the role of the cell cycle at very early stages of embryonic development. Here, we show that molecular pathways involving the cell cycle can be engineered to strongly affect embryonic stem cell differentiation at early stages in vitro. Strategies based on perturbing these pathways can shorten the rate and simplify the lineage path of ES differentiation. These results make it likely that pathways involving cell proliferation intersect at various points with pathways that regulate cell lineages in embryos and demonstrate that this knowledge can be used profitably to guide the path and effectiveness of cell differentiation of pluripotent cells.

Deep functional redundancy between FAMA and FOUR LIPS in stomatal development

Functional redundancy arises between gene paralogs as well as non-homologous genes that play a common role at a shared node. The bHLH transcription factor FAMA, along with the paralogous MYB genes, FOUR LIPS (FLP) and MYB88 all ensure that Arabidopsis stomata contain just two guard cells (GCs) by enforcing a single symmetric precursor cell division before stomatal maturity. Consistent with this function, FLP and FAMA exhibit the same expression pattern in which both translational GFP fusions emit fluorescence just before and after symmetric division; however, FAMA but not FLP is required to confer GC fate. Strikingly, swapping the genes and promoters of the FLP and FAMA genes results in the reciprocal complementation of respective loss-of-function mutants. Thus, an FLP transgene can restore GC fate to a fama mutant background. FAMA, FLP and the FLP paralog MYB88 were previously shown to influence higher order functions in stomatal development, including maintaining and stabilizing stomatal fate. Here we show that these overlapping functions are likely to also involve interactions between FLP and FAMA with the RETINOBLASTOMA-RELATED (RBR) protein.

Osteosarcoma: mouse models, cell of origin and cancer stem cell

Osteosarcoma (OS) is the most common non-hematologic primary tumor of bone in children and adults. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for non-metastatic disease approaching 70%. However, most OS tumors are high grade and tend to rapidly develop pulmonary metastases. Despite clinical advances, patients with metastatic disease or relapse have a poor prognosis. Here the cell biology of OS is reviewed with a special emphasis on mouse models as well as the roles of the cell of origin and cancer stem cells. A better understanding of the molecular pathogenesis of human OS is essential for the development of improved prognostic and diagnostic markers as well as targeted therapies for both primary and metastatic OS.

Immune response to RB1-regulated senescence limits radiation-induced osteosarcoma formation

Ionizing radiation (IR) and germline mutations in the retinoblastoma tumor suppressor gene (RB1) are the strongest risk factors for developing osteosarcoma. Recapitulating the human predisposition, we found that Rb1+/- mice exhibited accelerated development of IR-induced osteosarcoma, with a latency of 39 weeks. Initial exposure of osteoblasts to carcinogenic doses of IR in vitro and in vivo induced RB1-dependent senescence and the expression of a panel of proteins known as senescence-associated secretory phenotype (SASP), dominated by IL-6. RB1 expression closely correlated with that of the SASP cassette in human osteosarcomas, and low expression of both RB1 and the SASP genes was associated with poor prognosis. In vivo, IL-6 was required for IR-induced senescence, which elicited NKT cell infiltration and a host inflammatory response. Mice lacking IL-6 or NKT cells had accelerated development of IR-induced osteosarcomas. These data elucidate an important link between senescence, which is a cell-autonomous tumor suppressor response, and the activation of host-dependent cancer immunosurveillance. Our findings indicate that overcoming the immune response to senescence is a rate-limiting step in the formation of IR-induced osteosarcoma.

T antigen transformation reveals Tp53/RB-dependent route to PLAC1 transcription activation in primary fibroblasts

PLAC1 (placenta-specific 1) is a gene that is placenta specific and transcribed very little, if at all, in any somatic tissue. It is nevertheless expressed in many cancer cell lines. To understand how cancer cells may activate the gene in nonexpressing cells, we found that a model is provided by classical transformation of normal fibroblasts by SV40 T antigen. T antigen derepressed the PLAC1 P1 promoter, with Tp53 and RB exerting critical and opposing actions and nuclear receptors, retinoid X receptor and  liver X receptor, sharply increasing the level of expression.

Loss of the osteogenic differentiation potential during senescence is limited to bone progenitor cells and is dependent on p53

DNA damage can lead to the induction of cellular senescence. In particular, we showed that exposure to ionizing radiation (IR) leads to the senescence of bone marrow-derived multipotent stromal cells (MSC) and osteoblast-like stromal cells (OB–SC), a phenotype associated with bone loss. The mechanism by which IR leads to bone dysfunction is not fully understood. One possibility involves that DNA damage-induced senescence limits the regeneration of bone progenitor cells. Another possibility entails that bone dysfunction arises from the inability of accumulating senescent cells to fulfill their physiological function. Indeed, we show here that exposure to IR prevented the differentiation and mineralization functions of MSC, an effect we found was limited to this population as more differentiated OB–SC could still form mineralize nodules. This is in contrast to adipogenesis, which was inhibited in both IR-induced senescent MSC and 3T3-L1 pre-adipocytes. Furthermore, we demonstrate that IR-induced loss of osteogenic potential in MSC was p53-dependent, a phenotype that correlates with the inability to upregulate key osteogenic transcription factors. These results are the first to demonstrate that senescence impacts osteogenesis in a cell type dependent manner and suggest that the accumulation of senescent osteoblasts is unlikely to significantly contribute to bone dysfunction in a cell autonomous manner.

Hutchinson-Gilford progeria syndrome through the lens of transcription

Lamins are nuclear intermediate filaments. In addition to their structural roles, they are implicated in basic nuclear functions such as chromatin organization, DNA replication, transcription, DNA repair, and cell-cycle progression. Mutations in human LMNA gene cause several diseases termed laminopathies. One of the laminopathic diseases is Hutchinson-Gilford progeria syndrome (HGPS), which is caused by a spontaneous mutation and characterized by premature aging. HGPS phenotypes share certain similarities with several apparently comparable medical conditions, such as aging and atherosclerosis, with the conspicuous absence of neuronal degeneration and cancer rarity during the short lifespan of the patients. Cell lines from HGPS patients are characterized by multiple nuclear defects, which include abnormal morphology, altered histone modification patterns, and increased DNA damage. These cell lines provide insight into the molecular pathways including senescence that require lamins A and B1. Here, we review recent data on HGPS phenotypes through the lens of transcriptional deregulation caused by lack of functional lamin A, progerin accumulation, and lamin B1 silencing.

Tumor suppressor gene Rb is required for self-renewal of spermatogonial stem cells in mice

The retinoblastoma tumor suppressor gene Rb is essential for maintaining the quiescence and for regulating the differentiation of somatic stem cells. Inactivation of Rb in somatic stem cells typically leads to their overexpansion, often followed by increased apoptosis, defective terminal differentiation, and tumor formation. However, Rb's roles in germ-line stem cells have not been explored. We conditionally disrupted the Rb gene in mouse germ cells in vivo and discovered unanticipated consequences for GFRa1-protein-expressing A(single) (GFRa1(+) A(s)) spermatogonia, the major source of male germ-line stem cells. Rb-deficient GFRa1(+) A(s) spermatogonia were present at normal density in testes 5 d after birth, but they lacked the capacity for self-renewal, resulting in germ cell depletion by 2 mo of age. Rb deficiency did not affect the proliferative activity of GFRa1(+) A(s) spermatogonia, but their progeny were exclusively transit-amplifying progenitor spermatogonia and did not include GFRa1(+) A(s) spermatogonia. In addition, Rb deficiency caused prolonged proliferation of progenitor spermatogonia, transiently enlarging this population. Despite these defects, Rb deficiency did not block terminal differentiation into functional sperm; offspring were readily obtained from young males whose germ cell pool was not yet depleted. We conclude that Rb is required for self-renewal of germ-line stem cells, but contrary to its critical roles in somatic stem cells, it is dispensable for their proliferative activity and terminal differentiation. Thus, this study identifies an unexpected function for Rb in maintaining the stem cell pool in the male germ line.

Emerging roles of RB family: new defense mechanisms against tumor progression

The retinoblastoma (RB) family of proteins, including RB1/p105, retinoblastoma-like 1 (RBL1/p107), and retinoblastoma-like 2 (RBL2/p130), is principally known for its central role on cell cycle regulation. The inactivation of RB proteins confers a growth advantage and underlies multiple types of tumors. Recently, it has been shown that RB proteins have other important roles, such as preservation of chromosomal stability, induction and maintenance of senescence and regulation of apoptosis, cellular differentiation, and angiogenesis. RB proteins are involved in many cellular pathways and act as transcriptional regulators able to bind several transcription factors, thus antagonizing or potentiating their functions. Furthermore, RB proteins might control the expression of specific target genes by recruiting chromatin remodeling enzymes. Although many efforts have been made to dissect the different functions of RB proteins, it remains still unclear which are necessary for cancer suppression and the role they play at distinct steps of carcinogenesis. Moreover, RB proteins can behave differently in various cell types or cell states. Elucidating the intricate RB protein network in regulating cell fate might provide the knowledge necessary to explain their potent tumor suppressor activity and to design novel therapeutic strategies.

Cooperative activation of tissue-specific genes by pRB and E2F1

The retinoblastoma tumor suppressor protein pRB is conventionally regarded as an inhibitor of the E2F family of transcription factors. Conversely, pRB is also recognized as an activator of tissue-specific gene expression along various lineages including osteoblastogenesis. During osteoblast differentiation, pRB directly targets Alpl and Bglap, which encode the major markers of osteogenesis alkaline phosphatase and osteocalcin. Surprisingly, p130 and repressor E2Fs were recently found to cooccupy and repress Alpl and Bglap in proliferating osteoblast precursors before differentiation. This raises the further question of whether these genes convert to E2F activation targets when differentiation begins, which would constitute a remarkable situation wherein pRB and E2F would be cotargeting genes for activation. Chromatin immunoprecipitation analysis in an osteoblast differentiation model shows that Alpl and Bglap are indeed targeted by an activator E2F, i.e., is E2F1. Promoter occupation of Alpl and Bglap by E2F1 occurs specifically during activation, and depletion of E2F1 severely impairs their induction. Mechanistically, promoter occupation by E2F1 and pRB is mutually dependent, and without this cooperative effect, activation steps previously shown to be dependent on pRB, including recruitment of RNA polymerase II, are impaired. Myocyte- and adipocyte-specific genes are also cotargeted by E2F1 and pRB during differentiation along their respective lineages. The finding that pRB and E2F1 cooperate to activate expression of tissue-specific genes is a paradigm distinct from the classical concept of pRB as an inhibitor of E2F1, but is consistent with the observed roles of these proteins in physiological models.

Development of the endochondral skeleton

Much of the mammalian skeleton is composed of bones that originate from cartilage templates through endochondral ossification. Elucidating the mechanisms that control endochondral bone development is critical for understanding human skeletal diseases, injury response, and aging. Mouse genetic studies in the past 15 years have provided unprecedented insights about molecules regulating chondrocyte formation, chondrocyte maturation, and osteoblast differentiation, all key processes of endochondral bone development. These include the roles of the secreted proteins IHH, PTHrP, BMPs, WNTs, and FGFs, their receptors, and transcription factors such as SOX9, RUNX2, and OSX, in regulating chondrocyte and osteoblast biology. This review aims to integrate the known functions of extracellular signals and transcription factors that regulate development of the endochondral skeleton.

Relevance of brown adipose tissue in infancy and adolescence

Brown adipose tissue (BAT) was thought to disappear after infancy. Recent findings of BAT in patients undergoing positron emission tomography/computed tomography (PET/CT) have renewed the interest in deciphering the relevance of this tissue in humans. Available data suggest that BAT is more prevalent in children than in adults and that its activation during adolescence is associated with significantly lower gains in weight and adiposity. Data also show that pediatric patients with metabolically active BAT on PET/CT examinations have significantly greater muscle volume than patients without identifiable BAT. Both the activity and the amount of BAT increase during puberty. The magnitude of the increase is higher in boys as compared with girls and is closely related to gains in muscle volume. Hence, concurrent with the gains in skeletal muscle during infancy and puberty, all infants and adolescents accumulate large amounts of BAT. These observations are consistent with in vitro investigations suggesting close interactions between brown adipocytes, white adipocytes, and myocytes. In this review, we discuss the potential role of this tissue in regulating weight and musculoskeletal development in children.

RUNX family: Regulation and diversification of roles through interacting proteins

The Runt-related transcription factors (RUNX) belong to an ancient family of metazoan genes involved in developmental processes. Through multiple protein-interacting partners, RUNX proteins have been implicated in diverse signaling pathways and cellular processes. The frequent inactivation of RUNX genes in cancer indicates crucial roles for RUNX in tumor suppression. This review discusses the abilities of RUNX proteins, in particular RUNX3, to integrate oncogenic signals or environmental cues and to initiate appropriate tumor suppressive responses.

The differentiation stage of p53-Rb-deficient bone marrow mesenchymal stem cells imposes the phenotype of in vivo sarcoma development

Increasing evidence suggests that mesenchymal stem/stromal cells (MSCs) carrying specific mutations are at the origin of some sarcomas. We have reported that the deficiency of p53 alone or in combination with Rb (Rb(-/-) p53(-/-)) in adipose-derived MSCs (ASCs) promotes leiomyosarcoma-like tumors in vivo. Here, we hypothesized that the source of MSCs and/or the cell differentiation stage could determine the phenotype of sarcoma development. To investigate whether there is a link between the source of MSCs and sarcoma phenotype, we generated p53(-/-) and Rb(-/-)p53(-/-) MSCs from bone marrow (BM-MSCs). Both genotypes of BM-MSCs initiated leiomyosarcoma formation similar to p53(-/-) and Rb(-/-)p53(-/-) ASCs. In addition, gene expression profiling revealed transcriptome similarities between p53- or Rb-p53-deficient BM-MSCs/ASCs and muscle-associated sarcomagenesis. These data suggest that the tissue source of MSC does not seem to determine the development of a particular sarcoma phenotype. To analyze whether the differentiation stage defines the sarcoma phenotype, BM-MSCs and ASCs were induced to differentiate toward the osteogenic lineage, and both p53 and Rb were excised using Cre-expressing adenovectors at different stages along osteogenic differentiation. Regardless the level of osteogenic commitment, the inactivation of Rb and p53 in BM-MSC-derived, but not in ASC-derived, osteogenic progenitors gave rise to osteosarcoma-like tumors, which could be serially transplanted. This indicates that the osteogenic differentiation stage of BM-MSCs imposes the phenotype of in vivo sarcoma development, and that BM-MSC-derived osteogenic progenitors rather than undifferentiated BM-MSCs, undifferentiated ASCs or ASC-derived osteogenic progenitors, represent the cell of origin for osteosarcoma development.

Effector mechanisms of sunitinib-induced G1 cell cycle arrest, differentiation, and apoptosis in human acute myeloid leukaemia HL60 and KG-1 cells

Acute myeloid leukaemia (AML) is a heterogeneous disease with dismal outcome. Sunitinib is an orally active inhibitor of multiple tyrosine kinase receptors approved for renal cell carcinoma and gastrointestinal stromal tumour that has also been studied for AML in several clinical trials. However, the precise mechanism of sunitinib action against AML remains unclear and requires further investigation. For this purpose, this study was conducted using human AML cell lines (HL60 and KG-1) and AML patients' mononucleated cells. Sunitinib induced G1 phase arrest associated with decreased cyclin D1, cyclin D3, and cyclin-dependent kinase (Cdk)2 and increased p27(Kip1), pRb1, and p130/Rb2 expression and phosphorylated activation of protein kinase C alpha and beta (PKCα/β). Selective PKCα/β inhibitor treatment abolished sunitinib-elicited AML differentiation, suggesting that PKCα/β may underlie sunitinib-induced monocytic differentiation. Furthermore, sunitinib increased pro-apoptotic molecule expression (Bax, Bak, PUMA, Fas, FasL, DR4, and DR5) and decreased anti-apoptotic molecule expression (Bcl-2 and Mcl-1), resulting in caspase-2, caspase-3, caspase-8, and caspase-9 activation and both death receptor and mitochondria-dependent apoptosis. Taken together, these findings provide evidence that sunitinib targets AML cells through both differentiation and apoptosis pathways. More clinical studies are urgently needed to demonstrate its optimal clinical applications in AML.

Loss of pRB and p107 disrupts cartilage development and promotes enchondroma formation

The pocket proteins pRB, p107 and p130 have established roles in regulating the cell cycle through the control of E2F activity. The pocket proteins regulate differentiation of a number of tissues in both cell cycle-dependent and -independent manners. Prior studies showed that mutation of p107 and p130 in the mouse leads to defects in cartilage development during endochondral ossification, the process by which long bones form. Despite evidence of a role for pRB in osteoblast differentiation, it is unknown whether it functions during cartilage development. Here, we show that mutation of Rb in the early mesenchyme of p107-mutant mice results in severe cartilage defects in the growth plates of long bones. This is attributable to inappropriate chondrocyte proliferation that persists after birth and leads to the formation of enchondromas in the growth plates as early as 8 weeks of age. Genetic crosses show that development of these tumorigenic lesions is E2f3 dependent. These results reveal an overlapping role for pRB and p107 in cartilage development, endochondral ossification and enchondroma formation that reflects their coordination of cell-cycle exit at appropriate developmental stages.

Loss of wnt/β-catenin signaling causes cell fate shift of preosteoblasts from osteoblasts to adipocytes

Wnt signaling is essential for osteogenesis and also functions as an adipogenic switch, but it is not known if interrupting wnt signaling via knockout of β-catenin from osteoblasts would cause bone marrow adiposity. Here, we determined whether postnatal deletion of β-catenin in preosteoblasts, through conditional cre expression driven by the osterix promoter, causes bone marrow adiposity. Postnatal disruption of β-catenin in the preosteoblasts led to extensive bone marrow adiposity and low bone mass in adult mice. In cultured bone marrow-derived cells isolated from the knockout mice, adipogenic differentiation was dramatically increased, whereas osteogenic differentiation was significantly decreased. As myoblasts, in the absence of wnt/β-catenin signaling, can be reprogrammed into the adipocyte lineage, we sought to determine whether the increased adipogenesis we observed partly resulted from a cell-fate shift of preosteoblasts that had to express osterix (lineage-committed early osteoblasts), from the osteoblastic to the adipocyte lineage. Using lineage tracing both in vivo and in vitro we showed that the loss of β-catenin from preosteoblasts caused a cell-fate shift of these cells from osteoblasts to adipocytes, a shift that may at least partly contribute to the bone marrow adiposity and low bone mass in the knockout mice. These novel findings indicate that wnt/β-catenin signaling exerts control over the fate of lineage-committed early osteoblasts, with respect to their differentiation into osteoblastic versus adipocytic populations in bone, and thus offers potential insight into the origin of bone marrow adiposity.

Abnormal expression of seven myogenesis-related genes in extraocular muscles of patients with concomitant strabismus

Hyperplasia or hypoplasia of muscles gradually leads to strabismus. Myogenesis-related genes are involved in extraocular muscle development, including myogenic differentiation 1 (MYOD1), myogenin (MYOG), retinoblastoma 1 (RB1), cyclin-dependent kinase inhibitor 1A (P21), cyclin‑dependent kinase inhibitor 1C (P57), insulin-like growth factor 1 (IGF1) and muscle creatine kinase (MCK). This study evaluated the expression of the above seven myogenesis-related genes by real-time quantitative RT-PCR in 18 resected extrocular muscles of patients with concomitant strabismus and 12 normal control muscle samples from one presumably healthy male 6 h after sudden mortality. We found that although there was a great divergence among the expression levels of 6 myogenesis-related regulatory factors, the relative expression patterns were similar in all the normal muscles, including the synergistic, antagonistic and yoke muscles. However, their expression levels in the 18 diseased extraocular muscles were abnormal; the expression levels of all the genes, with the exception of P57, were reduced in most of the diseased muscle tissues. These results imply that the abnormal expression of these myogenesis-related genes may contribute to concomitant strabismus.

The role for runt related transcription factor 2 (RUNX2) as a transcriptional repressor in luteinizing granulosa cells

Transcription factors induced by the LH surge play a vital role in reprogramming the gene expression in periovulatory follicles. The present study investigated the role of RUNX2 transcription factor in regulating the expression of Runx1, Ptgs2, and Tnfaip6 using cultured granulosa cells isolated from PMSG-primed immature rats. hCG or forskolin+PMA induced the transient increase in Runx1, Ptgs2, and Tnfaip6 expression, while the expression of Runx2 continued to increase until 48 h. The knockdown of the agonist-stimulated Runx2 expression increased Runx1, Ptgs2, and Tnfaip6 expression and PGE(2) levels in luteinizing granulosa cells. Conversely, the over-expression of RUNX2 inhibited the expression of these genes and PGE(2) levels. The mutation of RUNX binding motifs in the Runx1 promoter enhanced transcriptional activity of the Runx1 promoter. The knockdown and overexpression of Runx2 increased and decreased Runx1 promoter activity, respectively. ChIP assays revealed the binding of RUNX2 in the Runx1 and Ptgs2 promoters. Together, these novel findings provide support for the role of RUNX2 in down-regulation of Runx1, Ptgs2, and Tnfaip6 during the late ovulatory period to support proper ovulation and/or luteinization.

Genetically engineered mouse models and human osteosarcoma

Osteosarcoma is the most common form of bone cancer. Pivotal insight into the genes involved in human osteosarcoma has been provided by the study of rare familial cancer predisposition syndromes. Three kindreds stand out as predisposing to the development of osteosarcoma: Li-Fraumeni syndrome, familial retinoblastoma and RecQ helicase disorders, which include Rothmund-Thomson Syndrome in particular. These disorders have highlighted the important roles of P53 and RB respectively, in the development of osteosarcoma. The association of OS with RECQL4 mutations is apparent but the relevance of this to OS is uncertain as mutations in RECQL4 are not found in sporadic OS. Application of the knowledge or mutations of P53 and RB in familial and sporadic OS has enabled the development of tractable, highly penetrant murine models of OS. These models share many of the cardinal features associated with human osteosarcoma including, importantly, a high incidence of spontaneous metastasis. The recent development of these models has been a significant advance for efforts to improve our understanding of the genetics of human OS and, more critically, to provide a high-throughput genetically modifiable platform for preclinical evaluation of new therapeutics.

The Rb/E2F pathway modulates neurogenesis through direct regulation of the Dlx1/Dlx2 bigene cluster

During brain morphogenesis, the mechanisms through which the cell cycle machinery integrates with differentiation signals remain elusive. Here we show that the Rb/E2F pathway regulates key aspects of differentiation and migration through direct control of the Dlx1 and Dlx2 homeodomain proteins, required for interneuron specification. Rb deficiency results in a dramatic reduction of Dlx1 and Dlx2 gene expression manifested by loss of interneuron subtypes and severe migration defects in the mouse brain. The Rb/E2F pathway modulates Dlx1/Dlx2 regulation through direct interaction with a Dlx forebrain-specific enhancer, I12b, and the Dlx1/Dlx2 proximal promoter regions, through repressor E2F sites both in vitro and in vivo. In the absence of Rb, we demonstrate that repressor E2Fs inhibit Dlx transcription at the Dlx1/Dlx2 promoters and Dlx1/2-I12b enhancer to suppress differentiation. Our findings support a model whereby the cell cycle machinery not only controls cell division but also modulates neuronal differentiation and migration through direct regulation of the Dlx1/Dlx2 bigene cluster during embryonic development.

The retinoblastoma tumor suppressor and stem cell biology

Stem cells play a critical role during embryonic development and in the maintenance of homeostasis in adult individuals. A better understanding of stem cell biology, including embryonic and adult stem cells, will allow the scientific community to better comprehend a number of pathologies and possibly design novel approaches to treat patients with a variety of diseases. The retinoblastoma tumor suppressor RB controls the proliferation, differentiation, and survival of cells, and accumulating evidence points to a central role for RB activity in the biology of stem and progenitor cells. In some contexts, loss of RB function in stem or progenitor cells is a key event in the initiation of cancer and determines the subtype of cancer arising from these pluripotent cells by altering their fate. In other cases, RB inactivation is often not sufficient to initiate cancer but may still lead to some stem cell expansion, raising the possibility that strategies aimed at transiently inactivating RB might provide a novel way to expand functional stem cell populations. Future experiments dedicated to better understanding how RB and the RB pathway control a stem cell's decisions to divide, self-renew, or give rise to differentiated progeny may eventually increase our capacity to control these decisions to enhance regeneration or help prevent cancer development.

Regulation of transcription and chromatin structure by pRB: here, there and everywhere

Commitment to divide is one of the most crucial steps in the mammalian cell division cycle. It is critical for tissue and organismal homeostasis, and consequently is highly regulated. The vast majority of cancers evade proliferative control, further emphasizing the importance of the commitment step in cell cycle regulation. The Retinoblastoma (RB) tumor suppressor pathway regulates this decision-making step. Since being the subject of Knudson's 'two hit hypothesis', there has been considerable interest in understanding pRB's role in cancer. It is best known for repressing E2F dependent transcription of cell cycle genes. However, pRB's role in controlling chromatin structure is expanding and bringing it into new regulatory paradigms. In this review we discuss pRB function through protein-protein interactions, at the level of transcriptional regulation of individual promoters and in organizing higher order chromatin domains.

RBF binding to both canonical E2F targets and noncanonical targets depends on functional dE2F/dDP complexes

The retinoblastoma (RB) family of proteins regulate transcription. These proteins lack intrinsic DNA-binding activity but are recruited to specific genomic locations through interactions with sequence-specific DNA-binding factors. The best-known target of RB protein (pRB) is the E2F transcription factor; however, many other chromatin-associated proteins have been described that may allow RB family members to act at additional sites. To gain a perspective on the scale of E2F-dependent and E2F-independent functions, we generated genome-wide binding profiles of RBF1 and dE2F proteins in Drosophila larvae. RBF1 and dE2F2 associate with a large number of binding sites at genes with diverse biological functions. In contrast, dE2F1 was detected at a smaller set of promoters, suggesting that it overrides repression by RBF1/dE2F2 at a specific subset of targets. Approximately 15% of RBF1-bound regions lacked consensus E2F-binding motifs. To test whether RBF1 action at these sites is E2F independent, we examined dDP mutant larvae that lack any functional dE2F/dDP heterodimers. As measured by chromatin immunoprecipitation-microarray analysis (ChIP-chip), ChIP-quantitative PCR (qPCR), and cell fractionation, the stable association of RBF1 with chromatin was eliminated in dDP mutants. This requirement for dDP was seen at classic E2F-regulated promoters and at promoters that lacked canonical E2F-binding sites. These results suggest that E2F/DP complexes are essential for all genomic targeting of RBF1.

Brown adipose tissue and its relationship to bone structure in pediatric patients

CONTEXT

Emerging evidence suggests a possible link between brown adipose tissue (BAT) and bone metabolism.

OBJECTIVE

The objective of this study was to examine the relationships between BAT and bone cross-sectional dimensions in children and adolescents.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at a pediatric referral center.

PATIENTS

Patients included 40 children and teenagers (21 males and 19 females) successfully treated for pediatric malignancies.

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

The volume of BAT was determined by fluorodeoxyglucose-positron emission tomography/computed tomography. Measures of the cross-sectional area and cortical bone area and measures of thigh musculature and sc fat were determined at the midshaft of the femur.

RESULTS

Regardless of sex, there were significant correlations seen between BAT volume and the cross-sectional dimensions of the bone (r values between 0.68 and 0.77; all P ≤ 0 .001). Multiple regression analyses indicated that the volume of BAT predicted femoral cross-sectional area and cortical bone area, even after accounting for height, weight, and gender. The addition of muscle as an independent variable increased the predictive power of the model but significantly decreased the contribution of BAT.

CONCLUSIONS

The volume of BAT is positively associated with the amount of bone and the cross-sectional size of the femur in children and adolescents. This relation between BAT and bone structure could, at least in part, be mediated by muscle.

Strontium ranelate rebalances bone marrow adipogenesis and osteoblastogenesis in senescent osteopenic mice through NFATc/Maf and Wnt signaling

With aging, bone marrow mesenchymal stromal cell (MSC) osteoblast differentiation decreases whereas MSC differentiation into adipocytes increases, resulting in increased adipogenesis and bone loss. Here, we investigated whether activation of cell signaling by strontium ranelate (SrRan) can reverse the excessive adipogenic differentiation associated with aging. In murine MSC cultures, SrRan increased Runx2 expression and matrix mineralization and decreased PPARγ2 expression and adipogenesis. This effect was associated with increased expression of the Wnt noncanonical representative Wnt5a and adipogenic modulator Maf and was abrogated by Wnt- and nuclear factor of activated T-cells (NFAT)c antagonists, implying a role for Wnt and NFATc/Maf signaling in the switch in osteoblastogenesis to adipogenesis induced by SrRan. To confirm this finding, we investigated the effect of SrRan in SAMP6 senescent mice, which exhibit decreased osteoblastogenesis, increased adipogenesis, and osteopenia. SrRan administration at a clinically relevant dose level increased bone mineral density, bone volume, trabecular thickness and number, as shown by densitometric, microscanning, and histomorphometric analyses in long bones and vertebrae. This attenuation of bone loss was related to increased osteoblast surface and bone formation rate and decreased bone marrow adipocyte volume and size. The restoration of osteoblast and adipocyte balance induced by SrRan was linked to increased Wnt5a and Maf expression in the bone marrow. The results indicate that SrRan acts on lineage allocation of MSCs by antagonizing the age-related switch in osteoblast to adipocyte differentiation via mechanisms involving NFATc/Maf and Wnt signaling, resulting in increased bone formation and attenuation of bone loss in senescent osteopenic mice.

Perspectives on cancer stem cells in osteosarcoma

Osteosarcoma is an aggressive pediatric tumor of growing bones that, despite surgery and chemotherapy, is prone to relapse. These mesenchymal tumors are derived from progenitor cells in the osteoblast lineage that have accumulated mutations to escape cell cycle checkpoints leading to excessive proliferation and defects in their ability to differentiate appropriately into mature bone-forming osteoblasts. Like other malignant tumors, osteosarcoma is often heterogeneous, consisting of phenotypically distinct cells with features of different stages of differentiation. The cancer stem cell hypothesis posits that tumors are maintained by stem cells and it is the incomplete eradication of a refractory population of tumor-initiating stem cells that accounts for drug resistance and tumor relapse. In this review we present our current knowledge about the biology of osteosarcoma stem cells from mouse and human tumors, highlighting new insights and unresolved issues in the identification of this elusive population. We focus on factors and pathways that are implicated in maintaining such cells, and differences from paradigms of epithelial cancers. Targeting of the cancer stem cells in osteosarcoma is a promising avenue to explore to develop new therapies for this devastating childhood cancer.

Loss of the retinoblastoma tumor suppressor protein in murine calvaria facilitates immortalization of osteoblast-adipocyte bipotent progenitor cells characterized by low expression of N-cadherin

The retinoblastoma gene, RB1, is frequently inactivated in a subset of tumors, including retinoblastoma and osteosarcoma (OS). One characteristic of OS, as well as other tumors in which RB1 is frequently inactivated, is the lack of N-cadherin-mediated cell-cell adhesions. The frequent inactivation of RB1 and parallel loss of N-cadherin expression in OS prompted us to ask whether these observations are directly related to each other. In this study, we observed reduced N-cadherin expression in RB1(-/-) calvarial osteoblasts. In addition, RB1(-/-) cell lines had increased migration potential compared to their RB1(+/+) counterparts. These properties of RB1(-/-) cell lines correlated with an adipogenic potential lacking in RB1(+/+) cell lines, suggesting that each property is present in an immature progenitor cell. The isolation of a cell population with low surface expression of N-cadherin and enhanced adipogenic ability supports this view. Interestingly, the acute loss of pRb does not affect N-cadherin expression or migration or confer adipogenic potential to immortalized RB1(+/+) calvarial cells, suggesting that these traits are not a direct consequence of pRb loss; rather, pRb loss leads to the expansion and immortalization of an immature progenitor pool characterized by these properties.