p53 functions as a negative regulator of osteoblastogenesis, osteoblast-dependent osteoclastogenesis, and bone remodeling

The role of the BH3-only protein Noxa in bone homeostasis

Bone homeostasis is maintained by a dynamic balance between bone resorption by osteoclasts and bone formation by osteoblasts. Since excessive osteoclast activity is implicated in pathological bone resorption, understanding the mechanism underlying osteoclast differentiation, function and survival is of both scientific and clinical importance. Osteoclasts are monocyte/macrophage lineage cells with a short life span that undergo rapid apoptosis, the rate of which critically determines the level of bone resorption in vivo. However, the molecular basis of rapid osteoclast apoptosis remains obscure. Here we report the role of a BH3-only protein, Noxa (encoded by the Pmaip1 gene), in bone homeostasis using Noxa-deficient mice. Among the Bcl-2 family members, Noxa was selectively induced during osteoclastogenesis. Mice lacking Noxa exhibit a severe osteoporotic phenotype due to an increased number of osteoclasts. Noxa deficiency did not have any effect on the number of osteoclast precursor cells or the expression of osteoclast-specific genes, but led to a prolonged survival of osteoclasts. Furthermore, adenovirus-mediated Noxa overexpression remarkably reduced bone loss in a model of inflammation-induced bone destruction. This study reveals Noxa to be a crucial regulator of osteoclast apoptosis, and may provide a molecular basis for a new therapeutic approach to bone diseases.

Time series gene expression profiling and temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization

Background

BMP6 mediated osteoblast differentiation plays a key role in skeletal development and bone disease. Unfortunately, the signaling pathways regulated by BMP6 are largely uncharacterized due to both a lack of data and the complexity of the response.

Results

To better characterize the signaling pathways responsive to BMP6, we conducted a time series microarray study to track BMP6 induced osteoblast differentiation and mineralization. These temporal data were analyzed using a customized gene set analysis approach to identify temporally coherent sets of genes that act downstream of BMP6. Our analysis identified BMP6 regulation of previously reported pathways, such as the TGF-beta pathway. We also identified previously unknown connections between BMP6 and pathways such as Notch signaling and the MYB and BAF57 regulatory modules. In addition, we identify a super-network of pathways that are sequentially activated following BMP6 induction.

Conclusion

In this work, we carried out a microarray-based temporal regulatory pathway analysis of BMP6 induced osteoblast differentiation and mineralization using GAGE method. This novel temporal analysis is more informative and powerful than the classical static pathway analysis in that: (1) it captures the interconnections between signaling pathways or functional modules and demonstrates the even higher level organization of molecular biological systems; (2) it describes the temporal perturbation patterns of each pathway or module and their dynamic roles in osteoblast differentiation. The same set of experimental and computational strategies employed in our work could be useful for studying other complex biological processes.

Ascorbic acid regulates osterix expression in osteoblasts by activation of prolyl hydroxylase and ubiquitination-mediated proteosomal degradation pathway

Mouse genetic studies reveal that ascorbic acid (AA) is essential for osteoblast (OB) differentiation and that osterix (Osx) was a key downstream target of AA action in OBs. To determine the molecular pathways for AA regulation of Osx expression, we evaluated if AA regulates Osx expression by regulating production and/or actions of local growth factors and extracellular matrix (ECM) proteins. Inhibition of actions of IGFs by inhibitory IGFBP-4, BMPs by noggin, and ECM-mediated integrin signaling by RGD did not block AA effects on Osx expression in OBs. Furthermore, blockade of components of MAPK signaling pathway had no effect on AA-induced Osx expression. Because AA is required for prolyl hydroxylase domain (PHD) activity and because PHD-induced prolyl-hydroxylation targets proteins to proteosomal degradation, we next tested if AA effect on Osx expression involves activation of PHD to hydroxylate and induce ubiquitin-proteosome-mediated degradation of transcriptional repressor(s) of Osx gene. Treatment of OBs with dimethyloxallyl glycine and ethyl 3, 4-dihydroxybenzoate, known inhibitors of PHD, completely blocked AA effect on Osx expression and OB differentiation. Knockdown of PHD2 expression by Lentivirus-mediated shRNA abolished AA-induced Osx induction and alkaline phosphatase activity. Furthermore, treatment of OBs with MG115, inhibitor of proteosomal degradation, completely blocked AA effects on Osx expression. Based on these data, we conclude that AA effect on Osx expression is mediated via a novel mechanism that involves PHD2 and proteosomal degradation of a yet to be identified transcriptional repressor that is independent of BMP, IGF-I, or integrin-mediated signaling in mouse OBs.

The ARF tumor suppressor regulates bone remodeling and osteosarcoma development in mice

The ARF tumor suppressor regulates p53 as well as basic developmental processes independent of p53, including osteoclast activation, by controlling ribosomal biogenesis. Here we provide evidence that ARF is a master regulator of bone remodeling and osteosarcoma (OS) development in mice. Arf^-/- mice displayed increased osteoblast (OB) and osteoclast (OC) activity with a significant net increase in trabecular bone volume. The long bones of Arf^-/- mice had increased expression of OB genes while Arf^-/- OB showed enhanced differentiation in vitro. Mice transgenic for the Tax oncogene develop lymphocytic tumors with associated osteolytic lesions, while Tax+Arf^-/- mice uniformly developed spontaneous OS by 7 months of age. Tax+Arf^-/- tumors were well differentiated OS characterized by an abundance of new bone with OC recruitment, expressed OB markers and displayed intact levels of p53 mRNA and reduced Rb transcript levels. Cell lines established from OS recapitulated characteristics of the primary tumor, including the expression of mature OB markers and ability to form mineralized tumors when transplanted. Loss of heterozygosity in OS tumors arising in Tax+Arf^+/- mice emphasized the necessity of ARF-loss in OS development. Hypothesizing that inhibition of ARF-regulated bone remodeling would repress development of OS, we demonstrated that treatment of Tax+Arf^-/- mice with zoledronic acid, a bisphosphonate inhibitor of OC activity and repressor of bone turnover, prevented or delayed the onset of OS. These data describe a novel role for ARF as a regulator of bone remodeling through effects on both OB and OC. Finally, these data underscore the potential of targeting bone remodeling as adjuvant therapy or in patients with genetic predispositions to prevent the development of OS.

The effects of Sp7/Osterix gene silencing in the chondroprogenitor cell line, ATDC5

Chondrocytes are known to express Sp7/Osterix (Osx) to varying degrees, but the role of Osx in chondrocytes is still unknown. In the current study, we investigated the role of the Osx gene using the clonal mouse embryonic cell line ATDC5, which retains the properties of the chondroprogenitor. ATDC5 cells express Osx; therefore, the effects of Osx gene silencing with shRNA lentiviral particles on chondrocyte marker gene expression and alkaline phosphatase (ALP) activity were investigated. At confluency, gene silencing down-regulated expression of the Sox trio (Sox5, 6, 9), Dlx5 and Alp mRNA, as well as ALP enzyme activity. Bone morphogenetic protein 2 (BMP2) is known to induce Osx gene expression in chondrocytes, and stimulation with BMP2 rescued Osx expression accompanied by up-regulation of Alp expression and ALP enzyme activity in a dose-dependent manner. To clarify the role of Osx in chondrocyte differentiation, cells induced to differentiate by 10μg/ml insulin for 21days were examined. Gene silencing inhibited the expression of the hypertrophic chondrocyte marker gene, type X collagen (Col X), and attenuated up-regulation of Dlx5 and Alp mRNA, as well as ALP enzyme activity. Taken together, these results suggest that Osx is involved in chondrogenic gene activation and is a positive regulator of the chondrocyte differentiation.

Extracellular microfibrils control osteoblast-supported osteoclastogenesis by restricting TGF{beta} stimulation of RANKL production

Mutations in fibrillin-1 or fibrillin-2, the major structural components of extracellular microfibrils, cause pleiotropic manifestations in Marfan syndrome and congenital contractural arachnodactyly, respectively. We recently found that fibrillin-1 and fibrillin-2 control bone formation by regulating osteoblast differentiation through the differential modulation of endogenous TGFβ and bone morphogenetic protein signals. Here, we describe in vivo and ex vivo experiments that implicate the fibrillins as negative regulators of bone resorption. Adult Fbn2(-/-) mice display a greater than normal osteolytic response to locally implanted lipopolysaccharide-coated titanium particles. Although isolated cultures of Fbn2(-/-) preosteoclasts exhibited normal differentiation and activity, these features were substantially augmented when mutant or wild-type preosteoclasts were co-cultured with Fbn2(-/-) but not wild-type osteoblasts. Greater osteoclastogenic potential of Fbn2(-/-) osteoblasts was largely accounted for by up-regulation of the Rankl gene secondary to heightened TGFβ activity. This conclusion was based on the findings that blockade of TGFβ signaling blunts Rankl up-regulation in Fbn2(-/-) osteoblasts and bones and that systemic TGFβ antagonism improves locally induced osteolysis in Fbn2(-/-) mice. Abnormally high Rankl expression secondary to elevated TGFβ activity was also noted in cultured osteoblasts from Fbn1(-/-) mice. Collectively our data demonstrated that extracellular microfibrils balance local catabolic and anabolic signals during bone remodeling in addition to implying distinct mechanisms of bone loss in Marfan syndrome and congenital contractural arachnodactyly.

Fibrillin-1 and -2 differentially modulate endogenous TGF-β and BMP bioavailability during bone formation

Extracellular microfibrils composed of fibrillin-1 and -2 regulate bone formation through modulation of TGF-β and BMP signaling.

Extracellular regulation of signaling by transforming growth factor (TGF)–β family members is emerging as a key aspect of organ formation and tissue remodeling. In this study, we demonstrate that fibrillin-1 and -2, the structural components of extracellular microfibrils, differentially regulate TGF-β and bone morphogenetic protein (BMP) bioavailability in bone. Fibrillin-2–null (Fbn2^−/−) mice display a low bone mass phenotype that is associated with reduced bone formation in vivo and impaired osteoblast maturation in vitro. This Fbn2^−/− phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization. Cultured osteoblasts from Fbn1^−/− mice exhibit improper latent TGF-β activation as well, but mature faster because of increased availability of otherwise matrix-bound BMPs. Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition. Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

Coordination of chondrogenesis and osteogenesis by hypertrophic chondrocytes in endochondral bone development

Mammalian bones have three distinct origins (paraxial mesoderm, lateral plate mesoderm, and neural crest) and undergo two different modes of formation (intramembranous and endochondral). Bones derived from the paraxial mesoderm and lateral plate mesoderm mainly form through the endochondral process. During this process, hypertrophic chondrocytes play a vital role in inducing osteogenesis. So far, a number of published papers have provided evidence that chondrocyte hypertrophy and osteoblast differentiation are controlled by a variety of signaling pathways and factors; however, little is known about their hierarchy (which are upstream? which are most potent?). In this review, we discuss the signaling pathways and transcriptional factors regulating chondrocyte hypertrophy and osteoblast differentiation based on the evidence that has been reported and confirmed by multiple independent groups. We then discuss which factor would provide the most coherent evidence for its role in endochondral ossification.

Sensitivity to cadmium-chloride-induced forelimb ectrodactyly is independent of the p53 gene-dosage in the C57BL/6J mouse

BACKGROUND

The p53 pathway plays an important role in the regulation of apoptosis, osteoblast differentiation, skeletal development, and teratogenic sensitivity. The administration of cadmium chloride (CdCl(2)) on gestational day 9 in susceptible mouse strains causes postaxial forelimb ectrodactyly in a percentage of fetuses through unknown mechanisms. In this study, the hypothesis that the p53 gene dosage might affect the incidence or severity of CdCl(2)-induced forelimb ectrodactyly was examined.

METHODS

Heterozygous p53-null female mice, on the C57BL/6J background known to be sensitive to CdCl(2)-induced forelimb ectrodactyly, were mated with heterozygous males and then treated with a single intraperitoneal (ip) dose of CdCl(2) (4 mg x kg(-1)) at embryonic day (ED) 9. Embryos and fetuses, genotyped using DNA isolated from the yolk sacs, were collected at ED10 and examined for the pattern of cell death in the limb buds or collected at ED18 and examined for limb malformations.

RESULTS

In the wild type and heterozygous p53 embryonic limb buds, CdCl(2)-induced apoptosis involved mesenchymal cells as well as the apical ectodermal ridge (AER), whereas CdCl(2)-induced apoptosis was restricted mainly to the AER in the homozygous p53-null limb buds. No difference in the incidence or severity of forelimb ectrodactyly in the embryos of different p53 genotypes was observed.

CONCLUSION

Despite the fact that CdCl(2) induced both p53-dependent (in the mesenchyme) and p53-independent (in the AER) cell death in the developing limb bud, CdCl(2)-induced ectrodactyly was independent of the p53 gene dosage at the studied time point.

Mouse models of p53 functions

Studies in mice have yielded invaluable insight into our understanding of the p53 pathway. Mouse models with activated p53, no p53, and mutant p53 have queried the role of p53 in development and tumorigenesis. In these models, p53 is activated and stabilized via redundant posttranslational modifications. On activation, p53 initiates two major responses: inhibition of proliferation (via cell-cycle arrest, quiescence, senescence, and differentiation) and induction of apoptosis. Importantly, these responses are cell-type and tumor-type-specific. The analysis of mutant p53 alleles has established a gain-of-function role for p53 mutants in metastasis. The development of additional models that can precisely time the oncogenic events in single cells will provide further insight into the evolution of tumors, the importance of the stroma, and the cooperating events that lead to disruption of the p53 pathway. Ultimately, these models should serve to study the effects of novel drugs on tumor response as well as normal homeostasis.

Runx2, p53, and pRB status as diagnostic parameters for deregulation of osteoblast growth and differentiation in a new pre-chemotherapeutic osteosarcoma cell line (OS1)

Osteosarcomas are the most prevalent primary bone tumors found in pediatric patients. To understand their molecular etiology, cell culture models are used to define disease mechanisms under controlled conditions. Many osteosarcoma cell lines (e.g., SAOS-2, U2OS, MG63) are derived from Caucasian patients. However, patients exhibit individual and ethnic differences in their responsiveness to irradiation and chemotherapy. This motivated the establishment of osteosarcoma cell lines (OS1, OS2, OS3) from three ethnically Chinese patients. OS1 cells, derived from a pre-chemotherapeutic tumor in the femur of a 6-year-old female, were examined for molecular markers characteristic for osteoblasts, stem cells, and cell cycle control by immunohistochemistry, reverse transcriptase-PCR, Western blotting and flow cytometry. OS1 have aberrant G-banded karyotypes, possibly reflecting chromosomal abnormalities related to p53 deficiency. OS1 had ossification profiles similar to human fetal osteoblasts rather than SAOS-2 which ossifies ab initio (P < 0.05). Absence of p53 correlates with increased Runx2 expression, while the slow proliferation of OS1 cells is perhaps attenuated by pRB retention. OS1 express mesenchymal stem cell markers (CD44, CD105) and differ in relative expression of CD29, CD63, and CD71 to SAOS-2. (P < 0.05). Cell cycle synchronization with nocodazole did not affect Runx2 and CDK1 levels but decreased cyclin-E and increased cyclin-A (P < 0.05). Xenotransplantion of OS1 in SCID mice yields spontaneous tumors that were larger and grew faster than SAOS-2 transplants. Hence, OS1 is a new osteosarcoma cell culture model derived from a pre-chemotherapeutic ethnic Chinese patient, for mechanistic studies and development of therapeutic strategies to counteract metastasis and deregulation of mesenchymal development.

Tumour suppression by p53: a role for the DNA damage response?

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

p53 regulates the proliferation, differentiation and spontaneous transformation of mesenchymal stem cells

Mesenchymal stem cells (MSC) have been extensively studied and gained wide popularity due to their therapeutic potential. Spontaneous transformation of MSC, from both human and murine origin, has been reported in many studies. MSC transformation depends on the culture conditions, the origin of the cells and the time on culture; however, the precise biological characteristics involved in this process have not been fully defined yet. In this study, we investigated the role of p53 in the biology and transformation of murine bone marrow (BM)-derived MSC. We demonstrate that the MSC derived from p53KO mice showed an augmented proliferation rate, a shorter doubling time and also morphologic and phenotypic changes, as compared to MSC derived from wild-type animals. Furthermore, the MSC devoid of p53 had an increased number of cells able to generate colonies. In addition, not only proliferation but also MSC differentiation is controlled by p53 since its absence modifies the speed of the process. Moreover, genomic instability, changes in the expression of c-myc and anchorage independent growth were also observed in p53KO MSC. In addition, the absence of p53 implicates the spontaneous transformation of MSC in long-term cultures. Our results reveal that p53 plays a central role in the biology of MSC.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

p53 Deficiency leads to compensatory up-regulation of p16INK4a

p53-p21-cyclin-dependent kinase and p16(INK4a)-cyclin-dependent kinase pathways have parallel functions in preventing tumorigenesis. In cancer patients, tumor suppressor p53 is frequently inactivated through mutations, whereas p16(INK4a) is silenced through promoter methylation. However, the interaction between these two pathways is less well understood. Here, we report that p53 controls p16(INK4a) expression in a unique way. p53 deficiency led to up-regulation of p16(INK4a) in primary mouse embryonic fibroblasts, osteoblasts, and various mouse organs, and an increase in the p16(INK4a) promoter activity, without affecting the half-life of p16(INK4a). Reconstitution of p53, but not mutant p53, restored the proper expression of p16(INK4a). These results indicate that p53 is necessary in repressing p16(INK4a) expression. However, up-regulation of p53 in response to genotoxic stress or nutlin-3 treatment did not down-regulate p16(INK4a). p53 did not repress the p16(INK4a) promoter activity either. These findings suggest that p53 has a necessary but not sufficient role in repressing p16(INK4a) expression. p16(INK4a) elevation in p53(-/-) cells is, at least partially, mediated by Ets1, a known positive regulator of p16(INK4a), as p53 deficiency up-regulated Ets1 through protein stabilization and knockdown of Ets1 down-regulated p16(INK4a) expression in p53(-/-) mouse embryonic fibroblasts. These studies uncover a compensatory mechanism for the loss of p53 and provide a basis for targeting both p53 and p16(INK4a) in cancer therapy.

Ethanol impairs estrogen receptor signaling resulting in accelerated activation of senescence pathways, whereas estradiol attenuates the effects of ethanol in osteoblasts

Epidemiological and animal studies have suggested that chronic alcohol consumption is a major risk factor for osteoporosis. Using bone from cycling female rats infused chronically with ethanol (EtOH) in vivo and osteoblastic cells in vitro, we found that EtOH significantly increased estrogen receptor alpha (ERalpha) and beta (ERbeta) mRNA and ERalpha protein levels. Treatment with 17beta-estradiol (E2) in vivo and in vitro interfered with these effects of EtOH on bone and osteoblastic cells. ERalpha agonist propylpyrazoletriol (PPT) and ERbeta agonist diarylpropionitrile (DPN) attenuated EtOH-induced ERalpha and ERbeta gene overexpression, respectively. Similar to the ER antagonist ICI 182780, EtOH blocked nuclear translocation of ERalpha-ECFP in the presence of E2 in UMR-106 osteoblastic cells. EtOH also downregulated ERE-luc reporter activity. On the other hand, EtOH by itself upregulated some common ERalpha- and ERbeta-mediated genes apparently by an ER-independent pathway. EtOH also transactivated the luciferase activity of the p21 promoter region independent of additional exogenous ERalpha, activated p21 and p53, and stimulated senescence-associated beta-galactosidase activity in rat stromal osteoblasts. E2 treatment attenuated these EtOH actions. We conclude that inhibitory cross-talk between EtOH and E2 in osteoblasts on ERs, p53/p21, and cell senescence provides a pathophysiologic mechanism underlying bone loss and the protective effects of estrogens in alcohol-exposed females.

Protein palmitoylation regulates osteoblast differentiation through BMP-induced osterix expression

Osteoporosis is one of the most common diseases and can be treated by either anti-resorption drugs, anabolic drugs, or both. To search for anabolic drug targets for osteoporosis therapy, it is crucial to understand the biology of bone forming cells, osteoblasts, in terms of their proliferation, differentiation, and function. Here we found that protein palmitoylation participates in signaling pathways that control osterix expression and osteoblast differentiation. Mouse calvarial osteoblasts express most of the 24 palmitoyl transferases, with some being up-regulated during differentiation. Inhibition of protein palmitoylation, with a substrate-analog inhibitor, diminished osteoblast differentiation and mineralization, but not proliferation or survival. The decrease in differentiation capacity is associated with a reduction in osterix, but not Runx2 or Atf4. Inhibition of palmitoyl transferases had little effect in p53(-/-) osteoblasts that show accelerated differentiation due to overexpression of osterix, suggesting that osterix, at least partially, mediated the effect of inhibition of palmitoyl transferases on osteoblast differentiation. BMPs are the major driving force of osteoblast differentiation in the differentiation assays. We found that inhibition of palmitoyl transferases also compromised BMP2-induced osteoblast differentiation through down-regulating osterix induction. However, palmitoyl transferases inhibitor did not inhibit Smad1/5/8 activation. Instead, it compromised the activation of p38 MAPK, which are known positive regulators of osterix expression and differentiation. These results indicate that protein palmitoylation plays an important role in BMP-induced MAPK activation, osterix expression, and osteoblast differentiation.

p53 plays a role in mesenchymal differentiation programs, in a cell fate dependent manner

Background

The tumor suppressor p53 is an important regulator that controls various cellular networks, including cell differentiation. Interestingly, some studies suggest that p53 facilitates cell differentiation, whereas others claim that it suppresses differentiation. Therefore, it is critical to evaluate whether this inconsistency represents an authentic differential p53 activity manifested in the various differentiation programs.

Methodology/Principal Findings

To clarify this important issue, we conducted a comparative study of several mesenchymal differentiation programs. The effects of p53 knockdown or enhanced activity were analyzed in mouse and human mesenchymal cells, representing various stages of several differentiation programs. We found that p53 down-regulated the expression of master differentiation-inducing transcription factors, thereby inhibiting osteogenic, adipogenic and smooth muscle differentiation of multiple mesenchymal cell types. In contrast, p53 is essential for skeletal muscle differentiation and osteogenic re-programming of skeletal muscle committed cells.

Conclusions

These comparative studies suggest that, depending on the specific cell type and the specific differentiation program, p53 may exert a positive or a negative effect, and thus can be referred as a “guardian of differentiation” at large.

The retinoblastoma protein tumor suppressor is important for appropriate osteoblast differentiation and bone development

Mutation of the retinoblastoma (RB) tumor suppressor gene is strongly linked to osteosarcoma formation. This observation and the documented interaction between the retinoblastoma protein (pRb) and Runx2 suggests that pRb is important in bone development. To assess this hypothesis, we used a conditional knockout strategy to generate pRb-deficient embryos that survive to birth. Analysis of these embryos shows that Rb inactivation causes the abnormal development and impaired ossification of several bones, correlating with an impairment in osteoblast differentiation. We further show that Rb inactivation acts to promote osteoblast differentiation in vitro and, through conditional analysis, establish that this occurs in a cell-intrinsic manner. Although these in vivo and in vitro differentiation phenotypes seem paradoxical, we find that Rb-deficient osteoblasts have an impaired ability to exit the cell cycle both in vivo and in vitro that can explain the observed differentiation defects. Consistent with this observation, we show that the cell cycle and the bone defects in Rb-deficient embryos can be suppressed by deletion of E2f1, a known proliferation inducer that acts downstream of Rb. Thus, we conclude that pRb plays a key role in regulating osteoblast differentiation by mediating the inhibition of E2F and consequently promoting cell cycle exit.

Elevated expression of Runx2 as a key parameter in the etiology of osteosarcoma

To understand the molecular etiology of osteosarcoma, we isolated and characterized a human osteosarcoma cell line (OS1). OS1 cells have high osteogenic potential in differentiation induction media. Molecular analysis reveals OS1 cells express the pocket protein pRB and the runt-related transcription factor Runx2. Strikingly, Runx2 is expressed at higher levels in OS1 cells than in human fetal osteoblasts. Both pRB and Runx2 have growth suppressive potential in osteoblasts and are key factors controlling competency for osteoblast differentiation. The high levels of Runx2 clearly suggest osteosarcomas may form from committed osteoblasts that have bypassed growth restrictions normally imposed by Runx2. Interestingly, OS1 cells do not exhibit p53 expression and thus lack a functional p53/p21 DNA damage response pathway as has been observed for other osteosarcoma cell types. Absence of this pathway predicts genomic instability and/or vulnerability to secondary mutations that may counteract the anti-proliferative activity of Runx2 that is normally observed in osteoblasts. We conclude OS1 cells provide a valuable cell culture model to examine molecular events that are responsible for the pathologic conversion of phenotypically normal osteoblast precursors into osteosarcoma cells.

Polyclonal antibody to porcine p53 protein: a new tool for studying the p53 pathway in a porcine model

Although the tumor suppressor protein p53 is important in the control of various cellular activities, the analysis of p53 in the porcine model has been hampered by a lack of a suitable antibody that is specific for porcine p53. Using a recombinant porcine p53, we generated a rabbit polyclonal antibody (designated SH0797) that is directed against porcine p53. The results of the study show that the antibody is capable of detecting recombinant p53 protein expressed in Escherichia coli, as well as FLAG-tagged p53 that is expressed in the transfected cells. This demonstrates that the antibody is specific for the porcine p53 protein. The antibody also showed the ability to immunoprecipitate p53 protein from extracts of porcine cells and to cross-react with human p53 protein. In addition, expression of porcine p53 could be induced readily in porcine cells and detected using this new tool. This antibody is a useful tool for use in studies of the cellular pathways that involve p53 in the porcine model.

Nicotine induces cell proliferation in association with cyclin D1 up-regulation and inhibits cell differentiation in association with p53 regulation in a murine pre-osteoblastic cell line

Recent studies have suggested that nicotine critically affects bone metabolism. Many studies have examined the effects of nicotine on proliferation and differentiation, but the underlying molecular mechanisms remain unclear. We examined cell cycle regulators involved in the proliferation and differentiation of MC3T3-E1 cells. Nicotine induced cell proliferation in association with p53 down-regulation and cyclin D1 up-regulation. In differentiated cells, nicotine reduced alkaline phosphatase activity and mineralized nodule formation in dose-dependent manners. Furthermore, p53 expression was sustained in nicotine-treated cells during differentiation. These findings indicate that nicotine promotes the cell cycle and inhibits differentiation in association with p53 regulation in pre-osteoblastic cells.

Metastatic osteosarcoma induced by inactivation of Rb and p53 in the osteoblast lineage

Mutation of the RB-1 and p53 tumor suppressors is associated with the development of human osteosarcoma. With the goal of generating a mouse model of this disease, we used conditional and transgenic mouse strains to inactivate Rb and/or p53 specifically in osteoblast precursors. The resulting Rb;p53 double mutant (DKO) animals are viable but develop early onset osteosarcomas with complete penetrance. These tumors display many of the characteristics of human osteosarcomas, including being highly metastatic. We established cell lines from the DKO osteosarcomas to further investigate their properties. These immortalized cell lines are highly proliferative and they retain their tumorigenic potential, as judged by their ability to form metastatic tumors in immunocompromised mice. Moreover, they can be induced to differentiate and, depending on the inductive signal, will adopt either the osteogenic or adipogenic fate. Consistent with this multipotency, a significant portion of these tumor cells express Sca-1, a marker that is typically associated with stem cells/uncommitted progenitors. By assaying sorted cells in transplant assays, we demonstrate that the tumorigenicity of the osteosarcoma cell lines correlates with the presence of the Sca-1 marker. Finally, we show that loss of Rb and p53 in Sca-1-positive mesenchymal stem/progenitor cells is sufficient to yield transformed cells that can initiate osteosarcoma formation in vivo.

Age-related intrinsic changes in human bone-marrow-derived mesenchymal stem cells and their differentiation to osteoblasts

In vivo and in vitro studies indicate that a subpopulation of human marrow-derived stromal cells (MSCs, also known as mesenchymal stem cells) has potential to differentiate into multiple cell types, including osteoblasts. In this study, we tested the hypothesis that there are intrinsic effects of age in human MSCs (17-90 years). We tested the effect of age on senescence-associated beta-galactosidase, proliferation, apoptosis, p53 pathway genes, and osteoblast differentiation in confluent monolayers by alkaline phosphatase activity and osteoblast gene expression analysis. There were fourfold more human bone MSCs (hMSCs) positive for senescence-associated beta-galactosidase in samples from older than younger subjects (P < 0.001; n = 17). Doubling time of hMSCs was 1.7-fold longer in cells from the older than the younger subjects, and was positively correlated with age (P = 0.002; n = 19). Novel age-related changes were identified. With age, more cells were apoptotic (P = 0.016; n = 10). Further, there were age-related increases in expression of p53 and its pathway genes, p21 and BAX. Consistent with other experiments, there was a significant age-related decrease in generation of osteoblasts both in the STRO-1+ cells (P = 0.047; n = 8) and in adherent MSCs (P < 0.001; n = 10). In sum, there is an age-dependent decrease in proliferation and osteoblast differentiation, and an increase in senescence-associated beta-galactosidase-positive cells and apoptosis in hMSCs. Up-regulation of the p53 pathway with age may have a critical role in mediating the reduction in both proliferation and osteoblastogenesis of hMSCs. These findings support the view that there are intrinsic alterations in human MSCs with aging that may contribute to the process of skeletal aging in humans.

PAX6 is expressed in pancreatic adenocarcinoma and is downregulated during induction of terminal differentiation

Tumors of the exocrine pancreas are a major cause of cancer death and have among the poorest prognosis of any malignancy. Following the "cancer stem cell hypothesis," where tumors are believed to originate in tissue specific stem cells, we screened primary ductal pancreatic carcinomas and cell lines for the expression of possible stem cell factors. We find 32/46 (70%) of primary tumors and 9/10 (90%) of cell lines express PAX6. PAX6 is a transcription factor expressed throughout the pancreatic bud during embryogenesis but not in the mature exocrine pancreas. PAX proteins have also been implicated in maintaining stem cells in a committed but undifferentiated state but a role for PAX proteins in putative pancreas stem cells is not known. We induced a pancreatic carcinoma cell line, Panc-1, to differentiate by transfecting wild-type p53 and treating the cells with differentiation agents gastrin or butyrate. This treatment induces cells to terminally differentiate into a growth-arrested cell with neurite-like processes, express the terminal differentiation marker somatostatin and downregulate PAX6. This phenotype can be replicated by directly inhibiting PAX6 expression. These data support a model where PAX proteins are aberrantly expressed in tumors and downregulation leads to differentiation.

Transcription factors controlling osteoblastogenesis

The recent development of molecular biology and mouse genetics and the analysis of the skeletal phenotype induced by genetic mutations in humans led to a better understanding of the role of transcription factors that govern bone formation. This review summarizes the role of transcription factors in osteoblastogenesis and provides an integrated perspective on how the activities of multiple classes of factors are coordinated for the complex process of developing the osteoblast phenotype. The roles of Runx2, the principal transcriptional regulator of osteoblast differentiation, Osterix, beta-Catenin and ATF which act downstream of Runx2, and other transcription factors that contribute to the control of osteoblastogenesis including the AP1, C/EBPs, PPARgamma and homeodomain, helix-loop-helix proteins are discussed. This review also updates the regulation of transcription factor expression by signaling factors and hormones that control osteoblastogenesis.

BMP-2 induces Osterix expression through up-regulation of Dlx5 and its phosphorylation by p38

Osterix, a zinc-finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osterix null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that bone morphogenetic protein-2 (BMP-2) induces an increase in Osterix expression, which is mediated through a homeodomain sequence located in the proximal region of the Osterix promoter. Our results demonstrate that induction of Dlx5 by BMP-2 mediates Osterix transcriptional activation. First, BMP-2 induction of Dlx5 precedes the induction of Osterix. Second, Dlx5 binds to the BMP-responsive homeodomain sequences both in vitro and in vivo. Third, Dlx5 overexpression and knock-down assays demonstrate its role in activating Osterix expression in response to BMP-2. Furthermore, we show that Dlx5 is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-34 and Ser-217 are the sites phosphorylated by p38. Phosphorylation at Ser-34/217 increases the transactivation potential of Dlx5. Thus, we propose that BMP activates expression of Osterix through the induction of Dlx5 and its further transcriptional activation by p38-mediated phosphorylation.

Genetic studies of bone diseases: evidence for involvement of DNA damage response proteins in bone remodeling

Bone remodeling is carried out by bone marrow mesenchymal stem cell derived osteoblasts, which form the bones, and hematopoeitic stem cell derived osteoclasts, which absorb the bones. Their actions are coordinated in two ways: osteoblasts and their precursors synthesize and secrete cytokines such as RANKL and M-CSF to regulate osteoclastogenesis; bone resorption releases matrix associated TGF-β and BMPs to stimulate bone formation at the same sites. Recent studies on transgenic mouse models revealed that several proteins involved in the DNA damage response play important roles in bone remodeling. DNA damage response is triggered by double stranded DNA breaks, single stranded DNA breaks as well as other types of lesions, which recruit and activate Ser/Thr kinases such as Atm to the damaged sites, where Atm activates p53 to promote apoptosis, cell cycle arrest, and DNA repair. Atm also activates c-Abl, a non-receptor tyrosine kinase, to promote apoptosis. Studies from our and other laboratories have shown that c-Abl and Atm positively regulate osteoblast differentiation and bone formation and mice deficient for either of them show osteoporosis, whereas p53 negatively regulates osteoblast proliferation/differentiation and bone formation and the knockout mouse shows osteosclerosis. These three proteins have osteoblast autonomous effect without directly affecting osteoclast differentiation or resorption activity. Furthermore, they appear to regulate osteoblast differentiation through controlling the expression of osterix, an osteoblast specific transcription factor essential for osteoblast differentiation. These results establish a functional link between osteoblast differentiation and DNA damage response.

Attenuation of WNT signaling by DKK-1 and -2 regulates BMP2-induced osteoblast differentiation and expression of OPG, RANKL and M-CSF

Background

Enhanced osteoblast-dependent osteoclastogenesis due to inhibition of Wnt/β-catenin signaling in bone morphogenic protein (BMP)-driven osteoprogenitors has been repeatedly implicated in the natural history of cancer-associated osteolytic lesions, but the mechanism of this bone loss is poorly understood.

Methods

We examined the impact of secreted Wnt inhibitors from the Dickkopf (Dkk) family on pluripotent mesenchymal cells undergoing BMP2-induced osteoblastic differentiation.

Results

We found that Dkk1 and -2 restored the Wnt3a-dependent reduction of alkaline phosphatase (ALP), Osterix and p53, indicating that mitigated Wnt/β-catenin signaling promotes certain aspects of early osteoblastogenesis through the BMP-p53-Osterix-ALP axis. Dkk1 and -2 increased the expression of the osteoclast differentiation factors, receptor activator of NF-κB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF), upon stimulation with Wnt3a/1,25-dihydroxyvitamine D₃ and Wnt3a/BMP2, respectively. The decoy receptor of RANKL, osteoprotegerin (OPG), was down regulated under the latter conditions. These findings indicated that Dkk1 and -2 facilitate osteoclastogenesis by enhancing RANKL/RANK and M-CSF/c-Fms interactions. Dkk4 weakly shared activities of Dkk-1 and -2, whereas Dkk3 was ineffective.

Conclusion

Our results suggest that inhibited Wnt/β-catenin signaling in BMP2-induced osteoprogenitors in vivo promotes, on balance, the heightened formation of osteoclasts. Focally increased Dkk1 production by tumor cells in the bone may thus lead to focal bone loss.

MDM2 antagonist Nutlin-3 suppresses the proliferation and differentiation of human pre-osteoclasts through a p53-dependent pathway

Exposure of human pre-osteoclasts to the MDM2 antagonist Nutlin-3 activated the p53 pathway and significantly decreased the entry of pre-osteoclasts in the S phase in response to RANKL. Moreover, repeated exposure to Nutlin-3 suppressed osteoclastic differentiation, without affecting cell survival at any culture time.

INTRODUCTION

The p53 oncosuppressor coordinates an intracellular network involved in protection from malignant transformation and cell cycle control; its activation is tightly regulated by the murine double minute 2 (MDM2) gene and p53-MDM2 interaction can be disrupted by selective small molecule inhibitors, the Nutlins. Although the ability of Nutlins to suppress the growth of wildtype p53 tumors has been clearly established, their biological activity in normal cells and tissues has not been extensively studied.

MATERIALS AND METHODS

Peripheral blood mononuclear cell pre-osteoclasts were cultured with macrophage-colony stimulating factor (M-CSF) + RANKL or co-cultured with SaOS-2 osteosarcoma cells in the presence of IL-1beta to induce osteoclastic differentiation. Cell cycle was analyzed by BrdU incorporation. The degree of osteoclastic differentiation was monitored at different culture times by TRACP and DAPI staining, as well as by TRACP-5b ELISA. Finally, the role of p53 in mediating the biological activity of Nutlin-3 was studied using specific siRNA.

RESULTS

Exposure of human pre-osteoclasts to RANKL induced an early (24 h) increase in the percentage of cells in the S phase, followed by the exit from the cell cycle at later time-points. The simultaneous addition of Nutlin-3 and RANKL dose-dependently decreased the percentage of pre-osteoclasts in the S phase and induced a rapid accumulation of p53 protein coupled with the induction of p53 target genes. Unexpectedly, the administration of Nutlin-3 to pre-osteoclasts at early culture times significantly suppressed the final output of osteoclasts at day 14 of culture. The role of p53 in mediating this biological activity of Nutlin-3 was underscored by gene knockdown experiments, in which the anti-osteoclastic activity of Nutlin-3 was significantly counteracted by siRNA specific for p53. Nutlin-3 also significantly decreased the formation of osteoclasts in a co-culture system of SaOS-2 osteosarcoma and pre-osteoclastic cells.

CONCLUSIONS

These findings indicate that Nutlin-3 abrogates both pre-osteoclastic proliferation and differentiation through a p53-dependent pathway and may have therapeutic implications for those neoplastic diseases characterized by an abnormal osteoclastic activity.

Mesenchymal stem cells: molecular targets for tissue engineering

Mesenchymal stem cells (MSCs) represent an adherent, fibroblast-like population present not only in the bone marrow, but in a number of tissues, including blood, adipose tissue, muscle, and dermis. Their extensive proliferation and transdifferentiation potential makes them best suited for tissue engineering applications. Identification of growth factors and signaling pathways involved in self-renewal and differentiation is important for designing strategies to overcome replicative senescence and attain directed differentiation. Wnt, bone morphogenetic protein (BMP), and Notch pathways have been implicated to play key roles in self-renewal and differentiation of hematopoietic, intestinal, and epidermal stem cells. They are also involved in regulating MSC differentiation. However, MSC self-renewal has not received much attention, with Nucleostemin being the only recently identified proliferation molecule. Although immortalization using viral oncogenes and telomerase has been achieved, transformation in long-term cultures is a potential risk. Understanding of the mechanisms governing osteogenic differentiation of MSCs is expanding with the recent identification of two major transcription factors, Osterix and Runx2. Enhanced expansion as well as osteogenic differentiation of MSCs can be attained using a combinatorial approach involving co-expression of proliferation and differentiation genes. However, a thorough understanding of the molecular mechanism is necessary for enhancing the self-renewal ability and osteogenic potential in vitro.

p53 in health and disease

As a component of the response to acute stress, p53 has a well established role in protecting against cancer development. However, it is now becoming clear that p53 can have a much broader role and can contribute to the development, life expectancy and overall fitness of an organism. Although the function of p53 as a tumour suppressor ensures that we can't live without it, an integrated view of p53 suggests that not all of its functions are conducive to a long and healthy life.

Signaling and transcriptional regulation in osteoblast commitment and differentiation

The major event that triggers osteogenesis is the transition of mesenchymal stem cells into bone forming, differentiating osteoblast cells. Osteoblast differentiation is the primary component of bone formation, exemplified by the synthesis, deposition and mineralization of extracellular matrix. Although not well understood, osteoblast differentiation from mesenchymal stem cells is a well-orchestrated process. Recent advances in molecular and genetic studies using gene targeting in mouse enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. Osteoblast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. We review Wnt signaling pathway and Runx2 regulation network, which are critical for osteoblast differentiation. Many other factors and signaling pathways have been implicated in regulation of osteoblast differentiation in a network manner, such as the factors Osterix, ATF4, and SATB2 and the TGF-beta, Hedgehog, FGF, ephrin, and sympathetic signaling pathways. This review summarizes the recent advances in the studies of signaling transduction pathways and transcriptional regulation of osteoblast cell lineage commitment and differentiation. The knowledge of osteoblast commitment and differentiation should be applied towards the development of new diagnostic and therapeutic alternatives for human bone diseases.

Sensitization of osteosarcoma cells to apoptosis by oncostatin M depends on STAT5 and p53

Oncostatin M (OSM), a cytokine of the interleukin-6 family, induces growth arrest and differentiation of osteoblastic cells into glial-like/osteocytic cells. Here, we asked whether OSM regulates apoptosis of normal or transformed (osteosarcoma) osteoblasts. We show that OSM sensitizes cells to apoptosis induced by various death inducers such as staurosporine, ultraviolet or tumor necrosis factor-alpha. Apoptosis is mediated by the mitochondrial pathway, with release of cytochrome c from the mitochondria to the cytosol and activation of caspases-9 and -3. DNA micro-arrays revealed that OSM modulates the expression of Bax, Bad, Bnip3, Bcl-2 and Mcl-1. Pharmacological inhibitors, dominant-negative signal transducer and activator of transcriptions (STATs), stable RNA interference and knockout cells indicated that the transcription factors p53 and STAT5, which are activated by OSM, are implicated in the sensitization to apoptosis, being responsible for Bax induction and Bcl-2 reduction, respectively. These results indicate that, in addition to growth arrest and induced differentiation, OSM also sensitizes normal and transformed osteoblasts to apoptosis by a mechanism implicating (i) activation and nuclear translocation of STAT5 and p53 and (ii) an increased Bax/Bcl-2 ratio. Therefore, association of OSM with kinase inhibitors such as Sts represents new therapeutic opportunities for wild-type p53 osteosarcoma.

Advances in defining regulators of cementum development and periodontal regeneration

Substantial advancements have been made in defining the cells and molecular signals that guide tooth crown morphogenesis and development. As a result, very encouraging progress has been made in regenerating crown tissues by using dental stem cells and recombining epithelial and mesenchymal tissues of specific developmental ages. To date, attempts to regenerate a complete tooth, including the critical periodontal tissues of the tooth root, have not been successful. This may be in part due to a lesser degree of understanding of the events leading to the initiation and development of root and periodontal tissues. Controversies still exist regarding the formation of periodontal tissues, including the origins and contributions of cells, the cues that direct root development, and the potential of these factors to direct regeneration of periodontal tissues when they are lost to disease. In recent years, great strides have been made in beginning to identify and characterize factors contributing to formation of the root and surrounding tissues, that is, cementum, periodontal ligament, and alveolar bone. This review focuses on the most exciting and important developments over the last 5 years toward defining the regulators of tooth root and periodontal tissue development, with special focus on cementogenesis and the potential for applying this knowledge toward developing regenerative therapies. Cells, genes, and proteins regulating root development are reviewed in a question-answer format in order to highlight areas of progress as well as areas of remaining uncertainty that warrant further study.

Osteosarcoma in Paget's disease of bone

Paget's disease of bone (PDB) is a focal disorder of bone metabolism first described by Sir James Paget in 1876. It is presumed benign in nature and mediated by abnormal osteoclast function. The incidence of osteosarcomas complicating PDB is estimated at <1%. These cancers occur mostly in persons with long-standing, polyostotic disease and affect patients in their seventh decade or when osteosarcoma is remarkably rare in the general population. Epidemiological studies suggest that this late peak of osteosarcomas is absent in regions where Paget's is infrequently reported. Whereas PDB has a predilection for the axial skeleton, skull, femurs, and tibias, pagetic osteosarcoma tend to spare the spine, and are reported more commonly in the pelvis, femur, humerus, and skull. A molecular basis for the association of osteosarcoma with Paget's disease is unclear. These osteosarcomas are osteogenic in origin, consistently arise in sites of pagetic bone, and may present as metachronous, multifocal lesions. On histopathology, the lesions are usually osteoblastic, and the tumor phenotype is sometimes characterized as an exaggerated, chaotic form of the accelerated bone remodeling that characterizes PDB. New insights from the biology of adolescent osteosarcomas, VCP and SQSTM1 mutations now defined in patients with Paget's disease, and emerging evidence that stromal lesions are present in patients with Paget's disease are changing the way we think about the pathogenesis of PDB and the rare complication of pagetic osteosarcomas.

Adenovirus-mediated p53 tumor suppressor gene therapy of osteosarcoma

The clinical outcome for osteosarcoma (OS) remains discouraging despite efforts to optimize treatment using conventional modalities including surgery, radiotherapy and chemotherapy. Novel therapeutic approaches based on our expanding understanding of the mechanisms of tumor cell killing have the potential to alter this situation. Tumor suppressor gene therapy aims to restore the function of a tumor suppressor gene lost or functionally inactivated in cancer cells. One such molecule, the p53 tumor suppressor gene plays a critical role in safeguarding the integrity of the genome and preventing tumorigenesis. Introduction of wild-type (wt) p53 into transformed cells has been shown to be lethal for most cancer cells in vitro, but clinical trials of p53 gene replacement have had limited success. Analysis of these clinical trials highlighted the insufficient efficacy of current vectors and low proapoptotic activity of wt p53 as a single agent in vivo. In this review, a contemporary summarization of the current status of adenovirus-mediated p53 gene therapy of OS is presented. Advancement in our understanding of p53 tumor suppressor activity, the molecular biology of chemoresistant OS, and recent advances in tumor targeting with adenoviral vectors are also addressed. Based on these parameters, prospects for future investigations are proposed.

Networks and hubs for the transcriptional control of osteoblastogenesis

We present an overview of the concepts of tissue-specific transcriptional control mechanisms essential for development of the bone cell phenotype. BMP2 induced transcription factors constitute a network of activities and molecular switches for bone development and osteoblast differentiation. Among these regulators are Runx2 (Cbfa1/AML3), the principal osteogenic master gene for bone formation, as well as homeodomain proteins and osterix. Runx2 has multiple regulatory activities, including activation or repression of gene expression, and integration of biological signals from developmental cues, such as BMP/TGFbeta, Wnt and Src signaling pathways. Runx2 provides a new paradigm for transcriptional control by functioning as a principal scaffolding protein in nuclear microenvironments to control gene expression in response to physiologic signals (growth factors, cytokines and hormones). The protein serves as a hub for the coordination of activities essential for the expansion and differentiation of osteogenic lineage cells through the formation of co-regulatory protein complexes organized in subnuclear domains. Mechanisms by which Runx2 supports commitment to osteogenesis and determines cell fate involve its retention on mitotic chromosomes. Disruption of a unique protein module, the subnuclear targeting signal of Runx2, has profound effects on osteoblast differentiation and metastasis of cancer cells in the bone microenvironment. Runx2 target genes include regulators of cell growth control, components of the bone extracellular matrix, angiogenesis, and signaling proteins for development of the osteoblast phenotype and bone turnover. The specificity of Runx2 regulatory activities provides a basis for novel therapeutic strategies to correct bone disorders.

Skeletons in the p53 tumor suppressor closet: genetic evidence that p53 blocks bone differentiation and development

A series of in vitro tissue culture studies indicated that the p53 tumor suppressor promotes cellular differentiation, which could explain its role in preventing cancer. Quite surprisingly, however, two new in vivo studies (Lengner et al., 2006; Wang et al., 2006) provide genetic evidence that p53 blocks osteoblast differentiation and bone development. These interesting results and their biological and clinical implications are the focus of this comment.

Atm-deficient mice: an osteoporosis model with defective osteoblast differentiation and increased osteoclastogenesis

Atm is a Ser/Thr kinase involved in DNA damage response and is required for genome integrity and stem cell renewal. Here, we report an additional role for Atm in bone remodeling. Atm-/- mice showed reduced bone mass, especially at the trabecular bones, accompanied by a decrease in bone formation rate and defective differentiation of osteoblasts, but normal numbers of osteoprogenitor cells and osteoblasts. Atm might affect osteoblast differentiation by modulating the expression of osterix, a lineage-specific transcription factor essential for osteoblast maturation, likely via the bone morphogenetic proteins pathway. Atm-/- mice also displayed a marked increase in osteoclastogenesis and bone resorption, although Atm had no cell-autonomous effect on osteoclast differentiation and resorption. Increased osteoclastogenesis could be caused by a substantial reduction in testosterone and estradiol levels in male and female mice, respectively. The steroid hormone deficiency is a result of gonad developmental defects, which led to an increase in serum gonadotrophic hormone, FSH via a feedback regulation. Overall, these results indicate that Atm deficiency leads to osteoporosis mainly as a result of hypogonadism-induced bone resorption together with compromised osteoblast differentiation, and that Atm plays a positive role in regulating expression of osteoblast-specific transcription factor, osterix.