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

Beyond the cell cycle: a new role for Cdk6 in differentiation

Over 10 years ago, cdk6 was identified as a new member in a family of vertebrate cdc-2 related kinases. This novel kinase was found to partner with the D-type cyclins and to possess pRb kinase activity in vitro and has since been understood to function solely as a pRb kinase in the regulation of the G(1) phase of the cell cycle. In the past 2 years, several independent studies in multiple cell types have indicated a novel role for cdk6 in differentiation. For example, cdk6 expression must be reduced to allow proper osteoblast and osteoclast differentiation, forced cdk6 expression blocked differentiation of mouse erythroid leukemia cells and cdk6 expression in primary astrocytes favors the expression of progenitor cell markers. Since exit from the cell cycle is a necessary step in terminal differentiation, down-regulation of a mitogenic factor may be expected in this process, however it is surprising that this association has not been previously uncovered and that it is apparently not shared with cdk4, long understood to be a functional homolog of cdk6. The mechanism of cdk6 function in differentiation is not understood, but it may extend beyond the established role of cdk6 as a pRb kinase. As this story unfolds it will be important to discover if the function of cdk6 in differentiation is pRb-dependent or pRb-independent, since pRb has long been established as a key factor in initiating and maintaining cell cycle exit during differentiation.

An E2F binding-deficient Rb1 protein partially rescues developmental defects associated with Rb1 nullizygosity

Rb1 is essential for normal embryonic development, as null mice die in midgestation with widespread unscheduled cell proliferation. Rb1 protein (pRb) mediates cell cycle control by binding E2F transcription factors and repressing expression from E2F-dependent promoters. An increasing amount of evidence suggests that pRb loss also compromises cellular differentiation. Since differentiation is often dependent on cell cycle exit, it is currently unclear whether the effects of pRb on differentiation are an indirect consequence of pRb/E2F-mediated cell cycle control or whether they reflect direct cell-type-specific pRb functions. We have mutated Rb1 in the mouse to express a protein (R654W) specifically deficient in binding E2F1, E2F2, and E2F3. R654W mutant embryos exhibit cell cycle defects the same as those of Rb1 null embryos, reinforcing the importance of the interactions of pRb with E2F1, E2F2, and E2F3 for cell cycle control. However, R654W embryos survive at least 2 days longer than Rb1 null embryos, and increased life span is associated with improved erythrocyte and fetal liver macrophage differentiation. In contrast, R654W pRb does not rescue differentiation defects associated with pRb-deficient retinae. These data indicate that Rb1 makes important cell-type-specific contributions to cellular differentiation that are genetically separable from its general ability to stably bind E2F1, E2F2, and E2F3 and regulate the cell cycle.

A retinoblastoma ortholog controls stalk/spore preference in Dictyostelium

We describe rblA, the Dictyostelium ortholog of the retinoblastoma susceptibility gene Rb. In the growth phase, rblA expression is correlated with several factors that lead to 'preference' for the spore pathway. During multicellular development, expression increases 200-fold in differentiating spores. rblA-null strains differentiate stalk cells and spores normally, but in chimeras with wild type, the mutant shows a strong preference for the stalk pathway. rblA-null cells are hypersensitive to the stalk morphogen DIF, suggesting that rblA normally suppresses the DIF response in cells destined for the spore pathway. rblA overexpression during growth leads to G1 arrest, but as growing Dictyostelium are overwhelmingly in G2 phase, rblA does not seem to be important in the normal cell cycle. rblA-null cells show reduced cell size and a premature growth-development transition; the latter appears anomalous but may reflect selection pressures acting on social ameba.

PTH/PTHrP receptor delays chondrocyte hypertrophy via both Runx2-dependent and -independent pathways

The transcription factor, Runx2, promotes chondrocyte hypertrophy, whereas parathyroid hormone-related protein (PTHrP) delays this process. To examine whether PTHrP suppresses chondrocyte hypertrophy via Runx2-dependent or -independent pathways, Runx2 expression and chondrocyte differentiation were analyzed using bones from embryonic limbs of wild type and Runx2(-/-) mice. Treatment of cultured rudiments with PTH dramatically suppresses Runx2 mRNA levels in hypertrophic chondrocytes. PTH-induced delay of chondrocyte hypertrophy was observed in cultured tibiae from both Runx2(-/-) and wild-type embryos. This delay was also seen after PTH administration to limbs from wild type and Runx2(-/-) mice expressing Runx2 in chondrocytes via a collagen 2 promoter-driven transgene. To further explore Runx2-dependent and -independent effects of PTHrP, we examined embryonic tibiae and femurs from littermates null for PTHrP, Runx2, or both genes. Runx2(-/-) femurs exhibited no vascular invasion or chondrocytes expressing collagen type X or osteopontin mRNA. In contrast, Runx2(-/-)/PTHrP(-/-) mice exhibited limited vascular invasion and some chondrocytes expressing collagen X or osteopontin mRNA. In both tibia and femur, Runx2(-/-)/PTHrP(-/-) mice exhibited expanded regions of proliferating chondrocytes when compared to the same regions in PTHrP(-/-) mice. These data indicate that the delayed hypertrophy induced by PTHrP is mediated by both Runx2-dependent and -independent mechanisms.

Transcriptional regulation by human papillomaviruses

Human papillomaviruses are the causative agent of cancers in stratified epithelial surfaces. They replicate in the upper parts of the epithelium, where cells would normally be dying to produce a cornified layer. Therefore, they need to inhibit or delay differentiation and stimulate cell cycle progression to create an environment conducive for replication of the viral genome. The alterations both in differentiation and in the cell cycle are achieved by the viral proteins E6 and E7, which modulate cellular transcription mainly through their effects on p53 and the retinoblastoma family.

The retinoblastoma gene family in cell cycle regulation and suppression of tumorigenesis

Since its discovery in 1986, as the first tumor suppressor gene, the retinoblastoma gene (Rb) has been extensively studied. Numerous biochemical and genetic studies have elucidated in great detail the function of the Rb gene and placed it at the heart of the molecular machinery controlling the cell cycle. As more insight was gained into the genetic events required for oncogenic transformation, it became clear that the retinoblastoma gene is connected to biochemical pathways that are dysfunctional in virtually all tumor types. Besides regulating the E2F transcription factors, pRb is involved in numerous biological processes such as apoptosis, DNA repair, chromatin modification, and differentiation. Further complexity was added to the system with the discovery of p107 and p130, two close homologs of Rb. Although the three family members share similar functions, it is becoming clear that these proteins also have unique functions in differentiation and regulation of transcription. In contrast to Rb, p107 and p130 are rarely found inactivated in human tumors. Yet, evidence is accumulating that these proteins are part of a "tumor-surveillance" mechanism and can suppress tumorigenesis. Here we provide an overview of the knowledge obtained from studies involving the retinoblastoma gene family with particular focus on its role in suppressing tumorigenesis.

Heparan sulfate regulates the anabolic activity of MC3T3-E1 preosteoblast cells by induction of Runx2

The transcription factor Runx2 can be controlled by a number of upstream regulators involved in intracellular signalling, including the activation ERK1/2 signaling by fibroblast growth factor-2 (FGF-2). FGFs interact with their cell surface receptors (FGFRs) through an obligate cross-binding interaction with heparan sulfate proteoglycan (HSPG) co-receptors; exogenous HS sugar chains have been shown to potently modulate changes in cell phenotype depending on the stage of tissue differentiation when the HS is harvested, suggesting that HS chain structure and function varies depending on the stage of cell maturity. This study examined the potential of bone-derived heparan sulfate (HS), harvested from differentiating osteoblasts, for the enhancement of preosteoblast growth and differentiation. HS was harvested from conditioned media, cell surface and matrix compartments of postconfluent (differentiating) MC3T3-E1 osteoblasts and dosed back onto preconfluent MC3T3-E1 cells. We show that HS can increase the expression Runx2, ALP, and OPN in preosteoblast cells, suggesting the potential for exogenous HS to shift cells from proliferative to differentiative phenotypes. In line with their structural differences, only HS released into the media was found to co-stimulate the mitogenic effect of FGF-2, whilst exogenous application of all the HSs together with FGF-2 served to increase the expression of OPN. Only the application of cell surface-derived HS triggered a synergistic increase in FGFR1 expression together with FGF-2, although all three HS preparations could trigger transient increases in PI3K, ERK1/2, and stat3 phosphorylation levels. These findings demonstrate that the compartmentally distinct HS species expressed by differentiating MC3T3-E1 cells act in complex ways to coordinate the extracellular conditions that lead to osteoblast differentiation, with the cell surface species coordinating the FGF response.

The related retinoblastoma (pRb) and p130 proteins cooperate to regulate homeostasis in the intestinal epithelium

pRb, p107, and p130 are related proteins that play a central role in the regulation of cell cycle progression and terminal differentiation in mammalian cells. Nevertheless, it is still largely unclear how these proteins achieve this regulation in vivo. The intestinal epithelium is an ideal in vivo system in which to study the molecular pathways that regulate proliferation and differentiation because it exists in a constant state of development throughout an animal's lifetime. We studied the phenotypic effects on the intestinal epithelium of mutating Rb and p107 or p130. Although mutating these genes singly had little or no effect, loss of pRb and p107 or p130 together produced chronic hyperplasia and dysplasia of the small intestinal and colonic epithelium. In Rb/p130 double mutants this hyperplasia was associated with defects in terminal differentiation of specific cell types and was dependent on the increased proliferation seen in the epithelium of mutant animals. At the molecular level, dysregulation of the Rb pathway led to an increase in the expression of Math1, Cdx1, Cdx2, transcription factors that regulate proliferation and differentiation in the intestinal epithelium. The absence of Cdx1 function in Rb/p130 double mutant mice partially reverted the histologic phenotype by suppressing ectopic mitosis in the epithelium. These studies implicate the Rb pathway as a regulator of epithelial homeostasis in the intestine.

Tumour suppressor retinoblastoma protein Rb: a transcriptional regulator

Rb was the first tumour suppressor identified through human genetic studies. The most significant achievement after almost twenty years since its cloning is the revelation that Rb possesses functions of a transcription regulator. Rb serves as a transducer between the cell cycle machinery and promoter-specific transcription factors. In this capacity, Rb is best known as a repressor of the E2F/DP family of transcription factors, which regulate expression of genes involved in cell proliferation and survival. An equally important aspect of Rb as a transcription regulator is that Rb also activates certain differentiation transcription factors to promote cellular differentiation. The molecular mechanisms behind the repressive effects of Rb on E2Fs have come to light in significant details, while those relating to Rb activation of differentiation transcription factors are much less understood. Finally, it has become clear that there are other aspects to Rb function that are not immediately related to transcription regulation.

Regulation of skeletal development by the Runx family of transcription factors

The Runx (runt-related protein) family of transcription factors plays important roles in different tissues and cell lineages. Runx1 determines commitment to the hematopoietic cell lineage and Runx2 determines commitment to the osteoblastic lineage. Cbfbeta is required for Runx1- and Runx2-dependent transcriptional regulation. Runx2 interacts with many other transcription factors and co-regulators in the transcriptional regulation of its target genes. Runx2 is essential for the commitment of multipotent mesenchymal cells into the osteoblastic lineage and inhibits adipocyte differentiation. Runx2 induces the gene expression of bone matrix proteins, while keeping the osteoblastic cells in an immature stage. Runx2 and Runx3 have redundant functions in chondrocytes, and they are essential for chondrocyte maturation. Runx2 directly induces Indian hedgehog (Ihh) expression and co-ordinates the proliferation and differentiation of chondrocytes. Therefore, elucidation of the signaling pathways through Runx2 and Runx3 will unravel the complex mechanism of skeletal development.

Binding of pRB to the PHD protein RBP2 promotes cellular differentiation

pRB can enforce a G1 block by repressing E2F-responsive promoters. It also coactivates certain non-E2F transcription factors and promotes differentiation. Some pRB variants activate transcription and promote differentiation despite impaired E2F binding and transcriptional repression capabilities. We identified RBP2 in a screen for proteins that bind to such pRB variants. RBP2 resembles other chromatin-associated transcriptional regulators and RBP2 binding tracked with pRB's ability to activate transcription and promote differentiation. RBP2 and pRB colocalize and pRB/RBP2 complexes were detected in chromatin isolated from differentiating cells. RBP2 siRNA phenocopied restoration of pRB function in coactivation and differentiation assays, suggesting that pRB prevents RBP2 from repressing genes required for differentiation. In addition, two bromodomain-containing proteins were identified as RBP2 targets that are transcriptionally activated by pRB in an RBP2-dependent manner. Our results suggest that promotion of differentiation by pRB involves neutralization of free RBP2 and transcriptional activation of RBP2 targets linked to euchromatin maintenance.

Cooperative interactions between activating transcription factor 4 and Runx2/Cbfa1 stimulate osteoblast-specific osteocalcin gene expression

The role of ATF4 (activating transcription factor 4) in osteoblast differentiation and bone formation was recently described using ATF4-deficient mice (Yang, X., Matsuda, K., Bialek, P., Jacquot, S., Masuoka, H. C., Schinke, T., Li, L., Brancorsini, S., Sassone-Corsi, P., Townes, T. M., Hanauer, A., and Karsenty, G. (2004) Cell 117, 387-398). However, the mechanisms of ATF4 in bone cells are still not clear. In this study, we determined the molecular mechanisms through which ATF4 activates the mouse osteocalcin (Ocn) gene 2 (mOG2) expression and mOG2 promoter activity. ATF4 increased the levels of Ocn mRNA and mOG2 promoter activity in Runx2-containing osteoblasts but not in non-osteoblastic cells that lack detectable Runx2 protein. However, ATF4 increased Ocn mRNA and mOG2 promoter activity in non-osteoblastic cells when Runx2 was co-expressed. Mutational analysis of the OSE1 (ATF4-binding site) and the two OSE2s (Runx2-binding sites) in the 657-bp mOG2 promoter demonstrated that ATF4 and Runx2 activate Ocn via cooperative interactions with these sites. Pull-down assays using nuclear extracts from osteoblasts or COS-7 cells overexpressing ATF4 and Runx2 showed that both factors are present in either anti-ATF4 and anti-Runx2 immunoprecipitates. In contrast, pull-down assays using purified glutathione S-transferase fusion proteins were unable to demonstrate a direct physical interaction between ATF4 and Runx2. Thus, accessory factors are likely involved in stabilizing interactions between these two molecules. Regions within Runx2 required for ATF4 complex formation and activation were identified. Deletion analysis showed that the leucine zipper domain of ATF4 is critical for Runx2 activation. This study is the first demonstration that cooperative interactions between ATF4 and Runx2/Cbfa1 stimulate osteoblast-specific Ocn expression and suggests that this regulation may represent a novel intramolecular mechanism regulating Runx2 activity and, thereby, osteoblast differentiation and bone formation.

Retinoblastoma protein acts as Pax 8 transcriptional coactivator

Control of cell proliferation and differentiation by the retinoblastoma protein (pRb) depends on its interactions with key cellular substrates. Available data indicate that pRb and the transcription factor Pax 8 play a crucial role in the differentiation of thyroid follicular cells. In this study, we show that pRb takes part in the complex assembled on the thyroperoxidase gene promoter acting as a transcriptional coactivator of Pax 8. Accordingly, pRb interacts with and potentiates Pax 8 transcriptional activity. In addition, we show that the downregulation of pRb gene expression, in thyrocytes, through RNA interference results in a reduction of the thyroperoxidase gene promoter activity mediated by the Pax 8-binding site. In agreement with these results and with the ability of the adenoviral protein E1A to bind pRb, we show that E1A downregulates Pax 8 activity and that such inhibition requires the E1A-Rb interaction. Furthermore, we show that the Pax 8/pRb synergy plays a role on the sodium/iodide symporter gene expression as well.

Viral oncoproteins E1A and E7 and cellular LxCxE proteins repress SUMO modification of the retinoblastoma tumor suppressor

The retinoblastoma tumor suppressor protein (pRB) is a major regulator of cell-cycle progression and cellular differentiation. Central to pRB function is the pocket domain, which serves as the main binding region for cellular regulators. In tumors pRB is frequently inactivated by mutations in the pocket domain or by binding of viral oncoproteins to this region. A characteristic feature of these viral oncoproteins and many cellular pRB-binding partners is an LxCxE sequence motif, which interacts with pRB's pocket domain. Here, we show that the ubiquitin-like modifier SUMO is covalently attached to a distinct residue (K720) of pRB within the B-box of the pocket region that binds LxCxE-motif proteins. We provide evidence that SUMO preferentially targets the active, hypophosphorylated form of pRB and show that tumorigenic mutations of pRB in the pocket domain lead to a loss of SUMOylation. Notably, the level of pRB SUMOylation is controlled by the interaction of pRB with viral and cellular LxCxE-motif proteins. Inhibitors of pRB function, including the viral oncoproteins E1A and E7 and the cellular E1A-like inhibitor of differentiation EID-1, completely abolish SUMO modification of pRB. Conversely, pRB mutants deficient in binding of LxCxE-motif proteins exhibit a drastically enhanced modification by SUMO. Finally, we provide evidence that SUMOylation can influence pRB function, as the SUMO-deficient pRB(K720R) mutant exerts a slightly higher repressive potential on an E2F-responsive reporter gene than wild-type pRB. Taken together, these data identify SUMO modification as a novel post-translational modification of pRB that may control pRB activity by modulating LxCxE-pocket interactions.

Master or slave: the complex relationship of RBP2 and pRb

The retinoblastoma protein or its regulators are altered in most human cancers. Although commonly thought of as solely a repressor of E2F-dependent transcription and cell cycle progression, pRb has gained notoriety in recent years as a key actor in cellular differentiation programs. In the June issue of Molecular Cell, Benevolenskaya et al. report that a long-known but poorly understood pRb interactor, RBP2, acts as an inhibitor of differentiation contributing to pRb's role as a coordinator of differentiation and cell cycle exit. Loss of pRb may unleash RBP2, maintaining cells in a poorly differentiated progenitor state that is prerequisite to tumor formation.

Selective ablation of retinoblastoma protein function by the RET finger protein

The retinoblastoma tumor suppressor protein (Rb) affects gene transcription both negatively and positively and through this regulates distinct cellular responses. Although cell cycle regulation requires gene repression, Rb's ability to promote differentiation and part of its antiproliferative activity appears to rely on the activation of gene transcription. We present evidence here that the RET finger protein (RFP)/tripartite motif protein 27 (TRIM 27) inhibits gene transcription activation by Rb but does not affect gene repression. RFP binds to Rb and prevents the degradation of the EID-1 inhibitor of histone acetylation and differentiation. Furthermore, ablation of RFP in U2OS osteosarcoma cells augments a transcriptional program indicative of lineage-specific differentiation in response to Rb. These findings provide precedent for a regulatory pathway that uncouples different Rb-dependent activities and thus silences specific cellular responses to Rb in a selective way.

Pocket proteins and cell cycle control

The retinoblastoma protein (pRB) and the pRB-related p107 and p130 comprise the 'pocket protein' family of cell cycle regulators. These proteins are best known for their roles in restraining the G1-S transition through the regulation of E2F-responsive genes. pRB and the p107/p130 pair are required for the repression of distinct sets of genes, potentially due to their selective interactions with E2Fs that are engaged at specific promoter elements. In addition to regulating E2F-responsive genes in a reversible manner, pocket proteins contribute to silencing of such genes in cells that are undergoing senescence or differentiation. Pocket proteins also affect the G1-S transition through E2F-independent mechanisms, such as by inhibiting Cdk2 or by stabilizing p27(Kip1), and they are implicated in the control of G0 exit, the spatial organization of replication, and genomic rereplication. New insights into pocket protein regulation have also been obtained. Kinases previously thought to be crucial to pocket protein phosphorylation have been shown to be redundant, and new modes of phosphorylation and dephosphorylation have been identified. Despite these advances, much remains to be learned about the pocket proteins, particularly with regard to their developmental and tumor suppressor functions. Thus continues the story of the pocket proteins and the cell cycle.

Cyclins and cdks in development and cancer: a perspective

A fundamental aspect of cancer is dysregulated cell cycle control. Unlike normal cells that only proliferate when compelled to do so by developmental or other mitogenic signals in response to tissue growth needs, the proliferation of cancer cells proceeds essentially unchecked. This does not mean that cancer cell cycles are necessarily different from those found in normal cycling cells, but rather implies that cancer cells proliferate because they are no longer subject to proliferation-inhibitory influences arising from the stroma or from gene expression pattern changes consequent to 'terminal' differentiation, nor do they necessarily require extrinsic growth factors to recruit them into or maintain their proliferative state. Finally, cancer cells have also often avoided normal controls linked to cell cycle progression that halt proliferation in the presence of damaged DNA or other physiological insults. The result of these alterations is the inappropriate proliferation commonly associated with cancerous tumor formation. This review will summarize the current understanding of dysregulation of the G0/G1-to-S-phase transition in cancer cells, with particular emphasis on recent in vivo studies that suggest a need to rethink existing models of cell cycle control in development and tumorigenesis.

Bone morphogenic protein (Smad)-mediated repression of proopiomelanocortin transcription by interference with Pitx/Tpit activity

The signaling molecules bone morphogenic protein (BMP) 4 and 2 have been implicated in early organogenesis and cell differentiation of the pituitary. However, the use of different experimental paradigms has led to conflicting interpretations with regard to the action of these factors on differentiation of corticotroph cells and on expression of the proopiomelanocortin (POMC) gene. We have now directly assessed the action of BMP signaling on POMC expression and found that BMP4 represses POMC mRNA levels and promoter activity. This repression appears to be dependent on the classical BMP signaling pathway that involves the activin-like kinase 3/6 receptors and the Smad1/4 transcription factors. The repression is reversed by overexpression of the inhibitory Smads, Smad6 or Smad7. Collectively, the evidence suggests that autocrine BMP signaling may be acting upon AtT-20 cells to set the level of POMC expression. Upon BMP4 stimulation, activated phospho-Smad1 is recruited to the POMC promoter, where it apparently acts through interactions with the Pitx and Tpit transcription factors. It is postulated that these interactions interfere with the transcriptional activity of Pitx and/or Tpit, thus resulting in transcriptional repression.

An immunohistochemical analysis to evaluate an inverse correlation between Runx2/Cbfa1 and NF kappa B in human osteosarcoma

BACKGROUND

Dominant negative inhibition of nuclear factor kappa B (NF kappa B) signalling activity in a human osteosarcoma cell line (Saos2) results in malignant reversion and the induction of the osteoblast differentiating transcription factor, Runx2/Cbfa1. This observation suggests that there is an inverse relation between a transcription factor associated with malignant progression and chemoresistance (NF kappa B) and an osteoblast differentiating transcription factor (Runx2/Cbfa1).

AIMS

To assess and correlate Runx2/Cbfa1 and NF kappa B (p65) immunoreactivity in human osteosarcoma.

METHODS

Runx2/Cbfa1 and NFkappaB (p65) immunoreactivity was assessed on 11 paraffin wax embedded archival specimens of human primary osteosarcoma by standard immunohistochemical methods and scored on a scale of 0-3. A Pearson correlation analysis between Runx2/Cbfa1 and NF kappa B (p65) scores was established.

RESULTS

Runx2/Cbfa1 was expressed constitutively in all pathology specimens of human osteosarcoma. Of note, a chondroblastic osteosarcoma showed the highest Runx2/Cbfa1 immunoreactivity. A Pearson correlation did not support an inverse correlation between Runx2/Cbfa1 and NF kappa B (p65) scores (r = 0.57) in human osteosarcoma.

CONCLUSION

Runx2/Cbfa1 immunoreactivity does not inversely correlate with NF kappa B immunoreactivity, and thus cannot serve as an indirect measure of NF kappa B activity or an independent predictive or prognostic indicator.

The bone-specific expression of Runx2 oscillates during the cell cycle to support a G1-related antiproliferative function in osteoblasts

The Runx2 (CBFA1/AML3/PEBP2alphaA) transcription factor promotes skeletal cell differentiation, but it also has a novel cell growth regulatory activity in osteoblasts. We addressed here whether Runx2 activity is functionally linked to cell cycle-related mechanisms that control normal osteoblast proliferation and differentiation. We found that the levels of Runx2 gene transcription, mRNA and protein, are each up-regulated with cessation of cell growth (i.e. G(0)/G(1) transition) in preconfluent MC3T3 osteoblastic cells that do not yet express mature bone phenotypic gene expression. Cell growth regulation of Runx2 is also observed in primary calvarial osteoblasts and other osteoblastic cells with relatively normal cell growth characteristics, but not in osteosarcoma cells (e.g. SAOS-2 and ROS17/2.8). Runx2 levels are cell cycle-regulated in MC3T3 cells with respect to the G(1)/S and M/G(1) transitions: oscillates from maximal expression levels during early G(1) to minimal levels during early S phase and mitosis. However, in normal or immortalized (e.g. ATDC5) chondrocytic cells, Runx2 expression is suppressed during quiescence, and Runx2 levels are not regulated during G(1) and S phase in ATDC5 cells. Antisense or small interfering RNA-mediated reduction of the low physiological levels of Runx2 in proliferating MC3T3 cells does not accelerate cell cycle progression. However, forced expression of Runx2 suppresses proliferation of MC3T3 preosteoblasts or C2C12 mesenchymal cells which have osteogenic potential. Forced elevation of Runx2 in synchronized MC3T3 cells causes a delay in G(1). We propose that Runx2 levels and function are biologically linked to a cell growth-related G(1) transition in osteoblastic cells.

Rb enhances p160/SRC coactivator-dependent activity of nuclear receptors and hormone responsiveness

The retinoblastoma tumor suppressor protein (Rb) is best known as a repressor of genes involved in cell cycle progression. Rb has also been implicated in activation of transcription, in particular by nuclear receptors (NRs) and by differentiation-related transcription factors, but the relevance of this activity is unclear. We show that Rb and the related proteins p107 and p130 enhance the activity of NRs related to NGFI-B (Nur factors) through direct interactions with NGFI-B and SRC-2. Although recruitment of SRC/p160 coactivators to the NGFI-B AF1 domain is independent of Rb, its presence enhances SRC-dependent transcription. Rb potentiation of SRC coactivators is exerted on a subset (Nur factors, hepatocyte nuclear factor-4 (HNF-4), SF-1, and ER) but not all NRs. The levels of Rb-related proteins modulate hormone responsiveness of the NGFI-B-dependent pituitary proopiomelanocortin gene and HNF-4-dependent transcription during enterocyte differentiation. Increased Rb expression upon cell differentiation may promote differentiated functions, at least in part, by potentiation of NR activity.

Proliferation of Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein

In mammals, hair cell loss causes irreversible hearing and balance impairment because hair cells are terminally differentiated and do not regenerate spontaneously. By profiling gene expression in developing mouse vestibular organs, we identified the retinoblastoma protein (pRb) as a candidate regulator of cell cycle exit in hair cells. Differentiated and functional mouse hair cells with a targeted deletion of Rb1 undergo mitosis, divide, and cycle, yet continue to become highly differentiated and functional. Moreover, acute loss of Rb1 in postnatal hair cells caused cell cycle reentry. Manipulation of the pRb pathway may ultimately lead to mammalian hair cell regeneration.

Retinoblastoma and the related pocket protein p107 act as coactivators of NeuroD1 to enhance gene transcription

Gene inactivation studies have suggested that the product of the retinoblastoma gene, Rb, is particularly limiting in pituitary pro-opiomelanocortin (POMC)-expressing cell lineages. Indeed, in Rb knock-out mice, these cells develop tumors with high frequency. To understand the implication of limiting Rb expression in these cells, we investigated the action of Rb and its related pocket proteins, p107 and p130, on POMC gene transcription. This led to the identification of the neurogenic basic helix-loop-helix transcription factor, NeuroD1, as a target of Rb action. Rb and to a lesser extent p107, but not p130, enhance NeuroD1-dependent transcription, and this activity appears to depend on direct protein interactions between the Rb pocket and the helix-loop-helix domain of NeuroD1. In vivo, NeuroD is found in a complex that includes Rb and also the orphan nuclear receptor NGFI-B, which mediates corticotropin-releasing hormone activation of POMC transcription. The formation of a similar complex in vitro requires the presence of Rb as a bridge between NeuroD and NGFI-B. In POMC-expressing AtT-20 cells, Rb and p107 are present on the POMC promoter and inhibition of their expression through small interfering RNA decreases POMC mRNA levels. The action of Rb and its related proteins on POMC transcription may contribute to the establishment and/or maintenance of the differentiation phenotype.

Terminal osteoblast differentiation, mediated by runx2 and p27KIP1, is disrupted in osteosarcoma

The molecular basis for the inverse relationship between differentiation and tumorigenesis is unknown. The function of runx2, a master regulator of osteoblast differentiation belonging to the runt family of tumor suppressor genes, is consistently disrupted in osteosarcoma cell lines. Ectopic expression of runx2 induces p27^KIP1, thereby inhibiting the activity of S-phase cyclin complexes and leading to the dephosphorylation of the retinoblastoma tumor suppressor protein (pRb) and a G1 cell cycle arrest. Runx2 physically interacts with the hypophosphorylated form of pRb, a known coactivator of runx2, thereby completing a feed-forward loop in which progressive cell cycle exit promotes increased expression of the osteoblast phenotype. Loss of p27^KIP1 perturbs transient and terminal cell cycle exit in osteoblasts. Consistent with the incompatibility of malignant transformation and permanent cell cycle exit, loss of p27^KIP1 expression correlates with dedifferentiation in high-grade human osteosarcomas. Physiologic coupling of osteoblast differentiation to cell cycle withdrawal is mediated through runx2 and p27^KIP1, and these processes are disrupted in osteosarcoma.

Conserved and specific functions of mammalian ssu72

We describe the cloning and characterization of a human homolog of the yeast transcription/RNA-processing factor Ssu72, following a yeast two-hybrid screen for pRb-binding factors in the prostate gland. Interaction between hSsu72 and pRb was observed in transfected mammalian cells and involved multiple domains in pRb; however, so far, mutual effects of these two factors could not be demonstrated. Like the yeast counterpart, mammalian Ssu72 associates with TFIIB and the yeast cleavage/polyadenylation factor Pta1, and exhibits intrinsic phosphatase activity. Mammals contain a single ssu72 gene and a few pseudogenes. During mouse embryogenesis, ssu72 was highly expressed in the nervous system and intestine; high expression in the nervous system persisted in adult mice and was also readily observed in multiple human tumor cell lines. Both endogenous and ectopically expressed mammalian Ssu72 proteins resided primarily in the cytoplasm and only partly in the nucleus. Interestingly, fusion to a strong nuclear localization signal conferred nuclear localization only in a fraction of transfected cells, suggesting active tethering in the cytoplasm. Suppression of ssu72 expression in mammalian cells by siRNA did not reduce proliferation/survival, and its over-expression did not affect transcription of candidate genes in transient reporter assays. Despite high conservation, hssu72 was unable to rescue an ssu72 lethal mutation in yeast. Together, our results highlight conserved and mammalian specific characteristics of mammalian ssu72.

Genetic interaction between Rb and K-ras in the control of differentiation and tumor suppression

Although the retinoblastoma protein (pRb) has been implicated in the processes of cellular differentiation, there is no compelling genetic or in vivo evidence that such activities contribute to pRb-mediated tumor suppression. Motivated by cell culture studies suggesting that Ras is a downstream effector of pRb in the control of differentiation, we have examined the tumor and developmental phenotypes of Rb and K-ras double-knockout mice. We find that heterozygosity for K-ras (i) rescued a unique subset of developmental defects that characterize Rb-deficient embryos by affecting differentiation but not proliferation and (ii) significantly enhanced the degree of differentiation of pituitary adenocarcinomas arising in Rb heterozygotes, leading to their prolonged survival. These observations suggest that Rb and K-ras function together in vivo, in the contexts of both embryonic and tumor development, and that the ability to affect differentiation is a major facet of the tumor suppressor function of pRb.

The Interferon-inducible p204 Protein Acts as a Transcriptional Coactivator of Cbfa1 and Enhances Osteoblast Differentiation

The differentiation of uncommitted mesenchymal cells into osteoblasts is a fundamental molecular event governing both embryonic development and bone repair. The bone morphogenetic proteins (BMPs) are important regulators of this process; they function by binding to cell surface receptors and signaling by means of Smad proteins. Core binding factor alpha-1 (Cbfa1), a member of the runt family of transcription factors, is an essential transcriptional regulator of osteoblast differentiation and bone formation, and this process is positively or negatively regulated by a variety of coactivators and corepressors. We report that p204, an interferon-inducible protein that was previously shown to inhibit cell proliferation and promote the differentiation of myoblasts to myotubes, is a novel regulator in the course of osteogenesis. p204 is expressed in embryonic osteoblasts and hypertrophic chondrocytes in the growth plate as well as in the calvaria osteoblasts of neonatal mice. Its level is increased in the course of the BMP-2-triggered osteoblast differentiation of pluripotent C2C12 cells. This increase is probably due to the activation of the gene encoding 204 (Ifi204) by Smad transcription factor, including Smad1, -4, and -5. Overexpression of p204 enhances the BMP-2-induced osteoblast differentiation in vitro, as revealed by elevated alkaline phosphatase activity and osteocalcin production. p204 acts as a cofactor of Cbfa1: 1) high levels of p204 augment, whereas the lowering of p204 level decreases, the Cbfa1-dependent transcription, and 2) p204 associates with Cbfa1 both in vitro and in vivo. Two nonoverlapping segments in p204 bind to Cbfa1, and the N-terminal 88-amino acid segment of Cbfa1 is required for binding to p204. p204, which is the first interferon-inducible protein found to associate with Cbfa1, functions as a novel regulator of osteoblast differentiation.

Role of the retinoblastoma tumor suppressor protein in cellular differentiation

The retinoblastoma protein (pRb105) is a true tumor suppressor as deregulation of the Rb pathway by either mutation of pRb105 itself or other proteins in the pathway, such as p16INK4a, occur in most cancers. This prototypical family member, along with the related p107 and p130, are involved in the control of cell cycle regulation, but pRb105 has also been shown to be involved in tissue development and differentiation. This prospective will discuss the increasing evidence for a role of pRb105 in cellular differentiation and the fact that various cancers, which contain mutant pRb105, or mutations in proteins in the pRb105 pathway, are perhaps a result of deregulation of differentiation.

Runx2: A master organizer of gene transcription in developing and maturing osteoblasts

Runx2 is essential for osteoblast development and proper bone formation. A member of the Runt domain family of transcription factors, Runx2 binds specific DNA sequences to regulate transcription of numerous genes and thereby control osteoblast development from mesenchymal stem cells and maturation into osteocytes. Although necessary for gene transcription and osteoblast development, Runx2 is not sufficient for optimal gene expression or bone formation. Runx2 cooperates with numerous proteins, including transcription factors and cofactors, is posttranslationally modified, and associates with the nuclear matrix to integrate a variety of signals and organize crucial events during osteoblast development and maturation. Consistent with its role as a master organizer, alterations in Runx2 expression levels are associated with skeletal diseases. Runx2 haploinsufficiency causes cleidocranial dysplasia, while Runx2 overexpression is common in many bone-metastatic cancers. In this review, we summarize the molecular mechanisms by which Runx2 integrates signals through coregulatory interactions, and discuss how its role as a master organizer may shift depending on promoter structure, developmental cues, and cellular context.

A common set of gene regulatory networks links metabolism and growth inhibition

Using genome-wide analysis of transcription factor occupancy, we investigated the mechanisms underlying three mammalian growth arrest pathways that require the pRB tumor suppressor family. We found that p130 and E2F4 cooperatively repress a common set of genes under each growth arrest condition and showed that growth arrest is achieved through repression of a core set of genes involved not only in cell cycle control but also mitochondrial biogenesis and metabolism. Motif-finding algorithms predicted the existence of nuclear respiratory factor-1 (NRF1) binding sites in E2F target promoters, and genome-wide factor binding analysis confirmed our predictions. We showed that NRF1, a factor known to regulate expression of genes involved in mitochondrial function, is a coregulator of a large number of E2F target genes. Our studies provide insights into E2F regulatory circuitry, suggest how factor occupancy can predict the expression signature of a given target gene, and reveal pathways deregulated in human tumors.

Protein-protein interactions and transcriptional antagonism between the subfamily of NGFI-B/Nur77 orphan nuclear receptors and glucocorticoid receptor

Glucocorticoids (Gc) act through the glucocorticoid receptor (GR) to enhance or repress transcription of glucocorticoid-responsive genes depending on the promoter and cellular context. Repression of proopiomelanocortin (POMC) gene expression by Gc was proposed to use different mechanisms. We described the POMC promoter Nur response element (NurRE) as a target for Gc repression. NGFI-B (Nur77), an orphan nuclear receptor, and two related factors, Nurr1 and NOR1, bind the NurRE as homo- or heterodimers to enhance POMC gene expression in response to CRH. Gc antagonize CRH-stimulated as well as NGFI-B-dependent transcription. We now show that GR antagonizes NurRE-dependent transcription induced by all members of the Nur77 subfamily and that these nuclear receptors can all interact directly with GR. Transcriptional antagonism as well as direct protein-protein interaction between NGFI-B and GR take place primarily via their respective DNA binding domains, although DNA binding itself and the GR homodimerization interface are not involved. In vivo, GR and Nur factors can be coimmunoprecipitated whereas GR is recruited to the POMC promoter upon glucocorticoid action. Thus, our data suggest a mechanism for transrepression between two nuclear receptors, GR and NGFI-B, that is unique, although quite similar to that proposed for transrepression between GR and activator protein 1 (AP-1) or nuclear factor-kappaB (NFkappaB).

Tissue-specific tumor suppressor activity of retinoblastoma gene homologs p107 and p130

The retinoblastoma gene family consists of three genes: RB, p107, and p130. While loss of pRB causes retinoblastoma in humans and pituitary gland tumors in mice, tumorigenesis in other tissues may be suppressed by p107 and p130. To test this hypothesis, we have generated chimeric mice from embryonic stem cells carrying compound loss-of-function mutations in the Rb gene family. We found that Rb/p107- and Rb/p130-deficient mice were highly cancer prone. We conclude that in a variety of tissues tumor development by loss of pRB is suppressed by its homologs p107 and p130. The redundancy of the retinoblastoma proteins in vivo is reflected by the behavior of Rb-family-defective mouse embryonic fibroblasts in vitro.

Inhibition of oncogenic transformation by mammalian Lin-9, a pRB-associated protein

Genetic studies in Caenorhabditis elegans identified lin-9 to function together with the retinoblastoma homologue lin-35 in vulva differentiation. We have now identified a human homologue of Lin-9 (hLin-9) and provide evidence about its function in the mammalian pRB pathway. hLin-9 binds to pRB and cooperates with pRB in flat cell formation in Saos-2 cells. In addition, hLin-9 synergized with pRB and Cbfal to transactivate an osteoblast-specific reporter gene. In contrast, hLin-9 was not involved in pRB-mediated inhibition of cell cycle progression or repression of E2F-dependent transactivation. Consistent with these data, hLin-9 was able to associate with partially penetrant pRB mutants that do not bind to E2F, but retain the ability to activate transcription and to promote differentiation. hLin-9 can also inhibit oncogenic transformation, dependent on the presence of a functional pRB protein. RNAi-mediated knockdown of Lin-9 can substitute for the loss of pRB in transformation of human primary fibroblasts. These data suggest that hLin-9 has tumor-suppressing activities and that the ability of hLin-9 to inhibit transformation is mediated through its association with pRB.

Cell-cell interaction mediated by cadherin-11 directly regulates the differentiation of mesenchymal cells into the cells of the osteo-lineage and the chondro-lineage

We studied cadherin-11 function in the differentiation of mesenchymal cells. Teratomas harboring the cadherin-11 gene generated bone and cartilage preferentially. Cadherin-11 transfectants of C2C12 cells and cadherin-11 and/or N-cadherin transfectants of L cells showed that cadherin-11 together with N-cadherin-induced expression of ALP and FGF receptor 2. These results suggest that cadherin-11 directly regulates the differentiation of mesenchymal cells into the cells of the osteo-lineage and the chondro-lineage in a different manner from N-cadherin.

INTRODUCTION

Cell-cell interaction is an essential event for tissue formation; however, the role of cell-cell adhesion in mesenchymal tissue formation as well as in cell differentiation in this tissue remains unclear. cadherins, which are calcium-dependent cell adhesion receptors, form adherence junctions after adherence and aggregation of cells. Because cadherin-11 as well as N-cadherin has been reported to be a mesenchyme-related cadherin, we examined the cadherin-11 action in teratomas and in the cell lines C2C12 and L cell. Herein, we show that cell-cell interaction mediated by cadherin-11 is responsible for bone and cartilage formation.

MATERIALS AND METHODS

It has been previously reported that N-cadherin-expressing E-cadherin-/- ES transfectants formed neuroepithelium and cartilage in teratomas. Thus, we transfected the E-cadherin-/- ES cell line with the cadherin-11 gene. Moreover, we also transfected C2C12 cells and L cells with the cadherin-11 gene for morphological analysis and study of the induced differentiation at the molecular level.

RESULTS AND CONCLUSION

Teratomas derived from embryonic stem cells in which the cadherin-11 gene had been expressed exogenously contained bone and cartilage preferentially, showing that cadherin-11 is involved in mesenchymal tissue formation, specifically in controlling the differentiation of these cells into osteoblasts and chondrocytes. Therefore, we further examined the functional difference between cadherin-11 and N-cadherin. The expression patterns of cadherin-11 and N-cadherin in cells of the mouse osteoblastic cell line MC3T3-E1 showed that each cadherin was located independently of the cell-cell adhesion site and acted individually. In hanging drop cultures, cadherin-11 L cell transfectants aggregated in a sheet-like structure, whereas N-cadherin transfectants aggregated in a spherical form, indicating that each cadherin confers a different 3D architecture because of its individual adhesive property. To investigate the molecular mechanism of cadherin-11 action in cell differentiation, we analyzed cadherin-11 transfectants of C2C12 cells and cadherin-11 and/or N-cadherin transfectants of L cells and showed that cadherin-11, together with N-cadherin, induced expression of alkaline phosphatase (ALP) and fibroblast growth factor receptor 2. These results suggest that cadherin-11 directly regulates the differentiation of mesenchymal cells into the cells of the osteo-lineage and the chondro-lineage in a different manner from N-cadherin.

Parathyroid hormone uses multiple mechanisms to arrest the cell cycle progression of osteoblastic cells from G1 to S phase

Parathyroid hormone (PTH) plays a major role in bone remodeling and has the ability to increase bone mass if administered daily. In vitro, PTH inhibits the growth of osteoblastic cell lines, arresting them in G(1) phase. Here, we demonstrate that PTH regulates the expression of at least three genes to achieve the following: inducing expression of MAPK phosphatase 1 (MKP-1) and p21(Cip1) and decreasing expression of cyclin D1 at both mRNA and protein levels. The induction of MKP-1 causes the dephosphorylation of extracellular signal-regulated kinase and therefore the decrease in cyclin D1. Overexpression of MKP-1 arrests UMR cells in G(1) phase. The mechanisms involved in PTH regulation of these genes were studied. Most importantly, PTH administration produces similar effects on expression of these genes in rat femoral metaphyseal primary spongiosa. Analyses of p21(Cip1) expression levels in bone indicate that repeated daily PTH injections make the osteoblast more sensitive to successive PTH treatments, and this might be an important feature for the anabolic functions of PTH. In summary, our data suggest that one mechanism for PTH to exert its anabolic effect is to arrest the cell cycle progression of the osteoblast and hence increase its differentiation.

Comparison of TGF-beta/BMP pathways signaled by demineralized bone powder and BMP-2 in human dermal fibroblasts

Demineralized bone induces chondrogenic differentiation of human dermal fibroblasts in vitro. Analyses of signaling gene expression showed that DBP and BMP-2 regulate common and distinct pathways. Although BMP-2 was originally isolated as a putative active factor in DBP, rhBMP-2 and DBP do not affect all the same genes or in the same ways.

INTRODUCTION

Demineralized bone powder (DBP) induces chondrogenic differentiation of human dermal fibroblasts (hDFs) in 3D culture, but the initiating mechanisms have not been identified. We tested the hypotheses that DBP would affect expression of signaling genes and that DBP's effects would differ from the effects of bone morphogenetic proteins (BMPs).

MATERIALS AND METHODS

A chondroinduction model was used in which hDFs were cultured with and without DBP in a porous collagen sponge. BMP-2 was delivered in a square of absorbable collagen felt inserted into a collagen sponge. Total RNA was isolated after 3 days of culture, a time that precedes expression of the chondrocyte phenotype. Gene expression was evaluated with two targeted macroarray screens. Effects of DBP and rhBMP-2 were compared by macroarray, RT-PCR, and Northern hybridization analysis of selected genes in the transforming growth factor (TGF)-beta/BMP signaling pathways.

RESULTS

By macroarray analysis of 16 signal transduction pathways, the following pathways were modulated in hDFs by DBP: TGF-beta, insulin/LDL, hedgehog, PI3 kinase/AKT, NF-kappaB, androgen, retinoic acid, and NFAT. There was convergence and divergence in DBP and rhBMP-2 regulation of genes in the TGF-beta/BMP signaling pathway. Smad target genes were the predominant group of DBP- or rhBMP-2-regulated genes. Several genes (IGF-BP3, ID2, and ID3) showed similar responses (increased expression) to DBP and rhBMP-2. In contrast, many of the genes that were greatly upregulated by DBP (TGFBI/betaig-h3, Col3A1, TIMP1, p21/Waf1/Cip1) were barely affected by rhBMP-2.

CONCLUSION

These findings indicate that multiple signaling pathways are regulated in fibroblasts by DBP, that one of the major pathways involves Smad target genes, and that DBP and rhBMP-2 elicit different gene expression responses in hDFs. Although BMP-2 was originally isolated as a putative inductive factor in DBP, rhBMP-2 and DBP do not affect all the same genes or in the same ways.

Dexamethasone inhibits insulin-induced chondrogenesis of ATDC5 cells by preventing PI3K-Akt signaling and DNA binding of Runx2

Glucocorticoids play important roles in cell growth and differentiation. In this study, we investigated the effect of application of dexamethasone (DEX) at the early stage of chondrogenesis using the prechondrogenic cell line, ATDC5, which differentiates into chondrocytes in the presence of insulin. When ATDC5 cells were cultured in the presence of DEX and insulin, DEX inhibited insulin-induced cellular condensation and subsequent cartilaginous nodule formation, and reduced proteoglycan synthesis and type II collagen expression dose-dependently. Pretreatment with 10(-8) M DEX for 1 day inhibited insulin-induced Akt phosphorylation, but not ERK1/2 phosphorylation, in ATDC5 cells. Treatment of ATDC5 cells with insulin for more than 2 days upregulated the levels of phosphatidylinositol 3-kinase (PI3K) subunit proteins, p85 and p110, and Akt, whereas the upregulation was inhibited in the presence of 10(-8) M DEX. In electrophoresis mobility shift assays (EMSAs), treatment with 10(-8) M DEX inhibited DNA binding of Runx2 during culture of ATDC5 cells with insulin. Reporter assays using osteocalcin promoter showed that DEX inhibited Runx2-dependent transcription dose-dependently. Adenoviral introduction of dominant-negative (dn)-Akt or dn-Runx2 into ATDC5 cells inhibited cellular condensation and reduced proteoglycan synthesis upon incubation with insulin, whereas adenoviral introduction of Akt or Runx2 prevented the inhibition of chondrogenesis by DEX. These findings indicate that DEX inhibits chondrogenesis of ATDC5 cells at the early stage by downregulating Akt phosphorylation as well as the protein levels of PI3K subunits and Akt, thereby suppressing PI3K-Akt signaling, and by inhibiting DNA binding of Runx2 and Runx2-dependent transcription.

Quantitative signature for architectural organization of regulatory factors using intranuclear informatics

Regulatory machinery for replication and gene expression is punctately organized in supramolecular complexes that are compartmentalized in nuclear microenvironments. Quantitative approaches are required to understand the assembly of regulatory machinery within the context of nuclear architecture and to provide a mechanistic link with biological control. We have developed 'intranuclear informatics' to quantify functionally relevant parameters of spatially organized nuclear domains. Using this informatics strategy we have characterized post-mitotic reestablishment of focal subnuclear organization of Runx (AML/Cbfa) transcription factors in progeny cells. By analyzing point mutations that abrogate fidelity of Runx intranuclear targeting, we establish molecular determinants for the spatial order of Runx domains. Our novel approach provides evidence that architectural organization of Runx factors may be fundamental to their tissue-specific regulatory function.

Aged mice require full transcription factor, Runx2/Cbfa1, gene dosage for cancellous bone regeneration after bone marrow ablation

Runx2 is prerequisite for the osteoblastic differentiation in vivo. To elucidate Runx2 gene functions in adult bone metabolism, we conducted bone marrow ablation in Runx2 heterozygous knockout mice and found that aged (but not young) adult Runx2 heterozygous knockout mice have reduced new bone formation capacity after bone marrow ablation. We also found that bone marrow cells from aged Runx2 heterozygous knockout mice have reduced ALP(+) colony-forming potential in vitro. This indicates that full Runx2 dosage is needed for the maintenance of osteoblastic activity in adult mice.

INTRODUCTION

Null mutation of the Runx2 gene results in total loss of osteoblast differentiation, and heterozygous Runx2 deficiency causes cleidocranial dysplasia in humans and mice. However, Runx2 gene functions in adult bone metabolism are not known. We therefore examined the effects of Runx2 gene function in adult mice with heterozygous loss of the Runx2 gene.

MATERIALS AND METHODS

Bone marrow ablation was conducted in young adult (2.5 +/- 0.5 months old) or aged adult (7.5 +/- 0.5 months old) Runx2 heterozygous knockout mice and wildtype (WT) littermates. Cancellous bone regeneration was evaluated by 2D microCT.

RESULTS

Although new bone formation was observed after bone marrow ablation in the operated bone marrow cavity of WT mice, such bone formation was significantly reduced in Runx2 heterozygous knockout mice. Interestingly, this effect was observed specifically in aged but not young adult mice. Runx2 heterozygous deficiency in aged mice significantly reduced the number of alkaline phosphatase (ALP)(+) cell colonies in the bone marrow cell cultures, indicating a reduction in the numbers of osteoprogenitor cells. Such effects of heterozygous Runx2 deficiency on osteoblasts in vitro was specific to the cells from aged adult mice, and it was not observed in the cultures of marrow cells from young adult mice.

CONCLUSION

These results indicate that full gene dosage of Runx2 is required for cancellous bone formation after bone marrow ablation in adult mice.

Bone morphogenetic protein 2-induced osteoblast differentiation requires Smad-mediated down-regulation of Cdk6

Because a temporal arrest in the G(1) phase of the cell cycle is thought to be a prerequisite for cell differentiation, we investigated cell cycle factors that critically influence the differentiation of mouse osteoblastic MC3T3-E1 cells induced by bone morphogenetic protein 2 (BMP-2), a potent inducer of osteoblast differentiation. Of the G(1) cell cycle factors examined, the expression of cyclin-dependent kinase 6 (Cdk6) was found to be strongly down-regulated by BMP-2/Smads signaling, mainly via transcriptional repression. The enforced expression of Cdk6 blocked BMP-2-induced osteoblast differentiation to various degrees, depending on the level of its overexpression. However, neither BMP-2 treatment nor Cdk6 overexpression significantly affected cell proliferation, suggesting that the inhibitory effect of Cdk6 on cell differentiation was exerted by a mechanism that is largely independent of its cell cycle regulation. These results indicate that Cdk6 is a critical regulator of BMP-2-induced osteoblast differentiation and that its Smads-mediated down-regulation is essential for efficient osteoblast differentiation.

Rb family proteins differentially regulate distinct cell lineages during epithelial development

pRb, p107 and p130 are important regulators of cell cycle and have extensive overlapping functions; however, only Rb has been shown to be a bone fide tumor suppressor. Defining the overlapping versus distinct pocket protein functions is therefore an important step to understanding the unique role of Rb. Using lung as a model, the present studies demonstrate that pocket proteins are important not only in regulating cell cycle and survival but also in cell lineage specification. An inducible lung-specific Rb knockout strategy was used to demonstrate that Rb is specifically required for restricting neuroendocrine cell fate despite functional compensation for Rb deficiency in other cell types. Ablation of total Rb family function resulted in opposing effects in specification along distinct cell lineages, providing evidence that pocket proteins inhibit neuroendocrine cell fate while being required for differentiation in other cell types. These findings identify a novel role for pocket proteins in cell fate determination, and establish a unique cell lineage-specific function for Rb that explains, at least in part, why Rb and p16 are inactivated in phenotypically distinct carcinomas.

ChIP Display: novel method for identification of genomic targets of transcription factors

Novel protein-DNA interactions in mammalian cells are traditionally discovered in the course of promoter studies. The genomic era presents opportunities for the reverse; namely, the discovery of novel target genes for transcription factors of interest. Chromatin immunoprecipitation (ChIP) is typically used to test whether a protein binds to a candidate promoter in living cells. We developed a new method, ChIP Display (CD), which allows genome-wide unbiased identification of target genes occupied by transcription factors of interest. Initial CD experiments pursuing target genes for RUNX2, an osteoblast master transcription factor, have already resulted in the identification of four genes that had never been reported as targets of RUNX2. One of them, Osbpl8, was subjected to mRNA and promoter-reporter analyses, which provided functional proof for its regulation by RUNX2. CD will help to assemble the puzzle of interactions between transcription factors and the genome.

Runx2 control of organization, assembly and activity of the regulatory machinery for skeletal gene expression

We present an overview of Runx involvement in regulatory mechanisms that are requisite for fidelity of bone cell growth and differentiation, as well as for skeletal homeostasis and the structural and functional integrity of skeletal tissue. Runx-mediated control is addressed from the perspective of support for biological parameters of skeletal gene expression. We review recent findings that are consistent with an active role for Runx proteins as scaffolds for integration, organization and combinatorial assembly of nucleic acids and regulatory factors within the three-dimensional context of nuclear architecture.

New roles for the RB tumor suppressor protein

For a gene whose existence was first postulated in 1971, was cloned in 1986 and whose functions have been extensively characterized ever since, you might be inclined to think there was not much new to report regarding the retinoblastoma tumor suppressor gene (RB)--but you would be wrong to make such an assumption. RB is still piquing our interest with several activities defined over the past year that reveal new and exciting roles for this key tumor suppressor gene. These functions include regulation of senescence through specific gene silencing mechanisms, control of developmental processes in extra-embryonic tissues, maintaining tissue homeostasis and determining survival responses to chemotherapy.

Cell cycle development

The Keystone Symposium on the Cell Cycle and Development brought together biologists with an interest in how cell cycle control is integrated into the ontogenetic program of multicellular organisms, and showcased research using a wide variety of systems from both animals and plants. A clear indication from the meeting is that this research is changing the conventional wisdom on both cell cycle control and development.

Retinoblastoma protein and CCAAT/enhancer-binding protein beta are required for 1,25-dihydroxyvitamin D3-induced monocytic differentiation of HL60 cells

Derivatives of vitamin D (deltanoids) are well known to have the ability to induce differentiation of a variety of malignant cells, including human leukemia cells, but the signaling pathways that lead to such an outcome are unclear. In this study we investigated the role of the retinoblastoma protein (pRb) and the CCAAT/enhancer-binding protein (C/EBP) beta in 1,25-dihydroxyvitamin D(3) (1,25D(3))-induced monocytic differentiation of human leukemia HL60 cells. It was found that in this system, pRb is up-regulated within 12 h of exposure to the inducer, and the kinetics of its increase parallel the appearance of the early markers of differentiation, CD14 and monocyte-specific esterase. The increase in pRb expression was accompanied by a similar increase in C/EBPbeta protein, and these two proteins coimmunoprecipitated, suggesting formation of a complex. Oligonucleotides antisense to pRb or C/EBPbeta (but not to C/EBPalpha) or containing the C/EBP-binding sequence ("decoys"), all inhibited 1,25D(3)-induced differentiation. Inhibition of signaling by vitamin D receptor or by mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase pathways using pharmacological inhibitors ZK159222, PD98059, or SP600125, respectively, inhibited pRb and C/EBPbeta expression and differentiation in a coordinate manner. In contrast, inhibition of the p38MAPK pathway by SB202190 potentiated differentiation and the up-regulation of pRb and C/EBPbeta. We suggest that 1,25D(3) may signal monocytic differentiation of HL60 cells in a vitamin D receptor-dependent manner that includes activation of extracellular signal-regulated kinase and c-Jun-NH(2)-terminal kinase MAPK pathways, which then up-regulate pRb and C/EBPbeta expression and in turn initiate the differentiation process.

Differences in stability of repressor complexes at promoters underlie distinct roles for Rb family members

Oncogenic transformation of cells can induce the cyclin-dependent kinase inhibitor, p16, which leads to hypophosphorylation and activation of retinoblastoma (Rb). Rb is capable of causing permanent growth arrest, which may underlie its role as a tumor suppressor. We show that repression by Rb at E2F target gene promoters involves the establishment of a stable repressor complex that is not displaced by the overexpression of E2F-1. Rather than displacing Rb, excess E2F-1 instead recruits more Rb, leading to direct transcriptional repression. In contrast, the Rb family members, p130 and p107, which have not been demonstrated to be tumor suppressors, bind preferentially to target promoters in the absence of growth factors and in proliferating cells, respectively, and these repressor complexes are displaceable by E2F-1. Heterochromatin protein 1 (HP1), which interacts with Rb, is associated with these distinct repressor complexes and follows a similar pattern of stability/displaceability. Efficient growth arrest by p16/Rb is dependent on histone H3 lysine 9 methylation, which provides a binding site for HP1. We propose that these differences in the stability of repressor complexes at promoters may, in part, underlie the different roles of Rb vs p130 and p107 in cell cycle regulation and tumor suppression.

Cell-cycle control and the cartilage growth plate

The longitudinal growth of endochondral bones is governed by proliferation and hypertrophic differentiation of growth plate chondrocytes. Numerous growth factors and hormones have been implicated in the regulation of these processes, but the intracellular mechanisms involved remain much less understood. We had suggested a role of cell-cycle genes in the integration of these diverse extracellular signals and their translation into coordinated proliferation and differentiation of chondrocytes. Numerous recent studies have provided support for such a scenario and provide novel insights into the regulation and function of cell-cycle genes in chondrocytes. This review article summarizes recent progress in the field.