Epigenetic modifications in osteogenic differentiation and transformation

Aberrant LETM1 elevation dysregulates mitochondrial functions and energy metabolism and promotes lung metastasis in osteosarcoma

Osteosarcoma is a differentiation-deficient disease, and despite the unique advantages and great potential of differentiation therapy, there are only a few known differentiation inducers, and little research has been done on their targets. Cell differentiation is associated with an increase in mitochondrial content and activity. The metabolism of some tumor cells is characterized by impaired oxidative phosphorylation, as well as up-regulation of aerobic glycolysis and pentose phosphate pathways. Leucine-containing zipper and EF-hand transmembrane protein 1 (LETM1) is involved in the maintenance of mitochondrial morphology and is closely associated with tumorigenesis and progression, as well as cancer cell stemness. We found that MG63 and 143B osteosarcoma cells overexpress LETM1 and exhibit abnormalities in mitochondrial structure and function. Knockdown of LETM1 partially restored the mitochondrial structure and function, inhibited the pentose phosphate pathway, promoted oxidative phosphorylation, and led to osteogenic differentiation. It also inhibited spheroid cell formation, proliferation, migration, and invasion in an in vitro model. When LETM1 was knocked down in vivo, there was reduced tumor formation and lung metastasis. These data suggest that mitochondria are aberrant in LETM1-overexpressing osteosarcoma cells, and knockdown of LETM1 partially restores the mitochondrial structure and function, inhibits the pentose phosphate pathway, promotes oxidative phosphorylation, and increases osteogenic differentiation, thereby reducing malignant biological behavior of the cells.

Effective role of Curcumin on expression regulation of EZH2 histone methyltransferase as a dynamic epigenetic factor in osteogenic differentiation of human mesenchymal stem cells

BACKGROUND

Efficient differentiation of mesenchymal stem cells (MSCs) into a desired cell lineage remains challenging in cell-based therapy and regenerative medicine. Numerous efforts have been made to efficiently promote differentiation of MSCs into osteoblast lineage. Accordingly, epigenetic signatures emerge as a key conductor of cell differentiation. Among them, Enhancer of Zeste Homolog 2 (EZH2), a histone methyltransferase appears to suppress osteogenesis. Curcumin is an osteoinductive natural polyphenol compound which supposedly modulates epigenetic mechanisms. Hence, the current study aims to address the role of the EZH2 epigenetic factor in osteogenic activity of MSCs after Curcumin treatment.

METHODS

The effect of Curcumin on viability and osteogenic differentiation was evaluated at different time points in vitro. The expression level of EZH2 was assessed using quantitative real-time polymerase chain reaction (qRT-PCR) after 14 and 21 days.

RESULTS

MTT results showed no cytotoxic effects at concentrations of 10 and 15 μM of Curcumin and cells survived up to 70 % at all time-points. qRT-PCR results demonstrated that Curcumin significantly enhanced the expression levels of osteogenic markers that included Runx2, Osterix, Collagen type I, Osteopontin and Osteocalcin at day 21.

CONCLUSIONS

Interestingly, we observed that the expression level of the EZH2 gene was downregulated in the presence of Curcumin compared to the control group during osteogenesis. This study confirmed that Curcumin acts as an epigenetic switch to regulate osteoblast differentiation specifically through the EZH2 suppression.

Overexpression of BRINP3 Predicts Poor Prognosis and Promotes Cancer Cell Proliferation and Migration via MAP4 in Osteosarcoma

Osteosarcoma (OS) is a primary malignant bone tumor most commonly affecting children and adolescents and is characterized by loss of differentiation. Bone morphogenetic protein/retinoic acid inducible neural-specific 3 (BRINP3) has been reported to regulate the differentiation of osteoblasts. However, the role that BRINP3 plays in the progression of osteosarcoma remains unknown. We found in this study that BRINP3 was highly expressed in 64.13% of human osteosarcoma tissues and it was associated with histological grade, tumor recurrence, and poor clinical prognosis of osteosarcoma. In vitro, downregulation of BRINP3 was able to inhibit the proliferation and invasion of osteosarcoma cell lines. Furthermore, BRINP3 interacted with microtubule-associated protein 4 (MAP4) at the protein level, and overexpression of MAP4 could partially reverse the inhibitory effect of downregulated BRINP3 on the proliferation and invasion of osteosarcoma cells, which indicates that downregulation of BRINP3 might suppress the proliferation and invasion of osteosarcoma cells by inhibiting MAP4 expression. Overall, our results demonstrate that BRINP3 functions as an oncogene within osteosarcoma through MAP4 and could therefore be used as a potential biomarker for osteosarcoma diagnostics and therapeutics.

Collagen type I promotes osteogenic differentiation of amniotic membrane-derived mesenchymal stromal cells in basal and induction media

Collagen has been widely shown to promote osteogenesis of bone marrow mesenchymal stromal cells (BM-MSCs). Due to the invasive procedure of obtaining BM-MSCs, MSCs from other tissues have emerged as a promising alternative for regenerative therapy. MSCs originated from different sources, exhibiting different differentiation potentials. Therefore, the applicability of collagen type I (COL), combining with amniotic membrane (AM)-MSCs was examined through proliferation and differentiation assays together with the expression of surface markers and genes associated with stemness and differentiation under basal or induction conditions. No increase in cell growth was observed because AM-MSCs might be directed toward spontaneous osteogenesis. This was evidenced by the calcium deposition and elevated expression of osteogenic genes when AM-MSCs were cultured in collagen plate with basal media. Under the osteogenic condition, reciprocal expression of OCN and CEBPA suggested a shift toward adipogenesis. Surprisingly, adipogenic genes were not elevated upon adipogenic induction, although oil droplets deposition was observed. In conclusion, our findings demonstrated that collagen causes spontaneous osteogenesis in AM-MSCs. However, the presence of exogenous inductors could shift the direction of adipo-osteogenic gene regulatory network modulated by collagen.

Dehydroandrographolide Inhibits Osteosarcoma Cell Growth and Metastasis by Targeting SATB2-mediated EMT

BACKGROUND

The 12-hydroxy-14-dehydroandrographolide (DP) is a predominant component of the traditional herbal medicine Andrographis paniculata (Burm. f.) Nees (Acanthaceae). Recent studies have shown that DP exhibits potent anti-cancer effects against oral and colon cancer cells.

OBJECTIVE

This investigation examined the potential effects of DP against osteosarcoma cell.

METHODS

A cell analyzer was used to measure cell viability. The cell growth and proliferation were performed by Flow cytometry and BrdU incorporation assay. The cell migration and invasion were determined by wound healing and transwell assay. The expression of EMT related proteins was examined by Western blot analysis.

RESULTS

In this study, we found that DP treatment repressed osteosarcoma (OS) cell growth in a dose-dependent manner. DP treatment significantly inhibited OS cell proliferation by arresting the cell cycle at G2/M phase. In addition, DP treatment effectively inhibited the migration and invasion abilities of OS cells through wound healing and Transwell tests. Mechanistic studies revealed that DP treatment effectively rescued the epithelialmesenchymal transition (EMT), while forced expression of SATB2 in OS cells markedly reversed the pharmacological effect of DP on EMT.

CONCLUSION

Our data demonstrated that DP repressed OS cell growth through inhibition of proliferation and cell cycle arrest; DP also inhibited metastatic capability of OS cells through a reversal of EMT by targeting SATB2. These findings demonstrate DP's potential as a therapeutic drug for OS treatment.

ADAMTS7 degrades Comp to fuel BMP2-dependent osteogenic differentiation and ameliorate oncogenic potential in osteosarcomas

Osteosarcoma (OS) is the most common primary malignant bone tumor in children and adolescents, with a high metastatic potential. Despite dramatic changes in OS treatments over the past decades, their efficiency still remains limited, with severe complications and adverse side effects. Key mechanisms underlining tumorigenesis, metastasis and chemotherapy resistance are currently lacking, in turn hindering any progress with respect to developing effective and safe therapeutic strategies against OS. Recently, ADAMTS7, a member of the disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family, was shown to be involved in osteogenic differentiation‐related pathological processes. ADAMTS7 promotes vascular calcification via disturbing the balance between osteogenic bone morphogenetic protein (BMP)2 (regulating osteogenic differentiation and bone formation during development) and its natural inhibitor cartilage oligomeric matrix protein (Comp). Hence, in the present study, we aimed to investigate the role of ADAMTS7 in the pathological process of OS. We first revealed that ADAMTS7 was decreased in OS tissues. Lower expression of ADAMTS7 was correlated with poor histological differentiation and an advanced clinical stage of OS. Through loss‐ and gain‐function analysis, we further revealed that ADAMTS7 attenuated cell proliferation, migration and invasion, at the same time as promoting the expression of osteogenic differentiation markers in two OS cell lines: MG63 and SAOS2. Moreover, Comp was responsible for the effects of ADAMTS7 on OS pathogenesis by reinforcing cell osteogenic differentiation mediated by BMP2 in vitro. In conclusion, ADAMTS7‐mediated degradation of Comp may provide a potential therapeutic target for the treatment of OS.

Metabolomics uncovers a link between inositol metabolism and osteosarcoma metastasis

Cancer development and progression are characterized by complex molecular events. The acquisition of these events is primarily believed to result from alterations in gene and protein expression/function. Recent studies have also suggested the role of metabolic alterations, or "metabolic reprogramming," that may similarly contribute to these events. Indeed, our previous investigations in osteosarcoma (OS) identified metabolic changes uniquely linked to metastasis. Based on those findings, here we sought to build a more detailed understanding of the specific alterations in metabolites or metabolic pathways that may be responsible for the observed metastasis-associated metabolic alterations, suggested by gene expression data. This was pursued using a combination of high-throughput liquid- and gas-chromatography-based mass spectrometry (LC/MS and GC/MS) for a global metabolic profiling/subtraction of four pairs of high/low metastatic OS cell lines. By comparing the identity and level of the metabolites between high/low metastatic cells, several metabolic pathways were identified to be differentially activated, such as arginine, glutathione, inositol and fatty acid metabolic pathways. To further interrogate these results, we investigated the effects of inositol pathway dysregulation, through the exposure of metastatic OS cells to IP6 (inositol hexaphosphate). Although IP6 exposures had modest to minimal effects on cell proliferation, we observed reduced cellular glycolysis, down-regulation of PI3K/Akt signaling and suppression of OS metastatic progression. Collectively these data supported further investigation of metabolic sensitivities as anti-metastatic strategies in a clinical setting as well as investigation of altered metabolomics associated with metastatic progression.

Epigenetic Regulations in Neural Stem Cells and Neurological Diseases

Among the regulatory mechanisms of the renewal and differentiation of neural stem cells, recent evidences support that epigenetic modifications such as DNA methylation, histone modification, and noncoding RNAs play critical roles in the regulation on the proliferation and differentiation of neural stem cells. In this review, we discussed recent advances of DNA modifications on the regulative mechanisms of neural stem cells. Among these epigenetic modifications, DNA 5-hydroxymethylcytosine (5hmC) modification is emerging as an important modulator on the proliferation and differentiation of neural stem cells. At the same time, Ten-eleven translocation (Tet) methylcytosine dioxygenases, the rate-limiting enzyme for the 5-hydroxymethylation reaction from 5-methylcytosine to 5-hydroxymethylcytosine, play a critical role in the tumorigenesis and the proliferation and differentiation of stem cells. The functions of 5hmC and TET proteins on neural stem cells and their roles in neurological diseases are discussed.

Upregulation and biological function of transmembrane protein 119 in osteosarcoma

Osteosarcoma is suggested to be caused by genetic and molecular alterations that disrupt osteoblast differentiation. Recent studies have reported that transmembrane protein 119 (TMEM119) contributes to osteoblast differentiation and bone development. However, the level of TMEM119 expression and its roles in osteosarcoma have not yet been elucidated. In the present study, TMEM119 mRNA and protein expression was found to be up-regulated in osteosarcoma compared with normal bone cyst tissues. The level of TMEM119 protein expression was strongly associated with tumor size, clinical stage, distant metastasis and overall survival time. Moreover, gene set enrichment analysis (GSEA) of the Gene Expression Omnibus (GEO) GSE42352 dataset revealed TMEM119 expression in osteosarcoma tissues to be positively correlated with cell cycle, apoptosis, metastasis and TGF-β signaling. We then knocked down TMEM119 expression in U2OS and MG63 cells using small interfering RNA, which revealed that downregulation of TMEM119 could inhibit the proliferation of osteosarcoma cells by inducing cell cycle arrest in G0/G1 phase and apoptosis. We also found that TMEM119 knockdown significantly inhibited cell migration and invasion, and decreased the expression of TGF-β pathway-related factors (BMP2, BMP7 and TGF-β). TGF-β application rescued the inhibitory effects of TMEM119 knockdown on osteosarcoma cell migration and invasion. Further in vitro experiments with a TGF-β inhibitor (SB431542) or BMP inhibitor (dorsomorphin) suggested that TMEM119 significantly promotes cell migration and invasion, partly through TGF-β/BMP signaling. In conclusion, our data support the notion that TMEM119 contributes to the proliferation, migration and invasion of osteosarcoma cells, and functions as an oncogene in osteosarcoma.

Hes4: A potential prognostic biomarker for newly diagnosed patients with high-grade osteosarcoma

BACKGROUND

Prognostic biomarkers for osteosarcoma (OS) at the time of diagnosis are lacking. Necrotic response of OS to preoperative chemotherapy correlates with survival and is determined 3-4 months after diagnosis. The purpose of this study is to identify biomarkers that will stratify patients into good or poor responders to chemotherapy at diagnosis and determine the role of potential biomarkers in OS pathogenesis.

PROCEDURE

Because OS may be caused by disruptions of osteogenic differentiation, and the Notch pathway is one regulator of bone development, we examined the link between Notch effectors, OS differentiation, and OS outcome. We probed the R2: Genomics Analysis and Visualization Platform for RNA expression levels of Notch targets in mixed high-grade OS pretreatment biopsies. We used human OS cell lines in vitro and in mice to determine the role of the Notch target hairy/enhancer of split 4 (Hes4) in OS.

RESULTS

We found that in OS patients, high expression of Hes4 is correlated with decreased metastasis-free and overall survival. Human OS cells that overexpress Hes4 are more immature and have an increased invasive capacity in vitro. This was not universal to all Notch effectors, as Hes1 overexpression induced opposing effects. When injected into NSG mice, Hes4-overexpressing OS cells produced significantly larger, more lytic tumors and significantly more metastases than did control cells.

CONCLUSIONS

Hes4 overexpression promotes a more aggressive tumor phenotype by preventing osteoblastic differentiation of OS cells. Hes4 expression may allow for the stratification of patients into good or poor responders to chemotherapy at diagnosis.

Galangin inhibits human osteosarcoma cells growth by inducing transforming growth factor-β1-dependent osteogenic differentiation

Osteosarcoma is the most common primary malignancy of the musculoskeletal system, and is associated with excessive proliferation and poor differentiation of osteoblasts. Currently, despite the use of traditional chemotherapy and radiotherapy, no satisfactory and effective agent has been developed to treat the disease. Herein, we found that a flavonoid natural product, galangin, could significantly attenuate human osteosarcoma cells proliferation, without causing obvious cell apoptosis. Moreover, galangin enhanced the expression of osteoblast differentiation markers (collagen type I, alkaline phosphatase, osteocalcin and osteopontin) remarkably and elevated the alkaline phosphatase activity in human osteosarcoma cells. And galangin could also attenuated osteosarcoma growth in vivo. These bioactivities of galangin resulted from its selective activation of the transforming growth factor (TGF)-β1/Smad2/3 signaling pathway, which was demonstrated by pathway blocking experiments. These findings suggested that galangin could be a promising agent to treat osteosarcoma. In addition, targeting TGF-β1 to induce osteogenic differentiation might represent a novel therapeutic strategy to treat osteosarcoma with minimal side effects.

In Vitro Differentiation of Preosteoblast-Like Cells, MC3T3-E1, to Adipocytes Is Enhanced by 1,25(OH)2 Vitamin D3

Osteoblasts and adipocytes originate from common mesenchymal progenitor cells and are controlled by specific transcription factors. While 1,25-dihydroxyvitamin D₃ (vitamin D) is known to be an important factor for osteoblast differentiation, there are conflicting reports regarding its effect on adipogenesis. In this study, we attempted to understand the effect of exposure of preosteoblasts (MC3T3-E1) to adipogenic media with and without vitamin D and determined the expression of adipogenic genes during this process. Our studies show that while transdifferentiation of preosteoblasts occurred on exposure to adipogenic media, the effect of vitamin D treatment was synergistic resulting in several hundred fold increase in adipocyte transcription factors C/EBPα and peroxisome proliferator-activated receptor-gamma (P < 0.001) along with an increase in markers of adipogenesis and accumulation of lipid droplets in cells. Vitamin D treatment was also accompanied by 100-fold to 700-fold increases in vitamin D receptor expression during the treatment period (P < 0.001). To determine how the effect of vitamin D might compare to other genetic manipulations that promote adipogenic differentiation, we stably knocked down retinoblastoma expression in MC3T3-E1 cells. Recent studies have suggested retinoblastoma (Rb1) tumor suppressor gene function to be critical to maintain osteoblasts function and inhibit adipocyte differentiation. We exposed MC3T3-E1 cells with reduced Rb1 expression to adipogenic media and found an increase in adipogenic differentiation when compared to cells with a full complement of Rb dosage. However, the extent of the change was not as dramatic as seen with vitamin D. These studies show that preosteoblasts are sensitive and respond to these manipulations that favor the adipocytic phenotype. While vitamin D is not known to directly affect targets in adipogenesis, our observations may have resulted from the malleability of preosteoblast genome in MC3T3-E1 cells, which allowed adipocyte specific gene expression under appropriate stimuli. Why this pathway is influenced and subverted by an anabolic bone factor such as vitamin D remains to be determined.

Establishment of Cancer Stem Cell Cultures from Human Conventional Osteosarcoma

The current improvements in therapy against osteosarcoma (OS) have prolonged the lives of cancer patients, but the survival rate of five years remains poor when metastasis has occurred. The Cancer Stem Cell (CSC) theory holds that there is a subset of tumor cells within the tumor that have stem-like characteristics, including the capacity to maintain the tumor and to resist multidrug chemotherapy. Therefore, a better understanding of OS biology and pathogenesis is needed in order to advance the development of targeted therapies to eradicate this particular subset and to reduce morbidity and mortality among patients. Isolating CSCs, establishing cell cultures of CSCs, and studying their biology are important steps to improving our understanding of OS biology and pathogenesis. The establishment of human-derived OS-CSCs from biopsies of OS has been made possible using several methods, including the capacity to create 3-dimensional stem cell cultures under nonadherent conditions. Under these conditions, CSCs are able to create spherical floating colonies formed by daughter stem cells; these colonies are termed "cellular spheres". Here, we describe a method to establish CSC cultures from primary cell cultures of conventional OS obtained from OS biopsies. We clearly describe the several passages required to isolate and characterize CSCs.

Mitotic Inheritance of mRNA Facilitates Translational Activation of the Osteogenic-Lineage Commitment Factor Runx2 in Progeny of Osteoblastic Cells

Epigenetic mechanisms mediate the acquisition of specialized cellular phenotypes during tissue development, maintenance and repair. When phenotype-committed cells transit through mitosis, chromosomal condensation counteracts epigenetic activation of gene expression. Subsequent post-mitotic re-activation of transcription depends on epigenetic DNA and histone modifications, as well as other architecturally bound proteins that "bookmark" the genome. Osteogenic lineage commitment, differentiation and progenitor proliferation require the bone-related runt-related transcription factor Runx2. Here, we characterized a non-genomic mRNA mediated mechanism by which osteoblast precursors retain their phenotype during self-renewal. We show that osteoblasts produce maximal levels of Runx2 mRNA, but not protein, prior to mitotic cell division. Runx2 mRNA partitions symmetrically between daughter cells in a non-chromosomal tubulin-containing compartment. Subsequently, transcription-independent de novo synthesis of Runx2 protein in early G1 phase results in increased functional interactions of Runx2 with a representative osteoblast-specific target gene (osteocalcin/BGLAP2) in chromatin. Somatic transmission of Runx2 mRNAs in osteoblasts and osteosarcoma cells represents a versatile mechanism for translational rather than transcriptional induction of this principal gene regulator to maintain osteoblast phenotype identity after mitosis.

Multiple levels of epigenetic control for bone biology and pathology

Multiple dimensions of epigenetic control contribute to regulation of gene expression that governs bone biology and pathology. Once confined to DNA methylation and a limited number of post-translational modifications of histone proteins, the definition of epigenetic mechanisms is expanding to include contributions of non-coding RNAs and mitotic bookmarking, a mechanism for retaining phenotype identity during cell proliferation. Together these different levels of epigenetic control of physiological processes and their perturbations that are associated with compromised gene expression during the onset and progression of disease, have contributed to an unprecedented understanding of the activities (operation) of the genomic landscape. Here, we address general concepts that explain the contribution of epigenetic control to the dynamic regulation of gene expression during eukaryotic transcription. This article is part of a Special Issue entitled Epigenetics and Bone.

Tumor-suppressing effects of miR-429 on human osteosarcoma

Osteosarcoma is the most common primary bone tumor. Recent data indicated miRNAs may be involved in the pathogenesis of osteosarcoma, suggesting some novel targets for therapy. It is known that miR-429 is down-regulated and functions as a tumor suppressor by targeting c-myc and PLGG1 in gastric and breast cancer. However, the exact role of miR-429 in osteosarcoma remained unknown. In our study, we found MiR-429 was down-regulated in primary osteosarcoma lesion and osteosarcoma cell lines. Moreover, MiR-429 can inhibit the proliferation of osteosarcoma cell lines and induce more cell apoptosis. Also, we discovered MiR-429 plays a role in osteosarcoma by binding the 3'UTR of zinc finger E-box-binding homeobox 1 (ZEB1) mRNA, and that overexpression of ZEB1 could reverse the proliferation, subsequently blocking effect of miR-429. In conclusion, miR-429 serves as a tumor suppressor via interaction with ZEB1. Our finding may provide a new target for osteosarcoma therapy.

miR-23a impairs bone differentiation in osteosarcoma via down-regulation of GJA1

Osteosarcoma is the most common type of bone cancer in children and adolescents. Impaired differentiation of osteoblast cells is a distinguishing feature of this aggressive disease. As improvements in survival outcomes have largely plateaued, better understanding of the bone differentiation program may provide new treatment approaches. The miRNA cluster miR-23a~27a~24-2, particularly miR-23a, has been shown to interact with genes important for bone development. However, global changes in gene expression associated with functional gain of this cluster have not been fully explored. To better understand the relationship between miR-23a expression and bone cell differentiation, we carried out a large-scale gene expression analysis in HOS cells. Experimental results demonstrate that over-expression of miR-23a delays differentiation in this system. Downstream bioinformatic analysis identified miR-23a target gene connexin-43 (Cx43/GJA1), a mediator of intercellular signaling critical to osteoblast development, as acutely affected by miR-23a levels. Connexin-43 is up-regulated in the course of HOS cell differentiation and is down-regulated in cells transfected with miR-23a. Analysis of gene expression data, housed at Gene Expression Omnibus, reveals that Cx43 is consistently up-regulated during osteoblast differentiation. Suppression of Cx43 mRNA by miR-23a was confirmed in vitro using a luciferase reporter assay. This work demonstrates novel interactions between microRNA expression, intercellular signaling and bone differentiation in osteosarcoma.

mTORC1 maintains the tumorigenicity of SSEA-4(+) high-grade osteosarcoma

Inactivation of p53 and/or Rb pathways restrains osteoblasts from cell-cycle exit and terminal differentiation, which underpins osteosarcoma formation coupled with dedifferentiation. Recently, the level of p-S6K was shown to independently predict the prognosis for osteosarcomas, while the reason behind this is not understood. Here we show that in certain high-grade osteosarcomas, immature SSEA-4⁺ tumor cells represent a subset of tumor-initiating cells (TICs) whose pool size is maintained by mTORC1 activity. mTORC1 supports not only SSEA-4⁺ cell self-renewal through S6K but also the regeneration of SSEA-4⁺ TICs by SSEA-4⁻ osteosarcoma cell dedifferentiation. Mechanistically, active mTORC1 is required to prevent a likely upregulation of the cell-cycle inhibitor p27 independently of p53 or Rb activation, which otherwise effectively drives the terminal differentiation of SSEA-4⁻ osteosarcoma cells at the expense of dedifferentiation. Thus, mTORC1 is shown to critically regulate the retention of tumorigenicity versus differentiation in discrete differentiation phases in SSEA-4⁺ TICs and their progeny.

Rewiring mesenchymal stem cell lineage specification by switching the biophysical microenvironment

The propensity of stem cells to specify and commit to a particular lineage program is guided by dynamic biophysical and biochemical signals that are temporally regulated. However, most in vitro studies rely on "snapshots" of cell state under static conditions. Here we asked whether changing the biophysical aspects of the substrate could modulate the degree of mesenchymal stem cell (MSC) lineage specification. We chose to explore two diverse differentiation outcomes: MSC osteogenesis and trans-differentiation to neuron-like cells. MSCs were cultured on soft (~0.5 kPa) or stiff (~40 kPa) hydrogels followed by transfer to gels of the opposite stiffness. MSCs on soft gels express elevated neurogenesis markers while MSCs on stiff substrates express elevated osteogenesis markers. Transfer of MSCs from soft to stiff or stiff to soft substrates led to a switch in lineage specification. However, MSCs transferred from stiff to soft substrates maintained elevated osteogenesis markers, suggesting a degree of irreversible activation. Transferring MSCs to micropatterned substrates reveal geometric cues that further modulate lineage reversal. Taken together, this study demonstrates that MSCs remain susceptible to the biophysical properties of the extracellular matrix--even after several weeks of culture--and can redirect lineage specification in response to changes in the microenvironment.

Separate and combined effects of DNMT and HDAC inhibitors in treating human multi-drug resistant osteosarcoma HosDXR150 cell line

Understanding the molecular mechanisms underlying multi-drug resistance (MDR) is one of the major challenges in current cancer research. A phenomenon which is common to both intrinsic and acquired resistance, is the aberrant alteration of gene expression in drug-resistant cancers. Although such dysregulation depends on many possible causes, an epigenetic characterization is considered a main driver. Recent studies have suggested a direct role for epigenetic inactivation of genes in determining tumor chemo-sensitivity. We investigated the effects of the inhibition of DNA methyltransferase (DNMT) and hystone deacethylase (HDAC), considered to reverse the epigenetic aberrations and lead to the re-expression of de novo methylated genes in MDR osteosarcoma (OS) cells. Based on our analysis of the HosDXR150 cell line, we found that in order to reduce cell proliferation, co-treatment of MDR OS cells with DNMT (5-Aza-dC, DAC) and HDAC (Trichostatin A, TSA) inhibitors is more effective than relying on each treatment alone. In re-expressing epigenetically silenced genes induced by treatments, a very specific regulation takes place which suggests that methylation and de-acetylation have occurred either separately or simultaneously to determine MDR OS phenotype. In particular, functional relationships have been reported after measuring differential gene expression, indicating that MDR OS cells acquired growth and survival advantage by simultaneous epigenetic inactivation of both multiple p53-independent apoptotic signals and osteoblast differentiation pathways. Furthermore, co-treatment results more efficient in inducing the re-expression of some main pathways according to the computed enrichment, thus emphasizing its potential towards representing an effective therapeutic option for MDR OS.

Coleusin factor, a novel anticancer diterpenoid, inhibits osteosarcoma growth by inducing bone morphogenetic protein-2-dependent differentiation

Coleusin factor is a diterpenoid compound isolated from the root of a tropical plant, Coleus forskohlii. Although Coleusin factor has been reported to suppress proliferation of and induce apoptosis in several types of cancer cells, the effects of Coleusin factor on osteosarcoma and the underlying mechanism are still not fully understood. In this study, we show that Coleusin factor treatment potently inhibits the growth of osteosarcoma cells associated with G(1) cell-cycle arrest. Interestingly, apoptosis and cell death are not induced. Instead, Coleusin factor causes osteosarcoma cells to exhibit typical properties of differentiated osteoblasts, including a morphologic alteration resembling osteoblasts, the expression of osteoblast differentiation markers, elevated alkaline phosphatase activity, and increased cellular mineralization. Coleusin factor treatment significantly increases the expression of bone morphogenetic protein-2 (BMP-2), a crucial osteogenic regulator, and runt-related transcription factor 2 (RUNX2), one of the key transcription factors of the BMP pathway. When BMP-2 signaling is blocked, Coleusin factor fails to inhibit cell proliferation and to induce osteoblast differentiation. Thus, upregulation of BMP-2 autocrine is critical for Coleusin factor to induce osteoblast differentiation and exert its anticancer effects on osteosarcoma. Importantly, administration of Coleusin factor inhibits the growth of osteosarcoma xenografted in nude mice without systemic or immunologic toxicity. Osteosarcoma is a highly aggressive cancer marked by the loss of normal differentiation. Coleusin factor represents a new type of BMP-2 inducer that restores differentiation in osteosarcoma cells. It may provide a promising therapeutic strategy against osteosarcoma with minimal side effects.

Understanding the role of Notch in osteosarcoma

The Notch pathway has been described as an oncogene in osteosarcoma, but the myriad functions of all the members of this complex signaling pathway, both in malignant cells and nonmalignant components of tumors, make it more difficult to define Notch as simply an oncogene or a tumor suppressor. The cell-autonomous behaviors caused by Notch pathway manipulation may vary between cell lines but can include changes in proliferation, migration, invasiveness, oxidative stress resistance, and expression of markers associated with stemness or tumor-initiating cells. Beyond these roles, Notch signaling also plays a vital role in regulating tumor angiogenesis and vasculogenesis, which are vital aspects of osteosarcoma growth and behavior in vivo. Further, osteosarcoma cells themselves express relatively low levels of Notch ligand, making it likely that nonmalignant cells, especially endothelial cells and pericytes, are the major source of Notch activation in osteosarcoma tumors in vivo and in patients. As a result, Notch pathway expression is not expected to be uniform across a tumor but likely to be highest in those areas immediately adjacent to blood vessels. Therapeutic targeting of the Notch pathway is likewise expected to be complicated. Most pharmacologic approaches thus far have focused on inhibition of gamma secretase, a protease of the presenilin complex. This enzyme, however, has numerous other target proteins that would be expected to affect osteosarcoma behavior, including CD44, the WNT/β-catenin pathway, and Her-4. In addition, Notch plays a vital role in tissue and organ homeostasis in numerous systems, and toxicities, especially GI intolerance, have limited the effectiveness of gamma secretase inhibitors. New approaches are in development, and the downstream targets of Notch pathway signaling also may turn out to be good targets for therapy. In summary, a full understanding of the complex functions of Notch in osteosarcoma is only now unfolding, and this deeper knowledge will help position the field to better utilize novel therapies as they are developed.

Dexamethasone shifts bone marrow stromal cells from osteoblasts to adipocytes by C/EBPalpha promoter methylation

Dexamethasone (Dex)-induced osteoporosis has been described as the most severe side effect in long-term glucocorticoid therapy. The decreased bone mass and the increased marrow fat suggest that Dex possibly shifts the differentiation of bone marrow stromal cells (BMSCs) to favor adipocyte over osteoblast, but the underlying mechanisms are still unknown. In this paper, we established a Dex-induced osteoporotic mouse model, and found that BMSCs from Dex-treated mice are more likely to differentiate into adipocyte than those from control mice, even under the induction of bone morphogenetic protein-2 (BMP2). We also discovered both in vitro and in vivo that the expression level of adipocyte regulator CCAAT/enhancer-binding protein alpha (C/EBPalpha) is significantly upregulated in Dex-induced osteoporotic BMSCs during osteoblastogenesis by a mechanism that involves inhibited DNA hypermethylation of its promoter. Knockdown of C/EBPalpha in Dex-induced osteoporotic cells rescues their differentiation potential, suggesting that Dex shifts BMSC differentiation by inhibiting C/EBPalpha promoter methylation and upregulating its expression level. We further found that the Wnt/beta-catenin pathway is involved in Dex-induced osteoporosis and C/EBPalpha promoter methylation, and its activation by LiCl rescues the effect of Dex on C/EBPalpha promoter methylation and osteoblast/adipocyte balance. This study revealed the C/EBPalpha promoter methylation mechanism and evaluated the function of Wnt/beta-catenin pathway in Dex-induced osteoporosis, providing a useful therapeutic target for this type of osteoporosis.

Sustained Low-Dose Treatment with the Histone Deacetylase Inhibitor LBH589 Induces Terminal Differentiation of Osteosarcoma Cells

Histone deacetylase inhibitors (HDACi) were identified nearly four decades ago based on their ability to induce cellular differentiation. However, the clinical development of these compounds as cancer therapies has focused on their capacity to induce apoptosis in hematologic and lymphoid malignancies, often in combination with conventional cytotoxic agents. In many cases, HDACi doses necessary to induce these effects result in significant toxicity. Since osteosarcoma cells express markers of terminal osteoblast differentiation in response to DNA methyltransferase inhibitors, we reasoned that the epigenetic reprogramming capacity of HDACi might be exploited for therapeutic benefit. Here, we show that continuous exposure of osteosarcoma cells to low concentrations of HDACi LBH589 (Panobinostat) over a three-week period induces terminal osteoblast differentiation and irreversible senescence without inducing cell death. Remarkably, transcriptional profiling revealed that HDACi therapy initiated gene signatures characteristic of chondrocyte and adipocyte lineages in addition to marked upregulation of mature osteoblast markers. In a mouse xenograft model, continuous low dose treatment with LBH589 induced a sustained cytostatic response accompanied by induction of mature osteoblast gene expression. These data suggest that the remarkable capacity of osteosarcoma cells to differentiate in response to HDACi therapy could be exploited for therapeutic benefit without inducing systemic toxicity.

The decreased metastatic potential of rhabdomyosarcoma cells obtained through MET receptor downregulation and the induction of differentiation

Rhabdomyosarcoma (RMS) is the most common type of pediatric soft tissue sarcoma. The MET receptor has an important role in the biology of RMS, and its overexpression and hyperactivation correlate with the metastatic ability of RMS. Consequently, interfering with MET expression or functionality may constitute a sound strategy for reducing the progression and metastatic potential of RMS. Our study reveals that downregulation of the MET receptor leads to changes in the morphology of ARMS cell in vivo. Tumors acquire a spindle shape that is characteristic of muscle fibers. Inhibition of MET expression or function leads to (i) a decreased expression of the early myogenic marker MyoD, (ii) a decreased ability of ARMS cells to metastasize to bone marrow cavities, (iii) downregulation of CXCR4 receptor expression and (iv) a decreased migration of MET-depleted cells towards gradients of HGF and SDF-1. Finally, we demonstrate that in vitro differentiation of alveolar RMS cells decreases their metastatic behavior by reducing both the expression of the MET and CXCR4 receptors and their migratory response to HGF and SDF-1. These findings suggest that blockers of MET receptor function and inducers of RMS cells differentiation may be clinically useful for reducing the aggressiveness and metastatic potential of RMS and may have significant implications for its treatment.

Human osteosarcoma CD49f(-)CD133(+) cells: impaired in osteogenic fate while gain of tumorigenicity

The biological relationships among self-renewal, tumorigenicity, and lineage differentiation of human osteosarcoma-initiating cells (OSIC) remain elusive, making it difficult to identify and distinguish OSIC from osteosarcoma-forming cells (OSFC) for developing OSIC-targeted therapies. Using a new inverse lineage tracking strategy coupled with serial human-to-mouse xenotransplantation, we identified a subpopulation of osteosarcoma cells with OSIC-like properties and sought to distinguish them from their progeny, OSFC. We found that serial transplantation of cells from different osteosarcoma cell lines and primary osteosarcoma tissues progressively increased the CD49f⁺ subpopulation composing the bulk of the osteosarcoma mass. These CD49f⁺ cells displayed characteristics of OSFC: limited in vivo tumorigenicity, weak lineage differentiation, more differentiated osteogenic feature, and greater chemo-sensitivity. By contrast, their parental CD49f⁻CD133⁺ cells had an inhibited osteogenic fate, together with OSIC-like properties of self-renewal, strong tumorigenicity, and differentiation to CD49f⁺ progeny. Hence, the CD49f⁻CD133⁺ phenotype appears to identify OSIC-like cells that possess strong tumorigenicity correlated with an impaired osteogenic fate and the ability to initiate tumor growth through generation of CD49f⁺ progeny. These findings advance our understanding of OSIC-like properties and, for the first time, provide a much-needed distinction between OSIC and OSFC in this cancer.

Prevailing importance of the hedgehog signaling pathway and the potential for treatment advancement in sarcoma

The hedgehog signaling pathway is important in embryogenesis and post natal development. Constitutive activation of the pathway due to mutation of pathway components occurs in ~25% of medulloblastomas and also in basal cell carcinomas. In many other malignancies the therapeutic role for hedgehog inhibition though intriguing, based on preclinical data, is far from assured. Hedgehog inhibition is not an established part of the treatment paradigm of sarcoma but the scientific rationale for a possible benefit is compelling. In chondrosarcoma there is evidence of hedgehog pathway activation and an ontologic comparison between growth plate chondrocyte differentiation and different chondrosarcoma subtypes. Immunostaining epiphyseal growth plate for Indian hedgehog is particularly positive in the zone of pre-hypertrophic chondrocytes which correlates ontologically with conventional chondrosarcoma. In Ewing sarcoma/PNET tumors the Gli1 transcription factor is a direct target of the EWS-FLI1 oncoprotein present in 85% of cases. In many cases of rhabdomyosarcomas there is increased expression of Gli1 (Ragazzini et al., 2004). Additionally, a third of embryonal rhabdomyosarcomas have loss of Chr.9q22 that encompasses the patched locus (Bridge et al., 2000). The potential to treat osteosarcoma by inhibition of Gli2 and the role of the pathway in ovarian fibromas and other connective tissue tumors is also discussed (Nagao et al., 2011; Hirotsu et al., 2010). Emergence of acquired secondary resistance to targeted therapeutics is an important issue that is also relevant to hedgehog inhibition. In this context secondary resistance of medulloblastomas to treatment with a smoothened antagonist in two tumor mouse models is examined.

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

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

DNA methylation restricts spontaneous multi-lineage differentiation of mesenchymal progenitor cells, but is stable during growth factor-induced terminal differentiation

The progressive restriction of differentiation potential from pluripotent embryonic stem cells, via multipotent progenitor cells to terminally differentiated, mature somatic cells, involves step-wise changes in transcription patterns that are tightly controlled by the coordinated action of key transcription factors and changes in epigenetic modifications. While previous studies have demonstrated tissue-specific differences in DNA methylation patterns that might function in lineage restriction, it is unclear at what exact developmental stage these differences arise. Here, we have studied whether terminal, multi-lineage differentiation of C2C12 myoblasts is accompanied by lineage-specific changes in DNA methylation patterns. Using bisulfite sequencing and genome-wide methylated DNA- and chromatin immunoprecipitation-on-chip techniques we show that in these cells, in general, myogenic genes are enriched for RNA polymerase II and hypomethylated, whereas osteogenic genes show lower polymerase occupancy and are hypermethylated. Removal of DNA methylation marks by 5-azacytidine (5AC) treatment alters the myogenic lineage commitment of these cells and induces spontaneous osteogenic and adipogenic differentiation. This is accompanied by upregulation of key lineage-specific transcription factors. We subsequently analyzed genome-wide changes in DNA methylation and polymerase II occupancy during BMP2-induced osteogenesis. Our data indicate that BMP2 is able to induce the transcriptional program underlying osteogenesis without changing the methylation status of the genome. We conclude that DNA methylation primes C2C12 cells for myogenesis and prevents spontaneous osteogenesis, but still permits induction of the osteogenic transcriptional program upon BMP2 stimulation. Based on these results, we propose that cell type-specific DNA methylation patterns are established prior to terminal differentiation of adult progenitor cells. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Cancer-related ectopic expression of the bone-related transcription factor RUNX2 in non-osseous metastatic tumor cells is linked to cell proliferation and motility

INTRODUCTION

Metastatic breast cancer cells frequently and ectopically express the transcription factor RUNX2, which normally attenuates proliferation and promotes maturation of osteoblasts. RUNX2 expression is inversely regulated with respect to cell growth in osteoblasts and deregulated in osteosarcoma cells.

METHODS

Here, we addressed whether the functional relationship between cell growth and RUNX2 gene expression is maintained in breast cancer cells. We also investigated whether the aberrant expression of RUNX2 is linked to phenotypic parameters that could provide a selective advantage to cells during breast cancer progression.

RESULTS

We find that, similar to its regulation in osteoblasts, RUNX2 expression in MDA-MB-231 breast adenocarcinoma cells is enhanced upon growth factor deprivation, as well as upon deactivation of the mitogen-dependent MEK-Erk pathway or EGFR signaling. Reduction of RUNX2 levels by RNAi has only marginal effects on cell growth and expression of proliferation markers in MDA-MB-231 breast cancer cells. Thus, RUNX2 is not a critical regulator of cell proliferation in this cell type. However, siRNA depletion of RUNX2 in MDA-MB-231 cells reduces cell motility, while forced exogenous expression of RUNX2 in MCF7 cells increases cell motility.

CONCLUSIONS

Our results support the emerging concept that the osteogenic transcription factor RUNX2 functions as a metastasis-related oncoprotein in non-osseous cancer cells.

Determination of the minimal melatonin exposure required to induce osteoblast differentiation from human mesenchymal stem cells and these effects on downstream signaling pathways

The purpose of this study was to determine the critical time periods of melatonin treatment required to induce human mesenchymal stem cells (hAMSCs) into osteoblasts and to determine which osteogenic genes are involved in the process. The study design consisted of adding melatonin for different times (2, 5, 10, 14 or 21 days) toward the end of a 21-day treatment containing osteogenic (OS+) medium or at the beginning of the 21-day treatment and then withdrawn. The results show that a 21-day continuous melatonin treatment was required to induce both alkaline phosphatase (ALP) activity and calcium deposition and these effects were mediated through MT₂Rs. Functional analysis revealed that peak ALP levels induced by melatonin were accompanied by attenuation of melatonin-mediated inhibition of forskolin-induced cAMP accumulation. Immunoprecipitation and western blot analyses, respectively, showed that MT₂R/β-arrestin scaffolds complexed to Gi, MEK1/2 and ERK1/2 formed in these differentiated hAMSCs (i.e., when ALP levels were highest) where ERK1/2 resided primarily in the cytosol. It is hypothesized that these complexes form to modulate the subcellular localization of ERK1/2 to affect osteogenic gene expression. Using real-time RT-PCR, chronic melatonin exposure induced the expression of osteogenic genes RUNX-2, osteocalcin and BMP-2, through MT₂Rs. No melatonin-mediated changes in the mRNA expression of ALP, BMP-6 or in the oxidative enzymes MtTFA, PGC-1α, Polγ, NRF-1, PDH, PDK and LDH occurred. These data show that a continuous 21-day melatonin exposure is required to induce osteoblast differentiation from hAMSCs through the formation of MT₂R/Gi/β-arrestin/MEK/ERK1/2 complexes to induce osteogenesis.

The role of CCAAT/enhancer binding protein (C/EBP)-alpha in osteogenesis of C3H10T1/2 cells induced by BMP-2

The balance between osteogenesis and adipogenesis of mesenchymal stem cells is disrupted in various human diseases. Investigating the mechanisms that fine-tune this balance is of medical importance. Identification of potential target gene which can be used to study the relationship between them could be really helpful for this purpose. In the current study, we used C3H10T1/2 as model cells and through which two models of both osteogenesis induced by bone-morphogenetic protein (BMP)-2 and transdifferentiation from osteogenesis to adipogenesis were established. We investigated the role of CCAAT/enhancer binding protein (C/EBP)-alpha in these two systems. Then from epigenetic point of view, we elucidated the underlying molecular mechanisms preliminarily. The results showed that down-regulations of both C/EBP-alpha expression and its inducibility in response to insulin, fetal bovine serum, methylisobutylxanthine and dexamethasone (IFMD) adipogenic hormonal cocktail were observed in terminal stage of osteogenesis of C3H10T1/2 cells induced by BMP-2. And overexpression of C/EBP-alpha could lead to inhibition of osteogenesis differentiation and rescue attenuation of potential of adipogenic conversion in this process. Furthermore, we provided evidence that remarkable DNA hypermethylation and histones 3 and 4 hypoacetylation in -1286 bp/1065 bp promoter region of C/EBP-alpha were involved in both of down-regulations. Our data suggest that C/EBP-alpha functions as regulator in the balance between osteogenesis and adipogenesis of C3H10T1/2 cells and may be a therapeutic target.

Expression analysis of genes associated with human osteosarcoma tumors shows correlation of RUNX2 overexpression with poor response to chemotherapy

BACKGROUND

Human osteosarcoma is the most common pediatric bone tumor. There is limited understanding of the molecular mechanisms underlying osteosarcoma oncogenesis, and a lack of good diagnostic as well as prognostic clinical markers for this disease. Recent discoveries have highlighted a potential role of a number of genes including: RECQL4, DOCK5, SPP1, RUNX2, RB1, CDKN1A, P53, IBSP, LSAMP, MYC, TNFRSF1B, BMP2, HISTH2BE, FOS, CCNB1, and CDC5L.

METHODS

Our objective was to assess relative expression levels of these 16 genes as potential biomarkers of osteosarcoma oncogenesis and chemotherapy response in human tumors. We performed quantitative expression analysis in a panel of 22 human osteosarcoma tumors with differential response to chemotherapy, and 5 normal human osteoblasts.

RESULTS

RECQL4, SPP1, RUNX2, and IBSP were significantly overexpressed, and DOCK5, CDKN1A, RB1, P53, and LSAMP showed significant loss of expression relative to normal osteoblasts. In addition to being overexpressed in osteosarcoma tumor samples relative to normal osteoblasts, RUNX2 was the only gene of the 16 to show significant overexpression in tumors that had a poor response to chemotherapy relative to good responders.

CONCLUSION

These data underscore the loss of tumor suppressive pathways and activation of specific oncogenic mechanisms associated with osteosarcoma oncogenesis, while drawing attention to the role of RUNX2 expression as a potential biomarker of chemotherapy failure in osteosarcoma.

Retinoid-suppressed phosphorylation of RARalpha mediates the differentiation pathway of osteosarcoma cells

Although retinoic acid (RA) is a potent agent that coordinates inhibition of proliferation with differentiation of many cell types, RA-mediated signaling pathways in osteosarcoma cell differentiation are uncharacterized. In this study, we show that in human U2OS osteosarcoma cells, decreased phosphorylation of RA receptor alpha (RARalpha) by RA treatment or overexpressing a phosphorylation-defective mutant RARalphaS77A results in the inhibition of proliferation and induction of differentiation, and that U2OS cells transduced with RARalphaS77A suppresses tumor formation in nude mice. Moreover, using different human primary osteosarcoma cells and human mesenchymal stem cells for gene expression analysis, we found that either RA or RARalphaS77A induces many of the same differentiation response pathways and signaling molecules involved in U2OS cell differentiation. In addition, overexpression of the fibroblast growth factor 8f (FGF8f), one of the downstream targets induced by both RA and RARalphaS77A in U2OS cells, inhibits proliferation and induces expression of osteoblastic differentiation regulators. Hence, these data strongly suggest that RA-suppressed phosphorylation of RARalpha induces FGF8f expression to mediate differentiation response pathway in U2OS osteosarcoma cells.

Impaired cell cycle regulation of the osteoblast-related heterodimeric transcription factor Runx2-Cbfbeta in osteosarcoma cells

Bone formation and osteoblast differentiation require the functional expression of the Runx2/Cbfbeta heterodimeric transcription factor complex. Runx2 is also a suppressor of proliferation in osteoblasts by attenuating cell cycle progression in G(1). Runx2 levels are modulated during the cell cycle, which are maximal in G(1) and minimal beyond the G(1)/S phase transition (S, G(2), and M phases). It is not known whether Cbfbeta gene expression is cell cycle controlled in preosteoblasts nor how Runx2 or Cbfbeta are regulated during the cell cycle in bone cancer cells. We investigated Runx2 and Cbfbeta gene expression during cell cycle progression in MC3T3-E1 osteoblasts, as well as ROS17/2.8 and SaOS-2 osteosarcoma cells. Runx2 protein levels are reduced as expected in MC3T3-E1 cells arrested in late G(1) (by mimosine) or M phase (by nocodazole), but not in cell cycle arrested osteosarcoma cells. Cbfbeta protein levels are cell cycle independent in both osteoblasts and osteosarcoma cells. In synchronized MC3T3-E1 osteoblasts progressing from late G1 or mitosis, Runx2 levels but not Cbfbeta levels are cell cycle regulated. However, both factors are constitutively elevated throughout the cell cycle in osteosarcoma cells. Proteasome inhibition by MG132 stabilizes Runx2 protein levels in late G(1) and S in MC3T3-E1 cells, but not in ROS17/2.8 and SaOS-2 osteosarcoma cells. Thus, proteasomal degradation of Runx2 is deregulated in osteosarcoma cells. We propose that cell cycle control of Runx2 gene expression is impaired in osteosarcomas and that this deregulation may contribute to the pathogenesis of osteosarcoma.

Wnt inhibitory factor 1 is epigenetically silenced in human osteosarcoma, and targeted disruption accelerates osteosarcomagenesis in mice

Wnt signaling increases bone mass by stimulating osteoblast lineage commitment and expansion and forms the basis for novel anabolic therapeutic strategies being developed for osteoporosis. These strategies include derepression of Wnt signaling by targeting secreted Wnt pathway antagonists, such as sclerostin. However, such therapies are associated with safety concerns regarding an increased risk of osteosarcoma, the most common primary malignancy of bone. Here, we analyzed 5 human osteosarcoma cell lines in a high-throughput screen for epigenetically silenced tumor suppressor genes and identified Wnt inhibitory factor 1 (WIF1), which encodes an endogenous secreted Wnt pathway antagonist, as a candidate tumor suppressor gene. In vitro, WIF1 suppressed beta-catenin levels in human osteosarcoma cell lines, induced differentiation of human and mouse primary osteoblasts, and suppressed the growth of mouse and human osteosarcoma cell lines. Wif1 was highly expressed in the developing and mature mouse skeleton, and, although it was dispensable for normal development, targeted deletion of mouse Wif1 accelerated development of radiation-induced osteosarcomas in vivo. In primary human osteosarcomas, silencing of WIF1 by promoter hypermethylation was associated with loss of differentiation, increased beta-catenin levels, and increased proliferation. These data lead us to suggest that derepression of Wnt signaling by targeting secreted Wnt antagonists in osteoblasts may increase susceptibility to osteosarcoma.

Osteosarcoma development and stem cell differentiation

Osteosarcoma is the most common nonhematologic malignancy of bone in children and adults. The peak incidence occurs in the second decade of life, with a smaller peak after age 50. Osteosarcoma typically arises around the growth plate of long bones. Most osteosarcoma tumors are of high grade and tend to develop pulmonary metastases. Despite clinical improvements, patients with metastatic or recurrent diseases have a poor prognosis. Here, we reviewed the current understanding of human osteosarcoma, with an emphasis on potential links between defective osteogenic differentiation and bone tumorigenesis. Existing data indicate osteosarcoma tumors display a broad range of genetic and molecular alterations, including the gains, losses, or arrangements of chromosomal regions, inactivation of tumor suppressor genes, and the deregulation of major signaling pathways. However, except for p53 and/or RB mutations, most alterations are not constantly detected in the majority of osteosarcoma tumors. With a rapid expansion of our knowledge about stem cell biology, emerging evidence suggests osteosarcoma should be regarded as a differentiation disease caused by genetic and epigenetic changes that interrupt osteoblast differentiation from mesenchymal stem cells. Understanding the molecular pathogenesis of human osteosarcoma could ultimately lead to the development of diagnostic and prognostic markers, as well as targeted therapeutics for osteosarcoma patients.

Recent advances in cancer stem/progenitor cell research: therapeutic implications for overcoming resistance to the most aggressive cancers

Overcoming intrinsic and acquired resistance of cancer stem/progenitor cells to current clinical treatments represents a major challenge in treating and curing the most aggressive and metastatic cancers. This review summarizes recent advances in our understanding of the cellular origin and molecular mechanisms at the basis of cancer initiation and progression as well as the heterogeneity of cancers arising from the malignant transformation of adult stem/progenitor cells. We describe the critical functions provided by several growth factor cascades, including epidermal growth factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), stem cell factor (SCF) receptor (KIT), hedgehog and Wnt/beta-catenin signalling pathways that are frequently activated in cancer progenitor cells and are involved in their sustained growth, survival, invasion and drug resistance. Of therapeutic interest, we also discuss recent progress in the development of new drug combinations to treat the highly aggressive and metastatic cancers including refractory/relapsed leukaemias, melanoma and head and neck, brain, lung, breast, ovary, prostate, pancreas and gastrointestinal cancers which remain incurable in the clinics. The emphasis is on new therapeutic strategies consisting of molecular targeting of distinct oncogenic signalling elements activated in the cancer progenitor cells and their local microenvironment during cancer progression. These new targeted therapies should improve the efficacy of current therapeutic treatments against aggressive cancers, and thereby preventing disease relapse and enhancing patient survival.

The epigenetics of adult (somatic) stem cells

While genetic studies have provided a wealth of information about health and disease, there is a growing awareness that individual characteristics are also determined by factors other than genetic sequences. These "epigenetic" changes broadly encompass the influence of the environment on gene regulation and expression and in a more narrow sense, describe the mechanisms controlling DNA methylation, histone modification and genetic imprinting. In this review, we focus on the epigenetic mechanisms that regulate adult (somatic) stem cell differentiation, beginning with the metabolic pathways and factors regulating chromatin structure and DNA methylation and the molecular biological tools that are currently available to study these processes. The role of these epigenetic mechanisms in manipulating adult stem cells is followed by a discussion of the challenges and opportunities facing this emerging field.