Fidelity of Runx2 Activity in Breast Cancer Cells Is Required for the Generation of Metastases-Associated Osteolytic Disease

Functional role of Runx3 in the regulation of aggrecan expression during cartilage development

Runx2 and Runx3 are known to be expressed in the growth plate during endochondral bone formation. Here we addressed the functional role of Runx3 as distinct from Runx2 by using two models of postnatal bone repair: fracture healing that proceeds by an endochondral process and marrow ablation that proceeds by only an intramembranous process. Both Runx2 and Runx3 mRNAs were differentially up regulated during fracture healing. In contrast, only Runx2 showed increased expression after marrow ablation. During fracture healing, Runx3 was expressed earlier than Runx2, was concurrent with the period of chondrogenesis, and coincident with maximal aggrecan expression a protein associated with proliferating and permanent cartilage. Immunohistological analysis showed Runx3 protein was also expressed by chondrocytes in vivo. In contrast, Runx2 was expressed later during chondrocyte hypertrophy, and primary bone formation. The functional activities of Runx3 during chondrocyte differentiation were assessed by examining its regulatory actions on aggrecan gene expression. Aggrecan mRNA levels and aggrecan promoter activity were enhanced in response to the over-expression of either Runx2 and Runx3 in ATDC5 chondrogenic cell line, while sh-RNA knocked down of each Runx protein showed that only Runx3 knock down specifically suppressed aggrecan mRNA expression and promoter activity. ChIP assay demonstrated that Runx3 interactions were selective to sites within the aggrecan promoter and were only observed during early periods of chondrogenesis before hypertrophy. Our studies suggest that Runx3 positively regulates aggrecan expression and suggest that its function is more limited to cartilage development than to bone. In aggregate these data further suggest that the various members of the Runx transcription factors are involved in the coordination of chondrocyte development, maturation, and hypertrophy during endochondral bone formation.

Oncogenic cooperation between PI3K/Akt signaling and transcription factor Runx2 promotes the invasive properties of metastatic breast cancer cells

The serine/threonine kinase Akt/PKB promotes cancer cell growth and invasion through several downstream targets. Identification of novel substrates may provide new avenues for therapeutic intervention. Our study shows that Akt phosphorylates the cancer-related transcription factor Runx2 resulting in stimulated DNA binding of the purified recombinant protein in vitro. Pharmacological inhibition of the PI3K/Akt pathway in breast cancer cells reduces DNA-binding activity of Runx2 with concomitant reduction in the expression of metastasis-related Runx2 target genes. Akt phosphorylates Runx2 at three critical residues within the runt DNA-binding domain to enhance its in vivo genomic interactions with a target gene promoter, MMP13. Mutation of these three phosphorylation sites reduces Runx2 DNA-binding activity. However, Akt signaling does not appear to interefere with CBFβ-Runx2 interactions. Consequently, expression of multiple metastasis-related genes is decreased and Runx2-mediated cell invasion is supressed. Thus, our work identifies Runx2 as a novel and important downstream mediator of the PI3K/Akt pathway that is linked to metastatic properties of breast cancer cells.

The cancer-related transcription factor Runx2 modulates cell proliferation in human osteosarcoma cell lines

Runx2 regulates osteogenic differentiation and bone formation, but also suppresses pre-osteoblast proliferation by affecting cell cycle progression in the G(1) phase. The growth suppressive potential of Runx2 is normally inactivated in part by protein destabilization, which permits cell cycle progression beyond the G(1)/S phase transition, and Runx2 is again up-regulated after mitosis. Runx2 expression also correlates with metastasis and poor chemotherapy response in osteosarcoma. Here we show that six human osteosarcoma cell lines (SaOS, MG63, U2OS, HOS, G292, and 143B) have different growth rates, which is consistent with differences in the lengths of the cell cycle. Runx2 protein levels are cell cycle-regulated with respect to the G(1)/S phase transition in U2OS, HOS, G292, and 143B cells. In contrast, Runx2 protein levels are constitutively expressed during the cell cycle in SaOS and MG63 cells. Forced expression of Runx2 suppresses growth in all cell lines indicating that accumulation of Runx2 in excess of its pre-established levels in a given cell type triggers one or more anti-proliferative pathways in osteosarcoma cells. Thus, regulatory mechanisms controlling Runx2 expression in osteosarcoma cells must balance Runx2 protein levels to promote its putative oncogenic functions, while avoiding suppression of bone tumor growth.

Estrogen Receptor α (ERα) and Estrogen Related Receptor α (ERRα) are both transcriptional regulators of the Runx2-I isoform

Runx2 is a master regulator of bone development and has also been described as an oncogene. Estrogen Receptor α (ERα) and Estrogen Related Receptor α (ERRα), both implicated in bone metabolism and breast cancer, have been shown to share common transcriptional targets. Here, we show that ERα is a positive regulator of Runx2-I transcription. Moreover, ERRα can act as a transcriptional activator of Runx2-I in presence of peroxisome proliferator activated receptor gamma coactivator-1 alpha (PGC-1α). In contrast, ERRα behaves as a negative regulator of Runx2-I transcription in presence of PGC-1β. ERα and ERRα cross-talk via a common estrogen receptor response element on the Runx2-I promoter. In addition, estrogen regulates PGC-1β that in turn is able to modulate both ERα and ERRα transcriptional activity.

The RUNX family in breast cancer: relationships with estrogen signaling

The three RUNX family members are lineage specific master regulators, which also have important, context-dependent roles in carcinogenesis as either tumor suppressors or oncogenes. Here we review evidence for such roles in breast cancer (BCa). RUNX1, the predominant RUNX family member in breast epithelial cells, has a tumor suppressor role reflected by many somatic mutations found in primary tumor biopsies. The classical tumor suppressor gene RUNX3 does not consist of such a mutation hot spot, but it too seems to inhibit BCa; it is often inactivated in human BCa tumors and its haploinsufficiency in mice leads to spontaneous BCa development. The tumor suppressor activities of RUNX1 and RUNX3 are mediated in part by antagonism of estrogen signaling, a feature recently attributed to RUNX2 as well. Paradoxically, however RUNX2, a master osteoblast regulator, has been implicated in various aspects of metastasis in general and bone metastasis in particular. Reciprocating the anti-estrogenic tumor suppressor activity of RUNX proteins, inhibition of RUNX2 by estrogens may help explain their context-dependent anti-metastatic roles. Such roles are reserved to non-osseous metastasis, because ERα is associated with increased, not decreased skeletal dissemination of BCa cells. Finally, based on diverse expression patterns in BCa subtypes, the successful use of future RUNX-based therapies will most likely require careful patient selection.

RUNX2 in mammary gland development and breast cancer

Runx2 is best known as an essential factor in osteoblast differentiation and bone development but, like many other transcription factors involved in development, is known to operate over a much wider tissue range. Our understanding of these other aspects of Runx2 function is still at a relatively early stage and the importance of its role in cell fate decisions and lineage maintenance in non-osseous tissues is only beginning to emerge. One such tissue is the mammary gland, where Runx2 is known to be expressed and participate in the regulation of mammary specific genes. Furthermore, differential and temporal expression of this gene is observed during mammary epithelial differentiation in vivo, strongly indicative of an important functional role. Although the precise nature of that role remains elusive, preliminary evidence hints at possible involvement in the regulation of mammary stem and/or progenitor cells. As with many genes important in regulating cell fate, RUNX2 has also been linked to metastatic cancer where in some established breast cell lines, retention of expression is associated with a more invasive phenotype. More recently, expression analysis has been extended to primary breast cancers where high levels of RUNX2 align with a specific subtype of the disease. That RUNX2 expression correlates with the so called "Triple Negative" subtype is particularly interesting given the known cross talk between Runx2 and estrogen receptor signaling pathways. This review summaries our current understanding of Runx2 in mammary gland development and cancer, and postulates a role that may link both these processes.

Recruitment of coregulator G9a by Runx2 for selective enhancement or suppression of transcription

Runx2, best known for its role in regulating osteoblast-specific gene expression, also plays an increasingly recognized role in prostate and breast cancer metastasis. Using the C4-2B/Rx2(dox) prostate cancer cell line that conditionally expressed Runx2 in response to doxycycline treatment, we identified and characterized G9a, a histone methyltransferase, as a novel regulator for Runx2 activity. G9a function was locus-dependent. Whereas depletion of G9a reduced expression of many Runx2 target genes, including MMP9, CSF2, SDF1, and CST7, expression of others, such as MMP13 and PIP, was enhanced. Physical association between G9a and Runx2 was indicated by co-immunoprecipitation, GST-pulldown, immunofluorescence, and fluorescence recovery after photobleaching (FRAP) assays. Since G9a makes repressive histone methylation marks and is primarily known as a corepressor, we further investigated the mechanism by which G9a functioned as a positive regulator for Runx2 target genes. Transient reporter assays indicated that the histone methyltransferase activity of G9a was not required for transcriptional activation by Runx2. Chromatin immunoprecipitation assays for Runx2 and G9a showed that G9a was recruited to endogenous Runx2 binding sites. We conclude that a subset of cancer-related Runx2 target genes require recruitment of G9a for their expression, but do not depend on its histone methyltransferase activity.

Integrin αvβ3 and CD44 pathways in metastatic prostate cancer cells support osteoclastogenesis via a Runx2/Smad 5/receptor activator of NF-κB ligand signaling axis

BACKGROUND

Bone loss and pathological fractures are common skeletal complications associated with androgen deprivation therapy and bone metastases in prostate cancer patients. We have previously demonstrated that prostate cancer cells secrete receptor activator of NF-kB ligand (RANKL), a protein essential for osteoclast differentiation and activation. However, the mechanism(s) by which RANKL is produced remains to be determined. The objective of this study is to gain insight into the molecular mechanisms controlling RANKL expression in metastatic prostate cancer cells.

RESULTS

We show here that phosphorylation of Smad 5 by integrin αvβ3 and RUNX2 by CD44 signaling, respectively, regulates RANKL expression in human-derived PC3 prostate cancer cells isolated from bone metastasis. We found that RUNX2 intranuclear targeting is mediated by phosphorylation of Smad 5. Indeed, Smad5 knock-down via RNA interference and inhibition of Smad 5 phosphorylation by an αv inhibitor reduced RUNX2 nuclear localization and RANKL expression. Similarly, knockdown of CD44 or RUNX2 attenuated the expression of RANKL. As a result, conditioned media from these cells failed to support osteoclast differentiation in vitro. Immunohistochemistry analysis of tissue microarray sections containing primary prostatic tumor (grade2-4) detected predominant localization of RUNX2 and phosphorylated Smad 5 in the nuclei. Immunoblotting analyses of nuclear lysates from prostate tumor tissue corroborate these observations.

CONCLUSIONS

Collectively, we show that CD44 signaling regulates phosphorylation of RUNX2. Localization of RUNX2 in the nucleus requires phosphorylation of Smad-5 by integrin αvβ3 signaling. Our results suggest possible integration of two different pathways in the expression of RANKL. These observations imply a novel mechanistic insight into the role of these proteins in bone loss associated with bone metastases in patients with prostate cancer.

Early membrane initiated transcriptional effects of estrogens in breast cancer cells: First pharmacological evidence for a novel membrane estrogen receptor element (ERx)

The complexity of estrogen actions mainly relies to the presence of different identified receptors (ERα, ERβ, their isoforms, and GPR30/GPER) and their discrete cellular distribution. Depending on the localization of the receptor that mediates estrogen effects, nuclear and extra-nuclear actions have been described. The latter can trigger a number of signaling events leading also to transcriptional modifications. In an attempt to clarify the nature of the receptor(s) involved in the membrane initiated effect of estrogens on gene expression, we performed a whole transcriptome analysis of breast cancer cell lines with different receptor profiles (T47D, MCF7, MDA-MB-231, SK-BR-3). A pharmacological approach was conducted with the use of estradiol (E(2)) or membrane-impermeable E(2)-BSA in the absence or presence of a specific ERα-β or GPR30/GPER antagonist. Our results clearly show that in addition to the ERα isoforms and/or GPR30/GPER that mainly mediate the transcriptional effect of E(2)-BSA, there is a specific transcriptional signature (found in T47D and MCF-7 cells) suggesting the presence of an unidentified membrane ER element (ERx). Analysis of its signature and phenotypic verification revealed that important cell function such as apoptosis, transcriptional regulation, and growth factor signaling are associated with ERx.

Glucose-activated RUNX2 phosphorylation promotes endothelial cell proliferation and an angiogenic phenotype

The runt-related protein-2 (RUNX2) is a DNA-binding transcription factor that regulates bone formation, tumor cell metastasis, endothelial cell (EC) proliferation, and angiogenesis. RUNX2 DNA binding is glucose and cell cycle regulated. We propose that glucose may activate RUNX2 through changes in post-translational phosphorylation that are cell cycle-specific and will regulate EC function. Glucose increased cell cycle progression in EC through both G2/M and G1 phases with entry into S-phase occurring only in subconfluent cells. In the absence of nutrients and growth factors (starvation), subconfluent EC were delayed in G1 when RUNX2 expression was reduced. RUNX2 phosphorylation, activation of DNA binding, and pRb phosphorylation were stimulated by glucose and were necessary to promote cell cycle progression. Glucose increased RUNX2 localization at focal subnuclear sites, which co-incided with RUNX2 occupancy of the cyclin-dependent kinase (cdk) inhibitor p21(Cip1) promoter, a gene normally repressed by RUNX2. Mutation of the RUNX2 cdk phosphorylation site in the C-terminal domain (S451A.RUNX2) reduced RUNX2 phosphorylation and DNA binding. Expression of this cdk site mutant in EC inhibited glucose-stimulated differentiation (in vitro tube formation), monolayer wound healing, and proliferation. These results define a novel relationship between glucose-activated RUNX2 phosphorylation, cell cycle progression, and EC differentiation. These data suggest that inhibition of RUNX2 expression or DNA binding may be a useful strategy to inhibit EC proliferation in tumor angiogenesis.

Regulation of RUNX2 transcription factor-DNA interactions and cell proliferation by vitamin D3 (cholecalciferol) prohormone activity

The fat-soluble prohormone cholecalciferol (Vitamin D3) is a precursor of the circulating 25-OH Vitamin D3, which is converted by 1α-hydroxylase to the biologically active 1,25-OH Vitamin D3. Active Vitamin D3 interacts with the Vitamin D receptor (VDR), a transcription factor that plays an important role in calcium mobilization and bone formation. RUNX2 is a DNA-binding transcription factor that regulates target genes important in bone formation, angiogenesis, and cancer metastasis. Using computer-assisted drug design (CADD) and a microtiter plate-based DNA-binding enzyme-linked immunosorbent assay (D-ELISA) to measure nuclear RUNX2 DNA binding, we have found that Vitamin D3 prohormones can modulate RUNX2 DNA binding, which was dose-dependent and sensitive to trypsin, salt, and phosphatase treatment. Unlabeled oligonucleotide or truncated, dominant negative RUNX2 proteins were competitive inhibitors of RUNX2 DNA binding. The RUNX2 heterodimeric partner, Cbfβ, was detected in the binding complexes with specific antibodies. Evaluation of several RUNX2:DNA targeted small molecules predicted by CADD screening revealed a previously unknown biological activity of the inactive Vitamin D3 precursor, cholecalciferol. Cholecalciferol modulated RUNX2:DNA binding at nanomolar concentrations even in cells with low VDR. Cholecalciferol and 25-OH Vitamin D3 prohormones were selective inhibitors of RUNX2-positive endothelial, bone, and breast cancer cell proliferation, but not of cells lacking RUNX2 expression. These compounds may have application in modulating RUNX2 activity in an angiogenic setting, in metastatic cells, and to promote bone formation in disease-mediated osteoporosis. The combination CADD discovery and D-ELISA screening approaches allows the testing of other novel derivatives of Vitamin D and/or transcriptional inhibitors with the potential to regulate DNA binding and biological function.

Metastatic breast cancer cells inhibit osteoblast differentiation through the Runx2/CBFβ-dependent expression of the Wnt antagonist, sclerostin

INTRODUCTION

Breast cancers frequently metastasise to the skeleton where they cause osteolytic bone destruction by stimulating osteoclasts to resorb bone and by preventing osteoblasts from producing new bone. The Runt-related transcription factor 2, Runx2, is an important determinant of bone metastasis in breast cancer. Runx2 is known to mediate activation of osteoclast activity and inhibition of osteoblast differentiation by metastatic breast cancer cells. However, while Runx2-regulated genes that mediate osteoclast activation have been identified, how Runx2 determines inhibition of osteoblasts is unknown.

METHODS

The aim of this study was to determine how Runx2 mediates the ability of metastatic breast cancer cells to modulate the activity of bone cells. We have previously demonstrated that Runx2 requires the co-activator core binding factor beta (CBFβ) to regulate gene expression in breast cancer cells. We, therefore, performed independent microarray analyses to identify target genes whose expression is dependent upon both Runx2 and CBFβ. Common target genes, with a role in modulating bone-cell function, were confirmed using a combination of siRNA, quantitative reverse transcriptase PCR (qRT-PCR), ELISA, promoter reporter analysis, Electrophoretic Mobility Shift Assay (EMSA) and chromatin immunoprecipitation (ChIP) assays. The function of Runx2/CBFβ-regulated genes in mediating the ability of MDA-MB-231 to inhibit osteoblast differentiation was subsequently established in primary bone marrow stromal cell cultures and MC-3T3 osteoblast cells.

RESULTS

We show that Runx2/CBFβ mediates inhibition of osteoblast differentiation by MDA-MB-231 cells through induction of the Wnt signaling antagonist, sclerostin. We demonstrate that MDA-MB-231 cells secrete sclerostin and that sclerostin-expression is critically dependent on both Runx2 and CBFβ. We also identified the osteoclast activators IL-11 and granulocyte-macrophage colony-stimulating factor (GM-CSF) as new target genes of Runx2/CBFβ in metastatic breast cancer cells.

CONCLUSIONS

This study demonstrates that Runx2 and CBFβ are required for the expression of genes that mediate the ability of metastatic breast cancer cells to directly modulate both osteoclast and osteoblast function. We also show that Runx2-dependent inhibition of osteoblast differentiation by breast cancer cells is mediated through the Wnt antagonist, sclerostin.

RUNX1T1: a novel predictor of liver metastasis in primary pancreatic endocrine neoplasms

OBJECTIVES

Using gene expression profiling on frozen primary pancreatic endocrine tumors (PETs), we discovered RUNX1T1 as a leading candidate progression gene. This study was designed (1) to validate the differential expression of RUNX1T1 protein on independent test sets of metastatic and nonmetastatic PETs and (2) to determine if RUNX1T1 underexpression in primary tumors was predictive of liver metastases.

METHODS

Immunohistochemical expression of RUNX1T1 protein was quantified using Allred scores on archival metastatic (n = 13) and nonmetastatic (n = 24) primary adult PET tissues using custom-designed tissue microarrays. Wilcoxon rank sum/Fisher exact tests and receiver operating characteristic curves were used in the data analysis.

RESULTS

Median RUNX1T1 scores were 2 (2-7) and 6 (3-8) in metastatic versus nonmetastatic primaries (P < 0.0001). Eleven of 13 metastatic and 1 of 24 nonmetastatic primaries exhibited RUNX1T1-scores of 4 or less (P < 0.0001). Low RUNX1T1 expression was highly associated with hepatic metastases (P < 0.0001), whereas conventional histological criteria (Ki-67 index, mitotic rate, necrosis) were weakly associated with metastases (P = 0.08-0.15). Considering RUNX1T1 expression (Allred) score of 4 or less to be predictive, the sensitivity to predict hepatic metastases was 85%, with a specificity of 96%.

CONCLUSIONS

RUNX1T1 protein is underexpressed in well-differentiated metastatic primary PETs relative to nonmetastatic primaries and emerges as a promising novel biomarker for prediction of liver metastases.

ODAM Expression Inhibits Human Breast Cancer Tumorigenesis

We have posited that Odontogenic Ameloblast Associated Protein (ODAM) serves as a novel prognostic biomarker in breast cancer and now have investigated its potential role in regulating tumor growth and metastasis. Human breast cancer MDA-MB-231 cells were transfected with a recombinant ODAM plasmid construct (or, as a control, the plasmid vector alone). ODAM expression increased adhesion and apoptosis of the transfected MDA-MB-231 cells and suppressed their growth rate, migratory activity, and capability to invade extracellular matrix-coated membranes. Implantation of such cells into mouse mammary fat pads resulted in significantly smaller tumors than occurred in animals that received control cells; furthermore, ODAM-expressing cells, when injected intravenously into mice, failed to metastasize, whereas the control-transfected counterparts produced extensive lung lesions. Our finding that induction of ODAM expression in human breast cancer cells markedly inhibited their neoplastic properties provides further evidence for the regulatory role of this molecule in tumorigenesis and, consequently, is of potential clinical import.

An architectural genetic and epigenetic perspective

The organization and intranuclear localization of nucleic acids and regulatory proteins contribute to both genetic and epigenetic parameters of biological control. Regulatory machinery in the cell nucleus is functionally compartmentalized in microenvironments (focally organized sites where regulatory factors reside) that provide threshold levels of factors required for transcription, replication, repair and cell survival. The common denominator for nuclear organization of regulatory machinery is that each component of control is architecturally configured and every component of control is embedded in architecturally organized networks that provide an infrastructure for integration and transduction of regulatory signals. It is realistic to anticipate emerging mechanisms that account for the organization and assembly of regulatory complexes within the cell nucleus can provide novel options for cancer diagnosis and therapy with maximal specificity, reduced toxicity and minimal off-target complications.

Advances in the biology of bone metastasis: how the skeleton affects tumor behavior

It is increasingly evident that the microenvironment of bone can influence the cancer phenotype in many ways that favor growth in bone. The ability of cancer cells to adhere to bone matrix and to promote osteoclast formation are key requirements for the establishment and growth of bone metastases. Several cytokine products of breast cancers (e.g. PTHrP, IL-11, IL-8) have been shown to act upon host cells of the bone microenvironment to promote osteoclast formation, allowing for excessive bone resorption. The increased release of matrix-derived growth factors, especially TGF-β, acts back upon the tumor to facilitate further tumor expansion and enhance cytokine production, and also upon osteoblasts to suppress bone formation. This provides a self-perpetuating cycle of bone loss and tumor growth within the skeleton. Other contributing factors favoring tumor metastasis and colonization in bone include the unique structure and stiffness of skeletal tissue, along with the diverse cellular composition of the marrow environment (e.g. bone cells, stromal fibroblasts, immune cells), any of which can contribute to the phenotypic changes that can take place in metastatic deposits that favor their survival. Additionally, it is also apparent that breast cancer cells begin to express different bone specific proteins as well as proteins important for normal breast development and lactation that allow them to grow in bone and stimulate bone destruction. Taken together, these continually emerging areas of study suggest new potential pathways important in the pathogenesis of bone metastasis and potential areas for targeting therapeutics.

Metastatic bone disease: role of transcription factors and future targets

Progression of cancer from the earliest event of cell transformation through stages of tumor growth and metastasis at a distal site involves many complex biological processes. Underlying the numerous responses of cancer cells to the tumor microenvironment which support their survival, migration and metastasis are transcription factors that regulate the expression of genes reflecting properties of the tumor cell. A number of transcription factors have been identified that play key roles in promoting oncogenesis, tumor growth, metastasis and tissue destruction. Relevant to solid tumors and leukemias, tissue-specific transcription factors that are deregulated resulting from mutations, being silenced or aberrantly expressed, have been well characterized. These are the master transcription factors of the Runx family of genes, the focus of this review, with emphasis placed on Runx2 that is abnormally expressed at very high levels in cancer cell lines that are metastatic to bone. Recent evidence has identified a correlation of Runx2 levels in advanced stages of prostate and breast cancer and demonstrated that effective depletion of Runx2 by RNA interference inhibits migration and invasive properties of the cells prevents metastatic bone disease. This striking effect is consistent with the broad spectrum of Runx2 properties in activating many genes in tumor cells that have already been established as indicators of bone metastasis in poor prognosis. Potential strategies to translate these findings for therapeutic applications are discussed.

The Role of RUNX2 in Osteosarcoma Oncogenesis

Osteosarcoma is an aggressive but ill-understood cancer of bone that predominantly affects adolescents. Its rarity and biological heterogeneity have limited studies of its molecular basis. In recent years, an important role has emerged for the RUNX2 "platform protein" in osteosarcoma oncogenesis. RUNX proteins are DNA-binding transcription factors that regulate the expression of multiple genes involved in cellular differentiation and cell-cycle progression. RUNX2 is genetically essential for developing bone and osteoblast maturation. Studies of osteosarcoma tumours have revealed that the RUNX2 DNA copy number together with RNA and protein levels are highly elevated in osteosarcoma tumors. The protein is also important for metastatic bone disease of prostate and breast cancers, while RUNX2 may have both tumor suppressive and oncogenic roles in bone morphogenesis. This paper provides a synopsis of the current understanding of the functions of RUNX2 and its potential role in osteosarcoma and suggests directions for future study.

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.

The cancer-related Runx2 protein enhances cell growth and responses to androgen and TGFbeta in prostate cancer cells

Prostate cancer cells often metastasize to bone where osteolytic lesions are formed. Runx2 is an essential transcription factor for bone formation and suppresses cell growth in normal osteoblasts but may function as an oncogenic factor in solid tumors (e.g., breast, prostate). Here, we addressed whether Runx2 is linked to steroid hormone and growth factor signaling, which controls prostate cancer cell growth. Protein expression profiling of prostate cell lines (i.e., PC3, LNCaP, RWPE) treated with 5alpha-dihydrotestosterone (DHT) or tumor growth factor beta (TGFbeta) revealed modulations in selected cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors that are generally consistent with mitogenic responses. Endogenous elevation of Runx2 and diminished p57 protein levels in PC3 cells are associated with faster proliferation in vitro and development of larger tumors upon xenografting these cells in bone in vivo. To examine whether TGFbeta or DHT signaling modulates the transcriptional activity of Runx2 and vice versa, we performed luciferase reporter assays. In PC3 cells that express TGFbetaRII, TGFbeta and Runx2 synergize to increase transcription of synthetic promoters. In LNCaP cells that are DHT responsive, Runx2 stimulates the androgen receptor (AR) responsive expression of the prostate-specific marker PSA, perhaps facilitated by formation of a complex with AR. Our data suggest that Runx2 is mechanistically linked to TGFbeta and androgen responsive pathways that support prostate cancer cell growth.

Runx2 in normal tissues and cancer cells: A developing story

The Runx transcription factors are essential for mammalian development, most notably in the haematopoietic and osteogenic lineages. Runx1 and its binding partner, CBFbeta, are frequently targeted in acute leukaemia but evidence is accumulating that all three Runx genes may have a role to play in a wider range of cancers, either as tumour promoters or tumour suppressors. Whilst Runx2 is renowned for its role as a master regulator of bone development we discuss here its expression pattern and putative functions beyond this lineage. Furthermore, we review the evidence that RUNX2 promotes neoplastic development in haematopoietic lineages and in advanced mammary and prostate cancer.

Proteasome inhibition with bortezomib suppresses growth and induces apoptosis in osteosarcoma

Osteosarcomas are primary bone tumors of osteoblastic origin that mostly affect adolescent patients. These tumors are highly aggressive and metastatic. Previous reports indicate that gain of function of a key osteoblastic differentiation factor, Runx2, leads to growth inhibition in osteosarcoma. We have previously established that Runx2 transcriptionally regulates expression of a major proapoptotic factor, Bax. Runx2 is regulated via proteasomal degradation, and proteasome inhibition has a stimulatory effect on Runx2. In this study, we hypothesized that proteasome inhibition will induce Runx2 and Runx2-dependent Bax expression sensitizing osteosarcoma cells to apoptosis. Our data showed that a proteasome inhibitor, bortezomib, increased Runx2 and Bax in osteosarcoma cells. In vitro, bortezomib suppressed growth and induced apoptosis in osteosarcoma cells but not in nonmalignant osteoblasts. Experiments involving intratibial tumor xenografts in nude mice demonstrated significant tumor regression in bortezomib-treated animals. Immunohistochemical studies revealed that bortezomib inhibited cell proliferation and induced apoptosis in osteosarcoma xenografts. These effects correlated with increased immunoreactivity for Runx2 and Bax. In summary, our results indicate that bortezomib suppresses growth and induces apoptosis in osteosarcoma in vitro and in vivo suggesting that proteasome inhibition may be effective as an adjuvant to current treatment regimens for these tumors. Published 2009 UICC. This article is a US Government work and, as such, is in the public domain in the United States of America.

Runx2 regulates survivin expression in prostate cancer cells

Previously we described that bone morphogenetic protein-7 (BMP7) could protect prostate cancer C4-2B cells from serum starvation-induced apoptosis via survivin induction. Here, for the first time, we identify Runx2 as a key regulator of survivin transcription. In C4-2B cells grown normally, suppression of Runx2 reduced survivin expression. Using ChIP assays, two regions of the survivin promoter, -1953 to -1812 (I) and -1485 to -1119 (II) encompassing consensus Runx-binding sites were examined. Runx2 was found to be associated with both regions, with a stronger affinity to region-I. In serum-starved cells neither region was occupied, but BMP7 restored association to region-II and not region-I. In reporter assays, transcription activity by BMP7 was significantly reduced when sequences including binding sites of region-II were deleted. Additionally, Runx2 expression was enhanced by BMP7 in these cells. Along with a strong survivin expression, a trend in increased Runx2 expression in human prostate cancer cells and tissues was noted. In the conditional Pten-knockout mouse, Runx2 level increased with growth of prostate tumor. The data define a novel role of Runx2 in regulating survivin expression in malignant epithelial cells and identify it as a critical factor in BMP signaling that protects cancer cells against apoptosis.

Transforming growth factor-beta1 regulation of ATF-3 and identification of ATF-3 target genes in breast cancer cells

Transforming growth factor-beta1 (TGF-beta1) is a crucial molecule for stimulation of breast cancer invasion and formation of bone metastases. The molecular mechanisms of how TGF-beta1 mediates these effects have yet to be completely determined. We have found that activating transcription factor-3 (ATF-3) is strongly stimulated and its level is sustained by TGF-beta1 in highly invasive and metastatic human breast cancer (MDA-MB231) and in mouse mammary pad tumor cells (r3T). ATF-3 is also overexpressed in human primary breast cancer tissue. Overexpression of ATF-3 increased normal human mammary epithelial cell number and DNA synthesis suggesting a role for ATF-3 in cell proliferation. The functional role of ATF-3 in breast cancer progression was determined by the RNA interference technique. Knockdown of ATF-3 by ATF-3 shRNA in MDA-MB231 cells decreased expression of cell cycle gene, cyclin A1 in MDA-MB231 cells. ATF-3 shRNA also decreased expression of an invasive and metastatic gene, matrix metalloproteinase-13 (MMP-13; collagenase-3) in these cells. Chromatin immunoprecipitation experiments identified the direct physical interaction of ATF-3 protein on the human MMP-13 promoter. Thus, the dysregulation of ATF-3 by TGF-beta1 is likely to activate cyclin A1 and MMP-13 genes in breast cancer cells and that would be key to the subsequent cancer cell invasion and metastasis.

Identification and functional characterization of ERK/MAPK phosphorylation sites in the Runx2 transcription factor

The Runx2 transcription factor is required for commitment of mesenchymal cells to bone lineages and is a major regulator of osteoblast-specific gene expression. Runx2 is subject to a number of post-transcriptional controls including selective proteolysis and phosphorylation. We previously reported that Runx2 is phosphorylated and activated by the ERK/MAPK pathway (Xiao, G., Jiang, D., Thomas, P., Benson, M. D., Guan, K., Karsenty, G., and Franceschi, R. T. (2000) J. Biol. Chem. 275, 4453-4459). In this study, we used a combination of in vitro and in vivo phosphorylation analysis, mass spectroscopy, and functional assays to identify two sites at Ser(301) and Ser(319) within the proline/serine/threonine domain of Runx2 that are required for this regulation. These sites are phosphorylated by activated ERK1 in vitro and in cell culture. In addition to confirming ERK-dependent phosphorylation at Ser(319), mass spectroscopy identified two other ERK-phosphorylated sites at Ser(43) and Ser(510). Furthermore, introduction of S301A,S319A mutations rendered Runx2 resistant to MAPK-dependent activation and reduced its ability to stimulate osteoblast-specific gene expression and differentiation after transfection into Runx2-null calvarial cells and mesenchymal cells. In contrast, S301E,S319E Runx2 mutants had enhanced transcriptional activity that was minimally dependent on MAPK signaling, consistent with the addition of a negative charge mimicking serine phosphorylation. These results emphasize the important role played by Runx2 phosphorylation in the control of osteoblast gene expression and provide a mechanism to explain how physiological signals acting on bone through the ERK/MAPK pathway can stimulate osteoblast-specific gene expression.

Runx2 association with progression of prostate cancer in patients: mechanisms mediating bone osteolysis and osteoblastic metastatic lesions

Runx2, a bone-specific transcriptional regulator, is abnormally expressed in highly metastatic prostate cancer cells. Here we identified the functional activities of Runx2 in facilitating tumor growth and osteolysis. Our studies demonstrate that negligible Runx2 is found in normal prostate epithelial and non-metastatic LNCaP prostate cancer cells. In the intra-tibial metastasis model, high Runx2 levels are associated with development of large tumors, increased expression of metastasis-related genes (MMP9, MMP13, VEGF, Osteopontin), and secreted bone resorbing factors (PTHrP, IL-8) promoting osteolytic disease. Runx2 siRNA treatment of PC3 cells decreased cell migration and invasion through Matrigel in vitro, and in vivo shRunx2 expression in PC3 cells blocked their ability to survive in the bone microenvironment. Mechanisms of Runx2 function were identified in co-culture studies demonstrating that PC3 cells promote osteoclastogenesis and inhibit osteoblast activity. The clinical significance of these findings is supported by human tissue microarray studies of prostate tumors at stages of cancer progression, where Runx2 is expressed in both adenocarcinomas and metastatic tumors. Together these findings indicate that Runx2 is a key regulator of events associated with prostate cancer metastatic bone disease.

Prostate cancer regulatory networks

Although the timing with which common epithelial malignancies arise and become established remains a matter of debate, it is clear that by the time they are detected these tumors harbor hundreds of deregulated, aberrantly expressed or mutated genes. This enormous complexity poses formidable challenges to identify gene pathways that are drivers of tumorigenesis, potentially suitable for therapeutic intervention. An alternative approach is to consider cancer pathways as interconnected networks, and search for potential nodal proteins capable of connecting multiple signaling networks of tumor maintenance. We have modeled this approach in advanced prostate cancer, a condition with current limited therapeutic options. We propose that the integration of three signaling networks, including chaperone-mediated mitochondrial homeostasis, integrin-dependent cell signaling, and Runx2-regulated gene expression in the metastatic bone microenvironment plays a critical role in prostate cancer maintenance, and offers novel options for molecular therapy.

Ectopic runx2 expression in mammary epithelial cells disrupts formation of normal acini structure: implications for breast cancer progression

The transcription factor Runx2 is highly expressed in breast cancer cells compared with mammary epithelial cells and contributes to metastasis. Here we directly show that Runx2 expression promotes a tumor cell phenotype of mammary acini in three-dimensional culture. Human mammary epithelial cells (MCF-10A) form polarized, growth-arrested, acini-like structures with glandular architecture. The ectopic expression of Runx2 disrupts acini formation, and electron microscopic ultrastructural analysis revealed the absence of lumens. Characterization of the disrupted acini structures showed increased cell proliferation (Ki-67 positive cells), decreased apoptosis (Bcl-2 induction), and loss of basement membrane formation (absence of beta(4) integrin expression). In complementary experiments, inhibition of Runx2 function in metastatic MDA-MB-231 breast cancer cells by stable expression of either short hairpin RNA-Runx2 or a mutant Runx2 deficient in subnuclear targeting resulted in reversion of acini to more normal structures and reduced tumor growth in vivo. These novel findings provide direct mechanistic evidence for the biological activity of Runx2, dependent on its subnuclear localization, in promoting early events of breast cancer progression and suggest a molecular therapeutic target.

Positive association between nuclear Runx2 and oestrogen-progesterone receptor gene expression characterises a biological subtype of breast cancer

PURPOSE

The runt-related transcription factor, Runx2 may have an oncogenic role in mediating metastatic events in breast cancer, but whether Runx2 has a role in the early phases of breast cancer development is not clear. We examined the expression of Runx2 and its relationship with oestrogen receptor (ER) and progesterone receptor (PR) in breast cancer cell lines and tissues.

METHODS

Two human breast cancer cell lines, MCF-7 and MDA-MB-231 were transiently transfected with vectors expressing either Runx2 or ER and the levels of both proteins and mRNA were examined by Western blot analysis and quantitative real-time PCR, respectively. Runx2 expression was also examined in tissue microarray sections of 123 breast cancer patients by immunohistochemistry and results were correlated with clinico-pathological characteristics.

RESULTS

Expression of Runx2 and ER was reciprocal in the breast cell culture models and Runx2 suppressed ERbeta but not ERalpha mRNA levels. In contrast, functional expression of Runx2 was evident in the nucleus in 28% of the breast cancer tissues and in both early and late stages of tumour growth. Importantly, Runx2 expression was significantly more frequent in Grade 2 compared to Grade 1 and Grade 3 tumours (48% versus 39% versus 13%) and the expression was significantly associated with ER (p=0.005), PR (p=0.008) expressions in Grade 2 & Grade 3 tumours than Grade 1 tumours.

CONCLUSION

We propose that Runx2, ER and PR triple positivity in Grades 2 and 3 defines a biological subtype in breast cancer.

Transcription-factor-mediated epigenetic control of cell fate and lineage commitment

Epigenetic control is required to maintain competency for the activation and suppression of genes during cell division. The association between regulatory proteins and target gene loci during mitosis is a parameter of the epigenetic control that sustains the transcriptional regulatory machinery that perpetuates gene-expression signatures in progeny cells. The mitotic retention of phenotypic regulatory factors with cell cycle, cell fate, and tissue-specific genes supports the coordinated control that governs the proliferation and differentiation of cell fate and lineage commitment.

Transcriptional co-repressors of Runx2

Runx2 is an essential transcription factor for skeletal mineralization because it stimulates osteoblast differentiation of mesenchymal stem cells, promotes chondrocyte hypertrophy, and contributes to endothelial cell migration and vascular invasion of developing bones. Runx2 is also expressed during mouse embryo development in nascent mammary gland epithelium. Recent evidence implicates deregulation of Runx2 as a contributing factor in breast cancer-induced osteolysis and invasion, as well as in ectopic vascular calcification. Like other Runt domain proteins, Runx2 is a context-dependent transcriptional activator and repressor of genes that regulate cellular proliferation and differentiation. Proteins that temporally and spatially associate with Runx2 dictate these opposing transcriptional activities. Recent studies have identified several co-repressor proteins that bind to Runx2 to regulate gene expression. These co-factors include histone deacetylases (HDACs), transducin-like enhancer of split (TLE) proteins, mSin3a, and yes-associated protein (YAP). These proteins do not bind DNA themselves and appear to act by preventing Runx2 from binding DNA, altering chromatin structure, and/or by possibly blocking co-activator complexes. The nuclear localization of several of these factors is regulated by extracellular signaling events. Understanding the mechanisms whereby co-repressor proteins affect Runx2 activity during normal cellular development and tumor progression will identify new therapeutic targets for skeletal disorders such as osteoporosis and for bone metastatic cancers.

Repression of Runx2 by androgen receptor (AR) in osteoblasts and prostate cancer cells: AR binds Runx2 and abrogates its recruitment to DNA

Runx2 and androgen receptor (AR) are master transcription factors with pivotal roles in bone metabolism and prostate cancer (PCa). We dissected AR-mediated repression of Runx2 in dihydrotestosterone (DHT)-treated osteoblastic and PCa cells using reporter assays and endogenous Runx2 target genes. Repression required DHT, but not AR's transactivation function, and was associated with nuclear colocalization of the two proteins. Runx2 and AR coimmunoprecipitated and interacted directly in glutathione-S-transferase pull-down assays. Interaction was ionic in nature. Intact AR DNA-binding domain (DBD) was necessary and sufficient for both interaction with Runx2 and its repression. Runx2 sequences required for interaction were the C-terminal 132 amino acid residues together with the Runt DBD. Runx2 DNA binding was abrogated by endogenous AR in chromatin immunoprecipitation assays and by recombinant AR-DBD in gel shift assays. Furthermore, AR caused increased nuclear mobility of Runx2 as indicated by faster fluorescence recovery after photobleaching. Thus, AR binds Runx2 and abrogates its binding to DNA and possibly to other nuclear components. Clinical relevance of our results was suggested by an inverse correlation between expression of AR-responsive prostate-specific antigen and osteocalcin genes in PCa biopsies. Given the tumor suppressor properties of Runx2, its repression by AR may constitute a mechanism of hormone carcinogenesis. Attenuation of Runx2 by AR in osteoblasts may play a role in skeletal metabolism: the bone-sparing effect of androgens is attributable, in part, to keeping Runx2 activity in check and preventing high-turnover bone disease such as seen after castration and in transgenic mice overexpressing Runx2 in osteoblasts.

Organization, integration, and assembly of genetic and epigenetic regulatory machinery in nuclear microenvironments: implications for biological control in cancer

There is growing awareness that the fidelity of gene expression necessitates coordination of transcription factor metabolism and organization of genes and regulatory proteins within the three-dimensional context of nuclear architecture. The regulatory machinery that governs genetic and epigenetic control of gene expression is compartmentalized in nuclear microenvironments. Temporal and spatial parameters of regulatory complex organization and assembly are functionally linked to biological control and are compromised with the onset and progression of tumorigenesis. High throughput imaging of cells, tissues, and tumors, including live cell analysis, is expanding research's capabilities toward translating components of nuclear organization into novel strategies for cancer diagnosis and therapy.

RUNX1 and RUNX2 upregulate Galectin-3 expression in human pituitary tumors

Galectin-3 is expressed in a cell-type specific manner in human pituitary tumors and may have a role in pituitary tumor development. In this study, we hypothesized that Galectin-3 is regulated by RUNX proteins in pituitary tumors. Transcription factor prediction programs revealed several putative binding sites in the LGALS3 (Galectin-3 gene) promoter region. A human pituitary cell line HP75 was used as a model to study LGALS3 and RUNX interactions using Chromatin immunoprecipitation assay and electrophoresis mobility shift assay. Two binding sites for RUNX1 and one binding site for RUNX2 were identified in the LGALS3 promoter region. LGALS3 promoter was further cloned into a luciferase reporter, and the experiments showed that both RUNX1 and RUNX2 upregulated LGALS3. Knock-down of either RUNX1 or RUNX2 by siRNA resulted in a significant downregulation of Galectin-3 expression and decreased cell proliferation in the HP 75 cell line. Immunohistochemistry showed a close correlation between Galectin-3 expression and RUNX1/RUNX2 level in pituitary tumors. These results demonstrate a novel binding target for RUNX1 and RUNX2 proteins and suggest that Galectin-3 is regulated by RUNX1 and RUNX2 in human pituitary tumor cells by direct binding to the promoter region of LGALS3 and thus may contribute to pituitary tumor progression.

Targeted overexpression of BSP in osteoclasts promotes bone metastasis of breast cancer cells

Bone is one of the most common sites of breast cancer metastasis while bone sialoprotein (BSP) is thought to play an important role in bone metastasis of malignant tumors. The objective of this study is to determine the role of BSP overexpression in osteolytic metastasis using two homozygous transgenic mouse lines in which BSP expression is elevated either in all the tissues (CMV-BSP mice) or only in the osteoclasts (CtpsK-BSP mice). The results showed that skeletal as well as systemic metastases of 4T1 murine breast cancer cells were dramatically increased in CMV-BSP mice. In CtpsK-BSP mice, it was found that targeted BSP overexpression in osteoclasts promoted in vitro osteoclastogenesis and activated osteoclastic differentiation markers such as Cathepsin K, TRAP and NFAT2. MicroCT scan demonstrated that CtpsK/BSP mice had reduced trabecular bone volume and bone mineral density (BMD). The real-time IVIS Imaging System showed that targeted BSP overexpression in osteoclasts promoted bone metastasis of breast cancer cells. The osteolytic lesion area was significantly larger in CtpsK/BSP mice than in the controls as demonstrated by both radiographic and histomorphometric analyses. TRAP staining demonstrated a twofold increase in the number of osteoclasts in the bone lesion area from CtpsK/BSP mice compared with that from wild type mice. We conclude that host tissue-derived BSP also plays important roles in breast cancer metastasis through inducing tumor cell seeding into the remote host tissues. Furthermore, osteoclast-derived BSP promotes osteoclast differentiation in an autocrine manner and consequently promotes osteolytic bone metastasis of breast cancer.

Modulation of Runx2 activity by estrogen receptor-alpha: implications for osteoporosis and breast cancer

The transcription factors Runx2 and estrogen receptor-alpha (ERalpha) are involved in numerous normal and disease processes, including postmenopausal osteoporosis and breast cancer. Using indirect immunofluorescence microscopy and pull-down techniques, we found them to colocalize and form complexes in a ligand-dependent manner. Estradiol-bound ERalpha strongly interacted with Runx2 directly through its DNA-binding domain and only indirectly through its N-terminal and ligand-binding domains. Runx2's amino acids 417-514, encompassing activation domain 3 and the nuclear matrix targeting sequence, were sufficient for interaction with ERalpha's DNA-binding domain. As a consequence of the interaction, Runx2's transcriptional activation activity was strongly repressed, as shown by reporter assays in COS7 cells, breast cancer cells, and late-stage MC3T3-E1 osteoblast cultures. Metaanalysis of gene expression in 779 breast cancer biopsies indicated negative correlation between the expression of ERalpha and Runx2 target genes. Selective ER modulators (SERM) induced ERalpha-Runx2 interactions but led to various functional outcomes. The regulation of Runx2 by ERalpha may play key roles in osteoblast and breast epithelial cell growth and differentiation; hence, modulation of Runx2 by native and synthetic ERalpha ligands offers new avenues in selective ER modulator evaluation and development.

Runx2 transcriptional activation of Indian Hedgehog and a downstream bone metastatic pathway in breast cancer cells

Runx2, required for bone formation, is ectopically expressed in breast cancer cells. To address the mechanism by which Runx2 contributes to the osteolytic disease induced by MDA-MB-231 cells, we investigated the effect of Runx2 on key components of the "vicious cycle" of transforming growth factor beta (TGFbeta)-mediated tumor growth and osteolysis. We find that Runx2 directly up-regulates Indian Hedgehog (IHH) and colocalizes with Gli2, a Hedgehog signaling molecule. These events further activate parathyroid hormone-related protein (PTHrP). Furthermore, Runx2 directly regulates the TGFbeta-induced PTHrP levels. A subnuclear targeting deficient mutant Runx2, which disrupts TGFbeta-induced Runx2-Smad interactions, failed to induce IHH and downstream events. In addition, Runx2 knockdown in MDA-MB-231 inhibited IHH and PTHrP expression in the presence of TGFbeta. In vivo blockade of the Runx2-IHH pathway in MDA-MB-231 cells by Runx2 short hairpin RNA inhibition prevented the osteolytic disease. Thus, our studies define a novel role of Runx2 in up-regulating the vicious cycle of metastatic bone disease, in addition to Runx2 regulation of genes related to progression of tumor metastasis.

Genetic and epigenetic regulation in nuclear microenvironments for biological control in cancer

The regulatory machinery that governs genetic and epigenetic control of gene expression is compartmentalized in nuclear microenvironments. Temporal and spatial parameters of regulatory complex organization and assembly are functionally linked to biological control and are compromised with the onset and progression of tumorigenesis providing a novel platform for cancer diagnosis and treatment.

Runx2 is expressed in human glioma cells and mediates the expression of galectin-3

Runx2 is a member of the Runx family of transcription factors (Runx1-3) with a restricted expression pattern. It has so far been detected predominantly in skeletal tissues where, inter alia, it regulates the expression of the beta-galactoside-specific lectin galectin-3. Here we show that, in contrast to Runx3, Runx1 and Runx2 are expressed in a variety of human glioma cells. Runx2 expression pattern in these cells correlated completely with that of galectin-3, but not with that of other galectins. A similar correlation in the expression pattern of galectin-3 and Runx2 transcripts was detected in distinct types of 70 primary neural tumors, such as glioblastoma multiforme, but not in others, such as gangliocytomas. In glioma cells, Runx2 is directly involved in the regulation of galectin-3 expression, as shown by RNAi and transcription factor binding assays demonstrating that Runx2 interacts with a Runx2-binding motif present in the human galectin-3 promoter. Knockdown of Runx2 was thus accompanied by a reduction of both galectin-3 mRNA and protein levels by at least 50%, dependent on the glial tumor cell line tested. Reverse transcriptase-polymerase chain reaction analyses, aimed at finding other potential target genes of Runx2 in glial tumor cells, revealed the presence of bone sialoprotein, osteocalcin, osteopontin, and osteoprotegerin. However, their expression patterns only partially overlap with that of Runx2. These data suggest a functional contribution of Runx-2-regulated galectin-3 expression to glial tumor malignancy.

Distribution of murine adipose-derived mesenchymal stem cells in vivo following transplantation in developing mice

Systemic delivery of mesenchymal stem cells (MSCs) or stromal cells in vivo is attractive because it offers means of disseminating therapeutic cells to various tissues and organs in vivo. In the present study, we investigated the distribution and engraftment of the murine adipose-derived mesenchymal stem cells (ADSCs) without exposure to or exposed to bone microenvironment or transforming growth factor-beta1 (TGF-beta1) prior to transplantation into developing mice. The ADSCs harvested from the murine inguinal fat pad exhibited potential for differentiation toward osteogenic and adipogenic cell lineages in vitro. Fourteen days after systemic transplantation of the ADSCs marked with enhanced green fluorescent protein (EGFP) into developing mice, minimal donor GFP(+) cells were detected in the skeletal tissues in a limited number of the recipient mice. Exposure of the ADSCs to bone microenvironment for 7 or 14 days prior to transplantation into developing mice enhanced their migration and survival in the bones of the recipient mice. Exposure of ADSCs to TGF-beta1 prior to systemic transplantation exerted similar effects on cell migration and engraftment in various tissues, including the bones of the recipient developing mice. At 28 days following systemic transplantation, the ADSCs exposed to bone microenvironment were restricted mostly to the skeletal tissues of the recipient mice. Donor cells retrieved from the bones of the recipient mice at 28 days following cell transplantation expressed the differentiation markers Runx2 and Osterix (Osx). These data suggest that exposure of ADSCs to bone microenvironment or to TGF-beta1 prior to transplantation enhances their survival in the skeletal tissues following transplantation.

TWIST modulates prostate cancer cell-mediated bone cell activity and is upregulated by osteogenic induction

TWIST, a helix-loop-helix transcription factor, is highly expressed in many types of human cancer. We have previously found that TWIST confers prostate cancer cells with an enhanced metastatic potential through promoting epithelial-mesenchymal transition (EMT) and a high TWIST expression in human prostate cancer is associated with an increased metastatic potential. The predilection of prostate cancer cells to metastasize to bone may be due to two interplaying mechanisms (i) by increasing the rate of bone remodeling and (ii) by undergoing osteomimicry. We further studied the role of TWIST in promoting prostate cancer to bone metastasis. TWIST expression in PC3, a metastatic prostate cancer cell line, was silenced by small interfering RNA and we found that conditioned medium from PC3 with lower TWIST expression had a lower activity on stimulating osteoclast differentiation and higher activity on stimulating osteoblast mineralization. In addition, we found that these effects were, at least partly, associated with TWIST-induced expression of dickkopf homolog 1 (DKK-1), a factor that promotes osteolytic metastasis. We also examined TWIST and RUNX2 expressions during osteogenic induction of an organ-confined prostate cancer cell, 22Rv1. We observed increased TWIST and RUNX2 expressions upon osteogenic induction and downregulation of TWIST through short hairpin RNA reduced the induction level of RUNX2. In summary, our results suggest that, in addition to EMT, TWIST may also promote prostate cancer to bone metastasis by modulating prostate cancer cell-mediated bone remodeling via regulating the expression of a secretory factor, DKK-1, and enhancing osteomimicry of prostate cancer cells, probably, via RUNX2.

Osteoactivin promotes breast cancer metastasis to bone

The skeleton is a preferred site of metastasis in patients with disseminated breast cancer. We have used 4T1 mouse mammary carcinoma cells, which metastasize to bone from the mammary fat pads of immunocompetent mice, to identify novel genes involved in this process. In vivo selection of parental cells resulted in the isolation of independent, aggressively bone metastatic breast cancer populations with reduced metastasis to the lung. Gene expression profiling identified osteoactivin as a candidate that is highly and selectively expressed in aggressively bone metastatic breast cancer cells. These cells displayed enhanced migratory and invasive characteristics in vitro, the latter requiring sustained osteoactivin expression. Osteoactivin depletion in these cells, by small interfering RNA, also lead to a loss of matrix metalloproteinase-3 expression, whereas forced osteoactivin expression in parental 4T1 cells was sufficient to elevate matrix metalloproteinase-3 levels, suggesting that this matrix metalloproteinase may be an important mediator of osteoactivin function. Overexpression of osteoactivin in an independent, weakly bone metastatic breast cancer cell model significantly enhanced the formation of osteolytic bone metastases in vivo. Finally, high levels of osteoactivin expression in primary human breast cancers correlate with estrogen receptor-negative status and increasing tumor grade. Thus, we have identified osteoactivin as a protein that is expressed in aggressive human breast cancers and is capable of promoting breast cancer metastasis to bone.

A cathepsin K inhibitor reduces breast cancer induced osteolysis and skeletal tumor burden

Osteoclasts mediate bone destruction in breast cancer skeletal metastases. Cathepsin K is a proteinase that is secreted by osteoclasts and degrades bone. Here, immunohistochemistry revealed that cathepsin K was expressed not only by osteoclasts but also by breast cancer cells that metastasize to bone. Following intratibial injection with cathepsin K-expressing human BT474 breast cancer cells, tumor-bearing mice treated with a clinical dosing regimen of cathepsin K inhibitor (CKI; 50 mg/kg, twice daily) had osteolytic lesions that were 79% smaller than those of tumor-bearing mice treated with the vehicle. The effect of CKI was also studied in a mouse model in which the i.v. inoculation of human B02 breast cancer cells expressing cathepsin K leads to bone metastasis formation. Drug administration was started before (preventive protocol) or after (treatment protocol) the occurrence of osteolytic lesions. In treatment protocols, CKI (50 mg/kg, twice daily) or a single clinical dose of 100 microg/kg zoledronic acid (osteoclast inhibitor) reduced the progression of osteolytic lesions by 59% to 66%. CKI therapy also reduced skeletal tumor burden by 62% compared with vehicle, whereas zoledronic acid did not decrease the tumor burden. The efficacy of CKI at inhibiting skeletal tumor burden was similar in the treatment and preventive protocols. By contrast, CKI did not block the growth of s.c. B02 tumor xenografts in animals. Thus, CKI may render the bone a less favorable microenvironment for tumor growth by inhibiting bone resorption. These findings raise the possibility that cathepsin K could be a therapeutic target for the treatment of bone metastases.

Bone morphogenetic protein 7 expression associates with bone metastasis in breast carcinomas

BACKGROUND

We recently showed that bone morphogenetic protein 7 (BMP7) is overexpressed in primary breast tumors. Here we explored the clinical significance of BMP7 expression in breast cancer.

MATERIALS AND METHODS

This study included 483 breast cancer patients with complete clinicopathological information and up to 15 years of follow-up. Samples contained 241 lobular carcinomas, 242 ductal carcinomas, and 40 local recurrences. BMP7 protein expression was determined using immunohistochemistry.

RESULTS

BMP7 was expressed in 47% of the primary tumor samples and 13% of the local recurrences. The primary tumors expressed BMP7 more often than the corresponding local recurrences (P = 0.004). BMP7 expression was dependent on the tumor subtype; 57% of the lobular carcinomas but only 37% of the ductal carcinomas were BMP7 positive (P = 0.0001). BMP7 expression was associated with accelerated bone metastasis formation (P = 0.040), especially in ductal carcinomas (P = 0.033), and multivariate analysis confirmed that BMP7 is an independent prognostic indicator for early bone metastasis development (P = 0.032).

CONCLUSION

BMP7 is clearly associated with bone metastasis formation and thus might have clinical utility in identification of patients with increased risk of bone metastasis. This is the first time that bone inducing factor BMP7 has been linked to the bone metastasis process in breast cancer.

Metastasis: the seed and soil theory gains identity

The metastatic spread of tumor cells to distant sites represents the major cause of cancer-related deaths. Cancer metastasis involves a series of complex interactions between tumor cells and microenvironment that influence its biological effectiveness and facilitate tumor cell arrest to distant organs. More than a century since Paget developed the theory of seed and soil, the enigma of tissue specificity observed in metastatic colonization of tumor cells begins to unfold itself. The advent of new technologies has led to the discovery of novel molecules and pathways that confer metastasis-associated properties to the cancer cells, mediating organ specificity and unique genetic signatures have been developed using microarray studies. Future clinical studies and new antimetastatic compounds aiming to improve survival of patients with metastasis will most probably be based on these signatures. This review summarizes the plethora of old and new molecules that are strongly correlated with organ-specific metastases and which provide now an identity to the theory of seed and soil.

Organotropism of breast cancer metastasis

Breast cancer causes mortality by metastasizing to a variety of vital organs, such as bone, lung, brain and liver. Effective therapeutic intervention of this deadly process relies on a better mechanistic understanding of metastasis organotropism. Recent studies have confirmed earlier speculations that metastasis is a non-random process and is dependent on intricate tumor-stroma interactions at the target organ. Both the intrinsic properties of breast cancer cells and the host organ microenvironment are important in determining the efficiency of organ-specific metastasis. Advances in animal modeling, in vivo imaging and functional genomics have accelerated the discovery of important molecular mediators of organ-specific metastasis. A conceptual framework of breast cancer organotropism is emerging and will be instrumental in guiding future efforts in this exciting research field.

Host-derived RANKL is responsible for osteolysis in a C4-2 human prostate cancer xenograft model of experimental bone metastases

BACKGROUND

C4-2 prostate cancer (CaP) cells grown in mouse tibiae cause a mixed osteoblastic/osteolytic response with increases in osteoclast numbers and bone resorption. Administration of osteoprotegerin (OPG) blocks these increases, indicating the critical role of RANKL in osteolysis in this model. The objective of our study was to investigate whether RANKL expressed by tumor cells (human origin) directly stimulates osteolysis associated with the growth of these cells in bone or whether the increased osteolysis is caused by RANKL expressed by the host environment cells (murine origin). The relative contribution of tumor-vs. host-derived RANKL has been difficult to establish, even with human xenografts, because murine and human RANKL are both capable of stimulating osteolysis in mice, and the RANKL inhibitors used to date (OPG and RANK-Fc) inhibit human and murine RANKL.

METHODS

To address this question we used a neutralizing, antibody (huRANKL MAb), which specifically neutralizes the biological activities of human RANKL and thereby the contribution of C4-2 derived RANKL in this tibial injection model of experimental bone metastases.

RESULTS

Administration of huRANKL MAb did not inhibit the osteolytic response of the bone to these cells, or affect the establishment and growth of the C4-2 tumors in this environment.

CONCLUSION

In conclusion, our results suggest that in this model, murine RANKL and not the tumor-derived human RANKL is the mediator of the osteolytic reaction associated with C4-2 growth in bone. We hypothesize that C4-2 cells express other factor/s inducing host production of RANKL, thereby driving tumor-associated osteolysis.

Nuclear microenvironments in biological control and cancer

Nucleic acids and regulatory proteins are compartmentalized in microenvironments within the nucleus. This subnuclear organization may support convergence and the integration of physiological signals for the combinatorial control of gene expression, DNA replication and repair. Nuclear organization is modified in many cancers. There are cancer-related changes in the composition, organization and assembly of regulatory complexes at intranuclear sites. Mechanistic insights into the temporal and spatial organization of machinery for gene expression within the nucleus, which is compromised in tumours, provide a novel platform for diagnosis and therapy.