Both activating and null mutations of proteins required for canonical WNT signaling have revealed the importance of this pathway for normal skeletal development. However, tissue-specific transcriptional mechanisms through which WNT signaling promotes the differentiation of bone-forming cells have yet to be identified. Here, we address the hypothesis that canonical WNT signaling and the bone-related transcription factor RUNX2/CBFA1/ AML3 are functionally linked components of a pathway required for the onset of osteoblast differentiation. Our findings show that, in bone of the SFRP1 (secreted frizzled-related protein-1)-null mouse, which exhibits activated WNT signaling and a high bone mass phenotype, there is a significant increase in expression of T-cell factor (TCF)-1, Runx2, and the RUNX2 target gene osteocalcin. We demonstrate by mutational analysis that a functional TCF regulatory element responsive to canonical WNT signaling resides in the promoter of the Runx2 gene (؊97 to ؊93). By chromatin immunoprecipitation, recruitment of ␤-catenin and TCF1 to the endogenous Runx2 gene is shown. Coexpression of TCF1 with canonical WNT proteins resulted in a 2-5-fold activation of Runx2 promoter activity and a 7-8-fold induction of endogenous mRNA in mouse pluripotent mesenchymal and osteoprogenitor cells. This enhancement was abrogated by SFRP1. Taken together, our results provide evidence for direct regulation of Runx2 by canonical WNT signaling and suggest that Runx2 is a target of ␤-catenin/TCF1 for the stimulation of bone formation. We propose that WNT/TCF1 signaling, like bone morphogenetic protein/transforming growth factor-␤ signaling, activates Runx2 gene expression in mesenchymal cells for the control of osteoblast differentiation and skeletal development.During development of the skeleton and formation of bone tissue, several morphogenic growth factor and hormone signaling pathways impinge upon transcriptional regulators to induce the osteogenic phenotype (1, 2). The challenge is to identify how developmental cues and regulatory factors are integrated to accommodate the requirements for biological control of cell differentiation and tissue formation. Here, we have addressed the interaction of two key signals for osteogenesis: the WNT pathway, which contributes to the development of skeletal structures (3, 4), and the transcription factor RUNX2 (CBFA1/AML3), which is required for embryonic bone formation (5, 6).WNT signaling comprises a family of 19 secreted glycoproteins that have functions related to cell specification, formation of the body plan, cell growth, differentiation and apoptosis (7,8). WNT proteins function through Frizzled receptors, which transduce the signal through either the canonical ␤-catenin pathway or non-canonical pathway (7, 8). Activation of the Frizzled receptor complex results in inhibition of a phosphorylation cascade that stabilizes intracellular ␤-catenin levels. ␤-Catenin is subsequently translocated to the nucleus to form a transcriptionally active heterodimeric ␤-catenin TCF 2...

MicroRNAs (miRNAs) repress cellular protein levels to provide a sophisticated parameter of gene regulation that coordinates a broad spectrum of biological processes. Bone organogenesis is a complex process involving the differentiation and crosstalk of multiple cell types for formation and remodeling of the skeleton. Inhibition of mRNA translation by miRNAs has emerged as an important regulator of developmental osteogenic signaling pathways, osteoblast growth and differentiation, osteoclast-mediated bone resorption activity and bone homeostasis in the adult skeleton. miRNAs control multiple layers of gene regulation for bone development and postnatal functions, from the initial response of stem/progenitor cells to the structural and metabolic activity of the mature tissue. This Review brings into focus an emerging concept of bone-regulating miRNAs, the evidence for which has been gathered largely from in vivo mouse models and in vitro studies in human and mouse skeletal cell populations. Characterization of miRNAs that operate through tissue-specific transcription factors in osteoblast and osteoclast lineage cells, as well as intricate feedforward and reverse loops, has provided novel insights into the supervision of signaling pathways and regulatory networks controlling normal bone formation and turnover. The current knowledge of miRNAs characteristic of human pathologic disorders of the skeleton is presented with a future goal towards translational studies.

Previous studies have associated activation of canonical Wnt signaling in osteoblasts with elevated bone formation. Here we report that deletion of the murine Wnt antagonist, secreted frizzled-related protein (sFRP)-1, prolongs and enhances trabecular bone accrual in adult animals. sFRP-1 mRNA was expressed in bones and other tissues of +/+ mice but was not observed in -/- animals. Despite its broad tissue distribution, ablation of sFRP-1 did not affect blood and urine chemistries, most nonskeletal organs, or cortical bone. However, sFRP-1-/- mice exhibited increased trabecular bone mineral density, volume, and mineral apposition rate when compared with +/+ controls. The heightened trabecular bone mass of sFRP-1-/- mice was observed in adult animals between the ages of 13-52 wk, occurred in multiple skeletal sites, and was seen in both sexes. Mechanistically, loss of sFRP-1 reduced osteoblast and osteocyte apoptosis in vivo. In addition, deletion of sFRP-1 inhibited osteoblast lineage cell apoptosis while enhancing the proliferation and differentiation of these cells in vitro. Ablation of sFRP-1 also increased osteoclastogenesis in vitro, although changes in bone resorption were not observed in intact animals in vivo. Our findings demonstrate that deletion of sFRP-1 preferentially activates Wnt signaling in osteoblasts, leading to enhanced trabecular bone formation in adults.

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

The WW domain-containing oxidoreductase (WWOX) gene encodes a tumor suppressor. We have previously shown that targeted ablation of the Wwox gene in mouse increases the incidence of spontaneous and chemically induced tumors. To investigate WWOX function in vivo, we examined Wwox-deficient (Wwox ؊/؊ ) mice for phenotypical abnormalities. Wwox ؊/؊ mice are significantly reduced in size, die at the age of 2-3 weeks, and suffer a metabolic disorder that affects the skeleton. Wwox ؊/؊ mice exhibit a delay in bone formation from a cell autonomous defect in differentiation beginning at the mineralization stage shown in calvarial osteoblasts ex vivo and supported by significantly decreased bone formation parameters in Wwox ؊/؊ mice by microcomputed tomography analyses. Wwox ؊/؊ mice develop metabolic bone disease, as a consequence of reduced serum calcium, hypoproteinuria, and hypoglycemia leading to increased osteoclast activity and bone resorption. Interestingly, we find WWOX physically associates with RUNX2, the principal transcriptional regulator of osteoblast differentiation, and on osteocalcin chromatin. We show WWOX functionally suppresses RUNX2 transactivation ability in osteoblasts. In breast cancer MDA-MB-242 cells that lack endogenous WWOX protein, restoration of WWOX expression inhibited Runx2 and RUNX2 target genes related to metastasis. Affymetrix mRNA profiling revealed common gene targets in multiple tissues. In Wwox ؊/؊ mice, genes related to nucleosome assembly and cell growth genes were down-regulated, and negative regulators of skeletal metabolism exhibited increased expression. Our results demonstrate an essential requirement for the WWOX tumor suppressor in postnatal survival, growth, and metabolism and suggest a central role for WWOX in regulation of bone tissue formation. WW domain-containing oxidoreductase (WWOX)3 is a 46-kDa protein that contains two N-terminal WW domains and a central short-chain dehydrogenase/reductase domain (1, 2). WWOX was identified as a putative tumor suppressor in cancer cells because it lies in a genomic region that is frequently altered in pre-neoplastic and neoplastic lesions (1, 2). Indeed, expression of WWOX is deregulated in several types of cancer, including breast, prostate, lung, stomach, and pancreatic carcinomas (3, 4). Ectopic expression of WWOX in cancer cells lacking expression of endogenous WWOX results in significant growth inhibition and prevents the development of tumors in athymic nude mice (5, 6). Recently, we generated a mouse carrying a targeted deletion of the Wwox gene (7). We reported that loss of both alleles of Wwox resulted in the formation of frequent juvenile osteosarcomas, whereas loss of one allele increased the incidence of spontaneous and chemically induced tumors (7, 8) thus confirming that Wwox is a bona fide tumor suppressor.The identification of WWOX-interacting proteins has provided insights into the potential roles of WWOX in cell signaling and its impact on cell fate. WWOX cytosolic interactions, through its first WW domain that b...