Regulatory controls for osteoblast growth and differentiation: role of Runx/Cbfa/AML factors.

Formation of skeletal elements during embryogenesis and the dynamic remodeling of bone in the adult involve an exquisite interplay of developmental cues, signaling proteins, transcription factors, and their coregulatory proteins that support differentiation of osteogenic lineage cells from the initial mesenchymal progenitor cell to the mature osteocyte in mineralized connective tissue. As regulatory factors continue to be identified, the complexity of the molecular mechanisms that control gene expression in osteoblast lineage cells and drive the osteoblast maturation process are being further appreciated. A central regulator of bone formation is the Runx2 (Cbfa1/AML3) transcription factor which fulfills its role as a master regulatory switch through unique properties for mediating the temporal activation and/or repression of cell growth and phenotypic genes as osteoblasts progress through stages of differentiation. This review examines the multifunctional roles of Runx2 during osteogenesis. Runx2 functions as a "platform protein" that interacts with a spectrum of coregulatory proteins to provide a combinatorial mechanism for integrating cell signaling pathways required for osteoblast differentiation and the tissue-specific regulation of gene expression. In a broader context, it has recently been appreciated that the Runx1 hematopoietic factor and the Runx3 gene associated with neural and gut development are also expressed in the skeleton, although at present our knowledge of their roles in bone formation is limited. Here we discuss the biological functions of Runx factors in promoting cell fate determination and lineage progression, which include (1) regulating gene activation and repression through coregulatory protein interactions and by supporting chromatin remodeling; (2) integrating ECM signaling and cues from developmental, hormonal, and signal transduction pathways by formation of complexes organized in subnuclear domains; and (3) mediating cell growth control. Last, a comprehensive understanding of Runx functions in the skeleton must consider the regulatory mechanisms that control Runx2 transcription and its functional activity through posttranslational modifications.

Nuclear microenvironments support assembly and organization of the transcriptional regulatory machinery for cell proliferation and differentiation.

The temporal and spatial organization of transcriptional regulatory machinery provides microenvironments within the nucleus where threshold concentrations of genes and cognate factors facilitate functional interactions. Conventional biochemical, molecular, and in vivo genetic approaches, together with high throughput genomic and proteomic analysis are rapidly expanding our database of regulatory macromolecules and signaling pathways that are requisite for control of genes that govern proliferation and differentiation. There is accruing insight into the architectural organization of regulatory machinery for gene expression that suggests signatures for biological control. Localized scaffolding of regulatory macromolecules at strategic promoter sites and focal compartmentalization of genes, transcripts, and regulatory factors within intranuclear microenvironments provides an infrastructure for combinatorial control of transcription that is operative within the three dimensional context of nuclear architecture.

Identification of HiNF-P, a key activator of cell cycle-controlled histone H4 genes at the onset of S phase.

At the G(1)/S phase cell cycle transition, multiple histone genes are expressed to ensure that newly synthesized DNA is immediately packaged as chromatin. Here we have purified and functionally characterized the critical transcription factor HiNF-P, which is required for E2F-independent activation of the histone H4 multigene family. Using chromatin immunoprecipitation analysis and ligation-mediated PCR-assisted genomic sequencing, we show that HiNF-P interacts with conserved H4 cell cycle regulatory sequences in vivo. Antisense inhibition of HiNF-P reduces endogenous histone H4 gene expression. Furthermore, we find that HiNF-P utilizes NPAT/p220, a substrate of the cyclin E/cyclin-dependent kinase 2 (CDK2) kinase complex, as a key coactivator to enhance histone H4 gene transcription. The biological role of HiNF-P is reflected by impeded cell cycle progression into S phase upon antisense-mediated reduction of HiNF-P levels. Our results establish that HiNF-P is the ultimate link in a linear signaling pathway that is initiated with the growth factor-dependent induction of cyclin E/CDK2 kinase activity at the restriction point and culminates in the activation of histone H4 genes through HiNF-P at the G(1)/S phase transition.

Cell growth regulatory role of Runx2 during proliferative expansion of preosteoblasts.

The Runx2 (CBFA1/AML3/PEBP2alphaA) transcription factor promotes lineage commitment and differentiation by activating bone phenotypic genes in postproliferative osteoblasts. However, the presence of Runx2 in actively dividing osteoprogenitor cells suggests that the protein may also participate in control of osteoblast growth. Here, we show that Runx2 is stringently regulated with respect to cell cycle entry and exit in osteoblasts. We addressed directly the contribution of Runx2 to bone cell proliferation using calvarial osteoblasts from wild-type and Runx2-deficient mice (i.e., Runx2(-/-) and Runx2(DeltaC/DeltaC)). Runx2(DeltaC/DeltaC) mice express a protein lacking the Runx2 COOH terminus, which integrates several cell proliferation-related signaling pathways (e.g., Smad, Yes/Src, mitogen-activated protein kinase, and retinoblastoma protein). Calvarial cells but not embryonic fibroblasts from Runx2(-/-) or Runx2(DeltaC/DeltaC) mutant mice exhibit increased cell growth rates as reflected by elevations of DNA synthesis and G(1)-S phase markers (e.g., cyclin E). Reintroduction of Runx2 into Runx2(-/-) calvarial cells by adenoviral delivery restores stringent cell growth control. Thus, Runx2 regulates normal osteoblast proliferation, and the COOH-terminal region is required for this biological function. We propose that Runx2 promotes osteoblast maturation at a key developmental transition by supporting exit from the cell cycle and activating genes that facilitate bone cell phenotype development.

Nuclear microenvironments support physiological control of gene expression.

There is growing recognition that the organization of nucleic acids and regulatory proteins is functionally linked to the assembly, localization and activity of gene regulatory machinery. Cellular, molecular, biochemical and in-vivo genetic evidence support an obligatory relationship between nuclear microenvironments where regulatory complexes reside and fidelity of transcriptional control. Perturbations in mechanisms governing the intranuclear trafficking of transcription factors and the temporal/spatial organization of regulatory proteins within the nucleus occur with compromised gene expression that abrogates skeletal development and mediates leukemogenesis.

Runx2/Cbfa1 functions: diverse regulation of gene transcription by chromatin remodeling and co-regulatory protein interactions.

Development of the osteoblast phenotype requires transcriptional mechanisms that regulate induction of a program of temporally expressed genes. Key components of gene activation, repression, and responsiveness to physiologic mediators require remodeling of the chromatin structure of a gene that renders promoter elements competent for the assembly of macromolecular transcriptional complexes. Here we review evidence that the Runx transcription factors support tissue-specific gene expression and bone formation by contributing to promoter structure, chromatin remodeling, and the integration of independent signaling pathways. In addition, we discuss the role of Runx2 in both activation and negative regulation of gene promoters (osteocalcin, bone sialoprotein, and Runx2/Cbfa1) in relation to the interaction of Runx with co-regulatory proteins in distinct subnuclear foci. The modifications in chromatin organization and transcription of the osteocalcin gene that are influenced by the activities of Runx2/Cbfa1 mediated by interacting proteins (YAP, TLE, SMAD, C/EBP) are emphasized. These functional properties of Runx2 provide novel insights into the requirements for multiple levels of transcriptional control within the context of nuclear architecture to support the convergence of regulatory signals that control tissue-specific gene expression.

Temporal and spatial parameters of skeletal gene expression: targeting RUNX factors and their coregulatory proteins to subnuclear domains.

Key components of the basal transcription machinery and several tissue-specific transcription factor complexes are functionally compartmentalized as specialized subnuclear domains. We have identified a unique 31-38 amino acid targeting signal (NMTS) that directs the Runx (Cbfa/AML) transcription factors to distinct nuclear matrix-(NM) associated sites within the nucleus that support gene expression. Our determination of the NMTS crystal structure, yeast 2 hybrid screens to identify NM interacting proteins, and in situ colocalization studies with Runx interacting factors (YAP, Smad, TLE) suggest that localization of Runx transcription factors at intranuclear sites facilitates the assembly and activity of regulatory complexes that mediate activation and suppression of target genes. Mice homozygous for the deletion of the intranuclear Runx2 targeting signal in a homologous recombination (Runx2 deltaC) do not form bone due to maturational arrest of osteoblasts, demonstrating the importance of fidelity of subnuclear localization for tissue-differentiating activity. These results provide evidence that Runx2 subnuclear targeting and the associated regulatory functions are essential for a spatiotemporal placement that facilitates activation of Runx-dependent genes involved in tissue differentiation during embryonic development.

Intranuclear organization of RUNX transcriptional regulatory machinery in biological control of skeletogenesis and cancer.

RUNX (AML/CBFA/PEBP2) transcription factors serve as paradigms for obligatory relationships between nuclear structure and physiological control of phenotypic gene expression. The RUNX proteins contribute to tissue restricted transcription by sequence-specific binding to promoter elements of target genes and serving as scaffolds for the assembly of coregulatory complexes that mediate biochemical and architectural control of activity. We will present an overview of approaches we are pursuing to address: (1) the involvement of RUNX proteins in governing competency for protein/DNA and protein/protein interactions at promoter regulatory sequences; (2) the recruitment of RUNX factors to subnuclear sites where the machinery for expression or repression of target genes is organized; and (3) the trafficking and integration of regulatory signals that control RUNX-mediated transcription.

Intranuclear trafficking of transcription factors: Requirements for vitamin D-mediated biological control of gene expression.

The architecturally associated subnuclear organization of nucleic acids and cognate regulatory factors suggest functional interrelationships between nuclear structure and gene expression. Mechanisms that contribute to the spatial distribution of transcription factors within the three-dimensional context of nuclear architecture control the sorting of regulatory information as well as the assembly and activities of sites within the nucleus that support gene expression. Vitamin D control of gene expression serves as a paradigm for experimentally addressing mechanisms that govern the intranuclear targeting of regulatory factors to nuclear domains where transcription of developmental and tissue-specific genes occur. We will present an overview of molecular, cellular, genetic, and biochemical approaches that provide insight into the trafficking of regulatory factors that mediate vitamin D control of gene expression to transcriptionally active subnuclear sites. Examples will be presented that suggest modifications in the intranuclear targeting of transcription factors abrogate competency for vitamin D control of skeletal gene expression during development and fidelity of gene expression in tumor cells.

Runx2 (Cbfa1, AML-3) interacts with histone deacetylase 6 and represses the p21(CIP1/WAF1) promoter.

Runx2 (Cbfa1, AML-3) is multifunctional transcription factor that is essential for osteoblast development. Runx2 binds specific DNA sequences and interacts with transcriptional coactivators and corepressors to either activate or repress transcription of tissue-specific genes. In this study, the p21(CIP/WAF1) promoter was identified as a repressible target of Runx2. A carboxy-terminal repression domain distinct from the well-characterized TLE/Groucho-binding domain contributed to Runx2-mediated p21 repression. This carboxy-terminal domain was sufficient to repress a heterologous GAL reporter. The repressive activity of this domain was sensitive to the histone deacetylase inhibitor trichostatin A but not to trapoxin B. HDAC6, which is insensitive to trapoxin B, specifically interacted with the carboxy terminus of Runx2. The HDAC6 interaction domain of Runx2 was mapped to a region overlapping the nuclear matrix-targeting signal. The Runx2 carboxy terminus was necessary for recruitment of HDAC6 from the cytoplasm to chromatin. HDAC6 also colocalized and coimmunoprecipitated with the nuclear matrix-associated protein Runx2 in osteoblasts. Finally, we show that HDAC6 is expressed in differentiating osteoblasts and that the Runx2 carboxy terminus is necessary for maximal repression of the p21 promoter in preosteoblasts. These data identify Runx2 as the first transcription factor to interact with HDAC6 and suggest that HDAC6 may bind to Runx2 in differentiating osteoblasts to regulate tissue-specific gene expression.