Identification of a nuclear matrix targeting signal in the leukemia and bone-related AML/CBF-alpha transcription factors

Phylogenetic analysis of the SAP30 family of transcriptional regulators reveals functional divergence in the domain that binds the nuclear matrix

Background

Deacetylation of histones plays a fundamental role in gene silencing, and this is mediated by a corepressor complex containing Sin3 as an essential scaffold protein. In this report we examine the evolution of two proteins in this complex, the Sin3-associated proteins SAP30L and SAP30, by using an archive of protein sequences from 62 species.

Results

Our analysis indicates that in tetrapods SAP30L is more similar than SAP30 to the ancestral protein, and the two copies in this group originated by gene duplication which occurred after the divergence of Actinopterygii and Sarcopterygii about 450 million years ago (Mya). The phylogenetic analysis and biochemical experiments suggest that SAP30 has diverged functionally from the ancestral SAP30L by accumulating mutations that have caused attenuation of one of the original functions, association with the nuclear matrix. This function is mediated by a nuclear matrix association sequence, which consists of a conserved motif in the C-terminus and the adjacent nucleolar localization signal (NoLS).

Conclusion

These results add further insight into the evolution and function of proteins of the SAP30 family, which share many characteristic with nuclear scaffolding proteins that are intimately involved in regulation of gene expression. Furthermore, SAP30L seems essential to eukaryotic biology, as it is found in animals, plants, fungi, as well as some taxa of unicellular eukaryotes.

Subnuclear targeting of the Runx3 tumor suppressor and its epigenetic association with mitotic chromosomes

Runx proteins are tissue-specific transcriptional scaffolds that organize and assemble regulatory complexes at strategic sites of target gene promoters and at intranuclear foci to govern activation or repression. During interphase, fidelity of intranuclear targeting supports the biological activity of Runx1 and Runx2 proteins. Both factors regulate genes involved in cell cycle control and cell growth (e.g., rRNA genes), as well as lineage commitment. Here, we have examined the subcellular regulatory properties of the third Runx member, the tumor suppressor protein Runx3, during interphase and mitosis. Using in situ cellular and biochemical approaches we delineated a subnuclear targeting signal that directs Runx3 to discrete transcriptional foci that are nuclear matrix associated. Chromatin immunoprecipitation results show that Runx3 occupies rRNA promoters during interphase. We also find that Runx3 remains associated with chromosomes during mitosis and localizes with nucleolar organizing regions (NORs), reflecting an interaction with epigenetic potential. Taken together, our study establishes that common mechanisms control the subnuclear distribution and activities of Runx1, Runx2, and Runx3 proteins to support RNA polymerase I and II mediated gene expression during interphase and mitosis.

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.

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.

Co-stimulation of the bone-related Runx2 P1 promoter in mesenchymal cells by SP1 and ETS transcription factors at polymorphic purine-rich DNA sequences (Y-repeats)

Transcriptional control of Runx2 gene expression through two alternative promoters (P1 and P2) is critical for the execution of its function as an osteogenic cell fate determining factor. In all vertebrates examined to date, the bone related P1 promoter contains a purine-rich region (-303 to -128 bp in the rat) that separates two regulatory domains. The length of this region differs dramatically between species even within the same order. Using deletion analysis, we show that part of this purine-rich region (-200 to -128) containing a duplicated element (Y-repeat) positively regulates Runx2 P1 transcription. Electrophoretic mobility assays and chromatin immunoprecipitations reveal that Y-repeat binds at least two different classes of transcription factors related to GC box binding proteins (e.g. SP1 and SP7/Osterix) and ETS-like factors (e.g. ETS1 and ELK1). Forced expression of SP1 increases Runx2 P1 promoter activity through the Y-repeats, and small interfering RNA depletion of SP1 decreases Runx2 expression. Similarly, exogenous expression of wild type ELK1, but not a defective mutant that cannot be phosphorylated, enhances Runx2 gene expression. SP1 is most abundant in proliferating cells, and ELK1 is most abundant in postconfluent cells; during MC3T3-E1 osteoblast differentiation, both proteins are transiently co-expressed when Runx2 expression is enhanced. Taken together, our data suggest that basal Runx2 gene transcription is regulated by dynamic interactions between SP1 and ETS-like factors during progression of osteogenesis.

The leukemogenic t(8;21) fusion protein AML1-ETO controls rRNA genes and associates with nucleolar-organizing regions at mitotic chromosomes

RUNX1/AML1 is required for definitive hematopoiesis and is frequently targeted by chromosomal translocations in acute myeloid leukemia (AML). The t(8;21)-related AML1-ETO fusion protein blocks differentiation of myeloid progenitors. Here, we show by immunofluorescence microscopy that during interphase, endogenous AML1-ETO localizes to nuclear microenvironments distinct from those containing native RUNX1/AML1 protein. At mitosis, we clearly detect binding of AML1-ETO to nucleolar-organizing regions in AML-derived Kasumi-1 cells and binding of RUNX1/AML1 to the same regions in Jurkat cells. Both RUNX1/AML1 and AML1-ETO occupy ribosomal DNA repeats during interphase, as well as interact with the endogenous RNA Pol I transcription factor UBF1. Promoter cytosine methylation analysis indicates that RUNX1/AML1 binds to rDNA repeats that are more highly CpG methylated than those bound by AML1-ETO. Downregulation by RNA interference reveals that RUNX1/AML1 negatively regulates rDNA transcription, whereas AML1-ETO is a positive regulator in Kasumi-1 cells. Taken together, our findings identify a novel role for the leukemia-related AML1-ETO protein in epigenetic control of cell growth through upregulation of ribosomal gene transcription mediated by RNA Pol I, consistent with the hyper-proliferative phenotype of myeloid cells in AML patients.

Unravelling the nuclear matrix proteome

The nuclear matrix (NM) model posits the presence of a protein/RNA scaffold that spans the mammalian nucleus. The NM proteins are involved in basic nuclear function and are a promising source of protein biomarkers for cancer. Importantly, the NM proteome is operationally defined as the proteins from cells and tissue that are extracted following a specific biochemical protocol; in brief, the soluble proteins and lipids, cytoskeleton, and chromatin elements are removed in a sequential fashion, leaving behind the proteins that compose the NM. So far, the NM has not been sufficiently verified as a biological entity and only preliminary at the molecular level. Here, we argue for a combined effort of proteomics, immunodetection and microscopy to unravel the composition and structure of the NM.

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.

Histone deacetylase 7 associates with Runx2 and represses its activity during osteoblast maturation in a deacetylation-independent manner

HDAC7 associates with Runx2 and represses Runx2 transcriptional activity in a deacetylase-independent manner. HDAC7 suppression accelerates osteoblast maturation. Thus, HDAC7 is a novel Runx2 co-repressor that regulates osteoblast differentiation.

INTRODUCTION

Runx2 is a key regulator of gene expression in osteoblasts and can activate or repress transcription depending on interactions with various co-factors. Based on previous observations that several histone deacetylases (HDACs) repress Runx2 activity and that HDAC inhibitors accelerate osteoblast differentiation in vitro, we hypothesized that additional HDACs may also affect Runx2 activity.

MATERIALS AND METHODS

A panel of HDACs was screened for repressors of Runx2 activity. Immunofluorescence, co-immunoprecipitation, GST-pulldowns, and chromatin immunoprecipitations were used to characterize the interactions between Runx2 and HDAC7. Expression of osteoblast markers was examined in a C2C12 cell osteoblast differentiation model in which HDAC7 levels were reduced by RNAi.

RESULTS

Runx2 activity was repressed by HDAC7 but not by HDAC9, HDRP, HDAC10, or HDAC11. HDAC7 and Runx2 were found co-localized in nuclei and associated with Runx2-responsive promoter elements in osseous cells. A carboxy-terminal domain of Runx2 associated with multiple regions of HDAC7. Although direct interactions with Runx2 were confined to the carboxy terminus of HDAC7, this region was dispensable for repression. In contrast, the amino terminus of HDAC7 bound Runx2 indirectly and was necessary and sufficient for transcriptional repression. Treatment with HDAC inhibitors did not decrease inhibition by HDAC7, indicating that HDAC7 repressed Runx2 by deacetylation-independent mechanism(s). Suppression of HDAC7 expression in C2C12 multipotent cells by RNAi accelerated their BMP2-dependent osteoblast differentiation program. Consistent with this observation, BMP2 decreased nuclear localization of HDAC7.

CONCLUSIONS

These results establish HDAC7 as a regulator of Runx2's transcriptional activity and suggest that HDAC7 may be an important regulator of the timing and/or rate of osteoblast maturation.

RUNX genes in development and cancer: regulation of viral gene expression and the discovery of RUNX family genes

Mouse embryonal carcinoma (EC) cells, also called teratocarcinoma stem cells, are nonpermissive for polyomavirus growth, whereas differentiated derivatives of the cells are permissive. Mutant viruses capable of growing in EC cells can be isolated. They have genomic alterations within the viral enhancer, which is required for viral gene expression and DNA replication. This viral regulatory region was considered as a potential probe for mouse cell differentiation. The 24-bp-long A element within the enhancer was identified as a minimum element, which also shows a lower activity in EC cells compared with the differentiated cells. Transcription factors PEA1/AP1, PEA2/PEBP2, and PEA3/ETS were identified as A element-binding proteins. All of them are absent in EC cells and induced to be expressed when the cells are differentiated. Although PEBP2 has a weaker transactivation activity compared with other two, it is essential for the enhancer function of the A element. Purification and cDNA cloning revealed that PEBP2 has two subunits, DNA-binding alpha (PEBP2alpha) and non-DNA-binding beta (PEBP2beta). PEBP2alpha was found to be highly homologous to a Drosophila segmentation gene, runt, and a human gene AML1 that was identified as a part of the fusion gene, AML1/ETO (MTG8) generated by t(8;21) chromosome translocation associated with acute myelogenous leukemia (AML). Core-binding factor (CBF), which interacts with a murine retrovirus enhancer, was found to be identical to PEBP2. runt, PEBP2alpha and AML1 are now termed RUNX family, which are involved in cell specification during development. There are three mammalian RUNX genes, RUNX1, RUNX2, and RUNX3. RUNX1 is essential for generation of hematopoietic stem cells and is involved in human leukemia. RUNX2 is essential for skeletal development and has an oncogenic potential. RUNX3 is expressed in wider ranges of tissues and has multiple roles. Among others, RUNX3 is a major tumor suppressor of gastric and many other solid tumors.

Scaffold/matrix attachment regions (S/MARs): relevance for disease and therapy

There is increasing awareness that processes, such as development, aging and cancer, are governed, to a considerable extent, by epigenetic processes, such as DNA and histone modifications. The sites of these modifications in turn reflect their position and role in the nuclear architecture. Since epigenetic changes are easier to reverse than mutations, drugs that remove or add the chemical tags are at the forefront of research for the treatment of cancerous and inflammatory diseases. This review will use selected examples to develop a unified view that might assist the systematic development of novel therapeutic regimens.

Runx3 interacts with DNA repair protein Ku70

Recent studies have suggested that Runt-related transcription factor 3 (Runx3) is associated with genesis and progression of gastric carcinoma. A proteomic approach was used to search for Runx3-interacting proteins to elucidate the molecular mechanisms of gastric carcinogenesis. Runx3 bound with myc and flag tags (MEF tags) is expressed in HEK293T cells, and the protein complex formed with Runx3 was purified and identified by mass spectrometry. Ku70 and Ku80, members of the DNA repair protein complex, were identified as Runx3-interacting proteins. Runx3, Ku70, and Ku80 associate in vivo, and in vitro interaction between Runx3 and Ku70 was confirmed via His-tag pull-down assay. The amino acids 241-322 of Runx3, which correspond to the transcriptional activation domain, and the amino acids 1-116 of Ku70 were necessary for binding with each other, and immunocytochemistry under confocal laser microscopy demonstrated that Runx3 and Ku70 localized throughout the nucleus excluding the nucleoli. Furthermore, Runx3 highly activated the transcription of p21, the target gene of Runx3, in Ku70 knockdown cells. These results suggest a possible link between a tumor suppressor function and DNA repair.

NFkappaB is persistently activated in continuously stimulated human neutrophils

Increased activation of the transcription factor NFkappaB in the neutrophils has been associated with the pathogenesis of sepsis, acute lung injury (ALI), bronchopulmonary dysplasia (BPD), and other neutrophil-mediated inflammatory disorders. Despite recent progress in analyzing early NFkappaB activation in human neutrophils, activation of NFkappaB in persistently stimulated neutrophils has not been previously studied. Because it is the persistent NFkappaB activation that is thought to be involved in the host response to sepsis and the pathogenesis of ALI and BPD, we hypothesized that continuously stimulated human neutrophils may exhibit a late phase of NFkappaB activity. The goal of this study was to analyze the NFkappaB activation and expression of IkappaB and NFkappaB proteins during neutrophil stimulation with inflammatory signals for prolonged times. We demonstrate that neutrophil stimulation with lipopolysaccharide (LPS) and tumor necrosis factor-alpha (TNFalpha) induces, in addition to the early activation at 30-60 min, a previously unrecognized late phase of NFkappaB activation. In LPS-stimulated neutrophils, this NFkappaB activity typically had a biphasic character, whereas TNFalpha-stimulated neutrophils exhibited a continuous NFkappaB activity peaking around 9 h after stimulation. In contrast to the early NFkappaB activation that inversely correlates to the nuclear levels of IkappaBalpha, however, in continuously stimulated neutrophils, NFkappaB is persistently activated despite considerable levels of IkappaBalpha present in the nucleus. Our data suggest that NFkappaB is persistently activated in human neutrophils during neutrophil-mediated inflammatory disorders, and this persistent NFkappaB activity may represent one of the underlying mechanisms for the continuous production of proinflammatory mediators.

Identification of a common subnuclear localization signal

Proteins share peptidic sequences, such as a nuclear localization signal (NLS), which guide them to particular membrane-bound compartments. Similarities have also been observed within different classes of signals that target proteins to membrane-less subnuclear compartments. Common localization signals affect spatial and temporal subcellular organization and are thought to allow the coordinated response of different molecular networks to a given signaling cue. Here we identify a higher-order and predictive code, {[RR(I/L)X(3)r]((n, n > or = 1))+[L(phi/N)(V/L)]((n,n>1))}, that establishes high-affinity interactions between a group of proteins and the nucleolus in response to a specific signal. This position-independent code is referred to as a nucleolar detention signal regulated by H(+) (NoDS(H+)) and the class of proteins includes the cIAP2 apoptotic regulator, VHL ubiquitylation factor, HSC70 heat shock protein and RNF8 transcription regulator. By identifying a common subnuclear targeting consensus sequence, our work reveals rules governing the dynamics of subnuclear organization and ascribes new modes of regulation to several proteins with diverse steady-state distributions and dynamic properties.

Transcriptional partners in regulatory T cells: Foxp3, Runx and NFAT

A general theme in gene regulation is that transcription factors never function alone. Recent studies have emphasized this concept for regulatory T cells, a unique lineage of CD4+ T cells that exert active immune suppression and are essential to maintaining self-tolerance.

TEL-AML1 preleukemic activity requires the DNA binding domain of AML1 and the dimerization and corepressor binding domains of TEL

The t(12;21)(p13;q22) translocation generates the TEL-AML1 (TEL, translocation-Ets-leukemia; AML1, acute myeloid leukemia-1) (ETV6-RUNX1) fusion product and is the most common chromosomal abnormality in pediatric leukemia. Our previous studies using a murine fetal liver transplantation model demonstrated that TEL-AML1 promotes the self-renewal of B-cell precursors in vitro and enhances the expansion of hematopoietic stem cells (HSCs) in vivo. This is consistent with the hypothesis that TEL-AML1 induces expansion of a preleukemic clone. Several studies have described domains within TEL-AML1 involved in the transcriptional regulation of specific target genes. However, it is unclear which of these domains is important for the activity of TEL-AML1 in preleukemic hematopoiesis. In order to examine this, we have generated a panel of deletion mutants and expressed them in HSCs. These experiments demonstrate that TEL-AML1 requires multiple domains from both TEL and AML1 to alter hematopoiesis. Furthermore, mutation of a single amino-acid residue within the runt homology domain of AML1, required for DNA binding, was sufficient to abrogate TEL-AML1 activity. These data suggest that TEL-AML1 acts as an aberrant transcription factor to perturb multiple pathways during hematopoiesis.

NFkappaB is persistently activated in continuously stimulated human neutrophils

Increased activation of the transcription factor NFkappaB in the neutrophils has been associated with the pathogenesis of sepsis, acute lung injury (ALI), bronchopulmonary dysplasia (BPD), and other neutrophil-mediated inflammatory disorders. Despite recent progress in analyzing early NFkappaB activation in human neutrophils, activation of NFkappaB in persistently stimulated neutrophils has not been previously studied. Because it is the persistent NFkappaB activation that is thought to be involved in the host response to sepsis and the pathogenesis of ALI and BPD, we hypothesized that continuously stimulated human neutrophils may exhibit a late phase of NFkappaB activity. The goal of this study was to analyze the NFkappaB activation and expression of IkappaB and NFkappaB proteins during neutrophil stimulation with inflammatory signals for prolonged times. We demonstrate that neutrophil stimulation with lipopolysaccharide (LPS) and tumor necrosis factor-alpha (TNFalpha) induces, in addition to the early activation at 30-60 min, a previously unrecognized late phase of NFkappaB activation. In LPS-stimulated neutrophils, this NFkappaB activity typically had a biphasic character, whereas TNFalpha-stimulated neutrophils exhibited a continuous NFkappaB activity peaking around 9 h after stimulation. In contrast to the early NFkappaB activation that inversely correlates to the nuclear levels of IkappaBalpha, however, in continuously stimulated neutrophils, NFkappaB is persistently activated despite considerable levels of IkappaBalpha present in the nucleus. Our data suggest that NFkappaB is persistently activated in human neutrophils during neutrophil-mediated inflammatory disorders, and this persistent NFkappaB activity may represent one of the underlying mechanisms for the continuous production of proinflammatory mediators.

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.

Mutual transactivational repression of Runx2 and the androgen receptor by an impairment of their normal compartmentalization

Steroid hormones play important roles not only in the reproductive system but also in bone metabolism. We examined the functional relationship between steroid hormone receptors and the Runx2 transcription factor that is essential for osteoblast differentiation and proliferation. A functional reporter assay using promoters carrying steroid hormone-responsive elements revealed that Runx2 suppressed ligand-dependent transcriptional activation mediated by receptors. To examine intracellular localization of these proteins, a three-dimensional imaging study was performed by laser scanning confocal microscopy of green fluorescent protein (GFP)-fused proteins. As previously reported, ligand-bound human androgen receptor (AR) was translocated from the cytoplasm to the nucleus and formed subnuclear fine foci. Coexpression of human Runx2 disrupted the AR subnuclear fine foci formation, and the intranuclear fluorescent pattern of AR became similar to that of Runx2. On the other hand, ligand-bound ARs repressed the Runx2-mediated transactivation function. Runx2 was also extracted from its original compartment by ligand-bound ARs. These results suggest that both Runx2 and ARs repress the transactivation function of the other protein by extracting it from its original compartment. The AR and Runx2 may play a mutual role in transcriptional activation in osteoblasts.

Transcription factor Runx1 recruits the polyomavirus replication origin to replication factories

Eukaryotic DNA replication takes place in the replication factories, where replication proteins are properly assembled to form replication forks. Thus, recruitment of DNA replication origins to the replication factories must be the key step for the regulation of DNA replication. The transcription factor Runx1 associates with the nuclear matrix, the putative substructure of DNA replication factories. An earlier report from our laboratory showed that Runx1 activates polyomavirus DNA replication, and that this requires its nuclear matrix-binding activity. Here, we show that Runx1 activates polyomavirus DNA replication by stimulating the binding of the viral-encoded replication initiator/helicase, large T antigen, to its replication origin. We found that newly replicated polyomavirus DNA is associated with the nuclear matrix and that large T antigen is targeted to replication factories, suggesting that polyomavirus is replicated in replication factories on the nuclear matrix. Although Runx1 did not co-localize with large T antigen-containing foci by itself, it co-localized with large T antigen-containing replication factories during Runx1-dependent polyomavirus DNA replication. These observations together suggest that Runx1 recruits the polyomavirus replication origin to the replication factory on the nuclear matrix, and that this requires the nuclear matrix-binding activity of Runx1.

RUNX1-RUNX1 Homodimerization Modulates RUNX1 Activity and Function

RUNX1 (AML1, CBFalpha2, PEBP2alphaB) is a transcription factor essential for the establishment of the hematopoietic stem cell. It is generally thought that RUNX1 exists as a monomer that regulates hematopoietic differentiation by interacting with tissue-specific factors and its DNA consensus through its N terminus. RUNX1 is frequently altered in human leukemia by gene fusions or point mutations. In general, these alterations do not affect the N terminus of the protein, and it is unclear how they consistently lead to hematopoietic transformation and leukemia. Here we report that RUNX1 homodimerizes through a mechanism involving C terminus-C terminus interaction. This RUNX1-RUNX1 interaction regulates the activity of the protein in reporter gene assays and modulates its ability to induce hematopoietic differentiation of hematopoietic cell lines. The promoters of genes regulated by RUNX1 often contain multiple RUNX1 binding sites. This arrangement suggests that RUNX1 could homodimerize to bring and hold together distant chromatin sites and factors and that if the dimerization region is removed by gene fusions or is altered by point mutations, as observed in leukemia, the ability of RUNX1 to regulate differentiation could be impaired.

Alterations in intranuclear localization of Runx2 affect biological activity

The transcription factor Runx2 controls osteoblast proliferation and differentiation. Runx2 organizes and assembles gene-regulatory complexes in nuclear microenvironments where target genes are activated or suppressed in a context-dependent manner. Intranuclear localization of Runx2 is mediated by the nuclear matrix-targeting signal (NMTS), an autonomous motif with a loop (L1)-turn-loop (L2) structure that forms predicted protein-protein interaction surfaces. Here we examined the functional consequences of introducing mutations in the L1 and L2 loops of the NMTS. These mutant proteins enter the nucleus, interact with the hetero-dimeric partner Cbfbeta, and bind to DNA in vitro and in vivo. In addition, these mutants retain interaction with the carboxy-terminus interacting co-regulatory proteins that include TLE, YAP, and Smads. However, two critical mutations in the L2 domain of the NMTS decrease association of Runx2 with the nuclear matrix. These subnuclear targeting defective (STD) mutants of Runx2 compromise target gene activation or repression. The biological significance of these findings is reflected by decreased osteogenic differentiation of mesenchymal progenitors, concomitant with major changes in gene expression profiles, upon expression of the STD Runx2 mutant. Our results demonstrate that fidelity of temporal and spatial localization of Runx2 within the nucleus is functionally linked with biological activity.

C-terminal domains deliver the DNA replication factor Ciz1 to the nuclear matrix

Cip1-interacting zinc finger protein 1 (Ciz1) stimulates DNA replication in vitro and is required for mammalian cells to enter S phase. Here, we show that a significant proportion of Ciz1 is retained in nuclear foci following extraction with nuclease and high salt. This suggests that Ciz1 is normally immobilized by interaction with non-chromatin nuclear structures, consistent with the nuclear matrix. Furthermore, matrix-associated Ciz1 foci strikingly colocalize with sites of newly synthesized DNA in S phase nuclei, suggesting that Ciz1 is present in DNA replication factories. Analysis of green fluorescent protein-tagged fragments indicates that nuclear immobilization of Ciz1 is mediated by sequences in its C-terminal third, encoded within amino acids 708-830. Immobilization occurs in a cell-cycle-dependent manner, most probably during late G1 or early S phase, to coincide with its reported point of action. Although C-terminal domains are sufficient for immobilization, N-terminal domains are also required to specify focal organization. Combined with previous work, which showed that the DNA replication activity of Ciz1 is encoded by N-terminal sequences, we suggest that Ciz1 is composed of two functionally distinct domains: an N-terminal replication domain and a C-terminal nuclear matrix anchor. This could contribute to the formation or function of DNA replication factories in mammalian cells.

Dynamic expression of Runx1 in skin affects hair structure

The three mammalian Runx transcription factors, some of which are known to be involved in human genetic diseases and cancer, are pivotal players in embryo development and function as key regulators of cell fate determination and organogenesis. Here, we report the expression of Runx1 during the development of hair and other skin appendages in the mouse and describe the effect of Runx1 on the structural hair output. In hair follicles, where the three Runx proteins are expressed, Runx1 expression is most prominent in both mesenchymal and epithelial compartments. The epithelial expression includes the hair keratin forming layers of the hair shaft and the bulge, where interestingly, Runx1 is co-expressed with keratin 15, a putative hair follicle stem cell marker. In the hair mesenchyme, during early stages of hair morphogenesis, Runx1 is expressed in a discrete dermal sub-epithelial layer, while at later stages it is found in a hair cycle dependent pattern in the dermal papilla. To elucidate the function of Runx1 in the hair follicle we have generated a Runx1 epidermal conditional knockout and found that the mutant mice display a remarkable structural deformation of the zigzag hair type. The data delineate Runx1 as a novel specific marker of several hair follicle cell types and sheds light on its role in hair morphogenesis and differentiation.

Networks and hubs for the transcriptional control of osteoblastogenesis

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.

Sp2 localizes to subnuclear foci associated with the nuclear matrix

We have reported that extracts prepared from many human and mouse cell lines show little or no Sp2 DNA-binding activity and that Sp2 has little or no capacity to stimulate transcription of promoters that are activated by Sp1, Sp3, and Sp4. Using an array of chimeric Sp1/Sp2 proteins we showed further that Sp2 DNA-binding activity and trans-activation are each negatively regulated in mammalian cells. As part of an ongoing effort to study Sp2 function and regulation we characterized its subcellular localization in comparison with other Sp-family members in fixed and live cells. We report that 1) Sp2 localizes largely within subnuclear foci associated with the nuclear matrix, and 2) these foci are distinct from promyelocytic oncogenic domains and appear to be stable during an 18-h time course of observation. Deletion analyses identified a 37 amino acid sequence spanning the first zinc-"finger" that is sufficient to direct nuclear matrix association, and this region also encodes a bipartite nuclear localization sequence. A second nuclear matrix targeting sequence is encoded within the Sp2 trans-activation domain. We conclude that Sp2 preferentially associates with the nuclear matrix and speculate that this subcellular localization plays an important role in the regulation of Sp2 function.

Runx1 binds positive transcription elongation factor b and represses transcriptional elongation by RNA polymerase II: possible mechanism of CD4 silencing

Runx1 binds the silencer and represses CD4 transcription in immature thymocytes. In this study, we found that Runx1 inhibits P-TEFb, which contains CycT1, CycT2, or CycK and Cdk9 and stimulates transcriptional elongation by RNA polymerase II (RNAPII) in eukaryotic cells. Indeed, its inhibitory domain, spanning positions 371 to 411, not only bound CycT1 but was required for silencing CD4 transcription in vivo. Our chromatin immunoprecipitation assays revealed that Runx1 inhibits the elongation but not initiation of transcription and that RNAPII is engaged at the CD4 promoter but is unable to elongate in CD4(-) CD8(+) thymoma cells. These results suggest that active repression by Runx1 occurs by blocking the elongation by RNAPII, which may contribute to CD4 silencing during T-cell development.

Subnuclear compartmentalization of sequence-specific transcription factors and regulation of eukaryotic gene expression

To date, the majority of the research regarding eukaryotic transcription factors has focused on characterizing their function primarily through in vitro methods. These studies have revealed that transcription factors are essentially modular structures, containing separate regions that participate in such activities as DNA binding, protein-protein interaction, and transcriptional activation or repression. To fully comprehend the behavior of a given transcription factor, however, these domains must be analyzed in the context of the entire protein, and in certain cases the context of a multiprotein complex. Furthermore, it must be appreciated that transcription factors function in the nucleus, where they must contend with a variety of factors, including the nuclear architecture, chromatin domains, chromosome territories, and cell-cycle-associated processes. Recent examinations of transcription factors in the nucleus have clarified the behavior of these proteins in vivo and have increased our understanding of how gene expression is regulated in eukaryotes. Here, we review the current knowledge regarding sequence-specific transcription factor compartmentalization within the nucleus and discuss its impact on the regulation of such processes as activation or repression of gene expression and interaction with coregulatory factors.

Phosphorylation of AML1/RUNX1 regulates its degradation and nuclear matrix association

The acute myeloid leukemia 1 (AML1) transcription factors are key regulators of hematopoietic differentiation. Cellular AML1c protein is found in the nucleus and can be separated into two fractions, one soluble in buffers containing salt and nonionic detergent and the other insoluble and tightly bound to the nuclear matrix. We find that the AML1c protein is modified by both phosphorylation and ubiquitination. Our studies show that the majority of the ubiquitinated AML1c is associated with the insoluble nuclear matrix. Treatment of cells with the proteasome inhibitor PS341 (Velcade, Bortezomib) increases the levels of ubiquitinated AML1c. Mutation of the four phosphorylation sites necessary for transcriptional regulation (serine 276, serine 293, serine 303, and threonine 300) mimics the effects of the proteasome inhibitor, increasing the levels of ubiquitinated, matrix-bound AML1c. We find that the soluble and insoluble forms of AML1c are degraded at a similar rate. However, mutation of these four serine/threonine residues statistically increases the half-life of the matrix-associated AML1c. Thus, phosphorylation of AML1c on specific serine/threonine residues controls both transcriptional activity and rate of degradation.

Combinatorial organization of the transcriptional regulatory machinery in biological control and cancer

The architecturally associated subnuclear organization of nucleic acids and cognate regulatory factors suggests 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 and integration of regulatory information as well as the combinatorial assembly, organization and activities of transcriptional machinery at scaffold-associated subnuclear sites that support gene expression. During the past several years our laboratory has been addressing intranuclear trafficking mechanisms that direct transcription factors to transcriptionally active nuclear microenvironments. We are pursuing these studies using the AML/Runx/Cbfa transcription factors that govern hematopoietic and bone-specific transcription as a paradigm. Our objective is to gain insight into linkage of intranuclear organization of genes, transcripts, and regulatory proteins with fidelity of biological control and contributions of aberrant nuclear structure/function relationships to the onset and progression of tumorigenesis.

Smad function and intranuclear targeting share a Runx2 motif required for osteogenic lineage induction and BMP2 responsive transcription

The coordinated activity of Runx2 and BMP/TGFbeta-activated Smads is critical for formation of the skeleton, but the precise structural basis for the Runx2/Smad interaction has not been resolved. By deletion mutagenesis, we have defined the Runx2 motif required for physical and functional interaction with either BMP or TGFbeta responsive Smads. Smad responsive transcriptional activity was retained upon deletion of the C-terminus to amino acid (aa) 432 but lost with deletion to aa 391. Thus the Smad interacting domain (SMID) of Runx2 (432-391) is embedded in the well-defined nuclear matrix targeting signal (NMTS) that mediates intranuclear trafficking. The SMID suffices as an interacting module when fused to the heterologous Gal-4 protein. Formation of the Runx2 and Smad complex is dependent on Runx2 phosphorylation through the MAPK signaling pathway, as determined by co-immunoprecipitation studies. We established that all SMID/NMTS deficient Runx2 mutants do not show in situ association with Smad in the nucleus nor do they support BMP2-mediated osteogenic induction of the mesenchymal C2C12 cell line. Thus, we provide direct evidence that the SMID/NMTS domain (391-432) of Runx2 is essential for BMP2-mediated osteoblast differentiation. Our findings suggest that TGFbeta/ BMP2 signaling, MAPK dependent phosphorylation, and Runx2 subnuclear targeting converge to induce the osteogenic phenotype.

The dynamic organization of gene-regulatory machinery in nuclear microenvironments

Nuclear components are functionally linked with the dynamic temporal and spatial compartmentalization, sorting and integration of regulatory information to facilitate its selective use. For example, the subnuclear targeting of transcription factors to punctate sites in the interphase nucleus mechanistically couples chromatin remodelling and the execution of signalling cascades that mediate gene expression with the combinatorial assembly of the regulatory machinery for biological control. In addition, a mitotic cycle of selective partitioning and sequential restoration of the transcriptional machinery provides a basis for the reassembly of regulatory complexes to render progeny cells competent for phenotypic gene expression. When this intranuclear targeting and localization of regulatory proteins is compromised, diseases, such as cancer, can occur. A detailed understanding of this process will provide further options for diagnosis and treatment.

Hairless contains a novel nuclear matrix targeting signal and associates with histone deacetylase 3 in nuclear speckles

Hair follicle cycling is a highly regulated and dynamic cellular process consisting of phases of growth, regression, and quiescence. The hairless (hr) gene encodes a nuclear factor that is highly expressed in the skin, where it appears to be an essential regulator during the regression in the catagen hair follicle. In hairless mice, as well as humans with congenital atrichia, the absence of hr protein initiates a premature and abnormal catagen due to defects in the signaling required for hair follicle remodeling. Here, we report that hr protein is a nuclear protein that is tightly associated with the nuclear matrix scaffold. Using a series of deletion constructs of the mouse hr gene, we monitored the sub-cellular localization of the recombinant protein by in situ immunolocalization and biochemical fractionation after nuclear matrix extraction of transiently transfected cells. We identified a novel nuclear matrix-targeting signal (NMTS) in the hr protein and mapped the domain to amino acid residues 111-186 of the mouse hr sequence. Furthermore, we provide evidence that this region not only mediates the interaction of hr with components of the nuclear architecture, but also specifies the sub-nuclear location of the hr protein to nuclear domains containing deacetylase activity. The N-terminal region directs hr to a speckled nuclear pattern that co-localizes with the histone deacetylase 3 (HDAC), but not with HDAC1 or HDAC7. Based on our findings, we propose that hr protein is part of a specific multi-protein repressor complex and that hr may be involved in chromatin remodeling.

Identification and characterisation of two runx2 homologues in zebrafish with different expression patterns

Genome and gene duplications are considered to be the impetus to generate new genes, as the presence of multiple copies of a gene allows for paralogues to adopt novel function. After at least two rounds of genome/gene duplication, the Runt gene family consists of three members in vertebrates, instead of one in invertebrates. One of the family members, Runx2, plays a key role in the development of bone, a tissue that first occurs in vertebrates. The family has thus gained new gene function in the course of evolution. Two Runx2 genes were cloned in the vertebrate model system the zebrafish (Danio rerio). The expression patterns of the two genes differ and their kinetics differ up to four fold. In addition, splice forms exist that are novel when compared with mammals. Together, these findings comprise opportunities for selection and retention of the paralogues towards divergent and possibly new function.

Point mutation in AML1 disrupts subnuclear targeting, prevents myeloid differentiation, and effects a transformation-like phenotype

The multifunctional C terminus of the hematopoietic AML1 transcription factor interacts with coregulatory proteins, supports the convergence and integration of physiological signals, and contains the nuclear matrix targeting signal, the protein motif that is necessary and sufficient to target AML1 to subnuclear sites. The (8;21) chromosomal translocation, which replaces the C terminus of AML1 with the ETO protein, modifies subnuclear targeting of AML1 in acute myeloid leukemia (AML) and results in defective myelopoiesis. We therefore addressed the relevance of AML1 subnuclear targeting and associated functions that reside in the C terminus to myeloid differentiation. A single amino acid substitution that abrogates intranuclear localization was introduced in the AML1 subnuclear targeting signal. Expression of the mutant AML1 protein blocks differentiation of myeloid progenitors to granulocytes in the presence of endogenous AML1 protein, as also occurs in the (8;21) chromosomal translocation, where only one allele of the AML1 gene is affected. The cells expressing the mutant AML1 protein continue to proliferate, maintain an immature blast-like morphology, and exhibit transformed properties that are hallmarks of leukemogenesis. These findings functionally link AML1 subnuclear targeting with competency for myeloid differentiation and expression of the transformed/leukemia phenotype.

The RUNX genes: gain or loss of function in cancer

The RUNX genes have come to prominence recently because of their roles as essential regulators of cell fate in development and their paradoxical effects in cancer, in which they can function either as tumour-suppressor genes or dominant oncogenes according to context. How can this family of transcription factors have such an ambiguous role in cancer? How and where do these genes impinge on the pathways that regulate growth control and differentiation? And what is the evidence for a wider role for the RUNX genes in non-haematopoietic cancers?

A T lymphocyte-specific transcription complex containing RUNX1 activates MHC class I expression

MHC class I expression is subject to both tissue-specific and hormonal regulatory mechanisms. Consequently, levels of expression vary widely among tissues, with the highest levels of class I occurring in the lymphoid compartment, in T cells and B cells. Although the high class I expression in B cells is known to involve the B cell enhanceosome, the molecular basis for high constitutive class I expression in T cells has not been explored. T cell-specific genes, such as TCR genes, are regulated by a T cell enhanceosome consisting of RUNX1, CBFbeta, LEF1, and Aly. In this report, we demonstrate that MHC class I gene expression is enhanced by the T cell enhanceosome and results from a direct interaction of the RUNX1-containing complex with the class I gene in vivo. T cell enhanceosome activation of class I transcription is synergistic with CIITA-mediated activation and targets response elements distinct from those targeted by CIITA. These findings provide a molecular basis for the high levels of MHC class I in T cells.

Impaired intranuclear trafficking of Runx2 (AML3/CBFA1) transcription factors in breast cancer cells inhibits osteolysis in vivo

Runx transcription factors comprise a family of proteins that are essential for organogenesis. A unique nuclear matrix-targeting signal in the C terminus directs these factors to their appropriate subnuclear domains. At these sites, they interact with coregulatory proteins and target genes. We have previously shown that aberrant expression of the Runx2 DNA binding domain in metastatic breast cancer cells can prevent production of osteolytic lesions in bone. Here, we show that proper Runx2 subnuclear targeting is required for osteolysis. We have identified point mutations of the Runx2 nuclear matrix-targeting signal sequence that impair its targeting to nuclear matrix sites. These mutations block the invasive and osteolytic properties of MDA-MB-231 breast cancer cells in vivo. Cell lines expressing this Runx2 mutant protein inhibit the osteogenic properties of bone marrow stromal cells in coculture assays. The mutant breast cancer cells also exhibit reduced invasiveness in vitro and do not express genes involved in invasion and angiogenesis (VEGF and MMP13). Our findings suggest that fidelity of Runx2 intranuclear organization is necessary for expression of target genes that mediate the osteolytic activity of metastatic breast cancer cells.

Physical and functional link of the leukemia-associated factors AML1 and PML

The AML1-CBFβ transcription factor complex is the most frequent target of specific chromosome translocations in acute myeloid leukemia (AML). The promyelocytic leukemia (PML) gene is also frequently involved in AML-associated translocation. Here we report that a specific isoform PML I forms a complex with AML1. PML I was able to recruit AML1 and coactivator p300 in PML nuclear bodies and enhance the AML1-mediated transcription in the presence of p300. A specific C-terminal region of PML I and a C-terminal region of AML1 were found to be required for both their association and colocalization in the nuclear bodies. Overexpression of PML I stimulates myeloid cells to differentiate. These results suggest that PML I could act as a mediator for AML1 and its coactivator p300/CBP to assemble into functional complexes and, consequently, activate AML1-dependent transcription and myeloid cell differentiation. (Blood. 2005;105:292-300)

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

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

Subnuclear targeting of Runx1 Is required for synergistic activation of the myeloid specific M-CSF receptor promoter by PU.1

Many types of acute myelogenous leukemia involve chromosomal translocations that target the C-terminus of Runx1/AML1 transcription factor, a master regulator of hematopoiesis. The C-terminus of Runx1/AML1 that includes the nuclear matrix targeting signal (NMTS) is essential for embryonic development, hematopoiesis, and target gene regulation. During the onset and normal progression of hematopoiesis, several lineage-specific factors such as C/EBPalpha and PU.1 interact with Runx1 to regulate transcription combinatorially. Here we addressed the functional interplay between subnuclear targeting of Runx1 and gene activation during hematopoiesis. Point mutations were generated in the NMTS of the human Runx1 protein and tested for their effect on transcriptional cooperativity with C/EBPalpha and PU.1 at myeloid-specific promoters. We characterized five mutants that do not alter nuclear import, DNA binding or C/EBPalpha-dependent synergistic activation of the target gene promoters. However a critical tyrosine in the NMTS is required for subnuclear targeting and activation of the granulocyte-macrophage colony stimulating factor (GM-CSF) promoter. Furthermore, this point mutation is defective for transcriptional synergism with PU.1 on the macrophage colony stimulating factor (MCSF) receptor c-FMS promoter. Our results indicate that the NMTS region of Runx1 is required for functional interactions with PU.1. Taken together, our findings establish that subnuclear targeting of Runx1 is a critical component of myeloid-specific transcriptional control.

Microtubule-dependent nuclear-cytoplasmic shuttling of Runx2

RUNX/AML transcription factors are critical regulators of cell growth and differentiation in multiple lineages and have been linked to human cancers including acute myelogenous leukemia (RUNX1), as well as breast (RUNX2) and gastric cancers (RUNX3). RUNX proteins are targeted to gene regulatory micro-environments within the nucleus via a specific subnuclear targeting signal. However, the dynamics of RUNX distribution and compartmentalization between the cytoplasm and nucleus is minimally understood. Here we show by immunofluorescence microscopy that RUNX2 relocates from the nucleus to the cytoplasm when microtubules are stabilized by the chemotherapeutic agent taxol. The taxol-dependent cytoplasmic accumulation of RUNX2 is inhibited by leptomycin B, which blocks CRM-1 dependent nuclear export, and is not affected by the protein synthesis inhibitor cycloheximide. Using biochemical assays, we show that endogenous RUNX2 associates with stabilized microtubules in a concentration-dependent manner and that the RUNX2 amino terminus mediates the microtubule association. In soluble fractions of cells, RUNX2 co-immunoprecipitates alpha tubulin suggesting that microtubule binding involves the alpha/beta tubulin subunits. We conclude that RUNX2 associates with microtubules and shuttles between the nucleus and the cytoplasm. We propose that nuclear-cytoplasmic shuttling of RUNX2 may modulate its transcriptional activity, as well as its ability to interface with signal transduction pathways that are integrated at RUNX2 containing subnuclear sites. It is possible that taxol-induced acute depletion of the nuclear levels of RUNX2 and/or other cell growth regulatory factors may represent an alternative pathway by which taxol exerts its biological effects during cancer chemotherapies.

Bone-specific transcription factor Runx2 interacts with the 1alpha,25-dihydroxyvitamin D3 receptor to up-regulate rat osteocalcin gene expression in osteoblastic cells

Bone-specific transcription of the osteocalcin (OC) gene is regulated principally by the Runx2 transcription factor and is further stimulated in response to 1alpha,25-dihydroxyvitamin D3 via its specific receptor (VDR). The rat OC gene promoter contains three recognition sites for Runx2 (sites A, B, and C). Mutation of sites A and B, which flank the 1alpha,25-dihydroxyvitamin D3-responsive element (VDRE), abolishes 1alpha,25-dihydroxyvitamin D3-dependent enhancement of OC transcription, indicating a tight functional relationship between the VDR and Runx2 factors. In contrast to most of the members of the nuclear receptor family, VDR possesses a very short N-terminal A/B domain, which has led to the suggestion that its N-terminal region does not contribute to transcriptional enhancement. Here, we have combined transient-overexpression, coimmunoprecipitation, in situ colocalization, chromatin immunoprecipitation, and glutathione S-transferase pull-down analyses to demonstrate that in osteoblastic cells expressing OC, VDR interacts directly with Runx2 bound to site B, which is located immediately adjacent to the VDRE. This interaction contributes significantly to 1alpha,25-dihydroxyvitamin D3-dependent enhancement of the OC promoter and requires a region located C terminal to the runt homology DNA binding domain of Runx2 and the N-terminal region of VDR. Together, our results indicate that Runx2 plays a key role in the 1alpha,25-dihydroxyvitamin D3-dependent stimulation of the OC promoter in osteoblastic cells by further stabilizing the interaction of the VDR with the VDRE. These studies demonstrate a novel mechanism for combinatorial control of bone tissue-specific gene expression. This mechanism involves the intersection of two major pathways: Runx2, a "master" transcriptional regulator of osteoblast differentiation, and 1alpha,25-dihydroxyvitamin D3, a hormone that promotes expression of genes associated with these terminally differentiated bone cells.

Growth hormone attenuates the transcriptional activity of Runx2 by facilitating its physical association with Stat3beta

We document that GH controls osteoblast function by modulating the biological activity of the osteospecific transcription factor Runx2. Evidence is provided for a physical interaction between Runx2 and Stat3beta, which is enhanced by GH and downregulates the transcriptional properties of this key osteogenic regulator.

INTRODUCTION

Growth hormone (GH) signals to bone either through insulin-like growth factor-1 or directly by influencing the function of osteoblasts, the bone-forming cells. This study aimed at exploring the molecular events that underlie the direct biological action of GH on osteoblastic cells, and specifically, the effects that it might exert on the function of the bone-specific transcriptional regulator Runx2.

MATERIALS AND METHODS

The GH-responsive human osteoblastic cell line Saos-2 was used as our experimental system. Western blot analyses were used to monitor the presence of several parameters known to be affected by GH in these cells (i.e., downregulation of GH receptor, induction of STATs, and extracellular signal-regulated kinase [ERK] mitogen-activated protein kinase [MAPK] pathways). Electrophoretic mobility shift assays were used to assess Runx2 and Stat3 binding activity on an osteoblast-specific element (OSE2) after GH treatment. A combination of yeast two-hybrid and co-immunoprecipitation assays were performed to test for the existence of a physical Runx2.Stat3beta association. Finally, co-transfection experiments were used to investigate the interplay of the two transcription factors on the activity of a p6OSE2-Luc promoter after GH stimulation.

RESULTS

We show that GH signaling through Stat3/ERK MAPK potentiates the DNA binding activity of Runx2 but, at the same time, restrains its transcriptional potential. Moreover, a novel physical interaction of Runx2 with transcription factor Stat3beta, which is enhanced by GH stimulation, was documented both in vitro and in vivo. Importantly, this interaction impairs the transcriptional activity of Runx2 without affecting its DNA binding capacity.

CONCLUSION

Our data provide the first evidence that GH modulates the transcriptional function of Runx2 in osteoblastic cells by promoting its inhibitory interaction with Stat3beta. Shedding light on such mechanisms will contribute to a better understanding of GH effects on skeletal homeostasis that may impact on decisions at the clinical level, especially in diseases affecting bone quantity and quality (e.g., osteoporosis).

Quantitative signature for architectural organization of regulatory factors using intranuclear informatics

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

Localization of the domains in Runx transcription factors required for the repression of CD4 in thymocytes

The runt family transcription factors Runx1 and Runx3 are expressed in developing murine thymocytes. We show that enforced expression of full-length Runx1 in CD4(-)CD8(-) thymocytes results in a profound suppression of immature CD4/CD8 double-positive thymocytes and mature CD4 single-positive thymocytes compared with controls. This effect arises from Runx1- or Runx3-mediated repression of CD4 expression, and is independent of positively selecting signals. Runx1 is able to repress CD4 in CD4/CD8 double-positive thymocytes, but not in mature splenic T cells. Runx-mediated CD4 repression is independent of association with the corepressors Groucho/TLE or Sin3. Two domains are required for complete Runx-mediated CD4 repression. These are contained within Runx1 aa 212-262 and 263-360. The latter region contains the nuclear matrix targeting sequence, which is highly conserved among runt family transcription factors across species. The presence of the nuclear matrix targeting sequence is required for Runx-mediated CD4 repression, suggesting that Runx transcription factors are stabilized on the CD4 silencer via association with the nuclear matrix.

Intranuclear trafficking: organization and assembly of regulatory machinery for combinatorial biological control

The molecular logistics of nuclear regulatory processes necessitate temporal and spatial regulation of protein-protein and protein-DNA interactions in response to physiological cues. Biochemical, in situ, and in vivo genetic evidence demonstrates the requirement for intranuclear localization of regulatory complexes that functionally couple cellular responses to signals that mediate combinatorial control of gene expression. We have summarized evidence that subnuclear targeting of transcription factors mechanistically links gene expression with architectural organization and assembly of nuclear regulatory machinery for biological control. The compromised intranuclear targeting of regulatory proteins under pathological conditions provides options for the diagnosis and treatment of disease.

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

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