The protooncogene product, PEBP2beta/CBFbeta, is mainly located in the cytoplasm and has an affinity with cytoskeletal structures

CBFβ and the leukemogenic fusion protein CBFβ-SMMHC associate with mitotic chromosomes to epigenetically regulate ribosomal genes

Mitotic bookmarking is an epigenetic control mechanism that sustains gene expression in progeny cells; it is often found in genes related to the maintenance of cellular phenotype and growth control. RUNX transcription factors regulate a broad spectrum of RNA Polymerase (Pol II) transcribed genes important for lineage commitment but also regulate RNA Polymerase I (Pol I) driven ribosomal gene expression, thus coordinating control of cellular identity and proliferation. In this study, using fluorescence microscopy and biochemical approaches we show that the principal RUNX co-factor, CBFβ, associates with nucleolar organizing regions (NORs) during mitosis to negatively regulate RUNX-dependent ribosomal gene expression. Of clinical relevance, we establish for the first time that the leukemogenic fusion protein CBFβ-SMMHC (smooth muscle myosin heavy chain) also associates with ribosomal genes in interphase chromatin and mitotic chromosomes to promote and epigenetically sustain regulation of ribosomal genes through RUNX factor interactions. Our results demonstrate that CBFβ contributes to the transcriptional regulation of ribosomal gene expression and provide further understanding of the epigenetic role of CBFβ-SMMHC in proliferation and maintenance of the leukemic phenotype.

Core binding factor β (CBFβ) is retained in the midbody during cytokinesis

Core Binding Factor β (CBFβ) is complexed with the RUNX family of transcription factors in the nucleus to support activation or repression of genes related to bone (RUNX2), hematopoiesis (RUNX1) and gastrointestinal (RUNX3) development. Furthermore, RUNX proteins contribute to the onset and progression of different types of cancer. Although CBFβ localizes to cytoskeletal architecture, its biological role in the cytoplasmic compartment remains to be established. Additionally, the function and localization of CBFβ during the cell cycle are important questions relevant to its biological role. Here we show that CBFβ dynamically distributes in different stages of cell division and importantly is present during telophase at the midbody, a temporal structure important for successful cytokinesis. A functional role for CBFβ localization at the midbody is supported by striking defects in cytokinesis that include polyploidy and abscission failure following siRNA-mediated downregulation of endogenous CBFβ or overexpression of the inv(16) fusion protein CBFβ-SMMHC. Our results suggest that CBFβ retention in the midbody during cytokinesis reflects a novel function that contributes to epigenetic control.

Spectrin isoforms: differential expression in normal hematopoiesis and alterations in neoplastic bone marrow disorders

Spectrins are large, rod-like, multifunctional molecules that participate in maintaining cell structure, signal transmission, and DNA repair. Because little is known about the role of spectrins in normal hematopoiesis and leukemogenesis, we immunohistochemically stained bone marrow biopsy specimens from 81 patients for αI, αII, βI, and βII spectrin isoforms in normal reactive marrow (NRM), myelodysplastic syndrome, myeloproliferative neoplasm, acute myeloid leukemia (AML) with well-characterized cytogenetic abnormalities, acute erythroid leukemia (EryL), and acute megakaryoblastic leukemia (MegL). In NRM, spectrin isoforms were differentially expressed according to cell lineage: αI and βI in erythroid precursors; αII and βII in granulocytes; and βI and βII in megakaryocytes. In contrast, 18 (44%) of 41 AMLs lacked αII spectrin and/or aberrantly expressed βI spectrin (P = .0398; Fisher exact test) and 5 (100%) of 5 EryLs expressed βII spectrin but lacked βI spectrin. The frequent loss and/or gain of spectrin isoforms in AMLs suggests a possible role for spectrin in leukemogenesis.

Dysregulation of the transcription factors SOX4, CBFB and SMARCC1 correlates with outcome of colorectal cancer

The aim of this study was to identify deregulated transcription factors (TFs) in colorectal cancer (CRC) and to evaluate their relation with the recurrence of stage II CRC and overall survival. Microarray-based transcript profiles of 20 normal mucosas and 424 CRC samples were used to identify 51 TFs displaying differential transcript levels between normal mucosa and CRC. For a subset of these we provide in vitro evidence that deregulation of the Wnt signalling pathway can lead to the alterations observed in tissues. Furthermore, in two independent cohorts of microsatellite-stable stage II cancers we found that high SOX4 transcript levels correlated with recurrence (HR 2.7; 95% CI, 1.2–6.0; P=0.01). Analyses of ∼1000 stage I–III adenocarcinomas, by immunohistochemistry, revealed that patients with tumours displaying high levels of CBFB and SMARCC1 proteins had a significantly better overall survival rate (P=0.0001 and P=0.0275, respectively) than patients with low levels. Multivariate analyses revealed that a high CBFB protein level was an independent predictor of survival. In conclusion, several of the identified TFs seem to be involved in the progression of CRC.

Integrative analysis of RUNX1 downstream pathways and target genes

Background

The RUNX1 transcription factor gene is frequently mutated in sporadic myeloid and lymphoid leukemia through translocation, point mutation or amplification. It is also responsible for a familial platelet disorder with predisposition to acute myeloid leukemia (FPD-AML). The disruption of the largely unknown biological pathways controlled by RUNX1 is likely to be responsible for the development of leukemia. We have used multiple microarray platforms and bioinformatic techniques to help identify these biological pathways to aid in the understanding of why RUNX1 mutations lead to leukemia.

Results

Here we report genes regulated either directly or indirectly by RUNX1 based on the study of gene expression profiles generated from 3 different human and mouse platforms. The platforms used were global gene expression profiling of: 1) cell lines with RUNX1 mutations from FPD-AML patients, 2) over-expression of RUNX1 and CBFβ, and 3) Runx1 knockout mouse embryos using either cDNA or Affymetrix microarrays. We observe that our datasets (lists of differentially expressed genes) significantly correlate with published microarray data from sporadic AML patients with mutations in either RUNX1 or its cofactor, CBFβ. A number of biological processes were identified among the differentially expressed genes and functional assays suggest that heterozygous RUNX1 point mutations in patients with FPD-AML impair cell proliferation, microtubule dynamics and possibly genetic stability. In addition, analysis of the regulatory regions of the differentially expressed genes has for the first time systematically identified numerous potential novel RUNX1 target genes.

Conclusion

This work is the first large-scale study attempting to identify the genetic networks regulated by RUNX1, a master regulator in the development of the hematopoietic system and leukemia. The biological pathways and target genes controlled by RUNX1 will have considerable importance in disease progression in both familial and sporadic leukemia as well as therapeutic implications.

TheC. elegansCBFβ homologue BRO-1 interacts with the Runx factor, RNT-1, to promote stem cell proliferation and self-renewal

In this report, we investigate the C. elegans CBFβ homologue,BRO-1. bro-1 mutants have a similar male-specific sensory ray loss phenotype to rnt-1 (the C. elegans homologue of the mammalian CBFβ-interacting Runx factors), caused by failed cell divisions in the seam lineages. Our studies indicate that BRO-1 and RNT-1 form a cell proliferation-promoting complex, and that BRO-1 increases both the affinity and specificity of RNT-1-DNA interactions. Overexpression of bro-1,like rnt-1, leads to an expansion of seam cell number and co-overexpression of bro-1 and rnt-1 results in massive seam cell hyperplasia. Finally, we find that BRO-1 appears to act independently of RNT-1 in certain situations. These studies provide new insights into the function and regulation of this important cancer-associated DNA-binding complex in stem cells and support the view that Runx/CBFβ factors have oncogenic potential.

Filamin A-bound PEBP2beta/CBFbeta is retained in the cytoplasm and prevented from functioning as a partner of the Runx1 transcription factor

The heterodimeric transcription factor PEBP2/CBF is composed of a DNA-binding subunit, called Runx1, and a non-DNA-binding subunit, called PEBP2beta/CBFbeta. The Runx1 protein is detected exclusively in the nuclei of most cells and tissues, whereas PEBP2beta is located in the cytoplasm. We addressed the mechanism by which PEBP2beta localizes to the cytoplasm and found that it is associated with filamin A, an actin-binding protein. Filamin A retains PEBP2beta in the cytoplasm, thereby hindering its engagement as a Runx1 partner. The interaction with filamin A is mediated by a region within PEBP2beta that includes amino acid residues 68 to 93. The deletion of this region or the repression of filamin A enables PEBP2beta to translocate to the nucleus. Based on these observations, we propose that PEBP2beta has two distinct domains, a newly defined regulatory domain that interacts with filamin A and the previously identified Runx1-binding domain.

Overexpression of the Runx3 Transcription Factor Increases the Proportion of Mature Thymocytes of the CD8 Single-Positive Lineage

The Runx family of transcription factors is thought to regulate the differentiation of thymocytes. Runx3 protein is detected mainly in the CD4(-)8(+) subset of T lymphocytes. In the thymus of Runx3-deficient mice, CD4 expression is de-repressed and CD4(-)8(+) thymocytes do not develop. This clearly implicates Runx3 in CD4 silencing, but does not necessarily prove its role in the differentiation of CD4(-)8(+) thymocytes per se. In the present study, we created transgenic mice that overexpress Runx3 and analyzed the development of thymocytes in these animals. In the Runx3-transgenic thymus, the number of CD4(-)8(+) cells was greatly increased, whereas the numbers of CD4(+)8(+) and CD4(+)8(-) cells were reduced. The CD4(-)8(+) transgenic thymocytes contained mature cells with a TCR(high)HSA(low) phenotype. These cells were released from the thymus and contributed to the elevated level of CD4(-)8(+) cells relative to CD4(+)8(-) cells in the spleen. Runx3 overexpression also increased the number of mature CD4(-)8(+) thymocytes in mice with class II-restricted, transgenic TCR and in mice with a class I-deficient background, both of which are favorable for CD4(+)8(-) lineage selection. Thus, Runx3 can drive thymocytes to select the CD4(-)8(+) lineage. This activity is likely to be due to more than a simple silencing of CD4 gene expression.

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.

Morpholino antisense oligonucleotide-mediated gene knockdown during thymocyte development reveals role for Runx3 transcription factor in CD4 silencing during development of CD4-/CD8+ thymocytes

During thymic T cell development, immature CD4(+)/CD8(+) thymocytes develop into either CD4(+)/CD8(-) helper or CD4(-)/CD8(+) CTLs. The molecular mechanisms governing the complex selection and differentiation steps during thymic T cell development are not well understood. Here we developed a novel approach to investigate gene function during thymocyte development. We transfected ex vivo isolated immature thymocytes with gene-specific morpholino antisense oligonucleotides and induced differentiation in cell or organ cultures. A morpholino oligonucleotide specific for CD8alpha strongly reduces CD8 expression. To our knowledge, this is the first demonstrated gene knockdown by morpholino oligonucleotides in primary lymphocytes. Using this approach, we show here that the transcription factor Runx3 is involved in silencing of CD4 expression during CD8 T cell differentiation. Runx3 protein expression appears late in thymocyte differentiation and is confined to mature CD8 single-positive thymocytes, whereas Runx3 mRNA is transcribed in mature CD4 and CD8 thymocytes. Therefore, Runx3 protein expression is regulated at a post-transcriptional level. The knockdown of Runx3 protein expression through morpholino oligonucleotides inhibited the development of CD4(-)/CD8(+) T cells. Instead, mature cells with a CD4(+)/CD8(+) phenotype accumulated. Potential Runx binding sites were identified in the CD4 gene silencer element, which are bound by Runx protein in EMSAs. Mutagenesis of potential Runx binding sites in the CD4 gene silencer abolished silencing activity in a reporter gene assay, indicating that Runx3 is involved in CD4 gene silencing. The experimental approach developed here should be valuable for the functional analysis of other candidate genes in T cell differentiation.

The Runx1 transcription factor inhibits the differentiation of naive CD4+ T cells into the Th2 lineage by repressing GATA3 expression

Differentiation of naive CD4+ T cells into helper T (Th) cells is controlled by a combination of several transcriptional factors. In this study, we examined the functional role of the Runx1 transcription factor in Th cell differentiation. Naive T cells from transgenic mice expressing a dominant interfering form of Runx1 exhibited enhanced interleukin 4 production and efficient Th2 differentiation. In contrast, transduction of Runx1 into wild-type T cells caused a complete attenuation of Th2 differentiation and was accompanied by the cessation of GATA3 expression. Furthermore, endogenous expression of Runx1 in naive T cells declined after T cell receptor stimulation, at the same time that expression of GATA3 increased. We conclude that Runx1 plays a novel role as a negative regulator of GATA3 expression, thereby inhibiting the Th2 cell differentiation.

The role of a Runt domain transcription factor AML1/RUNX1 in leukemogenesis and its clinical implications

A Runt domain transcription factor AML1/RUNX1 is essential for generation and differentiation of definitive hematopoietic stem cells. AML1 is the most frequent target of chromosomal translocations in acute leukemias. Several chimeric proteins such as AML1-MTG8 and TEL-AML1 have transdominant properties for wild-type AML1 and acts as transcriptional repressors. The transcriptional repression in AML1 fusion proteins is mediated by recruitment of nuclear corepressor complex that maintains local histone deacetylation. Inhibition of the expression of AML1-responsive genes leads to a block in hematopoietic cell differentiation and consequent leukemic transformation. On the other hand, mutations in the Runt domain of the AML1 are identified in both sporadic acute myeloblastic leukemia (AML) without AML1 translocation and familial platelet disorder with predisposition to AML. These observations indicate that a decrease in AML1 dosage resulting from chromosomal translocations or mutations contributes to leukemogenesis. Furthermore, dysregulated chromatin remodeling and transcriptional control appears to be a common pathway in AML1-associated leukemias that could be an important target for the development of new therapeutic agents.

Multimerization via its myosin domain facilitates nuclear localization and inhibition of core binding factor (CBF) activities by the CBFbeta-smooth muscle myosin heavy chain myeloid leukemia oncoprotein

In CBFbeta-SMMHC, core binding factor beta (CBFbeta) is fused to the alpha-helical rod domain of smooth muscle myosin heavy chain (SMMHC). We generated Ba/F3 hematopoietic cells expressing a CBFbeta-SMMHC variant lacking 28 amino acids homologous to the assembly competence domain (ACD) required for multimerization of skeletal muscle myosin. CBFbeta-SMMHC(DeltaACD) multimerized less effectively than either wild-type protein or a variant lacking a different 28-residue segment. In contrast to the control proteins, the DeltaACD mutant did not inhibit CBF DNA binding, AML1-mediated reporter activation, or G(1) to S cell cycle progression, the last being dependent upon activation of CBF-regulated genes. We also linked the CBFbeta domain to 149 or 83 C-terminal CBFbeta-SMMHC residues, retaining 86 or 20 amino acids N-terminal to the ACD. CBFbeta-SMMHC(149C) multimerized and slowed Ba/F3 proliferation, whereas CBFbeta-SMMHC(83C) did not. The majority of CBFbeta-SMMHC and CBFbeta-SMMHC(149C) was detected in the nucleus, whereas the DeltaACD and 83C variants were predominantly cytoplasmic, indicating that multimerization facilitates nuclear retention of CBFbeta-SMMHC. When linked to the simian virus 40 nuclear localization signal (NLS), a significant fraction of CBFbeta-SMMHC(DeltaACD) entered the nucleus but only mildly inhibited CBF activities. As NLS-CBFbeta-SMMHC(83C) remained cytoplasmic, we directed the ACD to CBF target genes by linking it to the AML1 DNA binding domain or to full-length AML1. These AML1-ACD fusion proteins did not affect Ba/F3 proliferation, in contrast to AML1-ETO, which markedly slowed G(1) to S progression dependent upon the integrity of its DNA-binding domain. Thus, the ACD facilitates inhibition of CBF by mediating multimerization of CBFbeta-SMMHC in the nucleus. Therapeutics targeting the ACD may be effective in acute myeloid leukemia cases associated with CBFbeta-SMMHC expression.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Overexpression of AML1 transcription factor drives thymocytes into the CD8 single-positive lineage

To understand the gene regulation involved in the development of single-positive (SP) thymocytes, we generated transgenic mice in which the AML1 transcription factor is overexpressed. In these mice the number of CD8 SP thymocytes was greatly increased, and this continued to be true even when MHC class I was absent. This promotion to the CD8 SP lineage was not, however, observed when both class I and class II were absent. Furthermore, even thymocytes carrying MHC class II-restricted TCR differentiated into the CD8 SP lineage when AML1 was overexpressed. The selected CD8 SP cells were, however, unable to mature, as judged by the expression level of heat-stable Ag. Thus, overexpression of AML1 is able to skew class II-restricted thymocytes into the CD8 SP lineage, but not to drive the maturation of resulting selected CD8 SP cells.

Diminution of the AML1 transcription factor function causes differential effects on the fates of CD4 and CD8 single-positive T cells

In the thymic cortex, T lymphocytes are positively selected to survive and committed either to the CD4 single-positive (SP) or the CD8 SP lineage. The SP cells then pass through a step of maturation in the medulla and are delivered to peripheral lymphoid tissues. We examined the role of AML1, the gene encoding a transcription factor, in the above processes by using the transgenic mice expressing a dominant interfering form of AML1 as well as mice targeted heterozygously for AML1. One phenotypic change seen in the AML1-diminished mice was the reduction in the numbers of both CD4 SP and CD8 SP thymocytes, reflecting the partial impairment of the transition from the double-positive to SP stage. In addition, distinct from the above abnormality, perturbed were several aspects of SP cells, including the maturation of SP thymocytes, the recent thymic emigration, and the proliferative responsiveness of peripheral T cells to TCR stimulation. Interestingly, the AML1 diminution caused inhibitory and enhancing effects on the CD4 SP and CD8 SP cells, respectively. These differential effects are most likely related to the reduction in the peripheral CD4 SP/CD8 SP ratio observed in the AML1-diminished mice. The AML1 transcription factor thus maintains the homeostasis of each SP subset by functioning at the later stages of T lymphocyte differentiation.

Association between the calcium-binding protein calretinin and cytoskeletal components in the human colon adenocarcinoma cell line WiDr

Calretinin (CR) is a Ca(2+)-binding protein (CaBP) of the EF-hand family expressed in a cell-type-specific manner and thought to act as a Ca(2+) buffer. Based upon previous studies, CR can undergo Ca(2+)-induced conformational changes, suggesting that it may also belong to the subfamily of Ca(2+)-sensor proteins that are characterized by their ability to interact with target ligands. To elucidate the role of CR, we used the undifferentiated colon adenocarcinoma cell line WiDr, which expresses significant amounts of CR. It has been shown previously that combined treatment with an inducer of differentiation sodium butyrate (NaBt) and a cell growth inhibitor hexamethylene bisacetamide (HMBA) or treatment with CR antisense oligonucleotides is down-regulating CR in parallel with a decrease of cell growth, suggesting a possible involvement of CR in maintaining the undifferentiated phenotype of WiDr cells. Furthermore, CR is absent from normal colon cells and from well-differentiated colon adenocarcinoma cell lines (e.g., Caco-2). Since members of the EF-hand family of proteins are interacting with cytoskeletal components, we investigated the possible association of CR with the cytoskeleton in WiDr cells. With double immunofluorescence stainings and immunoprecipitation experiments, we show close association of CR with intermediate filaments or microtubules in WiDr cells. Treatment with NaBt either disrupted or strongly diminished this interaction, respectively. The same effect was observed after elevation of [Ca(2+)](i) by applying the ionophore A-23187. These data suggest that CR may contribute to the transformation of enterocytes by interfering with the differentiation process, i.e., acting at both levels: cell shape dynamics and mitosis.

Exogenous cdk4 overcomes reduced cdk4 RNA and inhibition of G1 progression in hematopoietic cells expressing a dominant-negative CBF - a model for overcoming inhibition of proliferation by CBF oncoproteins

Core Binding Factor (CBF) is required for the development of definitive hematopoiesis, and the CBF oncoproteins AML1-ETO, TEL-AML1, and CBFbeta-SMMHC are commonly expressed in subsets of acute leukemia. CBFbeta-SMMHC slows the G1 to S cell cycle transition in hematopoietic cells, but the mechanism of this effect is uncertain. We have sought to determine whether inhibition of CBF-mediated trans-activation is sufficient to slow proliferation. We demonstrate that activation of KRAB-AML1-ER, a protein containing the AML1 DNA-binding domain, the KRAB repression domain, and the Estrogen receptor ligand binding domain, also slows G1, if its DNA-binding domain is intact. Also, exogenous AML1 overcame CBFbeta-SMMHC-induced inhibition of proliferation. Representational difference analysis (RDA) identified cdk4 RNA expression as an early target of KRAB-AML1 activation. Inhibition of CBF activities by KRAB-AML1-ER or CBFbeta-SMMHC rapidly reduced endogenous cdk4 mRNA levels, even in cells proliferating at or near control rates as a result of exogenous cdk4 expression. Over-expression of cdk4, especially a variant which cannot bind p16INK4a, overcame cell cycle inhibition resulting from activation of KRAB-AML1-ER, although cdk4 did not accelerate proliferation when expressed alone. These findings indicate that mutations which alter the expression of G1 regulatory proteins can overcome inhibition of proliferation by CBF oncoproteins. Oncogene (2000).

Indispensable role of the transcription factor PEBP2/CBF in angiogenic activity of a murine endothelial cell MSS31

Mice lacking the AML1/PEBP2alphaB/CBFa2 gene or PEBP2beta/CBFb gene exhibit a defect in definitive hematopoiesis and die in utero because of hemorrhage in the central nervous system. Hematopoiesis in the embryo is considered to be tightly associated with vascular development. Here we examined whether PEBP2/CBF plays any role in angiogenesis besides that in definitive hematopoiesis. We found that AML1/PEBP2alphaB/CBFa2, PEBP2alphaA/CBFa1, and PEBP2beta/CBFb were expressed in a murine endothelial cell line MSS31. The expression of these molecules as well as the DNA binding activity of PEBP2/CBF were augmented by angiogenic growth factors such as bFGF and VEGF. Moreover, the expression of PEBP2 alpha/CBFa protein in endothelial cells was confirmed at the site of angiogenesis in vivo. To further clarify the role of PEBP2/CBF in angiogenesis, we established permanent transfectants of PEBP2 beta-MYH11 gene, one that interacts with the runt domain of the alpha subunit and deregulates PEBP2/CBF in a dominant interfering manner. Proliferation, migration, and tube formation of the PEBP2 beta-MYH11 transfectants were significantly reduced in comparison with those activities of the mock transfectants. These results suggest that transcription factor PEBP2/CBF plays an important role in angiogenesis.

MSF (MLL septin-like fusion), a fusion partner gene of MLL, in a therapy-related acute myeloid leukemia with a t(11;17)(q23;q25)

MLL (ALL1, Htrx, HRX), which is located on chromosome band 11q23, frequently is rearranged in patients with therapy-related acute myeloid leukemia who previously were treated with DNA topoisomerase II inhibitors. In this study, we have identified a fusion partner of MLL in a 10-year-old female who developed therapy-related acute myeloid leukemia 17 months after treatment for Hodgkin's disease. Leukemia cells of this patient had a t(11;17)(q23;q25), which involved MLL as demonstrated by Southern blot analysis. The partner gene was cloned from cDNA of the leukemia cells by use of a combination of adapter reverse transcriptase-PCR, rapid amplification of 5' cDNA ends, and BLAST database analysis to identify expressed sequence tags. The full-length cDNA of 2.8 kb was found to be an additional member of the septin family, therefore it was named MSF (MLL septin-like fusion). Members of the septin family conserve the GTP binding domain, localize in the cytoplasm, and interact with cytoskeletal filaments. A major 4-kb transcript of MSF was expressed ubiquitously; a 1.7-kb transcript was found in most tissues. An additional 3-kb transcript was found only in hematopoietic tissues. By amplification with MLL exon 5 forward primer and reverse primers in MSF, the appropriately sized products were obtained. MSF is highly homologous to hCDCrel-1, which is a partner gene of MLL in leukemias with a t(11;22)(q23;q11.2). Further analysis of MSF may help to delineate the function of MLL partner genes in leukemia, particularly in therapy-related leukemia.

The leukemic protein core binding factor beta (CBFbeta)-smooth-muscle myosin heavy chain sequesters CBFalpha2 into cytoskeletal filaments and aggregates

The fusion gene CBFB-MYH11 is generated by the chromosome 16 inversion associated with acute myeloid leukemias. This gene encodes a chimeric protein involving the core binding factor beta (CBFbeta) and the smooth-muscle myosin heavy chain (SMMHC). Mouse model studies suggest that this chimeric protein CBFbeta-SMMHC dominantly suppresses the function of CBF, a heterodimeric transcription factor composed of DNA binding subunits (CBFalpha1 to 3) and a non-DNA binding subunit (CBFbeta). This dominant suppression results in the blockage of hematopoiesis in mice and presumably contributes to leukemogenesis. We used transient-transfection assays, in combination with immunofluorescence and green fluorescent protein-tagged proteins, to monitor subcellular localization of CBFbeta-SMMHC, CBFbeta, and CBFalpha2 (also known as AML1 or PEBP2alphaB). When expressed individually, CBFalpha2 was located in the nuclei of transfected cells, whereas CBFbeta was distributed throughout the cell. On the other hand, CBFbeta-SMMHC formed filament-like structures that colocalized with actin filaments. Upon cotransfection, CBFalpha2 was able to drive localization of CBFbeta into the nucleus in a dose-dependent manner. In contrast, CBFalpha2 colocalized with CBFbeta-SMMHC along the filaments instead of localizing to the nucleus. Deletion of the CBFalpha-interacting domain within CBFbeta-SMMHC abolished this CBFalpha2 sequestration, whereas truncation of the C-terminal-end SMMHC domain led to nuclear localization of CBFbeta-SMMHC when coexpressed with CBFalpha2. CBFalpha2 sequestration by CBFbeta-SMMHC was further confirmed in vivo in a knock-in mouse model. These observations suggest that CBFbeta-SMMHC plays a dominant negative role by sequestering CBFalpha2 into cytoskeletal filaments and aggregates, thereby disrupting CBFalpha2-mediated regulation of gene expression.

Smad2 overexpression enhances Smad4 gene expression and suppresses CBFA1 gene expression in osteoblastic osteosarcoma ROS17/2.8 cells and primary rat calvaria cells

Mothers against decapentaplegic-related proteins (Smads) are essential intracellular components for the signal transduction of transforming growth factor-beta (TGF-beta) family members. Smad1 mediates bone morphogenetic protein (BMP) signals, whereas Smad2 functions downstream of TGF-beta. TGF-beta is expressed in osteoblastic cells and acts as an autocrine and/or paracrine factor in regulation of osteoblastic functions. In this study, we examined the levels and functions of Smad2 in osteoblastic cells. Smad2 mRNA expression was hardly detectable by Northern blot analysis in an osteoblast-like cell line, ROS17/2.8, as well as in primary rat calvaria (PRC) cells. Overexpression of Smad2 gene enhanced endogenous Smad4 gene expression in both ROS17/2.8 and PRC cells, while Smad3 levels were not altered. Smad2 overexpression suppressed osteocalcin mRNA expression in ROS17/2.8 cells. Furthermore, Smad2 overexpression also suppressed transcriptional activity of the 1-kilobase pair osteocalcin gene promoter, which was linked to chloramphenicol acetyltransferase reporter gene in both ROS and PRC cells. Since core binding factor A1 (CBFA1) is involved in osteocalcin gene expression, we further examined CBFA1 expression in the Smad2-overexpressing ROS17/2.8 and PRC cells. The levels of CBFA1 mRNA were suppressed by the overexpression of Smad2 by about 50% in both ROS17/2.8 and PRC cells. TGF-beta treatment enhanced Smad4 expression in PRC cells, and this TGF-beta effect was blocked by the cotreatment with BMP, indicating that TGF-beta signaling pathway is interfered by BMP. These data indicate that Smad2 regulates Smad4 specifically and that CBFA1 gene is one of the downstream targets of Smad2.

Involvement of actin microfilaments in the replication of human parainfluenza virus type 3

Several studies indicate that paramyxoviruses require a specific cellular factor(s) for transcription of their genomic RNAs. We previously reported that the cellular cytoskeletal protein actin, in its polymeric form, participates in the transcription of human parainfluenza virus type 3 (HPIV3) in vitro. In the present study, we investigated the role of the polymeric form of actin, i.e., the actin microfilaments of the cytoskeletal framework, in the reproduction of HPIV3 in vivo. Pulse-chase labeling analyses indicate that the viral nucleocapsid-associated proteins, NP and P, are present predominantly in the cytoskeletal framework during infection. By in situ hybridization, we found that viral mRNAs and genomic RNA were synthesized from the nucleocapsids that were bound to the cytoskeletal framework. Double immunofluorescent labeling and confocal microscopy of the cytoarchitecture revealed that the viral nucleocapsids are specifically localized on the actin microfilaments. Treatment of cells with the actin-depolymerizing agent, cytochalasin D, resulted in the inhibition of viral RNA synthesis and ribonucleoprotein accumulation. These results strongly suggest that actin microfilaments play an important role in the replication of HPIV3.