Contribution of increased ISG15, ISGylation and deregulated type I IFN signaling in Usp18 mutant mice during the course of bacterial infections.

Host genetics has a key role in susceptibility to Salmonella Typhimurium infection. We previously used N-ethyl-N-nitrosourea (ENU) mutagenesis to identify a loss-of-function mutation within the gene ubiquitin-specific peptidase 18 (Usp18(Ity9)), which confers increased susceptibility to Salmonella Typhimurium. USP18 functions to regulate type I interferon (IFN) signaling and as a protease to remove ISG15 from substrate proteins. Usp18(Ity9) mice are susceptible to infection with Salmonella Typhimurium and have increased expression and function of ISG15, but Usp18(Ity9) mice lacking Isg15 do not show improved survival with Salmonella challenge. Type I IFN signaling is increased in Usp18(Ity9) mice and inhibition of type I IFN signaling is associated with improved survival in mutant mice. Hyperactivation of type I IFN signaling leads to increased IL-10, deregulated expression of autophagy markers and elevated interleukin (IL)-1ß and IL-17. Furthermore, Usp18(Ity9) mice are more susceptible to infection with Mycobacterium tuberculosis, have increased bacterial load in the lung and spleen, elevated inflammatory cytokines and more severe lung pathology. These findings demonstrate that regulation of type I IFN signaling is the predominant mechanism affecting the susceptibility of Usp18(Ity9) mice to Salmonella infection and that hyperactivation of signaling leads to increased IL-10, deregulation of autophagic markers and increased proinflammatory cytokine production.

Embryonic lethality and impairment of haematopoiesis in mice heterozygous for an AML1-ETO fusion gene.

Acute myeloid leukaemia (AML) is a major haematopoietic malignancy characterized by the proliferation of a malignant clone of myeloid progenitor cells. A reciprocal translocation, t(8;21)(q22;q22), observed in the leukaemic cells of approximately 40% of patients with the M2 subtype of AML disrupts both the AML1 (CBFA2) gene on chromosome 21 and the ETO (MTG8) gene on chromosome 8 (refs 3-5). A chimaeric protein is synthesized from one of the derivative chromosomes that contains the N terminus of the AML1 transcription factor, including its DNA-binding domain, fused to most of ETO, a protein of unknown function. We generated mice that mimic human t(8;21) with a "knock-in' strategy. Mice heterozygous for an AML1-ETO allele (AML1-ETO/+) die in midgestation from haemorrhaging in the central nervous system and exhibit a severe block in fetal liver haematopoiesis. This phenotype is very similar to that resulting from homozygous disruption of the AML1 (Cbfa2) or Cbfb genes, indicating that AML1-ETO blocks normal AML1 function. However, yolk sac cells from AML1-ETO/+ mice differentiated into macrophages in haematopoietic colony forming unit (CFU) assays, unlike Cbfa2-/- or Cbfb-/-cells, which form no colonies in vitro. This indicates that AML1-ETO may have other functions besides blocking wild-type AML1, a property that may be important in leukaemogenesis.

Measuring population differentiation using GST or D? A simulation study with microsatellite DNA markers under a finite island model and nonequilibrium conditions.

The genetic differentiation of populations is a key parameter in population genetic investigations. Wright's F(ST) (and its relatives such as G(ST) ) has been a standard measure of differentiation. However, the deficiencies of these indexes have been increasingly realized in recent years, leading to some new measures being proposed, such as Jost's D (Molecular Ecology, 2008; 17, 4015). The existence of these new metrics has stimulated considerable debate and induced some confusion on which statistics should be used for estimating population differentiation. Here, we report a simulation study with neutral microsatellite DNA loci under a finite island model to compare the performance of G(ST) and D, particularly under nonequilibrium conditions. Our results suggest that there exist fundamental differences between the two statistics, and neither G(ST) nor D operates satisfactorily in all situations for quantifying differentiation. D is very sensitive to mutation models but G(ST) noticeably less so, which limits D's utility in population parameter estimation and comparisons across genetic markers. Also, the initial heterozygosity of the starting populations has some important effects on both the individual behaviours of G(ST) and D and their relative behaviours in early differentiation, and this effect is much greater for D than G(ST) . In the early stages of differentiation, when initial heterozygosity is relatively low (<0.5, if the number of subpopulations is large), G(ST) increases faster than D; the opposite is true when initial heterozygosity is high. Therefore, the state of the ancestral population appears to have some lasting impacts on population differentiation. In general, G(ST) can measure differentiation fairly well when heterozygosity is low whatever the causes; however, when heterozygosity is high (e.g. as a result of either high mutation rate or high initial heterozygosity) and gene flow is moderate to strong, G(ST) fails to measure differentiation. Interestingly, when population size is not very small (e.g. N ≥ 1000), G(ST) measures differentiation quite linearly with time over a long duration when gene flow is absent or very weak even if mutation rate is not low (e.g. µ = 0.001). In contrast, D, as a differentiation measure, performs rather robustly in all these situations. In practice, both indexes should be calculated and the relative levels of heterozygosities (especially H(S) ) and gene flow taken into account. We suggest that a comparison of the two indexes can generate useful insights into the evolutionary processes that influence population differentiation.

AML1-ETO downregulates the granulocytic differentiation factor C/EBPalpha in t(8;21) myeloid leukemia.

The transcription factor CCAAT/enhancer binding protein alpha, or C/EBPalpha, encoded by the CEBPA gene, is crucial for the differentiation of granulocytes. Conditional expression of C/EBPalpha triggers neutrophilic differentiation, and Cebpa knockout mice exhibit an early block in maturation. Dominant-negative mutations of CEBPA have been found in some patients with acute myeloid leukemia (AML), but not in AML with the t(8;21) translocation which gives rise to the fusion gene RUNX1-CBF2T1 (also known as AML1-ETO) encoding the AML1-ETO fusion protein. RUNX1-CBF2T1 positive-AML blasts had eight-fold lower CEBPA RNA levels and undetectable C/EBPalpha protein levels compared with other subgroups of AML patients. Conditional expression of RUNX1-CBF2T1 in U937 cells downregulated CEBPA mRNA, protein and DNA binding activity. AML1-ETO appears to suppress C/EBPalpha expression indirectly by inhibiting positive autoregulation of the CEBPA promoter. Conditional expression of C/EBPalpha in AML1-ETO-positive Kasumi-1 cells results in neutrophilic differentiation. We suggest that restoring C/EBPalpha expression will have therapeutic implications in RUNX1-CBF2T1-positive leukemias.

The multi-functional cellular adhesion molecule CD44 is regulated by the 8;21 chromosomal translocation.

The 8;21 translocation is a common chromosomal abnormality in acute myeloid leukemia (AML). We recently identified a naturally occurring leukemogenic splice variant, AML1-ETO9a (acute myeloid leukemia-1 transcription factor and the eight-twenty-one corepressor-9a), of t(8;21). To understand the leukemic potential of AML1-ETO9a, we performed microarray analysis with the murine multipotential hematopoietic FDCP-mix A4 cell line. We identified changes in expression of various genes including CD44. CD44 is a type I transmembrane protein and functions as the major cellular adhesion molecule for hyaluronic acid, a component of the extracellular matrix. CD44 is expressed in most human cell types and is implicated in myeloid leukemia pathogenesis. We show that the presence of AML1-ETO9a significantly increased the expression of CD44 at both RNA and protein levels. Furthermore, the CD44 promoter is bound by AML1-ETO9a and AML1-ETO at the chromatin level. In addition, in the AML1-ETO9a leukemia mouse model CD44 is regulated in a cell context-dependent manner. Thus, our observations suggest that AML1-ETO and its splice variant AML1-ETO9a are able to regulate the expression of the CD44 gene, linking the 8;21 translocation to the regulation of a cell adhesion molecule that is involved in the growth and maintenance of the AML blast/stem cells.

The macrophage transcription factor PU.1 directs tissue-specific expression of the macrophage colony-stimulating factor receptor.

The macrophage colony-stimulating factor (M-CSF) receptor is expressed in a tissue-specific fashion from two distinct promoters in monocytes/macrophages and the placenta. In order to further understand the transcription factors which play a role in the commitment of multipotential progenitors to the monocyte/macrophage lineage, we have initiated an investigation of the factors which activate the M-CSF receptor very early during the monocyte differentiation process. Here we demonstrate that the human monocytic M-CSF receptor promoter directs reporter gene activity in a tissue-specific fashion. Since one of the few transcription factors which have been implicated in the regulation of monocyte genes is the macrophage- and B-cell-specific PU.1 transcription factor, we investigated whether PU.1 binds and activates the M-CSF receptor promoter. Here we demonstrate that both in vitro-translated PU.1 and PU.1 from nuclear extracts bind to a specific site in the M-CSF receptor promoter just upstream from the major transcription initiation site. Mutations in this site which eliminate PU.1 binding decrease M-CSF receptor promoter activity significantly in macrophage cell lines only. Furthermore, PU.1 transactivates the M-CSF receptor promoter in nonmacrophage cells. These results suggest that PU.1 plays a major role in macrophage gene regulation and development by directing the expression of a receptor for a key macrophage growth factor.

Multiple functional domains of AML1: PU.1 and C/EBPalpha synergize with different regions of AML1.

Control elements of many genes are regulated by multiple activators working in concert to confer the maximal level of expression, but the mechanism of such synergy is not completely understood. The promoter of the human macrophage colony-stimulating factor (M-CSF) receptor presents an excellent model with which we can study synergistic, tissue-specific activation for two reasons. First, myeloid-specific expression of the M-CSF receptor is regulated transcriptionally by three factors which are crucial for normal hematopoiesis: PU.1, AML1, and C/EBPalpha. Second, these proteins interact in such a way as to demonstrate at least two examples of synergistic activation. We have shown that AML1 and C/EBPalpha activate the M-CSF receptor promoter in a synergistic manner. As we report here, AML1 also synergizes, and interacts physically, with PU. 1. Detailed analysis of the physical and functional interaction of AML1 with PU.1 and C/EBPalpha has revealed that the proteins contact one another through their DNA-binding domains and that AML1 exhibits cooperative DNA binding with C/EBPalpha but not with PU.1. This difference in DNA-binding abilities may explain, in part, the differences observed in synergistic activation. Furthermore, the activation domains of all three factors are required for synergistic activation, and the region of AML1 required for synergy with PU.1 is distinct from that required for synergy with C/EBPalpha. These observations present the possibility that synergistic activation is mediated by secondary proteins contacted through the activation domains of AML1, C/EBPalpha, and PU.1.

PU.1 (Spi-1) and C/EBP alpha regulate expression of the granulocyte-macrophage colony-stimulating factor receptor alpha gene.

Growth factor receptors play an important role in hematopoiesis. In order to further understand the mechanisms directing the expression of these key regulators of hematopoiesis, we initiated a study investigating the transcription factors activating the expression of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor alpha gene. Here, we demonstrate that the human GM-CSF receptor alpha promoter directs reporter gene activity in a tissue-specific fashion in myelomonocytic cells, which correlates with its expression pattern as analyzed by reverse transcription PCR. The GM-CSF receptor alpha promoter contains an important functional site between positions -53 and -41 as identified by deletion analysis of reporter constructs. We show that the myeloid and B cell transcription factor PU.1 binds specifically to this site. Furthermore, we demonstrate that a CCAAT site located upstream of the PU.1 site between positions -70 and -54 is involved in positive-negative regulation of the GM-CSF receptor alpha promoter activity. C/EBP alpha is the major CCAAT/enhancer-binding protein (C/EBP) form binding to this site in nuclear extracts of U937 cells. Point mutations of either the PU.1 site or the C/EBP site that abolish the binding of the respective factors result in a significant decrease of GM-CSF receptor alpha promoter activity in myelomonocytic cells only. Furthermore, we demonstrate that in myeloid and B cell extracts, PU.1 forms a novel, specific, more slowly migrating complex (PU-SF) when binding the GM-CSF receptor alpha promoter PU.1 site. This is the first demonstration of a specific interaction with PU.1 on a myeloid PU.1 binding site. The novel complex is distinct from that described previously as binding to B cell enhancer sites and can be formed by addition of PU.1 to extracts from certain nonmyeloid cell types which do not express PU.1, including T cells and epithelial cells, but not from erythroid cells. Furthermore, we demonstrate that the PU-SF complex binds to PU.1 sites found on a number of myeloid promoters, and its formation requires an intact PU.1 site adjacent to a single-stranded region. Expression of PU.1 in nonmyeloid cells can activate the GM-CSF receptor alpha promoter. Deletion of the amino-terminal region of PU.1 results in a failure to form the PU-SF complex and in a concomitant loss of transactivation, suggesting that formation of the PU-SF complex is of functional importance for the activity of the GM-CSF receptor alpha promoter. Finally, we demonstrate that C/EBP alpha can also active the GM-CSF receptor alpha promoter in nonmyeloid cells. These results suggest that PU.1 and C/EBP alpha direct the cell-type-specific expression of GM-CSF receptor alpha, further establish the role of PU.1 as a key regulator of hematopoiesis, and point to C/EBP alpha as an additional important factor in this process.

Dichotomy of AML1-ETO functions: growth arrest versus block of differentiation.

The fusion gene AML1-ETO is the product of t(8;21)(q22;q22), one of the most common chromosomal translocations associated with acute myeloid leukemia. To investigate the impact of AML1-ETO on hematopoiesis, tetracycline-inducible AML1-ETO-expressing cell lines were generated using myeloid cells. AML1-ETO is tightly and strongly induced upon tetracycline withdrawal. The proliferation of AML1-ETO(+) cells was markedly reduced, and most of the cells eventually underwent apoptosis. RNase protection assays revealed that the amount of Bcl-2 mRNA was decreased after AML1-ETO induction. Enforced expression of Bcl-2 was able to significantly delay, but not completely overcome, AML1-ETO-induced apoptosis. Prior to the onset of apoptosis, we also studied the ability of AML1-ETO to modulate differentiation. AML1-ETO expression altered granulocytic differentiation of U937T-A/E cells. More significantly, this change of differentiation was associated with the down-regulation of CCAAT/enhancer binding protein alpha (C/EBPalpha), a key regulator of granulocytic differentiation. These observations suggest a dichotomy in the functions of AML1-ETO: (i) reduction of granulocytic differentiation correlated with decreased expression of C/EBPalpha and (ii) growth arrest leading to apoptosis with decreased expression of CDK4, c-myc, and Bcl-2. We predict that the preleukemic AML1-ETO(+) cells must overcome AML1-ETO-induced growth arrest and apoptosis prior to fulfilling their leukemogenic potential.

CCAAT enhancer-binding protein (C/EBP) and AML1 (CBF alpha2) synergistically activate the macrophage colony-stimulating factor receptor promoter.

Transcription factors play a key role in the development and differentiation of specific lineages from multipotential progenitors. Identification of these regulators and determining the mechanism of how they activate their target genes are important for understanding normal development of monocytes and macrophages and the pathogenesis of a common form of adult acute leukemia, in which the differentiation of monocytic cells is blocked. Our previous work has shown that the monocyte-specific expression of the macrophage colony-stimulating factor (M-CSF) receptor is regulated by three transcription factors interacting with critical regions of the M-CSF receptor promoter, including PU.1 and AML1.PU.1 is essential for myeloid cell development, while the AML1 gene is involved in several common leukemia-related chromosome translocations, although its role in hematopoiesis has not been fully identified. Along with AML1, a third factor, Mono A, interacts with a small region of the promoter which can function as a monocyte-specific enhancer when multimerized and linked to a heterologous basal promoter. Here, we demonstrate by electrophoretic mobility shift assays with monocytic nuclear extracts, COS-7 cell-transfected factors, and specific antibodies that the monocyte-enriched factor Mono A is CCAAT enhancer-binding protein (C/EBP). C/EBP has been shown previously to be an important transcription factor involved in hepatocyte and adipocyte differentiation; in hematopoietic cells, C/EBP is specifically expressed in myeloid cells. In vitro binding analysis reveals a physical interaction between C/EBP and AML1. Further transfection studies show that C/EBP and AML1 in concert with the AML1 heterodimer partner CBF beta synergistically activate M-CSF receptor by more then 60 fold. These results demonstrate that C/EBP and AML1 are important factors for regulating a critical hematopoietic growth factor receptor, the M-CSF receptor, suggesting a mechanism of how the AML1 fusion protein could contribute to acute myeloid leukemia. Furthermore, they demonstrate physical and functional interactions between AML1 and C/EBP transcription factor family members.

Identification of a region which directs the monocytic activity of the colony-stimulating factor 1 (macrophage colony-stimulating factor) receptor promoter and binds PEBP2/CBF (AML1).

The receptor for the macrophage colony-stimulating factor (or colony-stimulating factor 1 [CSF-1]) is expressed from different promoters in monocytic cells and placental trophoblasts. We have demonstrated that the monocyte-specific expression of the CSF-1 receptor is regulated at the level of transcription by a tissue-specific promoter whose activity is stimulated by the monocyte/B-cell-specific transcription factor PU.1 (D.-E. Zhang, C.J. Hetherington, H.-M. Chen, and D.G. Tenen, Mol. Cell. Biol. 14:373-381, 1994). Here we report that the tissue specificity of this promoter is also mediated by sequences in a region II (bp -88 to -59), which lies 10 bp upstream from the PU.1-binding site. When analyzed by DNase footprinting, region II was protected preferentially in monocytic cells. Electrophoretic mobility shift assays confirmed that region II interacts specifically with nuclear proteins from monocytic cells. Two gel shift complexes (Mono A and Mono B) were formed with separate sequence elements within this region. Competition and supershift experiments indicate that Mono B contains a member of the polyomavirus enhancer-binding protein 2/core-binding factor (PEBP2/CBF) family, which includes the AML1 gene product, while Mono A is a distinct complex preferentially expressed in monocytic cells. Promoter constructs with mutations in these sequence elements were no longer expressed specifically in monocytes. Furthermore, multimerized region II sequence elements enhanced the activity of a heterologous thymidine kinase promoter in monocytic cells but not other cell types tested. These results indicate that the monocyte/B-cell-specific transcription factor PU.1 and the Mono A and Mono B protein complexes act in concert to regulate monocyte-specific transcription of the CSF-1 receptor.

A novel ubiquitin-specific protease, UBP43, cloned from leukemia fusion protein AML1-ETO-expressing mice, functions in hematopoietic cell differentiation.

Using PCR-coupled subtractive screening-representational difference analysis, we have cloned a novel gene from AML1-ETO knockin mice. This gene is highly expressed in the yolk sac and fetal liver of the knockin mice. Nucleotide sequence analysis indicates that its cDNA contains an 1,107-bp open reading frame encoding a 368-amino-acid polypeptide. Further protein sequence and protein translation analysis shows that it belongs to a family of ubiquitin-specific proteases (UBP), and its molecular mass is 43 kDa. Therefore, we have named this gene UBP43. Like other ubiquitin proteases, the UBP43 protein has deubiquitinating enzyme activity. Protein ubiquitination has been implicated in many important cellular events. In wild-type adult mice, UBP43 is highly expressed in the thymus and in peritoneal macrophages. Among nine different murine hematopoietic cell lines analyzed, UBP43 expression is detectable only in cell lines related to the monocytic lineage. Furthermore, its expression is regulated during cytokine-induced monocytic cell differentiation. We have investigated its function in the hematopoietic myeloid cell line M1. UBP43 was introduced into M1 cells by retroviral gene transfer, and several high-expressing UBP43 clones were obtained for further study. Morphologic and cell surface marker examination of UBP43/M1 cells reveals that overexpression of UBP43 blocks cytokine-induced terminal differentiation of monocytic cells. These data suggest that UBP43 plays an important role in hematopoiesis by modulating either the ubiquitin-dependent proteolytic pathway or the ubiquitination state of another regulatory factor(s) during myeloid cell differentiation.

Restoration of MYC-repressed targets mediates the negative effects of GM-CSF on RUNX1-ETO leukemogenicity.

Granulocyte macrophage-colony-stimulating factor (GM-CSF) signaling regulates hematopoiesis and immune responses. CSF2RA, the gene encoding the α-subunit for GM-CSF, is significantly downregulated in t(8;21) (RUNX1-ETO or RE) leukemia patients, suggesting that it may serve as a tumor suppressor. We previously reported that GM-CSF signaling is inhibitory to RE leukemogenesis. Here we conducted gene expression profiling of primary RE hematopoietic stem/progenitor cells (HSPCs) treated with GM-CSF to elucidate the mechanisms mediating the negative effects of GM on RE leukemogenicity. We observed that GM treatment of RE HSPCs resulted in a unique gene expression profile that resembles primary human cells undergoing myelopoiesis, which was not observed in control HSPCs. Additionally, we discovered that GM-CSF signaling attenuates MYC-associated gene signatures in RE HSPCs. In agreement with this, a functional screen of a subset of GM-CSF-responsive genes demonstrated that a MYC inhibitor, MXI1 (Max interactor 1), reduced the leukemic potential of RE HSPCs and t(8;21) acute myeloid leukemia (AML) cells. Furthermore, MYC knockdown and treatment with the BET (bromodomain and extra terminal domain) inhibitor JQ1 reduced the leukemic potential of t(8;21) cell lines. Altogether, we discovered a novel molecular mechanism mediating the GM-CSF-induced reduction in leukemic potential of RE cells, and our findings support MYC inhibition as an effective strategy for reducing the leukemogenicity of t(8;21) AML.

Fatty acid-binding protein FABP4 mechanistically links obesity with aggressive AML by enhancing aberrant DNA methylation in AML cells.

Obesity is becoming more prevalent worldwide and is a major risk factor for cancer development. Acute myeloid leukemia (AML), the most common acute leukemia in adults, remains a frequently fatal disease. Here we investigated the molecular mechanisms by which obesity favors AML growth and uncovered the fatty acid-binding protein 4 (FABP4) and DNA methyltransferase 1 (DNMT1) regulatory axis that mediates aggressive AML in obesity. We showed that leukemia burden was much higher in high-fat diet-induced obese mice, which had higher levels of FABP4 and interleukin (IL)-6 in the sera. Upregulation of environmental and cellular FABP4 accelerated AML cell growth in both a cell-autonomous and cell-non-autonomous manner. Genetic disruption of FABP4 in AML cells or in mice blocked cell proliferation in vitro and induced leukemia regression in vivo. Mechanistic investigations showed that FABP4 upregulation increased IL-6 expression and signal transducer and activator of transcription factor 3 phosphorylation leading to DNMT1 overexpression and further silencing of the p15INK4B tumor-suppressor gene in AML cells. Conversely, FABP4 ablation reduced DNMT1-dependent DNA methylation and restored p15INK4B expression, thus conferring substantial protection against AML growth. Our findings reveal the FABP4/DNMT1 axis in the control of AML cell fate in obesity and suggest that interference with the FABP4/DNMT1 axis might be a new strategy to treat leukemia.

PU.1 (Spi-1) autoregulates its expression in myeloid cells.

PU.1 (Spi-1), a member of the Ets transcription factor family, is predominantly expressed in myeloid (granulocytes, monocytes and macrophages) and B cells. PU.1 is upregulated early during commitment of multipotential progenitors to the myeloid lineages and inhibition of PU.1 function in human CD34+ progenitors prior to this upregulation blocks myeloid colony formation. Since PU.1 expression appears to play a role in hematopoietic development, we characterized the PU.1 promoter. Here we report that the murine PU.1 promoter, as well as the human promoter, demonstrate tissue-specific reporter gene expression in myeloid cell lines but not in T cells and HeLa (non-hematopoietic cells) cells. Deletion analysis of the PU.1 promoter indicates that tissue-specific functional elements are encoded in the -61 to -39 bp and -7 to +34 bp regions. The first region contains a functional octamer (Oct) site at -54 bp and an Sp1 site at -39 bp. The second contains a binding site at +20 bp for both PU.1 itself and the related ets family member Spi-B. In vivo footprinting assays demonstrate that a hypersensitive band was detected at the PU.1 site in myeloid cells but not in HeLa. A mutation of the PU.1 site which abolished PU.1 binding caused a significant decrease in promoter activity. Mutation of the Oct and/or Sp1 site results in a lesser decrease of promoter activity in myeloid cells. Co-transfection of PU.1 or Spi-B in cells lacking PU.1 and Spi-B specifically transactivated a minimal promoter containing the PU.1 binding site, indicating that PU.1 can activate its own promoter elements in an autoregulatory loop. Positive autoregulation of the PU.1 promoter may play an important role in the function of PU.1 in myeloid cells.

Analysis of the mechanism of glucocorticoid-mediated down regulation of the mouse alpha-fetoprotein gene.

Regulation of alpha-fetoprotein gene expression by dexamethasone was examined in vivo and in vitro using primary mouse fetal liver cell cultures. Dexamethasone accelerates the developmental down regulation of AFP mRNA pools. However, treatment of primary fetal liver cells in culture does not reduce the AFP mRNA pool and may stabilize both AFP and albumin gene expression. These results indicate that in vivo the effect of dexamethasone may require interaction with another tissue or cell type. The mechanism of the dexamethasone mediated inhibition of AFP was examined by DNase I footprinting and transient expression assays. Two protein-binding regions of the proximal promoter (III and IV) show significant homology to the GRE consensus sequence. DNase I footprinting shows that only region IV can bind purified GR and competition with GRE oligonucleotides indicate that, using adult liver nuclear proteins, no GR is bound in either region. Nuclear protein from adrenalectomized mice show the same protection as controls. These results indicate that GR may not bind to the AFP proximal promoter in the adult. AFP promoter-CAT expression vectors were used to further examine the effect of dexamethasone on AFP expression. AFP promoter-CAT constructs were inhibited by 10(-6) M dexamethasone; while linking of an AFP enhancer to the promoter abolished the effect. We conclude that the in vitro effects on transiently expressed AFP directed expression vectors may be a function of vector structure and/or characteristics of the cells used whereas the in vivo effect may reflect normal regulatory mechanisms.

Rearrangement of the AML1/CBFA2 gene in myeloid leukemia with the 3;21 translocation: expression of co-existing multiple chimeric genes with similar functions as transcriptional repressors, but with opposite tumorigenic properties.

Several recurring chromosomal translocations involve the AML1 gene at 21q22 in myeloid leukemias resulting in fusion mRNAs and chimeric proteins between AML1 and a gene on the partner chromosome. AML1 corresponds to CBFA2, one of the DNA-binding subunits of the enhancer core binding factor CBF. Other CBF DNA-binding subunits are CBFA1 and CBFA3, also known as AML3 and AML2. AML1, AML2 and AML3 are each characterized by a conserved domain at the amino end, the runt domain, that is necessary for DNA-binding and protein dimerization, and by a transactivation domain at the carboxyl end. AML1 was first identified as the gene located at the breakpoint junction of the 8;21 translocation associated with acute myeloid leukemia. The t(8;21)(q22;q22) interrupts AML1 after the runt homology domain, and fuses the 5' part of AML1 to almost all of ETO, the partner gene on chromosome 8. AML1 is an activator of several myeloid promoters; however, the chimeric AML1/ETO is a strong repressor of some AML1-dependent promoters. AML1 is also involved in the t(3;21)(q26;q22), that occurs in myeloid leukemias primarily following treatment with topoisomerase II inhibitors. We have studied five patients with a 3;21 translocation. In all cases, AML1 is interrupted after the runt domain, and is translocated to chromosome band 3q26. As a result of the t(3;21), AML1 is consistently fused to two separate genes located at 3q26. The two genes are EAP, which codes for the abundant ribosomal protein L22, and MDS1, which encodes a small polypeptide of unknown function. In one of our patients, a third gene EVI1 is also involved. EAP is the closest to the breakpoint junction with AML1, and EVI1 is the furthest away. The fusion of EAP to AML1 is not in frame, and leads to a protein that is terminated shortly after the fusion junction by introduction of a stop codon. The fusion of AML1 to MDS1 is in frame, and adds 127 codons to the interrupted AML1. Thus, in the five cases that we studied, the 3;21 translocation results in expression of two coexisting chimeric mRNAs which contain the identical runt domain at the 5' region, but differ in the 3' region. In addition, the chimeric transcript AML1/MDS1/EVI1 has also been detected in cells from one patient with the 3;21 translocation as well as in one of our patients. Several genes necessary for myeloid lineage differentiation contain the target sequence for AML1 in their regulatory regions. One of them is the CSF1R gene. We have compared the normal AML1 to AML1/MDS1, AML1/EAP and AML1/MDS1/EVI1 as transcriptional regulators of the CSF1R promoter. Our results indicate that AML1 can activate the promoter, and that the chimeric proteins compete with the normal AML1 and repress expression from the CSF1R promoter. AML1/MDS1 and AML1/EAP affect cell growth and phenotype when expressed in rat fibroblasts. However, the pattern of tumor growth of cells expressing the different chimeric genes in nude mice is different. We show that when either fusion gene is expressed, the cells lose contact inhibition and form foci over the monolayer. In addition, cells expressing AML1/MDS1 grow larger tumors in nude mice, whereas cells expressing only AML1/EAP do not form tumors, and cells expressing both chimeric genes induce tumors of intermediate size. Thus, although both chimeric genes have similar effects in transactivation assays of the CSF1R promoter, they affect cell growth differently in culture and have opposite effects as tumor promoters in vivo. Because of the results obtained with cells expressing one or both genes, we conclude that MDS1 seems to have tumorigenic properties, but that AML1/EAP seems to repress the oncogenic property of AML1/MDS1.

Function of PU.1 (Spi-1), C/EBP, and AML1 in early myelopoiesis: regulation of multiple myeloid CSF receptor promoters.

Our studies of the promoters of the myeloid CSF receptors (M, GM, and G) in cell lines have led to the findings that the promoters are small, and are all activated by the PU.1 and C/EBP proteins. To date, we have only found evidence for involvement of C/EBP alpha, although further experiments will be needed to exclude the role of C/EBP beta and C/EBP delta in receptor gene expression. These studies suggest a model of hematopoiesis (Fig. 2) in which the lineage commitment decisions of multipotential cells are made by the alternative patterns of expression of certain transcription factors, which then activate growth factor receptors which allow those cells to respond to the appropriate growth factor to proliferate and survive. For example, expression of GATA-1 activates its own expression, as well as that of the erythropoietin receptor, inducing these cells to be capable of responding to erythropoietin. Similarly, expression of PU.1 activates its own promoter, and turns on the three myeloid CSF receptors (M, GM, and G), pushing these cells along the pathway of myeloid differentiation. C/EBP proteins, particularly C/EBP alpha, are also critical for myeloid receptor promoter function, and may also act via autoregulatory mechanisms. Murine C/EBP alpha has a C/EBP binding site in its own promoter. Human C/EBP alpha autoregulates its own expression in adipocytes by activating the USF transcription factor. Myeloid genes expressed later during differentiation, such as CD11b, are also activated by PU.1, which is expressed at highest levels in mature myeloid cells, but not by C/EBP alpha, which is downregulated in a differentiated murine myeloid cell line. Consistent with this model are the findings that overexpression of PU.1 in erythroid cells blocks erythroid differentiation, leading to erythroleukemia, and overexpression of GATA-1 in a myeloid line blocks myeloid differentiation. While these findings have provided some framework for understanding myeloid gene regulation, there are a number of critical questions to be addressed in the near future: What is the pattern of expression of the C/EBP proteins during the course of myeloid differentiation and activation of human CD34+ cells? What is the effect of targeted disruption and other mutations of the C/EBP and AML1 proteins on myeloid development and receptor expression? What are the interactions among these three different types of factors (ets, basic region-zipper, and Runt domain proteins) to activate the promoters? What is the effect of translocations, mutations, and alterations in expression of these factors, particularly in different forms of AML?

AML1/ETO cooperates with HIF1α to promote leukemogenesis through DNMT3a transactivation.

The mechanisms by which AML1/ETO (A/E) fusion protein induces leukemogenesis in acute myeloid leukemia (AML) without mutagenic events remain elusive. Here we show that interactions between A/E and hypoxia-inducible factor 1α (HIF1α) are sufficient to prime leukemia cells for subsequent aggressive growth. In agreement with this, HIF1α is highly expressed in A/E-positive AML patients and strongly predicts inferior outcomes, regardless of gene mutations. Co-expression of A/E and HIF1α in leukemia cells causes a higher cell proliferation rate in vitro and more serious leukemic status in mice. Mechanistically, A/E and HIF1α form a positive regulatory circuit and cooperate to transactivate DNMT3a gene leading to DNA hypermethylation. Pharmacological or genetic interventions in the A/E-HIF1α loop results in DNA hypomethylation, a re-expression of hypermethylated tumor-suppressor p15(INK4b) and the blockage of leukemia growth. Thus high HIF1α expression serves as a reliable marker, which identifies patients with a poor prognosis in an otherwise prognostically favorable AML group and represents an innovative therapeutic target in high-risk A/E-driven leukemia.

Two promoters direct expression of the murine Spi-B gene, an Ets family transcription factor.

Spi-B and PU.1 (Spi-1) comprise the most divergent subfamily of the Ets transcription factor family. Spi-B and PU.1 bind to similar DNA sequences, and can activate the same B-cell and myeloid promoters in vitro. However, PU.1 knockout mice demonstrate defective hematopoietic development of multiple hematopoietic lineages, indicating that Spi-B was not able to compensate for loss of PU.1. One explanation for these results is that, in contrast to PU.1, which is expressed in myeloid and B-cell lines, Spi-B expression is restricted to B-cells. In order to begin to understand the control of regulation of the Spi-B gene, murine Spi-B cDNA and genomic clones were isolated. The exon/intron organization and transcriptional start sites were determined; two major transcriptional start sites were detected. The two Spi-B promoters were isolated and characterized, and displayed differential activity in B-cell lines matching that of the endogenous gene. Further study of the two Spi-B promoters will provide insight into the molecular events regulating the tissue-specific and developmental stage-specific expression of Spi-B in B-cells.