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RUNX3 is one of three mammalian Runt-domain transcription factors (TFs) that regulate gene expression in several types of immune cells. Runx3-deficiency in mice is associated with a multitude of defects in the adaptive and innate immunity systems, including the development of early onset colitis. Our study reveals that conditional deletion of Runx3 specifically in mononuclear phagocytes (MNP) (MNPRunx3−/−) but not in T cells, recapitulates the early onset spontaneous colitis seen in Runx3−/− mice.We show that Runx3 is expressed in colonic MNP, including resident macrophages (RM) and the dendritic cell cDC2 subsets and its loss results in impaired differentiation/maturation of both cell types. At the transcriptome level, loss of Runx3 in RM and cDC2 was associated with upregulation of pro-inflammatory genes similar to those in the early onset IBD murine model of RMIl10r−/−. The impaired RM maturation in the absence of Runx3 was associated with a marked decrease in expression of anti-inflammatory and TGFβ-regulated genes. Similarly, the decreased expression of β-catenin signaling associated genes in Runx3-deficient cDC2 indicates their impaired differentiation/maturation. Analysis of ChIP-seq data suggests that in both MNP cell types a significant fraction of these differentially expressed genes are high confidence Runx3 directly regulated genes. Interestingly, several of these putative Runx3 target genes harbor SNPs associated with IBD susceptibility in humans. Remarkably, the impaired maturation and pro-inflammatory phenotype of MNP lacking Runx3 was associated with a substantial reduction in the prevalence of colonic lamina propria Foxp3+ regulatory T cells and an increase in IFNγ-producing CD4+ T cells, underscoring Runx3 critical role in establishing tolerogenic MNP.Together, these data emphasize the dual role of Runx3 in colonic MNP, as a transcriptional repressor of pro-inflammatory genes and an activator of maturation-associated genes including anti-inflammatory genes. Our study highlights the significance of the current MNPRunx3−/− model for understanding of human MNP-associated colitis. It provides new insights into the crucial involvement of Runx3 in intestinal immune tolerance by regulating colonic MNP maturation through TGFβR signaling and anti-inflammatory functions by Il10R signaling, befitting the identification of RUNX3 as a genome-wide associated risk gene for various immune-related diseases in humans including gastrointestinal tract diseases such as celiac and Crohn’s disease.

SummaryThe t(8;21) and inv(16) chromosomal aberrations generate the oncoproteins AML1-ETO (A-E) and CBFβ-SMMHC (C-S). The role of these oncoproteins in acute myeloid leukemia (AML) etiology has been well studied. Conversely, the function of native RUNX1 in promoting A-E- and C-S-mediated leukemias has remained elusive. We show that wild-type RUNX1 is required for the survival of t(8;21)-Kasumi-1 and inv(16)-ME-1 leukemic cells. RUNX1 knockdown in Kasumi-1 cells (Kasumi-1RX1-KD) attenuates the cell-cycle mitotic checkpoint, leading to apoptosis, whereas knockdown of A-E in Kasumi-1RX1-KD rescues these cells. Mechanistically, a delicate RUNX1/A-E balance involving competition for common genomic sites that regulate RUNX1/A-E targets sustains the malignant cell phenotype. The broad medical significance of this leukemic cell addiction to native RUNX1 is underscored by clinical data showing that an active RUNX1 allele is usually preserved in both t(8;21) or inv(16) AML patients, whereas RUNX1 is frequently inactivated in other forms of leukemia. Thus, RUNX1 and its mitotic control targets are potential candidates for new therapeutic approaches.

The Runx3 transcription factor regulates development of T cells during thymopoiesis and TrkC sensory neurons during dorsal root ganglia neurogenesis. It also mediates transforming growth factor-beta signaling in dendritic cells and is essential for development of skin Langerhans cells. Here, we report that Runx3 is involved in the development of skin dendritic epidermal T cells (DETCs); an important component of tissue immunoregulation. In developing DETCs, Runx3 regulates expression of the alphaEbeta7 integrin CD103, known to affect migration and epithelial retention of DETCs. It also regulates expression of IL-2 receptor beta (IL-2Rbeta) that mediates cell proliferation in response to IL-2 or IL-15. In the absence of Runx3, the reduction in CD103 and IL-2Rbeta expression on Runx3(-/-) DETC precursors resulted in impaired cell proliferation and maturation, leading to complete lack of skin DETCs in Runx3(-/-) mice. The data demonstrate the requirement of Runx3 for DETCs development and underscore the importance of CD103 and IL-2Rbeta in this process. Of note, while Runx3(-/-) mice lack both DETCs and Langerhans cells, the two most important components of skin immune surveillance, the mice did not develop skin lesions under pathogen-free (SPF) conditions.

Runx3 transcription factor regulates cell lineage decisions in thymopoiesis and neurogenesis. Here we report that Runx3 knockout (KO) mice develop spontaneous eosinophilic lung inflammation associated with airway remodeling and mucus hypersecretion. Runx3 is specifically expressed in mature dendritic cells (DC) and mediates their response to TGF-beta. In the absence of Runx3, DC become insensitive to TGF-beta-induced maturation inhibition, and TGF-beta-dependent Langerhans cell development is impaired. Maturation of Runx3 KO DC is accelerated and accompanied by increased efficacy to stimulate T cells and aberrant expression of beta2-integrins. Lung alveoli of Runx3 KO mice accumulate DC characteristic of allergic airway inflammation. Taken together, abnormalities in DC function and subset distribution may constitute the primary immune system defect, which leads to the eosinophilic lung inflammation in Runx3 KO mice. These data may help elucidate the molecular mechanisms underlying the pathogenesis of allergic airway inflammation in humans.

The RUNX transcription factors are important regulators of lineage-specific gene expression. RUNX are bifunctional, acting both as activators and repressors of tissue-specific target genes. Recently, we have demonstrated that Runx3 is a neurogenic transcription factor, which regulates development and survival of proprioceptive neurons in dorsal root ganglia. Here we report that Runx3 and Runx1 are highly expressed in thymic medulla and cortex, respectively, and function in development of CD8 T cells during thymopoiesis. Runx3-deficient (Runx3 KO) mice display abnormalities in CD4 expression during lineage decisions and impairment of CD8 T cell maturation in the thymus. A large proportion of Runx3 KO peripheral CD8 T cells also expressed CD4, and in contrast to wild-type, their proliferation ability was largely reduced. In addition, thein vitrocytotoxic activity of alloimmunized peritoneal exudate lymphocytes was significantly lower in Runx3 KO compared with WT mice. In a compound mutant mouse, null for Runx3 and heterozygous for Runx1 (Runx3-/-;Runx1+/-), all peripheral CD8 T cells also expressed CD4, resulting in a complete lack of single-positive CD8+T cells in the spleen. The results provide information on the role of Runx3 and Runx1 in thymopoiesis and suggest that both act as transcriptional repressors ofCD4expression during T cell lineage decisions.

The RUNX3 gene belongs to the runt domain family of transcription factors that act as master regulators of gene expression in major developmental pathways. In mammals the family includes three genes, RUNX1, RUNX2 and RUNX3. Here, we describe a comparative analysis of the human chromosome 1p36.1 encoded RUNX3 and mouse chromosome 4 encoded Runx3 genomic regions. The analysis revealed high similarities between the two genes in the overall size and organization and showed that RUNX3/Runx3 is the smallest in the family, but nevertheless exhibits all the structural elements characterizing the RUNX family. It also revealed that RUNX3/Runx3 bears a high content of the ancient mammalian repeat MIR. Together, these data delineate RUNX3/Runx3 as the evolutionary founder of the mammalian RUNX family. Detailed sequence analysis placed the two genes at a GC-rich H3 isochore with a sharp transition of GC content between the gene sequence and the downstream intergenic region. Two large conserved CpG islands were found within both genes, one around exon 2 and the other at the beginning of exon 6. RUNX1, RUNX2 and RUNX3 gene products bind to the same DNA motif, hence their temporal and spatial expression during development should be tightly regulated. Structure/function analysis showed that two promoter regions, designated P1 and P2, regulate RUNX3 expression in a cell type-specific manner. Transfection experiments demonstrated that both promoters were highly active in the GM1500 B-cell line, which endogenously expresses RUNX3, but were inactive in the K562 myeloid cell line, which does not express RUNX3.

The mammalian AML/CBFα runt domain (RD) transcription factors regulate hematopoiesis and osteoblast differentiation. Like their Drosophila counterparts, most mammalian RD proteins terminate in a common pentapeptide, VWRPY, which serves to recruit the corepressor Groucho (Gro). Using a yeast two-hybrid assay, in vitro association and pull-down experiments, we demonstrate that Gro and its mammalian homolog TLE1 specifically interact with AML1 and AML2. In addition to the VWRPY motif, other C-terminal sequences are required for these interactions with Gro/TLE1. TLE1 inhibits AML1-dependent transactivation of the T cell receptor (TCR) enhancers α and β, which contain functional AML binding sites, in transfected Jurkat T cells. LEF-1 is an additional transcription factor that mediates transactivation of TCR enhancers. LEF-1 and its Drosophila homolog Pangolin (Pan) are involved in the Wnt/Wg signaling pathway through interactions with the coactivator β-catenin and its highly conserved fly homolog Armadillo (Arm). We show that TLE/Gro interacts with LEF-1 and Pan, and inhibits LEF-1:β-catenin-dependent transcription. These data indicate that, in addition to their activity as transcriptional activators, AML1 and LEF-1 can act, through recruitment of the corepressor TLE1, as transcriptional repressors in TCR regulation and Wnt/Wg signaling.

Runx3 protein products that are translated from the distal (P1)- and proximal (P2)-promoter transcripts appear on Western blots as a 47-46kDa doublet corresponding to full-length proteins bearing the P1- and P2-N-termini respectively. An additional 44kDa protein band, the origin and nature of which was unclear, is also detected. Transfection of full-length Runx3 cDNA bearing the P2 N-terminus (P2-cDNA) into HEK293 cells resulted in expression of both 46 and 44kDa proteins. Sequence analysis of the P2-cDNA revealed an in-frame ATG 90bp downstream (+90ATG) of the proximal +1ATG. Insertion of an N-terminal HA-tag into P2-cDNA immediately downstream of the +1ATG produced HA-tagged 46kDa and untagged 44kDa proteins, consistent with the possibility that the latter was translated through initiation at the internal +90ATG site. Deleting or blocking the activity of the +1ATG, the natural cap-dependent translation initiation site in P2-cDNA, abrogated production of the 46kDa Runx3 protein while facilitating production of the 44kDa product. These findings supported the notion that Runx3 44kDa protein resulted from internal translation initiation at the +90ATG. Northern blot and RT-PCR analyses performed on RNA from P2-cDNA transfected cells showed a single transcript and product respectively, of the expected size, ruling out the possibility that the 44kDa protein was translated from transcripts originating at a cryptic promoter or produced by alternative splicing. Taken together, the data indicate that the 44kDa protein results from translation initiation at the internal ATG and that Runx3, like its family members Runx1 and Runx2, contains a mechanism for internal mRNA translation initiation.

Primary immune response to pathogens involves the maturation of antigen-presenting dendritic cells (DC). Bacterial lipopolysacharride (LPS) is a potent inducer of DC maturation, whereas the transforming growth factor beta (TGFbeta) attenuates much of this process. Here, we analyzed the global gene expression pattern in LPS-treated bone marrow derived DC during inhibition of their maturation process by TGFbeta. Exposure of DC to LPS induces a pronounced cell response, manifested in altered expression of a large number of genes. Interestingly, TGFbeta did not affect most of the LPS responding genes. Nevertheless, analysis identified a subset of genes that did respond to TGFbeta, among them the two inflammatory cytokines interleukin (IL)-12 and IL-18. Expression of IL-12, the major proinflammatory cytokine secreted by mature DC, was downregulated by TGFbeta, whereas the expression level of the proinflammatory cytokine IL-18, known to potentiate the IL-12 effect, was upregulated. Expression of the peroxisome proliferator-activated receptor gamma (PPARgamma) increased in response to TGFbeta, concomitantly with reduced expression of chemokine receptor 7 (CCR7). This finding supports the possibility that TGFbeta-dependent inhibition of CCR7 expression in DC is mediated by PPARgamma.

Background Alternative promoters usage is an important paradigm in transcriptional control of mammalian gene expression. However, despite the growing interest in alternative promoters and their role in genome diversification, very little is known about how and on what occasions those promoters are differentially regulated. Runx1 transcription factor is a key regulator of early hematopoiesis and a frequent target of chromosomal translocations in acute leukemias. Mice deficient in Runx1 lack definitive hematopoiesis and die in mid-gestation. Expression of Runx1 is regulated by two functionally distinct promoters designated P1 and P2. Differential usage of these two promoters creates diversity in distribution and protein-coding potential of the mRNA transcripts. While the alternative usage of P1 and P2 likely plays an important role in Runx1 biology, very little is known about the function of the P1/P2 switch in mediating tissue and stage specific expression of Runx1 during development. Results We employed mice bearing a hypomorphic Runx1 allele, with a largely diminished P2 activity, to investigate the biological role of alternative P1/P2 usage. Mice homozygous for the hypomorphic allele developed to term, but died within a few days after birth. During embryogenesis the P1/P2 activity is spatially and temporally modulated. P2 activity is required in early hematopoiesis and when attenuated, development of liver hematopoietic progenitor cells (HPC) was impaired. Early thymus development and thymopoiesis were also abrogated as reflected by thymic hypocellularity and loss of corticomedullary demarcation. Differentiation of CD4/CD8 thymocytes was impaired and their apoptosis was enhanced due to altered expression of T-cell receptors. Conclusion The data delineate the activity of P1 and P2 in embryogenesis and describe previously unknown functions of Runx1. The findings show unequivocally that the role of P1/P2 during development is non redundant and underscore the significance of alternative promoter usage in Runx1 biology.

AML1/RUNX1 belongs to the runt domain transcription factors that are important regulators of hematopoiesis and osteogenesis. Expression of AML1 is regulated at the level of transcription by two promoters, distal (D) and proximal (P), that give rise to mRNAs bearing two distinct 5′ untranslated regions (5′UTRs) (D-UTR and P-UTR). Here we show that these 5′UTRs act as translation regulators in vivo. AML1 mRNAs bearing the uncommonly long (1,631-bp) P-UTR are poorly translated, whereas those with the shorter (452-bp) D-UTR are readily translated. The low translational efficiency of the P-UTR is attributed to its length and the cis-acting elements along it. Transfections and in vitro assays with bicistronic constructs demonstrate that the D-UTR mediates cap-dependent translation whereas the P-UTR mediates cap-independent translation and contains a functional internal ribosome entry site (IRES). The IRES-containing bicistronic constructs are more active in hematopoietic cell lines that normally express the P-UTR-containing mRNAs. Furthermore, we show that the IRES-dependent translation increases during megakaryocytic differentiation but not during erythroid differentiation, of K562 cells. These results strongly suggest that the function of the P-UTR IRES-dependent translation in vivo is to tightly regulate the translation of AML1 mRNAs. The data show that AML1 expression is regulated through usage of alternative promoters coupled with IRES-mediated translation control. This IRES-mediated translation regulation adds an important new dimension to the fine-tuned control of AML1 expression.

The human AML1 gene encodes a heterodimeric transcription factor which plays an important role in mammalian hematopoiesis. Several alternatively spliced AML1 mRNA species were identified, some of which encode short protein products that lack the transactivation domain. When transfected into cells these short isoforms dominantly suppress transactivation mediated by the full length AML1 protein. However, their biological function remains obscure. To investigate the role of these short species in cell proliferation and differentiation we generated embryonic stem (ES) cells overexpressing one of the short isoforms, AML1-d, as well as cells expressing the full length isoforms AML1-b and AML2. The in vitro growth rate and differentiation of the transfected ES cells were unchanged. However, overexpression of AML1-d significantly affected the ES cells' ability to form teratocarcinomas in vivo in syngeneic mice, while a similar overexpression of AML1-b and AML2 had no effect on tumor formation. Histological analysis revealed that the AML1-d derived tumors were poorly differentiated and contained numerous apoptotic cells. These data highlight the pleiotropic effects of AML1 gene products and demonstrate for the first time an in vivo growth regulation function for the short isoform AML1-d.

A sensitive fluorimetric assay was developed for bacterial aminopeptidase P, based on intramolecularly quenched fluorogenic substrates. Two substrates were synthesized. Phe(NO2)-Pro-HN-CH2-CH2-NH-ABz (substrate I) and Phe(NO2)-Pro-Pro-HN-CH2-CH2-NH-ABz (substrate II), in which the Phe(NO2) group (rho-nitro-L-phenylalanyl) quenches the fluorescence of the ABz group (omicron-aminobenzoyl). Both substrates were readily cleaved by aminopeptidase P from Escherichia coli, releasing rho-nitro-L-phenylalanine and causing a proportional increase in fluorescence. Complete hydrolysis of the two substrates resulted in a 7.5-fold and 3.4-fold fluorescence increase, respectively. Applying this fluorogenic assay, we were able to detect and measure quantitatively amino-peptidase P-like activity in the human serum and calf-lung extracts. Substrate II was shown to be specifically cleaved by aminopeptidase P in these preparations, while substrate I was apparently cleaved by other enzymes as well. In both preparations, the enzyme activity was independent of Co2+ ions, and Pro-HN-CH2-CH2-NH-ABz (Cbz) was inhibitory. The kinetic constant Km was determined as 0.35 mM and 0.28 mM for the human serum and the calf-lung enzymes respectively. The enzyme activity was only slightly dependent on pH in the range 7.0-8.4.