Lupus susceptibility gene Pbx1 controls the development, stability, and function of regulatory T cells via Rtkn2 expression.

The maintenance of regulatory T (Treg) cells critically prevents autoimmunity. Pre-B cell leukemia transcription factor 1 (Pbx1) variants are associated with lupus susceptibility, particularly through the expression of a dominant negative isoform Pbx1-d in CD4+ T cells. Pbx1-d overexpression impaired Treg cell homeostasis and promoted inflammatory CD4+ T cells. Here, we showed a high expression of Pbx1 in human and murine Treg cells, which is decreased in lupus patients and mice. Pbx1 deficiency or Pbx1-d overexpression reduced the number, stability, and suppressive activity of Treg cells, which increased murine responses to immunization and autoimmune induction. Mechanistically, Pbx1 deficiency altered the expression of genes implicated in cell cycle and apoptosis in Treg cells. Intriguingly, Rtkn2, a Rho-GTPase previously associated with Treg homeostasis, was directly transactivated by Pbx1. Our results suggest that the maintenance of Treg cell homeostasis and stability by Pbx1 through cell cycle progression prevent the expansion of inflammatory T cells that otherwise exacerbates lupus progression in the hosts.

PBX1 as a novel master regulator in cancer: Its regulation, molecular biology, and therapeutic applications.

PBX1 is a critical transcription factor at the top of various cell fate-determining pathways. In cancer, PBX1 stands at the crossroads of multiple oncogenic signaling pathways and mediates responses by recruiting a broad repertoire of downstream targets. Research thus far has corroborated the involvement of PBX1 in cancer proliferation, resisting apoptosis, tumor-associated neoangiogenesis, epithelial-mesenchymal transition (EMT) and metastasis, immune evasion, genome instability, and dysregulating cellular metabolism. Recently, our understanding of the functional regulation of the PBX1 protein has advanced, as increasing evidence has depicted a regulatory network consisting of transcriptional, post-transcriptional, and post-translational levels of control mechanisms. Furthermore, accumulating studies have supported the clinical utilization of PBX1 as a prognostic or therapeutic target in cancer. Preliminary results showed that PBX1 entails vast potential as a targetable master regulator in the treatment of cancer, particularly in those with high-risk features and resistance to other therapeutic strategies. In this review, we will explore the regulation, protein-protein interactions, molecular pathways, clinical application, and future challenges of PBX1.

Regulation of the STAT3 pathway by lupus susceptibility gene Pbx1 in T cells.

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease in which poorly characterized genetic factors lead to the production of proinflammatory or autoreactive T cells. Pre-B cell leukemia homeobox 1 (PBX1) is a transcription factor whose dominant negative isoform (PBX1-D) is overexpressed in the CD4+ T cells of SLE patients and lupus-prone mice. Pbx1-D overexpression favors the expansion of proinflammatory T cells and impairs regulatory T (Treg) cell development. Here we show that Pbx1 deficiency and Pbx1-D overexpression decreased STAT3 expression and activation in T cells. Accordingly, Pbx1 deficiency in T cells and Pbx1-D overexpression reduced STAT3-dependent TH17 cell polarization in vitro, but it had no effect in vivo at steady state. STAT3-dependent follicular helper T (TFH) cell polarization in vitro and splenic TFH cell frequency were not affected by either Pbx1 deficiency or Pbx1-D overexpression. Pbx1 deficiency also increased the expression of cell cycle arrest and pro-apoptotic genes, with an increased apoptosis in T cells. Our results suggest a complex interplay between PBX1 and STAT3, which may contribute to lupus pathogenesis through dysregulation of the cell cycle and apoptosis.

A molecular signature for IL-10-producing Th1 cells in protozoan parasitic diseases.

Control of visceral leishmaniasis (VL) depends on proinflammatory Th1 cells that activate infected tissue macrophages to kill resident intracellular parasites. However, proinflammatory cytokines produced by Th1 cells can damage tissues and require tight regulation. Th1 cell IL-10 production is an important cell-autologous mechanism to prevent such damage. However, IL-10-producing Th1 (type 1 regulatory; Tr1) cells can also delay control of parasites and the generation of immunity following drug treatment or vaccination. To identify molecules to target in order to alter the balance between Th1 and Tr1 cells for improved antiparasitic immunity, we compared the molecular and phenotypic profiles of Th1 and Tr1 cells in experimental VL caused by Leishmania donovani infection of C57BL/6J mice. We also identified a shared Tr1 cell protozoan signature by comparing the transcriptional profiles of Tr1 cells from mice with experimental VL and malaria. We identified LAG3 as an important coinhibitory receptor in patients with VL and experimental VL, and we reveal tissue-specific heterogeneity of coinhibitory receptor expression by Tr1 cells. We also discovered a role for the transcription factor Pbx1 in suppressing CD4+ T cell cytokine production. This work provides insights into the development and function of CD4+ T cells during protozoan parasitic infections and identifies key immunoregulatory molecules.

Involvement of Transcriptional Factor Pbx1 in Peripheral B Cell Homeostasis to Constrain Lupus Autoimmunity.

OBJECTIVE: Disruption of B cell homeostasis and subsequent dominance of effector B cell subsets are critical for the development of systemic lupus erythematosus (SLE). Revealing the key intrinsic regulators involved in the homeostatic control of B cells has important therapeutic value for SLE. This study was undertaken to determine the regulatory role of the transcription factor Pbx1 in B cell homeostasis and lupus pathogenesis. METHODS: We constructed mice with B cell-specific deletion of Pbx1. T cell-dependent and T cell-independent humoral responses were induced by intraperitoneal injection of nitrophenyl-containing hapten (NP) conjugated to keyhole limpet hemocyanin or NP-Ficoll. The regulatory effects of Pbx1 on autoimmunity were observed in a Bm12-induced lupus murine model. We investigated mechanisms of Pbx1 using RNA sequencing, the cleavage under targets and tagmentation assay, and chromatin immunoprecipitation-quantitative polymerase chain reaction assay. We transduced B cells from SLE patients with plasmids that overexpressed PBX1 to explore the in vitro therapeutic efficacy of PBX1. RESULTS: Pbx1 was specifically down-regulated in autoimmune B cells and negatively correlated with disease activity. The deficiency of Pbx1 in B cells resulted in excessive humoral responses following immunization. In the Bm12-induced lupus model, mice with B cell-specific Pbx1 deficiency displayed enhancements in germinal center responses, plasma cell differentiation, and autoantibody production. Pbx1-deficient B cells had increased survival and proliferative advantages after activation. Pbx1 regulated genetic programs by directly targeting critical components of the proliferation and apoptosis pathways. In SLE patients, PBX1 expression was negatively correlated with effector B cell expansion; when PBX1 expression was enforced, the survival and proliferative capacity of SLE B cells were attenuated. CONCLUSION: Our study reveals the regulatory function and mechanism of Pbx1 in adjusting B cell homeostasis and highlights Pbx1 as a therapeutic target in SLE.

Overexpression of lipocalin 2 in PBX1-deficient decidual NK cells promotes inflammation at the maternal-fetal interface.

PROBLEM: Impairment of PBX1 expression in decidual natural killer (dNK) cells is associated with the pathogenesis of unexplained recurrent spontaneous abortion, which results in fetal growth restriction (FGR) by affecting the secretion of downstream growth factors. However, whether other mechanisms limit embryo growth in decidua containing PBX1-deficient natural killer (NK) cells is unknown. METHOD OF STUDY: Pbx1f/f ; Ncr1Cre mice were employed to explore the underlying mechanisms by which PBX1- NK cells affect embryonic development. To simulate the clinical testing of pregnant women, Doppler ultrasound imaging was used to detect embryo implantation and development. Differentially expressed genes (DEGs) in PBX1- NK cells that may affect normal pregnancy were screened using RNA-sequencing and real-time PCR. Immune cell changes caused by DEGs were detected by flow cytometry. Finally, the mechanism of FGR was explored by injecting the protein LCN2, corresponding to the selected DEG, into mice. RESULTS: We verified the embryonic dysplasia in pregnant Pbx1f/f ; Ncr1Cre mice by Doppler ultrasound imaging and found that LCN2 was upregulated in dNK cells. We also observed higher infiltration of neutrophils and macrophages in the decidua of Pbx1f/f ; Ncr1Cre mice. Finally, we found an increase in the number and activation of neutrophils at the maternal-fetal interface after injecting LCN2 into pregnant mice and observed that these mice showed signs of FGR. CONCLUSION: Excessive LCN2 secreted by PBX1- dNK cells at the maternal-fetal interface recruit neutrophils and causes an inflammatory response, which is related to FGR.

PBX1 attenuates H2O2-induced oxidant stress in human trabecular meshwork cells via promoting NANOG-mediated PI3K/AKT signaling pathway.

Oxidative stress-induced excessive extracellular matrix (ECM) deposition in trabecular meshwork (TM) tissue is considered the major pathological procedure of glaucoma. This study aimed to explore the role and regulatory mechanism of pre-B-cell leukemia transcription factor 1 (PBX1) in H2O2-induced human trabecular meshwork cells (HTMCs). Expressions of PBX1, NANOG, ECM, and pathway-related factors were detected by qRT-PCR and western blot. Cell viability and apoptosis of HTMCs were measured using CCK-8 and flow cytometry assays. Reactive oxygen species (ROS), superoxide dismutase (SOD), and L-glutathione (GSH) levels were detected to evaluate oxidative stress. Through luciferase reporter assay, the association between PBX1 and NANOG was verified. Results presented that PBX1 was significantly upregulated in H2O2-induced HTMCs. Functionally, PBX1 and NANOG promoted cell viability, inhibited cell apoptosis and ECM deposition, suppressed ROS accumulation, and enhanced the productions of SOD and GSH in H2O2-stimulated HTMCs, while PBX1 inhibition showed the opposite effects. In addition, PBX1 promoted the transcription of NANOG by upregulating the promoter activity of NANOG which activated the PI3K-AKT signaling pathway. What's more, the inhibitions of PI3K-AKT signaling pathway or NANOG reversed the protective effect of PBX1 on H2O2-stimulated HTMCs. In summary, our study firstly revealed that PBX1 attenuated the oxidative damage in HTMCs via regulating NANOG-mediated PI3K/AKT signaling, suggesting that PBX1 might be a potential treatment target for glaucoma patients.

The TALE never ends: A comprehensive overview of the role of PBX1, a TALE transcription factor, in human developmental defects.

PBX1 is a highly conserved atypical homeodomain transcription factor (TF) belonging to the TALE (three amino acid loop extension) family. Dimerized with other TALE proteins, it can interact with numerous partners and reach dozens of regulating sequences, suggesting its role as a pioneer factor. PBX1 is expressed throughout the embryonic stages (as early as the blastula stage) in vertebrates. In human, PBX1 germline variations are linked to syndromic renal anomalies (CAKUTHED). In this review, we summarized available data on PBX1 functions, PBX1-deficient animal models, and PBX1 germline variations in humans. Two types of genetic alterations were identified in PBX1 gene. PBX1 missense variations generate a severe phenotype including lung hypoplasia, cardiac malformations, and sexual development defects (DSDs). Conversely, truncating variants generate milder phenotypes (mainly cryptorchidism and deafness). We suggest that defects in PBX1 interactions with various partners, including proteins from the HOX (HOXA7, HOXA10, etc.), WNT (WNT9B, WNT3), and Polycomb (BMI1, EED) families are responsible for abnormal proliferation and differentiation of the embryonic mesenchyme. These alterations could explain most of the defects observed in humans. However, some phenotype variability (especially DSDs) remains poorly understood. Further studies are needed to explore the TALE family in greater depth.

Transcriptional network orchestrating regional patterning of cortical progenitors.

We uncovered a transcription factor (TF) network that regulates cortical regional patterning in radial glial stem cells. Screening the expression of hundreds of TFs in the developing mouse cortex identified 38 TFs that are expressed in gradients in the ventricular zone (VZ). We tested whether their cortical expression was altered in mutant mice with known patterning defects (Emx2, Nr2f1, and Pax6), which enabled us to define a cortical regionalization TF network (CRTFN). To identify genomic programming underlying this network, we performed TF ChIP-seq and chromatin-looping conformation to identify enhancer-gene interactions. To map enhancers involved in regional patterning of cortical progenitors, we performed assays for epigenomic marks and DNA accessibility in VZ cells purified from wild-type and patterning mutant mice. This integrated approach has identified a CRTFN and VZ enhancers involved in cortical regional patterning in the mouse.

Integrated RNAi screening identifies the NEDDylation pathway as a synergistic partner of azacytidine in acute myeloid leukemia.

Treatment of acute myeloid leukemia (AML) remains challenging and novel targets and synergistic therapies still need to be discovered. We performed a high-throughput RNAi screen in three different AML cell lines and primary human leukemic blasts to identify genes that synergize with common antileukemic therapies. We used a pooled shRNA library that covered 5043 different genes and combined transfection with exposure to either azacytidine or cytarabine analog to the concept of synthetic lethality. Suppression of the chemokine CXCL12 ranked highly among the candidates of the cytarabine group. Azacytidine in combination with suppression of genes within the neddylation pathway led to synergistic results. NEDD8 and RBX1 inhibition by the small molecule inhibitor pevonedistat inhibited leukemia cell growth. These findings establish an in vitro synergism between NEDD8 inhibition and azacytidine in AML. Taken together, neddylation constitutes a suitable target pathway for azacytidine combination strategies.

Transcriptional cooperation of PBX1 and PAX6 in adult neural progenitor cells.

PAX6 is a highly conserved transcription factor and key regulator of several neurogenic processes, including the continuous generation of dopaminergic/GABAergic interneurons in the adult ventricular-subventricular (V-SVZ) neurogenic system in mice. Here we report that PAX6 cooperates with the TALE-homeodomain transcription factor PBX1 in this context. Chromatin-immunoprecipitation showed that PBX1 and PAX6 co-occupy shared genomic binding sites in adult V-SVZ stem- and progenitor cell cultures and mouse embryonic stem cells, while depletion of Pbx1 revealed that association of PAX6 with these sites requires the presence of PBX1. Expression profiling together with viral overexpression or knockdown of Pax6 or Pbx1 identified novel PBX1-PAX6 co-regulated genes, including several transcription factors. Computational modeling of genome wide expression identified novel cross-regulatory networks among these very transcription factors. Taken together, the results presented here highlight the intimate link that exists between PAX6 and TALE-HD family proteins and contribute novel insights into how the orchestrated activity of transcription factors shapes adult V-SVZ neurogenesis.

PBX1-directed stem cell transcriptional program drives tumor progression in myeloproliferative neoplasm.

PBX1 regulates the balance between self-renewal and differentiation of hematopoietic stem cells and maintains proto-oncogenic transcriptional pathways in early progenitors. Its increased expression was found in myeloproliferative neoplasm (MPN) patients bearing the JAK2V617F mutation. To investigate if PBX1 contributes to MPN, and to explore its potential as therapeutic target, we generated the JP mouse strain, in which the human JAK2 mutation is induced in the absence of PBX1. Typical MPN features, such as thrombocythemia and granulocytosis, did not develop without PBX1, while erythrocytosis, initially displayed by JP mice, gradually resolved over time; splenic myeloid metaplasia and in vitro cytokine independent growth were absent upon PBX1 inactivation. The aberrant transcriptome in stem/progenitor cells from the MPN model was reverted by the absence of PBX1, demonstrating that PBX1 controls part of the molecular pathways deregulated by the JAK2V617F mutation. Modulation of the PBX1-driven transcriptional program might represent a novel therapeutic approach.

rs6426881 in the 3'-UTR of PBX1 is involved in breast and gastric cancers via altering the binding potential of miR-522-3p.

BACKGROUND: Breast and gastric cancers are the most important diseases that lead to cancer death and social healthcare challenge. Overexpression of PBX1, a proto-oncogene, is correlated with the progression and metastasis of various cancers. For the first time, in this study the researchers evaluated the relationship between rs6426881, affecting miR-522-3p binding to the PBX1, with breast and gastric cancers. METHODS AND RESULTS: The Microarray analysis was performed for finding the relative expression level of PBX1 and hsa-miR-522-3p, based on high throughput experiments. The GSE54397, GSE112369, GSE10810, GSE241585.ER, GSE24185.PR, GSE68373, and GSE38167 datasets were analyzed. A case-control study was carried out in 123 Iranian suffering from breast cancer and 132 participants as control samples as well as 130 people suffering from gastric cancer and 54 people as control group members. SNP rs6426881 in the 3'-UTR of PBX1 was genotyped by the High-Resolution Melting (HRM) method. Association analysis revealed that rs6426881 is correlated with Estrogen and Progesterone receptors, grade, and stage of breast cancer. Furthermore, a significant relationship was observed between the genotypes and blood groups in gastric cancer, while the distribution of alleles was significantly related to smoking, status of the primary tumor, and metastasis (Chi-Square P < 0.05). Finally, Bioinformatics analyses suggested that rs6426881 contains binding sites for miR-522-3p in the 3'-UTR of PBX1 transcript. The finding suggested that TT genotype is associated with poor prognosis in breast and gastric cancer. CONCLUSIONS: The rs6426881 T allele at PBX1 3'-UT is significantly related to breast and gastric cancers by altering the regulatory affinity of miR-522-3p to PBX1 3'-UTR and may be suggested as a novel prognostic biomarker for the diseases.

Analysis of lamprey meis genes reveals that conserved inputs from Hox, Meis and Pbx proteins control their expression in the hindbrain and neural tube.

Meis genes are known to play important roles in the hindbrain and neural crest cells of jawed vertebrates. To explore the roles of Meis genes in head development during evolution of vertebrates, we have identified four meis genes in the sea lamprey genome and characterized their patterns of expression and regulation, with a focus on the hindbrain and pharynx. Each of the lamprey meis genes displays temporally and spatially dynamic patterns of expression, some of which are coupled to rhombomeric domains in the developing hindbrain and select pharyngeal arches. Studies of Meis loci in mouse and zebrafish have identified enhancers that are bound by Hox and TALE (Meis and Pbx) proteins, implicating these factors in the direct regulation of Meis expression. We examined the lamprey meis loci and identified a series of cis-elements conserved between lamprey and jawed vertebrate meis genes. In transgenic reporter assays we demonstrated that these elements act as neural enhancers in lamprey embryos, directing reporter expression in appropriate domains when compared to expression of their associated endogenous meis gene. Sequence alignments reveal that these conserved elements are in similar relative positions of the meis loci and contain a series of consensus binding motifs for Hox and TALE proteins. This suggests that ancient Hox and TALE-responsive enhancers regulated expression of ancestral vertebrate meis genes in segmental domains in the hindbrain and have been retained in the meis loci during vertebrate evolution. The presence of conserved Meis, Pbx and Hox binding sites in these lamprey enhancers links Hox and TALE factors to regulation of lamprey meis genes in the developing hindbrain, indicating a deep ancestry for these regulatory interactions prior to the divergence of jawed and jawless vertebrates.

A Computer-Based Methodology to Design Non-Standard Peptides Potentially Able to Prevent HOX-PBX1-Associated Cancer Diseases.

In the last decades, HOX proteins have been extensively studied due to their pivotal role in transcriptional events. HOX proteins execute their activity by exploiting a cooperative binding to PBX proteins and DNA. Therefore, an increase or decrease in HOX activity has been associated with both solid and haematological cancer diseases. Thus, inhibiting HOX-PBX interaction represents a potential strategy to prevent these malignancies, as demonstrated by the patented peptide HTL001 that is being studied in clinical trials. In this work, a computational study is described to identify novel potential peptides designed by employing a database of non-natural amino acids. For this purpose, residue scanning of the HOX minimal active sequence was performed to select the mutations to be further processed. According to these results, the peptides were point-mutated and used for Molecular Dynamics (MD) simulations in complex with PBX1 protein and DNA to evaluate complex binding stability. MM-GBSA calculations of the resulting MD trajectories were exploited to guide the selection of the most promising mutations that were exploited to generate twelve combinatorial peptides. Finally, the latter peptides in complex with PBX1 protein and DNA were exploited to run MD simulations and the ΔGbinding average values of the complexes were calculated. Thus, the analysis of the results highlighted eleven combinatorial peptides that will be considered for further assays.

Combined genomic and proteomic approaches reveal DNA binding sites and interaction partners of TBX2 in the developing lung.

BACKGROUND: Tbx2 encodes a transcriptional repressor implicated in the development of numerous organs in mouse. During lung development TBX2 maintains the proliferation of mesenchymal progenitors, and hence, epithelial proliferation and branching morphogenesis. The pro-proliferative function was traced to direct repression of the cell-cycle inhibitor genes Cdkn1a and Cdkn1b, as well as of genes encoding WNT antagonists, Frzb and Shisa3, to increase pro-proliferative WNT signaling. Despite these important molecular insights, we still lack knowledge of the DNA occupancy of TBX2 in the genome, and of the protein interaction partners involved in transcriptional repression of target genes. METHODS: We used chromatin immunoprecipitation (ChIP)-sequencing and expression analyses to identify genomic DNA-binding sites and transcription units directly regulated by TBX2 in the developing lung. Moreover, we purified TBX2 containing protein complexes from embryonic lung tissue and identified potential interaction partners by subsequent liquid chromatography/mass spectrometry. The interaction with candidate proteins was validated by immunofluorescence, proximity ligation and individual co-immunoprecipitation analyses. RESULTS: We identified Il33 and Ccn4 as additional direct target genes of TBX2 in the pulmonary mesenchyme. Analyzing TBX2 occupancy data unveiled the enrichment of five consensus sequences, three of which match T-box binding elements. The remaining two correspond to a high mobility group (HMG)-box and a homeobox consensus sequence motif. We found and validated binding of TBX2 to the HMG-box transcription factor HMGB2 and the homeobox transcription factor PBX1, to the heterochromatin protein CBX3, and to various members of the nucleosome remodeling and deacetylase (NuRD) chromatin remodeling complex including HDAC1, HDAC2 and CHD4. CONCLUSION: Our data suggest that TBX2 interacts with homeobox and HMG-box transcription factors as well as with the NuRD chromatin remodeling complex to repress transcription of anti-proliferative genes in the pulmonary mesenchyme.

Establishment of the TALE-code reveals aberrantly activated homeobox gene PBX1 in Hodgkin lymphoma.

Homeobox genes encode transcription factors which regulate basic processes in development and cell differentiation and are grouped into classes and subclasses according to sequence similarities. Here, we analyzed the activities of the 20 members strong TALE homeobox gene class in early hematopoiesis and in lymphopoiesis including developing and mature B-cells, T-cells, natural killer (NK)-cells and innate lymphoid cells (ILC). The resultant expression pattern comprised eleven genes and which we termed TALE-code enables discrimination of normal and aberrant activities of TALE homeobox genes in lymphoid malignancies. Subsequent expression analysis of TALE homeobox genes in public datasets of Hodgkin lymphoma (HL) patients revealed overexpression of IRX3, IRX4, MEIS1, MEIS3, PBX1, PBX4 and TGIF1. As paradigm we focused on PBX1 which was deregulated in about 17% HL patients. Normal PBX1 expression was restricted to hematopoietic stem cells and progenitors of T-cells and ILCs but absent in B-cells, reflecting its roles in stemness and early differentiation. HL cell line SUP-HD1 expressed enhanced PBX1 levels and served as an in vitro model to identify upstream regulators and downstream targets in this malignancy. Genomic studies of this cell line therein showed a gain of the PBX1 locus at 1q23 which may underlie its aberrant expression. Comparative expression profiling analyses of HL patients and cell lines followed by knockdown experiments revealed NFIB and TLX2 as target genes activated by PBX1. HOX proteins operate as cofactors of PBX1. Accordingly, our data showed that HOXB9 overexpressed in HL coactivated TLX2 but not NFIB while activating TNFRSF9 without PBX1. Further downstream analyses showed that TLX2 activated TBX15 which operated anti-apoptotically. Taken together, we discovered a lymphoid TALE-code and identified an aberrant network around deregulated TALE homeobox gene PBX1 which may disturb B-cell differentiation in HL by reactivation of progenitor-specific genes. These findings may provide the framework for future studies to exploit possible vulnerabilities of malignant cells in therapeutic scenarios.

Mapping the native interaction surfaces of PREP1 with PBX1 by cross-linking mass-spectrometry and mutagenesis.

Both onco-suppressor PREP1 and the oncogene MEIS1 bind to PBX1. This interaction stabilizes the two proteins and allows their translocation into the nucleus and thus their transcriptional activity. Here, we have combined cross-linking mass-spectrometry and systematic mutagenesis to detail the binding geometry of the PBX1-PREP1 (and PBX1-MEIS1) complexes, under native in vivo conditions. The data confirm the existence of two distinct interaction sites within the PBC domain of PBX1 and unravel differences among the highly similar binding sites of MEIS1 and PREP1. The HR2 domain has a fundamental role in binding the PBC-B domain of PBX1 in both PREP1 and MEIS1. The HR1 domain of MEIS1, however, seem to play a less stringent role in PBX1 interaction with respect to that of PREP1. This difference is also reflected by the different binding affinity of the two proteins to PBX1. Although partial, this analysis provides for the first time some ideas on the tertiary structure of the complexes not available before. Moreover, the extensive mutagenic analysis of PREP1 identifies the role of individual hydrophobic HR1 and HR2 residues, both in vitro and in vivo.

PBX1 promotes development of natural killer cells by binding directly to the Nfil3 promoter.

The transcription factor nuclear factor interleukin-3-regulated protein (NFIL3, also called E4BP4) is crucial for commitment of natural killer (NK) cells from common lymphoid progenitors (CLPs). However, the identity of the factor that can regulate NFIL3 directly during the NK-cell development is not known. Here, we reveal that pre-B-cell leukemia transcription factor 1 (PBX1) can upregulate the NFIL3 expression directly. We used conditional knockout mice in which PBX1 in hematopoietic cells was specifically absent. The number of NK-committed progenitor pre-NKP cells and rNKP cells was reduced significantly in the absence of PBX1, which was consistent with NFIL3 deficiency. Also, the NFIL3 expression in NK cells was decreased if PBX1 was absent. We demonstrated that PBX1 was bound directly to the promoter of Nfil3 and facilitated transcription. Upon knockout of the binding site of PBX1 in the Nfil3 promoter, mice showed fewer NK-precursor cells and NK cells, just like that observed in Nfil3 knockout mice. Furthermore, asparagine N286 in the homeodomain of PBX1 controlled the binding of PBX1 to the Nfil3 promoter. Collectively, these findings demonstrate that the transcription factor PBX1 promotes the early development of NK cells by upregulating the Nfil3 expression directly.