Aberrant regulation of hematopoiesis by T cells in BAZF-deficient mice

BCL6B-dependent suppression of ETV2 hampers endothelial cell differentiation

BACKGROUND

B-cell CLL/lymphoma 6 member B (BCL6B) operates as a sequence-specific transcriptional repressor within the nucleus, playing crucial roles in various biological functions, including tumor suppression, immune response, stem cell self-renew, and vascular angiogenesis. However, whether BCL6B is involved in endothelial cell (EC) development has remained largely unknown. ETS variant transcription factor 2 (ETV2) is well known to facilitate EC differentiation. This study aims to determine the important role of BCL6B in EC differentiation and its potential mechanisms.

METHODS

Doxycycline-inducible human induced pluripotent stem cell (hiPSC) lines with BCL6B overexpression or BCL6B knockdown were established and subjected to differentiate into ECs and vessel organoids (VOs). RNA sequencing analysis was performed to identify potential signal pathways regulated by BCL6B during EC differentiation from hiPSCs. Quantitative real-time PCR (qRT-PCR) was used to detect the expression of pluripotency and vascular-specific marker genes expression. EC differentiation efficiency was determined by Flow cytometry analysis. The performance of EC was evaluated by in vitro Tube formation assay. The protein expression and the vessel-like structures were assessed using immunofluorescence analysis or western blot. Luciferase reporter gene assay and chromatin immunoprecipitation (ChIP)-PCR analysis were used to determine the regulatory relationship between BCL6B and ETV2.

RESULTS

Functional ECs and VOs were successfully generated from hiPSCs. Notably, overexpression of BCL6B suppressed while knockdown of BCL6B improved EC differentiation from hiPSCs. Additionally, the overexpression of BCL6B attenuated the capacity of derived hiPSC-ECs to form a tubular structure. Furthermore, compared to the control VOs, BCL6B overexpression repressed the growth of VOs, whereas BCL6B knockdown had little effect on the size of VOs. RNA sequencing analysis confirmed that our differentiation protocol induced landscape changes for cell/tissue/system developmental process, particularly vascular development and tube morphogenesis, which were significantly modulated by BCL6B. Subsequent experiments confirmed the inhibitory effect of BCL6B is facilitated by the binding of BCL6B to the promoter region of ETV2, led to the suppression of ETV2's transcriptional activity. Importantly, the inhibitory effect of BCL6B overexpression on EC differentiation from hiPSCs could be rescued by ETV2 overexpression.

CONCLUSIONS

BCL6B inhibits EC differentiation and hinders VO development by repressing the transcriptional activity of ETV2.

BCL6 maintains survival and self-renewal of primary human acute myeloid leukemia cells

B-cell lymphoma 6 (BCL6) is a transcription repressor and proto-oncogene that plays a crucial role in the innate and adaptive immune system and lymphoid neoplasms. However, its role in myeloid malignancies remains unclear. Here, we explored the role of BCL6 in acute myeloid leukemia (AML). BCL6 was expressed at variable and often high levels in AML cell lines and primary AML samples. AMLs with higher levels of BCL6 were generally sensitive to treatment with BCL6 inhibitors, with the exception of those with monocytic differentiation. Gene expression profiling of AML cells treated with a BCL6 inhibitor revealed induction of BCL6-repressed target genes and transcriptional programs linked to DNA damage checkpoints and downregulation of stem cell genes. Ex vivo treatment of primary AML cells with BCL6 inhibitors induced apoptosis and decreased colony-forming capacity, which correlated with the levels of BCL6 expression. Importantly, inhibition or knockdown of BCL6 in primary AML cells resulted in a significant reduction of leukemia-initiating capacity in mice, suggesting ablation of leukemia repopulating cell functionality. In contrast, BCL6 knockout or inhibition did not suppress the function of normal hematopoietic stem cells. Treatment with cytarabine further induced BCL6 expression, and the levels of BCL6 induction were correlated with resistance to cytarabine. Treatment of AML patient-derived xenografts with BCL6 inhibitor plus cytarabine suggested enhanced antileukemia activity with this combination. Hence, pharmacologic inhibition of BCL6 might provide a novel therapeutic strategy for ablation of leukemia-repopulating cells and increased responsiveness to chemotherapy.

Expression of the alternative splicing variants of bcl6b in medaka Oryzias latipes

Bcl6B, also known as BAZF, plays important roles in the immune response, repression of cancers, and maintenance of spermatogonial stem cells in mammals. In this study, the homologous gene bcl6b and its 5 alternative splicing variants, namely bcl6bX1 to bcl6bX5, were isolated from medaka fish, Oryzias latipes. Medaka bcl6b possesses conserved domains such as BTB domain, RD2 domain and four zinc fingers. Medaka bcl6bX1 to bcl6bX3 possess all three previously mentioned domains with minor differences in sequences. Medaka bcl6bX4 possesses only the BTB domain due to premature stopping, and bcl6bX5 possesses both the BTB domain and zinc fingers without the RD2 domain. Medaka bcl6b was expressed in the tissues including the brain, heart, gill, muscle, spleen, kidney, intestine, ovary and testes of adult fish. Medaka bcl6b was expressed in the embryos from very early stage, and could be detected clearly in the developing eyes by RT-PCR and in situ hybridization. Medaka bcl6b could respond to the stimuli of polyI:C and LPS in the kidney and spleen. Medaka bcl6bX1 to bcl6bX3 were the majority of the variants expressed in the adult tissues and the embryos, and were the major response to the stimulation of polyI:C and LPS in the spleen. These results suggested that bcl6b, including its isoforms, could function in various tissues and embryogenesis. Moreover, bcl6b might be a factor for immune response in medaka.

Functional analysis of Cullin 3 E3 ligases in tumorigenesis

Cullin 3-RING ligases (CRL3) play pivotal roles in the regulation of various physiological and pathological processes, including neoplastic events. The substrate adaptors of CRL3 typically contain a BTB domain that mediates the interaction between Cullin 3 and target substrates to promote their ubiquitination and subsequent degradation. The biological implications of CRL3 adaptor proteins have been well described where they have been found to play a role as either an oncogene, tumor suppressor, or can mediate either of these effects in a context-dependent manner. Among the extensively studied CRL3-based E3 ligases, the role of the adaptor protein SPOP (speckle type BTB/POZ protein) in tumorigenesis appears to be tissue or cellular context dependent. Specifically, SPOP acts as a tumor suppressor via destabilizing downstream oncoproteins in many malignancies, especially in prostate cancer. However, SPOP has largely an oncogenic role in kidney cancer. Keap1, another well-characterized CRL3 adaptor protein, likely serves as a tumor suppressor within diverse malignancies, mainly due to its specific turnover of its downstream oncogenic substrate, NRF2 (nuclear factor erythroid 2-related factor 2). In accordance with the physiological role the various CRL3 adaptors exhibit, several pharmacological agents have been developed to disrupt its E3 ligase activity, therefore blocking its potential oncogenic activity to mitigate tumorigenesis.

The survival decrease in gastric cancer is associated with the methylation of B-cell CLL/lymphoma 6 member B promoter

The methylation of B-cell CLL/lymphoma 6 member B (BCL6B) DNA promoter was detected in several malignancies. Here, we quantitatively detect the methylated status of CpG sites of BCL6B DNA promoter of 459 patients with gastric cancer (GC) by using bisulfite gene sequencing. We show that patients with three or more methylated CpG sites in the BCL6B promoter were significantly associated with poor survival. Furthermore, by using the Akaike information criterion value calculation, we show that the methylated count of BCL6B promoter was identified to be the optimal prognostic predictor of GC patients.

CD137 ligand reverse signaling skews hematopoiesis towards myelopoiesis during aging

CD137 is a costimulatory molecule expressed on activated T cells. Its ligand, CD137L, is expressed on the surface of hematopoietic progenitor cells, and upon binding to CD137 induces reverse signaling into hematopoietic progenitor cells promoting their activation, proliferation and myeloid differentiation. Since aging is associated with an increasing number of myeloid cells we investigated the role of CD137 and CD137L on myelopoiesis during aging. Comparing 3 and 12 months old WT, CD137^−/− and CD137L^−/− mice we found significantly more granulocytes and monocytes in the bone marrow of older WT mice, while this age-dependent increase was absent in CD137^−/− and CD137L^−/− mice. Instead, the bone marrow of 12 months old CD137^−/− and CD137L^−/− mice was characterized by an accumulation of hematopoietic progenitor cells, suggesting that the differentiation of hematopoietic progenitor cells became arrested in the absence of CD137L signaling. CD137L signaling is initiated by activated CD137-expressing, CD4⁺ T cells. These data identify a novel molecular mechanisms underlying immune aging by demonstrating that CD137-expressing CD4⁺ T cells in the bone marrow engage CD137L on hematopoietic progenitor cells, and that this CD137L signaling biases hematopoiesis towards myelopoiesis during aging.

The role of BTB-zinc finger transcription factors during T cell development and in the regulation of T cell-mediated immunity

The proper regulation of the development and function of peripheral helper and cytotoxic T cell lineages is essential for T cell-mediated adaptive immunity. Progress made during the last 10-15 years led to the identification of several transcription factors and transcription factor networks that control the development and function of T cell subsets. Among the transcription factors identified are also several members of the so-called BTB/POZ domain containing zinc finger (ZF) transcription factor family (BTB-ZF), and important roles of BTB-ZF factors have been described. In this review, we will provide an up-to-date overview about the role of BTB-ZF factors during T cell development and in peripheral T cells.

Role of SOX17 in hematopoietic development from human embryonic stem cells

To search for genes that promote hematopoietic development from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs), we overexpressed several known hematopoietic regulator genes in hESC/iPSC-derived CD34(+)CD43(-) endothelial cells (ECs) enriched in hemogenic endothelium (HE). Among the genes tested, only Sox17, a gene encoding a transcription factor of the SOX family, promoted cell growth and supported expansion of CD34(+)CD43(+)CD45(-/low) cells expressing the HE marker VE-cadherin. SOX17 was expressed at high levels in CD34(+)CD43(-) ECs compared with low levels in CD34(+)CD43(+)CD45(-) pre-hematopoietic progenitor cells (pre-HPCs) and CD34(+)CD43(+)CD45(+) HPCs. Sox17-overexpressing cells formed semiadherent cell aggregates and generated few hematopoietic progenies. However, they retained hemogenic potential and gave rise to hematopoietic progenies on inactivation of Sox17. Global gene-expression analyses revealed that the CD34(+)CD43(+)CD45(-/low) cells expanded on overexpression of Sox17 are HE-like cells developmentally placed between ECs and pre-HPCs. Sox17 overexpression also reprogrammed both pre-HPCs and HPCs into HE-like cells. Genome-wide mapping of Sox17-binding sites revealed that Sox17 activates the transcription of key regulator genes for vasculogenesis, hematopoiesis, and erythrocyte differentiation directly. Depletion of SOX17 in CD34(+)CD43(-) ECs severely compromised their hemogenic activity. These findings suggest that SOX17 plays a key role in priming hemogenic potential in ECs, thereby regulating hematopoietic development from hESCs/iPSCs.

The BTB-ZF transcription factors

The BTB-ZF (broad-complex, tramtrack and bric-à-brac--zinc finger) proteins are encoded by at least 49 genes in mouse and man and commonly serve as sequence-specific silencers of gene expression. This review will focus on the known physiological functions of mammalian BTB-ZF proteins, which include essential roles in the development of the immune system. We discuss their function in terminally differentiated lymphocytes and the progenitors that give rise to them, their action in hematopoietic malignancy and roles beyond the immune system.

Enhancing engraftment of cord blood cells via insight into the biology of stem/progenitor cell function

Cord blood (CB) transplantation has been used over the last 24 years to treat patients with malignant and nonmalignant disorders. CB has its advantages and disadvantages compared with other sources of hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) for transplantation. More knowledge of the cytokines and intracellular signaling molecules regulating HSCs and HPCs could be used to modulate these regulators for clinical benefit. This review provides information about the general field of CB transplantation and about studies from the author's laboratory that focus on regulation of HSCs and HPCs by CD26/DPPIV, SDF-1/CXCL12, the Rheb2-mTOR pathway, SIRT1, DEK, cyclin-dependent kinase inhibitors, and cytokines/growth factors. Cryopreservation of CB HSCs and HPCs is also briefly discussed.

POK/ZBTB proteins: an emerging family of proteins that regulate lymphoid development and function

The germinal center (GC) is a unique histological structure found in peripheral lymphoid organs. GCs provide an important source of humoral immunity by generating high affinity antibodies against a pathogen. The GC response is tightly regulated during clonal expansion, immunoglobulin modification, and affinity maturation, whereas its deregulation has a detrimental effect on immune function, leading to development of diseases, such as lymphoma and autoimmunity. LRF (lymphoma/leukemia-related factor), encoded by the ZBTB7A gene, is a transcriptional repressor belonging to the POK (POZ and Krüppel)/ZBTB (zing finger and BTB) protein family. LRF was originally identified as a PLZF (promyelocytic leukemia zinc finger) homolog that physically interacts with BCL6 (B-cell lymphoma 6), whose expression is required for GC formation and associated with non-Hodgkin's lymphoma. Recently, our group demonstrated that LRF plays critical roles in regulating lymphoid lineage commitment, mature B-cell development, and the GC response via distinct mechanisms. Herein, we review POK/ZBTB protein function in lymphoid development, with particular emphasis on the role of LRF in GC B cells.

CD1d expression on and regulation of murine hematopoietic stem and progenitor cells

In the present study, surface CD1d, which is involved in immune cell interactions, was assessed for effects on hematopoiesis. Mouse BM hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) express CD1d. The numbers and cycling status of HPCs in the BM and spleen of different strains of cd1d(-/-) mice were enhanced significantly, suggesting that CD1d is a negative regulator of HPCs. In support of this, CD1d was required for the SCF and Flt3 ligand synergistic enhancement of CSF induction of HPC colony formation and for HPC response to myelosuppressive chemokines. Colony formation by immature subsets of HPCs was greatly enhanced when normal, but not cd1d(-/-), BM cells were pretreated with CD1d Abs in vitro. These effects required the full CD1d cytoplasmic tail. In contrast, long-term, but not short-term, repopulating HSC engraftment was impaired significantly, an effect that was minimally influenced by the presence of a truncated CD1d cytoplasmic tail. Pretreatment of normal BM cells with CD1d Abs greatly enhanced their engraftment of HSCs. The results of the present study implicate CD1d in a previously unrecognized regulatory role of normal and stressed hematopoiesis.

BAZF, a novel component of cullin3-based E3 ligase complex, mediates VEGFR and Notch cross-signaling in angiogenesis

Angiogenic homeostasis is maintained by a balance between vascular endothelial growth factor (VEGF) and Notch signaling in endothelial cells (ECs). We screened for molecules that might mediate the coupling of VEGF signal transduction with down-regulation of Notch signaling, and identified B-cell chronic lymphocytic leukemia/lymphoma6-associated zinc finger protein (BAZF). BAZF was induced by VEGF-A in ECs to bind to the Notch signaling factor C-promoter binding factor 1 (CBF1), and to promote the degradation of CBF1 through polyubiquitination in a CBF1-cullin3 (CUL3) E3 ligase complex. BAZF disruption in vivo decreased endothelial tip cell number and filopodia protrusion, and markedly abrogated vascular plexus formation in the mouse retina, overlapping the retinal phenotype seen in response to Notch activation. Further, impaired angiogenesis and capillary remodeling were observed in skin-wounded BAZF(-/-) mice. We therefore propose that BAZF supports angiogenic sprouting via BAZF-CUL3-based polyubiquitination-dependent degradation of CBF1 to down-regulate Notch signaling.

The BTB-ZF family of transcription factors: key regulators of lineage commitment and effector function development in the immune system

Successful immunity depends upon the activity of multiple cell types. Commitment of pluripotent precursor cells to specific lineages, such as T or B cells, is obviously fundamental to this process. However, it is also becoming clear that continued differentiation and specialization of lymphoid cells is equally important for immune system integrity. Several members of the BTB-ZF family have emerged as critical factors that control development of specific lineages and also of specific effector subsets within these lineages. For example, BTB-ZF genes have been shown to control T cell versus B cell commitment and CD4 versus CD8 lineage commitment. Others, such as PLZF for NKT cells and Bcl-6 for T follicular helper cells, are necessary for the acquisition of effector functions. In this review, we summarize current findings concerning the BTB-ZF family members with a reported role in the immune system.

BCL6-mediated repression of p53 is critical for leukemia stem cell survival in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is induced by the oncogenic BCR-ABL1 tyrosine kinase and can be effectively treated for many years with tyrosine kinase inhibitors (TKIs). However, unless CML patients receive life-long TKI treatment, leukemia will eventually recur; this is attributed to the failure of TKI treatment to eradicate leukemia-initiating cells (LICs). Recent work demonstrated that FoxO factors are critical for maintenance of CML-initiating cells; however, the mechanism of FoxO-dependent leukemia initiation remained elusive. Here, we identified the BCL6 protooncogene as a critical effector downstream of FoxO in self-renewal signaling of CML-initiating cells. BCL6 represses Arf and p53 in CML cells and is required for colony formation and initiation of leukemia. Importantly, peptide inhibition of BCL6 in human CML cells compromises colony formation and leukemia initiation in transplant recipients and selectively eradicates CD34⁺ CD38⁻ LICs in patient-derived CML samples. These findings suggest that pharmacological inhibition of BCL6 may represent a novel strategy to eradicate LICs in CML. Clinical validation of this concept could limit the duration of TKI treatment in CML patients, which is currently life-long, and substantially decrease the risk of blast crisis transformation.

Gene expression programs of mouse endothelial cells in kidney development and disease

Endothelial cells are remarkably heterogeneous in both morphology and function, and they play critical roles in the formation of multiple organ systems. In addition endothelial cell dysfunction can contribute to disease processes, including diabetic nephropathy, which is a leading cause of end stage renal disease. In this report we define the comprehensive gene expression programs of multiple types of kidney endothelial cells, and analyze the differences that distinguish them. Endothelial cells were purified from Tie2-GFP mice by cell dissociation and fluorescent activated cell sorting. Microarrays were then used to provide a global, quantitative and sensitive measure of gene expression levels. We examined renal endothelial cells from the embryo and from the adult glomerulus, cortex and medulla compartments, as well as the glomerular endothelial cells of the db/db mutant mouse, which represents a model for human diabetic nephropathy. The results identified the growth factors, receptors and transcription factors expressed by these multiple endothelial cell types. Biological processes and molecular pathways were characterized in exquisite detail. Cell type specific gene expression patterns were defined, finding novel molecular markers and providing a better understanding of compartmental distinctions. Further, analysis of enriched, evolutionarily conserved transcription factor binding sites in the promoters of co-activated genes begins to define the genetic regulatory network of renal endothelial cell formation. Finally, the gene expression differences associated with diabetic nephropathy were defined, providing a global view of both the pathogenic and protective pathways activated. These studies provide a rich resource to facilitate further investigations of endothelial cell functions in kidney development, adult compartments, and disease.

FoxP3+ regulatory T cells restrain splenic extramedullary myelopoiesis via suppression of hemopoietic cytokine-producing T cells

Extramedullary myelopoiesis occurs in peripheral organs such as spleen and produces many types of myeloid cells with diverse functions in response to inflammation and infection. It is increased during immune responses and chronic inflammation and is a significant factor in regulating inflammatory diseases and immunity. Increased myeloid cells are found in FoxP3-deficient mice but the mechanism has been unclear. We investigated the mechanism by which FoxP3(+) regulatory T cells regulate the extramedullary myelopoiesis. We found that Ab or genetic depletion of FoxP3(+) regulatory T cells greatly increased the number of the myeloid progenitors in spleen during immune responses. Consistently, the splenic myelopoiesis was effectively suppressed by increased numbers of natural or induced FoxP3(+) regulatory T cells. We demonstrated that myelopoiesis is positively regulated by splenic CD4(+) T cells that produce myelopoietic cytokines (GM-CSF and IL-3), and these effector CD4(+) T cells are induced from naive CD4(+) T cells in response to antigenic stimulation. FoxP3(+) regulatory T cells were able to effectively suppress the differentiation of naive T cells into myelopoietic cytokine-producing T cells. This suppression was found to be dependent on cell contact but independent of TGFbeta. Unlike splenic myelopoiesis, marrow myelopoiesis is not significantly affected by FoxP3(+) regulatory T cells. We conclude that FoxP3(+) T cells can negatively regulate splenic extramedullary myelopoiesis by suppressing the naive T cell differentiation into myelopoietic cytokine-producing CD4(+) T cells. Our results provide new insights into regulation of extramedullary myelopoiesis.

Kaposi's sarcoma-associated herpesvirus encodes an ortholog of miR-155

MicroRNAs (miRNAs) are small noncoding RNAs that posttranscriptionally regulate gene expression by binding to 3'-untranslated regions (3'UTRs) of target mRNAs. Kaposi's sarcoma-associated herpesvirus (KSHV), a virus linked to malignancies including primary effusion lymphoma (PEL), encodes 12 miRNA genes, but only a few regulatory targets are known. We found that KSHV-miR-K12-11 shares 100% seed sequence homology with hsa-miR-155, an miRNA frequently found to be up-regulated in lymphomas and critically important for B-cell development. Based on this seed sequence homology, we hypothesized that both miRNAs regulate a common set of target genes and, as a result, could have similar biological activities. Examination of five PEL lines showed that PELs do not express miR-155 but do express high levels of miR-K12-11. Bioinformatic tools predicted the transcriptional repressor BACH-1 to be targeted by both miRNAs, and ectopic expression of either miR-155 or miR-K12-11 inhibited a BACH-1 3'UTR-containing reporter. Furthermore, BACH-1 protein levels are low in cells expressing either miRNA. Gene expression profiling of miRNA-expressing stable cell lines revealed 66 genes that were commonly down-regulated. For select genes, miRNA targeting was confirmed by reporter assays. Thus, based on our in silico predictions, reporter assays, and expression profiling data, miR-K12-11 and miR-155 regulate a common set of cellular targets. Given the role of miR-155 during B-cell maturation, we speculate that miR-K12-11 may contribute to the distinct developmental phenotype of PEL cells, which are blocked in a late stage of B-cell development. Together, these findings indicate that KSHV miR-K12-11 is an ortholog of miR-155.