Repression of PML nuclear body-associated transcription by oxidative stress-activated Bach2

BACH2: The Future of Induced T-Regulatory Cell Therapies

BACH2 (BTB Domain and CNC Homolog 2) is a transcription factor that serves as a central regulator of immune cell differentiation and function, particularly in T and B lymphocytes. A picture is emerging that BACH2 may function as a master regulator of cell fate that is exquisitely sensitive to cell activation status. In particular, BACH2 plays a key role in stabilizing the phenotype and suppressive function of transforming growth factor-beta (TGF-β)-derived human forkhead box protein P3 (FOXP3)+ inducible regulatory T cells (iTregs), a cell type that holds great clinical potential as a cell therapeutic for diverse inflammatory conditions. As such, BACH2 potentially could be targeted to overcome the instability of the iTreg phenotype and suppressive function that has hampered their clinical application. In this review, we focus on the role of BACH2 in T cell fate and iTreg function and stability. We suggest approaches to modulate BACH2 function that may lead to more stable and efficacious Treg cell therapies.

An autocrine Vitamin D-driven Th1 shutdown program can be exploited for COVID-19

Pro-inflammatory immune responses are necessary for effective pathogen clearance, but cause severe tissue damage if not shut down in a timely manner ^1,2 . Excessive complement and IFN-γ-associated responses are known drivers of immunopathogenesis ³ and are among the most highly induced immune programs in hyper-inflammatory SARS-CoV2 lung infection ⁴ . The molecular mechanisms that govern orderly shutdown and retraction of these responses remain poorly understood. Here, we show that complement triggers contraction of IFN-γ producing CD4 ⁺ T helper (Th) 1 cell responses by inducing expression of the vitamin D (VitD) receptor (VDR) and CYP27B1, the enzyme that activates VitD, permitting T cells to both activate and respond to VitD. VitD then initiates the transition from pro-inflammatory IFN-γ ⁺ Th1 cells to suppressive IL-10 ⁺ Th1 cells. This process is primed by dynamic changes in the epigenetic landscape of CD4 ⁺ T cells, generating superenhancers and recruiting c-JUN and BACH2, a key immunoregulatory transcription factor ^5-7 . Accordingly, cells in psoriatic skin treated with VitD increased BACH2 expression, and BACH2 haplo-insufficient CD4 ⁺ T cells were defective in IL-10 production. As proof-of-concept, we show that CD4 ⁺ T cells in the bronchoalveolar lavage fluid (BALF) of patients with COVID-19 are Th1-skewed and that VDR is among the top regulators of genes induced by SARS-CoV2. Importantly, genes normally down-regulated by VitD were de-repressed in CD4 ⁺ BALF T cells of COVID-19, indicating that the VitD-driven shutdown program is impaired in this setting. The active metabolite of VitD, alfacalcidol, and cortico-steroids were among the top predicted pharmaceuticals that could normalize SARS-CoV2 induced genes. These data indicate that adjunct therapy with VitD in the context of other immunomodulatory drugs may be a beneficial strategy to dampen hyperinflammation in severe COVID-19.

Binding partners of NRF2: Functions and regulatory mechanisms

NRF2 is a redox-sensitive transcription factor that plays an important role in protecting organisms against diverse types of electrophiles or oxidants. The level of NRF2 is maintained low in normal cells, but highly elevated in cancer provoking chemoresistance or radioresistance. It is now recognized that NRF2 does not merely maintain the redox balance, but also plays significant roles in autophagy, apoptosis, cell cycle progression, and stem cell differentiation, all of which could be possibly attributable to the existence of multiple binding proteins. In the present manuscript, we summarize direct binding partners of NRF2 and illustrate how they bind to NRF2 and regulate its stability or activity.

PML modulates H3.3 targeting to telomeric and centromeric repeats in mouse fibroblasts

Targeted deposition of histone variant H3.3 into chromatin is paramount for proper regulation of chromatin integrity, particularly in heterochromatic regions including repeats. We have recently shown that the promyelocytic leukemia (PML) protein prevents H3.3 from being deposited in large heterochromatic PML-associated domains (PADs). However, to what extent PML modulates H3.3 loading on chromatin in other areas of the genome remains unexplored. Here, we examined the impact of PML on targeting of H3.3 to genes and repeat regions that reside outside PADs. We show that loss of PML increases H3.3 deposition in subtelomeric, telomeric, pericentric and centromeric repeats in mouse embryonic fibroblasts, while other repeat classes are not affected. Expression of major satellite, minor satellite and telomeric non-coding transcripts is altered in Pml-null cells. In particular, telomeric Terra transcripts are strongly upregulated, in concordance with a marked reduction in H4K20me3 at these sites. Lastly, for most genes H3.3 enrichment or gene expression outcomes are independent of PML. Our data argue towards the importance of a PML-H3.3 axis in preserving a heterochromatin state at centromeres and telomeres.

Age-related changes in the BACH2 and PRDM1 genes in lymphocytes from healthy donors and chronic lymphocytic leukemia patients

Background

Age-related genetic changes in lymphocyte subsets are not currently well documented. BACH2 is a transcription factor that plays an important role in immune-mediated homeostasis by tightly regulating PRDM1 expression in both B-cells and T-cells. BACH2 gene expression is highly sensitive to DNA damage in aged mice. This concept led us to investigate the variation in BACH2 and also PRDM1 expression in major lymphocyte subsets with age.

Methods

Lymphocyte subsets from 60 healthy donors, aged from 20 to 90 years, and 41 untreated chronic lymphocytic leukemia patients were studied. BACH2 and PRDM1 gene expression was analyzed by real-time quantitative PCR. BACH2 gene expression was correlated with its protein expression. Lymphocyte apoptosis was evaluated after intracellular oxidative stress-inducing etoposide treatment of T and B cells.

Results

Our analysis shows BACH2 mRNA downregulation with age in healthy donor CD4+, CD8+ T-cells and CD19+ B-cells. Decreased BACH2 expression was also correlated with an age-related reduction in CD8 + CD28+ T-cells. We found a strong correlation between age-related BACH2 downregulation and decreased CD4+ T-cell and CD19+ B-cell apoptosis. PRDM1, as expected, was significantly upregulated in CD4+ T-cells, CD8+ T-cells and CD19+ B-cells, and inversely correlated with BACH2. A comparison of untreated chronic lymphocytic leukemia patients with age-matched healthy donors reveals that BACH2 mRNA expression was further reduced in CD4+ T-cells, CD8+ T-cells and leukemic-B cells. PRDM1 gene expression was consequently significantly upregulated in CD4+ and CD8+ T-cells in chronic lymphocytic leukemia patients but not in their leukemic B-cells.

Conclusion

Overall, our data suggest that BACH2 and PRDM1 genes are significantly correlated with age in human immune cells and may be involved in immunosenescence.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5276-2) contains supplementary material, which is available to authorized users.

RETRACTED: Bach2 regulates aberrant activation of B cell in systemic lupus erythematosus and can be negatively regulated by BCR-ABL/PI3K

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). The article has been retracted at the request of the Editor-in-Chief and the authors. The journal is retracting this paper after the authors reached out to the journal with a statement that several images contained duplications from another published paper. Part of the panels in Figure 1D and Figure 4D are duplications of panels in Figure 7 of Wang et al., Int J Mol Sci (2016), DOI: 10.3390/ijms17060969. The β-actin panel in Figure 2E is same as the α-tubulin panel in Figure 4B. In addition, the corresponding author informed the journal that “there were serious conflicts of personal interest, part of co-authors of this paper were not involved in the study and not aware of the submission, and they did not authorize their names to appear in the article”.

Herpesvirus Latency: On the Importance of Positioning Oneself

The nucleus is composed of multiple compartments and domains, which directly or indirectly influence many cellular processes including gene expression, RNA splicing and maturation, protein post-translational modifications, and chromosome segregation. Nuclear-replicating viruses, especially herpesviruses, have co-evolved with the cell, adopting strategies to counteract and eventually hijack this hostile environment for their own benefit. This allows them to persist in the host for the entire life of an individual and to ensure their maintenance in the target species. Herpesviruses establish latency in dividing or postmitotic cells from which they can efficiently reactivate after sometimes years of a seemingly dormant state. Therefore, herpesviruses circumvent the threat of permanent silencing by reactivating their dormant genomes just enough to escape extinction, but not too much to avoid life-threatening damage to the host. In addition, herpesviruses that establish latency in dividing cells must adopt strategies to maintain their genomes in the daughter cells to avoid extinction by dilution of their genomes following multiple cell divisions. From a biochemical point of view, reactivation and maintenance of viral genomes in dividing cells occur successfully because the viral genomes interact with the nuclear architecture in a way that allows the genomes to be transmitted faithfully and to benefit from the nuclear micro-environments that allow reactivation following specific stimuli. Therefore, spatial positioning of the viral genomes within the nucleus is likely to be essential for the success of the latent infection and, beyond that, for the maintenance of herpesviruses in their respective hosts.

Nucleus accumbens associated 1 is recruited within the promyelocytic leukemia nuclear body through SUMO modification

Nucleus accumbens associated 1 (NACC1) is a cancer-associated BTB/POZ (pox virus and zinc finger/bric-a-brac tramtrack broad complex) gene, and is involved in several cellular functions in neurons, cancer and stem cells. Some of the BTB/POZ proteins associated with cancer biology are SUMOylated, which appears to play an important role in transcription regulation. We show that NACC1 is SUMOylated on a phylogenetically conserved lysine (K167) out of three consensus SUMOylation motif sites. Amino acid substitution in the SIM sequence (SIM/M) within the BTB/POZ domain partially reduced K167 SUMOylation activity of NACC1. Overexpression of GFP-NACC1 fusion protein leads to formation of discrete nuclear foci similar to promyelocytic leukemia nuclear bodies (PML-NB), which colocalized with SUMO paralogues (SUMO1/2/3). Both NACC1 nuclear body formation and colocalization with SUMO paralogues were completely suppressed in the GFP-NACC1-SIM/M mutant, whereas they were partially maintained in the NACC1 K167R mutant. Confocal immunofluorescence analysis showed that endogenous and exogenous NACC1 proteins colocalized with endogenous PML protein. A pull-down assay revealed that the consensus motifs of the SUMO acceptor site at K167 and the SIM within the BTB/POZ domain were both necessary for efficient binding to PML protein. Our study demonstrates that NACC1 can be modified by SUMO paralogues, and cooperates with PML protein.

Association investigation of BACH2 rs3757247 and SOD2 rs4880 polymorphisms with the type 1 diabetes and diabetes long-term complications risk in the Polish population

Genetic factors are indicated in the development of type 1 diabetes (DM1). Recently, nucleotide variants of BACH2 and SOD2 have been associated with this chronic condition. Therefore, the purpose of the present study was to investigate the contribution of BACH2 rs3757247 and SOD2 rs4880 (Ala16Val) polymorphisms to the risk of DM1 and diabetes long-term complications. Selected polymorphic variants of BACH2 and SOD2 were investigated in a group of 141 patients with DM1 and in a group of age, gender-matched healthy subjects (n=369) using a high-resolution melting curve method. There was no evidence for either allelic or genotypic association with the risk of DM1 and diabetes chronic complications for analysed polymorphisms. In addition, no interaction between BACH2 and SOD2 variants in the development of this condition was observed. However, the frequency of BACH2 rs3757247 AG and AA genotypes was statistically different between DM1 patients with retinopathy and healthy individuals (odds ratio, 2.455; 95% confidence interval, 0.999-6.035; P=0.044), but this result did not survive multiple testing corrections. The present study did not confirm the involvement of BACH2 rs3757247 and SOD2 rs4880 polymorphisms in the development of DM1 and diabetes long-term complications. Further studies in a larger population sample are required.

Comparative ultrastructure of CRM1-Nucleolar bodies (CNoBs), Intranucleolar bodies (INBs) and hybrid PML/p62 bodies uncovers new facets of nuclear body dynamic and diversity

In order to gain insights on the nuclear organization in mammalian cells, we characterized ultrastructurally nuclear bodies (NBs) previously described as fluorescent foci. Using high resolution immunoelectron microscopy (I-EM), we provide evidence that CNoBs (CRM1-Nucleolar bodies) and INBs (Intranucleolar bodies) are distinct genuine nucleolar structures in untreated HeLa cells. INBs are fibrillar and concentrate the post-translational modifiers SUMO1 and SUMO-2/3 as strongly as PML bodies. In contrast, the smallest CRM1-labeled CNoBs are vitreous, preferentially located at the periphery of the nucleolus and, intricately linked to the chromatin network. Upon blockage of the CRM1-dependent nuclear export by leptomycin B (LMB), CNoBs disappear while p62/SQSTM1-containing fibrillar nuclear bodies are induced. These p62 bodies are enriched in ubiquitinated proteins. They progressively associate with PML bodies to form hybrid bodies of which PML decorates the periphery while p62/SQSTM1 is centrally-located. Our study is expanding the repertoire of nuclear bodies; revealing a previously unrecognized composite nucleolar landscape and a new mode of interactions between ubiquitous (PML) and stress-induced (p62) nuclear bodies, resulting in the formation of hybrid bodies.

Association between BACH2 expression and clinical prognosis in diffuse large B-cell lymphoma

BACH2, a B cell-specific transcriptional repressor, plays a significant role in B cell maturation. Despite a number of previous studies, the clinicopathological significance of BACH2 expression in diffuse large B cell lymphoma (DLBCL) remains to be established. The present study was performed to validate the significance of BACH2 expression as a predictor of prognosis in DLBCL. A total of 94 DLBCL cases were included in the present study. All were diagnosed between 2008 and 2011, and thorough clinical and pathological investigations were possible, including immunohistochemical analysis of BACH2. Eighteen cases were selected by positive MYC gene alteration (MYC+ group) according to cytogenetic study. The remaining 76 cases were subclassified into germinal center B cell phenotype (GCB group, 38 cases) or non-GCB phenotype (non-GCB group, 38 cases). There were no significant differences between the two groups with regard to clinical characteristics and outcomes. In the GCB group, 21 cases were judged to have high BACH2 expression, with 19 cases in the non-GCB group. In cases with high BACH2 expression in GCB and non-GCB groups, the 3-year overall survival (OS) rate was significantly shorter than that with low expression (71.7% vs 91.3%, P = 0.0256). In the MYC+ group, 15 cases had high BACH2 expression levels. Although overall the MYC+ group showed short survival time (3-year OS 35.0%), 3 out of 4 cases with low BACH2 expression are alive without disease relapse at the time of publication of this paper. In conclusion, BACH2 expression level is a promising predictor of prognosis for DLBCL.

BACH2: a marker of DNA damage and ageing

DNA damage and ageing share expression changes involving alterations in many aspects of metabolism, suppression of growth and upregulation of defence and genome maintenance systems. "Omics" technologies have permitted large-scale parallel measurements covering global cellular constituents and aided the identification of specific response pathways that change during ageing and after DNA damage. We have set out to identify genes with highly conserved response patterns through meta-analysis of mRNA expression datasets collected during natural ageing and accelerated ageing caused by a Transcription-Coupled Nucleotide Excision Repair (TC-NER) defect in a diverse set of organs and tissues in mice, and from in vitro UV-induced DNA damage in a variety of murine cells. The identified set of genes that show similar expression patterns in response to organ ageing (accelerated and normal), and endogenously and exogenously induced DNA damage, consists of genes involved in anti-oxidant systems and includes the transcription factor Bach2 as one of the most consistent markers. BACH2 was originally identified as a partner of the small Maf proteins and antagonist of the NRF2 anti-oxidant defence pathway and has been implicated in B-cell differentiation and immune system homeostasis. Although BACH2 has never before been associated with UV-induced damage or ageing, it shows a strong downregulation in both conditions. We have characterized the dynamics of Bach2 expression in response to DNA damage and show that it is a highly sensitive responder to transcription-blocking DNA lesions. Gene expression profiling using Affymetrix microarray analysis after siRNA-mediated silencing of Bach2 identified cell cycle and transcription regulation as the most significantly altered processes consistent with a function as transcription factor affecting proliferation.

Nuclear translocation of B-cell-specific transcription factor, BACH2, modulates ROS mediated cytotoxic responses in mantle cell lymphoma

BACH2, a B-cell specific transcription factor, plays a critical role in oxidative stress-mediated apoptosis. Bortezomib (Velcade^TM) is widely used to treat relapsed mantle cell lymphoma (MCL) patients despite varying clinical outcomes. As one of the potential mechanisms of action, bortezomib was reported to elicit endoplasmic reticulum (ER) stress which triggers reactive oxygen species (ROS). In the present study, we investigated the redox-sensitive intracellular mechanism that might play a critical role in bortezomib response in MCL cells. We demonstrated that in MCL cells that are sensitive to bortezomib treatments, BACH2 was translocated to the nucleus in response to bortezomib and induced apoptotic responses through the modulation of anti-oxidative and anti-apoptotic genes. On the other hand, in bortezomib resistant cells, BACH2 expression was confined in the cytoplasm and no suppression of antiapoptotic or antioxidative genes, Nrf2, Gss, CAT, HO-1 and MCL1, was detected. Importantly, levels of BACH2 were significantly higher in bortezomib sensitive MCL patient cells, indicating that BACH2 levels could be an indicator for clinical bortezomib responses. BACH2 translocation to the cytoplasm after phosphorylation was inhibited by PI3K inhibitors and combinatory regimens of bortezomib and PI3K inhibitors sensitized MCL cells to bortezomib. These data suggest that cellular distribution of BACH2 in response to ROS determines the threshold for the induction of apoptosis. Therapies that inhibit BACH2 phosphorylation could be the key for increasing bortezomib cytotoxic response in patients.

Over-expression of BACH2 is related to ongoing somatic hypermutation of the immunoglobulin heavy chain gene variable region of de novo diffuse large B-cell lymphoma

The basic region-leucine zipper (bZip) factor BTB, CNC homology 2 (BACH2) is known to have important roles in class switch recombination and somatic hypermutation (SHM) of the immunoglobulin (Ig) gene. In this study, we investigated the relationship between the expression of BACH2 and the status of SHM of the Ig heavy chain gene variable region (IgHV) for SHM in diffuse large B-cell lymphoma (DLBCL). We examined 20 cases of DLBCL, 13 of which were germinal center B-cell (GCB) DLBCL and 7 were non-GCB DLBCL. Seven cases were negative, 6 were positive (cytoplasmic expression) and 7 were strongly positive (both nuclear and cytoplasmic expression) for BACH2. Confirmed mutation (CM) was identified in 8 cases and the CM index (number of confirmed mutations per 10 subclones) was distributed from 0 to 5. A CM index of 7 strongly positive (over-expression) cases with BACH2 were distributed from 0 to 5, and that of 7 negative and 6 positive cases were distributed from 0 to 1. Over-expression of BACH2 was statistically related to CM index (P = 0.008). In conclusion, over-expression of BACH2 is critical for ongoing SHM of IgHV in DLBCL, and our data suggest that BACH2 may play an essential role for SHM of the Ig gene in B-cell lymphoma.

Bach2 maintains T cells in a naive state by suppressing effector memory-related genes

The transcriptional repressor BTB and CNC homology 2 (Bach2) is thought to be mainly expressed in B cells with specific functions such as class switch recombination and somatic hypermutation, but its function in T cells is not known. We found equal Bach2 expression in T cells and analyzed its function using Bach2-deficient (-/-) mice. Although T-cell development was normal, numbers of peripheral naive T cells were decreased, which rapidly produced Th2 cytokines after TCR stimulation. Bach2(-/-) naive T cells highly expressed genes related to effector-memory T cells such as CCR4, ST-2 and Blimp-1. Enhanced expression of these genes induced Bach2(-/-) naive T cells to migrate toward CCR4-ligand and respond to IL33. Forced expression of Bach2 restored the expression of these genes. Using Chromatin Immunoprecipitation (ChIP)-seq analysis, we identified S100 calcium binding protein a, Heme oxigenase 1, and prolyl hydroxylase 3 as Bach2 direct target genes, which are highly expressed in effector-memory T cells. These findings indicate that Bach2 suppresses effector memory-related genes to maintain the naive T-cell state and regulates generation of effector-memory T cells.

The structure of the Bach2 POZ-domain dimer reveals an intersubunit disulfide bond

Bach2 is a transcriptional repressor that is expressed during specific stages of B-cell development and in neuronal cells. It plays a critical role in modulating class-switch recombination during the differentiation of mature B cells to antibody-secreting plasma cells and it is also an important regulator of apoptotic responses to oxidative stress. Bach2 has been implicated both as an oncogene and as a tumour suppressor in human malignancy. The interaction of Bach2 with its target genes is mediated via its basic leucine-zipper region, whereas the N-terminal POZ domain recruits transcriptional co-repressors and class II histone deacetylases. Here, the crystal structure of the human Bach2 POZ domain is reported at 2.1 Å resolution. The Bach2 POZ-domain dimer resembles the POZ-domain dimers of the POZ zinc finger transcription factors and dimerization is independent of an N-terminal region that has previously been implicated in the dimerization of the POZ basic leucine-zipper protein Bach1. The Bach2 POZ domain crystallized in two forms which differed by the presence of an intersubunit disulfide bond. The intersubunit disulfide bond is present both in bacterially expressed Bach2 POZ domain in solution and in protein expressed in transfected eukaryotic cells. These crystal structures will be relevant for understanding the regulation of Bach2 in response to oxidative stress and for the design of therapeutics that target the Bach2 POZ domain in human malignancy.

The Xanthomonas type III effector XopD targets the Arabidopsis transcription factor MYB30 to suppress plant defense

Plant and animal pathogens inject type III effectors (T3Es) into host cells to suppress host immunity and promote successful infection. XopD, a T3E from Xanthomonas campestris pv vesicatoria, has been proposed to promote bacterial growth by targeting plant transcription factors and/or regulators. Here, we show that XopD from the B100 strain of X. campestris pv campestris is able to target MYB30, a transcription factor that positively regulates Arabidopsis thaliana defense and associated cell death responses to bacteria through transcriptional activation of genes related to very-long-chain fatty acid (VLCFA) metabolism. XopD specifically interacts with MYB30, resulting in inhibition of the transcriptional activation of MYB30 VLCFA-related target genes and suppression of Arabidopsis defense. The helix-loop-helix domain of XopD is necessary and sufficient to mediate these effects. These results illustrate an original strategy developed by Xanthomonas to subvert plant defense and promote development of disease.

Stress-activated cap'n'collar transcription factors in aging and human disease

Cap'n'collar (Cnc) transcription factors are conserved in metazoans and have important developmental and homeostatic functions. The vertebrate Nrf1, Nrf2, and Nrf3; the Caenorhabditis elegans SKN-1; and the Drosophila CncC comprise a subgroup of Cnc factors that mediate adaptive responses to cellular stress. The most studied stress-activated Cnc factor is Nrf2, which orchestrates the transcriptional response of cells to oxidative stressors and electrophilic xenobiotics. In rodent models, signaling by Nrf2 defends against oxidative stress and aging-associated disorders, such as neurodegeneration, respiratory diseases, and cancer. In humans, polymorphisms that decrease Nrf2 abundance have been associated with various pathologies of the skin, respiratory system, and digestive tract. In addition to preventing disease in rodents and humans, Cnc factors have life-span-extending and anti-aging functions in invertebrates. However, despite the pro-longevity and antioxidant roles of stress-activated Cnc factors, their activity paradoxically declines in aging model organisms and in humans suffering from progressive respiratory disease or neurodegeneration. We review the roles and regulation of stress-activated Cnc factors across species, present all reported instances in which their activity is paradoxically decreased in aging and disease, and discuss the possibility that the pharmacological restoration of Nrf2 signaling may be useful in the prevention and treatment of age-related diseases.

Molecular mechanisms of Nrf2-mediated antioxidant response

Nrf2 is the key transcription factor regulating the antioxidant response. Nrf2 signaling is repressed by Keap1 at basal condition and induced by oxidative stress. Keap1 is recently identified as a Cullin 3-dependent substrate adaptor protein. A two-sites binding "hinge & latch" model vividly depicts how Keap1 can efficiently present Nrf2 as substrate for ubiquitination. Oxidative perturbation can impede Keap1-mediated Nrf2 ubiquitination but fail to disrupt Nrf2/Keap1 binding. Nrf2 per se is a redox-sensitive transcription factor. A new Nrf2-mediated redox signaling model is proposed based on these new discoveries. Free floating Nrf2 protein functions as a redox-sensitive probe. Keap1 instead functions as a gate keeper to control the availability of Nrf2 probes and thus regulates the overall sensitivity of the redox signaling.

Identification of novel Bach2 transcripts and protein isoforms through tagging analysis of retroviral integrations in B-cell lymphomas

BACKGROUND

The Bach2 gene functions as a transcriptional repressor in B-cells, showing high expression level only before the plasma cell stage. Several lines of evidence indicate that Bach2 is a B-cell specific tumor suppressor. We here address patterns of insertional mutagenesis and expression of Bach2 is a murine retroviral model of B-cell lymphoma induction.

RESULTS

We report that the Bach2 gene is a target of proviral integrations in B-cell lymphomas induced by murine leukemia virus. An alternative Bach2 promoter was identified within intron 2 and this promoter was activated in one of the tumors harboring proviral integration. The alternative promoter was active in both normal and tumor tissue and the tissue specificity of the two Bach2 promoters was similar. Three different alternatively used Bach2 terminal exons were identified to be located in intron 4. The inclusion of these exons resulted in the generation of Bach2 mRNA with open reading frames lacking the bZIP DNA binding domain present in the normal Bach2 protein, but retaining a partial BTB protein dimerization domain. Such Bach2 protein was excluded from the cell nucleus.

CONCLUSION

We have identified an alternative promoter and new protein isoforms of Bach2. Our data imply that activation of an alternative promoter by proviral integration serves as a possible mechanism of up-regulation of the Bach2 gene with a potential role in B-cell lymphomagenesis. The finding of novel Bach2 transcripts and protein isoforms will facilitate a better insight into the normal and pathophysiological regulation of the Bach2 gene.

14-3-3 regulates the nuclear import of class IIa histone deacetylases

Class IIa histone deacetylases (HDACs) form complexes with a class of transcriptional repressors in the nucleus. While screening for compounds that could block the association of HDAC4 with the BTB domain-containing transcriptional repressor Bach2, we discovered that phorbol 12-myristate 13-acetate (PMA) induced the cytoplasmic retention of HDAC4 mutants lacking a nuclear export signal (NES). Although PMA treatment and PKD overexpression has been proposed to facilitate the nuclear export of class IIa HDACs by creating 14-3-3 binding sites containing phosphoserines, our experiments using HDAC mutants demonstrated that PMA greatly reduces nuclear import. PMA treatment repressed the NLS activity in a manner dependent on 14-3-3 binding. These results suggest that nuclear HDAC4 is not tethered in the nucleus, but instead shuttles between the nucleus and the cytoplasm. Phosphorylation-induced 14-3-3 binding biases the balance of nucleo-cytoplasmic shuttling toward the cytoplasm by inhibiting nuclear import.

Crystal structure of the Bach1 BTB domain and its regulation of homodimerization

The BTB/POZ domain is known as a protein-protein interaction motif that mediates homodimer and higher order self-associations. Proteins containing the BTB domain exist throughout eukaryotes; however, there is little information about the mechanism that determines the oligomeric state of the BTB domain. To address this question, we have determined the X-ray structure of the mouse Bach1 BTB domain. The present structure is similar to the previously determined BTB domain folds, including the human Bach1 BTB domain; however, distinct structural features are present, such as a novel homodimer interaction surface. The homodimer formation was found to involve a novel hydrogen bond network and interactions between hydrophobic surfaces of the kinked N-terminus (N-hook) and the partner's C-terminal residues. The deletion of the N-hook resulted in the conversion of the homodimer into a monomer in solution, indicating that the N-hook promotes the homodimerization of the mBach1 BTB domain. We have also found that the BTB domain of Bach2, a protein highly related to Bach1, is present as a monomer due to a short peptide insertion at the N-hook. These results represent the first example of the key modulatory element of BTB domain homodimerization.

The mobility of Bach2 nuclear foci is regulated by SUMO-1 modification

The small ubiquitin-like modifier-1 (SUMO-1) modulates the functions of nuclear proteins by changing their structure and/or subnuclear localization. Several nuclear proteins form dynamic higher order nuclear structures, termed non-chromatin nuclear domains, which are involved in the regulation of nuclear function. However, the role that SUMO modification of the component proteins plays in the regulation of the activity and function of nuclear domains is unclear. Here we demonstrate that nuclear domains formed by Bach2, a transcription repressor, show restricted movement and undergo fusion events upon oxidative stress. Mutation of the SUMO-acceptor lysines in Bach2 alters the behavior of these nuclear foci and results in a decreased frequency of fusion events. We propose that SUMO modification is an important regulatory system for the mobility of the nuclear domains formed by Bach2.

Structure, dynamics and functions of promyelocytic leukaemia nuclear bodies

The promyelocytic leukaemia (PML) tumour suppressor protein epitomizes the PML-nuclear body (PML-NB) and is crucially required for the proper assembly of this macromolecular nuclear structure. Unlike other, more specialized subnuclear structures such as Cajal and Polycomb group bodies, PML-NBs are functionally promiscuous and have been implicated in the regulation of diverse cellular functions. PML-NBs are dynamic structures that favour the sequestration and release of proteins, mediate their post-translational modifications and promote specific nuclear events in response to various cellular stresses. Recent data suggest that PML-NBs may be heterogeneous in composition, mobility and function.

The role of Bach2 in nucleic acid-triggered antiviral innate immune responses

Bach2, a bZIP transcription factor originally identified as interacting with the small Maf family of bZIP proteins, has been shown to play important roles in oxidative stress-mediated cell death. Here, we examine the role of Bach2 in cell death during double-stranded (ds)RNA- and dsDNA-triggered antiviral innate immune responses. Bach2 expression was induced in HeLa cells upon dsRNA/dsDNA treatment and the suppression of Bach2 expression by siRNA treatment alleviated cell death triggered by dsRNA and dsDNA. Unexpectedly, DNA microarray analysis revealed that siRNA-mediated suppression of Bach2 resulted in the attenuated activation of genes involved in the antiviral innate immune response after dsRNA treatment. Our study thus demonstrates a novel role for Bach2 as a key regulator of nucleic acid-triggered antiviral responses in human cells.

Co-repressor SMRT and class II histone deacetylases promote Bach2 nuclear retention and formation of nuclear foci that are responsible for local transcriptional repression

Bach2 is a member of the BTB-basic region leucine zipper factor family and represses transcription activity directed by the TPA response element, the Maf recognition element (MARE) and the antioxidant-responsive element. Recently, it was reported that upon oxidative stress Bach2 forms nuclear foci surrounding the promyelocytic leukaemia (PML) bodies and specifically represses the transcription around the PML bodies. Here we report that expression of the silencing mediator of retinoid and thyroid receptor (SMRT) and histone deacetylase4 (HDAC4) enhances the formation of the Bach2 foci in the nuclear matrix. SMRT mediates the HDAC4 binding to Bach2, and HDAC4 facilitates the retention of Bach2 in the foci. Scratch transcription labelling and 3D-reconstruction from the confocal images demonstrated that transcription is suppressed in and around the Bach2 foci. Indeed, Bach2 bound MARE and repressed the expression from the chromosomally integrated MARE-driven reporter gene when co-expressed with SMRT and HDAC4. Our observations suggest that both SMRT and HDAC4 play an important role in nuclear retention and the Bach2 focus formation in the mammalian cell nucleus, which may contribute to the local transcription repression.

Plasmacytic transcription factor Blimp-1 is repressed by Bach2 in B cells

Bach2 is a B cell-specific transcription repressor whose deficiency in mice causes a reduced class switch recombination and a reduced somatic hypermutation of immunoglobulin genes. Little is known about the direct target genes of Bach2 in B cells. By analyzing various B cell and plasma cell lines, we showed that the expression patterns of Bach2 and Blimp-1 (B lymphocyte-induced maturation protein 1), a master regulator of plasma cell differentiation, are mutually exclusive. The reporter gene of the Blimp-1 gene (Prdm1) was repressed by the overexpression of Bach2 in B cell lines. The heterodimer of Bach2/MafK bound to the Maf recognition element located upstream of the Prdm1 promoter in an electrophoretic mobility shift assay. The binding of MafK in B cells to the Prdm1 Maf recognition element was confirmed by chromatin immunoprecipitation assays. When MafK was purified from the BAL17 B cell line, a significant portion of it was present as a heterodimer with Bach2, with no apparent formation of MafK homodimer. These results strongly suggest that Bach2 represses the expression of Blimp-1 together with MafK in B cells prior to plasma cell differentiation. Accordingly, the knockdown of Bach2 mRNA using short hairpin RNA in BAL17 cells resulted in higher levels of Prdm1 expression after the stimulation of B cell receptor by surface IgM cross-linking. Induction of Prdm1 was more robust and faster in primary Bach2-deficient B cells than in wild-type control B cells upon lipopolysaccharide stimulation. Therefore, the Prdm1 regulation in B cells involves the repression by Bach2, which may be cancelled upon terminal plasma cell differentiation.

Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the Bcr-Abl oncoprotein. We have previously reported that expression of the Bach2 transcription factor, which induces apoptosis in response to oxidative stress, is greatly reduced in CML cells. Because these cells are resistant to apoptosis, we tested whether Bach2 could also be regulated through posttranslational mechanisms that promote inhibition of the apoptotic response to mutagenic stimuli in CML. We found that Bach2 is phosphorylated on S521 via the phosphatidylinositol-3/S6 kinase pathway, and substitution of this site to alanine leads to nuclear accumulation of the protein, indicating that this phosphorylation is important for its subcellular localization. Ectopic expression of the S521 mutant imparts greater impairment to CML cell growth than the wild-type factor. Furthermore, we showed that Bach2 transcriptionally represses heme oxygenase-1, an antiapoptotic factor up-regulated in CML. Because CML cells are known to produce high levels of intracellular reactive oxygen species, overexpression of heme oxygenase-1 resulting from inhibition of Bach2 activity may contribute to their genomic instability and leukemic phenotype.

The heme-Bach1 pathway in the regulation of oxidative stress response and erythroid differentiation

Heme--as a prosthetic group of proteins required for oxygen transport and storage, respiration, and biosynthetic pathways--is essential for practically all forms of life. Additionally, the degradation products of heme (i.e., carbon monoxide, biliverdin, and bilirubin) produced by the enzymatic actions of heme oxygenase (HO) and biliverdin reductase, possess various biological activities in vivo. In mammalian cells, heme also functions as an intracellular regulator of gene expression by virtue of its ability to bind to Bach1, a transcription factor that functions in association with small Maf proteins. Normally, such complexes function as repressors by binding to specific target sequences, the Maf recognition element (MARE), within enhancers of genes encoding proteins such as HO-1 and beta-globin. By binding to Bach1, heme induces selective removal of the repressor from the gene enhancers permitting subsequent occupancy of the MAREs by activators that, interestingly, also contain small Maf proteins. Thus small Maf proteins play dual functions in gene expression: complexes with Bach1 repress MARE-dependent gene expression, whereas heterodimers with NF-E2 p45 or related factors (Nrf1, Nrf2, and Nrf3) activate MARE-driven genes. By modulating the equilibrium of the small Maf heterodimer network, heme regulates expression of the cytoprotective enzyme HO-1 during the stress response and of beta-globin during erythroid differentiation. Implications of such heme-regulated gene expression in human diseases including atherosclerosis are discussed.

Prognostic significance of BACH2 expression in diffuse large B-cell lymphoma: a study of the Osaka Lymphoma Study Group

PURPOSE

BACH2, a B-cell-specific transcription repressor, is abundantly expressed in lymphocytes of B-cell lineage as well as B-cell lymphoma cell lines. BACH2 possesses an inhibitory effect on proliferation of Raji cell lines derived from Burkitt's lymphoma. In this study, the prognostic significance of BACH2 expression was examined in diffuse large B-cell lymphoma (DLBCL).

PATIENTS AND METHODS

BACH2 expression was immunohistochemically examined on the paraffin-embedded sections obtained by biopsy from 108 patients (62 males and 46 females; age range, 23 to 85 years) with DLBCL. Staining intensity in the cytoplasm of the tumor cells was categorized as equal to or stronger (level 1) or weaker (level 2) than that in the endothelial cells in the same specimens.

RESULTS

Level 1 and 2 expression of BACH2 was found in 32.4% and 67.6% of patients, respectively. Patients with level 1 expression showed significantly better disease-free and overall survival rate than those with level 2 expression (both P < .05). Multivariate analysis revealed BACH2 expression level together with performance status, elevated serum level of lactate dehydrogenase, and treatment response to be independent factors for prognosis of the patients.

CONCLUSION

BACH2 expression level is a useful marker to predict disease-free and overall survival of patients with DLBCL.

Sumoylation of the novel protein hRIP{beta} is involved in replication protein A deposition in PML nuclear bodies

Replication protein A (RPA) is a single-stranded-DNA-binding protein composed of three subunits with molecular masses of 70, 32, and 14 kDa. The protein is involved in multiple processes of eukaryotic DNA metabolism, including DNA replication, repair, and recombination. In Xenopus, Xenopus RPA-interacting protein alpha has been identified as a carrier molecule of RPA into the nucleus. In this study, human RPA-interacting protein alpha (hRIPalpha) and five novel splice isoforms (named hRIPalpha, hRIPbeta, hRIPgamma, hRIPdelta1, hRIPdelta2, and hRIPdelta3 according to the lengths of their encoding peptides) were cloned. Among hRIP isoforms, hRIPalpha and hRIPbeta were found to be the major splice isoforms and to show different subcellular localizations. While hRIPalpha localized to the cytoplasm, hRIPbeta was found in the PML nuclear body. Modification of hRIPbeta by sumoylation was found to be required for localization to the PML nuclear body. The results of the present work demonstrate that hRIPbeta transports RPA into the PML nuclear body and releases RPA upon UV irradiation. hRIPbeta thus plays an important role in RPA deposition in PML nuclear bodies and thereby supplements RPA for DNA metabolism.

Identification of a novel BTB-zinc finger transcriptional repressor, CIBZ, that interacts with CtBP corepressor

The transcriptional corepressor C-terminal binding protein (CtBP) is thought to be involved in development and oncogenesis, but the regulation of its corepressor activity is largely unknown. We show here that a novel BTB-zinc finger protein, CIBZ (CtBP-interacting BTB zinc finger protein; a mouse ortholog of rat ZENON that was recently identified as an e-box/dyad binding protein), redistributes CtBP to pericentromeric foci from a diffuse nuclear localization in interphase cells. CIBZ physically associates with CtBP via a conserved CtBP binding motif, PLDLR. When heterologously targeted to DNA, CIBZ represses transcription via two independent repression domains, an N-terminal BTB domain and a PLDLR motif-containing RD2 region, in a histone deacetylase-independent and -dependent manner, respectively. Mutation in the PLDLR motif abolishes the CIBZ-CtBP interaction and transcriptional repression activity of RD2, but does not affect the repression activity of the BTB domain. Furthermore, this PLDLR-mutated CIBZ cannot target CtBP to pericentromeric foci, although it is localized to the pericentromeric foci itself. These results suggest that at least one repression mechanism mediated by CIBZ is recruitment of the CtBP/HDAC complex to pericentromeric foci, and that CIBZ may regulate pericentromeric targeting of CtBP.

RGS14 is a centrosomal and nuclear cytoplasmic shuttling protein that traffics to promyelocytic leukemia nuclear bodies following heat shock

RGS14, a member of the regulator of G-protein signaling (RGS) protein family, possesses an N-terminal RGS domain, two Raf-like Ras-binding domains, and a GoLoco motif, which has GDP dissociation inhibitor activity. In this study we show that unique among the known mammalian RGS proteins, RGS14 localizes in centrosomes. Its first Ras-binding domain is sufficient to target RGS14 to centrosomes. RGS14 also shuttles between the cytoplasm and nucleus, and its nuclear export depends on the CRM-1 nuclear export receptor. Mutation of a nuclear export signal or treatment with leptomycin B causes nuclear accumulation of RGS14 and its association with promyelocytic leukemia protein nuclear bodies. Furthermore, a point mutant defective in nuclear export fails to target to centrosomes, suggesting that nuclear cytoplasmic shuttling is necessary for its proper localization. Mild heat stress, but not proteotoxic or transcription-linked stresses, re-localizes the RGS14 from the cytoplasm to promyelocytic leukemia nuclear bodies. Expression of RGS14, but not point mutants that disrupt the functional activity of its RGS domain or GoLoco motif, enhances the reporter gene activity. The multifunctional domains and the dynamic subcellular localization of RGS14 implicate it in a diverse set of cellular processes including centrosome and nuclear functions and stress-induced signaling pathways.

Establishment of papillomavirus infection is enhanced by promyelocytic leukemia protein (PML) expression

Previous studies have suggested that most papillomaviruses enter the host cell via clathrin-dependent receptor-mediated endocytosis but have not addressed later steps in viral entry. To examine these events, we followed the localization of L2 and packaged DNA after entry of infectious virions or L1/L2 pseudovirions. Confocal microscopic analyses of HeLa cells showed a time-dependent uncoating of capsids in cytoplasmic vesicles and the accumulation of both L2 and viral DNA at distinct nuclear domains identified as nuclear domain 10 (ND10). Both L2 and the pseudogenome had a punctate distribution and localized to ND10 in promyelocytic leukemia protein (PML)-expressing cells, whereas L2 had a diffuse nuclear distribution in PML-/- cells. The number of pseudovirus-infected cells was an order of magnitude higher in the PML+ cells compared with the PML-/- cells, and viral genome transcription after infection with authentic bovine papillomavirus virions was similarly elevated in PML+ cells. The results identify a role for PML in the enhancement of viral infectivity in the early part of the life cycle. We propose a model in which L2 chaperones the viral genome to ND10 to efficiently initiate viral transcription.