Heme-mediated inhibition of Bach1 regulates the liver specificity and transience of the Nrf2-dependent induction of zebrafish heme oxygenase 1

The induction of the gene encoding heme oxygenase 1 (Hmox1, HO-1) by Nrf2 is unique compared with other Nrf2 targets. We previously showed that the Nrf2a-mediated induction of zebrafish hmox1a was liver specific and transient. We screened transcription factors that could repress the induction of hmox1a but not other Nrf2a targets and concluded that Bach1b was a prime candidate. In bach1b-knocked-down larvae, the induction of hmox1a was observed ectopically in nonliver tissues and persisted longer than normal fish, suggesting that Bach1 is the only regulator for both the liver-specific and transient induction of hmox1a. Co-knockdown of bach1b with its co-ortholog bach1a enhanced these effects. To determine why Bach1 could not repress the hmox1a induction in the liver, we analyzed the effects of a heme biosynthesis inhibitor, succinylacetone, and a heme precursor, hemin. Succinylacetone decreased the Nrf2a-mediated hmox1a induction, whereas pre-treatment with hemin caused ectopic induction of hmox1a in nonliver tissues, implying that the high heme levels in the liver may release the repressive activity of Bach1. Our results suggested that Bach1 regulates the liver specificity and transience of the Nrf2a-dependent induction of hmox1a and that heme mediates this regulation through Bach1 inhibition based on its level in each tissue

Nitro-fatty acids and cyclopentenone prostaglandins share strategies to activate the Keap1-Nrf2 system: a study using green fluorescent protein transgenic zebrafish

Nitro-fatty acids are electrophilic fatty acids produced in vivo from nitrogen peroxide that have many physiological activities. We recently demonstrated that nitro-fatty acids activate the Keap1-Nrf2 system, which protects cells from damage owing to electrophilic or oxidative stresses via transactivating an array of cytoprotective genes, although the molecular mechanism how they activate Nrf2 is unclear. A number of chemical compounds with different structures have been reported to activate the Keap1-Nrf2 system, which can be categorized into at least six classes based on their sensing pathways. In this study, we showed that nitro-oleic acid (OA-NO2), one of major nitro-fatty acids, activates Nrf2 in the same manner that of a cyclopentenone prostaglandin 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) using transgenic zebrafish that expresses green fluorescent protein (GFP) in response to Nrf2 activators. In transgenic embryos, GFP was induced in the whole body by treatment with OA-NO2, 15d-PGJ2 or diethylmaleate (DEM), but not with hydrogen peroxide (H2O2), when exogenous Nrf2 and Keap1 were co-overexpressed. Induction by OA-NO2 or 15d-PGJ2 but not DEM was observed, even when a C151S mutation was introduced in Keap1. Our results support the contention that OA-NO2 and 15d-PGJ2 share an analogous cysteine code as electrophiles and also have similar anti-inflammatory roles

Tissue-Restricted Expression of Nrf2 and Its Target Genes in Zebrafish with Gene-Specific Variations in the Induction Profiles

The Keap1-Nrf2 system serves as a defense mechanism against oxidative stress and electrophilic toxicants by inducing more than one hundred cytoprotective proteins, including antioxidants and phase 2 detoxifying enzymes. Since induction profiles of Nrf2 target genes have been studied exclusively in cultured cells, and not in animal models, their tissue-specificity has not been well characterized. In this paper, we examined and compared the tissue-specific expression of several Nrf2 target genes in zebrafish larvae by whole-mount in situ hybridization (WISH). Seven zebrafish genes (gstp1, mgst3b, prdx1, frrs1c, fthl, gclc and hmox1a) suitable for WISH analysis were selected from candidates for Nrf2 targets identified by microarray analysis. Tissue-restricted induction was observed in the nose, gill, and/or liver for all seven genes in response to Nrf2-activating compounds, diethylmaleate (DEM) and sulforaphane. The Nrf2 gene itself was dominantly expressed in these three tissues, implying that tissue-restricted induction of Nrf2 target genes is defined by tissue-specific expression of Nrf2. Interestingly, the induction of frrs1c and gclc in liver and nose, respectively, was quite low and that of hmox1a was restricted in the liver. These results indicate the existence of gene-specific variations in the tissue specificity, which can be controlled by factors other than Nrf2

New Insights into PLZF/Rara Mechanism during APL Onset: EZH2 on the Road

International audienceINTRODUCTION Inappropriate recruitment of functional Polycomb-Group proteins (PcG) may trigger epigenetic unbalance at very specific genomic loci that substantially contribute to the pathogenesis of Acute Myeloid Leukemia (AML). This concept was first described in Acute Promyelocytic Leukemia (APL) in which PcG proteins were abnormally addressed due to the expression of X-RARA fusion proteins and were involved in the treatment response of the disease. For instance, in the context of APL with t(11;17)(q23;q21) translocation, the resulted oncogenic fusion protein PLZF/RARA leads to abnormal recruitment of PcG at the promoters of genes involved in acid-trans-retinoic acid (ATRA) response (Boukarabila et al.). As a consequence of that and compared to other APL subtypes (e-g: PML/RARA), APL with PLZF/RARA are insensitive to ATRA. In the recent years, a repertoire of cis-regulatory enhancer elements has been dissected to reveal important insight about leukemia onset and define new subsets of the disease with different treatment responses (Bhagwat et al). As we previously reported that PLZF displayed epigenetic specificity on enhancers (Poplineau et al.) we questioned the role of PLZF/RARA on these regulatory regions during APL onset. METHODS We performed in vivo comparative epigenomic profiling (H3K27ac, H3K4me1, H3K27me3 and H3K4me3 ChIPseq) between normal myeloid progenitors (granulocyte-monocyte progenitors purified from wild-type mice) and PLZF/RARA transformed mouse progenitors (late promyelocytes purified from mice developing APL). To question the role of PcG in APL onset, we used retroviral overexpression of PLZF/RARA and transduced Lineage negative cells from a conditional KO EZH2 mouse model. Transformation was tested by replating assay and cells were characterized by FACS and morphology analyses. We also performed EZH2 pharmacological inhibition using GSK126 and UNC1999 on a human cell line expressing the fusion protein PLZF/RARA. We analyzed the impact of this inhibition on their transcriptomic signature (RNAseq) and their proliferative capacity. RESULTS Upon PLZF/RARA expression and APL progression, specific cis-regulatory enhancer elements were targeted by the H3K27me3 PcG repressive mark. This gain in poised enhancer regions, upon PLZF/RARA expression reflected a reoriented PcG activity, from enhancers regulating developmental processes to those regulating stress and immune responses. To demonstrate the importance of this H3K27me3 switch for APL progression, we investigated the effect of EZH2 loss during PLZF/RARA transformation. Using a conditional KO EZH2 mouse model, we demonstrated that PLZF/RARA required EZH2 activity to efficiently transform progenitors since EZH2 loss promoted differentiation that altered the replating capacity of the PLZF/RARA expressing cells. In addition, EZH2 inhibition by GSK126 revealed some interesting benefits since it sensitized PLZF/RARA transformed progenitors to ATRA treatment. Moreover, inhibition of EZH2 with GSK126 or UNC1999 induced a decrease in the proliferation advantage of a human PLZF/RARA-inducible cell line. This was linked to a change of its transcriptomic signature towards an expression pattern closer to the one observed in the parent cell line. CONCLUSION Taken together, our data showed that PLZF/RARA modifies H3K27me3 profiles at enhancer regions and requires EZH2 activity for APL onset. Finally, our results suggest that EZH2 inhibition could be a new promising therapeutic approach for retinoic-acid resistant APL. Disclosures No relevant conflicts of interest to declare

Ezh1 Targets Bivalent Genes to Maintain Self-Renewing Stem Cells in Ezh2-Insufficient Myelodysplastic Syndrome

Summary: Polycomb repressive complex (PRC) 2 represses transcription through histone H3K27 trimethylation (H3K27me3). We previously reported that the hematopoietic-cell-specific deletion of Ezh2, encoding a PRC2 enzyme, induced myelodysplastic syndrome (MDS) in mice, whereas the concurrent Ezh1 deletion depleted hematopoietic stem and progenitor cells (HSPCs). We herein demonstrated that mice with only one Ezh1 allele (Ezh1+/-Ezh2Δ/Δ) maintained HSPCs. A chromatin immunopreciptation sequence analysis revealed that residual PRC2 preferentially targeted genes with high levels of H3K27me3 and H2AK119 monoubiquitination (H2AK119ub1) in HSPCs (designated as Ezh1 core target genes), which were mostly developmental regulators, and maintained H3K27me3 levels in Ezh1+/-Ezh2Δ/Δ HSPCs. Even upon the complete depletion of Ezh1 and Ezh2, H2AK119ub1 levels were largely retained, and only a minimal number of Ezh1 core targets were de-repressed. These results indicate that genes marked with high levels of H3K27me3 and H2AK119ub1 are the core targets of polycomb complexes in HSPCs as well as MDS stem cells. : Biological Sciences; Genetics; Molecular Biology; Immunology; Cell Biology; Developmental Biology Subject Areas: Biological Sciences, Genetics, Molecular Biology, Immunology, Cell Biology, Developmental Biolog

Non-Lethal Ionizing Radiation Promotes Aging-Like Phenotypic Changes of Human Hematopoietic Stem and Progenitor Cells in Humanized Mice.

Precise understanding of radiation effects is critical to develop new modalities for the prevention and treatment of radiation-induced damage. We previously reported that non-lethal doses of X-ray irradiation induce DNA damage in human hematopoietic stem and progenitor cells (HSPCs) reconstituted in NOD/Shi-scid IL2rγnull (NOG) immunodeficient mice and severely compromise their repopulating capacity. In this study, we analyzed in detail the functional changes in human HSPCs in NOG mice following non-lethal radiation. We transplanted cord blood CD34+ HSPCs into NOG mice. At 12 weeks post-transplantation, the recipients were irradiated with 0, 0.5, or 1.0 Gy. At 2 weeks post-irradiation, human CD34+ HSPCs recovered from the primary recipient mice were transplanted into secondary recipients. CD34+ HSPCs from irradiated mice showed severely impaired reconstitution capacity in the secondary recipient mice. Of interest, non-lethal radiation compromised contribution of HSPCs to the peripheral blood cells, particularly to CD19+ B lymphocytes, which resulted in myeloid-biased repopulation. Co-culture of limiting numbers of CD34+ HSPCs with stromal cells revealed that the frequency of B cell-producing CD34+ HSPCs at 2 weeks post-irradiation was reduced more than 10-fold. Furthermore, the key B-cell regulator genes such as IL-7R and EBF1 were downregulated in HSPCs upon 0.5 Gy irradiation. Given that compromised repopulating capacity and myeloid-biased differentiation are representative phenotypes of aged HSCs, our findings indicate that non-lethal ionizing radiation is one of the critical external stresses that promote aging of human HSPCs in the bone marrow niche

The Antioxidant Defense System Keap1-Nrf2 Comprises a Multiple Sensing Mechanism for Responding to a Wide Range of Chemical Compounds▿ †

Animals have evolved defense systems for surviving in a chemically diverse environment. Such systems should demonstrate plasticity, such as adaptive immunity, enabling a response to even unknown chemicals. The antioxidant transcription factor Nrf2 is activated in response to various electrophiles and induces cytoprotective enzymes that detoxify them. We report here the discovery of a multiple sensing mechanism for Nrf2 activation using zebrafish and 11 Nrf2-activating compounds. First, we showed that six of the compounds tested specifically target Cys-151 in Keap1, the ubiquitin ligase for Nrf2, while two compounds target Cys-273. Second, in addition to Nrf2 and Keap1, a third factor was deemed necessary for responding to three of the compounds. Finally, we isolated a zebrafish mutant defective in its response to seven compounds but not in response to the remaining four. These results led us to categorize Nrf2 activators into six classes and hypothesize that multiple sensing allows enhanced plasticity in the system