H2B ubiquitylation and the histone chaperone Asf1 cooperatively mediate the formation and maintenance of heterochromatin silencing

Activation of the FOXM1/ASF1B/PRDX3 axis confers hyperproliferative and antioxidative stress reactivity to gastric cancer

Nucleosome assembly during DNA replication is dependent on histone chaperones. Recent studies suggest that dysregulated histone chaperones contribute to cancer progression, including gastric cancer (GC). Further studies are required to explore the prognostic and therapeutic implications of histone chaperones and their mechanisms of action in GC progression. Here we identified histone chaperone ASF1B as a potential biomarker for GC proliferation and prognosis. ASF1B was significantly upregulated in GC, which was associated with poor prognosis. In vitro and in vivo experiments demonstrated that the inhibition of ASF1B suppressed the malignant characteristics of GC, while overexpression of ASF1B had the opposite effect. Mechanistically, transcription factor FOXM1 directly bound to the ASF1B-promoter region, thereby regulating its transcription. Treatment with thiostrepton, a FOXM1 inhibitor, not only suppressed ASF1B expression, but also inhibited GC progression. Furthermore, ASF1B regulated the mitochondrial protein peroxiredoxin 3 (PRDX3) transcription in a FOXM1-dependent manner. The crucial role of ASF1B-regulated PRDX3 in GC cell proliferation and oxidative stress balance was also elucidated. In summary, our study suggests that the FOXM1-ASF1B-PRDX3 axis is a potential therapeutic target for treating GC.

Histone chaperone HIRA complex regulates retrotransposons in embryonic stem cells

BACKGROUND

Histone cell cycle regulator (HIRA) complex is an important histone chaperone that mediates the deposition of the H3.3 histone variant onto chromatin independently from DNA synthesis. However, it is still unknown whether it participates in the expression control of retrotransposons and cell fate determination.

METHODS

We screened the role of HIRA complex members in repressing the expression of retrotransposons by shRNA depletion in embryonic stem cells (ESCs) followed by RT-qPCR. RNA-seq was used to study the expression profiles after depletion of individual HIRA member. RT-qPCR and western blot were used to determine overexpression of HIRA complex members. Chromatin immunoprecipitation (ChIP)-qPCR was used to find the binding of H3.3, HIRA members to chromatin. Co-immunoprecipitation was used to identify the interaction between Hira mutant and Ubn2. ChIP-qPCR was used to identify H3.3 deposition change and western blot of chromatin extract was used to validate the epigenetic change. Bioinformatics analysis was applied for the analysis of available ChIP-seq data.

RESULTS

We revealed that Hira, Ubn2, and Ubn1 were the main repressors of 2-cell marker retrotransposon MERVL among HIRA complex members. Surprisingly, Ubn2 and Hira targeted different groups of retrotransposons and retrotransposon-derived long noncoding RNAs (lncRNAs), despite that they partially shared target genes. Furthermore, Ubn2 prevented ESCs to gain a 2-cell like state or activate trophectodermal genes upon differentiation. Mechanistically, Ubn2 and Hira suppressed retrotransposons by regulating the deposition of histone H3.3. Decreased H3.3 deposition, that was associated with the loss of Ubn2 or Hira, caused the reduction of H3K9me2 and H3K9me3, which are known repressive marks of retrotransposons.

CONCLUSIONS

Overall, our findings shed light on the distinct roles of HIRA complex members in controlling retrotransposons and cell fate conversion in ESCs.

Studying PAR-Dependent Chromatin Remodeling to Tackle PARPi Resistance

Histone eviction and chromatin relaxation are important processes for efficient DNA repair. Poly(ADP) ribose (PAR) polymerase 1 (PARP1) is a key mediator of this process, and disruption of PARP1 activity has a direct impact on chromatin structure. PARP inhibitors (PARPis) have been established as a treatment for BRCA1- or BRCA2-deficient tumors. Unfortunately, PARPi resistance occurs in many patients and the underlying mechanisms are not fully understood. In particular, it remains unclear how chromatin remodelers and histone chaperones compensate for the loss of the PARylation signal. In this Opinion article, we summarize currently known mechanisms of PARPi resistance. We discuss how the study of PARP1-mediated chromatin remodeling may help in further understanding PARPi resistance and finding new therapeutic approaches to overcome it.

Redox Regulation of the NOR Transcription Factor Is Involved in the Regulation of Fruit Ripening in Tomato

Transcription factors (TFs) are important regulators of plant growth and development and responses to stresses. TFs themselves are also prone to multiple posttranslational modifications (PTMs). However, redox-mediated PTM of TFs in plants remains poorly understood. Here, we established that NON-RIPENING (NOR), a master TF regulating tomato (Solanum lycopersicum) fruit ripening, is a target of the Met sulfoxide reductases A and B, namely E4 and SlMsrB2, respectively, in tomato. Met oxidation in NOR, i.e. sulfoxidation, or mimicking sulfoxidation by mutating Met-138 to Gln, reduces its DNA-binding capacity and transcriptional regulatory activity in vitro. E4 and SlMsrB2 partially repair oxidized NOR and restore its DNA-binding capacity. Transgenic complementation of the nor mutant with NOR partially rescues the ripening defects. However, transformation of nor with NOR-M138Q, containing mimicked Met sulfoxidation, inhibits restoration of the fruit ripening phenotype, and this is associated with the decreased DNA-binding and transcriptional activation of a number of ripening-related genes. Taken together, these observations reveal a PTM mechanism by which Msr-mediated redox modification of NOR regulates the expression of ripening-related genes, thereby influencing tomato fruit ripening. Our report describes how sulfoxidation of TFs regulates developmental processes in plants.

Poly(ADP-ribosyl)ation mediates early phase histone eviction at DNA lesions

Nucleosomal histones are barriers to the DNA repair process particularly at DNA double-strand breaks (DSBs). However, the molecular mechanism by which these histone barriers are removed from the sites of DNA damage remains elusive. Here, we have generated a single specific inducible DSB in the cells and systematically examined the histone removal process at the DNA lesion. We found that histone removal occurred immediately following DNA damage and could extend up to a range of few kilobases from the lesion. To examine the molecular mechanism underlying DNA damage-induced histone removal, we screened histone modifications and found that histone ADP-ribosylation was associated with histone removal at DNA lesions. PARP inhibitor treatment suppressed the immediate histone eviction at DNA lesions. Moreover, we examined histone chaperones and found that the FACT complex recognized ADP-ribosylated histones and mediated the removal of histones in response to DNA damage. Taken together, our results reveal a pathway that regulates early histone barrier removal at DNA lesions. It may also explain the mechanism by which PARP inhibitor regulates early DNA damage repair.

The Wolbachia cytoplasmic incompatibility enzyme CidB targets nuclear import and protamine-histone exchange factors

Intracellular Wolbachia bacteria manipulate arthropod reproduction to promote their own inheritance. The most prevalent mechanism, cytoplasmic incompatibility (CI), traces to a Wolbachia deubiquitylase, CidB, and CidA. CidB has properties of a toxin, while CidA binds CidB and rescues embryonic viability. CidB is also toxic to yeast where we identified both host effects and high-copy suppressors of toxicity. The strongest suppressor was karyopherin-α, a nuclear-import receptor; this required nuclear localization-signal binding. A protein-interaction screen of Drosophila extracts using a substrate-trapping catalytic mutant, CidB*, also identified karyopherin-α; the P32 protamine-histone exchange factor bound as well. When CidB* bound CidA, these host protein interactions disappeared. These associations would place CidB at the zygotic male pronucleus where CI defects first manifest. Overexpression of karyopherin-α, P32, or CidA in female flies suppressed CI. We propose that CidB targets nuclear-protein import and protamine-histone exchange and that CidA rescues embryos by restricting CidB access to its targets.

The interplay of histone H2B ubiquitination with budding and fission yeast heterochromatin

Mono-ubiquitinated histone H2B (H2B-Ub) is important for chromatin regulation of transcription, chromatin assembly, and also influences heterochromatin. In this review, we discuss the effects of H2B-Ub from nucleosome to higher-order chromatin structure. We then assess what is currently known of the role of H2B-Ub in heterochromatic silencing in budding and fission yeasts (S. cerevisiae and S. pombe), which have distinct silencing mechanisms. In budding yeast, the SIR complex initiates heterochromatin assembly with the aid of a H2B-Ub deubiquitinase, Ubp10. In fission yeast, the RNAi-dependent pathway initiates heterochromatin in the context of low H2B-Ub. We examine how the different silencing machineries overcome the challenge of H2B-Ub chromatin and highlight the importance of using these microorganisms to further our understanding of H2B-Ub in heterochromatic silencing pathways.

Recruitment and allosteric stimulation of a histone-deubiquitinating enzyme during heterochromatin assembly

Heterochromatin formation in budding yeast is regulated by the silent information regulator (SIR) complex. The SIR complex comprises the NAD-dependent deacetylase Sir2, the scaffolding protein Sir4, and the nucleosome-binding protein Sir3. Transcriptionally active regions present a challenge to SIR complex-mediated de novo heterochromatic silencing due to the presence of antagonistic histone post-translational modifications, including acetylation and methylation. Methylation of histone H3K4 and H3K79 is dependent on monoubiquitination of histone H2B (H2B-Ub). The SIR complex cannot erase H2B-Ub or histone methylation on its own. The deubiquitinase (DUB) Ubp10 is thought to promote heterochromatic silencing by maintaining low H2B-Ub at sub-telomeres. Here, we biochemically characterized the interactions between Ubp10 and the SIR complex machinery. We demonstrate that a direct interaction between Ubp10 and the Sir2/4 sub-complex facilitates Ubp10 recruitment to chromatin via a co-assembly mechanism. Using hydrolyzable H2B-Ub analogs, we show that Ubp10 activity is lower on nucleosomes compared with H2B-Ub in solution. We find that Sir2/4 stimulates Ubp10 DUB activity on nucleosomes, likely through a combination of targeting and allosteric regulation. This coupling mechanism between the silencing machinery and its DUB partner allows erasure of active PTMs and the de novo transition of a transcriptionally active DNA region to a silent chromatin state.