CK2-mediated phosphorylation of SUZ12 promotes PRC2 function by stabilizing enzyme active site

LncRNA-MEG3 Regulates Muscle Mass and Metabolic Homeostasis by Facilitating SUZ12 Liquid-Liquid Phase Separation

Skeletal muscle plays a crucial role in maintaining motor function and metabolic homeostasis, with its loss or atrophy leading to significant health consequences. Long non-coding RNAs (lncRNAs) have emerged as key regulators in muscle biology; however, their precise roles in muscle function and pathology remain to be fully elucidated. This study demonstrates that lncRNA maternally expressed gene 3 (MEG3) is preferentially expressed in slow-twitch muscle fibers and dynamically regulated during muscle development, aging, and in the context of Duchenne muscular dystrophy (DMD). Using both loss- and gain-of-function mice models, this study shows that lncRNA-MEG3 is critical for preserving muscle mass and function. Its depletion leads to muscle atrophy, mitochondrial dysfunction, and impaired regenerative capacity, while overexpression enhances muscle mass, increases oxidative muscle fiber content, and improves endurance. Notably, lncRNA-MEG3 overexpression in MDX mice significantly alleviates muscle wasting and adipose tissue infiltration. Mechanistically, this study uncovers a novel interaction between lncRNA-MEG3 and the polycomb repressive complex 2 (PRC2), where lncRNA-MEG3 binds to SUZ12 polycomb repressive complex 2 subunit (Suz12), stabilizes PRC2, facilitates SUZ12 liquid-liquid phase separation (LLPS), and regulates the epigenetic modulation of four and a half lim domains 3 (Fhl3) and ring finger protein 128 (Rnf128). These findings not only highlight the crucial role of lncRNA-MEG3 in muscle homeostasis but also provide new insights into lncRNA-based therapeutic strategies for muscle-related diseases.

CBX2 suppresses interferon signaling to diminish tumor immunogenicity via a noncanonical corepressor complex

Chromobox 2 (CBX2), a crucial component of the polycomb repressive complex (PRC), has been implicated in the development of various human cancers. However, its role in the regulation of tumor immunogenicity and immune evasion remains inadequately understood. In this study, we found that ablation of CBX2 led to tumor growth inhibition, activation of the tumor immune microenvironment, and enhanced therapeutic efficacy of anti-PD1 or adoptive T cell therapies by using murine syngeneic tumor models. By analysis of the CBX2-regulated transcriptional program coupled with mass spectrometry screening of CBX2-interacting proteins, we found that CBX2 suppresses interferon signaling independent of its function in the canonical PRC. Mechanistically, CBX2 directly interacts with RACK1 and facilitates the recruitment of HDAC1, which attenuates the H3K27ac modification on the promoter regions of interferon-stimulated genes, thereby suppressing interferon signaling. Consequently, CBX2 reduces tumor immunogenicity and enables immune evasion. Moreover, a high expression level of CBX2 is associated with immune suppressive tumor microenvironment and reduced efficacy of immunotherapy across various human cancer types. Our study identifies a noncanonical CBX2-RACK1-HDAC1 corepressor complex in suppression of tumor immunogenicity, thereby presenting a potential target and biomarker for tumor immunotherapy.

SUZ12-Increased NRF2 Alleviates Cardiac Ischemia/Reperfusion Injury by Regulating Apoptosis, Inflammation, and Ferroptosis

Nuclear factor erythroid 2-related factor 2 (NRF2) is a redox-sensitive transcriptional factor that enables cells to resist oxidant responses, ferroptosis and inflammation. Here, we set out to probe the effects of NRF2 on cardiomyocyte injury under acute myocardial infarction (AMI) condition and its potential mechanism. Human cardiomyocytes were exposed to hypoxia/reoxygenation (H/R) to induce cell injury. qRT-PCR and western blot assays were used to detect the levels of mRNAs and proteins. Cardiomyocyte injury was determined by detecting the levels of lactate dehydrogenase and creatine Kinase MB (CK-MB). Cell apoptosis was investigated by flow cytometry and related markers. Levels of IL-6, IL-10, and TNF-α were measured by ELISA. Cell ferroptosis was assessed by detecting the production of reactive oxygen species (ROS), malonaldehyde (MDA), reduced glutathione/oxidized glutathione disulfide (GSH/GSSG) ratio, Fe + content, and related regulators. The interaction between NRF2 and the suppressor of zest 12 (SUZ12) was analyzed by using dual-luciferase reporter and RNA immunoprecipitation assays. AMI rat models were established for in vivo analysis. NRF2 was lowly expressed in AMI patients and H/R-induced cardiomyocytes. Forced expression of NRF2 reduced H/R-induced cardiomyocyte injury, apoptosis, inflammation, and ferroptosis. Moreover, NRF2 overexpression improved cardiac function and injury in vivo. Mechanistically, SUZ12 bound to the promoter of NRF2 and promoted its expression. Further functional analyses showed that SUZ12 overexpression reduced H/R-induced cardiomyocyte injury, apoptosis, inflammation, and ferroptosis, which were reversed by NRF2 silencing. SUZ12-increased NRF2 suppressed H/R-induced cardiomyocyte injury, apoptosis, inflammation, and ferroptosis in vitro and improved cardiac functions in rats with I/R injury, suggesting the potential cardioprotective effect of NRF2 in cardiac injury during AMI.

EPOP Restricts PRC2.1 Targeting to Chromatin by Directly Modulating Enzyme Complex Dimerization

Polycomb repressive complex 2 (PRC2) mediates developmental gene repression as two classes of holocomplexes, PRC2.1 and PRC2.2. EPOP is an accessory subunit specific to PRC2.1, which also contains PCL proteins. Unlike other accessory subunits that collectively facilitate PRC2 targeting, EPOP was implicated in an enigmatic inhibitory role, together with its interactor Elongin BC. We report an unusual molecular mechanism whereby EPOP regulates PRC2.1 by directly modulating its oligomerization state. EPOP disrupts the PRC2.1 dimer and weakens its chromatin association, likely by disabling the avidity effect conferred by the dimeric complex. Congruently, an EPOP mutant specifically defective in PRC2 binding enhances genome-wide enrichments of MTF2 and H3K27me3 in mouse epiblast-like cells. Elongin BC is largely dispensable for the EPOP-mediated inhibition of PRC2.1. EPOP defines a distinct subclass of PRC2.1, which uniquely maintains an epigenetic program by preventing the over-repression of key gene regulators along the continuum of early differentiation.

Mono-methylation of lysine 27 at histone 3 confers lifelong susceptibility to stress

Histone post-translational modifications are critical for mediating persistent alterations in gene expression. By combining unbiased proteomics profiling and genome-wide approaches, we uncovered a role for mono-methylation of lysine 27 at histone H3 (H3K27me1) in the enduring effects of stress. Specifically, mice susceptible to early life stress (ELS) or chronic social defeat stress (CSDS) displayed increased H3K27me1 enrichment in the nucleus accumbens (NAc), a key brain-reward region. Stress-induced H3K27me1 accumulation occurred at genes that control neuronal excitability and was mediated by the VEFS domain of SUZ12, a core subunit of the polycomb repressive complex-2, which controls H3K27 methylation patterns. Viral VEFS expression changed the transcriptional profile of the NAc, led to social, emotional, and cognitive abnormalities, and altered excitability and synaptic transmission of NAc D1-medium spiny neurons. Together, we describe a novel function of H3K27me1 in the brain and demonstrate its role as a "chromatin scar" that mediates lifelong stress susceptibility.

Diverse RNA Structures Induce PRC2 Dimerization and Inhibit Histone Methyltransferase Activity

Methyltransferase PRC2 (Polycomb Repressive Complex 2) introduces histone H3K27 trimethylation, a repressive chromatin mark, to tune the differential expression of genes. PRC2 is precisely regulated by accessory proteins, histone post-translational modifications and, notably, RNA. Research on PRC2-associated RNA has mostly focused on the tight-binding G-quadruplex (G4) RNAs, which inhibit PRC2 enzymatic activity in vitro and in cells. Our recent cryo-EM structure provided a molecular mechanism for G4 RNA inactivating PRC2 via dimerization, but it remained unclear how diverse RNAs associate with and regulate PRC2. Here, we show that a single-stranded G-rich RNA and an atypical G4 structure called pUG-fold unexpectedly also mediate near-identical PRC2 dimerization resulting in inhibition of PRC2 methyltransferase activity. The conformational flexibility of arginine-rich loops within subunits EZH2 and AEBP2 of PRC2 can accommodate diverse RNA secondary structures, resulting in protein-RNA and protein-protein interfaces similar to those observed previously with G4 RNA. Furthermore, we address a recent report that failed to detect PRC2-associated RNAs in living cells by demonstrating the insensitivity of PRC2-RNA interaction to photochemical crosslinking. Our results support the significance of RNA-mediated PRC2 regulation by showing that this interaction is not limited to a single RNA secondary structure, consistent with the broad PRC2 transcriptome containing many G-tract RNAs incapable of folding into G4 structures.

H3K27me3-mediated epigenetic regulation in pluripotency maintenance and lineage differentiation

Cell fate determination is an intricate process which is orchestrated by multiple regulatory layers including signal pathways, transcriptional factors, epigenetic modifications, and metabolic rewiring. Among the sophisticated epigenetic modulations, the repressive mark H3K27me3, deposited by PRC2 (polycomb repressive complex 2) and removed by demethylase KDM6, plays a pivotal role in mediating the cellular identity transition through its dynamic and precise alterations. Herein, we overview and discuss how H3K27me3 and its modifiers regulate pluripotency maintenance and early lineage differentiation. We primarily highlight the following four aspects: 1) the two subcomplexes PRC2.1 and PRC2.2 and the distribution of genomic H3K27 methylation; 2) PRC2 as a critical regulator in pluripotency maintenance and exit; 3) the emerging role of the eraser KDM6 in early differentiation; 4) newly identified additional factors influencing H3K27me3. We present a comprehensive insight into the molecular principles of the dynamic regulation of H3K27me3, as well as how this epigenetic mark participates in pluripotent stem cell-centered cell fate determination.

The emerging roles of CEACAM6 in human cancer (Review)

Carcinoembryonic antigen (CEA)‑related cell adhesion molecule 6 (CEACAM6) is a cell adhesion protein of the CEA family of glycosyl phosphatidyl inositol anchored cell surface glycoproteins. A wealth of research has demonstrated that CEACAM6 is generally upregulated in pancreatic adenocarcinoma, breast cancer, non‑small cell lung cancer, gastric cancer, colon cancer and other cancers and promotes tumor progression, invasion and metastasis. The transcriptional expression of CEACAM6 is regulated by various factors, including the CD151/TGF‑β1/Smad3 axis, microRNA (miR)‑146, miR‑26a, miR‑29a/b/c, miR‑128, miR‑1256 and DNA methylation. In addition, the N‑glycosylation of CEACAM6 protein at Asn256 is mediated by α‑1,6‑mannosylglycoptotein 6‑β‑N‑acetylglucosaminyltransferase. In terms of downstream signaling pathways, CEACAM6 promotes tumor proliferation by increasing levels of cyclin D1 and cyclin‑dependent kinase 4 proteins. CEACAM6 can activate the ERK1/2/MAPK or SRC/focal adhesion kinase/PI3K/AKT pathways directly or through EGFR, leading to stimulation of tumor proliferation, invasion, migration, resistance to anoikis and chemotherapy, as well as angiogenesis. This article provides a review of the expression pattern, biological function and relationship with prognosis of CEACAM6 in cancer. In summary, CEACAM6 may be a valuable diagnostic biomarker and potential therapeutic target for human cancers exhibiting overexpression of CEACAM6.

The molecular basis of cell memory in mammals: The epigenetic cycle

Cell memory refers to the capacity of cells to maintain their gene expression program once the initiating environmental signal has ceased. This exceptional feature is key during the formation of mammalian organisms, and it is believed to be in part mediated by epigenetic factors that can endorse cells with the landmarks required to maintain transcriptional programs upon cell duplication. Here, we review current literature analyzing the molecular basis of epigenetic memory in mammals, with a focus on the mechanisms by which transcriptionally repressive chromatin modifications such as methylation of DNA and histone H3 are propagated through mitotic cell divisions. The emerging picture suggests that cellular memory is supported by an epigenetic cycle in which reversible activities carried out by epigenetic regulators in coordination with cell cycle transition create a multiphasic system that can accommodate both maintenance of cell identity and cell differentiation in proliferating stem cell populations.

Dual phosphorylation of DGK5-mediated PA burst regulates ROS in plant immunity

Phosphatidic acid (PA) and reactive oxygen species (ROS) are crucial cellular messengers mediating diverse signaling processes in metazoans and plants. How PA homeostasis is tightly regulated and intertwined with ROS signaling upon immune elicitation remains elusive. We report here that Arabidopsis diacylglycerol kinase 5 (DGK5) regulates plant pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). The pattern recognition receptor (PRR)-associated kinase BIK1 phosphorylates DGK5 at Ser-506, leading to a rapid PA burst and activation of plant immunity, whereas PRR-activated intracellular MPK4 phosphorylates DGK5 at Thr-446, which subsequently suppresses DGK5 activity and PA production, resulting in attenuated plant immunity. PA binds and stabilizes the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD), regulating ROS production in plant PTI and ETI, and their potentiation. Our data indicate that distinct phosphorylation of DGK5 by PRR-activated BIK1 and MPK4 balances the homeostasis of cellular PA burst that regulates ROS generation in coordinating two branches of plant immunity.

Targeting polycomb repressor complex 2-mediated bivalent promoter epigenetic silencing of secreted frizzled-related protein 1 inhibits cholangiocarcinoma progression

BACKGROUND

Cholangiocarcinoma (CCA) refers to a collection of malignancies that are associated with a dismal prognosis. Currently, surgical resection is the only way to cure patients with CCA. Available systemic therapy is limited to gemcitabine plus cisplatin; however, this treatment is palliative in nature. Therefore, there is still a need to explore new effective therapeutic targets to intervene against CCA.

METHODS

We analyzed the expression of EZH2 and the prognosis of patients in CCA. The proliferation, migration and invasion of CCA cells after gene knockdown and overexpression were examined and validated by a xenograft model and a primary CCA mouse model with corresponding gene intervention. Targeting DNA methylation, and RNA-sequencing-based transcriptomic analysis in EZH2 and SUZ12 knockout CCA cells was performed. Bisulfite sequencing polymerase chain reaction (PCR), chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) and reverse-ChIP assays were performed for research purposes.

RESULTS

Increased expression of EZH2 in CCA exhibited a significantly poorer prognosis. DNA hypomethylation of the promoter and increased mRNA levels of secreted frizzled-related protein 1 (SFRP1) were observed in CCA cells following the inhibition of polycomb repressor complex 2 (PRC2), which was achieved through a knockout of EZH2, SUZ12 and EED, respectively, or treatment with GSK126 and GSK343. Targeting the SFRP1 promoter DNA hypermethylation with dCas9-DNMT3a decreased the mRNA level of SFRP1. The expression of SFRP1 is regulated by both H3K27me3 and DNA methylation and H3K27me3 plays a crucial role in promoting SFRP1 promotor DNA methylation. GSK343 is a small molecule inhibitor that targets the catalytic activity of EZH2. It effectively inhibits the progression and development of subcutaneous xenografts and primary CCA mouse models.

CONCLUSION

Overall, our data strongly suggested that targeting PRC2 promotes the expression of SFRP1, thereby inhibiting the progression of CCA.

KEY POINTS/HEADLIGHTS

Cholangiocarcinoma (CCA) exhibits elevated expression of EZH2, SUZ12 and EED, resulting in increased levels of H3K27me3. Targeting polycomb repressor complex 2 (PRC2) leads to the removal of H3K27me3 from the secreted frizzled-related protein 1 (SFRP1) promoter and DNA hypomethylation, thereby activating the transcription of SFRP1. Inhibiting PRC2, including the use of EZH2 inhibitors, holds promise as a potential strategy for developing anti-cancer drugs for CCA.

Diverse modes of regulating methyltransferase activity by histone ubiquitination

Post-translational modification of histones plays a central role in regulating transcription. Methylation of histone H3 at lysines 4 (H3K4) and 79 (H3K79) play roles in activating transcription whereas methylation of H3K27 is a repressive mark. These modifications, in turn, depend upon prior monoubiquitination of specific histone residues in a phenomenon known as histone crosstalk. Earlier work had provided insights into the mechanism by which monoubiquitination histone H2BK120 stimulates H3K4 methylation by COMPASS/MLL1 and H3K79 methylation by DOT1L, and monoubiquitinated H2AK119 stimulates methylation of H3K27 by the PRC2 complex. Recent studies have shed new light on the role of individual subunits and paralogs in regulating the activity of PRC2 and how additional post-translational modifications regulate yeast Dot1 and human DOT1L, as well as provided new insights into the regulation of MLL1 by H2BK120ub.

Structural basis for inactivation of PRC2 by G-quadruplex RNA

Polycomb repressive complex 2 (PRC2) silences genes through trimethylation of histone H3K27. PRC2 associates with numerous precursor messenger RNAs (pre-mRNAs) and long noncoding RNAs (lncRNAs) with a binding preference for G-quadruplex RNA. In this work, we present a 3.3-Å-resolution cryo-electron microscopy structure of PRC2 bound to a G-quadruplex RNA. Notably, RNA mediates the dimerization of PRC2 by binding both protomers and inducing a protein interface composed of two copies of the catalytic subunit EZH2, thereby blocking nucleosome DNA interaction and histone H3 tail accessibility. Furthermore, an RNA-binding loop of EZH2 facilitates the handoff between RNA and DNA, another activity implicated in PRC2 regulation by RNA. We identified a gain-of-function mutation in this loop that activates PRC2 in zebrafish. Our results reveal mechanisms for RNA-mediated regulation of a chromatin-modifying enzyme.

Structural basis for inactivation of PRC2 by G-quadruplex RNA

The histone methyltransferase PRC2 (Polycomb Repressive Complex 2) silences genes via successively attaching three methyl groups to lysine 27 of histone H3. PRC2 associates with numerous pre-mRNA and lncRNA transcripts with a binding preference for G-quadruplex RNA. Here, we present a 3.3Ã…-resolution cryo-EM structure of PRC2 bound to a G-quadruplex RNA. Notably, RNA mediates the dimerization of PRC2 by binding both protomers and inducing a protein interface comprised of two copies of the catalytic subunit EZH2, which limits nucleosome DNA interaction and occludes H3 tail accessibility to the active site. Our results reveal an unexpected mechanism for RNA-mediated inactivation of a chromatin-modifying enzyme. Furthermore, the flexible loop of EZH2 that helps stabilize RNA binding also facilitates the handoff between RNA and DNA, an activity implicated in PRC2 regulation by RNA.

One-Sentence Summary

Cryo-EM structure of RNA-bound PRC2 dimer elucidates an unexpected mechanism of PRC2 inhibition by RNA.