MAPKAP Kinase 3pK Phosphorylates and Regulates Chromatin Association of the Polycomb Group Protein Bmi1

O-GlcNAcylation: the sweet side of epigenetics

Histones display a wide variety of post-translational modifications, including acetylation, methylation, and phosphorylation. These epigenetic modifications can influence chromatin structure and function without altering the DNA sequence. Histones can also undergo post-translational O-GlcNAcylation, a rather understudied modification that plays critical roles in almost all biological processes and is added and removed by O-linked N-acetylglucosamine transferase and O-GlcNAcase, respectively. This review provides a current overview of our knowledge of how O-GlcNAcylation impacts the histone code both directly and by regulating other chromatin modifying enzymes. This highlights the pivotal emerging role of O-GlcNAcylation as an essential epigenetic marker.

Sticky, Adaptable, and Many-sided: SAM protein versatility in normal and pathological hematopoietic states

With decades of research seeking to generalize sterile alpha motif (SAM) biology, many outstanding questions remain regarding this multi-tool protein module. Recent data from structural and molecular/cell biology has begun to reveal new SAM modes of action in cell signaling cascades and biomolecular condensation. SAM-dependent mechanisms underlie blood-related (hematologic) diseases, including myelodysplastic syndromes and leukemias, prompting our focus on hematopoiesis for this review. With the increasing coverage of SAM-dependent interactomes, a hypothesis emerges that SAM interaction partners and binding affinities work to fine tune cell signaling cascades in developmental and disease contexts, including hematopoiesis and hematologic disease. This review discusses what is known and remains unknown about the standard mechanisms and neoplastic properties of SAM domains and what the future might hold for developing SAM-targeted therapies.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

The role of BMI1 in endometrial cancer and other cancers

Endometrial cancer (EC) is the third leading gynecological malignancy, and its treatment remains challenging. B cell-specific Moloney murine leukemia virus integration site-1 (BMI1) is one of the core members of the polycomb group (PcG) family, which plays a promoting role in the occurrence and development of various tumors. Notably, BMI1 has been found to be frequently upregulated in endometrial cancer (EC) and promote the occurrence of EC through promoting epithelial-mesenchymal transition (EMT) and AKT/PI3K pathways. This review summarizes the structure and upstream regulatory mechanisms of BMI1 and its role in EC. In addition, we focused on the role of BMI1 in chemoradiotherapy resistance and summarized the current drugs that target BMI1.

Bmi-1: A master regulator of head and neck cancer stemness

Head and neck cancers are composed of a diverse group of malignancies, many of which exhibit an unacceptably low patient survival, high morbidity and poor treatment outcomes. The cancer stem cell (CSC) hypothesis provides an explanation for the substantial patient morbidity associated with treatment resistance and the high frequency of tumor recurrence/metastasis. Stem cells are a unique population of cells capable of recapitulating a heterogenous organ from a single cell, due to their capacity to self-renew and differentiate into progenitor cells. CSCs share these attributes, in addition to playing a pivotal role in cancer initiation and progression by means of their high tumorigenic potential. CSCs constitute only a small fraction of tumor cells but play a major role in tumor initiation and therapeutic evasion. The shift towards stem-like phenotype fuels many malignant features of a cancer cell and mediates resistance to conventional chemotherapy. Bmi-1 is a master regulator of stem cell self-renewal as part of the polycomb repressive complex 1 (PRC1) and has emerged as a prominent player in cancer stem cell biology. Bmi-1 expression is upregulated in CSCs, which is augmented by tumor-promoting factors and various conventional chemotherapies. Bmi-1⁺ CSCs mediate chemoresistance and metastasis. On the other hand, inhibiting Bmi-1 rescinds CSC function and re-sensitizes cancer cells to chemotherapy. Therefore, elucidating the functional role of Bmi-1 in CSC-mediated cancer progression may unveil an attractive target for mechanism-based, developmental therapeutics. In this review, we discuss the parallels in the role of Bmi-1 in stem cell biology of health and disease and explore how this can be leveraged to advance clinical treatment strategies for head and neck cancer.

The Crucial Roles of Bmi-1 in Cancer: Implications in Pathogenesis, Metastasis, Drug Resistance, and Targeted Therapies

B-cell-specific Moloney murine leukemia virus integration region 1 (Bmi-1, also known as RNF51 or PCGF4) is one of the important members of the PcG gene family, and is involved in regulating cell proliferation, differentiation and senescence, and maintaining the self-renewal of stem cells. Many studies in recent years have emphasized the role of Bmi-1 in the occurrence and development of tumors. In fact, Bmi-1 has multiple functions in cancer biology and is closely related to many classical molecules, including Akt, c-MYC, Pten, etc. This review summarizes the regulatory mechanisms of Bmi-1 in multiple pathways, and the interaction of Bmi-1 with noncoding RNAs. In particular, we focus on the pathological processes of Bmi-1 in cancer, and explore the clinical relevance of Bmi-1 in cancer biomarkers and prognosis, as well as its implications for chemoresistance and radioresistance. In conclusion, we summarize the role of Bmi-1 in tumor progression, reveal the pathophysiological process and molecular mechanism of Bmi-1 in tumors, and provide useful information for tumor diagnosis, treatment, and prognosis.

MAPK Activated Protein Kinase 3 Is a Prognostic-Related Biomarker and Associated With Immune Infiltrates in Glioma

Glioma is the most common primary brain tumor that causes significant morbidity and mortality. MAPK activated protein kinase 3 (MAPKAPK3/MK3) is a serine/threonine protein kinase regulating various cellular responses and gene expression. However, the role of MK3 in tumor progress, prognosis, and immunity for glioma remains unclear. Here, we determined the expression and prognostic values of MK3. We further analyzed the correlation of MK3 expression with immune infiltrations by using the biochemical methods and bioinformatic approaches with available databases. We find that MK3 is aberrantly upregulated in glioma. In addition, the higher MK3 expression is closely linked to the poor clinicopathologic features of glioma patients. Importantly, MK3 expression is negatively correlated with the prognosis of patients with glioma. Mechanistically, we demonstrated that the correlated genes of MK3 were mainly enriched in pathways that regulate tumor immune responses. The MK3 level was significantly associated with tumor-infiltrating immune cells and positively correlated with the majority of tumor immunoinhibitors, chemokines, and chemokine receptors in glioma. Thus, these findings suggest the novel prognostic roles of MK3 and define MK3 as a promising target for glioma immunotherapy.

Transcriptomic Analysis of HCN-2 Cells Suggests Connection among Oxidative Stress, Senescence, and Neuron Death after SARS-CoV-2 Infection

According to the neurological symptoms of SARS-CoV-2 infection, it is known that the nervous system is influenced by the virus. We used pediatric human cerebral cortical cell line HCN-2 as a neuronal model of SARS-CoV-2 infection, and, through transcriptomic analysis, our aim was to evaluate the effect of SARS-CoV-2 in this type of cells. Transcriptome analyses revealed impairment in TXN gene, resulting in deregulation of its antioxidant functions, as well as a decrease in the DNA-repairing mechanism, as indicated by the decrease in KAT5. Western blot analyses of SOD1 and iNOS confirmed the impairment of reduction mechanisms and an increase in oxidative stress. Upregulation of CDKN2A and a decrease in CDK4 and CDK6 point to the blocking of the cell cycle that, according to the deregulation of repairing mechanism, has apoptosis as the outcome. A high level of proapoptotic gene PMAIP1 is indeed coherent with neuronal death, as also supported by increased levels of caspase 3. The upregulation of cell-cycle-blocking genes and apoptosis suggests a sufferance state of neurons after SARS-CoV-2 infection, followed by their inevitable death, which can explain the neurological symptoms reported. Further analyses are required to deeply explain the mechanisms and find potential treatments to protect neurons from oxidative stress and prevent their death.

The Role of Polycomb Group Protein BMI1 in DNA Repair and Genomic Stability

The polycomb group (PcG) proteins are a class of transcriptional repressors that mediate gene silencing through histone post-translational modifications. They are involved in the maintenance of stem cell self-renewal and proliferation, processes that are often dysregulated in cancer. Apart from their canonical functions in epigenetic gene silencing, several studies have uncovered a function for PcG proteins in DNA damage signaling and repair. In particular, members of the poly-comb group complexes (PRC) 1 and 2 have been shown to recruit to sites of DNA damage and mediate DNA double-strand break repair. Here, we review current understanding of the PRCs and their roles in cancer development. We then focus on the PRC1 member BMI1, discussing the current state of knowledge of its role in DNA repair and genome integrity, and outline how it can be targeted pharmacologically.

Manganese exposure in juvenile C57BL/6 mice increases glial inflammatory responses in the substantia nigra following infection with H1N1 influenza virus

Infection with Influenza A virus can lead to the development of encephalitis and subsequent neurological deficits ranging from headaches to neurodegeneration. Post-encephalitic parkinsonism has been reported in surviving patients of H1N1 infections, but not all cases of encephalitic H1N1 infection present with these neurological symptoms, suggesting that interactions with an environmental neurotoxin could promote more severe neurological damage. The heavy metal, manganese (Mn), is a potential interacting factor with H1N1 because excessive exposure early in life can induce long-lasting effects on neurological function through inflammatory activation of glial cells. In the current study, we used a two-hit model of neurotoxin-pathogen exposure to examine whether exposure to Mn during juvenile development would induce a more severe neuropathological response following infection with H1N1 in adulthood. To test this hypothesis, C57BL/6 mice were exposed to MnCl₂ in drinking water (50 mg/kg/day) for 30 days from days 21–51 postnatal, then infected intranasally with H1N1 three weeks later. Analyses of dopaminergic neurons, microglia and astrocytes in basal ganglia indicated that although there was no significant loss of dopaminergic neurons within the substantia nigra pars compacta, there was more pronounced activation of microglia and astrocytes in animals sequentially exposed to Mn and H1N1, as well as altered patterns of histone acetylation. Whole transcriptome Next Generation Sequencing (RNASeq) analysis was performed on the substantia nigra and revealed unique patterns of gene expression in the dual-exposed group, including genes involved in antioxidant activation, mitophagy and neurodegeneration. Taken together, these results suggest that exposure to elevated levels of Mn during juvenile development could sensitize glial cells to more severe neuro-immune responses to influenza infection later in life through persistent epigenetic changes.

Polycomb represses a gene network controlling puberty via modulation of histone demethylase Kdm6b expression

Female puberty is subject to Polycomb Group (PcG)-dependent transcriptional repression. Kiss1, a puberty-activating gene, is a key target of this silencing mechanism. Using a gain-of-function approach and a systems biology strategy we now show that EED, an essential PcG component, acts in the arcuate nucleus of the hypothalamus to alter the functional organization of a gene network involved in the stimulatory control of puberty. A central node of this network is Kdm6b, which encodes an enzyme that erases the PcG-dependent histone modification H3K27me3. Kiss1 is a first neighbor in the network; genes encoding glutamatergic receptors and potassium channels are second neighbors. By repressing Kdm6b expression, EED increases H3K27me3 abundance at these gene promoters, reducing gene expression throughout a gene network controlling puberty activation. These results indicate that Kdm6b repression is a basic mechanism used by PcG to modulate the biological output of puberty-activating gene networks.

Genistein nanoformulation promotes selective apoptosis in oral squamous cell carcinoma through repression of 3PK-EZH2 signalling pathway

BACKGROUND

Overexpression of polycomb protein contributes to epigenetic repression in oral squamous cell carcinoma (OSCC) ensuing in poor prognosis and aggressive phenotype. Several plant-based compounds could help prevent epigenome alteration and cancer progression, but their low bioavailability limits their therapeutic activity.

HYPOTHESIS

In this study, we have synthesized genistein nanoformulation (GLNPs) and evaluated its epigenetic regulation mechanism for selective apoptosis induction in OSCC.

METHODS

Lactalbumin was used to prepare nanoformulation of Genistein. The mechanism of epigenetic regulation and selective apoptosis by Genistein loaded nanoparticles was studied in OSCC cell line JHU011 and fibroblast cell line L929 using immunofluorescence, Western blotting and ChIP-qPCR assay.

RESULTS

We have found that GLNPs treatment selectively induced apoptosis in OSCC compared to the normal fibroblast cells. This selective effect in OSCC is achieved through enhanced reactive oxygen species (ROS) generation followed by Bax mitochondrial translocation and caspase 3 activation. Further, GLNPs induced withdrawal of epigenetic transcription repression through concurrent downregulation of the polycomb group proteins (PcG) Bmi 1 and EZH2 along with their successive targets, UbH2AK119 and H3K27me3, which have immense therapeutic implications in the treatment of OSCC. Last, we have established that GLNPs regulate EZH2expression through proteasomal mediated degradation and 3PK inhibition; 3PK protein was found physically linked with EZH2 protein and its promoter region (-1107 to -1002). This event indicates that 3PK might play some crucial role in EZH2 expression and epigenetic control of OSCC. Moreover, the formulation showed improved biodistribution, aqueous dispersibility and enhanced biocompatibility In-vivo.

CONCLUSIONS

These results provide evidence that GLNPs may withdraw epigenetic transcriptional repression and selectively induce apoptosis in human oral squamous cell carcinoma.

Multifaceted behavior of PEST sequence enriched nuclear proteins in cancer biology and role in gene therapy

The amino acid sequence enriched with proline (P), glutamic acid (E), serine (S), and threonine (T) (PEST) is a signal-transducing agent providing unique features to its substrate nuclear proteins (PEST-NPs). The PEST motif is responsible for particular posttranslational modifications (PTMs). These PTMs impart distinct properties to PEST-NPs that are responsible for their activation/inhibition, intracellular localization, and stability/degradation. PEST-NPs participate in cancer metabolism, immunity, and protein transcription as oncogenes or as tumor suppressors. Gene-based therapeutics are getting the attention of researchers because of their cell specificity. PEST-NPs are good targets to explore as cancer therapeutics. Insights into PTMs of PEST-NPs demonstrate that these proteins not only interact with each other but also recruit other proteins to/from their active site to promote/inhibit tumors. Thus, the role of PEST-NPs in cancer biology is multivariate. It is hard to obtain therapeutic objectives with single gene therapy. An especially designed combination gene therapy might be a promising strategy in cancer treatment. This review highlights the multifaceted behavior of PEST-NPs in cancer biology. We have summarized a number of studies to address the influence of structure and PEST-mediated PTMs on activation, localization, stability, and protein-protein interactions of PEST-NPs. We also recommend researchers to adopt a pragmatic approach in gene-based cancer therapy.

B lymphoma Moloney murine leukemia virus insertion region 1: An oncogenic mediator in prostate cancer

B lymphoma Moloney murine leukemia virus insertion region 1 (BMI1), a core member of polycomb repressive complex 1 (PRC1), has been intensely investigated in the field of cancer epigenetics for decades. Widely known as a critical regulator in cellular physiology, BMI1 is essential in self-renewal and differentiation in different lineages of stem cells. BMI1 also plays a significant role in cancer etiology for its involvement in pathological progress such as epithelial–mesenchymal transition (EMT) and cancer stem cell maintenance, propagation, and differentiation. Importantly, overexpression of BMI1 is predictive for drug resistance, tumor recurrence, and eventual therapy failure of various cancer subtypes, which renders the pharmacological targeting at BMI1 as a novel and promising therapeutic approach. The study on prostate cancer, a prevalent hormone-related cancer among men, has promoted enormous research advancements in cancer genetics and epigenetics. This review summarizes the role of BMI1 as an oncogenic and epigenetic regulator in tumor initiation, progression, and relapse of prostate cancer.

Muscle stem cell aging: identifying ways to induce tissue rejuvenation

Aging is characterized by the functional and regenerative decline of tissues and organs. This regenerative decline is a consequence of the numerical and functional loss of adult stem cells, which are the corner stone of tissue homeostasis and repair. A palpable example of this decline is provided by skeletal muscle, a specialized tissue composed of postmitotic myofibers that contract to generate force. Skeletal muscle stem cells (satellite cells) are long-lived and support muscle regeneration throughout life, but at advanced age they fail for largely undefined reasons. Here, we discuss recent advances in the understanding of how satellite cells integrate diverse intrinsic and extrinsic processes to ensure optimal homeostatic function and how this integration is perturbed during aging, causing regenerative failure. With this increased understanding, it is now feasible to design and test interventions that delay satellite cell aging. We discuss the exciting new therapeutic potential of integrating and combining distinct anti-aging strategies for regenerative medicine.

Regulation of senescence traits by MAPKs

A phenotype of indefinite growth arrest acquired in response to sublethal damage, cellular senescence affects normal aging and age-related disease. Mitogen-activated protein kinases (MAPKs) are capable of sensing changes in cellular conditions, and in turn elicit adaptive responses including cell senescence. MAPKs modulate the levels and function of many proteins, including proinflammatory factors and factors in the p21/p53 and p16/RB pathways, the main senescence-regulatory axes. Through these actions, MAPKs implement key traits of senescence-growth arrest, cell survival, and the senescence-associated secretory phenotype (SASP). In this review, we summarize and discuss our current knowledge of the impact of MAPKs in senescence. In addition, given that eliminating or suppressing senescent cells can improve health span, we discuss the function and possible exploitation of MAPKs in the elimination (senolysis) or suppression (senostasis) of senescent cells.

Differential regulation of cytokine and chemokine expression by MK2 and MK3 in airway smooth muscle cells

BACKGROUND

Airway smooth muscle (ASM) contributes to local inflammation and plays an immunomodulatory role in airway diseases. This is partially regulated by p38 mitogen-activated protein kinase (MAPK), which further activates two closely related isoforms of the MAPK-activated protein kinases (MKs), MK2 and MK3. The MKs have similar substrate specificities but less is known about differences in their functional responses. This study was undertaken to identify differential downstream inflammatory targets of MK2 and MK3 signaling and assess cross-talk between the MAPK pathway and NF-κB signaling relevant to ASM function.

METHODS

Wild-type and kinase-deficient MK2 (MK2^WT, MK2^KR) and MK3 (MK3^WT, MK3^3A) were expressed in human ASM cells stimulated for 20 h with 10 ng/ml each interleukin (IL)-1β, tumor necrosis factor (TNF)-α and interferon (IFN)-γ. Inflammatory mediator secretion was assessed by Luminex assays and ELISA. Signaling pathway activation was monitored by Western blotting.

RESULTS

Expression of these MKs and stimulation with 10 ng/ml IL-1β, TNFα and IFNγ for 20 h did not affect secretion of multiple cytokines including IL-4, IL-5, IL-13 and monocyte chemotactic protein (MCP)-1/CCL2 but did differentially affect the secretion of regulated upon activation, normal T cell expressed and secreted (RANTES)/CCL5, IL-6 and granulocyte macrophage-colony stimulating factor (GM-CSF). RANTES/CCL5 secretion was decreased by MK2^WT or MK3^WT and stimulated by inhibition of MK2 or MK3 activity with expression of the kinase-deficient enzymes MK2^KR or MK3^3A. IL-6 and GM-CSF secretion was decreased by inhibition of MK2 activity with MK2^KR and while MK3^WT had no effect, the kinase-deficient MK3^3A further decreased secretion of these mediators. Cross-talk of the MKs with other signaling pathways was investigated by examining NF-κB activation, which was inhibited by expression of MK3 but not affected by MK2.

CONCLUSIONS

These results suggest an inhibitory role for MK2 and MK3 activity in RANTES/CCL5 secretion and cross-talk of MK3 with NF-κB to regulate IL-6 and GM-CSF. These findings differentiate MK2 and MK3 function in ASM cells and provide insight that may enable selective targeting of MKs in ASM to modulate local inflammation in airway disease.

Chromatin remodeling controls Kaposi's sarcoma-associated herpesvirus reactivation from latency

Kaposi's sarcoma-associated herpesvirus (KSHV) is the etiologic agent of three human malignancies, the endothelial cell cancer Kaposi's sarcoma, and two B cell cancers, Primary Effusion Lymphoma and multicentric Castleman's disease. KSHV has latent and lytic phases of the viral life cycle, and while both contribute to viral pathogenesis, lytic proteins contribute to KSHV-mediated oncogenesis. Reactivation from latency is driven by the KSHV lytic gene transactivator RTA, and RTA transcription is controlled by epigenetic modifications. To identify host chromatin-modifying proteins that are involved in the latent to lytic transition, we screened a panel of inhibitors that target epigenetic regulatory proteins for their ability to stimulate KSHV reactivation. We found several novel regulators of viral reactivation: an inhibitor of Bmi1, PTC-209, two additional histone deacetylase inhibitors, Romidepsin and Panobinostat, and the bromodomain inhibitor (+)-JQ1. All of these compounds stimulate lytic gene expression, viral genome replication, and release of infectious virions. Treatment with Romidepsin, Panobinostat, and PTC-209 induces histone modifications at the RTA promoter, and results in nucleosome depletion at this locus. Finally, silencing Bmi1 induces KSHV reactivation, indicating that Bmi1, a member of the Polycomb repressive complex 1, is critical for maintaining KSHV latency.

The polycomb group protein Yaf2 regulates the pluripotency of embryonic stem cells in a phosphorylation-dependent manner

The polycomb group (PcG) proteins are key epigenetic regulators in stem cell maintenance. PcG proteins have been thought to act through one of two polycomb repressive complexes (PRCs), but more recent biochemical analyses have challenged this model in the identification of noncanonical PRC1 (nc-PRC1) complexes characterized by the presence of Rybp or Yaf2 in place of the canonical Chromobox proteins. However, the biological significance of these nc-PRC1s and the potential mechanisms by which they mediate gene repression are largely unknown. Here, we explore the functional consequences of Yaf2 disruption on stem cell regulation. We show that deletion of Yaf2 results in compromised proliferation and abnormal differentiation of mouse embryonic stem cells (mESCs). Genome-wide profiling indicates Yaf2 functions primarily as a transcriptional repressor, particularly impacting genes associated with ectoderm cell fate in a manner distinct from Rybp. We confirm that Yaf2 assembles into a noncanonical PRC complex, with deletion analysis identifying the region encompassing amino acid residues 102-150 as required for this assembly. Furthermore, we identified serine 166 as a Yaf2 phosphorylation site, and we demonstrate that mutation of this site to alanine (S166A) compromises Ring1B-mediated H2A monoubiquitination and in turn its ability to repress target gene expression. We therefore propose that Yaf2 and its phosphorylation status serve as dual regulators to maintain the pluripotent state in mESCs.

Mitotic bookmarking in development and stem cells

The changes imposed on the nucleus, chromatin and its regulators during mitosis lead to the dismantlement of most gene regulatory processes. However, an increasing number of transcriptional regulators are being identified as capable of binding their genomic targets during mitosis. These so-called ‘mitotic bookmarking factors’ encompass transcription factors and chromatin modifiers that are believed to convey gene regulatory information from mother to daughter cells. In this Primer, we review mitotic bookmarking processes in development and stem cells and discuss the interest and potential importance of this concept with regard to epigenetic regulation and cell fate transitions involving cellular proliferation.

O-GlcNAcylation modulates Bmi-1 protein stability and potential oncogenic function in prostate cancer

The Polycomb group transcriptional repressor Bmi-1 often overexpressed and participated in stem cells self-renewal and tumorigenesis initiating of prostate cancer. In this progression, Bmi-1 protein was regulated by transcription and post-translational modifications (PTMs). Nobly, the underlying PTMs regulation of Bmi-1 is poorly known. Here we use co-immunoprecipitation show that in C4-2 cell line, Bmi-1 directly interacted with OGT which is the only known enzyme catalyzed the O-GlcNAcylation in human. Furthermore, we identified that Ser255 is the site for Bmi-1 O-GlcNAcylation, and O-GlcNAcylation promoted Bmi-1 protein stability and its oncogenic activity. Finally, microarray analysis has characterized potential oncogenes associated pathway subject to repression via the OGT-Bmi-1 axis. Taken together, these results indicate that OGT-mediated O-GlcNAcylation at Ser255 stabilizes Bmi-1 and hence inhibits the TP53, PTEN and CDKN1A/CDKN2A pathway. The study not only uncovers a novel functional PTMs of Bmi-1 but also reveals a unique oncogenic role of O-GlcNAcylation in prostate cancer.

p38 (Mapk14/11) occupies a regulatory node governing entry into primitive endoderm differentiation during preimplantation mouse embryo development

During mouse preimplantation embryo development, the classically described second cell-fate decision involves the specification and segregation, in blastocyst inner cell mass (ICM), of primitive endoderm (PrE) from pluripotent epiblast (EPI). The active role of fibroblast growth factor (Fgf) signalling during PrE differentiation, particularly in the context of Erk1/2 pathway activation, is well described. However, we report that p38 family mitogen-activated protein kinases (namely p38α/Mapk14 and p38β/Mapk11; referred to as p38-Mapk14/11) also participate in PrE formation. Specifically, functional p38-Mapk14/11 are required, during early-blastocyst maturation, to assist uncommitted ICM cells, expressing both EPI and earlier PrE markers, to fully commit to PrE differentiation. Moreover, functional activation of p38-Mapk14/11 is, as reported for Erk1/2, under the control of Fgf-receptor signalling, plus active Tak1 kinase (involved in non-canonical bone morphogenetic protein (Bmp)-receptor-mediated PrE differentiation). However, we demonstrate that the critical window of p38-Mapk14/11 activation precedes the E3.75 timepoint (defined by the initiation of the classical ‘salt and pepper’ expression pattern of mutually exclusive EPI and PrE markers), whereas appropriate lineage maturation is still achievable when Erk1/2 activity (via Mek1/2 inhibition) is limited to a period after E3.75. We propose that active p38-Mapk14/11 act as enablers, and Erk1/2 as drivers, of PrE differentiation during ICM lineage specification and segregation.

BMI1, a new target of CK2α

BACKGROUND

The polycomb group protein, BMI1 plays important roles in chromatin modification, stem cell function, DNA damage repair and mitochondrial bioenergetics. Such diverse cellular functions of BMI1 could be, in part, due to post-translational modifications, especially phosphorylation. To date, AKT has been reported as a kinase that by site specific phosphorylation of BMI1 modulates its oncogenic functions.

METHODS

Immunoprecipitation in conjunction with kinase assay and mass spectrometry was used to determine association with and site specific phosphorylation of BMI1 by CK2α. Functional implications of the BMI1/CK2α axis was examined in cancer cells utilizing siRNA and exogenous gene expression followed by biochemical and phenotypic studies. Correlations between expression of CK2α and BMI1 were determined from cell lines and formalin fixed paraffin embedded tissues representing the normal fallopian tube epithelium and high grade serous ovarian cancer samples.

RESULTS

Here we report that CK2α, a nuclear serine threonine kinase, phosphorylates BMI1 at Serine 110 as determined by in-vitro/ex-vivo kinase assay and mass spectrometry. In ovarian cancer cell lines, expression of CK2α correlated with the phospho-species, as well as basal BMI1 levels. Preventing phosphorylation of BMI1 at Serine 110 significantly decreased half-life and stability of the protein. Additionally, re-expression of the phosphorylatable but not non-phosphorylatable BMI1 rescued clonal growth in endogenous BMI1 silenced cancer cells leading us to speculate that CK2α-mediated phosphorylation stabilizes BMI1 and promotes its oncogenic function. Clinically, compared to normal fallopian tube epithelial tissues, the expression of both BMI1 and CK2α were significantly higher in tumor tissues obtained from high-grade serous ovarian cancer patients. Among tumor samples, the expression of BMI1 and CK2α positively correlated (Spearman coefficient = 0.62, P = 0.0021) with each other.

CONCLUSION

Taken together, our findings establish an important regulatory role of CK2α on BMI1 phosphorylation and stability and implicate the CK2α/BMI1 axis in ovarian cancer.

Compositional and functional diversity of canonical PRC1 complexes in mammals

The compositional complexity of Polycomb Repressive Complex 1 (PRC1) increased dramatically during vertebrate evolution. What is considered the "canonical" PRC1 complex consists of four subunits originally identified as regulators of body segmentation in Drosophila. In mammals, each of these four canonical subunits consists of two to six paralogs that associate in a combinatorial manner to produce over a hundred possible distinct PRC1 complexes with unknown function. Genetic studies have begun to define the phenotypic roles for different PRC1 paralogs; however, relating these phenotypes to unique biochemical and transcriptional function for the different paralogs has been challenging. In this review, we attempt to address how the compositional diversity of canonical PRC1 complexes relates to unique roles for individual PRC1 paralogs in transcriptional regulation. This review focuses primarily on PRC1 complex composition, genome targeting, and biochemical function.

Repression of the Antioxidant NRF2 Pathway in Premature Aging

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, invariably fatal premature aging disorder. The disease is caused by constitutive production of progerin, a mutant form of the nuclear architectural protein lamin A, leading, through unknown mechanisms, to diverse morphological, epigenetic, and genomic damage and to mesenchymal stem cell (MSC) attrition in vivo. Using a high-throughput siRNA screen, we identify the NRF2 antioxidant pathway as a driver mechanism in HGPS. Progerin sequesters NRF2 and thereby causes its subnuclear mislocalization, resulting in impaired NRF2 transcriptional activity and consequently increased chronic oxidative stress. Suppressed NRF2 activity or increased oxidative stress is sufficient to recapitulate HGPS aging defects, whereas reactivation of NRF2 activity in HGPS patient cells reverses progerin-associated nuclear aging defects and restores in vivo viability of MSCs in an animal model. These findings identify repression of the NRF2-mediated antioxidative response as a key contributor to the premature aging phenotype.

Mitochondrial BMI1 maintains bioenergetic homeostasis in cells

The polycomb complex proto-oncogene BMI1 [B lymphoma Mo-MLV insertion region 1 homolog (mouse)] is essential for self-renewal of normal and cancer stem cells. BMI1-null mice show severe defects in growth, development, and survival. Although BMI1 is known to exert its effect in the nucleus via repression of 2 potent cell-cycle regulators that are encoded by the Ink4a/Arf locus, deletion of this locus only partially rescues BMI1-null phenotypes, which is indicative of alternate mechanisms of action of BMI1. Here, we show that an extranuclear pool of BMI1 localizes to inner mitochondrial membrane and directly regulates mitochondrial RNA (mtRNA) homeostasis and bioenergetics. These mitochondrial functions of BMI1 are independent of its previously described nuclear functions because a nuclear localization-defective mutant BMI1 rescued several bioenergetic defects that we observed in BMI1-depleted cells, for example, mitochondrial respiration, cytochrome c oxidase activity, and ATP production. Mechanistically, BMI1 coprecipitated with polynucleotide phosphorylase, a ribonuclease that is responsible for decay of mtRNA transcripts. Loss of BMI1 enhanced ribonuclease activity of polynucleotide phosphorylase and reduced mtRNA stability. These findings not only establish a novel extranuclear role of BMI1 in the regulation of mitochondrial bioenergetics, but also provide new mechanistic insights into the role of this proto-oncogene in stem cell differentiation, neuronal aging, and cancer.-Banerjee Mustafi, S., Aznar, N., Dwivedi, S. K. D., Chakraborty, P. K., Basak, R., Mukherjee, P., Ghosh, P., Bhattacharya, R. Mitochondrial BMI1 maintains bioenergetic homeostasis in cells.

Diffusible Factors Secreted by Glioblastoma and Medulloblastoma Cells Induce Oxidative Stress in Bystander Neural Stem Progenitors

Harmful effects that alter the homeostasis of neural stem or progenitor cells (NSPs) can affect regenerative processes in the central nervous system. We investigated the effect of soluble factors secreted by control or ¹³⁷Cs-γ-irradiated glioblastoma or medulloblastoma cells on redox-modulated endpoints in recipient human NSPs. Growth medium harvested from the nonirradiated brain tumor cells, following 24 h of growth, induced prominent oxidative stress in recipient NSPs as judged by overall increases in mitochondrial superoxide radical levels (p < .001), activation of c-jun N-terminal kinase, and decrease in the active form of FoxO3a. The induced oxidative stress was associated with phosphorylation of p53 on serine 15, a marker of DNA damage, induction of the cyclin-cyclin dependent kinase inhibitors p21^Waf1 and p27^Kip1, and perturbations in cell cycle progression (p < .001). These changes were also associated with increased apoptosis as determined by enhanced annexin V staining (p < .001) and caspase 8 activation (p < .05) and altered expression of critical regulators of self-renewal, proliferation, and differentiation. Exposure of the tumor cells to radiation only slightly altered the induced oxidative changes in the bystander NSPs, except for medium from irradiated medulloblastoma cells that was more potent at inducing apoptosis in the NSPs than medium from nonirradiated cells (p < .001). The elucidation of such stressful bystander effects provides avenues to understand the biochemical events underlying the development or exacerbation of degenerative outcomes associated with brain cancers. It is also relevant to tissue culture protocols whereby growth medium conditioned by tumor cells is often used to support the growth of stem cells.

Beyond transcription factors: how oncogenic signalling reshapes the epigenetic landscape

Cancer, once thought to be caused largely by genetic alterations, is now considered to be a mixed genetic and epigenetic disease. The epigenetic landscape, which is dictated by covalent DNA and histone modifications, is profoundly altered in transformed cells. These abnormalities may arise from mutations in, or altered expression of, chromatin modifiers. Recent reports on the interplay between cellular signalling pathways and chromatin modifications add another layer of complexity to the already complex regulation of the epigenome. In this Review, we discuss these new studies and how the insights they provide can contribute to a better understanding of the molecular pathogenesis of neoplasia.

Polycomb Protein BMI1 Regulates Osteogenic Differentiation of Human Adipose Tissue-Derived Mesenchymal Stem Cells Downstream of GSK3

Polycomb proteins such as the B lymphoma Mo-MLV insertion region 1 homolog (BMI1) are essential chromatin factors for the self-renewal and differentiation of embryonic and adult stem cells. BMI1 also plays a critical role in osteogenesis as Bmi1-deficient mice display a skeletal phenotype caused by the exhaustion of the mesenchymal stem cell pool. In this study, we have studied the role of BMI1 in the osteogenic differentiation of human adipose tissue-derived mesenchymal stem cells (hASCs). BMI1 protein, but not RNA levels, increases during in vitro osteogenic differentiation of hASCs. Overexpression of BMI1 leads to an osteogenic priming of hASCs under nondifferentiating conditions and enhanced osteogenesis upon differentiation, along with increased BMP2 and WNT11 expressions. Conversely, knockdown of BMI1 expression reduces osteogenic differentiation. Furthermore, our studies indicate that during osteogenic differentiation of hASCs, BMI1 is a downstream target of GSK3 signaling. BMI1, therefore, acts as a pro-osteogenic differentiation factor in hASCs and hence it is a promising target for active modulation of hASC-derived osteogenesis.

BMI1: A Biomarker of Hematologic Malignancies

BMI1 oncogene is a catalytic member of epigenetic repressor polycomb group proteins. It plays a critical role in the regulation of gene expression pattern and consequently several cellular processes during development, including cell cycle progression, senescence, aging, apoptosis, angiogenesis, and importantly self-renewal of adult stem cells of several lineages. Preponderance of evidences indicates that deregulated expression of PcG protein BMI1 is associated with several human malignancies, cancer stem cell maintenance, and propagation. Importantly, overexpression of BMI1 correlates with therapy failure in cancer patients and tumor relapse. This review discusses the diverse mode of BMI1 regulation at transcriptional, posttranscriptional, and posttranslational levels as well as at various critical signaling pathways regulated by BMI1 activity. Furthermore, this review highlights the role of BMI1 as a biomarker and therapeutic target for several subtypes of hematologic malignancies and the importance to target this biomarker for therapeutic applications.

The interaction between AMPKβ2 and the PP1-targeting subunit R6 is dynamically regulated by intracellular glycogen content

Breakdown of intracellular glycogen enhances interaction of the AMPKβ2 subunit and the R6 glycogen-targeting subunit of protein phosphatase type 1 (PP1), which occurs in conjunction with increased β2-Thr-148 phosphorylation.

Expression Analysis of Genes Involved in the RB/E2F Pathway in Astrocytic Tumors

Astrocytic gliomas, which are derived from glial cells, are considered the most common primary neoplasias of the central nervous system (CNS) and are histologically classified as low grade (I and II) or high grade (III and IV). Recent studies have shown that astrocytoma formation is the result of the deregulation of several pathways, including the RB/E2F pathway, which is commonly deregulated in various human cancers via genetic or epigenetic mechanisms. On the basis of the assumption that the study of the mechanisms controlling the INK4/ARF locus can help elucidate the molecular pathogenesis of astrocytic tumors, identify diagnostic and prognostic markers, and help select appropriate clinical treatments, the present study aimed to evaluate and compare methylation patterns using bisulfite sequencing PCR and evaluate the gene expression profile using real-time PCR in the genes CDKN2A, CDKN2B, CDC6, Bmi-1, CCND1, and RB1 in astrocytic tumors. Our results indicate that all the evaluated genes are not methylated independent of the tumor grade. However, the real-time PCR results indicate that these genes undergo progressive deregulation as a function of the tumor grade. In addition, the genes CDKN2A, CDKN2B, and RB1 were underexpressed, whereas CDC6, Bmi-1, and CCND1 were overexpressed; the increase in gene expression was significantly associated with decreased patient survival. Therefore, we propose that the evaluation of the expression levels of the genes involved in the RB/E2F pathway can be used in the monitoring of patients with astrocytomas in clinical practice and for the prognostic indication of disease progression.

Polycomb Group (PcG) Proteins and Human Cancers: Multifaceted Functions and Therapeutic Implications

Polycomb group (PcG) proteins are transcriptional repressors that regulate several crucial developmental and physiological processes in the cell. More recently, they have been found to play important roles in human carcinogenesis and cancer development and progression. The deregulation and dysfunction of PcG proteins often lead to blocking or inappropriate activation of developmental pathways, enhancing cellular proliferation, inhibiting apoptosis, and increasing the cancer stem cell population. Genetic and molecular investigations of PcG proteins have long been focused on their PcG functions. However, PcG proteins have recently been shown to exert non-classical-Pc-functions, contributing to the regulation of diverse cellular functions. We and others have demonstrated that PcG proteins regulate the expression and function of several oncogenes and tumor suppressor genes in a PcG-independent manner, and PcG proteins are associated with the survival of patients with cancer. In this review, we summarize the recent advances in the research on PcG proteins, including both the Pc-repressive and non-classical-Pc-functions. We specifically focus on the mechanisms by which PcG proteins play roles in cancer initiation, development, and progression. Finally, we discuss the potential value of PcG proteins as molecular biomarkers for the diagnosis and prognosis of cancer, and as molecular targets for cancer therapy.

Bmi-1: At the crossroads of physiological and pathological biology

Bmi-1 is a member of the Polycomb repressor complex 1 that mediates gene silencing by regulating chromatin structure and is indispensable for self-renewal of both normal and cancer stem cells. Despite three decades of research that have elucidated the transcriptional regulation, post-translational modifications and functions of Bmi-1 in regulating the DNA damage response, cellular bioenergetics, and pathologies, the entire potential of a protein with such varied functions remains to be realized. This review attempts to synthesize the current knowledge on Bmi-1 with an emphasis on its role in both normal physiology and cancer. Additionally, since cancer stem cells are emerging as a new paradigm for therapy resistance, the role of Bmi-1 in this perspective is also highlighted. The wide spectrum of malignancies that implicate Bmi-1 as a signature for stemness and oncogenesis also make it a suitable candidate for therapy. Nonetheless, new approaches are vitally needed to further characterize physiological roles of Bmi-1 with the long-term goal of using Bmi-1 as a prognostic marker and a therapeutic target.

MK3 modulation affects BMI1-dependent and independent cell cycle check-points

Although the MK3 gene was originally found deleted in some cancers, it is highly expressed in others. The relevance of MK3 for oncogenesis is currently not clear. We recently reported that MK3 controls ERK activity via a negative feedback mechanism. This prompted us to investigate a potential role for MK3 in cell proliferation. We here show that overexpression of MK3 induces a proliferative arrest in normal diploid human fibroblasts, characterized by enhanced expression of replication stress- and senescence-associated markers. Surprisingly, MK3 depletion evokes similar senescence characteristics in the fibroblast model. We previously identified MK3 as a binding partner of Polycomb Repressive Complex 1 (PRC1) proteins. In the current study we show that MK3 overexpression results in reduced cellular EZH2 levels and concomitant loss of epigenetic H3K27me3-marking and PRC1/chromatin-occupation at the CDKN2A/INK4A locus. In agreement with this, the PRC1 oncoprotein BMI1, but not the PCR2 protein EZH2, bypasses MK3-induced senescence in fibroblasts and suppresses P16^INK4A expression. In contrast, BMI1 does not rescue the MK3 loss-of-function phenotype, suggesting the involvement of multiple different checkpoints in gain and loss of MK3 function. Notably, MK3 ablation enhances proliferation in two different cancer cells. Finally, the fibroblast model was used to evaluate the effect of potential tumorigenic MK3 driver-mutations on cell proliferation and M/SAPK signaling imbalance. Taken together, our findings support a role for MK3 in control of proliferation and replicative life-span, in part through concerted action with BMI1, and suggest that the effect of MK3 modulation or mutation on M/SAPK signaling and, ultimately, proliferation, is cell context-dependent.

The recruitment of AMP-activated protein kinase to glycogen is regulated by autophosphorylation

The mammalian AMP-activated protein kinase (AMPK) is an obligatory αβγ heterotrimeric complex carrying a carbohydrate-binding module (CBM) in the β-subunit (AMPKβ) capable of attaching AMPK to glycogen. Nonetheless, AMPK localizes at many different cellular compartments, implying the existence of mechanisms that prevent AMPK from glycogen binding. Cell-free carbohydrate binding assays revealed that AMPK autophosphorylation abolished its carbohydrate-binding capacity. X-ray structural data of the CBM displays the central positioning of threonine 148 within the binding pocket. Substitution of Thr-148 for a phospho-mimicking aspartate (T148D) prevents AMPK from binding to carbohydrate. Overexpression of isolated CBM or β1-containing AMPK in cellular models revealed that wild type (WT) localizes to glycogen particles, whereas T148D shows a diffuse pattern. Pharmacological AMPK activation and glycogen degradation by glucose deprivation but not forskolin enhanced cellular Thr-148 phosphorylation. Cellular glycogen content was higher if pharmacological AMPK activation was combined with overexpression of T148D mutant relative to WT AMPK. In summary, these data show that glycogen-binding capacity of AMPKβ is regulated by Thr-148 autophosphorylation with likely implications in the regulation of glycogen turnover. The findings further raise the possibility of regulated carbohydrate-binding function in a wider variety of CBM-containing proteins.

MAPKAP kinase 3 suppresses Ifng gene expression and attenuates NK cell cytotoxicity and Th1 CD4 T-cell development upon influenza A virus infection

MK2 and MK3 are downstream targets of p38 and ERK1/2. They control the mRNA stability of several inflammatory cytokines, including TNF-α and IL-10. Whereas MK2 is expressed ubiquitously, the expression of MK3 is restricted to muscle, liver, and heart tissues and T and NK cells. Using Mk-deficient and wild-type (WT) mice, we demonstrated an inhibitory effect of MK3, but not of MK2, on interferon (IFN)-γ expression in T and NK lymphocytes. The results provided evidence that the inhibitory effect of MK3 is based on negative feedback phosphorylation of p38 and ERK1/2, which causes decreased binding of Stat4 to the IFN-γ promoter and reduced expression of IFN-γ mRNA and protein. Consequently, all Mk3(-/-) mice challenged with the Th1-inducing influenza A virus (IAV) survived the WT LD50 virus dose. The reduced disease severity in the Mk3(-/-) mice was accompanied by a >10-fold reduction in viral lung titer and an increase in the number of activated NK cells and enhanced Th1 activation of CD4 T cells. Thus, our data describe the protein kinase MK3 as a novel regulator of the innate and adaptive immune responses.-Köther, K., Nordhoff, C., Masemann, D., Varga, G., Bream, J. H., Gaestel, M., Wixler, V., Ludwig, S. MAPKAP kinase 3 suppresses Ifng gene expression and attenuates NK cell cytotoxicity and Th1 CD4 T-cell development upon influenza A virus infection.

Sensing cellular states--signaling to chromatin pathways targeting Polycomb and Trithorax group function

Cells respond to extra- and intra-cellular signals by dynamically changing their gene expression patterns. After termination of the original signal, new expression patterns are maintained by epigenetic DNA and histone modifications. This represents a powerful mechanism that enables long-term phenotypic adaptation to transient signals. Adaptation of epigenetic landscapes is important for mediating cellular differentiation during development and allows adjustment to altered environmental conditions throughout life. Work over the last decade has begun to elucidate the way that extra- and intra-cellular signals lead to changes in gene expression patterns by directly modulating the function of chromatin-associated proteins. Here, we review key signaling-to-chromatin pathways that are specifically thought to target Polycomb and Trithorax group complexes, a classic example of epigenetically acting gene silencers and activators important in development, stem cell differentiation and cancer. We discuss the influence that signals triggered by kinase cascades, metabolic fluctuations and cell-cycle dynamics have on the function of these protein complexes. Further investigation into these pathways will be important for understanding the mechanisms that maintain epigenetic stability and those that promote epigenetic plasticity.

OsFIE2 plays an essential role in the regulation of rice vegetative and reproductive development

Polycomb group (PcG) proteins are gene repressors that help to maintain cellular identity during development via chromatin remodeling. Fertilization-independent endosperm (FIE), a member of the PcG complex, operates extensively in plant development, but its role in rice has not been fully investigated to date. We report the isolation and characterization of a PcG member in rice, which was designated OsFIE2 for Oryza sativa Fertilization-Independent Endosperm 2. OsFIE2 is a single-copy gene in the rice genome and shows a universal expression pattern. The OsFIE2 RNAi lines displayed pleiotropic phenotypes in vegetative and reproductive organ generation. In unfertilized lines, endosperm formation could be triggered without embryo formation, which indicates that FIE is indeed involved in the suppression of autonomous endosperm development in rice. Furthermore, lateral root generation was promoted early in the roots of OsFIE2 RNAi lines, whereas the primary root was premature and highly differentiated. As the root tip stem cell differentiated, QHB, the gene required for stem cell maintenance in the quiescent center, was down-regulated. Our data suggest that the OsFIE2-PcG complex is vital for rice reproduction and endosperm formation. Its role in stem cell maintenance suggests that the gene is functionally conserved in plants as well as animals.

A new world of Polycombs: unexpected partnerships and emerging functions

Polycomb group (PcG) proteins are epigenetic repressors that are essential for the transcriptional control of cell differentiation and development. PcG-mediated repression is associated with specific post-translational histone modifications and is thought to involve both biochemical and physical modulation of chromatin structure. Recent advances show that PcG complexes comprise a multiplicity of variants and are far more biochemically diverse than previously thought. The importance of these new PcG complexes for normal development and disease, their targeting mechanisms and their shifting roles in the course of differentiation are now the subject of investigation and the focus of this Review.

Mitogen-activated protein kinase signaling mediates phosphorylation of polycomb ortholog Cbx7

Cbx7 is one of five mammalian orthologs of the Drosophila Polycomb. Cbx7 recognizes methylated lysine residues on the histone H3 tail and contributes to gene silencing in the context of the Polycomb repressive complex 1 (PRC1). However, our knowledge of Cbx7 post-translational modifications remains limited. Through combined biochemical and mass spectrometry approaches, we report a novel phosphorylation site on mouse Cbx7 at residue Thr-118 (Cbx7T118ph), near the highly conserved Polycomb box. The generation of a site-specific antibody to Cbx7T118ph demonstrates that Cbx7 is phosphorylated via MAPK signaling. Furthermore, we find Cbx7T118 phosphorylation in murine mammary carcinoma cells, which can be blocked by MEK inhibitors. Upon EGF stimulation, Cbx7 interacts robustly with other members of PRC1. To test the role of Cbx7T118 phosphorylation in gene silencing, we employed a RAS-induced senescence model system. We demonstrate that Cbx7T118 phosphorylation moderately enhances repression of its target gene p16. In summary, we have identified and characterized a novel MAPK-mediated phosphorylation site on Cbx7 and propose that mitogen signaling to the chromatin template regulates PRC1 function.

Pivotal role for ROS activation of p38 MAPK in the control of differentiation and tumor-initiating capacity of glioma-initiating cells

Reactive oxygen species (ROS) are involved in various aspects of cancer cell biology, yet their role in cancer stem cells (CSCs) has been poorly understood. In particular, it still remains unclear whether and how ROS control the self-renewal/differentiation process and the tumor-initiating capacity of CSCs. Here we show that ROS-mediated activation of p38 MAPK plays a pivotal role in the control of differentiation and tumor-initiating capacity of glioma-initiating cells (GICs) derived from human glioblastomas. Mechanistically, ROS triggered p38-dependent Bmi1 protein degradation and FoxO3 activation in GICs, which were shown to be responsible for the loss of their self-renewal capacity and differentiation, respectively. Thus, the results suggest that Bmi1 and FoxO3 govern distinct phases of transition from undifferentiated to fully differentiated cells. Furthermore, we also demonstrate in this study that oxidative stress deprives GICs of their tumor-initiating capacity through the activation of the ROS-p38 axis. As such, this is the first study to the best of our knowledge to delineate how ROS control self-renewal/differentiation and the tumor-initiating capacity of stem-like cancer cells. This study also suggests that targeting of the ROS-p38 axis could be a novel approach in the development of therapeutic strategies against gliomas, represented by glioblastoma.

Lineage-inappropriate PAX5 expression in t(8;21) acute myeloid leukemia requires signaling-mediated abrogation of polycomb repression

The activation of B-cell-specific genes, such as CD19 and PAX5, is a hallmark of t(8;21) acute myeloid leukemia (AML) which expresses the translocation product RUNX1/ETO. PAX5 is an important regulator of B-lymphoid development and blocks myeloid differentiation when ectopically expressed. To understand the molecular mechanism of PAX5 deregulation, we examined its chromatin structure and regulation in t(8;21) AML cells, non-t(8;21) myeloid precursor control cells, and pre-B cells. In non-t(8;21) myeloid precursors, PAX5 is poised for transcription, but is repressed by polycomb complexes. In t(8;21) AML, PAX5 is not directly activated by RUNX1/ETO, but expression requires constitutive mitogen-activated protein (MAP) kinase signaling. Using a model of t(8;21) carrying an activating KIT mutation, we demonstrate that deregulated MAP kinase signaling in t(8;21) AML abrogates the association of polycomb complexes to PAX5 and leads to aberrant gene activation. Our findings therefore suggest a novel role of activating tyrosine kinase mutations in lineage-inappropriate gene expression in AML.

Akt phosphorylates the transcriptional repressor bmi1 to block its effects on the tumor-suppressing ink4a-arf locus

The Polycomb group protein Bmi1 is a transcriptional silencer of the Ink4a-Arf locus, which encodes the cell cycle regulator p16(Ink4a) and the tumor suppressor p19(Arf). Bmi1 plays a key role in oncogenesis and stem cell self-renewal. We report that phosphorylation of human Bmi1 at Ser³¹⁶ by Akt impaired its function by triggering its dissociation from the Ink4a-Arf locus, which resulted in decreased ubiquitylation of histone H2A and the inability of Bmi1 to promote cellular proliferation and tumor growth. Moreover, Akt-mediated phosphorylation of Bmi1 also inhibited its ability to promote self-renewal of hematopoietic stem and progenitor cells. Our study provides a mechanism for the increased abundance of p16(Ink4a) and p19(Arf) seen in cancer cells with an activated phosphoinositide 3-kinase to Akt signaling pathway and identifies crosstalk between phosphorylation events and chromatin structure.

MK3 controls Polycomb target gene expression via negative feedback on ERK

Background

Gene-environment interactions are mediated by epigenetic mechanisms. Polycomb Group proteins constitute part of an epigenetic cellular transcriptional memory system that is subject to dynamic modulation during differentiation. Molecular insight in processes that control dynamic chromatin association and dissociation of Polycomb repressive complexes during and beyond development is limited. We recently showed that MK3 interacts with Polycomb repressive complex 1 (PRC1). The functional relevance of this interaction, however, remained poorly understood. MK3 is activated downstream of mitogen- and stress-activated protein kinases (M/SAPKs), all of which fulfill crucial roles during development. We here use activation of the immediate-early response gene ATF3, a bona fide PRC1 target gene, as a model to study how MK3 and its effector kinases MAPK/ERK and SAPK/P38 are involved in regulation of PRC1-dependent ATF3 transcription.

Results

Our current data show that mitogenic signaling through ERK, P38 and MK3 regulates ATF3 expression by PRC1/chromatin dissociation and epigenetic modulation. Mitogenic stimulation results in transient P38-dependent H3S28 phosphorylation and ERK-driven PRC1/chromatin dissociation at PRC1 targets. H3S28 phosphorylation by itself appears not sufficient to induce PRC1/chromatin dissociation, nor ATF3 transcription, as inhibition of MEK/ERK signaling blocks BMI1/chromatin dissociation and ATF3 expression, despite induced H3S28 phosphorylation. In addition, we establish that concomitant loss of local H3K27me3 promoter marking is not required for ATF3 activation. We identify pERK as a novel signaling-induced binding partner of PRC1, and provide evidence that MK3 controls ATF3 expression in cultured cells via negative regulatory feedback on M/SAPKs. Dramatically increased ectopic wing vein formation in the absence of Drosophila MK in a Drosophila ERK gain-of-function wing vein patterning model, supports the existence of MK-mediated negative feedback regulation on pERK.

Conclusion

We here identify and characterize important actors in a PRC1-dependent epigenetic signal/response mechanism, some of which appear to be nonspecific global responses, whereas others provide modular specificity. Our findings provide novel insight into a Polycomb-mediated epigenetic mechanism that dynamically controls gene transcription and support a direct link between PRC1 and cellular responses to changes in the microenvironment.

Regulation of oncogene-induced cell cycle exit and senescence by chromatin modifiers

Oncogene activation leads to dramatic changes in numerous biological pathways controlling cellular division, and results in the initiation of a transcriptional program that promotes transformation. Conversely, it also triggers an irreversible cell cycle exit called cellular senescence, which allows the organism to counteract the potentially detrimental uncontrolled proliferation of damaged cells. Therefore, a tight transcriptional control is required at the onset of oncogenic signal, coordinating both positive and negative regulation of gene expression. Not surprisingly, numerous chromatin modifiers contribute to the cellular response to oncogenic stress. While these chromatin modifiers were initially thought of as mere mediators of the cellular response to oncogenic stress, recent studies have uncovered a direct and specific regulation of chromatin modifiers by oncogenic signals. We review here the diverse functions of chromatin modifiers in the cellular response to oncogenic stress, and discuss the implications of these findings on the regulation of cell cycle progression and proliferation by activated oncogenes.