MRG15 activates the cdc2 promoter via histone acetylation in human cells

Structural and functional insights into the epigenetic regulator MRG15

MORF4-related gene on chromosome 15 (MRG15), a chromatin remodeller, is evolutionally conserved and ubiquitously expressed in mammalian tissues and cells. MRG15 plays vital regulatory roles in DNA damage repair, cell proliferation and division, cellular senescence and apoptosis by regulating both gene activation and gene repression via associations with specific histone acetyltransferase and histone deacetylase complexes. Recently, MRG15 has also been shown to rhythmically regulate hepatic lipid metabolism and suppress carcinoma progression. The unique N-terminal chromodomain and C-terminal MRG domain in MRG15 synergistically regulate its interaction with different cofactors, affecting its functions in various cell types. Thus, how MRG15 elaborately regulates target gene expression and performs diverse functions in different cellular contexts is worth investigating. In this review, we provide an in-depth discussion of how MRG15 controls multiple physiological and pathological processes.

Targeting CDK1 in cancer: mechanisms and implications

Cyclin dependent kinases (CDKs) are serine/threonine kinases that are proposed as promising candidate targets for cancer treatment. These proteins complexed with cyclins play a critical role in cell cycle progression. Most CDKs demonstrate substantially higher expression in cancer tissues compared with normal tissues and, according to the TCGA database, correlate with survival rate in multiple cancer types. Deregulation of CDK1 has been shown to be closely associated with tumorigenesis. CDK1 activation plays a critical role in a wide range of cancer types; and CDK1 phosphorylation of its many substrates greatly influences their function in tumorigenesis. Enrichment of CDK1 interacting proteins with Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was conducted to demonstrate that the associated proteins participate in multiple oncogenic pathways. This abundance of evidence clearly supports CDK1 as a promising target for cancer therapy. A number of small molecules targeting CDK1 or multiple CDKs have been developed and evaluated in preclinical studies. Notably, some of these small molecules have also been subjected to human clinical trials. This review evaluates the mechanisms and implications of targeting CDK1 in tumorigenesis and cancer therapy.

MRG15 orchestrates rhythmic epigenomic remodelling and controls hepatic lipid metabolism

The rhythmic regulation of transcriptional processes is intimately linked to lipid homeostasis, to anticipate daily changes in energy access. The Rev-erbα-HDAC3 complex was previously discovered to execute the rhythmic repression of lipid genes; however, the epigenetic switch that turns on these genes is less clear. Here, we show that genomic recruitment of MRG15, which is encoded by the mortality factor on chromosome 4 (MORF4)-related gene on chromosome 15, displays a significant diurnal rhythm and activates lipid genes in the mouse liver. RNA polymerase II (Pol II) recruitment and histone acetylation correspond to MRG15 binding, and the rhythm is impaired upon MRG15 depletion, establishing MRG15 as a key modulator in global rhythmic transcriptional regulation. MRG15 interacts with the nuclear receptor LRH-1, rather than with known core clock proteins, and is recruited to genomic loci near lipid genes via LRH-1. Blocking of MRG15 by CRISPR targeting or by the FDA-approved drug argatroban, which is an antagonist to MRG15, attenuates liver steatosis. This work highlights MRG15 as a targetable master regulator in the rhythmic regulation of hepatic lipid metabolism.

Therapeutic approach of Cancer stem cells (CSCs) in gastric adenocarcinoma; DNA methyltransferases enzymes in cancer targeted therapy

Cancer stem cells (CSCs) show a remarkable sub class of cancer cells population which have a potential to organize and regulate stemness properties which possess a main particular responsibility for uncontrolled growth in carcinogenesis, production of different cancers in differentiated situation and also resistancy to radiotherapy and chemotherapy. Correspondingly, gastric cancer (GC) as a very serious type in cancer mortality in the world, has received a deep attention in molecular therapy recently. Besides the main characteristics of CSCs like differentiation, epithelial mesenchymal transition, self-renewal and metastasis, they are so effective in expression of stemness genes resistancy in radiotherapy and chemotherapy. In this way, the regulation of epigenetic elements including DNA methylation and the performance of DNA methyltransferase (DNMT) which is a notable epigenetic trait in GC, is of great importance. Inhibitors of DNA methylation are the first epigenetic drugs in cancer therapy. Considerably, recent studies indicate that low doses of DNMT inhibitors have a high potential in sustaining reduced DNA methylation and related with re-expression of silenced genes in tumorigenesis. Importantly, these certain doses have the ability to decrease the carcinogenesis and tumorigenesis in CSC populations within GC. Meaningly, the inhibition of DNMTs are able to reduce the accumulation of tumorigenic ability of GC CSCs. Furthermore, many epigenetic drugs have a great potential in cancer therapy, including histone methyltransferases, lysine demethylases, histone deacetylasesand, bromodomain and extra-terminal domain proteins and DNA methyltransferases inhibitors. In this review article, we try to focus on the therapeutic mechanism of DNMTs alongside with their impact on CSCs in GC.

A saga of cancer epigenetics: linking epigenetics to alternative splicing

The discovery of an increasing number of alternative splicing events in the human genome highlighted that ∼94% of genes generate alternatively spliced transcripts that may produce different protein isoforms with diverse functions. It is now well known that several diseases are a direct and indirect consequence of aberrant splicing events in humans. In addition to the conventional mode of alternative splicing regulation by 'cis' RNA-binding sites and 'trans' RNA-binding proteins, recent literature provides enormous evidence for epigenetic regulation of alternative splicing. The epigenetic modifications may regulate alternative splicing by either influencing the transcription elongation rate of RNA polymerase II or by recruiting a specific splicing regulator via different chromatin adaptors. The epigenetic alterations and aberrant alternative splicing are known to be associated with various diseases individually, but this review discusses/highlights the latest literature on the role of epigenetic alterations in the regulation of alternative splicing and thereby cancer progression. This review also points out the need for further studies to understand the interplay between epigenetic modifications and aberrant alternative splicing in cancer progression.

Mortality factor 4 like 1 protein mediates epithelial cell death in a mouse model of pneumonia

Unchecked epithelial cell death is fundamental to the pathogenesis of pneumonia. The recognition of unique signaling pathways that preserve epithelial cell viability may present new opportunities for interventional strategies. We describe that mortality factor 4 like 1 (Morf4l1), a protein involved in chromatin remodeling, is constitutively expressed at low levels in the lung because of its continuous degradation mediated by an orphan ubiquitin E3 ligase subunit, Fbxl18. Expression of Morf4l1 increases in humans with pneumonia and is up-regulated in lung epithelia after exposure to Pseudomonas aeruginosa or lipopolysaccharide. In a mouse model of pneumonia induced by P. aeruginosa, Morf4l1 is stabilized by acetylation that protects it from Fbxl18-mediated degradation. After P. aeruginosa infection of mice, overexpression of Morf4l1 resulted in lung epithelial cell death, whereas its depletion restored cell viability. Using in silico modeling and drug-target interaction studies, we identified that the U.S. Food and Drug Administration-approved thrombin inhibitor argatroban is a Morf4l1 antagonist. Argatroban inhibited Morf4l1-dependent histone acetylation, reduced its cytotoxicity, and improved survival of mice with experimental lung injury at doses that had no anticoagulant activity. These studies uncover a previously unrecognized biological mechanism whereby pathogens subvert cell viability by extending the life span of a cytotoxic host protein. Morf4l1 may be a potential molecular target for non-antibiotic pharmacotherapy during severe pulmonary infection.

Proteasome regulation of the chromodomain protein MRG-1 controls the balance between proliferative fate and differentiation in the C. elegans germ line

The level of stem cell proliferation must be tightly controlled for proper development and tissue homeostasis. Multiple levels of gene regulation are often employed to regulate stem cell proliferation to ensure that the amount of proliferation is aligned with the needs of the tissue. Here we focus on proteasome-mediated protein degradation as a means of regulating the activities of proteins involved in controlling the stem cell proliferative fate in the C. elegans germ line. We identify five potential E3 ubiquitin ligases, including the RFP-1 RING finger protein, as being involved in regulating proliferative fate. RFP-1 binds to MRG-1, a homologue of the mammalian chromodomain-containing protein MRG15 (MORF4L1), which has been implicated in promoting the proliferation of neural precursor cells. We find that C. elegans with reduced proteasome activity, or that lack RFP-1 expression, have increased levels of MRG-1 and a shift towards increased proliferation in sensitized genetic backgrounds. Likewise, reduction of MRG-1 partially suppresses stem cell overproliferation. MRG-1 levels are controlled independently of the spatially regulated GLP-1/Notch signalling pathway, which is the primary signal controlling the extent of stem cell proliferation in the C. elegans germ line. We propose a model in which MRG-1 levels are controlled, at least in part, by the proteasome, and that the levels of MRG-1 set a threshold upon which other spatially regulated factors act in order to control the balance between the proliferative fate and differentiation in the C. elegans germ line.

Phosphoproteomics screen reveals akt isoform-specific signals linking RNA processing to lung cancer

The three Akt isoforms are functionally distinct. Here we show that their phosphoproteomes also differ, suggesting that their functional differences are due to differences in target specificity. One of the top cellular functions differentially regulated by Akt isoforms is RNA processing. IWS1, an RNA processing regulator, is phosphorylated by Akt3 and Akt1 at Ser720/Thr721. The latter is required for the recruitment of SETD2 to the RNA Pol II complex. SETD2 trimethylates histone H3 at K36 during transcription, creating a docking site for MRG15 and PTB. H3K36me3-bound MRG15 and PTB regulate FGFR-2 splicing, which controls tumor growth and invasiveness downstream of IWS1 phosphorylation. Twenty-one of the twenty-four non-small-cell-lung carcinomas we analyzed express IWS1. More importantly, the stoichiometry of IWS1 phosphorylation in these tumors correlates with the FGFR-2 splicing pattern and with Akt phosphorylation and Akt3 expression. These data identify an Akt isoform-dependent regulatory mechanism for RNA processing and demonstrate its role in lung cancer.

Alternative splicing of the chromodomain protein Morf4l1 pre-mRNA has implications on cell differentiation in the developing chicken retina

The proliferation, cell cycle exit and differentiation of progenitor cells are controlled by several different factors. The chromodomain protein mortality factor 4-like 1 (Morf4l1) has been ascribed a role in both proliferation and differentiation. Little attention has been given to the existence of alternative splice variants of the Morf4l1 mRNA, which encode two Morf41l isoforms: a short isoform (S-Morf4l1) with an intact chromodomain and a long isoform (L-Morf4l1) with an insertion in or in the vicinity of the chromodomain. The aim of this study was to investigate if this alternative splicing has a function during development. We analysed the temporal and spatial distribution of the two mRNAs and over-expressed both isoforms in the developing retina. The results showed that the S-Morf4l1 mRNA is developmentally regulated. Over-expression of S-Morf4l1 using a retrovirus vector produced a clear phenotype with an increase of early-born neurons: retinal ganglion cells, horizontal cells and cone photoreceptor cells. Over-expression of L-Morf4l1 did not produce any distinguishable phenotype. The over-expression of S-Morf4l1 but not L-Morf4l1 also increased apoptosis in the infected regions. Our results suggest that the two Morf4l1 isoforms have different functions during retinogenesis and that Morf4l1 functions are fine-tuned by developmentally regulated alternative splicing. The data also suggest that Morf4l1 contributes to the regulation of cell genesis in the retina.

[Research advance of dosage compensation and MSL complex]

Dosage compensation effect, which exists widely in eukaryotes with sexual reproduction, is an essential biological process that equalizes the level of gene expression between genders based on sex determination. In Drosophila, the male-specific lethal (MSL) complex mediates dosage compensation by acetylating histone H4 lysine K16 on nucleosome of some specific sites on the male X chromosome, globally upregulates twofold expression of active X-linked genes from the single X chromosome, and makes up for the shortage that the male has only one single X chromosome in male Drosophila. Up to date, the structure of basic components of MSL complex, which consists of at least five protein subunits and two non-coding RNAs, has already been revealed, and the interaction sites among these components have also been generally identified. Furthermore, abundant researches on recognition mechanism of the complex have been published. In contrast, many studies have revealed that mammalian dosage compensation functions by silencing gene expression from one of the two X chromosomes in females. The main components of mammalian MSL complex have already been identified, but the knowledge of their function is limited. Up to now, research of MSLs in teleosts is scarcely studied. This review summarizes the similarities and differences among dosage compensation mechanisms of nematodes, fruit flies and mammals, introduces the recent research advances in MSL complex, as well as molecular mechanism of dosage compensation in fruit fly, and finally addresses some problems to be resolved. Meanwhile, the diversity of msl3 gene in fishes is found by synteny analysis. This information might provide insightful directions for future research on the mechanisms of dosage compensation in various species.

A network of genes antagonistic to the LIN-35 retinoblastoma protein of Caenorhabditis elegans

The Caenorhabditis elegans pRb ortholog, LIN-35, functions in a wide range of cellular and developmental processes. This includes a role of LIN-35 in nutrient utilization by the intestine, which it carries out redundantly with SLR-2, a zinc-finger protein. This and other redundant functions of LIN-35 were identified in genetic screens for mutations that display synthetic phenotypes in conjunction with loss of lin-35. To explore the intestinal role of LIN-35, we conducted a genome-wide RNA-interference-feeding screen for suppressors of lin-35; slr-2 early larval arrest. Of the 26 suppressors identified, 17 fall into three functional classes: (1) ribosome biogenesis genes, (2) mitochondrial prohibitins, and (3) chromatin regulators. Further characterization indicates that different categories of suppressors act through distinct molecular mechanisms. We also tested lin-35; slr-2 suppressors, as well as suppressors of the synthetic multivulval phenotype, to determine the spectrum of lin-35-synthetic phenotypes that could be suppressed following inhibition of these genes. We identified 19 genes, most of which are evolutionarily conserved, that can suppress multiple unrelated lin-35-synthetic phenotypes. Our study reveals a network of genes broadly antagonistic to LIN-35 as well as genes specific to the role of LIN-35 in intestinal and vulval development. Suppressors of multiple lin-35 phenotypes may be candidate targets for anticancer therapies. Moreover, screening for suppressors of phenotypically distinct synthetic interactions, which share a common altered gene, may prove to be a novel and effective approach for identifying genes whose activities are most directly relevant to the core functions of the shared gene.

Stress-induced cell-cycle activation in Tip60 haploinsufficient adult cardiomyocytes

Background

Tat-interactive protein 60 (Tip60) is a member of the MYST family of histone acetyltransferases. Studies using cultured cells have shown that Tip60 has various functions including DNA repair, apoptosis and cell-cycle regulation. We globally ablated the Tip60 gene (Htatip), observing that Tip60-null embryos die at the blastocyst stage (Hu et al. Dev.Dyn.238:2912;2009). Although adult heterozygous (Tip60^+/−) mice reproduce normally without a haploinsufficient phenotype, stress caused by Myc over-expression induced B-cell lymphoma in Tip60^+/− adults, suggesting that Tip60 is a tumor suppressor (Gorrini et al. Nature 448:1063;2007). These findings prompted assessment of whether Tip60, alternative splicing of which generates two predominant isoforms termed Tip60α and Tip60β, functions to suppress the cell-cycle in adult cardiomyocytes.

Methodology/Principal Findings

Western blotting revealed that Tip60α is the predominant Tip60 isoprotein in the embryonic heart, transitioning at neonatal stages to Tip60β, which is the only isoprotein in the adult heart wherein it is highly enriched. Over-expression of Tip60β, but not Tip60α, inhibited cell proliferation in NIH3T3 cells; and, Tip60-haploinsufficient cultured neonatal cardiomyocytes exhibited increased cell-cycle activity. To address whether Tip60β suppresses the cardiomyocyte cell-cycle in the adult heart, hypertrophic stress was induced in Tip60^+/+ and Tip^+/− littermates via two methods, Myc over-expression and aortic banding. Based on immunostaining cell-cycle markers and western blotting cyclin D, stress increased cardiomyocyte cell-cycle mobilization in Tip60^+/− hearts, in comparison with Tip60^+/+ littermates. Aortic-banded Tip60^+/− hearts also exhibited significantly decreased apoptosis.

Conclusions/Significance

These findings provide evidence that Tip60 may function in a tumor suppressor pathway(s) to maintain adult cardiomyocytes in replicative senescence.