Increased genomic instability and altered chromosomal protein phosphorylation timing inHRAS-transformed mouse fibroblasts

Chromatin organization and behavior in HRAS-transformed mouse fibroblasts

In higher eukaryotic cells, a string of nucleosomes, where long genomic DNA is wrapped around core histones, are rather irregularly folded into a number of condensed chromatin domains, which have been revealed by super-resolution imaging and Hi-C technologies. Inside these domains, nucleosomes fluctuate and locally behave like a liquid. The behavior of chromatin may be highly related to DNA transaction activities such as transcription and repair, which are often upregulated in cancer cells. To investigate chromatin behavior in cancer cells and compare those of cancer and non-cancer cells, we focused on oncogenic-HRAS (Gly12Val)-transformed mouse fibroblasts CIRAS-3 cells and their parental 10T1/2 cells. CIRAS-3 cells are tumorigenic and highly metastatic. First, we found that HRAS-induced transformation altered not only chromosome structure, but also nuclear morphology in the cell. Using single-nucleosome imaging/tracking in live cells, we demonstrated that nucleosomes are locally more constrained in CIRAS-3 cells than in 10T1/2 cells. Consistently, heterochromatin marked with H3K27me3 was upregulated in CIRAS-3 cells. Finally, Hi-C analysis showed enriched interactions of the B-B compartment in CIRAS-3 cells, which likely represents transcriptionally inactive chromatin. Increased heterochromatin may play an important role in cell migration, as they have been reported to increase during metastasis. Our study also suggests that single-nucleosome imaging provides new insights into how local chromatin is structured in living cells.

Mitogen-induced distinct epialleles are phosphorylated at either H3S10 or H3S28, depending on H3K27 acetylation

Stimulation of the MAPK pathway results in mitogen- and stress-activated protein kinase 1/2 (MSK1/2)-catalyzed phosphorylation of histone H3 at serine 10 or 28 and expression of immediate-early (IE) genes. In 10T1/2 mouse fibroblasts, phosphorylation of H3S10 and H3S28 occurs on different H3 molecules and in different nuclear regions. Similarly, we show that mitogen-induced H3S10 and H3S28 phosphorylation occurs in separate pools in human primary fibroblasts. High-resolution imaging studies on both cell types reveal that H3S10 and H3S28 phosphorylation events can be induced in a single cell but on different alleles, giving rise to H3S10ph and H3S28ph epialleles. Coimmunoprecipitation and inhibition studies demonstrate that CBP/p300-mediated H3K27 acetylation is required for MSK1/2 to phosphorylate S28. Although the K9ac and S10ph marks coexist on H3, S10 phosphorylation is not dependent on K9 acetylation by PCAF. We propose that random targeting of H3S10 or H3S28 results from the stochastic acetylation of H3 by CBP/p300 or PCAF, a process comparable to transcriptional bursting causing temporary allelic imbalance. In 10T1/2 cells expressing Jun, at least two of three alleles per cell were induced, a sign of high expression level. The redundant roles of H3S10ph and H3S28ph might enable rapid and efficient IE gene induction.

The potential for chemical mixtures from the environment to enable the cancer hallmark of sustained proliferative signalling

The aim of this work is to review current knowledge relating the established cancer hallmark, sustained cell proliferation to the existence of chemicals present as low dose mixtures in the environment. Normal cell proliferation is under tight control, i.e. cells respond to a signal to proliferate, and although most cells continue to proliferate into adult life, the multiplication ceases once the stimulatory signal disappears or if the cells are exposed to growth inhibitory signals. Under such circumstances, normal cells remain quiescent until they are stimulated to resume further proliferation. In contrast, tumour cells are unable to halt proliferation, either when subjected to growth inhibitory signals or in the absence of growth stimulatory signals. Environmental chemicals with carcinogenic potential may cause sustained cell proliferation by interfering with some cell proliferation control mechanisms committing cells to an indefinite proliferative span.

Regulation of chromatin structure via histone post-translational modification and the link to carcinogenesis

The loss of genome integrity contributes to the development of tumors. Although genome instability is associated with virtually all tumor types including both solid and liquid tumors, the aberrant molecular origins that drive this instability are poorly understood. It is now becoming clear that epigenetics and specific histone post-translational modifications (PTMs) have essential roles in maintaining genome stability under normal conditions. A strong relationship exists between aberrant histone PTMs, genome instability, and tumorigenesis. Changes in the genomic location of specific histone PTMs or alterations in the steady-state levels of the PTM are the consequence of imbalances in the enzymes and their activities catalyzing the addition of PTMs ("writers") or removal of PTMs ("erasers"). This review focuses on the misregulation of three specific types of histone PTMs: histone H3 phosphorylation at serines 10 and 28, H4 mono-methylation at lysine 20, and H2B ubiquitination at lysine 120. We discuss the normal regulation of these PTMs by the respective "writers" and "erasers" and the impact of their misregulation on genome stability.

Histone H3 phosphorylation, immediate-early gene expression, and the nucleosomal response: a historical perspective1This article is part of Special Issue entitled Asilomar Chromatin and has undergone the Journal’s usual peer review process

Histone H3 is modified at serines 10 and 28 in interphase cells following activation of the RAS-MAPK or p38-MAPK pathways by growth factors or stress. These modifications are involved in the regulation of immediate-early genes, including Jun and Fos, whose increased expression is a trademark of various cancers. This review outlines the series of discoveries that led to the characterization of these modifications, the kinase, MSK1/2, which is activated by both MAPK pathways and directs phosphorylation of H3, and the mechanistic function of these modifications in transcriptional activation. Research examining the effect of deregulated MSK1/2 in human disorders, namely cancer, is evaluated. Recently, a number of reports proposed novel, intervening pathways leading to enrichment of phosphorylated serine 10 and 28 and the activation of MSK1/2. These novel pathways predict an even more complicated signalling mechanism for cell growth, apoptosis, and the immune response, suggesting that MSK1/2 is intrinsically responsible for an even greater number of biological processes. This review proposes that MSK1/2 is an optimal target for cancer therapy, based on its fundamental role in transmitting external signals into varied responses involved in cancer development.

Combination of a ptgs2 inhibitor and an epidermal growth factor receptor-signaling inhibitor prevents tumorigenesis of oligodendrocyte lineage-derived glioma-initiating cells

Recent findings have demonstrated that malignant tumors, including glioblastoma multiforme (GBM), contain cancer-initiating cells (CICs; also known as cancer stem cells), which self-renew and are malignant. However, it remains controversial whether such CICs arise from tissue-specific stem cells, committed precursor cells, or differentiated cells. Here, we sought to examine the origin of the CICs in GBM. We first showed that the overexpression of oncogenic HRas(L61) transformed p53-deficient oligodendrocyte precursor cells (OPCs) and neural stem cells (NSCs) into glioma-initiating cell (GIC)-like cells in mice. When as few as 10 of these GIC-like cells were transplanted in vivo, they formed a transplantable GBM with features of human GBM, suggesting that these GIC-like cells were enriched in CICs. DNA microarray analysis showed that widespread genetic reprogramming occurred during the OPCs' transformation: they largely lost their OPC characteristics and acquired NSC ones, including the expression of prominin1, hmga2, ptgs2, and epiregulin. In addition, the combination of a Ptgs2 inhibitor and an epidermal growth factor receptor (EGFR)-signaling inhibitor prevented the tumorigenesis of transformed OPCs and human GICs (hGICs) obtained from anaplastic oligodendroglioma, but not of transformed NSCs or hGICs obtained from GBM. Together, these findings suggest that GBM can arise from either OPCs or NSCs and that the therapeutic targets for GBM might be different, depending on each GIC's cell-of-origin.

Role of MSK1 in the malignant phenotype of Ras-transformed mouse fibroblasts

Activated by the RAS-MAPK signaling pathway, MSK1 is recruited to immediate-early gene (IEG) regulatory regions, where it phosphorylates histone H3 at Ser-10 or Ser-28. Chromatin remodelers and modifiers are then recruited by 14-3-3 proteins, readers of phosphoserine marks, leading to the occupancy of IEG promoters by the initiation-engaged form of RNA polymerase II and the onset of transcription. In this study, we show that this mechanism of IEG induction, initially elucidated in parental 10T1/2 murine fibroblast cells, applies to metastatic Hras1-transformed Ciras-3 cells. As the RAS-MAPK pathway is constitutively activated in Ciras-3 cells, MSK1 activity and phosphorylated H3 steady-state levels are elevated. We found that steady-state levels of the IEG products AP-1 and COX-2 were also elevated in Ciras-3 cells. When MSK1 activity was inhibited or MSK1 expression was knocked down in Ciras-3 cells, the induction of IEG expression and the steady-state levels of COX-2, FRA-1, and JUN were greatly reduced. Furthermore, MSK1 knockdown Ciras-3 cells lost their malignant phenotype, as reflected by the absence of anchorage-independent growth.

p53 functions and cell lines: have we learned the lessons from the past?

p53 has a determinant role in cancer prevention and is among the most studied proteins in the world. The majority of studies devoted to this protein are carried out in cell lines because they are easy to use and have naturally emerged as the main research tool in laboratories. However, the p53 pathway is commonly deregulated in cancer cells, from which the experimental cell lines are generally derived. The fact that the pathway is deregulated challenges the relevance of using cancer-derived cell lines to study wild-type p53 activities, or, in a broader sense, to study any normal cellular process. In the present article, we identify and discuss a number of limitations of cell lines using examples related to p53. Finally, we point out the general limitations of cell lines and propose solutions as alternatives to these cells.

Promoter chromatin remodeling of immediate-early genes is mediated through H3 phosphorylation at either serine 28 or 10 by the MSK1 multi-protein complex

Upon activation of the ERK and p38 MAPK pathways, the MSK1/2-mediated nucleosomal response, including H3 phosphorylation at serine 28 or 10, is coupled with the induction of immediate-early (IE) gene transcription. The outcome of this response, varying with the stimuli and cellular contexts, ranges from neoplastic transformation to neuronal synaptic plasticity. Here, we used sequential co-immunoprecipitation assays and sequential chromatin immunoprecipitation (ChIP) assays on mouse fibroblast 10T1/2 and MSK1 knockdown 10T1/2 cells to show that H3 serine 28 and 10 phosphorylation leads to promoter remodeling. MSK1, in complexes with phospho-serine adaptor 14-3-3 proteins and BRG1 the ATPase subunit of the SWI/SNF remodeler, is recruited to the promoter of target genes by transcription factors such as Elk-1 or NF-kappaB. Following MSK1-mediated H3 phosphorylation, BRG1 associates with the promoter of target genes via 14-3-3 proteins, which act as scaffolds. The recruited SWI/SNF remodels nucleosomes at the promoter of IE genes enabling the binding of transcription factors like JUN and the onset of transcription.

H3 phosphorylation: dual role in mitosis and interphase

Chromatin condensation and subsequent decondensation are processes required for proper execution of various cellular events. During mitosis, chromatin compaction is at its highest, whereas relaxation of chromatin is necessary for DNA replication, repair, recombination, and gene transcription. Since histone proteins are directly complexed with DNA in the form of a nucleosome, great emphasis is put on deciphering histone post-translational modifications that control the chromatin condensation state. Histone H3 phosphorylation is a mark present in mitosis, where chromatin condensation is necessary, and in transcriptional activation of genes, when chromatin needs to be decondensed. There are four characterized phospho residues within the H3 N-terminal tail during mitosis: Thr3, Ser10, Thr11, and Ser28. Interestingly, H3 phosphorylated at Ser10, Thr11, and Ser28 has been observed on genomic regions of transcriptionally active genes. Therefore, H3 phosphorylation is involved in processes requiring opposing chromatin states. The level of H3 phosphorylation is mediated by opposing actions of specific kinases and phosphatases during mitosis and gene transcription. The cellular contexts under which specific residues on H3 are phosphorylated in mitosis and interphase are known to some extent. However, the functional consequences of H3 phosphorylation are still unclear.

Genetic and epigenetic heterogeneity in cancer: a genome-centric perspective

Genetic and epigenetic heterogeneity (the main form of non-genetic heterogeneity) are key elements in cancer progression and drug resistance, as they provide needed population diversity, complexity, and robustness. Despite drastically increased evidence of multiple levels of heterogeneity in cancer, the general approach has been to eliminate the "noise" of heterogeneity to establish genetic and epigenetic patterns. In particular, the appreciation of new types of epigenetic regulation like non-coding RNA, have led to the hope of solving the mystery of cancer that the current genetic theories seem to be unable to achieve. In this mini-review, we have briefly analyzed a number of mis-conceptions regarding cancer heterogeneity, followed by the re-evaluation of cancer heterogeneity within a framework of the genome-centric concept of evolution. The analysis of the relationship between gene, epigenetic and genome level heterogeneity, and the challenges of measuring heterogeneity among multiple levels have been discussed. Further, we propose that measuring genome level heterogeneity represents an effective strategy in the study of cancer and other types of complex diseases, as emphasis on the pattern of system evolution rather than specific pathways provides a global and synthetic approach. Compared to the degree of heterogeneity, individual molecular pathways will have limited predictability during stochastic cancer evolution where genome dynamics (reflected by karyotypic heterogeneity) will dominate.

Epigenetic control

Epigenetics refers to mitotically and/or meiotically heritable variations in gene expression that are not caused by changes in DNA sequence. Epigenetic mechanisms regulate all biological processes from conception to death, including genome reprogramming during early embryogenesis and gametogenesis, cell differentiation and maintenance of a committed lineage. Key epigenetic players are DNA methylation and histone post-translational modifications, which interplay with each other, with regulatory proteins and with non-coding RNAs, to remodel chromatin into domains such as euchromatin, constitutive or facultative heterochromatin and to achieve nuclear compartmentalization. Besides epigenetic mechanisms such as imprinting, chromosome X inactivation or mitotic bookmarking which establish heritable states, other rapid and transient mechanisms, such as histone H3 phosphorylation, allow cells to respond and adapt to environmental stimuli. However, these epigenetic marks can also have long-term effects, for example in learning and memory formation or in cancer. Erroneous epigenetic marks are responsible for a whole gamut of diseases including diseases evident at birth or infancy or diseases becoming symptomatic later in life. Moreover, although epigenetic marks are deposited early in development, adaptations occurring through life can lead to diseases and cancer. With epigenetic marks being reversible, research has started to focus on epigenetic therapy which has had encouraging success. As we witness an explosion of knowledge in the field of epigenetics, we are forced to revisit our dogma. For example, recent studies challenge the idea that DNA methylation is irreversible. Further, research on Rett syndrome has revealed an unforeseen role for methyl-CpG-binding protein 2 (MeCP2) in neurons.