Increased histone H1 phosphorylation and relaxed chromatin structure in Rb-deficient fibroblasts

Histone renegades: Unusual H2A histone variants in plants and animals

H2A variants are histones that carry out specialized nucleosome function during the eukaryote genome packaging. Most genes encoding H2A histone variants arose in the distant past, and have highly conserved domains and structures. Yet, novel H2A variants have continued to arise throughout the radiation of eukaryotes and disturbed the apparent tranquility of nucleosomes. These species-specific H2A variants contributed to the functional diversification of nucleosomes through changes in both their structure and expression patterns. In this short review, we discuss the evolutionary trajectories of these histone renegades in plants and animal genomes.

WHSC1 monomethylates histone H1 and induces stem-cell like features in squamous cell carcinoma of the head and neck

Squamous cell carcinoma of the head and neck (SCCHN) is a malignancy with poor outcomes, thus novel therapies are urgently needed. We recently showed that WHSC1 is necessary for the viability of SCCHN cells through H3K36 di-methylation. Here, we report the identification of its novel substrate, histone H1, and that WHSC1-mediated H1.4K85 mono-methylation may enhance stemness features in SCCHN cells. To identify proteins interacting with WHSC1 in SCCHN cells, WHSC1 immunoprecipitation and mass spectrometry identified H1 as a WHSC1-interacting candidate. In vitro methyltransferase assays showed that WHSC1 mono-methylates H1 at K85. We generated an H1K85 mono-methylation-specific antibody and confirmed that this methylation occurs in vivo. Sphere formation assays using SCC-35 cells stably expressing either wild-type (FLAG-H1.4-WT) or mutated (FLAG-H1.4K85A) vector with lysine 85 to alanine substitution which is not methylated, indicated a higher number of spheres in SCC-35 cells expressing the wild type than those with the mutant vector. SCC-35 cells expressing the wild type H1.4 proliferated faster than those expressing the mutated vector. RNA sequencing, RT-PCR and Western blotting of the FLAG-H1.4-WT or FLAG-H1.4K85A SCC-35 cells revealed that OCT4 levels were higher in wild type compared to mutant cells. These results were reproduced in SCC-35 cells genetically modified with CRISPR to express H1.4K85R. Chromatin immunoprecipitation showed that FLAG-H1.4K85A had decreased occupancy in the OCT4 gene compared to FLAG-H1.4-WT. This study supports that WHSC1 mono-methylates H1.4 at K85, it induces transcriptional activation of OCT4 and stemness features in SCCHN cells, providing rationale to target H1.4K85 mono-methylation through WHSC1 in SCCHN.

An RB-Condensin II Complex Mediates Long-Range Chromosome Interactions and Influences Expression at Divergently Paired Genes

Interphase chromosomes are organized into topologically associated domains in order to establish and maintain integrity of transcriptional programs that remain poorly understood. Here, we show that condensin II and TFIIIC are recruited to bidirectionally transcribed promoters by a mechanism that is dependent on the retinoblastoma (RB) protein. Long-range chromosome contacts are disrupted by loss of condensin II loading, which leads to altered expression at bidirectional gene pairs. This study demonstrates that mammalian condensin II functions to organize long-range chromosome contacts and regulate transcription at specific genes. In addition, RB dependence of condensin II suggests that widespread misregulation of chromosome contacts and transcriptional alterations are a consequence of RB mutation.

Aberrant Function of the C-Terminal Tail of HIST1H1E Accelerates Cellular Senescence and Causes Premature Aging

Histones mediate dynamic packaging of nuclear DNA in chromatin, a process that is precisely controlled to guarantee efficient compaction of the genome and proper chromosomal segregation during cell division and to accomplish DNA replication, transcription, and repair. Due to the important structural and regulatory roles played by histones, it is not surprising that histone functional dysregulation or aberrant levels of histones can have severe consequences for multiple cellular processes and ultimately might affect development or contribute to cell transformation. Recently, germline frameshift mutations involving the C-terminal tail of HIST1H1E, which is a widely expressed member of the linker histone family and facilitates higher-order chromatin folding, have been causally linked to an as-yet poorly defined syndrome that includes intellectual disability. We report that these mutations result in stable proteins that reside in the nucleus, bind to chromatin, disrupt proper compaction of DNA, and are associated with a specific methylation pattern. Cells expressing these mutant proteins have a dramatically reduced proliferation rate and competence, hardly enter into the S phase, and undergo accelerated senescence. Remarkably, clinical assessment of a relatively large cohort of subjects sharing these mutations revealed a premature aging phenotype as a previously unrecognized feature of the disorder. Our findings identify a direct link between aberrant chromatin remodeling, cellular senescence, and accelerated aging.

Disruption of CDK-resistant chromatin association by pRB causes DNA damage, mitotic errors, and reduces Condensin II recruitment

Organization of chromatin structure is indispensible to the maintenance of genome integrity. The retinoblastoma tumor suppressor protein (pRB) mediates both transcriptional repression and chromatin organization, but the independent contributions of these functions have been difficult to study. Here, we utilize a synthetic Rb1 mutant allele (F832A) that maintains pRB association at cell cycle gene promoters, but disrupts a cyclin-dependent kinase (CDK)-resistant interaction with E2F1 to reduce occupancy of pRB on intergenic chromatin. Reduced pRB chromatin association increases spontaneous γH2AX deposition and aneuploidy. Our data indicates that the CDK-resistant pRB-E2F1 scaffold recruits Condensin II to major satellite repeats to stabilize chromatin structure in interphase and mitosis through mechanisms that are distinct from silencing of repetitive sequence expression.

Linker histone H1 and protein-protein interactions

Linker histones H1 are ubiquitous chromatin proteins that play important roles in chromatin compaction, transcription regulation, nucleosome spacing and chromosome spacing. H1 function in DNA and chromatin structure stabilization is well studied and established. The current paradigm of linker histone mode of function considers all other cellular roles of linker histones to be a consequence from H1 chromatin compaction and repression. Here we review the multiple processes regulated by linker histones and the emerging importance of protein interactions in H1 functioning. We propose a new paradigm which explains the multi functionality of linker histones through linker histones protein interactions as a way to directly regulate recruitment of proteins to chromatin.

Misexpression of cyclin D1 in embryonic germ cells promotes testicular teratoma initiation

Testicular teratomas result from anomalies in embryonic germ cell development. In the 129 family of inbred mouse strains, teratomas arise during the same developmental period that male germ cells normally enter G1/G0 mitotic arrest and female germ cells initiate meiosis (the mitotic:meiotic switch). Dysregulation of this switch associates with teratoma susceptibility and involves three germ cell developmental abnormalities seemingly critical for tumor initiation: delayed G1/G0 mitotic arrest, retention of pluripotency, and misexpression of genes normally restricted to embryonic female and adult male germ cells. One misexpressed gene, cyclin D1 (Ccnd1), is a known regulator of cell cycle progression and an oncogene in many tissues. Here, we investigated whether Ccnd1 misexpression in embryonic germ cells is a determinant of teratoma susceptibility in mice. We found that CCND1 localizes to teratoma-susceptible germ cells that fail to enter G1/G0 arrest during the mitotic:meiotic switch and is the only D-type cyclin misexpressed during this critical developmental time frame. We discovered that Ccnd1 deficiency in teratoma-susceptible mice significantly reduced teratoma incidence and suppressed the germ cell proliferation and pluripotency abnormalities associated with tumor initiation. Importantly, Ccnd1 expression was dispensable for somatic cell development and male germ cell specification and maturation in tumor-susceptible mice, implying that the mechanisms by which Ccnd1 deficiency reduced teratoma incidence were germ cell autonomous and specific to tumorigenesis. We conclude that misexpression of Ccnd1 in male germ cells is a key component of a larger pro-proliferative program that disrupts the mitotic:meiotic switch and predisposes 129 inbred mice to testicular teratocarcinogenesis.

The role of linker histone H1 modifications in the regulation of gene expression and chromatin dynamics

BACKGROUND

Linker histone H1 is a structural component of chromatin. It exists as a family of related proteins known as variants and/or subtypes. H1.1, H1.2, H1.3, H1.4 and H1.5 are present in most somatic cells, whereas other subtypes are mainly expressed in more specialized cells.

SCOPE OF REVIEW

H1 subtypes have been shown to have unique functions in chromatin structure and dynamics. This can occur at least in part via specific post-translational modifications of distinct H1 subtypes. However, while core histone modifications have been extensively studied, our knowledge of H1 modifications and their molecular functions has remained for a long time limited to phosphorylation. In this review we discuss the current state of knowledge of linker histone H1 modifications and where possible highlight functional differences in the modifications of distinct H1 subtypes.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

H1 histones are intensely post-translationally modified. These modifications are located in the N- and C-terminal tails as well as within the globular domain. Recently, advanced mass spectrometrical analysis revealed a large number of novel histone H1 subtype specific modification sites and types. H1 modifications include phosphorylation, acetylation, methylation, ubiquitination, and ADP ribosylation. They are involved in the regulation of all aspects of linker histone functions, however their mechanism of action is often only poorly understood. Therefore systematic functional characterization of H1 modifications will be necessary in order to better understand their role in gene regulation as well as in higher-order chromatin structure and dynamics.

Diversification of histone H2A variants during plant evolution

Among eukaryotes, the four core histones show an extremely high conservation of their structure and form nucleosomes that compact, protect, and regulate access to genetic information. Nevertheless, in multicellular eukaryotes the two families, histone H2A and histone H3, have diversified significantly in key residues. We present a phylogenetic analysis across the green plant lineage that reveals an early diversification of the H2A family in unicellular green algae and remarkable expansions of H2A variants in flowering plants. We define motifs and domains that differentiate plant H2A proteins into distinct variant classes. In non-flowering land plants, we identify a new class of H2A variants and propose their possible role in the emergence of the H2A.W variant class in flowering plants.

Linker histone partial phosphorylation: effects on secondary structure and chromatin condensation

Linker histones are involved in chromatin higher-order structure and gene regulation. We have successfully achieved partial phosphorylation of linker histones in chicken erythrocyte soluble chromatin with CDK2, as indicated by HPCE, MALDI-TOF and Tandem MS. We have studied the effects of linker histone partial phosphorylation on secondary structure and chromatin condensation. Infrared spectroscopy analysis showed a gradual increase of β-structure in the phosphorylated samples, concomitant to a decrease in α-helix/turns, with increasing linker histone phosphorylation. This conformational change could act as the first step in the phosphorylation-induced effects on chromatin condensation. A decrease of the sedimentation rate through sucrose gradients of the phosphorylated samples was observed, indicating a global relaxation of the 30-nm fiber following linker histone phosphorylation. Analysis of specific genes, combining nuclease digestion and qPCR, showed that phosphorylated samples were more accessible than unphosphorylated samples, suggesting local chromatin relaxation. Chromatin aggregation was induced by MgCl₂ and analyzed by dynamic light scattering (DLS). Phosphorylated chromatin had lower percentages in volume of aggregated molecules and the aggregates had smaller hydrodynamic diameter than unphosphorylated chromatin, indicating that linker histone phosphorylation impaired chromatin aggregation. These findings provide new insights into the effects of linker histone phosphorylation in chromatin condensation.

Histone H1 phosphorylation in breast cancer

Breast cancer is the second leading cause of cancer-related deaths in women. The need for new clinical biomarkers in breast cancer is necessary to further predict prognosis and therapeutic response. In this article, the LC-MS histone H1 phosphorylation profiles were established for three distinct breast cancer cell lines. The results show that the extent of H1 phosphorylation can distinguish between the different cell lines. The histone H1 from the metastatic cell line, MDA-MB-231, was subjected to chemical derivitization and LC-MS/MS analysis. The results suggest that the phosphorylation at threonine 146 is found on both histone H1.2 and histone H1.4. Cell lines were then treated with an extracellular stimulus, estradiol or kinase inhibitor LY294002, to monitor changes in histone H1 phosphorylation. The data show that histone H1 phosphorylation can increase and decrease in response to extracellular stimuli. Finally, primary breast tissues were stained for the histone H1 phosphorylation at threonine 146. Variable staining patterns across tumor grades and subtypes were observed with pT146 labeling correlating with tumor grade. These results establish the potential for histone H1 phosphorylation at threonine 146 as a clinical biomarker in breast cancer.

Host-virus interactions: from the perspectives of epigenetics

Chromatin structure and histone modifications play key roles in gene regulation. Some virus genomes are organized into chromatin-like structure, which undergoes different histone modifications facilitating complex functions in virus life cycles including replication. Here, we present a comprehensive summary of recent research in this field regarding the interaction between viruses and host epigenetic factors with emphasis on how chromatin modifications affect viral gene expression and virus infection. We also describe the strategies employed by viruses to manipulate the host epigenetic program to facilitate virus replication as well as the underlying mechanisms. Together, knowledge from this field not only generates novel insights into virus life cycles but may also have important therapeutic implications.

Pin1 promotes histone H1 dephosphorylation and stabilizes its binding to chromatin

The prolyl isomerase Pin1 stimulates the dephosphorylation of histone H1, stabilizing its binding to chromatin at transcriptionally active chromatin.

Histone H1 plays a crucial role in stabilizing higher order chromatin structure. Transcriptional activation, DNA replication, and chromosome condensation all require changes in chromatin structure and are correlated with the phosphorylation of histone H1. In this study, we describe a novel interaction between Pin1, a phosphorylation-specific prolyl isomerase, and phosphorylated histone H1. A sub-stoichiometric amount of Pin1 stimulated the dephosphorylation of H1 in vitro and modulated the structure of the C-terminal domain of H1 in a phosphorylation-dependent manner. Depletion of Pin1 destabilized H1 binding to chromatin only when Pin1 binding sites on H1 were present. Pin1 recruitment and localized histone H1 phosphorylation were associated with transcriptional activation independent of RNA polymerase II. We thus identify a novel form of histone H1 regulation through phosphorylation-dependent proline isomerization, which has consequences on overall H1 phosphorylation levels and the stability of H1 binding to chromatin.

H1 histones: current perspectives and challenges

H1 and related linker histones are important both for maintenance of higher-order chromatin structure and for the regulation of gene expression. The biology of the linker histones is complex, as they are evolutionarily variable, exist in multiple isoforms and undergo a large variety of posttranslational modifications in their long, unstructured, NH₂- and COOH-terminal tails. We review recent progress in understanding the structure, genetics and posttranslational modifications of linker histones, with an emphasis on the dynamic interactions of these proteins with DNA and transcriptional regulators. We also discuss various experimental challenges to the study of H1 and related proteins, including limitations of immunological reagents and practical difficulties in the analysis of posttranslational modifications by mass spectrometry.

Stat6 cooperates with Sp1 in controlling breast cancer cell proliferation by modulating the expression of p21(Cip1/WAF1) and p27 (Kip1)

BACKGROUND

The signal transducer and activator of transcription 6 (Stat6), a member of the family of DNA-binding proteins, has been identified as a critical cell differentiation modulator in breast cancer cells. As of yet, the mechanisms underlying this function have remained largely unknown. To further elucidate the role of Stat6 in breast cancer development, we investigated the consequences of exogenous Stat6 expression.

METHODS

Proliferation assays and flow cytometry assays were conducted to evaluate the putative role of Stat6 on cell proliferation. To this end, we produced synchronized cells after a double thymidine block, as confirmed by FACS analysis. mRNA levels of Stat6 were measured by RNase protection analysis. To confirm the interaction among proteins, we employed GST pull-down assays and immunoprecipitation assays. Luciferase assays and ChIP assays were used to assess the transcriptional activity.

RESULTS

Compared to control breast cancer cells, we found that exogenous Stat6 expression plays a critical role in controlling cell proliferation. Also in different breast tumor cell lines, endogenous Stat6 expression was found to be positively related to a lower proliferation rate. Interestingly, in human breast cancer cells Stat6 functions in G1/S cell cycle progression, and the growth-inhibitory effect of Stat6 was shown to be mediated by induction of the G1 cyclin-dependent kinase inhibitors p21(Cip1/WAF1) (p21) and p27(Kip1) (p27). Simultaneously, G1-related cyclin/cyclin-dependent kinase activities and pRB phosphorylation were markedly reduced, and cell cycle progression was blocked in the G1 phase. Stat6 knockdown resulted in enhanced cell proliferation and a decrease in p21 and p27 mRNA levels in the steroid-responsive and non-responsive T-47D and MDA-MB-231 cell lines, respectively. In addition, the stimulatory effect of Stat6 on p21 and p27 gene transcription was found to be associated with interaction of Stat6 with the transcription factor Sp1 at the proximal Sp1-binding sites in their respective promoters.

CONCLUSIONS

Together, these results identify Stat6 as an important cell differentiation regulatory protein functioning, at least in part, by interacting with Sp1 to activate the p21 and p27 gene promoters in breast cancer cells.

Germ cell pluripotency, premature differentiation and susceptibility to testicular teratomas in mice

Testicular teratomas result from anomalies in germ cell development during embryogenesis. In the 129 family of inbred strains of mice, teratomas initiate around embryonic day (E) 13.5 during the same developmental period in which female germ cells initiate meiosis and male germ cells enter mitotic arrest. Here, we report that three germ cell developmental abnormalities, namely continued proliferation, retention of pluripotency, and premature induction of differentiation, associate with teratoma susceptibility. Using mouse strains with low versus high teratoma incidence (129 versus 129-Chr19(MOLF/Ei)), and resistant to teratoma formation (FVB), we found that germ cell proliferation and expression of the pluripotency factor Nanog at a specific time point, E15.5, were directly related with increased tumor risk. Additionally, we discovered that genes expressed in pre-meiotic embryonic female and adult male germ cells, including cyclin D1 (Ccnd1) and stimulated by retinoic acid 8 (Stra8), were prematurely expressed in teratoma-susceptible germ cells and, in rare instances, induced entry into meiosis. As with Nanog, expression of differentiation-associated factors at a specific time point, E15.5, increased with tumor risk. Furthermore, Nanog and Ccnd1, genes with known roles in testicular cancer risk and tumorigenesis, respectively, were co-expressed in teratoma-susceptible germ cells and tumor stem cells, suggesting that retention of pluripotency and premature germ cell differentiation both contribute to tumorigenesis. Importantly, Stra8-deficient mice had an 88% decrease in teratoma incidence, providing direct evidence that premature initiation of the meiotic program contributes to tumorigenesis. These results show that deregulation of the mitotic-meiotic switch in XY germ cells contributes to teratoma initiation.

Diversity within the pRb pathway: is there a code of conduct?

The failure of cell proliferation to be properly regulated is a hallmark of tumourigenesis. The retinoblastoma protein (pRb) pathway represents a key component in the regulation of the cell cycle and tumour suppression. Recent findings have revealed new levels of complexity reflecting a repertoire of post-translational modifications that occur on pRb together with its key effector E2F-1. Here we provide an overview of the modifications and consider the possibility of a 'code' that endows pRb with the ability to function in diverse physiological settings.

Cell-cycle phases and genetic profile of bone marrow-derived mesenchymal stromal cells expanded in vitro from healthy donors

Human mesenchymal stromal cells (MSCs) expanded in vitro for cell therapy approaches need to be carefully investigated for genetic stability, by employing both molecular and conventional karyotyping. Reliability of cytogenetic analysis may be hampered in some MSC samples by the difficulty of obtaining an adequate number of metaphases. In an attempt to overcome this problem, a methodology apt to evaluate the cell-cycle structure on synchronous MSCs was optimised. Results obtained in five independent experiments by comparing cell-cycle analysis of synchronous and asynchronous MSC populations evaluated at early and late culture passages documented that in synchronous MSCs, 30% of cells entered G2/M phase after about 27-28 h of culture, while in asynchronous MSCs only 8% of cells in G2/M phase could be observed at the same time point. Cytogenetic analysis on synchronous MSCs allowed us to obtain 20-25 valuable metaphases/slide, whereas only 0-4 metaphases/slide were detectable in asynchronous preparations.

Histone H1 interphase phosphorylation becomes largely established in G1 or early S phase and differs in G1 between T-lymphoblastoid cells and normal T cells

Background

Histone H1 is an important constituent of chromatin, and is involved in regulation of its structure. During the cell cycle, chromatin becomes locally decondensed in S phase, highly condensed during metaphase, and again decondensed before re-entry into G₁. This has been connected to increasing phosphorylation of H1 histones through the cell cycle. However, many of these experiments have been performed using cell-synchronization techniques and cell cycle-arresting drugs. In this study, we investigated the H1 subtype composition and phosphorylation pattern in the cell cycle of normal human activated T cells and Jurkat T-lymphoblastoid cells by capillary electrophoresis after sorting of exponentially growing cells into G₁, S and G₂/M populations.

Results

We found that the relative amount of H1.5 protein increased significantly after T-cell activation. Serine phosphorylation of H1 subtypes occurred to a large extent in late G₁ or early S phase in both activated T cells and Jurkat cells. Furthermore, our data confirm that the H1 molecules newly synthesized during S phase achieve a similar phosphorylation pattern to the previous ones. Jurkat cells had more extended H1.5 phosphorylation in G₁ compared with T cells, a difference that can be explained by faster cell growth and/or the presence of enhanced H1 kinase activity in G₁ in Jurkat cells.

Conclusion

Our data are consistent with a model in which a major part of interphase H1 phosphorylation takes place in G₁ or early S phase. This implies that H1 serine phosphorylation may be coupled to changes in chromatin structure necessary for DNA replication. In addition, the increased H1 phosphorylation of malignant cells in G₁ may be affecting the G₁/S transition control and enabling facilitated S-phase entry as a result of relaxed chromatin condensation. Furthermore, increased H1.5 expression may be coupled to the proliferative capacity of growth-stimulated T cells.

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.

Assaying pharmacodynamic endpoints with targeted therapy: flavopiridol and 17AAG induced dephosphorylation of histone H1.5 in acute myeloid leukemia

Histone H1 is commonly used to assay kinase activity in vitro. As many promising targeted therapies affect kinase activity of specific enzymes involved in cancer transformation, H1 phosphorylation can serve as potential pharmacodynamic marker for drug activity within the cell. In this study we utilized a phosphoproteomic workflow to characterize histone H1 phosphorylation changes associated with two targeted therapies in the Kasumi-1 acute myeloid leukemia cell line. The phosphoproteomic workflow was first validated with standard casein phosphoproteins and then applied to the direct analysis of histone H1 from Kasumi-1 nuclear lysates. Ten H1 phosphorylation sites were identified on the H1 variants, H1.2, H1.3, H1.4, H1.5 and H1.x. LC MS profiling of intact H1s demonstrated global dephosphorylation of H1.5 associated with therapy by the cyclin-dependent kinase inhibitor, flavopiridol and the Heat Shock Protein 90 inhibitor, 17-(Allylamino)-17-demethoxygeldanamycin. In contrast, independent treatments with a nucleotide analog, proteosome inhibitor and histone deacetylase inhibitor did not exhibit decreased H1.5 phosphorylation. The data presented herein demonstrate that potential of histones to assess the cellular response of reagents that have direct and indirect effects on kinase activity that alters histone phosphorylation. As such, this approach may be a highly informative marker for response to targeted therapies influencing histone phosphorylation.

Minireview: role of kinases and chromatin remodeling in progesterone signaling to chromatin

Steroid hormones regulate gene expression by interaction of their receptors with hormone-responsive elements on DNA or with other transcription factors, but they can also activate cytoplasmic signaling cascades. Rapid activation of Erk by progestins via an interaction of the progesterone receptor (PR) with the estrogen receptor is critical for transcriptional activation of the mouse mammary tumor virus (MMTV) promoter and other progesterone target genes. Erk activation leads to the phosphorylation of PR, activation of mitogen- and stress-activated protein kinase 1, and the recruitment of a complex of the three activated proteins and of P300/CBP-associated factor (PCAF) to a single nucleosome, resulting in the phosphoacetylation of histone H3 and the displacement of heterochromatin protein 1γ. Hormone-dependent gene expression requires ATP-dependent chromatin remodeling complexes. Two switch/sucrose nonfermentable-like complexes, Brahma-related gene 1-associated factor (BAF) and polybromo-BAF are present in breast cancer cells, but only BAF is recruited to the MMTV promoter and cooperates with PCAF during activation of hormone-responsive promoters. PCAF acetylates histone H3 at K14, an epigenetic mark recognized by BAF subunits, thus anchoring the complex to chromatin. BAF catalyzes localized displacement of histones H2A and H2B, facilitating access of nuclear factor 1 and additional PR complexes to the hidden hormone-responsive elements on the MMTV promoter. The linker histone H1 is a structural component of chromatin generally regarded as a general repressor of transcription. However, it contributes to a better regulation of the MMTV promoter by favoring a more homogeneous nucleosome positioning, thus reducing basal transcription and actually enhancing hormone induced transcription. During transcriptional activation, H1 is phosphorylated and displaced from the promoter. The kinase cyclin-dependent kinase 2 is activated after progesterone treatment and could catalyze progesterone-induced phosphorylation of histone H1 by chromatin remodeling complexes. The initial steps of gene induction by progestins involve changes in the chromatin organization of target promoters that require the activation of several kinase signaling pathways initiated by membrane anchored PR. Because these pathways also respond to other external signals, they serve to integrate the hormonal response in the global context of the cellular environment.

Histone H1 phosphorylation is associated with transcription by RNA polymerases I and II

Histone H1 phosphorylation affects chromatin condensation and function, but little is known about how specific phosphorylations impact the function of H1 variants in higher eukaryotes. In this study, we show that specific sites in H1.2 and H1.4 of human cells are phosphorylated only during mitosis or during both mitosis and interphase. Antisera generated to individual H1.2/H1.4 interphase phosphorylations reveal that they are distributed throughout nuclei and enriched in nucleoli. Moreover, interphase phosphorylated H1.4 is enriched at active 45S preribosomal RNA gene promoters and is rapidly induced at steroid hormone response elements by hormone treatment. Our results imply that site-specific interphase H1 phosphorylation facilitates transcription by RNA polymerases I and II and has an unanticipated function in ribosome biogenesis and control of cell growth. Differences in the numbers, structure, and locations of interphase phosphorylation sites may contribute to the functional diversity of H1 variants.

The cell cycle and Myc intersect with mechanisms that regulate pluripotency and reprogramming

Pluripotent stem cells have long-term proliferative capacity and an unusual mode of cell-cycle regulation and can divide independently of extrinsic mitogenic signals. The last few years has seen evidence emerge that links cell-cycle regulation to the maintenance and establishment of pluripotency. Myc transcription factors appear to be central to this regulation. This review addresses these links and discusses how cell-cycle controls and Myc impact on the maintenance and establishment of pluripotency.

Tumor suppressor function of BCSC-1 in nasopharyngeal carcinoma

BCSC-1 is dramatically upregulated in CNE-2L2 human nasopharyngeal carcinoma cells with reduced malignancy (AS cells) and is proposed to be a candidate tumor suppressor gene. We therefore examined the effect of BCSC-1 expression on malignant behaviors of CNE-2L2 cells. Growth in vitro and tumorigenesis in nude mice of wild-type CNE-2L2 cells (W cells) were inhibited by ectopic BCSC-1, and those of AS cells were promoted by BCSC-1 suppression. The tumor suppressor function of BCSC-1 was further confirmed by a study showing that intratumor BCSC-1 injection caused growth suppression of the tumor from W cells inoculated in nude mice. Immunohistochemistry exhibited marked reduction of BCSC-1 expression in 11 of 39 human nasopharyngeal carcinoma specimens. Because BCSC-1 expression was as rich as that in normal cells in the rest of the carcinoma specimens and was poor in CNE-2L2 cells, HNE-1 human nasopharyngeal carcinoma cells with rich BCSC-1 expression were used as a control in the study. No effect of BCSC-1 transfection on growth of the cells was observed. The data suggest that BCSC-1 suppression might play roles in tumorigenesis of some nasopharyngeal carcinomas and that BCSC-1 might be a potential gene therapy target in nasopharyngeal carcinomas with poor BCSC-1 expression. Enhanced aggregation of cells together with increased E-cadherin and alpha-catenin expression and reduced Wnt signaling might be involved in the mechanisms of tumor suppressor function of BCSC-1.

pRb, a local chromatin organizer with global possibilities

The retinoblastoma (pRb) family of proteins are well known for their tumor suppressor properties and for their ability to regulate transcription. The action of pRb family members correlates with the appearance of repressive chromatin marks at promoter regions of genes encoding key regulators of cell proliferation. Recent studies raise the possibility that pRb family members do not simply act by controlling the activity of individual promoters but that they may also function by promoting the more general organization of chromatin. In several contexts, pRb family members stimulate the compaction or condensation of chromatin and promote the formation of heterochromatin. In this review, we summarize studies that link pRb family members to the condensation or compaction of DNA.

Molecular dynamics of histone H1This paper is one of a selection of papers published in this Special Issue, entitled CSBMCB’s 51st Annual Meeting – Epigenetics and Chromatin Dynamics, and has undergone the Journal’s usual peer review process

The histone H1 family of nucleoproteins represents an important class of structural and architectural proteins that are responsible for maintaining and stabilizing higher-order chromatin structure. Essential for mammalian cell viability, they are responsible for gene-specific regulation of transcription and other DNA-dependent processes. In this review, we focus on the wealth of information gathered on the molecular kinetics of histone H1 molecules using novel imaging techniques, such as fluorescence recovery after photobleaching. These experiments have shed light on the effects of H1 phosphorylation and core histone acetylation in influencing chromatin structure and dynamics. We also delineate important concepts surrounding the C-terminal domain of H1, such as the intrinsic disorder hypothesis, and how it affects H1 function. Finally, we address the biochemical mechanisms behind low-affinity H1 binding.

RBF1 promotes chromatin condensation through a conserved interaction with the Condensin II protein dCAP-D3

The Drosophila retinoblastoma family of proteins (RBF1 and RBF2) and their mammalian homologs (pRB, p130, and p107) are best known for their regulation of the G1/S transition via the repression of E2F-dependent transcription. However, RB family members also possess additional functions. Here, we report that rbf1 mutant larvae have extensive defects in chromatin condensation during mitosis. We describe a novel interaction between RBF1 and dCAP-D3, a non-SMC component of the Condensin II complex that links RBF1 to the regulation of chromosome structure. RBF1 physically interacts with dCAP-D3, RBF1 and dCAP-D3 partially colocalize on polytene chromosomes, and RBF1 is required for efficient association of dCAP-D3 with chromatin. dCap-D3 mutants also exhibit chromatin condensation defects, and mutant alleles of dCap-D3 suppress cellular and developmental phenotypes induced by the overexpression of RBF1. Interestingly, this interaction is conserved between flies and humans. The re-expression of pRB into a pRB-deficient human tumor cell line promotes chromatin association of hCAP-D3 in a manner that depends on the LXCXE-binding cleft of pRB. These results uncover an unexpected link between pRB/RBF1 and chromatin condensation, providing a mechanism by which the functional inactivation of RB family members in human tumor cells may contribute to genome instability.

Dissection of mechanisms of Chinese medicinal formula Realgar-Indigo naturalis as an effective treatment for promyelocytic leukemia

To enhance therapeutic efficacy and reduce adverse effects, practitioners of traditional Chinese medicine (TCM) prescribe a combination of plant species/minerals, called formulae, based on clinical experience. Nearly 100,000 formulae have been recorded, but the working mechanisms of most remain unknown. In trying to address the possible beneficial effects of formulae with current biomedical approaches, we use Realgar-Indigo naturalis formula (RIF), which has been proven to be very effective in treating human acute promyelocytic leukemia (APL) as a model. The main components of RIF are realgar, Indigo naturalis, and Salvia miltiorrhiza, with tetraarsenic tetrasulfide (A), indirubin (I), and tanshinone IIA (T) as major active ingredients, respectively. Here, we report that the ATI combination yields synergy in the treatment of a murine APL model in vivo and in the induction of APL cell differentiation in vitro. ATI causes intensified ubiquitination/degradation of promyelocytic leukemia (PML)-retinoic acid receptor alpha (RARalpha) oncoprotein, stronger reprogramming of myeloid differentiation regulators, and enhanced G(1)/G(0) arrest in APL cells through hitting multiple targets compared with the effects of mono- or biagents. Furthermore, ATI intensifies the expression of Aquaglyceroporin 9 and facilitates the transportation of A into APL cells, which in turn enhances A-mediated PML-RARalpha degradation and therapeutic efficacy. Our data also indicate A as the principal component of the formula, whereas T and I serve as adjuvant ingredients. We therefore suggest that dissecting the mode of action of clinically effective formulae at the molecular, cellular, and organism levels may be a good strategy in exploring the value of traditional medicine.

E2F4 function in G2: maintaining G2-arrest to prevent mitotic entry with damaged DNA

Mammalian cells undergo cell cycle arrest in response to DNA damage through multiple checkpoint mechanisms. One such checkpoint pathway maintains genomic integrity by delaying mitotic progression in response to genotoxic stress. Transition though the G2 phase and entry into mitosis is considered to be regulated primarily by cyclin B1 and its associated catalytically active partner Cdk1. While not necessary for its initiation, the p130 and Rb-dependent target genes have emerged as being important for stable maintenance of a G2 arrest. It was recently demonstrated that by interacting with p130, E2F4 is present in the nuclei and plays a key role in the maintenance of this stable G2 arrest. Increased E2F4 levels and its translocation to the nucleus following genotoxic stress result in downregulation of many mitotic genes and as a result promote a G0-like state. Irradiation of E2F4-depleted cells leads to enhanced cellular DNA double-strand breaks that may be measured by comet assays. It also results in cell death that is characterized by caspase activation, sub-G1 and sub-G2 DNA content, and decreased clonogenic cell survival. Here we review these recent findings and discuss the mechanisms of G2 phase checkpoint activation and maintenance with a particular focus on E2F4.

Loss of RB compromises specific heterochromatin modifications and modulates HP1alpha dynamics

Heterochromatin domains are important for gene silencing, centromere organization, and genomic stability. These genomic domains are marked with specific histone modifications, heterochromatin protein 1 (HP1) binding and DNA methylation. The retinoblastoma tumor suppressor, RB mediates transcriptional repression and functionally interacts with a number of factors that are involved in heterochromatin biology including HP1, Suv39h1, DNMT1, and components of the SWI/SNF chromatin remodeling complex. To analyze the specific influence of RB loss on chromatin modification, mouse adult fibroblasts (MAFs) derived from Rb(loxP/loxP) mice were utilized to acutely knockout RB. In this setting, target genes of RB are deregulated. Additionally, changes in histone modifications were observed. Specifically, histone H4 lysine 20 trimethylation was absent from heterochromatin domains following loss of RB and there were changes in the relative levels of histone modifications between RB-proficient and deficient cells. While RB loss significantly altered the modifications associated with heterochromatin domains, these domains were readily identified and efficiently mediated the recruitment of HP1alpha. Kinetic analyses of HP1alpha within the heterochromatin domains present in RB-deficient cells indicated that loss of RB retarded HP1alpha dynamics, indicating that HP1alpha is paradoxically more tightly associated with heterochromatin in the absence of RB function. Combined, these analyses demonstrate that loss of RB has global effects on chromatin modifications and dynamics.

Analyses of linker histone--chromatin interactions in situ

Recent studies, using cytometric techniques based on fluorescence microscopy, have provided new information on how linker histones interact with chromatin in vivo or in situ. In particular, the use of green fluorescent proteins (GFPs) has enabled detailed studies of how individual H1 subtypes, and specific motifs in them, interact with chromatin in vivo. Furthermore, the development of cytochemical methods to study the interaction between linker histones and chromatin using DNA-binding fluorochromes as indirect probes for linker histone affinity in situ, in combination with highly sensitive and specific analytical methods, has provided additional information on the interactions between linker histones and chromatin in several cell systems. Such results verified that linker histones have a substantially higher affinity for chromatin in mature chicken erythrocytes than in frog erythrocytes, and they also indicated that the affinity decreased during differentiation of the frog erythrocytes. Furthermore, in cultured human fibroblasts, the linker histones showed a relatively high affinity for chromatin in interphase, whereas it showed a significantly lower affinity in highly condensed metaphase chromosomes. This method also enables the analysis of linker histone affinity for chromatin in H1-depleted fibroblasts reconstituted with purified linker histones. No consistent correlation between linker histone affinity and chromatin condensation has so far been detected.

Retinoblastoma tumor suppressor: where cancer meets the cell cycle

The retinoblastoma tumor suppressor gene, Rb, was the first tumor suppressor identified and plays a fundamental role in regulation of progression through the cell cycle. This review details facets of RB protein function in cell cycle control and focuses on specific questions that remain intensive areas of investigation.

Phosphorylation of the linker histone H1 by CDK regulates its binding to HP1alpha

Two key components of mammalian heterochromatin that play a structural role in higher order chromatin organization are the heterochromatin protein 1alpha (HP1alpha) and the linker histone H1. Here, we show that these proteins interact in vivo and in vitro through their hinge and C-terminal domains, respectively. The phosphorylation of H1 by CDK2, which is required for efficient cell cycle progression, disrupts this interaction. We propose that phosphorylation of H1 provides a signal for the disassembly of higher order chromatin structures during interphase, independent of histone H3-lysine 9 (H3-K9) methylation, by reducing the affinity of HP1alpha for heterochromatin.

MEK-ERK signaling is involved in interferon-gamma-induced death of oligodendroglial progenitor cells

Oligodendrocytes are exposed to various cytokines in inflammatory lesions in the central nervous system. In this study, we focused on the direct effects of interferon-gamma (IFNG) on highly purified rat oligodendroglial cultures at different developmental stages. Among the three stages tested, IFNG had direct cytotoxic effects on actively proliferating oligodendrocyte progenitors but much less on immature oligodendrocytes and none on mature oligodendrocytes. This stage-specific susceptibility of progenitors to IFNG-induced cytotoxicity consisted of two components, delay in the G(1)/S transition of the cell cycle and increased cell death at least partly mediated by apoptosis, suggesting that progression of the cell cycle was tightly linked to this toxic mechanism. There was no functional difference in the signal transducers and activators of transcription (STAT) pathways between progenitors and mature oligodendrocytes as determined by induction of IRF1 mRNA in response to IFNG. We found that partial inhibition of the MEK-ERK pathway, one of the mitogen-activated protein kinase phosphorelay modules, by U0126 partially reversed the IFNG-induced cytotoxicity in progenitors. In addition, ERK activity was quickly down-regulated after in vitro differentiation of progenitors to immature oligodendrocytes. Therefore, we concluded that simultaneous activation of the STAT pathway by IFNG and of the ERK pathway by exogenous trophic factors played a role in the stage-specific IFNG-induced cytotoxicity in oligodendroglial progenitors. Our study has implications with respect to the mechanisms of periventricular leukomalacia in infants and of persistent demyelination in multiple sclerosis lesions in adults.

Histone H1 Phosphorylation Occurs Site-specifically during Interphase and Mitosis

H1 histones, isolated from logarithmically growing and mitotically enriched human lymphoblastic T-cells (CCRF-CEM), were fractionated by reversed phase and hydrophilic interaction liquid chromatography, subjected to enzymatic digestion, and analyzed by amino acid sequencing and mass spectrometry. During interphase the four H1 subtypes present in these cells differ in their maximum phosphorylation levels: histone H1.5 is tri-, H1.4 di-, and H1.3 and H1.2, only monophosphorylated. The phosphorylation is site-specific and occurs exclusively on serine residues of SP(K/A)K motifs. The phosphorylation sites of histone H1.5 from mitotically enriched cells were also examined. In contrast to the situation in interphase, at mitosis there were additional phosphorylations, exclusively at threonine residues. Whereas the tetraphosphorylated H1.5 arises from the triphosphosphorylated form by phosphorylation of one of two TPKK motifs in the C-terminal domain, namely Thr137 and Thr154, the pentaphosphorylated H1.5 was the result of phosphorylation of one of the tetraphosphorylated forms at a novel nonconsensus motif at Thr10 in the N-terminal tail. Despite the fact that histone H1.5 has five (S/T)P(K/A)K motifs, all of these motifs were never found to be phosphorylated simultaneously. Our data suggest that phosphorylation of human H1 variants occurs nonrandomly during both interphase and mitosis and that distinct serine- or threonine-specific kinases are involved in different cell cycle phases. The order of increased phosphorylation and the position of modification might be necessary for regulated chromatin decondensation, thus facilitating processes of replication and transcription as well as of mitotic chromosome condensation.

Abnormalities of chromatin in tumor cells

Nuclear morphometric descriptors such as nuclear size, shape, DNA content and chromatin organization are used by pathologists as diagnostic markers for cancer. Tumorigenesis involves a series of poorly understood morphological changes that lead to the development of hyperplasia, dysplasia, in situ carcinoma, invasive carcinoma, and in many instances finally metastatic carcinoma. Nuclei from different stages of disease progression exhibit changes in shape and the reorganization of chromatin, which appears to correlate with malignancy. Multistep tumorigenesis is a process that results from alterations in the function of DNA. These alterations result from stable genetic changes, including those of tumor suppressor genes, oncogenes and DNA stability genes, and potentially reversible epigenetic changes, which are modifications in gene function without a change in the DNA sequence. DNA methylation and histone modifications are two epigenetic mechanisms that are altered in cancer cells. The impact of genetic (e.g., mutations in Rb and ras family) and epigenetic alterations with a focus on histone modifications on chromatin structure and function in cancer cells are reviewed here.

Role of H1 phosphorylation in rapid GR exchange and function at the MMTV promoter

Photobleaching technology has demonstrated in live cells that the glucocorticoid receptor (GR) exchanges rapidly at the mouse mammary tumor virus (MMTV) promoter. GR rapid exchange at MMTV depends on chaperone and proteasome activity, and as suggested by several in vitro and in vivo biochemical approaches, may also involve chromatin remodeling activity. Inhibition of H1 phosphorylation, chromatin remodeling and transcription from MMTV can be accomplished by long-term blocking of Cdk2 protein kinase activity. We find that Cdk2 is recruited by a tandem array of MMTV promoters, strengthening the model that this kinase has a specific role in MMTV transcription. We also demonstrate that following a brief Cdk2 inhibition by a selective cyclin-dependent kinase inhibitor (Roscovitine), transcription from MMTV drops and GR exchange at MMTV becomes slower, with a fraction of GR molecules now tightly bound at the promoter. This immobile fraction is absent elsewhere in the nucleus, suggesting a specific effect of Cdk2 inhibition on GR-MMTV interactions. These are the first live cell data suggesting a role for H1 phosphorylation, and by implication chromatin remodeling, in rapid exchange of GR at MMTV.

Histone modifications as a platform for cancer therapy

Tumorigenesis and metastasis are a progression of events resulting from alterations in the processing of the genetic information. These alterations result from stable genetic changes (mutations) involving tumor suppressor genes and oncogenes (e.g., ras, BRAF) and potentially reversible epigenetic changes, which are modifications in gene function without a change in the DNA sequence. Mutations of genes coding for proteins that directly or indirectly influence epigenetic processes will alter the cell's gene expression program. Epigenetic mechanisms often altered in cancer cells are DNA methylation and histone modifications (acetylation, methylation, phosphorylation). This article will review the potential of these reversible epigenetic processes as targets for cancer therapies.

The H1 phosphorylation state regulates expression of CDC2 and other genes in response to starvation in Tetrahymena thermophila

In Tetrahymena thermophila, highly phosphorylated histone H1 of growing cells becomes partially dephosphorylated when cells are starved in preparation for conjugation. To determine the effects of H1 phosphorylation on gene expression, PCR-based subtractive hybridization was used to clone cDNAs that were differentially expressed during starvation in two otherwise-isogenic strains differing only in their H1s. H1 in A5 mutant cells lacked phosphorylation, and H1 in E5 cells mimicked constitutive H1 phosphorylation. Sequences enriched in A5 cells included genes encoding proteases. Sequences enriched in E5 cells included genes encoding cdc2 kinase and a Ser/Thr kinase. These results indicate that H1 phosphorylation plays an important role in regulating the pattern of gene expression during the starvation response and that its role in transcription regulation can be either positive or negative. Treatment of starved cells with a phosphatase inhibitor caused CDC2 gene overexpression. Expression of the E5 version of H1 in starved cells containing endogenous, wild-type H1 caused the wild-type H1 to remain highly phosphorylated. These results argue that Cdc2p is the kinase that phosphorylates Tetrahymena H1, establish a positive feedback mechanism between H1 phosphorylation and CDC2 expression, and indicate that CDC2 gene expression is regulated by an H1 phosphatase.

The Ras-MAPK signal transduction pathway, cancer and chromatin remodeling

Stimulation of the Ras-mitogen-activated protein kinase (MAPK) signal transduction pathway results in a multitude of events including expression of the immediate-early genes, c-fos and c-myc. Downstream targets of this stimulated pathway are the mitogen- and stress-activated protein kinases (MSK) 1 and 2, which are histone H3 kinases. In chromatin immunoprecipitation assays, it has been shown that the mitogen-induced phosphorylated H3 is associated with the immediate-early genes and that MSK1/2 activity and H3 phosphorylation have roles in chromatin remodeling and transcription of these genes. In oncogene-transformed fibroblasts in which the Ras-MAPK pathway is constitutively active, histone H1 and H3 phosphorylation is increased and the chromatin of these cells has a more relaxed structure than the parental cells. In this review we explore the deregulation of the Ras-MAPK pathway in cancer, with an emphasis on breast cancer. We discuss the features of MSK1 and 2 and the impact of a constitutively activated Ras-MAPK pathway on chromatin remodeling and gene expression.Key words: Ras, mitogen-activated protein kinase signal transduction pathway, histone H3 phosphorylation, MSK1, breast cancer.

The dynamic mobility of histone H1 is regulated by cyclin/CDK phosphorylation

The linker histone H1 is involved in maintaining higher-order chromatin structures and displays dynamic nuclear mobility, which may be regulated by posttranslational modifications. To analyze the effect of H1 tail phosphorylation on the modulation of the histone's nuclear dynamics, we generated a mutant histone H1, referred to as M1-5, in which the five cyclin-dependent kinase phosphorylation consensus sites were mutated from serine or threonine residues into alanines. Cyclin E/CDK2 or cyclin A/CDK2 cannot phosphorylate the mutant in vitro. Using the technique of fluorescence recovery after photobleaching, we observed that the mobility of a green fluorescent protein (GFP)-M1-5 fusion protein is decreased compared to that of a GFP-wild-type H1 fusion protein. In addition, recovery of H1 correlated with CDK2 activity, as GFP-H1 mobility was decreased in cells with low CDK2 activity. Blocking the activity of CDK2 by p21 expression decreased the mobility of GFP-H1 but not that of GFP-M1-5. Finally, the level and rate of recovery of cyan fluorescent protein (CFP)-M1-5 were lower than those of CFP-H1 specifically in heterochromatic regions. These data suggest that CDK2 phosphorylates histone H1 in vivo, resulting in a more open chromatin structure by destabilizing H1-chromatin interactions.

Direct activation of cyclin-dependent kinase 2 by human papillomavirus E7

Addition of human papillomavirus (HPV) E7 CDK2/cyclin A or CDK2/cyclin E, purified from either insect cells or bacteria, dramatically upregulates histone H1 kinase activity. Activation is substrate specific, with a smaller effect noted for retinoblastoma protein (Rb). The CDK2 stimulatory activity is equivalent in high-risk (HPV type 16 [HPV16] and HPV31) and low-risk (HPV6b) E7. Mutational analyses of HPV16 E7 indicate that the major activity resides in amino acids 9 to 38, spanning CR1 and CR2, and does not require casein kinase II or Rb-binding domain functions. Synthetic peptides spanning HPV16 amino acid residues 9 to 38 also activate CDK2. Peptides containing this sequence that carry biotin on the carboxy terminus, as well as a photoactivated cross-linking group (benzophenone), also activate the complex and covalently associate with the CDK2/cyclin A complex in a specific manner requiring UV. Cross-linking studies that use protein monomers detect association of the E7 peptides with cyclin A but not CDK2. Together, our results indicate a novel mechanism whereby E7 promotes HPV replication by directly altering CDK2 activity and substrate specificity.

Functional potential of P2P-R: a role in the cell cycle and cell differentiation related to its interactions with proteins that bind to matrix associated regions of DNA?

P2P-R is the alternately spliced product of the P2P-R/PACT gene in that P2P-R lacks one exon encoding 34 amino acids. The 250 kDa P2P-R protein is the predominate product expressed in multiple murine cell lines. It is a highly basic protein that contains multiple domains including an N-terminal RING type zinc finger, a proline rich domain, an RS region, and a C-terminal lysine-rich domain. P2P-R binds the p53 and the Rb1 tumor suppressors and is phosphorylated by the cdc2 and SRPK1a protein kinases. P2P-R also interacts with scaffold attachment factor-B (SAF-B), a well characterized MARs (for matrix attachment regions) binding factor, and may interact with nucleolin, another MARs binding factor. In addition, P2P-R binds single strand DNA (ssDNA). The expression of P2P-R is regulated by differentiation and cell cycle events. P2P-R mRNA is markedly repressed during differentiation, whereas immunoreactive P2P-R protein levels are >10-fold higher in mitotic than in G(0) cells. The localization of P2P-R also is modulated during the cell cycle. During interphase, P2P-R is present primarily in nucleoli and nuclear speckles whereas during mitosis, P2P-R associates with the periphery of chromosomes. Overexpression of near full length P2P-R induces mitotic arrest in prometaphase and mitotic apoptosis, and overexpression of selected P2P-R segments also can promote apoptosis. This compendium of data supports the possibility that P2P-R may form complexes with the Rb1 and/or p53 tumor suppressors and MARs-related factors, in a cell cycle and cell differentiation-dependent manner, to influence gene transcription/expression and nuclear organization.

The dual activity of pyruvate kinase type M2 from chromatin extracts of neoplastic cells

Pyruvate kinase type M(2) from Morris hepatoma 7777 tumour cell nuclei and cytosol, in contrast to types L and M(2) from nuclei and cytosol of normal rat liver, shows the histone H(1) kinase activity. Moreover, in the presence of L-cysteine and without ADP it converts 2-phosphoenolpyruvate (PEP) to pyruvate while in the presence of L-arginine or L-histidine does not. L-Cysteine markedly stimulates the activity of histone H(1) kinase transferring a phosphate group from PEP to, as results suggested, the epsilon -amino group of L-lysine of histone H(1). This, L-cysteine which is known to inhibit the activity of pyruvate kinase type M(2) from neoplastic cells transfering a phosphate from PEP to ADP, can act as a control factor champing the direction of enzymatic reaction in cancer cells.

Histone H1(S)-3 phosphorylation in Ha-ras oncogene-transformed mouse fibroblasts

Phosphorylation of linker histone H1(S)-3 (previously named H1b) and core histone H3 is elevated in mouse fibroblasts transformed with oncogenes or constitutively active mitogen-activated protein kinase (MAPK) kinase (MEK). H1(S)-3 phosphorylation is the only histone modification known to be dependent upon transcription and replication. Our results show that the increased amounts of phosphorylated H1(S)-3 in the oncogene Ha-ras-transformed mouse fibroblasts was a consequence of an elevated Cdk2 activity rather than the reduced activity of a H1 phosphatase, which our studies suggest is PP1. Induction of oncogenic ras expression results in an increase in H1(S)-3 and H3 phosphorylation. However, in contrast to the phosphorylation of H3, which occurred immediately following the onset of Ras expression, there was a lag of several hours before H1(S)-3 phosphorylation levels increased. We found that there was a transient increase in the levels of p21(cip1), which inhibited the H1 kinase activity of Cdk2. Cdk2 activity and H1(S)-3 phosphorylated levels increased after p21(cip1) levels declined. Our studies suggest that persistent activation of the Ras-MAPK signal transduction pathway in oncogene-transformed cells results in deregulated activity of kinases phosphorylating H3 and H1(S)-3 associated with transcribed genes. The chromatin remodelling actions of these modified histones may result in aberrant gene expression.