Rapid dephosphorylation of H1 histones after apoptosis induction

The Highest Density of Phosphorylated Histone H1 Appeared in Prophase and Prometaphase in Parallel with Reduced H3K9me3, and HDAC1 Depletion Increased H1.2/H1.3 and H1.4 Serine 38 Phosphorylation

Background: Variants of linker histone H1 are tissue-specific and are responsible for chromatin compaction accompanying cell differentiation, mitotic chromosome condensation, and apoptosis. Heterochromatinization, as the main feature of these processes, is also associated with pronounced trimethylation of histones H3 at the lysine 9 position (H3K9me3). Methods: By confocal microscopy, we analyzed cell cycle-dependent levels and distribution of phosphorylated histone H1 (H1ph) and H3K9me3. By mass spectrometry, we studied post-translational modifications of linker histones. Results: Phosphorylated histone H1, similarly to H3K9me3, has a comparable level in the G1, S, and G2 phases of the cell cycle. A high density of phosphorylated H1 was inside nucleoli of mouse embryonic stem cells (ESCs). H1ph was also abundant in prophase and prometaphase, while H1ph was absent in anaphase and telophase. H3K9me3 surrounded chromosomal DNA in telophase. This histone modification was barely detectable in the early phases of mitosis. Mass spectrometry revealed several ESC-specific phosphorylation sites of H1. HDAC1 depletion did not change H1 acetylation but potentiated phosphorylation of H1.2/H1.3 and H1.4 at serine 38 positions. Conclusions: Differences in the level and distribution of H1ph and H3K9me3 were revealed during mitotic phases. ESC-specific phosphorylation sites were identified in a linker histone.

Extra-nuclear histones: origin, significance and perspectives

Histones are classically known to organize the eukaryotic DNA into chromatin. They are one of the key players in regulating transcriptionally permissive and non-permissive states of the chromatin. Nevertheless, their context-dependent appearance within the cytoplasm and systemic circulation has also been observed. The past decade has also witnessed few scientific communications on the existence of vesicle-associated histones. Diverse groups have attempted to determine the significance of these extra-nuclear histones so far, with many of those studies still underway. Of note amongst these are interactions of extra-nuclear or free histones with cellular membranes, mediated by mutual cationic and anionic natures, respectively. It is here aimed to consolidate the mechanism of formation of extra-nuclear histones; implications of histone-induced membrane destabilization and explore the mechanisms of their association/release with extracellular vesicles, along with the functional aspects of these extra-nuclear histones in cell and systemic physiology.

The pro-apoptotic actions of 2-methoxyestradiol against ovarian cancer involve catalytic activation of PKCδ signaling

Background: 2-methoxyestradiol (2MeOE₂) is a natural metabolite of estradiol, which is generated by the action of CYP1A1 enzyme in the liver. We have previously shown that a flaxseed-supplemented diet decreases both the incidence and severity of ovarian cancer in laying hens, also induces CYP1A1 expression in liver. Recently, we have shown that as a biologically derived active component of flax diet, 2MeOE₂ induces apoptosis in ovarian cancer cells which is partially dependent on p38 MAPK. The objective of this study was to elucidate the molecular mechanism of actions of 2MeOE₂, a known microtubule disrupting agent, in inducing apoptosis in ovarian tumors.

Results: 2MeOE₂ induces γH2Ax expression and apoptotic histone modifications in ovarian cancer cells, which are predicted downstream targets of protein kinase Cδ (PKCδ) during apoptosis. Overexpressing full length PKCδ alone does not induce apoptosis but potentiates 2MeOE₂-mediated apoptosis. C3-domain mutated dominant-negative PKCδ (PKCδ^DN) significantly reduces 2MeOE₂-induced caspase-3 cleavage and apoptotic histone modification. Silencing PKCδ diminishes 2MeOE₂-mediated apoptosis. The catalytic fragment of PKCδ (PKCδ^CAT) evokes pro-apoptotic effects which are principally dependent on p38 MAPK phosphorylation.

Conclusions: The pro-apoptotic actions of 2MeOE₂ are in part dependent on catalytic activation of PKCδ. Catalytic activation of PKCδ accelerates the 2MeOE₂-induced apoptotic cascade. This study describes a novel molecular action of flaxseed diet in ovarian cancer.

Moving Pieces in a Cellular Puzzle: A Cryptic Peptide from the Scorpion Toxin Ts14 Activates AKT and ERK Signaling and Decreases Cardiac Myocyte Contractility via Dephosphorylation of Phospholamban

Cryptic peptides (cryptides) are biologically active peptides formed after proteolysis of native precursors present in animal venoms, for example. Proteolysis is an overlooked post-translational modification that increases venom complexity. The tripeptide KPP (Lys-Pro-Pro) is a peptide encrypted in the C-terminus of Ts14-a 25-mer peptide from the venom of the Tityus serrulatus scorpion that has a positive impact on the cardiovascular system, inducing vasodilation and reducing arterial blood pressure of hypertensive rats among other beneficial effects. A previous study reported that KPP and its native peptide Ts14 act via activation of the bradykinin receptor B2 (B2R). However, the cellular events underlying the activation of B2R by KPP are unknown. To study the cell signaling triggered by the Ts14 cryptide KPP, we incubated cardiac myocytes isolated from C57BL/6 mice with KPP (10^-7 mol·L^-1) for 0, 5, or 30 min and explored the proteome and phosphoproteome. Our results showed that KPP regulated cardiomyocyte proteins associated with, but not limited to, apoptosis, muscle contraction, protein turnover, and the respiratory chain. We also reported that KPP led to AKT phosphorylation, activating AKT and its downstream target nitric oxide synthase. We also observed that KPP led to dephosphorylation of phospholamban (PLN) at its activation sites (S16 and T17), leading to reduced contractility of treated cardiomyocytes. Some cellular targets reported here for KPP (e.g., AKT, PLN, and ERK) have already been reported to protect the cardiac tissue from hypoxia-induced injury. Hence, this study suggests potential beneficial effects of this scorpion cryptide that needs to be further investigated, for example, as a drug lead for cardiac infarction.

Group A Streptococcus-Mediated Host Cell Signaling

In the past decade, the field of the cellular microbiology of group A Streptococcus (S. pyogenes) infection has made tremendous advances and touched upon several important aspects of pathogenesis, including receptor biology, invasive and evasive phenomena, inflammasome activation, strain-specific autophagic bacterial killing, and virulence factor-mediated programmed cell death. The noteworthy aspect of S. pyogenes-mediated cell signaling is the recognition of the role of M protein in a variety of signaling events, starting with the targeting of specific receptors on the cell surface and on through the induction and evasion of NETosis, inflammasome, and autophagy/xenophagy to pyroptosis and apoptosis. Variations in reports on S. pyogenes-mediated signaling events highlight the complex mechanism of pathogenesis and underscore the importance of the host cell and S. pyogenes strain specificity, as well as in vitro/in vivo experimental parameters. The severity of S. pyogenes infection is, therefore, dependent on the virulence gene expression repertoire in the host environment and on host-specific dynamic signaling events in response to infection. Commonly known as an extracellular pathogen, S. pyogenes finds host macrophages as safe havens wherein it survives and even multiplies. The fact that endothelial cells are inherently deficient in autophagic machinery compared to epithelial cells and macrophages underscores the invasive nature of S. pyogenes and its ability to cause severe systemic diseases. S. pyogenes is still one of the top 10 causes of infectious mortality. Understanding the orchestration of dynamic host signaling networks will provide a better understanding of the increasingly complex mechanism of S. pyogenes diseases and novel ways of therapeutically intervening to thwart severe and often fatal infections.

Interphase H1 phosphorylation: Regulation and functions in chromatin

Many metazoan cell types differentially express multiple non-allelic amino acid sequence variants of histone H1. Although early work revealed that H1 variants, collectively, are phosphorylated during interphase and mitosis, differences between individual H1 variants in the sites they possess for mitotic and interphase phosphorylation have been elucidated only relatively recently. Here, we review current knowledge on the regulation and function of interphase H1 phosphorylation, with a particular emphasis on how differences in interphase phosphorylation among the H1 variants of mammalian cells may enable them to have differential effects on transcription and other chromatin processes.

Interplay between histone H1 structure and function

H1 linker histones are involved both in the maintenance of higher-order chromatin structure and in gene regulation. Histone H1 exists in multiple isoforms, is evolutionarily variable and undergoes a large variety of post-translational modifications. We review recent progress in the understanding of the folding and structure of histone H1 domains with an emphasis on the interactions with DNA. The importance of intrinsic disorder and hydrophobic interactions in the folding and function of the carboxy-terminal domain (CTD) is discussed. The induction of a molten globule-state in the CTD by macromolecular crowding is also considered. The effects of phosphorylation by cyclin-dependent kinases on the structure of the CTD, as well as on chromatin condensation and oligomerization, are described. We also address the extranuclear functions of histone H1, including the interaction with the β-amyloid peptide.

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.

The crucial elements of the 'last step' of programmed cell death induced by kinetin in root cortex of V. faba ssp. minor seedlings

Kinetin-induced programmed cell death, manifested by condensation, degradation and methylation of DNA and fluctuation of kinase activities and ATP levels, is an autolytic and root cortex cell-specific process. The last step of programmed cell death (PCD) induced by kinetin in the root cortex of V. faba ssp. minor seedlings was explained using morphologic (nuclear chromatin/aggregation) and metabolic (DNA degradation, DNA methylation and kinases activity) analyses. This step involves: (1) decrease in nuclear DNA content, (2) increase in the number of 4',6-diamidino-2-phenylindole (DAPI)-stained chromocenters, and decrease in chromomycin A3 (CMA3)-stained chromocenters, (3) increase in fluorescence intensity of CMA3-stained chromocenters, (4) condensation of DAPI-stained and loosening of CMA3-stained chromatin, (5) fluctuation of the level of DNA methylation, (6) fluctuation of activities of exo-/endonucleolytic Zn(2+) and Ca(2+)/Mg(2+)-dependent nucleases, (7) changes in H1 and core histone kinase activities and (8) decrease in cellular ATP amount. These results confirmed that kinetin-induced PCD was a specific process. Additionally, based on data presented in this paper (DNA condensation and ATP depletion) and previous studies [increase in vacuole, increase in amount of cytosolic calcium ions, ROS production and cytosol acidification "in Byczkowska et al. (Protoplasma 250:121-128, 2013)"], we propose that the process resembles autolytic type of cell death, the most common type of death during development of plants. Lastly, the observations also suggested that regulation of these processes might be under control of epigenetic (methylation/phosphorylation) mechanisms.

The C-terminal domain (CTD) in linker histones antagonizes anti-apoptotic proteins to modulate apoptotic outcomes at the mitochondrion

The loss of mitochondrial integrity as a consequence of apoptogenic complexes formed on the outer membrane constitutes a key step in controlling progression of apoptotic cascades. Here, we show that multiple members of the linker histone (LH) family of proteins modify apoptotic cascades initiated by the Bcl-2 protein Bak, and impart resistance to its endogenous antagonist Bcl-xL. Our experiments reveal apoptogenic capabilities equivalent to those documented for H1.2 in H1.1 and H1.3 isoforms. Deletion mutants of H1.2 and site-directed mutagenesis of H1.1 and H1.2 implicated the C-terminal domain in apoptogenic activity. In this context, disruption of protein kinase-C activity using chemical inhibitors, dominant-negative approaches and RNA interference coupled with site-directed modifications in H1.1, identified the protein kinase-Cβ1 isoform as a repressor of H1.1/H1.3 apoptogenic activity. Finally, a H1.2 C-terminal tail recombinant attenuated Bcl-xl inhibition of Bak-induced apoptosis, suggesting that the C-terminal domain was necessary and sufficient for apoptogenic functions. Thus, integration with apoptotic intermediates (via C-terminal tail interactions) may constitute a more generalized function of LH isoforms in apoptotic cascades.

Serine/threonine phosphatase (SP-STP), secreted from Streptococcus pyogenes, is a pro-apoptotic protein

This investigation illustrates an important property of eukaryote-type serine/threonine phosphatase (SP-STP) of group A Streptococcus (GAS) in causing programmed cell death of human pharyngeal cells. The secretory nature of SP-STP, its elevated expression in the intracellular GAS, and the ability of wild-type GAS but not the GAS mutant devoid of SP-STP to cause apoptosis of the host cell both in vitro and in vivo suggest that GAS deploys SP-STP as an important virulence determinant to exploit host cell machinery for its own advantage during infection. The exogenously added SP-STP is able to enter the cytoplasm and subsequently traverses into the nucleus in a temporal fashion to cause apoptosis of the pharyngeal cells. The programmed cell death induced by SP-STP, which requires active transcription and de novo protein synthesis, is also caspase-dependent. Furthermore, the entry of SP-STP into the cytoplasm is dependent on its secondary structure as the catalytically inactive SP-STP with an altered structure is unable to internalize and cause apoptosis. The ectopically expressed wild-type SP-STP was found to be in the nucleus and conferred apoptosis of Detroit 562 pharyngeal cells. However, the catalytically inactive SP-STP was unable to cause apoptosis even when intracellularly expressed. The ability of SP-STP to activate pro-apoptotic signaling cascades both in the cytoplasm and in the nucleus resulted in mitochondrial dysfunctioning and perturbation in the phosphorylation status of histones in the nucleus. SP-STP thus not only functions as a virulence regulator but also as an important factor responsible for host-related pathogenesis.

Chromatin structure following UV-induced DNA damage-repair or death?

In eukaryotes, DNA is compacted into a complex structure known as chromatin. The unravelling of DNA is a crucial step in DNA repair, replication, transcription and recombination as this allows access to DNA for these processes. Failure to package DNA into the nucleosome, the individual unit of chromatin, can lead to genomic instability, driving a cell into apoptosis, senescence, or cellular proliferation. Ultraviolet (UV) radiation damage causes destabilisation of chromatin integrity. UV irradiation induces DNA damage such as photolesions and subjects the chromatin to substantial rearrangements, causing the arrest of transcription forks and cell cycle arrest. Highly conserved processes known as nucleotide and base excision repair (NER and BER) then begin to repair these lesions. However, if DNA repair fails, the cell may be forced into apoptosis. The modification of various histones as well as nucleosome remodelling via ATP-dependent chromatin remodelling complexes are required not only to repair these UV-induced DNA lesions, but also for apoptosis signalling. Histone modifications and nucleosome remodelling in response to UV also lead to the recruitment of various repair and pro-apoptotic proteins. Thus, the way in which a cell responds to UV irradiation via these modifications is important in determining its fate. Failure of these DNA damage response steps can lead to cellular proliferation and oncogenic development, causing skin cancer, hence these chromatin changes are critical for a proper response to UV-induced injury.

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.

Cracking the death code: apoptosis-related histone modifications

The degradation and compaction of chromatin are long-standing hallmark features of apoptosis. The histones, chief protein components of chromatin, are subjected to a wide range of post-translational modifications. An increasing body of evidence suggests that combinations of epigenetic histone modifications influence the overall chromatin structure and have clear functional consequences in cellular processes including apoptosis. This review describes the work to date on the post-translational modification of histones during apoptosis, their regulation by enzymatic complexes and discusses the existence of the apoptotic histone code.

Reciprocal epigenetic modification of histone H2B occurs in chromatin during apoptosis in vitro and in vivo

Histone H2B phosphorylation at Serine 14 (phosS14) has been proposed as an epigenetic marker of apoptotic cells whereas acetylation at the adjacent Lysine 15 (acK15) is a property of non-dying cells. We investigated the relationship and the potential regulatory mechanisms between these two epigenetic histone modifications and internucleosomal DNA degradation during apoptosis. Using rat primary thymocytes induced to undergo apoptosis with glucocorticoids we found that H2B phosphorylated at Ser14 was associated with soluble, cleaved DNA in apoptotic nuclei. In contrast acK15 was prevalent in non-apoptotic nuclei and scarce in apoptotic nuclei. This switch between K15 acetylation and S14 phosphorylation on H2B was also observed in apoptotic thymocytes from animals treated in vivo with glucocorticoids and in a rat hepatoma cell line (HTC) induced to die by UV-C or Fas ligand. Interestingly the combined use of a histone deacetylase inhibitor and glucocorticoid suppressed both S14 phosphorylation and internucleosomal DNA degradation without inhibiting apoptosis in thymocytes. Using synthetic peptides and a PKC phosphorylation assay system, we show that the deacetylation of K15 was necessary to allow the S14 phosphorylation. These findings suggest that selective chromatin post-translational modifications are associated with DNA degradation during apoptosis.

Autoacetylation regulates P/CAF nuclear localization

Acetylation is a posttranslational modification that alters the biological activities of proteins by affecting their association with other proteins or DNA, their catalytic activities, or their subcellular distribution. The acetyltransferase P/CAF is autoacetylated and acetylated by p300 in vivo. P/CAF autoacetylation is an intramolecular or intermolecular event. Intramolecular acetylation targets five lysines within the nuclear localization signal at the P/CAF C terminus. We analyzed how the subcellular distribution of P/CAF is regulated by intramolecular autoacetylation and found that a P/CAF mutant lacking histone acetyltransferase activity accumulated primarily in the cytoplasm. This cytoplasmic fraction of P/CAF is enriched for nonautoacetylated P/CAF. In addition, P/CAF deacetylation by HDAC3 and in a minor degree by HDAC1, HDAC2, or HDAC4 leads to cytoplasmic accumulation of P/CAF. Importantly, our data show that P/CAF accumulates in the cytoplasm during apoptosis. These results reveal the molecular mechanism of autoacetylation control of P/CAF nuclear translocation and suggest a novel pathway by which P/CAF activity is controlled in vivo.

Caspase-mediated changes in histone H1 in early apoptosis: prolonged caspase activation in developing olfactory sensory neurons

Programmed cell death or apoptosis is required for the patterning and development of multicellular organisms. However, apoptosis is a difficult process to measure because the dead cells are rapidly degraded by their neighbors within a few hours. The post-caspase activation events that determine whether a cell will undergo apoptosis remain elusive. Here we report that apoptosis-specific nuclear events that occur before DNA fragmentation can be distinguished by monitoring the histone H1 status. In both mammals and Drosophila, dying cells failed to be immunolabeled with an anti-H1 monoclonal antibody, AE-4. Real-time imaging of caspase activation and H1 dynamics in mammalian neural cells revealed that H1 changed its location in the nucleus after caspase activation. In addition, the timing of this re-localization was largely dependent on the apoptotic stimulus used. From the staining patterns of AE-4 and anti-active caspase-3 antibodies, cells undergoing the transition from caspase activation to the apoptotic H1 change could be identified as H1-positive caspase-activated cells, providing a novel criterion for early apoptosis and making it possible to characterize caspase-activated cells in tissues. On the basis of these staining patterns, we found that many olfactory sensory neurons in the developing mouse olfactory epithelium showed sustained caspase activity without the H1 change, suggesting a unique caspase function in these neurons.

Inactivation of Cdk1/Cyclin B in metaphase-arrested mouse FT210 cells induces exit from mitosis without chromosome segregation or cytokinesis and allows passage through another cell cycle

It is well known that inactivation of Cdk1/Cyclin B is required for cells to exit mitosis. The work reported here tests the hypothesis that Cdk1/Cyclin B inactivation is not only necessary but also sufficient to induce mitotic exit and reestablishment of the interphase state. This hypothesis predicts that inactivation of Cdk1 in metaphase-arrested cells will induce the M to G1-phase transition. It is shown that when mouse FT210 cells (in which Cdk1 is temperature-sensitive) are arrested in metaphase and then shifted to their non-permissive temperature, they rapidly exit mitosis as evidenced by reassembly of interphase nuclei, decondensation of chromosomes, and dephosphorylation of histones H1 and H3. The resulting interphase cells are functionally normal as judged by their ability to progress through another cell cycle. However, they have double the normal number of chromosomes because they previously bypassed anaphase, chromosome segregation, and cytokinesis. These results, taken together with other observations in the literature, strongly suggest that in mammalian cells, inactivation of Cdk1/cyclin B is the trigger for mitotic exit and reestablishment of the interphase state.

Topotecan and methotrexate alter expression of the apoptosis-related genes BCL2, FAS and BCL2L12 in leukemic HL-60 cells

The BCL2 family of genes (B-cell CLL/lymphoma 2; Bcl-2) plays a pivotal role in the highly regulated process of apoptosis. We have recently cloned a newly identified member of this family, BCL2L12, which was found to be differentially expressed in many tumors. It is known that topotecan and methotrexate act through induction of apoptosis in cancer cells. In the present study we investigated the expression profile of the novel apoptotic gene BCL2L12 in relation to other apoptotic genes in the human leukemic cell line HL-60, after treatment with topotecan or methotrexate. The kinetics of apoptosis induction and cell toxicity were investigated by DNA laddering and the MTT method, respectively. Gene expression levels were analyzed by RT-PCR using gene-specific primers. Downregulation of BCL2L12, BCL2 and FAS was observed after treatment of HL-60 cells with topotecan, while treatment with methotrexate led to downregulation of BCL2 and FAS, with no change in BCL2L12 expression. Our results support the significance of mRNA modulations in the expression of apoptosis-related genes during treatment of human leukemic cells with anticancer drugs.

Hypoxia-induced and stress-specific changes in chromatin structure and function

Cellular adaptation to stress relies on specific, regulated responses to evoke changes in gene expression. Stresses such as hypoxia, heat shock, oxidative stress and DNA-damage activate signaling cascades that ultimately lead to either induction or repression of stress-responsive genes. In this review, we concentrate on the mechanisms by which stress-induced signaling promotes alterations in chromatin structure, whether the read-out is activation or repression of transcription. Specific alterations in chromatin are highly regulated and dictated by the type of imposed stress. Our primary focus is on the types of chromatin alterations that occur under hypoxic conditions, which exist within a majority of tumors, and to compare these to changes in chromatin structure that occur in response to a wide variety of cellular stresses.

Beyond the walls of the nucleus: the role of histones in cellular signaling and innate immunity

Although they are one of the oldest family of proteins known (first described in 1884 by Kossel), histones continue to surprise researchers with their ever expanding roles in biology. In the past 25 years, the view of core histone octamers as a simple spool around which DNA in the nucleus is wound and linker histones as mere fasteners clipping it all together has transformed into the realization that histones play a vital role in transcriptional regulation. Through post-translational modifications, histones control the accessibility of transcription factors and a host of other proteins to multiple, conceivably thousands of, genes at once. While researchers have spent decades deciphering the role of histones in the overall structure of chromatin, it might surprise some to find that an entirely separate faction of scientists have focused on the role of histones beyond the confines of the nuclear envelope. In the past decade, there has been an accumulation of observations that suggest that histones can be found at the mitochondrion during the onset of apoptotic signaling and even at the cell surface, acting as a receptor for bacterial and viral proteins. More provocatively, immunologists are becoming convinced that they can also be found in the lumen of several tissues, acting as antimicrobial agents--critical components of an ancient innate immune system. Perhaps nowhere is this observation as dramatic as in the ability of neutrophils to entrap bacterial pathogens by casting out "nets" of DNA and histones that not only act as a physical barrier, but also display bactericidal activity. As our views regarding the role of histones inside and outside the cell evolve, some have begun to develop therapies that either utilize or target histones in the fight against cancer, microbial infection, and autoimmune disease. It is our goal here to begin the process of merging the dichotomous lives of histones both within and without the nuclear membrane.

Apoptotic DNA fragmentation is not related to the phosphorylation state of histone H1

Changes in chromatin structure, histone phosphorylation and cleavage of DNA into nucleosome-size fragments are characteristic features of apoptosis. Since H1 histones bind to the site of DNA cleavage between nucleosomal cores, the question arises as to whether the state of H1 phosphorylation influences the rate of internucleosomal cleavage. Here, we tested the relation between DNA fragmentation and H1 phosphorylation both in cultured cells and in vitro. In Jurkat cells, hyperosmotic mannitol concentration resulted in apoptosis, including nucleosomal fragmentation, whereas apoptosis induction by increased NaCl concentration was not accompanied by DNA fragmentation. However, both treatments induced dephosphorylation of H1 histones. In contrast, treatment of Raji cells with alkylphosphocholine led to induction of apoptosis with internucleosomal fragmentation, albeit without notable histone H1 dephosphorylation. These results demonstrate that dephosphorylation of H1 histones is neither a prerequisite for nor a consequence of internucleosomal cleavage. Moreover, we observed with an in vitro assay that the known enhancing effect of H1 histones on the activity of the apoptosis-induced endonuclease DFF40 is independent of the subtype or the phosphorylation state of the linker histone.

Cleavage and cytoplasmic relocalization of histone deacetylase 3 are important for apoptosis progression

The apoptotic process is accompanied by major changes in chromatin structure and gene expression. The apoptotic genetic program is progressively set up with the inhibition of antiapoptotic genes and the activation of proapoptotic ones. Here, we show that the histone deacetylase 3 (HDAC-3), which is a known co-repressor of many proapoptotic genes, is subjected to proteolytic cleavage during apoptosis in a cell type- and species-independent manner. This cleavage is caspase dependent and leads to the loss of the C-terminal part of HDAC-3. The cleaved form of HDAC-3 accumulates in the cytoplasm. Furthermore, we found that forced nuclear localization of HDAC-3 decreases the efficiency of apoptosis induction, indicating that HDAC-3 cytoplasmic relocalization is important for the apoptotic process. Finally, we observed that HDAC-3 cleavage allowed increased histone acetylation and transcriptional activation on a proapoptotic HDAC-3-target gene, the Fas-encoding gene. Altogether, our results thus indicate that HDAC-3 cleavage is crucial for efficient apoptosis induction because it allows the activation of some proapoptotic genes during apoptosis progression.

Hypophosphorylation of the architectural chromatin protein DEK in death-receptor-induced apoptosis revealed by the isotope coded protein label proteomic platform

During apoptosis nuclear morphology changes dramatically due to alterations of chromatin architecture and cleavage of structural nuclear proteins. To characterize early events in apoptotic nuclear dismantling we have performed a proteomic study of apoptotic nuclei. To this end we have combined a cell-free apoptosis system with a proteomic platform based on the differential isotopic labeling of primary amines with N-nicotinoyloxy-succinimide. We exploited the ability of this system to produce nuclei arrested at different stages of apoptosis to analyze proteome alterations which occur prior to or at a low level of caspase activation. We show that the majority of proteins affected at the onset of apoptosis are involved in chromatin architecture and RNA metabolism. Among them is DEK, an architectural chromatin protein which is linked to autoimmune disorders. The proteomic analysis points to the occurrence of multiple PTMs in early apoptotic nuclei. This is confirmed by showing that the level of phosphorylation of DEK is decreased following apoptosis induction. These results suggest the unexpected existence of an early crosstalk between cytoplasm and nucleus during apoptosis. They further establish a previously unrecognized link between DEK and cell death, which will prove useful in the elucidation of the physiological function of this protein.

Release of Hypoacetylated and Trimethylated Histone H4 Is an Epigenetic Marker of Early Apoptosis

Nuclear events such as chromatin condensation, DNA cleavage at internucleosomal sites, and histone release from chromatin are recognized as hallmarks of apoptosis. However, there is no complete understanding of the molecular events underlying these changes. It is likely that epigenetic changes such as DNA methylation and histone modifications that are involved in chromatin dynamics and structure are also involved in the nuclear events described. In this report we have shown that apoptosis is associated with global DNA hypomethylation and histone deacetylation events in leukemia cells. Most importantly, we have observed a particular epigenetic signature for early apoptosis defined by a release of hypoacetylated and trimethylated histone H4 and internucleosomal fragmented DNA that is hypermethylated and originates from perinuclear heterochromatin. These findings provide one of the first links between apoptotic nuclear events and epigenetic markers.

HP1 binds specifically to Lys26-methylated histone H1.4, whereas simultaneous Ser27 phosphorylation blocks HP1 binding

Histone lysine methylation can have positive or negative effects on transcription, depending on the precise methylation site. According to the "histone code" hypothesis these methylation marks can be read by proteins that bind them specifically and then regulate downstream events. Hetero-chromatin protein 1 (HP1), an essential component of heterochromatin, binds specifically to methylated Lys(9) of histone H3 (K9/H3). The linker histone H1.4 is methylated on Lys(26) (K26/H1.4), but the role of this methylation in downstream events remains unknown. Here we identify HP1 as a protein specifically recognizing and binding to methylated K26/H1.4. We demonstrate that the Chromo domain of HP1 is mediating this binding and that phosphorylation of Ser(27) on H1.4 (S27/H1.4) prevents HP1 from binding. We suggest that methylation of K26/H1.4 could have a role in tethering HP1 to chromatin and that this could also explain how HP1 is targeted to those regions of chromatin where it does not colocalize with methylated K9/H3. Our results provide the first experimental evidence for a "phospho switch" model in which neighboring phosphorylation reverts the effect of histone lysine methylation.

Topoisomerase inhibitor induced dephosphorylation of H1 and H3 histones as a consequence of cell cycle arrest

Posttranslational modifications of histones have an integral function in the structural and functional organization of chromatin. Several changes in the modification state of histones could be observed after induction of apoptosis with topoisomerase inhibitors and other inducers. Most of these studies include the analysis of the state of phosphorylation of histones, and the results are to some extent controversial, depending on cell lines and agents used. In the present study we compared the kinetics of the dephosphorylation of H1 and H3 histones with apoptosis markers after treatment of leukemic cell lines with topoisomerase inhibitors. In parallel, we determined cell cycle parameters in detail. Dephosphorylation of both histone classes started within 1 h of induction, and no direct correlation with timing and intensity of the investigated apoptotic features could be observed. In contrast, we show that the effect of topoisomerase inhibitors on the state of H1 and H3 phosphorylation is not directly related to apoptosis, but reflects the changes in the cell cycle distribution of cells treated with these inducers.

Caspase-dependent regulation of histone deacetylase 4 nuclear-cytoplasmic shuttling promotes apoptosis

Histone deacetylases (HDACs) are important regulators of gene expression as part of transcriptional corepressor complexes. Here, we demonstrate that caspases can repress the activity of the myocyte enhancer factor (MEF)2C transcription factor by regulating HDAC4 processing. Cleavage of HDAC4 occurs at Asp 289 and disjoins the carboxy-terminal fragment, localized into the cytoplasm, from the amino-terminal fragment, which accumulates into the nucleus. In the nucleus, the caspase-generated fragment of HDAC4 is able to trigger cytochrome c release from mitochondria and cell death in a caspase-9-dependent manner. The caspase-cleaved amino-terminal fragment of HDAC4 acts as a strong repressor of the transcription factor MEF2C, independently from the HDAC domain. Removal of amino acids 166-289 from the caspase-cleaved fragment of HDAC4 abrogates its ability to repress MEF2 transcription and to induce cell death. Caspase-2 and caspase-3 cleave HDAC4 in vitro and caspase-3 is critical for HDAC4 cleavage in vivo during UV-induced apoptosis. After UV irradiation, GFP-HDAC4 translocates into the nucleus coincidentally/immediately before the retraction response, but clearly before nuclear fragmentation. Together, our data indicate that caspases could specifically modulate gene repression and apoptosis through the proteolyic processing of HDAC4.

The telomeric region is excluded from nucleosomal fragmentation during apoptosis, but the bulk nuclear chromatin is randomly degraded

Characteristic steps during cellular apoptosis are the induction of chromatin condensation and subsequent DNA fragmentation, finally leading to the formation of oligomers of nucleosomes. We have examined the kinetics and local distribution of this nucleosomal fragmentation within different genomic regions. For the induction of apoptosis, HL60 cells were treated with the water-soluble camptothecin derivative topotecan (a topoisomerase I inhibitor). The genomic origin of the fragments was analysed by Southern blot hybridisation of the cleaved DNA. In these experiments we observed similar hybridisation patterns of the fragmented DNA, indicating a random and synchronous cleavage of the nuclear chromatin. However, hybridisation with a telomeric probe revealed that, in contrast to the other analysed genomic regions, the telomeric chromatin was not cleaved into nucleosomal fragments despite our observation that the telomeric DNA in HL60 cells is organised in nucleosomes. We determined just a minor shortening of the telomeric repeats early during apoptosis. These observations suggest that telomeric chromatin is excluded from internucleosomal cleavage during apoptosis.

Induction of histone acetylation and inhibition of growth of mouse erythroleukemia cells by S-allylmercaptocysteine

Growth-inhibitory effects on DS19 mouse erythroleukemia cells were seen in the micromolar concentration range with allicin and S-allylmercaptocysteine and in the millimolar range with allyl butyrate, allyl phenyl sulfone, and S-allyl cysteine. Increased acetylation of histones was induced by incubation of cells with the allyl compounds at concentrations similar to those that resulted in the inhibition of cell proliferation. The induction of histone acetylation by S-allylmercaptocysteine was also observed in Caco-2 human colon cancer cells and T47D human breast cancer cells. In contrast to the effect on histone acetylation, there was a decrease in the incorporation of phosphate into histones when DS19 cells were incubated with 25 microM S-allylmercaptocysteine. Histone deacetylase activity was inhibited by allyl butyrate, but there was little or no effect with the allyl sulfur compounds examined in this study. A similar degree of downregulation of histone deacetylase and histone acetyltransferase was observed when DS19 cells were incubated with S-allylmercaptocysteine or allyl isothiocyanate. The induction of histone acetylation by S-allylmercaptocysteine was not blocked by a proteasome inhibitor. The mechanism by which S-allylmercaptocysteine induces histone acetylation remains to be characterized. It may be related in part to metabolism to allyl mercaptan, which is a more effective inhibitor of histone deacetylase.

Triggers for anti-chromatin autoantibody production in SLE

The formation of autoantibodies against chromatin is the main feature of systemic lupus erythematosis (SLE), an autoimmune disease, which is T-cell dependent and autoantigen-driven. Historically, antibodies against dsDNA, one of the components of chromatin, are considered as a hallmark of SLE. However, dsDNA is poorly immunogenic. Nucleosome-specific T helper cells have been identified. These T cells propagate not only nucleosome-specific antibodies, but also anti-dsDNA antibodies. Nucleosomes are formed during apoptosis by cleavage of chromatin, and evidence of disturbed apoptosis has been found especially in certain murine models of lupus. In addition to an increased rate of apoptosis, autoimmunity against chromatin might also result from an impaired phagocytosis of apoptotic material, for which strong evidence has been provided by studies in certain knock-out mice (C1q, SAP, Dnase I). The induction of an immune response to nucleosomes could be enhanced by modifications of histones or DNA during apoptosis, altered presentation by antigen presenting cells or a viral infection. The release of nucleosomes and the formation of anti-chromatin autoantibodies result in formation of complexes, which bind to the glomerular basement membrane via heparan sulfate. This deposition incites glomerulonephritis, the most serious manifestation of SLE.

Apoptotic release of histones from nucleosomes

Chromatin structure is influenced by histone modification, and this may help direct chromatin behavior to facilitate transcription, DNA replication, and DNA repair. Chromatin condensation and DNA fragmentation are the classic nuclear features but remain poorly characterized. It is highly probable that nucleosomal structure must be altered to allow these features to become apparent, but data to support this construct are lacking. We report here that in response to apoptotic signals from a death receptor (CD95 and tumor necrosis factor-alpha) or mitochondrial (staurosporine) apoptotic stimulus, the core nucleosomal histones H2A, H2B, H3, and H4 become separated from DNA during apoptosis in Jurkat and HeLa cells and are consequently detectable in the cell lysate prepared using a non-ionic detergent. The timing of this histone release from DNA correlates well with the progression of apoptosis. We also show expression of a caspase cleavage-resistant form of ICAD (ICAD-DM) in Jurkat and HeLa cells abolished DNA fragmentation and also dramatically reduced histone release in apoptotic cells. However, we demonstrate that apoptotic histone release is not an inevitable consequence of CAD/DFF-40-mediated DNA destruction as DNA fragmentation but not histone release occurs efficiently in tumor necrosis factor-alpha- and etoposide-treated NIH3T3 cells. Furthermore, in an in vitro apoptotic assay, incubation of apoptotic Jurkat cellular extract with non-apoptotic Jurkat nuclei led to nuclear DNA fragmentation without obvious histone release. Taken together, these data demonstrate that CAD/DFF-40 functions indirectly in mediating nucleosomal destruction during apoptosis.

Nuclear apoptotic changes: an overview

Apoptosis is a form of active cell death essential for morphogenesis, development, differentiation, and homeostasis of multicellular organisms. The activation of genetically controlled specific pathways that are highly conserved during evolution results in the characteristic morphological features of apoptosis that are mainly evident in the nucleus. These include chromatin condensation, nuclear shrinkage, and the formation of apoptotic bodies. The morphological changes are the result of molecular alterations, such as DNA and RNA cleavage, post-translational modifications of nuclear proteins, and proteolysis of several polypeptides residing in the nucleus. During the last five years our understanding of the process of apoptosis has dramatically increased. However, the mechanisms that lead to apoptotic changes in the nucleus have been only partially clarified. Here, we shall review the most recent findings that may explain why the nucleus displays these striking modifications. Moreover, we shall take into consideration the emerging evidence about apoptotic events as a trigger for the generation of autoantibodies to nuclear components.

Nuclear envelope and chromatin compositional differences comparing undifferentiated and retinoic acid- and phorbol ester-treated HL-60 cells

The human leukemic cell line (HL-60) can be induced to differentiate in vitro to granulocytic form with retinoic acid (RA), or to monocytic/macrophage form with phorbol ester (TPA). The granulocytic form acquires nuclear lobulation, nuclear envelope-limited chromatin sheets (ELCS), and cytoskeletal polarization, none of which are acquired following treatment with TPA. Immunoblotting analyses and capillary zone electrophoresis demonstrated that following RA treatment: lamins A/C and B1, and vimentin decreased to negligible amounts; LAP2 beta, lamin B2 and emerin remained essentially unchanged; lamin B receptor (LBR) increased markedly; histone subtypes H1.4 and 1.5 exhibited dephosphorylation. Following TPA treatment: lamins A/C and B1, B2 and vimentin increased in amount; LAP2 beta and emerin remained essentially unchanged; LBR increased markedly; histone subtypes H1.4 and 1.5 exhibited dephosphorylation. Emerin, which was cytoplasmic in undifferentiated or granulocytic cells, localized into the nuclear envelope following TPA. Normal human granulocytes revealed compositional differences compared to granulocytic forms of HL-60, namely increased vimentin and appearance of histone subtype H1.3. A working hypothesis for nuclear lobulation postulates a combination of: increased nuclear envelope deformability due to lamins A/C and B1 deficiency; an increase in nuclear surface area/volume; an increase in chromatin-nuclear envelope interactions.

A compendium of the histone H1 family of somatic subtypes: An elusive cast of characters and their characteristics

The last 35 years has seen a substantial amount of information collected about the somatic H1 subtypes, yet much of this work has been overshadowed by research into highly divergent isoforms of H1, such as H5. Reports from several laboratories in the past few years have begun to call into question some of the traditional views regarding the general function of linker histones and their heterogeneity. Hence, the impression in some circles is that less is known about these ubiquitous nuclear proteins as compared with the core histones. The goal of the following review is to acquaint the reader with the ubiquitous somatic H1s by categorizing them and their characteristics into several classes. The reasons for our current state of misunderstanding is put into a historical context along with recent controversies centering on the role of H1 in the nucleus. Finally, we propose a model that may explain the functional role of H1 heterogeneity in chromatin compaction.Key words: histone H1, linker histones, chromatin organization, chromatin compaction, heat shock.

Linker DNA destabilizes condensed chromatin

The contribution of the linker region to maintenance of condensed chromatin was examined in two model systems, namely sea urchin sperm nuclei and chicken red blood cell nuclei. Linkerless nuclei, prepared by extensive digestion with micrococcal nuclease, were compared with Native nuclei using several assays, including microscopic appearance, nuclear turbidity, salt stability, and trypsin resistance. Chromatin in the Linkerless nuclei was highly condensed, resembling pyknotic chromatin in apoptotic cells. Linkerless nuclei were more stable in low ionic strength buffers and more resistant to trypsin than Native nuclei. Analysis of histones from the trypsinized nuclei by polyacrylamide gel electrophoresis showed that specific histone H1, H2B, and H3 tail regions stabilized linker DNA in condensed nuclei. Thermal denaturation of soluble chromatin preparations from differentially trypsinized sperm nuclei demonstrated that the N-terminal regions of histones Sp H1, Sp H2B, and H3 bind tightly to linker DNA, causing it to denature at a high temperature. We conclude that linker DNA exerts a disruptive force on condensed chromatin structure which is counteracted by binding of specific histone tail regions to the linker DNA. The inherent instability of the linker region may be significant in all eukaryotic chromatins and may promote gene activation in living cells.Key words: chromatin condensation, sea urchin sperm, chicken red blood cell, nuclei, linker DNA, histone variants, micrococcal nuclease, nucleosome, trypsin, gel electrophoresis.

Histone modifications and apoptosis: Cause or consequence?

Since the first description of apoptosis, genetic and biochemical studies have led to a greater understanding of the multiple pathways that eukaryotic cells can take to terminate their existence. These findings have also proven useful in understanding the development of various diseases such as AIDS, Alzheimer's, and Parkinson's and have provided potential targets for possible therapies. Despite all these studies, the mechanism of chromatin condensation, a morphological hallmark of apoptosis, remains elusive. This review describes the work to date on the post-translational modifications of histones during apoptosis and discusses the models that have been presented to explain the apoptotic condensation of chromatin.Key words: histones, nucleosomes, chromatin, apoptosis.

A link between apoptosis and degree of phosphorylation of high mobility group A1a protein in leukemic cells

Nuclear phosphoprotein HMGA1a, high mobility group A1a, (previously HMGI) has been investigated during apoptosis. A change in the degree of phosphorylation of HMGA1a has been observed during apoptosis induced in four leukemic cell lines (HL60, K562, NB4, and U937) by drugs (etoposide, camptothecin) or herpes simplex virus type-1. Both hyper-phosphorylation and de-phosphorylation of HMGA1a have been ascertained by liquid chromatography-mass spectrometry. Hyper-phosphorylation (at least five phosphate groups/HMGA1a molecule) occurs at the early apoptotic stages and is probably related to HMGA1a displacement from DNA and chromatin release from the nuclear scaffold. De-phosphorylation (one phosphate or no phosphate groups/HMGA1a molecule) accompanies the later formation of highly condensed chromatin in the apoptotic bodies. We report for the first time a direct link between the degree of phosphorylation of HMGA1a protein and apoptosis according to a process that involves the entire amount of HMGA1a present in the cells and, consequently, whole chromatin. At the same time we report that variously phosphorylated forms of HMGA1a protein are also mono-methylated.

Deacetylase activity associates with topoisomerase II and is necessary for etoposide-induced apoptosis

DNA topoisomerase II (topo II) is a ubiquitous nuclear enzyme that is involved in DNA replication, transcription, chromosome segregation, and apoptosis. Here we show by immunoprecipitation, pull down with glutathione S-transferase fusion proteins, and yeast two-hybrid analysis that both topo IIalpha and -beta physically interact with the histone deacetylase HDAC1. The in vitro DNA decatenation activity of recombinant topo IIalpha and -beta is inhibited by association with catalytically inactive, recombinant HDAC1. We provide evidence for the in vivo significance of the topo II-HDAC1 association, showing that inhibition of HDAC activity with trichostatin A suppresses apoptosis induced by the topo II poison etoposide, but not by the topoisomerase I inhibitor camptothecin. We suggest that chromatin remodeling by an HDAC-containing complex facilitates both topo II-catalyzed DNA rearrangement and etoposide-induced DNA damage in vivo.