Specific site of histone H3 phosphorylation related to the maintenance of premature chromosome condensation. Evidence for catalytically induced interchange of the subunits

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.

Silver nanoparticle-induced phosphorylation of histone H3 at serine 10 is due to dynamic changes in actin filaments and the activation of Aurora kinases

The phosphorylation of histone H3 at serine 10 (p-H3S10) has been closely correlated with mitotic chromosome condensation. We previously reported that silver nanoparticles (AgNPs) significantly induced p-H3S10 independent of mitosis. In the present study, we examined the mechanisms underlying the induction of p-H3S10 by AgNPs. A treatment with AgNPs markedly induced p-H3S10 in a dose-dependent manner in three types of cell lines, and this was dependent on the cellular incorporation of AgNPs. The immunofluorescent staining of AgNP-induced p-H3S10 was thin and solid throughout the nucleus, and differed from that normally associated with mitosis. AgNPs induced the formation of globular actin in a dose-dependent manner. Latrunculin B (LatB) and phalloidin, inhibitors of actin polymerization and depolymerization, respectively, inhibited p-H3S10, suggesting that dynamic changes in actin filaments are related to AgNP-induced p-H3S10. Furthermore, p-H3S10 was mediated by Aurora kinase (AURK) pathways, which were suppressed by LatB and siRNA for cofilin 1, an actin-depolymerizing protein. AgNO₃ (Ag ions) exerted similar effects to those of AgNPs. These results suggest that Ag ions released from AgNPs incorporated into inner cells changed the dynamics of actin filaments, and this was followed by the activation of AURKs, leading to the induction of p-H3S10.

Rapid Discovery of Pyrido[3,4-d]pyrimidine Inhibitors of Monopolar Spindle Kinase 1 (MPS1) Using a Structure-Based Hybridization Approach

Monopolar spindle 1 (MPS1) plays a central role in the transition of cells from metaphase to anaphase and is one of the main components of the spindle assembly checkpoint. Chromosomally unstable cancer cells rely heavily on MPS1 to cope with the stress arising from abnormal numbers of chromosomes and centrosomes and are thus more sensitive to MPS1 inhibition than normal cells. We report the discovery and optimization of a series of new pyrido[3,4-d]pyrimidine based inhibitors via a structure-based hybridization approach from our previously reported inhibitor CCT251455 and a modestly potent screening hit. Compounds in this novel series display excellent potency and selectivity for MPS1, which translates into biomarker modulation in an in vivo human tumor xenograft model.

Axial contraction and short-range compaction of chromatin synergistically promote mitotic chromosome condensation

The segregation of eukaryotic chromosomes during mitosis requires their extensive folding into units of manageable size for the mitotic spindle. Here, we report on how phosphorylation at serine 10 of histone H3 (H3 S10) contributes to this process. Using a fluorescence-based assay to study local compaction of the chromatin fiber in living yeast cells, we show that chromosome condensation entails two temporally and mechanistically distinct processes. Initially, nucleosome-nucleosome interaction triggered by H3 S10 phosphorylation and deacetylation of histone H4 promote short-range compaction of chromatin during early anaphase. Independently, condensin mediates the axial contraction of chromosome arms, a process peaking later in anaphase. Whereas defects in chromatin compaction have no observable effect on axial contraction and condensin inactivation does not affect short-range chromatin compaction, inactivation of both pathways causes synergistic defects in chromosome segregation and cell viability. Furthermore, both pathways rely at least partially on the deacetylase Hst2, suggesting that this protein helps coordinating chromatin compaction and axial contraction to properly shape mitotic chromosomes.

DOI:http://dx.doi.org/10.7554/eLife.10396.001

DNA in humans, yeast and other eukaryotic organisms is packaged in structures called chromosomes. When a cell divides these chromosomes are copied and then the matching pairs are separated so that each daughter cell has a full set of its genome. To enable these events to take place, the DNA must become more tightly packed so that the chromosomes become rigid units with projections called arms. Any failure in this chromosome “condensation” leads to the loss of chromosomes during cell division.

Within a chromosome, sections of DNA are wrapped around groups of proteins to make a series of linked units called nucleosomes, which resemble beads on a string. These units and other scaffold proteins together make a structure called chromatin and establish the overall shape of the chromosome. However, it is not exactly clear how the nucleosomes and scaffold proteins are rearranged during condensation.

Kruitwagen et al. used microscopy to study chromosome condensation in budding yeast. The experiments reveal that condensation involves two separate processes. First, modifications to the nucleosomes result in these units becoming more tightly packed in a process called short-range compaction. Second, a group of proteins called condensin is responsible for rearranging the compacted chromatin to enforce higher-order structure on the arms of the condensed chromosome (long-range contraction). Further experiments suggest that an enzyme called Hst2 may help to co-ordinate these processes to ensure that chromosomes adopt the right shape before the cell divides. For example, Hst2 ensures that longer chromosomes condense more than shorter ones.

A future challenge will be to find out whether chromosome condensation works in a similar way in humans and other large eukaryotes, which form much larger chromosomes with more complicated structures than yeast.

DOI:http://dx.doi.org/10.7554/eLife.10396.002

AMG 900, a potent inhibitor of aurora kinases causes pharmacodynamic changes in p-Histone H3 immunoreactivity in human tumor xenografts and proliferating mouse tissues

BACKGROUND

The Aurora family of serine-threonine kinases are essential regulators of cell division in mammalian cells. Aurora-A and -B expression and kinase activity is elevated in a variety of human cancers and is associated with high proliferation rates and poor prognosis. AMG 900 is a highly potent and selective pan-aurora kinase inhibitor that has entered clinical evaluation in adult patients with advanced cancers. In mice, oral administration of AMG 900 blocks the phosphorylation of histone H3 on serine-10 (p-Histone H3), a proximal substrate of aurora-B and inhibits the growth of multiple human tumor xenografts, including multidrug-resistant models.

METHODS

In order to establish a preclinical pharmacokinetic-pharmacodynamic (PK-PD) relationship for AMG 900 that could be translated to the clinic, we used flow cytometry and laser scanning cytometry detection platforms to assess the effects on p-Histone H3 inhibition in terms of sensitivity, precision, and specificity, in human tumor xenografts in conjunction with mouse skin and bone marrow tissues. Mice with established COLO 205 tumors were administered AMG 900 at 3.75, 7.5, and 15 mg/kg and assessed after 3 hours.

RESULTS

Significant suppression of p-Histone H3 in mouse skin was only observed at 15 mg/kg (p <0.0001), whereas in mouse bone marrow and in tumor a dose-dependent inhibition was achieved at all three doses (p ≤ 0.00015). These studies demonstrate that AMG 900 inhibits p-Histone H3 in tumors and surrogate tissues (although tissues such as skin may be less sensitive for assessing PD effects). To further extend our work, we evaluated the feasibility of measuring p-Histone H3 using fine-needle aspirate (FNA) tumor xenograft biopsies. Treatment with AMG 900 significantly inhibited p-Histone H3 (>99% inhibition, p <0.0001) in COLO 205 tumors. Lastly, we illustrate this LSC-based approach can detect p-Histone H3 positive cells using mock FNAs from primary human breast tumor tissues.

CONCLUSION

Phosphorylation of histone H3 is a useful biomarker to determine the pharmacodynamics (PD) activity of AMG 900. FNA biopsies may be a viable approach for assessing AMG 900 PD effects in the clinic.

Histone modifications: implications in renal cell carcinoma

In 2012, an estimated 64,770 men and women were diagnosed with malignancy of the kidney and renal pelvis, of which 13,570 succumbed to their disease. Common genetic aberrations in renal cell carcinomas (RCCs) include loss of function of the VHL gene in clear-cell RCC, overexpression of the c-MET gene in papillary RCC type I, deficiency in the FH gene in papillary RCC type II and loss of heterozygozity of the BHD gene in chromophobe RCC. Recent studies illustrate epigenetic silencing of VHL, as well as alterations in histone modifications and their governing enzymes. The possibility of reversing these epigenetic marks has resulted in efforts to target these changes by utilizing inhibitors of HDACs, DNA methyltransferases and, recently, histone methyltransferases in preclinical and clinical studies. This article focuses on potential therapeutic interventions, and the implications of histone modifications and related enzyme alterations in RCC.

Useof anti-phosphohistone H3 immunohistochemistry to determine mitotic rate in thin melanoma

The seventh edition of the American Joint Committee on Cancer (AJCC) melanoma staging system, slated for release in 2010, will introduce mitotic rate (MR) as one of the primary criteria for staging thin melanoma (< or = 1.0 mm). Accurate counts are essential because the finding of a single mitotic figure (MF) will alter the staging and management of these patients. The traditional manner of counting of mitotic figures (MFs) using a X40 objective is time consuming and prone to inter- and intraobserver variability. We employed an antibody to phosphohistone H3 (pHH3, ser10) that labels MFs in all stages of mitosis, to evaluate mitotic counts at X20 in tissue sections from 30 melanoma patients with thin lesions 0.45 to 1.2 mm in depth, and compared results with routine hematoxylin and eosin (H&E) in a double-blind fashion. The mean MR was 1.63 by antipHH3, and 0.67 for H&E, representing a mean increase of 243%. The Spearman correlation coefficient for MR in H&E and anti-pHH3 sections was 0.88 (P < 0.0001). When melanomas were designated as "mitotically active," if the MR by anti-pHH3 was > or = 2 and > or = 1 by H&E, the correlation coefficient increased to 1.0. No thin melanomas were mitotically inactive on anti-pHH3 but active on H&E. Results indicate that anti-pHH3 is a useful immunostain for labeling melanocytes in mitosis. Subsequent studies will be needed confirm the accuracy of this staining technique, which has the potential to be used as a screening method for counting MFs before conventional H&E methodology in the microstaging of thin melanoma.

Monovalent ions control proliferation of Ehrlich Lettre ascites cells

Channels and transporters of monovalent ions are increasingly suggested as putative anticarcinogenic targets. However, the mechanisms involved in modulation of proliferation by monovalent ions are poorly understood. Here, we investigated the role of K+, Na+, and Cl− ions for the proliferation of Ehrlich Lettre ascites (ELA) cells. We measured the intracellular concentration of each ion in G0, G1, and S phases of the cell cycle following synchronization by serum starvation and release. We show that intracellular concentrations and content of Na+ and Cl− were reduced in the G0–G1 phase transition, followed by an increased content of both ions in S phase concomitant with water uptake. The effect of substituting extracellular monovalent ions was investigated by bromodeoxyuridine incorporation and showed marked reduction after Na+ and Cl− substitution. In spectrofluorometric measurements with the pH-sensitive dye BCECF, substitution of Na+ was observed to upregulate the activity of the Na+/H+ exchanger NHE1 as well as of Na+-independent acid extrusion mechanisms, facilitating intracellular pH (pHi) recovery after acid loading and increasing pHi. Results using the potential sensitive dye DiBaC4( 3 ) showed a reduced Cl− conductance in S compared with G1 followed by transmembrane potential ( Em) hyperpolarization in S. Cl− substitution by impermeable anions strongly inhibited proliferation and increased free, intracellular Ca2+ ([Ca2+]i), whereas a more permeable anion had little effect. Western blots showed reduced chloride intracellular channel CLIC1 and chloride channel ClC-2 expression in the plasma membrane in S compared with G1. Our results suggest that Na+ regulates ELA cell proliferation by regulating intracellular pH while Cl− may regulate proliferation by fine-tuning of Em in S phase and altered Ca2+ signaling.

SWI/SNF chromatin remodeling enzyme ATPases promote cell proliferation in normal mammary epithelial cells

The ATPase subunits of the SWI/SNF chromatin remodeling enzymes, Brahma (BRM) and Brahma-related gene 1 (BRG1), can induce cell cycle arrest in BRM and BRG1 deficient tumor cell lines, and mice heterozygous for Brg1 are pre-disposed to breast tumors, implicating loss of BRG1 as a mechanism for unregulated cell proliferation. To test the hypothesis that loss of BRG1 can contribute to breast cancer, we utilized RNA interference to reduce the amounts of BRM or BRG1 protein in the nonmalignant mammary epithelial cell line, MCF-10A. When grown in reconstituted basement membrane (rBM), these cells develop into acini that resemble the lobes of normal breast tissue. Contrary to expectations, knockdown of either BRM or BRG1 resulted in an inhibition of cell proliferation in monolayer cultures. This inhibition was strikingly enhanced in three-dimensional rBM culture, although some BRM-depleted cells were later able to resume proliferation. Cells did not arrest in any specific stage of the cell cycle; instead, the cell cycle length increased by approximately 50%. Thus, SWI/SNF ATPases promote cell cycle progression in nonmalignant mammary epithelial cells.

Detection of mitotic cells

In preparation for cell division, nuclear chromatin undergoes a vital rearrangement required for the organization of chromosomes and their allocation to daughter cells. This process is initiated during G(2) phase with the most remarkable morphological manifestation being chromatin condensation. This unit provides protocols for identification and quantification of mitotic cells based on immunocytochemical detection of histone H3 phosphorylated on Ser 10 (H3-P), the critical event occurring during the G(2) to M transition (essential for chromatin condensation), using anti-H3-P, a commercially available antibody to which apoptotic cells are not reactive, concurrently with differential staining of cellular DNA. Additionally an adaptation of this method used to stain cells mounted on microscope slides for analysis by multiparameter laser scanning cytometry is also presented.

Protein kinase D isozymes activation and localization during mitosis

The protein kinase D (PKD) family consists of three serine/threonine protein kinases involved in the regulation of fundamental biological processes in response to their activation and intracellular redistribution. Although a substantial amount of information is available describing the mechanisms regulating the activation and intracellular distribution of the PKD isozymes during interphase, nothing is known of their activation status, localization and role during mitosis. The results presented in this study indicate that during mitosis, PKD3 and PKD are phosphorylated at Ser(731) and Ser(744) within their activation loop by a mechanism that requires protein kinase C. Mitosis-associated PKD3 Ser(731) and PKD Ser(744) phosphorylation is related to the catalytic activation of these kinases as evidenced by in vivo phosphorylation of histone deacetylase 5, a substrate of PKD and PKD3. Activation loop-phosphorylated PKD3 and PKD, as well as PKD2, associate with centrosomes, spindles and midbody suggesting that these activated kinases establish dynamic interactions with the mitotic apparatus. Thus, this study reveals a connection between the PKD isozymes and cell division, suggesting a novel role for this family of serine/threonine kinases.

Acinus-provoked protein kinase C δ isoform activation is essential for apoptotic chromatin condensation

Histone H2B phosphorylation tightly correlates with chromatin condensation during apoptosis. The caspase-cleaved acinus (apoptotic chromatin condensation inducer in the nucleus) provokes chromatin condensation in the nucleus, but the molecular mechanism accounting for this effect remains elusive. Here, we report that the active acinus p17 fragment initiates H2B phosphorylation and chromatin condensation by activating protein kinase C delta isoform (PKC-delta). We show that p17 binds to both Mst1 and PKC-delta, which is upregulated by apoptotic stimuli, enhancing their kinase activities. Acinus mutant susceptible to degradation elicits stronger chromatin condensation and higher H2B phosphorylation than wild-type acinus. Dominant-negative PKC-delta but not Mst1 robustly blocks acinus-initiated H2B phosphorylation. Surprisingly, depletion of Mst1 triggers caspase-3 activation, provoking H2B phosphorylation through activating PKC-delta. Further, acinus-elicited H2B phosphorylation and chromatin condensation are abrogated in PKC-delta-deficient mouse embryonic fibroblast cells and siRNA-knocked down PC12 cells. Thus, PKC-delta but not Mst1 acts as a physiological downstream kinase of acinus in promoting H2B phosphorylation and chromatin condensation.

Ros-induced histone modifications and their role in cell survival and cell death

Much is known about the distal DNA damage repair response. In particular, many of the enzymes and auxiliary proteins that participate in DNA repair have been characterized. In addition, knowledge of signaling pathways activated in response to DNA damage is increasing. In contrast, comparatively less is known of DNA damage-sensing molecules or of the specific alterations to chromatin structure recognized by such DNA damage sensors. Thus, precisely how chromatin structure is altered in response to DNA damage and how such alterations regulate DNA repair processes remain important unanswered questions. In vertebrates, phosphorylation of the histone variant H2A.X occurs rapidly after double-strand break formation, extends over megabase chromatin domains, and is required for stable accumulation of repair proteins at damage foci. We have shown that reactive oxygen species (ROS)-induced DNA single-strand breaks induce the incorporation of 32P specifically into histone H3. ADP-Ribosylation of histones may stimulate local chromatin relaxation to facilitate the repair process, and, indeed, histone ribosylation preceded DNA damage-induced histone H3 phosphorylation. However, H3 phosphorylation occurred concomitant with overall chromatin condensation, as revealed by decreased sensitivity of chromatin to digestion by micrococcal nuclease and by DAPI staining of nuclei. Inhibitors of the ERK and p38MAPK pathways and inhibition of poly(ADP-ribose) polymerase all reduced ROS-induced H3 phosphorylation, chromatin condensation, and cell death. Precisely how changes in the post-translational modification of histone H3 regulate the survival response remains unclear. Attempts to determine the precise site of histone H3 phosphorylation, putative histone H3 kinases, and histone H3 interacting proteins are underway.

Sequential phosphorylation of Ser-10 on histone H3 and ser-139 on histone H2AX and ATM activation during premature chromosome condensation: relationship to cell-cycle phase and apoptosis

BACKGROUND

Histone H1 and H3 phosphorylation associated with chromatin condensation during mitosis has been studied extensively. Less is known on histone modifications that occur during premature chromosome condensation (PCC). The aim of the present study was to reveal the status of histone H3 and H2AX phosphorylation on Ser-10 and Ser-139, respectively, as well as ATM activation through phosphorylation on Ser-1981, during PCC, and relate these events to cell-cycle phase and to initiation of apoptosis.

MATERIALS AND METHODS

To induce PCC, A549 and HL-60 cells were exposed to the phosphatase inhibitor calyculin A (Cal A). Phosphorylation of histone H3 and H2AX as well as ATM activation were detected immunocytochemically concurrent with analysis of cellular DNA content and activation of caspase-3, a marker of apoptosis. The intensity of cellular fluorescence was measured by flow- or laser scanning cytometry.

RESULTS

Induction of PCC led to rapid histone H3 phosphorylation, followed by activation of ATM and then H2AX phosphorylation in both, HL-60 and A549 cells. All these events occurred sequentially, prior to caspase-3 activation, and affected cells in all phases of the cell cycle. ATM activation and H2AX phosphorylation was seen during mitosis of A549 but not HL-60 cells.

CONCLUSIONS

Because the Cal A-induced phosphorylation of histone H3 and H2AX, and of ATM, precede caspase-3 activation these modifications are pertinent to PCC and not to apoptosis-associated chromatin condensation. The sequence of histone H3 and H2AX phosphorylation and ATM activation during PCC is compatible with a role of ATM in mediating phosphorylation of H2AX but not H3. Mitosis in some cell types may proceed without ATM activation and H2AX phosphorylation.

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.

Measuring the mitotic index in chemically-treated human lymphocyte cultures by flow cytometry

In the human lymphocyte chromosome aberration assay, the mitotic index (MI) is the standard cytotoxic parameter for determining which test concentrations will be evaluated for chromosome aberrations. Assessment of the MI is performed microscopically by determining the frequency of mitotic cells in a population of 1000 cells. With the commercial availability of antibodies to the mitosis-specific marker, phosphorylated-histone H3 at serine 10, automating the assessment of the MI using flow cytometry is now possible [Cytometry 32 (1998) 71]. Our laboratory has utilized and validated this technology to measure the mitotic index of chemically-treated human lymphocyte cultures. Comparisons between the microscopic and flow MI frequencies from 24h treatments with mitomycin-C, aphidicolin, eugenol, etoposide, hydroxyrurea, potassium cyanide, staurosporine, ethyl alcohol, noscapine and colcemid((R)) are presented. Our results show that the mitosis specific H3-P marker is excellent for measuring the MI frequency in human lymphocyte cultures treated up to toxic concentrations. In addition, this study demonstrates that automation of analysis by flow cytometry is an excellent alternative to the microscopic method of analysis producing less variability than the microscopic scoring and a more complete dose response curve.

Distinct cell cycle timing requirements for extracellular signal-regulated kinase and phosphoinositide 3-kinase signaling pathways in somatic cell mitosis

Mitogen-activated protein (MAP) kinase and phosphoinositide 3-kinase (PI3K) pathways are necessary for cell cycle progression into S phase; however the importance of these pathways after the restriction point is poorly understood. In this study, we examined the regulation and function of extracellular signal-regulated kinase (ERK) and PI3K during G(2)/M in synchronized HeLa and NIH 3T3 cells. Phosphorylation and activation of both the MAP kinase kinase/ERK and PI3K/Akt pathways occur in late S and persist until the end of mitosis. Signaling was rapidly reversed by cell-permeable inhibitors, indicating that both pathways are continuously activated and rapidly cycle between active and inactive states during G(2)/M. The serum-dependent behavior of PI3K/Akt versus ERK pathway activation indicates that their mechanisms of regulation differ during G(2)/M. Effects of cell-permeable inhibitors and dominant-negative mutants show that both pathways are needed for mitotic progression. However, inhibiting the PI3K pathway interferes with cdc2 activation, cyclin B1 expression, and mitotic entry, whereas inhibiting the ERK pathway interferes with mitotic entry but has little effect on cdc2 activation and cyclin B1 and retards progression from metaphase to anaphase. Thus, our study provides novel evidence that ERK and PI3K pathways both promote cell cycle progression during G(2)/M but have different regulatory mechanisms and function at distinct times.

Influence of irradiation and pentoxifylline on histone H3 phosphorylation in human tumour cell lines

Phosphorylation of histone H3 at Ser-10 correlates with chromatin condensation and this amino terminal modification is now recognized as a specific marker of mitosis. We have monitored the appearance of cells showing histone H3 phosphorylation in four human tumour cell lines to identify cell cycle progression after irradiation. In the human melanoma cell lines Be11 and MeWo and in the squamous cell carcinoma lines 4197 and 4451 a dose of 7 Gy of Co-gamma irradiation increases the number of cells binding anti-histone H3-P antibody 1-8-fold in a p53-independent manner. In the p53 mutant cell lines MeWo and 4451 H3-P phosphorylated cells can be detected as early as 30 min and show a maximum 1 h post-irradiation. In the cell lines Be11, 4197 and 4451 the early wave of H3 phosphorylated cells is followed by a second wave, which reaches a maximum 4.5-7 h post-irradiation and then declines. These events are attributed to damage-induced cell cycle blocks in the G1 and G2 phase of the cell cycle. Addition of the dose modifying drug pentoxifylline before irradiation increases the appearance of cells showing early and the late H3 phosphorylation. When pentoxifylline is added 12-24 h post-irradiation when the cell cycle blocks have reached their maximum the appearance of cells with phosphorylated H3 increases 3-5-fold in the p53 mutant cell lines MeWo and 4451. These observations are consistent with the function of the drug as a G2 block abrogator. The large H3 phosphorylation signal in p53 mutant cells is consistent with early entry of a cohort of G2 cells into mitosis. The smaller H3-P signal in p53 wild type cells correlates with the lower proportion of stable G2 populations in G1 blocked cells. These results indicate that pentoxifylline influences the appearance of histone H3 phosphorylated cells in a manner strongly dependent on the number of cells in G2 phase. This suggests that addition of pentoxifylline indeed abrogates the G2 block and thereby facilitates early entry into mitosis.

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.

Identification of some heat-induced genes of Trichinella spiralis

Three heat-induced genes of the infective-stage larvae of Trichinella spiralis were successfully identified by the suppression subtractive hybridization (SSH) technique. As indicated by reverse Northern blotting, 19 of 25 clones were scored as differentially transcribed in the heat-shocked infective-stage larvae. The sequencing data showed the presence of 12 different genes. Three were homologous to histone H3, histone H2B and translationally controlled tumour protein (TCTP). A 0.6 kb cDNA of histone H3 was generated by the RACE method and sequenced. It contained an open reading frame of 136 amino acids that demonstrated 94% identity with genes from Drosophila hydei. Semi-quantitative RT-PCR indicated that after heat-shock treatment, the expression levels of histone H3, histone H2B and TCTP increased 4.8, 27 and 5.7-fold, respectively. Northern analysis confirmed the upregulation of histone H3, histone H2B and TCTP transcripts. The upregulation of these genes during stress conditions has not been reported in parasitic organisms. The stress proteins may play an active role to sustain the parasite after exposure to hostile host factors.

Ultraviolet B-induced phosphorylation of histone H3 at serine 28 is mediated by MSK1

N-terminal tail phosphorylation of histone H3 plays an important role in gene expression, chromatin remodeling, and chromosome condensation. Phosphorylation of histone H3 at serine 10 was shown to be mediated by RSK2, mitogen- and stress-activated protein kinase-1 (MSK1), and mitogen-activated protein kinases depending on the specific stimulation or stress. Our previous study showed that mitogen-activated protein kinases MAP kinases are involved in ultraviolet B-induced phosphorylation of histone H3 at serine 28 (Zhong, S., Zhong, Z., Jansen, J., Goto, H., Inagaki, M., and Dong, Z., J. Biol. Chem. 276, 12932-12937). However, downstream effectors of MAP kinases remain to be identified. Here, we report that H89, a selective inhibitor of the nucleosomal response, totally inhibits ultraviolet B-induced phosphorylation of histone H3 at serine 28. H89 blocks MSK1 activity but does not inhibit ultraviolet B-induced activation of MAP kinases p70/85(S6K), p90(RSK), Akt, and protein kinase A. Furthermore, MSK1 markedly phosphorylated serine 28 of histone H3 and chromatin in vitro. Transfection experiments showed that an N-terminal mutant MSK1 or a C-terminal mutant MSK1 markedly blocked MSK1 activity. Compared with wild-type MSK1, cells transfected with N-terminal or C-terminal mutant MSK1 strongly blocked ultraviolet B-induced phosphorylation of histone H3 at serine 28 in vivo. These data illustrate that MSK1 mediates ultraviolet B-induced phosphorylation of histone H3 at serine 28.

Histone H3 phosphorylation is coupled to poly-(ADP-ribosylation) during reactive oxygen species-induced cell death in renal proximal tubular epithelial cells

Although the cellular response to chemical-induced stress is relatively well characterized, particularly the response to DNA damage, factors that govern the outcome of the stress response (cell survival or cell death) are less clearly defined. In this context, the mitogen-activated protein kinase (MAPK) family responds to a variety of physical and chemical stresses. The activation of MAPKs, especially the extracellular-regulated protein kinase subfamily, seems to play a causal role in death of renal proximal tubular epithelial cells (LLC-PK1) induced by reactive oxygen species (ROS). In this study, we show that extracellular signal receptor-activated kinase (ERK) activation may be coupled with LLC-PK1 cell death via changes in chromatin structure, which is mediated by increases in the phosphorylation of histone H3 (a post-translational modification required for both chromosome condensation and segregation during mitosis) and premature chromatin/chromosomal condensation, leading to cell death. In support of this view, 2,3,5-tris-(glutathione-S-yl)hydroquinone (TGHQ)-induced phosphorylation of histone H3 is accompanied by increases in chromatin condensation, as observed with the use of 4,6-diamidino-2-phenylindole-fluorescent staining, and by decreases in the sensitivity of chromatin to digestion by micrococcal nuclease. Changes in chromatin structure precede cell death. TGHQ-induced histone H3 phosphorylation and chromatin condensation are inhibited by PD098059, which selectively inhibits MAPK kinase, an upstream regulator of ERKs. Moreover, histone phosphorylation is modulated by poly(ADP-)ribosylation. Thus, the inhibition of poly(ADP-ribose)polymerase with 3-aminobenzamide prevents histone H3 phosphorylation and increases cell survival, suggesting that ADP-ribosylation and histone H3 phosphorylation are coupled in this model of ROS-induced DNA damage and cell death. The coupling of histone phosphorylation with ribosylation has not been previously demonstrated.

ERKs and p38 kinases mediate ultraviolet B-induced phosphorylation of histone H3 at serine 10

Histone H3 is the core protein of the nucleosome. Phosphorylation of H3 involves immediate early gene expression, chromatin remodeling, and chromosome condensation during mitosis. Very recently, Rsk2 or MSK1 kinase-mediated phosphorylation of H3 at serine 10 was reported. In the present study, we show that both ERKs and p38 kinase may mediate ultraviolet B-induced phosphorylation of H3 at serine 10. PD 98059, a MEK1 inhibitor, and SB 202190, a p38 kinase inhibitor, efficiently inhibited ultraviolet B-induced phosphorylation of H3. Phosphorylation of H3 was also inhibited in cells expressing dominant negative mutant (DNM) ERK2 and DNM p38 kinase. In contrast, no inhibition of H3 phosphorylation in Jnk1 or Jnk2 knockout cells (Jnk1(-/-) or Jnk2(-/-)) and cells expressing DNM JNK1 was observed. More importantly, incubation of active ERK2 or p38 kinase with H3 protein resulted in phosphorylation of H3 at serine 10 in vitro. These results suggest that ERK and p38 kinase are at least two important mediators of phosphorylation of H3 at serine 10.

Histone H2B phosphorylation in mammalian apoptotic cells. An association with DNA fragmentation

Histone phosphorylation was investigated in several mammalian cells undergoing apoptosis (human HL-60 and HeLa, mouse FM3A and N18 cells, and rat thymocytes). Among the four nucleosomal core histones (H2A, H2B, H3, and H4), H2B, which is not usually phosphorylated in quiescent or growing cells, was found to be phosphorylated after treatment with various apoptotic inducers. The H2B was phosphorylated around the time when nucleosomal DNA fragmentation was initiated and, like this fragmentation, was completely blocked with Z-Asp-CH(2)-DCB, an inhibitor of ICE or ICE-like caspase. The involved single phosphopeptide of H2B proved to be phosphorylatable in vitro with a protein kinase C, and the site Ser-32 was tentatively identified. Despite typical apoptotic chromatin condensation, the H3 phosphorylation was at a low level, and the sites where phosphorylation did occur did not include any mitosis-specific phosphopeptides. Phosphorylation of H4 was increased, but the other two histone proteins (H1 and H2A) were not appreciably changed. These observations imply that 1) H2B phosphorylation occurs universally in apoptotic cells and is associated with apoptosis-specific nucleosomal DNA fragmentation, 2) chromatin condensation in apoptosis occurs by a different biochemical mechanism from those operating during mitosis or premature chromosome condensation, and 3) this unique phosphorylation of H2B is a useful biochemical hallmark of apoptotic cells.

Influence of novobiocin on <FONT FACE=Symbol>g</font>-irradiation G0-lymphocytes as analyzed by cytogenetic endpoints

Experiments with novobiocin (NB) post-treatment were performed to verify its effect on the frequencies of micronuclei (MN) and chromosomal aberrations (CA) induced by <FONT FACE="Symbol">g</font>-irradiation (0.75, 1.5 and 3.0 Gy) in human lymphocytes at G0-phase. The frequencies of MN significantly decreased by 44 and 50%, for the treatment with NB 50 µg/ml (30-min pulse) after radiation doses of 1.5 and 3.0 Gy, respectively. However, CA frequencies were not significantly affected. No significant effect on CA was observed when lymphocyte cultures were exposed to a single dose of 2.0 Gy at the G0-phase and posttreated with 25 µg/ml NB for three hours either immediately after irradiation (G0-phase) or after 24 h (S-phase). The significant suppressive effect of NB on MN frequencies supports the hypothesis that NB interaction with chromatin increases access to DNA repair enzymes.

Histone H3 phosphorylation in human monocytes and during HL-60 cell differentiation

Phosphorylation of the nucleosome core histone H3 (H3) on Ser-10 is thought to be a prerequisite for chromatin condensation at mitosis. Although during interphase, cell differentiation, or mitogenic activation of quiescent cells, changes in chromatin structure that involve local chromatin condensation/decondensation also occur, little is known about H3 phosphorylation during these transitions. Using the recently developed sensitive marker to monitor H3 phosphorylation, namely, the mAb that recognizes the phosphorylated epitope of H3 (anti-H3-P mAb), the status of H3 phosphorylation was assayed in individual human lymphocytes after their mitogenic stimulation (G0 to G1 transition) and in human leukemic HL-60 cells induced to differentiate by all-trans-retinoic acid (RA), 1,25-dihydroxyvitamin D3 (vit D3), dimethyl sulfoxide (DMSO), or phorbol myristate acetate (PMA). Multiparameter flow cytometry was used to correlate H3 phosphorylation with cell cycle position. The specificity of the anti-H3-P mAb was confirmed by the loss of its binding following cell treatment with alkaline phosphatase. The presence of phosphorylated H3 was detected during interphase in HL-60 cells and in normal lymphocytes at a level severalfold lower than during mitosis. No significant changes in H3 phosphorylation were observed during lymphocyte stimulation. Unexpectedly, the level of H3 phosphorylation was over fourfold higher in monocytes than in lymphocytes or granulocytes from peripheral blood. The punctate pattern of labeling with anti-H3-P mAb in monocyte nuclei suggests that H3 is phosphorylated in small clusters of adjacent nucleosomes. Differentiation of HL-60 cells was accompanied by a rise in H3 phosphorylation, which was higher after induction by RA, vit D3, and PMA (approx. threefold) than after DMSO (approximately 20%). The data indicate that in addition to being a critical event during chromatin condensation at mitosis, H3 phosphorylation plays a role during chromatin changes accompanying differentiation of HL-60 cells, in particular, along the monocytic lineage. The high level of H3 phosphorylation in monocytes may serve as a marker of these cells and is being explored as a possible diagnostic and prognostic tool in monocytic leukemias.

Follicle-stimulating hormone promotes histone H3 phosphorylation on serine-10

FSH promoted the rapid phosphorylation of the nuclear protein histone H3 in immature rat ovarian granulosa cells under experimental conditions that lead to cellular differentiation and not proliferation. FSH-stimulated histone H3 phosphorylation correlated with cAMP-dependent protein kinase A (PKA) activation and translocation of the PKA catalytic subunit to a nuclear-enriched fraction and was inhibited by the PKA inhibitor H89, and histone H3 phosphorylation was stimulated in cells treated with agents that raise intracellular cAMP levels such as forskolin and 8-bromo-cAMP. FSH-stimulated histone H3 phosphorylation in granulosa cells mapped to ser-10, a site previously identified as the PKA phosphorylation site in various mitotically active cells as the mitosis-specific phosphorylation site. Injection of the FSH analog PMSG to immature rats, which is known to stimulate granulosa cell proliferation as well as differentiation, also promoted histone H3 phosphorylation on ser-10 in granulosa cells. These results establish that FSH-stimulated histone H3 phosphorylation in granulosa cells is linked not only to granulosa cell mitosis but also to granulosa cell differentiation and that FSH-stimulated histone H3 phosphorylation on ser-10 in isolated granulosa cells is mediated by PKA. These results also identify the PKA-dependent histone H3 phosphorylation as an early nuclear protein marker for FSH-stimulated differentiation of granulosa cells. Based on the recently described function of histone H3 as a coactivator of transcription, these results are consistent with the hypothesis that phosphorylated histone H3 may facilitate PKA-dependent gene transcription in granulosa cells leading to the preovulatory phenotype.

Neocentromere activity of structurally acentric mini-chromosomes in Drosophila

Chromosome fragments that lack centromeric DNA (structurally acentric chromosomes) are usually not inherited in mitosis and meiosis. We previously described the isolation, after irradiation of a Drosophila melanogaster mini-chromosome, of structurally acentric mini-chromosomes that display efficient mitotic and meiotic transmission despite their small size (under 300 kb) and lack of centromeric DNA. Here we report that these acentric mini-chromosomes bind the centromere-specific protein ZW10 and associate with the spindle poles in anaphase. The sequences in these acentric mini-chromosomes were derived from the tip of the X chromosome, which does not display centromere activity or localize ZW10, even when separated from the rest of the X. We conclude that the normally non-centromeric DNAs present in these acentric mini-chromosomes have acquired centromere function, and suggest that this example of 'neocentromere' formation involves appropriation of a self-propagating centromeric chromatin structure. The potential relevance of these observations to the identity, propagation and function of normal centromeres is discussed.

Biochemical differences between staurosporine-induced apoptosis and premature mitosis

Apoptosis is morphologically related to premature mitosis, an aberrant form of mitosis. Staurosporine, a potent protein kinase inhibitor, induces not only apoptotic cell death in a wide variety of mammalian cells but also premature initiation of mitosis in hamster cells that are arrested in S phase by DNA synthesis inhibitors. Here we report on the biochemical differences between the two phenomena commonly caused by staurosporine. Rat 3Y1 fibroblasts that had been arrested in S phase with hydroxyurea underwent apoptosis by treatment with staurosporine, whereas S-phase-arrested CHO cells initiated mitosis prematurely when similarly treated with a low concentration of staurosporine. Chromosome condensation occurred in both apoptosis (3Y1) and premature mitosis (CHO). However, neither formation of mitotic spindles nor mitosis-specific phosphorylation of MPM-2 antigens was observed in apoptosis of 3Y1 cells, unlike premature mitosis of CHO cells. The p34cdc2 kinase activated in normal and prematurely mitotic cells remained inactive in the apoptotic cells, probably because the active cyclin B/p34cdc2 complex was almost absent in the S-phase-arrested 3Y1 cells. The absence of intracellular activation of p34cdc2 in apoptosis was confirmed by immunohistochemical analyses using a specific antibody raised against Ser55-phosphorylated vimentin which is specifically phosphorylated by p34cdc2 during M phase. Furthermore, phosphorylation of histones H1 and H3, which is associated with mitotic chromosome condensation, did not occur in the apoptotic cells. These results indicate that the two phenomena, staurosporine-induced apoptosis and premature mitosis, are different in their requirement for p34cdc2 kinase activation and histone phosphorylation.

Vanadate triggers the transition from chromosome condensation to decondensation in a mitotic mutant (tsTM13) inactivation of p34cdc2/H1 kinase and dephosphorylation of mitosis-specific histone H3

At the nonpermissive temperature (39 degrees C), chromosomes remain condensed in a temperature-sensitive cell mutant (tsTM13) arrested in the late stage of mitosis. Highly increased activity of histone H1 kinase, hyperphosphorylation of histone H1, and mitosis-specific histone H3 phosphorylation are maintained, even in telophase. In the present study, the defect of chromosome decondensation in tsTM13 cells was found to be partially normalized by a tyrosine phosphatase inhibitor, vanadate, with induction of chromosome decondensation and the formation of multinucleated cells. In the presence of vanadate, the H1 kinase activity dropped to near normal levels and the amount of the inactive from of p34cdc2 protein phosphorylated at a tyrosine residue was increased. H1 and H3 were also extensively de- phosphorylated, the latter being tightly associated with chromosome decondensation. Serine/threonine-protein phosphatase in late mitosis of the mutant works normally at 39 degrees C. The results indicate that (a) the genetic defect in the mutant may be involved in the control mechanism of the p34cdc2/H1 kinase activity in the late M phase rather than the phosphatase, (b) normalization of the defect of the mutant by vanadate results from inactivation of H1 kinase, and (c) late mitosis-specific events (p34cdc2/H1 kinase inactivation, mitosis-specific dephosphorylation of histone H1 and H3) are closely operating with chromosome decondensation.

Alteration of cell cycle-dependent histone phosphorylations by okadaic acid. Induction of mitosis-specific H3 phosphorylation and chromatin condensation in mammalian interphase cells

Effects of okadaic acid (OA), a protein phosphatase inhibitor, on chromatin structure and phosphorylation of histones were examined using HeLa and N18 cells. The chromatin condensation in HeLa cells was mild and resemble prometaphase nuclei, while the condensation in N18 cells was extensive and chromatin became a compact body. H2A in HeLa cells was extensively and consistently phosphorylated at the same site throughout the cell cycle, and H3 was demonstrated to be phosphorylated at the mitosis-specific site Ser10. In contrast, H1 phosphorylation was rapidly decreased in most sites within 3 h. The reduction of H1 phosphorylation was accompanied by a quantitative change in the set of H1 phosphopeptides. During the early phase of the OA treatment, H1 phosphorylation was transiently elevated in tandem, whereas H3 phosphorylation reached a maximum somewhat later. The results suggest that mitosis-specific events (cdc2/H1 kinase activation, H1 superphosphorylation, mitosis-specific H3 phosphorylation and chromatin condensation) induced by OA are sequentially associated. The changes appear to reflect a molecular mechanism similar to that operating in normal mitosis.

The nuclear matrix and the regulation of chromatin organization and function

Nuclear DNA is organized into loop domains, with the base of the loop being bound to the nuclear matrix. Loops with transcriptionally active and/or potentially active genes have a DNase I-sensitive chromatin structure, while repressed chromatin loops have a condensed configuration that is essentially invisible to the transcription machinery. Core histone acetylation and torsional stress appear to be responsible for the generation and/or maintenance of the open potentially active chromatin loops. The transcriptionally active region of the loop makes several dynamic attachments with the nuclear matrix and is associated with core histones that are dynamically acetylated. Histone acetyltransferase and deacetylase, which catalyze this rapid acetylation and deacetylation, are bound to the nuclear matrix. Several transcription factors are components of the nuclear matrix. Histone acetyltransferase, deacetylase, and transcription factors may contribute to the dynamic attachment of the active chromatin domains with the nuclear matrix at sites of ongoing transcription.

Molecular characterization and transcription of the histone H2B gene from the protozoan parasite Trypanosoma cruzi

The structure, genomic organization and transcription of the gene encoding histone H2B in the protozoan parasite Trypanosoma cruzi have been studied. This gene consists of a 746-nucleotide unit, tandemly repeated at least 18 times in each of two clusters. DNA probes corresponding to histones H2B and H3 hybridized to different chromosomes revealing that the genes coding for these two histones are not physically linked in the genome of T. cruzi. The primary transcription product of the H2B gene is processed by trans-splicing and polyadenylation. Inhibition of DNA synthesis with aphidicolin resulted in the reduction of histone H2B mRNA to undetectable levels in about two hours, suggesting that its abundance is regulated throughout the cell cycle as it occurs in other eukaryotes. In addition, a concomitant inhibition of translation by cycloheximide reverted this effect indicating that de novo protein synthesis is required for RNA instability. Histone mRNA abundance was dependent on the life-cycle stage of T. cruzi: abundant in amastigotes and epimastigotes, the dividing forms in the host cell and the insect vector, respectively, while undetected in trypomastigotes, the parasite's non-dividing life stage.

Sequence differences between histones of procyclic Trypanosoma brucei brucei and higher eukaryotes

Four histones, a, b, c, d from procyclic Trypanosoma brucei brucei, which show similarities with the amino acid composition of the core histones H3, H2A, H2B and H4, were isolated and cleaved with Endoproteinase Glu-C. The fragments were separated by FPLC reversed phase chromatography and a subset of the fragments (a5, a9, b6, c8, d3, d9, d11) was subjected to sequence analysis. A 54-71% identity was found in the sequences of the fragment c8 and the C-terminal half of H2B and of three fragments of protein d covering the N-terminal half as well as the C-terminal region of H4. The amino acid sequence of the fragment a9 showed a 57 and 54% identity with H3 sequences of Saccharomyces cerevisiae and Xenopus laevis. Neither the a5 nor the b6 sequence could be aligned with histone sequences of other eukaryotes. The significant differences of 21-48% between the T.b. brucei histone sequences and those of calf thymus histones, which are more pronounced than the differences of Tetrahymena pyriformis and the higher eukaryote, resulted partially from replacements of amino acids with different properties and indicate specific patterns of histone-histone and/or histone-DNA contact sites in the nucleosome of T.b. brucei. These differences, together with the lack of a functional histone H1, may be sufficient to explain the lack of a salt-dependent formation of the nucleosome filament into the 30 nm fibre, which reflects alternative methods of organizing and processing the genetic information in the nucleus of the protozoan parasite and which may be of chemotherapeutic significance.

Modulation by novobiocin of sister-chromatid exchanges induced by tumor necrosis factor in human lymphocytes

The effect of the antibiotic novobiocin on human recombinant tumor necrosis factor (TNF)-induced sister-chromatid exchanges (SCEs) were examined in human peripheral blood lymphocytes. TNF, when introduced in a dose range of 10-1000 U/ml at the initiation of culture, was found to cause a significant increase in SCE frequency. The simultaneous addition of TNF and novobiocin (25 micrograms/ml) in the assay resulted in no increase of SCE frequency. Delayed (for 24 h) addition of novobiocin suppressed the induction of SCEs by 50, 100 and 500 U/ml but not by 1000 U/ml of TNF.

A temperature-sensitive CHO-K1 cell mutant (tsTM13) defective in chromosome decondensation and spindle deconstruction in M phase

A temperature-sensitive CHO-K1 cell mutant, tsTM13, exhibited a delayed cell cycle progression from metaphase to telophase at a nonpermissive temperature and was finally arrested from anaphase to telophase. Metaphase chromosomes were overcondensed and chromosome disjunction in anaphase was uncoordinated. In telophase, sister chromatids were segregated and cytokinesis was completed, but chromosome structure remained in a condensed state and the spindle was not deconstructed. The level of phosphorylation of histones H1 and H3 remained high in the later stages of mitosis and the activity of histone H1 kinase was also maintained at a high level. These results strongly suggest that the pleiotropic defects of tsTM13 cells in mitosis are associated with a lack of inactivation of activated histone H1 kinase.

Biochemical properties of histone-like proteins of procyclic Trypanosoma brucei brucei

Four histone-like proteins a, b, c, d were extracted with 0.2 M H2SO4 from soluble nuclear chromatin of Trypanosoma brucei brucei procyclic culture forms and purified by FPLC reversed phase chromatography. The amino acid composition of these proteins and their electrophoretic mobilities in three different gel systems strongly indicated their core histone nature. Similarities were found between a, b, c and d with the core histones H3, H2A, H2B and H4 of higher eukaryotes, respectively. On the other hand, these proteins also showed differences as compared to higher eukaryotes; proteins a and d clearly differed from their counterparts H3 and H4 on the basis of their hydrophobic properties. The results indicate the occurrence of core histone variants in T.b. brucei which may influence DNA-histone and histone-histone interactions as well as the chromatin compaction in the nucleus of this protozoan parasite.

Mitosis-specific histone H3 phosphorylation in vitro in nucleosome structures

A mechanism of mitosis-specific enhancement of histone H3 phosphorylation was analyzed in vitro in terms of nucleosome structure. The incorporation of [32P]phosphate into DNA-bound H3 was approximately 5-7 times higher than in DNA-free H3 using the catalytic subunit of cAMP-dependent protein kinase. The two major N-terminal serine sites, including the mitosis-specific site (Ser10) and Ser28, were extensively phosphorylated in the DNA-bound forms. These phosphorylation patterns were identical to those of nucleosomal H3. In contrast, the H3 in DNA-free octamers was very slightly phosphorylated. The major site of H3 phosphorylation in DNA-free H3 was Thr118 in the C-terminus. Results indicate that DNA-binding is essential for the high level of mitosis-specific H3 phosphorylation, and that the nucleosome structure promotes H3 N-terminal phosphorylation in vitro. It also suggests the possibility that H1 prevents H3 phosphorylation during interphase of the cell cycle.

Effects of novobiocin on heat shock protection against chromatid aberration induction by triethylenemelamine (TEM) and maleic hydrazide (MH) in Vicia faba

Heat shock (10 min 40 degrees C) prior to challenge treatment with triethylenemelamine (TEM) or maleic hydrazide (MH) significantly reduced the frequency of induced chromatid aberrations in Vicia faba main root meristems. Novobiocin treatment before heat shock did not prevent heat shock protection against both clastogens; novobiocin application after heat shock prevented protective effects. These results and those obtained earlier for heat shock protection against X-ray challenge are used to discuss possible causes underlying the protective effects triggered by heat shock.

Effects of G2 treatments with inhibitors of DNA synthesis and repair on chromosome damage induced by X-rays and chemical clastogens in root tips of Vicia faba. Comparison with corresponding effects in cultured human lymphocytes

The frequencies of chromatid aberrations produced in roots of Vicia faba by clastogenic (chromosome-damaging) agents were strongly enhanced by exposing the root-tip cells to inhibitors of DNA synthesis during the G2 phase. Chromosome damage produced by both S-dependent (maleic hydrazide, methyl methanesulfonate, thio-TEPA) and S-independent (X-rays, streptonigrin) mechanisms was enhanced by the inhibitor treatments. The types of aberrations affected by the inhibitors were mainly chromatid gaps and breaks and isochromatid breaks of the non-union type. Most effective among the inhibitors tested were hydroxyurea (HU) and 5-fluorodeoxyuridine (FdUrd). Post-treatments with caffeine were effective in enhancing clastogen-induced chromosome damage when given during the S phase. All types of aberrations, exchanges as well as breaks, were enhanced by the post-treatments. When given during the G2 phase, caffeine enhanced only the frequency of chromatid aberrations produced by X-rays. The enhancement was slight and obtained only when the cells were irradiated in the G2 phase and immediately post-treated with caffeine. Clastogen-treated cultures of human lymphocytes responded to post-treatments with inhibitors of DNA synthesis in very much the same way as clastogen-treated root-tip cells of Vicia faba. Thus, the frequencies of chromatid gaps and breaks and isochromatid breaks of the non-union type were strongly enhanced by exposing clastogen-treated lymphocytes to inhibitors of DNA synthesis during the G2 phase. The efficiency of the inhibitors, however, varied considerably in the two materials. On the whole, the number of inhibitors capable of enhancing induced chromosome damage was much larger in lymphocytes than in bean root tips. Only HU was equally effective in both materials. The most striking difference between the two materials was found when caffeine was given as a post-treatment. Thus, in human lymphocytes the frequencies of chromatid aberrations induced by most clastogenic agents were strongly enhanced when caffeine was given during the G2 phase, but little affected by post-treatments with caffeine during the S phase.

Isolation and characterization of novobiocin-resistant BHK cells

We isolated two novobiocin-resistant mutants which were stable and approximately three and four times more resistant than the parent cells to novobiocin. Both mutants (Novr A2, Novr A41) were more sensitive than the wild-type cells to nalidixic acid, and cold sensitive for cell growth. When we isolated derivatives of Novr A2 and Novr A41 cells which are resistant to nalidixic acid, those are found to be phenotypically reverted to novobiocin sensitivity like wild-type cells, thereby suggesting the relationship between the targets for novobiocin and for nalidixic acid. But the cold sensitivity did not always revert to wild type, with accompanying resistance to nalidixic acid. The DNA and RNA syntheses of Novr mutants were more resistant to novobiocin but more sensitive to nalidixic acid, than those of wild-type cells. However, in vitro assays of wild-type and Novr cell extracts were unable to demonstrate any differences in the sensitivity of topoisomerase II activity to inhibition by novobiocin. While the targets of novobiocin and nalidixic acid show a mutual interaction in vivo and play a role in DNA replication and transcription, our results suggest that these targets are probably not topoisomerase II.