Sensitization to UV-induced apoptosis by the histone deacetylase inhibitor trichostatin A (TSA)

UV-induced apoptosis is a protective mechanism that is primarily caused by DNA damage. Cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts are the main DNA adducts triggered by UV radiation. Because the formation of DNA lesions in the chromatin is modulated by the structure of the nucleosomes, we postulated that modification of chromatin compaction could affect the formation of the lesions and consequently apoptosis. To verify this possibility we treated human colon carcinoma RKO cells with the histone deacetylase inhibitor trichostatin A (TSA) prior to exposure to UV radiation. Our data show that pre-treatment with TSA increased UV killing efficiency by more than threefold. This effect correlated with increased formation of CPDs and consequently apoptosis. On the other hand, TSA treatment after UV exposure rather than before had no more effect than UV radiation alone. This suggests that a primed (opened) chromatin status is required to sensitize the cells. Moreover, TSA sensitization to UV-induced apoptosis is p53 dependent. p53 and acetylation of the core histones may thus contribute to UV-induced apoptosis by modulating the formation of DNA lesions on chromatin.     

Lysine residues in N-terminal and C-terminal regions of human histone H2A are targets for biotinylation by biotinidase

In eukaryotic cell nuclei, DNA associates with the core histones H2A, H2B, H3 and H4 to form nucleosomal core particles. DNA binding to histones is regulated by posttranslational modifications of N-terminal tails (e.g., acetylation and methylation of histones). These modifications play important roles in the epigenetic control of chromatin structure. Recently, evidence that biotinidase and holocarboxylase synthetase (HCS) catalyze the covalent binding of biotin to histones has been provided. The primary aim of this study was to identify biotinylation sites in histone H2A and its variant H2AX. Secondary aims were to determine whether acetylation and methylation of histone H2A affect subsequent biotinylation and whether biotinidase and HCS localize to the nucleus in human cells. Biotinylation sites were identified using synthetic peptides as substrates for biotinidase. These studies provided evidence that K9 and K13 in the N-terminus of human histones H2A and H2AX are targets for biotinylation and that K125, K127 and K129 in the C-terminus of histone H2A are targets for biotinylation. Biotinylation of lysine residues was decreased by acetylation of adjacent lysines but was increased by dimethylation of adjacent arginines. The existence of biotinylated histone H2A in vivo was confirmed by using modification-specific antibodies. Antibodies to biotinidase and HCS localized primarily to the nuclear compartment, consistent with a role for these enzymes in regulating chromatin structure. Collectively, these studies have identified five novel biotinylation sites in human histones; histone H2A is unique among histones in that its biotinylation sites include amino acid residues from the C-terminus.     

K8 and K12 are biotinylated in human histone H4.

Folding of DNA into chromatin is mediated by binding to histones such as H4; association of DNA with histones is regulated by covalent histone modifications, e.g. acetylation, methylation, and biotinylation. We sought to identify amino-acid residues that are biotinylated in histone H4, and to determine whether acetylation and methylation of histones affect biotinylation. Synthetic peptides spanning fragments of human histone H4 were biotinylated enzymatically using biotinidase. Peptide-bound biotin was probed with streptavidin-peroxidase. Peptides based on the N-terminal sequence of histone H4 were effectively recognized by biotinidase as substrates for biotinylation; in contrast, peptides based on the C-terminal sequences were not biotinylated. Substitution of K8 or K12 with alanine or arginine decreased biotinylation, suggesting that these lysines are targets for biotinylation; K8 and K12 are also known targets for acetylation. Chemical acetylation or methylation of a given lysine decreased subsequent enzymatic biotinylation of neighboring lysines, consistent with cross-talk among histone modifications. Substitution of a given lysine (positive charge) with glutamate (negative charge) abolished biotinylation of neighboring lysines, providing evidence that the net charge of histones has a role in biotinylation. An antibody was generated that specifically recognized histone H4 biotinylated at K12. This antibody was used to detect biotinylated histone H4 in nuclear extracts from human cells. These studies suggest that K8 and K12 in histone H4 are targets for biotinylation, that acetylation and biotinylation compete for the same binding sites, and that acetylation and methylation of histones affect biotinylation of neighboring lysines.     

Biotinylation of histones in human cells. Effects of cell proliferation.

An enzymatic mechanism has been proposed by which biotinidase may catalyze biotinylation of histones. Here, human cells were found to covalently bind biotin to histones H1, H2A, H2B, H3, and H4. Cells respond to proliferation with increased biotinylation of histones; biotinylation increases early in the cell cycle and remains increased during the cycle. Notwithstanding the catalytic role of biotinidase in biotinylation of histones, mRNA encoding biotinidase and biotinidase activity did not parallel the increased biotinylation of histones in proliferating cells. Biotinylation of histones might be regulated by enzymes other than biotinidase or by the rate of histone debiotinylation.     

K4, K9 and K18 in human histone H3 are targets for biotinylation by biotinidase

Histones are modified post-translationally, e.g. by methylation of lysine and arginine residues, and by phosphorylation of serine residues. These modifications regulate processes such as gene expression, DNA repair, and mitosis and meiosis. Recently, evidence has been provided that histones are also modified by covalent binding of the vitamin biotin. The aims of this study were to identify biotinylation sites in histone H3, and to investigate the crosstalk among histone biotinylation, methylation and phosphorylation. Synthetic peptides based on the sequence of human histone H3 were used as substrates for enzymatic biotinylation by biotinidase; biotin in peptides was probed using streptavidin peroxidase. These studies provided evidence that K4, K9 and K18 in histone H3 are good targets for biotinylation; K14 and K23 are relatively poor targets. Antibodies were generated to histone H3, biotinylated either at K4, K9 or K18. These antibodies localized to nuclei in human placental cells in immunocytochemistry and immunoblotting experiments, suggesting that lysines in histone H3 are biotinylated in vivo. Dimethylation of R2, R8 and R17 increased biotinylation of K4, K9 and K18, respectively, by biotinidase; phosphorylation of S10 abolished biotinylation of K9. These observations are consistent with crosstalk between biotinylation of histones and other known modifications of histones. We speculate that this crosstalk provides a link to known roles for biotin in gene expression and cell proliferation.     

组蛋白的生物素酰化修饰

组蛋白修饰对基因表达的表观遗传学调控起着重要作用。组蛋白生物素酰化修饰是近年来新发现的一种组蛋白修饰,具有重要的生物学功能。有证据表明组蛋白生物素酰化在细胞增殖、DNA修复、维持基因组稳定等方面发挥作用。组蛋白的生物素酰化修饰是由羧化全酶合成酶与组蛋白直接相互作用的结果。本文主要介绍了组蛋白生物素酰化发现的过程,并对近年来在组蛋白生物素酰化催化机制和组蛋白生物素酰化功能方面的研究进展进行了综述,最后对组蛋白生物素酰化研究领域存在的问题进行了探讨。     

UV Damage in DNA Promotes Nucleosome Unwrapping

The association of DNA with histones in chromatin impedes DNA repair enzymes from accessing DNA lesions. Nucleosomes exist in a dynamic equilibrium in which portions of the DNA molecule spontaneously unwrap, transiently exposing buried DNA sites. Thus, nucleosome dynamics in certain regions of chromatin may provide the exposure time and space needed for efficient repair of buried DNA lesions. We have used FRET and restriction enzyme accessibility to study nucleosome dynamics following DNA damage by UV radiation. We find that FRET efficiency is reduced in a dose-dependent manner, showing that the presence of UV photoproducts enhances spontaneous unwrapping of DNA from histones. Furthermore, this UV-induced shift in unwrapping dynamics is associated with increased restriction enzyme accessibility of histone-bound DNA after UV treatment. Surprisingly, the increased unwrapping dynamics is even observed in nucleosome core particles containing a single UV lesion at a specific site. These results highlight the potential for increased “intrinsic exposure” of nucleosome-associated DNA lesions in chromatin to repair proteins.     

Analysis of the biotin-binding site on acetyl-CoA carboxylase from rat

The biotin-binding site of acetyl-CoA carboxylase from rat was characterized as to its amino acid sequence and relative position in the enzyme molecule. Biotin binds to the lysyl residue in the tetrapeptide Val-Met-Lys-Met; this tetrapeptide is located in close proximity to the NH2 terminus. In all other biotin-containing enzymes, the conserved tetrapeptide Ala-Met-Lys-Met is the counterpart to that of rat acetyl-CoA carboxylase; and the lysyl residue is 35 residues from the COOH terminus. To examine the significance of these unusual features of the biotinylation site of animal acetyl-CoA carboxylase, cDNA fragments were expressed in a bacterial system and the effects of specific site-directed mutagenesis were examined. Replacement of Val by Ala in the conserved tetrapeptide abolished biotinylation of the expressed protein. However, introduction of a termination codon at residue 36, in such a way that the distance between the lysine on which biotin binds and the COOH-terminal amino acid was 35 residues and the penultimate amino acid was the hydrophobic residue leucine, increased the efficiency of biotinylation, provided a substantial portion of the NH2-terminal peptide was removed.     

The ability to develop an activity that transfers histones onto sperm chromatin is acquired with meiotic competence during oocyte growth.

Following fertilization, the oocyte remodels the sperm chromatin into the male pronucleus. As a component of this process, during meiotic maturation, oocytes develop an activity that transfers histones onto sperm DNA. To further characterize this activity, we tested whether oocytes at different stages of growth could, upon entry into metaphase of maturation, transfer histones onto sperm DNA, as judged by chromatin morphology and immunocytochemistry. Meiotically competent growing oocytes, which spontaneously enter metaphase upon culture, transferred histones onto sperm chromatin, whereas incompetent oocytes did not, even when treated with okadaic acid to induce germinal vesicle breakdown (GVBD) and chromosome condensation. When incompetent oocytes were cultured until they acquired the ability to undergo GVBD, only a small proportion also developed histone-transfer activity during maturation. However, this proportion significantly increased when the oocytes were cultured as granulosa-oocyte complexes. The failure of histone-transfer activity to develop in incompetent oocytes treated with okadaic acid was not linked to low H1 kinase activity nor rescued by injected histones. Because competent, but not incompetent, oocytes produce natural calcium oscillations, incompetent oocytes were exposed to SrCl2. One-third of treated oocytes produced at least one Ca2+ oscillation and, following insemination, the same proportion transferred histones onto sperm DNA. Histone transfer did not occur in oocytes pretreated with the Ca2+ chelator, BAPTA-AM. These results indicate that the ability to develop histone-transfer activity is acquired by growing oocytes near the time of meiotic competence, that it is separable from this event, and that it may be regulated through a Ca2+-dependent process.     

Artifactual detection of biotin on histones by streptavidin

Biotinylation is a recent addition to the list of reported posttranslational modifications made to histones. Holocarboxylase synthetase (HCS) and biotinidase have been implicated as biotinylating enzymes. However, the details of the mechanism and the regulation of biotin transfer on and off histones remains unclear. Here we report that in a cell culture system low biotin availability reduces biotinylation of carboxylases, yet apparent biotinylation of histones is unaffected. This is despite biotin depletion having detrimental effects on cell viability and proliferation. Further analysis of the widely used method for detecting biotin on histones, streptavidin blotting, revealed that streptavidin interacts with histones independently of biotin binding. Preincubation of streptavidin with free biotin reduced binding to biotinylated carboxylases but did not block binding to histones. To investigate biotinylation of histones using an alternative detection method independent of streptavidin, incorporation of 14C biotin into biotinylated proteins was analyzed. Radiolabeled biotin was readily detectable on carboxylases but not on histones, implying very low levels of biotin in the nucleus attached to histone proteins (< 0.03% biotinylation). In conclusion, we would caution against the use of streptavidin for investigating histone biotinylation.     

Effect of Protease Inhibitors on Early Events of Apoptosis

Proteolysis is an early event of apoptosis which appears to be associated with activation of the endonuclease which is responsible for internucleosomal DNA cleavage. The present study was designed to reveal the possible role of proteolysis in other early events, such as chromatin condensation, nuclear breakdown, and destabilization ofin situDNA double-stranded structure. Apoptosis of human leukemic HL-60 cells and rat thymocytes was induced by different agents, including DNA topoisomerase inhibitors, an RNA antimetabolite, and the glucocorticosteroid, prednisolone. DNA degradation was evaluated by pulsed field and conventional gel electrophoresis and by the presence ofin situDNA strand breaks. DNA stability was estimated by the measure of its sensitivityin situto denaturation. Chromatin condensation, nuclear breakdown, and other morphological changes were monitored by interference contrast and UV microscopy following cell staining with the DNA-specific fluorochrome 4′,6-diamidino-2-phenylindole. Several irreversible or reversible serine protease inhibitors prevented internucleosomal DNA degradation, nuclear breakdown, and destabilization of DNA double-stranded structure. The effective inhibitors, however, did not prevent the onset of chromatin condensation, nor the loss of the fine structural framework, nor the initial step of DNA cleavage generating DNA fragments of ≥50 kb in size. The data indicate that in both cell systems the activity of proteases sensitive to the inhibitors tested is needed for internucleosomal DNA cleavage to occur. The data also suggest that these proteases may be involved in dissolution of the nuclear envelope. Because nuclear matrix proteins and histones stabilize DNAin situ,and the decrease in DNA stability which occurs during apoptosis is precluded by the inhibitors, it is likely that serine proteases may degrade DNA stabilizing proteins. The activity of these proteases, however, appears needed neither for DNA cleavage to ≥50-kb fragments nor for the onset of chromatin condensation which is associated with dissolution of the structural framework of the nucleus.     

DNA repair in Proteus mirabilis. IV. Post-irradiation DNA degradation as influenced by a function inducible in rec+ cells.

Summary Post-irradiation DNA degradation in P. mirabilis rec ⁺ strains after UV irradiation is found to be more extensive in starvation buffer than in growth medium. In growth medium restriction of protein synthesis, but not DNA synthesis, largely prevents the expression of breakdown limitation. By the addition of chloramphenicol during post-irradiation incubation in growth medium the expression of break-down limitation was followed and found to occur 20 to 40 min after UV irradiation. Pre-irradiation by a low dose of UV leads after a corresponding time of post-irradiation incubation to breakdown limitation even in starvation buffer after a second UV exposure.Post-irradiation DNA degradation is presumed to be initiated at the sites of DNA lesions which arise at replication points damaged by UV. While pre-starvation restricts the efficiency of postirradiation DNA degradation by the reduction of the number of replication points active at the time of irradiation, caffeine as well as 2,4-dinitrophenol inhibit DNA degradation even in rec ^- cells probably by the interference with nicking or exonucleoltytic events initiated at those sites in the absence of breakdown limitation.Breakdown limitation is postulated to be due to inducible derepression of REC-functions which lead to the protection and, probably, repair of DNA lesions arising at the replication points following UV exposure.     

The phospholipid scramblase PLSCR1 increases UV induced apoptosis primarily through the augmentation of the intrinsic apoptotic pathway and independent of direct phosphorylation by protein kinase C delta

Cell death by apoptosis can be caused by the DNA mutagen UV light whose exposure causes the direct activation of both the caspase 9 regulated cell damage intrinsic pathway and the caspase 8 regulated plasma membrane extrinsic pathway. We determined that increased activity of the plasma membrane phospholipid scramblase, PLSCR1, amplified UV mediated apoptosis primarily through the activation of the intrinsic apoptotic pathway. The caspase 8 inhibitor z-IETD-fmk was not as effective an inhibitor of PLSCR1 augmented UV induced apoptosis compared to treatment with caspase 3, caspase 9, or pan-caspase inhibitors. The inability of the caspase 8 inhibitor to decrease UV induced apoptosis was dependent on PLSCR1, as UV induced apoptosis was decreased by a similar amount in the control cells in the presence of inhibitors of caspase 8, caspase 9, caspase 3, or the pan-caspase inhibitor. PKC-delta directly phosphorylates human PLSCR1 resulting in increased PLSCR1 scramblase activity. PKC-delta can also be activated by caspase mediated cleavage resulting in the release of a constitutively active kinase domain. We observed that replacing the PKC-delta phosphorylation site of PLSCR1 with an alanine did not affect the ability of PLSCR1 to enhance UV induced apoptosis implying that PKC-delta does not directly phosphorylate PLSCR1 to increase plasma membrane scramblase activity during apoptosis. Cells transfected with a PLSCR1 mutant that contained an alanine substitution at its known PKC-delta phosphorylation site underwent UV induced apoptosis at a level similar to those transfected with wild type PLSCR1. The combined results indicate that UV exposure in cells possessing PLSCR1 increases apoptosis primarily by enhancement of the intrinsic apoptotic pathway, and also imply that the increased apoptosis observed upon exposure to UV light is not through direct phosphorylation of PLSCR1 by PKC-delta.     

Marginal biotin deficiency is teratogenic

Recent studies of biotin status during pregnancy provide evidence that a marginal degree of biotin develops in a substantial proportion of women during normal pregnancy. Several lines of evidence suggest that, although the degree of biotin deficiency is not severe enough to produce the classic cutaneous and behavioral manifestations of biotin deficiency, the deficiency is severe enough to produce metabolic derangements in women and that characteristic fetal malformations occur at a high rate in some mammals. Moreover, our analysis of data from a published multivitamin supplementation study provide significant albeit indirect evidence that the marginal degree of biotin deficiency that occurs spontaneously in normal human gestation is teratogenic. Investigation of potential mechanisms provides evidence that biotin transport by the human placenta is weak. Further, proliferating cells accumulate biotin at a rate five times faster than quiescent cells; this observation suggests that there is an increased biotin requirement associated with cell proliferation. Perhaps this requirement arises from the need to synthesize additional biotin-dependent holocarboxylases or provide additional biotin as a substrate for biotinylation of cellular histones. Reduced activity of the biotin-dependent enzymes acetyl-CoA carboxylase and propionyl-CoA carboxylase can cause alterations of lipid metabolism and might theoretically lead to alterations of polyunsaturated fatty acid and prostaglandin metabolism that derange normal skeletal development.     

Ultraviolet B radiation-induced cell death: critical role of ultraviolet dose in inflammation and lupus autoantigen redistribution

The nuclear self-Ags targeted in systemic lupus erythematosus translocate to the cell membrane of UV-irradiated apoptotic keratinocytes and may represent an important source of self-immunization. It is hard to understand how the noninflammatory milieu accompanying most apoptosis might provoke an immunogenic response leading to autoantibodies. We have found that the precise amount of keratinocyte UV exposure is crucial in determining the rate of apoptosis, the amount of inflammatory cytokine production, and the degree of autoantigen translocation. Low doses of UVB (相似文献