Chromosomal protein HMGN1 modulates histone H3 phosphorylation

Here we demonstrate that HMGN1, a nuclear protein that binds to nucleosomes and reduces the compaction of the chromatin fiber, modulates histone posttranslational modifications. In Hmgn1-/- cells, loss of HMGN1 elevates the steady-state levels of phospho-S10-H3 and enhances the rate of stress-induced phosphorylation of S10-H3. In vitro, HMGN1 reduces the rate of phospho-S10-H3 by hindering the ability of kinases to modify nucleosomal, but not free, H3. During anisomycin treatment, the phosphorylation of HMGN1 precedes that of H3 and leads to a transient weakening of the binding of HMGN1 to chromatin. We propose that the reduced binding of HMGN1 to nucleosomes, or the absence of the protein, improves access of anisomysin-induced kinases to H3. Thus, the levels of posttranslational modifications in chromatin are modulated by nucleosome binding proteins that alter the ability of enzymatic complexes to access and modify their nucleosomal targets.     

Chromosomal protein HMGN1 modulates the expression of N-cadherin

HMGN1 is a nuclear protein that binds to nucleosomes and alters the accessibility of regulatory factors to their chromatin targets. To elucidate its biological function and identify specific HMGN1 target genes, we generated Hmgn1-/- mice. DNA microarray analysis of Hmgn1+/+ and Hmgn1-/- embryonic fibroblasts identified N-cadherin as a potential HMGN1 gene target. RT-PCR and western blot analysis confirmed a linkage between HMGN1 expression and N-cadherin levels. In both transformed and primary mouse embryonic fibroblasts (MEFs), HMGN1 acted as negative regulator of N-cadherin expression. Likewise, the N-cadherin levels in early embryos of Hmgn1-/- mice were higher than those of their Hmgn1+/+ littermates. Loss of HMGN1 increased the adhesiveness, motility and aggregation potential of Hmgn1-/- MEFs, a phenotype consistent with increased levels of N-cadherin protein. Re-expression of wild-type HMGN1, but not of the mutant HMGN1 protein that does not bind to chromatin, in Hmgn1-/- MEFs, decreased the levels of N-cadherin and restored the Hmgn1+/+ phenotype. These studies demonstrate a role for HMGN1 in the regulation of specific gene expression. We suggest that in MEFs, and during early mouse development, the interaction of HMGN1 with chromatin down-regulates the expression of N-cadherin.     

Sodium arsenite modulates histone acetylation, histone deacetylase activity and HMGN protein dynamics in human cells

Extensive epidemiological data indicate that inorganic arsenic is associated with several types of human cancer. Nevertheless, the underlying mechanisms are poorly understood. Among its mode of action are the alterations on DNA methylation, which provoke aberrant gene expression. However, beyond DNA methylation, little is known about arsenic’s effects on chromatin. In this study, we investigated the effects of sodium arsenite (NaAsO₂) on global histone modifications and nucleosome-associated proteins. Our findings revealed that NaAsO₂ exposure significantly increases global histone acetylation. This effect was related to the inhibition of histone deacetylase (HDAC) activity because NaAsO₂ was able to inhibit HDACs comparable to the well-known HDAC inhibitor trichostatin A (TSA). Furthermore, analyses of the dynamic properties of the nucleosome-associated high mobility group N proteins demonstrate that NaAsO₂ elevates their mobility. Thus, our data suggest that NaAsO₂ induces chromatin opening by histone hyperacetylation due to HDAC inhibition and increase of the mobility of nucleosome-associated proteins. As the chromatin compaction is crucial for the regulation of gene expression as well as for genome stability, we propose that chromatin opening by NaAsO₂ may play a significant role to impart its genotoxic effects. Tzutzuy Ramirez and Jan Brocher contributed equally.     

The enhancement of histone H4 and H2A serine 1 phosphorylation during mitosis and S-phase is evolutionarily conserved

Histone phosphorylation has long been associated with condensed mitotic chromatin; however, the functional roles of these modifications are not yet understood. Histones H1 and H3 are highly phosphorylated from late G2 through telophase in many organisms, and have been implicated in chromatin condensation and sister chromatid segregation. However, mutational analyses in yeast and biochemical experiments with Xenopus extracts have demonstrated that phosphorylation of H1 and H3 is not essential for such processes. In this study, we investigated additional histone phosphorylation events that may have redundant functions to H1 and H3 phosphorylation during mitosis. We developed an antibody to H4 and H2A that are phosphorylated at their respective serine 1 (S1) residues and found that H4S1/H2AS1 are highly phosphorylated in the mitotic chromatin of worm, fly, and mammals. Mitotic H4/H2A phosphorylation has similar timing and localization as H3 phosphorylation, and closely correlates with the chromatin condensation events during mitosis. We also detected a lower level of H4/H2A phosphorylation in 5-bromo-2-deoxyuridine-positive S-phase cells, which corroborates earlier studies that identified H4S1 phosphorylation on newly synthesized histones during S-phase. The evolutionarily conserved phosphorylation of H4/H2A during the cell cycle suggests that they may have a dual purpose in chromatin condensation during mitosis and histone deposition during S-phase.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00412-004-0281-9Communicated by G. Almouzni     

p34cdc2 phosphorylation sites in histone H1 are dephosphorylated by protein phosphatase 2A1.

The protein phosphatase activity in rat liver cytosol or nuclear extracts that dephosphorylates histone H1 which has been phosphorylated by p34cdc2 is inhibited completely by okadaic acid, but unaffected by inhibitor-2 or magnesium ions, demonstrating that the only enzyme in this tissue capable of dephosphorylating this substrate is a type 2A phosphatase. Fractionation of the cytosol by anion-exchange chromatography and gel filtration demonstrated that histone H1 phosphatase activity coeluted with the major species of protein phosphatase 2A, termed PP2A1 and PP2A2. PP2A1 was the most active histone H1 phosphatase, its histone phosphatase phosphorylase phosphatase activity ratio being 6-fold higher than PP2A2 and 30-fold higher than the free catalytic subunit PP2AC. It is concluded that PP2A1 is likely to be the enzyme which dephosphorylates p34cdc2-labelled histone H1 in vivo and that the A and B subunits which interact with PP2AC in this species each play a key role in facilitating dephosphorylation of this substrate. The results demonstrate that PP2A, in addition to being involved in suppressing the activation of p34cdc2 in vivo, can also function to reverse at least one of its actions.     

Non-histone chromosomal protein HMG1 modulates the histone H1-induced condensation of DNA

Circular dichroic spectra revealed that the previously known regular, asymmetric condensation of DNA by H1 histone was modulated by HMG1, a nonhistone chromosomal protein. Under approximately physiological salt and pH conditions (150 mM NaCl, pH 7), ellipticities at 270 nm were observed as follows: DNA, 9 X 10(3) degree, cm2/dmol nucleotide; DNA X H1 histone complex (1:0.4, w/w), -37 X 10(3) degree, cm2/dmol nucleotide, and DNA X H1 X HMG1 complex (1:0.4:0.4 w/w/w), -52 X 10(3) degree, cm2/dmol. HMG1 by itself did not distort the spectrum of DNA, showing that the effect of HMG1 on the DNA X H1 complex was not simply the summation of individual effects of HMG1 and H1 on the DNA spectrum. The effect of added HMG1 on the spectrum of the preformed DNA X H1 complex depended on the amount of HMG1 added and developed slowly (a day) as if a structure required annealing. The ternary complex, DNA X HMG1 X 1, seemed to represent a specific structure, since its formation depeNded on the reduced sulfhydryl state of HMG1; the disulfide form of HMG1, which was shown by circular dichroism to contain more random coil than did the reduced form, had no effect on the circular dichroic spectrum of the DNA X H1 complex.     

Critical role of monoubiquitination of histone H2AX protein in histone H2AX phosphorylation and DNA damage response

DNA damage response is an important surveillance mechanism used to maintain the integrity of the human genome in response to genotoxic stress. Histone variant H2AX is a critical sensor that undergoes phosphorylation at serine 139 upon genotoxic stress, which provides a docking site to recruit the mediator of DNA damage checkpoint protein 1 (MDC1) and DNA repair protein complex to sites of DNA breaks for DNA repair. Here, we show that monoubiquitination of H2AX is induced upon DNA double strand breaks and plays a critical role in H2AX Ser-139 phosphorylation (γ-H2AX), in turn facilitating the recruitment of MDC1 to DNA damage foci. Mechanistically, we show that monoubiquitination of H2AX induced by RING finger protein 2 (RNF2) is required for the recruitment of active ataxia telangiectasia mutated to DNA damage foci, thus affecting the formation of γ-H2AX. Importantly, a defect in monoubiquitination of H2AX profoundly enhances ionizing radiation sensitivity. Our study therefore suggests that monoubiquitination of H2AX is an important step for DNA damage response and may have important clinical implications for the treatment of cancers.     

Chromatin-associated protein phosphatase 1 regulates aurora-B and histone H3 phosphorylation

Proper chromosome condensation requires the phosphorylation of histone and nonhistone chromatin proteins. We have used an in vitro chromosome assembly system based on Xenopus egg cytoplasmic extracts to study mitotic histone H3 phosphorylation. We identified a histone H3 Ser(10) kinase activity associated with isolated mitotic chromosomes. The histone H3 kinase was not affected by inhibitors of cyclin-dependent kinases, DNA-dependent protein kinase, p90(rsk), or cAMP-dependent protein kinase. The activity could be selectively eluted from mitotic chromosomes and immunoprecipitated by specific anti-X aurora-B/AIRK2 antibodies. This activity was regulated by phosphorylation. Treatment of X aurora-B immunoprecipitates with recombinant protein phosphatase 1 (PP1) inhibited kinase activity. The presence of PP1 on chromatin suggested that PP1 might directly regulate the X aurora-B associated kinase activity. Indeed, incubation of isolated interphase chromatin with the PP1-specific inhibitor I2 and ATP generated an H3 kinase activity that was also specifically immunoprecipitated by anti-X aurora-B antibodies. Nonetheless, we found that stimulation of histone H3 phosphorylation in interphase cytosol does not drive chromosome condensation or targeting of 13 S condensin to chromatin. In summary, the chromosome-associated mitotic histone H3 Ser(10) kinase is associated with X aurora-B and is inhibited directly in interphase chromatin by PP1.     

Competition between histone H1 and HMGN proteins for chromatin binding sites

The ability of regulatory factors to access their nucleosomal targets is modulated by nuclear proteins such as histone H1 and HMGN (previously named HMG-14/-17 family) that bind to nucleosomes and either stabilize or destabilize the higher-order chromatin structure. We tested whether HMGN proteins affect the interaction of histone H1 with chromatin. Using microinjection into living cells expressing H1–GFP and photobleaching techniques, we found that wild-type HMGN, but not HMGN point mutants that do not bind to nucleosomes, inhibits the binding of H1 to nucleosomes. HMGN proteins compete with H1 for nucleosome sites but do not displace statically bound H1 from chromatin. Our results provide evidence for in vivo competition among chromosomal proteins for binding sites on chromatin and suggest that the local structure of the chromatin fiber is modulated by a dynamic interplay between nucleosomal binding proteins.     

MAP kinases mediate UVB-induced phosphorylation of histone H3 at serine 28

Histone H3 phosphorylation is related closely to chromatin remodeling and chromosome condensation. H3 phosphorylation at serine 28 is coupled with mitotic chromosome condensation in diverse mammalian cell lines. However, the pathway that mediates phosphorylation of H3 at serine 28 is unknown. In the present study, ERK1, ERK2, or p38 kinase strongly phosphorylated H3 at serine 28 in vitro. JNK1 or JNK2 was able also to phosphorylate H3 at serine 28 in vitro but to a lesser degree. UVB irradiation markedly induced phosphorylation of H3 at serine 28 in JB6 Cl 41 cells. PD 98059, a MEK1 inhibitor, and SB 202190, a p38 kinase inhibitor, efficiently repressed UVB-induced H3 phosphorylation at serine 28. Expression of dominant negative mutant (DNM) ERK2 in JB6 Cl 41 cells totally blocked UVB-induced phosphorylation of H3 at serine 28. Additionally, DNM p38 kinase or DNM JNK1 partially blocked UVB-induced H3 phosphorylation at serine 28. Furthermore, UVB-induced H3 phosphorylation at serine 28 was inhibited in Jnk1(-/-) cells but not in Jnk2(-/-) cells. These results suggest that UVB-induced H3 phosphorylation at serine 28 may be mediated by mitogen-activated protein kinases.     

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.     

Initiation of DNA fragmentation during apoptosis induces phosphorylation of H2AX histone at serine 139

Histone H2AX is a ubiquitous member of the H2A histone family that differs from the other H2A histones by the presence of an evolutionarily conserved C-terminal motif, -KKATQASQEY. The serine residue in this motif becomes rapidly phosphorylated in cells and animals when DNA double-stranded breaks are introduced into their chromatin by various physical and chemical means. In the present communication we show that this phosphorylated form of H2AX, referred to as gamma-H2AX, appears during apoptosis concurrently with the initial appearance of high molecular weight DNA fragments. gamma-H2AX forms before the appearance of internucleosomal DNA fragments and the externalization of phosphatidylserine to the outer membrane leaflet. gamma-H2AX formation is inhibited by N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone and the inhibitor of caspase-activated DNase, and it is induced when DNase I and restriction enzymes are introduced into cells, suggesting that any apoptotic endonuclease is sufficient to induce gamma-H2AX formation. These results indicate that gamma-H2AX formation is an early chromatin modification following initiation of DNA fragmentation during apoptosis.     

MST1 promotes apoptosis through phosphorylation of histone H2AX

MST1 (mammalian STE20-like kinase 1) is a serine/threonine kinase that is cleaved and activated by caspases during apoptosis. Overexpression of MST1 induces apoptotic morphological changes such as chromatin condensation, but the mechanism is not clear. Here we show that MST1 induces apoptotic chromatin condensation through its phosphorylation of histone H2AX at Ser-139. During etoposide-induced apoptosis in Jurkat cells, the cleavage of MST1 directly corresponded with strong H2AX phosphorylation. In vitro kinase assay results showed that MST1 strongly phosphorylates histone H2AX. Western blot and kinase assay results with a mutant S139A H2AX confirmed that MST1 phosphorylates H2AX at Ser-139. Direct binding of MST1 and H2AX can be detected when co-expressed in HEK293 cells and was also confirmed by an endogenous immunoprecipitation study. When overexpressed in HeLa cells, both the MST1 full-length protein and the MST1 kinase domain (MST1-NT), but not the kinase-negative mutant (MST1-NT-KN), could induce obvious endogenous histone H2AX phosphorylation. The caspase-3 inhibitor benzyloxycarbonyl-DEVD-fluoromethyl ketone (Z-DEVD-fmk) attenuates phosphorylation of H2AX by MST1 but cannot inhibit MST1-NT-induced histone H2AX phosphorylation, indicating that cleaved MST1 is responsible for H2AX phosphorylation during apoptosis. Histone H2AX phosphorylation and DNA fragmentation were suppressed in MST1 knockdown Jurkat cells after etoposide treatment. Taken together, our data indicated that H2AX is a substrate of MST1, which functions to induce apoptotic chromatin condensation and DNA fragmentation.     

Contribution of the serine 129 of histone H2A to chromatin structure

Phosphorylation of a yeast histone H2A at C-terminal serine 129 has a central role in double-strand break repair. Mimicking H2A phosphorylation by replacement of serine 129 with glutamic acid (hta1-S129E) suggested that phosphorylation destabilizes chromatin structures and thereby facilitates the access of repair proteins. Here we have tested chromatin structures in hta1-S129 mutants and in a C-terminal tail deletion strain. We show that hta1-S129E affects neither nucleosome positioning in minichromosomes and genomic loci nor supercoiling of minichromosomes. Moreover, hta1-S129E has no effect on chromatin stability measured by conventional nuclease digestion, nor does it affect DNA accessibility and repair of UV-induced DNA lesions by nucleotide excision repair and photolyase in vivo. Similarly, deletion of the C-terminal tail has no effect on nucleosome positioning and stability. These data argue against a general role for the C-terminal tail in chromatin organization and suggest that phosphorylated H2A, gamma-H2AX in higher eukaryotes, acts by recruitment of repair components rather than by destabilizing chromatin structures.