The Secret Life of Histones

Molecular and Cellular Functions of the Linker Histone H1.2

Linker histone H1.2, which belongs to the linker histone family H1, plays a crucial role in the maintenance of the stable higher-order structures of chromatin and nucleosomes. As a critical part of chromatin structure, H1.2 has an important function in regulating chromatin dynamics and participates in multiple other cellular processes as well. Recent work has also shown that linker histone H1.2 regulates the transcription levels of certain target genes and affects different processes as well, such as cancer cell growth and migration, DNA duplication and DNA repair. The present work briefly summarizes the current knowledge of linker histone H1.2 modifications. Further, we also discuss the roles of linker histone H1.2 in the maintenance of genome stability, apoptosis, cell cycle regulation, and its association with disease.

Altered expression of nuclear and cytoplasmic histone H1 in pulmonary artery and pulmonary artery smooth muscle cells in patients with IPAH

The pathogenesis of idiopathic pulmonary hypertension is poorly understood. This paper utilized histology-based Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI MS) to identify as-yet unknown proteins that may be associated with the structural changes in the pulmonary arterial walls of patients with IPAH. The technology identified significant increases in two fragments of histone H1 in the IPAH cases compared to controls. This finding was further examined using immunofluorescence techniques applied to sections from IPAH and control pulmonary arteries. In addition, cultured pulmonary artery smooth muscle cells (PASMCs) were utilized for Western analysis of histone H1 and importin β and importin 7, immunoprecipitation and assessment of nucleosomal repeat length (NRL). Immunofluorescence techniques revealed that nuclear expression of histone H1 was decreased and the chromatin was less compact in the IPAH cases than in the controls; furthermore, some cases showed a marked increase in cytoplasmic histone H1 expression. Using nuclear and cytoplasmic fractions of cultured PASMCs, we confirmed the reduction in histone H1 in the nucleus and an increase in the cytoplasm in IPAH cells compared to controls. Immunoprecipitation demonstrated a decreased association of histone H1 with importin β while importin 7 was unchanged in the IPAH cells compared to controls. The assessment of NRL revealed that the distance between nucleosomes was increased by ~20 bp in IPAH compared to controls. We conclude that at least two factors contribute to the reduction in nuclear histone H1-fragmentation of the protein and decreased import of histone H1 into the nucleus by importins. We further suggest that the decreased nuclear H1 contributes the less compact nucleosomal pattern in IPAH and this, in turn, contributes to the increase in NRL.

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

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

Nuclear proteins acting on mitochondria

An important mechanism in apoptotic regulation is changes in the subcellular distribution of pro- and anti-apoptotic proteins. Among the proteins that change in their localization and may promote apoptosis are nuclear proteins. Several of these nuclear proteins such as p53, Nur77, histone H1.2, and nucleophosmin were reported to accumulate in the cytosol and/or mitochondria and to promote the mitochondrial apoptotic pathway in response to apoptotic stressors. In this review, we will discuss the functions of these and other nuclear proteins in promoting the mitochondrial apoptotic pathway, the mechanisms that regulate their accumulation in the cytosol and/or mitochondria and the potential role of Bax and Bak in this process. This article is part of a Special Issue entitled Mitochondria: the deadly organelle.

Post-translational modifications of the linker histone variants and their association with cell mechanisms

In recent years, a considerable amount of research has been focused on establishing the epigenetic mechanisms associated with DNA and the core histones. This effort is driven by the fact that epigenetics is intimately involved with genomics in a whole range of molecular processes. However, there is now a consensus that the epigenetics of the linker histones are just as important. The result of that consensus is that the post-translational modifications (PTMs) for most of the linker histone variants in human and mouse have now been established by a number of experimental techniques, foremost of which is mass spectrometry (MS). MS was also used by our group to establish the PTMs of the linker histone variants in chicken erythrocytes. Although it is now known which types of PTM occur at particular locations on the linker histone variants, there is still a large gap in the knowledge of how this data relates to function. The focus of this review is an analysis of the PTM data for the linker histones from several species, but with an emphasis on human, mouse, and chicken. Our analysis reveals that certain PTMs can be clearly correlated with specific functions of the linker histones in particular cell types, and that unique PTM patterns exist for different cell types.

Larger than life: Mitochondria and the Bcl-2 family

The intrinsic pathway of apoptosis relies on mitochondrial membrane permeabilization, with Bcl-2 proteins serving as its master regulators. They form a complex network of interactions both within the family and with multiple cellular factors outside the family. The understanding of the processes that regulate mitochondrial breach, and mechanisms that direct the pro- and anti-apoptotic functions of Bcl-2 proteins, should assist the development of novel anticancer therapies. Thus, it is of no surprise that research in the field is gaining momentum. In this review we outline the current concepts on regulatory circuits governing mitochondrial rupture and action of Bcl-2 proteins during cell death, and how this burgeoning knowledge is being translated into the clinics with the hope to combat cancer.

DNA-PK phosphorylates histone H2AX during apoptotic DNA fragmentation in mammalian cells

The phosphorylation of histone H2AX at serine 139 is one of the earliest responses of mammalian cells to ionizing radiation-induced DNA breaks. DNA breaks are also generated during the terminal stages of apoptosis when chromosomal DNA is cleaved into oligonucleosomal pieces. Apoptotic DNA fragmentation and the consequent chromatin condensation are important for efficient clearing of genomic DNA and nucleosomes and for protecting the organism from auto-immmunization and oncogenic transformation. In this study, we demonstrate that H2AX is phosphorylated during apoptotic DNA fragmentation in mouse, Chinese hamster ovary, and human cells. We have previously shown that ataxia telangiectasia mutated kinase (ATM) is primarily responsible for H2AX phosphorylation in murine cells in response to ionizing radiation. Interestingly, we find here that DNA-dependent protein kinase (DNA-PK) is solely responsible for H2AX phosphorylation during apoptosis while ATM is dispensable for the process. Moreover, the kinase activity of DNA-PKcs (catalytic subunit of DNA-PK) is specifically required for the induction of gammaH2AX. We further show that DNA-PKcs is robustly activated in apoptotic cells, as evidenced by autophosphorylation at serine 2056, before it is inactivated by cleavage. In contrast, ATM is degraded well before DNA fragmentation and gammaH2AX induction resulting in the predominance of DNA-PK during the later stages of apoptosis. Finally, we show that DNA-PKcs autophosphorylation and gammaH2AX induction occur only in apoptotic nuclei with characteristic chromatin condensation but not in non-apoptotic nuclei from the same culture establishing the most direct link between DNA fragmentation, DNA-PKcs activation, and H2AX phosphorylation. It is well established that DNA-PK is inactivated by cleavage late in apoptosis in order to forestall DNA repair. Our results demonstrate, for the first time, that DNA-PK is actually activated in late apoptotic cells and is able to initiate an early step in the DNA-damage response, namely H2AX phosphorylation, before it is inactivated by proteolysis.

Activity of histone H1.2 in infected burn wounds

OBJECTIVES

Infections with multidrug-resistant microorganisms (e.g. Pseudomonas aeruginosa and Staphylococcus aureus) cause immense complications in wound care and in the treatment of immunosuppressed patients. Like most antimicrobial peptides, histones are relatively small polycationic proteins located in each eukaryotic nucleus, which naturally supercoil DNA. The aim of this study was to investigate the in vitro and in vivo activity of histone H1.2 in infected burn wounds and its potential toxicity.

METHODS

To characterize the antimicrobial properties of histone H1.2 against potential causative organisms of burn wound infections, the in vitro radial diffusion assay and modified NCCLS microbroth dilution MIC assay were carried out. Haemolytic and cytotoxic properties were determined in human red blood cells and primary human keratinocytes. In vivo antimicrobial activity was tested in an infected rat burn model with P. aeruginosa (ATCC 27853). All results were compared with the naturally occurring broad-spectrum antimicrobial peptide protegrin-1 and with antibiotics clinically used against the corresponding bacteria.

RESULTS

Human histone H1.2 exerted good antimicrobial activity against all tested microorganisms without significant haemolytic activity. Surprisingly, histone H1.2 showed cytotoxicity with an LD50 of 7.91 mg/L in primary human keratinocytes. The in vivo burn model data revealed a significant three-fold higher reduction in bacterial counts within 4 h compared with carrier control.

CONCLUSIONS

These findings indicate that histone H1.2 is a potential candidate for use as a local and, because of its low haemolytic activity, systemic antimicrobial agent. However, further investigations are needed to specify the cytotoxicity and the dose-response relationship for histone H1.2.

DNA damage-induced activation of ATM and ATM-dependent signaling pathways

Ataxia-telangiectasia mutated (ATM) plays a key role in regulating the cellular response to ionizing radiation. Activation of ATM results in phosphorylation of many downstream targets that modulate numerous damage response pathways, most notably cell cycle checkpoints. In this review, we describe recent developments in our understanding of the mechanism of activation of ATM and its downstream signaling pathways, and explore whether DNA double-strand breaks are the sole activators of ATM and ATM-dependent signaling pathways.

Chromatin modifiers and carcinogenesis

Access of gene regulatory factors to the eukaryotic genome is modulated by chromatin. The organization of this nucleoprotein complex is highly dynamic and tightly regulated. The control of wide-ranging nuclear processes through the configuration of chromatin is achieved by the concerted actions of ATP-dependent chromatin-remodeling complexes and histone-modifying enzymes, and by the incorporation of specialized histone variants. It is becoming clear that perturbation of these chromatin modifiers can lead to cancer. Recent findings illustrate the mechanisms by which chromatin influences cancer development, and aid understanding of the regulation of chromatin organization, cellular transformation and the connections between tumor suppressor and oncogene function.