Selective in vitro methylation of rat chromatin associated histone after partial hepatectomy

H1.0 Linker Histone as an Epigenetic Regulator of Cell Proliferation and Differentiation

H1 linker histones are a class of DNA-binding proteins involved in the formation of supra-nucleosomal chromatin higher order structures. Eleven non-allelic subtypes of H1 are known in mammals, seven of which are expressed in somatic cells, while four are germ cell-specific. Besides having a general structural role, H1 histones also have additional epigenetic functions related to DNA replication and repair, genome stability, and gene-specific expression regulation. Synthesis of the H1 subtypes is differentially regulated both in development and adult cells, thus suggesting that each protein has a more or less specific function. The somatic variant H1.0 is a linker histone that was recognized since long ago to be involved in cell differentiation. Moreover, it has been recently found to affect generation of epigenetic and functional intra-tumor heterogeneity. Interestingly, H1.0 or post-translational forms of it have been also found in extracellular vesicles (EVs) released from cancer cells in culture, thus suggesting that these cells may escape differentiation at least in part by discarding H1.0 through the EV route. In this review we will discuss the role of H1.0 in development, differentiation, and stem cell maintenance, also in relation with tumorigenesis, and EV production.

Immunohistochemical demonstration of histone H10 in human breast carcinoma

Histone H1(0) is a linker histone subvariant present in tissues of low proliferation rate. It is supposed to participate in the expression and maintenance of the terminal differentiation phenotype. The aim of this work was to study histone H1(0) distribution in human breast carcinoma and its relationship with the processes of proliferation and differentiation. Most of the cells in carcinomas of moderate and high level of differentiation expressed histone H1(0) including cells invading connective and adipose tissues. In low differentiated tumours, the number of H1(0) expressing cells was considerably lower. Staining of myoepithelial cells, when seen, and of stromal fibroblasts was variable. The metastatic malignant cells in the lymph nodes also accumulated H1(0) but lymphocytes were always negative. All immunopositive malignant cells exhibited signs of polymorphism. Double H1(0)/Ki-67 staining showed that the growth fraction in more differentiated tumours belonged to the H1(0)-positive cells, while in poorly differentiated carcinomas it also included a cell subpopulation not expressing H1(0). If expressed, p27Kip1 was always found in H1(0)-positive cells. These findings are inconsistent with the widespread view that histone H1(0) is expressed only in terminally differentiated cells. Rather, they suggest that the protein is expressed in cells in a prolonged intermitotic period irrespective of their level of differentiation. Double H1(0)/Ki-67 immunostaining could be a useful tool in studying the growth fraction in tumours.

Modulation in proportions of H1 histone subfractions by differential changes in synthesis and turnover during butyrate treatment of neuroblastoma cells

Mouse neuroblastoma cells treated with millimolar concentrations of butyrate adjusted their recipe of histone H1 subfractions over the course of several days, eventually attaining an enrichment of about 5-fold in H1(0). This adjustment in the proportions of the H1's, which was essentially complete by 4 days, was brought about by changes in synthesis and turnover that were different for each of the three H1 subfractions. As the cells stopped dividing, the synthesis of all histones slowed substantially, but core histones were affected more than the H1's. Transiently, therefore, there was an overproduction of H1's relative to core histones, but the excess H1 was eventually removed by turnover. The very slow turnover of H1(0) and the rapid turnover of H1c were not substantially affected by butyrate treatment, but the turnover of H1ab was greatly accelerated by butyrate. Acetylation of the core histones was not necessary for maintenance of elevated H1(0) levels in the nondividing cells, although we did not rule out its involvement in the initial accumulation of H1(0).

Dynamics of H1(0) content in rat liver after partial hepatectomy

The changes in the lysine-rich histone subfraction H1(0) have been quantitatively studied in rat liver during the regeneration period after partial hepatectomy. A gradual decrease in this protein was found early after operation with a minimal value around the time of maximal mitotic activity. The reduction in the H1(0) content paralleled well the increasing number of cells in the cell cycle.

Conformation studies of histone H1(0) in comparison with histones H1 and H5

The class of lysine-rich histones, H1, found in most eukaryotic cells is largely replaced by another class of lysine-rich histones, H5, in avian and other erythrocytes. Erythrocytes are transcriptionally inert and this state has been attributed to the presence of H5. Although there are many sequence differences between H1 and H5 both molecules have very similar structures with three well-defined domains: a flexible basic N-terminal region, an apolar globular central region and a flexible basic C-terminal region. The lengths of the N-terminal regions are different for H1 and H5 whereas the lengths of the central and C-terminal regions are very similar. Considerable interest attaches to the findings that another type of mammalian lysine-rich histone H1(0) has an apolar region exhibiting considerable sequence homology (70%) with the central globular region of H5. The abundance of H1 in cells has been found to correlate inversely with their mitotic activities. Conformational studies using high-resolution nuclear magnetic resonance and optical spectroscopy have been made of H1 and its conformational behaviour has been compared with those of H1 and H5. H1 has been found to contain a central globular region of similar size to those found in H1 and H5. However, the conformation and stability of the globular domain of H1 are very similar to the globular region of H5 rather than H1. H1 appears to be a hybrid containing a major feature of the H5 histone. The globular regions of H1 and H5 are known to bind to a specific site on the nucleosome sealing off two turns of DNA. It is proposed that H1 binds to the same site.

Effect of L-ethionine on macromolecular synthesis in mitogen-stimulated lymphocytes

2--4 mM L-ethionine completely inhibits DNA synthesis in phytohaemagglutinin- or concanavalin A-stimulated lymphocytes even though it does not prevent the morphological changes characteristic of blast formation. Evidence is presented which indicates that complete commitment to DNA synthesis as well as a substantial increase in the rates of RNA and protein synthesis can occur in the presence of ethionine. Ethionine, however, does inhibit methylation of tRNA and prevents mitogen-induced increase in the activity of histone-modifying enzymes. All of these effects of exposure to ethionine are completely reversible. Removal of ethionine after 24 h or more of exposure results in a rapid, synchronous wave of DNA synthesis, an increase in the rate of methylation of RNA and an increase in activity of histone-modifying enzymes.

Pineal protein kinase: effect of enzymic phosphorylation on actinomycin D binding by, and template activity of, chromatin

The ability of protein kinase from bovinepineal gland to phosphorylate calf-thymus chromatin and thereby to alter the association between chromatin DNA and histones was investigated. Phosphorylation of calf-thymus chromatin by pineal protein kinase results in an apparent decreased binding between the histones and DNA in chromatin, as indicated by (i) an increase in actinomycin D-binding sites after phosphorylation and (ii) an increase in the template capacity of the calf-thymus chromatin after phosphorylation. F(1) histone and F(3) histone are the major histone classes in the chromatin that are phosphorylated by the protein kinase. These results support the hypothesis that pineal protein kinase may function at the transcriptional level.