Histone H1(0): a maintainer of the differentiated cell state?

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.

Effects of antineoplastic phospholipids on parameters of cell differentiation in U937 cells

The proliferation of the human promonocytic leukemia cell line U937 is inhibited by several ether lipids, ether lipid analogues and by phorbol esters. An early effect of this retardation of cell growth is the induction of a basic chromosomal protein, histone H1(0). Northern blot analysis of H1(0) mRNA levels reveals an increase of the mRNA concentration within a few hours after addition of hexadecylphosphocholine and 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine. This early effect on the synthesis of a subtype of H1 proteins precedes the expression of several parameters of the monocytic differentiation of U937 cells.

The expression of the histone H1 (0) gene in the human hepatoma cell line HepG2 is independent of the state of cell proliferation

The H1 histone subtype H1 (0) is a characteristic component of the chromatin of several mammalian tissues. Since H1 (0) is synthesized in nondividing cells upon terminal differentiation, it has been mostly considered either as a prerequisite for or as a consequence of an arrest of DNA replication during the process of differentiation. In several H1 (0)-expressing systems studied until now, inducers of differentiation or inhibitors of DNA synthesis cause an increase of the ratio between H1 (0) and the other H1 proteins. We have studied the steady-state levels of histone H1 (0) mRNA under varied growth conditions in the human hepatoma cell lines HepG2 and Hep3B, and we show in the HepG2 system that H1 (0) is not confined to resting cells, that the H1 (0) gene appears to be expressed throughout the cell cycle and that established inducers of de novo H1 (0) synthesis fail to cause a further increase of the high H1 (0) level. This constitutive expression of H1 (0) appears to reflect the chromatin structure of the liver cells, from which the HepG2 hepatoblastoma cells initially may have evolved. In contrast to the situation in nondividing adult liver cells, the H1 (0) gene is transcribed in HepG2 at a high level, and this expression is compatible with DNA replication.