VARIATION OF H1(0) CONTENT THROUGHOUT THE CELL-CYCLE IN REGENERATING RAT-LIVER

Histone H1(0), a differentiation-specific member of the histone H1 fam ily, accumulates in cells during the terminal phase of cell differenti ation, in tissues composed of arrested cells or cells exhibiting littl e proliferation. Moreover, the induction of cell proliferation in. viv o, i.e., after partial hepatectomy, is accompanied by a decrease in H1 (0) content. These observations suggest that H1(0) may be involved in the arrest of cell proliferation in vivo. In order to investigate this possibility, we took advantage of the fact that after partial hepatec tomy the initiation of cell division is not synchronous. The strategy was to know, at the level of a single cell, whether H1(0) decreases pr ior to the initiation of the S phase or whether a cell can initiate DN A replication having a significant amount of H1(0) in the nucleus. We defined new protocols to analyze H1(0) content and cell proliferation at the level of a single cell, both in situ and by flow cytometry. The simultaneous determination of the relative amount of H1(0) and the po sition of cells in the cell cycle showed that no significant differenc e in H1(0) content was detected in cells actively replicating their DN A compared to nondividing cells. These observations have been confirme d by the successive immunodetections of H1(0) and BrdU in situ on the same cells. Therefore, we show here that in vivo, cells can initiate D NA replication with significant amounts of H1(0) and that the decrease of H1(0) is not a prerequisite of cell division. We propose that the accumulation of H1(0) is not related to the arrest of cell proliferati on, but is controlled in such a manner that the protein accumulates in slowly dividing cells and decreases in rapidly growing cells. (C) 199 5 Academic Press, Inc.