The effects of x-irradiation on histone acetylation and methylation in cultured mammalian cells

Abstract

Random populations of mammalian cells in suspension culture with generation times ranging from 16–17 hr were subjected to 1600 rads of x-irradiation. Irradiated cells displayed a postirradiation division delay period of approximately 16 hr, during which period protein accumulation continued. DNA and histone biosynthesis ceased between 10 and 12 hr postirradiation. Approximately 15% of the irradiated population completed their first postirradiation division 16 hr after irradiation and very few were able to complete a second division. The DNA, protein and histone contents of the cell population rose during the division delay period, approximating levels attained by normal cells in the G 2 portion of their life cycle. Accumulation of labeled acetate in histone fractions F1, F2b and F3 of irradiated cells paralleled control values, while accumulation of incorporated acetate into histone fraction F2a of irradiated cells was depressed below control values between 16 and 24 hr after irradiation. Accumulation of labeled methyl groups as methyl-lysine derivatives in histone fractions F2a, F2b and F3 of irradiated cells was depressed below control values 24 hr postirradiation. No methylation of the lysine residues of histone fraction F1 was observed in either control or irradiated cultures during this period. Histone acetyltransferase and methyltransferase activity levels increased during the division delay period to follow the accumulation of cellular protein in irradiated cells. However, enzyme activity increased more rapidly than total protein, reached G 2 levels at 16 hr and decreased with resumption of cell division. These observations provide confirmatory evidence that mammalian cells irradiated under these conditions are arrested within the cell cycle in G 2 or a G 2-like state.