Response of CHO cell proliferation and histone phosphorylation to sodium arsenite

Abstract

Because of the causal association between arsenic and cancer in humans, studies were initiated to determine whether low levels of arsenic affect cell proliferation and chromatin constituents. Cultured CHO cells were treated with As(lll) or As(V) in the form of sodium arsenite (NaAsO₂) or sodium arsenate (Na₂HAsO₄), respectively. Arsenite was 10 times more toxic than arsenate, 50 μM arsenite causing irreversible inhibition of culture growth. At lower concentrations, 10 μM NaAsO_i induced culture growth kinetics that were radiomimetic‐that is, caused a cell division delay period followed by spontaneous recovery of cell division in the continued presence of the arsenite. During this division delay, histone HI phos‐phorylation was reduced to 40% of the control level. It recovered to the control level just before recovery of cell division. Histone H2A phosphorylation was stimulated in the middle of the delay period, followed by stimulation of H3 phosphorylation just before recovery of cell division. A TP pool levels were unaffected at these low arsenite concentrations. The DNA synthesis rate was also reduced during division delay, but it did not recover immediately before resumption of cell division. Thus, arsenite uncoupled H1 phosphorylation from DNA synthesis. Flow microfluorometric analysis showed that cell cycle progression was inhibited in G₁, S, and G₂ during the division delay period. On recovery of cell division, cells in S and G₂ regained the capacity to resume cycle traverse earlier than did cells in G₁. Thus, part of the reduced rate of DNA synthesis observed on resumption of cell division results from the failure of G₁ cells to resume initiation of S phase at the same time that S and G₂ cells begin cell cycle traverse. Cell survival studies indicated that 89% of the cells survived 24‐h treatment with 10 μM NaAsO₂. The spontaneous recovery of cell proliferation and high survival rate of arsenite‐treated cells indicate a need to determine whether the perturbations in histone and DNA metabolism caused by arsenite result in any genetic damage that could be transferred to future cell generations.