Dissimilar effects of β-lapachone- and hydroxyurea-induced DNA replication stress in root meristem cells of Allium cepa

Two anticancer drugs, beta-lapachone (beta-lap, a naphthoquinone) and hydroxyurea (HU, an inhibitor of ribonucleotide reductase), differently affect nuclear morphology and cell cycle control mechanisms in root meristem cells of Allium cepa. The 18 h treatment with 100 mu M beta-lap results in a lowered number of M-phase cells, increased occurrence of mitotic abnormalities, including over-condensation of chromosomes, their enhanced stickiness, formation of anaphase bridges, micronucleation and reduced mitotic spindles. Following prolonged incubations using high doses of beta-lap, cell nuclei reveal dark-red fluorescence evenly distributed in chromatin surrounding the unstained regions of nucleoli. Both drugs generate H2O2 and induce DNA double strand breaks, which is correlated with gamma-phoshorylation of H2AX histones. However, the extent of H2AX phosphorylation (including the frequency of gamma-H2AX foci and the relative number cells creating phospho-H2AX domains) is considerably reduced in root meristem cells treated jointly with the beta-lap/HU mixture. Furthermore, various effects of caffeine (an inhibitor of ATM/ATR cell cycle checkpoint kinases) on beta-lap- and HU-induced gamma-phoshorylation of H2AX histones and the protective activity of HU against beta-lap suggest that their genotoxic activities are largely dissimilar. beta-Lap treatment results in the induction of apoptosis-like programmed cell death, while HU treatment leads to cell adaptation to replication stress and promotion of abnormal nuclear divisions with biphasic interphase/mitotic states of chromatin condensation. (C) 2013 Elsevier Masson SAS. All rights reserved.