Abrogation of cell cycle checkpoint controls during malignant transformation of syrian hamster embryo cells is associated with decreased sensitivity to apoptosis

Abstract

Malachite green (MG), consisting of green crystals with a metallic luster, is highly soluble in water, cytotoxic to various mammalian cells, and may act as a liver tumor promoter. In view of its industrial importance and possible exposure to human beings, MG poses a potential environmental health hazard. We have reported earlier the malignant transformation of Syrian hamster embryo (SHE) cells by MG. In this study, we investigated the effects of MG on flow cytometric cell cycle phase distribution of normal and MG-transformed SHE cells in asynchronous and synchronous cell populations. DNA flow cytometric analysis indicated that culturing cells for 48 hours in a medium containing MG 0.1 microg/mL induced G2/M arrest in normal cells. Malignant-transformed cells showed no such accumulation of cells at the G2/M phase of the cell cycle in response to MG. Synchronization studies indicated that in the control, both in the presence and absence of MG, cells followed a normal cell cycle pattern up to 16 hours. After 16 hours, in the absence of MG, cells continued a normal cell cycle, whereas in the presence of MG they accumulated at the G2/M phase of the cell cycle. This pattern of accumulation of cells at the G2/M checkpoint control was not observed in either untreated or MG-treated transformed cells. We also studied the effects of MG on the induction of apoptosis using flow cytometric FSC/SSC scatter plots in normal and transformed SHE cells. Flow cytometric analysis showed a dose- and time-dependent induction of apoptosis by MG in control cells, whereas induction of apoptosis by MG was marginal in transformed cells. In the present study, we demonstrated the efficient operation of the G2/M checkpoint control, apoptosis in control SHE cells, the abrogation of checkpoint controls, and decreased sensitivity to apoptosis in transformed SHE cells.