SO₂-induced Yeast Cell Apoptosis And Its Regulation

Sulfur dioxide (SO2) is a common air pollutant, SO2at high concentrations in environment are toxic to humer, animals and plants. When entered into cells, SO2became SO32-and HSO3-, which had harmful effects to tissue, cells and biomacromolecules. SO2and its derivatives can restrain the cell division, cause genetic damage, increase the cell count in root which have nuclear or chromosome aberration, at last appear micronucleus and cell death. SO2can cause the content of ROS increasing in plant tissue, induce expressing the antioxidant enzymes genes and protective genes.The results of the present study indicated that ROS in cells activated Ca2+channels in plasma membrane, lead to Ca2+influx and intracellular Ca2+level increase, and mediated cell death. However, the toxic effects of SO2to microbe have not been reported.Saccharomyces cerevisiae has been investigated intensively as it has simple, well understood genetics and is amenable to modification by gene technology. The related research indicated, environmental physical or chemical factors such as UV-B, high osmotic, hydrogen(H2O2) and NaCl can induce yeast cell apoptosis. In this study, the effects of SO2derivatives (a mixture of sodium sulfite and sodium bisulfite,3:1, mmol·L-1) on cell death in yeast were analyzed.The expermential results showed that SO2derivatives induced cell death in a concentration-and time-dependent manner, and cell death rate increased after yeast cells exposed to25to100mmol·L-1, in which ROS generation, Ca2+level increase and caspase-like proteases taken part in the death procedure and played important roles. Detected with ROS fluorescence probe DCFH-DA, Ca2+fluorescence probe Fluo-3AM and NO fluorescence probe DAF-FM DA, the intracellular ROS and Ca2+levels significantly increased in yeast cells exposed to50mmol·L-1SO2derivatives compared to the untreated cells. But NO levels have little variations. The death rate decreased in yeast cells after exposure to SO2derivatives in the presence of1mmol·L-1AsA or 0.5mmol·L-1EGTA (Ca2+chelator), respectively, which indicated that intracellular ROS and Ca2+play important roles in SO2-induced cell death. Treated with100,250and500mmol·L-1CaCl2respectively, yeast cells also dead.Bcl-2and Ced-9are two of the most important antiapoptotic members in mammals and nematodes, respectively. They can prevent many forms of apoptosis in a variety of cell types. In our study, SO2-induced yeast cell death could be alleviated by Bcl-2and Ced-9, which indirectly supported the idea that SO2-induced cell death occurred in the PCD pathway. In this study, we also found that SO2-induced cell death is accompanied by△(?)m disruption and cyt c released. Moreover,2umol·L-1caspase inhibitors Z-Asp-CH2-DCB could significantly reduce SO2-induced cell death, demonstrating that SO2induced mitochondrial-mediated caspase-dependent cell death. The death rate decreased in Ced-9yeast cells after exposure to100mmol·L-1SO2derivatives in the presence of1mmol·L-1AsA or0.5mmol·L-1EGTA. However, the death rate did not decrease in Bcl-2yeast cells after exposure to100mmol·L-1SO2derivatives in the presence of1mmol·L-1AsA, but it significantly decreased after exposure to100mmol·L-1SO2derivatives in the presence of0.5mmol·L-1EGTA.The results of this study indicated that SO2-induced yeast cell death at high concentrations. SO2exposure induced intracellular ROS generation. Increased ROS level in yeast cells activated Ca2+channels in plasma membrane, lead to Ca2+influx and intracellular Ca2+level increase, and mediated a series of signal responses to regulate stomatal aperture and cell death. SO2-induced yeast cell death could be alleviated by Bcl-2, Ced-9and caspase inhibitors. So our results also suggest that SO2exposure can induce yeast cell apoptosis, and caspase-like proteases in leaf guard cells may involve in the procedure.