A comparison of hepatocyte cytotoxic mechanisms for chromate and arsenite

In the following, we have compared the cytotoxic mechanisms of the chromate CrO42− and arsenite AsO2−. Chromate (Cr (VI)) cytotoxicity was associated with reactive oxygen species (ROS) formation, lipid peroxidation and loss of mitochondrial membrane potential, which were prevented by catalase, antioxidants and ROS scavengers. Hepatocyte glutathione was also rapidly oxidized. Chromate reduction was inhibited in glutathione depleted hepatocytes, and glutathione depleted hepatocytes were also much more resistant to chromate induced cytotoxicity, ROS formation and lipid peroxidation. This suggests that chromate is reductively activated by glutathione. Chromate cytotoxicity also involved lysosomal injury and protease activation, which were prevented by lysosomotropic agents, endocytosis inhibitors, protease inhibitors and ROS scavengers. On the other hand, arsenite cytotoxicity was associated with much less oxidative stress, and lysosomal damage did not occur. However, arsenite cytotoxicity was also associated with loss of mitochondrial membrane potential, which in contrast to chromate cytotoxicity was inhibited by the ATP generators fructose, xylitol and glutamine. Arsenite induced cytotoxicity, mitochondrial membrane potential decline and also ROS formation were significantly increased by inactivating hepatocyte methionine synthase or hepatocyte methyl transferase. However, methyl donors such as betaine, methionine or folic acid prevented arsenite but not chromate cytotoxicity, and this suggests that arsenite is detoxified by reductive methylation. In conclusion, chromate induced cytotoxicity could be attributed to oxidative stress and lysosomal damage, whereas arsenite induced cytotoxicity could be attributed to mitochondrial toxicity and ATP depletion.