Mitochondria, reactive oxygen species and cadmium toxicity in the kidney

The heavy metal cadmium accumulates in kidney cells, particularly those of the proximal tubular epithelium, and the damage this causes is associated with development of chronic kidney disease. One of the causative mechanisms of chronic kidney disease is thought to be oxidative stress. Cadmium induces oxidative stress, but the molecular mechanisms involved in the cell damage from oxidative stress in cadmium-induced chronic kidney disease are not well understood. Mitochondrial damage is likely, given that dysfunctional mitochondria are central to the formation of excess reactive oxygen species (ROS), and are known key intracellular targets for cadmium. Normally, ROS are balanced by natural anti-oxidant enzymes. When mitochondria become dysfunctional, for example, through long term exposure to environmental toxicants like cadmium, they produce less cell energy and more ROS. The imbalance between these ROS and the natural anti-oxidants creates the condition of oxidative stress. The outcomes of mitochondrial injury are manyfold: injured mitochondria perpetuate oxidative stress; the loss of mitochondrial membrane potential causes release of cytochrome-c and activation of caspase pathways that lead to apoptotic deletion of renal cells; and attempts by cells to remove dysfunctional mitochondria through autophagy lead to “autophagic cell death” or apoptosis. Three pathways of mitochondrial regulation (upstream signalling pathways, direct mitochondrial targeting, and downstream cell death effector pathways) are therefore all promising targets for effective anti-oxidant treatment of cadmium toxicity in the kidney.