Mechanism (S) of Metal-Induced Apoptosis in Saccharomyces Cerevisiae

Heavy metals, such as copper and cadmium have been linked to a number of cellular dysfunctions in single and multicellular organisms that are associated with apoptosis. The yeast, Saccharomyces cerevisiae, provides a valuable model for elucidating apoptosis mechanisms, and this study extends that capability to Cu and Cd-induced apoptosis. We demonstrate that S. cerevisiae undergoes a glucose-dependent, programmed cell death in response to low cadmium concentrations, which is initiated within the first hour of Cd exposure. The response was associated with induction of the yeast caspase, Yca1p, and was abolished in YCA1∆ mutant. Other apoptotic markers, including sub-G1 DNA fragmentation and hyper-polarization of mitochondrial membranes, were also evident among Cd-exposed cells. We also show that low levels of copper can induce a similar apoptotic response in yeast within the first hour of exposure. Such cellular responses were verified by analyzing mitochondrial perturbation, generation of superoxide ions, activation of the yeast caspase1, and the eventual fragmentation of nuclear DNA (through TUNEL). In analyzing the response of yeast to the different metals, we also demonstrated that the metal-induced PCD is instigated through the sequential activity of at least two caspase-like proteins (i.e., Yca1 and Atg4), both of which appear to be in involved in the process of inducing mitochondrial stress. The additional caspase-like activity is shown to be derived from an enzyme involved in the latter stages of autophagy (Atg4), and provides an intriguing association of apoptosis with autophagy. Here we also demonstrate that metals such as copper and cadmium causes oxidative damage to mitochondrial proteins. Such oxidative attack is targeted and we show that oxidation of certain crucial proteins is required for apoptosis upon metal exposure. By showing that such targeted protein oxidation is dependent on YCA1 and ATG, we also confirm the finding that in yeast that have been exposed to a heavy metal, YCA1 and ATG are essential for damaging mitochondria and to initiate apoptosis. These novel findings highlight several new perspectives about the mechanism of metal-dependent apoptosis, while opening up future analyses to the power of the yeast model system