Ammonium is toxic for amino acid-starved yeast cells under extreme calorie restriction, inducing cell death through the regulation of PKA, TOR and Sch9 activities

Abstract

We have previously shown that ammonium, a nitrogen source commonly used for yeast growth, stimulates cell death in amino acid-deprived auxotrophic Saccharomyces cerevisiae cells (aa-starved cells) after they are transferred to water. Ammonium induced cell death was accompanied by an initial small increase of apoptotic cells followed by extensive necrosis. The occurrence of necrosis was confirmed by the observation of the nucleo-cytosolic translocation of Nhp6Ap, which nuclear release is considered a marker of necrosis. Ammonium toxicity decreased CLS and induced cell death in aa-starved cells but not in nitrogen-starved cells (N-starved cells), suggesting that the ammonium effect was dependent on the inappropriate arrest of the aa-starved cells. Autophagy was inhibited by ammonium, but this blockage did not cause decrease in cell viability. Activation of PKA stimulated ammonium induced decrease of CLS consistent with the observation that deletion of MEP1, MEP2, RAS2 or TPK1 partially reverted the ammonium effect. Deletion of TOR1 also significantly rescued the ammonium effect and decreased PKA activation. However, PKA activation in response to ammonium was completely abolished by SCH9 deletion, but even so, could not revert the shortening in CLS, indicating that PKA inactivation cannot extend CLS in the absence of Sch9p, that plays a pro-survival role in the process.