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The protein kinase Akt/protein kinase B (PKB) is implicated in insulin signaling in mammals and functions in a pathway that regulates longevity and stress resistance in Caenorhabditis elegans. We screened for long-lived mutants in nondividing yeast Saccharomyces cerevisiae and identified mutations in adenylate cyclase and SCH9, which is homologous to Akt/PKB, that increase resistance to oxidants and extend life-span by up to threefold. Stress-resistance transcription factors Msn2/Msn4 and protein kinase Rim15 were required for this life-span extension. These results indicate that longevity is associated with increased investment in maintenance and show that highly conserved genes play similar roles in life-span regulation in S. cerevisiae and higher eukaryotes.
Mutations that extend life-span in C. elegans, Drosophila melanogaster, and mice are associated with increased resistance to oxidative stress (1, 2). However, the mechanisms that regulate aging in these multicellular organisms are poorly understood. As in higher eukaryotes, the unicellular yeast Saccharomyces cerevisiae undergoes an age-dependent increase in cell dysfunction and mortality rates (3, 4). Aging in yeast is associated with an enlargement of the cell and a slowing in the budding rate, and is commonly measured by counting the number of buds generated by a single mother cell (replicative lifespan) (5, 6). The replicative life-span of yeast is regulated by the Sir2 protein, which mediates chromatin silencing in a nicotinamide adenine dinucleotide-dependent manner (6, 7). However, yeast can also age chronologically as a population of nondividing cells (2, 4, 6). Saccharomyces cerevisiae grown in complete glucose medium [synthetic complete (SC) medium] stop dividing after 24 to 48 hours and survive for 5 to 7 days while maintaining high metabolic rates (2, 8, 9), a situation more akin to their experience in nature where they are likely to survive as nondividing populations exposed to scarce nutrients. For these reasons, and to avoid extended growth and entry into the hypometabolic stationary phase induced by incubation in the nutrient-richer yeast extract/ peptone/dextrose (YPD) medium (10), our studies were performed exclusively in SC medium. The survival of nondividing yeast is shortened by null mutations in either or both superoxide dismutases (SODs) (2, 11, 12) and is modestly extended by overexpressing the antiapoptotic protein Bcl-2 (8).
To understand the molecular mechanism that regulates yeast longevity, we transposonmutagenized yeast cells...