Chronological and replicative life-span extension in Saccharomyces cerevisiae by increased dosage of alcohol dehydrogenase 1

Abstract

Alcohol dehydrogenase 1 (Adh1)p catalyses the conversion of acetaldehyde to ethanol, regenerating NAD + . In Saccharomyces cerevisiae, Adh1p is oxidatively modified during ageing and, consequently, its activity becomes reduced. To analyse whether maintaining this activity is advantageous for the cell, a yeast strain with an extra copy of the ADH1 gene (2¾ADH1) was constructed, and the effects on chronological and replicative ageing were analysed. The strain showed increased survival in stationary phase (chronological ageing) due to induction of antioxidant enzymes such as catalase and superoxide dismutases. In addition, 2¾ADH1 cells displayed an increased activity of silent information regulator 2 (Sir2)p, an NAD + -dependent histone deacetylase, due to a higher NAD + /NADH ratio. As a consequence, a 30 % extension in replicative life span was observed. Taken together, these results suggest that the maintenance of enzymes that participate in NAD + /NADH balancing is important to chronological and replicative life-span parameters. INTRODUCTIONOne of the features of ageing in Saccharomyces cerevisiae, as well as in other organisms, is the oxidative modification of specific protein targets, whose functions are impaired by such modification. A mild oxidation contributes to marking the protein for degradation (Stadtman & Oliver, 1991), while strong oxidation can promote protein aggregation, making proteins unavailable for proteasome degradation (Grune et al., 2004). The accumulation of these modified proteins in a cell contributes to the ageing phenotype (Harman, 1981;Stadtman, 1992;Stadtman & Levine, 2000;Levine, 2002; Nyström, 2005). In yeasts, replicative ageing refers to the number of cell divisions occurring in a mother yeast cell . Chronological ageing refers to the ability of cells to maintain viability in stationary phase (Herman, 2002).During the last decade investigations of ageing have uncovered physiological and molecular mechanisms involved in this process, as well as providing some clues towards understanding life-span lengthening (Bordone & Guarente, 2005). Calorie restriction is one of the mechanisms that has been consistently proven to extend life span in organisms ranging from yeasts to mammals (Sohal & Weindruch, 1996). In yeast, the replicative life span can be increased by shifting the cells from 2 to 0.5 % glucose (calorie restriction) (Lin et al., , 2004. One of the key molecules is the silent information regulator 2 (Sir2)p, a class III NAD + -dependent histone deacetylase, which is involved in several physiologically important functions such as silencing telomeres and rDNA, maintaining genome integrity, and ageing (Bitterman et al., 2003). Strains that have an extra copy of the SIR2 gene have a life span extended by 40 %, while deletion of SIR2 shortens life span by 50 % (Kaeberlein et al., 1999). Under calorie restriction, the oxygen consumption increases, thus raising the NAD + /NADH ratio by lowering the concentration of NADH. Since NADH has been described as a Sir2p inhibitor, calorie res...

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