A study of the roles of reactive oxygen species and superoxide dismutase in aspirin-induced apoptosis in yeast

Abstract

The principal objective of this study was to acquire a better understanding of the mechanism by which aspirin, a promising cancer-preventive agent, induces apoptosis in yeast cells deficient in mitochondrial manganese superoxide dismutase (MnSOD), during aerobic growth in ethanol medium (YPE). In this project, exogenous Escherichia coli iron superoxide dismutase (FeSOD) was targeted to the mitochondria of MnSOD-deficient Saccharomyces cerevlszae EG 11 0 cells, using YEpIPGK-S plasmicl DNA (encoding E. coli FeSOD with the S. cerevisiae MnSOD mitochondrial targeting signal). This was found to rescue the resulting recombinant EGllOF yeast cells (expressing E. coli FeSOD in the mitochondria) from aspirin-induced apoptosis during cultivation in YPE medium. Similarly, wildtypc EGI03 yeast cells, which express endogenous mitochondrial MnSOD, survived in the presence of aspirin during growth in YPE medium. However, MnSOD-ueficienl S. cerevisiae EG 11 OYEp cells (EG 11 0 cells having YEplPGK plasmid DNA and used as a control) underwent apoptosis in YPE medium in the presence of aspirin, as confirmed by dual fluorescence staining of whole yeast cells with FITC-Annexin V and propidium iodide. Subsequent investigation of potential aspirin-induced changes in the levels of different types of reactive oxygen species (ROS) in whole yeast cells grown in YPE medium, was carried out. In parallel to this, a study of potential aspirin-induced changes of yeast cytosolic and mitochondrial SOD activity was also performed. In MnSODdeficient EG 110YEp yeast cells, aspirin-induced apoptosis was accompanied by a sustained and highly significant accumulation of mitochondrial and overall cellular superoxide radicals (02•} This was not prevented by the concomitant aspirin-induced increase of cytosolic copper-zinc superoxide dismutase (CuZnSOD) activity, which failed to compensate for the deficiency of mitochondrial MnSOD and did not provide effective protection against the adverse pro-apoptotic effect of aspirin. In the recombinant EG 11 OF yeast cells (expressing E. coli FeSOD in the mitochondria) growing in YPR l1lodium in the presence of aspirin, an increase in the activity of mitochondrial FeSOD significantly reduced the levels of mitochondrial O2.- and rescued the apoptotic phenotype. In EG 103 wildtype yeast cells (which have endogenous MnSOD) there was a decrease in the level of mitochondrial 02•- throughout growth ill YPE medium in the presence of aspirin. This was due to the highly efficient antioxidant activity of mitochondrial MnSOD, the level of which did not change in response to aspirin. Comparative measurement of near UV-autoflourescence of whole yeast cells grown in YPE medium showed that, in MnSOD-deficient EG 11 OYEp yeast cells, sustained depletion of reduced mitochondrial NA(D)PH in the presence of aspirin took place, thus providing evidence of a positive correlation between aspirin-induced mitochondrial 02•- accumulation and the oxidation of mitochondrial NAD(P)H. In EGllOF yeast cells growing in YPE medium in the presence of aspirin, a significant increase of mitochondrial NAD(P)H was observed, coinciding with the decrease of mitochondrial 02•-. Also, in wildtype EG 103 yeast cells, where the presence of MnSOD reduced the levels of mitochondrial 02•- throughout growth, there was a significant increase of mitochondrial NAD(P)H in the presence of aspirin. IV Therefore, aspirin exerts a pro-oxidant effect on MnSOD-deficient yeast cells grown in YPE medium and causes early accumulation of mitochondrial 02'-, oxidation of mitochondrial NAD(P)H and apoptosis of the yeast cells, which can only be prevented by the presence of active mitochondrial SOD.

Keywords: aspirin, apoptosis, yeast, superoxide dismutase, reactive oxygen species, mitchondria, NADPII