Valishkevych B. TOR signaling pathway in the regulation of biochemical mechanisms of resistance of yeast Saccharomyces cerevisiaе to carbonyl/oxidative stress

TOR (target of rapamycin) signaling pathway first described in the yeast Sассharomyces cerevisiae is the highly conserved regulator of eukaryotic cell growth, aging and stress resistance. The effect of nitrogen sources, in particular amino acids, on the activity of TOR signaling pathway is well studied, however its relation to carbohydrates and carbonyl stress is poor understood. The aim of the present study is expanding of our understanding of potential role of TOR regulatory complexes in development of carbonyl/oxidative stress that can result by reducing carbohydrates (glucose and fructose), reactive carbonyl species (RCS, e.g. glyoxal and methylglyoxal), and reactive oxygen species (ROS, e.g. hydrogen peroxide). It is well documented that mentioned above compounds may lead to carbonyl/oxidative stress. Reducing carbohydrates, initiating glycation or other non-enzymatic processes, generate various RSC and ROS. It has been shown that the level of glycated proteins and activity of antyglycation enzymes (glyoxalase 1 and glyoxalase 2) were higher in cells grown on fructose that demonstrated carbonyl oxidative stress development as compared with cells grown on glucose. The above mentioned is consistent with somewhat better survival of S. cerevisiae after stress induced by high and stressful concentrations of glucose than those of fructose. Overall, yeast sensitivity to high concentrations of glucose somewhat changes in the direction: wild type ≈ ∆tor1 ≥ ∆tor2 ≈ ∆tor1∆tor2. At the same time, yeast sensitivity to high concentrations of fructose decreases like that: wild type ≥ ∆tor1 > ∆tor2 ≤ ∆tor1∆tor2. It has been found that growth of the single mutant Δtor2 was slower and growth of the double mutant Δtor1Δtor2 was faster in the both studied cell groups (glucose- and fructose-grown) then parental strain and single mutant Δtor1. It seems that simultaneous loss of both TOR1 and TOR2 genes can be suggested to activate some additional compensatory mechanism(s). The strains defective in TOR proteins cultivated in the presence of glucose as well as fructose demonstrated lower markers of the stress and aging than parental strain. Thus these data confirmed the previous conclusion on fructose having more potent ability to cause carbonyl/oxidative stress and accelerated aging in S. cerevisiae as compared with glucose. Defensive enzymes were found to play an important role in yeast protection against stress induced by the high concentration of hexoses. It has been found that the activity of glyoxalase 1 (GLO1), glutathione reductase (GR), and glucose-6-phosphate dehydrogenase (G6PDH) were determined by the type of monosaccharide as well as its concentration in the cultivation medium. Besides this the activity of the defensive enzymes were TOR-dependent. Mild stress can stimulate organisms’ biological functions and result in an acquisition of their resistance to high doses of the same stressor (preadaptation) as well as other stressing factors (cross-protection or cross-adaptation). This phenomenon also known as hormesis. Exposure to glyoxal, methylglyoxal and hydrogen peroxide resulted in carbohydrate-dependent hermetic effect in yeast.