Oxidative stress is involved in heat-induced cell death in Saccharomyces cerevisiae.

Abstract

The cause for death after lethal heat shock is not well understood. A shift from low to intermediate temperature causes the induction of heat-shock proteins in most organisms. However, except for HSP104, a convincing involvement of heat-shock proteins in the development of stress resistance has not been established in Saccharomyces cerevisiae. This paper shows that oxidative stress and antioxidant enzymes play a major role in heat-induced cell death in yeast. Mutants deleted for the antioxidant genes catalase, superoxide dismutase, and cytochrome c peroxidase were more sensitive to the lethal effect of heat than isogenic wild-type cells. Overexpression of catalase and superoxide dismutase genes caused an increase in thermotolerance. Anaerobic conditions caused a 500-to 20,000-fold increase in thermotolerance. The thermotolerance of cells in anaerobic conditions was immediately abolished upon oxygen exposure. HSP104 is not responsible for the increased resistance of anaerobically grown cells. The thermotolerance of anaerobically grown cells is not due to expression of heat-shock proteins. By using an oxidation-dependent fluorescent molecular probe a 2-to 3-fold increase in fluorescence was found upon heating. Thus, we conclude that oxidative stress is involved in heat-induced cell death.Most living cells are sensitive to sudden heat exposure. A shift in temperature from a low to an intermediate temperature induces the stress response or heat-shock response (1-3), which is considered to be an evolutionarily conserved genetic system advantageous to living organisms. After a temperature shift from 23 to 37°C in cells of the yeast Saccharomyces cerevisiae, 80 proteins were transiently induced; 20 of these proteins are now classified as major heat-shock proteins (HSPs) (2). Some of these HSPs have been characterized, but the function of many of them is still unclear (4).Initial studies suggested that HSPs play an essential role in the acquisition of stress tolerance. On the other hand, a convincing involvement of HSPs in the development of stress resistance has not been established in yeast. Except for HSP104, none of the other HSP disruption mutants show any block in the acquisition of stress resistance in yeast (5). Furthermore, a yeast strain with a temperature-sensitive mutation in the heat-shock factor (hsfl-m3) that leads to a general block in heat-shock-induced protein synthesis was not affected in the acquisition of thermotolerance (6). Therefore, HSPs may not be important for stress tolerance acquisition but rather for a rapid recovery after heat shock (4).The main factors causing death after heat exposure are still unknown. Thus, the heat-shock response may not elucidate why cells die in response to heat exposure but rather how they repair the damage afterwards. To investigate why cells die in response to heat exposure, we completely avoided the induction of the heat-shock response by exposing cells immediately to lethal heat. In particular, we investigated the possible involvement of oxidative stres...

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