Heat shock induces the cellular antioxidant defenses peroxiredoxin, glutathione and glucose 6-phosphate dehydrogenase through Nrf2.

Hyperthermia is a promising anticancer treatment used in combination with radiotherapy and/or chemotherapy. Heat (42-45 °C) can kill cancer cells. Low doses of heat at milder temperatures (39-41 °C) induce thermotolerance, an adaptive survival response that upregulates defense molecules to protect cells against subsequent exposure to toxic stress. Although hyperthermia has proven effective in clinical trials, there is still much to learn about its cellular mechanisms. This study aims to understand the role of reactive oxygen species (ROS), antioxidants and the antioxidant transcription factor Nrf2 in cellular stress responses to mild and lethal heat shock. Mild thermotolerance (40 °C) and hyperthermia (42-43 °C) caused increased expression of the antioxidants peroxiredoxin-3 (Prx3) and Prx2, and its hyperoxidized form Prx-SO₃. Cellular levels of superoxide and peroxides increased at 40 °C and 42 °C. Heat shock (42 °C)-induced increases in Prx3 and Prx-SO₃ were inhibited by antioxidants (PEG-catalase, MnTBAP) and a Nrf2 shRNA. Glucose metabolism by the pentose phosphate pathway produces NADPH, which maintains the antioxidant glutathione in its reduced form, GSH. Heat shock (40°C-42 °C) increased GSH levels, expression of glucose transporter GLUT1, and enzymatic activity and expression of glucose 6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose cycle. Heat-induced increases in GSH levels and G6PD expression were inhibited by antioxidants and Nrf2 knockdown. These results suggest that heat shock-generated ROS were involved in induction of cellular defense molecules Prxs, GSH and G6PD through Nrf2 activation. Our study sheds new light on the role of Nrf2 and antioxidants in cellular responses to heat shock at mild and lethal temperatures.