Thermotolerance induced by non-lethal heat shock at 40 °C is activated by mitochondrial ROS and is Nrf2-dependent

Hyperthermia is generally administered as an adjuvant to chemotherapy/radiotherapy and sensitizes tumors to these anticancer treatments. Repeated heat treatments (≥42 °C) cause development of transient thermotolerance, an adaptive survival response. This response can be mediated by upregulation of cellular defense pathways and remains unclear. We aim to clarify the mechanistic explanations behind activation of this response. In vitro, thermotolerance can be induced by mild heat stress at 40 °C and protects cells against subsequent lethal heat shock (≥42 °C). When HeLa cells were heated at 42 °C, cellular and mitochondrial superoxide and peroxide levels increased. Treatment with mitochondrial antioxidant MitoQ, or NADPH oxidase (NOX) inhibitor apocynin, decreased levels of reactive oxygen species (ROS) and apoptotic cell death, indicating that mitochondria and NOX are important sources of ROS at 42 °C. Mild heat stress at 40 °C increased production of ROS, which are thought to activate the adaptive response, and reduce subsequent cell death at 42 °C. Our results show that heat-derived ROS are linked to expression of master antioxidant regulator Nrf2. When Nrf2 was overexpressed or knocked down, Nrf2 expression was directly associated with protective ability of the adaptive response induced by mild heat stress (40 °C). Mitochondrial ROS were found to be essential in mediating Nrf2-dependent thermotolerance, because MitoQ treatment prior to exposure to 40 °C reduced Nrf2 levels and dissipated the subsequent protective effect of thermotolerance against toxicity at 42 °C. Our study demonstrates that specific sources of ROS had biologically different implications in activating Nrf2, underlining potential therapeutic targets that may contribute to thermotolerance in anticancer treatments.