Role of Endoplasmic Reticulum Stress Response in Hyperthermia and Thermotolerance

Hyperthermia (HT) combined with chemotherapy, radiotherapy or both has been considered a promising approach in cancer therapy. However, one of the problems with the use of HT is the acquisition of thermotolerance (TT), which makes HT less effective. The endoplasmic reticulum (ER) is the central intracellular organelle responsible for the quality control of newly synthesized proteins. ER stress―defined as the accumulation of unfolded proteins in the ER―induces a cytoprotective program, the ER stress response, also known as the unfolded protein response (UPR). ER stress can be triggered by various pathophysiological conditions, such as heat shock, glucose starvation, hypoxia, and calcium deprivation. This response is mediated through three distinct sensor molecules, IRE1 (inositol requiring enzyme-1), PERK (protein kinase R-like ER kinase), and ATF6 (activating transcription factor 6), which locate at the ER membrane. Under nonstress conditions, BiP (HSPA5: heat shock protein family A (Hsp70) member 5) interacts with these sensor molecules. In contrast, under ER stress conditions, BiP dissociates from the protein conjugate, leading to the activation of three sensor molecules. Interestingly, IRE1, PERK and/or ATF6 signaling pathways are found to be activated in the cells treated with heat stress. In this review, the physiological roles of ER stress response in HT and TT are summarized.