Thermotolerance induced at a mild temperature of 40 °C alleviates heat shock-induced ER stress and apoptosis in HeLa cells

Hyperthermia (39-45 degrees C) has emerged as an alternate prospect for cancer therapy in combination with radiation and chemotherapy. Despite promising progress in the clinic, molecular mechanisms involved in hyperthermia-induced cell death are not clear. Hyperthermia causes protein denaturation/aggregation, which results in cell death by apoptosis and/or necrosis. Hyperthermia also induces thermotolerance, which renders cells resistant to subsequent exposure to lethal heat shock. This study investigates the role of both lethal (42-43 degrees C) and mild (40 degrees C) hyperthermia in regulating ER stress and ER stress-induced apoptosis in HeLa cells. The ability of mild thermotolerance induced at 40 degrees C to alleviate either or both of these processes is also determined. Hyperthermia (42-43 degrees C) induced ER stress, revealed by phosphorylation of PERK, eIF2 alpha. and IRE1 alpha, cleavage of ATF6 and increased expression of BiP and sXBP1. Real-time PCR revealed that mRNA levels of ATF6, ATF4, BiP, sXBP1 and CHOP increased in cells exposed to hyperthermia. Moreover, hyperthermia caused disruption of calcium homeostasis and activated the calpain-calpastatin proteolytic system and ER resident caspase 4. Pre-exposure to mild hyperthermia (40 degrees C) alleviated the induction of cytotoxicity and ER stress by hyperthermia (42-43 degrees C) and protected cells against ER stress-induced apoptosis. ShRNA-mediated depletion of Hsp72 abrogated protective effects of mild thermotolerance (40 degrees C) against heat-shock induced ER stress and sensitized cells to ER stress-mediated apoptosis. Our findings show that Hsp72 contributes to the protective effects of mild hyperthermia (40 degrees C) against hyperthermia-induced ER stress and apoptosis. (C) 2014 Elsevier B.V. All rights reserved.