Previous studies reported that a mild, non-protein-denaturing, fever-like temperature
increase induced the unfolded protein response (UPR) in mammalian cells. Our dSTORM
super-resolution microscopy experiments revealed that the master regulator of the
UPR, the IRE1 (inositol-requiring enzyme 1) protein, is clustered as a result of UPR
activation in a human osteosarcoma cell line (U2OS) upon mild heat stress. Using ER
thermo yellow, a temperature-sensitive fluorescent probe targeted to the endoplasmic
reticulum (ER), we detected significant intracellular thermogenesis in mouse embryonic
fibroblast (MEF) cells. Temperatures reached at least 8 °C higher than the external
environment (40 °C), resulting in exceptionally high ER temperatures similar to those
previously described for mitochondria. Mild heat-induced thermogenesis in the ER of
MEF cells was likely due to the uncoupling of the Ca2+/ATPase (SERCA) pump. The high
ER temperatures initiated a pronounced cytosolic heat-shock response in MEF cells,
which was significantly lower in U2OS cells in which both the ER thermogenesis and
SERCA pump uncoupling were absent. Our results suggest that depending on intrinsic
cellular properties, mild hyperthermia-induced intracellular thermogenesis defines
the cellular response mechanism and determines the outcome of hyperthermic stress.
