The Endoplasmic Reticulum Stress Sensor IRE1α Regulates the UV DNA Repair Response through the Control of Intracellular Calcium Homeostasis

The unfolded protein response is activated by UVB irradiation, but the role of a key mediator, IRE1 alpha, is not clear. In this study, we show that mice with an epidermal IRE1 alpha deletion are sensitized to UV with increased apoptosis, rapid loss of UV-induced cyclopyrimidine dimer.positive keratinocytes, and sloughing of the epidermis. In vitro, Ire1 alpha-deficient keratinocytes have increased UVB sensitivity, reduced cyclopyrimidine dimer repair, and reduced accumulation of gamma H2AX and phosphorylated ATR, suggesting defective activation of nucleotide excision repair. Knockdown of XBP1 or pharmacologic inhibition of the IRE1 alpha ribonuclease did not phenocopy Ire1 alpha deficiency. The altered UV response was linked to elevated intracellular calcium levels and ROS, and this was due to dysregulation of the endoplasmic reticulum calcium channel InsP3R. Pharmacologic, genetic, and biochemical studies linked the regulation of the Ins3PR, intracellular calcium, and normal UV DNA damage response to CIB1 and the IRE1 alpha-TRAF2-ASK1 complex. These results suggest a model where IRE1 alpha activation state drives CIB1 binding either to the InsP3R or ASK1 to regulate endoplasmic reticulum calcium efflux, ROS, and DNA repair responses after UV irradiation.