Loss of Ca2+ entry via Orai–TRPC1 induces ER stress, initiating immune activation in macrophages

Activation of cellular stresses is associated with inflammation; however, the mechanisms are not well identified. Here, we provide evidence that loss of Ca2+ influx induces endoplasmic reticulum (ER) stress in primary macrophage and in murine macrophage cell line Raw 264.7, which initiates the unfolded protein response to modulate cytokine production thereby activating the immune response. Stressors that initiates the ER stress response blocks store-dependent Ca2+ entry in macrophage cells prior to the activation of the unfolded protein response. The endogenous Ca2+ entry channel was dependent on the Orai1-TRPC1-STIM1 complex and the presence of ER stressors decreased TRPC1/Orai1/STIM1 expression. Additionally, blocking Ca2+ entry with SKF-96365 also induced ER stress, promoted cytokine production, activation of autophagy, increased caspase activation and induced apoptosis. Furthermore, ER stress inducers inhibited cell cycle progression, promoted inflammatory M1 phenotype, and increased phagocytosis. Mechanistically, restoration of Orai1-STIM1 expression inhibited ER stress-mediated loss of Ca2+ entry that prevents ER stress, inhibits cytokine production, and induced cell survival. These results suggest an unequivocal role of Ca2+ entry in modulating ER stress and in the induction of inflammation.