IRE1 is the most conserved endoplasmic reticulum (ER)-resident stress sensor. Its activation not only splices XBP1 but also participates in a variety of cell signaling. We elucidated the role of IRE1α in Neuro2a cells by establishing IRE1α-deficient cells and applying four IRE1 inhibitors. IRE1α deficiency prevented almost all spliced XBP1 (sXBP1) protein expression by treatment with thapsigargin (Tg) and tunicamycin (Tm); these phenomena paralleled the values measured by our two Nanoluciferase-based IRE1 assays. However, cell viability and protein expression of other ER stress-responsive factors in the IRE1α-deficient cells were comparable to those in the parental wild-type cells with or without Tm treatment. Next, we elucidated the IRE1 inhibitory actions and cytotoxicity of four compounds: STF083010, KIRA6, 4μ8C, and toyocamycin. KIRA6 attenuated IRE1 activity in a dose-dependent manner, but it showed severe cytotoxicity even in the IRE1α-deficient cells at a low concentration. The IRE1α-deficient cells were slightly resistant to KIRA6 at 0.1 μM in both the presence and absence of ER stress; however, resistance was not observed at 0.02 μM. Treatment with only KIRA6 at 0.1 μM for 12 h remarkably induced LC3 II, an autophagic marker, in both parental and IRE1α-deficient cells. Co-treatment with KIRA6 and Tm induced LC3 II, cleaved caspase-9, and cleaved caspase-3; however, IRE1α-deficiency did not abolish the expression of these two cleaved caspases. On the other hand, KIRA6 prohibited Tm-induced ATF4 induction in an IRE1-independent manner; however, co-treatment with KIRA6 and Tm also induced LC3 II and two cleaved caspases in the ATF4-deficient Neuro2a cells. Thus, we demonstrate that IRE1α deficiency has little impact on cell viability and expression of ER stress-responsive factors in Neuro2a cells, and the pharmacological actions of KIRA6 include IRE1-independent ways.
Keywords: ER stress; IRE1; XBP1.