Background: We previously showed that nAChR signaling regulates activation of IRE1α and markers of terminal-unfolded protein response (T-UPR) under endoplasmic reticulum stress (ERS) in β-cells. However, it is still unknown if nAChR signaling affects the apoptosis under ERS or specific activation of IRE1α in β-cells. We also previously reported that IRE1α specific inhibitor, KIRA-Kinase Inhibiting RNase Attenuator-, suppresses apoptosis induced by ERS in INS1 cells and β-cells in Akita mice, which suggest that IRE1α plays a critical role in ERS-induced apoptosis. The aim of this study is to determine if and how nAChR signaling regulates the apoptosis induced by ERS or specific activation of IRE1α in β-cells.
Methods: We investigated the role of nAChR signaling on IRE1α signaling under ERS inducers or doxycycline-inducible IRE1α-overexpression (OE) in INS-1 cells. Cells were treated with Nicotine as an agonist of nAChR, and Tunicamycin or Thapsigargin as ERS inducers. Activation of IRE1α was confirmed by XBP1 mRNA splicing and phosphorylation of IRE1α. That of T-UPR was determined by Insulin1/proinsulin and TXNIP mRNA and protein expression. Translocation of phosphatidylserine to the external cell surface was detected by flow cytometry with Annexin V-FITC staining. Activation of caspase-3 and -7, markers of activation of caspase cascade, was detected by the active caspase-3/-7 specific substrate.
Results: Nicotine reduced XBP1 mRNA splicing and phosphorylation of IRE1α induced by ERS. Nicotine reversed increase of TXNIP and decrease of Insulin1/proinsulin expressions induced by both ERS and IRE1α OE. Increased Annexin positive cells and caspase-3/7 activation by ERS were reduced by Nicotine. Finally, increased caspase-3/7 activation by IRE1α OE was reversed by Nicotine as well as KIRA.
Conclusion: nAChR signaling could regulate IRE1α signaling and protect β-cells from apoptosis induced by ERS and specific activation of IRE1α in INS1 cells.
T. Ishibashi: None. S. Morita: None. A. Doi: None. H. Iwakura: None. H. Ariyasu: None. M. Nishi: None. H. Furuta: None. T. Akamizu: None.