Spontaneous Development of Endoplasmic Reticulum Stress That Can Lead to Diabetes Mellitus Is Associated with Higher Calcium-independent Phospholipase A2 Expression

• Published in: The Journal of biological chemistry 2010-02, Vol.285 (9), p.6693-6705
• Main Authors: Lei, Xiaoyong, Zhang, Sheng, Barbour, Suzanne E., Bohrer, Alan, Ford, Eric L., Koizumi, Akio, Papa, Feroz R., Ramanadham, Sasanka
• Format: Article
• Language: English
• Subjects: Apoptosis; Diabetes; ER Stress; Mitochondria; Phospholipase; Signal Transduction
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M109.084293

Our recent studies indicate that endoplasmic reticulum (ER) stress causes INS-1 cell apoptosis by a Ca2+-independent phospholipase A2 (iPLA2β)-mediated mechanism that promotes ceramide generation via sphingomyelin hydrolysis and subsequent activation of the intrinsic pathway. To elucidate the association between iPLA2β and ER stress, we compared β-cell lines generated from wild type (WT) and Akita mice. The Akita mouse is a spontaneous model of ER stress that develops hyperglycemia/diabetes due to ER stress-induced β-cell apoptosis. Consistent with a predisposition to developing ER stress, basal phosphorylated PERK and activated caspase-3 are higher in the Akita cells than WT cells. Interestingly, basal iPLA2β, mature SREBP-1 (mSREBP-1), phosphorylated Akt, and neutral sphingomyelinase (NSMase) are higher, relative abundances of sphingomyelins are lower, and mitochondrial membrane potential (ΔΨ) is compromised in Akita cells, in comparison with WT cells. Exposure to thapsigargin accelerates ΔΨ loss and apoptosis of Akita cells and is associated with increases in iPLA2β, mSREBP-1, and NSMase in both WT and Akita cells. Transfection of Akita cells with iPLA2β small interfering RNA, however, suppresses NSMase message, ΔΨ loss, and apoptosis. The iPLA2β gene contains a sterol-regulatory element, and transfection with a dominant negative SREBP-1 reduces basal mSREBP-1 and iPLA2β in the Akita cells and suppresses increases in mSREBP-1 and iPLA2β due to thapsigargin. These findings suggest that ER stress leads to generation of mSREBP-1, which can bind to the sterol-regulatory element in the iPLA2β gene to promote its transcription. Consistent with this, SREBP-1, iPLA2β, and NSMase messages in Akita mouse islets are higher than in WT islets.