Zeynep Dogusan

Selective inhibition of eukaryotic translation initiation factor 2 alpha dephosphorylation potentiates fatty acid-induced endoplasmic reticulum stress and causes pancreatic beta-cell dysfunction and apoptosis.

Free fatty acids cause pancreatic β-cell apoptosis and may contribute to β-cell loss in type 2 diabetes via the induction of endoplasmic reticulum stress. Reductions in eukaryotic translation initiation factor (eIF) 2α phosphorylation trigger β-cell failure and diabetes. Salubrinal selectively inhibits eIF2α dephosphorylation, protects other cells against endoplasmic reticulum stress-mediated apoptosis, and has been proposed as a β-cell protector. Unexpectedly, salubrinal induced apoptosis in primary β-cells, and it potentiated the deleterious effects of oleate and palmitate. Salubrinal induced a marked eIF2α phosphorylation and potentiated the inhibitory effects of free fatty acids on protein synthesis and insulin release. The synergistic activation of the PERK-eIF2α branch of the endoplasmic reticulum stress response, but not of the IRE1 and activating transcription factor-6 pathways, led to a marked induction of activating transcription factor-4 and the pro-apoptotic transcription factor CHOP. Our findings demonstrate that excessive eIF2α phosphorylation is poorly tolerated by β-cells and exacerbates free fatty acid-induced apoptosis. This modifies the present paradigm regarding the beneficial role of eIF2α phosphorylation in β-cells and must be taken into consideration when designing therapies to protect β-cells in type 2 diabetes.

Toll-like receptor 3 and STAT-1 contribute to double-stranded RNA+ interferon-gamma-induced apoptosis in primary pancreatic beta-cells

Viral infections and local production of cytokines probably contribute to the pathogenesis of Type 1 diabetes. The viral replicative intermediate double-stranded RNA (dsRNA, tested in the form of polyinosinic-polycytidylic acid, PIC), in combination with the cytokine interferon-γ (IFN-γ), triggers β-cell apoptosis. We have previously observed by microarray analysis that PIC induces expression of several mRNAs encoding for genes downstream of Toll-like receptor 3 (TLR3) signaling pathway. In this report, we show that exposure of β-cells to dsRNA in combination with IFN-α, -β, or -γ significantly increases apoptosis. Moreover, dsRNA induces TLR3 mRNA expression and activates NF-κB and the IFN-β promoter in a TRIF-dependent manner. dsRNA also induces an early (1 h) and sustained increase in IFN-β mRNA expression, and blocking IFN-β with a specific antibody partially prevents PIC plus IFN-γ-induced β-cell death. On the other hand, dsRNA plus IFN-γ does not induce apoptosis in INS-1E cells, and expression of TLR3 and type I IFNs mRNAs is not detected in these cells. Of note, disruption of the STAT-1 signaling pathway protects β-cells against dsRNA plus IFN-γ-induced β-cell apoptosis. This study suggests that dsRNA plus IFN-γ triggers β-cell apoptosis by two complementary pathways, namely TLR3-TRIF-NF-κB and STAT-1.

Double-stranded RNA induces pancreatic beta-cell apoptosis by activation of the toll-like receptor 3 and interferon regulatory factor 3 pathways.

OBJECTIVE— Viral infections contribute to the pathogenesis of type 1 diabetes. Viruses, or viral products such as double-stranded RNA (dsRNA), affect pancreatic β-cell survival and trigger autoimmunity by unknown mechanisms. We presently investigated the mediators and downstream effectors of dsRNA-induced β-cell death. RESEARCH DESIGN AND METHODS— Primary rat β-cells and islet cells from wild-type, toll-like receptor (TLR) 3, type I interferon receptor (IFNAR1), or interferon regulatory factor (IRF)-3 knockout mice were exposed to external dsRNA (external polyinosinic-polycytidylic acid [PICex]) or were transfected with dsRNA ([PICin]). RESULTS— TLR3 signaling mediated PICex-induced nuclear factor-κB (NF-κB) and IRF-3 activation and β-cell apoptosis. PICin activated NF-κB and IRF-3 in a TLR3-independent manner, induced eukaryotic initiation factor 2α phosphorylation, and triggered a massive production of interferon (IFN)-β. This contributed to β-cell death, as islet cells from IFNAR1−/− or IRF-3−/− mice were protected against PICin-induced apoptosis. CONCLUSIONS— PICex and PICin trigger β-cell apoptosis via the TLR3 pathway or IRF-3 signaling, respectively. Execution of PICin-mediated apoptosis depends on autocrine effects of type I IFNs.