C/EBP homologous protein contributes to cytokine-induced pro-inflammatory responses and apoptosis in β-cells

Abstract

Induction of the C/EBP homologous protein (CHOP) is considered a key event for endoplasmic reticulum (ER) stress-mediated apoptosis. Type 1 diabetes (T1D) is characterized by an autoimmune destruction of the pancreatic b-cells. Pro-inflammatory cytokines are early mediators of b-cell death in T1D. Cytokines induce ER stress and CHOP overexpression in b-cells, but the role for CHOP overexpression in cytokine-induced b-cell apoptosis remains controversial. We presently observed that CHOP knockdown (KD) prevents cytokine-mediated degradation of the anti-apoptotic proteins B-cell lymphoma 2 (Bcl-2) and myeloid cell leukemia sequence 1 (Mcl-1), thereby decreasing the cleavage of executioner caspases 9 and 3, and apoptosis. Nuclear factor-jB (NF-jB) is a crucial transcription factor regulating b-cell apoptosis and inflammation. CHOP KD resulted in reduced cytokine-induced NF-jB activity and expression of key NF-jB target genes involved in apoptosis and inflammation, including iNOS, FAS, IRF-7, IL-15, CCL5 and CXCL10. This was due to decreased IjB degradation and p65 translocation to the nucleus. The present data suggest that CHOP has a dual role in promoting b-cell death: (1) CHOP directly contributes to cytokine-induced b-cell apoptosis by promoting cytokine-induced mitochondrial pathways of apoptosis; and (2) by supporting the NF-jB activation and subsequent cytokine/chemokine expression, CHOP may contribute to apoptosis and the chemo attraction of mononuclear cells to the islets during insulitis. Type 1 diabetes (T1D) is a severe chronic disease resulting from an autoimmune destruction of the pancreatic b-cells. The incidence of T1D has been rising steadily in developed countries from the 1950s to the present day, with the recent, alarming prediction that it will double in children under the age of 5 years by 2020. 1 b-cell loss in T1DM occurs slowly over years and 480% of the b-cell mass is usually lost at the time of diagnosis. Because of the excessive mortality associated with complications of T1D and the increasing incidence of childhood diabetes, 2 there is an ongoing effort to develop novel strategies for a better treatment and hopefully, prevention of T1D.In T1D, b-cells cooperate with the immune system to its own destruction by activating pro-apoptotic pathways and secreting chemokines/cytokines that contribute to islet inflammation. 3 These responses are mostly triggered via the secretion of the pro-inflammatory cytokines interleukin-1b (IL-1b), tumor necrosis factor-a (TNF-a) and interferon-g (IFN-g) by the infiltrated immune cells. The mechanisms regulating cytokine-mediated b-cell apoptosis and pro-inflammatory responses are intricate and include, but are not restricted to, the activation of the transcription factors nuclear factor-kB (NF-kB) and STAT-1, the c-Jun N-terminal kinases (JNK), endoplasmic reticulum (ER) stress pathways and the intrinsic mitochondrial apoptotic pathways. [3][4][5][6][7] NF-kB activation is due to cytokine-dependent activation of the inhibitor of k-light polypeptide gene enha...

Cytokines induce endoplasmic reticulum stress in human, rat and mouse beta cells via different mechanisms

“…4e, f). These findings were confirmed by an independent series of experiments using a previously tested siRNA targeting CHOP (also known as DDIT3) [14] (ESM Fig. 7).…”

“…rat/mouse islet cells [11,12,38] or purified beta cells [11,14,39]. These cells were thus used for the initial comparisons with human islets exposed to cytokines with or without concomitant treatment with the iNOS blocker L-NMMA ( Fig.…”

“…For human islets we used the combination of TNF-α+ IFN-γ, which induces a marked ER stress activation [14], and also a combination of three cytokines, namely IL-1β, TNF-α and IFN-γ. The combination TNF-α+IFN-γ induced (Fig.…”

“…It was subsequently shown that IL-1β+ of NO production in the time period studied [12]. In human islets TNF-α+IFN-γ induces a more marked activation of the ER stress response [14], in spite of the fact that IL-1β+IFN-γ induces a nearly tenfold higher increase in NO production [22,47]. These conflicting observations caused confusion in the field, as it was not clear whether they were due to species differences or other reasons.…”

“…In spite of these differences, we observed that cytokineinduced ER stress is also important for human beta cell death. Thus, the chemical chaperone tauroursodeoxycholic acid (TUDCA), previously shown to mitigate ER stress [13] and protect rat and mouse beta cells against ER stress-induced apoptosis [9], or a small interfering (si)RNA blocking CHOP, a key executioner of ER stress-mediated apoptosis [2,14], partially protected human islet cells against cytokine-induced apoptosis. Finally, we identified JNK as a crucial downstream mediator leading to human beta cell apoptosis.…”