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

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 beta-cells. Pro-inflammatory cytokines are early mediators of beta-cell death in T1D. Cytokines induce ER stress and CHOP overexpression in beta-cells, but the role for CHOP overexpression in cytokine-induced beta-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-kappa B (NF-kappa B) is a crucial transcription factor regulating beta-cell apoptosis and inflammation. CHOP KD resulted in reduced cytokine-induced NF-kappa B activity and expression of key NF-kappa B target genes involved in apoptosis and inflammation, including iNOS, FAS, IRF-7, IL-15, CCL5 and CXCL10. This was due to decreased I kappa B degradation and p65 translocation to the nucleus. The present data suggest that CHOP has a dual role in promoting beta-cell death: (1) CHOP directly contributes to cytokine-induced beta-cell apoptosis by promoting cytokine-induced mitochondrial pathways of apoptosis; and (2) by supporting the NF-kappa B activation and subsequent cytokine/chemokine expression, CHOP may contribute to apoptosis and the chemo attraction of mononuclear cells to the islets during insulitis. Cell Death and Differentiation (2012) 19, 1836-1846; doi:10.1038/cdd.2012.67; published online 1 June 2012