A tight control of endoplasmic reticulum homeostasis is crucial for beta cell function and survival. We recently described that IL-1β plus IFN-γ deplete endoplasmic reticulum Ca²⁺ stores in beta cells, leading to endoplasmic reticulum stress and apoptosis. IL-1β alone induced endoplasmic reticulum stress but failed to induce beta cell death, while IFN-γ alone neither caused endoplasmic reticulum stress nor induced beta cell death. This suggests that IFN-γ aggravates endoplasmic reticulum stress induced by IL-1β, eventually triggering apoptosis. Here we tested this hypothesis and the mechanisms involved, by investigating the effects of IFN-γ on endoplasmic reticulum-stress-induced beta cell apoptosis caused by a specific blocker of the sarcoendoplasmic-reticulum pump Ca²⁺-ATPase (SERCA).

INS-1E cells or beta cells were pretreated with IFN-γ and then exposed to the SERCA blocker cyclopiazonic acid (CPA) for induction of endoplasmic reticulum stress. Cell death was evaluated by Hoechst 342 and propidium iodide staining. Expression of genes related to endoplasmic reticulum stress was determined by real-time RT-PCR, while activation of the endoplasmic reticulum stress response was determined by analysing X-box binding protein-1 (Xbp1) splicing and using a reporter construct containing five copies of the unfolded protein response element (UPRE).

CPA induces endoplasmic reticulum stress and apoptosis in insulin-producing cells. Pretreatment with IFN-γ decreased the basal level of spliced Xbp1 mRNA, the basal and CPA-induced activity of the UPRE reporter, and the mRNA expression of several endoplasmic reticulum chaperones (Bip, Grp94 and Orp 150) and Sec61a. Furthermore, CPA-induced Chop mRNA expression and beta cell apoptosis were potentiated in cells that had been pretreated with IFN-γ.

CPA-induced endoplasmic reticulum stress and apoptosis is enhanced in IFN-γ-treated beta cells. These effects are mediated via downregulation of the expression of genes involved in beta cell defence against endoplasmic reticulum stress.