Oleate rescues INS-1E β-cells from palmitate-induced apoptosis by preventing activation of the unfolded protein response

Background: Saturated free fatty acids (FFAs), such as palmitate, cause beta-cell apoptosis whereas unsaturated FFAs, e.g. oleate, are not harmful. The toxicity of palmitate could be mediated through endoplasmic reticulum (ER) stress which triggers the activation of a signal responding cascade also called unfolded protein response (UPR). We investigated whether or not palmitate induced beta-cell apoptosis through UPR activation and whether or not oleate as a monounsaturated fatty acid could counteract these effects. Methods: INS-1E beta-cells were incubated with palmitate [0.5 mM], oleate [1 mM] or the combination [0.5/1 mM] for 1, 6 and 24 h. Viability and induction of apoptosis were measured by WST-1 assay and FITC-Annexin/PI-staining, respectively. Western blot analyses were performed for UPR specific proteins and mRNA expression of target molecules was determined by qPCR. Results: Palmitate significantly decreased viability (29 +/- 8.8%) of INS-1E beta-cells compared to controls after 24 h. Stimulation with oleate showed no effect on viability but the combination of oleate and palmitate improved viability compared to palmitate treated cells (55 +/- 9.3%) or controls (26 +/- 5.3%). The number of apoptotic cells was increased 2-fold after 24 h incubation with palmitate compared to controls. Again, oleate showed no effect but in combination ameliorated the effect of palmitate to control level. Phosphorylation of eIF2 alpha was increased after 6 and 24 h incubation with palmitate. In contrast, oleate had no effect and in combination prevented phosphorylation of eIF2 alpha. Increased Xbp1 splicing was visible already 6 h after palmitate treatment and remained elevated at 24 h. The combination with oleate abolished Xbp1 splicing. Interestingly, mRNA expression of the chaperones Bip, Pdi, Calnexin and Grp94 was not altered by FFA treatment. Only the proapoptotic transcription factor Chop was significantly enhanced by palmitate incubation. In accordance with sustained cell survival the combination as well as oleate alone, did not result in increased Chop levels compared to controls. In summary, we showed that oleate protects INS-1E beta-cells from palmitate-induced apoptosis by the suppression of ER stress which was independent of chaperone activation. (C) 2013 The Authors. Published by Elsevier Inc. All rights reserved.