Abstract
The role of NR4A1 in apoptosis is controversial. Pancreatic β-cells often face endoplasmic reticulum (ER) stress under adverse conditions such as high free fatty acid (FFA) concentrations and sustained hyperglycemia. Severe ER stress results in β-cell apoptosis. The aim of this study was to analyze the role of NR4A1 in ER stress-mediated β-cell apoptosis and to characterize the related mechanisms. We confirmed that upon treatment with the ER stress inducers thapsigargin (TG) or palmitic acid (PA), the mRNA and protein levels of NR4A1 rapidly increased in both MIN6 cells and mouse islets. NR4A1 overexpression in MIN6 cells conferred resistance to cell loss induced by TG or PA, as assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, and TUNEL assays indicated that NR4A1 overexpression also protected against ER stress-induced apoptosis. This conclusion was further confirmed by experiments exploiting siRNA to knockdown NR4A1 expression in MIN6 cells or exploiting NR4A1 knock-out mice. NR4A1 overexpression in MIN6 cells reduced C/EBP homologous protein (CHOP) expression and Caspase3 activation induced by TG or PA. NR4A1 overexpression in MIN6 cells or mouse islets resulted in Survivin up-regulation. A critical regulatory element was identified in Survivin promoter (-1872 bp to -1866 bp) with a putative NR4A1 binding site; ChIP assays demonstrated that NR4A1 physically associates with the Survivin promoter. In conclusion, NR4A1 protects pancreatic β-cells against ER stress-mediated apoptosis by up-regulating Survivin expression and down-regulating CHOP expression, which we termed as "positive and negative regulation."
Highlights
It is not known whether NR4A1 plays a role in endoplasmic reticulum (ER) stress-induced -cell apoptosis
NR4A1 Gene Expression Is Induced in MIN6 Cells upon TG or palmitic acid (PA) Treatment—We examined NR4A1 mRNA expression in MIN6 cells treated with TG (0.5 M) or PA (0.4 mM) over several time points and found that TG treatment induced NR4A1 mRNA expression, which reached its peak at 2 h (Fig. 1A)
C/EBP homologous protein (CHOP) is regarded as a marker of TG- or PA-induced ER stress, and we found that both inducers increased CHOP expression (Fig. 1, C and D)
Summary
It is not known whether NR4A1 plays a role in ER stress-induced -cell apoptosis. Results: NR4A1 expression in -cells or islets correlated positively with Survivin expression and negatively with CHOP expression and apoptosis rate upon treatment with ER stress inducers. Conclusion: NR4A1 protects -cells from ER stress-induced apoptosis by up-regulating Survivin and down-regulating CHOP expression. NR4A1 overexpression in MIN6 cells conferred resistance to cell loss induced by TG or PA, as assessed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay, and TUNEL assays indicated that NR4A1 overexpression protected against ER stressinduced apoptosis. This conclusion was further confirmed by experiments exploiting siRNA to knockdown NR4A1 expression in MIN6 cells or exploiting NR4A1 knock-out mice. NR4A1 protects pancreatic -cells against ER stress-mediated apoptosis by up-regulating Survivin expression and down-regulating CHOP expression, which we termed as “positive and negative regulation.”
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