Sigma-1 receptor overexpression promotes proliferation and ameliorates cell apoptosis in β-cells

Sigma-1 receptor (Sig-1R) is a class of orphan receptors, the potential role of which in pancreatic islet cells remains poorly understood. The present study aimed to investigate the role of Sig-1R in islet β-cell proliferation and examine the effects of Sig-1R on islet β-cell injury under lipotoxic conditions. Sig-1R-overexpressing MIN6 cells were generated by lentiviral vector transfection. The effect of Sig-1R overexpression on cell proliferation detected by EdU staining, cell cycle progression by propidium iodide (PI), apoptosis by Annexin V-APC/PI, mitochondrial membrane potential by Mitolite Red and cytoplasmic Ca(2+) levelsby Fura-2/AM in islet β-cells were measured by flow cytometry. Western blot analysis was used to measure protein expression levels of endoplasmic reticulum (ER) stress markers glucose-regulated protein 78 and C/EBP homologous protein, mitochondrial apoptotic proteins Bcl-2-associated X and Bcl-2 and cytochrome c. In addition, ATP levels and insulin secretion were separately measured using ATP Assay and mouse insulin ELISA. Mitochondria-associated ER membrane (MAM) structures in MIN6 cells were then detected using transmission electron microscopy. Protein disulfide isomerase expression and possible colocalization between inositol 1,4,5-trisphosphate receptor and voltage-dependent anion channel 1 were examined using immunofluorescence. Sig-1R overexpression was found to promote β-cell proliferation by accelerating cell cycle progression. Furthermore, Sig-1R overexpression ameliorated the apoptosis rate whilst impairing insulin secretion induced by palmitic acid by relieving ER stress and mitochondrial dysfunction in MIN6 cells. Sig-1R overexpression also promoted Ca(2+) transport between mitochondria and ER by increasing the quantity of ER adjacent to mitochondria in the 50-nm range. It was concluded that Sig-1R overexpression conferred protective effects on β-cells against lipotoxicity as a result of the promotion of cell proliferation and inhibition of ER stress and oxidative stress, by regulating the structure of MAM.