A novel secretagogin/ATF4 pathway is involved in oxidized LDL-induced endoplasmic reticulum stress and islet ß-cell apoptosis.

Excessive accumulation of cholesterol in ß cells initiates endoplasmic reticulum (ER) stress and associated apoptosis. We have reported that excessive uptake of cholesterol by MIN6 cells decreases the expression of secretagogin (SCGN) and then attenuates insulin secretion. Here, we aimed to determine whether cholesterol-induced SCGN decrease is involved in the modulation of ER stress and apoptosis in pancreatic ß cells. In this study, MIN6 cells were treated with oxidized low-density lipoprotein (ox-LDL) for 24 h, and then intracellular lipid droplets and cell apoptosis were quantified, and SCGN and ER stress markers were identified by western blot analysis. Furthermore, small interfer RNA (siRNA)-mediated SCGN knockdown and recombinant plasmid-mediated SCGN restoration experiments were performed to confirm the role of SCGN in ER stress and associated cell apoptosis. Finally, the interaction of SCGN with ATF4 was computationally predicted and then validated by a co-immunoprecipitation assay. We found that ox-LDL treatment increased the levels of ER stress markers, such as phosphorylated protein kinase-like endoplasmic reticulum kinase, phosphorylated eukaryotic initiation factor 2 alpha, activating transcription factor 4 (ATF4), and transcription factor CCAAT-enhancer-binding protein homologous protein, and promoted MIN6 cell apoptosis; in addition, the expression of SCGN was downregulated. siRNA-mediated SCGN knockdown exacerbated ß-cell ER stress by increasing ATF4 expression. Pretreatment of MIN6 cells with the recombinant SCGN partly antagonized ox-LDL-induced ER stress and apoptosis. Furthermore, a co-immunoprecipitation assay revealed an interaction between SCGN and ATF4 in MIN6 cells. Taken together, these results demonstrated that pancreatic ß-cell apoptosis induced by ox-LDL treatment can be attributed, in part, to an SCGN/ATF4-dependent ER stress response.

A microRNA­24­to­secretagogin regulatory pathway mediates cholesterol­induced inhibition of insulin secretion.

Hypercholesterolemia is a key factor leading to ß­cell dysfunction, but its underlying mechanisms remain unclear. Secretagogin (Scgn), a Ca2+ sensor protein that is expressed at high levels in the islets, has been shown to play a key role in regulating insulin secretion through effects on the soluble N­ethylmaleimide­sensitive factor attachment receptor protein complexes. However, further studies are required to determine whether Scgn plays a role in hypercholesterolemia­associated ß­cell dysfunction. The present study investigated the involvement of a microRNA­24 (miR­24)­to­Scgn regulatory pathway in cholesterol­induced ß­cell dysfunction. In the present study, MIN6 cells were treated with increasing concentrations of cholesterol and then, the cellular functions and changes in the miR­24­to­Scgn signal pathway were observed. Excessive uptake of cholesterol in MIN6 cells increased the expression of miR­24, resulting in a reduction in Sp1 expression by directly targeting its 3' untranslated region. As a transcriptional activator of Scgn, downregulation of Sp1 decreased Scgn levels and subsequently decreased the phosphorylation of focal adhesion kinase and paxillin, which is regulated by Scgn. Therefore, the focal adhesions in insulin granules were impaired and insulin exocytosis was reduced. The present study concluded that a miR­24­to­Scgn pathway participates in the mechanism regulating cholesterol accumulation­induced ß­cell dysfunction.