Differentially Expressed Genes Regulating Glutathione Metabolism, Protein-Folding, and Unfolded Protein Response in Pancreatic β-Cells in Type 2 Diabetes Mellitus

Abstract

Impaired redox homeostasis in the endoplasmic reticulum (ER) may contribute to proinsulin misfolding and thus to activate the unfolded protein response (UPR) and apoptotic pathways, culminating in pancreatic β-cell loss and type 2 diabetes (T2D). The present study was designed to identify differentially expressed genes (DEGs) encoding enzymes for glutathione metabolism and their impact on the expression levels of genes regulating protein folding and UPR in β-cells of T2D patients. The GEO transcriptome datasets of β-cells of diabetics and non-diabetics, GSE20966 and GSE81608, were analyzed for 142 genes of interest using limma and GREIN software, respectively. Diabetic β-cells showed dataset-specific patterns of DEGs (FDR ≤ 0.05) implicated in the regulation of glutathione metabolism (ANPEP, PGD, IDH2, and CTH), protein-folding (HSP90AB1, HSP90AA1, HSPA1B, HSPA8, BAG3, NDC1, NUP160, RLN1, and RPS19BP1), and unfolded protein response (CREB3L4, ERP27, and BID). The GCLC gene, encoding the catalytic subunit of glutamate–cysteine ligase, the first rate-limiting enzyme of glutathione biosynthesis, was moderately down-regulated in diabetic β-cells from both datasets (p ≤ 0.05). Regression analysis established that genes involved in the de novo synthesis of glutathione, GCLC, GCLM, and GSS affect the expression levels of genes encoding molecular chaperones and those involved in the UPR pathway. This study showed for the first time that diabetic β-cells exhibit alterations in the expression of genes regulating glutathione metabolism, protein-folding, and UPR and provided evidence for the molecular crosstalk between impaired redox homeostasis and abnormal protein folding, underlying ER stress in type 2 diabetes.

The link between theANPEPgene and type 2 diabetes mellitus may be mediated by the disruption of glutathione metabolism and redox homeostasis

“…In pancreatic beta cells, knockdown of mitochondrial isocitrate dehydrogenase enzyme reduces insulin secretion [85]. It has been recently found [23] that pancreatic beta cells in T2D patients exhibit decreased expression of IDH2 , which strongly correlates with increased expression of CTH (cystathionine gamma-lyase), an enzyme in the transsulfuration pathway. It can be assumed that the correlations between the ANPEP polymorphisms and expression levels of ARPIN, MESP1, U7, PEX11A , and IDH2 genes may be attributed to shared enhancers co-regulating the expression of these genes co-localized at the 15q26.1 region.…”

“…The causes and mechanisms of changes in these enzymes are the subject of debate, but there is still no consensus on whether they are primary or secondary in the development of diabetes or if they accompany the disease. The same can be said for ANPEP, whose expression level is increased both in the plasma and beta cells of the pancreatic islets in patients with type 2 diabetes [23,50,96]. The main assumption is that increased expression of amino acid and peptide metabolism enzymes is required to provide amino acid precursors for the biosynthesis of glutathione, the deficit of which is considered a primary cause of type 2 diabetes [21,31,35].…”

“…Cystathionine gamma-lyase (CTH) is an enzyme that catalyzes the last step in the transsulfuration pathway by cleaving the L,L-cystathionine molecule into L-cysteine, part of which is utilized for the biosynthesis of glutathione [109]. Our recent study showed up-regulated expression of the CTH gene in pancreatic β-cells of T2D patients compared to non-diabetic controls, and the levels of CTH were correlated with genes regulating glutathione metabolism and protein folding [23]. Thus, the increased levels of enzymes in the transsulfuration pathway may be of great importance in providing the cell with cysteine for glutathione synthesis.…”

“…We recently proposed that a primary pathological condition responsible for oxidative stress and induces cellular stress responses, leading to pancreatic β-cell death and type 2 diabetes mellitus, is an endogenous deficiency of glutathione [21][22][23]. Glutathione, a tripeptide consisting of three amino acids: glutamate, cysteine, and glycine, is the most abundant non-protein thiol in cells, acting as the principal reducing agent and potent antioxidant, protecting the cell from oxidative damage [24].…”

“…In addition, the depletion of endogenous glutathione is correlated with increased blood glucose levels and contributes to the complications of type 2 diabetes [33,34,38]. Finally, additional evidence for glutathione deficiency in T2D is the results of genetic studies demonstrating a significant role of loss-of-function variants of genes encoding enzymes of glutathione metabolism in determining disease susceptibility [21,22,44–46], and the link of these genes to T2D risk is explained by decreased activity and/or expression of enzymes involved in glutathione biosynthesis, including in pancreatic beta cells [23,47].…”