GRP78, but Not Protein-disulfide Isomerase, Partially Reverses Hyperglycemia-induced Inhibition of Insulin Synthesis and Secretion in Pancreatic β-Cells

Abstract

Chronic hyperglycemia contributes to pancreatic ␤-cell dysfunction during the development of type 2 diabetes. Treatment of pancreatic ␤-cells with prolonged high glucose concentrations has been shown to reduce insulin promoter activity and insulin gene expression. Here, we examined the effect of high glucose on endoplasmic reticulum (ER) stress pathway activation and insulin production in INS-1 832/13 pancreatic ␤-cells. Treatment of cells with 25 mM glucose for 24 -48 h decreased insulin mRNA and protein levels and reduced the proinsulin translation rate, which was accompanied by enhanced unfolded protein response pathway activation (XBP-1 mRNA splicing and increased phospho-eIF2␣, CHOP, and active ATF6 levels). Overexpressing the ER chaperone GRP78 partially rescued high glucose-induced suppression of proinsulin levels and improved glucose-stimulated insulin secretion with no effect on insulin 2 mRNA levels. Under these conditions, there was little effect of GRP78 overexpression on ER stress markers. Knockdown of GRP78 expression under basal glucose conditions reduced cellular insulin levels and glucose-stimulated insulin secretion. Thus, GRP78 is essential for insulin biosynthesis, and enhancing chaperone capacity can improve ␤-cell function in the presence of prolonged hyperglycemia. In contrast, overexpression of the ER chaperone and oxidoreductase protein-disulfide isomerase (PDI) reduced glucose-stimulated insulin secretion and induced ER stress resulting from the accumulation of proinsulin in the ER. These results suggest a role for both GRP78 and PDI in insulin biosynthesis, although an excess of PDI disrupts normal proinsulin processing.

Biosynthesis and secretion of mouse cysteine‐rich with EGF‐like domains 2

“…Very recently, Hosur et al [16] suggest that CRELD2 would have an antagonistic activity in nicotine-induced up-regulation of acetylcholine receptor a4/b2. In their report, the induction of CRELD2 mRNA by nicotine was not parallel with those of other ER stress-inducible genes, GRP78 (HSPA5) [17,[23][24][25][26][27][28] and HERPUD1 [29]. These results suggest that CRELD2 is expected to function under ER stressunrelated conditions and that both the secreted and intracellular CRELD2 are likely to contribute to some neuronal events via the nicotinic signaling pathways.…”

GRP78 overproduction in pancreatic beta cells protects against high-fat-diet-induced diabetes in mice

“…Slides were then permeabilised with 0.1% Triton X-100/PBS. For antigen retrieval, slides were transferred into boiled sodium citrate buffer and microwaved to boil for 30 Western blotting Western blotting was performed as described previously [19]. Antibodies used: GRP78 and Myc tag (details provided above); phospho-EIF2α (catalogue no.…”

“…Glucose-regulated protein 78 kDa (GRP78, also known as BiP or HSPA5) is an abundant ER chaperone that has numerous roles in the ER [18], among which is an essential function in the folding of newly synthesised proteins, including proinsulin in pancreatic beta cells [19,20]. In addition, GRP78 is an important regulator of the UPR [21].…”

“…In addition, GRP78 is an important regulator of the UPR [21]. GRP78 overproduction has been shown to reduce ER stress in cultured cell lines and in tissues in vivo [19,[22][23][24] and also to protect pancreatic beta cells from lipotoxicity in vitro [10,25]. To test the hypothesis that ER stress contributes to beta cell dysfunction in obesity-induced diabetes we examined glucose homeostasis in response to high-fat diet (HFD) in transgenic mice overproducing GRP78 in pancreatic beta cells.…”

Protein Disulfide Isomerase: Structure, Mechanism of Oxidative Protein Folding and Multiple Functional Roles

“…PDI has been implicated in detrimental activities in cardiovascular diseases. Since over-expression of PDI in myocytes decreases the levels of misfolded pro-insulin molecule as well as insulin secretion, thereby it induces ER stress leading to apoptosis [86] .…”