Hypoxia reduces ER-to-Golgi protein trafficking and increases cell death by inhibiting the adaptive unfolded protein response in mouse beta cells

Abstract

Aims/hypothesis Hypoxia may contribute to beta cell failure in type 2 diabetes and islet transplantation. The adaptive unfolded protein response (UPR) is required for endoplasmic reticulum (ER) homeostasis. Here we investigated whether or not hypoxia regulates the UPR in beta cells and the role the adaptive UPR plays during hypoxic stress. Methods Mouse islets and MIN6 cells were exposed to various oxygen (O 2 ) tensions. DNA-damage inducible transcript 3 (DDIT3), hypoxia-inducible transcription factor (HIF)1α and HSPA5 were knocked down using small interfering (si)RNA; Hspa5 was also overexpressed. db/db mice were used. Results Hypoxia-response genes were upregulated in vivo in the islets of diabetic, but not prediabetic, db/db mice. In isolated mouse islets and MIN6 cells, O 2 deprivation (1-5% vs 20%; 4-24 h) markedly reduced the expression of adaptive UPR genes, including Hspa5, Hsp90b1, Fkbp11 and spliced Xbp1. Coatomer protein complex genes (Copa, Cope, Copg [also known as Copg1], Copz1 and Copz2) and ER-to-Golgi protein trafficking were also reduced, whereas apoptotic genes (Ddit3, Atf3 and Trb3 [also known as Trib3]), c-Jun N-terminal kinase (JNK) phosphorylation and cell death were increased. Inhibition of JNK, but not HIF1α, restored adaptive UPR gene expression and ER-to-Golgi protein trafficking while protecting against apoptotic genes and cell death following hypoxia. DDIT3 knockdown delayed the loss of the adaptive UPR and partially protected against hypoxia-induced cell death. The latter response was prevented by HSPA5 knockdown. Finally, Hspa5 overexpression significantly protected against hypoxia-induced cell death. Conclusions/interpretation Hypoxia inhibits the adaptive UPR in beta cells via JNK and DDIT3 activation, but independently of HIF1α. Downregulation of the adaptive UPR contributes to reduced ER-to-Golgi protein trafficking and increased beta cell death during hypoxic stress.

Human mesenchymal‐stem‐cells‐derived exosomes are important in enhancing porcine islet resistance to hypoxia

“…In contrast to the NICCs in normoxic conditions, there was a marked reduction in the viability and function of NICCs after exposure to hypoxia in our study. Beta cells are sensitive to hypoxic stress because their function depends heavily on the acceleration of mitochondrial metabolism upon glucose stimulation to generate ATP . As a result, hypoxia indeed compromises the viability and function of islets .…”

“…Other studies have reported some disadvantages for MSCs‐based therapy, such as poor survival, limited differentiation, and dedifferentiation of cells with passaging and donor site, which limits the clinical application of MSC therapy. Therefore, some studies have turned more attention to the concept of cell‐free therapy .…”

“…Beta cells are sensitive to hypoxic stress because their function depends heavily on the acceleration of mitochondrial metabolism upon glucose stimulation to generate ATP. 39 As a result, Reducing hypoxic damage is an important factor to protect the islet function and improve the effect of IT.…”

Chemical hypoxia‐induced integrated stress response activation in oligodendrocytes is mediated by the transcription factor nuclear factor (erythroid‐derived 2)‐like 2 (NRF2)

“…Comparably, exposure of human diploid fibroblasts or HeLa cells to hypoxia led to phosphorylation of eIF2a and ATF4 induction in a PERK-dependent manner (Koumenis et al 2002;Blais et al 2004). Increase in Ddit3 expression during experimental hypoxia has as well been reported to occur, for example, in mouse islets cells (Bensellam et al 2016) and cardiomyocytes (Gao et al 2016). This is, however, to the best of our knowledge, the first report showing ISR induction caused by 'chemical hypoxia', and linking this induction to the KEAP1/NRF2 pathway.…”

“…Increase in Ddit3 expression during experimental hypoxia has as well been reported to occur, for example, in mouse islets cells (Bensellam et al . ) and cardiomyocytes (Gao et al . ).…”