Hypoxia induces beta cell death by inhibiting the adaptive UPR.
(2015) 51st Annual Meeting of the European Association for the Study of Diabetes — Location: Stockholm, Sweden (14.September.2015)
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- Background and aims: Hypoxia is implicated in the loss of functional beta cell mass in type 2 diabetes and with islet transplantation, although the mechanisms are unknown. The adaptive unfolded protein response (UPR) is required for endoplasmic reticulum (ER) homeostasis and beta cell integrity. Here we investigated the influence of hypoxia on the adaptive UPR and the role it plays in apoptosis. Materials and methods: Isolated mouse islets and MIN6 cells were exposed to various O2 tensions. Ddit3 (Chop) and Hif1α were inhibited using siRNA. Hspa5 (Bip) was overexpressed using a plasmid vector. JNK was inhibited using SP600125. UPR and hypoxia-response gene expression was assessed in islets from prediabetic and diabetic db/db mice and age-matched lean control mice. mRNA and protein levels were measured by real-time RT-PCR and western blot. Apoptosis was measured by DNA fragmentation ELISA. Results: Deprivation of O2 (1% vs 20%) for 4-24h markedly reduced the mRNA and protein levels of adaptive UPR genes, including Hspa5, Hsp90b1 and Fkbp11 as well as the activation of Xbp1 (splicing) and PERK (phosphorylation). Opposing effects were observed in MEF cells suggesting that hypoxia specifically inhibits the adaptive UPR in beta cells. This was accompanied by upregulation of integrated stress response (ISR) genes, including Ddit3, Atf3 and Trb3 along with increased phosphorylated EIF2A. Interestingly, Ddit3 knockdown significantly increased adaptive UPR gene expression in association with partial protection against hypoxia-induced apoptosis (p<0.05). Moreover, Hspa5 overexpression alone partially protected against hypoxia-induced apoptosis (p<0.01). JNK inhibition, but not Hif1α knockdown, partially prevented the hypoxia-mediated loss of adaptive UPR gene expression and protected against hypoxia-induced apoptosis (p<0.01). Finally, mRNA levels of hypoxia-response genes, including Hyou1, Tpi1, Gapdh and Eno1, were markedly upregulated in vivo in the islets of diabetic db/db mice, but not in prediabetic db/db mice, suggesting that islet hypoxia correlates with beta cell failure. Interestingly, the upregulation of hypoxia-response genes further correlate with downregulation of adaptive UPR gene expression in diabetic db/db islets. Conclusion: Hypoxia inhibits the adaptive UPR in beta cells partially via Ddit3 and JNK activation, but independently of Hif1α. Downregulation of the adaptive UPR contributes to hypoxia-induced beta cell apoptosis and may play a role in the loss of functional beta cell mass in type 2 diabetes.
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Bensellam, M., Maxwell, E., Jonas, J.-C., Chan, J., & Laybutt, D. R. (2015). Hypoxia induces beta cell death by inhibiting the adaptive UPR. Diabetologia : clinical and experimental diabetes and metabolism, 58(Suppl. 1), S235. https://hdl.handle.net/2078.5/91661 (Original work published 2015)