Diabetes is caused by loss or dysfunction of beta cells. Endoplasmic reticulum (ER) stress has been implicated in beta cell loss in both type 1 and type 2 diabetes. Hyperglycemia and increased insulin demand cause ER stress, activating the unfolded protein response (UPR). The initial beta cell response to stress increases cell number through proliferation, but prolonged stress leads to beta cell demise. Precisely characterizing the beta cell UPR could lead to a better understanding of how beta cells are lost in diabetes. In this study, mouse primary islet cells were treated with ER stressors Thapsigargin (Tg) or tunicamycin (Tm), which activate all three arms of the UPR: Atf6, Ire1 and Perk. In mouse beta cells, Tg (1uM) activated the Ire1 (tested by splicing of Xbp1) and Perk (tested by phosphorylation of eIF2-alpha) pathways within 1 hour, and induced Atf6 transcriptional targets by 4 hours. Specific roles of Atf6 in the beta cell ER stress response are not well understood, with both positive and negative roles reported in the literature. Therefore, we tested how Atf6 participates in the acute beta cell response to severe stress, including UPR pathway activation and cell death. We found that knockdown of Atf6 leads to a blunted ER stress response, with delayed activation of Atf6 target genes Grp78, Hyou1 and Pdia4, but not HerpUD1. The Ire pathway was also negatively impacted by Atf6 knockdown, with blunted activation of Xbp1 target genes Sec24D, Erdj4 and Ssr3. Perk pathway activation was not affected by knockdown of Atf6 (p-eIF2-alpha, Chop mRNA). Finally, acute knockdown of Atf6 increased beta cell death even in the absence of stress (n=4) and made beta cells more prone to stress-mediated cell death (n=2). On the other hand, overexpression of ATF6 had no impact on Tg-mediated beta cell death suggesting Atf6 was not sufficient to protect (n=2) against irreversible ER stress. In sum, our data suggest a nuanced role for Atf6 in the beta cell stress response and the cell fate choice between life and death.
R.B. Sharma: None. C.O. Darko: None. B. Gablaski: None. L.C. Alonso: None.
National Institutes of Health (R01DK114686); National Institute of Diabetes and Digestive and Kidney Disease (R01DK113300)