Authors: Seyed Mojtaba Ghiasi, Tina Dahlby, Caroline Hede Andersen, Leena Haataja, Sólrun Petersen, Muhmmad Omar-Hmeadi, Mingyu Yang, Celina Pihl, Sophie Emilie Bresson, Muhammad Saad Khilji, Kristian Klindt, Oana Cheta, Marcelo J. Perone, Björn Tyrberg, Clara Prats, Sebastian Barg, Anders Tengholm, Peter Arvan, Thomas Mandrup-Poulsen, Michal Tomasz Marzec
Summary: Although endoplasmic reticulum (ER) chaperone binding to mutant proinsulin has been reported, the role of protein chaperones in the handling of wild-type proinsulin is under-investigated. Here, we have explored the importance of glucose regulated protein 94 (GRP94), a prominent ER chaperone known to fold insulin-like growth factors, in proinsulin handling within β-cells. We found that GRP94 co-immunoprecipitated with proinsulin and that inhibition of GRP94 function and/or expression reduced glucose-dependent insulin secretion, shortened proinsulin half-life and lowered intracellular proinsulin and insulin levels. This phenotype was accompanied by post-ER proinsulin misprocessing and higher numbers of enlarged insulin granules that contained amorphic material with reduced immunogold staining for mature insulin. Insulin granule exocytosis was two-fold accelerated but the secreted insulin had diminished bioactivity. Moreover, GRP94 knockdown or knockout in β-cells selectively activated Protein Kinase R-like Endoplasmic Reticulum Kinase (PERK), without increasing apoptosis levels. Finally, GRP94 mRNA was overexpressed in islets from T2D patients. We conclude that GRP94 is a chaperone crucial for proinsulin handling and insulin secretion.
Source: Diabetes, 2019; db180671