Reg2 upregulates GRP78 in mouse insulinoma cells and attenuates experimentally induced unfolded protein response

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SAT 834-867-Islet Biology
Bench to Bedside
Saturday, June 15, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SAT-851
Lu Liu*1, Jun-Li Liu2 and Coimbatore B Srikant2
1McGill University, Montreal, QC, Canada, 2McGill University Health Centre, Montreal, QC, Canada
Members of the Reg family of proteins are not detected in adult pancreatic islet cells, but some members like Reg2 and Reg3b are known to be induced in b-cells of mice in response to autoimmune destruction, streptozotocin-induced diabetes and partial pancreatectomy-induced islet cell regeneration. These observations suggest that Reg proteins may promote islet cell growth and survival. We have previously shown that Reg2, a member of the Reg family of proteins, protects mouse insulinoma cells against streptozotocin-induced apoptosis (1). In this study we compared thapsigargin-induced ER stress-mediated unfolded protein response (UPR) in MIN6 cells overexpressing Reg2 (MIN6-Reg2) or the empty transfection vector (MIN6-VC) given that persistent insulin-secretory demand imposed by insulin resistance in type 2 diabetes causes UPR in pancreatic b-cells. Thapsigargin-induced UPR was assessed by the phosphorylation and activation of the canonical UPR signal transducers IRE1a and eIF2a. We found a significant reduction in the level of phosphorylation of both IRE1a and eIF2a in MIN6-Reg2 cells compared to that seen in MIN6-VC cells indicating that Reg2 overexpression attenuates UPR. This protective action of Reg2 seems to be mediated by the upregulation of the ER chaperone protein GRP78 (also known as BiP) and Raptor, a key component of mTORC1 signaling complex but not of Rictor, the required constituent of mTORC2 signaling complex. While Reg-2 did not alter mTOR itself, the enhanced presence of Raptor suggested an increased mTORC1 activity under basal conditions which was confirmed by the increase in the phosphorylation of p70 S6K, a downstream target of mTORC1 in MIN6-Reg2 cells. Additionally Akt, which is known to regulate mTORC1 activity, displayed a significant increase in Ser473phosphorylation in MIN6-Reg2, but not in MIN6-VC cells. Inhibition of Akt signaling using the specific inhibitor MK-2206 in MIN6-Reg2 cells attenuated Reg2-induced expression of GRP78/BiP both at the mRNA and protein levels. Collectively, our results suggest that ectopically introduced Reg2 protein protects insulin-producing cells against UPR by inducing GRP78 expression and by potentiating Akt and mTORC1 signaling. Although much of the mechanism of action of Reg2 remains to be elucidated in detail, Reg2 was found to be secreted into the culture medium, raising the possibility that it may act either as an autocrine growth factor through a putative, yet to be identified, cell surface receptor functionally coupled to AkT-mTORC1 signaling pathway and/or as an intracellular modulator of Akt-mTORC1 signaling.

1. Liu L, Liu JL, and Srikant CB. Reg2 protects mouse insulinoma cells from streptozotocin-induced mitochondrial disruption and apoptosis. Growth Factors 28: 370-378, 2010.

Nothing to Disclose: LL, JLL, CBS

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

Sources of Research Support: This work is supported by the Canadian Diabetes Association.