ER stress increases expression of intracellular calcium channel RyR1 to modify Ca2+ homeostasis in pancreatic beta cells

Abstract

Pancreatic beta cells maintain glucose homeostasis by secreting pulses of insulin in response to a rise in plasma glucose. Pulsatile insulin secretion occurs as a result of glucose-induced oscillations in beta-cell cytosolic Ca²⁺. The endoplasmic reticulum (ER) helps regulate beta-cell cytosolic Ca²⁺, and ER stress can lead to ER Ca²⁺ reduction, beta-cell dysfunction, and an increased risk of type 2 diabetes. However, the mechanistic effects of ER stress on individual calcium channels are not well understood. To determine the effects of tunicamycin-induced ER stress on ER inositol 1,4,5-triphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) and their involvement in subsequent Ca²⁺ dysregulation, we treated INS-1 832/13 cells and primary mouse islets with ER stress inducer tunicamycin (TM). We showed TM treatment increased RyR1 mRNA without affecting RyR2 mRNA and decreased both IP3R1 and IP3R3 mRNA. Furthermore, we found stress reduced ER Ca²⁺ levels, triggered oscillations in cytosolic Ca²⁺ under subthreshold glucose conditions, and increased apoptosis and that these changes were prevented by cotreatment with the RyR1 inhibitor dantrolene. In addition, we demonstrated silencing RyR1-suppressed TM-induced subthreshold cytosolic Ca²⁺ oscillations, but silencing RyR2 did not affect these oscillations. In contrast, inhibiting IP3Rs with xestospongin-C failed to suppress the TM-induced cytosolic Ca²⁺ oscillations and did not protect beta cells from TM-induced apoptosis although xestospongin-C inclusion did prevent ER Ca²⁺ reduction. Taken together, these results show changes in RyR1 play a critical role in ER stress-induced Ca²⁺ dysfunction and beta-cell apoptosis.

Keywords: beta cells; calcium channels; endoplasmic reticulum stress (ER stress); inositol 1,4,5-triphosphate (IP(3)) receptor (IP(3)Rs); ryanodine receptor (RyRs).