Streptozotocin induces endoplasmic reticulum stress and apoptosis via disruption of calcium homeostasis in mouse pancreas

Calcium homeostasis refers to the regulation of calcium ion concentration in the body. This concentration is tightly controlled by a stabilizing system consisting of calcium channels and calcium buffering proteins. Calcium homeostasis is crucial for cell survival. Various forms of cell death (e.g., necrosis and apoptosis) also share calcium signaling pathways and molecular effectors. Calcium acts not only as a ubiquitous second messenger involved in apoptosis along with various cell death inducers but also a regulator for the synthesis of enzymes/hormones such as insulin. We hypothesized that streptozotocin disrupts calcium homeostasis and the altered intracellular calcium levels may induce cell death. After streptozotocin administration, blood glucose level was increased while insulin levels decreased. The expression of insulin response markers also decreased relative to the vehicle group. L-type voltage-gated calcium channel expression and sarcoplasmic reticulum Ca2+ ATPase were increased by streptozotocin. Calcium buffering protein calbindin-D9k and calmodulin family members were also increased. The expression of genes involved in transporting calcium ions to the endoplasmic reticulum (ER) was decrease while the expression of those affecting the removal of calcium from the ER was increased. Depletion of calcium from the ER leads to ER-stress and can induce apoptosis. In the streptozotocin-treatment group, apoptosis markers were increased. Taken together, these results imply that the disruption of calcium homeostasis by streptozotocin induces ER-stress and leads to the apoptosis of pancreatic cells. Additionally, findings from this study suggest that imbalances in calcium homeostasis could promote pancreatic beta cell death and result in type I diabetes.