Tributyltin-induced endoplasmic reticulum stress and its Ca2 +-mediated mechanism

Abstract

Organotin compounds, especially tributyltin chloride (TBT), have been widely used in antifouling paints for marine vessels, but exhibit various toxicities in mammals. The endoplasmic reticulum (ER) is a multifunctional organelle that controls post-translational modification and intracellular Ca2+ signaling. When the capacity of the quality control system of ER is exceeded under stress including ER Ca2+ homeostasis disruption, ER functions are impaired and unfolded proteins are accumulated in ER lumen, which is called ER stress. Here, we examined whether TBT causes ER stress in human neuroblastoma SH-SY5Y cells. We found that 700nM TBT induced ER stress markers such as CHOP, GRP78, spliced XBP1 mRNA and phosphorylated eIF2α. TBT also decreased the cell viability both concentration- and time-dependently. Dibutyltin and monobutyltin did not induce ER stress markers. We hypothesized that TBT induces ER stress via Ca2+ depletion, and to test this idea, we examined the effect of TBT on intracellular Ca2+ concentration using fura-2 AM, a Ca2+ fluorescent probe. TBT increased intracellular Ca2+ concentration in a TBT-concentration-dependent manner, and Ca2+ increase in 700nM TBT was mainly blocked by 50μM dantrolene, a ryanodine receptor antagonist (about 70% inhibition). Dantrolene also partially but significantly inhibited TBT-induced GRP78 expression and cell death. These results suggest that TBT increases intracellular Ca2+ concentration by releasing Ca2+ from ER, thereby causing ER stress.