An acidic pH environment increases cell death and pro-inflammatory cytokine release in osteoblasts: the involvement of BAX inhibitor-1

Abstract

BAX Inhibitor-1 (BI-1), a transmembrane protein on the endoplasmic reticulum, has been studied previously in various physio/pathological conditions, but not in bone cells. In this study, using the MG63 osteoblast cell line and osteoblasts differentiated from stem cells, the role of BI-1 was studied. First, expression of BI-1 was confirmed in osteoblasts, as well as osteoclasts, in mouse tibiae bone immunohistochemistry. For evaluation of a recently published property of BI-1, an acidic pH-dependent Ca²⁺ channel-like effect in osteoblasts, acidic pH-associated cell death, and pro-inflammatory cytokine release were examined. In MG63 osteoblasts, acidic pH induced a pH-dependent increase in cell death and ER stress, as determined by elevated expression of GRP78, CHOP, phospho-eIF2α, IRE-1α, spliced XBP-1, and phospho-JNK. In osteoblasts, mitochondrial Ca²⁺ also showed a strong pH-dependent increase. BI-1 knock-down using siRNA protected cells against acidic pH, regulating mitochondrial Ca²⁺ accumulation, possibly via the acidic pH-dependent Ca²⁺ channel-like effect of BI-1. BI-1 knock-down also resulted in inhibition of acidic pH-induced release of pro-inflammatory cytokines, including IL-1β, IL-6, and TNF-α. In addition, bone marrow stem cells were differentiated into human osteoblasts, which showed increased expression of BI-1 mRNA and protein. In differentiated primary human osteoblasts, acidic pH-associated cell death, mitochondrial Ca²⁺ accumulation, and pro-inflammatory cytokine release were more significant than in non-differentiated stem cells. In summary, endogenous expression of BI-1 is associated with acidic pH-induced Ca²⁺ release, cell death, and pro-inflammatory cytokine release in human osteoblasts.