High doses of dexamethasone induce endoplasmic reticulum stress-mediated apoptosis by promoting calcium ion influx-dependent CHOP expression in osteoblasts

BACKGROUND: The long-term use of dexamethasone (Dex), a well-known immunosuppressant, leads to an imbalance in bone metabolism and rapid decline of bone mineral density due to apoptosis of osteoblasts. The molecular mechanisms by which Dex induces osteoblast apoptosis remain unclear. MATERIALS AND METHODS: MC3T3-E1 cells were treated with 0, 10(−8), 10(−6), and 10(−4) M Dex for 24 h. ATF6, phosphorylated PERK, PERK, phosphorylated IRE1, and IRE1 expression, cell apoptosis, and caspase-12 and caspase-3 activity were measured. CHOP expression and calcium ion influx rate were measured in cells treated with 0 and 10(−4) M Dex for 24 h. The effect of 2-APB treatment was assessed in cells treated with 0 or 10(−4) M Dex. RESULTS: Levels of ATF6 and phosphorylated PERK and IRE1 increased in a dose-dependent manner in MC3T3-E1 cells treated with 10(−8), 10(−6), and 10(−4) M Dex, compared to the control group (P < 0.05). Cells treated with 10(−6) and 10(−4) M Dex had significantly increased apoptotic rates and caspase-12 and caspase-3 activities (P < 0.05). Cells treated with 10(−4) M Dex had significantly increased CHOP levels and calcium ion influx rates (P < 0.05). Combined treatment with 10(−4) M Dex and 2-APB abrogated the observed increases in cell apoptosis and caspase-12 and caspase-3 activities (P < 0.05). CONCLUSIONS: High doses of Dex induce CHOP expression by promoting calcium ion influx-dependent induction of ATF6, phosphorylated PERK and phosphorylated IRE1, which induce endoplasmic reticulum stress-mediated apoptosis in osteoblasts. 2-APB protects the osteoblasts from the effects of Dex, preventing endoplasmic reticulum stress-mediated apoptosis.