Apoptosis of osteoblasts induced by glucocorticoid (GC) has been identified as a main cause of osteoporosis, bone loss and fractures, and the oxidative stress was found as an important contributor. Therefore, natural or synthetic agents with antioxidant activities can antagonize GCs-induced apoptosis in osteoblasts, and thus demonstrate the potential application to reverse osteoporosis. In this study, we showed that, indole-3-carbinol (I3C), a natural product found in broadly consumed plants of the Brassica genus, could block the cytotoxic effects of dexamethasone (Dex), and elucidated the underlying molecular mechanisms. Firstly, we showed that, I3C could effectively suppress Dex-induced cytotoxicity and apoptotic cell death in osteoblastic cells, as evidenced by the decrease in Sub-G1 cell population. Treatment of the cells with Dex resulted in activation of caspase-3/-8/-9 and subsequent cleavage of PARP, which was also effectively blocked by co-incubation of I3C. Moreover, exposure to Dex triggered a rapid onset and time-dependent superoxide overproduction in osteoblastic cells, which was effectively suppressed by addition of I3C. Excess intracellular ROS induced by Dex significantly suppressed the expression levels of Nrf2 and the downstream effectors, HO1 and NQO1, but these changes could be reversed by I3C. Knockdown of Nrf2 using siRNA silencing technique significantly reversed the protective effects of I3C against Dex-induced apoptosis and ROS generation. Taken together, I3C can reverse cytotoxicity of Dex through blocking ROS overproduction and enhancement of Nrf2 expression. This study may provide a safe and good strategy for molecular intervention of GCs-induced osteoporosis by using natural products. (C) 2015 Elsevier Inc. All rights reserved.