Bisphenol A induces apoptosis and autophagy in murine osteocytes MLO-Y4: Involvement of ROS-mediated mTOR/ULK1 pathway

Bisphenol A (BPA) is a widely environmental endocrine disruptor. The accumulated BPA in humans is toxic to osteoblasts and osteoclasts, but few studies focused on the effects of BPA on osteocytes, the most abundant bone cell type, contributing to the development and metabolism of bone. Here, we reported that BPA (50, 100, 200 μmol/L) inhibited the cell viability of osteocytes MLO-Y4, promoted G0/G1 phase arrest and apoptosis in a dose-dependent manner. BPA treatment significantly increased the levels of autophagy-regulated proteins including Beclin-1 and LC3-II along with the decrease of p62, accompanied by the elevation of autophagy flux and the accumulation of acidic vacuoles, which was blocked by the autophagy inhibitor bafilomycin A1 (BafA1). Furthermore, BPA significantly inhibited the mammalian target of rapamycin (mTOR) and activated Unc-51 like autophagy activating kinase 1 (ULK1) signaling, leading to the decreased p-mTOR/mTOR ratio and the increased p-ULK1/ULK1 ratio. The mTOR activator MHY1485 (MHY) or the ULK1 inhibitor SBI-0206965 (SBI) prevented autophagy and enhanced apoptosis caused by BPA, respectively. In addition, BPA increased the levels of intracellular reactive oxygen species (ROS) and malondialdehyde (MDA) and decreased antioxidant enzymes nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) levels, resulting in oxidative stress. The ROS scavenger N-acetylcysteine (NAC) attenuated BPA-induced the mTOR/ULK1 pathway activation, apoptosis and autophagy. Collectively, ROS-mediated mTOR/ULK1 signaling is involved in BPA-induced apoptosis and autophagy in osteocytes MLO-Y4. Our data first provide in vitro evidence that apoptosis and autophagy as cellular mechanisms for the toxic effect of BPA on osteocytes, thereby advancing our understanding of the potential role of osteocytes in the adverse effect of BPA on bone health.