Alendronate induces osteoclast precursor apoptosis via peroxisomal dysfunction mediated ER stress.

Nitrogen-containing bisphosphonates including alendronate (ALN) are the current first line antiresorptive drug in treating osteoporosis. In our study, we found that ALN administration impaired the secretion of platelet derived growth factor-BB (PDGF-BB), the most important angiogenic cytokines produced by preosteoclast (POC), in both sham and ovariectomized (OVX) mice. To further understand this phenomenon, we induced bone marrow macrophages (BMMs) to POCs in vitro and detected the effects of ALN particularly in POCs. The proapoptotic effect of ALN in POCs was confirmed by flow cytometry. On the molecular level, we found that farnesyl diphosphate synthase (FDPS) inhibition of ALN led to peroxisomal dysfunction and up regulation of cytoprotective protein glucose-regulated protein (GRP) 78. Peroxisomal dysfunction further induced endoplasmic reticulum (ER) stress in POCs and finally resulted in cell apoptosis marked by reduced expression of B-cell lymphoma 2 (Bcl-2) and increased expressions of CCAAT/enhancer binding protein homologous protein (CHOP), Bcl2 associated X (Bax), and cleaved caspase-3. We concluded that ALN has no selectivity in inhibiting POC and mature osteoclast. For POCs, ALN inhibition of FDPS leads to peroxisomal dysfunction, which further mediates ER stress and finally causes cell apoptosis. Considering that decreased angiogenesis is also an important issue in treating osteoporosis, how to preserve pro-angiogenic POCs while depleting mature osteoclasts is a problem worthy to be solved.