FAP and breast cancer: Bone-tropic breast cancer cells expressed abundantly V-ATPase and MCT4, leading to intratumoral acidosis, which induced α-SMA+FAP+ CAFs, osteoblasts and MSCs to express inflammatory and nociceptive mediators including nerve growth factor, brain-derived neurotropic factor, IL-1β, IL-6, CXCL8, and CCL5 [114], which may be responsible for bone pain.