SPP1 and neoplasm: BMSCs, osteoclasts, osteoblasts and endothelial cells secrete several factors (Table 1), including vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), tumor necrosis factor alpha (TNF-α), hepatocyte growth factor (HGF), interleukin-6 and -8 (IL-6 and IL-8), osteopontin (OPN), angiopoietin-1 (Ang-1), B-cell activating factor, stromal cell-derived factor 1α (SDF1-α, also known as CXCL12), and various Notch family members, which are further up-regulated by tumor cell adhesion to extracellular matrix proteins and/or BMSCs [13,14].