SPP1 and hepatocellular carcinoma: Osteopontin up-regulates MMP-2 through activating the SDF-1α/CXCR4 axis, mediated by binding to integrin αvβ3 and CD44v6 and activating the PI3K/Akt and JNK pathways in hepatocellular carcinoma cells (HepG2 and SMMC7721).