CXCL12 and hepatocellular carcinoma: HSCs have a special role in the secretion of soluble factors, such as IL-6, vascular endothelial growth factor (VEGF), and stromal cell-derived factor 1 (SDF-1), and in increasing the level of reactive oxygen species (ROS) to modulate expression of the hypoxia-inducible factor 1-alpha (HIF-1α) in promoting the process of angiogenesis, epithelial-mesenchymal transition (EMT), HCC invasion, and progression.