CD44: More than a mere stem cell marker.

CD44 is a cell adhesion molecule that plays an important role in tumor progression and metastasis. The role of CD44 in tumorigenesis is due to its binding to extracellular matrix components, including hyaluronan (HA) and osteopontin (OPN), and to messenger molecules, such as growth factors present in the tumor microenvironment. HA and OPN are highly abundant in the leukemic stem cell niche and in solid tumors of various cancer types, where they contribute to the maintenance of the stemness of malignant cells. CD44 has consequently been recognized as a cancer stem cell marker in several types of cancers, which has been a topic of much recent research. In this review we have addressed the question of how CD44 might promote cancer cell stemness by interacting with extracellular matrix components, growth factors and cytokines.