The role of CD44 and cancer stem cells

Solid tumors are composed of mutually interacting cancer cells and tumor microenvironment. Many environmental components, such as extracellular matrix (ECM), mesenchymal stem cells, endothelial and immune cells, and various growth factors and cytokines, provide signals, either stimulatory or inhibitory, to cancer cells and determine their fates. Meanwhile, cancer cells can also educate surrounding cells or tissues to undergo changes that are in favorable of tumor progression. CD44, as a transmembrane receptor for hyaluronic acid (HA) and many other ECM components and a coreceptor for growth factors and cytokines, is a critical cell surface molecule that can sense, integrate, and transduce cellular microenvironmental signals to membrane-associated cytoskeletal proteins or to cell nucleus to regulate a variety of gene expressions that govern cell behaviors. Mounting evidence suggests that CD44, particularly CD44v isoforms, are cancer stem cell (CSC) markers and critical regulators of cancer stemness, including self-renewal, tumor initiation, and metastasis. Thus, CD44 is widely used alone or in combination with other cell surface markers to isolate or enrich CSCs through fluorescence-activated cell sorting of dissociated single cells that originate from the patient, xenograft tumor tissues, or tumor cell cultures. Sorted cells are cultured in a specialized culture medium for spheroid formation or inoculated into immunodeficient mice for the analysis of tumorigenic or metastatic potential. In this chapter, detailed experimental methods regarding CD44⁺ tumor cell isolation, spheroid culture, and characterization will be described.