THE ROLE OF CD133 IN THE IDENTIFICATION AND MAINTENANCE OF CANCER STEM CELLS DERIVED FROM HUMAN GLIOBLASTOMA

Glioblastoma multiforme (GBM) is the most common and lethal type of brain tumor in adults. There are some evidences that it is maintained by a population of cells, namely cancer stem cells, able to self-renew and propagate the tumor. Despite recent advances in surgical and therapeutic treatment, no tangible improvements have been made to prolong the patients’ survival. The role of the cell surface CD133 as a cancer stem cell marker in brain tumors has been widely investigated, since it identifies cells within glioblastomas able to initiate neurosphere growth and form heterogeneous tumors when transplanted in immune-compromised mice. However, evidences of CD133-negative cells exhibiting similar properties have been reported, making definitive conclusions on the correlation between tumor-initiating capabilities of glioma stem cells and CD133 expression difficult to drawn. Moreover, the functional role of CD133 in cancer stem/progenitor cells is not known. We analyzed the efficiency of CD133 in the identification and isolation of glioblastoma stem cells and we investigated its functional role, studying the biological effects of CD133 down-regulation in GBM-derived neurospheres in vitro and in vivo. We observed that CD133 is homogenously expressed in the cytoplasm of the cells composing the neurospheres, while its expression on the cell surface is highly variable. Cloning of single CD133-positive and CD133-negative cells demonstrated that CD133 shuttles dynamically between the plasmamembrane and the cell cytoplasm and no hierarchical relation can be established. Knockdown of CD133 by lentiviral-mediated short hairpin RNAs (shRNA) reduces the self-renewal and tumorigenic capacity of the GBM-derived neurosphere cells. Taken together, our data suggest that cell surface CD133 is not useful for the isolation of glioblastoma stem cells, since a complex regulation of its expression in terms of protein levels and cellular localization exists. However, CD133 may contribute to the maintenance and the tumorigenic potential of glioblastoma stem cells. This implies that CD133 could be used as a therapeutic target in glioblastomas, regardless of its expression on cell surface.