Cancer stem cells and therapeutic perspectives

The cancer stem cell hypothesis suggests that neoplastic clones are maintained exclusively by a rare fraction of cells with stem cell proprieties. Stem cells are defined as cells which are able to both extensively self-renew and differentiate into progenitors. Furthermore, stem cells are also attractive candidates as origin of cancers, as in their long lifespan mutations and epigenetic changes they can increase allowing for increasing evolution toward malignancy. Herein, we discuss the evidences reported in literature on existence of cancer stem cells in several tumors and mechanisms of the extrinsic and intrinsic circuitry controlling stem cell fate as well as their possible connections to cancer. In particular, the review will focus on recent results on conserved Polycomb Group (PcG) gene family, an epigenetic chromatin modifiers involved in cancer development and also in the maintenance of embryonic and adult stem cells. There are two distinct multiprotein PcG complexes identified, Polycomb repressive complex (PRC) 1 and 2. The fact that either PRC1 Bmi1 than PRC2 SU(Z)12 components are implicated in self-renewal stem cells and up-regulated in several kind of human cancer, confirm the importance of (de)regulation of the PcG genes in cancer and stem cell biology. Moreover, Bmi1 and SU(Z)12 are downstream target of Sonic hedgehog (Shh) and Wnt signaling respectively, providing for a connection between epigenetic change regulators (PcG) and developmental-signaling pathways. Finally, potential therapies using inhibitors acting on cancer stem cell population such as cyclopamine, an inhibitor of hedgehog signalling, 6-bromoindirubin-3'-oxime (BIO) which acts on GSK3 and inhibitors of beta-catenin signaling such as exisulind and the tyrosine-kinase inhibitor STI571/Gleevac/imatinib will also discuss.