Cancer stem cells and novel targets for antitumor strategies

Cancer stem cells (CSCs) were identified in human leukemias in landmark studies of John Dick and his colleagues. Subsequently, similar cancer stem-like cells were identified in solid tumors of the breast, colon, brain and other sites. CSCs have distinct markers and are highly tumorigenic compared to other subsets. They can differentiate into all the cell phenotypes of the parental tumor. Other key features include activation of pluripotency genes (Oct4, Sox2, Nanog), self-renewal, formation of tumor spheres in low-adherence cultures, and multi-drug resistance. Clinically, drug resistance is probably the most important feature, because CSCs resist conventional cancer therapies and are likely to play a major role in cancer relapse. Based on their properties, several molecules have been targeted for therapy with drugs as follows. 1) The self-renewal pathways Wnt/β-catenin, Hedgehog and Notch. 2) The aryl hydrocarbon receptor (AHR), with tranilast and other AHR agonists. 3) Cytokines and inflammatory pathways (e.g., IL-6, IL-8, NF-κB). 4) TGF-β and epithelial- to-mesenchymal transition (EMT) pathways. 5) Homing molecules involved in metastasis; most notably CXCR4 or its ligand CXCL12. 6) Growth factors, their receptors and coreceptors (such as neuropilin-1), and signaling components (e.g., tyrosine kinases). 7) Cell-surface markers (CD44 and integrins). Several drugs have been identified by screening or other observations (salinomycin, metformin, tesmilifene, sulforaphane, curcumin, piperine and others). Some of these drugs are at preclinical or early clinical phases of development, and it remains to be seen how many will progress to clinical application. This review focuses on some promising new developments in anti-CSC drug therapy.