Signaling pathways in cancer stem cells (Notch, Hedgehog, Wnt)

The key regulators in the embryonic life, and later in the differentiation of tissues and organs are the evolutionary reserved signalling pathways, as Notch, Hedgehog and Wnt. Mutations of these pathways have been identifi ed in many tumor types, increasing the risk  to the appearance of cancer stem cells (CSC), with very similar geno- and phenotype as normal stem cells have. Such CSCs with stemness functions can be developed not only from normal stem cells, but also from progenitor and differentiated cells. The main characteristics of CSC are the self maintenance, slow growth rate, very effective DNA-repair system, etc. All of these can contribute to the resistance. Further problems are the low number of CSC in the whole tumor mass, which makes rather diffi cult to achieve the effective drug concentration in CSC. The mentioned ancient pathways interact with many other pathways to form a network, which can infl uence the strategy of therapy. No doubt, that these pathways are promising targets, however, till now the clinical effectiveness is very low due to some reasons mentioned above. Nevertheless, some drugs are already in clinical use, either as monotherapy or part of the combinations. Little is known about the relationship between the pathways and the microenvironment, which has an outstanding role in the cellular activities, sometimes resulting opposite output. It is a great challenge to design effective drugs against CSC, similarly to fi nd reliable predictive biomarkers, which unfortunately still missing, since a reasonable drug-marker interactions would speed up the personalized treatment.