This study suggests two forms of cancer stem cells, a localized form and a migratory form

This study suggests two forms of cancer stem cells, a localized form and a migratory form. thus selectively drive tumor growth. Current chemotherapeutic agents and radiation therapy target the rapidly dividing cells that form the bulk of the tumor but do not target the relatively quiescent cancer stem cell (CSC), thus possibly accounting for treatment failures. Furthermore, both SCs and CSCs appear to have multidrug resistance, antiapoptotic machinery, and enhanced DNA repair allowing them to evade conventional treatments [14]. Thus, to target tumors effectively and with minimal toxicity, drugs need to be identified that specifically target the relatively rare CSC population while sparing normal tissue stem cells. == 2. Biologic Properties of Cancer Stem Cells == The properties that CSCs are postulated to exhibit are: (1)tumorigenic capacity or self-renewal, (2)the potential for multilineage differentiation (such that they can recapitulate the multiple tumor cell types found in the parent tumor), (3)serial passage, and (4)expression of a unique repertoire of surface markers that allow for their reliable identification and purification [5]. Specifically, the CSC hypothesis posits that within a given tumor, only a distinct phenotypic subset of cells has tumorigenic capacity, that is, injection of this cell fraction into nude mice is able to fully recapitulate the parent tumor. Currently, serial passage in xenotransplantation models is the gold standard assay for defining the CSC fraction [6]. Normal tissue SCs tightly regulate the balance between self-renewal and differentiation, and quiescence and proliferation (reviewed in [7,8]). SC number, in the context of the stem cell niche, is precisely maintained via the ratio PCDH9 of symmetric and asymmetric cell divisions. Dysregulation of self-renewal pathways appears to result in CSC overpopulation [9,10]. This may be due to an increase in symmetric divisions of the CSC and may represent a potential drug target Bibf1120 (Nintedanib) [11,12]. As has been well characterized in the hematopoietic system, the cells within solid organs appear to demonstrate a hierarchy in which stem cells give rise to committed progenitor cells which Bibf1120 (Nintedanib) give rise to rapidly proliferating cells which give rise to terminally differentiated cells. As SCs mature from a self-renewable stem cell to a terminally differentiated cell, they gradually shed their ability for self-renewal and pluripotency but gain mitotic activity. By analogy to normal cells SCs, CSCs must able to give rise to all the different cell types that comprise a given tumor, lending credence to the idea that tumors can be viewed as aberrantly functioning complex organs. Tumor heterogeneity is likely a consequence of ongoing build up of mutations over time that result in variable examples of Bibf1120 (Nintedanib) cellular differentiation. == 3. Dysregulation of Self-Renewal Pathways in CSCs: Hedgehog Pathway Involvement == Mutations in fundamental self-renewal pathways, such as the Hh pathway, likely cause normal stem cells to become self-employed of regulatory signals that are generated by their microenvironment. These pathways play a critical role in development and are usurped by transformed cells for tumor initiation, progression, and metastasis. The hedgehog proteins are secreted signaling proteins that were in the beginning recognized inDrosophilaas section polarity genes. In multiple cells types, the Hh pathway takes on a key part in organogenesis, patterning, and stem cell maintenance. Depending upon context, Hh can function both like a morphogen and a mitogen. Like a morphogen, Hh induces cell differentiation inside a concentration-dependent manner. Like a mitogen, it drives the proliferation of precursor cells and mediates the connection between the epithelial and mesenchymal compartments [13]. In multiple cells types, it has been shown that mesenchymal cells and progenitor cells are Hh responsive. In contrast, mature epithelial cells are typically nonresponsive to Hh. The components of the Hh signal transduction machinery have been elucidated, and while there are some important variations across species, the pathway remains relatively well conserved betweenDrosophilaand higher organisms. Three secreted Hh ligands have been recognized in humans, Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh) and are known to transmission in an autocrine and paracrine manner. In the absence of Hh ligand, Patch (PTCH), the hedgehog receptor binds and inhibits Smoothened (SMO) and thus prevents target gene transcription. Binding of the Hh ligand to PTCH releases.