br Transplantation of the non
Transplantation of the non-tumorigenic human embryonic kidney (HEK) 293 Brefeldin-A that stably express CD133 into SCID mice resulted in tumor formation (Canis et al., 2013). It clearly showed that CD133 is suﬃcient to initiate tumorigenesis. Overexpression of CD133 in cul-tured human pancreatic cancer cell line MIA PaCa-2 that has 0.1% endogenous CD133 upregulated several gene expressions associated with stemness (Nomura et al., 2015). These upregulated stemness genes include KITLG, LIN28B, c-MYC, KLF4, GLI1, SOX2, NANOG, SIRT1, POU5F1, and CXCR4. Moreover, the MIA PaCa-2 cells that ectopically express high levels of CD133 were greatly tumorigenic as a very low number of the cells (10 or 1000 cells) was able to induce tumor for-mation in athymic nude mice. In several pancreatic cancer cell lines as
well as human PDAC patient xenografts, FACS sorted CD133+ popu-lation was DCLK1high and acetylated α-tubulinhigh (Bailey et al., 2014).
In addition to forming tumor sphere structures in vitro, these cells had an increased tumor-initiating ability as judged by the number of cells implanted into immunodeficient mice that develop cancer.
In head and neck cancer initiating cells (HNCIC), knockdown of CD133 reduced the stemness gene expressions of OCT4 and NANOG, enhanced epithelial diﬀerentiation and promoted apoptosis (Chen et al., 2011b). In addition, these eﬀects were also observed in the tumor tissues from the shCD133 derived xenograft mice, indicating that CD133 initiates tumor formation via upregulation of cell stemness and downregulation of cell diﬀerentiation as well as cell death. Using tumor sphere formation ability as an indicator of cancer stemness, shCD133 lentivirally infected-NHCICs decreased cancer stemness through in-activation of Src signaling.
Primary heterogeneous glioblastoma can be derived from either CD133+ or CD133− CSCs. These two types of glioblastoma CSCs al-though possess distinct features of growth and diﬀerentiation, both of them were able to induce tumor formation at a comparable level in nude mice (Beier et al., 2007). Between these two types, SOX2 was the most up-regulated stemness gene in the CD133+ glioblastoma cells and controlled tumorigenesis of this disease (Song et al., 2016). Knockdown of SOX2 hindered CD133+-mediated tumor formation abilities in vivo. Activation of Notch has been reported in glioblastoma CSCs (Castro et al., 2006; Stockhausen et al., 2010). Blockade of Notch through its gene silencing or a γ secretase inhibitor suppressed the glioblastoma tumor formation in the xenograted mice (Fan et al., 2010). In addition, this Notch inhibition decreased several CSC markers including CD133, nestin, Bmi1 and OLIG2.
3. CD133 in cancer development and progression
In a xenograft mouse model, the size of tumor derived from the high CD133+ HEK293 cells is dramatically larger than that from the low CD133+ HEK293 cells (Canis et al., 2013), suggesting that a role of CD133 in regulating cell growth during cancer development. Over-expression of CD133 increased cell proliferation, cell cycle progression and telomerase activity in pancreatic cancer AsPC-1 cells (Weng et al., 2016), suggesting that CD133 promotes tumor progression through upregulation of cell growth. Pancreatic intraepithelial neoplasia (PanIN) are precursors of pancreatic ductal adenocarcinoma (PDAC), the most common form of pancreatic cancer. Enrichment of DCLK1 and acetylated α-tubulin in the FACS-sorted CD133+ population of PDAC
cells was reported (Bailey et al., 2014). Expression of CD133, DCLK1 and CD44 were present in PanIN cells of oncogenic KrasG12D mouse
pancreas (Fig. 2, (Bailey et al., 2014; Delgiorno et al., 2014; Liou et al., 2017)). However, these 3 CSC markers were expressed in diﬀerent subpopulations of murine PanIN cells (Fig. 2), suggesting a diverse CSC population during PDAC development. Knockout of DCLK1 specifically in the pancreas reduced KrasG12D-induced PanIN formations and the size of the formed PanIN lesions (Westphalen et al., 2016). To-date, the function of CD133 and CD44 on PanIN development remains unclear.
Medulloblastoma in Group 3 is the most common malignant pe-diatric brain cancer that has an upregulation of c-MYC and persist ac-tivation of STAT3. The enriched CD133+ cancer cells of medullo-blastoma in Group 3 promote tumor growth through activation of STAT3 when implanted in the mouse brains of NOD/SCID (Garg et al., 2017). Moreover, blockade of activated STAT3 through shSTAT3 len-tivirus to knock down STAT3 or STAT3 inhibitors significantly reduced the tumor burden of the mouse brain that is caused by CD133+ me-dulloblastoma cells. Likewise, activation of STAT3 signaling has been International Journal of Biochemistry and Cell Biology 106 (2019) 1–7