br reported in the CD ALDH cell
reported in the CD133+/ALDH+ cell population of human colon cancer (Lin et al., 2011). Furthermore, inhibition of STAT3 by short hairpin RNA or pharmacological compounds, Stattic and LLL12 in this popu-lation decreased cancer cell metabolic activity, CD133 protein level and gene expressions that are associated with cell proliferation including cyclin D1, survivin, Bcl-2, and Notch. The STAT3 inhibition also led to a smaller size of the CD133+/ALDH+-generated xenograft tumors. A higher expression of hormone gastrin precursor was detected in human colorectal cancer cells that have high CD133 (Ferrand et al., 2009). Furthermore, in xenografted mice, implantation of the CD133high/ CD44high/progastrinhigh cells resulted in bigger tumors than the CD133low/CD44low/progastrinlow cells due to the upregulation of JAK2, STAT3, ERK, and Akt.
4. CD133 and its signaling in cancer metastasis
A little more than a decade ago after discovering CD133 as a marker of Brefeldin-A tumor stem cells, accuVmulating evidence suggested that CD133 modulates cancer cell invasion, metastasis and drug resistance in many types of cancer (Fig. 3).
In metastatic ovarian cancer, increased mRNA expression of CD133 is regulated by transcription factor ARID3B. Knockdown of CD133 in ARID3B overexpressed cancer cells leads to quicker tumor-caused deaths in the xenograft mice as compared to these of CD133+ARID3B+ cancer cells (Roy et al., 2018). In addition, overexpressed CD133 pro-motes the attachment of ovarian adenocarcinoma cells including Kur-amochi, OVCA429 and Skov3IP cells to mesothelial cells in vitro and the mesothelium in ex vivo.
The CD133 expression has been associated with increased lymph node metastasis, upregulated expression of vascular endothelial growth factor C and a lower 5-year survival rate in pancreatic cancer patients (Maeda et al., 2008). CD133+ cancer cells selected from cultured human pancreatic cancer cell lines KP-2 and SUIT-2 showed elevated anchorage-independent growth as compared to the CD133− population in the same cell lines, implicating that CD133 promotes cancer cell transformation, invasiveness, and metastasis (Moriyama et al., 2010). In addition, CXCR4 was highly expressed in these CD133+ isolated cells and knockdown of CXCR4 by small interfering RNA diminished CD133-mediated cell migration and invasion. Similarly, as compared to the CD133+/CXCR4− cells isolated from a highly metastatic pancreatic cancer cell line L3.6 pl, CD133+/CXCR4+ cells from the same parental cells were able to intravasate into the portal vein in a xenograft mouse model (Hermann et al., 2007). Treating L3.6 pl generated xenograft tumors with a CXCR4 inhibitor AMD3100 completely abolished pan-creatic cancer metastasis.
Silencing of CD133 in Capan1M9 cells that have high levels of
Fig. 2. Cancer stem cell markers in PanIN lesions of mouse pancreas. Expression of cancer stem cell markers CD133 and CD44 in the PanIN1A lesions of p48cre:KrasG12D mice at the age of 14 weeks is evaluated by immunohistochemistry (A, B). In addition, tuft cells which express acetylated α-tubulin and possess cancer stem cell properties are also present in the PanIN lesions. (C) The H&E stain for visualizing PanIN1A structures in the same area shown in A and B. Scale bar: 50 μm.
Fig. 3. Signaling pathways mediated by CD133 to mod-ulate cancer metastasis. Reported cell signaling activated by CD133 or through CD133 to modulate cancer metastasis. These signaling pathways either regulate epithelial-mesenchymal transition (EMT) or cell migration. In the events that lead to cell migration, CD133 immunoprecipitates with either EGFR or Src to activate Akt and FAK respectively. In addition, CD133 physically interacts with HDAC6, α-tubulin, and β-catenin leading to activation of the Wnt signaling.
endogenous CD133 inhibits Capan1M9-induced lung and liver metas-tases in mice possibly through downregulation of genes that modulate epithelial-mesenchymal transition (EMT) process such as Slug and N-Cadherin (Ding et al., 2014). Moreover, ERK and Src inhibitors atte-nuated expression of CD133 and N-Cadherin in Capan1M9 cells, sug-gesting that activation of ERK and Src signaling leads to increased levels of CD133 as well as N-Cadherin. It also has been shown that ectopically expressed CD133 induced EMT and more invasive cells of MIA PaCa-2 through activation of NF-κB (Nomura et al., 2015). Furthermore, either silencing of PROM1 by shRNA technique or inhibition of NF-κB acti-vation by introduction of an IKKβ mutant or by a pharmacological BAY 11–7085 treatment, all of them abolished CD133 mediated invasiveness of MIA PaCa-2 cells. Recently, it has been demonstrated that activation of NF-κB by CD133 was mediated by cytokine IL-1β that can be secreted from either CD133+ CSCs or tumor-associated macrophages (Nomura et al., 2018). Overexpression of CD133 in pancreatic cancer AsPC-1 cells promoted cancer cell migration, invasion and angiogenesis (Weng et al., 2016). Furthermore, CD133 was immunoprecitated with EGFR. Knockdown of EGFR reduced CD133-mediated activation of Akt. Treating AsPC-1 cells with the EGFR inhibitor Gefitinib reversed the eﬀect on cancer cell migration induced by ectopically expressed CD133.