endogenous CD inhibits Capan M induced lung and
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 Sorafenib 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.
As mentioned previously in the cancer initiation section, all colon cancer initiating cells purified from a xenograft NOD/SCID mouse model expressed CD133 (O'Brien et al., 2007). Of note, it has been shown that upon using the lineage tracing technique to track en-dogenous CD133+ cells in a transgenic mouse model during the de-velopment of colon cancer metastasis, CD133 is expressed in the colon cancer epithelium (Shmelkov et al., 2008). It suggested a role of CD133 in the initiation of colon cancer metastasis. Knockdown of CD133 in SW620 human colon cancer cells impaired cell migration through re-duced phosphorylations of Src-focal adhesion kinase (FAK) and this is due to a failure of forming a complex of CD133, Src, and FAK (Liu et al., 2016). Furthermore, the interaction between CD133 and Src is required for CD133-induced cell motility by activation of Src downstream target FAK.
A large body of evidence demonstrated that activation of Wnt sig-naling is involved in controlling the malignant features of CSC through increased EMT, which leads to cancer invasion and metastasis (Katoh, 2017; Webster et al., 2015; Zhan et al., 2017). Knockdown of CD133 by the virally delivered short hairpins of RNA in human metastatic mela-noma FEMX-I cells impeded cell motility and their ability to form spheroids (Rappa et al., 2008). In addition, downregulation of CD133 in FEMX-I cells resulted in increased expression of several Wnt inhibitors such as DKK1. Besides plasma membrane, CD133 is also localized at
intracellular compartments such as Golgi apparatus and extracellular membrane vesicles. It has been shown that shCD133 expressing FEMX-1 cells produced less CD133 containing lipid droplets (Rappa et al., 2013). In addition, decreased nuclear β-catenin was detected in these cells, suggesting a reduced activation of Wnt signaling. These data implicated that CD133-containing membrane vesicles are involved in the activation of Wnt signaling to promote cancer metastasis of mela-noma. It has been demonstrated that in several types of cancer cells, histone deacetylase HDAC6 can physically interact with CD133 at the endosomes, α-tubulin, and β-catenin to form a tertiary complex (Mak et al., 2012). The stabilized β-catenin then translocated to the nucleus where it interacts with TCF/LEF transcription factors to upregulate gene expressions that modulate cancer cell migration and metastasis. Blockade of this tertiary complex formation by inhibiting HDAC6 leads to cancer cell diﬀerentiation via a deacetylation of α-tubulin, protea-somal degradation of β-catenin and endocytosis of CD133 followed by its lysosomal degradation. In colon cancer, CD133 and nuclear β-ca-tenin are biomarkers for disease progression and patient survival (Horst et al., 2009a). Treating CD133 expressing colon cancer HT29 and DLD1 cells with celecoxib reduced both CD133 expression and Wnt activation (Deng et al., 2013). Celecoxib inhibits TCF/LEF transcription factor activities and suppresses Wnt/β-catenin targeted gene expressions of cyclin D1 and survivin (Tang et al., 2018). Loss of E-Cadherin expres-sion at membranes not only impairs cell-cell adhesion but also has in-creased accumulation of nuclear β-catenin. Nuclear E-Cadherin was reported to be a negative regulator of Wnt pathway-induced cell inva-siveness in CD133+ lung cancer cells (Su et al., 2015). It reduced the Wnt/ β-catenin mediated transcriptional activity via disruption of the interaction between β-catenin and TCF4 transcription factor.