​Background

​Normal tissues contain stem cell populations that renew through asymmetrical division and give rise to progenitors committed to functional differentiation.  Cancer stem-like cells (CSCs) are similarly capable of self-renewal and have the capacity for generating diverse progeny that comprise the tumor.  Stem-like cells from hematologic malignancies and solid tumors have thus been defined in part by their self-renewal and tumor-initiating potential.  Nonetheless, debate remains about the definition of cancer stem-like cells as a result of their heterogeneity and plasticity.  In this context, cell surface markers, such as CD44 and CD133, have been used to isolate subsets enriched for stem-like cells in solid tumors.  In addition, aldehyde dehydrogenase (ALDH) activity has been a focus of study as a marker for both normal and cancer stem cells.  With regard to functional characteristics, a hallmark of cancer stem-like cells is their relative resistance to apoptosis in the response to genotoxic stress induced by anti-cancer drugs and radiation.  Moreover, cancer stem-like cells have been characterized by their ability to undergo the epithelial-mesenchymal transition (EMT), a process that endows more differentiated epithelial cells with stem cell characteristics.  The induction of EMT results in both the acquisition of mesenchymal traits necessary for invasion and metastases, and the expression of CSC markers.  EMT also increases the capacity of mammary epithelial cells to form mammospheres in non-adherent serum-free culture, a characteristic that is dependent on the presence of self-renewing stem cells.

MUC1-C is a Target for Breast Cancer Stem-Like Cells

Figure 1.
Schema depicting MUC1-C-induced activation of NF-κB p65 and thereby induction of IL-8/CXCR1 expression and mammosphere formation.  Targeting MUC1-C with (i) silencing, (ii) expressing a MUC1-C(CQC→AQA) mutant, or (iii) GO-203 treatment suppresses NF-κB activity, decreases IL-8/CXCR1 expression and inhibits mammosphere formation.  MUC1-C also activates (i) NF-κB p65-mediated induction of ZEB1 and EMT, and (ii) ERK→C/EBPβ signaling and ALDH1A1 expression (10).  Our results demonstrate that silencing ALDH1A1 has no detectable effect on mammosphere formation, indicating that the MUC1-C→NF-κB and MUC1-C→ALDH1A1 pathways confer distinct functions.

Discussion

​Targeting MUC1-C with silencing or an inhibitor downregulates p-HER2 activation in HER2-overexpressing breast cancer cells.  Moreover, MUC1-C has been linked to regulation of downstream RTK signaling, such as the PI3K→AKT and MEK→ERK pathways.  MUC1-C is also imported into the nucleus by importin-β, where it interacts with transcription factors and contributes to their transactivating function.  In this regard, MUC1-C associates with NF-κB p65 and induces activation of the ZEB1 gene by a NF-κB-mediated mechanism.  In turn, MUC1-C binds to ZEB1 and suppresses miR-200c expression with the resulting induction of EMT.  In addition, MUC1-C interacts with the CCAAT/enhancer-binding protein β (C/EBPβ) on the ALDH1A1 gene promoter and induces C/EBPβ-mediated ALDH1A1 expression.  The available evidence thus links MUC1-C to the induction of EMT and ALDH activity, both characteristics of breast cancer stem-like cell populations.  Overexpression of MUC1, as found in breast cancer cells, is also associated with resistance to apoptosis in response to genotoxic anti-cancer agents.  Other work has demonstrated that MUC1-C is upregulated under nonadherent culture conditions, which select for self-renewing breast cancer cells.  Moreover, silencing MUC1-C blocks the capacity of luminal, HER2-overexpressing and triple-negative breast cancer cells to form mammospheres.  Targeting MUC1-C homodimerization by expression of a MUC1-C(CQC→AQA) mutant or the MUC1-C inhibitor GO-203 also blocks self-renewal of breast cancer cells.  The mechanistic basis for these results is supported by the demonstration that MUC1-C activates NF-κB and thereby expression of IL-8 and CXCR1.  These findings indicate that targeting MUC1-C represents an approach to inhibit the self-renewal capacity of breast cancer stem-like cells.

​MUC1-C is a Target for Lung Cancer Stem-Like Cells



​​Figure 2.
Schema depicting the proposed pathway in which MUC1-C activates AKT and thereby the coordinate induction of ZEB1 and suppression of miR-200c.  In turn, MUC1-C drives EMT, self-renewal and KRAS independence.

KRAS mutant NSCLC (Figure 2) - Discussion

​KRAS mutant NSCLC -  Non-small cell lung cancers (NSCLCs) that harbor an oncogenic KRAS mutation are often associated with resistance to targeted therapies.  The MUC1-C transmembrane protein is aberrantly overexpressed in NSCLCs and confers a poor outcome.  Studies have demonstrated that silencing MUC1-C in H23/KRAS(G12C) and H460/KRAS(G12C) NSCLC cells is associated with downregulation of AKT signaling and inhibition of growth.  Overexpression of a MUC1-C(CQC→AQA) mutant, which inhibits MUC1-C homodimerization and function, suppressed both AKT and MEK activation.  Moreover, treatment with GO-203, an inhibitor of MUC1-C homodimerization, blocked AKT and MEK signaling and decreased cell survival.  Targeting MUC1-C also suppresses expression of the ZEB1 transcriptional repressor by an AKT-mediated mechanism, and in turn induces miR-200c.  In concert with these effects on the ZEB1/miR-200c regulatory loop, targeting MUC1-C was associated with reversal of EMT and inhibition of self-renewal capacity.  Loss of MUC1-C function also attenuates KRAS independence and inhibits growth of KRAS mutant NSCLC cells as tumors in mice.  These findings demonstrate that targeting MUC1-C inhibits mutant KRAS signaling in NSCLC cells and thereby reverses the EMT phenotype and decreases self-renewal. 


Figure 3.
Schema depicting the proposed MUC1-C → ⇑LIN28B → ⇓let-7 →  ⇑HMGA2 → ⇑TGFBR3 pathway that drives EMT and self-renewal.

EGFR mutant NSCLC (Figure 3) - Discussion

​EGFR mutant NSCLC -  Other studies in NSCLC cells harboring EGFR mutations have shown that targeting the MUC1-C pathway also inhibits their growth as tumors in mice.  Inhibiting MUC1-C function therefore represents a potential approach for targeting EMT and self-renewing NSCLC cell populations.  LIN28B has been functionally linked to stem-like cells and oncogenesis.  Studies have shown that MUC1-C induces LIN28B expression and thereby downregulates let-7 signaling in NSCLC cells.  In turn, MUC1-C increases HMGA2 and TGFBR3 expression, and promotes EMT and stemness.  These findings demonstrate that targeting MUC1-C function suppresses activation of the LIN28B→let-7→HMGA2 axis, reverses EMT and decreases self-renewal capacity of NSCLC cells.