Towards understanding cancer stem cell heterogeneity in the tumor microenvironment

Abstract

The Epithelial-Mesenchymal Transition (EMT) and Cancer Stem Cell (CSC) formation are two paramount processes driving tumor progression, therapy resistance and cancer metastasis. Some recent experiments show that cells with varying EMT and CSC phenotypes are spatially segregated in the primary tumor. The underlying mechanisms generating such spatiotemporal dynamics and heterogeneity in the tumor micro-environment, however, remain largely unexplored. Here, we show through a mechanism-based dynamical model that the diffusion of EMT-inducing signals such as TGF-β in a tumor tissue, together with non-cell autonomous control of EMT and CSC decisionmaking via juxtacrine signaling mediated via the Notch signaling pathway, can explain experimentally observed disparate localization of subsets of CSCs with varying EMT states in the tumor. Our simulations show that the more mesenchymal CSCs lie at the invasive edge, while the hybrid epithelial/mesenchymal (E/M) CSCs reside in the tumor interior. Further, motivated by the role of Notch-Jagged signaling in mediating EMT and stemness, we investigated the microenvironmental factors that promote Notch-Jagged signaling. We show that many inflammatory cytokines that can promote Notch-Jagged signaling such as IL-6 can (a) stabilize a hybrid E/M phenotype, (b) increase the likelihood of spatial proximity of hybrid E/M cells, and (c) expand the fraction of CSCs. To validate the predicted connection between Notch-Jagged signaling and stemness, we knocked down JAG1 in hybrid E/M SUM149 human breast cancer cells in vitro. JAG1 knockdown significantly restricted organoid formation, confirming the key role that Notch-Jagged signaling can play in tumor progression. Together, our integrated computational-experimental framework reveals the underlying principles of spatiotemporal dynamics of EMT and CSCs in the tumor microenvironment. Significance statementThe presence of heterogeneous subsets of cancer stem cells (CSCs) remains a clinical challenge. These subsets often occupy different regions in the primary tumor and have varied epithelialmesenchymal phenotypes. Here, we device a theoretical framework to investigate how the tumor microenvironment (TME) modulates this spatial patterning. We find that a spatial gradient of EMTinducing signal, coupled with juxtacrine Notch-JAG1 signaling triggered by inflammatory cytokines in TME, explains this spatial heterogeneity. Finally, in vitro JAG1 knockdown experiments in triple negative breast cancer cells severely restricts the growth of tumor organoid, hence validating the association between JAG1 and CSC fraction. Our results offer insights into principles of spatiotemporal patterning in TME, and identifies a relevant target to alleviate multiple CSC subsets -JAG1.

NRF2 activates a partial Epithelial-Mesenchymal Transition and is maximally present in a hybrid Epithelial/Mesenchymal phenotype

“…Our earlier efforts at quantifying the association between EMT and stemness have predicted that cells in hybrid E/M states are more inclined to gain stem-like properties [27,42,43], a prediction that has been recently validated in vitro and in vivo [2, 44,45]. Reinforcing these predictions, NRF2 has been related to cancer stem-like traits and chemo-resistance [35].…”

A biophysical model of Epithelial-Mesenchymal Transition uncovers the frequency and size distribution of Circulating Tumor Cell clusters across cancer types

“…Previous efforts to couple EMT with intercellular signaling have disregarded the loss of cell-cell adhesion leading to a reduced cell-to-cell signaling between mesenchymal cells (58). Also, we recently showed that JAG1, a Jagged ligand subtype, promotes the growth of tumor organoids in breast cancer cells in vitro (12). Overall, these observations suggest that Jagged can facilitate a 'plasticity window' that maximizes tumor aggressiveness in terms of EMT, proliferation and therapeutic resistance (45).…”

“…This replacement represents the turnover of cancer cells, owing to their enhanced proliferation potential, that may also reach the periphery of the tumor. In fact, cells at the periphery of a tumor can be more exposed to EMT-inducing signals whereas cells in the interior exhibit more epithelial traits (11,12).…”

Deciphering the Dynamics of Epithelial-Mesenchymal Transition and Cancer Stem Cells in Tumor Progression

“…Furthermore, this model proposed a strong overlap between a hybrid E/M phenotype, CSC properties and Notch-Jagged signaling [13], a pathway implicated in both drug resistance and in clusters of CTCs, the key drivers of metastasis [33]. Consistently, knockdown of Jagged was shown to restrict the growth of tumor emboli in SUM149 inflammatory breast cancer cells [82]. Given the role of Notch signaling in pattern formation in multiple contexts [83,84], Notch signaling coupled with EMT circuitry may underlie the spatial segregation of different subsets of cells with stem-like traits, as observed experimentally in a breast cancer tissue [85].…”

“…Given the role of Notch signaling in pattern formation in multiple contexts [83,84], Notch signaling coupled with EMT circuitry may underlie the spatial segregation of different subsets of cells with stem-like traits, as observed experimentally in a breast cancer tissue [85]. Secretion of a diffusive EMT-inducing signal at the tumor-stroma interface (such as TGF-), along with cell-cell signaling through Notch, was shown to give rise to mesenchymal CSCs at the invasive edge of the tumor and a population of hybrid epithelial/mesenchymal (E/M) CSCs in the tumor interior [82]. The idea that cell-cell signaling and the microenvironment can shape the spatial distribution of a cell population has been examined in different biological contexts, such as bacterial colonies [86,87] or eukaryotic chemotaxis [88,89], but remains largely unexplored in cancer biology, and thus demands further attention.…”