Abstract
The ability of breast cancer cells to interconvert between epithelial and mesenchymal states contributes to their metastatic potential. As opposed to cell autonomous effects, the impact of epithelial–mesenchymal plasticity (EMP) on primary and metastatic tumor microenvironments remains poorly characterized. Herein we utilize global gene expression analyses to characterize a metastatic model of EMP as compared to their non-metastatic counterparts. Using this approach, we demonstrate that upregulation of the extracellular matrix crosslinking enzyme tissue transglutaminase-2 (TG2) is part of a novel gene signature that only emerges in metastatic cells that have undergone induction and reversion of epithelial–mesenchymal transition (EMT). Consistent with our model system, patient survival is diminished when primary tumors demonstrate enhanced levels of TG2 in conjunction with its substrate, fibronectin. Targeted depletion of TG2 inhibits metastasis, while overexpression of TG2 is sufficient to enhance this process. In addition to being present within cells, we demonstrate a robust increase in the amount of TG2 and crosslinked fibronectin present within extracellular vesicle (EV) fractions derived from metastatic breast cancer cells. Confocal microscopy of these EVs suggests that FN undergoes fibrillogenesis on their surface via a TG2 and Tensin1-dependent process. Upon in vivo administration, the ability of tumor-derived EVs to induce metastatic niche formation and enhance subsequent pulmonary tumor growth requires the presence and activity of TG2. Finally, we develop a novel 3D model of the metastatic niche to demonstrate that conditioning of pulmonary fibroblasts via pretreatment with tumor-derived EVs promotes subsequent growth of breast cancer cells in a TG2-dependent fashion. Overall, our studies illustrate a novel mechanism through which EMP contributes to metastatic niche development and distant metastasis via tumor-derived EVs containing aberrant levels of TG2 and fibrillar FN.
Highlights
Metastatic progression is the major driver of lethality in breast cancer[1]
To characterize these two epithelial populations, we performed RNA sequencing analyses on the parental HME2 cells, the purely mesenchymal population that resulted immediately following transforming growth factor (TGF)-β1 treatment (HME2-TGFβ), and the HME2-BM cells (GSE115255). Analysis of these gene expression data clearly indicated that longterm TGF-β1 treatment induced a gene expression profile that is characteristic of epithelial–mesenchymal transition (EMT) and very unique from the related epithelial states of the HME2 parental and HME2BM populations (Fig. 1a)
These data suggest that TG2 in conjunction with FN are clinically relevant markers of epithelial–mesenchymal plasticity (EMP) whose enhanced expression within the primary tumor is consistent with metastatic disease progression
Summary
Metastatic progression is the major driver of lethality in breast cancer[1]. Overt manifestation of macroscopic metastases is the culminating event in the multistep process of disease progression. Following dissemination and adaptation to the new microenvironment, return to an epithelial state is consistent with an enhanced ability of cells to overcome dormancy and undergo metastatic outgrowth[3,7] In addition to these tumor cell autonomous effects of epithelial–mesenchymal plasticity (EMP) that take place at various steps in the metastatic process, differential EMP conversion rates within the Shinde et al Oncogenesis (2020)9:16 primary tumor contribute to dynamic paracrine relationships between tumor cell populations of varying epithelial or mesenchymal status. We recently termed this concept, epithelial–mesenchymal heterogeneity (EMH)[8,9].
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