An epigenetically distinct breast cancer cell subpopulation promotes collective invasion

Abstract

induced the invasion of non-trailblazer cells, thus revealing a new type of commensal relationship among naturally existing tumor subpopulations. Together, these results demonstrate how the epigenetic alteration of the signaling circuitry in a subpopulation of tumor cells can promote collective invasion through cellautonomous and non-cell-autonomous mechanisms. Results A distinct subpopulation of trailblazer cells has enhanced invasive ability.To begin defining the molecular traits that confer tumor cells with invasive ability, we analyzed spheroid invasion in an organotypic culture system that reconstitutes key features of collective invasion that are conserved in vivo (9, 13). Normal mammary epithelial cells form duct-like spheroids in this system (Supplemental Figure 1A; supplemental material available online with this article; doi:10.1172/JCI77767DS1), indicating that our model was testing for unique traits of tumor cells that promote cell-autonomous invasion, potentially during the transition from ductal carcinoma in situ (DCIS) to invasive breast cancer (13). To begin defining traits that promote collective invasion, we determined the percentage of invasive trailblazer spheroids that were formed in 7 different breast cancer cell lines that represent key known features of intertumor molecular diversity. Invasive spheroids were detected in 3 of the 7 cell lines evaluated, with the percentage of invasive spheroids ranging between 8% and 75% of the total population ( Figure 1, A and B). None of the cell lines contained a 100% pure population of invasive spheroids ( Figure 1B). The 3 cell lines that contained invasive spheroids were derived from patients with TNBC (no detectable estrogen receptor [ER], progesterone receptor, or human epidermal growth factor receptor 2 [HER2] expression) ( Figure 1A). TNBC accounts for 10% to 20% of diagnosed breast cancers and has a relatively worse outcome compared with that of ER + breast cancer (21). Importantly, the strand-like organization of the collectively invading cells observed in organotypic culture was also detected in primary breast tumors ( Figure 1C). Thus, our results indicate that there can be a distinct subpopulation within a community of tumor cells that has an enhanced capacity to lead collective invasion. We refer to this intrinsically invasive subpopulation as trailblazer cells to distinguish them from other types of leader cells, such as KRT14-expressing breast cancer cells, that are unable to invade under these conditions. The noninvasive subpopulation, which may require additional extrinsic factors to invade, is referred to as "opportunist" cells herein.Immunofluorescence analysis and time-lapse imaging showed that the leader trailblazer cells formed long cellular protrusions (LCPs) into the ECM before invading away from the main mass of cells (Figure 1, D and E, and Supplemental Video 1), similar to previous reports (9, 10). Additional trailblazer cells could then migrate into the space within the ECM created by the first invading cell, indicating tha...

Distinctive properties of metastasis-initiating cells

“…This kind of cooperation has been further characterized in other models proposing a leading invasive cell followed by "opportunistic" cells ( Fig. 5B; Chapman et al 2014;Westcott et al 2015). This synergy is in agreement with the observation that CTC clusters have dramatically increased metastatic potential compared with single CTCs (Aceto et al 2014) and the polyclonal nature of metastases (Gundem et al 2015;Maddipati and Stanger 2015).…”

The integrin–collagen connection – a glue for tissue repair?

“…2). Recently, however, ITGA11 was identified as an invasionpromoting gene within a seven-gene signature that had been established for the leading invasive 'trailblazer' cells in a spheroidbased model for collective invasion of breast cancer cells (Westcott et al, 2015). These data indicate that the role of α11 in tumors is not restricted to CAFs but could also extend to tumor cells that assume a mesenchymal invasive phenotype (Fig.…”

“…(7) α2β1 promotes prostate cancer invasion through RhoC activation (Hall et al, 2008). α11β1 is one of the seven genes identified as a signature for the 'trailblazer' cell phenotype (Westcott et al, 2015). (8) (Chen et al, 2005).…”

“…(7) α2β1 promotes prostate cancer invasion through RhoC activation (Hall et al, 2008). α11β1 is one of the seven genes identified as a signature for the 'trailblazer' cell phenotype (Westcott et al, 2015). (8) Trailblazer cells lead collective invasion and generate pathways within collagen matrices that also allow opportunist follower cells to invade.…”

Making Heads or Tails of It: Cell–Cell Adhesion in Cellular and Supracellular Polarity in Collective Migration

“…Although intrinsic factors contribute to leader cell selection, leader cells are often overtaken and replaced by follower cells, suggesting plasticity in leader cell fate (Poujade et al 2007;Prasad and Montell 2007;Ewald et al 2008;Jakobsson et al 2010). On the other hand, stable epigenetically distinct subpopulations exist in breast cancer cell lines that have a higher propensity to become leader cells (Westcott et al 2015), but it remains to be determined whether such stable populations result from tumor heterogeneity or reflect a general physiological mechanism.…”