Mesenchymal Cancer Cell-Stroma Crosstalk Promotes Niche Activation, Epithelial Reversion, and Metastatic Colonization

Abstract

During metastatic colonization, tumor cells must establish a favorable microenvironment or niche that will sustain their growth. However, both the temporal and molecular details of this process remain poorly understood. Here, we found that metastatic initiating cells (MICs) exhibit a high capacity for lung fibroblast activation as a result of Thrombospondin 2 (THBS2) expression. Importantly, inhibiting the mesenchymal phenotype of MICs by blocking the epithelial-to-mesenchymal transition (EMT)-associated kinase AXL reduces THBS2 secretion, niche-activating ability, and, consequently, metastatic competence. Subsequently, disseminated metastatic cells revert to an AXL-negative, more epithelial phenotype to proliferate and decrease the phosphorylation levels of TGF-β-dependent SMAD2-3 in favor of BMP/SMAD1-5 signaling. Remarkably, newly activated fibroblasts promote this transition. In summary, our data reveal a crosstalk between cancer cells and their microenvironment whereby the EMT status initially triggers and then is regulated by niche activation during metastatic colonization.

Graphical abstract

Figure 1

The Mesenchymal Features of MICs Are Part of Their Metastasis-Initiating Potential (A) Circos plot displaying selected gene signatures correlating positively with MIC-upregulated genes (see also Figures S1A and S1B). Black blocks highlight metastasis-promoting genes either known (white) or described in this study (red). (B and C) Representative immunofluorescence images (B) of non-MICs (CD24⁺CD90⁻) and MICs (CD24⁺CD90⁺) stained with E-cadherin, Vimentin, and AXL antibodies. Scale bar, 40 μm. Staining quantification is shown in (C). Data are from one representative experiment of five. (D) Spheroid invasion assay. Shown are representative fluorescence images of MIC or non-MIC spheroids from actin-GFP/PyMT tumors in Matrigel:collagen I. Scale bar, 100 μm. (E) Mixed spheroid invasion assay. The histogram shows the number of cells invading as single cell (1:9 actin/GFP MICs:non-labeled non-MICs) in Matrigel:collagen I (see also Figure S1C). (F and G) Schematic showing a collective cell challenge for primary tumor growth into mammary fat pads (F) or a single-cell challenge for lung metastasis via the tail vein (G). (H) Box plot showing the primary tumor burden in grams generated by the indicated sorted cells after overnight culture. Shown are data from three experiments using the indicated cell numbers (n = 4/group). IV, intravenous. (I) Box plot showing the lung metastatic burden 5 weeks after tail vein injection. Two independent experiments are shown (n = 5–6/group). Shown are representative images of lungs with GFP⁺ metastases. Error bars represent SD in (C) and SEM in all other plots. ^∗p < 0.05; ^∗∗∗p < 0.001; ^∗∗∗∗p < 0.0001; ns, not significant.

Figure 2

MICs Have a Higher Capacity to Activate Fibroblasts (A–D) Representative pictures showing single GFP⁺ cells (green) from the indicated subpools 72 hr after intravenous injection into RAG1-deficient mice. (A) DAPI staining (blue) and SMA (red); quantified in (C). (B) DAPI staining (blue) and Endomucin (red); quantified in (D). Scale bars, 10 μm. n = 32–50 cells. (E) Schematic of the cancer cell and GFP⁺ normal lung fibroblast cell line (NLF3) co-culture setting. (F and G) Representative images of (F) early YAP nuclear translocation in NLF3 or (G) FAP levels in NLF3 cells 24 hr after plating. Scale bars, 40 μm (F), 70 μm (G). (H) Chart indicating the levels of FAP in NLF3 cells from (G). Data are from one representative experiment of three. (I) Schematic displaying the cancer cell and NLF3 co-culture setting. (J) Representative images of the gel contraction by NLF3 assayed as in (I). The histogram shows the gel area of three independent experiments (n = 8/group). (K) Quantitative real-time PCR analysis of the levels of CAF-defining genes in primary NF after co-culture with MICs (NF-MIC) or nonMICs (NF-nonMIC). Data are from at least three experiments in triplicate normalized to Gapdh. p = 0.0235 by ANOVA comparing differences between the NF, NF-non-MIC, and NF-MIC gene signatures. Error bars represent SD in (H) and SEM in all other plots. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗∗p < 0.0001. ns, non-significant.

Figure 3

Cancer Cell-Derived THBS2 Enhances Fibroblast Activation (A) Gel contraction induced by NLF3 after 3 days of exposure to recombinant CCL7 or THBS2. Top: representative NLF3-gels. Bottom: gel area quantification of three independent experiments (n = 12/group). (B and C) Levels of the active isoform of integrin β1 (clone 9EG7) and its downstream effector FAK (total or phosphorylated) in NLF3 for the indicated groups. Data are from one representative experiment of two. Quantification is shown in (B) and representative images in (C). Scale bars, 20 μm. (D) Gel contraction induced by NLF3 after co-culture with recombinant THBS2 with or without integrin β1-blocking antibody. Top: representative NLF3-gels. Bottom: gel area quantification of two independent experiments (n = 4–16/group). (E) Quantitative real-time PCR analysis of the mRNA expression level of Thbs2 by the indicated cells. Data are from three different experiments in triplicate normalized to Gapdh. (F) Gel contraction induced by NLF3 after co-culture with the indicated cancer cells. Top: representative NLF3-gels. Bottom: gel area quantification of two independent experiments (n = 10–12/group). (G) Box plot showing early primary tumor growth of primary PyMT cells expressing shCONTROL or shTHBS2 constructs in an orthotopic challenge (n = 6/group). (H) Box plot showing the metastatic burden from PyMT cells expressing shCONTROL or shTHBS2 constructs 5 weeks after tail vein injection (n = 7–11/group). The superficial lung metastasis number was evaluated. (I) Box plot displaying the metastatic cell content by flow cytometry 20 days after tail vein injection of PyMT control or THBS2ORF-expressing cells (n = 8/group). Error bars represent SD in (B) and SEM in all other plots. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001, ^∗∗∗∗p < 0.0001.

Figure 4

The Mesenchymal Status of Cancer Cells Drives Fibroblast Activation (A) Representative images showing the expression of E-cadherin, Vimentin, and AXL in MICs 24 hr after treatment with DMSO or R428. Scale bar, 20 μm. See also Figure S4A. (B) Axl gene expression levels in PyMT cells pre-treated with or without R428 as determined by quantitative real-time PCR analysis. Data are from three different experiments in triplicate (normalized to Gapdh). (C) THBS2 secretion in the media collected from the indicated cancer cells 24 hr after R428 pre-treatment as measured by ELISA. Left: PyMT subpools, non-MIC data from three experiments in duplicate; MIC ± R428 treatment, data from seven experiments in duplicate. Right: MDA-MB-231 cell line (MDA231), data from four experiments in duplicate. (D) Gel contraction induced by NLF3 after exposure to MDA231 pre-treated with DMSO or R428. Top: representative NLF3 gels. Bottom: gel area quantification of three independent experiments (n = 12/group). (E) THBS2 secretion in the media collected from the indicated cancer cells 24 hr after plating by ELISA. Data are from three different experiments in duplicate. (F) Gel contraction induced by NLF3 after exposure to the indicated cells. Top: representative NLF3 gels. Bottom: gel area quantification of two independent experiments (n = 7–9/group). Error bars represent SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001. ns, non-significant.

Figure 5

The Mesenchymal Status Is Regulated Temporally during Metastatic Colonization (A) Flow cytometry analysis of the percentage of AXL⁺ MICs (red, left y axis) and total MICs (black, right y axis) among total GFP⁺ cancer cells in the lung 4 and 12 days after tail vein injection (n = 8–9/group from two independent experiments). (B) Flow cytometry analysis of the percentage of AXL⁺GFP⁺ MDA-MB-231 cells (MDA231) among total GFP⁺ cancer cells in the lung 4 and 12 days after tail vein injection (n = 3/group). (C) Schematic of the R428 treatment setting for PyMT or MDA231 cells. IV, intravenous. (D) Box plot showing the metastatic burden of MDA231 cells exposed to DMSO (n = 13) or R428 (n = 8) during the first week of colonization. The superficial lung metastasis number was evaluated. Shown are representative pictures of metastatic lungs. Scale bar, 300 μm. (E) Box plot displaying the metastatic burden of PyMT cells pre-treated with DMSO (n = 14) or R428 (n = 5). In the last group (light blue, n = 9) mice were additionally treated with R428 during the first week of colonization. The superficial lung metastasis number was evaluated. (F) Representative H&E staining of metastatic lung sections from the indicated mice. Scale bar, 100 μm. (G) Box plot showing the metastatic burden of control (Venus) or THBS2-expressing MDA231 cells treated as in (C) (n = 7–5/group). Micrometastases were evaluated in histological sections of one lung lobe (see also Figure S5J). (H) Box plot displaying early primary tumor growth after orthotopic transplantation of the indicated primary PyMT cells (n = 6/group). (I) Box plot showing the metastatic burden of the indicated PyMT cells after tail vein injection (n = 5–8/group). The superficial lung metastasis number was evaluated. Error bars represent SEM. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗∗p < 0.0001. ns, non-significant.

Figure 6

Attenuation of the Metastatic Cell Mesenchymal Phenotype (A) Flow cytometry analysis changes in AXL expression in AXL⁺ cancer cells seeded in the lungs after the indicated times points (n = 8–9/group). Data are from one representative experiment of two. (B and C) ImageStream analysis changes in Twist1 expression in AXL⁺ cancer cells seeded in the lungs after the indicated times points (n = 4–5/group). (B) Quantification of Twist1⁺ cells from one representative experiment of three. (C) Contour plot of representative lungs from (B) showing Twist1 staining intensity. (D and E) MIC changes after 3–5 days of co-culture with NLF3 as in Figure 2E. (D) Flow cytometry quantification of AXL expression levels. Data are from one representative experiment (n = 4/group) of two. (E) ImageStream analysis of the percentage of Twist1⁺ cells. Data are from one representative experiment (n = 4/group) of two. (F and G) Spheroid invasion assay. Labeled MIC or non-MIC (green) spheroids were imaged 48 hr after seeding into Matrigel:collagen I with or without CAFs. (F) Quantification of single (left) and collective (right) cell invasion. Data are from one representative experiment of two. (G) Representative spheroid images. Arrows indicate single invading cells. Scale bar, 100 μm. (H–K) ImageStream analysis of pSMAD2-3⁺ (H), pSMAD1-5⁺ (I), ID1⁺ (J), and ID1⁺Twist1⁻ (K) cell percentages in the lung 2 or 12 days post-seeding (n = 3–5/group). (L and M) Expression levels of ID1 in primary CD24⁺AXL⁺ tumor cells 24 hr after exposure to the indicated media and in the presence of LDN193189 inhibitor determined by (L) western blot using Actin as a loading control (one representative experiment of two) or (M) immunofluorescence staining quantification (one representative experiment of two). (N) Schematic showing the in vivo LDN193189 treatment setting. (O) Box plot displaying the metastatic burden of PyMT cells in mice treated with vehicle or LDN193189. Shown are representative H&E-stained lung sections (n = 9–12/group). Scale bar, 100 μm. Error bars represent SD in (M) and SEM in all other plots. ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p<0.001, ^∗∗∗∗p < 0.0001. ns, non-significant.

Figure 7

EMT-Stabilized Cells Do Not Metastasize Efficiently (A) Schematic showing the R428 treatment setting. (B) Box plot displaying the metastatic burden of mice injected with primary PyMT cells for the indicated groups: DMSO, n = 11; R428, days 10–15, n = 10; and R428, days 25–35, n = 7. The superficial lung metastasis number was evaluated. (C) Representative images of the indicated lung sections. Scale bar, 100 μm. (D) Schematic of the LTR-driven lentiviral plasmid expressing the human Axl open reading frame (ORF) (top) and control plasmid (bottom). (E and F) Flow cytometry analysis showing the percentage of total cancer cells (black) and AXL⁺ cells (blue) in the lung 3 or 4 days after intravenous injection of MDA231 expressing the control-GFP construct (E) or LTR-AXL construct (F). Data are displayed as the group average relative to day 3 (n = 4–6/group). (G) Representative flow cytometry plot showing the expression levels of AXL in the indicated primary PyMT cells. (H) Box plot showing superficial lung metastasis quantification after intravenous injection of PyMT cells expressing control-GFP or LTR-AXL constructs (n = 10/group). (I) Representative H&E staining of lung sections of mice injected with MDA231 of the indicated groups. Scale bar, 100 μm. (J) Representative flow cytometry plot showing the expression levels of AXL in the indicated MDA231 cells. (K) Box plot showing the superficial metastasis quantification after intravenous injection of MDA231 cells expressing the control-GFP or LTR-AXL constructs (n = 8/group). Error bars represent SEM. ^∗p < 0.05, ^∗∗∗∗p < 0.0001.