Breast cancer stem cells transition between epithelial and mesenchymal states reflective of their normal counterparts

Abstract

Previous studies have suggested that breast cancer stem cells (BCSCs) mediate metastasis, are resistant to radiation and chemotherapy, and contribute to relapse. Although several BCSC markers have been described, it is unclear whether these markers identify the same or independent BCSCs. Here, we show that BCSCs exist in distinct mesenchymal-like (epithelial-mesenchymal transition [EMT]) and epithelial-like (mesenchymal-epithelial transition [MET]) states. Mesenchymal-like BCSCs characterized as CD24(-)CD44(+) are primarily quiescent and localized at the tumor invasive front, whereas epithelial-like BCSCs express aldehyde dehydrogenase (ALDH), are proliferative, and are located more centrally. The gene-expression profiles of mesenchymal-like and epithelial-like BCSCs are remarkably similar across different molecular subtypes of breast cancer, and resemble those of distinct basal and luminal stem cells found in the normal breast. We propose that the plasticity of BCSCs that allows them to transition between EMT- and MET-like states endows these cells with the capacity for tissue invasion, dissemination, and growth at metastatic sites.

Figure 1

CD24⁻CD44⁺ and ALDH⁺ Identify Anatomically Distinct BCSCs with Distinct EMT and MET Gene-Expression Profiles in Primary Breast Cancers (A) Localization of CD24 (magenta), CD44 (green), ALDH1 (red), and DAPI (blue) in clinical samples of human invasive breast carcinoma as assessed by immunofluorescence staining. Bar: 100 μm. (B) Venn diagram of the intersection between genes elevated in CD24⁻CD44⁺ compared with other flow-sorted cells (others) and genes diminished in ALDH⁺ compared with ALDH⁻ cells (|fold change| > 1.5 for each comparison). (C) Venn diagram of the intersection between genes diminished in CD24⁻CD44⁺ versus others and genes elevated in ALDH⁺ versus ALDH⁻. (D) Heatmap of genes with opposite expression patterns between CD24⁻CD44⁺ and ALDH⁺ (from B and C). Each row represents an RNA transcript; each column represents a sample (red, high expression). The molecular subtype based on expression of ER, PR, and HER2 is displayed below the heatmap for each patient sample. See also Figure S1.

Figure 2

Specific EMT Markers Have Opposite Expression Patterns in CD24⁻CD44⁺ and ALDH⁺ Cells from Clinical Patient Breast Tumors (A) Expression of EMT/MET markers in CD24⁻CD44⁺ versus others and ALDH⁺ versus ALDH⁻. Except for MMP9 (probe 203926_s_at p = 0.138), Ki67 (probe 224713_at p = 0.229), all other EMT markers listed are significant at p < 0.05 for CD24⁻CD44⁺ versus others. Except for ZEB1 (probe 212764_at p = 0.106), ZEB2 (probe 228333_at p = 0.336), CDH1 (probe 201131_s_at p = 0.209), OCLN (probe 209925_at p = 0.203, probe 231022_at p = 0.203), CLDN8 (probe 214598_at p = 0.15), and DSP (probe 200606_at p = 0.46), all other EMT markers listed are significant with p < 0.05 for ALDH⁺ versus ALDH⁻. (B) For CD24⁻CD44⁺ versus others flow-sorted profile data sets, heatmap of EMT markers (|fold change| > 1.5). (C) As in (A), but for ALDH⁺ versus ALDH⁻ profile data sets. (D) Gene-expression levels measured by qRT-PCR confirm the results obtained with Affymetrix array HU133 Plus 2.0. Error bars represent mean ± SD. The fold changes are significant with p < 0.05 for CD24⁻CD44⁺ versus others. See also Figure S2 and S3.

Figure 3

BCSC Display a Cellular Plasticity that Allows Them to Transit between EMT and MET States Basal breast cancer cell lines SUM149 and MCF7 were immunostained with CD24 and CD44 antibodies, and subsequently with ALDEFLUOR. The four cell subpopulations defined by the ALDEFLUOR and CD24⁻CD44⁺ phenotypes were separated by fluorescence-activated cell sorting (FACS). (A) The percentages shown in the diagram show the representation of the cell subpopulations in the total tumor cell population and the overlap between the ALDEFLUOR phenotype and the CD24⁻CD44⁺ phenotype. (B) The unsorted SUM149 cells and the sorted CD24⁻CD44⁺ and ALDH⁺ populations as described in (A) were placed in culture for 10 days, and the resulting cells were reanalyzed by FACS for ALDEFLUOR, CD24, and CD44. ^∗p < 0.05 (percentage of ALDH⁺ cells generated from the ALDH⁺ population compared with that generated from parental cells). ^&p < 0.05 (percentage of CD24⁻CD44⁺ cells generated from the CD24⁻CD44⁺ population compared with that generated from parental cells). (C) The unsorted MCF7 cells and the sorted CD24⁻CD44⁺ and ALDH⁺ populations as described in (A) were placed in culture for 10 days, and the resulting cells were reanalyzed by FACS for ALDEFLUOR, CD24, and CD44. ^∗p < 0.05 (percentage of ALDH⁺ cells generated from ALDH⁺ population compared with that generated from parental cells). ^&p < 0.05 (percentage of CD24⁻CD44⁺ cells generated from the CD24⁻CD44⁺ population compared with that generated from parental cells). (D) The sorted four populations of SUM149 as described in (A) were assessed for invasive capacity utilizing the Matrigel invasion assay. Scale bar, 200 μm. (E) Hypothetical models show the characteristics of two different states of BCSCs. ^∗p < 0.05; error bars represent mean ± SD. See also Figure S6.

Figure 4

Identification of Mesenchymal-like and Epithelial-like States in Nontransformed MCF10A Cells (A) Cells were immunostained with EPCAM and CD49f antibodies. EPCAM⁺CD49f⁺ and EPCAM⁻CD49f⁺ cells were separated by FACS and plated in culture for 10 days. FACS analysis for EPCAM and CD49f was then repeated. Scale bar, 200 μm. (B and C) Total RNA was extracted from the sorted EPCAM⁺CD49f⁺ cells and EPCAM⁻CD49f⁺ cells, and the expression level of E-cadherin, vimentin, and TATA-binding protein (TBP) was measured by qRT-PCR. ^∗p < 0.05; error bars represent mean ± SD. (D) Cells were immunostained with EPCAM and CD49f antibodies, and subsequently stained with CD24/CD44 or ALDEFLUOR. Inset displays the negative control; cells incubated with DEAB, the specific inhibitor of ALDH, were used to establish the baseline fluorescence of these cells. (E) Cells were immunostained with CD24 and CD44 antibodies, and subsequently with ALDEFLUOR. The four cell subpopulations defined by the ALDEFLUOR and CD24⁻CD44⁺ phenotypes were separated by FACS. The percentages shown in the diagram depict cell subpopulations and the overlap between the ALDEFLUOR-positive phenotype and the CD24⁻CD44⁺ phenotype. Inset displays the negative control; cells incubated with DEAB, the specific inhibitor of ALDH, were used to establish the baseline fluorescence of these cells. (F) Total RNA was extracted from the four populations as described in (E) and used for Affymetrix array (HU133 Plus 2.0) analysis. The fold change for EMT/MET markers was compared among ALDH⁺, ALDH⁻, CD24⁻CD44⁺, and others. See also Figure S4.

Figure 5

Characterization of CD24⁻CD44⁺ Cells and ALDH⁺ Epithelial Cells Isolated from Normal Breast Tissues (A) Cells were immunostained with CD24 and CD44 antibodies, and subsequently stained with ALDEFLUOR. The four cell subpopulations defined by the ALDEFLUOR and CD24⁻CD44⁺ phenotypes were separated by FACS. The percentages in the diagram represent the cell subpopulations as a representative of the total tumor cell population and the overlap between the ALDEFLUOR phenotype and the CD24⁻CD44⁺ phenotype. Inset displays the negative control; cells incubated with DEAB, the specific inhibitor of ALDH, were used to establish the baseline fluorescence of these cells. (B) Expression of EMT/MET markers in CD24⁻CD44⁺ versus others and ALDH⁺ versus ALDH⁻ of normal breast as assessed by Affymetrix array HU133 Plus 2.0. (C) To confirm the gene-expression results for EMT/MET markers, we measured the mRNA expression level for Vimentin, ZEB1, Claudin 3, and Ki67 in a set of breast tumor cells sorted for ALDH⁺/ALDH⁻, CD24⁻CD44⁺/others by qRT-PCR. Gene-expression levels measured by qRT-PCR confirm the results obtained with the Affymetrix array HU133 Plus 2.0. The fold changes are significant with p < 0.05 for CD24⁻CD44⁺ versus others or ALDH⁺ versus ALDH⁻. Error bars represent mean ± SD.

Figure 6

Normal Human Breast Contains Distinct Basal and Luminal Stem Cells that Express EMT and MET Markers, Respectively (A) Cells isolated from reduction mammoplasty tissue were immunostained with lineage markers (Lin⁻), EPCAM, and CD49f antibodies, and were first gated based on viability (DAPI⁻) and lineage markers. The sorted cells (Lin⁻EPCAM⁺CD49f⁺ and Lin⁻EPCAM⁻CD49f⁺ cells) were injected into the No. 4 mammary gland fat pads, which were previously humanized with normal human breast fibroblasts. After about 2 months, the mice were sacrificed and the outgrowths that were generated in the fat pads were assessed by hematoxylin and eosin staining. Scale bar, 100 μm. (B) Cells were immunostained with EPCAM and CD49f antibodies followed by CD24/CD44 or ALDEFLUOR. Inset displays the negative control: cells incubated with DEAB, the specific inhibitor of ALDH, were used to establish the baseline fluorescence of these cells. (C) Cells were immunostained with Lin⁻, EPCAM, and CD49f antibodies, and subsequently with ALDEFLUOR. The cells were first gated based on viability (DAPI⁻) and lineage markers. The sorted cells (Lin⁻EPCAM⁺CD49f⁺ALDH⁺ cells and Lin⁻EPCAM⁺CD49f⁺ALDH⁻ cells) were grown in differentiating conditions on collagen-coated plates for 12 days and the number of colonies generated was assessed. (D) Sorted cells (Lin⁻EPCAM⁺CD49f⁺ALDH⁺ and Lin⁻EPCAM⁺CD49f⁺ALDH⁻ cells) were cultured in 3D Matrigel culture for 3 weeks and the number of branched structures generated was assessed. (E) Localization of CD24 (magenta), CD44 (green), ALDH1 (red), and DAPI (blue) in normal breast tissue as assessed by immunofluorescence staining. Yellow arrow, CD24⁻CD44⁺ cells; white arrow, ALDH1⁺ cells. Scale bar, 100 μm. See also Figure S5.