ABCG2 is a potential marker of tumor-initiating cells in breast cancer
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The existence of tumor-initiating cells (TICs) within solid tumors has been hypothesized to explain tumor heterogeneity and resistance to cancer therapy. In breast cancer, the expression of CD44 and CD24 and the activity of aldehyde dehydrogenase 1 (ALDH1) can be used to selectively isolate a cell population enriched in TICs. However, the ideal marker to identify TICs has not been established. The aim of this study was to evaluate the expression of novel potential markers for TIC in breast carcinoma. We prospectively analyzed the expression of CD44, CD24, ABCG2, and CXCR4, and the activity of ALDH1 by using flow cytometry in 48 invasive ductal carcinomas from locally advanced and metastatic breast cancer patients who were administered primary chemotherapy. A mammosphere assay was employed in 30 samples. The relationship among flow cytometric analyses, ABCG2 gene expression, and clinical and pathological responses to therapy was analyzed. The GSE32646 database was analyzed in silico to identify genes associated with tumors with low and high ABCG2 expression. We observed that the presence of ABCG2+ cells within the primary tumor was the only marker to predict the formation of mammospheres in vitro (R 2 = 0.15, p = 0.029). Quantitative polymerase chain reaction (qPCR) revealed a positive correlation between ABCG2 expression and the presence of ABCG2+ cells within the primary tumor. The expression of ABCG2 was predictive of the response to neoadjuvant chemotherapy in our experiments and in the GSE32646 dataset (p = 0.04 and p = 0.002, respectively). The in silico analysis demonstrated that ABCG2Up breast cancer samples have a slower cell cycle and a higher expression of membrane proteins but a greater potential for chromosomal instability, metastasis, immune evasion, and resistance to hypoxia. Such genetic characteristics are compatible with highly aggressive and resistant tumors. Our results support the hypothesis that the presence of ABCG2+ cells in breast carcinomas is a marker of resistance to chemotherapy, and based on in vitro assays and the genetic profile, we show, for the first time, that ABCG2 protein can be used as an independent marker for TIC identification in breast cancer.
KeywordsBreast cancer Tumor-initiating cells Chemotherapy Resistance to treatment
Aldehyde dehydrogenase 1
Chemokine receptor type 4
G protein-coupled receptors
Human epidermal growth factor receptor 2
Invasive ductal carcinomas
Minimum essential medium
Quantitative real-time polymerase chain reaction
TATA box binding protein
Triple-negative breast cancer
The authors wish to thank Stanislas Du Manoir. PhD, from the IRCM (Montpellier Cancer Research Institute) for his useful advices in the writing of the draft.
This research was supported by a grant from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (process 2011/13020-4).
Conflicts of interest
DGT participated in the design of the study, recruitment, and treatment of patients, draft the manuscript, and performed statistical analysis. JMA participated in the design and coordination of the study and helped to draft the manuscript. RDS was responsible for experimental procedures and helped to draft the manuscript. WAS was responsible for in silico analysis. LRMM and HHAC helped with the data collection and were responsible for patients’ treatment. TMGO and VFM were responsible for ultrasound-guided core-needle biopsy. All authors read and approved the final manuscript.
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