TGFβ1 Promotes Breast Cancer Local Invasion and Liver Metastasis by Increasing the CD44(high)/CD24(−) Subpopulation

OBJECTIVE: Previous studies have shown that the transforming growth factor β1 pathway plays an important role in breast cancer metastasis to the liver. However, the mechanism of this metastasis has not been fully clarified. Cancer stem cells are essential for the initiation and propagation of tumor metastasis. The objective of our current study was to define the role of cancer stem cells in transforming growth factor β1-mediated breast cancer hepatic metastases. METHOD: Hematoxylin and eosin staining was used to assess the formation of breast cancer liver metastases and local invasion. Cancer stem cells surface markers (CD44, CD24, and Epithelial cell adhesion molecule [EpCAM]), luminal/mesenchymal markers (keratin8 and alpha smooth muscle actin), and proliferation markers (Ki-67 and cyclinD1) were detected by immunohistochemistry assays. Flow cytometry was used to evaluate the effect of transforming growth factor β1 on the CD44(+)/CD24(−) cancer stem cell population. Quantitative real-time polymerase chain reaction was employed to assess the gene expression of the stem cell self-renewal markers nanog, pou5f1 (coding for Oct4), and sox2. RESULTS: Transforming growth factor β1 increased the formation of liver metastases by the MDA-MB231 (MDA) breast cancer cell line but did not affect the liver metastasis of CD44(+)/CD24(+) noncancer stem cells. Transforming growth factor β1 treatment did not significantly affect tumor proliferation in vitro or in vivo. Transforming growth factor β1 promoted mammary tumor local invasion. Furthermore, the CD44(high)/CD24(−) cancer stem cell population was also significantly increased by transforming growth factor β1 treatment. Besides, the gene expression of the stem cell self-renewal markers (nanog, pou5f1, and sox2) and another stem cell surface marker (EpCAM) was increased by transforming growth factor β1 treatment. Finally, clusters of CD44-positive breast cancer cells were observed in the livers of mice from the control and transforming growth factor β1 pretreatment groups. CONCLUSION: Our results indicate that transforming growth factor β1 increases the local invasive capacity and liver metastasis of breast cancer cells by inducing the CD44(high)/CD24(−) cancer stem cell population.