Effect of Her2 Overexpression on Cancer Stem-Like Characteristics of Triple-Negative Breast Cancer Cells

Abstract

ABSTRACT Aim: Cancer stem cells (CSCs) have been proposed to constitute a subpopulation of undifferentiated tumor cells that are resistant to both radio- and chemotherapy. The population exhibiting CD44+/CD24- is generally accepted to be human breast CSCs. However the overexpression of CD24 in breast cancer is associated with a poor patient prognosis. HER2-positive breast cancers are predominantly positive for CD24, which has been identified as a marker of differentiated normal mammary epithelial cells. We aimed to investigate the relationship between HER2 and CD24. Methods: We established stable HER2-expressing cell lines by introducing a wild type HER2 gene into 231-Luc triple-negative (ER- PgR- HER2-) breast cancer cells, having stem-like characteristics. Then, we determined the functional relevance of CD24 expression in HER2-positive breast cancer cells. We also investigated the expression of CD44, CD24 and HER2 by flow cytometry analysis. Results: Overexpression of HER2 decreased representative CSC properties, such as the sphere-forming ability and ALDH1 activity, and did not affect the tumorigenicity of 231-Luc triple-negative breast cancer cells. Flow cytometry analysis revealed that stable overexpression of HER2 up-regulates CD24 in triple-negative breast cancer. Positive relationship between HER2 and CD24 expression was also confirmed in HER2-positive breast cancer cell lines: BT-474, HCC202, SKBR3, and HCC1569. Given that overexpression of HER2 increases the level of Akt phosphorylation, we examined if CD24 depletion might suppress Akt phosphorylation in HER2-expressing cells. Successfully, the phosphorylation of Akt was down-regulated by CD24 depletion in the HER2-expressing cells. Moreover, knockdown of CD24 increased the sensitivity of HER2-expressing cells to lapatinib, a dual tyrosine kinase inhibitor for both EGFR and HER2. Conclusions: HER2 may have some contribution to stem-like characteristics of breast cancer cells but other factors are required to regulate CSC functions. CD24 supports both the expression of HER2 and the consequent activation of the PIK3/AKT signaling pathway so as to promote cell survival. CD24 appears to contribute to the resistance to HER2-targeted therapy and is itself a potential therapeutic target in HER2-positive breast cancer. Disclosure: All authors have declared no conflicts of interest.