Increasing evidence has shown that chemokines and chemokine receptors are associated with tumor growth and metastasis. CCR4, an important chemokine receptor for regulating immune homeostasis, is thought to be involved in hematologic malignancies and has also recently implicated in some solid tumors, such as gastric cancer. The possible role of CCR4 in breast cancer has not been well elucidated. In this study, we show that CCR4 is differentially expressed in human breast cancer cell lines. Specifically, we find that CCR4 is overexpressed in breast cancer cell lines with high metastatic potential. More importantly, we used a combination of overexpression and RNA interference to demonstrate that CCR4 promotes breast tumor growth and lung metastasis in mice. Furthermore, we find that microvessel density is significantly increased in tumors formed by CCR4-overexpressing cells and decreased in those formed by CCR4-knockdown cells. We find that overexpression of CCR4 can enhance the chemotactic response of breast cancer cells to CCL17. However, the expression of CCR4 does not affect the proliferation of breast cancer cells in vitro. Furthermore, we show that CCR4 expression is positively correlated with HER2 expression, tumor recurrence and lymph node, lung and bone metastasis (P < 0.05). Multivariate analysis showed that CCR4 expression is a significant independent prognostic factor for overall survival (P = 0.036) but not for disease-free survival in patients with breast cancer (P = 0.071). Survival analysis indicated a strong association between CCR4 expression and lower overall survival (P = 0.0001) and disease-free survival (P = 0.016) in breast cancer.

Metastasis represents the major remaining cause of mortality in human breast cancer. Interleukin-8 (IL-8), a proinflammatory chemokine, plays an important role during tumor angiogenesis and metastasis. In this study, we found that IL-8 and ERβ showed positive association. Overexpression of ERβ or PEA3 could up-regulate IL-8 promoter activity, mRNA and secretion; silencing of ERβ or PEA3 decreased IL-8 mRNA and secretion. ERβ and PEA3 increased IL-8 expression through binding to the IL-8 promoter and increased cell invasion. HER2 could increase ERβ and PEA3 expression and their binding to the IL-8 promoter. We conclude that ERβ and PEA3 play important roles in tumor invasion by regulating IL-8 expression, and HER2 maybe the upstream of ERβ and PEA3 - IL-8 pathway.