The roles of FOXP3 and CXCR4 in breast cancer

Abstract

The X-chromosome linked transcription factor, FOXP3, is expressed by epithelial cells from several organs. In these cells it is considered a potent tumour suppressor directly regulating the expression of many important oncogenes. The chemokine receptor, CXCR4, can also direct a number of processes relevant to the development of breast cancer. These include chemotaxis towards the sole ligand for CXCR4, CXCL12, which is expressed at the most common sites of metastatic spread. This study was designed to define further the role FOXP3 plays in metastatic breast cancer, with a particular focus on its potential to regulate CXCR4. In comparison with normal primary breast epithelial cells, FOXP3 was downregulated at both transcript and protein levels in the breast cancer cell lines, MCF-7 and MDA-MB-231. In the more invasive MDA-MB-231, the remaining FOXP3 was located predominately within the cytoplasm and lacked the natural Δ3 isoform. Following stable FOXP3 overexpression in MDA-MB-231, significant decreases were observed in the expression of ErbB2, SKP2, c-MYC and CXCR4. In contrast, an increase in p21 expression led to inhibition of cell proliferation, with a greater proportion in the G1 phase of the cell cycle suggesting the induction of senescence. Specific knockdown of FOXP3 in normal breast epithelial cells with siRNA significantly increased ErbB2, SKP2, c-MYC and CXCR4, and decreased p21 expression. These cells also showed a significantly increased chemotactic response towards CXCL12, suggesting a role for FOXP3 influencing cell migration. The expression of FOXP3 and CXCR4 in normal breast tissue (n=2), and both lymph node negative (n=7) and lymph node positive (n=9) breast cancer samples was investigated by immunohistochemistry. Both epithelial and cancer cells in all the breast cancer samples showed increased CXCR4 expression in comparison to normal tissue. However, the expression of FOXP3 by epithelial or cancer cells did not appear different between all the samples examined. Results from this study are consistent with FOXP3 functioning as an important tumour suppressor in breast cancer. Indeed, the functions of FOXP3 in breast epithelium can now potentially be extended to include influencing the expression of CXCR4 and response to the pro-metastatic chemokine, CXCL12. Further studies should be directed to investigation of the molecular mechanisms involved in this influence of chemokine responsiveness by FOXP3.