HER2 reduces breast cancer radiosensitivity by activating focal adhesion kinase in vitro and in vivo

// Jing Hou 1, * , Zhirui Zhou 2, 3, * , Xingxing Chen 2, 3, * , Ruping Zhao 4 , Zhaozhi Yang 2, 3 , Na Wei 1 , Qing Ni 1 , Yan Feng 2, 3 , Xiaoli Yu 2, 3 , Jinli Ma 2, 3 , Xiaomao Guo 2, 3 1 Department of Breast surgery, Guizhou Provincial People's Hospital, Guiyang 550002, Guizhou Province, China 2 Department of Radiation Oncology, Fudan University Shanghai Cancer Center, Shanghai 200032, China 3 Department of Oncology, Shanghai Medical College, Fudan University, Shanghai 200032, China 4 Department of Radiation Oncology, Hangzhou Cancer Hospital, Hangzhou 310006, Zhejiang, China * These authors contributed equally to this work Correspondence to: Xiaomao Guo, email: guoxm1800@163.com Jinli Ma, email: jinlima_c@yahoo.com Keywords: breast cancer, radiosensitivity, HER2, focal adhesion kinase, anoikis Received: March 06, 2016      Accepted: May 17, 2016      Published: June 07, 2016 ABSTRACT Growing evidence has demonstrated that human epidermal growth factor receptor 2 (HER2) is involved in the radiation response to breast cancer. However, the underlying mechanism remains elusive. Therefore, we investigated if HER2 overexpression is associated with radiosensitivity of breast cancer. We constructed breast cancer cell lines differing in HER2 expression by transducing HER2 cDNA or short hairpin RNA against HER2. We then assessed the radiosensitivity and investigated the potential mechanism by using cell proliferation assay, cell adhesion assays, anoikis assays, colony formation assays, and western blotting analyses. We found that HER2 introduction in breast cancer cell lines MCF-7 (low HER2 expression) and MDA-MB-231 (HER2 is not expressed) promoted cell proliferation and invasion and enhanced cell adhesion and resistance to anoikis. Moreover, HER2 reduced radiosensitivity in these two cells compared with the corresponding control. The opposite results were observed when HER2 was silenced in breast cancer cell lines ZR-7530 and SK-BR-3 (both cells with high expression of HER2) using HER2 shRNA. In addition, animal experiment results showed HER2 could enhance the radioresistance of xenograft tumors. Further studies showed HER2 promoted the phosphorylation of focal adhesion kinase (Fak) and thereby up-regulated the expression of proteins associated with the epithelial-to-mesenchymal transition such as Claudin-1, ZO-1, and ZEB-1. The inhibition of Fak activity using the Fak inhibitor (PF-562281) restored the radiosensitivity in HER2-overexpressing cells. In conclusion, HER2 reduces the radiosensitivity of breast cancer by activating Fak in vitro and in vivo . Fak might be a potential target for the radiosensitization of HER2-overexpressed breast cancer.