Sensitization of breast cancer cells to radiation by trastuzumab

HER2, a member of the human epidermal growth factor (EGF) receptor family, not only plays important roles in the progression of breast cancer tumorigenesis and metastasis, but may protect cancer cells from conventional cytotoxic therapies as well. In the current study, we evaluated the effect of targeting HER2 on radiosensitization of human breast cancer cells. Using six breast cancer cell lines with various levels of HER2 (BT474, SKBR3, MDA453, MCF7, ZR75B, and MDA468), we found that trastuzumab (Herceptin), a humanized monoclonal antibody that may inhibit breast cancer cell proliferation but does not induce apoptosis when used alone, enhanced radiation-induced apoptosis of the cells in a HER2 level-dependent manner. We furthered this study in MCF7 cells transfected for high levels of HER2 (MCF7HER2). Compared with parental or control vector-transfected MCF7 cells, MCF7HER2 cells showed increased phosphorylation of at least two important HER2 downstream molecules, protein kinase B/Akt and mitogen-activated protein kinase (MAPK), and increased resistance to radiotherapy, as shown by reduced induction of apoptosis and increased cell clonogenic survival after radiation. Exposure of the cells to trastuzumab down-regulated the levels of HER2 and reduced phosphorylation levels of Akt and MAPK in MCF7HER2 cells, and sensitized these cells to radiotherapy. When specific inhibitors of the phosphatidylinositol 3-kinase (PI3-K) and MAPK kinase (MEK) pathways were used, we found that exposure of MCF7HER2 cells to the PI3-K inhibitor LY294002 inhibited Akt phosphorylation and radiosensitized the cells, whereas the radiosensitization effect by the MEK inhibitor PD98059 was relatively weaker, albeit the phosphorylation of MAPK was reduced by PD98059 treatment. Our results indicate that the PI3-K pathway might be the major pathway for trastuzumab-mediated radiosensitization of breast cancer cells.