Targeting HER2 signaling pathway for radiosensitization: alternative strategy for therapeutic resistance.

Several studies have indicated the potential value of targeting HER-2 signaling to enhance the anti-tumor activity of ionizing radiation. However, therapeutic resistance resulting from several factors, including activation of the downstream pathway, represents a major obstacle to treatment. Here, we investigated whether inhibitors targeting downstream of HER-2 signaling would radiosensitize SKBR3 breast cancer cells that exhibit overamplification of HER2. Selective inhibition of MEK-ERK signaling using pharmacologic inhibitors (PD98059, UO126) did not increase the radiosensitivity of SKBR3 cells. Selective inhibition of the PI3K-AKT-mTOR pathway using pharmacologic inhibitors (LY294002, AKT inhibitor VIII, Rapamycin) significantly attenuated expression of p-AKT and p-70S6K, respectively and radiosensitized SKBR3 cells. MCF-7 cells those did not overexpress HER-2, showed less radiosensitization compared to SKBR3 cells by inhibition of this pathway. Pre-treatment with these inhibitors also caused significant abrogation of typical G(2) arrest following ionizing radiation and induced marked prolongation of gammaH2AX foci indicating impairment of DNA damage repair. A dual inhibitor of Class I PI3K and mTOR, PI103 effectively radiosensitized SKBR3 cells and showed significant prolongation of gammaH2AX foci. Inhibition of PI3K-AKT signaling was associated with downregulation of DNA-PKs, respectively. While apoptosis was the major mode of cell death when the cells were pretreated with LY294002 or AKT inhibitor VIII, the cells were pretreated by rapamycin or PI103 showed mixed mode of cell death including autophagy. Our results suggest possible mechanisms to counteract the HER-2 prosurvival signaling implicated in radioresistance, and offer an alternative strategy to overcome resistance to HER-2 inhibitors combined with radiation.