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.
PurposeHistone deacetylase (HDAC) inhibitors radiosensitize tumor cells. To elucidate mechanisms underlying radiosensitization by HDAC inhibition, understanding of differential contributions of HDAC isotypes is needed. The aim of this study was to investigate involvement of known HDAC isotypes in modulation of cellular radiosensitivity.Materials and MethodsBecause pharmacologic HDAC inhibitors lack isotype-specificity, RNA interference against 11 HDAC isotypes was used to inhibit HDAC in an isotype-specific manner. Radiation cell survival was evaluated using a clonogenic assay in SQ20B cells transfected with small interfering RNA specifically targeting HDAC isotypes. Immunocytochemistry was performed for detection of γH2AX foci. Protein expression was measured using Western blotting.ResultsAmong 11 HDAC isotypes tested, specific inhibition of 7 isotypes (HDAC1, HDAC3, HDAC4, HDAC6, HDAC7, HDAC10, and HDAC11) enhanced radiation lethality in SQ20B cells. Radiosensitization by inhibition of these HDAC isotypes was accompanied by delay of DNA double strand break repair. Radiosensitivity of SQ20B cells was not altered by selective inhibition of the remaining four isotypes (HDAC2, HDAC5, HDAC8, and HDAC9). Inhibition of HDAC isotypes resulted in downregulation of various proteins involved in pro-survival and DNA damage repair pathways.ConclusionIsotype-specificity exists in HDAC inhibition-induced radiosensitization. Different HDAC isotypes are differentially involved in modulation of cellular radiosensitivity.
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