Radiosensitization by inhibition of Hsp90 and microenvironmental factors

已知激活磷脂酰肌醇3-激酶(PI3K)-Akt通路可诱导肿瘤放射抵抗。在目前的研究中，我们检测了17-烯丙基氨基-17-去甲氧基格尔达霉素（17AAG）降低Hsp90客户蛋白水平的能力，包括PI3K-Akt通路的成分，使放射耐药的人鳞状细胞癌细胞对x射线敏感。17AAG （0.2 μM）对辐射耐药细胞SQ20B和scc13的辐射敏感性比辐射敏感细胞SCC61更有效。然而，在所有三种细胞系中，17AAG通过降低表皮生长因子受体和ErbB-2的表达以及抑制Akt的磷酸化而增加辐射诱导的细胞凋亡。此外，17AAG （1 μM）使SQ20B球体对辐射敏感，17AAG抑制Akt激活增加了辐射诱导的球体凋亡。研究结果表明，17AAG通过抑制PI3K-Akt通路有效地使辐射耐药细胞对辐射敏感。我们还测试了GA和辐射联合对人类肿瘤和正常细胞的细胞存活、PI3K/ akt相关蛋白和凋亡诱导的影响。与正常细胞相比，GA使肿瘤细胞对辐射更敏感。此外，辐射和GA联合抑制Akt活性，并强烈增强肿瘤细胞凋亡的诱导作用，肿瘤细胞的存活依赖于Akt蛋白活性。目前的数据支持GA通过调节有丝分裂、抗增殖和凋亡通路之间的平衡而使肿瘤细胞增敏的假设。因此，靶向肿瘤细胞中的Hsp90可能会导致放射治疗中新的放射增敏策略的发展。

Activation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway is known to induce tumor radioresistance. In the current studies, we examined the ability of 17-Allylamino-17-demethoxygeldanamycin (17AAG), which decreases the levels of Hsp90 client proteins including components of the PI3K-Akt pathway, to sensitize radioresistant human squamous cell carcinoma cells to X-irradiation. 17AAG (0.2 μM) enhanced the radiosensitivity more effectively in radioresistant SQ20B and SCC 13 cells than in radiosensitive SCC61 cells. However, in all three cell lines, 17AAG increased radiation-induced apoptosis by reducing the expression of the epidermal growth factor receptor and ErbB-2 and inhibiting the phosphorylation of Akt. Furthermore, 17AAG (1 μM) sensitized SQ20B spheroids to radiation, and inhibition of Akt activation by 17AAG increased radiation-induced apoptosis in spheroids. The findings suggest that 17AAG effectively sensitizes radioresistant cells to radiation by inhibiting the PI3K-Akt pathway.We also tested the effect of a combination of GA and radiation on cell survival, PI3K/Akt-related proteins and apoptosis induction in human tumor and normal cells. GA sensitized tumour cells in preference to normal cells to radiation. In addition, a combination of radiation and GA abolished Akt activities and strongly enhanced the induction of apoptosis in tumour cells which depend on Akt protein activities for cell survival. The present data support the hypothesis that GA sensitizes tumour cells by modulating the balance among mitogenic, antiproliferative and apoptotic pathways. Thus, targeting Hsp90 in tumour cells may lead to the development of new radiosensitizing strategies in radiotherapy.