Department of Radiology, University of Texas Health Science Center at San Antonio, San Antonio, TX, USA.
Apoptosis. 2014 May;19(5):816-28. doi: 10.1007/s10495-013-0962-z.
The signaling pathways via mTOR (mammalian target of rapamycin) and AMPK (AMP-activated protein kinase) play key roles in transcription, translation and carcinogenesis, and may be activated by light exposure. These pathways can be modulated by naturally occurring compounds, such as the triterpenoid, ursolic acid (UA). Previously, the transcription factors p53 and NF-κB, which transactivate mitochondrial apoptosis-related genes, were shown to be differentially modulated by UA. UA-modulated apoptosis, following exposure to UV-VIS radiation (ultraviolet to visible light broadband radiation, hereafter abbreviated to UVR), is observed to correspond to differential levels of oxidative stress in retinal pigment epithelial (RPE) and skin melanoma (SM) cells. The cellular response to this phytochemical was characterized using western blot, flow cytometry, microscopy with reactive oxidative species probes MitoTracker and dihydroethidium, and membrane permeability assay. UA pretreatment potentiated cell cycle arrest and UVR-induced apoptosis selectively in SM cells while reducing photo-oxidative stress in the DNA of RPE cells presumably by antioxidant activity of UA. Mechanistically, the nuclear transportation of p65 and p53 was reduced by UA administration prior to UVR exposure while the levels of p65 and p53 nuclear transportation in SM cells were sustained at a substantially higher level. Finally, the mitochondrial functional assay showed that UVR induced the collapse of the mitochondrial membrane potential, and this effect was exacerbated by rapamycin or UA pretreatment in SM preferentially. These results were consistent with reduced proliferation observed in the clonogenic assay, indicating that UA treatment enhanced the phototoxicity of UVR, by modulating the activation of p53 and NF-κB and initiating a mitogenic response to optical radiation that triggered mitochondria-dependent apoptosis, particularly in skin melanoma cells. The study indicates that this compound has multiple actions with the potential for protecting normal cells while sensitizing skin melanoma cells to UV irradiation.
mTOR（哺乳动物雷帕霉素靶蛋白）和 AMPK（AMP 激活的蛋白激酶）信号通路在转录、翻译和致癌作用中发挥着关键作用，并且可能会受到光照的激活。这些通路可以被天然化合物如三萜类化合物熊果酸（UA）调节。此前，被证明 p53 和 NF-κB 转录因子可差异调节线粒体凋亡相关基因的激活，这些转录因子的激活可以通过 UA 来实现。UA 调节的细胞凋亡，在暴露于 UV-VIS 辐射（紫外线到可见光宽带辐射，以下简称 UVR）后，在视网膜色素上皮（RPE）和皮肤黑色素瘤（SM）细胞中观察到与氧化应激水平的差异相关。使用 Western blot、流式细胞术、用活性氧物种探针 MitoTracker 和二氢乙啶进行显微镜观察以及膜通透性测定来表征细胞对这种植物化学物质的反应。UA 预处理增强了 SM 细胞的细胞周期停滞和 UVR 诱导的细胞凋亡，同时减少了 RPE 细胞中 DNA 的光氧化应激，这可能是由于 UA 的抗氧化活性。在机制上，UA 给药可减少 UVR 暴露前 p65 和 p53 的核转运，而 SM 细胞中 p65 和 p53 的核转运水平则保持在更高的水平。最后，线粒体功能测定表明 UVR 诱导了线粒体膜电位的崩溃，而在 SM 中，雷帕霉素或 UA 预处理可加剧这种作用。这些结果与克隆形成测定中观察到的增殖减少一致，表明 UA 处理通过调节 p53 和 NF-κB 的激活，以及引发对光学辐射的有丝分裂反应，从而触发依赖线粒体的凋亡，从而增强了 UVR 的光毒性，特别是在皮肤黑色素瘤细胞中。该研究表明，该化合物具有多种作用，具有保护正常细胞的潜力，同时使皮肤黑色素瘤细胞对紫外线照射敏感。
