Department of Therapeutic Research and Medicines Evaluation, Unit of Antitumor Drugs, Istituto Superiore di Sanità, Rome 00161, Italy.
J Biol Chem. 2012 Aug 31;287(36):30664-76. doi: 10.1074/jbc.M112.339127. Epub 2012 Jul 3.
Cyclic hypoxia and alterations in oncogenic signaling contribute to switch cancer cell metabolism from oxidative phosphorylation to aerobic glycolysis. A major consequence of up-regulated glycolysis is the increased production of metabolic acids responsible for the presence of acidic areas within solid tumors. Tumor acidosis is an important determinant of tumor progression and tumor pH regulation is being investigated as a therapeutic target. Autophagy is a cellular catabolic pathway leading to lysosomal degradation and recycling of proteins and organelles, currently considered an important survival mechanism in cancer cells under metabolic stress or subjected to chemotherapy. We investigated the response of human melanoma cells cultured in acidic conditions in terms of survival and autophagy regulation. Melanoma cells exposed to acidic culture conditions (7.0 < pH < 6.2) promptly accumulated LC3+ autophagic vesicles. Immunoblot analysis showed a consistent increase of LC3-II in acidic culture conditions as compared with cells at normal pH. Inhibition of lysosomal acidification by bafilomycin A1 further increased LC3-II accumulation, suggesting an active autophagic flux in cells under acidic stress. Acute exposure to acidic stress induced rapid inhibition of the mammalian target of rapamycin signaling pathway detected by decreased phosphorylation of p70S6K and increased phosphorylation of AMP-activated protein kinase, associated with decreased ATP content and reduced glucose and leucine uptake. Inhibition of autophagy by knockdown of the autophagic gene ATG5 consistently reduced melanoma cell survival in low pH conditions. These observations indicate that induction of autophagy may represent an adaptation mechanism for cancer cells exposed to an acidic environment. Our data strengthen the validity of therapeutic strategies targeting tumor pH regulation and autophagy in progressive malignancies.
周期性缺氧和致癌信号的改变导致癌细胞代谢从氧化磷酸化转变为有氧糖酵解。糖酵解上调的一个主要后果是产生更多的代谢酸，导致实体瘤中出现酸性区域。肿瘤酸中毒是肿瘤进展的一个重要决定因素，目前正在研究肿瘤 pH 调节作为一种治疗靶点。自噬是一种细胞分解代谢途径，导致溶酶体降解和蛋白质和细胞器的回收，目前被认为是代谢应激或化疗下癌细胞的重要生存机制。我们研究了在酸性条件下培养的人类黑色素瘤细胞的存活和自噬调节反应。暴露于酸性培养条件（7.0<pH<6.2）的黑色素瘤细胞迅速积累 LC3+自噬小泡。免疫印迹分析显示，与正常 pH 下的细胞相比，酸性培养条件下 LC3-II 持续增加。通过巴弗洛霉素 A1 抑制溶酶体酸化进一步增加了 LC3-II 的积累，表明在酸性应激下细胞中存在活跃的自噬流。急性暴露于酸性应激通过减少 p70S6K 的磷酸化和增加 AMP 激活的蛋白激酶的磷酸化来快速抑制哺乳动物雷帕霉素靶蛋白信号通路，这与 ATP 含量降低以及葡萄糖和亮氨酸摄取减少有关。通过敲低自噬基因 ATG5 抑制自噬一致降低了低 pH 条件下黑色素瘤细胞的存活率。这些观察结果表明，自噬的诱导可能代表暴露于酸性环境的癌细胞的一种适应机制。我们的数据加强了针对肿瘤 pH 调节和自噬的治疗策略在进行性恶性肿瘤中的有效性。
