Acidic extracellular pH neutralizes the autophagy-inhibiting activity of chloroquine: Implications for cancer therapies

Acidic pH is an important feature of tumor microenvironment and a major determinant of tumor progression. We reported that cancer cells upregulate autophagy as a survival mechanism to acidic stress. Inhibition of autophagy by administration of chloroquine (CQ) in combination anticancer therapies is currently evaluated in clinical trials. We observed in 3 different human cancer cell lines cultured at acidic pH that autophagic flux is not blocked by CQ. This was consistent with a complete resistance to CQ toxicity in cells cultured in acidic conditions. Conversely, the autophagy-inhibiting activity of Lys-01, a novel CQ derivative, was still detectable at low pH. The lack of CQ activity was likely dependent on a dramatically reduced cellular uptake at acidic pH. Using cell lines stably adapted to chronic acidosis we could confirm that CQ lack of activity was merely caused by acidic pH. Moreover, unlike CQ, Lys-01 was able to kill low pH-adapted cell lines, although higher concentrations were required as compared with cells cultured at normal pH conditions. Notably, buffering medium pH in low pH-adapted cell lines reverted CQ resistance. In vivo analysis of tumors treated with CQ showed that accumulation of strong LC3 signals was observed only in normoxic areas but not in hypoxic/acidic regions. Our observations suggest that targeting autophagy in the tumor environment by CQ may be limited to well-perfused regions but not achieved in acidic regions, predicting possible limitations in efficacy of CQ in antitumor therapies.     

Modulation of autophagic activity by extracellular pH

Reprogramming energy metabolism from oxidative phosphorylation to aerobic glycolysis, a common feature of human cancer, is associated with a relative acidic tumor microenvironment which can sometimes be further accentuated by hypoxia operating within most solid tumors. We found that alteration of extracellular pH induces marked and rapid changes of autophagic activity. Interestingly, acidic and basic conditions induced completely opposite effect on autophagy, with its activity suppressed at lower pH whereas stimulated at higher pH. Gene knockdown experiments indicated that pH induced-autophagy requires Beclin 1, Vps34 and Atg5, key components of the autophagy pathway. Of note, an acidic condition not only inhibits the basal but also blocks the starvation-induced autophagy activity. Significantly, examination of different areas of tumor mass revealed a lower autophagic activity within the inner region than the outer region. These findings have important implications on the connections between autophagy and cancer as well as a wide range of other physiological and pathological processes.     

自噬抑制剂氯奎对低氧诱导肺动脉平滑肌细胞增殖的影响

目的：探讨自噬抑制刺氯喹（cQ）在低氧（hypoxia）调节肺动脉平滑肌细胞（PASMCs）增殖中的作用。方法：将体外培养的大鼠PASMCs分为4组：正常对照组、1％低氧组、50μmol／L氯喹＋1％低氧组、50ttmol／L氯喹组。MTF方法检测各组的PASMCs增殖率;MDC染色检测细胞自噬空泡的变化;Westernblot方法检测微管相关蛋白轻链3（LC3）蛋白的表达变化;划痕法检测细胞迁移的变化。结果：与对照组比较,氯喹组的PASMCs细胞增殖率无明显变化。与对照组比较,1％低氧组PASMCs增殖率明显增加,细胞内出现大量自噬空泡,细胞迁移速度明显增加。细胞LC3-Ⅱ蛋白表达增强。与1％低氧组比较,氯喹与低氧联合作用时细胞自噬空泡的积聚以及rE3．II蛋白表达增强,但细胞增殖率和迁移明显降低。结论：低氧激活自噬过程并促进了PASMCs增殖和迁移,而自噬抑制剂氯喹在一定程度上通过抑制自噬进程,达到抑制肺动脉平滑肌细胞增殖和迁移的作用。     

Regulation of autophagy and chloroquine sensitivity by oncogenic RAS in vitro is context-dependent

Chloroquine (CQ) is an antimalarial drug and late-stage inhibitor of autophagy currently FDA-approved for use in the treatment of rheumatoid arthritis and other autoimmune diseases. Based primarily on its ability to inhibit autophagy, CQ and its derivative, hydroxychloroquine, are currently being investigated as primary or adjuvant therapy in multiple clinical trials for cancer treatment. Oncogenic RAS has previously been shown to regulate autophagic flux, and cancers with high incidence of RAS mutations, such as pancreatic cancer, have been described in the literature as being particularly susceptible to CQ treatment, leading to the hypothesis that oncogenic RAS makes cancer cells dependent on autophagy. This autophagy “addiction” suggests that the mutation status of RAS in tumors could identify patients who would be more likely to benefit from CQ therapy. Here we show that RAS mutation status itself is unlikely to be beneficial in such a patient selection because oncogenic RAS does not always promote autophagy addiction. Moreover, oncogenic RAS can have opposite effects on both autophagic flux and CQ sensitivity in different cells. Finally, for any given cell type, the positive or negative effect of oncogenic RAS on autophagy does not necessarily predict whether RAS will promote or inhibit CQ-mediated toxicity. Thus, although our results confirm that different tumor cell lines display marked differences in how they respond to autophagy inhibition, these differences can occur irrespective of RAS mutation status and, in different contexts, can either promote or reduce chloroquine sensitivity of tumor cells.     

Regulation of autophagy and chloroquine sensitivity by oncogenic RAS in vitro is context-dependent

Chloroquine (CQ) is an antimalarial drug and late-stage inhibitor of autophagy currently FDA-approved for use in the treatment of rheumatoid arthritis and other autoimmune diseases. Based primarily on its ability to inhibit autophagy, CQ and its derivative, hydroxychloroquine, are currently being investigated as primary or adjuvant therapy in multiple clinical trials for cancer treatment. Oncogenic RAS has previously been shown to regulate autophagic flux, and cancers with high incidence of RAS mutations, such as pancreatic cancer, have been described in the literature as being particularly susceptible to CQ treatment, leading to the hypothesis that oncogenic RAS makes cancer cells dependent on autophagy. This autophagy “addiction” suggests that the mutation status of RAS in tumors could identify patients who would be more likely to benefit from CQ therapy. Here we show that RAS mutation status itself is unlikely to be beneficial in such a patient selection because oncogenic RAS does not always promote autophagy addiction. Moreover, oncogenic RAS can have opposite effects on both autophagic flux and CQ sensitivity in different cells. Finally, for any given cell type, the positive or negative effect of oncogenic RAS on autophagy does not necessarily predict whether RAS will promote or inhibit CQ-mediated toxicity. Thus, although our results confirm that different tumor cell lines display marked differences in how they respond to autophagy inhibition, these differences can occur irrespective of RAS mutation status and, in different contexts, can either promote or reduce chloroquine sensitivity of tumor cells.     

自噬抑制剂氯喹对急性酒精诱导小鼠肝损伤的影响

目的：探讨自噬抑制剂氯喹（CQ）对急性酒精诱导肝损伤的影响及其作用机制。方法：将雄性C57BL/6小鼠随机分为3组：正常对照组、酒精组、氯喹干预组（n=7）,其中酒精组按4.5 g/kg剂量给予33%（V/V）酒精灌胃。HE和油红O染色检测各组小鼠肝组织脂滴变化;检测肝组织甘油三酯（TG）含量变化;检测血清谷草转氨酶（AST）和谷丙转氨酶（ALT）活性;免疫荧光法检测微管相关蛋白轻链3（LC3）蛋白变化;Western blot法检测LC3蛋白和核蛋白P65表达的变化;ELISA法检测促炎因子TNF-α、IL-6的变化。结果：与对照组比较,酒精组脂滴形成、TG含量、血清AST和ALT活性明显增高。与对照组比较,酒精组LC3-Ⅱ蛋白表达明显增加;与酒精组比较,氯喹干预组使酒精诱导的LC3-Ⅱ蛋白表达增强进一步加剧,使酒精诱导的TG含量、血清AST和ALT活性进一步增高,同时增加了酒精诱导的p65入核及TNFα、IL-6释放。结论：急性酒精能引起小鼠肝脏脂肪变化及炎症,而自噬抑制剂氯喹抑制自噬进程,加剧酒精诱导的肝损伤,说明自噬在酒精诱导肝损伤中可能具有保护效应。     

How to teach an old dog new tricks: Autophagy-independent action of chloroquine on the tumor vasculature

Chloroquine (CQ) is exploited in clinical trials as an autophagy blocker to potentiate anticancer therapy, but it is unknown if it solely acts by inhibiting cancer cell-autonomous autophagy. Our recent study shows that besides blocking cancer cell growth, CQ also affects endothelial cells (ECs) and promotes tumor vessel normalization. This vessel normalizing effect of CQ reduces tumor hypoxia, cancer cell intravasation, and metastasis, while improving the delivery and response to chemotherapy. By compromising autophagy in melanoma cells or using mice with a conditional knockout of ATG5 in ECs, we found that the favorable effects of CQ on the tumor vasculature do not rely on autophagy. CQ-induced vessel normalization relies mainly on altered endolysosomal trafficking and sustained NOTCH1 signaling in ECs. Remarkably these CQ-mediated effects are abrogated when tumors are grown in mice harboring EC-specific deletion of NOTCH1. The autophagy-independent vessel normalization by CQ leading to improved delivery and tumor response to chemotherapy further advocates its clinical use in combination with anticancer treatments.     

How to teach an old dog new tricks: Autophagy-independent action of chloroquine on the tumor vasculature

Chloroquine (CQ) is exploited in clinical trials as an autophagy blocker to potentiate anticancer therapy, but it is unknown if it solely acts by inhibiting cancer cell-autonomous autophagy. Our recent study shows that besides blocking cancer cell growth, CQ also affects endothelial cells (ECs) and promotes tumor vessel normalization. This vessel normalizing effect of CQ reduces tumor hypoxia, cancer cell intravasation, and metastasis, while improving the delivery and response to chemotherapy. By compromising autophagy in melanoma cells or using mice with a conditional knockout of ATG5 in ECs, we found that the favorable effects of CQ on the tumor vasculature do not rely on autophagy. CQ-induced vessel normalization relies mainly on altered endolysosomal trafficking and sustained NOTCH1 signaling in ECs. Remarkably these CQ-mediated effects are abrogated when tumors are grown in mice harboring EC-specific deletion of NOTCH1. The autophagy-independent vessel normalization by CQ leading to improved delivery and tumor response to chemotherapy further advocates its clinical use in combination with anticancer treatments.     

Inhibition of autophagy enhances the anticancer activity of silver nanoparticles

Silver nanoparticles (Ag NPs) are cytotoxic to cancer cells and possess excellent potential as an antitumor agent. A variety of nanoparticles have been shown to induce autophagy, a critical cellular degradation process, and the elevated autophagy in most of these situations promotes cell death. Whether Ag NPs can induce autophagy and how it might affect the anticancer activity of Ag NPs has not been reported. Here we show that Ag NPs induced autophagy in cancer cells by activating the PtdIns3K signaling pathway. The autophagy induced by Ag NPs was characterized by enhanced autophagosome formation, normal cargo degradation, and no disruption of lysosomal function. Consistent with these properties, the autophagy induced by Ag NPs promoted cell survival, as inhibition of autophagy by either chemical inhibitors or ATG5 siRNA enhanced Ag NPs-elicited cancer cell killing. We further demonstrated that wortmannin, a widely used inhibitor of autophagy, significantly enhanced the antitumor effect of Ag NPs in the B16 mouse melanoma cell model. Our results revealed a novel biological activity of Ag NPs in inducing cytoprotective autophagy, and inhibition of autophagy may be a useful strategy for improving the efficacy of Ag NPs in anticancer therapy.     

Inhibition of autophagy enhances the anticancer activity of silver nanoparticles

Silver nanoparticles (Ag NPs) are cytotoxic to cancer cells and possess excellent potential as an antitumor agent. A variety of nanoparticles have been shown to induce autophagy, a critical cellular degradation process, and the elevated autophagy in most of these situations promotes cell death. Whether Ag NPs can induce autophagy and how it might affect the anticancer activity of Ag NPs has not been reported. Here we show that Ag NPs induced autophagy in cancer cells by activating the PtdIns3K signaling pathway. The autophagy induced by Ag NPs was characterized by enhanced autophagosome formation, normal cargo degradation, and no disruption of lysosomal function. Consistent with these properties, the autophagy induced by Ag NPs promoted cell survival, as inhibition of autophagy by either chemical inhibitors or ATG5 siRNA enhanced Ag NPs-elicited cancer cell killing. We further demonstrated that wortmannin, a widely used inhibitor of autophagy, significantly enhanced the antitumor effect of Ag NPs in the B16 mouse melanoma cell model. Our results revealed a novel biological activity of Ag NPs in inducing cytoprotective autophagy, and inhibition of autophagy may be a useful strategy for improving the efficacy of Ag NPs in anticancer therapy.     

Autophagic,apoptotic, and necrotic cancer cell fates triggered by acidic pH microenvironment

Cancer as a multifactorial and smart disease is now considered a challenging problem. Despite many investigations on drug discovery, it remains incurable, in part, due to insufficient understanding of its special mechanisms. For the first time, we collaterally investigated the effect of acidosis on the contribution of apoptosis, necrosis, and autophagy in MDA-MB 231 cells. Our data showed that necrosis, apoptosis, and intracellular reactive oxygen species production drastically decreased from 48 to 72 hr while cell viability and autophagy increased along with a gap between the percentages. Eventually, the decrease of necrosis and apoptosis was related to upregulation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and fatty acid synthetase, respectively. It seems that at the early stage of cancer progression, apoptosis is the main mechanism of cell mortality and afterward autophagy would be the main mechanism of cell survival. Therefore, at the acute phase of cancer, apoptotic inducer medications would be effective while at the chronic phase of cancer progression, autophagy inhibitor medication would be added as well. This eventually means that autophagy acts as both cell death and survival mechanisms at the onset of cancer progression with the approach towards cell survival. Besides other unknown cell survival mechanisms are involved in cell viability, except for apoptosis and necrosis inhibition and autophagy improvement. This study reiterates the inefficaciousness of autophagy inhibitor's medication at the onset of disease. It also emphasizes discovering other cell death mechanisms for cancer cell adaptation at the onset of disease with the aim of their targeting in cancer invasion therapy.     

The effects of metabolic inhibition on intracellular pH and Ca

We have investigated the effects of inhibiting aerobic and/or anaerobic metabolism on contraction, intracellular calcium and pH in single rat ventricular myocytes. Inhibition of aerobic metabolism alone (with CN) had little effect on these variables. However, if anaerobic glycolysis was also inhibited, then the application of CN decreased systolic [Ca³⁺]_i and increased diastolic [Ca²⁺]_i. There was also a development of a diastolic contracture which lagged behind the increase of diastolic [Ca²⁺]_i. These events were accompanied by an intracellular acidosis. The acidosis was shown to depress contraction and perhaps to account for the fact that diastolic [Ca²⁺]_i increased before the contracture.     

Revisiting autophagy addiction of tumor cells

Inhibition of autophagy has been widely explored as a potential therapeutic intervention for cancer. Different factors such as tumor origin, tumor stage and genetic background can define a tumor's response to autophagy modulation. Notably, tumors with oncogenic mutations in KRAS were reported to depend on macroautophagy in order to cope with oncogene-induced metabolic stress. Our recent report details the unexpected finding that autophagy is dispensable for KRAS-driven tumor growth in vitro and in vivo. Additionally, we clarify that the antitumorigenic effects of chloroquine, a frequently used nonspecific inhibitor of autophagy, are not connected to the inhibition of macroautophagy. Our data suggest that caution should be exercised when using chloroquine and its analogs to decipher the roles of autophagy in cancer.     

Dual role of chloroquine in liver ischemia reperfusion injury: reduction of liver damage in early phase,but aggravation in late phase

The anti-malaria drug chloroquine is well known as autophagy inhibitor. Chloroquine has also been used as anti-inflammatory drugs to treat inflammatory diseases. We hypothesized that chloroquine could have a dual effect in liver ischemia/reperfusion (I/R) injury: chloroquine on the one hand could protect the liver against I/R injury via inhibition of inflammatory response, but on the other hand could aggravate liver I/R injury through inhibition of autophagy. Rats (n=6 per group) were pre-treated with chloroquine (60 mg/kg, i.p.) 1 h before warm ischemia, and they were continuously subjected to a daily chloroquine injection for up to 2 days. Rats were killed 0.5, 6, 24 and 48 h after reperfusion. At the early phase (i.e., 0–6 h after reperfusion), chloroquine treatment ameliorated liver I/R injury, as indicated by lower serum aminotransferase levels, lower hepatic inflammatory cytokines and fewer histopathologic changes. In contrast, chloroquine worsened liver injury at the late phase of reperfusion (i.e., 24–48 h after reperfusion). The mechanism of protective action of chloroquine appeared to involve its ability to modulate mitogen-activated protein kinase activation, reduce high-mobility group box 1 release and inflammatory cytokines production, whereas chloroquine worsened liver injury via inhibition of autophagy and induction of hepatic apoptosis at the late phase. In conclusion, chloroquine prevents ischemic liver damage at the early phase, but aggravates liver damage at the late phase in liver I/R injury. This dual role of chloroquine should be considered when using chloroquine as an inhibitor of inflammation or autophagy in I/R injury.     

Confocal pH Topography in Plant Cells — Acidic Layers in the Peripheral Cytoplasm and the Apoplast

The distribution of intracellular pH was studied in cultured cells of Gossypium hirsutum by con-focal pH topography using the fluorescent probe car-boxy SNARF1 and a ratio imaging procedure. The resulting pH maps can visualize pH differences with an accuracy of 0.1 unit in the investigated range between 7.5 and 5.6. They reveal the following characteristic features of the Gossypium cells: – the pH of the cytoplasmic core regions ranges from near 7.4 in younger to near 6.0 in older cells; – vacuoles show the expected acidity with pH < 5.6; – the cell wall/apoplastic region is acidic with a pH near 5.6 or below, especially in young, growing cells; – interestingly, acidic areas appear also at the periphery of the cytoplasm, i.e. beneath the plasma membrane. They remain stable in the presence of 5/μmol/I of the protonophore CCCP. Acidic layers of peripheral cytoplasm were also detected in protoplasts of Penicillium cyclopium, i.e. eukaryotic cells of simpler structure, which served as a reference object. This ronfirms earlier findings obtained with classical fluorescence microscopy and another fluoroprobe (fluorescein diacetate). Though additional experimental support is needed, low pH regions at the cytoplasm/plasma membrane interface should be considered a real contribution to the pH control of plant and fungal cells, facilitating e.g. the maintenance of cytosolic pH in acidic environments.