The acidic tumor pH neutralizes the autophagy inhibiting activity of chloroquine: Implications for cancer therapies
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1
Karolinska Institute, Department of Oncology-Pathology, Sweden
Background and aim. Tumor acidosis represents an important mechanism involved in malignant progression and resistance to therapy (1). It has been reported that human tumor cells activate autophagy as a protective and adaptive response to acidic stress, suggesting that autophagy inhibition may represent a therapeutic strategy to selectively kill cells adapted to low pH conditions (2). Inhibition of autophagy by administration of Chloroquine (CQ) in combination with anticancer therapies is currently evaluated in several clinical trials (3). Despite single-agent CQ treatment has shown antitumor effects in some models (4), CQ was also reported to have no effect or even slightly increase tumor growth (5). We evaluated the ability of CQ and Lys-01, a novel CQ derivative, to block autophagy under acidic conditions.
Methods. Different cell lines (HCT116, Me30966, Hos) were treated with acidic media for 24 hours (pH 6.8 and pH 6.5) and autophagy inhibitors (BafA1, CQ, HCQ and Lys-01). Autophagy was detected by measuring LC3-II levels by Western Blot. Cell viability was evaluated by the acid phosphatase assay. CQ and Lys-01 intracellular content was analyzed by HPLC. Autophagy inhibition and viability assays were also performed cell lines adapted to low pH.
Results. Acute acidic stress inhibited the cytotoxic and autophagy-blocking activity of CQ in melanoma, colon carcinoma and osteosarcoma cells. However, autophagy-inhibiting activity of Lys-01 is still detectable at pH 6.8. While low pH-adapted cells were completely resistant to CQ cytotoxic effect, these cells showed a lower viability after treatment with Lys-01. HPLC analysis showed that at low pH conditions CQ but not Lys-01 entry in to the cells was dramatically reduced. Using cell lines stably adapted to chronic acidosis we could confirm that CQ lack of activity was merely caused by acidic pH because the buffering of the medium pH restored CQ activity as cytotoxic agent and autophagy blocker.
Conclusions / Discussion. We observed in several in human cancer cell lines cultured at acidic pH that autophagy is not blocked by CQ, consistent with lack of sensitivity to CQ toxicity. Acidification of tumor pH is known to reduce entry and activity of many weakly basic chemotherapeutics including CQ, which may represent an obstacle to obtain proper antiautophagic activity in vivo. These 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 CQ efficacy.
Acknowledgements
This work was supported by grants from the Association for International Cancer Research and the Swedish Cancer Society
References
1. Gerweck LE Semin Radiat Oncol. 1998
2. Marino M, et al. Cell Death Dis 2010
3. Amaravadi RK, et al. Clin Cancer Res 2011
4. Amaravadi RK, et al J Clin Invest 2007
5. Bristol ML,et al. J Pharmacol Exp Ther 2013
Keywords:
acidic ph,
Chloroquine,
Autophagy,
Cell Survival,
chemoresistance,
cancer therapies
Conference:
4th Annual Meeting of the International Society of Proton Dynamics in Cancer, Garching, Germany, 10 Oct - 12 Oct, 2013.
Presentation Type:
Abstract
Topic:
5. pH control of cell survival
Citation:
Pellegrini
P,
Strambi
A,
Zipoli
C,
Linder
S and
De Milito
A
(2014). The acidic tumor pH neutralizes the autophagy inhibiting activity of chloroquine: Implications for cancer therapies.
Front. Pharmacol.
Conference Abstract:
4th Annual Meeting of the International Society of Proton Dynamics in Cancer.
doi: 10.3389/conf.fphar.2014.61.00036
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Received:
16 Jan 2014;
Published Online:
07 Feb 2014.
*
Correspondence:
Miss. Paola Pellegrini, Karolinska Institute, Department of Oncology-Pathology, Stockholm, Sweden, paola.pellegrini@ki.se