Abstract
BackgroundDrug screening for the identification of compounds with anticancer activity is commonly performed using cell lines cultured under normal oxygen pressure and physiological pH. However, solid tumors are characterized by a microenvironment with limited access to nutrients, reduced oxygen supply and acidosis. Tumor hypoxia and acidosis have been identified as important drivers of malignant progression and contribute to multicellular resistance to different forms of therapy. Tumor acidosis represents an important mechanism mediating drug resistance thus the identification of drugs active on acid-adapted cells may improve the efficacy of cancer therapy.MethodsHere, we characterized human colon carcinoma cells (HCT116) chronically adapted to grow at pH 6.8 and used them to screen the Prestwick drug library for cytotoxic compounds. Analysis of gene expression profiles in parental and low pH-adapted cells showed several differences relating to cell cycle, metabolism and autophagy.ResultsThe screen led to the identification of several compounds which were further selected for their preferential cytotoxicity towards acid-adapted cells. Amongst 11 confirmed hits, we primarily focused our investigation on the benzoporphyrin derivative Verteporfin (VP). VP significantly reduced viability in low pH-adapted HCT116 cells as compared to parental HCT116 cells and normal immortalized epithelial cells. The cytotoxic activity of VP was enhanced by light activation and acidic pH culture conditions, likely via increased acid-dependent drug uptake. VP displayed the unique property to cause light-dependent cross-linking of proteins and resulted in accumulation of polyubiquitinated proteins without inducing inhibition of the proteasome.ConclusionsOur study provides an example and a tool to identify anticancer drugs targeting acid-adapted cancer cells.
Highlights
Drug screening for the identification of compounds with anticancer activity is commonly performed using cell lines cultured under normal oxygen pressure and physiological pH
In an attempt to overcome acidosis-mediated drug resistance, we developed a drug screening assay to target the colon cancer HCT116 cell line chronically adapted to acidosis
Acid‐adapted HCT116 cells are resistant towards many clinically used drugs It has been previously shown that cancer cells are less sensitive and/or completely resistant to several clinically used drugs in conditions of acidosis, especially when the compounds are basic [8, 31,32,33]
Summary
Drug screening for the identification of compounds with anticancer activity is commonly performed using cell lines cultured under normal oxygen pressure and physiological pH. Tumor acidosis represents an important mechanism mediating drug resistance the identification of drugs active on acid-adapted cells may improve the efficacy of cancer therapy. Tumor cells within a tissue are constantly under the selective pressure of the surrounding environment, characterized by heterogeneity in terms of cellular components, availability of nutrients, fluctuating oxygen levels, poor tissue perfusion and low pH [1, 2]. Such a complex environment is poorly reproduced by standard in vitro culture conditions, which provide excessive levels of Pellegrini et al Cancer Cell Int (2018) 18:147 better reproduces the oxygen, nutrients and pH gradients observed in human tumor tissues [5, 6]. We discovered several aspects of the Verteporfin mechanism of action that raise several cautions with regards to interpretation of experimental results with this drug
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