Selective ascorbate toxicity in malignant mesothelioma: a redox Trojan mechanism

Our plain-English summary of Selective ascorbate toxicity in malignant mesothelioma: a redox Trojan mechanism, by E. Ranzato, S. Biffo, B. Burlando (American Journal of Respiratory Cell and Molecular Biology, 2011).

The study

• Authors: E. Ranzato, S. Biffo, B. Burlando
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 2011;44(1):108-17
• Cited by: 35 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

We studied the mechanism of ascorbate toxicity in malignant mesothelioma (MMe) cells. Neutral red uptake showed that ascorbate, but not dehydroascorbate, was highly toxic in the MMe cell lines REN and MM98, and less toxic in immortalized (human mesothelial cells-htert) and primary mesothelial cells. Ascorbate transport inhibitors phloretin, sodium azide, and ouabain did not reduce ascorbate toxicity. Ascorbate promoted the formation of H(2)O(2) in the cell medium, and its toxicity was suppressed by extracellular catalase, but the concentration of endogenous catalase was higher in MMe cells than in normal cells. The confocal imaging of cells loaded with the dihydrhodamine 123 reactive oxygen species probe showed that ascorbate caused a strong increase of rhodamine fluorescence in MMe cells, but not in mesothelial cells. MMe cells showed a higher production of superoxide and NADPH oxidase (NOX)4 expression than did mesothelial cells. Two inhibitors of cellular superoxide sources (apocynin and rotenone) reduced ascorbate toxicity and the ascorbate-induced rise in rhodamine fluorescence. NOX4 small interfering RNA also reduced ascorbate toxicity in REN cells. Taken together, the data indicate that ascorbate-induced extracellular H(2)O(2) production induces a strong oxidative stress in MMe cells because of their high rate of superoxide production. This explains the selective toxicity of ascorbate in MMe cells, and suggests its possible use in the clinical treatment of malignant mesothelioma.

In plain English

This laboratory study used cells grown in dishes; no patients, animals or asbestos exposures were studied. The authors tested ascorbate (vitamin C) on two malignant mesothelioma cell lines, REN and MM98, and on immortalized (HMC-htert) and primary mesothelial cells. After 1 hour of exposure and 23 hours of recovery, the concentration that cut cell viability by half (EC50) was 0.81 mM (95% confidence interval 0.75 to 0.89) for REN and 0.6 mM (0.4 to 0.7) for MM98. For HMC-htert it was 32 mM (26 to 38) in DMEM and 22 mM (16 to 30) in M199; for primary mesothelial cells, 10 mM (9.4 to 10.8) in DMEM and 9.0 mM (7.6 to 10.7) in F10-Ham. Dehydroascorbate showed almost no toxicity in REN cells. Catalase, which breaks down hydrogen peroxide, raised the EC50 more than 10-fold in both tumor lines. Mesothelioma cells produced more superoxide and NOX4 than mesothelial cells. Blocking superoxide sources or NOX4 reduced ascorbate toxicity in REN cells. The authors concluded this mechanism gives a rationale for using ascorbate to treat mesothelioma.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: in vitro laboratory study of cultured human cell lines (cell viability, enzyme, and imaging assays)
• Who was studied: Cultured cells: two malignant mesothelioma cell lines (REN, a p53 mutant epithelial subtype; MM98, from the pleural effusion of a sarcomatous mesothelioma), an immortalized human mesothelial cell line (HMC-htert), and primary mesothelial cells taken from surgically removed hernial sacs; Italy
• Exposure: Ascorbate (vitamin C) applied to cells at a range of concentrations, either for 24 hours or for 1 hour followed by 23 hours of recovery; dehydroascorbate was also tested. This was not an asbestos exposure study.
• Main results:
  • After 24-hour exposure, ascorbate EC50 was 1.2 mM (95% CI 1.1 to 1.3) in REN, 1.0 (0.8 to 1.1) in MM98, 9.1 (8.8 to 9.3) in HMC-htert, and 4.3 (2.5 to 7.6) in primary mesothelial cells
  • After 1 hour of exposure plus 23 hours of recovery, EC50 was 0.81 mM (95% CI 0.75 to 0.89) in REN and 0.6 (0.4 to 0.7) in MM98, both in DMEM. For HMC-htert it was 32 (26 to 38) in DMEM and 22 (16 to 30) in M199. For primary mesothelial cells it was 10 (9.4 to 10.8) in DMEM and 9.0 (7.6 to 10.7) in F10-Ham.
  • Dehydroascorbate showed almost no toxicity in REN cells (EC50 33 mM, 95% CI 31 to 36)
  • Ascorbate transport inhibitors did not substantially change toxicity. The authors stated that toxicity does not depend, or only minimally depends, on cell uptake of ascorbate.
  • Catalase reduced ascorbate toxicity in REN and MM98 cells, with a more than 10-fold increase in EC50
  • Primary mesothelial cells had lower catalase activity than the mesothelioma cells yet were less sensitive to ascorbate, which the authors called an apparent paradox
  • Ascorbate at 1 or 5 mM produced almost no rise in the reactive oxygen species signal in mesothelial cells, while REN cells showed a rapid increase
  • Mesothelioma cells produced significantly more superoxide than non-neoplastic cells
  • NOX4 expression was markedly higher in REN and MM98 cells than in mesothelial cells
  • In REN cells, superoxide-source inhibitors raised ascorbate EC50 from 0.81 mM to 18 (13 to 26) with apocynin and to 15 (12 to 18) with rotenone
  • NOX4 knockdown by siRNA progressively shifted the ascorbate EC50 higher in REN cells compared with scramble siRNA
• The authors' conclusion: The authors concluded that ascorbate's selective toxicity to mesothelioma cells is explained by a 'redox Trojan mechanism': extracellular hydrogen peroxide combined with high superoxide production in the tumor cells. They stated that this provides a rationale for using ascorbate in mesothelioma treatment.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Malignant Mesothelioma, Mesothelial Cells, Reactive Oxygen Species (ROS), Superoxide, Oxidative Stress.

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