Increasing Superoxide Production and the Labile Iron Pool in Tumor Cells may Sensitize Them to Extracellular Ascorbate

Abstract

Low millimolar concentrations of ascorbate are capable of inflicting lethal damage on a high proportion of cancer cells lines, yet leave non-transformed cell lines unscathed. Extracellular generation of hydrogen peroxide, reflecting reduction of molecular oxygen by ascorbate, has been shown to mediate this effect. Although some cancer cell lines express low catalase activity, this cannot fully explain the selective sensitivity of cancer cells to hydrogen peroxide. Ranzato and colleagues have presented evidence for a plausible new explanation of this sensitivity – a high proportion of cancers, via NADPH oxidase complexes or dysfunctional mitochondria, produce elevated amounts of superoxide. This superoxide, via a transition metal-catalyzed transfer of an electron to the hydrogen peroxide produced by ascorbate, can generate deadly hydroxyl radical (Haber–Weiss reaction). It thus can be predicted that concurrent measures which somewhat selectively boost superoxide production in cancers will enhance their sensitivity to i.v. ascorbate therapy. One way to achieve this is to increase the provision of substrate to cancer mitochondria. Measures which inhibit the constitutive hypoxia-inducible factor-1 (HIF-1) activity in cancers (such as salsalate and mTORC1 inhibitors, or an improvement of tumor oxygenation), or that inhibit the HIF-1-inducible pyruvate dehydrogenase kinase (such as dichloroacetate), can be expected to increase pyruvate oxidation. A ketogenic diet should provide more lipid substrate for tumor mitochondria. The cancer-killing activity of 42°C hyperthermia is to some degree contingent on an increase in oxidative stress, likely of mitochondrial origin; reports that hydrogen peroxide synergizes with hyperthermia in killing cancer cells suggest that hyperthermia and i.v. ascorbate could potentiate each other’s efficacy. A concurrent enhancement of tumor oxygenation might improve results by decreasing HIF-1 activity while increasing the interaction of ascorbic acid with oxygen. An increased pool of labile iron in cancer cells may contribute to the selective susceptibility of many cancers to i.v. ascorbate; antagonism of NF-kappaB activity with salicylate, and intravenous iron administration, could be employed to further elevate free iron in cancers.

Molecular mechanisms of pharmacological doses of ascorbate on cancer cells

“…In human A375 melanoma cells treatment with ascorbate induces a cell cycle arrest and apoptosis [50]. Similar antiproliferative effects of ascorbate were observed in various mesenchymal cancer cells, such as chronic myeloid leukemia cells [51,52], adult T-cell lymphoma cells [53] and in adequate tissue oxygen levels necessary for ascorbatedriven ROS generation [20]. and concluded that ascorbate substitution in physiological doses should be considered for late-stage melanoma patients [59].…”

“…This provides a mechanistic basis to apply pro-oxidative therapeutic strategies (such as high-dose ascorbate) to kill cancer cells using ROS-mediated mechanisms [18,19]. Thus, it seems plausible to assume that increasing ROS (superoxide) production and the labile iron pool in cancer cells may sensitize them to ascorbate [20]; however, extracellular iron at physiological conditions may also diminish the anticancer effects of ascorbate [21]. In line with this assumption, Levine and his group demonstrated in in vitro studies on 43 different cancer cell lines and five nontransformed somatic cell populations a preferential toxicity of ascorbate toward the cancer cells: an average IC 50 of < 10 mM ascorbate was detected in the cancer cells at incubation times of < 2 h, while the benign cells tolerated much higher doses of ascorbate [7].…”

Ascorbic acid in the prevention and treatment of cancer

“…34 At low millimolar concentrations, ascorbic acid is able to "kill" some cell lines in vitro, while in vivo it generates superoxide radicals, hydrogen peroxide, and extracellular ascorbyl responsible for its cytotoxic activity; however, concentrations as high as 20 mM did not pose any risk to the lineage of non-malignant cells. 19 Other studies confirm that high doses of ascorbic acid are effective in cell death as seen in in vitro studies as well as in vivo tumor growth inhibition. 20 Corroborating this study, researchers describe that vitamin C can be toxic in a selective manner in some types of tumor cells as a pro-oxidant, since concentrations above physiological (0.1 mM), between 1 mM and 10 mM, are toxic for neoplastic cells in vitro, for example, for melanoma and neuroblastoma cells, where concentrations from 10 nM to 1 mM can induce apoptosis.…”

Biphasic effects of l-ascorbate on the tumoricidal activity of non-thermal plasma against malignant mesothelioma cells

“…Previous studies have described the chemical mechanisms involved in the pro-oxidant activities of L-ascorbate [41] [42]. These reactions include: (1) autoxidation of L-ascorbate in the absence of catalytic metals, leading to O 2 À formation (L-ascorbate 2À þ O 2 / L- [41]; and (2) in the presence of a catalytic metal ion, L-ascorbate is an excellent one-electron reducing agent that can reduce Fe(III) to Fe(II), while being oxidized to ascorbate radical [42].…”

“…These reactions include: (1) autoxidation of L-ascorbate in the absence of catalytic metals, leading to O 2 À formation (L-ascorbate 2À þ O 2 / L- [41]; and (2) in the presence of a catalytic metal ion, L-ascorbate is an excellent one-electron reducing agent that can reduce Fe(III) to Fe(II), while being oxidized to ascorbate radical [42]. Subsequently, Fe(II) can reduce O 2 to O 2 À , which then dismutates to H 2 O 2 and O 2 , and this is followed by the Fenton reaction (Fe(II) þ H 2 O 2 / Fe(III) þ $ OH þ OH À ) that generates hydroxyl radicals [42].…”