Mechanistic Insights into Pharmacological Ascorbate Toxicity in Pancreatic Cancer Cells

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is a devastating cancer with a 5-year survival rate of only 6%. Conventional therapy increases survival by a few months at the expense of toxic side effects and undue financial burden on patients due to its high cost. Emerging as an inexpensive and safe cancer therapy, pharmacologic ascorbate, or vitamin C, has already been shown in both preclinical and clinical studies to have a robust anti-tumor effect. However, to fully uncover the therapeutic potential of ascorbate, more about the mechanism underlying cancer-cell selective ascorbate toxicity must be understood. One study found that dehydroascorbate (DHA), the oxidized form of ascorbate, generates intracellular reactive oxygen species (ROS) and deemed DHA to be the active agent of toxicity due to the upregulation of its transporter GLUT1 in many glycolytically addicted cancers. Another study reported that elevated levels of free iron in cancers reacts with ROS generated by ascorbate oxidation and thereby predispose them to DNA-damaging Fenton chemistry. To expand upon these models, liquid-chromatography mass spectrometry was used to measure oxidative stress in the PDAC cell line PANC-1 in response to ascorbate and DHA treatment with GLUT inhibition and iron chelation. Found to be more potent and fast acting than ascorbate, DHA was validated as the active agent of toxicity; blocking ascorbate oxidation to DHA using extracellular glutathione also prevented oxidative stress. Surprisingly, multiple strategies of GLUT inhibition failed to protect cells from oxidative stress induced by either ascorbate or DHA. Iron chelation, on the other hand, protected cells from oxidative stress induced by ascorbate but not DHA. These findings mediate the existing models of ascorbate toxicity by suggesting that even though DHA may be the active toxic agent, altered iron metabolism may render cancers more susceptible to ascorbate treatment than glycolytic addiction. Further investigation into the relationship between iron metabolism and ascorbate toxicity is well-warranted and could yield important findings that increase the therapeutic potential of ascorbate.