In Vivo Electron Paramagnetic Resonance Molecular Profiling of Tumor Microenvironment upon Tumor Progression to Malignancy in an Animal Model of Breast Cancer

PURPOSE

Hypoxia and acidosis are recognized tumor microenvironment (TME) biomarkers of cancer progression. Alterations in cancer redox status and metabolism are also associated with elevated levels of intracellular glutathione (GSH) and interstitial inorganic phosphate (Pi). This study aims to evaluate the capability of these biomarkers to discriminate between stages and inform on a switch to malignancy.

PROCEDURES

These studies were performed using MMTV-PyMT( +) female transgenic mice that spontaneously develop breast cancer and emulate human tumor staging. In vivo assessment of oxygen concentration (pO2), extracellular acidity (pHe), Pi, and GSH was performed using L-band electron paramagnetic resonance spectroscopy and multifunctional trityl and GSH-sensitive nitroxide probes.

RESULTS

Profiling of the TME showed significant deviation of measured biomarkers upon tumor progression from pre-malignancy (pre-S4) to the malignant stage (S4). For the combined marker, HOP: (pHe × pO2)/Pi, a value > 186 indicated that the tumors were pre-malignant in 85% of the mammary glands analyzed, and when < 186, they were malignant 42% of the time. For GSH, a value < 3 mM indicated that the tumors were pre-malignant 74% of the time, and when > 3 mM, they were malignant 80% of the time. The only marker that markedly deviated as early as stage 1 (S1) from its value in pre-S1 was elevated Pi, followed by a decrease of pHe and pO2 and increase in GSH at later stages.

CONCLUSION

Molecular TME profiling informs on alteration of tumor redox and metabolism during tumor staging. Early elevation of interstitial Pi at S1 may reflect tumor metabolic alterations that demand elevated phosphorus supply in accordance with the high rate growth hypothesis. These metabolic changes are supported by the following decrease of pHe due to a high tumor reliance on glycolysis and increase of intracellular GSH, a major intracellular redox buffer. The appreciable decrease in TME pO2 was observed only at malignant S4, apparently as a consequence of tumor mass growth and corresponding decrease in perfusion efficacy and increase in oxygen consumption as the tumor cells proliferate.

Roles of intracellular and extracellular ROS formation in apoptosis induced by cold atmospheric helium plasma and X-irradiation in the presence of sulfasalazine

Sulfasalazine (SSZ) is a well-known anti-inflammatory drug and also an inhibitor of the cystine-glutamate antiporter that is known to reduce intracellular glutathione (GSH) level and increase cellular oxidative stress, indicating its anti-tumor potential. However, the combination of SSZ with other physical modalities remains unexplored. Here, the effects of SSZ on cold atmospheric helium plasma (He-CAP), which produces approximately 24 x higher concentration of hydroxyl radicals (. OH) compared to X-irradiation (IR) in aqueous solution, and on IR-induced apoptosis in human leukemia Molt-4 cells were studied to elucidate the mechanism of apoptosis enhancement. Both the Annexin V-FITC/PI and DNA fragmentation assay revealed that pre-treatment of cells with SSZ significantly enhanced He-CAP and IR-induced apoptosis. Similar enhancement was observed during the loss of mitochondrial membrane potential, intracellular Ca²⁺ ions, and mitochondria- and endoplasmic reticulum-related proteins. The concentration of intracellular reactive oxygen species (ROS) was much higher in He-CAP treated cells than in X-irradiated cells. On the other hand, strong enhancement of Fas expression and caspase-8 and -3 activities were only observed in X-irradiated cells. It might be possible that the higher concentration of intracellular and extracellular ROS suppressed caspase activities and Fas expression in He-CAP-treated cells. Notably, pretreating the cells with an antioxidant N-acetyl-L-cysteine (NAC) dramatically decreased apoptosis in cells treated by He-CAP, but not by IR. These results suggest that IR-induced apoptosis is due to specific and effective ROS distribution since intracellular ROS formation is marginal and the high production of ROS inside and outside of cells plays unique roles in He-CAP induced apoptosis. We conclude that our data provides efficacy and mechanistic insights for SSZ, which might be helpful for establishing SSZ as a future sensitizer in He-CAP or IR therapy for cancer.

N-acetylcysteine ethyl ester as GSH enhancer in human primary endothelial cells: A comparative study with other drugs

Several drugs are currently in use as glutathione (GSH) enhancers in clinical, pre-clinical and experimental research. Here we compare the ability of N-acetylcysteine (NAC), 2-oxothiazolidine-4-carboxylic acid (OTC), glutathione ethyl ester (GSH-EE) and N-acetylcysteine ethyl ester (NACET) to increase the intracellular concentration of GSH using primary human umbilical vein endothelial cells (HUVEC) as in vitro model. Our experiments highlighted that NACET is largely the most efficient molecule in increasing the intracellular levels of GSH, cysteine, and γ-glutamylcysteine. This is because NACET is lipophilic and can freely cross plasma membrane but, inside the cell, it is de-esterified to the more hydrophilic NAC, which, in turn, is trapped into the cell and slowly transformed into cysteine. The higher availability of cysteine is matched by an increase in GSH synthesis, cysteine availability being the rate limiting step for this reaction. Surprisingly, the increase in GSH concentration was not linear but peaked at 0.5 mM NACET and gradually decreased when cells were treated with higher concentrations of NACET. We demonstrated that this puzzling ceiling effect was due to the fact that NAC released from NACET turned out to be a competitive inhibitor of the enzyme glutamate-cysteine ligase, with a Ki value of 3.2 mM. By using a cell culture medium lacking of cysteine and methionine, we could demonstrate that the slight increase in intracellular levels of cysteine and GSH induced by NAC in HUVEC grown in standard medium was due to the reduction of the cystine present in the medium itself there rather than to the action of NAC as Cys pro-drug. This fact may explain why NAC works well as GSH enhancer at very high concentrations in pre-clinical and in vitro studies, whereas it failed in most clinical trials.