Marie E. Varnes

The role of thiols in cellular response to radiation and drugs.

Cellular nonprotein thiols (NPSH) consist of glutathione (GSH) and other low molecular weight species such as cysteine, cysteamine, and coenzyme A. GSH is usually less than the total cellular NPSH, and with thiol reactive agents, such as diethyl maleate (DEM), its rate of depletion is in part dependent upon the cellular capacity for its resynthesis. If resynthesis is blocked by buthionine-S,R-sulfoximine(BSO), the NPSH, including GSH, is depleted more rapidly, Cellular thiol depletion by diamide, N-ethylmaleimide, and BSO may render oxygenated cells more sensitive to radiation. These cells may or may not show a reduction in the oxygen enhancement ratio (OER). Human A549 lung carcinoma cells depleted of their NPSH either by prolonged culture or by BSO treatment do not show a reduced OER but do show increased aerobic responses to radiation. Some nitroheterocyclic radiosensitizing drugs also deplete cellular thiols under aerobic conditions. Such reactivity may be the reason that they show anomalous radiation sensitization (i.e., better than predicted on the basis of electron affinity). Other nitrocompounds, such as misonidazole, are activated under hypoxic conditions to radical intermediates. When cellular thiols are depleted peroxide is formed. Under hypoxic conditions thiols are depleted because metabolically reduced intermediates react with GSH instead of oxygen. Thiol depletion, under hypoxic conditions, may be the reason that misonidazole and other nitrocompounds show an extra enhancement ratio with hypoxic cells. Thiol depletion by DEM or BSO alters the radiation response of hypoxic cells to misonidazole. In conclusion, we propose an altered thiol model which includes a mechanism for thiol involvement in the aerobic radiation response of cells. This mechanism involves both thiol-linked hydrogen donation to oxygen radical adducts to produce hydroperoxides followed by a GSH peroxidase-catalyzed reduction of the hydroperoxides to intermediates entering into metabolic pathways to produce the original molecule.

Biochemistry of reduction of nitro heterocycles.

Misonidazole is a metabolically active drug. Its addition to cells causes an immediate alteration in cellular electron transfer pathways. Under aerobic conditions the metabolic alterations can result in futile cycling with electron transfer to oxygen and production of peroxide. Thiol levels are extremely important in protecting the cell against the peroxide formation and potentially hazardous conditions for hydroxyl radical production. Nevertheless such electron shunting out of cellular metabolism will result in alterations in pentose cycle, glycolysis and cellular capacity to reduce metabolites to essential intermediates needed in DNA metabolism (i.e. deoxyribonucleotides). Glutathione must be depleted to very low levels before toxic effects of misonidazole and other nitro compounds are manifested in cell death via peroxidative damage. Under hypoxic conditions misonidazole also diverts the pentose cycle via its own reduction; however, unlike the aerobic conditions, there are a number of reductive intermediates produced that react with non-protein thiols such as GSH as well as protein thiols. The reaction with protein thiols results in the inhibition of glycolysis and other as yet undetermined enzyme systems. The consequences of the hypoxic pretreatment of cells with nitro compounds are increased vulnerability to radiation and chemotherapeutic drugs such as L-PAM, cis-platinum and bleomycin. The role that altered enzyme activity has in the cellular response to misonidazole and chemotherapeutic agents remains to be determined. It is also clear that the GSH depleted state not only makes cells more vulnerable to oxidative stress but also to hypoxic intermediates produced by the reduction of misonidazole beyond the one electron stage. The relevancy of the present work to the proposed use of thiol depletion in vivo to enhance the radiation or chemotherapeutic response of tumor tissue lies with the following considerations. Apparently, spontaneous peroxidative damage to normal tissue such as liver can occur with GSH depletion to 10-20% of control and with other normal tissue when GSH reaches 50% of control. This situation can obviously become more critical if peroxide producing drugs are administered. The only advantage to such combined drug treatments would lie in the possibility that tumors vary in their catalase and peroxidase activity and consequently may be more vulnerable to oxidative stress (cf. review by Meister. Our tumor model, the A549 human lung carcinoma cell in vitro, appears to be an exception because it has catalase, peroxidase and a high content of GSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonprotein Thiols and the Radiation Response of A549 Human Lung Carcinoma Cells

Glutathione (GSH)-depletion by buthionine sulphoximine (BSO) altered both the aerobic and anaerobic radiation response of A549 human lung cancer cells grown in vitro. The oxygen enhancement ratio (o.e.r) was increased slightly from 3.0-3.3. The lack of an effect of GSH-depletion on o.e.r. reduction, provides a system whereby the mechanism of action of the thiol reactive reagent diethylmaleate (DEM) can be investigated. Pretreatment of cells with DEM, under non-toxic concentrations, removed 13 per cent of the intracellular NPSH and resulted in an o.e.r. of 2. When BSO followed by DEM was used, so that both GSH and NPSH were reduced to zero, an o.e.r. of 1.5 was obtained. Cells treated with 1 mM BSO for 24 hours contained 10 per cent NPSH and no GSH. When these cells were exposed to 0.5 or 1 mM DEM briefly, during irradiation, the o.e.r. was 2.4 and 1.7 respectively. In some cases altered o.e.r.s occurred in combination with increased aerobic responses. This was especially true for aerobic irradiations of BSO-treated cells in the presence or absence of DEM. However, the increased aerobic response was offset by a more dramatic increase in the hypoxic response. These results indicate (a) that GSH plays a significant role in aerobic radiation response but is not a principal factor in o.e.r.-reduction, and (b) that reduction of the o.e.r. by DEM is not due primarily to GSH-removal. The preferential radiosensitization of hypoxic cells by DEM may involve reactions of this compound with NPSH or protein SH, or may be related to the ability of DEM to mimic oxygen as a hypoxic cell radiosensitizer.

Factors Influencing the Oxidation of Cysteamine and Other Thiols: Implications for Hyperthermic Sensitization and Radiation Protection

Some of the factors influencing the oxygen uptake and peroxide formation for cysteamine (MEA) and other thiols in serum-supplemented modified McCoys 5A, a well-known medium used to cultivate a variety of cells in vitro, have been studied. The oxidation of MEA and cysteine in modified McCoys 5A has been compared with that in Hams F-12, MEM, and phosphate-buffered saline. All of the growth media were supplemented with 10% calf serum and 5% fetal calf serum. The rate of oxygen uptake for all of the studied thiols was greatest in McCoys 5A. The data indicate that this medium may contain more copper than the other preparations. MEA and cysteine were found to be more effective at 0.4 mM at producing peroxide than dithiothreitol (DTT). N-acetylcysteine was the least reactive. The ability to produce peroxide is dependent upon the temperature, the concentration of thiol, the presence of copper ions, and pH of the medium. MEA and other thiol oxidation is inhibited by the copper chelator diethyldithiocarbamate. Catalase also reduces the oxygen uptake for all thiols. This inhibition involves the recycling of peroxide to oxygen. Superoxide dismutase (SOD) was found to stimulate the oxygen uptake in the case of MEA and cysteine, but had little or no effect with DTT and glutathione. The combined presence of SOD and catalase resulted in less inhibition of oxygen uptake than that obtained by catalase alone. Alkaline pH was found to enhance the oxidation of cysteine and MEA. An important observation was the inhibition of MEA oxidation at 0 degrees C and the stimulation at 42 degrees C. The results indicate that many problems may arise when thiols are added to various media. A major consideration is concerned with the production of peroxide, superoxide, and reduced trace metal intermediates. The presence of these intermediates may result in the production of hydroxyl radical intermediates as well as the eventual oxygen depletion from the medium. Oxygen depletion may alter the results of radiation sterilization and carcinogen activation. Radical production will cause cell damage that is temperature dependent. Therefore, careful consideration must be given to changes in oxygen tension when thiols are added to cells growing in complicated growth medium to protect against either chemical or radiation damage.

Sensitivity to chemical oxidants and radiation in CHO cell lines deficient in oxidative pentose cycle activity

In this paper we examine the susceptibility of a series of G6PD- CHO cell lines to a variety of chemical oxidants. Addition of these drugs to K1D, the parental cell line, results in as much as a 20-fold increase in pentose cycle (PC) activity over control values. In two of our mutant lines, E16 and E48, little or no stimulation of PC activity is seen. These lines are shown to be much more susceptible to the toxic effects of the chemical oxidants t-butyl hydroperoxide and diamide. PC activity is also stimulated by ionizing radiation in K1D cells. One of the G6PD- cell lines has an increased aerobic radiation response compared to the parental line. However, since this is not the case with the other G6PD- cell lines, it is unclear whether this represents a difference in the absolute value of PC activity or some additional variable that may be influencing the results.

Non-protein thiols and cellular response to drugs and radiation

Abstract Our results have enabled us to make a number of observations concerning the reactivity of nitrocompounds and non-nitrocompounds with glutathione (GSH) and cellular nonprotein thiols (NPSH), which may be as much as 90 % GSH. We have summarized our observations as follows with respect to the different types of reactions responsible for part or all of the cellular NPSH depletion by hypoxic cell radiosensitizing drugs. (A) Some nitrocompounds, such as 4-nitroimidazoles containing a 5-sulfonamide group, react spontaneously with GSH; (B) A number of hypoxic cell radiosensitizing drugs such as chlorodinitrobenzene (CDNB), dimethylfumarate (DMF) and diethylmaleate (DEM) are substrates for the enzyme glutathione-S-transferase and form covalent bonds with GSH; (C) NPSH may be oxidized by diamide; (D) form covalent bonds with N-ethylmaleimide; (E) or be converted by thiol-reactive drug intermediates formed under anaerobic conditions. The latter reaction occurs with misonidazole, Ro-05–9963, SR 2508 and SR 255:5; its mechanism is still unknown. It is obvious from the above that there are a variety of means by which radiosensitiizing drugs can alter cellular metabolism as reflected by changes in the NPSH. It remains to be determined whether a relationship exists between altered NPSH, metabolism and the radiosensitizing capacity of nitrocompounds when used alone or in combination with other drugs. Our studies strongly suggest that potential new sensitizers be routinely examined for their capacity to react spontaneously with GSH or to remove cellular NPSH under aerobic as well as anaerobic conditions. This is especially true for radiosensitizing drugs showing anomalous behavior, i.e., better sensitization than predicted by their one-electron reduction potentials. Such screening would pay dividends insofar as drugs that are too reactive could be excluded from further in vivo study.

Catalase Abrogates β-Lapachone–Induced PARP1 Hyperactivation–Directed Programmed Necrosis in NQO1-Positive Breast Cancers

Improving patient outcome by personalized therapy involves a thorough understanding of an agents mechanism of action. β-Lapachone (clinical forms, Arq501/Arq761) has been developed to exploit dramatic cancer-specific elevations in the phase II detoxifying enzyme NAD(P)H:quinone oxidoreductase (NQO1). NQO1 is dramatically elevated in solid cancers, including primary and metastatic [e.g., triple-negative (ER−, PR−, Her2/Neu−)] breast cancers. To define cellular factors that influence the efficacy of β-lapachone using knowledge of its mechanism of action, we confirmed that NQO1 was required for lethality and mediated a futile redox cycle where ∼120 moles of superoxide were formed per mole of β-lapachone in 2 minutes. β-Lapachone induced reactive oxygen species (ROS), stimulated DNA single-strand break-dependent poly(ADP-ribose) polymerase-1 (PARP1) hyperactivation, caused dramatic loss of essential nucleotides (NAD+/ATP), and elicited programmed necrosis in breast cancer cells. Although PARP1 hyperactivation and NQO1 expression were major determinants of β-lapachone–induced lethality, alterations in catalase expression, including treatment with exogenous enzyme, caused marked cytoprotection. Thus, catalase is an important resistance factor and highlights H2O2 as an obligate ROS for cell death from this agent. Exogenous superoxide dismutase enhanced catalase-induced cytoprotection. β-Lapachone–induced cell death included apoptosis-inducing factor (AIF) translocation from mitochondria to nuclei, TUNEL+ staining, atypical PARP1 cleavage, and glyceraldehyde 3-phosphate dehydrogenase S-nitrosylation, which were abrogated by catalase. We predict that the ratio of NQO1:catalase activities in breast cancer versus associated normal tissue are likely to be the major determinants affecting the therapeutic window of β-lapachone and other NQO1 bioactivatable drugs. Mol Cancer Ther; 12(10); 2110–20. ©2013 AACR.

The effect of L-buthionine sulfoximine on the aerobic radiation response of A549 human lung carcinoma cells

Our data show that A549 cells are increasingly radiosensitive with prolonged exposure to L-BSO. The resulting glutathione and protein thiol depleted cells show both loss of shoulder and slope modification. Furthermore, there is an increase in single strand DNA breaks and irrepairable cross-linking. The aerobic radiation damage in the thiol depleted state appears to be different from that obtained with hypoxic cells. Any postulated role for GSH in reducing or preventing peroxidative radiation damage must also include protection against single strand DNA breaks as well as involvement in repairing DNA-protein cross-links. The latter effect may be related to decreased protein thiol content as reflected in a decreased enzyme capacity to repair DNA damage.

Nitroheterocycle metabolism in mammalian cells. Stimulation of the hexose monophosphate shunt.

Misonidazole, SR-2508, nitrofurazone and other nitroheterocycles stimulated release of 14CO2 from [1-14C]glucose but not from [6-14C]glucose when incubated with mouse Ehrlich ascites cells or human A549 lung carcinoma cells in vitro. This demonstrated that the nitro compounds activated the hexose monophosphate shunt and is evidence that an important pathway of nitro reduction in these cell lines is electron transfer from NADPH-dependent cytochrome c reductase to the nitro group. Shunt activity was stimulated under both aerobic and anaerobic conditions. For catalase-free Ehrlich cells, aerobic effects were greater than anaerobic, indicating that NADPH was used for reduction of H2O2, via GSH peroxidase and reductase, as well as for one-electron nitro reduction, under aerobic conditions. Several of the compounds tested stimulated 14CO2 release from [2-14C]glucose as well as from [1-14C]-glucose. This shows that the cellular requirement for NADPH, in the presence of nitro drug, was great enough to cause recycling of pentose phosphates. Recycling could decrease the availability of ribose-5-P needed for nucleic acid synthesis, which could partly explain the inhibition of DNA synthesis observed upon prolonged aerobic incubation of cells with nitro compounds. Comparison of the rate of disappearance of nitrofurazone from anaerobic A549 cell suspensions with the rate of 14CO2 release suggests that the drug reduction in this cell line was catalyzed almost entirely by NADPH-requiring enzymes.

THE OXIDATION OF ASCORBATE BY ELECTRON AFFINIC DRUGS AND CARCINOGENS

Abstract— The nitrobenzenes, the carcinogens 4‐nitropyridine‐N‐oxide and 4‐nitro‐quinoline‐N‐oxide as well as the nitrofurans, also known to be carcinogenic, have been found to enhance the reaction of ascorbate with oxygen. The reaction results in the oxidation of ascorbate, the production of dehydroascorbate, superoxide radical, peroxide and water. The drugs are not reduced to stable intermediates during the oxidation but are recycled to their original state. The oxygen consumption is partially inhibited by either superoxide dismutase or catalase. If both superoxide dismutase and catalase are included in the reaction mixture, total oxygen consumption was equal to the amount expected for oxidation of ascorbate to dehydroascorbate and reduction of oxygen to water. The oxygen consumption was inhibited by ferricytochrome c. Semiquinones, nitro and hydroxylamine radicals, produced by electron transfer from ascorbate, reduce ferricytochrome c. These oxygen reactive radicals are responsible for the stimulation of oxygen utilization and ascorbate oxidation. In addition we have found that Ehrlich cells, containing catalase and superoxide dismutase, inhibit the drug catalyzed oxidation of ascorbate. The presence of cyanide, known to inhibit catalase and superoxide dismutase, abolished the cell effect for most of the drugs tested.