Tyrosinase-induced phenoxyl radicals of etoposide (VP-16): interaction with reductants in model systems, K562 leukemic cell and nuclear homogenates

Oxidative Stress Mediated Idiosyncratic Drug Toxicity

The following describes a novel screening method for "new chemical entities" (NCEs), suitable for ADMET studies, that measures ability to form prooxidant radicals on metabolism and their ability to induce oxidative stress in intact cells. The accelerated molecular cytotoxic mechanism screening (ACMS) techniques used with isolated rat hepatocytes showed that cytotoxicity is usually initiated as a result of macromolecular covalent binding or macromolecular oxidative stress. While P450 is likely responsible for drug metabolic activation in the liver, intestine, lung, and in other nonhepatic tissues, where P450 levels are low, peroxidases including prostaglandin synthetase peroxidase can catalyze xenobiotic one-electron oxidation to form prooxidant free radicals that may cause toxicity or carcinogenesis. Inflammation markedly activates H2O2, generating NADPH oxidase and peroxidase of certain immune cells when they infiltrate tissues including the liver. Myeloperoxidase and NADPH oxidase in the Kupffer cells (resident macrophages of the liver) also become activated during inflammation. The addition of noncytotoxic concentrations of peroxidase/H2O2 to the hepatocyte incubate markedly increased drug cytotoxicity and prooxidant radical formation as shown by glutathione or lipid oxidation. Many drugs that have hepato- or gastrointestinal (GI) toxicity problems or were withdrawn from the market for safety problems, e.g., troglitazone, tolcapone, mefenamic acid, diclofenac, and phenylbutazone, were markedly more toxic and prooxidant in this inflammation model system, whereas other drugs, e.g., entacapone, were not toxic in this inflammation model. Some of the idiosyncratic hepatotoxicity responsible for recent drug withdrawals may therefore result from commonplace sporadic inflammatory episodes during drug therapy.

Prolonged quercetin administration diminishes the etoposide-induced DNA damage in bone marrow cells of rats

The DNA damage in bone marrow cells induced by etoposide (E) injected intraperitoneally to rats (100 mg/kg b.w.) decreased to the control level when quercetin (Q) was administered subcutaneously for 10 consecutive days (40 mg/kg b.w.per day) before E was injected. The antioxidant power (FRAP assay) increased significantly after Q or E compared with control rats but did not change when Q preceded the E injection. The superoxide dismutase activity significantly increased in Q+E-treated rats compared with quercetin given alone. The study provides evidence that Q protects bone marrow cells against long-lived E-induced DNA damage and alters the redox balance in lung tissue.

Mitochondrial Thioredoxin System

Thioredoxin-2 (Trx2) is a mitochondrial protein-disulfide oxidoreductase essential for control of cell survival during mammalian embryonic development. This suggests that mitochondrial thioredoxin reductase-2 (TrxR2), responsible for reducing oxidized Trx2, may also be a key player in the regulation of mitochondria-dependent apoptosis. With this in mind, we investigated the effects of overexpression of TrxR2, Trx2, or both on mammalian cell responses to various apoptotic inducers. Stable transfectants of mouse Neuro2A cells were generated that overexpressed TrxR2 or an EGFP-TrxR2 fusion protein. EGFP-TrxR2 was enzymatically active and was localized in mitochondria. TrxR2 protein level and TrxR activity could be increased up to 6-fold in mitochondria. TrxR2 and EGFP-TrxR2 transfectants showed reduced growth rates as compared with control cells. This growth alteration was not due to cytotoxic effects nor related to changes in basal mitochondrial transmembrane potential (DeltaPsi(m)), reactive oxygen species production, or to other mitochondrial antioxidant components such as Trx2, peroxyredoxin-3, MnSOD, GPx1, and glutathione whose levels were not affected by increased TrxR2 activity. In response to various apoptotic inducers, the extent of DeltaPsi(m) dissipation, reactive oxygen species induction, caspase activation, and loss of viability were remarkably similar in TrxR2 and control transfectants. Excess TrxR2 did not prevent trichostatin A-mediated neuronal differentiation of Neuro2A cells nor did it protect them against beta-amyloid neurotoxicity. Neither massive glutathione depletion nor co-transfection of Trx2 and TrxR2 in Neuro2A (mouse), COS-7 (monkey), or HeLa (human) cells revealed any differential cellular resistance to prooxidant or non-oxidant apoptotic stimuli. Our results suggest that neither Trx2 nor TrxR2 gain of function modified the redox regulation of mitochondria-dependent apoptosis in these mammalian cells.

Thiol protecting agents and antioxidants inhibit the mitochondrial permeability transition promoted by etoposide: implications in the prevention of etoposide-induced apoptosis

Etoposide (VP-16) is known to promote cell apoptosis either in cancer or in normal cells as a side effect. This fact is preceded by the induction of several mitochondrial events, including increase in Bax/Bcl-2 ratio followed by cytochrome c release and consequent activation of caspase-9 and -3, reduction of ATP levels, depolarization of membrane potential (DeltaPsi) and rupture of the outer membrane. These events are apoptotic factors essentially associated with the induction of the mitochondrial permeability transition (MPT). VP-16 has been shown to stimulate the Ca2+-dependent MPT induction similarly to prooxidants and to promote apoptosis by oxidative stress mechanisms, which is prevented by glutathione (GSH) and N-acetylcysteine (NAC). Therefore, the aim of this work was to study the effects of antioxidants and thiol protecting agents on MPT promoted by VP-16, attempting to identify the underlying mechanisms on VP-16-induced apoptosis. The increased sensitivity of isolated mitochondria to Ca2+-induced swelling, Ca2+ release, depolarization of DeltaPsi and uncoupling of respiration promoted by VP-16, which are prevented by cyclosporine A proving that VP-16 induces the MPT, are also efficiently prevented by ascorbate, the primary reductant of the phenoxyl radicals produced by VP-16. The thiol reagents GSH, dithiothreitol and N-ethylmaleimide, which have been reported to prevent the MPT induction, also protect this event promoted by VP-16. The inhibition of the VP-16-induced MPT by antioxidants agrees with the prevention of etoposide-induced apoptosis by GSH and NAC and suggests the generation of oxidant species as a potential mechanism underlying the MPT that may trigger the release of mitochondrial apoptogenic factors responsible for apoptotic cascade activation.

p53 and redox state in etoposide-induced acute myeloblastic leukemia cell death

We investigated whether p53, being a redox-sensitive protein, has a role in the responsiveness of AML cells to etoposide. Two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), were used as models. Sensitivity to etoposide was measured by trypan blue and annexin V assays. Etoposide-induced peroxide formation was associated with the induction of cell death. Evident expression of mutated p53 was observed in both subclones in basal growth conditions as analysed by Western blotting and flow cytometry. After etoposide exposure for up to 24 hours, some nuclear accumulation of p53 was observed in the ER subclone, as analysed by Western blotting. The conformation of p53, however, was not changed from mutated toward wild-type during exposure in either of the subclones as analysed by flow cytometry. In conclusion, etoposide-induced change in cellular redox state was associated with apoptosis, but was not a sufficient stimulus for p53 to make its conformation active. Thus, mutated p53 seems to have no role in etoposide-induced apoptosis.

Induction of mitochondrial manganese superoxide dismutase confers resistance to apoptosis in acute myeloblastic leukaemia cells exposed to etoposide

We investigated the possible roles of mitochondrial manganese superoxide dismutase (MnSOD) and bcl-2 in etoposide-induced cell death in acute myeloblastic leukaemia (AML) using two subclones of the OCI/AML-2 cell line, the etoposide-sensitive (ES) and the etoposide-resistant (ER), as models. Cell death after 24 h exposure to 10 micromol/l etoposide was about 60% and 70% in the ES subclone and about 20% and 25% in the ER subclone, when analysed by trypan blue and annexin V respectively. Cytochrome c efflux from mitochondria to cytosol was observed after 4 h of exposure in both subclones, whereas the activation of caspase-3 was not detectable until after 12 h of exposure in the ES subclone and 24 h of exposure in the ER subclone, using Western blotting. The decrease in mitochondrial membrane potential, when analysed by the JC-1 probe fluorocytometrically, also appeared to take place later in the ER than in the ES subclone. Both subclones showed evident basal expression of MnSOD and bcl-2 by Western blotting. Etoposide caused a potent induction of MnSOD, more than 400% at 12 h, in the ER but not in the ES subclone. No significant change in bcl-2 expression could be observed in either of the subclones during exposure to etoposide when analysed by Western blotting or flow cytometry. In conclusion, we suggest that MnSOD might have a special role in the protection of AML cells against etoposide-induced cell death. Although unable to influence the cytochrome c efflux to cytosol, MnSOD might prevent the disruption of mitochondrial membrane potential, which evidently leads to cell death by releasing various activators of apoptosis.

Protection of acute myeloblastic leukemia cells against apoptotic cell death by high glutathione and gamma-glutamylcysteine synthetase levels during etoposide-induced oxidative stress

BACKGROUND

Etoposide mediates its cytotoxicity by inducing apoptosis. Thus, mechanisms which regulate apoptosis should also affect drug resistance. Oxidants and antioxidants have been shown to participate in the regulation of apoptosis. We were interested in studying whether responsiveness of acute myeloblastic leukemia (AML) cells to etoposide is mediated by oxidative stress and glutathione levels.

PATIENTS AND METHODS

Two subclones of the OCI/AML-2 cell line which are etoposide-sensitive (ES), and etoposide-resistant (ER), were established by the authors at the University of Oulu, and used as models. Assays for apoptosis included externalization of phosphatidylserine (as evidenced by annexin V binding), and caspase activation as indicated by cleavage of poly(ADP-ribose)polymerase (Western blotting). Peroxide formation was analyzed by flow cytometry. Glutathione and gamma-glutamylcysteine synthetase (gamma-GCS) levels were determined spectrophotometrically and by Western blotting, respectively.

RESULTS

Etoposide-induced apoptosis was evident 12 hours after treatment in the ES subclone, but was apparent in the ER subclone only after 24 hours. The basal glutathione and gamma-GCS levels were higher in the ER than the ES subclone. Etoposide increased peroxide formation in both subclones after 12-hour exposure. Significant depletion of glutathione was observed in the ES subclone during etoposide exposure, while glutathione levels were maintained in the ER subclone. In neither of the subclones was induction of gamma-GCS observed during 24-hour exposure to etoposide. Furthermore, the catalytic subunit of gamma-GCS was cleaved during apoptosis, concurrent with depletion of intracellular glutathione. When glutathione was depleted by treatment with buthionine sulfoximine, a direct inhibitor of gamma-GCS, the sensitivity to etoposide was increased, particularly in the ER subclone.

CONCLUSIONS

The results underline the significance of glutathione biosynthesis in the responsiveness of AML cells to etoposide. The molecular mechanisms mediating glutathione depletion during etoposide exposure might include the cleavage of the catalytic subunit of gamma-GCS.

Development and partial characterization of a human T-lymphoblastic leukemic (CCRF-CEM) cell line resistant to etoposide. Analysis of possible circumventing approaches

We have selected an etoposide-resistant variant (CCRF-CEM/VP-16) of the human T-lymphoblastic CCRF-CEM leukemia for study. Resistance to the topoisomerase II (topo II) inhibitor was about 11-fold and stable. Other data revealed that the new cell line had acquired an atypical, non-P-glycoprotein overexpressing multidrug resistant (MDR) phenotype with cross-resistance to other topo II inhibitors (amsacrine, doxorubicin, and mitoxantrone) and to glucocorticoids, but not to novobiocin, ICRF-187, vincristine or cisplatin. In a first instance, we assumed that altered drug-topo II interactions, based on quantitative and/or qualitative modifications of the enzyme, are a cause of resistance in the cell line. We tried to modify the drug sensitivity of the cells by means of various agents and cytokines. Positive results were obtained with verapamil and, to a lesser extent, cyclosporin A, but they were not specific for the drug resistant variant and occurred in the parental CCRF-CEM as well. Other attempts with buthionine sulfoximine, novobiocin, pentoxifylline, interleukin-1, interferon-alpha, retinoic acid, TNF-alpha, bryostatin 1 or phorbol myristate acetate were substantially unsuccessful, thus confirming the difficulty of pharmacologically overcoming atypical MDR. More encouragingly, however, CCRF-CEM/VP-16 cells exhibited hypersensitivity to other agents, including actinomycin D and taxol.

Inhibition of Na+/K(+)-ATPase by phenoxyl radicals of etoposide (VP-16): role of sulfhydryls oxidation

In the present work, we studied the effects of phenoxyl radicals, generated by tyrosinase-catalyzed oxidation of a phenolic antitumor drug, Etoposide (VP-16), on a purified dog kidney Na+/K(+)-ATPase by characterizing interactions of VP-16 phenoxyl radicals with the enzyme's SH-groups by ESR and correlating the loss of the enzymatic activity with the oxidation of its SH-groups, and oxidation of VP-16. VP-16/tyrosinase caused inhibition of Na+/K(+)-ATPase which was dependent on the incubation time and concentration of tyrosinase. The inhibition of Na+/K(+)-ATPase was accompanied by a decrease of DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)-titratable SH-groups. In the presence of Na+/K(+)-ATPase, a typical ESR signal of the VP-16 phenoxyl radical could be observed only following a lag period the duration of which was proportional to the concentration of the Na+/K(+)-ATPase added. Our HPLC measurements demonstrated that Na+/K(+)-ATPase protected VP-16 against tyrosinase-catalyzed oxidation. Combined these results suggest that redox-cycling of VP-16/VP-16 phenoxyl radical by SH-groups of Na+/K(+)-ATPase occurred. Ascorbate which is known to reduce the VP-16 phenoxyl radicals, protected the enzyme against inactivation, prevented oxidation of the enzyme's SH-groups. Reduction of VP-16 phenoxyl radicals by ascorbate was directly observed by the semidehydroascorbyl radical signal in the ESR spectra. VP-16 phenoxyl radical-induced oxidation of sulfhydryls and inhibition of the Na+/K(+)-ATPase may be responsible for at least some of its clinical side effects (e.g., cardiotoxicity) which can be prevented by ascorbate.

Myocardial adaptation to ischemia by oxidative stress induced by endotoxin

In this study, we examined the effects of oxidative stress adaptation on myocardial ischemic reperfusion injury. Oxidative stress was induced by injecting endotoxin (0.5 mg/kg) into the rat. After 24 h, rats were killed, hearts were isolated, and the effects of ischemia-reperfusion were studied using an isolated working heart preparation. The development of oxidative stress was examined by assessing malonaldehyde production in the heart. The antioxidant defense system was studied by estimating antioxidant enzyme activities and ascorbate- as well as thiol-dependent antioxidant reserve. The results of our study indicated that endotoxin induced oxidative stress within 1 h of treatment; the stress was reduced progressively and steadily up to 24 h. The antioxidant enzymes superoxide dismutase, catalase, glutathione (GSH) peroxidase, and GSH reductase were lowered up to 2 h and then increased. Both thiol- and ascorbate-dependent antioxidant reserve were enhanced, but the enhancement of the former was only transitory. After 24 h, endotoxin provided adequate protection to the heart from the ischemic-reperfusion injury, as evidenced by improved left ventricular function and aortic flow. Our results suggest that the induction of oxidative stress by endotoxin-induced adaptive modification of the antioxidant defense in the heart, thereby reducing ischemic-reperfusion injury.