Redox activities of antitumor anthracyclines determined by microsomal oxygen consumption and assays for superoxide anion and hydroxyl radical generation

Carminomycin I is an apoptosis inducer that targets the Golgi complex in clear cell renal carcinoma cells

Clear cell renal cell carcinoma (CCRCC) evolves due to mutations in the Von Hippel-Lindau (VHL) tumor suppressor gene. Although the loss of VHL enables survival and proliferation of CCRCC cells, it is also expected to introduce vulnerabilities that may be exploited for therapeutics discovery. To this end, we developed a high-throughput screen to identify small molecules derived from plants, microorganisms, and marine organisms to which CCRCC cells are sensitive. Screening over 8,000 compounds using this approach, we report here the identification of the microbially derived compound carminomycin I (CA) as an effective inhibitor of VHL-defective (VHL(-/-)) CCRCC cell proliferation. CA also induced apoptosis in CCRCC cells by a mechanism independent of p53 or hypoxia-inducible factor 2. We found that P-glycoprotein (P-gp) sequestered CA within the Golgi complex. Interestingly, Golgi sequestration was critical for the antiproliferative effects of CA and P-gp inhibitors abrogated this activity. Furthermore, CA induced cleavage of the Golgi protein p115 and the translocation of its C-terminal fragment to the nucleus. Finally, examination of the activity of the VHL-interacting Golgi protein, endoplasmic reticulum-Golgi intermediate compartment, ERGIC-53 showed that VHL could mediate protection from CA in CCRCC cells. Our natural product-based screening approach has revealed the P-gp-mediated localization of anticancer compounds within the Golgi in CCRCC cells as a potential strategy of targeting VHL-deficient CCRCC cells.

Shrinking the biologic world--nanobiotechnologies for toxicology

Although toxicologic effects need to be considered at the organismal level, the adverse events originate from interactions and alterations at the molecular level. Cellular structures and functions can be disrupted by modifications of the nanometer structure of critical molecules; therefore, devices used to assess biologic and toxicologic processes at the nanoscale will allow important new research pursuits. In order to properly assess alterations at these dimensions, nanofabricated tools are needed to detect, separate, analyze, and manipulate cells or biologic molecules of interest. The emergence of laser tweezers, surface plasmon resonance (SPR), laser capture microdissection (LCM), atomic force microscopy (AFM), and multi-photon microscopes have allowed for these assessments. Micro- and nanobiotechnologies will further advance biologic, clinical, and toxicologic endeavors with the aid of miniaturized, more sensitive devices. Miniaturized table-top laboratory equipment incorporating additional innovative technologies can lead to new advances, including micro total analysis systems (microTAS) or "lab-on-a-chip" and "sentinel sensor" devices. This review will highlight several devices, which have been made possible by techniques originating in the microelectronics industry. These devices can be used for toxicologic assessment of cellular structures and functions, such as cellular adhesion, signal transduction, motility, deformability, metabolism, and secretion.

Daunorubicin cardiotoxicity: evidence for the importance of the quinone moiety in a free-radical-independent mechanism

Anthracyclines, such as daunorubicin (Daun), and other quinone-containing compounds can stimulate the formation of toxic free radicals. The present study tests the hypothesis that the quinone moiety of Daun, by increasing free-radical production, disrupts sarcoplasmic reticulum (SR) function and thereby inhibits myocardial contractility in vitro. We compared Daun with its quinone-deficient analogue, 5-iminodaunorubicin (5-ID), using experimental interventions to produce various contractile states that depend on SR function. At concentrations of Daun or 5-ID that did not alter contractility (dF/dt) of steady-state contractions (1 Hz) in electrically paced atria isolated from adult rabbits, only Daun significantly attenuated the positive inotropic effects on dF/dt of increased rest intervals (PRP; post-rest potentiation) or increased stimulation frequencies. Attenuation was to 98+/-6% at 1 Hz, and 73+/-8 and 67+/-8% for 30 and 60 sec PRP, respectively, and 73+/-3 and 63 +/-3% at 2 and 3 Hz, respectively, for 88 microM Daun (P<0.05, vs pre-drug baseline values, mean +/- SEM). These effects of Daun were similar to those of caffeine (2 mM), an agent well known to deplete cardiac SR calcium. We also examined the effect of Daun in isolated neonatal rabbit atria, which lack mature, functional SR; Daun did not alter the force-frequency relationship or PRP contractions. Additional studies in Ca(2+)-loaded SR microsomes indicated that both Daun and 5-ID opened Ca(2+) release channels, with Daun being 20-fold more potent than 5-ID in this respect. Neither anthracycline, however, induced free-radical formation in SR preparations (assayed via nicking of supercoiled DNA) prior to stimulating Ca(2+) release. Thus, our results indicate that Daun impairs myocardial contractility in vitro by selectively interfering with SR function; the quinone moiety of Daun appears to mediate this cardiotoxic effect, acting through a mechanism that does not involve free radicals.

Polarized Caco-2 cells. Effect of reactive oxygen metabolites on enterocyte barrier function

Reactive oxygen metabolites are implicated in gastrointestinal disease and enterocyte injury associated with ischemia-reperfusion, bacterial translocation, inflammatory bowel disease, and necrotizing enterocolitis. The ileal-like, human colon carcinoma cell line, Caco-2, was used to investigate oxidative damage. After challenging Caco-2 cells with reactive oxygen metabolites, the permeability, viability, and energy charge of Caco-2 cells were assessed. Permeability was determined by transepithelial electrical potential and flux of small molecules. Viability was determined by release of 51Cr. Cell energy was evaluated by determining adenylate energy charge. The source of reactive oxygen metabolites, with the exception of menadione, did not affect viability of Caco-2 cells; cell permeability was increased. The increased varied with the source and location of the reactive oxygen metabolite. There was no change in energy charge. This study suggests that reactive oxygen metabolites could cause enterocyte damage and that the source of the reactive oxygen metabolite is an important variable in determining the extent of damage. Antioxidants might prevent injury.

Interactions of adriamycin aglycones with mitochondria may mediate adriamycin cardiotoxicity

Adriamycin and related anthracyclines are potent oncolytic agents, the clinical utility of which is limited by severe cardiotoxicity. Aglycone metabolites of Adriamycin (5-20 microM) induce a Ca(2+)-dependent increase in the permeability of the inner mitochondrial membrane of both heart and liver mitochondria to small (< 1,500 Da) solutes; this phenomenon is accompanied by release of mitochondrial Ca2+, mitochondrial swelling, collapse of the membrane potential, oxidation of mitochondrial pyridine nucleotides [NAD(P)H], uncoupling, and a transition from the condensed to the orthodox conformation and is inhibited by ATP, dithiothreitol, the immunosuppressant cyclosporin A, and the ubiquitous polyamine spermine. Aglycones also modify mitochondrial sulfhydryl groups and induce a Ca2+ independent oxidation of mitochondrial NAD(P)H which appears to reflect electron transport from NADH to oxygen, mediated by the aglycones and resulting in the production of superoxide (O2-). Selenium deficiency and butylated hydroxytoluene inhibit aglycone-induced Ca2+ release from liver, but not heart, mitochondria, suggesting that the interactions of the aglycones with mitochondria differ in these two tissues. It can be proposed that the effects of Adriamycin aglycones on heart mitochondria are responsible for the cardiotoxicity of the parent drug.

The consequences of nitrofurantoin-induced oxidative stress in isolated rat hepatocytes: evaluation of pathobiochemical alterations

Oxidative stress was induced in isolated rat hepatocytes by incubation with nitrofurantoin in the absence and presence of the GSSG reductase inhibitor BCNU. In both cases nitrofurantoin markedly reduced glutathione but exerted cytotoxicity as measured by LDH release and loss of intracellular potassium only in BCNU pretreated cells. The onset of cytotoxicity was accompanied by an increase of lipid peroxidation. Oxidation of protein thiols, however, could not be detected in the early phase of cell damage. The cytoprotective activity of N-acetyl-cysteine > dithiothreitol = deferoxamine revealed the substantial importance of glutathione for cellular defence and the sensitivity of not yet identified thiol-dependent targets of oxidative stress.

Free radical modes of cytotoxicity of adriamycin and streptonigrin

Free radical modes of cytotoxicity of streptonigrin (STN) and Adriamycin (ADR) in Chinese hamster V79 cells under aerobic conditions were evaluated using 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TP), a low molecular weight stable nitroxide free radical with antioxidant properties and desferrioxamine (DF), a transition metal chelator. In addition, exogenous superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6), were tested for cytoprotective effects. EPR studies showed that TP reacts with the semiquinones of both ADR and STN and also with O2- radicals generated during aerobic redox cycling of the respective semiquinone radicals. Pulsed field gel electrophoresis studies confirmed that DNA double-strand breaks (dsb) induced by STN in V79 cells were inhibited completely by TP, whereas ADR-induced DNA dsb were not affected by TP. Clonogenic cell survival studies showed that STN-induced cytotoxicity could be inhibited completely by DF or TP. Both agents were ineffective in inhibiting ADR-induced cytotoxicity. SOD and CAT were ineffective in protecting against both STN and ADR cytotoxicity. Our results are consistent with a mechanism requiring the semiquinone radical intermediate of STN for cytotoxicity and minimal free radical involvement in ADR-induced V79 cell cytotoxicity.

Quantitation of the hydroxyl radical adducts of salicylic acid by micellar electrokinetic capillary chromatography: oxidizing species formed by a Fenton reaction

There has been controversy concerning the products formed by a Fenton reaction. We determined the hydroxyl radical (.OH) generated in a Fenton reaction system with no iron chelator using micellar electrokinetic capillary chromatography (MECC). The hydroxyl radical generated in this Fenton system attacked salicylic acid to produce major products of 2,3- and 2,5-dihydroxybenzoic acid (DHB), 2,3-DHB being prominent. Hydroxyl radical scavengers, such as mannitol, ethanol, thiourea and a ferric chelator, Desferal, significantly diminished the peaks for DHBs, showing production of .OH. We compared the MECC method with the electron paramagnetic resonance (EPR) spin trapping technique. The quantity of DHBs obtained by MECC increased dose-dependently up to 1 microM Fe2+ at a fixed concentration of H2O2, whereas that of the spin adduct by EPR showed a bell-shaped curve. This quantitation of .OH adducts by MECC supports the proposal that the oxidizing species formed by a Fenton reaction with no chelator is .OH. The EPR spin trapping method appears to be erroneous, particularly when iron is present at a higher concentration than hydrogen peroxide. The application of this method to the paraquat effect in vitro is demonstrated, and the possibility for analysis of .OH in vivo is also discussed.

Caco-2 cell metabolism of oxygen-derived radicals

Reactive oxygen metabolites have been associated with gastrointestinal injury and may play a role as mediators of inflammation. The effect of oxygen metabolites on Caco-2 cell viability (trypan blue exclusion and 51Cr release), hexose monophosphate shunt activity, and glutathione was assessed. Caco-2 cells were incubated with amino acid oxidase, xanthine oxidase, menadione, and t-butylhydroperoxide in the presence and absence of superoxide dismutase, catalase, mannitol, and butylated hydroxytoluene. With amino acid oxidase, trypan blue exclusion decreased (P < 0.01) and 51Cr release, oxidized glutathione, and shunt activity increased (P < 0.05). The addition of catalase attenuated these changes. Trypan blue exclusion decreased (P < 0.005) and 51Cr release, oxidized glutathione, and shunt activity increased (P < 0.01) with xanthine oxidase. The addition of superoxide dismutase caused a further increase in 51Cr release, oxidized glutathione, and shunt activity (P < 0.01), which was prevented by the addition of catalase or mannitol. t-Butylhydroperoxide did not effect 51Cr release or trypan blue exclusion, but oxidized glutathione and shunt activity increased (P < 0.01). The increase in shunt activity was prevented by preincubation with butylated hydroxytoluene (P < 0.01). Menadione did not alter trypan blue exclusion or 51Cr release, but caused an increase in oxidized glutathione and shunt activity (P < 0.001). The increase in shunt activity was attenuated by preincubation with butylated hydroxytoluene (P < 0.001). Menadione also caused a depletion of total glutathione.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of adriamycin aglycones with isolated mitochondria. Effect of selenium deficiency

Adriamycin (AdM) aglycones have dramatic effects on isolated heart mitochondria, oxidizing pyridine nucleotides, modifying sulfhydryl groups, and triggering a permeability transition of the inner membrane that results in free passage of solutes smaller than 1500 Da. In this investigation, the role of glutathione (GSH) peroxidase in these actions of the aglycones was evaluated, by comparing mitochondria from selenium-deficient and selenium-supplemented rats, with the following results. Selenium deficiency was without effect on the permeability transition of heart mitochondria, followed via Ca2+ release and triggered by AdM aglycone or by t-butyl hydroperoxide (TBH) or H2O2, both of which are authentic substrates of the peroxidase. The permeability transition of liver mitochondria was delayed by selenium deficiency regardless of the triggering agent; however, substantial triggering by the aglycone and TBH persisted in mitochondria from selenium-deficient animals. Selenium deficiency inhibited thiol modification elicited by AdM aglycone and H2O2 in heart mitochondria and by the aglycone, TBH, and possibly H2O2 in liver mitochondria. It would thus appear that AdM aglycone, TBH, and H2O2 can induce the permeability transition of isolated heart mitochondria via a process (or processes) distinct from the catalytic activity of the peroxidase. Furthermore, even in liver, where involvement of the peroxidase is observed, mechanisms other than the GSH cycle can contribute to transition induction by the aglycone and by TBH. Finally, mitochondrial-SH group modification by the aglycones appeared not to be causally linked to induction of the permeability transition. This laboratory has suggested that the effects of aglycone metabolites of AdM on mitochondria mediate the cardiotoxicity that limits use of the parent drug. The data presented in this paper argue against the involvement of GSH peroxidase in that process. They are in agreement with in vivo studies, which have generally failed to find evidence for amelioration of AdM cardiotoxicity in selenium-deficient animals.

Antioxidant enzymes in the differentiated Caco-2 cell line

Injury to the gastrointestinal tract by oxygen dependent processes is important in ischemia, inflammatory bowel disease, and necrotizing enterocolitis. The Caco-2 cell line is an important tool in assessing various gastrointestinal functions and offers a unique opportunity to assess gastrointestinal oxidant metabolism on a cellular level. However, some Caco-2 cell functions change with time after confluence. To determine if antioxidant enzyme activity changes during differentiation, Caco-2 cells were grown to confluence, and superoxide dismutase, glutathione peroxidase, glutathione reductase, and catalase activities and specific mRNA content were quantitated. With time after confluence the enzymes demonstrated a small, but statistically significant increase in activity. Neither superoxide dismutase nor glutathione peroxidase mRNA levels correlated with enzyme activity changes. Catalase mRNA levels increased as catalase activity increased. Thus, differentiated Caco-2 cells express superoxide dismutase, glutathione peroxidase, glutathione reductase, and catalase activities and the superoxide dismutase, glutathione peroxidase, and catalase genes. Superoxide dismutase activity and glutathione peroxidase activity do not correlate with mRNA levels, and suggest that regulation may be at a level other than transcription. The correlation between catalase activity and catalase mRNA suggests differentiation may occur at transcription. If Caco-2 cells are used to elucidate oxidative metabolism, changes in activities of antioxidant enzymes as a function of cell differentiation should be considered.

One-electron reductive bioactivation of 2,3,5,6-tetramethylbenzoquinone by cytochrome P450

Bioreductive activation of quinones in mammalian liver has generally been attributed to NADPH-cytochrome P450 reductase. However, in view of the 20-30-fold molar excess of cytochrome P450 over NADPH-cytochrome P450 reductase on the endoplasmic reticulum of the rat liver cell and the capability of cytochrome P450 to bind and reduce xenobiotics, it was considered of interest to investigate the possible role of cytochrome P450 in the bioreduction of quinones. In the present study, 2,3,5,6-tetramethyl-1,4-benzoquinone (TMQ) was chosen as a model quinone. First, TMQ was found to bind at the metabolic active site of phenobarbital (PB)-inducible cytochrome P450s of rat liver microsomes, indicating that TMQ is a potential substrate for cytochrome P450-mediated biotransformation. Second, with electron spin resonance, one-electron reduction of TMQ to a semiquinone free radical (TMSQ) was found to occur in these microsomal fractions. SK&F 525-A, a well-known inhibitor of cytochrome P450, strongly inhibited TMSQ formation in these subcellular fractions without affecting NADPH-cytochrome P450 reductase activity. One-electron reductive bioactivation of TMQ was further investigated with purified NADPH-cytochrome P450 reductase alone and in reconstituted systems of purified cytochrome P450-IIB1 and NADPH-cytochrome P450 reductase. As measured by ESR, purified cytochrome P450-IIB1 in the presence of NADPH-cytochrome P450 reductase was able to reduce TMQ to TMSQ at a much greater rate than in the presence of NADPH-cytochrome P450 reductase alone. Reduction of TMQ was also investigated by measuring the initial rate of NADPH oxidation by TMQ under anaerobic conditions. Inhibitors of cytochrome P450, namely SK&F 525-A and antibodies against PB-inducible cytochrome P450s, caused a substantial decrease in reductive metabolism in PB-treated microsomes. These antibodies were also effective in the inhibition of TMQ-induced NADPH oxidation in a complete reconstituted system of equimolar concentrations of cytochrome P450-IIB1 and NADPH-cytochrome P450 reductase, indicating that the reaction was specific for cytochrome P450-IIB1. Finally, initial rates of NADPH oxidation were determined in reconstituted systems containing varying amounts of NADPH-cytochrome P450 reductase and cytochrome P450-IIB1 to determine the contribution of either enzyme in the reduction of TMQ. As expected, NADPH-cytochrome P450 reductase was able to reduce TMQ to a small extent. However, reconstitution in the presence of increasing amounts of cytochrome P450-IIB1 (relative to NADPH-cytochrome P450 reductase) resulted in increasing rates of TMQ-induced NADPH oxidation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ferritin as a source of iron for oxidative damage

The generation of deleterious activated oxygen species capable of damaging DNA, lipids, and proteins requires a catalyst such as iron. Once released, ferritin iron is capable of catalyzing these reactions. Thus, agents that promote iron release may lead to increased oxidative damage. The superoxide anion formed enzymatically, radiolytically, via metal-catalyzed oxidations, or by redox cycling xenobiotics reductively mobilizes ferritin iron and promotes oxidative damage. In addition, a growing list of compounds capable of undergoing single electron oxidation/reduction reactions exemplified by paraquat, adriamycin, and alloxan have been reported to release iron from ferritin. Because the rapid removal of iron from ferritin requires reduction of the iron core, it is not surprising that the reduction potential of a compound is a primary factor that determines whether a compound will mobilize ferritin iron. The reduction potential does not, however, predict the rate of iron release. Therefore, ferritin-dependent oxidative damage may be involved in the pathogenesis of diseases where increased superoxide formation occurs and the toxicity of chemicals that increase superoxide production or have an adequate reduction potential to mobilize ferritin iron.

Oxidation of mitochondrial pyridine nucleotides by aglycone derivatives of adriamycin

Adriamycin (AdM) and related anthracyclines are potent antineoplastic agents, the clinical utility of which is limited by severe cardiotoxicity. Aglycone derivatives of AdM have recently been reported to trigger the release of Ca2+ from isolated, preloaded rat heart mitochondria and to modify mitochondrial sulfhydryl (-SH) groups. Both mitochondrial Ca2+ retention and -SH status are sensitive to mitochondrial NAD(P)+/NAD(P)H ratios. This investigation examined the effects of AdM and its aglycone derivatives on the pyridine nucleotide redox status of isolated, intact heart mitochondria with the following results. (i) AdM aglycones induced the slow, Ca2(+)-independent oxidation of mitochondrial NAD(P)H. Oxidation was proportional to aglycone concentration between 5 and 60 microM. (ii) In terms of potency, 7-deoxy AdM aglycone greater than or equal to 7-hydroxy AdM aglycone much greater than AdM. (iii) Inhibitor data suggested that NAD(P)H oxidation reflects the rotenone-insensitive reduction of AdM aglycone and subsequent electron transfer to O2 generating superoxide. (iv) NAD(P)H oxidation mediated by AdM aglycone could be distinguished from the Ca2(+)-dependent NAD(P)H oxidation associated with mitochondrial Ca2+ release. This communication is the first to describe redox interactions of AdM with intact mitochondria.

The mechanism of the quinone reductase reaction of pig heart lipoamide dehydrogenase

The relationship between the NADH:lipoamide reductase and NADH:quinone reductase reactions of pig heart lipoamide dehydrogenase (EC 1.6.4.3) was investigated. At pH 7.0 the catalytic constant of the quinone reductase reaction (kcat.) is 70 s-1 and the rate constant of the active-centre reduction by NADH (kcat./Km) is 9.2 x 10(5) M-1.s-1. These constants are almost an order lower than those for the lipoamide reductase reaction. The maximal quinone reductase activity is observed at pH 6.0-5.5. The use of [4(S)-2H]NADH as substrate decreases kcat./Km for the lipoamide reductase reaction and both kcat. and kcat./Km for the quinone reductase reaction. The kcat./Km values for quinones in this case are decreased 1.85-3.0-fold. NAD+ is a more effective inhibitor in the quinone reductase reaction than in the lipoamide reductase reaction. The pattern of inhibition reflects the shift of the reaction equilibrium. Various forms of the four-electron-reduced enzyme are believed to reduce quinones. Simple and 'hybrid ping-pong' mechanisms of this reaction are discussed. The logarithms of kcat./Km for quinones are hyperbolically dependent on their single-electron reduction potentials (E1(7]. A three-step mechanism for a mixed one-electron and two-electron reduction of quinones by lipoamide dehydrogenase is proposed.

Detection of free radicals during the cellular metabolism of adriamycin

Experiments were conducted to determine which free radicals are generated during the metabolism of adriamycin (ADM) by canine tracheal epithelial (CTE) cells, guinea pig enterocytes, and rat hepatocytes. The technique employed in this study was spin trapping; the spin trap utilized was 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). The spin adduct 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OH) was observed during the metabolism of ADM by CTE cells. However, the addition of dimethyl sulfoxide to the in vitro system suggested that superoxide is initially spin trapped by the nitrone, and that the adduct 2-hydroperoxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OOH) is rapidly bioreduced to afford DMPO-OH. The addition of superoxide dismutase to the system indicated that superoxide generation was primarily intracellular. The adriamycin semiquinone free radical (ADM-SQ) was produced during the metabolism by enterocytes and hepatocytes. The rate of the production of ADM-SQ was enhanced under anaerobic conditions, suggesting that molecular oxygen was responsible for the degradation of this carbon-centered free radical. However, spin trapping of oxygen radicals was not observed; this observation suggests that these reactive intermediates are not produced at concentrations sufficient for detection by spin-trapping experiments.

Role of the glutathione-glutathione peroxidase cycle in the cytotoxicity of the anticancer quinones

Recent studies have suggested that the selenoenzyme glutathione peroxidase, in the presence of reducing equivalents from the tripeptide glutathione, is responsible for detoxifying hydrogen peroxide and lipid hydroperoxides generated as a consequence of the cyclic reduction and oxidation of quinone-containing anticancer agents including doxorubicin, daunorubicin, mitomycin C, diaziquone, and menadione. Alterations in the intracellular levels of glutathione peroxidase or glutathione can significantly affect the activity of these drugs against human tumor cells and the expression of their normal tissue toxicity, especially with respect to the heart. Furthermore, augmentation of the glutathione peroxidase pathway appears to render certain human tumor cells relatively resistant to the anticancer quinones; therefore, the glutathione peroxidase system may, at least in part, modulate certain forms of acquired drug resistance in man. Thus, the glutathione peroxidase cycle appears to play a central role in maintaining intracellular peroxide homeostasis during quinone-induced oxidative stress.

Investigation of the role of serum lipoprotein-associated peroxides in Adriamycin cardiotoxicity. Release of reduced glutathione from rat hearts perfused with lipase-hydrolyzed very low density lipoprotein fractions obtained from Adriamycin-treated and control rats

In a previous study, we demonstrated that the serum of rats treated chronically with the anticancer agent Adriamycin contains lipid peroxides associated with neutral lipids (W. S. Thayer, Biochem Pharmacol 33: 2259-2263, 1984). In the present study, hearts from untreated control rats were perfused with medium containing serum or very low density lipoprotein (VLDL) fractions obtained from either Adriamycin-treated rats or control rats. Release of endogenous glutathione from the perfused heart was tested to evaluate possible metabolism of the serum lipid peroxides through the glutathione peroxidase/glutathione reductase redox cycle. Perfusion with lipoprotein lipase-hydrolyzed serum or VLDL caused glutathione release, the extent of which increased with increasing VLDL concentration in the perfusate. The effect was not unique for VLDL from Adriamycin-treated rats, but instead appeared to be a more general phenomenon since it was also observed with VLDL from control rats. Glutathione was released in the reduced form (GSH), rather than the oxidized form (GSSG) observed during perfusions with model peroxides. Pretreatment of the VLDL with lipoprotein lipase in vitro prior to perfusion was necessary in order to obtain GSH release. Neither lipase alone nor palmitate in the absence of lipase was as effective in promoting GSH release. Simultaneous release of lactate dehydrogenase was quantitatively less than that of GSH. The results suggest an action of components of serum VLDL on cardiac membrane permeability. Peroxide metabolism-linked perturbation of the cardiac glutathione redox cycle does not appear to be the mode of action for the serum lipid peroxides found in Adriamycin-treated rats.

Enzymatic oxidative activation of 5-iminodaunorubicin. Spectrophotometric and electron paramagnetic resonance studies

Horseradish peroxidase catalyzed oxidation of the antitumor agent 5-iminodaunorubicin by hydrogen peroxide was studied with both spectrophotometric and electron paramagnetic resonance methods. Kinetics of oxidation of the drug at pH 3, 6 and 8 were determined. Rapid formation of a nitrogen-centered free radical metabolite was demonstrated with electron paramagnetic resonance employing the 15N-labeled drug and by deuterium exchange techniques. This enzymatic oxidative activation of 5-iminodaunorubicin suggests an alternative mode of metabolism and mechanism of action of this less cardiotoxic anticancer agent. By contrast, the parent compound, daunorubicin, did not undergo oxidation by the horseradish peroxidase-hydrogen peroxide system.

Free radical formation by antitumor quinones

Quinones are among the most frequently used drugs to treat human cancer. All of the antitumor quinones can undergo reversible enzymatic reduction and oxidation, and form semiquinone and oxygen radicals. For several antitumor quinones enzymatic reduction also leads to formation of alkylating species but whether this involves reduction to the semiquinone or the hydroquinone is not always clear. The antitumor activity of quinones is frequently linked to DNA damage caused by alkylating species or oxygen radicals. Some other effects of the antitumor quinones, such as cardiotoxicity and skin toxicity, may also be related to oxygen radical formation. The evidence for a relationship between radical formation and the biological activity of the antitumor quinones is evaluated.

Release of iron from ferritin by semiquinone, anthracycline, bipyridyl, and nitroaromatic radicals

The cytotoxicity of many xenobiotics is related to their ability to undergo redox reactions and iron dependent free radical reactions. We have measured the ability of a number of redox active compounds to release iron from the cellular iron storage protein, ferritin. Compounds were reduced to their corresponding radicals with xanthine oxidase/hypoxanthine under N2 and the release of Fe2+ was monitored by complexation with ferrozine. Ferritin iron was released by a number of bipyridyl radicals including those derived from diquat and paraquat, the anthracycline radicals of adriamycin, daunorubicin and epirubicin, the semiquinones of anthraquinone-2-sulphonate, 1,5 and 2,6-dihydroxyanthraquinone, 1-hydroxyanthraquinone, purpurin, and plumbagin, and the nitroaromatic radicals of nitrofurantoin and metronidazole. In each case, iron release was more efficient than with an equivalent flux of superoxide. Introduction of air decreased the rate of iron release, presumably because the organic radicals reacted with O2 to form superoxide. In air, iron release was inhibited by superoxide dismutase. Semiquinones of menadione, benzoquinone, duroquinone, anthraquinone 1,5 and 2.6-disulphonate, 1,4 naphthoquinone-2-sulphonate and naphthoquinone, when formed under N2, were unable to release ferrin iron. In air, these systems gave low rates of superoxide dismutase-inhibitible iron release. Of the compounds investigated, those with a single electron reduction potential less than that of ferritin were able to release ferritin iron.

Mitochondrial sulfhydryl group modification by adriamycin aglycones

Induction of Ca2+ release from isolated, preloaded rat heart mitochondria by low concentrations (less than 5 micrM) of adriamycin aglycones, has recently been reported [(1988) Biochem. Pharmacol. 37, 803]. Ca2+ release occurs via a generalized, Ca2+-dependent increase in the permeability of the inner mitochondrial membrane to small molecules. The process is antagonized by dithiothreitol, suggesting thiol involvement. This communication demonstrates modification of mitochondrial sulfhydryl groups, detected as decreased 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) reactivity, by adriamycin aglycones. Ca2+ release and sulfhydryl modification are shown to depend similarly on aglycone concentration and on the C-7 substituent of the anthracycline ring. In addition, DTNB elicits Ca2+ release. It can therefore be proposed that adriamycin aglycones alter mitochondrial membrane permeability by altering mitochondrial thiol status.

Na+-independent release of Ca2+ from rat heart mitochondria. Induction by adriamycin aglycone

The effect of adriamycin aglycones on Ca2+ retention by isolated, preloaded rat heart mitochondria was assessed. After an initial lag, which decreased with increasing drug concentration, the 7-hydroxy-aglycone (5-20 microM) triggered Ca2+ release. Aglycone-induced Ca2+ release was correlated with Ca2+-dependent mitochondrial swelling, Ca2+-dependent collapse of the mitochondrial membrane potential, Ca2+-dependent oxidation of mitochondrial pyridine nucleotides, and a transition from the condensed to the orthodox configuration. Aglycone-induced Ca2+ release was inhibited by dibucaine, dithiothreitol, ATP, and bovine serum albumin. It can be concluded, therefore, that aglycone-induced Ca2+ release reflects the Ca2+-dependent increase in the permeability of the inner mitochondrial membrane to solutes of molecular weight less than 1000 which has been observed with other triggering agents [R. A. Haworth and D. R. Hunter, Archs Biochem. Biophys. 195, 460 (1979); I. Al-Nasser and M. Crompton, Biochem. J. 239, 19 (1986)]. In particular, the 7-hydroxy-aglycone decreased the amount of Ca2+ required to trigger the permeability increase. No effect of the aglycone on Ca2+ uptake could be discerned. 7-Deoxy-adriamycin aglycone, the more prominent biological metabolite of adriamycin, was similarly effective in inducing Ca2+ release, and both aglycones were substantially more effective than the parent drug. Adriamycin and related anthracyclines are potent antineoplastic agents, the clinical use of which is limited by severe cardiotoxicity. These results suggest that aglycone formation and the resultant disruption of both cellular Ca2+ homeostasis and metabolite compartmentation may mediate anthracycline cardiotoxicity.

Increased anti-tumor effect of adriamycin-loaded albumin microspheres is associated with anaerobic bioreduction of drug in tumor tissue

Anti-tumor activity and fate of adriamycin incorporated into biodegradable albumin microspheres was examined in vivo after direct intratumoral injection. Adriamycin in microspherical form displayed superior anti-tumor activity to a comparable dose of drug in solution. This was associated at later time points (40 hr, 50 hr and 72 hr after injection) with higher median parent drug concentrations in tumor tissue (4.1, 3.6, 2.6 micrograms/g respectively for microspheres and 1.6, 1.7 and 1.0 micrograms/g for solution) and the consistent detection of 7-deoxyaglycone metabolites, end products of reduction of adriamycin under anaerobic conditions (1.1, 1.0, 1.0 micrograms/g respectively for microspheres and less than 0.1 micrograms/g at all time points for solution). It is generally considered that the redox properties of anthracyclines are responsible for their toxicity to normal tissues whereas other mechanisms are responsible for antineoplastic activity. In this study we show that inducing metabolism of Adriamycin via reductive pathways is associated with increased anti-tumor effect.

Mechanism of action of some inhibitors of endothelium-derived relaxing factor

The mechanism of the inhibitory action of phenidone, 3-amino-1-[m-(trifluoromethyl)phenyl]-2-pyrazoline (BW 755C), dithiothreitol, hydroquinone, and pyrogallol on the vascular relaxation induced by endothelium-derived relaxing factor (EDRF) was investigated. EDRF was released from porcine aortic endothelial cells in culture and bioassayed on a cascade of superfused rabbit aortic strips. These compounds inhibited EDRF-induced relaxation of vascular strips, without affecting the relaxation induced by glyceryl trinitrate, and their inhibitory potency was markedly attenuated (by more than 1 order of magnitude) by the addition of superoxide dismutase (5-15 units/ml) or oxidized cytochrome c (20-40 microM) but not by catalase (30 units/ml) or heat-inactivated superoxide dismutase. These data indicate that the above five inhibitors inactivate EDRF through the formation of superoxide ions, which have recently been shown to destroy EDRF. The inhibition of EDRF by these compounds is therefore attributable to their redox properties rather than to any specific biological action.