Lipid hydroperoxide generation, turnover, and effector action in biological systems

Lipid peroxidation is a well known example of oxidative damage in cell membranes, lipoproteins, and other lipid-containing structures. Peroxidative modification of unsaturated phospholipids, glycolipids, and cholesterol can occur in reactions triggered by i) free radical species such as oxyl radicals, peroxyl radicals, and hydroxyl radicals derived from iron-mediated reduction of hydrogen peroxide or ii) non-radical species such as singlet oxygen, ozone, and peroxynitrite generated by the reaction of superoxide with nitric oxide. Lipid hydroperoxides (LOOHs) are prominent non-radical intermediates of lipid peroxidation whose identification can often provide valuable mechanistic information, e.g., whether a primary reaction is mediated by singlet oxygen or oxyradicals. Certain cholesterol-derived hydroperoxides (ChOOHs) have been used very effectively in this regard, both in model systems and cells. Being more polar than parent lipids, LOOHs perturb membrane structure/function and can be deleterious to cells on this basis alone. However, LOOHs can also participate in redox reactions, the nature and magnitude of which often determines whether peroxidative injury is exacerbated or prevented. Exacerbation may reflect iron-catalyzed one-electron reduction of LOOHs, resulting in free radical-mediated chain peroxidation, whereas prevention may reflect selenoperoxidase-catalyzed two-electron reduction of LOOHs to relatively non-toxic alcohols. LOOH partitioning between these two pathways in an oxidatively stressed cell is still poorly understood, but recent cell studies involving various ChOOHs have begun to shed light on this important question. An aspect of related interest that is under intensive investigation is lipid peroxidation/LOOH-mediated stress signaling, which may evoke a variety of cellular responses, ranging from induction of antioxidant enzymes to apoptotic death. Ongoing exploration of these processes will have important bearing on our understanding of disease states associated with peroxidative stress.

Cholesterol Hydroperoxide Co-trafficking in Testosterone-generating Leydig Cells: GPx4 Inhibition of Cytotoxic and Anti-steroidogenic Effects

Trafficking of intracellular cholesterol (Ch) to and into mitochondria of steroidogenic cells is required for steroid hormone biosynthesis. This trafficking is typically mediated by one or more proteins of the steroidogenic acute regulatory (StAR) family. Our previous studies revealed that 7-OOH, a redox-active cholesterol hydroperoxide, could be co-trafficked with Ch to/into mitochondria of MA-10 Leydig cells, thereby inducing membrane lipid peroxidation (LPO) which impaired progesterone biosynthesis. These negative effects of 7-OOH were inhibited by endogenous selenoperoxidase GPx4, indicating that this enzyme could protect against 7-OOH-induced oxidative damage/dysfunction. In the present study, we advanced our Leydig focus to cultured murine TM3 cells and then to primary cells from rat testis, both of which produce testosterone. Using a fluorescent probe, we found that extensive free radical-mediated LPO occurred in mitochondria of stimulated primary Leydig cells during treatment with liposomal Ch+7-OOH, resulting in a significant decline in testosterone output relative to that with Ch alone. Strong enhancement of LPO and testosterone shortfall by RSL3 (a GPx4 inhibitor) and reversal thereof by Ebselen (a GPx4 mimetic), suggested that endogenous GPx4 was playing a key antioxidant role. 7-OOH in increasing doses was also cytotoxic to these cells, RSL3 exacerbating this in Ebselen-reversable fashion. Moreover, GPx4 knockdown increased cell sensitivity to LPO with reduced testosterone output. These findings, particularly with primary Leydigs (which best represent cells in intact testis) suggest that GPx4 plays a key protective role against peroxidative damage/dysfunction induced by 7-OOH co-trafficking with Ch.

The Negative Impact of Cancer Cell Nitric Oxide on Photodynamic Therapy

Numerous studies have shown that low-flux nitric oxide (NO) in tumors produced mainly by inducible nitric oxide synthase (iNOS/NOS2) can signal for angiogenesis, inhibition of apoptosis, and promotion of cell growth, migration, and invasion. Studies in the authors' laboratory have revealed that iNOS-derived NO in various cancer cell types elicits resistance to cytotoxic photodynamic therapy (PDT) and moreover endows PDT-surviving cells with more aggressive proliferation and migration/invasion. In this chapter, we describe how cancer cell iNOS/NO in vitro can be monitored in different PDT model systems (e.g., a targeted cell-bystander cell model) and how pharmacologic interference with basal and PDT-upregulated iNOS/NO can significantly improve PDT outcomes.

Protective action of phospholipid hydroperoxide glutathione peroxidase against membrane-damaging lipid peroxidation. In situ reduction of phospholipid and cholesterol hydroperoxides

The general reactivity of membrane lipid hydroperoxides (LOOHs) with the selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX) has been investigated. When human erythrocyte ghosts (lipid content: 60 wt % phospholipid; 25 wt % cholesterol) were treated with GSH/PHGPX subsequent to rose bengal-sensitized photoperoxidation, iodometrically measured LOOHs were totally reduced to alcohols. Similar treatment with the classic glutathione peroxidase (GPX) produced no effect unless the peroxidized membranes were preincubated with phospholipase A2 (PLA2). However, under these conditions, no more than approximately 60% of the LOOH was reduced; introduction of PHGPX brought the reaction to completion. Thin layer chromatographic analyses revealed that the GPX-resistant (but PHGPX-reactive) LOOH was cholesterol hydroperoxide (ChOOH) consisting mainly of the 5 alpha (singlet oxygen-derived) product. Membrane ChOOHs were reduced by GSH/PHGPX to species that comigrated with borohydride reduction products (diols). Sensitive quantitation of PHGPX-catalyzed ChOOH reduction was accomplished by using [14C]cholesterol-labeled ghosts. Kinetic analyses indicated that the rate of ChOOH decay was approximately 1/6 that of phospholipid hydroperoxide decay. Photooxidized ghosts underwent a large burst of free radical-mediated lipid peroxidation when incubation with ascorbate/iron or xanthine/xanthine oxidase/iron. These reactions were only partially inhibited by PLA2/GSH/GPX treatment, but totally inhibited by GSH/PHGPX treatment, consistent with complete elimination of LOOHs in the latter case. These findings provide important clues as to how ChOOHs are detoxified in cells and add new insights into PHGPX's protective role.

Delayed hyperresistance of endothelial cells to photodynamic inactivation after contact with hemin

Hemin (ferriprotoporphyrin IX), the oxidized prosthetic group of hemoglobin, is a potential source of prooxidant iron in heavily vascularized tumors. We have evaluated hemin's effects on photodynamic inactivation of bovine artery endothelial cells, using a partially purified oligomeric fraction of hematoporphyrin derivative (HPD-A) as the sensitizing agent. Confluent cells in 5% serum/RPMI medium showed a progressive loss of thiazolyl blue (MTT)-detectable viability when irradiated with broadband visible light in the presence of HPD-A. Cells pretreated with desferrioxamine (DFO) were substantially less sensitive to photokilling, implying that non-heme iron plays a role in cytotoxic activity. Hemin (10-20 microM) had remarkably different effects on photokilling, depending on the time interval between adding it to cells and exposing them to photodynamic action. For example, cells were more sensitive when photostressed immediately after 1 h hemin treatment and washing but much more resistant when photostressed 23 h later. Similar responses were observed when cells were challenged with glucose oxidase. Immunoblot analysis following hemin treatment revealed a progressive induction of the heavy (H) subunit of ferritin that paralleled the development of hyperresistance. After incubation with saturating levels of the synthetic iron donor [55Fe]ferric-8-hydroxyquinoline, hemin-stimulated cells contained about four times more immunoprecipitable ferritin 55Fe than controls. This is consistent with the notion that sequestration of toxic iron as a result of induction of H-chain-enriched ferritin is a key factor in hyperresistance. Inflammatory injury in tumor vasculatures could expose endothelial and neoplastic cells to chronic hemoglobin-derived iron. Consequent upregulation of ferritin could impact negatively on the efficacy of photodynamic therapy and other oxidant-based cancer therapies.

Singlet oxygen adducts of cholesterol: photogeneration and reductive turnover in membrane systems

Identification of signature products provides a powerful means for establishing whether singlet molecular oxygen (1O2) is a reactive intermediate in a photodynamic process. This approach is particularly attractive for biological systems in which direct physical measurement is difficult because of the short lifetime of 1O2. Among the many possible reporter molecules in a target system, cholesterol (Ch) has the advantage of affording a limited number of readily distinguishable oxidation products, among which are the hydroperoxides 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), 3 beta-hydroxycholest-4-ene-6 alpha-hydroperoxide (6 alpha-OOH) and 3 beta-hydroxycholest-4-ene-6 beta-hydroperoxide (6 beta-OOH) that derive specifically from 1O2 addition. The purpose of this study was to compare these species in terms of (1) rates of accumulation in photodynamically treated liposomal membranes; (2) susceptibility to iron-mediated 1 e- reduction that triggers chain peroxidative damage; (3) susceptibility to selenoperoxidase (phospholipid hydroperoxide glutathione peroxidase [PHGPX])-mediated 2 e- reduction that protects against such damage and (4) relative toxicity to mammalian cells. Our results indicate that 5 alpha-OOH is photogenerated at a much greater initial rate than 6 alpha-OOH or 6 beta-OOH. Although liposomal 5 alpha-OOH, 6 alpha-OOH, and 6 beta-OOH exhibit similar first-order decay kinetics during iron/ascorbate treatment, the former decays much more slowly during GSH/PHGPX treatment, and is more toxic to L1210 cells. These and related findings suggest that 5 alpha-OOH is potentially the most damaging ChOOH to arise in photodynamically treated cells.

Lethal damage to endothelial cells by oxidized low density lipoprotein: role of selenoperoxidases in cytoprotection against lipid hydroperoxide- and iron-mediated reactions

Oxidized low density lipoprotein (LDLox) is believed to be an important contributor to endothelial cytodamage and atherogenesis. The purpose of this study was to examine the role of glutathione (GSH) and GSH-dependent selenoperoxidases in cytoprotection against the damaging effects of LDLox. When irradiated in the presence of a phthalocyanine sensitizing dye, human LDL accumulated chromatographically detectable and iodometrically measurable lipid hydroperoxides (LOOHs). Photogenerated LDLox caused lethal damage to bovine aortic endothelial (BAE) cells in vitro, as determined by lactate dehydrogenase release and inhibition of thiazolyl blue reduction. When depleted of GSH by buthionine sulfoximine treatment, BAE cells became more sensitive to LDLox. Cells grown in 2% serum/DME-HAM's F-12 medium without added selenium [Se(-) cells] exhibited far lower GSH-peroxidase and phospholipid hydroperoxide GSH-peroxidase activities than selenium-supplemented controls [Se(+) cells], and were much more sensitive to oxidative injury induced by t-butyl hydroperoxide, liposomal cholesterol hydroperoxides, and LDLox. Preincubation of LDLox with GSH and Ebselen (a selenoperoxidase mimetic) resulted in a dramatic reduction in both LOOH content and cytotoxicity. Moreover, treating Se(-) cells themselves with Ebselen substantially restored their resistance to LDLox-induced damage. LDLox toxicity to Se(-) cells was strongly inhibited by desferrioxamine and stimulated by ferric-8-hydroxyquinoline (a lipophilic chelate), indicating that iron is an active participant in oxidative damage. These results demonstrate that the GSH-dependent selenoperoxidase(s) play(s) an important role in cellular defense against oxidized low density lipoprotein, presumably by detoxifying lipid hydroperoxides and thereby preventing their iron-catalyzed decomposition to damaging free radical intermediates.

Abstract 132: Cytoprotective induction of nitric oxide synthase in a cellular model of 5-aminolevulinic acid-based photodynamic therapy

Photodynamic therapy (PDT) employs a photosensitizing agent, molecular oxygen, and visible light to produce reactive oxygen species that kill tumor and tumor vasculature cells. Nitric oxide (NO) produced by these cells could be pro-carcinogenic by inhibiting apoptosis or promoting angiogenesis and tumor growth. Our previous studies showed that NO from a chemical donor made COH-BR1 breast tumor cells hyperresistant to apoptotic photokilling induced by photoactivation of 5-aminolevulinic acid (ALA)-generated protoporphyrin IX localized in mitochondria. Hyperresistance was associated with inhibition of proapoptotic JNK and p38α MAP kinase phosphorylation-activation. The purpose of this study was to determine whether tumor cells upregulate NOS-generated NO as a cytoprotective measure during PDT.COH-BR1 cells sensitized in mitochondria with ALA-derived protoporphyrin IX died apoptotically following irradiation as measured by Hoechst staining. Western analysis revealed that inducible nitric oxide synthase (iNOS) was upregulated >3-fold within 4 h after ALA/light treatment, while other NOS isoforms were unaffected. Exposing cells to the iNOS inhibitor, 1400W during photochallenge enhanced caspase-3/7 activation and apoptotic killing up to 2-3-fold while substantially reducing chemiluminescence-assessed NO production, suggesting that this NO was cytoprotective. Consistently, the NO scavenger cPTIO enhanced ALA/light-induced caspase-3/7 activation and apoptotic kill by >2.5-fold. Of added significance, cells could be rescued from 1400W-exacerbated apoptosis by an exogenous NO donor, spermine-NONOate. Using Hoechst and TUNEL staining, we found that short hairpin RNA (shRNA)-induced knockdown of iNOS enhanced the apoptotic kill of ALA/light treated COH-BR1 cells. ALA/light-treated COH-BR1cells exhibited a transient post-irradiation activation of JNK and p38α as measured by Western blot analysis. Consistently, both effects were intensified and prolonged by 1400W. The survival MAP kinase ERK1/2 was deactivated more rapidly when 1400W was present during a PpIX/light challenge. Similar effects on MAP kinase activation/deactivation were observed for iNOS knockdown cells under photostress cinfirming iNOS's protective role. As demonstrated for non-photodynamic stress systems, NO could have interfered with apoptosis by inactivating participating MAP kinases and/or caspases. This is the first reported evidence for increased tumor cell resistance due to iNOS upregulation in a PDT model. Our findings indicate that stress-elicited NO in PDT-treated tumors could compromise therapeutic efficacy, and suggest NOS-based pharmacologic interventions for preventing this. (Supported by NIH Grant CA70823)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 132.

Hyper-Aggressiveness of Bystander Cells in an Anti-Tumor Photodynamic Therapy Model: Role of Nitric Oxide Produced by Targeted Cells

When selected tumor cells in a large in vitro population are exposed to ionizing radiation, they can send pro-survival signals to non-exposed counterparts (bystander cells). If there is no physical contact between irradiated and bystander cells, the latter respond to mediators from targeted cells that diffuse through the medium. One such mediator is known to be nitric oxide (NO). It was recently discovered that non-ionizing anti-tumor photodynamic therapy (PDT) can also elicit pro-survival/expansion bystander effects in a variety of human cancer cells. A novel silicone ring-based approach was used for distinguishing photodynamically-targeted cells from non-targeted bystanders. A key finding was that NO from upregulated iNOS in surviving targeted cells diffused to the bystanders and caused iNOS/NO upregulation there, which in turn stimulated cell proliferation and migration. The intensity of these responses depended on the extent of iNOS/NO induction in targeted cells of different cancer lines. Moreover, the responses could be replicated using NO from the chemical donor DETA/NO. This review will focus on these and related findings, their negative implications for clinical PDT, and how these might be averted by using pharmacologic inhibitors of iNOS activity or transcription.

Role of lipid peroxidation in hematoporphyrin derivative-sensitized photokilling of tumor cells: protective effects of glutathione peroxidase

The ability of cells to detoxify lipid hydroperoxides (LOOHs) generated by hematoporphyrin derivative (HPD)-sensitized photooxidation was investigated for the first time. The general importance of glutathione in cytoprotection was confirmed by showing that murine L1210 cells were more sensitive to the lethal effects of HPD plus red light after being treated with buthionine sulfoximine. The specific role of Se-dependent glutathione peroxidase was investigated by using L1210 cells that were grown in Se-deficient media. Glutathione peroxidase activity of such cells was typically less than 5% of that exhibited by Se-replete cells. When examined by means of dye exclusion or clonogenic assay, Se-deficient cells were dramatically more sensitive to HPD-mediated photokilling than normal counterparts. Impaired metabolism of hydrogen peroxide was ruled out as a possible cause of enhanced photokilling, since added catalase had no protective effect on Se-deficient cells. Iodometric analysis of lipid extracts from photooxidized cells indicated a significantly greater rate of LOOH accumulation as a result of Se depletion. Moreover, when depleted cells were incubated in the dark after a short period of photo-peroxidation, LOOH decay was markedly slower than in controls. Similar results were obtained with human CaSki cells derived from cervical carcinoma. It is apparent from these results that lipid peroxidation plays an important role in tumor cell eradication by HPD/phototherapy, and that glutathione peroxidase serves as a natural protectant against photokilling by catalyzing the reduction of LOOHs.

Purification of calciferol-binding proteins from kidney: physicochemical and immunological properties

The calciferol-binding system of rat kidney cytosol has been purified and is shown to consist of two proteins, each capable of binding either 25-hydroxy-vitamin D3 (25-OH-D3) or 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The two proteins, designated A and B, have similar sedimentation coefficients (S20w) of 5.2 S. Component A binds 25-OH-D3 with a dissociation constant (Kd) of 10(-7) M while component B binds 1,25-(OH)2D3 with a Kd of 1.6 x 10(-8) M. The estimated molecular weights (Mr) of the two proteins are 105,000 for component A and 250,000 for component B. Amino acid analyses revealed that glutamic acid is the most abundant residue in both proteins, comprising 12% of the total number of amino acid residues. Immunodiffusion test using commercial anti-human serum group-specific protein antiserum gave a precipitin reaction when purified rat serum calciferol-binding protein was used as an antigen, but no reactions could be detected with proteins A and B. This result significantly eliminated the possibility of the presence of the rat serum binding protein in either of the purified kidney proteins. In contrast, anti-rat serum calciferol-binding protein antiserum prepared in rabbits interacted with the rat serum and kidney proteins. This result suggests that the antigenic determinants recognized by the antiserum against the rat serum calciferol-binding protein appear to be similar to those recognized in the kidney proteins A and B. Immunoelectrophoresis of the three rat proteins demonstrated dissimilar electrophoretic mobilities with the serum protein showing the least mobility, a property consistent with its higher lysine content relative to proteins A and B.

Interaction of inhibitors with muscle phosphofructokinase

The interaction of several inhibitors with muscle phosphofructokinase has been studied by both equilibrium binding measurements and kinetic analysis. At low concentrations of citrate a maximum of 1 mol is bound per mol of enzyme protomer. Tight binding requires MgATP and very weak binding is observed in the absence of either magnesium ion or ATP. ITP at low concentrations cannot replace ATP. In the presence of MgATP and at pH 7.0, the dissociation constant for the enzyme-citrate complex is 20 muM. At 50 muM citrate and excess magnesium ion, the concentration of ATP required to give half-maximal binding of citrate is approximately 3 muM . Both P-enolpyruvate and 3-P-glycerate compete for the binding of citrate and the estimated Ki values are 480 and 52 muM, respectively. Creatine-P, another inhibitor of muscle phosphofructokinase, does not compete with the binding of citrate. Measurement of the equilibrium binding of ATP shows that citrate, 3-P-glycerate, P-enolpyruvate, and creatine-P all increase the affinity of enzyme for MgATP with the concentration required to give an effect increasing in the order given. In kinetic studies, citrate, 3-P-glycerate and P-enolpyruvate each act synergistically with ATP to inhibit the phosphofructokinase reaction. This is indicated by the observation that the three metabolites do not inhibit the enzyme with ITP as the phosphoryl donor and that they inhibit at ATP concentrations that are not themselves inhibitory. Furthermore, the sensitivity to the inhibitors increases with increasing ATP concentrations. Striking differences in the extent of inhibition can be seen by varying the order of addition of assay components. Preincubation of the enzyme with ATP and citrate, 3-P-glycerate, or P-enolpyruvate results in greater inhibition than when the inhibitor is added after the reaction is started with fructose-6-P. Furthermore, the inhibition is reversed partially 10 to 15 min after the addition of fructose-6-P. This phenomenon is particularly striking with creatine-P as the inhibitor. Very high concentrations of this inhibitor are required to show any effect if the inhibitor is added after fructose-6-P. These effects are interpreted as reflecting slow conformational changes between an active form with high affinity for fructose-6-P and an inactive, or less active, conformation that binds the inhibitors. Citrate, 3-P-glycerate, P-enolpyruvate, and creatine-P increase the rate of the phosphofructokinase at subsaturating concentrations of MgITP. The results indicate a common binding site on the enzyme for citrate, 3-P-glycerate, and P-enolpyruvate that is distinct from the ATP inhibitory site. An additional site (or sites) for creatine-P is indicated. All four inhibitors act synergistically with ATP by increasing the affinity of the enzyme for MgATP at an inhibitory site. The inhibitors appear also to increase the affinity of the catalytic nucleoside triphosphate site for substrate.

Role of nitric oxide in hyper-aggressiveness of tumor cells that survive various anti-cancer therapies

Low level nitric oxide (NO) produced by inducible NO synthase (iNOS) in many malignant tumors is known to play a key role in the survival and proliferation of tumor cells. NO can also induce or augment resistance to anti-tumor treatments such as platinum-based chemotherapy (CT), ionizing radiotherapy (RT), and non-ionizing photodynamic therapy (PDT). In each of these treatments, tumor cells that survive the challenge may exhibit a striking increase in NO-dependent proliferative, migratory, and invasive aggressiveness compared with non-challenged controls. Moreover, NO from cells directly targeted by PDT can often stimulate aggressiveness in non- or poorly targeted bystander cells. Although NO-mediated resistance to many of these therapies is fairly-well recognized by now, the hyper-aggressiveness of surviving cells and bystander counterparts is not. We will focus on these negative aspects in this review, citing examples from the PDT, CT, and RT publications. Increased aggressiveness of cells that escape therapeutic elimination is a concern because it could enhance tumor progression and metastatic dissemination. Pharmacologic approaches for suppressing these negative responses will also be discussed, e.g., administering inhibitors of iNOS activity or iNOS expression as therapeutic adjuvants.

Upregulation of iNOS/NO in Cancer Cells That Survive a Photodynamic Challenge: Role of No in Accelerated Cell Migration and Invasion

Anti-tumor photodynamic therapy (PDT) is a unique modality that employs a photosensitizer (PS), PS-exciting light, and O2 to generate cytotoxic oxidants. For various reasons, not all malignant cells in any given tumor will succumb to a PDT challenge. Previous studies by the authors revealed that nitric oxide (NO) from inducible NO synthase (iNOS/NOS2) plays a key role in tumor cell resistance and also stimulation of migratory/invasive aggressiveness of surviving cells. iNOS was the only NOS isoform implicated in these effects. Significantly, NO from stress-upregulated iNOS was much more important in this regard than NO from preexisting enzymes. Greater NO-dependent resistance, migration, and invasion was observed with at least three different cancer cell lines, and this was attenuated by iNOS activity inhibitors, NO scavengers, or an iNOS transcriptional inhibitor. NO diffusing from PDT-targeted cells also stimulated migration/invasion potency of non-targeted bystander cells. Unless counteracted by appropriate measures, all these effects could seriously compromise clinical PDT efficacy. Here, we will review specific examples of these negative side effects of PDT and how they might be suppressed by adjuvants such as NO scavengers or inhibitors of iNOS activity or expression.

Participation of lipids in the tumor response to photodynamic therapy and its exploitation for therapeutic gain

Hydroperoxides of unsaturated membrane lipids (LOOHs) are the most abundant non-radical intermediates generated by photodynamic therapy (PDT) of soft tissues such as tumors and have far longer average lifetimes than singlet oxygen or oxygen radicals formed during initial photodynamic action. LOOH-initiated post-irradiation damage to remaining membrane lipids (chain peroxidation) or to membrane-associated proteins remains largely unrecognized. Such after-light processes could occur during clinical oncological PDT, but this is not well-perceived by practitioners of this therapy. In general, the pivotal influence of lipids in tumor responses to PDT needs to be better appreciated. Of related importance is the fact that most malignant tumors have dramatically different lipid metabolism compared with healthy tissues, and this too is often ignored. The response of tumors to PDT appears especially vulnerable to manipulations within the tumor lipid microenvironment. This can be exploited for therapeutic gain with PDT, as exemplified here by the combined treatment with the antitumor lipid edelfosine.

Damaging effects of oxygen radicals on resealed erythrocyte ghosts

Resealed ghosts of human erythrocytes are sensitive to oxidative damage induced by xanthine oxidase acting on xanthine in the presence of iron. Damage was assessed in terms of lipid peroxidation and increased permeation of trapped markers, Na+ and glucose-6-P. Key findings are as follows. (a) Marker efflux from xanthine/xanthine oxidase/iron-treated ghosts accelerated after a lag, Na+ emerging far ahead of glucose-6-P. (b) Both effluxes and lipid peroxidation were stimulated by Fe(III) in a dose-dependent fashion and inhibited by chelating agents. (c) The antioxidant butylated hydroxytoluene effectively halted lipid peroxidation and net glucose-6-P efflux, but slowed Na+ efflux only partially. (d) Lipid peroxidation and marker release could be completely inhibited by superoxide dismutase or catalase, indicating that O2- and H2O2 are both required, possibly as precursors of OH. via the iron-catalyzed Haber-Weiss reaction (O2- + H2O2 leads to OH- + OH. + O2). (e) OH. scavengers, e.g. ethanol, mannitol, choline, had no protective effect against marker efflux and lipid peroxidation. Yet these agents did intercept OH. in the bulk medium, since they inhibited the degradation of 2-deoxyribose added as an extramembranous OH. probe. It is proposed that OH. produced on the membrane at iron binding sites reacts so rapidly with target molecules that scavengers cannot compete. (f) Desferrioxamine abolished all effects, including net egress of Na+. EDTA, while totally inhibitory toward lipid peroxidation and glucose-6-P release, diminished Na+ release partially, changing it to first order, approximately 3-fold faster than background. The latter response was totally inhibited by catalase, but only marginally by superoxide dismutase. This and other evidence suggests that different forms of membrane damage are responsible for enhanced permeation of the two markers; although glucose-6-P depends on lipid peroxidation, Na+ does not, certainly when EDTA is present.

Hyperresistance of leukemia cells to photodynamic inactivation after long-term exposure to hemin

Merocyanine 540 (MC540)-mediated photodynamic action is a novel approach for purging tumor cells from autologous remission bone marrow explants. The purpose of this study was to evaluate the effects of hemin (ferriprotoporphyrin IX), a potential source of pro-oxidant iron in bone marrow, on in vitro photodynamic inactivation of leukemia cells. Murine L1210 cells exhibited a progressive loss of clonogenicity when irradiated with broad-band visible light in the presence of MC540. Hemin had strikingly different effects on photokilling, depending on its contact time with cells, eliciting a sizable decrease in resistance after short-term (30-min) contact but a marked increase in resistance after long-term (24-h) contact. Similar trends were observed when cells were challenged with glucose/glucose oxidase, indicating that the responses apply to more than one type of oxidative stress. Immunoblot analyses revealed that the levels of inducible heme oxygenase (HO-1) and ferritin heavy (H) chain were substantially elevated 24 h after hemin addition. HO-1 increased relatively rapidly and maximized within 4 h after adding hemin, whereas H-ferritin increased more slowly in parallel with the development of hyperresistance, maximizing after 24-36 h. Desferrioxamine, an avid iron chelator, had no effect on HO-1 induction but inhibited both ferritin induction and the increase in cell resistance, suggesting that HO-mediated release of iron from hemin was necessary for triggering these responses. Spleen apoferritin was taken up by L1210 cells and strongly inhibited photokilling, further implicating ferritin involvement in hyperresistance. Photokilling was accompanied by free radical-mediated lipid peroxidation (thiobarbituric acid reactivity), which could be suppressed substantially by 24-h hemin preincubation. A plausible explanation for the long-term effects of hemin is that excess H-ferritin generated as a result of iron-regulatory protein deactivation sequesters toxic iron, which might otherwise catalyze damaging lipid peroxidation. Chronic oxidative release of hemin from bone marrow erythroid cells could compromise the efficacy of photopurging by making tumor cells more tolerant to photooxidative insult.