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.

Pro-Tumor Activity of Endogenous Nitric Oxide in Anti-Tumor Photodynamic Therapy: Recently Recognized Bystander Effects

Various studies have revealed that several cancer cell types can upregulate inducible nitric oxide synthase (iNOS) and iNOS-derived nitric oxide (NO) after moderate photodynamic treatment (PDT) sensitized by 5-aminolevulinic acid (ALA)-induced protoporphyrin-IX. As will be discussed, the NO signaled cell resistance to photokilling as well as greater growth and migratory aggressiveness of surviving cells. On this basis, it was predicted that diffusible NO from PDT-targeted cells in a tumor might enhance the growth, migration, and invasiveness of non- or poorly PDT-targeted bystander cells. This was tested using a novel approach in which ALA-PDT-targeted cancer cells on a culture dish were initially segregated from non-targeted bystander cells of the same type via impermeable silicone-rimmed rings. Several hours after LED irradiation, the rings were removed, and both cell populations were analyzed in the dark for various responses. After a moderate extent of targeted cell killing (~25%), bystander proliferation and migration were evaluated, and both were found to be significantly enhanced. Enhancement correlated with iNOS/NO upregulation in surviving PDT-targeted cancer cells in the following cell type order: PC3 > MDA-MB-231 > U87 > BLM. If occurring in an actual PDT-challenged tumor, such bystander effects might compromise treatment efficacy by stimulating tumor growth and/or metastatic dissemination. Mitigation of these and other negative NO effects using pharmacologic adjuvants that either inhibit iNOS transcription or enzymatic activity will be discussed.

Trafficking of oxidative stress-generated lipid hydroperoxides: pathophysiological implications

Lipid hydroperoxides (LOOHs) are reactive intermediates that arise during peroxidation of unsaturated phospholipids, glycolipids and cholesterol in biological membranes and lipoproteins. Non-physiological lipid peroxidation (LPO) typically occurs under oxidative stress conditions associated with pathologies such as atherogenesis, neurodegeneration, and carcinogenesis. As key intermediates in the LPO process, LOOHs are susceptible to one-electron versus two-electron reductive turnover, the former exacerbating membrane or lipoprotein damage/dysfunction and the latter diminishing it. A third possible LOOH fate is translocation to an acceptor membrane/lipoprotein, where one- or two-electron reduction may then ensue. In the case of cholesterol (Ch)-derived hydroperoxides (ChOOHs), translocation can be specifically stimulated by StAR family trafficking proteins, which are normally involved in Ch homeostasis and Ch-mediated steroidogenesis. In this review, we discuss how these processes can be impaired by StAR-mediated ChOOH and Ch co-trafficking to mitochondria of vascular macrophages and steroidogenic cells, respectively. The protective effects of endogenous selenoperoxidase, GPx4, are also discussed. This is the first known example of detrimental ChOOH transfer via a natural Ch trafficking pathway and inhibition thereof by GPx4.

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 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.

Anti-steroidogenic effects of cholesterol hydroperoxide trafficking in MA-10 Leydig cells: Role of mitochondrial lipid peroxidation and inhibition thereof by selenoperoxidase GPx4

Steroid hormone synthesis in steroidogenic cells requires cholesterol (Ch) delivery to/into mitochondria via StAR family trafficking proteins. In previous work, we discovered that 7-OOH, an oxidative stress-induced cholesterol hydroperoxide, can be co-trafficked with Ch, thereby causing mitochondrial redox damage/dysfunction. We now report that exposing MA-10 Leydig cells to Ch/7-OOH-containing liposomes (SUVs) results in (i) a progressive increase in fluorescence probe-detected lipid peroxidation in mitochondrial membranes, (ii) a reciprocal decrease in immunoassay-detected progesterone generation, and ultimately (iii) loss of cell viability with increasing 7-OOH concentration. No significant effects were observed with a phospholipid hydroperoxide over the same concentration range. Glutathione peroxidase GPx4, which can catalyze lipid hydroperoxide detoxification, was detected in mitochondria of MA-10 cells. Mitochondrial lipid peroxidation and progesterone shortfall were exacerbated when MA-10 cells were treated with Ch/7-OOH in the presence of RSL3, a GPx4 inhibitor. However, Ebselen, a selenoperoxidase mimetic, substantially reduced RSL3's negative effects, thereby partially rescuing the cells from peroxidative damage. These findings demonstrate that co-trafficking of oxidative stress-induced 7-OOH can disable steroidogenesis, and that GPx4 can significantly protect against this.

Intermembrane Translocation of Photodynamically Generated Lipid Hydroperoxides: Broadcasting of Redox Damage†

Lipid hydroperoxides (LOOHs), including cholesterol- and phospholipid-derived species, are reactive intermediates that arise during photosensitized peroxidation of unsaturated lipids in biological membranes. These intermediates may appear in cancer cell membranes during anti-tumor photodynamic therapy (PDT). Photodynamically generated LOOHs have several different fates, including (a) iron-catalyzed one-electron reduction to free radical species which trigger damaging chain peroxidation reactions, (b) selenoperoxidase-catalyzed two-electron reduction to redox-inert alcohols (LOHs), and (c) spontaneous or protein-mediated translocation to other lipid membrane compartments where (a) or (b) may take place. These different LOOH fates will be described in this review, but with special attention to category (c), which the authors were the first to describe and characterize. Seminal early findings on cholesterol hydroperoxide (ChOOH) translocation and its potential negative consequences will be discussed. In reviewing this work, we wish to congratulate Jean Cadet, for his many outstanding accomplishments as a photobiologist and P&P editor.

Pathophysiological potential of lipid hydroperoxide intermembrane translocation: Cholesterol hydroperoxide translocation as a special case

Peroxidation of unsaturated phospholipids, glycolipids, and cholesterol in biological membranes under oxidative stress conditions can underlie a variety of pathological conditions, including atherogenesis, neurodegeneration, and carcinogenesis. Lipid hydroperoxides (LOOHs) are key intermediates in the peroxidative process. Nascent LOOHs may either undergo one-electron reduction to exacerbate membrane damage/dysfunction or two-electron reduction to attenuate this. Another possibility is LOOH translocation to an acceptor site, followed by either of these competing reductions. Cholesterol (Ch)-derived hydroperoxides (ChOOHs) have several special features that will be highlighted in this review. In addition to being susceptible to one-electron vs. two-electron reduction, ChOOHs can translocate from a membrane of origin to another membrane, where such turnover may ensue. Intracellular StAR family proteins have been shown to deliver not only Ch to mitochondria, but also ChOOHs. StAR-mediated transfer of free radical-generated 7-hydroperoxycholesterol (7-OOH) results in impairment of (a) Ch utilization in steroidogenic cells, and (b) anti-atherogenic reverse Ch transport in vascular macrophages. This is the first known example of how a peroxide derivative can be recognized by a natural lipid trafficking pathway with deleterious consequences. For each example above, we will discuss the underlying mechanism of oxidative damage/dysfunction, and how this might be mitigated by antioxidant intervention.

Negative effects of tumor cell nitric oxide on anti-glioblastoma photodynamic therapy

Glioblastomas are highly aggressive brain tumors that can persist after exposure to conventional chemotherapy or radiotherapy. Nitric oxide (NO) produced by inducible NO synthase (iNOS/NOS2) in these tumors is known to foster malignant cell proliferation, migration, and invasion as well as resistance to chemo- and radiotherapy. Minimally invasive photodynamic therapy (PDT) sensitized by 5-aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) is a highly effective anti-glioblastoma modality, but it is also subject to NO-mediated resistance. Studies by the authors have revealed that glioblastoma U87 and U251 cells use endogenous iNOS/NO to not only resist photokilling after an ALA/light challenge, but also to promote proliferation and migration/invasion of surviving cells. Stress-upregulated iNOS/NO was found to play a major role in these negative responses to PDT-like treatment. Our studies have revealed a tight network of upstream signaling events leading to iNOS induction in photostressed cells and transition to a more aggressive phenotype. These events include activation or upregulation of pro-survival/ pro-expansion effector proteins such as NF-κB, phosphoinositide-3-kinase (PI3K), protein kinase-B (Akt), p300, Survivin, and Brd4. In addition to this upstream signaling and its regulation, pharmacologic approaches for directly suppressing iNOS at its activity vs. transcriptional level are discussed. One highly effective agent in the latter category is bromodomain and extra-terminal (BET) inhibitor, JQ1, which was found to minimize iNOS upregulation in photostressed U87 cells. By acting similarly at the clinical level, a BET inhibitor such as JQ1 should markedly improve the efficacy of anti-glioblastoma PDT.

Nitric oxide-elicited resistance to anti-glioblastoma photodynamic therapy

Glioblastoma multiforme is a highly aggressive primary brain malignancy that resists most conventional chemo- and radiotherapeutic interventions. Nitric oxide (NO), a short lived free radical molecule produced by inducible NO synthase (iNOS) in glioblastomas and other tumors, is known to play a key role in tumor persistence, progression, and chemo/radiotherapy resistance. Site-specific and minimally invasive photodynamic therapy (PDT), based on oxidative damage resulting from non-ionizing photoactivation of a sensitizing agent, is highly effective against glioblastoma, but resistance also exists in this case. Studies in the authors’ laboratory have shown that much of the latter is mediated by iNOS/NO. For example, when glioblastoma U87 or U251 cells sensitized in mitochondria with 5-aminolevulinic acid -induced protoporphyrin IX were exposed to a moderate dose of visible light, the observed apoptosis was strongly enhanced by an iNOS activity inhibitor or NO scavenger, indicating that iNOS/NO had increased cell resistance to photokilling. Moreover, cells that survived the photochallenge proliferated, migrated, and invaded more aggressively than controls, and these responses were also driven predominantly by iNOS/NO. Photostress-upregulated iNOS rather than basal enzyme was found to be responsible for all the negative effects described. Recognition of NO-mediated hyper-resistance/hyper-aggression in PDT-stressed glioblastoma has stimulated interest in how these responses can be prevented or at least minimized by pharmacologic adjuvants such as inhibitors of iNOS activity or transcription. Recent developments along these lines and their clinical potential for improving anti-glioblastoma PDT are discussed.

Upstream signaling events leading to elevated production of pro-survival nitric oxide in photodynamically-challenged glioblastoma cells

Nitric oxide (NO) generated endogenously by inducible nitric oxide synthase (iNOS) promotes growth and migration/invasion of glioblastoma cells and also fosters resistance to chemotherapy and ionizing radiotherapy. Our recent studies revealed that glioblastoma cell iNOS/NO also opposes the cytotoxic effects of non-ionizing photodynamic therapy (PDT), and moreover stimulates growth/migration aggressiveness of surviving cells. These negative responses, which depended on PI3K/Akt/NF-κB activation, were strongly suppressed by blocking iNOS transcription with JQ1, a BET bromodomain inhibitor. In the present study, we sought to identify additional molecular events that precede iNOS transcriptional upregulation. Akt activation, iNOS induction, and viability loss in PDT-challenged glioblastoma U87 cells were all strongly inhibited by added l-histidine, consistent with primary involvement of photogenerated singlet oxygen (¹O₂). Transacetylase p300 not only underwent greater Akt-dependent activation after PDT, but greater interaction with NF-κB subunit p65, which in turn exhibited greater K310 acetylation. In addition, PDT promoted intramolecular disulfide formation and inactivation of tumor suppressor PTEN, thereby favoring Akt and p300 activation leading to iNOS upregulation. Importantly, deacetylase Sirt1 was down-regulated by PDT stress, consistent with the observed increase in p65-acK310 level, which fostered iNOS transcription. This study provides new mechanistic insights into how glioblastoma tumors can exploit iNOS/NO to not only resist PDT, but to attain a more aggressive survival phenotype.

Cholesterol Peroxidation as a Special Type of Lipid Oxidation in Photodynamic Systems

Like other unsaturated lipids in cell membranes and lipoproteins, cholesterol (Ch) is susceptible to oxidative modification, including photodynamic oxidation. There is a sustained interest in the pathogenic properties of Ch oxides such as those generated by photooxidation. Singlet oxygen (¹ O₂ )-mediated Ch photooxidation (Type II mechanism) gives rise to three hydroperoxide (ChOOH) isomers: 5α-OOH, 6α-OOH and 6β-OOH, the 5α-OOH yield far exceeding that of the others. 5α-OOH detection is relatively straightforward and serves as a definitive indicator of ¹ O₂ involvement in a reaction, photochemical or otherwise. Like all lipid hydroperoxides (LOOHs), ChOOHs can disrupt membrane or lipoprotein structure/function on their own, but subsequent light-independent reactions may either intensify or attenuate such effects. Such reactions include (1) one-electron reduction to redox-active free radical intermediates, (2) two-electron reduction to redox-silent alcohols and (3) translocation to other lipid compartments, where (1) or (2) may take place. In addition to these effects, ChOOHs may act as signaling molecules in reactions that affect cell fates. Although processes a-c have been well studied for ChOOHs, signaling activity is still poorly understood compared with that of hydrogen peroxide. This review focuses on these various aspects Ch photoperoxidation and its biological consequences.

Cholesterol Hydroperoxide Generation, Translocation, and Reductive Turnover in Biological Systems

Cholesterol is like other unsaturated lipids in being susceptible to peroxidative degradation upon exposure to strong oxidants like hydroxyl radical or peroxynitrite generated under conditions of oxidative stress. In the eukaryotic cell plasma membrane, where most of the cellular cholesterol resides, peroxidation leads to membrane structural and functional damage from which pathological states may arise. In low density lipoprotein, cholesterol and phospholipid peroxidation have long been associated with atherogenesis. Among the many intermediates/products of cholesterol oxidation, hydroperoxide species (ChOOHs) have a number of different fates and deserve special attention. These fates include (a) damage-enhancement via iron-catalyzed one-electron reduction, (b) damage containment via two-electron reduction, and (c) inter-membrane, inter-lipoprotein, and membrane-lipoprotein translocation, which allows dissemination of one-electron damage or off-site suppression thereof depending on antioxidant location and capacity. In addition, ChOOHs can serve as reliable and conveniently detected mechanistic reporters of free radical-mediated reactions vs. non-radical (e.g., singlet oxygen)-mediated reactions. Iron-stimulated peroxidation of cholesterol and other lipids underlies a newly discovered form of regulated cell death called ferroptosis. These and other deleterious consequences of radical-mediated lipid peroxidation will be discussed in this review.

Bystander effects of nitric oxide in anti-tumor photodynamic therapy

Ionizing radiation of specifically targeted cells in a given population is known to elicit pro-death or pro-survival responses in non-targeted bystander cells, which often make no physical contact with the targeted ones. We have recently demonstrated a similar phenomenon for non-ionizing photodynamic therapy (PDT), showing that prostate cancer cells subjected to targeted photodynamic stress stimulated growth and migration of non-stressed, non-contacting bystander cells. Diffusible nitric oxide (NO) generated by stress-upregulated inducible nitric oxide synthase (iNOS) was shown to play a dominant role in these responses. Moreover, target-derived NO stimulated iNOS/NO induction in bystanders, suggesting a NO-mediated feed-forward field effect driven by targeted cells surviving the photodynamic challenge. In this research highlight, we will review these findings and discuss their potential negative implications on clinical PDT outcomes and how these might be mitigated through pharmacologic use of select iNOS inhibitors.

Multiple Means by Which Nitric Oxide can Antagonize Photodynamic Therapy

Photodynamic therapy (PDT) is a unique site-specific treatment for eradicating a variety of solid tumors, including prostate, lung, bladder, and brain tumors. PDT is a three-component modality involving (i) administration of a photosensitizing agent (PS), (ii) PS photoexcitation by visible or near-infrared light, and (iii) molecular oxygen. Upon photoexcitation, PS gives rise to tumor-damaging reactive oxygen species, most prominently singlet oxygen (1O2). Previous studies revealed that endogenous nitric oxide (NO) in various mouse tumor models significantly reduced PDT effectiveness. Recent studies in the authors' laboratory indicated that NO produced by photostressed tumor cells per se can elicit anti-PDT effects. For example, breast cancer COH-BR1 and prostate cancer PC3 cells exhibited a rapid and prolonged upregulation of inducible nitric oxide synthase (iNOS) after sensitization with 5- aminolevulinic acid (ALA)-induced protoporphyrin-IX, followed by broad-band visible irradiation. Use of iNOS inhibitors and NO scavengers demonstrated that iNOS/NO played a key role in cell resistance to apoptotic photokilling. Moreover, cells surviving an ALA/light challenge proliferated, migrated, and invaded more rapidly than controls, again in iNOS/NOdependent fashion. Thus, NO was found to play a crucial role in various manifestations of enhanced aggressiveness exhibited by remaining live cells. Recent work has revealed that induced NO in PDT-targeted PC3 cells can also translocate and increase aggressiveness of non-targeted bystander cells. These negative and potentially tumor-promoting side effects of NO in PDT may be averted through use of iNOS inhibitors as adjuvants. Each of the above aspects of PDT antagonism by NO will be discussed in this review.

Enhanced aggressiveness of bystander cells in an anti-tumor photodynamic therapy model: Role of nitric oxide produced by targeted cells

The bystander effects of anti-cancer ionizing radiation have been widely studied, but far less is known about such effects in the case of non-ionizing photodynamic therapy (PDT). In the present study, we tested the hypothesis that photodynamically-stressed prostate cancer PC3 cells can elicit nitric oxide (NO)-mediated pro-growth/migration responses in non-stressed bystander cells. A novel approach was used whereby both cell populations existed on a culture dish, but made no physical contact with one other. Visible light irradiation of target cells sensitized with 5-aminolevulinic acid-induced protoporphyrin IX resulted in a striking upregulation of inducible nitric oxide synthase (iNOS) along with NO, the level of which increased after irradiation. Slower and less pronounced iNOS/NO upregulation was also observed in bystander cells. Activation of transcription factor NF-κB was implicated in iNOS induction in both targeted and bystander cells. Like surviving targeted cells, bystanders exhibited a significant increase in growth and migration rate, both responses being strongly attenuated by an iNOS inhibitor (1400W), a NO scavenger (cPTIO), or iNOS knockdown. Incubating bystander cells with conditioned medium from targeted cells failed to stimulate growth/migration, ruling out involvement of relatively long-lived stimulants. The following post-irradiation changes in pro-survival/pro-growth proteins were observed in bystander cells: upregulation of COX-2 and activation of protein kinases Akt and ERK1/2, NO again playing a key role. This is the first reported evidence for NO-enhanced bystander aggressiveness in the context of PDT. In the clinical setting, such effects could be averted through pharmacologic use of iNOS inhibitors as PDT adjuvants.

Antagonistic Effects of Endogenous Nitric Oxide in a Glioblastoma Photodynamic Therapy Model

Gliomas are aggressive brain tumors that are resistant to conventional chemotherapy and radiotherapy. Much of this resistance is attributed to endogenous nitric oxide (NO). Recent studies revealed that 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT) has advantages over conventional treatments for glioblastoma. In this study, we used an in vitro model to assess whether NO from glioblastoma cells can interfere with ALA-PDT. Human U87 and U251 cells expressed significant basal levels of neuronal NO synthase (nNOS) and its inducible counterpart (iNOS). After an ALA/light challenge, iNOS level increased three- to fourfold over 24 h, whereas nNOS remained unchanged. Elevated iNOS resulted in a large increase in intracellular NO. Extent of ALA/light-induced apoptosis increased substantially when an iNOS inhibitor or NO scavenger was present, implying that iNOS/NO was acting cytoprotectively. Moreover, cells surviving a photochallenge exhibited a striking increase in proliferation, migration and invasion rates, iNOS/NO again playing a dominant role. Also observed was a large iNOS/NO-dependent increase in matrix metalloproteinase-9 activity, decrease in tissue inhibitor of metalloproteinase-1 expression and increase in survivin and S100A4 expression, each effect being consistent with accelerated migration/invasion as a prelude to metastasis. Our findings suggest introduction of iNOS inhibitors as pharmacologic adjuvants for glioblastoma PDT.

Impairment of Macrophage Cholesterol Efflux by Cholesterol Hydroperoxide Trafficking: Implications for Atherogenesis Under Oxidative Stress

OBJECTIVE

Oxidative stress associated with cardiovascular disease can produce various oxidized lipids, including cholesterol oxides, such as 7-hydroperoxide (7-OOH), 7-hydroxide (7-OH), and 7-ketone (7=O). Unlike 7=O and 7-OH, 7-OOH is redox active, giving rise to the others via potentially toxic-free radical reactions. We tested the novel hypothesis that under oxidative stress conditions, steroidogenic acute regulatory (StAR) family proteins not only deliver cholesterol to/into mitochondria of vascular macrophages, but also 7-OOH, which induces peroxidative damage that impairs early stage reverse cholesterol transport.

APPROACH AND RESULTS

Stimulation of human monocyte-derived THP-1 macrophages with dibutyryl-cAMP resulted in substantial upregulation of StarD1 and ATP-binding cassette (ABC) transporter, ABCA1. Small interfering RNA-induced StarD1 knockdown before stimulation had no effect on StarD4, but reduced ABCA1 upregulation, linking the latter to StarD1 functionality. Mitochondria in stimulated StarD1-knockdown cells internalized 7-OOH slower than nonstimulated controls and underwent less 7-OOH-induced lipid peroxidation and membrane depolarization, as probed with C11-BODIPY (4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-inda-cene-3-undecanoic acid) and JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-benzimidazolylcarbocyanine iodide), respectively. Major functional consequences of 7-OOH exposure were (1) loss of mitochondrial CYP27A1 activity, (2) reduced 27-hydroxycholesterol (27-OH) output, and (3) downregulation of cholesterol-exporting ABCA1 and ABCG1. Consistently, 7-OOH-challenged macrophages exported less cholesterol to apoA-I or high-density lipoprotein than did nonchallenged controls. StarD1-mediated 7-OOH transport was also found to be highly cytotoxic, whereas 7=O and 7-OH were minimally toxic.

CONCLUSIONS

This study describes a previously unrecognized mechanism by which macrophage cholesterol efflux can be incapacitated under oxidative stress-linked disorders, such as chronic obesity and hypertension. Our findings provide new insights into the role of macrophage redox damage/dysfunction in atherogenesis.

Macrophage mitochondrial damage from StAR transport of 7-hydroperoxycholesterol: implications for oxidative stress-impaired reverse cholesterol transport

StAR family proteins in vascular macrophages participate in reverse cholesterol transport (RCT). We hypothesize that under pathophysiological oxidative stress, StARs will transport not only cholesterol to macrophage mitochondria, but also pro-oxidant cholesterol hydroperoxides (7-OOHs), thereby impairing early-stage RCT. Upon stimulation with dibutyryl-cAMP, RAW264.7 macrophages exhibited a strong time-dependent induction of mitochondrial StarD1 and plasma membrane ABCA1, which exports cholesterol. 7α-OOH uptake by stimulated RAW cell mitochondria (like cholesterol uptake) was strongly reduced by StarD1 knockdown, consistent with StarD1 involvement. Upon uptake by mitochondria, 7α-OOH (but not redox-inactive 7α-OH) triggered lipid peroxidation and membrane depolarization while reducing ABCA1 upregulation. These findings provide strong initial support for our hypothesis.

Deleterious cholesterol hydroperoxide trafficking in steroidogenic acute regulatory (StAR) protein-expressing MA-10 Leydig cells: implications for oxidative stress-impaired steroidogenesis

Steroidogenic acute regulatory (StAR) proteins in steroidogenic cells are implicated in the delivery of cholesterol (Ch) from internal or external sources to mitochondria (Mito) for initiation of steroid hormone synthesis. In this study, we tested the hypothesis that under oxidative stress, StAR-mediated trafficking of redox-active cholesterol hydroperoxides (ChOOHs) can result in site-specific Mito damage and dysfunction. Steroidogenic stimulation of mouse MA-10 Leydig cells with dibutyryl-cAMP (Bt2cAMP) resulted in strong expression of StarD1 and StarD4 proteins over insignificant levels in nonstimulated controls. During incubation with the ChOOH 3β-hydroxycholest-5-ene-7α-hydroperoxide (7α-OOH) in liposomes, stimulated cells took up substantially more hydroperoxide in Mito than controls, with a resulting loss of membrane potential (ΔΨm) and ability to drive progesterone synthesis. 7α-OOH uptake and ΔΨm loss were greatly reduced by StarD1 knockdown, thus establishing the role of this protein in 7α-OOH delivery. Moreover, 7α-OOH was substantially more toxic to stimulated than nonstimulated cells, the former dying mainly by apoptosis and the latter dying by necrosis. Importantly, tert-butyl hydroperoxide, which is not a StAR protein ligand, was equally toxic to stimulated and nonstimulated cells. These findings support the notion that like Ch itself, 7α-OOH can be transported to/into Mito of steroidogenic cells by StAR proteins and therein induce free radical damage, which compromises steroid hormone synthesis.

Permeabilization of the mitochondrial outer membrane by Bax/truncated Bid (tBid) proteins as sensitized by cardiolipin hydroperoxide translocation: mechanistic implications for the intrinsic pathway of oxidative apoptosis

Cytochrome c (cyt c) release upon oxidation of cardiolipin (CL) in the mitochondrial inner membrane (IM) under oxidative stress occurs early in the intrinsic apoptotic pathway. We postulated that CL oxidation mobilizes not only cyt c but also CL itself in the form of hydroperoxide (CLOOH) species. Relatively hydrophilic CLOOHs could assist in apoptotic signaling by translocating to the outer membrane (OM), thus promoting recruitment of the pro-apoptotic proteins truncated Bid (tBid) and Bax for generation of cyt c-traversable pores. Initial testing of these possibilities showed that CLOOH-containing liposomes were permeabilized more readily by tBid plus Ca(2+) than CL-containing counterparts. Moreover, CLOOH translocated more rapidly from IM-mimetic to OM-mimetic liposomes than CL and permitted more extensive OM permeabilization. We found that tBid bound more avidly to CLOOH-containing membranes than to CL counterparts, and binding increased with increasing CLOOH content. Permeabilization of CLOOH-containing liposomes in the presence of tBid could be triggered by monomeric Bax, consistent with tBid/Bax cooperation in pore formation. Using CL-null mitochondria from a yeast mutant, we found that tBid binding and cyt c release were dramatically enhanced by transfer acquisition of CLOOH. Additionally, we observed a pre-apoptotic IM-to-OM transfer of oxidized CL in cardiomyocytes treated with the Complex III blocker, antimycin A. These findings provide new mechanistic insights into the role of CL oxidation in the intrinsic pathway of oxidative apoptosis.

Surprising inability of singlet oxygen-generated 6-hydroperoxycholesterol to induce damaging free radical lipid peroxidation in cell membranes

Singlet oxygen attack on cholesterol (Ch), a prominent monounsaturated lipid of mammalian cell plasma membranes, gives rise to three hydroperoxide (ChOOH) isomers, 5alpha-OOH, 6alpha-OOH and 6beta-OOH, the latter two in lower yield than 5alpha-OOH, and 6alpha-OOH in lowest yield. A third possible positional isomer, 7alpha-OOH and 7beta-OOH, is produced by free radical attack. In the presence of iron and ascorbate (Fe/AH), 5alpha-OOH or 6beta-OOH in phosphatidylcholine/Ch/ChOOH (20:15:1 by mol) liposomes was reduced to its corresponding alcohol, the rate constant being approximately the same for both ChOOHs. Using [(14)C]Ch as an in situ probe, we found that liposomal 5alpha-OOH readily set off free radical-mediated (chain) peroxidation reactions when exposed to Fe/AH, whereas 6beta-OOH under the same conditions did not. Moreover, liposomal 5alpha-OOH triggered robust chain peroxidation in [(14)C]Ch-labeled L1210 cells, leading to cell death, whereas 6beta-OOH was essentially inert in this regard. Thus, 5alpha-OOH and 6beta-OOH undergo iron-catalyzed reductive turnover, but only the former can provoke toxic membrane damage. These novel findings have important implications for UVA-induced photodamage in Ch-rich tissues like skin and eye, where (1)O(2) often plays a major role.

Apoptosis-accommodating effect of nitric oxide in photodynamically stressed tumor cells

Using a 5-aminolevulinic acid (ALA)-photodynamic therapy model, we have discovered a new effect of nitric oxide (NO)-the ability to accommodate apoptosis. When sensitized by disseminated ALA-generated protoporphyrin IX, COH-BR1 tumor cells in glucose-containing medium died mainly by necrosis with a low level of apoptosis. Introduced before light at a nontoxic concentration, the NO donor SPNO inhibited necrosis, but supported apoptosis such that the latter became predominant in the remaining cell death. Accompanying this was a large increase in caspase-3/7 activation. SPNO-supported apoptosis was more pronounced when glucose-deprived cells were compared with glucose-replenished, SPNO-treated counterparts. SPNO plus glucose also suppressed plasma membrane-damaging lipid peroxidation and loss of cellular ATP under photostress. The NO effect is attributed to membrane protection with maintenance of sufficient glycolytic ATP to sustain apoptosis.

StarD4-mediated translocation of 7-hydroperoxycholesterol to isolated mitochondria: deleterious effects and implications for steroidogenesis under oxidative stress conditions

StAR family proteins, including StarD4, play a key role in steroidogenesis by transporting cholesterol (Ch) into mitochondria for conversion to pregnenolone. Using a model system consisting of peroxidized cholesterol (7 alpha-OOH)-containing liposomes as donors, we showed that human recombinant StarD4 accelerates 7 alpha-OOH transfer to isolated liver mitochondria, and to a greater extent than Ch transfer. StarD4 had no effect on transfer of non-oxidized or peroxidized phosphatidylcholine, consistent with sterol ring specificity. StarD4-accelerated 7 alpha-OOH transfer to mitochondria resulted in greater susceptibility to free radical lipid peroxidation and loss of membrane potential than in a non-StarD4 control. The novel implication of these findings is that in oxidative stress states, inappropriate StAR-mediated trafficking of peroxidized Ch in steroidogenic tissues could result in damage and dysfunction selectively targeted to mitochondria.

Intracellular Dissemination of Peroxidative Stress

Sterol carrier protein-2 (SCP-2) plays a crucial role in the trafficking and metabolism of cholesterol and other lipids in mammalian cells. Lipid hydroperoxides generated under oxidative stress conditions are relatively long-lived intermediates that damage cell membranes and play an important role in redox signaling. We hypothesized that SCP-2-facilitated translocation of lipid hydroperoxides in oxidatively stressed cells might enhance cytolethality if highly sensitive sites are targeted and detoxification capacity is insufficient. We tested this using a clone (SC2A) of rat hepatoma cells that overexpress mature immunodetectable SCP-2. When challenged with liposomal cholesterol-7alpha-hydroperoxide (7alpha-OOH), SC2A cells were found to be much more sensitive to viability loss than vector control (VC) counterparts. Correspondingly, SC2A cells imported [14C]7alpha-OOH more rapidly. The clones were equally sensitive to tert-butyl hydroperoxide, suggesting that the 7alpha-OOH effect was SCP-2-specific. Fluorescence intensity of the probes 2',7'-dichlorofluorescein and C11-BODIPY increased more rapidly in SC2A than VC cells after 7alpha-OOH exposure, consistent with more rapid internalization and oxidative turnover in the former. [14C]7alpha-OOH radioactivity accumulated much faster in SC2A mitochondria than in VC, whereas other subcellular fractions showed little rate difference. In keeping with this, 7alpha-OOH-stressed SC2A cells exhibited a faster loss of mitochondrial membrane potential and development of apoptosis. This is the first reported evidence that peroxidative stress damage can be selectively targeted and exacerbated by an intracellular lipid transfer protein.

Nitric oxide-induced resistance to lethal photooxidative damage in a breast tumor cell line

The long-term effects of nitric oxide (NO) on cell susceptibility to photodynamic killing have been studied, using a human breast tumor line (COH-BR1). Subconfluent cells were exposed to a nonlethal dose of spermine NONOate (SPNO, 0.2 mM) and 20 h later were metabolically sensitized with protoporphyrin IX (PpIX) by incubating with 5-aminolevulinic acid. PpIX overproduced in mitochondria was allowed to diffuse to peripheral sites, including plasma membrane, after which a photooxidative challenge was imposed. Active (but not decomposed) SPNO made cells substantially more resistant to necrotic photokilling than non-SPNO-treated controls. A similar response to a tert-butyl hydroperoxide challenge was observed. Hyperresistance was detected approximately 8 h post-SPNO, maximized after approximately 20 h, and reflected diminished oxidant accumulation, as determined with 2',7'-dichlorofluorescein. Intracellular free iron determined with the fluorescent probe calcein rose to approximately 160% of the control level 6 h after SPNO, but declined to approximately 70% after 24 h. Immunoblot analyses revealed a rapid early (approximately 2 h post-NO) increase in heme oxygenase-1 level, followed by a gradual (4-20 h post-NO) increase in ferritin. Upregulation of these proteins is consistent with a cytoprotective mechanism involving mobilization of "signaling" iron. Preactivated RAW 264.7 macrophages on microporous inserts also induced a long-term photoresistance in underlying PpIX-sensitized COH-BR1 cells. This response was abolished by L-NAME, indicating that NO from induced nitric oxide synthase was involved. The NO effects described are entirely novel in the context of photooxidative stress and provide new insights into how NO might affect antitumor photodynamic therapy (PDT).

Self-sensitized photodegradation of membrane-bound protoporphyrin mediated by chain lipid peroxidation: inhibition by nitric oxide with sustained singlet oxygen damage

In the presence of exciting light, iron and reductants, the singlet oxygen (1O2)-generating sensitizer protoporphyrin IX (PpIX) induces free radical lipid peroxidation in membranes, but gradually degrades in the process. We postulated that NO, acting as a chain-breaking antioxidant, would protect PpIX against degradation and consequently prolong its ability to produce 1O2. This idea was tested by irradiating PpIX-containing liposomes (LUVs) in the presence of iron and ascorbate, and monitoring the cholesterol hydroperoxides 5alpha-OOH and 7alpha/beta-OOH as respective 1O2 and free radical reporters. 5alpha-OOH accumulation, initially linear with light fluence, slowed progressively after prolonged irradiation, whereas 7alpha/beta-OOH accumulation only accelerated after an initial lag. The active, but not spent, NO donor spermine NONOate (0.4 mM) virtually abolished 7alpha/beta-OOH buildup as well as 5alpha-OOH slowdown. Increasing membrane phospholipid unsaturation hastened the onset of rapid chain peroxidation and 5alpha-OOH slowdown. Accompanying the 5alpha-OOH effect was a steady decrease in 1O2 quantum yield and PpIX fluorescence at 632 nm, both of which were inhibited by NO. An NO-inhibitable decay of PpIX fluorescence was also observed during dark incubation of 5alpha-OOH-bearing LUVs with iron and ascorbate, confirming a link between chain peroxidation and PpIX loss. By protecting PpIX in irradiated membranes, NO might select for and prolong purely 1O2-mediated damage. Supporting this was our observation that 1O2-mediated photoinactivation of a nonmembrane target, lactate dehydrogenase, slowed concurrently with 5alpha-OOH accumulation and that spermine NONOate prevented this. Thus, NO not only protected membrane lipids against PpIX-sensitized free radical damage, but PpIX itself, thereby extending its 1O2-generating lifetime. Consistent findings were obtained using porphyrin-sensitized COH-BR1 cells. These previously unrecognized effects of NO could have important bearing on 5-aminolevulinate-based photodynamic therapy in which PpIX is metabolically deposited in tumor cells.

Separation and quantitation of phospholipid hydroperoxide families using high-performance liquid chromatography with mercury cathode electrochemical detection

High-performance liquid chromatography with mercury cathode electrochemical detection (HPLC-EC(Hg)) was used to separate and quantify various phospholipid hydroperoxide (PLOOH) families. Under the conditions used, baseline separation of four major biologically relevant PLOOH classes was achieved. Responsiveness was linear up to at least 1 nmol of PLOOH with a detection limit in the subpicomolar range (0.1-0.5 pmol). Applying this method to photodynamically stressed murine leukemia cells and mitochondria isolated from these cells, we identified and quantified PLOOHs derived from phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and cardiolipin. In terms of high sensitivity, specificity, and reliability, HPLC-EC(Hg) has a clear advantage over all other existing techniques for determining PLOOHs in complex biological systems.

Merocyanine 540-sensitized photokilling of leukemia cells: role of post-irradiation chain peroxidation of plasma membrane lipids as revealed by nitric oxide protection

The lipophilic dye merocyanine 540 (MC540) localizes primarily in the plasma membrane (PM) of tumor cells, where it can sensitize lethal photoperoxidative damage of potential therapeutic importance. We postulated (i) that chain peroxidation triggered by iron-catalyzed turnover of nascent hydroperoxides (LOOHs) generated by singlet oxygen ((1)O(2)) attack on PM lipids contributes significantly to overall cytolethality, and (ii) that nitric oxide (NO), a known scavenger of organic free radicals, would suppress this and, thus, act cytoprotectively. In accordance, irradiation of MC540-sensitized L1210 cells produced 5alpha-OOH, a definitive (1)O(2) adduct of PM cholesterol, which decayed during subsequent dark incubation with appearance of other signature peroxides, viz. free-radical-derived 7alpha/beta-OOH. Whereas chemical donor (SPNO or SNAP)-derived NO had little or no effect on post-irradiation 5alpha-OOH disappearance, it dose-dependently inhibited 7alpha/beta-OOH accumulation, consistent with interception of chain-carrying radicals arising from one-electron reduction of primary LOOHs. Using [(14)C]cholesterol as an L1210 PM probe, we detected additional after-light products of chain peroxidation, including diols (7alpha-OH, 7beta-OH) and 5,6-epoxides, the yields of which were enhanced by iron supplementation, but strongly suppressed by NO. Correspondingly, photoinitiated cell killing was significantly inhibited by NO introduced either immediately before or after light exposure. These findings indicate that prooxidant LOOH turnover plays an important role in photokilling and that NO, by intercepting propagating radicals, can significantly enhance cellular resistance.

Role of mitochondrial cardiolipin peroxidation in apoptotic photokilling of 5-aminolevulinate-treated tumor cells

In 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), ALA taken up by tumor cells is metabolized to protoporphyrin IX (PpIX), which sensitizes photodamage leading to apoptotic or necrotic cell death. Since lipophilic PpIX originates in mitochondria, we postulated that photoperoxidation of highly unsaturated cardiolipin (CL), which anchors cytochrome c (cyt c) to the inner membrane, is an early proapoptotic event. As initial evidence, PpIX-sensitized photooxidation of liposomal CL to hydroperoxide (CLOOH) species precluded cyt c binding, but this could be reinstated by GSH/selenoperoxidase (GPX4) treatment. Further support derived from site-specific effects observed using (i) a mitochondrial GPX4-overexpressing clone (7G4) of COH-BR1 tumor cells, and (ii) an ALA treatment protocol in which most cellular PpIX is either inside (Pr-1) or outside (Pr-2) mitochondria. Sensitized cells were exposed to a lethal light dose, and then analyzed for death mechanism and lipid hydroperoxide (LOOH) levels. Irradiated Pr-1 vector control (VC) cells died apoptotically following cyt c release and caspase-3 activation, whereas 7G4 cells were highly resistant. Irradiated Pr-2 VC and 7G4 cells showed negligible cyt c release or caspase-3 activation, and both types died via necrosis. CLOOH (detected long before cyt c release) accumulated approximately 70% slower in Pr-1 7G4 cells than in Pr-1 VC, and this slowdown exceeded that of all other LOOHs. These and related findings support the hypothesis that CL is a key upstream target in mitochondria-dependent ALA-PDT-induced apoptosis.

Sterol carrier protein-2-facilitated intermembrane transfer of cholesterol- and phospholipid-derived hydroperoxides

Sterol carrier protein-2 (SCP-2) facilitates cholesterol (Ch) and phospholipid (PL) transfer/exchange between membranes and appears to play a key role in intracellular lipid trafficking. Whether SCP-2 can also facilitate lipid hydroperoxide (LOOH) transfer between membranes and thereby potentially enhance dissemination of peroxidative damage was examined in this study. Transfer kinetics of photochemically generated cholesterol hydroperoxide (ChOOH) species (5alpha-OOH, 6alpha/6beta-OOH, 7alpha/7beta-OOH) and phospholipid hydroperoxide (PLOOH) families (PCOOH, PEOOH, PSOOH) were determined, using HPLC with electrochemical detection for peroxide analysis. LOOH donor/acceptor pairs employed in transfer experiments included (i) all liposomes (e.g., agglutinable SUVs/ nonagglutinable LUVs); (ii) photoperoxidized erythrocyte ghosts/SUVs or vice versa; and (iii) SUVs/mitochondria. In a SUV/ghost system at 37 degrees C, the rate constant for total ChOOH spontaneous transfer was approximately 8 times greater than that for unoxidized Ch. Purified bovine liver and human recombinant SCP-2 exhibited an identical ability to stimulate overall ChOOH transfer, 0.5 unit/mL (based on [(14)C]Ch transfer) increasing the first-order rate constant (k) approximately 7-fold. SCP-2-enhanced translocation of individual ChOOHs increased with increasing hydrophilicity in the following order: 6beta-OOH < 6alpha-OOH < 5alpha-OOH < 7alpha/7beta-OOH. Likewise, SCP-2 stimulated PCOOH, PEOOH, or PSOOH transfer approximately 6-fold, but the net k was 1/5 that of 5alpha-OOH and 1/10 that of 7alpha/7beta-OOH. Donor membrane properties favoring SCP-2-enhanced LOOH transfer included (i) increasing PL unsaturation and (ii) increasing net negative charge imposed by phosphatidylserine. Cytotoxic relevance was demonstrated by showing that SCP-2 accelerates 7alpha-OOH transfer from SUVs to isolated mitochondria and that this enhances peroxide-induced loss of the mitochondrial membrane potential. On the basis of these findings, we postulate that SCP-2, by trafficking ChOOHs and PLOOHs in addition to parent lipids, might exacerbate cell injury under oxidative stress conditions.

Cooperation of antioxidants in protection against photosensitized oxidation

The aim of this study was to determine whether alpha-tocopherol and zeaxanthin offer synergistic protection against photosensitized lipid peroxidation mediated by singlet oxygen and free radicals. The antioxidant action of zeaxanthin and alpha-tocopherol was studied in liposomes made of phosphatidylcholine and cholesterol. Progress of lipid peroxidation, induced by aerobic photoexcitation of rose bengal, was monitored by the detection of lipid hydroperoxides and by electron spin resonance oximetry. In addition, cholesterol was employed as a mechanistic reporter molecule, which forms characteristic products of the interaction with singlet oxygen or free radicals. Cholesterol hydroperoxides were quantitatively determined by HPLC/electrochemical detection. HPLC/ultraviolet-visible (UV-VIS) absorption detection was used to measure concentrations of zeaxanthin and alpha-tocopherol. Zeaxanthin, even at concentrations of 2.5 microM, effectively protected against singlet oxygen-mediated lipid peroxidation but was rapidly consumed due to interaction with free radicals. alpha-Tocopherol alone was not effective in protecting against lipid peroxidation, even at concentration of 0.1 mM. Combinations of zeaxanthin and alpha-tocopherol exerted a synergistic protection against lipid peroxidation. The synergistic effect may be explained in terms of prevention of carotenoid consumption by effective scavenging of free radicals by alpha-tocopherol therefore allowing zeaxanthing to quench the primary oxidant-singlet oxygen effectively.

Nitric oxide inhibition of free radical-mediated lipid peroxidation in photodynamically treated membranes and cells

Of the numerous biological activities attributed to nitric oxide ((*)NO), relatively little is known about its ability to intercept lipid-derived free radicals and thus protect cells against the damaging effects of lipid peroxidation, particularly in photodynamic settings. To address this, we asked how the (*)NO donor spermine-NONOate (SPER/NO) would affect porphyrin (PpIX)-photosensitized, iron/ascorbate-amplified chain peroxidation in cholesterol (Ch)/phospholipid (0.8:1.0, mol/mol) liposomes. Several Ch oxidation products (ChOX) were monitored by high performance chromatographic techniques. When added immediately before irradiation, SPER/NO (0.4 mM) had no effect on accumulation of 5alpha-hydroperoxide, a primary singlet oxygen-derived ChOX, but strongly suppressed the secondary species arising from postphotooxidation chain reactions, including 7alpha/7beta-hydroperoxides, 7alpha/7beta-hydroxides, and 5,6-epoxides. Metabolism of exogenous 5-aminolevulinate to PpIX in COH-BR1 tumor cells sensitized them to ChOX photogeneration and necrotic photokilling. When present during irradiation, active (but not decomposed) SPER/NO strongly inhibited both effects. These findings support the hypothesis that suitably presented NO, by intercepting lipid-derived radicals, can antagonize the antitumor effects of photodynamic therapy and other oxidative therapies.

Loss of melanin from human RPE with aging: possible role of melanin photooxidation

The pigment melanin, which is believed to play a photoprotective role, was quantified here in human RPE cells from donors of different age. Electron spin resonance (ESR) spectroscopy was shown to provide a quantitative measure of melanin and was used as a non-destructive measure of melanin content. Results indicated an age-related melanin loss in RPE cells, with melanin content diminishing 2.5-fold between the first and the ninth decade of life. To determine whether photo-oxidation may contribute to age-related changes in RPE melanin, RPE in human eyecups, isolated human and bovine RPE cells, purified melanin granules, or synthetic dopa melanin were irradiated with various wavelengths and intensities of visible light. Samples were analysed for changes in melanin content by ESR spectroscopy, and by absorption and emission spectrophotometry. The concentration of hydrogen peroxide was measured in some samples, and some human eyecups were examined by transmission electron microscopy. Irradiation of RPE in eyecups with intense visible light was found to produce a time-dependent photobleaching of melanosomes that was accompanied by the formation of hydrogen peroxide. Photobleaching of isolated RPE melanosomes and synthetic dopa melanin resulted in enhanced melanin fluorescence, as previously shown for melanin from aged donors by others, and significantly reduced ESR signal intensity, resembling the changes in melanin with aging observed here. We conclude that the content of melanin in RPE cells undergoes an age-related change to which photo-oxidation may contribute. This observation raises the question of whether age-related changes in melanin reduce the photoprotective role of the pigment in aging RPE cells.

Chain-breaking Antioxidant and Cytoprotective Action of Nitric Oxide on Photodynamically Stressed Tumor Cells¶

Nitric oxide (.NO) has a multitude of physiological roles, including the ability to protect cells against oxidant-induced killing, e.g. by inhibiting caspase-mediated apoptosis or by intercepting damaging free radicals derived from membrane lipids. The purpose of this study was to test the hypothesis that low flux .NO acting in the latter fashion can enhance tumor-cell resistance to photodynamic killing, specifically that sensitized by 5-aminolevulinic acid (ALA)-derived protoporphyrin IX (PpIX). Preliminary model experiments with iron-ascorbate-treated, PpIX-sensitized liposomes showed that spermine NONOate (SPER/NO)-derived .NO had no effect on photoinduced accumulation of primary singlet oxygen adducts, e.g. the cholesterol hydroperoxide 5 alpha-OOH, but dose-dependently inhibited the buildup of free radical-generated oxidation products arising from one-electron turnover of primary peroxides. In subsequent studies, breast tumor COH-BR1 cells in serum-free medium were treated with 1 mM ALA for 15 min and then without ALA for 3.75 h, allowing biogenerated PpIX to diffuse to extramitochondrial sites, including plasma membrane. Cells were irradiated in the absence or presence of SPER/NO and compared for peroxidative damage and Hoechst-assessed viability after 5 h in the dark. Iron-stimulated necrotic photo-killing and accumulation of chain lipid peroxidation products were observed, and this was inhibited strongly by SPER/NO, but not by decomposed SPER/NO, confirming that .NO was the active agent. When introduced after irradiation, .NO became progressively less inhibitory, consistent with ongoing but waning free-radical activity. These findings provide new insights into the possible role of .NO in tumor resistance to ALA-photodynamic therapy and other photo-dynamic treatments.

Spontaneous transfer of phospholipid and cholesterol hydroperoxides between cell membranes and low-density lipoprotein: assessment of reaction kinetics and prooxidant effects

Under oxidative pressure in the vascular circulation, erythrocytes and phagocytic cells may accumulate membrane lipid hydroperoxides (LOOHs), including cholesterol- and phospholipid-derived species (ChOOHs, PLOOHs). LOOH translocation from cells to low-density lipoprotein (LDL) might sensitize the latter to free radical-mediated oxidative modification, an early event associated with atherogenesis. To test this, we examined the spontaneous transfer kinetics of various ChOOH species (5 alpha-OOH, 6 alpha-OOH, 6 beta-OOH, 7 alpha/7 beta-OOH) and various PLOOH groups (PCOOH, PEOOH, PSOOH, SMOOH) using photoperoxidized erythrocyte ghosts as model donors and freshly prepared LDL as an acceptor. LOOH departure or uptake was monitored by reverse-phase HPLC with reductive electrochemical detection. Mildly peroxidized ghost membranes transferred overall ChOOH and PLOOH to LDL with apparent first-order rate constants approximately 60 and approximately 35 times greater than those of the respective parent lipids. Individual ChOOH rate constants decreased in the following order: 7 alpha/7 beta-OOH > 5 alpha-OOH > 6 alpha-OOH > 6 beta-OOH. Kinetics for reverse transfer from LDL to ghosts followed the same trend, but rates were significantly higher for all species and their combined activation energy was lower (41 vs 85 kJ/mol). PLOOH transfer rate constants ranged from 4- to 15-fold lower than the composite ChOOH constant, their order being as follows: PCOOH approximately PEOOH approximately PSOOH > SMOOH. Similar PLOOH transfer kinetics were observed when LDL acceptor was replaced by unilamellar liposomes, consistent with desorption from the donor membrane being the rate-limiting step. The susceptibility of transfer LOOH-enriched LDL to Cu2+-induced chain peroxidative damage was assessed by monitoring the accumulation of conjugated dienes and products of free radical-mediated cholesterol oxidation. In both cases, transfer-acquired LOOHs significantly reduced the lag time for chain initiation relative to that observed using nonperoxidized ghosts. These findings are consistent with the idea that LDL can acquire significant amounts of "seeding" LOOHs via translocation from various donors in the circulation.

Hyperresistance to photosensitized lipid peroxidation and apoptotic killing in 5-aminolevulinate-treated tumor cells overexpressing mitochondrial GPX4

Antitumor photodynamic therapy (PDT) with administered 5-aminolevulinic acid (ALA) is based on metabolism of ALA to protoporphyrin IX (PpIX), which acts as a sensitizer of photo-oxidative damage leading to apoptotic or necrotic cell death. An initial goal of this study was to ascertain how the PpIX-sensitized death mechanism for a breast tumor line (COH-BR1 cells) might be influenced by the conditions of ALA exposure in vitro. Two different treatment protocols were developed for addressing this question: (i) continuous incubation with 1 mM ALA for 90 min; and, (ii) discontinuous incubation, i.e., 15 min with 1 mM ALA followed by 225 min without it. Following exposure to 2 J/cm2 of visible light, cell viability, death mechanism, and lipid hydroperoxide (LOOH) level were evaluated for each protocol using thiazolyl blue, Hoechst staining, and HPLC with electrochemical detection assays, respectively. PpIX was found to sensitize apoptosis when it existed mainly in mitochondria (protocol-1), but necrosis when it diffused to other sites, including plasma membrane (protocol-2). Experiments with a transfectant clone, 7G4, exhibiting approximately 85 times greater activity of the LOOH-detoxifying selenoenzyme GPX4 than parental cells, provided additional information about death mechanism. Located predominantly in mitochondria of 7G4 cells, GPX4 strongly inhibited both LOOH accumulation and apoptosis under protocol-1 conditions, but had no significant effect under protocol-2 conditions. These findings support the hypothesis that LOOHs produced by attack of photogenerated singlet oxygen on mitochondrial membrane lipids play an important early role in the apoptotic death cascade.

Photoreactivity of aged human RPE melanosomes: a comparison with lipofuscin

PURPOSE

To determine whether aging is accompanied by changes in aerobic photoreactivity of retinal pigment epithelial (RPE) melanosomes isolated from human donors of different ages, and to compare the photoreactivity of aged melanosomes with that of RPE lipofuscin.

METHODS

Human RPE pigment granules were isolated from RPE cells pooled into groups according to the age of the donors. Photoreactivity was determined by blue-light-induced oxygen uptake and photogeneration of reactive oxygen species. Short-lived radical intermediates were detected by spin-trapping, hydrogen peroxide by an oxidase electrode, singlet oxygen by cholesterol assay, and lipid hydroperoxides by iodometric assay.

RESULTS

Blue-light photoexcitation of melanosomes resulted in age-related increases in both oxygen uptake and the accumulation of superoxide anion spin adducts. The efficiencies of these processes, however, were still significantly lower than that induced by photoexcited lipofuscin. During irradiation of melanosomes, a substantial amount of oxygen was converted into hydrogen peroxide, whereas for lipofuscin, hydrogen peroxide accounted for not more than 3% of oxygen consumed. In contrast to lipofuscin, photoexcited melanosomes did not substantially increase the rate of oxidative reactions in the presence of polyunsaturated lipids or albumin. However, oxygen uptake was significantly elevated in the presence of ascorbate. Thus, the rate of photo-induced oxygen uptake in samples containing both ascorbate and melanosomes approached that observed in lipofuscin samples.

CONCLUSIONS

Blue-light-induced photoreactivity of melanosomes increases with age, perhaps providing a source of reactive oxygen species and leading to depletion of vital cellular reductants, which, together with lipofuscin, may contribute to cellular dysfunction.

Spontaneous intermembrane transfer of various cholesterol-derived hydroperoxide species: kinetic studies with model membranes and cells

Whereas spontaneous and protein-mediated transfer/exchange of cholesterol (Ch) between membranes has been widely studied, relatively little is known about the translocation of Ch oxidation products, particularly hydroperoxide species (ChOOHs), which can act as cytotoxic prooxidants. A major aim of the present study was to examine and compare the intermembrane transfer characteristics of several biologically relevant ChOOH isomers, including singlet oxygen-derived 5alpha-OOH, 6alpha-OOH, and 6beta-OOH and free radical-derived 7alpha-OOH and 7beta-OOH. These species were generated in [(14)C]Ch-labeled donor membranes [erythrocyte ghosts or unilamellar DMPC/Ch (1.0:0.8 mol/mol) liposomes] by means of dye-sensitized photoperoxidation. Spontaneous transfer to nonoxidized acceptor membranes (DMPC liposomes or ghosts, respectively) at 37 degrees C was monitored by thin-layer chromatography with phosphorimaging radiodetection (HPTLC-PI) or liquid chromatography with mercury cathode electrochemical detection [HPLC-EC(Hg)]. The former allowed measurement of total (unresolved) ChOOH along with parent Ch, whereas the latter allowed measurement of individual ChOOHs. Ghost membranes in which approximately 4% of the Ch had been peroxidized, giving mainly 5alpha-OOH, transferred total ChOOH and Ch to liposomes in apparent first-order fashion, the rate constant for ChOOH being approximately 65 times greater. Like Ch desorption, ChOOH desorption from donor membranes was found to be rate limiting, and rate varied inversely with size when liposomal donors were used. For individual ChOOHs, rate constant magnitude (7alpha/7beta-OOH > 5alpha-OOH > 6alpha-OOH > 6beta-OOH) correlated inversely with reverse-phase HPLC retention time, suggesting that faster transfer reflects greater hydrophilicity. Liposome-borne ChOOHs exhibited the same order of toxicity toward COH-BR1 cells, which are deficient in ability to detoxify these peroxides. The prospect of disseminating oxidative cell injury via translocation of ChOOHs and other lipid hydroperoxides is readily apparent from these findings.

Hyperresistance to cholesterol hydroperoxide-induced peroxidative injury and apoptotic death in a tumor cell line that overexpresses glutathione peroxidase isotype-4

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX; GPX4) plays a key role in eukaryotic defense against potentially lethal peroxidative injury and also regulation of physiological peroxide tone. In this work we focused on the cytoprotective antiperoxidant effects of GPX4, using a breast tumor epithelial cell line that over-expresses the enzyme. Wild-type COH-BR1 cells, which exhibit little (if any) GPX4 activity, were transfected with a construct encoding the mitochondrion-targeted long (L) form of the enzyme. Several transfectant clones were selected which expressed relatively large amounts of GPX4, as determined by both Northern and Western analysis. Enzyme activity ranged from 15-fold to 190-fold greater than that of wild-type or null-transfected cells. The functional ramifications of GPX4 overexpression were tested by challenging cells with photochemically generated cholesterol hydroperoxides (ChOOHs) in liposomal form. Compared with vector controls, overexpressing clones were found to be substantially more resistant to ChOOH-induced killing, as determined by annexin-V (early apoptotic) and thiazolyl blue (mitochondrial dehydrogenase) reactivity. Concomitantly, the clones exhibited a striking hyper-resistance to free radical-mediated lipid peroxidation, as assessed by labeling cell membranes with [(14)C]cholesterol and measuring a family of radiolabeled oxidation products (ChOX). L-form GPX4's antiperoxidant and cytoprotective effects could reflect its ability to detoxify ChOOHs as they enter cells and/or cell-derived lipid hydroperoxides arising from ChOOH one-electron turnover.

Inhibition of free radical-mediated cholesterol peroxidation by diazeniumdiolate-derived nitric oxide: effect of release rate on mechanism of action in a membrane system

Nitric oxide ((*)NO) flux in relation to antiperoxidant action has been studied, using large unilamellar liposomes (LUVs) as target membranes. LUVs consisting of an oxidizable phosphatidylcholine (PC), [(14)C]cholesterol (Ch) as a reaction probe, and 5alpha-hydroperoxycholesterol (5alpha-OOH) as a nonregenerable primer underwent chain peroxidation when exposed to a lipophilic iron chelate [Fe(HQ)(3), 1 microM] and ascorbate (AH(-), 1 mM) at 37 degrees C. Reaction progress was monitored by (i) HPLC with reductive-mode electrochemical detection to assess the decay of 5alpha-OOH and the formation and/or turnover of free radical-derived 7alpha- and 7beta-hydroperoxycholesterol (7alphabeta-OOH) and (ii) HPTLC with phosphorimaging to track all major (14)C-labeled oxidation products (ChOX), including 7alphabeta-OOH, 7alpha-OH, 7beta-OH, and 5,6-epoxide. Three diazeniumdiolate (*)NO donors with different half-lives were tested for their ability to interfere with peroxidation: MANO ( approximately 1 min), PANO (15 min), and SPNO (38 min). At starting concentrations of < or =200 microM, none of the donors slowed 5alpha-OOH exponential decay, ruling out any interference with redox-active iron. However, SPNO and to a greater extent PANO (but not the decomposed donors) decreased both the initial rate and steady state of 7alphabeta-OOH accumulation in a strong dose-dependent fashion. In contrast, MANO completely inhibited 7alphabeta-OOH formation over the first 5 min of reaction, but thereafter, the peroxide accumulated rapidly, albeit more slowly than without MANO and independently of the MANO dose. The latter response diminished with increasing Fe(HQ)(3) concentration, coincident with more rapid 5alpha-OOH loss. The same general trends with MANO, PANO, and SPNO were observed when the entire population of [(14)C]ChOX species was monitored. These effects are attributed to interception of Ch- and PC-derived free radicals by (*)NO, high-flux (*)NO from MANO acting mainly on 5alpha-OOH-derived radicals (chain prevention), low-flux (*)NO from SPNO mainly on downstream radicals (chain termination), and intermediate-flux (*)NO from PANO by a combination of these mechanisms. Thus, delivery rate can be an important determinant of how (*)NO inhibits peroxide-induced lipid peroxidation.

Cholesterol as a singlet oxygen detector in biological systems

In cells under oxidative attack, membrane Ch, through the formation of its signature hydroperoxide and diol products, can serve as a unique detector in situ, allowing discrimination between 1O2 and free radical intermediacy. Of the two techniques described for analyzing Ch oxidation products, TLC with color development suffices for preliminary, mainly qualitative product screening, whereas a high-performance approach such as HPLC-EC(Hg) is advised when maximum resolution and sensitivity of quantitation are necessary. By using these strategies, one can monitor the formation of 1O2, for example, in a biologically relevant milieu (membrane), thus avoiding the difficulties associated with external detection, e.g., by physical means. These approaches would be valuable for assessing reaction mechanisms for various oxidative agents of biomedical importance, including environmental phototoxins and the rapidly emerging family of phototherapeutic drugs. Although photodynamic stress has been emphasized, the methods described should have broad applicability in the elucidation of oxidative mechanisms.

Dissemination of peroxidative stress via intermembrane transfer of lipid hydroperoxides: model studies with cholesterol hydroperoxides

Lipid hydroperoxides (LOOHs) can be generated in cells when cholesterol (Ch) and other unsaturated lipids in cell membranes are degraded under conditions of oxidative stress. If LOOHs escape reductive detoxification by glutathione-dependent selenoperoxidases, they may undergo iron-catalyzed one-electron reduction to free radical species, thus triggering peroxidative chain reactions which exacerbate oxidative membrane damage. LOOHs are more polar than parent lipids and much longer-lived than free radical precursors or products. Accordingly, intermembrane transfer of LOOHs (analogous to that of unoxidized precursors) might be possible, and this could jeopardize acceptor membranes. We have investigated this possibility, using photoperoxidized [(14)C]Ch-labeled erythrocyte ghosts as cholesterol hydroperoxide (ChOOH) donors and unilamellar liposomes [e.g., dimyristoyl-phosphatidylcholine/Ch, 9:1 mol/mol] as acceptors. ChOOH material consisted mainly of 5alpha-hydroperoxide, a singlet oxygen adduct. Time-dependent transfer of ChOOH versus Ch at 37 degrees C was determined, using high-performance liquid and thin-layer chromatographic methods to analyze liposomal extracts for these species. A typical experiment in which the starting ChOOH/Ch mol ratio in ghosts was approximately 0.05 showed that the initial transfer rate of ChOOH was approximately 16 times greater than that of parent Ch. Using [(14)C]Ch as a reporter in liposome acceptors, we found that transfer-acquired ChOOHs, when exposed to a lipophilic iron chelate and ascorbate, could trigger strong peroxidative chain reactions, as detected by accumulation of [(14)C]Ch oxidation products. These findings support the hypothesis that intermembrane transfer of ChOOHs can contribute to their prooxidant membrane damaging and cytotoxic potential.

Nitric oxide inhibition of free radical-mediated cholesterol peroxidation in liposomal membranes

The ability of nitric oxide ((*)NO) to inhibit propagative lipid peroxidation was investigated using unilamellar liposomes (LUVs) constituted with egg phosphatidylcholine (PC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), [(14)C]cholesterol (Ch), and a nonregenerable singlet oxygen-derived primer, 5alpha-hydroperoxycholesterol (5alpha-OOH). Exposing LUVs to ascorbate and a lipophilic iron chelate at 37 degrees C resulted in an exponential decay of 5alpha-OOH and accumulation of free radical-derived 7alpha- and 7beta-hydroperoxycholesterol (7alphabeta-OOH), as detected by high-performance liquid chromatography with electrochemical detection. Thiobarbituric acid-reactive species (TBARS) were generated concurrently in egg PC-containing LUVs. Including the (*)NO donor spermine NONOate (SPNO, 5-50 microM) or S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 50-100 microM) in the reaction mixture had no effect on 5alpha-OOH decay (suggesting that iron was not redox-inhibited) but slowed TBARS and 7alphabeta-OOH accumulation in a strongly dose-dependent fashion. Decomposed SPNO or SNAP had no such effects, implying that (*)NO was the responsible agent. Accumulation of several [(14)C]Ch oxidation products, detected by high-performance thin-layer chromatography with phosphorimaging, was similarly diminished by active SPNO or SNAP. Concomitantly, a new band referred to as RCh.4 appeared, the radioactivity of which increased as a function of incubation time and (*)NO donor concentration. RCh.4 material was also generated via direct iron/ascorbate reduction of 7alpha-OOH in the presence of (*)NO, consistent with 7alpha-nitrite (7alpha-ONO) identity. However, various other lines of evidence suggest that RCh.4 is not 7alpha-ONO, but rather 5alpha-hydroxycholesterol (5alpha-OH) generated by reduction of 5alpha-ONO arising from 7alpha-ONO rearrangement. 5alpha-OH was only detected when (*)NO was present in the reaction system, thus providing indirect evidence for the existence of nitrosated Ch intermediates arising from (*)NO chain-breaking activity.