Radiolabeled cholesterol as a reporter for assessing one-electron turnover of lipid hydroperoxides

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.

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.

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.

Photodynamic Therapy as an Oxidative Anti-Tumor Modality: Negative Effects of Nitric Oxide on Treatment Efficacy

Anti-tumor photodynamic therapy (PDT) is a unique oxidative stress-based modality that has proven highly effective on a variety of solid malignancies. PDT is minimally invasive and generates cytotoxic oxidants such as singlet molecular oxygen (¹O₂). With high tumor site-specificity and limited off-target negative effects, PDT is increasingly seen as an attractive alternative or follow-up to radiotherapy or chemotherapy. Nitric oxide (NO) is a short-lived bioactive free radical molecule that is exploited by many malignant tumors to promote cell survival, proliferation, and metastatic expansion. Typically generated endogenously by inducible nitric oxide synthase (iNOS/NOS2), low level NO can also antagonize many therapeutic interventions, including PDT. In addition to elevating resistance, iNOS-derived NO can stimulate growth and migratory aggressiveness of tumor cells that survive a PDT challenge. Moreover, NO from PDT-targeted cells in any given population is known to promote such aggressiveness in non-targeted counterparts (bystanders). Each of these negative responses to PDT and their possible underlying mechanisms will be discussed in this chapter. Promising pharmacologic approaches for mitigating these NO-mediated responses will also be discussed.

Nitric Oxide-elicited Resistance to Antitumor Photodynamic Therapy via Inhibition of Membrane Free Radical-mediated Lipid Peroxidation

This review focuses on the ability of nitric oxide (NO) to antagonize antitumor photodynamic therapy (PDT). NO's anti-PDT effects were recognized relatively recently and require a better mechanistic understanding for developing new strategies to improve PDT efficacy. Many PDT sensitizers (PSs) are amphiphilic and tend to localize in membrane compartments of tumor cells. Unsaturated lipids in these compartments can undergo peroxidative degradation after PS photoactivation. Primary Type I (free radical) vs. Type II (singlet oxygen) photochemistry of lipid peroxidation is discussed, along with light-independent turnover of primary lipid hydroperoxides to free radical species. Chain lipid peroxidation mediated by the latter exacerbates membrane damage and cytotoxicity after a PDT challenge. Our studies have shown that NO from chemical donors can suppress chain peroxidation by intercepting lipid-derived free radical intermediates, thereby protecting cancer cells against photokilling. More recent evidence has revealed that inducible NO synthase (iNOS) is dramatically upregulated in several cancer cell types after a photodynamic challenge, and that iNOS-derived NO enhances resistance as well as growth and migratory aggressiveness of surviving cells. Chain breaking by NO and other possible NO-based resistance mechanisms are discussed, along with novel pharmacologic approaches for overcoming these negative effects.

5-aminolevulinic acid photodynamic therapy enhance the effect of acitretin on squamous cell carcinoma cells: An in vitro study

Squamous cell carcinoma (SCC) remains the second most common nonmelanoma skin cancer (NMSC) worldwide. Both acitretin and 5-Aminolevulinic acid mediated photodynamic therapy (ALA-PDT) have validated effect on SCC. However, the effects of both treatmens remain limited, and there has been no report concerning the potential synergistic effect of both treatments for SCC.

OBJECTIVE

To investigate the cytotoxic effect of acitretin on SCL-1 cells, and whether ALA-PDT enhances this effect.

METHODS

CCK-8 and trypan blue exclusion array were used to detect the cell cytotoxicity after acitretin treatment with different concentrations (1.6 × 10^-4mg/mL, 1.6 × 10^-3 mg/mL, 1.6 × 10^-2mg/mL and 1.6 × 10^-1mg/mL) for 24 h, 48 h and 72 h. Flow cytometry and trypan blue exclusion assay were used to detect the apoptosis and viability of SCL-1 cells after treated with acitretin, ALA-PDT and ALA-PDT immediately followed by acitretin. Independent sample t test was used to analyze the different incubation time of acitretin and acitretin combined with ALA-PDT on SCL-1 cells. Bonferroni Test One-way Anova method was used to analyze the effect of different treatment on the SCL-1 cells.

RESULTS

A significant cytotoxic effect was observed after acitretin treatment, in an acitretin concentration-dependent manner within the range of 1.6 × 10^-4mg/mL to 1.6 × 10^-1mg/mL and an acitretin incubation time-dependent manner within 24 h-72 h. The total apoptosis rate and dead cells rate in group of ALA-PDT combined with acitretin were both significantly higher than that of acitretin, ALA-PDT group. A stronger apoptotic and cytotoxic effect detected 24 h after treated with acitretin than that of 12 h was observed in this study.

CONCLUSION

Acitretin has a cytotoxic effect on SCL-1 cells, and ALA-PDT treatment enhances the the cytotoxic effect of acitretin.

Nitric Oxide Inhibition of Chain Lipid Peroxidation Initiated by Photodynamic Action in Membrane Environments

Iron-catalyzed, free radical-mediated lipid peroxidation may play a major role in tumor cell killing by photodynamic therapy (PDT), particularly when membrane-localizing photosensitizers are employed. Many cancer cells exploit endogenous iNOS-generated NO for pro-survival/expansion purposes and for hyper-resistance to therapeutic modalities, including PDT. In addition to inhibiting the pro-oxidant activity of Fe(II) via nitrosylation, NO may intercept downstream lipid oxyl and peroxyl radicals, thereby acting as a chain-breaking antioxidant. We investigated this for the first time in the context of PDT by using POPC/Ch/PpIX (100:80:0.2 by mol) liposomes (LUVs) as a model system. Cholesterol (Ch or [¹⁴C]Ch) served as an in-situ peroxidation probe and protoporphyrin IX (PpIX) as photosensitizer. PpIX-sensitized lipid peroxidation was monitored by two analytical methods that we developed: HPLC-EC(Hg) and HPTLC-PI. 5α-hydroperoxy-Ch (5α-OOH) accumulated rapidly and linearly with irradiation time, indicating singlet oxygen (¹O₂) intermediacy. When ascorbate (AH^-) and trace lipophilic iron [Fe(HQ)₃] were included, 7α/7β-hydroperoxy-Ch (7-OOH) accumulated exponentially, indicating progressively greater membrane-damaging chain lipid peroxidation. With AH^-/Fe(HQ)₃ present, the NO donor SPNO had no effect on 5α-OOH formation, but dose-dependently inhibited 7-OOH formation due to NO interception of chain-carrying oxyl and peroxyl radicals. Similar results were obtained when cancer cells were PpIX/light-treated, using SPNO or activated macrophages as the NO source. These findings implicate chain lipid peroxidation in PDT-induced cytotoxicity and NO as a potent antagonist thereof by acting as a chain-breaking antioxidant. Thus, unless NO formation in aggressive tumors is suppressed, it can clearly compromise PDT efficacy.

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.

Biochemical properties of cholesterol aldehyde secosterol and its derivatives

Elevated levels of cholesterol aldehyde, 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol-A, also called 5,6-secosterol), and its aldolization product (secosterol-B) have been detected in human atherosclerotic plaques and tissues samples of brains affected by neurodegeneration, such as Alzheimer’s disease and Lewy body dementia suggesting that increased formation of these compounds may be associated with inflammation-related diseases. Secosterol-A and secosterol-B, and also further oxidized products seco-A-COOH and seco-B-COOH induce several pro-inflammatory activities in vitro. Accumulating evidences demonstrate that the covalent bindings of these secosterols to target proteins seem to be critical to trigger their pro-inflammatory activities. One of the molecular mechanisms of protein adduct formations is that aldehydic function of secosterol-A and secosterol-B is reactive and form Schiff bases with ε- or N-terminal amino groups of proteins. In other cases, it is recently suggested that Michael acceptor moiety formed by the dehydration of not only secosterol-A and secosterol-B but also seco-A-COOH may react with nucleophilic site on target proteins. In this review, I summarize and provide an overview of formation mechanism of secosterols in in vitro and in vivo, patho- or physiological concentrations in biological and clinical samples, and molecular mechanisms of pro-inflammatory activities of secosterols.

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.

Nitroxide free radicals protect macular carotenoids against chemical destruction (bleaching) during lipid peroxidation

Macular xanthophylls (MXs) lutein and zeaxanthin are dietary carotenoids that are selectively concentrated in the human eye retina, where they are thought to protect against age-related macular degeneration (AMD) by multiple mechanisms, including filtration of phototoxic blue light and quenching of singlet oxygen and triplet states of photosensitizers. These physical protective mechanisms require that MXs be in their intact structure. Here, we investigated the protection of the intact structure of zeaxanthin incorporated into model membranes subjected to oxidative modification by water- and/or membrane-soluble small nitroxide free radicals. Model membranes were formed from saturated, monounsaturated, and polyunsaturated phosphatidylcholines (PCs). Oxidative modification involved autoxidation, iron-mediated, and singlet oxygen-mediated lipid peroxidation. The extent of chemical destruction (bleaching) of zeaxanthin was evaluated from its absorption spectra and compared with the extent of lipid peroxidation evaluated using the thiobarbituric acid assay. Nitroxide free radicals with different polarity (membrane/water partition coefficients) were used. The extent of zeaxanthin bleaching increased with membrane unsaturation and correlated with the rate of PC oxidation. Protection of the intact structure of zeaxanthin by membrane-soluble nitroxides was much stronger than that by water-soluble nitroxides. The combination of zeaxanthin and lipid-soluble nitroxides exerted strong synergistic protection against singlet oxygen-induced lipid peroxidation. The synergistic effect may be explained in terms of protection of the intact zeaxanthin structure by effective scavenging of free radicals by nitroxides, therefore allowing zeaxanthin to quench the primary oxidant, singlet oxygen, effectively by the physical protective mechanism. The redox state of nitroxides was monitored using electron paramagnetic resonance spectroscopy. Both nitroxide free radicals and their reduced form, hydroxylamines, were equally effective. Obtained data were compared with the protective effects of α-tocopherol, which is the natural antioxidant and protector of MXs within the retina. The new strategies employed here to maintain the intact structure of MXs may enhance their protective potential against AMD.

Natural product derived immune-regulatory agents

We can now declare that the clinical goal of immune intervention as a therapeutic strategy for neoplastic, infectious, autoimmune and inflammatory diseases, has been achieved and in many instances obtained regulatory approval. Although, interest in and optimism for this approach has fluctuated, in the last 20years, immunotherapy has progressed from trials with crude microbial mixtures and extracts to the sophisticated use of pure cultured bacterial, synthetized active moieties identified from crude extracts, analogues therefrom and agonists and antagonists identified during screening resulting in reproducible pharmacologically active compounds with multiple mechanisms of action. Our current understanding of the mechanism of action for immunoregulatory agents contributes to the future discovery of improved strategies to use these and future immunotherapies. In this review we have identified and discussed, those drugs that have been approved and or are in clinical development as immunoregulatory agents, emphasizing those derived from or associated with natural product.

Cholesterol as a natural probe for free radical-mediated lipid peroxidation in biological membranes and lipoproteins

We describe a relatively convenient and reliable procedure for assessing the magnitude of free radical-mediated (chain) lipid peroxidation in biological systems. The approach is based on use of radiolabeled cholesterol ([(14)C]Ch) as a probe and determination of well-resolved oxidation intermediates/products ([(14)C]ChOX species), using high performance thin layer chromatography with phorphorimaging detection (HPTLC-PI). In a lipid hydroperoxide-primed liposomal test system treated with ascorbate and a lipophilic iron chelate, the following well-resolved [(14)C]ChOX are detected and quantified: 7α/7β-OOH, 7α/7β-OH, and 5,6-epoxide, their levels increasing with incubation time at 37°C. [(14)C]Ch also serves as an excellent probe for lipid peroxidation in lipoproteins and plasma membranes of mammalian cells. Because this approach utilizes Ch as a natural in situ probe, it eliminates potential artifacts associated with artificial probes such as spin traps and fluorophores.

Binding and cytotoxic trafficking of cholesterol hydroperoxides by sterol carrier protein-2

Redox-active cholesterol hydroperoxides (ChOOHs) generated by oxidative stress in eukaryotic cells may propagate cytotoxic membrane damage by undergoing one-electron reduction or, at low levels, act as mobile signaling molecules like H2O2. We discovered that ChOOHs can spontaneously translocate between membranes or membranes and lipoproteins in model systems, and that this can be accelerated by sterol carrier protein-2 (SCP-2), a nonspecific lipid trafficking protein. We found that cells overexpressing SCP-2 were more susceptible to damage/toxicity by 7α-OOH (a free radical-generated ChOOH) than control cells, and that this correlated with 7α-OOH delivery to mitochondria. The methods used for obtaining these results and for establishing that cellular SCP-2 binds and traffics 7α-OOH are described in this chapter.

Pathways of cholesterol oxidation via non-enzymatic mechanisms

Cholesterol has many functions, including those that affect biophysical properties of membranes, and is a precursor to hormone synthesis. These actions are governed by enzymatic pathways that modify the sterol nucleus or the isooctyl tail. The addition of oxygen to the cholesterol backbone produces its derivatives known as oxysterols. In addition to having an enzymatic origin, oxysterols can be formed in the absence of enzymatic catalysis in a pathway usually termed "autoxidation," which has been known for almost a century and observed under various experimental conditions. Autoxidation of cholesterol can occur through reactions initiated by free radical species, such as those arising from the superoxide/hydrogen peroxide/hydroxyl radical system and by non-radical highly reactive oxygen species such as singlet oxygen, HOCl, and ozone. The susceptibility of cholesterol to non-enzymatic oxidation has raised considerable interest in the function of oxysterols as biological effectors and potential biomarkers for the non-invasive study of oxidative stress in vivo.

Sterol carrier protein-2 (SCP-2) involvement in cholesterol hydroperoxide cytotoxicity as revealed by SCP-2 inhibitor effects

Sterol carrier protein-2 (SCP-2) plays an important role in cholesterol trafficking and metabolism in mammalian cells. The purpose of this study was to determine whether SCP-2, under oxidative stress conditions, might also traffic hydroperoxides of cholesterol, thereby disseminating their cytotoxic effects. Two inhibitors, SCPI-1 and SCPI-3, known to block cholesterol binding by an insect SCP-2, were used to investigate this. A mouse fibroblast transfectant clone (SC2F) overexpressing SCP-2 was found to be substantially more sensitive to apoptotic killing induced by liposomal 7α-hydroperoxycholesterol (7α-OOH) than a wild-type control. 7α-OOH uptake by SC2F cells and resulting apoptosis were both inhibited by SCPI-1 or SCPI-3 at a subtoxic concentration. Preceding cell death, reactive oxidant accumulation and loss of mitochondrial membrane potential were also strongly inhibited. Similar SCPI protection against 7α-OOH was observed with two other types of SCP-2-expressing mammalian cells. In striking contrast, neither inhibitor had any effect on H(2)O(2)-induced cell killing. To learn whether 7α-OOH cytotoxicity is due to uptake/transport by SCP-2, we used a fluorescence-based competitive binding assay involving recombinant SCP-2, NBD-cholesterol, and SCPI-1/SCPI-3 or 7α-OOH. The results clearly showed that 7α-OOH binds to SCP-2 in SCPI-inhibitable fashion. Our findings suggest that cellular SCP-2 not only binds and translocates cholesterol but also cholesterol hydroperoxides, thus expanding their redox toxicity and signaling ranges under oxidative stress conditions.

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.

Spin-labelled lutein as a new antioxidant in protection against lipid peroxidation

A new potentially antioxidant compound, spin-labelled lutein (SL-lut), was synthesized and incorporated into egg yolk phosphatidylcholine (EYPC) liposome membrane. The approximate location of nitroxide free radical groups of SL-lut was determined based on electron paramagnetic resonance (EPR) spectra. Then the ability of SL-lut to protect EYPC liposomes against lipid peroxidation (LPO) was compared to the antioxidant effects of lutein and a nitroxide spin label 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-1-yloxy (3-CP). Two free radical generation systems were used - a thermal decomposition of 2,2'-azobis (2,4 dimethyl-valeronitrile) (AMVN) and a modified Fenton reaction using ferric-8-hydroxyquinoline (Fe(HQ)(3)). Determination of the amount of thiobarbituric acid reactive species (TBARS) was used as a measure of LPO. SL-lut was the most powerful antioxidant, reducing LPO by about 6-times in AMVN-treated liposomes and 7-times in Fe(HQ)(3)-treated liposomes. Lutein alone gave only a moderate protection in both systems, while 3-CP was as efficient as SL-lut in the presence of AMVN, but not efficient whatsoever in the presence of Fe(HQ)(3). The results suggest that a nitroxide part of SL-lut plays an important role in enhancing the antioxidant activity of lutein and makes SL-lut a powerful antioxidant efficient under different conditions.

Translocation as a means of disseminating lipid hydroperoxide-induced oxidative damage and effector action

Lipid hydroperoxides (LOOHs) generated in cells and lipoproteins under oxidative pressure may induce waves of damaging chain lipid peroxidation near their sites of origin if O2 is readily available and antioxidant capacity is overwhelmed. However, recent studies have demonstrated that chain induction is not necessarily limited to a nascent LOOH's immediate surroundings but can extend to other cell membranes or lipoproteins by means of LOOH translocation through the aqueous phase. Mobilization and translocation can also extend the range of LOOHs as redox signaling molecules and in this sense they could act like the small, readily diffusible inorganic analogue H2O2, which has been studied much more extensively in this regard. In this article, basic mechanisms of free-radical- and singlet-oxygen-mediated LOOH formation and one-electron and two-electron LOOH reduction pathways and their biological consequences are reviewed. The first studies to document spontaneous and protein-assisted LOOH transfer in model systems and cells are described. Finally, LOOH translocation is discussed in the context of cytotoxicity vs detoxification and expanded effector action, i.e., redox signaling activity.

Phospholipase Action of Platelet-activating Factor Acetylhydrolase, but Not Paraoxonase-1, on Long Fatty Acyl Chain Phospholipid Hydroperoxides

Phospholipid hydroperoxide (PLOOH) degrading activity of high density lipoprotein (HDL)-derived paraoxonase-1 (PON1) was investigated, using peroxidized 1-palmitoyl-2-oleoyl phosphatidylcholine (PCOOH) as substrate and high performance thin layer chromatography for quantitative peroxide analysis. Incubation of PCOOH with PON1 resulted in decay of the latter and reciprocal buildup of oleic acid hydroperoxide (OAOOH) at rates unaffected by GSH or other reductants. A serine esterase inhibitor blocked this activity and a recombinant PON1 was devoid of it, raising the possibility that the activity represents platelet-activating factor acetylhydrolase (PAF-AH), an esterase that co-purifies with PON1 from HDL. This was verified by showing that a recombinant PAF-AH recapitulates the ability of natural PON1 to hydrolyze PCOOH and release OAOOH while having essentially no effect on parental PC. Furthermore, recombinant PAF-AH and natural PON1 were shown to have similar K(m) values for PCOOH hydrolysis. Finally, we found that recombinant PAF-AH, but not PON1, catalyzes PLOOH hydrolysis in peroxidized low density lipoprotein. We conclude from this study that PON1 is neither a PLOOH peroxidase nor hydrolase and that the phospholipase A(2)-like activity previously attributed to PON1 in natural enzyme preparations was actually due to novel PLOOH hydrolytic activity of contaminating PAF-AH.

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.

Lipid peroxides promote large rafts: effects of excitation of probes in fluorescence microscopy and electrochemical reactions during vesicle formation

Raft formation and enlargement was investigated in liposomes and supported bilayers prepared from sphingomyelin (SM), cholesterol, and unsaturated phospholipids; NBD-DPPE and rhodamine-(DOPE) were employed as fluorescent probes. Rafts were created by lowering temperature. Maintaining 20 mol % SM, fluorescence microscopy showed that, in the absence of photooxidation, large rafts did not form in giant unilamellar vesicles (GUVs) containing 20 or more mol % cholesterol. But if photooxidation was allowed to proceed, large rafts were readily observed. In population, cuvette experiments, small rafts formed without photooxidation at high cholesterol concentrations. Thus, photooxidation was the cause of raft enlargement during microscopy experiments. Because photooxidation results in peroxidation at lipid double bonds, photosensitization experiments were performed to explicitly produce peroxides of SM and an unsaturated phospholipid. GUVs of high cholesterol content containing the breakdown products of SM-peroxide, but not phospholipid-peroxide, resulted in large rafts after lowering temperature. In addition, GUV production by electroswelling can result in peroxides that cause large raft formation. The use of titanium electrodes eliminates this problem. In conclusion, lipid peroxides and their breakdown products are the cause of large raft formation in GUVs containing biological levels of cholesterol. It is critical that experiments investigating rafts in bilayer membranes avoid the production of peroxides.

Oxidation of Cholesterol by a Biomimetic Oxidant, Cetyltrimethylammonium Dichromate

The oxidation of cholesterol by cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM) yielded 7-dehydrocholesterol, while with addition of acetic acid in DCM the product was found to be 5-cholesten-3-one. The kinetics of oxidation of cholesterol by CTADC in DCM, in the presence of acid, was investigated with change in [acid], [cholesterol], [CTADC], [surfactant], temperature, and solvents. The reaction was found to be first order with acetic acid and fractional order with CTADC and cholesterol. Michaelis-Menten-type kinetics was observed with respect to cholesterol. The solvent isotope effect was found to be k(D2O)/k(H2O) = 0.72. The observed experimental data suggest that the reaction occurs in reversed micellar system, akin to an enzymatic environment, and the reaction path involves the intermediate formation of an ester complex, which undergoes decomposition to give the product.

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.

Zeaxanthin in combination with ascorbic acid or alpha-tocopherol protects ARPE-19 cells against photosensitized peroxidation of lipids

The antioxidant action of carotenoids is believed to involve quenching of singlet oxygen and scavenging of reactive oxygen radicals. However, the exact mechanism by which carotenoids protect cells against oxidative damage, particularly in the presence of other antioxidants, remains to be elucidated. This study was carried out to examine the ability of exogenous zeaxanthin alone and in combination with vitamin E or C, to protect cultured human retinal pigment epithelium cells against oxidative stress. The survival of ARPE-19 cells, subjected to merocyanine 540-mediated photodynamic action, was determined by the MTT test and the content of lipid hydroperoxides in photosensitized cells was analyzed by HPLC with electrochemical detection. We found that zeaxanthin-supplemented cells, in the presence of either alpha-tocopherol or ascorbic acid, were significantly more resistant to photoinduced oxidative stress. Cells with added antioxidants exhibited increased viability and accumulated less lipid hydroperoxides than cells without the antioxidant supplementation. Such a synergistic action of zeaxanthin and vitamin E or C indicates the importance of the antioxidant interaction in efficient protection of cell membranes against oxidative damage induced by photosensitized reactions.

Role of lipid hydroperoxides in photo-oxidative stress signaling

Photosensitized peroxidation of membrane lipids has been implicated in skin pathologies such as phototoxicity, premature aging, and carcinogenesis, and may play a role in the antitumor effects of photodynamic therapy. Lipid hydroperoxides (LOOHs) are prominent early products of photoperoxidation that typically arise via singlet oxygen ((1)O(2)) attack. Nascent LOOHs can have several possible fates, including (i) iron-catalyzed one-electron reduction to chain-initiating free radicals, which exacerbate peroxidative damage, (ii) selenoperoxidase-catalyzed two-electron reduction to relatively innocuous alcohols, and (iii) translocation to other membranes, where reactions noted in (i) or (ii) might take place. In addition, LOOHs, like other stress-associated lipid metabolites/peroxidation products (e.g., arachidonate, diacylglycerol, ceramide, 4-hydroxynonenal), may act as signaling molecules. Intermembrane transfer of LOOHs may greatly expand their signaling range. When photogenerated rapidly and site-specifically, e.g., in mitochondria, LOOHs may act as early mediators of apoptotic cell death. This review will focus on these various aspects, with special attention to the role of LOOHs in photooxidative signaling.

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.

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.

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.

Photosensitized oxidation of membrane lipids: reaction pathways, cytotoxic effects, and cytoprotective mechanisms

Unsaturated lipids in cell membranes, including phospholipids and cholesterol, are well-known targets of oxidative modification, which can be induced by a variety of stresses, including ultraviolet A (UVA)- and visible light-induced photodynamic stress. Photodynamic lipid peroxidation has been associated with pathological conditions such as skin phototoxicity and carcinogenesis, as well as therapeutic treatments such as antitumor photodynamic therapy (PDT). Lipid hydroperoxides (LOOHs), including cholesterol hydroperoxides (ChOOHs), are important non-radical intermediates of the peroxidative process which can (i) serve as in situ reporters of type I vs. type II chemistry; (ii) undergo one-electron or two-electron reductive turnover which determines whether peroxidative injury is respectively intensified or suppressed; and (iii) mediate signaling cascades which either fortify antioxidant defenses of cells or evoke apoptotic death if oxidative pressure is too great. The purpose of this article is to review current understanding of photodynamic (UVA- or visible light-induced) lipid peroxidation with a special focus on LOOH generation and reactivity. Future goals in this area, many of which depend on continued development of state-of-the-art analytical techniques, will also be discussed.

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.