Nitric oxide-mediated resistance to photodynamic therapy in a human breast tumor xenograft model: Improved outcome with NOS2 inhibitors

Many malignant tumors employ iNOS-derived NO to resist eradication by chemotherapeutic agents or ionizing radiation. In this study, we determined whether human breast carcinoma MDA-MB-231 cells in vitro and in vivo as tumor xenografts would exploit endogenous iNOS/NO to resist the cytotoxic effects of 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT). Broad band visible irradiation of ALA-treated cells resulted in a marked after-light upregulation of iNOS protein which persisted for at least 24 h. Apoptotic killing of ALA/light-challenged cells was significantly enhanced by iNOS inhibitors (1400W, GW274150) and a NO trap (cPTIO), implying that stress-induced iNOS/NO was acting cytoprotectively. We found that cells surviving the photostress proliferated and migrated more rapidly than controls in 1400W- and cPTIO-inhibitable fashion, indicating iNOS/NO involvement. Female SCID mice bearing MDA-MB-231 tumors were used for animal model experiments. ALA-PDT with a 633 nm light source caused a significant reduction in post-irradiation tumor growth relative to light-only controls, which was further reduced by administration of 1400W or GW274150, whereas 1400W had little or no effect on controls. Immunoblot analyses of tumor samples revealed a progressive post-PDT upregulation of iNOS, which reached >5-times the control level after six days. Correspondingly, the nitrite/nitrate level in post-PDT tumor samples was substantially higher than that in controls. In addition, a 1400W-inhibitable upregulation of pro-survival/progression effector proteins such as Bcl-xL, Survivin, and S100A4 was observed after in vitro and in vivo ALA-PDT. This is the first known study to demonstrate iNOS/NO-induced resistance to PDT in an in vivo human tumor model.

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.

Modulation of the Anti-Tumor Efficacy of Photodynamic Therapy by Nitric Oxide

Nitric oxide (NO) produced by nitric oxide synthase (NOS) enzymes is a free radical molecule involved in a wide variety of normophysiologic and pathophysiologic processes. Included in the latter category are cancer promotion, progression, and resistance to therapeutic intervention. Animal tumor photodynamic therapy (PDT) studies several years ago revealed that endogenous NO can reduce PDT efficacy and that NOS inhibitors can alleviate this. Until relatively recently, little else was known about this anti-PDT effect of NO, including: (a) the underlying mechanisms; (b) type(s) of NOS involved; and (c) whether active NO was generated in vascular cells, tumor cells, or both. In addressing these questions for various cancer cell lines exposed to PDT-like conditions, the author's group has made several novel findings, including: (i) exogenous NO can scavenge lipid-derived free radicals arising from photostress, thereby protecting cells from membrane-damaging chain peroxidation; (ii) cancer cells can upregulate inducible NOS (iNOS) after a PDT-like challenge and the resulting NO can signal for resistance to photokilling; (iii) photostress-surviving cells with elevated iNOS/NO proliferate and migrate/invade more aggressively; and (iv) NO produced by photostress-targeted cells can induce greater aggressiveness in non-targeted bystander cells. In this article, the author briefly discusses these various means by which NO can interfere with PDT and how this may be mitigated by use of NOS inhibitors as PDT adjuvants.

Role of Endogenous Nitric Oxide in Hyperaggressiveness of Tumor Cells that Survive a Photodynamic Therapy Challenge

Many malignant tumors exploit nitric oxide (NO) for a survival, growth, and migration/invasion advantage, and also to withstand the cytotoxic effects of chemo- and radiotherapies. Endogenous NO has also been shown to antagonize photodynamic therapy (PDT), a unique minimally invasive modality involving a photosensitizing (PS) agent, PS-exciting light in the visible- to near-infrared range, and molecular oxygen. The anti-PDT effects of NO were discovered about 20 years ago, but the underlying mechanisms are still not fully understood. More recent studies in the author's laboratory using breast, prostate, and brain cancer cell lines have shown that inducible NO synthase (iNOS/NOS2) is dramatically upregulated after a PDT challenge using 5-aminolevulinic acid (ALA-) -induced protoporphyrin IX as the PS. The parallel increase in NO resulted not only in a greater resistance to cell killing but also in a striking increase in the growth and migration/invasion rate of surviving cells. These in vitro findings and their recent recapitulation at the in vivo level are discussed in this article, along with how iNOS/NO's negative effects on PDT can be attenuated by the use of select iNOS inhibitors as PDT adjuvants.

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.

Accelerated migration and invasion of prostate cancer cells after a photodynamic therapy-like challenge: Role of nitric oxide

Employing an in vitro model for 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), we recently reported that human prostate cancer PC3 cells rapidly and persistently overexpressed inducible nitric oxide synthase (iNOS) and nitric oxide (NO) after a moderate ALA/light challenge. The upregulated iNOS/NO was shown to play a key role in cell resistance to apoptotic photokilling and also in the dramatic growth spurt observed in surviving cells. In the present study, we found that PC3 cells surviving an ALA/light insult not only proliferated faster than non-stressed controls, but migrated and invaded faster as well, these effects being abrogated by an iNOS inhibitor or NO scavenger. Photostressed prostate DU145 cells exhibited similar behavior. Using in-gel zymography, we showed that PC3 extracellular matrix metalloproteinase-9 (MMP-9) was strongly activated 24 h after ALA/light treatment and that MMP-9 inhibitor TIMP-1 was downregulated, consistent with MMP-9 involvement in enhanced invasiveness. We also observed a photostress-induced upregulation of α6 and β1 integrins, implying their involvement as well. The MMP-9, TIMP-1, and integrin effects were strongly attenuated by iNOS inhibition, confirming NO's role in photostress-enhanced migration/invasion. This study reveals novel, potentially tumor-promoting, side-effects of prostate cancer PDT which may be averted through use of iNOS inhibitors as PDT adjuvants.

Photodynamic therapy (PDT) for malignant brain tumors--where do we stand?

INTRODUCTION

What is the current status of photodynamic therapy (PDT) with regard to treating malignant brain tumors? Despite several decades of effort, PDT has yet to achieve standard of care.

PURPOSE

The questions we wish to answer are: where are we clinically with PDT, why is it not standard of care, and what is being done in clinical trials to get us there.

METHOD

Rather than a meta-analysis or comprehensive review, our review focuses on who the major research groups are, what their approaches to the problem are, and how their results compare to standard of care. Secondary questions include what the effective depth of light penetration is, and how deep can we expect to kill tumor cells.

CURRENT RESULTS

A measurable degree of necrosis is seen to a depth of about 5mm. Cavitary PDT with hematoporphyrin derivative (HpD) results are encouraging, but need an adequate Phase III trial. Talaporfin with cavitary light application appears promising, although only a small case series has been reported. Foscan for fluorescence guided resection (FGR) plus intraoperative cavitary PDT results were improved over controls, but are poor compared to other groups. 5-Aminolevulinic acid-FGR plus postop cavitary HpD PDT show improvement over controls, but the comparison to standard of care is still poor.

CONCLUSION

Continued research in PDT will determine whether the advances shown will mitigate morbidity and mortality, but certainly the potential for this modality to revolutionize the treatment of brain tumors remains. The various uses for PDT in clinical practice should be pursued.

Tumor-generated nitric oxide as an antagonist of photodynamic therapy

Nitric oxide (NO) is a multifunctional free radical molecule produced naturally by nitric oxide synthase (NOS) enzymes. Many tumors exploit NO for survival and growth signaling, and also to thwart the effects of therapeutic treatments, including PDT. The anti-PDT effects of NO were discovered using animal tumor models, but the mechanisms involved are still not fully understood. Recent in vitro studies on breast and prostate cancer cells have shown that inducible NOS (iNOS) along with NO is dramatically upregulated after an ALA-PDT-like challenge. Cells were more resistant to apoptosis after a photochallenge and survivors grew, migrated, and invaded more rapidly, iNOS/NO playing a key role in all these effects. This perspective briefly reviews what is currently known about NO's negative effects on PDT and some of the signaling mechanisms involved. It also provides insights into how these effects may be attenuated by pharmacologic use of iNOS inhibitors.

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.

Regulation of ferroptotic cancer cell death by GPX4

Ferroptosis is a form of nonapoptotic cell death for which key regulators remain unknown. We sought a common mediator for the lethality of 12 ferroptosis-inducing small molecules. We used targeted metabolomic profiling to discover that depletion of glutathione causes inactivation of glutathione peroxidases (GPXs) in response to one class of compounds and a chemoproteomics strategy to discover that GPX4 is directly inhibited by a second class of compounds. GPX4 overexpression and knockdown modulated the lethality of 12 ferroptosis inducers, but not of 11 compounds with other lethal mechanisms. In addition, two representative ferroptosis inducers prevented tumor growth in xenograft mouse tumor models. Sensitivity profiling in 177 cancer cell lines revealed that diffuse large B cell lymphomas and renal cell carcinomas are particularly susceptible to GPX4-regulated ferroptosis. Thus, GPX4 is an essential regulator of ferroptotic cancer cell death.

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.

Development of a tumor-specific photoactivatable doxorubicin prodrug

This is a retrospective highlight on the publication by Ibsen and coworkers: Localized In Vivo Activation of a Photoactivatable Doxorubicin Prodrug in Deep Tumor Tissue, which appeared in a preceding issue of Photochem. Photobiol. (2013, 89:698-708). The authors describe the synthesis and properties of a novel doxorubicin (DOX) prodrug, DOX-PCB, which contains a photocleavable linker group. Systemic administration of the prodrug to a tumor-bearing animal followed by LED/fiber optic 365 nm light delivery allowed active DOX to be released site specifically in the tumor area. This elegant and timely study provides compelling evidence that photocleavable DOX-PCB can eliminate many of the toxic side effects of DOX that have plagued clinical use of this highly effective antitumor drug for many years.

Cytoprotective signaling associated with nitric oxide upregulation in tumor cells subjected to photodynamic therapy-like oxidative stress

Photodynamic therapy (PDT) employs photoexcitation of a sensitizer to generate tumor-eradicating reactive oxygen species. We recently showed that irradiating breast cancer COH-BR1 cells after treating with 5-aminolevulinic acid (ALA, a pro-sensitizer) resulted in rapid upregulation of inducible nitric oxide (NO) synthase (iNOS). Apoptotic cell killing was strongly enhanced by an iNOS inhibitor (1400W), iNOS knockdown (kd), or a NO scavenger, suggesting that NO was acting cytoprotectively. Stress signaling associated with these effects was examined in this study. ALA/light-stressed COH-BR1 cells, and also breast adenocarcinoma MDA-MB-231 cells, mounted an iNOS/NO-dependent resistance to apoptosis that proved to be cGMP-independent. Immunocytochemistry and subcellular Western analysis of photostressed COH-BR1 cells revealed a cytosol-to-nucleus translocation of NF-κB which was negated by the NF-κB activation inhibitor Bay11. Bay11 also enhanced apoptosis and prevented iNOS induction, consistent with NF-κB involvement in the latter. JNK and p38 MAP kinase inhibitors suppressed apoptosis, implicating these kinases in death signaling. Post-irradiation extent and duration of JNK and p38 phosphorylation were dramatically elevated by 1400 W or iNOS-kd, suggesting that these activations were suppressed by NO. Regarding pro-survival stress signaling, rapid activation of Akt was unaffected by 1400 W, but prevented by Wortmannin, which also enhanced apoptosis. Thus, a link between upstream Akt activation and iNOS induction was apparent. Furthermore, p53 protein expression under photostress was elevated by iNOS-kd, whereas robust Survivin induction was abolished, consistent with p53 and Survivin being negatively and positively regulated by NO, respectively. Collectively, these findings enhance our understanding of cytoprotective signaling associated with photostress-induced NO and suggest iNOS inhibitor-based approaches for improving PDT efficacy.

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.

Abstract A45: Signaling Events Associated with Cytoprotective Induction of Nitric Oxide Synthase in a Photodynamic Therapy Model

Photodynamic therapy (PDT) employs a photosensitizing agent, molecular oxygen, and photosensitizer-exciting light to produce reactive oxygen species that lead to tumor eradication. Nitric oxide (NO) produced by tumor or tumor vasculature cells could be pro-carcinogenic by inhibiting apoptosis and/or promoting angiogenesis and tumor growth. We recently showed that photoactivation of 5-aminolevulinic acid (ALA)-generated protoporphyrin IX in mitochondria of COH-BR1 breast tumor cells strongly upregulated inducible nitric oxide synthase (iNOS) and steady state NO level in these cells. Including the iNOS inhibitor 1400W during photochallenge dramatically enhanced caspase-9 activation and Annexin-assessed apoptotic cell killing while reducing NO production assessed by the intracellular NO probe DAF-2DA. Short hairpin RNA (ShRNA)-based iNOS knockdown (kd) produced similar results, confirming iNOS involvement. An apoptosis-promoting effect of iNOS inhibition was also observed when breast MDA-MB231 and prostate PC-3 cancer cells were subjected to ALA/light stress. We have now examined the signaling events associated with iNOS-mediated hyperresistance of COH-BR1 cells using a combination of immunocytochemistry, western blotting, immunoprecipitaion, and iNOS-kd methods. A soluble guanylyl cyclase inhibitor failed to stimulate ALA/light-provoked apoptosis, ruling out cGMP involvement in stress resistance. ALA/light activated PI3-kinase-dependent signaling via phosphorylation-inactivation of the tumor suppressor PTEN, leading to phosphorylation-activation of pro-survival Akt. Inhibition of PI3K by Wortmannin prevented ALA/light-induced Akt activation as well as iNOS upregulation while enhancing apoptotic photokilling. Moreover, photostress activation of PI3K/Akt was accompanied by a cytosol-to-nucleus translocation of the iNOS transcription factor NF-κB, and an inhibitor of NF-kB activation prevented iNOS induction while stimulating apoptosis. Furthermore, iNOS-kd as well as 1400W treatment resulted in intensified and more prolonged activation of MAP kinases JNK and p38α by ALA/light stress. In addition, photostressed iNOS-kd cells exhibited p53 upregulation and Survivin inactivation/downregulation. The following general mechanism of NO-based cytoprotection is suggested from these and related findings: Photostress activation of PI3K/Akt suppresses pro-apoptotic MAP kinases and p53 while upregulating/activating Survivin via NF-κB-mediated induction of iNOS/NO. This study provides important new insights into photostress-elicited pro-survival signaling that could reduce PDT effectiveness, and suggests iNOS inhibitor-based interventions for counteracting this.

(Supported by NIH Grant CA70823 and a grant from the MCW Cancer Center.)

Relationship between oxidizable fatty acid content and level of antioxidant glutathione peroxidases in marine fish

Biological membranes can be protected from lipid peroxidation by antioxidant enzymes including catalase (CAT) and selenium-dependent glutathione peroxidases 1 and 4 (GPx1 and GPx4). Unlike GPx1, GPx4 can directly detoxify lipid hydroperoxides in membranes without prior action of phospholipase A(2). We hypothesized that (1) GPx4 is enhanced in species that contain elevated levels of highly oxidizable polyunsaturated fatty acids (PUFA) and (2) activities of antioxidant enzymes are prioritized to meet species-specific oxidative stresses. In this study we examined (i) activities of the oxidative enzyme citrate synthase (CS) and antioxidant (CAT, GPx1 and GPx4) enzymes, (ii) GPx4 protein expression, and (iii) phospholipid composition in livers of five species of marine fish (Myxine glutinosa, Petromyzon marinus, Squalus acanthias, Fundulus heteroclitus and Myoxocephalus octodecemspinosus) that contain a range of PUFA. GPx4 activity was, on average, 5.8 times higher in F. heteroclitus and S. acanthias than in the other three marine fish species sampled. Similarly, activities of CAT and GPx1 were highest in S. acanthias and F. heteroclitus, respectively. GPx4 activity for all species correlates with membrane unsaturation, as well as oxidative activity as indicated by CS. These data support our hypothesis that GPx4 level in marine fish is a function, at least in part, of high PUFA content in these animals. GPx1 activity was also correlated with membrane unsaturation, indicating that marine species partition resources among glutathione-dependent defenses for protection from the initial oxidative insult (e.g. H(2)O(2)) and to repair damaged lipids within biological membranes.

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.

Photodynamic therapy of cancer: an update

Photodynamic therapy (PDT) is a clinically approved, minimally invasive therapeutic procedure that can exert a selective cytotoxic activity toward malignant cells. The procedure involves administration of a photosensitizing agent followed by irradiation at a wavelength corresponding to an absorbance band of the sensitizer. In the presence of oxygen, a series of events lead to direct tumor cell death, damage to the microvasculature, and induction of a local inflammatory reaction. Clinical studies revealed that PDT can be curative, particularly in early stage tumors. It can prolong survival in patients with inoperable cancers and significantly improve quality of life. Minimal normal tissue toxicity, negligible systemic effects, greatly reduced long-term morbidity, lack of intrinsic or acquired resistance mechanisms, and excellent cosmetic as well as organ function-sparing effects of this treatment make it a valuable therapeutic option for combination treatments. With a number of recent technological improvements, PDT has the potential to become integrated into the mainstream of cancer treatment.

Rapid upregulation of cytoprotective nitric oxide in breast tumor cells subjected to a photodynamic therapy-like oxidative challenge

Many tumor cells produce nitric oxide (NO) as an antiapoptotic/progrowth molecule which also promotes antiogenesis and tumor expansion. This study was designed to examine possible antagonistic effects of endogenous NO on tumor eradication by photodynamic therapy (PDT). Using COH-BR1 breast cancer cells sensitized in mitochondria with 5-aminolevulinic acid (ALA)-generated protoporphyrin IX as a model for ALA-based PDT, we found that caspase-9 activation and apoptotic death following irradiation were strongly enhanced by 1400W, an inhibitor of inducible nitric oxide synthase (iNOS). RT-PCR and Western analyses revealed a substantial upregulation of both iNOS mRNA and protein, beginning ca 4 h after irradiation and persisting for at least 20 h. Accompanying this was a strong 1400W-inhibitable increase in intracellular NO, as detected with the NO probe, DAF-2-DA. Short hairpin RNA-based iNOS knockdown in COH-BR1 cells dramatically reduced NO production under photostress while enhancing caspase-9 activation and apoptosis. These findings suggest that cytoprotective iNOS/NO induction in PDT-treated tumor cells could reduce treatment efficacy, and point to pharmacologic intervention with iNOS inhibitors for counteracting this.

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.

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

Photodynamic therapy (PDT) employs a photosensitizing agent, molecular oxygen, and visible light to generate reactive species that kill tumor and tumor vasculature cells. Nitric oxide produced by these cells could be procarcinogenic by inhibiting apoptosis or promoting angiogenesis and tumor growth. The purpose of this study was to determine whether tumor cells upregulate NO as a cytoprotective measure during PDT. Breast tumor COH-BR1 cells sensitized in their mitochondria with 5-aminolevulinic acid (ALA)-derived protoporphyrin IX died apoptotically after irradiation, ALA- and light-only controls showing no effect. Western analysis revealed that inducible nitric oxide synthase (iNOS) was upregulated >3-fold within 4 h after ALA/light treatment, whereas other NOS isoforms were unaffected. Exposing cells to a NOS inhibitor (L-NAME or 1400W) during photochallenge enhanced caspase-3/7 activation and apoptotic killing up to 2- to 3-fold while substantially reducing chemiluminescence-assessed NO production, suggesting that this NO was cytoprotective. Consistently, the NO scavenger cPTIO enhanced ALA/light-induced caspase-3/7 activation and apoptotic kill by >2.5-fold. Of added significance, cells could be rescued from 1400W-exacerbated apoptosis by an exogenous NO donor, spermine-NONOate. This is the first reported evidence for increased tumor cell resistance due to iNOS upregulation in a PDT model. Our findings indicate that stress-elicited NO in PDT-treated tumors could compromise therapeutic efficacy and suggest NOS-based pharmacologic interventions for preventing this.

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

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

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

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.