Quenching of singlet oxygen by biomolecules from L1210 leukemia cells

Singlet oxygen lifetime changes in dying glioblastoma cells

Time-resolved phosphorescence detection was employed to determine the lifetime of singlet oxygen in live cells. Using hypericin as a photosensitizer, singlet oxygen was generated in U87MG glioblastoma cells. The phosphorescence of singlet oxygen was detected in aqueous cell suspensions following pulsed laser excitation. Our goal was to eliminate or reduce the problems associated with lifetime measurements in water-based cell suspensions. The apparatus enabled simultaneous singlet oxygen phosphorescence and transient absorption measurements, reducing uncertainty in lifetime estimation. The changes in singlet oxygen lifetime were observed during early and late apoptosis induced by photodynamic action. Our findings show that the effective lifetime of singlet oxygen in the intracellular space of the studied glioblastoma cells is 0.4 μs and increases to 1.5 μs as apoptosis progresses. Another group of hypericin, presumably located in the membrane blebs and the plasma membrane of apoptotic cells, generates singlet oxygen with a lifetime of 1.9 μs.

M6L12 Nanospheres with Multiple C70 Binding Sites for 1O2 Formation in Organic and Aqueous Media

Singlet oxygen is a potent oxidant with major applications in organic synthesis and medicinal treatment. An efficient way to produce singlet oxygen is the photochemical generation by fullerenes which exhibit ideal thermal and photochemical stability. In this contribution we describe readily accessible M₆L₁₂ nanospheres with unique binding sites for fullerenes located at the windows of the nanospheres. Up to four C₇₀ can be associated with a single nanosphere, presenting an efficient method for fullerene extraction and application. Depending on the functionality located on the outside of the sphere, they act as vehicles for ¹O₂ generation in organic or in aqueous media using white LED light. Excellent productivity in ¹O₂ generation and consecutive oxidation of ¹O₂ acceptors using C₇₀⊂[Pd₆L₁₂], C₆₀⊂[Pd₆L₁₂] or fullerene soot extract was observed. The methodological design principles allow preparation and application of highly effective multifullerene binding spheres.

Prostate-specific membrane antigen (PSMA) targeted singlet oxygen delivery via endoperoxide tethered ligands

Singlet oxygen is the primary agent responsible for the therapeutic effects of photodynamic therapy (PDT). In this work, we demonstrate that singlet oxygen release due to thermal endoperoxide cycloreversion can be targeted towards specific features of selected cancer cells, and this targeted singlet oxygen delivery can serve as an effective therapeutic tool. Thus, cytotoxic singlet oxygen can be delivered regioselectively into prostate specific membrane antigen (PSMA) overexpressing lymph node carcinoma (LNCaP) cells. However, unlike typical photodynamic processes, there is no need for light or oxygen. The potential of the approach is exciting, considering the limitations on the availability of light and oxygen in deep-seated tumors.

Heme is responsible for enhanced singlet oxygen deactivation in cytochrome c

The deactivation of singlet oxygen, the lowest electronic excited state of molecular oxygen, by proteins is usually described through the interaction of singlet oxygen with certain amino acids. Changes in accessibility of these amino acids influence the quenching rate and the phosphorescence kinetics of singlet oxygen. In the cellular environment, however, numerous proteins with covalently bound or encapsulated cofactors are present. These cofactors could also influence the deactivation of singlet oxygen, and these have received little attention. To confront this issue, we used cytochrome c (cyt c) and apocytochrome c (apocyt c) to illustrate how the heme prosthetic group influences the rate constant of singlet oxygen deactivation upon acidic pH-induced conformational change of cyt c. Photo-excited flavin mononucleotide (FMN) was used to produce singlet oxygen. Our data show that the heme group has a significant and measurable effect on singlet oxygen quenching when the heme is exposed to solvents and is therefore more accessible to singlet oxygen. The effect of amino acids and heme accessibility on the FMN triplet state deactivation was also investigated.

Photodynamic Therapy-Current Limitations and Novel Approaches

Photodynamic therapy (PDT) mostly relies on the generation of singlet oxygen, via the excitation of a photosensitizer, so that target tumor cells can be destroyed. PDT can be applied in the settings of several malignant diseases. In fact, the earliest preclinical applications date back to 1900’s. Dougherty reported the treatment of skin tumors by PDT in 1978. Several further studies around 1980 demonstrated the effectiveness of PDT. Thus, the technique has attracted the attention of numerous researchers since then. Hematoporphyrin derivative received the FDA approval as a clinical application of PDT in 1995. We have indeed witnessed a considerable progress in the field over the last century. Given the fact that PDT has a favorable adverse event profile and can enhance anti-tumor immune responses as well as demonstrating minimally invasive characteristics, it is disappointing that PDT is not broadly utilized in the clinical setting for the treatment of malignant and/or non-malignant diseases. Several issues still hinder the development of PDT, such as those related with light, tissue oxygenation and inherent properties of the photosensitizers. Various photosensitizers have been designed/synthesized in order to overcome the limitations. In this Review, we provide a general overview of the mechanisms of action in terms of PDT in cancer, including the effects on immune system and vasculature as well as mechanisms related with tumor cell destruction. We will also briefly mention the application of PDT for non-malignant diseases. The current limitations of PDT utilization in cancer will be reviewed, since identifying problems associated with design/synthesis of photosensitizers as well as application of light and tissue oxygenation might pave the way for more effective PDT approaches. Furthermore, novel promising approaches to improve outcome in PDT such as selectivity, bioengineering, subcellular/organelle targeting, etc. will also be discussed in detail, since the potential of pioneering and exceptional approaches that aim to overcome the limitations and reveal the full potential of PDT in terms of clinical translation are undoubtedly exciting. A better understanding of novel concepts in the field (e.g. enhanced, two-stage, fractional PDT) will most likely prove to be very useful for pursuing and improving effective PDT strategies.

Autophagy Regulation and Photodynamic Therapy: Insights to Improve Outcomes of Cancer Treatment

Cancer is considered an age-related disease that, over the next 10 years, will become the most prevalent health problem worldwide. Although cancer therapy has remarkably improved in the last few decades, novel treatment concepts are needed to defeat this disease. Photodynamic Therapy (PDT) signalize a pathway to treat and manage several types of cancer. Over the past three decades, new light sources and photosensitizers (PS) have been developed to be applied in PDT. Nevertheless, there is a lack of knowledge to explain the main biochemical routes needed to trigger regulated cell death mechanisms, affecting, considerably, the scope of the PDT. Although autophagy modulation is being raised as an interesting strategy to be used in cancer therapy, the main aspects referring to the autophagy role over cell succumbing PDT-photoinduced damage remain elusive. Several reports emphasize cytoprotective autophagy, as an ultimate attempt of cells to cope with the photo-induced stress and to survive. Moreover, other underlying molecular mechanisms that evoke PDT-resistance of tumor cells were considered. We reviewed the paradigm about the PDT-regulated cell death mechanisms that involve autophagic impairment or boosted activation. To comprise the autophagy-targeted PDT-protocols to treat cancer, it was underlined those that alleviate or intensify PDT-resistance of tumor cells. Thereby, this review provides insights into the mechanisms by which PDT can be used to modulate autophagy and emphasizes how this field represents a promising therapeutic strategy for cancer treatment.

Confocal fluorescence imaging to evaluate the effect of antimicrobial photodynamic therapy depth on P. gingivalis and T. denticola biofilms

BACKGROUND

Porphyromonas gingivalis and Treponema denticola are both principally implicated in the incidence of both periodontal disease and peri-implantitis. Recent studies have demonstrated that these bacteria exhibit symbiotic growth in vitro and a synergistic virulence in co-infection of animal models. Found at varying depths throughout the biofilm, these bacteria present a significant challenge to traditional antimicrobial treatment modalities. Antimicrobial photodynamic therapy (aPDT) has yielded high success against bacterial biofilms, namely those found in the oral cavity. Data on the use of aPDT against these particular periodontal pathogens is, however, scarce. Here, we studied the qualitative killing efficacy and depth of drug and laser penetration into defined P. gingivalis and T. denticola biofilms.

METHODS

P. gingivalis and T. denticola were incubated under anaerobic (10%CO2, 10%H2, 80%N2) conditions for two days in diluted TSB with PBS (TYGVS for T. denticola maintenance) to elicit biofilm growth on coverslip-modified polystyrene dishes. Treated biofilms were exposed to a purpurin-based sensitizer (25 μg/mL in DMSO) for 30 min, and then aPDT was carried out using a diode laser at 664 nm. Light doses of 15 and 45 J/cm2 were used. All biofilms were then exposed to Filmtracer™ LIVE/DEAD® Biofilm Viability Kit (Cat No. L10316). Qualitative analysis was performed using a Zeiss LSM 510 Meta NLO Confocal Microscope with attached Zeiss Axioimager Z1 and Axiovert 200 M for visual data collection, and images were processed using the ZEN Digital Imaging for Light Microscopy software suite. Analysis was performed in 2 × 3 stacks to assess the entire depth of both the biofilm and presumed drug/laser penetration.

RESULTS

Initial planktonic studies confirmed that the bacteria in question were present in the grown cultures and susceptible to aPDT exposure. Biofilm control groups were found to have significant levels of surviving bacterial colonies. Both treatment groups featured complete bacterial kill throughout the entirety of the biofilm (average: 23.17 μm; range: 18.13-27.20 μm).

CONCLUSIONS

The efficacy of the purpurin-based PS and aPDT is demonstrated to be effective at both high and low light doses. Bacterial kill was fully efficacious at each visualized biofilm layer (1.01 μm/z-level). This study serves as a proof of concept for future studies that must consider appropriate treatment parameters, including the amount of applied PS, and laser dose. These findings indicate that aPDT is a method that can be used to eliminate microorganisms associated with biofilms implicated in the etiology of peri-implantitis and periodontitis at large.

Laminin targeting of a peripheral nerve-highlighting peptide enables degenerated nerve visualization

Target-blind activity-based screening of molecular libraries is often used to develop first-generation compounds, but subsequent target identification is rate-limiting to developing improved agents with higher specific affinity and lower off-target binding. A fluorescently labeled nerve-binding peptide, NP41, selected by phage display, highlights peripheral nerves in vivo. Nerve highlighting has the potential to improve surgical outcomes by facilitating intraoperative nerve identification, reducing accidental nerve transection, and facilitating repair of damaged nerves. To enable screening of molecular target-specific molecules for higher nerve contrast and to identify potential toxicities, NP41's binding target was sought. Laminin-421 and -211 were identified by proximity-based labeling using singlet oxygen and by an adapted version of TRICEPS-based ligand-receptor capture to identify glycoprotein receptors via ligand cross-linking. In proximity labeling, photooxidation of a ligand-conjugated singlet oxygen generator is coupled to chemical labeling of locally oxidized residues. Photooxidation of methylene blue-NP41-bound nerves, followed by biotin hydrazide labeling and purification, resulted in light-induced enrichment of laminin subunits α4 and α2, nidogen 1, and decorin (FDR-adjusted P value < 10^-7) and minor enrichment of laminin-γ1 and collagens I and VI. Glycoprotein receptor capture also identified laminin-α4 and -γ1. Laminins colocalized with NP41 within nerve sheath, particularly perineurium, where laminin-421 is predominant. Binding assays with phage expressing NP41 confirmed binding to purified laminin-421, laminin-211, and laminin-α4. Affinity for these extracellular matrix proteins explains the striking ability of NP41 to highlight degenerated nerve "ghosts" months posttransection that are invisible to the unaided eye but retain hollow laminin-rich tubular structures.

Photodynamic Efficiency: From Molecular Photochemistry to Cell Death

Photodynamic therapy (PDT) is a clinical modality used to treat cancer and infectious diseases. The main agent is the photosensitizer (PS), which is excited by light and converted to a triplet excited state. This latter species leads to the formation of singlet oxygen and radicals that oxidize biomolecules. The main motivation for this review is to suggest alternatives for achieving high-efficiency PDT protocols, by taking advantage of knowledge on the chemical and biological processes taking place during and after photosensitization. We defend that in order to obtain specific mechanisms of cell death and maximize PDT efficiency, PSes should oxidize specific molecular targets. We consider the role of subcellular localization, how PS photochemistry and photophysics can change according to its nanoenvironment, and how can all these trigger specific cell death mechanisms. We propose that in order to develop PSes that will cause a breakthrough enhancement in the efficiency of PDT, researchers should first consider tissue and intracellular localization, instead of trying to maximize singlet oxygen quantum yields in in vitro tests. In addition to this, we also indicate many open questions and challenges remaining in this field, hoping to encourage future research.

Singlet oxygen triplet energy transfer-based imaging technology for mapping protein-protein proximity in intact cells

Many cellular processes are carried out by large protein complexes that can span several tens of nanometres. Whereas forster resonance energy transfer has a detection range of <10 nm, here we report the theoretical development and experimental demonstration of a new fluorescence-imaging technology with a detection range of up to several tens of nanometres: singlet oxygen triplet energy transfer. We demonstrate that our method confirms the topology of a large protein complex in intact cells, which spans from the endoplasmic reticulum to the outer mitochondrial membrane and the matrix. This new method is thus suited for mapping protein proximity in large protein complexes.

Real-time monitoring of oxidative stress in live mouse skin

Oxidative stress is involved in many age-associated diseases, as well as in the aging process itself. The development of interventions to reduce oxidative stress is hampered by the absence of sensitive detection methods that can be used in live animals. We generated transgenic mice expressing ratiometric redox-sensitive green fluorescent protein (roGFP) in the cytosol or mitochondria of several tissues, including skin epidermal keratinocytes. Crossbreeding into hairless albino mice allowed noninvasive optical measurement of skin oxidative state. Topical application of hydrogen peroxide emulsion shifted the keratinocyte redox state toward oxidation within minutes and could be observed in real time by fluorescence ratio imaging. Exposing skin to 365 nm UVA radiation oxidized roGFP localized in keratinocyte mitochondria, but not when roGFP was localized in the cytosol. This suggests that significant amounts of the endogenous photosensitizers that mediate UVA-induced oxidative stress are located in the mitochondria. UVR is the major environmental cause of skin aging and UVA-mediated oxidative stress has been associated with the development of wrinkles in humans. Direct measurements of redox state in defined cell compartments of live animals should be a powerful and convenient tool for evaluating treatments that aim to modulate oxidative stress.

Monitoring oxygen concentration during photodynamic therapy using prompt photosensitizer fluorescence

A novel technique is described that uses either time-resolved or steady state prompt photosensitizer fluorescence to measure local oxygen concentration. Solution experiments conducted with Al(III) phthalocyanine chloride tetrasulfonic acid confirmed that the steady state fluorescence signal is dependent on the oxygen concentration and fluence rate. A relationship between prompt sensitizer fluorescence and sensitizer triplet quenching efficiency is derived which does not require knowledge of the Stern-Volmer constant. Similar relationships are also derived for sensitizer delayed fluorescence and phosphorescence. An explicit photodynamic therapy (PDT) dose metric that incorporates light dosimetry, sensitizer dosimetry, and triplet quenching efficiency is introduced. All components of this metric can be determined by optical measurements.

Photodynamic inactivation of mammalian viruses and bacteriophages

Photodynamic inactivation (PDI) has been used to inactivate microorganisms through the use of photosensitizers. The inactivation of mammalian viruses and bacteriophages by photosensitization has been applied with success since the first decades of the last century. Due to the fact that mammalian viruses are known to pose a threat to public health and that bacteriophages are frequently used as models of mammalian viruses, it is important to know and understand the mechanisms and photodynamic procedures involved in their photoinactivation. The aim of this review is to (i) summarize the main approaches developed until now for the photodynamic inactivation of bacteriophages and mammalian viruses and, (ii) discuss and compare the present state of the art of mammalian viruses PDI with phage photoinactivation, with special focus on the most relevant mechanisms, molecular targets and factors affecting the viral inactivation process.

Irradiation- and sensitizer-dependent changes in the lifetime of intracellular singlet oxygen produced in a photosensitized process

Singlet oxygen, O(2)(a(1)Δ(g)), was produced upon pulsed-laser irradiation of an intracellular photosensitizer and detected by its 1275 nm O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence in time-resolved experiments using (1) individual mammalian cells on the stage of a microscope and (2) suspensions of mammalian cells in a 1 cm cuvette. Data were recorded using hydrophilic and, independently, hydrophobic sensitizers. The microscope-based single cell results are consistent with a model in which the behavior of singlet oxygen reflects the environment in which it is produced; nevertheless, the data also indicate that a significant fraction of a given singlet oxygen population readily crosses barriers between phase-separated intracellular domains. The singlet oxygen phosphorescence signals reflect the effects of singlet-oxygen-mediated damage on cell components which, at the limit, mean that data were collected from dead cells and, in some cases, reflect contributions from both intracellular and extracellular populations of singlet oxygen. Despite the irradiation-induced changes in the environment to which singlet oxygen is exposed, the "inherent" intracellular lifetime of singlet oxygen does not appear to change appreciably as the cell progresses toward death. The results obtained from cell suspensions reflect key features that differentiate cell ensemble from single cell experiments (e.g., the ensemble experiment is more susceptible to the effects of sensitizer that has leaked out of the cell). Overall, the data clearly indicate that measuring the intracellular lifetime of singlet oxygen in a O(2)(a(1)Δ(g)) → O(2)(X(3)Σ(g)(-)) phosphorescence experiment is a challenging endeavor that involves working with a dynamic system that is perturbed during the measurement. The most important aspect of this study is that it establishes a useful framework through which future singlet oxygen data from cells can be interpreted.

Singlet molecular oxygen and primary mechanisms of photo-oxidative damage of chloroplasts. Studies based on detection of oxygen and pigment phosphorescence

Photogeneration of singlet oxygen molecules (1O2), their vibrationally excited stateand dimols (1O2)2has been shown by measuring photosensitised delayed luminescence in pigment-containing media. All singlet oxygen species are formed as a result of energy transfer to O2from triplet pigment molecules. Monomeric pigment molecules are the most efficient singlet oxygen generators. The1O2quantum yields are 40–80% in aerobic solutions of monomeric chlorophylls and pheophytins. Pigment aggregation causes a strong decrease in singlet oxygen production. The1O2quantum yield in chloroplasts has been estimated using literature and experimental data on formation of the chlorophyll triplet states in the photosynthetic apparatus. The most probable value is 0.1%. One of the major sources of singlet oxygen is likely to be the triplet states of newly formed pigment molecules which are not quenched by carotenoids and can be detected by measuring low-temperature pigment phosphorescence. Quenching of singlet oxygen by the thylakoid components has been analysed and the1O2lifetime estimated. The data suggest that carotenoids and chlorophylls are the most efficient physical1O2quenchers and the1O2lifetime is about 70 ns in thylakoids. The quantum yield of1O2-induced pigment photodestruction was estimated to be about 10−6–10−5. This value is close to the quantum yield of chlorophyll photobleaching experimentally observed in aerobic suspensions of isolated chloroplasts. The intensity of pigment phosphorescence at 77 K correlates with the rate of chlorophyll photobleaching in plant materials. The data suggest that1O2generation by the pigment triplet states is the most likely reason for chloroplast photodamage. The intensity of pigment phosphorescence can be used as an index of the degree of plant photo-oxidative stress.

The enzyme-mediated activation of radical source reaction: a new approach to identify partners of a given molecule in membrane microdomains

Important biological events associated with plasma membranes, such as signal transduction, cell adhesion, and protein trafficking, are mediated through the membrane microdomains. However, it is difficult to assess the issue of how they assemble under physiological conditions. We developed a new approach to identify partners of a given molecule on the cell surface in living cells. The important feature of this system, termed as enzyme-mediated activation of radical source, is that activation of cross-linking reagent arylazide-biotin tag can be accomplished not by ultraviolet light, but by an enzyme, horseradish peroxidase. By using this method, we found that many kinds of receptor tyrosine kinases are associated with β1 integrin whereas a few receptor tyrosine kinases are associated with ganglioside GM1 in HeLa S3 cells. This system is a comprehensive approach to identify interactions between cell surface molecules under living conditions. The advantages of this approach are as follows: (i) easy, high throughput, and without the need for special equipment, (ii) applicable to systematic approaches such as proteomic analysis, (iii) applicable to studies on the interactions among not only proteins but also glycans and lipids. The biochemical approach although the enzyme-mediated activation of radical source reaction will provide a new insight into a wide range of research concerning cis-interaction between biomolecules on the cell surface in living cells.

Measurement of singlet-oxygen in vivo: progress and pitfalls

This article is a highlight of the paper by Jarvi et al. in this issue of Photochemistry and Photobiology as well as a brief overview of the state of the field of singlet-oxygen ((1) O(2) ) detection in vivo. The in vivo detection of (1) O(2) using its characteristic 1270 nm phosphorescence is technically challenging. Nevertheless, substantial progress has been made in this area. Major advances have included the commercial development of photomultiplier tubes sensitive to 1270 nm light, techniques for spatially resolving the location of (1) O(2) at a subcellular level and more complex mathematical models for interpreting the kinetics of (1) O(2) emission from living cells. It is now recognized that oxygen consumption, photosensitizer bleaching, oxidation of biological molecules and diffusion of (1) O(2) can significantly change the kinetics of (1) O(2) emission from living cells.

Singlet oxygen: there is indeed something new under the sun

Singlet oxygen, O(2)(a(1)Delta(g)), the lowest excited electronic state of molecular oxygen, has been known to the scientific community for approximately 80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen, O(2)(X(3)Sigma), and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a "mature citizen", singlet oxygen nevertheless remains at the cutting-edge of modern science. In this critical review, recent work on singlet oxygen is summarized, focusing primarily on systems that involve light. It is clear that there is indeed still something new under the sun (243 references).

Chromophore-assisted laser inactivation in cell biology

Chromophore-assisted laser inactivation (CALI) is a technique whereby engineered proteins and dye molecules that produce substantial amounts of reactive oxygen species upon absorption of light are used to perturb biological systems in a spatially and temporally defined manner. CALI is an important complement to conventional genetic and pharmacological manipulations. In this review, we examine the applications of CALI to cell biology and discuss the underlying photochemical mechanisms that mediate this powerful technique.

Kinetics of singlet oxygen photosensitization in human skin fibroblasts

The roles played by singlet oxygen ((1)O(2)) in photodynamic therapy are not fully understood yet. In particular, the mobility of (1)O(2) within cells has been a subject of debate for the last two decades. In this work, we report on the kinetics of (1)O(2) formation, diffusion, and decay in human skin fibroblasts. (1)O(2) has been photosensitized by two water-soluble porphyrins targeting different subcellular organelles, namely the nucleus and lysosomes, respectively. By recording the time-resolved near-IR phosphorescence of (1)O(2) and that of its precursor the photosensitizer's triplet state, we find that the kinetics of singlet oxygen formation and decay are strongly dependent on the site of generation. (1)O(2) photosensitized in the nucleus is able to escape out of the cells while (1)O(2) photosensitized in the lysosomes is not. Despite showing a lifetime in the microsecond time domain, (1)O(2) decay is largely governed by interactions with the biomolecules within the organelle where it is produced. This observation may reconcile earlier views that singlet oxygen-induced photodamage is highly localized, while its lifetime is long enough to diffuse over long distances within the cells.

Biochemical visualization of cell surface molecular clustering in living cells

Many plasma membrane-resident molecules cluster with other molecules to collaborate in a variety of biological events. We herein report a sensitive and simple method to identify components of cell surface molecular clusters in living cells. This method includes a recently established reaction, called the enzyme-mediated activation of radical source (EMARS), to label molecules within a limited distance ( approximately 200-300 nm) from the probed molecule on which HRP is set. Because the size of this active area is close to that of the reported membrane clusters, it is suggested that the labeled molecules cluster with the probed molecule in the same membrane domain. A combination of the EMARS reaction and antibody array analysis demonstrated that many kinds of receptor tyrosine kinases (RTKs) formed clusters with beta1 integrin in HeLa S3 cells. A similar antibody array analysis after the EMARS reaction with three HRP-labeled antibodies against growth factor receptors showed the patterns of biotinylated RTKs to be substantially different from each other. These results suggest that different types of cell surface molecular clusters can thus be distinguished using the EMARS reaction. Therefore, the present "biochemical visualization" method is expected to be a powerful tool to elucidate molecular clustering on the cell surface of living cells in various contexts.

Ascorbate enhances the toxicity of the photodynamic action of Verteporfin in HL-60 cells

As a reducing agent, ascorbate serves as an antioxidant. However, its reducing function can in some settings initiate an oxidation cascade, i.e., seem to be a "pro-oxidant." This dichotomy also seems to hold when ascorbate is present during photosensitization. Ascorbate can react with singlet oxygen, producing hydrogen peroxide. Thus, if ascorbate is present during photosensitization the formation of highly diffusible hydrogen peroxide could enhance the toxicity of the photodynamic action. On the other hand, ascorbate could decrease toxicity by converting highly reactive singlet oxygen to less reactive hydrogen peroxide, which can be removed via peroxide-removing systems such as glutathione and catalase. To test the influence of ascorbate on photodynamic treatment we incubated leukemia cells (HL-60 and U937) with ascorbate and a photosensitizer (Verteporfin; VP) and examined ascorbic acid monoanion uptake, levels of glutathione, changes in membrane permeability, cell growth, and toxicity. Accumulation of VP was similar in each cell line. Under our experimental conditions, HL-60 cells were found to accumulate less ascorbate and have lower levels of intracellular GSH compared to U937 cells. Without added ascorbate, HL-60 cells were more sensitive to VP and light treatment than U937 cells. When cells were exposed to VP and light, ascorbate acted as an antioxidant in U937 cells, whereas it was a pro-oxidant for HL-60 cells. One possible mechanism to explain these observations is that HL-60 cells express myeloperoxidase activity, whereas in U937 cells it is below the detection limit. Inhibition of myeloperoxidase activity with 4-aminobenzoic acid hydrazide (4-ABAH) had minimal influence on the phototoxicity of VP in HL-60 cells in the absence of ascorbate. However, 4-ABAH decreased the toxicity of ascorbate on HL-60 cells during VP photosensitization, but had no affect on ascorbate toxicity in U937 cells. These data demonstrate that ascorbate increases hydrogen peroxide production by VP and light. This hydrogen peroxide activates myeloperoxidase, producing toxic oxidants. These observations suggest that in some settings, ascorbate may enhance the toxicity of photodynamic action.

Formation of singlet oxygen from solutions of vitamin E

Vitamin E offers protection against oxidative stress and is an efficient quencher of singlet oxygen. A recent report suggests that photo-excitation of vitamin E results in the formation of a triplet state (Naqvi et al. Photochem Photobiol Sci 2, 381 (2003)). This leads to the possibility of the triplet state of vitamin E being able to sensitize singlet oxygen and if this is the case it would be counter productive in terms of the biological protective function of vitamin E. We report the production of singlet oxygen, detected by 1270 nm luminescence, from pulsed laser excitation (308 nm) of vitamin E and an analogue, 2,2,5,7,8-pentamethyl-6-hydroxy-chroman (PMHC), with quantum yields between ~0.1 and 0.2. The luminescence was identified as singlet oxygen from self-quenching by vitamin E with solvent-dependent rate constants similar to published values. Whilst the beneficial antioxidant aspects of vitamin E are well established, these results indicate that vitamin E when directly excited can sensitize singlet oxygen formation and may, therefore, be capable of inducing biochemical and biological damage. The results are discussed in relation to recent reports on the deleterious effects of vitamin E dietary supplementation and pro-oxidant effects of vitamin E.

5,10,15,20-Tetrakis(N-Methyl-4-Pyridyl)-21H,23H-Porphine (TMPyP) as a Sensitizer for Singlet Oxygen Imaging in Cells: Characterizing the Irradiation-dependent Behavior of TMPyP in a Single Cell†

Singlet molecular oxygen, a1Delta(g), can be detected from a single cell by its weak 1270 nm phosphorescence (a1Delta(g)-->X3Sigma(g)-) upon irradiation of the photosensitizer 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) incorporated into the cell. The behavior of this sensitizer in a cell, and hence the behavior of the associated singlet oxygen phosphorescence signal, depends on the conditions under which the sample is exposed to light. Upon irradiation of a neuron freshly incubated with TMPyP, the intensity of TMPyP fluorescence initially increases and there is a concomitant increase in the singlet oxygen phosphorescence intensity from the cell. These results appear to reflect a photoinduced release of TMPyP bound to DNA in the nucleus of the cell, where TMPyP tends to localize, and the subsequent relocalization of TMPyP to a different microenvironment in the cell. Upon prolonged irradiation of the cell, TMPyP photobleaches and there is a corresponding decrease in the singlet oxygen phosphorescence intensity from the cell. The data reported herein provide insight into key factors that can influence photosensitized singlet oxygen experiments performed on biological samples.

Physical and chemical properties of pyropheophorbide-a methyl ester in ethanol, phosphate buffer and aqueous dispersion of small unilamellar dimyristoyl-l-α-phosphatidylcholine vesicles

The aggregation process of pyropheophorbide-a methyl ester (PPME), a second-generation photosensitizer, was investigated in various solvents. Absorption and fluorescence spectra showed that the photosensitizer was under a monomeric form in ethanol as well as in dimyristoyl-L-alpha-phosphatidylcholine liposomes while it was strongly aggregated in phosphate buffer. A quantitative determination of reactive oxygen species production by PPME in these solvents has been undertaken by electron spin resonance associated with spin trapping technique and absorption spectroscopy. In phosphate buffer, both electron spin resonance and absorption measurements led to the conclusion that singlet oxygen production was not detectable while hydroxyl radical production was very weak. In liposomes and ethanol, singlet oxygen and hydroxyl radical production increased highly; the singlet oxygen quantum yield was determined to be 0.2 in ethanol and 0.13 in liposomes. The hydroxyl radical production origin was also investigated. Singlet oxygen was formed from PPME triplet state deactivation in the presence of oxygen. Indeed, the triplet state formation quantum yield of PPME was found to be about 0.23 in ethanol, 0.15 in liposomes (too small to be measured in PBS).

Spatially Resolved Cellular Responses to Singlet Oxygen

Singlet oxygen (1O2) is unique amongst reactive oxygen species formed in cells in that it is an excited state molecule with an inherent upper lifetime of 4 micros in water. Whether the lifetime of 1O2 in cells is shortened by reactions with cellular molecules or reaches the inherent maximum value is still unclear. However, even with the maximum lifetime, the diffusion radius is only approximately 220 nm during three lifetimes (approximately 5% 1O2 remaining), much shorter than cellular dimensions indicating that the primary reactions of 1O2 will be subcellularly localized near the site of 1O2 formation. This fact has raised the question of whether spatially resolved cellular responses to 1O2 occur, i.e. whether responses can be initiated by generation and reaction of 1O2 at a particular subcellular location that would not have been produced by 1O2 generation at other subcellular sites. In this paper, we discuss examples of spatially resolved responses initiated by 1O2 as a function of distance from the site of generation of 1O2. Three levels are recognized, namely, a molecular level where the primary oxidation product directly modifies the behavior of a cell, an organelle level where the initial photo-oxidation products initiate mechanisms that are unique to the organelle and the cellular level where mediators diffuse from their site of formation to the target molecules that initiate the response. These examples indicate that, indeed, spatially resolved responses to 'O2 occur in cells.

Synthetic carotenoid derivatives prevent photosensitised killing of retinal pigment epithelial cells more effectively than lutein

We studied the photosensitised killing of retinal pigment epithelial (RPE) cells using two photosensitisers that localise in lysosomes. The ARPE-19 cell line was photosensitised using either acridine orange or cis-di(4-sulfonatophenyl)diphenylporphine. We then measured the amount of photoprotection provided to RPE cells by five synthetic carotenoid derivatives and by lutein. The synthetic carotenoid derivatives studied were the Girard's reagent P derivative (GRP) of retinal (GRP-retinal), the GRP derivative of beta-apo-8'-carotenal (GRP-carotenal), the Girard's reagent T derivative of beta-apo-8'-carotenal (GRT-carotenal), the GRP derivative of canthaxanthin ((GRP)2-canthaxanthin) and the dansyl hydrazine derivative of beta-apo-8'-carotenal (dansyl-carotenal). We found that GRP-carotenal, GRT-carotenal (GRP)2-canthaxanthin and dansyl-carotenal were effective photoprotectors. All of these carotenoids had large singlet-oxygen quenching constants and had chemical structures designed to localise either in mitochondria or in lysosomes. In contrast, lutein and GRP-retinal were not effective photoprotectors. The failure of GRP-retinal to provide significant photoprotection may have been due to its relatively low singlet-oxygen quenching constant. Lutein is a potent singlet-oxygen quencher, but may not have provided significant photoprotection in this model because the lutein may have had a different subcellular distribution than the photosensitisers used.

Methylene blue in photodynamic therapy: From basic mechanisms to clinical applications

Methylene blue (MB) is a molecule that has been playing important roles in microbiology and pharmacology for some time. It has been widely used to stain living organisms, to treat methemoglobinemia, and lately it has been considered as a drug for photodynamic therapy (PDT). In this review, we start from the fundamental photophysical, photochemical and photobiological characteristics of this molecule and evolved to show in vitro and in vivo applications related to PDT. The clinical cases shown include treatments of basal cell carcinoma, Kaposi's Sarcoma, melanoma, virus and fungal infections. We concluded that used together with a recently developed continuous light source (RL50(®)), MB has the potential to treat a variety of cancerous and non-cancerous diseases, with low toxicity and no side effects.

Time-Resolved Investigations of Singlet Oxygen Luminescence in Water, in Phosphatidylcholine, and in Aqueous Suspensions of Phosphatidylcholine or HT29 Cells

Singlet oxygen was generated by energy transfer from the photoexcited sensitizer, Photofrin or 9-acetoxy-2,7,12,17-tetrakis-(beta-methoxyethyl)-porphycene (ATMPn), to molecular oxygen. Singlet oxygen was detected time-resolved by its luminescence at 1270 nm in an environment of increasing complexity, water (H2O), pure phosphatidylcholine, phosphatidylcholine in water (lipid suspensions), and aqueous suspensions of living cells. In the case of the lipid suspensions, the sensitizers accumulated in the lipids, whereas the localizations in the cells are the membranes containing phosphatidylcholine. By use of Photofrin, the measured luminescence decay times of singlet oxygen were 3.5 +/- 0.5 micros in water, 14 +/- 2 micros in lipid, 9 +/- 2 micros in aqueous suspensions of lipid droplets, and 10 +/- 3 micros in aqueous suspensions of human colonic cancer cells (HT29). The decay time in cell suspensions was much longer than in water and was comparable to the value in suspensions of phosphatidylcholine. That luminescence signal might be attributed to singlet oxygen decaying in the lipid areas of cellular membranes. The measured luminescence decay times of singlet oxygen excited by ATMPn in pure lipid and lipid suspensions were the same within the experimental error as for Photofrin. In contrast to experiments with Photofrin, the decay time in aqueous suspension of HT29 cells was 6 +/- 2 micros when using ATMPn.

Ratiometric Singlet Oxygen Nano-optodes and Their Use for Monitoring Photodynamic Therapy Nanoplatforms

Ratiometric photonic explorers for bioanalysis with biologically localized embedding (PEBBLE) nanoprobes have been developed for singlet oxygen, using organically modified silicate (ORMOSIL) nanoparticles as the matrix. A crucial aspect of these ratiometric singlet-oxygen fluorescent probes is their minute size. The ORMOSIL nanoparticles are prepared via a sol-gel-based process and the average diameter of the resultant particles is about 160 nm. These sensors incorporate the singlet-oxygen-sensitive 9,10-dimethyl anthracene as an indicator dye and a singlet-oxygen-insensitive dye, octaethylporphine, as a reference dye for ratiometric fluorescence-based analysis. We have found experimentally that these nanoprobes have much better sensitivity than does the conventional singlet-oxygen-free dye probe, anthracene-9,10-dipropionic acid disodium salt. The much longer lifetime of singlet oxygen in the ORMOSIL matrix, compared to aqueous solutions, in addition to the relatively high singlet oxygen solubility because of the highly permeable structure and the hydrophobic nature of the outer shell of the ORMOSIL nanoparticles, results in an excellent overall response to singlet oxygen. These nanoprobes have been used to monitor the singlet oxygen produced by "dynamic nanoplatforms" that were developed for photodynamic therapy. The singlet oxygen nanoprobes could potentially be used to quantify the singlet oxygen produced by macrophages.

Genetically targeted chromophore-assisted light inactivation

Studies of protein function would be facilitated by a general method to inactivate selected proteins in living cells noninvasively with high spatial and temporal precision. Chromophore-assisted light inactivation (CALI) uses photochemically generated, reactive oxygen species to inactivate proteins acutely, but its use has been limited by the need to microinject dye-labeled nonfunction-blocking antibodies. We now demonstrate CALI of connexin43 (Cx43) and alpha1C L-type calcium channels, each tagged with one or two small tetracysteine (TC) motifs that specifically bind the membrane-permeant, red biarsenical dye, ReAsH. ReAsH-based CALI is genetically targeted, requires no antibodies or microinjection, and inactivates each protein by approximately 90% in <30 s of widefield illumination. Similar light doses applied to Cx43 or alpha1C tagged with green fluorescent protein (GFP) had negligible to slight effects with or without ReAsH exposure, showing the expected molecular specificity. ReAsH-mediated CALI acts largely via singlet oxygen because quenchers or enhancers of singlet oxygen respectively inhibit or enhance CALI.