Cellular photodestruction induced by hypericin in AY-27 rat bladder carcinoma cells

In a recent clinical study we showed that hypericin accumulates selectively in urothelial lesions following intravesical administration of the compound to patients. In the present study the efficacy of hypericin as a photochemotherapeutic tool against urinary bladder carcinoma was investigated using the AY-27 cells (chemically induced rat bladder carcinoma cells). The uptake of hypericin by the cells increased by prolonging the incubation time and increasing the extracellular hypericin concentration. Photodynamic treatment of the cells incubated with 0.8 and 1.6 microM hypericin concentrations resulted in remarkable cytotoxic effects the extent of which depended on the fluence rates. Photoactivation of 1.6 microM hypericin by 0.5, 1.0 or 2.0 mW/cm2 for 15 min resulted in 3, 30 and 95% of the antiproliferative effect, respectively. Increasing the photoactivating light dose from 0.45 to 3.6 J/cm2 resulted in a five-fold increase in hypericin photodynamic activity. Irrespective of the fluence rates and irradiation times incubation of the cells with 10 microM hypericin induced rapid and extensive cell death in all conditions. The type of cell death (apoptosis or necrosis) induced by photoactivated hypericin depended largely on the hypericin concentration and the postirradiation time. At lower hypericin concentrations and shorter postirradiation times apoptosis was the prominent mode of cell death; increasing the hypericin concentration and/or prolonging the postirradiation time resulted in increased necrotic cell death. Cell pretreatment with the singlet oxygen quencher histidine, but not with the free-radical quenchers, significantly protected the cells from photoactivated hypericin-induced apoptosis, at least when a relatively low concentration (1.25 microM) was used. This result suggests the involvement of a Type-II photosensitization process. However, cells treated with higher hypericin concentrations (2.5-5 microM) were inadequately protected by histidine. Since hypericin is thus shown to be a potent and efficient photosensitizer, and since the conditions used were the same as when hypericin is used clinically to locate early-stage urothelial carcinoma lesions, hypericin may well become very important for the photodynamic treatment of superficial bladder carcinoma.

Photodynamically induced cytotoxicity of hypericin dye on human fibroblast cell line MRC5

The possible application of hypericin (hyp) in the photodynamic therapy (PDT) of cancer was investigated using the human fibroblast cell line MRC5. In aerobic conditions, at pH 7.4, irradiation of MRC5 cells was carried out with different doses of visible light and different doses of hyp. A low concentration of hyp (5 x 10(-9) M) was highly toxic to MRC5 cells, producing 15% survival for an irradiation period of 40 min. In the dark, no cytotoxicity was observed in the range 10(-9)-10(-7) M hyp. The mechanism of cell killing by hyp was also examined. Significant inhibition of MRC5 killing was observed on addition of 1,4-diazabicyclo[2,2,2]octane (DABCO) or histidine, known quenchers of type II mechanisms. In addition, the photodynamic effect of hyp was enhanced by deuterium oxide. The addition of desferrioxamine, catalase or superoxide dismutase (SOD), known scavenging agents of the type I mechanism, had a significant inhibitory effect on the rate of photodynamic action of hyp. The experimental results suggest that hyp has considerable potential for use as a sensitizer in the PDT of cancer.

Cell membrane, a target for PUVA therapy

The photochemical reactions occurring in the cell membranes, sensitized photo-oxidation and psoralen photoaddition to lipids, are briefly reviewed. Phospholipid dynamics in the membrane structure, based on erythrocyte lipid organization, are described. Evidence for alterations of cell membrane functions under psoralen plus UVA radiation (PUVA) treatment in a variety of mammalian cells is presented. Cell receptor dysfunctions under PUVA treatment are demonstrated in a number of biological investigations. The purpose of this survey is to illustrate the feasibility of studying psoralen photobiology with phospholipids. The reaction of psoralens with phospholipids is considered to be one of the triggering mechanisms of the subsequent physiological responses, which may be relevant to PUVA photochemotherapy.

Direct exposure of mammalian cells to pure exogenous singlet oxygen (1 delta gO2)

Mammalian cells attached to membrane filters or deposited on filters without attachment were exposed to gas-phase singlet oxygen (1O2) in the absence of any other reactants. Cells were exposed in a monolayer or less, in the absence of external medium, during steady-state 1O2 generation, ensuring that singlet oxygen impinged directly and equally on all cells simultaneously. The current methodology for cell exposure ensures that 1O2 is initially the only reactive species to which the cells are exposed. Results seen with this system can therefore be attributed solely and unambiguously to events initiated by 1O2. Further, all cells in the sample receive the same magnitude of exposure per surface area per time interval, which supports calculations of the amount of 1O2 required for irreversible cell damage, based on measured 1O2 flux and exposed cell surface area. Exposure to pure 1O2 irreversibly damaged a variety of cell types, including rat basophilic leukemia, human squamous carcinoma and Chinese hamster lung fibroblast cell lines, and murine primary hepatocytes. Cell survival curves following exposure to 1O2 followed apparent first-order kinetics. A large number of singlet oxygen collisions (approximately 10(12)-10(13) were required to inactivate a cell, on average, indicating a low probability that singlet oxygen collision will reduce cell survival. Regardless of cell type or the survival endpoint measured, lethal toxicity required a fairly constant number of 1O2 collisions per cell. This poses a serious caveat in the assignment of causality in correlating 1O2-initiated cellular damage with mechanism of death, i.e. most damage observed will not be related to death.(ABSTRACT TRUNCATED AT 250 WORDS)

Genotoxicity of volatile and secondary reactive oxygen species generated by photosensitization

Reactive oxygen species were generated in the gas phase by photosensitization involving illumination of Rose Bengal. Depending on whether the chromophore is dry or solubilized, this system produces either energy-transfer reactions leading to generation of singlet oxygen specifically, or a combination of energy-transfer and electron-transfer reactions, providing both singlet oxygen and reduced forms of oxygen, such as superoxide anion and hydrogen peroxide. In neither case were the reactive species mutagenic in strain TA104 of Salmonella typhimurium, which had been previously shown to be reverted by oxygen species generated by the hypoxanthine-xanthine oxidase system in aqueous medium. However, mixed oxygen species induced an increased lethality in a variety of DNA repair-deficient Escherichia coli strains. This genotoxic effect, mainly reparable by the uvrA and recA mechanisms, was efficiently prevented by the thiol N-acetyl-L-cysteine. Singlet oxygen itself failed to exert direct genotoxic effects, although secondary reactants produced by its reaction with cell components enhanced lethality in some repair-deficient bacteria. Distance-dependence analyses provided measurements of the lifetimes of the oxygen species generated in the gas phase.