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Singlet oxygen, an electronically excited form of molecular oxygen, is a primary mediator of the activation of stress-activated protein kinases elicited by ultraviolet A (UVA; 320-400 nm). Here, the effects of singlet oxygen (1O2) on the extracellular signal-regulated kinase (ERK) 1/2 and Akt/protein kinase B pathways were analyzed in human dermal fibroblasts. While basal ERK 1/2 phosphorylation was lowered in cells exposed to either 1O2, UVA or photodynamic treatment, Akt was moderately activated by photochemically generated 1O2 in a phosphoinositide 3-kinase (PI3K)-dependent fashion, resulting in the phosphorylation of glycogen synthase kinase-3 (GSK3). The activation of ERK 1/2 and Akt as induced by stimulation with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) was inhibited by 1O2 generated intracellularly upon photoexcitation of rose Bengal (RB). Photodynamic therapy (PDT)-induced apoptosis is known to be associated with increased formation of ceramides. Likewise, both 1O2 and UVA induced ceramide generation in human skin fibroblasts. The attenuation of EGF- and PDGF-induced activation of ERK 1/2 and Akt by 1O2 was mimicked by stimulation of fibroblasts with the cell-permeable C2-ceramide. Interestingly, EGF-induced tyrosine phosphorylation of the EGF receptor was strongly attenuated by 1O2 but unimpaired by C2-ceramide, implying that, although ceramide formation may mediate the above attenuation of ERK and Akt phosphorylation induced by 1O2, mechanisms beyond ceramide formation exist that mediate impairment of growth factor signaling by singlet oxygen. In summary, these data point to a novel mechanism of 1O2 toxicity: the known 1O2-induced activation of proapoptotic kinases such as JNK and p38 is paralleled by the prevention of activation of growth factor receptor-dependent signaling and of anti-apoptotic kinases, thus shifting the balance towards apoptosis.

Selenoprotein P (SeP) is an extracellular glycoprotein with 8-10 selenocysteines per molecule, containing approximately 50% of total selenium in human serum. An antioxidant function of SeP has been postulated. In the present study, we show that SeP protects low-density lipoproteins (LDL) against oxidation in a cell-free in-vitro system. LDL were isolated from human blood plasma and oxidized with CuCl2, 2,2'-azobis(2-amidinopropane) (AAPH) or peroxynitrite in the presence or absence of SeP, using the formation of conjugated dienes as parameter for lipid peroxidation. SeP delayed the CuCl2- and AAPH-induced LDL oxidation significantly and more efficiently than bovine serum albumin used as control. In contrast, SeP was not capable of inhibiting peroxynitrite-induced LDL oxidation. The protection of LDL against CuCl2- and AAPH-induced oxidation provides evidence for the antioxidant capacity of SeP. Because SeP associates with endothelial membranes, it may act in vivo as a protective factor inhibiting the oxidation of LDL by reactive oxygen species.

The trace elements copper, zinc and selenium are linked together in cytosolic defense against reactive oxygen and nitrogen species. Copper, zinc-superoxide dismutase catalyzes the dismutation of superoxide to oxygen and hydrogen peroxide. The latter and other hydroperoxides are subsequently reduced by the selenoenzyme glutathione peroxidase (GPx). Cytosolic GPx can also act as a peroxynitrite reductase. The antioxidative functions of these trace elements are not confined to being constituents of enzymes: 1) copper and zinc ions may stimulate protective cellular stress-signaling pathways such as the antiapoptotic phosphoinositide-3-kinase/Akt cascade and may stabilize proteins, thereby rendering them less prone to oxidation; and 2) selenium does not only exist in the cell as selenocysteine (as in GPx) but also as selenomethionine, which is regularly present in low amounts in proteins in place of methionine. Selenomethionine catalyzes the reduction of peroxynitrite at the expense of glutathione. Also, low-molecular-weight organoselenium and organotellurium compounds of pharmacologic interest catalyze the reduction of hydroperoxides or peroxynitrite with various cellular reducing equivalents.

Peroxynitrite, a potent oxidizing and nitrating species, induces covalent modifications of biomolecules in a number of pathological conditions. In previous studies with S. cerevisiae, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was identified as being especially susceptible to nitration by peroxynitrite. The activity of this enzyme was strongly inhibited by low doses of peroxynitrite in yeast and in cultured rat astrocytes. Here, the sequence of modifications of isolated mammalian GAPDH induced by increasing concentrations of peroxynitrite is demonstrated to be as follows: (i) oxidation, leading to inactivation and to enhanced susceptibility of GAPDH for proteasomal degradation, (ii) oligomer formation, and (iii) nitration. In our study the susceptibility for degradation by isolated 20S proteasome was by far the most sensitive parameter for peroxynitrite-induced damage to GAPDH, implying that this might also occur under pathological conditions where peroxynitrite is generated at low concentrations in vivo.

2-Methyl-1,4-naphthoquinone, vitamin K(3) (menadione), which is frequently used as a model quinone in cell culture and in vivo studies, was tested for its effects on gap-junctional intercellular communication (GJC). Exposure of WB-F344 rat liver epithelial cells to menadione (50-100 micro M) led to a 50-75% decrease in GJIC. Different from the phorbol ester 12-O-tetradecanoylphorbol 13-acetate, menadione did not induce internalization of gap junctions. Rather, the decreased GJIC was found to be because of phosphorylation of connexin 43, the major connexin in the used cell line, which was mediated by MAPK/ERK kinase (MEK) 1 and MEK 2 as well as by activation of their direct substrates, extracellular signal-regulated kinase (ERK) 1 and ERK 2. Activation of ERK 1/2 was demonstrated to be independent of NAD(P)H:quinone oxidoreductase using the inhibitor dicoumarol, thus excluding redox cycling as the major mechanism causing these menadione effects. A substantial increase in tyrosine phosphorylation was detected in the cell membrane immunocytochemically upon exposure to menadione, consistent with arylation by menadione bearing the responsibility for the signaling events induced and consistent with the fact that protein tyrosine phosphatases are known targets of arylation reactions. ERK activation was attenuated using specific inhibitors of the epidermal growth factor receptor tyrosine kinase. Similarly, these inhibitors as well as inhibitors of MEK 1/2 counteracted the loss in gap-junctional communication elicited by menadione. This is of interest for chemotherapeutic approaches exploiting the bystander-effect, which is based upon intact GJIC.

The flavanol (-)-epicatechin has been found to protect against damage inflicted by peroxynitrite, an inflammatory intermediate. Here, epicatechin was tested in systems of increasing complexity. The compound efficiently protected against nitration of protein tyrosine residues by peroxynitrite (IC(50) approximately 0.02 mol epicatechin/mol peroxynitrite). However, at epicatechin concentrations completely preventing nitration of tyrosine by peroxynitrite, protection against the oxidative inactivation of glyceraldehyde-3-phosphate dehydrogenase or soybean lipoxygenase-1 was marginal (IC(50) > 1 mol epicatechin/mol peroxynitrite), approximately two orders of magnitude less. Likewise, epicatechin was relatively ineffective against oxidation of thiols in cell lysates, and against the oxidation of 2',7'-dichlorodihydrofluorescein in cultured cells. The activation of the kinases Akt/protein kinase B, ERK1/2 and p38-MAPK by peroxynitrite in murine aorta endothelial cells was not altered by epicatechin, suggesting that activation of these kinases is due to processes other than tyrosine nitration., (Copyright 2001 Academic Press.)

Photodynamic therapy (PDT) is being used clinically for the treatment of skin cancers. One concept of delivering the employed photosensitizer directly to target cells is to stimulate cellular synthesis of sensitizers such as porphyrins. ALA (5-aminolevulinate) is applied as a precursor of porphyrins which then serve as endogenous photosensitizers. Upon irradiation, reactive oxygen species, predominantly singlet oxygen, are generated, leading to cell death. ALA-PDT using red light (550-750 nm) is known to lead to the activation of stress kinases, such as c-Jun-N-terminal kinase and p38. These kinases are also activated by UVA (320-400 nm), whose biological effects are mediated in part by singlet oxygen. In the present study, the efficiency of a combination of both treatment strategies, ALA-PDT and UVA, in cytotoxicity and activation of stress kinases was investigated taking human skin fibroblasts as a model. Compared with the commonly used ALA-PDT with red light (LD(50) = 13.5 J/cm(2)), UVA-ALA-PDT was 40-fold more potent in killing cultured human skin fibroblasts (LD(50) = 0.35 J/cm(2)) and still 10-fold more potent than ALA-PDT with green light (LD(50) = 4.5 J/cm(2)). Its toxicity relied on the formation of singlet oxygen, as was shown employing modulators of singlet oxygen lifetime. In line with these data, strong activation of the stress kinase p38 was obtained in ALA-pretreated cells irradiated with UVA at doses two orders of magnitude lower than necessary for a comparable activation of p38 by UVA in control cells. Taken together, these data suggest UVA-ALA-PDT as a potentially interesting new approach in the photodynamic treatment of skin diseases.

Peroxynitrite (ONOO-), a potent oxidizing and nitrating species, has been linked to covalent modifications of biomolecules in a number of pathological conditions. In S. cerevisiae, a model eukaryotic cell system, ONOO- was found to be more potent than hydrogen peroxide in oxidizing thiols, inducing heat shock proteins (Hsp70) and enhancing the ubiquitination of proteins. As identified by microsequence analysis following immunoprecipitation with anti-nitrotyrosine antibodies, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was especially susceptible to nitration by ONOO- in yeast cells. The activity of this enzyme was strongly inhibited upon steady-state exposure of the cells to low doses of ONOO- in yeast and in cultured rat astrocytes. Thus, ONOO- is a potent stressor in yeast capable of inducing oxidative damage and protein nitration, with GAPDH being a preferential target protein that is efficiently inactivated.