Background— Oxidative damage of vascular endothelium represents an important initiation step in the development of atherosclerosis. Recently, we reported about protection of inducible nitric oxide synthase (iNOS)-derived high-output NO in endothelial cells. Because iNOS activity critically depends on the availability of its substrate l -arginine, the present study aims at elucidating iNOS-mediated effects on H ₂ O ₂ -induced apoptosis of cytokine-activated rat aortic endothelial cells (AECs) subject to medium l -arginine concentrations. Methods and Results— In cytokine-activated AECs, iNOS activity was found to be half-maximal at 60 μmol/L arginine, which represents the medium serum level in rats but also in humans. Maximal activity is seen at and above 200 μmol/L arginine. Activated cells grown in the absence of arginine with minimal iNOS activity are highly sensitive toward H ₂ O ₂ -induced apoptosis, and increases in medium arginine concentrations result in increased cell survival. Moreover, competition experiments show that iNOS activity is completely dependent on cationic amino acid transporter-mediated arginine uptake. We also find that the arginine-dependent protection includes inhibition of endothelial lipid peroxidation and increases in the expression of vasoprotective stress response genes. Conclusions— Our data demonstrate that arginine concentrations corresponding to physiological serum levels do not allow for optimal endothelial iNOS activity. Thus, decreases in systemic arginine concentrations, or locally within atherosclerotic plaques, will impair the endothelial iNOS-mediated stress response and will significantly increase the risk of endothelial dysfunction.

Human skin is exposed to solar ultraviolet radiation comprising UVB (280–315 nm) and UVA (315–400 nm) on a daily basis. Within the last two decades, the molecular and cellular response to UVA/UVB and the possible effects on human health have been investigated extensively. It is generally accepted that the mutagenic and carcinogenic properties of UVB is due to the direct interaction with DNA. On the other hand, by interaction with non-DNA chromophores as endogenous photosensitizers, UVA induces formation of reactive oxygen species (ROS), which play a pivotal role as mediators of UVA-induced injuries in human skin. This review gives a short overview about relevant findings concerning the molecular mechanisms underlying UVA/UVB-induced cell death. Furthermore, we will highlight the potential role of cutaneous antioxidants and photolabile nitric oxide derivates (NODs) in skin physiology. UVA-induced decomposition of the NODs, like nitrite, leads not only to non-enzymatic formation of nitric oxide (NO), but also to toxic reactive nitrogen species (RNS), like peroxynitrite. Whereas under antioxidative conditions the generation of protective amounts of NO is favored, under oxidative conditions, less injurious reactive nitrogen species are generated, which may enhance UVA-induced cell death.

Nitrite occurs ubiquitously in biological fluids such as blood and sweat, representing an oxidation product of nitric oxide. Nitrite has been associated with a variety of adverse effects such as mutagenicity, carcinogenesis, and toxicity. In contrast, here we demonstrate that the presence of nitrite, but not nitrate, during irradiation of endothelial cells in culture exerts a potent and concentration‐dependent protection against UVA‐induced apoptotic cell death. Protection is half‐maximal at a concentration of 3 mM, and complete rescue is observed at 10 mM. Nitrite‐ mediated protection is mediated via inhibition of lipid peroxidation in a similar manner as seen with butylated hydroxytoluene, a known inhibitor of lipid peroxidation. Interestingly, nitrite‐ mediated protection is completely abolished by coincubation with the NO scavenger cPTIO. Using electron paramagnetic resonance (EPR) spectroscopy or Faraday modulation spectroscopy, we directly prove UVA‐induced NO formation in solutions containing nitrite. In conclusion, evidence is presented that nitrite represents a protective agent against UVA‐induced apoptosis due to photodecomposition of nitrite and subsequent formation of NO.

The generation of nitric oxide (NO) in the skin plays a critical role in wound healing and the response to several stimuli, such as UV exposure, heat, infection, and inflammation. Furthermore, in the human body, NO is involved in vascular homeostasis and the regulation of blood pressure. Physiologically, a family of enzymes termed nitric oxide synthases (NOS) generates NO. In addition, there are many methods of non-enzymatic/NOS-independent NO generation, e.g., the reduction of NO derivates (NODs) such as nitrite, nitrate, and nitrosylated proteins under certain conditions. The skin is the largest and heaviest human organ and contains a comparatively high concentration of these NODs; therefore, it represents a promising target for many therapeutic strategies for NO-dependent pathological conditions. In this review, we give an overview of how the cutaneous NOD stores can be targeted and modulated, leading to a further accumulation of NO-related compounds and/or the local and systemic release of bioactive NO, and eventually, NO-related physiological effects with a potential therapeutical use for diseases such as hypertension, disturbed microcirculation, impaired wound healing, and skin infections.