Modulation of LDL oxidation by 7,8-dihydroneopterin

Interferons as Essential Modulators of Atherosclerosis

Interferons (IFNs) are key regulators of both innate and adaptive immune responses. The family of IFN cytokines can be divided into 3 main subtypes of which type I and type II IFNs are most well-defined. IFNs are known to be important mediators in atherosclerosis. Evidence from both in vitro and in vivo studies shows that the IFNs are generally proatherosclerotic. However, their role in atherosclerosis is complex, with distinct roles for these cytokines throughout different stages of the disease. In this review, we will discuss the current knowledge on the role of type I and type II IFNs in atherosclerosis development, specifically focusing on their role in endothelial activation, cell recruitment, foam cell formation, and regulation of apoptosis. Furthermore, we will discuss whether IFNs could be considered as new molecular targets for therapeutic intervention in atherosclerosis.

Oxidant production, oxLDL uptake, and CD36 levels in human monocyte–derived macrophages are downregulated by the macrophage-generated antioxidant 7,8-dihydroneopterin

The severity of atheroma burden in patients strongly correlates to increasing levels of plasma neopterin, the oxidation product of 7,8-dihydroneopterin. Interferon-γ stimulation of macrophages causes the synthesis of 7,8-dihydroneopterin, a potent antioxidant that inhibits oxidative damage to cells, and the cytotoxicity of oxidized low-density lipoprotein (oxLDL) to monocyte-like U937 cells but not THP-1 cells. With human monocyte-derived macrophages (HMDMs), oxLDL triggered a large oxidative stress, causing the rapid loss of cellular glutathione, glyceradehyde-3-phosphate dehydrogenase (GAPDH) inhibition, and eventual loss of viability without caspase-3 activation. Inhibition of oxLDL cytotoxicity to HMDMs occurred at 7,8-dihydroneopterin concentrations >100 μM. The oxLDL-mediated glutathione loss and GAPDH inactivation was inhibited by 7,8-dihydroneopterin. 7,8-Dihydroneopterin rapidly entered the HMDMs, suggesting that much of the protective effect was scavenging of intracellular oxidants generated in response to oxLDL. OxLDL uptake by HMDMs was reduced by 30% by 7,8-dihydroneopterin. Immunoblot analysis suggests that this decrease in oxLDL uptake was due to a significant downregulation in the levels of CD36. These results imply that 7,8-dihydroneopterin protects human macrophages both by scavenging oxidants generated in response to oxLDL and by decreasing CD36-mediated uptake of oxLDL.

Potential to inhibit growth of atherosclerotic plaque development through modulation of macrophage neopterin/7,8-dihydroneopterin synthesis

The rise in plasma neopterin observed with increasing severity of vascular disease is a strong indicator of the inflammatory nature of atherosclerosis. Plasma neopterin originates as the oxidation product of 7,8-dihydroneopterin secreted by gamma-interferon stimulated macrophages within atherosclerotic plaques. Neopterin is increasingly being used as a marker of inflammation during clinical management of patients with a range of disorders including atherosclerosis. Yet the role of 7,8-dihydroneopterin/neopterin synthesis during the inflammatory process and plaque formation remains poorly understood and controversial. This is partially due to the unresolved role oxidants play in atherosclerosis and the opposing roles of 7,8-dihydroneopterin/neopterin. Neopterin can act as pro-oxidant, enhancing oxidant damage and triggering apoptosis in a number of different cell types. Neopterin appears to have some cellular signalling properties as well as being able to chelate and enhance the reactivity of transition metal ions during Fenton reactions. In contrast, 7,8-dihydroneopterin is also a radical scavenger, reacting with and neutralizing a range of reactive oxygen species including hypochlorite, nitric oxide and peroxyl radicals, thus protecting lipoproteins and various cell types including macrophages. This has led to the suggestion that 7,8-dihydroneopterin is synthesized to protect macrophages from the oxidants released during inflammation. The oxidant/antioxidant activity observed in vitro appears to be determined both by the relative concentration of these compounds and the specific chemistry of the in vitro system under study. How these activities might influence or modulate the development of atherosclerotic plaque in vivo will be explored in this review.

Interference of 7,8-dihydroneopterin with peroxynitrite-mediated reactions

By in vitro studies 7,8-dihydroneopterin, which is secreted by macrophages stimulated by interferon-gamma, was reported to be a radical scavenger as well as a prooxidative agent depending on the experimental settings. In this study, we investigated the interference of 7,8-dihydroneopterin with peroxynitrite mediated reactions by different analytical procedures. Luminol chemiluminescence and oxidation of the spin probe 1-hydroxy-2,2,6,6-tetramethyl-4-oxo-piperidine induced by peroxynitrite were inhibited by 7,8-dihydroneopterin. On the other hand, we found that 7,8-dihydroneopterin very efficiently inhibits nitration of tyrosine by peroxynitrite. Hydroxylation, however, was rather enhanced than inhibited, suggesting that 7,8-dihydroneopterin reacts in quite different manner with the intermediates generated from peroxynitrite. We provide the first evidence that a pterin radical is formed from a dihydropterin using EPR spectroscopy and 2,2,4-trimethyl-2H-imidazole-1-oxide as a spin trap. We conclude that 7,8-dihydroneopterin while being a weak scavenger of superoxide acts as a very efficient inhibitor of tyrosine nitration induced by peroxynitrite.