Inhibition of biotransformation by nitric oxide (NO) overproduction and toxic consequences

Pycnogenol: a blend of procyanidins with multifaceted therapeutic applications?

Great interest is currently centred on the biologic activities of pycnogenol a standardized plant extract obtained from the bark of the French maritime pine Pinus pinaster (formerly known as Pinus maritima), Aiton, subspecies Atlantica des Villar (Pycnogenol, Horphag Research Ltd., UK, Geneve, Switzerland), which grows in the coastal southwest France. The quality of this extract is specified in the United States Pharmacopeia (USP 28). Between 65% and 75% of Pycnogenol are procyanidins comprising of catechin and epicatechin subunits with varying chain lengths. Other constituents are polyphenolic monomers, phenolic or cinnamic acids and their glycosides. As many studies indicate, pycnogenol components are highly bioavailable. Uniquely, pycnogenol displays greater biologic effects as a mixture than its purified components do individually indicating that the components interact synergistically. Pycnogenol is now utilized throughout the world as a nutritional supplement and as a phytochemical remedy for various diseases ranging from chronic inflammation to circulatory dysfunction, including several impaired psycho-physiological functions. Owing to the basic chemical structure of its components, the most obvious feature of pycnogenol is its strong antioxidant activity. In fact, phenolic acids, polyphenols, and in particular flavonoids, are composed of one (or more) aromatic rings bearing one or more hydroxyl groups and are therefore potentially able to quench free radicals by forming resonance-stabilized phenoxyl radicals. In this review, emphasizing the molecular, cellular, and functional bases of therapy, data appearing in the peer-reviewed literature and focussing the main therapeutic applications of pycnogenol will be summarized and critically evaluated.

Nitric oxide increases toxicity of hydrogen peroxide against rat liver endothelial cells and hepatocytes by inhibition of hydrogen peroxide degradation

Nitric oxide (NO) and hydrogen peroxide (H(2)O(2)) show cooperativity in their cytotoxic action. The present study was performed to decipher the mechanisms underlying this phenomenon. In cultured liver endothelial cells and in cultured, glutathione-depleted hepatocytes, the combined exposure to NO (released by spermine NONOate, 1 mM) and H(2)O(2) (released by glucose oxidase) induced cell injury that was far higher than the injury elicited by NO or H(2)O(2) alone. In both cell types, the addition of the NO donor increased H(2)O(2) steady-state levels, although with different kinetics: in hepatocytes, the increase in H(2)O(2) levels was already evident at early time points while in liver endothelial cells it became evident after > or =2 h of incubation. NO exposure inhibited H(2)O(2) degradation, assessed after addition of 50 microM, 200 microM, or 4 mM authentic H(2)O(2), significantly in both cell types. However, again, early and delayed inhibition was observed. The late inhibition of H(2)O(2) degradation in endothelial cells was paralleled by a decrease in glutathione peroxidase activity. Glutathione peroxidase inactivation was prevented by hypoxia or by ascorbate, suggesting inactivation by reactive nitrogen oxide species (NO(x)). Early inhibition of H(2)O(2) degradation by NO, in contrast, could be mimicked by the catalase inhibitor azide. Together, these results suggest that the cooperative effect of NO and H(2)O(2) is due to inhibition of H(2)O(2) degradation by NO, namely to inhibition of catalase by NO itself (predominant in hepatocytes) and/or to inhibition of glutathione peroxidase by NO(x) (prevailing in endothelial cells).

Inducible nitric oxide production is an adaptation to cardiopulmonary bypass-induced inflammatory response

BACKGROUND

Cardiopulmonary bypass (CPB) increases nitric oxide (NO) production by the activation of NO synthases (NOS). However, the role of NO from inducible NOS (iNOS) in CPB-induced inflammatory response remains unclear. We examined the effect of a selective iNOS inhibitor, aminoguanidine, on CPB-induced inflammatory response in a rat-CPB model.

METHODS

Adult Sprague-Dawley rats underwent 60 minutes of CPB (100 mL x kg(-1) x min(-1), 34 degrees C). Group A (n = 10) received 100 mg/kg of aminoguanidine intraperitoneally 30 minutes before the initiation of CPB, and group B (n = 10) served as controls.

RESULTS

There were significant time-dependent changes in plasma interleukin (IL)-6, IL-8, nitrate + nitrite, the percentage ratio of nitrotyrosine to tyrosine (%NO2-Tyr, an indicator of peroxynitrite formation), and respiratory index (RI). Three hours after CPB termination, IL-6, IL-8, and RI were significantly higher in group A (IL-6, 397.5+/-80.6 pg/mL; IL-8, 26.99+/-6.57 ng/mL; RI, 1.87+/-0.31) than in group B (IL-6, 316.5+/-73.9 pg/mL, p <0.05; IL-8, 17.21+/-3.12 ng/mL, p < 0.01; RI, 1.57+/-0.24, p < 0.05) although nitrate + nitrite (31.8+/-4.1 micromol/L) and %NO2-Tyr (1.15%+/-0.20%) were significantly lower in group A than in group B (nitrate + nitrite, 50.2+/-5.0 micromol/L, p < 0.01; %NO2-Tyr, 1.46%+/-0.21%, p < 0.01). Western immunoblot analysis from lung tissue of group A identified marked iNOS inhibition without inhibiting endothelial-constitutive NOS, and neutrophil accumulation in the lung specimens was significantly greater in group A (6.5+/-0.7/alveoli) than in group B (4.4+/-0.4/alveoli, p < 0.01).

CONCLUSIONS

These results suggest that NO production from iNOS may be an adaptive response for attenuating the CPB-induced inflammatory response.

P-selectin participates in cardiopulmonary bypass-induced inflammatory response in association with nitric oxide and peroxynitrite production

OBJECTIVES

P-selectin participates in the development of inflammatory disorders. Cardiopulmonary bypass is thought to induce inflammatory response and increase nitric oxide production. To evaluate the role of P-selectin in bypass-induced inflammatory response and its association with nitric oxide production, we examined the effect of P-selectin monoclonal antibody in a rat model of cardiopulmonary bypass.

METHODS

Twenty adult Sprague-Dawley rats undergoing cardiopulmonary bypass for 60 minutes were divided into 2 groups. A 3-mg/kg dose of anti-rat specific P-selectin monoclonal antibody (ARP2-4; Sumitomo Pharmaceuticals, Osaka, Japan) was administered into the priming solution before bypass in group P (n = 10) and a 3-mg/kg dose of PNB1.6 (nonblocking monoclonal antibody) was added in group C for control (n = 10).

RESULTS

At the termination of bypass and 3 hours after the termination of bypass, plasma levels of interleukins 6 and 8, nitrate/nitrite, the percentage ratio of nitrotyrosine to tyrosine (an indicator of peroxynitrite formation), and the respiratory index were significantly higher than before bypass in both groups, and they were significantly lower in group P than in group C. Plasma P-selectin level in group C and exhaled nitric oxide concentration in both groups at termination of bypass were significantly lower than those before bypass, and they were significantly higher 3 hours after termination of bypass than before bypass in both groups. Plasma P-selectin level and exhaled nitric oxide concentration in group P were significantly higher than those in group C at the end of bypass, but they were significantly lower 3 hours after the termination of bypass.

CONCLUSIONS

These results demonstrate that P-selectin may participate in the augmentation of bypass-induced inflammatory response in association with nitric oxide and peroxynitrite production.

Triggering and modulation of apoptosis by oxidative stress

Cell survival requires multiple factors, including appropriate proportions of molecular oxygen and various antioxidants. Although most oxidative insults can be overcome by the cell's natural defenses, sustained perturbation of this balance may result in either apoptotic or necrotic cell death. Numerous, recent studies have shown that the mode of cell death that occurs depends on the severity of the insult. Oxidants and antioxidants can not only determine cell fate, but can also modulate the mode of cell death. Effects of oxidative stress on components of the apoptotic machinery may mediate this modulation. This review will address some of the current paradigms for oxidative stress and apoptosis, and discuss the potential mechanisms by which oxidants can modulate the apoptotic pathway.

Antioxidant activity and biologic properties of a procyanidin-rich extract from pine (Pinus maritima) bark, pycnogenol

There is growing interest in the biologic activities of plant extracts such as that obtained from the bark of the French maritime pine Pinus maritima, Pycnogenol. Pycnogenol (PYC) is a standardized extract composed of a mixture of flavonoids, mainly procyandins and phenolic acids. Studies indicate that PYC components are highly bioavailable. Uniquely PYC displays greater biologic effects as a mixture than its purified components do individually indicating that the components interact synergistically. PYC has been reported to have cardiovascular benefits, such as a vasorelaxant activity, angiotensin-converting enzyme (ACE) inhibiting activity, and the ability to enhance the microcirculation by increasing capillary permeability. Investigations of the cellular mechanisms of these therapeutic effects have demonstrated that PYC has strong free radical-scavenging activity against reactive oxygen and nitrogen species. The oligomeric components of PYC contribute significantly to the ESR free radical signal. PYC also participates in the cellular antioxidant network as indicated by its ability to regenerate the ascorbyl radical and to protect endogenous vitamin E and glutathione from oxidative stress. PYC modulates NO metabolism in activated macrophages by quenching the NO radical and inhibiting both iNOS mRNA expression and iNOS activity. The spectrum of different effects of NO in the circulation and the nervous system suggest the potential applications of PYC in immune and circulatory disorders as well as in neurodegenerative disease. PYC can bind to proteins, altering their structure and thereby modulating the activity of key enzymes and proteins involved in metabolic pathways. PYC effects redox-sensitive signal transduction pathways and alters gene expression. Aspects of PYC's activity are presented and discussed together with possible future implications and directions in the field of flavonoid research.

The possible role of nitric oxide in the physiopathology of pain associated with temporomandibular joint disorders

Temporomandibular joint disorders (TMD) pose a significant challenge to the practice of oral and maxillofacial surgery. When painful, TMD are generally associated with hyperthermia of the overlying skin. It is hypothesized that this skin hyperthermia, caused by regional vasodilation, is induced by nitric oxide (NO) produced in the extravascular space of the joint. Extravascular NO can be produced by osteoblasts, chondrocytes and macrophages, or by stimulated neurons. It is suggested that this kind of pain is associated with NO-enhanced sensitivity of the peripheral nociceptors. Verification and clinical implications of the proposed mechanism are discussed.

Role of nitric oxide in acetaminophen-induced hepatotoxicity in the rat

Acetaminophen is a mild analgesic and antipyretic agent known to cause centrilobular hepatic necrosis at toxic doses. Although this may be due to a direct interaction of reactive acetaminophen metabolites with hepatocyte proteins, recent studies have suggested that cytotoxic mediators produced by parenchymal and nonparenchymal cells also contribute to the pathophysiological process. Nitric oxide is a highly reactive oxidant produced in the liver in response to inflammatory mediators. In the present studies we evaluated the role of nitric oxide in the pathophysiology of acetaminophen-induced liver injury. Treatment of male Long Evans Hooded rats with acetaminophen (1 g/kg) resulted in damage to centrilobular regions of the liver and increases in serum transaminase levels, which were evident within 6 hours of treatment of the animals and reached a maximum at 24 hours. This was correlated with expression of inducible nitric oxide synthase (iNOS) protein in these regions. Hepatocytes isolated from both control and acetaminophen-treated rats were found to readily synthesize nitric oxide in response to inflammatory stimuli. Cells isolated from acetaminophen-treated rats produced more nitric oxide than cells from control animals. Production of nitric oxide by cells from both control and acetaminophen-treated rats was blocked by aminoguanidine, a relatively specific inhibitor of iNOS. Arginine uptake and metabolism studies revealed that the inhibitory effects of aminoguanidine were due predominantly to inhibition of iNOS enzyme activity. Pretreatment of rats with aminoguanidine was found to prevent acetaminophen-induced hepatic necrosis and increases in serum transaminase levels. This was associated with reduced nitric oxide production by hepatocytes. Inhibition of toxicity was not due to alterations in acetaminophen metabolism since aminoguanidine had no effect on hepatocyte cytochrome P4502E1 protein expression or N-acetyl-p-benzoquinone-imine formation. Taken together, these data demonstrate that nitric oxide is an important mediator of acetaminophen-induced hepatotoxicity.

Role of nitric oxide in the physiopathology of pain

Many painful disorders, including joint dysfunctions such as rheumatoid arthritis (RA) or temporomandibular joint disorders (TMD), are associated with hyperthermia of the overlying skin. The same is true of certain intractable chronic pain conditions, such as chronic orofacial pain, which may be associated with TMD. We suggest that this skin hyperthermia, caused by regional vasodilation, is induced by extravascular nitric oxide (NO). Extravascular NO can be produced in the affected joint by osteoblasts, chondrocytes, and macrophages, by mechanical stimulation of endothelial cells, or by stimulated neurons. In view of a strong correlation between pain and skin hyperthermia in these disorders, and the evidence that NO enhances the sensitivity of peripheral nociceptors, we also suggest that at least this kind of pain is associated with excessive local level of NO. This hypothesis can be verified by dynamic area telethermometry, assessing the effect of NO on the sympathetic nervous function. This mechanism, which is in line with the general role of NO as a mediator between different organ systems, also may be relevant to any pain associated with enhanced immune response. Clinical implications of the proposed mechanism are discussed.