Orally administered nitrite attenuates cardiac allograft rejection in rats

Anti-inflammatory effects of cold atmospheric plasma irradiation on the THP-1 human acute monocytic leukemia cell line

Cold atmospheric plasma (CAP) has been studied and clinically applied to treat chronic wounds, cancer, periodontitis, and other diseases. CAP exerts cytotoxic, bactericidal, cell-proliferative, and anti-inflammatory effects on living tissues by generating reactive species. Therefore, CAP holds promise as a treatment for diseases involving chronic inflammation and bacterial infections. However, the cellular mechanisms underlying these anti-inflammatory effects of CAP are still unclear. Thus, this study aimed to elucidate the anti-inflammatory mechanisms of CAP in vitro. The human acute monocytic leukemia cell line, THP-1, was stimulated with lipopolysaccharide and irradiated with CAP, and the cytotoxic effects of CAP were evaluated. Time-course differentiation of gene expression was analyzed, and key transcription factors were identified via transcriptome analysis. Additionally, the nuclear localization of the CAP-induced transcription factor was examined using western blotting. The results indicated that CAP showed no cytotoxic effects after less than 70 s of irradiation and significantly inhibited interleukin 6 (IL6) expression after more than 40 s of irradiation. Transcriptome analysis revealed many differentially expressed genes (DEGs) following CAP irradiation at all time points. Cluster analysis classified the DEGs into four distinct groups, each with time-dependent characteristics. Gene ontology and gene set enrichment analyses revealed CAP-induced suppression of IL6 production, other inflammatory responses, and the expression of genes related to major histocompatibility complex (MHC) class II. Transcription factor analysis suggested that nuclear factor erythroid 2-related factor 2 (NRF2), which suppresses intracellular oxidative stress, is the most activated transcription factor. Contrarily, regulatory factor X5, which regulates MHC class II expression, is the most suppressed transcription factor. Western blotting revealed the nuclear localization of NRF2 following CAP irradiation. These data suggest that CAP suppresses the inflammatory response, possibly by promoting NRF2 nuclear translocation.

Precise treatment of acute antibody-mediated cardiac allograft rejection in rats using C4d-targeted microbubbles loaded with nitric oxide

BACKGROUND

Antibody-mediated rejection (AMR) constitutes an important cause of cardiac allograft loss; however, all current therapeutic strategies represent systemic applications with unsatisfactory efficacy. Previously, we successfully non-invasively detected C4d, a specific marker for AMR diagnosis, in allografts using C4d-targeted microbubbles (MB_C4d). In this study, we extended this approach by incorporating nitric oxide (NO), as high NO levels manifest immunosuppressive and anti-thrombotic effects.

METHODS

We designed novel MB_C4d loaded with NO (NO-MB_C4d). A rat model of AMR was established by pre-sensitization with skin transplantation. Contrast-enhanced ultrasound (CEUS) images were obtained and quantitatively analyzed following NO-MB_C4d injection. Allograft survival and histologic features were analyzed to evaluate the therapeutic effect and underlying mechanism of NO-MB_C4d toward AMR.

RESULTS

We successfully obtained CEUS images following NO-MB_C4d injection and demonstrated that the ultrasound signal intensity of the myocardial area and clearance time of NO-MB_C4d both increased with increased C4d grade, thereby realizing non-invasive diagnosis of AMR. Furthermore, allograft survival was significantly prolonged, and rejection was obviously attenuated following NO-MB_C4d injection through significant suppression of thrombosis and reduction of inflammatory cell infiltrates. Overall, the therapeutic efficacy was significantly improved in the NO-MB_C4d group compared with the control NO-MB group, demonstrating that precise treatment could significantly improve the therapeutic efficacy compared with that afforded by systemic applications.

CONCLUSIONS

This study presented a novel tool to provide simultaneous non-invasive diagnosis and precise treatment of AMR using NO-MB_C4d CEUS imaging, which may be expected to provide a better option for recipients with AMR in clinic.

[Long-term dietary nitrite and nitrate deficiency causes metabolic syndrome, endothelial dysfunction, and cardiovascular death in mice]

Nitric oxide (NO) is synthesized not only from L-arginine by NO synthases (NOSs), but also from its inert metabolites, nitrite and nitrate. Green leafy vegetables are abundant in nitrate, however whether or not a deficiency in dietary nitrite/nitrate spontaneously causes disease remains to be clarified. In this study, we tested our hypothesis that long-term dietary nitrite/nitrate deficiency induces metabolic syndrome (MetS) in mice. To this end, we prepared a low nitrite/nitrate diet (LND) consisting of an amino acid-based low nitrite/nitrate chow in which the contents of L-arginine, fat, carbohydrates, protein, and energy were identical with a regular chow, and potable ultrapure water. Nitrite and nitrate were undetectable in both the chow and the water. Intriguingly, in comparison with a regular diet, 3 months of the LND significantly elicited visceral adiposity, dyslipidaemia, and glucose intolerance; 18 months of the LND significantly provoked increased body weight, hypertension, insulin resistance, and impaired endothelium-dependent relaxations to acetylcholine; and 22 months of the LND significantly led to death due to cardiovascular disease, including acute myocardial infarction. These abnormalities were reversed by simultaneous treatment with sodium nitrate, and were significantly associated with endothelial NOS down-regulation, adiponectin insufficiency, and gut microbiota dysbiosis. These results provide the first evidence that long-term dietary nitrite/nitrate deficiency gives rise to MetS, endothelial dysfunction, and cardiovascular death in mice, indicating a novel pathogenetic role of the exogenous NO production system in MetS and its vascular complications.

Long-term dietary nitrite and nitrate deficiency causes the metabolic syndrome, endothelial dysfunction and cardiovascular death in mice

AIMS/HYPOTHESIS

Nitric oxide (NO) is synthesised not only from L-arginine by NO synthases (NOSs), but also from its inert metabolites, nitrite and nitrate. Green leafy vegetables are abundant in nitrate, but whether or not a deficiency in dietary nitrite/nitrate spontaneously causes disease remains to be clarified. In this study, we tested our hypothesis that long-term dietary nitrite/nitrate deficiency would induce the metabolic syndrome in mice.

METHODS

To this end, we prepared a low-nitrite/nitrate diet (LND) consisting of an amino acid-based low-nitrite/nitrate chow, in which the contents of L-arginine, fat, carbohydrates, protein and energy were identical with a regular chow, and potable ultrapure water. Nitrite and nitrate were undetectable in both the chow and the water.

RESULTS

Three months of the LND did not affect food or water intake in wild-type C57BL/6J mice compared with a regular diet (RD). However, in comparison with the RD, 3 months of the LND significantly elicited visceral adiposity, dyslipidaemia and glucose intolerance. Eighteen months of the LND significantly provoked increased body weight, hypertension, insulin resistance and impaired endothelium-dependent relaxations to acetylcholine, while 22 months of the LND significantly led to death mainly due to cardiovascular disease, including acute myocardial infarction. These abnormalities were reversed by simultaneous treatment with sodium nitrate, and were significantly associated with endothelial NOS downregulation, adiponectin insufficiency and dysbiosis of the gut microbiota.

CONCLUSIONS/INTERPRETATION

These results provide the first evidence that long-term dietary nitrite/nitrate deficiency gives rise to the metabolic syndrome, endothelial dysfunction and cardiovascular death in mice, indicating a novel pathogenetic role of the exogenous NO production system in the metabolic syndrome and its vascular complications.

Nitrate/Nitrite as Critical Mediators to Limit Oxidative Injury and Inflammation

SIGNIFICANCE

Nitric oxide (NO) is a critical signaling molecule marked by complex chemistry and varied biological responses depending on the context of the redox environment. In the setting of inflammation, NO can not only contribute to tissue injury and be causative of oxidative damage but can also signal as an adaptive molecule to limit inflammatory signaling in multiple cell types and tissues.

RECENT ADVANCES

An advance in our understanding of NO biology was the recognition of the nitrate-nitrite-NO axis, whereby nitrate (predominantly from dietary sources) could be converted to nitrite and nitrite could be reduced to NO.

CRITICAL ISSUES

Intriguingly, the recognition of multiple enzymes that serve as nitrite reductases in the setting of hypoxia or ischemia established the concept of nitrite as a circulating endocrine reservoir of NO, with the selective release of NO at sites that were primed for this reaction. This review highlights the anti-inflammatory roles of nitrite in numerous clinical conditions, including ischemia/reperfusion, transplant, cardiac arrest, and vascular injury, and in gastrointestinal inflammation.

FUTURE DIRECTIONS

These preclinical and clinical investigations set up further clinical trials and studies that elucidate the endogenous role this pathway plays in protection against inflammatory signaling.

Inorganic nitrite supplementation for healthy arterial aging

Aging is the major risk factor for cardiovascular diseases (CVD). This is attributable primarily to adverse changes in arteries, notably, increases in large elastic artery stiffness and endothelial dysfunction mediated by inadequate concentrations of the vascular-protective molecule, nitric oxide (NO), and higher levels of oxidative stress and inflammation. Inorganic nitrite is a promising precursor molecule for augmenting circulating and tissue NO bioavailability because it requires only a one-step reduction to NO. Nitrite also acts as an independent signaling molecule, exerting many of the effects previously attributed to NO. Results of recent studies indicate that nitrite may be effective in the treatment of vascular aging. In old mice, short-term oral sodium nitrite supplementation reduces aortic pulse wave velocity, the gold-standard measure of large elastic artery stiffness, and ameliorates endothelial dysfunction, as indicated by normalization of NO-mediated endothelium-dependent dilation. These improvements in age-related vascular dysfunction with nitrite are mediated by reductions in oxidative stress and inflammation, and may be linked to increases in mitochondrial biogenesis and health. Increasing nitrite levels via dietary intake of nitrate appears to have similarly beneficial effects in many of the same physiological and clinical settings. Several clinical trials are being performed to determine the broad therapeutic potential of increasing nitrite bioavailability on human health and disease, including studies related to vascular aging. In summary, inorganic nitrite, as well as dietary nitrate supplementation, represents a promising therapy for treatment of arterial aging and prevention of age-associated CVD in humans.

The key role of nitric oxide in hypoxia: hypoxic vasodilation and energy supply-demand matching

SIGNIFICANCE

A mismatch between energy supply and demand induces tissue hypoxia with the potential to cause cell death and organ failure. Whenever arterial oxygen concentration is reduced, increases in blood flow--hypoxic vasodilation--occur in an attempt to restore oxygen supply. Nitric oxide (NO) is a major signaling and effector molecule mediating the body's response to hypoxia, given its unique characteristics of vasodilation (improving blood flow and oxygen supply) and modulation of energetic metabolism (reducing oxygen consumption and promoting utilization of alternative pathways).

RECENT ADVANCES

This review covers the role of oxygen in metabolism and responses to hypoxia, the hemodynamic and metabolic effects of NO, and mechanisms underlying the involvement of NO in hypoxic vasodilation. Recent insights into NO metabolism will be discussed, including the role for dietary intake of nitrate, endogenous nitrite (NO₂⁻) reductases, and release of NO from storage pools. The processes through which NO levels are elevated during hypoxia are presented, namely, (i) increased synthesis from NO synthases, increased reduction of NO₂⁻ to NO by heme- or pterin-based enzymes and increased release from NO stores, and (ii) reduced deactivation by mitochondrial cytochrome c oxidase.

CRITICAL ISSUES

Several reviews covered modulation of energetic metabolism by NO, while here we highlight the crucial role NO plays in achieving cardiocirculatory homeostasis during acute hypoxia through both vasodilation and metabolic suppression.

FUTURE DIRECTIONS

We identify a key position for NO in the body's adaptation to an acute energy supply-demand mismatch.

Nitrite and nitrite reductases: from molecular mechanisms to significance in human health and disease

Nitrite, previously considered physiologically irrelevant and a simple end product of endogenous nitric oxide (NO) metabolism, is now envisaged as a reservoir of NO to be activated in response to oxygen (O(2)) depletion. In the first part of this review, we summarize and compare the mechanisms of nitrite-dependent production of NO in selected bacteria and in eukaryotes. Bacterial nitrite reductases, which are copper or heme-containing enzymes, play an important role in the adaptation of pathogens to O(2) limitation and enable microrganisms to survive in the human body. In mammals, reduction of nitrite to NO under hypoxic conditions is carried out in tissues and blood by an array of metalloproteins, including heme-containing proteins and molybdenum enzymes. In humans, tissues play a more important role in nitrite reduction, not only because most tissues produce more NO than blood, but also because deoxyhemoglobin efficiently scavenges NO in blood. In the second part of the review, we outline the significance of nitrite in human health and disease and describe the recent advances and pitfalls of nitrite-based therapy, with special attention to its application in cardiovascular disorders, inflammation, and anti-bacterial defence. It can be concluded that nitrite (as well as nitrate-rich diet for long-term applications) may hold promise as therapeutic agent in vascular dysfunction and ischemic injury, as well as an effective compound able to promote angiogenesis.

Sodium nitrite protects against kidney injury induced by brain death and improves post-transplant function

Renal injury induced by brain death is characterized by ischemia and inflammation and limiting it is a therapeutic goal that could improve outcomes in kidney transplantation. Brain death resulted in decreased circulating nitrite levels and increased infiltrating inflammatory cell infiltration into the kidney. Since nitrite stimulates nitric oxide signaling in ischemic tissues, we tested whether nitrite therapy was beneficial in a rat model of brain death followed by kidney transplantation. Nitrite, administered over 2 hours of brain death, blunted the increased inflammation without affecting brain death-induced alterations in hemodynamics. Kidneys were transplanted after 2 hours of brain death and renal function followed over 7 days. Allografts collected from nitrite-treated brain dead rats showed significant improvement in function over the first 2 to 4 days post transplantation compared to untreated brain dead animals. Gene microarray analysis after 2 hours of brain death without or with nitrite therapy showed the latter significantly altered the expression of about 400 genes. Ingenuity Pathway analysis indicated multiple signaling pathways were affected by nitrite, including those related to hypoxia, transcription and genes related to humoral immune responses. Thus, nitrite-therapy attenuates brain death-induced renal injury by regulating responses to ischemia and inflammation, ultimately leading to better post-transplant kidney function.

Natural product nitric oxide chemistry: new activity of old medicines

The use of complementary and alternative medicine (CAM) as a therapy and preventative care measure for cardiovascular diseases (CVD) may prove to be beneficial when used in conjunction with or in place of conventional medicine. However, the lack of understanding of a mechanism of action of many CAMs limits their use and acceptance in western medicine. We have recently recognized and characterized specific nitric oxide (NO) activity of select alternative and herbal medicines that may account for many of their reported health benefits. The ability of certain CAM to restore NO homeostasis both through enhancing endothelial production of NO and by providing a system for reducing nitrate and nitrite to NO as a compensatory pathway for repleting NO bioavailability may prove to be a safe and cost-effective strategy for combating CVD. We will review the current state of science behind NO activity of herbal medicines and their effects on CVD.

Inorganic nitrite therapy: historical perspective and future directions

Over the past several years, investigators studying nitric oxide (NO) biology and metabolism have come to learn that the one-electron oxidation product of NO, nitrite anion, serves as a unique player in modulating tissue NO bioavailability. Numerous studies have examined how this oxidized metabolite of NO can act as a salvage pathway for maintaining NO equivalents through multiple reduction mechanisms in permissive tissue environments. Moreover, it is now clear that nitrite anion production and distribution throughout the body can act in an endocrine manner to augment NO bioavailability, which is important for physiological and pathological processes. These discoveries have led to renewed hope and efforts for an effective NO-based therapeutic agent through the unique action of sodium nitrite as an NO prodrug. More recent studies also indicate that sodium nitrate may also increase plasma nitrite levels via the enterosalivary circulatory system resulting in nitrate reduction to nitrite by microorganisms found within the oral cavity. In this review, we discuss the importance of nitrite anion in several disease models along with an appraisal of sodium nitrite therapy in the clinic, potential caveats of such clinical uses, and future possibilities for nitrite-based therapies.

Nitrite and nitrate: cardiovascular risk-benefit and metabolic effect

PURPOSE OF REVIEW

To review the most recent published literature on the biological effects of nitrite and nitrate in order to establish the context for potential health benefits vs. potential risks or adverse effects. Nitrite and nitrate are indigenous to our diet and are formed naturally within our body from the oxidation of nitric oxide. Emerging health benefits from dietary sources of nitrite and nitrate contradict decades of epidemiological research that have suggested an association of nitrite and nitrate in foods, primarily cured and processed meat, with certain cancers.

RECENT FINDINGS

The major source of exposure of nitrite and nitrate comes from the consumption of nitrate-enriched vegetables. The preponderance of epidemiological studies shows a very weak association with consumption of meats and certain cancers, which contain very little nitrite and nitrate. Nitrite and nitrate in certain foods and diets can be metabolized to nitric oxide and promote cardiovascular benefits and cytoprotection.

SUMMARY

The cardiovascular benefits of nitrite and nitrate are beginning to be translated in humans by the increasing number of clinical trials using nitrite and nitrate. The collective body of evidence suggests that foods enriched in nitrite and nitrate provide significant health benefits with very little risk.

Nitrate and nitrite in biology, nutrition and therapeutics

Inorganic nitrate and nitrite from endogenous or dietary sources are metabolized in vivo to nitric oxide (NO) and other bioactive nitrogen oxides. The nitrate-nitrite-NO pathway is emerging as an important mediator of blood flow regulation, cell signaling, energetics and tissue responses to hypoxia. The latest advances in our understanding of the biochemistry, physiology and therapeutics of nitrate, nitrite and NO were discussed during a recent 2-day meeting at the Nobel Forum, Karolinska Institutet in Stockholm.