Selective macrophage ascorbate deficiency suppresses early atherosclerosis

Cardiac toxicity of heavy metals (cadmium and mercury) and pharmacological intervention by vitamin C in rabbits

Mercury and cadmium are highly dangerous metals that can lead to disastrous effects in animals and humans. The aim of the current research was to elucidate the poisonous effects of mercuric chloride and cadmium chloride individually and in combination on biochemical profiles of plasma and their accumulation in heart. The therapeutic effect of vitamin C against these metals in rabbits was also studied. Mercuric chloride (1.2 μg/g), cadmium chloride (1.5 μg/g), and vitamin C (150 μg/g of body weight) were orally given to treatment groups of the rabbits (1-control; 2-vitamin; 3-CdCl₂; 4-HgCl₂; 5-vitamin + CdCl₂; 6-vitamin + HgCl₂; 7-CdCl₂ + HgCl₂, and 8-vitamin + CdCl₂ + HgCl₂. After the biometric determination of all intoxicated rabbits, biochemical parameters, viz low-density lipoproteins (LDL), high-density lipoproteins (HDL), cholesterol, creatine kinase, and troponin T (TnT) were analyzed using available kits. Levels of cholesterol (0.7 ± 0.1 mmol/l), creatine kinase (2985.2 ± 11 IU/L), LDL (20.35 ± 1.31 mg/dl), and troponin T (1.22 ± 0.03 μg/l) were significantly (P < 0.05) increased. HDL (84.78 ± 4.30 mg/dl) was significantly (P < 0.05) decreased, while supplementation of vitamin C decreased the adverse effects of CdCl₂ and HgCl₂ on biochemical parameters in all metal-exposed groups. A similar trend was also seen in rabbits treated with CdCl₂ + vitamin and vitamin + CdCl₂ + HgCl₂. Accumulation of Cd and Hg was higher in heart tissues. This study, therefore, provides awareness on the cardiac toxicity of mercury and cadmium chlorides in the rabbits and the possible protective role of vitamin C against the perturbations induced by metals.

Immunometabolic modulation of retinal inflammation by CD36 ligand

In subretinal inflammation, activated mononuclear phagocytes (MP) play a key role in the progression of retinopathies. Little is known about the mechanism involved in the loss of photoreceptors leading to vision impairment. Studying retinal damage induced by photo-oxidative stress, we observed that cluster of differentiation 36 (CD36)-deficient mice featured less subretinal MP accumulation and attenuated photoreceptor degeneration. Moreover, treatment with a CD36-selective azapeptide ligand (MPE-001) reduced subretinal activated MP accumulation in wild type mice and preserved photoreceptor layers and function as assessed by electroretinography in a CD36-dependent manner. The azapeptide modulated the transcriptome of subretinal activated MP by reducing pro-inflammatory markers. In isolated MP, MPE-001 induced dissociation of the CD36-Toll-like receptor 2 (TLR2) oligomeric complex, decreasing nuclear factor-kappa B (NF-κB) and NLR family pyrin domain containing 3 (NLRP3) inflammasome activation. In addition, MPE-001 caused an aerobic metabolic shift in activated MP, involving peroxisome proliferator-activated receptor-γ (PPAR-γ) activation, which in turn mitigated inflammation. Accordingly, PPAR-γ inhibition blocked the cytoprotective effect of MPE-001 on photoreceptor apoptosis elicited by activated MP. By altering activated MP metabolism, MPE-001 decreased immune responses to alleviate subsequent inflammation-dependent neuronal injury characteristic of various vision-threatening retinal disorders.

Self-defense of macrophages against oxidative injury: Fighting for their own survival

Activated macrophages play a central role in both the development and resolution of inflammation. These immune cells need to be functional in harmful conditions with high levels of reactive oxygen and nitrogen species that can damage their basic cell components, which may alter their metabolism. An excessive accumulation of these cell alterations drives macrophages inexorably to cell death, which has been associated to the development of several inflammatory diseases and even with aging in a process termed as "immunosenescence". Macrophages, however, exhibit a prolonged survival in this hostile environment because they equip themselves with a complex network of protective mechanisms. Here we provide an overview of these self-defense mechanisms with special attention being paid to bioactive lipid mediators, NRF2 signaling and metabolic reprogramming.

Vitamin C promotes wound healing through novel pleiotropic mechanisms

Vitamin C (VitC) or ascorbic acid (AscA), a cofactor for collagen synthesis and a primary antioxidant, is rapidly consumed post-wounding. Parenteral VitC administration suppresses pro-inflammatory responses while promoting anti-inflammatory and pro-resolution effects in human/murine sepsis. We hypothesised that VitC could promote wound healing by altering the inflammatory, proliferative and remodelling phases of wound healing. Mice unable to synthesise VitC (Gulo(-/-) ) were used in this study. VitC was provided in the water (sufficient), withheld from another group (deficient) and supplemented by daily intra-peritoneal infusion (200 mg/kg, deficient + AscA) in a third group. Full thickness excisional wounds (6 mm) were created and tissue collected on days 7 and 14 for histology, quantitative polymerase chain reaction (qPCR) and Western blotting. Human neonatal dermal fibroblasts (HnDFs) were used to assess effects of In conclusion, VitC favorably on proliferation. Histological analysis showed improved wound matrix deposition and organisation in sufficient and deficient +AscA mice. Wounds from VitC sufficient and deficient + AscA mice had reduced expression of pro-inflammatory mediators and higher expression of wound healing mediators. Supplementation of HnDF with AscA induced the expression of self-renewal genes and promoted fibroblast proliferation. VitC favourably impacts the spatiotemporal expression of transcripts associated with early resolution of inflammation and tissue remodelling.

Inhibition of Glutathione Production Induces Macrophage CD36 Expression and Enhances Cellular-oxidized Low Density Lipoprotein (oxLDL) Uptake

The glutathione (GSH)-dependent antioxidant system has been demonstrated to inhibit atherosclerosis. Macrophage CD36 uptakes oxidized low density lipoprotein (oxLDL) thereby facilitating foam cell formation and development of atherosclerosis. It remains unknown if GSH can influence macrophage CD36 expression and cellular oxLDL uptake directly. Herein we report that treatment of macrophages with l-buthionine-S,R-sulfoximine (BSO) decreased cellular GSH production and ratios of GSH to glutathione disulfide (GSH/GSSG) while increasing production of reactive oxygen species. Associated with decreased GSH levels, macrophage CD36 expression was increased, which resulted in enhanced cellular oxLDL uptake. In contrast, N-acetyl cysteine and antioxidant enzyme (catalase or superoxide dismutase) blocked BSO-induced CD36 expression as well as oxLDL uptake. In vivo, administration of mice with BSO increased CD36 expression in peritoneal macrophages and kidneys. BSO had no effect on CD36 mRNA expression and promoter activity but still induced CD36 protein expression in macrophages lacking peroxisome proliferator-activated receptor γ expression, suggesting it induced CD36 expression at the translational level. Indeed, we determined that BSO enhanced CD36 translational efficiency. Taken together, our study demonstrates that cellular GSH levels and GSH/GSSG status can regulate macrophage CD36 expression and cellular oxLDL uptake and demonstrate an important anti-atherogenic function of the GSH-dependent antioxidant system by providing a novel molecular mechanism.

Resolution of sterile inflammation: role for vitamin C

INTRODUCTION

Macrophage reprogramming is vital for resolution of acute inflammation. Parenteral vitamin C (VitC) attenuates proinflammatory states in murine and human sepsis. However information about the mechanism by which VitC regulates resolution of inflammation is limited.

METHODS

To examine whether physiological levels of VitC modulate resolution of inflammation, we used transgenic mice lacking L-gulono-γ-lactone oxidase. VitC sufficient/deficient mice were subjected to a thioglycollate-elicited peritonitis model of sterile inflammation. Some VitC deficient mice received daily parenteral VitC (200 mg/kg) for 3 or 5 days following thioglycollate infusion. Peritoneal macrophages harvested on day 3 or day 5 were examined for intracellular VitC levels, pro- and anti-inflammatory protein and lipid mediators, mitochondrial function, and response to lipopolysaccharide (LPS). The THP-1 cell line was used to determine the modulatory activities of VitC in activated human macrophages.

RESULTS

VitC deficiency significantly delayed resolution of inflammation and generated an exaggerated proinflammatory response to in vitro LPS stimulation. VitC sufficiency and in vivo VitC supplementation restored macrophage phenotype and function in VitC deficient mice. VitC loading of THP-1 macrophages attenuated LPS-induced proinflammatory responses.

CONCLUSION

VitC sufficiency favorably modulates macrophage function. In vivo or in vitro VitC supplementation restores macrophage phenotype and function leading to timely resolution of inflammation.

Ascorbic acid and the brain: rationale for the use against cognitive decline

This review is focused upon the role of ascorbic acid (AA, vitamin C) in the promotion of healthy brain aging. Particular attention is attributed to the biochemistry and neuronal metabolism interface, transport across tissues, animal models that are useful for this area of research, and the human studies that implicate AA in the continuum between normal cognitive aging and age-related cognitive decline up to Alzheimer’s disease. Vascular risk factors and comorbidity relationships with cognitive decline and AA are discussed to facilitate strategies for advancing AA research in the area of brain health and neurodegeneration.

Does vitamin C deficiency increase lifestyle-associated vascular disease progression? Evidence based on experimental and clinical studies

SIGNIFICANCE

Despite continuous advances in the prevention of cardiovascular disease (CVD), critical issues associated with an unhealthy lifestyle remain an increasing cause of morbidity and mortality in industrialized countries.

RECENT ADVANCES

A growing body of literature supports a specific role for vitamin C in a number of reactions that are associated with vascular function and control including, for example, nitric oxide bioavailability, lipid metabolism, and vascular integrity.

CRITICAL ISSUES

A large body of epidemiological evidence supports a relationship between poor vitamin C status and increased risk of developing CVD, and the prevalence of deficiency continues to be around 10%-20% of the general Western population although this problem could easily and cheaply be solved by supplementation. However, large intervention studies using vitamin C have not found a beneficial effect of supplementation. This review outlines the proposed mechanism by which vitamin C deficiency worsens CVD progression. In addition, it discusses problems with the currently available literature, including the discrepancies between the large intervention studies and the experimental and epidemiological literature.

FUTURE DIRECTIONS

Increased insights into vitamin C deficiency-mediated CVD progression will enable the design of future randomized controlled trials that are better suited to test the efficacy of vitamin C in disease prevention as well as the identification of high-risk individuals which could possibly benefit from supplementation.

Vitamin C promotes maturation of T-cells

AIMS

Vitamin C (ascorbic acid) is thought to enhance immune function, but the mechanisms involved are obscure. We utilized an in vitro model of T-cell maturation to evaluate the role of ascorbic acid in lymphocyte development.

RESULTS

Ascorbic acid was essential for the developmental progression of mouse bone marrow-derived progenitor cells to functional T-lymphocytes in vitro and also played a role in vivo. Ascorbate-mediated enhancement of T-cell development was lymphoid cell-intrinsic and independent of T-cell receptor (TCR) rearrangement. Analysis of TCR rearrangements demonstrated that ascorbic acid enhanced the selection of functional TCRαβ after the stage of β-selection. Genes encoding the coreceptor CD8 as well as the kinase ZAP70 were upregulated by ascorbic acid. Pharmacologic inhibition of methylation marks on DNA and histones enhanced ascorbate-mediated differentiation, suggesting an epigenetic mechanism of Cd8 gene regulation via active demethylation by ascorbate-dependent Fe(2+) and 2-oxoglutarate-dependent dioxygenases.

INNOVATION

We speculate that one aspect of gene regulation mediated by ascorbate occurs at the level of chromatin demethylation, mediated by Jumonji C (JmjC) domain enzymes that are known to be reliant upon ascorbate as a cofactor. JmjC domain enzymes are also known to regulate transcription factor activity. These two mechanisms are likely to play key roles in the modulation of immune development and function by ascorbic acid.

CONCLUSION

Our results provide strong experimental evidence supporting a role for ascorbic acid in T-cell maturation as well as insight into the mechanism of ascorbate-mediated enhancement of immune function.

Role of vitamin C in the function of the vascular endothelium

SIGNIFICANCE

Vitamin C, or ascorbic acid, has long been known to participate in several important functions in the vascular bed in support of endothelial cells. These functions include increasing the synthesis and deposition of type IV collagen in the basement membrane, stimulating endothelial proliferation, inhibiting apoptosis, scavenging radical species, and sparing endothelial cell-derived nitric oxide to help modulate blood flow. Although ascorbate may not be able to reverse inflammatory vascular diseases such as atherosclerosis, it may well play a role in preventing the endothelial dysfunction that is the earliest sign of many such diseases.

RECENT ADVANCES

Beyond simply preventing scurvy, evidence is mounting that ascorbate is required for optimal function of many dioxygenase enzymes in addition to those involved in collagen synthesis. Several of these enzymes regulate the transcription of proteins involved in endothelial function, proliferation, and survival, including hypoxia-inducible factor-1α and histone and DNA demethylases. More recently, ascorbate has been found to acutely tighten the endothelial permeability barrier and, thus, may modulate access of ascorbate and other molecules into tissues and organs.

CRITICAL ISSUES

The issue of the optimal cellular content of ascorbate remains unresolved, but it appears that low millimolar ascorbate concentrations are normal in most animal tissues, in human leukocytes, and probably in the endothelium. Although there may be little benefit of increasing near maximal cellular ascorbate concentrations in normal people, many diseases and conditions have either systemic or localized cellular ascorbate deficiency as a cause for endothelial dysfunction, including early atherosclerosis, sepsis, smoking, and diabetes.

FUTURE DIRECTIONS

A key focus for future studies of ascorbate and the vascular endothelium will likely be to determine the mechanisms and clinical relevance of ascorbate effects on endothelial function, permeability, and survival in diseases that cause endothelial dysfunction.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

Recent applications of engineered animal antioxidant deficiency models in human nutrition and chronic disease

Dietary antioxidants are essential nutrients that inhibit the oxidation of biologically important molecules and suppress the toxicity of reactive oxygen or nitrogen species. When the total antioxidant capacity is insufficient to quench these reactive species, oxidative damage occurs and contributes to the onset and progression of chronic diseases, such as neurodegenerative diseases, cardiovascular diseases, and cancer. However, epidemiological studies that examine the relationship between antioxidants and disease outcome can only identify correlative associations. Additionally, many antioxidants also have prooxidant effects. Thus, clinically relevant animal models of antioxidant function are essential for improving our understanding of the role of antioxidants in the pathogenesis of complex diseases as well as evaluating the therapeutic potential and risks of their supplementation. Recent progress in gene knockout mice and virus-based gene expression has potentiated these areas of study. Here, we review the current genetically modified animal models of dietary antioxidant function and their clinical relevance in chronic diseases. This review focuses on the 3 major antioxidants in the human body: vitamin C, vitamin E, and uric acid. We examine genetic models of vitamin C synthesis (guinea pig, Osteogenic Disorder Shionogi rat, Gulo(-/-) and SMP30(-/-) mouse mutants) and transport (Slc23a1(-/-) and Slc23a2(-/-) mouse mutants), vitamin E transport (Ttpa(-/-) mouse mutant), and uric acid synthesis (Uox(-/-) mouse mutant). The application of these models to current research goals is also discussed.

Actions of "antioxidants" in the protection against atherosclerosis

This review addresses the role of oxidative processes in atherosclerosis and its resulting cardiovascular disease by focusing on the outcome of antioxidant interventions. Although there is unambiguous evidence for the presence of heightened oxidative stress and resulting damage in atherosclerosis, it remains to be established whether this represents a cause or a consequence of the disease. This critical question is complicated further by the increasing realization that oxidative processes, including those related to signaling, are part of normal cell function. Overall, the results from animal interventions suggest that antioxidants provide benefit neither generally nor consistently. Where benefit is observed, it appears to be achieved at least in part via modulation of biological processes such as increase in nitric oxide bioavailability and induction of protective enzymes such as heme oxygenase-1, rather than via inhibition of oxidative processes and lipid oxidation in the arterial wall. Exceptions to this may be situations of multiple/excessive stress, the relevance of which for humans is not clear. This interpretation is consistent with the overall disappointing outcome of antioxidant interventions in humans and can be rationalized by the spatial compartmentalization of cellular oxidative signaling and/or damage, complex roles of oxidant-producing enzymes, and the multifactorial nature of atherosclerosis.

Bone marrow transplantation as an established approach for understanding the role of macrophages in atherosclerosis and the metabolic syndrome

PURPOSE OF REVIEW

Bone marrow transplantation (BMT) technology is a firmly established tool for studying atherosclerosis. Only recently it is helping us to understand the inflammatory mechanisms leading to the development of obesity, insulin resistance and type 2 diabetes. Here we review the use of BMT as a tool for studying the metabolic syndrome.

RECENT FINDINGS

Bone marrow-derived cells, and particularly monocytes and macrophages, have been a major subject in the study of atherogenesis, and they are highly amenable for research purposes because of their application in bone marrow transplantations. For example, the many pathways studied using BMT have helped unmask ABC transporters as the genes controlling reverse cholesterol transport and foam cell formation, as well as other genes like CCR2 and IκBα controlling leukocyte development, migration and activation. The invasion of leukocytes, not only in the vessel wall, but also in adipose tissue and liver, shares many common mechanisms relevant to atherosclerosis and metabolic diseases.

SUMMARY

BMT is an efficient and versatile tool for assessing the roles of specific genes that are restricted to hematopoietic cells, and especially the monocytes and macrophages in metabolic syndrome and its related pathologies.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.

Benzo[a]pyrene potentiates the pathogenesis of abdominal aortic aneurysms in apolipoprotein E knockout mice

The objective of this study was to determine the effect of benzo[a]pyrene (BaP), an abundant environmental polycyclic aromatic hydrocarbon compound, on the pathogenesis of abdominal aortic aneurysms (AAA). Earlier studies have shown that BaP promotes vasculopathy, including atherosclerosis, a predisposing factor for AAA development. In two experimental arms, 203 apolipoprotein E knockout (ApoE-/-) mice were evaluated in 4 groups: BaP, angiotensin II (AngII), BaP+AngII and control. Mice in the first arm were exposed to 5mg/kg/week of BaP for 42 days, and in the second arm to 0.71mg/kg daily for 60 days. In arm one, AAA incidence was higher in the BaP+AngII (14/28) versus AngII (8/27) group (p < 0.05), rupture (n=3) was observed only in BaP+AngII treated mice (p < 0.05). In the second arm, AAA incidence did not differ between AngII (17/30) and BaP+AngII (16/29) groups. However, intact AAA diameter was larger in the BaP+AngII (2.3 ± 0.1mm) versus AngII (1.9 ± 0.1mm) group (p < 0.05), but AAA rupture did not differ (p=NS). In both experimental arms, BaP+AngII mice showed increased expression of tumor necrosis factor alpha (TNF-α), cyclophilin A (Cyp A), and matrix metalloproteinase-9 (MMP9) (p < 0.05). No AAA occurred in control or BaP groups. These findings suggest the role of BaP exposure in potentiating AAA pathogenesis, which may have potential public health significance.

The SLC23 family of ascorbate transporters: ensuring that you get and keep your daily dose of vitamin C

The ascorbate transporters SVCT1 and SVCT2 are crucial for maintaining intracellular ascorbate concentrations in most cell types. Although the two transporter isoforms are highly homologous, they have different physiologic functions. The SVCT1 is located primarily in epithelial cells and has its greatest effect in reabsorbing ascorbate in the renal tubules. The SVCT2 is located in most non-epithelial tissues, with the highest expression in brain and neuroendocrine tissues. These transporters are hydrophobic membrane proteins that have a high affinity and are highly selective for ascorbate. Their ability to concentrate ascorbate inside cells is driven by the sodium gradient across the plasma membrane as generated by Na+/K+ ATPase. They can concentrate ascorbate 20 to 60-fold over plasma ascorbate concentrations. Ascorbate transport on these proteins is regulated at the transcriptional, translational and post-translational levels. Available studies show that transporter function is acutely regulated by protein kinases A and C, whereas transporter expression is increased by low intracellular ascorbate and associated oxidative stress. The knockout of the SVCT2 in mice is lethal on day 1 of life, and almost half of SVCT1 knockout mice do not survive to weaning. These findings confirm the importance both of cellular ascorbate and of the two transport proteins as key to maintaining intracellular ascorbate. LINKED ARTICLES This article is part of a themed section on Transporters. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2011.164.issue-7.