Oxidative Stress and Peroxisome Proliferator–Activated Receptors

An increase in reactive oxygen species underlies neonatal cerebellum repair

The neonatal mouse cerebellum shows remarkable regenerative potential upon injury at birth, wherein a subset of Nestin-expressing progenitors (NEPs) undergoes adaptive reprogramming to replenish granule cell progenitors that die. Here, we investigate how the microenvironment of the injured cerebellum changes upon injury and contributes to the regenerative potential of normally gliogenic-NEPs and their adaptive reprogramming. Single cell transcriptomic and bulk chromatin accessibility analyses of the NEPs from injured neonatal cerebella compared to controls show a temporary increase in cellular processes involved in responding to reactive oxygen species (ROS), a known damage-associated molecular pattern. Analysis of ROS levels in cerebellar tissue confirm a transient increased one day after injury at postanal day 1, overlapping with the peak cell death in the cerebellum. In a transgenic mouse line that ubiquitously overexpresses human mitochondrial catalase (mCAT), ROS is reduced 1 day after injury to the granule cell progenitors, and we demonstrate that several steps in the regenerative process of NEPs are curtailed leading to reduced cerebellar growth. We also provide evidence that microglia are involved in one step of adaptive reprogramming by regulating NEP replenishment of the granule cell precursors. Collectively, our results highlight that changes in the tissue microenvironment regulate multiple steps in adaptative reprogramming of NEPs upon death of cerebellar granule cell progenitors at birth, highlighting the instructive roles of microenvironmental signals during regeneration of the neonatal brain.

Molecular Thumbprints: Biological Signatures That Measure Loss of Identity

Each life is challenged to adapt to an ever-changing environment with integrity-simply put, to maintain identity. We hypothesize that this mission statement of adaptive homeostasis is particularly poignant in an adaptive response, like inflammation. A maladaptive response of unresolved inflammation can seed chronic disease over a lifetime. We propose the concept of a molecular thumbprint: a biological signature of loss of identity as a measure of incomplete return to homeostasis after an inflammatory response. Over time, personal molecular thumbprints can measure dynamic and precise trajectories to chronic inflammatory diseases and further loss of self to cancer. Why is this important? Because the phenotypes and molecular signatures of established complex inflammatory diseases are a far cry from the root of the complex problem, let alone the initial seed. Understanding the science behind key germinating seeds of disease helps to identify molecular factors of susceptibility, resilience, and early dietary or drug intervention. We pilot this hypothesis in a rat colitis model that is well-established for understanding molecular mechanisms of colonic health, disease, and transition of colitis to cancer.

Metabolites of endophytic fungi isolated from leaves of Bauhinia variegata exhibit antioxidant activity and agonist activity on peroxisome proliferator-activated receptors α, β/δ and γ

Diabetes mellitus is a metabolic disorder that affects millions of people worldwide and is linked to oxidative stress and inflammation. Thiazolidinediones (TZD) improve insulin sensitization and glucose homeostasis mediated by the activation of peroxisome proliferator-activated receptors γ (PPARγ) in patients with type 2 diabetes. However, their use is associated with severe adverse effects such as loss of bone mass, retention of body fluids, liver and heart problems, and increased risk of bladder cancer. Partial PPARγ agonists can promote the beneficial effects of thiazolidinediones with fewer adverse effects. Endophytic fungi colonize plant tissues and have a particularly active metabolism caused by the interaction with them, which leads to the production of natural products with significant biological effects that may be like that of the colonized plant. Here, we identify seven endophytic fungi isolated from Bauhinia variegata leaves that have antioxidant activities. Also, one of the extracts presented pan-agonist activity on PPAR, and another showed activity in PPARα and PPARβ/δ. A better understanding of this relationship could help to comprehend the mechanism of action of antioxidants in treating diabetes and its complications. Moreover, compounds with these capabilities to reduce oxidative stress and activate the receptor that promotes glucose homeostasis are promising candidates in treatment of diabetes.

Oxidative Stress Causes Masculinization of Genetically Female Medaka Without Elevating Cortisol

Medaka (Oryzias latipes) is a teleost fish with an XX/XY sex determination system. Sex reversal from female-to-male (masculinization of XX fish) can be induced through cortisol elevation from exposure to environmental stress such as high temperature during sexual differentiation. However, the effects of oxidative stress, generated via metabolic reactions and biological defense mechanisms, on the sexual differentiation of medaka are unclear. Here, we investigated the effect of oxidative stress on medaka sexual differentiation using hydrogen peroxide (H₂O₂), which induces oxidative stress in vertebrates. H₂O₂ treatment from 0 to 5 days post-hatching induced masculinization of wild-type XX medaka, but not of gonadal soma-derived growth factor (gsdf) or peroxisome proliferator-activated receptor alpha-a (pparaa) knockout XX fish. Co-treatment with an oxidative stress inhibitor caused masculinization recovery but co-treatment with a cortisol synthesis inhibitor did not. H₂O₂ treatment significantly upregulated gsdf and pparaa expression in XX medaka. However, H₂O₂ did not elevate cortisol levels in medaka larvae during sexual differentiation. These results strongly indicate that oxidative stress induces masculinization of XX medaka without causing elevation of cortisol.

Roles of Phase Separation for Cellular Redox Maintenance

The oxidation reaction greatly alters characteristics of various cellular components. In exchange for efficient energy production, mitochondrial aerobic respiration substantially increases the risk of excess oxidation of cellular biomolecules such as lipids, proteins, nucleic acids, and numerous small molecules. To maintain a physiologically balanced cellular reduction-oxidation (redox) state, cells utilize a variety of molecular machineries including cellular antioxidants and protein degradation complexes such as the ubiquitin-proteasome system or autophagy. In the past decade, biomolecular liquid-liquid phase separation (LLPS) has emerged as a subject of great interest in the biomedical field, as it plays versatile roles in the maintenance of cellular homeostasis. With regard to redox homeostasis, LLPS arose as a major player in both well-characterized and newly emerging redox pathways. LLPS is involved in direct redox imbalance sensing, signal transduction, and transcriptional regulation. Also, LLPS is at play when cells resist redox imbalance through metabolic switching, translational remodeling, activating the DNA damage response, and segregation of vulnerable lipids and proteins. On the other hand, chronic accumulation of phase-separated molecular condensates such as lipid droplets and amyloid causes neurotoxic outcomes. In this review we enumerate recent progress on understanding how cells utilize LLPS to deal with oxidative stress, especially related to cell survival or pathogenesis, and we discuss future research directions for understanding biological phase separation in cellular redox regulation.

The influence of alkaloids on oxidative stress and related signaling pathways

Alkaloids have always attracted scientific interest due to either their positive or negative effects on human beings. This review aims to summarize their antioxidant effects by both classical in vitro scavenging assay and at the cellular level. Since most in vitro studies used the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay, the results from those studies are summed up in the first part of the article. In the second part, available data on the effect of alkaloids on NADPH-oxidase, the key enzyme for reactive oxygen species production, at the cellular level, are summarized. More than 130 alkaloids were tested by DPPH assay. However, due to methodological differences, a direct comparison is hardly possible. It can be at least concluded that some of them were either similar to or even more active than standard antioxidants and the number of aromatic hydroxyl groups seems to be the major determinant for the activity. The data on inhibition of NADPH-oxidase activity by alkaloids demonstrated that there is little relationship to the DPPH assay. The mechanism seems to be based on inhibition of synthesis, activation or translocation of NADPH-oxidase subunits. In some alkaloids, activation of the nuclear factor Nrf2 pathway was documented to be the grounds for inhibition of NADPH-oxidase. Interestingly, many alkaloids can behave both as anti-oxidants and pro-oxidants depending on conditions and pro-oxidation might be the reason for activation of Nrf2. Available data on other "antioxidant" transcription factors FOXOs and PPARs are also mentioned.

Comparative Analysis of Toxic Responses of Organic Extracts from Diesel and Selected Alternative Fuels Engine Emissions in Human Lung BEAS-2B Cells

This study used toxicogenomics to identify the complex biological response of human lung BEAS-2B cells treated with organic components of particulate matter in the exhaust of a diesel engine. First, we characterized particles from standard diesel (B0), biodiesel (methylesters of rapeseed oil) in its neat form (B100) and 30% by volume blend with diesel fuel (B30), and neat hydrotreated vegetable oil (NEXBTL100). The concentration of polycyclic aromatic hydrocarbons (PAHs) and their derivatives in organic extracts was the lowest for NEXBTL100 and higher for biodiesel. We further analyzed global gene expression changes in BEAS-2B cells following 4 h and 24 h treatment with extracts. The concentrations of 50 µg extract/mL induced a similar molecular response. The common processes induced after 4 h treatment included antioxidant defense, metabolism of xenobiotics and lipids, suppression of pro-apoptotic stimuli, or induction of plasminogen activating cascade; 24 h treatment affected fewer processes, particularly those involved in detoxification of xenobiotics, including PAHs. The majority of distinctively deregulated genes detected after both 4 h and 24 h treatment were induced by NEXBTL100; the deregulated genes included, e.g., those involved in antioxidant defense and cell cycle regulation and proliferation. B100 extract, with the highest PAH concentrations, additionally affected several cell cycle regulatory genes and p38 signaling.

Antioxidant Stress and Anti-Inflammation of PPARα on Warm Hepatic Ischemia-Reperfusion Injury

Hepatic ischemia-reperfusion (IR) injury is a serious clinical problem. Minimizing the adverse effect of ischemia-reperfusion injury after liver surgery or trauma is an urgent need. It has been proved that besides the effect of regulating the lipid and lipoprotein metabolism, PPARα also undertakes the task of organ protection. In this paper, related literature has been summarized and we come to the conclusion that administration of PPARα agonists can strengthen the antioxidant and anti-inflammation defense system by the upregulation of the expression of antioxidant enzymes and inhibition of NF-κB activity. This may provide a potential clinical treatment for hepatic ischemia-reperfusion injury.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

Activation of PPARγ by Rosiglitazone does not negatively impact male sex steroid hormones in diabetic rats

Peroxisome proliferator-activated receptor gamma (PPARγ) activation decreased serum testosterone (T) in women with hyperthecosis and/or polycystic ovary syndrome and reduced the conversion of androgens to estradiol (E2) in female rats. This implies modulation of female sex steroid hormones by PPARγ. It is not clear if PPARγ modulates sex steroid hormones in diabetic males. Because PPARγ activation by thiazolidinedione increased insulin sensitivity in type 2 diabetes, understanding the long term impact of PPARγ activation on steroid sex hormones in males is critical. Our objective was to determine the effect of PPARγ activation on serum and intratesticular T, luteinizing hormone (LH), follicle stimulating hormone (FSH) and E2 concentrations in male Zucker diabetic fatty (ZDF) rats treated with the PPARγ agonist rosiglitazone (a thiazolidinedione). Treatment for eight weeks increased PPARγ mRNA and protein in the testis and elevated serum adiponectin, an adipokine marker for PPARγ activation. PPARγ activation did not alter serum or intratesticular T concentrations. In contrast, serum T level but not intratesticular T was reduced by diabetes. Neither diabetes nor PPARγ activation altered serum E2 or gonadotropins FSH and LH concentrations. The results suggest that activation of PPARγ by rosiglitazone has no negative impact on sex hormones in male ZDF rats.

Molecular characterization of novel and selective peroxisome proliferator-activated receptor alpha agonists with robust hypolipidemic activity in vivo

The nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha) is recognized as the primary target of the fibrate class of hypolipidemic drugs and mediates lipid lowering in part by activating a transcriptional cascade that induces genes involved in the catabolism of lipids. We report here the characterization of three novel PPARalpha agonists with therapeutic potential for treating dyslipidemia. These structurally related compounds display potent and selective binding to human PPARalpha and support robust recruitment of coactivator peptides in vitro. These compounds markedly potentiate chimeric transcription systems in cell-based assays and strikingly lower serum triglycerides in vivo. The transcription networks induced by these selective PPARalpha agonists were assessed by transcriptional profiling of mouse liver after short- and long-term treatment. The induction of several known PPARalpha target genes involved with fatty acid metabolism were observed, reflecting the expected pharmacology associated with PPARalpha activation. We also noted the down-regulation of a number of genes related to immune cell function, the acute phase response, and glucose metabolism, suggesting that these compounds may have anti-inflammatory action in the mammalian liver. Whereas these compounds are efficacious in acute preclinical models, extended safety studies and further clinical testing will be required before the full therapeutic promise of a selective PPARalpha agonist is realized.

Macrophage expression of peroxisome proliferator-activated receptor-alpha reduces atherosclerosis in low-density lipoprotein receptor-deficient mice

BACKGROUND

The peroxisome proliferator-activated receptor-alpha (PPARalpha) plays important roles in lipid metabolism, inflammation, and atherosclerosis. PPARalpha ligands have been shown to reduce cardiovascular events in high-risk subjects. PPARalpha expression by arterial cells, including macrophages, may exert local antiatherogenic effects independent of plasma lipid changes.

METHODS AND RESULTS

To examine the contribution of PPARalpha expression by bone marrow-derived cells in atherosclerosis, male and female low-density lipoprotein receptor-deficient (LDLR(-/-)) mice were reconstituted with bone marrow from PPARalpha(-/-) or PPARalpha(+/+) mice and challenged with a high-fat diet. Although serum lipids and lipoprotein profiles did not differ between the groups, the size of atherosclerotic lesions in the distal aorta of male and female PPARalpha(-/-) --> LDLR(-/-) mice was significantly increased (44% and 46%, respectively) compared with controls. Male PPARalpha(-/-) --> LDLR(-/-) mice also had larger (44%) atherosclerotic lesions in the proximal aorta than male PPARalpha(+/+) --> LDLR(-/-) mice. Peritoneal macrophages from PPARalpha(-/-) mice had increased uptake of oxidized LDL and decreased cholesterol efflux. PPARalpha(-/-) macrophages had lower levels of scavenger receptor B type I and ABCA1 protein expression and an accelerated response of nuclear factor-kappaB-regulated inflammatory genes. A laser capture microdissection analysis verified suppressed scavenger receptor B type I and increased nuclear factor-kappaB gene expression levels in vivo in atherosclerotic lesions of PPARalpha(-/-) --> LDLR(-/-) mice compared with the lesions of control PPARalpha(+/+) --> LDLR(-/-) mice.

CONCLUSIONS

These data demonstrate that PPARalpha expression by macrophages has antiatherogenic effects via modulation of cell cholesterol trafficking and inflammatory activity.

Peroxisome proliferator-activated receptors as transcriptional nodal points and therapeutic targets

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors involved in the transcriptional regulation of key metabolic pathways such as lipid metabolism, adipogenesis, and insulin sensitivity. More recent work implicates all 3 PPAR isotypes (alpha, gamma, and delta, also known as beta or beta/delta) in inflammatory and atherosclerotic pathways. Because these nuclear receptors are activated by extracellular signals and control multiple gene targets, PPARs can be seen as nodes that control multiple inputs and outputs involved in energy balance, providing insight into how metabolism and the vasculature may be integrated. The ongoing clinical use of fibrates, which activate PPARalpha, and thiazolidinediones, which activate PPARgamma, establishes these receptors as viable drug targets, whereas considerable in vitro animal model and human surrogate marker studies suggest that PPAR activation may limit inflammation and atherosclerosis. Together, these various observations have stimulated intense interest in PPARs as therapeutic targets and led to large-scale cardiovascular end-point trials with PPAR agonists. The first of these studies has generated mixed results that require careful review, especially in anticipation of additional clinical trial data and ongoing attempts to develop novel PPAR modulators. Such analysis of the existing PPAR data, the appropriate use of currently approved PPAR agonists, and continued progress in PPAR therapeutics will be predicated on a better understanding of PPAR biology.

Peroxisomes and oxidative stress

The discovery of the colocalization of catalase with H2O2-generating oxidases in peroxisomes was the first indication of their involvement in the metabolism of oxygen metabolites. In past decades it has been revealed that peroxisomes participate not only in the generation of reactive oxygen species (ROS) with grave consequences for cell fate such as malignant degeneration but also in cell rescue from the damaging effects of such radicals. In this review the role of peroxisomes in a variety of physiological and pathological processes involving ROS mainly in animal cells is presented. At the outset the enzymes generating and scavenging H2O2 and other oxygen metabolites are reviewed. The exposure of cultured cells to UV light and different oxidizing agents induces peroxisome proliferation with formation of tubular peroxisomes and apparent upregulation of PEX genes. Significant reduction of peroxisomal volume density and several of their enzymes is observed in inflammatory processes such as infections, ischemia-reperfusion injury and hepatic allograft rejection. The latter response is related to the suppressive effects of TNFalpha on peroxisomal function and on PPARalpha. Their massive proliferation induced by a variety of xenobiotics and the subsequent tumor formation in rodents is evidently due to an imbalance in the formation and scavenging of ROS, and is mediated by PPARalpha. In PEX5-/- mice with the absence of functional peroxisomes severe abnormalities of mitochondria in different organs are observed which resemble closely those in respiratory chain disorders associated with oxidative stress. Interestingly, no evidence of oxidative damage to proteins or lipids, nor of increased peroxide production has been found in that mouse model. In this respect the role of PPARalpha, which is highly activated in those mice, in prevention of oxidative stress deserves further investigation.