Marc Birringer

Antioxidants prevent health-promoting effects of physical exercise in humans

Exercise promotes longevity and ameliorates type 2 diabetes mellitus and insulin resistance. However, exercise also increases mitochondrial formation of presumably harmful reactive oxygen species (ROS). Antioxidants are widely used as supplements but whether they affect the health-promoting effects of exercise is unknown. We evaluated the effects of a combination of vitamin C (1000 mg/day) and vitamin E (400 IU/day) on insulin sensitivity as measured by glucose infusion rates (GIR) during a hyperinsulinemic, euglycemic clamp in previously untrained (n = 19) and pretrained (n = 20) healthy young men. Before and after a 4 week intervention of physical exercise, GIR was determined, and muscle biopsies for gene expression analyses as well as plasma samples were obtained to compare changes over baseline and potential influences of vitamins on exercise effects. Exercise increased parameters of insulin sensitivity (GIR and plasma adiponectin) only in the absence of antioxidants in both previously untrained (P < 0.001) and pretrained (P < 0.001) individuals. This was paralleled by increased expression of ROS-sensitive transcriptional regulators of insulin sensitivity and ROS defense capacity, peroxisome-proliferator-activated receptor gamma (PPARγ), and PPARγ coactivators PGC1α and PGC1β only in the absence of antioxidants (P < 0.001 for all). Molecular mediators of endogenous ROS defense (superoxide dismutases 1 and 2; glutathione peroxidase) were also induced by exercise, and this effect too was blocked by antioxidant supplementation. Consistent with the concept of mitohormesis, exercise-induced oxidative stress ameliorates insulin resistance and causes an adaptive response promoting endogenous antioxidant defense capacity. Supplementation with antioxidants may preclude these health-promoting effects of exercise in humans.

Vitamin E activates gene expression via the pregnane X receptor.

Tocopherols and tocotrienols are metabolized by side chain degradation via initial omega-oxidation and subsequent beta-oxidation. omega-Oxidation is performed by cytochrome P450 (CYP) enzymes which are often regulated by their substrates themselves. Results presented here show that all forms of Vitamin E are able to activate gene expression via the pregnane X receptor (PXR), a nuclear receptor regulating a variety of drug metabolizing enzymes. In HepG2 cells transfected with the human PXR and the chloramphenicol acetyl transferase (CAT) gene linked to two PXR responsive elements, CAT activity was most strongly induced by alpha- and gamma-tocotrienol followed by rifampicin, delta-, alpha- and gamma-tocopherol. The inductive efficacy was concentration-dependent; its specificity was underscored by a lower response when cotransfection with PXR was omitted. Up-regulation of endogenous CYP3A4 and CYP3A5 mRNA was obtained by gamma-tocotrienol, the most potent activator of PXR, with the same efficacy as with rifampicin. This points to a potential interference of individual forms of Vitamin E with the metabolism and efficacy of drugs.

Tocopherols are metabolized in HepG2 cells by side chain ω-oxidation and consecutive β-oxidation

Abstract The metabolism of tocopherols by ω- and β-oxidation of the phytyl side chain has been inferred from the identification of the final products carboxyethyl-hydroxychromans (CEHC) and immediate precursors, α- and γ-carboxymethylbutyl-hydroxychromans (CMBHCs). This hypothesis is here corroborated by the identification of a further α-tocopherol metabolite, α-carboxymethylhexyl-hydroxychroman (α-CMHHC), and evidence for the involvement of a P450-type cytochrome. HepG2 cells, when exposed to 100 μM all-rac-α-tocopherol, released α-CEHC, α-CMBHC, and α-CMHHC into the medium. The detection of those metabolites required pretreatment of the cells with α-tocopherol for 10 d. In contrast, analogous metabolites of γ and δ-tocopherol were detectable without any preconditioning, while corresponding metabolites of RRR-α-tocopherol could not be detected at all. The formation of α-CEHC from all-rac-α-tocopherol was enhanced up to 5-fold by pretreatment of the HepG2 cells with rifampicin, known to induce CYP3A-type cytochromes with the capability of catalyzing ω-oxidation. In contrast, clofibrate did not reveal any effect. This observation suggests that a CYP3A-type cytochrome initiates tocopherol metabolism by ω-oxidation. It further reveals that inducible ω-oxidation is the rate-limiting step in tocopherol metabolism. It is discussed that competition of microsomal ω-oxidation with specific binding by the α-tocopherol transfer protein (α-TTP) determines the metabolic fate of the individual tocopherols.

Role of Sirtuins in Lifespan Regulation is Linked to Methylation of Nicotinamide

Sirtuins, a family of histone deacetylases, have a fiercely debated role in regulating lifespan. In contrast with recent observations, here we find that overexpression of sir-2.1, the ortholog of mammalian SirT1, does extend Caenorhabditis elegans lifespan. Sirtuins mandatorily convert NAD(+) into nicotinamide (NAM). We here find that NAM and its metabolite, 1-methylnicotinamide (MNA), extend C. elegans lifespan, even in the absence of sir-2.1. We identify a previously unknown C. elegans nicotinamide-N-methyltransferase, encoded by a gene now named anmt-1, to generate MNA from NAM. Disruption and overexpression of anmt-1 have opposing effects on lifespan independent of sirtuins, with loss of anmt-1 fully inhibiting sir-2.1-mediated lifespan extension. MNA serves as a substrate for a newly identified aldehyde oxidase, GAD-3, to generate hydrogen peroxide, which acts as a mitohormetic reactive oxygen species signal to promote C. elegans longevity. Taken together, sirtuin-mediated lifespan extension depends on methylation of NAM, providing an unexpected mechanistic role for sirtuins beyond histone deacetylation.

Comparison of different selenocompounds with respect to nutritional value vs. toxicity using liver cells in culture.

The essential micronutrient selenium (Se) exerts its biological effects mainly through enzymatically active selenoproteins. Their biosynthesis depends on the 21st proteinogenic amino acid selenocysteine and thus on dietary Se supply. Hepatically derived selenoprotein P (SEPP) is the central selenoprotein in blood controlling Se transport and distribution. Kidney-derived extracellular glutathione peroxidase is another relevant serum selenoprotein depending on SEPP for biosynthesis. Therefore, secretion of SEPP by hepatocytes is crucial to convert nutritional sources into serum Se, supporting Se status and selenoprotein biosynthesis in other tissues. In order to compare the bioactivity of 10 different selenocompounds, their dose-dependent toxicities and nutritional qualities to support SEPP and glutathione peroxidase biosynthesis were determined in a murine and two human liver cell lines. Characteristic dose- and time-dependent effects on viability and SEPP production were observed. Incubations with 100 nM sodium selenite, l- or dl-selenocystine, selenodiglutathione or selenomethyl-selenocysteine increased SEPP concentrations in the culture medium up to 6.5-fold over control after 72 h. In comparison, sodium selenate, l- or dl-selenomethionine or methylseleninic acid was less effective and increased SEPP by 2.5-fold under these conditions. As expected, ebselen did not increase selenoprotein production, supporting its classification as a stable selenocompound. Methylseleninic acid, l-selenocystine, selenodiglutathione or selenite induced cell death in micromolar concentrations, whereas selenomethionine or ebselen was not toxic within the concentration range tested. Our results indicate that hepatic selenoprotein production and toxicity of selenocompounds do not correlate with and rather represent compound-specific properties. The favourable profile of selenomethylselenocysteine warrants its consideration as a promising option for supplementation purposes.

Vitamin E: Emerging aspects and new directions

Abstract The discovery of vitamin E will have its 100th anniversary in 2022, but we still have more questions than answers regarding the biological functions and the essentiality of vitamin E for human health. Discovered as a factor essential for rat fertility and soon after characterized for its properties of fat‐soluble antioxidant, vitamin E was identified to have signaling and gene regulation effects in the 1980s. In the same years the cytochrome P‐450 dependent metabolism of vitamin E was characterized and a first series of studies on short‐chain carboxyethyl metabolites in the 1990s paved the way to the hypothesis of a biological role for this metabolism alternative to vitamin E catabolism. In the last decade other physiological metabolites of vitamin E have been identified, such as &agr;‐tocopheryl phosphate and the long‐chain metabolites formed by the ω‐hydroxylase activity of cytochrome P‐450. Recent findings are consistent with gene regulation and homeostatic roles of these metabolites in different experimental models, such as inflammatory, neuronal and hepatic cells, and in vivo in animal models of acute inflammation. Molecular mechanisms underlying these responses are under investigation in several laboratories and side‐glances to research on other fat soluble vitamins may help to move faster in this direction. Other emerging aspects presented in this review paper include novel insights on the mechanisms of reduction of the cardiovascular risk, immunomodulation and antiallergic effects, neuroprotection properties in models of glutamate excitotoxicity and spino‐cerebellar damage, hepatoprotection and prevention of liver toxicity by different causes and even therapeutic applications in non‐alcoholic steatohepatitis. We here discuss these topics with the aim of stimulating the interest of the scientific community and further research activities that may help to celebrate this anniversary of vitamin E with an in‐depth knowledge of its action as vitamin. Graphical abstract Absorption, transport and metabolism of vitamin E and long‐chain metabolites of vitamin E. The route of all vitamin E forms follows in principle the pathway of other lipid species. In the intestine, vitamin E and other lipids are packed into micelles, which are taken up via receptors. In intestinal epithelial cells, vitamin E is incorporate into nascent chylomicrons or HDL via the ATP‐binding cassette transporter ABCA1. In the blood, vitamin E follows the lipoprotein transport route of other lipids and is transported either to extrahepatic tissues or to the liver. The transport of vitamin E occurs via chylomicron remnants, intermediate‐density, low‐density or high‐density lipoproteins. In the liver, vitamin E undergoes several sorting steps that direct the different forms of vitamin E either to the catabolic route or to nascent lipoproteins via partly unknown mechanisms. The &agr;‐tocopherol transfer protein (&agr;‐TTP) discriminates between the different forms of vitamin E in favor of &agr;‐tocopherol, thus protecting it from excessive cytochrome P450‐mediated catabolism and excretion as &agr;‐carboxyethyl‐hydroxychroman (&agr;‐CEHC). On the contrary, non‐&agr;‐tocopherol forms are preferentially handled as xenobiotics and final degradation products, namely CEHCs, are found as sulfate and glucuronide conjugates in urine and bile. The figure was modified from [20]. Figure. No Caption available. HighlightsBiological functions and the essentiality of vitamin E for human health are still questionable.RDA and AI values have been proposed, but the concept of optimal intake for this vitamin remains elusive.In the last decades physiological and bioactive metabolites of vitamin E have been identified.Recent studies reveal promising applications of vitamin E in prevention of chronic diseases such as cardiovascular disease and NASH.Other potential applications of vitamin E have been proposed in neuroprotection, immunomodulation and antiallergic interventions.

Impaired respiration is positively correlated with decreased life span in Caenorhabditis elegans models of Friedreich Ataxia

Impaired expression of mitochondrial genes causes alterations in life span of the nematode Caenorhabditis elegans. Intriguingly, although some of these genes have been shown to extend life expectancy and reduce aging processes, others are known to shorten life span in the same model organism. Reduced expression of a mitochondrial protein called frataxin causes a neurodegenerative disorder named Friedreich Ataxia, which decreases life span in humans. Surprisingly, reduced expression of the C. elegans frataxin homologue frh‐1 has been associated with both increased as well as decreased life span by different laboratories. To further elucidate these conflicting findings, here we show that different RNA interference (RNAi) constructs directed against frh‐1 reduce C. el‐egans life span. Moreover, we show that frh‐1‐inhibiting RNAi impairs oxygen consumption and that respiratory rate is positively correlated with life span in this multi‐cellular eukaryote (r= 0.8566), suggesting that >73% of life span variance in C. elegans is explained by changes in respiratory rate. Taken together, impaired mitochon‐drial metabolism due to RNAi‐mediated inhibition of the frataxin homologue frh‐1 causes both impaired respiration as well as decreased life span in the multi‐cellular eukaryote C. elegans.—Zarse, K., Schulz, T. J., Birringer, M., Ristow, M. Impaired respiration is positively correlated with decreased life span in Caenorhabditis elegans models of Friedreich Ataxia. FASEB J. 21, 1271–1275 (2007)

Complexity of vitamin E metabolism

Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.

Small-Molecule Targeting of the Mitochondrial Compartment with an Endogenously Cleaved Reversible Tag

Reversible mitochondrial shuttle: A novel concept in mitochondrial pharmacology allows the transport of bioactive compounds into the mitochondrial compartment and their subsequent release. A lipoic acid derivative containing a cleavable (“reversible”) triphenylphosphonium tag is endogenously cleaved by the mitochondrial aldehyde dehydrogenase (ALDH‐2) after mitochondrial accumulation.

Regulatory metabolites of vitamin E and their putative relevance for atherogenesis

Vitamin E is likely the most important antioxidant in the human diet and α-tocopherol is the most active isomer. α-Tocopherol exhibits anti-oxidative capacity in vitro, and inhibits oxidation of LDL. Beside this, α-tocopherol shows anti-inflammatory activity and modulates expression of proteins involved in uptake, transport and degradation of tocopherols, as well as the uptake, storage and export of lipids such as cholesterol. Despite promising anti-atherogenic features in vitro, vitamin E failed to be atheroprotective in clinical trials in humans. Recent studies highlight the importance of long-chain metabolites of α-tocopherol, which are formed as catabolic intermediate products in the liver and occur in human plasma. These metabolites modulate inflammatory processes and macrophage foam cell formation via mechanisms different than that of their metabolic precursor α-tocopherol and at lower concentrations. Here we summarize the controversial role of vitamin E as a preventive agent against atherosclerosis and point the attention to recent findings that highlight a role of these long-chain metabolites of vitamin E as a proposed new class of regulatory metabolites. We speculate that the metabolites contribute to physiological as well as pathophysiological processes.