Maria Lönn

Thiol redox control via thioredoxin and glutaredoxin systems

The Trx (thioredoxin) and Grx (glutaredoxin) systems control cellular redox potential, keeping a reducing thiol-rich intracellular state, which on generation of reactive oxygen species signals through thiol redox control mechanisms. Here, we give a brief overview of the human Trx and Grx systems. The main part focuses on our current knowledge about mitochondrial Grx2, which facilitates mitochondrial redox homoeostasis during oxidative stress-induced apoptosis.

Impaired nuclear Nrf2 translocation undermines the oxidative stress response in Friedreich ataxia.

Background Friedreich ataxia originates from a decrease in mitochondrial frataxin, which causes the death of a subset of neurons. The biochemical hallmarks of the disease include low activity of the iron sulfur cluster-containing proteins (ISP) and impairment of antioxidant defense mechanisms that may play a major role in disease progression. Methodology/Principal Findings We thus investigated signaling pathways involved in antioxidant defense mechanisms. We showed that cultured fibroblasts from patients with Friedreich ataxia exhibited hypersensitivity to oxidative insults because of an impairment in the Nrf2 signaling pathway, which led to faulty induction of antioxidant enzymes. This impairment originated from previously reported actin remodeling by hydrogen peroxide. Conclusions/Significance Thus, the defective machinery for ISP synthesis by causing mitochondrial iron dysmetabolism increases hydrogen peroxide production that accounts for the increased susceptibility to oxidative stress.

Identification, Expression Pattern, and Characterization of Mouse Glutaredoxin 2 Isoforms

Glutaredoxin 2 (Grx2) is a glutathione-dependent oxidoreductase involved in the maintenance of mitochondrial redox homeostasis. Grx2 was first characterized as mitochondrial protein, but alternative mRNA variants lacking the transit peptide-encoding first exon were demonstrated for human and proposed for mouse. We systematically screened for alternative transcript variants of mouse Grx2. We identified a total of six exons, three constitutive (II, III, and IV), two alternative first exons (exons Ia and Ic), and one single-cassette exon (exon IIIb) located between exons III and IV. Exons Ic and IIIb are not present in the human genome; mice lack human exon Ib. The six exons give rise to five transcript variants that encode three protein isoforms: mitochondrial Grx2a, a cytosolic isoform that is homologous to the cytosolic/nuclear human Grx2c and present in specific cells of many tissues and the testis-specific isoform Grx2d that is unique to mice. Mouse Grx2c can form an iron/sulfur cluster-bridged dimer, is enzymatically active as a monomer, and can donate electrons to ribonucleotide reductase. Testicular cells lack mitochondrial Grx2a but contain cytosolic Grx2. Prominent immunostaining was detected in spermatogonia and spermatids. These results provide evidence for additional functions of Grx2 in the cytosol, in cell proliferation, and in cellular differentiation.

Both Thioredoxin 2 and Glutaredoxin 2 Contribute to the Reduction of the Mitochondrial 2-Cys Peroxiredoxin Prx3 *

The proteins from the thioredoxin family are crucial actors in redox signaling and the cellular response to oxidative stress. The major intracellular source for oxygen radicals are the components of the respiratory chain in mitochondria. Here, we show that the mitochondrial 2-Cys peroxiredoxin (Prx3) is not only substrate for thioredoxin 2 (Trx2), but can also be reduced by glutaredoxin 2 (Grx2) via the dithiol reaction mechanism. Grx2 reduces Prx3 exhibiting catalytic constants (Km, 23.8 μmol·liter−1; Vmax, 1.2 μmol·(mg·min)−1) similar to Trx2 (Km, 11.2 μmol·liter−1; Vmax, 1.1 μmol·(mg·min)−1). The reduction of the catalytic disulfide of the atypical 2-Cys Prx5 is limited to the Trx system. Silencing the expression of either Trx2 or Grx2 in HeLa cells using specific siRNAs did not change the monomer:dimer ratio of Prx3 detected by a specific 2-Cys Prx redox blot. Only combined silencing of the expression of both proteins led to an accumulation of oxidized protein. We further demonstrate that the distribution of Prx3 in different mouse tissues is either linked to the distribution of Trx2 or Grx2. These results introduce Grx2 as a novel electron donor for Prx3, providing further insights into pivotal cellular redox signaling mechanisms.

Penultimate selenocysteine residue replaced by cysteine in thioredoxin reductase from selenium-deficient rat liver

Selenium is an essential micronutrient for humans and animals, and its deficiency can predispose to the development of pathological conditions. This study evaluates the effect of selenium deficiency on the thioredoxin system, consisting of NADPH, selenoprotein thioredoxin reductase (TrxR), and thioredoxin (Trx);and the glutathione system, including NADPH, glutathione reductase, glutathione, and glutaredoxin coupled with selenoprotein glutathione peroxidase (GPx). We particularly investigate whether inactive truncated TrxR is present under selenium‐ starvation conditions due to reading of the UGA codon as stop. Feeding rats a selenium‐deficient diet resulted in a large decrease in activity of TrxR and GPx in rat liver but not in the levels of Trx1 and Grx1. However, selenium deficiency induced mitochondrial Grx2 10fold and markedly changed the expression of some flavoproteins that are involved in the cellular folate, glucose, and lipid metabolism. Liver TrxR mRNA was nearly unchanged, but no truncated enzyme was found. Instead, a low‐activity form of TrxR with a cysteine substituted for the penultimate selenocysteine in the C‐terminal active site was identified in selenium‐deficient rat liver. These results show a novel mechanism for decoding the UGA stop codon, inserting cysteine to make a full‐length enzyme that may be required for selenium assimilation.— Lu, J., Zhong, L., Lonn, M. E., Burk, R. F., Hill, K. E., Holmgren, A. Penultimate selenocysteine residue replaced by cysteine in thioredoxin reductase from selenium‐deficient rat liver. FASEBJ. 23, 2394–2402 (2009)

Occupational exposure to particles and incidence of acute myocardial infarction and other ischaemic heart disease.

Background Ambient particulate air pollution has been linked to cardiovascular disease. Occupational particle exposure levels may be several times higher than ambient levels but has been less studied. Objectives The authors investigated the association between occupational exposure to particles and the incidence of ischaemic heart disease (IHD). Methods The cohort included all manual workers in the Swedish national census of 1980 with information on demographic data and occupation. Information on hospital admissions for acute myocardial infarction or other IHDs and cause of death were obtained from nation-wide registers. A job-exposure matrix for exposure to small (<1 μm) and large (>1 μm) particles was developed. HRs were calculated with Cox regression with adjustment for sex, age, socioeconomic group and urban/rural residential area. Results Exposure to small particles was associated with an increased HR for acute myocardial infarction of 1.12 (95% CI 1.09 to 1.15), and HR for exposure to large particles was 1.14 (95% CI 1.10 to 1.18). The association was somewhat stronger for workers exposed to small particles for more than 5 years, 1.21 (95% CI 1.11 to 1.31), but no trend with exposure intensity was found. The risk associated with exposure to small particles was higher among women than among men, 1.30 (95% CI 1.12 to 1.51) and 1.10 (95% CI 1.07 to 1.14), respectively. Findings were essentially similar for other IHDs. Conclusions This explorative study gives some support to the hypothesis that occupational exposure to particles increases the risk of acute myocardial infarction and other IHD. The findings must be interpreted cautiously due to lack of smoking data.

Myocardial infarction and occupational exposure to particles from various sources

Objectives Ambient particulate air pollution has been linked to cardiovascular disease. Occupational particle exposure levels are usually several times higher than ambient levels, but the effects have been less studied. We investigated the association between occupational exposure to particles from various sources and the incidence of myocardial infarction. Methods A cohort was created including all workers in the Swedish National Census in 1980, with information on occupation from censuses in 1980, 1985 and 1990. Information on hospital admissions or death from acute myocardial infarction was obtained from nation-wide registers. Information on exposure was obtained from a job-exposure matrix (JEM) including particulate matter of twenty different agents. The cohort was limited to workers in order to reduce confounding from tobacco smoking. Results Exposure to quartz dust, ferrous dust, Benzo(A)pyrene, paper dust and oil mist was associated with a significantly increased risk ratio (RR) for acute myocardial infarction. The highest RR was noted for exposure to Benzo(A)pyrene, RR 1.32 (95% CI 1.20 to 1.45) and oil mist, RR 1.32 (1.23 to 1.42). Women had a consistently higher RR of acute myocardial infarction than men. The highest RR for women occurred in occupations exposed to Benzo(A)pyrene, RR 2.09 (1.56 to 2.81). Conclusions Occupational exposure to several particulate agents was associated with an increased risk of acute myocardial infarction, with consistently higher relative risks in women than in men. No individual data on tobacco smoking were available, but the potential bias from this was reduced by limitation to workers.