L-PhGPx expression can be suppressed by antisense oligodeoxynucleotides

The phospholipase A2 activity of peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a Ca²⁺-independent intracellular phospholipase A₂ (called aiPLA₂) that is localized to cytosol, lysosomes, and lysosomal-related organelles. Activity is minimal at cytosolic pH but is increased significantly with enzyme phosphorylation, at acidic pH, and in the presence of oxidized phospholipid substrate; maximal activity with phosphorylated aiPLA₂ is ∼2 µmol/min/mg protein. Prdx6 is a "moonlighting" protein that also expresses glutathione peroxidase and lysophosphatidylcholine acyl transferase activities. The catalytic site for aiPLA₂ activity is an S32-H26-D140 triad; S32-H26 is also the phospholipid binding site. Activity is inhibited by a serine "protease" inhibitor (diethyl p-nitrophenyl phosphate), an analog of the PLA₂ transition state [1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol (MJ33)], and by two naturally occurring proteins (surfactant protein A and p67^phox), but not by bromoenol lactone. aiPLA₂ activity has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids, in the repair of peroxidized cell membranes, and in the activation of NADPH oxidase type 2 (NOX2). The enzyme has been implicated in acute lung injury, carcinogenesis, neurodegenerative diseases, diabetes, male infertility, and sundry other conditions, although its specific roles have not been well defined. Protein mutations and animal models are now available to further investigate the roles of Prdx6-aiPLA₂ activity in normal and pathological physiology.

The roles of peroxidase and phospholipase A2 activities of peroxiredoxin 6 in protecting pulmonary microvascular endothelial cells against peroxidative stress

AIMS

Peroxiredoxin 6 (Prdx6), a bifunctional enzyme with glutathione peroxidase and phospholipase A(2) (PLA(2)) activities, has been demonstrated as playing a critical role in antioxidant defense of the lung. Our aim was to evaluate the relative role of each activity in Prdx6-mediated protection of mouse pulmonary microvascular endothelial cells (PMVECs) against the peroxidative stress of treatment with tert-butyl hydroperoxide (tBOOH).

RESULTS

PMVEC from Prdx6 null mice showed increased lethality on tBOOH exposure (50-200 μM) compared with wild-type (WT) controls. Treatment with 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol (MJ33), a Prdx6 PLA(2) activity inhibitor, increased the sensitivity of WT cells to peroxidative stress, but did not further sensitize Prdx6 null cells. Lethality in Prdx6 null PMVEC was "rescued" by transfection with a construct leading to the expression of WT rat Prdx6. Expression of mutant Prdx6 with either peroxidase activity or PLA(2) activity alone each partially rescued the survival of Prdx6 null cells, while constructs with both active sites mutated failed to rescue. Co-transfection with two different constructs, each expressing one activity, rescued cells as well as the WT construct.

INNOVATION AND CONCLUSION

Contrary to the general assumption that the peroxidase activity is the main mechanism for Prdx6 antioxidant function, these results indicate that the PLA(2) activity also plays a substantial role in protecting cells against oxidant stress caused by an exogenous hydroperoxide.

Peroxiredoxin 6: a bifunctional enzyme with glutathione peroxidase and phospholipase A₂ activities

Peroxiredoxin 6 (Prdx6) is the prototype and the only mammalian 1-Cys member of the Prdx family. Major differences from 2-Cys Prdxs include the use of glutathione (GSH) instead of thioredoxin as the physiological reductant, heterodimerization with πGSH S-transferase as part of the catalytic cycle, and the ability either to reduce the oxidized sn-2 fatty acyl group of phospholipids (peroxidase activity) or to hydrolyze the sn-2 ester (alkyl) bond of phospholipids (phospholipase A(2) [PLA(2)] activity). The bifunctional protein has separate active sites for peroxidase (C47, R132, H39) and PLA(2) (S32, D140, H26) activities. These activities are dependent on binding of the protein to phospholipids at acidic pH and to oxidized phospholipids at cytosolic pH. Prdx6 can be phosphorylated by MAP kinases at T177, which markedly increases its PLA(2) activity and broadens its pH-activity spectrum. Prdx6 is primarily cytosolic but also is targeted to acidic organelles (lysosomes, lamellar bodies) by a specific targeting sequence (amino acids 31-40). Oxidant stress and keratinocyte growth factor are potent regulators of Prdx6 gene expression. Prdx6 has important roles in both antioxidant defense based on its ability to reduce peroxidized membrane phospholipids and in phospholipid homeostasis based on its ability to generate lysophospholipid substrate for the remodeling pathway of phospholipid synthesis.

Comparison of glutathione peroxidase 1 and peroxiredoxin 6 in protection against oxidative stress in the mouse lung

Peroxiredoxin 6 (Prdx6) and cytosolic GSH peroxidase (GPx1), both GSH-dependent peroxidases, were compared for the effects of their knockout on injury and lipid peroxidation in: (a) lungs of mice exposed to 0.85 or 1.0atm O(2), (b) isolated perfused mouse lungs exposed to 5mM tert-butylhydroperoxide (t-BOOH) or 1mM paraquat, and (c) primary mouse pulmonary microvascular endothelial cells exposed to 50muM t-BOOH. Derangements in GPx1 null were similar or slightly greater than in wild type for all parameters in the various models of oxidant stress, whereas Prdx6 null showed markedly increased effects. GSH peroxidase activity with phosphatidylcholine hydroperoxide as substrate in GPx1-null lung homogenate was decreased only slightly vs wild type, whereas activity in Prdx6-null lungs was decreased by ~95%, indicating that Prdx6 is the major enzyme for reduction of oxidized lung phospholipids. Expression levels of oxidant-related genes measured with a PCR-based gene array indicated no significant differences between the Prdx6 and the GPx1 null except for the target genes and IL-19. Thus, Prdx6-null mice are significantly more sensitive to oxidant stress compared to GPx1 null, suggesting that scavenging of phospholipid hydroperoxides by Prdx6 plays a major role in lung antioxidant defense.

Binding of peroxiredoxin 6 to substrate determines differential phospholipid hydroperoxide peroxidase and phospholipase A(2) activities

Peroxiredoxin 6 (Prdx6) differs from other mammalian peroxiredoxins both in its ability to reduce phospholipid hydroperoxides at neutral pH and in having phospholipase A(2) (PLA(2)) activity that is maximal at acidic pH. We previously showed an active site C47 for peroxidase activity and a catalytic triad S32-H26-D140 necessary for binding of phospholipid and PLA(2) activity. This study evaluated binding of reduced and oxidized phospholipid hydroperoxide to Prdx6 at cytosolic pH. Incubation of recombinant Prdx6 with 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine hydroperoxide (PLPCOOH) resulted in peroxidase activity, cys47 oxidation as detected with Prdx6-SO2(3)) antibody, and a marked shift in the Prdx6 melting temperature by circular dichroism analysis indicating that PLPCOOH is a specific substrate for Prdx6. Preferential Prdx6 binding to oxidized liposomes was detected by changes in DNS-PE or bis-Pyr fluorescence and by ultrafiltration. Site-specific mutation of S32 or H26 in Prdx6 abolished binding while D140 mutation had no effect. Treatment of A549 cells with peroxides led to lipid peroxidation and translocation of Prdx6 from the cytosol to the cell membrane. Thus, the pH specificity for the two enzymatic activities of Prdx6 can be explained by the differential binding kinetics of the protein; Prdx6 binds to reduced phospholipid at acidic pH but at cytosolic pH binds only phospholipid that is oxidized compatible with a role for Prdx6 in the repair of peroxidized cell membranes.

Peroxiredoxin 6 as an antioxidant enzyme: protection of lung alveolar epithelial type II cells from H2O2-induced oxidative stress

We evaluated the antioxidant role of peroxiredoxin 6 (Prdx6) in primary lung alveolar epithelial type II cells (AEC II) that were isolated from wild type (WT), Prdx6-/-, or Prdx6 transgenic (Tg) overexpressing mice and exposed to H(2)O(2) at 50-500 microM for 1-24 h. Expression of Prdx6 in Tg AEC II was sevenfold greater than WT. Prdx6 null AEC II exposed to H(2)O(2) showed concentration-dependent cytotoxicity indicated by decreased "live/dead" cell ratio, increased propidium iodide (PI) staining, increased annexin V binding, increased DNA fragmentation by TUNEL assay, and increased lipid peroxidation by diphenylpyrenylphosphine (DPPP) fluorescence. Compared to Prdx6 null cells, oxidant-mediated damage was significantly less in WT AEC II and was least in Prdx6 Tg cells. Thus, Prdx6 functions as an antioxidant enzyme in mouse AEC II. Prdx6 has been shown previously to reduce phospholipid hydroperoxides and we postulate that this activity is a major mechanism for the effectiveness of Prdx6 as an antioxidant enzyme.

Peroxiredoxins in the lung with emphasis on peroxiredoxin VI

All six mammalian peroxiredoxins are expressed in the lung. Peroxiredoxin (Prx) VI is the isoform expressed at the highest level and its lung expression exceeds that for other organs. The predominant location of Prx VI is the cytosol and acidic organelles of Clara cells of the conducting airways and type II epithelial cells and macrophages in the alveoli. Prx I and VI show developmental induction of transcription at birth. PrxVI shares structural homology with other peroxiredoxins exhibiting a thioredoxin fold and a conserved catalytic Cys residue in the N-terminus of the protein. This enzyme is highly inducible by oxidative stress in both the neonatal and adult lung consistent with a role in antioxidant defense. Prx VI has several properties that distinguish its peroxidase activity from other peroxiredoxins: it can reduce phospholipid hydroperoxides in addition to other organic hydroperoxides and H2O2; the electron donor that serves to reduce the oxidized peroxidatic cysteine is not thioredoxin but GSH; instead of homodimerization, heterodimerization with pi-glutathione S-transferase is required for regeneration of the active enzyme. Prx VI also expresses a phospholipase A2 activity that is Ca2+-independent, maximal at acidic pH, and dependent on a serine-based catalytic triad and nucleophilic elbow at the surface of the protein. Models of altered Prx VI expression at the cellular, organ and whole animal levels have demonstrated that Prx VI functions as an important anti-oxidant enzyme with levels of protection that exceed those ascribed to GSH peroxidase (GPx1). The phospholipase A2 activity plays an important role in lung surfactant homeostasis and is responsible for the bulk of the degradation of internalized phosphatidylcholine and its resynthesis by the reacylation pathway. Expression of peroxiredoxins is elevated in several lung diseases including lung cancer, mesothelioma and sarcoidosis, although the mechanism for these alterations is not known. The unique properties of Prx VI enable it to play an important role in lung cell function.

Transgenic mice overexpressing peroxiredoxin 6 show increased resistance to lung injury in hyperoxia

Peroxiredoxin 6 (Prd x 6) is a novel peroxidase enzyme that is expressed at a high level in the lung. We tested the hypothesis that transgenic (Tg) mice overexpressing Prd x 6 would exhibit increased resistance to hyperoxia-induced lung injury. Wild-type and Tg mice were exposed to 100% O(2) and evaluated for survival, lung histopathology, total protein, and nucleated cells in bronchoalveolar lavage fluid (BALF), and oxidation of lung protein and lipids. Prd x 6 protein expression and enzyme activity were approximately 3-fold higher in Tg lungs compared with wild-type. Tg mice survived longer during exposure to 100% O(2) (LT(50) 104+/-2.8 h in Tg versus 88.9+/-1.1 h for wild-type). Lung wet/dry weight ratio and total protein and nucleated cell count in lung lavage fluid were significantly greater in wild-type mice at 72 and 96 h of hyperoxia compared with Tg mice. At 96 h of hyperoxia, Tg mice had less epithelial cell necrosis, perivascular edema, and inflammatory cell recruitment by light microscopy, and lower TBARS and protein carbonyls in lung homogenate (P<0.05). These results show that Tg mice have increased defense against lung injury in hyperoxia, providing evidence that Prd x 6 functions as a lung antioxidant enzyme.

Photoreceptor preservation in the S334ter model of retinitis pigmentosa by a novel estradiol analog

The cytoprotective activity of MITO-4565, a novel, non-hormonal, estradiol derivative, was evaluated in the S334ter transgenic model of retinitis pigmentosa (RP). Progressive blindness in RP is due to apoptotic death of the photoreceptors, a process mimicked by the animal models [Portera-Cailliau C, Sung C-H, Nathans J, Adler R. Apoptotic photoreceptor cell death in mouse models of retinitis pigmentosa. Proc Natl Acad Sci USA 1994;91:974-8]. On postnatal day 9, 10 transgenic S334ter rats received a single intraocular injection of MITO-4565 in the left eye, and vehicle in the right eye. By postnatal day 20, the thickness of the outer nuclear layer (ONL) in the superior retina of the untreated eyes was 5.76 +/- 1.12 microm (N = 10), versus 10.72 +/- 1.52 microm (N = 10) for eyes treated with MITO-4565 (P < 0.0001, ANOVA F = 1671). Comparable cytoprotection was also observed for the inferior retina. Cytoprotection by MITO-4565 was also observed in primary cultures of rat retinal ganglion cells against NMDA excitotoxicity. Data from studies of hexose monophosphate shunt flux, mitochondrial stability, and in vitro lipid peroxidation, are in accord with previous reports [Green PS, Gridley KE, Simpkins JW. Nuclear estrogen receptor independent neuroprotection by estratrienes: a novel interaction with glutathione. Neuroscience 1997;84:7-10]; a likely mechanism of action entails moderation of membrane lipid peroxidation in a redox couple with glutathione. Such preservation of membrane integrity is particularly crucial to mitochondria, where collapse of membrane potential precipitates cell death, and where GSH is maintained at mM concentrations. Indeed, exposure to MITO-4565, but not a methoxy substituted negative control, allowed mitochondria to retain membrane potential (DeltaPsi(m)) under conditions of Ca(2+) overload that would normally induce complete mitochondrial failure. Mitochondrial interventions offer a novel therapeutic approach for RP, and other degenerative diseases of the retina.

Lung injury and mortality with hyperoxia are increased in peroxiredoxin 6 gene-targeted mice

Overexpression of peroxiredoxin 6 (Prdx6) has been shown to protect lungs of mice against hyperoxia-mediated injury. In this study, we evaluated whether genetic inactivation of Prdx6 in mice increases sensitivity to oxygen toxicity. We evaluated mouse survival, lung histopathology, total protein and nucleated cells in bronchoalveolar lavage fluid (BALF), and oxidation of lung protein and lipids by measurement of protein carbonyls and thiobarbituric reactive substances (TBARS), respectively. The duration of survival for Prdx6 -/- mice was significantly shorter than that observed in wild-type mice on exposure to 85 or 100% O(2); survival of Prdx6 +/- mice was intermediate. After 72-h exposure to 100% O(2), lungs of Prdx6-/- mice showed more severe injury than wild-type with increased wet/dry weight, epithelial cell necrosis and alveolar edema on microscopic examination, increased protein and nucleated cells in BALF, and higher content of TBARS and protein carbonyls in lung homogenate. These findings show that Prdx6 -/- mice have increased sensitivity to hyperoxia and provide in vivo evidence that Prdx6 is an important lung antioxidant enzyme.