Reactive oxygen species (ROS) are known mediators of intracellular signal cascades. Excessive production of ROS may lead to oxidative stress, loss of cell function, and cell death by apoptosis or ...necrosis. Lipid hydroperoxides are one type of ROS whose biological function has not yet been clarified. Phospholipid hydroperoxide glutathione peroxidase (PHGPx, GPx4) is a unique antioxidant enzyme that can directly reduce phospholipid hydroperoxide in mammalian cells. This contrasts with most antioxidant enzymes, which cannot reduce intracellular phospholipid hydroperoxides directly. In this review, we focus on the structure and biological functions of PHGPx in mammalian cells. Recently, molecular techniques have allowed overexpression of PHGPx in mammalian cell lines, from which it has become clear that lipid hydroperoxides also have an important function as activators of lipoxygenase and cyclooxygenase, participate in inflammation, and act as signal molecules for apoptotic cell death and receptor-mediated signal transduction at the cellular level.
Oxidative stress is implicated in the pathologies of photoreceptor cells, and the protective role of antioxidant enzymes for photoreceptor cells have been well understood. However, their essentiality ...has remained unknown. In this study we generated photoreceptor-specific conditional knock-out (CKO) mice of glutathione peroxidase 4 (GPx4) and showed the critical role of GPx4 for photoreceptor cells. In the wild-type retina the dominant GPx4 expression was in the mitochondria, indicating the mitochondrial variant was the major GPx4 in the retina. In the GPx4-CKO mice, although photoreceptor cells developed and differentiated into rod and cone cells by P12, they rapidly underwent drastic degeneration and completely disappeared by P21. The photoreceptor cell death in the GPx4-CKO mice was associated with the nuclear translocation of apoptosis-inducing factor (AIF) and TUNEL-positive cells. Photoreceptor cells before undergoing apoptosis (P11) exhibited decreased mitochondrial biomass, decreased number of connecting cilia, as well as disorganized structure of outer segments. These findings indicate that GPx4 is a critical antioxidant enzyme for the maturation and survival of photoreceptor cells.
The essentiality of an antioxidant enzyme for the development and maturation of photoreceptor cells has remained unclear.
While GPx4-abrogated photoreceptor cells develop and differentiate into rod and cone cells, their outer segments are structurally disorganized and they undergo rapid apoptosis in vivo.
GPx4 is essential for the maturation of photoreceptor cells.
A novel role of an antioxidant enzyme for photoreceptor cells is disclosed in this study.
The glutathione peroxidase (GPx) family is a major antioxidant enzyme family that catalyzes the reduction of a variety of hydroperoxides. GPxs are divided into selenium‐ and nonselenium‐containing ...GPxs. Because of their efficient antioxidant activity, which depends on the presence of the amino acid residue selenocysteine, selenium‐containing GPxs have been the subject of many studies. However, the physiological roles of the nonselenium GPxs remain unclear. Here, we report that the deletion of phospholipid hydroperoxide glutathione peroxidase (PHGPx) homologues causes accelerated aging that leads to a shortened lifespan in Caenorhabditis elegans. PHGPx is an antioxidant enzyme that directly reduces the phospholipid hydroperoxides generated in biomembranes. The quadruple phgpx mutant gpx‐1; gpx‐2; gpx‐6; gpx‐7 developed normally, reached adulthood and reproduced as well as the wild type. However, a lifespan analysis showed that the quadruple phgpx mutant had a short maximum lifespan, with an age‐related increase in its mortality rate. The intestine is the primary tissue expressing gpx‐1, gpx‐2, gpx‐6 and gpx‐7 in C. elegans, and the expression of gpx‐6 is greatly enhanced under starvation conditions. These results suggest that the C. elegans PHGPx homologues have important functions in the regulation of aging, probably by reducing oxidative damage in the intestine.
Phospholipid hydroperoxide glutathione peroxidase (GPx4) is an intracellular antioxidant enzyme that directly reduces peroxidized phospholipids. GPx4 is strongly expressed in the mitochondria of ...testis and spermatozoa. We previously found a significant decrease in the expression of GPx4 in spermatozoa from 30% of infertile human males diagnosed with oligoasthenozoospermia (Imai, H., Suzuki, K., Ishizaka, K., Ichinose, S., Oshima, H., Okayasu, I., Emoto, K., Umeda, M., and Nakagawa, Y. (2001) Biol. Reprod. 64, 674–683). To clarify whether defective GPx4 in spermatocytes causes male infertility, we established spermatocyte-specific GPx4 knock-out mice using a Cre-loxP system. All the spermatocyte-specific GPx4 knock-out male mice were found to be infertile despite normal plug formation after mating and displayed a significant decrease in the number of spermatozoa. Isolated epididymal GPx4-null spermatozoa could not fertilize oocytes in vitro. These spermatozoa showed significant reductions of forward motility and the mitochondrial membrane potential. These impairments were accompanied by the structural abnormality, such as a hairpin-like flagella bend at the midpiece and swelling of mitochondria in the spermatozoa. These results demonstrate that the depletion of GPx4 in spermatocytes causes severe abnormalities in spermatozoa. This may be one of the causes of male infertility in mice and humans.
: Overexpression of phospholipid hydroperoxide glutathione peroxidase (PHGPx) in mitochondria of RBL2H3 cells (M15 cells) prevented the release of cytochrome c (cyt.c), the activation of caspase‐3, ...and apoptosis caused by 2‐deoxyglucose (2DG), whereas cells overexpressing nonmitochondrial PHGPx(L9) and control (S1) cells were induced to apoptosis. Hydro‐peroxide levels in mitochondria of L9 and S1 cells were significantly enhanced by 2DG‐induced apoptosis. In contrast, generation of hydroperoxide in mitochondria was protected in M15 cells, which also showed resistance to apoptosis by etoposide, staurosporine, UV irradiation, cycloheximide, and actinomycin D, stimuli that induce apoptosis by the liberation of cyt.c from mitochondria. Cyt.c preferentially binds to the monolayer of cardiolipin (CL), the specific phospholipid of the inner membrane of mitochondria. The amount of cyt.c bound to the monolayer of cardiolipin hydroperoxide (CL‐OOH) was much lower than that bound to CL. Cyt.c bound to liposome containing CL was released by peroxidation with a radical initiator. Adenine nucleotide translocator (ANT), which regulates the opening and closing the permeability transition (PT) pore, potentially was inactivated in apoptosis‐induced S1 cells 4 h after the addition of 2DG, coincidentally with cyt.c release from mitochondria. ANT activity was suppressed by the fusion of isolated mitochondria with liposomes containing CL‐OOH. ANT activity was expressed in proteoliposomes containing 10% CL, but it was competitively inhibited by the addition of CL‐OOH. This study suggests that CL peroxidation might have an initiating role in the liberation of cyt.c from the inner membrane, and in the opening of the PT pore via inactivation of ANT. Mitochondrial PHGPx might play a role as an anti‐apoptotic factor by protecting CL and reducing CL‐OOH.
Cardiolipin (CL) is a major membrane phospholipid specifically localized in mitochondria. At the cellular level, CL has been shown to have a role in mitochondrial energy production, mitochondrial ...membrane dynamics, and the triggering of apoptosis. However, the in vivo role of CL in multicellular organisms is largely unknown. In this study, by analyzing deletion mutants of a CL synthase gene (crls-1) in Caenorhabditis elegans, we demonstrated that CL depletion selectively caused abnormal mitochondrial function and morphology in germ cells but not in somatic cell types such as muscle cells. crls-1 mutants reached adulthood but were sterile with reduced germ cell proliferation and impaired oogenesis. In the gonad of crls-1 mutants, mitochondrial membrane potential was significantly decreased, and the structure of the mitochondrial cristae was disrupted. Contrary to the abnormalities in the gonad, somatic tissues in crls-1 mutants appeared normal with respect to cell proliferation, mitochondrial function, and mitochondrial morphology. Increased susceptibility to CL depletion in germ cells was also observed in mutants of phosphatidylglycerophosphate synthase, an enzyme responsible for producing phosphatidylglycerol, a precursor phospholipid of CL. We propose that the contribution of CL to mitochondrial function and morphology is different among the cell types in C. elegans.
Background: Cardiolipin is required for maintaining optimal mitochondrial function.
Results: Cardiolipin depletion selectively obstructed proliferation, mitochondrial function, and morphology in germ cells.
Conclusion: The contribution of cardiolipin to mitochondrial function and morphology varies among the different cell types in vivo.
Significance: This provides a biological basis for understanding the different sensitivities of organelles to changes in the lipid environment.
Choline kinase is the first step enzyme for phosphatidylcholine (PC) de novo biosynthesis. Loss of choline kinase activity in muscle causes rostrocaudal muscular dystrophy (rmd) in mouse and ...congenital muscular dystrophy in human, characterized by distinct mitochondrial morphological abnormalities. We performed biochemical and pathological analyses on skeletal muscle mitochondria from rmd mice. No mitochondria were found in the center of muscle fibers, while those located at the periphery of the fibers were significantly enlarged. Muscle mitochondria in rmd mice exhibited significantly decreased PC levels, impaired respiratory chain enzyme activities, decreased mitochondrial ATP synthesis, decreased coenzyme Q and increased superoxide production. Electron microscopy showed the selective autophagic elimination of mitochondria in rmd muscle. Molecular markers of mitophagy, including Parkin, PINK1, LC3, polyubiquitin and p62, were localized to mitochondria of rmd muscle. Quantitative analysis shows that the number of mitochondria in muscle fibers and mitochondrial DNA copy number were decreased. We demonstrated that the genetic defect in choline kinase in muscle results in mitochondrial dysfunction and subsequent mitochondrial loss through enhanced activation of mitophagy. These findings provide a first evidence for a pathomechanistic link between de novo PC biosynthesis and mitochondrial abnormality.
Phospholipid hydroperoxide glutathione peroxidase (PHGPx) is the only known intracellular antioxidant enzyme that can directly reduce lipid hydroperoxide in membrane. Mitochondrial and ...non-mitochondrial PHGPx and sperm nuclei GPx are transcribed from one gene by alternative transcription using different first exons Ia and Ib, respectively. To examine the role of PHGPx in development, we generated mice deficient in PHGPx by a targeted disruption of all exons of the PHGPx gene. Heterozygotes are viable, fertile, and appear normal, despite having decreased levels of three types of PHGPx mRNA and protein. Embryos homozygous for PHGPx-null die between 7.5 and 8.5 days post coitum (dpc), probably developing distal apoptosis. We examined the expression of PHGPx in mouse embryos using immunohistochemical analysis with anti-PHGPx mAb. The expression of PHGPx was detected in the embryonic ectoderm and the yolk sac membrane at 7.5
dpc. The results demonstrated that PHGPx is expressed in early gastrulation stage at 7.5
dpc and that the expression of PHGPx was essential for normal mouse development.
Cytochrome c (cyt. c) is a proapoptotic factor that binds preferentially to cardiolipin (CL), a mitochondrial lipid, but not to cardiolipin hydroperoxide (CL-OOH). Cyt. c that had bound to CL ...liposomes was liberated on peroxidation of the liposomes by a radical. The generation of CL-OOH in mitochondria occurred before the release of cyt. c in rat basophile leukaemia (RBL)2H3 cells that had been induced to undergo apoptosis by exposure to hypoglycaemia with 2-deoxyglucose (2DG). The amount of cyt. c bound to CL prepared from the mitochondria of 2DG-treated cells was lower than that of untreated cells. The release of cyt. c was completely suppressed when the production of CL-OOH in mitochondria was inhibited by the overexpression of mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx). The fluorescence from CL-labelling dye (10-N-nonyl Acridine Orange) decreased on the induction of apoptosis by 2DG. However, no decrease in fluorescence was observed in PHGPx-overexpressing cells. Cyt. c was released from mitochondria that had been isolated from control cells on peroxidation by t-butylhydroperoxide, but no similar liberation of cyt. c from mitochondria isolated from mitochondrial PHGPx-overexpressing cells was observed. These findings suggest that the generation of CL-OOH in mitochondria might be a primary event that triggers the release of cyt. c from mitochondria in the apoptotic process in which mitochondrial PHGPx participates as an anti-apoptotic factor by preventing the formation of CL-OOH.
Acidic sphingomyelinase (ASMase) catalyses the generation of ceramide from sphingomyelin. Ceramide is a lipid mediator and is implicated in mediating and regulating various cellular processes ...including cell proliferation, differentiation, stress response and inflammation. We have previously reported that electrophiles including diethyl maleate (DEM), heavy metals and cigarette smoke extracts induced ASMase expression in human bladder carcinoma ECV-304 cells, but the mechanism of ASMase mRNA induction by electrophiles remains unknown. In this study, we clarified the involvement of NF-E2-related factor 2 (Nrf2) in the induction of ASMase mRNA by DEM. Promoter analysis using a series of deletion mutants of the human ASMase gene showed that ARE-like element1 located in a region between -200 and -160 bp upstream of the transcription start point is mainly a DEM-responsive element. Moreover, an electrophoretic mobility shift assay using ARE-like element1 revealed that Nrf2 is a candidate transcription factor that binds to ARE-like element1 in response to DEM. Finally, alteration of Nrf2 expression by overexpression and knockdown could regulate the induction of ASMase mRNA by DEM. This is the first evidence that supports the possibility that sphingolipid metabolism is affected via the induction of ASMase by the Nrf2 pathway.
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