Tour-de-Force

More documents

Recommendations

Info

Tour-de-Force: Interplay between Mitochondria and Cell Cycle Progression Fall 2007However, in this research the focus will lie on the balance between ROS and antioxidants. Therefore, inthis proposal, the definition of the redox cycle is the cyclic fluctuation in the “balance between the levels ofreactive oxygen species (ROS) produced during metabolism and the antioxidant system that scavengesthem” (Menon and Goswami, 2007).ROS are reactive radicals and hydrogen peroxide.Hydrogen peroxide itself is not a radical, but when reactingwith a metal it will form the very reactive and destructivehydroxyl radical. ROS are produced as a side effect ofseveral metabolic pathways. They can extract an electronfrom other compounds, destabilizing them and makingthem inactive or incapable of performing their normalfunction. (Foster et al., 2006, citing Dröge et al., 2002 andBoveris and Chance, 1973) There are several types ofROS, which are described in Table 1.1. However, as thisresearch concerns mitochondrial ROS production, it willfocus on the two ROS produced by mitochondria,hydrogen peroxide and superoxide.Subtypes of ROSStructureHydrogen peroxide H 2 O 2Hydroxyl radical HO .Hypochlorous acidHOCLNitric oxideNOPeroxyl radical ROO .Peroxynitrite anionSuperoxide anionSinglet oxigenTable 1.1: Different types of ROSONOO-. O 2 -1 O 2A redox cycle in mammalian cellsROS levels in different phases of the cell cycle have been measured. Havens and colleagues (2006)found that ROS levels were lowest in G1, increased during S and were highest in G2/M. However, Conourand colleagues (2004) measured ROS levels throughout the cell cycle and did not find significantdifferences between phases. They also measured the balance between the reduced and oxidized form ofglutathione (GSH/GSSG balance). Glutathione (GSH) is a tripeptide, consisting of cysteine, glycine andglutamic acid. It is an important player in the antioxidant system, because it is involved in a reductionreaction with hydrogen peroxide: one hydrogen peroxide react with two GSH molecules to yield two watermolecules and two oxidized glutathiones (GSSGs). This reaction is catalyzed by glutathione peroxidase(GPx). GSSG can be converted back into GSH by glutathione reductase (GR). The ratio betweenGSH/GSSG is an important and often used indicator of the oxidative/reductive state of a cell (Schafer andBuettner, 2001; Conour et al., 2004). According to Conour and colleagues (2004), reduced glutathione(GSH) levels were higher in the G2/M phase compared to the G1 phase, and cells in the S phase showedintermediate GSH levels. This suggests that in the progression from G1 to S to G2/M, less oxidants areproduced, and that therefore the state of the GSH/GSSG buffer shifts to the reduced side. Thiscontradiction needs to be resolved before final conclusions can be drawn.Various producers of ROSAn interesting question is to what extent this redox cycle is linked to mitochondrial function. If ROS levelsdirectly correlate to mitochondrial activity, these observations strongly suggest that mitochondrial activityfluctuates throughout the cell cycle. However, even though mitochondria are the main source of ROS,there are also other contributors, including the NADPH oxidase complex, peroxisomes (through fatty acidmetabolism), cytochrome P450 reductase, xanthine oxidase and myeloperoxidase (Menon and Goswami,2007). Furthermore, a change in redox state might also be regulated independently of ROS production, forexample by a change in the pro-oxidant/anti-oxidant balance.Not all of these are relevant for this research, mainly because some ROS producers are very celltypespecific. Myeloperoxidase, for example, is only present in leukocytes (Daugherty et al., 1994).Xanthine oxidase is only present intracellularly in the endothelial cells of the capillaries in the mammarygland, liver, intestine, heart and lung. (Adachi et al., 1993). For other cell types it is only localizedextracellularly. For superoxide and hydrogen peroxide production within the cell there are just threepossible generators: peroxisomes (Schrader and Fahimi, 2004), mitochondria (Boonstra and Post, 2004)and NADPH oxidase (Dröge, 2002). Furthermore, a change in antioxidant levels or activity can alsoinfluence cellular ROS levels. An overview of the various contributors to ROS levels can be seen in Figure1.1.SCI 332 Advanced Molecular Cell Biology Research Proposal 12
Tour-de-Force: Interplay between Mitochondria and Cell Cycle Progression Fall 2007Figure 1.1 Different producers of cellular ROS.ROS production by mitochondriaThe main product of oxidative phospohorylation by mitochondria is ATP. Normally with the help of theelectron transport chain a membrane potential is generated, which enables the ATPase to convert ADPinto ATP. However electrons might leak out of the transport chain occasionally to react with molecularoxygen in respiratory complex I or III. This will result in the generation of superoxide. Then at least part ofthe superoxide is converted within the mitochondria into the less reactive hydrogen peroxide, H 2 O 2, by themanganese superoxide dismutase, SOD2, which is present in mitochondria. Superoxide and hydrogenperoxide both diffuse out of the mitochondria (Boonstra and Post, 2004).ROS production by NADPH oxidaseNADPH oxidase exists in two isomers, one present in normal cells and another in neutrophils andmacrophages. The second isomer, called phagocytic NADPH oxidase, contributes to massive superoxiderelease during the respiratory burst (Dröge, 2002), whereas the first isomer is mainly involved in oxidizingNADPH. Interestingly, some reports suggest that growth factors might activate NADPH oxidase, providinga possible link between cell cycle progression and ROS production (Burch and Heintz, 2005).ROS production by peroxisomesIn peroxisomes, hydrogen peroxide is produced as a side effect of the removal of electrons from variousmetabolites (De Duve and Baudhuin, 1966). The main hydrogen peroxide producing processes within theperoxisomes are the β-oxidation of fatty acids, the enzymatic reactions of flavin oxydases, and thedisproportionation of superoxides. As a defense mechanism, the peroxisomes also contain a repertoire ofdifferent antioxidants. These antioxidants help in retaining the balance between ROS production andscavenging them by peroxisomes.Effect of antioxidants on ROS levelsWithin the cell several antioxidants are produced to prevent the damaging effect of high levels of ROS(Chen et al., 2007). The most influential and important antioxidants are shown in Table 1.2 together withthe corresponding ROS they scavenge. Superoxide dismutases (SOD) convert the very reactivesuperoxide into the less reactive hydrogen peroxide. Furthermore, catalase degrades hydrogen peroxideinto water and oxygen. Hydrogen peroxide itself is not that reactive, but by reacting with a metal it can beconverted into the reactive and toxic hydroxyl radicals. Another very important antioxidant is glutathione(GSH), which can act as a buffer by being oxidized by ROS to GSSG. Glutathione reductase plays a verySCI 332 Advanced Molecular Cell Biology Research Proposal 13
Page 1 and 2: Tour-de-ForceInterplay between mito
Page 3 and 4: Tour-de-Force: Interplay between Mi
Page 11: Tour-de-Force: Interplay between Mi
Page 63 and 64:
Tour-de-Force: Interplay between Mi
Page 65 and 66:
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
show all

Tour-de-Force

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?