bluuebunny/crossref_metadata_2025 huggingface dataset:
Upper mantle seismic structure beneath the Ethiopian hot spot: Rifting at the edge of the African low‐velocity anomaly
Authors: Bastow I. D. , Nyblade A. A. , Stuart G. W. , Rooney T. O. , Benoit M. H.
Published: December 2008

The Miocene‐Recent East African Rift in Ethiopia subaerially exposes the transitional stage of rifting within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hot spot tectonism. We combine teleseismic traveltime data from 108 seismic stations deployed during two spatially and temporally overlapping broadband networks to present detailed tomographic images of upper mantle P and S wave seismic velocity structure beneath Ethiopia. Tomographic images reveal a ∼500 km wide low P and S wave velocity zone at 75 to ≥400 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift (MER) westward beneath the uplifted and flood basalt‐capped NW Plateau. We interpret this broad low‐velocity region (LVR) as the upper mantle continuation of the African Superplume. Within the broad LVR, zones of particularly low velocity are observed with absolute delay times (δtP ∼ 4 s) that indicate the mantle beneath this region is amongst the slowest worldwide. We interpret these low velocities as evidence for partial melt beneath the MER and adjacent NW Plateau. Surprisingly, the lowest‐velocity region is not beneath Afar but beneath the central part of the study area at ∼9°N, 39°E. Whether this intense low‐velocity zone is the result of focused mantle upwelling and/or enhanced decompressional melting at this latitude is unclear. The MER is located toward the eastern edge of the broad low‐velocity structure, not above its center. This observation, along with strong correlations between low‐velocity zones and lithospheric structures, suggests that preexisting structural trends and Miocene‐to‐Recent rift tectonics play an important role in melt migration at the base of the lithosphere in this magmatic rift zone.
Shallow magma pathway geometry at Mt. Etna volcano
Authors: Patanè D. , Di Grazia G. , Cannata A. , Montalto P. , Boschi E.
Published: December 2008

A fundamental goal of volcano seismology is to understand the dynamics of active magmatic systems in order to assess eruptive behavior and the associated hazard. Imaging of magma conduits, quantification of magma transport, and investigation of long‐period seismic sources, together with their temporal variations, are crucial for the comprehension of eruption‐triggering mechanisms. At Mt. Etna volcano, several intense episodes of tremor activity were recorded during 2007, in association with strombolian activity and/or intense fire fountaining episodes occurring from the South East Crater (SEC). The locations of the tremor sources and of the long‐period seismic events are used here to constrain both the area and the depth range of magma degassing, highlighting the geometry of the shallow conduits feeding SEC. The imaged conduits consist of two connected resonating dike‐like bodies, NNW‐SSE and NW–SE oriented, extending from sea level to the surface. In addition, we show how tremor, long‐period (LP), and very‐long‐period (VLP) event locations and signatures reflect pressure fluctuations in the plumbing system associated with the ascent/discharge of gas‐rich magma linked to the lava fountains. The evidence here reported, also corroborated by ground deformation variations, can help develop a better prediction and early warning system for those eruptions (effusive or explosive) that apparently manifest no clear precursors.
Intermediate water ventilation change in the subarctic northwest Pacific during the last deglaciation
Authors: Sagawa Takuya , Ikehara Ken
Published: December 2008

The laminated sediments suggest significant depletion in dissolved oxygen of intermediate water in the North Pacific during the last deglaciation. However, the cause of depleted oxygen during deglaciation is still a matter of debate. Here, we present foraminiferal Mg/Ca‐based intermediate and surface temperatures and δ18O of seawater (δ18OW; a proxy of salinity) in the Oyashio region in order to investigate intermediate water circulation through the last deglaciation. Results indicate that the intermediate water temperature and δ18OW increased in association with ventilation age at the Bølling‐Allerød period, suggesting that the poor ventilation at that time. The surface record shows that the decrease of salinity preceded the slowdown of intermediate ventilation. The coupled variation of intermediate and surface water suggests that the supply of cold, fresh intermediate water diminished because of poor ventilation at the high latitude due to surface freshening.
Assessing the performance of 20–25 m footprint waveform lidar data collected in ICESat data corridors in Greenland
Authors: Hofton M. A. , Blair J. B. , Luthcke S. B. , Rabine D. L.
Published: December 2008

The precision and accuracy of surface elevation measurements of the Greenland ice sheet provided by 20–25 m‐footprint waveform laser altimetry is assessed using data collected by NASA's LVIS airborne lidar in September 2007. Repeated data acquisitions from two ∼850 km long transects over the ice sheet and a ∼30 km‐long transect in the Summit area are evaluated for consistency and compared to in situ data for validation. Comparisons to data collected by NASA's ICESat system reveal good agreement. In the Summit area, differences are better than 0.07 m (1σ) (except for L3C where it is 0.13 m), with mean offsets of −0.09 to 0.16 m depending on the campaign. Within the feeder zone to Jakobshavn Glacier, ice elevation decreases of up to 12 m, 2004–2008, are observed. The results suggest that 25 m waveform lidar can measure ice sheet elevation precisely and accurately and that the measurement scheme could be used to complement the legacy ICESat measurements.
Solar Mass Ejection Imager 3‐D reconstruction of the 27–28 May 2003 coronal mass ejection sequence
Authors: Jackson B. V. , Bisi M. M. , Hick P. P. , Buffington A. , Clover J. M. , Sun W.
Published: December 2008

The Solar Mass Ejection Imager (SMEI) has recorded the inner‐heliospheric response in white‐light Thomson scattering for many hundreds of interplanetary coronal mass ejections (ICMEs). Some of these have been observed by the Solar and Heliospheric Observatory (SOHO) Large‐Angle Spectroscopic Coronagraph (LASCO) instruments and also in situ by near‐Earth spacecraft. This article presents a low‐resolution three‐dimensional (3‐D) reconstruction of the 27–28 May 2003 halo CME event sequence observed by LASCO and later using SMEI observations; this sequence was also observed by all in situ monitors near Earth. The reconstruction derives its perspective views from outward flowing solar wind. Analysis results reveal the shape, extent, and mass of this ICME sequence as it reaches the vicinity of Earth. The extended shape has considerable detail that is compared with LASCO images and masses for this event. The 3‐D reconstructed density, derived from the remote‐sensed Thomson scattered brightness, is also compared with the Advanced Composition Explorer (ACE) and Wind spacecraft in situ plasma measurements. These agree well in peak and integrated total value for this ICME event sequence when an appropriately enhanced (∼20%) electron number density is assumed to account for elements heavier than hydrogen in the ionized plasma.
Thermal and structural models of the Sumatra subduction zone: Implications for the megathrust seismogenic zone
Authors: Hippchen S. , Hyndman R. D.
Published: December 2008

The 2004 Sumatra‐Andaman and 2005 Nias events provide unique geodetic and seismic data constraining the updip and downdip rupture extents of great thrust earthquakes. These limits are important for tsunami generation and earthquake shaking, respectively. There is a consistent downdip limit of rupture along strike in these great earthquakes at a depth of ∼30 km, and there is an updip limit near the trench that are interpreted to define the seismogenic limits. Temperature and downdip changes in formation compositions are controls proposed for these limits. To examine the thermal control, we developed 2‐D finite element models of the Sumatra subduction zone with smoothly varying subduction dip, variable thermal properties of the rock units, frictional heating along the rupture planes, and appropriate thermal state of the incoming plate. The common updip thermal limit for seismic behavior of 100–150°C occurs close to or at the trench in agreement with the great earthquake rupture limit. Off central Sumatra the common downdip thermal limit range of 350–450°C occurs at 30–60 km depth. The 350°C location is in agreement with the earthquake limits, but 450°C is deeper. North of Sumatra, 350°C occurs ∼14 km deeper than the earthquake rupture limit. The proposed composition control for the downdip limit, the intersection of the subduction thrust with the fore‐arc mantle, is at a depth of ∼30 km, 140–200 km from the trench, in good agreement with the earthquake limits. These results support the conclusion that the Sumatra updip seismogenic limit is thermally controlled but the downdip limit is governed by the intersection of the downgoing plate with the fore‐arc Moho.
Molecular mechanisms regulating hormone-sensitive lipase and lipolysis
Authors: Holm C.
Published: December 2008

HSL (hormone-sensitive lipase) is a key enzyme in the mobilization of fatty acids from acylglycerols in adipocytes as well as non-adipocytes. In adipocytes, catecholamines stimulate lipolysis mainly through PKA (protein kinase A)-mediated phosphorylation of HSL and perilipin, a protein coating the lipid droplet. The anti-lipolytic action of insulin is mediated mainly via lowered cAMP levels, accomplished through activation of phosphodiesterase 3B. Phosphorylation of HSL by PKA occurs at three sites, the serines 563, 659 and 660, both in vitro and in primary rat adipocytes. Phosphorylation of Ser-659 and -660 is required for in vitro activation as well as translocation from the cytosol to the lipid droplet, whereas the role of the third PKA site remains elusive. Adipocytes isolated from homozygous HSL-null mice, generated in our laboratory, exhibit completely blunted catecholamine-induced glycerol release and reduced fatty acid release, suggesting the presence of additional, although not necessarily hormone-activatable, triacylglycerol lipase(s). Basal hyperinsulinaemia, release of exaggerated amounts of insulin during glucose challenges and retarded glucose disposal during insulin tolerance tests suggest that HSL-null mice are insulin resistant. Liver, adipose tissue and skeletal muscle appear all to be sites of impaired insulin sensitivity in HSL-null mice.
Fatty acid recycling in adipocytes: a role for glyceroneogenesis and phosphoenolpyruvate carboxykinase
Authors: Forest C. , Tordjman J. , Glorian M. , Duplus E. , Chauvet G. , Quette J. , Beale E.G. , Antoine B.
Published: December 2008

FA (fatty acid) recycling in adipose tissue appears to be an important pathway for regulating FA release into the blood during fasting. Re-esterification requires G3P (glycerol 3-phosphate), which cannot be synthesized from glucose because glycolysis is much reduced under such circumstances. In addition, G3P can scarcely originate from glycerol since glycerol kinase has a very low activity in white adipose tissue. It was shown about 35 years ago that a metabolic pathway named glyceroneogenesis, which allows G3P synthesis from non-carbohydrate precursors like pyruvate, lactate or amino acids, is activated during fasting. The major enzyme in this pathway was shown to be PEPCK-C [cytosolic phosphoenolpyruvate carboxykinase (GTP); EC 4.1.1.32]. The present review analyses the mechanisms by which a series of hormones and nutrients affect PEPCK-C gene transcription and glyceroneogenesis and describes evidence for dysregulation of this pathway in type 2 diabetes.
Myocardial carnitine palmitoyltransferase I as a target for oxidative modification in inflammation and sepsis
Authors: Eaton S. , Fukumoto K. , Stefanutti G. , Spitz L. , Zammit V.A. , Pierro A.
Published: December 2008

CPT I (outer membrane carnitine palmitoyltransferase I) is a crucial enzyme in myocardial substrate selection. Two isoforms exist in the heart, the liver (L-) and muscle (M-) isoforms, which have different kinetic characteristics and alter in relative amounts during the neonatal/weaning/adult transition. CPT I is a point for control and regulation of fatty acid oxidation via modulation of its activity by malonyl-CoA, the concentration of which is set by acetyl-CoA carboxylase, AMP-activated protein kinase and malonyl-CoA decarboxylase in response to, for example, alterations in glucose supply. Systemic inflammatory responses and sepsis lead to myocardial dysfunction as part of multiple system organ failure. We have shown that: (i) myocardial CPT I activity is inhibited during neonatal sepsis; (ii) on the basis of inhibitor studies this inhibition appears to be of M-CPT I rather than L-CPT I; (iii) nitration of M-CPT I occurs, probably by peroxynitrite, and this may be responsible for the decrease in CPT I activity; (iv) myocardial CPT I activity is also inhibited in another model of systemic inflammatory response, namely intestinal ischaemia/reperfusion injury, but this can prevented by whole-body moderate hypothermia. Inhibition of M-CPT I would be predicted to alter myocardial substrate selection but there are several questions that remain to be answered.
Short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency associated with hyperinsulinism: a novel glucose–fatty acid cycle?
Authors: Eaton S. , Chatziandreou I. , Krywawych S. , Pen S. , Clayton P.T. , Hussain K.
Published: December 2008

Hyperinsulinism of infancy is caused by inappropriate insulin secretion in pancreatic β-cells, even when blood glucose is low. Several molecular defects are known to cause hyperinsulinism of infancy, such as KATP channelopathies and regulatory defects of glucokinase and glutamate dehydrogenase. Although defects of fatty acid oxidation have not previously been known to cause hyperinsulinism, patients with deficiency in SCHAD (short-chain 3-hydroxyacyl-CoA dehydrogenase; an enzyme of mitochondrial β-oxidation) have hyperinsulinism. A novel link between fatty acid oxidation and insulin secretion may explain hyperinsulinism in these patients.
Regulation of pyruvate dehydrogenase complex activity by reversible phosphorylation
Authors: Holness M.J. , Sugden M.C.
Published: December 2008

PDC (pyruvate dehydrogenase complex) catalyses the oxidative decarboxylation of pyruvate, linking glycolysis to the tricarboxylic acid cycle. Regulation of PDC determines and reflects substrate preference and is critical to the ‘glucose–fatty acid cycle’, a concept of reciprocal regulation of lipid and glucose oxidation to maintain glucose homoeostasis developed by Philip Randle. Mammalian PDC activity is inactivated by phosphorylation by the PDKs (pyruvate dehydrogenase kinases). PDK inhibition by pyruvate facilitates PDC activation, favouring glucose oxidation and malonyl-CoA formation: the latter suppresses LCFA (long-chain fatty acid) oxidation. PDK activation by the high mitochondrial acetyl-CoA/CoA and NADH/NAD+ concentration ratios that reflect high rates of LCFA oxidation causes blockade of glucose oxidation. Complementing glucose homoeostasis in health, fuel allostasis, i.e. adaptation to maintain homoeostasis, is an essential component of the response to chronic changes in glycaemia and lipidaemia in insulin resistance. We develop the concept that the PDKs act as tissue homoeostats and suggest that long-term modulation of expression of individual PDKs, particularly PDK4, is an essential component of allostasis to maintain homoeostasis. We also describe the intracellular signals that govern the expression of the various PDK isoforms, including the roles of the peroxisome proliferator-activated receptors and lipids, as effectors within the context of allostasis.
AZD7545 is a selective inhibitor of pyruvate dehydrogenase kinase 2
Authors: Morrell J.A. , Orme J. , Butlin R.J. , Roche T.E. , Mayers R.M. , Kilgour E.
Published: December 2008

The PDH (pyruvate dehydrogenase) multi-enzyme complex catalyses a key regulatory step in oxidative glycolysis. Phosphorylation of the E1 subunit of the complex on serine residues results in the inactivation of enzyme activity. A family of four dedicated PDH kinase isoenzymes exists, each of which displays a distinct tissue-specific expression profile. AZD7545 is one of a series of PDH kinase inhibitors developed for the treatment of type 2 diabetes. The isoenzyme-selectivity profile of AZD7545 and related compounds is described and the consequences for their in vivo mode of action are discussed.
Mechanisms controlling EGF receptor endocytosis and degradation
Authors: Dikic I.
Published: December 2008

Activated EGF (epidermal growth factor) receptors are removed from the cell surface via endocytosis and subsequent degradation in the lysosome. This ultimately attenuates EGF receptor signals and diminishes the level of cell activation. The mechanisms underlying the EGF receptor down-regulation are beginning to be elucidated at the molecular level. Recent reports have indicated that receptor monoubiquitination and networks of protein–protein interactions control distinct steps in EGF receptor internalization, endosomal trafficking and sorting for lysosomal degradation. The emerging importance of the ubiquitin ligase Cbl and the adaptor molecule CIN85 (Cbl-interacting protein of 85 kDa) in the regulation of these pathways is discussed in detail.
Metalloprotease-dependent ErbB ligand shedding in mediating EGFR transactivation and vascular remodelling
Authors: Eguchi S. , Frank G.D. , Mifune M. , Inagami T.
Published: December 2008

AngII (angiotensin II) and its G-protein-coupled AT1 receptor play critical roles in mediating cardiovascular diseases such as hypertension, atherosclerosis and restenosis after vascular injury. It is widely believed that AngII promotes these diseases by inducing vascular remodelling that involves hypertrophy, hyperplasia and migration of VSMCs (vascular smooth muscle cells). We have shown that transactivation of an ErbB family receptor, EGFR (epidermal growth factor receptor; ErbB1), is essential for VSMC hypertrophy and migration induced by AngII. However, the precise signal transduction mechanism by which AngII transactivates EGFR/ErbB1 and whether other ErbBs are also required for AngII function remains unclear. Recent studies suggest an involvement of a metalloprotease-dependent ErbB family ligand production in the transactivation. Here, we will discuss the roles and mechanisms of AngII/AT1 receptor in promoting ErbB receptors transactivation in VSMCs. Further elucidation of this ErbB activation machinery not only will give us a better understanding of the critical molecular mechanism underlying vascular remodelling stimulated by AngII, but will also contribute to development of novel treatment strategies for cardiovascular diseases.
EGFR signal transactivation in cancer cells
Authors: Fischer O.M. , Hart S. , Gschwind A. , Ullrich A.
Published: December 2008

The EGFR (epidermal growth factor receptor) plays a key role in the regulation of essential normal cellular processes and in the pathophysiology of hyperproliferative diseases such as cancer. Recent investigations have demonstrated that GPCRs (G-protein-coupled receptors) are able to utilize the EGFR as a downstream signalling partner in the generation of mitogenic signals. This cross-talk mechanism combines the broad diversity of GPCRs with the signalling capacities of the EGFR and has emerged as a general concept in a multitude of cell types. The molecular mechanisms of EGFR signal transactivation involve processing of transmembrane growth factor precursors by metalloproteases which have been recently identified as members of the ADAM (adisintegrin and metalloprotease) family of zinc-dependent proteases. Subsequently, the EGFR transmits signals to prominent downstream pathways, such as mitogen-activated protein kinases, the phosphoinositide 3-kinase/Akt pathway and modulation of ion channels. Analysis of GPCR-induced EGFR activation in more than 60 human carcinoma cell lines derived from different tissues has demonstrated the broad relevance of this signalling mechanism in cancer. Moreover, EGFR signal transactivation was linked to diverse biological processes in human cancer cells, such as cell proliferation, migration and anti-apoptosis. Together with investigations revealing the importance of this GPCR–EGFR cross-talk mechanism in cardiac hypertrophy, Helicobacter pylori-induced pathophysiological processes and cystic fibrosis, these findings support an important role for GPCR ligand-dependent EGFR signal transactivation in diverse pathophysiological disorders.
Lysophosphatidic acid: mitogen and motility factor
Authors: van Leeuwen F.N. , Giepmans B.N.G. , van Meeteren L.A. , Moolenaar W.H.
Published: December 2008

LPA (lysophosphatidic acid), the simplest of al glycerophospholipids, is a potent inducer of cell proliferation, migration and survival. It does so by activating its cognate G-protein-coupled receptors, four of which have been identified. LPA receptors couple to at least three distinct G-proteins and thereby activate multiple signal transduction pathways, particularly those initiated by the small GTPases Ras, Rho and Rac. Our recent work has shown that LPA signals Rac activation via the Tiam1 GDP/GTP exchange factor and thereby stimulates cell migration. Here we discuss recent progress in our understanding of LPA action.
Stress-induced ATP release from and growth modulation of human lens and retinal pigment epithelial cells
Authors: Eldred J.A. , Sanderson J. , Wormstone M. , Reddan J.R. , Duncan G.
Published: December 2008

ATP release has been shown to occur following stimulation in several cellular systems. This study was undertaken to determine if lens and retinal epithelial cells release ATP in response to physiological stresses and to elucidate a possible role for ATP. Analysis of human aqueous humour samples showed a mean ATP level of 37.8±7.7 nM. Hyper-osmotic stress induced a dose- and time-dependent release of ATP. Both cell types were found to proliferate in serum-free medium, and the addition of ATP and adenosine at concentrations as low as 0.1 nM inhibited growth. Gene profiling also demonstrated the presence of the ectonucleotidases CD39 and CD73 and the A1 adenosine receptor on both cell types.
Receptor tyrosine kinase–GPCR signal complexes
Authors: Pyne N.J. , Waters C. , Moughal N.A. , Sambi B.S. , Pyne S.
Published: December 2008

The formation of complexes between growth factor receptors and members of a family of G-protein-coupled receptors whose natural ligands are S1P (sphingosine 1-phosphate) and LPA (lysophosphatidic acid) represents a new signalling entity. This receptor complex allows for integrated signalling in response to growth factor and/or S1P/LPA and provides a mechanism for more efficient activation (due to integrated close-proximity signalling from both receptor classes) of the p42/p44 MAPK (mitogen-activated protein kinase) pathway. This article provides information on the molecular events at the interface between receptor tyrosine kinases and S1P/LPA receptors. Examples include the PDGF (platelet-derived growth factor)-induced tyrosine phosphorylation of Giα, released upon S1P1 receptor activation, which is required for initiation of the p42/p44 MAPK pathway. Critical to this event is the formation of endocytic vesicles containing functionally active PDGFβ receptor–S1P1 receptor complexes, which are internalized and relocated with components of the p42/p44 MAPK pathway. We also report examples of cross-talk signal integration between the Trk A (tropomyosin receptor kinase A) receptor and the LPA1 receptor in terms of the NGF (nerve growth factor)-dependent regulation of the p42/p44 MAPK pathway. NGF induces recruitment of the LPA1 receptor to the nucleus (delivery might be Trk A-dependent), whereupon the LPA1 receptor may govern gene expression via novel nuclear signalling processes.
Biogenesis of tail-anchored proteins
Authors: Borgese N. , Brambillasca S. , Soffientini P. , Yabal M. , Makarow M.
Published: December 2008

A group of integral membrane proteins, known as C-tail anchored, is defined by the presence of a cytosolic N-terminal domain that is anchored to the phospholipid bilayer by a single segment of hydrophobic amino acids close to the C-terminus. The mode of insertion into membranes of these proteins, many of which play key roles in fundamental intracellular processes, is obligatorily post-translational, is highly specific and may be subject to regulatory processes that modulate the protein's function. Recent work has demonstrated that tail-anchored proteins translocate their C-termini across the endoplasmic reticulum membrane by a mechanism different from that used for Sec61-dependent post-translational signal-peptide-driven translocation. Here we summarize recent results on the insertion of tail-anchored proteins and discuss possible mechanisms that could be involved.
Organization of translocon complexes in ER membranes
Authors: Nikonov A.V. , Kreibich G.
Published: December 2008

Protein translocation in the ER (endoplasmic reticulum) and N-glycosylation are fundamental processes essential for the normal functioning of eukaryotic cells. They are the initial steps in the intracellular pathway that are followed by secretory proteins and membrane proteins of the endomembrane system and the plasma membrane. The translocation and concurrent N-glycosylation of these proteins take place on a large molecular machine, the TC (translocon complex), which is associated with membrane-bound polysomes. Segregation of TCs into a differentiated domain of the ER, the rough ER, may increase the efficiency of protein synthesis on membrane-bound polysomes. Our research is concerned with the assembly, functional organization and dynamics of the TCs in the ER, and their contribution to the functioning and the morphological appearance of this organelle. We hypothesize that the TCs form higher-order structures defining the rough domain of the ER. These structures, which are immobilized or diffuse slowly in the plain of the ER membrane, may be formed and stabilized by mRNAs interconnecting the TCs, by cytoskeletal elements and/or by hypothetical proteins that form links between the TCs. We have established the M3/18 cell line, which expresses the GFP (green fluorescent protein)–Dad1 fusion protein quantitatively and functionally incorporated into the OST (oligosaccharyltransferase). GFP–Dad1 can be used as a reporter molecule for the lateral mobility of the TCs since the OST is tightly associated with the complex. As determined by FRAP (fluorescence recovery after photobleaching), the lateral mobility of GFP–Dad1-tagged TCs was much more restricted than expected from the estimated size of the TC and can be affected by the functional state of the TCs. Currently, we are studying the possible involvement of cytoskeletal elements in the organization of the TCs. Our data suggest that microtubules also play a role in the immobilization of the TCs.
Retrograde transport of toxins across the endoplasmic reticulum membrane
Authors: Lord J.M. , Deeks E. , Marsden C.J. , Moore K. , Pateman C. , Smith D.C. , Spooner R.A. , Watson P. , Roberts L.M.
Published: December 2008

Several protein toxins, including the A chain of the plant protein ricin (RTA), enter mammalian cells by endocytosis and catalytically modify cellular components to disrupt essential cellular processes. In the case of ricin, the process inhibited is protein synthesis. In order to reach their cytosolic substrates, several toxins undergo retrograde transport to the ER (endoplasmic reticulum) before translocating across the ER membrane. To achieve this export, these toxins exploit the ERAD (ER-associated protein degradation) pathway but must escape, at least in part, the normal degradative fate of ERAD substrates in order to intoxicate the cell. Toxins that translocate from the ER have an unusually low lysine content that reduces the likelihood of ubiquitination and ubiquitin-mediated proteasomal degradation. We have changed the two lysyl residues normally present in RTA to arginyl residues. Their replacement in RTA did not have a significant stabilizing effect on the protein, suggesting that the endogenous lysyl residues are not sites for ubiquitin attachment. However, when four additional lysyl residues were introduced into RTA in a way that did not compromise the activity, structure or stability of the toxin, degradation was significantly enhanced. Enhanced degradation resulted from ubiquitination that predisposed the toxin to proteasomal degradation. Treatment with the proteasomal inhibitor lactacystin increased the cytotoxicity of the lysine-enriched RTA to a level approaching that of wild-type RTA.
Experiment and computer-aided simulation: complementary tools to understand exercise metabolism
Authors: Beneke R.
Published: December 2008

During the last two decades the concept of the MLSS (maximal lactate steady state) has been established. The MLSS detects the highest level of the BLC (blood lactate concentration) and the corresponding workload (MLSS workload) that can be maintained over time without continual BLC accumulation. In spite of a lack of experimental and/or theoretical foundation, it has been speculated that the level of the MLSS may decrease with increasing performance capacity. The potential inter-relationship between performance capacity and BLC response to prolonged constant workload will be analysed based on a recent study, which provided evidence that the MLSS is independent of performance whereas MLSS workload increases with performance capacity, and by a computer-aided simulation. The simulated model modifies and combines previous theories put forward to explain the response of BLC to exercise and incorporates a theory about limiting factors of oxygen transport to the muscle cell. Simulations consider the BLC response to selected prolonged constant workloads while paying special respect to changes in body structure and substrate utilization, which are generally accepted as limiting factors of performance capacity. This complex modulation of appearance and disappearance of lactate during constant prolonged exercise seems to support the experimental results, which indicated independence between MLSS and performance capacity.
Modulation of carbohydrate and fat utilization by diet, exercise and environment
Authors: Jeukendrup A.E.
Published: December 2008

At rest and during exercise carbohydrate and fat are the predominant substrates. They are oxidized simultaneously but the relative contribution of these two substrates is dependent on a variety of factors including the exercise intensity and duration, diet, environmental conditions and training status. Changes in carbohydrate metabolism during the transition from rest to exercise and from low- to high-intensity exercise are mainly due to allosteric regulation. The factors that up-regulate fat metabolism in the transition to moderate-intensity exercise and the factors that result in a down-regulation of fat metabolism at higher intensities are incompletely understood. Substrate use is further modulated by the endocrine milieu (e.g. catecholamines, insulin, cortisol) and possibly cytokines (e.g. interleukin-6). With increasing duration of exercise there are marked increases in fat metabolism and decreases in carbohydrate metabolism and this has been ascribed mainly to substrate availability. Both acute food intake and chronic diets also have profound effects on substrate utilization. An increase in carbohydrate intake will rapidly suppress fat metabolism and increase carbohydrate metabolism whereas such an adaptation to a high-fat diet may take several days. The environmental conditions can also alter substrate use; high ambient temperatures can increase glycogen breakdown as a result of increased body core temperature and increased circulation catecholamines. Low temperatures can also increase carbohydrate metabolism, especially when shivering. In addition to these factors adaptation to training, in particular endurance training, will reduce the reliance on carbohydrate metabolism and increase fat oxidation, especially from intramuscular triacylglycerol stores.
Regulation of PDH activity and isoform expression: diet and exercise
Authors: Peters S.J.
Published: December 2008

During exercise in human skeletal muscle, the proportion of carbohydrate derived acetyl-CoA is determined at least in part by the activity of the PDH (pyruvate dehydrogenase) complex. Inhibition of the complex is achieved through reversible phosphorylation of the E1 subunit by a family of PDH kinase isoforms (PDK1–4) while dephosphorylation and activation of the complex is catalysed by a pair of intrinsic PDH phosphatases (PDP1 and 2). In general, the relative activity of the kinases and phosphatases is determined by a host of intramitochondrial effectors which signal energy charge, substrate and product accumulation, muscle contraction and nutritional status. This review focuses on advances in our understanding in human skeletal muscle of the regulatory signals and changes in gene expression which are important during acute exercise and exercise training, as well as in prolonged situations of altered nutritional status.
Transgenic models – a scientific tool to understand exercise-induced metabolism: the regulatory role of AMPK (5′-AMP-activated protein kinase) in glucose transport and glycogen synthase activity in skeletal muscle
Authors: Wojtaszewski J.F.P. , Nielsen J.N. , Jørgensen S.B. , Frøsig C. , Birk J.B. , Richter E.A.
Published: December 2008

The AMPK (5´AMP-activated protein kinase) is becoming recognized as a critical regulator of energy metabolism. However, many of these effects in muscle metabolism have been ascribed to AMPK based on the use of the unspecific activator AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside). Using mouse models in which AMPK activity has been specifically blocked (kinase dead) or knocked out we and others have been able to conduct studies gaining more conclusive data on the role of AMPK in muscle metabolism. In this mini-review focus is on AMPK and its regulatory role for glucose transport and GS (glycogen synthase) activity in skeletal muscle, indicating that AMPK is a GS kinase in vivo which might influence GS activity during exercise and that AMPK is involved in AICAR/hypoxia-induced glucose transport but not or only partially in contraction-stimulated glucose transport.
Mitochondrial matrix reactive oxygen species production is very sensitive to mild uncoupling
Authors: Miwa S. , Brand M.D.
Published: December 2008

Mitochondria produce ROS (reactive oxygen species) as a by-product of aerobic respiration. Several studies in mammals and birds suggest that the most physiologically relevant ROS production is from complex I following reverse electron flow, and is highly sensitive to membrane potential. A study of Drosophila mitochondria respiring glycerol 3-phosphate revealed that membrane potential-sensitive ROS production from complex I following reverse electron flow was on the matrix side of the inner membrane. A 10 mV decrease in membrane potential was enough to abolish around 70% of the ROS produced by complex I under these conditions. Another important ROS generator in this model, glycerol-3-phosphate dehydrogenase, produced ROS mostly to the cytosolic side; this ROS production was totally insensitive to a small decrease in membrane potential (10 mV). Thus mild uncoupling may be particularly significant for ROS production from complex I on the matrix side of the mitochondrial inner membrane.
Novel peptoids for the detection and suppression of reactive oxygen and nitrogen species
Authors: Fisher A.E.O. , Naughton D.P.
Published: December 2008

Novel peptoids useful for the detection and suppression of various components contributing to oxidative stress and for elucidation of the interplay between these species are presented. Oxidative stress involves redox-active metal ion activation/generation of RONS (reactive oxygen and nitrogen species). For detection of RONS, the peptoid probes consist of a conjugate designed to (1) complex redox-active and non-redox-active metal ions, and (2) differentiate between RONS based upon the reaction products following RONS attack on the probe. For suppression of RONS, subtle modifications in peptoid structure impart catalase and superoxide dismutase activities to the peptoids upon ferric or cupric ion complexation.
Cell respiration and formation of reactive oxygen species: facts and artefacts
Authors: Nohl H. , Kozlov A.V. , Gille L. , Staniek K.
Published: December 2008

It is generally taken as an established fact that mitochondrial respiration is associated with the generation of small amounts of ROS (reactive oxygen species). There are many arguments supporting this side activity. A major argument is the particular physico-chemical configuration of dioxygen, which prevents the transfer of a pair of electrons. Instead, oxygen is reduced by the successive transfer of single electrons, necessarily leading to intermediates with odd electrons. The high rate of turnover of oxygen in the respiratory chain in combination with the existence of single-electron carriers supports the concept of mitochondria as the major cellular ROS generator. Experimental evidence on the ability of mitochondria to generate ROS was, however, based essentially on in vitro experiments with isolated mitochondria. A variety of structural and functional alterations associated with the removal of mitochondria from the cell, as well as the routinely applied ROS detection methods, may lead to artefactual deviation of odd electrons to dioxygen. We therefore checked these correlations in view of ROS formation, including the often reported effect of the membrane potential on the establishment of a redox couple with oxygen out of sequence. For this purpose we developed novel methods to prove the authenticity of mitochondria for ROS generation in the living cell. Based on our experiments, we can exclude spontaneous release of ROS from mitochondria. However, we describe conditions under which mitochondria can be transformed to mild ROS generators. The site of single-electron deviation to dioxygen was found to be ubiquinol interacting with the Rieske iron–sulphur protein and low-potential cytochrome b of the bc1 complex.
Cytochrome c, an ideal antioxidant
Authors: Pereverzev M.O. , Vygodina T.V. , Konstantinov A.A. , Skulachev V.P.
Published: December 2008

Generation of Δψ (membrane potential) by cytochrome oxidase proteoliposomes oxidizing superoxide-reduced cytochrome c has been demonstrated. XO+HX (xanthine oxidase and hypoxanthine) were used to produce superoxide. It was found that the generation of Δψ is completely abolished by cyanide (an uncoupler) or by superoxide dismutase, and is enhanced by nigericin. Addition of ascorbate after XO+HX causes a further increase in Δψ. On the other hand, XO+HX added after ascorbate do not affect Δψ, indicating that superoxide does not have measurable protonophorous activity. The half-maximal cytochrome c concentration for Δψ generation supported by XO+HX was found to be approx. 1 μM. These data and the results of some other researchers can be rationalized as follows: (1) O2 accepts an electron to form superoxide; (2) cytochrome c oxidizes superoxide back to O2; (3) an electron removed from the reduced cytochrome c is transferred to O2 by cytochrome oxidase in a manner that generates ΔμH+ (transmembrane difference in electrochemical H+ potential). Thus cytochrome c mediates a process of superoxide removal, resulting in regeneration of O2 and utilization of the electron involved previously in the O2 reduction. It is important that cytochrome c is not damaged during the antioxidant reaction, in contrast with many other antioxidants.
Possible role of free oxidation processes in the regulation of reactive oxygen species production in plant mitochondria
Authors: Popov V.N.
Published: December 2008

The non-coupled substrate oxidation mediated by components of the electron transport chain that are not coupled to energy accumulation (such as plant alternative oxidase and rotenone-insensitive NADH dehydrogenases) and uncoupled respiration are peculiar features of plant mitochondria. The physiological significance of such energy-wasting oxidation processes is still debated. It is proposed that non-coupled oxidation could regulate the level of reduction of components of the electron transport chain and the rate of one-electron reduction of oxygen, thereby affecting the rate of formation of reactive oxygen species.
Iron superoxide dismutases: structure and function of an archaic enzyme
Authors: Schäfer G. , Kardinahl S.
Published: December 2008

Iron and manganese superoxide dismutases are phylogenetically closely related. They are compared by in silico analysis with regard to their metal specificity and their three-dimensional structure. Special attention is given to the structure and properties of superoxide dismutases from archaeal prokaryotes. The mechanism and the extreme thermostability of superoxide dismutase from Sulfolobus acidocaldarius are discussed on the basis of its high-resolution X-ray structure. An alternating-site mechanism and an evolutionary origin of superoxide dismutases under the environmental conditions on the early Earth are proposed.
Kinetics of superoxide scavenging by glutathione: an evaluation of its role in the removal of mitochondrial superoxide
Authors: Jones C.M. , Lawrence A. , Wardman P. , Burkitt M.J.
Published: December 2008

Superoxide radicals are produced in trace amounts by the mitochondrial respiratory chain. Most are removed rapidly by superoxide dismutase in the matrix. Superoxide is also known to react with glutathione. Reported values of the rate constant for this reaction range from 102 to in excess of 105 M−1·s−1. The magnitude of this rate constant has important physiological implications because, if it is at the upper end of the reported range, a significant proportion of mitochondrial superoxide will evade removal by superoxide dismutase, and will oxidize glutathione to the potentially harmful glutathionyl radical. Using EPR spectroscopy to monitor competition between glutathione and the spin trap 5,5-dimethyl-1-pyrroline N-oxide for reaction with superoxide, we have estimated that the rate constant for the reaction between superoxide and glutathione is only ~200 M−1·s−1. Hence superoxide dismutase will always out-compete glutathione for reaction with the superoxide radical, thereby preventing formation of the glutathionyl radical.
The enzymatic defence against glycation in health, disease and therapeutics: a symposium to examine the concept
Authors: Thornalley P.J.
Published: December 2008

Glycation of proteins, nucleotides and basic phospholipids by glucose, glyoxal, methylglyoxal, 3-deoxyglucosone and other saccharide derivatives is potentially damaging to the proteome and mutagenic. It is now recognized that there is an enzymatic defence against glycation – a group of enzymes that suppress the physiological levels of potent glycating agents and repair glycated proteins: glyoxalase I, aldehyde reductases and dehydrogenases, amadoriase and fructosamine 3-phosphokinase. The enzymatic defence against glycation influences morbidity and the efficiency of drug therapy in certain diseases. Improved understanding of the balance between glycation and the enzymatic anti-glycation defence will advance disease diagnosis and therapy.
Glyoxalase I inhibitors in cancer chemotherapy
Authors: Creighton D.J. , Zheng Z.-B. , Holewinski R. , Hamilton D.S. , Eiseman J.L.
Published: December 2008

Several recent developments suggest that the GSH-dependent glyoxalase enzyme system deserves renewed interest as a potential target for antitumour drug development. This summary focuses on the design and development of new classes of tumoricidal agents that specifically target this elementary detoxification pathway in order to induce elevated concentrations of cytotoxic methylglyoxal in tumour cells. Special emphasis is placed on structure- and mechanism-based inhibitors of GlxI (glyoxalase I), the first enzyme in the pathway. A new class of bivalent transition-state analogues is described that simultaneously bind the active site on each subunit of the homodimeric human GlxI, resulting in Ki values as low as 1 nM. Also described is a new family of bromoacyl esters of GSH that function as active-site-directed irreversible inhibitors of GlxI. Newer prodrugs for delivering the GSH-based inhibitors into tumour cells include reactive sulphoxide esters that undergo acyl exchange with endogenous GSH to give the inhibitors, and polymethacrylamide esters of the inhibitors that are potentially tumour-selective on the basis of the ‘enhanced permeability and retention effect’. Finally, a preliminary evaluation of the efficacy of selected GlxI inhibitors in tumour-bearing mice is given.
Renal clearance of glycation adducts: anti-glycation defence in uraemia and dialysis
Authors: Dawnay A.
Published: December 2008

Increased plasma concentrations of free AGEs (advanced glycation end products) in uraemia are due to impaired renal clearance exacerbated by dietary load and breakdown of the elevated protein-bound AGEs. Increased protein-bound AGEs most probably reflect increased synthesis due to the increased oxidative and carbonyl stress of uraemia, which is exacerbated by the dialysis procedure. Use of biocompatible peritoneal dialysis fluids and haemodialysis membranes will be beneficial, but strategies to further reduce AGE formation are required. These should focus on the hydroimidazalone AGEs, which are quantitatively the most important, and not only on the conventionally monitored AGEs N∊-carboxymethyl-lysine and pentosidine.
Toxicity of glyoxals – role of oxidative stress, metabolic detoxification and thiamine deficiency
Authors: Shangari N. , Bruce W.R. , Poon R. , O'Brien P.J.
Published: December 2008

Glyoxals are reactive α-oxoaldehydes that are formed endogenously from sugars, the levels of which are increased in various pathological conditions associated with hyperglycaemia and thiamine deficiency. However, the molecular cytotoxic mechanisms of glyoxal are not known. Results presented here and in the other studies cited provide a glimpse into the cytotoxicity mechanisms involved and their pathological implications. We found that glyoxal (10 μM) markedly increased the susceptibility of hepatocyte glutathione (GSH) to oxidation by hydrogen peroxide (H2O2) and markedly increased cytotoxicity by compromising the cellular antioxidant enzyme system. At higher concentrations, glyoxal was cytotoxic towards hepatocytes, which can be attributed to GSH depletion, oxidative stress and mitochondrial toxicity. Aminoguanidine or penicillamine protected the hepatocytes. Glyoxal cytotoxicity was prevented by increasing glyoxal metabolism with thiamine or NAD(P)H generators, and was increased in GSH- or thiamine-deficient hepatocytes. It was also found that feeding rats reduced thiamine levels in a diet high in simple sugars increased the number of aberrant crypt foci/colon in the absence of clinical evidence of beriberi. This was associated with decreased plasma thiamine and low erythrocyte transketolase activity. Western diets, which are frequently poor in thiamine and high in sugars, could result in increased levels of endogenous glyoxals, which in turn may lead to a predisposition to AGE (advanced glycation end-product)-related pathologies and neoplastic conditions.