Balancing of energy-consuming processes of K 562 cells

Mitochondrial temperature homeostasis resists external metabolic stresses

Based on studies with a fluorescent reporter dye, Mito Thermo Yellow (MTY), and the genetically encoded gTEMP ratiometric fluorescent temperature indicator targeted to mitochondria, the temperature of active mitochondria in four mammalian and one insect cell line was estimated to be up to 15°C above that of the external environment to which the cells were exposed. High mitochondrial temperature was maintained in the face of a variety of metabolic stresses, including substrate starvation or modification, decreased ATP demand due to inhibition of cytosolic protein synthesis, inhibition of the mitochondrial adenine nucleotide transporter and, if an auxiliary pathway for electron transfer was available via the alternative oxidase, even respiratory poisons acting downstream of oxidative phosphorylation (OXPHOS) complex I. We propose that the high temperature of active mitochondria is an inescapable consequence of the biochemistry of OXPHOS and is homeostatically maintained as a primary feature of mitochondrial metabolism.

Increasing Temperature Results in Higher Allocation of Energy to Protein Synthesis in Sea Urchin Larvae (Lytechinus pictus)

AbstractIt is well established that metabolic processes change with temperature and size. Yet the underlying physiological mechanisms are less well understood regarding how such processes covary within a species and particularly so for developmental stages. Physiological analysis of larvae of the sea urchin Lytechinus pictus revealed that protein was the major biochemical substrate supporting metabolism. The complex dynamics of protein synthesis, turnover, and accretion changed during growth, showing a sevenfold decrease in the ratio of protein accretion to protein synthesis (protein depositional efficiency). To test hypotheses of physiological variation with rising temperature, larvae were reared over a temperature range experienced by this species in its ambient habitat. The thermal sensitivity of protein synthesis was greater than respiration (thermal sensitivity values of 3.7 and 2.4, respectively). Bioenergetic calculations revealed a disproportionate increase in energy allocation toward protein synthesis with rising temperature. These differential temperature sensitivities result in metabolic trade-offs of energy acquisition and expenditure, thereby altering physiological homeostasis. Such insights are of value for improving predictions about limits of biological resilience in a warming ocean.

Differing thermal sensitivities of physiological processes alter ATP allocation

Changes in environmental temperature affect rate processes at all levels of biological organization. Yet the thermal sensitivity of specific physiological processes that affect allocation of the ATP pool within a species is less well understood. In this study of developmental stages of the Pacific oyster, Crassostrea gigas, thermal sensitivities were measured for growth, survivorship, protein synthesis, respiration and transport of amino acids and ions. At warmer temperatures, larvae grew faster but suffered increased mortality. An analysis of temperature sensitivity (Q ₁₀ values) revealed that protein synthesis, the major ATP-consuming process in larvae of C. gigas, is more sensitive to temperature change (Q ₁₀ value of 2.9±0.18) than metabolic rate (Q ₁₀ of 2.0±0.15). Ion transport by Na⁺/K⁺-ATPase measured in vivo has a Q ₁₀ value of 2.1±0.09. The corresponding value for glycine transport is 2.4±0.23. Differing thermal responses for protein synthesis and respiration result in a disproportional increase in the allocation of available ATP to protein synthesis with rising temperature. A bioenergetic model is presented illustrating how changes in growth and temperature affect allocation of the ATP pool. Over an environmentally relevant temperature range for this species, the proportion of the ATP pool allocated to protein synthesis increases from 35 to 65%. The greater energy demand to support protein synthesis with increasing temperature will compromise energy availability to support other essential physiological processes. Defining the trade-offs of ATP demand will provide insights into understanding the adaptive capacity of organisms to respond to various scenarios of environmental change.

Biochemical bases of growth variation during development: a study of protein turnover in pedigreed families of bivalve larvae (Crassostrea gigas)

Animal size is a highly variable trait regulated by complex interactions between biological and environmental processes. Despite the importance of understanding the mechanistic bases of growth, predicting size variation in early stages of development remains challenging. Pedigreed lines of the Pacific oyster (Crassostrea gigas) were crossed to produce contrasting growth phenotypes to analyze the metabolic bases of growth variation in larval stages. Under controlled environmental conditions, substantial growth variation of up to 430% in shell length occurred among 12 larval families. Protein was the major biochemical constituent in larvae, with an average protein-to-lipid content ratio of 2.8. On average, 86% of protein synthesized was turned over (i.e. only 14% retained as protein accreted), with a regulatory shift in depositional efficiency resulting in increased protein accretion during later larval growth. Variation in protein depositional efficiency among families did not explain the range in larval growth rates. Instead, changes in protein synthesis rates predicted 72% of growth variation. High rates of protein synthesis to support faster growth, in turn, necessitated greater allocation of the total ATP pool to protein synthesis. An ATP allocation model is presented for larvae of C. gigas that includes the major components (82%) of energy demand: protein synthesis (45%), ion pump activity (20%), shell formation (14%) and protein degradation (3%). The metabolic trade-offs between faster growth and the need for higher ATP allocation to protein synthesis could be a major determinant of fitness for larvae of different genotypes responding to the stress of environmental change.

Genome-wide RNAi Screen for Fat Regulatory Genes in C. elegans Identifies a Proteostasis-AMPK Axis Critical for Starvation Survival

Organisms must execute metabolic defenses to survive nutrient deprivation. We performed a genome-wide RNAi screen in Caenorhabditis elegans to identify fat regulatory genes indispensable for starvation resistance. Here, we show that opposing proteostasis pathways are principal determinants of starvation survival. Reduced function of cytoplasmic aminoacyl tRNA synthetases (ARS genes) increases fat mass and extends starvation survival, whereas reduced proteasomal function reduces fat and starvation survival. These opposing pathways converge on AMP-activated protein kinase (AMPK) as the critical effector of starvation defenses. Extended starvation survival in ARS deficiency is dependent upon increased proteasome-mediated activation of AMPK. When the proteasome is inhibited, neither starvation nor ARS deficiency can fully activate AMPK, leading to greatly diminished starvation survival. Thus, activity of the proteasome and AMPK are mechanistically linked and highly correlated with starvation resistance. Conversely, aberrant activation of the proteostasis-AMPK axis during nutritional excess may have implications for obesity and cardiometabolic diseases.

Metabolic Cost of Protein Synthesis in Larvae of the Pacific Oyster (Crassostrea gigas) Is Fixed Across Genotype, Phenotype, and Environmental Temperature

The energy made available through catabolism of specific biochemical reserves is constant using standard thermodynamic conversion equivalents (e.g., 24.0 J mg protein(-1)). In contrast, measurements reported for the energy cost of synthesis of specific biochemical constituents are highly variable. In this study, we measured the metabolic cost of protein synthesis and determined whether this cost was influenced by genotype, phenotype, or environment. We focused on larval stages of the Pacific oyster Crassostrea gigas, a species that offers several experimental advantages: availability of genetically pedigreed lines, manipulation of ploidy, and tractability of larval forms for in vivo studies of physiological processes. The cost of protein synthesis was measured in larvae of C. gigas for 1) multiple genotypes, 2) phenotypes with different growth rates, and 3) different environmental temperatures. For all treatments, the cost of protein synthesis was within a narrow range--near the theoretical minimum--with a fixed cost (mean ± one standard error, n = 21) of 2.1 ± 0.2 J (mg protein synthesized)(-1) We conclude that there is no genetic variation in the metabolic cost of protein synthesis, thereby simplifying bioenergetic models. Protein synthesis is a major component of larval metabolism in C. gigas, accounting for more than half the metabolic rate in diploid (59%) and triploid larvae (54%). These results provide measurements of metabolic cost of protein synthesis in larvae of C. gigas, an indicator species for impacts of ocean change, and provide a quantitative basis for evaluating the cost of resilience.

Evidence of endoplasmic reticulum stress and protein synthesis inhibition in the placenta of non-native women at high altitude

Pregnancy at high altitude is associated with a reduction in birth weight of ∼100 g/1000 m of ascent. The underlying mechanisms are unclear but may involve alteration in energy-demanding activities, such as protein synthesis. To test this hypothesis, both in vivo and in vitro approaches were used. Placental tissues from pregnant women residing at 3100 m were studied, and placental cells were incubated under hypoxia. In the 3100-m placentas, we observed dilation of endoplasmic reticulum (ER) cisternae, increased phosphorylation of eukaryotic initiation factor 2 subunit α (P-eIF2α), reduced AKT phosphorylation, and reduced P-4E-BP1 but increased 4E-BP1 protein compared to sea level controls. These findings suggest the presence of ER stress and protein synthesis inhibition. Hypoxia (1% O(2)) reduced proliferation of trophoblast-like JEG-3 cells, BeWo cells, and placental fibroblasts by ∼40, ∼60, and ∼18%, respectively. Sublethal dosage of salubrinal, an eIF2α phosphatase inhibitor, increased P-eIF2α and reduced BeWo cell and placental fibroblast proliferation by ∼50%. Administration of the PI-3K inhibitor LY294002 also reduced JEG-3 proliferation. Our results demonstrate that exposure to chronic hypobaric hypoxia causes mild placental ER stress, which, in turn, modulates protein synthesis and slows proliferation. These effects may account for the reduced placental villous volume, and contribute to the low birth weight that typifies high-altitude populations.

The energetics of embryonic growth

Embryos typically operate under much tighter energy constraints than older animals. This has had a profound impact on how energy is stored, mobilized and partitioned. The result is sometimes quite different ways of doing things. Growth, in particular, is a much more important activity during development. Compared with adults, specific growth rates (g) are extremely high (≥150%day(-1) for some fish). Production efficiencies are also much higher, particularly for early stages where values of 80-90% are not uncommon. Higher production efficiencies are possible, in part, because of lower unit costs at high g. Unlike in adults, the unit cost of growth does not appear to be fixed during early life. Energy also tends to be partitioned in a different manner, with compensatory partitioning being much more important during early life. Other differences include much higher routine metabolic intensities, smaller aerobic scopes and approximately isometric scaling of routine metabolism. The implications for ontogenetic growth models are discussed.

Regulation of Na+/K+ ATPase transport velocity by RNA editing

Editing of Na⁺/K⁺ ATPase mRNAs modulates the Na⁺/K⁺ pump's turnover rate by selectively targeting the release of the final sodium to the outside.

Because firing properties and metabolic rates vary widely, neurons require different transport rates from their Na⁺/K⁺ pumps in order to maintain ion homeostasis. In this study we show that Na⁺/K⁺ pump activity is tightly regulated by a novel process, RNA editing. Three codons within the squid Na⁺/K⁺ ATPase gene can be recoded at the RNA level, and the efficiency of conversion for each varies dramatically, and independently, between tissues. At one site, a highly conserved isoleucine in the seventh transmembrane span can be converted to a valine, a change that shifts the pump's intrinsic voltage dependence. Mechanistically, the removal of a single methyl group specifically targets the process of Na⁺ release to the extracellular solution, causing a higher turnover rate at the resting membrane potential.

In order for excitable cells like neurons and muscles to generate electrical signals, they require ion gradients across their plasma membranes. For example, sodium concentrations are much lower inside a cell than outside, and for potassium it is the opposite case. The job of maintaining these ion gradients falls squarely on a single protein: the Na⁺/K⁺ pump. During each transport cycle, this enzyme moves three sodium ions out of the cell and imports two of potassium. Because this process is the foundation for so many physiological processes, the Na⁺/K⁺ pump is costly to operate, using ∼30% of the ATP generated by an organism. Proper regulation of its turnover rate is vital. In this work, we use the giant nerve cell of squid as a model to show that the Na⁺/K⁺ pump can be regulated by an unsuspected mechanism. Although the gene that codes for this enzyme can make a perfectly functional pump, sometimes its information changes as it passes through the messenger RNA. This is achieved by editing RNA and as a result subtly different versions of the pump can be made, differing at only three amino acids out of more than a thousand. We demonstrate that RNA editing modulates the Na⁺/K⁺ pump's turnover rate and sodium release.

Molecular mechanisms of metabolic regulation by insulin in Drosophila

The insulin signalling pathway is highly conserved from mammals to Drosophila. Insulin signalling in the fly, as in mammals, regulates a number of physiological functions, including carbohydrate and lipid metabolism, tissue growth and longevity. In the present review, I discuss the molecular mechanisms by which insulin signalling regulates metabolism in Drosophila, comparing and contrasting with the mammalian system. I discuss both the intracellular signalling network, as well as the communication between organs in the fly.

The bioenergetics of cancer: is glycolysis the main ATP supplier in all tumor cells?

The molecular mechanisms by which tumor cells achieve an enhanced glycolytic flux and, presumably, a decreased oxidative phosphorylation are analyzed. As the O(2) concentration in hypoxic regions of tumors seems not limiting for oxidative phosphorylation, the role of this mitochondrial pathway in the ATP supply is re-evaluated. Drugs that inhibit glycoysis and oxidative phosphorylation are analyzed for their specificity toward tumor cells and effect on proliferation. The energy metabolism mechanisms involved in the use of positron emission tomography are revised and updated. It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Hierarchies of ATP-consuming processes: direct compared with indirect measurements, and comparative aspects

The original aim of the present study was to deal with two problems that had emerged from a study on hierarchies of ATP-consuming processes in cells [Buttgereit and Brand (1995) Biochem. J. 312, 163-167]. Firstly, we wanted to find out whether the results of that study had been influenced by the method used for the determination of process activity and, secondly, we wondered whether and to what extent the structure of the hierarchy established for cell suspensions under energy-limiting conditions might depend on the type of cell or on the lifestyle, ecology and phylogenetic status of the species from which the cells were derived. We confined our study to the two most prominent ATP consumers of cells: protein synthesis and the Na(+)/K(+)-ATPase, measuring their activity directly by [3H]leucine incorporation and Rb(+)-flux respectively. We found large differences in the sensitivity of protein synthesis to energy limitation between hepatocytes from an anoxia-tolerant fish species and an anoxia-sensitive fish species (goldfish and rainbow trout respectively). On the other hand, Na(+)/K(+)-ATPase activity was hardly affected by energy limitation in the hepatocytes from both fish species. We also studied the response of a human hepatoma cell line, HepG2, to energy limitation and found both protein synthesis and Na(+)/K(+)-ATPase activity to be equally sensitive to energy limitation, but more sensitive than the Na(+)/K(+)-ATPase of the two fish species. A comparison of the indirect and direct methods for measuring protein synthesis revealed the rate of oxygen consumption to be functionally related to the concentration of cycloheximide, the inhibitor used. It was found that at 15 mM cycloheximide [three orders of magnitude higher than the concentration at which the incorporation of free amino acids (FAA) into protein is inhibited] total oxygen consumption was suppressed by 71-75%, whereas the measured rate of [3H]leucine incorporation into protein suggested that the cycloheximide-sensitive fraction should have amounted to not more than approx. 10% of the total oxygen consumption. On the other hand, the amount of oxygen consumption suppressed with the high concentration of cycloheximide corresponded almost exactly to the increase in oxygen consumption of cells incubated in an FAA-enriched medium compared with cells incubated in a standard, FAA-free medium. Our major conclusions are, firstly, that high concentrations of cycloheximide disrupt cellular metabolism, bringing to a standstill all those processes that can be stimulated by incubating starved cells in an FAA-enriched medium, secondly, that the attempt to estimate the metabolic cost of protein synthesis by inhibiting oxygen consumption with cycloheximide leads to spurious results, and, thirdly, that the structure of a 'hierarchy' of ATP-consumers may reflect the lifestyle and physiology of the species studied.

High macromolecular synthesis with low metabolic cost in Antarctic sea urchin embryos

Assessing the energy costs of development in extreme environments is important for understanding how organisms can exist at the margins of the biosphere. Macromolecular turnover rates of RNA and protein were measured at -1.5 degrees C during early development of an Antarctic sea urchin. Contrary to expectations of low synthesis with low metabolism at low temperatures, protein and RNA synthesis rates exhibited temperature compensation and were equivalent to rates in temperate sea urchin embryos. High protein metabolism with a low metabolic rate is energetically possible in this Antarctic sea urchin because the energy cost of protein turnover, 0.45 joules per milligram of protein, is 1/25th the values reported for other animals.

Partial inhibition of Na,K-ATPase activity in cultured rabbit non-pigmented ciliary epithelium following an episode of cytoplasmic ATP depletion

Ouabain-sensitive ATP hydrolysis (Na,K-ATPase activity) was measured in digitonin-permeabilized monolayers of cultured cells derived from rabbit non-pigmented ciliary epithelium. Diminished Na,K-ATPase activity was observed in cells that had been pre-treated 10 min with the protein kinase C activator, PDBu, as well as in cells that had been cooled to 4 degrees C for 4 h then rewarmed to 37 degrees C for 30 min (cool-rewarm manoeuvre). In the intact cells, ouabain binding was not decreased either by PDBu treatment or the cool-rewarm manoeuvre. However, both PDBu and the cool-rewarm manoeuvre increased the rate of ouabain-sensitive potassium (86Rb) uptake measured in intact cells. Cell ATP content was diminished in PDBu-treated cells and cells subjected to the cool-rewarm manoeuvre. We suggest that an episode of ATP depletion might initiate a mechanism which causes lasting, partial inhibition of Na,K-ATPase activity. In keeping with this suggestion, diminished Na,K-ATPase activity was observed in cells that had been pre-treated 20 min with the metabolic inhibitors CCCP or rotenone and in cells pre-treated 2.5 h in dextrose-free medium. This study illustrates that Na,K-ATPase activity measured in the permeabilized cell is a complex parameter which is not necessarily a reliable indicator of sodium pump responses in the intact cell.

Protein synthesis in the liver of Bufo marinus: cost and contribution to oxygen consumption

While many estimates of the contribution of protein synthesis to metabolic rate exist for a variety of animals, most rely on theoretical costs of protein synthesis. The limitations of this approach are that theoretical costs depend upon variable estimates of ATP cost per peptide bond. In addition, they do not take into account the fact that there are protein-specific pre- and post-translational costs. By inhibiting, protein synthesis with cycloheximide and measuring the resultant decrease in oxygen consumption, we have measured the actual cost of protein synthesis and its contribution to metabolic rate in an in vitro system of tissue slices from Bufo marinus. Such measurements exist for endotherms, but there are few such measurements for ectotherms, and none have been done previously for amphibians. The cost of protein synthesis in liver slices from B. marinus was 7.32+/-1.19 mmol O2 x g(protein)(-1) (x +/- SE, n = 48) and protein synthesis accounted for 12% of the total metabolic rate of this tissue. This cost is comparable to values measured for other ectotherms although the contribution of protein synthesis to metabolic rate is at the lower end of the range of estimates for other ectotherms.

Energy metabolism of reticulocytes: two different sources of energy for Na+K(+)-ATPase activity

Total energy production in rabbit reticulocytes amounted to 136.52 +/- 6.50 mumol ATP h-1 ml-1 of reticulocytes: 88.3 per cent was provided by oxidative phosphorylation, whereas only 11.7 per cent by aerobic glycolysis. Na+K(+)-ATPase accounted for 23 per cent, i.e. 27.65 +/- 2.55 mumol ATP h-1ml-1 of reticulocytes, in the overall energy consumption in reticulocytes of rabbits. Under basal conditions ATP for Na+K(+)-ATPase activity was derived exclusively from oxidative phosphorylation. However, when the activity of Na+K(+)-ATPase was increased due to the stimulation of adenylate cyclase by (-)-isoprenaline, the additional energy required was provided by aerobic glycolysis. These results indicate that two different compartments, one cytosolic and the other mitochondrial, provide energy for Na+K(+)-ATPase activity in reticulocytes.

Estimation of metabolic flux rates in liver purine catabolism of tumour-bearing mice by computer simulation of radioactive tracer experiments

Mouse hepatocytes from healthy control mice and from Ehrlich ascites tumour-bearing mice were used for tracer-kinetic studies of purine catabolism of liver cells during different periods of tumour growth. The dynamics of the radioactive tracers were modelled mathematically by a system of differential equations. Computer simulations, i.e. direct fitting of numerical solutions of these equations to the observed time-courses of metabolites and specific radioactivities, enables one to estimate unknown kinetic parameters of a simplified model of pathways of hepatic purine catabolism in tumour-bearing mice. There occurred great differences of metabolic flux rates between control hepatocytes, hepatocytes of mice during the proliferating period of tumour growth (6th day after inoculation of the tumour) and hepatocytes of mice during the resting period of tumour growth (12th day after inoculation of the tumour). The final purine degradation of hepatocytes prepared during the proliferating period was lower in comparison with that of control hepatocytes, but it was markedly higher in hepatocytes prepared during the resting period of tumour growth. The changes in hepatocyte purine catabolism during the proliferating period of tumour growth argue for transitions which aim at the maintenance of high purine nucleotide levels in the liver itself rather than for an increased nucleoside and nucleobase supply for the tumour. This suggestion is in accordance with the increased ATP level of the liver during the proliferating phase of tumour growth.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the Crabtree effect and an endogenous fuel in the energy metabolism of resting and proliferating thymocytes

Rat thymocytes have been used to characterize the changes in energy metabolism that occur as cells undergo a resting/proliferation transition. In the resting state, anaerobic ATP production accounts for only 4% of ATP turnover. The remainder is fueled by the oxidation of a mixture of an unidentified endogenous fuel (62%), glucose (18%) and glutamine (16%). 48 h after mitogen stimulation, the ATP turnover has increased twofold. In these proliferating cells, glucose inhibits oxygen consumption by 58%, indicating a profound Crabtree effect which is not present in resting cells. Consequently, proliferating cells, in the presence of glucose and glutamine, fuel the majority (61%) of ATP turnover anaerobically, producing lactate from glucose. The development of a Crabtree effect may be the result of the 8-10-fold increase in glycolytic enzyme activities which occurs with proliferation. Possible advantages of such a proliferative metabolism are a sparing of endogenous fuel, and a minimizing of oxidative metabolism, with its concurrent production of free radicals.