Reversible Inhibition of Adenine Nucleotide Translocation by Long Chain Fatty Acyl Coenzyme A Esters in Liver Mitochondria of Diabetic and Hibernating Animals

Exploring the multifaceted role of adenosine nucleotide translocase 2 in cellular and disease processes: A comprehensive review

Adenosine nucleotide translocases (ANTs) are a family of proteins abundant in the inner mitochondrial membrane, primarily responsible for shuttling ADP and ATP across the mitochondrial membrane. Additionally, ANTs are key players in balancing mitochondrial energy metabolism and regulating cell death. ANT2 isoform, highly expressed in undifferentiated and proliferating cells, is implicated in the development and drug resistance of various tumors. We conduct a detailed analysis of the potential mechanisms by which ANT2 may influence tumorigenesis and drug resistance. Notably, the significance of ANT2 extends beyond oncology, with roles in non-tumor cell processes including blood cell development, gastrointestinal motility, airway hydration, nonalcoholic fatty liver disease, obesity, chronic kidney disease, and myocardial development, making it a promising therapeutic target for multiple pathologies. To better understand the molecular mechanisms of ANT2, this review summarizes the structural properties, expression patterns, and basic functions of the ANT2 protein. In particular, we review and analyze the controversy surrounding ANT2, focusing on its role in transporting ADP/ATP across the inner mitochondrial membrane, its involvement in the composition of the mitochondrial permeability transition pore, and its participation in apoptosis.

OXPHOS capacity is diminished and the phosphorylation system inhibited during diapause in an extremophile, embryos of Artemia franciscana

Diapause exhibited by embryos of Artemia franciscana is accompanied by severe arrest of respiration. A large fraction of this depression is attributable to downregulation of trehalose catabolism that ultimately restricts fuel to mitochondria. This study now extends knowledge on the mechanism by revealing metabolic depression is heightened by inhibitions within mitochondria. Compared with that in embryo lysates during post-diapause, oxidative phosphorylation (OXPHOS) capacity P is depressed during diapause when either NADH-linked substrates (pyruvate and malate) for electron transfer (electron transfer capacity, E) through respiratory Complex I or the Complex II substrate succinate are used. When pyruvate, malate and succinate were combined, respiratory inhibition by the phosphorylation system in diapause lysates was discovered as judged by P/E flux control ratios (two-way ANOVA; F1,24=38.78; P<0.0001). Inhibition was eliminated as the diapause extract was diluted (significant interaction term; F2,24=9.866; P=0.0007), consistent with the presence of a diffusible inhibitor. One candidate is long-chain acyl-CoA esters known to inhibit the adenine nucleotide translocator. Addition of oleoyl-CoA to post-diapause lysates markedly decreased the P/E ratio to 0.40±0.07 (mean±s.d.; P=0.002) compared with 0.79±0.11 without oleoyl-CoA. Oleoyl-CoA inhibits the phosphorylation system and may be responsible for the depressed P/E in lysates from diapause embryos. With isolated mitochondria, depression of P/E by oleoyl-CoA was fully reversed by addition of l-carnitine (control versus recovery with l-carnitine, P=0.338), which facilitates oleoyl-CoA transport into the matrix and elimination by β-oxidation. In conclusion, severe metabolic arrest during diapause promoted by restricting glycolytic carbon to mitochondria is reinforced by depression of OXPHOS capacity and the phosphorylation system.

Quantification of Mitochondrial Oxidative Phosphorylation in Metabolic Disease: Application to Type 2 Diabetes

Type 2 diabetes (T2D) is a growing health concern with nearly 400 million affected worldwide as of 2014. T2D presents with hyperglycemia and insulin resistance resulting in increased risk for blindness, renal failure, nerve damage, and premature death. Skeletal muscle is a major site for insulin resistance and is responsible for up to 80% of glucose uptake during euglycemic hyperglycemic clamps. Glucose uptake in skeletal muscle is driven by mitochondrial oxidative phosphorylation and for this reason mitochondrial dysfunction has been implicated in T2D. In this review we integrate mitochondrial function with physiologic function to present a broader understanding of mitochondrial functional status in T2D utilizing studies from both human and rodent models. Quantification of mitochondrial function is explained both in vitro and in vivo highlighting the use of proper controls and the complications imposed by obesity and sedentary lifestyle. This review suggests that skeletal muscle mitochondria are not necessarily dysfunctional but limited oxygen supply to working muscle creates this misperception. Finally, we propose changes in experimental design to address this question unequivocally. If mitochondrial function is not impaired it suggests that therapeutic interventions and drug development must move away from the organelle and toward the cardiovascular system.

Enhanced oxidative capacity of ground squirrel brain mitochondria during hibernation

During hibernation, thirteen-lined ground squirrels (Ictidomys tridecemlineatus) regularly cycle between bouts of torpor and interbout arousal (IBA). Most of the brain is electrically quiescent during torpor but regains activity quickly upon arousal to IBA, resulting in extreme oscillations in energy demand during hibernation. We predicted increased functional capacity of brain mitochondria during hibernation compared with spring to accommodate the variable energy demands of hibernation. To address this hypothesis, we examined mitochondrial bioenergetics in the ground squirrel brain across three time points: spring (SP), torpor (TOR), and IBA. Respiration rates of isolated brain mitochondria through complex I of the electron transport chain were more than twofold higher in TOR and IBA than in SP (P < 0.05). We also found a 10% increase in membrane potential between hibernation and spring (P < 0.05), and that proton leak was lower in TOR and IBA than in SP. Finally, there was a 30% increase in calcium loading in SP brain mitochondria compared with TOR and IBA (P < 0.01). To analyze brain mitochondrial abundance between spring and hibernation, we measured the ratio of copy number in a mitochondrial gene (ND1) vs. a nuclear gene (B2M) in frozen cerebral cortex samples. No significant differences were observed in DNA copies between SP and IBA. These data show that brain mitochondrial bioenergetics are not static across the year and suggest that brain mitochondria function more effectively during the hibernation season, allowing for rapid production of energy to meet demand when extreme physiological changes are occurring.

Carnitine supplementation in high-fat diet-fed rats does not ameliorate lipid-induced skeletal muscle mitochondrial dysfunction in vivo

Muscle lipid overload and the associated accumulation of lipid intermediates play an important role in the development of insulin resistance. Carnitine insufficiency is a common feature of insulin-resistant states and might lead to incomplete fatty acid oxidation and impaired export of lipid intermediates out of the mitochondria. The aim of the present study was to test the hypothesis that carnitine supplementation reduces high-fat diet-induced lipotoxicity, improves muscle mitochondrial function, and ameliorates insulin resistance. Wistar rats were fed either normal chow or a high-fat diet for 15 wk. One group of high-fat diet-fed rats was supplemented with 300 mg·kg(-1)·day(-1) L-carnitine during the last 8 wk. Muscle mitochondrial function was measured in vivo by (31)P magnetic resonance spectroscopy (MRS) and ex vivo by high-resolution respirometry. Muscle lipid status was determined by (1)H MRS (intramyocellular lipids) and tandem mass spectrometry (acylcarnitines). High-fat diet feeding induced insulin resistance and was associated with decreases in muscle and blood free carnitine, elevated levels of muscle lipids and acylcarnitines, and an increased number of muscle mitochondria that showed an improved capacity to oxidize fat-derived substrates when tested ex vivo. This was, however, not accompanied by an increase in muscle oxidative capacity in vivo, indicating that in vivo mitochondrial function was compromised. Despite partial normalization of muscle and blood free carnitine content, carnitine supplementation did not induce improvements in muscle lipid status, in vivo mitochondrial function, or insulin sensitivity. Carnitine insufficiency, therefore, does not play a major role in high-fat diet-induced muscle mitochondrial dysfunction in vivo.

Antagonism of P2Y1-induced vasorelaxation by acyl CoA: a critical role for palmitate and 3'-phosphate

BACKGROUND AND PURPOSE

Acyl derivatives of CoA have been shown to act as antagonists at human platelet and recombinant P2Y1 receptors, but little is known about their effects in the cardiovascular system. This study evaluated the effect of these endogenous nucleotide derivatives at P2Y1 receptors natively expressed in rat and porcine blood vessels.

EXPERIMENTAL APPROACH

Isometric tension recordings were used to evaluate the effects of CoA, acetyl CoA, palmitoyl CoA (PaCoA) and 3'-dephospho-palmitoyl-CoA on concentration relaxation-response curves to ADP and uridine triphosphate (UTP). A FlexStation monitored ADP- and UTP-evoked calcium responses in HEK293 cells.

KEY RESULTS

Acetyl CoA and PaCoA, but not CoA, inhibited endothelium-dependent relaxations to ADP with apparent selectivity for P2Y1 receptors (over P2Y(2/4) receptors) in rat thoracic aorta; PaCoA was more potent than acetyl CoA (331-fold vs. fivefold shift of ADP response curve evoked by 10 μM PaCoA and acetyl CoA, respectively); the apparent pA2 value for PaCoA was 6.44. 3'-dephospho-palmitoyl-CoA (10 μM) was significantly less potent than PaCoA (20-fold shift). In porcine mesenteric arteries, PaCoA and the P2Y1 receptor antagonist MRS2500 blocked ADP-mediated endothelium-dependent relaxations; in contrast, they were ineffective against ADP-mediated endothelium-independent relaxation in porcine coronary arteries (which does not involve P2Y1 receptors). Calcium responses evoked by ADP activation of endogenous P2Y1 receptors in HEK293 cells were inhibited in the presence of PaCoA, which failed to alter responses to UTP (acting at endogenous P2Y(2/4) receptors).

CONCLUSIONS AND IMPLICATIONS

Acyl derivatives of CoA can act as endogenous selective antagonists of P2Y1 receptors in blood vessels, and this inhibitory effect critically depends on the palmitate and 3'-ribose phosphate substituents on CoA.

Pioglitazone treatment restores in vivo muscle oxidative capacity in a rat model of diabetes

AIM

To determine the effect of pioglitazone treatment on in vivo and ex vivo muscle mitochondrial function in a rat model of diabetes.

METHODS

Both the lean, healthy rats and the obese, diabetic rats are Zucker Diabetic Fatty (ZDF) rats. The homozygous fa/fa ZDF rats are obese and diabetic. The heterozygous fa/+ ZDF rats are lean and healthy. Diabetic Zucker Diabetic Fatty rats were treated with either pioglitazone (30 mg/kg/day) or water as a control (n = 6 per group), for 2 weeks. In vivo ¹H and ³¹P magnetic resonance spectroscopy was performed on skeletal muscle to assess intramyocellular lipid (IMCL) content and muscle oxidative capacity, respectively. Ex vivo muscle mitochondrial respiratory capacity was evaluated using high-resolution respirometry. In addition, several markers of mitochondrial content were determined.

RESULTS

IMCL content was 14-fold higher and in vivo muscle oxidative capacity was 26% lower in diabetic rats compared with lean rats, which was, however, not caused by impairments of ex vivo mitochondrial respiratory capacity or a lower mitochondrial content. Pioglitazone treatment restored in vivo muscle oxidative capacity in diabetic rats to the level of lean controls. This amelioration was not accompanied by an increase in mitochondrial content or ex vivo mitochondrial respiratory capacity, but rather was paralleled by an improvement in lipid homeostasis, that is lowering of plasma triglycerides and muscle lipid and long-chain acylcarnitine content.

CONCLUSION

Diminished in vivo muscle oxidative capacity in diabetic rats results from mitochondrial lipid overload and can be alleviated by redirecting the lipids from the muscle into adipose tissue using pioglitazone treatment.

Mitochondrial and cellular mechanisms for managing lipid excess

Current scientific debates center on the impact of lipids and mitochondrial function on diverse aspects of human health, nutrition and disease, among them the association of lipotoxicity with the onset of insulin resistance in skeletal muscle, and with heart dysfunction in obesity and diabetes. Mitochondria play a fundamental role in aging and in prevalent acute or chronic diseases. Lipids are main mitochondrial fuels however these molecules can also behave as uncouplers and inhibitors of oxidative phosphorylation. Knowledge about the functional composition of these contradictory effects and their impact on mitochondrial-cellular energetics/redox status is incomplete. Cells store fatty acids (FAs) as triacylglycerol and package them into cytoplasmic lipid droplets (LDs). New emerging data shows the LD as a highly dynamic storage pool of FAs that can be used for energy reserve. Lipid excess packaging into LDs can be seen as an adaptive response to fulfilling energy supply without hindering mitochondrial or cellular redox status and keeping low concentration of lipotoxic intermediates. Herein we review the mechanisms of action and utilization of lipids by mitochondria reported in liver, heart and skeletal muscle under relevant physiological situations, e.g., exercise. We report on perilipins, a family of proteins that associate with LDs in response to loading of cells with lipids. Evidence showing that in addition to physical contact, mitochondria and LDs exhibit metabolic interactions is presented and discussed. A hypothetical model of channeled lipid utilization by mitochondria is proposed. Direct delivery and channeled processing of lipids in mitochondria could represent a reliable and efficient way to maintain reactive oxygen species (ROS) within levels compatible with signaling while ensuring robust and reliable energy supply.

Organ protective mechanisms common to extremes of physiology: a window through hibernation biology

Supply and demand relationships govern survival of animals in the wild and are also key determinants of clinical outcomes in critically ill patients. Most animals' survival strategies focus on the supply side of the equation by pursuing territory and resources, but hibernators are able to anticipate declining availability of nutrients by reducing their energetic needs through the seasonal use of torpor, a reversible state of suppressed metabolic demand and decreased body temperature. Similarly, in clinical medicine the majority of therapeutic interventions to care for critically ill or trauma patients remain focused on elevating physiologic supply above critical thresholds by increasing the main determinants of delivery of oxygen to the tissues (cardiac output, perfusion pressure, hemoglobin concentrations, and oxygen saturation), as well as increasing nutritional support, maintaining euthermia, and other general supportive measures. Techniques, such as induced hypothermia and preconditioning, aimed at diminishing a patient's physiologic requirements as a short-term strategy to match reduced supply and to stabilize their condition, are few and underutilized in clinical settings. Consequently, comparative approaches to understand the mechanistic adaptations that suppress metabolic demand and alter metabolic use of fuel as well as the application of concepts gleaned from studies of hibernation, to the care of critically ill and injured patients could create novel opportunities to improve outcomes in intensive care and perioperative medicine.

Acyl-CoA metabolism and partitioning

Long-chain fatty acyl-coenzyme As (CoAs) are critical regulatory molecules and metabolic intermediates. The initial step in their synthesis is the activation of fatty acids by one of 13 long-chain acyl-CoA synthetase isoforms. These isoforms are regulated independently and have different tissue expression patterns and subcellular locations. Their acyl-CoA products regulate metabolic enzymes and signaling pathways, become oxidized to provide cellular energy, and are incorporated into acylated proteins and complex lipids such as triacylglycerol, phospholipids, and cholesterol esters. Their differing metabolic fates are determined by a network of proteins that channel the acyl-CoAs toward or away from specific metabolic pathways and serve as the basis for partitioning. This review evaluates the evidence for acyl-CoA partitioning by reviewing experimental data on proteins that are believed to contribute to acyl-CoA channeling, the metabolic consequences of loss of these proteins, and the potential role of maladaptive acyl-CoA partitioning in the pathogenesis of metabolic disease and carcinogenesis.

Are long chain acyl CoAs responsible for suppression of mitochondrial metabolism in hibernating 13-lined ground squirrels?

Hibernation in 13-lined ground squirrels (Ictidomys tridecemlineatus) is associated with a substantial suppression of whole-animal metabolism. We compared the metabolism of liver mitochondria isolated from torpid ground squirrels with those from interbout euthermic (IBE; recently aroused from torpor) and summer euthermic conspecifics. Succinate-fuelled state 3 respiration, calculated relative to mitochondrial protein, was suppressed in torpor by 48% and 44% compared with IBE and summer, respectively. This suppression remains when respiration is expressed relative to cytochrome c oxidase activity. We hypothesized that this suppression was caused by inhibition of succinate transport at the dicarboxylate transporter (DCT) by long-chain fatty acyl CoAs that may accumulate during torpor. We predicted, therefore, that exogenous palmitoyl CoA would inhibit respiration in IBE more than in torpor. Palmitoyl CoA inhibited respiration ~70%, in both torpor and IBE. The addition of carnitine, predicted to reverse palmitoyl CoA suppression by facilitating its transport into the mitochondrial matrix, did not rescue the respiration rates in IBE or torpor. Liver mitochondrial activities of carnitine palmitoyl transferase did not differ among IBE, torpor and summer animals. Although palmitoyl CoA inhibits succinate-fuelled respiration, this suppression is likely not related exclusively to inhibition of the DCT, and may involve additional mitochondrial transporters such as the adenine-nucleotide transporter.

Changes in the mitochondrial phosphoproteome during mammalian hibernation

Mammalian hibernation involves periods of substantial suppression of metabolic rate (torpor) allowing energy conservation during winter. In thirteen-lined ground squirrels (Ictidomys tridecemlineatus), suppression of liver mitochondrial respiration during entrance into torpor occurs rapidly (within 2 h) before core body temperature falls below 30°C, whereas reversal of this suppression occurs slowly during arousal from torpor. We hypothesized that this pattern of rapid suppression in entrance and slow reversal during arousal was related to changes in the phosphorylation state of mitochondrial enzymes during torpor catalyzed by temperature-dependent kinases and phosphatases. We compared mitochondrial protein phosphorylation among hibernation metabolic states using immunoblot analyses and assessed how phosphorylation related to mitochondrial respiration rates. No proteins showed torpor-specific changes in phosphorylation, nor did phosphorylation state correlate with mitochondrial respiration. However, several proteins showed seasonal (summer vs. winter) differences in phosphorylation of threonine or serine residues. Using matrix-assisted laser desorption/ionization-time of flight/time of flight mass spectrometry, we identified three of these proteins: F1-ATPase α-chain, long chain-specific acyl-CoA dehydrogenase, and ornithine transcarbamylase. Therefore, we conclude that protein phosphorylation is likely a mechanism involved in bringing about seasonal changes in mitochondrial metabolism in hibernating ground squirrels, but it seems unlikely to play any role in acute suppression of mitochondrial metabolism during torpor.

Regulation of succinate-fuelled mitochondrial respiration in liver and skeletal muscle of hibernating thirteen-lined ground squirrels

Hibernating ground squirrels (Ictidomys tridecemlineatus) alternate between two distinct metabolic states throughout winter: torpor, during which metabolic rate (MR) and body temperature (Tb) are considerably suppressed, and interbout euthermia (IBE), during which MR and Tb briefly return to euthermic levels. Previous studies showed suppression of succinate-fuelled respiration during torpor in liver and skeletal muscle mitochondria; however, these studies used only a single, saturating succinate concentration. Therefore, they could not address whether mitochondrial metabolic suppression occurs under physiological substrate concentrations or whether differences in the kinetics of mitochondrial responses to changing substrate concentration might also contribute to mitochondrial metabolic regulation during torpor. The present study confirmed that succinate oxidation is reduced during torpor in liver and skeletal muscle at 37 and 10°C over a 100-fold range of succinate concentrations. At 37°C, this suppression resulted from inhibition of succinate dehydrogenase (SDH), which had a greater affinity for oxaloacetate (an SDH inhibitor) during torpor. At 10°C, SDH was not inhibited, suggesting that SDH inhibition initiates but does not maintain mitochondrial suppression during torpor. Moreover, in both liver and skeletal muscle, mitochondria from torpid animals maintained relatively higher respiration rates at low succinate concentrations, which reduces the extent of energy savings that can be achieved during torpor, but may also maintain mitochondrial oxidative capacity above some lower critical threshold, thereby preventing cellular and/or mitochondrial injury during torpor and facilitating rapid recruitment of oxidative capacity during arousal.

Mechanism of Free Fatty Acid Effects on Hepatocyte Insulin Receptor Binding and Processing

We determined whether the palmitate effects on hepatocyte insulin receptor binding and post-receptor trafficking were mediated by accelerated mitochondrial beta-oxidation or accumulation of intracellular fatty acyl-CoA derivatives and possibly protein acylation. Preincubation of hepatocytes with moderate concentrations of palmitate (0.5 mM) resulted in a 23% decline in cell-surface binding and proportional decreases in receptor-mediated insulin internalization and degradation. Brief pretreatment of hepatocytes with the carnitine palmityltransferase-I inhibitor, methyl palmoxirate (MP), prevented 70% of the palmitate effects. At higher palmitate concentrations (2.0 mM), cell-surface binding was reduced by 34%, whereas internalization of the receptor complex was reduced by 78%. These effects were only partially prevented by MP pretreatment. Receptor-mediated insulin degradation increased by 34% and was uninfluenced by MP pretreatment. Octanoate, which is rapidly shunted into mitochondrial oxidation, produced a dose-dependent reduction in insulin binding, with proportional decreases in internalization and degradation. Similarly preincubation with 2.0 mM oleate, which, unlike palmitate, is not known to produce protein acylation, resulted in proportional decreases in insulin receptor binding and receptor-mediated internalization and degradation. High concentrations of octanoate or oleate (2.0 mM) did not reproduce the additive post-receptor effects of palmitate. We conclude that the receptor and post-receptor effects of moderate palmitate concentrations are closely linked to accelerated fatty acid oxidation. The post-receptor effects observed at higher concentrations involve other mechanisms, possibly relating to intracellular levels of palmityl-CoA derivatives.

Genetic ablation of calcium-independent phospholipase A(2)γ (iPLA(2)γ) attenuates calcium-induced opening of the mitochondrial permeability transition pore and resultant cytochrome c release

Herein, we demonstrate that calcium-independent phospholipase A(2)γ (iPLA(2)γ) is a critical mechanistic participant in the calcium-induced opening of the mitochondrial permeability transition pore (mPTP). Liver mitochondria from iPLA(2)γ(-/-) mice were markedly resistant to calcium-induced swelling in the presence or absence of phosphate in comparison with wild-type littermates. Furthermore, the iPLA(2)γ enantioselective inhibitor (R)-(E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one ((R)-BEL) was markedly more potent than (S)-BEL in inhibiting mPTP opening in mitochondria from wild-type liver in comparison with hepatic mitochondria from iPLA(2)γ(-/-) mice. Intriguingly, low micromolar concentrations of long chain fatty acyl-CoAs and the non-hydrolyzable thioether analog of palmitoyl-CoA markedly accelerated Ca(2+)-induced mPTP opening in liver mitochondria from wild-type mice. The addition of l-carnitine enabled the metabolic channeling of acyl-CoA through carnitine palmitoyltransferases (CPT-1/2) and attenuated the palmitoyl-CoA-mediated amplification of calcium-induced mPTP opening. In contrast, mitochondria from iPLA(2)γ(-/-) mice were insensitive to fatty acyl-CoA-mediated augmentation of calcium-induced mPTP opening. Moreover, mitochondria from iPLA(2)γ(-/-) mouse liver were resistant to Ca(2+)/t-butyl hydroperoxide-induced mPTP opening in comparison with wild-type littermates. In support of these findings, cytochrome c release from iPLA(2)γ(-/-) mitochondria was dramatically decreased in response to calcium in the presence or absence of either t-butyl hydroperoxide or phenylarsine oxide in comparison with wild-type littermates. Collectively, these results identify iPLA(2)γ as an important mechanistic component of the mPTP, define its downstream products as potent regulators of mPTP opening, and demonstrate the integrated roles of mitochondrial bioenergetics and lipidomic flux in modulating mPTP opening promoting the activation of necrotic and necroapoptotic pathways of cell death.

Prolonged exposure to insulin induces mitochondrion-derived oxidative stress through increasing mitochondrial cholesterol content in hepatocytes

We addressed the link between excessive exposure to insulin and mitochondrion-derived oxidative stress in this study and found that prolonged exposure to insulin increased mitochondrial cholesterol in cultured hepatocytes and in mice and stimulated production of reactive oxygen species (ROS) and decreased the reduced glutathione to glutathione disulfide ratio in cultured hepatocytes. Exposure of isolated hepatic mitochondria to cholesterol alone promoted ROS emission. The oxidative stress induced by the prolonged exposure to insulin was prevented by inhibition of cholesterol synthesis with simvastatin. We further found that prolonged exposure to insulin decreased mitochondrial membrane potential and the increased ROS production came from mitochondrial respiration complex I. Finally, we observed that prolonged exposure to insulin decreased mitochondrial membrane fluidity in a cholesterol synthesis-dependent manner. Together our results demonstrate that excess exposure to insulin causes mitochondrion-derived oxidative stress through cholesterol synthesis in hepatocytes.

Insulin and insulin signaling play a critical role in fat induction of insulin resistance in mouse

The primary player that induces insulin resistance has not been established. Here, we studied whether or not fat can cause insulin resistance in the presence of insulin deficiency. Our results showed that high-fat diet (HFD) induced insulin resistance in C57BL/6 (B6) mice. The HFD-induced insulin resistance was prevented largely by the streptozotocin (STZ)-induced moderate insulin deficiency. The STZ-induced insulin deficiency prevented the HFD-induced ectopic fat accumulation and oxidative stress in liver and gastrocnemius. The STZ-induced insulin deficiency prevented the HFD- or insulin-induced increase in hepatic expression of long-chain acyl-CoA synthetases (ACSL), which are necessary for fatty acid activation. HFD increased mitochondrial contents of long-chain acyl-CoAs, whereas it decreased mitochondrial ADP/ATP ratio, and these HFD-induced changes were prevented by the STZ-induced insulin deficiency. In cultured hepatocytes, we observed that expressions of ACSL1 and -5 were stimulated by insulin signaling. Results in cultured cells also showed that blunting insulin signaling by the PI3K inhibitor LY-294002 prevented fat accumulation, oxidative stress, and insulin resistance induced by the prolonged exposure to either insulin or oleate plus sera that normally contain insulin. Finally, knockdown of the insulin receptor prevented the oxidative stress and insulin resistance induced by the prolonged exposure to insulin or oleate plus sera. Together, our results show that insulin and insulin signaling are required for fat induction of insulin resistance in mice and cultured mouse hepatocytes.

Dietary fatty acid composition influences tissue lipid profiles and regulation of body temperature in Japanese quail

Many avian species reduce their body temperature (T(b)) to conserve energy during periods of inactivity, and we recently characterized how ambient temperature (T(a)) and nutritional stress interact with one another to influence physiologically controlled hypothermic responses in Japanese quail (Coturnix japonica). In the present study, we examined how the fatty acid (FA) composition of the diet influences the FA composition of phospholipids in major organs and how these affect controlled hypothermic responses and metabolic rates in fasted birds. For 5 weeks prior to fasting, quail were fed a standard diet and gavaged each morning with 0.7 ml of water (control), or a vegetable oil comprising saturated fatty acids (SFA; coconut oil), or unsaturated fatty acids (UFA; canola oil). Birds were then fasted for 4 days at a T(a) of 15°C. We found that, while fasting, both photophase and scotophase T(b) decreased significantly more in the SFA treatment group than in the control group; apparently the former down-regulated their T(b) set point. This deeper hypothermic response was correlated with changes in the phospholipid composition of the skeletal muscle and liver, which contained significantly more oleic acid (18:1) and less arachidonic acid (20:4), respectively. Our data imply that these two FAs may be associated with thermoregulation.

Insulin is a stronger inducer of insulin resistance than hyperglycemia in mice with type 1 diabetes mellitus (T1DM)

Subjects with type 1 diabetes mellitus (T1DM) eventually develop insulin resistance and other features of T2DM such as cardiovascular disorders. The exact mechanism has been not been completely understood. In this study, we tested the hypothesis that excessive or inappropriate exposure to insulin is a primary mediator of insulin resistance in T1DM. We found that continuous exposure of mice with non-obese diabetes to insulin detemir, which is similar to some current conventional treatment of human T1DM, induced severe insulin resistance, whereas untreated hyperglycemia for the same amount of time (2 weeks) did not cause obvious insulin resistance. Insulin resistance was accompanied by decreased mitochondrial production as evaluated by mitochondrial DNA and levels of transcripts and proteins of mitochondrion-associated genes, increased ectopic fat accumulation in liver and skeletal muscle (gastrocnemius) evaluated by measurements of triglyceride content, and elevated oxidative stress detected by the GSH/GSSG ratio. Prolonged exposure of cultured hepatocytes to insulin induced significant insulin resistance, whereas the same length of exposure to a high level of glucose (33 mm) did not cause obvious insulin resistance. Furthermore, our results showed that prolonged exposure to insulin caused oxidative stress, and blockade of mitochondrion-derived oxidative stress by overexpression of manganese-superoxide dismutase prevented insulin resistance induced by the prolonged exposure to insulin. Together, our results show that excessive exposure to insulin is a primary inducer of insulin resistance in T1DM in mice.

Increased basal level of Akt-dependent insulin signaling may be responsible for the development of insulin resistance

A majority of subjects with insulin resistance and hyperinsulinemia can maintain their blood glucose levels normal for the whole life presumably through protein kinase B (Akt)-dependent insulin signaling. In this study, we found that the basal Akt phosphorylation level was increased in liver and gastrocnemius of mice under the high-fat diet (HFD). Levels of mitochondrial DNA and expression of some mitochondrion-associated genes were decreased by the HFD primarily in liver. Triglyceride content was increased in both liver and gastrocnemius by the HFD. Oxidative stress was induced by the HFD in both liver and gastrocnemius. Insulin sensitivity was decreased by the HFD. All of these changes were largely or completely reversed by treatment of animals with the phosphatidylinositol 3-kinase inhibitor LY-294002 during the time when animals usually do not eat. Consequently, the overall insulin sensitivity was increased by treatment with LY-294002. Together, our results indicate that increased basal Akt-dependent insulin signaling suppresses mitochondrial production, increases ectopic fat accumulation, induces oxidative stress, and desensitizes insulin signaling in subjects with insulin resistance and hyperinsulinemia.

Features of mitochondrial energetics in living unicellular eukaryote Tetrahymena pyriformis. A model for study of mammalian intracellular adaptation

Tetrahymena pyriformis is used in diverse studies as a non-mammalian alternative due to their resemblance in many main metabolic cycles. However, such basic features of mitochondrial energetics as Delta psi (electrical potential difference across the inner mitochondrial membrane) or maximal stimulation of respiration by uncouplers with different mechanisms of uncoupling, such as DNP (2,4-dinitrophenol) and FCCP (p-trifluoromethoxycarbonylcyanide phenylhydrazone), have not been studied in living ciliates. Tetrahymena pyriformis GL cells during stationary growth phase after incubation under selected conditions were used in this study. Maximal stimulation of cellular respiration by FCCP was about six-fold, thus the proton motive force was high. The DNP uncoupling effect was significantly lower. This suggests low activity of the ATP/ADP-antiporter, which performs not only exchange of intramitochondrial ATP to extramitochondrial ADP, but also helps in the uncoupling process. It participates by a similar mechanism in electrophoretic transport from matrix to cytosol of ATP(4-) and DNP anion, but not FCCP anion. Thus, in contrast with mammalian mitochondria, T. pyriformis mitochondria cannot rapidly supply the cytosol with ATP; possibly the cells need high intramitochondrial ATP. The difference between DNP and FCCP is hypothetically explained by low Delta psi value and/or an increase in concentration of long-chain acyl-CoAs, inhibitors of the ATP/ADP-antiporter. The first suggestion is confirmed by absence of mitochondria with bright fluorescence in T. pyriformis stained with the Delta psi-sensitive probe MitoTracker Red. These data suggest that T. pyriformis cells are useful as a model for study of mitochondrial role in adaptation at the intracellular level.

Deleterious action of FA metabolites on ATP synthesis: possible link between lipotoxicity, mitochondrial dysfunction, and insulin resistance

Insulin resistance is a characteristic feature of type 2 diabetes and obesity. Insulin-resistant individuals manifest multiple disturbances in free fatty acid (FFA) metabolism and have excessive lipid accumulation in insulin target tissues. Although much evidence supports a causal role for altered FFA metabolism in the development of insulin resistance, i.e., "lipotoxicity", the intracellular mechanisms by which elevated plasma FFA levels cause insulin resistance have yet to be completely elucidated. Recent studies have implicated a possible role for mitochondrial dysfunction in the pathogenesis of insulin resistance in skeletal muscle. We examined the effect of FFA metabolites [palmitoyl carnitine (PC), palmitoyl-coenzyme A (CoA), and oleoyl-CoA] on ATP synthesis in mitochondria isolated from mouse and human skeletal muscle. At concentrations ranging from 0.5 to 2 microM, these FFA metabolites stimulated ATP synthesis; however, above 5 microM, there was a dose-response inhibition of ATP synthesis. Furthermore, 10 microM PC inhibits ATP synthesis from pyruvate. Elevated PC concentrations (> or =10 microM) inhibit electron transport chain activity and decrease the mitochondrial inner membrane potential. These acquired mitochondrial defects, caused by a physiological increase in the concentration of FFA metabolites, provide a mechanistic link between lipotoxicity, mitochondrial dysfunction, and muscle insulin resistance.

Effects of dietary polyunsaturated fatty acids on mitochondrial metabolism in mammalian hibernation

Thirteen-lined ground squirrels (Spermophilus tridecemlineatus)were fed one of four isocaloric, isolipemic diets containing 16, 22, 35 or 55 mg linoleic acid (18:2n-6) per gram. Mitochondrial properties were compared between hibernating and summer active states, and between diet groups. As in other studies, state 3 respiration was significantly reduced in hibernation, but only in animals fed the 22 mg g–1 18:2 diet. In the other diet groups, there was no difference in state 3 respiration between the hibernating and summer active groups. In the 22 mg g–1 18:2 diet group, there was no difference in mitochondrial proton conductance between hibernating and summer active animals, again in agreement with earlier studies. However, for all other diet groups,mitochondrial proton conductance was significantly reduced during hibernation. Mitochondrial phospholipid fatty acids changed significantly with hibernation,including increases in unsaturation indices and n-6/n-3, but no differences were found among diet groups. Mitochondrial proton conductance in hibernation showed a positive correlation with the content of linoleic acid(18:2) and arachidonic acid (20:4) in mitochondrial phospholipids. Lipid peroxidation was higher in mitochondria from hibernating animals, probably due to higher unsaturation, but there was no effect of dietary 18:2 on this pattern. Despite the dietary effects on mitochondrial metabolism, all animals hibernated with no differences in bout durations, body temperatures or whole-animal metabolic rates among the diet groups. The reduced mitochondrial proton leak in the 15, 35 and 55 mg g–1 18:2 diet groups might compensate for the inability to suppress respiration, permitting whole-animal energy savings over the hibernation season.

The ADP and ATP transport in mitochondria and its carrier

Different from some more specialised short reviews, here a general although not encyclopaedic survey of the function, metabolic role, structure and mechanism of the ADP/ATP transport in mitochondria is presented. The obvious need for an "old fashioned" review comes from the gateway role in metabolism of the ATP transfer to the cytosol from mitochondria. Amidst the labours, 40 or more years ago, of unravelling the role of mitochondrial compartments and of the two membranes, the sequence of steps of how ATP arrives in the cytosol became a major issue. When the dust settled, a picture emerged where ATP is exported across the inner membrane in a 1:1 exchange against ADP and where the selection of ATP versus ADP is controlled by the high membrane potential at the inner membrane, thus uplifting the free energy of ATP in the cytosol over the mitochondrial matrix. Thus the disparate energy and redox states of the two major compartments are bridged by two membrane potential responsive carriers to enable their symbiosis in the eukaryotic cell. The advance to the molecular level by studying the binding of nucleotides and inhibitors was facilitated by the high level of carrier (AAC) binding sites in the mitochondrial membrane. A striking flexibility of nucleotide binding uncovered the reorientation of carrier sites between outer and inner face, assisted by the side specific high affinity inhibitors. The evidence of a single carrier site versus separate sites for substrate and inhibitors was expounded. In an ideal setting principles of transport catalysis were elucidated. The isolation of intact AAC as a first for any transporter enabled the reconstitution of transport for unravelling, independently of mitochondrial complications, the factors controlling the ADP/ATP exchange. Electrical currents measured with the reconstituted AAC demonstrated electrogenic translocation and charge shift of reorienting carrier sites. Aberrant or vital para-functions of AAC in basal uncoupling and in the mitochondrial pore transition were demonstrated in mitochondria and by patch clamp with reconstituted AAC. The first amino acid sequence of AAC and of any eukaryotic carrier furnished a 6-transmembrane helix folding model, and was the basis for mapping the structure by access studies with various probes, and for demonstrating the strong conformation changes demanded by the reorientation mechanism. Mutations served to elucidate the function of residues, including the particular sensitivity of ATP versus ADP transport to deletion of critical positive charge in AAC. After resisting for decades, at last the atomic crystal structure of the stabilised CAT-AAC complex emerged supporting the predicted principle fold of the AAC but showing unexpected features relevant to mechanism. Being a snapshot of an extreme abortive "c-state" the actual mechanism still remains a conjecture.

Roles of CTPL/Sfxn3 and Sfxn family members in pancreatic islet

Pancreatic AR42J cells have the feature of pluripotency of the precursor cells of the gut endoderm. Betacellulin (BTC) and activin A (Act) convert them into insulin-secreting cells. Using mRNA differential display techniques, we have identified a novel mitochondrial transporter, which is highly expressed during the course of differentiation, and have designated it citrate transporter protein-like protein (CTPL). Recently sideroflexin 1 (Sfxn1) was shown to be a susceptible gene of flexed-tail (f/f) mice, and CTPL has turned out to be a rat orthologous protein of Sfxn3, a member of sideroflexin family. CTPL/Sfxn3 was targeted to mitochondrial membrane like Sfxn1. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were upregulated in the early phase of differentiation into insulin-secreting cells but the expression levels of Sfxn1 and Sfxn3 did not change. All Sfxn family members were expressed in rat pancreatic islet. The expression levels of CTPL/Sfxn3, Sfxn2, and Sfxn5 were also upregulated in islets of streptozotocin-induced diabetic rats compared to normal rats. The downregulation of CTPL/Sfxn3 in a rat insulinoma cell line, INS-1, with the antisense oligonucleotide did not affect the insulin secretion. Taken together, CTPL/Sfxn3 and some other family members might be important in the differentiation of pancreatic beta-cells as a channel or a carrier molecule and be related to the regeneration of pancreatic endocrine cells.

Mitochondrial energy dissipation by fatty acids. Mechanisms and implications for cell death

For most cell types, fatty acids are excellent respiratory substrates. After being transported across the outer and inner mitochondrial membranes they undergo beta-oxidation in the matrix and feed electrons into the mitochondrial energy-conserving respiratory chain. On the other hand, fatty acids also physically interact with mitochondrial membranes, and possess the potential to alter their permeability. This occurs according to two mechanisms: an increase in proton conductance of the inner mitochondrial membrane and the opening of the permeability transition pore, an inner membrane high-conductance channel that may be involved in the release of apoptogenic proteins into the cytosol. This article addresses in some detail the mechanisms through which fatty acids exert their protonophoric action and how they modulate the permeability transition pore and discusses the cellular effects of fatty acids, with specific emphasis on their role as potential mitochondrial mediators of apoptotic signaling.

Relative importance of enhanced glucose uptake versus attenuation of long-chain acyl carnitines in protecting ischemic myocardium

BACKGROUND

A number of experimental studies have shown that increasing glucose use or decreasing accumulation of long-chain acyl carnitines (LCAC) protect ischemic hearts.

METHODS

To evaluate the relative importance of these two strategies in protecting ischemic myocardium, isolated rat hearts (n = 6 in each group) were paced at 300 bpm and subjected to 50 min of low-flow ischemia followed by 60 min of reperfusion. Buffer contained 0.4 m mol/l albumin, 0.4 m mol/l palmitate, and 70 mU/l insulin, and either normal glucose (5 m mol/l) (CON), high glucose (10 m mol/l total) (HG, known to increase glucose use), 5 m mol/l glucose and niacin (10 micromol/l) (NIA, known to increase glucose use and decrease LCAC) or carnitine (10 m mol/l) (CAR, known to increase glucose use and decrease LCAC). Separate groups of hearts were perfused in the presence of 10 micromol/l cytochalasin-B (CB), an inhibitor of insulin-sensitive glucose transporters.

RESULTS

Ischemic injury, as assessed by creatine kinase (CK) release was diminished by an average of 50% in HG, NIA, and CAR hearts, and the percentage recovery of left ventricular (LV) function with reperfusion was enhanced by approximately 20% compared with CON hearts (P < 0.05 for each comparison). Cytochalasin-B abolished all of the salutary effects. Long-chain acyl carnitines levels were higher in HG hearts compared with NIA- and CAR-treated hearts ( P < 0.05), but ischemic protection and functional recovery was greater in HG hearts.

CONCLUSIONS

The data support the adjunctive use of agents that promote glucose uptake during ischemia and suggest that increasing glucose use is more important than decreasing LCAC in the protection against ischemic injury or in the recovery of contractile function.

The potential role of adenosine in the pathophysiology of the insulin resistance syndrome

An increased intracellular availability of the co-enzyme A esters of long-chain fatty acids is thought to underlie many aspects of the insulin resistance syndrome. However, the cause of clustering of a hyperdynamic circulation, sympathetic activation, hypertension, hyperuricaemia, and a raised haematocrit in the insulin resistance syndrome remains to be elucidated. We propose a mechanism that expands the etiological role of long-chain fatty acids. By inhibiting adenine nucleotide translocators, elevated intracellular concentrations of the co-enzyme A esters of long-chain fatty acids impair mitochondrial oxidative phosphorylation. This is expected to result in a chronic systemic increase in extracellular adenosine concentrations. As adenosine stimulates the sympathetic nervous system, induces systemic vasodilatation, stimulates erythropoiesis, and induces renal vasoconstriction with renal sodium retention, increased extracellular ADO concentrations may be the common denominator explaining the above-mentioned and still unexplained phenomena associated with the insulin resistance syndrome. Along the same lines, hyperuricaemia can be explained by the fact that adenosine is broken down to urate and because of increased renal urate retention.

S-[(1 and 2)-phenyl-2-hydroxyethyl]cysteine-induced alterations in renal mitochondrial function in male Fischer-344 rats

Previous studies from our laboratory have shown that mitochondrial dysfunction may be an important early event in S-[(1 and 2)-phenyl-2-hydroxyethyl]cysteine (PHEC)-induced cytotoxicity in isolated rat renal proximal tubules. The present study has therefore examined in more detail PHEC-induced mitochondrial dysfunction, both in vivo and in vitro, using isolated renal cortical mitochondria. Renal cortical mitochondria isolated from PHEC-treated rats in vivo showed depressed effects on the mitochondrial respiration and oxidative phosphorylation in both a dose (0, 250, and 500 micromol/kg iv)- and time (0-24 h)-dependent manner in the presence of both succinate (Site 2) and malate plus alpha-ketoglutarate (Site 1) as respiratory substrates, with initial significant depression occurring as early as 4 h following treatment with 500 micromol PHEC/kg. Similar mitochondrial dysfunctions were observed in vitro in concentration- and time-dependent manners with both respiratory substrates. PHEC also caused a marked dose-dependent inhibition of mitochondrial succinate dehydrogenase and NADH cytochrome c reductase activities both in vivo and in vitro, with initial inhibition occurring as early as 4 h after in vivo administration and 45 min after exposure to PHEC in vitro, while the NADH dehydrogenase activity was not considerably inhibited. The mitochondrial ATPase activity was significantly decreased 4 and 24 h following treatment with PHEC (500 micromol/kg). These results suggest that PHEC exerts its inhibitory effect on the mitochondrial respiration and oxidative phosphorylation through the action on the mitochondrial electron transport chain. PHEC significantly reduced the activity of adenine nucleotide translocase as well as the net uptake of substrates by mitochondria without affecting their efflux within 2-4 h after its injection (500 micromol/kg). On the other hand, significant renal damage, as assessed by morphological study, appeared as early as 24 h following such treatment. The observation of similar effects after both in vivo and in vitro exposures may suggest that the effect on mitochondria may have a pathogenic role in PHEC-induced renal injury in rats. PHEC produces mitochondrial toxicity that results from an inactivation of mitochondrial anionic substrate transporters as well as from an inhibition of activities of adenine nucleotide translocase and dehydrogenases.

Fatty acyl CoA esters as regulators of cell metabolism

Long chain fatty acyl CoA esters have the ability to interact with certain proteins and thereby serve as effectors in cell metabolism. In particular, they can displace nucleotides from specific nucleotide dependent or binding proteins and interfere with their action. The ADP/ATP carrier and uncoupling protein are two examples where the interplay of nucleotide and acyl CoA binding to the proteins regulate their function. Other proteins such as glucokinase can be considered in this group. In certain tissues like liver they are affected during fasting and insulin deficiency, and when serum fatty acids and liver acyl CoA levels are elevated. More recently, an acyl CoA binding protein in E. coli has been found to be a transcription factor for gene regulation of fatty acid metabolism enzymes. There appears to be some consensus in the amino acid sequence for acyl CoA binding sites on these proteins which serve a variety of important roles in cellular metabolism.

Redox control of beta-oxidation in rat liver mitochondria

Coupled rat liver mitochondria were incubated with [U-14C]hexadecanoate and carnitine which resulted in the formation of acyl-, 2-enoyl- and 3-hydroxyacyl-CoA and carnitine esters. The production of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters was associated with a significant lowering of the NAD+/NADH ratio, in contrast to rat muscle mitochondria [Eaton, S., Bhuiyan, A. K. M. J., Kler, R. S., Turnbull, D. M. & Bartlett, K. (1993) Biochem. J. 289, 161-172], suggesting that control by the respiratory chain is important under normal conditions. When NAD+/NADH ratios were held low by succinate-induced reverse electron flow, 3-enoyl-CoA esters were also detected, probably formed by the action of 3,2-enoyl-CoA isomerase. Measurement of the flux of beta-oxidation at different osmolalities showed that flux was strongly dependent on osmolality changes in the physiological range. Measurement of the CoA and carnitine esters resulting from incubations made at different osmolalities showed that there was an increase in the amounts of the saturated acyl-CoA esters with respect to 2-enoyl-CoA and 3-hydroxyacyl-CoA esters, consistent with control by the electron-transfer flavoprotein-ubiquinone segment [Halestrap, A. P. & Dunlop, J. L. (1986) Biochem. J. 239, 559-565]. This however could not be the only factor operating as indicated by the continued presence of 2-enoyl-CoA and 3-hydroxyacyl-CoA esters at high osmolalities.

Effect of fatty acids on energy coupling processes in mitochondria

Long-chain fatty acids are natural uncouplers of oxidative phosphorylation in mitochondria. The protonophoric mechanism of this action is due to transbilayer movement of undissociated fatty acid in one direction and the passage of its anion in the opposite direction. The transfer of the dissociated form of fatty acid can be, at least in some kinds of mitochondrion, facilitated by adenine nucleotide translocase. Apart from dissipating the electrochemical proton gradient, long-chain fatty acids decrease the activity of the respiratory chain by mechanism(s) not fully understood. In intact cells and tissues fatty acids operate mostly as excellent respiratory substrates, providing electrons to the respiratory chain. This function masks their potential uncoupling effect which becomes apparent only under special physiological or pathological conditions characterized by unusual fatty acid accumulation. Short- and medium-chain fatty acids do not have protonophoric properties. Nevertheless, they contribute to energy dissipation because of slow intramitochondrial hydrolysis of their activation products, acyl-AMP and acyl-CoA. Long-chain fatty acids increase permeability of mitochondrial membranes to alkali metal cations. This is due to their ionophoric mechanism of action. Regulatory function of fatty acids with respect to specific cation channels has been postulated for the plasma membrane of muscle cells, but not demonstrated in mitochondria. Under cold stress, cold acclimation and arousal from hibernation the uncoupling effect of fatty acids may contribute to increased thermogenesis, especially in the muscle tissue. In brown adipose tissue, the special thermogenic organ of mammals, long-chain fatty acids promote operation of the unique natural uncoupling protein, thermogenin. As anionic amphiphiles, long-chain fatty acids increase the negative surface charge of biomembranes, thus interfering in their enzymic and transporting functions.

Cardioprotective effect of L-carnitine in rats submitted to permanent left coronary artery ligation

Several studies have suggested that L-carnitine may limit the cellular alterations induced by myocardial hypoxia or ischemia. In the present study, rats were subjected to chronic treatment with L-carnitine (0, 25, 50 or 200 mg/kg/day i.p.) for 9 days prior to being submitted to permanent regional myocardial ischemia by left coronary artery ligation in situ. Following 48 hours of coronary occlusion, infarct size was measured using planimetry of transverse sections of the hearts, which had been stained with nitro-blue tetrazolium. Various functional and metabolic parameters have also been measured in isolated perfused hearts. Treatment with L-carnitine at 200 mg/kg/day i.p. for 9 days led to a significant reduction in infarct size and a better preservation of residual cardiac function. However, none of the metabolic parameters measured were modified. In conclusion, we suggest that the preservation of cardiac contractile function observed with L-carnitine pretreatment is secondary to carnitine-induced infarct size limitation.

Relationship between structure and function of liver mitochondria from hibernating and active ground squirrels, Citellus undulatus

1. Electron microscopy of liver tissue preparations, obtained from hibernating ground squirrels, reveals mitochondria in a condensed state. 2. When kept on ice, mitochondria isolated from the livers of hibernating and active ground squirrels are in a shrunken state. 3. Incubation of mitochondria isolated from the livers of active ground squirrels in the presence of succinate, at 27 degrees C, results in mitochondrial swelling, while mitochondria from hibernating ground squirrels under the same conditions remain relatively shrunken. 4. The swollen mitochondria from active ground squirrels show high oxidative activity, while the shrunken mitochondria from hibernating animals show low oxidative activity. 5. Swelling of mitochondria from hibernating ground squirrels in a hypo-osmolar medium is accompanied by a significant increase in oxidative activity. 6. It is inferred that the shrinkage of hibernating ground squirrel mitochondria is one of the main causes of the inhibition of oxidative activity and other mitochondrial functions during hibernation.

Effects of high fat-feeding to rats on the interrelationship of body weight, plasma insulin, and fatty acyl-coenzyme A esters in liver and skeletal muscle

Rats fed a high-saturated fat diet consumed more energy, gained more weight, and displayed hyperinsulinemia (P less than .05) without an elevation in the fasting plasma glucose level, compared with animals on two different high-carbohydrate diets. The total fatty acyl-coenzyme A (CoA) concentration was 18% (P less than .0001) and 46% (P less than .0001) higher in liver and skeletal muscle, respectively, from rats fed the high-fat diet compared with each of the other diet groups. Major long-chain fatty acyl-CoA molecular species of both tissues in high fat-fed rats reflected the fatty acid profile of the diet. Approximately 29%, 21%, and 16% of total liver and skeletal muscle fatty acyl-CoAs were comprised of oleoyl-CoA, palmitoyl-CoA, and stearoyl-CoA, respectively. The amounts of these three fatty acyl-CoA esters were significantly higher in liver and skeletal muscle after high-fat feeding than with the other diet treatments (P less than .0001). In contrast, the concentration of linoleoyl-CoA was lower in both tissues after high-fat feeding (P less than .0001). In rats fed the high-fat diet, plasma insulin levels were significantly correlated with gain in body weight or body weight (r = .80, P less than .001 for insulin and gain in body weight; r = .73, P less than .001 for insulin and body weight). Total fatty acyl-CoA ester content in liver and skeletal muscle was also strongly correlated with the plasma insulin concentration in high fat-fed rats (r = .80, P less than .001 for liver; r = .78, P less than .001 for skeletal muscle).(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

Streptozotocin-induced alterations in the levels of functional mitochondrial anion transport proteins

The effect of streptozotocin-induced diabetes on the levels of functional mitochondrial anion transport proteins has been determined. The experimental approach utilized for these studies consisted of the extraction of each of four mitochondrial anion transport proteins from rat liver mitoplasts (isolated from diabetic and control animals) with the nonionic detergent Triton X-114, followed by the functional reconstitution of each transporter in a liposomal system via the freeze-thaw-sonication technique. This approach permitted the quantification of transporter function without the complications that occur when such measurements are carried out with intact mitochondria (or mitoplasts). We found that experimental diabetes caused an increase in the extractable and reconstitutable specific (and total) transport activities of the pyruvate and dicarboxylate transporters, a decrease in the activity of the citrate transporter, and no significant change in the activity of the phosphate transporter relative to control values. An examination of the time course of the appearance of changes in the reconstitutable activities of the pyruvate and citrate transporters following the injection of streptozotocin revealed differences. Thus, whereas the activity of the pyruvate transporter displayed the most pronounced increase (193%) 1 week following streptozotocin injection and then subsequently declined from this peak and plateaued at later times (99% and 96% increases at 3 and 8 weeks, respectively), the activity of the citrate transporter progressively decreased with time (31-51% decreases at 1-8 weeks). We suggest that the observed diabetes-induced changes in mitochondrial anion transporter function are predictable on the basis of diabetes-induced alterations in the activities of enzymes that constitute metabolic pathways to which these transporters either supply substrate or remove product. Furthermore, we speculate that mitochondrial anion transport proteins may be regulated in coordination with the enzymes of such associated metabolic pathways.

Cerebral mitochondrial respiration in diabetic and chronically hypoglycemic rats

The respiratory function of cerebral mitochondria harvested from genetically diabetic (BB/W) and streptozotocin-diabetic rats deprived of insulin for 3-4 weeks was found to be unchanged from control values. Furthermore, insulin-deprived BB/W rats subjected to 30 min of insulin-induced hypoglycemic coma demonstrated a normal mitochondrial respiration following a 60 min period of glucose restitution, a finding consistent with earlier results in non-diabetic rats. However, in rats exposed to 1 week of moderate hypoglycemia (plasma glucose = 3.0 mumol.ml-1), both state 3 respiration and the respiratory control ratio (RCR) were reduced from control. In fact, when the chronic hypoglycemia was imposed following a 3-4 week period of diabetic hyperglycemia, the state 3 rate and RCR were found to be reduced to a greater degree than in chronically hypoglycemic, non-diabetic, previously normoglycemic rats. Finally, when 1 week of moderate hypoglycemia preceded a 30 min period of insulin-induced hypoglycemic coma, a disturbed pattern of mitochondrial respiration (i.e. increased state 4, decreased RCR) was found at 60 min of recovery following coma. These results indicate that chronic increases in glucose (and insulin deprivation) have no effect on cerebral mitochondrial respiratory function, whereas prolonged, albeit moderate, reductions in cerebral glucose supply result in perturbations in mitochondrial respiration. These results demonstrate the importance of an adequate glucose supply for normal mitochondrial activity.

Regulation of the rate of respiration and oxidative phosphorylation in liver mitochondria from hibernating ground squirrels, Citellus undulatus

1. The rates of oxidation of various substrates (beta-hydroxybutyrate, succinate, ascorbate + TMPD) and the rate of ATP synthesis in liver mitochondria from active and hibernating ground squirrels were measured. 2. It was shown that the rate of mitochondrial respiration is significantly lower in hibernating animals than in active animals. 3. The degree of inhibition of mitochondrial respiration in hibernating ground squirrels was found to correlate with the length of the respiratory chain fragment involved in the oxidation of a given substrate. 4. The inhibition of mitochondrial respiration in hibernating animals was accompanied by a decrease in the rate of ATP synthesis. 5. The activity of phospholipase A2 in liver mitochondria from hibernating ground squirrels was found to be decreased. The activation of phospholipase A2 by Ca2+ ions eliminated the inhibition of respiration almost completely. 6. It was assumed that the inhibition of mitochondrial respiration during hibernation is (a) related to the suppression of phospholipase A2 activity and (b) caused by the reduced rates of electron transport through the respiratory chain and/or of substrate transport across the mitochondrial membrane.

Age-related changes in respiration coupled to phosphorylation. I. Hepatic mitochondria

Age-related changes affecting mitochondrial adenine nucleotide metabolism may underlie age-related decreases in hepatic metabolic activities. Oxidative activity coupled with phosphorylation, the apparent Km and Vmax of the adenine nucleotide translocase (AdNT), the adenine nucleotide pool size and membrane lipid composition were determined for hepatic mitochondria from young (3 months), mature (12 months) and aged (24 months) Fischer 344 male rats which had been fed NIH-31 diet. The age-related decreases in state 3 respiration supported by NAD-linked substrates were 2-4-fold greater than that of an FAD-linked substrate. The 32% (P less than 0.05) decrease in the AdNT Vmax calculated for the aged rats was accompanied by a 17% decrease in the AdNT Km. The exchangeable pool of adenine nucleotides in mitochondria from aged rats was 72% (P less than 0.05) that in the young rats. While the age-related increase in the cholesterol/phospholipid-Pi ratio and changes in the phospholipid head group pattern were not significant, the overall change in the fatty acid pattern effected a 20% (P less than 0.05) decrease in the n-6/n-3 fatty acid ratio. These data suggest that the reduced Vmax of the AdNT is a consequence of a diminished pool of exchangeable adenine nucleotides. The lower AdNT velocity may reflect the effect of changes in the lipid environment of the membrane in which it is embedded. The major shifts in these parameters occurred during the second year of life.

Reconstitution of adenosine 3'-phosphate 5'-phosphosulfate transporter from rat brain

Adenosine 3'-phosphate 5'-phosphosulfate (PAPS), the "active" sulfate donor for sulfated macromolecules, is synthesized in the cytosolic fraction of rat brains. This molecule is then translocated into the lumen of the Golgi apparatus so that it is available to the sulfotransferase enzymes. The protein responsible for the PAPS translocating activity has been solubilized from vesicles enriched in enzyme markers for the Golgi apparatus and reconstituted into liposomes. In reconstituted liposomes translocating activity has a pH optimum of 7.0 and activity was increased 3-fold by divalent cations, although EDTA produced no inhibition. The affinity of the reconstituted translocator for PAPS showed a Km of 1.2 mM with a Vmax of 14 pmol of PAPS translocated/min/mg of protein. Specificity of the translocator activity was tested with a number of nucleotide analogues and only 3',5'-adenosine diphosphate was a competitive inhibitor. Inhibitors of the mitochondrial ADP/ATP transporter and the red cell anion channel blocked transport of PAPS only at very high concentrations.

Regulation of fatty acid oxidation in rat brain mitochondria: inhibition of high rates of palmitate oxidation by ADP

Regulation of oxidation of [1-14C]palmitate in rat brain mitochondria has been investigated in purified mitochondria of nonsynaptic origin prepared by use of a Ficoll/sucrose density gradient. The mitochondrial preparation contained considerable Mg2+-ATPase activity, but was virtually free of contamination with nonmitochondrial fractions. Palmitate oxidation was inhibited by increasing the concentration of ATP in the assay system to near-physiological levels (2 mM), and the inhibition at 2 or 4 mM ATP was analyzed by comparing it with palmitate oxidation at near-maximal rates with low levels of ATP (0.5 or 1 mM). Inhibition was increased by the addition of ADP or by increasing the concentration of Mg2+ in the assay system, whereas inhibition was decreased by decreasing the concentration of mitochondrial protein or L-carnitine in the assay system. Increasing CoA concentration also had a deinhibitory effect. With 0.5 or 1 mM ATP, however, neither inhibition by added ADP nor protein concentration-dependent inhibition was observed, and the rate of oxidation was saturated with increasing concentrations of Mg2+, L-carnitine, or CoA. These results indicated that ADP was involved in the inhibition of high rates of palmitate oxidation in the presence of sufficient ATP and L-carnitine. The inhibitory effect of increasing the concentration of mitochondrial protein could be explained by the enhanced amounts of ADP present in the preparation; similarly, increased concentrations of Mg2+ would provide higher levels of ADP by stimulating the Mg2+-ATPase reaction. We discuss the possibility that the transport of ADP across the inner membrane of brain mitochondria is coupled to the inhibition of palmitate oxidation.

Effect of ovarian hormones upon liver mitochondrial function in diabetic rats

In the present study it is shown that streptozotocin (SZ)-induced chronic diabetes of female albino rats produced significant alterations in liver mitochondrial function after 30-35 days of diabetes. The disturbances were as follows: (1) a significant fall of the mean values of the respiratory control ratio and of state 3 of respiration using three substrates, 3-hydroxybutyrate, malate-glutamate and succinate, and (2) a significant increase of the mean damping factor of the oscillatory osmotic variations (with valinomycin as K+ ionophore and succinate as substrate). The same mitochondrial function parameters were analyzed for comparison in control non-diabetic rats (group N) and in the following groups of female rats with chronic diabetes: intact (group I), oophorectomized (6 days after the injection of SZ) (group O), and oophorectomized with restitution therapy of 17 beta-estradiol (from the operation until the day before killing) (group O + Eol). The O group showed significantly higher values of the respiratory control ratio and of state 3 of respiration and significantly lower damping factors than group I. The restitution treatment in the O + Eol group restored the mitochondrial functions assayed to values similar to those of group I. These data provide strong evidence that estrogens exert a negative effect at the molecular level upon impaired liver mitochondrial functions in SZ-induced diabetes.

Hepatic gluconeogenesis and mitochondrial function during hibernation

1. The aim of these studies was to investigate a mitochondrial basis for changes in gluconeogenesis during hibernation. 2. State 3 respiration rates in liver mitochondria from hibernating ground squirrels were reduced by 62-66%. The limiting reaction appeared to be electron transport, particularly in respiratory complex III. 3. The mitochondrial ATP + ADP + AMP content was reduced by 29% during hibernation; cellular adenine nucleotide content was unchanged. 4. Pyruvate carboxylation in intact mitochondria was decreased 75% during hibernation, although total pyruvate carboxylase activity was not lower. 5. Rates of gluconeogenesis in intact hepatocytes isolated from hibernators were lower than in cells from non-hibernators.

Reye's syndrome: current concepts

Despite greater than 23 years of study, an incomplete understanding of the etiology, epidemiology and pathogenesis of Reye's syndrome persists. Better understanding of the disease has been hampered by the lack of a good animal model on which hypotheses of its pathogenesis could be tested. Human studies indicate that a primary mitochondrial injury may lead to complex metabolic disturbances that produce the observed pathophysiology. Specific directions regarding avenues for future research should pursue two lines: a good animal model still needs to be developed in which the biochemical and morphologic alterations identified in Reye's syndrome are duplicated. This model should include an antecedent viral illness but may not require aspirin exposure as an essential ingredient. With the identification of a satisfactory model, specific questions about the roles of environmental toxins or medications may be answered. Study of noncomatose cases of Reye's syndrome should continue. The specific emphasis should be to delineate what factors (NH3, free fatty acids and dicarboxylic acids) may be implicated in the pathogenesis of the CNS disease with the hopes of devising strategies for more effective treatment of encephalopathy and its attendant morbidity and mortality.

The modes of action of long chain alkyl compounds on the respiratory chain-linked energy transducing system in submitochondrial particles

The interactions of long chain (greater than C7), alkyl compounds with tightly coupled, beef heart submitochondrial particles (SMP) have been investigated with respect to their effects upon respiratory chain-linked electron transfer and energy coupling capacity. Long chain alkyl alcohols, amines, free fatty acids, and methyl esters exhibit a general uncoupling effect, with stimulation of the succinate oxidase activity but inhibition of the NADH oxidase, in SMP. The degree of effectiveness is dependent on the nature of the functional group and the length of the alkyl chain. Submitochondrial particles depleted of F1 and the F1-inhibitor protein are similarly affected. Subsequent treatment with bovine serum albumin reverses the effects of free fatty acids and results in partial recovery of activity with alkyl amines, alcohols, and methyl esters. Differences between the effects of these alkyl compounds and those of sodium dodecyl sulfate, deoxycholate, palmitoyl carnitine, and palmitoyl CoA rule out detergent-like action as the explanation for these observations. These data suggest that specific lipophilic interactions with the membrane, modulated by the nature of the functional group, are responsible for the effects of these compounds on the energy transducing system of SMP. Analyses of the reduction kinetics of the cytochromes indicate that the sites of interaction of these compounds with the inner mitochondrial membrane are associated with the primary dehydrogenase of complex I and energy coupling site 2; alkyl amines possess an additional site of interaction in the region of complex III.