Impairment of adrenergically-regulated thermogenesis in brown fat of obesity-resistant mice is compensated by non-shivering thermogenesis in skeletal muscle

OBJECTIVE

Non-shivering thermogenesis (NST) mediated by uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) can be activated via the adrenergic system in response to cold or diet, contributing to both thermal and energy homeostasis. Other mechanisms, including metabolism of skeletal muscle, may also be involved in NST. However, relative contribution of these energy dissipating pathways and their adaptability remain a matter of long-standing controversy.

METHODS

We used warm-acclimated (30 °C) mice to characterize the effect of an up to 7-day cold acclimation (6 °C; CA) on thermoregulatory thermogenesis, comparing inbred mice with a genetic background conferring resistance (A/J) or susceptibility (C57BL/6 J) to obesity.

RESULTS

Both warm-acclimated C57BL/6 J and A/J mice exhibited similar cold endurance, assessed as a capability to maintain core body temperature during acute exposure to cold, which improved in response to CA, resulting in comparable cold endurance and similar induction of UCP1 protein in BAT of mice of both genotypes. Despite this, adrenergic NST in BAT was induced only in C57BL/6 J, not in A/J mice subjected to CA. Cold tolerance phenotype of A/J mice subjected to CA was not based on increased shivering, improved insulation, or changes in physical activity. On the contrary, lipidomic, proteomic and gene expression analyses along with palmitoyl carnitine oxidation and cytochrome c oxidase activity revealed induction of lipid oxidation exclusively in skeletal muscle of A/J mice subjected to CA. These changes appear to be related to skeletal muscle NST, mediated by sarcolipin-induced uncoupling of sarco(endo)plasmic reticulum calcium ATPase pump activity and accentuated by changes in mitochondrial respiratory chain supercomplexes assembly.

CONCLUSIONS

Our results suggest that NST in skeletal muscle could be adaptively augmented in the face of insufficient adrenergic NST in BAT, depending on the genetic background of the mice. It may provide both protection from cold and resistance to obesity, more effectively than BAT.

Age-Dependent Changes in the Function of Mitochondrial Membrane Permeability Transition Pore in Rat Liver Mitochondria

Mitochondria play an important role in the cell aging process. Changes in calcium homeostasis and/or increased reactive oxygen species (ROS) production lead to the opening of mitochondrial permeability transition pore (MPTP), depolarization of the inner mitochondrial membrane, and decrease of ATP production. Our work aimed to monitor age-related changes in the Ca2+ ion effect on MPTP and the ability of isolated rat liver mitochondria to accumulate calcium. The mitochondrial calcium retention capacity (CRC) was found to be significantly affected by the age of rats. Measurement of CRC values of the rat liver mitochondria showed two periods when 3 to 17-week old rats were tested. 3-week and 17-week old rats showed lower CRC values than 7-week old animals. Similar changes were observed while testing calcium-induced swelling of rat liver mitochondria. These findings indicate that the mitochondrial energy production system is more resistant to calcium-induced MPTP opening accompanied by the damaging effect of ROS in adult rats than in young and aged animals.

Age-Dependent Changes in the Function of Mitochondrial Membrane Permeability Transition Pore in Rat Liver Mitochondria

Mitochondria play an important role in the cell aging process. Changes in calcium homeostasis and/or increased reactive oxygen species (ROS) production lead to the opening of mitochondrial permeability transition pore (MPTP), depolarization of the inner mitochondrial membrane, and decrease of ATP production. Our work aimed to monitor age-related changes in the Ca2+ ion effect on MPTP and the ability of isolated rat liver mitochondria to accumulate calcium. The mitochondrial calcium retention capacity (CRC) was found to be significantly affected by the age of rats. Measurement of CRC values of the rat liver mitochondria showed two periods when 3 to17-week old rats were tested. 3-week and 17-week old rats showed lower CRC values than 7-week old animals. Similar changes were observed while testing calcium-induced swelling of rat liver mitochondria. These findings indicate that the mitochondrial energy production system is more resistant to calcium-induced MPTP opening accompanied by the damaging effect of ROS in adult rats than in young and aged animals.

Factors affecting the function of the mitochondrial membrane permeability transition pore and their role in evaluation of calcium retention capacity values

Values of the calcium retention capacity (CRC) of rat liver mitochondria are highly dependent on the experimental conditions used. When increasing amounts of added calcium chloride are used (1.25-10 nmol), the values of the CRC increase 3-fold. When calcium is added in 75 s intervals, the CRC values increase by 30 % compared with 150 s interval additions. CRC values are not dependent on the calcium/protein ratio in the measured sample in our experimental design. We also show that a more detailed evaluation of the fluorescence curves can provide new information about mitochondrial permeability transition pore opening after calcium is added.

Gradual cold acclimation induces cardioprotection without affecting β-adrenergic receptor-mediated adenylyl cyclase signaling

Novel strategies are needed that can stimulate endogenous signaling pathways to protect the heart from myocardial infarction. The present study tested the hypothesis that appropriate regimen of cold acclimation (CA) may provide a promising approach for improving myocardial resistance to ischemia/reperfusion (I/R) injury without negative side effects. We evaluated myocardial I/R injury, mitochondrial swelling, and β-adrenergic receptor (β-AR)-adenylyl cyclase-mediated signaling. Male Wistar rats were exposed to CA (8°C, 8 h/day for a week, followed by 4 wk at 8°C for 24 h/day), while the recovery group (CAR) was kept at 24°C for an additional 2 wk. The myocardial infarction induced by coronary occlusion for 20 min followed by 3-h reperfusion was reduced from 56% in controls to 30% and 23% after CA and CAR, respectively. In line, the rate of mitochondrial swelling at 200 μM Ca2+ was decreased in both groups. Acute administration of metoprolol decreased infarction in control group and did not affect the CA-elicited cardiprotection. Accordingly, neither β1-AR-Gsα-adenylyl cyclase signaling, stimulated with specific ligands, nor p-PKA/PKA ratios were affected after CA or CAR. Importantly, Western blot and immunofluorescence analyses revealed β2- and β3-AR protein enrichment in membranes in both experimental groups. We conclude that gradual cold acclimation results in a persisting increase of myocardial resistance to I/R injury without hypertension and hypertrophy. The cardioprotective phenotype is associated with unaltered adenylyl cyclase signaling and increased mitochondrial resistance to Ca2+-overload. The potential role of upregulated β2/β3-AR pathways remains to be elucidated.

NEW & NOTEWORTHY We present a new model of mild gradual cold acclimation increasing tolerance to myocardial ischemia/reperfusion injury without hypertension and hypertrophy. Cardioprotective phenotype is accompanied by unaltered adenylyl cyclase signaling and increased mitochondrial resistance to Ca2+-overload. The potential role of upregulated β2/β3-adrenoreceptor activation is considered. These findings may stimulate the development of novel preventive and therapeutic strategies against myocardial ischemia/reperfusion injury.

Developmental and sex differences in cardiac tolerance to ischemia-reperfusion injury: the role of mitochondria 1

Age and sex play an essential role in the cardiac tolerance to ischemia-reperfusion injury: cardiac resistance significantly decreases during postnatal maturation and the female heart is more tolerant than the male myocardium. It is widely accepted that mitochondrial dysfunction, and particularly mitochondrial permeability transition pore (MPTP) opening, plays a major role in determining the extent of cardiac ischemia-reperfusion injury. We have observed that the MPTP sensitivity to the calcium load differs in mitochondria isolated from neonatal and adult myocardium, as well as from adult male and female hearts. Neonatal and female mitochondria are more resistant both in the extent and in the rate of mitochondrial swelling induced by high calcium concentration. Our data further suggest that age- and sex-dependent specificity of the MPTP is not the result of different amounts of ATP synthase and cyclophilin D: neonatal and adult hearts, similarly as the male and female hearts, contain comparable amounts of MPTP and its regulatory protein cyclophilin D. We can speculate that the lower sensitivity of MPTP to the calcium-induced swelling may be related to the higher ischemic tolerance of both neonatal and female myocardium.

Modification of calcium retention capacity of rat liver mitochondria by phosphate and tert-butyl hydroperoxide

By determining the calcium retention capacity (CRC) of rat liver mitochondria, we confirmed and extended previous observations describing the activation of mitochondrial swelling by phosphate and tert-butyl hydroperoxide (t-BHP). Using CRC measurements, we showed that both phosphate and t-BHP decrease the extent of calcium accumulation required for the full mitochondrial permeability transition pore (MPTP) opening to 35 % of control values and to only 15 % when both phosphate and t-BHP are present in the medium. When changes in fluorescence were evaluated at higher resolution, we observed that in the presence of cyclosporine A fluorescence values return after each Ca(2+) addition to basal values obtained before the Ca(2+) addition. This indicates that the MPTP remains closed. However, in the absence of cyclosporine A, the basal fluorescence after each Ca(2+) addition continuously increased. This increase was potentiated both by phosphate and t-BHP until the moment when the concentration of intramitochondrial calcium required for the full opening of the MPTP was reached. We conclude that in the absence of cyclosporine A, the MPTP is slowly opened after each Ca(2+) addition and that this rate of opening can be modified by various factors such as the composition of the media and the experimental protocol used.

Mitochondrial Targeting of Metformin Enhances Its Activity against Pancreatic Cancer

Pancreatic cancer is one of the hardest-to-treat types of neoplastic diseases. Metformin, a widely prescribed drug against type 2 diabetes mellitus, is being trialed as an agent against pancreatic cancer, although its efficacy is low. With the idea of delivering metformin to its molecular target, the mitochondrial complex I (CI), we tagged the agent with the mitochondrial vector, triphenylphosphonium group. Mitochondrially targeted metformin (MitoMet) was found to kill a panel of pancreatic cancer cells three to four orders of magnitude more efficiently than found for the parental compound. Respiration assessment documented CI as the molecular target for MitoMet, which was corroborated by molecular modeling. MitoMet also efficiently suppressed pancreatic tumors in three mouse models. We propose that the novel mitochondrially targeted agent is clinically highly intriguing, and it has a potential to greatly improve the bleak prospects of patients with pancreatic cancer. Mol Cancer Ther; 15(12); 2875-86. ©2016 AACR.

Myocardial iron content and mitochondrial function in human heart failure: a direct tissue analysis

AIMS

Iron replacement improves clinical status in iron-deficient patients with heart failure (HF), but the pathophysiology is poorly understood. Iron is essential not only for erythropoiesis, but also for cellular bioenergetics. The impact of myocardial iron deficiency (MID) on mitochondrial function, measured directly in the failing human heart, is unknown.

METHODS AND RESULTS

Left ventricular samples were obtained from 91 consecutive HF patients undergoing transplantation and 38 HF-free organ donors (controls). Total myocardial iron content, mitochondrial respiration, citric acid cycle and respiratory chain enzyme activities, respiratory chain components (complex I-V), and protein content of reactive oxygen species (ROS)-protective enzymes were measured in tissue homogenates to quantify mitochondrial function. Myocardial iron content was lower in HF compared with controls (156 ± 41 vs. 200 ± 38 µg·g^-1 dry weight, P < 0.001), independently of anaemia. MID (the lowest iron tercile in HF) was associated with more extensive coronary disease and less beta-blocker usage compared with non-MID HF patients. Compared with controls, HF patients displayed reduced myocardial oxygen₂ respiration and reduced activity of all examined mitochondrial enzymes (all P < 0.001). MID in HF was associated with preserved activity of respiratory chain enzymes but reduced activity of aconitase and citrate synthase (by -26% and -15%, P < 0.05) and reduced expression of catalase, glutathione peroxidase, and superoxide dismutase 2.

CONCLUSION

Myocardial iron content is decreased and mitochondrial functions are impaired in advanced HF. MID in HF is associated with diminished citric acid cycle enzyme activities and decreased ROS-protecting enzymes. MID may contribute to altered myocardial substrate use and to worsening of mitochondrial dysfunction that exists in HF.

The effect of alpha-tocopheryl succinate on succinate respiration in rat liver mitochondria

We compared the effect of alpha-tocopheryl succinate (TOS) on succinate-dependent respiration in rat liver mitochondria, homogenate and permeabilized hepatocytes in both a coupled and uncoupled state. In isolated mitochondria, a significant inhibitory effect was observed at a concentration of 5 microM, in liver homogenate at 25 microM and in permeabilized hepatocytes at 50 microM. The inhibitory effect of TOS on succinate respiration in an uncoupled state was less pronounced than in a coupled state in all the experimental models tested. When the concentration dependence of the TOS inhibitory effect was tested, the most sensitive in both states were isolated mitochondria; the most resistant were permeabilized hepatocytes.

In vitro and in vivo activation of mitochondrial membrane permeability transition pore using triiodothyronine

Using a novel method for evaluating mitochondrial swelling (Drahota et al. 2012a) we studied the effect of calcium (Ca(2+)), phosphate (P(i)), and triiodothyronine (T(3)) on the opening of mitochondrial membrane permeability transition pore and how they interact in the activation of swelling process. We found that 0.1 mM P(i), 50 microM Ca(2+) and 25 microM T(3) when added separately increase the swelling rate to about 10 % of maximal values when all three factors are applied simultaneously. Our findings document that under experimental conditions in which Ca(2+) and P(i) are used as activating factors, the addition of T(3) doubled the rate of swelling. T(3) has also an activating effect on mitochondrial membrane potential. The T(3) activating effect was also found after in vivo application of T(3). Our data thus demonstrate that T(3) has an important role in opening the mitochondrial membrane permeability pore and activates the function of the two key physiological swelling inducers, calcium and phosphate ions.

Metformin prevents ischemia reperfusion-induced oxidative stress in the fatty liver by attenuation of reactive oxygen species formation

Nonalcoholic fatty liver disease is associated with chronic oxidative stress. In our study, we explored the antioxidant effect of antidiabetic metformin on chronic [high-fat diet (HFD)-induced] and acute oxidative stress induced by short-term warm partial ischemia-reperfusion (I/R) or on a combination of both in the liver. Wistar rats were fed a standard diet (SD) or HFD for 10 wk, half of them being administered metformin (150 mg·kg body wt(-1)·day(-1)). Metformin treatment prevented acute stress-induced necroinflammatory reaction, reduced alanine aminotransferase and aspartate aminotransferase serum activity, and diminished lipoperoxidation. The effect was more pronounced in the HFD than in the SD group. The metformin-treated groups exhibited less severe mitochondrial damage (markers: cytochrome c release, citrate synthase activity, mtDNA copy number, mitochondrial respiration) and apoptosis (caspase 9 and caspase 3 activation). Metformin-treated HFD-fed rats subjected to I/R exhibited increased antioxidant enzyme activity as well as attenuated mitochondrial respiratory capacity and ATP resynthesis. The exposure to I/R significantly increased NADH- and succinate-related reactive oxygen species (ROS) mitochondrial production in vitro. The effect of I/R was significantly alleviated by previous metformin treatment. Metformin downregulated the I/R-induced expression of proinflammatory (TNF-α, TLR4, IL-1β, Ccr2) and infiltrating monocyte (Ly6c) and macrophage (CD11b) markers. Our data indicate that metformin reduces mitochondrial performance but concomitantly protects the liver from I/R-induced injury. We propose that the beneficial effect of metformin action is based on a combination of three contributory mechanisms: increased antioxidant enzyme activity, lower mitochondrial ROS production, and reduction of postischemic inflammation.

Ubiquinone-binding site mutagenesis reveals the role of mitochondrial complex II in cell death initiation

Respiratory complex II (CII, succinate dehydrogenase, SDH) inhibition can induce cell death, but the mechanistic details need clarification. To elucidate the role of reactive oxygen species (ROS) formation upon the ubiquinone-binding (Qp) site blockade, we substituted CII subunit C (SDHC) residues lining the Qp site by site-directed mutagenesis. Cell lines carrying these mutations were characterized on the bases of CII activity and exposed to Qp site inhibitors MitoVES, thenoyltrifluoroacetone (TTFA) and Atpenin A5. We found that I56F and S68A SDHC variants, which support succinate-mediated respiration and maintain low intracellular succinate, were less efficiently inhibited by MitoVES than the wild-type (WT) variant. Importantly, associated ROS generation and cell death induction was also impaired, and cell death in the WT cells was malonate and catalase sensitive. In contrast, the S68A variant was much more susceptible to TTFA inhibition than the I56F variant or the WT CII, which was again reflected by enhanced ROS formation and increased malonate- and catalase-sensitive cell death induction. The R72C variant that accumulates intracellular succinate due to compromised CII activity was resistant to MitoVES and TTFA treatment and did not increase ROS, even though TTFA efficiently generated ROS at low succinate in mitochondria isolated from R72C cells. Similarly, the high-affinity Qp site inhibitor Atpenin A5 rapidly increased intracellular succinate in WT cells but did not induce ROS or cell death, unlike MitoVES and TTFA that upregulated succinate only moderately. These results demonstrate that cell death initiation upon CII inhibition depends on ROS and that the extent of cell death correlates with the potency of inhibition at the Qp site unless intracellular succinate is high. In addition, this validates the Qp site of CII as a target for cell death induction with relevance to cancer therapy.

Carnitine supplementation alleviates lipid metabolism derangements and protects against oxidative stress in non-obese hereditary hypertriglyceridemic rats

The aim of this study was to estimate the effect of carnitine supplementation on lipid disorders and peripheral tissue insulin sensitivity in a non-obese animal model of insulin resistance, the hereditary hypertriglyceridemic (HHTg) rat. Male HHTg rats were fed a standard diet, and half of them received daily doses of carnitine (500 mg·kg−1body weight) for 8 weeks. Rats of the original Wistar strain were used for comparison. HHTg rats exhibited increased urinary excretion of free carnitine and reduced carnitine content in the liver and blood. Carnitine supplementation compensated for this shortage and promoted urinary excretion of acetylcarnitine without any signs of (acyl)carnitine accumulation in skeletal muscle. Compared with their untreated littermates, carnitine-treated HHTg rats exhibited lower weight gain, reduced liver steatosis, lower fasting triglyceridemia, and greater reduction of serum free fatty acid content after glucose load. Carnitine treatment was associated with increased mitochondrial biogenesis and oxidative capacity for fatty acids, amelioration of oxidative stress, and restored substrate switching in the liver. In skeletal muscle (diaphragm), carnitine supplementation was associated with significantly higher palmitate oxidation and a more favorable complete to incomplete oxidation products ratio. Carnitine supplementation further enhanced insulin sensitivity ex vivo. No effects on whole-body glucose tolerance were observed. Our data suggest that some metabolic syndrome-related disorders, particularly fatty acid oxidation, steatosis, and oxidative stress in the liver, could be attenuated by carnitine supplementation. The effect of carnitine could be explained, at least partly, by enhanced substrate oxidation and increased fatty acid transport from tissues in the form of short-chain acylcarnitines.

Impairment of mitochondrial function of rat hepatocytes by high fat diet and oxidative stress

Fatty liver disease associated with obesity is an important medical problem and the mechanisms for lipid accumulation in hepatocytes are not fully elucidated yet. Recent findings indicate that mitochondria play an important role in this process. Our data on hepatocytes in which mitochondria are in contact with other cytosolic structures important for their function, extend observations obtained on isolated mitochondria and confirm inhibition of Complex I activity in hepatocytes isolated from rats fed by high fat diet (HFD) compared with controls fed by standard diet (STD). Furthermore we have found that HFD-hepatocytes are more sensitive to the peroxidative stress because under these conditions also Complex II activity is disturbed. Therefore in HFD animals decrease of Complex I activity cannot be compensated by Complex II substrates as in STD hepatocytes. Our data thus indicates that combination of HFD and peroxidative stress potentiates HFD damaging effect of mitochondria because both branches of the respiratory chain (NADH- and flavoprotein-dependent) are disturbed.

Biguanides inhibit complex I, II and IV of rat liver mitochondria and modify their functional properties

In this study, we focused on an analysis of biguanides effects on mitochondrial enzyme activities, mitochondrial membrane potential and membrane permeability transition pore function. We used phenformin, which is more efficient than metformin, and evaluated its effect on rat liver mitochondria and isolated hepatocytes. In contrast to previously published data, we found that phenformin, after a 5 min pre-incubation, dose-dependently inhibits not only mitochondrial complex I but also complex II and IV activity in isolated mitochondria. The enzymes complexes inhibition is paralleled by the decreased respiratory control index and mitochondrial membrane potential. Direct measurements of mitochondrial swelling revealed that phenformin increases the resistance of the permeability transition pore to Ca(2+) ions. Our data might be in agreement with the hypothesis of Schäfer (1976) that binding of biguanides to membrane phospholipids alters membrane properties in a non-specific manner and, subsequently, different enzyme activities are modified via lipid phase. However, our measurements of anisotropy of fluorescence of hydrophobic membrane probe diphenylhexatriene have not shown a measurable effect of membrane fluidity with the 1 mM concentration of phenformin that strongly inhibited complex I activity. Our data therefore suggest that biguanides could be considered as agents with high efficacy but low specifity.

Developmental changes of the sensitivity of cardiac and liver mitochondrial permeability transition pore to calcium load and oxidative stress

Opening of the mitochondrial membrane permeability transition pore (MPTP) is an important factor in the activation of apoptotic and necrotic processes in mammalian cells. In a previous paper we have shown that cardiac mitochondria from neonatal rats are more resistant to calcium load than mitochondria from adult animals. In this study we have analyzed the ontogenetic development of this parameter both in heart and in liver mitochondria. We found that the high resistance of heart mitochondria decreases from day 14 to adulthood. On the other hand, we did not observe a similar age-dependent sensitivity in liver mitochondria, particularly in the neonatal period. Some significant but relatively smaller increase could be observed only after day 30. When compared with liver mitochondria cardiac mitochondria were more resistant also to the peroxide activating effect on calcium-induced mitochondrial swelling. These data thus indicate that the MPTP of heart mitochondria is better protected against damaging effects of the calcium load and oxidative stress. We can only speculate that the lower sensitivity to calcium-induced swelling may be related to the higher ischemic tolerance of the neonatal heart.

Idebenone-induced recovery of glycerol-3-phosphate and succinate oxidation inhibited by digitonin

Digitonin solubilizes mitochondrial membrane, breaks the integrity of the respiratory chain and releases two mobile redox-active components: coenzyme Q (CoQ) and cytochrome c (cyt c). In the present study we report the inhibition of glycerol-3-phosphate- and succinate-dependent oxygen consumption rates by digitonin treatment. Our results show that the inhibition of oxygen consumption rates is recovered by the addition of exogenous synthetic analog of CoQ idebenone (hydroxydecyl-ubiquinone; IDB) and cyt c. Glycerol-3-phosphate oxidation rate is recovered to 148 % of control values, whereas succinate-dependent oxidation rate only to 68 %. We find a similar effect on the activities of glycerol-3-phosphate and succinate cytochrome c oxidoreductase. Our results also indicate that succinate-dependent oxidation is less sensitive to digitonin treatment and less activated by IDB in comparison with glycerol-3-phosphate-dependent oxidation. These findings might indicate the different mechanism of the electron transfer from two flavoprotein-dependent dehydrogenases (glycerol-3-phosphate dehydrogenase and succinate dehydrogenase) localized on the outer and inner face of the inner mitochondrial membrane, respectively.

Inhibitory effect of metformin on oxidation of NADH-dependent substrates in rat liver homogenate

Metformin is widely used in the treatment of Type 2 diabetes, however, mechanisms of its antihyperglycemic effect were not yet fully elucidated. Complex I of mitochondrial respiration chain is considered as one of the possible targets of metformin action. In this paper, we present data indicating that the inhibitory effect of metformin can be tested also in liver homogenate. Contrary to previous findings on hepatocytes or mitochondria under our experimental conditions, lower metformin concentrations and shorter time of preincubation give significant inhibitory effects. These conditions enable to study the mechanism of the inhibitory effect of metformin in small samples of biological material (50-100 mg wet weight) and compare more experimental groups of animals because isolation of mitochonria is unnecessary.

Knockdown of F1 epsilon subunit decreases mitochondrial content of ATP synthase and leads to accumulation of subunit c

The subunit epsilon of mitochondrial ATP synthase is the only F1 subunit without a homolog in bacteria and chloroplasts and represents the least characterized F1 subunit of the mammalian enzyme. Silencing of the ATP5E gene in HEK293 cells resulted in downregulation of the activity and content of the mitochondrial ATP synthase complex and of ADP-stimulated respiration to approximately 40% of the control. The decreased content of the epsilon subunit was paralleled by a decrease in the F1 subunits alpha and beta and in the Fo subunits a and d while the content of the subunit c was not affected. The subunit c was present in the full-size ATP synthase complex and in subcomplexes of 200-400 kDa that neither contained the F1 subunits, nor the Fo subunits. The results indicate that the epsilon subunit is essential for the assembly of F1 and plays an important role in the incorporation of the hydrophobic subunit c into the F1-c oligomer rotor of the mitochondrial ATP synthase complex.

Succinimidyl oleate, established inhibitor of CD36/FAT translocase inhibits complex III of mitochondrial respiratory chain

The functional role of CD36 protein detected in mitochondrial fractions in long chain fatty acid (LCFA) oxidation is unclear due to conflicting results obtained in Cd36 knockout mice and experiments using sulfo-N-succinimidyl oleate (SSO) for inhibition of CD36 mediated LCFA transport. We investigated effect of SSO on mitochondrial respiration and found that SSO substantially inhibits not only LCFA oxidation, but also oxidation of flavoprotein- and NADH-dependent substrates and generation of mitochondrial membrane potential. Experiments in rat liver, heart and kidney mitochondria demonstrated a direct effect on mitochondrial respiratory chain with the most pronounced inhibition of the complex III (IC(50) 4microM SSO). The results presented here show that SSO is a potent and irreversible inhibitor of mitochondrial respiratory chain.

Tissue specific sensitivity of mitochondrial permeability transition pore to Ca2+ ions

Ca(2+)-induced opening of the mitochondrial permeability transition pore (MPTP) is involved in induction of apoptotic and necrotic processes. We studied sensitivity of MPTP to calcium using the model of Ca(2+)-induced, cyclosporine A-sensitive mitochondrial swelling. Presented data indicate that the extent of mitochondrial swelling (dA520/4 min) induced by addition of 25 microM Ca2+ is seven-fold higher in liver than in heart mitochondria (0.564 +/- 0.08/0.077 +/- 0.01). The extent of swelling induced by 100 microM Ca2+ was in liver tree times higher than in heart mitochondria (0.508 +/- 0.05/ 0.173 +/- 0.02). Cyclosporine A sensitivity showed that opening of the MPTP is involved. We may thus conclude that especially at low Ca2+ concentration heart mitochondria are more resistant to damaging effect of Ca2+ than liver mitochondria. These finding thus support hypothesis that there exist tissue specific strategies of cell protection against induction of the apoptotic and necrotic processes.

Peroxidative damage of mitochondrial respiration is substrate-dependent

The concentration-dependence of tert-butyl hydroperoxide (BHP) inhibitory effect on oxygen consumption in isolated rat liver mitochondria was measured in the presence of various respiratory substrates. Strong inhibitory effect at low concentrations of BHP (15-30 microM) was found for oxoglutarate and palmitoyl carnitine oxidation. Pyruvate and glutamate oxidation was inhibited at higher concentrations of BHP (100-200 microM). Succinate oxidation was not affected even at 3.3 mM BHP. Determination of mitochondrial membrane potential has shown that in the presence of NADH-dependent substrates the membrane potential was dissipated by BHP but was completely restored after addition of succinate. Our data thus indicate that beside peroxidative damage of complex I also various mitochondrial NADH-dependent dehydrogenases are inhibited, but to a different extent and with different kinetics. Our data also show that succinate could be an important nutritional substrate protecting hepatocytes during peroxidative damage.

TMEM70 mutations cause isolated ATP synthase deficiency and neonatal mitochondrial encephalocardiomyopathy

We carried out whole-genome homozygosity mapping, gene expression analysis and DNA sequencing in individuals with isolated mitochondrial ATP synthase deficiency and identified disease-causing mutations in TMEM70. Complementation of the cell lines of these individuals with wild-type TMEM70 restored biogenesis and metabolic function of the enzyme complex. Our results show that TMEM70 is involved in mitochondrial ATP synthase biogenesis in higher eukaryotes.

High efficiency of ROS production by glycerophosphate dehydrogenase in mammalian mitochondria

We investigated hydrogen peroxide production in mitochondria with low (liver, heart, brain) and high (brown adipose tissue, BAT) content of glycerophosphate dehydrogenase (mGPDH). ROS production at state 4 due to electron backflow from mGPDH was low, but after inhibition of electron transport with antimycin A high rates of mGPDH-dependent ROS production were observed in liver, heart and brain mitochondria. When this ROS production was related to activity of mGPDH, many-fold higher ROS production was found in contrast to succinate- (39-, 28-, 3-fold) or pyruvate plus malate-dependent ROS production (32-, 96-, 5-fold). This specific rate of mGPDH-dependent ROS production was also exceedingly higher (28-, 66-, 22-fold) compared to that in BAT. mGPDH-dependent ROS production was localized to the dehydrogenase+CoQ and complex III, the latter being the highest in all mitochondria but BAT. Our results demonstrate high efficiency of mGPDH-dependent ROS production in mammalian mitochondria with a low content of mGPDH and suggest its endogenous inhibition in BAT.

Induction of muscle thermogenesis by high-fat diet in mice: association with obesity-resistance

The obesogenic effect of a high-fat (HF) diet is counterbalanced by stimulation of energy expenditure and lipid oxidation in response to a meal. The aim of this study was to reveal whether muscle nonshivering thermogenesis could be stimulated by a HF diet, especially in obesity-resistant A/J compared with obesity-prone C57BL/6J (B/6J) mice. Experiments were performed on male mice born and maintained at 30 degrees C. Four-week-old mice were randomly weaned onto a low-fat (LF) or HF diet for 2 wk. In the A/J LF mice, cold exposure (4 degrees C) resulted in hypothermia, whereas the A/J HF, B/6J LF, and B/6J HF mice were cold tolerant. Cold sensitivity of the A/J LF mice was associated with a relatively low whole body energy expenditure under resting conditions, which was normalized by the HF diet. In both strains, the HF diet induced uncoupling protein-1-mediated thermogenesis, with a stronger induction in A/J mice. Only in A/J mice: 1) the HF diet augmented activation of whole body lipid oxidation by cold; and 2) at 30 degrees C, oxygen consumption, total content, and phosphorylation of AMP-activated protein kinase (AMPK), and AICAR-stimulated palmitate oxidation in soleus muscle was increased by the HF diet in parallel with significantly increased leptinemia. Gene expression data in soleus muscle of the A/J HF mice indicated a shift from carbohydrate to fatty acid oxidation. Our results suggest a role for muscle nonshivering thermogenesis and lipid oxidation in the obesity-resistant phenotype of A/J mice and indicate that a HF diet could induce thermogenesis in oxidative muscle, possibly via the leptin-AMPK axis.

Evaluation of mitochondrial function in isolated rat hepatocytes and mitochondria during oxidative stress

The majority of toxic agents act either fully or partially via oxidative stress, the liver, specifically the mitochondria in hepatocytes, being the main target. Maintenance of mitochondrial function is essential for the survival and normal performance of hepatocytes, which have a high energy requirement. Therefore, greater understanding of the role of mitochondria in hepatocytes is of fundamental importance. Mitochondrial function can be analysed in several basic models: hepatocytes cultured in vitro; mitochondria in permeabilised hepatocytes; and isolated mitochondria. The aim of our study was to use all of these approaches to evaluate changes in mitochondria exposed in vitro to a potent non-specific peroxidating agent, tert-butylhydroperoxide (tBHP), which is known to induce oxidative stress. A decrease in the mitochondrial membrane potential (MMP) was observed in cultured hepatocytes treated with tBHP, as illustrated by a significant reduction in Rhodamine 123 accumulation and by a decrease in the fluorescence of the JC-1 molecular probe. Respiratory Complex I in the mitochondria of permeabilised hepatocytes showed high sensitivity to tBHP, as documented by high-resolution respirometry. This could be caused by the oxidation of NADH and NADPH by tBHP, followed by the disruption of mitochondrial calcium homeostasis, leading to the collapse of the MMP. A substantial decrease in the MMP, as determined by tetraphenylphosphonium ion-selective electrode measurements, also confirmed the dramatic impact of tBHP-induced oxidative stress on mitochondria. Swelling was observed in isolated mitochondria exposed to tBHP, which could be prevented by cyclosporin A, which is evidence for the role of mitochondrial permeability transition. Our results demonstrate that all of the above-mentioned models can be used for toxicity assessment, and the data obtained are complementary.

Direct linkage of mitochondrial genome variation to risk factors for type 2 diabetes in conplastic strains

Recently, the relationship of mitochondrial DNA (mtDNA) variants to metabolic risk factors for diabetes and other common diseases has begun to attract increasing attention. However, progress in this area has been limited because (1) the phenotypic effects of variation in the mitochondrial genome are difficult to isolate owing to confounding variation in the nuclear genome, imprinting phenomena, and environmental factors; and (2) few animal models have been available for directly investigating the effects of mtDNA variants on complex metabolic phenotypes in vivo. Substitution of different mitochondrial genomes on the same nuclear genetic background in conplastic strains provides a way to unambiguously isolate effects of the mitochondrial genome on complex traits. Here we show that conplastic strains of rats with identical nuclear genomes but divergent mitochondrial genomes that encode amino acid differences in proteins of oxidative phosphorylation exhibit differences in major metabolic risk factors for type 2 diabetes. These results (1) provide the first direct evidence linking naturally occurring variation in the mitochondrial genome, independent of variation in the nuclear genome and other confounding factors, to inherited variation in known risk factors for type 2 diabetes; and (2) establish that spontaneous variation in the mitochondrial genome per se can promote systemic metabolic disturbances relevant to the pathogenesis of common diseases.

Respiratory chain components involved in the glycerophosphate dehydrogenase-dependent ROS production by brown adipose tissue mitochondria

Involvement of mammalian mitochondrial glycerophosphate dehydrogenase (mGPDH, EC 1.1.99.5) in reactive oxygen species (ROS) generation was studied in brown adipose tissue mitochondria by different spectroscopic techniques. Spectrofluorometry using ROS-sensitive probes CM-H2DCFDA and Amplex Red was used to determine the glycerophosphate- or succinate-dependent ROS production in mitochondria supplemented with respiratory chain inhibitors antimycin A and myxothiazol. In case of glycerophosphate oxidation, most of the ROS originated directly from mGPDH and coenzyme Q while complex III was a typical site of ROS production in succinate oxidation. Glycerophosphate-dependent ROS production monitored by KCN-insensitive oxygen consumption was highly activated by one-electron acceptor ferricyanide, whereas succinate-dependent ROS production was unaffected. In addition, superoxide anion radical was detected as a mGPDH-related primary ROS species by fluorescent probe dihydroethidium, as well as by electron paramagnetic resonance (EPR) spectroscopy with DMPO spin trap. Altogether, the data obtained demonstrate pronounced differences in the mechanism of ROS production originating from oxidation of glycerophosphate and succinate indicating that electron transfer from mGPDH to coenzyme Q is highly prone to electron leak and superoxide generation.

Inhibition of palmityl carnitine oxidation in rat liver mitochondria by tert-butyl hydroperoxide

Mitochondria as an energy generating cell device are very sensitive to oxidative damage. Our previous findings obtained in hepatocytes demonstrated that Complex I of the respiratory chain is more sensitive to oxidative damage than other respiratory chain complexes. We present additional data on isolated mitochondria showing that palmityl carnitine oxidation is strongly depressed at a low (200 microM) tert-butyl hydroperoxide (tBHP) concentration, while oxidation of the flavoprotein-dependent substrate-succinate is not affected and neither is ATP synthesis inhibited by tBHP. In the presence of tBHP, the respiratory control index for palmityl carnitine oxidation is strongly depressed, but when succinate is oxidized the respiratory control index remains unaffected. Our findings thus indicate that flavoprotein-dependent substrates could be an important nutritional factor for the regeneration process in the necrotic liver damaged by oxidative stress.

Inhibitory effect of t-butyl hydroperoxide on mitochondrial oxidative phosphorylation in isolated rat hepatocytes

Using high-resolution oxygraphy, we tested the changes of various parameters characterizing the mitochondrial energy provision system that were induced by peroxidative damage. In the presence of succinate as respiratory substrate, 3 mM t-butyl hydroperoxide increased respiration in the absence of ADP, which indicated partial uncoupling of oxidative phosphorylation. Low activity of coupled respiration was still maintained as indicated by the ADP-activated and oligomycin-inhibited respiration. However, during the incubation the phosphorylative capacity decreased as indicated by the continuous decrease of the mitochondrial membrane potential. Under these experimental conditions the maximum capacity of the succinate oxidase system was inhibited by 50% in comparison with values obtained in the absence of t-butyl hydroperoxide. Our data thus indicate that the oxygraphic evaluation of mitochondrial function represents a useful tool for evaluation of changes participating in peroxidative damage of cell energy metabolism.

Occurrence of A-kinase anchor protein and associated cAMP-dependent protein kinase in the inner compartment of mammalian mitochondria

Evidence showing the existence in the inner compartment of rat-heart mitochondria of AKAP121 and associated PKA is presented. Immunoblotting analysis and trypsin digestion pattern show that 90% or more of mitochondrial C-PKA, R-PKA and AKAP121 is localized in the inner mitochondrial compartment, when prepared both from isolated mitochondria or cardiomyocyte cultures. This localization is verified by measurement of the specific catalytic activity of PKA, radiolabelling of R-PKA by (32)P-phosphorylated C-PKA and of AKAP by (32)P-phosphorylated R-PKA and electron microscopy of mitochondria exposed to gold-conjugated AKAP121 antibody.

Tetraphenylphosphonium-selective electrode as a tool for evaluating mitochondrial permeability transition pore function in isolated rat hepatocytes

The changes in mitochondrial membrane potential (Deltapsi(m)) were used as an indicator for evaluating the mitochondrial permeability transition pore (MPTP) function. We found that in situ mitochondria in digitonin-permeabilized hepatocytes were coupled and responded to the addition of substrates, inhibitors and uncouplers. Ca(2+)-induced Deltapsi(m) dissipation was caused by MPTP opening because this process was inhibited by cyclosporin A. MPTP opening was enhanced by the pro-oxidant tert-butyl hydroperoxide.

Evaluation of mitochondrial membrane potential using a computerized device with a tetraphenylphosphonium-selective electrode

Mitochondrial membrane potential (Deltapsi(m)) plays important roles in the normal function of cells and in pathobiochemical situations. The application of ion-selective electrodes for the measurement of Deltapsi(m) is important for studying normal biological reactions and pathways and mitochondrial diseases. We constructed and optimized a computerized device for real-time monitoring of the Deltapsi(m), which included modification of tetraphenylphosphonium (TPP(+))-selective membrane that improved reproducibility of the TPP(+)-selective electrode. Application of MATLAB software increased the sensitivity of the system. We tested our improved device for membrane potential measurements of isolated mitochondria (in absolute scale of millivolts). In addition, we assessed relative changes of Deltapsi(m) (as changes in TPP(+) concentration) of digitonin-permeabilized cells (hepatocytes, control transmitochondrial cybrids, HeLa G and BSC-40) after addition of substrates, inhibitors, and uncoupler of respiratory chain. Our system can be successfully used for studies of many aspects of the regulation of mitochondrial bioenergetics and as a diagnostic tool for mitochondrial oxidative phosphorylation disorders.

Specific Properties of Heavy Fraction of Mitochondria from Human-term Placenta – Glycerophosphate-dependent Hydrogen Peroxide Production

Mitochondrial respiratory chain enzyme Complexes are present in placenta at proportion similar to other tissues with exception of glycerophosphate dehydrogenase (mGPDH) which is expressed at a very high rate. As shown by Western blot quantification and respiratory chain enzyme activity measurements, the specific content of mGPDH is similar to that of succinate dehydrogenase or NADH dehydrogenase. Using fluorometric probe dichlorodihydrofluorescein diacetate we found that placental mitochondria display high rate of glycerophosphate-dependent hydrogen peroxide production. This was confirmed by oxygraphic detection of glycerophosphate-induced, KCN- or antimycin A-insensitive oxygen uptake. Hydrogen peroxide production by mGPDH was highly activated by one-electron acceptor, potassium ferricyanide and it was depressed by inhibitors of mGPDH and by cytochrome c. Our results indicate that mGPDH should be considered as an additional source of reactive oxygen species participating in induction of oxidative stress in placenta.

Inhibition of glycerophosphate-dependent H2O2 generation in brown fat mitochondria by idebenone

The established protective effect of coenzyme Q (CoQ) analogs is dependent on the location of reactive oxygen species (ROS) generation. One of these analogs--idebenone (hydroxydecyl-ubiquinone) is used as an antioxidative therapeutic drug. We tested its scavenging effect on the glycerophosphate (GP)-dependent ROS production as this enzyme was shown as a new site in the mitochondrial respiratory chain where ROS can be generated. We observed that idebenone inhibits both GP- and succinate-dependent ROS production. Idebenone and CoQ1 were found to be more efficient in the scavenging activity (IC50: 0.052 and 0.075 microM, respectively) than CoQ3 (IC50: 45.8 microM). Idebenone also inhibited ferricyanide (FeCN)-activated, GP-dependent ROS production. Our data thus extend previous findings on the scavenging effect of idebenone and show that it can also eliminate GP-dependent ROS generation.

Time-course of hormonal induction of mitochondrial glycerophosphate dehydrogenase biogenesis in rat liver

Thyroid hormones are important regulators of mitochondrial metabolism. Due to their complex mechanism of action, the timescale of different responses varies from minutes to days. In this work, we studied selective T3 induction of the inner mitochondrial membrane enzyme-glycerophosphate dehydrogenase (mGPDH) in liver of euthyroid rats. We correlated the kinetics of the T3 level in blood, the mRNA level in liver, the activity and amount of mGPDH in liver mitochondria after a single dose of T3. The T3 level reached maximum after 1 h (80 nmol/l) and subsequently rapidly decreased. mGPDH mRNA increased also relatively fast, reaching a maximum after 12 h and fell to the control level after 72 h. An increase of mGPDH activity could be already found after 6 h and reached a maximum after 24 h in accordance with the increase in mGPDH content (2.4-fold vs. 2.7-fold induction). After 72 h, the mGPDH activity showed a significant 30% decrease. When the rats received three subsequent doses of T3, the increase of mGPDH activity was 2-fold higher than after a single T3 dose. The results demonstrate that mGPDH displays rapid induction as well as decay upon disappearance of a hormonal stimulus, indicating a rather short half-life of this inner mitochondrial membrane enzyme.

Polyunsaturated fatty acids of marine origin upregulate mitochondrial biogenesis and induce beta-oxidation in white fat

AIMS/HYPOTHESIS

Intake of n-3 polyunsaturated fatty acids reduces adipose tissue mass, preferentially in the abdomen. The more pronounced effect of marine-derived eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids on adiposity, compared with their precursor alpha-linolenic acid, may be mediated by changes in gene expression and metabolism in white fat.

METHODS

The effects of EPA/DHA concentrate (6% EPA, 51% DHA) admixed to form two types of high-fat diet were studied in C57BL/6J mice. Oligonucleotide microarrays, cDNA PCR subtraction and quantitative real-time RT-PCR were used to characterise gene expression. Mitochondrial proteins were quantified using immunoblots. Fatty acid oxidation and synthesis were measured in adipose tissue fragments.

RESULTS

Expression screens revealed upregulation of genes for mitochondrial proteins, predominantly in epididymal fat when EPA/DHA concentrate was admixed to a semisynthetic high-fat diet rich in alpha-linolenic acid. This was associated with a three-fold stimulation of the expression of genes encoding regulatory factors for mitochondrial biogenesis and oxidative metabolism (peroxisome proliferator-activated receptor gamma coactivator 1 alpha [Ppargc1a, also known as Pgc1alpha] and nuclear respiratory factor-1 [Nrf1] respectively). Expression of genes for carnitine palmitoyltransferase 1A and fatty acid oxidation was increased in epididymal but not subcutaneous fat. In the former depot, lipogenesis was depressed. Similar changes in adipose gene expression were detected after replacement of as little as 15% of lipids in the composite high-fat diet with EPA/DHA concentrate, while the development of obesity was reduced. The expression of Ppargc1a and Nrf1 was also stimulated by n-3 polyunsaturated fatty acids in 3T3-L1 cells.

CONCLUSIONS/INTERPRETATION

The anti-adipogenic effect of EPA/DHA may involve a metabolic switch in adipocytes that includes enhancement of beta-oxidation and upregulation of mitochondrial biogenesis.

Development of Cytochrome-c Oxidase Activity in Rat Heart: Downregulation in Newborn Rats

Cytochrome-c oxidase (COX) activity of the rat heart was two- to sevenfold activated when the membrane integrity was disrupted by hypotonic swelling, freezing-thawing, or a detergent, indicating that a large portion of COX capacity in intact mitochondria is not active. The effect of detergent was tested in heart mitochondria isolated from 1-, 5-, 15-, 29-, and 60-d-old rats; activation by detergent was up to 20-fold in 1-d-old animals, whereas in mitochondria from 60-d-old rats it was only 7-fold. Our data indicate that the rat heart exhibits significant developmental changes dependent on downregulation of COX activity in the neonatal period.

Tert-butyl hydroperoxide selectively inhibits mitochondrial respiratory-chain enzymes in isolated rat hepatocytes

Sensitivity of various mitochondrial enzymes to oxidative damage was tested on isolated rat liver hepatocytes permeabilized by digitonin. In permeabilized hepatocytes normal respiratory control values were obtained and mitochondrial membranes remained intact. Respiratory rates of NADH-dependent (glutamate + malate, palmitylcarnitine + malate) and flavoprotein-dependent (succinate) substrates were determined in hepatocytes exposed for 5 min to 0.5-3 mM tert-butyl hydroperoxide before addition of digitonin. Our data showed that oxidation of NADH-dependent substrates is much more sensitive to oxidative stress than oxidation of flavoprotein-dependent ones, evidently due to the modification of iron-sulfur clusters or SH groups in the NADH dehydrogenase enzyme complex (Complex I).

Developmental changes of cytochrome c oxidase and citrate synthase in rat heart homogenate

Activity of cytochrome c oxidase and citrate synthase in rat heart homogenates was determined in 5-, 15-, 28- and 60-day-old rats. The activity of both enzymes increased during postnatal development but their changes followed different kinetics. The membrane-bound cytochrome c oxidase reached its adult values during the early postnatal period, i.e. between days 5 and 15, whereas soluble matrix-localized citrate synthase also continued to increase between days 15 and 60. Our data indicate a relative excess of cytochrome c oxidase in neonatal cardiocytes.

Glycerophosphate-dependent hydrogen peroxide production by rat liver mitochondria

We studied the extent to which hormonally-induced mitochondrial glycerophosphate dehydrogenase (mGPDH) activity contributes to the supply of reducing equivalents to the mitochondrial respiratory chain in the rat liver. The activity of glycerophosphate oxidase was compared with those of NADH oxidase and/or succinate oxidase. It was found that triiodothyronine-activated mGPDH represents almost the same capacity for the saturation of the respiratory chain as Complex II. Furthermore, the increase of mGPDH activity induced by triiodothyronine correlated with an increase of capacity for glycerophosphate-dependent hydrogen peroxide production. As a result of hormonal treatment, a 3-fold increase in glycerophosphate-dependent hydrogen peroxide production by liver mitochondria was detected by polarographic and luminometric measurements.

Developmental Changes of Cytochrome c Oxidase and Citrate Synthase in Rat Heart Homogenate

Activity of cytochrome c oxidase and citrate synthase in rat heart homogenates was determined in 5-, 15-, 28- and 60-day-old rats. The activity of both enzymes increased during postnatal development but their changes followed different kinetics. The membrane-bound cytochrome c oxidase reached its adult values during the early postnatal period, i.e. between days 5 and 15, whereas soluble matrix-localized citrate synthase also continued to increase between days 15 and 60. Our data indicate a relative excess of cytochrome c oxidase in neonatal cardiocytes.

Glycerophosphate-dependent hydrogen peroxide production by rat liver mitochondria

We studied the extent to which hormonally-induced mitochondrial glycerophosphate dehydrogenase (mGPDH) activity contributes to the supply of reducing equivalents to the mitochondrial respiratory chain in the rat liver. The activity of glycerophosphate oxidase was compared with those of NADH oxidase and/or succinate oxidase. It was found that triiodothyronine-activated mGPDH represents almost the same capacity for the saturation of the respiratory chain as Complex II. Furthermore, the increase of mGPDH activity induced by triiodothyronine correlated with an increase of capacity for glycerophosphate-dependent hydrogen peroxide production. As a result of hormonal treatment, a 3-fold increase in glycerophosphate-dependent hydrogen peroxide production by liver mitochondria was detected by polarographic and luminometric measurements.