Brown-adipose-tissue mitochondria: photoaffinity labelling of the regulatory site of energy dissipation

Atypical beta-adrenoceptor on brown adipocytes as target for anti-obesity drugs

Recent studies suggest that thermogenesis in brown adipose tissue has an important role in the regulation of energy balance. Thermogenesis is effected by noradrenaline released from sympathetic nerve endings; the noradrenaline stimulates beta-adrenoceptors, causing lipolysis, and the released fatty acids then promote the uncoupling of oxidative phosphorylation from electron transport. It has been widely accepted that mammalian beta-adrenoceptors exist as two subtypes, beta 1 and beta 2, and rat brown adipocyte beta-adrenoceptors have been classed as beta 1 or as a mixed beta 1/beta 2 population. The beta 1 subtype predominates in atria, whereas the beta 2 subtype predominates in trachea. However, we have now found a novel group of beta-adrenoceptor agonists that selectively stimulate lipolysis in brown adipocytes. In contrast, isoprenaline, fenoterol and salbutamol are less potent as stimulants of lipolysis than as stimulants of atrial rate or tracheal relaxation. Therefore, beta-adrenoceptors in rat brown adipocytes are of neither the beta 1 nor beta 2 subtypes. Compounds that selectively stimulate brown adipocyte beta-adrenoceptors should have potential as thermogenic anti-obesity agents and this has been demonstrated with BRL 26830A , BRL 33725A and BRL 35135A .

Molecular mechanism of uncoupling in brown adipose tissue mitochondria. The non-identity of proton and chloride conducting pathways

Specific permeability properties of the inner membrane of brown adipose tissue mitochondria were analysed with the aid of simultaneous pH measurements outside mitochondria and of mitochondria swelling. It was shown that valinomycin-induced potassium diffusion potential drives a parallel passive uptake of chlorides and extrusion of proton. Electrogenic H+ -extrusion was independent on anion transport, no competition was found between the two processes and the former process exerted a lower sensitivity to the inhibitory effect of GDP. The existence of two distinct, independent pathways for translocation of protons and halide anions across the membrane is suggested.

Mitochondrial membrane potential monitored in situ within isolated guinea pig brown adipocytes by a styryl pyridinium fluorescent indicator

2-( Dimethylaminostyryl )-l-methyl pyridinium, a fluorescent indicator of mitochondrial membrane potential, is accumulated up to 400-fold by intact, isolated brown adipocytes from cold-adapted guinea pigs. The indicator can be optically localized within the in situ mitochondria. Ionophores which would be predicted to lower or increase the in situ mitochondrial membrane potential lead to decreased and increased uptake, respectively, of the radio-labelled indicator. Noradrenaline induces a 15 mV decrease in the sum of mitochondrial and plasma membrane potentials. The results indicate that the mitochondria within isolated brown adipocytes maintain a substantial membrane potential in the resting state, rather than being artifactually depolarized.

Brown adipose tissue in the parametrial fat pad of the mouse

Cold acclimation has been shown to produce a substantial increase in the number of brown adipocytes in the parametrial fat pad of female BALB/c mice-a site normally thought to consist of typical white adipocytes. The brown adipocytes have been identified not only on the basis of their morphology using light and electron microscopy, but also on the basis of the content of the mitochondrial 'uncoupling protein' (Mr = 32000) which is characteristic of the proton conductance pathway of brown adipose tissue.

Modulation of beta-oxidation and proton conductance pathway of brown adipose tissue in hypo- and hyperinsulinemic states

The metabolic capacity of interscapular brown adipose tissue of hypoinsulinemic (diabetic) rats is decreased and a reduced beta-oxidative capacity contributes to this metabolic alteration. It was thus of interest to compare, in diabetic and in chronically (8 days) insulin-infused rats, the beta-oxidative capacity and indices of the thermogenic state (GDP-binding and 32 000 Mr protein) in this tissue. Mitochondrial GDP-binding and 32 000 Mr protein were both decreased in diabetic rats compared to appropriate controls and markedly increased as was also the beta-oxidative capacity in hyperinsulinemic rats.

Enzyme-linked immunosorbent assay (ELISA) studies of the interaction between mammalian and avian anti-thermogenin antibodies and brown-adipose-tissue mitochondria from different species

Three different antibody preparations, rabbit anti-hamster and rabbit anti-rat thermogenin sera and chicken anti-rat thermogenin IgG, were tested for cross-reactivity towards isolated thermogenin and BAT mitochondria from different mammalian species using an ELISA-technique. It was found that the antibody preparations readily cross-reacted with different species, but that the affinity of the antibody preparations was greater towards the homologous species than the other species. The reactivity of an antibody preparation towards mitochondria from different tissues from the homologous species was also tested, and the exclusive occurrence of thermogenin in BAT could be confirmed.

The possible proton translocating activity of the mitochondrial uncoupling protein of brown adipose tissue. Reconstitution studies in liposomes

Loose coupling of thermogenic mitochondria of brown adipose tissue is related to a high proton (or hydroxyl) conductance of the inner membrane and to the presence of a unique 32 kDa uncoupling protein. Reconstitution experiments of the purified protein in liposomes are reported which suggest that this component could form proton channels in the membrane.

Brown-adipose-tissue mitochondria: the regulation of the 32000-Mr uncoupling protein by fatty acids and purine nucleotides

The increased proton permeability induced by the addition of a synthetic proton translocator to non-respiring hamster brown-fat mitochondria is unaffected by purine nucleotide addition. In contrast the permeability induced by fatty acids is inhibited by nucleotide, indicating that fatty acids act at the 32000-Mr uncoupling protein. Fatty acids lower the affinity of nucleotide binding to the 32000-Mr protein, but not sufficiently to explain their uncoupling action. The sensitivity of the fatty acid modulation of permeability is dependent on chain length, extent of unsaturation and pH. There is a requirement for an unesterified carboxyl group. In respiring mitochondria fatty acids act in the presence of nucleotide by lowering the 'break-point' potential at which the conductance of the 32000-Mr protein increases. Fatty acids have no effect on the chloride uniport activity of the 32000-Mr protein, but decouple the interference between chloride and protons when the simultaneous transport of both ions is attempted.

Adaptive changes in the concentration of the mitochondrial 'uncoupling' protein in brown adipose tissue of hamsters acclimated at different temperatures

The effect of acclimation at different temperatures on the activity of interscapular brown adipose tissue has been investigated in the hamster, a hibernator. Between 31 degrees and 4 degrees C the cytochrome oxidase activity of the tissue increased 4- to 5-fold, mitochondrial GDP binding per mg of mitochondrial protein doubled, and the amount of uncoupling protein rose from 1.7% to 5.4% of total mitochondrial protein. It is concluded that there are clear adaptive changes induced by temperature in brown adipose tissue of the hamster, but the changes are limited in comparison with those in the mouse.

Uncoupling protein in human brown adipose tissue mitochondria. Isolation and detection by specific antiserum

A protein of Mr 32 000 has been isolated from human infant brown adipose tissue mitochondria following the procedure used to purify the uncoupling protein from rat brown adipose tissue mitochondria. A specific antiserum has been raised against the human 32 kDa protein, and used to detect it by probing mitochondrial proteins separated by SDS-PAGE. The protein is present in large amounts in brown adipose tissue but is undetectable in human liver, heart or white adipose tissue. It has strong immunological cross-reactivity with rat brown adipose tissue uncoupling protein.

The regulation of the proton conductance of brown fat mitochondria. Identification of functional and non-functional nucleotide-binding sites

The binding of purine nucleotides to intact brown fat mitochondria is re-examined. In addition to the previously reported high affinity binding site, a low-affinity site is found, which requires several minutes to saturate. Only the high affinity site is functional in regulating the proton and halide permeabilities of the mitochondria. The low affinity site can introduce errors in the use of nucleotide binding to quantitate the Mr 32000 uncoupling protein unique to these mitochondria.

Effect of acclimation temperature on the concentration of the mitochondrial 'uncoupling' protein measured by radioimmunoassay in mouse brown adipose tissue

The effect of acclimation temperature on the concentration of the mitochondrial 'uncoupling' protein (Mr 32000) from brown adipose tissue of mice has been investigated. The uncoupling protein was measured by a specific radioimmunoassay. Between 33 degrees C (thermoneutrality) and -2 degrees C there was a progressive increase with decreasing environmental temperature in the amount of uncoupling protein. For mice at -2 degrees C the mitochondrial concentration of the protein was 9-times higher than at 33 degrees C, while the total amount of the protein in interscapular brown adipose tissue was estimated to be nearly 80-times greater at -2 degrees C compared to 33 degrees C.

Mitochondria of adult human brown adipose tissue contain a 32 000-Mr uncoupling protein

Using both immunohistological study and photoaffinity labelling with radioactive azido-ATP, evidence is presented that the mitochondrial membranes of the brown adipose tissue of the human adult contain a 32 000-Mr uncoupling protein, which is probably similar to the uncoupling protein of BAT of rodents.

The influence of GDP on Ca2+ uptake by mitochondria of brown adipose tissue from lean and genetically obese (ob/ob) mice

The specific binding capacity for purine nucleotides in brown-adipose-tissue mitochondria is thought to indicate the capacity of the proton-conductance pathway which leads to uncoupled respiration. This functional relationship was investigated in studies measuring initial Ca2+-uptake rates and membrane potential in the presence or absence of GDP in brown-adipose-tissue mitochondria with different GDP-binding capacities. The mitochondria from pre-obese and obese ob/ob mice were less able than those from lean control mice to dissipate membrane potential in the absence of GDP. Mitochondria from the obese animals also maintained a higher Ca2+-uptake rate without GDP in comparison with the rate found with mitochondria from the lean mice. The GDP-dependence of Ca2+ uptake was greater in brown-adipose-tissue mitochondria from cold-adapted animals than in those from animals kept at 22 degrees C or at thermoneutrality (33 degrees C). It is concluded that Ca2+-uptake rate and membrane-potential values are depressed in the absence of GDP and indicate indirectly the influence of purine nucleotides on maintaining the proton electrochemical gradient in brown-adipose-tissue mitochondria. It is also apparent that the lower GDP-binding capacity in mitochondria from ob/ob mice is related to a decreased ability to dissipate the proton electrochemical gradient.

Identification of two mitochondrial GDP-binding sites in rat brown adipose tissue

Scatchard analysis of specific guanosine-diphosphate-([3H]GDP-) binding to rat brown-adipose-tissue mitochondria revealed two distinct binding sites with apparent dissociation constants (Kd) of approximately 0.05 and 2.0 microM. Binding to both sites was insensitive to atractyloside. Reducing the pH of the binding medium from 7.1 to 6.6 caused marked reductions in the Kd of both sites, but at pH 7.6, the dissociation constants were increased about 3-fold. Acute treatment of rats with noradrenaline, 1 h before sacrifice, increased the maximum number of binding sites (Bmax, pmol/mg mitochondrial protein) of both sites and also increased the dissociation constants. The Bmax of the lower-affinity site was elevated in rats exposed to 5 degrees C or fed a palatable cafeteria diet for 10 d, compared to control animals, with the greater changes occurring in the cold-adapted group. The high-affinity site was unaltered by cold adaptation or cafeteria feeding. These results indicate the presence of two distinct nucleotide-binding sites in brown-fat mitochondria, both of which may be involved in thermogenesis.

Measurement of rat brown-adipose-tissue mitochondrial uncoupling protein by radioimmunoassay: increased concentration after cold acclimation

A specific antiserum has been raised against the 32 000-mol.wt. uncoupling protein from the mitochondria of rat brown adipose tissue and a sensitive radioimmunoassay for the protein has been developed. The uncoupling protein is present in large amounts in brown adipose tissue; its concentration is increased substantially by cold acclimation. The protein has not been detected in the liver, heart, or parametrial white adipose tissue of rats.

The effect of adrenalectomy on GDP binding to brown-adipose-tissue mitochondria of obese rat

GDP binding to brown-adipose-tissue mitochondria of young obese Zucker rats (fa/fa) was significantly lower than in lean control rats, as a result of a decrease in the number of binding sites. Adrenalectomy of fa/fa rats restored GDP binding to control values. Corticosterone replacement suppressed GDP binding in adrenalectomized obese rats.

The acute regulation of mitochondrial proton conductance in cells and mitochondria from the brown fat of cold-adapted and warm-adapted guinea pigs

Cells and mitochondria were prepared from the brown adipose tissue of adult guinea-pigs adapted to either 4-7 degrees C or 22-25 degrees C. The cold-adapted cells displayed noradrenaline-stimulated, propranolol-sensitive respiration, but noradrenaline failed to increase the respiration of the warm-adapted cells. Purine-nucleotide-sensitive proton conductance was greater in cold-adapted mitochondria than in warm-adapted controls. At the same time cold-adapted mitochondria were extremely sensitive to the uncoupling effect of endogenous and infused fatty acids, and resembled the mitochondria from the brown adipose tissue of cold-adapted hamsters. Warm-adapted mitochondria were ninefold less sensitive, and resembled liver mitochondria. With cold-adapted, but not warm-adapted mitochondria, respiration increased proportionately to the rate of fatty acid infusion. It is concluded that the presence of the 32000-Mr proton conductance pathway is necessary for the expression of a high sensitivity to fatty acid uncoupling, suggesting that the fatty acids interact directly with this protein to modulate the proton conductance during the acute regulation of thermogenesis.

Fatty acids as acute regulators of the proton conductance of hamster brown-fat mitochondria

Possible mechanisms are evaluated for the acute regulation of the hamster brown-fat mitochondrial proton-conductance pathway which is active during non-shivering thermogenesis. Isolated mitochondria are incubated under conditions designed to approximate to the non-thermogenic state, and the effect of the steady infusion of fatty acids or acyl derivatives upon respiration, membrane potential and membrane proton conductance is monitored continuously. Fatty acids increase the proton conductance with no detectable threshold concentration, allowing the generated acyl carnitine to be rapidly oxidized. The extent of depolarization and of respiratory increase is a function of the rate of infusion. Immediately infusion is terminated the conductance decreases, the mitochondria repolarize and respiration returns to the initial rate. Infusion of acyl-CoA and acylcarnitine cause only a slight depolarization or respiratory increase after high concentrations of these derivatives have accumulated. Any factor which decreases the rate of conversion of fatty acid to acyl-CoA potentiates the conductance increase. An effect of acyl-CoA upon chloride permeability is not specific to brown-fat mitochondria. Fatty acids infused into rat liver mitochondrial incubations produced a small conductance increase, comparable to that of acyl-CoA or acylcarnitine. It is concluded that fatty acids are the most plausible acute regulators of the proton conductance. The relation to the brown-fat-specific 32000-Mr protein is discussed.

Exclusive occurrence of thermogenin antigen in brown adipose tissue

Thermogenin is the purine-nucleotide binding polypeptide in brown adipose tissue mitochondria (Mr 32 000) which confers upon these mitochondria the ability to produce heat. An enzyme-linked immunosorbent assay (ELISA) has been developed to demonstrate and quantitate the occurrence of thermogenin antigen in small amounts of tissue, and thus to characterize different depots of fat tissue as white or brown. The extreme sensitivity of the method allows determination of thermogenin in samples equivalent to less than 1 mg tissue. The results indicate that thermogenin seems to be exclusively localised in brown fat mitochondria (as compared to white fat, liver or heart muscle mitochondria), and thermogenin antigen could only be found in brown adipocytes (as compared to white adipocytes). Thus, brown and white adipose tissue are probably ontogenetically different.

Studies on the activity of brown adipose tissue in suckling, pre-obese, ob/ob mice

The properties and activity of brown adipose tissue have been investigated in suckling, pre-obese, ob/ob mice in order to determine whether decreased thermogenesis in the tissue precedes the development of obesity in this mutant. At 14 days of age there was no difference between the ob/ob and normal animals in the total amount of interscapular brown adipose tissue, and the DNA content, protein content, and cytochrome oxidase activity of the tissue were similar in the two groups of mice. Respiration rates of brown adipose tissue mitochondria in the presence of albumin were, however, greater in the normal than the ob/ob animals, although after the addition of GDP to recouple the mitochondria there was no difference between the two groups. The mitochondrial membrane potential, measured with [3H]methyltriphenylphosphonium, was less affected by exogenous GDP in ob/ob mice than in normal animals. GDP binding to brown adipose tissue mitochondria, an index of the proton conductance pathway, was much greater in normal than in ob/ob mice at both 10 and 14 days of age; the decreased GDP binding in the mutant animals was found to result from a reduction in the number of binding sites. It is concluded that brown adipose tissue mitochondria of pre-obese ob/ob mice are more tightly coupled than those of normal siblings, and that the activity of the 'thermogenic' proton conductance pathway is lower in the mutant animals. A decrease in thermogenesis in brown adipose tissue is therefore an early event in the development of the ob/ob mouse and precedes the appearance of obesity.

Abnormal brown adipose tissue in genetically obese mice (ob/ob): effect of thyroxine

Lean and genetically obese (ob/ob): mice were treated daily for 2 wk with thyroxine (T4), noradrenaline, or thyroxine plus noradrenaline. T4 treatment of obese mice increased the abnormally low binding of GDP to brown adipose tissue mitochondria and permitted a cold-induced increase to occur. It also brought about a return to a more normal ultrastructure of the mitochondria of the obese mice. T4 treatment did not alter the binding of GDP to brown adipose tissue mitochondria of lean mice. The binding of GDP to brown adipose tissue mitochondria is known to be to a 32,000-dalton polypeptide associated with the thermogenic proton conductance pathway. T4 treatment did not alter the proportion of this polypeptide in the mitochondrial membrane in either lean or obese mice. Treatment with noradrenaline did not alter the binding of GDP to brown adipose tissue mitochondria in either lean or obese mice. The effect of T4 is thought to be due to an improvement in the defective responsiveness of brown adipose tissue to endogenous noradrenaline in the obese mice, known to be related to their poor cold resistance and obesity. The improvement allows a more normal noradrenaline-induced unmasking of GDP binding sites, both in response to diet and in response to cold. Such treatment is known to improve cold resistance of the obese mice, and this appears to be correlated with an improvement in the functioning of their defective brown adipose tissue.

Identification of the protein responsible for pyruvate transport into rat liver and heart mitochondria by specific labelling with [3H]N-phenylmaleimide

1. N-Phenylmaleimide irreversibly inhibits pyruvate transport into rat heart and liver mitochondria to a much greater extent than does N-ethylmaleimide, iodoacetate or bromopyruvate. alpha-Cyanocinnamate protects the pyruvate transporter from attack by this thiol-blocking reagent. 2. In both heart and liver mitochondria alpha-cyanocinnamate diminishes labelling by [3H]N-phenylmaleimide of a membrane protein of subunit mol.wt. 15000 on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 3. Exposure of mitochondrial to unlabelled N-phenylmaleimide in the presence of alpha-cyanocinnamate, followed by removal of alpha-cyanocinnamate and exposure to [3H]N-phenylmaleimide, produced specific labelling of the same protein. 4. Both labelling and kinetic experiments with inhibitors gave values for the approximate amount of carrier present in liver and heart mitochondria of 100 and 450 pmol/mg of mitochondrial protein respectively. 5. The turnover numbers for net pyruvate transport and pyruvate exchange at 0 degrees C were 6 and 200 min-1 respectively.

GDP binding to brown-adipose-tissue mitochondria of diabetic--obese (db/db) mice. Decreased binding in both the obese and pre-obese states

GDP binding to brown-adipose-tissue mitochondria from adult diabetic--obese (db/db) mice was significantly less than with lean siblings. Binding was also decreased in the mutant mice before obesity had begun to develop. Decreased GDP binding was found to result from a decrease in the number of binding sites.

The number and localisation of adenine nucleotide-binding sites in beef-heart mitochondrial ATPase (F1) determined by photolabelling with 8-azido-ATP and 8-azido-ADP

1. When irradiated 8-azido-ATP becomes covalently bound (as the nitreno compound) to beef-heart mitochondrial ATPase (F1) as the triphosphate, either in the absence or presence of Mg2+, label covalently bound is not hydrolysed. 2. In the presence of Mg2+ the nitreno-ATP is bound to both the alpha and beta subunits, mainly (63%) to the alpha subunits. 3. After successive photolabelling of F1 with 8-azido-ATP (no Mg2+) and 8-azido-ADP (with Mg2+) 4 mol label is bound to F1, 2 mol to the alpha and 2 mol to the beta subunits. 4. When the order of photolabelling is reversed, much less 8-nitreno-ATP is bound to F1 previously labelled with 8-nitreno-ADP. It is concluded that binding to the alpha-subunits hinders binding to the beta subunits. 5. F1 that has been photolabelled with up to 4 mol label still contains 2 mol firmly bound adenine nucleotides per mol F1. 6. It is concluded that at least 6 sites for adenine nucleotides are present in isolated F1.

Abnormal brown adipose tissue in hamsters with muscular dystrophy

The size (wet weight, total protein, total cytochrome oxidase) of interscapular brown adipose tissue is reduced to about one-half of normal in the cardiomyopathic hamster (BIO 14.6). The mitochondria are normal in binding of purine nucleotides [guanosine 5'-diphosphate (GDP)] and in proportion of polypeptides in the region of 32,000, both indices of the thermogenic proton conductance pathway, and in specific activity of cytochrome oxidase. Brown adipose tissue of the cardiomyopathic hamster can grow during acclimation to 4 degrees C, but its size remains smaller than in cold-acclimated normal hamsters. Mitochondrial polypeptide composition is not altered by acclimation to cold, but a large increase in mitochondrial GDP binding occurs in both normal and cardiomyopathic hamsters. The reduced calorigenic response of cardiomyopathic hamsters to catecholamines (Horwitz, B.A., and G.E. Hanes, Proc. Soc. Exp. Biol. Med. 147: 393-395, 1974) may, at least in part, be explained by a reduction in the amount of brown adipose tissue, the major site of this response. A defect in control of growth of this tissue in the cardiomyopathic hamster is suggested.

Localisation of adenine nucleotide-binding sites on beef-heart mitochondrial ATPase by photolabelling with 8-azido-ADP and 8-azido-ATP

1. In addition to the previously studied 8-azido-ATP, 8-azido-ADP is a suitable photoaffinity label for beef-heart mitochondrial ATPase (F1). 2. Photolysis at 350 nm of 8-azido-ADP in the presence of isolated F1 leads to inactivation of ATPase activity. Both ATP and ADP (but not AMP) protect against the inactivation. 3. In the absence of Mg2+, 8-azido-ADP binds almost equally to the alpha and beta subunits of F1, whereas in the presence of Mg2+ the alpha subunits are predominantly labelled. 4. The ATPase activity is completely inhibited when two molecules of 8-azido-ADP are bound per molecule F1. 5. 8-Azido-ATP and ATP are competitive substrates for F1, indicating that in the presence of Mg2+ 8-azido-ATP binds to the same site as ATP. 6. The amount of tightly bound nucleotides in F1 is not significantly changed upon incubation with 8-azido-ATP either in the light or the dark. 7. 8-Azido-ATP is also a suitadrial particles, photolabelling leading to inactivation of ATPase activity. 9. Oxidative phosphorylation and the ATP-driven reduction of NAD+ by succinate are also inhibited by photolabelling Mg-ATP particles with 8-azido-ATP. 10. In contrast to the uncoupled ATPase activity, where the two ATP-binding sites do not interact, cooperation between the two sites is required for ATP hydrolysis coupled to reduction of NAD+ by succinate.