A new, powerful player in lipoprotein metabolism: brown adipose tissue

Important causes for modern epidemics such as obesity, diabetes, and cardiovascular disease are over- and malnutrition. Dietary as well as endogenous lipids are transported through the bloodstream in lipoproteins, and disturbances in lipoprotein metabolism are associated with atherosclerosis, heart disease, and diabetes. Recent findings reveal biological principles-how lipoproteins, in particular triglyceride-rich lipoproteins, are metabolized and what factors regulate their processing. The fate of triglycerides delivered by lipoproteins is quite simple: either they can be stored or they can be utilized for combustion or biosynthetic pathways. In the healthy state, fatty acids derived from triglycerides can be burned in the heart, muscle, and other organs for actual work load, or they can be stored in white adipose tissue. The combination of storage and combustion is realized in brown adipose tissue (BAT), a peripheral organ that was long thought to be only of relevance in small mammals: Recent data however prove that BAT plays an important role in human adults. Here, we will review recent insights on how BAT controls triglyceride clearance and the possible implications for the treatment of chronic diseases caused by lipid mishandling.     

Evidence for a modulating effect of NA+/H+ exchange on the metabolic response of rat brown adipose tissue

Membrane potential and intracellular pH (pHi) were simultaneously monitored in rat perifused brown adipose tissue fragments by means of double-barrelled microelectrodes. In parallel experiments, the respiratory rate was measured. The cytosolic pH of unstimulated brown adipocytes was about 0.5 units higher than the value expected for a passive transmembrane distribution of H ions. Isoproterenol (5·10⁻¹⁰ M) had no effect on pHi and membrane potential while it induced a 3.1±0.5-fold increase of the respiratory rate. Clonidine (10⁻⁷ M) alone was followed by a cytosolic alkalinization of 0.14±0.05 pH units with no concomitant increase in the respiratory rate. A mirror image of the intracellular alkalinization induced by clonidine, i.e. an acidification of 0.09±0.03, was noted by monitoring the extracellular pH. Addition of clonidine in the presence of isoproterenol induced an alkalinization of 0.17±0.03 pH units and a 7.7±1.0-fold increase of the respiratory rate. Thus the alkalinizing effect of clonidine developed despite a massive increase in CO₂ production. Pretreatment of the preparation with amiloride (10⁻³ M), an inhibitor of Na⁺/H⁺ exchange, completely prevented the alkalinization and markedly reduced the potentiating effect of clonidine on the isoproterenol-induced respiratory rate. The results obtained are compatible with the hypothesis of a modulating effect of Na⁺/H⁺ exchange on the brown adipocyte metabolic response to catecholamine stimulation.     

Differential responses of white adipose tissue and brown adipose tissue to caloric restriction in rats

Caloric restriction (CR) slows the aging process and extends longevity, but the exact underlying mechanisms remain debatable. It has recently been suggested that the beneficial action of CR may be mediated in part by adipose tissue remodeling. Mammals have two types of adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT). In this study, proteome analysis using two-dimensional gel electrophoresis combined with MALDI-TOF MS, and subsequent analyses were performed on both WAT and BAT from 9-month-old male rats fed ad libitum or subjected to CR for 6 months. Our findings suggest that CR activates mitochondrial energy metabolism and fatty acid biosynthesis in WAT. It is likely that in CR animals WAT functions as an energy transducer from glucose to energy-dense lipid. In contrast, in BAT CR either had no effect on, or down-regulated, the mitochondrial electron transport chain, but enhanced fatty acid biosynthesis. This suggests that in CR animals BAT may change its function from an energy consuming system to an energy reservoir system. Based on our findings, we conclude that WAT and BAT cooperate to use energy effectively via a differential response of mitochondrial function to CR.     

The action of insulin on brown adipose tissue in vivo

1. The action of insulin on the net exchange of glucose, free fatty acids and glycerol by brown adipose tissue of young rabbits was investigated.2. Infusion of insulin (10,000 muu./kg.min I.V. for 10 min) caused a large increase in the uptake of glucose by brown adipose tissue of fed week-old rabbits. The release of fatty acids fell but glycerol release was unchanged.3. The brown adipose tissue of 9-day-old rabbits fasted in a warm environment had a high initial rate of fatty acid and glycerol release and low rates of glucose uptake. Insulin infusions (10,000 and 100 muu./kg.min) greatly reduced fatty acid release but had little or no effect on glucose uptake.4. The brown adipose tissue of 9-day-old rabbits fasted in a cold environment was depleted of fat and took up free fatty acid as well as glucose from the circulation. Infusion of insulin (10,000 muu./kg.min) caused a large increase in glucose uptake accompanied by a reduction in fatty acid uptake.5. The experiments support the view that insulin has a direct effect on fatty acid transport across the cell membrane.     

Could a lowered level of uncoupling protein in brown adipose tissue mitochondria play a role in SIDS aetiology?

Previously published studies on the relationship between brown adipose tissue thermogenesis and sudden infant death were examined and evidence was collated to support the hypothesis that a decreased level of mitochondrial uncoupling protein (or thermogenin) from the brown fat in some brown fat depots may play a role in SIDS aetiology.     

The role of brown adipose tissue in the calorigenic effect of adrenaline and noradrenaline in cold-acclimated rats

1. Removal of the interscapular brown adipose tissue of the cold-acclimated rat has no immediate effect on the calorigenic response of the rat to adrenaline or noradrenaline.2. There is a progressive loss of the enhanced response to adrenaline and to noradrenaline during the 4 days following removal of the interscapular brown adipose tissue from cold-acclimated rats. There is no loss of response in sham-operated cold-acclimated rats.3. Removal of the interscapular brown adipose tissue from rats living at room temperature has no effect on the calorigenic response to adrenaline or noradrenaline, neither immediately afterwards nor 2-4 days later.4. It is concluded that interscapular brown adipose tissue is not the major site of oxygen consumption in the enhanced calorigenic response to adrenaline or noradrenaline in cold-acclimated rats. However, it does play an important role in this enhanced metabolic response, probably as an endocrine gland whose secretory product modifies the ability of other tissues to respond calorigenically to catecholamines.     

Neuropeptides in interscapular and perirenal brown adipose tissue in the rat: a plurality of innervation

Summary The occurrence of neuropeptides in rat brown adipose tissue has been investigated. Immunohistochemical studies on interscapular and perirenal brown fat have demonstrated unequivocally the presence of substance-P (SP)-like, neuropeptide-Y (NPY)-like and calcitonin gene related-peptide (CGRP)-like immunoreactive elements (putative nerves) in adventitial distribution on inter- and intralobular supply arteries and in accompanying nerve bundles. At a more peripheral level, some NPY-like immunoreactive elements and a greater number of CGRP-like immunoreactive elements were observed in the parenchymal field. Somatostatin, bombesin, neurotensin, enkephalin, and vasoactive-intestinal-polypeptide immunoreactivities were not detected. No differences in neuropeptide distribution were noted between interscapular and perirenal brown fat.There is a degree of coincident distribution of SP, NPY and CGRP with that of noradrenergic nervous elements as visualized by condensation histochemistry. Since after 6-hydroxydopamine treatment not all the nerve terminals in rat brown adipose tissue are stigmatized (earlier report), the present results have been discussed in the light of a possible pluralism in innervation of brown adipose tissue.     

Gangliosides and energy substrates in brown adipose tissue of cold-acclimated rat

Effect of cold acclimation on gangliosides as well as triglyceride and glycogen in brown adipose tissue was studied in rats. Ganglioside GM3 level per unit fresh weight and per unit fat-free-dry-matter was significantly higher in cold-acclimated rats (CA) than in warm controls, while the tissue triglyceride and glycogen levels were lower in CA. GM3 may be involved in cold-induced proliferation and differentiation of brown adipose tissue.     

Nitric oxide and thermogenic function of brown adipose tissue in rats

To clarify the effects of cold acclimation and immobilization stress adaptation of rats on nitric oxide (NO) activity in interscapular brown adipose tissue (BAT), we incubated neatly diced (1-mm(3) blocks) BAT in a metabolic chamber for respiration, measured oxygen consumption using a Clark electrode, and estimated NO release in the buffer medium by measuring nitrite plus nitrate (NO(x)) using the Griess method (diazotization reaction). The production of NO(x) in the buffer medium confirmed that BAT releases NO, as there is no other source of NO(x) in the system. The NO activity was observed in the basal condition and increased with noradrenaline stimulation, showing a correlation with oxygen consumption in the warm (25 degrees C)-acclimated control rats. Cold acclimation (5 degrees C, 5 weeks) or immobilization stress adaptation (3 h daily, 25 degrees C, 5 weeks) caused enhanced NO activity in the basal condition in comparison with the control. We suggest that NO is involved in enhancement of the thermogenic functions of BAT in rats.     

Effects of a chronic corticotropin treatment on brown adipose tissue of cold acclimated rats

1. This study deales with the effects of chronic corticotropin treatment (daily injection of 2U · kg^?1 i.p. for 10 days) upon the lipolytic action of norepinephrine (NE) on abdominal and interscapular brown adipose tissues (BAT) and on epididymal white fat of rats acclimated to either 28°C or 5°C and compared to controls.
2. In incubated BAT pieces from both 28°C and 5°C control animals no stimulation of lipolysis (release of glycerol and fatty acids) was induced by NE (10^?4 mol). A similar absence of effect was observed in BAT from corticotropin-treated rats. In epididymal fat, the in vitro enhancement of lipolysis by NE (300–400%) in controls was not modified in ACTH-treated rats.
3. The in vivo lipolysis in interscapular BAT was estimated by determining the arteriovenous differences in free glycerol and fatty acids levels and measuring blood flow. In control animals the blood flow stimulation by NE and the in situ utilization of the hydrolyzed fatty acids were larger in 5°C rats than in 28°C ones. In corticotropin-treated animals compared to controls, blood flow stimulation and fatty acid utilization were enhanced in 28°C group, and in opposite they were reduced in 5°C group; no significant difference was observed between the two groups.
4. It is concluded that chronic corticotropin treatment induces a “pseudo cold-acclimation” of BAT in 28°C rats and, inversely, a “loss of acclimation” in cold-acclimated ones. These opposite effects may be related to both the corticotropic and lipolytic action of the hormone.

     

Effects of ovariectomy on thermogenesis in brown adipose tissue and liver in Syrian hamsters

Weight gain in ovariectomized Syrian hamsters occurs without increased food intake, which suggests that metabolic efficiency may be enhanced through a reduction in energy expenditure. We examined the effect of ovariectomy on metabolic activity in brown adipose tissue and liver. Four groups of hamsters (n = 13, each) were killed 0, 2, 4, or 16 weeks following ovariectomy. Ovariectomized hamsters rapidly gained weight without overeating. Body weights stabilized after 8 weeks and remained 12-17% above sham-operated control weights for the duration of the experiment. Weight gain in the hamsters ovariectomized for 16 weeks was characterized by significant increases in retroperitoneal white adipose tissue weight and carcass lipid content. Similar trends were seen in 2-week and 4-week ovariectomized animals. There were no differences in interscapular brown adipose tissue weight, protein content, DNA content, or norepinephrine (NE) content among sham-operated and 2-, 4-, or 16-week ovariectomized hamsters, indicating that ovariectomy had no effect on brown adipose tissue growth. Similarly, there was no difference in either sympathetic nervous system activity (estimated by the rate of NE turnover) or mitochondrial GDP binding among the four groups of hamsters. In contrast, hepatic cytochrome P-450 activity was significantly reduced 2, 4, and 16 weeks after ovariectomy. These results suggest that reduced thermogenic activity in liver, but not in brown adipose tissue, could contribute to the weight gain in Syrian hamsters after ovariectomy.     

Interaction of adrenalectomy and fenfluramine treatment on body weight, food intake and brown adipose tissue

Three experiments have examined the interaction of adrenalectomy and fenfluramine on food intake, body weight and the binding of guanosine-5'-diphosphate (GDP) to interscapular brown adipose tissue (IBAT). In the first experiment, GDP-binding by IBAT mitochondria from adrenalectomized or sham-operated animals was measured for 3 hr after one of 3 doses of fenfluramine. Fenfluramine stimulated GDP-binding at lower doses in the adrenalectomized animals than in the controls. In the first chronic experiment, adrenalectomy prevented the restoration of normal food intake observed 8-10 days after the beginning of fenfluramine treatment. Adrenalectomy also increased weight loss and enhanced GDP binding to mitochondria from IBAT in rats treated with fenfluramine. In the second chronic experiment, the combination of fenfluramine and adrenalectomy led to a progressive weight loss, continuing hypophagia and stimulation of GDP-binding by IBAT, whereas rats treated with fenfluramine alone showed a recovery of food intake at a stabilized but lower body weight. These data are consistent with the hypothesis that adrenalectomy and fenfluramine disable two separate components of the food intake system and that when combined, produce a profound and persisting disturbance in energy or nutrient balance.