The biochemistry of white and brown adipocytes analysed from a selection of proteins

Lack of hexose-6-phosphate dehydrogenase impairs lipid mobilization from mouse adipose tissue

In adipose tissue, glucocorticoids regulate lipogenesis and lipolysis. Hexose-6-phosphate dehydrogenase (H6PDH) is an enzyme located in the endoplasmic reticulum that provides a cofactor for the enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), regulating the set point of its activity and allowing for tissue-specific activation of glucocorticoids. The aim of this study was to examine the adipose tissue biology of the H6PDH null (H6PDH/KO) mouse. Real-time PCR analysis confirmed similar mRNA levels of 11beta-HSD1 and glucocorticoid receptor-alpha in wild-type (WT) and H6PDH/KO mice in liver and gonadal fat depots. Microsomal 11beta-HSD1 protein levels shown by Western blot analysis corresponded well with mRNA expression in gonadal fat of WT and H6PDH/KO mice. Despite this, the enzyme directionality in these tissues changed from predominately oxoreductase in WT to exclusively dehydrogenase activity in the H6PDH/KO mice. In the fed state, H6PDH/KO mice had reduced adipose tissue mass, but histological examination revealed no difference in average adipocyte size between genotypes. mRNA expression levels of the key lipogenic enzymes, acetyl CoA carboxylase, adiponutrin, and stearoyl-coenzyme A desaturase-2, were decreased in H6PDH/KO mice, indicative of impaired lipogenesis. In addition, lipolysis rates were also impaired in the H6PDH/KO as determined by lack of mobilization of fat and no change in serum free fatty acid concentrations upon fasting. In conclusion, in the absence of H6PDH, the set point of 11beta-HSD1 enzyme activity is switched from predominantly oxoreductase to dehydrogenase activity in adipose tissue; as a consequence, this leads to impairment of fat storage and mobilization.

Brown adipocytes of sucrose-overfed rats treated with corticosterone: A stereological and ultrastructural study

The aim of this study was to examine the effects of short-term corticosterone treatment on brown adipo?cytes of rats overfed with sucrose. Ultrastructural and stereological analysis showed that brown adipocyte components responded to the applied treatment in conformity with their own dynamics and affinity. Although brown adipocytes generally corresponded to thermogenically active cells, some signs of supression of that function, such as mitochondrial degradation and a pattern of lipid accumulation, were noticeable. Taken together, the presented results indicate that a high carbohydrate diet delays the expected inhibitory influence of corticosterone on brown adipose tissue thermogenesis. For the full expression of corticosterone effects, longer treatment is needed.

Effect of the acute crowding stress on the rat brown adipose tissue metabolic function

Our previous results have shown that metabolic and thermal stressors influence interscapular brown adipose tissue (IBAT) metabolic activity by increasing oxygen consumption and, consequently, altering the toxic reactive oxygen species (ROS) production and the antioxidative system activity. Since there is not enough evidence about the effect of psychosocial stressors on these processes, we studied the effect of acute crowding stress on the IBAT and hypothalamic monoamine oxidase (MAO) activity as well as IBAT antioxidative enzymes, manganese (MnSOD), copper-zinc superoxide dismutase (CuZnSOD) and catalase (CAT), as the relevant indicators of IBAT metabolic alternations under the stress exposure and the returning of animals to control conditions. The results indicated that acute crowding stress did not change the hypothalamic and IBAT MAO activities, the generation of ROS and, consequently, the IBAT CuZnSOD and CAT activities. However, all three antioxidative enzymes were affected only after the recovery period. It seems that peripheral overheating of rats during acute crowding changes the stress nature, by becoming more thermal than psychosocial and by suppression the hypothalamic efferent pathways involved in the IBAT thermogenesis regulation. However, it seems that returning of the animals to the control conditions after the stress termination causes the reactivation of IBAT thermogenesis with tendency to normalise the body temperature.

Identification of possible protein machinery involved in the thermogenic function of brown adipose tissue

Brown adipose tissue (BAT) is believed to function by dissipating excess energy in mammals. It is very important to understand the energy metabolism held in BAT since disorder of its energy-dissipating function may cause obesity or lifestyle-related diseases such as hypertension and diabetes. This function in BAT is mainly attributable to uncoupling protein (UCP), specifically expressed in its mitochondria. This protein consumes excess energy as heat by dissipating the H+ gradient across the inner mitochondrial membrane that is utilized as a driving force for ATP synthesis. In this review article, in addition to providing a brief introduction to the functional properties of BAT and UCP, we also describe and discuss properties of cultured brown adipocytes and the results of our exploratory studies on protein components involved in the energy-dissipating function in BAT.

Body fat accumulation is greater in rats fed a beef tallow diet than in rats fed a safflower or soybean oil diet

The effects of dietary fats , consisting of different fatty acids, on body fat accumulation and uncoupling protein (UCP) in interscapular brown adipose tissue were studied in rats. Metabolisable energy in experimental diets based on safflower oil, soybean oil or beef tallow was measured strictly (experiment 1). Male Wistar rats were then meal-fed an isoenergetic diet for 8 weeks (experiment 2). Each group of rats showed the same weight gain during the 8-week experimental period. Carcass fat content was greater in rats fed the beef tallow diet than in those fed the with the safflower or soybean oil diets, whereas the weight of abdominal adipose tissue was the same for all three dietary groups. Gene expression of UCP1 and the UCP content of the interscapular brown adipose tissue was lower in the beef tallow diet group than in the other dietary groups. A negative correlation was observed between carcass fat content and n-6 unsaturated fatty acid content in dietary fats. These results suggest that the greater body fat accumulation in rats fed the beef tallow diet results from lower expression of UCP1 mRNA and lower UCP content in brown adipose tissue. n-6 Polyunsaturated fatty acids may be the most effective fatty acids in limiting body fat.

Thyrotropin receptors in brown adipose tissue: thyrotropin stimulates type II iodothyronine deiodinase and uncoupling protein-1 in brown adipocytes

It has been demonstrated that TSH receptors are expressed not only in thyroid gland but also in extrathyroidal tissues. Brown adipose tissue of guinea pig has been reported to express TSH receptor messenger RNA (mRNA), but the physiological roles of TSH receptors in brown adipose tissue have not been understood. We studied the expression and function of TSH receptors in rat brown adipose tissue and cultured rat brown adipocytes. Northern analysis demonstrated the expression of TSH receptor mRNA in rat brown adipose tissue and cultured rat brown adipocytes. TSH receptor mRNA in rat brown adipose tissue was decreased by cold exposure of the rat, and its mRNA in cultured rat brown adipocytes was also decreased by incubation with TSH or (Bu)(2)cAMP. TSH increased the intracellular cAMP concentration in cultured rat brown adipocytes in a dose dependent manner. Type II iodothyronine deiodinase mRNA, its activity, and uncoupling protein-1 mRNA in cultured rat brown adipocytes were significantly increased by incubation with TSH in a dose-dependent manner. These results suggest the expression of functional TSH receptors in brown adipose tissue, which may be involved in regulation of the expression of type II iodothyronine deiodinase and uncoupling protein-1.

The Trp64Arg polymorphism of the beta3-adrenergic receptor gene is associated with hypertension in men with type 2 diabetes mellitus

A missense mutation of the beta3-adrenergic receptor gene (ADRB3) resulting in a tryptophan/arginine exchange at position 64 (Trp64Arg polymorphism) has recently been associated with greater capacity to gain weight, a low resting metabolic rate, higher blood pressure, and an early onset of type 2 diabetes. These findings prompted us to examine the relationship between this mutation, blood pressure, and vascular complications in German patients with type 2 diabetes. White patients with type 2 diabetes mellitus (n = 417) were enrolled in the study. The Trp64Arg polymorphism of the ADRB3 gene was detected by polymerase chain amplification and subsequent restriction digest with BstN I. Stepwise logistic regression analysis of the entire study population revealed a significant interaction between gender and genotype (P = .019). We therefore performed separate analyses for men and women. There was a significant relationship between hypertension and the ADRB3 Trp64Arg variant in men (P = .015), but not in women. Furthermore, blood pressure levels in male patients with the minor allele had higher blood pressure levels (P < .05), despite a significantly greater number of antihypertensive medications (P = .01). There was no association between ADRB3 genotype and vascular complications in these patients. In conclusion, our data are compatible with a contribution of this genetic variant of ADRB3 to hypertension in male patients with type 2 diabetes. Further studies will be needed to determine the role of this polymorphism as a predictor of hypertension or vascular complications in patients with type 2 diabetes.

High expression of monoamine oxidases in human white adipose tissue: evidence for their involvement in noradrenaline clearance

The clearance of plasma adrenaline and noradrenaline by human adipose tissue suggests the expression of the catecholamine-degrading enzyme monoamine oxidases and of catecholamine transport systems in adipocytes. In the present study, we identified and characterized the monoamine oxidases and an extraneuronal noradrenaline transporter expressed in human adipocytes. Enzyme assays using the monoamine oxidase A/B substrate [14C]tyramine showed that abdominal and mammary human adipocytes contain one of the highest monoamine oxidase activities in the body. Characterization of the enzyme isoforms by inhibition profiles of [14C]tyramine oxidation and Western and Northern blot analyses showed that mRNAs and proteins related to both monoamine oxidases A and B were expressed in adipocytes. Quantification of each enzyme isoform performed by enzyme assay and Western blot showed that monoamine oxidase A was predominant, representing 70-80% of the total enzyme activity. In uptake experiments, the monoamine oxidase substrate [3H]noradrenaline was transported into white adipocytes (Vmax 0.81+/-0.3 nmol/30 min/100 mg of lipid, Km 235+/-104 microM). The inhibition of [3H]noradrenaline uptake by specific inhibitors indicated that white human adipocytes contain an extraneuronal-type noradrenaline transporter. Competition studies of [14C]tyramine oxidation showed that noradrenaline is metabolized by monoamine oxidases in intact cells. In conclusion, the concomitant expression of monoamine oxidases and of a noradrenaline transporter in human white adipocytes supports the role of the adipose tissue in the clearance of peripheral catecholamines. These results suggest that adipocytes should be considered as a previously unknown potential target of drugs acting on monoamine oxidases and noradrenaline transporters.

Phosphorylation of extracellular signal-regulated kinases 1 and 2 in 3T3-L1 adipocytes by stimulation of beta(3)-adrenoceptor

Recent studies have revealed that activated extracellular signal-regulated kinases (ERKs) 1 and 2 by the stimulation of beta(3)-adrenoceptors played a critical role in cell survival in brown adipocytes. On the other hand, phosphorylation of ERK1/2 via beta(3)-adrenoceptors and its physiological and pathological significance in white adipocyte has remained uncertain despite the increasing significance of functioning white adipocytes. Accordingly, we here studied phosphorylation of ERK1/2 caused by the stimulation of beta(3)-adrenoceptors in 3T3-L1 adipocytes, and the roles of phosphorylated ERK1/2 in lipolysis. Phosphorylation of ERK1/2 was induced by a selective beta(3)-adrenoceptor agonist, DL-4-[2'-¿2-hydroxy-2-(3-chlorophenyl)ethylamino¿propyl] phenoxyacetic acid sodium salt sesquihydrate (BRL37344), in 3T3-L1 adipocytes in a time- and dose-dependent manner. The phosphorylation of ERK1/2 by BRL37344 was sensitive to the cyclic AMP (cAMP)-dependent protein kinase inhibitor, N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide (H89). To elucidate the roles of phosphorylated ERK1/2 in lipolysis, the effect of a selective inhibitor of ERK1/2 phosphorylation, 2'-amino-3'-methoxyflavone (PD98059), was examined. This inhibitor did not alter the lipolytic action caused by BRL37344, even at concentrations sufficient to block phosphorylation of ERK1/2, suggesting that ERK1/2 play no role in the lipolysis caused by BRL37344 in 3T3-L1 adipocytes.

PPARalpha activation by Wy 14643 induces transactivation of the rat UCP-1 promoter without increasing UCP-1 mRNA levels and attenuates PPARgamma-mediated increases in UCP-1 mRNA levels induced by rosiglitazone in fetal rat brown adipocytes

Rodent brown adipocytes express peroxisome proliferator activated receptor-alpha (PPARalpha) and PPARgamma and while the rodent uncoupling protein-1 (UCP-1) gene contains a putative peroxisome proliferator response element (PPRE), only PPARgamma activation by thiazolidinediones increase UCP-1 mRNA levels. We have investigated this phenomenon in foetal rat brown adipocytes (FBA) and show that although transactivation occurs in FBA containing a plasmid encoding 4.5kb of the 5'-flanking region of the rat UCP-1 promoter ((-4551)-UCP-1-CAT) treated with either the selective PPARgamma agonist rosiglitazone (1.0 microM) or the selective PPARalpha agonist Wy 14643 (10.0 microM), only rosiglitazone induced transcription of UCP-1 mRNA. Furthermore, Wy 14643 (10 and 100.0 microM) abolished rosiglitazone-induced UCP-1 mRNA induction in spite of a transactivation event occurring with the combination treatment. Thus in FBA PPARalpha-activation with Wy 14643 elicits a "blind" transactivation of the UCP-1 promoter which can prevent PPARgamma-mediated UCP-1 mRNA transcription either by competition for the PPRE or by an unidentified post-transcriptional event.

Selective uptake of high density lipoprotein-associated cholesterylesters by differentiated Ob1771 adipocytes is modulated by endogenous and exogenous lipoprotein lipase

The present study aimed at investigating mechanisms of selective uptake of HDL3-associated cholesterylesters (HDL3-CEs) by differentiated Ob1771 adipocytes. Our findings indicate that Ob1771 cells are capable of pronounced selective uptake of HDL3-CEs in 2.6-fold excess of HDL3 holoparticle uptake. Selective uptake of HDL3-CEs into a releasable pool (presumably located in the cellular plasma membrane) was temperature insensitive while prominent internalization into a non-releasable and subsequent hydrolysis in a non-chloroquine sensitive compartment occurred at 37 degrees C. Release of membrane bound endogenous LPL by heparin resulted in decreased HDL3 holoparticle, total CE and selective CE uptake. Accordingly, the addition of exogenous LPL to the culture medium resulted in increased holoparticle, total CE and selective CE uptake.

Physiological approach to maturation of brown adipocytes in primary cell culture

Molecular and metabolic aspects of differentiation of brown adipocytes of the Djungarian hamster (Phodopus sungorus) were studied in primary culture. Expression of uncoupling protein and lipoprotein lipase were investigated by Western and Northern blotting and indirect immuno-fluorescence microscopy. The activity of 5'-deiodinase type II was determined by a radioactive enzyme assay. Activity of cytochrome-c-oxidase and cell respiration rates were measured with a Clark electrode. We evaluated functional differences of developmental stages by measuring the reaction to beta-adrenergic stimulation throughout the differentiation process. The results show that differentiation of hamster brown adipocytes is an at least two-step development with physiologically discriminable cell types. Generation of triiodothyronine (T3) from thyroxine by activation of the 5'deiodinase occurs in immature brown adipocytes and is mediated primarily by beta1- rather than beta3-adrenergic receptors. The thermogenic capacity is subsequently increased in mature brown adipocytes. beta-Adrenergic receptor stimulation increases UCP expression of mature adipocytes but is not able to recruit new brown adipocytes.

Activities of antioxidant enzymes and monoamine oxidase-A in the rat interscapular brown adipose tissue: effects of insulin and 6-hydroxydopamine

The effects of different doses of insulin (INS) (0.4 or 4.0 IU/kg body mass, i.p., for 3 hr) and 6-hydroxydopamine (6-HDA) (100 mg/kg., i.p.) on the activities of antioxidant enzymes--copper-zinc superoxide dismutase (CuZnSOD), manganese superoxide dismutase (MnSOD), catalase (CAT) and catecholamine degrading enzyme monoamine oxidase (MAO-A)--in the rat interscapular brown adipose tissue (IBAT) were studied. In vivo 6-HDA administration, which induces the destruction of sympathetic nerves, markedly reduced IBAT CuZnSOD activity but did not change MnSOD and CAT activities. However, the low dose of INS, which did not induce hypoglycemia, significantly increased the activity of both IBAT mitochondrial enzymes (MnSOD and MAO-A) of control rats. This INS effect on MnSOD was abolished by 6-HDA. On the contrary, CuZnSOD activity was markedly reduced under the influence of INS in both control and 6-HDA-treated rats, whereas for the maintenance of the control level of this enzyme activity, the intact sympathetic nervous system (SNS) is necessary. INS, independent of the dose applied, did not affect CAT activity in control rats, whereas only low INS dose increased the activity of this enzyme in 6-HDA-treated rats. The results indicate that the stimulatory effect of INS on the IBAT mitochondrial enzymes studied is dose dependent and in the case of MnSOD is mediated by SNS. However, the depression in the activity of CuZnSOD is independent of the above-mentioned factors.

A novel regulatory pathway of brown fat thermogenesis. Retinoic acid is a transcriptional activator of the mitochondrial uncoupling protein gene

The mitochondrial uncoupling protein (UCP) is responsible for the thermogenic function of brown fat, and it is a molecular marker of the brown adipocyte cell type. Retinoic acid (RA) increased UCP mRNA levels severalfold in brown adipocytes differentiated in culture. This induction was independent of adrenergic pathways or protein synthesis. RA stimulated ucp gene expression regardless of the stage of brown adipocyte differentiation. In transient transfection experiments RA induced the expression of chloramphenicol acetyltransferase vectors driven by 4.5 kilobases of the 5'-noncoding region of the rat ucp gene, and co-transfection of expression vectors for RA receptors enhanced the action of RA. Retinoic acid receptor alpha was more effective than retinoid X receptor in promoting RA action, whereas a mixture of the two was the most effective. The RA-responsive region in the ucp gene was located at -2469/-2318 and contains three motifs (between -2357 and -2330) of the consensus half-sites characteristic of retinoic acid response elements. This 27-base pair sequence specifically binds purified retinoic acid receptor alpha as well as related proteins from brown fat nuclei. In conclusion, a novel potential regulatory pathway of brown fat development and thermogenic function has been recognized by identifying RA as a transcriptional activator of the ucp gene.