Normal differentiation of rat brown adipocytes in primary culture judged by their expressions of uncoupling protein and the physiological isoform of glucose transporter

Adropin stimulates proliferation but suppresses differentiation in rat primary brown preadipocytes

Adropin is a peptide hormone encoded by Energy Homeostasis Associated (Enho) gene. Adropin modulates glucose and lipid metabolism, and adiposity. Recently, we found that adropin suppresses differentiation of rodent white preadipocytes into mature fat cells. By contrast, the role of adropin in controlling brown adipogenesis is largely unknown. Therefore, in the present study we evaluated the effects of adropin on proliferation and differentiation of adipocyte precursor cells in rats. Brown adipocyte precursor cells were isolated from male Wistar rats. Cell replication was measured by BrdU incorporation. Gene expression was studied using real time PCR. Protein phosphorylation and production was assessed by Western blot. Lipid accumulation was evaluated by Oil Red O staining. Colorimetric kits were used to evaluate glycerol and free fatty acids release. We report here that adropin stimulates proliferation of brown preadipocytes. Moreover, in brown preadipocytes, adropin suppresses mRNA expression of adipogenic genes (C/ebpα, C/ebpβ, Pgc1α, Pparγ and Prdm16) during differentiation process. In addition, adropin suppresses UCP1 protein production in brown adipocytes. Finally, adropin reduces intracellular lipid content in brown adipocytes. These results indicate that adropin stimulates proliferation of brown preadipocytes and suppresses their differentiation into mature adipocytes.

Glucocorticoids and Metabolic Control

In response to stress, the central nervous system initiates a signaling cascade, which leads to the production of glucocorticoids (GCs). GCs act through the glucocorticoid receptor (GR) to coordinate the appropriate cellular response with the primary goal of mobilizing the storage forms of carbon precursors to generate a continuous glucose supply for the brain. Although GCs are critical for maintaining energy homeostasis, excessive GC stimulation leads to a number of undesirable side effects, including hyperglycemia, insulin resistance, fatty liver, obesity, and muscle wasting leading to severe metabolic dysfunction. Summarized below are the diverse metabolic roles of glucocorticoids in energy homeostasis and dysregulation, focusing specifically on glucose, lipid, and protein metabolism.

Deconstructing the roles of glucocorticoids in adipose tissue biology and the development of central obesity

Central obesity is associated with insulin resistance and dyslipidemia. Thus, the mechanisms that control fat distribution and its impact on systemic metabolism have importance for understanding the risk for diabetes and cardiovascular disease. Hypercortisolemia at the systemic (Cushing's syndrome) or local levels (due to adipose-specific overproduction via 11β-hydroxysteroid dehydrogenase 1) results in the preferential expansion of central, especially visceral fat depots. At the same time, peripheral subcutaneous depots can become depleted. The biochemical and molecular mechanisms underlying the depot-specific actions of glucocorticoids (GCs) on adipose tissue function remain poorly understood. GCs exert pleiotropic effects on adipocyte metabolic, endocrine and immune functions, and dampen adipose tissue inflammation. GCs also regulate multiple steps in the process of adipogenesis. Acting synergistically with insulin, GCs increase the expression of numerous genes involved in fat deposition. Variable effects of GC on lipolysis are reported, and GC can improve or impair insulin action depending on the experimental conditions. Thus, the net effect of GC on fat storage appears to depend on the physiologic context. The preferential effects of GC on visceral adipose tissue have been linked to higher cortisol production and glucocorticoid receptor expression, but the molecular details of the depot-dependent actions of GCs are only beginning to be understood. In addition, increasing evidence underlines the importance of circadian variations in GCs in relationship to the timing of meals for determining their anabolic actions on the adipocyte. In summary, although the molecular mechanisms remain to be fully elucidated, there is increasing evidence that GCs have multiple, depot-dependent effects on adipocyte gene expression and metabolism that promote central fat deposition. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Quantitative comparison of adipocytokine gene expression during adipocyte maturation in non-obese and obese rats

Adipocytokines secreted from adipocytes have been extensively analyzed due to their role as key factors in various complications of obesity, including arterial sclerosis, liver steatosis, insulin resistance, and diabetes. Several in vivo and in vitro studies have suggested that adipocyte maturation is related to fluctuations in adipocytokine secretion. However, the relationship between adipocyte maturation and adipocytokine levels has not been fully elucidated. Therefore, we sought to clarify the link between adipocytokine gene expression and adipocyte maturation through systematic analysis. We quantified mRNA for six adipocytokine genes: adiponectin, resistin, leptin, plasminogen activator inhibitor 1 (PAI-1), heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF), and visfatin, in adipose tissue, in primary cultured adipocytes obtained from an obese Zucker rat, and in the preadipocyte cell line 3T3-L1. Moreover, to elucidate the role of adipocytokines in adipocyte maturation, adipocytokine expression levels were analyzed during maturation. Although fluctuations in adipocytokine gene expression were heterogeneous, gene expression was highly similar during maturation of primary cultured adipocytes from obese and non-obese rats, suggesting that the maturation process is independent from processes that lead to obesity. Moreover, the expression patterns of adiponectin, resistin and leptin mRNA in 3T3-L1 cells were highly similar to those in primary cultured adipocytes, indicating that these adipocytokines could be common maturation markers for primary cultured adipocytes obtained from obese and non-obese rats, and for preadipocyte cell lines.

Quantifying carbon sources for de novo lipogenesis in wild-type and IRS-1 knockout brown adipocytes

Studies were conducted to evaluate the flux of various carbon sources to lipogenesis during brown adipocyte differentiation. (13)C labeling and isotopomer spectral analysis quantified the contribution of metabolites to de novo lipogenesis in wild-type (WT) and insulin receptor substrate-1 knockout (KO) brown adipocytes. Both glucose (Glc) and glutamine (Gln) provided substantial fractions of the lipogenic acetyl CoA for both WT and KO cells in standard media, together contributing 60%. Adding acetoacetate (AcAc; 10 mM) to the medium resulted in a large flux of AcAc to lipid, representing 70% of the lipogenic acetyl CoA and decreasing the contribution of Glc plus Gln to 30%. For WT cells, the fractional synthesis of new fatty acids during 4 days of differentiation was 80% of the total. Similarly, 80% of the lipidic glycerol was derived from Glc in the medium; Gln was not a precursor for glycerol. When Gln was removed from the medium, the contribution of Glc to fatty acid synthesis doubled, replacing most of the contribution of Gln and maintaining total lipogenesis. Conversely, removal of Glc dramatically decreased lipogenesis. These results indicate that Glc's distinct role in lipid synthesis during differentiation cannot be replaced by other carbon sources, consistent with the role of Glc supplying NADPH and/or glycerol for triglyceride synthesis.

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.

Comparison of gene expression profiles between white and brown adipose tissues of rat by microarray analysis

To characterize the energy metabolism in brown adipose tissue (BAT), the differences in gene expression profiles between BAT and white adipose tissue (WAT) were analyzed using a high-density cDNA microarray. RNAs isolated from two adipose tissues were hybridized to an Agilent rat cDNA Microarray that contained about 14,500 cDNA probe sets. The expression levels of 499 cDNA/ESTs were found to be at least 5-fold higher or lower in BAT than in WAT. Consistent with our previous findings, high expression levels of genes encoding uncoupling protein 1, muscle-type carnitine palmitoyltransferase and some other proteins involved in energy metabolism in BAT were found. Most of the genes encoding mitochondrial proteins, such as subunits of ATP synthase, cytochrome c oxidase, and NADH dehydrogenase, were highly expressed, reflecting possible differences in the cellular content of mitochondria between BAT and WAT. However, the expression levels of several genes encoding mitochondrial protein, such as liver mitochondrial aldehyde dehydrogenase and dicarboxylate carrier, were remarkably lower in BAT. These results may give important clues to understand the unique energy metabolism in BAT.

Thyroid hormone augments GLUT4 expression and insulin-sensitive glucose transport system in differentiating rat brown adipocytes in culture

The effects of triiodothyronine (T3) on differentiation-dependent expression of GLUT and responses of glucose transport to insulin and norepinephrine (NE) were investigated. Precursor cells of brown adipocytes isolated from the interscapular brown adipose tissue of newborn rats were cultured in the absence or presence of various concentrations of T3. Western bolt analysis revealed that treatment with T3 resulted in an increased expression of GLUT4, in a dose-dependent manner, whereas GLUT1 contents were unchanged. In parallel with the increase in GLUT4 expression, T3 improved insulin sensitivity for glucose transport, being accompanied by an increase in maximal transport rate and a reduction of ED(50). In contrast, T3-treatment of the brown adipocytes during the differentiation process had little effect on NE-regulatable glucose transport system. These results suggest that T3 plays a predominant role in the development of insulin-sensitive glucose transport during differentiation of brown adipocytes.

Specific elevation of transcript levels of particular protein subtypes induced in brown adipose tissue by cold exposure

To understand the difference in metabolic flow in rat brown adipose tissue (BAT) from that in white adipose tissue (WAT) at the molecular level, we examined the steady-state transcript levels of 39 proteins in both adipose tissues with and without cold exposure by Northern blot analysis. In addition to the transcript levels of uncoupling protein isoforms, those of proteins involved in the transport and catabolism of fatty acids and glucose in BAT were elevated by cold exposure, suggesting the stimulation of utilization of fatty acids and glucose as fuels in BAT. As to these changes, the muscle-type subtypes were remarkable; and therefore, they were suggested to be responsible for the cold exposure-induced acceleration of energy expenditure in BAT. Furthermore, of the isoforms of beta-adrenergic receptor (beta-AR) and CCAAT enhancer binding protein (C/EBP), transcript levels of beta(1)-AR and C/EBPbeta in BAT were increased by the cold exposure. Possible roles of these proteins in energy metabolism in BAT were discussed.

Effects of nitric oxide on proliferation and differentiation of rat brown adipocytes in primary cultures

1. In the present work, we study the effect of NO on the proliferation and differentiation of brown fat cells in primary cultures. 2. Brown fat precursor cells isolated from rat brown adipose tissue were cultured for 8 days until confluence and treated daily with the NO donating agents, S-nitroso-acetyl penicillamine (SNAP) or S-nitroso-L-glutathione (GSNO). Both agents (300 microM) decreased cell proliferation approximately 8 fold on day 8. The inhibitory effect of NO was unlikely to be due to cytotoxicity since (i) cells never completely lost their proliferation capacity even after 8 days of exposure to repeated additions of SNAP or GSNO, and (ii) the inhibitory effect was reversible after removal of the media containing NO donors. 3. Daily treatment with nitric oxide synthase inhibitors, such as NG-nitro-L-arginine methyl ester (L-NAME, 300 microM), led to the stimulation of cell proliferation by 44+/-5%, n=3, suggesting that NO, endogenously produced in brown adipocytes, may be involved in modulating cell growth. 4. Daily treatment with both SNAP or GSNO induced significant mitochondriogenesis, measured as the mitochondrial conversion of 3-[4,5-dimethylthiazol-2-yl-]-2,5-diphenyl tetrazolium bromide (MTT) to formazan, whilst daily treatment with L-NAME was without effect. 5. The inhibition of cell proliferation by NO donors was accompanied by the expression of two genes coding for peroxisome proliferator activated receptor-gamma and uncoupling protein-1, which are upregulated during differentiation. 6. Increasing cyclic GMP in cells by 8-bromo-cyclic GMP (100-1000 microM) did not reproduce the observed NO effects on either cell number or gene expression. On the other hand, chronic treatment with the inhibitor of the NO-stimulated guanylyl cyclase, 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), reduced the expression of peroxisome proliferator activated receptor-gamma and uncoupling protein-1.

Quantitative determinations of the steady state transcript levels of hexokinase isozymes and glucose transporter isoforms in normal rat tissues and the malignant tumor cell line AH130

The steady state transcript levels of the four hexokinase (HK) isozymes and four glucose transporter (GLUT) isoforms were determined quantitatively by Northern analysis of RNA samples from rat tissues using synthetic fragments of the RNAs encoding the HK isozymes and GLUT isoforms. Results showed that the levels of HK isozyme transcripts were low in rat tissues, the level of that most highly expressed, the type I isozyme (HKI), in the brain being 0.025% of the total poly(A)+ RNA. A good correlation was found between the reported HK activities and the total amounts of transcripts encoding all HK isozymes in various tissues, showing that the HK activities in tissues can be estimated from the total amount of transcripts encoding HK isozymes. The proposed associated expressions of HK isozymes and GLUT isoforms in particular tissues were confirmed at their transcript levels. The steady state transcript levels of type II HK and the type 1 GLUT isoform in the malignant tumor cell line AH130 were also determined quantitatively.

Direct effects of leptin on brown and white adipose tissue

Leptin is thought to exert its actions on energy homeostasis through the long form of the leptin receptor (OB-Rb), which is present in the hypothalamus and in certain peripheral organs, including adipose tissue. In this study, we examined whether leptin has direct effects on the function of brown and white adipose tissue (BAT and WAT, respectively) at the metabolic and molecular levels. The chronic peripheral intravenous administration of leptin in vivo for 4 d resulted in a 1.6-fold increase in the in vivo glucose utilization index of BAT, whereas no significant change was found after intracerebroventricular administration compared with pair-fed control rats, compatible with a direct effect of leptin on BAT. The effect of leptin on WAT fat pads from lean Zucker Fa/ fa rats was assessed ex vivo, where a 9- and 16-fold increase in the rate of lipolysis was observed after 2 h of exposure to 0.1 and 10 nM leptin, respectively. In contrast, no increase in lipolysis was observed in the fat pads from obese fa/fa rats, which harbor an inactivating mutation in the OB-Rb. At the level of gene expression, leptin treatment for 24 h increased malic enzyme and lipoprotein lipase RNA 1.8+/-0.17 and 1.9+/-0.14-fold, respectively, while aP2 mRNA levels were unaltered in primary cultures of brown adipocytes from lean Fa/fa rats. Importantly, however, no significant effect of leptin was observed on these genes in brown adipocytes from obese fa/fa animals. The presence of OB-Rb receptors in adipose tissue was substantiated by the detection of its transcripts by RT-PCR, and leptin treatment in vivo and in vitro activated the specific STATs implicated in the signaling pathway of the OB-Rb. Taken together, our data strongly suggest that leptin has direct effects on BAT and WAT, resulting in the activation of the Jak/STAT pathway and the increased expression of certain target genes, which may partially account for the observed increase in glucose utilization and lipolysis in leptin-treated adipose tissue.

Dramatic enhancement of the specific expression of the heart-type fatty acid binding protein in rat brown adipose tissue by cold exposure

To understand the difference in energy metabolisms in brown (BAT) and white (WAT) adipose tissues, we examined the steady-state transcript levels of the heart-type and adipose-type fatty acid binding proteins (H-FABP and A-FABP, respectively) by Northern blot analysis. The transcript of H-FABP in rat BAT was increased about 100-fold by cold exposure, whereas that in WAT was negligible, and was increased only slightly by cold exposure. The transcript of A-FABP was observed in both BAT and WAT, the level being slightly greater in WAT. However, its transcript level was not affected by cold exposure in either adipose tissue. In addition, on treatment with norepinephrine (NE), transcript level of H-FABP was elevated markedly but that of A-FABP was not changed in rat brown adipocytes. Therefore, the stimulatory effect of cold exposure on the transcript of H-FABP in BAT was concluded to be mediated by NE, like that of the uncoupling protein (UCP). Thus, the expressions of H-FABP and UCP may be controlled by the same mechanism.

Adrenergic stimulation of lipoprotein lipase gene expression in rat brown adipocytes differentiated in culture: mediation via beta3- and alpha1-adrenergic receptors

In order to investigate whether the positive effect of adrenergic stimulation on lipoprotein lipase (LPL) gene expression in brown adipose tissue is a direct effect on the brown adipocytes themselves, the expression of the LPL gene was investigated by measuring LPL mRNA levels in brown adipocytes, isolated as precursors from the brown adipose tissue of rats and grown in culture in a fully defined medium before experimentation. Addition of noradrenaline led to an enhancement of LPL gene expression; the mRNA levels increased as a linear function of time for at least 5 h and were finally approx. 3 times higher than in control cells, an increase commensurate with that seen in vivo in both LPL mRNA levels and LPL activity during physiological stimulation. The increase was dependent on transcription. The effect of noradrenaline showed simple Michaelis-Menten kinetics with an EC50 of approx. 11 nM. beta3-Agonists (BRL-37344 and CGP-12177) could mimic the effect of noradrenaline; the beta1-agonist dobutamine and the beta2-agonist salbutamol could not; the alpha1-agonist cirazoline had only a weak effect. The effect of noradrenaline was fully inhibited by the beta-antagonist propranolol and was halved by the alpha1-antagonist prazosin; the alpha2-antagonist yohimbine was without effect. An increase in LPL mRNA level similar to (but not significantly exceeding) that caused by noradrenaline could also be induced by the cAMP-elevating agents forskolin and cholera toxin, and 8-Br-cAMP also increased LPL mRNA levels. The increase in LPL gene expression was not mediated via an increase in the level of an intermediary proteinaceous factor. It is concluded that the physiologically induced increase in LPL gene expression is a direct effect of noradrenaline on the brown adipocytes themselves, mediated via a dominant beta3-adrenergic pathway and an auxiliary alpha1-adrenergic pathway which converge at a regulatory point in transcriptional control.

Insulin-like growth factor I and insulin induce adipogenic-related gene expression in fetal brown adipocyte primary cultures

Fetal rat brown adipocytes show high-affinity binding sites for both insulin-like growth factor I (IGF-I) and insulin. Cell culture for 24 h in the presence of IGF-I or insulin, independently, up-regulated the mRNA expression of adipogenic-related genes, such as fatty acid synthase (FAS), glycerol-3-phosphate de-hydrogenase and insulin-regulated glucose transporter Glut4, and down-regulated the expression of phosphoenolpyruvate carboxykinase mRNA in a dose-dependent manner. Moreover, both IGF-I and insulin increased the FAS gene transcription rate at 2 h, producing a time-dependent accumulation of FAS mRNA. Furthermore IGF-I or insulin increased glucose uptake and lipid content throughout the 24 h culture period. Our results suggest that both IGF-I and insulin are major signals involved in initiating and/or maintaining the expression of adipogenic-related genes in fetal rat brown adipocytes.

High expression of a novel carnitine palmitoyltransferase I like protein in rat brown adipose tissue and heart: isolation and characterization of its cDNA clone

To characterize energy metabolism in rat brown adipose tissue (BAT), we carried out differential screening of a cDNA library of BAT with a cDNA probe of white adipose tissue (WAT) and isolated one cDNA clone. It contained a single open reading frame of 2,316 bases which encodes a protein of 88.2 kDa. The predicted amino acid sequence showed the highest homology (62.6%) with that of rat carnitine palmitoyltransferase I (CPTI). The transcript corresponding to this cDNA was found to be abundantly expressed in BAT and heart. Therefore, the isolated clone is concluded to encode a CPTI like protein expressed in BAT and heart.