A preadipocyte clonal line from mouse brown adipose tissue. Short- and long-term responses to insulin and beta-adrenergics

Whole Exome Sequencing Reveals Rare Variants in Genes Associated with Metabolic Disorders in Women with PCOS

Background

Polycystic ovary syndrome (PCOS) is a complex genetic trait, the pathogenesis of which is governed by an interplay of genetic and epigenetic factors. However, the aetiology of PCOS is not fully understood.

Aims

The objective of this study was to investigate the genetic causes of PCOS by identifying rare variants in genes implicated in its pathophysiology.

Settings and Design

This was a hospital-based observational study.

Materials and Methods

We used whole-exome sequencing for 52 PCOS women to identify the rare variants in genes related to PCOS pathogenesis. Subsequently, we analysed these variants using in silico prediction software to determine their functional effects. We then assessed the relationship between these variants and the clinical outcomes of the patients.

Statistical Analysis Used

Student's t-test and Fisher's exact test were used to compare clinical parameters and frequency differences amongst PCOS patients with and without variants.

Results

A total of four rare exonic variants in obesity- and hyperinsulinaemia-related genes including UCP1 (p.Thr227Ile), UCP2 (p.Arg88Cys), IRS1 (p.Ser892Gly) and GHRL (p.Leu72Met) were identified in eight patients. Significant differences were observed between the patients carrying variants and those without variants. PCOS patients with identified variants exhibited significantly higher average body mass index and fasting insulin levels of PCOS subjects with identified variants compared to those without variants (P < 0.05). Additionally, there were significant differences in the variant frequencies of four variants when compared to the population database (P < 0.05).

Conclusion

This study shows a prevalence of rare variants in obesity and hyperinsulinaemia-related genes in a cohort of PCOS women, thereby underscoring the impact of the identified rare variants on the development of obesity and associated metabolic derangements in PCOS women.

In vitro and ex vivo models of adipocytes

Adipocytes are specialized cells with pleiotropic roles in physiology and pathology. Several types of fat cells with distinct metabolic properties coexist in various anatomically defined fat depots in mammals. White, beige, and brown adipocytes differ in their handling of lipids and thermogenic capacity, promoting differences in size and morphology. Moreover, adipocytes release lipids and proteins with paracrine and endocrine functions. The intrinsic properties of adipocytes pose specific challenges in culture. Mature adipocytes float in suspension culture due to high triacylglycerol content and are fragile. Moreover, a fully differentiated state, notably acquirement of the unilocular lipid droplet of white adipocyte, has so far not been reached in two-dimensional culture. Cultures of mouse and human-differentiated preadipocyte cell lines and primary cells have been established to mimic white, beige, and brown adipocytes. Here, we survey various models of differentiated preadipocyte cells and primary mature adipocyte survival describing main characteristics, culture conditions, advantages, and limitations. An important development is the advent of three-dimensional culture, notably of adipose spheroids that recapitulate in vivo adipocyte function and morphology in fat depots. Challenges for the future include isolation and culture of adipose-derived stem cells from different anatomic location in animal models and humans differing in sex, age, fat mass, and pathophysiological conditions. Further understanding of fat cell physiology and dysfunction will be achieved through genetic manipulation, notably CRISPR-mediated gene editing. Capturing adipocyte heterogeneity at the single-cell level within a single fat depot will be key to understanding diversities in cardiometabolic parameters among lean and obese individuals.

Promotion of lipid storage rather than of thermogenic competence by fetal versus newborn calf serum in primary cultures of brown adipocytes

Much current understanding of brown adipocyte development comes from in-vitro cell models. Serum type may affect the behavior of cultured cells and thus conclusions drawn. Here, we investigate effects of serum type (“fetal bovine” versus “newborn calf”) on responses to differentiation inducers (the PPARγ agonist rosiglitazone or the neurotransmitter norepinephrine) in cultured primary brown adipocytes. Lipid storage was enhanced by fetal versus newborn serum. However, molecular adipose conversion (Pparg2 and Fabp4 expression) was not affected by serum type. Rosiglitazone-induced (7-days) expression of thermogenic genes (i.e. Ucp1, Pgc1a, Dio2 and Elovl3) was not systematically affected by serum type. However, importantly, acute (2 h) norepinephrine-induced thermogenic gene expression was overall markedly higher (and adipose genes somewhat lower) in cells cultured in newborn serum. Thus, newborn serum promotes thermogenic competence, and the use of fetal serum in brown adipocyte cultures (as is often routine) counteracts adequate differentiation. Agents that counteract this inhibition may therefore confoundingly be ascribed genuine thermogenic competence-inducing properties.

Brown adipose tissue activity as a target for the treatment of obesity/insulin resistance

Presence of brown adipose tissue (BAT), characterized by the expression of the thermogenic uncoupling protein 1 (UCP1), has recently been described in adult humans. UCP1 is expressed in classical brown adipocytes, as well as in “beige cells” in white adipose tissue (WAT). The thermogenic activity of BAT is mainly controlled by the sympathetic nervous system. Endocrine factors, such as fibroblast growth factor 21 (FGF21) and bone morphogenic protein factor-9 (BMP-9), predominantly produced in the liver, were shown to lead to activation of BAT thermogenesis, as well as to “browning” of WAT. This was also observed in response to irisin, a hormone secreted by skeletal muscles. Different approaches were used to delineate the impact of UCP1 on insulin sensitivity. When studied under thermoneutral conditions, UCP1 knockout mice exhibited markedly increased metabolic efficiency due to impaired thermogenesis. The impact of UCP1 deletion on insulin sensitivity in these mice was not reported. Conversely, several studies in both rodents and humans have shown that BAT activation (by cold exposure, β3-agonist treatment, transplantation and others) improves glucose tolerance and insulin sensitivity. Interestingly, similar results were obtained by adipose tissue-specific overexpression of PR-domain-containing 16 (PRDM16) or BMP4 in mice. The mediators of such beneficial effects seem to include FGF21, interleukin-6, BMP8B and prostaglandin D2 synthase. Interestingly, some of these molecules can be secreted by BAT itself, indicating the occurrence of autocrine effects. Stimulation of BAT activity and/or recruitment of UCP1-positive cells are therefore relevant targets for the treatment of obesity/type 2 diabetes in humans.

In vivo adeno-associated viral vector-mediated genetic engineering of white and brown adipose tissue in adult mice

Adipose tissue is pivotal in the regulation of energy homeostasis through the balance of energy storage and expenditure and as an endocrine organ. An inadequate mass and/or alterations in the metabolic and endocrine functions of adipose tissue underlie the development of obesity, insulin resistance, and type 2 diabetes. To fully understand the metabolic and molecular mechanism(s) involved in adipose dysfunction, in vivo genetic modification of adipocytes holds great potential. Here, we demonstrate that adeno-associated viral (AAV) vectors, especially serotypes 8 and 9, mediated efficient transduction of white (WAT) and brown adipose tissue (BAT) in adult lean and obese diabetic mice. The use of short versions of the adipocyte protein 2 or uncoupling protein-1 promoters or micro-RNA target sequences enabled highly specific, long-term AAV-mediated transgene expression in white or brown adipocytes. As proof of concept, delivery of AAV vectors encoding for hexokinase or vascular endothelial growth factor to WAT or BAT resulted in increased glucose uptake or increased vessel density in targeted depots. This method of gene transfer also enabled the secretion of stable high levels of the alkaline phosphatase marker protein into the bloodstream by transduced WAT. Therefore, AAV-mediated genetic engineering of adipose tissue represents a useful tool for the study of adipose pathophysiology and, likely, for the future development of new therapeutic strategies for obesity and diabetes.

Antiobesity and Antidiabetic β-agonists: Lessons Learned and Questions to be Answered

In several species of obese animals, a group of phenethanolamine beta-agonists stimulates lipolysis and thermogenesis, resulting in the loss of body fat and weight. Brown adipose tissue is considered to be the major target tissue for the antiobesity activity of these compounds. Independent of this antiobesity activity, some of these compounds are also antidiabetic and increase muscle mass. Based on the pharmacological profile of these compounds, a beta3-receptor was proposed and characterized in mouse, rat, and humans. The beta3-receptor in brown adipose tissue has been suggested to mediate the antiobesity activity of these beta-agonists. Whether this receptor is responsible for the antidiabetic activity and whether there is a linkage between the antiobesity/antidiabetic activity and the nutrient partitioning activity is not clear. Clinical trials with these mixed beta-agonists showed marginal antiobesity effects when caloric intake of subjects was restricted. Insulin sensitivity was also improved in some of the trials designed to test the antidiabetic activity of these compounds. Side effects included tachycardia and tremor. To eliminate these side effects, a second generation of compounds was selected for its agonist activity on rat beta3-receptors. Clinical trials with these compounds have shown little increase of energy expenditure even at high doses. Successful development of an antiobesity and antidiabetic drug from this class of compounds will require the elucidation of the physiological role of the human beta3-receptor and the regulatory mechanism between fuel efficiency and feeding behavior.

Changes in gene expression in a porcine preadipocyte cell line during differentiation

Adipocyte differentiation plays an important role in the formation of fat tissues in pigs and affects meat quality and productivity. Clarification of the nature of the pig genes that participate in adipocyte differentiation will provide a clue to the regulation of fat content and thickness in pig carcases by dietary control; it will also help to find target genes for exploring potentially useful polymorphisms for molecular breeding aimed at fat traits. We constructed a DNA oligomer microarray based on pig transcripts, and we used the array to investigate time-dependent changes in gene expression in the PSPA porcine preadipocyte cell line during differentiation into adipocytes. We selected genes with markedly altered expression (at least fivefold difference in comparison with expression in undifferentiated cells) and classified them into five groups according to gene expression pattern. In the early stage after stimulation of adipocyte differentiation, we observed up-regulation of many genes encoding proteins involved in regulating cell proliferation and transcription. Among the probes corresponding to transcripts that showed marked changes in expression, 27 were located within previously reported QTL regions for traits related to adipose tissues. These results will be valuable resources for finding the genes responsible for fat-related traits that have been identified in previous studies using various pig resource families.

Test systems to study the structure and function of uncoupling protein 1: a critical overview

The discovery of active brown adipose tissue (BAT) in healthy adult humans has renewed interest in the biology of this organ. BAT is capable of distributing nutrient energy in the form of heat allowing small mammals to efficiently defend their body temperature when acutely exposed to the cold. On the other hand BAT might be a target for the treatment of obesity and related diseases, as its pharmacological activation could allow release of excess energy stored in white adipose tissue depots. Energy dissipation in BAT depends on the activity of uncoupling protein 1 (UCP1), therefore a BAT-based obesity therapy requires a detailed understanding of structure and function of UCP1. Although UCP1 has been in the focus of research since its discovery, central questions concerning its mechanistic function and regulation are not yet resolved. They have been addressed in native mitochondria but also in several test systems, which are generally used to lower inter-experimental variability and to simplify analysis conditions. Different test systems have contributed to our current knowledge about UCP1 but of course all of them have certain limitations. We here provide an overview about research on UCP1 structure and function in test systems. So far, these have nearly exclusively been employed to study rodent and not human UCP1. Considering that the amino acid sequence of mouse and human UCP1 is only 79% identical, it will be essential to test whether the human version has a similarly high catalytic activity, allowing a relevant amount of energy dissipation in human BAT. Besides the issue of comparable mechanistic function a sufficiently high expression level of human UCP1 is a further prerequisite for anti-obesity therapeutic potential. Treatments which induce BAT hyperplasia and UCP1 expression in humans might therefore be equally important to discover as mere activators of the thermogenic process.

Tri-iodothyronine upregulates adiponutrin mRNA expression in rat and human adipocytes

Adiponutrin (PNPLA3) is expressed in adipose tissue. Although its precise function is unknown, some data suggest a dual role in lipid homeostasis. We have investigated the influence of thyroid hormone (TH) on PNPLA3 mRNA, in rat and human cultured white adipocytes and in rat white adipose tissue (WAT). Pnpla3 mRNA increased during differentiation of rat adipocytes in an insulin-dependent manner. Tri-iodothyronine further increased Pnpla3 expression at any day during differentiation and its effects were time and dose-dependent. The Pnpla3 mRNA half-life was stabilized by tri-iodothyronine, but a transcriptional component was also observed. Pnpla3 mRNA decreased in WAT of hypothyroid rats and was partially restored by treatment with TH. Taqman analysis showed that tri-iodothyronine also increased human PNPLA3 expression in cultured subcutaneous adipocytes from obese patients. In conclusion, PNPLA3 mRNA expression is upregulated by tri-iodothyronine in adipocytes in vitro, in humans and rats, and in vivo in rat WAT.

A combination of octanoate and oleate promotes in vitro differentiation of porcine intramuscular adipocytes

To understand the relationship between intramuscular adipogenesis in the pig and the supply fatty acids, we established a clonal porcine intramuscular preadipocyte (PIP) line from the marbling muscle tissue of female Duroc pig. Confluent PIP cells exhibited a fibroblastic appearance. Their adipogenic ability was investigated using confluent PIP cells after exchanging growth medium for adipogenic medium containing 50 ng/mL insulin, 0.25 microM dexamethasone, 2 mM octanoate, and 200 microM oleate. Appropriate concentrations of octanoate and oleate for the induction of adipogenesis were determined from the ability of cells to accumulate lipid and the toxicity of fatty acids. When cells were cultured in differentiation medium for 8 days, large numbers of lipid droplets were observed in differentiated PIP cells, and their cytosolic TG content increased in a time-dependent manner. While oleate only induced the expression of PPARgamma mRNA, but not that of C/EBPalpha, octanoate significantly induced the expression of both PPARgamma and C/EBPalpha mRNA. Octanoate and oleate accelerated the inducing effect of insulin and dexamethasone on the expression of aP2 mRNA. These results indicate that a combination of octanoate and oleate synergistically induced PIP adipogenesis, and that the stimulation of octanoate was essential to the trigger for the adipogenesis in PIP cells.

Optimization of in vitro conditions for bovine subcutaneous and intramuscular preadipocyte differentiation

The objective of these experiments was to develop an in vitro cell culture system for differentiation of bovine preadipocytes, which will permit examination of differences in differentiation between intramuscular (i.m.) and subcutaneous (s.c.) bovine preadipocytes. Stromal-vascular cells from bovine i.m. and s.c. adipose depots were isolated and cultured. Clonally derived s.c. preadipocytes were used to determine the ability of insulin, bovine serum lipids, octanoate, acetic acid, dexamethasone (DEX), and troglitazone (TRO) to elicit differentiation of these cells when added to serum-free medium. Addition of 10 and 20 microL/mL of a commercially available serum lipids supplement to low-glucose Dulbecco's modified Eagle's medium containing 280 nM insulin increased glycerol-3-phosphate dehydrogenase (GPDH) activity (P < 0.01). Inclusion of 1.25 to 10 microM TRO to medium containing 280 nM insulin and 20 microL/ mL serum lipids supplement also increased GPDH activity (P < 0.001) compared with 0 microM TRO. The combination of 280 nM insulin, 1 mM octanoate, and 10 mM acetic acid, with 48 h exposure to 0.25 microM DEX caused morphological differentiation in a small number of cells but did not stimulate GPDH activity (P = 0.99). When used together, 280 nM insulin, 20 microL/mL of serum lipids supplement, 40 microM TRO, and 0.25 microM DEX stimulated differentiation compared with the aforementioned treatment (P < 0.001). Omission of TRO or insulin from this medium reduced GPDH activity by 68% (P < 0.001), whereas removal of DEX tended to reduce GPDH activity (P = 0.06). Preadipocytes from s.c. (n = 3) and i.m. (n = 2) adipose tissues of 3 steers were used to determine the effects of TRO on differentiation using the established conditions. Forty to sixty microM TRO enhanced differentiation compared with 0 microM TRO (P < 0.02) in both depots. No depot differences in response to TRO were detected (P = 0.32). These data demonstrate that bovine preadipocytes are capable of differentiation in response to combinations of insulin, serum lipids, DEX, and TRO. Although TRO enhanced differentiation of bovine preadipocytes, no differential effects of TRO on the differentiation of s.c. and i.m. cells were detected.

Cytosolic aspartate aminotransferase, a new partner in adipocyte glyceroneogenesis and an atypical target of thiazolidinedione

We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [(14)C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor gamma (PPARgamma)/retinoid X receptor-alpha direct binding. Nevertheless, PPARgamma is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARgamma. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORalpha binding. RORalpha ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORalpha and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORalpha expression by PPARgamma in adipocytes.

Expression of macrophage-selective markers in human and rodent adipocytes

CD14, CD68 and/or mouse F4/80 or human epidermal growth factor module-containing mucin-like receptor 1 (EMR1) are widely used as macrophage-specific markers. Since macrophages infiltrate several tissues during inflammatory processes, CD14, CD68 and EMR1-F4/80 have been employed to discriminate between tissue-containing macrophages, like adipose tissue (AT), and other cells. Using real-time PCR experiments, we show that isolated adipocytes from humans and mice AT express high levels of CD14 and CD68 mRNA, whereas EMR1-F4/80 is mainly present in the macrophage-containing stroma-vascular fraction. Furthermore, fibroblasts-like cells (adipoblasts), preadipocytes and adipocytes from the murine cell lines, 3T3-F442A and BFC-1, express CD14 and CD68 mRNA and protein as determined by fluorescence-activated cell sorter, but not F4/80 which, as expected, is strongly expressed in the macrophage cell line RAW264.7. These results reinforce the view that EMR1-F4/80 is the best macrophage marker to date and show that CD14 and CD68 are not macrophage-specific proteins.

Growth arrest by octanoate is required for porcine preadipocyte differentiation

A preadipocyte clonal line has been established from porcine subcutaneous tissue. This line, designated PSPA, showed a fibroblastic phenotype and kept on growing under a preadipose condition even after reaching confluence. When confluent cultures were stimulated with insulin, dexamethasone, biotin, pantothenate, and octanoate, growth was arrested, and the cells exhibited a marked increase in lipogenesis. However, adipose conversion was not induced upon exposure of PSPA cells to a standard hormonal mixture of mouse 3T3-L1 cells, and they continued dividing as did the preadipocytes in growth medium. By serially omitting each individual adipogenic agent from the PSPA differentiation medium, it was determined that octanoate was one of the most essential but the only factor able to induce growth arrest. Octanoate supplementation to 3T3-L1 medium increased the triglyceride accumulation of PSPA cells accompanied by growth arrest. Both RT-PCR and Western blot analysis supported the idea of octanoate as a potential agent with the antiproliferative activity requisite for porcine preadipocytes to enter terminal differentiation.

Novel adipocyte lines from brown fat: a model system for the study of differentiation, energy metabolism, and insulin action

Adipose tissue has emerged as an important endocrine regulator of glucose metabolism and energy homeostasis. By virtue of the mitochondrial protein uncoupling protein-1 (UCP-1), brown fat additionally plays a unique role in thermoregulation. Interest has focused on this tissue not only as a target for pharmacotherapy of obesity and insulin resistance but also as an endocrine tissue with leptin secretion and high insulin sensitivity. Most studies of adipocytes have been limited either to primary cell culture or to a small number of established cell lines. Recently, we have generated immortalized brown adipocyte cell lines from single newborn mice of different knockout mouse models. These cell lines retain the main characteristics of primary cells including UCP-1 expression. They display sensitive and diverse metabolic responses to insulin and adrenergic stimulation and have proven to be useful in the characterization of UCP regulation and the role of key insulin signaling elements for insulin action. Here, we outline common approaches to the generation of adipose tissue cell lines. Furthermore, we propose that the novel technique of generating brown adipocyte lines from a single newborn mouse will be instrumental in gaining further insight into the role of a broad range of signaling molecules in adipose tissue biology and in the pathogenesis of insulin resistance.

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.

Organization of extracellular matrix components during differentiation of adipocytes in long-term culture

Scanning electron microscopy (SEM) observation showed that fully differentiated spherical adipocytes were embraced by a network of collagens and fibroblastic preadipocytes. The properties of both the collagen networks and the preadipocytes allow the adipocytes to be interconnected, forming a fat-cell cluster, which can anchor to the bottom of a culture dish. In this network structure, collagen fibrils and fibrillar bundles were closely arranged and stratified. We found that immunostained collagens appeared to form extracellular network structures, which can be observed by SEM. The extracellular network of fibronectin was the first to develop among the extracellular matrix (ECM) components, though it became degraded with the progress of adipocyte differentiation. The type I collagen network was the last to develop and remained well organized through the late stage of adipocyte differentiation. The extracellular networks of type III, V, and VI collagen developed by the mid-stage and remained in the late stage of adipocyte differentiation. The network structures of type IV collagen and laminin became degraded during the differentiation process and localized at the surface of spherical cells. In addition to these basement membrane components, types III, V, and VI collagens also showed pericellular spherical staining patterns. These results demonstrated that the constitution and distribution of the ECM are altered during adipocyte differentiation, suggesting that the organization of each ECM component into a suitable structure is a requirement for the differentiation and maintenance of unilocular adipocytes.

Organization of extracellular matrix components during differentiation of adipocytes in long-term culture

Scanning electron microscopy (SEM) observation showed that fully differentiated spherical adipocytes were embraced by a network of collagens and fibroblastic preadipocytes. The properties of both the collagen networks and the preadipocytes allow the adipocytes to be interconnected, forming a fat-cell cluster, which can anchor to the bottom of a culture dish. In this network structure, collagen fibrils and fibrillar bundles were closely arranged and stratified. We found that immunostained collagens appeared to form extracellular network structures, which can be observed by SEM. The extracellular network of fibronectin was the first to develop among the extracellular matrix (ECM) components, though it became degraded with the progress of adipocyte differentiation. The type I collagen network was the last to develop and remained well organized through the late stage of adipocyte differentiation. The extracellular networks of type III, V, and VI collagen developed by the mid-stage and remained in the late stage of adipocyte differentiation. The network structures of type IV collagen and laminin became degraded during the differentiation process and localized at the surface of spherical cells. In addition to these basement membrane components, types III, V, and VI collagens also showed pericellular spherical staining patterns. These results demonstrated that the constitution and distribution of the ECM are altered during adipocyte differentiation, suggesting that the organization of each ECM component into a suitable structure is a requirement for the differentiation and maintenance of unilocular adipocytes.

Identification of an adipocyte-specific negative glucose response region in the cytosolic aspartate aminotransferase gene

Cytosolic aspartate aminotransferase (cAspAT) participates in gluconeogenesis in the liver and is expected to exert a glyceroneogenic function in the adipose tissue when the supply of glucose is limited. Here we demonstrate that adipose cAspAT messenger RNA (mRNA) is increased when rats are fed a low carbohydrate diet. In the 3T3-F442A, BFC-1 adipocyte cell lines and differentiated adipocytes in primary culture, a 24 h glucose deprivation induces approximately a 4-fold increase in cytosolic AspAT (cAspAT) mRNA, whereas mitochondrial AspAT mRNA remains unchanged. cAspAT activity is also increased in a weaker but reproducible manner. Addition of glucose within a physiological range of concentrations reverses the increase of cAspAT mRNA in 8 h (EC50 = 1.25 g/liter). Such a regulation requires protein synthesis and is specific for adipocytes differentiated in culture. It does not occur in Fao or H4IIE hepatoma cells, in C2 muscle cells, or in 293 kidney cells. 2-deoxyglucose mimicks glucose, while 3-orthomethyl-glucose has no effect, suggesting that glucose-6-phosphate is the effector. cAspAT mRNA stability is not affected by glucose deprivation. To ascertain the transcriptional nature of the glucose effect, we have stably transfected 3T3-F442A adipoblasts with constructs containing the chloramphenicol acetyltransferase reporter gene under the control of either 5'-deletions of the cAspAT gene promoter or internal fragments in an heterologous context. We demonstrate that a glucose response element(s) is present in the region between -1838 and -1702 bp relative to the translation start site. In this region, three DNA sequences bind nuclear proteins from adipocytes as shown by footprinting experiments. Our results indicate that cAspAT gene transcription is repressed by glucose selectively in adipocytes.

High expression of leptin by human bone marrow adipocytes in primary culture

Adipocytes participate in the microenvironment of the bone marrow (BM), but their exact role remains to be determined. It has recently been shown that leptin, a hormone secreted from extramedullary adipocytes, could be involved in hematopoiesis. Therefore we have developed a primary culture system of human BM adipocytes to characterize their differentiation and determine whether leptin is also secreted from these adipocytes. BM cells were cultured with fetal calf and horse sera. In the presence of dexamethasone, cells with vesicles containing lipids appeared within 15 days. They expressed glycerol phosphate dehydrogenase activity and a lipolytic activity in response to isoproterenol, but expressed neither the adrenergic beta3 receptor nor the mitochondrial uncoupling protein UCP1. The addition of insulin alone to the culture media did not promote adipocyte differentiation. Leptin was expressed and secreted at high levels during adipocyte differentiation. Acute exposure of differentiated adipocytes to insulin had little effect on leptin expression whereas forskolin strongly inhibited it. These results show that although human BM adipocytes differ from extramedullary adipose tissues in their sensitivity to different effectors, they are a secondary source of leptin production. They suggest that BM adipocytes could contribute to hematopoiesis via the secretion of leptin in the vicinity of hematopoietic stem cells.

Functional consequences of constitutively active alpha2A-adrenergic receptor expression in 3T3F442A preadipocytes and adipocytes

The functional consequences of a constitutively active mutated (CAM) human alpha2C10-adrenergic receptor (AR) stably expressed in the 3T3F442A preadipose cell line were analysed at both preadipocyte and adipocyte stages. At the preadipocyte stage, CAMalpha2C10-AR reproduced (in the absence of agonist) and amplified (in the presence of agonist) most of the cellular responses promoted by agonist-stimulated wild type alpha2C10-AR (increased preadipocyte proliferation, tyrosyl-phosphorylation of the Mitogen Activated Protein Kinases, resistance to serum-deprivation-induced cell retraction, inhibition of differentiation). In contrast, at the adipocyte stage, CAMalpha2C10-AR expression did not reproduced nor amplified the alpha2-adrenergic-dependent antilipolysis, but conversely led to a down-regulation of alpha i subunits of the Gi proteins and to an increase in the maximal response to lipolytic agents. Our results indicate that long term activation of intracellular signals by CAM-receptors not only lead to the expected cellular responses normally generated by agonist-stimulated wild type receptors, but can also lead to unexpected responses resulting from long term compensatory adaptations.

Retinyl ester hydrolysis and retinol efflux from BFC-1beta adipocytes

Adipose tissue is an important storage depot for retinol, but there are no data regarding retinol mobilization from adipose stores. To address this, dibutyryl cAMP was provided to murine BFC-1beta adipocytes and its effects on retinol efflux assessed. High performance liquid chromatography analysis of retinol and retinyl esters in adipocytes and media indicated that cAMP stimulated, in a time- and dose-dependent manner, retinol accumulation in the culture media and decreased cellular retinyl ester concentrations. Study of adipocyte retinol-binding protein synthesis and secretion indicated that cAMP-stimulated retinol efflux into the media did not result from increased retinol-retinol-binding protein secretion but was dependent on the presence of fetal bovine serum in the culture media. Since our data suggested that retinyl esters can be hydrolyzed by a cAMP-dependent enzyme like hormone-sensitive lipase (HSL), in separate studies, we purified a HSL-containing fraction from BFC-1beta adipocytes and demonstrated that it catalyzed retinyl palmitate hydrolysis. Homogenates of Chinese hamster ovary cells overexpressing HSL catalyzed retinyl palmitate hydrolysis in a time-, protein-, and substrate-dependent manner, with an apparent Km for retinyl palmitate of 161 microM, whereas homogenates from control Chinese hamster ovary cells did not.

Functional differentiation of white and brown adipocytes

Adipose tissue plays an important role in mammalian energy equilibrium not only as a lipid-dissipating, i.e. energy-storing, tissue (white adipose tissue), but also as an energy-dissipating one (brown adipose tissue). Brown adipocytes have the ability of facultative heat production due to a unique mitochondrial protein, the uncoupling protein (UCP). Differentiation of white and (to a lesser extent) brown adipocytes has been studied in different cell culture systems, which has led to the identification of external inducers, second messenger pathways and transcription factors involved in adipocyte differentiation. Functional differentiation of white adipocytes implies adipose conversion, whereas in brown adipocytes it insinuates additionally the development of a thermogenic function. This review discusses recent advances in the elucidation of the pathways responsible for, and the molecular bases of, adipose conversion on the one hand and development of the thermogenic properties of brown adipocytes on the other.

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.

Inhibition of hormone-sensitive lipase gene expression by cAMP and phorbol esters in 3T3-F442A and BFC-1 adipocytes

Hormone-sensitive lipase (HSL) catalyses the rate-limiting step in adipocyte lipolysis. Short-term hormonal regulation of HSL activity is well characterized, whereas little is known about the control of HSL gene expression. We have measured HSL mRNA content of 3T3-F442A and BFC-1 adipocytes in response to the cAMP analogue 8-(4-chlorophenylthio)-cAMP (8-CPT-cAMP) and to the phorbol ester phorbol 12-myristate 13-acetate (PMA) by Northern blot, using a specific mouse cDNA fragment. Treatment of the cells for 12 or 6 h with, respectively, 0.5 mM 8-CPT-cAMP or 1 microM PMA produced a maximal decrease of about 60% in HSL mRNA. These effects were unaffected by the protein-synthesis inhibitor anisomycin, suggesting that cAMP and PMA actions were direct. The reduction in HSL mRNA was accompanied by a reduction in HSL total activity. The intracellular routes that cAMP and PMA follow for inducing such an effect seemed clearly independent. (i) After desensitization of the protein kinase C regulation pathway by a 24 h treatment of the cells with 1 microM PMA, PMA action was abolished whereas cAMP was still fully active. (ii) Treatment with saturating concentrations of both agents produced an additive effect. (iii) The synthetic glucocorticoid dexamethasone had no proper effect on HSL gene expression but potentiated cAMP action without affecting PMA action. cAMP inhibitory action on HSL is unexpected. Indeed, the second messenger of catecholamines is the main activator of HSL by phosphorylation. We envision that a long-term cAMP treatment of adipocytes induces a counter-regulatory process that reduces HSL content and, ultimately, limits fatty acid depletion from stored triacylglycerols.

Characterization of the novel brown adipocyte cell line HIB 1B. Adrenergic pathways involved in regulation of uncoupling protein gene expression

The HIB 1B cell line, derived from a brown fat tumor of a transgenic mouse, is the first established brown adipocyte cell line capable of expressing the brown fat-specific mitochondrial uncoupling protein (UCP). UCP gene expression, which was virtually undetectable under basic conditions, was stimulated by acute catecholamine or cyclic AMP treatment to levels comparable to primary cultures of brown adipocytes. Elevation of UCP mRNA levels following stimulation was very rapid but transient, decreasing after about 4 hours with a half-life between 9 and 13 hours. Immunoblotting showed the presence of UCP in HIB 1B mitochondria, but expression was much lower than observed in BAT or primary cultures of brown adipocytes. Upon transfection of HIB 1B cells with a reporter gene containing the UCP promoter, the activity of the transgene was regulatable by cAMP and norepinephrine. Investigation of the possible adrenergic receptors involved in UCP stimulation showed that specific beta 3-adrenergic agonists were much less effective than nonspecific beta-adrenergic agonists and that mRNA levels of the atypical, fat-specific beta 3-adrenoceptor were lower than those observed in brown adipocytes differentiated in primary culture. From pharmacological evidence we conclude that beta 3-adrenergic receptors account for approximately 30–40% of catecholamine induced UCP gene stimulation, whereas about 60–70% is stimulated via the classical beta 1/2 adrenergic pathway. We conclude that HIB 1B cells represent a functional system for the study of mechanisms related to brown adipose thermogenesis.

Metabolism of methylamine by semicarbazide-sensitive amine oxidase in white and brown adipose tissue of the rat

The metabolism of [14C]methylamine (MA) by amine oxidase activity in rat white and brown adipose tissue homogenates, and in mature adipocytes from these tissues has been studied. Oxidation of MA was completely inhibited by 0.1-1 mM semicarbazide, without being affected by the monoamine oxidase (MAO) inhibitor, pargyline (1 mM), indicating that MA is metabolized by semicarbazide-sensitive amine oxidase (SSAO) and not by MAO. The mean Km for MA deamination in all of these sources was around 250-300 microM. SSAO activity towards MA was also demonstrated in white and brown pre-adipocytes, transformed to the adipose phenotype by treatment in culture for 7 days with lipogenic agents. These results are similar to previous findings that SSAO in vascular smooth muscle is able to metabolize aliphatic amines such as MA, and furthermore suggest that SSAO may play a role in adipose tissue function and/or maturation.

Regulation of UCP gene expression in brown adipocytes differentiated in primary culture. Effects of a new beta-adrenoceptor agonist

Primary cultures of precursor cells from mouse and rat brown adipose tissue (BAT) were used to study the effect of a new beta-agonist (ICI D7114) on the uncoupling protein (UCP) gene expression. ICI 215001 (the active metabolite of D7114) increased the expression of UCP and its mRNA in brown adipocytes differentiating in vitro in a dose-dependent manner. This stimulating effect was not inhibited by propranolol, a non-specific beta-antagonist, but was partially reduced by bupranolol, a beta 3-antagonist. No expression of UCP mRNA was ever induced by ICI 215001 in white adipocytes differentiated in vitro. It was concluded that the drug could affect the brown adipose cells through a beta 3-pathway. It could clearly modulate the expression of UCP in brown adipocytes differentiated in vitro, but was not able by itself to turn on the gene.

Glycosylation, activity and secretion of lipoprotein lipase in cultured brown adipocytes of newborn mice. Effect of tunicamycin, monensin, 1-deoxymannojirimycin and swainsonine

The effect of inhibitors on the glycosylation, activity and secretion of lipoprotein lipase was studied in brown adipocytes cultured from newborn mice. Such cells synthesized and secreted active lipoprotein lipase. It is generally accepted that active lipoprotein lipase is a homodimer. Glycosylation of lipoprotein lipase was analysed by PAGE of endoglycosidase H (endo H)-digested subunits of lipoprotein lipase immunoprecipitated from cells incubated for 1-2 h with [35S]methionine. The most prevalent 35S-labelled lipase subunit (Mr 57,000-58,000) in these cells contained endo H-resistant oligosaccharide chains, the next most prevalent contained totally endo H-sensitive chains, and the least prevalent subunit contained partially endo H-sensitive chains. Complete blocking of the glycosylation of lipoprotein lipase with tunicamycin (1 microgram/ml) for 24 h resulted in synthesis of an inactive non-secretable form of lipase with a smaller subunit (Mr 51,000-52,000). Immunofluorescent studies showed that unglycosylated lipase in tunicamycin-treated cells was retained in the endoplasmic reticulum. Cells treated with 1 microM-monensin, an intra-Golgi transport inhibitor, synthesized an active form of lipase which was not secreted, but was retained in the Golgi. The lipase in monensin-treated cells contained only partially or totally endo H-sensitive chains. Blocking either Golgi mannosidase I with 4 mM-1-deoxymannojirimycin or Golgi mannosidase II with 10 microM-swainsonine resulted in production of a form of lipoprotein lipase which was active and secreted, and which contained only endo H-sensitive chains. Our findings demonstrate that core glycosylation of lipoprotein lipase in the endoplasmic reticulum is required for lipase activity and transport from the reticulum, whereas processing of the oligosaccharide chains to endo H-resistant (complex) type chains in the Golgi is not required for either the activity or the secretion of lipoprotein lipase.

Increase in membrane uptake of long-chain fatty acids early during preadipocyte differentiation

An increase in early rates of oleate uptake, which reflected fatty acid (FA) entry into the cells, was apparent 2-3 days after confluence of differentiating BFC-1 preadipocytes. The increase was measured in cells kept without glucose and with arsenate, where greater than 95% of intracellular radioactivity was recovered as free unesterified oleate. Uptake of retinoic acid, a molecule structurally similar to long-chain FA, remained unaltered during cell differentiation. Increase in oleate transport was related to increase in transport Vmax (determined under arsenate treatment) from 0.2 to 2 nmol/min per 10(6) cells, whereas Km remained unchanged (2 x 10(-7) M). Oleate transport was maximal at about day 6 after cell confluence (day 0), as FA metabolism (incorporation into lipids) began to gradually increase. The increase in transport preceded induction of mRNAs for both cytosolic FA-binding protein, which appeared at day 6, and for the FA synthase, which appeared at day 10. Data indicated that increases in activities of FA transport and of lipoprotein lipase, early during cell differentiation, favored increased availability of exogenous FA at a stage when endogenous FA synthesis is limited. This result would promote FA esterification and lipid deposition by supplying a rate-limiting substrate. Furthermore, oleate addition to BFC-1 preadipocytes at confluence potentiated the effect of dexamethasone in inducing mRNA for cytosolic FA-binding protein. In adipocytes, FA from exogenous or endogenous sources was necessary to maintain levels of cytosolic FA-binding protein mRNA. Thus, the increase in FA availability might contribute to, or modulate, induction of proteins necessary for preadipocyte differentiation.

Metabolism of oleic acid in differentiating BFC-1 preadipose cells

Incorporation of [3H]oleate and [14C]glucose into cellular lipids was studied in the preadipose cell line BFC-1 to determine flux changes that accompany the adipose conversion process. Dilution of oleate by intracellular fatty acids (FA) was estimated from the 3H/14C incorporation ratios and from relating steady-state radioactivity in diglycerides to their measured cellular levels. The data indicated that exogenous FA mixed with less than 1% of endogenous FA on its pathway to esterification. Conversion of preadipocytes to adipocytes increased uptake of FA and glucose by approximately 3-fold and synthesis of diglycerides and triglycerides by 5- and 16-fold, respectively, with little if any increase of phospholipid synthesis. A 50% drop in 3H/14C incorporation ratio indicated a doubling of the rate at which endogenous FA mixed with the exogenous FA that had entered the cell. Adipocytes compared with preadipocytes exhibited a 50% greater cell diameter and a doubling of intracellular water volume and of protein and phospholipid levels, reflecting cellular enlargement consequent to the arrest of cell division that precedes adipose conversion. Diglyceride levels were also increased in adipocytes, however, since their turnover was fast, as indicated by rapid equilibration of diglyceride labeling; the increase reflected changes in their relative rates of synthesis and disposal. Diglyceride levels related to cell phospholipid, and other indexes of cell size remained constant. This indicated that the supply of diglycerides was tightly coupled to the synthesis of triglycerides and phospholipids, which suggested feedback regulation of diglyceride formation. The studies provide a methodological approach to measurement and interpretation of rates of lipid deposition in cultured cells.

Effects of cholera toxin on gene expression in brown preadipocytes differentiating in culture

To investigate the cellular control of the recruitment process in brown adipose tissue, the ability of cholera toxin to influence the differentiation of brown preadipocytes developing in culture was investigated. Stromalvascular cells obtained from the brown adipose tissue of 3-wk-old rats were grown in culture for 6-7 days in the presence or absence of cholera toxin. It was found that cholera toxin treatment decreased the expression of the actin gene (indicating an increased degree of differentiation), while at the same time promoting the expression of the genes coding for the mitochondriogenesis marker cytochrome-c oxidase and for the adipocyte conversion marker lipoprotein lipase (all followed at the mRNA level). Chronic cholera toxin treatment also increased the total amount of protein per cell in culture, and a specific cholera toxin-induced 35-kDa protein was identified. It was concluded that (in contrast to the case suggested for white preadipocytes) cholera toxin treatment of brown preadipocytes may not only affect the activity of catabolic enzymes but may also directly promote the differentiation process, indicating that this process is under beta-adrenergic control in the adapting animal.

Brown adipocytes differentiated in vitro can express the gene for the uncoupling protein thermogenin: effects of hypothyroidism and norepinephrine

Expression of the gene for the brown-fat specific uncoupling protein thermogenin was investigated in cell cultures by hybridization of isolated RNA with a cDNA clone corresponding to mouse thermogenin. The RNA was isolated 3-4 days after confluence from cells differentiated in culture from precursors isolated from the interscapular brown adipose tissue of 5-week-old mice. Very low thermogenin mRNA levels were found in cells derived from untreated mice, and there was only little effect of added norepinephrine on thermogenin gene expression in these cells. However, in cells derived from hypothyroid (methimazole-treated) mice there was a higher expression of thermogenin, and norepinephrine had a marked augmenting effect on the thermogenin mRNA level in these cells. These effects of thermogenin mRNA levels were specific, in that they contrasted with the effects of hypothyroidism and norepinephrine on the level of other mRNA species in these cells (coding for beta-actin, lipoprotein lipase, cytochrome-c oxidase, and glycerol-3-phosphate dehydrogenase). It was concluded that brown-fat cells in culture can reach a differentiated state, sufficiently advanced that the unique properties of these cells can be expressed, and that thermogenin gene expression (i.e., the level of thermogenin mRNA) is under direct control of norepinephrine.

Retrieval of precursors for white-type adipose conversion in brown adipose tissue

A cellular compartment from brown adipose tissue (BAT) of newborn rats was isolated by Percoll-density-gradient centrifugation and was shown to proliferate and to undergo adipose conversion in vitro in primary culture. The features of the effector requirement for adipose conversion as well as the differentiated morphological and biochemical phenotype are almost identical with that of a compartment designated HCF, from white adipose tissue (WAT). A possible role for these precursors from BAT and WAT in the involution of BAT into WAT, on the one hand, and in the development of brown adipose cells among typical WAT deposits, on the other, is discussed.

Expression of the mitochondrial uncoupling protein in brown adipocytes. Absence in brown preadipocytes and BFC-1 cells. Modulation by isoproterenol in adipocytes

The expression of the uncoupling protein has been compared in cells of BFC-1 clonal line established from mouse brown adipose tissue (BAT) and in preadipocytes, as well as in adipocytes from mouse BAT, both in primary culture. The results of immunoblots show that, after one week in culture, adipocytes have a reduced level of the 32 kD protein. This level can be raised 2-3.5-fold by a 24-h exposure to isoproterenol. Thus a direct modulation by a beta-agonist drug in the expression of the uncoupling protein is observed. Under the same conditions as well as under various other conditions, preadipocytes in primary culture and BFC-1 cells do not express the uncoupling protein. At the same time these cells are able both to differentiate into adipose cells, as demonstrated by the emergence of enzyme markers and triglyceride accumulation, and to respond to isoproterenol. Thus isoproterenol is not sufficient to trigger the expression of the uncoupling protein and behaves as a mere modulator once the cells have acquired the capacity to express it. Injection of undifferentiated BFC-1 cells into athymic mice bearing catecholamine-containing mini-osmotic pumps, or co-cultures of BFC-1 cells and pheochromocytoma PC-12 cells do not allow BFC-1 cells to express the uncoupling protein. Taken together, the results suggest that the formation of brown preadipocytes is critically linked during development to the release by sympathetic nerves of specific trophic factors acting locally.