Mitogenic and antiadipogenic properties of human growth hormone in differentiating human adipocyte precursor cells in primary culture

Angiogenesis in adipose tissue and obesity

While most tissues exhibit their greatest growth during development, adipose tissue is capable of additional massive expansion in adults. Adipose tissue expandability is advantageous when temporarily storing fuel for use during fasting, but becomes pathological upon continuous food intake, leading to obesity and its many comorbidities. The dense vasculature of adipose tissue provides necessary oxygen and nutrients, and supports delivery of fuel to and from adipocytes under fed or fasting conditions. Moreover, the vasculature of adipose tissue comprises a major niche for multipotent progenitor cells, which give rise to new adipocytes and are necessary for tissue repair. Given the multiple, pivotal roles of the adipose tissue vasculature, impairments in angiogenic capacity may underlie obesity-associated diseases such as diabetes and cardiometabolic disease. Exciting new studies on the single-cell and single-nuclei composition of adipose tissues in mouse and humans are providing new insights into mechanisms of adipose tissue angiogenesis. Moreover, new modes of intercellular communication involving micro vesicle and exosome transfer of proteins, nucleic acids and organelles are also being recognized to play key roles. This review focuses on new insights on the cellular and signaling mechanisms underlying adipose tissue angiogenesis, and on their impact on obesity and its pathophysiological consequences.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Tartrate-resistant acid phosphatase type 5/ACP5 promotes cell cycle entry of 3T3-L1 preadipocytes by increasing IGF-1/Akt signaling

Tartrate-resistant acid phosphatase (TRAP, encoded by ACP5)-overexpressing mice exhibit hyperplastic obesity. As the molecular mechanism remains elusive, the aims were to characterize the effect of TRAP on preadipocyte proliferation. We investigated cell cycle entry and signal transduction, that is, insulin-like growth factor 1 (IGF-1)/ insulin receptor substrate 1 (IRS-1) and the Akt signaling pathways, in 3T3-L1 preadipocytes treated with the TRAP 5a isoform. Results show that TRAP 5a increases S-phase entry. TRAP 5a stimulation increases IGF-1 mRNA and IRS-1 activation, indicative of insulin-like growth factor 1 receptor (IGF1R) activation. Furthermore, TRAP 5a stimulation resulted in Akt signaling pathway activation and subsequent increased nuclear translocation of β-catenin. In conclusion, TRAP 5a increases proliferation of preadipocytes in a dose-dependent fashion by promoting entry into S-phase. Part of this effect is likely due to increased IGF-1 signaling through the Akt signaling pathway.

Pattern of Use of Biosimilar and Originator Somatropin in Italy: A Population-Based Multiple Databases Study During the Years 2009-2014

PURPOSE

Somatropin [recombinant growth hormone (rGH)] is approved in children and adults for several conditions involving growth disturbances and the corresponding biosimilar is available in Italy since 2006. No population-based data are available on the pattern of rGH use in Italian clinical practice. This study aimed at exploring the pattern of biosimilar and originator rGH use in six Italian centers, where different policy interventions promoted biosimilar use.

METHODS

This population-based, drug-utilization study was conducted in the years 2009-2014, using administrative databases of Umbria, Tuscany, and Lazio Regions and Local Health Units of Caserta, Treviso, and Palermo. Naïve rGH users were characterized, and prevalence of use and discontinuation were assessed over time.

RESULTS

Among 6,785 patients treated with rGH during the study years, 4,493 (66.2%) were naïve users (males/females = 1.3), mostly affected by GH deficiency. The prevalence of rGH use increased from 2009 to 2010, remaining stable thereafter, but it was heterogeneous across centers (twofold higher prevalence of use in center n.2 than centers n.4 and 1 in 2014). Biosimilar rGH uptake increased over time but was low (7.8% in 2014) and heterogeneous as well. Discontinuation of rGH therapy occurred in 54.0% of naïve users, more frequently in females than males (58.1 vs. 50.9%). During the first year of treatment, discontinuation was frequent (39.9%), but no statistically significant differences were observed in treatment persistence for biosimilar vs. originator rGH (p > 0.05).

CONCLUSION

Geographical heterogeneity in the prevalence of rGH use was observed. Similarly, the biosimilar rGH uptake was low and variable across centers. Post-marketing monitoring is required to continuously monitor the benefit-risk profile of rGH, thus guaranteeing greater savings than only promoting lowest cost rGH.

The Impact of Adipose Tissue on Insulin Resistance in Acromegaly

Adipose tissue (AT) is recognized as key contributor to the systemic insulin resistance and overt diabetes seen in metabolic syndrome. Acromegaly is a disease characterized by excessive secretion of growth hormone (GH) and insulin-like growth factor I (IGF-I). GH is known both for its action on AT and for its detrimental effect on glucose metabolism and insulin signaling. In active acromegaly, while body fat deports are diminished, insulin resistance is increased. Early studies have demonstrated defects in insulin action, both at the hepatic and extrahepatic (i.e., muscle and fat) levels, in active disease. This review discusses recent data suggesting that AT inflammation, altered AT distribution, and impaired adipogenesis are potential mechanisms contributing to systemic insulin resistance in acromegaly.

The modulation of adiponectin by STAT5-activating hormones

Adiponectin is a hormone secreted from adipocytes that plays an important role in insulin sensitivity and protects against metabolic syndrome. Growth hormone (GH) and prolactin (PRL) are potent STAT5 activators that regulate the expression of several genes in adipocytes. Studies have shown that the secretion of adiponectin from adipose tissue is decreased by treatment with PRL and GH. In this study, we demonstrate that 3T3-L1 adipocytes treated with GH or PRL exhibit a reduction in adiponectin protein levels. Furthermore, we identified three putative STAT5 binding sites in the murine adiponectin promoter and show that only one of these, located at -3,809, binds nuclear protein in a GH- or PRL-dependent manner. Mutation of the STAT5 binding site reduced PRL-dependent protein binding, and supershift analysis revealed that STAT5A and -5B, but not STAT1 and -3, bind to this site in response to PRL. Chromatin immunoprecipitation (IP) analysis demonstrated that only STAT5A, and not STAT1 and -3, bind to the murine adiponectin promoter in a GH-dependent manner in vivo. Adiponectin promoter/reporter constructs were responsive to GH, and chromatin IP analysis reveals that STAT5 binds the adiponectin promoter in vivo following GH stimulation. Overall, these data strongly suggest that STAT5 activators regulate adiponectin transcription through the binding of STAT5 to the -3,809 site that leads to decreased adiponectin expression and secretion. These mechanistic observations are highly consistent with studies in mice and humans that have high GH or PRL levels that are accompanied by lower circulating levels of adiponectin.

GH action influences adipogenesis of mouse adipose tissue-derived mesenchymal stem cells

GH influences adipocyte differentiation, but both stimulatory and inhibitory effects have been described. Adipose tissue-derived mesenchymal stem cells (AT-MSCs) are multipotent and are able to differentiate into adipocytes, among other cells. Canonical Wnt/β-catenin signaling activation impairs adipogenesis. The aim of the present study was to elucidate the role of GH on AT-MSC adipogenesis using cells isolated from male GH receptor knockout (GHRKO), bovine GH transgenic (bGH) mice, and wild-type littermate control (WT) mice. AT-MSCs from subcutaneous (sc), epididiymal (epi), and mesenteric (mes) AT depots were identified and isolated by flow cytometry (Pdgfrα+ Sca1+ Cd45- Ter119- cells). Their in vitro adipogenic differentiation capacity was determined by cell morphology and real-time RT-PCR. Using identical in vitro conditions, adipogenic differentiation of AT-MSCs was only achieved in the sc depot, and not in epi and mes depots. Notably, we observed an increased differentiation in cells isolated from sc-GHRKO and an impaired differentiation of sc-bGH cells as compared to sc-WT cells. Axin2, a marker of Wnt/β-catenin activation, was increased in mature sc-bGH adipocytes, which suggests that activation of this pathway may be responsible for the decreased adipogenesis. Thus, the present study demonstrates that (i) adipose tissue in mice has a well-defined population of Pdgfrα+ Sca1+ MSCs; (ii) the differentiation capacity of AT-MSCs varies from depot to depot regardless of GH genotype; (iii) the lack of GH action increases adipogenesis in the sc depot; and iv) activation of the Wnt/β-catenin pathway might mediate the GH effect on AT-MSCs. Taken together, the present results suggest that GH diminishes fat mass in part by altering adipogenesis of MSCs.

Growth hormone treatment and risk of malignancy

Growth hormone (GH) treatment has been increasingly widely used for children with GH deficiencies as the survival rate of pediatric patients with malignancies has increased. Both GH and insulin-like growth factor-I have mitogenic and antiapoptotic activity, prompting concern that GH treatment may be associated with tumor development. In this review, the authors examined the relationship between GH treatment and cancer risk in terms of de novo malignancy, recurrence, and secondary neoplasm. Although the results from numerous studies were not entirely consistent, this review of various clinical and epidemiological studies demonstrated that there is no clear evidence of a causal relationship between GH treatment and tumor development. Nonetheless, a small number of studies reported that childhood cancer survivors who receive GH treatment have a small increased risk of developing de novo cancer and secondary malignant neoplasm. Therefore, regular follow-ups and careful examination for development of cancer should be required in children who receive GH treatment. Continued surveillance for an extended period is essential for monitoring long-term safety.

FTO deficiency induces UCP-1 expression and mitochondrial uncoupling in adipocytes

Variants in the fat mass- and obesity-associated (FTO) gene are associated with obesity and body fat mass in genome-wide association studies. However, the mechanism by which FTO predisposes individuals to obesity is not clear so far. First mechanistic evidence was shown in Fto-negative mice. These mice are resistant to obesity due to enhanced energy expenditure, whereas the mass of brown adipose tissue remains unchanged. We hypothesize that FTO is involved in the induction of white adipose tissue browning, which leads to mitochondrial uncoupling and increases energy expenditure. Uncoupling protein 1 (Ucp-1) was significantly higher expressed in both gonadal and inguinal adipose depots of Fto(-/-) compared with Fto(+/+) littermates accompanied by the appearance of multivacuolar, Ucp-1-positive adipocytes in these tissues. By using lentiviral short hairpin RNA constructs, we established FTO-deficient human preadipocytes and adipocytes and analyzed key metabolic processes. FTO-deficient adipocytes showed an adipogenic differentiation rate comparable with control cells but exhibited a reduced de novo lipogenesis despite unchanged glucose uptake. In agreement with the mouse data, FTO-deficient adipocytes exhibited 4-fold higher expression of UCP-1 in mitochondria compared with control cells. The up-regulation of UCP-1 in FTO-deficient adipocytes resulted in enhanced mitochondrial uncoupling. We conclude that FTO deficiency leads to the induction of a brown adipocyte phenotype, thereby enhancing energy expenditure. Further understanding of the signaling pathway connecting FTO with UCP-1 expression might lead to new options for obesity and overweight treatment.

Dwarfism and increased adiposity in the gh1 mutant zebrafish vizzini

Somatic growth and adipogenesis are closely associated with the development of obesity in humans. In this study, we identify a zebrafish mutant, vizzini, that exhibits both a severe defect in somatic growth and increased accumulation of adipose tissue. Positional cloning of vizzini revealed a premature stop codon in gh1. Although the effects of GH are largely through igfs in mammals, we found no decrease in the expression of igf transcripts in gh1 mutants during larval development. As development progressed, however, we found overall growth to be progressively retarded and the attainment of specific developmental stages to occur at abnormally small body sizes relative to wild type. Moreover, both subcutaneous (sc) and visceral adipose tissues underwent precocious development in vizzini mutants, and at maturity, the sizes of different fat deposits were greatly expanded relative to wild type. In vivo confocal imaging of sc adipose tissue (SAT) expansion revealed that vizzini mutants exhibit extreme enlargement of adipocyte lipid droplets without a corresponding increase in lipid droplet number. These findings suggest that GH1 signaling restricts SAT hypertrophy in zebrafish. Finally, nutrient deprivation of vizzini mutants revealed that SAT mobilization was greatly diminished during caloric restriction, further implicating GH1 signaling in adipose tissue homeostasis. Overall, the zebrafish gh1 mutant, vizzini, exhibits decreased somatic growth, increased adipose tissue accumulation, and disrupted adipose plasticity after nutrient deprivation and represents a novel model to investigate the in vivo dynamics of vertebrate obesity.

Failure to generate bone marrow adipocytes does not protect mice from ovariectomy-induced osteopenia

A reciprocal association between bone marrow fat and bone mass has been reported in ovariectomized rodents, suggesting that bone marrow adipogenesis has a negative effect on bone growth and turnover balance. Mice with loss of function mutations in kit receptor (kit(W/W-v)) have no bone marrow adipocytes in tibia or lumbar vertebra. We therefore tested the hypothesis that marrow fat contributes to the development of osteopenia by comparing the skeletal response to ovariectomy (ovx) in growing wild type (WT) and bone marrow adipocyte-deficient kit(W/W-v) mice. Mice were ovx at 4 weeks of age and sacrificed 4 or 10 weeks post-surgery. Body composition was measured at necropsy by dual-energy X-ray absorptiometry. Cortical (tibia) and cancellous (tibia and lumbar vertebra) bone architecture were evaluated by microcomputed tomography. Bone marrow adipocyte size and density, osteoblast- and osteoclast-lined bone perimeters, and bone formation were determined by histomorphometry. Ovx resulted in an increase in total body fat mass at 10 weeks post-ovx in both genotypes, but the response was attenuated in the in kit(W/W-v) mice. Adipocytes were present in bone marrow of tibia and lumbar vertebra in WT mice and bone marrow adiposity increased following ovx. In contrast, marrow adipocytes were not detected in either intact or ovx kit(W/W-v) mice. However, ovx in WT and kit(W/W-v) mice resulted in statistically indistinguishable changes in cortical and cancellous bone mass, cortical and cancellous bone formation rate, and cancellous osteoblast and osteoclast-lined bone perimeters. In conclusion, our findings do not support a causal role for increased bone marrow fat as a mediator of ovx-induced osteopenia in mice.

Adipogenesis

Adipose tissue is an important site for lipid storage, energy homeostasis, and whole-body insulin sensitivity. It is important to understand the mechanisms involved in adipose tissue development and function, which can be regulated by the endocrine actions of various peptide and steroid hormones. Recent studies have revealed that white and brown adipocytes can be derived from distinct precursor cells. This review will focus on transcriptional control of adipogenesis and its regulation by several endocrine hormones. The general functions and cellular origins of adipose tissue and how the modulation of adipocyte development pertains to metabolic disease states will also be considered.

Differential function of Akt1 and Akt2 in human adipocytes

Adipose tissue mass is determined by both cell size and cell number. Mouse models suggest that Akt isoforms are involved in the determination of fat mass by interfering with preadipocyte-to-adipocyte transition and regulating lipid storage. Here, we took advantage of a lentiviral mediated shRNA approach to study the role of Akt1 and Akt2 in differentiation and metabolism of human SGBS adipocytes. Adipogenic differentiation as measured by lipid accumulation was robustly inhibited in Akt2 deficient cells, whereas it was not affected by knockdown of Akt1. The knockdown of Akt2 caused an almost complete inhibition of preadipocyte proliferation. Furthermore, Akt2 deficient preadipocytes were significantly more sensitive to apoptosis induction by death receptor stimulation compared to Akt1 deficient cells. Both the knockdown of Akt1 or Akt2 equally affected insulin-stimulated lipogenesis as well as the anti-lipolytic effect of insulin. We conclude that Akt2 is indispensable for the regulation of preadipocyte and adipocyte number, whereas Akt1 and Akt2 are equally important for the regulation of insulin-stimulated metabolic pathways in human adipocytes. Recently proposed as an attractive target for the treatment of cancer, modulating Akt2 activity might also be a new molecular strategy to control adipose tissue mass.

The insulin-like growth factors in adipogenesis and obesity

Adipose tissue has been recognized as a major target of growth hormone (GH) action. GH was shown to inhibit adipocyte differentiation but stimulated preadipocyte proliferation in vitro. GH acts directly via its receptor or via upregulating insulin-like growth factor (IGF)-I, which is a critical mediator of preadipocyte proliferation, differentiation, and survival. Results from clinical studies on GH treatment in patients with GH deficiency or GH insensitivity syndrome can be used to dissect GH and IGF as well as IGF-binding protein (IGFBP) actions in vivo. In this article, changes of the GH/IGF system during adipocyte differentiation in vitro as well as related signaling pathways and their impact on adipose tissue growth and function are discussed. Clinical considerations include the effects of GH and IGF-I on adipose tissue during treatment of GH deficiency, differences in the IGF system between visceral and subcutaneous adipose tissue depots as well as the recently emerging role for adipose tissue in the regulation of glucose homeostasis.

IGF-I activation of the AKT pathway is impaired in visceral but not subcutaneous preadipocytes from obese subjects

Obesity morbidity is associated with excess visceral adiposity, whereas sc adipose tissue is much less metabolically hazardous. Human abdominal sc preadipocytes have greater capacity for proliferation, differentiation, and survival than omental preadipocytes. IGF-I is a critical mediator of preadipocyte proliferation, differentiation, and survival through multiple signaling pathways. We investigated IGF-I action in primary cultures of human preadipocytes isolated from sc and omental adipose tissue of obese subjects. IGF-I-stimulated DNA synthesis was significantly lower in omental compared with sc preadipocytes. IGF-I phosphorylation of the IGF-I receptor and the ERK pathway was comparable in sc and omental cells. However, omental preadipocytes had decreased insulin receptor substrate (IRS)-1 protein associated with increased IRS-1-serine(636/639) phosphorylation and degradation. IGF-I-stimulated phosphorylation of AKT on serine(473) but not threonine(308) was decreased in omental cells, and activation of downstream targets, including S6Kinase, glycogen synthase kinase-3, and Forkhead box O1 was also impaired. CyclinD1 abundance was decreased in omental cells due to increased degradation. Over-expression of IRS-1 by lentivirus in omental preadipocytes increased IGF-I-stimulated AKT-serine(473) phosphorylation. The mammalian target of rapamycin (mTOR)-Rictor complex regulates phosphorylation of AKT-serine(473) in 3T3-L1 adipocytes, but knockdown of Rictor by lentivirus-delivered short hairpin RNA in sc preadipocytes did not affect AKT-serine(473) phosphorylation by IGF-I. These data reveal an intrinsic defect in IGF-I activation of the AKT pathway in omental preadipocytes from obese subjects that involves IRS-1 but probably not mTOR-Rictor complex. We conclude that impaired cell cycle regulation by AKT contributes to the distinct growth phenotype of preadipocytes in visceral fat of obese subjects.

A novel effect of growth hormone on macrophage modulates macrophage-dependent adipocyte differentiation

The GH receptor (GHR) is expressed on macrophages. However, the precise role of GH in regulation of macrophage function is unclear. We hypothesized that soluble factors including cytokines produced by macrophages in a GH-dependent manner regulate adipogenesis. We confirmed expression and functional integrity of the GHR in the J774A.1 macrophage cells. Conditioned medium (CM) from macrophages inhibited adipogenesis in a 3T3-L1 adipogenesis assay. CM from GH-treated macrophages decreased the inhibitory effect of CM from macrophages on adipogenesis. This effect on preadipocyte differentiation was active only during the first (early) phase of adipocyte differentiation. CM from stromal vascular compartment macrophages of mice with macrophage-specific deletion of the GHR exhibited more inhibitory effect on 3T3-L1 preadipocyte differentiation compared with CM from stromal vascular compartment macrophages of control mice, indicating that intact GH action in primary macrophages also increases preadipocyte differentiation. GH did not increase IGF-1 expression in macrophages. PCR array analysis identified IL-1beta as a candidate cytokine whose expression was altered by GH in macrophages. Levels of IL-1beta mRNA and protein were significantly decreased in GH-treated J774A.1 macrophages. Nuclear factor-kappaB stimulates IL-1beta gene expression, and GH induced a significant decrease in the levels of phosphorylated nuclear factor-kappaB in macrophages. IL-1beta is a known inhibitor of adipogenesis, and these results support GH-dependent down-regulation of macrophage IL-1beta expression as one mechanism for the observed increase in adipogenesis with CM from GH-treated macrophages. We conclude that GH decreases secretion of IL-1beta by the macrophage and thus in a paracrine manner increases adipocyte differentiation. These results provide a novel mechanism for GH's actions in the control of adipogenesis.

Structure and activity of the human growth hormone receptor (hGHR) gene V2 promoter

Human GH (hGH) has important effects on growth as well as carbohydrate, fat, and protein metabolism. These actions require the presence of normal levels of a functional hGH receptor (hGHR) on the surface of target cells. hGHR gene expression is characterized by the use of several 5'-noncoding exons and alternative splicing, resulting in the generation of multiple mRNA isoforms. The hGHR V2 transcript is predominant in most tissues, including human fat. However, factors regulating its ubiquitous expression have remained unidentified. The present study was aimed at characterizing the mechanisms regulating hGHR V2 transcription. Two major V2 transcriptional start sites were identified by primer extension assays. The V2 proximal promoter is TATA-less, with several characteristics of a housekeeping gene promoter. Transient transfection analyses of 2.6 kb of the 5'-flanking region of V2 confirmed its promoter activity in multiple primate cell lines. Similar promoter activity patterns were observed in human SGBS preadipocytes and mature adipocytes but with much higher V2 promoter activity in mature adipocytes, suggesting that changes in the availability of specific factors during adipocyte differentiation play a role in V2 promoter regulation. Serial deletion and mutation analyses revealed that transcription of hGHR V2 in different cell types, including adipocytes, is determined by a core promoter and distinct inhibitory and activation domains in the 5'-promoter region as well as within the V2 exon. Our data suggest that V2 transcription is the result of a complex interplay involving multiple factors, to ensure appropriate expression of hGHR in different hGH target cells.

Mild mitochondrial uncoupling induces 3T3-L1 adipocyte de-differentiation by a PPARgamma-independent mechanism, whereas TNFalpha-induced de-differentiation is PPARgamma dependent

Impairment of mitochondrial activity affects lipid-metabolizing tissues and mild mitochondrial uncoupling has been proposed as a possible strategy to fight obesity and associated diseases. In this report, we characterized the 3T3-L1-adipocyte ;de-differentiation' induced by carbonyl cyanide (p-trifluoromethoxy)-phenylhydrazone (FCCP), a mitochondrial uncoupler. We found a decrease in triglyceride (TG) content in adipocytes incubated with this molecule. We next analyzed the expression of genes encoding adipogenic markers and effectors and compared the differentially expressed genes in adipocytes treated with FCCP or TNFalpha (a cytokine known to induce adipocyte de-differentiation). Furthermore, a significant decrease in the transcriptional activity of PPARgamma and C/EBPalpha transcription factors was found in adipocytes with impaired mitochondrial activity. However, although these modifications were also found in TNFalpha-treated adipocytes, rosiglitazone and 9-cis retinoic acid (PPARgamma and RXR ligands) were unable to prevent triglyceride loss in FCCP-treated cells. Metabolic assays also revealed that TG reduction could be mediated by a downregulation of lipid synthesis rather than an upregulation of fatty acid oxidation. Finally, lipolysis stimulated by the uncoupler also seems to contribute to the TG reduction, a process associated with perilipin A downregulation. These results highlight some new mechanisms that might potentially be involved in adipocyte de-differentiation initiated by a mitochondrial uncoupling.

Current and future perspectives on recombinant growth hormone for the treatment of obesity

The similarities between patients with untreated growth hormone (GH) deficiency and those with the cardiometabolic syndrome and the beneficial effects of recombinant human GH (rhGH) on body composition have led to the hypothesis that rhGH treatment may have utility in obesity. GH release is reduced in the setting of obesity, primarily due to hyperinsulinism and increased free fatty acid levels. We reviewed the outcomes of 23 clinical studies carried out between 1987 and 2006 that examined the effects of rhGH administration in the obese state. Typically, changes in overall body weight do not occur with rhGH therapy; however, assessment of body composition demonstrates reductions in visceral abdominal fat. Data on the effects of rhGH on lipid and carbohydrate metabolic profiles in obese patients are less clear-cut, with a subset of studies showing a beneficial effect and others a neutral effect. Given the increasing burden of obesity in the general population and the current paucity of effective therapies, it is useful to consider the data on rhGH and obesity from a clinical perspective to highlight potential treatment strategies that harness the somatotropic axis.

Subcutaneous fat in normal and diseased states

The quest for effective strategies to treat obesity has propelled fat research into an exploration of the molecular processes that drive adipocyte formation, and hence body fat mass. The development of obesity is dependent on the coordinated interplay of adipocyte hypertrophy (increased fat cell size), adipocyte hyperplasia (increased fat cell number), and angiogenesis. Evidence suggests that adipocyte hyperplasia, or adipogenesis, occurs throughout life, both in response to normal cell turnover as well as in response to the need for additional fat mass stores that arises when caloric intake exceeds nutritional requirements. Adipogenesis involves two major events-the recruitment and proliferation of adipocyte precursor cells, called preadipocytes, followed by the subsequent conversion of preadipocytes, or differentiation, into mature fat cells. In vitro studies using experimental and primary preadipocyte cell lines have uncovered the mechanisms that drive the adipogenic process, a tightly controlled sequence of events guided by the strict temporal regulation of multiple inhibitory and stimulatory signaling events involving regulators of cell-cycle functions and differentiation factors. This article reviews the current understanding of adipogenesis with emphasis on the various stages of adipocyte development; on key hormonal, nutritional, paracrine, and neuronal control signals; as well as on the components involved in cell-cell or cell-matrix interactions that are pivotal in regulating fat cell formation. Special consideration is given to clinical applications derived from adipogenesis research with impact on medical, surgical and cosmetic fields.

The role of 'adipotropins' and the clinical importance of a potential hypothalamic–pituitary–adipose axis

Since adipocytes express specific receptors for pituitary hormones and hypothalamic releasing factors, adipose tissue has to be regarded as a fast-acting endocrine gland under the control of the brain. Expanding on this suggestion, the existence and clinical impact of a hypothalamic-pituitary-adipose axis is reviewed. The term 'adipotropins' is introduced in order to describe pituitary and hypothalamic hormones or releasing factors that directly target adipocytes by their specific receptors.

Growth hormone and prolactin receptors in adipogenesis: STAT-5 activation, suppressors of cytokine signaling, and regulation of insulin-like growth factor I

Growth hormone GH stimulates lipolysis in mature adipocytes and primary preadipocytes but promotes adipogenesis in preadipocyte cell lines. The lactogenic hormones (prolactin PRL and placental lactogen) also stimulate adipogenesis in preadipocyte cell lines but have variable lipolytic and lipogenic effects in mature adipose tissue. We hypothesized that differences in expression of GH receptors GHR and PRL receptors PRLR during adipocyte development might explain some of the differential effects of the somatogens and lactogens on fat metabolism. To that end, we compared: (a) the expression of GHR and PRLR mRNAs in 3T3-L1 preadipocytes during the course of adipocyte differentiation; (b) the induction of STAT-5 activity by GH and PRL during adipogenesis; and (c) the acute effects of GH and PRL on the suppressors of cytokine signaling (SOCS-1-3 and cytokine-inducible SH2-domain-containing protein CIS) and IGF-I. In confluent, undifferentiated 3T3-L1 cells, the levels of GHR mRNA were approximately 250-fold higher than the levels of PRLR mRNA. Following induction of adipocyte differentiation the levels of PRLR mRNA rose 90-fold but GHR mRNA increased only 0.8-fold. Expression of both full-length (long) and truncated (short) isoforms of the PRLR increased during differentiation but the long isoform predominated at all time points. Mouse GH mGH stimulated increases in STAT-5a and 5b activity in undifferentiated as well as differentiating 3T3-L1 cells; mouse PRL mPRL had little or no effect on STAT-5 activity in undifferentiated cells but stimulated increases in STAT-5a and 5b activity in differentiating cells. mGH stimulated increases in SOCS-2 and SOCS-3 mRNAs in undifferentiated cells and SOCS-1-3 and CIS mRNAs in differentiating cells; mPRL induced CIS in differentiating cells but had no effect on SOCS-1-3. mPRL and mGH stimulated increases in IGF-I mRNA in differentiating cells but not in undifferentiated cells; the potency of mGH (3-6-fold increase, p < 0.01) exceeded that of mPRL (40-90% increase, p < 0.05). Our findings reveal disparities in the expression of PRLR and GHR during adipocyte development and differential effects of the hormones on STAT-5, the SOCS proteins, CIS, and IGF-I. These observations suggest that somatogens and lactogens regulate adipocyte development and fat metabolism through distinct but overlapping cellular mechanisms.

The regulation of fatty acid synthase by STAT5A

Growth hormone (GH) diminishes adipose tissue mass in vivo and decreases expression and activity of fatty acid synthase (FAS) in adipocytes. GH and prolactin (PRL) are potent activators of STAT5 and exert adipogenic and antiadipogenic effects in adipocytes. In this study, we demonstrate that GH and PRL decrease the mRNA and protein levels of FAS in 3T3-L1 adipocytes. We present evidence that indicates that FAS is repressed at the level of transcription. In addition, PRL responsiveness was shown to exist between -1,594 and -700 of the rat FAS promoter. Moreover, responsiveness to PRL was abolished with mutation of a site at position -908 to -893, which we have shown to bind STAT5A in a PRL-dependent manner. Taken together, these data strongly suggest that PRL directly represses expression of FAS in adipocytes through STAT5A binding to the -908 to -893 site. Furthermore, our results indicate that STAT5A has an antilipogenic function in adipocytes and may contribute to the regulation of energy balance.

Regulation of development and metabolism of adipose tissue by growth hormone and the insulin-like growth factor system

White adipose tissue plays a key role in the regulation of the energy balance of vertebrates. This tissue is also now recognized to secrete a variety of factors such as leptin, which is thought to be involved in the modulation of adipose mass. Unlike other tissues, adipose tissue mass has considerable capacity to expand. The review deals primarily on the regulation of development and metabolism of adipose tissue by growth hormone (GH) and the insulin-like growth factor (IGF) system, with a special focus on the pig. The anti-insulin effects of GH are well-documented in pigs as in other species. In vitro exposure of adipose precursor cells to GH leads to a decrease in differentiation of those cells in pigs, in contrast to data obtained in murine cell lines. In vivo treatment and prolonged in vitro incubation of adipose tissue or isolated adipocytes with GH result in a decrease in glucose transport and lipogenesis, especially at the level of the fatty acid synthase gene, resulting in a reduction of the lipid content and adipose tissue mass. The mechanism by which GH antagonizes insulin stimulation of lipogenesis is still unresolved, as it is not mediated by protein kinase A, protein kinase C and Janus kinase-2 at the signaling level, or upstream stimulatory factor 1 or sterol regulatory element binding protein-1 at the transcriptional level. GH is apparently the main regulator of IGF-I mRNA expression in adipose tissue, however, the effects of IGF-I on this tissue are rather unclear.

Adipogenesis: usefulness of in vitro and in vivo experimental models

The use of experimental models is the foundation of experimental biology, so it is important to know how much the models can tell us about actual animals. Inconsistent or contradictory results from in vitro models are often associated with the perception that a particular model or results are somehow wrong and therefore cannot tell us anything important about how an animal works. In fact, in vitro conditions do not create new biology. Differences between in vitro and in vivo behavior can only result from the actual cellular repertoire, which provides a powerful tool to uncover new information. Adipose tissue research provides a useful context for examining this issue because the regulation of adipose growth and metabolism has important economic implications for livestock production. Examples are discussed in which either excess skepticism or narrow interpretation of results slowed progress toward our current understanding of adipose biology. Similarly, contemporary examples using genomics are used to suggest that large inconsistencies are still apparent with in vitro methods. Careful consideration of these inconsistencies may provide new insights.

Constitutively active signal transducer and activator of transcription 5 can replace the requirement for growth hormone in adipogenesis of 3T3-F442A preadipocytes

Although it is the best characterized in vitro model of GH action, the mechanisms used by GH to induce differentiation of murine 3T3-F442A preadipocytes remain unclear. Here we have examined the role of three transcriptional regulators in adipogenesis. These regulators are either rapidly induced in response to GH [Stra13, signal transducer and activator of transcription (Stat)3] or of central importance to GH signaling (Stat5). Retroviral transfection of 3T3-F442A preadipocytes was used to increase expression of Stra13, Stat3, and Stat5a. Only Stat5a transfection increased the expression of adipogenic markers peroxisome proliferator-activated receptor gamma, CCAAT enhancer binding protein (C/EBP)alpha, and adipose protein 2/fatty acid-binding protein in response to GH, as determined by quantitative RT-PCR. Transfection with constitutively active Stat3 and Stat5a revealed that constitutively active Stat5a but not Stat3 was able to replace the GH requirement for adipogenesis. Constitutively active Stat5a but not Stat3 was able to increase the formation of lipid droplets and expression of alpha-glycerol phosphate dehydrogenase toward levels seen in mature adipocytes. Constitutively active Stat5a was also able to increase the expression of transcripts for C/EBPalpha to similar levels as GH, and of C/EBPbeta, peroxisome proliferator-activated receptor gamma, and adipose protein 2/fatty acid-binding protein transcripts to a lesser extent. An in vivo role for GH in murine adipogenesis is supported by significantly decreased epididymal fat depot size in young GH receptor-deleted mice, before manifestation of the lipolytic actions of GH. We conclude that Stat5 is a critical factor in GH-induced, and potentially prolactin-induced, murine adipogenesis.

Visceral adipose tissue and metabolic complications of obesity are reduced in Prader-Willi syndrome female adults: evidence for novel influences on body fat distribution

Visceral obesity is detrimental to health, but the mechanisms controlling body fat distribution are not fully understood. In premenopausal adult females (30 nonobese, 14 obese [body mass index >30 kg/m(2)]), variance in fasting insulin, glucose, insulin/glucose ratio, C-peptide/insulin ratio, triglycerides, and high-density lipoprotein/low-density lipoprotein-cholesterol ratio, were independently influenced by visceral but not total sc or abdominal sc adipose tissue, as measured by whole-body magnetic resonance imaging. Adult females with Prader-Willi syndrome (n = 13) had significantly reduced visceral adiposity, compared with obese controls (visceral/total sc adipose tissue ratio: 0.067 +/- 0.017 vs. 0.108 +/- 0.021), independent of their total adiposity (P < 0.001), or use of exogenous sex steroids. This is in contrast to that expected by their physical inactivity, hypogonadism, adult GH deficiency, and psychiatric problems. Females with Prader-Willi syndrome not receiving sex steroids (n = 8) had significantly reduced fasting insulin, insulin/glucose ratio, and triglycerides and increased C-peptide/insulin ratio, compared with obese controls, adjusting for total (P < 0.05) but not visceral adiposity (P = 0.3-0.6), supporting their association. The cause of the reduced visceral adiposity in Prader-Willi syndrome may reflect novel hormonal, hypothalamic, and/or genetic influences on body fat distribution.

Characterization of a human preadipocyte cell strain with high capacity for adipose differentiation

OBJECTIVE

To develop and to characterize a human preadipocyte cell strain with high capacity for adipose differentiation serving as a model for studying human adipocyte development and metabolism in vitro.

METHODS

Cells were derived from the stromal cells fraction of subcutaneous adipose tissue of an infant with Simpson-Golabi-Behmel syndrome (SGBS). Adipose differentiation was induced under serum-free culture conditions by exposure to 10 nM insulin, 200 pM triiodothyronine, 1 microM cortisol and 2 microM BRL 49653, a PPAR gamma agonist.

RESULTS

During the differentiation process SGBS cells developed a gene expression pattern similar to that found in differentiating human preadipocytes with a characteristic increase in fat cell-specific mRNAs encoding lipoprotein lipase (LPL), glycero-3-phosphate dehydrogenase (GPDH), GLUT4, leptin and others. Differentiated SGBS cells exhibited an increase in glucose uptake upon insulin stimulation and in glycerol release upon catecholamine exposure. SGBS adipocytes were morphologically, biochemically and functionally identical to in vitro differentiated adipocytes from healthy subjects. However, while preadipocytes from healthy control infants rapidly lost their capacity to differentiate after a few cell divisions in culture, SGBS cells maintained their differentiation capacity over many generations: upon appropriate stimulation 95% of SGBS cells of generation 30 developed into adipocytes. A mutation in the glypican 3 gene was not detected in the patient. Thus, it remains unclear whether the molecular alteration in SGBS cells is also responsible for the high differentiation capacity and further investigations are required.

CONCLUSION

The human cell strain described here provides an almost unlimited source of human preadipocytes with high capacity for adipose differentiation and may, therefore, represent a unique tool for studying human fat cell development and metabolism. International Journal of Obesity (2001) 25, 8-15

LiSa-2, a novel human liposarcoma cell line with a high capacity for terminal adipose differentiation

LiSa-2 is a stable cell line derived from a poorly differentiated, pleomorphic liposarcoma. In serum-containing medium, LiSa-2 cells are fibroblastoid and rapidly dividing. In a serum-free, chemically defined culture medium containing physiological concentrations of insulin, triiodothyronine and cortisol, LiSa-2 cells divide slower and, extensively storing fat, acquire adipocyte morphology. In contrast to fibroblastoid LiSa-2 cells, these adipocyte-like LiSa-2 cells highly express transcripts for peroxisome proliferator-activated receptor-gamma, lipoprotein lipase, fatty acid synthetase, hormone-sensitive lipase, adipocyte most abundant gene transcript-1, glycerol-3-phosphate-dehydrogenase and the insulin-sensitive glucose transporter-4, all of which are specific for differentiated adipocytes. However, leptin mRNA expression was demonstrated only after preventing DNA methylation by incorporation of 5-aza-deoxycytidine into cellular DNA. Functionally, adipocyte-like LiSa-2 cells show increased insulin-dependent glucose uptake and lipid synthesis and are sensitive to lipolytic agents. This cell line may serve as an in vitro model for studying the regulation of human liposarcoma differentiation and for screening drugs for induction of differentiation-associated growth arrest in liposarcomas.

Expressions of leptin and insulin-like growth factor-I are highly correlated and region-specific in adipose tissue of growing rats

OBJECTIVE

Anatomically distinct adipose tissue regions differ in their predominant modality of growth (i.e., cellular hypertrophy vs. hyperplasia). We examined site-specific patterns of expression of two genes whose products, leptin and insulin-like growth factor-I (IGF-I), could be involved in mediating differential growth and metabolism of white adipose tissue. We also related these patterns of expression to measures of adipose depot cellularity.

RESEARCH METHODS AND PROCEDURES

Male Wistar rats were fed ad libitum and studied from ages 7 weeks to approximately 12 months. Terminal measures of body weights; weights, composition, and cellularity of four white adipose depots; circulating leptin and IGF-I; and adipose depot-specific expression levels of leptin and IGF-I were measured in subsets of rats at 7, 12, 22, 42, and 46 weeks of age.

RESULTS

Both leptin and IGF-I mRNAs are quantitatively expressed in a depot-specific manner, in the following order: retroperitoneal approximately equals epididymal > mesenteric > subcutaneous inguinal. Furthermore, there is a marked correlation between the expressions of these hormones in the various regions of adipose tissue of rats during the first year of life. The mechanisms that underlie the parallel expressions of leptin and IGF-I appear to be related to fat-cell volume.

DISCUSSION

Because both leptin and IGF-I have been implicated in the regulation of energy homeostasis and are both expressed in adipose tissue, the depot-specific linkage between the two genes suggests interaction at the autocrine level. This interaction may have an important role in determining functional properties particular to individual adipose depots.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

The acquisition of obesity: insights from cellular and genetic research

The acquisition of increased adipose tissue mass in man occurs during prolonged periods of positive energy balance. Normally, energy homeostasis in children and adults is regulated strictly and the energy stores are kept within the defined age-dependent physiological range. Susceptibility to definitive increases in the level of energy balance during times of reduced energy consumption or increased energy intake, leading to changes in body composition and/or changes in relative body weight, seems to be genetically determined. Although at present much information on the regulation of energy homeostasis and related unfavourable factors exists from animal studies, knowledge of the regulation of energy balance in human subjects is still insufficient. Some evidence on relevant factors involved in the regulation of energy balance in man has been obtained from epidemiological data, as well as from studies of patients with rare monogenetic forms of obesity. In the present article a special focus will be put on the regulation of body energy stores at the level of the adipose tissue, with emphasis on the regulation of human adipocyte differentiation. In addition to the currently intensive scientific interest in the central regulation of energy homeostasis in man, there is sufficient evidence to support the idea that the acquisition of an increased adipose tissue mass is also dependent on the susceptibility of pre-adipocytes to proliferate, to differentiate or to enter into apoptosis.

The effect of GH and IGF-I on preadipocytes from Large White and Meishan pigs in primary culture

Proliferation and differentiation of preadipocytes from 7-day-old Large White (LW) and Meishan (MS) pigs were studied in primary culture. The effects of porcine GH (pGH) and IGF-I as well as the expression of GH (GHR) and IGF-I (IGF-IR) receptors mRNA were examined. Preadipocytes were exposed to serum-supplemented and serum-free medium to determine proliferation and differentiation, respectively. Proliferation was higher in MS than in LW pigs. Treatment with pGH (2 nM) or IGF-I (10 nM) resulted in a similar decrease in proliferation in LW and MS pigs. Parameters assessing differentiation and the effects of pGH and IGF-I on differentiation did not differ between the two breeds. The percentage of differentiating cells and LPL and ME activities were markedly reduced by pGH. IGF-I did not reduce differentiation significantly. Both GHR and IGF-IR mRNA were expressed in adipose tissue, adipocytes, preadipocytes, and 6-day-cultured cells from LW and MS pigs. The similar action of pGH and IGF-I on preadipocyte proliferation and differentiation, associated with the similar expression of GHR and IGF-IR mRNA in LW and MS pigs, suggests that the GH/IGF-I axis is not impaired in MS pigs. The difference in preadipocyte proliferation observed between LW and MS pigs could account for their adiposity difference.

Somatotropin regulates adipose tissue metabolism in neonatal swine

Somatotropin (ST) reduces lipid deposition in growing and adult animals, but its effect in neonatal pigs is not clear. In this study, we tested the hypothesis that ST inhibits lipid deposition in neonatal pig adipose tissue. Four neonatal (2.9 +/- 0.1 kg, 7 d of age) and four growing (17.0 +/- 1.4 kg, 60 +/- 3 d of age) crossbred pigs were used. Subscapular adipose tissue fragments were cultured with or without ST (4.5 nmol/L) for 24 h in the absence or presence of insulin (7 nmol/L). After culture for 24 h with insulin alone, adipocytes from neonatal and growing pig adipose tissue maintained the capacity to incorporate glucose into total lipid at rates comparable to those in fresh tissue. Culture for 24 h with ST in the presence or absence of insulin decreased adipocyte glucose incorporation into fatty acids. Addition of ST, in the absence or presence of insulin, also increased the accumulation of glycerol in the medium during culture of neonatal and growing pig adipose tissue. Furthermore, culture for 24 h with ST resulted in higher basal lipolysis measured during incubation of isolated adipocytes in the presence of adenosine deaminase. In addition, culture with ST decreased adipose tissue lipoprotein lipase (LPL) activity and completely blocked the stimulatory effect of insulin on activity of this enzyme. The present study is the first to demonstrate in neonatal pigs that, as in growing pigs, ST regulates adipose tissue metabolism through decreasing lipid synthesis and LPL activity and increasing lipolysis. Thus, ST may play an important role in nutrient partitioning during the neonatal period.

Pancreastatin inhibits insulin action in rat adipocytes

Pancreastatin (PST), a regulatory peptide with a general inhibitory effect on secretion, is derived from chromogranin A, a glycoprotein present throughout the neuroendocrine system. We have previously demonstrated the counterregulatory role of PST on insulin action in rat hepatocytes. Here, we are reporting the PST effects on rat adipocytes. PST dose dependently inhibits basal and insulin-stimulated glucose transport, lactate production, and lipogenesis, impairing the main metabolic actions of insulin in adipocytes. These effects were observed in a wide range of insulin concentrations, leading to a shift to the right in the dose-response curve. Maximal effect was observed at 10 nM PST, and the IC50 value was approximately 1 nM. Moreover, PST has a lipolytic effect in rat adipocytes (ED50 0.1 nM), although it was completely inhibited by insulin. In contrast, PST dose dependently stimulated protein synthesis and enhanced insulin-stimulated protein synthesis. In summary, these data show the lipokinetic effect of PST and the inhibitory effect of PST on insulin metabolic action within a range of physiological concentrations. Therefore, these results give new pathophysiological basis for the association of PST with insulin resistance.

The acute effect of dexamethasone on plasma leptin concentrations and the relationships between fasting leptin, the IGF-I/IGFBP system, dehydroepiandrosterone, androstenedione and testosterone in an elderly population

OBJECTIVE

To investigate the acute effect of dexamethasone administration on serum leptin levels and the relationships between dehydroepiandrosterone (DHEAS), androstenedione, testosterone and the IGF-I/IGFBP system and leptin levels in healthy elderly humans.

METHODS

In 209 healthy elderly individuals (95 men, 114 women, aged 55-80 years) measurements were made in the fasting state (0800 h) and after an overnight dexamethasone suppression test (1 mg p.o. at 2300 h.

RESULTS

Mean leptin levels increased from 6.2 +/- 0.4 (SE) micrograms/l to 7.3 +/- 0.5 (SE) micrograms/l in men and from 18.9 +/- 1.4 (SE) micrograms/l to 23.9 +/- 1.8 (SE) micrograms/l in women after 1 mg dexamethasone overnight ('post treatment')(P < 0.001 for both sexes). There was a significant relationship between post-treatment leptin and dexamethasone levels (men: P = 0.002; women: P < 0.001). The increase in leptin levels after dexamethasone administration was only partially related to the increase in plasma insulin concentrations. Cortisol levels were not related to leptin. In multivariate analyses the relationship between post-treatment leptin and dexamethasone levels remained after adjustment for post-treatment insulin levels, BMI, waist:hip ratio (WHR) and age (men: P < 0.001; women: P = 0.001). Plasma (free and total) IGF-I and IGFBP-3 levels were not related to leptin levels in men or women. IGFBP-1 levels were inversely related to leptin levels (P = 0.02), but this relationship was lost after adjustment for insulin, and/or BMI. In multivariate analyses the relationship between leptin and DHEAS was inverse in women (P = 0.04) (after adjustment for BMI, WHR, insulin and glucose), while there was no relationship between leptin and DHEAS in men.

CONCLUSIONS

Administration of dexamethasone acutely increased leptin levels within 9 h in this elderly population. This increase was only partly related to changes in circulating insulin concentrations, but was independent of BMI and waist:hip ratio. No relation existed between leptin and (free or total) IGF-I and IGFBP-3 in men or women. Dehydroepiandrosterone was inversely related to leptin in women. These findings suggest a contributory regulatory role for corticosteroids in modulating circulating leptin concentrations in elderly healthy individuals of both sexes, which is at least in part independent of insulin, BMI and waist:hip ratio. Dehydroepiandrosterone might play a role in the gender-specific differences in serum leptin levels.