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

Activation of Focal Adhesion Kinase Restores Simulated Microgravity-Induced Inhibition of Osteoblast Differentiation via Wnt/Β-Catenin Pathway

Simulated microgravity (SMG) inhibits osteoblast differentiation (OBD) and induces bone loss via the inhibition of the Wnt/β-catenin pathway. However, the mechanism by which SMG alters the Wnt/β-catenin pathway is unknown. We previously demonstrated that SMG altered the focal adhesion kinase (FAK)-regulated mTORC1, AMPK and ERK1/2 pathways, leading to the inhibition of tumor cell proliferation/metastasis and promoting cell apoptosis. To examine whether FAK similarly mediates SMG-dependent changes to Wnt/β-catenin in osteoblasts, we characterized mouse MC3T3-E1 cells cultured under clinostat-modeled SMG (µg) conditions. Compared to cells cultured under ground (1 g) conditions, SMG reduces focal adhesions, alters cytoskeleton structures, and down-regulates FAK, Wnt/β-catenin and Wnt/β-catenin-regulated molecules. Consequently, protein-2 (BMP2), type-1 collagen (COL1), alkaline-phosphatase activity and matrix mineralization are all inhibited. In the mouse hindlimb unloading (HU) model, SMG-affected tibial trabecular bone loss is significantly reduced, according to histological and micro-computed tomography analyses. Interestingly, the FAK activator, cytotoxic necrotizing factor-1 (CNF1), significantly suppresses all of the SMG-induced alterations in MC3T3-E1 cells and the HU model. Therefore, our data demonstrate the critical role of FAK in the SMG-induced inhibition of OBD and bone loss via the Wnt/β-catenin pathway, offering FAK signaling as a new therapeutic target not only for astronauts at risk of OBD inhibition and bone loss, but also osteoporotic patients.

Developmental programming of offspring adipose tissue biology and obesity risk

Obesity is reaching epidemic proportions and imposes major negative health crises and an economic burden in both high and low income countries. The multifaceted nature of obesity represents a major health challenge, with obesity affecting a variety of different organs and increases the risk of many other noncommunicable diseases, such as type 2 diabetes, fatty liver disease, dementia, cardiovascular diseases, and even cancer. The defining organ of obesity is the adipose tissue, highlighting the need to more comprehensively understand the development and biology of this tissue to understand the pathogenesis of obesity. Adipose tissue is a miscellaneous and highly plastic endocrine organ. It comes in many different sizes and shades and is distributed throughout many different locations in the body. Though its development begins prenatally, quite uniquely, it has the capacity for unlimited growth throughout adulthood. Adipose tissue is also a highly sexually dimorphic tissue, patterning men and women in different ways, which means the risks associated with obesity are also sexually dimorphic. Recent studies show that environmental factors during prenatal and early stages of postnatal development have the capacity to programme the structure and function of adipose tissue, with implications for the development of obesity. This review summarizes the evidence for a role for early environmental factors, such as maternal malnutrition, hypoxia, and exposure to excess hormones and endocrine disruptors during gestation in the programming of adipose tissue and obesity in the offspring. We will also discuss the complexity of studying adipose tissue biology and the importance of appreciating nuances in adipose tissue, such as sexual dimorphism and divergent responses to metabolic and endocrine stimuli. Given the rising levels of obesity worldwide, understanding how environmental conditions in early life affects adipose tissue phenotype and the subsequent development of obesity is of absolute importance.

Effects of stanniocalcin hormones on rat brown adipose tissue metabolism under fed and fasted conditions

In this study we determined the effect of fed and fasting (48 h) states on the expression of stanniocalcin-1 (Stc1) and stanniocalcin-2 (Stc2) in rat brown adipose tissue (BAT), as well as the in vitro effects of human stanniocalcin 1 and 2 (hSTC-1 and hSTC-2) hormones on lipid and glucose metabolism. In addition, lactate, glycogen levels and hexokinase (HK) activity were determined. In fasting Stc2 expression increased markedly. The targets of action of hSTC-1 and hSTC-2 were glucose uptake and oxidation as well as glycogen storage, controlling the energetic metabolism in BAT. The reduction in glycogen concentration induced by hSTC-2 in fed state might have deleterious consequences in BAT, such as decreased thermogenic activity, FA esterification and other adipocyte functions. On the other hand, the increase of glucose uptake caused by hSTC-1 of fed rats could play a role as a plasma glucose-clearing hormone in the postprandial period.

Inflammation, mesenchymal stem cells and bone regeneration

Achieving satisfactory reconstruction of bone remains an important goal in orthopedic and dental conditions such as bone trauma, osteoporosis, arthritis, osteonecrosis, and periodontitis. Appropriate temporal and spatial differentiation of mesenchymal stem cells (MSCs) is essential for postnatal bone regeneration. Additionally, an acute inflammatory response is crucial at the onset of bone repair, while an adaptive immune response has important implications during late bone remodeling. Various reports have indicated bidirectional interactions between MSCs and inflammatory cells or molecules. For example, inflammatory cells can recruit MSCs, direct their migration and differentiation, so as to exert anabolic effects on bone repair. Furthermore, both pro-inflammatory and anti-inflammatory cytokines can regulate MSCs properties and subsequent bone regeneration. MSCs have demonstrated highly immunosuppressive functions, such as inhibiting the differentiation of monocytes/hematopoietic precursors and suppressing the secretion of pro-inflammatory cytokines. This review emphasizes the important interactions between inflammatory stimuli, MSCs, and bone regeneration as well as the underlying regulatory mechanisms. Better understanding of these principles will provide new opportunities for promoting bone regeneration and the treatment of bone loss associated with immunological diseases.

Refeeding-induced brown adipose tissue glycogen hyper-accumulation in mice is mediated by insulin and catecholamines

Brown adipose tissue (BAT) generates heat during adaptive thermogenesis through a combination of oxidative metabolism and uncoupling protein 1-mediated electron transport chain uncoupling, using both free-fatty acids and glucose as substrate. Previous rat-based work in 1942 showed that prolonged partial fasting followed by refeeding led to a dramatic, transient increase in glycogen stores in multiple fat depots. In the present study, the protocol was replicated in male CD1 mice, resulting in a 2000-fold increase in interscapular BAT (IBAT) glycogen levels within 4-12 hours (hr) of refeeding, with IBAT glycogen stores reaching levels comparable to fed liver glycogen. Lesser effects occurred in white adipose tissues (WAT). Over the next 36 hr, glycogen levels dissipated and histological analysis revealed an over-accumulation of lipid droplets, suggesting a potential metabolic connection between glycogenolysis and lipid synthesis. 24 hr of total starvation followed by refeeding induced a robust and consistent glycogen over-accumulation similar in magnitude and time course to the prolonged partial fast. Experimentation demonstrated that hyperglycemia was not sufficient to drive glycogen accumulation in IBAT, but that elevated circulating insulin was sufficient. Additionally, pharmacological inhibition of catecholamine production reduced refeeding-induced IBAT glycogen storage, providing evidence of a contribution from the central nervous system. These findings highlight IBAT as a tissue that integrates both canonically-anabolic and catabolic stimulation for the promotion of glycogen storage during recovery from caloric deficit. The preservation of this robust response through many generations of animals not subjected to food deprivation suggests that the over-accumulation phenomenon plays a critical role in IBAT physiology.

Construction of engineering adipose-like tissue in vivo utilizing human insulin gene-modified umbilical cord mesenchymal stromal cells with silk fibroin 3D scaffolds

We evaluated the use of a combination of human insulin gene-modified umbilical cord mesenchymal stromal cells (hUMSCs) with silk fibroin 3D scaffolds for adipose tissue engineering. In this study hUMSCs were isolated and cultured. HUMSCs infected with Ade-insulin-EGFP were seeded in fibroin 3D scaffolds with uniform 50-60 µm pore size. Silk fibroin scaffolds with untransfected hUMSCs were used as control. They were cultured for 4 days in adipogenic medium and transplanted under the dorsal skins of female Wistar rats after the hUMSCs had been labelled with chloromethylbenzamido-1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (CM-Dil). Macroscopical impression, fluorescence observation, histology and SEM were used for assessment after transplantation at 8 and 12 weeks. Macroscopically, newly formed adipose tissue was observed in the experimental group and control group after 8 and 12 weeks. Fluorescence observation supported that the formed adipose tissue originated from seeded hUMSCs rather than from possible infiltrating perivascular tissue. Oil red O staining of newly formed tissue showed that there was substantially more tissue regeneration in the experimental group than in the control group. SEM showed that experimental group cells had more fat-like cells, whose volume was larger than that of the control group, and degradation of the silk fibroin scaffold was greater under SEM observation. This study provides significant evidence that hUMSCs transfected by adenovirus vector have good compatibility with silk fibroin scaffold, and adenoviral transfection of the human insulin gene can be used for the construction of tissue-engineered adipose.

Review of Signaling Pathways Governing MSC Osteogenic and Adipogenic Differentiation

Mesenchymal stem cells (MSC) are multipotent cells, functioning as precursors to a variety of cell types including adipocytes, osteoblasts, and chondrocytes. Between osteogenic and adipogenic lineage commitment and differentiation, a theoretical inverse relationship exists, such that differentiation towards an osteoblast phenotype occurs at the expense of an adipocytic phenotype. This balance is regulated by numerous, intersecting signaling pathways that converge on the regulation of two main transcription factors: peroxisome proliferator-activated receptor- γ (PPAR γ ) and Runt-related transcription factor 2 (Runx2). These two transcription factors, PPAR γ and Runx2, are generally regarded as the master regulators of adipogenesis and osteogenesis. This review will summarize signaling pathways that govern MSC fate towards osteogenic or adipocytic differentiation. A number of signaling pathways follow the inverse balance between osteogenic and adipogenic differentiation and are generally proosteogenic/antiadipogenic stimuli. These include β -catenin dependent Wnt signaling, Hedgehog signaling, and NELL-1 signaling. However, other signaling pathways exhibit more context-dependent effects on adipogenic and osteogenic differentiation. These include bone morphogenic protein (BMP) signaling and insulin growth factor (IGF) signaling, which display both proosteogenic and proadipogenic effects. In summary, understanding those factors that govern osteogenic versus adipogenic MSC differentiation has significant implications in diverse areas of human health, from obesity to osteoporosis to regenerative medicine.

A new era for brown adipose tissue: New insights into brown adipocyte function and differentiation

Until quite recently, brown adipose tissue was considered of metabolic significance only in small mammals and human newborns, since it was thought to disappear rapidly after birth in humans. However, nowadays this tissue is known to play a role in the regulation of energy balance not only in rodents, but also in humans. In this review we highlight new features regarding brown adipose tissue origin and function and revise old paradigms about brown adipocyte differentiation.

Heifer nutrient intake during early- and mid-gestation programs adult offspring adiposity and mRNA expression of growth-related genes in adipose depots

Changes in maternal nutrient intake during gestation alter IGF receptor abundance and leptin (LEP) mRNA expression in fetal adipose tissue. It is not known whether such changes persist into adult life and whether they are associated with an effect on phenotype. We investigated the effect of high (240%) and low (70%) levels of recommended daily crude protein intake for beef heifers during the first and second trimesters of gestation on singleton progeny (n=68): subcutaneous (SC) adipose tissue depth at rump (P8) and rib (RF) sites from 65 until 657 days of age; plasma leptin concentrations from birth until 657 days and expression of IGF1 and IGF2, their receptors (IGF1R and IGF2R) and LEP mRNA in perirenal (PR), omental (OM) and SC adipose tissue at 680 days of age. High-protein diets during the first trimester increased LEP and IGF1 mRNA in PR of males and females, respectively, compared with low-protein diets, and decreased IGF1R mRNA in SC of all progeny but increased RF depth of males between 552 and 657 days. High-protein diets compared with low-protein diets during the second trimester increased IGF1R mRNA in PR and OM of all progeny; LEP mRNA in PR of males; and IGF2 and IGF2R mRNA in OM of all progeny. Conversely, LEP mRNA in OM and IGF2 mRNA in PR of all progeny were decreased following exposure to high- compared with low-protein diets during the second trimester. Heifer diet during gestation has permanent sex- and depot-specific effects on the expression of adipogenic and adipocytokine genes and offspring adiposity.

Control of Brown Adipose Tissue Glucose and Lipid Metabolism by PPARγ

Brown adipose tissue (BAT) non-shivering thermogenesis impacts energy homeostasis in rodents and humans. Mitochondrial uncoupling protein 1 in brown fat cells produces heat by dissipating the energy generated by fatty acid and glucose oxidation. In addition to thermogenesis and despite its small relative size, sympathetically activated BAT constitutes an important glucose, fatty acid, and triacylglycerol-clearing organ, and such function could potentially be used to alleviate dyslipidemias, hyperglycemia, and insulin resistance. To date, chronic sympathetic innervation and peroxisome proliferator-activated receptor (PPAR) γ activation are the only recognized inducers of BAT recruitment. Here, we review the major differences between these two BAT inducers in the regulation of lipolysis, fatty acid oxidation, lipid uptake and triacylglycerol synthesis, glucose uptake, and de novo lipogenesis. Whereas BAT recruitment through sympathetic drive translates into functional thermogenic activity, PPARγ-mediated recruitment is associated with a reduction in sympathetic activity leading to increased lipid storage in brown adipocytes. The promising therapeutic role of BAT in the treatment of hypertriglyceridemic and hyperglycemic conditions is also discussed.

Maternal parity and its effect on adipose tissue deposition and endocrine sensitivity in the postnatal sheep

Maternal parity influences size at birth, postnatal growth and body composition with firstborn infants being more likely to be smaller with increased fat mass, suggesting that adiposity is set in early life. The precise effect of parity on fat mass and its endocrine sensitivity remains unclear and was, therefore, investigated in the present study. We utilised an established sheep model in which perirenal-abdominal fat mass (the major fat depot in the neonatal sheep) increases approximately 10-fold over the first month of life and focussed on the impact of parity on glucocorticoid sensitivity and adipokine expression in the adipocyte. Twin-bearing sheep of similar body weight and adiposity that consumed identical diets were utilised, and maternal blood samples were taken at 130 days of gestation. One offspring from each twin pair was sampled at 1 day of age, coincident with the time of maximal recruitment of uncoupling protein 1 (UCP1), whilst its sibling was sampled at 1 month, when UCP1 had disappeared. Plasma leptin was lower in nulliparous mothers than in multiparous mothers, and offspring of nulliparous mothers possessed more adipose tissue with increased mRNA abundance of leptin, glucocorticoid receptor and UCP2, adaptations that persisted up to 1 month of age when gene expression for interleukin-6 and adiponectin was also raised. The increase in fat mass associated with firstborn status is therefore accompanied by a resetting of the leptin and glucocorticoid axis within the adipocyte. Our findings emphasise the importance of parity in determining adipose tissue development and that firstborn offspring have an increased capacity for adipogenesis which may be critical in determining later adiposity.

Sp1 coordinately regulates de novo lipogenesis and proliferation in cancer cells

Cancers express high levels of fatty acid synthase (FAS) from which they derive fatty acids for membrane biosynthesis to sustain cell proliferation. How cancer cells coordinate de novo lipogenesis and proliferation has not been investigated. Transcription factors Sp1, Sp3 and Sp4 are overexpressed in a variety of cancers and regulate gene expression by interacting with GC-rich Sp1 binding sites. Genes encoding FAS and cell cycle proteins such as CDC25A contain Sp1 binding sites in their promoters. We demonstrate by RNA interference that Sp1, Sp3 and Sp4 all play a role in regulating CDC25A expression and proliferation in human breast cancer cells. Only Sp1, however, also regulates FAS. Furthermore, mithramycin, which blocks Sp1 binding sites, decreased proliferation, inhibited CDC25A and FAS expression and reduced binding of Sp1 to the promoters of these genes as assessed by ChIP assays. Conversely, 17beta-estradiol (E(2)) increased proliferation and CDC25A and FAS expression along with increased binding of Sp1 to the promoters of the 2 genes. In addition, we showed that the expression of sterol regulatory element-binding protein-1c (SREBP-1c), the only transcription factor that has been shown to regulate genes of lipogenic enzymes in cancer cells, is also regulated by Sp1. Finally, we demonstrated that Sp1 plays a role in sustaining proliferation and FAS expression in colon as well as prostate cancer cells. Overall, these observations suggest that Sp1 coordinately regulates de novo lipogenesis and proliferation in cancer cells.

Beneficial effects of PTP1B deficiency on brown adipocyte differentiation and protection against apoptosis induced by pro- and anti-inflammatory stimuli

Insulin is an inducer of brown fat adipogenesis through the activation of a signalling network that involves positive/negative modulators. Given the importance of brown adipose tissue (BAT) for basal thermogenic energy expenditure, we investigated the role of PTP1B in the acquisition of terminal differentiated phenotype and in the apoptotic responses of brown adipocytes. Immortalized brown preadipocytes lacking (PTP1B(-/-)) or expressing (PTP1B(+/+)) PTP1B have been generated. PTP1B deficiency accelerated a full program of brown adipogenesis including induction of transcription factors, coactivators, adipogenic markers and signalling molecules. Fully differentiated PTP1B(-/-) brown adipocytes were resistant to tumor necrosis factor (TNFalpha)-induced apoptosis as these cells were protected against caspase-8 activation, FLIP degradation, Bid cleavage and caspase-3 activation compared to wild-type controls. These events were recovered by PTP1B rescue. Survival signalling including phosphorylation of IRS-1 and Akt/PKB and BclxL expression were decreased in TNFalpha-treated PTP1B(-/-) cells but not in the wild-type. Similarly, PTP1B(-/-) brown adipocytes were protected against resveratrol-induced apoptosis. Phosphorylation of Akt/PKB and Foxo1 phosphorylation/acetylation decreased exclusively in resveratrol-treated wild-type cells, leading to nuclear localization of Foxo1 and up-regulation of Bim. Thus, PTP1B inhibition could be of benefit against obesity by counteracting TNFalpha-induced brown fat atrophy, and combined with resveratrol might improve low-grade inflammation.

The PPARgamma agonist rosiglitazone enhances rat brown adipose tissue lipogenesis from glucose without altering glucose uptake

We investigated the mechanisms whereby peroxisome proliferator-activated receptor-gamma (PPARgamma) agonism affects glucose and lipid metabolism in brown adipose tissue (BAT) by studying the impact of PPARgamma activation on BAT glucose uptake and metabolism, lipogenesis, and mRNA levels plus activities of enzymes involved in triacylglycerol (TAG) synthesis. Interscapular BAT of rats treated or not with rosiglitazone (15 mg*kg(-1).day(-1), 7 days) was evaluated in vivo for glucose uptake and lipogenesis and in vitro for glucose metabolism, gene expression, and activities of glycerolphosphate acyltransferase (GPAT), phosphatidate phosphatase-1 (PAP or lipin-1), and diacylglycerol acyltransferase (DGAT). Rosiglitazone increased BAT mass without affecting whole tissue glucose uptake. BAT glycogen content (-80%), its synthesis from glucose (-50%), and mRNA levels of UDP-glucose pyrophosphorylase (-40%), which generates UDP-linked glucose for glycogen synthesis, were all reduced by rosiglitazone. In contrast, BAT TAG-glycerol synthesis in vivo and glucose incorporation into TAG-glycerol in vitro were stimulated by the agonist along with the activities and mRNA levels of glycerol 3-phosphate-generating phosphoenolpyruvate carboxykinase and glycerokinase. Furthermore, rosiglitazone markedly increased the activities of GPAT and DGAT but not those of lipin-1-mediated PAP-1, enzymes involved in the sequential acylation of glycerol 3-phosphate and TAG synthesis. Because an adequate supply of fatty acids is essential for BAT nonshivering thermogenesis, the enhanced ability of BAT to synthesize TAG under PPARgamma activation may constitute an important mechanism by which lipid substrates are stored in preparation for an eventual thermogenic activation.

Insulin resistance induced by tumor necrosis factor-alpha in myocytes and brown adipocytes

Insulin resistance is an important contributor to the pathogenesis of type 2 diabetes, and obesity is a risk factor for its development, in part because adipose tissue secretes proteins, called adipokines, that may influence insulin sensitivity. Among these molecules, tumor necrosis factor (TNF)-alpha has been proposed as a link between obesity and insulin resistance because TNF-alpha is overexpressed in adipose tissues of obese animals and humans, and obese mice lacking either TNF-alpha or its receptor show protection against developing insulin resistance. Direct exposure to TNF-alpha induces a state of insulin resistance in terms of glucose uptake in myocytes and brown adipocytes because of the activation of proinflammatory pathways that impair insulin signaling at the level of the insulin receptor substrate (IRS) proteins. In this regard, the Ser(307) residue in IRS-1 has been identified as a site for the inhibitory effects of TNF-alpha in myotubes, with p38 mitogen-activated protein kinase and inhibitor kB kinase being involved in the phosphorylation of this residue. Conversely, Ser phosphorylation of IRS-2 mediated by TNF-alpha activation of mitogen-activated protein kinase was the mechanism found in brown adipocytes. Protein-Tyr phosphatase (PTP)1B acts as a physiological, negative regulator of insulin signaling by dephosphorylating the phosphotyrosine residues of the insulin receptor and IRS-1, and PTP1B expression is increased in muscle and white adipose tissue of obese and diabetic humans and rodents. Moreover, up-regulation of PTP1B expression was recently found in cells treated with TNF-alpha Accordingly, myocytes and primary brown adipocytes deficient in PTP1B are protected against insulin resistance induced by this cytokine. Furthermore, down-regulation of PTP1B activity is possible by the use of pharmacological agonists of nuclear receptors that restore insulin sensitivity in the presence of TNF-alpha. In conclusion, the lack of PTP1B in muscle and brown adipocytes increases insulin sensitivity and glucose uptake and could confer protection against insulin resistance induced by adipokines.

IRS-3 mediates insulin-induced glucose uptake in differentiated IRS-2(-/-) brown adipocytes

IRS-2 mediates insulin-induced glucose uptake in brown preadipocytes. Upon differentiation, basal IRS-3 expression increased concurrently with an enhancement in the IRS-3-associated phosphatidylinositol (PI) 3-kinase activity in the Triton-insoluble fraction in wild-type and IRS-2-deficient brown adipocytes stimulated with insulin. Moreover, insulin induced protein kinase B (Akt) and protein kinase C (PKC) zeta phosphorylation in both kinds of cells. More importantly, insulin induced glucose uptake in differentiated IRS-2-deficient brown adipocytes in a wortmannin-dependent manner. However, while insulin induced Akt phosphorylation occurred mainly in the cytosolic fraction, PKC zeta activation was constrained to the Triton-insoluble fraction. The reduction of IRS-3 expression by siRNA inhibited insulin-induced glucose uptake and also PKC zeta activation in differentiated IRS-2(-/-) brown adipocytes. In addition, inhibition of PKC zeta totally blunted insulin-induced glucose uptake in those cells. Our results provide evidences suggesting that IRS-3/PI 3-kinase/PKC zeta signaling is the main responsible for the insulin-induced glucose uptake observed upon differentiation of brown adipocytes lacking IRS-2.

Timing of nutrient restriction and programming of fetal adipose tissue development

It is apparent from epidemiological studies that the timing of maternal nutrient restriction has a major influence on outcome in terms of predisposing the resulting offspring to adult obesity. The present review will consider the extent to which maternal age, parity and nutritional restriction at defined stages of gestation can have important effects on fat deposition and endocrine sensitivity of adipose tissue in the offspring. For example, in 1-year-old sheep the offspring of juvenile mothers have substantially reduced fat deposition compared with those born to adult mothers. Offspring of primiparous adult mothers, however, show increased adiposity compared with those born to multiparous mothers. These offspring of multiparous ewes show retained abundance of the brown adipose tissue-specific uncoupling protein 1 at 1 month of age. A stimulated rate of metabolism in brown fat of these offspring may act to reduce adipose tissue deposition in later life. In terms of defined windows of development that can programme adipose tissue growth, maternal nutrient restriction targetted over the period of maximal placental growth results in increased adiposity at term in conjunction with enhanced abundance of mRNA for the insulin-like growth factor-I and -II receptors. In contrast, nutrient restriction in late gestation, coincident with the period of maximal fetal growth, has no major effect on adiposity but results in greater abundance of specific mitochondrial proteins, i.e. voltage-dependent anion channel and/or uncoupling protein 2. These adaptations may increase the predisposal of these offspring to adult obesity. Increasing maternal nutrition in late gestation, however, can result in proportionately less fetal adipose tissue deposition in conjunction with enhanced abundance of uncoupling protein 1.

The relationship between the insulin-like growth factor-1 system and the oestrogen metabolising enzymes in breast cancer tissue and its adjacent non-cancerous tissue

AIMS

Previous studies have shown that oestrogen and Insulin-like Growth Factor-1 (IGF-1) act synergistically and cross-stimulatory while the oestrogen receptor (ER) and IGF-1R downstream signalling pathways interact at many levels. We investigate the relationship between the ER, and IGF-1R and their ligands in a series of human breast cancer tissue and adjacent non-cancerous tissue (ANCT).

METHODS

A series of 139 pairs of breast cancer tissue and ANCT were obtained and divided into ER positive and ER negative groups based on tumour ER alpha immunostaining. All samples were processed for real-time quantitative-PCR to measure IGF-1, IGF-1R, ER alpha, STS and Cyp-19 mRNA levels. In addition, ER positive MCF-7 and ER negative MDA-MB-231 cell lines were treated separately with IGF-1 and an IGF-1R inhibitor called Tyrphostin AG1024 to see the effects of stimulating and inhibiting the IGF-1R. MCF-7 cell line was also treated with 4-hydroxytamoxifen. The mRNA levels of IGF-1, IGF-1R, ER alpha, STS and Cyp-19 of treated cell lines were measured and compared to those of non-treated controls. Data generated was normalised to Cytokeratin-19 mRNA levels.

RESULTS

IGF-1R expression was higher in tumour tissue compared to ANCT (P = 0.038) while IGF-1 expression was marginally higher in ANCT compared to tumour tissue only in the ER positive samples (P = 0.098). ER positive tumours had a higher expression of IGF-1 compared to ER negative tumours (P = 0.001) while IGF-1R, STS and Cyp-19 expression were higher in ER negative tumours (P = 0.000, 0.000 and 0.006 respectively). There was no difference in STS or Cyp-19 expression in tumours or ANCT. Using Spearman's Correlation test, IGF-1 positively correlated with STS, Cyp-19 and ER alpha in ER positive and negative groups (Coefficient = +0.497, +0.662 and +0.651 respectively, P = 0.000 in all). IGF-1R correlated with IGF-1, STS, Cyp-19 and ER alpha only in the ER negative tumours (Coefficient = +0.620, +0.394, +0.692 and +0.662 respectively, P = 0.000, 0.012, 0.000 and 0.000 respectively). In cell lines, IGF-1 treatment led to an increase in the mean expression of IGF-1, IGF-1R, STS and Cyp-19 in both cell lines while ER alpha expression increased only in MCF-7. IGF-1R inhibition caused a decrease in expression of all five genes in MDA-MB-231 but not in the MCF-7 cell line. Treatment with 4-hydroxytamoxifen caused a decrease in expression of all five genes.

CONCLUSIONS

IGF-1R is over-expressed in malignant tissue. IGF-1 is expressed at higher levels in ER positive tumours probably as a result of oestrogen stimulation while IGF-1R expression is higher in ER negative samples as an adaptation to lower local IGF-1 levels. An IGF-1 paracrine relationship may exist between tumour and ANCT but for STS and Cyp-19, there may be an autocrine-paracrine relationship. The IGF-1 ligand-receptor system is an important regulator of oestrogen production while oestrogen may be involved in stimulating IGF-1 expression. The expression of oestrogen synthesising enzymes is higher in ER negative breast cancers which may be due to the lack of oestrogen negative feedback or contribution from the overexpression of IGF-1R.

Ceramide mediates TNF-alpha-induced insulin resistance on GLUT4 gene expression in brown adipocytes

Tumour necrosis factor (TNF)-alpha impaired insulin induction on GLUT4 mRNA in foetal brown adipocytes, as demonstrated by quantitative RT-PCR and Northern blot. We have explored the hypothesis that some effects of TNF-alpha could be mediated by the generation of ceramide, since TNF-alpha treatment induced the production of ceramide in these primary cells. A short-chain ceramide analogue, C2-ceramide, precluded insulin-induced GLUT4 mRNA accumulation and GLUT4-chloramphenicol acetyltransferase (CAT) full promoter activation. Moreover, inhibition of the ceramide biosynthesis with fumonisin B, which inhibits ceramide synthase, completely restored insulin-induced GLUT4 mRNA and protein accumulation as well as GLUT4-CAT transactivation in the presence of TNF-alpha. In consequence, TNF-alpha-induced insulin resistance on glucose uptake was completely alleviated. In addition, TNF-alpha down-regulated insulin-induced CCAAT/enhancer binding protein (C/EBP)-alpha gene expression and DNA binding activity, but fumonisin B precludes these effects. Furthermore, co-transfection with a wild-type C/EBP-alpha construct transactivates GLUT4-CAT construct. Our results indicate that de novo ceramide produced by TNF-alpha-induced insulin resistance on GLUT4 gene expression in brown adipocytes by interfering C/EBP-alpha expression, a transcription factor essential for the expression of GLUT4.

IGF-1 induces rat glomerular mesangial cells to accumulate triglyceride

Rat glomerular mesangial cells (MC) become lipid-laden foam cells when they are exposed to IGF-1. IGF-1 increased accumulation of triglyceride (TG) 2.5-fold in MC after 7 days. TG accumulation resulted from enhanced macropinocytosis and decreased efflux secondary to a 40-50% reduction in peroxisome proliferator-activated receptor (PPAR)-delta (PPARdelta). There was no evidence of primary or secondary changes in cholesterol or TG synthesis, increased uptake by LDL or scavenger receptors, or reduced efflux via ATP-binding cassette A-1. Although the lipid moiety taken up can be influenced by the concentration of cholesterol or TG in the medium, in standard medium MC preferentially accumulate TG. TG-rich MC foam cells fail to contract in response to angiotensin II (Berfield AK, Andress DL, and Abrass CK. Kidney Int 62: 1229-1237, 2002); however, their migratory response to IGF binding protein-5 is unaffected. This differs from cholesterol loading, which impairs both phagocytosis and migration. These findings have important implications for understanding the mechanisms that contribute to lipid accumulation in MC and the functional consequences of different forms of foam cells. These observations are relevant to understanding vascular disease and progressive renal diseases that are accelerated by abnormalities in lipid metabolism.

Rosiglitazone up-regulates lipoprotein lipase, hormone-sensitive lipase and uncoupling protein-1, and down-regulates insulin-induced fatty acid synthase gene expression in brown adipocytes of Wistar rats

AIMS/HYPOTHESIS

Although thiazolidinediones are now widely used to treat type 2 diabetes, their mechanism of action remains largely unknown. They are agonists for the transcription factor PPARgamma, and in addition to their insulin-sensitising effects, they can promote adipogenesis and control gene expression in adipose tissues. We have explored the effect of rosiglitazone on insulin-mediated induction of pivotal genes involved in lipid metabolism and thermogenesis in brown fat. The genes studied were: (1) lipoprotein lipase (lpl), which is involved in lipid uptake; (2) hormone-sensitive lipase (hsl), which mobilises fatty acids from stored triglycerides; (3) fatty acid synthase (fas), which regulates de novo lipogenesis; and (4) the uncoupling proteins (ucp) 1 and 3, which control thermogenesis.

METHODS

We used fetal rat primary brown adipocytes cultured with insulin, rosiglitazone or both combined. Then, we studied gene expression by northern and western blotting, as well as 'run-on' and gel-shift assays to identify binding of potential transcription factors to the fas promoter.

RESULTS

Exposure to rosiglitazone for 24 h induced ucp-1, lpl and hsl gene expression and when rosiglitazone was combined with insulin a synergistic effect on lpl and ucp-3 mRNA expression was produced. These effects were consistent with increased LPL and HSL activities as well as respiration rates, mainly in response to exogenous palmitate. In contrast, treatment with rosiglitazone did not alter FAS mRNA basal levels but prevented the induction elicited by insulin in a time- and dose-dependent manner. Correspondingly diminished FAS protein levels and activity, as well as cellular lipid content, were observed, indicating an antilipogenic action of rosiglitazone in brown adipocytes. Furthermore, rosiglitazone impaired insulin increase in the FAS transcription rate by antagonising insulin-induced binding of upstream stimulatory factors to the E-box consensus sequence in the FAS promoter and insulin-induced binding of activating protein-1.

CONCLUSIONS/INTERPRETATION

Rosiglitazone prevents insulin-induced up-regulation of the main lipogenic enzyme but increases the expression of those enzymes involved in lipid uptake and mobilisation, favouring fatty acid utilisation through uncoupled respiration.

The brown adipose cell: a model for understanding the molecular mechanisms of insulin resistance

Type 2 diabetes mellitus is a complex metabolic disease that occurs when insulin secretion can no longer compensate insulin resistance in peripheral tissues. At the molecular level, insulin resistance correlates with impaired insulin signalling. This review provides new insights into the molecular mechanisms of insulin action and resistance in brown adipose tissue and pinpoints the role of this tissue in the control of glucose homeostasis. Brown adipocytes are target cells for insulin and IGF-I action, especially during late foetal development when insulin supports survival and promotes both adipogenic and thermogenic differentiation. The main pathway involved in insulin induction of adipogenic differentiation, monitored by fatty acid synthase expression, is the cascade insulin receptor substrate (IRS)-1/phosphatidylinositol 3-kinase (PI3K)/Akt. Glucose transport in these cells is maintained mainly by the activity of GLUT4. Acute insulin treatment stimulates glucose transport largely by mediating translocation of GLUT4 to the plasma membrane, involving the activation of IRS-2/PI3K, and the downstream targets Akt and protein kinase C zeta. Tumour necrosis factor (TNF-alpha) caused insulin resistance on glucose uptake by impairing insulin signalling at the level of IRS-2. Activation of stress kinases and phosphatases by this cytokine contribute to insulin resistance. Furthermore, brown adipocytes are also target cells for rosiglitazone action since they show a high expression of peroxisome proliferator activated receptor gamma, and rosiglitazone increased the expression of the thermogenic uncoupling protein 1. Rosiglitazone ameliorates insulin resistance provoked by TNF-alpha, completely restoring insulin-stimulated glucose uptake in parallel to the insulin signalling cascade. Accordingly, foetal brown adipocytes represent a model for investigating insulin action, as well as for the mechanism by which rosiglitazone increase insulin sensitivity under situations that mimic insulin resistance.

Susceptibility to apoptosis in insulin-like growth factor-I receptor-deficient brown adipocytes

Fetal brown adipocytes are insulin-like growth factor-I (IGF-I) target cells. To assess the importance of the IGF-I receptor (IGF-IR) in brown adipocytes during fetal life, we have generated immortalized brown adipocyte cell lines from the IGF-IR(-/-) mice. Using this experimental model, we demonstrate that the lack of IGF-IR in fetal brown adipocytes increased the susceptibility to apoptosis induced by serum withdrawal. Culture of cells in the absence of serum and growth factors produced rapid DNA fragmentation (4 h) in IGF-IR(-/-) brown adipocytes, compared with the wild type (16 h). Consequently, cell viability was decreased more rapidly in fetal brown adipocytes in the absence of IGF-IR. Furthermore, caspase-3 activity was induced much earlier in cells lacking IGF-IR. At the molecular level, IGF-IR deficiency in fetal brown adipocytes altered the balance of the expression of several proapoptotic (Bcl-xS and Bim) and antiapoptotic (Bcl-2 and Bcl-xL) members of the Bcl-2 family. This imbalance was irreversible even though in IGF-IR-reconstituted cells. Likewise, cytosolic cytochrome c levels increased rapidly in IGF-IR-deficient cells compared with the wild type. A rapid entry of Foxo1 into the nucleus accompanied by a rapid exit from the cytosol and an earlier activation of caspase-8 were observed in brown adipocytes lacking IGF-IR upon serum deprivation. Activation of caspase-8 was inhibited by 50% in both cell types by neutralizing anti-Fas-ligand antibody. Adenoviral infection of wild-type brown adipocytes with constitutively active Foxol (ADA) increased the expression of antiapoptotic genes, decreased Bcl-xL and induced caspase-8 and -3 activities, with the final outcome of DNA fragmentation. Up-regulation of uncoupling protein-1 (UCP-1) expression in IGF-IR-deficient cells by transduction with PGC-1alpha or UCP-1 ameliorated caspase-3 activation, thereby retarding apoptosis. Finally, insulin treatment prevented apoptosis in both cell types. However, the survival effect of insulin on IGF-IR(-/-) brown adipocytes was elicited even in the absence of phosphatidylinositol 3-kinase/Akt signaling. Thus, our results demonstrate for the first time the unique role of IGF-IR in maintaining the balance of death and survival in fetal brown adipocytes.

Rosiglitazone ameliorates insulin resistance in brown adipocytes of Wistar rats by impairing TNF-alpha induction of p38 and p42/p44 mitogen-activated protein kinases

AIMS/HYPOTHESIS

TNF-alpha caused insulin resistance on glucose uptake and on insulin signalling in fetal brown adipocytes. Since treatment with TNF-alpha activates stress kinases, including c-jun NH2 terminal kinase (JNK), and p42/p44 and p38 mitogen-activated protein kinases (MAPK), we explored the contribution of these pathways to insulin resistance by TNF-alpha. Rosiglitazone is used to treat Type 2 diabetes as it improves insulin sensitivity in vivo. However, its ability to ameliorate TNF-alpha-induced insulin resistance in brown adipocytes remains to be explored.

METHODS

We used fetal rat primary brown adipocytes cultured with TNF-alpha, with or without stress kinase inhibitors or rosiglitazone, and further stimulated with insulin. Then, we measured glucose uptake and GLUT4 translocation. To determine the insulin signalling cascade, we submitted cells to lysis, immunoprecipitation and immunoblotting.

RESULTS

Exposure to TNF-alpha for 24 h impairs insulin stimulation of the phosphatidylinositol (PI) 3-kinase activity associated with IRS-2 and Akt activity. Pretreatment with PD98059 or PD169316, which inhibit p42/p44MAPK and p38MAPK respectively, restored insulin signalling and insulin-induced glucose uptake in the presence of TNF-alpha. However, in the presence of SP600125, an inhibitor of JNK, TNF-alpha still produced insulin resistance. Rosiglitazone ameliorated insulin resistance by TNF-alpha in brown adipocytes, restoring completely insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane in parallel to the insulin signalling cascade IRS-2/PI 3-kinase/Akt.

CONCLUSIONS/INTERPRETATION

Rosiglitazone treatment impaired TNF-alpha activation of p38 and p42/p44MAPK, restoring insulin signalling and leading to normalisation of glucose uptake.

Differential Roles of the Insulin and Insulin-like Growth Factor-I (IGF-I) Receptors in Response to Insulin and IGF-I

Insulin and insulin-like growth factor-I (IGF-I) receptors are highly homologous tyrosine kinase receptors that share many common steps in their signaling pathways and have ligands that can bind to either receptor with differing affinities. To define precisely the signaling specific to the insulin receptor (IR) or the IGF-I receptor, we have generated brown preadipocyte cell lines that lack either receptor (insulin receptor knockout (IRKO) or insulin-like growth factor receptor knockout (IGFRKO)). Control preadipocytes expressed fewer insulin receptors than IGF-I receptors (20,000 versus 60,000), but during differentiation, insulin receptor levels increased so that mature adipocytes expressed slightly more insulin receptors than IGF-I receptors (120,000 versus 100,000). In these cells, insulin stimulated IR homodimer phosphorylation, whereas IGF-I activated both IGF-I receptor homodimers and hybrid receptors. Insulin-stimulated IRS-1 phosphorylation was significantly impaired in IRKO cells but was surprisingly elevated in IGFRKO cells. IRS-2 phosphorylation was unchanged in either cell line upon insulin stimulation. IGF-I-dependent phosphorylation of IRS-1 and IRS-2 was ablated in IGFRKO cells but not in IRKO cells. In control cells, both insulin and IGF-I produced a dose-dependent increase in phosphorylated Akt and MAPK, although IGF-I elicited a stronger response at an equivalent dose. In IRKO cells, the insulin-dependent increase in phospho-Akt was completely abolished at the lowest dose and reached only 20% of the control stimulation at 10 nm. Most interestingly, the response to IGF-I was also impaired at low doses, suggesting that IR is required for both insulin- and IGF-I-dependent phosphorylation of Akt. Most surprisingly, insulin- or IGF-I-dependent phosphorylation of MAPK was unaltered in either receptor-deficient cell line. Taken together, these results indicate that the insulin and IGF-I receptors contribute distinct signals to common downstream components in response to both insulin and IGF-I.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Rosiglitazone produces insulin sensitisation by increasing expression of the insulin receptor and its tyrosine kinase activity in brown adipocytes

AIMS/HYPOTHESIS

Rosiglitazone is used to treat Type 2 diabetes because it improves insulin sensitivity. However, the specific molecular mechanism by which this compound acts has not yet been explained.

METHODS

We used fetal rat primary brown adipocytes cultured for 24 h with or without 10 micro mol/l rosiglitazone and further stimulated for 5 min with 10 nmol/l insulin. Next we measured glucose uptake and GLUT4 translocation and submitted the cells to lysis, immunoprecipitation and immunoblotting in order to measure the insulin signalling cascade.

RESULTS

Rosiglitazone noticeably activated basal glucose uptake in a manner dependent on p38-mitogen-activated protein kinase. Rosiglitazone also produced a 40% increase in insulin-stimulated glucose uptake as a result of increased GLUT4 translocation to the plasma membrane. This happened without changes in the expression of GLUT4 at the mRNA or protein level. This effect correlated with the potentiation by rosiglitazone of insulin-stimulated Tyr phosphorylation of insulin receptor substrate-1 and to a greater extent of insulin receptor substrate-2. It also correlated with the subsequent activation of phosphatidylinositol 3-kinase and Akt, without changes in protein kinase Czeta activity. Rosiglitazone treatment increased insulin receptor expression and insulin-stimulated Tyr phosphorylation of insulin receptor beta-chain, but decreased insulin-stimulated Ser phosphorylation. It also potentiated insulin-induced Tyr phosphorylation of insulin receptor beta-chain and protein tyrosine phosphatase 1B in co-immunoprecipitates and impaired insulin activation of protein tyrosine phosphatase 1B activity.

CONCLUSIONS/INTERPRETATION

At the insulin receptor level, rosiglitazone-induced improvements of insulin sensitivity result from two convergent mechanisms: increased insulin receptor expression and insulin receptor activation.

Role of IRS-3 in the insulin signaling of IRS-1-deficient brown adipocytes

Insulin receptor substrate-1 (IRS-1) plays an essential role in mediating the insulin signals that trigger mitogenesis, lipid synthesis, and uncoupling protein-1 gene expression in mouse brown adipocytes. Expression of IRS-3 is restricted mainly to white adipose tissue; expression of this IRS protein is virtually absent in brown adipocytes. We have tested the capacity of IRS-3 to mediate insulin actions in IRS-1-deficient brown adipocytes. Thus, we expressed exogenous IRS-3 in immortalized IRS-1-/- brown adipocytes at a level comparable with that of endogenous IRS-3 in white adipose tissue. Under these conditions, IRS-3 signaling in response to insulin was observed, as revealed by tyrosine phosphorylation of IRS-3, and the activation of phosphatidylinositol (PI) 3-kinase associated with this recombinant protein. However, although insulin promoted the association of Grb-2 with recombinant IRS-3 in IRS-1-/- cells, the exogenous expression of this IRS family member failed to activate p42/44 MAPK and mitogenesis in brown adipocytes lacking IRS-1. Downstream of PI 3-kinase, IRS-3 expression restored insulin-induced Akt phosphorylation, which is impaired by the lack of IRS-1 signaling. Whereas the generation of IRS-3 signals enhanced adipocyte determination and differentiation-dependent factor 1/sterol regulatory element-binding protein (ADD-1/SREBP-1c) and fatty acid synthase mRNA and protein expression, activation of this pathway was unable to reconstitute CCAAT/enhancer-binding protein alpha and uncoupling protein-1 transactivation and gene expression in response to insulin. Similar results were obtained following insulin-like growth factor-I stimulation. In brown adipocytes expressing the IRS-3F4 mutant, the association of the p85alpha regulatory subunit via Src homology 2 binding was lost, but insulin nevertheless induced PI 3-kinase activity and Akt phosphorylation in a wortmannin-dependent manner. In contrast, activation of IRS-3F4 signaling failed to restore the induction of ADD-1/SREBP-1c and fatty acid synthase gene expression in IRS-1-deficient brown adipocytes. These studies demonstrate that recombinant IRS-3 may reconstitute some, but not all, of the signals required for insulin action in brown adipocytes. Thus, our data further implicate a unique role for IRS-1 in triggering insulin action in brown adipocytes.

Bi-directional regulation of brown fat adipogenesis by the insulin receptor

Insulin is a potent inducer of adipogenesis, and differentiation of adipocytes requires many components of the insulin signaling pathway, including the insulin receptor substrate IRS-1 and phosphatidylinositol 3-kinase (PI3K). Brown pre-adipocytes in culture exhibit low levels of insulin receptor (IR), and during differentiation there is both an increase in total IR levels and a shift in the alternatively spliced forms of IR from the A isoform (-exon 11) to the B isoform (+exon 11). Brown pre-adipocyte cell lines from insulin receptor-deficient mice exhibit dramatically impaired differentiation and an inability to regulate alternative splicing of the insulin receptor. Surprisingly, re-expression of either splice isoform of IR in the IR-deficient cells fails to rescue differentiation in these cells. Likewise, overexpression of IR in control IRlox cells also results in inhibition of differentiation and a failure to accumulate expression of the adipogenic markers peroxisome proliferator-activated receptor gamma, Glut4, and fatty acid synthase, although cells overexpressing IR retain the ability to activate PI3K and down-regulate mitogen-activated protein kinase (MAPK) phosphorylation. Thus, differentiation of brown adipocytes requires a timed and regulated expression of IR, and either the absence or overabundance of insulin receptors in these cells dramatically inhibits differentiation.

Insulin and dexamethasone induce GLUT4 gene expression in foetal brown adipocytes: synergistic effect through CCAAT/enhancer-binding protein alpha

Treatment of foetal brown adipocytes in primary culture with either dexamethasone or insulin, at physiological concentrations, for 24 h up-regulates the expression of the GLUT4 gene, producing a synergistic effect on mRNA accumulation (20-fold increase), in the amount of protein in the total membrane fraction (8-fold increase) and in the transactivation of a full-promoter GLUT4 -chloramphenicol acetyltransferase gene ( CAT ) construct (7-fold increase). However, GLUT1 expression remains essentially unmodified regardless of the presence of the hormones. As a consequence, exposure of brown adipocytes to dexamethasone and insulin results in a dramatic increase of glucose uptake (12-fold). Dexamethasone induces the expression of CCAAT/enhancer-binding protein (C/EBP) alpha, insulin promotes myocyte enhancer factor-2 DNA-binding activity and both combined produces a significant increase in C/EBPalpha DNA-binding activity. Moreover, co-transfection with a wild-type C/EBPalpha construct transactivates a full-promoter GLUT4 - CAT fusion gene, whereas a dominant-negative C/EBPalpha expression vector impairs the hormonal effects. Our results show that the synergism between insulin and glucocorticoids on glucose uptake is a consequence of the activation of the GLUT4 promoter by the transcription factor C/EBPalpha.

Insulin-induced up-regulated uncoupling protein-1 expression is mediated by insulin receptor substrate 1 through the phosphatidylinositol 3-kinase/Akt signaling pathway in fetal brown adipocytes

To investigate the role of insulin receptor substrate-1 (IRS-1) and its downstream signaling in insulin-induced thermogenic differentiation of brown adipocytes, we have reconstituted IRS-1-deficient fetal brown adipocytes (IRS-1(-/-)) with wild-type IRS-1 (IRS-1(wt)). The lack of IRS-1 resulted in the inability of insulin to induce IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity and Akt phosphorylation in IRS-1(-/-) brown adipocytes. In addition, these cells showed an impairment in activating alpha-Akt, beta-Akt, and gamma-Akt isoforms upon insulin stimulation. Reconstitution of IRS-1(-/-) brown adipocytes with IRS-1(wt) restored the IRS-1/PI 3-kinase/Akt signaling pathway. Treatment of wild-type brown adipocytes with insulin for 24 h up-regulated uncoupling protein-1 (UCP-1) expression and transactivated the UCP-1 promoter; this effect was abolished in the absence of IRS-1 or in the presence of an Akt inhibitor and further recovered after IRS-1(wt) reconstitution. Neither UCP-2 nor UCP-3 was up-regulated by insulin in wild-type and IRS-1-deficient brown adipocytes. Insulin stimulated the expression of CCAAT/enhancer-binding protein alpha (C/EBPalpha) and its DNA binding activity in wild-type brown adipocytes but not in IRS-1(-/-) cells. However, insulin stimulation of both C/EBPalpha expression and binding activity was restored after IRS-1(wt) reconstitution of deficient cells. Retrovirus-mediated expression of C/EBPalpha and peroxisome proliferator-activated receptor gamma in IRS-1(-/-) brown adipocytes up-regulated UCP-1 protein content and transactivated UCP-1 promoter regardless of insulin stimulation. Both C/EBPalpha and peroxisome proliferator-activated receptor gamma reconstituted FAS mRNA expression, but only C/EBPalpha restored insulin sensitivity in the absence of IRS-1. Finally, reconstitution of IRS-1(-/-) brown adipocytes with the IRS-1 mutants IRS-1(Phe-895), which lacks IRS-1/growth factor receptor binding protein 2 binding but not IRS-1/p85-PI 3-kinase binding, or with IRS-1(Tyr-608/Tyr-628/Tyr-658), which only binds p85-PI 3-kinase, induced UCP-1 expression and transactivated the UCP-1 promoter. These data provide strong evidence for an essential role of IRS-1 through the PI 3-kinase/Akt signaling pathway inducing UCP-1 gene expression by insulin.

Essential role of protein kinase C zeta in the impairment of insulin-induced glucose transport in IRS-2-deficient brown adipocytes

Insulin receptor substrate-2-deficient (IRS-2(-/-)) mice develop type 2 diabetes. We have investigated the molecular mechanisms by which IRS-2(-/-) immortalized brown adipocytes showed an impaired response to insulin in inducing GLUT4 translocation and glucose uptake. IRS-2-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity was blunted in IRS-2(-/-) cells, total PI 3-kinase activity being reduced by 30%. Downstream, activation of protein kinase C (PKC) zeta was abolished in IRS-2(-/-) cells. Reconstitution with retroviral IRS-2 restores IRS-2/PI 3-kinase/PKC zeta signalling, as well as glucose uptake. Wild-type cells expressing a kinase-inactive mutant of PKC zeta lack GLUT4 translocation and glucose uptake. Our results support the essential role played by PKC zeta in the insulin resistance and impaired glucose uptake observed in IRS-2-deficient brown adipocytes.

Essential role of insulin-like growth factor I receptor in insulin-induced fetal brown adipocyte differentiation

To define the specific role of IGF-I receptor (IGF-IR) in adipogenic and thermogenic differentiation of brown adipocytes during late fetal life, we have established immortalized brown adipocyte cell lines from fetuses of IGF-IR-deficient mice (IGF-IR(-/-)) as well as from wild-type mice (IGF-IR(+/+)). IGF-IR(-/-) cells showed an increased insulin sensitivity regarding insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation despite a substantial reduction in IRS-1 protein content. Furthermore, insulin-induced total and IRS-1-associated phosphatidylinositol 3-kinase activities were augmented in IGF-IR-deficient cells compared with wild-type cells. Downstream phosphatidylinositol 3-kinase activation of Akt, but not p70s6 kinase, were elicited at lower doses of insulin in IGF-IR(-/-) brown adipocytes. Activation of protein kinase Czeta by insulin was similar in both cell types as was insulin-induced glucose uptake. Treatment of wild-type brown adipocytes with insulin for 12 h up-regulated fatty acid synthase (FAS) and adipocyte determination and differentiation (ADD1/SREBP) mRNAs; this effect was impaired in the absence of IGF-IR. At the protein level, insulin increased FAS content and the amount of the mature form of adipocyte determination and differentiation (ADD1/SREBP) in the nucleus in wild-type cells, but not in IGF-IR(-/-) cells. Furthermore, 24 h of insulin stimulation induced the expression of both uncoupling protein-1 and CCAAT/enhancer-binding protein alpha (C/EBPalpha) in wild-type brown adipocytes; these effects were abolished in IGF-I-R(-/-) cells. Retrovirus-mediated reexpression of peroxisomal proliferator-activated receptor gamma (PPARgamma) in IGF-IR(-/-) brown adipocytes could overcome FAS mRNA impairment, bypassing insulin signaling. However, insulin further increased FAS mRNA expression in C/EBPalpha-IGF-IR(-/-) cells, but not in PPARgamma-IGF-IR(-/-) cells. In addition, fetal brown adipocytes lacking IGF-IR up-regulated uncoupling protein-1 expression in the absence of insulin when PPARgamma, but not C/EBPalpha, was overexpressed. These data provide strong evidence for a critical role of IGF-IR in the differentiation of the brown adipocyte phenotype in fetal life; this effect is mimicked by PPARgamma in an insulin-independent manner.

Increased insulin sensitivity in IGF-I receptor--deficient brown adipocytes

Immortalized brown adipocyte cell lines have been generated from fetuses of mice deficient in the insulin-like growth factor I receptor gene (IGF-IR(-/-)), as well as from fetuses of wild-type mice (IGF-IR(+/+)). These cell lines maintained the expression of adipogenic- and thermogenic-differentiation markers and show a multilocular fat droplets phenotype. IGF-IR(-/-) brown adipocytes lacked IGF-IR protein expression; insulin receptor (IR) expression remained unchanged as compared with wild-type cells. Insulin-induced tyrosine autophosphorylation of the IR beta-chain was augmented in IGF-IR--deficient cells. Upon insulin stimulation, tyrosine phosphorylation of (insulin receptor substrate-1) IRS-1 was much higher in IGF-IR(-/-) brown adipocytes, although IRS-1 protein content was reduced. In contrast, tyrosine phosphorylation of IRS-2 decreased in IGF-IR--deficient cells; its protein content was unchanged as compared with wild-type cells. Downstream, the association IRS-1/growth factor receptor binding protein-2 (Grb-2) was augmented in the IGF-IR(-/-) brown adipocyte cell line. However, SHC expression and SHC tyrosine phosphorylation and its association with Grb-2 were unaltered in response to insulin in IGF-IR--deficient brown adipocytes. These cells also showed an enhanced activation of mitogen-activated protein kinase (MAPK) kinase (MEK1/2) and p42/p44 mitogen-activated protein kinase (MAPK) upon insulin stimulation. In addition, the lack of IGF-IR in brown adipocytes resulted in a higher mitogenic response (DNA synthesis, cell number, and proliferating cell nuclear antigen expression) to insulin than wild-type cells. Finally, cells lacking IGF-IR showed a much lower association between IR or IRS-1 and phosphotyrosine phosphatase 1B (PTP1B) and also a decreased PTP1B activity upon insulin stimulation. However, PTP1B/Grb-2 association remained unchanged in both cell types, regardless of insulin stimulation. Data presented here provide strong evidence that IGF-IR--deficient brown adipocytes show an increased insulin sensitivity via IRS-1/Grb-2/MAPK, resulting in an increased mitogenesis in response to insulin.

New research in breast reconstruction: adipose tissue engineering

In the human species, a major function of the breast is aesthetic. The soft-tissue volume within the breast displaces the overlying skin to create the protuberant contour of the female thorax, that is solidly associated with, and to some extent, definitive of, femininity in modern culture. Adipose tissue is the major contributor to the volume of the breast.

Ceramide mediates insulin resistance by tumor necrosis factor-alpha in brown adipocytes by maintaining Akt in an inactive dephosphorylated state

Tumor necrosis factor (TNF)-alpha causes insulin resistance on glucose uptake in fetal brown adipocytes. We explored the hypothesis that some effects of TNF-alpha could be mediated by the generation of ceramide, given that TNF-alpha treatment induced the production of ceramide in these primary cells. A short-chain ceramide analog, C2-ceramide, completely precluded insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane, as determined by Western blot or immunofluorescent localization of GLUT4. These effects were not produced in the presence of a biologically inactive ceramide analog, C2-dihydroceramide. Analysis of the phosphatidylinositol (PI) 3-kinase signaling pathway indicated that C2-ceramide precluded insulin stimulation of Akt kinase activity, but not of PI-3 kinase or protein kinase C-zeta activity. C2-ceramide completely abolished insulin-stimulated Akt/protein kinase B phosphorylation on regulatory residues Thr 308 and Ser 473, as did TNF-alpha, and inhibited insulin-induced mobility shift in Akt1 and Akt2 separated in PAGE. Moreover, C2-ceramide seemed to activate a protein phosphatase (PP) involved in dephosphorylating Akt because 1) PP2A activity was increased in C2-ceramide- and TNF-alpha-treated cells, 2) treatment with okadaic acid concomitantly with C2-ceramide completely restored Akt phosphorylation by insulin, and 3) transient transfection of a constitutively active form of Akt did not restore Akt activity. Our results indicate that ceramide produced by TNF-alpha induces insulin resistance in brown adipocytes by maintaining Akt in an inactive dephosphorylated state.

Brown adipose tissue-specific insulin receptor knockout shows diabetic phenotype without insulin resistance

Although insulin regulates metabolism in both brown and white adipocytes, the role of these tissues in energy storage and utilization is quite different. Recombination technology using the Cre-loxP approach allows inactivation of the insulin receptor in a tissue-specific manner. Mice lacking insulin receptors in brown adipocytes show an age-dependent loss of interscapular brown fat but increased expression of uncoupling protein-1 and -2. In parallel, these mice develop an insulin-secretion defect resulting in a progressive glucose intolerance, without insulin resistance. This model provides direct evidence for not only a role for the insulin receptors in brown fat adipogenesis, the data also suggest a novel role of brown adipose tissue in the regulation of insulin secretion and glucose homeostasis.

Akt mediates insulin induction of glucose uptake and up-regulation of GLUT4 gene expression in brown adipocytes

Insulin acutely stimulated glucose uptake in rat primary brown adipocytes in a PI3-kinase-dependent but p70S6-kinase-independent manner. Since Akt represents an intermediate step between these kinases, this study investigated the contribution of Akt to insulin-induced glucose uptake by the use of a chemical compound, ML-9, as well as by transfection with a dominant-negative form of Akt (DeltaAkt). Pretreatment with ML-9 for 10 min completely inhibited insulin stimulation of (1) Akt kinase activity, (2) Akt phosphorylation on the regulatory residue Ser473 but not on Thr308, and (3) mobility shift in Akt1 and Akt2. However, ML-9 did not affect insulin-stimulated PI3-kinase nor PKCzeta activities. In consequence, ML-9 precluded insulin stimulation of glucose uptake and GLUT4 translocation to plasma membrane (determined by Western blot), without any effect on the basal glucose uptake. Moreover, DeltaAkt impaired insulin stimulation of glucose uptake and GFP-tagged GLUT4 translocation to plasma membrane in transiently transfected immortalised brown adipocytes and HeLa cells, respectively. Furthermore, ML-9 treatment for 6 h down-regulated insulin-induced GLUT4 mRNA accumulation, without affecting GLUT1 expression, in a similar fashion as LY294002. Indeed, co-transfection of brown adipocytes with DeltaAkt precluded the transactivation of GLUT4-CAT promoter by insulin in a similar fashion as a dominant-negative form of PI3-kinase. Our results indicate that activation of Akt may be an essential requirement for insulin regulation of glucose uptake and GLUT4 gene expression in brown adipocytes.

Insulin signaling leading to proliferation, survival, and membrane ruffling in C2C12 myoblasts

We have recently shown that insulin induced myogenesis in the mouse C2C12 skeletal muscle cell line by activation of phosphatidylinositol (PI) 3-kinase/p70S6-kinase and p38-mitogen-activated protein kinase (MAPK) and downregulation of p42/p44-MAPK. This study investigated the insulin-signaling pathways involved in mitogenesis, survival, and membrane ruffling in C2C12 myoblasts, a cellular system that besides IGF-I receptors, expressed a high number of functional insulin receptors. Insulin (10 nM) rapidly stimulated beta-chain insulin receptor and IRS-1 tyrosine phosphorylation, IRS-2 being poorly and SHC not phosphorylated at all. However, an association of SHC with IRS-1 was found under insulin stimulation. Insulin stimulated IRS-1 association with p85alpha leading to the activation of PI3-kinase, and, subsequently AKT and p70S6-kinases. Moreover, both p42/p44- and p38-MAPKs resulted in phosphorylation after insulin stimulation. Insulin treatment for 24 h produced mitogenesis, as demonstrated by the increase in ((3)H)-thymidine incorporation, DNA content, the expression of PCNA and cyclin D1 proteins, and the proportion of cells in S + G2/M phases of the cell cycle. This mitogenic effect of insulin was precluded by inhibition of p70S6-kinase (either by rapamycin or by the PI3-kinase inhibitor LY294002) as well as by inhibition of p44/p42-MAPK with PD098059, but was not affected by inhibition of p38-MAPK. Serum deprivation of C2C12 myoblasts resulted in growth arrest at the GO/G1 phases of the cell cycle and apoptosis, as detected either by DNA laddering or by increase in the percentage of hypodiploid cells. Insulin rescued serum-deprived cells from apoptosis in an AKT-dependent manner, as demonstrated by the inhibition of AKT-activity by the use of LY294002 and ML-9, meanwhile neither inhibition of p70S6-kinase, nor MAPK affected insulin-induced survival. Finally, we evaluated the capacity of insulin to modulate actin cytoskeleton rearrangement. Insulin stimulation of myoblasts produced membrane ruffling and decreased actin stress fibers; this biological response being dependent of p38-MAPK, as demonstrated by the use of the p38-MAPK inhibitors SB203580 or PD169316, but independent of PI3-kinase and p42/p44-MAPK.

Association of insulin receptor substrate 1 (IRS-1) y895 with Grb-2 mediates the insulin signaling involved in IRS-1-deficient brown adipocyte mitogenesis

We have recently generated immortalized fetal brown adipocyte cell lines from insulin receptor substrate 1 (IRS-1) knockout mice and demonstrated an impairment in insulin-induced lipid synthesis as compared to wild-type cell lines. In this study, we investigated the consequences of IRS-1 deficiency on mitogenesis in response to insulin. The lack of IRS-1 resulted in the inability of insulin-stimulated IRS-1-deficient brown adipocytes to increase DNA synthesis and enter into S/G2/M phases of the cell cycle. These cells showed a severe impairment in activating mitogen-activated protein kinase kinase (MEK1/2) and p42-p44 mitogen-activated protein kinase (MAPK) upon insulin stimulation. IRS-1-deficient cells also lacked tyrosine phosphorylation of SHC and showed no SHC-Grb-2 association in response to insulin. The mitogenic response to insulin could be partially restored by enhancing IRS-2 tyrosine phosphorylation and its association with Grb-2 by inhibition of phosphatidylinositol 3-kinase activity through a feedback mechanism. Reconstitution of IRS-1-deficient brown adipocytes with wild-type IRS-1 restored insulin-induced IRS-1 and SHC tyrosine phosphorylation and IRS-1-Grb-2, IRS-1-SHC, and SHC-Grb-2 associations, leading to the activation of MAPK and enhancement of DNA synthesis. Reconstitution of IRS-1-deficient brown adipocytes with the IRS-1 mutant Tyr895Phe, which lacks IRS-1-Grb-2 binding, restored SHC-IRS-1 association and SHC-Grb-2 association. However, the lack of IRS-1-Grb-2 association impaired MAPK activation and DNA synthesis in insulin-stimulated mutant cells. These data provide strong evidence for an essential role of IRS-1 and its direct association with Grb-2 in the insulin signaling pathway leading to MAPK activation and mitogenesis in brown adipocytes.

Insulin produces myogenesis in C2C12 myoblasts by induction of NF-kappaB and downregulation of AP-1 activities

In the present study, we have examined the insulin-signaling pathways involved in myogenesis in mouse C2C12 skeletal muscle cell line, a cellular system that expresses high number of high affinity insulin receptors. Insulin (50 nM) rapidly (5 min) stimulated beta-chain insulin receptor, activated the phosphatidylinositol (PI) 3-kinase/Akt/p70S6-kinase signaling pathway, as well as phosphorylated both p44/p42- and p38-mitogen-activated protein kinases (MAPKs). Preconfluent cells were differentiated in a serum-free medium in response to 50 nM insulin for 72 h, as revealed by the formation of multinucleated myotubes and the induction of the creatine kinase activity. This differentiation process was also monitored by the inhibition of the PCNA content and induction of the cell cycle inhibitor p21. Furthermore, insulin induced nuclear factor-kappaB (NF-kappaB) DNA binding activity and down-regulated activating protein-1 (AP-1) DNA binding activity throughout the differentiation process. The use of specific inhibitors of the insulin-signaling pathways indicated that myogenesis was precluded by treatment for 72 h with LY294002 (an inhibitor of PI 3-kinase), rapamycin (a p70S6-kinase blocker), and SB203580 or PD169316 (p38-MAPK inhibitors). These inhibitors abolished insulin induction of NF-kappaB DNA binding activity and kappaB-chloramphenicol acetyltransferase (CAT) promoter activity, maintaining expressed cytosolic IkappaB-alpha protein, and increased AP-1 DNA binding activity and TRE-CAT promoter activity. These data suggest that insulin induces myogenesis in C2C12 through PI 3-kinase/ p70S6-kinase and p38-MAPK pathways, the signaling through p44/p42-MAPK being inhibited.

Noradrenaline induces brown adipocytes cell growth via beta-receptors by a mechanism dependent on ERKs but independent of cAMP and PKA

It has been well established that the key role of noradrenaline is the induction of uncoupling-protein-1 (UCP-1) expression, the unique marker of brown adipocytes. However, its implication on proliferation and the pathways involved are not as well characterized. By using rat fetal brown adipocytes as a model, we show that, although noradrenaline activates extracellular regulated kinases (ERKs) through beta-, alpha1-, and alpha2-receptors, only beta-receptors mediate cell growth by a mechanism that requires ERKs activation but is independent of cyclic-adenosine-monophosphate/protein kinase A (cAMP/PKA). Conversely, the cAMP/PKA cascade mediates noradrenaline-induced UCP-1 expression, whereas ERKs pathway attenuates thermogenic differentiation. On the other hand, alpha1- and alpha2-receptors have an antiproliferative effect that is enhanced by ERK inhibition.

The potentiation of estrogen on insulin-like growth factor I action in MCF-7 human breast cancer cells includes cell cycle components

To gain insight into the mechanisms involved in the cross-talk between IGF-1 receptor (IGF-1R) and estrogen receptor signaling pathways, we used MCF-7-derived cells (SX13), which exhibit a 50% reduction in IGF-1R expression. Growth of NEO cells (control MCF-7 cells) was stimulated by both IGF-1 and estradiol (E2), and the addition of both mitogens resulted in a synergistic response. Estrogen enhanced IGF-1R signaling in NEO cells, but this effect was markedly diminished in SX13 cells. Estrogen was also able to potentiate the IGF-1 effect on the expression of cyclin D1 and cyclin E and on the phosphorylation of retinoblastoma protein in control but not in SX13 cells. IGF-1 increased the protein level of p21 and the luciferase activity of the p21 promoter, whereas it only reduced the protein level of p27 without affecting p27 promoter activity. Estrogen did not affect the p21 inhibitor, but it decreased the protein level of p27 and the p27 promoter luciferase activity. These effects of both mitogens were also observed at the level of association of both cyclin-dependent kinase inhibitors with CDK2 suggesting that IGF-1 and E2 affect the activity of both p21 and p27. Taken together, these data suggest that in MCF-7 cells, estrogen potentiates the IGF-1 effect on IGF-1R signaling as well as on the cell cycle components. Moreover, IGF-1 and E2 regulate the expression of p21 and p27 and their association with CDK2 differently.

Okadaic acid inhibits insulin-induced glucose transport in fetal brown adipocytes in an Akt-independent and protein kinase C zeta-dependent manner

In the present study we have investigated the effect of increased serine/threonine phosphorylation of insulin receptor substrates-1 and -2 (IRS-1 and IRS-2) by okadaic acid pretreatment on brown adipocyte insulin signalling leading to glucose transport, an important metabolic effect of insulin in brown adipose tissue. Okadaic acid pretreatment before insulin stimulation decreased IRS-1 and IRS-2 tyrosine phosphorylation in parallel to a decrease in their sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobility. IRS-1/IRS-2-associated p85alpha and phosphatidylinositol (PI) 3-kinase enzymatic activity were partly reduced in brown adipocytes pretreated with okadaic acid upon stimulation with insulin. Furthermore, insulin-induced glucose uptake was totally abolished by the inhibitor in parallel with a total inhibition of insulin-induced protein kinase C (PKC) zeta activity. However, activation of Akt/PKB or p70 S6 kinase (p70(s6k)) by insulin remained unaltered. Our results suggest that downstream of PI 3-kinase, insulin signalling diverges into at least two independent pathways through Akt/PKB and PKC zeta, the PKC zeta pathway contributing to glucose transport induced by insulin in fetal brown adipocytes.

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

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

Insulin and insulin-like growth factor I up-regulate GLUT4 gene expression in fetal brown adipocytes, in a phosphoinositide 3-kinase-dependent manner

Fetal brown adipocytes cultured in a serum-free medium, containing 5 mM glucose, expressed both GLUT4 and GLUT1 glucose transporters at the mRNA and protein level. Treatment with either insulin or insulin-like growth factor (IGF)-I at physiological concentrations up-regulates the expression of the GLUT4 gene, producing a time-dependent mRNA accumulation (7-fold increase at 24 h) and a 2.5-fold increase in the amount of protein in the total membrane fraction. However, insulin treatment down-regulates GLUT1 mRNA and protein expression. Moreover, either insulin or IGF-I transactivates a full-promoter GLUT4-chloramphenicol acetyltransferase gene (CAT) construct transiently transfected to the cells, without affecting GLUT1-CAT activity. In consequence, insulin treatment for 24 h increased by 3-fold the basal glucose uptake. Inhibition of phosphoinositide (PI) 3-kinase activity with chemical agents such as wortmannin or LY294002 partially blocked insulin-induced GLUT4 mRNA accumulation, insulin-induced GLUT4 protein content, GLUT4-CAT transactivation and glucose uptake. Furthermore, co-transfection of brown adipocytes with a dominant-negative form of PI 3-kinase precluded the transactivation of the GLUT4 promoter by insulin. However, inhibition of p70S6 kinase (p70(s6k)) with rapamycin or of mitogen-activated protein kinase (MAPK) with PD098059 does not preclude insulin effects on GLUT4 gene expression or glucose uptake. Our results show for the first time a positive effect of insulin on GLUT4 gene expression in fetal brown adipocytes, suggesting the existence of insulin response element(s) in its promoter. Moreover, PI 3-kinase, but not p70(s6k) or MAPK, is an essential requirement for insulin regulation of GLUT4 gene expression.

Insulin/IGF-I rescues immortalized brown adipocytes from apoptosis down-regulating Bcl-xS expression, in a PI 3-kinase- and map kinase-dependent manner

Serum deprivation of immortalized brown adipocyte cell line resulted in growth arrest in G0/G1 phases of the cell cycle and apoptosis, as detected either by DNA laddering or by increase in the percentage of hypodiploid cells. Furthermore, apoptosis is concurrent with a dramatic increase in the expression of the proapoptotic protein Bcl-xS, the expression of Bcl-xL remaining almost undetectable. Insulin/insulin-like growth factor (IGF-I) rescued serum-deprived brown adipocytes from apoptosis, decreasing the number of hypodiploid cells and increasing the number of cells undergoing cell cycle progression throughout S and G2/M phases of the cell cycle. Moreover, insulin down-regulated Bcl-xS expression without inducing the expression of Bcl-xL. Both phosphatidylinositol (PI) 3-kinase and mitogen-activated protein kinase (MAPK) pathways are necessary for insulin/IGF-I full survival effect, since the use of specific inhibitors of PI 3-kinase activity (wortmannin or LY294002, at the dose that inhibits PI 3-kinase activity induced by insulin) or MAPK kinase activity inhibitor (PD098059, at the dose that inhibits insulin-induced phosphorylation of MAPK) totally blocked the antiapoptotic effect induced by insulin/IGF-I, respectively. In conclusion, insulin survival effect on immortalized brown adipocytes is associated with inhibition of the Bcl-xS content without changing Bcl-xL, in a PI 3-kinase- and MAP kinase-dependent manner.