The human adipose tissue is a source of multipotent stem cells

Multipotent stem cells constitute an unlimited source of differentiated cells that could be used in pharmacological studies and in medicine. Recently, several publications have reported that adipose tissue contains a population of cells able to differentiate into different cell types including adipocytes, osteoblasts, myoblasts, and chondroblasts. More recently, stem cells with a multi-lineage potential at the single cell level have been isolated from human adipose tissue. These cells, called human Multipotent Adipose-Derived Stem (hMADS) cells, have been established in culture and interestingly, maintain their characteristics with long-term passaging. The adipocyte differentiation of hMADS cells has been thoroughly studied and differentiated cells exhibit the unique feature of human adipocytes. Finally, potential applications of stem cells isolated from adipose tissue in medicine will be discussed.

[Development of adipose tissue and dietary lipids: from basic science to medicine]

Professor Gérard Ailhaud was awarded the 2005 Institut Roche de l'Obésité Prize, for his studies on adipocyte biology and the differential role of dietary lipids in the increasing prevalence of overweight and obesity. With his colleagues, he demonstrated that natural fatty acids play an hormonal role by favouring the differentiation of preadipocytes into adipocytes, showed the potent adipogenic role of arachidonic acid (C20:4, n-6) and depicted the signaling pathways involved in this process. More recently, his research has underscored the importance of the composition of dietary fats during the gestation/suckling period, i.e. an excess of the essential polyunsaturated fatty acids of the n-6 series, in the hypertrophic/hyperplasic development of adipose tissue which has been associated over the last decades with the increasing prevalence of childhood obesity. The main findings of his studies are summarised in this review.

Fatty acid composition of fats is an early determinant of childhood obesity: a short review and an opinion

The importance of dietary fat in human obesity remains a controversial issue as the prevalence of overweight and obesity has increased despite no dramatic change in the amount of ingested fats over the past few decades. However, qualitative changes (i.e. the fatty acid composition of fats) have been largely disregarded. In this review, we summarize experimental evidence which supports polyunsaturated fatty acids of the omega6 series as being potent promoters of both adipogenesis in vitro and adipose tissue development in vivo during the gestation/lactation period. This conclusion is also supported by epidemiological data from infant studies as well as by the assessment of the fatty acid composition of mature breast milk and formula milk. It is proposed that unnoticed changes in fatty acid composition of ingested fats over the last decades have been important determinants in the increasing prevalence of childhood overweight and obesity.

Effects of high-fat diet, angiotensinogen (agt) gene inactivation, and targeted expression to adipose tissue on lipid metabolism and renal gene expression

To address the role of angiotensinogen (agt) in lipid metabolism and its potential endocrine effects in vivo, we studied the effects of high-fat diet (HFD) on adult, 28-week-old agt knockout (KO) mice compared to wild type (WT) mice. Recent studies (Massiera et al., 2001) have demonstrated that reexpression of agt in adipose tissue of KO mice normalized adiposity, blood pressure, and kidney abnormalities. We therefore used microarray analysis to investigate changes in gene expression profile in kidneys of KO vs. Tg-KO mice, where agt expression is restricted to adipose tissue. Body weight, adiposity and insulin levels were significantly decreased (p < 0.05) in KO mice on a chow diet (CD) compared to WT mice, while circulating leptin levels were similar. On a high-fat diet, KO mice exhibited significantly lower bodyweight (p < 0.05), adiposity (p < 0.05), leptin, and insulin levels (p < 0.05) compared to WT mice. In agreement with previously reported changes in kidney histology, agt KO mice displayed altered expressions of genes involved in blood pressure regulation and renal function, but these levels were corrected by reexpression of agt in adipose tissue. Collectively, these findings further document important endocrine roles of adipocyte agt, in part via regulation of lipid metabolism and kidney homeostasis.

PPARgamma-dependent and PPARgamma-independent effects on the development of adipose cells from embryonic stem cells

Peroxisome proliferator-activated receptor (PPAR) gamma was shown to be required for adipocyte formation both in vivo and in vitro. However, the role of PPARgamma in the initial steps of adipose cell development was not distinguished from its role in the terminal steps. We now show that PPARgamma is expressed early in embryoid bodies (EBs) derived from embryonic stem cells and in E.8.5 mouse embryos. Addition of a specific ligand for PPARgamma in developing EBs over-expressing PPARgamma did not commit stem cells towards the adipose lineage. In differentiated PPARgamma(-/-) EBs, only markers characteristic of preadipocytes were found to be expressed. PPARdelta is present in EBs but did not compensate for the lack of PPARgamma in terminal differentiation. Taken together, these results favor a critical PPARgamma-independent phase culminating in preadipocyte formation that precedes a PPARgamma-dependent phase in the development of adipose cells from pluripotent stem cells.

Angiotensinogen-deficient mice exhibit impairment of diet-induced weight gain with alteration in adipose tissue development and increased locomotor activity

White adipose tissue is known to contain the components of the renin-angiotensin system, which gives rise to angiotensin II from angiotensinogen (AGT). Recent evidence obtained in vitro and ex vivo is in favor of angiotensin II acting as a trophic factor of adipose tissue development. To determine whether AGT plays a role in vivo in this process, comparative studies were performed in AGT-deficient (agt(-/-)) mice and control wild-type mice. The results showed that agt(-/-) mice gain less weight than wild-type mice in response to a chow or high fat diet. Adipose tissue mass from weaning to adulthood appeared altered rather specifically, as both the size and the weight of other organs were almost unchanged. Food intake was similar for both genotypes, suggesting a decreased metabolic efficiency in agt(-/-) mice. Consistent with this hypothesis, cellularity measurement indicated hypotrophy of adipocytes in agt(-/-) mice with a parallel decrease in the fatty acid synthase activity. Moreover, AGT-deficient mice exhibited a significantly increased locomotor activity, whereas metabolic rate and mRNA levels of uncoupling proteins remained similar in both genotypes. Thus, AGT appears to be involved in the regulation of fat mass through a combination of decreased lipogenesis and increased locomotor activity that may be centrally mediated.

Adipose angiotensinogen is involved in adipose tissue growth and blood pressure regulation

White adipose tissue and liver are important angiotensinogen (AGT) production sites. Until now, plasma AGT was considered to be a reflection of hepatic production. Because plasma AGT concentration has been reported to correlate with blood pressure, and to be associated with body mass index, we investigated whether adipose AGT is released locally and into the blood stream. For this purpose, we have generated transgenic mice either in which adipose AGT is overexpressed or in which AGT expression is restricted to adipose tissue. This was achieved by the use of the aP2 adipocyte-specific promoter driving the expression of rat agt cDNA in both wild-type and hypotensive AGT-deficient mice. Our results show that in both genotypes, targeted expression of AGT in adipose tissue increases fat mass. Mice whose AGT expression is restricted to adipose tissue have AGT circulating in the blood stream, are normotensive, and exhibit restored renal function compared with AGT-deficient mice. Moreover, mice that overexpress adipose AGT have increased levels of circulating AGT, compared with wild-type mice, and are hypertensive. These animal models demonstrate that AGT produced by adipose tissue plays a role in both local adipose tissue development and in the endocrine system, which supports a role of adipose AGT in hypertensive obese patients.

Activation of extracellular signal-regulated kinases and CREB/ATF-1 mediate the expression of CCAAT/enhancer binding proteins beta and -delta in preadipocytes

The essential role of CCAAT/enhancer binding proteins (C/EBPs) beta and delta for adipocyte differentiation has been clearly established. In preadipocytes, their expression is up-regulated by the activation of leukemia inhibitory factor receptor (LIF-R) and prostacyclin receptor (IP-R) via the extracellular signal-regulated kinase (ERK) pathway and cAMP production, respectively. However, the molecular mechanisms by which LIF and prostacyclin-induced signals are propagated to the nucleus and the transcription factors mediating ERK and cAMP-induced C/EBP gene expression were unknown. Here we report that both pathways share cAMP responsive element binding protein/activation transcription factor 1 (CREB/ATF-1) as common downstream effectors. LIF-R and IP-R activation induced binding of CREB and/or ATF-1 to C/EBP promoters and CREB-dependent transcription. Expression of dominant negative forms of CREB dramatically reduced the LIF- and prostacyclin-stimulated C/EBP beta and C/EBP delta expression. Upon stimulation of the IP-R, the ERK pathway was activated in a PKA-dependent manner. ERK activation by the PKA pathway was not required for CREB/ATF-1 phosphorylation but rather was necessary for CREB-dependent up-regulation of C/EBPs expression. Our findings suggest that ERK activation is required for CREB transcriptional activity, possibly by recruitment of a coactivator.

[Does obesity protect against diabetes? A new controversy]

Obesity is a risk factor for the appearance of type 2 diabetes: this notion, largely based on clinical experience, dictates the essential principles of the treatment of type 2 diabetes. At odds with this conventional wisdom, Elliot Danforth Jr. has recently proposed that "too few adipocytes predisposes to type 2 diabetes". A further thought on this controversy is discussed herein. In the context of the "orthodox" view which links obesity and diabetes, and its relation to the controversy, we analyse on one hand the effects of thiazolidine-diones on insulin sensitivity and on adipogenesis and, on the other hand, those of extreme situations represented by lipoatrophic diabetes and morbid obesity. This analysis shows that fat tissue indeed favors the appearance of diabetes but is also able to be anti-diabetogenic, and a dynamic solution of this paradox is put forward. We propose a dual evolutionist hypothesis leading to the selection of an intermediary adipogenic genotype based on the limitation of both insulin secretion and adipogenic potential, which would explain both the necessity of the existence of adipose tissue in man and the limitations of its development.

Compactin enhances osteogenesis in murine embryonic stem cells

Embryonic stem (ES) cells have the capacity to differentiate into various cell types in vitro. In this study, we show that retinoic acid is important for the commitment of ES cells into osteoblasts. Culturing retinoic acid treated ES cells in the presence of the osteogenic supplements ascorbic acid and beta-glycerophosphate resulted in the expression of several osteoblast marker genes, osteocalcin, alkaline phosphatase, and osteopontin. However, there was only a slight amount of mineralized matrix secretion. Addition of bone morphogenic protein-2 or compactin, a drug of the statin family of HMG-CoA reductase inhibitors, resulted in a greatly enhanced formation of bone nodules. Compactin did not modify the expression of osteogenic markers, but at the late stage of differentiation promoted an increase in BMP-2 expression. These results establish ES-cell derived osteogenesis as an effective model system to study the molecular mechanisms by which the statin compactin promotes osteoblastic differentiation and bone nodule formation.

Angiotensin II as a trophic factor of white adipose tissue: stimulation of adipose cell formation

White adipose tissue is known to contain the components of the renin-angiotensin system giving rise to angiotensin II (AngII). In vitro, prostacyclin is synthesized from arachidonic acid through the activity of cyclooxygenases 1 and 2 and is released from AngII-stimulated adipocytes. Prostacyclin, in turn, is able to favor adipocyte formation. Based upon in vivo and ex vivo experiments combined to immunocytochemical staining of glycerol-3-phosphate dehydrogenase (GPDH), an indicator of adipocyte formation, it is reported herein that AngII favors the appearance of GPDH-positive cells. In the presence of a cyclooxygenase inhibitor, this adipogenic effect is abolished, whereas that of (carba)prostacyclin, a stable analog of prostacyclin that bypasses this inhibition, appears unaltered. Taken together, these results are in favor of AngII acting as a trophic factor implicated locally in adipose tissue development. It is proposed that AngII enhances the formation of GPDH-expressing cells from preadipocytes in response to prostacyclin released from adipocytes.

Angiotensinogen, angiotensin II and adipose tissue development

Adipose tissue is an important source of angiotensinogen (AGT). Recent evidence shows that a local renin-angiotensinogen system (RAS) is present in human adipose tissue and may act as a distinct system from plasma RAS. In obese patients, the involvement of angiotensin II (angII) as a consequence of increased plasma AGT secreted from adipose tissue has been proposed in the development of hypertension. Another role of AGT via angII in the development of adipose tissue is supported by the following: (i) in vitro, angII stimulates the production and release of prostacyclin from adipocytes, which in turn promotes the differentiation of precursor cells into adipocytes; (ii) ex vivo and in vivo, both angII and (carba)prostacyclin promote the formation of new fat cells; and (iii) AGT -/- mice exhibit a slowing down of adipose tissue development, as compared to wild-type mice. Altogether the data are consistent with an autocrine/paracrine mechanism implicating AGT, angII and prostacyclin in adipose tissue development.

Adipose tissue as an endocrine organ

The objective is to present a brief overview of peptide and non-peptide factors secreted from adipocytes and to describe some studies on the postulated role of the locally active triad angiotensinogen/angiotensinII/ prostacyclin in the development/enlargement of adipose tissue mass and increased blood pressure. In addition to the role of adipose tissue as an endocrine organ, the results emphasize the autocrine/paracrine mechanisms which are postulated to play a role in adipose tissue development and enlargement.

Peroxisome proliferator-activated receptor-alpha enhances lipid metabolism in a skin equivalent model

Peroxisome proliferator-activated receptors are involved in certain cell types such as adipocytes and hepatocytes, in the control of several pathways of lipid synthesis or catabolism by regulating the gene expression level of key lipid metabolizing enzymes. As the epidermis exhibits an extensive lipid metabolism necessary for the establishment of the barrier function, we have examined the role of peroxisome proliferator-activated receptor-alpha activation in this process. Living skin equivalents were treated with Wy 14,643, a selective peroxisome proliferator- activated receptor-alpha ligand, which enhanced greatly the synthesis of membrane coating granules, the organelles specialized in the processing of stratum corneum lipids. Also, the overall stratum corneum neutral lipid content assessed by Oil red O staining was increased. A detailed analysis of the lipid species present in the reconstructed epidermis showed that peroxisome proliferator-activated receptor-alpha activation increased the synthesis of ceramides and cholesterol derivatives, thought to be essential structural components of the permeability barrier. A synergistic effect was observed on lipid synthesis when peroxisome proliferator-activated receptor-alpha and retinoid X receptor were simultaneously activated by selective ligands. Furthermore, activation of peroxisome proliferator-activated receptor-alpha led to increased mRNA expression of several key enzymes of ceramide and cholesterol metabolism. An increase of serine-palmitoyl transferase and of beta-glucocerebrosidase enzymatic activity was also demonstrated. Altogether, these results show that peroxisome proliferator-activated receptor-alpha is a key transcription factor involved in the control of the epidermal lipid barrier.

Prostacyclin IP receptor up-regulates the early expression of C/EBPbeta and C/EBPdelta in preadipose cells

Prostacyclin (PGI(2)) and its stable analogue carbacyclin (cPGI(2)) are known to trigger the protein kinase A pathway after binding to the cell surface IP receptor and to promote or enhance terminal differentiation of adipose precursor cells to adipose cells. The early expression of C/EBPbeta and C/EBPdelta is known to be critical for adipocyte differentiation in vitro as well as in vivo. We report herein that in Ob1771 and 3T3-F442A preadipose cells, activation of the IP receptor by specific agonists (PGI(2), cPGI(2) and BMY 45778) is sufficient to up-regulate rapidly the expression of C/EBPbeta and C/EBPdelta. Cyclic AMP-elevating agents are able to substitute for IP receptor agonists, in agreement with the coupling of IP receptor to adenylate cyclase. Consistent with the fact that PGI(2) is released from preadipose cells and behaves as a paracrine/autocrine effector of adipose cell differentiation, the present results favor a key role of prostacyclin by means of the IP receptor and its intracellular signaling pathway in eliciting the critical early expression of both transcription factors.

Cross talk between adipocytes and their precursors: relationships with adipose tissue development and blood pressure

Adipose tissue is an important source of angiotensinogen (AT). A possible involvement of increased plasma AT has been proposed in hypertension in obese patients, but growing evidence suggests also that the local renin-angiotensinogen system (RAS), giving rise to angiotensin II (AngII), may act as a distinct system from the plasma RAS. AngII stimulates in vitro the production and release of prostacyclin from adipocytes which in turn promotes the differentiation of precursor cells into new adipocytes (Darimont et al. 1994, Endocrinology 138: 1092-1096). Cross talk between adipocytes and precursor cells has been validated ex vivo and in vivo as (1) AngII stimulates specifically prostacyclin production, (2) both carbacyclin, a stable analogue of prostacyclin, and AngII promote the formation of new fat cells, and (3) AT (-/-) mice, which have decreased blood pressure (Tanimoto et al. 1994, J. Biol. Chem. 269: 31334-31336), exhibit both hypertrophy and hypoplasia of adipose tissue compared to wild-type mice (collaboration with Prof. A. Fukamizu, Tsukuba University, Japan). Altogether the data are consistent with an autocrine/paracrine mechanism implicating AT, AngII, and prostacyclin in adipose tissue development and suggest a new role for local AngII in addition to that of systemic AngII in blood pressure. Hormonal regulation of AT production from adipose tissue is also discussed.

Leukemia inhibitory factor and its receptor promote adipocyte differentiation via the mitogen-activated protein kinase cascade

Extracellular factors and intracellular signaling pathways involved in early events of adipocyte differentiation are poorly defined. It is shown herein that expression of leukemia inhibitory factor (LIF) and LIF receptor is developmentally regulated during adipocyte differentiation. Preadipocytes secrete bioactive LIF, and an antagonist of LIF receptor inhibits adipogenesis. Genetically modified embryonic stem (ES) cells combined with culture conditions to commit stem cells into the adipocyte lineage were used to examine the requirement of LIF receptor during in vitro development of adipose cells. The capacity of embryoid bodies derived from lifr(-/-) ES cells to undergo adipocyte differentiation is dramatically reduced. LIF addition stimulates adipocyte differentiation of Ob1771 and 3T3-F442A preadipocytes and that of peroxisome proliferator-activated receptor gamma2 ligand-treated mouse embryonic fibroblasts. Expression of the early adipogenic transcription factors C/EBPbeta and C/EBPdelta is rapidly stimulated following exposure of preadipose cells to LIF. The selective inhibitors of mitogen-activated protein kinase kinase, i.e. PD98059 and U0126, inhibit LIF-induced C/EBP gene expression and prevent adipocyte differentiation induced by LIF. These results are in favor of a model that implicates stimulation of LIF receptor in the commitment of preadipocytes to undergo terminal differentiation by controlling the early expression of C/EBPbeta and C/EBPdelta genes via the mitogen-activated protein kinase cascade.

Cell surface receptors, nuclear receptors and ligands that regulate adipose tissue development

Our knowledge of adipose tissue development has increased dramatically over the last two decades, through a combination of in vitro studies using cellular models and in vivo studies using mouse models with invalidated target genes. Critical early events of the differentiation programme appear to involve in preadipose cells (i) the entry of fatty acids and the production of fatty acid metabolites as activators/ligands of nuclear peroxisome proliferator-activated receptors (PPARs) and (ii) the very early expression of PPARdelta and CAAT/enhancer binding proteins (C/EBPs) beta and delta. Among fatty acids, prostacyclin produced from arachidonic acid enhances the expression of both C/EBPs through cell surface IP receptor and presumably activates PPARdelta. Together, these transcription factors up-regulate the expression of PPARgamma and C/EBPalpha which lead in turn to the acquisition of the adipocyte phenotype. Altogether, these studies have provided a molecular link between high-fat diets and excess of adipose tissue development through hyperplasia and hypertrophy.

Differential expression of prostaglandin receptor mRNAs during adipose cell differentiation

To clarify the molecular basis for the prostaglandin (PG) mediated effects in adipose cells at various stages of their development, expression of mRNAs encoding receptors specific for prostaglandin E2, F2alpha and I2 (i.e. EP, FP, and IP receptors) was investigated in differentiating clonal Ob1771 pre-adipocytes, as well as in mouse primary adipose precursor cells and mature adipocytes. We have further characterized the differential expression of mRNAs encoding three subtypes of the EP receptor, i.e. EP1, EP3, and EP4, and examined the expression of mRNAs encoding the three isoforms (alpha, beta, and gamma) of the EP3 receptor. Altogether the results show that the expression of IP, FP, EP1, and EP4 receptor mRNAs was considerably more pronounced in pre-adipose cells than in adipose cells, mRNAs encoding the alpha, beta, and gamma isoforms of the EP3 receptor were all exclusively expressed in freshly isolated mature adipocytes. These data may indicate that PGI2, PGF2alpha, and PGE2 may interact directly with specific receptors in pre-adipose cells, whose transduction mechanisms are known to affect maturation related changes. In mature adipocytes, however, the equipment of mRNAs encoding the EP3 receptor isoforms is in agreement with the well known effect of PGE2 on adenylate cyclase and lipolysis in mature adipocytes.

Differential expression of peroxisome proliferator-activated receptor subtypes during the differentiation of human keratinocytes

The expression of mRNA encoding peroxisome proliferator-activated receptor (PPAR) subtypes in human keratinocytes was determined by semiquantitative reverse transcriptase-polymerase chain reaction. When normal human keratinocytes were induced to differentiate by shifting the culture medium to high Ca2+ concentration, the expression of PPAR-alpha and -gamma mRNA was increased, whereas that of PPAR-delta remained unchanged. At the protein level, the expression of PPAR in cultured human keratinocytes was demonstrated by a DNA mobility shift assay and the functionality of the receptor subtypes was assessed by transactivation experiments. In epidermis reconstructed in vitro, the level of PPAR-alpha and -gamma mRNA was also associated with keratinocyte differentiation. In lesional compared with nonlesional psoriatic epidermis, the expression of PPAR-alpha and -gamma mRNA was reduced, indicating that these two subtypes are tightly linked to the epidermal differentiation process.

Insulin down-regulates angiotensinogen gene expression and angiotensinogen secretion in cultured adipose cells

Adipose tissue is an important source of angiotensinogen (AT) after liver. Since an association exists between body mass index, hypertension, and insulin-resistance, the role of insulin on the regulation of AT gene expression and AT secretion was examined in cultured Ob1771 and 3T3-F442A adipose cells. Within a physiological range of concentrations (1-17 nM), insulin exerted a negative effect on the abundance of AT mRNA and the secretion of AT. Alterations of insulin-resistance by treatment of adipose cells with TNF-alpha or the thiazolidinedione BRL49653 led respectively to a decrease or an increase in the potency of insulin to down-regulate AT gene expression, whereas maximal inhibition by insulin increased from 30% in TNFalpha-treated cells to 60% in BRL49653-treated cells. These results suggest that a potential link between insulin resistance and high blood pressure may exist by means of increased AT secretion from adipose tissue, especially in obese subjects.

Regulation by glucocorticoids of angiotensinogen gene expression and secretion in adipose cells

Adipose cells are an important source of angiotensinogen (AT). Its activation product, angiotensin II, stimulates in vitro and in vivo the production and release of prostacyclin which acts as a potent adipogenic signal in promoting the terminal differentiation of preadipocytes to adipocytes. Since glucocorticoids are known to promote adipose cell differentiation in vitro as well as in vivo, their role in the regulation of AT gene expression and secretion has been investigated in cultured Ob1771 mouse adipose cells. In contrast with liver cells, which are the major source of AT and the target of several hormones for the regulation of its expression, adipose cells are only responsive to glucocorticoids, which are able to up-regulate AT gene expression and AT secretion rapidly and dose-dependently. On exposure to glucocorticoids, accumulation of AT mRNA appears primarily to be due to transcriptional activation of the gene and is parallelled by secretion of the protein. Similar results on AT mRNA expression and AT secretion were obtained using explants of rat adipose tissue ex vivo demonstrating a major if not exclusive mechanism of regulation of AT production by glucocorticoids in mature adipose cells. Together these results provide a potential link between glucocorticoids, AT, the growth of adipose tissue and increased blood pressure.

Up-regulation of UCP-2 gene expression by PPAR agonists in preadipose and adipose cells

UCP-2 is a member of the emerging family of UCP homologues. Upon high-fat feeding, UCP-2 mRNA levels are increased in epididymal fat pads of A/J mice, suggesting that the flux of fatty acids entering adipose tissue may regulate UCP-2 gene expression. Since fatty acids act as positive transcriptional regulators of lipid-related genes by means of peroxisome proliferator-activated receptors (PPARs), the regulation of UCP-2 gene expression by PPAR agonists (carbacyclin, alpha-bromopalmitate, BRL49653) has been examined in mouse preadipose and adipose cells in primary cultures or from clonal lines (Ob1771, 3T3-F442A, 1B8). In preadipose cells, carbacyclin and alpha-bromopalmitate are active and BRL49653 shows no effect, whereas all these ligands are active in adipose cells. The stimulatory effect of PPAR agonists is potentiated by RXR agonists in adipose cells. In contrast to the UCP-1 gene, norepinephrine as a cAMP-elevating agent does not enhance the expression of UCP-2 gene. Altogether, the data favor a predominant role of PPARdelta in preadipose cells and the involvement of PPARgamma2 in adipose cells in up-regulating UCP-2 gene expression. Thus, a potential link between fatty acid metabolism and thermogenesis may exist in PPAR-expressing tissues.

Differentiation of embryonic stem cells into adipocytes in vitro

Embryonic stem cells, derived from the inner cell mass of murine blastocysts, can be maintained in a totipotent state in vitro. In appropriate conditions embryonic stem cells have been shown to differentiate in vitro into various derivatives of all three primary germ layers. We describe in this paper conditions to induce differentiation of embryonic stem cells reliably and at high efficiency into adipocytes. A prerequisite is to treat early developing embryonic stem cell-derived embryoid bodies with retinoic acid for a precise period of time. Retinoic acid could not be substituted by adipogenic hormones nor by potent activators of peroxisome proliferator-activated receptors. Treatment with retinoic acid resulted in the subsequent appearance of large clusters of mature adipocytes in embryoid body outgrowths. Lipogenic and lipolytic activities as well as high level expression of adipocyte specific genes could be detected in these cultures. Analysis of expression of potential adipogenic genes, such as peroxisome proliferator-activated receptors gamma and delta and CCAAT/enhancer binding protein beta, during differentiation of retinoic acid-treated embryoid bodies has been performed. The temporal pattern of expression of genes encoding these nuclear factors resembled that found during mouse embryogenesis. The differentiation of embryonic stem cells into adipocytes will provide an invaluable model for the characterisation of the role of genes expressed during the adipocyte development programme and for the identification of new adipogenic regulatory genes.

Leptin gene is expressed in rat brown adipose tissue at birth

The ob gene product leptin is secreted from adipose tissue. Leptin has dramatic effects on food intake and energy expenditure in rodents. Brown adipose tissue is the first form of adipose tissue to appear during development, and is present at birth in most species. The development of a leptin feedback system in early life and the relative role of the brown and white adipose tissues have not yet been revealed. We have investigated the expression of ob/leptin mRNA in brown adipose tissue around birth and with respect to feeding. Northern blotting analysis and in situ hybridization experiments demonstrated the presence of leptin mRNA in brown adipose tissue at 0, 18, and 24 h after birth. The leptin mRNA level was decreased at 8 h postpartum in fed animals and at 18 or 24 h in the absence of feeding. In addition, circulating leptin was detected in the plasma of newborn rats at 0, 10, or 24 h after birth, whereas it was not detectable in 10 h-old animals that did not suckle at their mother. The presence at birth of ob mRNA and circulating leptin, as well as the early effect of suckling on ob mRNA levels, suggests the precocious involvement of leptin in the control of food intake.

Influence of estrogenic status on the lipolytic activity of parametrial adipose tissue in vivo: an in situ microdialysis study

Ovarian hormones have been shown to modulate the metabolism of adipose cells obtained from adipose tissue of different animals. The aim of this study was to better understand the short- and long-term influences of estrogens on the in vivo lipolytic response of rat parametrial fat pads, determined by measurement of extracellular glycerol concentrations using in situ microdialysis. Possible direct effects of estrogens on lipolysis were studied by perfusion of a potent estrogenic analogue such as moxestrol. Moxestrol (10(-6) M) failed to increase glycerol concentrations in estrus, diestrus, or 8-day ovariectomized animals. However, the basal glycerol concentrations and the lipolytic responses stimulated by 10(-6) M isoproterenol were decreased in parametrial fat pads of diestrus, compared with estrus, rats. Greater decreases in basal and stimulated glycerol concentrations were observed in rats that had been ovariectomized for 8, 15, or 30 days. In ovariectomized rats, isoproterenol-induced lipolysis was restored to the levels observed in diestrus animals by a daily injection of 17 beta-estradiol for a period of 7 days. These results implicate estrogens as long-term modulators of in vivo basal and stimulated lipolytic responses of rat parametrial fat pad.

Regulation by fatty acids of angiotensinogen gene expression in preadipose cells

Adipocytes represent an important source of angiotensiongen (AT). Angiotensin II (A-II) stimulates in vitro and in vivo the formation and release of prostacyclin which acts as a potent adipogenic signal in triggering the terminal differentiation of preadipocytes into adipocytes [Darimont, Vassaux, Gaillard. Ailhaud and Négrel (1994) Int. J. Obes. 18, 783-788]. Since fatty acids have been reported to activate in preadipose cells the expression of various differentiation-dependent genes, the role of fatty acids in the regulation of AT gene expression was investigated. Long-chain natural and non-metabolized fatty acids as well as peroxisome proliferators behave as activators of AT gene expression. Accumulation of AT mRNA parallels that of the adipocyte fatty acid-binding protein gene and is primarily due to transcriptional activation of the AT gene. AT mRNA decreases after fatty acid removal (half-life approx. 8 h). Secretion of AT is also observed but appears mainly as a late differentiation-dependent phenomenon. Thus the AT gene appears to be a fatty acid-responsive gene; this regulation provides a potential link between the flux of fatty acids and the potential of adipose tissue to produce AT and possibly A-II.

[Hormones and adipocyte development]

Dormant preadipocytes isolated from adipose tissue are able to differentiate into adipocytes in vitro. A few adipogenic hormones (glucocorticoids or prostacyclin, IGF-I and insulin) are sufficient to trigger the differentiation program. In preadipose and adipose cells, glucocorticoids play a cardinal role by regulating the expression of numerous genes and by increasing the production of prostacyclin which acts as an intracrine/autocrine/paracrine effector. Fatty acids and metabolites (including prostacyclin) enhance adipocyte differentiation via the activation of nuclear peroxisome proliferator-activated receptors (PPARs). PPARs then modulate positively the expression of various lipid-related genes involved in triacylglycerol accumulation. These in vitro observations emphasize the importance of the hypothalamic-pituitary-adrenal axis and provide also a link which may take place in vivo between high-fat diets and the excess of adipose tissue development.

Evidence for a novel regulatory pathway activated by (carba)prostacyclin in preadipose and adipose cells

Prostacyclin, one of the major prostanoids generated in adipose tissue, has been previously described as an autocrine/paracrine adipogenic effector, acting, in preadipose cells, by means of cAMP and free Ca2+ as cell surface receptor-mediated messengers. The present study presents evidence for the first time that its stable analogue, carbaprostacyclin, is unique among prostanoids in regulating the expression of two differentiation-dependent genes in preadipose and adipose cells in a way distinct from that elicited by its cell surface receptor. This regulation is likely mediated by some member(s) of the peroxisome proliferator-activated receptor family and suggests that prostacyclin behaves as an intracrine effector of adipose cell differentiation.

Modulation of vascular tone and glycerol levels measured by in situ microdialysis in rat adipose tissue

Changes in blood flow induced by the beta-adrenoceptor agonist isoproterenol (Iso) and a stable analogue of the major metabolite of arachidonic acid in adipose tissue, carbaprostacyclin (cPGI2), have been studied in rat periepididymal fat pad with in situ microdialysis measuring the distribution ratio of 0.2% ethanol in the dialysate (outflow) to that in the perfusate (inflow) (O/I ratio). Local perfusions of 1 microM cPGI2 or 1 microM Iso led to reversible decreases of the O/I ratio that were similar to the decrease induced by the vasodilating reference drug hydralazine (Hydra, 630 microM). Interestingly, a continuous perfusion of Hydra at a submaximal vasodilating concentration (63 microM) was sufficient to prevent further vasodilatation induced by Iso or cPGI2. To take advantage of this observation, experiments were designed to evaluate the influence of the vasodilating effect of Iso or cPGI2 on the ability of either to induce lipolysis in vivo. The results showed that the vasodilating effect of Iso could contribute to glycerol removal from the extracellular fluid and demonstrate that cPGI2 was devoid of lipolytic activity.

Changes in adenosine A1- and A2-receptor expression during adipose cell differentiation

Two adenosine receptors A1 and A2 are associated with either stimulation (A2) or inhibition (A1) of adenylate cyclase. Using the clonal cell line Ob1771, we have studied the expression of the two receptors during the process of adipose conversion accelerated by exposure to dexamethasone and 3-isobutyl-l-methylxanthine (IBMX) during the first 3 days post-confluence. The effects mediated by the two receptors on preadipocyte differentiation and adipocyte metabolism were also investigated. The two adenosine agonists NECA and PIA were used as preferential agonists of the A2- and A1-receptor, respectively. In preadipose cells (just confluent), both of the mouse clonal line and human primary culture, NECA dose-dependently stimulated cAMP production with a significant higher potency (P < 0.01) than did PIA. In adipose cells (16-day post-confluent) NECA was found to exert a biphasic effect on forskolin-stimulated cAMP production: i.e., NECA was clearly inhibitory in the femto- to picomolar concentration range whereas this effect gradually diminished at higher concentrations. The effect of PIA in 16-day post-confluent adipose cells however, was purely inhibitory on both cAMP production (IC50: 33.52 +/- 0.44 fM) and lipolysis (64% +/- 7%; P < 0.01). These findings were corroborated by Northern blot analysis which revealed A1-receptor mRNA to be exclusively expressed in the mature adipocytes, whereas A2-receptor mRNA gradually declined during the differentiation process except in 16-day post-confluent cells. In addition, NECA significantly enhanced the effect of corticosterone-induced differentiation by 46.8% (P < 0.05) but failed to have any adipogenic potency acting either alone or in concert with carbaprostacyclin (cPGI2). Thus, endogenous adenosine may have a bimodal action on adipose tissue metabolism mediated through stimulatory A2- and inhibitory A1-receptors, respectively, as a function of adipose conversion.

Fatty acids regulate the expression of lipoprotein lipase gene and activity in preadipose and adipose cells

During fasting, a reduction in lipoprotein lipase (LPL) activity has been observed in rat fat pad with no change in enzyme mass, whereas LPL mRNA and synthesis are increased, suggesting that insulin and/or fatty acids (FA) regulate LPL activity post-translationaly [Doolittle, Ben-Zeev, Elovson, Martin and Kirchgessner (1990) J. Biol. Chem. 265, 4570-4577]. To examine the role of FA, either preadipose Ob1771 cells or Ob1771 and 3T3-F442A adipose cells were exposed to long-chain FA and to 2-bromopalmitate, a non-metabolized FA. A rapid (2-8 h) and dose-dependent increase (up to 6-fold) in LPL mRNA occurred, primarily due to increased transcription, which is accompanied by a decrease (down to 4-fold) in LPL cellular activity. Under these conditions, secretion of active LPL was nearly abolished. Removal of FA led to full recovery of LPL activity. LPL gene expression in 3T3-C2 fibroblasts was not affected by FA treatment. However fatty acid-activated receptor transfected-3T3-C2 cells, which show FA responsiveness, had increased LPL gene expression upon FA addition. LPL synthesis and cellular content appeared unaffected by FA treatment, whereas secretion of LPL was inhibited. These results indicate that FA regulate the post-translational processing of LPL. It is proposed that the regulation of LPL activity by FA is important with regard to the fine-tuning of FA entry into adipocytes during fasting/feeding periods.

Expression of ob gene in adipose cells. Regulation by insulin

The product of the recently cloned mouse obese (ob) gene is likely to play an important role in a loop regulating the size of the adipose tissue mass. The hormonal regulation of the ob gene could affect adiposity. To investigate this point, the effect of insulin on ob gene expression was examined in cells of the 3T3-F442A preadipocyte clonal line. ob mRNA is absent from exponentially growing, undifferentiated cells as well as from confluent preadipose cells. Terminal differentiation of preadipose to adipose cells leads to the expression of ob mRNA detected by a sensitive and quantitative ribonuclease protection assay. In adipose cells, the level of ob mRNA is sensitive to insulin in the nanomolar range of concentrations with an increase from an average of 1 copy to 5-10 copies/cell. The effect of insulin was fully reversible and takes place primarily at a transcriptional level. The ob mRNA shows a rapid turnover, with a half-life of approximately 2 h in the absence or presence of insulin. The level of secreted Ob protein is also regulated by insulin. These results indicate that the ob gene is expressed in mature fat cells only and support the possibility that insulin is an important regulator of ob gene expression.

Cholesteryl ester transfer protein (CETP) mRNA abundance in human adipose tissue: relationship to cell size and membrane cholesterol content

Cholesteryl ester transfer protein (CETP) has a well-defined role in plasma neutral lipid transport. CETP synthesized by human adipose tissue may contribute to the plasma CETP pool. CETP mRNA abundance increases in subcutaneous adipose tissue in response to cholesterol feeding and we have hypothesized that CETP gene expression is regulated by a specific pool of cellular sterol. In the present study, we have quantified CETP mRNA levels in subcutaneous adipose tissue of 10 female subjects using a solution hybridization RNase protection assay. Particulate (membrane cholesterol) and lipid droplet cholesterol (core cholesterol) were determined by gas chromatography. CETP mRNA abundance in these adipose tissue specimens correlated significantly with membrane cholesterol expressed as a fraction of membrane protein (r = 0.67, P = 0.031). There was also a linear relationship between CETP mRNA abundance and membrane cholesterol to core triglyceride ratio (r = 0.77, P = 0.009) and a strong correlation between the percentage of cellular cholesterol in the membrane fraction (ratio of membrane to core cholesterol) and CETP mRNA abundance (r = 0.91, P = 0.0002). In contrast, there was a negative relationship between each of lipid droplet cholesterol and triglyceride and CETP mRNA levels. Human adipose tissue maintained in organ culture for several days was shown to secrete CETP into the culture medium. Incubation with cholesterol-rich chylomicron remnants elicited a dose-dependent increase in both membrane and core cholesterol and a concomitant increase in the level of CETP mRNA. These studies demonstrate that adipose tissue CETP mRNA abundance is a function of membrane cholesterol concentration rather than lipid droplet cholesterol and that CETP mRNA increases with adipocyte cholesterol enrichment via chylomicron remnants. CETP gene expression is highest in small lipid-poor adipocytes, suggesting that CETP synthesized and secreted by adipocytes may have a role in promoting cellular cholesterol accumulation.

Increased expression in adipocytes of ob RNA in mice with lesions of the hypothalamus and with mutations at the db locus

The gene product of the recently cloned mouse obese gene (ob) is important in regulating adipose tissue mass. ob RNA is expressed specifically by mouse adipocytes in vivo in each of several different fat cell depots, including brown fat. ob RNA is also expressed in cultured 3T3-442A preadipocyte cells that have been induced to differentiate. Mice with lesions of the hypothalamus, as well as mice mutant at the db locus, express a 20-fold higher level of ob RNA in adipose tissue. These data suggest that both the db gene and the hypothalamus are downstream of the ob gene in the pathway that regulates adipose tissue mass and are consistent with previous experiments suggesting that the db locus encodes the ob receptor. In db/db and lesioned mice, quantitative differences in expression level of ob RNA correlated with adipocyte lipid content. The molecules that regulate expression level of the ob gene in adipocytes probably are important in determining body weight, as are the molecules that mediate the effects of ob at its site of action.

Cloning of a protein that mediates transcriptional effects of fatty acids in preadipocytes. Homology to peroxisome proliferator-activated receptors

Exposure of preadipocytes to long chain fatty acids induces expression of several gene markers of adipocyte differentiation. This report describes the cloning, from a preadipocyte library, of a cDNA encoding a fatty acid-activated receptor, FAAR. The cDNA had the characteristics and ligand-binding domains of nuclear hormone receptors and encoded a 440 amino acid protein related to peroxisome proliferator-activated receptors, PPAR. The deduced protein sequence was 88% homologous to that of hNUC I, isolated from human osteosarcoma cells. FAAR mRNA was abundant in adipose tissue, intestine, brain, heart, and skeletal muscles and less abundant in kidney, liver, testis, and spleen. The mRNA was undetectable in growing Ob1771 and 3T3-F442A preadipocytes, was strongly induced early during differentiation, and was increased by fatty acid. Transcription assays using hybrid receptor showed strong stimulation by fatty acid and weaker induction by fibrates. Transfection of 3T3-C2 fibroblasts, with a FAAR expression vector, conferred fatty acid inducibility of the adipocyte lipid-binding protein and the fatty acid transporter. Transcriptional induction of these genes exhibited inducer specificity identical to that described in preadipocytes. In summary, the data indicate that FAAR is likely a mediator of fatty acid transcriptional effects in preadipocytes.

Fatty acids and adipose cell differentiation

Fatty acids are important metabolic substrates for adipose tissue. In preadipose cells, fatty acids are also potent inducers of various genes encoding proteins directly involved in fatty acid metabolism. On a longer-term basis, fatty acids induce the terminal differentiation of preadipose to adipose cells. Fatty acids act primarily at a transcriptional level. A member of the steroid/thyroid hormone receptor superfamily has been identified by cDNA cloning from a mouse Ob1771 preadipose cell library. This receptor is likely the fatty acid-activated receptor implicated in the transcriptional effects of fatty acids in adipose cells. Thus fatty acids appear to play a new role as signal transducing molecules which are involved in adipose cell differentiation.

In situ microdialysis of prostaglandins in adipose tissue: stimulation of prostacyclin release by angiotensin II

In order to clarify the physiological roles that PGE2 and PGI2, the two major metabolites of arachidonic acid in adipose tissue could play in vivo, their measurements have been undertaken in the interstitial fluid of rat periepididymal adipose tissue using in situ microdialysis. This technique appears suitable with a rather high dialysis yield (70%) for the measurement of prostaglandins. The basal equilibrium extracellular concentrations of 6-keto-PGF1 alpha (the stable metabolite of PGI2) and PGE2 can be estimated to 1 and 0.3 nM, respectively. Experiments designed to study the hormonal regulation of in situ PGI2 and PGE2 production show that (i) lipolysis induced by local perfusion of 10(-6) M isoproterenol is not accompanied by an increase of either 6-keto-PGF1 alpha or PGE2 in the dialysate but, in contrast (ii) perfusion of angiotensin II (10(-7)-10(-5) M) leads, independently of triacylglycerol hydrolysis, to a transient dose-dependent and indomethacin-sensitive increase of released 6-keto-PGF1 alpha with no change in released PGE2. Therefore, in vivo PGI2 appears as the major hormonally regulated prostaglandin in rat adipose tissue, and angiotensin II appears as a physiological effector of its extracellular release.

Evidence for a common mechanism of action for fatty acids and thiazolidinedione antidiabetic agents on gene expression in preadipose cells

In diabetic rodents, thiazolidinediones are able to improve insulin sensitivity of target tissues and to reverse, at least partially, the diabetic state. The effects of these drugs on phenotypic expression in various tissues, including adipose tissue, have been reported. We report here that a new thiazolidinedione compound, BRL 49653, exerts, in preadipose cells, potent effects on the expression of genes encoding proteins involved in fatty acid metabolism. These effects of BRL 49653 in Ob 1771 preadipose cells are similar, in terms of kinetics, reversibility, specificity of genes affected, and requirement for protein synthesis, to those already described for natural or nonmetabolizable fatty acids. Moreover, when used at submaximally effective concentrations, BRL49653 and 2-bromopalmitate act in an additive manner to induce gene expression in preadipose cells, but this additivity of effects is lost when one of the compounds is used at a maximally effective concentration. These observations, suggesting similar mechanisms of action for thiazolidinediones and fatty acids, are strongly supported by the demonstration that (i) both molecules activate, in a heterogolous trans-activation assay, the same nuclear receptor of the steroid/thyroid hormone nuclear receptor superfamily and (ii) transfection of 3T3-C2 fibroblasts with an expression vector for this nuclear receptor confers thiazolidinedione inducibility of adipocyte lipid-binding protein gene expression.

Proliferation and differentiation of rat adipose precursor cells in chemically defined medium: differential action of anti-adipogenic agents

Primary rat adipose precursor cells, maintained in the minimal chemically defined medium (ITT medium) able to promote differentiation, have been used to investigate the ability of several agents to modulate their proliferation and their differentiation. Fetuin and fibroblast growth factor (FGF), which exhibited a strong and a weak mitogenic activity, respectively, do not significantly affect the proportion of differentiated cells as indicated by glycerol-3-phosphate dehydrogenase (GPDH) activity values. In contrast, carbaprostacyclin (cPGI2), a stable analogue of prostacyclin, behaves as a true adipogenic factor leading to a 4 to 5-fold increase in GPDH-specific activities with no significant effect on cell growth. Submaxillary gland kallikrein (SMGK), transforming growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) behave as growth-promoting agents but at the same time elicit a dose-dependent inhibition of differentiation. Epidermal growth factor (EGF) and prostaglandin F2 alpha (PGF2 alpha) do not show any effect on cell proliferation at concentrations which exert a maximal inhibitory action on differentiation. Upon removal of EGF from the culture medium, complete resumption of differentiation occurs, whereas upon removal of PGF2 alpha or SMGK, complete resumption only takes place when differentiation is triggered by cPGI2. Upon removal of TNF-alpha, a partial resumption of differentiation is observed, whereas no subsequent differentiation is observed upon TGF-beta removal. These results emphasize the adipogenic, nonmitogenic role of cPGI2 and also allow the distinction between the various adipogenic/mitogenic factors which affect adipose cell differentiation.