Microarray analyses during adipogenesis: understanding the effects of Wnt signaling on adipogenesis and the roles of liver X receptor alpha in adipocyte metabolism

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

A diet enriched in PUFAs, in particular of the n-3 family, decreases adipose tissue mass and suppresses development of obesity in rodents. Although several nuclear hormone receptors are identified as PUFA targets, the precise molecular mechanisms underlying the effects of PUFAs still remain to be elucidated. Here we review research aimed at elucidating molecular mechanisms governing the effects of PUFAs on the differentiation and function of white fat cells. This review focuses on dietary PUFAs as signaling molecules, with special emphasis on agonistic and antagonistic effects on transcription factors currently implicated as key players in adipocyte differentiation and function, including peroxisome proliferator activated receptors (PPARs) (alpha, beta and gamma), sterol regulatory element binding proteins (SREBPs) and liver X receptors (LXRs). We review evidence that dietary n-3 PUFAs decrease adipose tissue mass and suppress the development of obesity in rodents by targeting a set of key regulatory transcription factors involved in both adipogensis and lipid homeostasis in mature adipocytes. The same set of factors are targeted by PUFAs of the n-6 family, but the cellular/physiological responses are dependent on the experimental setting as n-6 PUFAs may exert either an anti- or a proadipogenic effect. Feeding status and hormonal background may therefore be of particular importance in determining the physiological effects of PUFAs of the n-6 family.

SIRT1 Deacetylation and Repression of p300 Involves Lysine Residues 1020/1024 within the Cell Cycle Regulatory Domain 1

The SIR2 family of nicotinamide adenosine dinucleotide (NAD)-dependent deacetylases modulates diverse biological functions in different species, including longevity, apoptosis, cell cycle exit, and cellular differentiation. SIRT1, the closest mammalian ortholog of the yeast SIR2 (silent information regulator 2) gene, represses several transcription factors, including p53, NFkappaB and forkhead proteins. The p300 protein serves as a rate-limiting transcriptional cointegrator of diverse transcription factors either to activate or to repress transcription through modular subdomains. Herein, SIRT1 physically interacted with and repressed p300 transactivation, requiring the NAD-dependent deacetylase activity of SIRT1. SIRT1 repression involved the CRD1 transcriptional repression domain of p300. Two residues within the CRD1 domain (Lys-1020 and Lys-1024) were required for SIRT1 repression and served as substrates for SIRT1 deacetylation. These residues also serve as acceptor lysines for modification by the ubiquitin-like SUMO protein. The SUMO-specific protease SSP3 relieved SIRT1 repression of p300. SSP3 antagonism of SIRT1 required the SUMO-deconjugating function of SSP3. Thus, p300 serves as a deacetylase substrate for SIRT1 through a conserved SUMO consensus motif. Because p300 is a limiting transcriptional cofactor, deacetylation and repression of p300 by SIRT1 may serve an important integration point during metabolism and cellular differentiation.

Cloning, expression, and differentiation-dependent regulation of SMAF1 in adipogenesis

With the aim of identifying novel molecular pathways in the adipocyte, we conducted differential screening of DNA filter arrays with probes from 3T3-L1 preadipocytes and adipocytes, and discovered a novel 0.7kb transcript we term small adipocyte factor 1 (SMAF1). SMAF1 encodes a wholly novel 10kDa protein. Transfection and localization studies of a SMAF1-EGFP fusion construct indicate nuclear localization, suggestive of a possible regulatory role. Northern blot analysis of various murine tissues indicates adipose tissue-restricted expression, and fractionation of adipose tissue reveals that SMAF1 is expressed soley in adipocytes and not in the stromal-vascular cell population. Northern blot analysis of brown and white adipogenic conversion reveals that expression of SMAF1 closely parallels emergence of an adipocyte phenotype and that TNFalpha-mediated dedifferentiation of 3T3-L1 adipocytes results in a rapid decline of SMAF1 transcript. These data indicate that SMAF1 is closely tied to the adipocyte phenotype and predict a novel and possibly regulatory role for this gene in adipocyte function.

Effects of Wnt signaling on brown adipocyte differentiation and metabolism mediated by PGC-1alpha

Activation of canonical Wnt signaling inhibits brown adipogenesis of cultured cells by impeding induction of PPARgamma and C/EBPalpha. Although enforced expression of these adipogenic transcription factors restores lipid accumulation and expression of FABP4 in Wnt-expressing cells, additional expression of PGC-1alpha is required for activation of uncoupling protein 1 (UCP1). Wnt10b blocks brown adipose tissue development and expression of UCP1 when expressed from the fatty acid binding protein 4 promoter, even when mice are administered a beta3-agonist. In differentiated brown adipocytes, activation of Wnt signaling suppresses expression of UCP1 through repression of PGC-1alpha. Consistent with these in vitro observations, UCP1-Wnt10b transgenic mice, which express Wnt10b in interscapular tissue, lack functional brown adipose tissue. While interscapular tissue of UCP1-Wnt10b mice lacks expression of PGC-1alpha and UCP1, the presence of unilocular lipid droplets and expression of white adipocyte genes suggest conversion of brown adipose tissue to white. Reciprocal expression of Wnt10b with UCP1 and PGC-1alpha in interscapular tissue from cold-challenged or genetically obese mice provides further evidence for regulation of brown adipocyte metabolism by Wnt signaling. Taken together, these data suggest that activation of canonical Wnt signaling early in differentiation blocks brown adipogenesis, whereas activating Wnt signaling in mature brown adipocytes stimulates their conversion to white adipocytes.

Effects of Wnt signaling on proliferation and differentiation of human mesenchymal stem cells

Mesenchymal stem cells are pluripotent cells from bone marrow, which can be differentiated into the osteogenic, chondrogenic, and adipogenic lineages in vitro and are a source of cells in bone and cartilage tissue engineering. An improvement in current tissue-engineering protocols requires more detailed insight into the molecular cues that regulate the distinct steps of osteochondral differentiation. Because Wnt signaling has been widely implicated in mesenchymal differentiation, we analyzed the role of Wnt signaling in human mesenchymal stem cell (hMSC) biology by stimulation of the pathway with lithium chloride and Wnt3A-conditioned medium. We demonstrate a role for low levels of Wnt signaling in proliferation of uncommitted hMSCs and confirm that Wnt signaling controls osteoprogenitor proliferation. On the other hand, at high Wnt levels we observed a block in adipogenic differentiation and an increase in the expression of alkaline phosphatase, suggesting a role in the initiation of osteogenesis. The results of this study suggest that bone tissue engineering could benefit from the activation of critical levels of Wnt signaling at defined stages of differentiation. Moreover, our data suggest that hMSCs provide a valid in vitro model to study the role of Wnt signaling in mesenchymal biology.

High extracellular calcium attenuates adipogenesis in 3T3-L1 preadipocytes

We studied the effect of extracellular Ca(2+) concentration ([Ca(2+)](e)) on adipocyte differentiation. Preadipocytes exposed to continuous [Ca(2+)](e) higher than 2.5 mmol/l accumulated little or no cytoplasmic lipid compared to controls in 1.8 mmol/l [Ca(2+)](e). Differentiation was monitored by Oil Red O staining of cytoplasmic lipid and triglyceride assay of accumulated lipid, by RT-PCR analysis of adipogenic markers, and by the activity of glycerol-3-phosphate dehydrogenase (GPDH). Elevated [Ca(2+)](e) inhibited expression of peroxisome proliferator-activated receptor gamma, CCAAT/enhancer binding protein alpha, and steroid regulatory binding element protein. High [Ca(2+)](e) significantly inhibited differentiation marker expression including adipocyte fatty acid binding protein, and GPDH. The decrease in Pref-1 expression that accompanied differentiation also was prevented by high [Ca(2+)](e). Treatment of 3T3-L1 cells with high [Ca(2+)](e) did not significantly affect cell number or viability and did not trigger apoptosis. Levels of intracellular Ca(+2) remained unchanged in various [Ca(2+)](e). Treatment of 3T3-L1 with pertussis toxin (PTX) partially restored lipid accumulation and increased differentiation markers in cells treated with 5 mmol/l [Ca(2+)](e). 'Classical' parathyroid cell Ca(2+) sensing receptors (CaSR) were not detected either by RT-PCR or by Western blotting. These results suggest that continuous exposure to high [Ca(2+)](e) inhibits preadipocyte differentiation and that this may involve a G-protein-coupled mechanism mediated by a novel Ca(2+) sensor or receptor.

Adipocytic differentiation and liver x receptor pathways regulate the accumulation of triacylglycerols in human vascular smooth muscle cells

Lipid accumulation by vascular smooth muscle cells (VSMC) is a feature of atherosclerotic plaques. In this study we describe two mechanisms whereby human VSMC foam cell formation is driven by de novo synthesis of fatty acids leading to triacylglycerol accumulation in intracellular vacuoles, a process distinct from serum lipoprotein uptake. VSMC cultured in adipogenic differentiation medium accumulated lipids and were induced to express the adipocyte marker genes adipsin, adipocyte fatty acid-binding protein, C/EBPalpha, PPARgamma, and leptin. However, complete adipocyte differentiation was not observed as numerous genes present in mature adipocytes were not detected, and the phenotype was reversible. The rate of lipid accumulation was not affected by PPARgamma agonists, but screening for the effects of other nuclear receptor agonists showed that activation of the liver X receptors (LXR) dramatically promoted lipid accumulation in VSMC. Both LXRalpha and LXRbeta were present in VSMC, and their activation with TO901317 resulted in induction of the lipogenic genes fatty acid synthetase, sterol regulatory element binding protein (SREBP1c), and stearoyl-CoA desaturase. 27-Hydroxycholesterol, an abundant oxysterol synthesized by VSMC acted as an LXR antagonist and, therefore, may have a protective role in preventing foam cell formation. Immunohistochemistry showed that VSMC within atherosclerotic plaques express adipogenic and lipogenic markers, suggesting these pathways are present in vivo. Moreover, the development of an adipogenic phenotype in VSMC is consistent with their known phenotypic plasticity and may contribute to their dysfunction in atherosclerotic plaques and, thus, impinge on plaque growth and stability.

Bile acid signaling to the nucleus: finding new connections in the transcriptional regulation of metabolic pathways

Recent findings indicate that the function of metabolically relevant genes is finely regulated at the level of gene transcription. Disturbances of these regulatory pathways often lead to metabolic unbalance and to the onset of socially relevant diseases, i.e. diabetes, metabolic syndrome, atherosclerosis and cardiovascular diseases. The ability of lipid metabolites, such as fatty acids and oxysterols, to signal to cells and tissues and to affect gene transcription by activating specific nuclear receptors has been known since several years. Bile acids have been known in the past as cholesterol end products, purely acting as detergents. Only recently new biological properties of bile acids as signaling molecules have been disclosed and appreciated. In this review, we will describe how bile acids can regulate their own synthesis and other metabolic pathways (i.e. glucose metabolism) by modulating gene transcription through multiple mechanisms. These findings also open new perspectives towards the exploitation of bile acid metabolism as a pharmacological target.

Synthesis and secretion of apoC-I and apoE during maturation of human SW872 liposarcoma cells

Little is known about the regulation of apolipoprotein (apo) C-I production by human adipocytes. The aim of the present study, therefore, was to investigate the effect of different tissue culture conditions on the synthesis and secretion of apoC-I and apoE in human SW872 liposarcoma cells. After 3-4 d in culture (0.5 x 10(6) cells/well, DMEM/F-12 medium with 10% fetal calf serum), cells reached confluence and became growth arrested. The molar ratio of apoE:apoC-I in the cell was 8.9 +/- 0.6 and in the medium was 6.6 +/- 0.5. After 17 d in culture, SW872 cells contained significantly more cholesterol (100%) and triglyceride (3-fold) and secreted more apoC-I [4 vs. 17 d: 0.11 +/- 0.01 vs. 0.23 +/- 0.01 pmol/(10(6) cells . 24 h), P < 0.001] and apoE [0.7 +/- 0.1 vs. 3.1 +/- 0.3 pmol/(10(6) cells . 24 h), P < 0.001]. Cellular apoC-I increased 7-fold and apoE increased 16-fold. Cell maturation was associated with significantly higher levels of apoE mRNA but not apoC-I mRNA. Increases in cell lipids, apoC-I, and apoE were not dependent on the presence of extracellular lipids because similar changes occurred in cells incubated with lipoprotein-deficient serum or in cells incubated without serum. Treatment (7 d) of cells during maturation with insulin (10 or 1000 nmol/L) significantly reduced the secretion of apoC-I and apoE. These results demonstrate that in maturing SW872 cells, cholesterol and triglyceride accumulation in the presence or absence of extracellular lipids, is associated with increased apoC-I and apoE production. Furthermore, apoC-I and apoE production are differentially regulated at the transcriptional level, and long-term treatment with insulin has an inhibitory rather than stimulatory effect on apoC-I and apoE production.

Divergent effects of peroxisome proliferator-activated receptor gamma agonists and tumor necrosis factor alpha on adipocyte ApoE expression

ApoE is expressed in multiple mammalian cell types in which it supports cellular differentiated function. In this report we demonstrate that apoE expression in adipocytes is regulated by factors involved in modulating systemic insulin sensitivity. Systemic treatment with pioglitazone increased systemic insulin sensitivity and increased apoE mRNA levels in adipose tissue by 2-3-fold. Treatment of cultured 3T3-L1 adipocytes with ciglitazone increased apoE mRNA levels by 2-4-fold in a dose-dependent manner and increased apoE secretion from cells. Conversely, treatment of adipocytes with tumor necrosis factor (TNF) alpha reduced apoE mRNA levels and apoE secretion by 60%. Neither insulin nor a peroxisome proliferator-activated receptor (PPAR) alpha agonist regulated adipocyte apoE gene expression. In addition, treatment of human monocyte-derived macrophages with ciglitazone did not regulate expression of apoE. Additional analyses using reporter genes indicated that the effect of TNFalpha and PPARgamma agonists on the apoE gene was mediated via distinct gene control elements. The TNFalpha effect was mediated by elements within the proximal promoter, whereas the PPARgamma effect was mediated by elements within a downstream enhancer. However, the addition of TNFalpha substantially reduced the absolute levels of apoE reporter gene response even in the presence of ciglitazone. These results indicate for the first time that adipose tissue expression of apoE is modulated by physiologic regulators of insulin sensitivity.

Tissue-specific autoregulation of the LXRalpha gene facilitates induction of apoE in mouse adipose tissue

The functions of the liver X receptors (LXRs) are not well documented in adipose tissue. We demonstrate here that expression of the LXRalpha gene is highly induced in vivo and in vitro in mouse and human adipocytes in the presence of the synthetic LXR agonist T0901317. This autoregulation is caused by an identified LXR-responsive element motif in the mouse LXRalpha promoter, which is conserved in the human LXRalpha promoter. Using different LXR-deficient mice, we demonstrate that the basal expression level of LXRalpha is increased in LXRbeta(-/-) mice, whereas the basal expression level of LXRbeta is unchanged in LXRalpha(-/-) mice. The two LXRs can compensate for each other in mediating ligand-activated regulation of LXR target genes involved in lipid homeostasis in adipose tissue. Sterol regulatory element binding protein-1 (SREBP-1), ATP binding cassette transporter A1 (ABCA1), ABCG1, as well as apolipoprotein E (apoE) are induced in vivo by T0901317 in wild-type, LXRalpha(-/-) or LXRbeta(-/-) mice but not in LXRalpha(-/-)beta(-/-) mice. Although SREBP-1 and ABCG1 are induced in liver, muscle, and adipose tissue, the apoE, glucose transporter-4 (GLUT4), and LXRalpha genes are specifically induced only in adipose tissue. We suggest that an important aspect of LXRalpha autoregulation in adipose tissue may be to increase the level of LXRalpha over a threshold level necessary to induce the expression of certain target genes.

Medium-chain Fatty acids attenuate agonist-stimulated lipolysis, mimicking the effects of starvation

OBJECTIVE

To test the hypothesis that incorporation of medium-chain fatty acids (FAs) into adipocyte triglycerides alters intracellular lipolysis.

RESEARCH METHODS AND PROCEDURES

3T3-L1 adipocytes were pretreated with octanoate for various incubation periods. After the removal of exogenous FAs, cells were incubated with different lipolytic agonists. To determine the effects on lipolysis, we measured the following: the release of glycerol and FAs, lipase activity, protein levels of hormone-sensitive lipase (HSL), and perilipin A; translocation of HSL; phosphorylation of perilipin A; and levels of cellular adenosine triphosphate, cyclic adenosine monophosphate, and H2O2. To compare the effects of starvation with those caused by octanoate pretreatment, we measured glycerol release and H2O2 generation in rat adipocytes of starved donors.

RESULTS

Pretreatment of adipocytes with octanoate in vitro increased basal lipolysis but decreased the cellular response for agonists. The same effects were seen in starvation in vivo. Preincubation with octanoate for 48 hours did not affect basal lipase activity, HSL, and perilipin protein levels, but it reduced agonist-stimulated perilipin phosphorylation and HSL translocation toward fat droplets. This was associated with a reduction in basal cellular adenosine triphosphate levels and agonist-stimulated cyclic adenosine monophosphate generation. Starvation and octanoate pretreatment both increased intracellular H2O2 concentrations, which might also contribute to the inhibition on agonist-stimulated lipolysis.

DISCUSSION

Pretreatment with octanoate seems to induce changes in adipocyte lipolysis in a pattern mimicking the effects of starvation. Such changes could contribute, in part, to weight loss in animals and humans associated with dietary medium-chain FAs.

Wnt10b inhibits development of white and brown adipose tissues

Wnt is a family of secreted signaling proteins that regulate diverse developmental processes. Activation of canonical Wnt signaling by Wnt10b inhibits differentiation of preadipocytes in vitro. To determine whether Wnt signaling blocks adipogenesis in vivo, we created transgenic mice in which Wnt10b is expressed from the FABP4 promoter. Expression of Wnt10b in adipose impairs development of this tissue throughout the body, with a decline of approximately 50% in total body fat and a reduction of approximately 60% in weight of epididymal and perirenal depots. FABP4-Wnt10b mice resist accumulation of adipose tissue when fed a high fat diet. Furthermore, transgenic mice are more glucose-tolerant and insulin-sensitive than wild type mice. Expression of Wnt10b from the FABP4 promoter also blocks development of brown adipose tissue. Interscapular tissue of FABP4-Wnt10b mice has the visual appearance of white adipose tissue but expresses neither brown (e.g. uncoupling protein 1) nor white adipocyte markers. Transgenic mice are unable to maintain a core body temperature when placed in a cold environment, providing further evidence that Wnt10b inhibits development of brown adipose tissue. Although food intake is not altered in FABP4-Wnt10b mice, oxygen consumption is decreased. Thus, FABP4-Wnt10b mice on a chow diet gain more weight than controls, largely because of an increase in weight of skin. In summary, inhibition by Wnt10b of white and brown adipose tissue development results in lean mice without lipodystrophic diabetes.

New insights into inhibitors of adipogenesis

PURPOSE OF REVIEW

Adipose tissue is a dynamic organ that changes mass throughout life in response to the metabolic needs of the animal. In the past three decades, significant advances have been made in delineating key extracellular and intracellular stimulators of fat cell formation or adipogenesis. In this review, the author focuses on new findings of specific inhibitors of adipogenesis. Understanding the balance between positive and negative regulators of adipogenesis has important health-related implications for anti-obesity medical therapy and lipodystrophy.

RECENT FINDINGS

Adipogenesis is a highly regulated process requiring coordinated expression and activation of two main groups of adipogenic transcription factors, CCAAT/enhancer binding proteins and peroxisome proliferators activated receptor gamma. In response to hormonal and nutrient stimuli, the increased expression and activation of these transcription factors induce the expression of adipocyte-specific genes. More recently, several groups have identified extracellular inhibitors of adipocyte formation, including cytokines, lipid molecules, genistein, and protease inhibitors. Intracellular signaling molecules, which negatively regulate adipogenesis, include Pref-1, Foxo1, Foxa2, SMAD-3, WNT-10b, GATA-2 and GATA-3.

SUMMARY

The prevalence of obesity is increasing in the United States and in other westernized societies. Understanding the mechanisms of excessive energy storage in adipose tissue is necessary to develop a comprehensive strategy to prevent and treat obesity. One potential, but unrealized, approach to obesity treatment is to target excessive adipose tissue enlargement. A number of promising extra- and intracellular inhibitors of fat cell formation have been identified, but the modulation of adipose tissue mass may have both advantageous and deleterious health effects.

Activated liver X receptors stimulate adipocyte differentiation through induction of peroxisome proliferator-activated receptor gamma expression

Liver X receptors (LXRs) are nuclear hormone receptors that regulate cholesterol and fatty acid metabolism in liver tissue and in macrophages. Although LXR activation enhances lipogenesis, it is not well understood whether LXRs are involved in adipocyte differentiation. Here, we show that LXR activation stimulated the execution of adipogenesis, as determined by lipid droplet accumulation and adipocyte-specific gene expression in vivo and in vitro. In adipocytes, LXR activation with T0901317 primarily enhanced the expression of lipogenic genes such as the ADD1/SREBP1c and FAS genes and substantially increased the expression of the adipocyte-specific genes encoding PPARgamma (peroxisome proliferator-activated receptor gamma) and aP2. Administration of the LXR agonist T0901317 to lean mice promoted the expression of most lipogenic and adipogenic genes in fat and liver tissues. It is of interest that the PPARgamma gene is a novel target gene of LXR, since the PPARgamma promoter contains the conserved binding site of LXR and was transactivated by the expression of LXRalpha. Moreover, activated LXRalpha exhibited an increase of DNA binding to its target gene promoters, such as ADD1/SREBP1c and PPARgamma, which appeared to be closely associated with hyperacetylation of histone H3 in the promoter regions of those genes. Furthermore, the suppression of LXRalpha by small interfering RNA attenuated adipocyte differentiation. Taken together, these results suggest that LXR plays a role in the execution of adipocyte differentiation by regulation of lipogenesis and adipocyte-specific gene expression.

Intrinsic heterogeneity in adipose tissue of fat-specific insulin receptor knock-out mice is associated with differences in patterns of gene expression

Mice with a fat-specific insulin receptor knock-out (FIRKO) have reduced adipose tissue mass, are protected against obesity, and have an extended life span. White adipose tissue of FIRKO mice is also characterized by a polarization into two major populations of adipocytes, one small (<50 microm) and one large (>100 microm), which differ with regard to basal triglyceride synthesis and lipolysis, as well as in the expression of fatty acid synthase, sterol regulatory element-binding protein 1c, and CCAAT/enhancer-binding protein alpha (C/EBP-alpha). Gene expression analysis using RNA isolated from large and small adipocytes of FIRKO and control (IR lox/lox) mice was performed on oligonucleotide microarrays. Of the 12,488 genes/expressed sequence tags represented, 111 genes were expressed differentially in the four populations of adipocytes at the p < 0.001 level. These alterations exhibited 10 defined patterns and occurred in response to two distinct regulatory effects. 63 genes were identified as changed in expression depending primarily upon adipocyte size, including C/EBP-alpha, C/EBP-delta, superoxide dismutase 3, and the platelet-derived growth factor receptor. 48 genes were regulated primarily by impairment of insulin signaling, including transforming growth factor beta, interferon gamma, insulin-like growth factor I receptor, activating transcription factor 3, aldehyde dehydrogenase 2, and protein kinase Cdelta. These data suggest an intrinsic heterogeneity of adipocytes with differences in gene expression related to adipocyte size and insulin signaling.

TCDD administration after the pro-adipogenic differentiation stimulus inhibits PPARgamma through a MEK-dependent process but less effectively suppresses adipogenesis

Hormone (IDMB)-induced adipogenesis in C3H10T1/2 cells is suppressed by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the aryl hydrocarbon receptor (AhR). We have previously reported that TCDD addition 48 h before the hormonal stimulation of IDMB suppresses a key mediator of adipogenesis, the peroxisome proliferator-activated receptor (PPARgamma), by a MEK/ERK dependent mechanism. Here we add to previous evidence that this synergism functions after IDMB addition but before increased PPARgamma1 transcription. Suppression remains effective and MEK/ERK dependent when TCDD is added 6-12 h after IDMB addition but not when delayed to 16-24 h, thus preceding the rise in PPARgamma mRNA. TCDD suppression of the number of committed adipocytes and of triglyceride formation is less effective with the delayed addition. TCDD therefore does not directly suppress the expression of the key mediator PPARgamma1. An alternative mediation of adipocyte commitment is apparently less sensitive to the 6-12 h of delayed TCDD addition. TCDD suppression potencies (EC(50) = 50 pM) match the potencies for stimulation of CYP1B1 protein and AhR-sensitive reporters. The AhR antagonist 3'-methoxy-4'-nitroflavone (3-MNF) inhibited both TCDD-mediated CYP1B1 induction and inhibition of PPARgamma protein expression. This antagonism was only effective when 3-MNF was present in the 24-h period after IDMB addition. TCDD activation of AhR in conjunction with MEK/ERK therefore generates PPARgamma1 suppression activity before the increase of PPARgamma1 synthesis. The potency and inhibition data are consistent with induction of one or more gene products that sustain suppression through the extended period of PPARgamma1 transcription.

Effects of oral exposure of bisphenol A on mRNA expression of nuclear receptors in murine placentae assessed by DNA microarray

Bisphenol A (BPA), a candidate endocrine disruptor (ED), is considered to bind to estrogen receptors and to regulate expressions of estrogen responsive genes. It has also shown evidence of affecting the reproductive, immunological and nervous systems of mammalian embryos. However, the effects of BPA on placentae, a central organ of feto-maternal interlocution, are still unclear. To reveal the mechanisms of BPA effects on placentae in mammals, we compared the mRNA expression of 20 nuclear receptors between placentae of vehicle controls and those of orally BPA exposed pregnant mice by a DNA microarray technique. In murine placentae, mRNAs of 11 nuclear receptors were not detected. However, greater than 1.5 fold changes in mRNA expression of nine nuclear receptors between vehicle control and BPA treated mice were noted. Moreover, remarkable changes in mRNA expression of six non-nuclear receptor proteins were induced by BPA exposure. There were various differences in the effects of BPA on the expression of these mRNAs between the placentae with male embryos and those with female embryos. Such embryo-sex dependent differences are interesting and important pointers to understanding of the endocrine disrupting effect of BPA. The present data indicate that BPA affects the expression of nuclear receptor mRNAs in placentae and may disrupt the physiological functions of placentae.

Glucocorticoid response and promoter occupancy of the mouse LXRalpha gene

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are members of the nuclear receptor superfamily of proteins which are highly expressed in metabolically active tissues. They regulate gene expression of critical genes involved in cholesterol catabolism and transport, lipid and triglyceride biosynthesis, and carbohydrate metabolism in response to distinct oxysterol intermediates in the cholesterol metabolic pathway. Several LXR target genes have been identified, but there is limited information on how expression of the LXRs themselves is controlled. In this study we have characterized the upstream flanking region of the mouse LXRalpha gene. Transient transfections show that the LXRalpha promoter is able to drive transcription of a luciferase reporter gene, however, the transcriptional potential of the promoter in the cell lines used was low. The -2143 to -1513 region of the promoter mediates repression of reporter gene activity in all cells analyzed and multiple DNA-protein interactions were detected in this region by DNase I footprinting. The Zta, Ets, and Hes1 transcription factors were all shown to mediate alterations in reporter gene activity driven by LXRalpha promoter deletion constructs. These factors have been linked to cell cycle and differentiation processes suggesting that expression of LXRalpha might be under control of signalling mechanisms regulating cell proliferation. Several putative binding sites of the glucocorticoid receptor (GR) were identified in the LXRalpha promoter and transient cotransfections of the GR and LXRalpha promoter deletion constructs induced reporter gene activity. Addition of dexamethasone, a GR agonist, abolished this effect suggesting cross talk between GR and LXR signalling.

The Peroxisome Proliferator-activated Receptor γ Regulates Expression of the Perilipin Gene in Adipocytes

Recent studies have shown that lipid droplets are covered with a proteinaceous coat, although the functions and identities of the component proteins have not yet been well elucidated. The first identified lipid droplet-specific proteins are the perilipins, a family of proteins coating the surfaces of lipid droplets of adipocytes. The generation of perilipin-null mice has revealed that although they consume more food than control mice, they have normal body weight and are resistant to diet-induced obesity. In one study (Martinez-Botas, J., Anderson, J. B., Tessier, D., Lapillonne, A., Chang, B. H. J., Quast, M. J., Gorenstein, D., Chen, K. H., and Chan, L. (2000) Nat. Genet. 26, 474-479) it was reported that in an animal model obesity was reversible by breeding perilipin -/- alleles into Lepr db/db obese mice, ostensibly by increasing the metabolic rate of the mice. To understand the exact mechanisms that drive the exclusive expression of the perilipin gene in adipocytes, we analyzed the 5'-flanking region of the mouse gene. Treatment of differentiating 3T3-L1 adipocytes with an agonist of proliferator-activated receptor (PPAR) gamma, the putative "master regulator" of adipocyte differentiation, significantly augmented perilipin gene expression. Reporter assays using the -2.0-kb promoter revealed that this region contains a functional PPARgamma-responsive element. Gel mobility shift and chromatin immunoprecipitation assays showed that endogenous PPARgamma protein binds to the perilipin promoter. PPARgamma2, an isoform exclusively expressed in adipocytes, was found to be the most potent regulator from among the PPAR family members including PPARalpha and PPARgamma1. These results make evident the fact that perilipin gene expression in differentiating adipocytes is crucially regulated by PPARgamma2, providing new insights into the adipogenic action of PPARgamma2 and adipose-specific gene expression, as well as potential anti-obesity pharmaceutical agents targeted to a reduction of the perilipin gene product.

Microarray analysis of differentiation-specific gene expression during 3T3-L1 adipogenesis

During cellular differentiation and development, it is recognized that many complex molecular mechanisms as well as precise patterns of differentially expressed genes occur in directing precursor cells toward a given lineage. Using microarray-based technology, we examined gene expression across the course of 3T3-L1 adipocyte differentiation. Total cellular RNA was isolated at times 0, 2, 8, 16, 24, 48, and 96 h following treatment with either standard hormonal inducers of differentiation; insulin, dexamethasone, isobutylmethylxanthine (IDX), or IDX plus trichostatin A (TsA), a histone deacetylase inhibitor and potent adipogenic inhibitor. cRNA was synthesized from cellular RNA and hybridized to high density Affymetrix MG_U74Av2 microarray gene chips containing 12,488 cDNA/Expressed Sequence Tags (ESTs) probe sets. From the IDX-only treated cells, all probe sets that were either unchanged or differentially expressed less than 2-fold throughout differentiation with respect to time 0 preadipocytes were excluded from further analyses. This selection resulted in a net of 1686 transcripts, 859 were increased in expression, and 827 were decreased in expression at least 2-fold across differentiation. To focus in on genes that were more specific to differentiation, the same analysis was performed on IDX plus TsA-treated non-differentiating cells and all probe sets from the IDX-only group that exhibited similar expression profiles in the non-differentiating TsA-treated group were excluded leaving a total of 1016 transcripts that were regulated only under differentiating conditions. Six hundred and thirty-six of these transcripts were elevated at least 2-fold and 380 exhibited a decrease in expression relative to time 0 preadipocytes. This group of genes was further analyzed using hierarchical clustering and self-organizing maps and resulted in the identification of numerous genes not previously known to be regulated during adipocyte differentiation. Many of these genes may well represent novel adipogenic mediators and markers of adipogenesis.

Putative metabolic effects of the liver X receptor (LXR)

The nuclear receptors liver X receptor (LXR)alpha and LXRbeta are sensors of cholesterol metabolism and lipid biosynthesis. They have recently been found to be regulators of inflammatory cytokines, suppressors of hepatic glucose production, and involved in different cell-signaling pathways. LXRalpha is a target gene of the peroxisome proliferator-activated receptor-gamma, a target of drugs used in treating elevated levels of glucose seen in diabetes. Furthermore, insulin induces LXRalpha in hepatocytes, resulting in increased expression of lipogenic enzymes and suppression of key enzymes in gluconeogenesis, including PEPCK. LXR seems to have an important role in the regulation of glucocorticoid action and a role in the overall energy homeostasis suggested by its putative regulatory effect on leptin and uncoupling protein 1. The physiological roles of LXR indicate that it is an interesting potential target for drug treatment of diabetes.

Adipogenic differentiation of human adult stem cells from bone marrow stroma (MSCs)

We assayed gene expressions during adipogenesis of human MSCs. Microarray assays demonstrated time-dependent increases in expression of 67 genes, including 2 genes for transcription factors that were not previously shown to be expressed during adipogenesis. INTRODUCTION Increased numbers of bone marrow adipocytes have been observed in patients with osteoporosis and aplastic anemia, but the pathological mechanisms remain unknown. Recently, microarray assays for mRNAs were used to follow adipogenic differentiation of the preadipocytic cell line, 3T3-L1, but adipogenic differentiation has not been examined in primary cells from bone marrow. Here we defined the sequence of gene expression during the adipogenesis ex vivo of human cells from bone marrow referred to as either mesenchymal stem cells or marrow stromal cells (MSCs).

MATERIALS AND METHODS

MSCs were plated at extremely low densities to generate single-cell derived colonies, and adipogenic differentiation of the colonies assayed by accumulation of fat vacuoles, time-lapse photomicroscopy, microarrays, and reverse transcriptase-polymerase chain reaction (RT-PCR) assays. RESULTS AND CONCLUSIONS About 30% of the colonies differentiated to adipocytes in 14 days and about 60% in 21 days. Cell proliferation was inhibited by approximately 50% in adipogenic medium. The differentiation occurred primarily at the center of the colonies, and a few adipocytes that formed near the periphery migrated toward the centers. RT-PCR assays demonstrated that the differentiation was accompanied by increases in a series of genes previously shown to increase with adipogenic differentiation: peroxisome proliferator activated receptor gamma, CCAAT enhancer-binding protein alpha, acylCoA synthetase, lipoprotein lipase, and fatty acid binding protein 4. We also followed differentiation with microarray assays. Sixty-seven genes increased more than 10-fold at day 1 and 20-fold at day 7, 14, or 21. Many of the genes identified were previously shown to be expressed during adipocytic differentiation. However, others, such as zinc finger E-box binding protein and zinc finger protein 145, were not. This study should serve as a basis for future study to clarify the mechanisms of adipocyte differentiation of MSCs.

Regulation of triacylglycerol hydrolase expression by dietary fatty acids and peroxisomal proliferator-activated receptors

Triacylglycerol hydrolase (TGH) is an enzyme that catalyzes the lipolysis of intracellular stored triacylglycerol (TG). Peroxisomal proliferator-activated receptors (PPAR) regulate a multitude of genes involved in lipid homeostasis. Polyunsaturated fatty acids (PUFA) are PPAR ligands and fatty acids are produced via TGH activity, so we studied whether dietary fats and PPAR agonists could regulate TGH expression. In 3T3-L1 adipocytes, TGH expression was increased 10-fold upon differentiation, compared to pre-adipocytes. 3T3-L1 cells incubated with a PPARgamma agonist during the differentiation process resulted in a 5-fold increase in TGH expression compared to control cells. Evidence for direct regulation of TGH expression by PPARgamma could not be demonstrated as TGH expression was not affected by a 24-h incubation of mature 3T3-L1 adipocytes with the PPARgamma agonist. Feeding mice diets enriched in fatty acids for 3 weeks did not affect hepatic TGH expression, though a 3-week diet enriched in fatty acids and cholesterol increased hepatic TGH expression 2-fold. Two weeks of clofibrate feeding did not significantly affect hepatic TGH expression or microsomal lipolytic activities in wild-type or PPARalpha-null mice, indicating that PPARalpha does not regulate hepatic TGH expression. Therefore, TGH expression does not appear to be directly regulated by PPARs or fatty acids in the liver or adipocytes.

Obesity is associated with macrophage accumulation in adipose tissue

Obesity alters adipose tissue metabolic and endocrine function and leads to an increased release of fatty acids, hormones, and proinflammatory molecules that contribute to obesity associated complications. To further characterize the changes that occur in adipose tissue with increasing adiposity, we profiled transcript expression in perigonadal adipose tissue from groups of mice in which adiposity varied due to sex, diet, and the obesity-related mutations agouti (Ay) and obese (Lepob). We found that the expression of 1,304 transcripts correlated significantly with body mass. Of the 100 most significantly correlated genes, 30% encoded proteins that are characteristic of macrophages and are positively correlated with body mass. Immunohistochemical analysis of perigonadal, perirenal, mesenteric, and subcutaneous adipose tissue revealed that the percentage of cells expressing the macrophage marker F4/80 (F4/80+) was significantly and positively correlated with both adipocyte size and body mass. Similar relationships were found in human subcutaneous adipose tissue stained for the macrophage antigen CD68. Bone marrow transplant studies and quantitation of macrophage number in adipose tissue from macrophage-deficient (Csf1op/op) mice suggest that these F4/80+ cells are CSF-1 dependent, bone marrow-derived adipose tissue macrophages. Expression analysis of macrophage and nonmacrophage cell populations isolated from adipose tissue demonstrates that adipose tissue macrophages are responsible for almost all adipose tissue TNF-alpha expression and significant amounts of iNOS and IL-6 expression. Adipose tissue macrophage numbers increase in obesity and participate in inflammatory pathways that are activated in adipose tissues of obese individuals.

Liver X receptors are regulators of adipocyte gene expression but not differentiation: identification of apoD as a direct target

The liver X receptors alpha and beta (LXRalpha and LXRbeta) have been shown to play important roles in lipid homeostasis in liver and macrophages, however, their function in adipose tissue is not well defined. Both LXRs are highly expressed in fat, and the expression of LXRalpha increases during adipogenesis. Furthermore, LXRalpha expression is induced by peroxisome proliferator-activated receptor gamma (PPARgamma), the master regulator of fat cell differentiation. Here we investigate the role of LXRs in adipocyte differentiation and gene expression and their potential crosstalk with the PPARgamma pathway. We demonstrate that LXR agonists have no significant effect on the differentiation of 3T3-F442A or 3T3-L1 preadipocytes in vitro and do not alter the expression of differentiation-linked PPARgamma target genes in vivo. Moreover, retroviral expression of LXRalpha in NIH-3T3 cells does not alter the adipogenic potential of these cells and neither augments nor inhibits the action of PPARgamma. However, transcriptional profiling studies reveal that LXRs are important regulators of adipocyte gene expression. We identify the multifunction lipid carrier protein apolipoprotein D and the lipogenic protein Spot 14 as LXR responsive genes both in vitro and in vivo. Thus, although LXRs do not influence adipocyte differentiation per se, these receptors are likely to play an important role in the modulation of lipid metabolism in adipocytes.

Adipose tissue energy metabolism: altered gene expression profile of mice subjected to long-term caloric restriction

We investigated the influences of short-term and lifespan-prolonging long-term caloric restriction (LCR) on gene expression in white adipose tissue (WAT). Over 11,000 genes were examined using high-density oligonucleotide microarrays in four groups of 10- to 11-month-old male C57Bl6 mice that were either fasted for 18 h before death (F), subjected to short-term caloric restriction for 23 days (SCR), or LCR for 9 months and compared with nonfasted control (CO) mice. Only a few transcripts of F and SCR were differentially expressed compared with CO mice. In contrast, 345 transcripts of 6,266 genes found to be expressed in WAT were altered significantly by LCR. The expression of several genes encoding proteins involved in energy metabolism was increased by LCR. Further, many of the shifts in gene expression after LCR are known to occur during adipocyte differentiation. Selected LCR-associated alterations of gene expression were supported by quantitative reverse transcriptase-polymerase chain reaction, histology, and histochemical examinations. Our data provide new insights on the metabolic state associated with aging retardation by LCR.

Expression of the insulin-responsive glucose transporter GLUT4 in adipocytes is dependent on liver X receptor alpha

The insulin-responsive glucose transporter GLUT4 plays a crucial role in insulin-mediated facilitated glucose uptake into adipose tissue and muscle, and impaired expression of GLUT4 has been linked to obesity and diabetes. In this study, we demonstrate that liver X receptors (LXRs) regulate the expression of GLUT4 through direct interaction with a conserved LXR response element in the GLUT4 promoter. The expression of GLUT4 in WAT is induced by a potent LXR agonist in wild type, LXR alpha-/-, and LXR beta-/- mice but not in LXR alpha-/-beta-/- mice, demonstrating that both LXRs are able to mediate ligand activated transcription of the GLUT4 gene. However, basal and insulin stimulated expression of GLUT4 in epididymal WAT is reduced only in mice carrying ablation of the LXR alpha isoform. The expression of GLUT4 is furthermore correlated to the induction of LXR alpha during mouse and human adipocyte differentiation. LXR beta is thus apparently not able to rescue basal expression of GLUT4 in the absence of LXR alpha. We have previously demonstrated that LXR alpha is down-regulated in animal models of obesity and diabetes, thus revealing a striking correlation between GLUT4 and LXR alpha expression in insulin-resistant conditions. This suggests that the LXR alpha isoform has a unique role in adipose expression of GLUT4 and suggests that alteration of adipose tissue expression of LXR alpha might be a novel tool to normalize the expression of a gene that is dysregulated in diabetic and insulin-resistant conditions.

Gene expression profiling in adipose tissue indicates different transcriptional mechanisms of liver X receptors alpha and beta, respectively

The nuclear receptors liver X receptors (LXR) alpha and beta are important regulators of genes involved in lipid, cholesterol, and carbohydrate metabolism and are highly expressed in mature adipocyte tissue. In this study we show that LXRalpha and LXRbeta are more expressed in brown adipose tissue and subcutaneous white adipose tissue than visceral (gonadal) adipose tissue. Furthermore, we report differences between LXRalpha and LXRbeta in their ability to alter expression of target genes. Gene expression profiling analysis of gonadal white adipose tissue from LXRalpha(-/-) mice and LXRbeta(-/-) mice shows different gene expression patterns in the two LXR-deficient mouse strains. Genes regulated similarly in both KO mouse strains as well as genes regulated in one, but not the other LXR-deficient mouse strain were seen. A number of genes were regulated in opposite directions by the respective LXR isoform. Taken together this suggests that the LXR isoforms might operate through different transcriptional mechanisms as well as common mechanisms. These results are in consonance with the growing body of evidence reporting differences in regulation of gene expression between the two isoforms. Furthermore, gene expression profiling shows altered gene expression patterns in primary mouse embryonic fibroblasts (MEFs) from wild type versus LXRbeta(-/-) mice; MEFs are pluripotent cells with the potential to differentiate into mature adipocytes. These results indicate a role of LXR in early developmental stages of adipose tissue.

The LXR ligand T0901317 induces severe lipogenesis in the db/db diabetic mouse

Liver X receptor (LXR) ligands are currently being evaluated as potential therapeutic agents for the treatment of low HDL. The LXR ligand T0901317 elevates ATP binding cassette transporter A1 (ABCA1) and HDL levels in animal models and induces moderate lipogenesis through upregulation of sterol regulatory element binding protein 1c (SREBP1c). Because insulin may also regulate lipogenesis through SREBP1c and fatty acid synthase (FAS), we investigated the effect of an LXR ligand in hyperinsulinemic mice. Administration of T0901317 to male db/db mice for 12 days resulted in a more severe hypertriacylglycerolemia and hepatic triacylglycerol accumulation than observed in nondiabetic mice. The LXR target genes ABCA1, SREBP1c, FAS, and stearoyl-CoA desaturase 1 were upregulated by T0901317 treatment in both diabetic db/db and nondiabetic C57BLKS mice. Changes in lipogenic gene expression were independent of mouse strain, indicating that the severe lipogenesis observed in LXR ligand-treated db/db mice was not due to additive effects of insulin on lipogenic gene expression. Phosphoenolpyruvate carboxykinase expression was suppressed, suggesting that a shift from gluconeogenesis toward lipogenesis could partially explain our observations in db/db mice. Our data suggest that LXR ligands that have effects on both fatty acid and carbohydrate metabolism should be carefully evaluated in obesity, insulin, and leptin resistance.

BMP-2 controls alkaline phosphatase expression and osteoblast mineralization by a Wnt autocrine loop

Wnt/beta-catenin signaling has recently been suggested to be involved in bone biology. The precise role of this cascade in osteoblast differentiation was examined. We show that a Wnt autocrine loop mediates the induction of alkaline phosphatase and mineralization by BMP-2 in pre-osteoblastic cells.

INTRODUCTION

Loss of function of LRP5 leads to osteoporosis (OPPG syndrome), and a specific point mutation in this same receptor results in high bone mass (HBM). Because LRP5 acts as a coreceptor for Wnt proteins, these findings suggest a crucial role for Wnt signaling in bone biology.

MATERIALS AND METHODS

We have investigated the involvement of the Wnt/LRP5 cascade in osteoblast function by using the pluripotent mesenchymal cell lines C3H10T1/2, C2C12, and ST2 and the osteoblast cell line MC3T3-E1. Transfection experiments were carried out with a number of elements of the Wnt/LRP5 pathway. Measuring osteoblast and adipocyte differentiation markers addressed the effect of this cascade on osteoblast differentiation.

RESULTS

In mesenchymal cells, only Wnt's capable of stabilizing beta-catenin induced the expression of alkaline phosphatase (ALP). Wnt3a-mediated ALP induction was inhibited by overexpression of either Xddl, dickkopf 1 (dkk1), or LRP5deltaC, indicating that canonical beta-catenin signaling is responsible for this activity. The use of Noggin, a bone morphogenic protein (BMP) inhibitor, or cyclopamine, a Hedgehog inhibitor, revealed that the induction of ALP by Wnt is independent of these morphogenetic proteins and does not require de novo protein synthesis. In contrast, blocking Wnt/LRP5 signaling or protein synthesis inhibited the ability of both BMP-2 and Shh to induce ALP in mesenchymal cells. Moreover, BMP-2 enhanced Wntl and Wnt3a expression in our cells. In MC3T3-E1 cells, where endogenous ALP levels are maximal, antagonizing the Wnt/LRP5 pathway led to a decrease of ALP activity. In addition, overexpression of dkkl reduced extracellular matrix mineralization in a BMP-2-dependent assay.

CONCLUSIONS

Our data strongly suggest that the capacity of BMP-2 and Shh to induce ALP relies on Wnt expression and the Wnt/LRP5 signaling cascade. Moreover the effects of BMP-2 on extracellular matrix mineralization by osteoblasts are mediated, at least in part, by the induction of a Wnt autocrine/paracrine loop. These results may help to explain the phenotype of OPPG patients and HBM.

T-cell factor 4N (TCF-4N), a novel isoform of mouse TCF-4, synergizes with beta-catenin to coactivate C/EBPalpha and steroidogenic factor 1 transcription factors

We have cloned T-cell factor 4N (TCF-4N), an alternative isoform of TCF-4, from developing pituitary and 3T3-L1 preadipocytes. This protein contains the N-terminal interaction domain for beta-catenin but lacks the DNA binding domain. While TCF-4N inhibited coactivation by beta-catenin of a TCF/lymphoid-enhancing factor (LEF)-dependent promoter, TCF-4N potentiated coactivation by beta-catenin of several non-TCF/LEF-dependent promoters. For example, TCF-4N synergized with beta-catenin to activate the alpha-inhibin promoter through functional and physical interactions with the orphan nuclear receptor steroidogenic factor 1 (SF-1). In addition, TCF-4N and beta-catenin synergized with the adipogenic transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) to induce leptin promoter activity. The mechanism by which beta-catenin and TCF-4N coactivated C/EBPalpha appeared to involve p300, based upon synergy between these important transcriptional regulators. Consistent with TCF-4N's redirecting the actions of beta-catenin in cells, ectopic expression of TCF-4N in 3T3-L1 preadipocytes partially relieved the block of adipogenesis caused by beta-catenin. Thus, we propose that TCF-4N inhibits coactivation by beta-catenin of TCF/LEF transcription factors and potentiates the coactivation by beta-catenin of other transcription factors, such as SF-1 and C/EBPalpha.

Activation of liver X receptor improves glucose tolerance through coordinate regulation of glucose metabolism in liver and adipose tissue

The control of lipid and glucose metabolism is closely linked. The nuclear receptors liver X receptor (LXR)alpha and LXR beta have been implicated in gene expression linked to lipid homeostasis; however, their role in glucose metabolism is not clear. We demonstrate here that the synthetic LXR agonist GW3965 improves glucose tolerance in a murine model of diet-induced obesity and insulin resistance. Analysis of gene expression in LXR agonist-treated mice reveals coordinate regulation of genes involved in glucose metabolism in liver and adipose tissue. In the liver, activation of LXR led to the suppression of the gluconeogenic program including down-regulation of peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase expression. Inhibition of gluconeogenic genes was accompanied by an induction in expression of glucokinase, which promotes hepatic glucose utilization. In adipose tissue, activation of LXR led to the transcriptional induction of the insulin-sensitive glucose transporter, GLUT4. We show that the GLUT4 promoter is a direct transcriptional target for the LXR/retinoid X receptor heterodimer and that the ability of LXR ligands to induce GLUT4 expression is abolished in LXR null cells and animals. Consistent with their effects on GLUT4 expression, LXR agonists promote glucose uptake in 3T3-L1 adipocytes in vitro. Thus, activation of LXR alters the expression of genes in liver and adipose tissue that collectively would be expected to limit hepatic glucose output and improve peripheral glucose uptake. These results outline a role for LXRs in the coordination of lipid and glucose metabolism.

Nutrigenomics: goals and strategies

Nutrigenomics is the application of high-throughput genomics tools in nutrition research. Applied wisely, it will promote an increased understanding of how nutrition influences metabolic pathways and homeostatic control, how this regulation is disturbed in the early phase of a diet-related disease and to what extent individual sensitizing genotypes contribute to such diseases. Ultimately, nutrigenomics will allow effective dietary-intervention strategies to recover normal homeostasis and to prevent diet-related diseases.

Adipocyte-specific gene expression and adipogenic steatosis in the mouse liver due to peroxisome proliferator-activated receptor gamma1 (PPARgamma1) overexpression

Peroxisome proliferator activated-receptor (PPAR) isoforms, alpha and gamma, function as important coregulators of energy (lipid) homeostasis. PPARalpha regulates fatty acid oxidation primarily in liver and to a lesser extent in adipose tissue, whereas PPARgamma serves as a key regulator of adipocyte differentiation and lipid storage. Of the two PPARgamma isoforms, PPARgamma1 and PPARgamma2 generated by alternative splicing, PPARgamma1 isoform is expressed in liver and other tissues, whereas PPARgamma2 isoform is expressed exclusively in adipose tissue where it regulates adipogenesis and lipogenesis. Since the function of PPARgamma1 in liver is not clear, we have, in this study, investigated the biological impact of overexpression of PPARgamma1 in mouse liver. Adenovirus-PPARgamma1 injected into the tail vein induced hepatic steatosis in PPARalpha(-/-) mice. Northern blotting and gene expression profiling results showed that adipocyte-specific genes and lipogenesis-related genes are highly induced in PPARalpha(-/-) livers with PPARgamma1 overexpression. These include adipsin, adiponectin, aP2, caveolin-1, fasting-induced adipose factor, fat-specific gene 27 (FSP27), CD36, Delta(9) desaturase, and malic enzyme among others, implying adipogenic transformation of hepatocytes. Of interest is that hepatic steatosis per se, induced either by feeding a diet deficient in choline or developing in fasted PPARalpha(-/-) mice, failed to induce the expression of these PPARgamma-regulated adipogenesis-related genes in steatotic liver. These results suggest that a high level of PPARgamma in mouse liver is sufficient for the induction of adipogenic transformation of hepatocytes with adipose tissue-specific gene expression and lipid accumulation. We conclude that excess PPARgamma activity can lead to the development of a novel type of adipogenic hepatic steatosis.

The forkhead transcription factor Foxo1 regulates adipocyte differentiation

An outstanding question in adipocyte biology is how hormonal cues are relayed to the nucleus to activate the transcriptional program that promotes adipogenesis. The forkhead transcription factor Foxo1 is regulated by insulin via Akt-dependent phosphorylation and nuclear exclusion. We show that Foxo1 is induced in the early stages of adipocyte differentiation but that its activation is delayed until the end of the clonal expansion phase. Constitutively active Foxo1 prevents the differentiation of preadipocytes, while dominant-negative Foxo1 restores adipocyte differentiation of fibroblasts from insulin receptor-deficient mice. Further, Foxo1 haploinsufficiency protects from diet-induced diabetes in mice. We propose that Foxo1 plays an important role in the integration of hormone-activated signaling pathways with the complex transcriptional cascade that promotes adipocyte differentiation.

FGF4, a direct target of LEF1 and Wnt signaling, can rescue the arrest of tooth organogenesis in Lef1(-/-) mice

Lymphoid enhancer factor (LEF1), a nuclear mediator of Wnt signaling, is required for the formation of organs that depend on inductive interactions between epithelial and mesenchymal tissues. In previous tissue recombination experiments with normal and Lef1(-/-) tooth germs, we found that the effect of LEF1 expression in the epithelium is tissue nonautonomous and transferred to the subjacent mesenchyme. Here we examine the molecular basis for LEF1 function and find that the epithelium of the developmentally arrested Lef1(-/-) tooth rudiments fails to express Fgf4, Shh, and Bmp4, but not Wnt10a. We identify the Fgf4 gene as a direct transcriptional target for LEF1 and show that beads soaked with recombinant FGF4 protein can fully overcome the developmental arrest of Lef1(-/-) tooth germs. In addition, we find that FGF4 beads induce rapidly the expression of Fgf3 in dental mesenchyme and that both epithelial and mesenchymal FGF proteins induce the delayed expression of Shh in the epithelium. Taken together, these data indicate that a single target of LEF1 can account for the function of LEF1 in tooth development and for a relay of a Wnt signal reception to a cascade of FGF signaling activities, allowing for a sequential and reciprocal communication between epithelium and mesenchyme.

Wnt signaling protects 3T3-L1 preadipocytes from apoptosis through induction of insulin-like growth factors

Ectopic expression of Wnt-1 in 3T3-L1 preadipocytes stabilizes beta-catenin, activates TCF-dependent gene transcription, and blocks adipogenesis. Here we report that upon serum withdrawal, Wnt-1 causes 3T3-L1 cells to resist apoptosis through a mechanism that is partially dependent on phosphatidylinositol 3-kinase. Although activation of Wnt signaling by inhibition of GSK-3 activity or ectopic expression of dominant stable beta-catenin blocks apoptosis, inhibition of Wnt signaling through expression of dominant negative TCF-4 increases apoptosis. Wnt-1 stimulates 3T3-L1 preadipocytes to secrete factors that increase PKB/Akt phosphorylation at levels comparable with treatment with 10% serum. With DNA microarrays, we identified several secreted antiapoptotic genes that are induced by Wnt-1, notably insulin-like growth factor I (IGF-I) and IGF-II. Consistent with IGFs mediating the antiapoptotic effects of Wnt-1 in preadipocytes, conditioned medium from Wnt-1 expressing 3T3-L1 cells was unable to promote protein kinase B phosphorylation after the addition of recombinant IGFBP-4. Thus, we demonstrated that Wnt-1 induces expression of antiapoptotic genes in 3T3-L1 preadipocytes such as IGF-I and IGF-II, which allows these cells to resist apoptosis in response to serum deprivation.