Induction of peroxisome proliferator-activated receptor gamma during the conversion of 3T3 fibroblasts into adipocytes is mediated by C/EBPbeta, C/EBPdelta, and glucocorticoids

The novel gene fad104, containing a fibronectin type III domain, has a significant role in adipogenesis

A novel gene named fad104 (factor for adipocyte differentiation-104), whose expression level quickly increased in the early stage of adipogenesis, was isolated and characterized. The deduced amino acid sequence of fad104 revealed the possible presence of a fibronectin type III domain and transmembrane domain. The expression of fad104 was detected in adipocyte differentiable 3T3-L1 cells but not observed in the non-adipogenic cell line NIH-3T3. Moreover, the ability of 3T3-L1 cells to differentiate declined with the knockdown of fad104 by RNA interference, strongly indicating that fad104 functions as a positive regulator of adipogenesis.

Involvement of C/EBP-alpha gene in in vitro activation of rat hepatic stellate cells

Hepatic stellate cells (HSCs) play key roles in hepatic fibrosis. One of the most striking alterations in activated HSCs is loss of cytoplasmic lipid droplets. However, the association of lipid storage with the activation of HSCs remains unclear. CCAAT/enhancer-binding proteins family (C/EBPs), especially C/EBP-alpha, controls differentiation of adipocytes. We suggested that C/EBP-alpha gene may be involved in HSCs activation. The present results showed that the expression levels of C/EBP-alpha and C/EBP-beta genes declined in activated HSCs. Over-expression of C/EBP-alpha gene in activated HSCs: (1) inhibited HSCs proliferation, extracellular matrix-producing, alpha-smooth muscle actin gene expression, and induced rebound of cytoplasmic lipid droplets; (2) reduced retinoic acid receptor-beta, C/EBP-delta and -beta gene expressions, but increased the active form C/EBP-beta PSer(105), and induced retinoid X receptor-alpha gene expression; and (3) did not affect the protein level of p16INK4a, p21Cip1/WAF1 or p27Kip1. In conclusions, C/EBP-alpha gene is involved in in vitro activation of rat HSCs.

Role of Krüppel-like factor 15 (KLF15) in transcriptional regulation of adipogenesis

Krüppel-like zinc finger transcription factors (KLFs) play diverse roles during cell differentiation and development in mammals. We have now shown by microarray analysis that expression of the KLF15 gene is markedly up-regulated during the differentiation of 3T3-L1 preadipocytes into adipocytes. Inhibition of the function of KLF15, either by expression of a dominant negative mutant or by RNA interference, both reduced the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) and blocked adipogenesis in 3T3-L1 preadipocytes exposed to inducers of adipocyte differentiation. However, the dominant negative mutant of KLF15 did not affect the expression of CCAAT/enhancer-binding protein beta (C/EBPbeta) elicited by inducers of differentiation in 3T3-L1 preadipocytes. In addition, ectopic expression of KLF15 in NIH 3T3 or C2C12 cells triggered both lipid accumulation and the expression of PPARgamma in the presence of inducers of adipocyte differentiation. Ectopic expression of C/EBPbeta, C/EBPdelta, or C/EBPalpha in NIH 3T3 cells also elicited the expression of KLF15 in the presence of inducers of adipocyte differentiation. Moreover, KLF15 and C/EBPalpha acted synergistically to increase the activity of the PPARgamma2 gene promoter in 3T3-L1 adipocytes. Our observations thus demonstrate that KLF15 plays an essential role in adipogenesis in 3T3-L1 cells through its regulation of PPAR gamma expression.

ETO/MTG8 is an inhibitor of C/EBPbeta activity and a regulator of early adipogenesis

The putative transcriptional corepressor ETO/MTG8 has been extensively studied due to its involvement in a chromosomal translocation causing the t(8;21) form of acute myeloid leukemia. Despite this, the role of ETO in normal physiology has remained obscure. Here we show that ETO is highly expressed in preadipocytes and acts as an inhibitor of C/EBPbeta during early adipogenesis, contributing to its characteristically delayed activation. ETO prevents both the transcriptional activation of the C/EBPalpha promoter by C/EBPbeta and its concurrent accumulation in centromeric sites during early adipogenesis. ETO expression rapidly reduces after the initiation of adipogenesis, and this is essential to the normal induction of adipogenic gene expression. These findings define, for the first time, a molecular role for ETO in normal physiology as an inhibitor of C/EBPbeta and a novel regulator of early adipogenesis.

Phosphorylation of C/EBPbeta at a consensus extracellular signal-regulated kinase/glycogen synthase kinase 3 site is required for the induction of adiponectin gene expression during the differentiation of mouse fibroblasts into adipocytes

Stimulation of adipogenesis in mouse preadipocytes requires C/EBPbeta as well as activation of the MEK/extracellular signal-regulated kinase (ERK) signaling pathway. In this study, we demonstrate that phosphorylation of C/EBPbeta at a consensus ERK/glycogen synthase kinase 3 (GSK3) site regulates adiponectin gene expression during the C/EBPbeta-facilitated differentiation of mouse fibroblasts into adipocytes. First, we show that exposure of 3T3-L1 preadipocytes to insulin, dexamethasone (DEX), and isobutylmethylxanthine (MIX) leads to the phosphorylation of C/EBPbeta at threonine 188. Pretreating the cells with a MEK1-specific inhibitor (U0126) significantly attenuates this activity. Similarly, these effectors activate the phosphorylation of T188 within an ectopic C/EBPbeta overexpressed in Swiss mouse fibroblasts, and this event involves both MEK1 and GSK3 activity. We further show that expression of C/EBPbeta (p34kD LAP isoform) in Swiss mouse fibroblasts exposed to DEX, MIX, and insulin induces expression of peroxisome proliferator-activated receptor gamma (PPARgamma) and some adiponectin but that it does not activate expression of FABP4/aP2. In fact, complete conversion of these fibroblasts into lipid-laden adipocytes, which includes activation of FABP4 and adiponectin expression, requires their exposure to a potent PPARgamma ligand such as troglitazone. Expression of a mutant C/EBPbeta in which threonine 188 has been modified to alanine (C/EBPbeta T188A) can induce PPARgamma production in the mouse fibroblasts, but it is incapable of stimulating adiponectin expression in the absence or presence of troglitazone. Interestingly, replacement of T188 with aspartic acid creates a C/EBPbeta molecule (C/EBPbeta T188D) that possesses adipogenic activity similar to that of the wild-type molecule. The absence of adiponectin expression correlates with a reduced amount of C/EBPalpha in the adipocytes expressing the T188A mutant suggesting that C/EBPalpha is required for expression of adiponectin. In fact, ectopic expression of PPARgamma in C/EBPalpha-deficient fibroblasts (NIH 3T3 cells) produces a modest amount of adiponectin, whereas expression of both PPARgamma and C/EBPalpha in NIH 3T3 cells facilitates production of abundant quantities of adiponectin. These data demonstrate that phosphorylation of C/EBPbeta at a consensus ERK/GSK3 site is required for both C/EBPalpha and adiponectin gene expression during the differentiation of mouse fibroblasts into adipocytes.

Transcription suppression of peroxisome proliferator-activated receptor gamma2 gene expression by tumor necrosis factor alpha via an inhibition of CCAAT/ enhancer-binding protein delta during the early stage of adipocyte differentiation

TNFalpha is known to inhibit adipocyte differentiation and induce insulin resistance. Moreover, TNFalpha is known to down-regulate peroxisome proliferator-activated receptor (PPAR)gamma2, an adipocyte-specific nuclear receptor of insulin-sensitizer thiazolidinediones. To clarify molecular mechanisms of TNFalpha- mediated PPARgamma2 down-regulation, we here examined the effect of TNFalpha on transcription regulation of PPARgamma2 gene expression during the early stage of adipocyte differentiation. 3T3-L1 preadipocytes (2 d after 100% confluent) were incubated in a differentiation mixture (dexamethasone, insulin, 3-isobutyl-1-methlxanthine), with or without 50 ng/ml TNFalpha, for 24 h. TNFalpha significantly decreased PPARgamma2 expression both at mRNA and protein levels (to approximately 40%), as well as aP2 mRNA expression. The mouse PPARgamma2 gene promoter region (2.2-kb) was isolated and was used for luciferase reporter assays by transient transfection. TNFalpha significantly suppressed PPARgamma2 gene transcription (to approximately 50%), and deletion analyses demonstrated that the suppression was mediated via CCAAT/enhancer-binding protein (C/EBP) binding elements at the -320/-340 region of the promoter. Moreover, TNFalpha significantly decreased expression of C/EBPdelta mRNA and protein levels (to approximately 40%). EMSA, using 3T3-L1 cells nuclear extracts with the -320/-340 region as a probe, demonstrated the binding of C/EBPdelta to the element, which was significantly decreased by TNFalpha treatment. Overexpression of CEBP/delta prevented the TNFalpha-mediated suppression of PPARgamma2 transactivation. Taken together, TNFalpha suppresses PPARgamma2 gene transcription by the inhibition of C/EBPdelta expression and its DNA binding during the early stage of adipocyte differentiation, which may contribute to the inhibition of adipocyte differentiation, as well as the induction of insulin resistance.

regulation of peroxisome proliferator-activated receptor-gamma activity by mammalian target of rapamycin and amino acids in adipogenesis

Adipocyte differentiation is a developmental process that is critical for metabolic homeostasis and nutrient signaling. The mammalian target of rapamycin (mTOR) mediates nutrient signaling to regulate cell growth, proliferation, and diverse cellular differentiation. It has been reported that rapamycin, the inhibitor of mTOR and an immunosuppressant, blocks adipocyte differentiation, but the mechanism underlying this phenomenon remains unknown. Here we show that mTOR plays a critical role in 3T3-L1 preadipocyte differentiation and that mTOR kinase activity is required for this process. Rapamycin specifically disrupted the positive transcriptional feedback loop between CCAAT/enhancer-binding protein-alpha and peroxisome proliferator-activated receptor-gamma (PPAR-gamma), two key transcription factors in adipogenesis, by directly targeting the transactivation activity of PPAR-gamma. In addition, we demonstrate for the first time that PPAR-gamma activity is dependent on amino acid sufficiency, revealing a molecular link between nutrient status and adipogenesis. The results of our further investigation have led us to propose a model in which the mTOR pathway and the phosphatidylinositol 3-kinase/Akt pathway act in parallel to regulate PPAR-gamma activation during adipogenesis by mediating nutrient availability and insulin signals, respectively. It is interesting that troglitazone (a thiazolidinedione drug) reversed the inhibitory effects of rapamycin and amino acid deprivation, implicating therapeutic values of thiazolidinedione drugs to counter certain side effects of rapamycin as an immunosuppressant.

STAT 5 activators can replace the requirement of FBS in the adipogenesis of 3T3-L1 cells

The 3T3-L1 cells differentiate into fat cells that have many properties of native adipocytes including: substantial lipid accumulation, insulin sensitivity, and the ability to secrete endocrine hormones. A substantial expense in using these cells is fetal bovine serum (FBS), a critical component of efficient adipogenesis. Our recent studies on STAT 5 proteins have revealed that these transcription factors are phosphorylated and translocate to the nucleus immediately after the initiation of differentiation. Studies by several other laboratories also suggest that STAT 5 proteins can have pro-adipogenic properties. Growth hormone (GH) and prolactin (PRL) are both potent activators of STAT 5A and STAT 5B proteins. Since, FBS has high concentrations of GH; we examined the ability of GH to replace FBS as a component of the differentiation cocktail for 3T3-L1 cells. Our studies revealed that FBS was not required for the adipogenesis of 3T3-L1 cells if GH or PRL was added to the differentiation cocktail. Adipogenesis was judged by Oil Red O staining and expression of adipocyte marker genes. Hence, we have developed a substantially less expensive method for differentiating 3T3-L1 cells without FBS, thiazolidinediones, or expensive cytokines.

GDF-3 is an adipogenic cytokine under high fat dietary condition

Growth differentiation factor 3 (GDF-3) is structurally a bone morphogenetic protein/growth differentiation factor subfamily member of the TGF-beta superfamily. GDF-3 exhibits highest level of expression in white fat tissue in mice and is greatly induced by high fat diet if fat metabolic pathway is blocked. To identify its biological function, GDF-3 was overexpressed in mice by adenovirus mediated gene transfer. Mice transduced with GDF-3 displayed profound weight gain when fed with high fat diet. The phenotypes included greatly expanded adipose tissue mass, increased body adiposity, highly hypertrophic adipocytes, hepatic steatosis, and elevated plasma leptin. GDF-3 stimulated peroxisome proliferator activated receptor expression in adipocytes, a master nuclear receptor that controls adipogenesis. However, GDF-3 was not involved in blood glucose homeostasis or insulin resistance, a condition associated with obesity. In contrast, similar phenotypes were not observed in GDF-3 mice fed with normal chow, indicating that GDF-3 is only active under high lipid load. Thus, GDF-3 is a new non-diabetic adipogenic factor tightly coupled with fat metabolism.

Adipogenesis: usefulness of in vitro and in vivo experimental models

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

Effect of a C/EBP gene replacement on mitochondrial biogenesis in fat cells

CCAAT/enhancer-binding proteins, C/EBPalpha and C/EBPbeta, are required for fat cell differentiation and maturation. Previous studies showed that replacement of C/EBPalpha with C/EBPbeta, generating the beta/beta alleles in the mouse genome, prevents lipid accumulation in white adipose tissue (WAT). In this study, beta/beta mice lived longer and had higher energy expenditure than their control littermates due to increased WAT energy oxidation. The WAT of beta/beta mice was enriched with metabolically active, thermogenic mitochondria known for energy burning. The beta/beta allele exerted its effect through the elevated expression of the G protein alpha stimulatory subunit (Galphas) in WAT. Galphas, when overexpressed in fat-laden 3T3-L1 cells, stimulated mitochondrial biogenesis similar to that seen in the WAT of beta/beta mice, and effectively diminished the stored lipid pool.

The forkhead transcription factor FoxC2 inhibits white adipocyte differentiation

In this study, we show that expression of FoxC2 blocks the capacity of 3T3-L1 preadipocytes to undergo adipogenesis in the presence of dexamethasone, isobutylmethylxanthine, and insulin. This block is characterized by an extensive decrease in the expression of proteins associated with the function of the mature fat cell, most notably C/EBPalpha, adiponectin, perilipin, and the adipose-specific fatty acid-binding protein, FABP4/aP2. Since the expression of these proteins lies downstream of PPARgamma, we overexpressed PPARgamma in Swiss mouse fibroblasts to promote adipocyte differentiation. We show that FoxC2 blocks the ability of PPARgamma to induce adipogenic gene expression in response to exposure of the cells to dexamethasone, isobutylmethylxanthine, insulin, and a PPARgamma ligand. Interestingly, the expression of aP2 escapes the inhibitory action of FoxC2 under conditions that promote maximum PPARgamma activity. In contrast, FoxC2 inhibits the expression of C/EBPalpha, perilipin, and adiponectin even in the presence of potent PPARgamma ligands. Finally, we show that FoxC2 does not affect the ability of PPARgamma to bind to or transactivate from a PPARgamma response element. These data suggest that FoxC2 blocks adipogenesis by inhibiting the capacity of PPARgamma to promote the expression of a subset of adipogenic genes.

DeltaFosB induces osteosclerosis and decreases adipogenesis by two independent cell-autonomous mechanisms

Osteoblasts and adipocytes may develop from common bone marrow mesenchymal precursors. Transgenic mice overexpressing DeltaFosB, an AP-1 transcription factor, under the control of the neuron-specific enolase (NSE) promoter show both markedly increased bone formation and decreased adipogenesis. To determine whether the two phenotypes were linked, we targeted overexpression of DeltaFosB in mice to the osteoblast by using the osteocalcin (OG2) promoter. OG2-DeltaFosB mice demonstrated increased osteoblast numbers and an osteosclerotic phenotype but normal adipocyte differentiation. This result firmly establishes that the skeletal phenotype is cell autonomous to the osteoblast lineage and independent of adipocyte formation. It also strongly suggests that the decreased fat phenotype of NSE-DeltaFosB mice is independent of the changes in the osteoblast lineage. In vitro, overexpression of DeltaFosB in the preadipocytic 3T3-L1 cell line had little effect on adipocyte differentiation, whereas it prevented the induction of adipogenic transcription factors in the multipotential stromal cell line ST2. Also, DeltaFosB isoforms bound to and altered the DNA-binding capacity of C/EBPbeta. Thus, the inhibitory effect of DeltaFosB on adipocyte differentiation appears to occur at early stages of stem cell commitment, affecting C/EBPbeta functions. It is concluded that the changes in osteoblast and adipocyte differentiation in DeltaFosB transgenic mice result from independent cell-autonomous mechanisms.

CCAAT/enhancer binding protein homologous protein (DDIT3) induces osteoblastic cell differentiation

CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP/DDIT3), a member of the C/EBP family of transcription factors, plays a role in cell survival and differentiation. CHOP/DDIT3 binds to C/EBPs to form heterodimers that do not bind to consensus Cebp sequences, acting as a dominant-negative inhibitor. CHOP/DDIT3 blocks adipogenesis, and we postulated it could induce osteoblastogenesis. We investigated the effects of constitutive CHOP/DDIT3 overexpression in murine ST-2 stromal cells transduced with retroviral vectors. ST-2 cells differentiated toward osteoblasts, and CHOP/DDIT3 accelerated and enhanced the appearance of mineralized nodules, and the expression of osteocalcin and alkaline phosphatase mRNAs, particularly in the presence of bone morphogenetic protein-2. CHOP/DDIT3 overexpression opposed adipogenesis, and did not cause substantial changes in cell number. CHOP/DDIT3 overexpression did not modify C/EBPalpha or -beta mRNA levels but decreased C/EBPdelta after 24 d of culture. Electrophoretic mobility shift and supershift assays demonstrated that overexpression of CHOP/DDIT3 decreased the binding of C/EBPs to their consensus sequence by interacting with C/EBPalpha and -beta, confirming its dominant-negative role. In addition, CHOP/DDIT3 enhanced bone morphogenetic protein-2/Smad signaling. In conclusion, CHOP/DDIT3 enhances osteoblastic differentiation of stromal cells, in part by interacting with C/EBPalpha and -beta and also by enhancing Smad signaling.

Deactivation of murine alveolar macrophages by peroxisome proliferator-activated receptor-γ ligands

Peroxisome proliferator-activated receptor-γ (PPAR-γ), a member of the nuclear hormone receptor family of ligand-dependent transcription factors, is a critical regulator of adipocyte differentiation and glucose metabolism. The expression, regulation, and functional significance of PPAR-γ in alveolar macrophages (AMs), the predominant resident immune effector cell within the alveolus, have not been previously examined. In this study, we show that, in contrast to peritoneal macrophages, resident murine AMs constitutively express high levels of PPAR-γ. Expression was primarily located in the nucleus by immunofluorescence staining. Quantitative real-time RT-PCR demonstrated that the predominant isoform was PPAR-γ2. Expression of PPAR-γ was induced by the anti-inflammatory cytokine IL-4. Treatment of murine AMs with PPAR-γ ligands suppresses PMA-stimulated oxidative burst activity and LPS + IFN-γ-mediated expression of inducible nitric oxide synthase. In addition, LPS-induced IL-12 mRNA and protein expression was inhibited by PPAR-γ ligands. These results support an important immunomodulatory role for PPAR-γ in AM responses.

Peroxisome-proliferator-activated receptor gamma suppresses Wnt/beta-catenin signalling during adipogenesis

The Wnt/beta-catenin signalling pathway appears to operate to maintain the undifferentiated state of preadipocytes by inhibiting adipogenic gene expression. To define the mechanisms regulating suppression of Wnt/beta-catenin signalling, we analysed the beta-catenin expression in response to activation of transcription factors that regulate adipogenesis. The results show an extensive down-regulation of nuclear beta-catenin that occurs during the first few days of differentiation of 3T3-L1 preadipocytes and coincides with the induction of the adipogenic transcription factors, C/EBPbeta (CCAAT-enhancer-binding protein) and PPARgamma (peroxisome-proliferator-activated receptor). To assess the role of each of these factors in this process, we conditionally overexpressed C/EBPbeta in Swiss mouse fibroblasts using the TET-off system. Abundant expression of C/EBPbeta alone had minimal effect on beta-catenin expression, whereas expression of C/EBPbeta, in the presence of dexamethasone, induced PPARgamma expression and caused a measurable decrease in beta-catenin. In addition, exposure of cells expressing both C/EBPbeta and PPARgamma to a potent PPARgamma ligand resulted in an even greater decrease in beta-catenin by mechanisms that involve the proteasome. Our studies also suggest a reciprocal relationship between PPARgamma activity and beta-catenin expression, since ectopic production of Wnt-1 in preadipocytes blocked the induction of PPARgamma gene expression. Moreover, by suppressing beta-catenin expression, ectopic expression of PPARgamma in Wnt-1-expressing preadipocytes rescued the block in adipogenesis after their exposure to the PPARgamma ligand, troglitazone.

Agouti expression in human adipose tissue: functional consequences and increased expression in type 2 diabetes

It is well recognized that the agouti/melanocortin system is an important regulator of body weight homeostasis. Given that agouti is expressed in human adipose tissue and that the ectopic expression of agouti in adipose tissue results in moderately obese mice, the link between agouti expression in human adipose tissue and obesity/type 2 diabetes was investigated. Although there was no apparent relationship between agouti mRNA levels and BMI, agouti mRNA levels were significantly elevated in subjects with type 2 diabetes. The regulation of agouti in cultured human adipocytes revealed that insulin did not regulate agouti mRNA, whereas dexamethasone treatment potently increased the levels of agouti mRNA. Experiments with cultured human preadipocytes and with cells obtained from transgenic mice that overexpress agouti demonstrated that melanocortin receptor (MCR) signaling in adipose tissue can regulate both preadipocyte proliferation and differentiation. Taken together, these results reveal that agouti can regulate adipogenesis at several levels and suggest that there are functional consequences of elevated agouti levels in human adipose tissue. The influence of MCR signaling on adipogenesis combined with the well-established role of MCR signaling in the hypothalamus suggest that adipogenesis is coordinately regulated with food intake and energy expenditure.

Notch 1 impairs osteoblastic cell differentiation

Notch receptors are single pass transmembrane receptors activated by membrane-bound ligands with a role in cell proliferation and differentiation. As Notch 1 and 2 mRNAs are expressed by osteoblasts and induced by cortisol, we postulated that Notch could regulate osteoblastogenesis. We investigated the effects of retroviral vectors directing the constitutive expression of the Notch 1 intracellular domain (NotchIC) in murine ST-2 stromal and in MC3T3 cells. NotchIC overexpression was documented by increased Notch 1 transcripts and activity of the Notch-dependent Hairy Enhancer of Split promoter. In the presence of bone morphogenetic protein-2 (BMP-2), ST-2 cells differentiated toward osteoblasts forming mineralized nodules, and Notch 1 opposed this effect and decreased the expression of osteocalcin, type I collagen, and alkaline phosphatase transcripts and Delta2Delta FosB protein. Further, NotchIC decreased Wnt/beta-catenin signaling. As cells differentiated in the presence of BMP-2, they underwent apoptosis, and Notch opposed this event. In the presence of cortisol, NotchIC induced the formation of mature adipocytes and enhanced the effect of cortisol on adipsin, peroxisome proliferator-activated receptor-gamma2 and CCAAT enhancer binding protein alpha and delta mRNA levels. NotchIC also opposed MC3T3 cell differentiation and the expression of a mature osteoblastic phenotype. In conclusion, NotchIC impairs osteoblast differentiation and enhances adipogenesis in stromal cell cultures.

Adipogenic differentiating agents regulate expression of fatty acid binding protein and CD36 in the J744 macrophage cell line

Adipocyte fatty acid binding protein (aP2) is a key mediator of intracellular transport and metabolism of fatty acids. Its expression during adipocyte differentiation is regulated through the actions of peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer binding protein alpha (C/EBPalpha). Macrophages also express aP2, and the lack of macrophage aP2 significantly reduces atherosclerotic lesion size in hypercholesterolemic mice. We investigated the regulation of expression of macrophage aP2 and CD36, a fatty acid membrane binding protein and scavenger receptor, in response to the adipogenic agents isobutylmethylxanthine (IBMX), insulin, and dexamethasone, a combination of agents shown to induce fibroblast-to-adipocyte differentiation. Treatment of J774 macrophages with adipogenic agents significantly induced aP2 mRNA expression, while CD36 expression was inhibited. Dexamethasone was essential and sufficient to induce aP2 expression, and insulin had a synergistic effect. However, IBMX antagonized induced-aP2 expression. aP2 protein expression and [14C]oleic acid uptake by macrophages were also increased by dexamethasone. Unlike what occurs in adipocytes, adipogenic agents had mixed effects on the expression of PPARgamma and C/EBPalpha in macrophages. Our data demonstrate differences in the regulation of aP2 in adipocytes and macrophages and show that macrophage aP2 expression by adipogenic agents is independent of the PPARgamma and/or C/EBPalpha signaling pathway.

The orphan nuclear receptor Rev-Erbalpha is a peroxisome proliferator-activated receptor (PPAR) gamma target gene and promotes PPARgamma-induced adipocyte differentiation

Rev-Erbalpha (NR1D1) is an orphan nuclear receptor encoded on the opposite strand of the thyroid receptor alpha gene. Rev-Erbalpha mRNA is induced during adipocyte differentiation of 3T3-L1 cells, and its expression is abundant in rat adipose tissue. Peroxisome proliferator-activated receptor gamma (PPARgamma) (NR1C3) is a nuclear receptor controlling adipocyte differentiation and insulin sensitivity. Here we show that Rev-Erbalpha expression is induced by PPARgamma activation with rosiglitazone in rat epididymal and perirenal adipose tissues in vivo as well as in 3T3-L1 adipocytes in vitro. Furthermore, activated PPARgamma induces Rev-Erbalpha promoter activity by binding to the direct repeat (DR)-2 response element Rev-DR2. Mutations of the 5' or 3' half-sites of the response element totally abrogated PPARgamma binding and transcriptional activation, identifying this site as a novel type of functional PPARgamma response element. Finally, ectopic expression of Rev-Erbalpha in 3T3-L1 preadipocytes potentiated adipocyte differentiation induced by the PPARgamma ligand rosiglitazone. These results identify Rev-Erbalpha as a target gene of PPARgamma in adipose tissue and demonstrate a role for this nuclear receptor as a promoter of adipocyte differentiation.

Expression of peroxisome proliferator-activated receptor and CCAAT/enhancer binding protein transcription factors in cultured human sebocytes

Lipid synthesis and accumulation represent a major step in sebocyte differentiation and it may be of importance for sebocytes to express two families of transcription factors, CCAAT/enhancer binding proteins (c/EBPs) and peroxisome proliferator-activated receptors (PPARs), which were found to play a crucial role in the differentiation of adipocytes. Using the immortalized human sebaceous gland cell line SZ95 we examined the expression of the molecules before and after treatment with testosterone, 5alpha-dihydrotestosterone, dexamethasone, 17beta-estradiol and genistein, at 6, 12, 24, and 48 h, respectively. Reverse transcription-PCR analysis showed expression of peroxisome proliferator-activated receptors -alpha, -delta, -gamma1, -gamma2 and CCAAT/enhancer binding proteins-alpha, -beta, -gamma-delta in native SZ95 sebocytes. In western blot studies, high levels of CCAAT/enhancer binding proteins-alpha and -beta, and peroxisome proliferator-activated receptors-gamma were expressed at 6, 24, and 12 h, respectively. Immunostaining of the cultured sebocytes showed the CCAAT/enhancer binding proteins-alpha and -beta mainly localized within nuclei, whereas peroxisome proliferator-activated receptors-gamma in the cytoplasm. Strong staining of sebocytes was immunohistochemically revealed in the basal layer of sebaceous glands in human scalp and sebaceous nevus. Genistein down-regulated the expression of CCAAT/enhancer binding proteins-alpha and -beta, and peroxisome proliferator-activated receptors-gamma on the protein level. Treatment with linoleic acid for 48 h induced further differentiation of sebocytes leading to abundant lipid synthesis.

Trans10, cis12-conjugated linoleic acid prevents triacylglycerol accumulation in adipocytes by acting as a PPARgamma modulator

A group of polyunsaturated fatty acids called conjugated linoleic acids (CLAs) are found in ruminant products, where the most common isomers are cis9, trans11 (c 9,t11) and trans10, cis12 (t10,c12) CLA. A crude mixture of these isomers has been shown in animal studies to alter body composition by a reduction in body fat mass as well as an increase in lean body mass, with the t10,c12 isomer having the most pronounced effect. The objective of this study was to establish the molecular mechanisms by which t10,c12 CLA affects lipid accumulation in adipocytes. We have shown that t10,c12 CLA prevents lipid accumulation in human and mouse adipocytes at concentrations as low as 5 microM and 25 microM, respectively. t10,c12 CLA fails to activate peroxisome proliferator-activated receptor gamma (PPARgamma) but selectively inhibits thiazolidinedione-induced PPARgamma activation in 3T3-L1 adipocytes. Treatment of mature adipocytes with t10,c12 CLA alone or in combination with Darglitazone down-regulates the mRNA expression of PPARgamma as well as its target genes, fatty acid binding protein (aP2) and liver X receptor alpha (LXRalpha). Taken together, our results suggest that the trans10, cis12 CLA isomer prevents lipid accumulation in adipocytes by acting as a PPARgamma modulator.

Mechanisms of glucocorticoid-induced osteoporosis

Glucocorticoids modify osteoblastic cell differentiation, number, and function. Glucocorticoids stimulate osteoclastogenesis and increase the expression of receptor activator of Nuclear factor-kappaB ligand and colony-stimulating factor-1, and decrease the expression of osteoprotegerin. However, the most significant effect of glucocorticoids in bone is an inhibition of bone formation. This inhibition is caused by a decrease in the number of osteoblasts secondary to a shift in the differentiation of mesenchymal cells away from the osteoblastic lineage, and an increase in the death of mature osteoblasts. Glucocorticoids decrease the function of the remaining osteoblasts directly and indirectly through the inhibition of insulin-like growth factor I expression. The stimulation of bone resorption is likely responsible for the initial bone loss after glucocorticoid exposure. Eventually, the inhibition of bone formation will cause a decrease in bone remodeling and a continued increased risk of fractures.

Activation of peroxisome proliferator-activated receptor-gamma inhibits the Runx2-mediated transcription of osteocalcin in osteoblasts

Mesenchymal cells are able to differentiate into several distinct cell types, including osteoblasts and adipocytes. The commitment to a particular lineage may be regulated by specific transcription factors. Peroxisome proliferator-activated receptor-gamma (PPARgamma), acting in conjunction with CCAAT/enhancer-binding protein-alpha, has been suggested as a key regulator of adipogenic differentiation. Previous studies have shown that the activation of PPARgamma in osteoblasts suppresses osteoblast differentiation and the expression of osteocalcin, an osteoblast-specific protein. However, the mechanism of this inhibition remains unclear. We investigated the effect of PPARgamma activation on the expression of osteocalcin and analyzed the molecular mechanism. Mouse osteoblastic MC3T3-E1 cells expressed PPARgamma, which was transcriptionally active, whereas rat osteosarcoma ROS 17/2.8 cells did not. Treatment of MC3T3-E1 osteoblasts and ROS 17/2.8 cells stably transfected with PPARgamma2 with the PPARgamma activator 15-deoxy-Delta12,14-prostaglandin J2 inhibited the mRNA expression of osteocalcin and Runx2, the latter of which is a key transcription factor in osteoblast differentiation. This decreased expression of osteocalcin and Runx2 was partly explained by the decreased level of Runx2 resulting from the suppressed transcription from the Runx2 promoter. However, in addition to this indirect effect, the activation of PPARgamma by 15-deoxy-Delta12,14-prostaglandin J2 directly suppressed the Runx2-mediated induction of the activities of the osteocalcin promoter and the artificial promoter p6OSE2, which contains six tandem copies of osteoblast-specific element-2, the Runx2-binding promoter sequence. This inhibition was mediated by a physical interaction between PPARgamma and Runx2 and the subsequent repression of the transcriptional activity at the osteoblast-specific element-2 sequence. Thus, this study demonstrates that the activation of PPARgamma inhibits osteocalcin expression both by suppressing the expression of Runx2 and by interfering with the transactivation ability of Runx2.

Peroxisome proliferator-activated receptor gamma (PPARgamma) expression is decreased in pulmonary hypertension and affects endothelial cell growth

PPARgamma is a member of a family of nuclear receptors/ligand-dependent transcription factors, which bind to hormone response elements on target gene promoters. An antiproliferative and proapoptotic action profile of PPARgamma has been described and PPARgamma may function as a tumor suppressor gene, but little is known about the role of PPARgamma in vascular remodeling. One group of human diseases that shows impressive vascular remodeling exclusively in the lungs is the group of severe pulmonary hypertensive disorders, which is characterized by complex, endothelial cell-proliferative lesions of lung precapillary arterioles composed of clusters of phenotypically altered endothelial cells that occlude the vessel lumen and contribute to the elevation of the pulmonary arterial pressure and reduce local lung tissue blood flow. In the present study, we report the ubiquitous PPARgamma expression in normal lungs, and in contrast, a reduced lung tissue PPARgamma gene and protein expression in the lungs from patients with severe PH and loss of PPARgamma expression in their complex vascular lesions. We show that fluid shear stress reduces PPARgamma expression in ECV304 endothelial cells, that ECV304 cells that stably express dominant-negative PPARgamma (DN-PPARgamma ECV304) form sprouts when placed in matrigel and that DN-PPARgamma ECV304 cells, after tail vein injection in nude mice, form lumen-obliterating lung vascular lesions. We conclude that fluid shear stress decreases the expression of PPARgamma in endothelial cells and that loss of PPARgamma expression characterizes an abnormal, proliferating, apoptosis-resistant endothelial cell phenotype.

AhR- and ERK-dependent pathways function synergistically to mediate 2,3,7,8-tetrachlorodibenzo-p-dioxin suppression of peroxisome proliferator-activated receptor-gamma1 expression and subsequent adipocyte differentiation

Activation of the aryl-hydrocarbon receptor (AhR) by pretreatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) blocks hormone (IDM/BRL)-induced adipocyte differentiation of C3H10T1/2 cells in proportion to the suppression of the elevation of the key mediator, peroxisome proliferator-activated receptor (PPARgamma1). Inhibition of MEK-induced ERK phosphorylation had no effect on adipogenesis but prevented this TCDD suppression. Initiation of MEK inhibition up to 6 h after IDM/BRL stimulation in combination with serum addition completely reversed the TCDD-mediated suppression but declined to ineffectiveness when delayed to 24 h after stimulation. This period occurs well after the decline of serum-induced ERK activation, at a time when ERK phosphorylation is low, and prior to the onset of IDM/BRL-stimulated PPARgamma1 expression. This temporal separation of ERK activation from the affected PPARgamma1 expression suggests that ERK does not act directly on either PPARgamma1 transcription or receptor function. Thus, ERK activation and TCDD/AhR stimulation work synergistically to inhibit adipocyte differentiation. Nonrenewal of serum at the time of IDM/BRL addition removed most of the ERK activation and also the TCDD-mediated suppressions of PPARgamma1 expression and adipocyte differentiation. Transfection of a vector expressing constitutively active MEK1 generated a constant, high level of phosphorylated ERK comparable to the peak serum-induced level and fully restored TCDD suppression without a TCDD-mediated effect on ERK phosphorylation. We conclude that low levels of activated MEK and ERK cooperate with AhR-induced factor(s) to generate a suppressor that prevents PPARgamma1 transcription and then differentiation.

Transcription cooperation by NFAT.C/EBP composite enhancer complex

The nuclear factor of activated T cells (NFAT) group of transcription factors regulates gene expression in immune and non-immune cells. NFAT-mediated gene transcription is orchestrated, in part, by formation of a composite regulatory element. Here we demonstrate that NFAT interacts with transcription factor CCAAT/enhancer-binding protein (C/EBP) to form a composite enhancer complex, to potentiate expression of the peroxisome proliferator-activated receptor-gamma2 gene. Formation of a ternary NFAT.C/EBP.DNA complex is required for the transcriptional cooperation. A similar NFAT.C/EBP composite element is found in the regulatory region of the insulin-like growth factor 2, angiotensin-converting enzyme homolog, and transcription factor POU4F3 genes. Thus, the NFAT.C/EBP composite element represents a novel regulatory enhancer to direct NFAT-mediated gene transcription.

Bone morphogenetic proteins, their antagonists, and the skeleton

Skeletal homeostasis is determined by systemic hormones and local factors. Bone morphogenetic proteins (BMP) are unique because they induce the differentiation of mesenchymal cells toward cells of the osteoblastic lineage and also enhance the differentiated function of the osteoblast. However, the activity of BMPs needs to be tempered by intracellular and extracellular antagonists. BMPs bind to specific receptors and signal by phosphorylating the cytoplasmic proteins mothers against decapentaplegic (Smad) 1 and 5, which form heterodimers with Smad 4, and after nuclear translocation regulate transcription. BMP antagonists can be categorized as pseudoreceptors that compete with signaling receptors, inhibitory Smads that block signaling, intracellular binding proteins that bind Smad 1 and 5, and factors that induce ubiquitination and proteolysis of signaling Smads. In addition, a large number of extracellular proteins that bind BMPs and prevent their binding to signaling receptors have emerged. They are the components of the Spemann organizer, noggin, chordin, and follistatin, members of the Dan/Cerberus family, and twisted gastrulation. The antagonists tend to be specific for BMPs and are regulated by BMPs, indicating the existence and need of local feedback mechanisms to temper BMP cellular activities.

Transforming growth factor-beta inhibits adipocyte differentiation by Smad3 interacting with CCAAT/enhancer-binding protein (C/EBP) and repressing C/EBP transactivation function

Transforming growth factor (TGF)-beta is a potent inhibitor of adipocyte differentiation. To identify which adipocyte transcription factors might be targeted by TGF-beta, we overexpressed key adipogenic transcription factors, C/EBPbeta, C/EBPdelta, or peroxisome proliferator-activated receptor (PPAR) gamma in NIH3T3 cells and tested the ability of TGF-beta to block adipogenesis. We show that TGF-beta inhibits adipocyte differentiation driven by either C/EBPbeta or C/EBPdelta without affecting C/EBP protein expression levels, suggesting that these C/EBPs are a direct target of TGF-beta action. Because TGF-beta inhibits adipogenesis by signaling through Smad3, we examined physical and functional interactions of Smad3 and Smad4 with C/EBPbeta, C/EBPdelta, and PPARgamma2. C/EBPbeta and C/EBPdelta were found to physically interact with Smad3 and Smad4, and Smad3 cooperated with Smad4 and TGF-beta signaling to repress the transcriptional activity of C/EBPs. Thus, repression of the activity of C/EBPs by Smad3/4 at C/EBP binding sites inhibited transcription from the PPARgamma2 and leptin promoters. In contrast, PPARgamma interacted only very weakly with Smad3 and its transcriptional activity was not repressed by Smad3/4 or in response to TGF-beta. Smad3/4 did not reduce the ability of C/EBP to bind to its cognate DNA sequence, but repressed transcription by inhibiting the transactivation function of C/EBP.

STAT5A promotes adipogenesis in nonprecursor cells and associates with the glucocorticoid receptor during adipocyte differentiation

The differentiation of adipocytes is regulated by the activity of a variety of transcription factors, including peroxidase proliferator-activated receptor (PPAR)-gamma and C/EBPalpha. Our current study demonstrates that ectopic expression of STAT5A, such as that of PPAR-gamma and C/EBPalpha, promotes adipogenesis in two nonprecursor fibroblast cell lines. Using morphologic and biochemical criteria, we have demonstrated that STAT5A and the combination of STAT5A and STAT5B are sufficient to induce the expression of early and late adipogenic markers in BALB/c and NIH-3T3 cells. Yet, the ectopic expression of STAT5B alone does not induce the expression of adipocyte genes, but enhances the induction of these genes in cells also expressing STAT5A. This finding suggests that STAT5A and STAT5B do not function identically in adipocytes. In addition, these studies demonstrate that the phosphorylation of STAT5 proteins may play a role in adipogenesis. Moreover, we have shown that STAT5A is associated with the glucocorticoid receptor during adipogenesis in a highly regulated manner.

Arachidonic acid and prostacyclin signaling promote adipose tissue development: a human health concern?

High fat intake is associated with fat mass gain through fatty acid activation of peroxisome proliferator-activated receptors delta and gamma, which promote adipogenesis. We show herein that, compared to a combination of specific agonists to both receptors or to saturated, monounsaturated, and omega-3 polyunsaturated fatty acids, arachidonic acid (C20:4, omega-6) promoted substantially the differentiation of clonal preadipocytes. This effect was blocked by cyclooxygenase inhibitors and mimicked by carbacyclin, suggesting a role for the prostacyclin receptor and activation of the cyclic AMP-dependent pathways that regulate the expression of the CCAAT enhancer binding proteins beta and delta implicated in adipogenesis. During the pregnancy-lactation period, mother mice were fed either a high-fat diet rich in linoleic acid, a precursor of arachidonic acid (LO diet), or the same isocaloric diet enriched in linoleic acid and alpha-linolenic acid (LO/LL diet). Body weight from weaning onwards, fat mass, epididymal fat pad weight, and adipocyte size at 8 weeks of age were higher with LO diet than with LO/LL diet. In contrast, prostacyclin receptor-deficient mice fed either diet were similar in this respect, indicating that the prostacyclin signaling contributes to adipose tissue development. These results raise the issue of the high content of linoleic acid of i) ingested lipids during pregnancy and lactation, and ii) formula milk and infant foods in relation to the epidemic of childhood obesity.

Noggin arrests stromal cell differentiation in vitro

Noggin is a glycoprotein that binds bone morphogenetic proteins (BMPs) selectively and, when added to osteoblasts, it opposes the effects of BMPs. However, the consequences of its continued expression in stromal cells are not known. We investigated the effects of noggin overexpression under the control of a constitutive promoter, on murine ST-2 stromal cells, and its impact on stromal cells from transgenic mice overexpressing noggin under the control of the osteocalcin promoter. ST-2 cells were transduced with a retroviral vector (pLPCX) or a vector driving noggin (pLPCX noggin). Untreated (pLPCX) ST-2 cells developed the appearance of mineralized nodules and expressed osteocalcin. pLPCX noggin delayed the appearance of mineralized nodules and prevented the expression of osteocalcin. Noggin also prevented the cortisol-dependent induction of peroxisome proliferator-activated receptor gamma2 and adipsin transcripts, indicating a generalized inhibition of cell differentiation. Primary stromal cells from noggin transgenic mice displayed impaired differentiation when compared to cells from wild-type animals and did not express osteocalcin mRNA. In conclusion, noggin arrests the differentiation of stromal cells, preventing cellular maturation.

Calcineurin mediates the calcium-dependent inhibition of adipocyte differentiation in 3T3-L1 cells

Recent studies have revealed that the calcium-dependent serine/threonine phosphatase calcineurin mediates the effects of intracellular calcium in many different cell types. In this study we investigated the role of calcineurin in the regulation of adipocyte differentiation. We found that the specific calcineurin inhibitors cyclosporin A and FK506 overcame the antiadipogenic effect of calcium ionophore on the differentiation of 3T3-L1 preadipocytes. This finding suggests that calcineurin is responsible for mediating the previously documented Ca(2+)-dependent inhibition of adipogenesis. We further demonstrate that the expression of a constitutively active calcineurin mutant potently inhibits the ability of 3T3-L1 cells to undergo adipocyte differentiation by preventing expression of the proadipogenic transcription factors peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding protein alpha (C/EBPalpha). This calcineurin-mediated block in adipocyte differentiation is rescued by ectopic expression of PPARgamma1. Finally, we demonstrate that inhibition of endogenous calcineurin activity with either FK506 or a specific calcineurin inhibitory peptide enhances differentiation of 3T3-L1 cells in response to suboptimal adipogenic stimuli, suggesting that endogenous calcineurin activity normally sets a signaling threshold that antagonizes efficient adipocyte differentiation. Collectively, these data indicate that calcineurin acts as a Ca(2+)-dependent molecular switch that negatively regulates commitment to adipocyte differentiation by preventing the expression of critical proadipogenic transcription factors.

Early B-cell factor (O/E-1) is a promoter of adipogenesis and involved in control of genes important for terminal adipocyte differentiation

Olf-1/early B-cell factor (O/E-1) is a transcription factor important for B-lymphocyte and neuronal gene regulation. Here we report that all three known O/E genes (O/E-1, -2, and -3) are expressed in mouse adipose tissue and are upregulated during adipocyte differentiation. Forced expression of O/E-1 in either the preadipocyte cell line 3T3-L1 or mouse embryonic fibroblasts augmented adipogenesis, and constitutive expression of O/E-1 in uncommitted NIH 3T3 fibroblasts led to initiation of adipocyte differentiation. Furthermore, a dominant negative form of O/E-1 partially suppressed 3T3-L1 adipogenesis, indicating that expression from endogenous O/E target genes is required for 3T3-L1 terminal differentiation. Thus, our data point to the importance of O/E target genes for adipocyte differentiation and suggest a novel role for O/E-1 as an initiator and stimulator of adipogenesis.

Bone morphogenetic protein and retinoic acid signaling cooperate to induce osteoblast differentiation of preadipocytes

Mesenchymal cells can differentiate into osteoblasts, adipocytes, myoblasts, or chondroblasts. Whether mesenchymal cells that have initiated differentiation along one lineage can transdifferentiate into another is largely unknown. Using 3T3-F442A preadipocytes, we explored whether extracellular signals could redirect their differentiation from adipocyte into osteoblast. 3T3-F442A cells expressed receptors and Smads required for bone morphogenetic protein (BMP) signaling. BMP-2 increased proliferation and induced the early osteoblast differentiation marker alkaline phosphatase, yet only mildly affected adipogenic differentiation. Retinoic acid inhibited adipose conversion and cooperated with BMP-2 to enhance proliferation, inhibit adipogenesis, and promote early osteoblastic differentiation. Expression of BMP-RII together with BMP-RIA or BMP-RIB suppressed adipogenesis of 3T3-F442A cells and promoted full osteoblastic differentiation in response to retinoic acid. Osteoblastic differentiation was characterized by induction of cbfa1, osteocalcin, and collagen I expression, and extracellular matrix calcification. These results indicate that 3T3-F442A preadipocytes can be converted into fully differentiated osteoblasts in response to extracellular signaling cues. Furthermore, BMP and retinoic acid signaling cooperate to stimulate cell proliferation, repress adipogenesis, and promote osteoblast differentiation. Finally, BMP-RIA and BMP-RIB induced osteoblast differentiation and repressed adipocytic differentiation to a similar extent.

Longitudinal evolution of HIV-1-associated lipodystrophy is correlated to serum cortisol:DHEA ratio and IFN-alpha

BACKGROUND

We have previously shown that lipid alterations in HIV-1-associated lipodystrophy (LD) are correlated with decreased serum dehydroepiandosterone (DHEA) and increased cortisol:DHEA ratio and IFN-alpha levels.

OBJECTIVE

To evaluate in a longitudinal study whether steroid and cytokine modifications are associated with the evolution of physical changes and lipid alterations associated with LD.

METHODS

Thirty-four HIV-1-positive men were followed during 32.5 +/- 4.0 months and tested at four time-points. The patients were subdivided into five groups according to physical changes and anthropometric measurements: LD-negative, initially LD-negative becoming LD-positive, LD-positive unchanged, aggravated or improved. Serum lipids, apolipoproteins, adrenal steroids and cytokines were measured and compared with baseline values.

RESULTS

(1) LD aggravation is associated with persistent elevated lipids, a decrease in serum DHEA, an increase in cortisol:DHEA ratio and persistent high levels of IFN-alpha. (2) LD improvement is associated with normalization of serum lipids, an increase in serum DHEA leading to normalization in cortisol:DHEA ratio, and normalization of IFN-alpha levels. (3) In LD-positive men evolution of VLDL cholesterol is negatively correlated with DHEA (r = -0.56, P < 0.01) and positively with cortisol:DHEA ratio (r = 0.62, P < 0.004) and with IFN-alpha (r = 0.57, P < 0.01). (4) The switch to LD is associated with a decrease in serum DHEA. (5) Patients who remained LD-negative maintained normal lipids, elevated cortisol and DHEA, and normal cortisol:DHEA ratio and normal levels of IFN-alpha.

CONCLUSIONS

This study indicates that cortisol:DHEA ratio and serum IFN-alpha levels are closely associated with clinical evolution and atherogenic lipid alterations in LD.

Activation of MEK/ERK signaling promotes adipogenesis by enhancing peroxisome proliferator-activated receptor gamma (PPARgamma ) and C/EBPalpha gene expression during the differentiation of 3T3-L1 preadipocytes

We demonstrate that exposure of post-confluent 3T3-L1 preadipocytes to insulin, isobutylmethylxanthine (MIX), dexamethasone (DEX), and fetal bovine serum induces a rapid but transient activation of MEK1 as indicated by extensive phosphorylation of ERK1 and ERK2 during the initial 2 h of adipogenesis. Inhibition of this activity by treating the cells with a MEK1-specific inhibitor (U0126 or PD98059) prior to the induction of differentiation significantly attenuated the expression of peroxisome proliferator-activated receptor (PPAR) gamma, CCAAT/enhancer-binding protein (C/EBP) alpha, perilipin, and adipocyte-specific fatty acid-binding protein (aP2). Treating the preadipocytes with troglitazone, a potent PPARgamma ligand, could circumvent the inhibition of adipogenic gene expression by U0126. Fibroblast growth factor-2 (FGF-2), in the presence of dexamethasone, isobutylmethylxanthine, and insulin, induces a prolonged activation of the MEK/ERK signaling pathway, which lasts for at least 12 h post-induction, and this activity is less sensitive to the MEK inhibitors. Consequently, preadipocytes treated with U0126 in the presence of fibroblast growth factor-2 (FGF-2) express normal post-induction levels of MEK activity, and, in so doing, are capable of undergoing adipogenesis. We further show that activation of MEK1 significantly enhances the transactivation of the C/EBPalpha minimal promoter during the early phase of the differentiation process. Our results suggest that activation of the MEK/ERK signaling pathway during the initial 12 h of adipogenesis enhances the activity of factors that regulate both C/EBPalpha and PPARgamma expression.

Reduced IRS-2 and GLUT4 expression in PPARgamma2-induced adipocytes derived from C/EBPbeta and C/EBPdelta-deficient mouse embryonic fibroblasts

In adipose tissue, the ability of cells to respond to insulin and to express genes such as those encoding fatty-acid-binding protein (422/aP2), lipoprotein lipase (LPL), adipsin and glucose transporter 4 (GLUT4) is acquired during their differentiation into mature adipocytes. It has been recognized that peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding proteins (C/EBPs) play critical roles in adipocyte differentiation. However, it remained uncertain whether PPARgamma or which C/EBP is involved in the acquisition of these characteristics. We introduced PPARgamma2 into C/EBPbeta/delta-double deficient mouse embryonic fibroblasts (MEFs), followed by stimulation with its ligands, in order to define the roles of C/EBPbeta and C/EBPdelta in phenotypic acquisition during adipocyte differentiation. This procedure resulted in differentiation of these MEFs into mature adipocytes morphologically similar to wild-type MEFs. However, the adipocytes derived from the C/EBPbeta/delta-deficient MEFs showed lower expression of GLUT4 and adipsin mRNA than those derived from wild-type MEFs, although aP2 and LPL mRNA levels were similar in both types. The C/EBPbeta/delta-deficient adipocytes also expressed lower amounts of insulin receptor substrate 2 (IRS-2) than the adipocytes derived from wild-type MEFs, whereas the amounts of insulin receptor and IRS-1 were similar. Finally, insulin-responsive 2-deoxyglucose uptake was lower in the C/EBPbeta/delta-deficient cells. It could thus be demonstrated that C/EBPbeta and C/EBPdelta are involved in the acquisition of IRS-2 and GLUT4 expression as well as in insulin-sensitive glucose uptake during adipocyte differentiation.

Differential expression of NAT1 translational repressor during development of bovine intramuscular adipocytes

This study was undertaken to test for differential gene expression in intramuscular adipocytes during fat deposition of feedlot steers. Angus x Hereford steers (n = 50) were fed a high-energy concentrate ration ad libitum for 20 (n = 5), 86 (n = 15), 121 (n = 15), and 146 days (n = 15) to obtain various degrees of intramuscular adipocyte development. Carcass traits were significantly different (P < 0.05) between the groups. Intramuscular adipose tissue was excised from the longissimus dorsi and snap frozen in liquid nitrogen. Pooled samples of total RNA representing each group were analyzed by differential-display polymerase chain reaction using 200 primer combinations comprising 20 arbitrary (5') and 10 anchor (3') oligonucleotides. Bands (n = 70) representing putative differences among treatment groups were excised, sequenced, and subjected to BLAST homology search. From these, 40 contained significant homology to known genes. One was of particular interest, the translational repressor NAT1 (novel APOBEC-1 target-1). NAT1 mRNA was quantified in individual animals to confirm differential expression among treatment groups. Results indicate that NAT1 message is more abundant (P < 0.05) in intramuscular adipocytes of younger/leaner animals.

CCAAT/enhancer-binding proteins: structure, function and regulation

CCAAT/enhancer binding proteins (C/EBPs) are a family of transcription factors that all contain a highly conserved, basic-leucine zipper domain at the C-terminus that is involved in dimerization and DNA binding. At least six members of the family have been isolated and characterized to date (C/EBP alpha[bond]C/EBP zeta), with further diversity produced by the generation of different sized polypeptides, predominantly by differential use of translation initiation sites, and extensive protein-protein interactions both within the family and with other transcription factors. The function of the C/EBPs has recently been investigated by a number of approaches, including studies on mice that lack specific members, and has identified pivotal roles of the family in the control of cellular proliferation and differentiation, metabolism, inflammation and numerous other responses, particularly in hepatocytes, adipocytes and haematopoietic cells. The expression of the C/EBPs is regulated at multiple levels during several physiological and pathophysiological conditions through the action of a range of factors, including hormones, mitogens, cytokines, nutrients and certain toxins. The mechanisms through which the C/EBP members are regulated during such conditions have also been the focus of several recent studies and have revealed an immense complexity with the potential existence of cell/tissue- and species-specific differences. This review deals with the structure, biological function and the regulation of the C/EBP family.

Cortisol regulates the expression of Notch in osteoblasts

Glucocorticoids have important effects on osteoblastic replication, differentiation, and function, and the Notch family of receptors is considered to play a role in osteoblastic cell differentiation. We postulated that cortisol could regulate Notch and Notch ligand expression in osteoblastic cells, providing an additional mechanism by which glucocorticoids could regulate osteoblastic differentiation. We examined the expression and regulation of Notch1, 2, 3, and 4 and their ligands Jagged 1 and 2 and Delta 1 and 3 by cortisol in cultures of osteoblastic MC3T3-E1 cells. Cortisol caused a time-dependent increase in Notch1 and 2 mRNA levels in MC3T3 cells. Notch3 and 4 were not detected in the presence or absence of cortisol. MC3T3 cells expressed Delta 1 and Jagged 1 but not Jagged 2 or Delta 3 mRNAs, and cortisol did not have a substantial effect on the expression of any of these ligands. Cortisol increased the rate of Notch1 and 2 transcription and, in transcriptionally arrested cells, did not modify the decay of the transcripts, indicating a transcriptional level of control. In conclusion, cortisol stimulates Notch1 and 2 transcription in osteoblasts. Since Notch signaling appears to play a negative role in osteoblastic differentiation, its increased expression could be relevant to the actions of cortisol in bone.

Involvement of matrix metalloproteinases in the adipose conversion of 3T3-L1 preadipocytes

When mouse 3T3-L1 preadipocytes are induced to differentiate into adipocytes, they change from an extended fibroblast-like morphology to a rounded one. This change most likely occurs through extracellular matrix remodelling, a process known to be mediated in part by matrix metalloproteinases (MMPs). In this study, we have shown by semi-quantitative reverse transcriptase-PCR, zymographic and immunoblot analysis that MMP-2, MMP-9 and membrane type 1 (MT1)-MMP are regulated during adipose conversion. To assess the importance of MMPs for adipocytic differentiation we have used MMP-specific inhibitors as well as neutralizing antibodies. Treatment of 3T3-L1 preadipocytes with the broad MMP inhibitor Ilomastat or the more restricted MMP-2 Inhibitor I prevented their differentiation into adipocytes in a dose-dependent manner, as evidenced by absence of triglyceride accumulation. Inhibitor treatment prevented the fibronectin-network degradation, as well as the induction of the genes for peroxisome-proliferator-activated receptor gamma and adipsin, two adipocyte phenotype markers. Inhibitor treatment was effective when applied during the early stages of adipocytic conversion, whereas inhibitor treatment during later stages had little effect. Inhibitor treatment did not inhibit clonal mitotic expansion; nor did it affect the expression pattern of the adipogenic transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) or its nuclear translocation. It did, however, markedly reduce C/EBPbeta DNA-binding capacity. Taken together, these results suggest that MMPs, and notably MMP-2 and MMP-9, may be necessary mediators of adipocytic differentiation of 3T3-L1 cells.

Adipogenesis and aging: does aging make fat go MAD?

In advanced old age, fat depot size declines while lipid is redistributed to muscle, bone marrow, and other tissues. Decreased fat depot size is related to reduced fat cell size and function and impaired differentiation of preadipocytes into fat cells. Reduced differentiation-dependent gene expression results from decreased abundance of the adipogenic transcription factors, CCAAT/enhancer binding alpha (C/EBPalpha) and peroxisome proliferator activated receptor gamma (PPARgamma). Increased expression of anti-adipogenic C/EBP family members contributes, perhaps due to cellular stress response pathway activation with aging. Hence, dysfunctional adipocyte-like cells appear in adipose tissue that are smaller and less insulin responsive than fully differentiated fat cells. Adipogenesis can be restored by overexpressing adipogenic transcription factors in preadipocytes from old animals. Redistribution of lipid to extra-adipose sites with aging could result from loss of lipid storage capacity in fat depots, altered fatty acid handling resulting in lipid accumulation, dysdifferentiation of mesenchymal precursors, such as muscle satellite cells and osteoblast precursors, into a partial adipocyte phenotype, or a combination of these mechanisms. Thus, accumulation of mesenchymal adipocyte-like default (MAD) cells in fat depots, muscle, bone marrow, and elsewhere is a potentially reversible process that could contribute to maldistribution of fat in old age.

Effects of cortisol and bone morphogenetic protein-2 on stromal cell differentiation: correlation with CCAAT-enhancer binding protein expression

Bone marrow stroma contain pluripotential cells with the potential to differentiate into various mesenchymal cell lineages. We compared the effect of cortisol and bone morphogenetic protein-2 (BMP-2) on the differentiation of murine ST-2 stromal cells into mature osteoblasts or adipocytes. ST-2 cells were cultured for 3-27 days in the presence of 10% fetal bovine serum, 100 microg/mL ascorbic acid, and 5 mmol/L beta-glycerolphosphate in the presence or absence of cortisol at 1 micromol/L or BMP-2 at 1 nmol/L. Untreated ST-2 cells expressed high levels of alkaline phosphatase activity (APA) 15 days after confluence, and this was followed by the appearance of mineralized nodules after 24 days. BMP-2 accelerated and intensified the appearance of cells expressing APA and the presence of mineralized nodules. In contrast, cortisol decreased APA, prevented the formation of mineralized nodules, and induced a cellular phenotype characteristic of adipocytes. Untreated stromal cells expressed osteocalcin, Cbfa1, type I collagen, and alkaline phosphatase mRNA. BMP-2 increased osteocalcin and alkaline phosphatase mRNA, whereas cortisol suppressed their expression, as well as Cbfa1 and type I collagen transcripts. Cortisol enhanced, and BMP-2 downregulated, peroxisome proliferator-activated receptor gamma 2 and adipsin transcripts. The C/EBP transcription factors regulate genes critical for adipocytic and osteoblastic differentiation. Cortisol increased the expression of C/EBP alpha, beta, delta, and gamma mRNA levels, whereas BMP-2 had minor effects on C/EBP expression. In conclusion, BMP-2 accelerates the differentiation of stromal cells toward an osteoblastic phenotype, whereas glucocorticoids induce their differentiation toward an adipocytic phenotype.

Octanoate attenuates adipogenesis in 3T3-L1 preadipocytes

Preadipocytes exposed to octanoate accumulate less lipid than cells exposed to long-chain fatty acids. This effect of octanoate involves significant attenuation of expression of key adipogenic transcription factors, including peroxisome proliferator-activated receptor (PPAR)gamma, steroid regulatory binding element protein (SREBP)-1c and CCAAT element binding protein (C/EBPalpha) at both the mRNA and protein levels. Expression of differentiation markers, including adipocyte fatty acid binding protein (ALBP), glycerol-3-phosphate dehydrogenase (GPDH) and leptin, was also significantly diminished by octanoate. However, octanoate did not prevent the decrease in preadipocyte factor-1 (Pref-1) expression that occurs during adipogenesis, nor did it inhibit the early induction of C/EBPbeta,delta. Treatment with synthetic PPARgamma ligands partially offset the inhibitory effect of octanoate on differentiation. Ectopic expression of PPARgamma2 in 3T3-L1 cells partially restored lipid accretion and GPDH activity in octanoate-treated cells. Adding octanoate together with troglitazone attenuated the effects of troglitazone on adipocyte differentiation in both normal 3T3-L1 cells and engineered 3T3-L1 cells that expressed ectopic PPARgamma2, implying that octanoate might compete against troglitazone for its binding to PPARgamma. These results suggest that octanoate may block adipogenesis at least in part by its influence on the expression/activation of PPARgamma.

Requirement of fibroblast growth factor 10 in development of white adipose tissue

Fibroblast growth factors (FGFs) are important intercellular signaling molecules in developmental processes. Here, we show that FGF10 is secreted by cultured preadipocytes and that prevention of FGF10 signaling inhibits the expression of C/EBPbeta and the subsequent differentiation of these cells. An active form of C/EBPbeta rescued differentiation of the cells in which FGF10 signaling was blocked. Development of white adipose tissue and the expression of C/EBPbeta in this tissue of FGF10 knockout mice were markedly reduced, and the ability of embryonic fibroblasts derived from FGF10 knockout mice to differentiate into adipocytes was impaired. Therefore, FGF10 plays an important role in adipogenesis, at least partly by contributing to the expression of C/EBPbeta through an autocrine/paracrine mechanism.

Proprotein convertases are important mediators of the adipocyte differentiation of mouse 3T3-L1 cells

Mouse 3T3-L1 cells are widely used to study adipocyte differentiation in vitro. When treated with insulin, dexamethasone and isobutylmethylxanthine these fibroblastic cells differentiate into round triglyceride-rich adipocytes. Because several proteins implicated in adipocyte differentiation(e.g. type 1 IGF receptors) are proteolytically activated by endoproteinases of the proprotein convertase family, we sought to determine whether these endoproteinases are crucial for adipose conversion. In this study, we show that expression of the proprotein convertases PACE4, PC7 and furin increases when 3T3-L1 cells are induced to differentiate into adipocytes. The differentiation was blocked in transfected cells expressingα1-antitrypsin Portland or in normal cells pre-treated with the synthetic inhibitor decanoyl-RVKR-chloromethylketone. Both inhibitors are known to specifically inactivate proprotein convertases. The block was associated with impaired proteolytic activation of proIGF-1 receptor, absence of induction of the adipogenic transcriptional factor PPARγ and marked reduction of the nuclear translocation of the C/EBPβ factor. Taken together, these data constitute evidence that proprotein convertases are crucial mediators of adipogenesis.

Effects of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane (p,p'-DDT) on 3T3-L1 and 3T3-F442A adipocyte differentiation

Based upon our initial observations that 1,1,1-trichloro-2,2-bis(p-chlorophenyl)-ethane (p,p'-DDT) induces a concentration-dependent increase in 3T3-L1 adipocyte differentiation, the mechanism of the p,p'-DDT-induced adipocyte differentiation was studied, using 3T3-L1 and 3T3-F442A cells. Since, it is known that the differentiation of the 3T3-L1 adipocyte cell line involves the induction of the transcription factors CCAAT enhancer binding protein beta (C/EBPbeta), peroxisome proliferator-activated receptor gamma (PPARgamma), and C/EBPalpha, the possible role of these factors in p,p'-DDT-induced adipocyte differentiation had to be examined. It was found that p,p'-DDT-treated 3T3-L1 cells showed a concentration-dependent increase in the nuclear levels of both PPARgamma and C/EBPalpha protein. On the other hand, treatment with p,p'-DDT (20 microM) did not affect the expression pattern of C/EBPbeta protein during differentiation. Gel shift analysis of nuclear proteins for binding to the C/EBP recognition site of DNA showed an increase in binding activity at day 2 of differentiation in p,p'-DDT-treated cells. Supershift analysis revealed that this rise was caused mainly by a dramatic increase in the abundance of the C/EBPalpha-DNA complex. Similar increases were observed at days 4 and 7 after the induction of differentiation. Tumor necrosis factor alpha induced a strong inhibition of adipocyte differentiation, which was reversed by co-treatment with troglitazone, an activator of PPARgamma. p,p'-DDT was unable to reverse the inhibitory effect of tumor necrosis factor alpha on adipocyte differentiation in 3T3-L1 cells. 3T3-F442A is another preadipocyte cell line that can be induced to differentiate into adipocytes in the presence of insulin and fetal bovine serum. p,p'-DDT (20 microM) induced an alteration in the morphology of these cells at day 2 after the induction of differentiation. These cells however, were unable to become fully differentiated adipocytes. These data showed, therefore, the ability of p,p'-DDT to alter the differentiation process of adipocyte cell lines through the modification of transcription factors regulating this event.