Repressive effect of Sp1 on the C/EBPalpha gene promoter: role in adipocyte differentiation

Targeted disruption of the basic Krüppel-like factor gene (Klf3) reveals a role in adipogenesis

Krüppel-like factors (KLFs) recognize CACCC and GC-rich sequences in gene regulatory elements. Here, we describe the disruption of the murine basic Krüppel-like factor gene (Bklf or Klf3). Klf3 knockout mice have less white adipose tissue, and their fat pads contain smaller and fewer cells. Adipocyte differentiation is altered in murine embryonic fibroblasts from Klf3 knockouts. Klf3 expression was studied in the 3T3-L1 cellular system. Adipocyte differentiation is accompanied by a decline in Klf3 expression, and forced overexpression of Klf3 blocks 3T3-L1 differentiation. Klf3 represses transcription by recruiting C-terminal binding protein (CtBP) corepressors. CtBPs bind NADH and may function as metabolic sensors. A Klf3 mutant that does not bind CtBP cannot block adipogenesis. Other KLFs, Klf2, Klf5, and Klf15, also regulate adipogenesis, and functional CACCC elements occur in key adipogenic genes, including in the C/ebpalpha promoter. We find that C/ebpalpha is derepressed in Klf3 and Ctbp knockout fibroblasts and adipocytes from Klf3 knockout mice. Chromatin immunoprecipitations confirm that Klf3 binds the C/ebpalpha promoter in vivo. These results implicate Klf3 and CtBP in controlling adipogenesis.

Cyclic AMP (cAMP)-mediated stimulation of adipocyte differentiation requires the synergistic action of Epac- and cAMP-dependent protein kinase-dependent processes

Cyclic AMP (cAMP)-dependent processes are pivotal during the early stages of adipocyte differentiation. We show that exchange protein directly activated by cAMP (Epac), which functions as a guanine nucleotide exchange factor for the Ras-like GTPases Rap1 and Rap2, was required for cAMP-dependent stimulation of adipocyte differentiation. Epac, working via Rap, acted synergistically with cAMP-dependent protein kinase (protein kinase A [PKA]) to promote adipogenesis. The major role of PKA was to down-regulate Rho and Rho-kinase activity, rather than to enhance CREB phosphorylation. Suppression of Rho-kinase impaired proadipogenic insulin/insulin-like growth factor 1 signaling, which was restored by activation of Epac. This interplay between PKA and Epac-mediated processes not only provides novel insight into the initiation and tuning of adipocyte differentiation, but also demonstrates a new mechanism of cAMP signaling whereby cAMP uses both PKA and Epac to achieve an appropriate cellular response.

TG-interacting factor is required for the differentiation of preadipocytes

The accumulation of visceral adipose tissue is closely associated with insulin resistance and metabolic syndrome. Therefore, it is important to identify genes that are required for adipocyte differentiation. To identify genes that are required for the differentiation of 3T3-L1 preadipocytes into mature adipocytes, we used retrovirus insertion-mediated random mutagenesis to generate 3T3-L1 cell lines that lose their ability to differentiate into mature adipocytes. One of the genes identified was TG-interacting factor (TGIF), a DNA binding homeodomain protein that has been demonstrated to suppress Smad-mediated activation of transforming growth factor beta (TGF-beta)-regulated transcription. In the TGIF-disrupted clone of 3T3-L1 preadipocytes, the rate of differentiation into mature adipocytes was clearly reduced compared with that in the wild-type clone. Suppression of TGIF by lentivirus-mediated RNAi also inhibited the differentiation of 3T3-L1 cells. Insulin specifically increased the abundance of TGIF protein, primarily by enhancing its stability. In addition, insulin caused the rapid accumulation of TGIF in the nuclei. Forced expression of exogenous TGIF repressed both endogenous and overexpressed Smad2/3-mediated promoter activity in 3T3-L1. These findings suggest that insulin specifically antagonizes TGF-beta signaling in preadipocytes by stabilizing the putative Smad transcriptional corepressor TGIF and regulates adipocyte differentiation.

Sequential regulation of diacylglycerol acyltransferase 2 expression by CAAT/enhancer-binding protein beta (C/EBPbeta) and C/EBPalpha during adipogenesis

Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step of triacylglycerol (TG) synthesis. Despite the existence of an alternative acyltransferase (DGAT1), mice lacking DGAT2 have a severe deficiency of TG in adipose tissue, indicating a nonredundant role for this enzyme in adipocyte TG synthesis. We have studied the regulation of DGAT2 expression during adipogenesis. In both isolated murine preadipocytes and 3T3-L1 cells the temporal pattern of DGAT2 expression closely mimicked that of genes whose expression is regulated by CAAT/enhancer-binding protein beta (C/EBPbeta). Inhibition of C/EBPbeta expression in differentiating preadipocytes reduced DGAT2 expression, and electrophoretic mobility shift assay and chromatin immunoprecipitation experiments identified a promoter element in the DGAT2 gene that is likely to mediate this effect. The importance of C/EBPbeta in adipocyte expression of DGAT2 was confirmed by the finding of reduced DGAT2 expression in the adipose tissue of C/EBPbeta-null animals. However, DGAT2 expression is maintained at high levels during the later stages of adipogenesis, when C/EBPbeta levels decline. We show that, at these later stages of differentiation, C/EBPalpha is capable of substituting for C/EBPbeta at the same promoter element. These observations provide novel insight into the transcriptional regulation of DGAT2 expression. Moreover, they further refine the complex and serial roles of the C/EBP family of transcription factors in inducing and maintaining the metabolic properties of mature adipocytes.

CCAAT/enhancer binding protein-alpha is down-regulated by toll-like receptor agonists in microglial cells

The transcription factor CCAAT/enhancer binding protein-alpha (C/EBPalpha) can regulate the expression of important genes in the inflammatory response, but little is known about its role in glial activation. By using primary cortical murine glial cultures, we show that C/EBPalpha is expressed by microglial cells in vitro. Lipopolysaccharide (LPS) down-regulates C/EBPalpha mRNA at 2 hr and all C/EBPalpha protein isoforms at 4 hr. This effect is elicited by LPS concentrations >/=100 pg/ml. LPS-induced C/EBPalpha down-regulation occurs in microglial cells both in mixed glial and in microglial-enriched cultures. As seen with LPS, other toll-like receptor agonists (polyinosinic-polycytidylic acid, peptidoglycan from Staphylococcus aureus, and the oligonucleotide CpG1668) also down-regulate C/EBPalpha whereas cytokines such as interleukin-1beta, interleukin-6, macrophage-colony stimulating factor, and interferon-gamma do not. These findings suggest that C/EBPalpha down-regulation in activated microglia could play an important role in the increased expression of genes that are potentially pathogenic in a variety of neurological disorders.

Role of cdk2 in the sequential phosphorylation/activation of C/EBPbeta during adipocyte differentiation

Upon induction of differentiation, growth-arrested (G(1) phase) 3T3-L1 preadipocytes express CCAAT/enhancer binding protein-beta (C/EBPbeta), initiating a transcriptional cascade. C/EBPbeta immediately undergoes a priming phosphorylation (on Thr(188)) by MAPK/ERK. However, the acquisition of DNA binding and transactivation capacity of C/EBPbeta is delayed until further phosphorylation (on Ser(184) or Thr(179)) by GSK3beta occurs. Phosphorylation by glycogen synthase kinase-3beta (GSK3beta) induces S phase entry and thereby mitotic clonal expansion (MCE), a requirement for terminal differentiation. Because MAPK activity is down-regulated before S phase is completed, we sought to identify the kinase that maintains C/EBPbeta in the primed phosphorylated state throughout S phase and MCE. We show here that cdk2/cyclinA, whose expression is activated at the onset of S phase, functions in this capacity. Ex vivo and in vitro experiments show that cdk2/cyclinA catalyzes this delayed priming phosphorylation. Mass spectrometric analysis revealed that cdk2/cyclinA phosphorylates C/EBPbeta on Thr(188) and is required for phosphorylation (on Ser(184) or Thr(179)) of C/EBPbeta by GSK3beta and maintenance of DNA binding activity. Suppression of cdk2 activity by RNA interference or pharmacologic inhibitor disrupts subsequent events in the differentiation program. Thus, MAPK and cdk2/cyclinA act sequentially to maintain Thr(188) of C/EBPbeta in the primed phosphorylated state during MCE and thereby progression of terminal differentiation.

-391 C to G substitution in the regulator of G-protein signalling-2 promoter increases susceptibility to the metabolic syndrome in white European men: consistency between molecular and epidemiological studies

BACKGROUND

The regulator of G-protein signalling-2 (RGS2) is a key factor in adipogenesis. We hypothesized that the metabolic syndrome, of which obesity is an important component, might be related to genetic variation in RGS2.

METHODS AND RESULTS

We screened the human RGS2 gene. We tested the functionality of a common genetic variant in vitro, ex vivo, and in epidemiological study involving six European populations. The C to G substitution at position -391 in the RGS2 promoter was associated with enhanced RGS2 expression in vitro in transfected 3T3-L1 adipocytes and Chinese hamster cells and ex vivo in adipocytes from male, but not female, volunteers. In 2732 relatives from 512 families and 348 unrelated individuals, randomly recruited from six European populations, the prevalence of GG homozygosity was 54.1%. The metabolic syndrome score, a composite of six continuous traits making up this clinical entity, was 0.27 standardized units higher (P < 0.001) in 795 GG homozygous men compared with 683 men carrying the C allele. Transmission of the -391 G allele to male offspring was associated with a 0.20 unit increase in the score (P=0.039). These epidemiological relations were not significant in 1602 women.

CONCLUSIONS

The C to G substitution at position -391 in the RGS2 promoter increases RGS2 expression in adipocytes and is associated with the metabolic syndrome in white European men. Further experimental and clinical research should establish whether this common polymorphism might be a target for preventive or therapeutic intervention.

Subcutaneous fat in normal and diseased states

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

Upstream stimulatory factors regulate the C/EBP alpha gene during differentiation of 3T3-L1 preadipocytes

During adipocyte differentiation, CCAAT/enhancer-binding protein alpha (C/EBPalpha) functions as a pleiotropic transcriptional activator of numerous adipocyte genes. The promoter of the C/EBPalpha gene has an E-box upstream of C/EBP binding site. Deletion or mutation of the E-box decreases promoter activity, suggesting that the E-box participates in the regulation of C/EBPalpha expression. Protein binding to the E-box during the adipocyte differentiation is increased as indicated by EMSA and UV cross-linking. Purification of the E-box binding proteins from differentiated 3T3-L1 adipocytes, showed that USF and AP-4 are associated with the E-box. Supershift analysis showed that USF1 and USF2 bind to this element as heterodimers, whereas the addition of anti-AP-4 antibody enhanced the binding complex, suggesting that AP-4 negatively regulates the promoter activity. The expression of AP-4 is reciprocally regulated with USF-1 during adipocyte differentiation. These findings suggest that USF-1 and 2 play roles in C/EBPalpha expression, whereas the AP-4 represses it.

CCAAT/enhancer-binding protein (C/EBP) beta is acetylated at multiple lysines: acetylation of C/EBPbeta at lysine 39 modulates its ability to activate transcription

Transcription factor function can be modulated by post-translational modifications. Because the transcription factor CCAAT/enhancer-binding protein (C/EBP) beta associates with the nuclear coactivator p300, which contains acetyltransferase activity, acetylation of C/EBPbeta was examined to understand its regulation and function. C/EBPbeta is acetylated by acetyltransferases p300 and p300/CREB-binding protein associated factor. Endogenous C/EBPbeta in 3T3-F442A preadipocytes is also recognized by an acetyl-lysine-specific antibody. Analysis of truncations of C/EBPbeta and peptides based on C/EBPbeta sequences identified multiple lysines within C/EBPbeta that can be acetylated. Among these, a novel acetylation site at lysine 39 of C/EBPbeta was identified. Mutation of Lys-39 to arginine or alanine impairs its acetylation and the ability of C/EBPbeta to activate transcription at the promoters for C/EBPalpha and c-fos. Different C/EBPbeta-responsive promoters require different patterns of acetylated lysines in C/EBPbeta for transcription activation. Furthermore, C/EBPbeta acetylation was increased by growth hormone, and mutation of Lys-39 impaired growth hormone-stimulated c-fos promoter activation. These data suggest that acetylation of Lys-39 of C/EBPbeta, alone or in combination with acetylation at other lysines, may play a role in C/EBPbeta-mediated transcriptional activation.

Lactacystin inhibits 3T3-L1 adipocyte differentiation through induction of CHOP-10 expression

Hormonal induction triggers a cascade leading to the expression of CCAAT/enhancer-binding protein(C/EBP)alpha and peroxisome proliferator-activated receptor (PPAR) gamma, C/EBPalpha, and PPARgamma turns on series of adipocyte genes that give rise to the adipocyte phenotype. Previous findings indicate that C/EBPbeta, a transcriptional activator of the C/EBPalpha and PPARgamma genes, is rapidly expressed after induction, but lacks DNA-binding activity and therefore cannot activate transcription of the C/EBPalpha and PPARgamma genes early in the differentiation program. Acquisition of DNA-binding activity of C/EBPbeta occurs when CHOP-10, a dominant-negative form of C/EBP family members, is down-regulated and becomes hyperphosphorylated as preadipocytes traverse the G1-S checkpoint of mitotic clonal expansion. Evidences are presented in this report that lactacystin, a proteasome inhibitor, up-regulated the CHOP-10 expression, blocked the DNA-binding activity of C/EBPbeta, and subsequently inhibited MCE as well as adipocyte differentiation.

Mechanism of plasma glutathione peroxidase production in bovine adipocytes

Plasma glutathione peroxidase (pGPx) is an anti-oxidative enzyme. Using the polymerase chain reaction subtraction method, we have previously identified pGPx as a large part of the genes that are expressed following adipocyte differentiation in a bovine intramuscular preadipocyte (BIP) line. Therefore, we have analyzed the mechanism of production of pGPx in adipocytes. The expression of pGPx and C/EBPdelta increases during adipogenesis, with dexamethasone being the main effector of these genes. The expression of pGPx gene has been clearly detected in BIP cells and human adipocytes, but hardly in 3T3-L1 cells. The production of pGPx in bovine tissues is greatest in kidney and in intraperitoneal fat. We consider that the transcriptional control of pGPx in cattle might be carried out by C/EBPdelta and that the expression of pGPx might be a characteristic phenomenon of bovine adipogenesis.

Importance of cAMP-response element-binding protein in regulation of expression of the murine cyclic nucleotide phosphodiesterase 3B (Pde3b) gene in differentiating 3T3-L1 preadipocytes

Incubation of 3T3-L1 preadipocytes with isobutylmethylxanthine (IBMX), dexamethasone, and insulin, alone or in combination, demonstrated that IBMX, which increased cAMP-response element-binding protein (CREB) phosphorylation, was the predominant regulator of Pde3b expression. Real time PCR and immunoblotting indicated that in 3T3-L1 preadipocytes, IBMX-stimulated induction of Pde3b mRNA and protein was markedly inhibited by dominant-negative CREB proteins. By transfecting preadipocytes, differentiating preadipocytes, and HEK293A cells with luciferase reporter vectors containing different fragments of the 5'-flanking region of the Pde3b gene, we identified a distal promoter that contained canonical cis-acting cAMP-response elements (CRE) and a proximal, GC-rich promoter region, which contained atypical CRE. Mutation of the CRE sequences dramatically reduced distal promoter activity; H89 inhibited IBMX-stimulated CREB phosphorylation and proximal and distal promoter activities. Distal promoter activity was stimulated by IBMX and phorbol ester (PMA) in Raw264.7 monocytes, but only by IBMX in 3T3-L1 preadipocytes. Chromatin immunoprecipitation analyses with specific antibodies against CREB, phospho-CREB, and CBP/p300 (CREB-binding protein) showed that these proteins associated with both distal and proximal promoters and that interaction of phospho-CREB, the active form of CREB, with both Pde3b promoter regions was increased in IBMX-treated preadipocytes. These results indicate that CRE in distal and proximal promoter regions and activation of CREB proteins play a crucial role in transcriptional regulation of Pde3b expression during preadipocyte differentiation.

Cytokines Promote Wnt Signaling and Inflammation and Impair the Normal Differentiation and Lipid Accumulation in 3T3-L1 Preadipocytes

Obesity with enlarged fat cells is associated with high local concentrations of interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFalpha) in the adipose tissue. We examined the effects of this inflammatory state on 3T3-L1 preadipocyte development and differentiation to mature adipose cells. Both IL-6 and TNFalpha impaired the normal differentiation pattern and lipid accumulation. However, IL-6 allowed a normal early induction of differentiation with inhibition of Wnt10b and Pref-1, whereas expression of CCAAT/enhancer-binding protein alpha, in contrast to peroxisome proliferator-activated receptor gamma, was markedly reduced. TNFalpha also allowed a normal early induction of differentiation, whereas the terminal differentiation to adipose cells was completely prevented. However, both cytokines induced an inflammatory phenotype of the cells but with different profiles. Remarkably, both IL-6 and TNFalpha maintained and augmented the canonical Wnt signaling associated with low axin and high low density lipoprotein receptor-related protein (LRD), Dishevelled, and beta-catenin levels. TNFalpha, but not IL-6, activated Wnt10b expression, whereas IL-6 increased the apparent phosphorylation of Dishevelled. Thus, both IL-6 and TNFalpha prevent the normal development of preadipocytes to fully differentiated adipose cells and, instead, promote an inflammatory phenotype of the adipocytes. These results provide an explanation as to why obesity and diabetes are associated with both local and systemic inflammation, insulin resistance, and ectopic lipid accumulation.

Activation of CCAAT/enhancer-binding protein (C/EBP) alpha expression by C/EBP beta during adipogenesis requires a peroxisome proliferator-activated receptor-gamma-associated repression of HDAC1 at the C/ebp alpha gene promoter

Studies have shown that CCAAT/enhancer-binding protein beta (C/EBP beta) can stimulate adipogenesis in noncommitted fibroblasts by activating expression of peroxisome proliferator-activated receptor-gamma (PPARgamma). Other investigations have established a role for C/EBP alpha as well as PPARgamma in orchestrating the complex program of adipogenic gene expression during terminal preadipocyte differentiation. Consequently, it is important to identify factors regulating transcription of the C/ebp alpha gene. In this study, we demonstrated that inhibition of PPARgamma activity by exposure of 3T3-L1 preadipocytes to a potent and selective PPARgamma antagonist inhibits adipogenesis but also blocks the activation of C/EBP alpha expression at the onset of differentiation. Ectopic expression of C/EBP beta in Swiss 3T3 mouse fibroblasts (Swiss-LAP cells) induces PPARgamma expression without any significant enhancement of C/EBP alpha expression. Treatment of Swiss-LAP cells with a PPARgamma agonist induces adipogenesis, which includes activation of C/EBP alpha expression. To further establish a role for PPARgamma in regulating C/EBP alpha expression, we expressed C/EBP beta in PPARgamma-deficient mouse embryo fibroblasts (MEFs). The data show that C/EBP beta is capable of inducing PPARgamma in Ppar gamma+/- MEFs, which leads to activation of adipogenesis, including C/EBP alpha expression following exposure to a PPARgamma ligand. In contrast, C/EBP beta is not able to induce C/EBP alpha expression or adipogenesis in Ppar gamma-/- MEFs. Chromatin immunoprecipitation analysis reveals that C/EBP beta is bound to the minimal promoter of the C/ebp alpha gene in association with HDAC1 in unstimulated Swiss-LAP cells. Exposure of the cells to a PPARgamma ligand dislodges HDAC1 from the proximal promoter of the C/ebp alpha gene, which involves degradation of HDAC1 in the 26 S proteasome. These data suggest that C/EBP beta activates a single unified pathway of adipogenesis involving its stimulation of PPARgamma expression, which then activates C/EBP alpha expression by dislodging HDAC1 from the promoter for degradation in the proteasome.

Effect of serum on the down-regulation of CHOP-10 during differentiation of 3T3-L1 preadipocytes

Hormonal induction of growth-arrested 3T3-L1 preadipocytes in medium containing fetal bovine serum (FBS) triggers a signaling cascade that culminates in adipogenesis. The transcription factor, C/EBPbeta, is expressed early in this differentiation program, but lacks DNA-binding activity until much later as the preadipocytes traverse the G(1)-S checkpoint of mitotic clonal expansion. Dominant-negative CHOP-10 is initially expressed by growth-arrested preadipocytes and sequesters/inactivates C/EBPbeta by heterodimerization with its leucine zipper. After a lag period, CHOP-10 undergoes down-regulation releasing C/EBPbeta from inhibitory constraint allowing transactivation of the C/EBPalpha and PPARgamma genes, transcription factors required for terminal differentiation. We verify that following induction of differentiation in FBS-containing medium, CHOP-10 undergoes down-regulation and differentiation occurs normally. However, when differentiation is induced in calf serum-containing medium, CHOP-10 is not down-regulated resulting in delayed and incomplete differentiation. Under these conditions the expression of C/EBPalpha and PPARgamma and the accumulation of cytoplasmic triglyceride are attenuated. It appears that a factor(s) present in FBS is required to affect the down-regulation of CHOP-10 necessary for successful terminal differentiation.

Adipose development: from stem cell to adipocyte

Cell culture models have been developed to study commitment and subsequent differentiation of preadipocytes into adipocytes. Bone morphogenetic protein 4 commits mesenchymal stem cells to the adipose lineage. Other factors, including Wnt signaling, cell density, and cell shape, play a role in lineage commitment. Following commitment to the adipose lineage, growth-arrested preadipocytes can differentiate to adipocytes by treatment with insulin-like growth factor 1, glucocorticoid and an agent that increases cAMP level. This process is characterized by a rapid and transient increase in CCAAT/enhancer binding protein (C/EBP) beta and synchronous re-entry into the cell cycle. Acquisition of DNA-binding by C/EBPbeta occurs after the transcription factor becomes phosphorylated. The cells enter a growth-arrested state and begin terminal differentiation. C/EBPalpha, peroxisome proliferator-activated receptor gamma, and adipocyte determination, and differentiation-dependent factor 1 coordinate the expression of genes that create and maintain the adipocyte phenotype.

Changes in integrin expression during adipocyte differentiation

3T3-L1 preadipocytes require cAMP for maximal differentiation. Microarray analysis reveals that the integrins alpha5 and alpha6 are coordinately regulated by cAMP. alpha5 expression is gradually diminished during adipogenesis, whereas alpha6 is increased. Overexpression of alpha5 in preadipocytes results in enhanced proliferation and attenuated differentiation. Conversely, alpha6 overexpression is without effect. The GTPase Rac is normally inhibited during differentiation. However, overexpression of integrin alpha5 increases Rac activity. Constitutively active but not dominant-negative Rac inhibits differentiation when overexpressed in preadipocytes, implying its role downstream of alpha5 integrin in maintaining preadipocytes in an undifferentiated state. Moreover, alpha6 integrin is critically involved in clustering growth-arrested preadipocytes on basement membrane Matrigel. Perturbation of such clustering enhances Rho activity and promotes growth-arrested preadipocytes to reenter the cell cycle. These findings demonstrate a role for integrin alpha6 in connecting morphogenesis with signaling processes leading to terminal differentiation.

Sequential phosphorylation of CCAAT enhancer-binding protein beta by MAPK and glycogen synthase kinase 3beta is required for adipogenesis

CCAAT enhancer-binding protein (C/EBP)beta, C/EBPalpha, and peroxisome proliferator activated receptor (PPAR)gamma act in a cascade where C/EBPbeta activates expression of C/EBPalpha and PPARgamma, which then function as pleiotropic activators of genes that produce the adipocyte phenotype. When growth-arrested 3T3-L1 preadipocytes are induced to differentiate, C/EBPbeta is rapidly expressed but still lacks DNA-binding activity. After a long (14-hour) lag, glycogen synthase kinase 3beta enters the nucleus, which correlates with hyperphosphorylation of C/EBPbeta and acquisition of DNA-binding activity. Concurrently, 3T3-L1 preadipocytes synchronously enter S phase and undergo mitotic clonal expansion, a prerequisite for terminal differentiation. Ex vivo and in vitro experiments with C/EBPbeta show that phosphorylation of Thr-188 by mitogen-activating protein kinase "primes" C/EBPbeta for subsequent phosphorylation on Ser-184 and Thr-179 by glycogen synthase kinase 3beta, acquisition of DNA-binding function, and transactivation of the C/EBPalpha and PPARgamma genes. The delayed transactivation of the C/EBPalpha and PPARgamma genes by C/EBPbeta appears necessary to allow mitotic clonal expansion, which would otherwise be prevented, because C/EBPalpha and PPARgamma are antimitotic.

An early event in adipogenesis, the nuclear selection of the CCAAT enhancer-binding protein {beta} (C/EBP{beta}) mRNA by HuR and its translocation to the cytosol

HuR is a ligand for nuclear mRNAs containing adenylate-uridylate-rich elements in the 3'-untranslated region. Once bound to the mRNA, HuR is recognized by adapter proteins that then facilitate nuclear export of the complex. In the cytosol, HuR is thought to function to control stability and translation of its ligand message. In the 3T3-L1 cells HuR is constitutively expressed and localized predominantly to the nucleus in the preadipocytes. However, within 30 min of exposure to the differentiation stimulus the HuR content in the cytosol increases, consistent with HuR regulating the availability of relevant mRNAs for translation. Using in vitro RNA gel shifts, we have demonstrated that the CCAAT enhancer-binding protein beta (C/EBPbeta) message is a ligand for HuR. Within 2 h of initiation of the differentiation process, HuR complexes containing C/EBPbeta mRNA could be isolated from the cytosolic compartment. Importantly, the process appears to be highly selective, as cyclin D1, which contains a putative HuR binding site and is expressed on the same time frame as C/EBPbeta, was not found in the immunoprecipitated messenger ribonucleoprotein complexes. The proximity of this event to adipogenic stimuli and the importance of C/EBPbeta to the differentiation process have led us to hypothesize a role for HuR in the regulation of the onset of adipogenesis. In support of this hypothesis, small interfering RNA suppression of HuR protein content resulted in an inhibition of C/EBPbeta protein expression and an attenuation of the differentiation process.

Signal transducers and activators of transcription 3 augments the transcriptional activity of CCAAT/enhancer-binding protein alpha in granulocyte colony-stimulating factor signaling pathway

The Janus kinase (Jak)-Stat pathway plays an essential role in cytokine signaling. Granulocyte colony-stimulating factor (G-CSF) promotes granulopoiesis and granulocytic differentiation, and Stat3 is the principle Stat protein activated by G-CSF. Upon treatment with G-CSF, the interleukin-3-dependent cell line 32D clone 3(32Dcl3) differentiates into neutrophils, and 32Dcl3 cells expressing dominant-negative Stat3 (32Dcl3/DNStat3) proliferate in G-CSF without differentiation. Gene expression profile and quantitative PCR analysis of G-CSF-stimulated cell lines revealed that the expression of C/EBPalpha was up-regulated by the activation of Stat3. In addition, activated Stat3 bound to CCAAT/enhancer-binding protein (C/EBP)alpha, leading to the enhancement of the transcription activity of C/EBPalpha. Conditional expression of C/EBPalpha in 32Dcl3/DNStat3 cells after G-CSF stimulation abolishes the G-CSF-dependent cell proliferation and induces granulocytic differentiation. Although granulocyte-specific genes, such as the G-CSF receptor, lysozyme M, and neutrophil gelatinase-associated lipocalin precursor (NGAL) are regulated by Stat3, only NGAL was induced by the restoration of C/EBPalpha after stimulation with G-CSF in 32Dcl3/DNStat3 cells. These results show that one of the major roles of Stat3 in the G-CSF signaling pathway is to augment the function of C/EBPalpha, which is essential for myeloid differentiation. Additionally, cooperation of C/EBPalpha with other Stat3-activated proteins are required for the induction of some G-CSF responsive genes including lysozyme M and the G-CSF receptor.

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.

Vascular smooth muscle cell-specific regulation of cyclin-dependent kinase inhibitor p21WAF1/Cip1 transcription by Sp1 is mediated via distinctcis-acting positive and negative regulatory elements in the proximal p21WAF1/Cip1 promoter

Smooth muscle cells (SMC) play a central role in common vascular pathologies such as atherosclerosis and restenosis. Understanding the molecular regulation of SMC proliferation at a transcriptional level may provide important clues for the targeted control of vascular hyperplasia. We recently reported the capacity of the transcription factor Sp1 to down-regulate p21(WAF1/Cip1) production thereby reducing p21(WAF1/Cip1)-cyclin D1-Cdk4 complex formation and inhibiting vascular SMC proliferation (Kavurma and Khachigian [2003] J. Biol. Chem. 278, 32537-32543). We have now localized the Sp1-response elements in the p21(WAF1/Cip1) promoter responsible for p21(WAF1/Cip1) repression in WKY12-22 SMCs. The proximal region of the p21(WAF1/Cip1) promoter contains five distinct Sp1-binding elements that we have termed A, B, C, D, and E. Electrophoretic mobility shift analysis revealed that SMC nuclear Sp1 interacts with all five Sp1-binding sites, and each of these sites is critical for Sp1 repression of the p21(WAF1/Cip1) promoter, since mutation in any one element ablates repression, and in some cases results in activation. In contrast, only elements C, D, and E are bound by Sp1 in endothelial cells. Sp1 overexpression activates the p21(WAF1/Cip1) promoter in this cell type. Furthermore, mutation in any of these five elements is not sufficient to prevent activation of the p21(WAF1/Cip1) promoter by Sp1 in endothelial cells. Surprisingly, double mutations of elements C and E facilitates superactivation by Sp1 in both cell types, whereas triple mutations of C, D, and E inactivate the promoter. These findings demonstrate cell type-specific regulation of p21(WAF1/Cip1) transcription by Sp1 via distinct cis-acting positive and negative regulatory elements in the proximal p21(WAF1/Cip1) promoter.

Sequential gene promoter interactions of C/EBPbeta, C/EBPalpha, and PPARgamma during adipogenesis

Treatment of 3T3-L1 preadipocytes with differentiation inducers triggers a cascade in which C/EBPbeta is rapidly expressed, followed by C/EBPalpha and PPARgamma. C/EBPalpha and PPARgamma then activate the expression of adipocyte genes that produce the differentiated phenotype. Circumstantial evidence indicates that C/EBPbeta activates transcription of the C/EBPalpha and PPARgamma genes, both of which possess C/EBP regulatory elements in their proximal promoters. Although C/EBPbeta is expressed immediately upon induction of differentiation, acquisition of DNA binding activity is delayed for approximately 14h. Chromatin immunoprecipitation (ChIP) analysis conducted 24h after induction revealed that C/EBPbeta binds to C/EBP regulatory elements in the proximal promoters of the C/EBPalpha and PPARgamma genes. After an additional delay ChIP analysis showed that C/EBPalpha binds to its own promoter and to the promoters of the PPARgamma and 422/aP2 genes. These findings support the view that once expressed, C/EBPalpha is responsible for maintaining the expression of PPARgamma and C/EBPalpha, as well as adipocyte proteins (e.g., 422/aP2) in the terminally differentiated state. Together these findings provide compelling evidence that C/EBPbeta, C/EBPalpha, and PPARgamma participate in a cascade during adipogenesis.

Role of specificity protein-1, PPARγ, and pituitary protein transcription factor-1 in transcriptional regulation of the murine CORS-26 promoter

The collagenous repeat-containing sequence of 26-kDa protein (CORS-26) was recently described as a new gene that is induced during adipocyte differentiation. Since the transcription factors specificity protein-1 (SP-1) and PPARgamma have been demonstrated to modulate transcriptional activation of adipocytic genes, we investigated the putative role of SP-1 and PPARgamma in the regulation of the murine CORS-26 promoter. Computer-based sequence analysis revealed two putative SP-1 binding sites and binding sites for PPARgamma and Pit-1 within the TATA-box containing promoter. Electrophoretic mobility shift assays (EMSA) with nuclear extracts from 3T3-L1 adipocytes and appropriate promoter fragments demonstrated that SP-1 binds specifically to both SP-1 binding sites. Specificity was demonstrated by (i) the appearance of supershift bands, (ii) competition experiments and, (iii) by using oligonucleotides carrying mutated SP-1 binding sites. Functional promoter activity was analyzed by Luciferase reporter gene assays and SP-1 was shown to exert inhibitory effects on the transcriptional activation of the murine CORS-26 gene. Additionally, specific binding activity of PPARgamma and Pit-1 to the CORS-26 promoter was demonstrated. Taken together, the present data demonstrate the functionality of the proximal murine CORS-26 promoter, which is regulated specifically by two SP-1 binding sites via SP-3-independent repressive effects of SP-1 on transcriptional activation. Pit-1 and PPARgamma can bind specifically to the promoter and might play an additive functional role in gene regulation of murine CORS-26.

Sequential gene promoter interactions by C/EBPβ, C/EBPα, and PPARγ during adipogenesis

Treatment of 3T3-L1 preadipocytes with differentiation inducers triggers a cascade in which C/EBPbeta is rapidly expressed, followed by C/EBPalpha and PPARgamma. C/EBPalpha and PPARgamma then activate the expression of adipocyte genes that produce the differentiated phenotype. Circumstantial evidence indicates that C/EBPbeta activates transcription of the C/EBPalpha and PPARgamma genes, both of which possess C/EBP regulatory elements in their proximal promoters. Although C/EBPbeta is expressed immediately upon induction of differentiation, acquisition of DNA binding activity is delayed for approximately 14h. Chromatin immunoprecipitation (ChIP) analysis conducted 24h after induction revealed that C/EBPbeta binds to C/EBP regulatory elements in the proximal promoters of the C/EBPalpha and PPARgamma genes. ChIP analysis showed that after an additional delay C/EBPalpha binds to its own promoter and to the promoters of the PPARgamma and 422/aP2 genes. These findings support the view that once expressed, C/EBPalpha is responsible for maintaining the expression of PPARgamma, and C/EBPalpha, as well as adipocyte proteins (e.g., 422/aP2) in the terminally differentiated state. Together these findings provide compelling evidence that C/EBPbeta, C/EBPalpha, and PPARgamma participate in a cascade during adipogenesis.

An evolutionary and molecular analysis of Bmp2 expression

The coding regions of many metazoan genes are highly similar. For example, homologs to the key developmental factor bone morphogenetic protein (BMP) 2 have been cloned by sequence identity from arthropods, mollusks, cnidarians, and nematodes. Wide conservation of protein sequences suggests that differential gene expression explains many of the vast morphological differences between species. To test the hypothesis that the regulatory mechanisms controlling this evolutionarily ancient and critical gene are conserved, we compared sequences flanking Bmp2 genes of several species. We identified numerous conserved noncoding sequences including some retained because the fish lineage separated 450 million years ago. We tested the function of some of these sequences in the F9 cell model system of Bmp2 expression. We demonstrated that both mouse and primate Bmp2 promoters drive a reporter gene in an expression pattern resembling that of the endogenous transcript in F9 cells. A conserved Sp1 site contributes to the retinoic acid responsiveness of the Bmp2 promoter, which lacks a classical retinoic acid response element. We have also discovered a sequence downstream of the stop codon whose conservation between humans, rodents, deer, chickens, frogs, and fish is striking. A fragment containing this region influences reporter gene expression in F9 cells. The conserved region contains elements that may mediate the half-life of the Bmp2 transcript. Together, our molecular and evolutionary analysis has identified new regulatory elements controlling Bmp2 expression.

Role of CREB in Transcriptional Regulation of CCAAT/Enhancer-binding Protein β Gene during Adipogenesis

The proximal promoter of the C/EBPbeta gene possesses dual cis regulatory elements (TGA1 and TGA2), both of which contain core CREB binding sites. Comparison of the activities of C/EBPbeta promoter-reporter genes with 5'-truncations or site-directed mutations in the TGA elements showed that both are required for maximal promoter function. Electrophoretic mobility shift and chromatin immunoprecipitation (ChIP) analyses with antibodies specific to CREB and ATF1 showed that these CREB family members associate with the proximal promoter both in vitro and ex vivo. Immunoblotting and ChIP analysis revealed that other CREB family members, CREM and ATF1, are up-regulated and associate with the proximal C/EBPbeta promoter in mouse embryonic fibroblasts (MEFs) from CREB(-/-) mice. ChIP analysis of wild-type MEFs and 3T3-L1 preadipocytes revealed that interaction of phospho-CREB, the active form of CREB, with the C/EBPbeta gene promoter occurs only after induction of differentiation of 3T3-L1 preadipocytes and MEFs. Consistent with the interaction of CREB and ATF1 at the TGA regulatory elements, expression of constitutively active CREB strongly activated C/EBPbeta promoter-reporter genes, induced expression of endogenous C/EBPbeta, and caused adipogenesis in the absence of the hormonal inducers normally required. Conversely, expression of a dominant-negative CREB blocked promoter-reporter activity, expression of C/EBPbeta, and adipogenesis. When subjected to the standard adipocyte differentiation protocol, wild-type MEFs differentiate into adipocytes at high frequency, whereas CREB(-/-) MEFs exhibit greatly reduced expression of C/EBPbeta and differentiation. The low level of expression of C/EBPbeta and differentiation in CREB(-/-) MEFs appears to be due to up-regulation of other CREB protein family members, i.e. ATF1 and CREM.

Dominant-negative C/EBP disrupts mitotic clonal expansion and differentiation of 3T3-L1 preadipocytes

Hormonal induction of growth-arrested 3T3-L1 preadipocytes rapidly activates expression of CCAAT/enhancer-binding protein (C/EBP) beta. Acquisition of DNA-binding activity by C/EBPbeta, however, is delayed until the cells synchronously enter the S phase of mitotic clonal expansion (MCE). After MCE, C/EBPbeta activates expression of C/EBPalpha and peroxisome proliferator-activated receptor gamma, which then transcriptionally activate genes that give rise to the adipocyte phenotype. A-C/EBP, which possesses a leucine zipper but lacks functional DNA-binding and transactivation domains, forms stable inactive heterodimers with C/EBPbeta in vitro. Infection of 3T3-L1 preadipocytes with an adenovirus A-C/EBP expression vector interferes with C/EBPbeta function after induction of differentiation. A-C/EBP inhibited events associated with hormone-induced entry of S-phase of the cell cycle, including the turnover of p27/Kip1, a key cyclin-dependent kinase inhibitor, expression of cyclin A and cyclin-dependent kinase 2, DNA replication, MCE, and, subsequently, adipogenesis. Although A-C/EBP blocked cell proliferation associated with MCE, it did not inhibit normal proliferation of 3T3-L1 preadipocytes. Immunofluorescent staining of C/EBPbeta revealed that A-C/EBP prevented the normal punctate nuclear staining of centromeres, an indicator of C/EBPbeta binding to C/EBP regulatory elements in centromeric satellite DNA. The inhibitory effects of A-C/EBP appear to be due primarily to interference with nuclear import of C/EBPbeta caused by obscuring its nuclear localization signal. These findings show that both MCE and adipogenesis are dependent on C/EBPbeta.

Cell cycle arrest by Kaposi's sarcoma-associated herpesvirus replication-associated protein is mediated at both the transcriptional and posttranslational levels by binding to CCAAT/enhancer-binding protein alpha and p21(CIP-1)

Lytic-cycle replication of Kaposi's sarcoma-associated herpesvirus (KSHV) in PEL cells causes G(1) cell cycle arrest mediated by the virus-encoded replication-associated protein (RAP) (or K8 protein), which induces high-level expression of the cellular C/EBPalpha and p21 proteins. Here we have examined the mechanism of this induction at both the transcriptional and posttranslational levels. RAP proved to bind very efficiently to both C/EBPalpha and p21 and stabilized them by up to 10-fold from proteasome-mediated degradation in vitro. Cross-linking revealed that RAP itself forms stable dimers and tetramers in solution and forms higher-order complexes but not heterodimers with C/EBPalpha. Cotransfection of RAP with C/EBPalpha cooperatively stimulated both the C/EBPalpha and p21 promoters in luciferase reporter gene assays. Only the basic/leucine zipper region of RAP was needed for interaction with and stabilization of C/EBPalpha, but both the N-terminal and C-terminal domains were required for transcriptional augmentation. In vitro-translated RAP interfered with DNA binding by C/EBPalpha in electrophonetic mobility shift assay (EMSA) experiments but did not itself bind to the target C/EBPalpha sites or form supershifted bands. However, in endogenous chromatin immunoprecipitation (ChIP) assays with tetradecanoyl phorbol acetate-induced PEL cells, RAP proved to specifically associate with the C/EBPalpha promoter in vivo, but only in a C/EBPalpha-dependent manner, implying an in vivo piggyback interaction with DNA-bound C/EBPalpha. Expression of exogenous RAP (Ad-RAP) caused G(1)/S cell cycle arrest in human dermal microvascular endothelial cells and also induced both the C/EBPalpha and p21 proteins, which formed punctate nuclear patterns that colocalized with RAP in PML nuclear bodies. In the presence of RAP, C/EBPalpha was also efficiently recruited into viral DNA replication compartments in both infected and cotransfected cells. In support of a direct role for this interaction in viral DNA replication, three C/EBPalpha binding sites were identified by in vitro EMSA experiments within a 220-bp core segment of the duplicated KSHV Ori-Lyt region, and although RAP did not bind to Ori-Lyt DNA directly in vitro, both endogenous RAP and C/EBPalpha were found to be associated with the Ori-Lyt region by ChIP assays in lytically induced PEL cells. Finally, we found that the KSHV lytic cycle could not be triggered by either synchronizing KSHV latently infected PEL cells in G(1) phase or inducing p21 in a C/EBPalpha-independent process.

Adjacent sequence controls the response polarity of nitric oxide-sensitive Sp factor binding sites

Nitric oxide (NO*) and cAMP-dependent protein kinase (PKA) inhibitors up-regulate tumor necrosis factor alpha (TNFalpha) by decreasing Sp1 binding to a proximal GC box element. Here, elements flanking GC boxes were tested for their role in determining whether Sp sites act as activators or repressors. Promoter studies in receptive human cell lines demonstrated that NO* down-regulated endothelial NO* synthase (eNOS) but up-regulated TNFalpha. Like TNFalpha, Sp1 binding to the eNOS promoter was decreased by NO* and a PKA inhibitor, H89, and increased by a PKA activator, dibutyryl cAMP (Bt2cAMP). For either promoter, mutation of Sp sites abolished NO* responses. In contrast, mutation of an upstream AP1 site in the TNFalpha promoter (not present in eNOS) maintained NO* responsiveness, but reversed the direction of NO* and cAMP effects. Using artificial constructs, NO* increased transcription when Sp and AP1 sites were both present (TNFalpha-like response), but decreased it when the adjacent AP1 site was disrupted (eNOS-like response). NO*, H89, and Bt2cAMP were found to produce reciprocal protein binding changes at contiguous AP1 and Sp sites (p < 0.0001 for an interaction). Chromatin immunoprecipitation assays demonstrated that Sp1 and to a lesser extent Sp3 bound to the GC box regions of eNOS and TNFalpha in intact cells. Thus, this NO*- and cAMP-responsive regulatory module has a Sp site sensor variably coupled to an adjacent element that determines response polarity. These results define a composite element that can utilize secondary inputs to convert off signals to on, thereby conferring complex functionalities to the same DNA binding motif.

Dual regulation of ets-activated gene expression by SP1

The human folate receptor (hFR) type gamma gene is driven by a TATA-less promoter that uses a canonical Sp1 element for basal transcription. Using nuclear extract from 293 (human embryonic) cells, we mapped a second (non-canonical) Sp1 element to which Sp1 bound with a comparable affinity and which overlaps a functional ets binding site (EBS). Mutagenesis experiments revealed that the binding of ets to the EBS activates the promoter synergistically with Sp1 bound at the downstream site; however, binding of Sp1 to the EBS does not contribute to promoter activity. A further increase in Sp1 by inducible expression in recombinant 293 cells resulted in a small but significant decrease in the hFR-gamma promoter activity, but the decrease was abolished when the EBS was deleted from the promoter. In 293 cells, which do not express hFR-gamma, the Sp1 level was relatively high whereas in the hFR-gamma-positive HL60 leukemia cells, the Sp1 level was low and the EBS predominantly bound an ets protein. To account for the above observations, we propose a model in which when the Sp1 level is low, ets out competes Sp1 for binding to the EBS and synergistically enhances the hFR-gamma promoter activity by interacting with Sp1 bound at the canonical site whereas at higher levels, Sp1 represses the promoter by competitively inhibiting the binding of ets. As a partial extension of this model to the regulation of other ets activated genes, we show that Sp1 can predictably bind to a variety of ets elements including those responsive to Ets1 and Spi.1/Pu.1. A dual concentration-dependent action of Sp1 as an activator or a repressor offers a potential mechanism contributing to tissue-specific regulation of ets-dependent genes by Sp1.

CCAAT/enhancer-binding protein beta is required for mitotic clonal expansion during adipogenesis

Hormonal induction of growth-arrested 3T3-L1 preadipocytes triggers a signaling cascade that culminates in adipogenesis. CCAATenhancer-binding protein (CEBP)beta is expressed immediately but gains DNA-binding activity only after a long lag as the cells synchronously begin mitotic clonal expansion (MCE). After MCE, a process required for adipogenesis, CEBPbeta activates expression of CEBPalpha and peroxisome proliferator-activated receptor gamma, which then transcriptionally activate genes that produce the adipocyte phenotype. When mouse embryo fibroblasts (MEFs) are subjected to the same differentiation protocol, a subset of the MEFs undergoes a similar program of events. Similar to 3T3-L1 preadipocytes, the MEFs reenter the cell cycle (as indicated by the synchronous expression of cyclin A) and undergo MCE as evidenced by the incorporation of BrdUrd into DNA and the formation of mitotic foci of cells that undergo adipogenesis. CEBPbeta is expressed immediately after induction but exhibits delayed acquisition of DNA-binding activity followed by expression of adipocyte markers and the accumulation of cytoplasmic triglyceride. MEFs from CEBPbeta(-/-) mice, however, neither undergo MCE nor differentiate into adipocytes. Forced expression of CEBPbeta (LAP) but not dominant-negative CEBPbeta (LIP) in CEBPbeta(-/-) MEFs restores MCE, expression of adipocyte markers, and the capacity to form mitotic foci of cells that undergo adipogenesis. These findings demonstrate that expression of CEBPbeta is a prerequisite for MCE in the adipocyte-differentiation program.

When translation meets metabolism: multiple links to diabetes

Type 2 diabetes is a polygenic disorder characterized by multiple biochemical defects including transcriptional, translational, and posttranslational abnormalities. Although major progress has been made in elucidation of factors at the transcriptional and posttranslational levels, defects at the translational level remain elusive. Mutation of a kinase that regulates translation initiation has been implicated in the etiology of a monogenic form of diabetes known as Wolcott-Rallison syndrome. Characterization of mice rendered deficient in eukaryotic initiation factors has provided model systems to study the involvement of translation in regulating insulin synthesis and secretion, hepatic function, peripheral insulin resistance, and diabetic complications. Recent progress in the understanding of endoplasmic reticulum overload by unfolded proteins has begun to uncover mechanisms leading to pancreatic beta-cell exhaustion. Future advances in this area may lead to identification of the missing links in the pathogenesis of beta-cell failures due to conditions such as hyperinsulinemia, hyperglycemia, and long-term treatment with sulfonylureas, and thus may identify novel therapeutic targets for diabetes.

Down-regulation of acyl-CoA oxidase gene expression and increased NF-kappaB activity in etomoxir-induced cardiac hypertrophy

Activation of nuclear factor-kappaB (NF-kappaB) is required for hypertrophic growth of cardiomyocytes. Etomoxir is an irreversible inhibitor of carnitine palmitoyltransferase I (CPT-I) that activates peroxisome proliferator-activated receptor alpha (PPARalpha) and induces cardiac hypertrophy through an unknown mechanism. We studied the mRNA expression of genes involved in fatty acid oxidation in the heart of mice treated for 1 or 10 days with etomoxir (100 mg/kg/day). Etomoxir administration for 1 day significantly increased (4.4-fold induction) the mRNA expression of acyl-CoA oxidase (ACO), which catalyzes the rate-limiting step in peroxisomal beta-oxidation. In contrast, etomoxir treatment for 10 days dramatically decreased ACO mRNA levels by 96%. The reduction in ACO expression in the hearts of 10-day etomoxir-treated mice was accompanied by an increase in the mRNA expression of the antioxidant enzyme glutathione peroxidase and the cardiac marker of oxidative stress bax. Moreover, the activity of the redox-regulated transcription factor NF-kappaB was increased in heart after 10 days of etomoxir treatment. Overall, the findings here presented show that etomoxir treatment may induce cardiac hypertrophy via increased cellular oxidative stress and NF-kappaB activation.

Mitotic clonal expansion: a synchronous process required for adipogenesis

When induced to differentiate, growth-arrested 3T3-L1 preadipocytes synchronously reenter the cell cycle and undergo mitotic clonal expansion (MCE) followed by expression of genes that produce the adipocyte phenotype. The preadipocytes traverse the G(1)S checkpoint synchronously as evidenced by the expressionactivation of cdk2-cyclin-EA, turnover of p27kip1, hyperphosphorylation of Rb, translocation of cyclin D(1) from nuclei to cytoplasm and GSK-3beta from cytoplasm to nuclei, and incorporation of [(3)H]thymidine into DNA. As the cells cross the G(1)S checkpoint, CEBPbeta acquires DNA-binding activity, initiating a cascade of transcriptional activation that culminates in the expression of adipocyte proteins. The mitogen-activated protein kinaseextracellular signal-regulated kinase kinase (MEK) inhibitor PD98059 delays, but does not block, MCE and differentiation, the extent of the delay causing a comparable delay in the expression of cell-cycle markers, MCE, and adipogenesis. The more potent and specific MEK inhibitor UO126 and the cyclin-dependent kinase inhibitor roscovitine, which inhibit the cell cycle at different points, block MCE, expression of cell cycle and adipocyte markers, as well as adipogenesis. These results show that MCE is a prerequisite for differentiation of 3T3-L1 preadipocytes into adipocytes.

Asthma and the CCAAT-enhancer binding proteins: a holistic view on airway inflammation and remodeling

Asthma is an airway disease with increasing prevalence characterized by intermittent reversible airway obstruction, airway inflammation, and airway wall remodeling. The disease is generally triggered by inhalation of allergens, but nonallergic asthma triggers are quite common. The pathogenesis of asthma is well documented, and a great deal of research has been carried out to elucidate the underlying mechanisms. A multitude of articles have focused on cells alleged to be involved in the pathogenesis, including circulating cells from the immunologic compartment (ie, eosinophils and T lymphocytes) and resident cells, such as fibroblasts, airway smooth muscle cells, and, more recently, the airway epithelium. Despite the enormous amount of research, it is still unclear what exactly causes asthma. A general feature of most studies is an enhanced activation status of asthmatic cells, suggesting a general defect with respect to regulation of cellular responses. Here we discuss the ubiquitous transcription factor family of CCAAT-enhancer binding proteins (C/EBPs) and its involvement in inflammation and proliferation. We propose that an imbalance of C/EBP isoform expression might lead to an enhanced activity of asthmatic cells and provide an overall hypothesis that both airway inflammation and remodeling can be conceived as the result of an imbalance of C/EBP isoform expression.

Genistein inhibits CCAAT/enhancer-binding protein beta (C/EBPbeta) activity and 3T3-L1 adipogenesis by increasing C/EBP homologous protein expression

The tyrosine kinase inhibitor genistein inhibits 3T3-L1 adipogenesis when present during the first 72 h of differentiation. In this report, we investigated the underlying mechanisms involved in the anti-adipogenic effects of genistein. We found that genistein blocked the DNA binding and transcriptional activity of CCAAT/enhancer-binding protein beta (C/EBPbeta) during differentiation by promoting the expression of C/EBP homologous protein, a dominant-negative member of the C/EBP family. Loss of C/EBPbeta activity was manifested as a loss of differentiation-induced C/EBPalpha and peroxisome-proliferator-activated receptor gamma protein expression and a dramatic reduction in lipid accumulation. Further, we documented for the first time that C/EBPbeta was tyrosine-phosphorylated in vivo during differentiation and in vitro by activated epidermal growth factor receptor. Genistein inhibited both of these events. Collectively, these results indicate that genistein blocks adipogenesis and C/EBPbeta activity by increasing the level of C/EBP homologous protein and possibly by inhibiting the tyrosine phosphorylation of C/EBPbeta.

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.

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.

Calpain function in the differentiation of mesenchymal stem cells

Calpain is a calcium-activated non lysosomal neutral thiol protease (EC 3.4.22.17) present in a wide variety of eukaryotic cells. Calpain is usually present as an inactive form and is activated by calcium ions and phospholipids. The ability of calpain to alter, by limited proteolysis, the activity or function of numerous cytoskeletal proteins, enzymes, and receptors suggests its involvement in various Ca2+-regulated cellular functions. In this review we focus on the differentiation of mesenchymal stem cells, such as the myoblastic, osteoblastic, chondrocytic, and adipocytic lineages, and the biological significance of calpain in its regulation. Calpain has been implicated in the differentiation of myoblasts through the turnover of glycoproteins. In preosteoblastic cells, calpain is important in mediating the proliferative and prodifferentiating effects of parathyroid hormone and bone morphogenetic proteins. For the differentiation of chondrocytes, calpain is involved in cartilage-matrix mineralization. Furthermore, calpain is required for the differentiation of 3T3-L1 preadipocytes into adipocytes, involving the transcriptional activation of the C/EBP alpha gene and the degradation of the cyclin-dependent kinase inhibitor p27 during the mitotic clonal expansion phase of adipocyte differentiation. We summarize these regulatory effects of calpain on the differentiation of mesenchymal stem cells and speculate on the function and location of calpain in the differentiation processes.

Paradoxical regulation of Sp1 transcription factor by glucagon

Insulin is a potent regulator of Sp1 transcription factor. To examine if glucagon, which usually antagonizes insulin, regulates Sp1, we assessed the levels of Sp1 by Western blotting from H-411E cells exposed to glucagon with or without insulin. Glucagon alone (1.5 x 10(-9) to 1.5 x 10(-5) M) stimulated Sp1 accumulation but inhibited insulin's (10,000 microU/ml) stimulatory effect on Sp1. We also assessed the effect of TNF-alpha, wortmannin, a PI3K inhibitor, and cAMP-dependent protein kinase inhibitor on Sp1 accumulation. While TNF-alpha (5 ng/ml) blocked insulin-stimulated Sp1, it failed to block stimulation of Sp1 by glucagon (1.5 x 10(-5) M). Similarly, wortmannin inhibited insulin- but not glucagon-stimulated Sp1, whereas protein kinase inhibitor had an opposite effect. Thus, insulin acts primarily via PI3K, and glucagon apparently stimulates through a cAMP-dependent pathway. Insulin increased the staining intensity of Sp1 seen exclusively in the nuclei of H-411E cells. Sp1 was demonstrable in both nucleus and cytoplasm after glucagon treatment. Finally, as judged by immunoblotting to specific antibody, insulin but not glucagon, stimulated O-glycosylation of Sp1. Thus, unique signaling mechanisms mediate the response of Sp1 to glucagon in the presence or absence of insulin.

Downregulation of repressive CUP/AP-2 isoforms during adipocyte differentiation

Activation of transcription of the C/EBPalpha (CCAAT/enhancer binding protein alpha) gene is a critical event in the differentiation of 3T3-L1 preadipocytes into adipocytes. The kinetics of this process parallels a decline of AP-2alpha protein (also referred to as CUP, C/EBP undifferentiated protein) and decreased binding of CUP/AP-2alpha to the C/EBPalpha promoter. Mutation of the CUP/AP-2 binding sites in the C/EBPalpha promoter results in increased C/EBPalpha expression. Based on these findings, it appears that decline in AP-2alpha expression is an important early event in the adipocyte differentiation program. In the studies presented here, we identify three mRNAs that encode the repressive CUP/AP-2alpha isoforms expressed in undifferentiated 3T3-L1 preadipocytes. We demonstrate that the kinetics of the decline of these isoforms' expression over the course of differentiation parallels both the decrease in CUP/AP-2alpha DNA binding activity and the increase in C/EBPalpha protein observed in previous studies.

Functional Interaction of bZIP Proteins and the Large Subunit of Replication Factor C in Liver and Adipose Cells

The transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPalpha) has a vital role in cell growth and differentiation. To delineate further a mechanism for C/EBPalpha-mediated differentiation, we screened C/EBPalpha-interacting proteins through far-Western screening. One of the strongest interactions was with RFC140, the large subunit of the replication factor C complex. C/EBPalpha specifically interacted with RFC140 from rat liver nuclear extract as determined by a combination of affinity chromatography and co-immunoprecipitation. Subsequent far-Western blotting showed that the bZIP domain of C/EBPalpha interacted with the DNA-binding region of RFC140. Overexpression of RFC140 in mammalian cells increased the transactivation activity of C/EBPalpha on both minimal and native promoters. Consistent with the enhanced transactivation, a complex of C/EBPalpha and RFC140 proteins with the cognate DNA element was detected in vitro. The specific interaction between C/EBPalpha and RFC140 was detected in the terminal differentiation of 3T3-L1 preadipocytes to adipocytes. The synergistic transcription effect of these two proteins increased the promoter activity and protein expression of peroxisome proliferator-activated receptor-gamma, which is a main regulator of adipocyte differentiation. Our results demonstrate that the specific transcription factor C/EBPalpha and the general DNA replication factor RFC140 interact functionally and physically. This observation highlights a unique mechanism by which the levels of the general replication factor can strongly modulate the functional activity of the specific transcription factor as a coactivator.

Nuclear factor 1 interferes with Sp1 binding through a composite element on the rat poly(ADP-ribose) polymerase promoter to modulate its activity in vitro

Poly(ADP-ribose) polymerase-1 (PARP-1) catalyzes the rapid and extensive poly(ADP-ribosyl)ation of nuclear proteins in response to DNA strand breaks, and its expression, although ubiquitous, is modulated from tissue to tissue and during cellular differentiation. PARP-1 gene promoters from human, rat, and mouse have been cloned, and they share a structure common to housekeeping genes, as they lack a functional TATA box and contain multiple GC boxes, which bind the transcriptional activator Sp1. We have previously shown that, although Sp1 is important for rat PARP1 (rPARP) promoter activity, its finely tuned modulation is likely dependent on other transcription factors that bind the rPARP proximal promoter in vitro. In this study, we identified one such factor as NF1-L, a rat liver isoform of the nuclear factor 1 family of transcription factors. The NF1-L site on the rPARP promoter overlaps one of the Sp1 binding sites previously identified, and we demonstrated that binding of both factors to this composite element is mutually exclusive. Furthermore, we provide evidence that NF1-L has no effect by itself on rPARP promoter activity, but rather down-regulates the Sp1 activity by interfering with its ability to bind the rPARP promoter in order to modulate transcription of the rPARP gene.

A role for C/EBPbeta in regulating peroxisome proliferator-activated receptor gamma activity during adipogenesis in 3T3-L1 preadipocytes

The differentiation of 3T3-L1 preadipocytes is regulated in part by a cascade of transcriptional events involving activation of the CCAAT/enhancer-binding proteins (C/EBPs) and peroxisome proliferator-activated receptor gamma (PPARgamma) by dexamethasone (DEX), 3-isobutyl-1-methylxanthine (MIX), and insulin. In this study, we demonstrate that exposure of 3T3-L1 preadipocytes to DEX and insulin fails to induce adipogenesis as indicated by a lack of C/EBPalpha, PPARgamma2, and adipose protein 2/fatty acid-binding protein expression; however, PPARgamma1 is expressed. Treatment of these MIX-deficient cells with a PPARgamma ligand, troglitazone, induces C/EBPalpha expression and rescues the block in adipogenesis. In this regard, we also show that induction of C/EBPalpha gene expression by troglitazone in C3H10T1/2 cells ectopically expressing PPARgamma occurs in the absence of ongoing protein synthesis, suggesting a direct transactivation of the C/EBPalpha gene by PPARgamma. Furthermore, ectopic expression of a dominant negative isoform of C/EBPbeta (liver-enriched transcriptional inhibitory protein (LIP)) inhibits the induction of C/EBPalpha, PPARgamma2, and adipose protein 2/fatty acid-binding protein by DEX, MIX, and insulin in 3T3-L1 cells without affecting the induction of PPARgamma1 by DEX. Exposure of LIP-expressing preadipocytes to troglitazone along with DEX, MIX, and insulin induces differentiation into adipocytes. Additionally, we show that sustained expression of C/EBPalpha in these LIP-expressing adipocytes requires constant exposure to troglitazone. Taken together, these observations suggest that inhibition of C/EBPbeta activity not only blocks C/EBPalpha and PPARgamma2 expression, but it also renders the preadipocytes dependent on an exogenous PPARgamma ligand for their differentiation into adipocytes. We propose, therefore, an additional role for C/EBPbeta in regulating PPARgamma activity during adipogenesis, and we suggest an alternative means of inducing preadipocyte differentiation that relies on the dexamethasone-associated induction of PPARgamma1 expression.

AML1-ETO downregulates the granulocytic differentiation factor C/EBPalpha in t(8;21) myeloid leukemia

The transcription factor CCAAT/enhancer binding protein alpha, or C/EBPalpha, encoded by the CEBPA gene, is crucial for the differentiation of granulocytes. Conditional expression of C/EBPalpha triggers neutrophilic differentiation, and Cebpa knockout mice exhibit an early block in maturation. Dominant-negative mutations of CEBPA have been found in some patients with acute myeloid leukemia (AML), but not in AML with the t(8;21) translocation which gives rise to the fusion gene RUNX1-CBF2T1 (also known as AML1-ETO) encoding the AML1-ETO fusion protein. RUNX1-CBF2T1 positive-AML blasts had eight-fold lower CEBPA RNA levels and undetectable C/EBPalpha protein levels compared with other subgroups of AML patients. Conditional expression of RUNX1-CBF2T1 in U937 cells downregulated CEBPA mRNA, protein and DNA binding activity. AML1-ETO appears to suppress C/EBPalpha expression indirectly by inhibiting positive autoregulation of the CEBPA promoter. Conditional expression of C/EBPalpha in AML1-ETO-positive Kasumi-1 cells results in neutrophilic differentiation. We suggest that restoring C/EBPalpha expression will have therapeutic implications in RUNX1-CBF2T1-positive leukemias.

Molecular regulation of adipogenesis

Adipogenesis, or the development of fat cells from preadipocytes, has been one of the most intensely studied models of cellular differentiation. In part this has been because of the availability of in vitro models that faithfully recapitulate most of the critical aspects of fat cell formation in vivo. More recently, studies of adipogenesis have proceeded with the hope that manipulation of this process in humans might one day lead to a reduction in the burden of obesity and diabetes. This review explores some of the highlights of a large and burgeoning literature devoted to understanding adipogenesis at the molecular level. The hormonal and transcriptional control of adipogenesis is reviewed, as well as studies on a less well known type of fat cell, the brown adipocyte. Emphasis is placed, where possible, on in vivo studies with the hope that the results discussed may one day shed light on basic questions of cellular growth and differentiation in addition to possible benefits in human health.