Differentiation-induced gene expression in 3T3-L1 preadipocytes: CCAAT/enhancer binding protein interacts with and activates the promoters of two adipocyte-specific genes

C/EBPalpha is required to maintain postmitotic growth arrest in adipocytes

Terminal differentiation is often coupled with irreversible loss of proliferative potential. The CCAAT enhancer binding protein alpha (C/EBPalpha) preferentially accumulates in postmitotic, differentiated 3T3-L1 adipocytes but declines during tumor necrosis factor alpha (TNFalpha)-induced dedifferentiation. We have discovered that this decline in C/EBPalpha correlates with an increased mitotic growth potential. In order to further investigate the antimitotic activity of C/EBPalpha, we introduced antisense C/EBPalpha RNA into 3T3-L1 cells to block endogenous C/EBPalpha expression. When treated according to the standard differentiation protocol, stable cells lines harboring antisense C/EBPalpha RNA did not differentiate into fat-laden adipocytes, consistent with previous findings (Lin F, Lane MD, Genes Dev 1992;6:533-544). We found that these undifferentiated cells expressing antisense-C/EBPalpha can reenter the cell cycle after mitogenic stimulation at a time in development when parental 3T3-L1 cells cannot. Moreover, the expression profiles of the growth-arrest-associated genes gas1 and gas2 revealed that the antisense C/EBPalpha-expressing cells withdrew from the cell cycle after the period of clonal expansion but failed to progress to the state of least proliferative potential characteristic of terminally differentiated adipocytes.

Role of the CCAAT enhancer binding proteins (C/EBPs) in adipocyte differentiation

Members of the C/EBP family of transcription factors play essential roles in the adipocyte differentiation program. Treatment of growth-arrested 3T3-L1 preadipocytes with appropriate hormonal agents causes the cells to synchronously reenter the cell cycle and to undergo mitotic clonal expansion. Expression of C/EBPbeta and delta occur early in clonal expansion, later followed by C/EBPalpha (which is anti-mitotic) as the cells exit the cell cycle begin to express adipocyte genes. C/EBPalpha serves as transcriptional activator of many adipocyte genes whose expression produce the adipocyte phenotype. Recent work in this laboratory has focussed on the roles of C/EBPbeta and delta in the differentiation program, in particular the mechanisms by which they activate transcription of the C/EBPalpha gene. Several regulatory elements, both repressive and activating, in proximal promoter of the gene have been identified. The cognate transacting factors that interact with these elements have been characterized and their functions elucidated. These factors have been incorporated into a model for a cascade that leads to transcriptional activation of the C/EBPalpha gene and the terminal steps in the differentiation program.

Glycogen synthase kinase 3 is an insulin-regulated C/EBPalpha kinase

CCAAT/enhancer binding protein alpha (C/EBPalpha) is a transcription factor involved in creating and maintaining the adipocyte phenotype. We have shown previously that insulin stimulates dephosphorylation of C/EBPalpha in 3T3-L1 adipocytes. Studies to identify the insulin-sensitive sites of phosphorylation reveal that a C/EBPalpha peptide (amino acids H215 to K250) is phosphorylated on T222, T226, and S230 in vivo. The context of these phosphoamino acids implicates glycogen synthase kinase 3 (GSK3), whose activity is known to be repressed in response to insulin, as a potential kinase for phosphorylation of T222 and T226. Accordingly, GSK3 phosphorylates the predicted region of C/EBPalpha on threonine in vitro, and GSK3 uses C/EBPalpha as a substrate in vivo. In addition, the effect of pharmacological agents on GSK3 activity correlates with regulation of C/EBPalpha phosphorylation. Treatment of 3T3-L1 adipocytes with the phosphatidylinositol 3-kinase inhibitor wortmannin results in phosphorylation of C/EBPalpha, whereas treatment with the GSK3 inhibitor lithium results in dephosphorylation of C/EBPalpha. Collectively, these data indicate that insulin stimulates dephosphorylation of C/EBPalpha on T222 and T226 through inactivation of GSK3. Since dephosphorylation of C/EBPalpha in response to lithium is blocked by okadaic acid, strong candidates for the T222 and T226 phosphatase are protein phosphatases 1 and 2a. Treatment of adipocytes with insulin alters the protease accessibility of widespread sites within the N terminus of C/EBPalpha, consistent with phosphorylation causing profound conformational changes. Finally, phosphorylation of C/EBPalpha and other substrates by GSK3 may be required for adipogenesis, since treatment of differentiating preadipocytes with lithium inhibits their conversion to adipocytes.

CCAAT/enhancer-binding protein beta (C/EBPbeta) and C/EBPdelta contribute to growth hormone-regulated transcription of c-fos

Using the c-fos enhancer as a model to analyze growth hormone (GH)-promoted gene expression, the roles of CCAAT/enhancer-binding proteins (C/EBPs) in GH-regulated transcription were investigated. In 3T3-F442A fibroblasts stably expressing the c-fos promoter mutated at the C/EBP binding site upstream of luciferase, c-fos promoter activity is stimulated by GH 6-7-fold; wild type c-fos promoter shows only a 2-fold induction by GH. This suggests that C/EBP restrains GH-stimulated expression of c-fos. Electrophoretic mobility shift assays with nuclear extracts from 3T3-F442A cells indicate that GH rapidly (2-5 min) increases binding of C/EBPbeta and C/EBPdelta, to the c-fos C/EBP binding site. Both liver activating protein (LAP) and liver inhibitory protein (LIP), forms of C/EBPbeta, are detected in 3T3-F442A cells by immunoblotting. GH increases the binding of LAP/LAP and LAP/LIP dimers. Overexpression of LIP interferes with GH-promoted reporter expression in CHO cells expressing GH receptors, consistent with the possibility that LIP restrains GH-stimulated c-fos expression. Overexpression of LAP elevates basal luciferase activity but does not influence promoter activation by GH, while overexpressed C/EBPdelta elevates basal promoter activity and enhances the stimulation by GH. GH stimulates the expression of mRNA for C/EBPbeta and -delta and increases levels of C/EBPdelta. Although C/EBPbeta is not detectably altered, GH induces a shift to more rapidly migrating forms of LIP and LAP upon immunoblotting. Treatment of extracts from GH-treated cells with alkaline phosphatase causes a shift of the slower migrating form to the rapidly migrating form, consistent with GH promoting dephosphorylation of LIP and LAP. These studies implicate C/EBPbeta and -delta in GH-regulated gene expression. They also indicate that GH stimulates the binding of C/EBPbeta and -delta to the c-fos promoter and promotes the dephosphorylation of LIP and LAP. These events may contribute to the ability of C/EBPbeta and -delta to regulate GH-stimulated expression of c-fos.

The v-Myb oncoprotein activates C/EBPbeta expression by stimulating an autoregulatory loop at the C/EBPbeta promoter

Previous studies have implicated the CCAAT box/enhancer binding protein beta (C/EBPbeta) in the regulation of cell-type specific gene expression in myelomonocytic cells and in the activation of target genes by the transcription factor v-Myb. To better understand the role of C/EBPbeta in myelomonocytic cells we have cloned the chicken C/EBPbeta gene and studied its regulation. The chicken C/EBPbeta promoter contains a number of C/EBP binding sites and is activated by C/EBPbeta, suggesting that the C/EBPbeta gene is autoregulated by its own protein product. Interestingly, the C/EBPbeta promoter is not activated by C/EBPalpha, another C/EBP family member highly expressed in myelomonocytic cells, indicating that the autoregulation is specific for C/EBPbeta. Comparison of different C/EBP inducible promoters shows that the relative transactivation potential of C/EBPalpha and beta is extremely dependent on the promoter context. By using the promoters of the mim-1 and C/EBPbeta genes and by exchanging the DNA-binding domains between C/EBPalpha and beta we show that the observed promoter preferences of C/EBPalpha and beta are not due to differential DNA-binding but instead depend on the transactivation domains of these proteins. The C/EBPbeta promoter also contains several Myb binding motifs, suggesting that the C/EBPbeta gene is also myb-inducible. We show that the C/EBPbeta promoter is activated synergistically by v-Myb and C/EBPbeta and that transcription of the endogenous C/EBPbeta gene is increased by v-Myb. Thus, our results identify the C/EBPbeta gene as a novel v-Myb target gene. Taken together, our data suggest a model for the regulation of C/EBPbeta expression in which v-Myb stimulates the synthesis of C/EBPbeta by enhancing an autoregulatory loop acting on the C/EBPbeta promoter.

Activation and centromeric localization of CCAAT/enhancer-binding proteins during the mitotic clonal expansion of adipocyte differentiation

Hormonal induction of 3T3-L1 preadipocytes triggers a cascade of events that initiate differentiation into adipocytes. CCAAT/enhancer-binding proteins beta and delta (C/EBPbeta/delta) are expressed early in the differentiation program, but are not immediately active. After a long lag, C/EBPbeta/delta become competent to bind to the C/EBP regulatory element in the C/EBPalpha gene promoter, C/EBPalpha being a transcriptional activator of numerous adipocyte genes. As C/EBPbeta/delta acquire binding activity, they become localized to centromeres as preadipocytes synchronously enter S phase at the onset of mitotic clonal expansion. Localization to centromeres occurs through C/EBP consensus-binding sites in centromeric satellite DNA. C/EBPalpha, which is antimitotic, becomes centromere-associated much later in the differentiation program as mitotic clonal expansion ceases and the cells become terminally differentiated.

Regulation of peroxisome proliferator-activated receptor gamma expression by adipocyte differentiation and determination factor 1/sterol regulatory element binding protein 1: implications for adipocyte differentiation and metabolism

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor implicated in adipocyte differentiation and insulin sensitivity. We investigated whether PPARgamma expression is dependent on the activity of adipocyte differentiation and determination factor 1/sterol regulatory element binding protein 1 (ADD-1/SREBP-1), another transcription factor associated with both adipocyte differentiation and cholesterol homeostasis. Ectopic expression of ADD-1/SREBP-1 in 3T3-L1 and HepG2 cells induced endogenous PPARgamma mRNA levels. The related transcription factor SREBP-2 likewise induced PPARgamma expression. In addition, cholesterol depletion, a condition known to result in proteolytic activation of transcription factors of the SREBP family, induced PPARgamma expression and improved PPRE-driven transcription. The effect of the SREBPs on PPARgamma expression was mediated through the PPARgamma1 and -3 promoters. Both promoters contain a consensus E-box motif that mediates the regulation of the PPARgamma gene by ADD-1/SREBP-1 and SREBP-2. These results suggest that PPARgamma expression can be controlled by the SREBP family of transcription factors and demonstrate new interactions between transcription factors that can regulate different pathways of lipid metabolism.

Expression of CCAAT/enhancer binding protein C/EBPalpha, beta and delta in rat adipose stromal-vascular cells in vitro

Stromal-vascular (S-V) cells from rat inguinal fat depots were isolated and cultured in medium containing fetal bovine serum (FBS) and differentiated in defined medium until lipid accumulation was apparent. C/EBPalpha, beta and delta levels were evaluated for different growth conditions and at different times using Western blots. Immediately after isolation C/EBPalpha, beta and delta could not be detected in S-V cells. After seeding for 24 h in Dulbecco's modified Eagle's medium (DMEM) with FBS, C/EBPalpha, beta and delta could all be detected. Cells at day 1 of culture in insulin, transferrin, triiodothyronine and selenium (ITTS) had increased levels of C/EBPalpha and continued steady high levels to day 6 of culture. Cultures grown in DMEM alone, with no ITTS, showed C/EBPalpha levels similar to ITTS cultures at day 1 and day 3; however, levels diminished after day 3. DMEM cultures also showed lipid accumulation at day 6; however, the number of cells and the amount of lipid cell were reduced from levels observed in ITTS cultures. C/EBPbeta was expressed uniformly throughout the culture period in either DMEM or ITTS cultures while C/EBPdelta expression was higher with DMEM treatment than with ITTS. Treatment of 2 day DMEM cultures with FBS increased levels of C/EBPbeta and delta but significantly reduced levels of C/EBPalpha. Immunocytochemical analysis of S-V cells at day 1 of culture showed a similar percentage of cells stained in DMEM cultures and ITTS cultures. However, by day 6 of culture the percentage of cells staining positively for C/EBPalpha in DMEM had been reduced by one half while in ITTS the percent positive cells remained about the same. Our results indicate that ITTS is not necessary for the induction of C/EBPalpha and accumulation of lipid in S-V cells. However, ITTS is responsible for maintaining C/EBPalpha and enhanced lipid accumulation. Because C/EBPalpha, beta and delta expression occurs very early in cell culture and C/EBPalpha and delta expression continues to increase in DMEM without any apparent inducing agents, our results suggest that these factors may be expressed by the same cells in vivo before being placed in culture. Thus, a large fraction of S-V cells may be further along in the differentiation program than 3T3 cells are when they begin differentiation.

Modulation of CCAAT/enhancer-binding protein-alpha gene expression by metabolic signals in rodent adipocytes

The transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPalpha) is a positive modulator of transcription for several adipocyte-specific genes that play a role in energy metabolism. However, there is little information available regarding the regulation of its expression by metabolic signals. Exposure to insulin for 5-24 h attenuated C/EBPalpha expression when 3T3-L1 adipocytes were incubated in 24 mM glucose, but not in 5.7 mM glucose. Nuclear run-on transcription assays indicated a transcriptional repression of C/EBPalpha gene, but not that of C/EBPbeta. Glucosamine, a product of the hexosamine pathway, in the presence of low glucose mimicked high glucose's ability to reduce C/EBPalpha messenger RNA expression in insulin-treated cells. Similar results were obtained with xylitol, an activator of the pentose phosphate pathway. There was no correlation between the accumulation of hexosamine pathway metabolites (e.g. UDP-N-acetylhexosamines) and/or changes in intracellular protein glycosylation with the ability of high glucose, glucosamine, or xylitol to down-regulate C/EBPalpha gene expression. None of these treatments caused a reduction in intracellular ATP levels. Stable transfection of 3T3-L1 cells with the 5'-flanking 468-bp sequence of the mouse C/EBPalpha gene fused to luciferase demonstrated that promoter activity was also reduced by these nutrients. Of interest, treatment of rats with glucose or glucosamine led to a reduction in C/EBPalpha messenger RNA levels in epididymal, but not omental, fat. Taken together, these results suggest that metabolic signals serve to down-regulate C/EBPalpha expression both in vitro and in vivo.

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

Expression of C/EBPalpha is required for differentiation of 3T3-L1 preadipocytes into adipocytes. Previous investigations indicated that transcription of the C/EBPalpha gene is sequentially activated during differentiation, initially by C/EBPbeta and C/EBPdelta and later by C/EBPalpha (autoactivation). These events are mediated by a C/EBP regulatory element in the promoter of the C/EBPalpha gene. This article presents evidence that members of the Sp family, notably Sp1, act repressively on the C/EBPalpha promoter prior to the induction of differentiation. Sp1 was shown to bind to a GC box at the 5' end of the C/EBP regulatory element in the C/EBPalpha promoter and, in so doing, to competitively prevent binding to and transactivation of the promoter by the C/EBPs. One of the differentiation inducers methylisobutylxanthine (a cAMP phosphodiesterase inhibitor) or Forskolin, both of which increase the cellular cAMP level, causes down-regulation of Sp1. This decrease in Sp1 level early in the differentiation program appears to facilitate access of C/EBPbeta and/or C/EBPdelta to the C/EBP regulatory element and, thereby, derepression of the C/EBPalpha gene.

Separation of C/EBPalpha-mediated proliferation arrest and differentiation pathways

Cell proliferation and terminal differentiation are mutually exclusive in most cell lineages. The b-zip transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) induces proliferation arrest and differentiation in many cell types, suggesting that both activities are linked. Here we show that C/EBPalpha-mediated proliferation arrest and differentiation pathways can be separated by the E7 oncoprotein of the "high-risk" human papilloma virus 16. The E7 oncoprotein overrides C/EBPalpha-mediated cell cycle withdrawal without compromising the transactivation activity of C/EBPalpha or its ability to participate in differentiation. Uncoupling of both pathways depends on the casein kinase II site of the oncoprotein but not on its ability to neutralize pocket proteins or the cyclin-dependent kinase inhibitor protein p21. Our results suggest a bifurcation of C/EBPalpha-mediated proliferation arrest and differentiation pathways.

Cloning and characterization of a novel zinc finger transcriptional repressor. A direct role of the zinc finger motif in repression

We have identified a novel transcriptional repressor, AEBP2, that binds to a regulatory sequence (termed AE-1) located in the proximal promoter region of the aP2 gene that encodes the adipose fatty acid-binding protein. Sequence analysis of AEBP2 cDNA revealed that it encodes a protein containing three Gli-Krüppel (Cys2-His2)-type zinc fingers. Northern blot analysis revealed two transcripts (4.5 and 3.5 kilobases) which were ubiquitously expressed in every mouse tissue examined. In co-transfection assays, AEBP2 repressed transcription from the homologous aP2 promoter containing multiple copies of the AE-1 sequence. Moreover, a chimeric construct encoding a fusion AEBP2 protein with the Gal4 DNA-binding domain was able to repress the transcriptional activity of a heterologous promoter containing the Gal4-binding sequence. The transcriptional repression function of AEBP2 was completely abolished when one of the conserved histidine residues and a flanking serine residue in the middle zinc finger were replaced with an arginine residue. The defective transcriptional repression function of the mutant derivative was due neither to lack of expression nor to a failure to localize to the nucleus. Moreover, both the wild-type and mutant derivative of either the histidine-tagged recombinant AEBP2 proteins or the in vitro translated Gal4-AEBP2 fusion proteins were equally able to bind to the target DNA. These results suggest that a portion of the zinc finger structure may play a direct role in transcriptional repression function, but not in DNA binding.

The transcription factor CCAAT/enhancer-binding protein beta regulates gluconeogenesis and phosphoenolpyruvate carboxykinase (GTP) gene transcription during diabetes

CCAAT/enhancer-binding protein (C/EBP) beta and C/EBPalpha are members of the c/ebp gene family and are highly expressed in mammalian liver and adipose tissue. C/EBPalpha is essential for adipogenesis and neonatal gluconeogenesis, as shown by the C/EBPalpha knockout mouse. C/EBPbeta binds to several sequences of the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter with high affinity, and C/EBPbeta protein is increased 200% in the livers of streptozotocin-diabetic mice, concurrent with increased PEPCK mRNA. To elucidate the role of C/EBPbeta in the control of gluconeogenesis during diabetes, we studied the levels of plasma metabolites and hormones related to energy metabolism during diabetes in adult mice heterozygous and homozygous for a null mutation of the gene for C/EBPbeta. We also examined the expression of PEPCK and glucose 6-phosphatase mRNAs and regulation of blood glucose, including the contribution of gluconeogenesis to blood glucose in c/ebpbeta-/- mice. C/EBPbeta was not essential to basal PEPCK mRNA levels. However, C/EBPbeta deletion affected streptozotocin-diabetic response by: (a) delaying hyperglycemia, (b) preventing the increase of plasma free fatty acids, (c) limiting the full induction of PEPCK and glucose 6-phosphatase genes, and (d) preventing the increase in gluconeogenesis rate. Gel supershifts of transcription factor C/EBPalpha, bound to CRE, P3I, and AF-2 sites of the PEPCK promoter, was not increased in diabetic c/ebpbeta-/- mouse liver nuclei, suggesting that C/EBPalpha does not substitute for C/EBPbeta in the diabetic response of liver gene transcription. These results link C/EBPbeta to the metabolic and gene regulatory responses to diabetes and implicate C/EBPbeta as an essential factor underlying glucocorticoid-dependent activation of PEPCK gene transcription in the intact animal.

PPARgamma activators down-regulate the expression of PPARgamma in 3T3-L1 adipocytes

Transcriptional activation of PPARgamma by the anti-diabetic compound troglitazone enhances the rate of 3T3-L1 adipocyte differentiation. In this study, we examined the effects of troglitazone, a specific PPARgamma ligand, on the expression of PPARgamma during and after 3T3-L1 adipocyte differentiation. Troglitazone treatment caused a significant decrease in PPARgamma proteins and DNA binding activity. This reduction was associated with a similar decrease in transcription of PPARgamma mRNA. These data suggest that in 3T3-L1 cells, the expression of PPARgamma is auto-regulated.

Expression of the dominant-negative regulator Id4 is induced during adipocyte differentiation

Expression of the Id4 gene was investigated during differentiation of 3T3-L1 preadipocytes into mature adipocytes. Id4 is a member of a family encoding non-DNA binding helix-loop-helix proteins proposed to inhibit the activity of basic HLH (bHLH) proteins important in many developmental processes. We show here that Id4 expression is low in confluent preadipocytes and rapidly induced by treatment with the combination of hormones which causes differentiation into mature adipocytes. Id4 expression is also induced by treatment with individual hormones, especially dexamathasone. Id4 mRNA can be detected in mouse and human adipose tissue. Genes encoding E-proteins (bHLH proteins known to interact with and be regulated by Id proteins) are expressed and regulated during differentiation in 3T3-L1 cells. These data suggest that the Id4 transcriptional regulator is playing a role in adipose cell differentiation and suggest that DNA-binding HLH proteins may also be important in regulation of differentiation of these cells.

Cross-regulation of C/EBP alpha and PPAR gamma controls the transcriptional pathway of adipogenesis and insulin sensitivity

Mice deficient in C/EBP alpha have defective development of adipose tissue, but the precise role of C/EBP alpha has not been defined. Fibroblasts from C/EBP alpha(-/-) mice undergo adipose differentiation through expression and activation of PPAR gamma, though several clear defects are apparent. C/EBP alpha-deficient adipocytes accumulates less lipid, and they do not induce endogenous PPAR gamma, indicating that cross-regulation between C/EBP alpha and PPAR gamma is important in maintaining the differentiated state. The cells also show a complete absence of insulin-stimulated glucose transport, secondary to reduced gene expression and tyrosine phosphorylation for the insulin receptor and IRS-1. These results define multiple roles for C/EBP alpha in adipogenesis and show that cross-regulation between PPAR gamma and C/EBP alpha is a key component of the transcriptional control of this cell lineage.

Reciprocal regulation of gadd45 by C/EBP alpha and c-Myc

The CCAAT/enhancer-binding protein-alpha (C/EBP alpha) drives the differentiation of murine 3T3-L1 cells to adipocytes through transcriptional activation of phenotype-associated genes via proximal promoter elements. In addition, C/EBP alpha suppresses mitotic growth. We report here that C/EBP alpha directly regulates gadd45 through a C/EBP-binding site in the proximal promoter. A 3 basepair substitution, directed at the most conserved residues of the sequence, reduced C/EBP alpha-mediated transactivation and impaired binding of C/EBP alpha in adipocyte nuclear extracts. We also found that c-Myc antagonized C/EBP alpha-mediated transactivation of gadd45. Analysis of systematically altered forms of C/EBP alpha revealed that c-Myc antagonism targeted the antimitotic, transcriptional activation domain of C/EBP alpha. In addition, we localized the regulatory sequences in the gadd45 promoter that are required for c-Myc antagonism of C/EBP alpha transactivation. Our findings reveal that C/EBP alpha coordinates cellular differentiation and mitotic growth arrest through direct, coordinate regulation of phenotype-associated and growth-arrest-associated genes. In addition, our findings reveal that the reciprocal relation between C/EBP alpha and c-Myc in 3T3-L1 cells includes antagonistic transcriptional control of a growth-arrest-associated gene.

Conjugated linoleic acid inhibits proliferation but stimulates lipid filling of murine 3T3-L1 preadipocytes

This study documented the effects of conjugated linoleic acid (CLA) on the proliferation and differentiation of 3T3-L1 preadipocytes. During proliferation, preadipocytes were cultured in Dulbecco's modified Eagle's medium (DMEM), 100 g/L fetal bovine serum (FBS), 0. 584 g/L L-glutamine and 0 (control), 0.5, 1.0, 5.0 or 10.0 mg/L CLA. Proliferation of 3T3-L1 preadipocytes was measured directly by cell counting and indirectly by radiolabeled thymidine incorporation into DNA at 96 h postinoculation. Conjugated linoleic acid was not cytotoxic during proliferation or differentiation. The 0.5, 1.0, 5.0 or 10.0 mg/L CLA treatments inhibited proliferation by 8, 12, 31 and 36%, respectively (all P < 0.05). Treatment with 10 mg/L CLA or 10 mg/L linoleic acid (cis-9,12) reduced the incorporation of 3H-thymidine into DNA by 56 and 35%, respectively, suggesting that some portion of the effect of CLA on preadipocyte proliferation was nonspecific. After the initiation of differentiation, preadipocytes were cultured in DMEM, 100 g/L FBS, 0.584 g/L L-glutamine, 1.7 micromol/L insulin and 0 (control), 0.5, 1.0, 5.0 or 10.0 mg/L CLA. Radiolabeled glucose incorporation into cellular lipids was increased from 7.4 to 11.1, 11.1, 17.4 and 22.5 nmol/(h.10(6 )cells) (all P < 0.05) by 0.5, 1.0, 5.0 and 10.0 mg/L CLA, respectively. A media concentration of 10 mg/L CLA increased total cellular CLA (from 0 to 0.16 +/- 0.01 micromol/10(6 )cells), palmitic acid (from 0.47 to 1.10 +/- 0.03 micromol/10(6 )cells) and palmitoleic acid (from 0.24 to 0.81 +/- 0.03 micromol/10(6 )cells) (means +/- pooled SEM; all P < 0.05). Conjugated linoleic acid had no effect on arachidonic acid content, but decreased its proportion (g arachidonic acid/100 g total fatty acids) by >50% (P < 0.05). These data indicate that CLA inhibited proliferation and promoted de novo lipogenesis and lipid filling in 3T3-L1 preadipocytes, suggesting that CLA may reduce overall fat accumulation in growing animals by inhibiting stromal vascular preadipocyte hyperplasia.

Mechanism of minimally modified LDL-mediated induction of serum amyloid A gene in monocyte/macrophage cells

Minimally modified low-density lipoprotein (MM-LDL) is regarded as a major risk factor for the development of atherosclerosis. In this report, we show that this lipoprotein complex can induce expression of an inflammatory protein, serum amyloid A (SAA), in monocyte/macrophage cells, a key cell type implicated in the pathogenesis of atherosclerosis. By promoter function analysis and site-directed mutagenesis, we have located promoter regions responsive to MM-LDL action. Using electrophoretic mobility shift, antibody ablation/supershift, and Western blot assays, we showed that induction of SAA by MM-LDL is mediated via activation of SAS binding factor (SAF) and C/EBP transcription factors. We further show that tamoxifen, a downregulator of CD36, one of the major scavenger receptors which binds MM-LDL, can inhibit MM-LDL-mediated SAA induction in THP-1 cells. This finding suggests that CD36 participates in the manifestation of the inflammatory effects of MM-LDL. Our experiments provide the first evidence for transcription factor activation by MM-LDL.

A potential role for the nuclear factor of activated T cells family of transcriptional regulatory proteins in adipogenesis

NFAT (nuclear factor of activated T cells) is a family of transcription factors implicated in the control of cytokine and early immune response gene expression. Recent studies have pointed to a role for NFAT proteins in gene regulation outside of the immune system. Herein we demonstrate that NFAT proteins are present in 3T3-L1 adipocytes and, upon fat cell differentiation, bind to and transactivate the promoter of the adipocyte-specific gene aP2. Further, fat cell differentiation is inhibited by cyclosporin A, a drug shown to prevent NFAT nuclear localization and hence function. Thus, these data suggest a role for NFAT transcription factors in the regulation of the aP2 gene and in the process of adipocyte differentiation.

Expression of CCAAT/enhancer binding proteins during porcine preadipocyte differentiation

The expression of three CCAAT/enhancer-binding proteins (C/EBPs) was examined with immunocytochemistry and Western blot analysis during preadipocyte differentiation in porcine stromal vascular (S-V) cell cultures. Regardless of treatment and time in culture, immunoreactivity for all three C/EBP isoforms was restricted to cell nuclei. At day 1, 50 +/- 6% of S-V cells were C/EBPdelta positive, whereas 13 +/- 3 and 11.7 +/- 3% of S-V cells were AD-3 and C/EBPalpha positive, respectively. After 3 days of seeding in fetal bovine serum (FBS) and dexamethasone (DEX), C/EBPdelta; AD-3, and C/EBPalpha-positive cells increased to 67 +/- 5, 42 +/- 4, and 32 +/- 3%, respectively. Double staining clearly showed that most of the C/EBPalpha reactive cells had not accumulated appreciable lipid after 3 days of FBS + DEX. Following 3 days of insulin treatment, the percentage of C/EBPdelta cells was 50 +/- 6, whereas the percentage of AD-3- and C/EBPalpha-positive cells was 41 +/- 4 and 31 +/- 3, respectively. After insulin treatment all fat cells were AD-3, C/EBPalpha, and C/EBPdelta positive. Double staining demonstrated that fat cells were C/EBPdelta reactive throughout the culture period. Western blotting showed changes in C/EBP isoform expression that were consistent with the immunocytochemical results. We conclude that C/EBPalpha is a terminal differentiation marker which is expressed later than AD-3 but further studies are needed to determine the relationship between C/EBPdelta and adipogenesis in porcine S-V cultures.

The C/EBP family of transcription factors in the liver and other organs

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are pivotal regulators of liver functions such as nutrient metabolism and its control by hormones, acute-phase response and liver regeneration. Recent progress in clarification of regulatory mechanisms for the C/EBP family members gives insight into understanding the liver functions at the molecular level.

Transcriptional control of the pref-1 gene in 3T3-L1 adipocyte differentiation. Sequence requirement for differentiation-dependent suppression

Preadipocyte factor-1 (Pref-1) is a transmembrane epidermal growth factor-like domain-containing protein highly expressed in 3T3-L1 preadipocytes, but is undetectable in mature fat cells; this down-regulation is required for adipocyte differentiation. We show here that pref-1 transcription is markedly suppressed during adipose conversion and results in decreased Pref-1 RNA levels. Using 3T3-L1 cells stably transfected with Pref-1 5'-deletion constructs truncated at -6000, -2100, -1300, -692, -300, -235, -193, -183, -170, -93, and -45 base pairs, we determined that the -183 to -170 region is responsible for the suppression of the pref-1 gene during adipogenesis. This is distinct from the -93 to -45 sequence important for pref-1 promoter activity in preadipocytes. The placement of a 40-base pair -193 to -154 pref-1 sequence containing the putative SAD (suppression in adipocyte differentiation) element upstream of the SV40 promoter decreased promoter activity by 85% upon adipocyte differentiation, compared with 40% observed with the SV40 promoter alone. The SAD element is therefore sufficient for adipocyte differentiation-dependent down-regulation of a heterologous promoter. A DNA-protein complex was observed when the -193 to -174 sequence was used with 3T3-L1 nuclear extracts in gel mobility shift assays. Competition with oligonucleotides harboring base substitution mutations identified a core sequence of -183AAAGA-179 as crucial for DNA-protein complex formation. UV cross-linking predicts that an approximately 63-kDa protein specifically binds the SAD element.

Down-regulation of CCAAT/enhancer binding proteins alpha, beta and delta in adipose tissue by intracerebroventricular leptin in rats

In our previous report, intracerebroventricular (i.c.v.) administration of leptin caused fat depletion by an induced adipocyte apoptosis in addition to influencing lipid metabolism. To uncover the biochemical mechanisms that mediate this response, the present study was designed to determine whether CCAAT/enhancer binding proteins (C/EBP)alpha, -beta and -delta play a role in the leptin-induced fat depletion. Expressions of C/EBPalpha, -beta and -delta in epididymal fat tissues were examined by Western immunoblot and in situ immunocytochemical analysis after 5 days of i.c.v. treatment. Young and old rats (3 and 8 months old) were treated with or without 5 micrograms/day leptin. The expression of C/EBPalpha, -beta and -delta was decreased by i.c.v. leptin treatment in young rats as compared with controls (P<0.05). However, leptin did not influence the expression of C/EBPalpha, -beta and -delta in adipose tissues of 8-month-old rats. The basal level of expression of C/EBPbeta was greater in 8-month-old rats than in 3-month-old rats, (P<0.05) whereas the basal expression of C/EBPalpha and -delta was not different between age groups. These results were confirmed by in situ immunocytochemical analysis. The present study suggests that leptin-induced down-regulation of C/EBPalpha, -beta and -delta might influence adipocyte differentiation and growth in a number of ways.

Characterization of novel parent-specific epigenetic modifications upstream of the imprinted mouse H19 gene

Genomic imprinting results in parent-specific monoallelic expression of a small number of genes in mammals. The identity of imprints is unknown, but much evidence points to a role for DNA methylation. The maternal alleles of the imprinted H19 gene are active and hypomethylated; the paternal alleles are inactive and hypermethylated. Roles for other epigenetic modifications are suggested by allele-specific differences in nuclease hypersensitivity at particular sites. To further analyze the possible epigenetic mechanisms determining monoallelic expression of H19, we have conducted in vivo dimethylsulfate and DNase I footprinting of regions upstream of the coding sequence in parthenogenetic and androgenetic embryonic stem cells. These cells carry only maternally and paternally derived alleles, respectively. We observed the presence of maternal-allele-specific dimethylsulfate and DNase I footprints at the promoter indicative of protein-DNA interactions at a CCAAT box and at binding sites for transcription factors Sp1 and AP-2. Also, at the boundary of a region further upstream for which existent differential methylation has been suggested to constitute an imprint, we observed a number of strand-specific dimethylsulfate reactivity differences specific to the maternal allele, along with an unusual chromatin structure in that both strands of maternally derived DNA were strongly hypersensitive to DNase I cutting over a distance of 100 nucleotides. We therefore reveal the existence of novel parent-specific epigenetic modifications, which in addition to DNA methylation, could constitute imprints or maintain monoallelic expression of H19.

Leptin and its receptors: regulators of whole-body energy homeostasis

Leptin is the adipocyte-specific product of the ob gene. Expression of leptin in fully fed animals reflects adipocyte size and body-fat mass. Leptin signals the status of body energy stores to the brain, where signals emanate to regulate food intake and whole-body energy expenditure. The leptin gene was identified in the leptin-deficient, obese ob/ob mouse by positional cloning techniques. Recently, leptin has been cloned in domestic species including pigs, cattle, and chickens. The leptin receptor has at least five splice variants; the long form of the receptor is primarily expressed in the hypothalamus and is thought to be the predominant signaling isoform. Leptin receptors are members of the cytokine family of receptors and signal via janus-activated kinases (JAK)/signal transducers and activators of transcription (STAT) and mitogen-activated protein kinase (MAPK) pathways. Mutations in the leptin or leptin receptor genes results in morbid obesity, infertility, and insulin resistance in rodents and humans. Leptin regulates food intake and energy expenditure via central and peripheral mechanisms. Leptin receptors are expressed in most tissues, and in vitro evidence suggests that leptin may have direct effects on some tissues such as adipose tissue, the adrenal cortex, and the pancreatic beta-cell. Leptin is thought to influence whole-body glucose homeostasis and insulin action. Studies are underway to determine the role that leptin plays in the biology of domestic animals.

Posttranscriptional control of adipocyte differentiation through activation of phosphoinositide 3-kinase

Differentiation of adipocytes is an important aspect of energy homeostasis. Although the transcriptional regulation of adipocyte differentiation is relatively well characterized, the subsequent molecular events remain unclear. The activity of phosphoinositide (PI) 3-kinase precipitated with antibodies to phosphotyrosine has now been shown to increase transiently during adipocyte differentiation of 3T3-F442A and of 3T3-L1 cells. PI 3-kinase activity precipitated with antibodies to insulin receptor substrate 1 (IRS1) and association of subunits of PI 3-kinase with IRS1 were also increased at this stage of differentiation, suggesting that IRS1 contributes to PI 3-kinase activation. Inhibition of the activation of PI 3-kinase by expression of dominant negative mutant subunits of the enzyme prevented adipogenesis, as assessed by lipid accumulation and expression of key adipocyte proteins such as GLUT4, adipsin, and aP2, suggesting that PI 3-kinase activation is essential for adipocyte differentiation. However, these mutant proteins did not affect either the expression of the transcription factor PPARgamma at the mRNA or protein level or the increase in the abundance of mRNAs encoding the adipocyte marker proteins. These results demonstrate that adipocyte differentiation is regulated at the posttranscriptional level and that activation of PI 3-kinase is required for this regulation.

Cloning and chromosomal location of the murine keratinocyte lipid-binding protein gene

The keratinocyte lipid-binding protein (KLBP) is a member of a large multigene family of intracellular fatty-acid-binding proteins. It is expressed in skin and tongue epithelia, adipose, lung and mammary tissue and has been found upregulated in several skin cell carcinomas and papillomas (Krieg et al., 1993). In order to study the regulation of KLBP expression, the murine gene has been cloned. Southern analysis using an exon 2 specific cDNA probe indicated the presence of multiple copies of the gene in the murine genome. Based on the highly conserved structure of the fatty-acid-binding protein genes, the third intron of the KLBP gene was PCR-amplified utilizing murine genomic DNA. Southern analysis with the intron 3 probe identified one unique gene in the murine genome. A full-length genomic clone of KLBP was obtained from a P1 library, and the structural gene was sequenced. Similar to the other FABP genes, the functional KLBP gene contains four exons separated by three introns and maintains the conservation of size and placement of each exon. A functional minimal promoter was demonstrated by transient transfections of 5' upstream KLBP-luciferase reporter constructs into line 308 keratinocyte cells as well as in primary adipocytes. RT-PCR on primary adipocyte RNA demonstrated expression of this KLBP gene by amplification of intron 3 from the primary transcript. Fluorescence in-situ hybridization identified the murine KLBP gene as the fourth FABP gene on chromosome 3, along with myelin P2, ALBP, and intestinal FABP. These studies provide a framework for analysis of KLBP expression in normal and pathophysiological conditions.

Life without white fat: a transgenic mouse

We have generated a transgenic mouse with no white fat tissue throughout life. These mice express a dominant-negative protein, termed A-ZIP/F, under the control of the adipose-specific aP2 enhancer/promoter. This protein prevents the DNA binding of B-ZIP transcription factors of both the C/EBP and Jun families. The transgenic mice (named A-ZIP/F-1) have no white adipose tissue and dramatically reduced amounts of brown adipose tissue, which is inactive. They are initially growth delayed, but by week 12, surpass their littermates in weight. The mice eat, drink, and urinate copiously, have decreased fecundity, premature death, and frequently die after anesthesia. The physiological consequences of having no white fat tissue are profound. The liver is engorged with lipid, and the internal organs are enlarged. The mice are diabetic, with reduced leptin (20-fold) and elevated serum glucose (3-fold), insulin (50- to 400-fold), free fatty acids (2-fold), and triglycerides (3- to 5-fold). The A-ZIP/F-1 phenotype suggests a mouse model for the human disease lipoatrophic diabetes (Seip-Berardinelli syndrome), indicating that the lack of fat can cause diabetes. The myriad of consequences of having no fat throughout development can be addressed with this model.

Analysis of differentiation-induced expression mechanisms of thyrotropin receptor gene in adipocytes

Rat adipose tissue, as well as differentiated 3T3-L1 cells, has been shown to express TSH receptor (TSHR) mRNA in amounts approaching those in the thyroid. We investigated the molecular mechanisms of TSHR gene expression in adipose cells. Primer extension and cloned cDNA sequences showed that transcription of the TSHR gene in rat adipose tissue was from multiple start sites clustered between -89 to -68 bp and almost identical to those in FRTL-5 thyroid cells. By transient expression analysis, we localized, between -146 and -90 bp, a positive regulatory element, the activity of which was markedly increased after the differentiation of 3T3-L1 cells. Deoxyribonuclease I protection showed that nuclear extracts from differentiated 3T3-L1 cells strongly protected two sequences, from -146 to -127 bp, including a cAMP response element-like sequence and from -112 to -106 bp containing a putative Ets-binding sequence. In differentiated 3T3-L1 cells, disruption or deletion of either sequence was found to result in the loss of enhancer activity, suggesting both elements may synergistically activate the TSHR promoter. Electrophoretic mobility shift analysis revealed the induction of new protein/DNA complexes formed either with the cAMP response element-like site or with putative Ets elements after the differentiation into adipocytes. In contrast, nuclear proteins, whose binding to DNA was diminished after the differentiation of 3T3-L1 cells, were found to interact with the site contiguous to the 5'-end of the putative Ets-binding sequence. Mutations of this binding site, which reduced the protein/DNA complex formation, increased TSHR promoter activity in undifferentiated cells. These observations suggested that differentiation-induced diminution of suppressor interactions may allow the enhancers to synergistically activate the transcription of TSHR gene in adipocytes.

Involvement of the retinoblastoma protein in brown and white adipocyte cell differentiation: functional and physical association with the adipogenic transcription factor C/EBPalpha

We investigated the expression of the retinoblastoma protein (pRB) in adipocytes and its possible interaction with the adipogenic transcription factor CCAAT/enhancer-binding protein alpha (C/EBPalpha) in controlling the acquisition of the terminally differentiated adipocyte phenotype. The pRB was expressed (as measured by immunoblotting and/or immunofluorescence) in mice brown and white adipose tissue and in cultured adipocytes that showed lipid accumulation and expressed specific differentiation markers such as aP2 (measured using a specific cDNA probe) and in the case of brown adipocytes UCP-1 (measured using specific antibodies), but was undetectable in proliferative undifferentiated preadipocytes. Transient transfection experiments revealed a functional interaction between pRB and C/EBPalpha affecting transcription from the ucp-1 gene promoter. Thus, in immortalized brown adipocytes, co-transfection of both a C/EBPalpha and a pRB expression vectors maximally enhanced the expression of reporter chloramphenicol acetyltransferase driven by the ucp-1 promoter. Interestingly, C/EBPalpha inhibited reporter gene expression in CHO cells in an effect that was also potentiated in the presence of pRB. A positive effect of pRB on transcription from the ucp-1 promoter could be detected in C/EBPalpha-/-fibroblasts only after forced to overexpress C/EBPalpha, suggesting that the effect of pRB is dependent on its interaction with C/EBPalpha. We also found evidence that pRB and C/EBPalpha can directly bind to each other in vitro. Our results show that the expression of pRB is restricted to differentiated adipocytes, and provide evidence of a physical and functional interaction between pRB and C/EBPalpha that affects the transcriptional activity of the later on a brown adipocyte-specific gene.

Upregulation of interleukin 6 and granulocyte colony-stimulating factor receptors by transcription factor CCAAT enhancer binding protein alpha (C/EBP alpha) is critical for granulopoiesis

Cytokines stimulate granulopoiesis through signaling via receptors whose expression is controlled by lineage-specific transcription factors. Previously, we demonstrated that granulocyte colony-stimulating factor (G-CSF) receptor mRNA was undetectable and granulocyte maturation blocked in CCAAT enhancer binding protein alpha (C/EBPalpha)-deficient mice. This phenotype is distinct from that of G-CSF receptor-/- mice, suggesting that other genes are likely to be adversely affected by loss of C/EBPalpha. Here we demonstrate loss of interleukin 6 (IL-6) receptor and IL-6-responsive colony-forming units (CFU-IL6) in C/EBPalpha-/- mice. The observed failure of granulopoiesis could be rescued by the addition of soluble IL-6 receptor and IL-6 or by retroviral transduction of G-CSF receptors, demonstrating that loss of both of these receptors contributes to the absolute block in granulocyte maturation observed in C/EBPalpha-deficient hematopoietic cells. The results of these and other studies suggest that additional C/EBPalpha target genes, possibly other cytokine receptors, are also important for the block in granulocyte differentiation observed in vivo in C/EBPalpha-deficient mice.

IRE-ABP (insulin response element-A binding protein), an SRY-like protein, inhibits C/EBPalpha (CCAAT/enhancer-binding protein alpha)-stimulated expression of the sex-specific cytochrome P450 2C12 gene

In primary hepatocytes, overexpression of an insulin response element-A binding protein (IRE-ABP), a member of the SRY family of high-mobility group (HMG) proteins, inhibits CCAAT/enhancer-binding protein alpha (C/EBPalpha)-mediated activation of the female-specific cytochrome P450 2C12 (CYP2C12) gene, but not the male-specific cytochrome P450 2C11 (CYP2C11) gene. IRE-ABP and C/EBPalpha have overlapping specificity for the C/EBPalpha target site in the CYP2C12 promoter and compete for binding to CYP2C12 DNA in vitro. In contrast, IRE-ABP and C/EBPalpha bind distinct sequences in the CYP2C11 promoter. A single amino acid substitution in the HMG domain of IRE-ABP impairs its ability to bind DNA and to inhibit the effect of C/EBPalpha on CYP2C12 gene expression. Therefore, the ability of IRE-ABP to inhibit C/EBPalpha-stimulated CYP2C12 gene expression requires a functional DNA-binding domain. Taken together, our findings suggest that SRY-like proteins can bind to a subset of sequences recognized by the C/EBP family of DNA-binding proteins and modulate gene transcription in a context-specific manner.

Dehydroepiandrosterone reduces proliferation and differentiation of 3T3-L1 preadipocytes

The purpose of these studies was to determine whether the antiobesity actions of dehydroepiandrosterone (DHEA) and DHEA-sulfate (DHEAS) observed in vivo are due to an influence on proliferation and/or differentiation in monolayer cultures of 3T3-L1 preadipocytes. For the proliferation study (Exp. 1), cells were grown in plating medium containing DHEA at 0, 5, 25, 50, or 100 microM for 1-4 d. DHEAS was added at the 100 microM level only. For the differentiation study (Exp. 2), cultures were grown in plating medium containing DHEA at 0, 5, 30, 60, 120, or 240 microM for 2-6 d. DHEAS was added at the 240 microM level only. In Exp. 3, the effect of DHEA on mature adipocytes was determined by exposing adipocytes grown in plating medium to DHEA at 0, 75, 125, and 250 microM for 1-4 d. In Exp. 1, preadipocyte proliferation decreased as the level of DHEA increased in cultures of 3T3-L1 cells. DHEAS had no effect on preadipocyte proliferation. The antiproliferative effect of DHEA was partially reversed by the addition of 1 microM mevalonic acid to proliferating cultures containing 25 microM DHEA. In Exp. 2, preadipocyte differentiation decreased as the level of DHEA in the cultures increased. In contrast, neither DHEAS nor mevalonic acid treatment influenced preadipocyte differentiation decreased as the level and duration of DHEA treatment increased in cultures of mature adipocytes. These data support the hypothesis that DHEA, but not DHEAS, is the active form of the steroid that attenuates obesity via altering preadipocyte proliferation and differentiation. The addition of 1 microM mevalonic acid to cultures of 3T3-L1 preadipocytes partially reversed DHEA's antiproliferative effects.

Understanding adipocyte differentiation

The adipocyte plays a critical role in energy balance. Adipose tissue growth involves an increase in adipocyte size and the formation of new adipocytes from precursor cells. For the last 20 years, the cellular and molecular mechanisms of adipocyte differentiation have been extensively studied using preadipocyte culture systems. Committed preadipocytes undergo growth arrest and subsequent terminal differentiation into adipocytes. This is accompanied by a dramatic increase in expression of adipocyte genes including adipocyte fatty acid binding protein and lipid-metabolizing enzymes. Characterization of regulatory regions of adipose-specific genes has led to the identification of the transcription factors peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein (C/EBP), which play a key role in the complex transcriptional cascade during adipocyte differentiation. Growth and differentiation of preadipocytes is controlled by communication between individual cells or between cells and the extracellular environment. Various hormones and growth factors that affect adipocyte differentiation in a positive or negative manner have been identified. In addition, components involved in cell-cell or cell-matrix interactions such as preadipocyte factor-1 and extracellular matrix proteins are also pivotal in regulating the differentiation process. Identification of these molecules has yielded clues to the biochemical pathways that ultimately result in transcriptional activation via PPAR-gamma and C/EBP. Studies on the regulation of the these transcription factors and the mode of action of various agents that influence adipocyte differentiation will reveal the physiological and pathophysiological mechanisms underlying adipose tissue development.

Multiple functional domains of AML1: PU.1 and C/EBPalpha synergize with different regions of AML1

Control elements of many genes are regulated by multiple activators working in concert to confer the maximal level of expression, but the mechanism of such synergy is not completely understood. The promoter of the human macrophage colony-stimulating factor (M-CSF) receptor presents an excellent model with which we can study synergistic, tissue-specific activation for two reasons. First, myeloid-specific expression of the M-CSF receptor is regulated transcriptionally by three factors which are crucial for normal hematopoiesis: PU.1, AML1, and C/EBPalpha. Second, these proteins interact in such a way as to demonstrate at least two examples of synergistic activation. We have shown that AML1 and C/EBPalpha activate the M-CSF receptor promoter in a synergistic manner. As we report here, AML1 also synergizes, and interacts physically, with PU. 1. Detailed analysis of the physical and functional interaction of AML1 with PU.1 and C/EBPalpha has revealed that the proteins contact one another through their DNA-binding domains and that AML1 exhibits cooperative DNA binding with C/EBPalpha but not with PU.1. This difference in DNA-binding abilities may explain, in part, the differences observed in synergistic activation. Furthermore, the activation domains of all three factors are required for synergistic activation, and the region of AML1 required for synergy with PU.1 is distinct from that required for synergy with C/EBPalpha. These observations present the possibility that synergistic activation is mediated by secondary proteins contacted through the activation domains of AML1, C/EBPalpha, and PU.1.

Human translocation liposarcoma-CCAAT/enhancer binding protein (C/EBP) homologous protein (TLS-CHOP) oncoprotein prevents adipocyte differentiation by directly interfering with C/EBPbeta function

Human translocation liposarcoma (TLS)-CCAAT/enhancer binding protein (C/EBP) homologous protein (CHOP) is a fusion oncoprotein found specifically in a malignant tumor of adipose tissue and results from a t(12;16) translocation that fuses the amino-terminal part of TLS to the entire coding region of CHOP. Being that CHOP is a member of the C/EBP transcription factor family, proteins that comprise part of the adipocyte differentiation machinery, we examined whether TLS-CHOP blocked adipocyte differentiation by directly interfering with C/EBP function. Using a single-step retroviral infection protocol, either wild-type or mutant TLS-CHOP were co-expressed along with C/EBPbeta in naïve NIH3T3 cells, and their ability to inhibit C/EBPbeta-driven adipogenesis was determined. TLS-CHOP was extremely effective at blocking adipocyte differentiation when expressed at a level comparable to that observed in human myxoid liposarcoma. This effect of TLS-CHOP required a functional leucine zipper domain and correlated with its ability to heterodimerize with C/EBPbeta and inhibit C/EBPbeta DNA binding and transactivation activity in situ. In contrast, the TLS-CHOP basic region was dispensable, making it unlikely that the inhibitory effect of TLS-CHOP is attributable to unscheduled gene expression resulting from TLS-CHOP's putative transactivation activity. Another adipogenic transcription factor, PPARgamma2, was able to rescue TLS-CHOP-inhibited cells, indicating that TLS-CHOP interferes primarily with C/EBPbeta-driven adipogenesis and not with other requisite events of the adipocyte differentiation program. Together, the results demonstrate that TLS-CHOP blocks adipocyte differentiation by directly preventing C/EBPbeta from binding to and transactivating its target genes. Moreover, they provide strong support for the thesis that a blockade to normal differentiation is an important aspect of the cancer process.

Nuclear factor 1 is essential for the expression of stearoyl-CoA desaturase 1 gene during preadipocyte differentiation

Stearoyl CoA desaturase gene 1 (SCD1) is a delta 9 desaturase that is transcriptionally activated during the differentiation of 3T3-L1 preadipocytes into adipocytes. We have demonstrated that a SCD1/BP region in SCD1 proximal promoter (-114 to -86 bp) is essential for the transcriptional activation of this gene during differentiation. Mutation in this region abolished the basal activity of the proximal promoter of SCD1, and also failed to induce transcription in response to the differentiation cocktail in transfected cells. The SCD1/BP region contains a TGGCA sequence at -90 bp from the transcription start site. Using competitor oligonucleotides and nuclear factor 1 (NF1)-specific antibodies in gel shift assays, we have shown that in preadipocytes, a NF1 protein binds to this TGGCA sequence. On MDI-induced differentiation of preadipocyte into adipocyte, an additional DNA-protein complex appeared. The appearance of a new NF1 complex is related to the differentiation-specific transcriptional activation of the SCD1 gene. This is the first report to show a differentiation-related change in NF1 protein binding during preadipocyte differentiation.

Characterization of CCAAT/enhancer-binding protein alpha as a cyclic AMP-responsive nuclear regulator

The alpha isoform of CCAAT/enhancer-binding protein (C/EBPalpha) is a transcription factor that regulates expression of genes linked to adipose differentiation and hepatic nutrient metabolism. Recently, our laboratory has characterized a role for C/EBPalpha in mediating hormonal responsiveness. For example, the cAMP responsiveness of the phosphoenolpyruvate carboxykinase gene promoter in liver requires synergism among the cAMP response element-binding protein (CREB), C/EBPalpha, and activator protein-1. In the present study, we show that C/EBPalpha can functionally substitute for CREB in this cAMP response unit, i.e. cAMP responsiveness can occur in the absence of CREB. This observation is physiologically relevant since both CREB and C/EBPalpha have been shown to bind with high affinity to the cAMP response element in this particular promoter. Structure/function analysis of C/EBPalpha identified specific mutations that differentially affected its constitutive and protein kinase A-inducible activities. This finding suggests that the mechanism whereby C/EBPalpha mediates constitutive transactivation is distinct from that whereby it mediates cAMP responsiveness. These data support the hypothesis that C/EBPalpha plays a critical role in metabolism, in part by participating in the hormonal regulation of expression of metabolically important genes.

Expression of CCAAT/enhancer binding proteins (C/EBP) is associated with squamous differentiation in epidermis and isolated primary keratinocytes and is altered in skin neoplasms

The epidermis is a stratified squamous epithelium composed primarily of keratinocytes that undergo sequential changes in gene expression during differentiation. CCAAT/enhancer binding proteins (C/EBP) are members of the bZIP family of DNA binding proteins/transcription factors. Northern analysis demonstrated that C/EBPalpha, C/EBPbeta, and C/EBPdelta mRNA are expressed in mouse epidermis and their mRNA levels were generally greater than those observed in other tissues known to express high levels of C/EBP. Western analysis of isolated epidermal cell nuclei demonstrated the presence of a 42 and 30 kDa C/EBPalpha protein and 35 kDa C/EBPbeta protein. Immunohistochemical localization of C/EBPalpha and C/EBPbeta in intact interfollicular epidermis revealed that C/EBPbeta expression is exclusive to the nuclei of a three-cell cluster of suprabasal keratinocytes that is morphologically consistent with the central column of the epidermal proliferative unit, and that C/EBPalpha is expressed in the nuclei and cytoplasm of suprabasal keratinocytes and weakly expressed in a perinuclear manner in some basal keratinocytes. In squamous cell carcinomas the expression of C/EBPalpha and C/EBPbeta was greatly diminished as both the intensity of nuclear staining and the number of cells expressing C/EBPalpha and C/EBPbeta were reduced. In isolated primary mouse keratinocytes, calcium-induced differentiation was accompanied by specific temporal changes in the expression of C/EBPalpha, C/EBPbeta, and C/EBPdelta mRNA and C/EBPalpha and C/EBPbeta protein. These results implicate a role for the C/EBP family in the regulation of genes involved in or specifically expressed during the process of squamous differentiation in epidermis.

Regulation of fat synthesis and adipose differentiation

Adipocytes have highly specialized function of accumulating fat as stored energy that can be used during periods of food deprivation. The process of fat synthesis and development of adipose tissue are under hormonal and nutritional control. This review first describes transcription of the two critical enzymes involved in fat synthesis, fatty acid synthase and mitochondrial glycerol-3-phosphate acyltransferase, is decreased to an undetectable level during fasting. Food intake, especially a high carbohydrate, fat-free diet, subsequent to fasting causes dramatic increase in transcription of these genes. Insulin secretion is increased during feeding, having a positive effect, whereas cAMP, which mediates the effect of glucagon which increases during fasting, has a negative effect on transcription of these genes. Using adipocytes in culture and in transgenic mice that express liciferase driven by the fatty acid synthase promoter, cis-acting and trans-acting factors that may mediate the transcriptional regulation were examined. Upstream stimulatory factors (USFs) that bind to -65 E-box are required for insulin-mediated transcriptional activation of the fatty acid synthase gene. This review next describes how pref-1 is a novel inhibitor of adipose differentiation and is a plasma membrane protein containing six EGF-repeats in the extracellular domain. Pref-1 is highly expressed in 3T3-L1 preadipocytes, but is not detectable in mature fat cells. Down regulation of pref-1 is required for adipose differentiation, and constitutive expression of pref-1 inhibits adipogenesis. Moreover, the ectodomain of pref-1 is cleaved to generate a biologically active 50 kDa soluble form. There are four major forms of membrane pref-1 resulting from alternate splicing, but two of the forms with a larger deletion do not produce biologically active soluble form, indicating that alternate splicing determines the range of action, juxtacrine or paracrine, of the pref-1.

Suppression of C/EBPalpha and induction of C/EBPbeta by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mouse adipose tissue and liver

We examined the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on two transcription factors, CAAT/enhancer binding protein-alpha (C/EBPalpha) and beta (C/EBPbeta), involved in the coordination of gene expression in adipose and liver. A single dose of TCDD (100 microg/kg) to male C57BL mice resulted in a time- and dose-dependent decrease in the level of C/EBPalpha mRNA in adipose tissue and liver, and a reciprocal increase in C/EBPbeta mRNA. Gel shift analysis using hepatic nuclear extracts from control and TCDD-treated mice and an oligonucleotide containing a C/EBP recognition element revealed a time-dependent change in DNA-protein complexes formed. Bands corresponding to C/EBPalpha, as determined by supershift analysis, diminished in TCDD-treated animals over a 7-day time period, whereas two new bands corresponding to C/EBPbeta, not present in control extracts, were increased significantly in treated samples. TCDD induced C/EBPbeta mRNA in wild-type mouse hepatoma cells, but not in aryl hydrocarbon receptor (AhR) nuclear translocator-deficient hepatoma cells. Induction in wild-type hepatoma cells was antagonized effectively by a molar excess of alpha-naphthoflavone. These results showed that TCDD caused rapid, reciprocal changes in C/EBPalpha and C/EBPbeta mRNAs and DNA binding in the adipose and liver of male C57BL mice and induced C/EBPbeta in hepatoma cells in an AhR-dependent manner. C/EBPs play vital roles in the coordination of energy homeostasis, and their alteration by TCDD may provide insight into the mechanism by which TCDD perturbs energy storage and utilization in vivo.

Inhibition of CCAAT/enhancer-binding protein alpha and beta translation by upstream open reading frames

CCAAT/enhancer-binding protein (C/EBP) alpha is a bZIP transcription factor whose expression is restricted to specific cell types. Analysis of C/EBPalpha mRNA and protein levels in various mammalian cells indicates that expression of this gene is controlled both transcriptionally and post-transcriptionally. We report here that C/EBPalpha translation is repressed in several cell lines by an evolutionarily conserved upstream open reading frame (uORF), which acts in cis to inhibit C/EBPalpha translation. Mutations that disrupt the uORF completely abolished translational repression of C/EBPalpha. The related c/ebpbeta gene also contains an uORF that suppresses translation. The length of the spacer sequence between the uORF terminator and the ORF initiator codon (7 bases in all c/ebpalpha genes and 4 bases in c/ebpbeta homologs) is precisely conserved. The effects of insertions, deletions, and base substitutions in the C/EBPalpha spacer showed that both the length and nucleotide sequence of the spacer are important for efficient translational repression. Our data indicate that the uORFs regulate translation of full-length C/EBPalpha and C/EBPbeta and do not play a role in generating truncated forms of these proteins, as has been suggested by start site multiplicity models.

Transcriptional control of adipogenesis

Adipocyte differentiation is coordinatedly regulated by several transcription factors. C/EBP beta, C/EBP delta and ADD-1/SREBP-1 are active early during the differentiation process and induce the expression and/or activity of the peroxisome proliferator activated receptor-gamma (PPAR gamma), the pivotal coordinator of the adipocyte differentiation process. Activated PPAR gamma induces exit from the cell cycle and triggers the expression of adipocyte-specific genes, resulting in increased delivery of energy to the cells. C/EBP alpha, whose expression coincides with the later stages of differentiation, cooperates with PPAR gamma in inducing additional target genes and sustains a high level of PPAR gamma in the mature adipocyte as part of a feedforward loop. Altered activity and/or expression of these transcription factors might underlie the pathogenesis of disorders characterized by increased or decreased adipose tissue depots.

Intestinal maturation in mice lacking CCAAT/enhancer-binding protein alpha (C/EPBalpha)

In rodents, there is a surge of intestinal expression of CCAAT/enhancer-binding protein alpha (C/EBPalpha) in the late fetal phase just before morphological maturation and the onset of expression of numerous epithelial genes. To investigate directly the hypothesis that C/EBPalpha plays a causal role in the latter phenomena, we have assessed both structural and functional maturation in neonatal intestine from C/EBPalpha-null mice and their littermates. No effects of C/EBPalpha genotype were observed on mucosal architecture or on the size of the proliferative zone in the intestinal crypts. Likewise, the mRNA levels for the glucose transporter 2 (GLUT2), intestinal and liver fatty acid-binding proteins, and apolipoprotein A-IV in newborn intestine were similar in all genotypes. Paradoxically, Na+/glucose co-transporter (SGLT1), lactase phlorizin-hydrolase and apolipoprotein B mRNAs were more abundant in the C/EBPalpha-deficient animals. In wild-type intestines, C/EBPbeta and C/EBPdelta mRNAs were detectable throughout the late fetal period and increased toward term in parallel with C/EBPalpha mRNA. In newborn intestine, there was no compensatory up-regulation of these isoforms in the C/EBPalpha-deficient mice. We conclude that C/EBPalpha has no essential role in morphological maturation of the intestine, the pattern of proliferation of the epithelium, or the onset of expression of this cluster of epithelial mRNAs. However, since other C/EBP isoforms are present in the developing intestine, it is possible that there is a generic requirement for a member of the C/EBP family.

Adipocyte differentiation and leptin expression

Adipose tissue has long been known to house the largest energy reserves in the animal body. Recent research indicates that in addition to this role, the adipocyte functions as a global regulator of energy metabolism. Adipose tissue is exquisitely sensitive to a variety of endocrine and paracrine signals, e.g. insulin, glucagon, glucocorticoids, and tumor necrosis factor (TNF), that combine to control both the secretion of other regulatory factors and the recruitment and differentiation of new adipocytes. The process of adipocyte differentiation is controlled by a cascade of transcription factors, most notably those of the C/EBP and PPAR families, which combine to regulate each other and to control the expression of adipocyte-specific genes. One such gene, i.e. the obese gene, was recently identified and found to encode a hormone, referred to as leptin, that plays a major role in the regulation of energy intake and expenditure. The hormonal and transcriptional control of adipocyte differentiation is discussed, as is the role of leptin and other factors secreted by the adipocyte that participate in the regulation of adipose homeostasis.

PPARgamma induces the insulin-dependent glucose transporter GLUT4 in the absence of C/EBPalpha during the conversion of 3T3 fibroblasts into adipocytes

To define the molecular mechanisms that control GLUT4 expression during adipogenesis, NIH-3T3 fibroblasts ectopically expressing different adipogenic transcription factors (C/EBPbeta, C/EBPdelta, C/EBPalpha, and PPARgamma) under the control of a tetracycline-responsive inducible (C/EBPs) or a constitutive retroviral (PPARgamma) expression system were used. Enhanced production of C/EBPbeta (beta2 cell line), C/EBPbeta together with C/EBPdelta (beta/delta39 cell line), C/EBPalpha (alpha1 cell line), or PPARgamma (Pgamma2 cell line) in cells exposed to dexamethasone and the PPARgamma ligand ciglitazone (a thiazolidinedione) resulted in expression of GLUT4 mRNA as well as other members of the adipogenic gene program, including aP2 and adipsin. Focusing our studies on the beta/delta39 cells, we have demonstrated that C/EBPbeta along with C/EBPdelta in the presence of dexamethasone induces PPARgamma, adipsin, and aP2 mRNA production; however, GLUT4 mRNA is only expressed in cells exposed to ciglitazone. In addition, enhanced expression of a ligand-activated form of PPARgamma in the beta/delta39 fibroblasts stimulates synthesis of GLUT4 protein and gives rise to a population of adipocytic cells that take up glucose in direct response to insulin. C/EBPalpha is not expressed in the beta/delta39 cells under conditions that stimulate the adipogenic program. This observation suggests that PPARgamma alone or in combination with C/EBPbeta and C/EBPdelta is capable of activating GLUT4 gene expression.

Expression of C/EBP alpha, beta and delta in fetal and postnatal subcutaneous adipose tissue

The C/EBP (CCAAT/enhancer binding protein) family of transcription factors (C/EBP alpha, beta, and delta) has been implicated in the development and the metabolic regulation of adipocytes from in vitro studies, yet the function of these factors, particularly CEBP beta and delta, in vivo has not been characterized. To assess the role of these factors in vivo, subcutaneous adipose, tissue from fetal and postnatal pigs was examined for C/EBP alpha, beta, and delta expression in developing and mature adipocytes. Western blot analysis of fetal adipose tissue showed a progressive increase of C/EBP alpha expression in 50, 75 and 95 day old fetuses. C/EBP beta and delta proteins were not observed in fetal adipose tissue. These results were confirmed with immunohistochemical studies of fetal adipose showing enhanced C/EBP alpha expression in the nuclei of adipocytes and cells closely associated with adipose cell clusters from 75 and 95 day old fetuses. For the same tissues only light background staining with no differential enhancement was found for C/EBP beta and delta. In postnatal adipose tissue C/EBP alpha and C/EBP beta protein were expressed in both 8 day old postnatal and mature (180 day) pigs. C/EBP delta reactive products were found in postnatal tissues however, their molecular weights were lower than that found in fetal pig liver. Our data suggest that adipose cell terminal differentiation proceeds in the pig fetus without the expression of C/EBP beta and delta and that these factors may have a more important role in fully differentiated adipose cells in postnatal tissue.