Selective induction of CCAAT/enhancer binding protein isoforms occurs during rat liver development

Ribosome rescue factor PELOTA modulates translation start site choice for C/EBPα protein isoforms

Translation initiation at alternative start sites can dynamically control the synthesis of two or more functionally distinct protein isoforms from a single mRNA. Alternate isoforms of the developmental transcription factor CCAAT/enhancer-binding protein α (C/EBPα) produced from different start sites exert opposing effects during myeloid cell development. This choice between alternative start sites depends on sequence features of the CEBPA transcript, including a regulatory uORF, but the molecular basis is not fully understood. Here, we identify the factors that affect C/EBPα isoform choice using a sensitive and quantitative two-color fluorescent reporter coupled with CRISPRi screening. Our screen uncovered a role of the ribosome rescue factor PELOTA (PELO) in promoting the expression of the longer C/EBPα isoform by directly removing inhibitory unrecycled ribosomes and through indirect effects mediated by the mechanistic target of rapamycin kinase. Our work uncovers further links between ribosome recycling and translation reinitiation that regulate a key transcription factor, with implications for normal hematopoiesis and leukemogenesis.

The Dual Role of C/EBPδ in Cancer

CCAAT/Enhancer-Binding Protein delta (C/EBPδ) is a transcription factor involved in differentiation and inflammation. While sparsely expressed in adult tissues, aberrant expression of C/EBPδ has been associated with different cancers. Initially, re-expression of C/EBPδ in cell cultures limited tumor cell proliferation, assigning it a tumor suppressor role. However, opposing observations were made in pre-clinical models and patients, suggesting that C/EBPδ not only mediates cell proliferation but dictates a broader spectrum of tumorigenesis-related effects. It is now widely accepted that C/EBPδ contributes to an inflammatory, tumor-promoting microenvironment, aids hypoxia adaption and contributes to the recruitment of blood vessels for improved nutrient supply to tumor cells and facilitated extravasation. This review summarizes the work published on this transcription factor in the field of cancer over the past decade. It points out areas in which a consensus on C/EBPδ's role appears to emerge and seek to explain seemingly contradictory results.

Ribosome rescue factor PELOTA modulates translation start site choice and protein isoform levels of transcription factor C/EBPα

Translation initiation at alternative start sites can dynamically control the synthesis of two or more functionally distinct protein isoforms from a single mRNA. Alternate isoforms of the hematopoietic transcription factor CCAAT-enhancer binding protein α (C/EBPα) produced from different start sites exert opposing effects during myeloid cell development. This alternative initiation depends on sequence features of the CEBPA transcript, including a regulatory upstream open reading frame (uORF), but the molecular basis is not fully understood. Here we identify trans-acting factors that affect C/EBPα isoform choice using a sensitive and quantitative two-color fluorescence reporter coupled with CRISPRi screening. Our screen uncovered a role for the ribosome rescue factor PELOTA (PELO) in promoting expression of the longer C/EBPα isoform, by directly removing inhibitory unrecycled ribosomes and through indirect effects mediated by the mechanistic target of rapamycin (mTOR) kinase. Our work provides further mechanistic insights into coupling between ribosome recycling and translation reinitiation in regulation of a key transcription factor, with implications for normal hematopoiesis and leukemiagenesis.

Hepatitis B Virus X Protein Induces Hepatic Steatosis by Enhancing the Expression of Liver Fatty Acid Binding Protein

Hepatitis B virus (HBV) has been implicated as a potential trigger of hepatic steatosis although molecular mechanisms involved in the pathogenesis of HBV-associated hepatic steatosis still remain elusive. Our prior work has revealed that the expression level of liver fatty acid binding protein 1 (FABP1), a key regulator of hepatic lipid metabolism, was elevated in HBV-producing hepatoma cells. In this study, the effects of HBV X protein (HBx) mediated FABP1 regulation on hepatic steatosis and the underlying mechanism were determined. mRNA and protein levels of FABP1 were measured by quantitative RT-PCR (qPCR) and Western blotting. HBx-mediated FABP1 regulation was evaluated by luciferase assay, coimmunoprecipitation, and chromatin immunoprecipitation. Hepatic lipid accumulation was measured by using Oil-Red-O staining and the triglyceride level. It was found that expression of FABP1 was increased in HBV-producing hepatoma cells, the sera of HBV-infected patients, and the sera and liver tissues of HBV-transgenic mice. Ectopic overexpression of HBx resulted in upregulation of FABP1 in HBx-expressing hepatoma cells, whereas HBx abolishment reduced FABP1 expression. Mechanistically, HBx activated the FABP1 promoter in an HNF3β-, C/EBPα-, and PPARα-dependent manner, in which HBx increased the gene expression of HNF3β and physically interacted with C/EBPα and PPARα. On the other hand, knockdown of FABP1 remarkably blocked lipid accumulation both in long-chain free fatty acids treated HBx-expressing HepG2 cells and in a high-fat diet-fed HBx-transgenic mice. Therefore, FABP1 is a key driver gene in HBx-induced hepatic lipid accumulation via regulation of HNF3β, C/EBPα, and PPARα. FABP1 may represent a novel target for treatment of HBV-associated hepatic steatosis.

IMPORTANCE

Accumulating evidence from epidemiological and experimental studies has indicated that chronic HBV infection is associated with hepatic steatosis. However, the molecular mechanism underlying HBV-induced pathogenesis of hepatic steatosis still remains to be elucidated. In this study, we found that expression of liver fatty acid binding protein (FABP1) was dramatically increased in the sera of HBV-infected patients and in both sera and liver tissues of HBV-transgenic mice. Forced expression of HBx led to FABP1 upregulation, whereas knockdown of FABP1 remarkably diminished lipid accumulation in both in vitro and in vivo models. It is possible that HBx promotes hepatic lipid accumulation through upregulating FABP1 in the development of HBV-induced nonalcoholic fatty liver disease. Therefore, inhibition of FABP1 might have therapeutic value in steatosis-associated chronic HBV infection.

Secretion of adipocytes and macrophages under conditions of inflammation and/or insulin resistance and effect of adipocytes on preadipocytes under these conditions

The purpose of the present study was to examine changes in preadipocytes following the coculture of preadipocytes and adipocytes and the effects on the secretion of adipocytes and macrophages following induction of inflammation and insulin resistance. Mature adipocytes and RAW264.7 macrophages were treated with lipopolysaccharide and insulin to establish models of inflammation and insulin resistance, respectively. The mRNA expression levels of IL-6, MCP-1, and TNF-α in all adipocyte treatment groups were significantly greater compared with the control, and that of adiponectin was less (P<0.05). In the RAW264.7 macrophages, the mRNA expression levels of IL-6 and TNF-α were greater than those in the control group (P<0.05). Moreover, the results of this study confirmed that adipocytes and macrophages increased the secretion of inflammatory factors under conditions of induced inflammation and insulin resistance. In addition, 3T3-L1 adipocytes inhibited the proliferation and differentiation of preadipocytes when cocultured with adipocytes under conditions of inflammation and/or insulin resistance, and the phenotype of preadipocytes did not change.

Mechanism of differentiation-enhanced photodynamic therapy for cancer: upregulation of coproporphyrinogen oxidase by C/EBP transcription factors

The efficacy of photodynamic therapy (PDT) for epithelial cancers is increased when PDT is combined with calcitriol (Vit D), a form of differentiation therapy (DT). Here, we describe an underlying mechanism for this effect. Differentiation-promoting agents are known to upregulate CCAAT/enhancer-binding proteins (C/EBP), powerful regulators of cellular differentiation. In subcutaneous A431 tumors in mice, pretreatment with Vit D induced the expression of C/EBPβ isoforms, and of coproporphyrinogen oxidase (CPO), a heme pathway enzyme responsible for the conversion of 5-aminolevulinic acid (ALA) into protoporphyrin IX (PpIX), the principal light-absorbing molecule during PDT. To further investigate this apparent link between C/EBPs and CPO, two cell lines (MEL and LNCaP) were exposed to differentiating agents, and levels of PpIX, C/EBPs, and CPO were measured. Differentiating agents, or transfection of C/EBP expression vectors, increased C/EBP and CPO levels in parallel. Focusing on approximately 1,300 bp of upstream CPO gene promoter, we tested the ability of recombinant C/EBPα, C/EBPβ, C/EBPδ, and C/EBPζ to bind to CPO gene sequences [electrophoretic mobility shift assay (EMSA) assays] and to affect transcriptional activity (luciferase assays). Multiple C/EBP consensus binding sites were identified (15 for mouse, 18 for human). Individual probes representing each site bound to C/EBPs with characteristic affinities (strong, moderate, or weak), but when sites were inactivated in the context of the native promoter, transcriptional activity was reduced nearly equally for strong or weak sites. Cooperative interactions between regularly spaced C/EBP sites seem critical for CPO transcriptional regulation by differentiation therapy. These results provide a mechanistic rationale for DT/PDT combination therapy for cancer.

Human liver fatty acid binding protein (hFABP1) gene is regulated by liver-enriched transcription factors HNF3β and C/EBPα

The human liver fatty acid binding protein (hFABP1) participates in cellular long-chain fatty acid trafficking and regulation of lipid metabolism and changes in hFABP1 are associated with an increased risk for type 2 diabetes, cardiovascular disease (CVD), and metabolic syndromes. Gene regulation of hFABP1 is not fully understood. Therefore, in the present study, the full length hFABP1 promoter (nucleotides -2125 to +51) and a series of truncated promoter regions were cloned. A luciferase reporter assay revealed that nucleotides -255 to +50 in the promoter region contained full of maximum hFABP1 promoter activity compared with the full length promoter. Furthermore high activity was shown when the plasmid was transfected into liver-derived cells such as the human hepatoblastoma cell line HepG2 and the hepatoma cell line Huh7. TFSEARCH and TESS programs were used to predict potential transcription factor binding sites. Two putative binding sites for the liver-enriched transcription factors hepatocyte nuclear factor 3β (HNF3β) and CCAAT/enhancer binding protein α (C/EBPα) were identified in the -255 nt to -155 nt hFABP1 promoter region. Site-directed mutagenesis of these two sites reduced dramatically hFABP1 promoter activity. In addition, the electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation assay (ChIP) revealed that these binding sites were recognized by HNF3β and C/EBPα respectively. Overexpression of HNF3β and C/EBPα enhanced the transcription of hFABP1 and consequently improved the protein level of hFABP1 in HepG2 cells, while knockdown of HNF3β and C/EBPα showed the inverse effects. Taken together, the hFABP1 gene is highly transcribed in liver-derived cells, and regulated predominantly by liver-enriched transcription factors HNF3β and C/EBPα.

Visualization by BiFC of different C/EBPβ dimers and their interaction with HP1α reveals a differential subnuclear distribution of complexes in living cells

How the co-ordinated events of gene activation and silencing during cellular differentiation are influenced by spatial organization of the cell nucleus is still poorly understood. Little is known about the molecular mechanisms controlling subnuclear distribution of transcription factors, and their interplay with nuclear proteins that shape chromatin structure. Here we show that C/EBPβ not only associates with pericentromeric heterochromatin but also interacts with the nucleoskeleton upon induction of adipocyte differentiation of 3T3-L1 cells. Different C/EBPβ dimers localize in different nuclear domains. Using BiFC in living cells, we show that LAP (Liver Activating Protein) homodimers localize in euchromatin and heterochromatin. In contrast, LIP (Liver Inhibitory Protein) homodimers localize exclusively in heterochromatin. Importantly, their differential subnuclear distribution mirrors the site for interaction with HP1α. HP1α inhibits LAP transcriptional capacity and occupies the promoter of the C/EBPβ-dependent gene c/ebpα in 3T3-L1 preadipocytes. When adipogenesis is induced, HP1α binding decreases from c/ebpα promoter, allowing transcription. Thus, the equilibrium among different pools of C/EBPβ associated with chromatin or nucleoskeleton, and dynamic changes in their interaction with HP1α, play key roles in the regulation of C/EBP target genes during adipogenesis.

CCAAT/enhancer-binding protein mediates carbon monoxide-induced suppression of cyclooxygenase-2

Cyclooxygenase-2 (COX-2) is a key enzyme involved in the inflammatory process that is rapidly induced in macrophages in response to LPS. Carbon monoxide (CO), a byproduct of heme oxygnease-1, can suppress proinflammatory response in various in vitro and in vivo models of inflammation. This study was undertaken to examine whether CO can regulate (and if so, to delineate the mechanism by which CO regulates) LPS-induced COX-2 expression in macrophages. RAW 264.7 murine macrophages were stimulated with LPS (0-10 ng/ml) with or without CO (500 ppm). Northern and Western blot analysis was done. Progstaglandin E(2) and nitrite concentration was measured from cell culture supernatant. Electrophoretic mobility shift assay was performed to assess nuclear factor binding. CO downregulated LPS-induced COX-2 mRNA and protein expression. CO also inhibited LPS-induced prostaglandin E(2) secretion (P < 0.05). CO also decreased LPS-induced CCAAT/enhancer-binding protein (C/EBP) beta and delta protein expression in LPS-treated RAW 264.7 cells. Gel shift analysis revealed that CO treatment decreased LPS-induced activation of protein binding to C/EBP consensus oligonucleotides of murine cyclooxygenase-2 promoter. CO also decreased LPS-induced nitric oxide synthase-2 protein expression and nitrite production, and decreased LPS-induced activation of protein binding to C/EBP consensus oligonucleotides of murine nitric oxide synthase-2 promoter. CO may act as an important regulator of inflammation by virtue of its ability to regulate C/EBPs.

C/EBPalpha activates the transcription of triacylglycerol hydrolase in 3T3-L1 adipocytes

TGH (triacylglycerol hydrolase) catalyses the lipolysis of intracellular stored triacylglycerol. To explore the mechanisms that regulate TGH expression in adipose tissue, we studied the expression of TGH during the differentiation of 3T3-L1 adipocytes. TGH mRNA and protein levels increased dramatically in 3T3-L1 adipocytes compared with pre-adipocytes. Electrophoretic mobility shift assays demonstrated enhanced binding of nuclear proteins of adipocytes to the distal murine TGH promoter region (-542/-371 bp), yielding one adipocyte-specific migrating complex. Competitive and supershift assays demonstrated that the distal TGH promoter fragment bound C/EBPalpha (CCAAT/enhancer-binding protein alpha). Transient transfections of different mutant TGH promoter-luciferase constructs into 3T3-L1 adipocytes and competitive electromobility shift assays showed that the C/EBP-binding elements at positions -470/-459 bp and -404/-390 bp are important for transcriptional activation. Co-transfection with C/EBPalpha cDNA and TGH promoter constructs in 3T3-L1 pre-adipocytes demonstrated that C/EBPalpha increased TGH promoter activity. Ectopic expression of C/EBPalpha in NIH 3T3 cells activated TGH mRNA expression without causing differentiation into adipocytes. These experiments directly link increased TGH expression in adipocytes to transcriptional regulation by C/EBPalpha. This is the first evidence that C/EBPalpha participates directly in the regulation of an enzyme associated with lipolysis.

CCAAT/enhancer binding protein beta binds to and activates the P670 promoter of human papillomavirus type 16

The P670 promoter of HPV16 directs transcription of the virus late genes in the differentiating epithelium. We found that CCAAT/enhancer binding protein beta (C/EBPbeta), a key transcription factor that induces the terminal differentiation of keratinocytes, enhanced the P670-driven transcription in transient reporter assays in HeLa cells and human primary keratinocytes, whereas it inhibited, as reported previously, the transcription from the early P97 promoter. An electrophoretic mobility shift analysis identified two binding sites in the upstream region of P670 for a bacterially expressed C/EBPbeta. A chromatin immunoprecipitation analysis demonstrated that C/EBPbeta bound to these sites of the P670 reporter plasmid in HeLa cells. Nucleotide substitutions in these sites in the reporter plasmid abrogated the enhancement by C/EBPbeta in the transient HeLa and keratinocyte assays, indicating that the C/EBPbeta-binding to these sites is required for the enhancement of transcription from P670. These results suggest that C/EBPbeta is involved in enhancing transcription from the P670 during keratinocyte differentiation.

Metabolic response of mice to a postnatal ablation of CCAAT/enhancer-binding protein alpha

Although CCAAT/enhancer-binding protein alpha (C/EBPalpha) is essential for initiating or sustaining several metabolic processes during the perinatal period, the consequences of total ablation of C/EBPalpha during postnatal development have not been investigated. We have created a conditional knock-out model in which the administration of poly(I:C) caused a virtually total deletion of c/ebpalpha (C/EBPalpha(Delta/-) mice) in the liver, spleen, white and brown adipose tissues, pancreas, lung, and kidney of the mice. C/EBPalpha itself was completely ablated in the liver by day 4 after the injection of poly(I:C). There was no noticeable change in phenotype during the first 15 days after the injection. The mice maintained a normal level of fasting blood glucose and responded to the diabetogenic action of streptozotocin. From day 16 onward, the mice developed hypophagia, exhibited severe weight loss, lost triglyceride in white but not brown adipose tissue, became hypoglycemic and hypoinsulinemic, depleted their hepatic glycogen, and developed fatty liver. They also exhibited lowered plasma levels of free fatty acid, triglyceride, and cholesterol, as well as marked changes in hepatic mRNA for C/EBPdelta, peroxisome proliferator-activated receptor alpha, sterol regulatory element-binding protein 1, hydroxymethylglutaryl-coenzyme A reductase, and apolipoproteins. Although basal levels of hepatic mRNA for the cytosolic isoform of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase were reduced, transcription of the genes for these enzymes was inducible by dibutyryl cyclic AMP in C/EBPalpha(Delta/-) mice. The animals died about 1 month after the injection of poly(I:C). These findings demonstrate that C/EBPalpha is essential for the survival of animals during postnatal life and that its ablation leads to distinct biphasic change in metabolic processes.

Characterization of liver function in transdifferentiated hepatocytes

We previously demonstrated that dexamethasone (Dex) induces the transdifferentiation (or conversion) of the pancreatic progenitor cell line AR42J-B13 (B13) to hepatocytes based on the expression of liver proteins. We have extended our original observations to determine: (1) the effects of Dex on pancreatic gene expression; (2) the time course of expression of liver enriched transcription factors during conversion from pancreatic to hepatic phenotype; (3) the functional potential of transdifferentiated hepatocytes; (4) the proliferative capacity of transdifferentiated hepatocytes; and (5) whether ectopic expression of transcription factors can induce the hepatic phenotype in pancreatic B13 cells. The results were as follows. The B13 cell markers amylase, synaptophysin, and neurofilament were lost in transdifferentiated hepatocytes compared to control cells and the liver enriched transcription factors C/EBPbeta and C/EBPalpha were induced first, followed by HNF4alpha and then RXRalpha. Using RT-PCR analysis and immunolocalisation studies, we detected hepatic markers (e.g., apolipoprotein B) in Dex-treated cells. In transdifferentiated hepatocytes albumin was secreted, insulin stimulated lipid deposition and ciprofibrate enhanced the expression of catalase. Proliferation of transdifferentiated hepatocytes is promoted in the presence of HGF and NEAA as indicated by the co-expression of the cell cycle markers cyclin D and phosphohistone H3 with liver proteins. Lastly, ectopic expression of C/EBPalpha or C/EBPbeta in AR42J-B13 cells was sufficient to induce transdifferentiation, based on nuclear localization of HNF4alpha and induction of UDP-glucuronosyltransferase expression. These results indicate that the B13 progenitor cell model is suitable for studying liver function and for understanding the molecular and cellular events that occur during transdifferentiation.

Liver-enriched transcription factors in liver function and development. Part II: the C/EBPs and D site-binding protein in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation

In the first part of our review (see Pharmacol Rev 2002;54:129-158), we discussed the basic principles of gene transcription and the complex interactions within the network of hepatocyte nuclear factors, coactivators, ligands, and corepressors in targeted liver-specific gene expression. Now we summarize the role of basic region/leucine zipper protein family members and particularly the albumin D site-binding protein (DBP) and the CAAT/enhancer-binding proteins (C/EBPs) for their importance in liver-specific gene expression and their role in liver function and development. Specifically, regulatory networks and molecular interactions were examined in detail, and the experimental findings summarized in this review point to pivotal roles of DBP and C/EBPs in cell cycle control, carcinogenesis, circadian gene regulation, liver regeneration, apoptosis, and liver-specific gene regulation. These regulatory proteins are therefore of great importance in liver physiology, liver disease, and liver development. Furthermore, interpretation of the vast data generated by novel genomic platform technologies requires a thorough understanding of regulatory networks and particularly the hierarchies that govern transcription and translation of proteins as well as intracellular protein modifications. Thus, this review aims to stimulate discussions on directions of future research and particularly the identification of molecular targets for pharmacological intervention of liver disease.

Suppression of C/EBP alpha expression in biliary cell differentiation from hepatoblasts during mouse liver development

BACKGROUND/AIMS

Intrahepatic biliary cell differentiation takes place in periportal hepatoblasts under the influence of the subjacent mesenchyme, which leads to the suppression of mature hepatocyte marker expression. This study was undertaken to analyze C/EBP alpha and beta expression, which may govern transcription of mature hepatocyte marker genes, during mouse liver development with special attention given to biliary differentiation.

METHODS

Expression of C/EBP alpha and beta was immunohistochemically examined. Expression of alpha-fetoprotein, albumin and urea cycle enzymes, the genes of which have CCAAT motifs in their upstream regulatory sequences, was examined immunohistochemically or by using in situ hybridization.

RESULTS

C/EBP alpha started to be expressed in endodermal cells of 9.5-day liver primordium, and continued to be expressed in hepatoblasts and hepatocytes throughout development. Although biliary cell progenitors transiently expressed mature hepatocyte markers, their expression of C/EBP alpha was weak or totally absent. The signals of C/EBP beta in hepatocytes were weak in fetal liver, but became stronger with postnatal development. Differentiated epithelial cells of intrahepatic biliary structures did not express C/EBP alpha.

CONCLUSIONS

These data suggest that the suppression of C/EBP alpha expression may be prerequisite to biliary cell differentiation in the hepatoblast population and one of its earliest signs.

C/EBPalphap30, a myeloid leukemia oncoprotein, limits G-CSF receptor expression but not terminal granulopoiesis via site-selective inhibition of C/EBP DNA binding

Heterozygous mutations of the CEBPA gene are present in 5% of acute myeloid leukemia (AML) cases and often lead to the expression of an N-terminally truncated, 30 kDa isoform, C/EBPalphap30, from an internal translation start site. We have assessed the effect of C/EBPalphap30 on granulopoiesis utilizing C/EBPalphap30-ER, containing the estradiol receptor ligand-binding domain. In contrast to C/EBPalpha-ER, C/EBPalphap30-ER did not induce 32Dcl3 myeloid cell differentiation in IL-3. However, both isoforms, when expressed at high levels, were capable of inhibiting E2F activity in 32Dcl3 cells and of slowing their G1 to S progression. C/EBPalphap30 repressed expression of the endogenous G-CSF receptor several-fold. To facilitate investigation of the effect of C/EBPalphap30-ER on granulopoiesis downstream of G-CSF signalling, we coexpressed exogenous G-CSF receptor. C/EBPalphap30-ER/GR cells expressed several granulocytic markers in G-CSF and demonstrated nuclear maturation. Rat C/EBPalpha-ER and C/EBPalphap30-ER, expressed in 293T cells, bound the C/EBP site from the NE gene with similar affinity, as did human C/EBPalpha and C/EBPalphap30. In contrast, C/EBPalphap30 bound the C/EBP sites in the PU.1 or GR gene with 3-6-fold reduced affinity. Thus, the selective inhibition of GR expression by C/EBPalphap30-ER is due in part to its variable affinity for C/EBP sites. Variation in affinity for selected cis elements among isoforms may affect the biology of basic region-leucine zipper (bZIP) proteins.

Differential activation of a C/EBP beta isoform by a novel redox switch may confer the lipopolysaccharide-inducible expression of interleukin-6 gene

C/EBP beta, a member of the CCAAT/enhancer binding protein (C/EBP) family, is one of the key transcription factors responsible for the induction of a wide array of genes, some of which play important roles in innate immunity, inflammatory response, adipocyte and myeloid cell differentiation, and the acute phase response. Three C/EBP beta isoforms (i.e. LAP*, LAP, and LIP) were known to arise from differential translation initiation and display different functions in gene regulation. C/EBP beta is known to induce interleukin (IL)-6 gene when P388D1 cells are treated with lipopolysaccharide (LPS). Exactly how the transcriptional activities of C/EBP beta isoforms are involved in the regulation of the IL-6 gene remains unclear. Here we report that LPS-induced expression of IL-6 gene in P388D1 cells is mediated by a redox switch-activated LAP*. The intramolecular disulfide bonds of LAP* and LAP have been determined. Among the cysteine residues, amino acid 11 (Cys11) of LAP* plays key roles for determining the overall intramolecular disulfide bonds that form the basis for redox switch regulation. The DNA binding activity and transcriptional activity of LAP* are enhanced under reducing condition. LAP and LIP, lacking 21 and 151 amino acids, respectively, in the N-terminal region, are not regulated in a similar redox-responsive manner. Our results indicate that LAP* is the primary isoform of C/EBP beta that regulates, through a redox switch, the LPS-induced expression of the IL-6 gene.

Activation domains of CCAAT enhancer binding protein delta: regions required for native activity and prostaglandin E2-dependent transactivation of insulin-like growth factor I gene expression in rat osteoblasts

In osteoblasts, hormones such as prostaglandin E2 that activate protein kinase A increase the translocation of transcription factor CCAAT/enhancer binding protein delta (C/EBPdelta) from the cytoplasm to the nucleus where it rapidly induces IGF-I gene expression. In this study, we identified activation and suppression domains in C/EBPdelta using native and heterologous gene promoter assay systems. We demonstrated functional interactions between C/EBPdelta and trans-gene-expressed cAMP response element binding protein-binding protein, and showed that the ability of C/EBPdelta to promote gene expression was suppressed by viral protein E1A, which blocks the activity of native cAMP response element binding protein-binding protein. Site-directed mutations at serines 62 or 191 within C/EBPdelta reduced its basal transcriptional activity, whereas mutation at serine 191 suppressed the stimulatory effect of prostaglandin E2 on C/EBPdelta function as well as its DNA binding potential. These results are consistent with the location of serine 191 in the DNA binding domain of C/EBPdelta. Our studies provide the first evidence for regions of C/EBPdelta that are important for basal and for hormone-induced transcriptional activity, and for its interactions with other enhancers and suppressers of gene expression.

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.

CCAAT/enhancer binding proteins in normal mammary development and breast cancer

CCAAT/enhancer binding proteins (C/EBPs) are a family of leucine zipper, transcription factors that bind to DNA as homodimers and heterodimers. They regulate cellular proliferation, differentiation and apoptosis in the mammary gland. Multiple protein isoforms, including truncated, dominant negatives, are generated by translation of the C/EBPbeta transcript or via proteolytic cleavage of the full-length C/EBPbeta protein. Gene deletion of individual C/EBP family members has demonstrated an essential role for C/EBPbeta in normal mammary development, while transgenic and overexpression studies provide evidence that the dominant-negative C/EBPbeta-liver-enriched inhibitory protein isoform induces proliferation in mammary epithelial cells. Mounting evidence suggests that alterations in the ratio of the C/EBPbeta-liver-enriched inhibitory protein isoform and the C/EBPbeta-liver-enriched activating protein isoform may play a role in the development of breast cancer. This review will consequently focus on C/EBP actions in normal mammary development and on the emerging data that supports a role in breast cancer.

Contribution of cyclooxygenase 2 to liver regeneration after partial hepatectomy

Partial hepatectomy (PH) triggers a rapid regenerative response in the remaining tissue to reinstate the organ function and the cell numbers. Among the molecules that change in the course of regeneration is an accumulation of prostaglandin E2 in the sera of rats with PH. Analysis of the cyclooxygenase (COX) isoenzymes in the remnant liver showed the preferential expression of COX-2 in hepatocytes. Cultured regenerating hepatocytes expressed significant levels of COX-2, a process that was not observed in the sham counterparts. Maximal expression of COX-2 was detected 16 h after PH with increased levels present even at 96 h. Pharmacological inhibition of COX-2 activity with NS398 shunted the up-regulation of cell proliferation after PH, which suggests a positive interaction of prostaglandins with the progression of the cell cycle. Similar results were obtained after PH of mice lacking the COX-2 gene. The expression of COX-2 in regenerating liver was concomitant with a decrease in CCAAT-enhancer binding protein (C/EBP-a) level and an increase in the expression of C/EBP-b and C/EBP-d. These results suggest a contribution of the enhanced synthesis of prostaglandins to liver regeneration observed after PH.

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.

Epimorphin mediates mammary luminal morphogenesis through control of C/EBPbeta

We have shown previously that epimorphin (EPM), a protein expressed on the surface of myoepithelial and fibroblast cells of the mammary gland, acts as a multifunctional morphogen of mammary epithelial cells. Here, we present the molecular mechanism by which EPM mediates luminal morphogenesis. Treatment of cells with EPM to induce lumen formation greatly increases the overall expression of transcription factor CCAAT/enhancer binding protein (C/EBP)β and alters the relative expression of its two principal isoforms, LIP and LAP. These alterations were shown to be essential for the morphogenetic activities, since constitutive expression of LIP was sufficient to produce lumen formation, whereas constitutive expression of LAP blocked EPM-mediated luminal morphogenesis. Furthermore, in a transgenic mouse model in which EPM expression was expressed in an apolar fashion on the surface of mammary epithelial cells, we found increased expression of C/EBPβ, increased relative expression of LIP to LAP, and enlarged ductal lumina. Together, our studies demonstrate a role for EPM in luminal morphogenesis through control of C/EBPβ expression.

Structural and functional characterization of liver cell-specific activity of the human sodium/taurocholate cotransporter

Bile salts are rapidly removed from the circulation by the liver-specific sodium/taurocholate cotransporter (SLC10A1). To understand factors controlling its liver-specific expression, we isolated human SLC10A1 from a YAC chromosomal clone. SLC10A1 spans approximately 23 kb distributed over five exons. The major transcription start site is at 299 bp, and a minor start site is at 395 bp from the translational start site. A 1.2-kb portion of the 5' flanking region was sequenced and shown to contain a number of liver-enriched elements, but no TATA box. Using secreted alkaline phosphatase reporter constructs liver-specific expression was examined. Transient transfection demonstrated that SLC10A1 promoter expression was selectively expressed eightfold in FAO and rat hepatocytes, while deletion mutants demonstrated liver-specific expression in a region extending from -5 to +198 bp, which contained putative sites for C/EBP and HNF3. Mutations of the C/EBP site resulted in loss of 77% of transcriptional activity. Cotransfection of C/EBP, but not other putative liver-enriched binding factors, increased SLC10A1 promoter activity. Electrophoretic mobility shift assays demonstrated specific protein-DNA interactions that involved C/EBPalpha and beta. These studies demonstrate that the TATA-less human SLC10A1 promoter exhibits liver-specific activity and its regulatory elements contain binding sites for C/EBP, which contributes specifically to its transcriptional regulation.

Regulation of cyclooxygenase 2 expression in hepatocytes by CCAAT/enhancer-binding proteins

BACKGROUND & AIMS

Expression of cyclooxygenase (COX)-2 in response to lipopolysaccharide (LPS) challenge has been analyzed in cultured fetal, neonatal, and adult hepatocytes and in hepatoma cell lines.

METHODS

To study the mechanisms of LPS-dependent expression of COX-2 in these cells, the activity of the COX-2 promoter and the levels of CCAAT/enhancer-binding proteins (C/EBPs) were determined.

RESULTS

COX-2 was induced in fetal hepatocytes, but this response declined rapidly after birth. This loss of inducibility of COX-2 paralleled the expression of C/EBP-alpha in neonatal hepatocytes. Transfection of fetal and adult hepatocytes with sequences corresponding to the 5'-flanking region of the rat COX-2 gene confirmed the absence of promoter activity in adult hepatocytes. Moreover, transient expression of C/EBP-alpha, but not C/EBP-delta, in the hepatoma cell line AT3F cells abolished the COX-2 promoter activity. Prolonged culture of adult hepatocytes restored the induction of COX-2 after complete disappearance of C/EBP-alpha.

CONCLUSIONS

These results suggest that the presence of high levels of C/EBP-alpha is involved in the impairment of COX-2 expression in adult hepatocytes challenged with proinflammatory stimuli.

Translational control of C/EBPα and C/EBPβ isoform expression

Transcription factors derived from CCAAT/enhancer binding protein (C/EBP)α and C/EBPβ genes control differentiation and proliferation in a number of cell types. Various C/EBP isoforms arise from unique C/EBPβ and C/EBPα mRNAs by differential initiation of translation. These isoforms retain different parts of the amino terminus and therefore display different functions in gene regulation and proliferation control. We show that PKR and mTOR signaling pathways control the ratio of C/EBP isoform expression through the eukaryotic translation initiation factors eIF-2α and eIF-4E, respectively. An evolutionary conserved upstream open reading frame in C/EBPα and C/EBPβ mRNAs is a prerequisite for regulated initiation from the different translation initiation sites and integrates translation factor activity. Deregulated translational control leading to aberrant C/EBPα and C/EBPβ isoform expression or ectopic expression of truncated isoforms disrupts terminal differentiation and induces a transformed phenotype in 3T3-L1 cells. Our results demonstrate that the translational controlled ratio of C/EBPα and C/EBPβ isoform expression determines cell fate.

The protein composition of the hepatocyte nuclear matrix is differentiation-stage specific

The protein composition of hepatocyte nuclear matrices was examined in rats from the 16th day of gestation to 75 days after birth (adult). An overall increase in size of the nuclear matrix was accompanied by quantitative and qualitative changes in its protein content. Quantitative changes of the major proteins of the peripheral lamina surrounding the isolated nuclear matrix were detected. By Western analysis we established that in pre- and postnatal nuclear matrices the relative concentrations of lamin C were greater than lamin A. After birth, the relative concentrations of both lamins progressively increased. In the adult nuclear matrix, the concentration of lamin A was greater than lamin C. In contrast, the relative concentrations of lamin B remained unchanged throughout development and growth. The relative concentrations of two nuclear matrix-associated regulatory proteins studied changed with development and growth: transcription factor C/EBPalpha isoforms, which were detected during the gestation period, increased notably after the first postnatal day, attaining a maximum at the adult stage; the high concentrations of the proliferating cell nuclear antigen (PCNA) perceptibly decreased after the 21st prenatal day. Changes in the composition of the nuclear matrix protein suggest that this structure coordinates nuclear functioning during cell differentiation.

Gluconeogenesis in the fetus and neonate

Gluconeogenesis (GNG), a key metabolic process, involves the formation of glucose and glycogen from non-glucose precursors via pyruvate. In the strict sense, it also includes the contribution of glycerol as well as recycled glucose carbon (Cori's cycle). The developmental expression of GNG in the fetus and newborn and the quantitative contribution of GNG to glucose has been extensively investigated in humans and other mammalian species. Data from studies in rodents, rabbits, and sheep fetuses show that the development of GNG is a well-orchestrated process that is regulated by the expression of specific factors involved in the transcription of the genes for specific regulating enzymes, which catalyze GNG. These transcription factors and the genes for gluconeogenic enzymes are expressed at specific time periods during development. Although the fetus has the potential for GNG, the actual formation of glucose from pyruvate is not apparent until after birth because the rate limiting enzyme phosphoenolpyruvate carboxykinase appears only after birth in the immediate newborn period. Several tracer isotope methods have been employed to quantify the contribution of GNG to glucose. Of these, the recently developed stable isotope techniques with deuterium labeled water and the mass isotopomer distribution analysis appear to be the most precise and easily applicable in human studies. The available data show that in the human newborn, GNG appears soon after birth and contributes 30% to 70% to glucose produced. Application of new molecular biology techniques, in combination with sensitive tracer isotopic methods, will allow us to identify and examine metabolic disorders that impact GNG and help develop intervention strategies.

Interleukin-1beta suppresses retinoid transactivation of two hepatic transporter genes involved in bile formation

Cytokines have been implicated in the pathogenesis of inflammatory cholestasis. This is due to transcriptional down-regulation of hepatic transporters including the Na(+)/bile acid cotransporter, ntcp, and the multispecific organic anion exporter, mrp2. We have recently shown that ntcp suppression by lipopolysaccharide in vivo is caused by down-regulation of transactivators including the previously uncharacterized Footprint B-binding protein. Both the ntcp FpB element and the mrp2 promoter contain potential retinoid-response elements. We hypothesized that retinoic acid receptor (RAR) and retinoid X receptor (RXR) heterodimers would activate these two genes and that cytokines that reduce bile flow might do so by suppressing nuclear levels of these transactivators. Retinoid transactivation and interleukin-1beta down-regulation of the ntcp and mrp2 promoters were mapped to RXRalpha:RARalpha-response elements. Gel mobility shift assays demonstrated specific binding of RXRalpha:RARalpha heterodimers to the ntcp and mrp2 retinoid-response elements. The RXRalpha:RARalpha complex was down-regulated by IL-1beta in HepG2 cells. An unexpected finding was that an adjacent CAAT-enhancer-binding protein element was required for maximal transactivation of the ntcp promoter by RXRalpha:RARalpha. Taken together, these studies demonstrate regulation of two hepatobiliary transporter genes by RXRalpha:RARalpha and describe a mechanism which likely contributes to their down-regulation during inflammation.

C/EBPalpha regulates generation of C/EBPbeta isoforms through activation of specific proteolytic cleavage

C/EBPalpha and C/EBPbeta are intronless genes that can produce several N-terminally truncated isoforms through the process of alternative translation initiation at downstream AUG codons. C/EBPbeta has been reported to produce four isoforms: full-length 38-kDa C/EBPbeta, 35-kDa LAP (liver-enriched transcriptional activator protein), 21-kDa LIP (liver-enriched transcriptional inhibitory protein), and a 14-kDa isoform. In this report, we investigated the mechanisms by which C/EBPbeta isoforms are generated in the liver and in cultured cells. Using an in vitro translation system, we found that LIP can be generated by two mechanisms: alternative translation and a novel mechanism-specific proteolytic cleavage of full-length C/EBPbeta. Studies of mice in which the C/EBPalpha gene had been deleted (C/EBPalpha-/-) showed that the regulation of C/EBPbeta proteolysis is dependent on C/EBPalpha. The induction of C/EBPalpha in cultured cells leads to induced cleavage of C/EBPbeta to generate the LIP isoform. We characterized the cleavage activity in mouse liver extracts and found that the proteolytic cleavage activity is specific to prenatal and newborn livers, is sensitive to chymostatin, and is completely abolished in C/EBPalpha-/- animals. The lack of cleavage activity in the livers of C/EBPalpha-/- mice correlates with the decreased levels of LIP in the livers of these animals. Analysis of LIP production during liver regeneration showed that, in this system, the transient induction of LIP is dependent on the third AUG codon and most likely involves translational control. We propose that there are two mechanisms by which C/EBPbeta isoforms might be generated in the liver and in cultured cells: one that is determined by translation and a second that involves C/EBPalpha-dependent, specific proteolytic cleavage of full-length C/EBPbeta. The latter mechanism implicates C/EBPalpha in the regulation of posttranslational generation of the dominant negative C/EBPbeta isoform, LIP.

Acupuncture for gastrointestinal and hepatobiliary disorders

Acupuncture has been used for various gastrointestinal (GI) conditions. Voluminous data support the effect of acupuncture on the physiology of the GI tract, including acid secretion, motility, neurohormonal changes, and changes in sensory thresholds. Much of the neuroanatomic pathway of these effects has been identified in animal models. A large body of clinical evidence supports the effectiveness of acupuncture for suppressing nausea associated with chemotherapy, postoperative state, and pregnancy. Prospective randomized controlled trials have also shown the efficacy of acupuncture for analgesia for endoscopic procedures, including colonoscopy and upper endoscopy. Acupuncture has also been used for a variety of other conditions including postoperative ileus, achalasia, peptic ulcer disease, functional bowel diseases (including irritable bowel syndrome and nonulcer dyspepsia), diarrhea, constipation, inflammatory bowel disease, expulsion of gallstones and biliary ascariasis, and pain associated with pancreatitis. Although there are few prospective randomized clinical studies, the well-documented physiological basis of acupuncture effects on the GI tract, and the extensive history of successful clinical use of acupuncture, makes this a promising modality that warrants further investigation.

Hypoglycemia and impaired hepatic glucose production in mice with a deletion of the C/EBPbeta gene

The transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) is enriched in liver and adipose tissue and controls the expression of a wide variety of genes coding for important metabolic pathways, including gluconeogenesis and lipid synthesis. To investigate the role of C/EBPbeta on glucose homeostasis, we studied mice with a targeted deletion of the gene for C/EBPbeta-/- mice. Adult C/EBPbeta-/- mice have hypoglycemia after an 18-hour fast, accompanied by lower hepatic glucose production (40% of that of wild-type mice), with no change in plasma insulin and a lower concentration of plasma free fatty acids (FFA). Glucagon infusion during a pancreatic clamp acutely stimulated hepatic glucose production by 38% in wild-type animals, with no change detected in C/EBPbeta-/- mice. Unexpectedly, both the basal and glucagon-stimulated hepatic cyclic adenosine monophosphate (cAMP) levels were lower in C/EBPbeta-/- mice, indicating an essential role for C/EBPbeta in controlling proximal signal transduction. Fasting hypoglycemia was associated with normal levels of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) gene expression, however net liver glycogenolysis was impaired in C/EBPbeta-/- mice. FFA release from isolated adipose tissue in response to epinephrine was 68% lower in C/EBPbeta-/- mice than in control animals; however, N6,O2'-dibutyryladenosine (Bt2) cAMP stimulated a twofold increase in FFA release in C/EBPbeta-/- compared with no further increase in wild-type mice. Because a deletion in the gene for C/EBPbeta reduces blood glucose and circulating FFA, it could be an important therapeutic target for the treatment of non-insulin-dependent diabetes and possibly obesity, based on designing antagonists that decrease C/EBPbeta activity.

Cholesterol 7alpha-hydroxylase (CYP7A): patterns of messenger RNA expression during rat liver development

Cholesterol 7-hydroxylase is a rate-limiting enzyme in bile acid synthesis, a major pathway for cholesterol catabolism. It plays a crucial role in postnatal development and survival. In an adult liver, its activity and messenger RNA (mRNA) are heterogeneously distributed with concentration in the pericentral area. We defined how the pattern of cholesterol 7-hydroxylase mRNA evolves during rat liver development, correlated this with its total liver mRNA levels, and determined when its heterogeneous pattern of expression is established. Cholesterol 7-hydroxylase mRNA was undetectable in 18-day-old fetal livers by Northern blot. It was increased markedly in newborns with a homogeneous liver lobular distribution as determined by in situ hybridization. At postnatal day four, mRNA levels were markedly decreased with concomitant appearance of a lobular gradient: mRNA was detected only in a few hepatocytes located around efferent venules. At 22 days, the time of highest mRNA expression, a marked extension of the gradient towards the periportal area was observed, indicating that the increase in total liver cholesterol 7-hydroxylase mRNA level was a result of recruitment of hepatocytes upstream from the central vein area. By 28 days, the adult pattern was observed. Thus, expression of cholesterol 7-hydroxylase mRNA is tightly regulated during rat liver development, both temporally and spatially supporting its critical role in normal postnatal development.

Regulation of LPS-mediated induction of C/EBP delta gene expression in livers of young and aged mice

The C/EBP family of transcription factors plays a major role in the regulation of families of stress response genes, in particular, the acute phase response genes. We have examined expression of the C/EBP delta gene during the bacterial lipopolysaccharide mediated induction of the acute phase response in livers of young (4 months) and aged (24-28 months) male C57B1/6 mice by Northern, Western, and Southwestern analyses. C/EBP delta mRNA is present at a low constitutive level, is induced by lipopolysaccharide, and reaches the same induced level in young and aged mice. Aged mice, however, show a higher constitutive, uninduced mRNA pool level and a delay in recovery to uninduced levels after lipopolysaccharide treatment. C/EBP delta mRNA is observable 30 min after lipopolysaccharide in total RNA, cytoplasmic and polysomal fractions. Specific full length 28-kDa nascent peptides are detectable in polysomes 90 min after lipopolysaccharide. mRNA and nascent peptides cosediment with large polysomes and C/EBP delta mRNA is shifted to larger polysomes in lipopolysaccharide treated aged mice, consistent with an increased rate of initiation. Specific DNA-binding activity of C/EBP delta protein in nuclear extracts was examined by electromobility shift and antibody supershift assay. The levels of C/EBP delta binding-activity, are consistent with the changes in mRNA levels in young lipopolysaccharide treated livers. These studies support our hypothesis that aged mice exhibit a state of chronic inflammation or stress in the absence of a stressor.

C/EBPbeta, but not C/EBPalpha, is essential for ductal morphogenesis, lobuloalveolar proliferation, and functional differentiation in the mouse mammary gland

The CCAAT/enhancer binding proteins (C/EBPs) are differentially expressed throughout mammary gland development and interact with binding sites within the promoter of a milk protein gene, beta-casein. The specific roles of C/EBPbeta and C/EBPalpha in mouse mammary gland development and differentiation have been investigated in mice that carry targeted deletions of these genes. C/EBPbeta-/- virgin mice exhibited cystic, enlarged mammary ducts with decreased secondary branching. Transplantation of C/EBPbeta-/- mammary epithelium into the cleared mammary fat pads of nude mice confirmed that this defect in ductal morphogenesis was intrinsic to the epithelium. When treated with estrogen/progesterone (E+P) to simulate pregnancy, C/EBPbeta-/- mammary glands displayed only limited lobuloalveolar development and ductal side branching. Primary mammary epithelial cells obtained from E+P-treated C/EBPbeta-/- mice that were cultured on extracellular matrix gels did not functionally differentiate in response to lactogenic hormones despite their organization into three-dimensional structures. Expression of beta-casein protein was inhibited 85%-100% and whey acidic protein (WAP) was undetectable. In contrast, no detectable alterations in mammary development or beta-casein expression were observed in mammary outgrowths derived from newborn C/EBPalpha-/- mammary epithelium transplanted into the cleared mammary fat pads of syngeneic hosts. These results demonstrate that C/EBPbeta, but not C/EBPalpha, is required for ductal morphogenesis, lobuloalveolar development, and functional differentiation of mammary epithelial cells.

Function of cis-acting elements in human alcohol dehydrogenase 4 (ADH4) promoter and role of C/EBP proteins in gene expression

The ADH4 gene, which encodes human pi-alcohol dehydrogenase, is expressed in a tissue-specific manner, with the highest level in liver and lower levels in the gastrointestinal tract. We examined the location and function of the cis-acting elements that regulate ADH4 transcription. Liver contains proteins that bound to seven sites in the proximal promoter (from bp -387 to bp +17). Proteins from other tissues bound to subsets of these sites and to two additional sites, one of which is a negative cis-acting element. Members of two important transcription factor families, C/EBP and AP-1, bound to several sites in this promoter. The proximal ADH4 promoter functioned in a hepatoma cell line (H4IIE-C3) and a kidney cell line (CV-1). Coexpression of members of the C/EBP family strongly enhanced promoter activity, which can in part explain the high level of expression of ADH4 in liver. At one site that can be bound by both C/EBP and c-Jun, a mutation that abolished binding by C/EBP but not by c-Jun decreased promoter activity in both cell lines. This mutation had a stronger effect in the context of a longer promoter, suggesting interaction among cis-acting elements.

Maturation-dependent gene expression in a conditionally transformed liver progenitor cell line

We have isolated a conditionally transformed liver progenitor cell line with phenotypic similarities to both hepatoblasts (bipotent embryonic liver cells that give rise to hepatocytes and intrahepatic biliary epithelial cells) and liver epithelial cells (primitive hepatic cells isolated from adult livers capable of generating both hepatocytic and biliary lineages). Cell line L2039 was derived from E14 fetal mouse liver after transformation with temperature-sensitive SV-40 large T antigen. At 33 degrees C, these cells have an epithelial morphology with a high nucleocytoplasmic ratio and express both hepatocytic and biliary genes, including albumin, alpha-fetoprotein, glutamine synthetase, insulinlike growth factor II receptor, fibronectin and laminin, and cytokeratins 8 and 19, a set of markers characteristic for hepatoblasts. The presence of cytokeratin 14, vimentin, and several oval-cell antigens link cell line L2039 to nonparenchymal liver epithelial cell populations thought to contain progenitor cells. Serum-free, hormonally defined media conditions and extracellular matrix requirements were determined for growth and differentiation of this cell line. During culture on type IV collagen at 39 degrees C, L2039 cells cease dividing and demonstrate hepatocytic differentiation with the assumption of a hepatocytelike morphology and glucocorticoid-dependent regulation of liver-specific genes, including albumin, alpha-fetoprotein, phosphoenolpyruvate carboxykinase, and liver-enriched transcription factors. The number of albumin-positive cells increases during culture at 39 degrees C, indicating that L2039 cells convert from a prehepatocytic to a hepatocytic phenotype. Under conditions specific for hepatocytic differentiation, C/EBPs were expressed and differentially regulated, with C/EBPbeta and C/EBPdelta upregulated early and C/EBPalpha only slightly expressed after 7 d, indicating that C/EBPalpha may not be a crucial factor in commitment to the hepatocytic phenotype.

Lack of C/EBP alpha gene expression results in increased DNA synthesis and an increased frequency of immortalization of freshly isolated mice [correction of rat] hepatocytes

The CCAAT/enhancer binding protein alpha (C/EBP alpha) binds to specific promoter sequences and directs transcription of many genes expressed in the liver. Overexpression of C/EBP alpha in established cell lines inhibits cell proliferation. Primary hepatocytes from newborn C/EBP alpha(-/-) mice and normal littermates were used to determine whether the absence of C/EBP alpha increased proliferation and/or transformation of these cells in vitro. DNA synthesis, as measured by bromodeoxyuridine (BrdU) incorporation 24 hours postharvest, was fourfold higher in cells from C/EBP alpha(-/-) pups. Established cell lines were derived from 7 of 8 hepatocyte cultures initiated from null mutants, 4 of 23 cultures from heterozygotes, and 0 of 12 cultures from wild-type animals. C/EBP alpha(-/-) cultures had epithelial morphology, showed bile canaliculi, and expressed albumin messenger RNA (mRNA). When cultured on Matrigel, which promotes differentiation, cell lines derived from C/EBP alpha(-/-) mice formed cords and increased albumin mRNA expression by 1.7- to 3.8-fold. C/EBP alpha(-/-) cell lines exhibited rapid growth and rapid accumulation of chromosomal abnormalities, and were capable of forming nodules when inoculated into the abdominal subcutaneous tissue of nude mice. Our data show that C/EBP alpha is an important regulator of hepatocyte proliferation and participates in the maintenance of the nontransformed hepatic phenotype in vitro.

Role of the CCAAT/enhancer binding protein-alpha transcription factor in the glucocorticoid stimulation of p21waf1/cip1 gene promoter activity in growth-arrested rat hepatoma cells

The preceding paper (Cha, H. H., Cram, E. J., Wang, E. C., Huang, A. J., Kasler, H. G., and Firestone, G. L. (1998) J. Biol. Chem. 273, 0000-0000(478563) defined a glucocorticoid responsive region within teh promoter of the p21 CDK inhibitor gene that contains a putative DNA-binding site for the transcription factor CCAAT/ enhancer binding protein-alpha (C/EBP alpha). Wild type rat BDS1 hepatoma cells as well as as4 hepatoma cells, which express antisense sequences to C/EBP alpha and ablate its protein production, were utilized to investigate the role of this transcription factor in the glucocorticoid regulation of p21 gene expression. The stimulation of p21 protein levels and promoter activity, as well as inhibition of CDK2-mediated retinoblastoma protein phosphorylation, by the synthetic glucocorticoid, dexamethasone, required the expression of C/EBP alpha. Overexpression of C/EBP alpha in as4 cells rescued the dexamethasone responsiveness of the p21 promoter. Site-directed mutagenesis of the p21 promoter revealed that dexamethasone stimulation of p21 promoter activity required the C/EBP consensus DNA-binding site. Furthermore, in glucocorticoid receptor-defective EDR1 hepatoma cells, dexamethasone failed to stimulate C/EBP alpha and p21 protein expression and promoter activities. Our results have established a functional link between the glucocorticoid receptor signaling pathway that mediates a G1 cell cycle arrest of rat hepatoma cells and the transcriptional control of p21 by a cascade that requires the steroid induction of C/EBP alpha gene expression.

Changes in expression of CCAAT/enhancer binding protein alpha (C/EBP alpha) and C/EBP beta in rat liver after partial hepatectomy but not after treatment with cyproterone acetate

BACKGROUND/AIMS

The proliferation rate of adult rat liver is normally very low. It is markedly enhanced during compensatory regeneration, e.g. after partial hepatectomy, or after administration of certain growth promoters, e.g. cyproterone acetate. These two types of liver cell proliferation appear to differ, since the expression of immediate early genes is induced during compensatory regeneration but not after cyproterone acetate treatment. The transcription factor C/EBP alpha, which has been associated with hepatocyte differentiation and growth arrest, is suppressed during compensatory regeneration. In contrast, C/EBP beta, associated with acute phase reaction, is increased during regeneration. We have investigated the effects of the liver growth promoter cyproterone acetate on the hepatic expression of C/EBP alpha and C/EBP beta.

METHODS

Adult male rats received either cyproterone acetate treatment or were subjected to partial hepatectomy. Livers were obtained at different time intervals for measurement of C/EBP alpha and C/EBP beta mRNA with solution hybridization/RNAse protection assay, and C/EBP alpha and C/EBP beta content with immunoblotting.

RESULTS

The levels of both C/EBP alpha and C/EBP beta mRNA and the corresponding immunoreactivities were unchanged 2-48 h after injection of cyproterone acetate. The levels of C/EBP alpha mRNA and immunoreactivity were significantly suppressed 10-18 h and 18-26 h after partial hepatectomy, respectively. The levels of C/EBP beta mRNA and immunoreactivity were enhanced during compensatory regeneration 2 h after partial hepatectomy.

CONCLUSIONS

Liver cell proliferation during regeneration, but not in response to cyproterone acetate treatment, is associated with changes in C/EBP alpha and C/EBP beta expression. This further supports the notion that changes in expression of transcription factors during liver growth in vivo are dependent on the growth inducer.

5' sequences direct developmental expression and hormone responsiveness of tyrosine aminotransferase in primary cultures of fetal rat hepatocytes

Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and N6,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP), whereas less mature hepatocytes (<E15) do not [Shelly, L. L. & Yeoh, G. C. T. (1991) Eur. J. Biochem. 199, 475-481]. Therefore, we consider >E15 hepatocytes, and not <E15 hepatocytes, to be determined. This study reports that 11.1 kb of sequences upstream of the TyrAT transcription start site, which include a cAMP-responsive element (CRE) and a glucocorticoid-responsive element (GRE), are required for correct developmental regulation of gene expression in determined fetal hepatocytes. In contrast, the TyrAT CRE alone does not have this capability. Dexamethasone augments basal and Bt2cAMP-stimulated activity of the TyrAT CRE alone, suggesting that synergism may be due to interaction between the glucocorticoid and cAMP-signaling pathways. However, Bt2cAMP does not further increase dexamethasone-induced activity of the 11.1 kb 5' sequences when the TyrAT CRE is removed, thus excluding interaction of Bt2cAMP with the glucocorticoid pathway. Finally, insulin inhibition of dexamethasone-induced gene transcription is shown to be conferred by TyrAT 5' sequences. This study shows that cellular components, other than those which mediate hormonal regulation of genes, are required for determination of hepatocytes with respect to TyrAT. Since this phenomenon is observed with transient transfections, it is unlikely to involve higher-order chromatin structure.

Role of the isoforms of CCAAT/enhancer-binding protein in the initiation of phosphoenolpyruvate carboxykinase (GTP) gene transcription at birth

The gene for phosphoenolpyruvate carboxykinase (PEPCK), a target of CCAAT/enhancer-binding protein-alpha (C/EBPalpha) and -beta (C/EBPbeta), begins to be expressed in the liver at birth. Mice homozygous for a deletion in the gene for CEBPalpha (C/EBPalpha-/- mice) die shortly after birth of hypoglycemia, with no detectable hepatic PEPCK mRNA and negligible hepatic glycogen stores. Half of the mice homozygous for a deletion in the gene for CEBPbeta (C/EBPbeta-/- mice) have normal glucose homeostasis (phenotype A), and the other half die at birth of hypoglycemia due to a failure to express the gene for PEPCK and to mobilize hepatic glycogen (phenotype B). Insulin deficiency induces C/EBPalpha and PEPCK gene transcription in the livers of 19-day fetal rats, whereas dibutyryl cyclic AMP (Bt2cAMP) increases the expression of the gene for C/EBPbeta and causes a transient burst of PEPCK mRNA. Bt2cAMP induces PEPCK mRNA in the livers of fetal C/EBPalpha-/- mice, but at only 20% of the level of control animals; however, there is no induction of PEPCK mRNA if the cyclic nucleotide is injected into C/EBPalpha-/- mice immediately after delivery. The expression of the gene for C/EBPbeta is markedly induced in the livers of C/EBPalpha-/- mice within 2 h after the administration of Bt2cAMP. C/EBPbeta-/- mice injected at 20 days of fetal life with Bt2cAMP have a normal pattern of induction of hepatic PEPCK mRNA. In C/EBPbeta-/- mice with phenotype B, the administration of Bt2cAMP immediately after delivery induces PEPCK mRNA, causes the mobilization of hepatic glycogen, and maintains normal glucose homeostasis for up to 4 h (duration of the experiment). We conclude that C/EBPalpha is required for the cAMP induction of PEPCK gene expression in the liver and that C/EBPbeta can compensate for the loss of C/EBPalpha if its concentration is induced to appropriate levels.

Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression

Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK) is a key enzyme in the synthesis of glucose in the liver and kidney and of glyceride-glycerol in white adipose tissue and the small intestine. The gene for the cytosolic form of PEPCK (PEPCK-C) is acutely regulated by a variety of dietary and hormonal signals, which result in alteration of synthesis of the enzyme. Major factors that increase PEPCK-C gene expression include cyclic AMP, glucocorticoids, and thyroid hormone, whereas insulin inhibits this process. PEPCK-C is absent in fetal liver but appears at birth, concomitant with the capacity for gluconeogenesis. Regulatory elements that control transcription of the PEPCK-C gene in liver, kidney, and adipose tissue have been delineated, and many of the transcription factors that bind to these elements have been identified. Transgenic mice have been especially useful in elucidating the physiological roles of specific sequence elements in the PEPCK-C gene promoter and in demonstrating the key role played at these sites by the isoforms of CAAT/enhancer binding protein in patterning of PEPCK-C gene expression during the perinatal period. The PEPCK-C gene provides a model for the metabolic control of gene transcription.

Glucocorticoid-stimulated CCAAT/enhancer-binding protein alpha expression is required for steroid-induced G1 cell cycle arrest of minimal-deviation rat hepatoma cells

By genetic correlation with the growth-suppressible phenotype and direct functional tests, we demonstrate that the glucocorticoid-stimulated expression of the CCAAT/enhancer-binding protein alpha (C/EBP alpha) transcription factor is required for the steroid-mediated G1 cell cycle arrest of minimal-deviation rat hepatoma cells. Comparison of C/EBP alpha transcript and active protein levels induced by the synthetic glucocorticoid dexamethasone in glucocorticoid growth-suppressible (BDS1), nonsuppressible receptor-positive (EDR1) and nonsuppressible receptor-deficient (EDR3) hepatoma cell proliferative variants revealed that the stimulation of C/EBP alpha expression is a rapid, glucocorticoid receptor-mediated response associated with the G1 cell cycle arrest. Consistent with the role of C/EBP alpha as a critical intermediate in the growth suppression response, maximal induction of transcription factor mRNA occurred within 2 h of dexamethasone treatment whereas maximal inhibition of [3H] thymidine incorporation was observed 24 h after steroid treatment. As a direct functional approach, ablation of C/EBP alpha protein expression and DNA-binding activity by transfection of an antisense C/EBP alpha expression vector blocked the dexamethasone-induced G1 cell cycle arrest of hepatoma cells but did not alter general glucocorticoid responsiveness. Transforming growth factor beta induced a G1 cell cycle arrest in C/EBP alpha antisense transfected cells, demonstrating the specific involvement of C/EBP alpha in the glucocorticoid growth suppression response. Constitutive expression of a conditionally activated form of C/EBP alpha caused a G1 cell cycle arrest of BDS1 hepatoma cells in the absence of glucocorticoids. In contrast, overexpression of C/EBP beta or C/EBP delta had no effect on hepatoma cell growth. Taken together, these results demonstrate that the steroid-induced expression of C/EBP alpha is necessary to mediate the glucocorticoid G1 cell cycle arrest of rat hepatoma cells and implicates a role for this transcription factor in the growth control of liver-derived epithelial tumor cells.

Tumor necrosis factor alpha promotes nuclear localization of cytokine-inducible CCAAT/enhancer binding protein isoforms in hepatocytes

Hepatocytes were cultured in the presence of recombinant tumor necrosis factor (TNF) alpha or mutated TNF alpha peptides that specifically activate either p55 or p75 TNF receptors to determine if TNF alpha can activate cytokine-inducible CCAAT/enhancer binding protein (C/EBP) isoforms by post-transcriptional mechanisms that are initiated by TNF receptors. Within 5-10 min after treatment with any of these agents, nuclear concentrations of C/EBP beta and C/EBP delta double and remain 2-4-fold greater than control cultures for 30 min (p < 0.01). Consistent with these results, gel mobility shift assays demonstrate 3-fold increased nuclear C/EBP beta- and C/EBP delta-DNA binding activity in TNF alpha-treated cells, and immunocytochemistry confirms rapid redistribution of these C/EBP isoforms into the nucleus. In contrast, mRNA and whole cell protein concentrations of C/EBP beta and delta are not altered by TNF alpha exposure, and nuclear concentrations of another C/EBP isoform, C/EBP alpha, are decreased by 80%. This novel evidence that TNF alpha initiates post-transcriptional activation of cytokine-inducible C/EBP isoforms identifies a mechanism that enables hepatocytes to respond immediately to inflammatory stress.

CCAAT/enhancer-binding protein alpha (C/EBP alpha) inhibits cell proliferation through the p21 (WAF-1/CIP-1/SDI-1) protein

C/EBPalpha has a role in growth arrest and differentiation of mouse preadipocytes. To study the mechanism of C/EBPalpha-induced growth arrest, we developed a cell line, HT1, that contained the human C/EBPalpha gene under Lac repressor control. IPTG-induced C/EBPalpha caused inhibition of cell proliferation and DNA synthesis as measured by colony growth assays, cell counting, and BrdU uptake. A number of proteins that are known to be involved in the regulation of the cell cycle, such as cyclin-dependent kinase (CDK)2 and CDK4, proliferating cell nuclear antigen (PCNA), p53, c-fos, and the CDK inhibitor p16 and p27 were investigated by Western analysis. No change in their expression was observed. However, the p21 (WAF-1/CIP-1/SDI-1) protein was significantly elevated in growth-arrested HT1 cells. Elevation of p21/SDI-1 mRNA (threefold) and activation of the p21/SDI-1 promoter by C/EBPalpha did not account for the 12- to 20-fold increase in p21/SDI-1 protein. Protein synthesis inhibition by cycloheximide (CHX) treatment indicated that the half-life of p21/SDI-1 in dividing HT1 cells was approximately 30 min. However, in C/EBPalpha growth-arrested cells, the level of the p21/SDI-1 did not change for > 80 min after CHX addition. Our studies demonstrate that C/EBPalpha activates p21/SDI-1 by increasing p21/SDI-1 gene expression and by post-translational stabilization of p21/SDI-1 protein. Furthermore, induction of p21/SDI-1 is responsible for the ability of C/EBPalpha to inhibit proliferation because transcription of antisense p21/SDI-1 mRNA eliminated growth inhibition by C/EBPalpha.