CREB controls LAP/C/EBP beta transcription

Identification of transcription factor binding sites upstream of human genes regulated by the phosphatidylinositol 3-kinase and MEK/ERK signaling pathways

We have taken an integrated approach in which expression profiling has been combined with the use of small molecule inhibitors and computational analysis of transcription factor binding sites to characterize regulatory sequences of genes that are targets of specific signaling pathways in growth factor-stimulated human cells. T98G cells were stimulated with platelet-derived growth factor (PDGF) and analyzed by DNA microarrays, which identified 74 immediate-early gene transcripts. Cells were then treated with inhibitors to identify subsets of genes that are targets of the phosphatidylinositol 3-kinase (PI3K) and MEK/ERK signaling pathways. Four groups of PDGF-induced genes were defined: independent of PI3K and MEK/ERK signaling, dependent on PI3K signaling, dependent on MEK/ERK signaling, and dependent on both pathways. The upstream regions of all genes in the four groups were scanned using TRANSFAC for putative cis-elements as compared with a background set of non-induced genes. Binding sites for 18 computationally predicted transcription factors were over-represented in the four groups of co-expressed genes compared with the background sequences (p < 0.01). Many of the cis-elements identified were conserved in orthologous mouse genes, and many of the predicted elements and their cognate transcription factors were consistent with previous experimental data. In addition, chromatin immunoprecipitation assays experimentally verified nine predicted SRF binding sites in T98G cells, including a previously unknown SRF site upstream of DUSP5. These results indicate that groups of human genes regulated by discrete intracellular signaling pathways share common cis-regulatory elements.

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

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

Dioxin increases C/EBPbeta transcription by activating cAMP/protein kinase A

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD = dioxin) has been shown to increase the expression of C/EBPbeta. The modulated expression of C/EBPbeta has been suggested to be associated with toxic responses of TCDD such as wasting syndrome, diabetes, and inhibition of adipocyte differentiation. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of C/EBPbeta. Elevated C/EBPbeta mRNA and protein levels in mouse embryonic fibroblasts (C3H10T(1/2)) and in mouse hepatoma cells (Hepa1c1c7) were correlated with increased binding affinity of the C/EBPbeta protein. Transfection studies with different deletion constructs of the CCAAT/enhancer-binding protein promoter indicated that a small region located 60-120 bp upstream of the start site of transcription is required for activation of the C/EBPbeta gene by TCDD in both cell lines tested. Further analysis using mutation constructs of the C/EBPbeta promoter demonstrated that activation of the C/EBPbeta promoter is mediated through incomplete cAMP-response element-binding protein (CREB) sites located close to the TATA box of the C/EBPbeta gene. The protein kinase A (PKA) inhibitor H89 completely blocks the TCDD-dependent effect on C/EBPbeta promoter activity, indicating that TCDD activates CREB binding via a cAMP/PKA pathway, which is supported by the increased cAMP level and PKA activity observed after TCDD treatment. Gel shift analyses demonstrated that CREB itself binds to the putative CREB motif that mediates the TCDD-dependent effect on C/EBPbeta gene transcription. Cotransfection experiments with CREB and PKA expression plasmids further supported our conclusions that the TCDD-dependent effect on C/EBPbeta transcription is mediated via PKA-dependent CREB activation.

The human caspase-8 promoter sustains basal activity through SP1 and ETS-like transcription factors and can be up-regulated by a p53-dependent mechanism

Caspase-8, also known as MACH/FLICE/Mch5, is the most upstream-located cysteine-aspartyl-protease (caspase) in a caspase cascade involved in apoptosis triggered by members of the tumor necrosis factor receptor superfamily or other stimuli such as chemotherapeutic agents. Regulation of caspase-8 expression on a post-translational level has been studied in detail, whereas only little information is available on its control by gene transcription. We identified and cloned the human caspase-8 promoter, determined the transcriptional start site of the caspase-8 gene, and examined the regulatory mechanisms of the promoter with respect to its basal activity as well as to its inducibility upon apoptotic stimuli in human hepatoma cells. We identified two minimal sequences essential for basal transcription of caspase-8 and demonstrate that a single SP1 and an ETS-like binding motif mediate this effect. We further show that the caspase-8 promoter is inducible and demonstrate that adenoviral infection increases caspase-8 mRNA levels. However, the increase in caspase-8 gene transcription after adenoviral infection absolutely depends on the p53 status of the hepatoma cell line, implying that caspase-8 is a target gene of p53. We show that delivery of exogenous p53 alone is sufficient to induce the caspase-8 promoter even in p53-deficient Hep3B hepatoma cells. Subsequent promoter deletion analysis in combination with luciferase reporter assays identified a p53-responsive element downstream of the transcriptional start site. We demonstrate that this p53-responsive sequence overlaps with the ETS-like binding site and suggest that an additional p53-inducible, yet unknown factor interacts with this region of the caspase-8 promoter. In summary, our study contributes to the understanding of the transcriptional regulation of the caspase-8 gene by basal (SP1- and ETS-dependent) and inducible (p53-dependent) mechanisms.

C/EBPbeta regulation in lipopolysaccharide-stimulated macrophages

C/EBP family members contribute to the induction of the interleukin-12 p40 gene and the genes encoding several other mediators of inflammation. Here, we show by chromatin immunoprecipitation that C/EBPbeta binds the p40 promoter following lipopolysaccharide stimulation of peritoneal macrophages. However, three modes of C/EBPbeta regulation reported in other cell types were not detected, including alternative translation initiation, nuclear translocation, and increased DNA binding following posttranslational modification. In contrast, C/EBPbeta concentrations greatly increased following stimulation via MAP kinase-dependent induction of C/EBPbeta gene transcription. Increased C/EBPbeta concentrations were unimportant for p40 induction, however, as transcription of the p40 gene initiated before C/EBPbeta concentrations increased. Furthermore, disruption of C/EBPbeta upregulation by a MAP kinase inhibitor only slightly diminished p40 induction. Phosphopeptide mapping revealed that endogenous C/EBPbeta in macrophages is phosphorylated on only a single tryptic peptide containing 14 potential phosphoacceptors. This peptide was constitutively phosphorylated in primary and transformed macrophages, in contrast to its inducible phosphorylation in other cell types in response to Ras and growth hormone signaling. Altered-specificity experiments supported the hypothesis that C/EBPbeta activity in macrophages does not require an inducible posttranslational modification. These findings suggest that, although C/EBPbeta contributes to the induction of numerous proinflammatory genes, it is fully active in unstimulated macrophages and poised to stimulate transcription in conjunction with other factors whose activities are induced.

Distinct involvement of cAMP-response element-dependent transcriptions in functional and morphological maturation during retinoid-mediated human myeloid differentiation

We evaluated the involvement of cyclic adenosine monophosphate-response element (CRE)-dependent transcriptions in all-trans retinoic acid (ATRA)-induced myeloid differentiation using human monoblastic U937 cells. ATRA treatment caused an increment in the CRE-dependent transcription activity and induced a wide variety of differentiation phenotypes including functional and morphological maturation. Indeed, ATRA treatment induced the expression of CCAAT/enhancer-binding protein beta (C/EBPbeta), a CRE-dependent transcription factor important in monocytic differentiation, and the inhibition of CRE-enhancer activity by the expression of a dominant-negative CRE-binding protein (dn-CREB) abolished the induction of C/EBPbeta. Functional maturation, such as the enhancement of cell adhesion and respiratory burst activity, was dramatically suppressed by the expression of dn-CREB. In addition, the differentiation-dependent induction of an adhesion molecule (CD11b), the phagocyte oxidase required for respiratory burst, and the transcription factor PU.1 responsible for phagocyte oxidase induction were all abolished by dn-CREB. Surprisingly, morphological maturation, including nuclear convolution and cytoplasmic vacuolar formation, was augmented by dn-CREB. Under the same conditions, the differentiation-associated cell-growth arrest was not affected by the expression of dn-CREB. Our results clearly indicate that CRE-driven transcription plays at least three distinct roles during myeloid differentiation: It stimulates functional maturation but suppresses morphological maturation and has no effects on cell-growth arrest.

The role of C/EBPbeta in mammary gland development and breast cancer

The CCAAT/enhancer binding protein (C/EBP) family of bZIP transcription factors control the proliferation and differentiation of a variety of tissues. While C/EBPalpha and -delta are also expressed in the mammary gland, the multiple protein isoforms of C/EBPbeta appear to play a critical role in mammary gland development and breast cancer. Targeted deletion of all the C/EBPbeta isoforms results in a severe inhibition of lobuloalveolar development and a block to functional differentiation, as well as more subtle changes in ductal morphogenesis. The altered expression of a number of molecular markers, including the progesterone, estrogen, and prolactin receptors, the transporter proteins (NKCC1 and aquaporin 5), and several markers of skin differentiation (Sprr2A and keratin 6), suggests that germline deletion of C/EBPbeta results in an altered cell fate. Thus, C/EBPbeta appears to play a role in the specification of progenitor cell fate not only in the mammary gland, but also in a number of other tissues.

Analysis of DNase I-hypersensitive sites in the chromatin of the chicken C/EBPbeta gene reveals multiple cis-regulatory elements

CCAAT box/enhancer binding protein beta (C/EBPbeta), a member of the basic region-leucine zipper (bzip) class of transcription factors, plays important roles during differentiation of certain cell types, such as liver cells, fat cells and myelomonocytic cells. C/EBPbeta is highly expressed in these cell types, and activates specific genes during their differentiation. In the hematopoietic system, C/EBPbeta expression is restricted to the myelomonocytic lineage. To investigate the molecular basis of the cell-type specific expression of the C/EBPbeta gene in hematopoietic cells we have cloned the chicken C/EBPbeta gene and mapped DNase I hypersensitive sites (DHS) in the vicinity of the gene, using myelomonocytic as well as other cell types. We show that there are multiple nuclease-sensitive sites, most of which are cell-type specific, suggesting that they might act as cell-type specific cis-regulatory DNA elements. To study the possible function of these elements we have constructed reporter genes containing these sequences and analyzed their activity in different cell types. Our results show that several of the nuclease-sensitive regions act as cis-acting stimulatory elements in myelomonocytic but not in other cells. Taken together, our data suggest that the expression of the C/EBPbeta gene in myelomonocytic cells is controlled by multiple cell-type specific cis-acting sequences located both upstream and downstream of the gene.

Nongenomic actions of androgen in Sertoli cells

Nongenomic actions mediated by androgens have now been described in more than 10 cell types. Some of these cells transduce androgen signals using surface receptors that await final characterization, whereas other cells employ the classical AR. Various second messengers can be activated by androgens, including cAMP, IP3, phospholipase C, DAG, and Ca2+. Each of these second messengers is capable of activating multiple kinases. One of the most important kinase networks to be regulated by androgens is the MAP kinase cascade. This series of kinase reactions is capable of altering the activity of many transcription factors with important implications for the regulation of gene expression. Because there is evidence that androgen is capable of regulating CREB-mediated gene expression via the MAP kinase pathway, it is now somewhat misleading to characterize androgen actions in Sertoli cells as nongenomic. Instead, it may be more appropriate to label these activities as independent of AR-DNA interactions, or more simply as nonclassical. The nonclassical regulation of gene expression in Sertoli cells is particularly relevant for providing an answer to the paradox of how testosterone can support spermatogenesis yet regulate few genes via AR-promoter interactions. It is expected that with the increasing use of microarray and related technologies, additional AR-regulated genes will be identified. However, the androgen-induced increases in [Ca2+]i, the activation of Src kinase, and the MAP kinase cascade that have been characterized thus far have the potential to regulate the expression of many more genes than is possible by direct AR-promoter interactions. Thus, it is likely that nonclassical actions of testosterone in Sertoli cells will be found to be a necessary complement to the classical actions that are required to maintain spermatogenesis.

Transcriptional regulation of the rat type IIA phospholipase A2 gene by cAMP and interleukin-1beta in vascular smooth muscle cells: interplay of the CCAAT/enhancer binding protein (C/EBP), nuclear factor-kappaB and Ets transcription factors

The abundant secretion of type IIA secreted phospholipase A(2) (sPLA(2)) is a major feature of the inflammatory process of atherosclerosis. sPLA(2) is crucial for the development of inflammation, as it catalyses the production of lipid mediators and induces the proliferation of smooth muscle cells. We have analysed the activation of sPLA(2) transcription by cAMP and interleukin-1beta (IL-1beta), and shown that the 500 bp region upstream of the transcription start site of the rat sPLA(2) gene is implicated in activation by synergistically acting cAMP and IL-1beta. We transiently transfected and stimulated rat smooth muscle cells in primary culture and measured the promoter activities of serial and site-directed deletion mutants of sPLA(2)-luciferase constructs. A distal region, between -488 and -157 bp, bearing a CAAT/enhancer binding protein (C/EBP)-responsive element (-242 to -223) was sufficient for cAMP/protein kinase A-mediated sPLA(2) promoter activation. We find evidence for the first time that activation of the sPLA(2) promoter by IL-1beta requires activation of an Ets-responsive element in the -184 to -180 region of the distal promoter via the Ras pathway and a nuclear factor-kappaB site at positions -141 to -131 of the proximal promoter. We also used electrophoretic mobility shift assays to identify five binding sites for the Sp1 factor; a specific inhibitor of Sp1, mithramycin A, showed that this factor is crucial for the basal activity of the sPLA(2) promoter.

Different mechanisms for loss and recovery of binocularity in the visual cortex

Diverse molecular mechanisms have been discovered that mediate the loss of responses to the deprived eye during monocular deprivation. cAMP/Ca2+ response element-binding protein (CREB) function, in particular, is thought to be essential for ocular dominance plasticity during monocular deprivation. In contrast, we have very little information concerning the molecular mechanisms of recovery from the effects of monocular deprivation, even though this information is highly relevant for understanding cortical plasticity. To test the involvement of CREB activation in recovery of responses to the deprived eye, we used herpes simplex virus (HSV) to express in the primary visual cortex a dominant-negative form of CREB (HSV-mCREB) containing a single point mutation that prevents its activation. This mutant was used to suppress CREB function intracortically during the period when normal vision was restored in two protocols for recovery from monocular deprivation: reverse deprivation and binocular vision. In the reverse deprivation model, inhibition of CREB function prevented loss of responses to the newly deprived eye but did not prevent simultaneous recovery of responses to the previously deprived eye. Full recovery of cortical binocularity after restoration of binocular vision was similarly unaffected by HSV-mCREB treatment. The HSV-mCREB injections produced strong suppression of CREB function in the visual cortex, as ascertained by both DNA binding assays and immunoblot analysis showing a decrease in the expression of the transcription factor C/EBPbeta, which is regulated by CREB. These results show a mechanistic dichotomy between loss and recovery of neural function in visual cortex; CREB function is essential for loss but not for recovery of deprived eye responses.

Nopp140 is a mediator of the protein kinase A signaling pathway that activates the acute phase response alpha1-acid glycoprotein gene

The acute phase response (APR) in liver during inflammation is one of the well known examples for elucidating the signaling pathways that lead to the combinatorial regulation of gene expression. The APR is exemplified by alpha(1)-acid glycoprotein gene (agp) expression. A number of transcription factors, including CCAAT/enhancer-binding protein beta (C/EBPbeta), glucocorticoid receptor, cAMP-response element-binding protein (CREB), and Nopp140, are known to participate in its induction. The underlying mechanism of Nopp140 and other factors for regulating agp expression remains unclear. Here we demonstrate that protein kinase A (PKA)-dependent phosphorylation of Nopp140, together with C/EBPbeta, induces agp gene expression synergistically. The cooperative activation of the agp gene by Nopp140 and forskolin is sensitive to inhibition by PKI. Results from biochemical and functional characterizations of Nopp140 mutants defective in PKA phosphorylation sites suggest that PKA-dependent Nopp140 phosphorylation is important for its role in agp gene activation. Furthermore, maximal activation of the agp gene by PKA-phosphorylated Nopp140 depends on the presence of CREB and C/EBPbeta. The participation of CREB in the activation is, however, independent of its PKA-mediated phosphorylation. In summary, we demonstrate the existence of a novel Nopp140-mediated PKA signaling pathway that leads to the activation of agp, one of the major acute phase response genes.

Different mechanisms for loss and recovery of binocularity in the visual cortex

Diverse molecular mechanisms have been discovered that mediate the loss of responses to the deprived eye during monocular deprivation. cAMP/Ca2+ response element-binding protein (CREB) function, in particular, is thought to be essential for ocular dominance plasticity during monocular deprivation. In contrast, we have very little information concerning the molecular mechanisms of recovery from the effects of monocular deprivation, even though this information is highly relevant for understanding cortical plasticity. To test the involvement of CREB activation in recovery of responses to the deprived eye, we used herpes simplex virus (HSV) to express in the primary visual cortex a dominant-negative form of CREB (HSV-mCREB) containing a single point mutation that prevents its activation. This mutant was used to suppress CREB function intracortically during the period when normal vision was restored in two protocols for recovery from monocular deprivation: reverse deprivation and binocular vision. In the reverse deprivation model, inhibition of CREB function prevented loss of responses to the newly deprived eye but did not prevent simultaneous recovery of responses to the previously deprived eye. Full recovery of cortical binocularity after restoration of binocular vision was similarly unaffected by HSV-mCREB treatment. The HSV-mCREB injections produced strong suppression of CREB function in the visual cortex, as ascertained by both DNA binding assays and immunoblot analysis showing a decrease in the expression of the transcription factor C/EBPbeta, which is regulated by CREB. These results show a mechanistic dichotomy between loss and recovery of neural function in visual cortex; CREB function is essential for loss but not for recovery of deprived eye responses.

Insulin and sterol-regulatory element-binding protein-1c (SREBP-1C) regulation of gene expression in 3T3-L1 adipocytes. Identification of CCAAT/enhancer-binding protein beta as an SREBP-1C target

We evaluated the hypothesis of sterol-regulatory element-binding protein (SREBP)-1c being a general mediator of the transcriptional effects of insulin, with a focus on adipocytes, in which insulin profoundly influences specific gene expression. Using real time quantitative reverse transcriptase-PCR to monitor changes in the expression of about 50 genes that cover a wide range of adipocyte functions, we have compared the impact of insulin treatment with that of adenoviral overexpression of either dominant positive or dominant negative SREBP-1c mutants in 3T3-L1 adipocytes. As expected, insulin up-regulated, dominant positive stimulated, and dominant negative decreased previously characterized direct SREBP targets (FAS, SCD-1, and low density lipoprotein receptor). We also identified three novel SREBP-1c transcriptional targets in adipocytes, which were confirmed by run-on assays: plasminogen activator inhibitor 1, CCAAT/enhancer-binding protein delta (C/EBPdelta), and C/EBPbeta. Because most insulin-regulated genes were also modulated by SREBP-1c mutants, our data establish that 1) SREBP-1c is an important mediator of insulin transcriptional effects in adipocytes, and 2) C/EBPbeta is under the direct control of SREBP-1c, as demonstrated by the ability of SREBP-1c to activate the transcription from C/EBPbeta promoter through canonical SREBP binding sites. Thus, some of the effects of insulin and/or SREBP-1c in mature fat cells might require C/EBPbeta or C/EBPdelta as transcriptional relays.

Inhibition of the NF-kappaB transcriptional activity by protein kinase A

The cAMP-dependent protein kinase (PKA) signaling pathway plays a major role in a number of pathophysiological conditions. However, there have been conflicting evidences regarding the action of cAMP/PKA on nuclear factor- kappaB (NF-kappaB). In this study, we have explored the effect of cAMP/PKA on NF-kappaBeta activity and determined its molecular mechanism. PKA activating agents or expression of the catalytic subunit of PKA (PKAc) inhibited the NF-kappaBeta-dependent reporter gene expression induced by tumor necrosis factor alpha (TNFalpha). PKA activators affected neither IkappaBalpha phosphorylation, IkappaBetaalpha degradation, nor the NF-kappaBeta/DNA binding. Expression of PKAc inhibited the transactivation potential of Gal4-p65 (286-551) suggesting that the inhibitory action of PKA is through the C-terminal transactivation domain of p65 but not by phosphorylation of the consensus PKA recognition site containing serine at position 276. Overexpression of coactivators, CBP (CREB-binding protein) and p300, failed to reverse the PKA-mediated inhibition of p65 transactivation. Thus, the inhibitory action of the cAMP/PKA pathway on the transcriptional activity of NF-kappaB appears to be exhibited by modifying the C-terminal transactivation domain of p65, either directly or indirectly.

Organization of the promoter region of the human NF-IL6 gene

In monocyte/macrophages, the human NF-IL6 gene was activated by LPS or PMA. However, a robust response required stimulation of cells with both LPS and PMA. To examine the molecular basis of this response, we isolated human genomic DNA and determined the nucleotide sequence of a segment (6.4 kb) that included the transcription initiation site of the gene. The unique sequences in the 6.4-kb DNA include several potential transcription factor-binding elements that may explain the molecular basis of the activation of the human NF-IL6 gene by signaling molecules that control the immune and inflammatory responses. Deletion analysis localized an LPS+PMA responsive region downstream position -287, with respect to the transcription initiation site of the NF-IL6 gene. The responsive region includes a potential site for interactions with CREB and a region (-287 to -247) that interacts with SP1 and SP3. In functional assays, the potential CREB site responded to cellular stimulation. The region that interacted with SP1 and SP3 augmented the overall level of activity produced in response to LPS+PMA.

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

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

Transcriptional activation of C/EBPbeta gene by c-Jun and ATF2

C/EBPbeta is one of the key transcription factors responsible for the induction of a wide array of genes. Like many proto-oncogenes and transcription factors, transcription of C/EBPbeta gene can be induced by multiple extracellular signals. Using nuclear extracts from lipopolysaccharide (LPS)-stimulated mouse liver, five trans-acting factor-binding motifs, URE1 (-376 to -352), URE2 (-253 to -223), URE3 (-220 to -190), URE4 (-123 to -103), and URE5 (-72 to -45) were identified by DNAse I footprinting assays. Competition and supershift analysis of the complexes formed at the URE2 and URE4 indicated that they contain CREB/ATF and AP-1 family factors. Furthermore, recombinant ATF2 and c-Jun proteins from mammalian and bacterial cells can bind to URE2 and URE4 but not URE1. Cotransfection experiments showed that ATF2 and c-Jun activate the C/EBPbeta gene expression cooperatively through URE2 and URE4, and this activation was greatly increased under the treatment of low concentration of anisomycin. During acute phase response, the phosphorylation of c-Jun and ATF2 was found to correlate with C/EBPbeta gene expression. Taken together, our results provide the evidences that both c-Jun and ATF2 are the regulators of C/EBPbeta gene.

Jun kinase modulates tumor necrosis factor-dependent apoptosis in liver cells

Tumor necrosis factor (TNF) triggers distinct pathways in liver cells through TNF receptor 1 (TNF-R1) via adapter molecules, including the intracellular cascades leading to apoptosis, nuclear factor-kappa B (NF-kappa B), and Jun kinase (JNK) activation. TNF-dependent activation of NF-kappa B induces the transcription of antiapoptotic genes that renders liver cells resistant against TNF-induced apoptosis. In contrast, the role of JNK during TNF-induced apoptosis is less clear, so we studied its role during this process. Hepatoma cells treated with TNF and cycloheximide undergo apoptosis, which is proceeded by a strong activation of JNK. Adenoviral vectors (adv) were tested to block TNF-dependent JNK activation selectively. An adv expressing dominant-negative (dn) TRAF2 inhibited only JNK and not ERK or NF-kappa B activation. However, the effect of inhibiting JNK activation with a dn TAK1 virus was also specific but was stronger than that via dn TRAF2. In further experiments, the inhibitory effect of dn TAK1 on JNK was used to define its role during TNF-dependent apoptosis. Inhibition of JNK by adv dn TAK1 resulted in an earlier and stronger induction of apoptosis. Interestingly, TAM67, a dn form of c-Jun, did not mediate the JNK-dependent effect on TNF-dependent apoptosis, indicating that other molecular targets are essential to confer this mechanism. However, the modified apoptosis pattern could be inhibited by adv expressing Bcl-2 or dn FADD. In conclusion, we define TAK1 as a kinase specifically involved in TNF-induced JNK activation in hepatoma cells and show that JNK transduces antiapoptotic signals, which modulate the strength and time course of FADD-dependent cell death involving mitochondrial permeability transfer.

The induction of cyclooxygenase-2 mRNA in macrophages is biphasic and requires both CCAAT enhancer-binding protein beta (C/EBP beta ) and C/EBP delta transcription factors

Prostaglandins are important mediators of activated macrophage functions, and their inducible synthesis is mediated by cyclooxygenase-2 (COX-2). Here, we make use of the murine macrophage cells RAW264 as well as of immortalized macrophages derived from mice deficient for the transcription factor CCAAT enhancer-binding protein beta (C/EBP beta) to explore the molecular mechanisms regulating COX-2 induction in activated macrophages. We demonstrate that lipopolysaccharide-mediated COX-2 mRNA induction is biphasic. The initial phase is independent of de novo protein synthesis, correlates with cAMP-response element-binding protein (CREB) activation, is inhibited by treatments that abolish CREB phosphorylation and reduce NF-kappa B-mediated gene activation, and requires the presence of the transcription factor C/EBP beta. On the other hand, C/EBP delta appears to be essential in addition to C/EBP beta to effect the second phase of COX-2 gene transcription, which is important for maintaining the induced state and requires de novo protein synthesis. Indeed, both phases of COX-2 induction were defective in C/EBP beta-/- macrophages. Moreover, the synthesis of C/EBP delta was increased dramatically by treatment with lipopolysaccharide and, like COX-2 induction, repressed by combined inhibition of the MAPK and of the SAPK2/p38 cascades. Taken together, these data identify CREB, NF-kappa B, and both C/EBP beta and -delta as key factors in coordinately orchestrating transcription from the COX-2 promoter in activated macrophages.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

Anti-mycotics suppress interleukin-4 and interleukin-5 production in anti-CD3 plus anti-CD28-stimulated T cells from patients with atopic dermatitis

It is reported that anti-mycotic agents are effective for the treatment of patients with atopic dermatitis. We studied the in vitro effects of anti-mycotics on T helper-1 and T helper-2 cytokine production in anti-CD3 plus anti-CD28-stimulated T cells from atopic dermatitis patients and normal donors. The amounts of interleukin-4 and interleukin-5 secreted by anti-CD3/CD28-stimulated T cells were higher in atopic dermatitis patients than in normal donors. Azole derivatives, ketoconazole, itraconazole, miconazole, and nonazole terbinafine hydrochloride, and tolnaftate reduced interleukin-4 and interleukin-5 secretion without altering that of interferon-gamma and interleukin-2 in anti-CD3/CD28-stimulated T cells from both atopic dermatitis patients and normal donors. The azole derivatives were more inhibitory than nonazole anti-mycotics. These anti-mycotics reduced the anti-CD3/CD28-induced mRNA expression and promoter activities for interleukin-4 and interleukin-5. The 3',5'-cyclic adenosine monophosphate analog dibutyryl 3',5'-cyclic adenosine monophosphate reversed the inhibitory effects of the anti-mycotics on interleukin-4 and interleukin-5 secretion, mRNA expression, and promoter activities. Anti-CD3/CD28 transiently (< or = 5 min) increased intracellular 3',5'-cyclic adenosine monophosphate in T cells, and the increase was greater in atopic dermatitis patients than in normal donors. The increase of 3',5'-cyclic adenosine monophosphate by anti-CD3/CD28 correlated with interleukin-4 and interleukin-5 secretion by anti-CD3/CD28. The transient 3',5'-cyclic adenosine monophosphate increase was suppressed by anti-mycotics, and azole derivatives were more suppressive than nonazoles. Azole derivatives inhibited the activity of cyclic adenosine monophosphate-synthesizing adenylate cyclase whereas terbinafine hydrochloride and tolnaftate enhanced the activity of 3',5'-cyclic adenosine monophosphate-hydrolyzing cyclic nucleotide phosphodiesterase in atopic dermatitis and normal T cells. These results suggest that the anti-mycotics may suppress interleukin-4 and interleukin-5 production by reducing 3',5'-cyclic adenosine monophosphate signal, and stress their potential use for the suppression of T helper-2-mediated allergic reactions.

Ras adenoviruses modulate cyclin E protein expression and DNA synthesis after partial hepatectomy

Ras-genes encode for proteins important for transmitting extracellular signals from the cytoplasm to the nucleus. In this study we investigated the impact of Ras on cell cycle progression after hepatectomy by using adenoviral vectors (adv) expressing beta-galactosidase (beta-gal), a dominant-negative (Ras N17) or a dominant-active (Ras 61L) form of H-Ras. Partial hepatectomy was performed in mice treated with the different adenoviruses and cell cycle progression was studied by analysing factors involved in cell cycle control during liver regeneration. After hepatectomy, adv Ras 61L increases DNA synthesis significantly in comparison to the other treatment groups. Higher Ras activity results in an early increase of transcriptional active E2F-3, which is associated with higher cyclin E, but almost unchanged cyclin D protein expression. However, Northern blot analysis and cyclin E promoter experiments indicate that, besides transcriptional mechanisms also post-transcriptional mechanisms are involved in regulating cyclin E protein expression after partial hepatectomy in mice treated with adv Ras 61L. Cyclin E phosphorylation studies demonstrate that adv Ras 61L results in hypophosphorylation of cyclin E compared to the control group at early time points after hepatectomy, when cyclin E protein expression strongly increases and there is only a minor effect on cyclin E mRNA levels. Our experiments indicate adv Ras 61L in vivo increases Cyclin E expression by higher transcription via E2F and a post-transcriptional mechanism. These mechanisms result in an earlier activation of an active CDK2/Cyclin E complex which, in turn, triggers DNA synthesis.

Gangliosides GD1b, GT1b, and GQ1b suppress the growth of human melanoma by inhibiting interleukin-8 production: the inhibition of adenylate cyclase

We studied the effects of various gangliosides on in vitro growth of human metastatic melanoma WM266-4. GD1b, GT1b, and GQ1b inhibited 3H-thymidine uptake and growth rate of WM266-4 whereas the other gangliosides were ineffective. The growth inhibition by GD1b, GT1b, and GQ1b was counteracted by interleukin-8 but not by the other growth factors. The growth inhibition by gangliosides was not detected in the presence of anti-interleukin-8 antibody. GD1b, GT1b, and GQ1b reduced the constitutive interleukin-8 secretion and mRNA levels in WM266-4. Transient transfection showed that GD1b, GT1b, and GQ1b inhibited the constitutive chloramphenicol acetyltransferase expression driven by interleukin-8 promoter in WM266-4. Transfection with a series of 5'-deleted mutants demonstrated that the sequences between -98 and -62 bp on interleukin-8 promoter may be involved in the transcriptional repression by these gangliosides. Cyclic AMP analog dibutyryl cAMP counteracted GD1b, GT1b, and GQ1b-induced inhibition of interleukin-8 production at the levels of protein secretion, mRNA expression, and promoter activity. GD1b, GT1b, and GQ1b reduced cAMP level and protein kinase A activity in WM266-4. These gangliosides suppressed adenylate cyclase activity without altering that of cyclic nucleotide phosphodiesterase in WM266-4. The data indicate that GD1b, GT1b, and GQ1b may suppress the growth of melanoma by inhibiting interleukin-8 production via the inhibition of adenylate cyclase.

Prostaglandins are required for CREB activation and cellular proliferation during liver regeneration

The liver responds to multiple types of injury with an extraordinarily well orchestrated and tightly regulated form of regeneration. The response to partial hepatectomy has been used as a model system to elucidate the molecular basis of this regenerative response. In this study, we used cyclooxygenase (COX)-selective antagonists and -null mice to determine the role of prostaglandin signaling in the response of liver to partial hepatectomy. The results show that liver regeneration is markedly impaired when both COX-1 and COX-2 are inhibited by indocin or by a combination of the COX-1 selective antagonist, SC-560, and the COX-2 selective antagonist, SC-236. Inhibition of COX-2 alone partially inhibits regeneration whereas inhibition of COX-1 alone tends to delay regeneration. Neither the rise in IL-6 nor the activation of signal transducer and activator of transcription-3 (STAT3) that is seen during liver regeneration is inhibited by indocin or the selective COX antagonists. In contrast, indocin treatment prevents the activation of CREB by phosphorylation that occurs during hepatic regeneration. These data indicate that prostaglandin signaling is required during liver regeneration, that COX-2 plays a particularly important role but COX-1 is also involved, and implicate the activation of CREB rather than STAT3 as the mediator of prostaglandin signaling during liver regeneration.

Molecular characterization of the Xenopus CCAAT-enhancer binding protein beta gene promoter

Transcription factors belonging to the CCAAT-enhancer binding protein (C/EBP) family play key roles in the regulation of genes implicated in the control of growth, differentiation, metabolism, and inflammation. The recent limited studies on the promoter regions of C/EBP genes, particularly C/EBPalpha, have indicated the potential existence of species-specific regulatory mechanisms. It is therefore essential that the promoter regions of different C/EBP genes from a wide range of species are investigated in detail. As an important step toward this goal, we report here the characterization of the Xenopus laevis C/EBPbeta gene promoter. Sequence analysis showed that the 1.6-kb promoter region contained putative binding sites for several transcription factors that have previously been implicated in the regulation of the C/EBPs, including C/EBP, CREB, Myb, STAT, and USF. The -288/+91 promoter region was capable of directing high levels of expression in the hepatoma Hep3B cell line. In addition, this minimal promoter could be autoregulated by both C/EBPalpha and C/EBPbeta and activated by lipopolysaccharide, interleukin-6 and CREB. These results therefore demonstrate that several aspects of C/EBPbeta regulation in mammals have been highly conserved in amphibians. However, a comparison of C/EBPbeta gene promoters characterized to date does indicate the existence of species-specific differences in autoregulation.

Autoregulation enables different pathways to control CCAAT/enhancer binding protein beta (C/EBP beta) transcription

CCAAT/enhancer binding protein beta (C/EBP beta) also named liver-enriched transcriptional activating protein (LAP) is a member of the C/EBP family of transcription factors and is involved in hepatocyte-specific gene expression and in the process of tissue differentiation. The activity of LAP/C/EBP beta can be regulated at the transcriptional and posttranslational level or by protein-protein interaction with other transcription factors. In this study we show that LAP/C/EBP beta can stimulate its own transcription. Deletion analysis of the rat LAP/C/EBP beta promoter in luciferase reporter gene experiments demonstrated that the region located between nucleotide -121 to -71, comprising two recently characterized cAMP responsive element (CRE)-like elements, is important for autoregulation. Gel shift experiments using oligonucleotides with overlapping point mutations identified the sequence GCAATGA (beta-site) adjacent to and partially overlapping the first CRE-like site as core motif for LAP/C/EBP beta binding. Analysis of a mutated beta-site in reporter gene experiments showed the functional relevance of this site for autoregulation. The composite C/EBP beta-CRE-element in the promoter enables synergistic activation of transcription by LAP/C/EBP beta and the protein kinase A (PKA)/cAMP responsive element binding protein (CREB) pathway in a cell-type specific manner. In hepatoma cells nuclear factor kappa B (NF-kappa B) increased autoregulation and therefore could mediate enhanced activation during inflammatory responses. In summary, our results demonstrated that the assembly of the three binding sites in the promoter and thus the interaction between LAP/C/EBP beta and members of the CREB or NF-kappa B family allows the control of LAP/C/EBP beta gene transcription as a response to different stimuli in a tissue specific manner.

Interleukin-6-induced tethering of STAT3 to the LAP/C/EBPbeta promoter suggests a new mechanism of transcriptional regulation by STAT3

LAP/C/EBPbeta is a member of the C/EBP family of transcription factors and contributes to the regulation of the acute phase response in hepatocytes. Here we show that IL-6 controls LAP/C/EBPbeta gene transcription and identify an IL-6 responsive element in the LAP/C/EBPbeta promoter, which contains no STAT3 DNA binding motif. However, luciferase reporter gene assays showed that STAT3 activation through the gp130 signal transducer molecule is involved in mediating IL-6-dependent LAP/C/EBPbeta transcription. Southwestern analysis indicated that IL-6 induces binding of a 68-kDa protein to the recently characterized CRE-like elements in the LAP/C/EBPbeta promoter. Transfection experiments using promoter constructs with mutated CRE-like elements revealed that these sites confer IL-6 responsiveness. Further analysis using STAT1/STAT3 chimeras identified specific domains of the protein that are required for the IL-6-dependent increase in LAP/C/EBPbeta gene transcription. Overexpression of the amino-terminal domain of STAT3 blocked the IL-6-mediated response, suggesting that the STAT3 amino terminus has an important function in IL-6-mediated transcription of the LAP/C/EBPbeta gene. These data lead to a model of how tethering STAT3 to a DNA-bound complex contributes to IL-6-dependent LAP/C/EBPbeta gene transcription. Our analysis describes a new mechanism by which STAT3 controls gene transcription and which has direct implication for the acute phase response in liver cells.

Links among growth factors, hormones, and nuclear factors with essential roles in bone formation

Research performed during the last several years implicates important roles for a variety of growth factors that affect osteoblasts or their precursors during bone development, remodeling, or repair. Of these, three families of growth factors in particular-the transforming growth factor betas (TGF-betas), insulin-like growth factors (IGFs), and bone morphogenetic proteins (BMPs)-are considered to be principal local regulators of osteogenesis, although none is specific for cells of the osteoblast lineage. Therefore, mechanisms to induce skeletal tissue specificity might occur through interactions among these growth factors, with circulating hormones, or through specific intracellular mediators. In the latter case, even more recent studies point to two nuclear transcription factors, termed Core Binding Factor a1 (CBFa1) and CCAAT/Enhancer Binding Protein delta (C/EBPdelta), as significant regulators of the expression or activity of specific bone growth factors or their receptors. Perhaps more importantly, events that link these growth factors to nuclear proteins occur in response to glucocorticoids, sex steroids, parathyroid hormone (PTH), or prostaglandin E2 (PGE2), which themselves have well-known effects on bone biology. In this review, we discuss the situations and processes that initially suggested growth-factor- and hormone-specific interactions on cells within the osteoblast lineage, and present evidence for roles that CBFa1 and C/EBPdelta have on osteoblast function. Finally, we offer examples for how these factors integrate events that are associated with various aspects of bone formation.

HIV-1-associated central nervous system dysfunction

Despite more than 15 years of extensive investigative efforts, a complete understanding of the neurological consequences of HIV-1 CNS infection remains elusive. Although the resources of numerous investigators have been focused on studies of HIV-1-associated CNS disease, the complex nature of the disease processes that underlie the clinical, pathological, and cellular manifestations of HIV-1 CNS infection have required a larger volume of studies than was initially envisioned. Several major areas remain as the focus of current research efforts. One of the more pressing issues facing researchers and clinicians alike is the search for correlates to the development of HIV-1-associated CNS neuropathology and the onset of HIVD. Although numerous parameters have been studied, none have been shown to be absolute predictors or markers of HIV-1-related CNS dysfunction. The identification of solid correlates of HIVD is an important goal that would permit clinical identification of individuals at risk for developing potentially crippling, life-threatening CNS abnormalities and would facilitate early treatment of nascent neurological problems. A more complete comprehension of the cellular foundations of CNS dysfunction and HIVD is also a fundamental part of strategies designed to treat or prevent HIV-1-associated CNS disease. Future investigations will strive to expand the body of knowledge concerning the complex interactions between infected and uninfected neuroglial cells and the roles of numerous cytokines, chemokines, and other soluble agents that are deregulated during HIV-1 CNS infection. In particular, a thorough understanding of the mechanisms of neurotoxicity may facilitate the development of new therapies that alleviate or eliminate the clinical consequences of CNS infection. Finally, investigators will continue to study HIVD within the context of single and combination drug therapies used in the treatment of HIV-1 infection and AIDS. As newer and more effective systemic treatments for HIV-1 infection and AIDS are introduced, the effects of these treatments on the onset, incidence, and severity of HIVD will also require intensive study. The impact of drug therapies on the ability of the CNS to act as an HIV-1 reservoir will also need to be addressed. Introduction of each new drug or drug combination will necessitate studies of drug penetration into the CNS and efficacy against the development of CNS abnormalities. Furthermore, as more effective treatments prolong the lifespan of individuals infected with HIV-1, the impact of extended survival on the occurrence and severity of HIVD will also require further investigations. The quest for answers to these and other questions will be complicated by the diversity of experimental systems used to study different aspects of HIV-1 CNS infection and HIVD. Each system has its own unique strengths and weaknesses. Clinical observations provide a continuous spectrum of symptomatic findings but reveal little about the underlying mechanisms of disease. In vivo imaging techniques, such as CT and MRI, also provide a continuum of observations, but the images are limited in their resolution. Neuropathological examinations of postmortem HIV-1-infected brains offer gross, cellular, and molecular views (including phenotypic and genotypic analyses of CNS viral isolates) of the diseased brain, but only provide a snapshot of the end-stage neurologic dysfunction. Studies that rely on animal surrogates for HIV-1, including SIV, simian-HIV (SHIV), feline immunodeficiency virus (FIV), visna virus, and HIV-1 SCID-hu models, permit experimental protocols that cannot be carried out in humans, but are limited by the fidelity with which each virus and animal model emulates the conditions and events observed in the human host. Finally, in vitro techniques, which include the use of primary cells and cell lines, adult or fetal human cell cultures, and BBB barrier model systems, are also convenient means by which aspe

Fornix-dependent induction of hippocampal CCAAT enhancer-binding protein [beta] and [delta] Co-localizes with phosphorylated cAMP response element-binding protein and accompanies long-term memory consolidation

The cAMP response element-binding protein (CREB) is an evolutionarily conserved transcription regulator essential for long-term memory formation. It is not known, however, whether the molecular events downstream of CREB activation are also conserved. An early, cAMP-dependent event necessary for learning-related long-term synaptic plasticity in the invertebrate Aplysia californica is the induction of the transcription factor CCAAT enhancer-binding protein (C/EBP). Here we show that two homologs in the rat, C/EBPbeta and C/EBPdelta, are induced at discrete times after inhibitory avoidance learning and co-localize with phosphorylated CREB in the hippocampus. This induction is blocked by fornix lesions, which are known to disrupt activation of CREB in the hippocampus and to impair memory consolidation. These results indicate that C/EBPs are evolutionarily conserved components of the CREB-dependent gene cascade activated in long-term memory.

Fornix-dependent induction of hippocampal CCAAT enhancer-binding protein [beta] and [delta] Co-localizes with phosphorylated cAMP response element-binding protein and accompanies long-term memory consolidation

The cAMP response element-binding protein (CREB) is an evolutionarily conserved transcription regulator essential for long-term memory formation. It is not known, however, whether the molecular events downstream of CREB activation are also conserved. An early, cAMP-dependent event necessary for learning-related long-term synaptic plasticity in the invertebrate Aplysia californica is the induction of the transcription factor CCAAT enhancer-binding protein (C/EBP). Here we show that two homologs in the rat, C/EBPbeta and C/EBPdelta, are induced at discrete times after inhibitory avoidance learning and co-localize with phosphorylated CREB in the hippocampus. This induction is blocked by fornix lesions, which are known to disrupt activation of CREB in the hippocampus and to impair memory consolidation. These results indicate that C/EBPs are evolutionarily conserved components of the CREB-dependent gene cascade activated in long-term memory.

The human PER1 gene is transcriptionally regulated by multiple signaling pathways

The mammalian period (Per) genes are components of the circadian clock and appear to be regulated via an autoregulatory feedback loop. Here we show that the human PER1 (hPER1) gene is synergistically activated by protein kinases A and C (PKA, PKC) and cAMP responsive element binding protein. Activators and inhibitors of PKA as well as PKC modulate endogenous hPER1 expression and hPER1 promoter-driven reporter gene activity in a dose-dependent manner. Our results suggest that the hPER1 promoter acts as a sensor for multiple signaling molecules thereby integrating different physiological parameters. This regulation of hPER1 appears to be significant for rapid adaptation to changing environmental conditions.

Protein kinase A-dependent stimulation of rat type II secreted phospholipase A(2) gene transcription involves C/EBP-beta and -delta in vascular smooth muscle cells

Type II secreted phospholipase A(2) (sPLA(2)) releases precursors of important inflammatory lipid mediators from phospholipids. Some observations have indicated that the sPLA(2), which has been implicated in chronic inflammatory conditions such as arthritis, contributes to atherosclerosis in the arterial wall. sPLA(2) was not detected in control vascular smooth muscle cells (VSMC). Treatment of VSMC with agents that increase intracellular cAMP (eg, forskolin, dibutyryl [db]-cAMP) resulted in a time- and concentration-dependent increase in sPLA(2) gene expression. Semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) showed a marked dose-dependent inhibition of forskolin-induced mRNA by protein kinase A inhibitor. Electrophoretic mobility shift analysis of nuclear proteins from forskolin-treated and db-cAMP-treated VSMC with C/EBP consensus oligonucleotides and C/EBP oligonucleotides from the rat promoter revealed greater binding than in control VSMC. Incubation of VSMC with H89, a specific protein kinase inhibitor, also blocked the binding of nuclear C/EBP to the C/EBP site of the rat promoter induced by db-cAMP and forskolin. Binding was unchanged with the use of CRE consensus oligonucleotides. Antibodies revealed the specific formation of C/EBP/DNA complexes, the majority of which were supershifted by C/EBP-ss and -delta antibodies. Functional activation of C/EBP was confirmed by a luciferase reporter gene assay. A construct comprising 4 tandem repeat copies of the C/EBP element from the rat sPLA(2) promoter linked to luciferase was transcriptionally activated in VSMC by cotransfection with expression vector for the protein kinase A catalytic subunit. It was also significantly activated in transfected VSMC treated by forskolin or db-cAMP. H89 inhibited this activations. We therefore conclude that the increases in sPLA(2) mRNA and enzyme activity produced by cAMP-elevating agents is controlled by a mechanism involving nuclear C/EBP-ss and -delta acting through a protein kinase A signaling pathway.

cAMP increases the expression of human angiotensinogen gene through a combination of cyclic AMP responsive element binding protein and a liver specific transcription factor

Angiotensinogen is the glycoprotein precursor of one of the most potent vasoactive hormones angiotensin-II which plays an important role in the regulation of blood pressure. We show here that the promoter activity of reporter constructs containing human angiotensinogen promoter is increased by cAMP treatment on transient transfection in HepG2 cells. We have identified a composite cAMP responsive element, located around 840 bases upstream from the transcriptional initiation site, in the promoter of human angiotensinogen gene. This element is recognized by members of CREB/ATF as well as C/EBP family of transcription factors. Another C/EBP binding site that is not recognized by CREB is located 10 bases upstream from this site. We show that co-transfection of CREB increases the promoter activity of reporter constructs containing human angiotensinogen gene promoter attached to the CAT gene. We also show that co-transfection of DBP (which is a member of C/EBP family of transcription factors) increases promoter activity of these reporter constructs.

Hyperstimulation with interleukin 6 inhibits cell cycle progression after hepatectomy in mice

Interleukin 6 (IL-6) is an important mediator of hepatocyte proliferation after hepatectomy. However, elevated IL-6 levels are found in patients with chronic liver disease. Therefore, it is unclear if hyperstimulation with IL-6 may have an influence on liver regeneration. We investigated whether a strong activation of IL-6-dependent pathways may change the course of hepatocyte proliferation after hepatectomy. Transgenic mice overexpressing the human soluble IL-6 receptor/gp80 (hsgp80) in hepatocytes were stimulated with or without hepatectomy with human IL-6 (hIL-6). Nuclear extracts were prepared and activation of gp130-dependent pathways was studied by Western blot and gel shift experiments. Cell cycle progression of hepatocytes after hepatectomy was investigated by monitoring cell cycle-specific factors. hIL-6 strongly activates Stat3 for more than 48 hours in human soluble hsgp80 transgenic mice. In contrast, no major differences were evident in the regulation of the Ras/MAP kinase pathway compared with wild-type (wt) mice. Also when hsgp80 mice were stimulated with hIL-6 3 hours before hepatectomy Stat3 is activated for more than 72 hours, whereas in unstimulated mice this event is restricted to the early hours. Strong activation of Stat3 resulted in a delay and inhibition of hepatocyte proliferation as measured by 5-bromo-2'-deoxyuridine (BrdU) staining and Cyclin A and E expression. This observation directly correlates with the induction of the cell cycle inhibitor p21. In summary, strong IL-6-dependent activation of Stat3 before hepatectomy results in delay and inhibition of cell cycle progression after hepatectomy. Therefore our results suggest that hyperstimulation with IL-6 can inhibit liver regeneration.

Runt domain factor (Runx)-dependent effects on CCAAT/ enhancer-binding protein delta expression and activity in osteoblasts

Transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta) is normally associated with acute-phase gene expression. However, it is expressed constitutively in primary osteoblast cultures where it increases insulin-like growth factor I synthesis in a cAMP-dependent way. Here we show that the 3' proximal region of the C/EBPdelta gene promoter contains a binding sequence for Runt domain factor Runx2, which is essential for osteogenesis. This region of the C/EBPdelta promoter directed high reporter gene expression in osteoblasts, and specifically bound Runx2 in osteoblast-derived nuclear extract. C/EBPdelta gene promoter activity was reduced by mutating the Runx binding sequence or by co-transfecting with Runx2 antisense expression plasmid, and was enhanced by overexpression of Runx-2. Exposure to prostaglandin E(2) increased Runx-dependent gene transactivation independently of Runx2 binding to DNA. Runx2 bound directly to the carboxyl-terminal region of C/EBPdelta itself, and its ability to drive C/EBPdelta expression was suppressed when C/EBPdelta or its carboxyl-terminal fragment was increased by overexpression. Consistent effects also occurred on C/EBPdelta-dependent increases in gene expression driven by synthetic or insulin-like growth factor I gene promoter fragments. These interactions between Runx2 and C/EBPdelta, and their activation by prostaglandin E(2), provide new evidence for their importance during skeletal remodeling, inflammatory bone disease, or fracture repair.

Adenosine-induced expression of interleukin-6 in astrocytes through protein kinase A and NF-IL-6

In various neurologic diseases, astrocytes express interleukin-6 (IL-6), which is an endogenous pyrogen, a neuroprotective factor, and a regulator of the blood-brain barrier. The expression of IL-6 in astrocytes is stimulated by extracellular adenosine through A(2B) receptors. To investigate the signaling cascade that induces IL-6 gene transcription further, we transfected primary mouse astrocytes with a reporter gene construct, in which luciferase expression is directed by the human IL-6 promoter. Expression of PKI, an inhibitor of protein kinase A (PKA), interfered with IL-6 transcription indicating that PKA mediates the effect of adenosine. The CAAT box of the IL-6 promoter is necessary for the stimulation by adenosine as a mutation in this element reduced the stimulation by adenosine. Indeed, the cAMP agonist forskolin increased the binding of the transcription factors NF-IL-6 and C/EBPdelta to the CAAT box of the IL-6 promoter in nuclear extracts of astrocytes. Inhibition of the de novo synthesis of NF-IL-6 by cycloheximide or an antisense oligonucleotide reduced the enhancement of NF-IL-6 binding to the CAAT box and inhibited stimulation of IL-6 transcription by forskolin. In addition, overexpression of NF-IL-6 induced IL-6 transcription. This suggests that adenosine induces the de novo synthesis of NF-IL-6 through activation of PKA and thereby stimulates transcription of IL-6 in astrocytes.

The enhancer I core region contributes to the replication level of hepatitis B virus in vivo and in vitro

Chronic hepatitis B virus (HBV) infection can lead to liver cirrhosis and hepatocellular carcinoma. Long-term interaction of the immune system with the virus results in the selection of escape mutants and viral persistence. In this work we characterize mutations in the enhancer I region isolated prior to liver transplantation from the HBV genomes of 10 patients with chronic HBV infection. The HBV-genomes were sequenced, and the enhancer I region was cloned into luciferase reporter constructs to determine the transcriptional activity. Functional studies were performed by transfecting HBV replication-competent plasmids into hepatoma cells. Analyses of the replication fitness of the mutant strains were conducted by biochemical analysis. In all HBV genomes the enhancer I region was mutated. Most of these mutations resulted in decreased transcriptional activity. The strongest effects were detectable in strains with mutations in the hepatocyte nuclear factor 3 and 4 (HNF3 and HNF4) binding sites of the enhancer I core domain. Replication-competent HBV constructs containing these mutations demonstrated up to 10-fold-reduced levels of virus replication. Before liver transplantation, when the mutant strains were detected in the patients' sera, low HBV DNA levels were found. After transplantation and reinfection with a wild-type virus, the levels of replication were up to 240-fold higher. Our results show that mutations in the enhancer I region of HBV have a major impact on HBV replication. These mutations may also determine the switch from high to low levels of viral replication which is frequently observed during chronic HBV infection.

Hormonal regulation of the phosphoenolpyruvate carboxykinase gene. Role of specific CCAAT/enhancer-binding protein isoforms

The CCAAT/enhancer-binding protein alpha (C/EBP) is a transcription factor that trans-activates a number of metabolically important genes. Previous work has demonstrated that C/EBPalpha and C/EBPbeta have the potential to mediate the cAMP responsiveness of phosphoenolpyruvate carboxykinase (PEPCK) in liver cells. However, these studies used GAL4 fusion proteins and artificial promoter-reporter gene vectors in transfection experiments; as a result, these studies only indicated that both isoforms had the potential to mediate the hormonal response and not which isoform actually participated in vivo. To address this issue, we produced hepatoma cell lines that stably expressed either a dominant negative inhibitor or antisense RNA for these two main liver C/EBP isoforms. Inhibition of all C/EBP isoforms via expression of the dominant negative protein eliminated cAMP responsiveness, and reduced glucocorticoid responsiveness, of the endogenous PEPCK gene in hepatoma cells. Antisense directed against C/EBPalpha mRNA, which reduced C/EBPalpha protein levels by nearly 80%, also significantly reduced the cAMP responsiveness of the endogenous PEPCK promoter, whereas antisense directed against C/EBPbeta was without effect. There was no major alteration in cAMP signaling in the C/EBPalpha antisense cells, as cAMP induction of the C/EBPbeta gene was similar to that in wild-type H4IIE cells. These data suggest that the alpha-isoform of C/EBP is specifically utilized for mediating the cAMP responsiveness of the PEPCK gene.

CREB activation induces adipogenesis in 3T3-L1 cells

Obesity is the result of numerous, interacting behavioral, physiological, and biochemical factors. One increasingly important factor is the generation of additional fat cells, or adipocytes, in response to excess feeding and/or large increases in body fat composition. The generation of new adipocytes is controlled by several "adipocyte-specific" transcription factors that regulate preadipocyte proliferation and adipogenesis. Generally these adipocyte-specific factors are expressed only following the induction of adipogenesis. The transcription factor(s) that are involved in initiating adipocyte differentiation have not been identified. Here we demonstrate that the transcription factor, CREB, is constitutively expressed in preadipocytes and throughout the differentiation process and that CREB is stimulated by conventional differentiation-inducing agents such as insulin, dexamethasone, and dibutyryl cAMP. Stably transfected 3T3-L1 preadipocytes were generated in which we could induce the expression of either a constitutively active CREB (VP16-CREB) or a dominant-negative CREB (KCREB). Inducible expression of VP16-CREB alone was sufficient to initiate adipogenesis as determined by triacylglycerol storage, cell morphology, and the expression of two adipocyte marker genes, peroxisome proliferator activated receptor gamma 2, and fatty acid binding protein. Alternatively, KCREB alone blocked adipogenesis in cells treated with conventional differentiation-inducing agents. These data indicate that activation of CREB was necessary and sufficient to induce adipogenesis. Finally, CREB was shown to bind to putative CRE sequences in the promoters of several adipocyte-specific genes. These data firmly establish CREB as a primary regulator of adipogenesis and suggest that CREB may play similar roles in other cells and tissues.

NF-kappaB determines between apoptosis and proliferation in hepatocytes during liver regeneration

Tumor necrosis factor (TNF)-alpha is a potent inducer of apoptotic cell death in various tissues, whereas the transcription factor nuclear factor (NF)-kappaB is essential to protect against TNF-alpha-induced apoptosis. Human hepatoma cell lines were used to investigate the effectiveness and specificity of the fungal metabolite gliotoxin in inhibiting TNF-alpha-induced NF-kappaB activation in transformed cells. Gliotoxin-TNF-alpha cotreatment induced massive apoptosis in these otherwise TNF-alpha-resistant cell lines. With the use of the mouse partial hepatectomy model, we were also able to demonstrate in vivo the capacity of gliotoxin to act as inhibitor of NF-kappaB activation. Bromodeoxyuridine staining of liver sections showed that the lack of NF-kappaB activation correlated with 80% reduction of DNA synthesis 48 h after hepatectomy compared with untreated controls. Additionally, animals treated with gliotoxin showed nuclear condensation and DNA laddering of hepatocytes indicative of apoptosis 24 h after hepatectomy. In summary, our results demonstrate that NF-kappaB is essential in defining the fate of liver cells in response to TNF-alpha in vivo and furthermore implicate gliotoxin as a potential new response modifier for TNF-alpha-based therapy.

2-acetaminofluorene blocks cell cycle progression after hepatectomy by p21 induction and lack of cyclin E expression

In the Solt-Faber model DENA and 2-Acetaminofluorene (AAF) treatment combined with hepatectomy induces hepatocellular carcinoma in rats. In this model AAF blocks proliferation of hepatocytes, while oval cells restore liver mass. Here we studied the molecular mechanism involved in blocking AAF-dependent cell cycle progression of hepatocytes. AAF inhibits cell proliferation of hepatocytes shown by the lack of Cyclin E expression before the G1/S phase restriction point. Immunfluorescence studies revealed that Cyclin E positive signals were restricted to oval cells, while hepatocytes remained negative. Additionally, AAF treatment induces strong nuclear p53 expression which is associated with increased p21 mRNA levels. Inhibition of active Cyclin/CdK (cyclin dependent kinase) complexes is reflected in AAF-treated animals by decreased RB expression and phosphorylation. The decrease in RB expression and phosphorylation, which is essential in triggering DNA synthesis and Cyclin A expression, leads to a deficiency in transcriptionally active E2F complex formation after hepatectomy. Thus, two molecular explanations are evident to account for AAF-dependent cell cycle progression of hepatocytes in vivo: first, induction of p53 expression which leads to higher p21 mRNA levels, and second, a lack of Cyclin E expression at the G1/S phase restriction point after hepatectomy.

CCAAT/enhancer-binding proteins are mediators in the protein kinase A-dependent activation of the decidual prolactin promoter

In the course of decidualization, human endometrial stromal cells (ESC) activate the alternative upstream promoter of the decidual prolactin (dPRL) gene. The dPRL promoter is induced by the protein kinase A pathway in a delayed fashion via the region -332/-270 which contains two overlapping consensus binding sequences, B and D, for CCAAT/enhancer-binding proteins (C/EBP). Here we show that sites B and D both bind C/EBPbeta and -delta from ESC nuclear extracts. When decidualization of cultured ESC was induced by treatment with 8-Br-cAMP, complex formation on sites B and D was enhanced. Western blot analysis revealed an elevation of both C/EBPbeta isoforms, liver-enriched activator protein and liver-enriched inhibitory protein, with a delayed onset between 8 and 24 h of cAMP treatment, while C/EBPdelta expression remained unaffected. Cyclic AMP-mediated activation of dPRL promoter construct dPRL-332/luc3 was abrogated by mutation of sites B and D at -310/-285. An expression vector for liver-enriched activator protein potently induced transcription of dPRL-332/luc3 and further enhanced cAMP-mediated induction, while liver-enriched inhibitory protein expression vector abolished the cAMP response, implying that C/EBPs serve as mediators in the delayed cAMP signal transduction to the dPRL promoter. The ratio between activating and repressing isoforms is likely to dictate the transcriptional output.

Adrenergic stimulation of hepatocyte growth factor expression

Hepatocyte growth factor (HGF), a potent mitogen, is released into plasma at increased levels following injury to certain tissues, including the liver. Early increases in plasma HGF are not due to a release from the injured liver, but rather from distal organs, particularly the lung. We have investigated the ability of norepinephrine (NE), which rises rapidly in plasma after liver resection, to trigger elevated production of HGF in MRC-5 human embryonic lung fibroblasts. Levels of HGF released to culture media and of HGF mRNA increased when cultures were exposed to NE, or to other adrenergic agonists. While stimulation of either beta- or alpha(1)-adrenergic receptors increased HGF expression, responses to NE appear to be mediated primarily via beta receptors. Since NE has already been shown to act as a comitogen with HGF, our findings suggest that adrenergic hormones may act both to induce production of HGF at distal sites, and to enhance the response to HGF at target tissues.

Adenovirus-directed expression of a nonphosphorylatable mutant of CREB (cAMP response element-binding protein) adversely affects the survival, but not the differentiation, of rat granulosa cells

Although usually considered to be a constitutively expressed protein, in the primate ovary the expression of CREB (cAMP response element-binding protein) is extinguished after ovulation, and its loss is temporally associated with the cessation of proliferation of luteal cells and the ultimate commitment of the corpus luteum to undergo regression. To determine the cellular consequences of the loss of CREB expression, we expressed a nonphosphorylatable mutant of CREB (CREB M1) in primary cultures of rat granulosa cells using a replication-defective adenovirus vector. Expression of CREB M1 did not block granulosa cell differentiation as assessed by acquisition of the ability to produce estrogen and progesterone in response to FSH or forskolin. However, granulosa cells expressing CREB M1, but not adenovirus-directed beta-galactosidase or enhanced green fluorescent protein, exhibited a 35% reduction in viability that was further reduced to 65% after stimulation with 10 microM forskolin. These results demonstrate that the trophic effects of cAMP (proliferation and survival) on ovarian granulosa cells are functionally separate from the effects of cAMP on differentiation and provide novel evidence that CREB may function as a cell survival factor in the ovary. The separation of signaling pathways that govern differentiation and survival in the ovary thereby provides a mechanism by which progesterone production, which is absolutely essential for the maintenance of pregnancy, can continue despite the cessation of proliferation of luteal cells and their commitment to cell death (luteolysis).

The pre-S region determines the intracellular localization and appearance of hepatitis B virus

The functional role of the hepatitis B virus (HBV) pre-S region for assembly and appearance of the virus is not completely understood. In this study, 3 natural-occurring mutants were investigated. Three mutants of the pre-S region-a point mutation in the CCAAT box (MUT1), a 6-bp deletion (MUT2) 3' of the CCAAT box, and a 153-bp deletion (MUT3) in the preS2 domain-were cloned alone or in combinations in replication-competent HBV plasmids and transfected in hepatoma cells. The impact on HBV assembly and appearance was studied by Northern Blot, primer extension analysis, immunofluorescence studies, enzyme-linked immunosorbent assay, and electron microscopy. An inversed ratio of pre-S/S mRNA transcripts compared with wild-type (wt) HBV was found when either MUT1 or -2 were included into the plasmid. Intracellular localization with both mutants showed retention of large S-protein in the endoplasmic reticulum and nuclear accumulation of core protein. The extracellular amount of S-protein was reduced with MUT1 and -2 or combinations in which 1 of the mutants was included. However, the extracellular appearance of viral products was comparable with wtHBV. In contrast, MUT3 showed major changes. Virion-like particles had a fried-egg, and filaments a screw-like appearance. The S-promoter mutations MUT1 and MUT2 correlated with viral retention. MUT3 leads to malformed viral particles. Therefore, different regions in the pre-S domain are essential to determine the intracellular localization and extracellular appearance of HBV, and might contribute to the prognosis of chronic HBV infection.