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

Nuclear export of phosphorylated C/EBPbeta mediates the inhibition of albumin expression by TNF-alpha

Decreased albumin expression is a frequent feature of cachexia patients afflicted with chronic diseases, including cancer, and a major contributor to their morbidity. Here we show that tumor necrosis-alpha (TNF-alpha) treatment of primary mouse hepatocytes or TNF-alpha overexpression in a mouse model of cachexia induces oxidative stress, nitric oxide synthase (NOS) expression and phosphorylation of C/EBPbeta on Ser239, within the nuclear localization signal, thus inducing its nuclear export, which inhibits transcription from the albumin gene. SIN-1, a NO donor, duplicated the TNF-alpha effects on hepatocytes. We found similar molecular abnormalities in the liver of patients with cancer-cachexia. The cytoplasmic localization and association of C/EBPbeta-PSer239 with CRM1 (exportin-1) in TNF-alpha-treated hepatocytes was inhibited by leptomycin B, a blocker of CRM1 activity. Hepatic cells expressing the non-phosphorylatable C/EBPbeta alanine mutant were refractory to the inhibitory effects of TNF-alpha on albumin transcription since the mutant remained localized to the nucleus. Treatment of TNF-alpha mice with antioxidants or NOS inhibitors prevented phosphorylation of C/EBPbeta on Ser239 and its nuclear export, and rescued the abnormal albumin gene expression.

CCAAT/enhancer-binding proteins alpha and beta negatively influence the capacity of tumor necrosis factor alpha to up-regulate the human cytomegalovirus IE1/2 enhancer/promoter by nuclear factor kappaB during monocyte differentiation

Recently we demonstrated that the ability of tumor necrosis factor alpha (TNFalpha) to stimulate the human cytomegalovirus (HCMV) IE1/2 enhancer/promoter activity in myeloid progenitor-like cells decreases when these cells differentiate into promonocytic cells. In addition, TNFalpha stimulation in the progenitor-like cell line HL-60 was shown to be mediated by nuclear factor kappaB (NF-kappaB) activation and its binding to the 18-base pair sequence motifs of the IE1/2 enhancer. We demonstrate here that the cell differentiation-dependent reduction of TNFalpha stimulation is not due to insufficient NF-kappaB activation but correlates with increased synthesis of the monocyte differentiation-associated factors CCAAT/enhancer-binding protein (C/EBP) alpha and beta. Overexpression of C/EBPalpha/beta in HL-60 cells, which normally produce only very small amounts of C/EBP, stimulated the basal activity of the promoter in the absence of NF-kappaB but suppressed the stimulatory effect of TNFalpha. A novel C/EBP-binding site was identified in the IE1/2 enhancer directly downstream of a NF-kappaB site. In order to understand the mechanisms of interaction, we used an IE1/2 promoter mutant that failed to bind C/EBP at this position and several constructs that contained exclusively NF-kappaB- and/or C/EBP-binding sites upstream of the minimal IE1/2 promoter. We could demonstrate that C/EBPalpha/beta interacts with NF-kappaB p65 and displays inhibitory activity even in the absence of direct DNA binding by forming p65-C/EBP-containing protein complexes bound to the NF-kappaB site. Moreover, C/EBP binding to the DNA adjacent to NF-kappaB supports the down-regulatory effect of C/EBPs possibly due to stabilization of a multimeric NF-kappaB-C/EBP complex. Our results show that cell differentiation factors may interfere with TNFalpha-induced human cytomegalovirus gene (re)activation.

Hormonal control of insulin-like growth factor I gene transcription in human osteoblasts: dual actions of cAMP-dependent protein kinase on CCAAT/enhancer-binding protein delta

Insulin-like growth factor-I (IGF-I) is essential for somatic growth and promotes bone cell replication and differentiation. IGF-I production by rat osteoblasts is stimulated by activation of cAMP-dependent protein kinase (PKA). In this report, we define two interacting PKA-regulated pathways that control IGF-I gene transcription in cultured human osteoblasts. Stimulation of cAMP led to a 12-fold increase in IGF-I mRNA and enhanced IGF-I promoter activity through a DNA response element termed HS3D and the transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta). Under basal conditions, C/EBPdelta was found in osteoblast nuclei but was transcriptionally silent. Treatment with the PKA inhibitor H-89 caused redistribution of C/EBPdelta to the cytoplasm. After hormone treatment, the catalytic subunit of PKA accumulated in osteoblast nuclei. Inhibition of active PKA with targeted nuclear expression of PKA inhibitor had no effect on the subcellular location of C/EBPdelta but prevented hormone-induced IGF-I gene activation, while cytoplasmic PKA inhibitor additionally caused the removal of C/EBPdelta from the nucleus. These results show that IGF-I gene expression is controlled in human osteoblasts by two PKA-dependent pathways. Cytoplasmic PKA mediates nuclear localization of C/EBPdelta under basal conditions, and nuclear PKA stimulates its transcriptional activity upon hormone treatment. Both mechanisms are indirect, since PKA failed to phosphorylate human C/EBPdelta in vitro.

Selective suppression of CCAAT/enhancer-binding protein beta binding and cyclooxygenase-2 promoter activity by sodium salicylate in quiescent human fibroblasts

The anti-inflammatory actions of salicylates cannot be explained by inhibition of cyclooxygenase (COX) activity. This study demonstrates that sodium salicylate at a therapeutic concentration suppressed COX-2 gene transcription induced by phorbol 12-myristate 13-acetate and interleukin 1beta by inhibiting the binding of CCAAT/enhancer-binding protein beta to its promoter region of COX-2. By contrast, salicylate did not inhibit nuclear factor kappaB-dependent COX-2 induction by tumor necrosis factor alpha. The inhibitory effect of sodium salicylate was restricted to serum-deprived quiescent cells. These findings indicate that contrary to the current view that salicylate acts via inhibition of nuclear factor kappaB the pharmacological actions of aspirin and salicylates are mediated by inhibiting CCAAT/enhancer-binding protein beta binding and transactivation. These findings have a major impact on the conceptual understanding of the mechanism of action of salicylates and on new drug discovery and design.

Regulated nuclear-cytoplasmic localization of CCAAT/enhancer-binding protein delta in osteoblasts

Insulin-like growth factor I (IGF-I) plays a central role in skeletal growth by promoting bone cell replication and differentiation. Prostaglandin E2 (PGE2) and parathyroid hormone enhance cAMP production in cultured rat osteoblasts and stimulate IGF-I expression through a transcriptional mechanism mediated by cAMP-dependent protein kinase (PKA). We previously showed that PGE2 activated the transcription factor CCAAT/enhancer-binding protein delta (C/EBPdelta) in osteoblasts and induced its binding to a DNA element within the IGF-I promoter. We report here that a PKA-dependent pathway stimulates nuclear translocation of C/EBPdelta. Under basal conditions, C/EBPdelta was cytoplasmic but rapidly accumulated in the nucleus after PGE2 treatment (t(1/2) < 30 min). Nuclear translocation occurred without concurrent protein synthesis and was maintained in the presence of hormone. Nuclear localization required PKA and was blocked by a dominant-interfering regulatory subunit of the enzyme, even though C/EBPdelta was not a PKA substrate. Upon removal of hormonal stimulus, C/EBPdelta quickly exited the nucleus (t(1/2) < 12 min) through a pathway blocked by leptomycin B. Mutagenesis studies indicated that the basic domain of C/EBPdelta was necessary for nuclear localization and that the leucine zipper region permitted full nuclear accumulation. We thus define a pathway for PKA-mediated activation of C/EBPdelta through its regulated nuclear import.

Renal CCAAT/enhancer-binding proteins in experimental diabetes mellitus

There is evidence that mediators of inflammation including components of the cytokine system are present in human and experimental diabetic kidney disease. CCAAT/enhancer-binding proteins (C/EBPs) represent a family of cytokine-inducible transcription factors. C/EBPs themselves regulate cytokine expression and also the expression of acute-phase reactants and connective tissue proteins. At least three C/EBP isoforms (alpha, beta, delta) are known. Upon stimulation with cytokines or bacterial lipopolysaccharide, the expression of the alpha isoform typically decreases, and the expression of the beta and/or delta isoforms increases. In view of the fact that components of the inflammatory response are present in diabetic kidney disease, there is a potential that the expression and activity of renal C/EBPs are altered in the diabetic state. In this study we sought to examine the status of C/EBP proteins in kidneys of rats with streptozotocin-induced diabetes mellitus. Diabetes was induced in 5 male Sprague-Dawley rats. Eight weight-matched non-diabetic rats were used as controls. Animals were sacrificed after 4 weeks, and the whole kidney nuclear protein was extracted. An electrophoretic mobility shift assay showed that DNA-binding activity was present in all five kidney nuclear extracts of the diabetic animals, but in only 2 out of 8 control samples (p < 0.05). A supershift assay showed that the DNA-bound protein complex consisted mainly of the C/EBPbeta isoform. Western analysis showed an increase of the C/EBPbeta protein in renal nuclear extracts of the diabetic animals compared to controls (p < 0.05). There was a decrease of the C/EBPalpha protein in the kidney nuclear extracts of the diabetic animals compared to controls (p < 0.05). We conclude that renal C/EBP dynamics are altered in experimental diabetes mellitus and that the patterns of C/EBP changes resemble those observed after cytokine or lipopolysaccharide stimulation.

Differential regulation of macrophage CCAAT-enhancer binding protein isoforms by lipopolysaccharide and cytokines

The regulation of the C/EBP family in macrophages by LPS and cytokines is of potentially crucial importance in several pathophysiological conditions. The action of LPS and three cytokines on the expression of C/EBP mRNA, protein and functional DNA binding activity in the murine J774.2 cell line was therefore studied. Exposure of the cells to LPS, IL-1, IFN-gamma and TNF-alpha produced a reduction of C/EBP alpha mRNA levels and a corresponding increase in the expression of C/EBP beta and C/EBP delta. EMSA showed time-dependent changes in the DNA binding activity of individual C/EBP isoforms and demonstrated the participation of heterodimers between the different members in DNA-protein interactions. Additionally, mediator-specific changes in the kinetics and magnitude of C/EBP mRNA expression pattern and profile of DNA-protein interactions were observed. These studies provide novel insights into the potential mechanisms that may be responsible for the mediator-specific regulation of macrophage gene expression through the C/EBP family.

Stimulus- and cell-type-specific regulation of CCAAT-enhancer binding protein isoforms in glomerular mesangial cells by lipopolysaccharide and cytokines

Binding sites for the CCAAT-enhancer binding protein (C/EBP) family are present in the promoter regions of several genes that are known to be expressed by mesangial cells (MC) during the pathogenesis of glomerular inflammatory diseases. The precise regulation of the C/EBP family by agents that are known to activate MC is, however, poorly understood. We report here the action of interleukin-1 (IL)-1 and, for the first time, lipopolysaccharide (LPS), platelet-derived growth factor (PDGF), IL-6, interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) on the C/EBP expression profile and functional DNA binding activity in primary rat MC. Both cell-type- and stimulus-specific regulation of C/EBP mRNA expression and DNA binding activity were identified, with C/EBPalpha being induced by LPS, C/EBPbeta by LPS, IL-1, TNF-alpha and C/EBPdelta by LPS, IL-1, IFN-gamma, TNF-alpha and PDGF. Such differential regulation, particularly that of C/EBPbeta, may be responsible for the mediator-specific differences in the expression of C/EBP-regulated genes in MC. Additionally, the involvement of potential post-transcriptional mechanisms in the regulation of C/EBPdelta were identified. These studies provide novel insights into the stimulus-specific regulation of gene expression during renal diseases.

Tumor necrosis factor alpha down-regulates expression of the alpha1(I) collagen gene in rat hepatic stellate cells through a p20C/EBPbeta- and C/EBPdelta-dependent mechanism

Tumor necrosis factor alpha (TNF-alpha) is one of the key cytokines of the acute phase response and of many inflammatory processes. This cytokine has several antifibrogenic actions and down-regulates the expression of the type I collagen genes and induces the expression of metalloproteinases. Because TNF-alpha directly antagonizes some fibrogenic actions of transforming growth factor beta(1) (TGF-beta(1)), we considered it important to map the cis-acting regulatory element of the alpha1(I) collagen (col1a1) promoter involved in TNF-alpha responsiveness in hepatic stellate cells (HSC), to investigate the transcription factors that bind to it, and to establish possible mechanisms by which TNF-alpha down-regulates its expression. In this article, we show the presence of a functional TNF-alpha-responsive element (TaRE) in the -378 to -345 region of the col1a1 promoter. This element colocalizes with a previously reported TGF-beta(1)-responsive element. We further demonstrate that TNF-alpha induces nuclear translocation and binding of transcriptional complexes containing p20C/EBPbeta, p35C/EBPbeta, and C/EBPdelta to this sequence of the promoter. Transient overexpression of C/EBPdelta or p20C/EBPbeta, the natural dominant negative form of C/EBPbeta in HSC, down-regulated activity of a CAT reporter vector driven by -412 to +110 of the col1a1 promoter. Taken together, these data suggest that the -378 to -340 region of the col1a1 promoter is the site of convergence of different stimuli that ultimately modulate col1a1 gene transcription.

Mechanisms of cholestasis

From the multiple mechanisms of cholestasis presented in this article, a unifying hypothesis may be deduced by parsimony. The disturbance of the flow of bile must inevitably lead to the intracellular retention of biliary constituents. Alternatively, the lack of specific components of bile unmasks the toxic potential of other components, as in the case of experimental mdr2 deficiency. In the sequence of events that leads to liver injury, the cytotoxic action of bile salts is pivotal to all forms of cholestasis. The inhibition of the bsep by drugs, sex steroids, or monohydroxy bile salts is an example of direct toxicity to the key mediator in canalicular bile salt excretion. In other syndromes, the dysfunction of distinct hepatocellular transport systems is the primary pathogenetic defect leading to cholestasis. Such dysfunctions include the genetic defects in PFIC and the direct inhibition of gene transcription by cytokines. Perturbations in the short-term regulation of transport protein function are exemplified by the cholestasis of endotoxinemia. The effect of bile salts on signal transduction, gene transcription, and transport processes in hepatocytes and cholangiocytes has become the focus of intense research in recent years. The central role of bile salts in the pathogenesis of cholestasis has, ironically, become all the more evident from the improvement of many cholestatic syndromes with oral bile salt therapy.

Induction of the angiogenic modulator fibroblast growth factor-binding protein by epidermal growth factor is mediated through both MEK/ERK and p38 signal transduction pathways

Fibroblast growth factor-binding protein (FGF-BP) is a secreted protein that binds and activates fibroblast growth factors (FGF-1 and FGF-2) and induces angiogenesis in some human cancers. FGF-BP is expressed at high levels in squamous cell carcinoma (SCC) cell lines and tumor samples and has been shown to be rate-limiting for the growth of SCC tumors in vivo. In this study, we examine the regulation of FGF-BP by epidermal growth factor (EGF) and the signal transduction mechanisms that mediate this effect. We found that EGF treatment of the ME-180 SCC cell line caused a rapid induction of FGF-BP gene expression. This induction was mediated transcriptionally through the AP-1 (c-Fos/JunD) and CCAAT/enhancer-binding protein elements as well as through an E-box repressor site in the proximal regulatory region of the FGF-BP promoter. Pharmacological inhibition of protein kinase C and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1/2 (MEK1/2) completely blocked EGF induction of FGF-BP mRNA, whereas inhibition of phosphatidylinositol 3-kinase had no effect. Additionally, both EGF- and anisomycin-induced FGF-BP mRNA was abrogated by inhibition of p38 mitogen-activated protein kinase, demonstrating a role for p38 in the regulation of FGF-BP. Co-transfection of the FGF-BP promoter with dominant negative forms of MEK2, extracellular signal-regulated kinase 2, and p38 significantly decreased the level of EGF induction, whereas expression of a dominant negative c-Jun N-terminal kinase mutant or expression of c-Jun N-terminal kinase inhibitory protein had no effect. Similarly, activation of the p38 pathway by overexpression of wild-type p38 or MKK6 enhanced FGF-BP transcription. These results demonstrate that EGF induction of FGF-BP occurs selectively through dual activation of the stress-activated p38 and the MEK/extracellular signal-regulated kinase mitogen-activated protein kinase pathways, which ultimately leads to activation of the promoter through AP-1 and CCAAT/enhancer-binding protein sites.

Human herpesvirus 8 interleukin-6 (vIL-6) signals through gp130 but has structural and receptor-binding properties distinct from those of human IL-6

Human herpesvirus 8 (HHV-8) has been associated with classical, endemic (African), and AIDS-related Kaposi's sarcoma (KS), body cavity-based primary effusion lymphomas, and multicentric Castleman's disease (MCD). HHV-8 encodes a functional homologue of interleukin-6 (IL-6), a cytokine that promotes the growth of KS and myeloma cells and is found at elevated levels in MCD lesions and patient sera. We have previously reported that the viral IL-6 (vIL-6) gene product can support the growth of the IL-6-dependent murine hybridoma cell line, B9, and that the gp80 (IL-6 receptor [IL-6R]) component of the IL-6 receptor-signal transducer (gp180) complex plays a role in mediating this activity. However, it has been shown by others that vIL-6 can function in human cells independently of IL-6R. Here we have extended our functional studies of vIL-6 by identifying transcription factors and pathways used in human Hep3B cells, investigating the utilization of gp130 and IL-6R by vIL-6, and undertaking mutational analyses of vIL-6 and gp130. The data presented here establish that vIL-6, in common with its endogenous counterparts, can mediate signal transduction through gp130 and activate multiple transcription factors, map residues within the vIL-6 protein that are and are not important for vIL-6 signalling, and identify a gp130 mutant that is nonfunctional with respect to vIL-6 signalling in the absence of IL-6R but that retains the ability to mediate vIL-6 and human IL-6 (hIL-6) signal transduction when IL-6R is coexpressed. The data presented demonstrate functional and mechanistic similarities between vIL-6 and endogenous IL-6 proteins but also highlight differences in the structural and receptor-binding properties of vIL-6 relative to its human counterpart.

CCAAT/enhancer-binding protein delta regulates mammary epithelial cell G0 growth arrest and apoptosis

CCAAT/enhancer-binding proteins (C/EBPs) are a highly conserved family of DNA-binding proteins that regulate cell-specific growth, differentiation, and apoptosis. Here, we show that induction of C/EBPdelta gene expression during G0 growth arrest is a general property of mammary-derived cell lines. C/EBPdelta is not induced during G0 growth arrest in 3T3 or IEC18 cells. C/EBPdelta induction is G0-specific in mouse mammary epithelial cells; C/EBPdelta gene expression is not induced by growth arrest in the G1, S, or G2 phase of the cell cycle. C/EBPdelta antisense-expressing cells (AS1 cells) maintain elevated cyclin D1 and phosphorylated retinoblastoma protein levels and exhibit delayed G0 growth arrest and apoptosis in response to serum and growth factor withdrawal. Conversely, C/EBPdelta-overexpressing cells exhibited a rapid decline in cyclin D1 and phosphorylated retinoblastoma protein levels, a rapid increase in the cyclin-dependent kinase inhibitor p27, and accelerated G0 growth arrest and apoptosis in response to serum and growth factor withdrawal. When C/EBPdelta levels were rescued in AS1 cells by transfection with a C/EBPdelta "sense" construct, normal G0 growth arrest and apoptosis were restored. These results demonstrate that C/EBPdelta plays a key role in the regulation of G0 growth arrest and apoptosis in mammary epithelial cells.

C/EBP beta in rheumatoid arthritis: correlation with inflammation, not disease specificity

Rheumatoid arthritis synovial tissue was examined and compared with osteoarthritis tissue for the presence of the nuclear transcription factor C/EBP beta (NF-IL-6). The region (lining or sublining), cell type, and subcellular distribution (cytoplasmic or nuclear) of the expression of C/EBP beta was characterized. Rheumatoid arthritis synovial fluid and blood and normal peripheral blood were also examined. C/EBP beta was detected in the synovial lining and in sublining cells of synovial tissue from patients with both rheumatoid and osteoarthritis. A significant (P < 0.001 and < 0.05, respectively) increase in the percentage of cells with nuclear staining was seen in the lining layer, compared to cells in the sublining region, in rheumatoid and osteoarthritis. In both diseases a strong correlation (r = 0.79, P < 0.001) was observed between the percentage of cells in the synovial lining that were positive for nuclear C/EBP beta and lining cell depth. Two-color immunohistochemistry demonstrated that both macrophages and fibroblast-like synoviocytes were positive for nuclear C/EBP beta. The presence of C/EBP beta was confirmed by immunohistochemistry and Western blot analysis with isolated synovial fibroblasts. Nuclear C/EBP beta was also detected in rheumatoid synovial fluid monocytes/macrophages, but not in lymphocytes or neutrophils. Western blot analysis confirmed the presence of C/EBP beta in these cells. The intensity of C/EBP beta staining was greater (P < 0.001) in synovial fluid monocytes than in those from normal or rheumatoid peripheral blood. In conclusion, the enhanced nuclear staining for C/EBP beta in the synovial lining, compared to the sublining, suggesting activation in the lining, and the positive correlation of lining layer depth with the percentage of cells in the lining positive for nuclear C/EBP beta, suggest a potential role for C/EBP beta in chronic inflammation. The regulation of the production or activity of C/EBP beta, to inhibit inflammatory mediator expression by synovial macrophages and fibroblasts, offers a novel approach to therapeutic intervention.

Lipids up-regulate uncoupling protein 2 expression in rat hepatocytes

BACKGROUND & AIMS

Hepatic steatosis reflects the accumulation of triglycerides and free fatty acids in hepatocytes. Although lipids and their metabolites are potentially hepatotoxic, the absence of overt injury in fatty livers suggests that adaptive responses to lipid accumulation occur. Fatty acids induce mitochondrial uncoupling proteins (UCP) 2 and 3 in muscle and fat, providing a mechanism to dispose of excessive fatty acids. Although hepatocytes do not normally express uncoupling proteins, UCP-2 is expressed in hepatocytes of genetically obese mice with fatty livers, suggesting that lipids also induce UCP-2 in hepatocytes.

METHODS

To test whether lipids up-regulate hepatocyte UCP-2, cultures of rat hepatocytes were treated with lipid emulsions, linoleic or oleic acid, and UCP-2 expression was evaluated by Northern blotting and immunocytochemistry. Because increased reactive oxygen species (ROS) production may contribute to lipid-related UCP-2 induction, the DNA-binding activity of the ROS-activated transcription factor, NF-kappaB, was measured, and the effects of tert-butyl hydroperoxide (TBHP) and glutathione (GSH) on UCP-2 induction were also assessed.

RESULTS

Lipid emulsions increased the DNA-binding activity of NF-kappaB and resulted in a dose- and time-dependent induction of UCP-2 transcripts in cultured hepatocytes; after 24 hours, UCP-2 messenger RNA levels were increased 4.5-fold, and increased UCP-2 protein was shown by immunocytochemistry. Consistent with the possibility that ROS generated intracellularly during lipid metabolism participates in UCP-2 induction, addition of the cell-impermeable antioxidant GSH did not alter lipid-related induction of UCP-2. Furthermore, TBHP, which is known to increase hepatocyte mitochondrial ROS production, also increased UCP-2 messenger RNA levels.

CONCLUSIONS

Lipids increase ROS and induce UCP-2 in hepatocytes. Thus, the liver may adapt to an excessive supply of lipid substrates by inducing UCP-2 to facilitate substrate disposal while constraining ROS production.

Tumor necrosis factor increases mitochondrial oxidant production and induces expression of uncoupling protein-2 in the regenerating mice [correction of rat] liver

The growth-stimulatory actions of tumor necrosis factor alpha (TNF-alpha) after partial hepatectomy (PH) are difficult to reconcile with its well-established role in the genesis of liver injury. The lethal actions of TNF are thought to involve the induction of oxidant production by mitochondria. It is not known if TNF initiates mitochondrial oxidant production after PH. Furthermore, if this potentially toxic response follows PH, it is not clear how hepatocytes defend themselves sufficiently so that replication, rather than death, occurs. These studies test the hypothesis that TNF does increase mitochondrial oxidant production after PH but that these oxidants primarily promote the induction of antioxidant defenses in regenerating hepatocytes. Consistent with this concept, H2O2 production by liver mitochondria increases from 5 minutes to 3 hours after PH, beginning before the transient inductions of hepatic NF kB activity (which peaks at 30 minutes post-PH) and uncoupling protein-2 (UCP-2) (which begins around 30 minutes and peaks from 6-24 hours post-PH). Pretreatment with neutralizing anti-TNF antibodies, which inhibits hepatocyte DNA synthesis after PH, also reduces post-PH hepatic mitochondrial oxidant production by 80% and inhibits NF kappaB activation and UCP-2 induction by 50% and 80%, respectively. In contrast, pretreatment with D609, an agent that inhibits phosphatidylcholine-specific phospholipase C, neither inhibits regenerative induction of mitochondrial oxidant production, UCP-2 expression, nor hepatocyte DNA synthesis, although it inhibits NF kappaB activation by 50%. Given published evidence that NF kappaB is antiapoptotic and that UCP-2 may decrease mitochondrial oxidant production in some cells, these results suggest that TNF-dependent increases in oxidant production by liver mitochondria promote the induction of antioxidant defenses in the regenerating liver.

The expression of CCAAT/enhancer binding protein (C/EBP) in the human ovary in vivo: specific increase in C/EBPbeta during epithelial tumour progression

The CCAAT/enhancer binding protein (C/EBP) family of transcription factors is involved in metabolism and differentiation of cells, especially in rodent liver cells and adipocytes. Their roles in vivo and in particular during pathophysiological conditions in humans are largely unknown. We have investigated the presence of C/EBPalpha, -beta, -delta and -zeta in normal ovaries and in epithelial ovarian tumours of different stages. Immunohistochemical experiments demonstrated that C/EBPalpha and C/EBPbeta were preferentially expressed in epithelial/tumour cells irrespective of stage or grade of the tumour. C/EBPbeta was located in the nuclei of the cells, in contrast to C/EBPalpha, which was present only in the cytoplasm of these cells. The nuclear localization of C/EBPbeta indicates an active role of this transcription factor in tumour cells, whereas the cytoplasmic distribution suggests a more passive function of C/EBPalpha. C/EBPdelta and -zeta demonstrated a more diverse distribution with predominant localization to epithelial cells, but stromal distribution was also noted. The intracellular distribution was confined to both the nucleus and the cytoplasm for C/EBPdelta and -zeta. Western blotting demonstrated that C/EBPalpha, -beta, -delta and -zeta were present in a majority of the samples. The amount of C/EBPbeta increased markedly with malignancy, i.e. with degree of dedifferentiation, while the other members of the C/EBP family displayed a more constant expression level. These results demonstrate an association between the expression of members of the C/EBP family and the formation of epithelial ovarian tumours, with C/EBPbeta as a potential marker for these tumours. As C/EBPbeta is known to be expressed during proliferation of cells in vitro, it may participate in the proliferative process of ovarian epithelial tumour cells in vivo and play a central role in tumour progression.

Transforming growth factor beta1 induces the expression of alpha1(I) procollagen mRNA by a hydrogen peroxide-C/EBPbeta-dependent mechanism in rat hepatic stellate cells

Oxidative stress plays a key role in liver fibrosis. Both inflammatory cells and activated Kupffer cells produce H2O2, an oxidant involved in the activation of hepatic stellate cells (HSC). Increased production of reactive oxygen intermediates (ROIs) in fibrotic livers is associated in part with the up-regulation of transforming growth factor beta (TGF-beta), and this cytokine enhances collagen production by cultured HSC. However, the possible link between oxidative stress and the molecular mechanisms by which TGF-beta induces collagen gene expression in HSC remains to be elucidated. To address this question, we investigated whether H2O2 is a mediator of TGF-beta-elicited alpha1(I) collagen gene (col1a1) up-regulation. We demonstrated that TGF-beta induces the accumulation of H2O2, and that this oxidant is, in turn, directly involved in up-regulating the expression of the col1a1 gene. While the addition of H2O2 to HSC induced the expression of alpha1(I) procollagen mRNA, catalase, an H2O2 enzyme scavenger, abrogated TGF-beta-mediated col1a1 gene up-regulation. We transfected HSC with chimeric plasmids driven by different segments of the mouse col1a1 promoter and mapped a cis-acting element (-370 to -344) essential for TGF-beta responsiveness. We further showed that TGF-beta induced the activation and binding of a C/EBPbeta-containing transcriptional complex to this sequence, an effect that was also mimicked by the addition of H2O2. Taken together, these data demonstrate a direct connection between TGF-beta-mediated accumulation of H2O2 and the up-regulation of col1a1 gene in HSC.

Phenotypic abnormalities in macrophages from leptin-deficient, obese mice

Obesity is a complex syndrome that involves defective signaling by a number of different factors that regulate appetite and energy homeostasis. Treatment with exogenous leptin reverses hyperphagia and obesity in ob/ob mice, which have a mutation that causes leptin deficiency, proving the importance of this factor and its receptors in the obesity syndrome. Cells with leptin receptors have been identified outside of the appetite regulatory centers in the brain. Thus leptin has peripheral targets. Because macrophages express signaling-competent leptin receptors, these cells may be altered during chronic leptin deficiency. Consistent with this concept, the present study identifies several phenotypic abnormalities in macrophages from ob/ob mice, including decreased steady-state levels of uncoupling protein-2 mRNA, increased mitochondrial production of superoxide and hydrogen peroxide, constitutive activation of CCAAT enhancer binding protein (C/EBP)-beta, an oxidant-sensitive transcription factor, increased expression of interleukin-6 and cyclooxygenase (COX)-2, two C/EBP-beta target genes, and increased COX-2-dependent production of PGE2. Given the importance of macrophages in the general regulation of inflammation and immunity, these alterations in macrophage function may contribute to obesity-related pathophysiology.

Autocrine Signals Control CCAAT/Enhancer Binding Protein β Expression, Localization, and Activity in Macrophages

The transcription factor CCAAT/enhancer binding protein β (C/EBPβ, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1β. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBPβ. One factor was shown to be tumor necrosis factor- (TNF-), which increased C/EBPβ expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBPβ localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBPβ correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBPβ to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBPβ. These observations suggest that diffuse distribution of C/EBPβ in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor.

Autocrine Signals Control CCAAT/Enhancer Binding Protein β Expression, Localization, and Activity in Macrophages

The transcription factor CCAAT/enhancer binding protein β (C/EBPβ, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1β. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBPβ. One factor was shown to be tumor necrosis factor- (TNF-), which increased C/EBPβ expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBPβ localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBPβ correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBPβ to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBPβ. These observations suggest that diffuse distribution of C/EBPβ in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor.

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

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

Roles of CCAAT/enhancer-binding proteins in regulation of liver regenerative growth

The expressions and activities of several CCAAT/enhancer-binding proteins (C/EBP) isoforms fluctuate in the regenerating liver. The physiological implications of these variations in C/EBP function remain poorly characterized in the setting of regeneration. However, lessons learned in various hepatocyte cell lines and by studying primary hepatocytes from transgenic C/EBPalpha-deficient mice suggest that the C/EBP isoforms are likely to influence proliferation, differentiated gene expression, and survival in mature, adult hepatocytes. In addition, these factors are potentially important modulators of liver nonparenchymal cell genes, including those that encode matrix molecules and growth factors that are required for successful liver regeneration. The possibility that members of the C/EBP family of transcription factors actively participate in many aspects of the regenerative response to liver injury is strengthened by growing evidence that many hepatocyte mitogens and co-mitogens regulate C/EBP activity. Furthermore, the C/EBPs themselves appear to regulate the expression of some of these growth regulators.

Impaired liver regeneration in inducible nitric oxide synthasedeficient mice

The mechanisms that permit adult tissues to regenerate when injured are not well understood. Initiation of liver regeneration requires the injury-related cytokines, tumor necrosis factor (TNF) alpha and interleukin (IL) 6, and involves the activation of cytokine-regulated transcription factors such as NF-kappabeta and STAT3. During regeneration, TNFalpha and IL-6 promote hepatocyte viability, as well as proliferation, because interventions that inhibit either cytokine not only block hepatocyte DNA synthesis, but also increase liver cell death. These observations suggest that the cytokines induce hepatoprotective factors in the regenerating liver. Given evidence that nitric oxide can prevent TNF-mediated activation of the pro-apoptotic protease caspase 3 and protect hepatocytes from cytokine-mediated death, cytokine-inducible nitric oxide synthase (iNOS) may be an important hepatoprotective factor in the regenerating liver. In support of this hypothesis we report that the hepatocyte proliferative response to partial liver resection is severely inhibited in transgenic mice with targeted disruption of the iNOS gene. Instead, partial hepatectomy is followed by increased caspase 3 activity, hepatocyte death, and liver failure, despite preserved induction of TNFalpha, IL-6, NF-kappabeta, and STAT3. These results suggest that during successful tissue regeneration, injury-related cytokines induce factors, such as iNOS and its product, NO, that protect surviving cells from cytokine-mediated death.

Biological role of the CCAAT/enhancer-binding protein family of transcription factors

CCAAT/enhancer-binding proteins (C/EBPs) comprise a family of transcription factors that are critical for normal cellular differentiation and function in a variety of tissues. The prototypic C/EBP is a modular protein, consisting of an activation domain, a dimerization bZIP region, and a DNA-binding domain. All family members share the highly conserved dimerization domain, required for DNA binding, by which they form homo- and heterodimers with other family members. C/EBPs are least conserved in their activation domains and vary from strong activators to dominant negative repressors. The pleiotropic effects of C/EBPs are in part because of tissue- and stage-specific expression. Dimerization of different C/EBP proteins precisely modulates transcriptional activity of target genes. Recent work with mice deficient in specific C/EBPs underscores the effects of these factors in tissue development, function, and response to injury.

Transcriptional regulation of cyclooxygenase-2 in mouse skin carcinoma cells. Regulatory role of CCAAT/enhancer-binding proteins in the differential expression of cyclooxygenase-2 in normal and neoplastic tissues

Many studies have suggested that overexpression of cyclooxygenase-2 (COX-2) contributes to the development of tumors in several tissues. COX-2 expression tends to be up-regulated in various types of tumors and transformed cell lines, and the overexpression of COX-2 is caused by enhanced transcription of the gene. In an attempt to characterize the signaling pathway leading to the overexpression of COX-2 in the mouse skin carcinoma cell line JWF2, we investigated cis- and trans-acting factors required for COX-2 expression and demonstrated a molecular mechanism by which COX-2 is expressed differentially in normal and neoplastic tissues. Two regions of the COX-2 promoter containing an E-box and nuclear factor IL6 site were identified as the positive regulatory elements through transient transfections with luciferase reporter vectors containing the various 5'-flanking regions of the promoter. Moreover, electrophoretic mobility shift assays and cotransfection experiments showed that upstream stimulatory factors and CCAAT/enhancer-binding proteins (C/EBPs) bind to the E-box and nuclear factor IL6 site, respectively, and functionally transactivate the COX-2 promoter. We also found that C/EBP isoforms are expressed differentially during mouse skin carcinogenesis, suggesting that overexpression of COX-2 in tumors may be caused by a change in C/EBP expression levels.

Effects of chronic ethanol consumption on cytokine regulation of liver regeneration

Ethanol ingestion may interrupt the proregenerative signal transduction that is initiated by injury-related cytokines such as tumor necrosis factor (TNF)-alpha and TNF-alpha- inducible cytokines including interleukin (IL)-6. To test this theory, liver regeneration, TNF-alpha and IL-6 expression, and cytokine-regulated prereplicative events were compared in ethanol-fed rats and isocalorically fed controls after 70% partial hepatectomy (PH). Ethanol feeding inhibits hepatocyte replication and recovery of liver mass after PH but generally promotes induction of both cytokines in the liver and extrahepatic tissues (i.e., white adipose tissue). Cytokine-regulated events that occur early in the prereplicative period are influenced differentially. TNF-alpha-dependent increases in hepatic nuclear factor-kappaB (NF-kappaB) p50 and p65 expression and DNA binding activity are prevented, whereas IL-6-dependent inductions of hepatic Stat-3 phosphorylation and DNA binding activity occur normally. In contrast, events (e.g., induction of cyclin D1, cdk-1, cyclin D3, and p53 mRNA) that occur at the end of the prereplicative period are uniformly inhibited. These findings indicate that chronic ethanol ingestion arrests the regenerative process during the prereplicative period and demonstrate that increased TNF-alpha, IL-6 and Stat-3 are not sufficient to assure hepatocyte proliferation after PH.

Effects of age on the posttranscriptional regulation of CCAAT/enhancer binding protein alpha and CCAAT/enhancer binding protein beta isoform synthesis in control and LPS-treated livers

The CCAAT/enhancer binding protein alpha (C/EBPalpha) and CCAAT/enhancer binding protein beta (C/EBPbeta) mRNAs are templates for the differential translation of several isoforms. Immunoblotting detects C/EBPalphas with molecular masses of 42, 38, 30, and 20 kDa and C/EBPbetas of 35, 20, and approximately 8.5 kDa. The DNA-binding activities and pool levels of p42(C/EBPalpha) and p30(C/EBPalpha) in control nuclear extracts decrease significantly whereas the binding activity and protein levels of the 20-kDa isoforms increase dramatically with LPS treatment. Our studies suggest that the LPS response involves alternative translational initiation at specific in-frame AUGs, producing specific C/EBPalpha and C/EBPbeta isoform patterns. We propose that alternative translational initiation occurs by a leaky ribosomal scanning mechanism. We find that nuclear extracts from normal aged mouse livers have decreased p42(C/EBPalpha) levels and binding activity, whereas those of p20(C/EBPalpha) and p20(C/EBPbeta) are increased. However, translation of 42-kDa C/EBPalpha is not down-regulated on polysomes, suggesting that aging may affect its nuclear translocation. Furthermore, recovery of the C/EBPalpha- and C/EBPbeta-binding activities and pool levels from an LPS challenge is delayed significantly in aged mouse livers. Thus, aged livers have altered steady-state levels of C/EBPalpha and C/EBPbeta isoforms. This result suggests that normal aging liver exhibits characteristics of chronic stress and a severe inability to recover from an inflammatory challenge.

Evidence that protein kinase A activity is required for the basal and tax-stimulated transcriptional activity of human T-cell leukemia virus type-I long terminal repeat

The present study was undertaken to investigate the role of protein kinase A (PKA) in the control of human T-cell leukemia virus type-I (HTLV-I) long terminal repeat (LTR) expression, since this issue is still controversial. For this purpose we employed two human T-cell lines; the Jurkat cells in which long exposure to diBu-cAMP severely down-regulated the catalytic subunit of PKA (PKA-C), and H-9 cells in which such exposure markedly increased PKA-C level. Transient transfection assays revealed that addition of diBu-cAMP 1 h before or after transfection profoundly increased HTLV-I LTR directed CAT expression and synergistically enhanced its stimulation by the viral transactivator tax gene product in both cell lines. However longer exposure to diBu-cAMP before transfection reduced LTR-CAT expression to below its basal level and completely abolished its stimulation by tax in Jurkat cells, and this diBu-cAMP inhibitory effect could be abrogated by co-transfection of a PKA-C expressing vector. By contrast, in H-9 cells, this long exposure to diBu-cAMP continued enhancing LTR-CAT expression and its tax-mediated transactivation, and this stimulatory effect of diBu-cAMP could be diminished by the PKA-specific inhibitor N-12-(p-bromocinnamylamine)ethyll-5-isoquinolinsulfonamid e (H-89). Notably, in the absence of diBu-cAMP treatment H-89 reduced LTR-CAT expression to below its basal level and prevented its stimulation by tax in both cell lines. Together these findings indicate not only that cAMP-activated PKA stimulates HTLV-I LTR expression and its transactivation by tax, but even in the absence of PKA activating signals the basal HTLV-I LTR expression as well as its stimulation by tax are both dependent on a basal PKA activity.

Concanavalin A-induced liver injury triggers hepatocyte proliferation

Concanavalin A (Con A) injection into mice leads to immune-mediated liver injury. We studied whether after Con A-induced liver injury, TNF- and IL-6-dependent signaling pathways known to be related to hepatocyte proliferation are activated. 2 h after Con A injection, maximum TNF-alpha, and after 4-8 h, maximum IL-6 serum levels were found. The rise in aminotransferases and DNA fragmentation started after 4 h; maximum levels were evident after 8 h. 5-Bromo-2'-deoxyuridine staining and nuclear cyclin A expression as markers of the S-phase were first detected in hepatocyte nuclei after 24 h, peaking after 48 h. An increase in TNF-dependent nuclear expression of CCAAT/enhancer-binding protein-beta (C/EBP-beta)/liver-enriched activating protein (LAP) was detected after 1 h, whereas an increase in RNA expression was evident only after 4 h. C/EBP-beta/LAP expression returned to normal values before progression into the S-phase. DNA binding of signal transducer and activator of transcription (STAT) 3/acute phase response factor (APRF) increased for up to 8 h. As found by supershift experiments, in addition to STAT3/APRF, STAT1 also binds to the same sequence. During the course of time gel shift experiments, DNA binding of the apoptosis-related STAT1 started earlier than DNA binding of STAT3/APRF, which regulates hepatocyte proliferation. However, the subsequent decrease in DNA binding of both factors was comparable. This study demonstrates that after Con A injection, TNF- and IL-6- dependent signals trigger nuclear events regulating hepatocyte apoptosis and proliferation during liver injury.

CCAAT/enhancer-binding protein delta activates insulin-like growth factor-I gene transcription in osteoblasts. Identification of a novel cyclic AMP signaling pathway in bone

Insulin-like growth factor-I (IGF-I) plays a key role in skeletal growth by stimulating bone cell replication and differentiation. We previously showed that prostaglandin E2 (PGE2) and other cAMP-activating agents enhanced IGF-I gene transcription in cultured primary rat osteoblasts through promoter 1, the major IGF-I promoter, and identified a short segment of the promoter, termed HS3D, that was essential for hormonal regulation of IGF-I gene expression. We now demonstrate that CCAAT/enhancer-binding protein (C/EBP) delta is a major component of a PGE2-stimulated DNA-protein complex involving HS3D and find that C/EBPdelta transactivates IGF-I promoter 1 through this site. Competition gel shift studies first indicated that a core C/EBP half-site (GCAAT) was required for binding of a labeled HS3D oligomer to osteoblast nuclear proteins. Southwestern blotting and UV-cross-linking studies showed that the HS3D probe recognized a approximately 35-kDa nuclear protein, and antibody supershift assays indicated that C/EBPdelta comprised most of the PGE2-activated gel-shifted complex. C/EBPdelta was detected by Western immunoblotting in osteoblast nuclear extracts after treatment of cells with PGE2. An HS3D oligonucleotide competed effectively with a high affinity C/EBP site from the rat albumin gene for binding to osteoblast nuclear proteins. Co-transfection of osteoblast cell cultures with a C/EBPdelta expression plasmid enhanced basal and PGE2-activated IGF-I promoter 1-luciferase activity but did not stimulate a reporter gene lacking an HS3D site. By contrast, an expression plasmid for the related protein, C/EBPbeta, did not alter basal IGF-I gene activity but did increase the response to PGE2. In osteoblasts and in COS-7 cells, C/EBPdelta, but not C/EBPbeta, transactivated a reporter gene containing four tandem copies of HS3D fused to a minimal promoter; neither transcription factor stimulated a gene with four copies of an HS3D mutant that was unable to bind osteoblast nuclear proteins. These results identify C/EBPdelta as a hormonally activated inducer of IGF-I gene transcription in osteoblasts and show that the HS3D element within IGF-I promoter 1 is a high affinity binding site for this protein.

Antioxidant-induced nuclear translocation of CCAAT/enhancer-binding protein beta. A critical role for protein kinase A-mediated phosphorylation of Ser299

Alterations in intracellular oxidative status activate several signal transduction pathways resulting in distinct patterns of gene expression. Treatment of colorectal cancer cells with antioxidants can lead to apoptosis by induction of p21 through a mechanism involving CCAAT/enhancer-binding protein beta (C/EBPbeta). Herein, we demonstrate that the antioxidant pyrrolidinedithiocarbamate activates cAMP-dependent protein kinase (PKA) in a colorectal cancer cell line DKO-1. Activation of PKA phosphorylates Ser299 within C/EBPbeta, which is essential for protein translocation to the nucleus. Pharmacological inhibition of PKA and mutation of Ser299 to alanine blocks C/EBPbeta nuclear translocation and induction of p21. Our results indicate that a cAMP-dependent phosphorylation of C/EBPbeta at Ser299 is critical for nuclear translocation of this protein and its subsequent transactivation of genes in response to antioxidant treatment.

A novel transcriptional regulatory region within the core promoter of the hepatocyte growth factor gene is responsible for its inducibility by cytokines via the C/EBP family of transcription factors

Hepatocyte growth factor (HGF) is an inducible cytokine that is essential for the normal growth and development of various tissues, such as the liver. To decipher the molecular mechanisms that regulate HGF gene induction at the transcriptional level, we carried out in vitro and in vivo studies on the mouse HGF gene promoter. We have identified a novel regulatory element, located between -6 and +7 bp (from the transcription start site) in the HGF basal promoter region, which binds to inducible transcription factors and dictates responsiveness to extracellular stimuli that activate this gene. The core binding sequence for the inducible cis-acting factors was determined to be TTTGCAA (-4 to +3 bp) within the HGF promoter. Competition and gel mobility supershift assays showed that these binding complexes are composed of C/EBPbeta (CCAAT/enhancer-binding protein beta) and C/EBPdelta. DNA binding analysis also revealed that the binding site for the C/EBP family of transcription factors in the HGF promoter region overlaps that of another binding protein (complex C1), which binds specifically to a novel sequence with a core binding site of ACCGGT located adjacent to the C/EBP site (-9 to -4 bp). C1 binds to this region of the promoter and represses the inducible upregulation by C/EBP through direct competition for their individual binding sites. Partial hepatectomy, which is known to activate HGF gene expression in the liver, increased C/EBP (especially C/EBPbeta) binding activity to this region of the HGF promoter. Thus, our present results provide a mechanistic explanation for the transcriptional induction of the HGF gene by extracellular signals (i.e., cytokines) that induce tissue growth and regeneration.

Multiple control elements mediate activation of the murine and human interleukin 12 p40 promoters: evidence of functional synergy between C/EBP and Rel proteins

Interleukin 12 (IL-12) is a heterodimeric cytokine whose activity is critical for T-helper 1 responses. The gene for the IL-12 p40 subunit is expressed in macrophages following induction by bacterial products, and its expression is augmented by gamma interferon. In this study, we performed a functional analysis of the murine and human p40 promoters in the murine macrophage cell line RAW 264.7. Transcription from the murine p40 promoter was strongly induced by lipopolysaccharide and heat-killed Listeria monocytogenes (HKLM), but promoter activity was not enhanced by gamma interferon. Multiple cis-acting elements involved in activated transcription were identified through an extensive mutant analysis. The most critical element, whose activity is conserved in mice and humans, is located between positions -96 and -88 relative to the murine transcription start site. This element exhibits functional synergy with a previously described NF-kappaB half-site which interacts with Rel proteins. DNase I footprinting and electrophoretic mobility shift assays demonstrated that C/EBP proteins interact with the critical element, but in nuclear extracts, cooperative binding of C/EBP and Rel proteins to their respective sites was not observed. Interestingly, promoter activity was induced by HKLM in the presence of cycloheximide, consistent with induction by posttranslational mechanisms. The results suggest that C/EBP and Rel proteins play important roles in the activation of IL-12 p40 transcription by bacteria. However, many complex interactions will need to be clarified to fully understand p40 regulation.

CREB controls LAP/C/EBP beta transcription

LAP/C/EBP beta is a member of the C/EBP family of transcription factors and is involved in hepatocyte-specific gene expression. Recently we showed that, besides its posttranscriptional regulation, LAP/C/EBP beta mRNA is modulated during liver regeneration. Therefore, in this study we investigated mechanisms which control LAP/C/EBP beta gene transcription. Deletion analysis of the 5'-flanking region, located upstream of the start site of transcription in the LAP/C/EBP beta gene, demonstrated that a small region in close proximity to the TATA box is important in maintaining a high level of transcription of the luciferase reporter gene constructs. In gel shift experiments two sites were identified which are important for specific complex formation within this region. Further analysis by cross-linking, super shift, and competition experiments was performed with liver cell nuclear extracts, hepatoma cell nuclear extracts, or recombinant CREB protein. These experiments conclusively demonstrated that CREB binds to both sites in the LAP/C/EBP beta promoter with an affinity similar to that with the CREB consensus sequence. Transfection experiments with promoter constructs where the CREB sites were mutated showed that these sites are important to maintain both basal promoter activity and LAP/C/EBP beta inducibility through CREB. Northern blot analysis and runoff transcription assays demonstrated that the protein kinase A pathway not only stimulated the activity of the luciferase reporter construct but also the transcription of the endogenous LAP/C/EBP beta gene in different cell types. Western blot analysis of rat liver cell nuclear extracts and runoff transcription assays of rat liver cell nuclei after two-thirds hepatectomy showed a functional link between the induction of CREB phosphorylation and LAP/C/EBP beta mRNA transcription during liver regeneration. These results demonstrate that the two CREB sites are important to control LAP/C/EBP beta transcription in vivo. As several pathways control CREB phosphorylation, our results provide evidence for the transcriptional regulation of LAP/C/EBP beta via CREB under different physiological conditions.

Obesity increases sensitivity to endotoxin liver injury: implications for the pathogenesis of steatohepatitis

Genetically obese fatty/fatty rats and obese/obese mice exhibit increased sensitivity to endotoxin hepatotoxicity, quickly developing steatohepatitis after exposure to low doses of lipopolysaccharide (LPS). Among obese animals, females are more sensitive to endotoxin liver injury than males. LPS induction of tumor necrosis factor alpha (TNF alpha), the proven affecter of endotoxin liver injury, is no greater in the livers, white adipose tissues, or sera of obese animals than in those of lean controls. Indeed, the lowest serum concentrations of TNF occur in female obese rodents, which exhibit the most endotoxin-induced liver injury. Several cytokines that modulate the biological activity of TNF are regulated abnormally in the livers of obese animals. After exposure to LPS, mRNA of interferon gamma, which sensitizes hepatocytes to TNF toxicity, is overexpressed, and mRNA levels of interleukin 10, a TNF inhibitor, are decreased. The phagocytic activity of liver macrophages and the hepatic expression of a gene encoding a macrophage-specific receptor are also decreased in obesity. This new animal model of obesity-associated liver disease demonstrates that hepatic macrophage dysfunction occurs in obesity and suggests that this might promote steatohepatitis by sensitizing hepatocytes to endotoxin.