Regulation of CCAAT/enhancer-binding protein-beta isoform synthesis by alternative translational initiation at multiple AUG start sites

C/EBPB-dependent adaptation to palmitic acid promotes tumor formation in hormone receptor negative breast cancer

Epidemiological studies have established a positive association between obesity and the incidence of postmenopausal breast cancer. Moreover, it is known that obesity promotes stem cell-like properties of breast cancer cells. However, the cancer cell-autonomous mechanisms underlying this correlation are not well defined. Here we demonstrate that obesity-associated tumor formation is driven by cellular adaptation rather than expansion of pre-existing clones within the cancer cell population. While there is no correlation with specific mutations, cellular adaptation to obesity is governed by palmitic acid (PA) and leads to enhanced tumor formation capacity of breast cancer cells. This process is governed epigenetically through increased chromatin occupancy of the transcription factor CCAAT/enhancer-binding protein beta (C/EBPB). Obesity-induced epigenetic activation of C/EBPB regulates cancer stem-like properties by modulating the expression of key downstream regulators including CLDN1 and LCN2. Collectively, our findings demonstrate that obesity drives cellular adaptation to PA drives tumor initiation in the obese setting through activation of a C/EBPB dependent transcriptional network.

Obesity is linked to cancer risk in post-menopausal breast cancer. At the molecular level this is governed by cellular adaption to palmitic acid through epigenetic activation of a C/EBPB-dependent transcriptional network that drives tumor formation.

Metformin inhibits human non-small cell lung cancer by regulating AMPK-CEBPB-PDL1 signaling pathway

Metformin has been found to have inhibitory effects on a variety of tumors. However, its effects on non-small cell lung cancer (NSCLC) remain unclear. We demonstrated that metformin could inhibit the proliferation of A549 and H1299 cells. RNA transcriptome sequencing revealed that PDL1 was significantly downregulated in both cell types following treatment with metformin (P < 0.001). Jaspar analysis and chromatin immunoprecipitation showed that CEBPB could directly bind the promoter region of PDL1. Western blotting showed that protein expression of the isoforms CEBPB-LAP*, CEBPB-LAP, and CEBPB-LIP was significantly upregulated and the LIP/LAP ratio was increased. Gene chip analysis showed that PDL1 was significantly upregulated in A549-CEBPB-LAP cells and significantly downregulated in A549-CEBPB-LIP cells (P < 0.05) compared with CEBPB-NC cells. Dual-luciferase reporter gene assay showed that CEBPB-LAP overexpression could promote transcription of PDL1 and CEBPB-LIP overexpression could inhibit the process. Functional assays showed that the changes in CEBPB isoforms affected the function of NSCLC cells. Western blotting showed that metformin could regulate the function of NSCLC cells via AMPK-CEBPB-PDL1 signaling. Animal experiments showed that tumor growth was significantly inhibited by metformin, and atezolizumab and metformin had a synergistic effect on tumor growth. A total of 1247 patients were retrospectively analyzed, including 166 and 1081 patients in metformin and control groups, respectively. The positive rate of PDL1 was lower than that of the control group (HR = 0.338, 95% CI = 0.235-0.487; P < 0.001). In conclusion, metformin inhibited the proliferation of NSCLC cells and played an anti-tumor role in an AMPK-CEBPB-PDL1 signaling-dependent manner.

Perm1 promotes cardiomyocyte mitochondrial biogenesis and protects against hypoxia/reoxygenation-induced damage in mice

Normal contractile function of the heart depends on a constant and reliable production of ATP by cardiomyocytes. Dysregulation of cardiac energy metabolism can result in immature heart development and disrupt the ability of the adult myocardium to adapt to stress, potentially leading to heart failure. Further, restoration of abnormal mitochondrial function can have beneficial effects on cardiac dysfunction. Previously, we identified a novel protein termed Perm1 (PGC-1 and estrogen-related receptor (ERR)-induced regulator, muscle 1) that is enriched in skeletal and cardiac-muscle mitochondria and transcriptionally regulated by PGC-1 (peroxisome proliferator-activated receptor gamma coactivator 1) and ERR. The role of Perm1 in the heart is poorly understood and is studied here. We utilized cell culture, mouse models, and human tissue, to study its expression and transcriptional control, as well as its role in transcription of other factors. Critically, we tested Perm1's role in cardiomyocyte mitochondrial function and its ability to protect myocytes from stress-induced damage. Our studies show that Perm1 expression increases throughout mouse cardiogenesis, demonstrate that Perm1 interacts with PGC-1α and enhances activation of PGC-1 and ERR, increases mitochondrial DNA copy number, and augments oxidative capacity in cultured neonatal mouse cardiomyocytes. Moreover, we found that Perm1 reduced cellular damage produced as a result of hypoxia and reoxygenation-induced stress and mitigated cell death of cardiomyocytes. Taken together, our results show that Perm1 promotes mitochondrial biogenesis in mouse cardiomyocytes. Future studies can assess the potential of Perm1 to be used as a novel therapeutic to restore cardiac dysfunction induced by ischemic injury.

Everolimus restrains the IL-17A-dependent osteoclast-like transdifferentiation of dendritic cells in multiple myeloma

Interleukin-17A (IL-17A) promotes the osteoclast (OC)-like differentiation of dendritic cells (DCs) in multiple myeloma (MM) and contributes to the pathogenesis of myeloma bone disease (MBD). In our study, everolimus (EVR) abrogated the in vitro OC-like activity of DCs from 12 MM patients significantly. Exploring the EVR effects, we found that the inhibition of the osteoerosive activity of OC-DCs was mostly due to the blockade of signals driven by the IL-17A receptor toward the CCAAT/enhancer-binding protein beta/musculoaponeurotic fibrosarcoma oncogene homolog B axis Therefore, MM patients with MBD would probably benefit from mammalian target of rapamycin inhibition.

CCAAT/enhancer binding protein β is dispensable for development of lung adenocarcinoma

Lung cancer is the leading cause of cancer death worldwide. Although disruption of normal proliferation and differentiation is a vital component of tumorigenesis, the mechanisms of this process in lung cancer are still unclear. A transcription factor, C/EBPβ is a critical regulator of proliferation and/or differentiation in multiple tissues. In lung, C/EBPβ is expressed in alveolar pneumocytes and bronchial epithelial cells; however, its roles on normal lung homeostasis and lung cancer development have not been well described. Here we investigated whether C/EBPβ is required for normal lung development and whether its aberrant expression and/or activity contribute to lung tumorigenesis. We showed that C/EBPβ was expressed in both human normal pneumocytes and lung adenocarcinoma cell lines. We found that overall lung architecture was maintained in Cebpb knockout mice. Neither overexpression of nuclear C/EBPβ nor suppression of CEBPB expression had significant effects on cell proliferation. C/EBPβ expression and activity remained unchanged upon EGF stimulation. Furthermore, deletion of Cebpb had no impact on lung tumor burden in a lung specific, conditional mutant EGFR lung cancer mouse model. Analyses of data from The Cancer Genome Atlas (TCGA) revealed that expression, promoter methylation, or copy number of CEBPB was not significantly altered in human lung adenocarcinoma. Taken together, our data suggest that C/EBPβ is dispensable for development of lung adenocarcinoma.

TGR5 reduces macrophage migration through mTOR-induced C/EBPβ differential translation

The bile acid-responsive G protein-coupled receptor TGR5 is involved in several metabolic processes, and recent studies suggest that TGR5 activation may promote pathways that are protective against diet-induced diabetes. Here, we investigated the role of macrophage-specific TGR5 signaling in protecting adipose tissue from inflammation and associated insulin resistance. Examination of adipose tissue from obese mice lacking macrophage Tgr5 revealed enhanced inflammation, increased chemokine expression, and higher macrophage numbers compared with control obese animals. Moreover, macrophage-specific deletion of Tgr5 exacerbated insulin resistance in obese animals. Conversely, pharmacological activation of TGR5 markedly decreased LPS-induced chemokine expression in primary macrophages. This reduction was mediated by AKT-dependent activation of mTOR complex 1, which in turn induced the differential translation of the dominant-negative C/EBPβ isoform, liver inhibitory protein (LIP). Overall, these studies reveal a signaling pathway downstream of TGR5 that modulates chemokine expression in response to high-fat diet and suggest that targeting this pathway has the potential to be therapeutically exploited for prevention of chronic inflammatory diseases and type 2 diabetes mellitus.

A positive regulatory domain in CCAAT/enhancer binding protein β (C/EBPΒ) is required for the glucocorticoid-mediated displacement of histone deacetylase 1 (HDAC1) from the C/ebpα promoter and maximum adipogenesis

Glucocorticoids promote adipogenesis and contribute to the metabolic syndrome through a number of mechanisms. One of the effectors of glucocorticoid action is the CCAAT/enhancer binding protein β (C/EBPβ). C/EBPβ is a basic leucine-zipper transcription factor involved in diverse processes including differentiation, cellular proliferation, and inflammation. C/EBPβ transcriptional activity is regulated, in part, by its acetylation profile resulting from its dynamic interaction with either acetylases general control nonrepressed protein 5/p300/CBP associated factor (GCN5/PCAF) or deacetylase complexes (mSin3A/histone deacetylase 1 [HDAC1]). Glucocorticoid treatment of preadipocytes promotes C/EBPβ acetylation, leading to mSin3A/HDAC1 dissociation from C/EBPβ and resulting in C/ebpα promoter activation at the onset of adipogenesis, thus increasing the differentiation rate. We recently showed that the regulatory domain 1 (RD1) of C/EBPβ contains four residues (153-156) required for its interaction with HDAC1, therefore supporting RD1 proposed inhibitory role. In an attempt to further elucidate the intrinsic regulatory property of RD1, we sought to characterize the regulatory potential of the N terminus region of RD1 (residues 141-149). In this study, we show that C/EBPβΔ141-149 transcriptional activity was compromised on the C/ebpα, but not on the Pparγ, promoter. Additionally, the ability of C/EBPβΔ141-149 to induce adipogenesis in NIH 3T3 cells was compromised when compared with C/EBPβwt owing to a delayed expression of C/ebpα at the onset of differentiation. Furthermore, the data suggest that the reduced expression of C/ebpα in cells expressing C/EBPβΔ141-149 was due to a persistent recruitment of HDAC1 to the C/ebpα promoter after glucocorticoid treatment. Together, these results suggest that amino acids 141-149 of C/EBPβ act as a positive regulatory domain required for maximum transcriptional activity.

CCAAT/enhancer binding protein β (C/EBPβ) isoforms as transcriptional regulators of the pro-invasive CDH3/P-cadherin gene in human breast cancer cells

P-cadherin is a cell-cell adhesion molecule codified by the CDH3 gene, which expression is highly associated with undifferentiated cells in normal adult epithelial tissues, as well as with poorly differentiated carcinomas. In breast cancer, P-cadherin is frequently overexpressed in high-grade tumours and is a well-established indicator of aggressive tumour behaviour and poor patient prognosis. However, till now, the mechanisms controlling CDH3 gene activation have been poorly explored. Since we recently described the existence of several CCAAT/Enhancer Binding Protein β (C/EBPβ) transcription factor binding sites at the CDH3 promoter, the aim of this study was to assess if the distinct C/EBPβ isoforms were directly involved in the transcriptional activation of the CDH3 gene in breast cancer cells. DNA-protein interactions, mutation analysis and luciferase reporter assay studies have been performed. We demonstrated that C/EBPβ is co-expressed with P-cadherin in breast cancer cells and all the three isoforms function as transcriptional regulators of the CDH3 gene, directly interacting with specific regions of its promoter. Interestingly, this transcriptional activation was only reflected at the P-cadherin protein level concerning the LIP isoform. Taken together, our data show that CDH3 is a newly defined transcriptional target gene of C/EBPβ isoforms in breast cancer, and we also identified the binding sites that are relevant for this activation.

Identification of alternatively translated Tetherin isoforms with differing antiviral and signaling activities

Tetherin (BST-2/CD317/HM1.24) is an IFN induced transmembrane protein that restricts release of a broad range of enveloped viruses. Important features required for Tetherin activity and regulation reside within the cytoplasmic domain. Here we demonstrate that two isoforms, derived by alternative translation initiation from highly conserved methionine residues in the cytoplasmic domain, are produced in both cultured human cell lines and primary cells. These two isoforms have distinct biological properties. The short isoform (s-Tetherin), which lacks 12 residues present in the long isoform (l-Tetherin), is significantly more resistant to HIV-1 Vpu-mediated downregulation and consequently more effectively restricts HIV-1 viral budding in the presence of Vpu. s-Tetherin Vpu resistance can be accounted for by the loss of serine-threonine and tyrosine motifs present in the long isoform. By contrast, the l-Tetherin isoform was found to be an activator of nuclear factor-kappa B (NF-κB) signaling whereas s-Tetherin does not activate NF-κB. Activation of NF-κB requires a tyrosine-based motif found within the cytoplasmic tail of the longer species and may entail formation of l-Tetherin homodimers since co-expression of s-Tetherin impairs the ability of the longer isoform to activate NF-κB. These results demonstrate a novel mechanism for control of Tetherin antiviral and signaling function and provide insight into Tetherin function both in the presence and absence of infection.

Regulation of innate immunity is critical to maintain a balance between control of a perceived threat and immunopathology. The interferon induced cellular factor Tetherin has been shown to restrict budding of a broad range of enveloped viruses including the human immunodeficiency virus. Though Tetherin appears to be a bona fide viral restriction factor, additional cellular functions have been observed including an involvement in actin cytoskeleton organization in polarized cells, regulating interferon secretion and signaling through nuclear factor-kappa B (NF-κB). Our studies present a mechanism by which Tetherin function is regulated at the translational level through the production of alternatively translated isoforms. The short isoform of Tetherin was observed to be significantly more resistant to HIV-1 Vpu. In contrast, the longer isoform can induce NF-κB activity, a function lacking in the short isoform. Critical NF-κB signaling residues include a dual tyrosine motif, which is only present in the long isoform. Identification of these isoforms helps to illuminate how Tetherin functions, not only as a restriction factor, but also as a signaling molecule. These data highlight a previously unappreciated level of regulation and furthers our understanding of additional Tetherin functions.

Negative regulation of miR-145 by C/EBP-β through the Akt pathway in cancer cells

MicroRNAs are master gene regulators that can also be under the control of transcriptional regulation. We have previously shown that miR-145 is a tumor suppressor capable of silencing c-Myc and the tumor suppressor p53 induces miR-145 by directly binding to the miR-145 promoter, demonstrating the role of miR-145 in p53-mediated c-Myc repression. However, little is known as to why miR-145 is often downregulated in tumors. In this study, we identify CCAAT/enhancer binding protein beta (C/EBP-β) as a negative regulator for miR-145 expression by direct interaction with the putative C/EBP-β binding site in the miR-145 promoter. In the wild-type p53 background, C/EBP-β counteracts the ability of p53 to induce miR-145. Moreover, C/EBP-β is able to suppress miR-145 in the mutant p53 background, suggesting the p53 independent regulation of miR-145. Of interest, both the large isoform (LAP-2) and the small isoform (LIP) of C/EBP-β can exert suppressive function for miR-145. Finally, we further show that, like serum starvation and PI3K inhibitor LY29, the antioxidant resveratrol suppresses pAkt and phosphorylation of C/EBP-β and at the same time, it induces miR-145. Together, these results suggest a miR-145 regulatory system involving the Akt and C/EBP-β, which may contribute to the downregulation of miR-145 in cancer cells.

C/EBPβ binds the P1 promoter of the Runx2 gene and up-regulates Runx2 transcription in osteoblastic cells

The Runx2 factor is an essential component of the regulatory mechanisms that control transcription during skeletogenesis. Runx2/p57 expression in osteoblastic cells is controlled by the P1 promoter, which is recognized by key regulators of osteoblast differentiation including homeodomain factors and Wnt- and BMP-signaling mediators. Here, we report that the transcription factor C/EBPβ up-regulates Runx2/p57 expression by directly binding to the Runx2 P1 promoter in mesenchymal, pre-osteoblastic, and osteoblastic cells. This C/EBPβ-mediated up-regulation is principally dependent on C/EBP site II that is located within the first 180 bp of the proximal P1 promoter region and is highly conserved among mouse, rat, and human Runx2 genes. Our studies reveal how the C/EBPβ factor, known to have a key role during osteogenesis, contributes to regulating the expression of Runx2, the master regulator of osteoblast differentiation.

LIP expression is regulated by IGF-1R signaling and participates in suppression of anoikis

Background

The transcription factor, CCAAT enhancer binding protein-β (C/EBPβ), is expressed as several distinct protein isoforms (LAP1, LAP2 and LIP) that have opposing actions in cellular proliferation and differentiation. Increases in the ratio of LIP/LAP are associated with aggressive, metastatic breast cancer; however, little is known regarding the molecular mechanisms that regulate LIP expression or the biological actions of an increase in the LIP/LAP ratio. Metastasis is highly dependent upon the suppression of anoikis and the role of C/EBPβ and LIP in this anchorage-independent, survival process is currently not known in mammary epithelial cells. IGF-1R signaling is important for the survival of breast cancer cells and crosstalk between IGF-1R and EGFR signaling pathways have been implicated in the development of more aggressive disease. We therefore evaluated in mammary epithelial cells whether IGF-1R signaling regulates the LIP/LAP ratio, analyzed the potential interplay between EGFR and IGF-1R signaling and addressed the biological significance of increased LIP expression in cellular survival and suppression of anoikis.

Results

Our data provide the first evidence that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. Although crosstalk between IGF-1R signaling and EGFR signaling is detectable in MCF10A cells, this crosstalk is not required for the IGF-1 mediated regulation of LIP expression. Rather, the critical regulator of IGF-1 induced LIP expression appears to be EGFR-independent, Akt activity. Our data also demonstrate that increases in LIP expression promote cell survival via suppression of anoikis. Likewise, knockdown of total C/EBPβ leads to increased cell death and suggest that C/EBPβ expression is important for survival and resistance to anoikis. IGF-1 treatment can partially rescue vector control cells from anoikis; however, cells with reduced C/EBPβ expression do not survive anoikis.

Conclusions

Taken together, our data demonstrate that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. C/EBPβ expression and elevations in LIP play an important role in regulating cellular survival via suppression of anoikis, in an IGF-1R mediated context or in a manner independent of IGF-1R signaling.

The Ric-8B gene is highly expressed in proliferating preosteoblastic cells and downregulated during osteoblast differentiation in a SWI/SNF- and C/EBPbeta-mediated manner

The Ric-8 gene encodes a guanine exchange factor (GEF) that modulates G protein-mediated signaling, exhibiting a relevant role during regulation of cell division. In mammals, two Ric-8 homologues have been reported (Ric-8A and Ric-8B), and recent studies indicate equivalent roles for each protein. Here, we show that the Ric-8B gene is negatively regulated during osteoblast differentiation by the transcription factor C/EBPβ. Only the larger C/EBPβ isoform (C/EBPβ-LAP*) downregulates Ric-8B gene promoter activity in osteoblastic cells. Accordingly, knockdown of C/EBPβ expression by small intefering RNA in osteoblastic cells results in a significant increase of Ric-8B gene expression. Transient overexpression of Brg1 or Brm, the catalytic subunits of the SWI/SNF chromatin-remodeling complex, inhibits Ric-8B promoter activity. Also, the presence of inactive SWI/SNF complexes in osteoblastic cells results in increased endogenous Ric-8B transcription, indicating that SWI/SNF activity negatively regulates Ric-8B expression. During osteoblast differentiation, Ric-8B gene repression is accompanied by changes in nucleosome placement at the proximal Ric-8B gene promoter and reduced accessibility to regulatory sequences.

CCAAT/enhancer-binding protein beta inhibits proliferation in monocytic cells by affecting the retinoblastoma protein/E2F/cyclin E pathway but is not directly required for macrophage morphology

Monocytic differentiation is orchestrated by complex networks that are not fully understood. This study further elucidates the involvement of transcription factor CCAAT/enhancer-binding protein β (C/EBPβ). Initially, we demonstrated a marked increase in nuclear C/EBPβ-liver-enriched activating protein* (LAP*)/liver-enriched activating protein (LAP) levels and LAP/liver-enriched inhibiting protein (LIP) ratios in phorbol 12-myristate 13-acetate (PMA)-treated differentiating THP-1 premonocytic cells accompanied by reduced proliferation. To directly study C/EBPβ effects on monocytic cells, we generated novel THP-1-derived (low endogenous C/EBPβ) cell lines stably overexpressing C/EBPβ isoforms. Most importantly, cells predominantly overexpressing LAP* (C/EBPβ-long), but not those overexpressing LIP (C/EBPβ-short), exhibited a reduced proliferation, with no effect on morphology. PMA-induced inhibition of proliferation was attenuated in C/EBPβ-short cells. In C/EBPβ^WT macrophage-like cells (high endogenous C/EBPβ), we measured a reduced proliferation/cycling index compared with C/EBPβ^KO. The typical macrophage morphology was only observed in C/EBPβ^WT, whereas C/EBPβ^KO stayed round. C/EBPα did not compensate for C/EBPβ effects on proliferation/morphology. Serum reduction, an independent approach known to inhibit proliferation, induced macrophage morphology in C/EBPβ^KO macrophage-like cells but not THP-1. In PMA-treated THP-1 and C/EBPβ-long cells, a reduced phosphorylation of cell cycle repressor retinoblastoma was found. In addition, C/EBPβ-long cells showed reduced c-Myc expression accompanied by increased CDK inhibitor p27 and reduced cyclin D1 levels. Finally, C/EBPβ-long and C/EBPβ^WT cells exhibited low E2F1 and cyclin E levels, and C/EBPβ overexpression was found to inhibit cyclin E1 promoter-dependent transcription. Our results suggest that C/EBPβ reduces monocytic proliferation by affecting the retinoblastoma/E2F/cyclin E pathway and that it may contribute to, but is not directly required for, macrophage morphology. Inhibition of proliferation by C/EBPβ may be important for coordinated monocytic differentiation.

Liver-enriched inhibitory protein (LIP) actively inhibits preadipocyte differentiation through histone deacetylase 1 (HDAC1)

The CCAAT/enhancer-binding protein β (C/EBPβ) is expressed as three isoforms (LAP*, liver-enriched activating protein (LAP), and liver-enriched inhibitory protein (LIP)) that differentially regulate gene expression. The interplay between LAP*, LAP, and LIP in regulating cellular processes is largely unknown, and LIP has been largely regarded to repress transcription through a passive heterodimerization-dependent mechanism. Recently, we have shown that p300/GCN5 and mSin3A/HDAC1 differentially regulate the ability of C/EBPβ to stimulate preadipocyte differentiation through activation of C/ebpα transcription. Here, we have mapped requirements for binding of mSin3A/HDAC1 to LAP/LAP* and LIP to a 4-amino acid motif in the central region of LAP/LAP* (residues 153-156) and the N terminus of LIP. Reducing mSin3A/HDAC1 binding to LAP/LAP* and LIP through deletion of this motif reduced the recruitment of HDAC1 to the C/ebpα promoter and increased preadipocyte differentiation stimulated by insulin and 1-methyl-3-isobutylxanthine. Additional studies showed that the interaction of HDAC1 with LIP provides for active repression of C/ebpα transcription and is largely responsible for the ability of LIP and HDAC1 to repress preadipocyte differentiation. Thus, although mSin3A/HDAC1 interacted readily with LAP/LAP* in addition to LIP and that expression of LAP/LAP* was sufficient to recruit HDAC1 to the C/ebpα promoter, mutations in C/ebpβ that abrogated HDAC1 association to LAP/LAP* in the absence of LIP provided no additional stimulation of differentiation or transcription beyond the deletion of LIP alone. The implication of these results for the interaction between p300/GCN5 and mSin3A/HDAC1 in regulating C/EBPα transcription and preadipocyte differentiation are discussed.

[Progress of transcription factor CCAAT enhancer binding protein β]

CCAAT enhancer binding protein β (C/EBP β) belongs to CCAAT enhancer binding protein (C/EBP) family, which is a subfamily of basic leucine zipper (bZIP) protein family. C/EBP family plays important roles in many processes such as cell differentiation, metabolism, and development. In this paper, the structure, expression regulation, and function of C/EBP β were reviewed.

Upstream open reading frames: molecular switches in (patho)physiology

Conserved upstream open reading frames (uORFs) are found within many eukaryotic transcripts and are known to regulate protein translation. Evidence from genetic and bioinformatic studies implicates disturbed uORF-mediated translational control in the etiology of human diseases. A genetic mouse model has recently provided proof-of-principle support for the physiological relevance of uORF-mediated translational control in mammals. The targeted disruption of the uORF initiation codon within the transcription factor CCAAT/enhancer binding protein β (C/EBPβ) gene resulted in deregulated C/EBPβ protein isoform expression, associated with defective liver regeneration and impaired osteoclast differentiation. The high prevalence of uORFs in the human transcriptome suggests that intensified search for mutations within 5' RNA leader regions may reveal a multitude of alterations affecting uORFs, causing pathogenic deregulation of protein expression.

CCAAT/enhancer binding protein beta2 is involved in growth hormone-regulated insulin-like growth factor-II gene expression in the liver of rainbow trout (Oncorhynchus mykiss)

Previously, we showed that levels of different CCAAT/enhancer binding protein (C/EBP) mRNAs in the liver of rainbow trout were modulated by GH and suggested that C/EBPs might be involved in GH-induced IGF-II gene expression. As a step toward further investigation, we have developed monospecific polyclonal antibodies to detect rainbow trout C/EBPalpha, -beta1, -beta2, and -delta2 isoform proteins. Injection of GH into adult rainbow trout resulted in a significant increase of C/EBPbeta1, C/EBPbeta2, and C/EBPdelta2 proteins in the liver. Chromatin immunoprecipitation analysis revealed that C/EBPbeta2 binds to multiple sites at the 5' promoter/regulatory region, introns, and the 3' untranslated region of the IGF-II gene. GH treatment reduced C/EBPbeta2 binding to several of these regions at 6 h after injection. The decreased occupancy of C/EBPbeta2 coincided well with an increase of histone H4 acetylation at the proximal promoter and elevation of the IGF-II mRNA level. Immunoblotting analysis showed that C/EBPbeta2 existed predominately as a truncated form in the liver, and cotransfection analysis further showed that the truncated C/EBPbeta2 acted as a negative regulator on IGF-II proximal promoter. GH treatment caused deacetylation of C/EBPbeta2 in the liver. In addition, we observed a GH-dependent interaction of C/EBPbeta2 with a complex involving histone H1. All together, these results suggest that C/EBPbeta2 was regulated at multiple levels by GH, and C/EBPbeta2 may play a suppressive role in mediating GH-induced IGF-II expression in the liver of rainbow trout.

Age-dependent response of CCAAT/enhancer binding proteins following traumatic brain injury in mice

Exacerbated inflammatory responses have been reported following traumatic injury to the aged brain. The present study was designed to investigate the involvement of the transcription factors belonging to the CCAAT/enhancer binding protein (C/EBP) family that regulate expression of many of the pro-inflammatory genes which show increased expression following injury to the aged brain. Controlled cortical impact injury was induced in adult (5-6 months) and aged (22-24 months) C57/BL6 mice. C/EBP mRNA and protein expression were analyzed in injured cortex at 1, 3, and 7 days post-injury. Expression of C/EBPalpha was reduced relative to baseline at day 1 in both adult and aged mice, whereas, it increased at days 3 and 7 post-injury. No significant differences were observed between adult and aged brain. Upregulation of C/EBPbeta was observed 1 day following injury in both the adult and aged brain, but there were no major age-related differences in mRNA levels. However, there was higher C/EBPbeta protein in the aged brain. C/EBPdelta expression increased beginning 1 day post-injury in both adult and aged brain. In this case, the increase in C/EBPdelta expression was higher in the aged brain than in the adult at all time points studied. Expression of CCAAT/enhancer binding protein homologous protein (CHOP), a transcription factor involved in ER stress and protein unfolding responses, was also up-regulated in response to injury, but CHOP levels were significantly lower in the aged than the adult brain. Based on these results, we conclude that differential expression of C/EBP beta, delta and CHOP might contribute to the hyper-inflammatory response and poor prognosis following traumatic brain injury in the elderly patients. In addition elevated C/EBPdelta levels following TBI in the aged brain may play a role in the link between TBI and Alzheimer's disease.

Transcription factor C/EBPbeta isoform ratio regulates osteoclastogenesis through MafB

Disequilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts is central to many bone diseases. Here, we show that dysregulated expression of translationally controlled isoforms of CCAAT/enhancer-binding protein β (C/EBPβ) differentially affect bone mass. Alternative translation initiation that is controlled by the mammalian target of rapamycin (mTOR) pathway generates long transactivating (LAP^*, LAP) and a short repressive (LIP) isoforms from a single C/EBPβ transcript. Rapamycin, an inhibitor of mTOR signalling increases the ratio of LAP over LIP and inhibits osteoclastogenesis in wild type (WT) but not in C/EBPβ null (c/ebpβ^−/−) or in LIP knock-in (L/L) osteoclast precursors. C/EBPβ mutant mouse strains exhibit increased bone resorption and attenuated expression of MafB, a negative regulator of osteoclastogenesis. Ectopic expression of LAP and LIP in monocytes differentially affect the MafB promoter activity, MafB gene expression and dramatically affect osteoclastogenesis. These data show that mTOR regulates osteoclast formation by modulating the C/EBPβ isoform ratio, which in turn affects osteoclastogenesis by regulating MafB expression.

CCAAT/enhancer-binding protein beta: its role in breast cancer and associations with receptor tyrosine kinases

The CCAAT/enhancer-binding proteins (C/EBPs) are a family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Encoded by an intronless gene, C/EBPbeta is expressed as several distinct protein isoforms (LAP1, LAP2, LIP) whose expression is regulated by the differential use of several in-frame translation start sites. LAP1 and LAP2 are transcriptional activators and are associated with differentiation, whereas LIP is frequently elevated in proliferative tissue and acts as a dominant-negative inhibitor of transcription. However, emerging evidence suggests that LIP can serve as a transcriptional activator in some cellular contexts, and that LAP1 and LAP2 might also have unique actions. The LIP:LAP ratio is crucial for the maintenance of normal growth and development, and increases in this ratio lead to aggressive forms of breast cancer. This review discusses the regulation of C/EBPbeta activity by post-translational modification, the individual actions of LAP1, LAP2 and LIP, and the functions and downstream targets that are unique to each isoform. The role of the C/EBPbeta isoforms in breast cancer is discussed and emphasis is placed on their interactions with receptor tyrosine kinases.

Insulin regulates TRB3 and other stress-responsive gene expression through induction of C/EBPbeta

Pseudokinase TRB3 is an inducible gene whose expression is regulated by stress response and insulin and associated with insulin resistance and metabolic syndrome. In this report, we have investigated the mechanism under which insulin regulates TRB3 gene expression and demonstrated that insulin induces TRB3 expression via C/EBPbeta. We found that in Fao hepatoma and 3T3-L1 adipocytes, C/EBPbeta expression induced by insulin preceded that of TRB3 and that mutation of the C/EBPbeta binding site in TRB3 promoter abolished the responsiveness of the TRB3 gene to insulin. We further showed that ectopic expression of C/EBPbeta augmented, whereas knockdown of C/EBPbeta reduced, TRB3 expression induced by insulin. In addition, we presented data to show that insulin, through a similar mechanism under which insulin induces TRB3 expression, promotes the expression of genes such as ANAS, ATF3, BIP, and CHOP, which are typical stress-responsive genes. We also examined the impact of C/EBPbeta expression on Akt activation and found that inaction of C/EBPbeta not only augmented Akt activation but also obliterated the suppression of Akt activation due to prolonged insulin stimulation. We suggest, through induction of C/EBPbeta in hepatic cells and adipocytes, that insulin induces the expression of stress-responsive genes, which may represent a novel insulin action.

Sumoylation of LAP1 is involved in the HDAC4-mediated repression of COX-2 transcription

CEBPB, one of the CEBP family members, is a crucial regulator of gene expression during innate immunity, inflammatory responses and adipogenesis. In this study, the EGF-induced increase of CEBPB mRNA is shown to be coincident with the decrease of COX-2 mRNA. We identified that all of the individual CEBPB isoforms, LAP1, LAP2 and LIP, attenuate EGF-induced COX-2 promoter activity. Although increased sumoylation of both LAP1 and LAP2 is observed during the lagging stage of EGF treatment, only the sumoylated LAP1, but not the sumoylated LAP2, is responsible for COX-2 gene repression. In addition, EGF treatment can regulate the nucleocytoplasmic redistribution of HDAC4 and SUMO1. We further demonstrated by loss-of- and gain-of-function approaches that HDAC4 can be a negative regulator while inactivating COX-2 transcription. The sumoylation mutant LAP1, LAP1K174A, exhibits an attenuated ability to interact with HDAC4, and increased COX-2 promoter activity. Furthermore, the in vivo DNA binding assay demonstrated that LAP1K174A and CEBPDK120A, sumoylation-defective CEBPD mutants, attenuate the binding of HDAC4 on the COX-2 promoter. In light of the above, our data suggest that the suCEBPD and suLAP1 are involved in the repression of COX-2 transcription through the recruitment of HDAC4.

Distinct functions of CCAAT enhancer-binding protein isoforms in the regulation of manganese superoxide dismutase during interleukin-1beta stimulation

The mitochondrial antioxidant enzyme manganese superoxide dismutase (Mn-SOD) is crucial in maintaining cellular and organismal homeostasis. Mn-SOD expression is tightly regulated in a manner that synchronizes its cytoprotective functions during inflammatory challenges. Induction of Mn-SOD gene expression by the proinflammatory cytokine IL-1beta is mediated through a complex intronic enhancer element. To identify and characterize the transcription factors required for Mn-SOD enhancer function, a yeast one-hybrid assay was utilized, and two CCAAT enhancer-binding protein (C/EBP) members, C/EBP beta and C/EBP delta, were identified. These two transcription factors responded to IL-1beta treatment with distinct expression profiles, different temporal yet inducible interactions with the endogenous Mn-SOD enhancer, and also opposite effects on Mn-SOD transcription. C/EBP beta is expressed as three isoforms, LAP* (liver-activating protein), LAP, and LIP (liver-inhibitory protein). Our functional analysis demonstrated that only the full-length C/EBP beta/LAP* served as a true activator for Mn-SOD, whereas LAP, LIP, and C/EBP delta functioned as potential repressors. Finally, our systematic mutagenesis of the unique N-terminal 21 amino acids further solidified the importance of LAP* in the induction of Mn-SOD and emphasized the crucial role of this isoform. Our data demonstrating the physiological relevance of the N-terminal peptide also provide a rationale for revisiting the role of LAP* in the regulation of other genes and in pathways such as lipogenesis and development.

Glucocorticoid regulation of the promoter of 11beta-hydroxysteroid dehydrogenase type 1 is indirect and requires CCAAT/enhancer-binding protein-beta

11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) converts inert 11keto-glucocorticoids to active 11beta-hydroxy forms, thereby amplifying intracellular glucocorticoid action. Up-regulation of 11beta-HSD1 in adipose tissue and liver is of pathogenic importance in metabolic syndrome. However, the mechanisms controlling 11beta-HSD1 transcription are poorly understood. Glucocorticoids themselves potently increase 11beta-HSD1 expression in many cells, providing a potential feed-forward system to pathology. We have investigated the molecular mechanisms by which glucocorticoids regulate transcription of 11beta-HSD1, exploiting an A549 cell model system in which endogenous 11beta-HSD1 is expressed and is induced by dexamethasone. We show that glucocorticoid induction of 11beta-HSD1 is indirect and requires new protein synthesis. A glucocorticoid-responsive region maps to between -196 and -88 with respect to the transcription start site. This region contains two binding sites for CCAAT/enhancer-binding protein (C/EBP) that together are essential for the glucocorticoid response and that bind predominantly C/EBPbeta, with C/EBPdelta present in a minority of the complexes. Both C/EBPbeta and C/EBPdelta are rapidly induced by glucocorticoids in A549 cells, but small interfering RNA-mediated knockdown shows that only C/EBPbeta reduction attenuates the glucocorticoid induction of 11beta-HSD1. Chromatin immunoprecipitation studies demonstrated increased binding of C/EBPbeta to the 11beta-HSD1 promoter in A549 cells after glucocorticoid treatment. A similar mechanism may apply in adipose tissue in vivo where increased C/EBPbeta mRNA levels after glucocorticoid treatment were associated with increased 11beta-HSD1 expression. C/EBPbeta is a key mediator of metabolic and inflammatory signaling. Positive regulation of 11beta-HSD1 by C/EBPbeta may link amplification of glucocorticoid action with metabolic and inflammatory pathways and may represent an endogenous innate host-defense mechanism.

Adipogenic signaling in rat white adipose tissue: modulation by aging and calorie restriction

Alterations in adipogenesis could have significant impact on several aging processes. We previously reported that calorie restriction (CR) in rats significantly increases the level of circulating adiponectin, a distinctive marker of differentiated adipocytes, leading to a concerted modulation in the expression of key transcription target genes and, as a result, to increased fatty acid oxidation and reduced deleterious lipid accumulation in other tissues. These findings led us to investigate further the effects of aging on adipocytes and to determine how CR modulates adipogenic signaling in vivo. CR for 2 and 25 months, significantly increased the expression of PPARgamma, C/EBPbeta and Cdk-4, and partially attenuated age-related decline in C/EBPalpha expression relative to rats fed ad libitum (AL). As a result, adiponectin was upregulated at both mRNA and protein levels, resulting in activation of target genes involved in fatty acid oxidation and fatty acid synthesis, and greater responsiveness of adipose tissue to insulin. Moreover, CR significantly decreased the ratio of C/EBPbeta isoforms LAP/LIP, suggesting the suppression of gene transcription associated with terminal differentiation while facilitating preadipocytes proliferation. Morphometric analysis revealed a greater number of small adipocytes in CR relative to AL feeding. Immunostaining confirmed that small adipocytes were more strongly positive for adiponectin than the large ones. Overall these results suggest that CR increased the expression of adipogenic factors, and maintained the differentiated state of adipocytes, which is critically important for adiponectin biosynthesis and insulin sensitivity.

Age-specific CUGBP1-eIF2 complex increases translation of CCAAT/enhancer-binding protein beta in old liver

The RNA-binding protein CUGBP1 regulates translation of proteins in a variety of biological processes. In this study, we show that aging liver increases CUGBP1 translational activities by induction of a high molecular weight protein-protein complex of CUGBP1. The complex contains CUGBP1, subunits alpha, beta, and gamma of the initiation translation factor eIF2, and four proteins of the endoplasmic reticulum, eR90, CRT, eR60, and Grp78. The induction of the CUGBP1-eIF2 complex in old livers is associated with the elevation of protein levels of CUGBP1 and with the hyper-phosphorylation of CUGBP1 by a cyclin D3-cdk4 kinase, activity of which is increased with age. We have examined the role of the elevation of CUGBP1 and the role of cyclin D3-cdk4-mediated phosphorylation of CUGBP1 in the formation of the CUGBP1-eIF2 complex by using CUGBP1 transgenic mice and young animals expressing high levels of cyclin D3 after injection with cyclin D3 plasmid. These studies showed that both the increased levels of CUGBP1 and cdk4-mediated hyper-phosphorylation of CUGBP1 are involved in the age-associated induction of the CUGBP1-eIF2 complex. The CUGBP1-eIF2 complex is bound to C/EBPbeta mRNA in the liver of old animals, and this binding correlates with the increased amounts of liver-enriched activator protein and liver-enriched inhibitory protein. Consistent with these observations, the purified CUGBP1-eIF2 complex binds to the 5' region of C/EBPbeta mRNA and significantly increases translation of the three isoforms of C/EBPbeta in a cell-free translation system, in cultured cells, and in the liver. Thus, these studies demonstrated that age-mediated induction of the CUGBP1-eIF2 complex changes translation of C/EBPbeta in old livers.

Upregulation of CCAAT/enhancer binding protein β in activated astrocytes and microglia

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) regulates the expression of key genes in inflammation but little is known about the involvement of C/EBPbeta in glial activation. In this report, we have studied the patterns of astroglial and microglial C/EBPbeta expression in primary mouse cortical cultures. We show that both astrocytes and microglia express C/EBPbeta in untreated mixed glial cultures. C/EBPbeta is upregulated when glial activation is induced by lipopolysaccharide (LPS). The LPS-induced upregulation of glial C/EBPbeta is rapid (2 h at mRNA level, 4 h at protein level). It is elicited by low concentrations of LPS (almost maximal effect at 1 ng/mL) and it is reversed by the protein synthesis inhibitor cycloheximide. C/EBPbeta nuclear levels increase both in astrocytes and microglia after LPS treatment, and the response is more marked in microglia. The LPS-induced increase in microglial C/EBPbeta is prevented by coadministration of the MAP kinase inhibitors SB203580 (p38 inhibitor) + SP600125 (JNK inhibitor) or SB203580 + U0126 (ERK inhibitor). Systemic injection of LPS also increases brain nuclear levels of C/EBPbeta as shown by Western blot, and this increase is localized in microglial cells as shown by double immunofluorescence, in the first report to our knowledge of C/EBPbeta expression in activated glial cells in vivo. These findings support a role for C/EBPbeta in the activation of astrocytes and, particularly, microglia. Given the nature of the C/EBPbeta-regulated genes, we hypothesize that this factor participates in neurotoxic effects associated with glial activation. (c) 2006 Wiley-Liss, Inc.

Intracellular processing of the Sendai virus C' protein leads to the generation of a Y protein module: structure-functional implications

The Sendai virus "C-proteins" (C', C, Y1 and Y2) are a nested set of non-structural proteins. The shorter Y proteins arise in vivo both by de novo translation initiation and by proteolytic processing of C'. In this paper, we demonstrate that C' but not C (differing only by 11 N-terminal amino acid) serves as an efficient substrate for intracellular processing. However, processing can be mimicked in vitro by the addition of endopeptidases. Under conditions of limited proteolysis we observed that in a fraction of the C' protein the Y region exists as a proteinase resistant core. This core was conserved in the C protein. We propose that C' functions as a Pro-protein delivering the Y module to a specific intracellular location.

C/EBPbeta contributes to hepatocyte growth factor-induced replication of rodent hepatocytes

BACKGROUND/AIMS

Hepatocyte replication can be induced in vivo by hepatocyte growth factor (HGF), which might be used for gene therapy or to promote liver regeneration. However, the biochemical steps critical for this process are not clear. C/EBPbeta and C/EBPalpha are liver-enriched transcription factors that induce and inhibit hepatocyte replication, respectively. Because of their role in hepatocyte replication, this study examined the effect of HGF upon C/EBP proteins in vivo.

METHODS

Rats were treated with HGF, and the effect upon C/EBPs was evaluated in liver extracts. Normal or C/EBPbeta-deficient mice were treated with HGF, and the effect upon hepatocyte replication was determined.

RESULTS

HGF had no effect in rat liver upon C/EBPalpha or C/EBPbeta mRNA, nuclear protein, or nuclear DNA binding activity. However, HGF increased phosphorylated p90-RSK and ERK to 18- and 3-fold normal, respectively. These kinases phosphorylate C/EBPbeta and increase its transcriptional activity. The percentage of hepatocytes that replicated in C/EBPbeta-deficient mice after HGF administration was only 1.1%, which was lower than the value of 6.6% for hepatocytes from HGF-treated normal mice (P=0.005).

CONCLUSIONS

C/EBPbeta contributes to the induction of hepatocyte replication in response to HGF in rodents, which is likely due to post-translational modifications.

The amino terminus of Slob, Slowpoke channel binding protein, critically influences its modulation of the channel

The Drosophila Slowpoke calcium-dependent potassium channel (dSlo) binding protein Slob was discovered by a yeast two-hybrid screen using the carboxy-terminal tail region of dSlo as bait. Slob binds to and modulates the dSlo channel. We have found that there are several Slob proteins, resulting from multiple translational start sites and alternative splicing, and have named them based on their molecular weights (in kD). The larger variants, which are initiated at the first translational start site and are called Slob71 and Slob65, shift the voltage dependence of dSlo activation, measured by the whole cell conductance-voltage relationship, to the left (less depolarized voltages). Slob53 and Slob47, initiated at the third translational start site, also shift the dSlo voltage dependence to the left. In contrast, Slob57 and Slob51, initiated at the second translational start site, shift the conductance-voltage relationship of dSlo substantially to more depolarized voltages, cause an apparent dSlo channel inactivation, and increase the deactivation rate of the channel. These results indicate that the amino-terminal region of Slob plays a critical role in its modulation of dSlo.

Silencing of integrated human papillomavirus type 18 oncogene transcription in cells expressing SerpinB2

The serine protease inhibitor SerpinB2 (PAI-2), a major product of differentiating squamous epithelial cells, has recently been shown to bind and protect the retinoblastoma protein (Rb) from degradation. In human papillomavirus type 18 (HPV-18)-transformed epithelial cells the expression of the E6 and E7 oncoproteins is controlled by the HPV-18 upstream regulatory region (URR). Here we illustrate that PAI-2 expression in the HPV-18-transformed cervical carcinoma line HeLa resulted in the restoration of Rb expression, which led to the functional silencing of transcription from the HPV-18 URR. This caused loss of E7 protein expression and restoration of multiple E6- and E7-targeted host proteins, including p53, c-Myc, and c-Jun. Rb expression emerged as sufficient for the transcriptional repression of the URR, with repression mediated via the C/EBPbeta-YY1 binding site (URR 7709 to 7719). In contrast to HeLa cells, where the C/EBPbeta-YY1 dimer binds this site, in PAI-2- and/or Rb-expressing cells the site was occupied by the dominant-negative C/EBPbeta isoform liver-enriched transcriptional inhibitory protein (LIP). PAI-2 expression thus has a potent suppressive effect on HPV-18 oncogene transcription mediated by Rb and LIP, a finding with potential implications for prognosis and treatment of HPV-transformed lesions.

Interleukin-6 and mevastatin regulate plasminogen activator inhibitor-1 through CCAAT/enhancer-binding protein-delta

OBJECTIVE

We sought to determine the etiologic mechanism of proinflammatory cytokine, interleukin-6 (IL-6), and statin as regulators of synthesis of plasminogen activator inhibitor-1 (PAI-1), the physiological fibrinolysis inhibitor and an acute-phase reactant.

METHODS AND RESULTS

Transient transfection and luciferase assay in HepG2 human hepatoma-derived cells demonstrated that IL-6 increased PAI-1 promoter activity and mevastatin decreased IL-6-inducible response. Systematic deletion assay of the promoter demonstrated that the region (-239 to -210 bp) containing a putative CCAAT/enhancer-binding protein (C/EBP) binding site was necessary. Point mutation in this site abolished the IL-6-inducible response. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay demonstrated that C/EBPalpha, C/EBPbeta, and C/EBPdelta were involved in protein-DNA complex formation in intact cells. Deoxyribonuclease (DNase) I footprinting analysis revealed that 5' flanking region (-232 to -210 bp) is acute-phase response protein-binding site. C/EBPdelta binding activity was increased by IL-6 and attenuated by mevastatin. Mevastatin attenuated IL-6-mediated increase of C/EBPdelta protein in the nuclear extracts. IL-6 also increased PAI-1 and C/EBPdelta mRNA in mouse primary hepatocytes.

CONCLUSIONS

IL-6 increases hepatic PAI-1 expression mediated by the -232- to -210-bp region of the promoter containing a C/EBPdelta binding site. Vascular protection by statins may be partly mediated through regulation of CEBPdelta and consequent modulation of PAI-1 expression.

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

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

Phosphorylation of C/EBPbeta at a consensus extracellular signal-regulated kinase/glycogen synthase kinase 3 site is required for the induction of adiponectin gene expression during the differentiation of mouse fibroblasts into adipocytes

Stimulation of adipogenesis in mouse preadipocytes requires C/EBPbeta as well as activation of the MEK/extracellular signal-regulated kinase (ERK) signaling pathway. In this study, we demonstrate that phosphorylation of C/EBPbeta at a consensus ERK/glycogen synthase kinase 3 (GSK3) site regulates adiponectin gene expression during the C/EBPbeta-facilitated differentiation of mouse fibroblasts into adipocytes. First, we show that exposure of 3T3-L1 preadipocytes to insulin, dexamethasone (DEX), and isobutylmethylxanthine (MIX) leads to the phosphorylation of C/EBPbeta at threonine 188. Pretreating the cells with a MEK1-specific inhibitor (U0126) significantly attenuates this activity. Similarly, these effectors activate the phosphorylation of T188 within an ectopic C/EBPbeta overexpressed in Swiss mouse fibroblasts, and this event involves both MEK1 and GSK3 activity. We further show that expression of C/EBPbeta (p34kD LAP isoform) in Swiss mouse fibroblasts exposed to DEX, MIX, and insulin induces expression of peroxisome proliferator-activated receptor gamma (PPARgamma) and some adiponectin but that it does not activate expression of FABP4/aP2. In fact, complete conversion of these fibroblasts into lipid-laden adipocytes, which includes activation of FABP4 and adiponectin expression, requires their exposure to a potent PPARgamma ligand such as troglitazone. Expression of a mutant C/EBPbeta in which threonine 188 has been modified to alanine (C/EBPbeta T188A) can induce PPARgamma production in the mouse fibroblasts, but it is incapable of stimulating adiponectin expression in the absence or presence of troglitazone. Interestingly, replacement of T188 with aspartic acid creates a C/EBPbeta molecule (C/EBPbeta T188D) that possesses adipogenic activity similar to that of the wild-type molecule. The absence of adiponectin expression correlates with a reduced amount of C/EBPalpha in the adipocytes expressing the T188A mutant suggesting that C/EBPalpha is required for expression of adiponectin. In fact, ectopic expression of PPARgamma in C/EBPalpha-deficient fibroblasts (NIH 3T3 cells) produces a modest amount of adiponectin, whereas expression of both PPARgamma and C/EBPalpha in NIH 3T3 cells facilitates production of abundant quantities of adiponectin. These data demonstrate that phosphorylation of C/EBPbeta at a consensus ERK/GSK3 site is required for both C/EBPalpha and adiponectin gene expression during the differentiation of mouse fibroblasts into adipocytes.

Epidermal growth factor receptor stimulation activates the RNA binding protein CUG-BP1 and increases expression of C/EBPbeta-LIP in mammary epithelial cells

The transcription factor CCAAT/enhancer binding protein beta (C/EBP beta) is a key regulator of growth and differentiation in many tissues. C/EBP beta is expressed as several distinct protein isoforms (LAP1, LAP2, and LIP) whose expression is regulated by alternative translational initiation at downstream AUG start sites. The dominant-negative LIP isoform is predominantly expressed during proliferative cellular responses and is associated with aggressive tumors. In this study, we investigated a mechanism by which the LIP isoform is translationally regulated in mammary epithelial cells. We have demonstrated that LIP expression is increased in response to activation of the epidermal growth factor receptor (EGFR) signaling pathway and that the increased expression of LIP is regulated in part by an RNA binding protein referred to as CUG repeat binding protein (CUG-BP1). Our data demonstrate that EGFR signaling results in the phosphorylation of CUG-BP1 and this leads to an increase in the binding of CUG-BP1 to C/EBP beta mRNA and elevated expression of the LIP isoform. Phosphorylation is necessary for the binding activity of CUG-BP1 and the consequent increase in LIP expression, as determined by binding assays and a cell free, transcription-coupled translation system. CUG-BP1 is thus a previously unidentified downstream target of EGFR signaling and represents a new translational regulator of LIP expression in human mammary epithelial cells.

Challenges to environmental toxicology and epidemiology: where do we stand and which way do we go?

Modern toxicology investigates a wide array of both old and new health hazards. Priority setting is needed to select agents for research from the plethora of exposure circumstances. The changing societies and a growing fraction of the aged have to be taken into consideration. A precise exposure assessment is of importance for risk estimation and regulation. Toxicology contributes to the exploration of pathomechanisms to specify the exposure metrics for risk estimation. Combined effects of co-existing agents are not yet sufficiently understood. Animal experiments allow a separate administration of agents which can not be disentangled by epidemiological means, but their value is limited for low exposure levels in many of today's settings. As an experimental science, toxicology has to keep pace with the rapidly growing knowledge about the language of the genome and the changing paradigms in cancer development. During the pioneer era of assembling a working draft of the human genome, toxicogenomics has been developed. Gene and pathway complexity have to be considered when investigating gene-environment interactions. For a best conduct of studies, modern toxicology needs a close liaison with many other disciplines like epidemiology and bioinformatics.

5'-untranslated regions with multiple upstream AUG codons can support low-level translation via leaky scanning and reinitiation

Upstream AUGs (uAUGs) and upstream open reading frames (uORFs) are common features of mRNAs that encode regulatory proteins and have been shown to profoundly influence translation of the main ORF. In this study, we employed a series of artificial 5'-untranslated regions (5'-UTRs) containing one or more uAUGs/uORFs to systematically assess translation initiation at the main AUG by leaky scanning and reinitiation mechanisms. Constructs containing either one or two uAUGs in varying contexts but without an in-frame stop codon upstream of the main AUG were used to analyse the leaky scanning mechanism. This analysis largely confirmed the ranking of different AUG contextual sequences that was determined previously by Kozak. In addition, this ranking was the same for both the first and second uAUGs, although the magnitude of initiation efficiency differed. Moreover, approximately 10% of ribosomes exhibited leaky scanning at uAUGs in the most favourable context and initiated at a downstream AUG. A second group of constructs containing different numbers of uORFs, each with optimal uAUGs, were used to measure the capacity for reinitiation. We found significant levels of initiation at the main ORF even in constructs containing four uORFs, with nearly 10% of ribosomes capable of reinitiating five times. This study shows that for mRNAs containing multiple uORFs/uAUGs, ribosome reinitiation and leaky scanning are efficient mechanisms for initiation at their main AUGs.

The inflammation-induced down-regulation of plasma Fetuin-A (alpha2HS-Glycoprotein) in liver results from the loss of interaction between long C/EBP isoforms at two neighbouring binding sites

Fetuin-A is an hepatic protein whose mRNA transiently falls during the inflammatory acute phase via unknown transcriptional mechanisms. Various FETUA promoter/cat constructs transiently transfected in the Hep3B hepatoma cell line allowed us to identify four NF-1 and C/EBP binding sites (N, C) arranged in a 5'-N2-C2-N1-C1-3' order and required for basal promoter activity. Mutant constructs demonstrated that C1 and C2 but not N1 nor N2 are required for the cytokine-driven down-regulation of the promoter. A variable spacing between C2 and N1 showed that the alignment of the (C1+N1) and (C2+N2) areas is critical for the promoter activity in quiescent but not cytokine-stimulated cells. Co-transfection of a plasmid only producing either a long or short C/EBPbeta isoform prevented FETUA regulation by cytokines. Electromobility shift assays with liver nuclear extracts showed that during the acute phase the complexes formed over N1 and N2 are not modified whereas short C/EBPalpha and -beta isoforms replace the long isoforms bound to C1 and C2 in the quiescent liver. Therefore the basal promoter activity requires an interaction between the long C/EBP isoforms bound to C1 and C2 whereas the inflammation-induced down-regulation results from the loss of interaction between the cytokine-induced, short C/EBP isoforms.

Calcium channel and NMDA receptor activities differentially regulate nuclear C/EBPbeta levels to control neuronal survival

Insulin-like growth factor-1 (IGF-1) promotes the survival of cerebellar granule neurons by enhancing calcium influx through L-type calcium channels, whereas NMDA receptor-mediated calcium influx can lead to excitotoxic death. Here we demonstrate that L and NMDA receptor channel activities differentially regulate the transcription factor C/EBPbeta to control neuronal survival. Specifically, we show that L channel-dependent calcium influx results in increased CaMKIV activity, which acts to decrease nuclear C/EBPbeta levels. Conversely, NMDA receptor-mediated influx rapidly elevates nuclear C/EBPbeta and induces excitotoxic death via activation of the calcium-dependent phosphatase, calcineurin. Moderate levels of AMPA receptor activity stimulate L channels to improve survival, whereas higher levels stimulate NMDA receptors and reduce neuronal survival, suggesting differential synaptic effects. Finally, N-type calcium channel activity reduces survival, potentially by increasing glutamate release. Together, these results show that the L-type calcium channel-dependent survival and NMDA receptor death pathways converge to regulate nuclear C/EBPbeta levels, which appears to be pivotal in these mechanisms.

CSF-induced and HIV-1-mediated distinct regulation of Hck and C/EBPbeta represent a heterogeneous susceptibility of monocyte-derived macrophages to M-tropic HIV-1 infection

Granulocyte/macrophage colony-stimulating factor (GM-CSF)-induced monocyte-derived macrophages (GM-MPhi) are permissive to M-tropic HIV-1 entry, but inhibit viral replication at posttranscriptional and translational levels, whereas M-CSF-induced macrophages (M-MPhi) produce a large amount of HIV-1. M-MPhi express a high level of Hck and a large isoform of C/EBPbeta, and HIV-1 infection increases the expression of Hck but not of C/EBPbeta. GM-MPhi express a high level of C/EBPbeta and a low level of Hck, and HIV-1 infection drastically increases the expression of a short isoform of C/EBPbeta but decreases that of Hck. Treatment of M-MPhi with antisense oligonucleotide for Hck (AS-Hck) not only suppresses the expression of Hck, but also stimulates the induction of the short isoform of C/EBPbeta and inhibits the viral replication. Treatment of GM-MPhi with a moderate amount of AS-C/EBPbeta not only inhibits the expression of the small isoform of C/EBPbeta preferentially, but also stimulates the induction of Hck and stimulates the virus production at a high rate. These results suggest that CSF-induced and HIV-1-mediated distinct regulation of Hck and small isoform of C/EBPbeta represent the heterogeneous susceptibility of tissue MPhi to HIV-1 infection, and the regulation of Hck and C/EBPbeta are closely related and these two molecules affect one another.

Expression of the murine CB2 cannabinoid receptor using a recombinant Semliki Forest virus

A recombinant Semliki Forest virus (SFV) RNA construct, SFV1-mCB(2) RNA, was employed for the high-level expression of the murine CB(2) (mCB(2)) cannabinoid receptor in baby hamster kidney cells. Biosynthetic radiolabel incorporation studies in concert with urea-sodium dodecylsulfate-polyacrylamide gel electrophoresis (urea-SDS-PAGE) and western immunoblotting revealed that two major proteins of approximately 26 and 40kDa were produced by the construct. The 40kDa product, but not the 26kDa product, was glycosylated as determined by 2-deoxy-D-glucose incorporation and peptide-N-glycosidase F digestion analysis. Assessment of [3H]CP55940 ([3H]-(-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol) binding data for membranes of cells transfected with SFV1-mCB(2) RNA indicated a K(d) of 0.35+/-0.04nM and a B(max) of 24.4+/-2.7pmol/mg. A rank order of binding affinities for cannabinoids, which paralleled that reported for native mCB(2) receptors, was observed. The CB(2) receptor-specific antagonist SR144528 (N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide) blocked binding of [3H]CP55940, while the CB(1) receptor-specific antagonist SR141716A [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride] had a minimal effect. These results indicate that the recombinant receptor expressed from SFV1-mCB(2) RNA exhibits properties, including ligand binding features, that are consistent with those for the native mCB(2) receptor. However, the presence of both 26 and 40kDa receptor species is consistent with alternative translation from two AUG start sites using the SFV1-mCB(2) RNA expression system.

Expression of the CCAAT/enhancer-binding proteins C/EBPalpha, C/EBPbeta and C/EBPdelta in breast cancer: correlations with clinicopathologic parameters and cell-cycle regulatory proteins

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are involved in the regulation of proliferation and differentiation of the mammary gland. In order to investigate the role of C/EBPalpha, -beta and -delta in breast cancer, we performed western blot analysis and partly immunohistochemistry in 75 mammary carcinomas, 10 normal mammary tissue samples and four mammary cell lines. Expression levels of both C/EBPalpha isoforms, C/EBPbeta isoforms LAP1, LAP2 (liver-enriched transcriptional activating proteins), and LIP (liver-enriched transcriptional inhibitory protein), and C/EBPdelta in the tumors were correlated with clinicopathological tumor parameters, expression of estrogen and progesterone receptors (ER, PR), Ki67 immunostaining, and expression of seven cell-cycle regulatory proteins which had been analyzed before. High C/EBPalpha and -delta protein levels correlated significantly with expression of cell-cycle promoters (cyclin D1 and E) and cell-cycle inhibitory proteins (Rb, p27, p16), but with none of the established prognostic parameters. In contrast, statistically significant relationships of the full-length C/EBPbeta isoform LAP1 and a negative estrogen receptor status, high grading, nodal involvement, and high cyclin E and p16 expression were found. For the shorter isoform LIP, correlations with an ER-negative phenotype and high Ki67 immunostaining were detected, and high histological grading (G3) correlated with lower LAP/LIP ratio. These results suggest that high C/EBPbeta expression might be involved in tumor progression and indicative of an unfavorable prognosis.

CCAAT/enhancer binding protein beta (C/EBPbeta)-2 transforms normal mammary epithelial cells and induces epithelial to mesenchymal transition in culture

The transcription factor CCAAT/enhancer binding protein (C/EBP)beta is critical for normal growth and differentiation of the mammary gland. The intronless C/EBPbeta gene encodes a single mRNA that produces three protein isoforms, C/EBPbeta-1, -2, and -3, which share a common basic-leucine zipper domain at their C-terminus, but are distinguished at their N-termini by the in-frame methionine codon used to initiate translation. Although C/EBPbeta-1 and -2 are both transactivators, they likely perform distinct functions in mammary epithelial cells. C/EBPbeta-1 is the only isoform detected in normal human mammary tissue. In breast cancer cell lines, C/EBPbeta-1 is absent, and the C/EBPbeta-2 transactivator is expressed. Moreover, our data suggest that C/EBPbeta-2 is upregulated in human primary breast tumors. To assess C/EBPbeta-2's ability to participate in the transformation process, we generated recombinant retrovirus selectively encoding epitope-tagged C/EBPbeta-2. Strikingly, 10 days after infecting a normal human mammary epithelial cell line (MCF10A) with C/EBPbeta-2 virus, transformed subcultures were readily generated. Specifically, C/EBPbeta-2-overexpressing MCF10A cells form foci, gain anchorage independence, express markers associated with having undergone an epithelial-to-mesenchymal transition, and acquire an invasive phenotype. These studies, and our previous observations, provide supportive evidence that deregulated expression of C/EBPbeta-2 contributes to malignant conversion of the human breast.