Hypoglycemia-associated hyperammonemia caused by impaired expression of ornithine cycle enzyme genes in C/EBPalpha knockout mice

Datamining approaches for examining the low prevalence of N-acetylglutamate synthase deficiency and understanding transcriptional regulation of urea cycle genes

Ammonia, which is toxic to the brain, is converted into non-toxic urea, through a pathway of six enzymatically catalyzed steps known as the urea cycle. In this pathway, N-acetylglutamate synthase (NAGS, EC 2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetyl coenzyme A. NAGS deficiency (NAGSD) is the rarest of the urea cycle disorders, yet is unique in that ureagenesis can be restored with the drug N-carbamylglutamate (NCG). We investigated whether the rarity of NAGSD could be due to low sequence variation in the NAGS genomic region, high NAGS tolerance for amino acid replacements, and alternative sources of NAG and NCG in the body. We also evaluated whether the small genomic footprint of the NAGS catalytic domain might play a role. The small number of patients diagnosed with NAGSD could result from the absence of specific disease biomarkers and/or short NAGS catalytic domain. We screened for sequence variants in NAGS regulatory regions in patients suspected of having NAGSD and found a novel NAGS regulatory element in the first intron of the NAGS gene. We applied the same datamining approach to identify regulatory elements in the remaining urea cycle genes. In addition to the known promoters and enhancers of each gene, we identified several novel regulatory elements in their upstream regions and first introns. The identification of cis-regulatory elements of urea cycle genes and their associated transcription factors holds promise for uncovering shared mechanisms governing urea cycle gene expression and potentially leading to new treatments for urea cycle disorders.

Adipocyte Biology from the Perspective of In Vivo Research: Review of Key Transcription Factors

Obesity and type 2 diabetes are both significant contributors to the contemporary pandemic of non-communicable diseases. Both disorders are interconnected and associated with the disruption of normal homeostasis in adipose tissue. Consequently, exploring adipose tissue differentiation and homeostasis is important for the treatment and prevention of metabolic disorders. The aim of this work is to review the consecutive steps in the postnatal development of adipocytes, with a special emphasis on in vivo studies. We gave particular attention to well-known transcription factors that had been thoroughly described in vitro, and showed that the in vivo research of adipogenic differentiation can lead to surprising findings.

Untargeted analysis of the serum metabolome in cats with exocrine pancreatic insufficiency

Exocrine pancreatic insufficiency (EPI) causes chronic digestive dysfunction in cats, but its pathogenesis and pathophysiology are poorly understood. Untargeted metabolomics is a promising analytic methodology that can reveal novel metabolic features and biomarkers of clinical disease syndromes. The purpose of this preliminary study was to use untargeted analysis of the serum metabolome to discover novel aspects of the pathobiology of EPI in cats. Serum samples were collected from 5 cats with EPI and 8 healthy controls. The diagnosis of EPI was confirmed by measurement of subnormal serum feline trypsin-like immunoreactivity (fTLI). Untargeted quantification of serum metabolite utilized ultra-high-performance liquid chromatography-tandem mass spectroscopy. Cats with EPI had significantly increased serum quantities of long-chain fatty acids, polyunsaturated fatty acids, mevalonate pathway intermediates, and endocannabinoids compared with healthy controls. Diacylglycerols, phosphatidylethanolamines, amino acid derivatives, and microbial metabolites were significantly decreased in cats with EPI compared to healthy controls. Diacyclglycerols and amino acid metabolites were positively correlated, and sphingolipids and long-chain fatty acids were negatively correlated with serum fTLI, respectively. These results suggest that EPI in cats is associated with increased lipolysis of peripheral adipose stores, dysfunction of the mevalonate pathway, and altered amino acid metabolism. Differences in microbial metabolites indicate that feline EPI is also associated with enteric microbial dysbiosis. Targeted studies of the metabolome of cats with EPI are warranted to further elucidate the mechanisms of these metabolic derangements and their influence on the pathogenesis and pathophysiology of EPI in cats.

Hepatic transcriptional responses to fasting and feeding

Mammals undergo regular cycles of fasting and feeding that engage dynamic transcriptional responses in metabolic tissues. Here we review advances in our understanding of the gene regulatory networks that contribute to hepatic responses to fasting and feeding. The advent of sequencing and -omics techniques have begun to facilitate a holistic understanding of the transcriptional landscape and its plasticity. We highlight transcription factors, their cofactors, and the pathways that they impact. We also discuss physiological factors that impinge on these responses, including circadian rhythms and sex differences. Finally, we review how dietary modifications modulate hepatic gene expression programs.

Reverse Erythroblastosis Virus α Antagonism Promotes Homocysteine Catabolism and Ammonia Clearance

Metabolic homeostasis of amino acids is essential for human health. Here, we aimed to investigate a potential role for the clock component reverse erythroblastosis virus α (Rev-erbα) in circadian regulation of amino acid metabolism. RNA-seq with Rev-erbα^-/- mice showed expression changes in genes involved in amino acid metabolism, particularly, the urea cycle and methionine metabolism. Rev-erbα ablation increased hepatic mRNA, protein, and enzymatic activity of betaine homocysteine methyltransferase (Bhmt), cystathionine β-synthase (Cbs), and cystathionine γ-lyase (Cth) and decreased the levels of plasma and liver homocysteine in mice. Cell-based assays confirmed negative regulation of these three genes by Rev-erbα. Combined luciferase reporter, mobility-shift, and chromatin immunoprecipitation assays identified Rev-erbα as a transcriptional repressor of Bhmt, Cbs, and Cth. Rev-erbα ablation or antagonism alleviated chemical-induced hyperhomocysteinemia in mice. This was accompanied by elevated expressions of Bhmt, Cbs, and Cth. Moreover, Rev-erbα ablation or antagonism promoted urea production and ammonia clearance. Of urea cycle-related genes, arginase 1 (Arg1), ornithine transcarbamylase (Otc), and carbamoyl-phosphate synthase 1 (Cps1) expressions were up-regulated in Rev-erbα^-/- mice. Negative regulation of these urea cycle genes by Rev-erbα was validated using cell-based experiments. Mechanistic studies revealed that Rev-erbα inhibited CCAAT-enhancer-binding protein α transactivation to repress the transcription of Arg1, Cps1, and Otc. Conclusion: Rev-erbα antagonism alleviates hyperhomocysteinemia and promotes ammonia clearance. Targeting Rev-erbα represents an approach for the management of homocysteine- and ammonia-related diseases.

Optimizing combination of liver-enriched transcription factors and nuclear receptors simultaneously favors ammonia and drug metabolism in liver cells

The HepG2 cell line is widely used in studying liver diseases because of its immortalization, but its clinical application is limited by its low expression of the urea synthesis key enzymes and cytochromes P450 (CYPs). On the basis of our previous work, we investigated the transcriptional regulation of arginase 1 (Arg1) and ornithine transcarbamylase (OTC) in HepG2 cells. We also screened for the optimal combination of liver enrichment transcription factors (LETFs) and xenobiotic nuclear receptors that can promote the expression of key urea synthases and five major CYPs in HepG2 cells. Thus, recombinant HepG2 cells were established. Results showed that C/EBPβ, not C/EBPα, could upregulate expression of Arg1 and PGC1α and HNF4α cooperatively regulate the expression of OTC. The two optimal combinations C/EBPβ+HNF4α+HNF6+PXR and C/EBPβ+HNF4α+HNF6+CAR were selected. Compared with the control cells, the recombinant HepG2 cells modified by the two optimal combinations exhibited enhanced ammonia metabolism and CYP enzyme activity. Moreover, the HepG2/(C/EBPβ+HNF4α+HNF6+PXR) cells more strongly reduced ammonia than any other combination tested in this study. The present work indicated that optimizing the combination of transcription factors will simultaneously promote hepatocyte ammonia metabolism and drug metabolism. The recombinant HepG2 liver cell line constructed by the optimal combination provided an improved alternative means for bioartificial liver applications and drug toxicity testing.

Development of MTL-CEBPA: Small Activating RNA Drug for Hepatocellular Carcinoma

BACKGROUND

Oligonucleotide drug development has revolutionised the drug discovery field. Within this field, 'small' or 'short' activating RNAs (saRNA) are a more recently discovered category of short double-stranded RNA with clinical potential. saRNAs promote transcription from target loci, a phenomenon widely observed in mammals known as RNA activation (RNAa).

OBJECTIVE

The ability to target a particular gene is dependent on the sequence of the saRNA. Hence, the potential clinical application of saRNAs is to increase target gene expression in a sequence-specific manner. saRNA-based therapeutics present opportunities for expanding the "druggable genome" with particular areas of interest including transcription factor activation and cases of haploinsufficiency.

RESULTS AND CONCLUSION

In this mini-review, we describe the pre-clinical development of the first saRNA drug to enter the clinic. This saRNA, referred to as MTL-CEBPA, induces increased expression of the transcription factor CCAAT/enhancer-binding protein alpha (CEBPα), a tumour suppressor and critical regulator of hepatocyte function. MTL-CEBPA is presently in Phase I clinical trials for hepatocellular carcinoma (HCC). The clinical development of MTL-CEBPA will demonstrate "proof of concept" that saRNAs can provide the basis for drugs which enhance target gene expression and consequently improve treatment outcome in patients.

Generation of carbamoyl phosphate synthetase 1 reporter cell lines for the assessment of ammonia metabolism

Both primary hepatocytes and stem cells-derived hepatocyte-like cells (HLCs) are major sources for bioartificial liver (BAL). Maintenance of hepatocellular functions and induction of functional maturity of HLCs are critical for BAL's support effect. It remains difficult to assess and improve detoxification functions inherent to hepatocytes, including ammonia clearance. Here, we aim to assess ammonia metabolism and identify ammonia detoxification enhancer by developing an imaging strategy. In hepatoma cell line HepG2, and immortalized hepatic cell line LO2, carbamoyl phosphate synthetase 1 (CPS1) gene, the first enzyme of ammonia-eliminating urea cycle, was labelled with fluorescence protein via CRISPR/Cas9 system. With the reporter-based screening approach, cellular detoxification enhancers were selected among a collection of 182 small molecules. In both CPS1 reporter cell lines, the fluorescence intensity is positively correlated with cellular CPS1 mRNA expression, ammonia elimination and secreted urea, and reflected ammonia detoxification in a dose-dependent manner. Surprisingly, high-level CPS1 reporter clones also reserved many other critical hepatocellular functions, for example albumin secretion and cytochrome 450 metabolic functions. Sodium phenylbutyrate and resveratrol were identified to enhance metabolism-related gene expression and liver-enriched transcription factors C/EBPα, HNF4α. In conclusion, the CPS1-reporter system provides an economic and effective platform for assessment of cellular metabolic function and high-throughput identification of chemical compounds that improve detoxification activities in hepatic lineage cells.

Farnesoid X receptor: A "homeostat" for hepatic nutrient metabolism

The Farnesoid X receptor (FXR) is a nuclear receptor activated by bile acids (BAs). BAs are amphipathic molecules that serve as fat solubilizers in the intestine under postprandial conditions. In the post-absorptive state, BAs bind FXR in the hepatocytes, which in turn provides feedback signals on BA synthesis and transport and regulates lipid, glucose and amino acid metabolism. Therefore, FXR acts as a homeostat of all three classes of nutrients, fats, sugars and proteins. Here we re-analyze the function of FXR in the perspective of nutritional metabolism, and discuss the role of FXR in liver energy homeostasis in postprandial, post-absorptive and fasting/starvation states. FXR, by regulating nutritional metabolism, represses autophagy in conditions of nutrient abundance, and controls the metabolic needs of proliferative cells. In addition, FXR regulates inflammation via direct effects and via its impact on nutrient metabolism. These functions indicate that FXR is an attractive therapeutic target for liver diseases.

Epithelial Morphogenesis during Liver Development

Tissue stem/progenitor cells supply multiple types of epithelial cells that eventually acquire specialized functions during organ development. In addition, three-dimensional (3D) tissue structures need to be established for organs to perform their physiological functions. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes, which are derived from hepatoblasts, fetal liver stem/progenitor cells (LPCs), in mid-gestation. Hepatocytes performing many metabolic reactions form cord-like structures, whereas cholangiocytes, biliary epithelial cells, form tubular structures called intrahepatic bile ducts. Analyses for human genetic diseases and mutant mice have identified crucial molecules for liver organogenesis. Functions of those molecules can be examined in in vitro culture systems where LPCs are induced to differentiate into hepatocytes or cholangiocytes. Recent technical advances have revealed 3D epithelial morphogenesis during liver organogenesis. Therefore, the liver is a good model to understand how tissue stem/progenitor cells differentiate and establish 3D tissue architectures during organ development.

Cloning and comparative analysis the proximal promoter activities of arginase and agmatinase genes in Apostichopus japonicus

Our previous work demonstrated that Apostichopus japonicus arginase and agmatinase from l-arginine metabolism synergistically compete with NOS under pathogens challenge. Here we conducted a study to further investigate the mechanism in the regulation of arginase and agmatinase genes in l-arginine metabolism using EPC cell system. Luciferase analysis and progressive 5' deletion analysis suggested that Ajagmatinase promoter was a very robust promoter for its transcription, and the core region of Ajarginase promoter was located within -277 bp to -157 bp. Besides, their promoter activities were significantly activated by LPS and l-arginine challenge both in a time- and dose-dependent manners in EPC cells. When different truncated reporter vector and expression vector co-transfection experiment revealed transcription factor NF-κB/Rel and STAT5 could significantly inhibited Ajarginase promoter activity, but not Ajagmatinase. Our findings were provided novel insights into the transcriptional regulation of Ajarginase and Ajagmatinase, and selectively change their expressions might prevent pathogens infection.

Morphogenesis of liver epithelial cells

The mammalian liver is a physiologically important organ performing various types of metabolism, producing serum proteins, detoxifying bilirubin and ammonia, and protecting the body from infection. Those physiological functions are achieved with the 3D tissue architecture of liver epithelial cells. The liver contains two types of epithelial cells, namely, hepatocytes and cholangiocytes. They split from hepatoblasts (embryonic liver stem cells) in mid-gestation and differentiate into structurally and functionally mature cells. Analyses of mutant mice showing abnormal liver organogenesis have identified genes involved in liver development. In vitro culture systems have been used to examine the mechanism in which each molecule or signaling pathway regulates the morphogenesis and functional differentiation of hepatocytes and cholangiocytes. In addition, liver epithelial cells as well as mesenchymal, sinusoidal endothelial and hematopoietic cells can be purified from developing livers, which enables us to perform genome-wide screening to identify novel genes regulating epithelial morphogenesis in the liver. By combining these in vivo and in vitro systems, the liver could be a unique and suitable model for revealing a principle, governing epithelial morphogenesis. In this review, we summarize recent progress in the understanding of the development of liver epithelial tissue structures.

CCAAT/enhancer-binding protein α is required for hepatic outgrowth via the p53 pathway in zebrafish

CCAAT/enhancer-binding protein α (C/ebpα) is a transcription factor that plays important roles in the regulation of hepatogenesis, adipogenesis and hematopoiesis. Disruption of the C/EBPα gene in mice leads to disturbed liver architecture and neonatal death due to hypoglycemia. However, the precise stages of liver development affected by C/ebpα loss are poorly studied. Using the zebrafish embryo as a model organism, we show that inactivation of the cebpa gene by TALENs results in a small liver phenotype. Further studies reveal that C/ebpα is distinctively required for hepatic outgrowth but not for hepatoblast specification. Lack of C/ebpα leads to enhanced hepatic cell proliferation and subsequent increased cell apoptosis. Additional loss of p53 can largely rescue the hepatic defect in cebpa mutants, suggesting that C/ebpα plays a role in liver growth regulation via the p53 pathway. Thus, our findings for the first time demonstrate a stage-specific role for C/ebpα during liver organogenesis.

Ammonia-lowering activities and carbamoyl phosphate synthetase 1 (Cps1) induction mechanism of a natural flavonoid

OBJECTIVE

Ammonia detoxification is essential for physiological well-being, and the urea cycle in liver plays a predominant role in ammonia disposal. Nobiletin (NOB), a natural dietary flavonoid, is known to exhibit various physiological efficacies. In the current study, we investigated a potential role of NOB in ammonia control and the underlying cellular mechanism.

MATERIALS/METHODS

C57BL/6 mice were fed with regular chow (RC), high-fat (HFD) or high-protein diet (HPD) and treated with either vehicle or NOB. Serum and/or urine levels of ammonia and urea were measured. Liver expression of genes encoding urea cycle enzymes and C/EBP transcription factors was determined over the circadian cycle. Luciferase reporter assays were carried out to investigate function of CCAAT consensus elements on the carbamoyl phosphate synthetase (Cps1) gene promoter. A circadian clock-deficient mouse mutant, Clock (Δ19/Δ19) , was utilized to examine a requisite role of the circadian clock in mediating NOB induction of Cps1.

RESULTS

NOB was able to lower serum ammonia levels in mice fed with RC, HFD or HPD. Compared with RC, HFD repressed the mRNA and protein expression of Cps1, encoding the rate-limiting enzyme of the urea cycle. Interestingly, NOB rescued CPS1 protein levels under the HFD condition via induction of the transcription factors C/EBPα and C/EBPβ. Expression of other urea cycle genes was also decreased by HFD relative to RC and again restored by NOB to varying degrees, which, in conjunction with Cps1 promoter reporter analysis, suggested a C/EBP-dependent mechanism for the co-induction of urea cycle genes by NOB. In comparison, HPD markedly increased CPS1 levels relative to RC, yet NOB did not further enrich CPS1 to a significant extent. Using the circadian mouse mutant Clock (Δ19/Δ19) , we also showed that a functional circadian clock, known to modulate C/EBP and CPS1 expression, was required for NOB induction of CPS1 under the HFD condition.

CONCLUSION

NOB, a dietary flavonoid, exhibits a broad activity in ammonia control across varying diets, and regulates urea cycle function via C/EBP-and clock-dependent regulatory mechanisms.

Energy metabolism in the liver

The liver is an essential metabolic organ, and its metabolic function is controlled by insulin and other metabolic hormones. Glucose is converted into pyruvate through glycolysis in the cytoplasm, and pyruvate is subsequently oxidized in the mitochondria to generate ATP through the TCA cycle and oxidative phosphorylation. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis. Long-chain fatty acids are incorporated into triacylglycerol, phospholipids, and/or cholesterol esters in hepatocytes. These complex lipids are stored in lipid droplets and membrane structures, or secreted into the circulation as very low-density lipoprotein particles. In the fasted state, the liver secretes glucose through both glycogenolysis and gluconeogenesis. During pronged fasting, hepatic gluconeogenesis is the primary source for endogenous glucose production. Fasting also promotes lipolysis in adipose tissue, resulting in release of nonesterified fatty acids which are converted into ketone bodies in hepatic mitochondria though β-oxidation and ketogenesis. Ketone bodies provide a metabolic fuel for extrahepatic tissues. Liver energy metabolism is tightly regulated by neuronal and hormonal signals. The sympathetic system stimulates, whereas the parasympathetic system suppresses, hepatic gluconeogenesis. Insulin stimulates glycolysis and lipogenesis but suppresses gluconeogenesis, and glucagon counteracts insulin action. Numerous transcription factors and coactivators, including CREB, FOXO1, ChREBP, SREBP, PGC-1α, and CRTC2, control the expression of the enzymes which catalyze key steps of metabolic pathways, thus controlling liver energy metabolism. Aberrant energy metabolism in the liver promotes insulin resistance, diabetes, and nonalcoholic fatty liver diseases.

Novel RNA oligonucleotide improves liver function and inhibits liver carcinogenesis in vivo

Hepatocellular carcinoma (HCC) occurs predominantly in patients with liver cirrhosis. Here we show an innovative RNA-based targeted approach to enhance endogenous albumin production while reducing liver tumor burden. We designed short-activating RNAs (saRNA) to enhance expression of C/EBPα (CCAAT/enhancer-binding protein-α), a transcriptional regulator and activator of albumin gene expression. Increased levels of both C/EBPα and albumin mRNA in addition to a 3-fold increase in albumin secretion and 50% decrease in cell proliferation was observed in C/EBPα-saRNA transfected HepG2 cells. Intravenous injection of C/EBPα-saRNA in a cirrhotic rat model with multifocal liver tumors increased circulating serum albumin by over 30%, showing evidence of improved liver function. Tumor burden decreased by 80% (P = 0.003) with a 40% reduction in a marker of preneoplastic transformation. Since C/EBPα has known antiproliferative activities by way of retinoblastoma, p21, and cyclins, we used messenger RNA (mRNA) expression liver cancer-specific microarray in C/EBPα-saRNA-transfected HepG2 cells to confirm down-regulation of genes strongly enriched for negative regulation of apoptosis, angiogenesis, and metastasis. Up-regulated genes were enriched for tumor suppressors and positive regulators of cell differentiation. A quantitative polymerase chain reaction (PCR) and western blot analysis of C/EBPα-saRNA-transfected cells suggested that in addition to the known antiproliferative targets of C/EBPα, we also observed suppression of interleukin (IL)6R, c-Myc, and reduced STAT3 phosphorylation.

CONCLUSION

A novel injectable saRNA-oligonucleotide that enhances C/EBPα expression successfully reduces tumor burden and simultaneously improves liver function in a clinically relevant liver cirrhosis/HCC model.

Cellular and molecular basis of liver development

Liver is a prime organ responsible for synthesis, metabolism, and detoxification. The organ is endodermal in origin and its development is regulated by temporal, complex, and finely balanced cellular and molecular interactions that dictate its origin, growth, and maturation. We discuss the relevance of endoderm patterning, which truly is the first step toward mapping of domains that will give rise to specific organs. Once foregut patterning is completed, certain cells within the foregut endoderm gain competence in the form of expression of certain transcription factors that allow them to respond to certain inductive signals. Hepatic specification is then a result of such inductive signals, which often emanate from the surrounding mesenchyme. During hepatic specification bipotential hepatic stem cells or hepatoblasts become apparent and undergo expansion, which results in a visible liver primordium during the stage of hepatic morphogenesis. Hepatoblasts next differentiate into either hepatocytes or cholangiocytes. The expansion and differentiation is regulated by cellular and molecular interactions between hepatoblasts and mesenchymal cells including sinusoidal endothelial cells, stellate cells, and also innate hematopoietic elements. Further maturation of hepatocytes and cholangiocytes continues during late hepatic development as a function of various growth factors. At this time, liver gains architectural novelty in the form of zonality and at cellular level acquires polarity. A comprehensive elucidation of such finely tuned developmental cues have been the basis of transdifferentiation of various types of stem cells to hepatocyte-like cells for purposes of understanding health and disease and for therapeutic applications.

Artemisinic acid inhibits melanogenesis through downregulation of C/EBP α-dependent expression of HMG-CoA reductase gene

Cholesterol is associated with the regulation of melanogenesis which is the major physiological defense against solar irradiation. The present study was designed to determine the effects of artemisinic acid on melanogenesis and its mechanisms of action in human epidermal melanocytes. In this study, we found that artemisinic acid inhibited melanin content. The mRNA levels of microphthalmia-associated transcription factor (MITF) and its downstream genes tyrosinase, tyrosinase-related protein (TRP)-1, and TRP-2 were reduced by artemisinic acid treatment. Additionally, the mRNA levels of melanogenesis-related genes (c-KIT, stem cell factor (SCF), and macrophage migration inhibitory factor (MIF)) were down-regulated by artemisinic acid. Furthermore, cAMP production and protein kinase A (PKA) activity were suppressed by artemisinic acid. Moreover, attempts to elucidate a possible mechanism underlying the artemisinic acid-mediated effects revealed that artemisinic acid regulated melanogenesis by inhibiting cholesterol synthesis through downregulation of the hydroxymethylglutaryl CoA (HMG CoA) reductase gene, which was mediated through reduced expression of the CCAAT/enhancer-binding protein (C/EBP) α gene. Taken together, these findings indicate that the inhibition of melanogenesis by artemisinic acid occurs through reduced expression of the HMG CoA reductase gene, which is mediated by C/EBP α inhibition and suggest that artemisinic acid may be useful as a hyperpigmentation inhibitor.

New insights into urea and glucose handling by the kidney, and the urine concentrating mechanism

The mechanism by which urine is concentrated in the mammalian kidney remains incompletely understood. Urea is the dominant urinary osmole in most mammals and may be concentrated a 100-fold above its plasma level in humans and even more in rodents. Several facilitated urea transporters have been cloned. The phenotypes of mice with deletion of the transporters expressed in the kidney have challenged two previously well-accepted paradigms regarding urea and sodium handling in the renal medulla but have provided no alternative explanation for the accumulation of solutes that occurs in the inner medulla. In this review, we present evidence supporting the existence of an active urea secretion in the pars recta of the proximal tubule and explain how it changes our views regarding intrarenal urea handling and UT-A2 function. The transporter responsible for this secretion could be SGLT1, a sodium-glucose cotransporter that also transports urea. Glucagon may have a role in the regulation of this secretion. Further, we describe a possible transfer of osmotic energy from the outer to the inner medulla via an intrarenal Cori cycle converting glucose to lactate and back. Finally, we propose that an active urea transporter, expressed in the urothelium, may continuously reclaim urea that diffuses out of the ureter and bladder. These hypotheses are all based on published findings. They may not all be confirmed later on, but we hope they will stimulate further research in new directions.

Induction of a mature hepatocyte phenotype in adult liver derived progenitor cells by ectopic expression of transcription factors

BACKGROUND/AIMS

By ectopic expression of a distinct combination of transcription factors we aimed to induce a mature hepatocyte phenotype in an adult liver derived progenitor cell population (ALDPC).

METHODS

The open reading frames encoding murine Foxa2, Hnf4α and C/ebpα were cloned into lentivirus vectors and sequentially expressed in target cells. After seven days of culture, cells were analysed for expression of liver specific genes, and functional assays were performed. Fresh primary hepatocytes, twenty four hours in culture, served as positive controls.

RESULTS

Untransduced ALDPC under established differentiation conditions exhibited moderate signs of maturation, in particular in comparison with fresh hepatocyte controls. In transcription factor transduced cells, fifteen mRNA´s coding for secreted proteins, cytochrome p450 isoenzymes, liver metabolic enzymes were detected by RT-qPCR at levels close to controls. Albumin secretion increased incrementally in single (Foxa2), double (Foxa2, Hnf4α) and triple-transduced cells (Foxa2, Hnf4α, C/ebpα) and reached levels observed in primary hepatocytes. Glycogen storage as determined by PAS staining was detectable in double and triple transduced cells, comparable to controls. Ureagenesis was also induced in triple transduced cells, but at lower levels compared to primary hepatocytes.

CONCLUSIONS

Sequential expression of Foxa2, Hnf4α and C/ebpα induces a mature hepatocyte phenotype in an expandable liver derived progenitor cell line.

Effects of feeding diets naturally contaminated with Fusarium mycotoxins on protein metabolism in late gestation and lactation of first-parity sows

A study was conducted to assess the effects of feeding a blend of grains naturally contaminated with Fusarium mycotoxins to sows on the capacity for protein synthesis in skeletal muscle, the protein content per cellular unit, and the efficacy of a polymeric glucomannan adsorbent (GMA) to prevent these effects in late gestation and in lactation. Thirty-two Yorkshire sows were assigned to 4 treatment groups (8 per treatment) from 91 +/- 3 d of gestation up to weaning on d 21 after farrowing. Diets included 1) control, 2) contaminated grains, and 3) contaminated grains + 0.2% GMA. A fourth treatment of feeding sows the control diet at a restricted feed allowance was also included. The variables measured include ADFI, average daily BW change, serum total protein, urea, and ammonia, and skeletal muscle DNA, RNA, and protein. To assess the capacity for protein synthesis, ratios of RNA:DNA, and RNA:protein were compared among dietary treatments. To assess the degree of muscle protein mobilization in gestation and lactation, ratios of protein:DNA were compared among dietary treatments. Muscle samples were obtained from the triceps brachii. Blood and muscle samples were obtained 3 times: the first was obtained 1 d before the sows began to receive the experimental diets (90 +/- 3 d of gestation), a second sample was obtained 14 d later (104 +/- 3 d of gestation), and the third sample was obtained 10 d after farrowing. Serum ammonia concentrations were similar in sows fed the contaminated feed and sows fed the restricted feed compared with controls, but serum ammonia concentrations were greater in sows fed contaminated feed (P = 0.02) and restricted-fed sows (P = 0.008) compared with sows fed the contaminated grains plus GMA on 104 +/- 3 d of gestation. There were no reductions in the capacity for protein synthesis caused by mycotoxins or restricted feeding compared with controls. A reduction in ADFI (P = 0.003) was observed in sows fed the 2 contaminated diets in lactation. Muscle protein mobilization was not affected by diet, but a reduction (P = 0.04) in the content of protein per cellular unit was observed in lactation compared with gestation. Reduction in protein:DNA could be caused by the catabolic state in lactation, which was augmented by a low ADFI. The rate of muscle mobilization could be the result of the indirect effect of the reduction in ADFI in lactation rather than a direct effect of Fusarium mycotoxins in the capacity for protein synthesis.

Y-box binding protein-1 down-regulates expression of carbamoyl phosphate synthetase-I by suppressing CCAAT enhancer-binding protein-alpha function in mice

BACKGROUND & AIMS

Carbamoyl phosphate synthetase-I (CPS1) is a key enzyme in the urea cycle and patients with defects in the function or expression of CPS1 suffer from hyperammonemia. CPS1 is expressed in the liver at neonatal and adult stages in a CCAAT enhancer-binding protein-alpha (C/EBPalpha)-dependent manner. Despite expression of C/EBPalpha, CPS1 is not expressed in fetal liver, indicating an additional factor is involved in the regulation of CPS1 expression. The aim of this study was to elucidate the mechanism of CPS1 expression.

METHODS

Microarray was performed to find Y-box binding protein-1 (YB-1) that was expressed in mouse fetal liver. The role of YB-1 in CPS1 expression was investigated by overexpression of YB-1 in mouse fetal liver culture and luciferase reporter assays using the CPS1 promoter. Chromatin immunoprecipitation assay was used to examine recruitment of YB-1 to the CPS1 promoter in vivo.

RESULTS

Expression of YB-1 and CPS1 was inversely correlated in vivo, and YB-1 inhibited CPS1 expression and ammonia clearance in fetal liver culture. Although YB-1 was not expressed in adult liver, acute liver injury up-regulated YB-1 and down-regulated CPS1, accompanying an increase of the serum ammonia level. YB-1 inhibited C/EBPalpha-induced transcription from the CPS1 promoter via the Y-box near the C/EBPalpha-binding site. Chromatin immunoprecipitation assays demonstrated that YB-1 was recruited to the CPS1 promoter in fetal and injured adult liver, but not in normal adult liver.

CONCLUSIONS

YB-1 is a key regulator of ammonia detoxification by negatively regulating CPS1 expression via suppression of C/EBPalpha function.

Regulation of nitric oxide synthesis and apoptosis by arginase and arginine recycling

Nitric oxide (NO) is synthesized from arginine and O2 by NO synthase (NOS). Citrulline formed as a by-product of the NOS reaction can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). We found that AS and sometimes AL are coinduced with inducible NOS (iNOS) in various cells. In these cells, NO was synthesized from citrulline (via arginine) as well as from arginine, indicating operation of the citrulline-NO cycle. On the other hand, we found that arginase isoforms (types I and II) are coinduced with iNOS by LPS in rodent tissues and cultured macrophages. Km values for arginine of arginase I and II (approximately 10 mmol/L) are much higher than that of iNOS (approximately 5 micromol/L), whereas Vmax of arginase I and II were 10(3)-10(4) times higher than that of iNOS in activated macrophages. Thus, Vmax/Km values of arginases were close to that of iNOS, and these enzymes were expected to compete for arginine in the cells. In fact, NO production by iNOS in activated macrophages was decreased by coinduction of arginase I or arginase II. Low concentrations of NO protect cells from apoptosis, whereas excessive NO causes apoptosis. We found that NO causes endoplasmic reticulum (ER) stress, induces a transcription factor, CAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), and leads to apoptosis. These results suggest that the arginine metabolic enzymes and the ER stress-CHOP pathway can be good targets to regulate NO production and NO-mediated apoptosis in diseases associated with overproduction or impaired production of NO.

Distinct C/EBPalpha motifs regulate lipogenic and gluconeogenic gene expression in vivo

The C/EBPalpha transcription factor regulates hepatic nitrogen, glucose, lipid and iron metabolism. However, how it is able to independently control these processes is not known. Here, we use mouse knock-in mutagenesis to identify C/EBPalpha domains that specifically regulate hepatic gluconeogenesis and lipogenesis. In vivo deletion of a proline-histidine rich domain (PHR), dephosphorylated at S193 by insulin signaling, dysregulated genes involved in the generation of acetyl-CoA and NADPH for triglyceride synthesis and led to increased hepatic lipogenesis. These promoters bound SREBP-1 as well as C/EBPalpha, and the PHR was required for C/EBPalpha-SREBP transcriptional synergy. In contrast, the highly conserved C/EBPalpha CR4 domain was found to undergo liver-specific dephosphorylation of residues T222 and T226 upon fasting, and alanine mutation of these residues upregulated the hepatic expression of the gluconeogenic G6Pase and PEPCK mRNAs, but not PGC-1alpha, leading to glucose intolerance. Our results show that pathway-specific metabolic regulation can be achieved through a single transcription factor containing context-sensitive regulatory domains, and indicate C/EBPalpha phosphorylation as a PGC-1alpha-independent mechanism for regulating hepatic gluconeogenesis.

Dysregulation of C/EBPalpha by mutant Huntingtin causes the urea cycle deficiency in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a CAG trinucleotide expansion in the Huntingtin (Htt) gene. Using two mouse models of HD, we demonstrate that the urea cycle deficiency characterized by hyperammonemia, high blood citrulline and suppression of urea cycle enzymes is a prominent feature of HD. The resultant ammonia toxicity might exacerbate the neurological deficits of HD. Suppression of C/EBPalpha, a crucial transcription factor for the transcription of urea cycle enzymes, appears to mediate the urea cycle deficiency in HD. We found that in the presence of mutant Htt, C/EBPalpha loses its ability to interact with an important cofactor (CREB-binding protein). Moreover, mutant Htt recruited C/EBPalpha into aggregates, as well as suppressed expression of the C/EBPalpha gene. Consumption of protein-restricted diets not only led to the restoration of C/EBPalpha's activity, and repair of the urea cycle deficiency and hyperammonemia, but also ameliorated the formation of Htt aggregates, the motor deterioration, the suppression of striatal brain-derived neurotrophic factor and the normalization of three protein chaperones (Hsp27, Hsp70 and Hsp90). Treatments aimed at repairing the urea cycle deficiency may provide a new strategy for dealing with HD.

Suppression of C/EBPalpha expression in periportal hepatoblasts may stimulate biliary cell differentiation through increased Hnf6 and Hnf1b expression

The expression of C/EBPalpha, which may govern transcription of mature hepatocyte marker genes, was suppressed in periportal hepatoblasts in mouse liver development, leading to biliary cell differentiation. This study was undertaken to analyze how inactivation of the Cebpa gene affects biliary cell differentiation and gene expression of the regulatory genes for that differentiation, including Hnf1b and Hnf6. In the knockout mouse liver at midgestation stages, pseudoglandular structures were abundantly induced in the parenchyma with elevated expression of Hnf6 and Hnf1b mRNAs. The wild-type liver parenchyma expressed mRNAs of these transcription factors at low levels, though periportal biliary progenitors had strong expression of them. These results suggest that expression of Hnf6 and Hnf1b is downstream of C/EBPalpha action in fetal liver development, and that the suppression of C/EBPalpha expression in periportal hepatoblasts may lead to expression of Hnf6 and Hnf1b mRNAs. Immunohistochemical studies with biliary cell markers in knockout livers demonstrated that differentiated biliary epithelial cells were confined to around the portal veins. The suppression of C/EBPalpha expression may result in upregulation of Hnf6 and Hnf1b gene expression, but be insufficient for biliary cell differentiation. When liver fragments of Cebpa-knockout fetuses, in which hepatoblasts were contained as an endodermal component, were transplanted in the testis of Scid (Prkdc) male mice, almost all hepatoblasts gave rise to biliary epithelial cells. Wild-type hepatoblasts constructed mature hepatic tissue accompanied by biliary cell differentiation. These results also demonstrate that the suppression of C/EBPalpha expression may stimulate biliary cell differentiation.

Transcriptional regulation of metabolism

Our understanding of metabolism is undergoing a dramatic shift. Indeed, the efforts made towards elucidating the mechanisms controlling the major regulatory pathways are now being rewarded. At the molecular level, the crucial role of transcription factors is particularly well-illustrated by the link between alterations of their functions and the occurrence of major metabolic diseases. In addition, the possibility of manipulating the ligand-dependent activity of some of these transcription factors makes them attractive as therapeutic targets. The aim of this review is to summarize recent knowledge on the transcriptional control of metabolic homeostasis. We first review data on the transcriptional regulation of the intermediary metabolism, i.e., glucose, amino acid, lipid, and cholesterol metabolism. Then, we analyze how transcription factors integrate signals from various pathways to ensure homeostasis. One example of this coordination is the daily adaptation to the circadian fasting and feeding rhythm. This section also discusses the dysregulations causing the metabolic syndrome, which reveals the intricate nature of glucose and lipid metabolism and the role of the transcription factor PPARgamma in orchestrating this association. Finally, we discuss the molecular mechanisms underlying metabolic regulations, which provide new opportunities for treating complex metabolic disorders.

Transcription factor sumoylation and factor YY1 serve to modulate mouse amelogenin gene expression

Amelogenin proteins are essential in the control of enamel biomineralization and the amelogenin gene therefore is spatiotemporally regulated to ensure proper amelogenin protein expression. In this study, we examined the role of sumoylation to alter CCAAT/enhancer-binding protein alpha (C/EBPalpha) activity, and performed a search using a protein/DNA array system for other proteins that act co-operatively with C/EBPalpha to alter amelogenin expression. We observed that C/EBPalpha was modified by sumoylation, and that this modification played an indirect inhibitory role on the regulation of C/EBPalpha activity which appeared to act through other transcription factors. The protein/DNA array allowed us to single out the transcription factor, YY1, which acts in the absence of direct DNA binding to repress both the basal amelogenin promoter activity and C/EBPalpha-mediated transactivation. Taken together, these pathways may account for part of the physiological modulation of the amelogenin gene expression in accordance with tooth developmental and enamel biomineralization requirements.

NF-Y and CCAAT/enhancer-binding protein alpha synergistically activate the mouse amelogenin gene

Amelogenin is the major protein component of the forming enamel matrix. In situ hybridization revealed a periodicity for amelogenin mRNA hybridization signals ranging from low to high transcript abundance on serial sections of developing mouse teeth. This in vivo observation led us to examine the amelogenin promoter for the activity of transcription factor(s) that account for this expression aspect of the regulation for the amelogenin gene. We have previously shown that CCAAT/enhancer-binding protein alpha (C/EBPalpha) is a potent transactivator of the mouse X-chromosomal amelogenin gene acting at the C/EBPalpha cis-element located in the -70/+52 minimal promoter. The minimal promoter contains a reversed CCAAT box (-58/-54) that is four base pairs downstream from the C/EBPalpha binding site. Similar to the C/EBPalpha binding site, the integrity of the reversed CCAAT box is also required for maintaining the activity of the basal promoter. We therefore focused on transcription factors that interact with the reversed CCAAT box. Using electrophoretic mobility shift assays we demonstrated that NF-Y was directly bound to this reversed CCAAT site. Co-transfection of C/EBPalpha and NF-Y synergistically increased the promoter activity. In contrast, increased expression of NF-Y alone had only marginal effects on the promoter. A dominant-negative DNA binding-deficient NF-Y mutant (NF-YAm29) dramatically decreased the promoter activity both in the absence or presence of exogenous expression of C/EBPalpha. We identified protein-protein interactions between C/EBPalpha and NF-Y by a co-immunoprecipitation analysis. These results suggest that C/EBPalpha and NF-Y synergistically activate the mouse amelogenin gene and can contribute to its physiological regulation during amelogenesis.

C/EBPalpha: a tumour suppressor in multiple tissues?

The CCATT/enhancer binding protein alpha, C/EBPalpha, is a key transcription factor involved in late differentiation events of several cell types. Besides acting as a classical transcription factor, C/EBPalpha is also a well-characterized inhibitor of mitotic growth in most cell lines tested. In line with its anti-mitotic properties, C/EBPalpha has been shown to interact with, and alter the activities of, several cell cycle related proteins and a number of models as to the mechanistics of C/EBPalpha-mediated growth repression have been proposed. More recently, several reports have indicated that C/EBPalpha acts as a tumour suppressor in the hematopoietic system and that mutation within C/EBPalpha is sufficient to induce tumourigenesis. Here, we will review these data and probe the possibility that C/EBPalpha also act as a tumour suppressor in other C/EBPalpha-expressing tissues.

The proline-histidine-rich CDK2/CDK4 interaction region of C/EBPalpha is dispensable for C/EBPalpha-mediated growth regulation in vivo

The C/EBPalpha transcription factor regulates growth and differentiation of several tissues during embryonic development. Several hypotheses as to how C/EBPalpha inhibits cellular growth in vivo have been derived, mainly from studies of tissue culture cells. In fetal liver it has been proposed that a short, centrally located, 15-amino-acid proline-histidine-rich region (PHR) of C/EBPalpha is responsible for the growth-inhibitory function of the protein through its ability to interact with CDK2 and CDK4, thereby inhibiting their activities. Homozygous Cebpa(DeltaPHR/DeltaPHR) (DeltaPHR) mice, carrying a modified cebpa allele lacking amino acids 180 to 194, were born at the Mendelian ratio, reached adulthood, and displayed no apparent adverse phenotypes. When fetal livers from the DeltaPHR mice were analyzed for their expression of cell cycle markers, bromodeoxyuridine incorporation, cyclin-dependent kinase 2 kinase activity, and global gene expression, we failed to detect any cell cycle or developmental differences between the DeltaPHR mice and their control littermates. These in vivo data demonstrate that any C/EBPalpha-mediated growth repression via the PHR as well as the basic region is dispensable for proper embryonic development of, and cell cycle control in, the liver. Surprisingly, control experiments performed in C/EBPalpha null fetal livers yielded similar results.

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

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

Characterization of liver function in transdifferentiated hepatocytes

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

MACROPHAGE ARGINASE REGULATION BY CCAAT/ENHANCER-BINDING PROTEIN ??

Arginase activity is expressed by macrophages in healing wounds and other sites of inflammation and has been shown to modulate the synthesis of nitric oxide, polyamines, and collagen. The role of CCAAT/enhancer-binding protein beta (C/EBPbeta) in the regulation of macrophage arginase by different agonists was investigated using C/EBPbeta-/- and +/+ macrophage cell lines. 8-Bromo-cyclic adenosine monophosphate (8-Br-cAMP, 0.5 mM), recombinant murine interleukin 4 (rmIL-4, 20 U/mL), Escherichia coli lipopolysaccharide (100 ng/mL), and hypoxia (1% O2) induced arginase activity in C/EBPbeta+/+ macrophages, where enzyme activity correlated with arginase I protein. Only rmIL-4 increased arginase activity in C/EBPbeta-/- cells. Arginase II protein was expressed constitutively in wild-type and C/EBPbeta-/- cell lines and was unaltered by 8-Br-cAMP or rmIL-4. rmIL-4-stimulated immortalized C/EBPbeta-/- macrophages demonstrated higher nuclear signal transducer and activator of transcription-6 (STAT6) and phospho-STAT6 content than their +/+ counterparts. Validating the biological relevance of findings with the cell lines, additional experiments examined wound fluids and peritoneal macrophages from C/EBPbeta-/- mice and demonstrated that both contained less arginase activity than those from wild-type controls. Wounds in C/EBPbeta-/- animals showed signs of delayed maturation, as manifested by the persistence of neutrophils in the inflammatory infiltrate. Peritoneal macrophages from C/EBPbeta+/+ animals responded to 8-Br-cAMP and rmIL-4 with increased arginase activity, whereas those from C/EBPbeta-/- mice did not respond to cAMP. Results demonstrate a key mechanistic role for C/EBPbeta in the modulation of macrophage arginase I expression in vivo and in vitro.

Essential requirement of CCAAT/enhancer binding proteins in embryogenesis

The CCAAT/enhancer binding proteins C/EBPalpha and C/EBPbeta are related transcription factors that are important for the function of various organs in the postnatal mouse. Gene replacement and tissue culture experiments have suggested partial redundancy of both transcription factors. Here we show that mouse embryos deficient of both C/EBPalpha and C/EBPbeta (C/EBPalphabeta(-/-)) die between embryonic day 10 (E10) and E11 and display defective placentas. In situ hybridization revealed that C/EBPalpha and C/EBPbeta are coexpressed in the chorionic plate at E9.5 and later in the trophoblasts of the labyrinthine layer. In C/EBPalphabeta(-/-) placentas, allantoic blood vessels invaded the chorion; however, vessel expansion and development of the labyrinthine layer was impaired. Furthermore, a single copy of either C/EBPalpha in the absence of C/EBPbeta or C/EBPbeta in the absence of C/EBPalpha is sufficient to complete development, suggesting complementation of these C/EBPs during embryogenesis. A single copy of C/EBPalpha in the absence of C/EBPbeta, however, fails to rescue survival after birth, suggesting haploinsufficiency of C/EBPalpha in newborns. Our data thus reveal novel essential, redundant, and dosage dependent functions of C/EBPs.

Mechanisms of glucocorticoid signalling

It has become increasingly clear that glucocorticoid signalling not only comprises the binding of the glucocorticoid receptor (GR) to its response element (GRE), but also involves indirect regulation glucocorticoid-responsive genes by regulating or interacting with other transcription factors. In addition, they can directly regulate gene expression by binding to negative glucocorticoid response elements (nGREs), to simple GREs, to GREs, or to GREs and GRE half sites (GRE1/2s) that are part of a regulatory unit. A response unit allows a higher level of glucocorticoid induction than simple GREs and, in addition, allows the integration of tissue-specific information with the glucocorticoid response. Presumably, the complexity of such a glucocorticoid response unit (GRU) depends on the number of pathways that integrate at this unit. Because GRUs are often located at distant sites relative to the transcription-start site, the GRU has to find a way to communicate with the basal-transcription machinery. We propose that the activating signal of a distal enhancer can be relayed onto the transcription-initiation complex by coupling elements located proximal to the promoter.

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

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

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

BACKGROUND/AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Purification and expression of a processing protease on beta-alanine-oxoglutarate aminotransferase from rat liver mitochondria

GABA[arrow beta]AlaAT convertase is an endopeptidase that processes brain-type 4-aminobutyrate aminotransferase (GABA AT; EC 2.6.1.19) to liver-type beta-alanine-oxoglutarate aminotransferase (beta-AlaAT I) in rats. Its molecular mass was 180 kDa as determined by gel filtration. A subunit molecular mass of 97652 Da was measured using MALDI-TOF MS. The N-terminal sequence of the purified GABA[arrow beta]AlaAT convertase was SRVEVSKVLILGSGGLSIGQAGEFDYSGSQAV- and was identical to residues 418-449 of carbamoyl-phosphate synthetase I (CPS I; EC 1.2.1.27) purified from rat liver. The subunit molecular mass and the N-terminal amino acid sequence suggested that GABA[arrow beta]AlaAT convertase was the 418-1305 peptide of CPS I. An expression vector containing the coding region of the 418-1305 peptide of rat CPS I was transfected into NIH3T3 cells and the extract of the cells showed GABA[arrow beta]AlaAT convertase activity.

Disruption of hepatic C/EBPalpha results in impaired glucose tolerance and age-dependent hepatosteatosis

C/EBPalpha is highly expressed in liver and regulates many genes that are preferentially expressed in liver. Because C/EBPalpha-null mice die soon after birth, it is impossible to analyze the function of C/EBPalpha in the adult with this model. To address the function of C/EBPalpha in adult hepatocytes, liver-specific C/EBPalpha-null mice were produced using a floxed C/EBPalpha allele and the albumin-Cre transgene. Unlike whole body C/EBPalpha-null mice, mice lacking hepatic C/EBPalpha expression did not exhibit hypoglycemia, nor did they show reduced hepatic glycogen in adult. Expression of liver glycogen synthase, phosphoenolpyruvate carboxykinase, and glucose-6-phosphatase remained at normal levels. However, these mice exhibited impaired glucose tolerance due in part to reduced expression of hepatic glucokinase, and hyperammonemia from reduced expression of hepatic carbamoyl phosphate synthase-I. These mice also had reduced serum cholesterol and steatotic livers that was exacerbated with aging. This phenotype could be explained by increased expression of hepatic lipoprotein lipase and reduced expression of microsomal triglyceride transfer protein, apolipoproteins B100, and A-IV. These data demonstrate that hepatic C/EBPalpha is critical for ammonia detoxification and glucose and lipid homeostasis in adult mice.

C/EBPalpha deficiency results in hyperproliferation of hematopoietic progenitor cells and disrupts macrophage development in vitro and in vivo

CCAAT enhancer binding protein-alpha (C/EBPalpha) inhibits proliferation in multiple cell types; therefore, we evaluated whether C/EBPalpha-deficient hematopoietic progenitor cells (HPCs) have an increased proliferative potential in vitro and in vivo. In this study we demonstrate that C/EBPalpha(-/-) fetal liver (FL) progenitors are hyperproliferative, show decreased differentiation potential, and show increased self-renewal capacity in response to hematopoietic growth factors (HGFs). There are fewer committed bipotential progenitors in C/EBPalpha(-/-) FL, whereas multipotential progenitors are unaffected. HGF-dependent progenitor cell lines can be derived by directly culturing C/EBPalpha(-/-) FL cells in vitro Hyperproliferative spleen colonies and myelodysplastic syndrome (MDS) are observed in mice reconstituted with C/EBPalpha(-/-) FL cells, indicating progenitor hyperproliferation in vitro and in vivo. C/EBPalpha(-/-) FL lacked macrophage progenitors in vitro and had impaired ability to generate macrophages in vivo. These findings show that C/EBPalpha deficiency results in hyperproliferation of HPCs and a block in the ability of multipotential progenitors to differentiate into bipotential granulocyte/macrophage progenitors and their progeny.

Induction of arginase I and II in bleomycin-induced fibrosis of mouse lung

Arginase, which hydrolyzes arginine to urea and ornithine, is a precursor for the synthesis of polyamines and proline, which is abundant in collagen. The supply of proline can be a crucial factor in the process of lung fibrosis. We investigated the induction of arginine metabolic enzymes in bleomycin-induced mouse lung fibrosis. Histological studies and quantification of lung hydroxyproline showed that lung fibrosis develops in up to 14 days after bleomycin treatment. Under these conditions, collagen I mRNA was induced gradually in up to 15 days, and the content of hydroxyproline reached a maximum at 10 days. Arginase I mRNA was undetectable before bleomycin treatment but was induced 5-10 days after this treatment. Arginase I protein was induced at 7 days and remained little changed for up to 10 days and decreased at 14 days. On the other hand, arginase II mRNA that was detectable before treatment was increased gradually for up to 10 days and decreased at 14 days. Arginase II protein began to increase at day 5, increased for up to 10 days, and was decreased at day 14. mRNAs for cationic amino acid transporter-2 and ornithine decarboxylase were induced in a manner similar to that seen with collagen I mRNA. Immunohistochemical analysis showed that arginase I is induced in macrophages, whereas arginase II is induced in various cell types, including macrophages and myofibroblasts, and roughly colocalizes with the collagen-specific chaperone heat shock protein 47. Our findings suggest that arginine metabolic enzymes play an important role in the development of lung fibrosis, at least in mice.

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

Argininosuccinate synthetase (ASS, EC 6.3.4.5) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues. Indeed, discovery of the citrulline-NO cycle has increased interest in this enzyme that was found to represent a potential limiting step in NO synthesis. Depending on arginine utilization, location and regulation of ASS are quite different. In the liver, where arginine is hydrolyzed to form urea and ornithine, the ASS gene is highly expressed, and hormones and nutrients constitute the major regulating factors: (a) glucocorticoids, glucagon and insulin, particularly, control the expression of this gene both during development and adult life; (b) dietary protein intake stimulates ASS gene expression, with a particular efficiency of specific amino acids like glutamine. In contrast, in NO-producing cells, where arginine is the direct substrate in the NO synthesis, ASS gene is expressed at a low level and in this way, proinflammatory signals constitute the main factors of regulation of the gene expression. In most cases, regulation of ASS gene expression is exerted at a transcriptional level, but molecular mechanisms are still poorly understood.

Regulation of enzymes of the urea cycle and arginine metabolism

The urea cycle is comprised of five enzymes but also requires other enzymes and mitochondrial amino acid transporters to function fully. The complete urea cycle is expressed in liver and to a small degree also in enterocytes. However, highly regulated expression of several enzymes present in the urea cycle occurs also in many other tissues, where these enzymes are involved in synthesis of nitric oxide, polyamines, proline and glutamate. Glucagon, insulin, and glucocorticoids are major regulators of the expression of urea cycle enzymes in liver. In contrast, the "urea cycle" enzymes in nonhepatic cells are regulated by a wide range of pro- and antiinflammatory cytokines and other agents. Regulation of these enzymes is largely transcriptional in virtually all cell types. This review emphasizes recent information regarding roles and regulation of urea cycle and arginine metabolic enzymes in liver and other cell types.

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

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

Defective ureagenesis in mice carrying a liver-specific disruption of hepatocyte nuclear factor 4alpha (HNF4alpha ). HNF4alpha regulates ornithine transcarbamylase in vivo

Hepatocyte nuclear factor 4alpha (HNF4alpha) regulates the expression of many genes preferentially expressed in liver. HNF4alpha-null mice die during embryogenesis precluding the analysis of its function in the adult. To circumvent this problem, liver-specific HNF4alpha-null mice were produced. Mice lacking hepatic HNF4alpha expression exhibited increased serum ammonia and reduced serum urea. This disruption in ureagenesis may be explained by a marked decrease in expression and activity of hepatic ornithine transcarbamylase (OTC). To determine the molecular mechanisms involved in transcriptional regulation of the mouse OTC gene, the OTC promoter region was analyzed. Sequence analysis revealed the presence of two putative HNF4alpha-binding sites in the mouse OTC promoter region. By using transient transfection analysis, it was established that high levels of promoter activity were dependent on both HNF4alpha-binding sites and the expression of HNF4alpha. Furthermore, the proximal HNF4alpha-binding site was found to be more important than the distal one for transactivating OTC promoter. These data demonstrate that HNF4alpha is critical for urea homeostasis by direct regulation of the OTC gene in vivo.