Tissue-specific, developmental, hormonal, and dietary regulation of rat phosphoenolpyruvate carboxykinase-human growth hormone fusion genes in transgenic mice

Proximal Tubule-Specific Deletion of Angiotensin II Type 1a Receptors in the Kidney Attenuates Circulating and Intratubular Angiotensin II-Induced Hypertension in PT-Agtr1a-/- Mice

[Figure: see text].

New Insights into the Liver-Visceral Adipose Axis During Hepatic Resection and Liver Transplantation

In the last decade, adipose tissue has emerged as an endocrine organ with a key role in energy homeostasis. In addition, there is close crosstalk between the adipose tissue and the liver, since pro- and anti-inflammatory substances produced at the visceral adipose tissue level directly target the liver through the portal vein. During surgical procedures, including hepatic resection and liver transplantation, ischemia–reperfusion injury induces damage and regenerative failure. It has been suggested that adipose tissue is associated with both pathological or, on the contrary, with protective effects on damage and regenerative response after liver surgery. The present review aims to summarize the current knowledge on the crosstalk between the adipose tissue and the liver during liver surgery. Therapeutic strategies as well as the clinical and scientific controversies in this field are discussed. The different experimental models, such as lipectomy, to evaluate the role of adipose tissue in both steatotic and nonsteatotic livers undergoing surgery, are described. Such information may be useful for the establishment of protective strategies aimed at regulating the liver–visceral adipose tissue axis and improving the postoperative outcomes in clinical liver surgery.

LCAT Enzyme Replacement Therapy Reduces LpX and Improves Kidney Function in a Mouse Model of Familial LCAT Deficiency

Familial LCAT deficiency (FLD) is due to mutations in lecithin:cholesterol acyltransferase (LCAT), a plasma enzyme that esterifies cholesterol on lipoproteins. FLD is associated with markedly reduced levels of plasma high-density lipoprotein and cholesteryl ester and the formation of a nephrotoxic lipoprotein called LpX. We used a mouse model in which the LCAT gene is deleted and a truncated version of the SREBP1a gene is expressed in the liver under the control of a protein-rich/carbohydrate-low (PRCL) diet-regulated PEPCK promoter. This mouse was found to form abundant amounts of LpX in the plasma and was used to determine whether treatment with recombinant human LCAT (rhLCAT) could prevent LpX formation and renal injury. After 9 days on the PRCL diet, plasma total and free cholesterol, as well as phospholipids, increased 6.1 ± 0.6-, 9.6 ± 0.9-, and 6.7 ± 0.7-fold, respectively, and liver cholesterol and triglyceride concentrations increased 1.7 ± 0.4- and 2.8 ±0.9-fold, respectively, compared with chow-fed animals. Transmission electron microscopy revealed robust accumulation of lipid droplets in hepatocytes and the appearance of multilamellar LpX particles in liver sinusoids and bile canaliculi. In the kidney, LpX was found in glomerular endothelial cells, podocytes, the glomerular basement membrane, and the mesangium. The urine albumin/creatinine ratio increased 30-fold on the PRCL diet compared with chow-fed controls. Treatment of these mice with intravenous rhLCAT restored the normal lipoprotein profile, eliminated LpX in plasma and kidneys, and markedly decreased proteinuria. The combined results suggest that rhLCAT infusion could be an effective therapy for the prevention of renal disease in patients with FLD.

Intestinal CREBH overexpression prevents high-cholesterol diet-induced hypercholesterolemia by reducing Npc1l1 expression

Objective

The transcription factor cyclic AMP-responsive element-binding protein H (CREBH, encoded by Creb3l3) is highly expressed in the liver and small intestine. Hepatic CREBH contributes to glucose and triglyceride metabolism by regulating fibroblast growth factor 21 (Fgf21) expression. However, the intestinal CREBH function remains unknown.

Methods

To investigate the influence of intestinal CREBH on cholesterol metabolism, we compared plasma, bile, fecal, and tissue cholesterol levels between wild-type (WT) mice and mice overexpressing active human CREBH mainly in the small intestine (CREBH Tg mice) under different dietary conditions.

Results

Plasma cholesterol, hepatic lipid, and cholesterol crystal formation in the gallbladder were lower in CREBH Tg mice fed a lithogenic diet (LD) than in LD-fed WTs, while fecal cholesterol output was higher in the former. These results suggest that intestinal CREBH overexpression suppresses cholesterol absorption, leading to reduced plasma cholesterol, limited hepatic supply, and greater excretion. The expression of Niemann–Pick C1-like 1 (Npc1l1), a rate-limiting transporter mediating intestinal cholesterol absorption, was reduced in the small intestine of CREBH Tg mice. Adenosine triphosphate-binding cassette transporter A1 (Abca1), Abcg5/8, and scavenger receptor class B, member 1 (Srb1) expression levels were also reduced in CREBH Tg mice. Promoter assays revealed that CREBH directly regulates Npc1l1 expression. Conversely, CREBH null mice exhibited higher intestinal Npc1l1 expression, elevated plasma and hepatic cholesterol, and lower fecal output.

Conclusion

Intestinal CREBH regulates dietary cholesterol flow from the small intestine by controlling the expression of multiple intestinal transporters. We propose that intestinal CREBH could be a therapeutic target for hypercholesterolemia.

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Plasma cholesterol, hepatic lipid, and gallstones were lower in CREBH Tg mice.

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Expression of intestinal Npc1l1 was reduced in CREBH Tg mice.

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CREBH directly down-regulates mouse Npc1l1 promoter activity.

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Intestinal CREBH regulates dietary cholesterol flow from the small intestine.

Kielin/chordin-like protein attenuates both acute and chronic renal injury

The secreted kielin/chordin-like (KCP) protein, one of a family of cysteine-rich proteins, suppresses TGF-β signaling by sequestering the ligand from its receptor, but it enhances bone morphogenetic protein (BMP) signaling by promoting ligand-receptor interactions. Given the critical roles for TGF-β and BMP proteins in enhancing or suppressing renal interstitial fibrosis, respectively, we examined whether secreted KCP could attenuate renal fibrosis in mouse models of chronic and acute disease. Transgenic mice that express KCP in adult kidneys showed significantly less expression of collagen IV, α-smooth muscle actin, and other markers of disease progression in the unilateral ureteral obstruction model of renal interstitial fibrosis. In the folic acid nephrotoxicity model of acute tubular necrosis, mice expressing KCP survived high doses of folic acid that were lethal for wild-type mice. With a lower dose of folic acid, mice expressing KCP exhibited improved renal recovery compared with wild-type mice. Thus, these data suggest that extracellular regulation of the TGF-β/BMP signaling axis by KCP, and by extension possibly other cysteine-rich domain proteins, can attenuate both acute and chronic renal injury.

Interplay of the inflammatory and stress systems in a hepatic cell line: interactions between glucocorticoid receptor agonists and interleukin-6

The liver plays an important role in inflammation and stress by producing the acute phase proteins (APPs) required for resolution of inflammation as well as by delivering systemic glucose, through gluconeogenesis, required to fuel the stress response. Disruption of the interplay between interleukin 6 (IL-6) and glucocorticoids (GCs), the peripheral mediators of inflammation and stress, respectively, may lead to side-effects associated with the pharmacological use of GCs. The current study investigated the interplay between IL-6 and GCs in a hepatoma cell line (BWTG3) at protein (protein activity assays, Western blotting, and ELISA) and mRNA (qPCR) levels. Specifically, the action of dexamethasone (Dex), a known antiinflammatory drug and glucocorticoid receptor (GR) agonist, is compared to that of Compound A (CpdA), a selective glucocorticoid receptor agonist (SEGRA). CpdA, like IL-6, but unlike Dex, increases GR binding and decreases the metabolic enzymes, tyrosine aminotransferase, phosphoenolpyruvate carboxykinase, and gamma glutamyltransferase, at protein or mRNA level. Like Dex, both CpdA and IL-6 increase the positive APPs, serum amyloid A and C-reactive protein, and decrease the negative APP, corticosteroid binding globulin. The study shows that the GC, Dex, and IL-6 generally have divergent effects on the GR and metabolic enzymes, while their functions are convergent on the APPs. In contrast to Dex, CpdA has effects convergent to that of IL-6 on the GR, metabolic enzymes, and APPs. Thus these findings suggest that CpdA, like Dex, modulates APPs, leading to effective control of inflammation, while, in contrast to Dex, it is less likely to lead to GC-induced side-effects.

Exendin-4 and exercise improve hepatic glucose homeostasis by promoting insulin signaling in diabetic rats

Recently, it has been reported that a long-acting glucagon-like peptide-1 (exendin-4) and physical exercise improve hepatic insulin action in diabetic rats. However, this phenomenon remains poorly understood. We investigated the long-term effect that exendin-4 and exercise had on hepatic insulin resistance through the modulation of hepatic and/or hypothalamic insulin signaling in 90% pancreatectomized diabetic rats fed 40% energy fat diets. The rats were divided into 4 groups: exendin-4 only, exendin-4 plus exercise training, saline (control), or exercise training only. Rats in the exendin-4 groups were administered with 150 pmol/kg exendin-4 twice a day for 8 weeks, whereas those in the exercise groups ran on an uphill treadmill with a 15 degrees incline at 20 m/min for 30 minutes 5 days a week. Exendin-4 reduced serum glucagon levels in overnight-fasted rats. Exendin-4 treatment by itself decreased hepatic glucose output at hyperinsulinemic states, and exercise without exendin-4 treatment also had the same effect. Exendin-4 promoted hepatic insulin signaling by potentiating tyrosine phosphorylation of the insulin receptor substrate-2 without changing hypothalamic insulin signaling. Exendin-4 also enhanced hypothalamic glucose sensing. However, exercise improved both hepatic and hypothalamic insulin signaling by activating the phosphorylation of cyclic adenosine monophosphate-responding element binding proteins to induce insulin receptor substrate-2 expression. Exendin-4 and exercise decreased the expression of phosphoenolpyruvate carboxykinase, which in turn reduced hepatic glucose output. Exendin-4 in combination with exercise had no additive effects. In conclusion, exendin-4 and exercise improve hepatic glucose homeostasis by promoting hepatic insulin signaling in diabetic rats.

Monocyte chemoattractant protein-1 secreted by adipose tissue induces direct lipid accumulation in hepatocytes

For many years, adipose tissue has been mainly considered as an inert reservoir for storing triglycerides. Since the discovery that adipocytes may secrete a variety of bioactive molecules (hormones, chemokines, and cytokines), an endocrine and paracrine role for white adipose tissue (WAT) in the regulation of energy balance and other physiological processes has been established, particularly with regard to brain and muscle. In contrast, little is known about the interactions of WAT with liver. Hence, we examined the effect of the secretory products of WAT on hepatocytes. Conditioned medium of human WAT explants induced significant steatosis in hepatocyte cell lines. Factor(s) responsible for the conditioned medium-induced steatosis were screened by a battery of blocking antibodies against different cytokines/chemokines shown to be secreted by WAT. In contrast to interleukin-8 and interleukin-6, the monocyte chemoattractant protein-1 was capable of inducing steatosis in hepatocytes in a time-dependent manner at concentrations similar to those found in conditioned medium. Incubation of conditioned medium with antimonocyte chemoattractant protein-1 antibodies prevented triglyceride accumulation. Investigation of the mechanism leading to the triglyceride accumulation showed that both a diminution of apolipoprotein B secretion and an increase in phosphoenolpyruvate carboxykinase messenger RNA may be involved.

CONCLUSION

The monocyte chemoattractant protein-1 secreted by adipose tissue may induce steatosis not only recruiting macrophages but also acting directly on hepatocytes.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Immunocytochemical localization of glucose 6-phosphatase and cytosolic phosphoenolpyruvate carboxykinase in gluconeogenic tissues reveals unsuspected metabolic zonation

Immunohistochemical analysis was used to define the precise cell-specific localization of Glucose-6-phosphatase (Glc6Pase) and cytosolic form of the phosphoenolpyruvate carboxykinase (PEPCK-C) in the digestive system (liver, small intestine and pancreas) and the kidney. Co-expression of Glc6Pase and PEPCK-C was shown to take place in hepatocytes, in proximal tubules of the cortex kidney and at the top of the villi of the small intestine suggesting that these tissues are all able to perform complete gluconeogenesis. On the other hand, intrahepatic bile ducts, collecting tubes of the nephron and the urinary epithelium in the calices of the kidney, as well as the crypts of the small intestine, express Glc6Pase without significant levels of PEPCK-C. In such cases, the function of Glc6Pase could be related to the transepithelial transport of glucose characteristic of these tissues, rather than to the neoformation of glucose. Lastly, PEPCK-C expression in the absence of Glc6Pase was noted in both the exocrine pancreas and the endocrine islets of Langerhans. Possible roles of PEPCK-C in exocrine pancreas might be the provision of gluconeogenic intermediates for further conversion into glucose in the liver, whereas PEPCK-C would be instrumental in pyruvate cycling, which has been suggested to play a regulatory role in insulin secretion by the beta-cells of the islets.

Renal bone morphogenetic protein-7 protects against diabetic nephropathy

Longstanding diabetes causes renal injury with early dropout of podocytes, albuminuria, glomerular and tubulointerstitial fibrosis, and progressive renal failure. The renal pathology seems to be driven, in part, by TGF-beta and is associated with a loss of renal bone morphogenic protein-7 (BMP-7) expression. Here, the hypothesis that maintenance of renal (especially podocyte) BMP-7 by transgenic expression reduces diabetic renal injury was tested. Diabetic mice that expressed the phosphoenolpyruvate carboxykinase promoter-driven BMP-7 transgene and nondiabetic, transgenic mice as well as diabetic and nondiabetic wild-type controls were studied for up to 1 yr. Transgenic expression of BMP-7 in glomerular podocytes and proximal tubules prevents podocyte dropout and reductions in nephrin levels in diabetic mice. Maintenance of BMP-7 also reduces glomerular fibrosis and interstitial collagen accumulation as well as collagen I and fibronectin expression. Diabetic wild-type mice develop progressive albuminuria, which is substantially reduced in transgenic mice. These effects of the BMP-7 transgene occur without changing renal TGF-beta levels. It is concluded that maintenance of renal BMP-7 during the evolution of diabetic nephropathy reduces diabetic renal injury, especially podocyte dropout. The findings also establish a role for endogenous glomerular BMP-7 as an autocrine regulator of podocyte integrity in vivo.

Transgenic animal models for the study of adipose tissue biology

The traditional view of adipose tissue as a passive energy reservoir has changed. Adipose tissue is a complex, highly active metabolic and endocrine organ. With obesity as an increasingly important public health threat, a major development in the understanding of adipose tissue biology has come with observations in different biological spheres including whole-body physiology and application of transgenic animal models. Scientific progress has been made with the identification of several genes in spontaneous monogenic animal models of obesity, and in understanding the molecular mechanisms underlying phenotypes of altered body weight, adiposity and fat distribution by creating transgenic and knockout animal models. Mouse phenotypes resulting from inactivation or overexpression of molecules responsible for the regulation of adipose tissue metabolism have led to novel concepts in the understanding of adipocyte biology and development of obesity. This review presents an overview of transgenic animal models for the study of adipose tissue biology.

Glucocorticoids regulate transcription of the gene for phosphoenolpyruvate carboxykinase in the liver via an extended glucocorticoid regulatory unit

The hepatic transcriptional regulation by glucocorticoids of the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK-C) gene is coordinated by interactions of specific transcription factors at the glucocorticoid regulatory unit (GRU). We propose an extended GRU that consists of four accessory sites, two proximal AF1 and AF2 sites and their distal counterpart dAF1 (-993) and a new site, dAF2 (-1365); together, these four sites form a palindrome. Sequencing and gel shift binding assays of hepatic nuclear proteins interacting with these sites indicated similarity of dAF1 and dAF2 sites to the GRU proximal AF1 and AF2 sites. Chromatin immunoprecipitation assays demonstrated that glucocorticoids enhanced the binding of FOXO1 and peroxisome proliferator-activated receptor-alpha to AF2 and dAF2 sites and not to dAF1 site but enhanced the binding of hepatic nuclear transcription factor-4alpha only to the dAF1 site. Insulin inhibited the binding of these factors to their respective sites but intensified the binding of phosphorylated FOXO1. Transient transfections in HepG2 human hepatoma cells showed that glucocorticoid receptor interacts with several non-steroid nuclear receptors, yielding a synergistic response of the PEPCK-C gene promoter to glucocorticoids. The synergistic stimulation by glucocorticoid receptor together with peroxisome proliferator-activated receptor-alpha or hepatic nuclear transcription factor-4alpha requires all four accessory sites, i.e. a mutation of each of these markedly affects the synergistic response. Mice with a targeted mutation of the dAF1 site confirmed this requirement. This mutation inhibited the full response of hepatic PEPCK-C gene to diabetes by reducing PEPCK-C mRNA level by 3.5-fold and the level of circulating glucose by 25%.

Molecular mechanism of hypoxia-mediated hepatic gluconeogenesis by transcriptional regulation

Until now, it is known that hypoxia increases the glycolytic enzyme expression at the transcriptional level. Here, we show evidence that hypoxia increases hepatic glucose output and HIF-1 and ATF-2-mediated transactivation of phosphoenolpyruvate carboxykinase (PEPCK), which plays a critical role as a rate-limiting enzyme in gluconeogenesis, gene in liver. HIF-1 directly bound to the specific PEPCK promoter region through its cognate binding element and found as an active complex with coactivator CBP. Additionally, ATF-2 was also involved to regulate hypoxia-dependent PEPCK transcription in the transcriptional complex with HIF-1 and CBP. Interestingly, retinoic acid (RA) signaling induced the recruitment of HIF-1 on the PEPCK promoter, resulting from the functional interaction of HIF-1 and ATF-2 with coactivator CBP. Taken together, these results suggest that hypoxia signaling leads the hepatic glucose production and release via the increased gene expression of gluconeogenic enzymes, possibly playing a role in providing glucose to other tissues, such as endothelial, brain and muscle cells.

Long-Term Correction of Ornithine Transcarbamylase Deficiency by WPRE-Mediated Overexpression Using a Helper-Dependent Adenovirus

The urea cycle disorders (UCDs) are important models for developing gene replacement therapy for liver diseases. Long-term correction of the most common UCD, ornithine transcarbamylase (OTC) deficiency, has yet to be achieved in clinical or preclinical settings. The single human clinical trial using early-generation adenovirus (Ad) failed to show any biochemical correction. In adult OTC-deficient mice, an E1/E2-deleted Ad vector expressing the mouse OTC gene, but not the human, was only transiently therapeutic. By using post-transcriptional overexpression in the context of the less immunogenic helper-dependent adenoviral vector, we achieved metabolic correction of adult OTC-deficient mice for >6 months. Demonstrating this result were normalized orotic aciduria, normal hepatic enzyme activity, and elevated OTC RNA and protein levels in the absence of chronic hepatotoxicity. Overexpressing the human protein may have overcome two potential mechanisms accounting for poor cross-species complementation: a kinetic block at the level of mitochondrial import or a dominant negative effect by the mutant polypeptide. These data represent an important approach for treating human inborn errors of hepatocyte metabolism like the UCDs that require high-level transduction and gene expression for clinical correction.

Induction of hepatocyte proliferation and death by modulation of T-Antigen expression

Mice expressing SV40 T-Antigen in liver under control of the phosphoenolpyruvate carboxykinase promoter were generated. By altering the carbohydrate content of the diet, TAg expression, the rate of hepatocyte proliferation and apoptosis, and hence hepatocarcinogenesis, could be regulated. Carbohydrate-mediated suppression of TAg resulted in slow hepatic growth that progressed to focal hepatocellular carcinoma (HCC) after a long latency period. In contrast, induction of TAg by feeding mice a low carbohydrate diet resulted in massive hepatomegaly that progressed rapidly to diffuse multifocal HCC. Hepatic TAg expression could be efficiently repressed by switching mice from the low to the high-carbohydrate diet, which if instigated prior to the development of HCC, resulted in rapid regression through a p53-independent reduction in hepatocyte proliferation and an increase in hepatocyte apoptosis. Although liver growth was accompanied by compensatory hepatocyte apoptosis, an apoptotic deficit developed following chronic exposure to high levels of TAg. This was associated with Akt phosphorylation and increased expression of the antiapoptotic molecules bfl-1/A1, TIAP, and A20. Mice were resistant to Fas-induced hepatocellular apoptosis due to severely impaired caspase activation and failed activation of the mitochondrial amplification loop. This model will be useful to investigate oncogene-mediated disruption of the cell cycle and apoptosis, and to determine which processes constitute fixed, or reversible aspects of the tumorigenic process.

Activating transcription factor-2 mediates transcriptional regulation of gluconeogenic gene PEPCK by retinoic acid

All-trans-retinoic acid (RA) is known to increase the rate of transcription of the PEPCK gene upon engagement of the RA receptor (RAR). RA also mediates induction of specific gene transcription via several signaling pathways as a nongenomic effect. Here we show that RA upregulation of PEPCK promoter activity requires the cAMP response element (CRE)-1 in addition to the RA-response element and that activating transcription factor-2 (ATF-2) binds the CRE element to mediate this effect. Furthermore, we show that RA treatment potentiates ATF-2-dependent transactivation by inducing specific phosphorylation of ATF-2 by p38beta kinase. ATF-2 activation by RA blocked the inhibitory intramolecular interaction of ATF-2 amino and carboxyl terminal domains in a p38beta kinase-dependent manner. Consistent with these results, RA treatment increased the DNA binding activity of ATF-2 on the PEPCK CRE-1 sequence. Taken together, the data suggest that RA activates the p38beta kinase pathway leading to phosphorylation and activation of ATF-2, thereby enhancing PEPCK gene transcription and glucose production.

Role of vitamin A in mitochondrial gene expression

Diabetes-prone BHE/Cdb and Sprague-Dawley (SD) rats were studied with respect to mitochondrial (mt) function and mt gene expression. The BHE/Cdb rats carry mutations in the mt ATPase 6 gene that phenotype as decreased OXPHOS efficiency with subsequent development of impaired glucose tolerance. The base substitutions result in amino acid substitutions in the proton channel and this, in turn, affects the efficiency of energy capture in the ATP molecule. Feeding studies showed that BHE/Cdb rats required 10 times more vitamin E and three times more vitamin A in their diets than do normal SD rats. Vitamin A supplementation 'normalized' mt OXPHOS as well as increased the amount of ATPase subunit a protein in the mt compartment. Western blot analysis of retinoic acid receptors in the mitochondrial and nuclear compartments showed that these proteins were present in the mt compartment. The effect of the vitamin A supplementation plus the observation of retinoic acid receptors suggest that vitamin A functions to enhance the transcription of the ATPase 6 gene. Work with primary cultures of hepatocytes showed that not only does retinoic acid increase mitochondrial ATPase 6 gene expression but so too does the steroid hormone intermediate, dehydroepiandrosterone (DHEA). Triiodothyronine also plays a role in this process but not as an independent factor. Rather, this hormone potentiates the effects of retinoic acid and DHEA on ATPase gene expression. These results suggest that mt gene expression requires more than just the mt transcription factor A. More than likely the process requires a number of factors in much the same way as does nuclear gene expression.

Mechanism of increased renal gene expression during metabolic acidosis

Increased renal catabolism of plasma glutamine during metabolic acidosis generates two ammonium ions that are predominantly excreted in the urine. They function as expendable cations that facilitate the excretion of acids. Further catabolism of alpha-ketoglutarate yields two bicarbonate ions that are transported into the venous blood to partially compensate for the acidosis. In rat kidney, this adaptation is sustained, in part, by the induction of multiple enzymes and various transport systems. The pH-responsive increases in glutaminase (GA) and phosphoenolpyruvate carboxykinase (PEPCK) mRNAs are reproduced in LLC-PK(1)-fructose 1,6-bisphosphatase (FBPase) cells. The increase in GA activity results from stabilization of the GA mRNA. The 3'-untranslated region of the GA mRNA contains a direct repeat of an eight-base AU sequence that functions as a pH-response element. This sequence binds zeta-crystallin/NADPH:quinone reductase with high affinity and specificity. Increased binding of this protein during acidosis may initiate the pH-responsive stabilization of the GA mRNA. In contrast, induction of PEPCK occurs at the transcriptional level. In LLC-PK(1)-FBPase(+) kidney cells, a decrease in intracellular pH leads to activation of the p38 stress-activated protein kinase and subsequent phosphorylation of transcription factor ATF-2. This transcription factor binds to cAMP-response element 1 within the PEPCK promoter and may enhance its transcription during metabolic acidosis.

C/EBPbeta interacts with the P-enolpyruvate carboxykinase adipocyte-specific enhancer

CCAAT/enhancer binding protein (C/EBP) family members are known to transactivate the gene encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) in hepatocytes via promoter proximal C/EBP response elements. PEPCK is also expressed in adipocytes; however, fibroblasts that are homozygous null for C/EBPbeta cannot express PEPCK when induced to differentiate into adipocytes (Tanaka et al., EMBO J. 16, 7432-7443, 1997). This along with our previous observation that an upstream adipocyte-specific enhancer contains multiple putative C/EBP binding elements suggested the possibility that C/EBPbeta transactivates the PEPCK gene in adipocytes via distal elements. We report here that C/EBPbeta transactivates a PEPCK-luciferase chimera in transient transfection assays. C/EBPbeta acted independently of peroxisome proliferator-activated receptor gamma (PPARgamma) which is required for function of the enhancer. C/EBPbeta in nuclear extracts and recombinant C/EBPbeta bound three of the putative C/EBP-binding elements within the enhancer. C/EBPbeta binding to these three elements was strongly cooperative. However, mutation of all three elements did not affect reporter transactivation by C/EBPbeta suggesting that additional elements participate in PEPCK regulation or that the effects of C/EBPbeta are indirect.

Long-term stable correction of low-density lipoprotein receptor-deficient mice with a helper-dependent adenoviral vector expressing the very low-density lipoprotein receptor

BACKGROUND

Familial hypercholesterolemia (FH) that results from LDL receptor (LDLR) deficiency affects approximately 1 in 500 persons in the heterozygous state and approximately 1 in 1 million persons in the homozygous state. We tested a novel gene therapy strategy for the treatment of FH in a mouse model.

METHODS AND RESULTS

We delivered the VLDL receptor (VLDLR) to the liver of LDLR-deficient mice and compared the effect of a helper-dependent adenoviral vector with all viral coding sequences deleted (HD-Ad-mVLDLR) with a first-generation vector (FG-Ad-mVLDLR), an HD-Ad (HD-Ad-0) that contained no expression cassette, and dialysis buffer (DB). A single intravenous injection of HD-Ad-mVLDLR led to a lowering of plasma cholesterol that lasted >/=6 months. Acute liver toxicity (as measured with liver enzyme elevation) occurred after FG-Ad-mVLDLR but not after HD-Ad-mVLDLR, HD-Ad-0, or DB treatment. At 6 months, VLDLR was detected in the liver with Western blotting and with immunofluorescence staining only in HD-Ad-mVLDLR-treated mice. Aortic atherosclerosis was almost completely prevented in these animals.

CONCLUSIONS

HD-Ad-mediated intravenous delivery of VLDLR to hepatocytes is well tolerated. It produces long-term lowering of plasma cholesterol and prevents atherosclerosis development in LDLR-deficient mice. These data provide support for the feasibility and safety of this approach for therapy of human subjects.

A single regulatory module of the carbamoylphosphate synthetase I gene executes its hepatic program of expression

A 469-base pair (bp) upstream regulatory fragment (URF) and the proximal promoter of the carbamoylphosphate synthetase I (CPS) gene were analyzed for their role in the regulation of spatial, developmental, and hormone-induced expression in vivo. The URF is essential and sufficient for hepatocyte-specific expression, periportal localization, perinatal activation and induction by glucocorticoids, and cAMP in transgenic mice. Before birth, the transgene is silent but can be induced by cAMP and glucocorticoids, indicating that these compounds are responsible for the activation of expression at birth. A 102-bp glucocorticoid response unit within the URF, containing binding sites for HNF3, C/EBP, and the glucocorticoid receptor, is the main determinant of the hepatocyte-specific and hormone-controlled activity. Additional sequences are required for a productive interaction between this minimal response unit and the core CPS promoter. These results show that the 469-bp URF, and probably only the 102-bp glucocorticoid response unit, functions as a regulatory module, in that it autonomously executes a correct spatial, developmental and hormonal program of CPS expression in the liver.

CREB (cAMP response element binding protein) and C/EBPalpha (CCAAT/enhancer binding protein) are required for the superstimulation of phosphoenolpyruvate carboxykinase gene transcription by adenoviral E1a and cAMP

In the present study, we observed superstimulated levels of cAMP-stimulated transcription from the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter in cells infected with wild-type adenovirus expressing 12 S and 13 S E1a proteins, or in cells expressing 13 S E1a alone. cAMP-stimulated transcription was inhibited in cells expressing only 12 S E1a, but slightly elevated in cells expressing E1a proteins with mutations in conserved regions 1 or 2, leading us to conclude that the superstimulation was mediated by conserved region 3 of 13 S E1a. E1a failed to enhance cAMP-stimulated transcription from promoters containing mutations that abolish binding by cAMP response element binding protein (CREB) or CCAAT/enhancer binding proteins (C/EBPs). This result was supported by experiments in which expression of dominant-negative CREB and/or C/EBP proteins repressed E1a- and cAMP-stimulated transcription from the PEPCK gene promoter. In reconstitution experiments using a Gal4-responsive promoter, E1a enhanced cAMP-stimulated transcription when chimaeric Gal4-CREB and Gal4-C/EBPalpha were co-expressed. Phosphorylation of CREB on serine-133 was stimulated in cells treated with dibutyryl cAMP, whereas phosphorylation of C/EBPalpha was increased by E1a expression. Our data support a model in which cAMP agonists increase CREB activity and stimulate PEPCK gene transcription, a process that is enhanced by E1a through the phosphorylation of C/EBPalpha.

Direct binding of hepatitis B virus X protein and retinoid X receptor contributes to phosphoenolpyruvate carboxykinase gene transactivation

The X gene product of the human hepatitis B virus (HBx), a major factor responsible for hepatitis and hepatocellular carcinoma, modulates transactivation by a variety of transcription factors. Herein, expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene was found to be regulated transcriptionally by HBx through two distinct promoter regions. The cAMP response element (CRE)-1 site within the proximal promoter region mediated the HBx-induced transactivation of the PEPCK gene through C/EBP alpha and ATF-2. A retinoid X receptor (RXR) response element within the distal promoter region also contributed to the HBx-induced transactivation. Consistent with these results, HBx directly interacted with RXR, and the interaction interfaces were localized to the transactivation domain of HBx and the ligand binding domain of RXR.

A crucial role of sterol regulatory element-binding protein-1 in the regulation of lipogenic gene expression by polyunsaturated fatty acids

Dietary polyunsaturated fatty acids (PUFA) are negative regulators of hepatic lipogenesis that exert their effects primarily at the level of transcription. Sterol regulatory element-binding proteins (SREBPs) are transcription factors responsible for the regulation of cholesterol, fatty acid, and triglyceride synthesis. In particular, SREBP-1 is known to play a crucial role in the regulation of lipogenic gene expression in the liver. To explore the possible involvement of SREBP-1 in the suppression of hepatic lipogenesis by PUFA, we challenged wild-type mice and transgenic mice overexpressing a mature form of SREBP-1 in the liver with dietary PUFA. In the liver of wild-type mice, dietary PUFA drastically decreased the mature, cleaved form of SREBP-1 protein in the nucleus, whereas the precursor, uncleaved form in the membranes was not suppressed. The decreases in mature SREBP-1 paralleled those in mRNAs for lipogenic enzymes such as fatty acid synthase and acetyl-CoA carboxylase. In the transgenic mice, dietary PUFA did not reduce the amount of transgenic SREBP-1 protein, excluding the possibility that PUFA accelerated the degradation of mature SREBP-1. The resulting sustained expression of mature SREBP-1 almost completely canceled the suppression of lipogenic gene expression by PUFA in the SREBP-1 transgenic mice. These results demonstrate that the suppressive effect of PUFA on lipogenic enzyme genes in the liver is caused by a decrease in the mature form of SREBP-1 protein, which is presumably due to the reduced cleavage of SREBP-1 precursor protein.

Expression, DNA-binding specificity and transcriptional regulation of nuclear factor 1 family proteins from rat

Nuclear factor 1 (NF1) family proteins, which are encoded by four different genes (NF1-A, NF1-B, NF1-C and NF1-X), bind to the palindromic sequence and regulate the expression of many viral and cellular genes. We have previously purified NF1-A and NF1-B from rat liver as factors that bind to the silencer in the glutathione transferase P gene, and have also reported the repression domain of NF1-A. In the present study we cloned five cDNA species (NF1-B1, NF1-B2, NF1-B3, NF1-C2 and NF1-X1) and compared their expression profiles and the affinity and specificity of the DNA binding of these NF1 family members. By Northern blot analysis, we found that the expression profiles of the NF1s are indistinguishable in the various tissues of the rat. The DNA-binding affinities of NF1-A and NF1-X are higher than those of NF1-B and NF1-C, whereas all four NF1 proteins showed the same DNA-binding specificity. Transfection analyses revealed that the function of NF1-B on the transcriptional regulation differed between NF1-B isoforms and was affected by the factor(s) that bind to the promoter regions. In addition, we identified the transcriptional regulatory domain of NF1-B, which is enriched with proline and serine residues.

Insulin regulates expression of metabolic genes through divergent signaling pathways

The regulation of metabolic gene expression is a major mechanism by which insulin modulates glucose homeostasis. Defective transcription factors or signal transduction molecules that are required for insulin regulated gene expression could contribute to insulin resistance. The phosphoenolpyruvate carboxykinase (PEPCK) and hexokinase II (HKII) genes are involved in metabolic processes that represent opposing facets of glucose homeostasis, namely gluconeogenesis and glucose utilization. The regulation of the PEPCK and HKII genes by insulin has been studied in great detail at the level of both transcription and signal transduction. Recent work on the insulin signaling pathways that lead to down-regulation of PEPCK gene expression and upregulation of HKII gene expression has shown that they both require activation of phosphatidylinositol 3-kinase (PI3K) for the transmission of the insulin signal. However, the pathways diverge after PI3K and lead to activation of different downstream effectors. In this paper we review the results of studies on the transcriptional regulation of these genes by insulin and the signal transduction pathways that mediate these responses.

Analysis of genomic regions involved in regulation of the rabbit surfactant protein A gene in transgenic mice

The gene encoding surfactant protein (SP) A, a developmentally regulated pulmonary surfactant-associated protein, is expressed in a lung-specific manner, primarily in pulmonary type II cells. SP-A gene transcription in the rabbit fetal lung is increased by cAMP. To delineate the genomic regions involved in regulation of SP-A gene expression, lines of transgenic mice carrying fusion genes composed of various amounts of 5'-flanking DNA from the rabbit SP-A gene linked to the human growth hormone structural gene as a reporter were established. We found that as little as 378 bp of 5'-flanking DNA was sufficient to direct appropriate lung cell-selective and developmental regulation of transgene expression. The same region was also sufficient to mediate cAMP induction of transgene expression. Mutagenesis or deletion of either of two DNA elements, proximal binding element and a cAMP response element-like sequence, previously found to be crucial for cAMP induction of SP-A promoter activity in transfected type II cells, did not affect lung-selective or temporal regulation of expression of the transgene; however, overall levels of fusion gene expression were reduced compared with those of wild-type transgenes.

Characterization of a kidney-specific pattern of chromatin structure in the rat phosphoenolpyruvate carboxykinase gene

The kidney-specific chromatin structure of the phosphoenolpyruvate carboxykinase (PEPCK) gene was examined and compared to that of the liver. Kidney nuclear extracts were found to lack a liver-enriched factor, pepA, that binds to HSS A, a distal enhancer of the PEPCK gene that may be involved in opening the chromatin domain of the PEPCK gene in the liver. To begin the characterization of the kidney-specific chromatin structure of the PEPCK gene, nuclease hypersensitive sites (HSS) were mapped by indirect end-labeling analysis in proximal tubules from control rats, proximal tubules from acidotic rats which express induced levels of PEPCK, and NRK52E cells, a rat kidney epithelial cell line which does not express the PEPCK gene. A subset of HSS, at -400/+1 over the proximal promoter and at +1900 within the coding region, correlate with kidney-specific PEPCK expression. Two other HSS, at -3.1 kb and +6.2 kb, are detected in kidney cells regardless of PEPCK expression. The HSS at -4800, -1240, and +4650, previously identified in PEPCK-expressing liver cells, were not observed in the kidney. As in the liver, the pattern of hypersensitivity in the kidney does not change by altering the rate of transcription.

Adipose expression of the phosphoenolpyruvate carboxykinase promoter requires peroxisome proliferator-activated receptor gamma and 9-cis-retinoic acid receptor binding to an adipocyte-specific enhancer in vivo

A putative adipocyte-specific enhancer has been mapped to approximately 1 kilobase pair upstream of the cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene. In the present study, we used transgenic mice to identify and characterize the 413-base pair (bp) region between -1242 and -828 bp as a bona fide adipocyte-specific enhancer in vivo. This enhancer functioned most efficiently in the context of the PEPCK promoter. The nuclear receptors peroxisome proliferator-activated receptor gamma (PPARgamma) and 9-cis-retinoic acid receptor (RXR) are required for enhancer function in vivo because: 1) a 3-bp mutation in the PPARgamma-/RXR-binding element centered at -992 bp, PCK2, completely abolished transgene expression in adipose tissue; and 2) electrophoretic mobility supershift experiments with specific antibodies indicated that PPARgamma and RXR are the only factors in adipocyte nuclear extracts which bind PCK2. In contrast, a second PPARgamma/RXR-binding element centered at -446 bp, PCK1, is not involved in adipocyte specificity because inactivation of this site did not affect transgene expression. Moreover, electrophoretic mobility shift experiments indicated that, unlike PCK2, PCK1 is not selective for PPARgamma/RXR binding. To characterize the enhancer further, the rat and human PEPCK 5'-flanking DNA sequences were compared by computer and found to have significant similarities in the enhancer region. This high level of conservation suggests that additional transcription factors are probably involved in enhancer function. A putative human PCK2 element was identified by this sequence comparison. The human and rat PCK2 elements bound PPARgamma/RXR with the same affinities. This work provides the first in vivo evidence that the binding of PPARgamma to its target sequences is absolutely required for adipocyte-specific gene expression.

A mechanistic model for the development and maintenance of portocentral gradients in gene expression in the liver

In the liver, genes are expressed along a portocentral gradient. Based on their adaptive behavior, a gradient versus compartment type, and a dynamic versus stable type of gradient have been recognized. To understand at least in principle the development and maintenance of these gradients in gene expression in relation to the limited number of signal gradients, we propose a simple and testable model. The model uses portocentral gradients of signal molecules as input, while the output depends on two gene-specific variables, viz., the affinity of the gene for its regulatory factors and the degree of cooperativity that determines the response in the signal-transduction pathways. As a preliminary validity test for its performance, the model was tested on control and hormonally induced expression patterns of phosphoenolpyruvate carboxykinase (PCK), carbamoylphosphate synthetase I (CPS), and glutamine synthetase (GS). Affinity was found to determine the overall steepness of the gradient, whereas cooperativity causes these gradients to steepen locally, as is necessary for a compartment-like expression pattern. Interaction between two or more different signal gradients is necessary to ensure a stable expression pattern under different conditions. The diversity in sequence and arrangement of related DNA-response elements of genes appears to account for the gene-specific shape of the portocentral gradients in expression. The feasibility of testing the function of hepatocyte-specific DNA-response units in vivo is demonstrated by integrating such units into a ubiquitously active promoter/enhancer and analyzing the pattern of expression of these constructs in transgenic mice.

TGF-beta1 in liver fibrosis: an inducible transgenic mouse model to study liver fibrogenesis

Transforming growth factor-beta1 (TGF-beta1) is a powerful stimulus for collagen formation in vitro. To determine the in vivo effects of TGF-beta1 on liver fibrogenesis, we generated transgenic mice overexpressing a fusion gene [C-reactive protein (CRP)/TGF-beta1] consisting of the cDNA coding for an activated form of TGF-beta1 under the control of the regulatory elements of the inducible human CRP gene promoter. Two transgenic lines were generated with liver-specific overexpression of mature TGF-beta1. After induction of the acute phase response (15 h) with lipopolysaccharide (100 microgram ip), plasma TGF-beta1 levels reached >600 ng/ml in transgenic animals, which is >100 times above normal plasma levels. Basal plasma levels of uninduced transgenic animals were about two to five times above normal. As a consequence of hepatic TGF-beta1 expression, we could demonstrate marked transient upregulation of procollagen I and procollagen III mRNA in the liver 15 h after the peak of TGF-beta1 expression. Liver histology after repeated induction of transgene expression showed an activation of hepatic stellate cells in both transgenic lines. The fibrotic process was characterized by perisinusoidal deposition of collagen in a linear pattern. This transgenic mouse model gives in vivo evidence for the important role of TGF-beta1 in stellate cell activation and liver fibrogenesis. Due to the ability to control the level of TGF-beta1 expression, this model allows the study of the regulation and kinetics of collagen synthesis and fibrolysis as well as the degree of reversibility of liver fibrosis. The CRP/TGF-beta1 transgenic mouse model may finally serve as a model for the testing of antifibrogenic agents.

Isolation and characterization of the mouse cytosolic phosphoenolpyruvate carboxykinase (GTP) gene: evidence for tissue-specific hypersensitive sites

A 72 kilobase pair DNA fragment that contains the mouse phosphoenolpyruvate carboxykinase (PEPCK) gene locus, pck1, was isolated from a genomic bacterial artificial chromosome library. The region from approximately -5.5 to +6.6 kilobase pairs relative to the pck1 transcription start site was sequenced and exhibits a high degree of homology to the rat and human genes. Additionally, the chromatin structure of the PEPCK gene in mouse liver resembles that seen in rat. Backcross panel analysis of a microsatellite sequence confirms that the gene is located on chromosome 2. Hypersensitive site analysis was performed on nuclei isolated from the adipocyte cell line 3T3-F442A in the preadipose and adipose states. Several hypersensitive sites are present in the undifferentiated 3T3-F442A cells, before PEPCK mRNA is detected. The same sites are present after differentiation, however, the sensitivity of mHS 3 increases relative to the others. We conclude that the chromatin is open in 3T3-F442A cells and that factors are able to bind in the undifferentiated state but that something else is required for transcription.

Activating transcription factor-2 regulates phosphoenolpyruvate carboxykinase transcription through a stress-inducible mitogen-activated protein kinase pathway

Several protein-nucleic acid complexes are observed when nuclear extracts from hepatoma cells are assayed for binding to the cAMP response element found in the phosphoenolpyruvate carboxykinase-cytosolic (PEPCK-C) promoter. Although cAMP response element-binding protein and CCAAT/enhancer binding proteins alpha and beta have been identified as liver factors that bind this motif, an uncharacterized, slower migrating complex was also observed. We identify activating transcription factor-2 (ATF-2) as the factor in this complex and show that ATF-2 stimulates expression from the PEPCK-C promoter. ATF-2 is a basic-leucine zipper transcription factor and a target for stress-activated protein kinases. We demonstrate that p38beta mitogen-activated protein (MAP) kinase augments ATF-2 transactivation activity on the PEPCK-C promoter, which is consistent with the interpretation that PEPCK-C promoter activity is maintained under stress through a p38 MAP kinase dependent pathway. In this regard, we show that treatment with sodium arsenite, a known activator of p38 MAP kinases, also stimulates expression from the PEPCK promoter. These results show that ATF-2 can stimulate transcription of the PEPCK-C promoter and support a role for stress inducible kinases in the maintenance of PEPCK-C expression.

Differential regulation in the heart of mitochondrial carnitine palmitoyltransferase-I muscle and liver isoforms

Carnitine palmitoyltransferase-I (CPT-I) plays a crucial role in regulating cardiac fatty acid oxidation which provides the primary source of energy for cardiac muscle contraction. CPT-I catalyzes the transfer of long chain fatty acids into mitochondria and is recognized as the primary rate controlling step in fatty acid oxidation. Molecular cloning techniques have demonstrated that two CPT-I isoforms exist as genes encoding the 'muscle' and 'liver' enzymes. Regulation of fatty acid oxidation rates depends on both short-term regulation of enzyme activity and long-term regulation of enzyme synthesis. Most early investigations into metabolic control of fatty acid oxidation at the CPT-I step concentrated on the hepatic enzyme which can be inhibited by malonyl-CoA and can undergo dramatic amplification or reduction of its sensitivity to inhibition by malonyl-CoA. The muscle CPT-I is inherently more sensitive to malonyl-CoA inhibition but has not been found to undergo any alteration of its sensitivity. Short-term control of activity of muscle CPT-I is apparently regulated by malonyl-CoA concentration in response to fuel supply (glucose, lactate, pyruvate and ketone bodies). The liver isoform is the only CPT-I enzyme present in the mitochondria of liver, kidney, brain and most other tissues while muscle CPT-I is the sole isoform expressed in skeletal muscle as well as white and brown adipocytes. The heart is unique in that it contains both muscle and liver isoforms. Liver CPT-I is highly expressed in the fetal heart, but at birth its activity begins to decline whereas the muscle isoform, which is very low at birth, becomes the predominant enzyme during postnatal development. In this paper, the differential regulation of the two CPT-I isoforms at the protein and the gene level will be discussed.

Insulin stimulates cAMP-response element binding protein activity in HepG2 and 3T3-L1 cell lines

Earlier studies from our laboratory demonstrated an insulin-mediated increase in cAMP-response element binding protein (CREB) phosphorylation. In this report, we show that insulin stimulates both CREB phosphorylation and transcriptional activation in HepG2 and 3T3-L1 cell lines, models of insulin-sensitive tissues. Insulin stimulated the phosphorylation of CREB at serine 133, the protein kinase A site, and mutation of serine 133 to alanine blocked the insulin effect. Many of the signaling pathways known to be activated by insulin have been implicated in CREB phosphorylation and activation. The ability of insulin to induce CREB phosphorylation and activity was efficiently blocked by PD98059, a potent inhibitor of mitogen-activated protein kinase kinase (MEK1), but not significantly by rapamycin or wortmannin. Likewise, expression of dominant negative forms of Ras or Raf-1 completely blocked insulin-stimulated CREB transcriptional activity. Finally, we demonstrate an essential role for CREB in insulin activation of fatty-acid synthase and fatty acid binding protein (FABP) indicating the potential physiologic relevance of insulin regulation of CREB. In summary, insulin regulates CREB transcriptional activity in insulin-sensitive tissues via the Raf --> MEK pathway and has an impact on physiologically relevant genes in these cells.

Phosphoenolpyruvate carboxykinase (GTP) gene transcription and hyperglycemia are regulated by glucocorticoids in genetically obese db/db transgenic mice

The molecular mechanisms underlying increased hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene transcription and gluconeogenesis in type II diabetes are largely unknown. To examine the involvement of glucocorticoids and the cis-acting insulin response sequence (IRS, -416/-407) in the genetically obese db/db mouse model, we generated crosses between C57BL/KsJ-db/+ mice and transgenic mice that express -460 or -2000 base pairs of the rat PEPCK gene promoter containing an intact or mutated IRS, linked to a reporter gene. Transgenic mice expressing the intact PEPCK(460)-CRP (C-reactive protein) transgene bred to near homozygosity at the db locus were obese, hyperinsulinemic, and developed fasting hyperglycemia (389 +/- 26 mg/100 ml) between 4 and 10 weeks of age. Levels of CRP reporter gene expression were increased 2-fold despite severe hyperinsulinemia compared with non-diabetic non-obese transgenic mice. Reporter gene expression was also increased 2-fold in transgenic obese diabetic db/db mice bearing a mutation in the IRS, -2000(IRS)-hGx, compared with non-obese non-diabetic transgenic 2000(IRS)-hGx mice. Treatment of obese diabetic db/db transgenic mice with the glucocorticoid receptor blocker RU 486 decreased plasma glucose by 50% and reduced PEPCK, GLUT2, glucose-6-phosphatase, tyrosine aminotransferase, CRP, and hGx reporter gene expression to levels similar to those of non-obese normoglycemic transgenic mice. Taken together, these results establish that -460 bp of 5'-flanking sequence is sufficient to mediate the induction of PEPCK gene transcription in genetically obese db/db mice during the development of hyperglycemia. The results further demonstrate that the mechanism underlying increased expression of gluconeogenic enzymes in the db/db mouse requires the action of glucocorticoids and occurs independently of factors acting through the PEPCK IRS (-416/-407) promoter binding site.

Involvement of HNF-1 in the regulation of phosphoenolpyruvate carboxykinase gene expression in the kidney

The cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK) gene is differentially expressed in several tissues. A specific set of regulatory elements in the promoter are responsible for the control of PEPCK gene transcription and, in turn, determine its distinct metabolic role in each tissue. DNase I footprinting analysis of the PEPCK promoter, using nuclear proteins from tissues which express the gene for PEPCK, and transient expression assays in renal cell lines have demonstrated that the HNF-1 recognition motif (P2) in the PEPCK promoter characterizes kidney-specific expression. This site is required also for the response to acidosis. Since the P2 site is not involved in the expression of the PEPCK gene in the liver, we propose that its critical role in the kidney stems from a combination of abundance of HNF-1 together with low concentrations of members of the C/EBP family in this tissue.

A 346-base pair region of the mouse gamma-glutamyl transpeptidase type II promoter contains sufficient cis-acting elements for kidney-restricted expression in transgenic mice

The mouse gamma-glutamyl transpeptidase (GGT) gene encodes seven distinct mRNAs that are transcribed from seven separate promoters. Type II mRNA is the most abundant in kidney. We have developed a cell line with features of renal proximal tubular cells which expresses GGT mRNA types with a pattern similar to that of mouse kidney. Because a 346-bp sequence from the type II promoter directed the highest level of CAT activity in these cells, this region was used to drive the expression of a beta-galactosidase reporter gene in transgenic mice. Two transgenic mouse lines expressed beta-galactosidase limited to the renal proximal tubules. Site-directed deletions within this 346-bp promoter region demonstrated that cis-elements containing the consensus binding sites for AP2, a glucocorticoid response element (GRE)-like element, and the initiator region were required for transcriptional activity and were not additive. Purified AP2 bound and footprinted the AP2 consensus region, making it likely that transcription from the GGT type II promoter is regulated in part by AP2. These data suggest that transcription of the type II promoter requires multiple protein DNA interactions involving at least an AP2 element, and probably a GRE-like element and the initiator region.

Maternal dietary glucose modifies phosphoenolpyruvate carboxykinase (PEPCK) gene expression in the kidney of newborn rats

The consequence of low maternal dietary glucose on perinatal phosphoenolpyruvate carboxykinase (PEPCK EC 4.1.32) gene expression was investigated. Pregnant rats were fed isoenergetic diets containing graded levels of glucose (0, 12, 24, and 60%) from gestation day 2 to lactation day 15. The postnatal developmental profile of PEPCK mRNA in the neonatal kidney was analysed by Northern blot and presented as PEPCK/GAPDH mRNA ratios. In comparison with the 24 and 60% dietary groups, maternal dietary glucose restriction (0 or 12%) during pregnancy resulted in a significant delay in postnatal renal PEPCK gene expression. In these glucose restricted pups, renal PEPCK mRNA was barely detected at birth and was fully visualized only at 4-6 hr; it peaked 24 hr after birth, which was 12 hr later than pups born to dams fed 24 or 60% glucose diets. These results demonstrate for the first time that maternal dietary glucose can modify postnatal renal PEPCK gene expression during perinatal development when glucose homeostasis via gluconeogenesis is critical for neonatal survival.

Significant suppression of rat liver aldolase B by a toxic coplanar polychlorinated biphenyl, 3,3',4,4',5-pentachlorobiphenyl

A toxic coplanar polychlorinated biphenyl, 3,3',4,4',5-pentachlorobiphenyl (PenCB), significantly suppresses the expression of liver aldolase B in rats. Hepatic aldolase activity in PenCB-treated rats was significantly reduced to about 50% of that in free- and pair-fed control groups. The reduced aldolase activity following PenCB-treatment was due to the marked suppression of the expression of aldolase B shown by immunoblot analysis after SDS-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis. The suppression of rat liver aldolase B could be a key biochemical lesion caused by PenCB.

Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression

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

The upstream regulatory region of the carbamoyl-phosphate synthetase I gene controls its tissue-specific, developmental, and hormonal regulation in vivo

The carbamoyl-phosphate synthetase I gene is expressed in the periportal region of the liver, where it is activated by glucocorticosteroids and glucagon (via cyclic AMP), and in the crypts of the intestinal mucosa. The enhancer of the gene is located 6.3 kilobase pairs upstream of the transcription start site and has been shown to direct the hormone-dependent hepatocyte-specific expression in vitro. To analyze the function of the upstream region in vivo, three groups of transgenic mice were generated. In the first group the promoter drives expression of the reporter gene, whereas the promoter and upstream region including the far upstream enhancer drive expression of the reporter gene in the second group. In the third group the far upstream enhancer was directly coupled to a minimized promoter fragment. Reporter-gene expression was virtually undetectable in the first group. In the second group spatial, temporal, and hormonal regulation of expression of the reporter gene and the endogenous carbamoyl-phosphate synthetase gene were identical. The third group showed liver-specific periportal reporter gene expression, but failed to activate expression in the intestine. These results show that the upstream region of the carbamoyl-phosphate synthetase gene controls four characteristics of its expression: tissue specificity, spatial pattern of expression within the liver and intestine, hormone sensitivity, and developmental regulation. Within the upstream region, the far upstream enhancer at -6.3 kilobase pairs is the determinant of the characteristic hepatocyte-specific periportal expression pattern of carbamoyl-phosphate synthetase.

Adenovirus E1A proteins regulate phosphoenolpyruvate carboxykinase gene transcription through multiple mechanisms

Recently, Kalvakolanu et al. (Kalvakolanu, D. V. R., Liu, J., Hanson, R. W., Harter, M. L., and Sen, G. C. (1992) J. Biol. Chem. 267, 2530-2536) showed that E1A inhibited the basal and cAMP-stimulated transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK). This inhibition was mediated by the conserved region 1 (CR1) domain of E1A, which has been shown by other laboratories to bind to the cellular transcriptional adaptor proteins, p300 and cAMP response element binding protein (CREB)-binding protein (CBP). The PEPCK gene promoter contains a functional cAMP-response element, through which CREB and, therefore, CBP modulate transcription, and a consensus p300 DNA binding sequence is also present in a distal protein binding site of the promoter. We hypothesized that E1A might inhibit PEPCK gene transcription by binding to p300 and/or CBP. Surprisingly, we found that E1A consistently stimulated basal transcription from the PEPCK promoter in transfection assays in adenovirus (Ad)-infected HepG2 hepatoma cells or E1A-expressing, stably transfected 3T3 fibroblasts and nuclear run-on assays in Ad-infected H4IIE hepatoma cells. E1A also enhanced the stimulation of PEPCK gene transcription by Bt2cAMP. In transfection assays, wild type Ad5 expressing both 243R and 289R forms of E1A or a mutant virus expressing the 289R form alone stimulated transcription from the PEPCK promoter by approximately 5-fold 20 h postinfection. However, no stimulation was observed in cells infected with a virus expressing either the 243R protein alone or a 289R protein from which conserved region 3 (CR3) was mutated. Mutation or deletion of CR1 of E1A had no significant effect on transcription from the PEPCK promoter. Mutations within conserved region 2 (CR2) of E1A that inhibit the binding of E1A to the retinoblastoma gene product (pRb) further enhanced the stimulation of transcription from the PEPCK promoter by 2 3-fold compared with wild type E1A. These findings suggested that the normal function of pRb is to stimulate PEPCK gene transcription, and that this process is inhibited by the binding of E1A to pRb. This hypothesis was confirmed by overexpressing pRb in HepG2 cells, which stimulated transcription from the PEPCK promoter. Our findings indicate that Ad E1A regulates PEPCK gene transcription through a stimulatory mechanism involving CR3, and by attenuating a stimulatory effect of pRb through CR2.

Tissue-specific expression and dietary regulation of chimeric mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase/human growth hormone gene in transgenic mice

We have studied the role of the mitochondrial 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) synthase gene in regulating ketogenesis. The gene exhibits expression in various tissues and it is regulated in a tissue-specific manner. To investigate the underlying mechanisms of this expression, we linked a 1148-base-pair portion of the mitochondrial HMG-CoA synthase promoter to the human growth hormone (hGH) gene and analyzed the expression of the hGH reporter gene in transgenic mice. mRNA levels of hGH were observed in liver, testis, ovary, stomach, colon, cecum, brown adipose tissue, spleen, adrenal glands, and mammary glands from adult mice, and also in liver and stomach, duodenum, jejunum, brown adipose tissue, and heart of suckling mice. There was no expression either in kidney or in any other nonketogenic tissue. The comparison between these data and those of the endogenous mitochondrial HMG-CoA synthase gene suggests that the 1148 base pairs of the promoter contain the elements necessary for expression in liver and testis, but an enhancer is necessary for full expression in intestine of suckling animals and that a silencer prevents expression in stomach, brown adipose tissue, spleen, adrenal glands, and mammary glands in wild type adult mice. In starvation, transgenic mice showed higher expression in liver than did wild type. Both refeeding and insulin injection reduced the expression. Fat diets, composed in each case of different fatty acids, produced similar expression levels, respectively, to those found in wild type animals, suggesting that long-, medium-, and short-chain fatty acids may exert a positive influence on the transcription rate in this 1148-base-pair portion of the promoter. The ketogenic capacity of liver and the blood ketone body levels were equal in transgenic mice and in nontransgenic mice.