Transcriptional activation of the rat liver tyrosine aminotransferase gene by cAMP

Characteristics, tissue-specific expression, and hormonal regulation of expression of tyrosine aminotransferase in the avian female reproductive tract

Tyrosine aminotransferase (TAT) catalyzes the transamination of tyrosine to p-hydroxyphenylpyruvate. Accumulation of tyrosine in the body due to a genetic mutation in the TAT gene causes tyrosomia type II in humans. The TAT gene is regarded as a model for studying steroid-inducible factors regulating a variety of biological functions of TAT. However, little is known of the effects of estrogen on the expression of the TAT gene in chickens. Therefore, in the present study, we identified expression of the avian TAT gene in various organs. The results showed the TAT was detected predominantly in the liver and reproductive organs including testis, oviduct, and ovary. Specifically, TAT mRNA was expressed abundantly in the glandular and luminal epithelia of the oviducts in response to endogenous and exogenous estrogens which also induce dramatic morphological changes in the oviduct of chickens. In addition, target microRNAs of TAT (miR-1460, miR-1626-3p, miR-1690-5p, and miR-7442-3p) were found to modulate expression of the TAT gene. Especially, miR-1690-5p influenced TAT gene transcription by binding directly to its 3'-UTR region. Moreover, the expression of TAT was abundant in glandular epithelia of cancerous but not normal ovaries from laying hens. Taken together, our findings suggest that TAT plays an important role in the cytodifferentiation of oviducts in response to estrogen and in the progression of ovarian cancer in chickens.

Phenobarbital reduces blood glucose and gluconeogenesis through down-regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression in rats

The regulatory mechanism of phosphoenolpyruvate carboykinase (GTP) (EC 4.1.1.32) (PEPCK) gene expression and gluconeogenesis by phenobarbital (PB), which is known to induce drug-metabolizing enzymes, was investigated. Higher level of PEPCK mRNA was observed in spherical rat primary hepatocytes on EHS-gel than monolayer hepatocytes on TIC (type I collagen). We found that PB directly suppressed PEPCK gene expression in spherical hepatocytes on EHS-gel, but not in those on TIC. PB strongly suppressed cAMP-dependent induction of PEPCK gene expression. Tyrosine aminotransferase (TAT), another gluconeogenic enzyme, was induced by cAMP, but not suppressed by PB. Chronic administration of PB reduced hepatic PEPCK mRNA in streptozotocin-induced diabetic and nondiabetic rats, and PB reduced blood glucose level in diabetic rats. Increased TAT mRNA in diabetic rats was not suppressed by PB. These results indicated that PB-dependent reduction is specific to PEPCK. From pyrvate challenge test, PB suppressed the increased gluconeogenesis in diabetic rats. PEPCK gene promoter activity was suppressed by PB in HepG2 cells. In conclusion, we found that spherical hepatocytes cultured on EHS-gel are capable to respond to PB to suppress PEPCK gene expression. Moreover, our results indicate that hypoglycemic action of PB result from transcriptional repression of PEPCK gene and subsequent suppression of gluconeogenesis.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Chronic prednisolone treatment aggravates hyperglycemia in mice fed a high-fat diet but does not worsen dietary fat-induced insulin resistance

Synthetic glucocorticoids such as prednisolone have potent antiinflammatory actions. Unfortunately, these drugs induce severe adverse effects in patients, many of which resemble features of the metabolic syndrome, such as insulin resistance. In this study, we investigated whether adverse effects of prednisolone on glucose homeostasis are aggravated in mice with compromised insulin sensitivity due to a high-fat diet by applying various methods to analyze changes in insulin sensitivity in mice. C57BL/6J mice were fed a high-fat diet for 6 wk and treated with either prednisolone (10 mg/kg · d) or vehicle for the last 7 d. Insulin sensitivity and blood glucose kinetics were analyzed with state-of-the-art stable isotope procedures in different experimental conditions. Prednisolone treatment aggravated fasting hyperglycemia and hyperinsulinemia caused by high-fat feeding, resulting in a higher homeostatic assessment model of insulin resistance. In addition, prednisolone-treated high-fat diet-fed mice appeared less insulin sensitive by detailed analysis of basal glucose kinetics. Remarkably, using hyperinsulinemic-euglycemic or hyperglycemic clamp techniques, neither hepatic nor peripheral insulin resistance was worsened in the group that was treated with prednisolone. Yet analysis of hepatic glucose metabolism revealed that prednisolone did alter glycogen balance by reducing glycogen synthase flux under hyperinsulinemic as well as hyperglycemic conditions. In addition to elevated insulin levels, prednisolone-treated mice showed a major rise in plasma leptin and fibroblast growth factor 21 levels. Our data indicate that prednisolone-induced adverse effects on glucose metabolism in high-fat diet-fed mice do not reflect impaired insulin sensitivity but may be caused by other changes in the hormonal regulatory network controlling glucose metabolism such as fibroblast growth factor 21 and leptin.

Blood content of tyrosine is an index of glucocorticoid action on metabolism

Glucocorticoid hormones directly or indirectly control virtually all metabolic and physiological processes. Glucocorticoids are also shown to act on a multitude of genes, enzyme systems, and proinflammatory factors, but for these hormones there is no representative index of action on metabolism similar to glucose content in blood for insulin. The absence of such an index prevents the assessment of tissue provision with these hormones under various conditions and seems to be an essential cause of complications associated with the clinical use of glucocorticoid preparations. Considering specific features of tyrosine metabolism and data obtained experimentally and on a clinical model (adrenalectomy in rats and substitution therapy in endocrine disease), blood content of this amino acid seems promising as such an index. Based on comparing results of glucocorticoid treatment in patients with systemic lupus erythematosus with changes in their blood tyrosine contents, the pharmacological effect of glucocorticoid preparations is suggested to be mainly due to compensating a relative shortage of these hormones.

Dietary protein differentially regulates the kinetic behaviour of serine dehydratase and tyrosine aminotransferase of liver and white muscle of rainbow trout (Oncorhynchus mykiss)

We have determined the kinetic behaviour of serine dehydratase (SerDH) and tyrosine aminotransferase (TyrAT) in the liver and white muscle of juvenile rainbow trout (Oncorhynchus mykiss) fed on a low-protein/high-fat (LP/HF) and control diets. The relationship between the kinetic parameters and tissue-DNA concentration has also been determined. SerDH and TyrAT showed hyperbolic kinetics in all cases. The partial replacement of protein with fat significantly increased hepatic SerDH-specific activity, maximum velocity (V(max)) and Michaelis constant (K(m)) while no significant changes were detected in the values of these parameters in white muscle. Nevertheless, hepatic and white-muscle TyrAT specific activity and V(max) decreased in trout fed on LP/HF diet with respect to control. The K(m) of hepatic TyrAT was also lower in trout fed on LP/HF diet than in control. The TyrAT kinetic parameters expressed per cell unit also changed in the same sense as previously described. These results show that, in trout, SerDH and TyrAT are two enzymes regulated by the partial replacement of dietary protein by fat.

Serine dehydratase and tyrosine aminotransferase activities increased by long-term starvation and recovery by refeeding in rainbow trout (Oncorhynchus mykiss)

Here, we study a cycle of long-term starvation followed by refeeding in relation to the kinetics of serine dehydratase (SerDH) and tyrosine aminotransferase (TyrAT) in rainbow trout (Oncorhynchus mykiss). We determine SerDH- and TyrAT- specific activity at different substrate concentrations in liver and white muscle of juvenile trout starved for 70 days and then refed for 6 hr, 32 hr, 4 days, and 9 days. SerDH showed a hyperbolic kinetic with a K(m) for L-serine of 77.07+/-8.78 mM in the liver of control trout. After 70 days of starvation, the SerDH activity at saturate substrate concentration rose 100% over control. No significant changes were found in the K(m) values of the enzyme. After refeeding, the SerDH activity declined to control values. TyrAT also showed a hyperbolic kinetic with a K(m) for L-tyrosine of 1.86+/-0.12 and 2.55+/-0.57 mM in liver and white muscle, respectively. In starved trout, TyrAT activity in liver and white muscle was about 64 and 267%, respectively, higher than control. After 9 days of refeeding, the control values recovered, although, at 6 hr of refeeding, hepatic TyrAT activity was higher than that for starvation. This work shows that SerDH and TyrAT are present in rainbow trout and that the two enzymes have regulatory functions in the catabolism of their respective amino acids in this species.

Fifth-generation model for corticosteroid pharmacodynamics: application to steady-state receptor down-regulation and enzyme induction patterns during seven-day continuous infusion of methylprednisolone in rats

A fifth-generation model for receptor/gene-mediated corticosteroid effects was proposed based on results from a 50 mg/kg i.v. bolus dose of methylprednisolone (MPL) in male adrenalectomized rats, and confirmed using data from other acute dosage regimens. Steady-state equations for receptor down-regulation and tyrosine aminotransferase (TAT) enzyme induction patterns were derived. Five groups of male Wistar rats (n = 5/group) were subcutaneously implanted with Alzet mini-pumps primed to release saline or 0.05, 0.1, 0.2, and 0.3 mg/kg/hr of MPL for 7 days. Rats were sacrificed at the end of the infusion. Plasma MPL concentrations, blood lymphocyte counts, and hepatic cytosolic free receptor density, receptor mRNA, TAT mRNA, and TAT enzyme levels were quantitated. The pronounced steroid effects were evidenced by marked losses in body weights and changes in organ weights. All four treatments caused a dose-dependent reduction in hepatic receptor levels, which correlated with the induction of TAT mRNA and TAT enzyme levels. The 7 day receptor mRNA and free receptor density correlated well with the model predicted steady-state levels. However, the extent of enzyme induction was markedly higher than that predicted by the model suggesting that the usual receptor/gene-mediated effects observed upon single/intermittent dosing of MPL may be countered by alterations in other aspects of the system. A mean IC50 of 6.1 ng/mL was estimated for the immunosuppressive effects of methylprednisolone on blood lymphocytes. The extent and duration of steroid exposure play a critical role in mediating steroid effects and advanced PK/PD models provide unique insights into controlling factors.

Pharmacodynamics and pharmacogenomics of methylprednisolone during 7-day infusions in rats

An array of adverse steroid effects was examined on a whole body, tissue, and molecular level. Groups of male adrenalectomized Wistar rats were subcutaneously implanted with Alzet mini-pumps giving zero-order release rates of 0, 0.1, and 0.3 mg/kg/h methylprednisolone for 7 days. The rats were sacrificed at various times during the 7-day infusion period. A two-compartment model with a zero order input could adequately describe the kinetics of methylprednisolone upon infusion. Blood lymphocyte counts dropped to a minimum by 6 h and were well characterized by the cell trafficking model. The time course of changes in body and organ (liver, spleen, thymus, gastrocnemius muscle, and lungs) weights was described using indirect response models. Markers of gene-mediated steroid effects included hepatic cytosolic free receptor density, receptor mRNA, tyrosine aminotransferase (TAT) mRNA, and TAT levels. Our fifth-generation model of acute corticosteroid pharmacodynamics was used to predict the time course of receptor/gene-mediated effects. An excellent agreement between the expected and observed receptor dynamics suggested that receptor events and mRNA autoregulation are not altered upon 7-day methylprednisolone dosing. However, the model indicated a decoupling between the receptor and TAT dynamics with this infusion. The strong tolerance seen in TAT mRNA induction could be partly accounted for by receptor down-regulation. An amplification of translation of TAT mRNA to TAT and/or a reduction in the enzyme degradation rate could account for the observed exaggerated TAT activity. Our results exemplify the importance of biological signal transduction variables in controlling receptor/gene-mediated steroid responses during chronic dosing.

Cloning of a mouse glucocorticoid modulatory element binding protein, a new member of the KDWK family

A mouse cDNA that encodes a nuclear DNA binding protein was identified by yeast two-hybrid screening using the activation domain 2 of the nuclear receptor coactivator TIF2 as a bait. BLAST analysis revealed that the identified cDNA encodes a KDWK domain and contains sequences almost identical to three tryptic peptides of rat GMEB-1 which together with the GMEB-2 heterodimeric partner binds to the GME/CRE sequence (glucocorticoid modulatory element) of the tyrosine aminotransferase (TAT) promoter. Mouse GMEB-1 is ubiquitously expressed in all the tissues examined. In vitro translated mGMEB-1 bound specifically to GME oligonucleotides, either alone or as a heterodimer with rGMEB-2. Transient transfection experiments with TAT promoter reporter genes suggest a potential role for mGMEB-1 as a transcriptional regulator of the TAT promoter.

5' sequences direct developmental expression and hormone responsiveness of tyrosine aminotransferase in primary cultures of fetal rat hepatocytes

Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and N6,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP), whereas less mature hepatocytes (<E15) do not [Shelly, L. L. & Yeoh, G. C. T. (1991) Eur. J. Biochem. 199, 475-481]. Therefore, we consider >E15 hepatocytes, and not <E15 hepatocytes, to be determined. This study reports that 11.1 kb of sequences upstream of the TyrAT transcription start site, which include a cAMP-responsive element (CRE) and a glucocorticoid-responsive element (GRE), are required for correct developmental regulation of gene expression in determined fetal hepatocytes. In contrast, the TyrAT CRE alone does not have this capability. Dexamethasone augments basal and Bt2cAMP-stimulated activity of the TyrAT CRE alone, suggesting that synergism may be due to interaction between the glucocorticoid and cAMP-signaling pathways. However, Bt2cAMP does not further increase dexamethasone-induced activity of the 11.1 kb 5' sequences when the TyrAT CRE is removed, thus excluding interaction of Bt2cAMP with the glucocorticoid pathway. Finally, insulin inhibition of dexamethasone-induced gene transcription is shown to be conferred by TyrAT 5' sequences. This study shows that cellular components, other than those which mediate hormonal regulation of genes, are required for determination of hepatocytes with respect to TyrAT. Since this phenomenon is observed with transient transfections, it is unlikely to involve higher-order chromatin structure.

Bisecting GlcNAc structures act as negative sorting signals for cell surface glycoproteins in forskolin-treated rat hepatoma cells

The bisecting N-acetylglucosamine residue is formed by UDP-N-acetylglucosamine:beta-D-mannoside-beta-1, 4-N-acetylglucosaminyltransferase III (GnT-III), a key branching enzyme for N-glycans. We found that forskolin, an adenylyl cyclase activator, markedly enhanced GnT-III at the transcriptional level in various hepatoma cells and hepatocytes, resulting in an increase of bisecting GlcNAc residues in various glycoproteins, as judged from the lectin binding to erythroagglutinating phytohemagglutinin (E-PHA). In whole cell lysates, the E-PHA binding was increased, and leukoagglutinating phytohemagglutinin (L-PHA) binding was decreased at 12 h after forskolin treatment, by time, both GnT-III activity and mRNA had reached the maximum levels. In contrast, the binding capacity as to E-PHA, determined by fluorescence-activated cell sorting on the cell surface, was decreased, suggesting that bisecting GlcNAc structures in certain glycoproteins changed the expression levels of glycoproteins and decreased their sorting on the cell surface. Fractionated organelles of M31 cells showed that the binding capacity as to E-PHA was mainly localized in Golgi membranes and lysosomes. This was also supported by a fluorescence microscopy. In order to determine whether or not the bisecting GlcNAc residue acts as a sorting signal for glycoproteins, N-oligosaccharide structures of lysosomal-associated membrane glycoprotein 1 and beta-glucuronidase, gamma-glutamyltranspeptidase, and secretory glycoproteins such as ceruloplasmin and alpha-fetoprotein were measured by E-PHA and L-PHA blotting after immunoprecipitation. The expression levels of lysosomal membrane glycoprotein 1 and gamma-glutamyltranspeptidase on the cell surface were decreased at 12 h after forskolin treatment, indicating that the bisecting GlcNAc structure may act as a negative sorting signal for the cell surface glycoproteins and may alter the characteristics of hepatoma cells. This is the first report on glycoprotein sorting related to a specific structure of oligosaccharides, bisecting GlcNAc.

Regulation of tyrosine aminotransferase gene expression by glucocorticoids in quiescent and regenerating liver

Following 70% hepatectomy, the induction of tyrosine amino-transferase mRNA by glucocorticoids was marginal at 1.5 h, significantly impaired between 3 and 8 h and, at 16 h post-hepatectomy, reached a value approx. 5-fold the basal level, similar to the level observed in quiescent liver. The fold induction of the mRNA was accounted for by a similar fold activation of transcription of the gene by glucocorticoids in regenerating but not in quiescent liver; in the latter, activation of transcription was marginal in spite of glucocorticoid-induced hypersensitivity to cleavage by DNase I at the glucocorticoid-dependent enhancer of the gene. The possibility that in quiescent liver glucocorticoids act at a transcriptional step beyond initiation, increasing the rate of elongation or overcoming a blockage in elongation, was excluded. However, a similar fold induction was determined for total and nuclear tyrosine aminotransferase mRNA in the presence of glucocorticoids, suggesting that in quiescent liver glucocorticoids promote efficient maturation of the tyrosine aminotransferase primary transcript. Thus a glucocorticoid-induced nuclear post-transcriptional up-regulation apparently compensates for impaired activation of transcription of the tyrosine aminotransferase gene by glucocorticoids in quiescent liver.

Correction of diabetic alterations by glucokinase

Hyperglycemia is a common feature of diabetes mellitus. It results from a decrease in glucose utilization by the liver and peripheral tissues and an increase in hepatic glucose production. Glucose phosphorylation by glucokinase is an initial event in glucose metabolism by the liver. However, glucokinase gene expression is very low in diabetic animals. Transgenic mice expressing the P-enolpyruvate carboxykinase/glucokinase chimeric gene were generated to study whether the return of the expression of glucokinase in the liver of diabetic mice might prevent metabolic alterations. In contrast to nontransgenic mice treated with streptozotocin, mice with the transgene previously treated with streptozotocin showed high levels of both glucokinase mRNA and its enzyme activity in the liver, which were associated with an increase in intracellular levels of glucose 6-phosphate and glycogen. The liver of these mice also showed an increase in pyruvate kinase activity and lactate production. Furthermore, normalization of both the expression of genes involved in gluconeogenesis and ketogenesis in the liver and the production of glucose and ketone body by hepatocytes in primary culture were observed in streptozotocin-treated transgenic mice. Thus, glycolysis was induced while gluconeogenesis and ketogenesis were blocked in the liver of diabetic mice expressing glucokinase. This was associated with normalization of blood glucose, ketone bodies, triglycerides, and free fatty acids even in the absence of insulin. These results suggest that the expression of glucokinase during diabetes might be a new approach to the normalization of hyperglycemia.

An ATP-dependent As(III)-glutathione transport system in membrane vesicles of Leishmania tarentolae

Membrane preparations enriched in plasma membrane vesicles prepared from promastigotes of Leishmania tarentolae were shown to accumulate thiolate derivatives of 73As(III). Free arsenite was transported at a low rate, but rapid accumulation was observed after reaction with reduced glutathione (GSH) conditions that favor the formation of As(GS)3. Accumulation required ATP but not electrochemical energy, indicating that As(GS)3 is transported by an ATP-coupled pump. Pentostam, a Sb(V)-containing drug that is one of the first-line therapeutic agents for treatment of leishmaniasis, inhibited uptake after reaction with GSH. Vesicles prepared from a strain in which both copies of the pgpA genes were disrupted accumulated As(GS)3 at wild-type levels, demonstrating that the PgpA protein is not the As(GS)3 pump. These results have important implications for the mechanism of drug resistance in the trypanosomatidae, suggesting that a plasma membrane As(GS)3 pump catalyzes active extrusion of metal thiolates, including the Pentostam-glutathione conjugate.

Prevention of diabetic alterations in transgenic mice overexpressing Myc in the liver

Recent studies have demonstrated that the overexpression of the c-myc gene in the liver of transgenic mice leads to an increase in both utilization and accumulation of glucose in the liver, suggesting that c-Myc transcription factor is involved in the control of liver carbohydrate metabolism in vivo. To determine whether the increase in c-Myc might control glucose homeostasis, an intraperitoneal glucose tolerance test was performed. Transgenic mice showed lower levels of blood glucose than control animals, indicating that the overexpression of c-Myc led to an increase of blood glucose disposal by the liver. Thus, the increase in c-Myc might counteract diabetic hyperglycemia. In contrast to control mice, transgenic mice treated with streptozotocin showed normalization of concentrations of blood glucose, ketone bodies, triacylglycerols and free fatty acids in the absence of insulin. These findings resulted from the normalization of liver metabolism in these animals. While low glucokinase activity was detected in the liver of diabetic control mice, high levels of both glucokinase mRNA and enzyme activity were noted in the liver of streptozotocin-treated transgenic mice, which led to an increase in intracellular levels of glucose 6-phosphate and glycogen. The liver of these mice also showed an increase in pyruvate kinase activity and lactate production. Furthermore, normalization of both the expression of genes involved in the control of gluconeogenesis and ketogenesis and the production of glucose and ketone bodies was observed in streptozotocin-treated transgenic mice. Thus, these results suggested that c-Myc counteracted diabetic alterations through its ability to induce hepatic glucose uptake and utilization and to block the activation of gluconeogenesis and ketogenesis.

Analysis of perinatal gene expression: hormone response elements mediate activation of a lacZ reporter gene in liver of transgenic mice

The transcription of genes encoding gluconeogenic enzymes is tightly regulated during the perinatal period. These genes are induced by glucagon (cAMP) and glucocorticoids and repressed by insulin. To address the role of cAMP and glucocorticoids in the physiological activation of genes encoding gluconeogenic enzymes in the perinatal period, transgenic mice have been generated with chimeric constructs containing the reporter gene lacZ under the control of hormone response elements. The activity of the transgene is restricted to the liver by the presence of the enhancers from the alpha-fetoprotein gene and its transcription is driven by a promoter that contains a TATA box linked to either cAMP response elements (CREs) or glucocorticoid response elements (GREs). We demonstrate cAMP and glucocorticoid regulation, liver-specific expression, and perinatal activation of the reporter gene. These data indicate that the CRE and GRE are, independently, necessary and sufficient to mediate perinatal gene activation. Perinatal activation was not impaired when a CRE reporter transgene was assayed in mice that contain a targeted mutation of the CRE-binding protein (CREB) gene, providing further evidence for functional redundancy among the members of the CREB/ATF gene family.

Differential regulation of the cytochrome P450 3A1 gene transcription by dexamethasone in immature and adult rat liver

We have previously shown that the in vivo induction of cytochrome P450 3A1 by dexamethasone occurs through a sharp and early transcriptional activation in the immature rat liver that is drastically impaired in adults [Telhada, M. B., Pereira, T. M. & Lechner, M. C. (1992) Arch. Biochem. Biophys. 298, 714-725]. In the present study we investigate the relative importance of cytochrome P450 3A1 gene transcription on the adaptive response to the synthetic glucocorticoid dexamethasone, by measuring the time-course run-on transcription rate and concomitant mRNA accumulation in the male rat liver at two different ontological developmental stages. The primary (direct) or secondary (dependent on protein neo-synthesis) nature of the in vivo inductive response to dexamethasone and to pregnenolone 16 alpha-carbonitrile, is further investigated by inhibiting translation by cycloheximide pretreatment. The induction of cytochrome P450 3A1 gene transcription by the anti-glucocorticoid pregnenolone 16 alpha-carbonitrile is demonstrated to occur through a secondary mechanism, requiring ongoing protein biosynthesis, regardless of the developmental stage of the animals. Conversely, a significant developmentally controlled change is observed in the inductive response of the cytochrome P450 3A1 gene to dexamethasone, characterized by a markedly delayed transcriptional activation in the adult rat liver (90 day old) as compared to the immature rat liver (21 day old). This is consistent with the net primary response of the cytochrome P450 3A1 gene to dexamethasone demonstrated in this study to occur in the immature rat liver and almost lost at the adult stage, when protein neo-synthesis becomes essential for the inductive response. Our results demonstrate (a) a difference in the mechanisms underlying induction of the cytochrome P450 3A1 gene by the glucocorticoid agonist dexamethasone and by the antagonist pregnenolone 16 alpha-carbonitrile, and (b) an important change in the mechanisms of the inductive response to dexamethasone, associated with the immature/adult liver phenotype transition. This indicates the participation of specific labile transcription factors in the induction of cytochrome P450 3A1 gene by the synthetic glucocorticoids.

Development of dexamethasone-inducible tyrosine aminotransferase activity in WB-F344 rat liver epithelial stemlike cells cultured in the presence of sodium butyrate

Sodium butyrate acts as a differentiation-promoting agent for a wide variety of cell types, including some tumor cell lines. In this study, we examined the effects of sodium butyrate (SB) on the functional differentiation of cultured WB-F344 rat liver epithelial stemlike cells. Treatment of WB-F344 cells with 3.75 mM SB resulted in an inhibition of cellular proliferation, alterations to normal cellular morphology (increased cell size and decreased nuclear/cytoplasmic ratio), and significant increases in cellular protein synthesis. The SB-mediated changes in cell morphology, proliferative status, and protein catabolism were accompanied by development of dexamethasone-inducible tyrosine aminotransferase (TAT) enzyme activity. Culture of WB-F344 cells in growth medium containing SB and dexamethasone (DEX; 1 x 10(-6) M) resulted in greater than sevenfold increase in the basal TAT activity compared with control cultures. An additional sixfold increase in TAT activity was observed when cells cultured in medium containing SB and DEX were exposed to 1 x 10(-7) M DEX during the last 24 hours of culture. The DEX-inducible TAT activity developed by SB-treated WB-F344 cells responded to the modulating effects of insulin and L-tyrosine in a manner that closely resembled that reported for cultured hepatocytes and hepatoma cell lines. These studies show that treatment of WB-F344 rat liver epithelial stemlike cells with the differentiation-promoting agent SB in vitro leads to expression of the differentiation-specific hepatocyte enzyme TAT.

The efficiency of nuclear processing of the tyrosine aminotransferase mRNA transcript increases after partial hepatectomy

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotransferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operated livers from adrenalectomized rats, was established 16 h post-hepatectomy. The increase in mRNA levels was not accounted for by a comparable change in the rate of transcription of the gene which, at 0.5 h post-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quiescent and 16-h-regenerating liver, the rate of transcription of the gene in quiescent liver was threefold higher than the rate in 16-h-regenerating liver. The maintenance of a higher rate of gene transcription in quiescent liver, as compared to regenerating liver, was shown to depend on ongoing protein synthesis. The possibility that the high rate of gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the efficiency of nuclear processing of the primary transcript.

Regulation of lipogenic enzyme and phosphoenolpyruvate carboxykinase gene expression in cultured white adipose tissue. Glucose and insulin effects are antagonized by cAMP

In cultured adipose tissue of suckling rats, glucose alone is able to induce the appearance of fatty-acid synthase and acetyl-CoA carboxylase mRNA by a mechanism involving glucose-6-phosphate accumulation; insulin alone has no effect but potentiates the effect of glucose. In the present study, we have analysed in cultured adipose tissue the effects of other hormones on the expression of these enzymes as well as on phosphoenolpyruvate carboxykinase. Triiodothyronine has only a marginal effect on fatty-acid synthase expression, in the absence or presence of glucose and insulin. A synthetic glucocorticoid, dexamethasone, opposes the inductive effect of glucose and insulin on fatty-acid synthase expression but increases the expression of phosphoenolpyruvate carboxykinase. A beta-agonist, isoproterenol totally inhibits the inductive effect of glucose and insulin on acetyl-CoA carboxylase and fatty-acid synthase expression whereas it increases the expression of phosphoenolpyruvate carboxykinase. Similarly, glucagon and cAMP have antagonistic effects on glucose and insulin-induced fatty-acid synthase expression. These inhibitory effects cannot be explained only by a reduction in glucose-6-phosphate concentration. We conclude that, in adipose tissue, dexamethasone and cAMP-generating hormones are negative regulators of lipogenic enzyme expression. Finally, the regulation of phosphoenolpyruvate carboxykinase expression in adipose tissue is similar to that found in the liver, i.e. inhibition by insulin and glucose and activation by glucocorticoids and cAMP.

Expression of the neomycin-resistance (neo) gene induces alterations in gene expression and metabolism

The amino 3'-glycosyl phosphotransferase (neo) gene is the selectable marker most widely used in stable transfection or infection protocols. Because the neo gene product has phosphotransferase activity, it might modify the phosphorylation state when introduced in mammalian cells. NIH-3T3 fibroblast cells expressing the neo gene, after either infection with retroviral vectors or transfection with plasmids, showed a 50% reduction in both fructose 2,6-bisphosphate (Fru 2,6-P2) concentration and lactate production compared with control NIH-3T3 cells, indicating that these neo-expressing cells are less glycolytic. In addition, a marked decrease in the levels of mRNA for the procollagen 1 alpha and fibronectin genes was also observed in neo-expressing NIH-3T3 cells. This decrease was concomitant with an increase in the mRNA concentration of the endogenous c-myc gene. FTO-2B rat hepatoma cells also showed modifications in gene expression when the neo gene was introduced by stable transfection or infection. In these cells an increase in both P-enolpyruvate carboxykinase (PEPCK) and tyrosine aminotransferase (TAT) mRNA was observed. These results suggest that neo gene expression may induce changes in the cells, which should be considered when neo-selected cells are used to deliver specific genes in different therapy approaches and in embryo manipulation.

Calcium-mobilizing effectors inhibit P-enolpyruvate carboxykinase gene expression in cultured rat hepatocytes

Incubation of primary cultures of hepatocytes from fed and fasted rats with calcium ionophore strongly decreased glucose production from pyruvate. Like insulin, calcium ionophore A23187, phenylephrine, vasopressin, and prostaglandins E2 and F2 alpha caused a significant reduction (50-60%) in basal concentrations of mRNA for P-enolpyruvate carboxykinase (PEPCK), the main regulatory enzyme of gluconeogenesis. Phenylephrine, prostaglandin E2 and calcium ionophore A23187 were also able to counteract the induction of PEPCK gene expression by Bt2cAMP. These effects were similar to those exerted by both vanadate and phorbol ester TPA. The decrease in extracellular calcium by the addition of the calcium-chelating agent EGTA to the incubation medium caused an increase in PEPCK mRNA levels. This effect was additive to that of Bt2cAMP and was counteracted by vanadate.

The distal enhancer implicated in the developmental regulation of the tyrosine aminotransferase gene is bound by liver-specific and ubiquitous factors

Tyrosine aminotransferase gene expression is confined to parenchymal cells of the liver, is inducible by glucocorticoids and glucagon, and is repressed by insulin. Three enhancers control this tissue-specific and hormone-dependent activity, one of which, located at -11 kb, is implicated in establishing an active expression domain. We have studied in detail this important regulatory element and have identified a 221-bp fragment containing critical enhancer sequences which stimulated the heterologous thymidine kinase promoter more than 100-fold in hepatoma cells. Within this region, we have characterized two essential liver-specific enhancer domains, one of which was bound by proteins of the hepatocyte nuclear factor 3 (HNF3) family. Analyses with the dedifferentiated hepatoma cell line HTC suggested that HNF3 alpha and/or -gamma, but not HNF3 beta, are involved in activating the tyrosine aminotransferase gene via the -11-kb enhancer. Genomic footprinting and in vitro protein-DNA binding studies documented cell-type-specific binding of ubiquitous factors to the second essential enhancer domain, which by itself stimulated the thymidine kinase promoter preferentially in hepatoma cells. These results will allow further characterization of the role of these enhancer sequences in developmental activation of the tyrosine aminotransferase gene.

The distal enhancer implicated in the developmental regulation of the tyrosine aminotransferase gene is bound by liver-specific and ubiquitous factors

Tyrosine aminotransferase gene expression is confined to parenchymal cells of the liver, is inducible by glucocorticoids and glucagon, and is repressed by insulin. Three enhancers control this tissue-specific and hormone-dependent activity, one of which, located at -11 kb, is implicated in establishing an active expression domain. We have studied in detail this important regulatory element and have identified a 221-bp fragment containing critical enhancer sequences which stimulated the heterologous thymidine kinase promoter more than 100-fold in hepatoma cells. Within this region, we have characterized two essential liver-specific enhancer domains, one of which was bound by proteins of the hepatocyte nuclear factor 3 (HNF3) family. Analyses with the dedifferentiated hepatoma cell line HTC suggested that HNF3 alpha and/or -gamma, but not HNF3 beta, are involved in activating the tyrosine aminotransferase gene via the -11-kb enhancer. Genomic footprinting and in vitro protein-DNA binding studies documented cell-type-specific binding of ubiquitous factors to the second essential enhancer domain, which by itself stimulated the thymidine kinase promoter preferentially in hepatoma cells. These results will allow further characterization of the role of these enhancer sequences in developmental activation of the tyrosine aminotransferase gene.

Vanadate treatment restores the expression of genes for key enzymes in the glucose and ketone bodies metabolism in the liver of diabetic rats

Oral administration of vanadate to diabetic streptozotocin-treated rats decreased the high blood glucose and D-3-hydroxybutyrate levels related to diabetes. The increase in the expression of the P-enolpyruvate carboxykinase (PEPCK) gene, the main regulatory enzyme of gluconeogenesis, was counteracted in the liver and the kidney after vanadate administration to diabetic rats. Vanadate also counteracted the induction in tyrosine aminotransferase gene expression due to diabetes and was able to increase the expression of the glucokinase gene to levels even higher than those found in healthy animals. Similarly, an induction in pyruvate kinase mRNA transcripts was observed in diabetic vanadate-treated rats. These effects were correlated with changes on glucokinase and pyruvate kinase activities. Vanadate treatment caused a decrease in the expression of the liver-specific glucose transporter, GLUT-2. Thus, vanadate was able to restore liver glucose utilization and block glucose production in diabetic rats. The increase in the expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCoAS) gene, the key regulatory enzyme in the ketone bodies production pathway, observed in diabetic rats was also blocked by vanadate. Furthermore, a similar pattern in the expression of PEPCK, GLUT-2, HMGCoAS, and the transcription factor CCAAT/enhancer-binding protein alpha genes has been observed. All of these results suggest that the regulation of the expression of genes involved in the glucose and ketone bodies metabolism could be a key step in the normalization process induced by vanadate administration to diabetic rats.

Activation of the tyrosine aminotransferase gene is dependent on synergy between liver-specific and hormone-responsive elements

Tyrosine aminotransferase (TAT; L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) gene activity is stimulated by glucocorticoids and glucagon and is repressed by insulin. Expression and responsiveness to the different signal transduction pathways are restricted to the liver, in which the gene is activated shortly after birth. Here we provide a model for the basis of this tissue specificity of the hormonal control. In the two enhancers mediating hormone induction of TAT gene activity we find the hormone response elements in combination with binding sites for constitutive liver-enriched transcription factors: proteins of the hepatocyte nuclear factor 3 family bind in the vicinity of the glucocorticoid response element located 2.5 kb upstream of the transcription start site, while hepatocyte nuclear factor 4 interacts with an essential element in the cAMP-responsive enhancer at -3.6 kb. By juxtaposing the liver-specific element and the target sequence of the signal transduction pathway the regulatory properties of either enhancer can be reconstituted. Thus, the interdependence of the respective enhancer motifs restricts the hormonal activation of the TAT gene to the liver. The coincidence of the onset of TAT gene expression around birth with the perinatal changes in the concentrations of glucocorticoids, glucagon, and insulin suggests cooperation of signal transduction pathways and cell type-specific transcription factors in the developmental activation of the TAT gene.

Binding at an NFI site is modulated by cyclic AMP-dependent activation of myelin basic protein gene expression

Using stable cell lines containing a series of deletions of the myelin basic protein (MBP) promoter directing the bacterial chloramphenicol acetyltransferase gene in a peripheral neurinoma cell line, we have studied the sequences in the MBP promoter needed for induction by cyclic AMP. Stimulation of expression from the MBP promoter by cyclic AMP is not a rapid response. Expression begins after 24 h and reaches a maximum at approximately 72 h. The results from the stable transformants indicate at least one region that appears to be essential to the induction of transcription directed by the MBP promoter. The region that is necessary for induction does not contain a consensus cyclic AMP response element. A specific binding site involved in the induction by cyclic AMP was localized to an NFI binding site.

Epidermal growth factor inhibits phosphoenolpyruvate carboxykinase gene expression in rat hepatocytes in primary culture

Epidermal growth factor (EGF) decreased the basal, and blocked the dibutyryl cyclic AMP (Bt2cAMP)-induced, expression of P-enolpyruvate carboxykinase (GTP) (PEPCK) and tyrosine aminotransferase (TAT) genes in both rat hepatocytes in primary culture and the FTO-2B hepatoma cell line. Treatment of hepatocytes with EGF in combination with phorbol ester (TPA) resulted in an additive decrease of PEPCK mRNA levels. Overnight pretreatment of hepatocytes with TPA, which is known to downregulate protein kinase C, abolished the TPA and reduced the EGF-mediated inhibition of PEPCK gene expression. These results suggested that EGF caused its effect, at least in part, through protein kinase C.

Point mutations in the tyrosine aminotransferase gene in tyrosinemia type II

Tyrosinemia type II (Richner-Hanhart syndrome, RHS) is a disease of autosomal recessive inheritance characterized by keratitis, palmoplantar hyperkeratosis, mental retardation, and elevated blood tyrosine levels. The disease results from deficiency in hepatic tyrosine aminotransferase (TAT; L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5), a 454-amino acid protein encoded by a gene with 12 exons. To identify the causative mutations in five TAT alleles cloned from three RHS patients, chimeric genes constructed from normal and mutant TAT alleles were tested in directing TAT activity in a transient expression assay. DNA sequence analysis of the regions identified as nonfunctional revealed six different point mutations. Three RHS alleles have nonsense mutations at codons 57, 223, and 417, respectively. One "complex" RHS allele carries a GT----GG splice donor mutation in intron 8 together with a Gly----Val substitution at amino acid 362. A new splice acceptor site in intron 2 of the fifth RHS allele leads to a shift in reading frame.

Ablation of stimulation of a cAMP-responsive promoter in CHO cell lines defective in their cAMP-dependent protein kinase system

We have studied the requirement for an intact cAMP-dependent protein kinase (PKA) system to regulate cAMP-mediated gene transcription in Chinese hamster ovary (CHO) cells. Wild-type CHO cells and mutant CHO cell lines selected for their resistance to the growth inhibitory effect of 8-Br-cAMP and defective in their PKA system were transiently transfected with reporter plasmids containing 2.5 and 3.0 kb of the 5'-flanking sequence of the rat tyrosine aminotransferase (TAT) gene promoter. This segment of DNA contains no CRE-like sequences, yet wild-type transfectants exhibited a specific increase in TAT promoter activity following growth in medium containing 8-Br-cAMP. In CHO cell lines defective in their PKA, the transfected TAT promoter failed to respond to cAMP treatment. We conclude that an intact PKA system is necessary for the cAMP-mediated increase in TAT promoter activity in CHO cells and that there is no requirement for a CRE to see this effect.

The kinetics of mammalian gene expression

When rates of transcription from specific genes change, delays of variable length intervene before the corresponding mRNAs and proteins attain new levels. For most mammalian genes, the time required to complete transcription, processing, and transport of mRNA is much shorter than the period needed to achieve a new, steady-state level of protein. Studies of inducible genes have shown that the period required to attain new levels of individual mRNAs and proteins is related to their unique half-lives. The basis for this is a physical principle that predicts rates of accumulation of particles in compartmental systems. The minimum period required to achieve a new level is directly proportional to product half-lives because rates of decay control the ratio between the rate of synthesis and the concentration of gene products at steady state. This kinetic model suggests that sensitivity of gene products to degradation by ribonucleases and proteinases is an important determinant of the time scale of gene expression.

Effects of dexamethasone and cAMP on tyrosine aminotransferase expression in cultured fetal rat hepatocytes

Fetal hepatocyte cultures were used to investigate tyrosine aminotransferase (TyrAT) expression during development. Previous studies showed that TyrAT is synthesized by hepatocytes isolated from 15-day-gestation fetuses maintained in culture for two or more day, then exposed to dexamethasone. TyrAT expression was essentially undetectable on the first day of culture of hepatocytes derived from 15-day-gestation, or less mature, fetuses. Dexamethasone and cAMP are potent inducers of TyrAT and they synergistically induce TyrAT to extremely high levels when added simultaneously to cultured fetal hepatocytes. The effects of dibutyryl-cAMP (Bt2cAMP) alone and in combination with dexamethasone on TyrAT expression are investigated. Hepatocytes isolated from 15-day-gestation fetuses exposed to both inducers possessed detectable levels of TyrAT activity and mRNA on day 1 of culture, and this increased by day 3. In contrast, hepatocytes exposed to either inducing agent alone were essentially negative on day 1, but positive on day 3. This was shown to be a consequence of transcription. When 13-day-gestation hepatocytes were maintained in culture under identical conditions detectable levels of TyrAT mRNA were evident on day 1, and this increased by day 3. Immunocytochemical studies revealed that the appearance and subsequent increase in TyrAT production elicited by dexamethasone and Bt2cAMP were due to changes in the proportion of hepatocytes expressing the enzyme. Therefore, in the presence of both dexamethasone and Bt2cAMP, TyrAT expression can be detected in some cells at an earlier stage of liver development than reported previously.

Phosphorylation of the v-erbA protein is required for its function as an oncogene

The v-erbA oncogene of avian erythroblastosis virus (AEV) encodes a ligand-independent mutated version of the chicken c-erbA alpha-encoded thyroid hormone receptor. The v-erbA gene product, a 75-kD gag/v-erbA fusion protein, is phosphorylated on Ser-16/17 of its v-erbA-encoded domain, and phosphorylation at this site is increased in vivo after activation of either the PKA or PKC signal transduction pathways. To test the hypothesis that phosphorylation of Ser-16/17 regulates gag/v-erbA protein function, mutant proteins in which Ser-16/17 had been changed to alanine or threonine residues were analyzed for their ability to inhibit erythroid differentiation of ts v-erbB or ts v-sea-transformed erythroblasts at nonpermissive temperature. Conversion of Ser-16/17 into alanine, although not affecting nuclear localization or DNA binding of the gag/erbA protein, prevented phosphorylation of the v-erbA-encoded domain of the protein both in unstimulated cells or after stimulation by PKA and PKC activators. The nonphosphorylatable AA-gag/v-erbA protein proved unable to inhibit temperature-induced differentiation of ts v-erbB and ts v-sea-transformed erythroblasts and to block expression of the erythrocyte-specific genes band 3 and carbonic anhydrase II. Back mutation of these alanine residues to serine resulted in the recovery of both normal phosphorylation levels and wild-type biological activity. In contrast, substitution of Ser-16/17 for threonine, which preserved phosphorylation in unstimulated cells but not PKA- and PKC-enhanced phosphorylation, resulted in a partially active gag/v-erbA protein. These results, together with the fact that the protein kinase inhibitor H7 resulted in both a dose-dependent inhibition of gag/v-erbA protein phosphorylation and the induction of terminal differentiation of AEV-transformed erythroblasts show that phosphorylation of gag/v-erbA protein is required for full biological activity. These results support the hypothesis that phosphorylation of the gag/v-erbA protein is important for transcriptional repression of at least some of its target genes in erythroid cells.

In vivo monitoring of a cAMP-stimulated DNA-binding activity

The transcriptional activity of the tyrosine aminotransferase (TAT) gene is influenced by two major signal transduction pathways, by glucocorticoids and by glucagon acting via cAMP. We analyzed the effect of cAMP on protein-DNA interactions in vivo and on the transcription rate of the TAT gene. We demonstrate that a cAMP-responsive element (CRE) is located in a tissue-specific DNase I-hypersensitive region, 3.6 kb upstream of the start site of transcription. By using the genomic footprinting technique, we show that this sequence is occupied by protein in uninduced cells and that the in vivo footprint is transiently increased upon cAMP induction. Protein binding at the TAT-CRE correlates with the rate of transcription of the TAT gene. Cycloheximide treatment reveals that the genomic footprint is subject to rapid turnover; however, subsequent cAMP induction in the continued presence of cycloheximide restores the footprint partially. We conclude that as a part of the signal transduction pathway, a cAMP-dependent, post-translational modification increases the DNA-binding activity of a protein to the TAT-CRE and thereby stimulates the transcription rate of the TAT gene.

Chromatin structures of the rat tyrosine aminotransferase gene relate to the function of its cis-acting elements

The relationship between DNase I-hypersensitive sites (HSs) and transcriptional enhancers of the rat tyrosine aminotransferase (TAT) gene was examined by comparing HSs in and around the TAT gene with the activity of the corresponding DNA sequences in transient transfection assays. In this manner, we identified two HSs as liver-specific enhancers. Of three hepatoma cell lines examined, only one sustained TAT mRNA levels comparable to those of liver. In this cell line, both enhancers were strongly active, and strong hypersensitivity in chromatin over the enhancers was evident. The other two hepatoma cell lines had reduced levels of TAT mRNA and no or altered hypersensitivity over either the enhancers or the promoter. One of these lines carried a negative regulator of the TAT gene, the tissue specific extinguisher Tse-1. This cell line exhibited all HSs characteristic of the strongly active gene except at the promoter; however, one enhancer was inactive even though hypersensitive in chromatin. In a TAT-nonexpressing cell line, inactivity of both enhancers correlated with absence of the respective HSs. We conclude that although hypersensitivity in chromatin necessarily accompanies cell-type-specific enhancer activity, the occurrence of cell-type-specific HSs does not imply that the underlying sequences harbor enhancers active in transient transfection assays.

Isolation and characterization of the human tyrosine aminotransferase gene

Structure and sequence of the human gene for tyrosine aminotransferase (TAT) was determined by analysis of cDNA and genomic clones. The gene extends over 10.9 kbl and consists of 12 exons giving rise to a 2,754 nucleotide long mRNA (excluding the poly(A)tail). The human TAT gene is predicted to code for a 454 amino acid protein of molecular weight 50,399 dalton. The overall sequence identity within the coding region of the human and the previously characterized rat TAT genes is 87% at the nucleotide and 92% at the protein level. A minor human TAT mRNA results from the use of an alternative polyadenylation signal in the 3' exon which is present but not used at the corresponding position in the rat TAT gene. The non-coding region of the 3' exon contains a complete Alu element which is absent in the rat TAT gene but present in apes and old world monkeys. Two functional glucocorticoid response elements (GREs) reside 2.5 kb upstream of the rat TAT gene. The DNA sequence of the corresponding region of the human TAT gene shows the distal GRE mutated and the proximal GRE replaced by Alu elements.

Chromatin structures of the rat tyrosine aminotransferase gene relate to the function of its cis-acting elements

The relationship between DNase I-hypersensitive sites (HSs) and transcriptional enhancers of the rat tyrosine aminotransferase (TAT) gene was examined by comparing HSs in and around the TAT gene with the activity of the corresponding DNA sequences in transient transfection assays. In this manner, we identified two HSs as liver-specific enhancers. Of three hepatoma cell lines examined, only one sustained TAT mRNA levels comparable to those of liver. In this cell line, both enhancers were strongly active, and strong hypersensitivity in chromatin over the enhancers was evident. The other two hepatoma cell lines had reduced levels of TAT mRNA and no or altered hypersensitivity over either the enhancers or the promoter. One of these lines carried a negative regulator of the TAT gene, the tissue specific extinguisher Tse-1. This cell line exhibited all HSs characteristic of the strongly active gene except at the promoter; however, one enhancer was inactive even though hypersensitive in chromatin. In a TAT-nonexpressing cell line, inactivity of both enhancers correlated with absence of the respective HSs. We conclude that although hypersensitivity in chromatin necessarily accompanies cell-type-specific enhancer activity, the occurrence of cell-type-specific HSs does not imply that the underlying sequences harbor enhancers active in transient transfection assays.

Regulation of cytosolic aspartate aminotransferase mRNAs in the Fao rat hepatoma cell line by dexamethasone, insulin and cyclic AMP

Glucocorticoid hormones increase the activity of cytosolic aspartate aminotransferase (cAspAT) in the Fao rat hepatoma cell line. Maximal increase (6-10-fold) was observed 48 h following the addition of the glucocorticoid agonist dexamethasone at a concentration of 0.1 microM. The effect of dexamethasone was specific since it was not mimicked by sex steroids and was inhibited by the glucocorticoid antagonist RU 486. Insulin (0.1 microM) inhibited by more than 50% the induction of cAspAT by glucocorticoids. The cAMP analog, 8-bromoadenosine 3',5'-monophosphate (Br8cAMP, 0.5 mM), potentiated the effect of dexamethasone (2-3-fold) and partially relieved the inhibitory effect of insulin on the induction by dexamethasone. Both insulin and Br8-cAMP had no significant effect on basal activity. The mitochondrial isoenzyme was insensitive to the various hormonal treatments. Northern blot analysis revealed the presence of two major (2.1-kb and 1.8-kb) and one minor (4-kb) mRNA species hybridizing with a rat cAspAT probe. The regulation of these mRNAs by glucocorticoids, insulin and cAMP correlated with the variation of the cAspAT activity, suggesting that these hormones act at the pretranslational level. We compared the regulation of cAspAT mRNAs with those of tyrosine aminotransferase mRNA. Both were similarly increased by dexamethasone but the latter was also increased by cAMP even in the absence of the glucocorticoid agonist. In addition, the increase in tyrosine aminotransferase mRNA was inhibited by cycloheximide whereas the increase in cAspAT mRNAs was not. These results show that there are significant differences in the regulation of cAspAT and tyrosine aminotransferase by glucocorticoids and other hormones, although both enzymes probably contribute to the same metabolic pathway.

Hepatocyte differentiation in vitro: initiation of tyrosine aminotransferase expression in cultured fetal rat hepatocytes

A fetal rat hepatocyte culture system has been used to study the molecular mechanisms of tyrosine aminotransferase (TAT) gene expression during development. It has previously been shown that TAT activity can be detected in 19-d, but not 15-d, gestation hepatocytes on the first day of culture (Yeoh, G. C. T., F. A. Bennett, and I. T. Oliver. 1979. Biochem. J. 180:153-160). In this study enzyme activity, synthesis, and mRNA levels were determined in hepatocytes isolated from 13-, 15-, and 19-d gestation rats maintained in culture for 1, 2, or 3 d and exposed to dexamethasone. TAT expression is barely detectable in 13-d gestation hepatocytes even after 3 d in culture. Hepatocytes isolated from 15-d gestation fetuses have undetectable levels of enzyme activity and synthesis on the first day of culture; both can be assayed by days 2 and 3. TAT mRNA levels in these hepatocytes, measured by hybridization with a specific cDNA, increase substantially during culture. TAT activity, synthesis, and mRNA are evident on the first and subsequent days of culture in 19-d gestation hepatocytes. Transcription measurements in isolated nuclei indicate that the increase in TAT mRNA in 15- and 19-d gestation hepatocytes is associated with an increase in transcription of the gene. Immunocytochemical studies demonstrated that the increase in TAT expression correlated with an increase in the proportion of hepatocytes expressing the enzyme, rather than a simultaneous increase in all hepatocytes. These results support the proposal that a subpopulation of 15-d fetal hepatocytes undergo differentiation in culture with respect to TAT.

Highly inducible expression from vectors containing multiple GRE's in CHO cells overexpressing the glucocorticoid receptor

A conditional glucocorticoid-responsive expression vector system is described for highly inducible expression of heterologous genes in mammalian cells. This host-vector system requires high level expression of the glucocorticoid receptor (GR) protein in the host cell and multiple copies of the receptor binding site within the expression vector. Transfection and selection of Chinese hamster ovary cells with expression vectors encoding the rat GR yielded cell lines which express functional receptor at high levels. Insertion of multiple copies of the MMTV enhancer (glucocorticoid responsive element, GRE) into an Adenovirus major late promoter (AdMLP) based expression vector yielded greater than 1000-fold inducible expression by dexamethasone (dex) in transient DNA transfection assays. The induced expression level was 7-fold greater than that obtained with an AdMLP based vector containing an SV40 enhancer, but lacking GRE's. Vectors containing the SV40 enhancer in combination with multiple GRE's exhibited elevated basal expression in the absence of dex, but retained inducibility in both transient assays and after integration and amplification in the CHO genome. This expression system should be of general utility for studying gene regulation and for expressing heterologous genes in a regulatable fashion.