Isolation of cDNA clones coding for rat tyrosine aminotransferase

Identification of marker genes for lipid-lowering effect of a short-chain fructooligosaccharide by DNA microarray analysis

Administration of short-chain fructooligosaccharide (scFOS) is known to lower serum triglyceride levels in rats fed a high-fat diet, but the molecular mechanisms remain unclear. This study aimed to identify marker genes for lipid-lowering effect of scFOS administration. The changes in hepatic gene expressions in rats fed scFOS were investigated using DNA microarray and quantitative RT-PCR analysis. The DNA microarray showed that phytanoyl-CoA 2-hydroxylase 2 (Phyh2), lipoprotein lipase (Lpl) and tyrosine aminotransferase (Tat) were significantly affected by scFOS administration (p < .05). Since Lpl is involved in lipid metabolism, the up-regulation of Lpl in the liver can be a potential marker of the lipid-lowering effect of scFOS.

Comparison of the tyrosine aminotransferase cDNA and genomic DNA sequences of normal mink and mink affected with tyrosinemia type II

Type II tyrosinemia, designated Richner-Hanhart syndrome in humans, is a hereditary metabolic disorder with autosomal recessive inheritance characterized by a deficiency of tyrosine aminotransferase activity. Mutations occur in the human tyrosine aminotransferase gene, resulting in high levels of tyrosine and disease. Type II tyrosinemia occurs in mink, and our hypothesis was that it would also be associated with mutation(s) in the tyrosine aminotransferase gene. Therefore, the transcribed cDNA and the genomic tyrosine aminotransferase gene were sequenced from normal and affected mink. The gene extended over 11.9 kb and had 12 exons coding for a predicted 454-amino-acid protein with 93% homology with human tyrosine aminotransferase. FISH analysis mapped the gene to chromosome 8 using the Mandahl and Fredga (1975) nomenclature and chromosome 5 using the Christensen et al. (1996) nomenclature. The hypothesis was rejected because sequence analysis disclosed no mutations in either cDNA or introns that were associated with affected mink. This suggests that an unlinked gene regulatory mutation may be the cause of tyrosinemia in mink.

Generation and characterization of p53 null transformed hepatic progenitor cells: oval cells give rise to hepatocellular carcinoma

Oval cells are bipotential liver stem cells able to differentiate into hepatocytes and bile duct epithelia. In normal adult liver oval cells are quiescent, existing in low numbers around the periportal region, and proliferate following severe, prolonged liver trauma. There is evidence implicating oval cells in the development of hepatocellular carcinoma, and hence the availability of an immortalized oval cell line would be invaluable for the study of liver cell lineage differentiation and carcinogenesis. A novel approach in the generation of cell lines is the use of the p53 knockout mouse. Absence of p53 allows a cell to cycle past the normal Hayflick limit, rendering it immortalized, although subsequent genetic alterations are thought necessary for transformation. p53 knockout mice were fed a choline-deficient, ethionine-supplemented diet, previously shown to increase oval cell numbers in wild-type mice. The oval cells were isolated by centrifugal elutriation and maintained in culture. Colonies of hepatic cells were isolated and characterized with respect to phenotype, growth characteristics and tumorigenicity. Analysis of gene expression by Northern blotting and immunocytochemistry suggests they are oval-like cells by virtue of albumin and transferrin expression, as well as the oval cell markers alpha fetoprotein, M(2)-pyruvate kinase and A6. Injection into athymic nude mice shows the cell lines are capable of forming tumors which phenotypically resemble hepatocellular carcinoma. Thus, the use of p53 null hepatic cells successfully generated immortalized and tumorigenic hepatic stem cell lines. The results presented support the idea that deleting p53 allows immortalization and contributes to the transformation of the oval-like cell lines. Further, the tumorigenic status of the cell lines is direct evidence for the participation of oval cells in the formation of hepatocellular carcinoma.

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.

Distribution of tyrosine aminotransferase activity in mink (Mustela vison)

The distribution of the enzyme tyrosine aminotransferase in tissues of mink, Mustela vison, was investigated. High levels of enzymatic activity were detected only in liver, documenting the hepatic-specific nature of this enzyme in this species. Further studies disclosed that tyrosine aminotransferase is not absent from non-hepatic tissues because of the lack of the use of a stabilized buffer, sensitivity to temperature, or due to the presence of an inhibitor. Collectively, these results suggest that the enzymatic assay of tyrosine aminotransferase will be unlikely to be an efficacious approach for identifying mink that are heterozygous for the autosomal recessive deficiency of this enzyme that is common in dark mink.

Hepatocyte nuclear factor 4 expression overcomes repression of the hepatic phenotype in dedifferentiated hepatoma cells

The capacity of the liver-enriched transcription factor hepatocyte nuclear factor 4 (HNF4) to direct redifferentiation of dedifferentiated rat hepatoma cells was investigated by stable transfection of epitope-tagged HNF4 cDNA into H5 variant cells. HNF4-producing cells expressed the previously silent HNF1 gene and showed activation of some hepatic functions, including alpha1-antitrypsin, beta-fibrinogen, and transthyretin, but not of the endogenous HNF4 gene. Expression of the other hepatocyte-enriched transcription factors was not modified. Treatment of the HNF4tag-expressing cells with dexamethasone induced expression of the transgene by 10-fold, resulting in enhanced expression of target genes of both glucocorticoid hormones and HNF4. The set of activated hepatic genes was extended by treatment of cells with the demethylating agent 5-azacytidine followed by selection in dexamethasone-containing glucose-free medium. Some of the colonies that developed reexpressed the entire set of hepatic functions tested. Fusion of HNF4tag-producing H5 cells with well-differentiated Fao cells showed that only those hybrids which maintained expression of HNF4tag were protected from complete extinction, including that of the Fao HNF4 gene. Thus, H5 cells must produce an extinguisher of the HNF4 gene. In addition, this result implies that HNF4 itself, or its target HNF1, is a positive regulator of HNF4. In conclusion, HNF4tag expression overcomes repression of the hepatic phenotype of the H5 cell without abolishing its potential to extinguish an active genome. Taken together, these results predict that expression of HNF4 should be sufficient to establish heritable expression of many parameters of the hepatic differentiated state.

Molecular sites of regulation of expression of the rat cationic amino acid transporter gene

Cat-1 is a protein with a dual function, a high affinity, low capacity cationic amino acid transporter of the y+ system and the receptor for the ecotropic retrovirus. We have suggested that Cat-1 is required in the regenerating liver for the transport of cationic amino acids and polyamines in the late G1 phase, a process that is essential for liver cells to enter mitosis. In our earlier studies we had shown that the cat-1 gene is silent in the quiescent liver but is induced in response to hormones, insulin, and glucocorticoids, and partial hepatectomy. Here we demonstrate that cat-1 is a classic delayed early growth response gene in the regenerating liver, since induction of its expression is sensitive to cycloheximide, indicating that protein synthesis is required. The peak of accumulation of the cat-1 mRNA (9-fold) by 3 h was not associated with increased transcriptional activity of the cat-1 gene in the regenerating liver, indicating post-transcriptional regulation of expression of this gene. Induction of the cat-1 gene results in the accumulation of two mRNA species (7.9 and 3.4 kilobase pairs (kb)). Both mRNAs hybridize with the previously described rat cat-1/2.9-kb cDNA clone. However, the 3' end of a longer rat cat-1 cDNA (rat cat-1/6.5-kb) hybridizes only to the 7.9-kb mRNA transcript. Sequence analysis of this clone indicated that the two mRNA species result from the use of alternative polyadenylation signals. The 6. 5-kb clone contains a number of AT-rich mRNA destabilizing sequences which is reflected in the half-life of the cat-1 mRNAs (90 min for 7. 9-kb mRNA and 250 min for 3.4-kb mRNA). Treatment of rats with cycloheximide superinduces the level of the 7.9-kb cat-1 mRNA in the kidney, spleen, and brain, but not in the liver, suggesting that cell type-specific labile factors are involved in its regulation. We conclude that the need for protein synthesis for induction of the cat-1 mRNA, the short lived nature of the mRNAs, and the multiple sites for regulation of gene expression indicate a tight control of expression of the cat-1 gene within the regenerating liver and suggest that y+ cationic amino acid transport in liver cells is regulated at the molecular level.

Genetic analysis of a transcriptional activation pathway by using hepatoma cell variants

A hierarchy of liver-enriched transcription factors plays an important role in activating expression of many hepatic genes. In particular, hepatocyte nuclear factor 4 (HNF-4) is a major activator of the gene encoding HNF-1, and HNF-1 itself activates expression of more than 20 liver genes. To dissect this activation pathway genetically, we prepared somatic cell variants that were deficient in expression of the liver-specific alpha 1-antitrypsin (alpha 1AT) gene, which requires both HNF-1 and HNF-4 for high-level gene activity. This was accomplished in two steps. First, hepatoma transfectants that stably expressed two selectable markers under alpha 1AT promoter control were prepared; second, variant sublines that could no longer express either transgene were isolated by direct selection. In this report, we demonstrate that the variants contain defects in the HNF-4/HNF-1 activation pathway. These defects functioned in trans, as expression of many liver genes was affected, but the variant phenotypes were recessive to wild type in somatic cell hybrids. Three different variant classes could be discriminated by their phenotypic responses to ectopic expression of either HNF-4 or HNF-1. Two variant clones appeared specifically deficient in HNF-4 expression, as transfection with an HNF-4 expression cassette fully restored their hepatic phenotypes. Another line activated HNF-1 in response to forced HNF-4 expression, but activation of downstream genes failed to occur. One clone was unresponsive to either HNF-1 or HNF-4. Using the variants, we demonstrate further that the chromosomal genes encoding alpha 1AT, aldolase B, and alpha-fibrinogen display strict requirements for HNF-1 activation in vivo, while other liver genes were unaffected by the presence or absence of HNF-1 or HNF-4. We also provide evidence for the existence of an autoregulatory loop in which HNF-1 regulates its own expression through activation of HNF-4.

Genetic analysis of a transcriptional activation pathway by using hepatoma cell variants

A hierarchy of liver-enriched transcription factors plays an important role in activating expression of many hepatic genes. In particular, hepatocyte nuclear factor 4 (HNF-4) is a major activator of the gene encoding HNF-1, and HNF-1 itself activates expression of more than 20 liver genes. To dissect this activation pathway genetically, we prepared somatic cell variants that were deficient in expression of the liver-specific alpha 1-antitrypsin (alpha 1AT) gene, which requires both HNF-1 and HNF-4 for high-level gene activity. This was accomplished in two steps. First, hepatoma transfectants that stably expressed two selectable markers under alpha 1AT promoter control were prepared; second, variant sublines that could no longer express either transgene were isolated by direct selection. In this report, we demonstrate that the variants contain defects in the HNF-4/HNF-1 activation pathway. These defects functioned in trans, as expression of many liver genes was affected, but the variant phenotypes were recessive to wild type in somatic cell hybrids. Three different variant classes could be discriminated by their phenotypic responses to ectopic expression of either HNF-4 or HNF-1. Two variant clones appeared specifically deficient in HNF-4 expression, as transfection with an HNF-4 expression cassette fully restored their hepatic phenotypes. Another line activated HNF-1 in response to forced HNF-4 expression, but activation of downstream genes failed to occur. One clone was unresponsive to either HNF-1 or HNF-4. Using the variants, we demonstrate further that the chromosomal genes encoding alpha 1AT, aldolase B, and alpha-fibrinogen display strict requirements for HNF-1 activation in vivo, while other liver genes were unaffected by the presence or absence of HNF-1 or HNF-4. We also provide evidence for the existence of an autoregulatory loop in which HNF-1 regulates its own expression through activation of HNF-4.

Retinoids regulate expression of the retinol-binding protein gene in hepatoma cells in culture

The expression of the gene coding for retinol-binding protein has been studied in a system of cultured human hepatoma cells exposed to retinoids. We report that the gene is positively modulated by retinol and retinoic acid in a time- and dose-dependent fashion. The stimulation at the mRNA level is paralleled by an increase of the corresponding protein that is secreted in the presence of the physiological ligand. An RBP-CAT chimeric gene, introduced by transfection, is also responsive to the treatment, showing the gene dose-dependency as the endogenous gene. These results demonstrate that retinoids up-regulate the RBP gene and that the control takes place at transcriptional level.

Tissue-specific extinguisher loci in the murine genome: a screening study based on a rat/mouse microcell hybrid panel

Extinction of tissue-specific traits in intertypic somatic cell hybrids is a well-known phenomenon. In the past few years, microcell hybrids have been used in attempts to dissect this phenotype genetically, and tissue-specific extinguisher loci have been mapped to two different mouse chromosomes. When transferred from fibroblast into hepatoma cells by microcell fusion, these loci down-regulate expression of specific liver genes in trans. However, other liver genes that are extinguished in genotypically complete hybrids seem not to be extinguished in monochromosomal hybrids. To assess the generality of monochromosomal extinction phenotypes, we assembled a collection of rat hepatoma/mouse fibroblast microcell hybrids that represent most of the mouse chromosome complement, and we screened them for expression of a large number of liver-specific genes. Phosphoenolpyruvate carboxykinase gene expression was down-regulated in hybrids containing mouse chromosome 7 or mouse chromosome 11, but other extinction phenotypes were not readily apparent. These results indicate that extinction of many liver genes may be a polygenic trait.

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.

Hormonal regulation of the gene for the type C ecotropic retrovirus receptor in rat liver cells

The infectibility of the regenerating rat liver by ecotropic retroviruses was studied relative to the expression of the gene coding for the ecotropic retrovirus receptor (Ecor) that functions as a cationic amino acid transporter. It is known that the gene for the receptor is expressed in primary hepatocytes and hepatoma cells but is absent in adult liver cells. Isolation of a 2.85-kb cDNA for the rat Ecor suggested that the rat viral receptor is 97% homologous to the mouse viral receptor and that it contains the envelope-binding domain that determines the host range of ecotropic murine retroviruses. This explains the efficient infection of rat cells by ecotropic retroviruses. Since cell division is required for liver cells to be infected, we determined the susceptibility of the regenerating rat liver to infection at different time points after partial hepatectomy (0 to 24 h) in relation to the presence of receptor mRNA. Infection of the liver occurred only when the liver was exposed to virus 4 h after partial hepatectomy. This time course of infection paralleled expression of the gene for the Ecor, which was rapidly induced between 2 and 6 h during liver regeneration. However, expression of the dormant receptor gene in quiescent liver cells can be induced by insulin, dexamethasone, and arginine, indicating that cell division is not required for expression of the receptor gene in liver cells. A diet high in carbohydrate (low in protein) significantly increased the concentration of receptor mRNA in liver cells, indicating that hormones play a role in the regulation of expression of this gene in vivo. We conclude that the gene for the viral receptor is expressed in the regenerating and quiescent liver when the urea cycle enzymes are down regulated. The infection of the regenerating rat liver by ecotropic retroviruses at the time point of expression of the receptor gene supports the requirement of expression of this transporter for infection.

Phorbol esters inhibit the glucocorticoid-mediated stimulation of cytosolic aspartate aminotransferase gene transcription

The regulation of cytosolic aspartate aminotransferase (cAspAT) gene expression by phorbol esters was investigated in the highly differentiated hepatoma cell line Fao. Phorbol 12,13-dibutyrate (PdBu) had no effect on basal activity but partially inhibited the induction of cAspAT by dexamethasone. The extent of inhibition (40%) was similar to that obtained with insulin or vanadate. The inhibitory effects of PdBu and vanadate were additive. In the case of PdBu, the inhibitory effects could be eliminated by first incubating the cells with PdBu, which down-regulates protein kinase C. In contrast, inhibition by insulin was not modified by this treatment. The molecular mechanism of PdBu action was investigated. Northern blot analysis showed that the steady-state mRNA levels of cAspAT were decreased by PdBu in the presence of dexamethasone. In addition, the transcription rate, as measured by run-on experiments, was also decreased under the same conditions. Finally, a 2.4 kb promoter fragment driving the chloramphenicol acetyltransferase gene was stably transfected into the Fao cells. The regulation of the activity of this promoter fragment by dexamethasone and PdBu was similar to the regulation of the endogenous cAspAT activity. We conclude that PdBu acts by regulating the promoter activity of the cAsPAT gene.

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.

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.

Increase of beta-actin mRNA upon hypotonic perfusion of perfused rat liver

beta-Actin mRNA levels in livers exposed to hypotonic perfusion (from 305 to 225 mosmol/l) for one hour are increased 2-fold relative to albumin mRNA. Like albumin, glyceraldehyde-3-phosphate dehydrogenase and tyrosine aminotransferase mRNAs remain at the levels observed under normotonic conditions. The increase in beta-actin mRNA is interpreted as a cytoskeletal response due to cell swelling.

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.

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.

Precocious induction of tyrosine aminotransferase mRNA by hydrocortisone in cultured fetal rat hepatocytes at different developmental stages

Tyrosine-aminotransferase (TAT) is encoded by a liver-specific gene known to be expressed perinatally. Fetal rat hepatocytes (gestation day 19) in primary cultures, in which TAT gene expression is normally undetectable, are induced by hydrocortisone to express TAT-mRNA in a dose-dependent manner (greater than 10(-7) M). In hepatocytes incubated with hydrocortisone, TAT-mRNA levels were marginal after 24 hours, reaching maximal levels at 48 hours. After a pre-incubation of hepatocytes for 24 hours in the absence of hydrocortisone followed by exposure to hydrocortisone (24-48 hours). TAT-mRNA levels were high. Hepatocytes derived from fetuses of gestation days 14 and 17 displayed comparable levels of TAT-mRNA in response to hydrocortisone. These results demonstrate that cultured hepatocytes of gestational stages as early as day 14, which initially do not respond to hydrocortisone by TAT gene induction, undergo a "maturation" process during the initial 24 hours following cultivation, resulting in the acquisition of precocious competence for TAT gene transcription in response to hydrocortisone. This suggests that one or more factor(s), required for hydrocortisone-inducible TAT gene transcription, and not available in fetal liver until birth (Gluecksohn-Waelsch: Cell, 18:225-237, 1979) appear in fetal hepatocytes upon cultivation during this "maturation" period, thus permitting precocious TAT gene expression in vitro.

Predominant periportal expression of the phosphoenolpyruvate carboxykinase and tyrosine aminotransferase genes in rat liver. Dynamics during the daily feeding rhythm and starvation-refeeding cycle demonstrated by in situ hybridization

The zonal distribution of phosphoenolpyruvate carboxykinase (PCK) and tyrosine aminotransferase (TAT) mRNA in liver was studied by in situ hybridization with radiolabelled cRNA probes and the abundance of PCK and TAT mRNA was quantified by Northern blot analysis of total RNA with biotinylated cRNA probes. Livers were taken from rats during a normal 12 h day/night rhythm, when they had access to food only during the dark period from 7 pm to 7 am, or during refeeding, when they had access to food after having been starved for 60 h. 1. Daily feeding rhythm: High levels of PCK mRNA were distributed mainly in the periportal and intermediate zone during the fasting period at noon and 6 pm. Feeding caused a rapid decrease in PCK mRNA level and a restriction of PCK mRNA localization to the periportal area within the first 2 h. No further alterations were observed during the following hours of the feeding period. TAT mRNA was distributed also in the periportal and intermediate zone during the fasting period. Feeding first reduced the mRNA level without changing the distribution pattern. Then towards the end of the feeding period TAT mRNA increased again to half-maximal levels and became restricted mainly to the periportal area. 2. Starvation-refeeding cycle: High amounts of PCK mRNA as well as of TAT mRNA were localized predominantly in the periportal and intermediate zone after 60 h of starvation. PCK and TAT mRNA both decreased markedly during the first 2 h of refeeding and then remained almost constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Content of glucocorticoid receptor and arginase in gastric cancer and normal gastric mucosal tissues

The content of glucocorticoid receptor (GR) and arginase in human gastric cancer and the corresponding normal gastric mucosal tissues was determined. Among the 25 patients studied, the GR content in gastric cancer tissues was 33.2 +/- 10.2 fmol/mg protein versus 7.6 +/- 3.4 fmol/mg protein in gastric mucosal tissues. This difference is statistically significant (P less than 0.005). Of the 25 paired samples, 19 cancer tissues contained GR, whereas only seven of the normal mucosal tissues had GR. The level of arginase in gastric cancer tissues in 19 patients was assayed, it was 26.6 +/- 4.2 ng/mg protein which is also significantly higher than that in normal gastric mucosal tissues (13.5 +/- 1.8 ng/mg protein) (P less than 0.005). Since glucocorticoids and arginase are potent immune suppressive agents, the increased level of GR and arginase in gastric cancer tissue suggest that these glucocorticoid-related factors in gastric cancer tissue may play a partial role in regulating cellular immunity.

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.

Differential activity of a tissue-specific extinguisher locus in hepatic and nonhepatic cells

Tissue-specific extinguisher 1 (Tse-1) is a genetic locus on mouse chromosome 11 that can repress expression of several liver genes in trans. This locus is clearly active in fibroblasts, as hepatoma cells retaining fibroblast chromosome 11 are extinguished for both tyrosine aminotransferase and phosphoenolpyruvate carboxykinase gene expression. To assess the activity of Tse-1 in other tissues, we transferred mouse chromosome 11 from several different cell types into rat hepatoma recipients. Tse-1 was active in nonhepatic cell lines derived from each primary germ layer, but Tse-1 activity was not apparent in hybrids between hepatoma cells and primary mouse hepatocytes. These differences in the genetic activity of murine Tse-1 were apparently heritable in cis.

Biochemical characterization of chromatin fractions isolated from induced and uninduced Friend erythroleukemia cells

Chromatin fractions from Friend erythroleukemia cells after induction of differentiation by dimethylsulfoxide (DMSO) were compared in their biochemical characteristics to fractions from uninduced cells. Fractions were prepared by extracting chromatin from nuclei after mild micrococcal nuclease treatment with increasing concentrations of NaCl according to Sanders. This procedure has been found to release chromatin containing hyperacetylated histones preferentially. The fractions obtained by this procedure were analysed in respect to the amount of chromatin released, the amount of histone H1, the degree of acetylation of histone H4, the presence of non-histone proteins and the concentration of transcribed and non-transcribed sequences. It was found that the fractions differ in the amount of histone H1 present, in several non-histone proteins and in the acetylation of histone H4, regardless whether induced or uninduced cells were analysed. The distribution of transcribed sequences versus non-transcribed sequences among the fractions was the same, demonstrating that this fractionation procedure, although leading to fractions with biochemical differences, is not able to discriminate functional states of chromatin and that the biochemical characteristics of the fractions may be common to both, active as well as inactive states of chromatin.

Glucocorticoid- and progesterone-specific effects are determined by differential expression of the respective hormone receptors

Although glucocorticoids and progestins control vastly different physiological processes, the receptors mediating the effects of these hormones interact with the same DNA sequences. Transfer experiments involving synthetic genes and in vitro binding studies have shown that progesterone and glucocorticoid receptors both recognize the same 15-base pair DNA element (TGTACAGGATGTTCT), raising the question of how the two steroids affect gene expression selectively. We considered the possibility that their selectivity arises from either the differential expression of the receptors in target cells or the differential dependence of receptor function on additional transcription factors. To test these alternatives we introduced a progesterone-receptor expression plasmid into the rat hepatoma cell line Fto2B-3 which contains glucocorticoid receptor but is devoid of progesterone receptor. We report that expression of the progesterone receptor in Fto2B-3 cells renders endogenous glucocorticoid-regulated genes inducible by progestins. Our data show that the responsiveness of a cell to external stimuli can be reprogrammed by the expression of a single transcription factor and that differential expression of hormone receptors is at least one mechanism by which steroid-specific gene activation is achieved.

Insulin antagonism of dexamethasone induction of tyrosine aminotransferase in cultured fetal hepatocytes. A correlation between enzyme activity, synthesis, level of messenger RNA and transcription

Previous studies have shown that insulin depresses the induction of tyrosine aminotransferase by glucocorticoids in cultured fetal rat hepatocytes. However, the site at which this inhibitory effect is exerted was not elucidated, since only enzyme activity was determined in such studies. Therefore, the effect of insulin on tyrosine aminotransferase synthesis, the level of its mRNA as well as the rate of transcription of the gene in isolated nuclei have been determined. The results obtained indicate that in cultures exposed to dexamethasone, Bt2cAMP, insulin and combinations of these additives, there is an excellent correlation between the enzyme activity, enzyme synthesis and the level of mRNA. Run-on transcription experiments indicate that the reduction in the level of mRNA by insulin in dexamethasone-supplemented cultures is the result of a diminished rate of gene transcription.

Differential activity of a tissue-specific extinguisher locus in hepatic and nonhepatic cells

Tissue-specific extinguisher 1 (Tse-1) is a genetic locus on mouse chromosome 11 that can repress expression of several liver genes in trans. This locus is clearly active in fibroblasts, as hepatoma cells retaining fibroblast chromosome 11 are extinguished for both tyrosine aminotransferase and phosphoenolpyruvate carboxykinase gene expression. To assess the activity of Tse-1 in other tissues, we transferred mouse chromosome 11 from several different cell types into rat hepatoma recipients. Tse-1 was active in nonhepatic cell lines derived from each primary germ layer, but Tse-1 activity was not apparent in hybrids between hepatoma cells and primary mouse hepatocytes. These differences in the genetic activity of murine Tse-1 were apparently heritable in cis.

Conditional enhancement of liver-specific gene transcription

We sought to develop a cell line in which liver-specific transcription could be induced at will, to facilitate the study of factors that cause hepatocyte-specific transcription of the serum albumin gene in mice. We therefore created the H2.35 cell line from mouse hepatocytes infected with a temperature-sensitive strain of simian virus 40. During routine propagation at the permissive temperature, H2.35 cells exhibit extremely low levels of albumin transcription and mRNA. Albumin mRNA increases at least 100-fold when H2.35 cells are cultured at the restrictive temperature and in serum-free medium on a collagen substratum; the two latter conditions maintain the differentiated state of primary hepatocyte cultures. Although a major cause of the mRNA increase is posttranscriptional, the transcription rates of albumin and other liver-specific genes increase significantly. Transient-transfection experiments demonstrated that an induction of transcription is caused by activation of an albumin upstream sequence that was previously shown to enhance liver-specific transcription in transgenic mice. Thus, hepatocyte differentiation appears to be maintained in part by extracellular signals that stimulate the activity of a tissue-specific enhancer element.

mRNA-decapping enzyme from Saccharomyces cerevisiae: purification and unique specificity for long RNA chains

An enzyme that hydrolyzes one PPi bond of the cap structure of mRNA, yielding m7GDP and 5'-p RNA was purified from Saccharomyces cerevisiae to a stage suitable for characterization. The specificity of the enzyme was studied, using both yeast mRNA and synthetic RNAs labeled in the cap structure. A synthetic capped RNA (540 nucleotides) was not reduced in size, while as much as 80% was decapped. Yeast mRNA treated with high concentrations of RNase A, nuclease P1, or micrococcal nuclease was inactive as a substrate. The use of synthetic capped RNAs of different sizes (50 to 540 nucleotides) as substrates showed that the larger RNA can be a better substrate by as much as 10-fold. GpppG-RNA was hydrolyzed at a rate similar to that at which 5'-triphosphate end group were not hydrolyzed.

Glucocorticoid receptor-dependent inhibition of cellular proliferation in dexamethasone-resistant and hypersensitive rat hepatoma cell variants

Exposure of the Fu5 rat hepatoma cell line to glucocorticoids, such as dexamethasone and hydrocortisone, suppressed the growth rate and final density of cells grown in the presence of serum. This hormonal effect was proportional to receptor occupancy and affinity and, in addition, the glucocorticoid antagonist RU38486 prevented this response. Two classes of dexamethasone-resistant variants that failed to be growth inhibited were recovered from ethyl methylsulfonate-mutagenized populations by continuous culture in the presence of 1 microM dexamethasone. The first class, represented by the EDR3 subclone, was completely glucocorticoid unresponsive and failed to express receptor transcripts. The second class, represented by the EDR1, EDR5, and EDR7 subclones, possessed significant levels of glucocorticoid receptor but were only partially glucocorticoid responsive when stimulated with saturating levels of hormone. Introduction of functional glucocorticoid receptor genes into both classes of dexamethasone-resistant variants by a recombinant retrovirus expression vector restored glucocorticoid responsiveness and suppression of cell growth. A hypersensitive variant (BDS1), recovered by bromodeoxyuridine selection, was fully glucocorticoid responsive, and its inhibition of proliferation was more acutely regulated by dexamethasone. Taken together, our results established that the inhibition of proliferation in Fu5 rat hepatoma cells represents a new glucocorticoid response that requires the expression of a functional glucocorticoid receptor.

Efficient one-step selection of hepatoma cell variants of a variety of phenotypes by use of aflatoxin B1

We have developed a method to select for rat hepatoma cells that fail to express hepatocyte-specific functions. Well-differentiated cells descended from the H4IIEC3 hepatoma line express aldrin epoxidase (AE) activity, an indicator of the liver-specific forms of cytochromes P450 and, concurrently, are able to activate the procarcinogen aflatoxin B1 (AFB1) into highly toxic metabolites. Thus, differentiated hepatoma cells are highly sensitive to AFB1, while dedifferentiated derivatives, which fail to express AE activity, are resistant. Exposure of differentiated Fao cells to 10 microM AFB1 for 24 h permits the isolation, at a frequency of 5 x 10(-5), of resistant colonies that exhibit strongly reduced AE activity. Strikingly, various morphological types can be observed. In more than 90% of the colonies, cells are morphologically similar to the original differentiated cells and accumulate all liver-specific mRNAs examined in amounts comparable to Fao cells. Moreover, they are able to carry out gluconeogenesis, as judged by their capacity to grow in glucose-free medium. For a minor fraction of colonies, the cells exhibit nonhepatic morphology. These cells fail to express three or more of the liver functions and are not able to proliferate in glucose-free medium. Our results demonstrate that the use of AFB1 constitutes a simple and efficient single-step selective method for obtaining variant hepatoma cells of a wide variety of phenotypes.

Sodium butyrate preserves aspects of the differentiated phenotype of normal adult rat hepatocytes in culture

We have determined that sodium butyrate and, to a lesser extent, dimethylsulfoxide (DMSO) and 3-aminobenzamide (3-AB) preserve aspects of the differentiated phenotype of primary cultures of adult rat hepatocytes. The histone deacetylase inhibitor, butyrate, inhibits the increase in gamma-glutamyltranspeptidase (GGT) activity and the decrease in basal tyrosine aminotransferase (TAT) activity normally observed when hepatocytes are cultured under appropriate conditions. The effects of butyrate on GGT and TAT activities are accompanied by parallel changes in GGT and TAT mRNA levels. The poly(ADP)ribose-synthetase inhibitor, 3-aminobenzamide, has effects similar to butyrate on GGT activity and mRNA levels, while both 3-AB and DMSO increase basal TAT activity in cultured hepatocytes. Under appropriate conditions all three agents--butyrate, 3-AB, and DMSO--extend the length of time cultured hepatocytes can be maintained as confluent monolayers. However, under all the conditions studied, butyrate extended the length of time hepatocytes could be maintained as monolayers more than any other treatment used. Butyrate-treated hepatocytes maintained ultrastructural features that were more similar to those of hepatocytes in vivo than hepatocytes treated with any other of the agents tested. Histone acetylation levels of primary cultures of adult rat hepatocytes declined concomitant with the loss of the differentiated phenotype of the cells. These results suggest that histone acetylation may play a role in the changes in gene expression observed when hepatocytes are placed in culture.

Gene-specific acquisition of hormonal responsiveness in rat liver during development

Cloned cDNAs were used in hybridization analyses to assess hormonal responsiveness of two similarly regulated genes in livers of late-term fetal rats. Transcription of the tyrosine aminotransferase gene and of gene 33 (Lee et al.: J Biol Chem 260:16433-16438, 1985) is enhanced by glucocorticoids and by each of the usually antagonistic hormonal agents, insulin and cAMP, in adult liver, and that of both genes is developmentally activated at or just prior to birth. The mRNA of gene 33 was found to be significantly increased by each of the hormonal regulators in livers of fetuses treated in utero. Expression of the nearly silent aminotransferase gene in fetal liver was appreciably increased by cAMP but was refractory to control by either glucocorticoids or insulin; capacity of this gene to respond to insulin was not realized until several days postpartum. The data indicate specificity in the developmental acquisition of the capacity of individual genes to respond to hormonal regulators.

The mouse albumin promoter and a distal upstream site are simultaneously DNase I hypersensitive in liver chromatin and bind similar liver-abundant factors in vitro

In this paper we characterize the chromatin structure and nuclear proteins associated with different transcriptional states of the mouse serum albumin gene. We found the albumin gene to be transcribed in liver at rates 1000-fold or greater than in other tissues tested. We discovered seven DNase I hypersensitive sites encompassing the albumin gene only in liver chromatin, with strong hypersensitivity at the promoter and the enhancer, which is over 10 kb upstream. Using a gel retardation assay, we found a liver nuclear protein, or set of proteins, which binds specifically to DNA of a liver-specific hypersensitive site that maps 3.5 kb upstream, between the promoter and enhancer. Footprinting, heat insensitivity, and binding competition experiments indicate that the protein(s) have characteristics similar to a heat-stable, liver-abundant protein that binds to the albumin promoter and other enhancer and promoter sequences. Finally, we asked whether the liver-specific factors that cause DNase I hypersensitivity in vivo are present concurrently at the various sites in chromatin. We devised a simple new method to reveal that in liver, individual albumin genes are hypersensitive simultaneously at the promoter, the enhancer, and the -3.5-kb site. Thus, transcriptionally active albumin genes appear to contain tissue-abundant factors that are present at three widely spaced points in chromatin, yet at the same point in time. Similar factors binding simultaneously to at least two of these sites could create a specific structure in chromatin required for high-level albumin gene transcription.

mRNA-decapping enzyme from Saccharomyces cerevisiae: purification and unique specificity for long RNA chains

An enzyme that hydrolyzes one PPi bond of the cap structure of mRNA, yielding m7GDP and 5'-p RNA was purified from Saccharomyces cerevisiae to a stage suitable for characterization. The specificity of the enzyme was studied, using both yeast mRNA and synthetic RNAs labeled in the cap structure. A synthetic capped RNA (540 nucleotides) was not reduced in size, while as much as 80% was decapped. Yeast mRNA treated with high concentrations of RNase A, nuclease P1, or micrococcal nuclease was inactive as a substrate. The use of synthetic capped RNAs of different sizes (50 to 540 nucleotides) as substrates showed that the larger RNA can be a better substrate by as much as 10-fold. GpppG-RNA was hydrolyzed at a rate similar to that at which 5'-triphosphate end group were not hydrolyzed.

Glucocorticoid receptor-dependent inhibition of cellular proliferation in dexamethasone-resistant and hypersensitive rat hepatoma cell variants

Exposure of the Fu5 rat hepatoma cell line to glucocorticoids, such as dexamethasone and hydrocortisone, suppressed the growth rate and final density of cells grown in the presence of serum. This hormonal effect was proportional to receptor occupancy and affinity and, in addition, the glucocorticoid antagonist RU38486 prevented this response. Two classes of dexamethasone-resistant variants that failed to be growth inhibited were recovered from ethyl methylsulfonate-mutagenized populations by continuous culture in the presence of 1 microM dexamethasone. The first class, represented by the EDR3 subclone, was completely glucocorticoid unresponsive and failed to express receptor transcripts. The second class, represented by the EDR1, EDR5, and EDR7 subclones, possessed significant levels of glucocorticoid receptor but were only partially glucocorticoid responsive when stimulated with saturating levels of hormone. Introduction of functional glucocorticoid receptor genes into both classes of dexamethasone-resistant variants by a recombinant retrovirus expression vector restored glucocorticoid responsiveness and suppression of cell growth. A hypersensitive variant (BDS1), recovered by bromodeoxyuridine selection, was fully glucocorticoid responsive, and its inhibition of proliferation was more acutely regulated by dexamethasone. Taken together, our results established that the inhibition of proliferation in Fu5 rat hepatoma cells represents a new glucocorticoid response that requires the expression of a functional glucocorticoid receptor.

The induction, desensitization and de-induction of tyrosine aminotransferase by 8-bromo-cyclic AMP in rat hepatoma cells

Addition of 1-3 mM-8-bromo-cyclic AMP to monolayer cultures of H-4 rat hepatoma cells resulted in a rapid but short-lived increase in tyrosine aminotransferase (EC 2.6.1.5) activity. The transient nature of this induction is due to desensitization to 8-bromo-cyclic AMP. Throughout this time course of induction and desensitization, removal of 8-bromo-cyclic AMP resulted in a rapid and significant decrease in tyrosine aminotransferase activity, a process referred to as 'de-induction' in this study. We showed that the changes in tyrosine aminotransferase activity in its induction, desensitization and de-induction by 8-bromo-cyclic AMP were directly attributable to changes in the synthesis rate of the protein, and the amount of translatable and hybridizable mRNA encoding for tyrosine aminotransferase (mRNATAT). We further showed that this desensitization was specific to cyclic AMP. First, only active analogues of cyclic AMP and agents which increased cellular concentrations of cyclic AMP elicited this desensitization. Second, the desensitized cells were refractory only to the effects of 8-bromo-cyclic AMP; dexamethasone and insulin induced the tyrosine aminotransferase activity in the 8-bromo-cyclic AMP-desensitized cells in a manner similar to that of the controls. Studies on the metabolism of 8-bromo-cyclic AMP suggest that neither its degradation nor the accumulation of its primary metabolite, 8-bromoadenosine, played a significant role in modulating the expression of tyrosine aminotransferase during the time course of action of 8-bromo-cyclic AMP. These results provide evidence for a specific pretranslational mode of action of cyclic AMP in the control of tyrosine aminotransferase expression in its desensitization and de-induction, in addition to the early phase of induction.