Increase in hepatic tyrosine aminotransferase mRNA during enzyme induction by N6,O2'-dibutyryl cyclic AMP

Studies on the posttranscriptional site of cAMP action in the regulation of the synthesis of tyrosine aminotransferase

Synthesis of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) can be induced by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 cell cultures. Actinomycin D fails to block this induction which indicates a target for Bt2cAMP at a posttranscriptional level. We have determined the influence of Bt2cAMP on several translational events during the tyrosine aminotransferase synthesis with the following results. (1) The number of nascent tyrosine aminotransferase chains increased, whereas no effect was measured on the number of nascent total protein chains. (2) The rate of elongation along the tyrosine aminotransferase mRNA and total mRNA is not enhanced by Bt2cAMP. (3) The induced synthesis of tyrosine aminotransferase is more sensitive to the inhibition of elongation. We conclude from our results that Bt2cAMP induces the synthesis of tyrosine aminotransferase by an increase in the rate of initiation on the tyrosine aminotransferase mRNA.

Chemical reverse transformation of CHO-K1 cells induces changes in expression of a candidate tumour suppressor and of a gene not previously characterised as transformation related

Chemical reverse transformation of CHO-K1 and other cells is a well-established phenomenon, in which oncogenically transformed cells re-acquire fibroblastoid morphology, contact inhibition and anchorage-dependent growth, in response to cyclic AMP and other agents. A limited number of changes in gene transcription and enzyme activity have been demonstrated to coincide with these morphological and physiological changes. We have used a partial differential display to identify four genes that are transcriptionally modulated in reverse transformation. One of these, encoding ribosomal protein S18, is transcriptionally suppressed, probably as a result of the detransforming process. Three others are transcriptionally activated. One has homology to NADH-ubiquinone oxidoreductase chain 4 protein, and is also probably changed as a result of the detransforming process. Another is homologous to a human sequence which encodes a 27 kDa protein, p27(BBP/eIF6), that is involved in the biogenesis of 60S ribosomal subunit, and in cell lines of epithelial origin binds to beta integrin. This has not previously been described as transformation-related, and could have a causative role in reverse transformation. The third has homology, with transcriptional or processing variations, to a human genomic sequence, a positional candidate for a tumour suppressor gene, encoding the Krit1 protein which interacts with the Ras-family GTPase Krev-1.

Inhibition of cyclic AMP-dependent induction of ornithine aminotransferase by simple carbohydrates in cultured hepatocytes

Glucose administration inhibits the induction of ornithine aminotransferase (OAT) in both the whole animal and cultured hepatocytes. We have examined the ability of several hexoses and related molecules to inhibit the cAMP-dependent induction of OAT in primary cultures of adult rat hepatocytes. The hexoses (D-glucose, fructose, sorbitol, sorbose, and mannose) that were effective as inhibitors of OAT induction also resulted in accumulation of lactate in the culture medium, although lactate itself was not effective as an inhibitor. The hexoses and related 6-carbon structures (galactose, L-glucose, 2-deoxyglucose, 3-O-methylglucose, rhamnose, mannitol, and inositol) that were not effective as inhibitors of OAT induction did not result in accumulation of lactate in the culture medium. These results suggest that the carbohydrate repression of hepatic OAT requires metabolism of the carbohydrate by the liver cell. Upon addition to the culture medium of several compounds related to carbohydrate metabolism, many (ribose, xylitol, dihydroxyacetone, and glycerol) exhibited an inhibitory effect, with glycerol exhibiting the greatest effect. Fructose and glycerol inhibit OAT induction in the presence of 2-deoxyglucose, suggesting that the inhibitory effect of nonglucose carbohydrates is not occurring through conversion to glucose. The carbon sources observed to be most effective as inhibitors of OAT induction (glycerol, fructose, sorbitol, and sorbose result in more than 90% inhibition at 25 mM) all enter the glycolytic pathway at the triosephosphate level. The mechanism of the inhibitory effect of simple carbohydrates on OAT induction is not known but may involve an increase in certain glycolytic intermediates. Glucose and the related carbon sources exert their effect by inhibiting the cAMP-dependent increase in OAT synthesis. The cAMP-dependent increase in OAT mRNA was inhibited by fructose. These findings suggest that the carbohydrate inhibition of the cAMP-dependent increase in OAT synthesis occurs at a pretranslational level.

Pathway of urokinase-type plasminogen activator induction in the T47D and LLC-PK1 cell lines

Induction of urokinase-type plasminogen activator (uPA) in response to either reagents activating cAMP-dependent protein kinase (cAMP-PK) or the calcium ion phospholipid-dependent kinase (C-kinase) was compared in the LLC-PK1 and T47D cell lines. The two cell lines exhibited quantitatively different responses to calcitonin, to the phosphodiesterase inhibitor isobutylmethylxanthine, and to the adenylate cyclase activator forskolin. Both showed activation of cAMP-PK in response to all these reagents, with T47D cells displaying a greater extent of activation. T47D cells, however, failed to produce uPA in response to calcitonin, forskolin, or the cAMP analog 8-bromo-cAMP, whereas LLC-PK1 cells produced high levels of uPA in response to all these agents. Both cell lines responded to phorbol esters in terms of uPA induction, though to differing extents. Phorbol myristate acetate (PMA) was shown conclusively not to activate cAMP-PK in either cell line, even at concentrations 10-fold higher than those promoting maximal uPA induction. It was concluded that phorbol ester-mediated induction of uPA does not involve cAMP or cAMP-PK activation. These results are discussed in relation to proposed models concerning the role of cAMP-PK in uPA induction.

Regulation of lactate dehydrogenase gene expression by cAMP-dependent protein kinase subunits

The studies described in this report suggest a rather complex, albeit incomplete, sequence of molecular events that we believe form part of the cascade of reactions through which a series of hormones, via cAMP, regulates the expression of specific gene products. The majority of our own studies relate to cAMP-mediated induction of LDH. Some, if not all, of the molecular steps discussed in this paper may ultimately be recognized as part of a universal mechanism by which cAMP controls gene expression in higher eukaryotes. The idea of a functional role for cAMP-dependent protein kinase subunits in cAMP-mediated gene control has already had experimental support, but our identification of the regulatory subunit RII as a topoisomerase now more firmly points to a complex function for the kinase in regulating gene function at the DNA level. We look forward to the elucidation of the function of those nuclear proteins that serve as substrate for the catalytic subunit of cAMP-dependent protein kinase. Further studies related to the molecular interaction of RII with chromosomal DNA should be a fruitful area for future research.

Molecular genetic analysis of cAMP-dependent protein kinase

Genetic evidence has been obtained indicating that the responsiveness of CHO cells to cAMP requires an intact cADepPK system. DNA carrying mutant RI and C subunit genes has been transferred to wild-type cells. Recipient cells carrying the DNA have been detected by selecting for expression of the phenotype for cAMP resistance. We are in the process of cloning a functional mutant RI subunit to use as a moveable genetic element that can confer cAMP resistance on recipient cells. This cloned gene should allow us to test hypotheses concerning the mechanism of cAMP regulation of transcription in mammalian cells.

Insulin enhances transcription of the tyrosine aminotransferase gene in rat liver

The mechanism of insulin-mediated induction of tyrosine aminotransferase in rat liver was investigated using a cloned cDNA probe. The level of aminotransferase mRNA increases about fourfold following administration of the hormone. This induced mRNA accumulation does not require de novo protein synthesis. Nuclear runoff transcription assays in isolated liver nuclei demonstrate that insulin has a rapid and time-dependent stimulatory effect on aminotransferase gene transcription. The magnitude of enhanced transcription can fully account for the increase in the mRNA. We conclude that the induction of tyrosine aminotransferase in rat liver by insulin is primarily a consequence of a selective increase in the rate of transcription of the aminotransferase gene.

Effects of glucagon, dexamethasone and triiodothyronine on phosphoenolpyruvate carboxykinase (GTP) synthesis and mRNA level in rat liver cells

Acute hormonal effects on the synthesis rate of the cytosolic form of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (GTP), were investigated using rat hepatocytes maintained in short-term suspension culture. Cells were pulse-labeled with [3H]leucine or [35S]methionine and the rate of synthesis of phosphoenolpyruvate carboxykinase was estimated after immunoprecipitation of cell extracts with specific antibodies or following high-resolution two-dimensional gel electrophoresis of cell proteins. Total RNA was also extracted from cultured cells and subsequently translated in a wheat germ cell-free protein-synthesis system, in order to quantify the level of functional mRNA coding for phosphoenolpyruvate carboxykinase. Glucagon, the single most effective inducer, causes a 15--20-fold increase in the level of specific mRNA in 2 h, accompanied by a similar increase in enzyme synthesis rate. The extent of induction is further amplified about threefold when dexamethasone is added to the culture medium. The synergistic action of dexamethasone does not require pre-exposure of the cells to the glucocorticoid, but on the contrary occurs without lag upon simultaneous addition of glucagon and dexamethasone. The induction of phosphoenolpyruvate carboxykinase mRNA by glucagon is markedly depressed in hepatocytes inhibited for protein synthesis by cycloheximide. Cycloheximide-inhibited cells, however, display a considerable induction of the message after joint stimulation with dexamethasone and glucagon. Thus, the synergistic action of dexamethasone does not require concomitant protein synthesis. These data provide indirect evidence for a primary effect of the glucocorticoids on the expression of the phosphoenolpyruvate carboxykinase gene. Besides glucagon and dexamethasone, the thyroid hormones are shown to influence the rate of phosphoenolpyruvate carboxykinase synthesis in isolated liver cells. The stimulatory effect of 3,5,3'-triiodothyronine (T3) is best demonstrated as a twofold increase in relative rate of enzyme synthesis in cells supplied with T3 plus glucagon, as compared to cells challenged with glucagon alone. The effect of T3 relies on a pretranslational mechanism, as shown by a commensurate increase in functional mRNA coding for phosphoenolpyruvate carboxykinase. Dose-response experiments with T3 as well as dexamethasone demonstrate effects at very low hormone levels, consistent with a role for these hormones as physiological modulators of phosphoenolpyruvate carboxykinase expression.

Hormonal regulation of plasminogen activator mRNA production in porcine kidney cells

Plasminogen activator (PA) production in LLC-PK1 pig kidney cell culture is induced to high levels by calcitonin and vasopressin, both of which stimulate adenylate cyclase, or by other compounds that also raise intracellular cAMP levels. Enzyme induction is transiently sensitive to inhibition by actinomycin D, suggesting that increased concentrations of cAMP mediate the inducing effects of the hormones by enhancing the transcription of PA-mRNA sequences. We tested this hypothesis by measuring PA-mRNA sequences in the Xenopus oocyte translation system which showed a 15-20-fold enhanced PA-synthesizing capacity when supplied with poly(A)+RNA from induced cells, above that obtained from uninduced cell RNA. Changes in PA-mRNA levels measured by Northern hybridization using cloned PA-specific cDNA gave results that agreed well with those obtained from translation assays. Pretreatment with high concentrations of cycloheximide did not block calcitonin-induced PA-mRNA synthesis, indicating that PA gene activation was a primary transcriptional result of calcitonin stimulation and did not require new protein synthesis.

Development of tyrosine aminotransferase in perinatal rat liver: changes in functional messenger RNA and the role of inducing hormones

Expression of the hepatic enzyme tyrosine aminotransferase was analyzed in the perinatal period of development in the rat, when this expression undergoes significant changes associated with hepatocyte differentiation. In late prenatal liver both enzyme and functional mRNA gene products are present at levels 10- to 15-fold below those in the fully differentiated adult liver. This low level of expression in fetal liver is refractory to induction by glucocorticoids, but both gene products are increased to a limited extent by cyclic AMP. This induction by cyclic AMP (cAMP) does not confer glucocorticoid-responsiveness on expression. By 3 hr after birth both functional mRNA and enzyme levels are significantly increased, an increase which continues until a peak is reached at 12 hr that is appreciably above the adult levels. Both gene products then decline until adult levels are reached by 24 hr. The postnatal shift in aminotransferase expression is accompanied by acquisition of the capacity to respond to glucocorticoids. Treatment of newborns with an antiglucocorticoid steroid or with glucose suppresses the postnatal overshoot of expression, but neither treatment affects the increase from fetal to adult levels of expression. The results indicate that prior to birth, expression of the aminotransferase gene is partially repressed, a repression that is lifted essentially immediately upon birth. The hormones capable of inducing aminotransferase synthesis have no apparent necessary role in this process.

On the mechanism of a radiation-induced change in enzymic differentiation during development

In experiments on glucose-6-phosphatase and tyrosine aminotransferase it was shown that radiation induces changes in enzymic differentiation in perinatal rat liver. A study was made of the probable reasons for the observed changes. It was shown that the macromolecular system of the protein enzyme synthesis was not damaged by the radiation doses used. The observed decrease in glucose-6-phosphatase activity during late embryogenesis, after pre-irradiation at early organogenesis, is eliminated by administration of exogenous thyroxine. A radiation-induced rise in the tyrosine aminotransferase activity during the perinatal period correlated with the cyclic AMP system status. It is proposed that modification of enzymic differentiation after irradiation results from the change in the amount of inductors.

The molecules that initiate cardiac hypertrophy are not species-specific

Extracts from hypertrophying dog hearts perfused through isolated rat hearts increase the synthesis of messenger RNA and initiate hypertrophy in the treated hearts. Total RNA extracted from experimental and control hearts was translated in vitro and hybridized with polyuridylate. Synthesis of protein and polyadenylate-containing RNA was greater in rat hearts perfused with extracts of hypertrophying dog hearts than in control hearts. The results demonstrate that molecules from hypertrophying dog hearts are not species-specific since they are effective in stimulating transcription of messenger RNA in rat hearts as well as in dog hearts.

Correlation of the biosynthesis of prostaglandin and cyclic AMP in monolayer cultures of rabbit articular chondrocytes

We have utilized ionophores to test whether stimulation of chondrocyte prostaglandin biosynthesis is accompanied by an increase in cyclic nucleotide levels in these cells. Radioimmunoassay of prostaglandin E2, 6-oxo-prostaglandin F1 alpha (the stable metabolite of prostaglandin I2) and prostaglandin F2 alpha showed that synthesis of each was stimulated by the divalent-cation ionophore, A23187 after short-term incubation (1-7 min) in serum-free medium. No stimulation of thromboxane B2 was detected. Two monovalent ionophores, lasalocid and monensin failed to stimulate prostaglandin biosynthesis after short-term incubation. Ionophore A23187-stimulated prostaglandin biosynthesis was variably and partially inhibited by sodium meclofenamate, indomethacin and aspirin, but not by sodium salicylate. Ionophore A23187-stimulated prostaglandin biosynthesis was accompanied by a 7.5-fold increase in cyclic AMP levels after 15 min. Sodium meclofenamate, indomethacin and aspirin which inhibited prostaglandin E2 biosynthesis also reduced cyclic AMP levels. Exogenous prostaglandin E2 (1 microgram/ml) stimulated cyclic AMP biosynthesis, which was not inhibited by aspirin. These results indicated that prostaglandins can be considered as one of the local effectors controlling cyclic AMP production in articular cartilage.

Positive and negative cAMP-mediated control of tyrosine aminotransferase synthesis in Reuber H35 hepatoma cells

Induction of L-tyrosine:2-oxoglutarate aminotransferase (EC 2.6.1.5) by N6,O2'-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP) in Reuber H35 hepatoma cells reaches a maximum value between 3-5 h after addition of Bt2cAMP and subsequently decreases in the continuous presence of Bt2cAMP. We have investigated the kinetics of the increase, i.e. induction, and the decrease, i.e. the repressed state, of the tyrosine-aminotransferase-synthesizing system under these conditions. Our experimental results are as follows. 1. The repressed state of the tyrosine-aminotransferase-synthesizing system is not caused by a decrease in the intracellular cAMP concentration. 2. The repressed state is inhibited by actinomycin D (while induction is not inhibited). 3. During the repressed state no effect of dexamethasone on tyrosine aminotransferase synthesis is found, while during induction Bt2cAMP and dexamethasone act synergistically. 4. Longer starvation of the cells in serum-free medium has no influence on the kinetics of the induction/repressed state curve. From these results we have concluded that the mechanism of the transition to the repressed state of the tyrosine-aminotransferase-synthesizing system is essentially different from the mechanism of deinduction which occurs after removal of the inducer. Moreover, the repressed state of the system is a phenomenon which is induced by Bt2cAMP separately from induction at a different level of protein synthesis.

Cyclic AMP mediated modulation of complement protein production

We have investigated the mechanisms by which cAMP analogues and phosphodiesterase inhibitors, reduced the production of C2 by monocytes in culture. Pulse label studies with 3H-labelled aminoacids showed that dibutyryl cAMP (dbcAMP) impaired the secretion of newly synthesised protein, both total (acid-precipitable) and individual complement proteins (precipitated antibody by antisera to C4, C2, C3, C5, B, P, C3b inactivator and beta 1H). The intracellular degradation of newly synthesised protein was increased in dbcAMP-treated cultures and protein synthesis was reduced. Studies aimed at defining the temporal relationships between these changes showed that protein secretion was impaired on the first day of culture, and increased degradation of newly synthesised protein was obvious by day 2. Protein synthesis was not significantly reduced until day 3 of culture. It is proposed that changes in intracellular cAMP levels may act as a second signal in the control of protein production by monocytes.

The regulation of hepatic tyrosine aminotransferase

Tyrosine aminotransferase induction has been studied in hepatocytes from untreated, partially and fully glucocorticoid-induced rats: enzyme activities were initially 12.9 +/- 1.7 (n = 16), 41.4 +/- 3.2 (n = 6) and 117.9 +/- 10.5 (n = 7) munits/mg protein, respectively. Untreated or fully induced hepatocytes maintain initial levels, whereas partially induced hepatocytes increase their tyrosine aminotransferase activity even in the presence of actinomycin D. Fully induced hepatocytes possess a normal protein synthetizing machinery and the mechanisms to degrade selectively tyrosine aminotransferase. The effect of progesterone treatment is consistent with these cells retaining a high dexamethasone level. Glucagon induces tyrosine aminotransferase via its second messenger, cyclic AMP. This induction decreases dramatically with in vivo glucocorticoid treatment. Time courses and effects of inhibitors are consistent with these in vivo and in vitro treatments being alternative methods of inducing tyrosine aminotransferase by the same basic pretranslational step.

Further evidence for translational regulation of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP in Reuber H35 hepatoma cells

Cyclic AMP derivatives increase the rate of synthesis of tyrosine aminotransferase in Reuber H35 hepatoma cells. Various studies lend support to the hypothesis that cyclic AMP increases the synthesis of tyrosine aminotransferase by acting at a posttranscriptional site. The presence of a limited non-translatable pool of tyrosine aminotransferase mRNA prior to the formation of the translatable tyrosine aminotransferase mRNA implicates a possible site of action of cyclic AMP. We compared the capacity of N6,O2'-dibutyryl cyclic AMP to induce tyrosine aminotransferase synthesis when untranslatable tyrosine aminotransferase mRNA sequences are present or absent. The transition of a condition in which non-translatable tyrosine aminotransferase mRNA sequences were present to a condition in which they were absent was established by preinduction of Reuber H35 cells with dexamethasone, followed by addition of actinomycin D. In the time period thereafter, the amount of non-translatable mRNA decreased and 1.5-2 h after addition of actinomycin D, only translatable tyrosine aminotransferase mRNA was present. It can be seen that the induction of tyrosine aminotransferase synthesis by dibutyryl cyclic AMP follows the normal decrease of tyrosine aminotransferase mRNA. We present evidence that dibutyryl cyclic AMP in Reuber H35 hepatoma cells regulates tyrosine aminotransferase synthesis at a posttranscriptional site independent of the pool of non-translatable tyrosine aminotransferase mRNA sequences, but influencing the efficiency of translation of active tyrosine aminotransferase mRNA.

Protein synthesis with membrane-bound polysomes and albumin messenger RNA from livers of mutant mice

Investigation of deficiencies in serum protein synthesis resulting from deletion-mutations at the albino locus in mice was continued using in vitro conditions. Previous work showed that although total protein synthesis was only slightly lower in livers from albinos, newly synthesized protein appearing in plasma was 22% of that in controls. It was thought that the disorganized endoplasmic reticulum and Golgi apparatus, characteristic for the liver (and kidney) of these mutants, might be responsible for the observed deficiencies. In the present study membrane-bound polysomes isolated from the livers of newborn albinos were 55% (c3H/c3H strain) and 62% (c14CoS strain) as efficient as those from normal littermates in incorporating radioactive leucine into protein in a cell-free system. These differences could not be eliminated by the addition of excess liver mRNA, exogenous soluble factors or by the exchange of cell sap between albino and control polysomes. In an earlier study albino liver slices synthesized only 13% (or 17% per mg of total protein synthesized) as much albumin as controls. We have now found that the level of albumin poly(A)+-RNA isolated from albino livers assayed with a reticulocyte lysate, was almost as high (85%) as in controls. It was concluded that the very low level of albumin in albino livers did not result from a deficiency of albumin mRNA. Whether the rate-limiting step in synthesis of albumin in mutant livers is at the level of translation or processing for secretion requires further investigation.

Factors affecting induction of tyrosine aminotransferase in isolated rat liver cells

1. The response of tyrosine aminotransferase activity in isolated liver cells has been studied under several conditions. 2. Activity is increased over a 5 h period by both glucagon and glucocorticoids in cells from adrenalectomized rats. The results do not support the view that glucagon action is dependent on preexposure of cells to steroid. 3. In cells from fed animals, significant stimulation is seen only when both glucagon and steroid are present together. 4. In cells from 48 h fasted rats steroid is effective, but glucagon is not significant so. 5. These anomalies are attributed to the differences in hormonal and nutritional status between the animals from which the cells are isolated.

Translation of poly-A RNA from rat liver in vitro. Evidence for a high molecular weight subunit of tyrosine aminotransferase

Poly-A RNA extracted from the rat liver was translated in a cell-free wheat germ system and a rabbit reticulocyte lysate. The subunit of tryptophan pyrrolase precipitated by specific antiserum after synthesis in vitro has the same molecular weight as the corresponding subunit derived from the rat liver. With specific antiserum prepared against tyrosine aminotransferase, however, a radioactive protein from both the in vitro assays was precipitated with an about 5% higher molecular weight than the tyrosine aminotransferase subunit precipitated from rat liver. The immunological evidence and the comparison of the specific peptide patterns prepared by cyanogen bromide treatment showed that the in vitro product corresponds to tyrosine aminotransferase. Various concentrations of potassium or spermidine used in the wheat germ translation system did not alter the size of the enzyme subunit synthesized. The run of the tyrosine aminotransferase purified form the rat liver in the SDS-polyacrylamide gel electrophoresis was not influenced by treatment with Escherichia coli alkaline phosphatase. The possibility is discussed that the larger enzyme synthesized in vitro represents a precursor molecule which is cleaved proteolytically in vivo.

Regulation of rat-liver tyrosine-aminotransferase mRNA by hydrocortisone and by N6,O2'-dibutyryladenosine 3',5'-phosphate

The intraperitoneal injection of either hydrocortisone of N6,O2'-dibutyryladenosine 3',5'-phosphate (Bt2cAMP) results in a specific increase in functional tyrosine aminotransferase mRNA (mRNATAT) activity in rat liver that is proportional to the degree of enzyme induction. Both require continuous RNA synthesis. There are several differences in the response to these inducers: (a) the magnitude of the increase is greater following hydrocortisone injection than after Bt2cAMP; (b) the peak response is seen within 1 h following the injection of Bt2cAMP as compared to the 5 h required for the maximal response following hydrocortisone injection; (c) finally, although both responses are rapid, the lag period which precedes the accumulation of functional tyrosine aminotransferase mRNA activity following the injection of hydrocortisone is at least 20 min whereas following Bt2cAMP it is 5-10 min. The administration of actinomycin D to rats 5 h after they were treated with hydrocortisone causes an additional twofold increase in tyrosine aminotransferase enzymatic activity, a phenomenon known as superinduction, but does not prevent the normal decrease in its mRNA seen at this time. This dissociation of enzyme and mRNA activities indicates that superinduction of tyrosine aminotransferase is not due to a selective stabilization of the mRNA which codes for this protein.

Relative increase in polysomal mRNA for R1 cAMP-binding protein in neuroblastoma cells treated with 1,N6-dibutyryl-adenosine 3',-5'-phosphate

Polysomal RNAs were isolated from control neuroblastoma cells and those treated with 1,N6-dibutyrl-adenosine 3',5'-phosphate (Bt2cAMP) and translated in wheat germ lysates. Comparison of proteins synthesized in vitro on two-dimensional gel electrophoretograms showed that there was a specific induction in the synthesis of a protein, Mr 48000, by the polysomal RNAs from Bt2cAMP-treated cells. This protein was identified as the R1 cAMP-binding protein by its coelectrophoresis with unlabelled binding protein and by its specific retention on 8-(6-aminohexylamino)-adenosine 3',5'-phosphate linked to Sepharose. Quantification of the proteins synthesized in vitro with subsaturating inputs of polysomal RNAs showed that there was a 1.4--1.7-fold increase in the synthesis of the R1 cAMP-binding protein by polysomal RNAs isolated from Bt2cAMP-treated cells. There was a similar increase when purified polyadenylated mRNA populations were compared. showing there was no change in the ratio of adenylated to nonadenylated mRNAs in the induced mRNA population. There was no corresponding increase in the synthesis of the R2 cAMP-binding protein although the relative synthesis of several other proteins was also increased and the synthesis of actin and the alpha and beta-tubulin subunits was decreased. The increased levels of the R1 cAMP-binding protein found in Bt2cAMP-treated neuroblastoma cells are therefore partly caused by a specific accumulation of its mRNA on polysomes. The mRNA content of the cytoplasmic messenger ribonucleoprotein (mRNP) population of control cells was insufficient to account for this increase by a translocation of R1 mRNA from the mRNP to the polysome fraction in Bt2cAMP-treated cells. The increase in polysomal R1 mRNA is therefore caused by its increased transcription of post-transcriptional processing or its decreased rate of degradation in Bt2cAMP-treated cells. Although the R1 and R2 binding proteins have identical molecular weights and similar pI values, the specific induction of the mRNA for R1 cAMP-binding protein and the differential distribution of the R1 and R2 mRNAs between the polysomal and messenger ribonucleoprotein compartments show that these two cAMP-binding proteins are encoded by different mRNA populations.

Primary monolayer culture of liver parenchymal cells and kidney cortical tubules as a useful new model for biochemical pharmacology and experimental toxicology. Studies in vitro on hepatic membrane transport, induction of liver enzymes, and adaptive changes in renal cortical enzymes

Freshly isolated liver parenchymal cells were maintained in either short-term monolayer, suspension of long-term monolayer culture. Rapidly occurring processes through hepatocellular membrane, e.g., the enhanced amino acid transport and the concomitantly increased potassium influx following progressive starvation, were kinetically evaluated best in short-term monolayer culture. The inducibility of tyrosine aminotransferase by glucagon, dexamethasone, and a combination of both was compared in suspension and in monolayer culture. The induction of slowly inducible foreign compound-metabolizing enzymes, (e.g., ethoxycoumarin-O-dealkylase, p-nitroanisole-O-demethylase, and UDP-glucuronyltransferase) by phenobarbital, 3-methylcholanthrene, and dexamethasone were studied in long-term monolayer culture. The latter system was also used to maintain isolated kidney cortical tubules for the investigation of renal enzyme adaptation during progressive time in culture.

Acute ethanol effects on rat liver tryptophan oxygenase and tyrosine aminotransferase

In starved rats, ethanol administered acutely enhances tryptophan oxygenase (TO) and tyrosine aminotransferase (TAT) activities. Ethanol also inhibits the early phase of the cortisol-mediated TO and TAT induction. Ethanol administered at the same time as tryptophan does not modify the tryptophan-mediated TO and TAT induction. In cortisol-pretreated rats, ethanol enhances the subsequent TO and TAT induction whereas no additive effects are observed when ethanol is injected together with tryptophan. These results suggest that ethanol mimics the effects of tryptophan on TO and TAT activities. In fed animals, ethanol alone does not result in increased TO and TAT activities, but inhibits their cortisol induction. It increases TO activities when given together with a tryptophan dose which, when given alone, does not enhance these activities. It is suggested that the observed inhibitory effects of ethanol on cortisol-mediated TO and TAT induction in starved and fed animals are related to a defective cortisol transport in the liver cells.