Changes in hepatic levels of tyrosine aminotransferase messenger RNA during induction by hydrocortisone

Stepping stones in the path of glucocorticoid-driven apoptosis of lymphoid cells

Cumulative work on glucocorticoid (GC) regulation of genes in lymphoid cell cultures has revealed that apoptotic sensitivity to GCs depends on sufficient active GC receptors in the cells. The actions of the ligand-driven GC receptor that lead to apoptosis depend on interactions with other major cell-signaling systems, including the MAPK pathways, the cAMP/PKA pathway, the hedgehog pathway, the mTOR system and the c-myc system. The balance between these systems determines whether a given cell responds to GCs by undergoing apoptosis. A central core of networked genes may be found under GC control in many types of malignant, GC-sensitive cells. The partial core list identified should be tested in clinical cell samples from hematologic malignancies.

The effect of tyrosine-deficient total parenteral nutrition on the synthesis of dihydroxyphenylalanine in neural tissue and the activities of tyrosine and branched-chain aminotransferases

The poor solubility of tyrosine (Tyr) limits the amount of this amino acid in total parenteral nutrition (TPN). In rats maintained on a standard pediatric TPN mixture, plasma and brain concentrations of Tyr are reduced to about 25% of the levels in chow-fed controls. To determine whether these low concentrations of Tyr affect the synthesis of catecholamines in neural tissue, the rate-limiting step (conversion of Tyr to dihydroxyphenylalanine [DOPA]) is studied by administering NSD-1015 to block the pyridoxal phosphate (PLP)-dependent decarboxylation of DOPA. However, in TPN rats, plasma concentrations of Tyr are increased by drug treatment. Because brain Tyr is also increased, these and other experiments using NSD-1015 clearly overestimate the rate of DOPA synthesis for drug-free rats on TPN. Nevertheless, in TPN rats, there is less DOPA in the brain in one experiment and less DOPA in the olfactory bulbs in another, versus control rats. Further examination of the metabolic effects of NSD-1015 reveals that the drug also elevates the concentration of branched-chain amino acids (BCAAs) in the plasma of TPN rats. These findings result from inhibition by NSD-1015 of the PLP-dependent aminotransferases that initiate catabolism of Tyr in the liver and BCAAs in the muscle. Despite the pronounced reduction in plasma Tyr, TPN rats showed a marked increase in the activity of hepatic Tyr aminotransferase compared with chow-fed controls. Conversely, although TPN elevates BCAA concentrations in plasma, the activity of branched-chain aminotransferase (BCAT) in the heart muscle of TPN rats is not different from control values. Different values but the same relationships are seen in drug-free rats.

Evidence that protein kinase C is involved in delta-aminolevulinate synthase expression in rat hepatocytes

There are many factors that regulate the rate of synthesis of delta-aminolevulinate synthase (ALA-S), the enzyme which governs the rate-limiting step in heme biosynthesis. In rat hepatocytes, phenobarbital increases ALA-S gene transcription and dibutyryl cAMP potentiates this induction, whereas insulin and glucose have the opposite effect. The present report provides evidence that protein kinase C (PKC) activation negatively influences ALA-S mRNA levels, as measured by Northern and slot-blot analysis. The addition of 1,2-dioctanoyl-sn-glycerol (DOG) or 12-O-tetradecanoylphorbol 13-acetate (TPA), a PKC activator that mimics diacylglycerol function, to cultures led to a significant decrease of both basal and phenobarbital-induced ALA-S mRNA levels in a dose-dependent manner. This TPA effect depends on the specific activation of PKC because the analog 4 alpha-phorbol 12,13-diacetate, a nonstimulatory PKC phorbol ester, is unable to inhibit ALA-S mRNA. Furthermore, the effect of TPA is blocked by the PKC inhibitors staurosporine and calphostin C. Desensitization of the PKC pathway by prolonged exposure to TPA abolished the subsequent action of the phorbol ester. On the other hand, neither TPA nor DOG modified the half-life of ALA-S mRNA. The study of the combinatorial action of TPA and cAMP revealed that the inhibitory effect of TPA overcomes dibutyryl cAMP induction. Thus, these results indicate that PKC plays an essential role in regulating ALA-S expression, probably at a transcriptional level.

Differential dynamics of receptor down-regulation and tyrosine aminotransferase induction following glucocorticoid treatment

Autoregulation of glucocorticoid receptor (GR) concentration in vivo may be an important determinant of steroid sensitivity. The dynamics of GR regulation were assessed and compared to regulation of tyrosine aminotransferase (TAT) expression in liver tissue taken from rats treated with a single 50 mg/kg i.v. dose of methylprednisolone. Plasma methylprednisolone concentrations were determined by HPLC analysis. Receptor and TAT message levels were determined by quantitative Northern hybridization. Methylprednisolone plasma kinetics showed a half-life of 0.6 h. Receptor occupancy occurred rapidly and cytosolic GR reappeared over 2-12 h. TAT activity rose between 2 and 6 h and then dissipated. Reduction in receptor mRNA levels occurred very rapidly, being detectable by 30 min following steroid administration. A down-regulated steady-state in GR message expression was reached by 2 h post-injection, and was maintained throughout the 18 h examined in this study. Comparison of methylprednisolone kinetics demonstrated that down-regulation was maintained long after drug was eliminated. In contrast, TAT message induction occurred with a sharp peak; maximal induction occurred between 5-6 h and return to baseline at approx. 8-10 h post-induction. This study shows that unlike TAT induction, GR message repression in vivo does not require continual presence of hormone.

Third-generation model for corticosteroid pharmacodynamics: roles of glucocorticoid receptor mRNA and tyrosine aminotransferase mRNA in rat liver

A third-generation pharmacokinetic/pharmacodynamic model was proposed for receptor/gene-mediated corticosteroid effects. The roles of the messenger RNA (mRNA) for the glucocorticoid receptor (GR) in hepatic GR down-regulation and the mRNA for hepatic tyrosine aminotransferase (TAT) induction by methylprednisolone (MPL) were examined. Male adrenalectomized Wistar rats received 50 mg/kg MPL iv. Blood and liver samples were collected at various time points for a period of 18 hr. Plasma concentrations of MPL, free hepatic cytosolic GR densities, GR mRNA, TAT mRNA, and TAT activities in liver were determined. Plasma MPL profile was biexponential with a terminal t1/2 of 0.57 hr. Free hepatic GR density rapidly disappeared from cytoplasm after the MPL dose and then slowly returned to about 60% of starting level after 16 hr. Meanwhile, GR mRNA level fell to 45% of baseline within 2 hr postdosing, and remained at that level for at least 18 hr. The GR down-regulation of GR mRNA and protein turnover rate were modeled. The TAT mRNA began to increase at about 2 hr, reached a maximum at about 5 hr, and declined to baseline by 14 hr. TAT induction followed a similar pattern, except the induction was delayed about 0.5 hr. Pharmacodynamic parameters were obtained by fitting seven differential equations in a piecewise fashion. The cascade of corticosteroid steps were modeled by a series of inductions for steroid-receptor-DNA complex, two intermediate transit compartments, TAT mRNA, and TAT activity. Results indicate that GR mRNA and TAT mRNA are major controlling factors for the receptor/gene-mediated effects of corticosteroids.

Insulin increases transcription of rat gene 33 through cis-acting elements in 5'-flanking DNA

Gene 33 is a multihormonally-regulated rat gene whose transcription is rapidly and markedly enhanced by insulin in liver and cultured hepatoma cells. To examine the mechanism by which insulin regulates transcription, we have constructed chimeric plasmids in which expression of the bacterial cat gene, encoding chloramphenicol acetyltransferase (CAT), is governed by gene 33 promoter elements and contiguous sequences in DNA flanking the transcription start point (tsp). When transfected into H4IIE hepatoma cells, these constructs gave rise to stably transformed cell lines producing the bacterial CAT enzyme. This expression was increased by insulin treatment in a fashion resembling the effect of this hormone on transcription of the native gene. In vitro transcription assays in nuclear extracts also revealed increased transcription of the chimeric plasmids when the extracts were prepared from insulin-treated rat hepatoma cells. The results demonstrate that induction by insulin is mediated by cis-acting nucleotide sequences located between bp -480 to +27 relative to the tsp.

Receptor-mediated methylprednisolone pharmacodynamics in rats: Steroid-induced receptor down-regulation

Several approaches to receptor down-regulation were examined to extend previous receptor/gene-mediated pharmacokinetic/dynamic models of corticosteroids. Down-regulation of the glucocorticoid receptor was considered as an instantaneous event or as a gradual steroid-receptor-mediated process. Concentrations of plasma methylprednisolone, free hepatic cytosolic receptors, and the activity of hepatic tyrosine aminotransferase (TAT) enzyme were measured for 16 hr following administration of 0, 10, and 50 mg/kg methylprednisolone sodium succinate to 93 adrenalectomized rats. Receptor down-regulation was best described by a fractional decrement in the rate of return of free cytosolic glucocorticoid receptor. Predicted values for free receptor, bound receptor, nuclear bound receptor, and transfer compartments were in accord with the expected rank order values based on the high and low steroid doses. Model parameter estimates were independent of dose and described the rapid depletion of free cytosolic receptor, late-phase return of cytosolic receptor to a new baseline level that was 20-40% lower than control, and the TAT induction/dissipation pattern following steroid dosing. The microscopic association and dissociation constants describing the steroid-receptor interaction were 0.23 L/nmole per hr (k(on)) and 4.74 hr-1 (k(off)) for methylprednisolone compared to previously obtained values of 0.20 L/nmole per hr and 15.7 hr-1 for the related steroid prednisolone. The time course of TAT induction was similar to that observed previously for prednisolone. Efficiency of TAT induction was more closely related to steroid receptor occupancy than plasma methylprednisolone concentrations due to receptor saturability and receptor recycling.

The kinetics of mammalian gene expression

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

Induction of phosphoenolpyruvate carboxykinase gene expression by retinoic acid in an adult rat hepatocyte line

Regulation of expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene was examined in an adult rat hepatocyte line, RALA255-10G, that was immortalized with an SV40 temperature-sensitive (ts) A mutant. These hepatocytes express a transformed phenotype at the permissive temperature (33 degrees C) but a differentiated liver phenotype at the nonpermissive temperature (40 degrees C). We have shown previously that RALA255-10G cells express only low levels of liver-specific genes such as albumin and tyrosine aminotransferase at 33 degrees C. In the present study, we demonstrated that at 33 degrees C, PEPCK synthesis and mRNA expression could be detected only in the simultaneous presence of dexamethasone (DEX), retinoic acid, and dibutyryl-cAMP (Bt2cAMP). At 40 degrees C, PEPCK synthesis and mRNA expression were demonstrated in the presence of Bt2cAMP alone, but not in the presence of either DEX or retinoic acid. However, at 40 degrees C, PEPCK gene expression was stimulated by the combination of DEX plus retinoic acid; additionally, DEX and retinoic acid potentiated the Bt2cAMP-mediated PEPCK induction. In RALA255-10G cells, optimal PEPCK gene expression required the simultaneous presence of DEX, retinoic acid, and Bt2cAMP; DEX had to be present at all times. Triiodothyronine (T3) also potentiated the Bt2cAMP-mediated PEPCK gene expression but failed to increase further the induction by DEX/retinoic acid/Bt2cAMP. By performing nuclear runoff assays, we demonstrated that the PEPCK gene transcription rate in the absence or presence of inducing agents was closely related to the levels of the corresponding mRNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Description and analysis of differential sensitivity to glucocorticoids in Fao cells

This study shows that the derived hepatoma cell line Fao displays different sensitivities for glucocorticoid induction of tyrosine aminotransferase (TAT), alanine aminotransferase (AAT) and gamma-glutamyltransferase (GGT). This was seen in the different behaviors of nine steroids with respect to these three effects: (1) in the presence of full agonists (dexamethasone or deacylcortivazol), half-maximal induction of GGT occurred at approx 5- to 6-fold higher agonist concentrations than those required for half-maximal induction of AAT and TAT; (2) in the presence of full antagonists (RU 486, R5020, or progesterone) the GGT response induced by an equal agonist concentration was inhibited at concentrations approx 4- to 5-fold lower than those required for an equivalent inhibition of TAT response; (3) in the presence of cortexolone, deoxycorticosterone, 11 beta-hydroxyprogesterone and dexamethasone-3'-oxetanone, there was a partial agonistic effect (30-50%) on TAT and AAT responses, whereas there was a mainly antagonistic effect (very weak agonistic effect: 0-10%) on GGT response; (4) regardless of the steroid or its full or partial agonist activity, a given TAT induction level (50%, for example) always corresponded to the same AAT and GGT induction levels (50 and 10% respectively). We provide evidence showing that the three above-mentioned biological responses are mediated via the same type of glucocorticoid receptor binding site. Consequently, this differential behavior probably originates from a phenomenon occurring after the common steps (activation, translocation) that follow the formation of the steroid-receptor complex. This leads us to propose a model in which this phenomenon is assumed to originate from a difference in the affinities of the activated receptor for the nuclear acceptor sites of the TAT and GGT genes.

Regulators of fetal liver differentiation in vitro

Seventeen-day-old fetal rat hepatocytes were employed to examine factors required to promote differentiation in vitro. In the absence of effectors, primary fetal hepatocytes dedifferentiated, as characterized by the rapid decline in synthesis of fetal alpha-fetoprotein (AFP), albumin, and transferrin. On the other hand, cells maintained in the presence of glucocorticoid hormone produced high levels of albumin and transferrin. Glucocorticoid could not prevent the decline in fetal AFP synthesis, but induced synthesis of the 65K variant AFP--the major AFP species produced by adult rat liver. Fetal hepatocytes maintained in the presence of 8-bromo-cAMP (8-BrcAMP), or methyl isobutyl xanthine (MIX), an agent that increases intracellular cAMP levels, synthesized high levels of fetal AFP and albumin but reduced levels of transferrin. Both glucocorticoid and 8-BrcAMP or MIX induced expression of adult liver-specific genes such as tyrosine aminotransferase (TAT) and phosphoenolpyruvate carboxykinase (PEPCK), suggesting that these fetal hepatocytes have matured. Cells maintained in the presence of glucocorticoid hormone and MIX (or 8-BrcAMP) contained more albumin, TAT, and PEPCK mRNAs and synthesized increased amounts of the 65K variant AFP than those with either agent alone. However, the glucocorticoid/MIX cells produced intermediate levels of the fetal AFP and transferrin. Our data indicate that both glucocorticoid hormone and cAMP are necessary for optimal differentiation of fetal hepatocytes in vitro.

Different mechanisms control developmental activation of transcription of genes subject to identical hormonal regulation in adult liver

Developmental changes in expression of two genes subject to identical hormonal controls in adult liver were examined in livers of fetal and newborn rats. Both mRNA concentrations and transcription of tyrosine aminotransferase were very low throughout gestation and increased sharply at birth. The mRNA of gene 33 (Lee et al., J. Biol. Chem. 260: 16433-16438, 1985) and its transcription were also low in fetal liver until a significant increase occurred just prior to birth, followed by a further increase at birth. In mutant mice carrying a deletion that prevents developmental activation of aminotransferase transcription, that of gene 33 was not affected. The data indicate that different mechanisms control developmental activation of these genes, in contrast to hormonal regulation of their expression.

Receptor-mediated pharmacodynamics of prednisolone in the rat

A pharmacokinetic/pharmacodynamic model describing receptor-mediated effects of prednisolone is presented. The basis of the model is the generally accepted mechanism of action of steroid hormones in which corticosteroids bind to cytosolic receptors forming steroid-receptor complexes, which are activated and translocated into the nucleus. There the complexes associate with specific DNA sequences and modulate the rate of transcription of DNA into specific RNAs that code for the synthesis of proteins that elicit biological responses. Prednisolone, 5 or 50 mg/kg, was administered intravenously to adrenalectomized rats. Total plasma, free plasma, CBG-free plasma, and liver prednisolone concentrations were measured simultaneously with free hepatic cytosolic glucocorticoid receptor concentrations and tyrosine aminotransferase (TAT) activity of the liver as a function of time. The association/dissociation kinetics of prednisolone binding to the glucocorticoid receptor were measured separately in vitro at 37 degrees C. Total plasma, free plasma, and CBG-free plasma prednisolone concentrations could be used equally well in the model to account for the time course of receptor concentrations and TAT activity. However, use of liver steroid concentrations resulted in an overestimation of receptor depletion. Steroid concentrations in plasma increased 20 to 30-fold with a tenfold increase in dose, but receptor occupancy and TAT activity over time increased about threefold. While prednisolone pharmacokinetics were dose-dependent, parameters describing receptor kinetics and TAT activity were constant at each prednisolone dose. The major determinants of receptor-mediated glucocorticoid activity are confirmed to be the availability of the receptor, drug-receptor dissociation rate, and corticosteroid persistence in the biophase.

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.

Intracellular localization of bacterial lipopolysaccharide using the avidin biotin complex method at the electron microscopic level

The intracellular localization in 3T6 fibroblasts of Escherichia coli lipopolysaccharide (LPS) using the rapid avidin-biotin-immunoperoxidase technique at the electron microscopic level was studied. The role of bacterial endotoxin in the etiology of periodontal disease has been well documented previously. The purpose of the present study was to localize LPS within the cell, thereby determining which organelles concentrate the material and relate this to the cytologic pathophysiology. An increased concentration of LPS was found in the cell nuclei and, specifically, in association with nuclear chromatin and nucleoli. The concentration of LPS in the nucleus was directly related to the time of incubation, with some product appearing in that site within 2 minutes. There was no specific localization of endotoxin in mitochondria, lysosomes, Golgi, endoplasmic reticulum or ribosomes. These results imply that bacterial endotoxin may have a direct effect on nuclear components of fibroblasts. The relationship of these results to the etiologic mechanisms of periodontal disease is discussed.

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.

The effect of carcinogens on the accumulation of tyrosine aminotransferase by foetal rat hepatocytes in culture

The hepatocarcinogen 3'-methyl-4-dimethyl-aminoazobenzene (MDAB) suppresses the accumulation of tyrosine aminotransferase in cultured foetal hepatocytes. Experiments involving liver derived from foetuses of various ages reveals that a response is only obtained with rats older than 16-day gestation. It has been proposed that the lack of an effect in less mature hepatocytes is due to their inability to activate the carcinogen. Chemically synthesized analogues of MDAB which are considered likely to be activated forms of the procarcinogen are shown to be effective in the less mature cells. This supports the proposal that these cells may be unresponsive because they are unable to activate MDAB. Tests with other carcinogens reveal that the hepatocarcinogen dimethylbenzanthracene is also effective in 19-day gestation hepatocytes. However, the non-hepatocarcinogens azaserine and benz(a)pyrene are ineffective. Treatment with MDAB is shown not to alter the level of steroid receptor and reduce its translocation into the nucleus, suggesting that this is not the mechanism by which TAT is suppressed. The effect of the tumour promoter phorbol-myristate acetate (PMA) administered together with MDAB was shown not to modify the response to the carcinogen alone.

Selective inhibition by sodium butyrate of glucocorticoid-induced tyrosine aminotransferase synthesis in hepatoma tissue-cultured cells

Sodium butyrate in a 5 mM concentration prevents the induction of tyrosine aminotransferase in hepatoma culture cells, without affecting the basal level of the enzyme. This effect is reversible immediately after the removal of butyrate, or after a lag, if butyrate was present for more than 2 h. Neither the amount of cellular RNA nor the rate of total RNA synthesis were affected by sodium butyrate. Furthermore, butyrate does not inhibit protein synthesis: [35S]methionine incorporation into proteins, measured in a reticulocyte lysate system, shows no significant difference between the translation capacity of the RNAs from butyrate-treated cells and from dexamethasone-induced or uninduced cells. Nevertheless, when tyrosine aminotransferase was isolated from the translation products by its specific antiserum and analyzed by gel electrophoresis, we observed that the amount of the enzyme synthetized in the presence of RNAs from dexamethasone/butyrate-treated cells was strongly diminished relative to that synthesized in the presence of RNA from dexamethasone-induced cells. These experiments indicate that the treatment of the cells with butyrate decreases the activity of the specific messenger RNA for tyrosine aminotransferase to a level close to the basal level.

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.

Inhibition of tyrosine aminotransferase induction by UTP deficiency and its reversal by 5-fluorouridine in cultured hepatoma cells

Hepatoma tissue culture cells, grown in the presence of D-galactosamine and 6-azauridine, demonstrate a strong reduction of the intracellular UTP pool that can be replenished by formation of UTP from uridine and FUTP from 5-fluorouridine within 2 h. Concomitantly with the UTP deficiency, a decrease of dexamethasone-induced tyrosine aminotransferase activity occurs. 5-Fluorouridine, as compared to uridine, is even more efficient in restoring the activity of tyrosine aminotransferase. Treatment of the cells with D-galactosamine alone results in a minor lowering of UTP that is not followed by the inhibition of the enzyme induction. However, the administration of D-galactosamine, simultaneously or at any time up to 5 h before or after dexamethasone, leads to a 1.5- to 2-fold higher induction (superinduction) which appears 24 h later.

Tyrosine aminotransferase deficiency in mink (Mustela vision): a model for human tyrosinemia II

Mink pseudodistemper, a recessive disease associated with high blood tyrosine levels, is an animal analogue of the human inborn error of metabolism, tyrosinemia II. Affected mink and man have eye and skin lesions. Affected mink have no hepatic tyrosine aminotransferase (TAT) activity, as measured immunologically and biochemically. Hepatic mitochondrial aspartate aminotransferase is increased to 188% of control. This new genetic animal model of TAT deficiency should allow new studies of tyrosine metabolism.

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.

Stimulation of protein and RNA synthesis by methylmercury chloride in the liver of intact and adrenalectomized rats

(1) A single injection of methylmercury chloride in the rat (10-50 mg/kg) increased both in vivo and in vitro rates of 14C-leucine incorporation into the protein of the post-mitochondrial supernatant fraction of the liver. In contrast, no stimulation of protein synthesis was observed in the brain of the methylmercury-treated rats. (2) Methylmercury administration also stimulated RNA polymerase activities in isolated hepatic nuclei, stimulation of Mg-dependent activity being higher than that of Mn-dependent activity. (3) In experiments with adrenalectomized rats, it was found that the stimulatory effect of methylmercury on protein and RNA synthesis in the liver was mediated partly through the adrenal gland. (4) Analysis of serum by starch-block electrophoresis revealed that synthesis of all serum proteins, including albumin and alpha-gamma globulin fractions, was stimulated by methylmercury treatment. (5) These results suggest that the observed effects of methylmercury on the liver depend on mechanisms other than enhancement of the synthesis of acute-phase proteins.

Role of progesterone and glucocorticoids in the transcription of the beta-casein and 28-S ribosomal genes in the rabbit mammary gland

Isolated mammary nuclei were incubated in the presence of HgCTP and the neosynthesized RNA was isolated with a SH-Sepharose column. The concentration of beta-casein mRNA and 28-S ribosomal RNA in the neosynthesized RNA fractions was measured using [3H]cDNA probes complementary to beta-casein mRNA and 28-S rRNA respectively. Prolactin injected into pseudopregnant animals accelerates the transcription of both genes and increases the stability of the beta-casein mRNA but not of the 28-S rRNA. Progesterone injected simultaneously with prolactin reduced considerably all these effects of prolactin, with a lower efficiency when the highest doses of prolactin were injected. These observations suggest that progesterone attenuates the transfer of prolactin information related to the lactogenesis into the mammary cell. Glucocorticoids injected with prolactin amplify the prolactin action on the expression of the beta-casein gene but not of the 28-S rRNA genes. In the absence of prolactin (a situation obtained by injecting simultaneously CB 154, a drug which inhibits pituitary prolactin secretion) glucocorticoids exhibit no effect. In the lactating rabbit glucocorticoids do not delay significantly the drop of beta-casein gene transcription rate provoked by weaning or by prolactin withdrawal obtained by injecting CB 154. A comparison of the beta-casein mRNA accumulation and the transcription rate of the beta-casein gene indicates that glucocorticoids act essentially by amplifying the activation of beta-casein transcription supported by prolactin, but not by enhancing the stability of the beta-casein mRNA.

Does quantitative tRNA adaptation to codon content in mRNA optimize the ribosomal translation efficiency? Proposal for a translation system model

Neither a dynamic nor an energetic approach of the translation process has taken into account that intracellular levels of iso-tRNA species are adapted or adjusted to the codon frequency of mRNA being decoded (Bombyx mori silk gland, rabbit reticulocyte). A critical study of available experimental data suggests that the average elongation rate of a protein is maximized in the presence of an adapted tRNA population, usually an homologous tRNA. In addition, the amount of synthesized protein parallels that of corresponding mRNA. Other evidences--including in vitro and in vivo elongation assays with fibroin mRNA--show that individual elongation rates are not uniform. Pauses occur at certain sites of the mRNA chain. The relative lifetime of these pauses depends on the tRNA pool used. Finally, it appears that translation accuracy also depends on the balanced tRNA population. We propose to explain these different effects by using a codon-anticodon recognition model, called "trial and error system" based on a stochastic processing of the ribosome. Accordingly, various acylated tRNA species which surround a ribosome randomly encounter the receptor A site. Every trapped tRNA species is tested for a proper pairing with the codon to be recognized at the level of a comparator or discriminator function. If the pairing is correct, transpeptidation becomes irreversible. If not, the aminoacyl-tRNA is rejected and another randomly trapped tRNA is processed in turn. Mathematical analysis of this model shows that the mean number of trials used for translating the whole sequence of a mRNA is minimized when the proportion of different iso-tRNA species is correlated with the square root of codon frequency. Quantitations of reticulocyte tRNA support such a parabolic relation. Our translation system model brings some light into the role of tRNA adaptation for optimizing translation efficiency, i.e. maximizing both speed and accuracy. Some consequences of the model are discussed.

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.

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.