In vivo and in vitro studies on the effect of tryptophan on translocation of RNA from nuclei of rat liver

Hormonal influences on tryptophan binding to rat hepatic nuclei

We evaluated whether selected hormones, 3,5,3'-triiodothyronine (T3), hydrocortisone (HC) or insulin, would influence the binding (saturable, stereospecific, and of high affinity) of L-tryptophan to rat hepatic nuclei or nuclear envelopes. T3 (10(-14) to 10(-10) M) appreciably inhibited in vitro L-(5-3H) tryptophan binding to hepatic nuclei and T3 (10(-16) to 10(-4) M) appreciably ameliorated the inhibitory effect of unlabeled tryptophan (10(-4) M) on such binding. In vivo tryptophan administration (1 h) stimulated hepatic protein synthesis but the addition of T3 negated such stimulation. HC (10(-12) to 10(-4) M) did not affect and insulin (10(-16) to 10(-4) M) had only a small inhibitory effect on 3H-tryptophan binding to hepatic nuclei, but each (10(-12) to 10(-4) M) when added to unlabeled tryptophan (10(-4) M) diminished the inhibitory binding effect of unlabeled tryptophan alone. Thus, T3 competes with tryptophan for hepatic nuclear tryptophan binding and also negates tryptophan's stimulatory effect on hepatic protein synthesis.

Influence of lead acetate and selected metal salts on tryptophan binding to rat hepatic nuclei

This study evaluated whether lead acetate or other selected metal salts would influence the binding of L-tryptophan to rat hepatic nuclei. Lead salts and other salts of cadmium, zinc, mercury, and molybdenum, when added alone, had only small effects on 3H-tryptophan binding to hepatic nuclei in vitro. However, each of these salts, when added along with unlabeled L-tryptophan (excess, 10(-4) M), caused significantly less inhibition of 3H-tryptophan binding to hepatic nuclei than did unlabeled L-tryptophan alone. Lead acetate (10(-10) to 10(-4) M), when added along with unlabeled L-tryptophan, abrogated the inhibition of binding related to unlabeled L-tryptophan alone. Sodium arsenite (but not potassium arsenate) as well as sodium selenite (at 10(-4) M concentrations) inhibited to a moderate degree the in vitro 3H-tryptophan binding to hepatic nuclei, but addition of 10(-4) dithiothreitol, a protective agent for sulfhydryl groups, diminished this inhibition. Rats receiving a high dose of lead acetate before being tube-fed L-tryptophan displayed a decrease in hepatic protein synthesis compared with the stimulatory response connected with L-tryptophan alone. Thus, the addition of lead acetate and of other metal salts appears to have an inhibitory effect on L-tryptophan binding to hepatic nuclei. Lead acetate was investigated in in vivo experiments and was found to negate the stimulation of hepatic protein synthesis related to L-tryptophan alone.

Hormonal influences on tryptophan binding to rat hepatic nuclei

This study evaluated whether selected hormones, 3,5,3'-triiodothyronine (T3), hydrocortisone (HC), or insulin, would influence the binding of L-tryptophan to rat hepatic nuclei or nuclear envelopes. The first two hormones have nuclear receptors that belong to the same superfamily, while insulin belongs to a different unrelated superfamily of receptors. Previous reports have indicated that the binding of L-tryptophan to hepatic nuclear proteins was saturable, stereospecific, and of high affinity. Also, previous studies showed that administration of L-tryptophan rapidly stimulated hepatic protein synthesis. In this study, we investigated whether each hormone alone or together with unlabeled tryptophan would influence tryptophan binding to rat hepatic nuclei or nuclear envelopes as assayed by in vitro L-5-(3)H-tryptophan binding. Our results indicate that T3 10(-14) to 1(-10) mol/L appreciably inhibited in vitro 3H-tryptophan binding to hepatic nuclei and T3 10(-16) to 10(-4) mol/L appreciably ameliorated the inhibitory effect of unlabeled tryptophan (10(-4) mol/L) on in vitro 3H-tryptophan binding. In vivo administration (1 hour) of tryptophan alone stimulated hepatic protein synthesis, but addition of T3 negated such stimulation. Addition of HC 10(-12) to 10(-4) mol/L had no effect and addition of insulin 10(-16) to 10(-4) mol/L had only a small inhibitory effect on in vitro 3H-tryptophan binding to rat hepatic nuclei, but each (10(-12) to 10(-4) mol/L), when added to unlabeled tryptophan (10(-4) mol/L), diminished the inhibitory binding effect of unlabeled tryptophan alone. Our study indicates that T3 competes with tryptophan for hepatic nuclear tryptophan binding, and it also appears to negate tryptophan's stimulatory effect on hepatic protein synthesis.

Tryptophan and carcinogenesis: review and update on how tryptophan may act

This review covers the historical developments of the consideration that tryptophan may influence the induction of cancer in experimental studies. Studies relating to stimulatory effects, as well as to inhibitory effects, of tryptophan or tryptophan-related compounds are described. Also the effects of pyrolysis products of tryptophan on carcinogenesis are covered. In consideration that new L-tryptophan-related contaminants may be involved in a recently described human disease, a description is given of the eosinophilia-myalgia syndrome, which has been attributed to the ingestion of L-tryptophan-containing related contaminants. Whether these new L-tryptophan-related contaminants alone or together with L-tryptophan may prove to be carcinogenic remains to be determined. Lastly, recent developments relating to regulatory effects of L-tryptophan on liver metabolism are reviewed and then considered as possibly playing a role in carcinogenesis.

Differences in tryptophan binding to hepatic nuclei of NZBWF1 and Swiss mice: insight into mechanism of tryptophan's effects

We have observed that in NZBWF1 mice the affinity for L-tryptophan binding to hepatic nuclei in vitro is markedly less than that of Swiss mice. In vitro binding of [3H]tryptophan to hepatic nuclei from both strains was determined without and with unlabeled L-tryptophan (10(-4) mol/L). The relative specific binding of L-tryptophan to hepatic nuclei in vitro was 60.9 +/- 4.4% for Swiss mice and 35.8 +/- 5.4% (P < 0.01) in NZBWF1 mice. The total specific binding (bound radioactivity/mg nuclear protein) of L-tryptophan to hepatic nuclei in vitro was 74.9% (P < 0.05) lower in NZBWF1 mice than in Swiss mice. Other strains (DBA, SJL and BALB/c) had specific binding affinities similar to that of Swiss mice. Serum and hepatic free tryptophan concentrations and hepatic tryptophan dioxygenase activity in mice that were food-deprived overnight or 1 h after tube-feeding L-tryptophan (20 mg/100 g body weight) were similar in the strains of mice. In vitro [14C] leucine incorporation into protein using hepatic microsomes of mice 1 h after tube-feeding L-tryptophan (20 mg/100 g body weight) revealed a significantly greater (P < 0.05) increase relative to food-deprived controls in Swiss mice (76.8 +/- 19.2%) than the increase in NZBWF1 mice (26.5 +/- 2.6%). Nuclear [14C]-labeled RNA release in vitro was increased 77.2 +/- 18.0% by tube-feeding of L-tryptophan in Swiss but only 7.6 +/- 5.8% (P < 0.02) in NZBWF1 mice. Liver nuclear poly(A)-polymerase and nucleoside triphosphatase activities were variably increased by the administration of L-tryptophan in both strains. In summary, compared with Swiss mice, NZBWF1 mice have a lower specific binding affinity for L-tryptophan by hepatic nuclei, and this alteration may account for the other differences in responses to L-tryptophan by the two strains.

Effect of metyrapone on tryptophan binding to rat hepatic nuclei

This study evaluated whether metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), an inhibitor of endogenous adrenal corticosteroid synthesis via inhibition of cytochrome P-450-mediated steroid hydroxylation, would influence the binding of L-tryptophan to rat hepatic nuclei or nuclear envelopes. Previous publications have indicated that binding of L-tryptophan to hepatic nuclear envelope proteins was saturable, stereospecific, and of high affinity. In this study, we investigated whether metyrapone would influence L-tryptophan binding to rat hepatic nuclei or nuclear envelopes as assayed by in vitro L-(5-3H)tryptophan binding. Our results indicate that the addition of metyrapone in vitro has little influence on L-(5-3H)tryptophan binding to hepatic nuclei or nuclear envelopes. On the other hand, when metyrapone (1 mg/100 g body weight) is tube-fed 30 minutes before killing, the isolated hepatic nuclei show decreased specific L-tryptophan binding (total binding minus nonspecific binding [using 2,000-fold excess of unlabeled L-tryptophan]) compared with controls. Also, addition of metyrapone in vitro to rat liver before homogenization and preparation of nuclei caused the nuclei to show decreased specific tryptophan binding compared with controls. Under these in vitro conditions, SKF 525A, another inhibitor of hydroxylation, showed inhibitory effects similar to those of metyrapone. Thus, metyrapone can interfere with rat liver nuclear envelope receptor binding to L-tryptophan, and possibly acts via its effects on hydroxylation. At high doses, metyrapone (20 mg/100 g body weight) appears to inhibit tryptophan-induced stimulation of hepatic protein synthesis.

Effect of benzodiazepines on tryptophan binding to rat hepatic nuclei

This study evaluates whether or not some of the benzodiazepines would influence the binding of L-tryptophan to rat hepatic nuclei or nuclear envelopes. Previous publications have indicated that binding of L-tryptophan to hepatic nuclear envelope proteins was saturable, stereospecific, and of high affinity. In this study, we investigated whether some of the benzodiazepines would influence L-tryptophan binding to rat hepatic nuclei or nuclear envelopes as assayed by in vitro L-(5-3H) tryptophan binding. Our results indicate that the addition of chlordiazepoxide, diazepam, prazepam, flurazepam, nordazepam, N-desalkylflurazepam, temazepam, oxazepam, lorazepam, or 4-chlorodiazepam has little influence on the L-(5-3H) tryptophan binding to hepatic nuclei in vitro. However, the addition of demoxepam, the N-desalkylated compound of chlordiazepoxide, caused marked competition with 3H-tryptophan binding to hepatic nuclei in vitro. When chlordiazepoxide (1 mg/100 g body weight) is administered intraperitoneally 20 min before killing, the isolated hepatic nuclei reveal decreased specific L-tryptophan binding compared to controls. Also, rats pretreated with chlordiazepoxide intraperitoneally before tube-feeding L-tryptophan revealed diminished tryptophan-induced hepatic nuclear RNA efflux and protein synthesis. Our results suggest that chlordiazepoxide, possibly by itself or through a metabolite, can act to affect hepatic nuclear binding of L-tryptophan and to inhibit the stimulatory effect of L-tryptophan on hepatic protein synthesis.

Role of cytosol in the stimulation of RNA transport in vitro during cardiac hypertrophy in rats

The 100,000 g supernatant isolated from hypertrophic hearts on fractionation by (NH4)2SO4 and DEAE-cellulose chromatography showed an enhanced RNA-transport activity when incubated with isolated nuclei from sham-operated hearts in vitro. Proteins of Mr 73,000, 68,000, 43,000 and 32,000 are enriched in the DEAE-cellulose fractions exhibiting maximal transport activity, and they are phosphorylatable. Pretreatment of the cytosol with antibodies to the Mr-68,000 and -32,000 proteins decreases the transport activity of the cytosol from 14% to 4.25%. Proteins of Mr 73,000, 68,000, 43,000 and 32,000 are translocated from the cytosol to the nuclear envelope under conditions of RNA transport in vitro. Our results here suggest that at least two of these proteins, those of Mr 68,000 and 32,000, play an indispensible role in the nucleocytoplasmic RNA transport in vitro. By making use of a specific myosin heavy-chain B-gene probe and hybridization, we have also shown the effect of cytosol on the transport of myosin heavy-chain mRNA from nucleus to cytosol.

Effect of feeding a choline-deficient diet on the hepatic nuclear response to tryptophan in the rat

In earlier studies the acute administration of tryptophan (TRP) to rats was reported to induce enhanced in vivo [14C]orotate-labeled hepatic nuclear RNA release in vitro. This change was considered to possibly be related to the induction of more and larger gamma-glutamyl transpeptidase-positive foci in the livers of rats treated with diethylnitrosamine and fed long-term elevated TRP in a choline-supplemented (CS) but not in a choline-deficient (CD) diet (comparisons with respective controls). In this study we investigated whether feeding a CD compared to a CS diet for 1 week would affect selected hepatic nuclear responses to TRP. Rats fed the CS but not the CD diet and tube-fed TRP 10 min before being killed revealed enhanced labeled hepatic nuclear RNA release in vitro. In all experiments, comparisons were made with the control groups (rats fed the CS or stock diet). When rats were fed elevated TRP (2%) in the diets (CS or CD) for 1 week, labeled hepatic nuclear RNA release was increased with the CS + TRP but not with the CD + TRP diet group. [3H]TRP binding to hepatic nuclei in vitro revealed no change in the CS + TRP group, decreased in the CD group, and markedly increased in the CD + TRP group in comparison with the control (CS) group. Hepatic nuclear nucleoside triphosphatase activity was increased only in the CS + TRP group while hepatic nuclear poly(A) polymerase activity was increased in the CS + TRP and in the CD +/- TRP groups. Serum cholesterol and triglycerides were decreased in the CD group and increased to control levels in the CD + TRP group.

Identification and immunohistochemical localization of a tryptophan binding protein in nuclear envelopes of rat liver

A tryptophan binding protein which was identified by binding studies has been purified to apparent homogeneity from rat liver nuclear envelopes. Two affinity matrices, namely, concanavalin A-agarose and tryptophan-agarose, were utilized for purification of the binding protein. Findings with lectin affinity chromatography suggested that the binding entity was a glycoprotein since it could be eluted off the column with methyl alpha-D-mannopyranoside (0.2 M). Eluates from both columns, when electrophoresed separately (under denaturing conditions) on polyacrylamide gels, revealed the presence of a protein with an apparent molecular weight of approximately 33,000-34,000 which is the same as that observed when covalently bound (i.e., crosslinked) [3H]-tryptophan is analyzed on polyacrylamide gels under denaturing conditions and then autoradiographed. Polyclonal antibodies raised against the binding protein recognized polypeptides with molecular weights of 64,000 and 33,000-34,000 when analyzed by the Western blot technique, suggesting that the protein was probably a dimer. Immunohistochemical studies revealed that the antigen is localized in the nuclear membranes, thereby corroborating our biochemical premise that the binding protein was present in the nuclear envelopes.

Effect of tryptophan on toxic cirrhosis induced by intermittent carbon tetrachloride intoxication in the rat

The effects of the administration of tryptophan on toxic cirrhosis induced by intermittent carbon tetrachloride (CCl4) intoxication in the rat were investigated. Rats received CCl4 (0.45 ml/100 g body wt ip) twice weekly for 10-14 weeks. Tryptophan (30 mg/100 g body wt) by stomach tube was administered 1 hr before killing. Tryptophan improved hepatic polyribosomal aggregation and [14C]leucine incorporation into protein in vitro of control rats as well as long-term CCl4-treated rats that had developed toxic cirrhosis. However, the effects were more marked in control than in experimental rats. Tryptophan administration induced an increase in labeled nuclear RNA release in vitro and a decrease in labeled tryptophan binding to nuclear protein in vitro of livers of rats receiving long-term CCl4 and of control rats. The results indicate that the stimulatory effects of a single administration of tryptophan in toxic cirrhotic livers are similar to, but somewhat less than, those which occur in livers of normal, control rats.

Effect of tryptophan on isolated hepatocytes of rats

The addition of tryptophan to adult rat hepatocyte cultures stimulated DNA synthesis. The increase in DNA synthesis as measured by 3H-thymidine incorporation into DNA was observed on treatment of the cultures with tryptophan for 48 h but also as short as for 6 h in comparison with control cultures. An increase was also apparent at 30 h which was maintained for up to 48 h post treatment with tryptophan. The increase in DNA synthesis by tryptophan cannot be attributed to cell injury or to increased DNA degradation. Of the degradative enzymes added after harvesting the hepatocytes, only DNase decreased incorporation of 3H-thymidine. The observed effect was specific for tryptophan since treatment with kynurenine, isoleucine, methionine or serine failed to show a significant effect. Pretreatment of cultured hepatocytes with hydroxyurea prevented the tryptophan stimulated increase in DNA synthesis suggesting that the latter was due to replicative and not to reparative DNA synthesis. Experiments performed with the addition of diethylnitrosamine also alluded to tryptophan's role in replicative DNA synthesis. The mechanism of tryptophan-induced DNA synthesis is discussed.

Role of tryptophan in carcinogenesis

This paper reviews some of the earlier experimental studies concerning the role that tryptophan plays in enhancing tumorigenesis induced by selected chemical carcinogens. For many years, tryptophan has been implicated in carcinogenesis of the bladder. The evidence regarding tryptophan's effect on hepatic tumorigenesis is conflicting; an enhancing effect has been reported by some investigators, but a reduction in tumorigenesis has been reported by other workers. Some of the unique effects that tryptophan exerts upon the liver are reviewed. Also, experimental studies from our laboratory are reported in which we observed a potentiating effect of increased dietary tryptophan on the induction of gamma-glutamyltranspeptidase-positive foci in liver when rats were fed a choline-supplemented diet but no potentiation was found when rats were fed a choline-deficient diet for 10 weeks. The results suggest that increased dietary tryptophan has a promoting effect on liver carcinogenesis as measured by the induction of gamma-glutamyltranspeptidase-positive foci in the livers of rats exposed to diethylnitrosamine. The possible significance of these findings is reviewed.

The role of protein phosphokinase and protein phosphatase during the nuclear envelope nucleoside triphosphatase reaction

The activities of nuclear envelope-associated protein phosphokinase and protein phosphatase were determined in nuclear ghosts from liver and oviduct of quails. The protein kinase was found to be inhibited by poly(A) by 75%. During the kinase reaction proteins with molecular weights of 106 000 and 64 000 were phosphorylated. The phosphoprotein phosphatase from liver was stimulated to 190% by poly(A), whereas only a slight enhancing effect by this polymer was determined with the oviduct enzyme (to 125%). Comparative determinations of the nuclear ghost-associated enzyme activities revealed the following values (in nmol Pi/min per 10(8) ghosts); oviduct: phosphokinase, 0.015; phosphatase, 0.004 and nucleoside triphosphatase, 39.4; and liver: phosphokinase, 0.044; phosphatase, 0.012 and nucleoside triphosphatase, 11.7. These data indicate that phosphorylation/dephosphorylation proceeds independently of the nucleoside triphosphatase cycle. This assumption is supported by analytical results revealing that no marked dephosphorylation occurs after poly(A) binding to the nuclear envelope. Moreover, stoichiometrical data showed a nearly 1:1 molar ratio between ATP-binding and phosphorylation of nuclear envelope protein. From these findings a new model for the nucleoside triphosphatase-mediated poly(A)(+)mRNA efflux from nuclei is deducted, proposing phosphokinase and phosphatase only to modulate the affinity of the 'carrier structure' for poly(A) (+)mRNA, but not to constitute the nucleoside triphosphatase.

Tryptophan-induced stimulation of hepatic ornithine decarboxylase activity in the rat

The effect of a single tube feeding of L-tryptophan on hepatic ornithine decarboxylase (ODC) activity in rats was investigated. The levels of ODC activity in the livers of control and experimental rats were assayed in vitro by measuring the release of 14CO2 from DL-[1-14C]ornithine. Single tube feedings of varying levels of L-tryptophan (2.5-30 mg/100 g body wt) to overnight-fasted rats 1 hr before sacrifice exhibited increases in the hepatic ODC activities. L-Tryptophan (30 mg/100 g body wt) tube fed to overnight-fasted rats 1/6 to 12 hr before sacrifice induced hepatic ODC activities which were significantly elevated beginning at 1 hr and peaking at 2 hr (6.5-fold increase over controls). In vitro [14C]leucine incorporation into protein using hepatic microsomes of tryptophan-treated rats was significantly increased at 1 hr in comparison with that of controls. The tryptophan-induced stimulation of hepatic ODC activity was not affected by prior adrenalectomy but was abolished by pretreatment with cycloheximide. These studies demonstrate that a single feeding of L-tryptophan can significantly enhance in the rat the activity of ODC, a key enzyme in the biosynthesis of polyamines.

Effect of cytosol on transport of RNA in vitro during cardiac hypertrophy

An enhanced RNA-transport activity was observed in vitro from nuclei obtained from animals with cardiac hypertrophy as compared with that of sham-operated controls. The 100 000 g supernatants obtained from hypertrophic hearts stimulated the RNA transport from nuclei of sham-operated controls, and this stimulation was maximum with 40% supernatant. Ca2+- and nucleic acid-dependent ATPase and alkaline phosphatase activities, which may be involved in an energy-dependent transport, were high in nuclei from hypertrophic hearts, and the nuclei of sham-operated animals showed higher activities of these enzymes after incubation with supernatant from hypertrophic hearts, which stimulates the RNA transport in vitro from nuclei of sham-operated animals.

Transport of functional messenger RNA from liver nuclei in a reconstituted cell-free system

The reliability of a reconstituted cell-free system for messenger RNA processing and transport, consisting of isolated nuclei in fortified cytosol, has been evaluated in terms of the functionality and regulated release of the transported product. The poly(A) messenger RNA transport in vitro formed appropriate initiation complexes with ribosomes in an optimized translation system and had template activity comparable to that transported in vivo. The intra-nuclear origin of this messenger RNA is supported by pulse-labeling studies, its transport from detergent-treated nuclei and the absence of the release under non-transport conditions. Serum albumin was identified by immunoprecipitation and electrophoresis as one of the products synthesized when the transported RNA was translated in vitro. The transport of messenger RNA in the cell-free system was dependent on specific cytosol (soluble cytoplasmic) proteins. These proteins, which constitutes less than 0.1% of the total cytosol proteins, are precipitated wtih streptomycin with high specificity.

Regulated transport of messenger ribonucleic acid from isolated liver nuclei by nucleic acid binding proteins

Rat liver nucleocytosolic messenger ribonucleic acid (mRNA) transport is shown to be regulated by proteins with a high affinity for nucleic acids. In the cell-free system described, the energy-dependent transport of all RNA classes [transfer RNA (tRNA), mRNA, and ribosomal RNA (rRNA)] exhibited a dependence upon the availability of discrete minor sets of cytosol proteins. In addition to having a different level of saturation, only the mRNA "transport protein" activities are increased by adenosine cyclic 3',5'-phosphate (cAMP), an effect most likely mediated by a cAMP-dependent protein kinase. The mRNA transport proteins were isolated from cytosol by precipitation with streptomycin sulfate followed by deoxyribonucleic acid (DNA)-cellulose affinity chromatography, or from oligo-(thymidylate)-cellulose bound cytoplasmic messenger ribonucleoprotein (mRNP) particles by high-salt extraction. Either method yielded a protein fraction which exhibited a 1000-fold increase in mRNA transport activity as compared to cytosol. Over one-half of the mRNA transport activity is associated with the mRNP of the cell. A partial homology between the cytosol and mRNP-derived proteins was demonstrated by polyacrylamide gel electrophoresis. One major (20 000 daltons) and several minor proteins (23 000, 52 000, 54 000, and 72 000 daltons) were in common. Nuclear 4-5S exited from in vitro incubated nuclei in three phases, according to their differential in vivo rates of labeling and intranuclear pool sizes. The amount of nuclear RNA transported in vitro as mRNA (about 1.0%) agrees wtih the in vivo estimates. Additional evidence for in vivo equivalence was provided by the physicochemical characterization and bioassay of the RNA. The transported mRNA sedimented in urea-sucrose gradients as an 8-18S heterodisperse product. This RNA initiated cell-free translation with the synthesis of precursor peptides as diverse in size as those for albumin and alpha 2U-globulin. The relative abundancies of various transported mRNAs were different than the corresponding abundancies of liver cytoplasmic mRNAs.