Further evidence against post-transcriptional control of inducible tyrosine aminotransferase synthesis in cultured hepatoma cells

Superinduction of ornithine decarboxylase (ODC) by actinomycin D is due to stimulation of ODC mRNA translation

Inhibition of transcription by treatment with actinomycin D caused superinduction of the ornithine decarboxylase (ODC) activity in Ehrlich ascites tumor cells. Experiments with cycloheximide ruled out the possibility that this superinduction was due to stabilization of ODC. Instead the ODC activity exhibited a more rapid turnover in the presence of actinomycin D (t1/2 = 56 min). The superinduction was found to coincide with an increased rate of ODC synthesis, as determined by measuring the incorporation of [35S )methionine into immunoreactive ODC protein. The steady-state level of ODC mRNA was unchanged, indicating an effect on the translational efficiency.

Insulin-mediated regulation of tyrosine aminotransferase in rat hepatoma cells: inhibition of transcription and inhibition of enzyme degradation

Insulin induces the enzyme tyrosine aminotransferase (TAT) in Reuber H-35 rat hepatoma cells. A clone of these cells (KRC-7) was used to study the relationship between changes in enzyme activity and hybridizable mRNA, and rates of transcription for TAT in response to insulin. Our results indicate that enzyme activity is inducible by insulin in the presence of an inhibitor of RNA synthesis, suggesting that insulin functions post-transcriptionally to increase enzyme activity. Unexpectedly, insulin causes a decrease in the level of hybridizable TAT mRNA. Glucocorticoids cause an increase in TAT mRNA and insulin inhibits this increase when added either subsequent to or simultaneous with the addition of this agonist. Transcriptional runoffs demonstrate that insulin inhibits transcription of TAT to account for the aforementioned decrease in hybridizable mRNA. To examine the possibility that a post-translational mechanism is responsible for the increase in TAT activity caused by insulin, the rate of degradation of TAT protein was measured using polyclonal antibody. These experiments indicate that the rate of degradation of TAT is decreased about twofold in the presence of insulin, which suggests that part of the observed increase in TAT activity is due to selective post-translational stabilization of TAT. Therefore, insulin regulates TAT in KRC-7 cells by both transcriptional and post-translational mechanisms, the latter being responsible for the increase in activity.

Effect of cold exposure on the amylase activity of submaxillary gland of rats

Cold exposure of rats for 3 h (6 +/- 2 degrees C) caused an increase in amylase activity of the submaxillary gland. This effect was not observed in other salivary glands i.e. parotid and sublingual or in the pancreas. The increase of amylase activity during cold exposure was completely abolished by the beta-receptor antagonist, propranolol, and the alpha-receptor antagonist, phenoxybenzamine, reduced the effect. Administration of actinomycin D to the cold-exposed rats produced a tremendous increase of enzyme activity instead of abolition of the increase as had been expected.

Effects of protein-degradation inhibitors on the inactivation of tyrosine aminotransferase, tryptophan oxygenase and benzopyrene hydroxylase in isolated rat hepatocytes

The following three potent inhibitors of hepatocytic proteolysis were investigated to see if they would inhibit the intracellular inactivation of enzymes: chymostatin and leupeptin (proteinase inhibitors) and methylamine (a lysosomotropic weak base). Chymostatin inhibited the inactivation of two of the three enzymes tested: tyrosine aminotransferase (EC 2.6.1.5) and tryptophan oxygenase (tryptophan 2,3-dioxygenase, EC 1.13.11.11). Leupeptin had no effect on any of the enzymes, whereas methylamine had only a weak inhibitory effect on tyrosine aminotransferase inactivation. Apparently proteolytic cleavage (probably by a non-lysosomal proteinase, since only chymostatin is effective) is involved in the inactivation of tyrosine aminotransferase and tryptophan oxygenase. The third enzyme, benzopyrene hydroxylase (flavoprotein-linked mono-oxygenase, EC 1.14.14.1), is probably inactivated by a non-proteolytic mechanism.

Metabolism of poly(A)(+)RNA in toad bladder epithelial cells

Previous studies have characterized the induction of poly(A)(+)RNA synthesis by aldosterone during the latent period, preceding the increased active transepithelial sodium transport (measured as short-circuit current, SCC). To assess the role of aldosterone in the maintenance of the response in general and the metabolism of this RNA in particular, the decay of the increased SCC and of the newly synthesized poly(A)(+)RNA was monitored. On removal of the hormone, the SCC decayed with a half-life of 6.5 hr after a lag period of 2-3 hr. Studies on the disappearance from the cytoplasm of poly(A)(+)RNA synthesized in the first two hours after addition of aldosterone revealed a number of RNA species with diverse size decaying at a relatively slow rate after removal of aldosterone, and RNA sedimenting in the 10-14 S region decaying at a faster rate closely related to the decay in SCC. Maintenance of aldosterone in the media resulted in a much slower rate of decay of this 10-14 S. It is concluded that the decay of the 10-14 S poly(A)(+)RNA is closely related to the decay in SCC and the stability of this RNA is influenced by the retention of aldosterone in the medium.

Indirect evidence for a strict negative control of S-adenosyl-L-methionine decarboxylase by spermidine in rat hepatoma cells

1. Direct or indirect inhibitors of l-ornithine decarboxylase (EC 4.1.1.17), structurally related or unrelated to l-ornithine, including dl-alpha-difluoromethylornithine, alpha-methylornithine and 1,3-diaminopropane, used alone or in combination, decreased polyamine concentrations in rat hepatoma tissue culture (HTC) cells and increased S-adenosyl-l-methionine decarboxylase activity (EC 4.1.1.50). 2. Comparison of the catalytic properties of S-adenosyl-l-methionine from cells with elevated and normal activities revealed no apparent modification of the catalytic site as judged by affinity for the substrate, stimulation by di- and tri-amines and inhibition by methylglyoxal bis-(guanylhydrazone). 3. Actinomycin D and cycloheximide, and RNA and a proteinsynthesis inhibitor respectively, blocked the increase of S-adenosyl-l-methionine decarboxylase activity elicited by alpha-difluoromethylornithine. In polyamine-depleted cells the apparent half-life of elevated S-adenosyl-l-methionine decarboxylase activity, determined by inhibition of protein synthesis, was 2.5-fold longer than in control cells. The present results suggest that elevation of S-adenosyl-l-methionine decarboxylase activity by alpha-difluoromethylornithine is due to stabilization of the enzyme. 4. Restoration of the normal intracellular putrescine content, by addition of putrescine to the medium of polyamine-deficient cells, transiently increased S-adenosyl-l-methionine decarboxylase activity. Thereafter, intracellular conversion of putrescine into spermidine was accompanied by inactivation of the enzyme at a rate that was similar to that found on addition of spermidine itself. No relationship between total intracellular spermine content and S-adenosyl-l-methionine decarboxylase activity could be established. 5. Addition of 1mm-1,3-diaminopropane to polyamine-deficient cells did not cause a decrease in the activity of S-adenosyl-l-methionine decarboxylase, whereas addition of 1,5-diaminopentane (cadaverine) did. 1,3-Diamino-N-(3-aminopropyl)propane did not accumulate in cells treated with alpha-difluoromethylornithine and 1,3-diaminopropane, whereas addition of 1,5-diaminopentane led to the accumulation of 1,5-diamino-N-(3-aminopropyl)pentane. 1,3-Diamino-N-(3-aminopropyl)propane (10mum) was as effective as spermidine in decreasing S-adenosyl-l-methionine decarboxylase activity. Thus effectiveness of a diamine in decreasing enzyme activity is related to its capability of being converted into a closely structurally related homologue of spermidine by spermidine synthase. 6. The spermidine site of action appears to be post-translational since (a) the spermidine-induced decrease of S-adenosyl-l-methionine activity was not prevented by actinomycin D and (b) spermidine in the presence of cycloheximide led to a synergistic inactivation of the enzyme with a decay rate that progressively approached control values. Altogether these results are indirect evidence for a strict negative control of S-adenosyl-l-methionine decarboxylase by spermidine and substantiate previous findings [Mamont, Duchesne, Grove & Tardif (1978) Exp. Cell Res.115, 387-393]. Spermidine appears to act on some processes involved in denaturation and/or degradation of the enzyme protein. Putrescine appears to decrease the rate of these processes. The physiological significance of the regulatory control of S-adenosyl-l-methionine decarboxylase is discussed.

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.

Glucocorticoid induction of tyrosine aminotransferase in cultured cells

For over a decade, tyrosine aminotransferase induction in tissue culture cells has been a useful model system in which to study glucocorticosteroid action. In the 1960s, the establishment in culture of rat hepatomas expressing the inducible enzyme, already known to be induced in liver in vivo, provoked a wide-ranging series of experiments. The data from these experiments have provided considerable information regarding the mechanism of action of steroids. These include the fundamental facts that the steroids act directly on the induced cell in unmetablized form, that removal of steroid results in deinduction, that induction does not require DNA synthesis or massive changes in RNA synthesis, and that cytoplasmic receptor occupancy by active steroids correlates closely with the steroids' ability to affect inductions. Studies in tissue culture cells have led to the analysis of transcriptional and posttranscriptional models attempting to explain enzyme induction. The effects on enzyme induction of nonsteroid hormones and other factors have been studied through the use of tissue culture cells. Finally, cells and clones of cell variants are being used to study enzyme induction, through biochemical analysis and cell genetic approaches, including somatic cell hybridization.

Posttranscriptional regulation of glucocorticoid-regulated functions

Relying heavily on studies of TAT regulation in cultured rat hepatoma cell lines, we have attempted in this brief review to discuss possible mechanisms for posttranscriptional regulation of glucocorticoid-sensitive enzymes and to chronicle the evidence for and against posttranscriptional mechanisms for specific enzyme induction by glucocorticoids. Initially, mechanisms were considered that would reconcile results showing sensitivity of both induction and deinduction of TAT to inhibitors of RNA synthesis with studies demonstrating first that glucocorticoids regulate the rates of specific enzyme synthesis and, then, that glucocorticoids regulate levels of enzyme-specific mRNA. Such reconciliation proved unnecessary when it was demonstrated that inhibitors of RNA synthesis such as actinomycin D were not specific for RNA synthesis, but also had effects on mRNA turnover and protein metabolism. The bulk of evidence to date establishes that glucocorticoids promote the production of enzyme-specific mRNA for the proteins whose synthesis is regulated by thses steroids. Nevertheless, there is still very little direct evidence that steroids can modulate rates of specific gene transcription. The glucocorticoid stimulation of mouse mammary tumor virus RNA production in cultured cell lines is the only example to date where such a mechanism is supported by RNA-DNA hybridization studies. Posttranscriptional actions of steroids on the turnover, processing, or extranuclear transport of specific mRNA precursors remain potential steps at which glucocorticoids might function. The rapid turnover of some glucocorticoid-regulated enzymes and their mRNAs not only ensures a rapid response to steroid addition or withdrawal, but also subjects these proteins to relatively large fluctuations upon alterations in overall protein or mRNA metabolism. Thus many of the inductions and repressions of hepatic TAT and TO by mediators other than the glucocorticoids may be attributable entirely to nonspecific mechanisms.

Functional level of rat liver tryptophan 2,3-dixoygenase messenger RNA during superinduction of enzyme with actinomycin D

Tryptophan 2,3-dioxygenase [EC 1.13.11.11; L-tryptophan:oxygen 2,3-oxidoreductase (decyclizing)] activity is induced by glucocorticoid hormones and superinduced by actinomycin D. Previous experiments had shown that hormonal induction of the enzyme activity is accompanied by parallel increases in tryptophan 2,3-dioxygenase mRNA level. In this study, we measured the tryptophan 2,3-dioxygenase mRNA levels during superinduction as well as hormonal induction, to determine whether superinduction of the enzyme activity is also mediated through changes in mRNA concentration. Tryptophan 2,3-dioxygenase mRNA was measured in a Krebs ascites cell-free protein synthesizing system supplemented with rabbit reticulocyte initiation factors. We found that during superinduction of the enzyme activity by actinomycin D, the mRNA level is identical to that of the actinomycin D-free controls. Our results do not, therefore, support the hypothesis that hormonal induction and/or superinduction of tryptophan 2,3-dioxygenase mRNA are regulated by a rapidly turning over repressor.

The effects of metabolic inhibitors on the synthesis of inducible tyrosine aminotransferase in cultured hepatoma cells

The effects of actinomycin-D and 3'-deoxyadenosine (cordycepin) on the steroid-mediated induction of tyrosine aminotransferase (TAT) synthesis have been reexamined in view of recent reports that the primary inhibitory action of these compounds may affect synthesis of proteins as well as RNA. The present results confirm that cordycepin blocks the steroid-mediated induction of TAT in rat hepatoma cells (HTC), but unlike actinomycin-D, cordycepin neither increases nor maintains the levels of TAT found in HTC cells preinduced with dexamethasone. Indeed, cordycepin added to preinduced cells, either in the presence or absence of steroid, causes a prompt decline in TAT activity. These data also confirm that both actinomycin-D and cordycepin have an early inhibitory effect on protein synthesis, but the cordycepin effect is observed sooner and the extent of inhibition is greater. When actinomycin-D and cordycepin are added simultaneously to preinduced cells with the steroid removed, the actinomycin-td produced maintenance of preinduced levels of TAT persists. Also, the inhibition of protein synthesis in cultures with both inhibitors approaches that for the cells treated with actinomycin-D alone instead of cordycepin alone. These data suggest that cordycepin inhibits TAT synthesis in preinduced cells by its inhibition of protein synthesis, and this inhibitory effect of cordycepin is blocked by actinomycin-D. It is possible that actinomycin-D does this by preventing the incorporation of cordycepin into RNA. However, regardless of the correctness of this speculation, the multiple effects of cordycepin indicate that this inhibitor cannot be used either to prove or rule out the post-transcriptional model for regulation of gene expression. Also, this requirement that protein synthesis must continue in order to maintain pre-induced levels of TAT is inconsistent with the assumption that the maintenance of these induced TAT levels by actinomycin-D is due to inhibition of TAT degradation.

Mechanism of stimulation of murine type-C RNA tumor virus production by glucocorticoids: post-transcriptional effects

We have previously shown that dexamethasone stimulates production of type-C virus from seemingly normal murine fibroblasts (BALB/3T3) and from transformed (Kirsten sarcoma-leukemia virus) nonproducing cells (BALB/K3T3) induced by 5-iododeoxyuridine. In this report, we further examine the mechanism of this effect by using BALB/K3T3 cells. Several observations suggest that this effect is post-transcriptional. The optimal stimulation by dexamethasone is obtained when dexamethasone is given 24 to 48 h after 5-iododeoxyuridine induction. Although this effect is late, time course experiments suggest that dexamethasone does not act to promote release of preformed virions. The stimulation by dexamethasone is blocked when cells are treated with cordycepin (3'-deoxyadenosine) during the first 24 h of induction, but not when cordycepin is added later. Conversely, interferon, which inhibits virus production, interferes with dexamethasone when it is added late or after removal of the steroid. The results of molecular hybridization experiments show that there is no detectable increase in Kirsten sarcoma-leukemia virus-specific RNA in dexamethasone-treated cells (with or without 5-iododeoxyuridine). The results of the time course studies, and the cordycepin, interferon, and hybridization experiments, suggest that the effect of dexamethasone on type-C virus production in this system is post-transcriptional.

Evidence for the genetic control of the sodium pump density in HeLa cells

1. HeLa cells were grown in normal and altered growth solutions; the ion contents, volumes, K sensitive ouabain binding, the Na-K-ATPase and the Na and K transport measured.2. Cells grown in 1 x 10(-4)M ethacrynate or low-K media for 24 hr have a raised [Na](i), a decreased [K](i), and an increased ouabain binding. Those grown in low-K also have an increased Na-K-ATPase activity.3. When cells are put into low-K solutions the [Na](i) initially rises to a high value, and then starts to fall some 8 hours later as the ouabain binding increases, suggesting that these additional sites represent working Na pumps. Flux measurements on low-K cells provide some support for this view.4. Experiments in which sorbitol replaced [Na](o) showed that the increased ouabain binding and Na-K-ATPase was related to the increase in [Na](i) rather than the decrease in [K](i) and was not due to a non-specific effect of [K](o) change.5. The protein synthesis inhibitors cycloheximide and puromycin stopped the effect of ethacrynate and low-K solutions on increased ouabain binding. They also decreased the ouabain binding and K influx in normal cells over 24 hr. Cycloheximide had similar effects on Na-K-ATPase in low-K treated and normal cells. These results suggest that protein synthesis is required for the appearance of more ouabain sensitive sites in the cell membrane, both in response to ethacrynate and low-K treatment and for normal replacement during the cell's life.6. The RNA synthesis inhibitors actinomycin D (AMD) and cordycepin had complex effects on ouabain binding in fresh and ethacrynate treated cells. These inhibitors increased the ouabain binding but decreased the K influx. This discrepancy was due to the appearance of ouabain binding sites with different characteristics from normal sites. A limited investigation of this phenomenon was carried out. Probably AMD stops the normal replacement of sites in the membrane.7. These results are consistent with the hypothesis that HeLa cells have a system for controlling the number of Na pumps in their membranes. This system responds to the level of [Na](i) within the cell and involves protein synthesis. It is not clear to what extent the nucleus is normally involved in this process.