Control of degradation and synthesis of induced tyrosine aminotransferase studied in hepatoma cells in culture

Degradation of microinjected proteins: effects of lysosomotropic agents and inhibitors of autophagy

HeLa cells, injected with radioiodinated proteins by fusion with RBC ghosts, were exposed to inhibitors of lysosomal proteolysis and autophagy. The degradation of injected [125I]bovine serum albumin (BSA) was unaffected by chloroquine, NH4Cl, nocodazole, colcemid, puromycin, cycloheximide, or enucleation. Although degradation of [125I]lactate dehydrogenase (LDH) and [125I]pyruvate kinase (PK) was inhibited one-third by chloroquine or ammonia, their degradation was unaffected by the other compounds. In contrast, enhanced degradation of 125I-PK resulting from depriving injected HeLa cells of amino acids and serum was inhibited 70% by colcemid and abolished by chloroquine or ammonia. Similarly, degradation of [14C]sucrose-labeled BSA-polylysine conjugates that entered HeLa cells by endocytosis was inhibited as much as 80% by chloroquine and ammonia. Sensitivity of both enhanced proteolysis and degradation of exogenous proteins to ammonia or chloroquine indicates they are effective inhibitors of lysosomal proteolysis in HeLa cells. Failure of ammonia or chloroquine to inhibit degradation of injected 125I-BSA and the modest inhibition of degradation of injected 125I-LDH or 125I-PK indicates that virtually all BSA molecules and most PK or LDH molecules are degraded by a nonlysosomal proteolytic system. Components of this degradative system are present in vast excess or are long lived, since inhibition of protein synthesis for 20 hr had no effect on the degradation of injected proteins.

Induction and maintenance of 2',5'-oligoadenylate synthetase in interferon-treated chicken embryo cells

Treatment of primary cultures of chicken embryo cells with homologous interferon results in a substantial increase in the level of 2',5'-oligoadenylate synthetase activity that can be detected in cell extracts. This increase can be prevented by inhibitors of RNA or protein synthesis and is thus thought to represent the induction of an interferon-inducible gene, perhaps the 2',5'-oligoadenylate synthetase gene itself. To examine this response in greater detail, we studied its kinetics under the following conditions: (i) cessation of interferon treatment after different lengths of time, (ii) delayed inhibition of RNA or protein synthesis, and (iii) combinations of these treatments. The results showed that in cells treated continuously with interferon, the enzyme level reached a peak after 9 h of treatment and then decreased with a half-life of about 30 h, despite the continued presence of interferon. Removal of interferon during induction reduced the peak level of activity that was attained and somewhat accelerated its decline but did not otherwise affect the time-course of the response. On the other hand, removal of interferon after maximum induction clearly accelerated the decay of enzyme activity. This process could be delayed by inhibitors of protein synthesis, which effectively stabilized the induced enzyme. This behavior is reminiscent of other inducible enzymes, such as the steroid-induced tyrosine aminotransferase, and suggests that the level of 2',5'-oligoadenylate synthetase, which is also inducible by steroid hormones in some cell types, is subject to similar control mechanisms.

Induction and maintenance of 2',5'-oligoadenylate synthetase in interferon-treated chicken embryo cells

Treatment of primary cultures of chicken embryo cells with homologous interferon results in a substantial increase in the level of 2',5'-oligoadenylate synthetase activity that can be detected in cell extracts. This increase can be prevented by inhibitors of RNA or protein synthesis and is thus thought to represent the induction of an interferon-inducible gene, perhaps the 2',5'-oligoadenylate synthetase gene itself. To examine this response in greater detail, we studied its kinetics under the following conditions: (i) cessation of interferon treatment after different lengths of time, (ii) delayed inhibition of RNA or protein synthesis, and (iii) combinations of these treatments. The results showed that in cells treated continuously with interferon, the enzyme level reached a peak after 9 h of treatment and then decreased with a half-life of about 30 h, despite the continued presence of interferon. Removal of interferon during induction reduced the peak level of activity that was attained and somewhat accelerated its decline but did not otherwise affect the time-course of the response. On the other hand, removal of interferon after maximum induction clearly accelerated the decay of enzyme activity. This process could be delayed by inhibitors of protein synthesis, which effectively stabilized the induced enzyme. This behavior is reminiscent of other inducible enzymes, such as the steroid-induced tyrosine aminotransferase, and suggests that the level of 2',5'-oligoadenylate synthetase, which is also inducible by steroid hormones in some cell types, is subject to similar control mechanisms.

Studies on the role of protein synthesis and of sodium on the regulation of ornithine decarboxylase activity

The minimum requirements for eliciting or enhancing ornithine decarboxylase activity (EC. 4.1.1.17); L-ornithine carboxylase) in neuroblastoma cells incubated in salts-glucose solutions have been investigated. These incubation conditions permit the study of changes in ornithine decarboxylase activity independently of the growth-associated reactions that occur in cell culture media (Chen, K.Y. and Canellakis, E.S. (1977) Proc. Natl, Acad. Sci. U.S.A. 74, 3791-3795). Ornithine decarboxylase activity can be elicited by a variety of asparagine and other amino acid analogs, including alpha-aminoisobutyric acid, that cannot participate in protein synthesis. Of the eleven asparagine analogs tested, alpha-N-CH3-DL-asparagine is the most potent in eliciting ornithine decarboxylase activity and is equivalent to asparagine in this regard. Inclusion of polar groups into the asparagine molecule results in the loss of its ability to elicit ornithine decarboxylase activity. With the use of these analogs and of analogs of other amino acids it is shown that the rapid fall in ornithine decarboxylase activity that is noted following cycloheximide treatment may not be a consequence of the inhibition of protein synthesis. The rapid fall in ornithine decarboxylase activity is primarily due to the removal of the agent that elicits and stabilizes its activity. These results, the finding that alpha-aminoisobutyric acid stimulates ornithine decarboxylase activity and that sodium is required for the stimulation of ornithine decarboxylase activity are discussed in relation to the "A" amino acid transport system.

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.

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.

Role of the vacuolar apparatus in augmented protein degradation in cultured fibroblasts

Rat embryo fibroblasts, grown in Eagle's MEM with 10% serum, showed a rapid increase in autophagic vacuoles when placed in MEM with 0-1% serum. Concurrent with this response, degradation of cellular proteins showed a 2-fold increase. We did not find any increases in cathepsin D, beta-glucuronidase, beta-galactosidase, and beta-glucosidase, or proteolytic activity of cell homogenates at pH 3.7 towards endogenous substrates. Homogenates prepared in 250 mM sucrose at pH 7.0 showed a 40% increase in protein breakdown. These data support the hypothesis that the induced increase in proteolysis, characteristic of cells placed in a nutritionally deficient medium, is effected by an activated vacuolar apparatus (lysosomes and autophagic vacuoles). We suggest, however, that this mechanism is distinct from normal protein turnover in the cell, but can be rapidly induced by appropriate alterations in the cellular environment. Finally, this induced proteolytic mechanism is not dependent upon an increase in lysosomal enzymes, but rather a structural alteration within the cell which effects a transfer of cellular proteins into the vacuolar apparatus.

Biphasic effects of translational inhibitors on liver tyrosine aminotransferase

Low doses of cycloheximide or emetine cause rat liver tyrosine aminotransferase activity to rise up to twice the control levels in 2 h. By contrast, in the same interval no changes, or only a slight decrease, are produced by either drug at high dosage. Adrenalectomised animals display the same pattern of response. High doses of either antibiotic virtually afford a complete inhibition of 14C-labelled amino acid incorporation into liver and plasma proteins, whereas no more than a 30% decrease is observed with low doses. When administered in the course of the induction by cortisol, high doses of inhibitor prevent any further change in tyrosine aminotransferase activity, stabilising it at the level already attained; low doses, while slightly affecting the synthetic phase evoked by cortisol, drastically interfere with the deinduction. Six hours after various doses of either inhibitor the tyrosine aminotransferase activity is markedly increased, this late effect being largely dependent on the presence of adrenals. The amino acid incorporating actitivy of the liver may exceed that of controls, as observed particularly after small doses of emetine.

Induction of placental alkaline phosphatase in choriocarcinoma cells by 5-bromo-2'-deoxyuridine

Growth of choriocarcinoma cells in the presence of 5-bromo-2'-deoxyuridine (BrdUrd) results in a 30- to 40-fold increase in alkaline phosphatase activity. The effects of BrdUrd is specific for phosphatase with an alkaline pH optimum. The induction by BrdUrd is probably not due to the production of an altered enzyme, since the induced enzyme resembles the basal enzyme in thermal denaturation and kinetic properties. Enzyme induction can be prevented by thymidine but not by deoxycytidine or deoxyuridine. The induction of alkaline phosphatase appears to require incorporation of the BrdUrd into cellular DNA. The presence of BrdUrd in the growth medium is not necessary for alkaline phosphatase induction in proliferating cells containing BrdUrd-substituted genomes. However, enzyme induction and maintenance of the induced levels of alkaline phosphatase in nonproliferating cells containing BrdUrd-substituted DNA requires the presence of the analogues in the medium. The induction of alkaline phosphatase by BrdUrd in probably an indirect process.

The diverse effects of 5'-bromodeoxyuridine on enzyme activities in cultured H35 hepatoma cells

Reuber (H35) hepatoma cells were grown in medium containing 10(-5)M bromodeoxyuridine (BrdU), which was incorporated into their DNA. Cell growth rate was unaffected by BrdU for the first two generations, after which it was reduced by about 50%. The effect of BrdU incorporation on the activities of several enzymes with rapid turnover rates was examined to test the hypothesis that the synthesis of such enzymes will be preferentially inhibited by BrdU. Tyrosine amino-transferase (TAT) activity decreased by 70% within two generations whereas thymidine kinase activity remained at control values. PEP carboxykinase activity was unchanged during the first generation in BrdU-containing medium but, during the second, its activity increased by at least 30%. Ornithine decarboxylase levels decreased by about 50% only after two generations in the presence of BrdU. There appeared to be no simple relationship between turnover rates and the effect of BrdU on enzyme activity. Incorporation of BrdU was found to inhibit the induction of both TAT and PEP carboxykinase by dexamethasone and to enhance the inhibition of cell growth by this steroid. These results are discussed with respect to possible mechanisms of gene expression and development in both normal and neoplastic cells.

Regulation of accumulation and turnover of an inducible glutamate dehydrogenase in synchronous cultures of Chlorella

Earlier studies indicated that the gene of an ammonium-inducible glutamate dehydrogenase (GDH) was inducible throughout the cell cycle and was expressible shortly after replication early in the S-phase in synchronous Chlorella cells growing at a rate of 13% per h in the absence of inducer. In the present study, synchronous cells cultured at the same growth rate in the continuous presence of inducer accumulated this enzyme in a linear manner, with a positive rate change observed late instead of early in the S-phase. At a growth rate of 26% per h, the positive rate change appeared to be displaced to 1.5 h before the S-phase in the next cell cycle. With 2'-deoxyadenosine, an in vivo inhibitor of deoxyribonucleic acid (DNA) synthesis, the magnitude of the positive rate change was shown to be proportional to the relative increase in DNA in the previous cell cycle. Collectively, these data support the idea that expression of newly replicated genes of this enzyme can be delayed into the subsequent cell cycle in cells in the continuous presence of inducer. Studies with cycloheximide indicated that the inducible GDH and another GDH isozyme were stable in fully induced cells in the absence of protein synthesis. However, after ammonium was removed from the culture medium, the activity of the inducible GDH decreased rapidly in vivo, with a half-time of 5 to 10 min at 38.5 degrees C, whereas the rate of accumulation of the other GDH isozyme did not change. Addition of cycloheximide, at the time of inducer removal, prevented this loss in activity of the inducible GDH. The inability to rescue the activity of the inducible GDH, by readdition of ammonium during the deinduction period, indicates that this enzyme probably underwent irreversible inactivation and/or proteolytic degradation.

Cell protein degradation in cultured rat embryo fibroblasts. Suppression by vinblastine of the enhanced proteolysis by serum-deficient media

Rat embryo fibroblasts grown in Eagle's minimal essential medium with 10% serum were labeled with L-[14C]leucine. After a 24 h cold chase, rates of proteolysis were evaluated by measuring the appearance of trichloroacetic acid-soluble 14C in the media. Cells remaining in minimal essential medium with 10% serum (basal) showed a proteolysis rate of 1% per h, whereas cells placed in minimal essential medium alone (serum-deficient) showed a stimulation of proteolysis to 3--4% per h. This enhanced proteolysis was transitory, occurring only for the first 4--8 h after cells were placed in the serum-deficient media. Vinblastine 10-5 M inhibited the enhanced proteolysis 40% but had no effect on basal proteolysis. Control experiments showed no detectable hydrolysis of extracellular proteins, nor did vinblastine affect the rate of protein synthesis. These data suggest that basal and enhanced proteolysis have at least partially distinct mechanisms in the cell and that only enhanced proteolysis involves microtubules.

Reduction by cycloheximide of lysosomal proteolytic enzyme activity and rate of protein degradation in organ-cultured hearts

Cycloheximide, an agent whose primary action is inhibition of protein synthesis, also causes a decrease in the rate of protein degradation in cultured fetal mouse hearts. This is associated with marked decreases in the activities of cathespin D and other lysosomal hydrolases. It is suggested that reduced lysosomal proteolytic capacity may contribute to cycloheximide-induced inhibition of protein degradation.

Some paradoxical effects of inhibitors of protein synthesis on protein turnover in cultured human cells

Low concentrations of cycloheximide, sufficient to block net protein synthesis in growing normal and cancer cells, had no effect on protein turnover, i.e. either the incorporation of labeled amino acids from media lacking other amino acids essential for growth, or the loss to the medium of amino acids from prelabeled cells. At the concentrations that blocked growth, the rate of amino acid incorporation from complete medium was reduced to the "turnover level" i.e. the rate of incorporation seen in amino acid-deficient media. Protein turnover was inhibited only at higher concentrations of the inhibitor. Qualitatively similar results have been obtained with puromycin, anisomycin, emetin and tylocerebrine.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

Requirement for protein synthesis in the regulation of protein breakdown in cultured hepatoma cells

The modes of action of insulin and of inhibitors of protein synthesis on the degradation of labeled cellular proteins have been studied in cultured hepatoma (HTC) cells. Protein breakdown is accelerated upon the deprivation of serum (normally present in the culture medium), and this enhancement is inhibited by either insulin or cycloheximide. An exception is a limited class of rapidly turning over cellular proteins, the degradation of which is not influenced by insulin or cycloheximide. Alternative hypotheses to explain the relationship of protein synthesis to the regulation of protein breakdown, viz., control by the levels of precursors of protein synthesis, regulation by the state of the ribosome cycle, or requirement for a product of protein synthesis, have been examined. Protein breakdown was not influenced by amino acid deprivation, and measurements of valyl-tRNA levels in HTC cells subjected to various experimental conditions showed no correlation between the levels of charged tRNAVal and the rates of protein degradation. Three different inhibitors of protein synthesis (puromycin, pactamycin, and cycloheximide) suppressed enhanced protein breakdown in a similar fashion. A direct relationship was found between the respective potencies of these drugs to inhibit protein synthesis and to block enhanced protein breakdown. When cycloheximide and insulin were added following a prior incubation of HTC cells in a serum-free medium, protein breakdown was maximally suppressed within 15-30 min. Actinomycin D inhibited protein breakdown only after a time lag of about 90 min. It is suggested that the regulation of protein breakdown in hepatoma cells requires the continuous formation of a product of protein synthesis, in a manner analogous to the mode of the control of this process in bacteria.

The effect of inhibitors in vivo on protein synthesis and the amino acid pool in the sheep blowfly, Lucilia cuprina

1. The rates of detoxification of cycloheximide (33 mug/g fresh wt.), puromycin (167 mug/g fresh wt.) and actinomycin D (1 mug/g fresh wt.) were assessed in vivo on the basis of acid-insoluble [14C]leucine incorporation in the sheep blowfly, Lucilla cuprina; these were compared with quantitative estimates which took account not only of incorporation data but also of leucine pool size and turnover. Quantitatively, cycloheximide and puromycin were still at least 50% effective in inhibiting protein synthesis after 6.5 and 24.5h of exposure respectively, whereas values based only on incorporation data suggested that cycloheximide was 83% effective and puromycin completely ineffective after the respective periods. Quantitative estimates also showed that actinomycin D effectiveness increased with increasing exposure over 24.5h, in contrast with values based only on incorporation data, which suggested that it was completely ineffective after 24h.2. All inhibitors affected the dynamic state of the amino acid pool; there was a marked decrease in the rate of leucine-pool turnover as well as an increase in the half-life of leucine in the pool. 3. Inhibition of protein synthesis resulted in changes in leucine-pool size; the most pronounced increase occurred with cycloheximide and puromycin and the most pronounced decreases with actinomycin D. 4. Evidence is presented which suggests that proteolysis is functionally linked to protein synthesis, which determines its rate indirectly.

"Superinduction" of tyrosine aminotransferase by actinomycin D: a reevaluation

Reexamination of the effects of actinomycin D (AMD) on the intracellular level and rate of synthesis of tyrosine aminotransferase (TAT) in hepatoma tissue culture (HTC) cells reveals that much apparent controversy can be resolved with acknowledgment of the multi-faceted nature of this inhibitor's action. AMD can slow overall protein synthesis and inhibit the degradation of both TAT and its mRNA as well as block the synthesis of RNA. The extent of these secondary actions of the inhibitor depend somewhat upon the growth condition of the cells. The effects of cordycepin (3'-deoxyadenosine) on the metabolism of TAT and its mRNA are also complex, but differ in several respects from those of AMD.

Inactivation of tyrosine aminotransferase in neutral homogenates and rat liver slices

Inactivation of tyrosine aminotransferase induced in vivo by triamcinolone was studied in a homogenate incubated at neutral pH values. The integrity and the presence of subcellular particles together with a compartment of acidic pH are necessary for inactivation of tyrosine aminotransferase. It is suggested that tyrosine aminotransferase is inactivated inside lysosomes. The system responsible for inactivation of tyrosine aminotransferase was partially purified and identified with lysosomal cathepsins B and B(1). Inactivation of tyrosine aminotransferase in liver slices is controlled by the amino acid concentration and strongly stimulated by cysteine. 3,3',5-Tri-iodo-l-thyronine reversibly and strongly decreases the rate of inactivation of tyrosine aminotransferase. The effect is not due to an increased rate of tyrosine aminotransferase synthesis.

Expression of tyrosine aminotransferase activity in somatic-cell heterokaryons: evidence for negative control of enzyme expression

Cells of the tyrosine aminotransferase (EC 2.6.1.5)-inducible line, HTC, were fused with those of the BRL-62 line, which contain little or no enzyme and are unresponsive to the usual steroid inducers. Histochemical examination of the resulting heterokaryons within 24 hr of the fusion showed them to have little or no enzyme and to be unresponsive to steroids. These experiments disclose that the observed negative dominance is universal in the fused cells and that such dominance occurs promptly after fusion, when loss of chromosomes has not occurred.