Analysis of the induction and deinduction of tyrosine aminotransferase in enucleated HTC cells

Subcellular compartmentalization of MCF-7 estrogen receptor synthesis and degradation

Turnover of the estrogen receptor protein was studied by using enucleation of human breast cancer-derived MCF-7 cells, to examine receptor synthesis and receptor degradation in the separated cytoplasmic compartment (cytoplasts) and nuclear compartment (nucleoplasts). Cytoplasts synthesized estrogen receptors as measured by both hormone-binding and immunoassay, while estrogen receptors (but not progesterone or glucocorticoid receptors) were rapidly degraded in nucleoplasts with a half-life of 3-4 h. Little or no degradation of estrogen receptors in cytoplasts was observed under several conditions. Interestingly, MCF-7 cytoplasts contained approximately 15% of the cell's estrogen receptors, which were not 'translocated' by treatment with 17 beta-estradiol before enucleation. We conclude that the estrogen receptor can be synthesized at least to a hormone binding form in the cytoplasm alone without requiring processing in the nucleus, while the nucleus (or perinuclear cytoplasm) is the primary site of degradation of the estrogen receptor protein. In addition, the presence of a population of estrogen receptors that is cytoplasmic but nontranslocatable may need to be considered in the subcellular localization and actions of steroid receptors.

Alterations in the hepatic glucocorticoid response to mirex treatment

Corticosterone has been shown to be involved in the regulation of mirex-induced adaptive liver growth. To further investigate the role of corticosterone in this response, plasma corticosterone, hepatic tyrosine aminotransferase (TAT) activity, and hepatic cytosolic binding of glucocorticoids were determined in male Sprague-Dawley rats following a single oral dose of mirex (100 mg/kg body wt). Mirex stimulated a significant elevation in plasma corticosterone levels 12 and 24 hr after dosing; however, hepatic tyrosine aminotransferase activity was not induced above control levels 6, 12, or 24 hr after mirex dosing. Mirex does not appear to directly inhibit the enzyme because tyrosine aminotransferase activity was increased in a dose-dependent manner in both intact and adrenalectomized rats when corticosterone supplements (1-50 mg/kg body wt) were given after mirex dosing. In an effort to explain the lack of hepatic TAT induction, the concentration of cytosolic binding sites for [3H]dexamethasone in intact, adrenalectomized, and adrenalectomized corticosterone-supplemented rats was measured 12, 24, and 48 hr after mirex dosing. There was a significant decrease in the total concentration of cytosolic binding sites for [3H]dexamethasone 12 and 48 hr after mirex dosing in intact rats, 12 and 48 hr after mirex dosing in adrenalectomized rats, and 12 and 24 hr after mirex dosing in adrenalectomized corticosterone-supplemented rats. There was a significant increase in the apparent dissociation constant (Kd) in intact rats dosed with mirex as compared to the oil controls, but there was no difference in Kd after mirex dosing in the adrenalectomized (ADX) rats when compared to the Kd for the oil-dosed control rats. The maximal binding capacity (Bmax) was not significantly different from oil controls after mirex dosing in either intact or ADX rats. The lack of hepatic TAT induction in the presence of increased plasma levels of corticosterone appears to be related to glucocorticoid receptor alterations in the liver of intact rats.

Synthesis of globin RNA in enucleated differentiating murine erythroleukemia cells

In an earlier report (Volloch, V., 1986, Proc. Natl. Acad. Sci. USA., 83:1208-1212) we had presented evidence for the occurrence of the cytoplasmic synthesis of globin mRNA and of RNA complementary to globin mRNA which differed from DNA-dependent transcription by its insensitivity to actinomycin D. In this paper, we describe the use of enucleated differentiating mouse erythroleukemia cells to demonstrate directly the occurrence of cytoplasmic synthesis of both positive- and negative-strand globin RNA. For this purpose, we developed an enucleation procedure which yielded pure cytoplasts from differentiated mouse erythroleukemia cells in the absence of cytochalasin B and selectively permeabilized the cytoplasts to small molecules by treatment with dextran sulfate and saponin. The permeabilized cytoplasts incorporated [3H]dUTP into positive- and negative-strand globin RNA and experiments with mercurated nucleotide substrate suggested that this process involved de novo RNA synthesis rather than limited terminal nucleotide addition. Globin RNA synthesis required Mg++, was inhibited by Mn++, and was unaffected by the addition of Zn++. Studies of its response to inhibitors of DNA-dependent RNA synthesis showed that it differed from that process in its insensitivity to actinomycin D and alpha-amanitin, but that like many other macromolecular biosynthetic reactions it was inhibited by rifamycin AF/ABDP and aurintricarboxylic acid. These observations provide additional evidence for the occurrence of cytoplasmic RNA-dependent RNA synthesis in differentiated cells and show permeabilized enucleated cells to be a useful experimental system for further studies of the characteristics of that process.

Glucagon regulation of amino acid transport in hepatocytes: effect of cell enucleation

Glucagon and cAMP analogs stimulate amino acid transport in freshly isolated hepatocytes by inducing the synthesis of new transport proteins. The role of the cell nucleus in the glucagon regulation of amino acid transport has been studied in rat hepatocytes enucleated by centrifugation through a discontinuous Ficoll gradient in the presence of cytochalasin B. Enucleated hepatocytes take up alpha-aminoisobutyric acid (AIB) through a Na+-dependent transport component with kinetic properties similar to those found in intact hepatocytes. Cytoplasts prepared from glucagon-stimulated cells retain the increase AIB transport induced by the hormone in the intact cells. The direct addition of glucagon to cytoplasts has no effect on AIB transport, in spite of the fact that the cytoplasts exhibit a higher capacity to bind glucagon than their nucleated counterparts. These data indicate that the nucleus is required for the glucagon stimulation of amino acid transport in isolated hepatocytes.

Glucocorticoid regulation of amino acid transport in anucleate rat hepatoma (HTC) cells

The transport of alpha-aminoisobutyric acid (AIB) by rat hepatoma tissue culture (HTC) cells is rapidly and reversibly inhibited by dexamethasone and other glucocorticoids. To investigate the role of the nucleus in the regulation of transport and to determine whether steroid hormones or steroid-receptor complexes may have direct effects on cytoplasmic or membrane functions, we have examined the regulation of transport by dexamethasone in anucleate HTC cells. Cytoplasts prepared from suspension cultures of HTC cells fully retain active transport of AIB with the same kinetic properties as intact cells. However, the uptake of AIB is not inhibited by dexamethasone or other corticosteroids. Neither is the inhibited rate of transport, manifested by cytoplasts prepared from dexamethasone-treated cells, restored to normal upon removal of the hormone. Anucleate cells exhibit specific, saturable binding of [3H]dexamethasone; however, the binding is reduced compared with that of intact cells. The nucleus is thus required for the glucocorticoid regulation of amino acid transport in HTC cells.

Xeroderma pigmentosum variants have a slow recovery of DNA synthesis after irradiation with ultraviolet light

Human cells (normal and xeroderma pigmentosum variant) irradiated with ultraviolet light and pulse-labelled with [3H]thymidine underwent transient decline and recovery of molecular weights of newly synthesized DNA and rates of [3H]thymidine incorporation. The ability to synthesize normal-sized DNA recovered more rapidly in both cell types than thymidine incorporation. During recovery cells steadily increased in their ability to replicate normal-sized DNA on damaged templates. The molecular weight versus time curves fitted exponential functions with similar rate constants in normal and heterozygous xeroderma pigmentosum cells, but with a slower rate in two xeroderma pigmentosum variant cell lines. Caffeine added during the post-irradiation period eliminated the recovery of molecular weights in xeroderma pigmentosum variant but not in normal cells. The recovery of the ability to synthesize normal-sized DNA represents a combination of a number of cellular regulatory processes, some of which are constitutive, and one of which is altered in the xeroderma pigmentosum variant such that recovery becomes slow and caffeine sensitive.

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.

Glucocorticoids and the plasma membrane

Glucocorticoids affect the composition and function of the plasma membrane in a variety of cell types. Cultured rat hepatoma (HTC) cells in tissue culture provide an excellent model system for analysis of such effects. In these cells, dexamethasone rapidly and dramatically inhibits the influx of amino acids sharing the A or alanine-preferring transport system. Inhibition is half-maximal within 2 h, and maximal after 6 h incubation with the hormone. The inhibition is rapidly reversed by insulin, and more slowly by removing the steroid. Microtubules and microfilaments are not apparently involved in this hormonal effect, but continuous protein synthesis is required for the glucocorticoid inhibition of transport. Dexamethasone also decreases the number of microvilli on the surface of HTC cells, increases their adhesiveness to a substratum, and dramatically decreases the production of plasminogen activator, but it does not affect the growth rate or plating efficiency of the cells. Variant cell lines stably resistant to dexamethasone inhibition of plasminogen activator production have been isolated using an agar-fibrin overlay technique to detect protease production by individual colonies of HTC cells. The hormonal resistance to inhibition of protease production is associated witha maintenance of inducibility of other glucocorticoid-regulated functions and therefore is not apparently secondary to abnormal or absent glucocorticoid receptor, but due to a lesion in a later step in hormone action specific for plasminogen activator. Combined genetic and biochemical analysis of such dexamethasone-resistant variants should facilitate study of the hormonal regulation of specific membrane phenotypes and of the role of proteases in this regulation.

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.

Absence of interaction between X-rays and UV light in inducing ouabain- and thioguanine-resistant mutants in Chinese hamster cells

Chinese hamster ovary cells were irradiated with X-rays at times from 0 to 17 h before being irradiated with ultraviolet (UV) light. No synergism was observed between the two radiations for the production of mutants resistant to either ouabain or 6-thioguanine. These experiments were designed to test whether X-rays induced an error-prone repair system that would increase the frequency of mutations produced by UV light, but no such system was detected.

Cycloheximide causes increased accumulation of translatable mRNA for tyrosine aminotransferase and tryptophan oxygenase in livers of cortisol-treated rats

Messenger RNA activities for two cortisol-inducible enzymes, tyrosine aminotransferase and tryptophan oxygenase, have been determined by translation in a wheat germ system. The effects of cycloheximide on the two mRNA activities have been evaluated. Cortisol leads to an increase of the translatable mRNAs for tyrosine aminotransferase and tryptophan oxygenase with a maximum at approximately 6 h. Cycloheximide was administered 4 h after treatment with cortisol; 2 h later, the activities of tyrosine aminotransferase and tryptophan oxygenase mRNA had increased five-fold and two-fold, respectively, compared to the activities reached with cortisol alone. Thereafter the amount of the two translatable mRNAs declined, though 14 h after cortisol administration the mRNA activities were still several fold higher than in control animals. Application of alpha-amanitin together with cycloheximide did not prevent an increased accumulation of specific translatable mRNAs. The increase in tyrosine aminotransferase and tryptophan oxygenase activity by cortisol was immediately blocked by cycloheximide. Whereas tryptophan oxygenase activity rapidly declined after cycloheximide application, tyrosine aminotransferase activity remained at the same level. Approximately 4 h thereafter, both enzyme activities increased again.

The stability of tyrosine aminotransferase and other proteins in enucleated rat hepatoma tissue culture cells

The role of the nucleus in bringing about the induction of tyrosine aminotransferase (TAT) by glucocorticosteroid hormone and its deinduction upon steroid removal has been studied in enucleated rat hepatoma tissue culture cells (FU5-5). Both processes require the presence of the nucleus. However, cytoplasts from preinduced cells show an initial rapid decline in enzyme activity immediately after enucleation followed by maintenance of a constant level of activity. This initial decline in enzyme activity can be partially prevented by trypan blue, an inhibitor of lysosomal activity. This suggests that the early fall in enzyme activity could be due to an increase in the level of lysosomal activity immediately after enucleation. The subsequent constant level of activity seems due to maintenance rather than synthesis and degradation since it is not affected by cycloheximide. The absence of degradation applies to other kinds of proteins in enucleated FU5-5 cells and enucleated mouse fibroblast L cells. These experiments suggest that some kind of labile RNA or protein dependent on the presence of the nucleus is required for the degradation of all classes of proteins in different kinds of cells.

Evidence for genetic control of glycine uptake in cultured cells, regulated by the amino acid concentration of the growth medium

1. Cultured cells were grown in various concentrations of amino acids for periods up to 3 days and the characteristics of the glycine transport system measured under fixed experimental conditions. During this time, the effect of enucleation, using cytochalasin B, and the effects of protein synthesis inhibitors (cycloheximide and actinomycin D) were investigated. 2. Glycine influx is regulated by the prior growth concentration of similarly transported amino acids. 3. The modification in transport involves primarily a change in Vmax (but also a change in Km in HeLa cells) and is effected within 2-10 hr after media change. Increased transport activity is calculated to be sufficient to compensate for the reduction in extracellular amino acid concentration, so that nearly normal influx values from media are maintained. Regulation over the range of concentrations studied is shown to be very accurate. 4. The nucleus is essential for the regulatory mechanism to function. It seems probable that mRNA synthesis is required for acquisition of increased transport activity and mRNA translation required for maintenance of normal activity. 5. The controlling factor in the regulatory mechanism appears unlikely to be intracellular pool size. Other possible signals are discussed.