Effect of concanavalin A on tyrosine aminotransferase in rat hepatoma tissue culture cells. Rapid reversible inactivation of soluble enzyme

Receptor binding affinity and antiproliferative activity of new antiinflammatory antedrugs: 6-methoxycarbonyl prednisolone and its derivatives

Systemic side effects of antiinflammatory steroids may be minimized by incorporation of a metabolically labile group which is metabolized to make the steroid inactive upon entry into the systemic circulation (antedrug concept). In continuing efforts to minimize systemic adverse effects of potent antiinflammatory steroids, we have recently synthesized methyl 11 beta, 17 alpha, 21-trihydroxy-3,20-dioxopregna-1,4-diene-6-carboxylate (P6CM), its 21-acetoxys (P6CMa, P6CMb) and 17,21-acetonide (P6CMacet) derivatives. Structure-activity relationships have now been assessed and compared with prednisolone (P) for glucocorticoid receptor affinity (P IC50 = 28 nM), gluconeogenic activity as induction of tyrosine aminotransferase (EC50 = 4.4 nM) in H4-II-C3 HTC cells and antiproliferative effects (P = 48% inhibition of [3H]thymidine incorporation at 1 microM). Relative potencies for receptor binding (P = 1) were 0.12, 0.03, 0.004, and 0.0008 for P6CM, P6CMa, P6CMb, and P6CMacet, respectively, and enzyme induction relative potencies were 0.13, 0.05, 0.01, and 0.008, respectively. Antiproliferative effects of all derivatives were also less than that of P. These decreases suggest that addition of the 6-carboxymethyl group to prednisolone results in the general reduction of glucocorticoid activities. Taken together with previously reported results demonstrating retention of topical antiinflammatory activity of these novel steroids, P6CM and its derivatives may represent new locally active antiinflammatory steroids with reduced propensity to cause gluconeogenic and antiproliferative adverse effects.

Development of specific bioluminescent in vitro assays for selecting potential antimineralocorticoids

Efficient antimineralocorticoid selection requires a reliable, discriminating and easy assay for monitoring biological activity of not only the specific receptor, but also closely related receptors such as glucocorticoid and progestin. These related activities should be as low as possible to obtain specific antimineralocorticoid compounds. In this paper, we describe two cellular models used for easy and specific measurement of mineralocorticoid and progestin activities. These models involve the induction of firefly luciferase under hormonal control mediated by a chimeric receptor. The first model comprises transiently transfected MCF-7 cells, whereas the second uses stably transfected HeLa cells. Glucocorticoid activity was assayed with the classic tyrosine-aminotransferase induction method in HTC cells. Six compounds of a new family of 11 beta-substituted-17-spirolactone steroids were thus studied and compared to control compounds. Five of them showed antimineralocorticoid activity and one was active at a concentration lower than that of mespirenone.

Enzyme induction and receptor-binding affinity of steroidal 20-carboxamides in rat hepatoma tissue culture cells

The steroidal 20-carboxamides [(20R)- and (20S)-21-(N-substituted amino)-11 beta,17,20-trihydroxy-3,21-dioxo-1,4-pregnadiene] recently have been shown to possess anti-inflammatory activity in animal models of inflammation. These N-substituted methyl, ethyl, n-propyl, and benzyl derivatives also exhibited suppressive effects on plasma corticosterone and thymus function. Generally, the (20R)-hydroxy-20-carboxamides were more potent than the corresponding (20S)-epimers. In continuing investigations on the glucocorticoid effects of these compounds, we have studied their ability to induce tyrosine aminotransferase (TAT), inhibit uptake of [3H]thymidine into DNA, and complete with [3H] dexamethasone for binding to the hepatoma tissue culture glucocorticoid receptor. Results indicated that the N-substituted methyl, ethyl, and n-propyl derivatives were full glucocorticoid agonists in the three measurements. Receptor binding affinities of the N-substituted carboxamides correlated well with their ability to induce TAT activity and to inhibit thymocyte proliferation. Structure-activity relationships indicated that the larger the N-substituent, the weaker the agonist activity in this system, and 20R isomers exhibited higher glucocorticoid agonist activity than the corresponding 20S isomers. This investigation is part of our effort to elucidate structure-activity relationships of steroidal carboxamides synthesized on the basis of the antedrug concept.

Description and analysis of differential sensitivity to glucocorticoids in Fao cells

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

Inverse correlation between dexamethasone 21-mesylate agonist activity and sensitivity to dexamethasone for induction of tyrosine aminotransferase in rat hepatoma cells

Previous results demonstrated that both the level of induction of the liver specific enzyme tyrosine aminotransferase (TAT) by the irreversible antiglucocorticoid dexamethasone 21-mesylate (Dex-Mes) and the concentration of the reversible glucocorticoid dexamethasone (Dex) required for 50% of maximal TAT induction (i.e. EC50) were different in HTC and Fu5-5 rat hepatoma culture cells. In the present study, a retrospective analysis of these two parameters over an 8 yr period indicates that the absolute values of both parameters varied within each cell line over time in a reversible manner. The variation of both parameters appears to be causally related since a linear, reciprocal relationship exists between the amount of Dex-Mes agonist activity and log10 (Dex EC50) in both cell lines (correlation coefficient is -0.896 for n = 46). This relationship was independent of changes in basal TAT level, culture medium, and serum lot. Results with cloned HTC cells indicate that these temporal variations are not due to fluctuations in the relative abundance of two cell populations displaying either high or low amounts of agonist activity with Dex-Mes. While these analyses relied on the detection of enzyme levels, the amount of TAT mRNA is shown to parallel the enzyme levels. Thus the variation in parameters of TAT induction by Dex and by Dex-Mes appears to be modulated at a pre-translational step. Such variations have not previously been observed for the control of specific gene transcripts by other steroid hormones and may be related to the known differences in agonist activity seen for most antisteroids in various systems.

Analysis of the relation between receptor binding affinity and antagonist efficacy of antiglucocorticoids

The biological potencies of four antiglucocorticoids, RU486 (RU), dexamethasone-oxetanone (DOX), R5020, and progesterone have been studied with respect to dexamethasone induction of tyrosine aminotransferase (TAT) in rat hepatoma tissue culture (HTC) cells. Their inhibitory effects in whole-cell competition binding studies (at 37 degrees C) and in TAT induction studies were analyzed by Dixon plots and Schild plots, respectively. We show that: In both cases, there is an actual competition of each antiglucocorticoid with the agonist dexamethasone for the same binding site; the two Kd values derived from the two plots are almost identical for each antiglucocorticoid; RU486 can be distinguished from the three other antiglucocorticoids by its high biological efficacy and its high affinity for the glucocorticoid receptor in whole cells at 37 degrees C (identical to its affinity in cytosol at 0 degree C). These results imply that: There is a linear correlation between the antagonist efficacies of antiglucocorticoids and their affinities for the glucocorticoid receptor in whole cells at 37 degrees C; the antagonistic action is solely mediated by competition with the agonist for the receptor binding site; this is verified by the fact that in all cases, in the presence or absence of antiglucocorticoids, a specific TAT induction level was always related to the same level of receptor saturation by the agonist in whole cells; the phenomena responsible for the high antagonist efficacy of RU486 are also responsible for its high affinity in whole cells at 37 degrees C.

Antiglucocorticoid steroids have increased agonist activity in those hepatoma cell lines that are more sensitive to glucocorticoids

FU5-5 rat hepatoma (Reuber H35) cells are hypersensitive in that the same percentages of full induction of tyrosine aminotransferase (TAT) occur at much lower concentrations of glucocorticoids than in the related HTC rat hepatoma (Morris) cells. Unexpectedly, these hypersensitive FU5-5 cells also exhibited more agonist activity with the affinity labeling antiglucocorticoids cortisol 21-mesylate and dexamethasone 21-mesylate than did HTC cells (Mercier et al., Endocrinology 112, 601-609 [1983]). In the present study, several other antiglucocorticoids (11-desoxycortisone, progesterone, dexamethasone oxetanone, and RU 486 in addition to dexamethasone 21-mesylate) and the antiandrogen cyproterone acetate were examined to see if chemically unreactive, reversible antisteroids also would exhibit an altered activity (i.e. increased agonist activity) in FU5-5 cells. Each antiglucocorticoid examined did display a 2-fold increased amount of agonist activity in FU5-5 cells, as compared to HTC cells; only RU 486 was predominantly an antagonist in FU5-5 cells but the potency of RU 486 was about 9-fold less than in HTC cells. Dexamethasone, and especially progesterone, was metabolized in FU5-5 and HTC cells. However, differential metabolism in FU5-5 vs HTC cells cannot account for the increased induction of TAT in FU5-5 cells since the amount of agonist activity seen for dexamethasone mesylate (or its metabolites) depended not on the cell type used but rather on the glucocorticoid inducible enzyme monitored, i.e. TAT or glutamine synthetase. The combined data suggest that the hypersensitivity of FU5-5 cells towards glucocorticoid induction of TAT may be linked with the ability of both reversible and irreversible antiglucocorticoids to display increased TAT agonist activity in FU5-5 cells. This behavior was somewhat steroid specific since the antiandrogen cyproterone acetate did not display increased TAT agonist activity in FU5-5 cells compared to HTC cells and was only 2-fold less effective as an antiglucocorticoid in FU5-5.

Affinity-labeling steroids as biologically active probes of antiglucocorticoid hormone action

The role of the glucocorticoid receptor in the expression of antiglucocorticoid action has been investigated with a chemically-reactive derivative of three glucocorticoid steroids with differing biological potencies, i.e. the C-21 mesylates of cortisol, dexamethasone and deacylcortivazol. Dexamethasone 21-mesylate (Dex-Mes) was the most useful derivative due to its favorable balance of high receptor affinity and predominantly irreversible antiglucocorticoid activity. A number of criteria have been used to conclude that [3H]Dex-Mes covalently labels glucocorticoid receptors in the steroid-binding cavity. The available data indicate that covalent Dex-Mes-labeled receptors (mol. wt approximately equal to 98,000) are responsible for the irreversible antiglucocorticoid activity while the partial agonist activity of Dex-Mes is due to non-covalent Dex-Mes-bound receptors. Further support for this hypothesis comes from the observations that deacylcortivazol 21-mesylate was a full glucocorticoid and did not affinity label receptors (and marginally labeled cytosol proteins) although it was capable of covalently-labeling bovine serum albumin. Several mechanisms for the expression of irreversible antiglucocorticoid activity by covalent Dex-Mes-labeled receptors were examined and can be eliminated. Covalent receptor-Dex-Mes complexes formed in whole HTC cells were found to have a decreased capacity for nuclear binding. This decreased nuclear-binding capacity could be responsible for the whole-cell irreversible antiglucocorticoid activity of Dex-Mes.

Formation of a fluorescent glucocorticoid receptor-steroid complex in HTC cell cytosol

An intensely fluorescent rhodamine derivative of dexamethasone (i.e. Dex-C2-Rho) was synthesized. Dex-C2-Rho possessed high affinity for HTC cell glucocorticoid receptors in cell-free systems. Whole cell activity and receptor affinity of Dex-C2-Rho were both much lower, apparently due to problems with cell permeability and/or metabolism. A specific, fluorescent receptor-steroid complex at concentrations as low as 1 X 10(-9) M could readily be observed with crude HTC cell receptors after removal of the free Dex-C2-Rho. This appears to be the first report of a fluorescent glucocorticoid receptor-steroid complex.

The release of cytosolic proteins from cells treated with concanavalin A during scraping from plastic culture dishes

When rat hepatoma cells (HTC and R117-21B), treated with concanavalin A (conA) at 37 degrees C, were scraped from plastic culture dishes with a silicone-rubber policeman, the cell membranes were broken and the cytoplasm was released. This phenomenon was also observed in cells treated with conA at 4 degrees C, even though it took a longer time to show the same effect. The effect of 10 micrograms/ml of conA on the release of the cellular proteins reached a plateau when the treatment was carried out at 37 degrees C. Ninety percent of this effect was abolished by 10 mM of alpha-methyl-D-mannoside. The effect was completely nullified by 100 mM. At 4 degrees C, however, even 100 mM of this sugar could not abolish this effect. The apparent decrease in the cellular proteins with conA after scraping was observed not only in the logarithmic phase, but also in the stationary phase of cell growth. The breakdown of plasma membranes with conA eventually caused decrease in tyrosine aminotransferase activity, even though the lectin induced the enzyme activity in cultured cells.

Concanavalin A alters the turnover rate of tyrosine aminotransferase in cultured hepatoma cells

Concanavalin A added to monolayer cultures of Reuber H-35 hepatoma cells caused a rapid inactivation of tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, E.C. 2.6.1.5) and loss of reactivity with antibody against the native, dimeric enzyme. Analysis of treated cells with an antibody raised against carboxymethylated, denatured enzyme showed that the inactivated enzyme was reactive with this reagent, which does not react with the native enzyme. Subsequent addition of alpha-methyl-D-mannopyranoside to remove concanavalin A restored both enzyme activity and reactivity to antibody against native enzyme. After long-term treatment with concanavalin A, the restored enzyme levels were significantly higher than in controls treated with the sugar but not the lectin. Analysis of the turnover of the enzyme by two methods revealed that the rate of its degradation is reduced about 2-fold in concanavalin A-treated cells. Treatment with H-35 cells with concanavalin A thus effects an alteration in conformation of tyrosine aminotransferase, rendering it somewhat less sensitive to intracellular degradation.

Lectins mimic insulin in the induction of tyrosine aminotransferase

Various lectins were found to induce tyrosine aminotransferase in H-35 rat hepatoma cells grown in monolayer culture. Wheat germ agglutinin gave a maximal induction of tyrosine aminotransferase 6 hours after its addition. The induction time course was similar to that elicited by insulin. Fourteen micrograms of wheat germ agglutinin per milliliter gave half-maximal enzyme induction and 50 micrograms per milliliter gave the maximal response. The induction of tyrosine aminotransferase by wheat germ agglutinin was additive with the induction by either dexamethasone or dibutyryl adenosine 3',5'-monophosphate, but was not additive with the tyrosine amino transferase induction by insulin. Wheat germ agglutinin also mimicked insulin in the inhibition of cellular protein degradation in the absence of serum. The insulin-like effects of lectins should be considered in lectin-mediated manipulations such as agglutination.

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.

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.