Evidence that natural vs synthetic steroid hormones bind to physicochemically distinct cellular receptors

Physiological action and receptor binding of a newly synthesized and novel antiglucocorticoid

RU 38486, a newly synthesized molecule, reversed glucocorticoid mediated enzyme induction and gluconeogenesis in the liver, and RNA synthesis in rat thymocytes. The transfer of radiolabelled dexamethasone from the cytoplasm to the nucleus was also opposed by RU 38486 in intact thymocytes. Although RU 38486 saturated the same molecular species of the receptor as the hormone in the liver, differences seemed to appear when thymus was taken into account. Along with the ongoing clinical trials, an important new tool thus appears at hand to understand and harness the molecular action of glucocorticoid hormones in mammalian systems.

Nuclear binding of cortisol in intestinal mucosa and liver of a teleost fish (Salmo gairdnerii irideus)

Nuclear binding of corticosteroids in fish tissues was investigated in two target organs of the trout Salmo gairdnerii irideus: the liver and the intestinal mucosa. Incubation of intact nuclei with [3H]-cortisol or [3H]-dexamethasone failed to demonstrate high-affinity binding of these steroids to proteins. Exchange assay of [3H]-cortisol in high-salt nuclear extracts indicated an association constant Ka = 1.9 X 10(4) M-1 for intestinal mucosa and 2.1 X 10(4) M-1 for liver. In sea water-adapted trout, the association constant remained the same as in fresh water. These results extend previous observations obtained on the cytosol which showed that no high-affinity receptors could be disclosed in fish tissues using these two corticosteroids.

Corticosteroid receptors in the kidney of chick embryo. I. Nature and properties of corticosterone receptor

Cytosol from kidney of chick embryo (age 16-18 days) contained a corticosterone-binding site with the features of a putative receptor. This receptor was a thermolabile protein, readily digested by proteolytic enzymes, with a sedimentation coefficient of 7-8 S and with an apparent molecular weight greater than 100,000. Simultaneous studies with transcortin (CBG) revealed several differences between the renal- and serum-binding protein pertaining to the effect of temperature, the sedimentation coefficient, the charcoal "stripping" and, finally, the binding and competition of various steroids for the two proteins. Kinetic analysis showed a rapid association (10 min), which followed second-order reaction kinetics, and a dissociation of pseudo-first-order reaction kinetics with a t1/2 of 168 min at 0 degrees. The analysis of the Scatchard plot showed the presence of a single class of binding sites with an association constant (KA) of 1.3 X 10(8)M-1 and a binding capacity (nmax) of 500-700 fmol/mg protein. We obtained similar results when we used dexamethasone as a ligand. The association (ka) and dissociation (kd) rate constants were respectively 2.9 X 10(6)M-1 sec-1 and 6.86 X 10(-5) sec-1. From their ratio a KA value of 4.2 X 10(10) M-1 was obtained. Studies with various steroids demonstrated that only dexamethasone and, to a lesser degree, progesterone competed for the binding site. These data showed that the kidney of chick embryo possessed one type of receptor for the glucocorticoids, which was similar to the type II described in rat kidney.

Cellular factors which modulate hormone responses: glucocorticoid action in perspective

Virtually all glucocorticoid effects are mediated through changes in gene transcription. Steroid binding to the receptor and nuclear binding of the steroid-receptor complex are pivotal events in this process, but may be modified by the ability of a given steroid to traverse the cell membrane or cause receptor activation. The physical nature of the receptor, its precise subcellular location, and the process by which gene activation is accomplished are unknown. Preparation of purified receptor and further characterization of the nuclear binding sites will be crucial to a better understanding of this process.

Search for antiglucocorticoid activity in rat liver in vivo

Estradiol and testosterone both lowered endogenous liver glycogen and at 20-fold higher doses impaired triamcinolone acetonide mediated glyconeogenesis in adult adrenalectomized male rats. Neither steroid influenced liver tyrosine transaminase although tryptophan pyrrolase activity was depressed by testosterone. Progesterone increased liver tryptophan pyrrolase but did not influence other parameters. Cortexolone did not alter either of these processes whereas cortisol induced both enzymes and, at much higher dose levels, gluconeogenesis. Binding of 3H-triamcinolone acetonide to its cytoplasmic receptor in vitro was left unaffected in presence of 20-fold greater concentration of either sex steroid but almost totally abolished by cold, homologous molecules. Similar results were obtained by 3H-cortisol except that estradiol partially competed for 3H-cortisol binding sites even at 20-fold greater concentrations of cold estradiol. Separation on DEAE-cellulose-52 and Ultrogel 44 columns revealed binding of all steroids to macromolecules of comparable physicochemical properties although the ratios of binding to the various subpopulations of the receptor were a function of the steroid in question. These results are discussed in terms of sex steroid binding to different moieties of a complex, heterogeneous, polymorphic protein rather than inhibition of binding to the active configuration acquired in presence of an inducer.

Evidence for synergism in steroid hormone-receptor association

A number of glucocorticoids stimulated oestradiol binding to liver cytosol receptor; oestradiol activated glucocorticoid receptor association at a time when it reversed triamcinolone mediated increase in liver glycogen synthesis.

Corticosterone binding in AtT-20 pituitary tumor cell cytosol: evidence for one class of binding site for both natural and synthetic glucocorticoids

The AtT-20 mouse pituitary cell is an established, cloned cell line which produced adrenocorticotrophic hormone in a glucocorticoid-suppressible manner. A receptor for glucocorticoids was identified in cytosol prepared from these cells using the natural mouse glucocorticoid, corticosterone, as the labeled ligand. The question of whether this binding component is identical to the one detectable using labeled triamcinolone acetonide was addressed by comparing their physicochemical characteristics and by detailed studied of binding specificity using both ligands. The corticosterone and triamcinolone acetonide binding components behaved similarly on sucrose density gradient analysis and DEAE-cellulose ion-exchange chromatography. Scatchard analysis with corticosterone detected 30% fewer binding sites than a similar analysis with triamcinolone acetonide, probably because corticosterone binding was of lower affinity (Kd = 8.6 . 10(-9)M vs. 1.4 . 10(-9)M) and hence less stable. The relative glucocorticoid binding affinities of thirteen unlabeled steroids were obtained using either labeled steroid as ligand. Both ligands yielded similar results, suggesting that they both detected a similar binding site. The results suggest that AtT-20 cell cytosol contains a single class of binding site which detects both natural and synthetic glucocorticoids.

Heterogeneity of AtT-20 cell glucocorticoid binding sites: evidence for a membrane receptor

In previous studies we have found that intact AtT-20 cells contained two glucocorticoid binding sites with distinctly different affinities and specificity. In this paper, the nature of these sites was investigated by studying glucocorticoid binding to cytosol and to plasma membranes isolated from AtT-20 mouse pituitary tumor cells. Plasma membrane vesicles were isolated from AtT-20 cells and found to take up alpha-aminoisobutyric acid, indicating that they were properly oriented and functionally intact. Corticosterone bound to these vesicle in a time- and temperature-dependent manner. The binding exhibited a glucocorticoid preference since non-glucocorticoids such as progesterone, testosterone or estradiol were unable to inhibit binding. In addition, binding specificity differed from that of the cytoplasmic receptor since the synthetic glucocorticoids were also ineffective competitors. The major inhibitors of binding were corticosterone greater than 11-dehydrocorticosterone greater than 11-ketoprogesterone greater than cortisol. In other complementary studies, AtT-20 cell cytosol was tested to determine whether heterogenous soluble sites exhibiting a preference for the natural vs the synthetic steroid could also be identified. We found that binding sites for both steroid classes were approximately similar in number, specificity and behavior on ion-exchange chromatography. We conclude that, in addition to a classical soluble cytoplasmic glucocorticoid receptor, AtT-20 cells contain plasma membrane glucocorticoid binding sites. The affinity and specificity of these sites for the natural ligand, corticosterone, suggest that they play an important role in the subcellular mechanism of glucocorticoid action.

Glucocorticoid receptors

Glucocorticoid receptors are found in most mammalian tissues and have been studied in detail in a number of tissue culture systems. With cells that have not been exposed to steroids, the receptors are found in the cytoplasmic fraction from which they can be isolated and studied. Methods for studying glucocorticoid receptors depend on their high-affinity specific binding of radioactive steroids. The reversible interaction is intracellular. It follows Michaelian kinetics, at least in cell-free cytosol, and involves a thermodynamically homogeneous population of about 10 000 sites per cell. The receptor is an asymmetric, slightly acidic protein of about 100 000 daltons. It is very labile, especially in the unbound form. Binding activity depends on the integrity of thiol groups and perhaps on phosphorylation of amino acid residues. Although indirect, the evidence is overwhelmingly convincing that this protein is the physiologic glucocorticoid receptor. The time-kinetics of binding and dissociation are consistent with the sequence of events in glucocorticoid action. Various steroid analogs display binding characteristics predictable from their glucocorticoid activity. Loss of the binding protein from certain cultured cell lines is accompanied by unresponsiveness to glucocorticoids. The extensive tissue distribution of receptors parallels the extensive role of glucocorticoids in regulation. Finally, there is a strong correlation between nuclear binding of receptors and nuclear effects of the steroid. The glucocorticoid receptor can be distinguished from other glucocorticoid-binding proteins, based on their steroid specificity and physicochemical properties. There is no clear-cut demonstration that the receptor differs from tissue to tissue, and it is in fact very similar in various species. Unlike in other systems, receptor concentration does not seem to be regulated by its ligand or by other hormones. However, certain cases of hypo- as well as hypersensitivity to glucocorticoids appear to result from changes at the receptor level. The data indicate that the receptor can exist in inactive and active forms. The former predominate in the absence of steroid or when an angatonist is bound. Glucocorticoid agonists bind the active form, allowing it to be "activated" and subsequently bound to the nucleus. All of the receptors in isolated cytosol do not appear to be available for immediate occupancy by an agonist and this may be due to the time required for conversion of the receptors from inactive to active forms. The correlations between receptor binding and the glucocorticoid response indicate that the receptor is a rate-limiting factor in the magnitude and kinetics of the response, and this finding has important implications regarding mechanisms.

Heterogeneity of steroid hormone receptor in adult rat lung cytosol

Chromatography on cellulose DEAE-52 columns revealed that the glucocorticoid receptor for rat lung cytosol consisted of a component in the 0.001 M prewash, revealed with synthetic steroids and natural mineralocorticoids, a second component eluted with 0.04 M PO4, labelled with triamcinolone, dexamethasone, and a third moiety in the 0.06 M PO4 region, evident with natural glucocorticoids (corticosterone, cortisol) as well as mineralocorticoids (aldosterone, deoxycorticosterone). The third component coeluted with rat blood serum transcortin in double labelled experiments. Rate lung was devoid of another component in the 0.02 M PO4 found in rat liver supernate and of the mineralocorticoid receptor evident only in rat kidney. Chromatography on Sephadex G-200 columns revealed a shift of radioactivity from a higher to a lower molecular weight region in the presence of 0.4 M KCl. Collectively, these studies indicate the subunit nature of the lung receptor as evidenced in most tissues hitherto tested. Moreover, polymorphism within a given subunit component can not be revealed by competition alone, as attempted by others, but can be revealed under selected conditions of physical separation.