Modulators of the glucocorticoid receptor also regulate mineralocorticoid receptor function

The sea lamprey (Petromyzon marinus) has a receptor for androstenedione

The use of nuclear steroid receptors as ligand-activated transcription factors is a critical event in vertebrate evolution. It is believed that nuclear steroid receptors arose at or before the vertebrate radiation, except for an androgen receptor (Ar) that evolved only in the gnathostome line. We report an androgen-Ar complex in the male sea lamprey (Petromyzon marinus), an extant jawless vertebrate. The androgen with the highest affinity is not testosterone, but its direct precursor, androstenedione (Ad). To establish that the binding moiety in lamprey testis is a receptor-and not an "androgen-binding protein"-we have shown that it can be extracted from the nucleus as well as the cytosol, that the Ad-receptor complex binds to DNA, and that the receptor is approximately twice the size of an androgen-binding protein extracted from the Atlantic salmon testis. The capacity (and high affinity) of binding of the lamprey Ar is such that much of the Ad present in male lampreys becomes sequestered within the testis (as opposed to circulating in the plasma). Concentrations of Ad (but not of testosterone) in plasma and testis tissue are upregulated by injection of lamprey GnRH. Implantation of male lampreys with exogenous Ad significantly accelerates the development of the testis and growth of at least one secondary male characteristic. It appears that all classes of steroid hormones have contributed to the evolution of the regulatory complexity of steroid receptors found in modern vertebrates.

A low-molecular-weight cytosolic inhibitor of the specific testosterone binding in bovine seminal vesicles

Bovine seminal vesicle cytosol contains a low-molecular-weight and thermostable substance which specifically inhibits the binding of testosterone to its cognate receptor. The mass and the ether phospholipid structure of the inhibitor were elucidated by mass spectrometry. Saturation and binding experiments indicate that the inhibitor acts in a dose-dependent and competitive manner, altering the apparent dissociation constant (K(D)) while maintaining the number of androgen-binding sites (B(max)). Its possible role in the regulation of androgen binding activity is discussed.

Decreased apparent affinity of corticosterone for colonic crypt glucocorticoid receptors is dependent on the cellular milieu and is distinct from corticosterone metabolism

[3H]Steroid binding in intact colonic crypt cells and cytosol was compared to determine if 11beta hydroxysteroid dehydrogenase (11betaHSD) activity modulates access of corticosterone (B) to both glucocorticoid receptors (GR) and mineralocorticoid receptors (MR). Cytosol from non-adrenalectomized rat colonic crypt cells showed no 11betaHSD activity, and B bound with high affinity to both MR (Kd=0.47+/-0.03 nM; Bmax=177+/-34 fmol/mg protein) and GR (Kd=4.5+/-0.3 nM; Bmax=279+/-40 fmol/mg protein). In contrast, intact colonic crypt cells incubated with 25-30 nM [3H]B for 90 min converted 62% of B to 11-dehydrocorticosterone, with little binding to MR (14+/-3 fmol/mg protein) and GR (22+/-5 fmol/mg protein). When 11betaHSD activity was inhibited with carbenoxolone, and the same concentration of [3H]B used, binding of [3H]B to MR increased 10-fold to 122+/-12 fmol/mg protein, not significantly different from MR levels in colonic crypt cytosol. In contrast, [3H]B binding to GR in intact cells increased only 1.6-fold to 36+/-9 fmol/mg protein, significantly less than to GR in cytosol (212+/-24 fmol/mg protein). Scatchard analysis showed both lower levels of GR and an apparently lower affinity for [3H]B in colonic crypt cells (Kd=31+/-3 nM; Bmax=130+/-21 fmol/mg protein) compared with cytosol (Kd=4.5+/-0.3 nM; Bmax=279+/-40 fmol/mg protein. [3H]Dexamethasone similarly showed an apparently lower affinity and capacity for GR (Kd=8.8+/-1.3 nM; Bmax=232+/-32 fmol/mg protein) in intact cells compared with cytosol (two separate determinations, Kd=2.6 and 2.9 nM; Bmax=369 and 300 fmol/mg protein). In contrast, [3H]aldosterone displayed similar affinity and capacity for MR in both intact cells (Kd=2.0 nM; Bmax=121 fmol/mg protein) and cytosol (Kd=1.5 and 1.4nM; Bmax=115 and 93 fmol/mg protein). These findings demonstrate not only that 11betaHSD modulates binding to both MR and GR in colonic crypt cells, but also that an additional mechanism(s) operating in whole cells but not in cytosol selectively reduces the affinity and capacity of colonic GR for both natural and synthetic ligands.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Hsp90 mutants disrupt glucocorticoid receptor ligand binding and destabilize aporeceptor complexes

In order to attain competence to respond to hormone, certain steroid hormone receptors must be assembled into hetero-oligomeric aporeceptor complexes, containing Hsp90 and other proteins. Members of the Hsp90 gene family are highly conserved, strongly expressed, and required for viability in eukaryotic organisms. To elucidate the role of Hsp90 in the activity of steroid hormone receptors in vivo, four Hsp90 mutatns, which cause defects in glucocorticoid receptor (GR) signaling, but support the viability of Saccharomyces cerevisiae, were previously isolated (Bohen, S. P., and Yamamoto, K. R. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11424-11428). In this study, I characterize the effects of the Hsp90 mutants on GR ligand response, ligand binding activity, and aporeceptor complex stability. The mutants fall into two classes. Three of the Hsp90 mutants cause defects in GR ligand binding in vivo and form aporeceptor complexes that are unstable in vitro, relative to those containing wild-type Hsp90. The other mutant affects GR signaling, but aporeceptor complexes with this mutant are not defective for ligand binding or stability. These findings indicate that the binding of Hsp90 to GR in the aporeceptor complex is insufficient to induce a high ligand affinity conformation, rather the high ligand affinity to GR requires a specific interaction with Hsp90, which is altered by certain Hsp90 mutants.

Isolation of Hsp90 mutants by screening for decreased steroid receptor function

The 90-kDa heat shock protein Hsp90 represents a highly conserved strongly expressed gene family; in Saccharomyces cerevisiae, Hsp90 proteins are essential for cell viability. Hsp90 interacts with certain cellular proteins, including steroid hormone receptors, tyrosine and serine/threonine kinases, and other heat shock proteins, but its biological functions are not understood. The unliganded glucocorticoid receptor must interact with Hsp90 to acquire competence for high-affinity hormone binding and subsequent transcriptional regulation. By screening in yeast for defects in glucocorticoid receptor function, Hsp90 mutants were isolated. Four such mutants are described, all of which interact with the glucocorticoid receptor but display distinct defects in ligand responsiveness and differences in growth and resistance to high temperature.