Competitive inhibition of dexamethasone binding to the glucocorticoid receptor in HTC cells by tryptophan methyl ester

Inhibition of 3,5,3'-triiodothyronine binding to its receptor in rat liver by protease inhibitors and substrates

Various protease inhibitors (e.g. phenylmethanesulfonyl fluoride (PMSF), tosyl-phenylalanine chloromethyl ketone (TosPheCH2Cl)) and substrates (e.g., tosyl-arginine methyl ester (TosArgOMe), tryptophan methyl ester (TrpOMe)) inhibit the binding of adrenal and sex steroids to their cognate receptors (Hubbard and Kalimi (1985) Mol. Cell. Biochem. 66, 101-109). Here we extend this finding to the receptor for 3,5,3'-triiodothyronine (T3) in rat liver nuclei. We find that PMSF, TosPheCH2Cl and other protease inhibitors as well as TosArgOMe, TrpOMe, tyrosine methyl ester (TyrOMe) and tyrosine ethyl ester (TyrOEt) inhibit binding of 125I-T3 to its receptor in rat liver nuclei. Inhibition by protease substrates appears to be at or close to the hormone binding domain. By analogy with the known mechanism of binding of protease inhibitors and substrates to enzymes, we suggest that the T3 receptor contains a nucleophilic site at or close to the hormone binding domain.

Aprotinin inhibits the hormone binding of the estrogen receptor from calf uterus

Micromolar concentrations of the proteinase inhibitor, aprotinin, produced a dose-dependent inhibition in the binding capacity of the estrogen receptor from calf uterus. Aprotinin inhibition was greater at 28 degrees C than at 4 degrees C and only occurred when conditions allowed the receptor transformation. When aprotinin was tested in the presence of transformation inhibitors, its effect was no longer seen. The binding capacity of the highly purified estrogen-binding subunit was similarly inhibited.

Estradiol receptor has proteolytic activity that is responsible for its own transformation

We have investigated the effect of various protease inhibitors and substrates on the hormone- and temperature-dependent binding of partially purified estradiol-receptor complex to isolated nuclei. Only serine protease substrates and inhibitors significantly depressed estradiol receptor transformation. At 20 degrees C, we observed 50% inhibition with about 3 microM aprotinin or with 1.4 mM diisopropyl fluorophosphate. Aprotinin also blocked those size and charge modifications of receptor that are characteristic of the transformation process. The estradiol receptor was able to bind to aprotinin-agarose only under transforming conditions; i.e., the interaction was hormone- and temperature-dependent and inhibited by molybdate. Diisopropyl fluorophosphate, a covalent reagent for serine esterases, competitively inhibited the binding and specifically eluted the estradiol-receptor complex that had been bound to aprotinin-agarose. These results indicate that estradiol receptor transformation is due to the effect of a serine protease and that the receptor itself is endowed with this catalytic activity, which is triggered by the steroid.

Effects of protease inhibitors on nuclear binding of glucocorticoid hormones in C3H10T1/2 cells

We have measured incorporation of the glucocorticoid hormone cortisone into nuclear hormone-receptor complexes in the C3H10T1/2 cell line. As we had found cortisone to be capable of malignantly transforming these cells in vitro, and certain protease inhibitors have been shown to suppress transformation in this cell line, we investigated the effects of these protease inhibitors (antipain, chymostatin and the Bowman-Birk inhibitor) on the formation of nuclear cortisone-receptor complexes. All 3 inhibitors were found to suppress wholly or partially formation of nuclear cortisone-receptor complexes, suggesting that such complexes may be involved in the process of glucocorticoid-enhanced transformation.

Structure and regulation of the glucocorticoid hormone receptor

The glucocorticoid receptor is an intracellular protein which possesses three distinct domains, one that binds agonist and antagonist steroids, one that binds DNA, and one that binds anti-receptor antibodies and is required for glucocorticoid modulation of gene expression. In intact cells, receptor number, affinity and activity can change in response to factors that bind to the receptor, or that act indirectly through ill-defined mechanisms which may include resumption or arrest of cell cycling and variations in intracellular calcium ion concentrations. Some of these factors appear to exert their effect by controlling critical receptor properties such as ATP-dependent phosphorylation, integrity of thiol groups, and exposure of key amino acid residues. Glucocorticoid agonists promote the 'transformation' of the receptor into the DNA-binding state, which is competent for modulating gene expression. Glucocorticoid antagonists are steroids that interact with the receptor but either fail to produce a stable complex or produce a stable but inefficient complex. Although substituent groups that confer agonist or antagonist activity to the steroid have been identified, the molecular determinants of this difference at the receptor level remain unknown. Most in vitro and in vivo data on receptor regulation can be accommodated by postulating the existence of an intracellular cycle that involves five states of the receptor. The active free receptor is phosphorylated, reduced, and presumably oligomeric (state A). Following binding of an agonist (state B), it can become transformed by dissociation into its subunits and dephosphorylation (state C). The transformed receptor then interacts with chromatin (state D). Dissociation of the steroid and oxidation of receptor thiol group(s) lead to the inactive receptor form (state E). Reduction and rephosphorylation of the receptor enable it to bind steroids again so that the cycle is closed.

Factors affecting tryptophan-induced hypoglycaemia in rats

The mechanisms whereby tryptophan administration leads to hypoglycaemia in some groups of rats but not others have been investigated. Animals insensitive to tryptophan are rendered responsive by adrenalectomy. This effect is reversed by steroid replacement. Turnover studies with [2-3H]glucose show that hypoglycaemia in sensitive animals is associated with a decrease in glucose synthesis. Tryptophan administration causes a marked and sustained increase in plasma glucagon concentrations in all animals. The locus of the inhibition of gluconeogenesis in tryptophan-sensitive animals is the reaction catalysed by phosphoenolpyruvate carboxykinase. The sensitivities to tryptophan of gluconeogenesis in isolated hepatocytes from normal and adrenalectomized animals were similar. Cells from chronically streptozotocin-diabetic animals required higher concentrations of the amino acid for the same effect. These results are discussed in relation to previous discrepancies in the literature, and a unifying hypothesis for tryptophan-induced hypoglycaemia is proposed.

Inhibition of serine proteases by steroids

Proteolysis of 14C-labeled globin, as well as the hydrolysis of the specific substrate benzoyl tyrosine ethyl ester, by purified bovine chymotrypsin was found to be inhibited by several steroid hormones. The inhibition of chymotrypsin by the steroids was of a competitive nature, with Ki values of 9.9 x 10(-5) M for triamcinolone (9-fluoro-11 beta, 16 alpha, 17,21-tetrahydroxy-1,4-pregnadiene-3,20-dione), 1.6 x 10(-4) M for cortisol (11 beta, 17 alpha, 21-trihydroxypregn-4-ene-3,20-dione), 3.7 x 10(-4) M for testosterone (17 beta-hydroxy-4-androsten-3-one), 5.0 x 10(-4) M for dexamethasone (9-fluoro-11 beta, 17,21-trihydroxy-16 alpha-methyl-1,4-pregnadiene-3,20-dione) and 1.0 x 10(-4) M for epicortisol (11 alpha, 17,21-trihydroxy-4-pregnene-3,20-dione). The activity of purified bovine trypsin on its specific substrate, TAME (tosyl arginine methyl ester), also showed a similar pattern of inhibition by steroids. Both chymotrypsin and trypsin were found to bind 3H-labeled dexamethasone and cortisol. This binding was markedly inhibited by the general protease inhibitor, PMSF (phenylmethanesulfonyl fluoride), whereas the chymotrypsin-specific inhibitor, TPCK (L-[1-tosyl-amido-2-phenyl]ethylchloromethyl ketone), inhibited only the steroid binding to chymotrypsin but not to trypsin. These observations indicate that serine proteases recognize steroid hormones in a fashion similar to the recognition of their specific substrates and that the steroids inhibit activity of these enzymes at their binding sites.

Inhibition of estrogen binding to rat alpha-fetoprotein by tryptophan p-nitrophenyl esters

Rat alpha-fetoprotein (AFP) contains a site that both binds the protease substrate tryptophan methyl ester (TrpOMe) and influences estrogen binding. We have studied the effect of changing the amino acid and ester portions of this compound on its binding to AFP, as measured by the ability of the ester to inhibit binding of [3H]-estrone to AFP. We find that: (1) AFP binds tryptophan esters better than phenylalanine or tyrosine esters, (2) substitution of butyl or benzyl for the methyl group in TrpOMe increases binding for AFP by 30- to 100-fold, (3) substitution of p-nitrophenol at the ester position increases binding for AFP by 10(5), (4) p-nitrophenyl substitution in the ester position of tyrosine or phenylalanine methyl ester increases the affinity for AFP by 10(3) to 10(4), (5) inhibition of estrogen binding to AFP by tryptophan p-nitrophenyl esters is reversible and competitive, and (6) the hydrolysis products of tryptophan p-nitrophenyl ester are ineffective in inhibiting estrogen binding to AFP. Based on these results, we suggest that AFP contains a tryptophan ester recognition site which binds p-nitrophenyl esters with high affinity and influences estrogen binding. The ester binding site may be located spatially near the estrogen binding site.

Spironolactone: a glucocorticoid agonist or antagonist?

Spironolactone displaces 3H-dexamethasone from HTC cell nuclei and inhibits the induction of tyrosine aminotransferase (TAT) activity by dexamethasone in HTC cells. Spironolactone alone fails to increase TAT activity in HTC cells or in the liver of adrenalectomized rats. These actions indicate that spironolactone lacks glucocorticoid agonist activity but can be an antagonistic to the actions of glucocorticoid as well as mineralocorticoid steroid hormones.