Substrate-induced down-regulation of human type 2 deiodinase (hD2) is mediated through proteasomal degradation and requires interaction with the enzyme's active center.

Type 2 iodothyronine deiodinase (D2) catalyzes the first step in thyroid hormone action, the deiodination of T4 to T3. Endogenous D2 activity is posttranslationally regulated by substrate that accelerates its degradation through the ubiquitin-proteasome pathway. To understand how D2 activity correlates with D2 protein during its normal decay and rT3-induced down-regulation, HEK-293 cells, transiently expressing human D2, were labeled with Na75SeO3 and then treated with 100 microM cycloheximide (CX), 30 nM rT3, and/or 10 microM MG132, a specific proteasome inhibitor, for 2-4 h. D2 protein and enzyme activity changed in parallel, disappearing with a half-life of 2 h in the presence of CX, or 1 h when CX + rT3 were combined. Treatment with MG132 blocked these effects. We created selenocysteine (Sec) 133 to cysteine (Cys) or alanine (Ala) D2 mutants, without changing Sec 266. The CysD2 activity and protein levels were also parallel, with a similar half-life of approximately 2 h, whereas the rT3-induced D2 down-regulation required approximately 1000-fold higher rT3 concentration (30 microM) due to a proportionally higher Michaelis constant of CysD2. In similar experiments, the AlaD2 mutant retained the short half-life but was not catalytically active and not susceptible to rT3-accelerated degradation. We conclude that substrate-induced loss of D2 activity is due to proteasomal degradation of the enzyme and requires interaction with the catalytic center of the protein.

Type 2 iodothyronine deiodinase in rat pituitary tumor cells is inactivated in proteasomes.

The goal of these studies was to define the rate-limiting steps in the inactivation of type 2 iodothyronine deiodinase (D2). We examined the effects of ATP depletion, a lysosomal protease inhibitor, and an inhibitor of actin polymerization on D2 activity in the presence or absence of cycloheximide or 3,3', 5'-triiodothyronine (reverse T3, rT3) in rat pituitary tumor cells (GH4C1). We also analyzed the effects of the proteasomal proteolysis inhibitor carbobenzoxy- L-leucyl-L-leucyl-L-leucinal (MG132). The half-life of D2 activity in hypothyroid cells was 47 min after cycloheximide and 60 min with rT3 (3 nM). rT3 and cycloheximide were additive, reducing D2 half-life to 20 min. D2 degradation was partially inhibited by ATP depletion, but not by cytochalasin B or chloroquine. Incubation with MG132 alone increased D2 activity by 30-40% for several hours, and completely blocked the cycloheximide- or rT3-induced decrease in D2 activity. These results suggest that D2 is inactivated by proteasomal uptake and that substrate reduces D2 activity by accelerating degradation through this pathway. This is the first demonstration of a critical role for proteasomes in the post-translational regulation of D2 activity.

Metyrosine and pheochromocytoma.

BACKGROUND: Severe hemodynamic instability may occur during surgery for removal of pheochromocytoma, unless there is preoperative pharmacological treatment. OBJECTIVE: To evaluate the effects of metyrosine (alpha-methyl-p-tyrosine), a catecholamine synthesis inhibitor, and alpha-blockade with prazosin or phenoxybenzamine on cardiovascular morbidity during surgery for pheochromocytoma. METHODS: A retrospective analysis was made of patients followed up at the Medical College of Georgia, Augusta, during 28 years who received metyrosine and prazosin (n = 6), metyrosine and phenoxybenzamine alone (n = 14), phenoxybenzamine alone (n = 6), or no medication (n = 7) during 3 weeks before tumor removal. The percentage of patients not requiring pressors or phentolamine during the intraoperative period as well as the perioperative peak systolic pressures and peak heart rates were estimated in each group. RESULTS: There was a significant (P < .05) increase in intraoperative peak systolic pressures without preoperative treatment (mean +/- SD, 243 +/- 40 mm Hg) vs metyrosine (mean +/- SD, 168 +/- 27 mm Hg). Ninety-five percent of patients who received metyrosine did not require pressors intraoperatively vs 50% with phenoxybenzamine alone. Eighty-one percent of patients pretreated with metyrosine did not require intraoperative phentolamine vs 33% with phenoxybenzamine alone and 29% without medications. Two patients in the no medication group died as a results of hypertensive crisis. CONCLUSIONS: The combination of alpha-metyrosine and alpha-blockade results in better blood pressure control and less need for use of antihypertensive medication or pressors during surgery, compared with the classical method of single-agent adrenergic blockade. Preoperative treatment with metyrosine along with an alpha-blocker is a useful strategy for decreasing the surgical morbidity in patients with pheochromocytoma and assumes greater importance as long as the availability of phentolamine for intraoperative use is a problem.

Microsomal steroid receptors in target tissues.

Target tissues contain microsomal receptors for steroid hormones. High-affinity, low-capacity binding sites for steroids are located in the endoplasmic reticulum and do not bind DNA. Studies by diverse groups on the nature and function of these receptors using biochemical and morphological approaches are discussed. The findings indicate that microsomes can be the site of receptor synthesis. Microsomes can also play a role in the control of receptor recycling and/or receptor processing after the complexes exit the nucleus of target cells. Moreover, microsomal receptors may be involved in posttranscriptional actions of steroid hormones.

Effect of 5 alpha-dihydrotestosterone and dexamethasone on estrogen receptors of the anterior pituitary and uterus.

The administration of 5 alpha-dihydrotestosterone (5 alpha-DHT) and dexamethasone has been shown to attenuate estrogen-induced prolactin release in the estrogen-primed rat. Therefore, the effect of these compounds was studied on anterior pituitary and uterine estrogen receptors. Injection of 0.8 mg/kg body weight of 5 alpha-DHT to ovariectomized adult rats treated with 2 micrograms estradiol/d for 4 days resulted in a significant decrease in occupied nuclear estrogen receptors of the anterior pituitary but not the uterus. Estrogen priming was essential for 5 alpha-DHT effect on occupied nuclear anterior pituitary estrogen receptors because this effect did not occur in ovariectomized vehicle-treated control animals. The administration of 1 mg/kg body weight of dexamethasone brought about a decrease in uterine but not anterior pituitary nuclear estradiol receptors. These results provide further evidence that the regulation of estrogen receptor dynamics is different in the anterior pituitary and the uterus and that different steroids can exert tissue-specific effects.

Effects of androgen on intracellular calcium of LNCaP cells.

Mibolerone (dimethylnortestosterone) or 5 alpha-dihydrostestosterone (DHT) increase intracellular calcium (Ca2+i) of human prostate cancer cells (LNCaP) as early as 2 min after treatment. These effects were concentration-dependent (10(-6)-10(-12) M) and they were blocked by preincubation with hydroxyflutamide (10(-6) M). Verapamil (10(-6) M) also suppressed the mibolerone (10(-6) M)-induced increase in Ca2+i, in cells which were previously exposed to 1 mM CaCl2 introduced in a Ca(2+)-free media. The results indicate that androgens elicit changes in Ca2+i in LNCaP cells as a result of Ca2+ influx through L-type channels in the plasma membrane. Since androgens are involved in the regulation of prostate cell division and growth, these findings suggest that calcium is involved in metabolic and mitogenic responses to steroid hormone in target cells.

Effects of steroidal and non-steroidal antiandrogens on the androgen binding properties of the rat ventral prostate androgen receptor.

Steroidal (cyproterone acetate) and non-steroidal (RU23908 and hydroxyflutamide) antiandrogens are able to block testosterone-induced increases in nuclear androgen receptor (AR) in the prostate of 1-day orchidectomized rats, but when given alone, RU23908 and hydroxyflutamide increase nuclear AR (RU23908 greater than hydroxyflutamide) in the same animal model. The increases in nuclear AR induced by antiandrogen alone or with testosterone alone are blocked by cycloheximide 1 h after administration, suggesting that androgen or antiandrogens induce de novo AR synthesis. Concomitant to nuclear AR accumulation, testosterone is able to induce depletion of cytosol and microsomal AR. Blockade of testosterone-induced depletion of microsomal AR, but not of cytosol AR, occurs in the presence of antiandrogens. Cyproterone acetate has a higher relative binding affinity (RBA) for microsomal AR and cytosol AR than RU23908 or hydroxyflutamide. This phenomenon is in good agreement with the degree of inhibition by these compounds of the association rate of androgen for the microsomal AR. This correlation between RBA and inhibition of the initial rate of hormone binding to the receptor is not found for cytosol AR. The results show that antiandrogens are not 'pure' antagonists of androgen action and they are potent agonists in the absence of testosterone. Furthermore, testosterone alone or antiandrogens per se regulate AR levels acutely by protein-synthesis dependent mechanisms of action, in rat ventral prostate.

Role of microsomal receptors in steroid hormone action.

Ventral prostate was used as a system to study the nature and properties of microsomal androgen receptor. The endoplasmic reticulum from rat ventral prostate contains high-affinity, low-capacity binding sites for androgen that are intrinsic to this intracellular compartment. Microsomal androgen receptors are not due to plasma membrane or cytosol contamination and they display a fast turnover, with depletion after 1 hour and complete replenishment 6 hours after androgen stimuli. Cycloheximide, but not actinomycin D, inhibits microsomal androgen receptor replenishment, indicating that testosterone may control microsomal receptor levels acutely by posttranscriptional mechanisms. Microsomal androgen receptor is a 5S protein that has a higher stability than its cytosolic counterpart, regardless of the presence of ligand. It does not become activated after heat or salt treatment. After extraction of binding sites, microsomes are capable of accepting cytosol mibolerone-receptor complexes to a level similar to the concentration of depleted binding sites; microsomes from nontarget tissues do not manifest such capability. The results indicate the coexistence of a non-DNA-binding form of androgen receptor in the microsomal membranes with the typical DNA-binding form of androgen receptor present in the cytosol of ventral prostate homogenates. Microsomal androgen receptor may represent an additional level of regulation of androgen action in the intact target cell.

Specific binding of androgen and androgen-receptor complex by microsomes from rat ventral prostate.

Microsomes from rat ventral prostate show the presence of a high affinity-low capacity population of androgen-binding sites with affinity for ionic exchange resin similar to that of cytosol androgen receptor (AR), as manifested by similar results obtained with hydroxylapatite. The affinity for mibolerone was similar for both forms (Ka = 0.5-2.9 x 10(10) M-1). The membrane-bound form can be extracted in hypotonic buffer, with retention of binding properties. Isotonic sucrose allowed higher degree of extractability of the microsomal AR than 10% (v/v) glycerol. The presence of hormone lends stability to the microsomal AR, while high salt or nonionic detergents have a deleterious effect on their longevity. The microsomal receptor form is not sensitive to serine-proteases as opposed to the cytosol AR. After exhaustive extraction of binding sites, microsomes are capable of accepting cytosol mibolerone-receptor complexes to a level corresponding to the concentration of depleted binding sites; microsomes from non-target tissue do not manifest such capability. Microsomal AR complexes do not bind DNA and they are not activated after heat treatment. Mixed preparations of extracted microsomal complexes with cytosol complexes showed heat-induced increased ability to bind DNA to the same level of diluted cytosol complex alone, indicating the absence of a microsomal inhibitor of DNA binding. The results indicate the co-existence of a non-DNA binding form of the AR in the microsomal membranes with the classical DNA binding form of the AR present in the cytosol of ventral prostate homogenates.

Association of androgen binding sites with the endoplasmic reticulum of rat ventral prostate.

Microsomes from ventral prostate of 24-h castrated rats contain a single set of tissue-specific high-affinity, low-capacity androgen binding sites. These sites are indigenous to the endoplasmic reticulum, as shown by purification procedures associated with marker enzymes and electron microscopic analyses. When prostatic microsomal membranes are separated from plasma membranes using the nuclear or the mitochondrial pellets as the source of fractionation in sucrose gradients, the androgen binding activity is selectively associated with fractions rich in rough endoplasmic reticulum and ribosomes. Eighty-four percent of the total content of Na+/K+ adenosine triphosphatase (ATPase) and only 27% of the total binding capacity were concentrated in fractions rich in smooth-surfaced vesicular membranes, when nuclear suspensions constituted the membrane source. In contrast, the region of the same gradient when enriched in rough endoplasmic reticulum and deficient in plasma membrane content contained 73% of the androgen-binding capacity and only 14% of the ATPase. For fractions collected using mitochondrial suspensions as starting material, the ratio (total glucose-6-phosphatase/total binding capacity) was closer to 1.0 than similar ratios of ATPase/binding capacity, indicating co-sedimentation of binding sites with microsomal membranes and not with plasma membranes. Na+/K+ ATPase, but not 5' nucleotidase, is a valid plasma membrane marker for ventral prostate. Microsomal androgen receptors may constitute a new level of regulation of androgen action in target cells.

Relative binding properties of microsomal and cytosolic androgen receptor species of the ventral prostate.

A comparison between a microsomal and a cytosolic source of receptor-like androgen-binding activity was made in the rat ventral prostate. Microsomal binders remain in solution at 90% saturation with (NH4)2SO4 and display equal affinity for 5 alpha-dihydrotestosterone (DHT) and mibolerone. They have a half-life of dissociation of steroid-receptor complexes of 96 +/- 11 h and a 5S sedimentation coefficient for the untransformed moiety, with the appearance of a 3.5S species after incubation at 24 C for 30 min. They do not acquire DNA-binding capability after heat- or salt-attempted activation. Cytosol binders precipitate upon 90% saturation with (NH4)SO4 and have higher affinity for DHT than mibolerone. The half-life for dissociation of complexes is 85 +/- 14 h, and the complex sediments as an 8S moiety which is able to transform to a 4.4S form after heat activation correlated with increased DNA-binding ability of these species. Unactivated cytosol steroid-receptor complexes are also able to bind to DNA in the presence of molybdate. Salt-induced activation of cytosol moieties only occurred in the absence of molybdate. Microsomal androgen receptor is more stable than cytosol androgen receptor independently of the presence of hormone or partial purification of the moieties; the inactivation rates of the two forms of complexes differ 3-fold. Results indicate that androgen-binding sites associated with the microsomal and cytosolic fractions of the prostate are distinct entities.

Microsomal receptor for steroid hormones: functional implications for nuclear activity.

Target tissues for steroid hormones are responsive by virtue of and to the extent of their content of functional intracellular receptors. Recent years have seen a shift in considerations of the cellular dynamics and distribution of these receptors, with current views favoring predominant intranuclear localization in the intact cell. This paper summarizes our analyses of the microsomal estrogen and androgen binding capability of rat uterine and ventral prostate tissue, respectively; these studies have revealed a set of high affinity sites that may act as a conduit for estrogen traversing the cell en route to the nucleus. These sites have many properties in common with cytosolic receptors, with the salient difference of a failure to activate to a more avid DNA-binding form under conditions which permit such activation of cytosolic receptors. The microsomal estrogen-binding proteins also have appreciable affinity for progesterone, another distinction from other known cellular estrogen receptor species. Various experimental approaches were employed to demonstrate that the microsomal receptors were not simply cytosol contaminants; the most convincing evidence is the recent successful separation of the cytosolic and microsomal forms by differential ammonium sulfate precipitation. Discrete subfractionation of subcellular components on successive sucrose gradients, with simultaneous assessments of binding capability and marker enzyme concentrations, indicates that the major portion of the binding is localized within the vesicles of the endoplasmic reticulum free of significant plasma membrane contamination. The microsomal receptors are readily solubilized by extraction with high- or low-salt-containing buffers or with steroid. The residual microsomes following such extraction have the characteristics of saturable acceptor sites for cytosolic estrogen-receptor complexes. The extent to which these sites will accept the cytosolic complexes is equal to the concentration of microsomal binding sites extracted. These observations suggest three possible roles for the microsomal receptor-like proteins: (a) modulation of estrogen access to nuclear binding sites; (b) formation of functional complexes which diffuse to other extranuclear sites to alter non-genomic cellular processes; (c) regulation of nuclear concentration of estrogen-receptor complexes by virtue of producing microsomal acceptor sites for uptake of free or loosely associated nuclear complexes, previously thought to exist in the cytoplasm.

Androgen receptor dynamics in the rat ventral prostate.

Upon testosterone administration, a dose-dependent cytosolic depletion and nuclear accumulation of androgen receptors in the ventral prostate of 1-day-castrated male rats is observed. Replenishment in the cytosol is rapid with a return to control levels 3 h after testosterone stimulation. The process of nuclear retention (as measured 4-6 h post-injection) is both dose-dependent and time-dependent (there is no retention of the androgen receptor 15 h after testosterone). When assayed 1 h after testosterone, the increase in nuclear binding sites was not sufficient to conclude that the disappearance of cytosolic binding sites could be accounted for by translocation of cytosolic receptors to the nucleus. The cytosolic compartment contained more than 70% of the total cellular receptors whether testosterone was present (in the range 50-400 micrograms/100 g body wt.) or not. Nuclear processing of androgen receptors is extensive and it is dose-dependent. Turnover of prostatic androgen receptors was studied simultaneously in the cytosolic, microsomal and nuclear compartments 1, 2, 3, 4, 5 and 6 h after testosterone administration. Cytosolic and microsomal depletion-replenishment patterns are similar, displaying a nadir after 1 h and a full replenishment 3-4 h post-testosterone. Cycloheximide, but not actinomycin D, inhibits cytosolic and microsomal replenishment. Nuclear accumulation and retention of androgen receptors is insensitive to both drugs. The very rapid and RNA synthesis-independent turnover of androgen receptors in the ventral prostate suggests that testosterone regulates the receptor levels acutely by both a rapid post-transcriptional positive action and a similarly rapid negative effect on nuclear receptor half-life.

Effect of theophylline on nuclear retention of oestrogenreceptor complexes: correlation with oestrogen responses.

In the ovariectomized adult rat uterine oedema induced by 0.01 and 0.1 micrograms oestradiol-17 beta/100g body weight increased further in the presence of theophylline. Nuclear retention of oestrogen-receptor complexes also increased in response to theophylline both in vivo and in vitro. Theophylline decreased the number of eosinophils in the blood and concurrently decreased oestrogen-induced uterine eosinophilia at doses of 0.001, 0.01, 0.1, 1, 10 or 30 micrograms oestradiol/100 g body weight, through a mechanism independent of glucocorticoids. There was, therefore, no correlation between changes in the number of uterine eosinophils and changes in uterine wet weight induced by theophylline and oestrogen. It is suggested that the presence of oestrogen-receptor complexes in the nucleus for at least 4 h is a prerequisite for the induction of uterine oedema and growth in the presence of theophylline and oestradiol-17 beta.

Multiple mechanisms of regulation of estrogen action in the rat uterus: effects of insulin.

Eosinophils appear in the rat uterus in the presence of estrogen. The level of these cells in the uterus depends on the number of blood eosinophils. Insulin is an eosinopenic hormone in the blood and, therefore, could regulate estrogenic responses mediated by these cells in the uterus. Estrogen-induced uterine edema and eosinophilia at doses of 0.01,. 0.1, 1, 10, and 30 micrograms 17 beta-estradiol (E2)/100 g BW are inhibited by insulin. Estrogen binding by uterine eosinophils in vitro decreases in the presence of insulin, suggesting another explanation for the observations in the uterus in vivo. Injection of insulin alone or in combination with 0.01, 0.01, 0.1, or 1 microgram E2/100 g BW increases uterine RNA and protein contents by 6 h. Inactive insulin does not modify any of these stimulatory effects of estrogen. The results support the idea of two separate receptor systems for estrogens in the rat uterus: the eosinophil receptor system, which mediates estrogen-induced uterine edema, and the cytosol-nuclear receptor system, which mediates estrogen-induced uterine RNA and protein syntheses.

Effects of thyroid hormone on some uterine responses to estrogen.

Uterine edema induced by 0.1 microgram 17 beta-estradiol (E2)/100 g BW, as well as uterine eosinophilia induced by 0.01, 0.1, 1, 10, and 30 micrograms E2/100 g BW, decrease in the presence of 20 micrograms T3 or T4/100 g BW in the immature rat. Estrogen binding by uterine eosinophils and the number of eosinophils in the blood also decrease with T3 or T4, suggesting an explanation for the findings in the uterus. Thyroid hormones alone or in the presence of 0.001, 0.01, 0.1 and 1 microgram E2/100 g BW, increase the uterine RNA content 7 h after administration compared with the same doses of E2 alone. This response is not associated with any further increase in the urine protein content. The results show a dissociation between the effects of estrogen on RNA or protein levels, which are decreased by thyroid hormones, and some early estrogenic responses (uterine edema and eosinophilia), which are decreased by thyroid hormones.

Effect of insulin and epinephrine on some early oestrogenic responses in the rat uterus.

Oestradiol induces uterine eosinophilia and oedema, both in the intact immature rat and in the adult ovariectomized rat. These responses are decreased in ovariectomized-adrenalectomized adult rats, in adrenalectomized immature rats and in medullectomized immature rats. Pre-treatment with L-epinephrine restored both oestrogenic effects. Insulin inhibits oestrogen binding by uterine eosinophils and concurrently decreases oestrogen-induced uterine oedema and eosinophilia. These phenomena do not occur with inactive insulin. Insulin also induces blood eosinopaenia, suggesting an explanation for the findings in the uterus.

Theophylline-estrogen interaction in the rat uterus: role of the ovary.

Uterine edema induced by 0.1 microgram/100 g body wt of estradiol further increases in the presence of theophylline. Theophylline alone or in the presence of 0.001, 0.01, 0.1, and 1 microgram/100 g body wt of estradiol increases uterine RNA and protein content 6 h after its administration, as compared with the same doses of estradiol alone. Both phenomena disappear in the ovariectomized immature rat, suggesting that theophylline potentiates the action of gonadotropins, increasing the synthesis of endogenous ovarian estrogens by the immature ovary. Theophylline decreases the number of eosinophils in the blood and concurrently decreases estrogen-induced uterine eosinophilia at doses of 0.01, 0.1, 1, 10, and 30 micrograms/100 g body wt of estradiol. Estrogen binding by uterine eosinophils in vitro increases in the presence of theophylline. This effect of theophylline could also explain the increase in vivo in estrogen-induced uterine edema.

Interaction between theophylline and oestrogen in the rat uterus.

Theophylline alone or in the presence of 0.001, 0.01, 0.1 or 1 microgram oestradiol-17 beta/100 g body wt increase uterine RNA and protein content 6 h after administration. Uterine oedema induced by physiological doses of oestradiol-17 beta was increased further in the presence of theophylline. Theophylline decreased the number of eosinophils in the blood and concurrently decreased oestrogen-induced uterine oesinophilia at doses of 0.01, 0.1, 1, 10 or 30 micrograms oestradiol-17 beta/100 g body wt. Oestrogen binding by uterine eosinophils in vitro increased in the presence of theophylline. This effect of theophylline could explain the increase of oestrogen-induced uterine oedema in vivo.

Effect of theophylline and triiodothyronine on some early estrogenic responses in the rat uterus.

Theophylline increases and triodothyronine decreases uterine edema induced by physiological doses of estradiol-17 beta. Both of them decrease estrogen-induced uterine eosinophilia and the number of blood eosinophils, suggesting an explanation for the results in the uterus.