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PubMed1996

Thyrotropin-releasing hormone in critical illness: from a dopamine-dependent test to a strategy for increasing low serum triiodothyronine, prolactin, and growth hormone concentrations.

OBJECTIVE: The aim of this study was to examine the effect of dopamine infusion on the thyrotropin (TSH), thyroid hormone, prolactin, and growth hormone responses to thyrotropin-releasing hormone (TRH) in critically ill patients. DESIGN: Prospective, randomized, controlled, open-labeled clinical study. SETTING: The intensive care unit, University Hospital Gasthuisberg, Leuven, over a 1-month period. PATIENTS AND INTERVENTIONS: In 15 critically ill patients receiving dopamine treatment (5 micrograms/kg/min) for a mean of 43.3 +/- 1.2 (SEM) hrs after trauma or cardiac surgery, we studied the TSH, thyroid hormone, prolactin, and growth hormone responses to the administration of two consecutive intravenous TRH boluses of 200 micrograms, with a 6-hr interval. The dopamine infusion was continued in the control group and discontinued in the study group. Serum concentrations of TSH, prolactin, and growth hormone were measured before and 20, 40, 60, and 120 mins after TRH administration. Serum concentrations of thyroxine (T4), triiodothyronine (T3), reverse T3, and thyroid hormone binding globulin were determined before and 120 mins after each TRH injection. MEASUREMENTS AND MAIN RESULTS: There was a > 100-fold interindividual variation in the baseline TSH concentration and in the TSH peak value after TRH administration. Two consecutive doses of TRH evoked a mean 16% increase in serum T4 concentration (p = .003) and a mean 47% increase in T3 (p = .001), whereas serum reverse T3 and thyroid hormone binding globulin values remain unaltered. Each of the TRH boluses increased serum growth hormone concentrations in the continued dopamine and discontinued dopamine groups, by a median of 60% (p = .001) and 68% (p = .001), respectively. Three hours after dopamine withdrawal, there was a three-fold increase of the peak TSH response (p = .001), a higher T3 response (p = .01), and a ten-fold increase of the peak prolactin value (p = .001) in response to TRH administration. CONCLUSIONS: The TSH response to TRH administration in critical illness presents a striking interindividual variation and dopamine dependent. Repeated TRH administration results in a repetitive increase of TSH, prolactin, growth hormone, T4, and T3, without increasing reverse T3. These observations point toward a potential for TRH as a strategy for reversing the euthyroid sick syndrome, growth hormone deficiency, and immune dysfunction associated with critical illness.

Adolescent↗

PubMed1992

Functional properties of a novel mutant thyroid hormone receptor in a family with generalized thyroid hormone resistance syndrome.

OBJECTIVE: We wished to ascertain whether a mutation in the thyroid hormone receptor beta gene was present in a family with generalized thyroid hormone resistance syndrome and to characterize the functional properties of this mutant receptor. DESIGN: Blood samples were obtained from family members for hormone assays and genomic DNA was isolated from leucocytes for genetic analyses. PATIENTS: Three members (B,C,E) of a family with possible thyroid hormone resistance and two normal family members (A,D) were studied. MEASUREMENTS: Basal thyroid function tests together with serum sex hormone binding globulin (SHBG) levels were measured. The thyroid hormone receptor beta gene was amplified using the polymerase chain reaction and the receptor mutation identified by sequence analysis. The ability of mutant receptor to bind T3, interact with a specific DNA sequence and to modulate target gene expression was tested. The effects of mutant receptor on co-expressed wild type receptor action were determined. RESULTS: Patients with resistance had raised levels of T4 and T3 together with inappropriately normal serum TSH and SHBG whereas unaffected individuals had a normal hormone profile. A single nucleotide substitution corresponding to a glycine to serine mutation at codon 340 (G340S) in the hormone binding domain was identified in one of the two beta receptor gene alleles in patients with resistance, but not in the normal family members. When expressed in vitro, this receptor protein (G340S), as well as a related (G340R) mutant identified in another family, retained the ability to bind to a specific DNA sequence but were unable to bind ligand or to activate or repress target gene expression. In addition both receptor mutants were capable of inhibiting the function of wild type thyroid hormone receptor in a co-expression assay but differed in their inhibitory potential. CONCLUSIONS: We report a second type of mutation (Gly to Ser) in codon 340 of hTR beta in a family with generalized thyroid hormone resistance. Mutations at this site eliminate T3 binding, causing a loss of hormone-stimulated receptor function. However, the mutant receptors retain the ability to block normal receptor action. The occurrence of different mutations at the same site suggests that alterations in this region of the receptor may be important for generating the clinical phenotype of this disorder.

DNA Mutational Analysis↗

PubMed2004

Psychological functioning after growth hormone therapy in adult growth hormone deficient patients: endocrine and body composition correlates.

Growth hormone replacement in adult growth hormone deficient patients improves psychological well-being and the quality of life. The aim of this study was to investigate relationship between changes in mood, cognitive functioning, quality of life, changes in body composition and hormone concentration at baseline and six months after treatment with human recombinant growth hormone. Eighteen adult patients with growth hormone deficiency syndrome were recruited to the study. Growth hormone was administered in doses of 12 IU per week in an open design. After 6 months of growth hormone replacement therapy the psychological functioning improved significantly on mood scales (Profile of Mood State) and on a cognitive performance tests. Changes in quality of life scale were trivial. After growth hormone treatment serum concentration of Insulin like growth factor -1 (IGF-1) and triiodothyronine increased and concentration of serum free thyroxine decreased significantly in comparison to basal concentration. There were no significant differences in changes of plasma cortisol, thyrotropin and growth hormone concentrations. Improvement on Profile of Mood State global score as well as on Vigor-Activity subscale correlated significantly with increase in IGF-1 concentration. Improvement on Profile of Mood State Vigor-Activity subscale correlated with increase in water body mass and improvement on Hospital Anxiety and Depression scale depression subscale correlated with decrease in cortisol concentration. The study shows that growth hormone replacement improves mood and cognition in adult growth hormone deficient patients. This improvement is related to changes in water body mass as well as to endocrine changes.

Adult↗

PubMed1990

Pretranslational control by thyroid hormone of rat liver steroid 5 alpha-reductase and comparison to the thyroid dependence of two growth hormone-regulated CYP2C mRNAs.

The sexually differentiated microsomal enzyme steroid 5 alpha-reductase (NADPH: delta 4-3-oxosteroid 5 alpha-oxido-reductase, EC 1.3.99.5) catalyzes the NADPH-dependent conversion of testosterone to 5 alpha-dihydrotestosterone, a more potent androgen. In rat liver, this enzyme is expressed at a 10-fold higher level in adult females as compared to adult males. The pituitary regulation of this enzyme and its mRNA was studied in untreated and hypophysectomized rats and in rats rendered hypothyroid by treatment with the antithyroid drug methimazole. Hepatic 5 alpha-reductase activity was elevated 8-fold, to 85% of adult female levels, in adult male rats given growth hormone by continuous infusion. This same treatment was only partially effective in restoring 5 alpha-reductase in rats depleted of endogenous growth hormone by hypophysectomy, indicating that other pituitary-dependent factors contribute to the elevation observed in the inact animals. Further analysis revealed that thyroxine, but not adrenocorticotropic hormone (ACTH) or chorionic gonadotropin, could elevate 5 alpha-reductase activity and mRNA when given to the hypophysectomized rats and that this effect was enhanced by the presence of growth hormone. This thyroid hormone dependence was confirmed by the decrease in hepatic 5 alpha-reductase expression in hypothyroid rats and by its substantial restoration following thyroxine replacement. Thyroxine also stimulated expression of another female-predominant hepatic mRNA, encoding the steroid 16 alpha-hydroxylase cytochrome P-450f (IIC7), in a manner that was independent of the stimulatory effect of growth hormone on this transcript. In contrast, thyroid hormone did not significantly affect protein or mRNA levels of the growth hormone-stimulated, female-specific steroid sulfate 15 beta-hydroxylase P-450 2d (IIC12). These findings establish that thyroid hormones act at a pretranslational level to modulate the expression of some, but not all, growth hormone-stimulated hepatic mRNAs and demonstrate that both thyroxine and growth hormone can independently contribute to the sex-dependent expression of hepatic enzymes of steroid metabolism.

3-Oxo-5-alpha-Steroid 4-Dehydrogenase↗

PubMed1989

Thyroid hormone regulation of the rat glycoprotein hormone alpha-subunit gene promoter activity.

An important physiological control of the glycoprotein hormone alpha-subunit is the negative feedback by thyroid hormones in the thyrotrope. A region of the rat glycoprotein hormone alpha-subunit gene that is involved in transcriptional regulation by thyroid hormone has been identified by transient transfection studies, and sequence-specific binding of the thyroid hormone receptor to a site within this region has been demonstrated. Deletion-mutation studies using plasmid expression vectors containing either 246, 170, or 80 base pairs of the 5'-flanking region of the rat alpha-subunit gene fused to the coding region of the bacterial chloramphenicol acetyltransferase gene demonstrate 3,5,3'-triiodo-L-thyronine (T3)-regulated expression in GH3 cells, a T3-responsive somatotrophic cell line. In order to investigate the possibility of thyroid hormone receptor interaction with this segment of the rat alpha-subunit gene, the binding of the thyroid hormone receptor to synthetic oligodeoxynucleotides was analyzed using an avidin-biotin complex DNA binding assay. An oligodeoxyribonucleotide representing a fragment of the alpha-subunit gene from -74 to -38, relative to the transcriptional start site, shows significant binding to [125I]T3-receptor complex present in nuclear extracts of GH3 cells. This fragment binds receptor to a degree similar to that seen with a fragment of the rat growth hormone gene which contains a putative thyroid hormone-responsive element. In addition, this fragment of the rat alpha-subunit gene binds to the in vitro synthesized human c-erbA beta protein, which has been identified as a member of the family of putative T3 receptors. These data demonstrate that a cis-active thyroid hormone-responsive element resides in the 5'-flanking region of the rat alpha-subunit gene and that the mechanism involved in the suppression of expression of this gene by T3 could involve specific binding of the thyroid hormone receptor to this region of the gene.

Animals↗

PubMed1993

Effect of hypermagnesemia on the adenohypophyseal-gonadal function, parathyroid hormone secretion and some other hormonal indicators.

Hormonal secretory mechanisms are influenced positively or negatively by magnesium, the effect depending on its concentration. The submitted study is concerned with the effect of acute hypermagnesemia induced by intravenous infusion of 6 g MgSO4 on quiescent and LHRH-induced secretion of FSH, LH, prolactin and testosterone; on the levels of intact parathyroid hormone; and on other hormonal and mineral indicators in 10 healthy male subjects. Gonadotropin secretion was not altered by hypermagnesemia. Quiescent testosterone levels decreased moderately (p < 0.01), and the secretory kinetics of the hormone slightly changed. Parathyroid hormone levels markedly dropped in the 60th minute of hypermagnesemia duration (p < 0.01), the drop depending on the initial values of the hormone prior to the administration of magnesium (r = 0.923, p < 0.01). Although in hypermagnesemia no correlations between the levels of parathyroid hormone and those of magnesium, calcium and phosphorus could be found, in normomagnesemia significant correlations were confirmed between the levels of parathyroid hormone and magnesemia (r = -0.8068, p < 0.05), and the levels of parathyroid hormone and calcemia (r = -0.9451, p < 0.01). In hypermagnesemia there were no alterations in the levels of the plasma prolactin, cortisol or catecholamines, nor was there any alteration in the rate at which catecholamines were excreted in urine. In conclusion, magnesium in supraphysiologic concentrations does not significantly change the function of the adenohypophyseal-gonadal axis or other hormonal indicators studied. The slight decline of quiescent testosterone levels does not seem to have any clinical significance, although this must be verified by further studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed2005

Effects of growth hormone secretagogues on the release of adenohypophyseal hormones in young and old healthy dogs.

The effects of three growth hormone secretagogues (GHSs), ghrelin, growth hormone-releasing peptide-6 (GHRP-6), and growth hormone-releasing hormone (GHRH), on the release of adenohypophyseal hormones, growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinising hormone (LH), prolactin (PRL) and on cortisol were investigated in young and old healthy Beagle dogs. Ghrelin proved to be the most potent GHS in young dogs, whereas in old dogs GHRH administration was associated with the highest plasma GH concentrations. The mean plasma GH response after administration of ghrelin was significantly lower in the old dogs compared with the young dogs. The mean plasma GH concentration after GHRH and GHRP-6 administration was lower in the old dogs compared with the young dogs, but this difference did not reach statistical significance. In both age groups, the GHSs were specific for GH release as they did not cause significant elevations in the plasma concentrations of ACTH, cortisol, TSH, LH, and PRL. It is concluded that in young dogs, ghrelin is a more powerful stimulator of GH release than either GHRH or GHRP-6. Ageing is associated with a decrease in GH-releasing capacity of ghrelin, whereas this decline is considerably lower for GHRH or GHRP-6.

Adrenocorticotropic Hormone↗

PubMed1992

Human pituitary null cell adenomas and oncocytomas in vitro: effects of adenohypophysiotropic hormones and gonadal steroids on hormone secretion and tumor cell morphology.

Human pituitary null cell adenomas and oncocytomas are not associated with evidence of excess hormone secretion in vivo; their cellular derivation has not been clarified by morphologic investigation. In this study we examined 41 null cell adenomas and 58 oncocytomas in vitro to determine hormone release and its response to several adenohypophysiotropic hormones and gonadal steroids. In vitro, 96/99 tumors released LH, FSH, and/or alpha-subunit of glycoprotein hormones. TSH was released by 11 tumors. GH, PRL, and ACTH were found in small quantities in 11, 8, and 5 tumors, respectively. Only 3 tumors released no detectable hormones. Incubations with test substances were examined at 2- and 24-h periods for up to 72 h. All but 3 of 53 tumors showed marked and persistent increases in the release of LH, FSH, and/or alpha-subunit in response to GnRH in short and long duration experiments. Secretion of LH, FSH, or alpha-subunit was stimulated to more than 150% of control by TRH in 37/48 tumors, by CRH in 10/20, by GRH in 7/20. Estradiol, progesterone, and testosterone increased release of FSH, LH, and/or alpha-subunit in 23/32, 3/12, and 3/12 tumors, respectively, and reduced their release in 6/32, 5/12, and 7/12, respectively. In tumors which showed no response to gonadal steroids, GnRH in combination with estradiol, progesterone, or testosterone yielded the same result as GnRH alone; in tumors inhibited by gonadal steroids, GnRH in combination with one of those substances reduced the response to GnRH. No secretion of GH, PRL, ACTH, or TSH was detected after incubation with GRH, estradiol, CRH, or TRH except in the tumors which initially released GH, PRL, or TSH. Ultrastructural examination of cultured cells from 15 cases revealed morphologic alterations that correlated with changes in hormone release and could be quantified by morphometry. This study represents the largest analysis of hormone production and release in vitro and morphologic correlation of clinically nonfunctioning pituitary adenomas. The responsiveness of gonadotropin secretion by null cell adenomas and oncocytomas to GnRH and gonadal steroids resembles that of gonadotroph adenomas. However, the unexpected increases in gonadotropin release attributable to several other adenohypophysiotropic hormones and the release of multiple hormones suggests that null cell adenomas and oncocytomas may represent neoplasms derived from uncommitted or committed precursor cells that can undergo differentiation towards several cell lines.

Adenoma↗

PubMed1984

Effect of thyroxine and growth hormone on the hypercholesterolemia of growth hormone deficiency.

To study the selective effect of thyroid hormone and growth hormone, alone and in combination on plasma cholesterol, 12 children were monitored monthly during three 4-month treatment periods. Plasma cholesterol was unaltered by growth hormone therapy alone (mean +/- SD, basal cholesterol level: 202 +/- 26 mg/dL; cholesterol level after growth hormone 200 +/- 24 mg/dL). With thyroxine (T4) replacement alone, or combined with growth hormone, a significant (P less than 0.01) decrement in plasma cholesterol occurred. (Thyroxine alone: 172 +/- 11 mg/dL; thyroxine and growth hormone: 172 +/- 10 mg/dL). Basal thyroid function tests, including TRH stimulation, were normal. Serum thyroxine levels decreased and serum triiodothyronine (T3) levels increased in response to growth hormone therapy. Linear growth increased significantly in response to growth hormone alone and both therapies combined. Serum values of T4 and T3 were significantly (P less than 0.001) correlated with basal cholesterol, r = 0.84 and r = 0.83, respectively. Our experience with euthyroid growth hormone deficiency confirms that modest hypercholesterolemia is characteristic of that disorder, and that growth hormone therapy has no statistically significant effect on plasma cholesterol. The modest hypercholesterolemia that does respond to thyroid replacement is further evidence of a subtle abnormality of thyroid function in such individuals. The mechanism whereby thyroid hormone has its cholesterol lowering effect in these apparently euthyroid subjects is unclear.

Child↗

PubMed1990

Influence of different types of antiandrogens on luteinizing hormone-releasing hormone analogue-induced testosterone surge in patients with metastatic carcinoma of the prostate.

The long-term effect of the luteinizing hormone-releasing hormone analogue-induced initial testosterone surge in the treatment of patients with metastatic carcinoma of the prostate still is unknown. However, acute worsening of the disease has been reported in up to 10% of the patients. To prevent such tumor flare we investigated the endocrinological effects of different types of antiandrogens administered in addition to a luteinizing hormone-releasing hormone analogue. Patients with newly diagnosed metastatic prostate cancer were pre-treated with either the steroidal antiandrogen cyproterone acetate (6) or the nonsteroidal antiandrogen flutamide (5) for 1 week before the initial injection of the depot luteinizing hormone-releasing hormone analogue Zoladex. In another 5 patients flutamide was first given 24 hours before Zoladex therapy was started. Luteinizing hormone, testosterone and prostatic acid phosphatase during month 1 of luteinizing hormone-releasing hormone analogue therapy were compared to data obtained in 5 patients treated by Zoladex alone. Only pre-treatment with cyproterone acetate was capable of preventing the Zoladex-induced testosterone surge. However, both pre-treatment regimens with either cyproterone acetate or flutamide for 1 week prevented an initial increase in prostatic acid phosphatase beyond pre-treatment levels in all patients. In contrast, in 4 of 5 patients treated with Zoladex alone and in 2 of 5 pre-treated with flutamide for 1 day an initial increase in prostatic acid phosphatase beyond the pre-treatment values was seen. Our data indicate that pre-treatment with flutamide for only 1 day may not be sufficient to prevent a luteinizing hormone-releasing hormone analogue-induced tumor flare in all cases.

Acid Phosphatase↗

PubMed1983

The treatment of metastatic prostatic cancer with a potent luteinizing hormone releasing hormone analogue.

Metastatic prostatic cancer generally is treated by either castration or the administration of exogenous estrogens, both of which have significant clinical disadvantages. Luteinizing hormone releasing hormone analogues have been shown to suppress gonadal steroidogenesis in animals and humans. To assess the effect of the administration of a potent luteinizing hormone releasing hormone analogue, (D-Leu-6)luteinizing hormone releasing hormone (1-9) nonapeptide ethylamide, 9 patients with previously untreated stage D2 prostatic cancer were treated with this agent for 3 to 8 months. By 3 months of treatment all patients demonstrated a significant decrease in serum testosterone and a decreased peak serum luteinizing hormone and testosterone response to the acute administration of the analogue, with no change in baseline serum luteinizing hormone or prolactin. These data suggest that this analogue acts by decreasing the pituitary release of luteinizing hormone. No major adverse effects were noted with this treatment modality, and all patients were symptomatically improved and demonstrated a decrease or stabilization in tumor activity as measured by prostatic computerized tomography or ultrasound scan, prostatic acid phosphatase and bone scan. A representative case is presented. Luteinizing hormone releasing hormone analogues may prove to be valuable new agents in the treatment of advanced prostatic cancer.

Aged↗

PubMed2003

Regulation of growth-hormone-receptor gene expression by growth hormone and pegvisomant in human mesangial cells.

BACKGROUND: Mice transgenic for growth hormone develop mesangial proliferation, glomerular hypertrophy, and progressive glomerular sclerosis suggesting that the growth hormone-insulin-like growth factor I (IGF-I) pathway plays an important role. Therefore, we studied the impact of variable concentrations of 22 kD, 20 kD growth hormone, as well as of the growth hormone receptor antagonist pegvisomant (B2036-PEG), on both the growth hormone receptor (GHR/GHBP) gene expression and growth hormone binding protein (GHBP) formation in a human glomerular mesangial cell line. Further, the impact on collagen, IGF-I and IGF binding protein-1 (IGFBP-1) formation was studied. METHODS: In order to assess transcription, quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used. RESULTS: Physiologic doses of 22 kD or 20 kD growth hormone caused a dose-dependent and significant (P < 0.01) up-regulation of GHR/GHBP gene transcription, whereas supraphysiologic doses (50 and 500 ng/mL) resulted in down-regulation (P < 0.001). Whenever pegvisomant was used, there was no increase in GHR/GHBP expression. These data were confirmed using run-on experiments. Further, the assessment of GHBP presented a constant, dose-dependent increase, which was completely abolished in the experiments where pegvisomant was used. CONCLUSION: We present data showing that growth hormone has a direct impact on GHR/GHPB gene transcription and that pegvisomant is a potent growth hormone receptor antagonist in human mesangial cells. In addition, although the GHR/GHBP gene transcription is down-regulated by supraphysiologic growth hormone concentrations, this effect was not found when GHBP levels were measured. This finding may reflect a self-inhibitory effect of growth hormone on the level of GHR/GHBP gene transcription, which does not involve the regulation of the shedding of GHBP and may, therefore, be of physiologic interest.

Cells, Cultured↗

PubMed1990

Growth hormone and adipose differentiation: growth hormone-induced antimitogenic state in 3T3-F442A preadipose cells.

An additional activity for pituitary growth hormone is described--i.e., the in vitro induction of an antimitogenic state in murine 3T3-F442A preadipocyte fibroblasts. We previously developed a serum-free, hormonally defined medium permissive for the adipose differentiation of 3T3-F442A cells. When 3T3-F442A fibroblasts were maintained in serum-free medium without insulin but with growth hormone (2 nM), typical adipose differentiation did not occur. However, we found that growth hormone induced a state of cellular refractoriness to the mitogenic stimulus of fetal bovine serum as assayed by de novo DNA synthesis. The mitogen refractory condition (i.e., the antimitogenic state) was time-dependent (half maximal at approximately 2.5 days) and growth hormone concentration-dependent (half maximal and maximal at approximately 0.05 and 2.0 nM, respectively). The antimitogenic state was specifically induced by growth hormone and was not mediated by insulin-like growth factor I or prolactin. The growth hormone-induced antimitogenic state was completely reversible. The antimitogenic state was not induced by growth hormone in 3T3-C2 cells, a sister clone of 3T3 cells that exhibits essentially no adipose conversion. The kinetics for growth hormone-dependent commitment to adipose differentiation and induction of the antimitogenic state were similar. We suggest a relationship of growth hormone-induced antimitogenic state and the growth hormone-induced adipose differentiation of 3T3-F442A cells.

Adipose Tissue↗

PubMed1998

High affinity hormone binding to the extracellular N-terminal exodomain of the follicle-stimulating hormone receptor is critically modulated by exoloop 3.

The human follicle-stimulating hormone receptor (FSH-R) consists of two distinct domains of >330 amino acids, the N-terminal extracellular exodomain and membrane-associated endodomain. The exodomain alone binds hormone with high affinity, whereas the endodomain is the site of receptor activation. Coordination of these two domains is essential for successful hormone action but little is known about their functional and structural relationship. In this communication, we report that exoloop 3 of FSH-R constrains follicle-stimulating hormone binding to the exodomain. When the FSH-R exodomain was prepared by truncating its endodomain, the hormone binding affinity of the exodomain was slightly improved, compared with the wild type receptor. The binding affinity was further improved by >3-fold when the exodomain was attached to the membrane-associated domain of CD8. These results suggest that the FSH-R endodomain attenuates hormone binding at the exodomain. As a first step to test this hypothesis, the 11 amino acids except Ala589 of exoloop 3 were individually substituted with Ala. Ala substitution for Leu583 or Ile584 improved the hormone binding affinity by 4-6-fold while totally abolishing cAMP induction, indicating an inverse relationship. The Ala substitution for Lys580 or Pro582 had a similar trend but to a lesser extent. This significant improvement in the binding affinity suggests that the four residues at the N-terminal region of exoloop 3 interact with the exodomain and constrain the hormone binding in the wild type receptor. This effect is specific since substitutions for other than the 4 residues did not improve the hormone binding affinity. Computer modeling shows that the 4 residues can be positioned on one side of exoloop 3. This result and the apparent inverse relationship of hormone binding and cAMP induction suggest that these two essential functions may work against each other. Therefore, hormone binding might be compromised to preserve cAMP inducibility while maintaining a reasonably high, but below maximum, binding affinity.

Amino Acid Substitution↗

PubMed1992

Effect of growth hormone on the low level of growth hormone binding protein in idiopathic short stature.

OBJECTIVE: Growth hormone receptor status was assessed in children with idiopathic short stature by evaluating plasma growth hormone-binding protein before and under GH therapy. DESIGN: Among 22 children presenting idiopathic short stature, 15 were randomly selected to be treated with GH (1.2 IU/kg/week); they were studied before and under GH therapy. Untreated patients served as a control group for age and GH effect. PATIENTS: Twenty-two prepubertal children, aged 5-11 years, were studied. They presented growth retardation of -2.8 +/- 0.1 SDS (mean +/- SEM). All had normal GH secretion and their mean IGF-I plasma level was normal. MEASUREMENT: Growth hormone-binding protein was measured using high pressure liquid chromatography gel filtration. The specific binding of 125I-hGH to the growth hormone-binding protein was expressed as a percentage of the total radioactivity. RESULTS: Specific binding of 125I-hGH to the high affinity growth hormone-binding protein was low with a mean +/- SEM value of 11.1 +/- 0.9% of radioactivity. In the treated group, growth hormone-binding protein increased significantly after 3 months of treatment; it reached 21.1 +/- 1.0% of radioactivity (mean +/- SEM) in the eight children who have been treated for 18 months. In the seven untreated children, the growth hormone-binding protein value increased to 16.2 +/- 1.1% after 18 months; this value is significantly lower than that found in the GH-treated children, demonstrating that the GH effect is greater than the age-related increase in the growth hormone-binding protein. A positive correlation was found between IGF-I plasma levels and growth hormone-binding protein and also between growth velocity and growth hormone-binding protein. CONCLUSIONS: The low growth hormone-binding protein and the response to high doses of GH suggest partial GH resistance at the receptor level, in this group of children with idiopathic short stature.

Carrier Proteins↗

PubMed1988

Are corticosteroid-binding globulin and sex hormone-binding globulin hormones?

Because it no longer seemed reasonable to us that the sole function of the steroid-binding proteins in plasma was to serve as a buffer reservoir for steroid hormones, we conducted experiments which sought out other possibilities. Both CBG and SHBG bind to cell membranes, and this interaction partakes of the general characteristics of peptide hormone-membrane receptor systems. Additionally, human CBG has the ability to cause an increase in the activity of membrane-bound adenylate cyclase in MCF-7 cells, and this, in turn, results in an increase in cellular cAMP content. Thus, CBG appears to be a protein hormone. As a first consideration, one might presume that because CBG's half-life is measured in days, it would be counted among the hormones which, for the most part, are tonic in their effects, e.g., thyroid hormone. However, two important considerations tend to believe this presumption: (1) CBG which is unoccupied by steroid is not hormonally active (Figure 5): (2) Depending upon the time of day, circulating CBG is approximately 0-60% occupied in normal humans. These observations result in a circumstance in which a substantial portion of circulating CBG is available for activation by bursts of cortisol secretion. It seems prudent to speculate that, because steroids are essential for CBG's activity, the hormonal role of CBG may be entwined with, or complementary to the steroids which it binds. Finally, we should comment on the impact that our model of CBG as a hormone has on the view that only unbound steroid can be hormonally active. First, it should be stated that we have not addressed this question experimentally. Although there is evidence that CBG may be required for cortisol action, we feel that an obligate role for it is not documented adequately. At this time, we believe that CBG's hormonal role is compatible with a hypothesis that encompasses the view that unbound steroid hormones can diffuse into cells in some tissues and that both free and bound steroid can enter cells in others. Obviously, the final word on these important topics, as always, awaits the proper experiments.

Binding Sites↗

PubMed1997

A conformational switch in nuclear hormone receptors is involved in coupling hormone binding to corepressor release.

Nuclear hormone receptors are ligand-regulated transcription factors that modulate gene expression in response to small, hydrophobic hormones, such as retinoic acid and thyroid hormone. The thyroid hormone and retinoic acid receptors typically repress transcription in the absence of hormone and activate it in the presence of hormone. Transcriptional repression is mediated, in part, through the ability of these receptors to physically associate with ancillary polypeptides called corepressors. We wished to understand the mechanism by which corepressors are recruited to unliganded nuclear hormone receptors and are released on the binding of hormone. We report here that an alpha-helical domain located at the thyroid hormone receptor C terminus appears to undergo a hormone-induced conformational change required for release of corepressor and that amino acid substitutions that abrogate this conformational change can impair or prevent corepressor release. In contrast, retinoid X receptors appear neither to undergo an equivalent conformational alteration in their C termini nor to release corepressor in response to cognate hormone, consistent with the distinct transcriptional regulatory properties displayed by this class of receptors.

Amino Acid Sequence↗

PubMed2001

Isoform-specific transcriptional regulation by thyroid hormone receptors: hormone-independent activation operates through a steroid receptor mode of co-activator interaction.

Thyroid hormone receptors (T3Rs) are hormone-regulated transcription factors that play important roles in vertebrate homeostasis, differentiation, and development. T3Rs are synthesized as multiple isoforms that display tissue-specific expression patterns and distinct transcriptional properties. Most T3R isoforms associate with co-activator proteins and mediate transcriptional activation only in the presence of thyroid hormone. The pituitary-specific T3Rbeta-2 isoform departs from this general rule and is able to interact with p160 co-activators, and to mediate transcriptional activation in both the absence and presence of hormone. We report here that this hormone-independent activation is mediated by contacts between the unique N terminus of T3Rbeta-2 and an internal interaction domain in the SRC-1 (steroid receptor co-activator-1) and GRIP-1 (glucocorticoid receptor interacting protein 1) co-activators. These hormone-independent contacts between T3Rbeta-2 and the p160 co-activators are distinct in sequence and function from the LXXLL motifs that mediate hormone-dependent transcriptional activation and resemble instead a mode of co-activator recruitment previously observed only for the steroid hormone receptors and only in the presence of steroid hormone. Our results suggest that the transcriptional properties of the different T3R isoforms represent a combinatorial mixture of repression, antirepression, and hormone-independent and hormone-dependent activation functions that operate in conjunction to determine the ultimate transcriptional outcome.

Amino Acid Sequence↗