Enhancement of growth hormone bioactivity by zinc in the eluted stain assay system

The effects of ionic zinc (Zn2+) on human (h) GH bioactivity have been examined using a lactogenic bioassay. The potencies of pituitary-derived hGH (IRP 80/505), recombinant 22K hGH (IRP 88/624), pituitary-derived human PRL (IRP 84/500), and a recombinant methionyl 20-kilodalton variant of hGH in the presence of selected concentrations of ZnCl2 were investigated with an eluted stain assay that uses Nb2 rat lymphoma cells. This precise colorimetric bioassay is based upon the reduction of a yellow tetrazolium salt, 3-[4,5-dimethyl-thiazol-2-yl]2,5-di-phenyl-tetrazolium bromide, to its purple formazan by lactogen-activated Nb2 cells. Zinc (6-100 microM) enhanced the bioactivity of low doses (< 0.045 nM) of both pituitary-derived and recombinant 22K hGH, although the magnitude of enhancement was considerably less than might have been anticipated from previous binding studies (13). Higher concentrations of pituitary-derived hGH (> 0.045 nM) were inhibited by Zn2+. The bioactivity of recombinant methionyl 20K hGH was greatly enhanced by zinc (3-100 microM). In contrast to hGH, the bioactivity of hPRL was not potentiated by Zn2+. These discriminatory effects of Zn2+ when stimulating via the lactogenic receptor are in concordance with the results of previous radioligand binding studies (13). The striking enhancement of 20K hGH lactogenic bioactivity was observed at Zn2+ concentrations within the physiological range for normal human serum (5-20 microM).     

Importance of extracellular domains for ligand binding in the thyrotropin-releasing hormone receptor

The role of putative extracellular sequences for ligand binding in the TRH receptor was examined using deletion or substitution mutations. Each mutant receptor was transiently expressed in TRH receptor-minus GH(1)2C(1)b rat pituitary cells, and binding of 4 Nu Mu [3H]pGlu-N(tau)-MeHis-Pro-NH2 ([3H] MeTRH) was measured. When binding was not detected, signal transduction at 10 microM MeTRH was measured to assess receptor expression. Deletion of most of the N-terminal sequences (Glu(2)-Leu(22)), including two potential glycosylation sites, had no effect on the affinity of the receptor for MeTRH. Segmental deletions or simultaneous substitution of multiple amino acid residues in the first, second, or third extracellular loop (EL1, EL2, or EL3) resulted, however, in total loss of [3H]MeTRH binding, suggesting important roles for the loop sequences in either receptor expression or ligand binding. Individual substitutions were made to test further the role of the specific extracellular loop sequences in TRH binding. In EL1, conversion of Tyr93 to Ala resulted in more than 20-fold decrease in affinity for MeTRH. In EL2 and the top portion of the fifth transmembrane helix, conversion of Tyr181 to Phe, Tyr188 to Ala, and Phe199 to Ala resulted in a large ( > 100-fold) decrease in affinity for MeTRH, and conversion of Tyr 188 to Phe and Phe196 to Ala caused an agonist-specific 4- to 5-fold decrease in affinity. In EL3, conversion of Asn289 to Ala and of Ser290 to Ala caused a large ( > 100-fold) decrease in affinity for MeTRH. These results suggest important roles for the extracellular loops in high affinity TRH binding and lead us to propose a model in which TRH binds to the extra-cellular domain of its receptor.     

Identification of common ligand binding determinants of the insulin and insulin-like growth factor 1 receptors. Insights into mechanisms of ligand binding

Insulin and insulin-like growth factor 1 (IGF-1) are peptides that share nearly 50% sequence homology. However, although their cognate receptors also exhibit significant overall sequence homology, the affinity of each peptide for the non-cognate receptor is 2-3 orders of magnitude lower than for the cognate receptor. The molecular basis for this discrimination is unclear, as are the molecular mechanisms underlying ligand binding. We have recently identified a major ligand binding site of the insulin receptor by alanine scannning mutagenesis. These studies revealed that a number of amino acids critical for insulin binding are conserved in the IGF-1 receptor, suggesting that they may play a role in ligand binding. We therefore performed alanine mutagenesis of these amino acids to determine whether this is the case. cDNAs encoding alanine-substituted secreted recombinant IGF-1 receptors were expressed in 293 EBNA cells, and the ligand binding properties of the expressed proteins were evaluated. Mutation of Phe701 resulted in a receptor with undetectable IGF-1 binding; alanine substitution of the corresponding amino acid of the insulin receptor, Phe714, produces a 140-fold reduction in affinity for insulin. Mutation of Asp8, Asn11, Phe58, Phe692, Glu693, His697, and Asn698 produces a 3.5-6-fold reduction in affinity for IGF-1. In contrast, alanine mutation of the corresponding amino acids of the insulin receptor with the exception of Asp12 produces reductions in affinity that are 50-fold or greater. The affinity of insulin for these mutants relative to wild type receptor was similar to that of their relative affinity for IGF-1 with two exceptions; the IC50 values for insulin binding to the mutants of Arg10, which has normal affinity for IGF-1, and His697, which has a 6-fold reduction in affinity for IGF-1, were both at least 2 orders of magnitude greater than for wild type receptor. The Kd values for insulin of the corresponding alanine mutants of the insulin receptor, Arg14 and His710, are 2-3 orders of magnitude greater than for wild type receptor. However, in contrast, the relative affinity of des(25-30)[PheB25 alpha-carboxamide]insulin for these IGF-1 receptor mutants is reduced only 4- and 50-fold, respectively.     

Mode of the autocrine/paracrine mechanism of growth hormone action

GH is synthesized at multiple extrapituitary sites suggestive of an autocrine/paracrine mechanism of action. We have investigated a possible autocrine/paracrine mechanism of GH action, compared the cellular response to exogenous versus endogenously produced GH, and determined the nature of the interaction between external stimuli and endogenously produced GH. BRL cells expressing the GH receptor were transiently transfected with expression plasmids containing either the hGH or the bGH gene and the response of the cell was measured by CAT reporter plasmids requiring either STATs 1 and 3 or STAT5 for their response. Transient transfection of the hGH gene resulted in hGH accumulation in the cell and secretion into the media. The functional response through STATs 1 and 3 and STAT5 obtained with endogenously produced hGH was comparable or greater in magnitude to that obtained with the maximal stimulatory dose of exogenous hGH. Similar results were obtained with an expression plasmid containing the bGH gene. Endogenously produced hGH interacted in an additive manner when combined with submaximal doses of both exogenous hGH and serum. Such results were also observed in a more physiologically relevant mammary carcinoma cell line (MCF-7). The nonreceptor-dimerizing hGH antagonist, hGH-G120R, used in cells expressing the homologous receptor extracellular domain was able to only partially inhibit the response of the cell to endogenously produced hGH, in contrast to full inhibition of exogenous hGH. We therefore conclude that GH can function in an autocrine/paracrine manner, additive in effect to external stimuli.     

Identification of transmembrane domain residues determinant in the structure-function relationship of the human platelet-activating factor receptor by site-directed mutagenesis

Platelet-activating factor (PAF) is a potent phospholipid mediator that produces a wide range of biological responses. The PAF receptor is a member of the seven-transmembrane GTP-binding regulatory protein-coupled receptor superfamily. This receptor binds PAF with high affinity and couples to multiple signaling pathways, leading to physiological responses that can be inhibited by various structurally distinct PAF antagonists. We have used site-directed mutagenesis and functional expression studies to examine the role of the Phe97 and Phe98 residues located in the third transmembrane helix and Asn285 and Asp289 of the seventh transmembrane helix in ligand binding and activation of the human PAF receptor in transiently transfected COS-7 cells. The double mutant FFGG (Phe97 and Phe98 mutated into Gly residues) showed a 3-4-fold decrease in affinity for PAF, but not for the specific antagonist WEB2086, when compared with the wild-type (WT) receptor. The FFGG mutant receptor, however, displayed normal agonist activation, suggesting that these two adjacent Phe residues maintain the native PAF receptor conformation rather than interacting with the ligand. On the other hand, substitution of Ala for Asp289 increased the receptor affinity for PAF but abolished PAF-dependent inositol phosphate accumulation; it did not affect WEB2086 binding. Substitution of Asn for Asp289, however, resulted in a mutant receptor with normal binding and activation characteristics. When Asn285 was mutated to Ala, the resulting receptor was undistinguishable from the WT receptor. Surprisingly, substitution of Ile for Asn285 led to a loss of ligand binding despite normal cell surface expression levels of this mutant, as verified by flow cytometric analysis. Our data suggest that residues 285 and 289 are determinant in the structure and activation of the PAF receptor but not in direct ligand binding, as had been recently proposed in a PAF receptor molecular model.     

Interrelation of the therapeutic effects of growth hormone and testosterone on growth in hypopituitarism

The present study evaluates the modifying effect of growth hormone on the growth-promoting action of testosterone in boys at pubertal bone age. Growth and bone maturation were analyzed in 42 boys with primary or secondary Leydig cell insufficiency who had been treated with testosterone in an attempt to induce puberty and the accompanying growth spurt. The dosage given was considered normal or high for physiologic replacement therapy at puberty. Sixteen boys had normal GH secretion (seven had isolated gonadotropin deficiency, nine had congenital anorchia); 26 were GH and Gn deficient (20 idiopathic, six craniopharyngiomas). Of the GH-deficient patients, 12 received hGH simultaneously, while 14 received only testosterone. Results from each group were compared with the normal pubertal growth spurt in 15 untreated healthy boys. In isolated Gn deficiency and in congenital anorchia, the growth rates increased to above normal during the first six months of treatment, indicating that the testosterone dosage was probably too high for the beginning of puberty. During two subsequent six-month treatment periods, the rates leveled off close to normal. The same was true in the GH- and Gn-deficient patients on adequate hGH replacement. For contrast, there was minimal or no stimulation of growth when an even higher testosterone dose was given to GH- and Gn-deficient boys without hGH therapy. Bone maturation was normal in the boys with normal GH secretion or with hGH replacement, but was subnormal in the GH-deficient boys not treated with hGH. We conclude that testosterone exerts its full growth-promoting action only in the presence of normal endogenous GH secretion or with sufficient hGH replacement and that both hormones should be continued simultaneously until final adult height is achieved.     

A hydrophobic cluster between transmembrane helices 5 and 6 constrains the thyrotropin-releasing hormone receptor in an inactive conformation

We have studied the role of a highly conserved tryptophan and other aromatic residues of the thyrotropin-releasing hormone (TRH) receptor (TRH-R) that are predicted by computer modeling to form a hydrophobic cluster between transmembrane helix (TM)5 and TM6. The affinity of a mutant TRH-R, in which Trp279 was substituted by alanine (W279A TRH-R), for most tested agonists was higher than that of wild-type (WT) TRH-R, whereas its affinity for inverse agonists was diminished, suggesting that W279A TRH-R is constitutively active. We found that W279A TRH-R exhibited 3.9-fold more signaling activity than WT TRH-R in the absence of agonist. This increased basal activity was inhibited by the inverse agonist midazolam, confirming that the mutant receptor is constitutively active. Computer-simulated models of the unoccupied WT TRH-R, the TRH-occupied WT TRH-R, and various TRH-R mutants predict that a hydrophobic cluster of residues, including Trp279 (TM6), Tyr282, and Phe199 (TM5), constrains the receptor in an inactive conformation. In support of this model, we found that substitution of Phe199 by alanine or of Tyr282 by alanine or phenylalanine, but not of Tyr200 (by alanine or phenylalanine), resulted in a constitutively active receptor. We propose that a hydrophobic cluster including residues in TM5 and TM6 constrains the TRH-R in an inactive conformation via interhelical interactions. Disruption of these constraints by TRH binding or by mutation leads to changes in the relative positions of TM5 and TM6 and to the formation of an active form of TRH-R.     

Comparison of inversion-recovery gradient- and spin-echo and fast spin-echo techniques in the detection and characterization of liver lesions

For several G protein-coupled receptors, amino acids in the seventh transmembrane helix have been implicated in ligand binding and receptor activation. The function of this region in the AT1 angiotensin receptor was further investigated by mutation of two conserved polar residues (Asn294 and Asn295) and the adjacent Phe293 residue. Analysis of the properties of the mutant receptors expressed in COS-7 cells revealed that alanine replacement of Phe293 had no major effect on AT1 receptor function. Substitution of the adjacent Asn294 residue with alanine (N294A) reduced receptor binding affinities for angiotensin II, two nonpeptide agonists (L-162,313 and L-163,491), and the AT1-selective nonpeptide antagonist losartan but not that for the peptide antagonist [Sar1, Ile8]angiotensin II. The N294A receptor also showed impaired G protein coupling and severely attenuated inositol phosphate generation. In contrast, alanine replacement of Asn295 decreased receptor binding affinities for all angiotensin II ligands but did not impair signal transduction. Additional substitutions of Asn295 with a variety of amino acids did not identify specific structural elements for ligand binding. These findings indicate that Asn295 is required for the integrity of the intramembrane binding pocket of the AT1a receptor but is not essential for signal generation. They also demonstrate the importance of transmembrane helices in the formation of the binding site for nonpeptide AT1 receptor agonists. We conclude that the Asn294 residue of the AT1 receptor is an essential determinant of receptor activation and that the adjacent Asn295 residue is required for normal ligand binding.     

Delineation of amino acid residues within hTSHr 256-275 that participate in hormone binding

The amino acid sequence 256-275 of the human thyrotropin (TSH) receptor extracellular domain has previously been shown to participate in a high affinity TSH binding site by a synthetic peptide approach as well as by site-directed mutagenesis. To further investigate this binding site, we synthesized a series of peptides with alanine substitutions for each residue in the native sequence. Peptides were also synthesized containing truncations or deletions of the native sequence. Each peptide was tested for its ability to inhibit 125I-bTSH binding to porcine thyroid membrane preparations, and the concentration at which 50% inhibition of binding occurred was determined (EC50). Alanine substitution at residues Tyr258, Cys262, Cys263, Phe265, Lys266, Asn267, Lys269, Lys270, and Arg272 all resulted in statistically significant decreases in activity when compared to the native sequence (p < 0.05). Alanine substitution of the remaining residues did not alter their activity. Comparison of this sequence with the corresponding sequences of the remaining glycoprotein hormone receptors (human lutropin and human follitropin receptors) reveals that these residues lie within one of the most highly conserved regions of the extracellular domain. We conclude that 9 specific amino acids within the sequence 256-275 of hTSHr (-Y--CC-FKN-KK-R--) participate in the interaction of the hTSHr-extracellular domain with TSH. This may represent a site in which the nonconserved residues are involved in the binding of the beta-subunit and the conserved residues are involved in the binding of the common alpha-subunit or a region of the beta-subunit that is common to all glycoprotein hormones.     

COOH-terminal amino acids of the alpha subunit play common and different roles in human choriogonadotropin and follitropin

Human choriogonadotropin (hCG) and follitropin (FSH) belong to the glycoprotein hormone family. These hormones are heterodimers and composed of a common alpha subunit and a distinct beta subunit which confers receptor-binding specificities. In addition to this structural similarity, they share a similar signal pathway involving G protein, adenylyl-cyclase and induction of cAMP synthesis. Therefore, a presumptive relationship of these common structure and function has been the subject of extensive past investigations, but a definitive clue has been elusive. As a step to address this important issue, a series of recombinant mutants of hCG and human FSH were generated in which the COOH-terminal amino acids of the alpha subunit were successively removed or substituted. Furthermore, a set of peptides were synthesized with sequences corresponding to different regions of the alpha subunit. Deletion of the alpha COOH-terminal Ser92 had no effect on receptor-binding or cAMP induction by FSH and hCG. Truncation of alpha Lys91-Ser92 or alpha His90-Lys91-Ser92 abolished the ability of both hormones to induce cAMP synthesis. It significantly reduced receptor binding of FSH but not hCG. The different functions of the alpha COOH-terminal region are further noticed with a peptide corresponding to the last 10 amino acids of alpha. It failed to bind to the FSH receptor but was capable of binding to the LH/CG receptor and stimulating cAMP synthesis. These results are the first conclusive evidence that alpha His90-Lys91 play an essential role in cAMP induction of both hormones. In contrast to this common role, they are necessary for FSH binding to the FSH receptor but not for hCG binding to the LH/CG receptor. The hCG alpha COOH-terminal region makes direct contact with the LH/CG receptor, and this low affinity contact is necessary and sufficient to activate the receptor for signal generation. This conclusion is supported by the study using mutant hCGs in which either alpha His90 or Lys91 was substituted.     

Insulin-like growth factor binding proteins-2 and -3 stimulate growth hormone receptor binding and mitogenesis in rat osteosarcoma cells

GH exerts its biological actions on osteoblasts through a specific high affinity receptor expressed on these cells. GH receptor binding is positively modulated by a number of factors, including retinoic acid and dexamethasone, whereas fetal calf serum strongly decreases the binding. To identify responsible factors in serum, components of serum, the insulin-like growth factors (IGFs)-I and -II, and IGF binding proteins (IGFBPs)-2 and -3 were tested for a possible negative modulatory role. IGF-I and -II decreased [125I]hGH binding at an optimal concentration of 30 ng/ml for IGF-I and 100 ng/ml IGF-II, reducing the binding to 51% and 55%, respectively, of control values. A stimulation of [125I]hGH binding was observed with IGFBP-2 as well as IGFBP-3, inducing an increase to 148% and 151% of control binding at an optimal concentration of 3000 ng/ml for both peptides. The effects of all peptides were dependent on the incubation time, being significantly increased after 8 h of incubation and reaching the full effect thereafter. The effects were declined at 24 h compared with 16 h for IGFBP-2 and -3 but not for IGF-I and -II. Coincubation of the cells with IGF-I and -II and IGFBP-2 and -3 neutralized the effects of the factors alone. In conclusion, these results show that IGF-I and -II on the one hand and IGFBP-2 and -3 on the other hand exert opposite actions on [125I]hGH binding, IGFBP-2 and -3 exerting probably an IGF-independent effect. Further, IGF-I and -II decreased GH receptor messenger RNA (mRNA) levels, as quantified by a solution hybridization ribonuclease protection assay, from 8.65 +/- 1.78 attomoles (amol)/microgram DNA (control) to 2.4 +/- 0.68 and 2.16 +/- 0.92 amol/microgram DNA, respectively. IGFBP-2 increased GH receptor mRNA levels from 5.26 +/- 1.17 (control) to 13.19 +/- 3.48. Incubation with IGFBP-3 did not result in stimulation of GH receptor mRNA levels (8.59 +/- 2.91 amol/microgram DNA). This shows that the mechanism of regulation of the GH receptor is, except for IGFBP-3, at least in part on the mRNA level. Lastly, IGFBP-2 and IGFBP-3 are mitogenic for UMR-106.01 rat osteosarcoma cells, inducing an increase in cell number to 125% and 142% of control cell counts after 48 h of incubation with 1000 ng/ml IGFBP-2 and -3, whereas IGF-I, IGF-II and Long R3 IGF-I did not stimulate proliferation. IGFBP-2 and -3 potentiate hGH induced mitogenesis at low hGH concentrations of both factors, whereas at higher concentrations no such effect is observed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Toxicity and immunogenicity of a verotoxin 1 mutant with reduced globotriaosylceramide receptor binding in rabbits

The verotoxins (VT1 and VT2), produced by strains of enterohemorrhagic Escherichia coli, have been implicated in the pathogenesis of hemorrhagic colitis and the hemolytic uremic syndrome. To better understand the role of globotriaosylceramide (Gb3) receptor binding by the verotoxins in disease production, we examined the clinicopathologic effects of an intravenously (i.v.) administered verotoxin 1 mutant holotoxin (Phe30Ala) in rabbits. The substitution of alanine for phenylalanine 30 in the VT1 B subunit has been shown previously to reduce both Gb3 binding affinity and capacity in vitro. This reduction in receptor binding corresponded to a 10(5)-fold reduction in the toxic activity of VT1 on a Vero cell monolayer. In this study, purified 125I-labeled Phe30Ala was administered i.v. to rabbits to determine its specific distribution in rabbit tissues. In contrast to the rapid elimination of i.v. administered 125I-VT1 from the bloodstream, 125I-Phe30Ala had a 52-fold-longer half-life in serum and failed to localize preferentially in the gastrointestinal tract and central nervous system (CNS). Rabbits challenged with Phe30Ala at a dose equivalent to 10 times the 50% lethal dose (LD50) of VT1 showed no visible clinical symptoms typical of VT effect after 7 days. Administration of Phe30Ala at a dose equivalent to 100 times the LD50 of VT1, however, caused both clinical and histopathologic features indistinguishable from VT1 toxemia in rabbits, although the onset of symptoms was delayed. Rabbits were immunized with Phe30Ala and challenged i.v. with either 125I-VT1 or 125I-VT2. The specific uptake of 125I-VT1 in the gastrointestinal tract and CNS was totally inhibited in Phe30Ala immune rabbits. Only a partial decrease in target organ uptake was observed in Phe30Ala immune rabbits challenged with 125I-VT2. From this study, we conclude that Gb3 binding is responsible for target organ localization of VT1 and disease production in the rabbit. The ability of Phe30Ala to induce both strong antibody and protective responses against VT1 suggests that VT mutants with reduced receptor binding properties may be useful in vaccine strategies. A further reduction in the toxicity of Phe30Ala would be required for its use as a natural toxoid to protect against human verotoxigenic E. coli infections.     

Hormonal regulation of the estrogen receptor in primary cultures of hepatocytes from female rats

Estrogen treatment affects the hepatic synthesis and/or secretion of several proteins involved in clinically important pathological processes such as atherosclerosis, hypertension, and thrombosis. The endocrine regulation of the estrogen receptor (ER) concentration in primary cultures of rat hepatocytes was studied. Human growth hormone (hGH) and dexamethasone (DEX) in combination increased ER concentration 6-fold and ER mRNA levels 2.5-fold. These effects were not significantly different from those observed after treatment with the purely somatogenic bovine growth hormone (GH) in combination with DEX. Treatment with the lactogen ovine prolactin in the presence or absence of DEX did not significantly affect ER or ER mRNA concentrations. Triiodothyronine treatment at the most effective concentration (50 nM) increased ER and ER mRNA levels twofold. Medium supplementation with estradiol (0.1 nM) throughout the experiment did not affect the response to treatment with hGH and DEX. Treatment with high concentrations of ethinylestradiol in combination with hGH and DEX, however, increased the ER level twice as much as hGH and DEX without addition of estradiol or ethinylestradiol, whereas the ER mRNA concentration was the same in both the GH+DEX group and GH+ DEX+ (estradiol or ethinylestradiol) groups. These data indicate the importance of GH in combination with glucocorticoids for the maintenance of ER concentrations in the rat liver. Thyroid hormones may be of some, although minor importance, whereas the data suggest that prolactin is not directly involved in hepatic ER regulation.     

A case of cytophagic histiocytic panniculitis: successful treatment of recurrent attacks with steroid pulse therapy and oral cyclosporin A

Basic fibroblast growth factor (bFGF) is implicated in the pathogenesis of several vascular and connective diseases. A key step in the discovery of bFGF receptor antagonists to mitigate these actions is to define the functional epitope required for receptor binding of the growth factor. In previous studies, we identified Glu96 as an essential residue in this epitope using site-directed mutagenesis. Here we examined the role of solvent accessible neighboring residues of Glu96 of bFGF on receptor binding affinity. Wild-type bFGF and its muteins were cloned and expressed in Escherichia coli and evaluated for FGF receptor binding affinity. Replacement of Asn104 of bFGF by alanine reduced receptor binding affinity over 400-fold compared with wild-type bFGF. We next explored the effect of neighboring residues of Asn104 on receptor binding affinity-Muteins in which Arg97, Leu98, Glu99, Asn101, Asn102, Thr105 and Pro141 were individually replaced by alanine exhibited receptor binding similar to wild-type bFGF. By contrast, substitution of Tyr103 or Leu140 by alanine reduced receptor binding affinity about 400- and 150-fold, respectively, in accord with a previous report. We conclude that at least six solvent-accessible residues in bFGF are crucial for high-affinity receptor binding, as evidenced by at least a 10-fold diminution in the affinity of the corresponding alanine muteins. The polar residues Glu96 and Asn104 appear to form an area important for facilitating the initial contact between ligand and receptor, whereas Tyr24, Tyr103, Leu140 and Met142 form a hydrophobic patch that may stabilize the complex. The detailed structure of this functional epitope can be employed in the discovery and design of bFGF antagonists using computational methods.     

Out-of-hospital quantitative monitoring of end-tidal carbon dioxide pressure during CPR

BACKGROUND: Human growth hormone (hGH) binds to both the hGH and human prolactin (hPRL) receptors. Binding to the hPRL receptor, however, is approximately 50-fold tighter and requires a single Zn2+ cation, unlike binding of hGH to the hGH receptor. Previous mutational studies have identified putative ligands from hGH and the hPRL receptor responsible for coordinating the interfacial Zn2+. RESULTS: One of these ligands was introduced at a structurally analogous site in the extracellular domain of the hGH receptor by mutating Asn218 to His, and the resulting mutant protein showed a 20-fold increase in hGH binding in the presence of ZnCl2. Alanine-scanning mutagenesis showed that the binding site on hGH for the Asn218-->His hGH receptor in the presence of Zn2+ resembled that for the hPRL receptor. CONCLUSIONS: It is possible to introduce the metal-binding site from the hPRL receptor into the homologous hGH receptor. More generally, these studies indicate that affinity between two proteins may be enhanced by design of an interfacial metal-binding site.     

Growth hormone and its receptor are expressed in human thymic cells

GH has been shown to modulate various functions of the thymus. We now demonstrate the production of human GH (hGH) by human thymic cells, and the expression of GH receptors in thymic epithelial cells (TEC) and in thymocytes at different stages of differentiation. The presence of hGH messenger RNA was shown by RT-PCR in both human thymocytes and in primary cultures of TEC. Moreover, immunoreactive hGH material was detected in culture media of thymocytes and TEC with the use of a sensitive immunoradiometric assay. GH receptor gene expression was shown in TEC in primary cultures and in fetal and postnatal TEC lines as well as in thymocytes. By immunocytochemistry, the presence of GH receptors in the various TEC preparations was confirmed. In cytofluorometric studies with the use of a biotinylated anti-GH receptor monoclonal antibody, we could show that GH receptors are predominantly expressed by immature thymocytes: over 90% of CD3- CD4- CD8- CD19- CD34+ CD2- cells (a phenotype characterizing the most immature T cell progenitors in the thymus) were GH receptor positive. Our results provide a molecular basis for an autocrine/paracrine mode of action of GH in the human thymus.     

Key amino acids in the gamma subunit of the gamma-aminobutyric acidA receptor that determine ligand binding and modulation at the benzodiazepine site

Pharmacological analyses of gamma-aminobutyric acidA (GABAA) receptor subtypes have suggested that both the alpha and gamma subunits, but not the beta subunit, contribute to the benzodiazepine binding site. We took advantage of the different pharmacological properties conferred by the inclusion of different gamma subunits in the receptor macromolecule to identify amino acids gamma2Phe77 and gamma2Met130 as key determinants of the benzodiazepine binding site. gamma2Phe77 was required for high affinity binding of the benzodiazepine site ligands flumazenil, CL218,872, and methyl-beta-carboline-3-carboxylate but not flunitrazepam. This amino acid was, however, required for allosteric modulation by flunitrazepam, as well as other benzodiazepine site ligands. In contrast, gamma2Met130 was required for high affinity binding of flunitrazepam, clonazepam, and triazolam but not flumazenil, CL218, 872, or methyl-beta-carboline-3-carboxylate and did not affect benzodiazepine efficacy. Introduction of the phenylalanine and methionine into the appropriate positions of gamma1 was not sufficient to confer high affinity for the benzodiazepine site ligand zolpidem. These data show that gamma2Phe77 and gamma2Met130 are necessary for high affinity binding of a number of benzodiazepine site ligands. Although most previous studies have focused on the contribution of the alpha subunit, we demonstrated a critical role for the gamma subunit at the benzodiazepine binding site, indicating that this modulatory site is located at the interface of these two subunits. Furthermore, gamma2Phe77 is homologous to alpha1Phe64, which has been previously shown to be a key determinant of the GABA binding site, suggesting a conservation of motifs between different ligand binding sites on the GABAA receptor.     

Rapid disappearance of AML1/ETO fusion transcripts in patients with t(8;21) acute myeloid leukemia following bone marrow transplantation and chemotherapy

A new analogue of recombinant human growth hormone (hGH), hGH des(1-6,14) was expressed in Escherichia coli, refolded and purified to homogeneity. The mutation decreased the hormone's ability to bind lactogenic and somatogenic receptors through its site 1, and almost completely abolished its ability to bind these receptors through site 2, as evidenced by both binding and gel-filtration experiments. More specifically, the binding to prolactin receptors (PRLRs) from various species or their soluble recombinant extracellular domains (ECDs) was decreased 1.5-4-fold, whereas the binding to hGH receptor (hGHR) was decreased 10-85-fold. These changes caused an almost total loss of hormone agonistic activity in several in vitro bioassays and subsequently, the hGH des(1-6,14) analogue acquired antagonistic properties. This antagonistic activity was dependent upon modification of site 1. In those cases in which the binding was reduced only slightly, e.g. binding to rabbit PRLRs, hGH des(1-6,14) acted as a strong antagonist, whereas in others in which the binding of site 1 was reduced to a higher degree, such as other PRLRs and, in particular, hGHR, the antagonistic activity was correspondingly weaker. Circular dichroism spectra of the analogue suggested that these changes do not result from a decrease in overall alpha-helix content, but rather from minor local structural modifications at the N-terminus.     

Dynamics of growth hormone secretion in patients with primary hypothyroidism, before and after treatment with thyroideal hormones

Growth hormone (GH) secretion was studied in a group of alut controls and a series of patients with primary hypothyroidism before and after treatment with thyroid hormones. Intravenous insulin-induced hypoglycemia was the stimulus used (0,1 UI/kg body weight). It is concluded that in primary hypothyroidism, GH secretion is almost absent due to relative inactivity of the somatotropic cells and/or lack of GH-RH. This decreased secretory response becomes rapidly normal after treatment with physiological doses of thyroid hormones.