Identification and characterization of a heterogeneous population of growth hormone receptors in mouse hepatic membranes

Exon 3 of the growth hormone receptor (GH-R) is specific to eutherian mammals

Growth hormone receptor (GH-R) plays a critical role in the control of growth and metabolism in all vertebrates. GH-R consists of 9 coding exons (2-10) in all eutherian mammals, while the chicken only has 8 coding exons, and does not have an orthologous region to exon 3 in eutherians. To further understand the evolutionary origins of exon 3 of the GH-R in eutherians we cloned the full-length GH-R sequence in a marsupial, the tammar wallaby to determine whether exon 3 was present or absent in marsupial liver cDNA. There was no evidence for the presence of an exon 3 containing mRNA in sequence of tammar pouch young and adult livers. We next examined the genomes of the platypus (a monotreme mammal) and the grey short-tailed opossum (another marsupial). Like the tammar, the GH-R gene of neither species contained an exon 3. GH receptor can obviously function in the absence of this exon, raising speculation about the function of this domain, if any, in eutherians. A comparison of exon 3 protein sequences within 16 species of eutherian mammals showed that there was approximately 75% homology in the domain but only 3 of the 21 amino acids were identical (Leu12, Gln13 and Pro17). Interestingly, we detected greater evolutionary divergence in exon 3 sequences from species that have variants of GH or prolactin (PRL) in their placentas. These data show that exon 3 was inserted into the GH-R after the divergence of the marsupial and eutherian lineages at least 130 million years ago.

Mammary tumorigenesis in growth hormone deficient spontaneous dwarf rats; effects of hormonal treatments

This study was carried out to investigate mammary tumorigenesis in growth hormone (GH) deficient spontaneous dwarf rats (SDR). At 50-60 days of age, the rats were divided into five groups. Group 1 received bovine (b) GH (prolonged release formulation) administered at a dose of 40-50 mg/kg body wt. in 50 microl weekly injections; group 2 received recombinant human insulin-like growth factor-I (IGF-I) at a dose of 1 mg/kg body wt./day administered via osmotic pumps; animals in group 3 were fitted with subcutaneous silastic capsule containing 30 microg 17 beta-estradiol (E2) plus 30 mg progesterone (P4), replaced every 2 months; group 4 received both bGH and E2 plus P4 treatments at the same doses as above, and control animals (group 5) received sham treatments (vegetable oil injection, silastic capsules containing cellulose). After 1 week of treatment, all animals were injected intraperitoneally with the carcinogen N-methyl-N-nitrosourea (MNU) at a dose of 50 mg/kg body wt. Other groups of animals, receiving identical hormonal treatment to those exposed to MNU, were treated for 10 days only and then sacrificed for assessment of circulating concentrations of hormones and mammary gland characteristics at the time of carcinogen exposure. The hormonal treatments of the animals exposed to the MNU were continued for an additional 20 weeks and mammary tumor development monitored by weekly palpation and tumors collected as necessary. The rats were weighed weekly. At the end of the treatment period, all animals were sacrificed and remaining tumors were collected. Rats in all groups continued to gain weight throughout the experimental period, but the largest weight gain was see in animals receiving GH either alone or with E2 and P4. Animals treated with IGF-I also gained weight compared to controls, but this weight gain was less than that seen in GH-treated rats. GH treatment alone increased mammary tumor incidence from 4.8% in controls to 100%. Average tumor load and latency in the GH-treated rats were 7.0 +/- 0.8 tumors/tumor-bearing rat (mean +/- SEM) and 57.3 +/- 2.7 days (mean +/- SEM), respectively. As in intact Sprague-Dawley rats, approximately 90% of the tumors that developed in the GH-treated rats were ovarian dependent for growth. IGF-I treatment also increased mammary tumor development to 62.5%. Average tumor load and latency in the IGF-I-treated rats were 1.6 +/- 0.4 tumors/tumor-bearing rat (mean +/- SEM) and 96.2 +/- 14.5 days (mean +/- SEM), respectively. However E2 + P4 treatments did not significantly alter tumorigenesis and, surprisingly, simultaneous treatment with E2 + P4 and GH obliterated the GH-stimulated increase in tumor development. Prolactin (PRL) did not appear to influence mammary tumorigenesis in the SDRs, as untreated SDRs had significantly elevated serum concentration of PRL as compared with normal Sprague-Dawley (SD) rats, whereas GH-treated SDRs had PRL levels similar to that of normal SD rats. No obvious structural characteristics were associated with high or low susceptibility to mammary tumorigenesis, as assessed by mammary gland whole mounts from the different animal groups sacrificed at the time of carcinogen administration. Enhanced expression of the extracellular signal-regulated kinase 1/2 (ERK1/2), and activation (phosphorylation) of ERK1/2 were associated with an increase in mammary tumorigenesis. Similarly, the expression of the estrogen receptor-alpha (ER alpha) was significantly elevated in animal groups with the highest susceptibility to tumorigenesis, whereas the levels of cyclin D1 expression were not related to mammary tumorigenesis.

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.

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

Biochemistry and pharmacology of rabbit cardiac growth hormone (GH) receptors

In this report we present the first in-depth description of the biochemical and pharmacological properties of rabbit cardiac GH receptors. The apparent M(r)'s of the [125I]human (h) GH-receptor complexes were 380, 205, 90, 62, 52 and 38 kDa as demonstrated by an autoradiograph of affinity-labelled cardiac GH receptors separated under non-reducing conditions by SDS PAGE. The [125I]hGH-cardiac GH receptor complexes were disulfide-linked since the M(r)s of the complexes diminished to 170, 116, 97, 71, 45 and 38 kDa under reducing conditions, indicating the presence of multiple receptors, receptor-associated macromolecules or receptor and ligand in various ratios. The pharmacology of cardiac GH receptors is not typical of GH receptors present in other tissues. In radio receptor assays, both bovine GH and ovine prolactin were 50-fold and 100-fold less potent, respectively, than unlabelled hGH, in blocking the binding of [125I]hGH to cardiac binding sites and were, therefore, extremely weak antagonists. Similarly, neither bovine GH nor ovine prolactin blocked the [125I]hGH affinity-labelling of cardiac GH receptors compared to equivalent doses of unlabelled hGH. Parameters which characterize the kinetics for the association, dissociation and equilibrium binding of [125I]hGH to cardiac GH receptors were ascertained. Association kinetics for the binding of [125I]hGH to heart GH receptors exhibited a maximum specific binding at 17 h and 25 degrees C. The association of [125I]hGH to heart GH receptors was reversible with approximately 15 h required for half of the specifically bound [125I]hGH to dissociate. The coupling of [125I]hGH to heart GH receptors was optimum at pH 6 and the strength of the equilibrium binding, as measured by the ED50, was approximately 2 ng/ml. These data indicate that the cardiac GH receptors are pharmacologically distinct and that there is a M(r) heterogeneity in the [125I]hGH receptor complexes.

Pharmacokinetics of radioiodinated growth hormones in the turtle Chrysemys dorbigni

Growth hormone binding proteins (GHBP) have been identified in the blood of many species. The aim of the present work is to study the physiological role of the GHBP in the turtle serum which we recently described. Binding studies were carried out using in vivo pharmacokinetic and chromatographic techniques as well as in vitro methods. When (125)I-GH was injected in physiological concentration into Chrysemys dorbigni turtles, the first step of pharmacokinetics was the binding of a significant fraction of the labeled GH by the GHBPs present in serum. The decay curve followed a three compartments model and gave the equation: Ae(-alphat) + Be(-betat) + Ce(-gammat). The fast compartment with t(1/2) of 14.4 min or 25.2 min, for hGH and bGH represents 30.3% and 18.9% of total radioactivity, respectively, at hypothetical time zero (not experi mental). Chromatographic studies reveal that this rapid compartment represents free GH. The second and third compartments represent complex forms between GH and GHBPs present in the turtle serum, and represent 70% and 80% of total radioactivity for hGH and bGH, respectively. In vitro chromatographic studies showed direct evidence of the presence of GHBPs in the turtle serum. The presence of these GHBPs changed the pharmacokinetics of labeled GH in plasma and the subsequent liver uptake of GH. The labeled hGH or bGH binds to turtle serum in similar proportion, but maximal liver uptake of these hormones are completely different (L/B ratio of 9.2 +/- 0.6 (n = 5) for ( 125)I-hGH and 4.8 +/- 0.3 (n = 7) for (125)I-bGH). The reasons for these differences could be that human GH binds to lactogenic and somatotropic receptors and bovine GH binds only to somatotropic receptors.

Identification and modulation of a growth hormone-binding protein in rainbow trout (Oncorhynchus mykiss) plasma during seawater adaptation

A soluble protein that specifically bound 125I-human growth hormone (hGH) was identified in rainbow trout plasma, using HPLC-gel filtration. The binding affinity of the protein for hGH was 1.2 x 10(9)M-1. 125I-rainbow trout GH (tGH) was also able to bind to the protein albeit with a lower affinity (6.6 x 10(7)M-1) than hGH. Crosslinking experiments using 125I-hGH revealed two specific bands of 150 and 130 kDa. The complex 125I-hGH-BP could be precipitated by a monoclonal anti-GH receptor antibody, suggesting a close relationship between the plasma GH-BP and the GH receptor. A fourfold increase in the hGH binding to the GH-BP was shown 48 h after transfer of the fishes from freshwater to seawater. The increase in binding was related to a high binding capacity without significant changes in binding affinity. These results suggest a potential role of this related GH-BP as an index of GH effects during seawater adaptation in salmonids.

Identification and characterization of growth hormone receptors in snakehead fish (Ophiocephalus argus cantor) liver

The specific binding of 125I-labeled fish growth hormone (GH) to hepatic membranes prepared from several freshwater fish was assessed. A high level of growth hormone receptor (GHR) was detected on the hepatic membranes of the snakehead fish (Ophiocephalus argus Cantor). Scatchard analysis of the binding data showed a single class of high affinity binding site with a binding affinity (Ka) of 1.45 +/- 0.23 x 10(9) M-1 and a binding capacity (Bmax) of 198 +/- 57 fmol/mg protein. The binding was specific for fish GH and was saturable. In addition, the specific binding was temperature- and time-dependent, reaching a steady state after 16 hr of incubation at 25 degrees . The molecular weight of GHR as measured by Sephadex G-200 column chromatography and Western blot analysis using a monoclonal antibody (Mab263) against GHR was found to be 200-400 and 90-93 kDa, respectively. Two bands at 65 and 89 kDa were identified in ligand crosslinking studies of membrane receptors. A sensitive teleost GH radioreceptor assay (RRA) was developed, using recombinant fish GH and a membrane preparation from snakehead fish liver, capable of measuring bioactive GH in fish sera or other samples.

The effect of bovine somatotropin and diet on somatotropin binding sites in hepatic tissue of lactating dairy cows

In the lactating cow, galactopoiesis is stimulated by treatment with recombinant bovine somatotropin (bST) and by an improved plane of nutrition. The present study determined the interaction between these variables and examined whether a positive galactopoietic effect was accompanied by a change in hepatic binding sites for bST. Lactating dairy cows received one of three diets with increasing nutrient density; diet 1, 150 g/kg of dry matter (DM) of crude protein (CP) and 10.5 MJ/kg of DM of metabolizable energy; diet 2, 170 g/kg of DM of CP and 11.3 MJ/kg of DM of metabolizable energy; and diet 3, 190 g/kg of DM of CP and 12.1 MJ/kg of DM of metabolizable energy. At 90 d after calving, half of the cows in each dietary group were treated with bST every 14 d for the rest of the lactation. Both nutrient density and administration of bST increased milk yield significantly in mid and late lactation; there was no significant treatment by diet interaction. Treatment with bST significantly increased plasma concentrations of insulin-like growth factor (IGF)-I compared with IGF-I concentrations in controls in both mid and late lactation. Comparisons within diet revealed that concentrations of IGF-I were significantly higher in cows fed diet 3 than in cows fed diets 1 and 2 at both stages of lactation. Increases in plasma insulin were confined to cows in late lactation, and no changes were observed for nonesterified fatty acids. Liver biopsies showed that concentrations of hepatic binding sites for bST were not affected significantly by bST treatment but were increased in midlactation for cows fed diet 3. Concentration of hepatic binding sites per unit weight of tissue were greater for cows in midlactation than for cows in late lactation. In summary, exogenous bST treatment and increased nutrient density were associated with elevated plasma IGF-I concentrations and increased milk yield; however, only nutrient density in midlactation increased the number of hepatic binding sites for bST. Exogenous bST treatment had relatively little effect on the concentration of hepatic bST receptors compared with nutrient density.

Growth hormone-binding protein in normal mice and in transgenic mice expressing bovine growth hormone gene

The levels and characteristics of growth hormone (GH)-binding protein (GHBP) and the distribution of GH in peripheral circulation between the free and the bound fractions were studied in three lines of transgenic mice with various degrees of overexpression of bovine (b) GH gene. Two serum fractions bound GH specifically: one with low affinity and high capacity (GHBPI) and one with high affinity and low capacity (GHBPII). The GHBP binding capacity in normal mice (both sexes), transgenic male mice that express the metallothionein-I-hybrid bGH genes, transgenic female mice that express phosphoenolpyruvate carboxykinase (PEPCK)-bGH hybrid genes (PEPCK-bGH-1), and transgenic PEPCK-bGH-5 animals was 1.1 +/- 0.2, 2.0 +/- 0.1, 3.0 +/- 0.1, and 3.9 +/- 0.6 pmol/ml serum, respectively. The amount of GH bound to GHBP in transgenic animals vs. normal siblings was increased 1.8-, 2.5-, and 3.9-fold in these three lines. Consequently, the levels of GH-GHBP complexes in the circulation of PEPCK-bGH-1 transgenic mice were increased approximately 10-fold. Specific GHBP radioimmunoassay confirmed a threefold increase in GHBP in PEPCK-bGH-1 transgenic animals. The levels of GHBP were not significantly correlated to serum GH within or between lines, perhaps due to elevation of serum GH in PEPCK-bGH mice above the level producing maximal response. From these and previous studies, we conclude that life-long exposure to supranormal GH levels leads to major shifts in GH binding in the circulation and in the GH target organs.

In vitro mutagenesis of growth hormone receptor Asn-linked glycosylation sites

Site-directed mutagenesis was used to replace asparagine (Asn) residues with glutamine (Gln) at the five potential N-linked glycosylation sites located at positions 28, 97, 138, 143, and 182 in the extracellular domain of the porcine growth hormone receptor (pGHR). These mutated pGHR cDNAs were stably expressed in mouse L cells. Single substitution of the Asn residues did not alter growth hormone binding when compared to cells which express native pGHR (KD approximately 1 nM). However, substitution of the five potential Asn-linked sites together (pGHR delta 5) resulted in a 20-fold reduced GH binding affinity (KD = 20 nM). Residues Asn97, Asn138, and Asn182 were apparently glycosylated and upon cross-linking with 125I-labeled pGH migrated as a molecular complex of approximately 130 kDa. Native pGHR and pGHR analogs with substitutions of N28Q and N143Q when cross-linked to 125I-labeled pGH, migrated with a Mr of 138 kDa. The fully deglycosylated cross-linked receptor, pGHR delta 5, migrated as a complex of 108 kDa. Therefore, each carbohydrate moiety contributed approximately 10 kDa to the total molecular mass of the pGHR, in sum contributing 30 kDa to the total Mr of the glycosylated pGHR. pGHR delta 5 was able to internalize nearly all the bound 125I-labeled pGH within 10 min, whereas native pGHR and individual Asn substituted pGHR analogs internalized 25% of bound 125I-labeled pGH at 10 min. Also, mutagenesis of the pGHR five potential Asn-linked glycosylation sites, either singly or together, did not alter the ability of GH to induce tyrosine phosphorylation of a 95-kDa protein. Together, the results indicate that three of the five pGHR Asn residues are apparently glycosylated and are necessary for maintenance of a high affinity GH binding site and for GH internalization. However, glycosylation of the pGHR is not critical for eliciting tyrosine phosphorylated proteins following the GH/GHR interaction.

Characterization of striped bass growth hormone receptors by disulfide-bond reduction and cross-linking studies

Growth hormone (GH) receptors were analyzed in striped bass (Morone saxatilis) by addition of disulfide-bond reducing agents to radioreceptor assays and by cross-linking both striped bass and coho salmon (Oncorhynchus kisutch) crude membrane preparations to radiolabeled hormone. Dithiothreitol (DTT) caused a dose-dependent increase in specific binding of 125I-tilapia (Oreochromis mossambicus) GH to striped bass membrane preparations. Maximal enhancement of 3.4-fold was obtained with 1 mM DTT and 0.03 trypsin inhibitor units/ml of aprotinin. Addition of N-ethylmaleimide (NEM), which binds covalently to free sulfhydryl groups, decreased specific binding. Scatchard analysis of striped bass membrane preparations indicated a single class of GH receptors. Addition of DTT with aprotinin increased GH-binding site concentration from 278 to 507 fmol/mg, while the dissociation constant of 0.56 nM remained unchanged. Cross-linking 125I-tilapia GH to striped bass hepatic membrane preparations and 125I-salmon GH to coho salmon membrane preparations yielded two to three specifically labeled proteins on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Endoglycosidase H treatment was without effect on specifically labeled proteins from either species. Following digestion with N-glycosidase F, relative molecular weights of specifically labeled 125I-GH complexes were reduced, suggesting that hepatic GH-binding proteins in striped bass and salmon are N-linked glycoproteins.

Regulation of gonadotropin receptor gene expression

The receptors for the gonadotropins differ from the other G protein-coupled receptors by having a large extracellular hormone-binding domain, encoded by nine or ten exons. Alternative splicing of the large pre-mRNA of approximately 100 kb can result in mRNA species that encode truncated receptor proteins. In this review we discuss the regulation of gonadotropin receptor mRNA expression and the possible roles of alternative splicing in gonadotropin receptor function.

Growth hormone-receptor messenger RNA in the rat ovary: regulation and localization

The results of several reports indicate that GH can modulate ovarian function. In the present study, the expression of the growth hormone-receptor (GH-R) mRNA was studied in the rat ovary using an RNA probe corresponding to a part of the extracellular domain of the GH-R. The probe hybridized to two major transcripts with estimated sizes of 4.0 kb and 1.2 kb in RNA extracted from liver and ovary. Recently, these transcripts have been shown to encode the GH-R and the GH-binding protein (GH-BP). The ontogeny of the GH-R/GH-BP mRNA expression was studied using Northern blot analysis and a solution hybridization RNase protection assay. In the liver GH-R/GH-BP mRNA levels increased with age, while in the ovary, the levels decreased between 1 and 5 weeks of age. Hypophysectomy caused a decrease in GH-R/GH-BP gene expression in the ovary, an effect which could be partly reversed with a single injection of GH (2 mg/kg). No significant changes in the ovarian concentration of GH-R/GH-BP transcripts were seen during the estrus cycle. Using in situ hybridization GH-R/GH-BP transcripts were found to be most abundant in follicles. Northern blot analysis of RNA extracted from isolated granulosa cells and corpora lutea showed that both these compartments contained GH-R and GH-BP mRNA, although more abundant in granulosa cells. Immunoreactive GH-R was detected in granulosa cells of healthy follicles, corpus luteum, and in the germinal epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of somatogenic binding sites in liver microsomes from normal mice and transgenic mice expressing human growth hormone gene

Somatogenic binding sites were detected and characterized in microsomal preparations from livers of normal mice and mice expressing metallothionein-I/hGH (mMT/hGH) hybrid gene, using 125I-labelled bovine or human GH, or a photoreactive derivative of hGH (125I-AP-hGH1). Specific binding of 125I-bGH was detected in liver microsomes from both normal and transgenic mice with an apparent Kd of 2 nM. 125I-hGH was partially displaced by bGH. 125I-AP-hGH1 was covalently bound to the microsomal preparations, and bGH prevented the formation of the 130 kDa species with no appreciable effect on 63 kDa and 70 kDa lactogenic complexes.

Growth hormone binding proteins in pig adipose tissue: number, size and effects of pGH treatment on pGH and bGH binding

The present study was undertaken to determine the number and size of growth hormone binding proteins present in pig adipose tissue, determine if there were differences in binding of pGH and bGH to adipose tissue membranes and establish the effects of pGH treatment on GH binding. Administration of pGH (0, 25, 50 or 100 micrograms pGH/kg BW/d) for 7 d did not affect binding of [125I]bGH to adipose tissue microsomes. Maximum binding of bGH was approximately 8-fold higher than that observed for pGH. Half-maximal inhibition of [125I]bGH binding was observed at 11 ng/ml of bGH. In contrast, a more than 10-fold greater concentration of pGH was required to half-maximally inhibit [125I]pGH binding. bGH and pGH both bound to the same GH binding proteins (Mr of 92,000, 73,000 and 53,000). The GH binding proteins appear to be produced by post-translational modification of a single GH receptor transcript rather than alternative splicing of a primary transcript since only one GH receptor mRNA transcript (4.2 kb) was detected on Northern analysis. Our findings indicate that: 1) bGH is the preferred ligand to use to study GH binding in pig adipose tissue membranes (or adipocytes); 2) exogenous pGH does not alter GH binding; and 3) only one GH receptor mRNA transcript is present in pig adipose tissue.

Expression and physiological significance of growth hormone receptors and growth hormone binding proteins in rat and man

The molecular structure of the GH receptor has recently been characterized and the receptor identified as a member of a new receptor superfamily that includes the prolactin receptor and several cytokine receptors. No obvious signal transducing domain has been identified on any of these related receptors. One possible signalling mechanism involves receptor interaction with other membrane-associated proteins that function as mediators of signal transduction. Whether such a mechanism is involved in signal transduction of the GH receptor is not known. Another common feature of these receptors is the presence of soluble forms such as the GHBP. The functions of these proteins in the circulation and at the level of the target cell remain to be resolved.

Serum binding proteins for growth hormone: origins, regulation of gene expression and possible roles

The above discussion highlights the heterogeneity of the family of GH receptors/GHBPs and their mRNAs. Considerable uncertainty still exists as to the interrelationships between the various forms, the specific mechanisms for their generation and their possible significance in terms of modulating GH action and receptor function. Collectively, the regulatory data indicate that while the mRNAs encoding the membrane and soluble GH receptors/GHBPs in the rat are expressed by the same broad distribution of tissues, they can be differentially regulated. Such regulated expression implies a functional basis for production of GHBP. The wide tissue distribution of GHBP mRNA also suggests a role for GHBP as a paracrine/autocrine effector molecule, perhaps in addition to an endocrine role. Additional studies, both in vitro and in vivo, perhaps utilizing highly purified recombinant GHBP, will be required to provide more definitive information as to the true physiological role(s) of the circulating GHBPs.

Identification of growth hormone binding protein in rat serum

The present report describes the initial characterization of a specific, high-affinity growth hormone binding protein (GH-BP) in adult male rat serum. GH-BP activity was measured by incubation of rat serum with [125I]hGH and [125I]rGH and separation of bound from free GH by dextran-coated charcoal. [125I]hGH binding to rat serum was dependent on serum concentration and incubation time, equilibrium being reached within 10 min both at 4 and 37 degrees C. Binding was rapidly and completely reversible and specific for somatogenic (but not lactogenic) hormones. Scatchard analysis yielded a linear plot with an affinity (Ka) of 1.51 +/- 0.63 x 10(8) M-1. Preliminary data obtained in various physiological conditions showed that GH-BP activity in adult male rats was 5.95 +/- 0.20%/0.1 ml serum. Significantly higher values were obtained in sera of female (21.66 + 0.79%/0.1 ml serum) and pregnant rats (23.02 +/- 1.15%/0.1 ml serum). A closer analysis of these binding values by Scatchard analysis revealed that the binding capacity in pregnant rats (50.5 +/- 5.8 pmol/0.1 ml serum) was significantly higher than in adult female estrous rats (19.2 +/- 6.5 pmol/0.1 ml serum), both being much higher than in adult male rats (2.5 +/- 0.6 pmol/0.1 ml serum). The GH-BP activity of 10-day-old rats was only approximately 63% of the adult male rat value. The presence of high-affinity GH-specific binding protein in rat serum suggests a probable action in regulation of GH activity. The detailed physiological role of rat serum GH-BP is currently being investigated.

Decreased expression of the growth hormone receptor and growth hormone binding protein in rat liver nodules

The growth hormone receptor (GHR) and growth hormone-binding protein (GH-BP) expression were characterized in liver nodules and hepatomas from male Wistar rats. The mRNA levels of GHR and GH-BP, studied by northern blot analysis and solution hybridization, were 35-50% (in nodules) and 2-6% (in hepatomas) of the level found in liver from untreated, age-matched rats. The binding of 125I-labeled human growth hormone to a low-density membrane fraction (LDMF) containing Golgi and endosome membranes in the presence of excess ovine prolactin was 75-80% lower in nodules than in liver. When endogenous ligand was removed, the binding increased in nodules but not in liver. Affinity cross-linking experiments revealed identical specific receptor-binding protein complexes at Mr of 95,000, 55,000, and 43,000 in both nodules and liver, assuming stochiometric binding of ligand. The in vivo endocytosis of 125I-labeled bovine growth hormone correlated with the level of GH binding and was thus reduced 75-80% in nodules, compared with liver. The level of insulin-like growth factor-I (IGF-I) mRNA was reduced by 50% in nodules; however, GH administration resulted in a twofold induction of IGF-I mRNA in both nodules and liver. It is concluded that the greater proportion of occupied GHR in nodules could result from an impaired dissociation of endogenous GH, which might explain the reduced GHR mRNA expression. Furthermore the low GHR mRNA expression may reflect a de-differentiated phenotype in nodules.

Proteins associated with somatogenic and lactogenic receptors in microsomal membranes and intact rat hepatocytes

Lactogenic and somatogenic receptors present in rat liver have been examined by cross-linking with a derivative of human somatotropin (AP-hGH1) followed by SDS-polyacrylamide gel electrophoresis. AP-hGH1, which has a content of 2.2 azidophenacyl groups per molecule, mainly linked to half Cys-182 and half Cys-189, exerted a specificity similar to that of the native hormone (hGH), with an ability of 46% with respect to hGH to compete with the radiolabelled hormone for the binding sites of microsomal preparations. Photolysis of the 125I-labelled derivative bound to the lactogenic receptors present in either microsomal membranes or Triton X-100 solubilized preparations gave rise to a 63 kDa species. In addition, 30% of the covalent complexes formed in microsomal membranes belonged to a species with a molecular mass of 70 kDa. Incubation of viable rat hepatocytes with the radiolabelled derivative at either 0 degrees C for 3 h or 15 degrees C for 1.5 h and subjection to irradiation, yielded covalent complexes of molecular masses estimated at 130, 73, 63, 45 and 35 kDa. Experiments performed in the presence of 1 mM NaCN, gave rise to the previous species in a similar yield as that obtained in the absence of cyanide. The 130 kDa complex is related to the somatogenic binding sites, since it was not visualized in the presence of unlabelled bovine somatotropin, while the 70-73, 63, 45 and 35 kDa bands disappeared when the incubations were performed in the presence of unlabelled ovine prolactin.

Glycosylation characteristics of the mouse liver lactogenic receptor

The structural characteristics and glycosylation properties of the lactogenic receptor were examined in 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS)-solubilized plasma membranes from female mouse liver. The specific binding of the radioiodinated human growth hormone [( 125I]hGH) was displaced with an equivalent potency by both hGH and prolactin. After a mild neuraminidase treatment, this binding was increased by 40%, as a result of an increase in receptor affinity. Affinity chromatography on immobilized lectins revealed that the [125I]hGH-receptor complexes were specifically retained and eluted from ricin lectin-agarose, concanavalin A and lentil lectin, indicating the presence of N-linked glycans. Covalent cross-linking of solubilized [125I]hGH-receptor complexes with disuccinimidyl suberate, followed by analysis by sodium dodecyl sulfate-gel electrophoresis (SDS-PAGE) under reducing conditions, and autoradiography resulted in the appearance of two bands with apparent Mr approximately 62,000 and approximately 100,000. The labelling of these bands was prevented by unlabelled hGH or ovine prolactin (oPrl) but not by bovine growth hormone (bGH). Neuraminidase treatment of the two receptor forms resulted in increased electrophoretic mobility which was inhibited by simultaneous addition of sialyl-lactose, a neuraminidase substrate. The both cross-linked forms were unaffected by endoglycosidase H, while endoglycosidase F decreased the molecular weight of each of the forms by about 8000 Da, yielding bands at Mr approximately 54,000 and approximately 92,000. In conclusion, taking into account that hGH is a Mr 22,000 polypeptide, the two forms of the receptor correspond to glycoproteins of Mr approximately 40,000 and approximately 78,000, respectively. They contain polypeptide backbones of Mr approximately 32,000 and approximately 70,000, and complex N-linked oligosaccharide chains with terminal sialic acid residues which could be involved in receptor binding affinity.

Receptor-mediated endocytosis and degradation of bovine growth hormone in rat liver

Receptor-mediated endocytosis of radiolabelled bovine growth hormone (125I-bGH) via somatogenic receptors in the liver was studied following in vivo intraportal injection. At different times after injection, subcellular membrane fractions involved in binding (plasma membranes), endocytosis (endocytic vesicles) and degradation (lysosomes) of peptide hormones were isolated by sucrose density gradient centrifugation. These fractions were evaluated for the time-course accumulation of radiolabelled bGH and for the presence of internalized 125I-bGH-receptor complexes. These uptake studies indicate that after initial plasma membrane association of 125I-bGH, the ligand is transported in two successive endocytic compartments prior to arrival in lysosomes. The molecular weight of the somatogenic binders of male and female rat livers involved in internalization of 125I-bGH was determined to 95,000, 64,000, 55,000, 43,000 and 35,000, assuming a 1:1 binding of the hormone to the binder. These binders were seen in both endosomes and lysosomes, which suggests that growth hormone is transported to the lysosomes in a complex with its receptor. Binding and uptake of 125I-bGH was also compared in male and female rat livers, and endocytosis of 125I-bGH was compared to that of radiolabelled ovine prolactin (125I-oPrl). The specific uptake of 125I-bGH appeared not to be sexually differentiated in contrast to that of 125I-oPrl which showed a 35-fold higher uptake in female rat liver. Degradation of 125I-bGH was studied under in vitro binding assay conditions. A distinct 15,000 Da fragment was generated by plasma membrane, endosomal and lysosomal fractions. Based on protease inhibitor studies, a non-trypsin-like serine protease is suggested to be involved in the degradation of bGH. The 15,000 Da proteolytic fragment of GH can be affinity cross-linked to somatogenic binders of similar molecular weights as those involved in the binding of intact GH.

Growth hormone receptors

The receptor for rabbit liver growth hormone (GH) has recently been purified and cloned. Sequencing data demonstrates that this site represents a new class of transmembrane receptor proteins containing covalently linked ubiquitin and N-linked oligosaccharides. Homology with the human GH liver receptor is estimated to be 84%. GH receptors are also present on non-liver cells, however, their function is poorly understood, and it is not yet known if there is homology with the liver site. Direct stimulation of differentiation of preadipocytes to adipocytes by GH has been demonstrated. In bone, binding of GH to the resting cell zone, but not the proliferative layers has been reported. Taken together, these data support the hypothesis of Green and colleagues that GH has a dual mechanism of action which includes direct stimulation of differentiation of precursor cells and a secondary effect of enhancing the clonal proliferative response.