Subcellular trafficking of growth hormone receptor and Jak2 under ligand exposure

In CHO cells, growth hormone stimulation induces a rapid degradation of mature and precursor forms of its receptor, but does not affect Jak2 concentration. Confocal analysis of the receptor and of specific markers for subcellular localization shows that ligand exposure induced the disappearance of cell surface receptors, while some receptors seem to be sequestered in the endoplasmic reticulum (ER) and in the Golgi apparatus. Using a tagged version of Jak2 (HA-Jak2) and double immuno-fluorescence analysis with anti-HA and anti-Stat5 antibodies, we demonstrate that ligand stimulation induces Stat5 nuclear accumulation while Jak2 remains localized in the cytoplasm. Immunoblots of nuclear extracts confirm the Jak2 nuclear exclusion.

Growth hormone receptor signalling and actions in bone growth

Growth hormone (GH) acts by binding to a membrane receptor that is part of the cytokine receptor superfamily. Ligand binding induces receptor dimerization leading to activation of the associated tyrosine kinase, Janus kinase (Jak) 2. Transphosphorylation of Jak2 occurs followed by tyrosine phosphorylation of the receptor, and numerous cytoplasmic proteins. Among these are the signal transducers and activators of transcription (Stat) proteins, as well as adaptor proteins leading to the activation of the Ras/mitogen-activated protein (MAP) kinase and the phosphatidyl-inositol-3'-kinase (PI 3-kinase) pathways. Activation of the GH receptor system is relatively transient, with several mechanisms being involved in down-regulation: internalization and degradation of the receptor and recruitment of phosphatases or specific inhibitors of the Jak-Stat pathway, the suppressors of cytokine signalling (SOCS) proteins. Finally, the use of the GH receptor knock-out mouse model has allowed us to dissect the role of this hormone in post-natal body growth and homeostasis.

Stimulation of endogenous GH and interleukin-6 receptors selectively activates different Jaks and Stats, with a Stat5 specific synergistic effect of dexamethasone

The interaction of GH, interleukin (IL)-6 and glucocorticoids is likely to be important in regulating the GH-insulin-like growth factor (IGF)-I axis. The signalling cascades activated by GH and IL-6 appear to be very similar, as demonstrated by studies using overexpression of the receptor and other components of the Jak-Stat and mitogen-activated protein (MAP) kinase pathways. Here we show that the human embryonic kidney cell line 293 (HEK293) expresses GH and IL-6 receptors endogenously. To determine which specific pathways might be activated by the two cytokines, at physiological levels of all components, we studied GH and IL-6 mediated signal transduction both under basal conditions and in the presence of overexpressed receptors and Stat proteins. Our results suggest a receptor specificity of Jak2 for GH receptors, and Jak1 for IL-6 receptors. Stat activation in response to GH and IL-6 was determined by reporter gene induction. Both GH and IL-6 were able to induce the reporter gene containing the Stat5 responsive element (LHRE) but the IL-6 response appeared to be mediated mainly through Stat3 activation. In contrast, the reporter gene containing the Stat3 responsive element (SIE) was IL-6 specific. The levels of gene induction by GH and IL-6 were not altered by the co-stimulation with GH and IL-6, suggesting that there is little cross-talk at the Jak-Stat activation level between the two cytokines. Neither GH nor IL-6 activated the MAP-kinase responsive serum response element (SRE), unless GH receptors or gp130 were overexpressed. Transfection of Stat3 or Stat5 expression vectors enhanced the response to GH and IL-6. Stimulation with dexamethasone synergistically enhanced GH activation of the LHRE reporter gene but had no effect on the IL-6 activation of the same reporter or on the SIE reporter gene. Thus, our studies suggest that while each cytokine, GH and IL-6, may activate various members of the Jak-Stat pathway in overexpression studies, specific activation of Stat3 by IL-6 and of Jak2 and Stat5 by GH can be observed in HEK293 cells and that in this system the synergistic effect of dexamethasone appears specific for Stat5.

Regulators of growth hormone signaling

Growth hormone acts through binding to membrane receptors that belong to the cytokine receptor superfamily. Ligand binding induces receptor dimerization and activation of the receptor-associated kinase: JAK2; this results in phosphorylation of the kinase itself, of the receptor, and of many cellular proteins. Among these are the Stat proteins as well as adaptors leading to the activation of the Ras/MAP kinase pathway and of the PI-3 kinase pathway. Activation by growth hormone is very transient and several mechanisms are involved in this downregulation: internalization and degradation of the receptor and recruitment of phosphatases or of specific inhibitors of the JAK/Stat pathway, the SOCS proteins.

Activation of growth hormone receptor delivers an antiapoptotic signal: evidence for a role of Akt in this pathway

A signaling pathway was delineated by which GH promotes cell survival. Experiments were performed in human leukemic cells (HL-60) and Chinese hamster ovary (CHO) cells. In HL-60 cells, GH treatment reduced starvation-induced cell death. In contrast, when HL-60 cells were treated with an anti-GH antibody, cell survival was sharply reduced. In CHO cells stably expressing either the wild-type (wtGHR) or a truncated form (delta454GHR) of the GH receptor in which GH induces a sustained activation of the receptor-associated tyrosine kinase JAK2, we found that GH stimulation inhibited programmed cell death induced by withdrawal of survival factors. This effect was enhanced in cells expressing the truncated form. In contrast, GH did not affect cell survival in CHO cells transfected with either the empty vector or a mutated GHR unable to transduce the signal (4P/AGHR). We also showed that the inhibitory action of GH on apoptosis is probably mediated via stimulation of the serine-threonine kinase Akt, as 1) GH treatment induces a prompt phosphorylation of Akt; and 2) GH effects on cell survival are abolished by transfection of an Akt mutant that exhibits dominant negative function. Experiments with pharmacological inhibitors demonstrated that GH-induced Akt phosphorylation is dependent on phosphoinositide 3-kinase activation. In contrast, we found no changes in Bcl-2 levels secondary to GHR activation.

Dual effects of suppressor of cytokine signaling (SOCS-2) on growth hormone signal transduction

A family of suppressors of cytokine signaling (SOCS) has recently been identified of which two members have been shown to block growth hormone (GH) signaling. Dose-response experiments were conducted in 293 cells and SOCS-1 and SOCS-3 were shown to inhibit the transcriptional activation of a GH-responsive element and suppressed Jak2 tyrosine kinase activity. SOCS-2 had two opposite effects: at low concentrations it inhibited GH-induced STAT5-dependent gene transcription, but restoration of GH signaling was observed at higher concentrations. In cotransfection studies, SOCS-2 was able to block the inhibitory effect of SOCS-1 but not that of SOCS-3 on GH signaling. These findings suggest that a major function for SOCS-2 is to restore the sensitivity to GH by overcoming the initial inhibitory effects of other endogenous SOCS molecules.

Studies with a growth hormone antagonist and dual-fluorescent confocal microscopy demonstrate that the full-length human growth hormone receptor, but not the truncated isoform, is very rapidly internalized independent of Jak2-Stat5 signaling

We have investigated trafficking of two negative regulators of growth hormone receptor (GHR) signaling: a human, truncated receptor, GHR1-279, and a GH antagonist, B2036. Fluorescent-labeled growth hormone (GH) was rapidly internalized by the full-length GHR, with >80% of the hormone internalized within 5 min of exposure to GH. In contrast, <5% of labeled GH was internalized by cells expressing truncated GHR1-279. Using another truncated receptor, GHR1-317 fused to enhanced green fluorescent protein (EGFP), we have exploited fluorescence energy transfer to monitor the trafficking of ligand-receptor complexes. The data confirmed that internalization of this truncated receptor is very inefficient. It was possible to visualize the truncated GHR1-317-EGFP packaged in the endoplasmic reticulum, its rapid movement in membrane bound vesicles to the Golgi apparatus, and subsequent transport to the cell membrane. The GH antagonist, B2036, blocked Jak2-Stat5-mediated GHR signaling but was internalized with a similar time course to native GH.

THE RESULTS

1) demonstrate the rapid internalization of GH when studied under physiological conditions; 2) confirm the hypothesis that internalization of cytoplasmic domain truncated human GHRs is very inefficient, which explains their dominant negative action; and 3) show that the antagonist action of B2036 is independent of receptor internalization.

New growth hormone receptor exon 9 mutation causes genetic short stature

A novel form of congenital growth hormone insensitivity syndrome (GHIS), which lacks the classic phenotype associated with this condition, is described. Dominant inheritance is shown to result from a heterozygous 876-1 G to C transversion of the 3' splice acceptor site preceding exon 9 in the growth hormone receptor (GHR) gene. The result of this mutation is a severely truncated cytoplasmic domain of the GHR, which is incapable of transmitting a signal. The mutant receptor is shown to form a heterodimer with the wild-type GHR, the activity of which is inhibited in a dominant-negative manner.

Four contiguous amino acid substitutions, identified in patients with Laron syndrome, differently affect the binding affinity and intracellular trafficking of the growth hormone receptor

We have analyzed the GH receptor (GHR) gene in four individuals with Laron syndrome, and a missense mutation was identified for each patient in the extracellular domain of the GHR (D152H, I153T, Q154P, and V155G). The D152H mutation was previously reported. We have reproduced the three novel mutations in the GHR complementary DNA and analyzed their consequences in human 293 transfected cells. In cells expressing the I153T and V155G mutants, binding of [125I]human GH at the cell surface was very low, whereas binding to total membrane fractions was much less affected, suggesting impaired cell surface expression. Binding assays with cells expressing the Q154P mutant revealed severe defects both at the cell surface and in total particulate membrane fractions. Immunofluorescence experiments confirmed that cell surface expression of the three mutants was altered, and colocalization studies suggested that most of the mutant receptors are retained in the endoplasmic reticulum. Endoglycosidase H resistance tests also indicated that the majority of I153T and V155G GHRs are trapped in the endoplasmic reticulum. Thus, mutations on contiguous amino acids of the GHR result in various defects. The I153T, Q154P, and V155G mutations mainly affect intracellular trafficking and binding affinity of the receptor, whereas the D152H mutation affects receptor expression, dimerization, and signaling.

Prolactin and growth hormone signal transduction in lymphohaemopoietic cells

The peptide hormones, prolactin (PRL) and growth hormone (GH), are known to regulate numerous target tissues. Among such targets are cells of the immune system, including T cells, B cells, macrophages and natural killer cells. We have cloned a panel of PRL- and GH-inducible T cell genes for use in studies to understand how these hormones through the expression of these genes modulate the biology of immune function cells. This article focuses on the signalling pathways emanating from the PRL receptor (PRL-R) and GH receptor (GH-R), and the expression of PRL-inducible target genes.

The D152H mutation found in growth hormone insensitivity syndrome impairs expression and function of human growth hormone receptor but is silent in rat receptor

In two patients with growth hormone (GH) insensitivity syndrome (Laron syndrome), in whom the GH receptor is able to bind the hormone, the D152H mutation was identified, and lack of dimerization was proposed to explain GH resistance in these patients. To examine further the consequences of the substitution of conserved aspartate 152 on the function of the GH receptor (GHR), we reproduced the mutation in vitro on the full length GH receptor cDNA from man and rat. Effects of the mutation on expression and activity of the GHR were analyzed in 293 cells transfected with wild-type and mutant GHR cDNAs. Mutant human receptor protein was expressed at a lower level than wild-type receptor and its activity was reduced: GH-dependent signal transducer and activator of transcription 5 (Stat5)-mediated transactivation of a reporter gene was lower in 293 cells transfected with mutant GHR cDNA than in transfected cells expressing a comparable level of wild-type GHR. The membrane-bound form of the mutant and of the wild-type human GHR were able to homodimerize, as suggested by the size of the complexes detected in cross-linking experiments with 125I-human (h) GH, and also by the activity in the functional test. With the soluble GHR resulting from proteolysis of the wild-type membrane form, no dimeric complexes could be detected. However, when a soluble receptor lacking the transmembrane and cytoplasmic domains of the receptor was expressed, wild-type and not mutant GH binding protein (GHBP) was able to form dimers in the presence of hGH. The amino acid substitution has no effect on either expression or function of the rat receptor. Structural modeling of D152H soluble human and rat GHR (GHBP) supports the species-specific functional consequences of the mutation. Evaluation of the functional importance of the mutation strongly suggests that impairment in expression and activity of the mutant receptor, rather than complete lack of dimerization, explains the GH resistance of the patients.

Growth-hormone-receptor and cytokine-receptor-family signaling

Structure of growth-hormone receptor and the class I type of cytokine receptors: common structural features; cytokine-receptor isoforms; oligomerization of receptor components initiates cytokine signalling. Role of the Jak kinases in mediating specific functions of growth-hormone receptor and cytokine receptors. Role of signal transducer and activator of transcription (Stat) proteins in growth hormone and cytokine functions. Other pathways activated by cytokine receptors: the mitogen-activated protein kinase pathway; insulin-receptor substrates 1 and 2 and phosphatidylinositol-3-kinase pathways; the Src pathways and other tyrosine kinase pathways; the phospholipase C/protein kinase C/Ca2+ pathways. Regulation of growth-hormone receptor and cytokine receptor signaling: binding sites, internalization and ubiquitination; phosphatases and Janus kinase/Stat inhibitors. Conclusions and future prospects.

Grb10 identified as a potential regulator of growth hormone (GH) signaling by cloning of GH receptor target proteins

The cloning of receptor targets procedure, used so far to identify proteins associated with tyrosine kinase receptors was modified to clone SH2 proteins able to bind to the growth hormone receptor (GHR). The cytoplasmic region of GHR, a member of the cytokine receptor superfamily does not contain tyrosine kinase activity. It was thus phosphorylated in bacteria by the Elk tyrosine kinase and radiolabeled to screen a mouse expression library. With this probe, we identified Shc and the p85 subunit of phosphatidylinositol 3-kinase as direct targets of the receptor. The other proteins identified, Csk, Shb, Grb4, and Grb10 are new potential transducers for cytokine receptors. We show in Huh-7 hepatoma cells that Grb10 and GHR associate under GH stimulation. Co-transfections in 293 cells further show that Grb10 interacts with both the GHR and Jak2. Functional tests demonstrate that Grb10 inhibits transcription of two reporter genes containing, respectively, the serum response element of c-fos and the GH response element 2 of the Spi2.1 gene, whereas it has no effect on a reporter gene containing only Stat5 binding elements. Our results suggest that Grb10 is a new target for a member of the cytokine receptor family that down-regulates some GH signaling pathways downstream of Jak2 and independently of Stat5.

A short isoform of the human growth hormone receptor functions as a dominant negative inhibitor of the full-length receptor and generates large amounts of binding protein

The GH receptor (GHR) is a member of the cytokine receptor family. Short isoforms resulting from alternative splicing have been reported for a number of proteins in this family. RT-PCR experiments, in human liver and cultured IM-9 cells, using primers in exon 7 and 10 of the GHR, revealed three bands reflecting alternative splicing of GHR mRNA: the predicted product at 453 bp and two other products at 427 and 383 bp. The 427-bp product (GHR1-279) utilized an alternative 3'-acceptor splice site 26 bp downstream in exon 9; the predicted C-terminal residues are six frameshifted exon 9 codons ending in an inframe stop codon. The 383-bp product (GHR1-277) resulted from skipping of exon 9; the predicted C-terminal residues are three frame-shifted exon 10 codons ending in an in-frame stop codon. RNase protection experiments confirmed the presence of the GHR1-279 variant in IM-9 cells and human liver. The proportion of alternative splice to full length was 1-10% for GHR1-279 and less than 1% for GHR1-277. The function of GHR1-279 was examined after subcloning in an expression vector and transient transfection in 293 cells. Scatchard analysis of competition curves for [125l]-hGH bound to cells transfected either with GHR full length (GHRfl) or GHR1-279 revealed a 2-fold reduced affinity and 6-fold increased number of binding sites for GHR1-279. The increased expression of GHR1-279 was confirmed by cross-linking studies. The media of cells transfected with GHR1-279 contained 20-fold more GH-binding protein (GHBP) than that found in the media of cells transfected with the full-length receptor. Immunoprecipitation and Western blotting experiments, using a combination of antibodies directed against extracellular and intracellular GHR epitopes, demonstrated that GHRfl and GHR1-279 can form heterodimers and that the two forms also generate a 60-kDa GHBP similar in size to the GHBP in human serum. Functional tests using a reporter gene, containing Stat5-binding elements, confirmed that while the variant form was inactive by itself, it could inhibit the function of the full-length receptor. We have demonstrated the presence of a splice variant of the GHR in human liver encoding a short form of the receptor similar in size to a protein previously identified in human liver and choroid plexus. Expression studies in 293 cells support the hypothesis that while the expression of the splice variant accounts for only a small proportion of the total GHR transcript, it produces a short isoform that modulates the function of the full-length receptor, inhibits signaling, and generates large amounts of GHBP. The differential expression of GHR receptor short forms may regulate the production of GHBP, and truncated receptors may act as transport proteins or negative regulators of GHR signaling.

A novel growth hormone response element unrelated to STAT (signal transducer and activator of transcription)-binding sites is a bifunctional enhancer

Regulation of gene expression by GH has so far been shown to be mediated by a few cis-acting elements, most of which are signal transducer and activator of transcription (STAT)-binding sites. Here we have characterized a novel GH-response element present in the promoter of rat serine protease inhibitor (spi) genes. It consists of a 13 nucleotide-long GAGA box containing two GAGGAG repeats separated by a G, structurally unrelated to STAT-binding sites. In hepatocytes, the spi GAGA box behaves as a position-dependent bifunctional enhancer controlling basal and GH-dependent transcription. In addition, spi GAGA box oligonucleotides inhibit cell-free transcription driven by GAGA box-containing as well as GAGA box-less promoters, suggesting that the spi GAGA box interacts directly or indirectly with component(s) of the basic transcriptional machinery. Mobility shift assays showed that this GAGA box is specifically recognized by nuclear factors that are unrelated to previously characterized proteins binding to purine-rich elements or to GH-activated STATs. Finally, experiments performed with cells expressing wild type, truncated, or mutated forms of the GH receptor indicate that protein kinase Janus kinase 2 is involved in the GH-dependent activation of the spi GAGA box. These studies reveal the existence of an as yet unidentified Janus kinase-2-dependent, STAT-independent pathway in GH activation of gene expression.

Differential activation of Stat3 and Stat5 by distinct regions of the growth hormone receptor

The GH receptor (GHR) is a member of the cytokine receptor superfamily; its signaling involves the activation of Janus tyrosine kinases (JAK2) and Stat (signal transducers and activators of transcription) transcription factors. Using truncated and tyrosine mutants of the receptor, we show that different receptor domains are essential for the activation of Stat3 and Stat5. GH-dependent phosphorylation of JAK2, Stat3, and Stat5, as well as transactivation studies with reporter genes containing Stat3 and Stat5 DNA-binding elements, was performed in cells expressing the various GHR mutants. The membrane-proximal region of the receptor necessary for JAK2 activation is sufficient for Stat3 activation. In contrast, C-terminal tyrosine residues of GHR are absolutely required for Stat5 activation. The same residues are also involved in the regulation of JAK2 dephosphorylation, possibly through the activation of a phosphatase. Using in vitro experiments with glutathione-S-transferase fusion proteins, we demonstrate that the SH2 domain of Stat5 binds to the carboxy-terminal tyrosine-phosphorylated residues of GHR. Our results show that a cytokine receptor can mediate differently the activation of distinct Stat proteins that could be involved in cytokine-specific effects.

The proline-rich region of the GH receptor is essential for JAK2 phosphorylation, activation of cell proliferation, and gene transcription

Mutational analysis of the proximal transmembrane region of the cytoplasmic domain of the GH receptor (GHR) allowed us to characterize box 1, a proline-rich sequence of eight amino acids, which has been shown to be critical for signal transduction of many cytokine receptors. Mutants of the box 1 region of the rat GHR were studied for their ability to initiate the phosphorylation of JAK2 and the proliferation of stably transfected BAF B03 cells and also the activation of Spi 2.1 gene transcription in transiently transfected Chinese hamster ovary (CHO) cells. Convergence of effects of the box 1 mutants on JAK 2 phosphorylation, cell proliferation, and gene transcription was found. Our results suggest that no single amino acid in the box 1 sequence is essential for signaling and that the last two prolines (PXP motif) and the hydrophobic residues are necessary for integrity of box 1. Box 1 represents a structural determinant, potentially able to provide an interaction between JAK2 and the receptor; this interaction could be direct or indirect via an adaptor protein.

Growth hormone receptor: structure and signal transduction

The growth hormone receptor (GHR) belongs to the superfamily of transmembrane proteins that includes the prolactin receptor and a number of cytokine receptors. Two forms exist for the GHR: the full-length membrane-bound human receptor is a protein of 620 amino acids with a single transmembrane region; and the GH binding protein (GHBP) is a short soluble from corresponding to the extracellular domain of the full-length receptor. In rodents, GHBP is encoded by a specific mRNA of 1.2-1.5 kb, whereas in man and other species GHBP is believed to result from proteolytic cleavage of the membrane receptor. Growth hormone binding protein prolongs the half-life of GH but other functions for GHBP remain to be demonstrated. Recombinant GHBP complexed to human GH shows a 2:1 stoichiometric crystal structure. Growth hormone-induced dimerization of the cell surface GHR appears to be a prerequisite for biological activity of the hormone. JAK2 has been identified as a tyrosine kinase associated with GHR and other receptors of the superfamily. Binding of GH to its receptor results in dimerization of the GHR, phosphorylation of JAK2 and of the GHR. Other substrates for JAK2 have to be identified. Transcription factors belonging to the STAT (signal transducers and activators of transcriptions) family are involved in the transcriptional effects of GH. The activity of mutants of the GHR has been measured in functional tests to identify sequences of the cytoplasmic domain of the receptor that are important for signal transduction. A proline-rich sequence, called Box I, conserved among members of the receptor family has been shown to be crucial for GH effects on gene transcription. MAP kinase activity and cell proliferation. The C-terminal region of the GHR is required for tyrosine phosphorylation of the receptor and for a hormonal effect on gene transcription, whereas only 46 membrane proximal amino acids of the cytoplasmic domain are necessary for activation of JAK2 and transduction of the GH proliferative signal. Much work remains to be done to identify other protein kinases and signalling molecules involved in the mechanism of action of GH.

The membrane proximal region of the cytoplasmic domain of the growth hormone receptor is involved in the activation of Stat 3

Growth hormone receptor (GHR) signaling involves activation of the Janus Kinases (Jak) and of Stat proteins (signal transducers and activators of transcription). Growth hormone (GH) induces transcriptional activation of c-fos gene and the c-sis inducible element (SIE) of its promoter was shown to bind the Stat proteins. Using cells co-transfected with GHR and Stat 3 expression vectors, we directly demonstrate that GH induces tyrosine phosphorylation of Stat 3 and its binding to the SIE probe. We showed, using mutant forms of GHR, that only the cytoplasmic membrane proximal domain of the receptor, including a conserved proline rich region (box 1), is required for this effect.

Distinct cytoplasmic regions of the growth hormone receptor are required for activation of JAK2, mitogen-activated protein kinase, and transcription

The GH receptor (GHR) is a member of the cytokine/hematopoietic growth factor family, and protein tyrosine phosphorylation has been implicated in the signaling cascade of these receptors. It was recently shown that the tyrosine kinase JAK2 is associated with the GHR. GH induces the activation of JAK2, which phosphorylates itself and the receptor. Mitogen-activated protein (MAP) kinase activation and transcriptional stimulation of specific genes, such as Spi 2.1, have also been reported to be induced by GH. To identify functionally important regions in the cytoplasmic domain of the GHR, we compared the actions of the wild-type receptor, two truncated mutants, and one internal deletion mutant (similar to the intermediate Nb2 form of the PRL receptor) in transfectants of the Chinese hamster ovary cell line. A region of 46 amino acids adjacent to the membrane was found to be sufficient for activation of both JAK2 and MAP kinases. This region contains a proline-rich sequence (box 1) conserved in the cytokine receptor family that is important for signal transduction. For transcriptional activity, the C-terminal region of the GHR is required, and we found that the last 80 terminal residues contain sequences allowing activation of the Spi 2.1 promoter. Tyrosine phosphorylation of the receptor also requires the C-terminal portion of the GHR cytoplasmic domain, and we found that GHR tyrosine phosphorylation appears to be linked to activation of the Spi 2.1 transcription pathway. Thus, the GHR could be composed of at least 2 functional regions: the 46 proximal amino acids required for activation of JAK2 and sufficient to stimulate the MAP kinase pathway, and an additional carboxy-terminal region necessary for transcriptional activation.

Receptor domains involved in signal transduction of prolactin and growth hormone

Prolactin (PRL) and growth hormone (GH) receptors are members of a superfamily that include receptors for a number of cytokines. GH and its receptor form an unusual homodimer consisting of one molecule of GH and two molecules of receptor. A similar homodimer of the PRL receptor is probably required for biological effects to be seen. Using specific assays to measure the functional activity of PRL and GH receptors, a 25 amino acid juxtamembrane region has been identified as essential but not sufficient for normal action. More detailed studies have limited the region to eight amino acids, rich in prolines, that is highly conserved in many members of the receptor superfamily. Finally, GH and PRL have been shown to induce the rapid tyrosine phosphorylation of an associated kinase, Janus kinase 2, and of the receptor itself.

Expression of two isoforms of the human growth hormone receptor in normal liver and hepatocarcinoma

In man, two isoforms of growth hormone receptor (GHR) have been reported. The first, hGHR corresponds to the original form described by Leung et al. (J. Biol. Chem. 264 (1987) 9905-9910). The second, hGHRd3, missing 22 N-terminal amino acids is encoded by an mRNA derived from alternative splicing and deletion of the third exon. This latter form has been isolated from placenta and its tissue distribution, as well its function warrant further examination. We studied the relative expression of the two isoforms of GHR in liver at various stages of development of differentiation. Several samples from fetal livers and hepatocarcinomas were analysed and we observed that hGHR and hGHRd3 can be expressed solely or coordinately. Furthermore, we investigated the possibility that alternative splicing of exon 3 could be related to the specific use of different promoters. Our data show that both isoforms have identical 5'-untranslated regions in normal liver or in hepatoma cell lines. Finally, these experiments strongly suggest that hGHRd3 is efficiently translated as a functional receptor, with the soluble form of GHR being recovered in the cell media. The results of the present study support the concept that both isoforms are expressed in human liver, with exon 3 alternative splicing being neither tissue specific nor related to hepatic differentiation.

The GH receptor and signal transduction

The primary structure of the growth hormone (GH) receptor in rabbits and humans determined by complementary DNA cloning revealed a single membrane-spanning protein of approximately 620 amino acids. A binding protein (bp) specific for GH has been identified in the serum of a number of species. In rabbits and man, a single 4.5-kb transcript has been identified that encodes the full-length receptor. In rats and mice, however, a smaller transcript produced by alternative splicing has been reported which is specific for the GHbp. Recently, the X-ray crystallographic structure of GH and its receptor have clearly shown the formation of an unusual homodimer, consisting of one molecule of GH and two molecules of hGHbp. Formation of the GH dimer is a necessary prerequisite for biological activity. The transcriptional activity of wild-type and mutant forms of GH receptor has been determined by co-transfecting the promoter of a GH-responsive gene, coupled to CAT along with the receptor cDNA. A 25-amino acid region near the transmembrane domain has been shown to be important for functional activity, although 8 amino acids (known as Box 1), rich in prolines, is essential. Alanine scanning mutagenesis has revealed that individual substitution of each residue is without effect, while the replacement of the last 2 or all 4 of the prolines abolishes activity. Finally, GH has been shown to induce rapid tyrosine phosphorylation of several proteins in cells expressing the receptor, one of which has recently been identified as the kinase JAK2 and another as MAP kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for generation of the growth hormone-binding protein through proteolysis of the growth hormone membrane receptor

The growth hormone-binding protein (GHBP) which circulates in plasma is a soluble short form of the membrane growth hormone receptor (GHR). In rats and mice, GHR and GHBP originate from two alternatively spliced mRNAs (4.5 and 1.2 kb). In human and rabbit tissues, a single predominant mRNA of 4.5 kb was detected and it was hypothesized that GHBP could be produced by proteolytic cleavage of the GHR. Using gel filtration and HPLC, we have detected a high level of GH binding activity in media of cells transfected with rabbit GHR cDNA. The [125I]hGH-GHBP complex eluted at the same time as the plasma complex and both the binding affinity and specificity of the BP were comparable to that of rabbit plasma. Immunoprecipitation experiments and Western blots confirmed that GHBP in the media of transfected cells was a 55 kDa protein related to the extracellular domain of the GHR. In contrast, no BP was detected in the media of cells transfected with the cDNA encoding the rat GHR. These results strongly suggest that, in rabbit and probably in man, the GHBP could, at least in part, be produced by proteolytic cleavage of the GHR.

Lack of hormone binding in COS-7 cells expressing a mutated growth hormone receptor found in Laron dwarfism

A single point mutation in the growth hormone (GH) receptor gene generating a Phe-->Ser substitution in the extracellular binding domain of the receptor has been identified in one family with Laron type dwarfism. The mutation was introduced by site-directed mutagenesis into cDNAs encoding the full-length rabbit GH receptor and the extracellular domain or binding protein (BP) of the human and rabbit GH receptor, and also in cDNAs encoding the full length and the extracellular domain of the related rabbit prolactin (PRL) receptor. All constructs were transiently expressed in COS-7 cells. Both wild type and mutant full-length rabbit GH and PRL receptors, as well as GH and prolactin BPs (wild type and mutant), were detected by Western blot in cell membranes and concentrated culture media, respectively. Immunofluorescence studies showed that wild type and mutant full-length GH receptors had the same cell surface and intracellular distribution and were expressed with comparable intensities. In contrast, all mutant forms (full-length receptors or BPs), completely lost their modify the synthesis ligand. These results clearly demonstrate that this point mutation (patients with Laron syndrome) does not modify the synthesis or the intracellular pathway of receptor proteins, but rather abolishes ability of the receptor or BP to bind GH and is thus responsible for the extreme GH resistance in these patients.

An actin-binding site containing a conserved motif of charged amino acid residues is essential for the morphogenic effect of villin

The actin-binding protein villin induces microvillus growth and reorganization of the cytoskeleton in cells that do not normally produce this protein. Transfection of mutagenized villin cDNAs into CV-1 cells was used to show that a conserved, COOH-terminally located cluster of charged amino acid residues (KKEK) is crucial for the morphogenic activity of villin in vivo. In vitro experiments with a 22 amino acid synthetic peptide corresponding to this region of villin provide evidence that this motif is part of an F-actin-binding site that induces G-actin to polymerize. Chemical cross-linking of actin to this peptide, the effects of amino acid substitutions in peptides, and the behavior of villin variants further corroborate the participation of the KKEK sequence in actin contacts.

In vivo analysis of functional domains from villin and gelsolin

Transfected CV1 cells were used to compare the in vivo effects of various domains of villin and gelsolin. These two homologous actin modulating proteins both contain a duplicated severin-like sequence. Villin has in addition a carboxy-terminal domain, the headpiece, which accounts for its bundling activity. The effects of the villin-deleted mutants were compared with those of native villin. Our results show that essential domains of villin required to induce the growth of microvilli and F-actin redistribution are present in the first half of the core and in the headpiece. We also show that the second half of the villin core cannot be exchanged by its homolog in gelsolin. When expressed at high levels of CV1 cells, full length gelsolin completely disrupted stress fibers without change of the cell shape. Addition of the villin headpiece to gelsolin had no effect on the phenotype induced by gelsolin alone. Expression of the first half of gelsolin induced similar modifications as capping proteins and rapid cell mortality; this deleterious effect on the cell structure was also observed when the headpiece was linked to the first half of gelsolin. In cells expressing the second half of gelsolin, a dotted F-actin staining was often seen. Moreover elongated dorsal F-actin structures were observed when the headpiece was linked to the second gelsolin domain. These studies illustrate the patent in vivo severing activity of gelsolin as well as the distinct functional properties of villin core in contrast to gelsolin.

Sequence of human villin: a large duplicated domain homologous with other actin-severing proteins and a unique small carboxy-terminal domain related to villin specificity

Villin is a calcium-regulated actin-binding protein that caps, severs, and bundles actin filaments in vitro. This 92,500-D protein is a major constituent of the actin bundles within the microvilli of the brush border surface of intestinal and kidney proximal tubule cells. Villin is a very early marker of cells involved in absorption and its expression is highly increased during intestinal cell differentiation. The amino acid sequence deduced from the cDNA sequence revealed that human villin is composed of three domains. The first two domains appear as the result of a duplication: their structural organization is similar. We can then define a basic unit in which a slightly hydrophilic motif is followed by three hydrophobic motifs, similar between themselves and regularly spaced. The duplicated domain is highly homologous to three other actin-severing proteins and this basic structure represents the whole molecule in severin and fragmin, while two basic units compose gelsolin. The third domain which is carboxy terminal is villin specific: it is unique among actin modulating proteins so far known. It could account for its actin-binding properties (dual regulation by calcium of severing and bundling activities). We propose that it may also be related to the subcellular localization of villin in different epithelial cell types.

The influenza hemagglutinin insertion signal is not cleaved and does not halt translocation when presented to the endoplasmic reticulum membrane as part of a translocating polypeptide

The co-translational insertion of polypeptides into endoplasmic reticulum membranes may be initiated by cleavable amino-terminal insertion signals, as well as by permanent insertion signals located at the amino-terminus or in the interior of a polypeptide. To determine whether the location of an insertion signal within a polypeptide affects its function, possibly by affecting its capacity to achieve a loop disposition during its insertion into the membrane, we have investigated the functional properties of relocated insertion signals within chimeric polypeptides. An artificial gene encoding a polypeptide (THA-HA), consisting of the luminal domain of the influenza hemagglutinin preceded by its amino-terminal signal sequence and linked at its carboxy-terminus to an intact prehemagglutinin polypeptide, was constructed and expressed in in vitro translation systems containing microsomal membranes. As expected, the amino-terminal signal initiated co-translational insertion of the hybrid polypeptide into the membranes. The second, identical, interiorized signal, however, was not recognized by the signal peptidase and was translocated across the membrane. The failure of the interiorized signal to be cleaved may be attributed to the fact that it enters the membrane as part of a translocating polypeptide and therefore cannot achieve the loop configuration that is thought to be adopted by signals that initiate insertion. The finding that the interiorized signal did not halt translocation of downstream sequences, even though it contains a hydrophobic region and must enter the membrane in the same configuration as natural stop-transfer signals, indicates that the HA insertion signal lacks essential elements of halt transfer signals that makes the latter effective membrane-anchoring domains. When the amino-terminal insertion signal of the THA-HA chimera was deleted, the interior signal was incapable of mediating insertion, probably because of steric hindrance by the folded preceding portions of the chimera. Several chimeras were constructed in which the interiorized signal was preceded by polypeptide segments of various lengths. A signal preceded by a segment of 111 amino acids was also incapable of initiating insertion, but insertion took place normally when the segment preceding the signal was only 11-amino acids long.(ABSTRACT TRUNCATED AT 400 WORDS)

A human villin cDNA clone to investigate the differentiation of intestinal and kidney cells in vivo and in culture

Villin, a Ca2+-regulated actin-binding protein is a major component of microvilli of intestinal epithelial cells and kidney proximal tubule cells. Villin expression during assembly of the brush border can be investigated using a human colon adenocarcinoma cell line HT29-18. This cell line is able to differentiate under nutritional control and develops an enterocyte-like phenotype. A cDNA library from a subclone HT29-18-C1 was constructed in an expression vector and a cDNA specific for human villin was isolated. This cDNA codes for the 110 carboxy-terminal residues of villin. Within that region, the 76 carboxy-terminal residues present 65% homology with the chicken villin 'head piece'. We show that two mRNA species 4.0 kb and 3.2 kb long hybridize with this cDNA probe in humans, whereas in rat and chicken only one mRNA species can be detected. The two villin mRNA species are co-expressed in normal human small and large intestinal mucosa and tumoral HT29-18 cells as well as in normal kidney. No villin mRNAs were detected in other normal or malignant epithelial cell types. Finally, we observed an accumulation of the two mRNA species coding for villin when HT29-18 cells become differentiated, suggesting that control of villin expression during terminal differentiation can occur at the transcription level or by RNA stabilization.

Biosynthesis and intracellular sorting of growth hormone-viral envelope glycoprotein hybrids

Various aspects of the biogenetic mechanisms that are involved in the insertion of nascent plasma membrane proteins into the endoplasmic reticulum (ER) membrane and their subsequent distribution through the cell have been investigated. For these studies chimeric genes that encode hybrid proteins containing carboxy-terminal portions of the influenza virus hemagglutinin (154 amino acids) or the vesicular stomatitis virus envelope glycoprotein (G) (60 amino acids) linked to the carboxy terminus of a nearly complete secretory polypeptide, growth hormone (GH), were used. In in vitro transcription-translation experiments, it was found that the insertion signal in the GH portion of the chimeras led to incorporation of the membrane protein segments into the ER membrane. Effectively, GH became part of the luminal segment of membrane proteins of which only very small segments, corresponding to the cytoplasmic portions of the G or HA proteins, remained exposed on the surface of the microsomes. When the chimeric genes were expressed in transfected cells, the products, as expected, failed to be secreted and remained cell-associated. These results support the assignment of a halt transfer role to segments of the membrane polypeptides that include their transmembrane portions. The hybrid polypeptide containing the carboxy-terminal portion of HA linked to GH accumulated in a juxtanuclear region of the cytoplasm within modified ER cisternae, closely apposed to the Golgi apparatus. The location and appearance of these cisternae suggested that they represent overdeveloped transitional ER elements and thus may correspond to a natural way station between the ER and the Golgi apparatus, in which further transfer of the artificial molecules is halted. The GH-G hybrid could only be detected in transfected cells treated with chloroquine, a drug that led to its accumulation in the membranes of endosome or lysosome-like cytoplasmic vesicles. Although the possibility that the chimeric protein entered such vesicles directly from the Golgi apparatus cannot be ruled out, it appears more likely that it was first transferred to the cell surface and was then internalized by endocytosis.

Biosynthesis and processing of gamma-glutamyl transpeptidase in hepatoma tissue culture cells

The biosynthesis of gamma-glutamyl transpeptidase was investigated in hepatoma tissue culture cells. Pulse-chase experiments using [35S]methionine labeling have shown that the two glycosylated subunits of the enzyme (Mr = 58,000 and 29,000) derive from a single glycosylated precursor (Mr = 79,000 at early times). Only one polypeptide chain was immunoprecipitated from cell-free translation products and was shown to correspond to the nonglycosylated precursor (Mr = 64,000). Treatment with endoglycosidase H was used to probe for the transfer of the proteins from the endoplasmic reticulum to the Golgi and demonstrated: (i) that the precursor is at least partially cleaved in the endoplasmic reticulum; (ii) that part of the precursor is transferred to the Golgi where the processing of the oligosaccharide chains takes place. None of the precursor forms were detected at the surface of the cell where the mature enzyme was found. Tunicamycin, an inhibitor of protein glycosylation, did not prevent the proteolytic processing of the enzyme, but delayed the appearance of the mature enzyme at the cell surface. Monensin, which is known to alter Golgi functions, significantly delayed the acquisition of complex type oligosaccharides and the appearance of the enzyme at the cell surface. It did not, however, alter the proteolytic processing of the precursor of gamma-glutamyl transpeptidase. Taken together, these results show that gamma-glutamyl transpeptidase is synthetized as a single precursor which is at least partially cleaved in the endoplasmic reticulum. Part of the precursor is transferred to the Golgi where its oligosaccharide chains are processed.

In vitro biosynthesis and membrane insertion of gamma-glutamyl transpeptidase

gamma-Glutamyl transpeptidase consists of two polypeptide chains anchored to the kidney brush-border membrane only through a short hydrophobic domain near the NH2-terminal end of the heavy subunit. The two subunits were reported to derive from a single polypeptide precursor by tissue labeling experiments. We have investigated the first steps of GGT biosynthesis and processing in a cell-free system. mRNA was prepared from kidney and enriched in specific sequences by a preparative gel electrophoresis. In vitro translation resulted in the synthesis of a single polypeptide (Mr = 63,000) specifically immunoprecipitated by antibodies raised against the mature dimeric enzyme. Incubation with microsomal membranes resulted in the appearance of a glycosylated form of the propeptide (Mr = 78,000). This latter form was cotranslationally segregated into microsomes and was sensitive to endoglycosidase H. Purified Escherichia coli leader peptidase did not process the primary gamma-glutamyl transpeptidase chain. This ectoprotein therefore appears to be inserted in the phospholipid bilayer without cleavage of a signal peptide, similar to most integral membrane proteins so far studied.

Germinoma in a boy with precocious puberty: evidence of hCG secretion by the tumoral cells

The present report concerns a 10-year-old boy in whom diabetes insipidus and short stature were the first manifestations of a suprasellar germinoma. Neuroradiological investigations performed when these symptoms appeared were negative. 2 years later, a rapid and early pubertal development was observed and related to secretion by the germinoma of human chorionic gonadotropin (hCG) identified immunohistochemically. This clinical evolution is unusual and indicates that plasma hCG measurements in patients with so-called idiopathic diabetes insipidus can be of clinical value in predicting the presence of an hCG-secreting tumor.

Ethanol effects in a rat hepatoma cell line: induction of gamma-glutamyltransferase

The clone C2 derived from a rat hepatoma cell line was used to investigate the mechanism of the induction of gamma-glutamyltransferase by ethanol. gamma-glutamyltransferase activity was detected in the C2 cell (1.4 mU per mg protein), and its kinetic properties were similar to normal rat liver gamma-glutamyltransferase. Ethanol provoked a dose- and time-dependent increase in gamma-glutamyltransferase activity, the maximum (2- to 3-fold) occurring 48 hr after the addition of ethanol (180 mM). In contrast, the activity of five other enzymes tested were not markedly modified by ethanol. Propanol was more potent than ethanol in inducing gamma-glutamyltransferase (5-fold stimulation), whereas methanol had no effect. The release of the enzyme in the medium was increased by ethanol and propanol. Several observations argue in favor of an increase in the biosynthesis of gamma-glutamyltransferase after ethanol addition: (i) ethanol increased the maximal velocity of the enzyme and did not modify the affinity for its substrates. It did not alter gamma-glutamyltransferase subcellular distribution; (ii) ethanol had no immediate effect when added directly to the assay mixture; (iii) the lag period and the time course of the increase in gamma-glutamyltransferase activity were those expected for an induction process; (iv) the increase in gamma-glutamyltransferase activity was prevented by cycloheximide and actinomycin D suggesting that ethanol acted at the transcriptional level. The effect of ethanol was not mimicked by acetaldehyde. In conclusion, we have demonstrated that ethanol increases the biosynthesis of gamma-glutamyltransferase in a rat hepatoma cell line which provides a new in vitro system.

The hormonal induction of gamma glutamyltransferase in rat liver and in a hepatoma cell line

Gamma glutamyltransferase (GGT) is a membrane-bound enzyme that is involved in glutathione metabolism and aminoacids uptake. GGT activity is stimulated by a number of hormones and pharmacological agents in certain animal tissues. In rat liver, adrenalectomy causes a 2-fold decrease in GGT activity and hydrocortisone treatment of adrenalectomized animals specifically stimulates this enzyme activity. In a highly differentiated hepatoma cell line, Fao, GGT activity is similar to rat liver and is under glucocorticoids control. These hormones specifically stimulate GGT activity (2- to 3-fold). Translation and transcription inhibitors prevent the hormonal effect. The stimulation of GGT activity is therefore probably due to an increase in GGT mRNA synthesis. The results reported suggest that the Fao cell line is a very convenient system for the study of the molecular mechanisms of both the glucocorticoid effects on differentiated cells as well as the modulation of membrane-bound enzymes biosynthesis.

Adenylate cyclase in Xenopus laevis oocytes: characterization of the progesterone-sensitive, membrane-bound form

Progesterone-induced reinitiation of meiosis in Xenopus laevis oocytes involves a decrease in cAMP level. In these cells, adenylate cyclase is compartmentalized, with 25-30% in the plasma membrane fraction P-10000 (sedimenting at 10000 X g) and greater than 50% in the cytosol. Soluble adenylate cyclase appears not to be regulated via a GTP-binding regulatory protein (G/F) and is insensitive to progesterone. In contrast, membrane-bound adenylate cyclase seems to be linked to G/F, since it is stimulated by sodium fluoride, guanyl-5'-imidodiphosphate (Gpp(NH)p) and by cholera toxin; it is also inhibited by progesterone. The steroid inhibition is displayed towards basal and stimulated activities. The extent of progesterone inhibition of basal activity is dependent on Mg2+ and Mn2+ concentrations. The hormonal effect is independent of GTP concentration and is observed even in the presence of the non-hydrolysable analogue of GTP, Gpp(NH)p. The progesterone effect is not mediated by adenosine. Exposure of the P-10000 fraction to 5'-deoxy-5'-S-isobutylthioadenosine (a methyltransferase inhibitor) increases adenylate cyclase activity.

Glucocorticoid hormones increase the activity of gamma-glutamyltransferase in a highly differentiated hepatoma cell line

Gamma-Glutamyltransferase activity was detected in the plasma membrane of the highly differentiated hepatoma cell line Fao, (0.93 mU/mg cell protein). Dexamethasone (1 microM) provoked a 2-3-fold increase in the activity of the enzyme in the presence of fetal calf serum. Maximal induction occurred 48-72 h after addition of the glucocorticoid to the cell culture medium. The hormonal specificity was demonstrated by the relative potencies of several glucocorticoids and sex steroids: hydrocortisone and corticosterone increased gamma-glutamyltransferase activity while tetrahydrocorticosterone and all sex steroids tested were ineffective. The effect of dexamethasone on gamma-glutamyltransferase activity wa specific since the activities of several other plasma membrane enzymes were not modified. The mechanism of the dexamethasone-induced increase in gamma-glutamyltransferase activity was neither by modification of the affinity of the enzyme for its substrates nor by alteration of the subcellular distribution of the enzyme. This increase was prevented by cycloheximide and actinomycin D. The data presented are consistent with a specific glucocorticoid receptor-mediated induction of gamma-glutamyltransferase activity in Fao cells. The kinetic parameters of the induction process by glucocorticoids are very similar to those found in adult rat liver. These results suggest that the Fao cell line is a very convenient system for the study of the molecular mechanisms of glucocorticoid effects on differentiated cells.

Endocrine aspects and tumoral markers in intracranial germinoma: an attempt to delineate the diagnostic procedure in 14 patients

This work outlines the endocrine abnormalities associated with intracranial germinomas (14 patients before treatment). Diabetes insipidus of various intensity was present in all cases. Adipsia is often present as well. In six patients, the first neuroradiologic examination after the onset of DI was normal. Growth hormone secretion was deficient in 11 of 13 patients. Abnormalities of TSH response to TRH were present in nine of ten (four insufficient and five exaggerated responses to TRH). Anterior pituitary dysfunction could not be predicted by the tumor site as determined by radiologic criteria. Tumoral markers have also been studied. Elevated plasma tumoral markers were found in four patients of 11 studied. Tumoral germinoma cells were present in CSF in five patients of ten, in one of them before radiologic confirmation. Pertinent endocrine evaluation and search for tumoral markers was of great value in systematic follow-up of patients with central diabetes insipidus, and could lead to early diagnosis and treatment of the tumor.

Human erythrocyte monoester lipase: characterization and radiochemical assay of the cell-bound enzyme in normal subjects

A monoester lipase (MEL) activity (EC 3.1.1.3) is described in human red blood cells (RBC). The lipase acts as a cell-bound enzyme and is able to exert its catalytic activity in vitro toward an exogenously added emulsified substrate. The enzyme activity, which appears to be confined to the cell membrane, is inhibited by Triton X-100. The MEL activity of human RBC is assayed using intact RBC as the enzyme source, with an emulsion of ethyl [3H]oleate (2 mM) as the substrate. The optimum pH for the reaction is 7.8 at 37 C. Lipolytic rates are monitored by quantitation of the amount of [3H]oleic acid released during 20 min of incubation after extraction by means of a liquid-liquid partition system. Suspensions of purified RBC obtained from 161 healthy adult subjects had a MEL activity of 1022 +/- 134 microunits/10(12) RBC (mean +/- SD), with a normal range (+/- 2 SD) between 754-1290 microunits. The individual activity values varied from 733-1490 microunits. The median of the 161 subjects was 1010 microunits/10(12) RB. There was no significant difference between the mean activities of RBC samples from men and women. MEL activity in RBC from the cord blood of 16 normal infants was found to be 43% higher than that in adults, with an average activity of 1458 +/- 174 microunits (mean +/- SD).

Lactogenic and somatotropic binding sites in liver membranes of rats with renal insufficiency

Binding sites for human GH (hGH) were studied in liver membranes of rats with chronic renal insufficiency (CRI) associated with marked growth retardation. A subtotal nephrectomy was performed in young female rats. One month after the nephrectomy, the animals with a plasma creatinine level 3 times or more that of controls were studied; their mean statural gain was 56% that of controls. The specific binding of [125I]hGH to microsomal membranes of rats with CRI was low (40% that of controls). The number of binding sites rather than the affinity of the binding was affected; both the lactogenic and somatotropic sites were decreased, as judged from the binding of ovine [125I]PRL and bovine [125I]GH. The binding sites of the plasma membranes as well as those of the Golgi fractions, were reduced. In plasma membranes of rats with CRI, the specific binding of glucagon was low, and the specific binding of insulin was elevated; these modifications were associated with a high plasma glucagon level and a decreased insulinemia in rats with CRI, but no modification of plasma GH and PRL levels was found. Thus, the hormone level does not appear to regulate the GH-binding sites in this system. The link between the growth defect and the decreased number of GH-binding sites in the liver membranes of rats with CRI remains to be established.