Specificity of transcription enhancement via the STAT responsive element in the serine protease inhibitor 2.1 promoter

The growth hormone regulated serine protease inhibitor (SPI) 2.1 and 2.2 gene promoters have been shown to contain a response element similar to the gamma-interferon activated sequence (GAS) family of signal transducer and activator of transcription (STAT) response elements. We have investigated the STAT and cytokine specificity of the SPI 2.1 STAT responsive element using a luciferase (LUC) reporter construct and a cDNA complementation strategy in the COS 7 cell line. Growth hormone was found to stimulate SPI-LUC reporter gene expression via activation of STAT 5, but not STATs 1 or 3, which indicates that the SPI 2.1 STAT responsive element is STAT 5 specific. In addition to the growth hormone receptor, the receptors for prolactin and erythropoietin enhanced gene transcription via the SPI 2.1 STAT responsive element, which indicates that this element is, on the other hand, not cytokine specific. Activation of STAT 5 was also observed after growth hormone treatment of cells transfected with cDNA expression plasmids for several different truncated growth hormone receptor mutants, although this activation was less efficient than with the wild type receptor. Point mutation of individual tyrosines in the growth hormone receptor intracellular domain to phenylalanines had no significant effect on signal transduction via STAT 5. These data, taken together with results from experiments using the phosphatase inhibitor sodium orthovanadate, suggest that STAT 5 may not have an absolute requirement for specific phosphorylated receptor tyrosine docking sites. That receptor tyrosine residues in a variety of amino acid contexts, or phosphorylated Janus kinase (JAK) 2 alone, can facilitate STAT 5 activation could explain the observed lack of cytokine specificity in STAT 5 activation.

Crystal structure of an antagonist mutant of human growth hormone, G120R, in complex with its receptor at 2.9 A resolution

Human growth hormone binds two receptor molecules and thereby induces signal transduction through receptor dimerization. At high concentrations, growth hormone acts as an antagonist because of a large difference in affinities at the respective binding sites. This antagonist action can be enhanced further by reducing binding in the low affinity binding site. A growth hormone antagonist mutant Gly-120 --> Arg, has been crystallized with its receptor as a 1:1 complex and the crystal structure determined at 2.9 A resolution. The 1:1 complex is remarkably similar to the native growth hormone-receptor 1:2 complex. A comparison between the two structures reveals only minimal differences in the conformations of the hormone or its receptor in the two complexes, including the angle between the two immunoglobulin-like domains of the receptor. Further, two symmetry-related 1:1 complexes in the crystal form a 2:2 complex with a large solvent inaccessible area between two receptor molecules. In addition, we present here a native human growth hormone-human growth hormone-binding protein 1:2 complex structure at 2.5 A resolution. One important difference between our structure and the previously published crystal structure at 2.8 A is revealed. Trp-104 in the receptor, a key residue in the hormone-receptor interaction, has an altered conformation in the low affinity site enabling a favorable hydrogen bond to be formed with Asp-116 of the hormone.

Growth hormone, interferon-gamma, and leukemia inhibitory factor utilize insulin receptor substrate-2 in intracellular signaling

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is tyrosyl-phosphorylated following stimulation of 3T3-F442A fibroblasts with growth hormone (GH), leukemia inhibitory factor and interferon-gamma. In response to GH and leukemia inhibitory factor, IRS-2 is immediately phosphorylated, with maximal phosphorylation detected at 15 min; the signal is substantially diminished by 60 min. In response to interferon-gamma, tyrosine phosphorylation of IRS-2 was prolonged, with substantial signal still detected at 60 min. Characterization of the mechanism of signaling utilized by GH indicated that tyrosine residues in GH receptor are not necessary for tyrosyl phosphorylation of IRS-2; however, the regions of GH receptor necessary for IRS-2 tyrosyl phosphorylation are the same as those required for JAK2 association and tyrosyl phosphorylation. The role of IRS-2 as a signaling molecule for GH is further demonstrated by the finding that GH stimulates association of IRS-2 with the 85-kDa regulatory subunit of phosphatidylinositol 3'-kinase and with the protein-tyrosine phosphatase SHP2. These results are consistent with the possibility that IRS-2 is a downstream signaling partner of multiple members of the cytokine family of receptors that activate JAK kinases.

Constitutive nuclear localization of Janus kinases 1 and 2

Both GH and the GH receptor have been reported to undergo rapid nuclear translocation. Janus kinases (JAK) 1 and 2 have been implicated in GH receptor signaling, and both of these kinases are phosphorylated by GH stimulation. In this report, we have investigated the subcellular distribution of JAK1 and JAK2. Both JAK1 and JAK2 exhibit a nucleocytoplasmic distribution by immunocytochemistry in unstimulated serum deprived CHO cells stably transfected with rat GH receptor complementary DNA (cDNA). The nucleocytoplasmic localization of JAK2 was verified by immunogold electron microscopy in both rat liver hepatocytes and CHO cells stably transfected with rat GH receptor cDNA. Nucleocytoplasmic localization of JAK2 was also verified by transient tranfection of CHO cells with a Haemophilus influenzae haemagglutinin (HA) epitope tagged JAK2 expression plasmid and subsequent localization of HA immunoreactivity. Western blot analysis of purified nuclear extracts revealed the presence of immunoreactive JAK1 at 130 kDa and immunoreactive JAK2 at 128 kDa. No change in the nuclear content of JAK1 or JAK2 was observed upon ligand stimulation of GH receptor cDNA transfected cells with 100 nM human GH for 5, 10, 15, 30, or 60 min. GH stimulation caused, however, the appearance of tyrosine phosphorylated 42- and 44-kDa proteins as well as tyrosine phosphorylated JAK2 in the nucleus. The constitutive nuclear localization of the Janus Kinases is suggestive of a novel nuclear role for JAK family members, in addition to their described cytosolic function and presents an interesting challenge to the subcellular site of hormone action.

The role of the growth hormone (GH) receptor and JAK1 and JAK2 kinases in the activation of Stats 1, 3, and 5 by GH

GH has been shown to activate the GH receptor (GHR)-associated tyrosine kinase JAK2 and the Src homology 2 domain-containing transcription factors Stats (signal transducers and activators of transcription) 1, 3, and 5. The present work investigates the role of GHR and JAK2 in the activation of Stats 1, 3, and 5 by GH. The ability of GH to stimulate the tyrosyl phosphorylation of these Stats was assessed in Chinese hamster ovary (CHO) cells expressing truncated and mutated GHR. GH was observed to stimulate tyrosyl phosphorylation of Stats 1, 3, and 5 in CHO cells expressing GHRs that bind JAK2 [GHR1-638 (full-length) and GHR1-454 (lacks approximately half of the cytoplasmic domain)] but not in CHO cells expressing GHR that do not bind JAK2 (GHR1-318 or GHR1-294). GH-dependent tyrosyl phosphorylation of Stat5, but not Stats 1 or 3, was reduced in CHO cells expressing GHR1-454. GH-dependent tyrosyl phosphorylation of Stats 3 and 5 was severely reduced and undetectable for Stat1 in cells expressing GHR1-454 in which tyrosines 333 and 338 (the only tyrosines phosphorylated within 1-454) are mutated to phenylalanine (GHR1-454Y333, 338F). However, GH-dependent phosphorylation of Stats 1, 3, and 5 was observed in cells expressing full-length GHR in which tyrosines 333 and 338 are mutated to phenylalanine (GHR1-638Y333, 338F) GH, whose receptor lacks previously defined Stat1- or Stat3-binding sites, was found in 3T3-F442A fibroblasts and 2fTGH-GHR cells to stimulate tyrosyl phosphorylation of JAK2 to a substantially greater extent than, and JAK1 to a similar extent as, leukemia inhibitory factor (LIF) and/or interferon gamma (IFN gamma), ligands whose receptors contains Stat3- and Stat1-binding sites and activate Stat3 and Stat1, respectively, better than GH. These findings suggest that: 1) JAK2 is required for GH-dependent phosphorylation of Stats 1, 3, and 5; 2) tyrosines 333 and/or 338 are required for maximal tyrosyl phosphorylation of Stats 1, 3, and 5; 3) Stat5 binds to a phosphorylated tyrosine(s) within amino acids 454-638 in addition to tyrosines 333 and/or 338; 4) GH stimulates tyrosyl phosphorylation of JAK1 in addition to JAK2 with JAK2 having a much greater response; 5) some Stat3 and Stat5 (and possibly Stat1) may bind to nonphosphorylated amino acids in GHR or to phosphorylated tyrosines in proteins that bind to GHR (e.g. JAK22) to be maximally activated; and 6) if JAK2, which contains Stat3-binding motifs, does serve as a docking site for some Stat proteins, Stat-JAK2 binding is likely to be more important for GH than LIF or IFN gamma in 3T3-F442A cells since GH induces 15 times more tyrosyl-phosphorylated JAK2 than LIF or IFN gamma.

Requirement of tyrosine residues 333 and 338 of the growth hormone (GH) receptor for selected GH-stimulated function

We have examined the involvement of tyrosine residues 333 and 338 of the growth hormone (GH) receptor in the cellular response to GH. Stable Chinese hamster ovary (CHO) cell clones expressing a receptor with tyrosine residues at position 333 and 338 of the receptor substituted for phenylalanine (CHO-GHR1-638 Y333F, Y338F) were generated by cDNA transfection. Compared with the wild type receptor the Y333F,Y338F mutant possessed normal high affinity ligand binding, hormone internalization, and ligand-induced receptor down-regulation. GH activation of mitogen-associated protein kinase was also similar in CHO clones expressing similar wild type and Y333F,Y338F receptor number. However, two GH-regulated cellular events (lipogenesis, and protein synthesis) were deficient in the tyrosine substituted receptor. In contrast, transcriptional regulation by GH (as evidenced by chloramphenicol acetyltransferase cDNA expression driven by the GH-responsive region of the SPI 2.1 gene) was not affected by Y333F,Y338F substitution. Thus we provide the first experimental evidence that specific tyrosine residues of the GH receptor are required for selected cellular responses to GH.

Growth hormone (GH) treatment up-regulates GH receptor mRNA levels in adipocytes from patients with GH deficiency and Prader-Willi syndrome

We have investigated the effect of growth hormone (GH) treatment on GH receptor mRNA expression in five prepubertal children with Prader-Willi syndrome and in eight patients with GH deficiency. An adipose tissue needle biopsy was taken before and after 2-4 mo of GH treatment, and RNA was isolated from adipose tissue and from adipocytes. GH receptor mRNA levels were determined by an RNase protection/solution hybridization assay. To further assess the specificity of the assay for GH receptor mRNAs, RNA extracted from human adipose tissue was subjected to Northern blot analysis. GH treatment significantly increased GH receptor mRNA levels in adipose tissue and isolated adipocytes. Our results indicate that GH may have an important role in regulating the GH receptor in humans.

Growth hormone specifically stimulates the expression of low density lipoprotein receptors in human hepatoma cells

Human GH (hGH) has been shown to stimulate hepatic low density lipoprotein (LDL) receptor expression in man in vivo. To further characterize this effect in vitro, we determined the expression of LDL receptors in cultured human hepatoma (HepG2) cells exposed to hGH. After incubation with hGH, stimulation of LDL receptors appeared at a concentration of 0.25 nM hGH. The presence of hGH receptors on HepG2 cells could be demonstrated by immunocytochemistry using a hGH receptor-specific monoclonal antibody. Binding studies, using 125I-labeled hGH, revealed high affinity binding with the appropriate somatogenic specificity. The LDL receptor induction was specific for hGH, as both bovine GH and recombinant human PRL were without effect. The LDL receptor stimulation occurred in parallel with increased levels of LDL receptor messenger RNA. Inclusion of dexamethasone and thyroid hormone in the incubation medium enhanced the LDL receptor stimulation by hGH. Although incubation with insulin-like growth factor-I (IGF-I) stimulated LDL receptor expression, the hGH-induced stimulation was unaltered after preincubation of cells with a monoclonal mouse anti-IGF-I antibody, suggesting that the release of IGF-I is not involved in LDL receptor stimulation by hGH. We conclude that hGH specifically induces the LDL receptor in cultured HepG2 cells at both the protein and the messenger RNA level, and that the induction is independent of IGF-I release.

Hormonal regulation of the female enriched GH receptor/binding protein mRNA in rat liver

At least two classes of mRNA for the GH receptor (GHR) and GH binding protein (GH BP) with different 5' untranslated first exons exist in the rat. One such class, the GHR1 is predominantly expressed in the liver of female rats. The hepatic expression of the GHR1 mRNA in normal and hypophsectomized rats of both sexes was studied by employing an RNase protection/solution hybridization assay. Normal females expressed 10-fold more GHR1 mRNA than males, hypophysectomy of female rats decreased the GHR1 level to that observed in male rats. Continuous GH treatment of hypophysectomized male and female rats for 6 days increased the expression of GHR1 mRNA to levels found in normal females, whereas intermittent GH treatment without effect. Bovine GH(bGH) induced the GHR1 expression in a time- and dose-dependent manner in primary cultures of adult rat hepatocytes as determined by solution hybridization. Maximal induction was achieved after 72 h of treatment with 50 ng bGH/ml medium. Female enriched expression of receptor and binding protein mRNAs raises the possibility that they participate in determining the ability of the liver to respond differently to the male and female GH secretory patterns. Our in vitro model utilizing cultures of primary adult rat hepatocytes could be used to address this issue as well as explore a hormonal interplay in regulation of GHR1 expression.

Identification of phenylalanine 346 in the rat growth hormone receptor as being critical for ligand-mediated internalization and down-regulation

The functional significance of growth hormone (GH) receptor (GHR) internalization is unknown; therefore, we have analyzed domains and individual amino acids in the cytoplasmic region of the rat GHR required for ligand-mediated receptor internalization, receptor down-regulation, and transcriptional signaling. When various mutated GHR cDNAs were transfected stably into Chinese hamster ovary cells or transiently into monkey kidney (COS-7) cells, internalization of the GHR was found to be dependent upon a domain located between amino acids 318 and 380. Mutational analysis of aromatic residues in this domain revealed that phenylalanine 346 is required for internalization. Receptor down-regulation in transiently transfected COS-7 cells was also dependent upon the phenylalanine 346 residue of the GHR, since no GH-induced down-regulation was observed in cells expressing the F346A GHR mutant. In contrast, the ability to stimulate transcription of the serine protease inhibitor 2.1 promoter by the GHR was not affected by the phenylalanine 346 to alanine mutation. These results demonstrate that phenylalanine 346 is essential for GHR internalization and down-regulation but not for transcriptional signaling, suggesting that ligand-mediated endocytosis is not a prerequisite for GH-induced gene transcription.

Prolactin, growth hormone, erythropoietin and granulocyte-macrophage colony stimulating factor induce MGF-Stat5 DNA binding activity

The molecular components which mediate cytokine signaling from the cell membrane to the nucleus were studied. Upon the interaction of cytokines with their receptors, members of the janus kinase (Jak) family of cytoplasmic protein tyrosine kinases and of the signal transducers and activators of transcription (Stat) family of transcription factors are activated through tyrosine phosphorylation. It has been suggested that the Stat proteins are substrates of the Jak protein tyrosine kinases. MGF-Stat5 is a member of the Stat family which has been found to confer the prolactin response. MGF-Stat5 can be phosphorylated and activated in its DNA binding activity by Jak2. The activation of MGF-Stat5 is not restricted to prolactin. Erythropoietin (EPO) and growth hormone (GH) stimulate the DNA binding activity of MGF-Stat5 in COS cells transfected with vectors encoding EPO receptor and MGF-Stat5 or vectors encoding GH receptor and MGF-Stat5. The activation of DNA binding by prolactin, EPO and GH requires the phosphorylation of tyrosine residue 694 of MGF-Stat5. The transcriptional induction of a beta-casein promoter luciferase construct in transiently transfected COS cells is specific for the prolactin activation of MGF-Stat5; it is not observed in EPO- and GH-treated cells. In the UT7 human hematopoietic cell line, EPO and granulocyte-macrophage colony stimulating factor activate the DNA binding activity of a factor closely related to MGF-Stat5 with respect to its immunological reactivity, DNA binding specificity and molecular weight. These results suggest that MGF-Stat5 regulates physiological processes in mammary epithelial cells, as well as in hematopoietic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediation of growth hormone-dependent transcriptional activation by mammary gland factor/Stat 5

Previous observations have shown that binding of growth hormone to its receptor leads to activation of transcription factors via a mechanism involving phosphorylation on tyrosine residues. In order to establish whether the prolactin-activated transcription factor Stat 5 (mammary gland factor) is also activated by growth hormone, nuclear extracts were prepared from COS-7 cells transiently expressing transfected Stat 5 and growth hormone receptor cDNA. Gel electrophoresis mobility shift analyses revealed the growth hormone-dependent presence of specific DNA-binding proteins in these extracts. The complexes formed could be supershifted by polyclonal anti-Stat 5 antiserum. In other experiments nuclear extracts from growth hormone-treated Chinese hamster ovary cells stably expressing transfected growth hormone receptor cDNA and liver from growth hormone-treated hypophysectomized rats were used for gel electrophoresis mobility shift analyses. These also revealed the presence of specific DNA-binding proteins sharing antigenic determinants with Stat 5. Stat 5 cDNA was shown to be capable of complementing the growth hormone-dependent activation of transcription of a reporter gene in the otherwise unresponsive COS-7 cell line. This complementation was dependent on the presence of Stat 5 tyrosine 694, suggesting a role for phosphorylation of this residue in growth hormone-dependent activation of DNA-binding and transcription.

Growth hormone-promoted tyrosyl phosphorylation of SHC proteins and SHC association with Grb2

Growth hormone (GH) has been shown to stimulate the mitogen-activated protein (MAP) kinases designated ERKs (extracellular signal regulated kinases) 1 and 2. One pathway by which ERKs 1 and 2 are activated by tyrosine kinases involves the Src homology (SH)-2 containing proteins SHC and Grb2. To gain insight into pathways coupling GH receptor (GHR) to MAP kinase activation and signaling molecules that might interact with GHR and its associated tyrosine kinase JAK2, we examined whether SHC and Grb2 proteins serve as signaling molecules for GH. Human GH was shown to promote the rapid tyrosyl phosphorylation of 66-, 52-, and 46-kDa SHC proteins in 3T3-F442A fibroblasts. GH also promoted binding of GHR and JAK2 to the SH2 domain of 46/52-kDa SHC protein fused to glutathione S-transferase (GST). Constitutively phosphorylated JAK2, from COS-7 cells transiently transfected with murine JAK2 cDNA, bound to SHC SH2-GST fusion protein, demonstrating that the SHC SH2 domain can bind tyrosyl-phosphorylated JAK2 in the absence of GHR. Regions of GHR required for GH-dependent tyrosyl phosphorylation of SHC were examined using Chinese hamster ovary cells expressing mutated rat GHR. In cells expressing GHR1-638 and GHR1-638(Y333,338F), GH stimulated phosphorylation of all 3 SHC proteins whereas GH stimulated phosphorylation of only the 66- and 52-kDa SHC proteins in cells expressing GHR1-454. GH had no effect on SHC phosphorylation in cells expressing GHR1-294 or GHR delta P, the latter lacking amino acids 297-311 containing the proline-rich motif required for JAK2 activation by GH. In contrast to SHC, Grb2 appeared not to interact directly with GHR or JAK2. However, Grb2 was shown to associate rapidly with SHC proteins in a GH-dependent manner. These findings suggest that GH stimulates: 1) the association of SHC proteins with JAK2.GHR complexes via the SHC-SH2 domain, 2) tyrosyl phosphorylation of SHC proteins, and 3) subsequent Grb2 association with SHC proteins. These events are likely to be early events in GH activation of MAP kinases and possibly of other responses to GH.

Nuclear translocation and anchorage of the growth hormone receptor

The extracellular domain of the rabbit growth hormone (GH) receptor has previously been shown to be associated with the nucleus. However, in this species the GH binding protein (BP) is derived by proteolytic cleavage of the full-length receptor, and thus distinction between the receptor and BP is difficult. The intracellular domain of the GH receptor is required for GH-stimulated function. Thus a direct nuclear function of GH would presumably require the receptor intracellular domain in the nucleus. We have therefore characterized the rat nuclear GH receptor and BP based on their distinct antigenic identity. We show, in vivo, that the full-length receptor is associated with the nucleus, including the respective subnuclear fractions (nucleoplasm, outer nuclear membranes, inner nuclear membranes, and chromatin). In vivo, the receptor is also subject to ligand-dependent nuclear translocation. Cellular transfection of rat GH receptor cDNA resulted in the appearance of nuclear binding sites for 125I-labeled human GH not present in the untransfected parental cell line (Chinese hamster ovary (CHO), buffalo rat liver). To determine which portion of the receptor was responsible for nuclear anchorage, we examined the binding of 125I-labeled human GH to whole nuclei isolated from CHO cells expressing the full-length receptor, a receptor in which 184 amino acids had been deleted from the carboxyl-terminal intracellular domain (CHO-454) and a receptor containing only 5 of 349 amino acids in the intracellular domain (CHO-294). Nuclear binding above the level of the untransfected parental cell line was detected only in CHO-638 and CHO-454 cells, suggesting that amino acids 294-454 of the receptor are necessary for nuclear anchorage. This observation was not due to membrane contamination, as the CHO-294 cells express a membrane-bound receptor that was not anchored in the nucleus. The full-length GH receptor in receptor cDNA-transfected cells is nucleocytoplasmic in the absence of ligand but is also subject to rapid ligand-dependent nuclear translocation. The presence of the intracellular domain of the GH receptor in the nucleus allows the possibility of a direct nuclear response to GH.

Growth hormone specifically regulates serine protease inhibitor gene transcription via gamma-activated sequence-like DNA elements

Growth hormone activates gene transcription of the serine protease inhibitors (SPI) 2.1 and 2.2 by an unknown mechanism. In order to define the promoter regions responsible for this effect and to characterize the transcription factors involved, we have performed gel electrophoresis mobility shift assays on nuclear extracts from cell lines transfected with growth hormone receptor cDNA. We have identified a 9-base pair DNA element, the SPI-GLE 1, which forms a complex with nuclear proteins following activation by growth hormone and which, when placed upstream of a minimal thymidine kinase promoter, drives chloramphenicol acetyltransferase expression in a growth hormone-dependent fashion. This element is similar to those from several genes regulated by other cytokines including interferon. The growth hormone-induced complexes formed were dependent on tyrosine phosphorylation but did not contain the interferon-gamma-activated transcription factor Stat 91. Competition studies with oligonucleotides similar to the SPI-GLE 1 reveal the sequence of a consensus element that specifically binds growth hormone-regulated nuclear proteins.

Androgen regulation of the insulin-like growth factor-I and the estrogen receptor in rat uterus and liver

For the first time testosterone is shown to be an important regulator of the insulin-like growth factor-I (IGF-I) in the rat uterus under in vivo conditions. In this study the regulation of IGF-I and the estrogen receptor (ER) by gonadal steroids in the uterus and liver of female rats was monitored. The ER level was assayed by hormone binding after treatment with testosterone, 5 alpha-dihydrotestosterone or estradiol and specific mRNA species were analyzed by a solution hybridization/RNase protection assay using 35S-labeled RNA probes. Ovariectomized rats restored uterine weight after treatment with testosterone. Uterine IGF-I mRNA was more than 20-fold higher in testosterone treated rats compared to untreated ovariectomized controls after 48 h treatment. The effects of testosterone on ovariectomized animals was followed in a timecourse study. Testosterone administration increased uterine IGF-I mRNA expression during the first 48 h and the maximally induced level was maintained throughout the duration of the experiment (168 h). Since induction of IGF-I mRNA by estrogen is transient, these data indicate that androgen and estrogen increase IGF-I mRNA by different mechanisms. Regulation of IGF-I mRNA by gonadal steroids was also studied in hypophysectomized animals. The rats were given either testosterone, 5 alpha-dihydrotestosterone or estradiol, and uterine IGF-I mRNA was measured after 1 week of treatment. At this timepoint estrogen treated rats showed levels of IGF-I mRNA not significantly different from those of hypophysectomized controls. In contrast testosterone and 5 alpha-dihydrotestosterone increased the IGF-I mRNA level 30 and 40 times, respectively, relative to hypophysectomized control animals. Since 5 alpha-dihydrotestosterone is not convertable to estrogen, the induction by testosterone was considered to be a true androgenic phenomenon.

Domains of the growth hormone receptor required for association and activation of JAK2 tyrosine kinase

Growth hormone (GH) has recently been shown to activate the GH receptor (GHR)-associated tyrosine kinase JAK2. In the present study, regions of the GHR required for JAK2 association with GHR were identified. GH-dependent JAK2 association with GHR was detected in Chinese hamster ovary (CHO) cells expressing wild-type GHR (GHR1-638) or GHR truncated at amino acid 454 (GHR1-454) or 380 (GHR1-380). JAK2 did not associate with GHR in cells expressing GHR truncated at amino acid 294 (GHR1-294) or when amino acids 297-311 containing a proline-rich motif were deleted (GHR delta P) or prolines 300, 301, 303, and 305 in the proline-rich motif were mutated to alanines (GHR4P-->A). Cross-linking 125I-human GH to GHR demonstrated that GHR mutants migrated with the appropriate molecular weight, with the exception of GHR4P-->A which migrated as a protein similar in size to GHR1-294. In studies performed in CHO and RIN-5AH cells, the ability of JAK2 to associate with the mutated GHR was found to correlate with GH-dependent activation of JAK2, tyrosyl phosphorylation of GHR (in the case of GHR1-638 and GHR1-454), and the ability of the GHR to copurify with tyrosine kinase activity. In CHO cells expressing mutated GHR, GH-dependent tyrosyl phosphorylation of cellular proteins (p121, p97, p42, and p39) was dependent on the ability to activate JAK2. No proteins showed increased tyrosyl phosphorylation in CHO cells expressing GHR1-294, GHR4P-->A, or GHR delta P. Deletion of the C-terminal half (amino acids 455-638) of the GHR ablated GH-dependent tyrosyl phosphorylation of p97. Taken together, these results provide strong evidence that the N-terminal quarter of the cytoplasmic domain of GHR and within this region, the proline-rich motif, is required for association of JAK2 with GHR and GH-dependent activation of JAK2, and that tyrosines in the N-terminal half of the cytoplasmic domain of the GHR are phosphorylated by JAK2. The finding that a specific interaction with the C-terminal half of GHR appears to be necessary for p97 phosphorylation indicates that while JAK2 activation may be necessary for a full biological response to GH, it appears not to be sufficient.

Receptor-mediated nuclear translocation of growth hormone

We have previously shown that the growth hormone (GH) receptor-binding protein is associated with the nucleus. We show here both by electron microscopy and nuclear isolation that GH is subject to rapid nuclear translocation. The intracellular fate of intravenously injected 125I-bovine growth hormone (bGH) was examined in the rat hepatocyte by electron microscopic autoradiography. The hormone appeared rapidly at the plasma membrane, then sequentially in lysosomal and multivesicular bodies and/or the nuclear membrane before final translocation to the nuclear matrix. Maximal translocation to the nuclear matrix occurred within 30 min of injection. Nuclear translocation of 125I-hGH was also studied by isolation of nuclei from cells stably transfected with cDNAs encoding the GH receptor, GH-binding protein, and a membrane bound but cytoplasmic domain-deficient receptor. Specific internalization and nuclear translocation of hormone only occurred in cells transfected with the full-length receptor. The translocation was rapid and became saturated within 1 h after addition of hormone to the culture media. SDS-polyacrylamide gel electrophoresis of isolated nuclei showed that GH is transported to the nucleus as the intact molecule. Pretreatment of cells with lysosomotropic agents (chloroquine, ammonium chloride, and bacitracin) decreased hormone degradation and increased nuclear translocation of GH. The nuclear translocation of GH was achieved independent of the cytoskeletal system (microtubular, microfilament, and intermediate filament networks). Thus, GH is subject to rapid receptor-dependent nuclear translocation via the endosomal pathway.

Cell transfection as a tool to study growth hormone action

The isolation of growth hormone receptor (GHR) cDNA clones has made possible the transfection of GHRs into cultured cells. Our aim in this minireview is to show how the application of such approaches have benefited GHR research. GH stimulation of cells expressing GHR cDNAs can cause an alteration of cellular function that mimic those of the endogenous GHR. GHR cDNA transfected cells also offer a system where the mechanism of GH action can be studied. Such a system has been used to demonstrate that the GHR itself becomes tyrosine phosphorylated and that further phosphorylation of downstream proteins is important in GH action. The GH signals are transmitted to the nucleus and GH regulated genes have now begun to be characterized. The ability to use cell transfection for mechanistic studies of GH action will be instrumental to define domains within the receptor that are of functional importance and to determine pathways whereby GH signals are conveyed within the cell.

Identification of intracellular domains in the growth hormone receptor involved in signal transduction

The growth hormone (GH) receptor belongs to the GH/prolactin/cytokine super-family of receptors. The signal transduction mechanism utilized by this class of receptors remains largely unknown. In order to identify functional domains in the intracellular region of the GH receptor we generated a number of GH receptor mutants and analyzed their function after transfection into various cell lines. A truncated GH receptor missing 184 amino acids at the C-terminus was unable to mediate GH effects on transcription of the Spi 2.1 and insulin genes. However, this mutant was fully active in mediating GH-stimulated metabolic effects such as protein synthesis and lipolysis. Furthermore, this mutant GH receptor internalized rapidly following GH binding. Another truncated GH receptor lacking all but five amino acids of the cytoplasmic domain could not mediate any effects of GH nor did it internalize. Deletion of the proline-rich region or changing the four prolines to alanines also resulted in a GH receptor deficient in signaling. Mutation of phenylalanine 346 to alanine resulted in a GH receptor which did not internalize rapidly; however, this mutant GH receptor was capable of mediating GH-stimulated transcription as well as metabolic effects. These results indicate that the intracellular part of the GH receptor can be divided into at least three functional domains: (i) for transcriptional activity, two domains are involved, one located in the C-terminal 184 amino acids and the other in the proline-rich domain; (ii) for metabolic effects, a domain located in or near the proline-rich region is of importance; and (iii) for internalization, phenylalanine 346 is necessary.