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

Growth hormone-induced JAK2 signaling and GH receptor down-regulation: role of GH receptor intracellular domain tyrosine residues

GH receptor (GHR) mediates important somatogenic and metabolic effects of GH. A thorough understanding of GH action requires intimate knowledge of GHR activation mechanisms, as well as determinants of GH-induced receptor down-regulation. We previously demonstrated that a GHR mutant in which all intracellular tyrosine residues were changed to phenylalanine was defective in its ability to activate signal transducer and activator of transcription (STAT)5 and deficient in GH-induced down-regulation, but able to allow GH-induced Janus family of tyrosine kinase 2 (JAK2) activation. We now further characterize the signaling and trafficking characteristics of this receptor mutant. We find that the mutant receptor's extracellular domain conformation and its interaction with GH are indistinguishable from the wild-type receptor. Yet the mutant differs greatly from the wild-type in that GH-induced JAK2 activation is augmented and far more persistent in cells bearing the mutant receptor. Notably, unlike STAT5 tyrosine phosphorylation, GH-induced STAT1 tyrosine phosphorylation is retained and augmented in mutant GHR-expressing cells. The defective receptor down-regulation and persistent JAK2 activation of the mutant receptor do not depend on the sustained presence of GH or on the cell's ability to carry out new protein synthesis. Mutant receptors that exhibit resistance to GH-induced down-regulation are enriched in the disulfide-linked form of the receptor, which reflects the receptor's activated conformation. Furthermore, acute GH-induced internalization, a proximal step in down-regulation, is markedly impaired in the mutant receptor compared to the wild-type receptor. These findings are discussed in the context of determinants and mechanisms of regulation of GHR down-regulation.

Growth hormone receptors in zebrafish (Danio rerio): adult and embryonic expression patterns

Growth hormone receptor (GHR) is a critical regulator of growth and metabolism. Although two GHRs have been characterized in many fish species, their functional characteristics, mechanisms of regulation and roles in embryonic development remain unclear. The zebrafish (Danio rerio) is an excellent model organism to study both developmental and physiological processes. In the present work, we characterized the complete cDNA sequences of zebrafish GHRs, ghra and ghrb, and their gene structures. We studied the expression of both receptors in adult tissues, and during embryonic development and larval stages by means of RT-PCR and whole-mount in situ hybridization. We determined that both transcripts are maternal ones, with specific expression patterns during development. Both GHR transcripts are mainly expressed in the notochord, myotomes, anterior structures and in the yolk cell. Interestingly, their expression became undetectable at 96h post-fertilization. Unlike other reports in fish, ghrs expression could not be detected in brain when adult tissues were used, and we detected ghrb but not ghra transcripts in muscle. In addition, we determined alternative transcript sequences for ghra with specific domain deletions, and alternative transcripts for ghrb that generate a premature stop codon and codify for truncated isoforms. These isoforms lack intracellular regions necessary for the activation of signal transducers and activators of transcription (STAT) family transcription factors 5.

Medaka receptors for somatolactin and growth hormone: phylogenetic paradox among fish growth hormone receptors

Somatolactin (SL) in fish belongs to the growth hormone/prolactin family. Its ortholog in tetrapods has not been identified and its function(s) remains largely unknown. The SL-deficient mutant of medaka (color interfere, ci) and an SL receptor (SLR) recently identified in salmon provide a fascinating field for investigating SL's function(s) in vivo. Here we isolated a medaka ortholog of the salmon SLR. The mRNA is transcribed in variable organs. Triglycerides and cholesterol contents in the ci are significantly higher than those in the wild type, providing the first evidence of SL's function in suppressing lipid accumulation to organs. Interestingly, phylogenetic comparisons between the medaka SLR and growth hormone receptor (GHR), which is also isolated in this study, in relation to GHRs of other fish, suggested that all GHRs reported from nonsalmonid species are, at least phylogenetically, SLRs. An extra intron inserted in medaka and pufferfish SLRs and flounder and sea bream GHRs also supports their orthologous relationship, but not with tetrapod GHRs. These results may indicate lineage-specific diversification of SLR and GHR functions among fish or just an inappropriate naming of these receptors. Further functional and comparative reassessments are necessary to address this question.

Molecular cloning and characterization of gilthead sea bream (Sparus aurata) growth hormone receptor (GHR). Assessment of alternative splicing

The full-length growth hormone receptor (GHR) of gilthead sea bream (Sparus aurata) was cloned and sequenced by RT-PCR and rapid amplification of 5'and 3'ends. The open reading frame codes for a mature 609 amino acid protein with a hydrophobic transmembrane region and all the characteristic motifs of GHRs. Sequence analysis revealed a 96 and 76% of amino acid identity with black sea bream (Acanthopagrus schlegeli) and turbot (Scophthalmus maximus) GHRs, respectively, but this amino acid identity decreases up to 52% for goldfish (Carassius auratus) GHR. By means of real-time PCR assays, concurrent changes in the hepatic expression of GHRs and insulin-like growth factor-I (IGF-I) was evidenced. Moreover, their regulation occurred in conjunction with the summer spurt of growth rates and circulating levels of GH and IGF-I. Search of alternative splicing was carried out exhaustively for gilthead sea bream GHR, but Northern blot and 3' RACE failed to demonstrate the occurrence of short alternative messengers. Besides, RT-PCR screening did not reveal deletions or insertions that could lead to alternative reading frames. In agreement with this, cross-linking assays only evidenced two protein bands that match well with the size of glycosylated and non-glycosylated forms of the full-length GHR. If so, it appears that alternative splicing at the 3'end does not occur in gilthead sea bream, although different messengers for truncated or longer GHR variants already exist in turbot and black sea bream, respectively. The physiological relevance of this finding remains unclear, but perhaps it points out large inter-species differences in the heterogeneity of the GHR population.

Seabream growth hormone receptor: molecular cloning and functional studies of the full-length cDNA, and tissue expression of two alternatively spliced forms

A full-length clone of the growth hormone receptor (GHR) was isolated from a cDNA library constructed from the liver of black seabream (Acanthopagrus schlegeli). The seabream GHR (sbGHR) cDNA sequence encodes a transmembrane protein of 640 amino acids (aa) possessing the characteristic motifs and architectural design of GHRs of other species. When compared to the other fish GHRs, it is most homologous to another marine fish species, the turbot, where the aa identity is 79.3%. But the sbGHR sequence is more remotely related to the goldfish GHR (51.6% aa identity) and the salmonid GHRs (approximately 46-48% aa identities). Phylogenetic comparison with other known GHRs indicates that the fish GHRs constitute a distinct group among the different vertebrate classes. The aa identities between sbGHR and other GHRs are low, being around 40% with mammalian GHRs, around 45% with avian and reptilian GHRs, and less than 35% with Xenopus GHR. CHO cells transfected with the sbGHR cDNA can be stimulated to proliferate by recombinant seabream growth hormone (sbGH). In addition, the transfected cells can transactivate a co-expressed mammalian serine protease inhibitor (Spi) 2.1 promoter upon stimulation by sbGH. These functional assays indicated that the fish receptor can interact with its homologous ligand to evoke the downstream post-receptor events. Reverse transcription-polymerase chain reaction (RT-PCR) and genomic PCR using a pair of gene-specific primers revealed the expression of two alternatively spliced forms of sbGHR in various tissues of the fish. A 93-bp intron, unique to the sbGHR gene and not found in any other known GHR genes, is alternatively spliced to give rise to two forms of receptor mRNA transcripts. The two forms of the receptor are differentially expressed among the different tissues of the fish.

The ubiquitin-proteasome pathway and the regulation of growth hormone receptor availability

The number of growth hormone receptors (GHR) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. This article deals with the regulatory mechanisms underlying the availability of GHR for serum growth hormone. The availability of membrane proteins at the cell surface can be regulated at different locations within the cell: (1) The amount of protein synthesized in the endoplasmic reticulum (ER) is largely controlled by gene transcription. In addition, the ER quality control system regulates the exiting of properly folded proteins from the ER. (2) In the trans-Golgi network, proteins can either be diverted directly to the lysosomes or be transported to the cell surface. (3) At the plasma membrane, the endocytic machinery can select proteins for endocytosis via clathrin-coated pits or proteins may be subject to proteolysis, resulting in shedding of the extracellular domain. (4) In endosomes, internalized proteins are either recycled back to the plasma membrane or targeted to the lysosome for degradation. At each of these cellular locations the ubiquitin-proteasome pathway can specifically regulate protein levels via different mechanisms. In transfected Chinese hamster lung cells, GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). As outlined in this article the level of GHR at the cell surface, defined as GHR availability, is mainly regulated by the ubiquitin-proteasome pathway.

Skin fibroblasts of children with idiopathic short stature show an increased mitogenic response to IGF-I and secrete more IGFBP-3

OBJECTIVE AND PATIENTS

To study differences in cellular parameters of GH and IGF-I responsiveness in skin fibroblasts of 14 children with idiopathic short stature (ISS) treated with recombinant human GH and 13 children with normal height. Secondly, to investigate whether these cellular parameters can predict the growth response to GH treatment in children with ISS.

DESIGN AND MEASUREMENTS

The mitogenic responsiveness to GH and IGF-I was investigated by 3H-Thymidine incorporation. Insulin-like growth factor binding protein-3 (IGFBP-3) levels in the media were measured by radioimmunoassay (RIA).

RESULTS

No significant mitogenic responses were observed to various doses of GH (1000, 5000 or 50.000 ng/ml) in children with ISS or controls. ISS fibroblasts showed an increased mitogenic response to IGF-I (10 ng/ml) compared to controls (mean +/- SD 5.9 +/- 2.4- vs. 4.2 +/- 1.5-fold stimulation, P < 0.05), and GH enhanced this effect in both groups. IGFBP-3 secretion was increased in ISS fibroblasts when compared to controls under all conditions examined (basal, 200 and 5000 ng/ml GH, 10 ng/ml IGF-I for 24 and 48 h). High IGFBP-3 levels were related to low mitogenic responses to IGF-I or to GH + IGF-I in children with ISS (r = -0.7, P < 0.05), but not in controls. Within the ISS group, an enhanced mitogenic response to IGF-I in vitro was related to more extreme short stature before GH treatment (r = -0.70, P < 0.05) and to a relatively impaired response to high dose GH treatment in vivo (r = -0.52, P < 0.05).

CONCLUSION

The demonstration of high IGFBP-3 levels and enhanced mitogenic response to IGF-I shows that ISS fibroblasts have different cellular characteristics compared to controls of normal height. It is hypothesized that in ISS an alteration of the signal transduction pathway between the GH receptor and IGFBP-3 synthesis results in a local imbalance with high IGFBP-3 levels and lower IGF-I availability for the IGF-I receptor. This may be reflected by an increased IGF-I responsiveness in vitro which is associated with an impaired capacity to grow in vivo.

Impairment of liver GH receptor signaling by fasting

Fasting causes a state of GH resistance responsible for low circulating IGF-I levels. To investigate whether this resistance may result from alterations in the GH signaling pathway, we determined the effects of fasting on the GH transduction pathway in rat liver. Forty-eight-hour fasted or fed male rats were injected with recombinant rat GH via the portal vein. Liver was removed 0 and 15 min after injection. Although GH stimulated Janus kinase 2 (JAK2) phosphorylation in all animals, this was severely blunted in fasted animals. Similarly, the phosphorylation of the GH receptor, although observed in both fasted and fed rats after GH injection, was markedly reduced in fasted rats. A rapid signal transducer and activator of transcription 5 (STAT5) tyrosine phosphorylation was also induced in the liver of fed animals in response to GH. In contrast, in fasted rats only a slight phosphorylated STAT5 signal was observed. The inhibitory effect of fasting on these GH signaling molecules occurred without changes in their protein content. Furthermore, the impairment of the JAK-STAT pathway in fasted animals was associated with increased liver suppressor of cytokine signaling 3 mRNA levels. Although glucocorticoids, which are increased by fasting, may cause GH resistance, adrenalectomy failed to prevent alterations in the JAK-STAT pathway caused by fasting. In conclusion, the GH resistance induced by fasting is associated with impairment of the JAK-STAT signaling pathway. This might contribute to the decrease in liver IGF-I production observed in fasting.

Signal transduction via the growth hormone receptor

Rapid progress has been made recently in the definition of growth hormone (GH) receptor signal transduction pathways. It is now apparent that many cytokines, including GH, share identical or similar signalling components to exert their cellular effects. This review provides a brief discourse on the signal transduction pathways, which have been demonstrated to be utilized by GH. The identification of such pathways provides a basis for understanding the pleiotropic actions of GH. The mechanisms by which the specific cellular effects of GH are achieved remain to be elucidated.

Molecular cloning of a teleost growth hormone receptor and its functional interaction with human growth hormone

This paper reports the first full-length cDNA sequence of a growth hormone receptor (GHR) from a teleost fish and its functional expression in cultured eukaryotic cells. The cDNA sequence, from Carassius auratus (goldfish), encodes a protein of 602 amino acids (aa) akin in architecture to the GHRs of other species. Despite the presence of motifs characteristic of GHR, the overall homology between the goldfish GHR and other GHRs is very low ( approximately 40% aa identity). CHO cells transfected with this receptor cDNA can be stimulated to proliferate by human growth hormone (hGH). In addition, the transfected cells can transactivate a co-expressed mammalian serine protease inhibitor (Spi) 2.1 promoter upon stimulation by hGH, indicating the successful interaction of the fish receptor with the mammalian ligand to evoke the down-stream post-receptor events. Tissue distribution studies indicated that the receptor is mostly expressed in the liver and hypothalamus of goldfish. A single mRNA transcript of a size of about 4 kb was found in the goldfish liver.

The mechanism of effect of growth hormone on preadipocyte and adipocyte function

Growth hormone (GH) is not only the major regulator of postnatal somatic growth but also exerts profound effects on body composition through a combination of anabolic, lipolytic and antinatriuretic actions. GH enhancement of the lipolytic activity of adipose tissue in combination with a reduction of triglyceride accumulation via inhibition of lipoprotein lipase activity appears to be the major mechanism by which GH results in a reduction of the total fat mass. Recently, much progress has been made in understanding the molecular mechanism by which GH affects cellular function. This review provides a brief discourse and summary of the mechanism of effects of GH on preadipocyte/adipocyte function. It is intended to provide a functional understanding of the mechanism of action of GH as it relates to adipogenesis and adipocyte function.

Pulsatility of growth hormone (GH) signalling in liver cells: role of the JAK-STAT5b pathway in GH action

The intracellular signalling molecule and transcriptional activator STAT5b is a key mediator of the effects of intermittent plasma growth hormone (GH) pulses on the male-specific pattern of liver gene expression and pubertal body growth rates in rodents. Experiments with Stat5b gene-knockout mice have revealed that these GH-regulated, male-specific phenotypes are a direct consequence of GH pulse-dependent STAT5b activation and that loss of function of STAT5b cannot be compensated for by the closely related signalling molecule STAT5a. Physiological plasma GH pulses are required to obtain the high levels of activated STAT5b seen in the livers of males, and down-regulation of the GH receptor (GHR)-JAK-STAT5b pathway in hepatocytes exposed to GH in a near-continuous fashion underlies the low level of liver STAT5b activity that is characteristic of adult female rats. Termination of nuclear STAT5b signalling occurs at the conclusion of a plasma GH pulse, with STAT5b deactivation catalysed by a tyrosine phosphatase. In males, termination of the intracellular signalling stimulated by a plasma GH pulse is proposed to be additionally facilitated by GH-STAT5b-inducible SOCS-CIS proteins, which block the further activation of STAT5b by binding to and inhibiting the action of the GHR-JAK2 complex via multiple mechanisms. In this manner, the liver cell is rendered temporarily unresponsive to further GH-signalling events. SOCS-CIS proteins synthesized in liver cells stimulated continuously with GH may also contribute to the apparent down-regulation of STAT5b signalling that is observed in the female rat liver.

SOCS/CIS protein inhibition of growth hormone-stimulated STAT5 signaling by multiple mechanisms

The inhibition of growth hormone (GH) signaling by five members of the GH-inducible suppressor of cytokine signaling (SOCS/CIS) family was investigated in transfected COS cells. Complete inhibition of GH activation of the signal transducer STAT5b and STAT5b-dependent transcriptional activity was observed upon expression of SOCS-1 or SOCS-3, while partial inhibition (CIS, SOCS-2) or no inhibition (SOCS-6) was seen with other SOCS/CIS family members. SOCS-1, SOCS-2, SOCS-3, and CIS each strongly inhibited the GH receptor (GHR)-dependent tyrosine phosphorylation of JAK2 seen at low levels of transfected JAK2; however, only SOCS-1 strongly inhibited the GHR-independent tyrosine phosphorylation of JAK2 seen at higher JAK2 levels. To probe for interactions with GHR, in vitro binding assays were carried out using glutathione S-transferase-GHR fusion proteins containing variable lengths of GHR's COOH-terminal cytoplasmic domain. CIS and SOCS-2 bound to fusions containing as few as 80 COOH-terminal GHR residues, provided the fusion protein was tyrosine-phosphorylated. By contrast, SOCS-3 binding required tyrosine-phosphorylated GHR membrane-proximal sequences, SOCS-1 binding was tyrosine phosphorylation-independent, and SOCS-6 did not bind the GHR fusion proteins at all. Mutation of GHR's membrane-proximal tyrosine residues 333 and 338 to phenylalanine suppressed the inhibition by SOCS-3, but not by CIS, of GH signaling to STAT5b. SOCS/CIS proteins can thus inhibit GH signaling to STAT5b by three distinct mechanisms, distinguished by their molecular targets within the GHR-JAK2 signaling complex, as exemplified by SOCS-1 (direct JAK2 kinase inhibition), SOCS-3 (inhibition of JAK2 signaling via membrane-proximal GHR tyrosines 333 and 338), and CIS and SOCS-2 (inhibition via membrane-distal tyrosine(s)).

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Jak2-Stat5 interactions analyzed in yeast

Many cytokine receptors employ Janus protein tyrosine kinases (Jaks) and signal transducers and activators of transcription (Stats) for nuclear signaling. Here, we have established yeast strains in which an autoactivated Jak2 kinase induces tyrosine phosphorylation, dimerization, nuclear translocation, and DNA binding of a concomitantly expressed Stat5 protein. Transcriptional activity of Stat5 on a stably integrated, Stat-dependent reporter gene required the C-terminal fusion of the VP16 transactivation domain. In such yeast strains, the interaction between Jak2 and Stat5 was analyzed without interference by other mammalian proteins involved in regulating Jak-Stat signaling, and mutant versions of both proteins were analyzed for their ability to productively interact. Complexes between Jak2 and Stat5 were found to be stable under stringent co-immunoprecipitation conditions. Deletion of the Jak homology regions 2-7 (JH2-JH7) of Jak2, leaving only the kinase domain (JH1) intact, reduced the ability of the kinase to phosphorylate Stat5, whereas deletion of the JH2 domain caused an increased enzymatic activity. A site-directed R618K mutation in the Stat5 SH2 domain abolished the phosphorylation by Jak2, while deletion of the C terminus led to Stat5 hyperphosphorylation. A single phosphotyrosine-SH2 domain interaction was sufficient for the dimerization of Stat5, but such dimers bound to DNA very inefficiently. Together, our data show that yeast cells are appropriate tools for studying Jak-Stat or Stat-Stat interactions. Our mutational analysis suggests that the Stat5 SH2 domain is essential for the interaction with Jak2 and that the kinase domain of Jak2 is sufficient for Jak2-Stat5 interaction. Therefore, the Jak kinase domain may be all that is needed to cause Stat phosphorylation in situations where receptor docking is dispensable.

The molecular basis of growth hormone action

Recent studies have begun to elucidate the molecular actions of growth hormone, a major regulator of somatic growth and metabolic functions. The cell surface growth hormone receptor, a member of the cytokine receptor superfamily, binds as a dimer to a single growth hormone molecule. Receptor dimerization precedes signal transduction, which is predominantly mediated by the non-receptor tyrosine kinase, Jak2. Activation of Jak2 leads to mitogenic proliferation, phosphorylation of intracellular proteins, MAP kinase activation, activation of Stats 1, 3, and 5, and induction of target gene expression. Specific cytoplasmic domains of the growth hormone receptor mediate Jak2 activation, metabolic actions of growth hormone, Stat activation, and calcium influx.

The full agonistic effect of recombinant 20 kDa human growth hormone (hGH) on CHO cells stably transfected with hGH receptor cDNA

The agonistic effect of the recombinant 20 kilodalton human GH (20K-hGH) with authentic primary structure was studied using Chinese hamster ovary (CHO) cells stably transfected with hGH receptor (hGHR) cDNA and was compared with that of 22K-hGH. The binding affinities (dissociation constants) of 20K- and 22K-hGH were identical (0.41 +/- 0.11 nM and 0.41 +/- 0.04 nM, respectively). In addition, the two hGHs possessed the same potencies in activating the rat serine protease inhibitor (Spi) 2.1 gene promoter. 20K-hGH was similarly internalized as 22K-hGH but its internalization rate was a little slower than that of 22K-hGH. We also found that proliferation of CHO-hGHR cells stimulated by serum was remarkably inhibited by both hGHs to the same degree. In conclusion, both hGH isoforms exhibited the same binding affinities for hGHR and were potent enough to induce some hGHR-mediated cellular events. These suggest that 20K-hGH exerts a full agonistic activity for hGHR.

Multiple prolactin (PRL) receptor cytoplasmic residues and Stat1 mediate PRL signaling to the interferon regulatory factor-1 promoter

The Nb2 PRL receptor (PRL-R) is known to mediate PRL signaling to the interferon (IFN) regulatory factor-1 (IRF-1) gene via the family of signal transducers and activators of transcription or Stats. To analyze the components of the PRL-R/Stat/IRF-1 signaling pathway, various PRL-R, Stat, and IRF-1-CAT reporter constructs were transiently cotransfected into COS cells. First, mutations in the IFNgamma-activated sequence (GAS), either multimerized or in the context of the 1.7-kb IRF-1 promoter, failed to mediate a PRL response, showing that the IRF-1 GAS is a target of PRL signaling. Next, pairwise alanine substitutions into conserved residues in the proline-rich motif or Box 1 region and two tyrosine mutations, Y308F and Y382F, in the PRL-R intracellular domain all impaired PRL signaling to multimerized GAS or to the 1.7-kb IRF-1 promoter. Furthermore, these PRL-R mutants mediated reduced Stat1 binding to the IRF-1 GAS. Transfection of Stat1 further enhanced PRL signaling to the IRF-1 promoter, suggesting that Stat1 is a positive mediator of PRL action. These studies show that both membrane proximal and distal residues of the PRL-R are involved in signaling to the IRF-1 gene. Further, Stat1 and the GAS element are important for PRL activation of the IRF-1 gene.

Identification of tyrosine residues in the intracellular domain of the growth hormone receptor required for transcriptional signaling and Stat5 activation

The binding of growth hormone (GH) to its receptor results in its dimerization followed by activation of Jak2 kinase and tyrosine phosphorylation of the GH receptor itself, as well as Jak2 and the transcription factors Stat1, -3, and -5. In order to study the role of GH receptor tyrosine phosphorylation in intracellular signaling, we constructed GH receptors in which combinations of tyrosines were mutated to phenylalanines. We identified three tyrosine residues at positions 534, 566, and 627 that were required for activation of GH-stimulated transcription of the serine protease inhibitor (Spi) 2.1 promoter. Any of these three tyrosines is able to independently mediate GH-induced transcription, indicating redundancy in this part of the GH receptor. Tyrosine phosphorylation was not required for GH stimulation of mitogen-activated protein (MAP) kinase activity or for GH-stimulated Ca2+ channel activation since these pathways were normal in cells expressing a GH receptor in which all eight intracellular tyrosines were mutated to phenylalanines. Activation of Stat5 by GH was, however, abolished in cells expressing the GH receptor lacking intracellular tyrosines. This study demonstrates that specific tyrosines in the GH receptor are required for transcriptional signaling possibly by their role in the activation of transcription factor Stat5.

JAK2, Ras, and Raf are required for activation of extracellular signal-regulated kinase/mitogen-activated protein kinase by growth hormone

Protein-tyrosine kinases (PTKs) of the JAK family have been characterized on the basis of their ability to mediate the rapid induction of transcription of interferon-responsive genes through the stimulation of a class of latent cytoplasmic transcription factors known as signal transducers and activators of transcription (STATs). STAT activation, which has been described as being Ras-independent, requires tyrosine phosphorylation, but STAT transactivating activity is enhanced by phosphorylation on serine as well, probably by extracellular signal-regulated kinase/mitogen-activated protein kinase(s) (ERK/MAPK). STATs can be activated upon binding of ligands to receptor PTKs, to G-protein-linked receptors, and to cytokine receptors. Whether JAKs are required for the activation of signaling pathways other than that leading to STAT activation is not known. The binding of growth hormone (GH) to its receptor (GHR) activates JAK2 and STATs as well as ERK/MAP kinases. We have used a transient transfection system in 293 cells to evaluate the requirement for JAK2 in the activation of ERK2/MAPK by GH. We found that JAK2 is required for GH-simulated activation of ERK2/MAPK. Employing the transient expression of dominant negative forms of H-Ras and Raf-1, we determined that the GHR/JAK2-mediated activation of ERK2/MAPK is dependent on both Ras and Raf. Thus, JAK protein-tyrosine kinases may represent a common component in the activation of the ERK2/MAPK and STAT signaling pathways, which appear to bifurcate upstream of Ras activation but converge with ERK/MAPK phosphorylation of STATs.

Growth hormone-dependent phosphorylation of tyrosine 333 and/or 338 of the growth hormone receptor

Many signaling pathways initiated by ligands that activate receptor tyrosine kinases have been shown to involve the binding of SH2 domain-containing proteins to specific phosphorylated tyrosines in the receptor. Although the receptor for growth hormone (GH) does not contain intrinsic tyrosine kinase activity, GH has recently been shown to promote the association of its receptor with JAK2 tyrosine kinase, to activate JAK2, and to promote the tyrosyl phosphorylation of both GH receptor (GHR) and JAK2. In this work, we examined whether tyrosines 333 and/or 338 in GHR are phosphorylated by JAK2 in response to GH. Tyrosines 333 and 338 in rat full-length (GHR1-638) and truncated (GHR1-454) receptor were replaced with phenylalanines and the mutated GHRs expressed in Chinese hamster ovary cells. These substitutions caused a loss of GH-dependent tyrosyl phosphorylation of truncated receptor and a reduction of GH-dependent phosphorylation of the full-length receptor. Consistent with Tyr333 and/or Tyr338 serving as substrates of JAK2, these substitutions resulted in a loss of tyrosyl phosphorylation of truncated receptor in an in vitro kinase assay using substantially purified GH.GHR.JAK2 complexes. The Tyr to Phe substitutions did not substantially alter GH-dependent JAK2 association with GHR or tyrosyl phosphorylation of JAK2. These results suggest that Tyr333 and/or Tyr338 in GHR are phosphorylated in response to GH and may therefore serve as binding sites for SH2 domain-containing proteins in GH signal transduction pathways.