Regulators of growth hormone signaling

Increased SULT1E1 activity in HepG2 hepatocytes decreases growth hormone stimulation of STAT5b phosphorylation

Mouse models of cystic fibrosis (CF) display increased sulfotransferase 1E1 (SULT1E1) activity in hepatocytes of cystic fibrosis transmembrane receptor (CFTR)-deficient animals. SULT1E1 is responsible for the sulfation and inactivation of beta-estradiol (E2) at physiological concentrations. IGF-1 message levels in CFTR(-/-) mouse livers were positively correlated with body weight and negatively correlated with SULT1E1 activity. Growth hormone (GH) is important in the regulation of hepatic IGF-1 expression indicating that E2 levels are involved with GH signaling in hepatocytes. To investigate the effects of E2 and SULT1E1 activity on GH signal transduction in human hepatocytes, SULT1E1 was stably expressed in HepG2 cells. Effects of increased E2 sulfation on the GH signaling pathway and E2-regulated gene expression were examined. Pretreatment of HepG2 cells with 10nM E2 prior to GH stimulation increased STAT5b phosphorylation and IGF-1 expression. In SULT1E1-transfected HepG2 cells, GH-stimulated STAT5b phosphorylation was significantly decreased. E2 treatment had no effect on STAT5b phosphorylation in the absence of GH stimulation. E2 also had no effect on Jak-2 phosphorylation. E2 has an apparent rapid action on increasing GH-stimulated STAT5b phosphorylation that was not attenuated by the estrogen receptor antagonist, ICI 182,780. Physiological levels of E2 in HepG2 cells increase GH stimulation of IGF-1 production apparently through increased phosphorylated STAT5b levels and transcriptional activation of the IGF-1 gene. The enhanced SULT1E1 activity may have a role in inhibiting GH-stimulated STAT5b phosphorylation and IGF-1 synthesis via the sulfation and inactivation of E2.

The growth hormone receptor in growth

The growth hormone receptor (GHR) is a major effector of Human growth. Functional variants of the GHR include very rare loss-of-function mutations (pathology) and very common polymorphisms (physiology). Recent experimental data have clarified the mechanisms through which mutations of the GHR or Stat5 lead to growth hormone insensitivity and major monogenic growth defects. Recent pharmacogenetic studies support that the response to growth-promoting administration of growth hormone is influenced by exon 3 polymorphism of the GHR.

Estrogen receptor-alpha regulates SOCS-3 expression in human breast cancer cells

The suppressor of cytokine signalling (SOCS) protein family negatively regulates cytokine action. In this study, we investigated the effects of estrogen (E2) on SOCS-3 expression in T47D and MCF-7 human breast cancer cells. Real-time PCR analysis of E2-treated T47D cells revealed a ligand and time-dependent increase in of SOCS-3 mRNA levels. Cloning of a 1.7 kb fragment of the human SOCS-3 5' flanking sequence, and subsequent analysis of potential transcription factor-binding sites identified an incomplete ERE motif located -1493 to -1489 upstream of the start site. Transient transfection of the cloned fragment in MCF-7 cells showed that both E2 and genistein treatment caused an increase in reporter gene activity, which was inhibited by co-treatment with ICI 182,780. Chromatin immunoprecipitation analysis revealed an E2 and time-dependent recruitment of ERalpha to the E2 responsive region of the human SOCS-3 promoter. In summary, this study shows that ERalpha directly regulates human SOCS-3 promoter activity in human breast cancer cells, thus modulating cytokine activity.

Human growth hormone induces SOCS3 and CIS mRNA increase in the hypothalamic neurons of hypophysectomized rats

The activation of the growth hormone (GH) receptor is followed by activation of the JAK2-STAT system in peripheral tissues, which in turn induces the expression of suppressors of cytokine signaling (SOCS) and/or cytokine-inducible SH2 protein (CIS) to achieve the attenuation of the signaling. To examine whether GH involves the SOCS/CIS system as intracellular negative regulators in the hypothalamus, we observed the effects of human GH on the gene expression of SOCS/CIS in the rat hypothalamus. The mRNAs of CIS, SOCS2, and SOCS3 in the hypothalamus of hypophysectomized male rats were examined by Northern analysis following the intravenous administration of recombinant human GH (hGH), 50 microg/100 g BW. The SOCS3 and CIS mRNAs were increased transiently with maximum expression at 1 h after hGH administration. The intravenous hGH did not induce SOCS2 mRNA expression in the hypothalamus. In situ hybridization demonstrated the increase of SOCS3 and CIS mRNAs in the arcuate nucleus after hGH administration, and the increase of SOCS3 mRNA in the periventricular nucleus. The hGH applied to primary cultured hypothalamic neurons at 500 ng/ml induced transient increase of SOCS3 and CIS mRNAs, but not SOCS2 mRNA. The results show that hGH acts directly on the neurons in the hypothalamus, and increases SOCS3 and CIS mRNAs, suggesting that these negative regulators may be involved in the mechanism that turns off the hGH action in the hypothalamic neurons.

Tyrosine phosphorylation controls Runx2-mediated subnuclear targeting of YAP to repress transcription

Src/Yes tyrosine kinase signaling contributes to the regulation of bone homeostasis and inhibits osteoblast activity. Here we show that the endogenous Yes-associated protein (YAP), a mediator of Src/Yes signaling, interacts with the native Runx2 protein, an osteoblast-related transcription factor, and suppresses Runx2 transcriptional activity in a dose-dependent manner. Runx2, through its PY motif, recruits YAP to subnuclear domains in situ and to the osteocalcin (OC) gene promoter in vivo. Inhibition of Src/Yes kinase blocks tyrosine phosphorylation of YAP and dissociates endogenous Runx2-YAP complexes. Consequently, recruitment of the YAP co-repressor to subnuclear domains is abrogated and expression of the endogenous OC gene is induced. Our results suggest that Src/Yes signals are integrated through organization of Runx2-YAP transcriptional complexes at subnuclear sites to attenuate skeletal gene expression.

Estrogen inhibits GH signaling by suppressing GH-induced JAK2 phosphorylation, an effect mediated by SOCS-2

Oral estrogen administration attenuates the metabolic action of growth hormone (GH) in humans. To investigate the mechanism involved, we studied the effects of estrogen on GH signaling through Janus kinase (JAK)2 and the signal transducers and activators of transcription (STATs) in HEK293 cells stably expressing the GH receptor (293GHR), HuH7 (hepatoma) and T-47D (breast cancer) cells. 293GHR cells were transiently transfected with an estrogen receptor-alpha expression plasmid and luciferase reporters with binding elements for STAT3 and STAT5 or the beta-casein promoter. GH stimulated the reporter activities by four- to sixfold. Cotreatment with 17beta-estradiol (E(2)) resulted in a dose-dependent reduction in the response of all three reporters to GH to a maximum of 49-66% of control at 100 nM (P < 0.05). No reduction was seen when E(2) was added 1-2 h after GH treatment. Similar inhibitory effects were observed in HuH7 and T-47D cells. E(2) suppressed GH-induced JAK2 phosphorylation, an effect attenuated by actinomycin D, suggesting a requirement for gene expression. Next, we investigated the role of the suppressors of cytokine signaling (SOCS) in E(2) inhibition. E(2) increased the mRNA abundance of SOCS-2 but not SOCS-1 and SOCS-3 in HEK293 cells. The inhibitory effect of E(2) was absent in cells lacking SOCS-2 but not in those lacking SOCS-1 and SOCS-3. In conclusion, estrogen inhibits GH signaling, an action mediated by SOCS-2. This paper provides evidence for regulatory interaction between a sex steroid and the GHJAKSTAT pathway, in which SOCS-2 plays a central mechanistic role.

Growth hormone-induced diacylglycerol and ceramide formation via Galpha i3 and Gbeta gamma in GH4 pituitary cells. Potentiation by dopamine-D2 receptor activation

Growth hormone (GH) secretion is regulated by indirect negative feedback mechanisms. To address whether GH has direct actions on pituitary cells, lipid signaling in GH(4)ZR(7) somatomammotroph cells was examined. GH (EC(50) = 5 nm) stimulated diacylglycerol (DAG) and ceramide formation in parallel by over 10-fold within 15 min and persisting for >3 h. GH-induced DAG/ceramide formation was blocked by pertussis toxin (PTX) implicating G(i)/G(o) proteins and was potentiated 1.5-fold by activation of G(i)/G(o)-coupled dopamine-D2S receptors, which had no effect alone. Following PTX pretreatment, only PTX-resistant Galpha(i)3, not Galpha(o) or Galpha(i)2, rescued GH-induced DAG/ceramide signaling. GH-induced DAG/ceramide formation was also blocked in cells expressing Gbetagamma blocker GRK-ct. In GH(4)ZR(7) cells, GH induced phosphorylation of JAK2 and STAT5, which was blocked by PTX and mimicked by ceramide analogue C2-ceramide or sphingomyelinase treatment to increase endogenous ceramide. We conclude that in GH(4) pituitary cells, GH induces formation of DAG/ceramide via a novel Galpha(i)3/Gbetagamma-dependent pathway. This novel pathway suggests a mechanism for autocrine feedback regulation by GH of pituitary function.

Integration of Runx and Smad regulatory signals at transcriptionally active subnuclear sites

Runx factors control lineage commitment and are transcriptional effectors of Smad signaling. Genetic defects in these pathways interfere with normal development. The in situ localization of Runx and Smad proteins must impact the mechanisms by which these proteins function together in gene regulation. We show that the integration of Runx and Smad signals is mediated by in situ interactions at specific foci within the nucleus. Activated Smads are directed to these subnuclear foci only in the presence of Runx proteins. Smad-Runx complexes are associated in situ with the nuclear matrix, and this association requires the intranuclear targeting signal of Runx factors. The convergence of Smad and Runx proteins at these sites supports transcription as reflected by BrUTP labeling and functional cooperativity between the proteins. Thus, Runx-mediated intranuclear targeting of Smads is critical for the integration of two distinct pathways essential for fetal development.

Modeling the influence of growth hormone on lipolysis

Lipolysis (the breakdown of fat) is generally estimated using stable isotopes, where the rate of appearance (Ra) of glycerol is calculated using Steele's equations. These equations are based on single-compartment differential equations for tracer and tracee where rate of change is approximated by the change in concentration from one time point to the next. We demonstrate an alternative approach to estimate metabolic processes, and to determine relationships between hormones and their actions. Growth hormone (GH) or saline was administered in a double-blind randomized crossover design to eight normal weight (NW) and eight obese (OB) subjects, and differences in the effects of GH on lipolysis were investigated. The relationship between the plasma GH concentration and glycerol Ra (as an index of lipolysis) was described using PK/PD modeling. The model incorporated the plasma GH, glycerol and D5-glycerol concentration profiles, and two sequential effect compartments to account for the delay in response. The estimated time-profile of glycerol Ra was compared with estimates obtained using Steele's equations. NONMEM (Version V) FOCE was used for parameter estimation, four differential equations were used, and glycerol and D5-glycerol were estimated simultaneously. The model adequately described both primary variables (glycerol) and derived variables (glycerol Ra as obtained using Steele's equations). Modeling allowed the assessment of potential differences in GH sensitivity in the two groups, and indicated the importance of GH in lipolysis.

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.

Growth hormone receptor ubiquitination, endocytosis, and degradation are independent of signal transduction via Janus kinase 2

The ubiquitin-proteasome system is required in growth hormone receptor (GHR) endocytosis. For cytokine receptors, which lack intrinsic tyrosine kinase activity, signal transduction is initiated by the activation of a member of the Janus kinase (JAK) family. Previously, we have shown that GHR and JAK2 tyrosine (de) phosphorylation are regulated via the ubiquitin system. In this study, we examined the role of JAK2-mediated signal transduction in GHR internalization and down-regulation. Mutation of the attachment site for JAK2, box-1, in the GHR cytoplasmic tail resulted in the complete absence of GHR and JAK2 phosphorylation. This modification did not alter the rate and extent of receptor-bound growth hormone internalization as compared with a functional GHR, nor did it change its turnover and transport to the plasma membrane. In addition, the receptor was still normally ubiquitinated and remained dependent on both an intact ubiquitin system and proteasomal action for its internalization. Thus, GHR ubiquitination, endocytosis, and degradation occur independently of GHR signal transduction via JAK2. We conclude that whereas endocytosis and degradation require the ubiquitin system, they are independent of GHR signal transduction.

Signal transduction pathways in directed migration of human monocytes induced by human growth hormone in vitro

The human growth hormone (GH) was shown to modulate leukocyte functions such as stimulating directed migration of human monocytes in vitro. Dimerisation of GH-receptors leads to the activation of various signalling mechanisms. As transduction of GH signals to monocytes is unknown, we investigated GH signalling mechanisms in monocyte migration using a modified Boyden chamber chemotaxis assay. Inhibition of tyrosyl phosphorylation of GH receptor-associated tyrosine kinase by tyrphostin-23 or staurosporine blocked GH-stimulated monocyte migration down to random levels. Furthermore, pre-incubation with effective concentrations of 4B-phorbol-12-myristate-13-acetate (PMA), staurosporine and bisindolylmaleimide I, inhibitors of protein kinase C, significantly decreased GH-induced migration, suggesting that PKC is involved in the signalling cascade. Additionally, phosphatidylinositol 3-kinase and mitogen-activated protein kinase (MAPK) activation seems to be required. This study revealed signalling pathways in monocyte movement toward GH in vitro.