Development of a homologous radioimmunoassay for mouse growth hormone receptor

MAP dendrimer elicits antibodies for detecting rat and mouse GH-binding proteins

The membrane-bound rat GH-R and an alternatively spliced isoform, the soluble rat GH-BP, are comprised of identical N-terminal GH-binding domains; however, their C-terminal sequences differ. Immunological reagents are needed to distinguish between the two isoforms in order to understand their respective roles in mediating the actions of GH. Accordingly, a tetravalent MAP dendrimer with four identical branches of a C-terminal peptide sequence of the rat GH-BP (GH-BP(263-279)) was synthesized and used as an immunogen in rabbits. Solid-phase peptide synthesis of four GH-BP(263-279) segments onto a tetravalent Lys(2)-Lys-beta-Ala-OH core peptide was carried out using Fmoc chemistry. The mass of the RP-HPLC-purified synthetic product, 8398 Da, determined by ESI-MS, was identical to expected mass. Three anti-rat GH-BP(263-279) MAP antisera, BETO-8039, BETO-8040, and BETO-8041, at dilutions of 10(-3), recognized both the rat GH-BP(263-279) MAP and recombinant mouse GH-BP with ED(50)s within a range of 5-10 fmol, but did not cross-react with BSA in dot blot analyses. BETO-8041 antisera (10(-3) dilution) recognized GH-BPs of rat serum and liver having M(r)s ranging from 35 to 130 kDa, but did not recognize full-length rat GH-Rs. The antisera also detected recombinant mouse GH-BPs. In summary, the tetravalent rat GH-BP(263-279) MAP dendrimer served as an effective immunogenic antigen in eliciting high titer antisera specific for the C-termini of both rat and mouse GH-BPs. The antisera will facilitate studies aimed at improving our understanding of the biology of GH-BPs.

Effect of pregnancy on cytochrome P450 3a and P-glycoprotein expression and activity in the mouse: mechanisms, tissue specificity, and time course

The plasma concentrations of orally administered anti-human immunodeficiency virus protease inhibitors are significantly reduced during human and mouse pregnancy. We have shown that in the mouse, at gestational day 19, this reduction is due to increased hepatic cytochrome P450 3a (Cyp3a) protein expression and activity. In the current study, we investigated the mechanisms by which Cyp3a activity is increased by pregnancy and the time course of change in expression of Cyp3a and P-glycoprotein (P-gp) in various tissues. We found that hepatic transcripts of Cyp3a16, Cyp3a41, and Cyp3a44 were significantly increased during pregnancy, whereas those of Cyp3a11 and Cyp3a25 were significantly decreased. This resulted in a net increase in Cyp3a protein expression and activity in the liver during pregnancy. The increase in Cyp3a41 and Cyp3a44 transcripts was positively correlated (p < 0.05) with hepatocyte nuclear factor 6 and estrogen receptor-alpha transcripts. The pregnancy-related factors that transcriptionally activated mouse Cyp3a isoforms also activated the human CYP3A4 promoter in pregnant CYP3A4-promoter-luciferase transgenic (CYP3A4-tg) mice. In contrast, intestinal Cyp3a protein expression was not significantly affected by pregnancy. No change in P-gp protein expression was observed in the liver or kidney during pregnancy, although a significant decrease was observed in the placenta. Because hepatic CYP3A activity also seems to be induced during human pregnancy, the mouse (including CYP3A4-tg mouse) seems to be an excellent animal model to determine the molecular mechanisms for such an induction.

Desensitization of the JAK2/STAT5 GH signaling pathway associated with increased CIS protein content in liver of pregnant mice

Chronic exposure to growth hormone (GH) was related to the desensitization of the JAK2/STAT5 signaling pathway in liver, as demonstrated in cells, female rats, and transgenic mice overexpressing GH. The cytokine-induced suppressor (CIS) is considered a major mediator of this desensitization. Pregnancy is accompanied by an increment in GH circulating levels, which were reported to be associated with hepatic GH resistance, although the molecular mechanisms involved in this resistance are not clearly elucidated. We thus evaluated the JAK2/STAT5b signaling pathway and its regulation by the suppressors of cytokine signaling (SOCS)/CIS family and the JAK2-interacting protein SH2-Bbeta in pregnant mouse liver, a model with physiological prolonged exposure to high GH levels. Basal tyrosyl phosphorylation levels of JAK2 and STAT5b in pregnant mice were similar to values obtained for virgin animals, in spite of the important increment of GH they exhibit. Moreover, these signaling mediators were not phosphorylated upon GH stimulation in pregnant mice. A 3.3-fold increase of CIS protein content was found for pregnant mice, whereas the abundance of the other SOCS proteins analyzed and SH2-Bbeta did not significantly change compared with virgin animals. The desensitization of the JAK2/STAT5b GH signaling pathway observed in pregnant mice would then be mainly related to increased CIS levels rather than to the other regulatory proteins examined.

Negative regulation of growth hormone receptor signaling

GH has been of significant scientific interest for decades because of its capacity to dramatically change physiological growth parameters. Furthermore, GH interacts with a range of other hormonal pathways and is an established pharmacological agent for which novel therapeutical applications can be foreseen. It is easy to see the requirement for a number of postreceptor mechanisms to regulate and control target tissue sensitivity to this versatile hormone. In recent years, some of the components that take part in the down-regulatory mechanism targeting the activated GH receptor (GHR) have been defined, and the physiological significance of some of these key components has begun to be characterized. Down-regulation of the GHR is achieved through a complex mechanism that involves rapid ubiquitin-dependent endocytosis of the receptor, the action of tyrosine phosphatases, and the degradation by the proteasome. The suppressors of cytokine signaling (SOCS) protein family, particularly SOCS2, plays an important role in regulating GH actions. The aim of this review is to summarize collected knowledge, including very recent findings, regarding the intracellular mechanisms responsible for the GHR signaling down-regulation. Insights into these mechanisms can be of relevance to several aspects of GH research. It can help to understand growth-related disease conditions, to explain GH resistance, and may be used to develop pharmaceuticals that enhance some the beneficial actions of endogenously secreted GH in a tissue-specific manner.

Prolactin prevents acute stress-induced hypocalcemia and ulcerogenesis by acting in the brain of rat

Stress causes hypocalcemia and ulcerogenesis in rats. In rats under stressful conditions, a rapid and transient increase in circulating prolactin (PRL) is observed, and this enhanced PRL induces PRL receptors (PRLR) in the choroid plexus of rat brain. In this study we used restraint stress in water to elucidate the mechanism by which PRLR in the rat brain mediate the protective effect of PRL against stress-induced hypocalcemia and ulcerogenesis. We show that rat PRL acts through the long form of PRLR in the hypothalamus. This is followed by an increase in the long form of PRLR mRNA expression in the choroid plexus of the brain, which provides protection against restraint stress in water-induced hypocalcemia and gastric erosions. We also show that PRL induces the expression of PRLR protein and corticotropin-releasing factor mRNA in the paraventricular nucleus. These results suggest that the PRL levels increase in response to stress, and it moves from the circulation to the cerebrospinal fluid to act on the central nervous system and thereby plays an important role in helping to protect against acute stress-induced hypocalcemia and gastric erosions.

Inability of heterologous growth hormone (GH) to regulate GH binding protein in GH-transgenic swine

Transgenic pigs expressing bovine, ovine, or human growth hormone (GH) structural genes fused to mouse metallothionein-I (mMT-bGH), ovine MT (oMT-oGH), or mouse transferrin (mTf-hGH) promoters were used to study the effects of GH on the regulation of serum GH-binding protein (GHBP). In the 14 transgenic pigs studied, circulating concentrations of heterologous GH ranged from 15 to 2,750 ng/mL. Using chromatographic methods, specific binding of GH was detected in serum from normal pigs but was undetectable in serum from all the transgenic pigs used, probably as a result of the high serum concentrations of heterologous GH present in these animals. Thus, to avoid interference of binding by high GH concentrations, serum samples were subjected to immunoblotting using a specific anti-GHBP antibody. A specific 54-kDa band was detected in normal pig serum as well as in sera from mMT-bGH, oMT-oGH, and mTf-hGH pigs. Additionally, sera from transgenic mMT-bGH pigs and their sibling controls were subjected to immunoprecipitation with an anti-GHBP antibody followed by immunoblotting with the same antibody. With this technique, we detected two specific bands of 53 and 45 kDa that could represent different degrees of glycosylation of GHBP. As determined by densitometric analysis the amount of GHBP in transgenic pig sera was similar to that detected in sera of the respective control animals. The amount of circulating GHBP remained unchanged even in oMT-oGH and mTf-hGH pigs that were exposed from birth to circulating concentrations of GH as high as 2,750 ng/mL. Thus, we conclude that heterologous GH do not act as modulators ofthe serum GHBP in pigs.

Cytokine-inducible SH2 protein up-regulation is associated with desensitization of GH signaling in GHRH-transgenic mice

The effects of continuous high GH levels on GH signal transduction through the GH receptor (GHR)/Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) pathway as well as the desensitization of this pathway by suppressors of cytokine signaling (SOCS) were studied in transgenic mice overexpressing GHRH. In transgenic mice, hepatic GHR levels were 4.5-fold higher than in normal animals, whereas the protein contents of JAK2, STAT5a, and STAT5b did not vary. This same pattern was found for basal tyrosine phosphorylation (PY-): PY-GHR was 4.5-fold increased in transgenic mice, whereas there were no differences in PY-JAK2 and PY-STATs between normal and transgenic animals. After GH administration, tyrosine phosphorylation of GHR, JAK2, and STAT5s increased 3- to 7-fold in normal mice, but no significant changes were found in transgenic mice, indicating a decreased GH sensitivity in these animals. The content of cytokine-inducible SH2 protein, a member of the SOCS family, was 18-fold higher in GHRH-transgenic than in normal mice. Conversely, SOCS-3, present in normal mice, was hardly seen in transgenic animals, whereas SOCS-2 levels did not vary. These findings suggest that cytokine-inducible SH2 protein, significantly induced by continuously elevated GH levels, may be the SOCS protein responsible for the GH signaling desensitization in transgenic animals.

Demonstration of direct effects of growth hormone on neonatal cardiomyocytes

The cellular and molecular basis of growth hormone (GH) actions on the heart remain poorly defined, and it is unclear whether GH effects on the myocardium are direct or mediated at least in part via insulin-like growth factor (IGF-1). Here, we demonstrate that the cultured neonatal cardiomyocyte is not an appropriate model to study the effects of GH because of artifactual loss of GH receptors (GHRs). To circumvent this problem, rat neonatal cardiomyocytes were infected with a recombinant adenovirus expressing the murine GHR. Functional integrity of GHR was suggested by GH-induced activation of the cognate JAK2/STAT5, MAPK, and Akt intracellular pathways in the cells expressing GHR. Although exposure to GH resulted in a significant increase in the size of the cardiomyocyte and increased expression of c-fos, myosin light chain 2, and skeletal alpha-actin mRNAs, there were no significant changes in IGF-1 or atrial natriuretic factor mRNA levels in response to GH stimulation. In this model, GH increased incorporation of leucine, uptake of palmitic acid, and abundance of fatty acid transport protein mRNA. In contrast, GH decreased uptake of 2-deoxy-d-glucose and levels of Glut1 protein. Thus, in isolated rat neonatal cardiomyocytes expressing GHR, GH induces hypertrophy and causes alterations in cellular metabolic profile in the absence of demonstrable changes in IGF-1 mRNA, suggesting that these effects may be independent of IGF-1.

Growth hormone (GH) and estradiol regulation of membrane-associated GH binding protein and GH receptors in GH releasing hormone transgenic mice

Growth hormone (GH) releasing hormone (GHRH) transgenic mice were used to examine the influence of GH on GH receptor (GHR) and membrane-associated GH binding protein (MA-GHBP) levels by means of specific radioimmunoassays and Western blot analysis, since MA-GHBP was described as the major constituent of somatogenic binding to liver membranes in mice. In transgenic animals, a 10-fold increment over normal values was found for hepatic somatogenic binding that could be accounted for by a 3--4-fold increase in GHR and a 9-fold augmentation of MA-GHBP levels. The apparent molecular weight of MA-GHBP was smaller than that of serum GHBP, a difference that was partially abolished by endoglycosidase F digestion. In vivo treatment of female mice with 17 beta-estradiol led to an unexpected down-regulation of MA-GHBP and GHR by 60--75% only in transgenic animals. MA-GHBP and GHR levels are strongly up-regulated by GH, although MA-GHBP up-regulation is much more important than that of GHR.

Growth hormone (GH) and 17beta-estradiol regulation of the expression of mouse GH receptor and GH-binding protein in cultured mouse hepatocytes

In the present study, primary mouse hepatocytes from 8- to 10-week-old virgin female Swiss-Webster mice were perfused with collagenase (100 U/ml) using the two-step method. Isolated hepatocytes were plated in a rat tail type I collagen sandwich configuration to examine the regulation of GH receptor (GHR) and GH-binding protein (GHBP) expression by GH and 17beta-estradiol (E2). After 48 h of initial plating, hepatocytes were divided into groups of five replicates and treated for 24 h with medium containing no hormones (controls), GH (100 ng/ml), E2 (10(-9) M), E2 (10(-9) M) plus GH (100 ng/ml), or E2 plus GH and ICI 182-780 at different concentrations. Treatment of hepatocytes with GH or E2 alone did not have any effect on the cellular concentrations of GHBP and GHR. However, the combination of E2 and GH up-regulated the cellular concentrations of GHBP and GHR 2- to 3-fold. GHBP and GHR messenger RNA concentrations were also up-regulated 2- to 3-fold. ICI 182-780, a competitive inhibitor of E2 for the estrogen receptor (ER), at different concentrations inhibited the E2 and GH-induced stimulation of GHBP and GHR. Furthermore, ER concentrations increased 5- to 7-fold in hepatocytes treated with E2 and GH compared with those in untreated cells or cells treated with either E2 or GH alone. In the present study we have shown that in cultured hepatocytes from virgin female mice, GH or E2 alone did not affect the concentrations of GHBP and GHR. However, E2 and GH together significantly up-regulated GHR and GHBP expression.