Regulation of rat growth hormone receptor gene expression

Cardiac insulin-like growth factor I and growth hormone receptor expression in renal hypertension

The aim of the present study was to investigate the role of insulin-like growth factor I in the development of cardiac hypertrophy in two-kidney, one clip hypertension by relating growth hormone receptor and insulin-like growth factor I receptor mRNA levels to insulin-like growth factor I gene transcription using a solution hybridization/RNase protection assay. Two-kidney, one clip hypertension was induced in male Wistar rats, and experiments were performed 2, 4, 7, and 12 days after surgery. Systolic blood pressure was elevated 2, 7, and 12 days after clipping (P < .001). Left ventricular weights were increased 2, 4, 7, and 12 days after surgery (P < .01). Associated with the rise in blood pressure, left ventricular insulin-like growth factor I mRNA was increased 2, 7, and 12 days after surgery (P < .01). Furthermore, growth hormone receptor and insulin-like growth factor I receptor gene expression increased specifically in the left ventricle of renal hypertensive rats (P < .05 and P < .001, respectively). Left ventricular growth hormone receptor mRNA peaked 7 days after induction of renal artery stenosis. These results show that insulin-like growth factor I, growth hormone receptor, and insulin-like growth factor I receptor mRNA increase in the pressure-overloaded left ventricle of two-kidney, one clip rats, suggesting a role for insulin-like growth factor I and the growth hormone/insulin-like growth factor I axis in the development of cardiac hypertrophy.

Multihormonal control of vitellogenin mRNA expression in the liver of frog, Rana esculenta

In Rana esculenta in an in vitro system, hepatic vitellogenin synthesis can be induced by growth hormone in both sexes. In this study: (1) the ability of this hormone to induce transcription of the VTG gene was determined, and (2) this ability was compared with that of estradiol-17 beta. The results indicate that growth hormone stimulates VTG mRNA transcription both in vivo and in vitro, in both sexes. The levels of mRNA are related to protein levels in the medium. In addition, seasonal variation occurs in the VTG gene transcription under growth hormone and estradiol-17 beta; indeed the more active inducer was growth hormone during the reproductive period and estradiol-17 beta during the preproductive phase.

The role of growth hormone, insulin-like growth factors (IGFs), and IGF-binding proteins in experimental diabetic kidney disease

Early renal changes in type I diabetes are characterized by an increase in renal size, glomerular volume, and kidney function, and later by development of mesangial proliferation, accumulation of glomerular extracellular matrix, and increased urinary albumin excretion (UAE). Growth hormone (GH) and insulin-like growth factors (IGFs) have a long and distinguished history in diabetes mellitus, with possible participation in the development of long-term complications. In experimental diabetes in dwarf rats with isolated GH and IGF-I deficiency, a slower and lesser renal and glomerular hypertrophy is observed as compared with diabetic control animals with intact pituitary. Furthermore, diabetic dwarf rats with a diabetes duration of 6 months display a smaller increase in UAE, indicating that GH and IGF-I may be involved in the development of diabetic kidney changes. In line with this, administration of octreotide to streptozotocin (STZ)-diabetic animals with normal pituitary inhibits initial renal growth without affecting blood glucose levels, and 6 months' administration of octreotide to diabetic rats reduces long-term renal/glomerular hypertrophy and UAE. In addition, the initial increase in renal size and function in experimental diabetes is preceded by an increase in renal IGF-I, IGF-binding proteins (IGFBPs), and IGF-II/mannose-6-phosphate receptor (IGF-II/Man-6-P receptor) concentration. Finally, specific changes occur in renal GH-binding protein (GHBP) mRNA, IGF-I receptor mRNA, and IGFBP mRNA expression in long-term diabetes. In conclusion, the knowledge we have today indicates that GH and IGFs, through a complex system consisting of GHBP, IGFs, IGF receptors, and IGFBPs, may be responsible for both early and late renal changes in experimental diabetes.

The growth hormone (GH)-independent growth of the obese Zucker rat is not due to increased levels of GH receptor messenger RNA in the liver

Obese Zucker rats maintain normal rates of linear growth and circulating concentrations of insulin-like growth factor-I (IGF-I) and of IGF-binding protein-3 (IGFBP-3) in spite of low GH secretion. The mechanisms underlying this GH-independent growth in obesity are unknown. To assess whether the liver expression of the GH receptor (GHR) messenger RNA (mRNA) is increased and/or if the liver expression of IGFBP-3 mRNA is maintained in the obese, Zucker rats of both genders and phenotypes (four groups, n = 6/group) were studied at 12 weeks of age. By Northern analysis, mRNA levels for GHR and GHBP were not increased in obese rats compared to their sex-matched lean littermates; the expression of these two transcripts was sexually dimorphic and the changes in GHBP mRNA/GHR mRNA ratios associated with obesity were sex-specific. In both genders, IGFBP-1 and IGFBP-3 mRNAs were decreased in the obese. We concluded that the GH-independent growth of obese Zucker rats is not due to increased GHR mRNA or to maintained IGFBP-3 mRNA levels in the liver.

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.

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.

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.

Effects of long-term ovariectomy and ovarian steroids on somatogenic binding sites in rat brain and liver

The effects of long-term ovariectomy and replacement with ovarian steroids on the levels of brain and liver somatogenic binding sites as well as plasma and liver growth hormone (GH) were studied in sham-operated (Sham) and ovariectomized female rats receiving either, 17 beta-estradiol (OVX-E), progesterone (OVX-P), or vehicle (OVX). Long-term ovariectomy decreased the levels of somatogenic binding sites in the choroid plexus and liver as well as GH in plasma and liver. The levels of these sites in the choroid plexus were partially restored only by estradiol replacement. Moreover, exogenous estradiol but not progesterone restored the levels of plasma and liver GH as well as liver somatogenic binding sites. Our results suggest that estrogens regulate the levels of somatogenic binding sites in the liver and choroid plexus.

Growth hormone, interferon-gamma, and leukemia inhibitory factor promoted tyrosyl phosphorylation of insulin receptor substrate-1

The identification of JAK2 as a growth hormone (GH) receptor-associated, GH-activated tyrosine kinase has established tyrosyl phosphorylation as a signaling mechanism for GH. In the present study, GH is shown to stimulate tyrosyl phosphorylation of insulin receptor substrate 1 (IRS-1), the principle substrate of the insulin receptor. Tyrosyl phosphorylation of IRS-1 is a critical step in insulin signaling and provides binding sites for proteins with the appropriate Src homology 2 domains, including the 85-kDa regulatory subunit of phosphatidylinositol (PI) 3'-kinase. In 3T3-F442A fibroblasts, GH-dependent tyrosyl phosphorylation of IRS-1 was detected by 1 min and at GH concentrations as low as 5 ng/ml (0.23 nM). Tyrosyl phosphorylation of IRS-1 was transient, with maximal stimulation detected at 30 min and diminished signal detected at 60 min. The ability of GH receptor (GHR) to transduce the signal for IRS-1 tyrosyl phosphorylation is mediated by the intracellular region of GHR between amino acids 295 and 380 by a mechanism not involving the two tyrosines in this region. This region of GHR is required for GH-dependent JAK2 association and activation (VanderKuur, J. A., Wang, X., Zhang, L., Campbell, G. S., Allevato, G., Billestrup, N., Norstedt, G., and Carter-Su, C. (1994) J. Biol. Chem. 269, 21709-21717). When other cytokines that activate JAK2 were tested for the ability to stimulate the tyrosyl phosphorylation of IRS-1, stimulation was detected with interferon-gamma and leukemia inhibitory factor. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH, interferon-gamma, and leukemia inhibitory factor and in cells expressing different GHR mutants, provides evidence that IRS-1 may interact with JAK2 or an auxiliary molecule that binds to JAK2. GH is also shown to stimulate binding of IRS-1 to the 85-kDa regulatory subunit of PI 3'-kinase. The ability of GH to stimulate tyrosyl phosphorylation of IRS-1 and its association with PI 3'-kinase provides a biochemical basis for responses shared by insulin and GH including the well characterized insulin-like metabolic effects of GH observed in a variety of cell types.

Intermittent plasma growth hormone triggers tyrosine phosphorylation and nuclear translocation of a liver-expressed, Stat 5-related DNA binding protein. Proposed role as an intracellular regulator of male-specific liver gene transcription

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription that are regulated by the temporal pattern of pituitary GH release, which is intermittent in male rats and nearly continuous in females. To investigate the influence of these GH secretory patterns on intracellular hepatocyte signaling, we compared the pattern of liver nuclear protein tyrosine phosphorylation in male and female rats. An M(r) approximately 93,000 polypeptide, p93, was found to be tyrosine phosphorylated to a high level in male but not female rats. GH, but not prolactin, rapidly stimulated p93 tyrosine phosphorylation in hypophysectomized rats. Intermittent plasma GH pulses triggered repeated p93 phosphorylation, while continuous GH exposure led to desensitization and a dramatic decline in liver nuclear p93. p93 was cross-reactive with two monoclonal antibodies raised to mammary Stat 5, whose tyrosine phosphorylation is stimulated by prolactin. Intermittent GH pulsation translocated liver Stat 5/p93 protein from the cytosol to the nucleus and also activated its DNA binding activity, as demonstrated using a mammary Stat 5-binding DNA element derived from the beta-casein gene. p93 is thus a liver-expressed, Stat 5-related DNA binding protein that undergoes tyrosine phosphorylation and nuclear translocation in response to intermittent plasma GH stimulation and is proposed to be an intracellular mediator of the stimulatory effects of GH pulses on male-specific liver gene expression.

Evidence of a direct role for growth hormone (GH) in mammary gland proliferation and lactation

A panel of monoclonal antibodies to the growth hormone (GH) receptor/binding protein was used to demonstrate the existence and detail the expression of GH receptors in ductal and alveolar epithelial cells from rat and rabbit mammary glands by immunohistochemistry. Intense immunoreactivity was present in membrane, cytoplasm and some nuclei of epithelial cells during proliferation and lactation. Receptor expression decreased during weaning and was absent or weak in regressive mammary glands. Immunoreactivity was weak in ductal epithelial cells from virgin adult animals. Pronounced expression of GH receptor/binding protein was observed with two monoclonal antibodies and lesser reactivity was seen with others, paralleling their affinities for the receptor. The cytoplasmic presence of this putatively plasma membrane located GH receptor is accounted for by the existence of a soluble form on the GH receptor, namely the growth hormone binding protein derived from the membrane receptor by cleavage. Primary localization of the receptor in proliferating and lactating epithelial cells suggests that the rat and rabbit mammary gland is a GH target tissue. This finding is in contradiction to both classical GH action and the somatomedin hypothesis and challenges the widely held view that GH has no direct influence on mammary growth and function.

Growth hormone signaling leading to CYP2C12 gene expression in rat hepatocytes involves phospholipase A2

The expression of CYP2C12 is liver-specific and regulated at the transcriptional level by growth hormone (GH). In attempts to elucidate the nature of signaling molecules mediating the GH regulation of this gene in rat hepatocytes, a role for phospholipase A2 (PLA2) as a transducer of GH-induced levels of P4502C12 mRNA was investigated. GH was shown to induce tyrosyl-phosphorylation of p42 and p44 microtubule-associated protein (MAP) kinases and to reduce the electrophoretic mobility of a 100-kDa protein, immunologically related to cPLA2. These events were observed in parallel with GH-stimulated release of [3H]arachidonic acid ([3H]AA) from cellular phospholipids of rat hepatocytes labeled with [3H]AA. These rapid effects of GH action, as well as the GH-induced expression of CYP2C12, were inhibited in cells treated with the tyrosine kinase inhibitor herbimycin A. Similarly, when the GH-induced liberation of [3H]AA was blocked by the PLA2 inhibitor mepacrine or the Ca2+ channel blocker verapamil, GH-induced accumulation of P4502C12 mRNA was absent. These results suggest a correlation between PLA2 activity and GH regulation of the CYP2C12 gene. The inhibitory effect of mepacrine on GH induction of P4502C12 mRNA was reversed by AA addition, further supporting a role for eicosanoids in the regulation of CYP2C12. Finally, inhibitors of P450-mediated AA metabolism, SKF-525A and ketoconazole as well as eicosatetraynoic acid, blocked the GH-mediated induction of P4502C12 mRNA, whereas more specific inhibitors of cyclooxygenase or lipoxygenase metabolism did not. Based on these results, we suggest that GH signaling in rat hepatocytes, leading to increased expression of CYP2C12, involves PLA2 activation and subsequent P450-catalyzed formation of an active AA metabolite.

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.

Responsiveness of an SV40-immortalized hepatocyte cell line to growth hormone

The response of an SV40-immortalized hepatocyte cell line (CWSV-1) derived from adult male rat hepatocytes to human growth hormone (hGH) was investigated. CWSV-1 cells, which have been characterized extensively, retain certain differentiated functions of normal liver (Woodworth and Isom, Mol Cell Biol 7: 3740-3748, 1987). This cell line consists of tightly associated polygonal, mononucleated cells that grow as monolayers. These cells showed no significant morphological changes with the addition of hGH. Northern blot analysis showed that continuous treatment of the CWSV-1 cells with hGH induced the expression of insulin-like growth factor I (IGF-I) and 5 alpha-reductase RNAs. In addition, continuous exposure to hGH resulted in the induction of expression of the growth hormone receptor/growth hormone binding protein (GHR/GHBP) genes. This study indicates that the CWSV-1 cells may serve as a valuable in vitro model system for studying the signaling pathway of GH.

Cloning of the promoter-regulatory region of the murine growth hormone receptor gene. Identification of a developmentally regulated enhancer element

The growth hormone (GH) receptor is essential for the actions of GH on postnatal growth and metabolism. To identify DNA sequences involved in the regulation of transcription of the murine GH receptor gene, a 17-kilobase genomic clone containing the 5'-flanking region, exon 1, and part of intron 1 of the murine GH receptor gene was isolated. Utilizing primer extension and ribonuclease protection assays, two major transcription start sites were identified in RNA from liver of male, female, and pregnant mice. Transient transfection studies using a reporter gene demonstrated promoter activity in a variety of eukaryotic cells. Deletional analysis and DNA-protein binding assays led to the identification of a 30-base pair enhancer element located about 3.4 kilobases upstream of the transcription start sites. Computer analysis of the nucleotide sequence of the enhancer element did not reveal any potential DNA binding motifs for known transcription factors, and this DNA element failed to exhibit binding activity for some common transcription factors. Analysis of both functional activity and DNA-protein binding activity of this enhancer element in adult and fetal hepatocytes suggests that this DNA element may play a role in the developmental expression of the GH receptor gene.

Rat growth hormone receptor/growth hormone-binding protein mRNAs with divergent 5'-untranslated regions are expressed in a tissue-specific manner

In the rat, the growth hormone receptor (GH-R) gene generates two transcripts, one encoding the transmembrane GH-R, and a shorter one encoding the GH-binding protein (GH-BP). These transcripts exhibit a high degree of heterogeneity in their 5'-untranslated regions (5'-UTRs). Some of the exons encoding these 5'-UTR variants may be flanked by distinct promoter regions whose activity would result in the tissue-specific expression of the GH-R gene. To assess this possibility, we used single-sided polymerase chain reaction (PCR) amplification to characterize 5'-UTR variants in rat GH-R cDNAs, and by using 5'-UTR-specific probes, we determined their pattern of expression in several tissues. Besides two previously described variants (V1 and V2), three new 5'-UTR variants were identified, extending 56 nucleotides (V3), 135 nucleotides (V4), and 209 nucleotides (V5) upstream of the ATG translation initiation codon. The expression of GH-R and GH-BP transcripts was clearly tissue specific. In the liver, GH-BP mRNA was the predominant transcript, whereas in other tissues, there was equivalent expression of both transcripts or predominant expression of GH-R mRNA. With respect to the tissue distribution of the 5'-UTR variants in particular, variants V1 and V5 exhibited a pattern of expression closely resembling that seen with an exon 2 probe, with the overall expression of variant V1 being much higher than that of variant V5. The V2 species was exclusively expressed in liver. Variant V3 was expressed at low levels in liver, muscle, heart, and kidney; in muscle and heart, it was preferentially associated with GH-BP transcripts. Variant V4, although present in liver, was more abundant in extrahepatic tissues and predominantly found in GH-R mRNA transcripts. Southern blot analyses were consistent with exon 2 and the exons encoding the V1 and V2 sequences being in proximity, with the other 5'-UTR sequences being encoded by exons located further upstream of exon 2. These findings support the concept that different 5'-UTR variants are the result of the different promoters acting in a tissue-specific manner. The association of specific 5'-UTR variants with either GH-R or GH-BP transcripts raises the possibility that the alternative splicing process that generates GH-BP mRNA in the rat might be controlled by the 5'-flanking region regulating the expression of specific leader exons.

Localization of growth hormone receptor messenger RNA in human tissues

In order to identify GH target cells in human tissues, we investigated the celiuiar distribution of human GH receptor (GHR) messenger RNA. This was performed byin situ hybridization and Northern blot hybridization using complementary radioactive DNA probe encoding part of the extracellular domain of the GHR. Several tests were carried out to validate the detection of gene expression. Our results demonstrate that the GHR gene is expressed in all human tissues studied, including liver, muscle, kidney, heart, skin, thymus, adipose tissue, placenta, testis, ovary and mammary gland. The quantification of thein situ hybridization signal by densitometric analysis showed a greater expression of GHR transcript in liver, muscle, kidney, heart, and skin epidermis, and a reduced expression in thymus, mammary gland, testis, ovary, and dermis/ hypodermis. These results confirm the widespread distribution of the GHR gene expression in human tissues and identify the cells that might be responsive to GH.

Expression and regulation of growth hormone (GH) and prolactin (PRL) receptors in a rat insulin producing cell line (INS-1)

We have examined the expression of growth hormone (GH) and prolactin (PRL) receptors, and the binding parameters of human GH (hGH), on the rat insulin producing cell line INS-1. We found that, like normal insulin producing beta-cells, INS-1 cells express both GH and PRL receptors, and the majority of human GH (hGH) binding sites on this cell line are of lactogenic specificity. As calculated from Scatchard plots, about 6600 hGH binding sites with a Ka of 6.2 x 10(8) M-1 are present per cell. Northern blot analysis showed two mRNA species of 4 and 1.6 kb for the GH receptor and, one major species of 10.5 kb for the PRL receptor. The PRL receptor mRNA was up-regulated by bovine GH (bGH), rat PRL (rPRL), in a time- and dose-dependent manner. On the contrary, bGH and rPRL, down-regulated the expression of GH receptor gene. The importance of this differential regulation in in vivo and in vitro studies are discussed.

Endocrine, paracrine and autocrine regulation of testicular steroidogenesis

Testicular steroidogenesis takes place almost exclusively in Leydig cells. Some metabolism of the androgens produced by Leydig cells takes place in seminiferous tubules, especially in the immature animal (e.g. aromatization and 5 alpha- reduction). Luteinizing hormone (LH) is the main tropic regulator of Leydig cell function, without which quantitatively important androgen production is not possible. LH acts through a receptor that belongs to the seven times cell membrane spanning, G protein associated, receptor family, and cyclic AMP is the main second messenger of its signal transduction. Information about the involvement of other signal transduction systems in LH action has also emerged recently. The action of LH is under manyfold modulation by other hormones (e.g. prolactin, growth hormone and insulin), growth factors and bioactive peptides. In this modulation, various paracrine and autocrine mechanisms play an important role. Seminiferous tubules influence the development and function of adjacent Leydig cells through several growth factors. When germ cells are damaged, Leydig cells in the vicinity proliferate faster. Leydig cell morphology also depends on the germ cell composition in the neighbouring seminiferous tubules, and certain stages of the seminiferous epithelial cycle increase the Leydig cell capacity to produce testosterone. Also negative modulation of Leydig cells by Sertoli/germinal cell derived factors has been demonstrated. However, the physiological importance of the paracrine and modulatory influences of the different hormones and growth factors still remains obscure since almost all information has so far been obtained from in vitro studies. In the study of testicular steroidogenesis, the main switch of the function, LH action, is well known whereas the role of the "in house" circuits of paracrine and autocrine regulation remain to be elucidated.

Characterization of growth hormone binding sites in rat brain

The binding of 125I-labelled rat growth hormone (GH) to different areas in the brain was studied in male Sprague-Dawley rats. A high density of GH binding was found in the choroid plexus, hypothalamus, hippocampus, pituitary and spinal cord, whereas a lower binding density was observed in the cortex. Binding of the hormone to the various brain regions was age dependent. Binding was also dependent on time, pH and protein concentration. The binding affinity of the labelled hormone to choroid plexus was 4.3 per nmol/l and the binding capacity was 33.4 nmol/mg protein. The corresponding figures for binding of 125I-labelled GH to hypothalamus were 5.6 per nmol/l and 21.6 nmol/mg protein. By sodium dodecyl sulphate electrophoresis of the cross-linked hormone-receptor complexes, molecular weights of 60,000 and 61,000 were determined for the binding units in the choroid plexus and hypothalamus, respectively. It was further indicated that the binding unit for rat GH was distinct from that for prolactin.

In vitro mutagenesis of growth hormone receptor Asn-linked glycosylation sites

Site-directed mutagenesis was used to replace asparagine (Asn) residues with glutamine (Gln) at the five potential N-linked glycosylation sites located at positions 28, 97, 138, 143, and 182 in the extracellular domain of the porcine growth hormone receptor (pGHR). These mutated pGHR cDNAs were stably expressed in mouse L cells. Single substitution of the Asn residues did not alter growth hormone binding when compared to cells which express native pGHR (KD approximately 1 nM). However, substitution of the five potential Asn-linked sites together (pGHR delta 5) resulted in a 20-fold reduced GH binding affinity (KD = 20 nM). Residues Asn97, Asn138, and Asn182 were apparently glycosylated and upon cross-linking with 125I-labeled pGH migrated as a molecular complex of approximately 130 kDa. Native pGHR and pGHR analogs with substitutions of N28Q and N143Q when cross-linked to 125I-labeled pGH, migrated with a Mr of 138 kDa. The fully deglycosylated cross-linked receptor, pGHR delta 5, migrated as a complex of 108 kDa. Therefore, each carbohydrate moiety contributed approximately 10 kDa to the total molecular mass of the pGHR, in sum contributing 30 kDa to the total Mr of the glycosylated pGHR. pGHR delta 5 was able to internalize nearly all the bound 125I-labeled pGH within 10 min, whereas native pGHR and individual Asn substituted pGHR analogs internalized 25% of bound 125I-labeled pGH at 10 min. Also, mutagenesis of the pGHR five potential Asn-linked glycosylation sites, either singly or together, did not alter the ability of GH to induce tyrosine phosphorylation of a 95-kDa protein. Together, the results indicate that three of the five pGHR Asn residues are apparently glycosylated and are necessary for maintenance of a high affinity GH binding site and for GH internalization. However, glycosylation of the pGHR is not critical for eliciting tyrosine phosphorylated proteins following the GH/GHR interaction.

Reduced food intake is the main cause of low growth hormone receptor expression in uremic rats

To examine the respective effects of reduced food intake and of uremia on the growth defect in uremic rats, we have studied the expression of GH receptors in three groups of male rats: Group 1, rats fed ad libitum; Group 2, food-restricted to be pair-fed with uremic rats; Group 3, uremic rats. Animals were studied for a time period of 9 days starting 1 week after surgery (sham operation in rats of Groups 1 and 2, 5/6 nephrectomy in rats of Group 3). The gain in body length and weight of pair-fed controls and of uremic rats was comparable and significantly lower than that of rats fed ad libitum. IGF-1 plasma levels were low in rats of groups 2 and 3. Low food intake (50% that of rats fed ad libitum) resulted in a reduced number of GH receptors in liver membranes and a low plasma level of GH-binding protein (GHBP); GH receptor gene expression in the liver, as analyzed by Northern blots, was not significantly lower in normal food-restricted animals. In uremic rats, the low level of GH binding to liver membranes was comparable to that found in pair-fed controls; but the level of GHBP activity was normal, not different from the values found in rats fed ad libitum. However, expression of the liver GHBP mRNA was reduced in uremic rats. In uremia, the GH receptor dysfunction is not only at a transcriptional level but also at a post-transcriptional level. These findings suggest that uremia, as such, is not primarily responsible for the growth failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of GH and IGF-I in the regulation of IGF-I, IGF-I receptor and IGF binding protein gene expression in the rat spleen

To characterize the expression of the IGF-I system in the spleen and its role in spleen growth, we have studied the effect of hypophysectomy and the action of either GH or IGF-I treatment on the expression of several components of the IGF system in the rat. Female Sprague-Dawley rats were hypophysectomized (Hx) on postnatal day 50, and five animals each received twice-daily sc injections of saline, bovine GH (bGH; 84 micrograms/animal/day), or recombinant human IGF-I (rhIGF-I; 125 micrograms/animal/day) for 11 days. Compared to sham-operated controls, Hx animals exhibited a reduction in both body (192.6 +/- 5.6 g (mean +/- S.E.M.) vs. 268.6 +/- 6.0 g; P < 0.001) and spleen weights (0.42 +/- 0.03 g vs. 0.84 +/- 0.06 g; P < 0.001). The reduction in body and spleen weights in Hx animals was partially prevented by both bGH and rhIGF-I. Body weights were 234.2 +/- 5.3 g (P < 0.001) after bGH and 213.8 +/- 6.3 g (P < 0.05) after rhIGF-I. Spleen weights were 0.56 +/- 0.048 after bGH P < 0.01 and 0.53 +/- 0.05 g after rhIGF-I (P < 0.05). Serum GH and IGF-I levels were markedly reduced in Hx animals and bGH partially maintained IGF-I levels. Hypophysectomy reduced spleen IGF-I mRNA levels (30.6 +/- 7.5% of control values; P < 0.05) and this reduction was prevented by bGH (96.6 +/- 24.2%; NS) but not by rhIGF-I (39.9 +/- 5.0% NS vs. Hx). There were no changes in GH receptor or IGF-I receptor mRNA levels in Hx or bGH or rhIGF-I-treated animals. When IGF-I binding protein (IGFBP) mRNA levels were studied under these conditions, we found that IGFBP-1 mRNA was not detected in spleen; IGFBP-2 mRNA levels were reduced in Hx rats (67.9 +/- 7.4% of control values, P < 0.05) and bGH treatment prevented this reduction (95.5 +/- 12.2%, NS). IGFBP-3 mRNA levels were not affected by hypophysectomy or by bGH treatment, but were reduced in rhIGF-treated rats (69.6 +/- 3.0%, P < 0.05). On the other hand, IGFBP-4 mRNA levels were increased in Hx rats (136.4 +/- 15.9% of control values, P < 0.05) and bGH treatment prevented this increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired cardiac performance in GH-deficient adults and its improvement after GH replacement

Cardiac performance was investigated by radionuclide angiography in 11 patients with childhood-onset growth hormone (GH) deficiency and in 12 control subjects. Both at rest and during maximal physical exercise, systolic function was markedly depressed in GH-deficient patients. Ejection fraction rose from 66 +/- 6 to 76 +/- 7% during exercise in control subjects, whereas in GH-deficient patients it remained unchanged or even decreased (55 +/- 6 and 54 +/- 9% at rest and after exercise, respectively; P < 0.01 vs. controls). Cardiac index was significantly lower in GH-deficient patients than in controls, both at rest (2.7 +/- 0.6 vs. 3.7 +/- 0.5 l.min-1.m-2; P < 0.001) and during exercise (8 +/- 1.2 vs. 10 +/- 1.5 l.min-1.m-2; P < 0.01). Five GH-deficient patients were treated with recombinant human (rh) GH for 6 mo at a dose of 0.05 IU.kg-1.day-1. Cardiac index at rest improved from 2.8 +/- 0.6 to 3.3 +/- 0.8 l.min-1.m-2 (P < 0.01) after rhGH. Also, cardiac index response to exercise improved markedly and became similar to that of controls (7.5 +/- 1.2 and 10.1 +/- 1.1 l.min-1.m-2 before and after rhGH, respectively; P < 0.005). Exercise tolerance was impaired in GH-deficient patients and was restored by rhGH treatment. The data support the hypothesis that GH plays an important role in the maintenance of a normal cardiac performance in humans.

Estradiol inhibits growth hormone receptor gene expression in rabbit liver

We studied the ontogeny of GH receptor mRNA levels and the effect of exogenous estradiol administration on GH receptor mRNA levels in rabbit liver. A solution hybridization-RNase protection assay revealed a predominant 370-base long protected band corresponding to the mRNA encoding the transmembrane GH receptor, and a 241-base long protected band, representing about 9.0%, with the predicted size for the truncated form of the GH receptor. To study the developmental profile of GH receptor expression, we studied 12 female rabbits, at ages 1, 3, 5 and 7 months. Maximal GH receptor mRNA levels were observed in 3-month-old animals and decreased in 7-month-old animals. To investigate the effect of estradiol, 8-week-old immature female rabbits were randomly divided into five groups, and received subcutaneous pellets containing either placebo or estradiol at doses of 0.1, 0.5, 1.5 and 5.0 mg for 3 weeks. Exogenous administration of estradiol, at doses that resulted in physiological circulating levels, induced a reduction in GH receptor expression, measured both by GH binding (36 and 46%), and GH receptor mRNA levels (38 and 87%), in animals receiving pellets containing 1.5 and 5.0 mg of estradiol, respectively. We conclude that estradiol decreases GH receptor expression in rabbit liver. The results of our study suggest that there is an inverse relationship between circulating estrogen concentrations and liver GH receptor expression.

Identification of a liver-specific promoter for the ovine growth hormone receptor

Growth hormone (GH) receptor cDNA clones from several species are characterized by heterogeneity in the 5' untranslated region (5'UT). This has been attributed to different promoters directing the expression of the gene from exons encoding 5'UT's which are alternatively spliced onto a common splice acceptor 11 basepairs (bp) upstream of the initiating AUG on exon 2. The following study identifies exon 1A of the ovine (o) GH receptor gene, corresponding to the 5'UT of a developmentally regulated, liver-specific transcript. Exon 1A spans 206 bp at a position 17 kilobases (kb) upstream of exon 2. Sequencing of the 669 bp region 5' to the transcription initiation site (+1) reveals a TATA box at -31, a CCAAT box at -88, and putative binding sites for several transcription factors involved in liver-specific gene expression. Two repetitive sequence elements are located in the 5' and 3' flanking regions of exon 1A. Functional analysis of the 4.5 kb region upstream of exon 1A was performed by transfecting the human hepatoma cell line HuH7 with luciferase reporter gene constructs. Positive and negative regulatory regions are identified, with basal promoter activity within 473 bp of the transcription initiation site. A 47 bp region containing putative binding sites for the activated glucocorticoid receptor and C/EBP-like proteins, between -180 and -133, is essential for transcriptional activation.

The Hep G2 cell line in the study of growth hormone receptor/binding protein

This study identifies specific, high affinity GH-receptors (GH-R) in human hepatoma Hep G2 cells. The binding characteristics of GH-R in the Hep G2 cells are similar to those of human liver membranes, such as the high specificity for hGH, the binding affinity (Ka = 1.7 +/- 0.5 x 10[9] M[-1]) and the molecular weight of the membrane bound GH-R (apparent 125,000 and 71,000). In addition, lower molecular weight forms (approximately 94,000 and approximately 58,000) were identified as GH-binding protein (GH-BP) in Hep G2 conditioned medium, or following incubation of Hep G2 cells, in the presence of 10 mM N-ethylmaleimide for 90 min at 30 degrees C; the latter are presumed to be shed by a proteolytic cleavage of the GH-R. Exposure of Hep G2 cells to physiologic concentrations of hGH resulted in a concentration-dependent increase in 3H-thymidine incorporation, up to 48.4 +/- 7.9% above control. In summary, the demonstration of specific, high affinity GH-R in Hep G2 cells, as well as shedding of GH-BP, suggest these cells may provide a homologous human system to study the receptor-effector interrelationship of hGH and to further our understanding of hepatocyte production of soluble GH-BP.

Reduced hepatic growth hormone (GH) receptor gene expression and increased plasma GH binding protein in experimental uremia

In uremia, reduced longitudinal growth and decreased hepatic insulin-like growth factor-I (IGF-I) secretion despite elevated GH serum levels point to an insensitivity to the action of GH. The molecular basis that accounts for this insensitivity could comprise decreased GH receptor expression in the target organs for GH or binding of GH in the circulation to substances that compete with the receptor. To address this hypothesis, the abundance of hepatic GH receptor mRNA was measured by solution hybridization RNase protection assay in uremic female Sprague-Dawley rats, following two-stage 5/6 nephrectomy, and in pair-fed and in ad libitum-fed sham-operated controls; rat GH binding protein (GHBP) plasma concentration was measured by a sensitive direct RIA. Uremia was associated with a 50% decrease of hepatic GH receptor expression compared to pair-fed controls, which themselves showed a 25% reduction of hepatic GH receptor mRNA abundance when compared to ad libitum-fed controls. Plasma GHBP levels in uremia were markedly higher than in both control groups. Treatment with recombinant human GH (rhGH) (10 IU/kg body wt per day s.c. for 10 days) led to a comparable induction of IGF-I plasma levels and weight gain in uremia and pair-fed controls, indicating that the insensitivity to GH in uremia can be overcome by large rhGH doses. Subcutaneous rhGH injections did not significantly alter the hepatic GH receptor transcript abundance or plasma GHBP levels in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoreactive growth hormone receptor/binding protein is present on fibroblasts and in serum of Laron-type dwarfs

Laron-type dwarfism is an autosomal recessive disorder characterised by extreme growth retardation and growth hormone (GH) resistance and has been shown in some cases to be associated with mutations in the GH receptor gene. Limited data suggest that in this condition specific liver GH binding is absent. In the majority of reported cases specific GH binding is also absent in serum. However it is not known whether the GH receptor and/or the serum GH binding protein are expressed in this condition. Using the techniques of immunohistochemistry and Northern blotting we have demonstrated that in cultured skin fibroblasts derived from four patients with Laron-type dwarfism the GH receptor gene is transcribed and the GH receptor protein is expressed on the cell surface. Further study of one of these patients, who has not previously been reported, has also revealed low but detectable levels of GH binding protein in serum using a two-site immunradiometric assay which does not depend on GH binding. These results indicate that the growth hormone receptor/binding protein is expressed in Laron-type dwarfism.

Growth hormone regulation of lipid metabolism in cells transfected with growth hormone receptor cDNA

The functional properties of the growth hormone (GH) receptor was studied using cellular transfection of GH receptor cDNA. GH treatment (1.5-2 h) of Chinese hamster ovary cells, stably transfected with GH receptor cDNA (CHO4), resulted in increased cellular lipid synthesis (240% of control). This effect was blocked by staurosporine, suggesting a dependence on cellular kinases. However, if GH treatment of CHO4 cells was prolonged (16 h), this instead stimulated lipolysis (128% of control). The GH receptor in CHO4 cells was also shown to be functional in terms of ligand internalization. A GH receptor mutant, in which 183 amino acids had been deleted in the carboxyterminal of the intracellular domain was functionally active, while a receptor without its intracellular domain was shown to be inactive. In conclusion, GH receptors expressed in CHO cells are functional and GH was also shown to have both an acute insulin-like effect, which was kinase dependent, and a long-term anti-insulin-like effect on the lipid metabolism. This suggests that an approach using GH receptor cDNA transfected cells can be of value in understanding the mechanism of GH action.

Effects of rhGH and rhIGF-1 on renal growth and morphology

It is known that in rodents recombinant human growth hormone (rhGH) and recombinant human insulin-like growth factor (rhIGF-1) increase renal mass. It is uncertain, however, whether renal mass increases in proportion to body growth, or whether renal growth is stimulated selectively. In 120 to 150 g female Sprague-Dawley rats, we measured the effects of rhGH and rhIGF-1 and their combination by the following parameters: kidney weight/body weight ratio, DNA/protein ratio, mRNA of GH receptor and of IGF-1, mitosis index and PCNA (by immunohistology), zonal architecture and glomerular diameter by micromorphometry. Both rhGH and rhIGF-1 dose-dependently increased renal weight and body weight over vehicle treated controls. With rhGH, liver dry weight/body weight ratio increased, but kidney dry weight/body weight ratio remained unchanged (0.99 +/- 0.06 x 10(-3) vs. 1.02 +/- 0.07 in vehicle controls). In contrast, a significant increase of kidney dry weight/body weight ratio was seen in rats treated with rhIGF-1 (1.3 +/- 0.21 x 10(-3). Addition of high doses of rhGH to high doses of rhIGF-1 caused no further increase of the ratio despite a significant further increase of body weight. rhGH increased the abundance of renal GH receptor mRNA (0.46 +/- 0.32 amol/microgram DNA vs. 0.08 +/- 0.07 in controls) and of IGF-1 mRNA (1.35 +/- 0.5 pg/micrograms DNA vs. 0.35 +/- 0.17), whereas no change was seen with IGF-1 treatment. rhGH and rhIGF-1 increased kidney DNA/protein ratio, mitoses and PCNA expression in various renal structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors affecting circulating growth hormone binding protein in chickens

Growth hormone binding protein (GHBP) may be an important factor in the regulation of growth and might provide an indirect, relatively noninvasive means of predicting the status of hepatic growth hormone receptor (GHR) activity. Several factors have been reported to influence growth hormone (GH), GHR, or GHBP. Therefore, these studies were conducted to test how age, sex, nutritional status, and glucocorticoids (cortisone acetate, CA) influence serum concentrations of chicken GHBP. Serum GHBP activity was highest (mean percentage specific binding (%SB) = 12.43 +/- .80) at hatch and decreased linearly (P < .0001) to 5 wk of age (%SB = 1.99 +/- 1.13). There were no sex-related differences in serum GHBP activity from hatch to 5 wk of age (P > .08). Short-term nutrient deprivation (24-h) of 4-wk-old broilers also had a significant effect on serum GHBP activity (P < .0001). Measurement of serum GHBP activity with refeeding (after a 24-h period without feed) restored %SB to normal values. Feeding broilers a low-protein diet (12% CP) did not significantly affect serum GHBP activity when compared with that of broilers fed a commercial broiler diet (23% CP; P > .30). Administration of cortisone (1, 5, and 10 mg/day), every 24 h for 7 days, had no effect, at any dose, on serum GHBP activity at 48 h and 1 wk after the last injection. These results indicate that serum GHBP activity is influenced by factors such as age and feed deprivation. It remains to be determined whether these changes in GHBP are associated with changes in GHR as reported for mammalian species.

Growth hormone-receptor messenger RNA in the rat ovary: regulation and localization

The results of several reports indicate that GH can modulate ovarian function. In the present study, the expression of the growth hormone-receptor (GH-R) mRNA was studied in the rat ovary using an RNA probe corresponding to a part of the extracellular domain of the GH-R. The probe hybridized to two major transcripts with estimated sizes of 4.0 kb and 1.2 kb in RNA extracted from liver and ovary. Recently, these transcripts have been shown to encode the GH-R and the GH-binding protein (GH-BP). The ontogeny of the GH-R/GH-BP mRNA expression was studied using Northern blot analysis and a solution hybridization RNase protection assay. In the liver GH-R/GH-BP mRNA levels increased with age, while in the ovary, the levels decreased between 1 and 5 weeks of age. Hypophysectomy caused a decrease in GH-R/GH-BP gene expression in the ovary, an effect which could be partly reversed with a single injection of GH (2 mg/kg). No significant changes in the ovarian concentration of GH-R/GH-BP transcripts were seen during the estrus cycle. Using in situ hybridization GH-R/GH-BP transcripts were found to be most abundant in follicles. Northern blot analysis of RNA extracted from isolated granulosa cells and corpora lutea showed that both these compartments contained GH-R and GH-BP mRNA, although more abundant in granulosa cells. Immunoreactive GH-R was detected in granulosa cells of healthy follicles, corpus luteum, and in the germinal epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization and ontogeny of growth hormone receptor gene expression in the central nervous system

There is literature evidence that both growth hormone (GH) and its mediator, insulin-like growth factor 1 (IGF-1), are able to act upon neuronal and glial cells in the brain. We report here the location of the GH receptor in the brain of the rat and rabbit. Receptor distribution was determined by immunohistochemistry with GH receptor/binding protein (BP) specific monoclonal antibodies and by in situ hybridization with a [35S]riboprobe. GH receptor/BP immunoreactivity in the rat was most prominent in the neonate and declined with postnatal age. Receptor immunoreactivity was generalised with variation in immunoreactivity in regional areas. In the rat, strongest immunoreactivity was seen in layers 2, 3, 5 and especially layer 6 of the cerebral cortex, in neurones of the thalamus and hypothalamus, in Purkinje cells of the cerebellum, in neurones of the trapezoid body of the brainstem, and in retinal ganglion cells. Glial cells, notably astrocytes were also strongly reactive, along with ependyma of the choroid plexus, ventricular lining and pia mater. In the neonatal rabbit, strongest immunoreactivity was evident in layers 2 and 3 of the cerebral cortex, in pyramidal cells of the hippocampus, and in neurones of the inferior and superior colliculi, brain stem reticular formation, dorsal thalamus and hypothalamus. A similar distribution of GH receptor mRNA was seen by in situ hybridization. The ontogeny of GH receptor/BP mRNA in whole rat brain was quantified by solution hybridization-RNAse protection assay. Contrary to its ontogeny in the liver (Endocrinology, 113 (1983) 1325-1329) receptor mRNA decreased with postnatal age.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of the growth hormone receptor gene in the rat hypothalamus

The ontogeny of growth hormone receptor (GHR) gene expression was studied in the rat hypothalamus. Total RNA from the hypothalamus of rats at different developmental stages (embryonic day 15-56 days of age) was characterized using a 32P-labeled RNA probe derived from the extracellular domain of the rat GHR cDNA. Two RNA species, 4.5 kilobases (kb) encoding for GHR and 1.2 kb encoding for GH-binding protein, were detected in hypothalamic tissue from embryonic day 15 to 56 days of age. Their levels were low at embryonic day 15 and increased toward 3 days of age. The level of 4.5-kb transcript preferentially increased from 7 days after birth, and it was maintained until 35 days of age. Thereafter, the level of 4.5-kb transcript declined. The ratio between the 4.5- and 1.2-kb transcripts was less than 2.0 from embryonic day 15 to 3 days after birth, while it was larger than 4 after 7 days of age. There was no sex difference in the levels or the ratios of the transcripts of the GHR gene from 7 to 56 days of age. The findings indicate that the 4.5-kb transcript preferentially processed postnatally in the rat hypothalamus.

The turnover of growth hormone (GH)-binding protein and GH receptor in rabbit and rat

The present study was undertaken to further explore the comparative dynamics of growth hormone-binding protein (GH-BP) in relation to the turnover of the GH-receptor (GH-R) in vivo in rabbits and rats. The strategy used was to examine the time course of hepatic GH-R turnover over a 3 h period after cycloheximide treatment, with simultaneous measurements of serum GH-BP level. In the rabbit we sampled multiple liver biopsies and serum samples consecutively from each animal. In the rat, experiments on individual animals were conducted for each time point. In the rat, both liver GH-R and serum GH-BP declined after cycloheximide injection following first-order kinetics. The t 1/2 values for GH-R and GH-BP were 29.7-44.5 and 82.7-119.5 min (95% confidence limits), respectively. A significant positive correlation was found between rat liver GH-R and serum GH-BP (r = 0.85; p < 0.001). In contrast, the decline in rabbit liver GH-R, following cycloheximide treatment was accompanied by simultaneous time-dependent accumulation of serum GH-BP. The t 1/2 for rabbit serum GH-BP accumulation was 30.4-67.6 min. Scatchard analysis of [125I]hGH binding to rabbit GH-BP indicated that the binding capacity increased from 2818 +/- 538 fmol/ml, at time zero, to 5236 +/- 419 fmol/ml following 60 min cycloheximide treatment (p < 0.05). No significant changes in affinity were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Multifactorial regulation of IGF-I gene expression

Insulin-like growth factor-I (IGF-I) is a highly conserved 70-residue circulating peptide with diverse biological effects. In mammals IGF-I is an essential mediator of normal postnatal growth and its expression is influenced by hormonal, nutritional, tissue-specific, and developmental factors. Recent studies have demonstrated that the IGF-I gene is more complicated than might have been predicted from its simple protein sequence. In rats and in humans the single-copy six-exon gene is transcribed by adjacent promoters into nascent RNAs with different 5' leader sequences that undergo both alternative RNA splicing and differential polyadenylation to yield multiple mature transcripts. These observations suggest that trophic agents may modulate expression of IGF-I at any of several nodal points. In this report we review several of the mechanisms responsible for regulating production of IGF-I in the rat. During neonatal development IGF-I gene transcription is progressively activated, leading to a rise in both hepatic IGF-I mRNA and in serum IGF-I. The induction of IGF-I expression is limited to mRNAs directed by promoter 1, the more 5' of two rat IGF-I gene promoters, and precedes the ontogenic appearance of liver growth hormone (GH) receptors, indicating that mechanisms independent of GH activate IGF-I expression during early postnatal life. By contrast, in adult GH-deficient rats, a single intraperitoneal injection of GH causes a prompt rise in IGF-I gene transcription that is mediated equivalently by promoters 1 and 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone (GH) and reproduction: a review

Interaction between growth and reproduction occurs in many vertebrates and is particularly obvious at certain stages of the life cycle in fish. Endocrine interactions between the gonadotropic axis and the somatotropic axis are described, the potential role of GH being emphasised. A comparative analysis of these phenomena in mammals, amphibians and fish, suggests a specific role of GH in the physiology of puberty, gametogenesis and fertility. It also shows the original contribution made by studies on the fish model in this field of investigations.