Ontogeny of messenger RNA for the rat growth hormone receptor and serum binding protein

The tammar wallaby: a non-traditional animal model to study growth axis maturation

Maturation of the growth hormone (GH)/insulin-like growth factor 1 (IGF1) axis is a critical developmental event that becomes functional over the peripartum period in precocial eutherian mammals such as sheep. In mice and marsupials that give birth to altricial young, the GH/IGF1 axis matures well after birth, suggesting that functional maturation is associated with developmental stage, not parturition. Recent foster-forward studies in one marsupial, the tammar wallaby (Macropus eugenii), have corroborated this hypothesis. 'Fostering' tammar young not only markedly accelerates their development and growth rates, but also affects the timing of maturation of the growth axis compared with normal growing young, providing a novel non-traditional animal model for nutritional manipulation. This review discusses how nutrition affects the maturation of the growth axis in marsupials compared with traditional eutherian animal models.

Intermittent hypoxia suppression of growth hormone and insulin-like growth factor-I in the neonatal rat liver

OBJECTIVES

Extremely low gestational age neonates with chronic lung disease requiring oxygen therapy frequently experience fluctuations in arterial oxygen saturation or intermittent hypoxia (IH). These infants are at risk for multi-organ developmental delay, reduced growth, and short stature. The growth hormone (GH)/insulin-like growth factor-I (IGF-1) system, an important hormonal regulator of lipid and carbohydrate metabolism, promotes neonatal growth and development. We tested the hypothesis that increasing episodes of IH delay neonatal growth by influencing the GH/IGF-I axis.

DESIGN

Newborn rats were exposed to 2, 4, 6, 8, 10, or 12 hypoxic episodes (12% O₂) during hyperoxia (50% O₂) from P0-P7, P0-P14 (IH), or allowed to recover from P7-P21 or P14-P21 (IHR) in room air (RA). RA littermates at P7, P14, and P21 served as RA controls; and groups exposed to hyperoxia only (50% O₂) served as zero IH controls. Histopathology of the liver; hepatic levels of GH, GHBP, IGF-I, IGFBP-3, and leptin; and immunoreactivities of GH, GHR, IGF-I and IGF-IR were determined.

RESULTS

Pathological findings of the liver, including cellular swelling, steatosis, necrosis and focal sinusoid congestion were seen in IH, and were particularly severe in the P7 animals. Hepatic GH levels were significantly suppressed in the IH groups exposed to 6-12 hypoxic episodes per day and were not normalized during IHR. Deficits in the GH levels were associated with reduced body length and increase body weight during IHR suggesting increased adiposity and catchup fat. Catchup fat was also associated with elevations in GHBP, IGF-I, leptin.

CONCLUSIONS

IH significantly impairs hepatic GH/IGF-1 signaling during the first few weeks of life, which is likely responsible for hepatic GH resistance, increased body fat, and hepatic steatosis. These hormonal perturbations may contribute to long-term organ and body growth impairment, and metabolic dysfunction in preterm infants experiencing frequent IH and/or apneic episodes.

Growth Hormone Did Not Activate Its Intracellular Signaling Molecules in Rats' Liver Hepatocytes During Early Life Period

BACKGROUND

Although growth hormone (GH) has essential roles in the growth of animals, it has no growth-promoting effect during infancy period. The molecular mechanism underlying lack of growth-promoting effect of GH during infancy period remains unclear. Important signaling pathways are mediated by GH, including Janus kinase 2 (JAK2), extracellular signal-regulated kinase 1/2 (ERK1/2), signal transducers, and activators of transcription 5, 3, and 1 (STATs 5, 3 and 1).

OBJECTIVES

This study explored the underlying molecular mechanisms driving to the lack of growth-promoting effect of GH in the early stage of life by in vivo assessment of intracellular signal response (STAT5/ 3/ 1, JAK2 and ERK1/ 2) to GH at different physiological stages.

METHODS

In this study, five age groups of rats (1-, 4-day-old, and 1-, 2-, 3-week-old) were selected. The rats were anesthetized using pentobarbital (100 mg/kg) and then received the rat GH (2mg/kg) via inferior vena cava injection. The control rats were injected with normal saline during the same period. The intracellular signal response to GH was assessed by Western blot analysis.

RESULTS

JAK2 and STAT5 were expressed in 1-day and 4-day-old newborn rats and their expression levels were comparable with the levels of the 1-, 2-, and 3-week-old rats; however, JAK2/STAT5 phosphorylation was not observed in 1-day-old and 4-day-old newborn rats after stimulation with GH in the liver. Similar to JAK2 and STAT5, we did not detect STAT3/1 activation during infancy stages although basic STAT3 and STAT1 were also expressed in hepatocytes from newborn rats. In addition we detected ERK1/2 activation in 4-day-old, 1-, 2-, and 3-week-old rats but not in 1-day-old rats.

CONCLUSIONS

JAK2, STAT5, STAT3, STAT1, and ERK1/2 were not simultaneously activated by GH in newborn rats; this finding may be one of the underlying mechanism of GH insensitivity in newborn rats.

Growth axis maturation is linked to nutrition, growth and developmental rate

Maturation of the mammalian growth axis is thought to be linked to the transition from fetal to post-natal life at birth. However, in an altricial marsupial, the tammar wallaby (Macropus eugenii), this process occurs many months after birth but at a time when the young is at a similar developmental stage to that of neonatal eutherian mammals. Here we manipulate growth rates and demonstrate in slow, normal and fast growing tammar young that nutrition and growth rate affect the time of maturation of the growth axis. Maturation of GH/IGF-I axis components occurred earlier in fast growing young, which had significantly increased hepatic GHR, IGF1 and IGFALS expression, plasma IGF-I concentrations, and significantly decreased plasma GH concentrations compared to age-matched normal young. These data support the hypothesis that the time of maturation of the growth axis depends on the growth rate and maturity of the young, which can be accelerated by changing their nutritional status.

Growth hormone STAT5-mediated signaling and its modulation in mice liver during the growth period

Postnatal growth exhibits two instances of rapid growth in mice: the first is perinatal and independent of growth hormone (GH), the second is peripuberal and GH-dependent. Signal transducer and activator of transcription 5b (STAT5b) is the main GH-signaling mediator and it is related to IGF1 synthesis and somatic growth. The aim of this work was to assess differential STAT5 sensitivity to GH during the growth period in mouse liver of both sexes. Three representative ages were selected: 1-week-old animals, in the GH-independent phase of growth; 2.5-week-old mice, at the onset of the GH-dependent phase of growth; and 9-week-old young adults. GH-signaling mediators were assessed by immunoblotting, quantitative RT-PCR and immunohistochemistry. GH-induced STAT5 phosphorylation is low at one-week and maximal at 2.5-weeks of age when compared to young adults, accompanied by higher protein content at the onset of growth. Suppressor CIS and phosphatase PTP1B exhibit high levels in one-week animals, which gradually decline, while SOCS2 and SOCS3 display higher levels at adulthood. Nuclear phosphorylated STAT5 is low in one-week animals while in 2.5-week animals it is similar to 9-week control; expression of SOCS3, an early response GH-target gene, mimics this pattern. STAT5 coactivators glucocorticoid receptor (GR) and hepatic nuclear factor 1 (HNF1) abundance is higher in adulthood. Therefore, GH-induced STAT5 signaling presents age-dependent activity in liver, with its maximum coinciding with the onset of GH-dependent phase of growth, accompanied by an age-dependent variation of modulating factors. This work contributes to elucidate the molecular mechanisms implicated in GH responsiveness during growth.

Hormonal influences of early postnatal indomethacin and ibuprofen in neonatal rats

OBJECTIVE

Indomethacin and ibuprofen are administered to preterm neonates for symptomatic patent ductus arteriosus. The drugs suppress prostaglandins (PGs) which modulate growth and secretion of various hormones. We examined the hypothesis that early postnatal indomethacin and ibuprofen influence growth and GH-IGF-I-insulin and HPA axes in neonatal rats.

DESIGN

Rat pups received IP injections of saline (Sal) on P1, P2, and P3; 10mg/kg ibuprofen on P1 followed by 5mg/kg on P2 and P3; or 0.2mg/kg indomethacin on P1 followed by 0.1mg/kg on P2 and P3. Serum and hepatic GH, GHBP and IGF-I; and serum corticosterone and insulin levels were determined.

RESULTS

Ibuprofen suppressed somatic growth in the sucking rats, but the effect was transient, resolving by P14. Indomethacin had an opposite, latent effect on body weight and liver to body weight ratios in weanling rats. Both indomethacin and ibuprofen had profound hormonal effects that differed in magnitude and timing. Indomethacin resulted in a sustained elevation in corticosterone levels at P21, while ibuprofen increased serum and hepatic GH levels. Both drugs suppressed GHBP in serum at P7 and P14; and liver at P4 and P7, but a rebound increase in serum GHBP was noted at P21 with Ibuprofen only. Both drugs increased serum IGF-I at P7. The effect remained sustained with indomethacin.

CONCLUSIONS

These results provide evidence for an involvement of PGs in the regulation of growth as well as the GH-IGF and HPA axes. Therefore, early postnatal exposure to PG inhibitors may further exacerbate postnatal growth restriction and ability to cope with stress.

The acid-labile subunit is required for full effects of exogenous growth hormone on growth and carbohydrate metabolism

Normal postnatal growth is dependent in part on overlapping actions of GH and IGF-I. These actions reflect GH stimulation of IGF-I production in liver and extrahepatic tissues, representing respectively the endocrine and autocrine/paracrine arms of the IGF system. Recent experiments in genetically modified mice show that each source of IGF-I can compensate for absence of the other but do not resolve their relative role in postnatal growth. In an effort to address this issue, we studied the GH responsiveness of mice harboring a null mutation of the acid-labile subunit (ALS). Null ALS mice have a substantial reduction in endocrine IGF-I but, unlike other models of plasma IGF-I deficiency, have no obvious additional endocrine defects. Wild type and null ALS mice of both sexes received daily sc injections of saline or recombinant bovine GH between d 35 and 63 of postnatal age. The GH-stimulated body weight gain of null ALS mice was reduced by more than 30% relative to wild type mice, irrespective of sex. Reductions in GH responsiveness were also seen for kidney and linear growth. Absence of ALS eliminated the ability of GH to increase plasma IGF-I despite intact GH-dependent stimulation of IGF-I expression in liver, adipose tissue, and skeletal muscle. GH treatment was also less efficient in antagonizing insulin action in null ALS mice. Overall, these results suggest that the GH effects mediated by endocrine IGF-I depends on ALS, and accordingly null ALS mice are less responsive to exogenous GH therapy.

Pituitary mRNA expression of the growth hormone axis in the 1-year-old intrauterine growth restricted rat

Intrauterine growth restriction (IUGR) is one of the major causes of short stature in childhood. Abnormalities in the growth hormone (GH) axis have frequently been observed in children who are born intrauterine growth restricted and GH treatment is effective to improve final height. However, the way that the GH axis is involved is not fully understood. Previously, when investigating the effect of IUGR on the central somatotrophic axis, a hypothalamic effect was discovered with elevated somatostatin and decreased neuropeptide Y mRNA expression levels, whereas serum GH and insulin-like growth factor I (IGFI) were unaltered. These findings were thought to indicate a hypothalamic alteration of the GH axis due to IUGR, probably to compensate pituitary output, thereby normalising peripheral values of GH and IGFI. Therefore, the present study aimed to evaluate the effect of IUGR on the pituitary GH axis in this rat model. Pups from rats that underwent bilateral uterine artery ligation at day 17 of pregnancy were studied. Pituitary glands were collected from 1-year-old offspring for quantitative measurements of GH, GH-receptor (GH-R), GH-releasing hormone receptor (GHRH-R), somatostatin receptor subtype 2 and 5, IGFI and IGFI receptor mRNA levels using a real-time reverse transcriptase-polymerase chain reaction. In addition, liver GH-R and IGFI mRNA expression levels were measured and a radioimmunoassay was performed to determine serum IGFI levels. In the IUGR rat, levels of pituitary GH, GH-R and GHRH-R relative gene expression (RGE) were increased. No differences were found in the RGE level of all other pituitary growth factors, liver GH-R and IGFI, and serum IGFI concentration between IUGR and control rats. The present data show that intrauterine growth failure leads to changes in the pituitary that might counterbalance the effects found previously in the hypothalamus.

Characterization of the growth hormone receptor in human dermal fibroblasts and liver during development

Human tissues express growth hormone receptors (hGHR) by the 3rd mo of gestation. We assessed developmental changes in hGHR function in fibroblasts and liver, testing binding and hormonal response. Fetal cells showed low but reproducible hGH binding. No age-related changes occurred in fibroblasts (9 wk-34 yr). In contrast, there was a fourfold increase in hGH binding in postnatal liver, with a sixfold increase in hGHR mRNA. Both full-length and truncated hGHR mRNAs were detected in all livers. Cross-linking revealed a larger hGH/receptor complex in fetal liver. Fetal hepatocytes produced 10 times more insulin-like growth factor (IGF)-II than IGF-I, and responded to hGH (150 ng/ml) with a significant increase in IGF-II. Fetal hepatocytes secreted three IGF-binding proteins (IGFBPs), including IGFBP1, but not IGFBP3. hGH did not alter fetal hepatocyte IGFBPs but stimulated glucose uptake. Exposure of fibroblasts to hGH decreased hGH binding only in >1-yr postnatal fibroblasts, whereas treatment with dexamethasone (100-400 nM) increased binding only in postnatal cells. Thus, although fetal hepatocytes and fibroblasts possess functional hGHR, these receptors (and/or their signaling pathways) are immature or have adapted to the in utero environment.

Hepatic growth hormone signaling in the late gestation fetal rat

The role of GH in the developing fetus is poorly understood. Several studies have demonstrated a limited role for GH in late fetal life. In fact, few data are available regarding GH signal transduction in the late gestation fetus. We therefore focused on a comparison of hepatic GH signaling in near-term fetal rats [embryonic day 19 (E19)] and adult rats using a combination of in vitro studies employing hepatocytes in primary culture and in vivo studies. We found that GH receptor (GHr) binding was comparable in fetal liver and adult liver. The long isoform of the GHr underwent tyrosine phosphorylation in response to GH stimulation of E19 fetal hepatocytes in a manner similar to that seen in cultured adult hepatocytes. Furthermore, downstream signaling via the Janus kinase-2 tyrosine kinase, STAT1 (signal transducer and activator of transcription), and STAT5 was also intact in both, as demonstrated by the tyrosine phosphorylation of these signaling proteins. To confirm the relevance of these findings to the in vivo situation, GH was directly administered by ip injection to E 19 fetal and adult rats. In both cases, tyrosine phosphorylation of STAT5 was markedly and rapidly induced. Finally, transfection of E19 fetal hepatocytes with GH-responsive reporter elements [Spi2.1(-275/+85)-CAT and 8xGHRE-TKCAT] demonstrated intact transcriptional regulation. Our data indicate that GHr abundance and activity as well as downstream GH signaling are similar in the late gestation fetal rat and in the adult and that these mechanisms appear capable of supporting physiological GH functions in the developing liver.

Growth hormone action in hypothyroid infant rats

In neonatal rats, expression of serine protease inhibitors 2.1 and 2.3 mRNA peaks on d 2 of life and declines shortly thereafter, coinciding with levels of circulating GH. To evaluate the role of GH in this increase and to test the hypothesis that GH is active in perinatal life, we studied GH action in a model of GH deficiency. Maternal/neonatal hypothyroidism with consequent GH deficiency was induced by methimazole administration to pregnant dams. The resultant hypothyroid neonates were treated at d 2 or 7 of age with GH or saline for 1 h before exsanguination. In d-7 neonates, but not at d 2, GH administration resulted in significant serine protease inhibitors 2.1 and 2.3 mRNA induction. This treatment did not result in increased production of either GH receptor or IGF-I mRNA at either age. There was a slight GH-independent increase in GH receptor and IGF-I mRNA expression by d 7. Electromobility shift assays using hepatic nuclear extracts from these neonates and the GH response element from the serine protease inhibitor 2.1 promoter showed signal transducer and activator of transcription 5 (Stat5) binding in response to GH in extracts from d-7 rats only. Immunoblots of these extracts showed twice as much Stat5 in the nuclei of d-7 treated neonates compared with d-2 treated neonates. We conclude that there is apparent insensitivity to GH treatment in d-2 neonates that remits by d 7 and that this remission correlates with increased abundance of GH receptor and Stat5.

Developmental regulation of insulin-like growth factor-I and growth hormone receptor gene expression

During development, the insulin-like growth factor I (IGF-I) gene is expressed in a tissue specific manner; however, the molecular mechanisms governing its developmental regulation remain poorly defined. To examine the hypothesis that expression of the growth hormone (GH) receptor accounts, in part, for the tissue specific expression of the IGF-I gene during development, the developmental regulation of IGF-I and GH receptor gene expression in rat tissues was examined. The level of IGF-I and GH receptor mRNA was quantified in RNA prepared from rats between day 17 of gestation (E17) and 17 months of age (17M) using an RNase protection assay. Developmental regulation of IGF-I gene expression was tissue specific with four different patterns of expression seen. In liver, IGF-I mRNA levels increased markedly between E17 and postnatal day 45 (P45) and declined thereafter. In contrast, in brain, skeletal muscle and testis, IGF-I mRNA levels decreased between P5 and 4M but were relatively unchanged thereafter. In heart and kidney, a small increase in IGF-I mRNA levels was observed between the early postnatal period and 4 months, whereas in lung, minimal changes were observed during development. The changes in GH receptor mRNA levels were, in general, coordinate with the changes in IGF-I mRNA levels, except in skeletal muscle. Interestingly, quantification of GH receptor levels by Western blot analysis in skeletal muscle demonstrated changes coordinate with IGF-I mRNA levels. The levels of the proteins which mediate GH receptor signaling (STAT1, -3, and -5, and JAK2) were quantified by Western blot analysis. These proteins also are expressed in a tissue specific manner during development. In some cases, the pattern of expression was coordinate with IGF-I gene expression, whereas in others it was discordant. To further define molecular mechanisms for the developmental regulation of IGF-I gene expression, protein binding to IGFI-FP1, a protein binding site that is in the major promoter of the rat IGF-I gene and is important for basal promoter activity in vitro, was examined. Gel shift analyses using a 34-base pair oligonucleotide that contained IGFI-FP1 did not demonstrate changes in protein binding that paralleled those in IGF-I gene expression, suggesting that protein binding to IGFI-FP1 does not contribute to the developmental regulation of IGF-I gene expression, at least in brain and liver. In summary, the present studies demonstrate coordinate expression of the IGF-I gene and GH receptor during development and suggest that GH receptor expression contributes to the tissue specific expression of the IGF-I gene during development.

Differential expression of renal growth hormone receptor and its binding protein in experimental diabetes mellitus

Growth hormone (GH) may have a role in the development of diabetic nephropathy. The effect of experimental diabetes on renal expression of the growth hormone receptor gene products, including the receptor itself (GHR) and its binding protein (GHBP) was examined. Adult female rats received i.v. streptozotocin and were killed at 7, 30, 90 and 180 days after the induction of diabetes. Diabetic animals had a pronounced increase in kidney weight and progressive albuminuria. In renal cortex, no change was seen in GHR mRNA levels throughout the observation period of 6 months, while a significant increase in cortical GHBP mRNA levels was observed after 1 month of diabetes and sustained for the rest of the study period. Immunohistochemical analysis of kidney sections revealed a stronger staining for GHBP at the cortical and inner medullary areas in the diabetic animals. These data indicate that although the GHR and GHBP mRNAs originate from the same gene, their renal levels are differentially regulated during the development of experimental diabetic kidney disease, suggesting a functional role for GHBP.

Embryonic expression of the mRNA for the rat homologue of the fusin/CXCR-4 HIV-1 co-receptor

We have previously cloned a human receptor recently shown to be a cofactor for entry of T-tropic HIV-1 strains into CD4+ cells, now named fusin. Stromal derived factor-1 (SDF-1) is an endogenous ligand for fusin, also called CXCR-4. Here we show the distribution of fusin/CXCR-4 mRNA during ontogeny in the rat. The onset of mRNA expression is around embryonic day 9 and the mRNA expression is high in the thymus as well as proliferative areas of the brain during development. Our results suggest: (1) that fusin/CXCR-4 might have a dual role in both brain development and the immune system; (2) that SDF-1 has a role in brain development or that additional physiological ligands exist for this receptor; (3) co-expression of CD4 and fusin/CXCR-4 may make fetuses susceptible to HIV infection during development.

In situ gene expression of growth hormone (GH) receptor and GH binding protein in adult male rat tissues

Pituitary growth hormone (GH) acts as a growth promoter in a wide range of tissues after binding to its specific GH receptor (GHR) or to a cytosolic circulating GH binding protein (GHBP). To further characterize GH target cells in the rat, in situ hybridization was used to investigate the tissue and cell distribution of mRNAs encoding GHR and GHBP, and their hepatic developmental expression was examined. Cryostat sections of adult male rat tissue were hybridized with [35S]dATP-labeled oligonucleotide antisense probes, one directed against a specific sequence of the intracellular domain of rat GHR mRNA, the other against the hydrophilic tail of rat GHBP mRNA. Several tests were carried out to validate the in situ detection of mRNA. Co-expression of the two transcripts in liver, spleen, thymus, kidney, adrenal, skin, muscle, heart, and pituitary was autoradiographically detected. However, relative expression levels, as demonstrated by computer-assisted microdensitometry, appeared to be variable. Both transcripts showed higher levels of expression in the liver, anterior and intermediate pituitary lobes, outer kidney medulla, adrenal cortex, skin epidermis, heart and muscle, but lower levels in spleen, thymus, hypodermis, adrenal medulla and posterior pituitary lobe. As a physiological control, hepatic levels of expression were examined during development, and the two forms of mRNA were found to be present at low levels in fetal liver, increasing considerably after birth. These results permit the identification in the adult male rat of cells that might be directly responsive to GH, and demonstrate the differential expression of rGHR and rGHBP transcripts.

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.

Age-related reduction of human growth hormone-binding sites in the human brain

Previous studies have shown that alterations in various neuroendocrine functions occur with increasing age. We here report a study of growth hormone (GH)-binding sites in different areas of post-mortem human brains collected from individual males and females of different age. The results indicate that there exists a significant negative correlation between the density of GH-binding sites and increasing age. This phenomenon was observed in both sexes in brain areas such as choroid plexus, hippocampus, hypothalamus, pituitary and putamen but not in e.g. thalamus. In all tissues (except for choroid plexus), the GH binding was significantly higher in those originating from females than those from males. This discrepancy was found likely to be associated with the affinity of GH to lactogenic rather than to somatogenic sites as no pronounced sex difference in binding was observed in the presence of excessive amounts of human prolactin. Data also indicate that the putative GH receptors in the various brain regions differ with regard to binding constants and to the estimated molecular size of the hormone-binding units. The loss of GH receptors in brain of elderly people may have consequences in several physiological courses. The decrease in GH binding at hypothalamic and pituitary levels may be of importance for the mechanisms behind the release or secretion of the hormone.

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.

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)

Expression of growth hormone-responsive serpin mRNAs in perinatal rat liver

Hormonal mechanisms controlling growth of the fetus are poorly understood, and generally growth hormone (GH) is not thought to influence perinatal growth. To examine the influence of GH in the expression of genes in perinatal rat liver, we measured RNA levels of several GH responsive and growth axis genes. Spi 2.1, Spi 2.2, Spi 2.3, insulin-like growth factors (IGF) I and II, and GH receptor mRNAs were measured in rat liver total RNA from gestational days 19, 20, 21, and postnatal day 2. Spi 2.1 and 2.3 genes were faintly expressed on day 20, 6% and 13 +/- 1% of adult levels on gestation day 21, and 6% and 31 +/- 6% of adult levels on day 2. Deoxyribonuclease I (DNase I)-hypersensitive sites in the 5' flanking region of the Spi 2.1 gene, which are concordant with GH response, were not present in DNA extracted from livers at gestation day 19 but were present at days 20, 21, and 2, suggesting the gene is transcriptionally competent after day 19 and that the areas of chromatin vulnerable to DNase I digestion are the same in pre- and postnatal life. Low levels of GH receptor mRNAs (approximately 10% of adult) were present on all measured days. IGF-I mRNA was below quantitatable levels in day 19 or 20 fetal samples and was only 2.7 +/- 0.1% of adult levels on day 21. Levels on day 2 were 9.6 +/- 1.9% of adult. IGF-II mRNA was essentially constant throughout this period, with a minimal increase at day 21 of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of hepatic growth hormone receptor and insulin-like growth factor-I mRNA in the chicken

We have examined the ontogeny of expression of growth hormone (GH) receptor (GHR) and insulin-like growth factor-I (IGF-I) mRNA in chicken liver from day 13 of incubation until 31 weeks of age. The profiles of GHR and IGF-I mRNA levels were compared to developmental changes in body weight and plasma levels of GH and IGF-I. In the embryo, hepatic GHR mRNA was not detectable until day 15, highest on days 17 and 19, and then declined at hatching (day 21). Following an initial 2-week delay after hatching, there was a progressive increase in hepatic GHR mRNA which continued after the birds reached mature body weight. Plasma GH reached peak levels at 3-4 weeks of age and then fell sharply until maintenance of a low basal level after 10 weeks of age. Thus, there appears to be a strong inverse relationship between expression of the GHR and basal plasma GH levels in the prepubertal chicken. Although IGF-I mRNA was undetectable in embryonic liver by Northern blot analysis, there is a good correlation between expression of hepatic IGF-I mRNA and the plasma IGF-I profile during post-hatching development in the chicken. The highest levels of IGF-I mRNA were reached at 4 weeks of age which was followed by a slow decline to the basal levels maintained after 10 weeks of age. It appears that the decline in plasma IGF-I lags considerably behind the sharp fall in plasma GH levels and expression of hepatic IGF-I mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum binding proteins for growth hormone: origins, regulation of gene expression and possible roles

The above discussion highlights the heterogeneity of the family of GH receptors/GHBPs and their mRNAs. Considerable uncertainty still exists as to the interrelationships between the various forms, the specific mechanisms for their generation and their possible significance in terms of modulating GH action and receptor function. Collectively, the regulatory data indicate that while the mRNAs encoding the membrane and soluble GH receptors/GHBPs in the rat are expressed by the same broad distribution of tissues, they can be differentially regulated. Such regulated expression implies a functional basis for production of GHBP. The wide tissue distribution of GHBP mRNA also suggests a role for GHBP as a paracrine/autocrine effector molecule, perhaps in addition to an endocrine role. Additional studies, both in vitro and in vivo, perhaps utilizing highly purified recombinant GHBP, will be required to provide more definitive information as to the true physiological role(s) of the circulating GHBPs.