Regulation of rat growth hormone receptor gene expression

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)

Expression and binding properties of two isoforms of the human growth hormone receptor

Two isoforms of the human growth hormone receptor mRNA, one containing exon 3 (encoding an extracellular domain of the receptor), hGHR, and one excluding exon 3, hGHRd3, have been described. To study the cellular distribution of the two types of messengers we have analysed a panel of tissues. Both isoforms were expressed independently or simultaneously depending on the tissue studied. To investigate the binding properties of hGHRd3 we have cloned its cDNA in a eukaryotic expression vector; transient expression in COS-7 cells showed that the receptor without exon 3 was expressed on the plasma membrane and was able to bind human growth hormone (hGH) with the same high affinity as hGHR. Human lactogen (hCS) removed 125I-hGH bound to the full-length and exon 3-excluding receptors to the same extent. These results show that hGHR and hGHRd3 have tissue-specific expression and share identical binding properties for hGH and hCS and leave open the possibility that exon 3 might influence receptor signalling.

Establishment of a growth hormone responsive chondrogenic cell line from fetal rat tibia

Reproducible effects of growth hormone (GH) on primary isolated cells in monolayer are highly dependent on the culture conditions and/or the fraction of GH responsive cells. To study the effect of GH at the cellular level, a homogenous cell line with both GH responsiveness and chondrogenic properties was established. Primary isolated cells from 18-day-old fetal rat tibia were subcultured using a strict protocol for passages (every third day and a seeding density of 15,000/cm2). Of six established cell lines, one fetal tibia cell line No. 5 (FTC 5) expressed adipogenic and chondrogenic properties at a low frequency. Cells from FTC 5 were subcultured in soft agar suspension with the addition of bovine GH (100 ng/ml). After 14 days in culture eight monoclonal cell lines were established from individual large colonies. Two subclones, FTC 5:3 and FTC 5:6, expressed a chondrogenic phenotype as demonstrated by chondrocyte foci, alcian blue staining and production of type II collagen. Further characterization of FTC 5:3 revealed specific binding of bovine GH with an affinity of 1.7 x 10(9) M-1, and approximately 7300 receptors/cell. Northern blot analysis of FTC 5:3 with a 32P-labeled RNA probe complementary to an extracellular part of the rat GH receptor, revealed two major labeled bands (4.0 and 1.2 kilobases). Both GH and insulin-like growth factor-I (IGF-I) stimulated 3H-thymidine uptake in FTC 5:3 (194 +/- 28% and 405 +/- 127% over control, respectively), while proteoglycan synthesis, as measured by [35S]sulphate uptake, was stimulated by IGF-I only (101 +/- 18% over control).(ABSTRACT TRUNCATED AT 250 WORDS)

Structural, functional and evolutionary implications of the three-dimensional crystal structure of murine interferon-beta

The crystal structure of recombinant murine interferon-beta as elucidated by Senda et al. (Proc. Jap. Acad. 66B: 77-80 (1990); EMBO J. 11: 3193-3201 (1992)) appears to represent the basic structural framework of all Type I interferons including interferons-beta and all subtypes of interferons-alpha of various mammalian origin. Now the huge accumulated data on the structure-activity relationship of Type I interferons using various chemical and genetic techniques can be systematically evaluated in terms of the three-dimensional structure. Structural comparison with other cytokines, for which three-dimensional structures have been established, including interferon-gamma and considerations on the evolution of cytokines and cytokine receptors are also given.

Variants of the 5'-untranslated sequence of human growth hormone receptor mRNA

The human growth hormone receptor (GHR) gene was proposed to contain multiple 5'-noncoding exons (Leung et al., 1987). The exact number and structure of these exons are unknown. As a first step in investigating this point more closely, we decided to clone alternative 5'-noncoding sequences of human liver GHR mRNA. The ligation-mediated single-sided polymerase chain reaction (PCR) was applied for selective amplification of 5'-terminal sequences of human liver GHR cDNA. PCR products were cloned and sequenced. Eight different sequence variants diverging in the 5'-untranslated regions beginning 12 base pairs upstream from the initiating ATG codon were found. One variant seems to represent unspliced or partially spliced GHR mRNA. The remaining variants probably correspond to multiple alternatively spliced forms of GHR mRNA. Homologs for three of these variants were found among previously published 5'-noncoding sequences of GHR cDNA obtained from other species by conventional cDNA cloning. Most of the cloned human liver GHR cDNA variants contain one or more ATG preceding the main GHR open reading frame start of translation. Thus, the GHR genes appeared to be a striking example of a very complex transcription unit.

Regulation of liver-specific steroid metabolizing cytochromes P450: cholesterol 7α-hydroxylase, bile acid 6β-hydroxylase, and growth hormone-responsive steroid hormone hydroxylases

The hydroxylation of cholesterol, bile acids, and steroid hormones by liver cytochrome P450 (CYP) enzymes proceeds with a high degree of regiospecificity, and contributes to both biosynthetic and catabolic pathways of sterol metabolism. CYP 7-catalyzed cholesterol 7α-hydroxylation, a key control point of bile acid biosynthesis, is regulated at a pretranslational step, probably transcription initiation, by multiple factors, including liver bile acid and cholesterol levels, thyroid hormone status, and diurnal rhythm. Hydrophobic bile acids, such as lithocholic acid, are converted to less cholestatic derivatives by 6β-hydroxylation carried out by CYP 3A P450s, which also catalyze steroid hormone 6β-hydroxylation reactions. Complex, gender-dependent developmental patterns characterize the expression of steroid 5α-reductase and several rat liver steroid hydroxylase CYPs. Multiple pituitary-dependent factors regulate the expression of these enzymes; of greatest importance are the gonadal steroids and the sex-dependent secretory patterns of growth hormone (GH) that they impart. The continuous presence of GH in circulation, a characteristic of adult female rats, positively regulates expression of the female-specific steroid disulfate 15β-hydroxylase CYP 2C12, while expression of the male-specific steroid 16α- and 2α-hydroxylase CYP 2C11 is stimulated by the intermittent pituitary secretion of GH that occurs in adult male rats. Intermittent GH can stimulate CYP 2C11 gene expression even when the hormone presents to the hepatocyte at a non-physiological pulse amplitude, duration, and frequency, provided that an interpulse interval of no GH (obligatory recovery period) is maintained for at least 2.5 h. GH regulates the expression of the CYP 2C11 and CYP 2C12 genes at the level of transcription initiation. This process is probably mediated by sex-dependent and GH-regulated protein-DNA interactions, such as those observed in the 5'-flank of the CYP 2C12 gene. Thyroid hormone is a second major regulator of liver steroid hydroxylase P450 activity. It regulates these enzymes directly, at a pretranslational step, and indirectly, through its stimulation of pituitary GH secretion and by its positive effects on the expression of the flavoenzyme NADPH-P450 reductase, which catalyzes electron transfer that is obligatory for all microsomal steroid hydroxylation reactions.

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.

Regulation of hepatic growth hormone receptors in coho salmon (Oncorhynchus kisutch)

Factors potentially regulating hepatic growth hormone (GH) receptors in coho salmon (Oncorhynchus kisutch) have been investigated. From December to June of the first year, relative changes in hepatic 125I-sGH binding and 35SO4 incorporation by ceratobranchial cartilage were similar. Stunted salmon, which in seawater have elevated plasma GH yet fail to grow, showed lower hepatic 125I-sGH binding than did normally growing seawater salmon. However, MgCl2 treatment of stunts' membranes to reveal total specific binding of 125I-sGH indicated receptor occupation by endogenous sGH. Total specific 125I-sGH binding was low in seawater stunts and remained low if these fish remained unfed after return to fresh water, but increased approximately twofold upon feeding. Total specific binding in fasted salmon in fresh water showed a trend toward decreased levels by 1 week; by 3 weeks, binding was 40% lower than in fed fish. There was a positive correlation (r = 0.600) between condition factor and total specific binding in fed and fasted salmon in fresh water. Two weeks after hypophysectomy total specific binding was 50% lower than in sham-operated control salmon, indicating pituitary regulation of GH receptors. GH treatment reduced both free and total 125I-sGH binding in salmon examined 24 hr after treatment. Treatment with recombinant bovine insulin-like growth factor I, thyroxine, or cortisol did not affect free 125I-sGH binding. Both the pituitary and nutrition appear to be prime regulators of hepatic GH receptors in coho salmon.

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)

Growth hormone and prolactin stimulate androgen receptor, insulin-like growth factor-I (IGF-I) and IGF-I receptor levels in the prostate of immature rats

In this study we investigated the involvement of several different pituitary hormones on rat prostate development. 22-day-old Wistar rats, hypophysectomized (hypox) at 19 days of age were supplemented with highly purified human prolactin (hPRL), human luteinizing hormone (hLH), porcine follicle-stimulating hormone (pFSH), and bovine growth hormone (bGH) or with saline. Quantitative analysis of RNAs shows that treatment with either PRL or GH increases significantly steady-state mRNAs levels of the following genes in the prostate: androgen receptor (AR) (respectively 3.5- and 4.8-fold above hypox controls), IGF-I (5- and 2.7-fold), and IGF-I receptor (2.9- and 2.3-fold). LH and FSH, by contrast, have negative effects on these parameters. To test whether the enhancing effect of PRL and GH on AR-mRNA abundance was followed by increased content in the protein itself, binding assays were performed with the androgen agonist [3H]R1881 (131 and 153 fmol/mg protein while hypox controls contained 110 fmol/mg protein). In addition to the well-documented presence of prolactin receptors in prostatic tissues, we have further demonstrated, by means of nuclease S1 protection assays plus dot- and Northern-blot analyses, that a GH receptor mRNA is produced in the immature rat prostate. Moreover, we observed not only strong lactogenic but also purely somatogenic binding to be occurring in the immature prostates. Finally, we have studied IGF-I mRNA content in separated epithelial/stromal cell fractions and have concluded that IGF-I expression is principally located in the prostatic stroma. Taken together, these results suggest that PRL and GH are involved in regulating AR synthesis, at least partially by direct action on the organ. In this context IGF-I appears as a paracrine factor playing a role in epithelium/stroma interactions during prostatic development.

Cellular localization of IGF-I and IGF-II mRNAs in immature hypophysectomized rat testis and epididymis after in vivo hormonal treatment

IGF-I and II genes expression has been localized by in situ hybridization in testis and epididymis of immature hypophysectomized rats treated in vivo with either pFSH, hLH, bGH, hPRL or with saline. IGF-I mRNA expression was found in both Sertoli and Leydig cells after treatment with either FSH or LH. IGF-I mRNA was highly expressed in germ cells after FSH stimulation and to a lesser extent after GH or LH treatments. However, its expression was very low in hypophysectomized control or PRL treated rats. IGF-I mRNA was also expressed in stromal cells of epididymis after LH treatment and to a lesser extent after GH stimulation. In contrast, IGF-II mRNA expression was detected in all testicular cell types whatever the hormonal treatment (FSH, LH, GH, PRL). For each hormonal treatment testicular sections were examined after immunohistochemical staining with specific antisera against IGF-I and IGF-II. Both in situ hybridization and immunohistochemical data were examined in order to determine the testicular sites of synthesis of IGF-I and IGF-II.

Expression of growth hormone receptor, insulin-like growth factor 1 (IGF-1) and IGF-1 receptor mRNA and proteins in human skin

A cDNA corresponding to the membrane receptor for growth hormone (GH) was amplified by polymerase chain reaction (PCR) directly from human skin. The cDNA was cloned and found to have complete sequence homology to the extracellular domain of human liver GH receptor (GH-R). Northern analysis, using the cloned GH-R as probe, revealed relatively higher levels of GH-R transcripts in cultured human dermal fibroblasts compared to cultured keratinocytes or keratome biopsies. Semi-quantitative PCR analysis indicated that the level of GH-R mRNA in cultured melanocytes was similar to that in fibroblasts. The receptor protein encoded by GH-R mRNA in fibroblasts was shown by affinity cross-linking to have an apparent M(r) of 115-120 kDa, similar to that of 3T3-F442A fibroblasts used as a control. mRNA transcripts for the major mediator of GH actions, insulin-like growth factor 1 (IGF-1), were detected by PCR in fibroblasts, melanocytes, and keratome biopsies, but not in keratinocytes. In contrast, IGF-1 receptor mRNA were abundant in cultured keratinocytes and skin biopsies, as determined by Northern analysis. IGF-1 but not GH (5-50 ng/ml) promoted clonal proliferation of cultured keratinocytes. In contrast, GH (10 ng/ml) after 5 d markedly increased fibroblast cell numbers (70%, p less than 0.009) over 0.2% serum control. These data indicate that human skin cells possess the molecular elements necessary to respond to GH and raise the possibility that GH may influence skin growth in vivo.

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

Little is known of the regulation of gene expression for the family of growth hormone (GH) and prolactin (PRL) receptors (PRL-R). Furthermore, the relationship between expression of the GH receptor (GHR) and its soluble truncated form (GH-binding protein, GHBP) is unclear. The actions of both GH and PRL are developmentally regulated and several studies have examined the ontogeny of these receptors by classical hormone-binding techniques. In the current study we have examined the expression of GHR/GHBP and PRL-R mRNA in the male rat over a broad developmental range--fetal through to 110 days of age. The GHR mRNA (4.5 kb) was barely detectable in fetal and early (less than 20 days) postnatal livers, but was followed by a gradual increase up to 40 days of age by which time adult plateau levels were reached. In contrast, hepatic GHBP mRNA (1.2 kb) was clearly identifiable in the fetus and subsequently followed a similar pattern to the 4.5 kb GHR mRNA although there was a somewhat earlier rise. Hepatic membrane binding studies using 125I-bovine GH as ligand revealed no measurable binding activity at less than 20 days of age. Binding remained low thereafter. In contrast, the serum GHBP binding activity was detectable at 10 days of age and rose to adult levels by 50 days of age. These results indicate that mRNA species for GHR, GHBP, PRL-R and insulin-like growth factor I (IGF-I) are all developmentally regulated with the pattern for IGF-I correlating more closely with that of GHBP than GHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of the growth hormone receptor gene in rabbit tissues

Total RNA from several adult (6-18-month-old) rabbit tissues was characterized using an oligonucleotide probe derived from the extracellular domain of the nucleotide sequence of the rabbit growth hormone receptor (GH-R) cDNA. Multiple GH-R mRNA species of approximately 4.6, approximately 3.3, 2.1 and approximately 1.4 kb were detected. The major 4.6 kb transcript was detectable in all tissues examined but with quite marked abundance differences. The highest level of expression was observed in liver followed closely by muscle. A qualitative assessment of insulin-like growth factor I (IGF-I) mRNA abundance was made in these same tissues. The data showed that the tissue abundance of GH-R mRNA was not necessarily parallel to that of IGF-I mRNA. The ontogeny of GH-R mRNA was studied in rabbit liver, muscle, heart and kidney. Low levels of GH-R mRNA were detectable in all fetal tissues studied except kidney which showed relatively high levels, suggesting that GH may play an important role in early kidney development. The overall developmental pattern of GH-R mRNA was similar in heart, muscle and liver, being low in fetal and early neonatal (day 3) periods and reaching maximal levels between 2 and 6 months. However, in kidney the pattern contrasted markedly. Relatively high levels of GH-R mRNA were observed in fetal and early neonatal (day 3) kidney with little change throughout development. The developmental pattern of IGF-I gene expression was not necessarily co-ordinately regulated with the ontogenic pattern of GH-R gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone binding proteins in pig adipose tissue: number, size and effects of pGH treatment on pGH and bGH binding

The present study was undertaken to determine the number and size of growth hormone binding proteins present in pig adipose tissue, determine if there were differences in binding of pGH and bGH to adipose tissue membranes and establish the effects of pGH treatment on GH binding. Administration of pGH (0, 25, 50 or 100 micrograms pGH/kg BW/d) for 7 d did not affect binding of [125I]bGH to adipose tissue microsomes. Maximum binding of bGH was approximately 8-fold higher than that observed for pGH. Half-maximal inhibition of [125I]bGH binding was observed at 11 ng/ml of bGH. In contrast, a more than 10-fold greater concentration of pGH was required to half-maximally inhibit [125I]pGH binding. bGH and pGH both bound to the same GH binding proteins (Mr of 92,000, 73,000 and 53,000). The GH binding proteins appear to be produced by post-translational modification of a single GH receptor transcript rather than alternative splicing of a primary transcript since only one GH receptor mRNA transcript (4.2 kb) was detected on Northern analysis. Our findings indicate that: 1) bGH is the preferred ligand to use to study GH binding in pig adipose tissue membranes (or adipocytes); 2) exogenous pGH does not alter GH binding; and 3) only one GH receptor mRNA transcript is present in pig adipose tissue.

Mutation creating a new splice site in the growth hormone receptor genes of 37 Ecuadorean patients with Laron syndrome

Laron syndrome is an autosomal recessive condition characterized by resistance to growth hormone. We sought to determine the molecular basis of this condition in an Ecuadorean population with a high incidence of affected individuals. Growth hormone receptor gene sequences from an obligate heterozygote were amplified by the polymerase chain reaction and screened for mutations using denaturing gradient gel electrophoresis. Only exon 6 revealed homo- and heteroduplexes on denaturing gradient gels. Sequencing revealed a substitution of guanine for adenine in the third position of codon 180 that did not change the amino acid encoded. Sequencing of the exon 6-exon 7 splice junction from RNA-polymerase chain reaction amplified cellular RNA of an affected individual revealed that the substitution activates a 5' splice site 24 nucleotides upstream from the normal exon 6-intron 6 boundary. Splicing in two probands' lymphoblasts occurred virtually exclusively at the abnormal 5' splice site created by the codon 180 substitution. Exon 6 sequences from 38 patients and 47 relatives were amplified and analyzed by sequencing or dot-blot hybridization with allele-specific oligonucleotides. The substitution was detected in 74 of 76 Laron syndrome patients' GH-receptor alleles. All 26 parents and 12 of 21 unaffected siblings were heterozygous for this mutation. It was absent in 61 unrelated unaffected control individuals. We conclude that the codon 180 nucleotide substitution probably causes Laron syndrome as translation of the observed, abnormally spliced growth hormone receptor transcript would lead to the synthesis of a receptor protein with an 8 amino acid deletion from the extracellular domain.

Growth hormone regulation of hepatic cytochrome P450 expression in the rat

GH by means of its sexually differentiated secretory pattern is the predominant regulator of the expression of cytochrome P450 enzymes responsible for a sexual dimorphism of hepatic steroid metabolism. Other hormones, such as gonadal, thyroid and glucocorticoid hormones, as well as insulin appear to modulate the sexually differentiated expression of these enzymes. The major constitutively expressed sex specific forms of P450, belonging to the P4502C-subfamily, have been shown to be regulated by GH at the level of transcription. However, the GH postreceptor events leading to increased or decreased transcriptional activity are essentially unknown. Neither is the functional role of the soluble GH binding protein yet resolved. On-going protein synthesis is a prerequisite for GH transcriptional activation of the female specific P4502C12 but not for all GH effects in the hepatocyte. With regard to signalling mechanisms PKC activity appears to be permissive for the GH induction of P4502C12 but some as yet unidentified factor/kinase(s) may also be activated. The transcriptional control exerted on the rat P4502C-gene subfamily by the pattern of GH secretion offers a versatile tool to elucidate the molecular mechanisms of GH regulation of cytochrome P450 expression.

Expression and physiological significance of growth hormone receptors and growth hormone binding proteins in rat and man

The molecular structure of the GH receptor has recently been characterized and the receptor identified as a member of a new receptor superfamily that includes the prolactin receptor and several cytokine receptors. No obvious signal transducing domain has been identified on any of these related receptors. One possible signalling mechanism involves receptor interaction with other membrane-associated proteins that function as mediators of signal transduction. Whether such a mechanism is involved in signal transduction of the GH receptor is not known. Another common feature of these receptors is the presence of soluble forms such as the GHBP. The functions of these proteins in the circulation and at the level of the target cell remain to be resolved.

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.

Skeletal muscle regeneration in young rats is dependent on growth hormone

Skeletal muscle fibres have a well known ability to regenerate after different kinds of injury. This study was undertaken to establish if regenerating skeletal muscle is dependent on growth hormone (GH) in the same manner as normal, growing skeletal muscle in young rats. Muscle regeneration was achieved by injection of notexin into the soleus muscle. Initial necrosis, which included all muscle fibres, was followed by a rapid and uniform regeneration throughout the muscle. Cell proliferation was estimated by scintillation counting and autoradiography of incorporated [3H]thymidine, injected intravenously 1 h before killing, 7 or 27 days after the initiation of regeneration. GH deficiency was accomplished by hypophysectomy 4 days before the [3H]thymidine injection. Cell proliferation was diminished in both regenerating and normal muscle of the hypophysectomized rats compared to control and GH-substituted rats. After 7 days of regeneration the reduction of cell proliferation seen in hypophysectomized rats was less pronounced in the regenerating than in the normal muscle. These findings demonstrate that GH plays an important role for muscle regeneration in young rats, although other substances appear to be of greater importance during the early stages of regeneration.

Identification of a novel growth hormone binding protein mRNA in rat liver

The presence of highly specific serum binding proteins for growth hormone (GH) has been well characterized in many species. In the rat the major growth hormone-binding protein (GH-BP) is a truncated, variant form of the target tissue GH receptor and is derived by an alternative mRNA splicing event. The GHBP mRNA is coexpressed in all tissues expressing the full length GH receptor. In the present study, we have made an oligonucleotide probe to the unique hydrophilic tail of the rat GHBP mRNA (1.2kb) and identified a novel GHBP-like mRNA of 2.6 kb transcript in addition to the 1.2 kb transcript. This unique 2.6 kb transcript was expressed/detected only in rat liver. There was no significant difference in abundance between the sexes or during pregnancy, implying that this transcript may be regulated independently of the 1.2 kb mRNA. The 2.6 kb transcript was clearly identifiable in the fetus, as was the 1.2 kb transcript, but showed virtually no change in abundance with age, in sharp contrast to the 1.2 kb mRNA, which has a distinct developmental pattern, being low in the fetus and peaking early postnatally. RNAse H treatment suggested that this 2.6 kb transcript is polyadenylated. A corresponding 2.6 kb mRNA has been detected using a longer cDNA or cRNA probe for the GH-binding domain of the rat GHR/GHBP. These data collectively suggest that the 2.6 kb mRNA transcript is a bonafide but tissue-specific GHBP mRNA and that the 1.2 and 2.6 kb mRNAs are likely to differ primarily with respect to the length of the 3' untranslated region of the sequence.

Prolactin receptor gene expression in the rabbit: identification, characterization and tissue distribution of several prolactin receptor messenger RNAs encoding a unique precursor

The expression of the prolactin (PRL) receptor gene was studied in rabbit tissues by Northern blot and S1 mapping analysis of mRNA preparations. Rabbit mammary gland contained three major (10.5, 3.4, and 2.7 kb) and one minor (6.2 kb) prolactin receptor poly(A)+ RNA transcripts all of which contain the entire coding sequence of the long form of PRL receptor. Each of these mammary mRNAs hybridized equally well with cDNA sequences encoding either the NH2 terminal, middle, or COOH terminal part of the rabbit mammary PRL receptor. The four mRNAs differed only in their 5'- and 3'-untranslated regions. The 10.5 kb mammary transcript was further shown to represent a primary transcript of nuclear origin. Among the various rabbit tissues tested, male and female adrenals, mammary gland, ovaries, and jejunum contained the highest level of prolactin receptor mRNA. The prolactin receptor gene was also expressed at moderate to weak abundance in uterus, liver, kidney, pancreas, testis and seminal vesicles. No prolactin receptor mRNA species were detected in adult muscle, lung, total brain, placental cotyledons and spleen, and in thymus from young animals. In all the rabbit tissues examined, the same four PRL receptor poly(A)+ RNA transcripts identified in the mammary gland were expressed and no additional transcript(s) were detected. Variations in the relative proportion of the 10.5 kb transcript and the two smaller transcripts were observed, while the ratio of the 3.4 and 2.7 kb mRNAs remained unchanged. These findings ask for the role of these different transcripts generated in the rabbit, all of which encode the same long form of PRL receptor precursor but have heterogenous 5'- and 3'-untranslated regions. Moreover, they suggest that the various forms of PRL receptor mRNA originate through differential splicing of a single PRL receptor gene.

Contributions of growth hormone receptor and postreceptor defects to growth hormone resistance in malnutrition

Malnutrition results in poor growth and is associated with resistance to growth hormone (GH) action. The mechanisms involved in the GH resistance depend on the severity and the timing of the nutritional insult. Stringent dietary restrictions such as fasting may produce GH resistance by reducing the number of GH receptors. Less severe nutritional deprivation such a short-term protein restriction may cause GH insensitivity mainly through postreceptor mechanisms.

Growth hormone and insulin-like growth factor I: nutritional pathophysiology and therapeutic potential

The growth hormone--insulin-like growth factor I axis has been appreciated for more than 30 years and the effects of malnutrition on this axis for more than 20 years. Over the last decade, advances in molecular biology have permitted enhanced understanding of feedback regulation between growth hormone and IGF-I at the gene level, including limited information on nutritional influences. Similarly, the availability of recombinant human growth hormone has allowed controlled clinical studies demonstrating its net anabolic actions at hypocaloric dietary energy intake levels and its ability to enhance height velocity in children with various causes of diminished growth. Although investigational use of recombinant IGF-I in humans has been limited, its actions are likely to complement those of growth hormone during periods of profound dietary energy deficit. From the information presented, two hypotheses are developed. First, recombinant IGF-I administration will enhance substrate anabolic events during the acutely malnourished state when dietary intake is severely limited. Second, administration of recombinant human growth hormone will accelerate protein anabolism and catch-up growth during the period of recovery from protein-energy malnutrition. Given current clinical investigational tools and the availability of both recombinantly-produced hormones, these are testable hypotheses.

Regulation of human growth hormone receptor gene expression by human growth hormone in a human hepatoma cell line

We have investigated the effects of recombinant human growth hormone (r-hGH) on the expression of hGH-receptor in a human hepatoma cell line (HuH 7). Levels of hGH-receptor mRNA in HuH 7 cells treated with different doses of r-hGH were measured by means of an RNase protection assay. Treatment with r-hGH at physiological concentrations (12.5, 25 and 50 ng/ml) resulted in an increase in hGH-receptor mRNA levels within 1 h of addition of the hormone. A steady state was reached after 3-4 h and maintained for at least 48 h. In contrast, treatment with supraphysiological r-hGH concentrations (150 and 500 ng/ml) led to a down-regulation of hGH-receptor mRNA levels during the first 3 h after hormone addition followed by an increase in hGH-receptor mRNA levels thereafter. Nuclear run-off assays demonstrated that these changes in hGH-receptor mRNA levels were a result of changes in the rate of transcription of the hGH-receptor gene. Cycloheximide (10 micrograms/ml) did not affect these changes in hGH-receptor gene transcription significantly, indicating that they are mediated by pre-existing factors and do not require new protein synthesis. These data demonstrate that r-hGH specifically regulates the rate of transcription of the hGH-receptor gene in a human hepatoma cell line.

Neuroendocrine rhythms

Hormones are secreted with circhoral, circadian and seasonal periodicities. Circhoral pulsatility is a temporal code, many chronic and acute changes in neuroendocrine status being mediated by changes in the frequency of circhoral release. The identity of the neuronal circuits controlling circhoral release is not known. Circadian release of hormones occurs with a precise temporal order entrained to the light-dark cycle, synchronized to the activity/rest rhythm and generated by circadian oscillators, of which the suprachiasmatic nuclei are the most important. Seasonal rhythms are driven either by an endogenous circannual clock mechanism or by a process of photoperiodic time measurement which is dependent upon the duration of the nocturnal peak of the pineal hormone melatonin.

Identification and characterization of growth hormone receptor mRNA in the mammary gland

The present report describes the first characterization of growth hormone (GH) receptor (GH-R) mRNA in the rabbit mammary gland. Northern blot analysis of poly(A)+ RNA isolated from several tissues of rabbit probed with a rabbit liver GH-R cDNA fragment revealed hybridization to only one transcript of 4.2 kb. A specific hybridizing signal appears in the mammary gland mRNA during gestation, when three different probes derived from liver GH-R cDNA and encoding respectively for extracellular, transmembrane and intracellular regions, were used. The signal is lower than in the liver but highly significant. These results indicate that the three regions are present and well conserved in the GH-R transcript found in the mammary gland. By S1 nuclease mapping analysis we demonstrated that the extracellular and transmembrane domains of mammary gland GH-R mRNA are strongly homologous to the liver GH-R mRNA. In addition, mammary gland GH-R mRNA is probably generated by mammary epithelial cells as demonstrated by the hybridization signal obtained using mRNA extracted from purified acini. The increase in the concentration of GH-R mRNA occurs during epithelial cell proliferation associated with a decrease in the proportion of adipocytes and connective cells at late gestation. The 4.2 kb GH-R mRNA species was also detected in ovine and porcine mammary glands during gestation, suggesting a probable expression of the related form of GH-R in these species.

Molecular basis of Laron dwarfism

An autosomal recessive disorder, Laron-type dwarfism, results from peripheral unresponsiveness to growth hormone. Mutations in the growth hormone receptor have recently been identified in this syndrome. Analysis of patients with Laron-type dwarfism should provide insight into the mechanisms of hormone receptor binding and signal transduction pathways of this receptor, which belongs to a new class of transmembrane receptors.

Zinc mediation of the binding of human growth hormone to the human prolactin receptor

Human growth hormone (hGH) elicits a diverse set of biological activities including lactation that derives from binding to the prolactin (PRL) receptor. The binding affinity of hGH for the extracellular binding domain of the hPRL receptor (hPRLbp) was increased about 8000-fold by addition of 50 micromolar ZnCl2. Zinc was not required for binding of hGH to the hGH binding protein (hGHbp) or for binding of hPRL to the hPRLbp. Other divalent metal ions (Ca2+, Mg2+, Cu2+, Mn2+, and Co2+) at physiological concentrations did not support such strong binding. Scatchard analysis indicated a stoichiometry of one Zn2+ per hGH.hPRLbp complex. Mutational analysis showed that a cluster of three residues (His18, His21, and Glu174) in hGH and His188 from the hPRLbp (conserved in all PRL receptors but not GH receptors) are probable Zn2+ ligands. This polypeptide hormone.receptor "zinc sandwich" provides a molecular mechanism to explain why nonprimate GHs are not lactogenic and offers a molecular link between zinc deficiency and its association with altered functions of hGH.

The sheep growth hormone receptor: molecular cloning and ontogeny of mRNA expression in the liver

Two overlapping cDNA clones encoding the sheep growth hormone (GH) receptor were isolated from a sheep liver cDNA library. The translated amino acid sequence predicts a polypeptide precursor of 634 amino acids with a calculated molecular weight of 70,799. The mature GH receptor comprises an extracellular domain of 242 amino acids, a hydrophobic transmembrane region of 24 amino acids, and a cytoplasmic domain of 350 amino acids. The nucleotide and translated amino acid sequences display extensive similarity with sequences established for GH receptors from a number of other mammalian species. A prominent transcript of 4.5 kb and a minor transcript of 1.9 kb are detected following Northern blot hybridization of poly(A)+ RNA isolated from sheep liver. The onset of GH receptor mRNA expression in the liver is developmentally regulated: GH receptor transcripts are first detected by Northern blot hybridization in liver taken from a term (145 days of gestation) fetus and reach maximum levels within one week following birth. Ribonuclease protection assays reveal heterogeneity within the 5' untranslated region of GH receptor mRNA transcripts detected in liver and a number of other tissues. At least one transcript appears to be expressed in a liver-specific fashion, supporting a role for alternative RNA splicing in the tissue-specific regulation of sheep GH receptor expression.

Expression of the growth hormone-binding protein messenger RNA in the liver and extrahepatic tissues in the rat: co-expression with the growth hormone receptor

A cDNA encoding a growth hormone-binding protein (GH-BP) was recently cloned from mouse and rat liver. The GH-BP in these species is identical to the extracellular part of the GH receptor (GH-R) with the transmembrane and intracellular domain substituted for a hydrophilic tail. In the present study the expression of the GH-BP and GH-R was studied in rat liver and extrahepatic tissues. Specific transcripts with estimated sizes of 1.2 kb (GH-BP) and 4.0 kb (GH-R) were found in the liver from both sexes. The expression of GH-BP increased with age up to puberty suggesting that it is developmentally regulated in a similar manner as GH-R. GH-BP mRNA was found in all extrahepatic tissues examined that contained GH-R mRNA. The ratio between the 1.2 kb and 4.0 kb transcripts varied between tissues indicating that GH-R and GH-BP transcripts may be separately regulated. The co-expression of GH-BP and GH-R suggests a functional role for the GH-BP in the local regulation of GH action.

Cloning and in vivo expression of bovine growth hormone receptor mRNA

A cDNA for the bovine growth hormone (bGH) receptor has been cloned out of a cDNA library prepared from liver of a pregnant Holstein heifer. The cDNA clone hybridizes to a single 4.5 kb mRNA species and shares a high degree of sequence homology with growth hormone receptors cloned from other species. Utilizing the bGH receptor cDNA as a probe, a relatively high level of bGH-receptor mRNA was detected in bovine liver. In comparison to liver values, lower concentrations of bGH-receptor mRNA were detected in bovine kidney, anterior pituitary, and mammary gland. Because specific binding sites for bGH have not been convincingly demonstrated in isolated cell membranes from whole bovine mammary tissue, mammary tissue from two pregnant heifers (separate experiments) was separated into fractions enriched for epithelium, stroma, and blood components. These fractions were then probed for growth hormone receptor mRNA using solution hybridization-nuclease protection assays performed on isolated RNA. The assay results indicated that a low level of bGH-receptor mRNA is relatively evenly distributed throughout the mammary tissues of the two cows studied. In contrast, experiments using a probe to bovine insulin-like growth factor-I (IGF-I) indicate that the IGF-I mRNA is localized in the stromal/blood component of the mammary gland. These data suggest a possible paracrine mechanism for bGH action in the mammary gland.

Growth hormone regulation of the growth hormone receptor mRNA in cultured rat epiphyseal chondrocytes

This study was designed to investigate whether growth hormone (GH) influences the expression of its own receptor in chondrocytes. To investigate this possibility GH-receptor mRNA was measured in cultured rat epiphyseal chondrocytes in the absence or presence of GH under various experimental conditions. Chondrocytes were isolated enzymatically from epiphyseal growth plates of the proximal tibia of 20-day-old male rats and cultured in monolayer in Ham's F-12 medium supplemented with 10% calf serum and 1% of a serum substitute. The cells were seeded at various densities (100,000-1,000,000 cells per flask) and cultured for 14 days. Subsequently, the calf serum-containing medium and the cells cultured for various periods of time (0-24 h) before total nucleic acid preparation. GH-receptor mRNA was measured with a solution hybridization technique using [35S]UTP-labeled RNA growth hormone receptor cloned from rat liver cDNA. Human GH (hGH; 50 ng/ml) increased GH-receptor mRNA after 3 h and maximal levels were seen 12 h after GH addition. This effect of hGH was time and dose dependent with a significant effect of hGH at a concentration of 0.5 ng/ml and a maximal effect at 50 ng/ml. The hGH-stimulated increase of GH-receptor mRNA was completely blocked by actinomycin-C1 (1.0-0.1 micrograms/ml), while cycloheximide (10 micrograms/ml) only slightly counteracted the hGH effect. Rat and human GH were equally potent, and ovine prolactin was effective at 500 ng/ml but not 5 and 50 ng/ml. A high dose of insulin-like growth factor-I (IGF-I; 1 microgram/ml) caused a small stimulatory effect and addition of 10% calf serum caused a marked increase in GH-receptor mRNA. The level of GH receptor mRNA after 14 days of culture was inversely proportional to the cell density at the start of culture. These results show that GH specifically regulates mRNA levels for its own receptor in rat epiphyseal chondrocytes by interacting with somatogenic binding sites. These findings also suggest a transcription-dependent regulatory system between the GH-receptor and the GH-receptor gene.