Insulinlike growth factor I regulation of growth hormone gene transcription in primary rat pituitary cells

Radioimmunoassay of insulin-like growth factors I and II in the cerebrospinal fluid of patients with pituitary and other central nervous system tumors

Tumor cells are characterized by abnormalities in growth and metabolism, including the autocrine secretion of certain growth factors. On the basis of our previous demonstration of the production of insulin-like growth factors (IGFs) and their binding proteins by central nervous system (CNS) tumors, we asked whether the levels of IGFs in the CSF may be altered in patients with pituitary and other CNS tumors and may reflect autocrine secretion. We used specific radioimmuoassays for IGF-I and -II and measured these growth factors in the cerebrospinal fluid (CSF) from 26 patients with tumors located adjacent to the ventricular system. The tumors included were eight pituitary tumors (five nonsecreting, three growth hormone secreting), five gliomas, two meningiomas, five medulloblastomas, three metastases, and three other tumors. CSF from patients without tumors served as controls. For radioimmunoassay, CSF was treated with acetic acid overnight and IGF-binding proteins were separated from IGFs by C-2 solid phase cartridge extraction. The pituitary tumors were characterized by significantly elevated levels of IGFs in the CSF. In nonseceting pituitary tumors, the levels of IGF-I in the CSF were similar to normal levels, whereas IGF-II levels were significantly elevated. In acromegalic patients, levels of both IGF-I and -II in the CSF were significantly elevated compared with normal levels and compared with levels in patients with nonsecreting tumors. In contrast, the levels of IGFs in the CSF from most of the primary and metastatic CNS CNS tumors did not significantly differ from normal values. In summary, although IGFs may contribute to the regulation of cell growth in primary CNS tumors, CSF levels are not elevated.(ABSTRACT TRUNCATED AT 250 WORDS)

Human and murine pituitary expression of leukemia inhibitory factor. Novel intrapituitary regulation of adrenocorticotropin hormone synthesis and secretion

Leukemia inhibitory factor (LIF) gene expression was detected in human fetal pituitary tissue by expression of LIF mRNA transcripts, protein immunocytochemistry, and immunoelectron microscopy. Fetal LIF immunoreactivity colocalized with 30% of ACTH-expressing cells, approximately 20% of somatotrophs, and approximately 15% of non-hormone-expressing cells. LIF was also strongly expressed in normal adult pituitary and in four growth hormone-producing and two ACTH-producing adenomas, but not in eight nonfunctioning pituitary tumors. Culture of fetal cells expressing surface LIF-binding sites demonstrated predominance of in vitro ACTH secretion as compared with other pituitary hormones. In AtT-20 murine cells, LIF (ED50 10 pM) stimulated basal proopiomelanocortin mRNA levels by 40% and corticotropin-releasing hormone-induced ACTH secretion (two- to threefold), as did oncostatin M (ED50 30 pM), a related peptide. ACTH responses were not further enhanced by both cytokines together, which is consistent with their shared receptor. Anti-LIF antiserum neutralized basal and LIF-induced ACTH secretion, suggesting autocrine regulation of ACTH by LIF. The results show that human pituitary cells express the LIF gene and LIF-binding sites, predominantly in corticotrophs. Pituitary LIF expression and LIF regulation of proopiomelanocortin and ACTH reflect an intrapituitary role for LIF in modulating early embryonic determination of specific human pituitary cells and as a paracrine or autocrine regulator of mature ACTH.

The IGF-I receptor sub-membrane domain is intact in GH-secreting pituitary tumours

BACKGROUND AND OBJECTIVE

Clinical acromegaly is characterized by dysregulation of somatotroph GH secretion in the presence of high circulating serum IGF-I levels. Physiologically, IGF-I exerts a negative feedback on GH secretion at both the hypothalamic and the pituitary levels. We have previously shown that the 943 and 950 tyrosine residues in the IGF-I receptor beta-subunit are required for ligand signalling to the GH gene, as substitution of these residues abrogates IGF-I signal transduction. To determine whether a mutation within the IGF-I receptor submembrane domain may be involved in the pathogenesis of GH secreting tumours, we studied this region in these tumours.

DESIGN

Exon 15 of the IGF-I receptor containing both the 943 and 950 tyrosines was analysed in 19 GH-secreting tumours by single-strand conformation polymorphism (SSCP) analysis of polymerase chain reaction (PCR) products. Tumour DNA and patients' lymphocyte DNA, which served as normal controls, were analysed.

RESULTS

All samples exhibited normal migration patterns in the SSCP analysis which was further confirmed by direct DNA sequencing.

CONCLUSIONS

We conclude that mutations in the IGF-I receptor sub-membrane domain which disrupt the negative feedback loop are not involved in the pathogenesis of acromegaly.

Differential regulation of the expression of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase by thyroid hormone and insulin-like growth factor-I in the L6 muscle cell line

The aim of this study was to investigate the mechanism(s) underlying the thyroid-hormone (L-tri-iodothyronine, T3)-induced elevation of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase (SERCA1) levels in L6 myotubes and the potentiating effect of insulin-like growth factor-I (IGF-I) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 275, 35-40]. T3 increased the SERCA1 protein level (per microgram of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA levels (50%), whereas IGF-I + T3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that the more-than-additive action of T3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not altered by T3. In contrast, IGF-I prolonged the half-life of SERCA1 protein 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T3 or IGF-I alone. When T3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T3 and IGF-I were observed on the half-life of total protein (37.4 h) and total RNA (37.0 h). The absence of an effect of T3 on SERCA1 protein and mRNA stability, when it was present alone, suggested transcriptional regulation, which was confirmed by nuclear run-on experiments, showing a 3-fold increase in transcription frequency of the SERCA1 gene by T3. We conclude that the synergistic stimulating effects of T3 and IGF-I on SERCA1 expression are the result of both transcriptional and post-transcriptional regulation. T3 acts primarily at the transcriptional level by increasing the transcription frequency of the SERCA1 gene, whereas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, only in the presence of T3.

The effects of recombinant human insulin-like growth factor I on growth hormone secretion in adolescents with insulin dependent diabetes mellitus

OBJECTIVE

It has been proposed that low IGF-I levels and reduced IGF-I bioactivity may lead to elevated GH levels in adolescents with insulin dependent diabetes (IDDM). We have therefore studied the effects of human recombinant insulin-like growth factor I (rhIGF-I) administration on GH levels and GH secretion in adolescents with IDDM.

PATIENTS

Nine late pubertal adolescents (four male and five female) with IDDM.

DESIGN

A double-blind placebo controlled study of rhIGF-I administered subcutaneously in a dose of 40 micrograms/kg body weight at 1800 h.

MEASUREMENTS

IGF-I and GH concentrations were measured at regular intervals throughout the study. Twenty-two hour GH secretory rates were calculated by deconvolution analysis. Overnight GH profiles were analysed by distribution analysis, and Fourier transformations were performed on both overnight GH concentrations and GH secretory rates.

RESULTS

Mean IGF-I levels over the 22-hour study period were significantly elevated following rhIGF-I administration (350 +/- 26 vs 205 +/- 21 micrograms/l (mean +/- SEM), P < 0.01). Mean 22-hour GH levels were reduced following rhIGF-I administration (19.4 +/- 4.0 compared with 33.6 +/- 5.8 mU/l; P = 0.01). Distribution analysis demonstrated that the reduction in GH levels was due to changes in the proportion of values at both high and low concentrations. Deconvolution analysis also revealed a significant overall reduction in GH secretory rate following IGF-I administration (1.81 +/- 0.30 vs 2.98 +/- 0.47 mU/min, P = 0.01) which was still apparent during the final 5.5 hours of the study period (1.51 +/- 0.30 vs 2.76 +/- 0.61 mU/min, P = 0.02). The dominant periodicity of GH secretory episodes as determined by Fourier transformation was between 120 and 180 minutes after both IGF-I and placebo.

CONCLUSIONS

In late pubertal adolescents with IDDM the rise in IGF-I levels following rhIGF-I administration in a subcutaneous dose of 40 micrograms/kg body weight leads to a significant reduction in GH levels and GH secretory rate. The reduction in GH secretion is due to changes in pulse amplitude rather than frequency. A reduction in GH secretion was apparent at the beginning and also towards the end of the 22-hour study period.

Growth hormone secretion: the role of glucocorticoids

Glucocorticoids (GCs) modulate the somatotropic axis at a genomic and a non-genomic level. Critical concentrations of steroids not only determine somatotrope differentiation but also enhance growth hormone (GH) gene expression. At a cellular level GH-releasing hormone (GHRH) and somatostatin (SS) are the two principal neuropeptides involved in the release of GH. In vitro data indicates that steroids enhance GH release by altering the affinity and the density of GHRH receptors. In addition, they reduce the sensitivity of the somatotrope to SS and decrease IGF-1 induced negative feedback on GH secretion. The net effect is an enhancement of GH release.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

Insulin-like growth factor I modulates hypothalamic somatostatin through a growth hormone releasing factor increased somatostatin release and messenger ribonucleic acid levels

Insulin-like growth factor I (IGF-I) has been shown to participate in feedback inhibition of growth hormone (GH) secretion at the level of both the pituitary and hypothalamus. Therefore, we tested the possible involvement of IGF-I on somatostatin (SRIF) and GH-releasing factor (GRF) release in median eminence (ME) fragments and periventricular nucleus (PeN) of male rats. The levels of SRIF messenger ribonucleic acid (mRNA) were also determined in PeN incubated in vitro with IGF-I. The ME's were incubated in Krebs-Ringer bicarbonate glucose buffer in the presence of various concentrations of IGF-I (10(-7) to 10(-11) M) for 30 min. SRIF and GRF released into the medium were quantitated by RIA. The release of SRIF and GRF from the ME's was stimulated significantly (P < 0.025 and P < 0.05, respectively) by 10(-9) M IGF-I. To determine whether the effect of IGF-I on SRIF release is mediated by GRF release in the ME, a specific GRF antibody (ab) (1:500) was used concomitantly with IGF-I (10(-9) M). The release of SRIF induced by IGF-I was blocked by the GRF ab (P < 0.001), but not by normal rabbit serum used at the same dilution. To determine the effect of IGF-I on the regulation of SRIF mRNA levels, SRIF mRNA was determined in PeN explants incubated in the presence of IGF-I (10(-8) to 10(-10) M) for 2 to 6 h. Levels of SRIF mRNA were determined by a S1 nuclease protection assay using a 32P-labelled rat SRIF riboprobe.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone response to L-dopa and pyridostigmine in women with polycystic ovarian syndrome

OBJECTIVE

To evaluate the GH secretion and clarify the factors influencing the GH secretion in women with polycystic ovarian syndrome (PCOS).

DESIGN

Comparison of the GH response to L-dopa with or without pyridostigmine (inhibitor of acetylcholinesterase) pretreatment and insulin response to oral glucose tolerance test in patients with PCOS and matched controls.

SETTING

Outpatients and healthy volunteers studied at a clinical research unit of a university hospital.

PATIENTS, PARTICIPANTS

Ten women with PCOS and 9 controls with regular cycles were recruited.

INTERVENTIONS

After an overnight fast, each subject underwent a GH stimulation test with L-dopa with or without pyridostigmine pretreatment. Plasma insulin and glucose levels were measured after a 75-g glucose load.

MAIN OUTCOME MEASURES

Plasma GH, insulin-like growth factor I (IGF-I), insulin, and nonesterified fatty acids.

RESULTS

Growth hormone responses and GH area under the response curve (AUC) to L-dopa were significantly lower in PCOS than those in controls. Pyridostigmine enhanced the GH response to L-dopa significantly in PCOS. Insulin responses and insulin AUC to oral glucose load were significantly higher in PCOS than those in controls. Plasma IGF-I levels of PCOS were significantly higher than controls. Insulin AUC had a positive correlation with plasma IGF-I levels but an inverse correlation with GH AUC in PCOS and controls.

CONCLUSION

Our result indicated that decreased GH secretion of PCOS may be associated with a high somatostatin activity and a high plasma IGF-I level.

Pituitary and hypothalamic insulin-like growth factor-I (IGF-I) and IGF-I receptor expression in food-deprived rats

The present study was designed to evaluate a possible role for the insulin-like growth factor-I (IGF-I) system in mediating the suppression of growth hormone (GH) secretion observed in food-deprived rats by measuring IGF-I mRNA, receptor concentration and receptor mRNA in neuroendocrine tissues (hypothalamus and pituitary). Rats were deprived of food (food-deprived) for 72 h or had free access to food (fed). Tissues were processed for measurement of steady-state levels of: (a) IGF-I and IGF-I receptor mRNA (by solution hybridization/RNase protection assay); (b) IGF-I in serum and tissue extracts (by RIA) and (c) IGF-I displaceable [125I]IGF-I binding to plasma membrane preparations. Food deprivation resulted in decreased serum and liver levels of IGF-I. Kidney IGF-I mRNA levels were reduced 80% in food-deprived rats with a concomitant increase in IGF-I receptor concentration and mRNA levels. Refeeding of food-deprived rats fully normalized these perturbations. Pituitary IGF-I content was reduced 50% in food-deprived rats while IGF-I mRNA levels were unaffected. A modest increase was seen in pituitary IGF-I receptor concentration; however, IGF-I receptor mRNA levels were not changed. Hypothalamic IGF-I mRNA content was reduced in 72 h food-deprived rats while IGF-I receptor binding capacity and mRNA were unaffected. In conclusion, IGF-I mRNA levels are decreased in liver, kidney and hypothalamus together with a reduction in plasma IGF-I in food-deprived rats but is unaffected in anterior pituitary. IGF-I receptor gene expression and binding capacity are coordinately regulated in kidney and hypothalamus, but not in the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of pituitary insulin-like growth factor-I bindings in rats bearing somatomammotrophic tumors

To clarify the hormonal regulation of pituitary insulin-like growth factor-I (IGF-I) bindings, we examined the continuous effect of growth hormone (GH) on [125I] IGF-I binding sites, using rats bearing transplantable GH-secreting rat pituitary tumor cells. A total of 24 female Wistar-Furth rats (4-week old) was divided into four groups (n = 6). The first group rats were control (C). The second group rats were injected subcutaneously with 3 x 10(6) GH3 pituitary tumor cells (GH). The third group rats were thyroidectomized (Tx) and the fourth were dual-treated (GHTx). The brain, pituitary gland, liver, and kidney were immediately subjected to quantitative receptor autoradiography after a 4-week treatment period. Using kinetic experiments, GH rats had higher Bmax values of specific [125I] IGF-I binding sites in the anterior pituitary gland and Tx rats had lower Bmax values than control rats. Two-way analysis of variance among the 4 groups was examined. The effects of both GH and Tx treatment on [125I] IGF-I binding Bmax were observed only in the anterior pituitary gland and not in the other tissues examined. There were no differences in the Kd values of the binding sites. These data indicate that continuous GH excess selectively up-regulates the number of pituitary IGF-I binding sites in vivo, and that this may play a role in feedback regulation of the GH-IGF-I axis.

Human insulin-like growth factor I receptor function in pituitary cells is suppressed by a dominant negative mutant

Hybrid receptors were studied in GC rat pituitary cells overexpressing either wild-type 950Tyr (WT) human insulin-like growth factor I (IGF-I) receptors or mutant human IGF-I receptors truncated at position 952 in the beta subunit transmembrane region (952STOP). 125I-IGF-I binding was increased in both 950Tyr (WT) (14-fold) and truncated human IGF-I receptor (952STOP) stable transfectants (50-fold), when compared to untransfected cells that contained endogenous rat IGF-I receptors. Metabolic cell labeling followed by immunoprecipitation with monoclonal alpha and beta subunit-specific antibodies revealed the presence of hybrid rat/truncated human receptors, truncated transfected human receptors, and WT human IGF-I holotetramers. Both mutant and hybrid receptors were degraded slower than 950Tyr (WT) receptors (> 16 h). Despite their markedly increased ligand binding and prolonged receptor half-life, 952STOP transfectants failed to transduce the IGF-I signal to suppress growth hormone (GH). Also, they neither underwent autophosphorylation nor phosphorylated endogenous proteins. The expected suppression of GH by endogenous rat IGF-I receptors was completely abrogated in 952STOP transfectants (P < 0.001 compared to untransfected cells). Mutant 952STOP cells were therefore completely devoid of biological signaling to GH despite the presence of endogenous rat IGF-I receptors. Thus mutant IGF-I receptors block ligand-mediated endogenous rat IGF-I signaling by functioning as a dominant negative forming nonfunctional human/rat hybrid receptors. Defective IGF-I receptors may function therefore as dominant negative phenotypes which suppress normal receptor responses in pituitary cells.

Response of young broiler chickens to chronic injection of recombinant-derived human insulin-like growth factor-I

The purpose of this study was to determine if exogenous insulin-like growth factor-I (IGF-I) would improve growth rate or body composition of young broiler chickens. Broiler cockerels were given a daily intramuscular (im) injection of sodium acetate buffer (buffer control), 100 or 200 micrograms recombinant-derived human IGF-I (rhIGF-I) per kg body weight from 11 to 24 days of age. Exogenous IGF-I did not affect the average daily gain, average daily feed consumption, or the gain-to-feed ratio of broiler chickens. Although daily injection of 200 micrograms/kg of rhIGF-I reduced (P less than 0.05) body ash content, there was no significant effect of IGF-I treatment on either body fat or protein content. Plasma GH levels were depressed (P less than 0.05) by chronic treatment with rhIGF-I. In contrast, plasma levels of T3 and T4 were not affected by rhIGF-I treatment. The half-life of rhIGF-I in plasma was determined at 25 days of age in naive control or chronically-injected chickens after a single intravenous dose of 50 micrograms rhIGF-I/kg. We found a single compartment, first-order disappearance pattern of rhIGF-I from chicken plasma. The half-life (t1/2) of rhIGF-I in plasma was similar (t1/2 = 32.5 min) for naive controls (injected once) or chronically-treated chickens which had received a daily injection of rhIGF-I (100 or 200 micrograms/kg) for 14 d. These data indicate that daily injection of IGF-I cannot be used to enhance growth performance or body composition of broiler chickens when given during the early growth period. The depression of plasma GH levels in rhIGF-I-injected chickens supports a negative-feedback role of IGF-I on pituitary GH secretion.

Growth hormone and insulin-like growth factor-I measurements in high growth (hg) mice

Effects of a recessive gene causing high growth (hg) were studied on two major components of the growth axis in mice. Plasma and pituitary levels of growth hormone and plasma levels of insulin-like growth factor I (IGF-I) were measured in three lines homozygous for hg, each compared with a control line of alike genetic background but wild type for the hg locus (Hg). Line Gh (hghg) and line GH (HgHg) are from a line which had undergone long-term selection for high postweaning weight gain; line Ch (hghg) and line CH (HgHg) were extracted from the second backcross of Gh to C57BL/6J; line L54 (hghg) was from the sixth backcross to C57BL/6J (B6) (HgHg). Pituitary GH levels and plasma IGF-I levels were measured in both sexes at 3, 4.5, 6 and 9 wk of age. Plasma growth hormone was measured in 8- to 12-wk-old males at hourly intervals from 08.00 to 17.00. Body weight in lines homozygous for hg at 6 and 9 wk of age was 10-30% greater than in control lines. The ontogeny of this increased growth depended on genetic background. Pituitary growth hormone content was 52% lower in the two hghg lines measured (lines Ch and Gh) than in control lines at 4.5, 6 and 9 wk. Plasma growth hormone levels were also much lower in hg mice, with values only 20-30% of those in their respective controls. hg lines showed consistently low plasma growth hormone levels throughout the 9 hr sampling period, while control lines expressed the characteristic pulsatile hormone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of insulin-like growth factor 1 (IGF-1) and 2 (IGF-2) receptors in the hippocampal formation of rats and mice

This study demonstrated species differences in IGF-1 and IGF-2 receptor binding and localization in the hippocampus of the rat and mouse. Competition binding studies indicated that there were no differences in the relative binding affinities for the type 1 or type 2 receptors between the brains of these animals. These results suggested that the observed species differences were not attributable to alterations in IGF receptor kinetics. Receptor autoradiographic analyses demonstrated that IGF-1 binding differed in both the localization and overall receptor densities observed, with the rat demonstrating more specific localization and greater receptor density in the hippocampus than the mouse. The rat also exhibited a greater density of IGF-2 receptors in the hippocampus than the mouse. Despite differences in IGF receptor populations, both species exhibit similar hippocampal structure and lamination. Therefore, these results demonstrate a disparity in the localization of IGF receptor binding in the rat and mouse, suggesting that IGFs in these species are differentially regulated, with distinct neuromodulatory, neurotrophic, and/or developmental roles in this region of the brain. Previous comparative anatomical studies of the hippocampal formation of rats and mice fail to offer an explanation for the absence or reduction of binding of IGF-1 in the mouse. Although the mouse has a greater cell density in the s. granulosum than the rat, and both species exhibit similar glia and synaptic contact densities in the s. moleculare of the dentate gyrus, the mouse exhibits a complete absence of IGF-1 binding in this region. The lack of anatomical differences in the hippocampal formation of these species suggests that the patterns observed in IGF binding result from alterations in either neurochemical modulation of these neurons or specific neurotrophic requirements of the cells in this region. Differences have been reported on the concentrations and binding of various neurotransmitters in the hippocampus of these species, however these differences do not easily account for the variations observed in IGF binding in this study. IGFs are known to influence acetylcholine neurotransmission in the hippocampus as well as other brain areas in the rat. Recently, a truncated form of IGF-1, in which a tripeptide is cleaved from the N-terminus of the peptide, has been reported in brain. The cleaved tripeptide has been shown to activate glutamate receptors, which may dramatically influence excitatory neurotransmission in this region. Therefore, in addition to the possible neurotrophic actions of the peptide itself, subsequent processing of IGF-1 may be an important aspect of IGF-1 activity in the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation and action of insulin-like growth factors at the cellular level

Present understanding of IGF-1 as a growth factor mediating integration of nutritional-hormonal interactions indicates that IGF-1 acts in both an endocrine mode on distant targets and an autocrine-paracrine mode on local targets. In the liver, the combined presence of GH, insulin, and critical metabolic fuels such as essential amino acids results in increased levels of IGF-1 messenger RNA, increased production of a high-MW IGF-1 precursor, and increased release of IGF-1 into the circulation, permitting action on distant target tissues bearing specific receptors for IGF-1. The net effect is distant amplification of anabolic hormone action via IGF-1 acting in an endocrine mode. In extrahepatic tissues, both 'general' anabolic hormones (insulin and GH) as well as 'specific' hormones (e.g. gonadotropins) acting on a wide variety of targets (including fibroblasts and chondrocytes as well as granulosa and Leydig cells) promote both local secretion of IGF-1 and an increase in IGF-1 receptors. Local actions of IGF-1 then result in a secondary increase in both hormone receptors and hormone responses. The net effect is local amplification of hormone action via IGF-1 acting as a growth factor in an autocrine-paracrine mode.

Identification and localization of insulin-like growth factor receptors in human infant pituitary gland-similar distribution to somatotrophs

Abstract Insulin-like growth factors(IGF) are involved in feedback regulation of growth hormone(GH) secretion from the pituitary. Though receptors for IGF-I and IGF-II have been identified on particulate preparations of rat pituitary, their localization and relationship to GH-secreting acidophils has not been determined. We used quantitative in vitro autoradiography and immunocytochemistry to simultaneously determine the distribution of IGF receptors and GH-secreting cells in human infant pituitary gland. Frozen or fixed post-mortem human infant pituitary glands were sectioned for binding studies, or immunocytochemistry. Binding for IGF-I and IGF-II showed characteristic specificity for respectively Type I and Type II receptors. Binding sites were visualized throughout the pituitary gland, with similar density and distribution for IGF-I and IGF-II receptor sites. Receptor density was two-fold higher in anterior than posterior pituitary, with highest density in the lateral horns of the anterior pituitary. The distribution of GH-containing cells (acidophils) was similar to IGF receptor distribution. Increased density of IGF receptors in regions of GH-secreting cells may point to the mechanism whereby IGF uniquely inhibits synthesis of human GH in contrast to its promotion of synthetic processes in other cells.

Transcriptional stimulation of the delta 1-crystallin gene by insulin-like growth factor I and insulin requires DNA cis elements in chicken

Insulin-like growth factor I (IGF-I) and insulin regulate expression of the endogenous delta 1-crystallin gene in embryonic lens cells that express receptors for both peptides. To further analyze the transcriptional component of this hormonal effect, transient transfections of lens cells were prepared with DNA constructs containing deletions of the delta 1-crystallin promoter and the chloramphenicol acetyltransferase reporter gene. A 77-nucleotide DNA segment of the delta 1-crystallin promoter from nucleotide positions-120 to -43 confers sensitivity to insulin and IGF-I. The hormonal effect is dose-dependent, and maximal stimulation of promoter activity (2- to 2.5-fold induction) is obtained with 10(-8) M IGF-I and 10(-7) M insulin. Mobility-shift DNA-binding analysis shows specific binding of nuclear protein(s) to the delta 1-crystallin promoter DNA between positions -120 and +23, which appears to be regulated by IGF-I. An SP1-binding motif is involved in this DNA-protein interaction. The bivalent IgG fraction of an anti-insulin receptor antiserum (B-10), known to mimic insulin action in other systems, stimulates promoter activity to the same extent as insulin.

A close correlation between the inhibitory effects of insulin-like growth factor-I and SMS 201-995 on growth hormone release by acromegalic pituitary tumours in vitro and in vivo

In the present study we compared the in-vitro effects of IGF-I and SMS 201-995 on GH release by cultured tumour cells obtained from seven acromegalic patients with the preoperative in-vivo GH dynamics, including the acute response to 50 micrograms SMS 201-995 subcutaneously. IGF-I and SMS 201-995 inhibited GH release during a 24 h incubation in four and five of the seven tumour cell preparations, respectively. The inhibitory effect of SMS 201-995 was greater than that exerted by IGF-I (P less than 0.01). There was a close correlation between the in-vitro inhibitory effects of IGF-I and SMS 201-995 (P less than 0.01). In addition, the acute inhibitory effect of 50 micrograms SMS 201-995 on circulating GH levels in vivo correlated with the inhibitory effects in vitro of both SMS 201-995 (P less than 0.01) and IGF-I (P less than 0.05). The inhibitory effects of IGF-I and SMS 201-995 on GH release in vitro were shown to be additive in two of four tumours. There was no relation between the in-vitro effects of IGF-I and/or SMS 201-995 and several in-vivo parameters, including fluctuations in GH levels, sleep-induced GH release, a paradoxical increase of GH in response to TRH, and the circulating IGF-I and PRL levels.

IN CONCLUSION

(1) there is a close correlation between the sensitivity of GH release by cultured human adenoma cells to IGF-I and SMS 201-995. (2) There is also a close correlation between the in-vivo inhibitory effect on GH release of SMS 201-995 and the in-vitro inhibitory effects of both SMS 201-995 and IGF-I. (3) A subgroup of acromegalic patients harbour pituitary tumours in which the qualitative regulation of hormone secretion is similar to that of normal GH secretion.

Dexamethasone control of growth hormone mRNA levels in GH3 pituitary cells is cycloheximide-sensitive and primarily posttranscriptional

To clarify the mechanism of growth hormone (GH) gene activation by glucocorticoids in GH3 pituitary cells, GH mRNA accumulation in nuclear and cytoplasmic compartments was measured in the presence and absence of cycloheximide. In dexamethasone-treated cells, levels of GH mRNA were increased in the nucleus by 6 h and in the cytoplasm by 12 h. Dexamethasone treatment caused a 5- to 24-fold rise in total GH mRNA levels by 48-72 h. The differential elevation of nuclear levels of GH mRNA relative to the amount of cytoplasmic GH mRNA persisted for 48 h. A transient accumulation of GH mRNA in the nucleus was followed by a brief rise in cytoplasmic GH mRNA levels in GH3 cells treated simultaneously with dexamethasone and cycloheximide. In GH3 cells pretreated for 2 h with cycloheximide, the rise in nuclear and cytoplasmic GH mRNA levels mediated by dexamethasone was blocked completely. Levels of glucocorticoid receptor were unaffected by cycloheximide. These data suggest that the stimulation of GH mRNA levels by glucocorticoids is initiated within the nucleus and that cycloheximide-sensitive events are essential for this stimulation to occur. To assess the importance of GH gene transcriptional activation by glucocorticoids, nuclear transcription run-on reactions and assays of GH promoter activity in an aminoglycoside 3'-phosphotransferase (Neo) fusion gene within stably transformed GH3 cells were performed. Evidence for a weak, transient transcriptional activation of the GH gene by dexamethasone in nuclear run-on assays was obtained. Consistent with this idea, a 30-72 h exposure to dexamethasone raised levels of Neo mRNA in GH-Neo GH3 cell transformants by less than or equal to 2-fold. We conclude that glucocorticoid stimulation of GH mRNA in GH3 cells requires ongoing protein synthesis and can occur largely independently of GH gene transcriptional activation.

Processing of the primary transcript for the rat growth hormone gene in vivo

Processing of the rat growth hormone (rGH) gene primary transcript and the effects of thyroid and glucocorticoid hormones on rGH pre-mRNA levels have been studied using subcloned radiolabeled DNA fragments from each of the four introns of this gene as probes. Blot-hybridization analysis of poly(A)+RNA from GC cells, GH3 cells, and normal pituitary gland indicates that processing of intron sequences from the precursor transcript takes place in a qualitatively similar fashion in each of these cell types. The data indicate that, in general, those introns closest to the termini of the primary transcript are removed first followed by removal of the internal introns. The suggested order of removal is IA, ID, IC, and IB. This process is unaffected qualitatively by thyroid or glucocorticoid hormones, both of which increase the rate of transcription of the gene. In addition to the primary transcript and the partially processed intermediate transcripts, GC and GH3 cells were found to contain a heterogenous group of intron-containing polyadenylated rGH gene transcripts which cannot be accounted for by any combination of intron deletions. These transcripts could arise either from internal start sites in the gene, premature termination of transcription, or inefficient processing of rGH mRNA precursors in the transformed cells. Thyroid hormone rapidly increases the levels of intron C-containing transcripts with kinetics that parallel the binding of thyroid hormone receptor to nuclei, but does not alter the ratio of primary to partially processed transcripts. These data suggest that most of the stimulatory activity of this hormone is due to effects on rGH gene transcription and not on pre-mRNA processing.(ABSTRACT TRUNCATED AT 250 WORDS)