Insulin-like growth factor I regulates growth hormone secretion and messenger ribonucleic acid levels in human pituitary tumor cells

GH secretion and mRNA levels were measured in cultured human GH adenoma cells incubated in serum-free medium for up to 48 h. A human recombinant insulin-like growth factor I (IGF-I) analog, Thr-59-IGF-I (6.5 nM), inhibited basal GH secretion by up to 60% in tumor cell cultures. The 30-50% stimulation of GH secretion by GH-releasing hormone (GHRH) was prevented by simultaneous exposure of the cells to IGF-I (6.5 nM). Gel electrophoresis of total RNA derived from GH cell adenoma tissue, followed by transfer and hybridization with 32P-labeled human GH cDNA, revealed a distinct mRNA species of about 1.0 kilobases. Using cytoplasmic dot blot hybridization, IGF-I inhibited the levels of human GH mRNA sequences in these cells and also prevented the GHRH-induced stimulation of GH mRNA. A monoclonal antibody to the type I IGF-I receptor (alpha IR3) prevented the inhibitory effects of IGF-I on basal and GHRH-stimulated GH secretion. This antibody also prevented the IGF-I-induced suppression of GH mRNA sequences. PRL secretion in these cells was not altered by IGF-I. Furthermore, relative levels of beta-actin mRNA were unaltered by IGF-I. Thus, IGF-I suppresses basal and GHRH-stimulated GH secretion and GH mRNA levels in pituitary adenoma cells, indicating that IGF-I acts selectively on the somatotroph to directly regulate GH gene expression.     

Insulin-like growth factor I stimulates the synthesis and release of prolactin from human decidual cells

Recent studies suggest a role for insulin-like growth factor I (IGF-I) in the regulation of hormone release from placental, gonadal, and pituitary tissues. To examine whether IGF-I may also regulate the release of PRL from human decidual tissue, we have investigated the effect of recombinant human IGF-I on PRL release from monolayer cultures of human decidual cells exposed to IGF-I for up to 4 days. IGF-I (10-1000 ng/ml) stimulated a sustained dose-dependent increase in PRL release (half-maximal concentration, 25 ng/ml) beginning 48 h after initial exposure, but had no effect on the intracellular PRL content. The amounts of PRL released from maximally stimulated cultures on days 3 and 4 were 168 +/- 3% (mean +/- SEM) and 258 +/- 8% of control values, respectively. IGF-I-mediated effects were inhibited by cycloheximide (3.6 microM), suggesting that the increase in PRL was the result of newly synthesized hormone. The increase in PRL release was not due to a generalized effect on protein release, since IGF-I had no effect on the release of trichloroacetic acid-precipitable [35S]methionyl proteins. Radioligand competition studies indicate that the biological actions of IGF-I are mediated through interaction with the IGF-I receptor. Binding of radiolabeled IGF-I to decidual cells in suspension was specific, saturable, and displacable by unlabeled IGF-I, with a potency nearly 10 times greater than that of insulin. Furthermore, exposure of decidual cells to a monoclonal antibody to the IGF-I receptor (alpha-IR3) completely inhibited both IGF-I-mediated PRL release and specific binding of [125I]IGF-I to decidual cells. Since the actions of IGF-I occurred at physiological concentrations, these findings strongly support a role for IGF-I in the regulation of PRL secretion by human decidua.     

Insulin-like growth factor I and growth hormone in canine starvation

The roles of plasma insulin-like growth factor I (IGF I) and growth hormone (GH) were studied in 7 beagle dogs before and during starvation and during refeeding. IGF I levels significantly decreased from 75.2 +/- 5.9 ng/ml at 7 days prior to the start of starvation to 9 +/- 1.7 ng/ml at 19 days after the commencement of starvation (mean +/- SEM; P less than 0.0001). During refeeding IGF I significantly rose from 9 +/- 1.7 ng/ml to 55.5 +/- 7.5 ng/ml within 9 days (mean +/- SEM; P less than 0.002). During starvation plasma GH levels significantly increased (P less than 0.05) and these elevated levels returned to normal during refeeding. The dogs' GH secretory capacity significantly increased during starvation (P = 0.012) and became normal again during refeeding. The following conclusions can be drawn from this study: 1) starvation in the dog leads to a significant and drastic reduction of the circulating levels of IGF I, and 2) starvation in the dog, as in man, leads to increased circulating GH levels and to an increased GH-secretory capacity possibly brought about by a lack of a negative feedback normally exerted by IGF I.     

Insulin-like growth factor and growth hormone secretion in juvenile chronic arthritis.

Insulin-like growth factor (IGF-1) concentrations were determined in a series of 23 children with juvenile chronic arthritis in conjunction with anthropometric assessment. When standardised z scores were used significant decreases in height and weight were shown in comparison with the normal age/sex matched means. Severe growth disturbance was seen, particularly in those with prolonged disease duration, which was independent of corticosteroid treatment, indicating disease activity itself is a major factor in the growth retardation. Eight children had low IGF-1 z scores--that is, less than -2.00 from age/sex matched mean. Low IGF-1 z scores were associated with low weight z scores but not with low height z scores. Overnight growth hormone secretory profiles were determined in 10 patients, including seven with low IGF-1, and showed generally normal secretion in all but one patient, who subsequently attained normal concentrations coincident with catch up growth. Increased pulse frequency of overnight secretion was commonly seen. Low IGF-1 concentrations probably result from varying factors, particularly nutritional, but do not reflect marked endocrinological abnormalities in most patients.     

Hypothalamic control of prolactin and growth hormone secretion in the pituitary gland of the pigeon and the chicken: in vitro studies

Hypothalamic extracts stimulated the release of prolactin and growth hormone from pigeon and chicken pituitary glands incubated in vitro. Release of hormone was proportional to the amount of hypothalamic extract added. Pituitary glands from "lactating" pigeons released more prolactin and their hypothalami contained more prolactin-releasing activity compared with controls. Partial separation of prolactin releasing activity from growth hormone releasing activity in chicken hypothalamic extract was achieved using gel filtration chromatography. Co-incubation studies in vitro with hypothalamic tissue present showed that prolactin release from the pituitary was inhibited and growth hormone release was stimulated when dopamine was added to the medium. The effects of dopamine were blocked by the antagonist pimozide. The possible existence of hypothalamic releasing and inhibiting factors regulating secretion of prolactin and growth hormone is discussed.     

Oestradiol 17 beta modifies fowl pituitary prolactin and growth hormone secretion in vitro

Chicken pituitary glands were incubated in medium containing oestradiol 17 beta (E2), alone or together with single whole hypothalami. E2 stimulated prolactin release from the pituitary and increased the prolactin releasing activity of the hypothalamus, but did not affect growth hormone release. Preincubation of pituitaries with E2 dramatically stimulated subsequent prolactin release. Pituitaries primed with E2 were more responsive to the prolactin-stimulating effects of hypothalamic extract (HE) and thyrotrophin-releasing hormone (TRH) and more sensitive to the prolactin-inhibiting effect of dopamine. E2-primed pituitaries were much less sensitive to the growth hormone releasing activity of TRH and HE. These results show that E2 may regulate pituitary function by direct effects on hormone release by modifying pituitary sensitivity to stimulatory or inhibitory influences and by altering hypothalamic releasing activity.     

Adenosine inhibits prolactin and growth hormone secretion in a clonal pituitary cell line

Although purine nucleosides have been shown to regulate the secretion of several peptide and steroid hormones, effects on pituitary hormone release have not been reported. We show here that in the clonal GH4C1 pituitary cell line maximal concentrations of adenosine (greater than or equal to 50 microM) inhibited PRL and GH secretion by 40%. Adenosine deaminase abolished the inhibitory effect of adenosine but not that of SRIF or (-)N6(R-2-phenylisopropyl)adenosine (PIA), a nonhydrolyzable adenosine analog. Furthermore, this enzyme increased basal secretion by 50%, and analysis of the incubation medium by HPLC demonstrated that the cells secreted biologically effective concentrations of adenosine. These results indicate that adenosine produced in culture tonically inhibits hormone release. In other target cells, adenosine inhibition is mediated by two types of binding sites: an extracellular Ri-site requiring an intact ribose moiety or an intracellular P-site requiring an intact purine ring. Four lines of evidence indicate that in GH4C1 cells, adenosine acts at an Ri-site. PIA, an Ri-site-specific agonist, was a potent inhibitor of hormone release (ED50 = 30 nM). Theophylline, an Ri-site antagonist, competitively inhibited the action of PIA (Ki = 2.4 microM). 3) 2'5'-Dideoxyadenosine, a P-site-specific agonist, did not inhibit PRL release even at a concentration of 1 mM. 4) Dipyridamole, an adenosine uptake inhibitor, did not reduce adenosine inhibition. In addition to its effect on basal secretion, PIA inhibited stimulation of hormone release by vasoactive intestinal peptide and TRH. PIA also reduced vasoactive intestinal peptide-stimulated cAMP accumulation by 75%, consistent with its action to inhibit adenylate cyclase via Ri receptors in other targets. Since PIA inhibition of PRL release and cAMP accumulation was not additive with the effects of SRIF and carbamyl choline, these inhibitors may act via a common rate-limiting step. Our results demonstrate that adenosine activates an Ri-type of adenosine receptor in GH4C1 cells and that the production of adenosine under normal culture conditions causes autocrine inhibition of secretion.     

Insulin-like growth factor I, growth hormone and insulin in white adipose tissue

Maturation of adipose tissue results from both the expansion of mature adipocytes and the formation of new adipocytes from adipocyte precursor cells. A variety of hormones related to adipogenesis have been identified recently. Both growth hormone (GH) and insulin-like growth factor I (IGF-I) are of major significance in adipocyte differentiation. IGF-I has been suggested to be a major regulator of cell proliferation, differentiation and metabolism, thus regulating, among other biological processes, adipose tissue growth and differentiation of pre-adipocytes into adipocytes. GH exerts its effects by increasing the pool of adipocyte precursor cells capable of differentiating into mature adipocytes. In addition, GH seems to have the potential to reduce the volume of mature adipocytes, thus inhibiting the expansion of adipose tissue and reducing body fat. This chapter gives an overview of studies that have investigated the roles of insulin, GH and IGF-I in adipogenesis.     

Dynamic studies of growth hormone and prolactin secretion in the female rat.

Blood samples were removed via chronic intra-atrial cannulae every 15 min in female rats during the estrous cycle, the last week of pregnancy, parturition and suckling. Growth hormone (GH) secretion during the estrous cycle is characterized by episodic release, occurring approximately once hourly. The surges in GH increase during the last 3-4 days of gestation, and rise to high levels during delivery and with suckling. Prolactin (PRL) shows minimal fluctuations during the estrous cycle, except for a prominent pulsatile surge during proestrus. PRL rises 4-6 h prior to parturition and declines during delivery. These studies provide a basis for further studies on the dynamics of GH and PRL secretion in the female rat.     

Greater conservation of somatolactin, a presumed pituitary hormone of the growth hormone/prolactin family, than of growth hormone in teleost fish

From pituitary cDNA libraries of Atlantic cod and chum salmon, cDNA clones coding for somatolactin (SL), a presumed pituitary hormone belonging to the growth hormone (GH)/prolactin (PRL) family, were isolated and characterized. The 1.3-kb cod SL mRNA was composed of a greater than 0.25-kb 5' untranslated region, a coding region for the precursor of 235 amino acids (aa), a 0.14-kb 3' untranslated region, and a poly(A) tail. The 2.5-kb salmon SL mRNA had a less than 0.1-kb 5' untranslated region, a precursor (233 aa) coding region, a 1.6-kb 3' untranslated region, and a poly(A) tail. A signal peptide of 26 and 24 aa was found in the SL precursor of cod and salmon, respectively. Thus, the mature SLs of these fish are composed of 209 aa. Two potential N-glycosylation sites were identified in cod SL, whereas no site could be found in the salmon. A comparison of amino acid sequences of the three SLs so far isolated indicated six Cys residues to be in homologous positions to those in GH and PRL, and one Cys residue to be characteristically present in SL. Among cod, salmon, and flounder, greater colinearity of amino acid sequences was noted in SLs than in GHs. The identities of the SL amino acid sequences were between 73 and 81% as compared to 58-62% for the corresponding GHs, indicating greater conservation of SL than GH.     

Immunocytochemical identification of the prolactin- and growth hormone-secreting cells in the teleost pituitary with antisera to tilapia prolactin and growth hormone

Antisera raised to highly purified tilapia (Sarotherodon mossambicus) prolactin (PRL) and growth hormone (GH) were used to locate PRL and GH cells in the adenohypophysis of seven species of teleosts by immunoenzymological methods using horseradish peroxidase. The only pituitary cells in the tilapia that immunologically reacted with anti-tilapia PRL were the PRL cells located in the rostral pars distalis (RPD). In the pituitary glands of two species of salmonids and two species of marine fishes, the tilapia PRL antibodies bound to both PRL and GH cells; no prolactin-immunoreactive cells were identified in the goldfish and eel pituitaries. Antiserum to tilapia GH reacted specifically with the GH cells located in the proximal pars distalis of all teleosts examined in this study, but not with any presumed PRL cells in the RPD. These results indicate that tilapia PRL antisera are specific for tilapia PRL cells but not necessarily for these cells in other teleost species. Tilapia GH antisera showed no species specificity among the teleosts tested.     

Insulin-like growth factor I and daily growth hormone profile in the assessment of active acromegaly

A poor correlation exists between growth hormone levels and the severity of disease in patients with acromegaly. It has been suggested that insulin-like growth factor I correlates better with the clinical condition thaN GH itself, but the presence of a correlation between GH and IGF-I is still a matter of debate. We therefore studied the fasting concentration of acid-extracted IGF-I and the daily GH profile (at 08.00, 12.00, 16.00, 20.00 h) in 28 acromegalic patients, both in basal condition and at different times after treatment. All patients studied before treatment exhibited a GH mean daily concentration always above 5 micrograms/l (range 5.15-1.47 micrograms/l), and IGF-I values (1.99-6.85 U/l) always above the normal range (0.38-1.90 U/l). During the follow-up after medical, surgical or radiation therapy we did not observe normalization of the IGF-I concentration for a GH day curve value above 5 micrograms/l. Compared with the GH day curve, a single morning GH value was slightly less sensitive and definitely less specific. The analysis of covariance evidenced a good correlation (P less than 0.001) between the GH day curve and IGF-I concentration during the first 6 months of medical as well as during the first year of radiation therapy, and after surgical treatment. Our data indicate that a single determination of acid-extractable IGF-I is as reliable as GH multiple sampling to assess the acromegalic disease activity.     

Histologic identification and immunochemical studies of prolactin and growth hormone in the primate pituitary gland

The two acidophilic cell types were tinctorially differentiated in rhesus monkey pituitaries as well as in pituitaries of other species of monkeys and apes and were identified by immunofluorescence as the prolactin and the growth hormone (GH) cells. The number of prolactin cells was significantly greater in adult than in juvenile rhesus monkeys. During late pregnancy and lactation, these cells appeared to occupy 60–80% of the entire anterior lobe of the rhesus monkey. The concentration of prolactin and CH was measured by radioimmunoassay in pituitary extracts (PE) from pools of juvenile male, juvenile female, adult male and adult female rhesus monkey pituitaries. More prolactin was present in PEs from adult than from juvenile animals, with the highest concentration being present in adult females. A similar relationship was observed with GH. The qualitative immunologic relationship between ovine prolactin and prolactin in the rhesus monkey PEs, and between human GH and GH in the PEs, was established using agar gel diffusion. The two prolactin hormones were partially related immunochemically, while the two GHs were shown to be immunochemically identical. In intact juvenile female rhesus monkeys given estradiol benzoate, the concentrations of pituitary and serum prolactin were elevated over that of the controls, while the concentration of pituitary GH was decreased. An increase in the number of prolactin cells was also observed in the steroid-treated animals. These observations demonstrate further that prolactin and GH reside in separate cells in the primate pituitary and that the concentration of these two hormones varies depending upon age, sex, or stage of the reproductive cycle of the animal.