Differential effects on proliferation of GH and IGFs in sea bream (Sparus aurata) cultured myocytes

Primary culture of gilthead sea bream skeletal muscle cells was used to examine the effects of growth hormone (GH) and insulin-like growth factors (IGFs) in fish muscle proliferation and growth. Proliferation was measured as the percentage of positive cells expressing the proliferating cell nuclear antigen (PCNA) analyzed by immunocytochemistry. First, the effects of GH from two different origins (mammals and fish) were tested. GH from human (hGH) did not stimulate proliferation except at 3 h at the dose of 1 nM. On the other hand, sea bream GH (sbGH) significantly stimulated proliferation, without differences between the three incubation times studied (3, 6, and 18 h), at the dose of 10 nM, demonstrating that the homologous hormone has a more potent effect. In addition, the results with the IGFs indicated that both peptides, IGF-I and IGF-II significantly stimulated proliferation of sea bream myocytes, but IGF-II showed higher effects than IGF-I, and even than those of sbGH. Finally, the combinations of peptide treatments (GHs with IGFs) indicated that IGF-I has higher effects on proliferation when it is combined with GHs compared with IGF-I alone, while IGF-II has similar effects alone or combined with either GH. These results indicate that IGF-II may have an important role on muscle proliferation that appears to be independent of GH. On the contrary, IGF-I seems to play a synergistic action with GH stimulating myocyte proliferation.     

Effects of insulin-like growth factors (IGF-I and -II) on growth hormone and prolactin release and gene expression in euryhaline tilapia,Oreochromis mossambicus

We investigated in vitro effects of insulin-like growth factors (IGF-I and -II) on growth hormone (GH) and prolactin (PRL) release and gene expression in euryhaline tilapia, Oreochromis mossambicus. Pituitaries were removed from freshwater-acclimated adult males and incubated for 2-24h in the presence of human IGF-I or -II at doses ranging from 1-1000 ng/ml (0.13-130 nM). IGF-I at concentrations higher than 10 ng/ml and IGF-II higher than 100 ng/ml significantly inhibited GH release after 8, 16, and 24h. No effect of IGFs was seen during the first 4h of incubation. IGFs at the same concentrations also significantly attenuated GH gene expression after 24h, although no effect was seen at 2h. By contrast, PRL(188) release was stimulated significantly and in a dose-related manner by IGF-I at concentrations higher than 10 ng/ml and by IGF-II at concentrations higher than 100 ng/ml within 2h. No stimulation was observed after 4h. Similarly, both IGFs at concentrations higher than 10 ng/ml increased PRL(177) release within 2h. However, no significant effect of IGF-I or -II was observed on mRNA levels of both PRLs after 2 and 24h at all concentrations examined. These results clearly indicate differential regulation of GH and PRL release and synthesis by IGFs in the tilapia pituitary, i.e., rapid-acting, stimulatory effects of IGFs on PRL release and slow-acting, inhibitory effects on GH release and synthesis.     

Production of insulin-like growth factors and their binding proteins by rabbit articular chondrocytes: relationships with cell multiplication

Articular chondrocytes from 2- to 3-month-old rabbits were cultured in serum-free medium supplemented with fibroblast growth factor. The effects were studied of GH, insulin-like growth factors (IGFs), and insulin on the production of IGF-I, IGF-II, and their binding proteins (BPs) and on cell multiplication. In the control culture medium, IGF-I levels were about one fifth those of IGF-II. Western blot analysis of the BPs revealed a predominant 30K form and 24K and 20K forms which appeared inconsistently and in small quantities. Ten to 100 ng/ml human GH had no mitogenic effect, and even had a slightly inhibitory effect. IGF-I at 10 ng/ml stimulated cell multiplication above the control level by 41% and at 50 ng/ml by 74%, whereas the mean increase obtained with IGF-II (10 and 50 ng/ml) was only 19%. At the same doses, insulin had no effect, but at 5 micrograms/ml it stimulated cell multiplication by a mean of 67%. There was a positive correlation between cell number and release into the medium of both IGF-I (r = 0.86) and IGF-II (r = 0.77). Neither IGF-I nor IGF-II production was affected by GH. Insulin (5 micrograms/ml) increased IGF-I production by a factor of 2.6, but increased IGF-II production by a factor of only 1.4. Under the various conditions of culture with different doses of GH and insulin, cell multiplication, relative to the control value was positively correlated to the IGF-I/IGF-II production ratio (r = 0.77). It would, therefore, seem that IGF-I secreted by the chondrocytes may stimulate their own proliferation. When IGFs or insulin were added to the culture medium, changes in the electrophoretic profiles of the BPs included an increase in the 30K form and an increase in or the appearance of the 24K and 20K forms. Ten and 50 ng/ml IGF-I or IGF-II had effects equal to or greater than those induced by 5 micrograms/ml insulin. These results indicate that the syntheses of BPs and IGFs are coordinated and that IGFs may be implicated in the control of the synthesis of their BPs.     

Hormonal regulation of rat hypothalamic neuropeptide mRNAs: effect of hypophysectomy and hormone replacement on growth-hormone-releasing factor, somatostatin and the insulin-like growth factors.

Hormonal feedback regulation of hypothalamic peptides putatively involved in growth hormone (GH) regulation has been studied by measurement of steady-state mRNA levels in male hypophysectomized rats with or without thyroid hormone, corticosterone, testosterone or GH replacement. Hypothalamic GH-releasing factor (GRF) mRNA levels increased progressively following hypophysectomy to 420% of sham levels after 15 days while hypothalamic insulin-like growth factor I (IGF-I) and insulin-like growth factor II (IGF-II) mRNA levels decreased to less than 40% of sham levels. Whole hypothalamic somatostatin mRNA levels were not significantly different from sham. One week of continuous GH infusion restored hypothalamic IGF-I mRNA to levels (95%) indistinguishable from those in sham-operated controls but had no effect on either IGF-II or GRF mRNA. Thyroid hormone, corticosterone and testosterone treatment without GH had no effect on the hypophysectomy-induced reduction of either IGF-I or IGF-II mRNA levels but reversed the elevation of GRF mRNA. We conclude that hypothalamic IGF-I may be involved in GH feedback regulation and thus may function as a hypothalamic modulator of GH. In contrast, IGF-II may be regulated by one of the pituitary trophic hormones but not by GH or the target hormones tested. Finally, hypothalamic GRF mRNA regulation appears to be complex and may include target hormone feedback.     

New insulin-like growth factor (IGF)-precursor sequences from mammalian genomes: the molecular evolution of IGFs and associated peptides in primates

The insulin-like growth factors (IGF-I and IGF-II) and insulin are related proteins that play an important role in regulation of metabolism and growth. In mammals these proteins are generally strongly conserved, though the sequence of insulin underwent periods of rapid change during the evolution of hystricomorph rodents and new-world monkeys (NWM). The availability of genomic sequence information for a number of mammals provides gene sequences for insulin and IGF precursors from several new species, and this has been used here to study the evolution of these proteins in primates. The sequence of insulin is strongly conserved in primates except for the branch leading to NWM – the sequence of marmoset insulin confirms the episode of rapid evolution in this lineage. Strongly conserved sequences are also seen for IGF-I and IGF-II, though for IGF-I (but not IGF-II) the marmoset sequence again shows an episode of fairly rapid evolution, paralleling the changes seen in insulin. Thus in NWM the sequences of insulin and IGF-I show a co-evolution that may reflect a coordinated change in the functional properties of these two molecules. The other components of the insulin and IGF precursors (signal peptides, E-domains of IGFs, insulin C-peptide) are much less strongly conserved, though to a variable extent. Signal peptides are generally quite variable, but the sequence encoding the N-terminal region of the unusually long signal peptide of IGF-I is strongly conserved, suggesting specific function(s), at least partly associated with nucleotide rather than protein sequence. The Ea domain of proIGF-I and the N-terminal end of the E-domain of proIGF-II are quite strongly conserved, which accords with reports of a biologically active peptide (preptin) derived from the latter. However, the C-terminal parts of the Eb and Ec domains of proIGF-I (produced by alternative splicing) are very variable, which is of interest in view of reports of peptides with important biological activities deriving from these regions.     

Growth hormone and insulin-like growth factors have different effects on sebaceous cell growth and differentiation.

Several observations suggest that GH stimulates sebaceous gland growth and development. Therefore, we studied the effects of GH and insulin-like growth factors (IGFs), alone and with androgen, on sebaceous epithelial cell (sebocyte) growth and differentiation in vitro. The rat preputial cell culture model system was used to judge differentiation (induction of lipid-forming colonies, LFCs) and DNA synthesis. GH increased sebocyte differentiation. At a dose of 10(-8) M in the presence of micromolar insulin, GH was 3.8 times more potent than IGF-I (38.1+/-4.2%, SEM, vs. 10+/-1.5% LFCs) and 6 times more potent than IGF-II (6+/-0.5% LFCs). IGF-I 10(-8) M alone stimulated a similar amount of differentiation as insulin 10(-6) M, although it was less effective than insulin in augmenting the effect of GH on differentiation. GH had no effect on sebocyte uptake of 3H-thymidine at doses up to 10(-6) M. On the other hand, IGF-I was the most potent stimulus of DNA synthesis (168% of control; P < 0.001 vs. all others). IGF-II 10(-8) M stimulated 3H-thymidine incorporation similarly to insulin 10(-6) M. In the presence of insulin, dihydrotestosterone (DHT) 10(-6) M induced 31.4+/-1.7% LFCs, and there was a tendency of DHT and GH to interact in promoting differentiation. When insulin was omitted from the system, differentiation was decreased overall, but GH +/- DHT slightly improved differentiation. The IGFs had no effect on the response to DHT. DHT decreased DNA synthesis by 40%, an effect unaltered by GH or IGFs. These results suggest that GH and IGFs have different functions in sebaceous cell growth and differentiation: GH stimulated differentiation beyond that found with IGFs or insulin, yet had no effect on DNA synthesis, a parameter stimulated most potently by IGF-I. While GH augmented the effect of DHT on differentiation, the IGFs had no effect on the response of DHT. These data indicate that GH may in part act directly on sebocytes rather than indirectly through IGF production. These data are consistent with the concept that increases in GH and IGF production contribute in complementary ways to the increase in sebum production during puberty and in acromegaly.     

Quantification of common carp (Cyprinus carpio) IGF-I and IGF-II mRNA by real-time PCR: differential regulation of expression by GH

IGFs are potent mitogens for many different cell types and play important roles in growth and development. A multitude of regulatory factors modulate the expression of IGFs. In some teleosts, liver IGF-I expression has been demonstrated to depend on the presence of GH. However, the GH dependence of IGF-II expression in teleosts is controversial. Moreover, most IGF expression studies in bony fish have been focused on the liver, and information on extrahepatic tIssues are conflicting and inconsistent. This is partly due to the fact that the traditional methods of mRNA measurement such as Northern blot and RT-PCR are not sensitive enough to detect changes in IGF levels in extrahepatic tIssues because of the low levels of IGFs in these tIssues. In addition, there have been few studies on the IGF system of non-salmonid teleosts. Our laboratory has thus begun such studies on a local tropical fast-growing fish, the common carp (Cyprinus carpio). In this study, real-time quantitative PCR assays were developed for the accurate measurement of IGF-I and IGF-II mRNA levels in common carp tIssues. This quantitative method was based on the measurement of a fluorescent labeled probe, which was cleaved by Taq polymerase during PCR by the 5'-->3' nuclease activity. The signal generated was directly proportional to the starting copy number of the target molecules in the sample. Hence, it was possible to detect and quantify the mRNA levels of both IGF-I and IGF-II reliably in very small amounts of tIssues obtained from juvenile common carp. Using these assays, the expression pattern of IGF-I and IGF-II in various common carp tIssues was studied, and their differential response to GH stimulation was also investigated.     

Growth hormone and insulin-like growth factor-I: effects on the growth of glioma cell lines.

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are known to be mitogens for many types of neoplasms. To investigate their role in tumors of glial origin, in vitro and in vivo experiments were performed with a panel of immortalized glioma cell lines (D54, SNB-19, U87, U251 and U373). Initial analysis for mRNA expression demonstrated the following: GH receptor (5/5 cell lines positive), IGF-I (0/5), IGF-II (0/5), IGF-I receptor (5/5), IGF-II receptor (2/5). Thus, each cell line expressed the necessary receptors to respond to GH and the IGFs but there was no autocrine IGF production by the tumors themselves. IGF-I stimulated mitogenesis as measured by [(3)H]thymidine uptake experiments in U251 and U373 cells. However, when these two IGF-responsive cell lines were xenografted into mice, tumor development and growth rates were not significantly different in GH-deficient animals (despite having IGF-I serum concentrations only 31% of normal). Because our studies were performed in immunocompromised animals, GH or IGF effects on immune surveillance, known to be important from some syngeneic glioma models, would not be likely to contribute to our findings. Nevertheless, these studies are important because they demonstrate that the growth of glioma cell lines in an in vivo environment can remain robust in a GH/IGF-I-deficient setting, even if in vitro experiments indicate that IGF-I is mitogenic.     

Purification, amino acid sequences and assay cross-reactivities of porcine insulin-like growth factor-I and -II

Porcine insulin-like growth factor-I (IGF-I) and IGF-II have been characterized to help define the roles of these peptides in the growth process. The amino acid sequence of porcine IGF-I was found to be identical to the human and bovine peptides. Porcine IGF-II was more similar to human IGF-II than to forms of this growth factor in other mammalian species, differing only in the replacement of asparagine for serine at residue 36. In a biological assay that measures the stimulation of protein synthesis in rat L6 myoblasts, porcine IGF-I was approximately ninefold more potent than porcine IGF-II or bovine IGF-II, while recombinant human IGF-I and IGF-II had half the potency of the respective natural peptides. Porcine and recombinant human IGF-I showed essentially equal competition for binding in a human IGF-I radioimmunoassay while between 0.6 and 1.5% cross-reactivity was observed with human, bovine or porcine IGF-II. A receptor assay for IGF-II demonstrated similar potencies for the three IGF-II peptides, while the cross-reactivity of recombinant human IGF-I was only 0.05%. Porcine IGF-I exhibited a higher cross-reactivity, presumably due to very slight contamination with IGF-II.     

Differential regulation of insulin-like growth factor-I (IGF-I) and IGF-II release from cultured neonatal mouse calvaria by parathyroid hormone, transforming growth factor-beta, and 1,25-dihydroxyvitamin D3

In a previous study we found that PTH stimulated bone resorption and release of insulin-like growth factor-I (IGF-I) and IGF-II from cultured neonatal mouse calvaria. Since IGF-I and IGF-II stimulate osteoblast proliferation and collagen synthesis, these results suggested that increased release of IGFs during resorption could mediate in part coupling of bone formation to bone resorption. In the present study two other osteolytic agents, transforming growth factor-beta (TGF beta) and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3 were examined for effects on IGF release from neonatal mouse calvaria. Like PTH, TGF beta stimulated resorption and increased release of IGF-I and IGF-II. 1,25-(OH)2D3, however, stimulated resorption and IGF-II release comparable to PTH, but inhibited release of IGF-I. 1,25-(OH)2D3 (0.1-100 nM) inhibited basal release of IGF-I, and 10 nM 1,25-(OH)2D3 inhibited release of IGF-I induced by PTH or TGF beta. The effects of 1,25-(OH)2D3 were specific to this vitamin D metabolite and did not occur with 25-hydroxyvitamin D3 or 24,25-(OH)2D3 at the same concentration. Calcitonin (50 mU/ml) decreased 1,25-(OH)2D3 stimulation of resorption, but did not affect 1,25-(OH)2D3 stimulation of IGF-II release and inhibition of IGF-I release. This evidence that effects of 1,25-(OH)2D3 on release of the IGFs were independent of bone resorption supports the conclusion that 1,25-(OH)2D3 modulated the production and secretion of IGF-I and IGF-II in calvarial cells. The results of this and the previous study suggest that PTH, TGF beta, and 1,25-(OH)2D3 differentially regulate mouse calvarial cell IGF-I and IGF-II production.     

Short-term exposure to insulin-like growth factors stimulates testosterone production by testicular interstitial cells

The effect of short-term exposure to the insulin-like growth factors (IGF-I and IGF-II) on testosterone production by rat testicular interstitial cells in primary culture has been examined. Both peptides, when present during a 1-h pre-incubation period, increased human chorionic gonadotropin (hCG)-stimulated testosterone release over the following 16-h period. The effect of exposure to IGFs was most marked on maximally hCG-stimulated testosterone release. Maximal stimulation following IGF exposure was 80-85% above that seen without IGFs, and the IGF effect was half-maximal at 1.5-2 micrograms/l of IGF-I or IGF-II. Pre-incubation with IGFs did not alter the concentration of hCG (0.1 microgram/l) at which half-maximal stimulation of testosterone release was seen. Increasing cell density had a marked effect on the testosterone production rate per 10(5) cells, and the stimulatory effect of IGFs was only seen at relatively high cell density (2.8 X 10(5) cells/ml). Varying the period of pre-incubation with IGFs between 0.5 and 16 h, it was found that a 1-h period gave maximal stimulation. We conclude that a short exposure to IGFs is capable of increasing hCG-stimulated steroidogenesis in Leydig cells, and postulate that this effect may be part of an intratesticular paracrine control mechanism.     

Stimulation of steroidogenesis in immature rat Leydig cells evoked by interleukin-1alpha is potentiated by growth hormone and insulin-like growth factors

The cytokine IL-1alpha is produced constitutively by the intact testis, but its function in this organ remains largely unknown. In this study we examined cooperation between IL-1alpha and GH and IGFs with regard to stimulation of steroidogenesis by Leydig cells from 40-d-old rats in vitro. IL-1alpha alone stimulated testosterone (T) and dihydrotestosterone (DHT) production. GH, IGF-I, or IGF-II alone was without effect on T production, but they were found to elevate DHT release, albeit without an obvious dose-response effect. Costimulation with IL-1alpha and GH or with IL-1alpha and IGF-I or IGF-II elevated the rate of steroidogenesis (both T and DHT) above that observed with IL-1alpha alone. GH was found to increase the level of IGF-I in the cultured Leydig cells, an effect that was potentiated by IL-1alpha. The costimulatory effect of GH on steroidogenesis was abolished by treatment with picropodophyllin, a specific inhibitor of the IGF-I receptor, indicating that the action of GH is mediated via IGF-I. Moreover, cells costimulated with IL-1alpha and GH exhibited a marked decrease in the level of intact IGF-binding protein-3 in the culture medium due to the induction of proteolytic activity toward this binding protein. In contrast, secretion of IGF-binding protein-2 was increased by such costimulation. These findings suggest that the stimulation of steroidogenesis in Leydig cells evoked by GH and IGFs requires cooperation with IL-1alpha. This cooperation may play an important role in connection with postnatal Leydig cell maturation and steroidogenesis.     

The effects of insulin-like growth factors on tumorigenesis and neoplastic growth

Several decades of basic and clinical research have demonstrated that there is an association between the insulin-like growth factors (IGFs) and neoplasia. We begin with a brief discussion of the function and regulation of expression of the IGFs, their receptors and the IGF-binding proteins (IGFBPs). A number of investigational interventional strategies targeting the GH or IGFs are then reviewed. Finally, we have assembled the available scientific knowledge about this relationship for each of the major tumor types. The tumors have been grouped together by organ system and for each of the major tumors, various key elements of the relationship between IGFs and tumor growth are discussed. Specifically these include the presence or absence of autocrine IGF-I and IGF-II production; presence or absence of IGF-I and IGF-II receptor expression; the expression and functions of the IGFBPs; in vitro and in vivo experiments involving therapeutic interventions; and available results from clinical trials evaluating the effect of GH/IGF axis down-regulation in various malignancies.     

Developmental changes in growth hormone, insulin-like growth factors (IGF-I and IGF-II) and IGF-binding proteins in plasma of young growing pigs.

The relationship between plasma concentrations of normally secreted GH and insulin-like growth factor-I (IGF-I) was investigated in pigs after weaning. Frequent blood sampling for between 12 and 24 h showed that plasma GH was pulsatile in pigs of 10, 20 and 35 kg liveweight. Pulses were brief in duration, low in amplitude and variable in frequency. Basal and average daily plasma concentrations of GH changed significantly with development, increasing by about 50% between 10 and 20 kg liveweight. Concentrations of IGF-I in plasma showed little or no evidence of diurnal periodicity and were not increased by GH pulses. Average daily concentrations of both IGF-I and IGF-II in plasma progressively increased between 10 and 35 kg liveweight, as did the total desaturated IGF-binding protein (IGFBP) activity of plasma. A strong positive correlation was observed between the total concentration of IGFs (IGF-I plus IGF-II) in the circulation and plasma IGFBP activity. The developmental rise in IGFBP activity of plasma was associated with increased labelling with 125I-labelled human IGF-II in ligand blots of binding proteins of apparent molecular masses greater than 200, 50, 43 and 29 kDa. One class of binding proteins of 34.5 kDa decreased with development. This study of young growing pigs shows that normally secreted endogenous GH exerts no significant immediate control over plasma IGF-I concentrations, and that plasma levels of IGF-I and IGF-II increase with maturation in this species.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of growth hormone-releasing hormone with the insulin-like growth-factors during prenatal development in the rat.

The placenta is a chimeric organ that produces all the components of the hypothalamic-pituitary GH axis. We propose that placental GH-releasing hormone (GHRH) stimulates placental GH-like hormones which in turn stimulate production of the insulin-like growth factors (IGFs), IGF-I and IGF-II, and these placental IGFs are important for growth and development of the placenta as well as the fetus. To test this hypothesis, pregnant rats were given either GHRH antisera or preimmune sera ip from days 7-19 of gestation. Fetuses were killed on day 19, and IGF-I and IGF-II tissue and serum concentrations in the mother and fetus were measured by RIA. IGF-II receptor content was measured by Western analysis. IGF-I and IGF-II messenger (m) RNA levels were measured in the placentas as well as in the fetal livers. The GHRH antibody titer was highest at day 19 of gestation but continued to be present through day 20 of postnatal development. Although placental weights did not differ, antibody-treated animals had higher placental IGF-I and IGF-II levels (I, 108 +/- 6 (SD); II, 126 +/- 5 ng/g, respectively) vs. control animals (I, 88 +/- 2.5 (SD); II, 48 +/- 11 ng/g) in pooled specimens. The IGF-II receptor was also up-regulated in placentas from antibody-treated mothers. The fetuses of antibody-treated (A) mothers were larger than the controls (C) (A, 2.615 g; C, 2.49 g, P less than 0.05). Levels of both IGFs were significantly increased in livers of antibody treated fetuses (IGF-I: A, 15 +/- 1 (SD); C, 12 +/- 0.8 ng/g; and IGF-II: A, 295 +/- 10 (SD); C, 233 +/- 10 (SD) ng/g). In addition, the concentration of the IGF-II receptor in liver of antibody-treated fetuses was also increased. Further, pooled fetal sera from antibody-treated fetuses had higher levels of IGF-II than controls (A, 950 ng/ml; C, 700 ng/ml), and the circulating IGF-II receptor was increased as measured by Western analysis. In the liver, IGF-II mRNA levels of antibody-treated fetuses were increased to 117% of controls, whereas IGF-I mRNA levels were undetectable. The placenta showed no increase in placental lactogen or GH mRNA, whereas IGF-II and GHRH mRNA were slightly increased in antibody-treated animals. In conclusion, these data suggest that GHRH may interact with the IGFs in a different fashion during prenatal development then during postnatal development.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synergistic interaction between insulin-like growth factors-I and -II in central regulation of pulsatile growth hormone secretion.

Insulin-like growth factor (IGF)-I and -II peptides, receptors, mRNAs, and binding proteins are widely distributed in the central nervous system (CNS), yet their physiological role in the brain remains largely unknown. While earlier in vivo studies in the rat suggested that IGF-I may participate in feedback regulation of GH secretion at a CNS level, the preparations used were only partially pure. The recent availability of purified recombinant IGF-I and -II peptides prompted us to reexamine the involvement of the IGFs in vivo in central regulation of pulsatile GH secretion. Five groups of free-moving adult male rats bearing chronic intracerebroventricular (icv) and intracardiac venous cannulae were icv administered IGF-I (in doses of 0.5, 2, 3, and 10 micrograms) or the acid-saline vehicle; an additional group received 1 microgram of the potent IGF-I analog, long R3 IGF-I. Spontaneous 6-h plasma GH secretory profiles were obtained from all groups. Vehicle-injected control animals exhibited the typical pulsatile pattern of GH secretion, with most peak GH values above 150 ng/ml and trough levels below 1.2 ng/ml. Central administration of IGF-I alone or long R3 IGF-I at all doses tested failed to alter the pulsatile pattern of GH release; there were no significant differences in GH peak amplitude, GH trough level, GH interpeak interval, or mean 6-h plasma GH level compared to those in vehicle-injected controls. In a second study, designed to determine the effects of central administration of IGF-I and IGF-II, in combination, icv injection of 1 microgram IGF-I and 1 microgram IGF-II resulted in a marked suppression in the amplitude of spontaneous GH secretory bursts approximately 3 h after injection; both GH pulse amplitude (43.5 +/- 5.6 vs. 130.6 +/- 14.6 ng/ml; P less than 0.001) and mean plasma GH level (16.3 +/- 1.9 vs. 35.2 +/- 1.8 ng/ml; P less than 0.001) were severely reduced 3-6 h after injection compared to those in vehicle-injected controls. These results demonstrate that IGF-I alone does not play a physiologically important role in feedback regulation of GH secretion at the level of the CNS. Our findings suggest a synergistic interaction between IGF-I and -II in the brain for central control of pulsatile GH secretion.     

Paracrine regulation of insulin-like growth factor I (IGF-I) an IGF-II on prostaglandins F2alpha and E2 synthesis by human corpus luteum in vitro: a possible balance of luteotropic and luteolytic effects.

The existence of a complete intraovarian insulin-like growth factor (IGF) system replete with ligands, receptors, and binding proteins has been demonstrated as well as the ability of IGF-I to positively affect steroidogenesis in human granulosa cells. Furthermore, we recently showed that IGF-I and IGF-II stimulate progesterone secretion by human luteal cells. As the PGs, PGE2 and PGF2alpha, are classically known to have luteotropic and luteolytic effects, we wanted to determine whether the IGFs could affect the human luteal phase by influencing the PG system. For this reason, human luteal cells were cultured for different times (12, 24, and 48 h) with IGF-I, IGF-II (10-100 ng/mL), and GH (100 ng/mL), and both PGs were assayed in the medium culture. We found that both IGF-I and IGF-II were able to stimulate PGE2 synthesis in a time- and dose-dependent way, whereas they both inhibited PGF2alpha production. GH, too, significantly reduced PGF2alpha synthesis; this effect was IGF-I mediated because it was reverted by increasing dilutions of an anti-IGF-I antibody. On the contrary, no GH effect was observed on PGE2 production. In conclusion, based on these data and on our previous results, we speculate that IGFs could influence luteal steroidogenesis through PG system.