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

Effects of long-term cadmium exposure on growth, antioxidant defense and DNA methylation in juvenile Nile tilapia (Oreochromis niloticus)

Cadmium (Cd) is a ubiquitous environmental contaminant, posing serious threats to aquatic organisms. The aims of the present study were to investigate the effects of long-term Cd exposure on the growth, GH/IGF axis, antioxidant defense and DNA methylation in juvenile Nile tilapia (Oreochromis niloticus). To this end, juvenile Nile tilapia were exposed to 0, 10 and 50 µg∙L-1 Cd for 45 and 90 days. The obtained results revealed that exposure to high concentrations of Cd significantly decreased body mass and body length, and down-regulated mRNA levels of GHRs, IGF-I and IGF-II in the liver of Nile tilapia. Cd exposure induced oxidative stress including the reduction of antioxidant activities and increases of malondialdehyde (MDA) and 8-hydroxydeoxyguanosine (8-OHdG) contents. Beside, the global DNA methylation levels significantly decreased with increasing Cd concentration and exposure time, which might result from increased oxidative DNA damage, the down-regulated expression of DNMT3a and DNMT3b and up-regulated expression of TET1 and TET2. In conclusion, long-term Cd exposure could inhibit growth, reduce antioxidant capacity and lead to oxidative damages to lipid and DNA, and decrease global DNA methylation level in juvenile Nile tilapia.

Production of two recombinant insulin-like growth factor binding protein-1 subtypes specific to salmonids

Salmonids have four subtypes of insulin-like growth factor binding protein (IGFBP)-1, termed -1a1, -1a2, -1b1 and 1b2, owing to teleost- and a lineage-specific whole-genome duplications. We have previously produced recombinant proteins of masu salmon IGFBP-1a1 and -1b2 and conducted functional analysis. To further characterize salmonid-specific IGFBP-1s, we cloned cDNAs encoding mature proteins of IGFBP-1a2 and -1b1 from the liver of masu salmon (Oncorhynchus masou). IGFBP-1a2 and -1b1 shared a 56% amino acid sequence homology whereas their homologies with their counterparts (i.e. -1a1 and -1b2) were 77% and 82%, respectively. We next expressed recombinant masu salmon (rs) IGFBP-1a2 and -1b1 with fusion partners thioredoxin (Trx) and a His-tag using the pET-32a(+) vector system in Escherichia coli. Trx.His.rsIGFBP-1s were detected in the insoluble faction, solubilized in a buffer containing urea, and isolated by Ni-affinity chromatography. They were refolded by dialysis and cleaved from the fusion partners by enterokinase. rsIGFBP-1a2 and -1b1 were purified by reversed-phase high performance liquid chromatography. Purified rsIGFBP-1a2 and -1b1 had the ability to bind digoxigenin-labeled human IGF-I on ligand blotting. We then examined the effects of rsIGFBP-1a1, -1a2, -1b1 and -1b2 in combination with human IGF-I on growth hormone (GH) release from cultured pituitary cells of masu salmon. IGF-I alone reduced GH release while the addition of rsIGFBP-1a1, -1b1 or -1b2, but not rsIGFBP-1a2, diminished the suppressive effect of IGF-I. Addition of rsIGFBP-1s without IGF-I had no effect on GH release. These results show that rsIGFBP-1b1, along with rsIGFBP-1a1 and -1b2, inhibits IGF-I action on the pituitary in masu salmon. The lack of the effect by rsIGFBP-1a2 suggests that salmon IGFBP-1 subtypes underwent subfunction partitioning and have different degrees of IGF-inhibitory action.

Effects of feeding frequency on juvenile Chinese sturgeon Acipenser sinensis

In this study, the effects of different feeding frequencies on the growth and the expression of genes in the GH/IGF axis were assessed in juvenile Chinese sturgeon. The newly hatched Chinese sturgeons were bred for 38 days at three different feeding frequencies groups (feeding frequency of two times a day, TWD; three times a day, THD; and four times a day, FOD), and the expression levels of the GH/IGF axis responses to feeding frequency were determined by quantitative real-time PCR. In addition, the full-length of the Coding Sequences of IGF I and IGF II genes (489-bp and 660-bp, respectively), were cloned and analyzed from Chinese sturgeon the first time. Multiple sequence alignments of IGFs revealed that Chinese sturgeon are high sequence identity to IGFs from other species. The phylogenetic relationships based on the IGF I and IGF II amino acid sequences were consistent with the traditional classification. After 38 days of growth, the three different feeding frequencies groups of Chinese sturgeon had no significant difference of body length, body weight, specific growth rate, the survival rate, the rate of weight gain and the condition factor. However, the relative expression of Chinese sturgeon GH in the pituitary decreased with increasing feeding frequency. The relative expression of Chinese sturgeon GHR in liver and skeletal muscle was deceased with increasing feeding frequency, while the relative expression of GHR in stomach and intestines at THD group was significantly higher than that of at TWD group and FOD group (p < 0.05). The relative expression of Chinese sturgeon IGF I in liver increased significantly with increasing feeding frequency (p < 0.05). The relative expression of IGF I in stomach and skeletal muscle was similar at the three groups. The relative expression of IGF I in intestines was significantly higher at FOD group than at TWD group and THD group (p < 0.05). The relative expression of Chinese sturgeon IGF II in liver at TWD group was significantly higher than that at THD group and FOD group (p < 0.05). However, the relative expression of IGF II in stomach, intestines and skeletal muscle at THD group was higher than that at TWD group and FOD group. Based on these previous studies that liver IGF I is regarded as a biomarker of growth performance, this result suggested that the juvenile Chinese sturgeon is better for growth when feeding four times daily compared to twice and thrice daily.

Tilapia adropin: the localization and regulation of growth hormone gene expression in pituitary cells

The peptide hormone adropin, encoded by the energy homeostasis-associated (Enho) gene, plays a role in energy homeostasis and the control of vascular function. The aim of this study was to examine the role of adropin in growth hormone (GH) gene expression at the pituitary level in tilapia. As a first step, the antiserum for the tilapia adropin was produced, and its specificity was confirmed by antiserum preabsorption and immunohistochemical staining in the tilapia pituitary. Adropin could be detected immunocytochemically in the proximal pars distalis (PPD) of the tilapia pituitary. In primary cultures of tilapia pituitary cells, tilapia adropin was effective in increasing GH mRNA levels. However, removal of endogenous adropin by immunoneutralization using adropin antiserum inhibited GH gene expression. In parallel experiments, pituitary cells co-treated with ovine pituitary adenylate cyclase activating polypeptide 38 (oPACAP38) and adropin showed a similar increase level compared to those treated with oPACAP38 alone, whereas insulin-like growth factor 1 (IGF1) not only had an inhibitory effect on basal GH mRNA levels, but also could abolish adropin stimulation of GH gene expression. In pituitary cells pretreated with actinomycin D, the half-life of GH mRNA was enhanced by adropin. Taken together, these findings suggest that adropin may serve as a novel local stimulator for GH gene expression in tilapia pituitary.

Differential gene expression profiles in foetal skin of Rex rabbits with different wool density

<p>This study investigated the mechanisms controlling hair follicle development in the Rex rabbit. The Agilent rabbit gene expression microarray was used to determine differentially expressed genes in Rex rabbit foetuses with different wool densities. The expression patterns of selected differentially-expressed genes were further investigated by quantitative real-time PCR. Compared to low wool density rabbits, 1342 differentially expressed probes were identified in high wool density rabbits, including 950 upregulated probes and 392 downregulated probes. Gene ontology analysis revealed that the most upregulated differentially expressed probes belonged to receptors and the most downregulated differentially expressed probes belonged to DNA binding molecules. Kyoto Encyclopedia of Genes and Genomes pathway analysis revealed that the differentially expressed probes were mainly involved in the sonic hedgehog (Shh) and Eph signalling pathways. The results also suggest that transforming growth factor-beta 1, growth hormone receptor, and the keratin-associated protein 6.1 genes, as well as the Shh and Eph signalling pathways, may be involved in the regulation of hair follicle developmental in Rex rabbits.</p>

Autocrine Positive Feedback Regulation of Prolactin Release From Tilapia Prolactin Cells and Its Modulation by Extracellular Osmolality

Prolactin (PRL) is a vertebrate hormone with diverse actions in osmoregulation, metabolism, reproduction, and in growth and development. Osmoregulation is fundamental to maintaining the functional structure of the macromolecules that conduct the business of life. In teleost fish, PRL plays a critical role in osmoregulation in fresh water. Appropriately, PRL cells of the tilapia are directly osmosensitive, with PRL secretion increasing as extracellular osmolality falls. Using a model system that employs dispersed PRL cells from the euryhaline teleost fish, Oreochromis mossambicus, we investigated the autocrine regulation of PRL cell function. Unknown was whether these PRL cells might also be sensitive to autocrine feedback and whether possible autocrine regulation might interact with the well-established regulation by physiologically relevant changes in extracellular osmolality. In the cell-perfusion system, ovine PRL and two isoforms of tilapia PRL (tPRL), tPRL177 and tPRL188, stimulated the release of tPRLs from the dispersed PRL cells. These effects were significant within 5-10 minutes and lasted the entire course of exposure, ceasing within 5-10 minutes of removal of tested PRLs from the perifusion medium. The magnitude of response varied between tPRL177 and tPRL188 and was modulated by extracellular osmolality. On the other hand, the gene expression of tPRLs was mainly unchanged or suppressed by static incubations of PRL cells with added PRLs. By demonstrating the regulatory complexity driven by positive autocrine feedback and its interaction with osmotic stimuli, these findings expand upon the knowledge that pituitary PRL cells are regulated complexly through multiple factors and interactions.

Functional characterization of insulin-like growth factors in an ancestral fish species, the Shovelnose sturgeon Scaphirhynchus platorhynchus

Observations from the present study provide the first characterization of the GH-IGF axis in Shovelnose sturgeon Scaphirhynchus platorhynchus, an ancestral fish species. An initial characterization of steady-state IGF-I and IGF-II gene expression in multiple tissues was conducted using real-time RT-qPCR. Overall, the tissues had significantly different profiles of IGF-I gene expression, with the highest IGF-I expression observed in the liver. The highest IGF-II gene expression was also observed in the liver, with minimal or no detection in muscle. A comparison between IGF-I and IGF-II expression within individual tissues revealed higher levels of IGF-II than IGF-I mRNA in the spleen, stomach and trunk kidney, and higher levels of relative IGF-I mRNA expression in the intestine and muscle. The GH-IGF axis was further elucidated by observing the effects of exogenous GH on IGF-I and IGF-II expression in liver and muscle tissue. The results revealed a significant dose-dependent response of both hepatic IGF-I and IGF-II, and muscle IGF-I mRNA expression following rbGH administration. At the highest rbGH concentration (240μg/g BW), IGF-I mRNA levels in liver and muscle peaked significantly at 48h, indicating both hepatic and muscle IGF-I expression to be stimulated by GH. Hepatic IGF-II expression was also stimulated 48h following rbGH administration. Expression of IGF-II mRNA was not inducible in the muscle. Few studies have evaluated the effects of exogenous GH on IGF expression in ancestral vertebrate species, and as such, this research provides valuable insight into the evolution of the somatotropic axis in vertebrates.

Insulin-like growth factor 1 (IGF-1) regulates prolactin, growth hormone, and IGF-1 receptor expression in the pituitary gland of the gilthead sea bream Sparus aurata

The role of insulin-like growth factor 1 (IGF-1) on regulation of growth hormone (GH) and prolactin (PRL) as well as the possible involvement of IGF-1 receptor subtype a (IGF-1Ra) mRNA was assessed in juvenile specimens of Sparus aurata. IGF-1Ra was successfully cloned, and active receptor domains were localized in its mRNA precursor. Also, phylogenetic analysis of the protein sequence indicated a closer proximity to IGF-1Ra isoform found in zebrafish and other teleosts, than to the isoform IGF-1Rb. The most abundant presence of IGF-1Ra mRNA was detected in white muscle, whereas head kidney showed the lowest gene expression among 24 different studied tissues. Pituitaries of juvenile specimens of S. aurata were incubated in vitro with different doses of IGF-1 (0, 1, 100, and 1000 ng mL(-1)) during a period of 10 h. Total RNA with a high quality could be obtained from these pituitaries. PRL mRNA expression significantly increased with increasing IGF-1 doses. Similarly, IGF-1Ra mRNA increased its expression in response to IGF-1. However, GH mRNA levels decreased in a dose-dependent manner after IGF-1 treatment. The contradictory responses of GH and PRL expressions to IGF-1 in our experiment are possibly mediated by IGF-1Ra presence on the somatotrophs and prolactotrophs. The increase in IGF-1Ra mRNA levels may be related to the proper activation of the PI3-K/Akt signal transduction pathways which are normally involved in GH and PRL regulation.

The acute salinity changes activate the dual pathways of endocrine responses in the brain and pituitary of tilapia

To analyze and compare the stress and osmoregulatory hormones and receptors in pituitary during acute salinity changes, the expression patterns of corticotropin releasing hormone (crh) in hypothalamus, prolactin (prl) releasing peptide (pRrp) in telencephalon and diencephalon, glucocorticoid receptors 2 (gr2), and mineralocorticoid receptor (mr), crh-r, pro-opiomelanocorticotropin (pomc), pRrp, prl, dopamine 2 receptor (d2-r), growth hormone (gh), gh-receptor (gh-r) and insulin-like growth hormone (igf-1) transcripts in pituitary were characterized in euryhaline tilapia. The results indicate that the crh transcripts increased in the hypothalamus and rostral pars distalis of the pituitary after the transfer of fish to SW. Similarly, the pRrp transcripts were more abundant in SW acclimated tilapia forebrain and hypothalamus. The crh-r, gr2 and mr transcripts were more expressed in rostral pars distalis and pars intermedia of pituitary at SW than FW tilapia. The data indicate that the SW acclimation stimulates these transcripts in the specific regions of the brain and pituitary which may be related to the activation of the hypothalamic-pituitary-interrenal (HPI)-axis. The results of dual in situ hybridization reveal that the transcripts of crh-r, gr2 and mr with pomc are highly co-localized in corticotrophs of pituitary. Furthermore, we demonstrate high expression of pRrp in the brain and low expression of pRrp and prl transcripts in the pituitary of SW fish. No crh-r and corticosteroid receptors were co-localized with prl transcripts in the pituitary. The gh-r and igf-1 mRNA levels were significantly increased in SW acclimated tilapia pituitary whereas there was no difference in the gh mRNA levels. The data suggest that the locally produced pRrp and d2-r may control and regulate the expression of prl mRNA in pituitary. Therefore, the dual roles of pRrp are involved in the stress (via brain-pituitary) and osmoregulatory (via pituitary) pathways in tilapia exposed to acute salinity changes.

Nutritional status and growth hormone regulate insulin-like growth factor binding protein (igfbp) transcripts in Mozambique tilapia

Growth in teleosts is controlled in large part by the activities of the growth hormone (Gh)/insulin-like growth factor (Igf) system. In this study, we initially identified igf-binding protein (bp)1b, -2b, -4, -5a and -6b transcripts in a tilapia EST library. In Mozambique tilapia (Oreochromis mossambicus), tissue expression profiling of igfbps revealed that igfbp1b and -2b had the highest levels of expression in liver while igfbp4, -5a and -6b were expressed at comparable levels in most other tissues. We compared changes in hepatic igfbp1b, -2b and -5a expression during catabolic conditions (28days of fasting) along with key components of the Gh/Igf system, including plasma Gh and Igf1 and hepatic gh receptor (ghr2), igf1 and igf2 expression. In parallel with elevated plasma Gh and decreased Igf1 levels, we found that hepatic igfbp1b increased substantially in fasted animals. We then tested whether systemic Gh could direct the expression of igfbps in liver. A single intraperitoneal injection of ovine Gh into hypophysectomized tilapia specifically stimulated liver igfbp2b expression along with plasma Igf1 and hepatic ghr2 levels. Our collective data suggest that hepatic endocrine signaling during fasting may involve post-translational regulation of plasma Igf1 via a shift towards the expression of igfbp1b. Thus, Igfbp1b may operate as a molecular switch to restrict Igf1 signaling in tilapia; furthermore, we provide new details regarding isoform-specific regulation of igfbp expression by Gh.

Endocrine regulation of prolactin cell function and modulation of osmoreception in the Mozambique tilapia

Prolactin (PRL) cells of the Mozambique tilapia, Oreochromis mossambicus, are osmoreceptors by virtue of their intrinsic osmosensitivity coupled with their ability to directly regulate hydromineral homeostasis through the actions of PRL. Layered upon this fundamental osmotic reflex is an array of endocrine control of PRL synthesis and secretion. Consistent with its role in fresh water (FW) osmoregulation, PRL release in tilapia increases as extracellular osmolality decreases. The hyposmotically-induced release of PRL can be enhanced or attenuated by a variety of hormones. Prolactin release has been shown to be stimulated by gonadotropin-releasing hormone (GnRH), 17-β-estradiol (E2), testosterone (T), thyrotropin-releasing hormone (TRH), atrial natriuretic peptide (ANP), brain-natriuretic peptide (BNP), C-type natriuretic peptide (CNP), ventricular natriuretic peptide (VNP), PRL-releasing peptide (PrRP), angiotensin II (ANG II), leptin, insulin-like growth factors (IGFs), ghrelin, and inhibited by somatostatin (SS), urotensin-II (U-II), dopamine, cortisol, ouabain and vasoactive intestinal peptide (VIP). This review is aimed at providing an overview of the hypothalamic and extra-hypothalamic hormones that regulate PRL release in euryhaline Mozambique tilapia, particularly in the context on how they may modulate osmoreception, and mediate the multifunctional actions of PRL. Also considered are the signal transduction pathways through which these secretagogues regulate PRL cell function.

Influences of the environment on the endocrine and paracrine fish growth hormone-insulin-like growth factor-I system

Insulin-like growth factor-I (IGF-I) is a key component of the complex system that regulates differentiation, development, growth and reproduction of fishes. The IGF-I gene is mainly expressed in the liver that represents the principal source of endocrine IGF-I but also in numerous other organs where the hormone most probably acts in an autocrine-paracrine manner. The primary stimulus for synthesis and release of IGF-I is growth hormone (GH) from the anterior pituitary. Thus, in analogy to mammals, it is usual to speak of a fish 'GH-IGF-I axis'. The GH-IGF-I system is affected by changes in the environment and probably represents a target of endocrine disrupting compounds (EDC) that impair many physiological processes in fishes. Thus, the review deals with the influences of changes in different environmental factors, such as food availability, temperature, photoperiod, season, salinity and EDCs, on GH gene expression in pituitary, IGF-I gene expression in liver and extrahepatic sites and the physiological effects resulting from the evoked alterations in endocrine and local IGF-I. Environmental influences certainly interact with each other but for convenience of the reader they will be dealt with in separate sections. Current trends in GH-IGF-I research are analysed and future focuses are suggested at the end of the sections.

Effects of short- and long-term fasting on plasma and stomach ghrelin, and the growth hormone/insulin-like growth factor I axis in the tilapia, Oreochromis mossambicus

Ghrelin is a highly conserved peptide hormone secreted by the stomach, which is involved in the regulation of food intake and energy expenditure. Ghrelin stimulates growth hormone (GH) release, and increases appetite in a variety of mammalian and non-mammalian vertebrates, including several fish species. Studies were conducted to investigate the effect of feeding and fasting on plasma and stomach ghrelin, and the growth hormone/insulin-like growth factor I (IGF-I) axis in the Mozambique tilapia, a euryhaline teleost. No postprandial changes in plasma and stomach ghrelin levels or stomach ghrelin mRNA levels were observed. Plasma levels of GH, IGF-I and glucose all increased postprandially which agrees with the anabolic roles of these factors. Fasting for 4 and 8d did not affect ghrelin levels in plasma or stomach. Plasma GH was elevated significantly after 4 and 8d of fasting, while plasma IGF-I levels were reduced. Plasma ghrelin levels were elevated significantly after 2 and 4 wk of fasting, but no change was detected in stomach ghrelin mRNA levels. Four weeks of fasting did not affect plasma GH levels, although plasma IGF-I and glucose were reduced significantly, indicating that GH resistance exists during a prolonged nutrient deficit (catabolic state). These results indicate that ghrelin may not be acting as a meal-initiated signal in tilapia, although it may be acting as a long-term indicator of negative energy balance.

Pre- and postprandial effects on ghrelin signaling in the brain and on the GH/IGF-I axis in the Mozambique tilapia (Oreochromis mossambicus)

The discovery of ghrelin (GRLN) has broadened our understanding of the regulation of energy homeostasis in vertebrates. In addition to stimulating growth hormone release from the pituitary, GRLN has been implicated as a hunger signal stimulating food intake in mammals and goldfish. Indeed, GRLN levels rise preprandial and fall following a meal. The current study investigated pre- and postprandial changes (3 h before and after a meal) in GRLN signaling in the tilapia (Oreochromis mossambicus). Significant elevations in preprandial brain mRNA levels of the GRLN receptor (GHS-R1a) and GRLN were observed; though not significant brain neuropeptide Y (NPY) mRNA levels did increase preprandially. GHS-R1b, and NPY mRNA levels were reduced significantly 3 h after a meal; whereas GHS-R1a levels were unaltered postprandially. Brain ghrelin mRNA levels exhibited a transient significant increase 1 h postprandially. Tilapia that missed the scheduled feeding exhibited no changes in brain GHS-R1a, GRLN and NPY postprandial mRNA levels; whereas GHS-R1b mRNA levels were significantly reduced 1 and 3 h postprandially. Brain GHSR preprocessed RNA (heteronuclear mRNA) levels were significantly elevated 3 h preprandially. GHS-R hnRNA levels were significantly elevated 1h postprandial in fed and fasted tilapia. No preprandial rise in plasma GRLN was observed. Following a meal, plasma GRLN levels were significantly elevated; whereas there was no change in tilapia missing the scheduled feeding. Stomach mRNA levels of GRLN rose preprandially and remained unchanged following a meal. In animals that missed the scheduled feeding stomach GRLN levels dropped significantly 1 h following a meal. There was no change in plasma growth hormone levels in the fed fish, although there was a significant rise in the fasted fish 1h after the scheduled feeding. Postprandial levels of plasma IGF-I were elevated in both fed and fasted tilapia. These results suggest that brain derived GRLN is likely driving day-to-day appetite through GHS-R1a and NPY; while systemic GRLN may play a role in postprandial metabolism.

Absence of effects of short-term fasting on plasma ghrelin and brain expression of ghrelin receptors in the tilapia, Oreochromis mossambicus

Ghrelin is an important endocrine peptide that links the gastrointestinal system and brain in the regulation of food intake and energy expenditure. In human, rat, and goldfish plasma levels of ghrelin and GH are elevated in fasted animals, suggesting that ghrelin is an orexigenic signal and a driving force behind the elevated plasma levels of GH during fasting. Ghrelin's orexigenic action is mediated by the ghrelin receptor (GHS-R1a and GHS-R1b) which is localized on neuropeptide Y (NPY) neurons in the brain. Studies were undertaken to investigate the effect of short-term fasting on plasma ghrelin and brain expression of GHS-R1a, GHS-R1b, and NPY in the tilapia. Fasting for 7 days had no effect on plasma ghrelin concentrations, whereas significant increases in plasma levels of GH were observed on day 3. Fasting significantly reduced plasma levels of IGF-I on days 3 and 7, and of glucose on days 3, 5, and 7. Brain expression of ghrelin and GHS-R1b were significantly elevated in fasted fish on day 3, but were significantly reduced on day 5. This reduction was likely due to a significant increase in the expression in the fed controls on day 5 compared to day 0. No change was detected in the expression of GHS-R1a or NPY in the brain. These results indicate that ghrelin is not acting as a hunger signal in short-term fasted tilapia and is not responsible for the elevated levels of plasma GH.

IGF-I is distinctly located in the bony fish pituitary as revealed for Oreochromis niloticus, the Nile tilapia, using real-time RT-PCR, in situ hybridisation and immunohistochemistry

In bony fish, IGF-I released from the liver under the control of pituitary GH is the main endocrine regulator of growth, maintenance and development, and the amount of circulating IGF-I regulates synthesis and release of GH. In mammals and amphibia, evidence indicates that anterior pituitary endocrine cells also contain IGF-I. However, only preliminary and conflicting data exist on IGF-I gene expression in bony fish pituitary. Thus, we investigated the presence of IGF-I in the tilapia (Oreochromis niloticus) pituitary by quantitative real-time RT-PCR, in situ hybridisation and immunohistochemistry. The absolute amount of IGF-I mRNA in the whole pituitary (7.4+/-3.3 x 10(-3)pg/microg total RNA) was 1000-times lower than in liver (7.5+/-3.1 pg/microg total RNA). IGF-I peptide occurred in both neuro- and adenohypophysis but IGF-I gene expression was mainly restricted to the adenohypophysis. In the neurohypophysis, only few cells, probably pituicytes, contained IGF-I mRNA whereas IGF-I peptide was found also in numerous axons in the pars nervosa. In the adenohypophysis, both IGF-I mRNA and peptide were present in the majority of ACTH cells in all individuals investigated. In alpha-MSH cells, only IGF-I mRNA but no IGF-I peptide was detected likely suggesting an immediate release of IGF-I after synthesis. IGF-I mRNA and peptide were further observed in GH cells but their presence showed pronounced inter-individual differences likely due to the physiological, e.g., nutritional, status of the individual. IGF-I released from the GH cells may serve as auto/paracrine mediator of a negative feedback mechanism in addition to liver-derived endocrine IGF-I. Generally, the constitutive synthesis of IGF-I in ACTH cells and the varying content in GH and alpha-MSH cells suggest particular roles for IGF-I. Local IGF-I may regulate synthesis and release of pituitary hormones in an autocrine and/or paracrine manner as well as prevent apoptosis and stimulate proliferation of endocrine cells.

Regulation of the RAP1/RAF-1/extracellularly regulated kinase-1/2 cascade and prolactin release by the phosphoinositide 3-kinase/AKT pathway in pituitary cells

In pituitary cells, prolactin (PRL) synthesis and release are controlled by multiple transduction pathways. In the GH4C1 somatolactotroph cell line, we previously reported that MAPK ERK-1/2 are a point of convergence between the pathways involved in the PRL gene regulation. In the present study, we focused on the involvement of the phosphoinositide 3-kinase (PI3K)/Akt pathway in the MAPK ERK-1/2 regulation and PRL secretion in pituitary cells. Either specific pharmacological PI3K and Akt inhibitors (LY294002, Akt I, and phosphoinositide analog-6) or Akt dominant-negative mutant (K179M) enhanced ERK-1/2 phosphorylation in unstimulated GH4C1 cells. Under the same conditions, PI3K and Akt inhibition also both increased Raf-1 kinase activity and the levels of GTP-bound (active form) monomeric G protein Rap1, which suggests that a down-regulation of the ERK-1/2 cascade is induced by the PI3K/Akt signaling pathway in unstimulated cells. On the contrary, ERK-1/2 phosphorylation, Raf-1 activity, and Rap1 activation were almost completely blocked in IGF-I-stimulated cells previously subjected to PI3K or Akt inhibition. Although the PRL promoter was not affected by either PI3K/Akt inhibition or activation, PRL release increased in response to the pharmacological PI3K/Akt inhibitors in unstimulated GH4C1 and rat pituitary primary cells. The IGF-I-stimulated PRL secretion was diminished, on the contrary, by the pharmacological PI3K/Akt inhibitors. Taken together, these findings indicate that the PI3K/Akt pathway exerts dual regulatory effects on both the Rap1/Raf-1/ERK-1/2 cascade and PRL release in pituitary cells, i.e. negative effects in unstimulated cells and positive ones in IGF-I-stimulated cells.

Effects of fasting on growth hormone, growth hormone receptor, and insulin-like growth factor-I axis in seawater-acclimated tilapia, Oreochromis mossambicus

Effects of fasting on the growth hormone (GH)--growth hormone receptor (GHR)-insulin-like growth factor-I (IGF-I) axis were characterized in seawater-acclimated tilapia (Oreochromis mossambicus). Fasting for 4 weeks resulted in significant reductions in body weight and specific growth rate. Plasma GH and pituitary GH mRNA levels were significantly elevated in fasted fish, whereas significant reductions were observed in plasma IGF-I and hepatic IGF-I mRNA levels. There was a significant negative correlation between plasma levels of GH and IGF-I in the fasted fish. No effect of fasting was observed on hepatic GHR mRNA levels. Plasma glucose levels were reduced significantly in fasted fish. The fact that fasting elicited increases in GH and decreases in IGF-I production without affecting GHR expression indicates a possible development of GH resistance.

Feedback regulation of growth hormone synthesis and secretion in fish and the emerging concept of intrapituitary feedback loop

Growth hormone (GH) is known to play a key role in the regulation of body growth and metabolism. Similar to mammals, GH secretion in fish is under the control of hypothalamic factors. Besides, signals generated within the pituitary and/or from peripheral tissues/organs can also exert a feedback control on GH release by effects acting on both the hypothalamus and/or anterior pituitary. Among these feedback signals, the functional role of IGF is well conserved from fish to mammals. In contrast, the effects of steroids and thyroid hormones are more variable and appear to be species-specific. Recently, a novel intrapituitary feedback loop regulating GH release and GH gene expression has been identified in fish. This feedback loop has three functional components: (i) LH induction of GH release from somatotrophs, (ii) amplification of GH secretion by GH autoregulation in somatotrophs, and (iii) GH feedback inhibition of LH release from neighboring gonadotrophs. In this article, the mechanisms for feedback control of GH synthesis and secretion are reviewed and functional implications of this local feedback loop are discussed. This intrapituitary feedback loop may represent a new facet of pituitary research with potential applications in aquaculture and clinical studies.

Effects of aquaculture related stressors and nutritional restriction on circulating growth factors (GH, IGF-I and IGF-II) in Atlantic salmon and rainbow trout

The effects of aquaculture related stressors on circulating levels of GH, IGF-I and for the first time, IGF-II in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) were investigated. Specifically, circulating growth factor levels were measured in four different experiments. Two 24 h confinement stressor procedures, (one with Atlantic salmon, the other with rainbow trout); following a hypo-osmotic stressor (freshwater bath) in salt water acclimated, adult, Atlantic salmon; and during a 22 day starvation and re-feeding protocol with juvenile Atlantic salmon. Handling and confinement resulted in significant decreases in circulating levels of all three growth factors in Atlantic salmon, and IGF-I and IGF-II (but not GH) in rainbow trout. A 2-3 h freshwater bath to remove gill parasites on a commercial Atlantic salmon aquaculture operation caused a significant decrease in circulating GH and IGF-I concentrations, but no significant change in IGF-II concentration, 2 days post bathing. Starvation for a period of 15 days in Atlantic salmon resulted in a significant increase in circulating GH levels and a significant decrease in circulating IGF-I and IGF-II. Re-feeding of starved fish for 7 days resulted in a significant decrease in GH to the concentration measured in continually fed fish, however re-feeding did not change plasma levels of IGF-I and IGF-II relative to continually starved fish. The results presented here confirm previously observed handling and confinement stressor induced effects on GH and IGF-I and, for the first time, on IGF-II in salmonids. Furthermore this study confirms the nutritional regulation of GH, IGF-I and IGF-II in juvenile Atlantic salmon.

The Advantage of Absolute Quantification in Comparative Hormone Research as Indicated by a Newly Established Real-Time RT-PCR: GH, IGF-I, and IGF-II Gene Expression in the Tilapia,Oreochromis niloticus

We have developed a real-time RT-PCR that absolutely quantifies the gene expression of hormones using the standard curve method. The method avoids cloning procedures by using primer extension to create templates containing a T7 promoter gene sequence. It is rapid since neither separate reverse transcriptions nor postamplification steps are necessary, and its low detection level (2 pg/mug total RNA) allows precise absolute quantification. Using the method, we have quantified the gene expression of GH, IGF-I, and IGF-II in the tilapia.

Effects of salmon GnRH and sex steroid hormones on expression of genes encoding growth hormone/prolactin/somatolactin family hormones and a pituitary-specific transcription factor in masu salmon pituitary cells in vitro

Expression of genes encoding growth hormone (GH), prolactin (PRL), and somatolactin (SL) in growing and maturing salmon was stimulated by gonadotropin-releasing hormone (GnRH) analog during particular periods of the life cycle. GnRH therefore appears to directly and/or indirectly regulate gene expression for GH, PRL, and SL in combination with the pituitary-gonadal axis, such as sex steroid hormones. Direct effects of salmon GnRH (sGnRH), estradiol-17beta (E2), testosterone, and 11-ketotestosterone (11KT) on the amounts of GH, PRL, and SL mRNAs were thus examined using primary pituitary cell cultures of masu salmon at the four reproductive stages. We also determined the amounts of mRNA encoding pituitary specific POU homeodomain transcription factor (Pit-1) by real-time polymerase chain reactions. The amounts of GH, PRL, and SL mRNAs in the control cells elevated with gonadal maturation, coincidently with those of Pit-1 mRNA. sGnRH at 1.0 nM elevated the amounts of all mRNAs examined in the pre-spawning females, whereas significant effects were not observed with 100 nM sGnRH at any reproductive stages. Sex steroid hormones had no significant effects before initiation of gonadal maturation and at the maturing stage. In the males, E2 tended to decrease the amounts of SL mRNA in the pre-spawning stage. In the females, E2 and 11KT increased the amounts of PRL and SL mRNAs in the pre-spawning stage, but halved those of PRL mRNA in the spawning stage. The amounts of Pit-1 mRNA changed coincidently with those of PRL and SL mRNAs at all examined stages. The effects of E2 alone were abolished by 100 nM sGnRH. The present results indicated that both sGnRH and steroid hormones directly modulate synthesis of Pit-1, and further expression of PRL and SL genes. sGnRH may indirectly regulate GH/PRL/SL family hormone genes through the pituitary-gonadal axis, particularly in the late stage of gametogenesis.

Modulation of calmodulin gene expression as a novel mechanism for growth hormone feedback control by insulin-like growth factor in grass carp pituitary cells

Calmodulin (CaM), the Ca2+ sensor in living cells, is essential for biological functions mediated by Ca2+-dependent mechanisms. However, modulation of CaM gene expression at the pituitary level as a means to regulate pituitary hormone synthesis has not been characterized. In this study we examined the functional role of CaM in the feedback control of GH by IGF using grass carp pituitary cells as a cell model. To establish the structural identity of CaM expressed in the grass carp, a CaM cDNA, CaM-L, was isolated from the carp pituitary using 3'/5' rapid amplification of cDNA ends. The open reading frame of this cDNA encodes a 149-amino acid protein sharing the same primary structure with CaMs reported in mammals, birds, and amphibians. This CaM cDNA is phylogenetically related to the CaM I gene family, and its transcripts are ubiquitously expressed in the grass carp. In carp pituitary cells, IGF-I and IGF-II induced CaM mRNA expression with a concurrent drop in GH transcript levels. These stimulatory effects on CaM mRNA levels were not mimicked by insulin and appeared to be a direct consequence of IGF activation of CaM gene transcription without altering CaM transcript stability. CaM antagonism and inactivation of calcineurin blocked the inhibitory effects of IGF-I and IGF-II on GH gene expression, and CaM overexpression also suppressed the 5' promoter activity of the grass carp GH gene. These results, as a whole, provide evidence for the first time that IGF feedback on GH gene expression is mediated by activation of CaM gene expression at the pituitary level.

Insulin-like growth factor signaling in fish

The insulin-like growth factor (IGF) system plays a central role in the neuroendocrine regulation of growth in all vertebrates. Evidence from studies in a variety of vertebrate species suggest that this growth factor complex, composed of ligands, receptors, and high-affinity binding proteins, evolved early during vertebrate evolution. Among nonmammalian vertebrates, IGF signaling has been studied most extensively in fish, particularly teleosts of commercial importance. The unique life history characteristics associated with their primarily aquatic existence has fortuitously led to the identification of novel functions of the IGF system that are not evident from studies in mammals and other tetrapod vertebrates. Furthermore, the emergence of the zebrafish as a preferred model for development genetics has spawned progress in determining the requirements for IGF signaling during vertebrate embryonic development. This review is intended as a summary of our understanding of IGF signaling, as revealed through research into the expression, function, and evolution of IGF ligands, receptors, and binding proteins in fish.

Growth hormone secretion from the Arctic charr (Salvelinus alpinus) pituitary gland in vitro: effects of somatostatin-14, insulin-like growth factor-I, and nutritional status

This study investigated the influence of nutritional status on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis in Arctic charr (Salvelinus alpinus). The objectives were to study the regulation of GH secretion in vitro by somatostatin-14 (SRIF) and hIGF-I, and to determine whether pituitary sensitivity to these factors is dependent upon nutritional status. Arctic charr were fed at three different ration levels (0, 0.35, and 0.70% BWd(-1)), and pituitary glands were harvested at 1, 2, and 5 weeks for in vitro study. Both SRIF and hIGF-I inhibited GH secretion from Arctic charr pituitary tissue in long-term (18 h) static hemipituitary culture, as well as after acute exposure in a pituitary fragment perifusion system. This response appeared to be dose-dependent for SRIF in static culture over the range of 0.01-1 nM, but not for hIGF-I. The acute inhibitory action of hIGF-I on GH release in the perifusion system suggests an action that is initially independent of any effects on GH gene expression or protein synthesis. Nutritional status did not affect the sensitivity of Arctic charr pituitary tissue to either SRIF or hIGF-I in vitro, indicating that changes in abundance of pituitary SRIF or IGF-I receptors may not explain the alterations in plasma GH levels found during dietary restriction.

Growth hormone and insulin-like growth factor-1 differentially stimulate the expression of preprosomatostatin mRNAs in the Brockmann bodies of rainbow trout, Oncorhynchus mykiss

We previously characterized three cDNAs obtained from the endocrine pancreas (Brockmann body) of rainbow trout that encode for distinct preprosomatostatin (PPSS) molecules: PPSS I containing somatostain-14 (SS-14) at its C-terminus and two separate PPSS IIs, PPSS II' and PPSS II'', containing [Tyr7,Gly10]-SS-14 at their C-termini. In this study, we examined the control of PPSS I, PPSS II', and PPSS II'' mRNA expression by growth hormone (GH) and insulin-like growth factor-1 (IGF-1). Rainbow trout implanted with GH for 21 days displayed elevated pancreatic expression of all PPSS mRNAs compared to control animals. Growth hormone directly stimulated the expression of all pancreatic PPSS mRNAs in vitro in a dose-dependent manner; however, GH was a more potent stimulator of PPSS II' expression than of PPSS I or PPSS II'' expression. Insulin-like growth factor-1 also directly stimulated the expression of PPSS mRNAs in a dose-dependent manner in Brockmann bodies incubated in vitro; IGF-1 was a more potent stimulator of PPSS I and PPSS II' expression than of PPSS II'' expression. These results indicate that the expression of PPSS mRNAs in the Brockmann body of trout is differentially regulated by GH and IGF-1 and suggest that SS mediate the feedback regulation of GH and IGF-1.

In vitro effects of cortisol on the release and gene expression of prolactin and growth hormone in the tilapia, Oreochromis mossambicus

Exposure to cortisol inhibits prolactin (PRL) release from the tilapia pituitary within 10-20min through a plasma membrane-associated, non-genomic pathway. In the present study, in vitro effects of cortisol on the release and mRNA levels of two PRLs (PRL(188) and PRL(177)) and growth hormone (GH) were examined in the organ-cultured pituitary of the Mozambique tilapia, Oreochromis mossambicus. The PRL release was significantly greater in hyposmotic (300mOsmolal) than in hyperosmotic (350mOsmolal) medium during the 2-8h of incubation. The mRNA levels of two PRLs, as estimated by RNase protection assay, were increased after 8h in hyposmotic medium. Cortisol (200nM) inhibited the release of two PRLs under hyposmotic conditions within 1h, and the inhibitory effects lasted for 24h. Cortisol also reduced the gene transcription of both PRLs during 2-8h of incubation but not after 24h. No effect of cortisol was observed on PRL release or on its mRNA levels under hyperosmotic condition. There was no significant effect of medium osmolality on the release or mRNA levels of GH during 8h of incubation. However, GH release was significantly stimulated by cortisol after 4h, and the effect lasted for 24h under both hyposmotic and hyperosmotic conditions. Cortisol also caused a significant increase in GH mRNA levels at 8 and 24h. These results suggest that cortisol inhibits PRL release from the tilapia pituitary through non-genomic and also through transcriptional pathways, while stimulating GH release through classical genomically mediated glucocorticoid actions.

Effects of transfer from seawater to fresh water on the growth hormone/insulin-like growth factor-I axis and prolactin in the Tilapia, Oreochromis mossambicus

The effect of freshwater (FW) transfer on growth and on the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis was examined in the tilapia, Oreochromis mossambicus. Tilapia were raised in seawater (SW) for 5 months and then transferred to FW for an additional 40 days. The growth rate of the fish transferred to FW was significantly reduced compared with the growth rate of fish that remained in SW. Plasma levels of GH were significantly elevated in FW-transferred fish, as were plasma IGF-I levels. Pituitary GH and liver IGF-I mRNA levels, on the other hand, were significantly reduced in the fish transferred to FW. There was a significant correlation between body mass and mRNA levels of GH and IGF-I, but not with plasma levels of GH and IGF-I. Fish transferred to FW had significantly higher prolactin (PRL)(177) levels than the SW control fish, although there was no difference in plasma PRL(188) levels. Consistent with the hyperosmoregulatory effects of PRL, mRNA levels of both PRL(177) and PRL(188) were significantly higher in FW-transferred fish than in the fish in SW. These results suggest that transferring tilapia from SW to FW activates the GH/IGF-I axis, but growth is still inhibited, possibly due to the greater metabolic cost of osmoregulation in FW than in SW.

Effects of environmental temperature on IGF1, IGF2, and IGF type I receptor expression in rainbow trout (Oncorhynchus mykiss)

Recently, we have demonstrated in rainbow trout that environmental temperature may, independently of nutritional status, directly stimulate plasma growth hormone (GH) that is recognised as being an insulin-like growth factor (IGF) system regulator. The aim of this study was to determine whether temperature may directly regulate the IGF system or indirectly regulate it through plasma GH or nutritional status. For this purpose, rainbow trout were reared at 8, 12, or 16 degrees C and fed either ad libitum (similar nutritional status) to evidence the global effect of temperature, or with the same ration (1.2% body weight/day), to determine the temperature effect in fish with the same growth rate. Endocrine and autocrine/paracrine regulations of the IGF system were determined by measuring plasma IGF1 and IGF2, liver and muscle IGF1 and IGF2 mRNA as well as IGFRIa, IGFRIb mRNA, and the quantity of IGF type I receptor in muscle. Our results show that neither rearing temperature nor the nutritional status of fish affected the expression of both IGF receptor genes in muscle. Nevertheless, the quantity of IGF type I receptor determined by a binding study, appeared to be inversely proportional (P<0.05) to the rearing temperature without any relationship with nutritional status, suggesting a direct effect of temperature on its turnover. After 2 weeks of treatment, the levels of IGF1 mRNA in muscle at 8 degrees C were 2-fold higher (P<0.05) than at 16 degrees C in both ad libitum and restricted feed fish, whereas after 6 weeks, this difference was no longer observed. In both experiments, the levels of plasma IGF2 were 10-fold higher than the levels of plasma IGF1 (mean 105+/-3.0 versus 13.5+/-0.6 ng/ml), and plasma levels were correlated with their respective mRNA liver concentrations (r2=0.14 and 0.25, respectively; P<0.01). In the ad libitum feeding experiment, plasma and mRNA levels of IGF1 were related to the rearing temperature (P<0.05), while for IGF2 no effect was seen. In contrast, in the restricted feeding experiment, plasma and IGF2 mRNA levels were inversely proportional to the rearing temperature (P<0.0001) while plasma IGF1 was unaltered. Levels of plasma IGF1 were related to the growth rate in both experiments, while levels of plasma IGF2 appeared to be associated with the nutritional status of the fish. Our results suggest that the autocrine/paracrine expression of IGF1 and IGF2 in muscle is not a key regulator of the growth promoting effect of temperature. Conversely, temperature seems to promote growth through IGF1 secretion by the liver following GH stimulation, and impairment of nutritional status would prevent the IGF1 stimulation by temperature. In addition, the growth-promoting effect of temperature did not affect plasma IGF2, which appeared to be more related to the metabolic status of the fish.

Effects of fasting on growth hormone/insulin-like growth factor I axis in the tilapia, Oreochromis mossambicus

Effects of fasting on the growth hormone (GH)-insulin-like growth factor I (IGF-I) axis were examined in the tilapia (Oreochromis mossambicus) acclimated to fresh water. Fasting for 2 weeks resulted in significant reductions in body weight, specific growth rate and hepatosomatic index in both males and females. Significant reductions in specific growth rates were observed after 1 and 2 weeks in both sexes, although the decrease in body weight was not significant in the female. A significant reduction was also seen in the condition factor of females after 2 weeks. No change was seen in the gonadosomatic index in either sex. Two weeks of fasting also produced a significant reduction in plasma IGF-I but not in plasma GH, prolactin (PRL(188)) or cortisol. Significant reductions in the hepatic IGF-I mRNA were seen in both sexes. On the other hand, a significant increase was observed in cortisol receptor mRNA in the female liver. Plasma IGF-I levels were correlated significantly with specific growth rate, condition factor and hepatosomatic index, indicating that plasma IGF-I is a good indicator of growth in the tilapia. No change was seen in plasma glucose or osmolality after 2 weeks of fasting. During fasting, tilapia appears to convert metabolic energy from growth to basal metabolism including maintenance of ion and water balance.