Growth hormone (GH)-dependent hepatic GH receptors in the Japanese eel, Anguilla japonica: effects of hypophysectomy and GH injection

Endocrine regulation of compensatory growth in fish

Compensatory growth (CG) is a period of accelerated growth that occurs following the alleviation of growth-stunting conditions during which an organism can make up for lost growth opportunity and potentially catch up in size with non-stunted cohorts. Fish show a particularly robust capacity for the response and have been the focus of numerous studies that demonstrate their ability to compensate for periods of fasting once food is made available again. CG is characterized by an elevated growth rate resulting from enhanced feed intake, mitogen production, and feed conversion efficiency. Because little is known about the underlying mechanisms that drive the response, this review describes the sequential endocrine adaptations that lead to CG; namely during the precedent catabolic phase (fasting) that taps endogenous energy reserves, and the following hyperanabolic phase (refeeding) when accelerated growth occurs. In order to elicit a CG response, endogenous energy reserves must first be moderately depleted, which alters endocrine profiles that enhance appetite and growth potential. During this catabolic phase, elevated ghrelin and growth hormone (GH) production increase appetite and protein-sparing lipolysis, while insulin-like growth factors (IGFs) are suppressed, primarily due to hepatic GH resistance. During refeeding, temporal hyperphagia provides an influx of energy and metabolic substrates that are then allocated to somatic growth by resumed IGF signaling. Under the right conditions, refeeding results in hyperanabolism and a steepened growth trajectory relative to constantly fed controls. The response wanes as energy reserves are re-accumulated and homeostasis is restored. We ascribe possible roles for select appetite and growth-regulatory hormones in the context of the prerequisite of these catabolic and hyperanabolic phases of the CG response in teleosts, with emphasis on GH, IGFs, cortisol, somatostatin, neuropeptide Y, ghrelin, and leptin.

Regulation of growth hormone (GH) receptor (GHR1 and GHR2) mRNA level by GH and metabolic hormones in primary cultured tilapia hepatocytes

Growth hormone (GH) regulates essential physiological functions in teleost fishes, including growth, metabolism, and osmoregulation. Recent studies have identified two clades of putative receptors for GH (GHR1 clade and GHR2 clade) in fishes, both of which are highly expressed in the liver. Moreover, the liver is an important target for the anabolic effects of GH via endocrine IGFs, and liver sensitivity to GH is modulated by metabolic hormones. We investigated the effects of GH, insulin, glucagon, cortisol and triiodothyronine on GHR1 and GHR2 mRNA levels in primary cultured tilapia hepatocytes. Physiological concentrations of GH strongly stimulated GHR2 mRNA level (0.5-50×10(-9) M), but did not affect GHR1 mRNA level. Insulin suppressed stimulation of GHR2 mRNA level by GH (10(-8)-10(-6) M). Insulin increased basal GHR1 mRNA level (10(-8)-10(-6) M). Cortisol increased basal GHR2 mRNA level (10(-7)-10(-6) M), but did not consistently affect GH-stimulated GHR2 mRNA level. Cortisol increased basal GHR1 mRNA level (10(-9)-10(-6) M). Glucagon suppressed GH-stimulated GHR2 mRNA level and increased basal GHR1 mRNA level at a supraphysiological concentration (10(-6) M). A single injection of GH (5 μg/g) increased liver GHR2 mRNA level, and insulin injection (5 μg/g) decreased both basal and GH-stimulated GHR2 mRNA levels after 6 h. In contrast, insulin and GH injection had little effect on liver GHR1 mRNA level. This study shows that GHR1 and GHR2 gene expression are differentially regulated by physiological levels of GH and insulin in tilapia primary hepatocytes.

Hormone receptors in gills of smolting Atlantic salmon, Salmo salar: expression of growth hormone, prolactin, mineralocorticoid and glucocorticoid receptors and 11beta-hydroxysteroid dehydrogenase type 2

This is the first study to report concurrent dynamics in mRNA expression of growth hormone receptor (GHR), prolactin receptor (PRLR), gluco- and mineralocorticoid receptor (GR and MR) and the 11beta-hydroxysteroid dehydrogenase type-2 enzyme (11beta-HSD2) in Atlantic salmon (Salmo salar) gill during smoltification. Transcript levels were analysed by quantitative PCR in fresh water (FW) fish and after a 24-h salt water (SW) challenge. GHR transcript levels increased concurrent with gill Na(+),K(+)-ATPase activity in FW fish consistent with the SW-adaptive role of GH. SW-transfer induced an increased GHR expression levels in the early stages of smoltification but a decrease in expression at the peak of smoltification. PRLR transcript levels decreased steadily during smoltification in agreement with the recognized hyper-osmoregulatory role of PRL. Surprisingly, PRLR levels increased after SW transfer during the course of smoltification. GR mRNA levels were low early on during smoltification but increased at the peak of smoltification and remained high during de-smoltification, indicative of increased cortisol signalling at this point. Coherently, SW transfer increased GR levels to smolt levels prior to the smoltification peak. 11beta-HSD2 levels increased at the smoltification peak and MR levels increased during de-smoltification, suggesting a need for protection of MR from cortisol signalling during smoltification. This is supported by the fact that SW-transfer results in a profound up-regulation of 11beta-HSD2, whereas SW transfer down-regulates MR levels. The study concludes that GR and MR may have distinctive roles in developing hypo- and hyper-osmoregulatory mechanisms during smoltification and de-smoltification, respectively.

Comparative aspects of GH and metabolic regulation in lower vertebrates

In all vertebrates, the regulations of growth and energy balance are complex phenomena which involve elaborate interactions between the brain and peripheral signals. Most vertebrates adopt and maintain a life style after birth, but lower vertebrates may have complex life histories involving metamorphoses, migrations and long periods of fasting. In order to achieve the complex developmental programs associated with these changes, coordinated regulation of all aspects of energy metabolism is required. Somatotropic axis (somatostatin (SRIH) growth hormone (GH) and insulin-like growth factor 1 (IGF1), is known to be involved in the regulation of growth and energy balance. Interestingly, recent studies showed that additional factors such as pituitary adenylate cyclase-activated polypeptide (PACAP), corticotropin-releasing hormone (CRH), ghrelin and leptin could also have major roles in the control of growth and metabolism in lower vertebrates (fish, amphibians and reptiles). This mini-review will survey the function of GH and metabolic regulation in lower vertebrates.

Discordant regulation of hepatic IGF-I mRNA and circulating IGF-I during compensatory growth in a teleost, the hybrid striped bass (Morone chrysopsxMorone saxatilis)

Compensatory growth (CG) is a period of growth that exceeds normal rates after animals are alleviated of certain growth-stunting conditions. Little is known, however, about the endocrine control of CG in teleosts. So, our aim was to induce CG in juvenile hybrid striped bass (HSB, Morone chrysopsxMorone saxatilis) through manipulations in feeding regimen, and then determine whether changes in circulating insulin-like growth factor-I (IGF-I) and hepatic IGF-I gene expression accompany the CG response. A considerable catabolic state was induced in HSB fed a total of two times over 4 weeks (once each in the 2nd and 3rd week). Negative energy balance was evidenced through weight loss (-3.4% BW) and a significant drop in hepatosomatic index (HSI) from a value of 3.71 to 1.46. Upon realimentation, in which HSB were fed ad libitum 2x/day, a significant CG response was observed over a 4-week period. The CG response was characterized by an elevated specific growth rate, hyperphagia, restoration of the HSI and an improvement in feed conversion, all relative to controls that were fed ad libitum 2x/day throughout the experiment. Moreover, the CG response and catabolic state preceding it were marked by a discordant regulation in the expression of hepatic IGF-I mRNA and plasma IGF-I levels, the latter parameter paralleling changes in growth (r(2)=0.56, P<001). The catabolic state was accompanied by an 82% increase in hepatic IGF-I mRNA while levels of plasma IGF-I were significantly depressed relative to controls. During the subsequent CG response, however, hepatic IGF-I mRNA decreased by 61% while plasma IGF-I increased by 86%. The underlying mechanisms for this inverse regulation of hepatic IGF-I mRNA and circulating IGF-I are uncertain, but may reflect alterations in hepatic IGF-I mRNA production, stability, and translation such that hepatic IGF-I mRNA is accumulated during periods of catabolism and then rapidly translated and released into circulation when conditions improve. These results suggest that CG can be induced in HSB following a sufficient catabolic state and that systemic IGF-I may be an important mediator of the accelerated growth rate characteristic of CG.

Molecular cloning and characterization of growth hormone receptor and its homologue in the Japanese eel (Anguilla japonica)

Two cDNAs encoding growth hormone receptor (GHR)-like genes, eGHR1 and eGHR2, were isolated from Japanese eel (Anguilla japonica) liver tissue. The putative eel GHR proteins showed conserved structural features of vertebrate GHRs, including six cysteine residues and a YGEFS motif in the extracellular domain, a single transmembrane region, and proline-rich box 1 and box 2 domains. Northern blot analysis showed a single eGHR1 transcript in liver, while two sizes of eGHR2 transcripts, thought to be produced by alternative splicing, were present. RT-PCR revealed that eGHR1 and eGHR2 transcripts were widely distributed throughout the whole body of the Japanese eel. Moreover, the results of binding assays showed the specific binding of growth hormone to recombinant eGHR1. Since these putative eGHR proteins show all characteristics of the GHR family, we conclude that eGHR1 and eGHR2 cDNA encode two different GHRs in Japanese eel. We confirmed the ligand specificity of eGHR1 by binding assay, and further research is needed to allow characterization of the binding capability of eGHR2.

Time course of the GH/IGF axis response to fasting and increased ration in chinook salmon (Oncorhynchus tshawytscha)

Body growth in vertebrates is chiefly regulated by the GH/IGF axis. Pituitary growth hormone (GH) stimulates liver insulin-like growth factor-I (IGF-I) production. During fasting, plasma IGF-I levels decline due to the development of liver GH resistance, while GH levels generally increase. In mammals, decreased insulin during fasting is thought to cause liver GH resistance. However, the sequence of events in the GH/IGF axis response to fasting is not well characterized, especially in non-mammalian vertebrates. We assessed the time course of the GH/IGF axis response to fasting and increased ration in chinook salmon. Fish were placed on Fasting, Increased, or Control rations, and sampled daily for 4 days and at more widely spaced intervals through 29 days. Plasma IGF-I, GH, insulin, and 41 kDa IGF binding protein (putative salmon IGFBP-3), and liver IGF-I gene expression were measured. Control and Increased ration fish did not differ strongly. Plasma IGF-I and 41 kDa IGFBP were significantly lower in Fasted versus Control fish from day 4 onward, and liver IGF-I gene expression was significantly lower from day 6 onward. Liver IGF-I gene expression and plasma IGF-I levels were correlated. Plasma insulin was lower in Fasted fish from day 6 onward. There was a trend toward increased GH in Fasted fish on days 1-2, and GH was significantly increased Fasted fish from day 3 onward. Fasted GH first increased (days 1-3) to a plateau of 10-20 ng/ml (days 4-12) and then increased dramatically (days 15-29), suggesting that the GH response to fasting had three phases. The early increase in GH, followed by the decrease in plasma IGF-I after 4 days, suggests that GH resistance developed within 4 days.

Effects of food deprivation on expression of growth hormone receptor and proximate composition in liver of black seabream Acanthopagrus schlegeli

The effects of food deprivation on the hepatic level growth hormone receptor (GHR) were investigated in black seabream (Acanthopagrus schlegeli) both at the protein level (by radioreceptor assay) and at the mRNA level (by ribonuclease protection assay). Serum levels of growth hormone (GH) and triiodothyronine (T(3)) were also measured. Condition factor and hepatic proximate composition of the fish were also assessed. Significant decrease in hepatic GHR binding was recorded as early as on day 2 of starvation. On day 30 this decrease was even more pronounced, with the level in the starved fish reaching less than 20% the fed control level. A concomitant decrease in the hepatic GHR mRNA content was also noted during this period, with a progressive decrease from day 2 to day 30 of starvation. The extent of decrease in the mRNA content was less pronounced than the decrease in receptor binding, with the hepatic GHR mRNA content in the day 30 starved fish representing approximately 30% of the level in the fed control. In large contrast, serum GH level increased progressively during starvation. After 30 days of starvation, serum GH levels in the starved fish were more than three times the concentration found in the fed control. Serum T(3) levels, on the other hand, decreased during starvation, with the difference reaching significance on day 15 and day 30. After 30 days of starvation, serum T(3) levels in the starved fish were only approximately 40% the concentration found in the fed control. The hepatic lipid content exhibited an increasing trend during starvation. On day 30 the hepatic lipid content of the starved fish had doubled the level found in the fed control. However, the hepatic protein content did not exhibit much change during starvation. There was also a minor decrease in the moisture content of the liver during starvation, but the condition factor of the fish as a whole registered a gradual decrease during the course of food deprivation.

Environmental temperature increases plasma GH levels independently of nutritional status in rainbow trout (Oncorhynchus mykiss)

Like many poecilotherms, salmonids exhibit seasonal variations of growth rate in relation with seasonal temperatures and plasma GH level. However, temperature alters other parameters like food intake, which may directly modify the level of plasma GH. In order to determine whether temperature regulates plasma GH levels independently of nutritional status, fish were reared at 8, 12, or 16 degrees C and either fed ad libitum (fish with different food intake) to determine the global effect of temperature, or with the same ration (1.2%/body weight) to observe the temperature effect in fish with the same growth rate. Plasma insulin level was inversely proportional to the temperature (8, 12, and 16 degrees C) in fish fed ad libitum (12.1+/-0.3 ng/ml, 10.9+/-0.3 ng/ml, 9.5+/-0.4 ng/ml; P<0.001) and in restricted fish (14.0+/-0.3 ng/ml, 11.3+/-0.3 ng/ml, 10.0+/-0.2 ng/ml; P<0.0001), probably due to a prolonged nutrient absorption, and delayed recovery of basal insulin level at low temperature. Conversely, temperature did not affect plasma T3 level of fish fed ad libitum (2.5+/-0.2 ng/ml, 2.4+/-0.1 ng/ml, 2.5+/-0.1 ng/ml at 8, 12, and 16 degrees C) while fish fed with the same ration present less T3 at 16 degrees C than at 8 degrees C (1.83+/-0.1 ng/ml versus 1.2+/-0.1 ng/ml; P<0.001) throughout the experiment; these observations indicate that different plasma T3 levels reflect the different nutritional status of the fish. The levels of GH1 and GH2 mRNA, and GH1/GH2 ratio were not different for whatever the temperature or the nutritional status. Pituitary GH content, of fish fed ad libitum did not exhibit obvious differences at 8, 12, or 16 degrees C (254+/-9 ng/g bw, 237+/-18 ng/g bw, 236+/-18 ng/g bw), while fish fed with the same ration have higher pituitary GH contents at 16 degrees C than at 8 degrees C (401+/-30 ng/g bw versus 285+/-25 ng/g bw; P<0.0001). Interestingly, high temperature strongly increases plasma GH levels (2.5+/-0.3 ng/ml at 8 degrees C versus 4.8+/-0.6 ng/ml at 16 degrees C; P<0.0001) to the same extent in both experiments, since at a given temperature average plasma GH was similar between fish fed ad libitum or a restricted diet. Our results, demonstrate that temperature regulates plasma GH levels specifically but not pituitary GH content, nor the levels of GH1 and GH2 mRNA. In addition no differential regulation of both GH genes was evidenced whatever the temperature.

Characterization of growth hormone binding sites in the goldfish,Carassius auratus: effects of hypophysectomy and hormone injection

A recombinant carp growth hormone (rcGH) was used to develop for a GH radioreceptor binding assay in the goldfish (Carassius auratus). Specific binding of(125)I-rcGH to goldfish liver membranes was a pH, time, temperature, and membrane protein dependent process. Scatchard and LIGAND analysis indicated a single class of high affinity and low capacity binding site, with an association constant (Ka) of 1.9×10(10) M(-1) and a maximum binding capacity (Bmax) of 9 fmol mg(-1) protein. Liver tissue displayed the highest(125)I-rcGH binding of all the tissues examined. Displacement of(125)I-rcGH with various unlabeled teleost and mammalian GHs and prolactins revealed that the goldfish hepatic binding site was highly specific for teleost GH. Intraperitoneal administration of 0.1, 1.0, and 10 μg rcGH g(-1) body weight to hypophysectomized goldfish resulted in a 27, 52, and 68% decrease in total binding sites, respectively. Injection of a high dose of rat prolactin (rPRL) (5 μg rPRL g(-1) body weight) also resulted in a 32% decrease in total binding sites. These results suggest that endogenous GH may have a role in the regulation of its own receptors in the goldfish.

Characterization of striped bass growth hormone receptors by disulfide-bond reduction and cross-linking studies

Growth hormone (GH) receptors were analyzed in striped bass (Morone saxatilis) by addition of disulfide-bond reducing agents to radioreceptor assays and by cross-linking both striped bass and coho salmon (Oncorhynchus kisutch) crude membrane preparations to radiolabeled hormone. Dithiothreitol (DTT) caused a dose-dependent increase in specific binding of 125I-tilapia (Oreochromis mossambicus) GH to striped bass membrane preparations. Maximal enhancement of 3.4-fold was obtained with 1 mM DTT and 0.03 trypsin inhibitor units/ml of aprotinin. Addition of N-ethylmaleimide (NEM), which binds covalently to free sulfhydryl groups, decreased specific binding. Scatchard analysis of striped bass membrane preparations indicated a single class of GH receptors. Addition of DTT with aprotinin increased GH-binding site concentration from 278 to 507 fmol/mg, while the dissociation constant of 0.56 nM remained unchanged. Cross-linking 125I-tilapia GH to striped bass hepatic membrane preparations and 125I-salmon GH to coho salmon membrane preparations yielded two to three specifically labeled proteins on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Endoglycosidase H treatment was without effect on specifically labeled proteins from either species. Following digestion with N-glycosidase F, relative molecular weights of specifically labeled 125I-GH complexes were reduced, suggesting that hepatic GH-binding proteins in striped bass and salmon are N-linked glycoproteins.

Osmoregulatory actions of growth hormone and its mode of action in salmonids: A review

Osmoregulatory actions of growth hormone (GH) and its mode of action in salmonids are reviewed. We present evidence suggesting that insulin-like growth factor I (IGF-I) mediates some of the actions of GH on seawater acclimation. Plasma concentration and turnover of GH rise following exposure to seawater. Exogenous GH (in vivo) increases gill Na(+),K(+)-ATPase activity and the number of gill chloride cells, and inhibits an increase in plasma osmolarity and ions following transfer of fish to seawater. A single class of high affinity GH receptors is present in the liver, gill, intestine, and kidney. The levels of IGF-I mRNA in the liver, gill and kidney increased after GH-injection. After transfer to seawater, IGF-I mRNA increased in the gill and kidney following the rise in plasma GH, although no significant change was seen in the liver. Injection of IGF-I improved the ability of the fish to maintain plasma sodium levels after transfer to seawater. GH treatment also sensitizes the interrenal to adrenocorticotropin (ACTH), increasing cortisol secretion. Both cortisol and IGF-I may be involved in mediating the action of GH in seawater adaptation, although studies on the effect of GH on osmoregulatory physiology of non-salmonid species are limited. An integrated model of the osmoregulatory actions of GH is presented, and areas in need of research are outlined.

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

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