The GH/IGF axis in the sea lamprey during metamorphosis and seawater acclimation.

How the growth hormone (GH)/insulin-like growth factor (IGF) system affects osmoregulation in basal vertebrates remains unknown. We examined changes in the expression of components of the GH/IGF axis and gill ion transporters during metamorphosis and following seawater (SW) exposure of sea lamprey. During metamorphosis, increases in gill nka and nkcc1 and salinity tolerance were accompanied by increases in pituitary gh, liver igf1, gill ghr and igf1, but not liver ghr. SW exposure of fully metamorphosed sea lamprey resulted in slight increases in plasma chloride concentrations after SW exposure, indicating a high level of SW tolerance, but no major changes in mRNA levels of gill ion transporters or components of the GH/IGF axis. Our results indicate that metamorphosis is a critical point in the lifecycle of sea lamprey for stimulation of the GH/IGF axis and is temporally associated with and likely promotes metamorphosis and SW tolerance.

Environmental estrogens inhibit insulin-like growth factor (IGF) receptor mRNA expression, IGF binding, and IGF signaling ex vivo in rainbow trout (Oncorhynchus mykiss).

In this study, we used juvenile rainbow trout to examine the direct effects of selected environmental estrogens (EE), specifically, 17 ß-estradiol (E2), ß-sitosterol (ßS), and 4-n-nonylphenol (NP), on target tissue sensitivity to insulin-like growth factor (IGF) as assessed by expression of IGF receptor type 1 (IGFR1) mRNAs and IGF-1 binding capacity, as well as on the cell signaling pathways through which EE exert their effects. E2 and NP inhibited IGFR1A and IGFR1B mRNA expression in a time- and concentration-related manner in gill and muscle; however, ßS had no effect on expression of IGFR1 mRNAs in either tissue. NP reduced 125I-IGF binding in gill and E2 and NP reduced 125I-IGF in white muscle; ßS had no effect on 125I-IGF binding in either gill or white muscle. Treatment of gill filaments with either E2 or NP rapidly deactivated (via reduced proportion of phosphorylation) JAK2, STAT5, Akt, and ERK; ßS had no effect on the activation state of any cell signaling elements tested. The effects of EE on IGFR mRNA expression in gill were estrogen receptor (ER) dependent as the inhibitory effects were rescued by the ER antagonist, ICI 182,780. All EE tested blocked growth hormone (GH)-stimulated IGFR mRNA expression in gill filaments. GH-stimulated activation of JAK2, STAT5, Akt, and ERK were blocked by E2, ßS, and NP. Lastly, E2 and NP stimulated suppressor of cytokine signaling 2 (SOCS-2) mRNA expression, an effect that also was ER dependent. These results indicate that EE directly reduce the sensitivity of peripheral tissues to IGF by reducing mRNA and functional expression of IGFRs. Such inhibitory actions of EE are mediated, at least in part, by ER-dependent mechanisms that deactivate JAK, STAT, Akt, and ERK and enhance expression of SOCS-2. These findings together with our previous results show that EE retard growth of post-embryonic rainbow trout through widespread direct effects on the GH-IGF system, specifically, by reducing tissue sensitivity to GH, inhibiting IGF production, reducing tissue sensitivity to IGF, and by deactivating post-receptor IGF cell signaling pathways.

Discovery of prolactin-like in lamprey: Role in osmoregulation and new insight into the evolution of the growth hormone/prolactin family.

We used a representative of one of the oldest extant vertebrate lineages (jawless fish or agnathans) to investigate the early evolution and function of the growth hormone (GH)/prolactin (PRL) family. We identified a second member of the GH/PRL family in an agnathan, the sea lamprey (Petromyzon marinus). Structural, phylogenetic, and synteny analyses supported the identification of this hormone as prolactin-like (PRL-L), which has led to added insight into the evolution of the GH/PRL family. At least two ancestral genes were present in early vertebrates, which gave rise to distinct GH and PRL-L genes in lamprey. A series of gene duplications, gene losses, and chromosomal rearrangements account for the diversity of GH/PRL-family members in jawed vertebrates. Lamprey PRL-L is produced in the proximal pars distalis of the pituitary and is preferentially bound by the lamprey PRL receptor, whereas lamprey GH is preferentially bound by the lamprey GH receptor. Pituitary PRL-L messenger RNA (mRNA) levels were low in larvae, then increased significantly in mid-metamorphic transformers (stage 3); thereafter, levels subsided in final-stage transformers and metamorphosed juveniles. The abundance of PRL-L mRNA and immunoreactive protein increased in the pituitary of juveniles under hypoosmotic conditions, and treatment with PRL-L blocked seawater-associated inhibition of freshwater ion transporters. These findings clarify the origin and divergence of GH/PRL family genes in early vertebrates and reveal a function of PRL-L in osmoregulation of sea lamprey, comparable to a role of PRLs that is conserved in jawed vertebrates.

Repeated intraperitoneal injection of ovine growth hormone accelerates growth in sub-yearling Siberian sturgeon Acipenser baerii.

The role of growth hormone (GH) in chondrosteans is poorly understood, particularly with regard to its effects on growth. In this study, we examined the influence of exogenous GH on growth performance and body composition in juvenile Siberian sturgeon (Acipenser baerii). Fish with initial weight of 80.2 ± 0.1 g (mean ± S.E) were injected once every 10 days with either purified ovine GH (oGH) at 1, 2, 4, and 8 µg oGH/g body weight (BW) or with saline over a 50-day period. Treatment with the highest dose of oGH significantly enhanced growth performance (final body weight and length, body weight increase and specific growth rate, SGR). Notably, 8 µg oGH/g BW increased body weight by 33% and SGRw by 141% compared to control fish. GH-stimulated (8 µg oGH/g BW) growth was accompanied by increased crude protein content; however, oGH treatment did not affect levels of total protein, total lipid, cholesterol, triglyceride, or glucose in plasma. oGH decreased plasma levels of thyroxine (at 4 µg oGH/g BW), but had no significant effect on plasma levels of triiodothyronine or cortisol compared to controls. These findings indicate that 8 µg oGH/g BW enhances somatic growth and synthesis of body protein in juvenile Siberian sturgeon and demonstrate the feasibility of exogenous oGH treatment in conservation and aquaculture programs for this ancient species.

Roles of leptin in initiation of acquired growth hormone resistance and control of metabolism in rainbow trout.

Catabolic conditions often induce concomitant changes in plasma leptin (Lep), growth hormone (GH), and insulin growth factor I (IGF-I) levels in teleost fish, but it is unclear whether these parts of the endocrine system are responding independently or functionally linked. In this study, fasted rainbow trout was used to study the effects of Lep on the GH-IGF-I system and metabolism. Fish were implanted intraperitoneally with recombinant rainbow trout Lep pellets and remained unfed. After 4 days, plasma GH levels were elevated in the Lep-treated fish in a dose-dependent manner; the expression of hepatic igf1 and plasma IGF-I levels were suppressed accordingly. In vitro Lep treatment reversed ovine GH (oGH)-stimulated expression of igf1 and igf2 in hepatocytes isolated from fasted fish, similar to the inhibitory effects of the MEK1/2 inhibitor U0126 treatment. However, Lep treatment alone had no effect on the expression of igfs or oGH-stimulated ghr2a expression in the hepatocytes. These results demonstrate an additive effect of Lep on suppression of IGF-I under catabolic conditions, indicating that Lep is likely involved in initiation of acquired GH resistance. Although the Lep-implant treatment had no effect on standard metabolic rate, it significantly suppressed gene expression of hepatic hydroxyacyl-CoA dehydrogenase, phosphoenolpyruvate carboxykinase, and glucose 6-phosphatase, which are key enzymes in lipid utilization and gluconeogenesis, in different patterns. Overall, this study indicates that the Lep increase in fasting salmonids is an important regulatory component for physiological adaptation during periods of food deprivation, involved in suppressing growth and hepatic metabolism to spare energy expenditure.

Coordinate regulation of feeding, metabolism, and growth: Perspectives from studies in fish.

This paper develops a model for coordinate regulation of feeding, metabolism, and growth based on studies in fish. Many factors involved with the control of feeding [e.g., cholecystokinin (CCK) and ghrelin (GRLN)], energy metabolism [e.g., insulin (INS), glucagon (GLU), glucagon-like peptide (GLP), and somatostatins (SS), produced in the endocrine pancreas; and leptin (LEP) produced broadly], and growth [e.g., GRLN, growth hormone (GH), insulin-like growth factors (IGFs), GH receptors (GHR), IGF receptors (IGFR)] interact at various levels. Many such interactions serve to coordinate these systems to favor anabolic processes (i.e., lipid and protein synthesis, glycogenesis) and growth, including GH promotion of feeding and stimulation of INS production/secretion and the upregulation of GHR and IGFR by GRLN. As nutrient and stored energy status change, various feedbacks serve to curtail feeding and transition the animal from an anabolic/growth state to a catabolic state. Many factors, including LEP and IGF, promote satiety, whereas SS downregulates INS signaling as well as IGF production and GHR and IGFR abundance. As INS and IGF levels fall, GH becomes disconnected from growth as a result of altered linkage of GHR to cell signaling pathways. As a result, the catabolic actions of GH, GLU, GLP, LEP, and SS prevail, mobilizing stored energy reserves. Coordinate regulation involves relative abundances of blood-borne hormones as well as the ability to adjust responsiveness to hormones (via receptor and post-receptor events) in a cell-/tissue-specific manner that results from genetic and epigenetic programming and modulation by the local milieu of hormones, nutrients, and autocrine/paracrine interactions. The proposed model of coordinate regulation demonstrates how feeding, metabolism, and growth are integrated with each other and with other processes, such as reproduction, and how adaptive adjustments can be made to energy allocation during an animal's life history and/or in response to changes in environmental conditions.

Environmental estrogens inhibit the expression of insulin-like growth factor mRNAs in rainbow trout in vitro by altering activation of the JAK-STAT, AKT-PI3K, and ERK signaling pathways.

Environmental estrogens (EE) have been found to disrupt a host of developmental, reproductive, metabolic, and osmoregulatory process in a wide-range of animals, particularly those in aquatic ecosystems where such compounds concentrate. Previously, we showed that EE inhibited post-embryonic organismal growth of rainbow trout in vivo, but the precise mechanism(s) through which EE exert their growth inhibiting effects remain unknown. In this study, we used rainbow trout (Oncorhynchus mykiss) as a model to investigate the direct effects of 17ß-estradiol (E2), ß-sitosterol (ßS), and 4-n-nonylphenol (NP) on the synthesis of insulin-like growth factors (IGFs) and to elucidate the mechanism(s) by which EEs exert such effects. E2, ßS, and NP significantly inhibited the expression of both IGF-1 and IGF-2 mRNAs in liver and gill in a time- and concentration-related manner. Although the response evoked by each EEs on the expression of IGF mRNAs was similar, the potency and efficacy varied with EE; the rank order potency/efficacy was as follows: E2 > NP > ßS. The effects of EEs on the expression of IGF mRNAs was blocked by the estrogen receptor (ER) antagonist, ICI 182780. The mechanism(s) through which EEs inhibit IGF mRNA expression were investigated in isolated liver cells in vitro. EE treatment deactivated JAK, STAT, ERK, and AKT. Moreover, blockade of growth hormone (GH)-stimulated IGF expression by EE was accompanied by deactivation of JAK, STAT, ERK, and AKT. EEs also increased the expression of suppressor of cytokine signaling 2 (SOCS-2), a known inhibitor of JAK-2--an action that also was blocked by ICI 182780. These results indicate that EEs directly inhibit the expression of IGF mRNAs by disrupting GH post-receptor signaling pathways (e.g., JAK, STAT, ERK, and AKT) in an ER-dependent manner.

Divergent genes encoding the putative receptors for growth hormone and prolactin in sea lamprey display distinct patterns of expression.

Growth hormone receptor (GHR) and prolactin receptor (PRLR) in jawed vertebrates were thought to arise after the divergence of gnathostomes from a basal vertebrate. In this study we have identified two genes encoding putative GHR and PRLR in sea lamprey (Petromyzon marinus) and Arctic lamprey (Lethenteron camtschaticum), extant members of one of the oldest vertebrate groups, agnathans. Phylogenetic analysis revealed that lamprey GHR and PRLR cluster at the base of gnathostome GHR and PRLR clades, respectively. This indicates that distinct GHR and PRLR arose prior to the emergence of the lamprey branch of agnathans. In the sea lamprey, GHR and PRLR displayed a differential but overlapping pattern of expression; GHR had high expression in liver and heart tissues, whereas PRLR was expressed highly in the brain and moderately in osmoregulatory tissues. Branchial PRLR mRNA levels were significantly elevated by stage 5 of metamorphosis and remained elevated through stage 7, whereas levels of GHR mRNA were only elevated in the final stage (7). Branchial expression of GHR increased following seawater (SW) exposure of juveniles, but expression of PRLR was not significantly altered. The results indicate that GHR and PRLR may both participate in metamorphosis and that GHR may mediate SW acclimation.

The growth hormone signaling system: Insights into coordinating the anabolic and catabolic actions of growth hormone.

Although growth hormone (GH) is a multifunctional factor that coordinates various aspects of feeding, reproduction, osmoregulation, and immune system function, perhaps two of its most studied actions are the regulation of growth and metabolism, particularly lipid metabolism. In this review, we describe the major growth-promoting and lipid metabolic actions of GH and then discuss how the GH system regulates these actions. Numerous intrinsic and extrinsic factors provide information about the metabolic status of the organism and influence the production of release of GH. The actions of GH are mediated by GH receptors (GHR), which are widely distributed among tissues. Teleosts possess multiple forms of GHRs that arose through the evolution of this group. Modulation of tissue responsiveness to GH is regulated by molecular and functional expression of GHRs, and in teleosts GHR subtypes, by various factors that reflect the metabolic and growth status of the organism, including nutritional state. The action of GH is propagated by the linkage of GHRs to several cellular effector systems, including JAK-STAT, ERK, PI3K-Akt, and PKC. The differential activation of these pathways, which is governed by nutrient status, underlies GH stimulation of growth or GH stimulation of lipolysis. Taken together, the multi-functional actions of GH are determined by the distribution and abundance of GHRs (and GHR subtypes in teleosts) as well as by the GHR-effector system linkages.

Insulin and insulin-like growth factor-1 modulate the lipolytic action of growth hormone by altering signal pathway linkages.

Growth hormone (GH) has many actions in vertebrates, including the regulation of two disparate metabolic processes: growth promotion (anabolic) and the mobilization of stored lipids (catabolic). Our previous studies showed that GH stimulated IGF-1 production in hepatocytes from fed rainbow trout, but in cells from fasted fish GH stimulated lipolysis. In this study, we used rainbow trout (Oncorhynchus mykiss) to elucidate regulation of the mechanisms that enable cells to alter their lipolytic responsiveness to GH. In the first experiment, cells were removed from either fed or fasted fish, conditioned in medium containing serum (10%) from either fed or fasted fish, then challenged with GH. GH stimulated the expression of hormone sensitive lipase (HSL), the primary lipolytic enzyme, in cells from fasted fish conditioned with "fasted serum" but not in cells from fasted fish conditioned in "fed serum." Pretreatment of cells from fed fish with "fasted serum" resulted in GH-stimulated HSL expression, whereas GH-stimulated HSL expression in cells from fasted fish was blocked by conditioning in "fed serum." The nature of the conditioning serum governed the signaling pathways activated by GH irrespective of the nutritional state of the animals from which the cells were removed. When hepatocytes were pretreated with "fed serum," GH activated JAK2, STAT5, Akt, and ERK pathways; when cells were pretreated with "fasted serum," GH activated PKC and ERK. In the second study, we examined the direct effects of insulin (INS) and insulin-like growth factor (IGF-1), two nutritionally-regulated hormones, on GH-stimulated lipolysis and signal transduction in isolated hepatocytes. GH only stimulated HSL mRNA expression in cells from fasted fish. Pretreatment with INS and/or IGF-1 abolished this lipolytic response to GH. INS and/or IGF-1 augmented GH activation of JAK2 and STAT5 in cells from fed and fasted fish. However, INS and/or IGF-1 eliminated the ability of GH to activate PKC and ERK from fasted cells. These results indicate that INS and IGF-1 determine the signaling pathways activated by GH and whether or not a lipolytic response ensues. Such hormone-receptor-signal pathway linkages provide insight into the molecular basis of GH multifunctionality and into how cellular responses to GH can be adjusted to meet physiological (e.g., nutritional), developmental, or other conditions.

Environmental estrogens inhibit mRNA and functional expression of growth hormone receptors as well as growth hormone signaling pathways in vitro in rainbow trout (Oncorhynchus mykiss).

Fish in aquatic habitats are exposed to increasing concentrations and types of environmental contaminants, including environmental estrogens (EE). While there is growing evidence to support the observation that endocrine-disrupting compounds (EDCs) possess growth-inhibiting effects, the mechanisms by which these physiological effects occur are poorly understood. In this study, we examined the direct effects of EE, specifically 17ß-estradiol (E2), ß-sitosterol (ßS), and 4-n-nonylphenol (NP), on GH sensitivity as assessed by mRNA expression and functional expression of growth hormone receptor in hepatocytes, gill filaments, and muscle in rainbow trout (Oncorhynchus mykiss). Additionally, we examined the effects of EE on signaling cascades related to growth hormone signal transduction (i.e., JAK-STAT, MAPK, and PI3K-Akt). Environmental estrogens directly suppressed the expression of GHRs in a tissue- and compound-related manner. The potency and efficacy varied with EE; effects were most pronounced with E2 in liver. EE treatment deactivated the JAK-STAT, MAPK, and PI3K-Akt pathways in liver a time-, EE- and concentration-dependent manner. Generally, E2 and NP were most effective in deactivating pathway elements; maximum suppression for each pathway was rapid, typically occurring at 10-30min. The observed effects occurred via an estrogen-dependent pathway, as indicated by treatment with an ER antagonist, ICI 182,780. These findings suggest that EEs suppress growth by reducing GH sensitivity in terms of reduced GHR synthesis and reduced surface GHR expression and by repressing GH signaling pathways.

Nutritional state modulates growth hormone-stimulated lipolysis.

Growth hormone (GH) regulates several processes in vertebrates, including two metabolically disparate processes: promotion of growth, an anabolic action, and mobilization of stored lipid, a catabolic action. In this study, we used hepatocytes isolated from continuously fed and long-term (4weeks) fasted rainbow trout (Oncorhynchus mykiss) as a model to investigate the mechanistic basis of the anabolic and catabolic actions of GH. Our hypothesis was that nutritional state modulates the lipolytic responsiveness of cells by adjusting the signal transduction pathways to which GH links. GH stimulated lipolysis as measured by increased glycerol release in both a time- and concentration-related manner from cells of fasted fish but not from cells of fed fish. Expression of mRNAs that encode the lipolytic enzyme hormone-sensitive lipase (HSL), HSL1 and HSL2, also was stimulated by GH in cells from fasted fish and not in cells from fed fish. Activation of the signaling pathways that mediate GH action also was studied. In cells from fed fish, GH activated the JAK-STAT, PI3K-Akt, and ERK pathways, whereas in cells from fasted fish, GH activated the PLC/PKC and ERK pathways. In hepatocytes from fasted fish, blockade of PLC/PKC and of the ERK pathway inhibited GH-stimulated lipolysis and GH-stimulated HSL mRNA expression, whereas blockade of JAK-STAT or of the PI3K-Akt pathway had no effect on lipolysis or HSL expression stimulated by GH. These results indicate that during fasting GH activates the PLC/PKC and ERK pathways resulting in lipolysis but during periods of feeding GH activates a different complement of signal elements that do not promote lipolysis. These findings suggest that the responsiveness of cells to GH depends on the signal pathways to which GH links and helps resolve the growth-promoting and lipid catabolic actions of GH.

Differential regulation of the multiple insulin and insulin receptor mRNAs by somatostatin.

We used rainbow trout as a model to study the regulation of the multiple and distinct insulin (INS) and insulin receptor (IR) mRNAs by somatostatin (SS). Implantation of SS reduced growth of animals without affecting food intake. SS decreased INS1 and INS2 expression in Brockmann bodies, but increased INS1 and INS2 expression in adipose and INS1 expression in brain. SS reduced mRNA levels of IR 2 and IR 3 in adipose tissue; of IR1 and IR 4 in Brockmann bodies; of IR1, IR2, IR3, and IR4 in cardiac muscle; of IR2 and IR4 in liver; of IR 3 and IR 4 in gill; and of IR4 in skeletal muscle. The direct effects of SS were examined in Brockmann bodies and liver in vitro. SS decreased INS and IR mRNAs in both tissues in a concentration-, time-, and isoform/subtype-dependent manner. These results indicate that SS regulates the expression of INS- and IR-encoding mRNAs and that independent mechanisms may serve to regulate the various INS isoforms and IR subtypes.

Environmental estrogens inhibit growth of rainbow trout (Oncorhynchus mykiss) by modulating the growth hormone-insulin-like growth factor system.

Although environmental estrogens (EE) have been found to disrupt a wide variety of developmental and reproductive processes in vertebrates, there is a paucity of information concerning their effects on organismal growth, particularly postembryonic growth. In this study, we exposed juvenile rainbow trout (Oncorhynchus mykiss) to 17ß-estradiol (E2) ß-sitosterol (ßS), or 4-n-nonylphenol (NP) to assess the effects of EE on overall organismal growth and on the growth hormone-insulin-like-growth factor (GH-IGF) system. EE treatment significantly reduced food conversion, body condition, and body growth. EE-inhibited growth resulted from alterations in peripheral elements of the GH-IGF system, which includes multiple GH receptors (GHRs), IGFs, and IGF receptors (IGFRs). In general, E2, ßS, and NP reduced the expression of GHRs, IGFs, and IGFRs; however, the effects varied in an EE-, tissue-, element type-specific manner. For example, in liver, E2 was more efficacious than either ßS, and NP in reducing GHR expression, and the effect of E2 was greater on GHR 1 than GHR2 mRNA. By contrast, in gill, all EEs affected GHR expression in a similar manner and there was no difference in the effect on GHR1 and GHR 2 mRNA. With regard to IGF expression, all EEs reduced hepatic IGF1 and IGF2 mRNA levels, whereas as in gill, only E2 and NP significantly reduced IGF1 and IGF2 expression. Lastly, E2 and NP reduced the expression of IGFR1A and IGFR1B mRNA expression similarly in gill and red and white muscle, whereas ßS had no effect on expression of IGFR mRNAs. These findings indicate that EEs disrupt post-embryonic growth by reducing GH sensitivity, IGF production, and IGF sensitivity.

Characterization of a novel growth hormone receptor-encoding cDNA in rainbow trout and regulation of its expression by nutritional state.

To clarify the divergence of the growth hormone receptor (GHR) family, we characterized a novel GHR from a teleost fish (rainbow trout). A 2357-nt cDNA was isolated and found to contain a single initiation site 71 nt from the most 5' end, an open reading frame of 1971 nt encoding a 657-amino acid protein, and a single polyadenylation site 229 nt from the poly-A tail. Based on structural analysis, the protein was identified as a type 1 GHR (GHR1). The new GHR1 shares 42% and 43% amino acid identity, respectively, with GHR2a and GHR2b, the two type 2 GHRs isolated from trout previously. GHR1 mRNA was found in a wide array of tissues with the highest expression in the liver, red muscle, and white muscle. Fasting animals for 4 weeks reduced steady state levels of GHR1 in the liver, adipose, and red muscle. These findings help clarify the divergence and nomenclature of GHRs and provide insight into the function of duplicated GHR types.

PKC and ERK mediate GH-stimulated lipolysis.

GH regulates several physiological processes in vertebrates, including the promotion of growth, an anabolic process, and the mobilization of stored lipids, a catabolic process. In this study, we used hepatocytes isolated from rainbow trout (Oncorhynchus mykiss) as a model to examine the mechanism of GH action on lipolysis. GH stimulated lipolysis as measured by increased glycerol release in both a time- and a concentration-related manner. The promotion of lipolysis was accompanied by GH-stimulated phosphorylation of the lipolytic enzyme hormone-sensitive lipase (HSL). GH-stimulated lipolysis was also manifested by an increased expression of the two HSL-encoding mRNAs, HSL1 and HSL2. The signaling pathways that underlie GH-stimulated lipolysis were also studied. GH resulted in the activation of phospholipase C (PLC)/protein kinase C (PKC) and the MEK/ERK pathway, whereas JAK-STAT and the PI3K-Akt pathway were deactivated. The blockade of PLC/PKC and the MEK/ERK pathway inhibited GH-stimulated lipolysis and GH-stimulated phosphorylation of HSL as well as GH-stimulated HSL mRNA expression, whereas the blockade of JAK-STAT or the PI3K-Akt pathway had no effect on the activation of lipolysis or the expression of HSL stimulated by GH. These results indicate that GH promotes lipolysis by activating HSL and by enhancing the de novo expression of HSL mRNAs via the activation of PKC and ERK. These findings also suggest molecular mechanisms for activating the lipid catabolic actions of GH while simultaneously deactivating anabolic processes such as antilipolysis and the growth-promoting actions of GH.

Evolutionary origin and divergence of the growth hormone receptor family: insight from studies on sea lamprey.

Sea lamprey, one of the oldest extant lineages of vertebrates, Agnatha, was used to clarify the evolutionary origin and divergence of the growth hormone receptor (GHR) family. A single full-length cDNA encoding a protein that shares amino acid identity with GHRs and prolactin receptors (PRLRs) previously characterized from teleost fish was identified. Expression of the GHR/PRLR-like transcript was widespread among tissues, including brain, pituitary, heart, liver, and skeletal muscle, which is consistent with the broad physiological roles of GH-family peptides. Phylogenetic analysis suggests that the lamprey possess an ancestral gene encoding a common GHR/PRLR that diverged to give rise to distinct GHRs and PRLRs later in the course of vertebrate evolution. After the divergence of the Actinopterygian and Sarcopterygian lineages, the GHR gene was duplicated in the Actinopterygian lineage during the fish-specific genome duplication event giving rise to two GHRs in teleosts, type 1 GHR and type 2 GHR. A single GHR gene orthologous to the teleost type 1 GHR persisted in the Sarcopterygian lineage, including the common ancestor of tetrapods. Within the teleosts, several subsequent independent duplication events occurred that led to several GHR subtypes. A revised nomenclature for vertebrate GHRs is proposed that represents the evolutionary history of the receptor family. Structural features of the receptor influence ligand binding, receptor dimerization, linkage to signal effector pathways, and, ultimately, hormone function.

Peripheral regulation of the growth hormone-insulin-like growth factor system in fish and other vertebrates.

The growth hormone (GH)-insulin-like growth factor (IGF) system plays a major role in coordinating the growth of vertebrates including fish. Considerable research on the regulation of growth has focused on the production and secretion of GH from the pituitary. This review will synthesize recent work on regulating extrapituitary aspects of the GH-IGF system, which includes GH binding proteins (GHBP), GH receptors (GHR), IGF binding protein (IGFBP), and IGF receptors (IGFR). These components are widely distributed and they interact to coordinate growth as well as a host of other biological processes such as metabolism, osmoregulation, reproduction, behavior, and immunity. The GH-IGF system of fish is particularly interesting and complex because it consists of multiple subtypes of GHRs, IGFRs, and IGFBPs that arose through gene duplication events associated with the evolution of the teleost lineage. Peripheral regulation of the GH-IGF system results from adjusting peripheral sensitivity to GH and IGFs as well as from modulating the bioavailability and actions of GH and IGFs in target cells. Numerous chemicals, including hormones such as growth hormone, insulin, somatostatin, and sex steroids as well as a variety of transcription factors, proteases, and phosphatases, regulate the synthesis and activity of GHRs, GHBPs, IGFRs, and IGFBPs as well as the synthesis, secretion, and bioavailability of IGFs. In addition, numerous environmental factors such as nutritional state, photoperiod, stress, and temperature have dramatic effects on the expression and activity of peripheral components of the GH/IGF system. The complex regulation of these system components appears to be both organism- and tissue-specific.

Estrogenic compounds decrease growth hormone receptor abundance and alter osmoregulation in Atlantic salmon.

Exposure of Atlantic salmon smolts to estrogenic compounds is shown to compromise several aspects of smolt development. We sought to determine the underlying endocrine mechanisms of estrogen impacts on the growth hormone (GH)/insulin-like growth factor I (IGF-I) axis. Smolts in freshwater (FW) were either injected 3 times over 10 days with 2 µgg(-1) 17ß-estradiol (E2) or 150µgg(-1) 4-nonylphenol (NP). Seawater (SW)-acclimated fish received intraperitoneal implants of 30 µgg(-1) E2 over two weeks. Treatment with these estrogenic compounds increased hepatosomatic index and total plasma calcium. E2 and NP reduced maximum growth hormone binding by 30-60% in hepatic and branchial membranes in FW and SW, but did not alter the dissociation constant. E2 and NP treatment decreased plasma levels of IGF-I levels in both FW and SW. In FW E2 and NP decreased plasma GH whereas in SW plasma GH increased after E2 treatment. Compared to controls, plasma chloride concentrations of E2-treated fish were decreased 5.5mM in FW and increased 10.5mM in SW. There was no effect of NP or E2 on gill sodium-potassium adenosine triphosphatase (Na(+)/K(+)-ATPase) activity in FW smolts, whereas E2 treatment in SW reduced gill Na(+)/K(+)-ATPase activity and altered the number and size of ionocytes. Our data indicate that E2 downregulates the GH/IGF-I-axis and SW tolerance which may be part of its normal function for reproduction and movement into FW. We conclude that the mechanism of endocrine disruption of smolt development by NP is in part through alteration of the GH/IGF-I axis via reduced GH receptor abundance.