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The Journal of Nutrition Vol. 128 No. 2 February 1998,
pp. 306S-314S
Department of Biology, University of Michigan, Ann Arbor, MI 48109-1048
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ABSTRACT |
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Abstract
Introduction
References
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The insulin-like growth factors (IGF) are evolutionarily ancient growth factors present in all vertebrates. The central importance of IGF for normal development and growth has been illustrated by the severe growth-retarded phenotype exhibited by IGF-I, IGF-II or IGF-I receptor "knockout" mice. Although we know much about the gross effects of IGF on the overall size of the fetus and the clinical manifestations that result from fetal and neonatal deficiency of IGF (i.e., severe growth retardation leads to dwarfism), very little is known about the in vivo actions of IGF during embryogenesis at the cellular and molecular levels. Most research on the developmental role of IGF has relied on rodent models, and attempts to elucidate the molecular and cellular basis of IGF actions have been hampered by the inaccessibility of the mammalian fetus enclosed in the uterus. During the past decade, there has been growing support for the concept that the IGF have been highly conserved in all vertebrates. Both IGF-I and IGF-II are present in fish, and their structures are highly conserved. Human and fish IGF-I are equally potent in mammalian and fish bioassay systems. Insulin-like growth factor mRNA is found in all life stages of fish, ranging from unfertilized egg to adult. The temporal and spatial expression patterns of fish IGF-I seem to be similar to those in mammals. Nutritional status and growth hormone both have a profound effect on IGF-I expression in fish, as they do in mammals. These features suggest that the IGF system is highly conserved between teleost fish and mammals. Because fish embryos develop externally, they provide excellent animal models for understanding the regulatory roles of IGF, IGF receptor and IGF-binding proteins in vertebrate embryonic development. Current research on the developmental and nutritional roles of IGF in fish will undoubtedly contribute to knowledge of the basic physiology of vertebrates in general.
KEY WORDS: insulin-like growth factors · embryogenesis · nutrition · gene expression · fish
Animal growth, determined by the rate and duration of the growth process, is largely genetically controlled in vertebrates. Animal growth is also strongly influenced by environmental and nutritional factors, and this is especially true for ectotherms such as teleost fish, which rely on temperature, photoperiods and food availability to trigger developmental processes such as hatching, metamorphosis (flatfishes, eels) or smoltification (salmonids), sexual maturation and spawning. Information from both external stimuli and internal state is processed, integrated and responded to by the brain for appropriate modification of growth through hormonally mediated pathways. A central step in this endocrine pathway is the growth hormone (GH) and insulin-like growth factor-I (IGF-I) axis. Growth hormone is synthesized in the pituitary gland and secreted into the bloodstream under the regulation of neuronal, hormonal and nutritional factors. The critical importance of GH for postnatal growth has been illustrated by the gigantic animals resulting from transgenic animals overexpressing GH and the clinical manifestations that result from either deficiency or overproduction of GH (i.e., dwarfism and acromegaly, respectively). In most vertebrates, however, nutritional deprivation, which causes growth arrest in juvenile animals, leads to an increase rather than a decrease in circulating GH levels. This phenomenon has been documented in many vertebrate species including humans, sheep, dogs, chicken and fish. These observations indicated that the primary cause of the growth arrest is resistance to GH action at the tissue level and that this mechanism is common in most vertebrates. It is now fully appreciated that this growth arrest is primarily due to a decline in the production of IGF-I at the tissue level. During the past decade, there has been growing support for the concept that the basic endocrine mechanism underlying growth regulation has been highly conserved during evolution. The primary goal of this article is to review the research on the nutritional and developmental regulation of IGF in fishes. This article will deal with bony fish, because little is known about the IGF system in cartilaginous fishes. Although no sophisticated evolutionary picture can be expected to emerge from simple comparison of teleost fishes with mammals, recognition of components of the mammalian IGF system in teleost fish should provide a picture of possible conservation of fundamental mechanisms for growth regulation in the vertebrate lineage.
The IGF, including IGF-I and IGF-II, are single-chain polypeptides with structural homology to proinsulin. Both IGF-I and IGF-II are essential for both fetal and postnatal growth in mammals (Jones and Clemmons 1995 Earlier reports on IGF-like activity in fish presented conflicting results. The conflicting results were due largely to the use of human IGF immunoassays and receptor binding assays. These heterologous assays resulted in decreased sensitivity for fish IGF and therefore hampered detection of them in these earlier works. In addition, it was shown later that IGFBP present in the blood interfere with the measurement of IGF, and IGFBP are abundant in fish serum (see Bern et al. 1991
Genomic analysis indicates that salmonid fish have two IGF-I genes (Wallis and Devlin 1993
Many studies on the tissue distribution and hormonal regulation of IGF-I gene expression have been reported in fish. As is the case with mammals, IGF-I mRNA transcripts are found in many fish tissues. In juvenile and adult coho salmon, detectable amounts of IGF-I mRNA were found in virtually all tissues examined, but liver had the highest levels, suggesting that it is the major site of IGF-I mRNA production in salmon (Duan et al. 1993 Nutritional status has a profound effect on the GH-IGF-I axis in fish (Table 1). In salmonids, prolonged starvation causes cessation of growth but significant elevation of plasma GH concentrations (Duan and Plisetskaya 1993
In mammals, the expression of IGF-I and IGF-II genes is regulated in a temporal- and spatial-restricted manner. Insulin-like growth factor-II is expressed predominantly during the fetal stage in multiple tissues, and IGF-I is also expressed in a wide variety of tissues during fetal and postnatal stages. Shortly after birth, however, the liver becomes the predominant site for endocrine IGF-I production under the regulation of GH. The expression patterns of IGF in teleost fish seem to be similar to those in mammals. In the gilthead seabream, both IGF-I and IGF-II mRNA were detected in larva 1 d after hatching (Duguay et al. 1996 The biological actions of IGF in fish are diverse. Insulin-like growth factor-I has also been shown to stimulate DNA synthesis, cartilage sulfation and protein synthesis, enhance seawater adaptability, stimulate spermatogenesis and induce final oocyte maturation in fish. Both mammalian IGF-I and IGF-II stimulate proteoglycan synthesis by cultured fish cartilage (Cheng and Chen 1995 The functional importance of IGF during mammalian fetal growth has been conclusively demonstrated in a series of experiments using IGF-I, IGF-II and the IGF-I receptor "knock-out" mice (Baker et al. 1993 I wish to thank Erika M. Plisetskaya at the University of Washington for her encouragement and discussion.
INTRODUCTION
Abstract
Introduction
References
THE MAMMALIAN INSULIN-LIKE GROWTH FACTOR SYSTEM
), and IGF-I mediates many of the growth-promoting effects of GH during postnatal life. Originally, IGF-I was believed to be exclusively synthesized in the liver under the regulation of GH, to circulate in blood, and to act on distant target tissues in an endocrine fashion. It is now fully appreciated that IGF-I is also produced in a wide variety of cell types and acts locally in paracrine and autocrine manners. Both IGF-I and -II are important during prenatal growth; however, the role of IGF in fetal growth is GH independent. The central importance of IGF for normal growth has been conclusively illustrated by the severe growth-retarded phenotype exhibited by mice in which the IGF-I, IGF-II or IGF-I receptor genes have been inactivated by homologous gene targeting (Baker et al. 1993, DeChiara et al. 1990, Liu et al. 1993). As with other peptide growth factors, the biological actions of IGF are diverse and not limited to cell proliferation at the cellular level. Depending on the biological context, IGF may stimulate cell growth, promote cell differentiation (stimulate the expression of differentiated functions) and inhibit apoptosis (Jones and Clemmons 1995). Most, if not all, of the biological actions of IGF are mediated by the IGF-I receptors, which are widely expressed in a broad spectrum of cell types. Like the insulin receptor, the IGF-I receptor has a heterotetrameric structure with a tyrosine kinase domain in the cytoplasmic portion of the beta-subunit (Czech 1989). Both IGF can bind to the insulin receptor with low affinity. Hybrid insulin/IGF-I receptors have also been identified. It is not known at present if the hybrid receptor mediates any of the physiological actions of the IGF or insulin (Moxham et al. 1989). In mammals, a second transmembrane IGF receptor, the IGF-II/mannose 6-phosphate receptor, also exists and preferentially binds to IGF-II over IGF-I. Although the binding of IGF-II to the IGF-II/mannose 6-phosphate receptor has been shown to cause internalization and degradation of IGF-II (Oka et al. 1985), the role of this receptor in transducting the IGF signals remains elusive. Comparative studies indicate that chicken and amphibian mannose 6-phosphate receptors do not possess the IGF-binding capacity (Clairmont and Czech 1985, Yang et al. 1991). Therefore, the IGF-II binding property of this receptor as well as any of its physiological function in regard to IGF would have been a later acquisition during evolution.
, Oh et al. 1996). These proteins share relatively high amino acid sequence similarity, but each has distinct structural and biochemical properties. They can act as carrier proteins in the blood stream and control the efflux of IGF from the vascular space. The IGF/IGFBP complexes prolong the half-lives of IGF and buffer the acute hypoglycemic effects of IGF. More importantly, because IGF bind to IGFBP with higher affinities than to the IGF receptors, IGFBP may provide a means of localizing IGF on target cells and can alter their biological activity by modulating their interaction with IGF receptors (Jones and Clemmons 1995).
THE INSULIN-LIKE GROWTH FACTOR SYSTEM IN FISH
, Chan and Steiner 1994, Chen et al. 1994, Duan et al. 1994, Siharath and Bern 1993, for reviews). Strong evidence for the existence of IGF in fish, however, was obtained through classic physiological studies. Low levels of IGF activity were detected in trout and dogfish serums by a mammalian cartilage sulfation bioassay (Shapiro and Pimstone 1977). This result was confirmed by Komourdjian and Idler (1978), who found that sulfate uptake into trout gill arch cartilage was stimulated by coincubation with liver slices from normal fish but not with liver slices from hypophysectomized fish. Duan and co-authors identified a GH-dependent sulfation factor in the serum of eels and demonstrated that human IGF-I stimulates cartilage sulfation in fish (Duan and Hirano 1990, Duan and Inui 1990). These studies clearly indicate the existence of a functional IGF-I-like molecule in teleost fish.
). The deduced amino acid sequence of coho salmon preproIGF-I contains 176 amino acids (aa) and consists of a 44 aa signal peptide, 70-aa mature IGF-I, and a 62-aa E peptide. Since then, the nucleotide sequences of IGF-I cDNAs have been determined in coho salmon, Atlantic salmon, Chinook salmon, rainbow trout, carp, catfish, seabream and shark (Cao et al. 1989, Duguay et al. 1992, 1995 and 1996, Liang et al. 1996, McRory and Sherwood 1994, Shamblott and Chen 1992, Wallis and Devlin 1993). Amino acid sequence identities of the coding regions are well conserved among these fishes. Comparison of the sequence of mature fish IGF-I with those of frog, chicken, rat, and human indicates that IGF-I has been highly conserved throughout vertebrate evolution (Fig. 1). For instance, the predicted amino acid sequence of salmon IGF-I is 80% identical to that of human IGF-I. In addition to structural conservation, functional studies indicate that the biological potency of IGF-I is also remarkably conserved throughout several hundred million years of vertebrate evolution. Using a biological assay based on the incorporation of 35S-sulfate into fish branchial cartilage, Duan and Hirano (1990) showed that human IGF-I is biologically active in teleost fish. Similar results were also reported in other teleost species (Cheng and Chen 1995, Gray and Kelly 1991, Kelley et al. 1993, Marchant and Moroz 1993, McCormick et al. 1992b, Takagi and Bjonsson 1996, Tsai et al. 1994). The functional conservation was further demonstrated by our observation that human and salmon IGF-I are equally potent in stimulating cartilage sulfation in salmon (Moriyama et al. 1993). Similarly, Upton et al. (1996) reported that human and salmon IGF-I are equally potent in binding to the human IGF-I receptors and in stimulating protein synthesis in mammalian cells.
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Fig 1.
Amino acid sequence of vertebrate IGF-I. The amino acid sequences of 70-residue IGF-I are aligned. Dash lines represent amino acids that are identical to those found in human IGF-I.
and 1996, Shamblott and Chen 1992). Fish IGF-II is very similar to its mammalian homologues. The deduced amino acid sequence of mature trout IGF-II is ~80% identical to human IGF-II (Fig. 2). Additional sequence homology was also found when the trout E peptide was compared with human IGF-II E peptide. There is no report at present on the biological potency of fish IGF-II, and the structure of the IGF-II gene has yet to be characterized in any fish. Nevertheless, the high degree of structural homology predicts that fish IGF-II may have similar biological activity. With the availability of recombinant fish IGF-II (rainbow trout, Gentil et al. 1996), information on the functional aspects of fish IGF-II can be expected in the near future. The IGF-I receptor is present in fish and seems to be conserved both structurally and functionally. Insulin-like growth factor-I receptors binding to 125I-labeled human IGF-I probe and having tyrosine kinase activity have been found in brain, ovary, heart and skeletal muscles of teleost species (Drakenberg et al. 1993, Gutierrez et al. 1995, Leibush et al. 1996, Moon et al. 1996, Parrizas et al. 1995a and 1995b). Recently, cDNAs encoding the IGF-I receptor have been cloned from the gilhead seabream, Sparus aurata (Chan, S. J., personal communication). The structure of seabream IGF-I receptor is very similar to those of mammals, and the amino acid sequence identity between seabream and human IGF-I receptors is greater than 70%.
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Fig 2.
Amino acid sequence of vertebrate IGF-II. The amino acid sequences of 67-residue IGF-II are aligned. Dash lines represent amino acids that are identical to those found in human IGF-I.
, Fukuzawa et al. 1995, Kelley et al. 1992, Niu and LeBail 1993, Siharath et al. 1995 and 1996). Using labeled human IGF-I as the ligand, Kelley et al. (1992) identified three forms of IGFBP in the plasma of four teleost fish. The three forms of fish IGFBP were found to have estimated molecular sizes of 40-45, 31 and 29 kDa, respectively. The 40-45-kDa predominant form was also identified in rainbow trout and was shown to be up-regulated by GH and decreased by food deprivation (Kelley et al. 1992, Niu and LeBail 1993, Siharath et al. 1995). Because its molecular size is close to that of mammalian IGFBP-3 and because it is regulated by GH and food deprivation, this protein is likely to be a fish version of IGFBP-3. The structure of this GH-dependent IGFBP, however, has yet to be determined, and its role in coordinating IGF-I actions in fish is not clear. The 31-kDa and 29-kDa IGFBP are similar to mammalian IGFBP-2 and -1 in size, but their identities and structure remain undetermined.
DUPLICATED IGF-I GENES AND MULTIPLE
IGF-I mRNAs IN SALMONID FISH
). Indeed, two nonallelic salmon IGF-I genes have been cloned from chum salmon (Kavsan et al. 1993 and 1994). This is consistent with the tetraploid nature of the salmon genome. Each of the two IGF-I genes spans approximately 20 kb and is organized into four exons (Fig. 3). Mature salmon IGF-I is encoded in exon 2 and 3; the E peptide is encoded by exon 3 and exon 4. This structure is much more compact and simpler than those of the mammalian and avian IGF-I genes. The human IGF-I gene consists of six exons distributed over 100 kb of chromosomal DNA, and the chicken IGF-I gene spans over 50 kb. Therefore, it seems that the IGF-I gene arrangement is simpler in lower vertebrates and that additional leader exons have been acquired during evolution. A further comparison of the salmon, chicken and mammalian IGF-I genes suggests that the 5
flanking region shares considerable identity among these three classes of vertebrates (Kavsan et al. 1993). This is unusual and intriguing because the 5 and 3 noncoding regions are often the most divergent. This high degree of conservation of the 5 flanking sequence suggests that this region may have a conserved regulatory function.
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Fig 3.
Structure and expression of a salmon IGF-I gene. Exons are represented by boxes. Open boxes indicate untranslated regions, and shaded boxes contain coding sequences. Four alternatively spliced mRNAs identified in salmonid fishes are shown, and the proposed pattern of mRNA processing is indicated by the thin lines. Four IGF-I precursors as a result of RNA alternative splicing are shown in the bottom panel. See text for additional details.
). Alternative splicing of IGF mRNA also occurs in fish, at least in salmonid fish. Although Northern blot analysis of RNA from fish liver reveals one major band at approximately 4 kb, polymerase chain reaction (PCR) analysis and RNase protection assays suggested the existence of multiple IGF-I mRNA transcripts coding for different IGF-I prohormones in salmonids, and these mRNAs have been designated as salmon Ea-1, Ea-2, Ea-3 and Ea-4 (Duan et al. 1994, and see Fig. 3). Duguay and co-workers (1992) identified in coho salmon and Atlantic salmon three IGF-I mRNA transcripts, Ea-1, Ea-3 and Ea-4. Salmon Ea-1 mRNA codes for proIGF-IA-1 with a 35-amino acid E peptide, which shares 79% amino acid sequence identity with human proIGF-IA. Salmon Ea-3 and Ea-4 mRNA are homologous to Ea-1 but contain 81-bp and 117-bp inserts in the E-domains. These transcripts code for proIGF-IA-3, with a 62 aa E-peptide, and proIGF-IA-4, with a 74 aa E peptide. These findings were confirmed and extended by Wallis and Devlin (1993) and Shamblott and Chen (1993). By analysis of IGF-I transcripts using RT-PCR in another salmonid species, the chinook salmon, Wallis and Devlin identified another IGF-I transcript Ea-2, which contained a 36-bp insert. This transcript encodes for proIGF-1A-2 with a 47 aa E peptide. These authors further showed that the 81-bp insertion is contiguous with the exon coding for the first portion E-domain and is included by use of an alternative donor splicing sequence that is located 81 bp downstream from the splice site used by the mammalian gene. The 36-bp insertion was encoded by a distinct exon in the salmon IGF-I gene. Working independently with rainbow trout, Shamblott and Chen (1993) demonstrated the presence of all four IGF-I mRNAs in a salmonid species and therefore confirmed the existence of four IGF-I mRNA transcripts coding for four proIGF-I in salmonids.
TISSUE DISTRIBUTION AND HORMONAL REGULATION OF IGF mRNAs IN FISH
, Duguay et al. 1992, Shamblott and Chen 1993). Consistent with the mRNA expression pattern, immunoreactive IGF-I has been detected in multiple fish tissues (Berwert et al. 1995, Kagawa et al. 1995, Mack et al. 1995, Reinecke et al. 1992).
, Duan and Inui 1990, Gray and Kelley 1991, Marchant and Moroz 1993, Moriyama 1995). Injection of GH significantly increased the IGF-I mRNA levels in the liver of coho salmon, eel, rainbow trout and seabream (Cao et al. 1989, Duan and Hirano 1992, Duan et al. 1993, Duguay et al. 1994 and 1996, Shamblott et al. 1995). Two other pituitary hormones, prolactin and somatolactin, had no effect on the hepatic IGF-I mRNA level (Duan et al. 1993, Duguay et al. 1994). The GH-induced increase in IGF-I mRNA expression is accompanied by an increase in the circulating IGF-I peptide levels (Niu et al. 1993, Moriyama et al. 1994). Concentration-dependent stimulation of IGF-I mRNA expression by GH was also obtained in primary cultures of salmon hepatocytes (Duan et al. 1993, Shamblott et al. 1995). In coho salmon hepatocyte cultures, the minimum effective concentration for GH is approximately 10 ng/mL (Duan et al. 1993). This is well within the physiological range of circulating GH in salmonid fish, suggesting that the fish IGF-I gene is capable of responding to physiological concentrations of GH.
). Studies in intact animals suggest, however, that not all nonhepatic tissues are as sensitive as the liver to GH treatment in terms of increasing IGF-I mRNA. Likewise, the relative abundance of IGF-I mRNA in non-hepatic tissues in fish seems not to be regulated by GH. Although injection of GH increased hepatic IGF-I mRNA abundance in fish, it had no effect in other tissues, including heart, fat, brain, ovary and spleen (Duan et al. 1993, Duguay et al. 1994 and 1996). Insulin-like growth factor-I expression in the gill was reported to be increased by GH treatment in rainbow trout (Sakamoto and Hirano 1993). This liver-specific regulation by GH can be explained by the differential tissue distribution and regulation of different preproIGF-I mRNAs in salmonid fish. In coho salmon, the expression of salmon Ea-1 and Ea-3 transcripts is restricted to the liver, whereas the expression of the salmon Ea-4 transcript is universal. RNase protection assay analysis showed that GH treatment dramatically increased levels of salmon Ea-1 and Ea-3 transcripts but did not alter the abundance of the Ea-4 transcript (Duguay et al. 1994). The preferential increase in the levels of Ea-1 and Ea-3 transcripts over Ea-4 transcript in response to GH treatment has also been manifested in cultured hepatocytes in vitro (Duan et al. 1994). Thus GH-induced increase in IGF-I mRNA levels observed in the liver can be partially explained by the liver specific expression of GH-responsive salmon Ea-1 and Ea-3 transcripts. A possible reason that most nonhepatic tissues do not show a change in IGF-I mRNA levels after GH treatment is because they express only the unresponsive salmon Ea-4 transcript.
). Experiments using primary cultures of salmon hepatocytes indicated that insulin alone had no effect on steady-state IGF-I mRNA levels, but co-incubation of insulin with GH significantly enhanced the stimulatory effect of GH (Duan et al. 1994). Therefore insulin may act synergistically with GH to stimulate hepatic IGF-I expression in salmon. The effect of insulin on IGF-I expression in salmon is restricted to the liver. The IGF-I mRNA in nonhepatic tissues of streptozotocin-induced diabetic salmon was not significantly different from normal, despite a significant reduction in the hepatic IGF-I mRNA levels. Analysis of the alternatively spliced IGF-I mRNA transcripts indicated that the decrease in hepatic IGF-I mRNA levels in the diabetic fish was primarily due to the dramatic reduction of salmon Ea-1 and Ea-3 transcripts (Duan et al. 1994). The salmon Ea-4 mRNA transcript, which is expressed in both hepatic and nonhepatic tissues, showed little change.
, Shamblott and Chen 1993). There is conflicting information in regard to the role of GH in IGF-II expression in fish. Shamblott et al. (1995) showed that GH treatment significantly increased the IGF-II mRNA in the liver and in pyloric ceca in rainbow trout. These authors also demonstrated concentration-dependent stimulation of IGF-II mRNA expression by GH in primary culture hepatocytes. However, a study in seabream showed that GH treatment did not increase the tissue IGF-II mRNA levels (Duguay et al. 1996). This difference may reflect differences in species or experimental procedures.
NUTRITIONAL REGULATION OF INSULIN-LIKE GROWTH FACTORS IN FISH
, Sumpter et al. 1991). This phenomenon has been documented in many vertebrate species, including humans, sheep, dogs and chicken (Thissen et al. 1994). The only exception is rodents, in which starvation does not increase plasma GH concentrations. This starvation-induced rise in plasma GH concentrations is associated with a significant decrease in the hepatic binding sites for GH (Gray et al. 1992), suggesting a possible resistance to GH at tissue levels. As in mammals, food deprivation causes reduction of circulating levels of immunoreactive IGF-I in coho salmon (Moriyama et al. 1994). In a closely related species, rainbow trout (Oncorhynchus mykiss), 4 wk of starvation caused a significant decrease in the circulating levels of IGF-like peptide(s) (Niu et al. 1993). A 25-29-kDa IGFBP (IGFBP-I ?) has been shown to be increased by a 60-d food-deprivation treatment whereas a 35-42-kDa IGFBP does not change (Siharath et al. 1996). Food restriction also affects circulating GH and IGF-I concentrations in fish, as it does in mammals. Perez-Sanchez et al. (1995) showed a positive correlation between dietary protein content and plasma IGF-I concentrations in the gilthead seabream. These authors also showed that increasing feeding ration size resulted in an increase in plasma IGF-I concentrations. It seems, therefore, that both protein and energy intakes are critical in the regulation of circulating IGF-I concentrations in fish.
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Table 1.
Nutritional regulation of the growth hormone-insulin-like growth factor-I axis in fish
showed that hepatic IGF-I mRNA and IGF-I bioactivity are significantly lower in food-deprived eel compared with fed controls. Starvation decreased hepatic IGF-I mRNA levels, whereas refeeding of the starved fish led to a rise in hepatic IGF-I mRNA in coho salmon (Duan and Plisetskaya 1993). These findings suggest that nutritional status regulates IGF-I production at the mRNA level. Whether the decrease in steady-state mRNA levels is regulated at the transcriptional level or the posttranscriptional level or both in fish is presently unknown. Compared with rats, a much longer period of starvation is required to observe significant changes in IGF-I levels in fish. In rats, 3 d of starvation caused a significant decrease, and 2 d of refeeding partially restored it (Thissen et al. 1994). In salmon, however, 3 to 4 wk was required to obtain a significant change in hepatic IGF-I mRNA or circulating IGF-I peptide levels, and 2 wk of refeeding was necessary to restore it (Duan and Plisetskaya 1993). The slower responses in salmon may be a reflection of the generally slower metabolism of these ectothermal animals. In addition, many fish, including salmonids, live in cold water and are metabolically adapted to long periods of food deprivation during their life cycle.
). This pattern of salmon Ea-1 and Ea-3 response to nutritional status is similar to the patterns seen with GH treatment or streptozotocin injections. Elucidation of the underlying mechanism(s) controlling the tissue-specific expression of these alternatively spliced IGF-I mRNAs in salmon remains an area of future investigation.
DEVELOPMENTAL REGULATION OF INSULIN-LIKE GROWTH FACTORS IN FISH
). The IGF-II mRNA level was highest in larva 1 d after hatching and decreased thereafter. In contrast, IGF-I mRNA expression increased in 12- and 16-d-old larva. Insulin-like growth factor-I mRNAs were also detected by RT-PCR in larva and juvenile salmon (Duguay et al. 1992), but there are presently no quantitative data on the expression of IGF during early development in salmon. The ontogeny of IGF-I and insulin in the endocrine pancreas of the turbot fish was studied by Reinecke and colleagues (Berwert et al. 1995). They reported that IGF-I immunoreactivity appeared at d 11 after hatching in the pancreas.
). A moderate but significant increase in the IGF-I mRNA levels of the gills was reported in rainbow trout and coho salmon during smoltification or transfer to seawater (Duguay et al. 1994, Sakamoto and Hirano 1993). Circulating IGF-I concentrations were also elevated during May and June, during which time most fish went through the parr-smolt transformation. This increase occured following the spring-associated elevation in GH. After the spring peak, the levels declined rapidly and reached minimum levels in late October. Subsequently, IGF-I levels increased to intermediate levels in winter and maintained those levels until March. The changes in GH and IGF-I levels are closely correlated with the growth rate of the fish and reflect the coordination by the endocrine system of physiological responses with changes of environmental cues.
showed that IGF-I mRNA is present in mouse unfertilized eggs, in oocytes and in embryos during first cleavages. Avian and Xenopus oocytes and embryos have also been shown to contain IGF-I mRNAs (Scavo et al. 1991). It is not clear whether and when these mRNAs are translated into IGF-I peptides and begin to be functional in the embryonic stage.
BIOLOGICAL ACTIONS OF INSULIN-LIKE GROWTH FACTORS IN FISH
, Duan and Hirano 1990 and 1992, Gary and Kelley 1991, Kelley et al. 1993, Marchant and Moroz 1993, McCormick et al. 1992b, Takagi and Bjornsson 1996). Mack and co-authors (1995) showed that both IGF-I and IGF-II are present in the cichilid fish retina (Haplochromis burtoni). Administration of exogenous IGF-I and insulin stimulates proliferation of rod precursor cells in this system (Mack and Fernald 1993). These data suggest that IGF may play a role in regulating new neuron production in the fish retina. Negatu and Meier (1995) reported that mammalian IGF-I stimulates protein synthesis in muscles of the gulf killifish. Injection of bovine IGF-I into eels caused a decrease in the plasma amino acid nitrogen level (Duan, C., Inui, Y. and Hirano, T., unpublished data), indicating a role of IGF-I in protein metabolism in fish. In salmon, GH has been shown to increase seawater adaptation independent of its growth-promoting effects (Bolton et al. 1987). McCormick and others (1991) showed that injection of IGF-I improved the ability of rainbow trout to maintain plasma osmolarity and sodium levels. Further, IGF-I has been shown to act directly in stimulating gill Na+,K+-ATPase activity in salmon in vitro (Madsen and Bern 1993, McCormick 1996). Sakamoto and co-authors reported that IGF-I is required to mediate the osmoregulatory action of GH in salmonid fish (Sakamoto and Hirano 1993, Sakamoto et al. 1995). A role of IGF-I in regulating spermatogenesis and inducing final oocyte maturation has also been demonstrated in fish (Kagawa et al. 1994, Le Gac and Loir 1993, Loir and Le Gac 1994).
), this result was probably due to the hypoglycemic effect of exogenous IGF-I. High dosages of human IGF-I were given by direct intraperitoneal injection in that study. By infusing IGF-I through implanted mini-osmotic pumps, McCormick and co-authors (1992a) showed that IGF-I stimulates salmon somatic growth in vivo. Along the same line, we and others have demonstrated that IGF-I is a potent stimulator of DNA synthesis and proteoglycan synthesis of skeletal tissues in fish (Duan and Hirano 1992, Tsai et al. 1994). In addition, the hepatic IGF-I mRNA levels and plasma IGF-I concentration are significantly correlated with growth rate in juvenile coho salmon (Duan et al. 1995). The critical role of IGF-I in normal growth and development of fish was further illustrated by the recent study on stunted salmon (Duan et al. 1995). As mentioned above, smoltification is a critical period in development of juvenile salmon. Transfer of yearling coho salmon from fresh water to seawater before the completion of smoltification results in abnormal development ("stunting") in a fraction of the population. Stunted fish grow poorly and eventually die in seawater. Despite their retarded growth and abnormal development, these fish show dramatic increases in secretory activity of GH cells and elevated plasma GH concentrations. Measurement of IGF-I levels has shown that the hepatic IGF-I mRNA expression and circulating IGF-I concentrations in stunted fish were significantly reduced. The hepatic GH binding sites in stunted fish are also reduced (Gray et al. 1992). The reduced GH receptor and IGF-I expression in the liver and elevated plasma GH concentrations suggest that stunted salmon are GH-resistant. These changes in the GH-IGF-I axis in stunted salmon are similar to those in starved fish. Although stunted fish do eat when kept together with their normally growing counterparts, studies have shown that their intestinal nutrient uptake is significantly reduced (Collie 1985). Therefore, stunted fish may suffer from protein deficiency or energy deficiency or both. The malnutrition induces GH-resistance in tissues, reduces IGF-I production and causes retarded response to GH treatment, which leads to growth retardation in stunted salmon. This salmon stunt model is a demonstration of the interface between nutrients and hormones acting in concert to control animal growth and development.
CONCLUSIONS AND PERSPECTIVES
, DeChiara et al. 1990, Liu et al. 1993). During the past decade, there has been growing evidence that the IGF have been highly conserved during evolution. The structure and biological potency of fish IGF-I are very similar to those of mammalian homologues. Insulin-like growth factor-I is expressed in a wide variety of tissues with the highest level found in the liver in fish. Hepatic IGF-I expression is under the regulation of GH. Nutrition is another major regulator of IGF-I in fish. This nutritional regulation of the GH-IGF-I axis, which is an interface between nutrients and hormones acting in concert to control animal growth and development, seems to be important and well conserved throughout vertebrate evolution. These findings have extended our understanding of IGF biology and revealed an essential role of IGF in all vertebrates.
). Significant levels of IGF-I mRNA are detected in a variety of fish tissues other than the liver. Unlike in mammals, however, the IGF-I mRNA expression in these nonhepatic tissues is not regulated by GH or nutritional status, and the biological significance of the local IGF-I in fish is not clear. The structure, function and expression of IGFBP are almost entirely undefined in fish. Thus research activity is lacking in this area even though fish models can contribute immensely to our understanding of the role of the IGF system in early development. Unlike mammalian embryos, which live within the uterus and are supported by maternal contributions through the placenta, fish embryos and larva grow freely in water. The free-living, accessible embryos of teleost fish, in particular zebrafish, make them well suited for dissecting the mechanisms by which IGF act to regulate cell proliferation, differentiation and apoptosis in early development. The transparency of zebrafish embryos provides perhaps the greatest advantage for using them in these studies. One can literally observe every cell in every organ of the living embryos and analyze them with single-cell resolution over long periods of time. Sophisticated experimental techniques have been developed for fate mapping, time-lapse lineage tracing, and transplantation in zebrafish embryos. Furthermore, hundreds of zebrafish mutants affecting various aspects of embryonic and early larval development are available as a result of two large-scale chemical mutagenesis screens completed recently. A combination of these advantages makes the zebrafish a particularly attractive animal model for studying the role and mechanisms of IGF in controlling growth and development.
ACKNOWLEDGMENT
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FOOTNOTES |
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LITERATURE CITED |
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Abstract
Introduction
References
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