Duplication of the IGFBP-2 gene in teleost fish: protein structure and functionality conservation and gene expression divergence

New insights into the mechanism of lens development using zebra fish

On the basis of recent advances in molecular biology, genetics, and live-embryo imaging, direct comparisons between zebra fish and human lens development are being made. The zebra fish has numerous experimental advantages for investigation of fundamental biomedical problems that are often best studied in the lens. The physical characteristics of visible light can account for the highly coordinated cell differentiation during formation of a beautifully transparent, refractile, symmetric optical element, the biological lens. The accessibility of the zebra fish lens for direct investigation during rapid development will result in new knowledge about basic functional mechanisms of epithelia-mesenchymal transitions, cell fate, cell-matrix interactions, cytoskeletal interactions, cytoplasmic crowding, membrane transport, cell adhesion, cell signaling, and metabolic specialization. The lens is well known as a model for characterization of cell and molecular aging. We review the recent advances in understanding vertebrate lens development conducted with zebra fish.

Expression profiling and gene ontology analysis in fathead minnow (Pimephales promelas) liver following exposure to pulp and paper mill effluents

Many studies link pulp and paper mill effluent (PPME) exposure to adverse effects in fish populations present in the mill receiving environments. These impacts are often characteristic of endocrine disruption and may include impaired reproduction, development and survival. While these physiological endpoints are well-characterized, the molecular mechanisms causing them are not yet understood. To investigate changes in gene transcription induced by exposure to a PPME at several stages of treatment, male and female fathead minnows (FHMs) were exposed for 6 days to 25% (v/v) secondary (biologically) treated kraft effluent (TK) or 100% (v/v) combined mill outfall (CMO) from a mill producing both kraft pulp and newsprint. The gene expression changes in the livers of these fish were analyzed using a 22K oligonucleotide microarray. Exposure to TK or CMO resulted in significant changes in the expression levels of 105 and 238 targets in male FHMs and 296 and 133 targets in females, respectively. Targets were then functionally analyzed using gene ontology tools to identify the biological processes in fish hepatocytes that were affected by exposure to PPME after its secondary treatment. Proteolysis was affected in female FHMs exposed to both TK and CMO. In male FHMs, no processes were affected by TK exposure, while sterol, isoprenoid, steroid and cholesterol biosynthesis and electron transport were up-regulated by CMO exposure. The results presented in this study indicate that short-term exposure to PPMEs affects the expression of reproduction-related genes in the livers of both male and female FHMs, and that secondary treatment of PPMEs may not neutralize all of their metabolic effects in fish. Gene ontology analysis of microarray data may enable identification of biological processes altered by toxicant exposure and thus provide an additional tool for monitoring the impact of PPMEs on fish populations.

Expression profile analysis of mycotoxin-related genes in cartilage with endemic osteochondropathy Kashin-Beck Disease

BACKGROUND

Kashin-Beck Disease (KBD) is an endemic osteochondropathy. Mycotoxins are believed to play an important role in the pathogenesis of KBD. Because the molecular mechanism of mycotoxin-induced cartilage lesions remains unclear, there is not effective treatment for KBD now. To identify key genes involved in the mycotoxin-induced cartilage lesions, we compared the expression profiles of mycotoxin-related genes (MRG) between KBD cartilage and healthy cartilage.

METHODS

Total RNA was isolated from cartilage samples, following by being amplified, labeled and hybridized to Agilent human whole genome microarray chip. qRT-PCR was conducted to validate the microarray data. 1,167 MRG were derived from the environmentally related genomic database Toxicogenomics. The microarray data of MRG was subjected to single gene and gene ontology (GO) expression analysis for identifying differently expressed genes and GO.

RESULTS

We identified 7 up-regulated MRG and 2 down-regulated MRG in KBD cartilage, involved in collagen, apoptosis, metabolism and growth & development. GO expression analysis found that 4 apoptosis-related GO and 5 growth & development-related GO were significantly up-regulated in KBD cartilage.

CONCLUSIONS

Based on the results of previous and our studies, we suggest that mycotoxins might contribute to the development of KBD through dysfunction of MRG involved in collagen, apoptosis and growth & development in cartilage.

The paracrine effect of exogenous growth hormone alleviates dysmorphogenesis caused by tbx5 deficiency in zebrafish (Danio rerio) embryos

BACKGROUND

Dysmorphogenesis and multiple organ defects are well known in zebrafish (Danio rerio) embryos with T-box transcription factor 5 (tbx5) deficiencies, mimicking human Holt-Oram syndrome.

METHODS

Using an oligonucleotide-based microarray analysis to study the expression of special genes in tbx5 morphants, we demonstrated that GH and some GH-related genes were markedly downregulated. Zebrafish embryos microinjected with tbx5-morpholino (MO) antisense RNA and mismatched antisense RNA in the 1-cell stage served as controls, while zebrafish embryos co-injected with exogenous growth hormone (GH) concomitant with tbx5-MO comprised the treatment group.

RESULTS

The attenuating effects of GH in tbx5-MO knockdown embryos were quantified and observed at 24, 30, 48, 72, and 96 h post-fertilization. Though the understanding of mechanisms involving GH in the tbx5 functioning complex is limited, exogenous GH supplied to tbx5 knockdown zebrafish embryos is able to enhance the expression of downstream mediators in the GH and insulin-like growth factor (IGF)-1 pathway, including igf1, ghra, and ghrb, and signal transductors (erk1, akt2), and eventually to correct dysmorphogenesis in various organs including the heart and pectoral fins. Supplementary GH also reduced apoptosis as determined by a TUNEL assay and decreased the expression of apoptosis-related genes and proteins (bcl2 and bad) according to semiquantitative reverse-transcription polymerase chain reaction and immunohistochemical analysis, respectively, as well as improving cell cycle-related genes (p27 and cdk2) and cardiomyogenetic genes (amhc, vmhc, and cmlc2).

CONCLUSIONS

Based on our results, tbx5 knockdown causes a pseudo GH deficiency in zebrafish during early embryonic stages, and supplementation of exogenous GH can partially restore dysmorphogenesis, apoptosis, cell growth inhibition, and abnormal cardiomyogenesis in tbx5 knockdown zebrafish in a paracrine manner.

Zebrafish p53 protein enhances the translation of its own mRNA in response to UV irradiation and CPT treatment

p53 protein is an important regulatory factor involved in cell growth and development. In our previous study, we demonstrated that recombined zebrafish p53 protein could specifically bind to its own mRNA in vitro. To determine if a similar interaction exists in zebrafish and if this interaction affects zebrafish development, in the present study, we investigated the interaction of p53 protein and its mRNA in zebrafish embryos. Our results revealed that expressed zebrafish p53 protein could bind with its own mRNA in zebrafish embryos. Furthermore, the endogenous activated or ectopically expressed p53 protein could enhance the relative activity of Renilla luciferase fused with p53 3'UTR in response to UV irradiation and CPT treatment, and retarded development of zebrafish embryos was observed.

The IGF/IGFBP system in rainbow trout (Oncorhynchus mykiss) adipose tissue: expression related to regional localization and cell type

While in mammals, the IGF/IGFBP system is known to be involved in adipose tissue growth, the presence of such a system in fish is as yet undetermined. The present work aimed at investigating the influence of regional localization of adipose tissues and cell types on the expression of this system. Using quantitative real-time PCR (qPCR), the presence of IGFs, IGFBPs, IGFBP-rPs, and IGFR I (insulin-like growth factors, IGF-binding proteins, IGFBP-related proteins, type I IGF receptor) was studied in crude subcutaneous and visceral fat depots as well as in isolated stromal vascular (SV) cells and mature adipocytes from the latter depot in the prepubescent female rainbow trout. In adipose tissues, no differences were observed in the transcript expressions of IGFs and IGFBPs (1-6) and rP1 relative to their regional localization. On the other hand, the two paralogues of IGFR I were more strongly expressed in visceral than in subcutaneous depots which could be related to a differential receptivity to IGF between fat depots. The amount of IGF-I and IGFR Ia transcripts is larger in mature adipocytes than in SV cells, while a similar level of expression was observed for IGF-II and IGFR Ib. IGFBP-1, -2a, -4, -5, -6, and -rP1 were more strongly expressed in the SV cells than in adipocytes, while IGFBP-2b displayed comparable expression. These results indicate that the IGF/IGFBP system is expressed in rainbow trout fat depots, whatever their regional origin, and that cell types, i.e., SV cells or mature adipocytes, influence its expression.

Circulating salmon 28- and 22-kDa insulin-like growth factor binding proteins (IGFBPs) are co-orthologs of IGFBP-1

Circulating insulin-like growth factor binding proteins (IGFBPs) play pivotal roles in stabilizing IGFs and regulating their availability to target tissues. In the teleost circulation, three major IGFBPs are typically detected by ligand blotting with molecular masses around 20-25, 28-32 and 40-45kDa. However, their identity is poorly established and often confused. We previously identified salmon 22- and 41-kDa forms as IGFBP-1 and -2b, respectively. In the present study, we cloned the cDNA of 28-kDa IGFBP from Chinook salmon (Oncorhynchus tshawytscha) as well as rainbow trout (Oncorhynchus mykiss) based on the partial N-terminal amino acid sequence of purified protein and identified it as an ortholog of IGFBP-1. Structural and phylogenetic analyses revealed that the 28-kDa IGFBP is more closely related to human IGFBP-1 and zebrafish IGFBP-1a than the previously identified salmon IGFBP-1 (i.e. 22-kDa IGFBP). We thus named salmon 28- and 22-kDa forms as IGFBP-1a and -1b, respectively. Salmon IGFBP-1a contains a potential PEST region involved in rapid protein turnover and phosphorylation sites typically found in mammalian IGFBP-1, although the PEST and phosphorylation scores are not as high as those of human IGFBP-1. There was a striking difference in tissue distribution patterns between subtypes; Salmon igfbp-1a was expressed in a variety of tissues while igfbp-1b was almost exclusively expressed in the liver, suggesting that IGFBP-1a has more local actions. Direct seawater exposure (osmotic stress) of Chinook salmon parr caused increases in both IGFBP-1s in plasma, while IGFBP-1b appeared to be more sensitive. The presence of two co-orthologs of IGFBP-1 in the circulation in salmon, and most likely in other teleosts, provides a good opportunity to investigate subfunction partitioning of duplicated IGFBP-1 during postnatal growth.

Insulin-like growth factor (IGF) signalling and genome-wide transcriptional regulation in fast muscle of zebrafish following a single-satiating meal

Male zebrafish (Danio rerio) were fasted for 7 days and fed to satiation over 3 h to investigate the transcriptional responses to a single meal. The intestinal content at satiety (6.3% body mass) decreased by 50% at 3 h and 95% at 9 h following food withdrawal. Phosphorylation of the insulin-like growth factor (IGF) signalling protein Akt peaked within 3 h of feeding and was highly correlated with gut fullness. Retained paralogues of IGF hormones genes were regulated with feeding, with igf1a showing a pronounced peak in expression after 3 h and igf2b after 6 h. Igf-I receptor transcripts were markedly elevated with fasting, and decreased to their lowest levels 45 min after feeding. igf1rb transcripts increased more quickly than igf1ra transcripts as the gut emptied. Paralogues of the insulin-like growth factor binding proteins (IGFBPs) were constitutively expressed, except for igfbp1a and igfbp1b transcripts, which were significantly elevated with fasting. Genome-wide transcriptional responses were analysed using the Agilent 44K oligonucleotide microarray and selected genes validated by qPCR. Fasting was associated with the upregulation of genes for the ubiquitin-proteasome degradation pathway, anti-proliferative and pro-apoptotic genes. Protein chaperones (unc45b, hspd1, hspa5, hsp90a.1, hsp90a.2) and chaperone interacting proteins (ahsa1 and stip1) were upregulated 3 h after feeding along with genes for the initiation of protein synthesis and mRNA processing. Transcripts for the enzyme ornithine decarboxylase 1 showed the largest increase with feeding (11.5-fold) and were positively correlated with gut fullness. This study demonstrates the fast nature of the transcriptional responses to a meal and provides evidence for differential regulation of retained paralogues of IGF signalling pathway genes.

Characterization of three IGFBP mRNAs in Atlantic croaker and their regulation during hypoxic stress: potential mechanisms of their upregulation by hypoxia

Insulin-like growth factor-binding proteins (IGFBPs) play important roles in downregulating IGF activity and growth and development in vertebrates under hypoxic stress. However, the mechanisms of hypoxia regulation of IGFBPs in teleost fishes are unknown. The involvement of reactive oxygen species (ROS) and hypoxia-inducible factors (HIFs) in hypoxia upregulation of IGFBPs in Atlantic croaker were investigated. Three croaker IGFBPs, IGFBP-1, IGFBP-2, and IGFBP-5, were cloned and characterized. Chronic hypoxia exposure [dissolved oxygen (DO): 1.7 mg/l for 2-4 wk] caused significant increases in hepatic and neural IGFBP-1 mRNA expression compared with tissue mRNA levels in fish held under normoxic conditions (6.5 mg DO/l). Moreover, longer-term chronic hypoxia exposure (2-2.7 mg DO/l for 15-20 wk) caused significant increases in mRNA levels of all three IGFBPs in both liver and brain tissues. Hypoxia exposure also markedly increased superoxide radical (O(2)(·-), an index of ROS) production and HIF-1α mRNA and HIF-2α protein expression in croaker livers. Pharmacological treatment with an antioxidant attenuated the hypoxia-induced increases in O(2)(·-) production and HIFα mRNA and protein expression as well as the elevation of IGFBP-1 mRNA levels. These results suggest that the upregulation of IGFBP expression under hypoxia stress is due, in part, to alterations in the antioxidant status, which may involve ROS and HIFs.

Growth and the regulation of myotomal muscle mass in teleost fish

Teleost muscle first arises in early embryonic life and its development is driven by molecules present in the egg yolk and modulated by environmental stimuli including temperature and oxygen. Several populations of myogenic precursor cells reside in the embryonic somite and external cell layer and contribute to muscle fibres in embryo, larval, juvenile and adult stages. Many signalling proteins and transcription factors essential for these events are known. In all cases, myogenesis involves myoblast proliferation, migration, fusion and terminal differentiation. Maturation of the embryonic muscle is associated with motor innervation and the development of a scaffold of connective tissue and complex myotomal architecture needed to generate swimming behaviour. Adult muscle is a heterogeneous tissue composed of several cell types that interact to affect growth patterns. The development of capillary and lymphatic circulations and extramuscular organs--notably the gastrointestinal, endocrine, neuroendocrine and immune systems--serves to increase information exchange between tissues and with the external environment, adding to the complexity of growth regulation. Teleosts often exhibit an indeterminate growth pattern, with body size and muscle mass increasing until mortality or senescence occurs. The dramatic increase in myotomal muscle mass between embryo and adult requires the continuous production of muscle fibres until 40-50% of the maximum body length is reached. Sarcomeric proteins can be mobilised as a source of amino acids for energy metabolism by other tissues and for gonad generation, requiring the dynamic regulation of muscle mass throughout the life cycle. The metabolic and contractile phenotypes of muscle fibres also show significant plasticity with respect to environmental conditions, migration and spawning. Many genes regulating muscle growth are found as multiple copies as a result of paralogue retention following whole-genome duplication events in teleost lineages. The extent to which indeterminate growth, ectothermy and paralogue preservation have resulted in modifications of the genetic pathways regulating muscle growth in teleosts compared to mammals largely remains unknown. This review describes the use of compensatory growth models, transgenesis and tissue culture to explore the mechanisms of muscle growth in teleosts and provides some perspectives on future research directions.

Evolution of the insulin-like growth factor binding protein (IGFBP) family

The evolution of the IGF binding protein (IGFBP) gene family has been difficult to resolve. Both chromosomal and serial duplications have been suggested as mechanisms for the expansion of this gene family. We have identified and annotated IGFBP sequences from a wide selection of vertebrate species as well as Branchiostoma floridae and Ciona intestinalis. By combining detailed sequence analysis with sequence-based phylogenies and chromosome information, we arrive at the following scenario: the ancestral chordate IGFBP gene underwent a local gene duplication, resulting in a gene pair adjacent to a HOX cluster. Subsequently, the gene family expanded in the two basal vertebrate tetraploidization (2R) resulting in the six IGFBP types that are presently found in placental mammals. The teleost fish ancestor underwent a third tetraploidization (3R) that further expanded the IGFBP repertoire. The five sequenced teleost fish genomes retain 9-11 of IGFBP genes. This scenario is supported by the phylogenies of three adjacent gene families in the HOX gene regions, namely the epidermal growth factor receptors (EGFR) and the Ikaros and distal-less (DLX) transcription factors. Our sequence comparisons show that several important structural components in the IGFBPs are ancestral vertebrate features that have been maintained in all orthologs, for instance the integrin interaction motif Arg-Gly-Asp in IGFBP-2. In contrast, the Arg-Gly-Asp motif in IGFBP-1 has arisen independently in mammals. The large degree of retention of IGFBP genes after the ancient expansion of the gene family strongly suggests that each gene evolved distinct and important functions early in vertebrate evolution.

Circulating salmon 41-kDa insulin-like growth factor binding protein (IGFBP) is not IGFBP-3 but an IGFBP-2 subtype

In vertebrates, most circulating insulin-like growth factor (IGF) is bound to multiple forms of IGF-binding proteins (IGFBPs) that differ both structurally and functionally. In mammals, the largest reservoir of IGF in the circulation comes from a large (150kDa) ternary complex comprised of IGF bound to IGFBP-3, which is bound to an acid label subunit (ALS), and this variant of IGFBP is regulated by growth hormone (GH) and feed intake. Although multiple variants of IGFBPs ranging from 20 to 50kDa have been found in fishes, no ternary complex is present and it has been assumed that the majority of circulating IGF is bound to fish IGFBP-3. Consistent with this assumption is previous work in salmon showing the presence of a 41-kDa IGFBP that is stimulated by GH, decreases with fasting and increases with feeding. However, the hypothesis that the salmon 41-kDa IGFBP is structurally homologous to mammalian IGFBP-3 has not been directly tested. To address this issue, we cloned cDNAs for several Chinook salmon IGFBPs, and found that the cDNA sequence of the 41-kDa IGFBP is most similar to that of mammalian IGFBP-2 and dissimilar to IGFBP-3. We found an additional IGFBP (termed IGFBP-2a) with high homology to mammalian IGFBP-2. These results demonstrate that salmon 41-kDa IGFBP is not IGFBP-3, but a paralog of IGFBP-2 (termed IGFBP-2b). Salmon IGFBP-2s are also unique in terms of having potential N-glycosylation sites and splice variants. Additional research on non-mammalian IGFBPs is needed to fully understand the molecular/functional evolution of the IGFBP family and the significance of the ternary complex in vertebrates.

Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins: roles in skeletal muscle growth and differentiation

The insulin-like growth factor (IGF) signaling pathway consists of multiple IGF ligands, IGF receptors, and IGF-binding proteins (IGFBPs). Studies in a variety of animal and cellular systems suggest that the IGF signaling pathway plays a key role in regulating skeletal muscle growth, differentiation, and in maintaining homeostasis of the adult muscle tissues. Intriguingly, IGFs stimulate both myoblast proliferation and differentiation, which are two mutually exclusive biological events during myogenesis. Both of these actions are mediated through the same IGF-1 receptor. Recent studies have shed new insights into the molecular mechanisms underlying these paradoxical actions of IGFs in muscle cells. In this article, we provide a brief review of our current understanding of the IGF signaling system and discuss recent findings on how local oxygen availability and IGFBPs act to specify IGF actions in muscle cells.

Transcriptional regulation of the IGF signaling pathway by amino acids and insulin-like growth factors during myogenesis in Atlantic salmon

The insulin-like growth factor signalling pathway is an important regulator of skeletal muscle growth. We examined the mRNA expression of components of the insulin-like growth factor (IGF) signalling pathway as well as Fibroblast Growth Factor 2 (FGF2) during maturation of myotubes in primary cell cultures isolated from fast myotomal muscle of Atlantic salmon (Salmo salar). The transcriptional regulation of IGFs and IGFBP expression by amino acids and insulin-like growth factors was also investigated. Proliferation of cells was 15% d⁻¹ at days 2 and 3 of the culture, increasing to 66% d⁻¹ at day 6. Three clusters of elevated gene expression were observed during the maturation of the culture associated with mono-nucleic cells (IGFBP5.1 and 5.2, IGFBP-6, IGFBP-rP1, IGFBP-2.2 and IGF-II), the initial proliferation phase (IGF-I, IGFBP-4, FGF2 and IGF-IRb) and terminal differentiation and myotube production (IGF2R, IGF-IRa). In cells starved of amino acids and serum for 72 h, IGF-I mRNA decreased 10-fold which was reversed by amino acid replacement. Addition of IGF-I and amino acids to starved cells resulted in an 18-fold increase in IGF-I mRNA indicating synergistic effects and the activation of additional pathway(s) leading to IGF-I production via a positive feedback mechanism. IGF-II, IGFBP-5.1 and IGFBP-5.2 expression was unchanged in starved cells, but increased with amino acid replacement. Synergistic increases in expression of IGFBP5.2 and IGFBP-4, but not IGFBP5.1 were observed with addition of IGF-I, IGF-II or insulin and amino acids to the medium. IGF-I and IGF-II directly stimulated IGFBP-6 expression, but not when amino acids were present. These findings indicate that amino acids alone are sufficient to stimulate myogenesis in myoblasts and that IGF-I production is controlled by both endocrine and paracrine pathways. A model depicting the transcriptional regulation of the IGF pathway in Atlantic salmon muscle following feeding is proposed.

Duplicated zebrafish insulin-like growth factor binding protein-5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity

Insulin-like growth factor binding protein (IGFBP)-5 is a secreted protein that binds to IGF and modulates IGF actions. IGFBP-5 is also found in the nucleus of mammalian cells and has transactivation activity. The structural basis of this transactivation activity and its role in mediating IGF-independent actions are not clear. Here we report that there are 2 igfbp-5 genes in zebrafish and other teleost fish. In zebrafish, igfbp-5a and -5b are expressed in spatially restricted, mostly nonoverlapping domains during early development. The IGF binding site is conserved in both zebrafish IGFBP-5s, and they are both secreted and capable of IGF binding. Both proteins contain a consensus bipartite nuclear localization signal and were found in the nucleus when introduced into cultured cells. Although zebrafish IGFBP-5b possesses transactivation activity, zebrafish IGFBP-5a lacks this activity. Mutational analysis demonstrated that 2 unique amino acids in positions 22 and 56 of IGFBP-5a are responsible for its lack of transactivation activity. These findings suggest that the duplicated zebrafish IGFBP-5s have evolved divergent regulatory mechanisms and distinct biological properties by partitioning of ancestral structural domains and provide new evidence for a conserved role of the IGF binding, nuclear localization, and transactivation domain of this multifunctional IGFBP.

Zebrafish IGF genes: gene duplication, conservation and divergence, and novel roles in midline and notochord development

Insulin-like growth factors (IGFs) are key regulators of development, growth, and longevity. In most vertebrate species including humans, there is one IGF-1 gene and one IGF-2 gene. Here we report the identification and functional characterization of 4 distinct IGF genes (termed as igf-1a, -1b, -2a, and -2b) in zebrafish. These genes encode 4 structurally distinct and functional IGF peptides. IGF-1a and IGF-2a mRNAs were detected in multiple tissues in adult fish. IGF-1b mRNA was detected only in the gonad and IGF-2b mRNA only in the liver. Functional analysis showed that all 4 IGFs caused similar developmental defects but with different potencies. Many of these embryos had fully or partially duplicated notochords, suggesting that an excess of IGF signaling causes defects in the midline formation and an expansion of the notochord. IGF-2a, the most potent IGF, was analyzed in depth. IGF-2a expression caused defects in the midline formation and expansion of the notochord but it did not alter the anterior neural patterning. These results not only provide new insights into the functional conservation and divergence of the multiple igf genes but also reveal a novel role of IGF signaling in midline formation and notochord development in a vertebrate model.

Molecular characterization, tissue distribution patterns and nutritional regulation of IGFBP-1, -2, -3 and -5 in yellowtail, Seriola quinqueradiata

Insulin-like growth factor-binding proteins (IGFBPs) play a vital role in regulating the biological activities of IGFs. In this study, we cloned and determined full-length cDNA sequences of yellowtail IGFBP-1, -2, -3 and -5. Their tissue distribution was determined by real-time quantitative RT-PCR, which revealed that IGFBP-1, -2, -3 and -5 are widely distributed in yellowtail tissues. In yellowtail, both IGFBP-1 and -2 are highly expressed in the liver, IGFBP-3 is predominantly expressed in the heart and skin, with the lowest expression in the liver, and IGFBP-5 is highly expressed in the liver and kidneys. The widespread tissue expression of the yellowtail IGFBPs suggests that they may act in an autocrine and/or paracrine manner in the regulation of IGF activity. The effects of nutritional deprivation on yellowtail IGFBPs and IGF-I were also examined. During a 15-day starvation period, significant elevation was observed in hepatic yellowtail IGFBP-1. Refeeding restored its level to that of the control. No significant change was observed in the hepatic yellowtail IGFBP-2 mRNA levels in starved fish compared with control fish during the starvation period. Interestingly, during the early period of food deprivation, a significant increase was observed in hepatic yellowtail IGFBP-3 and -5 mRNA levels, concomitant to the significant elevation in hepatic IGF-I mRNA from day 3 to day 9. The unexpected increase in growth stimulatory IGFBPs and IGF-I during nutritional deprivation may represent a species-specific response to changes in nutritional condition.

Molecular and functional characterization of two distinct IGF binding protein-6 genes in zebrafish

Insulin-like growth factor binding proteins (IGFBPs) are high-affinity binding partners for IGFs and play important roles in modulating IGF activities. In this study, we have identified and characterized two functional IGFBP-6 genes in zebrafish. Structural, phylogenetic, and comparative genomic analyses indicate that they are co-orthologs of the human IGFBP-6 gene. To gain insight into how the duplicated genes may have evolved through partitioning of ancestral functions, gene expression and functional studies were carried out. In adult fish, IGFBP-6a mRNA was most abundantly expressed in the muscle. The levels of IGFBP-6a mRNA in nonmuscle tissues were very low or barely detectable. In comparison, the levels of IGFBP-6b mRNA were high in the brain, heart, and muscle, but very low or undetectable in other adult tissues. During embryogenesis, the IGFBP-6a mRNA levels were relatively low. The IGFBP-6b mRNA levels were low during the initial 48 h. They became significantly higher at 72 and 96 h postfertilization. Overexpression of zebrafish IGFBP-6a and IGFBP-6b caused a similar degree of reduction in body size and developmental rate. No notable effects were observed on cell fate or patterning in these transgenic fish. These data suggest that the duplicated igfbp-6 genes encode two functionally similar proteins, but they have evolved distinct spatial and temporal expression patterns. These findings are consistent with the notion of an additional gene duplication event in teleost fish and have provided novel insight into the structural and functional evolution of the IGFBP gene family.

Conditional ablation of mediator subunit MED1 (MED1/PPARBP) gene in mouse liver attenuates glucocorticoid receptor agonist dexamethasone-induced hepatic steatosis

Glucocorticoid receptor (GR) agonist dexamethasone (Dex) induces hepatic steatosis and enhances constitutive androstane receptor (CAR) expression in the liver. CAR is known to worsen hepatic injury in nonalcoholic hepatic steatosis. Because transcription coactivator MED1/PPARBP gene is required for GR- and CAR-mediated transcriptional activation, we hypothesized that disruption of MED1/PPARBP gene in liver cells would result in the attenuation of Dex-induced hepatic steatosis. Here we show that liver-specific disruption of MED1 gene (MED1(delta Liv)) improves Dex-induced steatotic phenotype in the liver. In wild-type mice Dex induced severe hepatic steatosis and caused reduction in medium- and short-chain acyl-CoA dehydrogenases that are responsible for mitochondrial beta-oxidation. In contrast, Dex did not induce hepatic steatosis in mice conditionally null for hepatic MED1, as it failed to inhibit fatty acid oxidation enzymes in the liver. MED1(delta Liv) livers had lower levels of GR-regulated CAR mRNA compared to wild-type mouse livers. Microarray gene expression profiling showed that absence of MED1 affects the expression of the GR-regulated genes responsible for energy metabolism in the liver. These results establish that absence of MED1 in the liver diminishes Dex-induced hepatic steatosis by altering the GR- and CAR-dependent gene functions.