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The GH (growth hormone)/IGFs (insulin-like growth factors) system has an important function in the regulation of growth. In this system, IGFBPs play a crucial regulatory role in IGF functions. As a member of the IGFBP family, IGFBP2 can bind to IGF and regulate IGF functions to regulate development and growth. In addition, IGFBP2 shows key regulatory functions in cell proliferation and metabolism. In this study, the igfbp2 gene was cloned from grass carp (Ctenopharyngodon idellus) liver. The ORF of grass carp igfbp2 is 834 bp long and encodes 277 amino acids. The tissue distribution results showed that igfbp2 is expressed in multiple tissues in grass carp and has a high expression level in the liver. In the OGTT, igfbp2 expression was significantly decreased in the liver and brain after 6 h of treatment with glucose. In vitro, igfbp2 expression in grass carp's primary hepatocytes was significantly suppressed by insulin after treatment for 6 and 12 h. Moreover, igfbp2 expression was markedly increased in a dose-dependent manner with glucagon incubation in grass carp's primary hepatocytes. To the best of our knowledge, this is the first report about Igfbp2 in grass carp. These results will provide a basis for the in-depth study of grass carp Igfbp2.

IGFBPs play a pivotal role in regulating the physiological function of IGFs (insulin-like growth factors). As an important member of IGFBPs, IGFBP3 is involved in the regulation of physiological functions of IGFs. To investigate the functional role of Igfbp3 in a herbivorous fish species, grass carp igfbp3 (GenBank accession no. MN251843) was isolated from the liver by molecular cloning. The ORF of grass carp igfbp3 was 882 bp, which encoded 293 amino acids, and the first 22 amino acids comprised the signal peptide. The predicted molecular weight of grass carp Igfbp3 is 31.95 kDa, and the theoretical isoelectric point is 8.32. Grass carp Igfbp3 displayed a high identity with its counterparts of common carp and zebrafish. And phylogenetic tree analysis showed that the grass carp Igfbp3 was clustered into the Igfbp3 subgroups of common carp and zebrafish. Tissue distribution study showed that igfbp3 was ubiquitously expressed in all tissues examined in the present study. High expression level of igfbp3 was detected in the heart, brain and fat of grass carp. The OGTT demonstrated that igfbp3 mRNA expression was significantly elevated in the liver of grass carp in response to glucose treatment. In vitro study showed that insulin could markedly stimulated igfbp3 mRNA expression in primary grass carp hepatocytes. Moreover, igfbp3 mRNA levels were also regulated by glucagon in grass carp primary hepatocytes. These results may provide the theoretical foundation for investigating the role of Igfbp3 in fish metabolism., Competing Interests: Declaration of Competing Interest All the authors declare that they have no conflict of interest., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Generally, fish are thought to have a limited ability to utilize carbohydrate. Postprandial blood glucose is cleared sluggishly in fish, resulting in prolonged hyperglycemia. Facilitative glucose transporters (GLUTs) play an important role in glucose utilization. In the present study, the expression levels of glut2 in different tissues were detected in grass carp. Furthermore, the effects of oral glucose administration on glut2 mRNA expression in the liver, intestine and kidney were investigated, and we also evaluated the response of glut2 mRNA to insulin and glucagon in the primary hepatocytes of grass carp. The expression level of glut2 mRNA was highest in the liver, followed by the intestine and kidney, but lower in other tissues. The result of glucose tolerance test (GTT) showed that serum glucose reached the highest level at 3 h after GTT and recovered to the basic level at 6 h. The glut2 mRNA in the intestine was up-regulated at 1 h after GTT. However, the glut2 mRNA expression in the liver of grass carp was unchanged after GTT for 1, 3, 6 h, and even decreased at 12 h after GTT. In addition, the expression of glut2 mRNA in the primary hepatocytes was enhanced by insulin and glucagon at 3 h post treatment. These results suggested that glut2 expression in the liver of grass carp was sensitive to insulin and glucagon, but not blood glucose. The up-regulation of glut2 by these hormones might be involved in the bi-directional transportation of glucose in the liver., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Glucose transporter 4 (GLUT4) is comprehensively investigated in mammals, while the comparative research of GLUT4 in common carp is deficient. To investigate the function of GLUT4, carp glut4 was first isolated. The open reading frame of carp glut4 was 1518 bp in length, encoding 505 amino acids. A high‐sequence homology was identified in carp and teleost, and the phylogenetic tree displayed that the carp GLUT4 was clustered with the teleost. A high level of glut4 mRNA was analysed in fat, red muscle and white muscle. After fasting treatment, glut4 mRNA expression was increased significantly in muscle. In the oral glucose tolerance test experiment, glut4 mRNA was also significantly elevated in muscle, gut and fat. Furthermore, intraperitoneal injection of insulin resulted in the upregulation of glut4 gene expression significantly in white muscle, gut and fat. On the contrary, the glut4 mRNA level in the white muscle, gut and fat was markedly downregulated after glucagon injection. These results suggest that GLUT4 might play important roles in food intake and could be regulated by nutrient condition, insulin and glucagon in common carp. Our study is the first to report on GLUT4 in common carp. These data provide a basis for further study on fish GLUT4. [ABSTRACT FROM AUTHOR]

Apelin plays pivotal roles in fluid homeostasis, food intake and energy metabolism in mammals. However, the regulatory role of apelin in glucose metabolism in teleosts is still unclear. In this study, we investigated the regulatory role of Pyr‐apelin‐13 in glucose metabolism of common carp (Cyprinuscarpio L.). The results showed that apelin mRNA abundance was significantly increased in the foregut and hepatopancreas after glucose injection. Insulin and glucagon also up‐regulated apelin mRNA levels in the foregut, hepatopancreas and hepatocytes. In vivo, intraperitoneal injection of Pyr‐apelin‐13 induced hypoglycaemia through the up‐regulation of mRNA expression and activity of hexokinase (HK) coupled with the down‐regulation of mRNA expression and activity of glucose‐6‐phosphatase (G6Pase) in the hepatopancreas. In vitro, upon incubation of the primary hepatocytes with Pyr‐apelin‐13 (0, 10, 100 and 1000 nM), the mRNA expression of glucokinase (GK) was increased, whereas the mRNA expression of phosphoenolpyruvate carboxykinase (PEPCK) and G6Pase was reduced. For glucose transport, the glucose transporter 2 (GLUT2) mRNA expression increased in the hepatopancreas, intestine, primary enterocytes and hepatocytes after Pyr‐apelin‐13 treatment. Collectively, our data may provide a basis for further studies on elucidation of the role and mechanism of apelin in glucose metabolism of teleosts. [ABSTRACT FROM AUTHOR]

It is widely acknowledged that glucose serves as the primary energy source for organisms. However, fish exhibit persistent postprandial hyperglycemia and are thought to have low glucose tolerance. Glycolysis serves as the ubiquitous pathway for glucose catabolism, with phosphofructokinase (PFK) acting as a crucial rate-limiting enzyme in this process and playing an indispensable role. PFK is classified into three isoforms based on their major expression sites, i.e., PFKM (skeletal muscle type), PFKL (liver type) and PFKP (platelet type). In this study, grass carp (Ctenopharyngodon idella) was used as animal model and the open reading frame (ORF) sequences of six PFK genetic isoforms of grass carp were cloned. Real-time PCR was used to detect its tissue distribution, and expression changes in oral glucose tolerance test (OGTT), insulin and glucagon injection experiments. The results showed that the ORF of pfkla , pfklb , pfkma , pfkmb , pfkpa and pfkpb genes was 2343, 2340, 2355, 2331, 2364 and 2349 bp in length, respectively. The results of tissue distribution showed that pfkla and pfklb , homologous to mammalian pfkl , exhibited low expression levels in the liver of grass carp, but were expressed at the highest level in the brain. Muscle-type pfkma and pfkmb mRNA were found to be highly expressed in both red and white muscle, with pfkmb also exhibiting high expression levels in the heart, while platelet type pfkpa and pfkpb showed high mRNA abundances in the brain and heart. Oral glucose administration stimulated pfkma and pfkmb mRNA expression in the red muscle, and up-regulated pfklb mRNA levels in the liver at 3 h post treatment, but it suppressed liver-type and platelet-type PFK genes expression in the brain. The expression of pfkmb in white muscle and pfkmb and pfkpb in heart were promoted by insulin, whereas the expression of pfkla and pfkpb in the brain, pfkma and pfkmb in the red muscle, pfkma in the white muscle, and pfklb in the liver was suppressed by insulin. As for glucagon, it inhibited pfkma and pfkmb mRNA expression in the red muscle, as well as pfklb in the liver, but it up-regulated PFK genes expression in most tissues detected, such as brain (pfklb , pfkpa and pfkpb), white muscle (pfkma and pfkmb), liver (pfkla) and heart (pfkmb and pfkpb). Our results suggest that PFK family genes have different or even opposite expression patterns in response to glucose, insulin and glucagon stimulation in various tissues of grass carp, which may contribute to glucose intolerance in fish. [Display omitted] • Brain, heart, red and white muscle are the main tissues for PFK expression. • Glucose promoted PFK expression in muscle (M-type), liver (pfklb), heart (pfkpb). • Insulin up-regulated expression of pfkmb in white muscle, pfkmb and pfkpb in heart. • Glucagon up-regulated PFK expression in most tissues, except for red muscle. • PFK family genes have different or even opposite expression patterns. [ABSTRACT FROM AUTHOR]

Adiponectin plays critical roles in energy metabolism and immunity of mammals by binding to its receptors (AdipoR1 and AdipoR2). However, they have not been fully understood in fish. The present study examined the distribution of adiponectin receptors (AdipoRs) mRNA in grass carp, and the changes of their expressions under glucose load, insulin or glucagon stimulation, lipopolysaccharide (LPS) or polyinosinic:polycytidylic acid [poly (I:C)] challenge. As for glucose load, fish were orally administrated with glucose (1.67 mg/g BW) for 1, 3, 6, 12 h. For insulin or glucagon stimulation, the primary hepatocytes of grass carp were incubated with 0, 10, 100, 1000 nM insulin or glucagon for 1, 3, 6, 12 h. In the immune challenge experiment, grass carp were intraperitoneally injected with LPS or poly (I:C) (5 μg/g BW) for 3, 6, 12, 24, 48 h. Finally, AdipoRs mRNA expression in all experiments was detected by Real-time quantitative PCR. The results showed that AdipoRs mRNA was widely expressed in various tissues of grass carp. The adipoR1a mRNA was highly expressed in the brain regions, and the highest level of adipoR2 mRNA was observed in the adipose tissue. Glucose load up-regulated adipoR1b and adipoR2 mRNA expressions in the liver at 6 h post administration and decreased adipoR1a expression in the muscle at 3 h. Insulin significantly enhanced adipoR1b and adipoR2 mRNA expressions in the hepatocytes at 1–12 h, and promoted adipoR1a mRNA expression at 1, 3 and 12 h , while glucagon had little effect on their expressions. LPS and poly (I:C) stimuli induced AdipoRs expression in the liver at 6 and 12 h, and their expressions in the muscle were decreased slightly or remained unchanged. In addition, LPS promoted AdipoRs expression in the kidney and spleen, whereas poly (I:C) only induced adipoR1a mRNA expression in the kidney, and increased adipoR2 mRNA level in the spleen. These results suggest that AdipoRs may act as one of the targets of insulin and immune response in grass carp, but whether they are direct target remains to be further studied. • AdipoRs were identified in grass carp and were widely expressed in various tissues. • Hepatic adipoR1b and adipoR2 expressions increased during glucose tolerance test. • Insulin, LPS and poly (I:C) induced hepatic AdipoRs expression. • LPS promoted kidney and spleen AdipoRs expression, poly (I:C) had limited effects. [ABSTRACT FROM AUTHOR]