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Abstract Insulin-responsive glucose transporter 4 (GLUT4), a member of the glucose transporter family, plays a vital role in mediating glucose movement across cell membranes in insulin-sensitive tissues. In this study, we aimed to compare the molecular characterization and expression analysis of GLUT4 from three fish species of different feeding habits: golden pompano (Trachinotus ovatus), cobia (Rachycentron canadum) and tilapia (Oreochromis niloticus) after being fed three different dietary carbohydrate-to-lipid (CHO:L) ratios for 8 weeks. The results showed that the full length of golden pompano GLUT4 (ToGLUT4) and cobia GLUT4 (RcGLUT4) gene cDNA are 1973 and 2241 bp, containing an open reading frame (ORF) encoding 506 and 507 amino acids, respectively. Sequence alignment and phylogenetic analysis of GLUT4 revealed a high degree of conservation among most fish and higher vertebrates, and high homology were found in GLUT4 of golden pompano, cobia and tilapia, which share several identical motifs but contain small substitutions in GLUT4 protein. In addition, the highest GLUT4 gene expression was observed in white muscle followed by heart in these three fish species. Furthermore, in all these three species, the expression of GLUT4 was regulated in heart and white muscle by dietary CHO:L ratios. Expression of GLUT4 in heart decreased when golden pompano and tilapia were fed medium or high CHO:L ratio diets. However, in cobia, GLUT4 was induced when fed medium CHO:L ratio diet but depressed when fed high CHO:L ratio diet. With respect to white muscle, high expression of GLUT4 was found in these three fish species when fed medium CHO:L ratio diet, and high CHO:L ratio diet intake depressed its abundance in white muscle. The results indicate that GLUT4 of golden pompano, cobia and tilapia share several functionally important structural characteristics but contain small substitutions in GLUT4 protein that may lead to differences in the traffic behavior and affect the efficiency of glucose transport. In accordance with mammalian expression pattern, GLUT4 of golden pompano, cobia and tilapia are all expressed primarily in heart and white muscle, which could be regulated by dietary CHO:L ratios but appear to be species-specific and tissue-specific. However, none of the parameters examined clarifies the differences in carbohydrate utilization between carnivorous (golden pompano, cobia) and omnivorous (tilapia) fish. Decreased GLUT4 mRNA in heart and white muscle caused by feeding a relatively high carbohydrate with low lipid diet might partially explain the poor utilization of carbohydrate in these three fish species. Highlights • Sequence alignment and phylogenetic analysis of GLUT4 revealed a high degree of conservation among most fish and higher vertebrates. • Heart and white muscle GLUT4 of golden pompano, cobia and tilapia are regulated by dietary carbohydrate-to-lipid ratios. • Decreased GLUT4 mRNA in heart and white muscle after a carbohydrate rich diet could be a partial explanation for the poor utilization of carbohydrate in these three fish species. [ABSTRACT FROM AUTHOR]

In the present study, we test the hypothesis that the ability of Ctenopharyngodon idellu to clear a glucose load is related to nutrient history of different dietary carbohydrate through the transport and glycolysis of glucose. Firstly, we obtained the complete coding sequence from C. idellus GLUT1 (ciGLUT1), composed by 2364 bp with an open reading frame of 1473 bp encoding 490 amino acids. Sequence alignment and phylogenetic analysis revealed a high degree of conservation (80–97%) among most fish and higher vertebrates. The analysis of the 5′ and 3′ UTRs showed the presence of several post-transcriptional regulatory elements. The highest ciGLUT1 expression was observed in the heart followed by brain, whereas relatively low values were detected in the muscle, gill, intestine, kidney, spleen and liver. Then, hepatic and intestine ciGLUT1 expressions and hepatic glycolytic enzymes activities were determined in fish subjected to a glucose load after being fed different dietary carbohydrate levels (20%, 40% and 60%) for 8 weeks. And the result showed that different dietary carbohydrate had no effect on ciGLUT1 mRNA of hepatic and intestine after 8 weeks trail. However, after the adaptation of carbohydrate levels, glucose administration caused a prompt increase of hepatic ciGLUT1 mRNA expressions in all treatments whereas it either had no effect or a negative effect on intestine ciGLUT1 mRNA. In terms of dietary carbohydrate levels, the ciGLUT1 expressions of fish fed high carbohydrate diet (60%) were significantly higher than the other groups after the glucose load. In additon, after the glucose administration, hepatic low Km hexokinases (HK) activities of fish fed high carbohydrate diets increased significantly and was also significantly higher than the other groups. The results indicated that the GLUT1 gene of C. idellus showed a typical structure of the glucose transporters family, supporting the concept that ciGLUT1 functions as a ubiquitous and constitutive basal level glucose transporter. Consistent with the function of the GLUT1 as a constitutive glucose transporter, ciGLUT1 expressions of hepatic and intestine were hardly affected by the changes in dietary carbohydrate. Furthermore, after the glucose load, the fish after a long-term high dietary carbohydrate adaptation showed a more efficient clearance of glucose through the more remarkable enhancements of hepatic ciGLUT1 expressions and HK activities. [ABSTRACT FROM AUTHOR]