Molecular characterization and expression analysis of glucose transporter 4 from Trachinotus ovatus, Rachycentron canadums and Oreochromis niloticus in response to different dietary carbohydrate-to-lipid ratios

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.