An evaluation of hepatic glucose metabolism at the transcription level for the omnivorous GIFT tilapia, Oreochromis niloticus during postprandial nutritional status transition from anabolism to catabolism

To better understand the regulatory mechanism of glucose metabolism, sub-adult Genetically Improved Farmed Tilapia (GIFT) were sampled at 0, 1, 3, 8 and 24 h after feeding (HAF) after 36 h of food deprivation, and the time course of changes in plasma nutrients and the transcription of a series of key genes involved with hepatic glucose transport and utilization were analyzed. Compared with the fasting state (0 HAF), plasma glucose increased at 1 HAF, peaked at 3 HAF, decreased at 8 HAF and returned to basal level at 24 HAF. Genes such as glucokinase (gck) and liver type of phosphofructokinase (pfkl) were sensitive indicators for hepatic glycolysis, and their transcription was active at 1-8 HAF, especially at 3 HAF. Genes such as glucose-6-phosphatase catalytic subunit a2 (g6pca2) and phosphoenolpyruvate carboxykinase 2 (pck2) were useful markers for hepatic gluconeogenesis, and their transcription was inhibited at 1-3 HAF, especially at 3 HAF. Hepatic gluconeogenesis was up-regulated with feeding time from 3 to 8 h, suggesting a nutritional status transition from anabolism to catabolism. The mRNA level of glycogen synthase 1 (gys1) increased at 3-8 HAF and liver glycogen accumulated at 8 HAF, but the principal expressed gys2 was unexpectedly down-regulated after feeding, suggesting that glycogen storage might be controlled by Gys1 rather than Gys2 in the liver of tilapia. In line with the increased plasma triglyceride and cholesterol concentrations, the transcription of hepatic lipogenic genes including acetyl-CoA carboxylase α (acca), ATP citrate lyase a (aclya) and fatty acid synthase (fas) were all up-regulated during 1-8 HAF (especially 3-8 HAF), suggesting that hepatic lipogenesis was an efficient way to dispose excess glucose in tilapia. Hepatic glucose transport was induced since the feeding time was over 8 h, indicating that hepatic glucose production was already transported to extra-hepatic tissues at 8 HAF, which further confirmed the nutritional status transition at this time. Taking the results together, it was suggested that the physiologically specific time point of postprandial nutritional status transition from anabolism to catabolism occurred at about or a little earlier than 8 HAF in tilapia.