Molecular cloning, tissue distribution and functional characterization of glucose-dependent insulinotropic peptide (GIP) gene in Ctenopharyngodon idella

Abstract As the incretin hormone, Glucose-dependent insulinotropic peptide (GIP) plays an important role in lipid metabolism, glucose uptake, and bone metabolism in mammals. To evaluate the roles of GIP in the teleost, the gip gene was isolated from the foregut of grass carp by RT-PCR. The ORF of grass carp gip was 327 bp and encodes 108 amino acids. And the mature peptide of grass carp was 31 amino acids. The tissue distribution results showed that gip mRNA was highly expressed in the gut, and was also at high levels in the spleen and brain region. In the oral glucose tolerance test (OGTT), the gip expression was significantly increased in foregut by glucose treatment for 1 and 3 h. The primary hepatocytes and intraperitoneal injection studies showed that the glut2, g6pase, pepck, acc1, fas, hsl, lpl expression were increased and the gk and cpt1 expression were decreased by GIP treatment. And the expression of pi3k, akt and pka were also increased by GIP treatment. In the primary hepatocytes, the effects of GIP on g6pase, glut2, lpl and cpt1 expression were relieved by Wortmannin (PI3K inhibitor), and the effects of GIP on pepck, gk and hsl expression were alleviated by H89•2HCl (PKA inhibitor). And glucose and triglyceride (TG) contents are promoted in primary hepatocytes by GIP treatment. In the intraperitoneal injection experiment, the serum glucose, total cholesterol (T-CHO) and low-density lipoprotein cholesterol (LDL-C) contents were reduced, and serum TG and high-density lipoprotein cholesterol (HDL-C) contents were elevated after GIP injection. Our studies for the first time unveil the potential role of GIP as a regulatory factor of glucose and lipid metabolism in teleost.