Hepatic Expression of the Mouse Angptl8 Gene Is Regulated by Insulin Through Increases in Hepatocyte Nuclear Factor-1

Abstract

Angiopoietin-like proteins (ANGPTLs) are a family of proteins structurally similar to angiopoietins. ANGPTL8 is an important regulator of circulating triglyceride (TG) levels in mammals. Increasing evidence revealed an association between ANGPTL8 expression and serum lipid profiles, especially in subjects with metabolic syndrome. Several mice studies demonstrated that Angptl8 is suppressed by fasting and induced by long term refeeding, however the detailed mechanism is still unclear. In humans, ANGPTL8 is mainly expressed in the liver. Therefore, this study aims to investigate the mechanisms that control the refeeding induced increase in the hepatic Angptl8 gene expression. Methods and Results: Twenty-week-old male C57/BL6 mice were used in this study. Mice were fasted for 12h during the dark cycle and re-fed for 30, 60, 120, 240 and 360 minutes during the light cycle. Mice were euthanized after each refeeding time course and tissues were collected. We found even short refeeding times (~60 min) enhanced the expression levels of hepatic Angptl8 in mice. We cloned the mouse Angptl8 gene promoter region. Promoter deletion analyses showed that the basal promoter activity was significantly attenuated by a deletion of -309/-60 region in hepatocytes. A computational motif search revealed the presence of a potential binding motif for hepatocyte nuclear factor 1α/1β (HNF-1α/β) at -84/-68 bp of the promoter. Mutations of the HNF-1 binging site significantly decreased the promoter activity in mouse hepatoma cells (Hepa1-6) and mouse primary hepatocytes, and the promoter carrying the mutated HNF-1 site was not transactivated by co-transfected HNF-1 in a non-hepatic cell line. These findings indicated that HNF-1 was essential and critical factor for the basal expression of Angptl8 in murine liver. In fact, knockdown of Hnf-1 using siRNA method in mouse Hepa1-6 and mouse primary hepatocytes reduced Angptl8 protein levels. We also performed Electrophoretic mobility-shift assays and confirmed the direct binding of Hnf-1 to its Angptl8 promoter binding motif. To elucidate whether refeeding could enhance HNF-1, we checked the expression levels of Hnf-1 in mouse liver. Hnf-1 expression levels of both mRNA and protein were increased after short-term refeeding, paralleling the enhanced expression of the Angptl8. Moreover, insulin-stimulated primary hepatocytes showed increased expression of Angptl8 protein, but knockdown of Hnf-1 completely abolished this enhancement by insulin. Chromatin immunoprecipitation (ChIP) analyses confirmed the recruitment of endogenous Hnf-1 to the Angptl8 promoter region and it was strongly induced by insulin. Conclusion: HNF-1 plays essential role in hepatocyte-specific and refeeding-induced rapid increases in Angptl8 expression via insulin.