Angiopoietin-like protein 8 (ANGPTL8) is a novel circulating protein synthesized by the liver and adipose tissue and plays a key role in regulating the plasma lipid profiles. Its expression is regulated acutely by fasting and refeeding in humans and animals. However, the molecular and physiological mechanisms, which regulate its expression, remain largely unknown. In this study, we investigated the regulation of ANPTL8 by insulin and glucose in multiple cellular and animal systems. Interestingly, insulin alone rapidly and dose-dependently increased ANGPTL8 gene expression in both cultured hepatocytes and mature adipocytes, while high glucose combined with insulin presented further stimulation on ANGPTL8 only in adipocytes. In mice, 2-hour euglycemic-hyperinsulinemic and hyperglycemic-hyperinsulinemic clamps markedly stimulated ANGPTL8 mRNA levels in liver to a similar extent. However, only combined hyperglycemia-/hyperinsulinemia caused a significant elevation of ANGPTL8 gene transcript in mouse adipose tissue. These findings imply that distinct pathways are responsible for the effects of insulin and glucose on ANGPTL8 expression in liver and adipose tissue. By cloning the ANGPTL8 gene promoter fragment into a luciferase reporter vector, we discovered that insulin-induced ANGPTL8 gene transcript was mediated by the CCAAT/enhancer-binding protein beta (C/EBPβ) transcription factor, which is likely through Janus Kinase 2 (JAK2) pathway. We also observed that AMPK activation had an inhibitory effect on insulin-induced ANGPTL8 gene expression; this could explain, in part, the low ANGPTL8 levels during fasting. Finally, in mice ANGPTL8 knockout did not alter body weight or food intake compared to wild type littermates, but significantly improved glucose tolerance in diet induced obese mice. This beneficial effect on glucose tolerance most likely is explained by the reduction in serum FFA and triglyceride levels.
L. Zhang: None. M. Abdul-Ghani: None. R.A. DeFronzo: Speaker's Bureau; Self; AstraZeneca, Novo Nordisk Inc.. Advisory Panel; Self; AstraZeneca, Novo Nordisk Inc., Janssen Pharmaceuticals, Inc., Boehringer Ingelheim Pharmaceuticals, Inc., Elcelyx Therapeutics, Inc., Intarcia Therapeutics, Inc.. Research Support; Self; AstraZeneca, Boehringer Ingelheim Pharmaceuticals, Inc., Janssen Pharmaceuticals, Inc., Takeda Pharmaceuticals U.S.A., Inc.. L. Norton: None.