Host Factors Alter Effects of Angiopoietin-Like Protein 8 on Glucose Homeostasis in Diabetic Mice

Angiopoietin-like protein 8 (betatrophin) inhibits hepatic gluconeogenesis through PI3K/Akt signaling pathway in diabetic mice

BACKGROUND & AIMS

Angiopoietin-like protein 8 (ANGPTL8) is a 198 amino-acid long, novel secreted protein that is mainly expressed in the liver and brown adipose tissues. At present, evidence supporting the involvement of ANGPTL8 in the regulation of glucose metabolism is inconclusive, along with its function in the liver. Previous studies mainly focused on the effect of ANGPTL8 on glucose metabolism in non-diabetic mice, and few relevant studies in diabetic mice exist. Therefore, this study aimed to investigate the role of ANGPTL8 on glucose homeostasis and elucidate the underlying mechanisms in diabetic mice.

METHODS

db/db diabetic and high-fat diet/streptozotocin-induced diabetic mice were injected with adenovirus expressing ANGPTL8 through the tail vein. Blood glucose levels were measured and glucose, insulin, and pyruvate tolerance tests were performed. To explore the molecular mechanism by which ANGPTL8 regulates hepatic glucose metabolism and manipulate mouse ANGPTL8 expression levels both in vivo and in vitro based on adenoviral transduction, gain- and loss-of-function strategies were adopted.

RESULTS

Adenovirus-mediated overexpression of ANGPTL8 decreased fasting blood glucose levels and improved glucose tolerance and insulin sensitivity in db/db and high-fat diet/streptozotocin-induced diabetic mice. ANGPTL8 knockdown yielded the opposite effects. ANGPTL8 was upregulated in the cAMP/Dex-induced hepatocyte gluconeogenesis model. Moreover, ANGPTL8 overexpression in primary hepatocytes and diabetic mouse livers inhibited the expression of gluconeogenesis-related genes, including PEPCK and G6PC, by activating the AKT signaling pathway and, thereby, reducing glucose production. Therefore, the results demonstrated that ANGPTL8 improved glucose metabolism via inhibition of hepatic gluconeogenesis in diabetic mice.

CONCLUSIONS

Current findings highlight a critical role of hepatic ANGPTL8 in glucose homeostasis, suggesting that increased ANGPTL8 expression could be an underlying factor for the inhibition of hepatic gluconeogenesis, which could be targeted for the prevention and treatment of type 2 diabetes.

Newly discovered endocrine functions of the liver

The liver, the largest solid visceral organ of the body, has numerous endocrine functions, such as direct hormone and hepatokine production, hormone metabolism, synthesis of binding proteins, and processing and redistribution of metabolic fuels. In the last 10 years, many new endocrine functions of the liver have been discovered. Advances in the classical endocrine functions include delineation of mechanisms of liver production of endocrine hormones [including 25-hydroxyvitamin D, insulin-like growth factor 1 (IGF-1), and angiotensinogen], hepatic metabolism of hormones (including thyroid hormones, glucagon-like peptide-1, and steroid hormones), and actions of specific binding proteins to glucocorticoids, sex steroids, and thyroid hormones. These studies have furthered insight into cirrhosis-associated endocrinopathies, such as hypogonadism, osteoporosis, IGF-1 deficiency, vitamin D deficiency, alterations in glucose and lipid homeostasis, and controversially relative adrenal insufficiency. Several novel endocrine functions of the liver have also been unraveled, elucidating the liver’s key negative feedback regulatory role in the pancreatic α cell-liver axis, which regulates pancreatic α cell mass, glucagon secretion, and circulating amino acid levels. Betatrophin and other hepatokines, such as fetuin-A and fibroblast growth factor 21, have also been discovered to play important endocrine roles in modulating insulin sensitivity, lipid metabolism, and body weight. It is expected that more endocrine functions of the liver will be revealed in the near future.