Leptin regulates the expression of angiopoietin-like 6

Highlights

• High-fat diet results in increased hepatic ANGPTL6 expression.

• Leptin treatment upregulates ANGPTL6 expression in hepatocytes.

• Exercise training decreases serum ANGPTL6 as well as leptin.

Abstract

Angiopoietin-like 6 (ANGPTL6) is a hepatokine that antagonizes obesity and insulin resistance by increasing energy expenditure. Despite its beneficial effects on metabolism, human studies have shown a paradoxical increase in ANGPTL6 level in the serum of patients with metabolic diseases, which has been interpreted as a compensatory upregulation. However, the regulatory mechanism of ANGPTL6 remains unclear. Since upregulation of ANGPTL6 is induced on metabolic stress, we investigated the hepatic expression of ANGPTL6 by leptin, a representative adipokine of obesity. Mice on a high-fat diet showed increased serum leptin levels and hepatic Angptl6 expression, which were attenuated by exercise training. A single leptin injection also induced hepatic ANGPTL6 expression and increased serum ANGPTL6 levels. In an in vitro model using primary hepatocytes, leptin treatment significantly upregulated ANGPTL6 expression at the mRNA and protein levels, as well as the amount of secreted ANGPTL6 protein in conditioned media. Similarly, exercise training on human participants also showed diminished serum levels of leptin and ANGPTL6. Altogether, these results strongly indicated that hepatic ANGPTL6 expression was determined by leptin.

Introduction

Angiopoietin-like proteins (ANGPTLs) are structurally similar to angiopoietin, which contains coiled-coil and fibrinogen-like domains [1]. Thus far, 8 members of the ANGPTL family (ANGPTL1 to 8) have been identified [2]. Despite their names, ANGPTLs do not bind to the TIE1 or TIE2 receptor tyrosine kinase, which binds angiopoietin [3]. Most ANGPTLs exert angiogenic effects, but their roles in metabolic regulation have been identified recently [4]. ANGPTL6, also known as angiopoietin-related growth factor (AGF), is a hepatocyte-derived angiogenic factor that promotes angiogenesis [3,5] and epidermal proliferation [6]. Although most ANGPTLs now have their own receptors including leukocyte immunoglobulin-like receptors (LILRs), toll-like receptor 4 (TLR4), and integrin alpha-5/beta-3 [4], ANGPTL6 remains as an orphan ligand. In addition to its angiogenic effects, ANGPTL6 exerts regulatory functions on energy metabolism. Angptl6-knockout mice developed obesity even on standard chow diet, with a decreased core temperature and oxygen consumption rate (VO₂) [7]. This phenotype was completely reversed in Angptl6-transgenic mice, which were resistant to diet-induced obesity with increased VO₂ and density of capillaries in muscle fiber. Exogenous ANGPTL6 by adenoviral transduction decreased body weight and increased insulin sensitivity in diet-induced obesity [7]. Furthermore, recombinant ANGPTL6 protein decreased the glucose output of hepatocytes [8]. However, despite its potential as a therapeutic target for obesity, diabetes, and metabolic syndrome [1], clinical studies have reported increased serum levels of ANGPTL6 in polycystic ovary syndrome [9], diabetes [10,11], gestational diabetes [12] and metabolic syndrome [13]. Although the concept of compensation or resistance of ANGPTL6 has been introduced for these paradoxical observations, the mechanism of ANGPTL6 upregulation remains unclear.

Positive energy balance, resulting from high calorie intake versus low energy expenditure, results in the accumulation of extra energy in the form of triglycerides in adipose tissue [14]. Besides the repository of excessive energy, adipose tissue secretes adipokines that can participate in interorgan metabolic communications in the muscle, gut, brain, and liver [[15], [16], [17]]. Dysregulated adipokine profiles due to obesity are closely linked to metabolic disorders [18]. Leptin, one of the characteristic adipokines, exerts an anorexigenic effect in the hypothalamus, resulting in a negative energy balance [19,20]. Serum levels of leptin correlate with body fat mass [21], and obese patients have increased leptin with decreased sensitivity, which has been interpreted as leptin resistance [22]. Thus, the upregulation of leptin has been regarded as a marker of metabolic stress with insulin resistance and obesity.

Since the mechanism for the upregulation of ANGPTL6 in metabolic stress has not yet been identified, we hypothesized that leptin, a representative adipokine in obesity, can modulate ANGPTL6 expression. In this study, we investigated the relationship of leptin and hepatic ANGPTL6 expression using in vivo and in vitro models. We also evaluated the changes in leptin and ANGPTL6 expression by exercise training in human participants.