br Funding The present study was funded

Funding The present study was funded by The Scientific and Technological Research Council of Turkey (TUBITAK) within Research Program 1001, unique project number: 214S349.
Introduction The apelin receptor, also called APJ or angiotensin receptor-like 1 was first cloned in 1993 due to its strong sequence homology with the angiotensin II receptor (AT1) (54% in transmembrane domains and 31% for the entire sequence). Nevertheless, APJ does not bind angiotensin II (O’Dowd et al., 1993). Human apelin receptor gene encodes for a protein of 380 Desfuroylceftiofur synthesis which belongs to the class A (rhodopsin-like receptor) G protein-coupled receptor (GPCR) family. Examining APJ protein sequence, Glu20 and Asp23 in the extracellular N-terminal tail were first identified as crucial residues for binding of its endogenous ligand called apelin (Langelaan et al., 2013, Zhou et al., 2003b). In addition, combining three-dimensional molecular modeling with site-directed mutagenesis, Gerbier et al. (2015) recently established that Asp94, Glu174 and Asp284 are also involved in apelin binding. Apelin and APJ are both widely expressed in human organism and can be detected in the central nervous system and in the periphery (heart, lung, kidney, adipose tissue, muscle…). Indeed, apelinergic system is notably expressed in hypothalamus where it participates to the regulation of fluid homeostasis, food intake and glucose metabolism (De Mota et al., 2004, Drougard et al., 2014, Duparc et al., 2011, Lv et al., 2013). APJ stimulation or inhibition in the brain may have consequences on behavior (memory, food and water intake) and physiology (neuroprotection, pain, metabolism). Moreover, it is well established that apelin signaling participates to the peripheral regulation of cardiovascular function playing an essential role in physiological (Cox et al., 2006, Kalin et al., 2007, Kang et al., 2013, Kasai et al., 2010, Kasai et al., 2008, Saint-Geniez et al., 2002) and pathological (Berta et al., 2015, Berta et al., 2010, Kalin et al., 2007, Liu et al., 2015, Sorli et al., 2007, Sorli et al., 2006) angiogenesis (for reviews, see Audigier et al., 2014). Furthermore, the apelin–APJ system has been extensively described as a major factor involved in energy metabolism. Consequently, dysregulation of apelin signaling is associated with pathological states such as cardiac hypertrophy, type 2 diabetes (T2D) and obesity (Castan-Laurell et al., 2011, Castan-Laurell et al., 2012). Given the broad range of pathophysiological actions of apelin, APJ represents a promising target for pharmacological agent design.
APJ signaling
Metabolic effects of apelin/APJ system The presence of APJ in pancreatic islets allowed to hypothesize a putative role of apelin on insulin production and more broadly on metabolism regulation (Ringstrom et al., 2010, Sorhede Winzell et al., 2005). Since 2005, different studies demonstrated dose depending effects of apelin on pancreas insulin production: a high dose (1µM) of apelin-36 causes a moderate increase in glucose-stimulated insulin secretion (30%), while lower concentrations (10–100nM) of apelin robustly reduce insulin secretion by 50% (Ringstrom et al., 2010, Sorhede Winzell et al., 2005). More recently, the presence of APJ on metabolic active tissues such as striated skeletal muscle cells and adipocytes led our group to investigate new potential roles of apelin on glucose and lipid metabolism. Thus, intravenously-injected physiological low doses of apelin-13 induced plasma glucose decrease in steady state conditions in mouse (Dray et al., 2008). Furthermore, apelin single intravenous injection performed 30minutes before an oral carbohydrate load in mouse was able to significantly improve glucose tolerance without modifying insulin production. To better characterize the effect of apelin on glucose homeostasis, intravenous infusion of the peptide was performed during hyperinsulinemic euglycemic clamps. These experiments showed that in spite of hyperinsulinemic conditions, apelin potentialized glucose uptake specifically in adipose tissues and skeletal muscle (Dray et al., 2008). This additive effect has also been confirmed both in isolated mouse skeletal muscle and in the 3T3-L1 adipocyte cell line (Dray et al., 2008, Zhu et al., 2011). In fact, apelin signaling is partially independent from the one stimulated by insulin. Involvement of apelin in glucose homeostasis has been further confirmed by studying the phenotype of apelin KO mice which displayed an amplified insulin-resistance during high fat and carbohydrate feeding (Yue et al., 2010). Moreover, these mice had an increase in visceral fat mass associated with a rise of plasma lipids such as triglycerides suggesting a role of apelin in lipids handling. Indeed, rescue of apelin KO mice by two weeks exogenous apelin supplementation demonstrated a potential role of the peptide in the inhibition of lipolysis since plasma glycerol amounts were decreased in apelin-treated knock-out mice. The interplay between apelin and lipolysis has further been confirmed in isolated and cultured adipocytes (Than et al., 2012, Yue et al., 2011). Metabolic effects of the apelin/APJ system led different teams to investigate new targets for apelin regarding glucose homeostasis. Apelin, in vivo, has been shown to increase myocardial glucose uptake and GLUT4 membrane translocation in C57BL/6J mice (Xu et al., 2012). Apelin also increases glucose transport in vitro, in cardiomyoblasts cell line H9C2 (Xu et al., 2012). In gut, a cycle between apelin and glucose has been described (Dray et al., 2013). After glucose ingestion, apelin is produced by enterocytes and seems to act in an autocrine loop on APJ to induce an increase of net glucose flux across the intestinal barrier (Dray et al., 2013). This physiological loop results in an increase of glucose in the portal vein and can be prevented by APJ antagonists given by oral gavage. Furthermore, this study demonstrates that apelin can induce AMPK phosphorylation that in turn increases glucose transporters amount such as GLUT2 and sodium/glucose cotransporter-1 on apical membrane of enterocytes (Dray et al., 2013). Taken together, these data show that apelin is a regulator of glucose homeostasis by acting first on glucose absorption in gut and then enhancing the glucose uptake/utilization in muscle or adipose tissues.