Overall the expression of both

Overall, the expression of both components (ligand and receptor) of the apelin signaling system allows for analysis of the direct role of apelin in CL function, and specifically P4 production. Although the CL secretes many different hormones, P4 is of predominant importance because it is necessary for transforming the endometrium to a state receptivity for MTEP hydrochloride implantation and for maintaining an early pregnancy. After ovulation, differentiation of follicular cells into luteal cells capable of producing P4, is accomplished by the increased activity of enzymes necessary for the conversion of cholesterol to P4. 3β-HSD converts 5-ene-3β–hydroxysteroids to the 4-ene-3-oxo configuration and, therefore, has an essential role in the biosynthesis of hormonally active steroids, such as P4 (Strauss and Miller et al., 1991). Any dysfunction of the CL can result in failure of embryo implantation or early abortion. In the present study, the first in vitro evidence was obtained for the direct stimulatory effect of apelin on luteal P4 synthesis in pigs, involving an increase in 3β-HSD activity. Previously published data demonstrated effects of apelin in male (Sandal et al., 2015) and female (Roche et al., 2016; Rak et al., 2017) steroid production. For example, chronic central infusion of apelin-13 resulted in a decreased concentration of testosterone in male rats (Sandal et al., 2015). The findings of the present study are, however, consistent with those from other research where it was documented in in vitro experiments with cattle (Roche et al., 2016) and human (Roche et al., 2017) granulosal cells, as well as pig ovarian follicles cells (Rak et al., 2017) that apelin stimulated P4 secretion. The mechanism of apelin action on P4 secretion in the present experiments was demonstrated by using selective blockers of the APJ receptor (ML221) and AMPKα kinase (Compound C). With these approaches, the stimulatory effect of apelin on P4 secretion was inhibited in response to the both APJ receptor and AMPKα kinase blockers, confirming that APJ and AMPKα kinase are involved in the apelin action in CL. The results obtained are consistent with results of studies conducted on molecular mechanism of apelin action on P4 secretion in human (Roche et al., 2016), pig (Rak et al., 2017) and cattle (Roche et al., 2017) ovarian follicular cells. Moreover, the positive effect of apelin on P4 production and the abundance of the 3β-HSD protein in human ovarian cells was dependent on activation of the MAPK3/1 kinase pathway as well as Akt (Roche et al., 2016). Based on results of the present experiments which indicated a stimulatory effect of apelin on luteal P4 secretion, it is suggested that apelin is another factor which can regulate CL development. Apelin as a hormone functioning through paracrine or autocrine pathways may affect blood vessel development and P4 secretion and thus support the maturation and function of the CL of pigs.
Declaration of interest
Acknowledgments This research was supported by Jagiellonian University in Krakow: DSC/MND/WBiNoZ/IZ/5/2015, K/ZDS/006310 and by Ministry of Science and Higher Education for the PHC project under the bilateral Polish-France Agreement "POLONIUM" (2016–2017) between Agnieszka Rak and Joelle Dupont.
Introduction The apelin receptor (the angiotensin receptor like-1, the angiotensin II protein J receptor, or APJR), a member of the class A γ-group of G-protein-coupled receptors (GPCRs), is involved in diverse (patho)physiological functions such as angiogenesis, vasoconstriction, heart muscle contractility, regulation of energy metabolism, and fluid homeostasis (Masri et al., 2005, O'Carroll et al., 2013, Pitkin et al., 2010). Human genetic studies demonstrated that polymorphisms of APJR genes were associated with increased risks of hypertension and coronary artery diseases (Falcone et al., 2012, Kadoglou et al., 2010). APJR was cloned in 1993 (O'Dowd et al., 1993) and subsequently deorphanized with the discovery of its first endogenous ligand apelin, followed by the recent discovery of its second endogenous ligand Elabela (ELA)/Toddler/Apela (Chng et al., 2013, Pauli et al., 2014). These endogenous peptide ligands of APJR vary in length and amino acid sequence. Apelin peptides of multiple lengths (apelin-36, -17, and -13) result from maturation and proteolysis from the same prepropeptide. Despite the apparent lack of sequence similarity, previous studies reported that apelin and ELA peptides occupy the same orthosteric binding site on APJR and both activate the receptor through the Gi/o pathway, as well as β-arrestin1/2 recruitment (Murza et al., 2016). The therapeutic potential of APJR agonists for cardiac diseases was evaluated in a clinical study which demonstrated that acute administration of apelin in heart-failure patients significantly improved overall cardiac functions (Japp et al., 2010). Thus, to understand the fundamental biology, we sought to gain structural insights into the molecular basis of APJR-peptide recognition and receptor signal transduction. Here we report the crystal structure of human APJR in complex with a designed apelin-17 mimetic peptide.