Apelin Receptor

In 1993, a gene encoding a novel class A G-protein-coupled receptor was discovered by homology cloning. It showed greatest sequence homology with the angiotensin AT₁ receptor (54% in the transmembrane regions) but did not bind angiotensin II. It was therefore designated an "orphan" GPCR, having no known ligand, and was named APJ by O'Dowd et al., (1993) [51]. The approved Human Genome Organization (HUGO) gene symbol for APJ is now APLNR. The endogenous ligand for this receptor was later identified as apelin, which led the International Union of Pharmacology (IUPHAR) to recommend “apelin receptor” as the nomenclature for the receptor protein [54]. This follows the convention of naming the receptor protein after its endogenous ligand.

The human apelin receptor comprises 380 amino acid residues and has the typical 7-transmembrane domain structure of a class A GPCR. It contains consensus sites for phosphorylation by cAMP-dependent protein kinase, palmitoylation, and glycosylation [51]. The apelin receptor has been identified in a number of other species, including mouse, rat, cow, rhesus macaque, Xenopus laevis, and Danio rerio.

To date, there is no evidence for multiple receptor subtypes in mammals. During the initial receptor identification, a polymerase chain reaction strategy using oligonucleotides based on the apelin receptor yielded no closely related genes [51]. In addition, saturation binding experiments in human tissues gave Hill coefficients close to unity, indicating that the radioligand bound to a single receptor population [30], although this does not exclude the possibility of two receptor subtypes with the same affinity.

Activation of apelin receptors expressed in cell lines inhibited forskolin-stimulated cAMP production, suggesting that the receptor is coupled to inhibitory G-proteins (G_i) [19], which is supported by the finding that apelin actions are pertussis toxin-sensitive [22,44]. A number of interactions between the apelin and angiotensin systems have been reported, including recent evidence that the apelin receptor forms heterodimers with the angiotensin AT₁ receptor [9].

Apelin Peptides

In 1998, the endogenous ligand for APJ was identified as a 36-amino acid peptide named apelin (for APJ endogenous ligand), isolated from bovine stomach extracts. This peptide induced extracellular acidification in CHO cells expressing apelin receptors [65]. cDNA encoding a 77-amino acid prepropeptide (preproapelin) was identified in human and bovine tissue [65], showing considerable sequence similarity across all species examined, with the last 23 residues of the C terminus being identical in mammals. Apelin peptides so far have been detected in vascular and endocardial endothelial cells [34-35], in cells from the epithelial layer of the stomach mucosa [61], and in neurones of the supraoptic and paraventricular nuclei of the hypothalamus [55].

Preproapelin contains a number of paired basic amino acids residues (Arg-Arg and Arg-Lys) that are possible cleavage sites for endopeptidases [19]. Cleavage at these sites would produce a predicted family of C-terminal fragments, including apelin-36, apelin-17, apelin-13, and the post-translationally modified (Pyr¹)apelin-13, which are all agonists at the apelin receptor. The degradative pathway for apelin peptides is unknown, but angiotensin-converting enzyme 2 (ACE2) cleaves the C-terminal phenylalanine from apelin-13 and apelin-36 [69]. All of these predicted isoforms have been shown to be present in vivo, but the predominant apelin isoform in human cardiac tissue is (Pyr¹)apelin-13 [42], which is not unexpected because the pyroglutamate moiety protects the N terminus of peptides from exopeptidase degradation [68]. The predominant isoforms in plasma are (Pyr¹)apelin-13, apelin-13 and apelin-17 [3,14,48]. The relative potency of the apelin peptides varies between experimental systems, (Pyr¹)apelin-13 and apelin-13 being the most potent activators of apelin receptors expressed in cell lines [19,31,47,65], whereas apelin-36 is the most potent inhibitor of HIV infection of cells in vitro [72]. However, (Pyr¹)apelin-13, apelin-13, and apelin-36 are equipotent mediators of vascular tone and cardiac contractility in human tissues in vitro [42].

A number of synthetic apelin analogs have biological activity. Three cyclic apelin analogues have been reported to show agonism at the apelin receptor, inhibiting cAMP accumulation in a pertussis toxin-sensitive manner, although binding data of these analogues are not available [20]. Another cyclic analogue, MM54, was designed and shown be the first specific antagonist at the apelin receptor showing high affinity at apelin receptor transfected in Chinese Hamster Ovary cells and at the native receptor in human left ventricle. MM54 displayed no agonist activity in cAMP accumulation assays, but shifted the (Pyr¹)apelin-13 dose–response curve to the right in a dose-dependent manner, with no change in maximum response, typical of a competitive antagonist [41]. Furthermore, apelin-13 with the C-terminal phenylalanine mutated to alanine (Apelin-13(F13A)) has been hown to act as an apelin-specific functional antagonist in rats in vivo, blocking the hypotensive effects of apelin-13 [39]. However, three conflicting reports including two alanine scanning studies showed that apelin-13(F13A) had comparable affinity and agonist activity at the native apelin receptor in human cardiovascular tissues in vitro and at the human apelin receptor expressed in human embryonic kidney 293 cells [16,47,53], indicating that this peptide is a full agonist in man.

A non-peptide ligand of the apelin receptor, E339–3D6, was reported to behave as a partial agonist with regard to cAMP production and a full agonist with regard to apelin receptor internalization. This ligand induced vasorelaxation of precontracted rat aorta and potently inhibited systemic vasopressin release in water-deprived mice when intracerebroventricularly injected [25]. Additionally, ALX40-4C, a small-molecule antagonist of the chemokine receptor CXCR4, has been shown to directly bind apelin receptors expressed in cell lines and to block ligand-induced receptor internalization and signaling, suggesting it is a nonspecific apelin receptor antagonist [71]. A recent high throughput screen has identified ML221, a potent functional small molecule antagonist in cell-based assays that is highly selective for the apelin receptor against other related GPCRs [43]. The discoverers of ML221 have also reported ML233 from the screen as a potent and selective small molecule apelin receptor functional agonist in cell-based assays, and claimed it as the first non-peptide based agonist and E339–3D6 as a peptidomimetic [32].

A number of radiolabeled ligands for the apelin receptor have been synthesized based on the structure of the endogenous ligands, but most are based on (Pyr¹)apelin-13. [¹²⁵I](Pyr¹)apelin-13 binds to receptors in human left ventricle with a K_D of 0.35 nM. It associates rapidly, with a half-time for association of 6 min, and dissociates with a half-time for dissociation of 53 min [30]. It is noteworthy that analogues of this radioligand have been made by others with modifications at position 75 [22], replacing the methionine at this position with norleucine to prevent possible oxidation during the radiolabeling process, because oxidized (Pyr¹)apelin-13 was found to be inactive. The resulting radioligand, [¹²⁵I](Pyr¹)[Nle⁷⁵,Tyr⁷⁷]apelin-13, is commercially available. Another group oxidized the methionine at this position, because the unoxidized form of the radioligand was very unstable [47].

Functions

Since the discovery of apelin as the endogenous ligand for APJ in 1998, a number of physiological roles for the receptor have emerged, including regulation of cardiovascular function, fluid homeostasis, the adipoinsular axis, gastrointestinal and immunomodulatory functions, reviewed by Pitkin et al., (2010) [53].

Apelin modulates vascular tone in vivo, causing a reduction in blood pressure when infused into rats [8,15,24,38-39,49,55,66] and vasodilation of resistance vessels when infused into the human forearm [26], both responses mediated primarily by nitric oxide. In vitro, apelin causes vasodilation of human splanchnic artery, largely via a nitric oxide-dependent mechanism [59]. Apelin also causes vasoconstriction of human saphenous vein [30] and mammary artery [42] in vitro by a direct action on vascular smooth muscle. These data support a role for the apelin system in modulation of vascular tone, where apelin released from endothelial cells would act on apelin receptors on the endothelium to cause vasodilation or on underlying smooth muscle cells to cause vasoconstriction. Apelin also modulates cardiac function. Apelin peptides have positive inotropic effects in rats [2,4,27] and mice [1] in vivo. In vitro studies have demonstrated that apelins are potent positive inotropic agents by a direct action on cardiac tissue in rat [12,17,62] and human [42]. It is noteworthy that apelins are the most potent endogenous inotropic agents yet reported in isolated cardiac tissue, with EC₅₀ values of 40 to 125 pM in human tissue [42] and 33 pM in rat tissue [62]. In addition, apelin is a potent angiogenic factor [10,29] and mitogen of endothelial cells [29,45] and vascular smooth muscle cells [40]. Mice lacking the apelin gene show retardation of retinal vascular development [28] and narrow blood vessels in intersomitic vessels during embryogenesis [33], whereas APLNR(−/−) mice have cardiac developmental defects [7]. It has recently been shown that apelin acts downstream of Cripto (official gene name tdgf-1) to specify murine embryonic stem cells toward the cardiac lineage [11].

The colocalization of apelin and its receptor with vasopressin in magnocellular neurons of the SON and PVN of the hypothalamus triggered investigation of their role in fluid homeostasis. Apelin, given to mice by intracerebroventricular injection, inhibits vasopressin neuron activity and vasopressin release, decreasing plasma vasopressin concentration and increasing diuresis [14]. Dehydration increases apelin [56] and apelin receptor [50] expression and decreases vasopressin expression in rat magnocellular neurons [56]. In addition to its central effects, apelin has direct actions on the microvasculature of the kidney [23]. APLNR(−/−) mice have abnormal fluid homeostasis and altered responses to osmotic stress [57-58]. Osmotic stimuli have been show to exert opposing effects on plasma apelin and vasopressin in man; increased plasma osmolality increases plasma vasopressin and decreases plasma apelin, and vice versa [3].

Apelin is expressed and released by cultured adipocytes, identifying it as a novel adipokine [5], and adipose tissue is a possible source of plasma apelin. Apelin expression in adipose tissue is regulated by factors such as fasting and refeeding [5], insulin [5,70], hypoxia [18,37], growth hormone [36], tumor necrosis factor α [13], and peroxisome proliferator-activated receptor γ coactivator 1α [46]. Whereas insulin stimulates adipose apelin expression [5,70], apelin inhibits insulin secretion [63], presenting an interesting interaction between the two systems. There is evidence for a role for apelin regulation of adiposity, peripherally administered apelin causing no change in food intake [21,60] but decreasing adiposity, possibly by up-regulating uncoupling proteins and increasing energy expenditure [21]. However, investigation of the role of central apelin on food intake and body weight has yielded disparate results [52,60,64,67]. Apelin may also be involved in vascularization of adipose tissue [37].

APJ has further been reported to be a co-receptor for the infection of CD4-positive cells with subtypes of human immunodeficiency virus in the central nervous system. Another interesting action of apelin peptides is their ability to effectively inhibit HIV infection by blocking the HIV co-receptor APJ, with apelin-36 being the most potent anti-infective form of apelin [6,16,72].

Further information is available from the Pharmacological Review [53] by the International Union of Basic and Clinical Pharmacology. LXXIV. Apelin receptor nomenclature, distribution, pharmacology, and function.

1. Ashley EA, Powers J, Chen M, Kundu R, Finsterbach T, Caffarelli A, Deng A, Eichhorn J, Mahajan R, Agrawal R et al.. (2005) The endogenous peptide apelin potently improves cardiac contractility and reduces cardiac loading in vivo. Cardiovasc Res, 65 (1): 73-82. [PMID:15621035]

2. Atluri P, Morine KJ, Liao GP, Panlilio CM, Berry MF, Hsu VM, Hiesinger W, Cohen JE, Joseph Woo Y. (2007) Ischemic heart failure enhances endogenous myocardial apelin and APJ receptor expression. Cell Mol Biol Lett, 12 (1): 127-38. [PMID:17119870]

3. Azizi M, Iturrioz X, Blanchard A, Peyrard S, De Mota N, Chartrel N, Vaudry H, Corvol P, Llorens-Cortes C. (2008) Reciprocal regulation of plasma apelin and vasopressin by osmotic stimuli. J Am Soc Nephrol, 19 (5): 1015-24. [PMID:18272843]

4. Berry MF, Pirolli TJ, Jayasankar V, Burdick J, Morine KJ, Gardner TJ, Woo YJ. (2004) Apelin has in vivo inotropic effects on normal and failing hearts. Circulation, 110 (11 Suppl 1): II187-93. [PMID:15364861]

5. Boucher J, Masri B, Daviaud D, Gesta S, Guigné C, Mazzucotelli A, Castan-Laurell I, Tack I, Knibiehler B, Carpéné C et al.. (2005) Apelin, a newly identified adipokine up-regulated by insulin and obesity. Endocrinology, 146 (4): 1764-71. [PMID:15677759]

6. Cayabyab M, Hinuma S, Farzan M, Choe H, Fukusumi S, Kitada C, Nishizawa N, Hosoya M, Nishimura O, Messele T et al.. (2000) Apelin, the natural ligand of the orphan seven-transmembrane receptor APJ, inhibits human immunodeficiency virus type 1 entry. J Virol, 74 (24): 11972-6. [PMID:11090199]

7. Charo DN, Ho M, Fajardo G, Kawana M, Kundu RK, Sheikh AY, Finsterbach TP, Leeper NJ, Ernst KV, Chen MM et al.. (2009) Endogenous regulation of cardiovascular function by apelin-APJ. Am J Physiol Heart Circ Physiol, 297 (5): H1904-13. [PMID:19767528]

8. Cheng X, Cheng XS, Pang CC. (2003) Venous dilator effect of apelin, an endogenous peptide ligand for the orphan APJ receptor, in conscious rats. Eur J Pharmacol, 470 (3): 171-5. [PMID:12798955]

9. Chun HJ, Ali ZA, Kojima Y, Kundu RK, Sheikh AY, Agrawal R, Zheng L, Leeper NJ, Pearl NE, Patterson AJ et al.. (2008) Apelin signaling antagonizes Ang II effects in mouse models of atherosclerosis. J Clin Invest, 118 (10): 3343-54. [PMID:18769630]

10. Cox CM, D'Agostino SL, Miller MK, Heimark RL, Krieg PA. (2006) Apelin, the ligand for the endothelial G-protein-coupled receptor, APJ, is a potent angiogenic factor required for normal vascular development of the frog embryo. Dev Biol, 296 (1): 177-89. [PMID:16750822]

11. D'Aniello C, Lonardo E, Iaconis S, Guardiola O, Liguoro AM, Liguori GL, Autiero M, Carmeliet P, Minchiotti G. (2009) G protein-coupled receptor APJ and its ligand apelin act downstream of Cripto to specify embryonic stem cells toward the cardiac lineage through extracellular signal-regulated kinase/p70S6 kinase signaling pathway. Circ Res, 105 (3): 231-8. [PMID:19574549]

12. Dai T, Ramirez-Correa G, Gao WD. (2006) Apelin increases contractility in failing cardiac muscle. Eur J Pharmacol, 553 (1-3): 222-8. [PMID:17055480]

13. Daviaud D, Boucher J, Gesta S, Dray C, Guigne C, Quilliot D, Ayav A, Ziegler O, Carpene C, Saulnier-Blache JS et al.. (2006) TNFalpha up-regulates apelin expression in human and mouse adipose tissue. FASEB J, 20 (9): 1528-30. [PMID:16723381]

14. De Mota N, Reaux-Le Goazigo A, El Messari S, Chartrel N, Roesch D, Dujardin C, Kordon C, Vaudry H, Moos F, Llorens-Cortes C. (2004) Apelin, a potent diuretic neuropeptide counteracting vasopressin actions through inhibition of vasopressin neuron activity and vasopressin release. Proc Natl Acad Sci USA, 101 (28): 10464-9. [PMID:15231996]

15. El Messari S, Iturrioz X, Fassot C, De Mota N, Roesch D, Llorens-Cortes C. (2004) Functional dissociation of apelin receptor signaling and endocytosis: implications for the effects of apelin on arterial blood pressure. J Neurochem, 90 (6): 1290-301. [PMID:15341513]

16. Fan X, Zhou N, Zhang X, Mukhtar M, Lu Z, Fang J, DuBois GC, Pomerantz RJ. (2003) Structural and functional study of the apelin-13 peptide, an endogenous ligand of the HIV-1 coreceptor, APJ. Biochemistry, 42 (34): 10163-8. [PMID:12939143]

17. Farkasfalvi K, Stagg MA, Coppen SR, Siedlecka U, Lee J, Soppa GK, Marczin N, Szokodi I, Yacoub MH, Terracciano CM. (2007) Direct effects of apelin on cardiomyocyte contractility and electrophysiology. Biochem Biophys Res Commun, 357 (4): 889-95. [PMID:17466269]

18. Glassford AJ, Yue P, Sheikh AY, Chun HJ, Zarafshar S, Chan DA, Reaven GM, Quertermous T, Tsao PS. (2007) HIF-1 regulates hypoxia- and insulin-induced expression of apelin in adipocytes. Am J Physiol Endocrinol Metab, 293 (6): E1590-6. [PMID:17878221]

19. Habata Y, Fujii R, Hosoya M, Fukusumi S, Kawamata Y, Hinuma S, Kitada C, Nishizawa N, Murosaki S, Kurokawa T et al.. (1999) Apelin, the natural ligand of the orphan receptor APJ, is abundantly secreted in the colostrum. Biochim Biophys Acta, 1452 (1): 25-35. [PMID:10525157]

20. Hamada J, Kimura J, Ishida J, Kohda T, Morishita S, Ichihara S, Fukamizu A. (2008) Evaluation of novel cyclic analogues of apelin. Int J Mol Med, 22 (4): 547-52. [PMID:18813863]

21. Higuchi K, Masaki T, Gotoh K, Chiba S, Katsuragi I, Tanaka K, Kakuma T, Yoshimatsu H. (2007) Apelin, an APJ receptor ligand, regulates body adiposity and favors the messenger ribonucleic acid expression of uncoupling proteins in mice. Endocrinology, 148 (6): 2690-7. [PMID:17347313]

22. Hosoya M, Kawamata Y, Fukusumi S, Fujii R, Habata Y, Hinuma S, Kitada C, Honda S, Kurokawa T, Onda H et al.. (2000) Molecular and functional characteristics of APJ. Tissue distribution of mRNA and interaction with the endogenous ligand apelin. J Biol Chem, 275 (28): 21061-7. [PMID:10777510]

23. Hus-Citharel A, Bouby N, Frugière A, Bodineau L, Gasc JM, Llorens-Cortes C. (2008) Effect of apelin on glomerular hemodynamic function in the rat kidney. Kidney Int, 74 (4): 486-94. [PMID:18509323]

24. Ishida J, Hashimoto T, Hashimoto Y, Nishiwaki S, Iguchi T, Harada S, Sugaya T, Matsuzaki H, Yamamoto R, Shiota N et al.. (2004) Regulatory roles for APJ, a seven-transmembrane receptor related to angiotensin-type 1 receptor in blood pressure in vivo. J Biol Chem, 279 (25): 26274-9. [PMID:15087458]

25. Iturrioz X, Alvear-Perez R, De Mota N, Franchet C, Guillier F, Leroux V, Dabire H, Le Jouan M, Chabane H, Gerbier R et al.. (2010) Identification and pharmacological properties of E339-3D6, the first nonpeptidic apelin receptor agonist. FASEB J, 24 (5): 1506-17. [PMID:20040517]

26. Japp AG, Cruden NL, Amer DA, Li VK, Goudie EB, Johnston NR, Sharma S, Neilson I, Webb DJ, Megson IL et al.. (2008) Vascular effects of apelin in vivo in man. J Am Coll Cardiol, 52 (11): 908-13. [PMID:18772060]

27. Jia YX, Pan CS, Zhang J, Geng B, Zhao J, Gerns H, Yang J, Chang JK, Tang CS, Qi YF. (2006) Apelin protects myocardial injury induced by isoproterenol in rats. Regul Pept, 133 (1-3): 147-54. [PMID:16278022]

28. Kasai A, Shintani N, Kato H, Matsuda S, Gomi F, Haba R, Hashimoto H, Kakuda M, Tano Y, Baba A. (2008) Retardation of retinal vascular development in apelin-deficient mice. Arterioscler Thromb Vasc Biol, 28 (10): 1717-22. [PMID:18599802]

29. Kasai A, Shintani N, Oda M, Kakuda M, Hashimoto H, Matsuda T, Hinuma S, Baba A. (2004) Apelin is a novel angiogenic factor in retinal endothelial cells. Biochem Biophys Res Commun, 325 (2): 395-400. [PMID:15530405]

30. Katugampola SD, Maguire JJ, Matthewson SR, Davenport AP. (2001) [(125)I]-(Pyr(1))Apelin-13 is a novel radioligand for localizing the APJ orphan receptor in human and rat tissues with evidence for a vasoconstrictor role in man. Br J Pharmacol, 132 (6): 1255-60. [PMID:11250876]

31. Kawamata Y, Habata Y, Fukusumi S, Hosoya M, Fujii R, Hinuma S, Nishizawa N, Kitada C, Onda H, Nishimura O, Fujino M. (2001) Molecular properties of apelin: tissue distribution and receptor binding. Biochim Biophys Acta, 1538: 162-171. [PMID:11336787]

32. Khan P, Maloney PR, Hedrick M, Gosalia P, Milewski M, Li L, Roth GP, Sergienko E, Suyama E, Sugarman E et al.. (2011) Functional Agonists of the Apelin (APJ) Receptor. Probe Reports from the NIH Molecular Libraries Program,. [PMID:22834038]

33. Kidoya H, Ueno M, Yamada Y, Mochizuki N, Nakata M, Yano T, Fujii R, Takakura N. (2008) Spatial and temporal role of the apelin/APJ system in the caliber size regulation of blood vessels during angiogenesis. EMBO J, 27 (3): 522-34. [PMID:18200044]

34. Kleinz MJ, Davenport AP. (2005) Emerging roles of apelin in biology and medicine. Pharmacol Ther, 107 (2): 198-211. [PMID:15907343]

35. Kleinz MJ, Skepper JN, Davenport AP. (2005) Immunocytochemical localisation of the apelin receptor, APJ, to human cardiomyocytes, vascular smooth muscle and endothelial cells. Regul Pept, 126 (3): 233-40. [PMID:15664671]

36. Kralisch S, Lossner U, Bluher M, Paschke R, Stumvoll M, Fasshauer M. (2007) Growth hormone induces apelin mRNA expression and secretion in mouse 3T3-L1 adipocytes. Regul Pept, 139 (1-3): 84-9. [PMID:17126924]

37. Kunduzova O, Alet N, Delesque-Touchard N, Millet L, Castan-Laurell I, Muller C, Dray C, Schaeffer P, Herault JP, Savi P et al.. (2008) Apelin/APJ signaling system: a potential link between adipose tissue and endothelial angiogenic processes. FASEB J, 22 (12): 4146-53. [PMID:18708591]

38. Lee DK, Cheng R, Nguyen T, Fan T, Kariyawasam AP, Liu Y, Osmond DH, George SR, O'Dowd BF. (2000) Characterization of apelin, the ligand for the APJ receptor. J Neurochem, 74 (1): 34-41. [PMID:10617103]

39. Lee DK, Saldivia VR, Nguyen T, Cheng R, George SR, O'Dowd BF. (2005) Modification of the terminal residue of apelin-13 antagonizes its hypotensive action. Endocrinology, 146 (1): 231-6. [PMID:15486224]

40. Li F, Li L, Qin X, Pan W, Feng F, Chen F, Zhu B, Liao D, Tanowitz H, Albanese C et al.. (2008) Apelin-induced vascular smooth muscle cell proliferation: the regulation of cyclin D1. Front Biosci, 13: 3786-92. [PMID:18508473]

41. Macaluso NJ, Pitkin SL, Maguire JJ, Davenport AP, Glen RC. (2011) Discovery of a competitive apelin receptor (APJ) antagonist. ChemMedChem, 6 (6): 1017-23. [PMID:21560248]

42. Maguire JJ, Kleinz MJ, Pitkin SL, Davenport AP. (2009) [Pyr1]apelin-13 identified as the predominant apelin isoform in the human heart: vasoactive mechanisms and inotropic action in disease. Hypertension, 54 (3): 598-604. [PMID:19597036]

43. Maloney PR, Khan P, Hedrick M, Gosalia P, Milewski M, Li L, Roth GP, Sergienko E, Suyama E, Sugarman E et al.. (2012) Discovery of 4-oxo-6-((pyrimidin-2-ylthio)methyl)-4H-pyran-3-yl 4-nitrobenzoate (ML221) as a functional antagonist of the apelin (APJ) receptor. Bioorg Med Chem Lett, 22 (21): 6656-60. [PMID:23010269]

44. Masri B, Lahlou H, Mazarguil H, Knibiehler B, Audigier Y. (2002) Apelin (65-77) activates extracellular signal-regulated kinases via a PTX-sensitive G protein. Biochem Biophys Res Commun, 290 (1): 539-45. [PMID:11779205]

45. Masri B, Morin N, Cornu M, Knibiehler B, Audigier Y. (2004) Apelin (65-77) activates p70 S6 kinase and is mitogenic for umbilical endothelial cells. FASEB J, 18 (15): 1909-11. [PMID:15385434]

46. Mazzucotelli A, Ribet C, Castan-Laurell I, Daviaud D, Guigné C, Langin D, Valet P. (2008) The transcriptional co-activator PGC-1alpha up regulates apelin in human and mouse adipocytes. Regul Pept, 150 (1-3): 33-7. [PMID:18501443]

47. Medhurst AD, Jennings CA, Robbins MJ, Davis RP, Ellis C, Winborn KY, Lawrie KW, Hervieu G, Riley G, Bolaky JE, Herrity NC, Murdock P, Darker JG. (2003) Pharmacological and immunohistochemical characterization of the APJ receptor and its endogenous ligand apelin. J Neurochem, 84: 1162-1172. [PMID:12603839]

48. Miettinen KH, Magga J, Vuolteenaho O, Vanninen EJ, Punnonen KR, Ylitalo K, Tuomainen P, Peuhkurinen KJ. (2007) Utility of plasma apelin and other indices of cardiac dysfunction in the clinical assessment of patients with dilated cardiomyopathy. Regul Pept, 140 (3): 178-84. [PMID:17223209]

49. Mitra A, Katovich MJ, Mecca A, Rowland NE. (2006) Effects of central and peripheral injections of apelin on fluid intake and cardiovascular parameters in rats. Physiol Behav, 89 (2): 221-5. [PMID:16839572]

50. O'Carroll AM, Lolait SJ. (2003) Regulation of rat APJ receptor messenger ribonucleic acid expression in magnocellular neurones of the paraventricular and supraopric nuclei by osmotic stimuli. J Neuroendocrinol, 15 (7): 661-6. [PMID:12787050]

51. O'Dowd BF, Heiber M, Chan A, Heng HH, Tsui LC, Kennedy JL, Shi X, Petronis A, George SR, Nguyen T. (1993) A human gene that shows identity with the gene encoding the angiotensin receptor is located on chromosome 11. Gene, 136 (1-2): 355-60. [PMID:8294032]

52. O'Shea M, Hansen MJ, Tatemoto K, Morris MJ. (2003) Inhibitory effect of apelin-12 on nocturnal food intake in the rat. Nutr Neurosci, 6 (3): 163-7. [PMID:12793520]

53. Pitkin SL, Maguire JJ, Bonner TI, Davenport AP. (2010) International Union of Basic and Clinical Pharmacology. LXXIV. Apelin receptor nomenclature, distribution, pharmacology, and function. Pharmacol Rev, 62 (3): 331-42. [PMID:20605969]

54. Read C, Nyimanu D, Williams TL, Huggins DJ, Sulentic P, Macrae RGC, Yang P, Glen RC, Maguire JJ, Davenport AP. (2019) International Union of Basic and Clinical Pharmacology. CVII. Structure and Pharmacology of the Apelin Receptor with a Recommendation that Elabela/Toddler Is a Second Endogenous Peptide Ligand. Pharmacol Rev, 71 (4): 467-502. [PMID:31492821]

55. Reaux A, De Mota N, Skultetyova I, Lenkei Z, El Messari S, Gallatz K, Corvol P, Palkovits M, Llorens-Cortès C. (2001) Physiological role of a novel neuropeptide, apelin, and its receptor in the rat brain. J Neurochem, 77 (4): 1085-96. [PMID:11359874]

56. Reaux-Le Goazigo A, Morinville A, Burlet A, Llorens-Cortes C, Beaudet A. (2004) Dehydration-induced cross-regulation of apelin and vasopressin immunoreactivity levels in magnocellular hypothalamic neurons. Endocrinology, 145 (9): 4392-400. [PMID:15166125]

57. Roberts EM, Newson MJ, Pope GR, Landgraf R, Lolait SJ, O'Carroll AM. (2009) Abnormal fluid homeostasis in apelin receptor knockout mice. J Endocrinol, 202 (3): 453-62. [PMID:19578099]

58. Roberts EM, Pope GR, Newson MJ, Landgraf R, Lolait SJ, O'Carroll AM. (2010) Stimulus-specific neuroendocrine responses to osmotic challenges in apelin receptor knockout mice. J Neuroendocrinol, 22 (4): 301-8. [PMID:20136689]

59. Salcedo A, Garijo J, Monge L, Fernández N, Luis García-Villalón A, Sánchez Turrión V, Cuervas-Mons V, Diéguez G. (2007) Apelin effects in human splanchnic arteries. Role of nitric oxide and prostanoids. Regul Pept, 144 (1-3): 50-5. [PMID:17628718]

60. Sunter D, Hewson AK, Dickson SL. (2003) Intracerebroventricular injection of apelin-13 reduces food intake in the rat. Neurosci Lett, 353 (1): 1-4. [PMID:14642423]

61. Susaki E, Wang G, Cao G, Wang HQ, Englander EW, Greeley Jr GH. (2005) Apelin cells in the rat stomach. Regul Pept, 129 (1-3): 37-41. [PMID:15927696]

62. Szokodi I, Tavi P, Földes G, Voutilainen-Myllylä S, Ilves M, Tokola H, Pikkarainen S, Piuhola J, Rysä J, Tóth M et al.. (2002) Apelin, the novel endogenous ligand of the orphan receptor APJ, regulates cardiac contractility. Circ Res, 91 (5): 434-40. [PMID:12215493]

63. Sörhede Winzell M, Magnusson C, Ahrén B. (2005) The apj receptor is expressed in pancreatic islets and its ligand, apelin, inhibits insulin secretion in mice. Regul Pept, 131 (1-3): 12-7. [PMID:15970338]

64. Taheri S, Murphy K, Cohen M, Sujkovic E, Kennedy A, Dhillo W, Dakin C, Sajedi A, Ghatei M, Bloom S. (2002) The effects of centrally administered apelin-13 on food intake, water intake and pituitary hormone release in rats. Biochem Biophys Res Commun, 291 (5): 1208-12. [PMID:11883945]

65. Tatemoto K, Hosoya M, Habata Y, Fujii R, Kakegawa T, Zou MX, Kawamata Y, Fukusumi S, Hinuma S, Kitada C et al.. (1998) Isolation and characterization of a novel endogenous peptide ligand for the human APJ receptor. Biochem Biophys Res Commun, 251 (2): 471-6. [PMID:9792798]

66. Tatemoto K, Takayama K, Zou MX, Kumaki I, Zhang W, Kumano K, Fujimiya M. (2001) The novel peptide apelin lowers blood pressure via a nitric oxide-dependent mechanism. Regul Pept, 99 (2-3): 87-92. [PMID:11384769]

67. Valle A, Hoggard N, Adams AC, Roca P, Speakman JR. (2008) Chronic central administration of apelin-13 over 10 days increases food intake, body weight, locomotor activity and body temperature in C57BL/6 mice. J Neuroendocrinol, 20 (1): 79-84. [PMID:18081555]

68. Van Coillie E, Proost P, Van Aelst I, Struyf S, Polfliet M, De Meester I, Harvey DJ, Van Damme J, Opdenakker G. (1998) Functional comparison of two human monocyte chemotactic protein-2 isoforms, role of the amino-terminal pyroglutamic acid and processing by CD26/dipeptidyl peptidase IV. Biochemistry, 37 (36): 12672-80. [PMID:9730840]

69. Vickers C, Hales P, Kaushik V, Dick L, Gavin J, Tang J, Godbout K, Parsons T, Baronas E, Hsieh F et al.. (2002) Hydrolysis of biological peptides by human angiotensin-converting enzyme-related carboxypeptidase. J Biol Chem, 277 (17): 14838-43. [PMID:11815627]

70. Wei L, Hou X, Tatemoto K. (2005) Regulation of apelin mRNA expression by insulin and glucocorticoids in mouse 3T3-L1 adipocytes. Regul Pept, 132 (1-3): 27-32. [PMID:16137778]

71. Zhou N, Fan X, Mukhtar M, Fang J, Patel CA, DuBois GC, Pomerantz RJ. (2003) Cell-cell fusion and internalization of the CNS-based, HIV-1 co-receptor, APJ. Virology, 307 (1): 22-36. [PMID:12667811]

72. Zou MX, Liu HY, Haraguchi Y, Soda Y, Tatemoto K, Hoshino H. (2000) Apelin peptides block the entry of human immunodeficiency virus (HIV). FEBS Lett, 473 (1): 15-8. [PMID:10802050]