Paragraph: Apelin is an inotropic and cardioprotective peptide that exhibits beneficial effects through activation of the APJ receptor in the pathology of cardiovascular diseases. Apelin induces the expression of angiotensin-converting enzyme 2 (ACE2) in failing hearts, thereby improving heart function in an angiotensin 1-7-dependent manner. Whether apelin antagonizes the over-activation of the renin-angiotensin system in the heart remains elusive. In this study we show that the detrimental effects of angiotensin II (Ang II) were exacerbated in the hearts of aged apelin-gene-deficient mice. Ang II-mediated cardiac dysfunction and hypertrophy were augmented in apelin knockout mice. The loss of apelin increased the ratio of angiotensin-converting enzyme (ACE) to ACE2 expression in the Ang II-stressed hearts, and Ang II-induced cardiac fibrosis was markedly enhanced in apelin knockout mice. mRNA expression of pro-fibrotic genes, such as transforming growth-factor beta (TGF-β) signaling, were significantly upregulated in apelin knockout hearts. Consistently, treatment with the ACE-inhibitor Captopril decreased cardiac contractility in apelin knockout mice. In vitro, apelin ameliorated Ang II-induced TGF-β expression in primary cardiomyocytes, accompanied with reduced hypertrophy. These results provide direct evidence that endogenous apelin plays a crucial role in suppressing Ang II-induced cardiac dysfunction and pathological remodeling. Int. J. Mol. Sci. 2019, 20, 239 2 of 12 dysfunctions [8, 10, 11] . By contrast, APJ was identified as a dual receptor to mediate cardiac hypertrophy [12, 13] via β-arrestin signaling [14, 15] . Thus, the precise role of endogenous apelin signaling in heart function remains elusive. Cardiac hypertrophy is an adaptive response to maintain cardiac function in the event of an increased workload, although prolonged cardiac hypertrophy can lead to heart failure via the ischemia of cardiomyocytes [16, 17] . In addition, cardiac fibrosis adversely increases tissue stiffness and impairs ventricular function. Consistently, cardiac hypertrophy and the development of fibrosis correlate with the deterioration of myocardial function in heart failure [18] [19] [20] . Thus, cardiac hypertrophy and fibrosis are key features of heart failure, and apelin's inotropic and protective roles in the heart are attractive as a potential therapeutic candidate for treating heart failure. Apelin has also been identified as a substrate for angiotensin-converting enzyme 2 (ACE2), which cleaves apelin with similar efficiency as its primary substrate angiotensin II (Ang II) [21] [22] [23] . ACE2 is a negative regulator of the renin-angiotensin system (RAS) that inactivates Ang II by converting it to angiotensin 1-7 (Ang 1-7) [21] [22] [23] , whereas angiotensin-converting enzyme 1 (or ACE1) activates RAS by converting Ang I to Ang II. ACE2 was also identified as a regulator of heart failure [24-26], diabetic nephropathy [27, 28] , acute lung failure [29, 30] , and the SARS (severe acute respiratory syndrome) coronavirus receptor [31, 32] . Apelin-APJ signaling has been shown to counteract the effects of Ang II on AT1R in defined physiological and pathophysiological settings. For instance, in hypertensive rat models, the continuous infusion of Ang II downregulates apelin and APJ expression [33] . We have shown that Apelin upregulates ACE2 expression. Apelin-induced ACE2 downregulates Ang II signaling while enhancing Ang 1-7-Mas receptor signaling in aged or failing hearts [34] . On the other hand, it has been suggested that the counter-balancing effects of the apelin-APJ axis on Ang II-AT1R signaling are mediated by receptor dimerization. The APJ receptor physically interacts with AT1 receptors, and exogenous apelin negatively regulates Ang II-AT1R signaling in in vitro cell culture systems and in an in vivo atherosclerosis model [35] . However, whether endogenous apelin counteracts the over-activation of RAS in the heart remains unexplored. In this study, we aimed to determine the effects of endogenous apelin on Ang II-induced pathology in the heart. We showed that Ang II-mediated cardiac dysfunction, hypertrophy, and fibrosis were augmented in apelin knockout (Apelin KO) mice. 
 Title: Loss of Apelin Augments Angiotensin II-Induced Cardiac Dysfunction and Pathological Remodeling
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