ACE2/Ang 1-7 axis: A critical regulator of epicardial adipose tissue inflammation and cardiac dysfunction in obesity

ABSTRACT

Obesity is characterized by an excessive fat accumulation in adipose tissues leading to weight gain and is increasing in prevalence and is strongly associated with metabolic and cardiovascular disorders. The renin-angiotensin system (RAS) has emerged as a key pathogenic mechanism for these disorders; activated RAS and angiotensin (Ang) II production results in worsening of cardiovascular diseases and angiotensin converting enzyme 2 (ACE2) negatively regulates RAS by metabolizing Ang II into Ang 1-7. ACE2 is expressed in the adipocytes and its expression is upregulated in response to high fat diet induced obesity in mice. Loss of ACE2 results in heart failure with preserved ejection fraction which is mediated in part by epicardial adipose tissue inflammation. Angiotensin 1-7 reduces the obesity associated cardiac dysfunction predominantly via its role in adiponectin expression and attenuation of epicardial adipose tissue inflammation. Human heart disease is also linked with inflammed epicardial adipose tissue. Here, we discuss the important interpretation of the novel of ACE2/Ang 1-7 pathway in obesity associated cardiac dysfunction.

KEYWORDS:

• Angiotensin converting enzyme 2
• Angiotensin 1-7
• epicardial adipose tissue
• obesity
• renin-angiotensin system

Introduction

Obesity is characterized by the excessive fat accumulation in the adipose tissues across the body leading to android type (males; central distribution) and gynoid type (females; gluteal/femoral accumulation) fat distributions. It is the most common nutritional disorder in industrialized countries and is associated with an increased mortality and morbidity of cardiovascular diseases (CVD). In fact, obesity itself is an independent risk factor for the development of heart failure with preserved ejection fraction (HFPEF) independent of other co-morbid conditions.^1-4 Human heart is surrounded by 2 different depots of adipose tissue; epicardial adipose tissue (EAT) and pericardial adipose tissue. Anatomically, epicardial and pericardial adipose tissue are very distinct in terms of their locations; while EAT is located between the myocardium and the visceral layer of pericardium, pericardial adipose tissue is located between visceral and parietal pericardium. However, due to its anatomical proximity to the myocardium and the lack of an absolute anatomical boundary between the EAT and the heart, EAT likely has a dominant role in cardiovascular biology.⁵ Furthermore, the location of EAT in respect to heart eg. peri-atrial, peri-ventricular, determines the transcriptomic signature of the EAT.⁶ The importance of EAT in the cardiac disease is illustrated by the association between EAT inflammation and cardiac dysfunction.^7,8

Renin-angiotensin system and its role in adipose tissue inflammation

The renin-angiotensin system (RAS) is a family of peptides with endocrine and paracrine functions. Angiotensinogen, the precursor of RAS, is converted to Ang I and Ang II by the enzymes renin and angiotensin converting enzyme (ACE) respectively, where Ang II is the active peptide which is involved in the regulations of blood pressure, fluid balance etc. ACE2, a newly discovered member of the RAS family,^9,10 is a monocarboxypeptidase which degrades (and inactivates) Ang II into Ang 1-7, a ligand for Mas receptor (MasR)¹¹ showing physiological antagonism to Ang II effects. Thus ACE2/Ang 1–7/MasR axis is a negative regulator of ACE/Ang II/AT1 receptor (AT1R) axis. Angiotensinogen is expressed and constitutively secreted from adipose tissues by mature adipocytes in both humans and experimental animal models.¹² Similarly, other components of RAS, including AT1R, ACE2 and MasR are also expressed in the white and brown adipose tissues.^13,14 The angiotensin type 1 (AT1R) and type 2 (AT2R) receptors may mediate the effect of Ang II and cause upregulation of adipose tissue lipogenesis (mediated via AT2R) and downregulation of lipolysis (mediated via AT1R).^15,16

Activated RAS, and resultant production of Ang II is critically involved in obesity associated inflammation.^17,18 Macrophages express the AT1 receptors and their activation by Ang II worsens the extent and complexity of end-organ inflammation by macrophage polarization toward the classically activated or CD11c⁺ M1 phenotype.¹⁷ Macrophages also express the MasR transcripts and their activation by Ang 1–7 treatment decreases LPS-induced expression of pro-inflammatory cytokines.¹⁹ Thus Ang II/AT1R activation is involved in the macrophage polarization resulting in increased inflammation, whereas Ang 1–7 decreases expression of the pro-inflammatory cytokines. Importantly, ACE2 expression in the bone marrow cells regulate macrophage polarization and adipose tissue inflammation,²⁰ providing further evidence for a crucial role of ACE2 in adipose tissue inflammation. In addition to the effects on adipose tissue and macrophages, we have also shown a promising cardioprotective role of ACE2/Ang 1–7 axis in murine models of different cardiac diseases.^21-26

In light of these previous reports, and our observed upregulation of ACE2 in EAT, we hypothesized that the ACE2/Ang 1–7 axis plays a critical role in obesity-associated EAT inflammation leading to impaired cardiac metabolism and heart failure with preserved ejection fraction.²⁷ We showed the critical role of ACE2 and Ang 1–7 in the regulation of obesity associated heart failure.²⁷ In this study, high fat diet feeding-induced obesity (DIO) in ACE2-null (ACE2KO) mice resulted in increased glucose intolerance in spite of reduced obesity. In addition, DIO worsened the EAT inflammation and the macrophage polarization leading to increased resident CD11c⁺ adipose tissue macrophages (ATM) and increased gene expressions of pro-inflammatory cytokines, TNF-α, IL-1β, IL-6 and iNOS (Fig. 1). In light of the previously illustrated fundamental role of macrophages in adipose tissue inflammation and regulation of insulin sensitivity,²⁸ our data provides an essential role of ACE2 in negative regulation obesity-induced EAT inflammation. Epicardial adipose tissue inflammation in obesity is associated with the progression of cardiac dysfunction.^29-32 Increased EAT inflammation in obese-ACE2KO mice also resulted in worsening of myocardial insulin resistance resulting in impaired cardiac metabolism, which was also associated with increased lipotoxicity, oxidative stress and impaired molecular signaling (Fig. 1).²⁷ Angiotensin 1–7 administration to obese-ACE2KO mice for 4 weeks attenuated the EAT inflammation, impaired cardiac metabolism, lipotoxicity, oxidative stress resulting in preserved cardiac function.³³ Ang 1–7 effects are predominantly mediated by the activation of its endogenous G-protein coupled receptor, Mas, which is widely expressed including adipocytes.^33,34 Importantly, in our study, Ang 1–7 administration for 4 weeks resulted in marked increase in myocardial adiponectin levels as assessed by Western blot and immunofluorescence analyses, in response to Ang 1–7 treatment,²⁷ which might be mediated via increased gene expression in response to Ang 1–7/MasR signaling or may be secondary to the anti-inflammatory effects of Ang 1–7/MasR. However, it is critical to assess the effect of Ang 1–7 on adiponectin gene/protein expression in the EAT. It is noteworthy that Ang 1–7/MasR activation in macrophages is known to mediate anti-inflammatory effects resulting in decreased expression of pro-inflammatory cytokines. Adiponectin is an anti-inflammatory adipokine, synthesized exclusively in the adipocytes. In addition to its classic endocrine effects on heart, adiponectin is also thought to have paracrine effects on the myocardium.^35,36 Due to the anatomical proximity between both tissues, increased myocardial adiponectin level in Ang 1–7 treated mice could have resulted from paracrine secretion from the EAT adipocytes. Moreover, the increased myocardial adiponectin levels could also be secreted from other adipose tissues leading to endocrine effects on the heart and assessment of plasma adiponectin levels is essential to understand the endocrine vs paracrine effects of adiponectin. Based on our study, we conclude that ACE2 and Ang 1–7 are negative regulators of obesity associated EAT inflammation and cardiac dysfunction. Our results, however, raise important questions which should be addressed in future studies such as whether the ACE2/Ang 1–7 mediated anti-inflammatory effects are limited to EAT or include other adipose tissue depots. Indeed, the role of ACE2/Ang 1–7 in the peri-renal³⁷ as well as peri-vascular adipose tissue are particularly important.

Figure 1. Role of ACE2 and Ang 1–7 in obesity associated epicardial adipose tissue (EAT) inflammation and cardiac dysfunction. Obesity results in ACE2 upregulation in the adipose tissue of WT mice along with the greater increase in alternatively activated CD206⁺ macrophages resulting in mild EAT inflammation, cardiac insulin resistance and metabolic impairment leading to mild heart failure with preserved ejection fraction (HFPEF). Obesity in ACE2KO mice results in increased EAT inflammation, worsened cardiac insulin resistance, lipotoxicity and metabolic impairment leading to worsened HFPEF. Angiotensin 1–7 suppresses the EAT inflammation, increases myocardial adiponectin levels and attenuates HFPEF by suppressing the lipotoxicity and correcting the metabolic alterations. ACE2KO: ACE2-null; AMPK: AMP-activated protein kinase; Ang 1–7: Angiotensin 1–7; IL: interleukin; iNOS: inducible nitric oxide synthase; PDH: pyruvate dehydrogenase; PDK4: pyruvate dehydrogenase kinase-4; TNFα: tumor necrosis factor-α; WT: wildtype.

The use of human epicardial adipose tissue

Murine models are not ideal models to study EAT inflammation especially as it relates to human diseases. Human tissues are very different than mice in terms of the amount and localization of EAT. In mice, EAT is localized to atrioventricular groove, whereas the human heart has a much greater abundance of EAT and is nearly completely surrounded by EAT (Fig. 2).³⁸ Importantly, obese patients with heart failure (preserved or reduced ejection fraction) show increased deposition of EAT around the heart when compared with non-obese non-failing controls (Fig. 2). Therefore, we think the human tissues are the ultimate candidate to assess translational nature of the scientific findings, particularly the investigations involving EAT. Therefore, in addition to mice experiments, we also assessed the ACE2 levels and inflammatory status in the EAT obtained from non-failing control and obese patients with HFPEF, which essentially showed similar results validating our experimental findings.²⁷

Figure 2. Epicardial adipose tissue in human hearts. Representative anterior and posterior views of human explanted hearts as a non-failing control (NFC) (weight = 290 g), heart failure with preserved ejection fraction (HFPEF) (ejection fraction = 55%; weight = 590 g) and heart failure with reduced ejection fraction (HFREF) (ejection fraction = 23%; weight = 678 g) depicting the extensive epicardial adipose tissue. Scalebar indicates 1 cm.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

VBP is supported by Alberta Innovates-Health Solutions (AI-HS) Fellowship. We acknowledge funding support from Canadian Institutes of Health Research, Heart and Stroke Foundation and AI-HS.