Macrophage immunometabolism in perivascular adipose tissue

Perivascular adipose tissue (PVAT), a distinct integral layer of adipose tissue surrounding most conduit blood vessels, is a physiologically and metabolically active endocrine tissue that plays an important role in maintaining vascular homeostasis [1]. In healthy, lean states, PVAT may resemble brown adipose tissue (BAT) and play a protective role in regulating vascular metabolism and function. Conversely, in the setting of obesity and other cardiovascular risk factors, PVAT becomes dysfunctional, exhibiting a “white-like” phenotype and losing its thermogenic capacity, thus contributing to diseases such as atherosclerosis, aneurysms, and hypertension [2]. PVAT consists of various cell types including mature adipocytes, preadipocytes, mesenchymal stem/precursor cells, endothelial cells and inflammatory cells, the balance and phenotype of which govern the extent of PVAT inflammation. Immune cells infiltrating into PVAT can communicate with adipocytes and vascular cells (e.g. endothelial cells, smooth muscle cells) via releasing proinflammatory chemocytokines to disrupt vasoreactivity and promote vascular fibrosis and arterial stiffness, hallmarks in the pathogenesis of hypertension [3,4].

Macrophages contained with PVAT have been demonstrated to orchestrate inflammatory processes by releasing cytokines such as tumor necrosis factor alpha (TNFα), interleukin 6 (IL-6) and IL-1β, expression of which is dependent on activation of the Nod-like receptor family, pyrin domain-containing 3 (NLRP3) inflammasome. Macrophages also can produce reactive oxygen species (ROS) through activated NADPH oxidase 2 (NOX2). Furthermore, macrophages in PVAT play a regulatory role in T cell activation through antigen presentation, which is also associated with PVAT inflammation. Dysregulation of macrophage polarization between the proinflammatory M1 and anti-inflammatory M2 phenotypes promotes inflammation and vascular injury, and a higher number of M1 macrophages has been linked to the development of hypertension [5]. Metabolic shifting between glycolysis and mitochondrial oxidative phosphorylation has been implicated in macrophage polarization, as M1 macrophages mainly rely on glycolysis while M2 macrophages primarily rely on the tricarboxylic acid cycle and oxidative phosphorylation [6]. However, mechanisms that regulate metabolism in macrophages residing in PVAT, and how this in turn impacts PVAT function and vascular disease, are not fully understood.

In this issue of ATVB, Wei et al. report the metabolic role of sirtuin 3 (SIRT3) expressed in macrophages in protecting against PVAT inflammation in the context of hypertension [7]. SIRT proteins are a highly conserved family of nicotinamide adenine dinucleotide (NAD)-dependent deacetylases that regulate cellular metabolism, DNA repair, inflammatory signaling and redox state [8]. SIRT3, a mitochondrial sirtuin that is highly expressed in BAT, has gained recognition as a significant regulator of cardiovascular disease and oxidative stress [9]. Previously, the authors reported that SIRT3 levels were decreased in BAT under high salt (HS) intake, and global ablation of the SIRT3 gene in mice provoked a conversion of BAT to a white-like phenotype, which was mechanistically explained by enhanced acetylation of pyruvate dehydrogenase E1a (PDHA1) at lysine 83 (K83) [10]. In the present study, using myeloid-specific SIRT3 knockout mice and angiotensin-II (AngII) infusion model of hypertension, the authors report that myeloid SIRT3 gene deletion aggravated PVAT dysfunction, as demonstrated by increased macrophage infiltration and IL-1β secretion, along with conversion to a white-like phenotype (Figure). Mechanistically, SIRT3-deficient macrophages exhibited a metabolic shift from oxidative phosphorylation to glycolysis, which was achieved through increased acetylation and deactivation of PDHA1 at lysine 83. This metabolic shift in SIRT3-deficient macrophages led to increased lactate production, which augmented NLRP3 inflammasome activation and IL-1β secretion. Elegant mechanistic experiments using PVAT-derived adipocytes incubated with conditioned medium from SIRT3-deficient macrophages confirmed a role for macrophage IL-1β and NLRP3 in promoting brown adipocyte “whitening” and fibrosis. Taken together, these results suggest that SIRT3 protects against PVAT inflammation and fibrosis by shifting the macrophages’ metabolic flux away from glycolysis towards oxidative phosphorylation via deacetylation of PDHA1. This decrease in glycolytic flux prevents downstream NLRP3 inflammasome activation and IL-1β secretion, thereby preserving healthy PVAT function. The findings of the current study provide direct evidence that endogenous myeloid SIRT3 can modulate PVAT inflammation via metabolic programming. Furthermore, this study identifies an immunometabolic pathway linking macrophage SIRT3, glycolysis and NLRP3 inflammasome in PVAT. However, the PDHA1 acetylation/deacetylation modification presented in this study represents only one possible post-translational modification, as phosphorylation could also contribute to the metabolic effects of PDHA1. Additionally, a potential feedback mechanism in which adipocyte-secreted factors impact the macrophage secretome to regulate adipocyte metabolism may exist and warrants further exploration.

Figure.

Proposed regulatory role of myeloid Sirt1 in PVAT function. Myeloid deletion of Sirt3 exacerbates perivascular BAT inflammation in response to AngII. Sirt3 deficiency in macrophages increases acetylation of PDHA1 at lysine 83, leading to activation of NLRP3 inflammasome and IL-1β production. Impaired UCP-1 expression and mitochondrial oxygen consumption ability by IL-1β further promotes PVAT whitening and fibrosis.

Clinical data indicate that SIRT3 expression is negatively correlated with cardiovascular risk factors such as age, smoking and hypertension [11,12]. Dekalova et al. previously detected decreased SIRT3 levels and increased superoxide dismutase 2 (SOD2) acetylation in peripheral blood mononuclear cells of hypertensive patients [13]. They also demonstrated that global SIRT3 gene deletion in mice promoted hypertension and oxidative stress through acetylation and inactivation of SOD2 at specific lysine residues [14]. Furthermore, overexpression of SIRT3 protected against endothelial dysfunction and vascular oxidative stress, while also improving blood pressure in hypertension models [14]. These findings suggest that reduced SIRT3 expression or activity may promote the development of hypertension and vascular dysfunction not only in animals but also in humans. The present study further identifies myeloid SIRT3’s key function to maintain healthy PVAT in the context of hypertension. However, the phenotype of PVAT differs depending on species and anatomical location, and brown-like phenotypes are relatively rare in human PVAT [2]. Thus, the relevance of myeloid SIRT3 signaling pathways in human PVAT function remains to be determined. Furthermore, this study focused on thoracic PVAT only; the role of SIRT3 in abdominal PVAT or in other PVAT depots was not studied. In rodents, abdominal PVAT is highly responsive to high fat diet and becomes more white-like, whereas thoracic PVAT is relatively resistant to these changes [15]. Finally, data from the present study showed that myeloid-specific SIRT3 deletion did not affect angiotensin-II-induced hypertension in mice, raising questions regarding the functional role of SIRT3 expression in PVAT in blood pressure regulation. Gain-of-function approaches to investigate the impact of SIRT3 overexpression in PVAT were also not applied, so the clinical-translational potential of the key findings presented in this study remain unclear.

With advances in vascular and adipose biology research, the molecular and cellular mechanisms that regulate PVAT function in the setting of cardiometabolic diseases have begun to be revealed. This study by Wei et al. provides a new piece to the puzzle with regard to the role that myeloid SIRT3 plays in regulating macrophage immunometabolism and preserving PVAT function in hypertension.