Sirtuin 1 as a potential therapeutic target in pulmonary artery hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vascular system, primarily influencing small pulmonary arterioles [1]. PAH is defined as mean pulmonary artery pressure at least 25 mmHg and pulmonary capillary artery pressure 15 mmHg or less, which is identified by the presence of precapillary pulmonary hypertension in the absence of lung and other diseases (including chronic thromboembolic pulmonary hypertension and other rare diseases) related to precapillary pulmonary hypertension [2]. As shown in Table 1, according to the hemodynamic and pathophysiologic characteristics, there are five major subgroups of PAH [1–5]. The majority of PAH cases are cases of idiopathic PAH (i.e. the PAH is because of unknown reasons) and secondary pulmonary hypertension disease. The latter is because of heritable/familial factors or associated with other medical conditions such as connective tissue disease, congenital heart disease, portal hypertension (liver disease), HIV infection, and sickle cell disease [1,4]. PAH has also been associated with toxin and drug exposures [5].TABLE 1: Updated clinical classification of pulmonary hypertension from the Fifth World Symposium at Nice, FranceIn all forms of pulmonary hypertension, it is well known that the progressive vasculopathy is complex with a broad imbalance of vasodilators (i.e. prostacyclin, nitric oxide, endothelin-1, and thromboxane A2) [1]. This condition likely precedes the development of secondary aberrant pulmonary artery cell proliferation [1]. In addition to the dysregulation of vasodilators and vasoconstrictor factors, there is a parallel induction of major oxidase enzymes (i.e. the nicotinamide adenine dinucleotide phosphate oxidase family, xanthine and aldehyde oxidase, an uncoupled endothelial nitric oxygen synthase, and dysfunctional mitochondria), which can produce reactive oxygen species (superoxide, hydrogen superoxide, and peroxynitrite) to stimulate the proliferation of pulmonary arterial smooth muscle cells (PASMCs) and endothelial cells (PAECs), and this vascular remodeling along with in situ thrombosis leads to a progressive narrowing of the blood vessels and elevated pulmonary artery resistance and pressure [6–8]. Finally, these changes can result in a reduction of cardiac output, heart failure, and ultimately death. The enormous progress in the understanding, diagnosis, and management of PAH patients has been covered by recent comprehensive reviews [4,9]. Many attempts – including pharmacological approaches (e.g. endothelin antagonists, phosphodiesterase-5 inhibitors, prostacycline/its analogs, and now a soluble guanylate cyclase activator, and more) and pulmonary endovascular balloon dilation/stent implantation – to prevent progressive alterations in pulmonary artery resistance and pulmonary artery pressure in animals and patients with PAH have been made that are directed at reversing pulmonary artery negative remodeling and fibrosis. In addition, basic and clinical potential therapeutic challenges are largely focused on targeting PASMC proliferation and PAEC dysfunction. Despite the advances that have been achieved, no potential nonpharmacological/pharmacological therapies are currently in clinical use that adequately reverse pulmonary artery negative remodeling, which would lead to improvements in the symptoms and mortality of PAH patients. Silent information regulator 1 (Sirt1, also known as Sirtuin 1) is a member of the sirtuin family of class III histone deacetylases [10]. Sirt1 is involved in gene silencing, differentiation, cell survival, and longevity [11]. Nicotinamide adenine dinucleotide (NAD+) is required to exhibit Sirt1 family enzyme activity [12]. Genetic and pharmacological Sirt1 activation prevented diabetes and metabolic damage in mice in response to various stresses [13–15]. Conversely, Sirt1 knockout mice exhibit abnormal sodium channel deacetylation-mediated cardiac electric ability [16]. A growing body of biological evidence has demonstrated that Sirt1 regulates various cellular biological events (i.e. proliferation, migration, and apoptosis) [17–20], suggesting its novel biological activity as a promising therapeutic drug candidate in the management of PAH in humans and animals. However, the exact roles of Sirt1 in PASMC proliferation and PAH initiation and progression are largely unknown. In this issue of the Journal of Hypertension, Zurlo et al. report their intriguing observation that modification of Sirt1 activity influences PASMC proliferation and pulmonary artery development in a mouse model of hypoxia-induced experimental PAH [21], and they provide three pieces of new information. First, PASMCs from patients with PAH displayed decreased protein levels of mitochondrial mass markers (i.e. voltage-dependent anion channel and citrate synthase) and gene levels of peroxisome proliferator-activated receptor-alpha (PPAR-α), PPAR-gamma co-activator-alpha (PGC-1α), and superoxide dismutase-2 (SOD2) without altered Sirt1 protein expression and with increased protein levels of glucose transporter 4 (GLU4) and acetylated histone H1. Second, Zurlo et al. report that pharmacological and genetic inhibitions of Sirt1 promoted cell proliferation via the alteration of the acetylation/deacetylation balance, as in the targeted histone H1 and forkhead box protein O1 (FOXO1) in rat and human PASMCs in response to platelet-derived growth factor (PDGF); these effects were mitigated by Sirt1 activator (Stac-3). Finally, Zurlo et al. note that Sirt1 deletion accelerated pulmonary artery remodeling and right ventricular remodeling in mice under hypoxia conditions. PASMC proliferation plays a vital role in various aspects of progressive vascular remodeling [1–3]. To the best of our knowledge, the Zurlo et al. study is the first to provide direct evidence of the role of Sirt1 in PASMC proliferation and PAH development [21]. In vitro, increased levels of acetylated histone H1 and FOXO1 proteins were observed in rat and human PASMCs treated by a Sirt1 inhibitor, and these changes were mimicked by Sirt1 silencing [21]. Both biological approaches induced the activation of PASMC proliferation, and the human PAH PASMCs had increased levels of acetylated histone H1 compared with the control cells. Sirt1 regulates the function of transcription factors and cofactors, including MyoD, p53, PGC1, and the FoxO family of transcription factors [10], through deacetylation. The acetylation/deacetylation imbalance at the histone H1 and FOXO1 protein levels can facilitate in cell proliferation [22]. Thus, Sirt1 modulate PASMC proliferation via the alteration of histone H1 and FOXO1 acetylation and deacetylation balance. This notion was further supported by the data of Sirt1 activator (Stac-3) experiments revealing that Sirt1 activation markedly mitigated targeted protein acetylation and PASMC proliferation [21]. In vivo, Sirt1-deficient mice exhibited more intense pulmonary artery remodeling as well as right ventricular hypertrophy and remodeling compared with their control littermates [21]. Conversely, Sirt1 genetic gain of function and pharmacological activation mitigated carotid artery neointimal formation and pulmonary artery remodeling in several animal models [17]. Collectively, the up-regulations of histone H1 and FOXO1 acetylation by Sirt1 inactivation could represent a common mechanism in the PASMC proliferation activation and PAH development in this experimental PAH model (Fig. 1).FIGURE 1: Schematic of the potential role of the Sirt1-mediated transcriptional factor acetylation/deacetylation balance in the pathogenesis of PAH. CS, citrate synthase; FOXO1, forkhead box protein O1; PAH, pulmonary arterial hypertension; PASMC, pulmonary artery smooth muscle; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator 1-alpha; PI, pharmacological inhibition; PPAR-α, peroxisome proliferator-activated receptor-alpha; Sirt1, sirtuin 1; SOD2, superoxide dismutase 2; TFAM, mitochondrial transcription factor A; VDAC, voltage-dependent anion channel.Another possible mechanism by which Sirt1 modulates PPAR-α/PGC-1α-mediated mitochondrial biogenesis in rat and human PASMCs was also examined by Zurlo et al.[21]. Here, human PAH PASMCs had decreased levels of not only voltage-dependent anion channels (VDACs) and citrate synthase but also PPAR-α and PGC-1α. The PPAR-α/PGC-1α axis regulates mitochondrial biogenesis and function in mammalian cells [23]. PPAR-α/PGC-1α signaling has been targeted in several studies of hyperproliferative angiogenesis and atherosclerosis under chronic psychological conditions [24–26]. In the Zurlo et al. study described in this issue of the Journal of Hypertension, Sirt1 inhibition suppressed the levels of PGC-1α protein as well as VDAC and citrate synthase proteins in vivo or/and in vitro, and these changes were ameliorated by Sirt1 activation. On the other hand, human PAH PASMCs exhibited lower levels of SOD2 protein compared with the control cells [21]. The antioxidant enzymes (SOD2 and TFAM) in cultured PASMCs were also sensitive to alteration by the Sirt1 inhibition and activation. The PPAR-α/PGC-1α signaling inactivation is associated with the imbalance of the oxidative and antioxidative enzyme abilities [25]. Although there was no direct evidence regarding the role of Sirt1 in mitochondrial oxidative stress and mitochondrial biogenesis in vivo and in vitro, Zurlo et al. proposed that Sirt1 acts as a key regulator of the protective actions of PPAR-α/PGC-1α-mediated oxidative stress production and mitochondrial damage/dysfunction to contribute to the amelioration of PASMC proliferation and pulmonary artery remodeling under these experimental PAH conditions (Fig. 1). Of course, the results presented by Zurlo et al.[21] are only the beginning of this exciting new direction for the therapeutic control of experimental PAH by the modification of Sirt1 activity. Given the potent antiproliferative effect of Sirt1, which is known as a class III histone deacetylase, several issues regarding Sirt1 as a promising therapeutic target must be investigated. In particular, the following must be assessed: the stabilities of Sirt1 gene and protein levels, aging-related and PAH type-related pathophysiological changes, genetic and nonpharmacological/pharmacological modifications of Sirt1 activity, the best delivery route of the Sirt1 gene, and the optimal duration of Sirt1-related therapy for the challenges in animals and patients with arteriosclerosis-based and/or thrombosis-based PAH. Zurlo and colleagues have focused on the role of Sirt1 in the proliferation of rat and human PASMCs. It was reported that Sirt1 regulates cell apoptosis in vivo and in vitro[10]. Thus, one critical limitation of the Zurlo et al. study is that the authors did not investigate whether Sirt1 regulates PASMC apoptosis via the alteration of histone H1 and FOXO1 deacetylation and PGC-1α expression in animal and human tissues. Another potential limitation is that in order to determine whether Sirt1 could be a promising therapeutic target for PAH treatment whenever using an animal model, a better approach would be to investigate the genetic gain of function and pharmacological activations of Sirt1 biological activity. In conclusion, in light of the findings suggesting Sirt1 as a new promising therapeutic target for the treatment of experimental PAH, further studies should focus on whether altered PASMC proliferation and mitochondrial biogenic activity (as the results of the recovery of the Sirt1 activation-mediated acetylation/deacetylation balance) contribute to the reversal of pulmonary artery remodeling and fibrosis in patients with PAH. Addressing the questions listed above will also better guide the clinical evaluation and care of patients with PAH. ACKNOWLEDGEMENTS Sources of funding: This work was supported in part by the Scientific Research Fund of the Chinese Ministry of Education (Grant Nos: 81260068, 81560240, and 81460082). Conflicts of interest There are no conflicts of interest.