Abstract
Salusin β is a bioactive peptide, detectable in many tissues and body fluids, first identified nearly 20 years ago. Since then, many studies have been performed to define the role of salusin β, concentrating on its role in atherosclerosis and conditions leading to vascular injury such as hypertension, diabetes and hyperlipidemia, in which salusin β seems to play a proatherogenic role. Previous literature has evaluated salusin as a predictor of atherosclerosis. Herein, we performed online research using 5 databases, namely PubMed, Ovid, Web of Science, Scopus, and Cochrane Library. Inclusion criteria were articles published in the years 2017–2022, concerning the association between salusin β and obesity, atherosclerosis, hypertension, and hyperglycemia. The aim of the review was to provide comprehensive data regarding the latest studies in this area. The latest research confirms that salusin β plays an important role in the development of vascular remodeling, inflammation, hypertension, and atherosclerosis. Additionally, the peptide is associated with hyperglycemia and lipid disorders, and its widespread activity makes it a potential therapeutic target. However, there is a need for additional studies to confirm the potential role of salusin β as a novel target for treatment. Many of the reports were performed in animal models, while research conducted in humans was generally based on small groups of patients and not always compared with healthy controls; studies enrolling children are rare.
Key words: salusin β, atherosclerosis, hypertension, obesity, type 2 diabetes mellitus
Introduction
Salusins are endogenous bioactive peptides first identified by Shichiri et al. in 2003. Salusin α consists of 28 amino acids, and salusin β consists of 20 amino acids. Both peptides are translated from alternative splicing of mRNA from the target of rapamycin 2A (TOR2A) gene. Preprosalusin mRNA is ubiquitously expressed and found in many tissues and organs, such as the nervous system, endothelium, muscles, liver, lungs, kidneys, bone marrow, lymph nodes, spleen, thymus, adrenal glands, small intestine, stomach, salivary glands, and testes, and in fluids, such as plasma and urine.1
It was discovered that when salusins were intravenously administered to rats, they affected cardiac function, causing both rapid bradycardia and hypotension.2 However, endogenous salusins play important roles in the development of atherosclerosis and foam cell formation by influencing cholesteryl ester accumulation and acetyl-CoA acetyltransferase 1 (ACAT-1) protein expression.3
Salusin α shows anti-atherogenic effects, as it reduces atherosclerotic plaques, and its expression is decreased in patients with hypertension and lipid disorders.4, 5 Conversely, salusin β plays a proatherogenic role. The available literature has evaluated salusin α and β as predictors for atherosclerosis, and salusin β seems to be a better indicator of atherosclerosis development than salusin α.4
Salusin β has been shown to increase nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase activity and reactive oxygen species (ROS) production in cells. It activates the release of inflammatory cytokines, such as interleukin (IL)-1β, IL-6 and tumor necrosis factor alpha (TNF-α).5, 6 Inflammation stimulates vascular smooth muscle cell (VSMC) proliferation and atherosclerotic lesion formation. Furthermore, high glucose levels seem to induce the production of salusin β,7 and increased salusin β expression is observed in conditions that are components of the metabolic syndrome, such as obesity, hypertension, diabetes mellitus (DM)/hyperglycemia, and lipid disorders.
Objectives
The aim of this review was to confirm the hypothesis that salusin β is a good predictor of atherosclerosis and components of the metabolic syndrome, and to provide comprehensive information about the latest studies in this area.
Materials and methods
The research was performed using 5 online databases, namely PubMed, Ovid, Web of Science, Scopus, and Cochrane Library. Only articles published in the years 2017–2022 and those concerning the association between salusin β and atherosclerosis, hypertension, obesity, hyperlipidemia, and hyperglycemia/DM were selected. Only original papers were included, with 33 articles meeting the inclusion criteria. Data were double-checked independently by 2 authors. The process of selection comprised the removal of duplicates, and the elimination by title, abstract and full-text review, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. The selection process is presented in Figure 1.
Results
Contrary to salusin α, the expression of salusin β is increased in patients with atherosclerosis, hypertension and metabolic syndrome. Increased levels of salusin β are observed in patients with cardiovascular disease (CVD) and cerebrovascular disease. Endogenous expression of salusin β is the lowest in the morning. Furthermore, the stimulation of the parasympathetic nervous system and insulin secretion decreases the level of free salusin β.8 In the central nervous system, salusin β stimulates or depresses sympathetic and vagal activity depending on the location within the brain. It reduces blood pressure and heart rate in the intermediate dorsal motor nucleus of the vagus.6
Salusin β has been shown to stimulate the proliferation of VSMCs and fibroblasts through the activation of immediate response genes such as c-Myc and Fos in rats and humans. It also activates the release of inflammatory cytokines, including IL-1β, IL-6 and TNF-α. Consequently, oxidative stress markers increase and monocyte-endothelial adhesion is promoted.9 This occurs within lesions of the blood vessel endothelium and in atherosclerosis.
Atherosclerosis
Salusin β is a pro-inflammatory agent. Human umbilical vein endothelial cells (HUVECs) incubated in salusin β increased the production of mRNA and protein levels of IL-6, IL-8, IL-18, and reduced the level of IL-1Ra.10 Furthermore, salusin β has an influence on the formation of macrophage foam cells. The peptide promotes the growth of atherosclerotic plaques and stimulates the adhesion of monocytes to the endothelium. Salusin β boosts the production of ACAT-1, the enzyme that breaks down fatty acids into acetyl coenzyme A in human monocytes/macrophages. In addition, salusin β encourages the storage of lipid droplets, increases the intracellular cholesterol content and stimulates monocyte adhesion. The proatherogenic role of the peptide was shown by silencing salusin β. The knockdown of the peptide improved cardiac function and cardiovascular remodeling in myocardial infarction-induced heart failure in rats and alleviated cardiac inflammation in diabetic rats.2, 9
The relationship between salusin β and atherosclerosis was also confirmed among patients suffering from coronary artery disease (CAD). A study by Awad et al. compared patients undergoing transcatheter therapy, and found that salusin β levels were significantly higher in patients with CVD before therapy compared to healthy controls. Moreover, after therapy, salusin β expression was significantly lower than before the intervention, or when compared to the control group. In addition, patients and controls varied in fasting blood glucose levels, insulin levels, body mass index (BMI), systolic blood pressure (SBP), and lipid profile.11 Salusin β levels were significantly lower in patients with stenosis or dilatation in coronary angiography than in healthy volunteers.12, 13 A similar finding was observed in patients with slow coronary flow that seemed to be caused by microvascular atherosclerosis.14
Risk factors of atherosclerosis are similar to abdominal aortic aneurysm (AAA). However, an evaluation of 48 patients with AAA revealed lower salusin β levels compared to 47 healthy controls. The levels of salusin β were also negatively correlated with abdominal aortic diameter.15 The role of salusin β in the pathogenesis of atherosclerosis is presented in Figure 2.
Diabetes mellitus
Many studies have confirmed that the serum level of salusin β is increased in DM. High glucose not only increases the level of salusin β mRNA but also upregulates its production by stimulating prosalusin protein expression. In cell culture experiments, the exposure of HUVECs to high glucose reduced proliferation and migration, and the knockdown of salusin β reversed these abilities. Adenosine monophosphate-activated protein kinase (AMPK) participates in the signaling pathway of salusin β.7 Parallel overexpression of salusin β in proximal tubular (HK-2) cells from both human and mouse models as well as human retinal capillary endothelial cells incubated in high glucose induced inflammatory cytokines and oxidative stress. In human research, inflammation and apoptosis are activated by ROS-dependent signaling pathways.16, 17, 18, 19
In the study performed by Argun et al., the group of patients suffering from type 2 diabetes mellitus (T2DM) had significantly higher levels of salusin β, but only when their hemoglobin A1C (HbA1C) level was higher than 9%.20 Deyekh et al. also confirmed higher serum salusin β levels in patients with T2DM compared to those with prediabetes or healthy controls, while there was no significant difference in salusin β levels between the prediabetes and control groups. Although each group consisted of 30 persons, a detailed description was not provided.21 Additionally, Aldulimya and Alaaraji revealed higher levels of salusin β in women with T2DM than in healthy volunteers, and the levels of salusin β correlated with fasting serum glucose. However, a limitation of this study was the significant difference in age between the examined groups.22 In a study by Wang et al., the level of salusin β was higher in patients with diabetic retinopathy than in healthy controls.16 Moreover, Yassien et al. revealed a correlation between the mean carotid intima-media thickness and left ventricular hypertrophy.23 These conclusions suggest that salusin β participates in the development of complications associated with chronic hyperglycemia.
Sipahi et al. reported that the serum levels of salusin β were elevated in patients undergoing hemodialysis compared to healthy controls.24 In patients also suffering from DM, the salusin β/salusin α ratio was higher, and the levels of salusin α were significantly lower. However, there was no evidence of a correlation between salusin β and diabetes in this group of patients.
An opposite outcome was observed in a study performed among diabetic patients with and without diabetic foot.25 Interestingly, among healthy volunteers, salusin β levels were significantly higher than in diabetic patients. Moreover, salusin α was significantly higher in healthy controls, which raises doubts regarding the methodology of the study.
Hypertension
Salusin β is elevated in patients with hypertension, inducing VSMC proliferation and fibrosis within the vascular wall. Moreover, it activates intimal hyperplasia after vascular injury and can trigger vascular constriction. This occurs via the activation of endothelial nitric oxide synthase (eNOS), the release of nitric oxide (NO) and an increase in NAD(P)H oxidase activity. Acute intravenous administration of salusin β increased the mean arterial pressure in spontaneously hypertensive rats (SHRs), while injections of anti-salusin β IgG reduced the mean blood pressure and heart rate. The important role of salusin β in the development of hypertension and the subsequent vascular remodeling was verified by salusin β knockdown, after which the vascular function was augmented in hypertensive rats.26, 27, 28
Contrary to patients that normally present with physiological night-time blood pressure reduction, those with newly diagnosed non-dipper hypertension demonstrate elevated levels of salusin β. In addition, salusin β positively correlates with the left ventricle mass index and negatively correlates with diastolic parameters of the left ventricle. These results were not compared with healthy controls, which is a limitation of the study.29
The interaction between salusin β and hypertension was confirmed in a study performed in patients treated with anti-hypertensive medicines.30 The therapy consisting of felodipine and enalapril was more effective and brought a more relevant reduction of salusin β level than felodipine alone. This is the first study that demonstrated practically this piece of theoretical knowledge.
The correlation between the level of salusin β and hypertension has been confirmed in both adults and children. The study by Kołakowska et al. performed in adolescents revealed that the serum level was significantly higher in patients with essential hypertension when compared to those with white coat hypertension.31
Finally, salusin β has an influence on the consequences of hypertension. Ageing SHRs have higher concentrations of the peptide in the hypothalamic paraventricular nucleus, myocardium and mesenteric artery. The knockdown of salusin β improved cardiac function and reduced the levels of p38 MAPK, ERK1/2, JNK, and NAD(P)H, which are likely to be involved in the signaling pathway.32 Furthermore, the level of this peptide is significantly increased in the calcified VSMCs of rats. The calcification has been shown to stimulate the expression of salusin β, and the overexpression of salusin β has been shown to promote spontaneous calcification. This process is probably mediated by ROS and NAD(P)H oxidase. Furthermore, the overexpression of salusin β decreases the levels of Klotho, which is an anti-inflammatory and anti-oxidative stress protein.33
Obesity/lipid disorders
Previous studies revealed that obesity is another condition that leads to increased serum salusin β levels. In patients with T2DM, salusin β correlated with both BMI and waist circumference, and was significantly higher than in healthy controls.23 The correlation between salusin β and BMI was also revealed in patients suffering from CVD.11
However, research performed on a group of 75 obese children aged 6–18 years did not confirm this theory.34 No correlation was found between salusin β and BMI, blood pressure, carotid intima-media thickness, and epicardial adipose tissue thickness. There was a negative correlation between salusin α and diastolic blood pressure (DBP). Additionally, in a group of 48 children with Down syndrome, the connection between salusin β and excessive body weight (obesity and overweight) was not proved.35 Nevertheless, physical training decreases the level of serum salusin β in patients with high body weight.
In this regard, moderate- and high-intensity interval training (HIIT) were compared between obese and overweight women. It was revealed that both types of training improved lipid parameters and decreased serum salusin β levels.36
Similar studies were performed among obese or overweight males with a mean age of 11 years. The levels of salusin β were reduced after 12 weeks of training, with HIIT demonstrating more significant results than aerobic training. Apart from salusin β and α, other parameters also improved, such as the lipid profile and markers of inflammation.37, 38 A summary of the studies on salusin β used in this review is included in Table 1.
Discussion
Our research encompassed 12 articles based on studies performed on animals or cell culture systems and 21 articles concerning humans; 15 of these included healthy controls, and only 5 articles involved children. Four studies enrolled more than 100 patients, and 11 enrolled more than 50 patients. Therefore, most studies were based on relatively small study groups, which hampers a definitive evaluation. Nonetheless, most of the studies confirmed the thesis that salusin β has a relevant function in atherosclerotic development, and is correlated with components of the metabolic syndrome. Furthermore, Genç Elden et al. demonstrated an association between sudden hearing loss, atherosclerosis and salusin β, and all groups presenting with similar atherosclerotic parameters. The collected material confirmed that salusin β is a prognostic factor in hearing loss. However, the results of the above study did not indicate sufficient evidence for the development of atherosclerosis in the study group, and hence it was not included in the review.39
The analyzed data indicate that there is much that remains to be discovered about salusin β. Preprosalusin mRNA is ubiquitously expressed, being found in many tissues and organs, such as the nervous system, endothelium, muscles, liver, lungs, kidneys, bone marrow, lymph nodes, spleen, thymus, adrenal glands, small intestine, stomach, salivary glands, and testes, as well as in fluids, such as plasma and urine. A significant number of recent studies on salusin β have concentrated on its role in biochemical pathways in hypertension, atherosclerosis, hyperglycemia, and obesity. Currently available data indicate that salusin β could be used in diagnostics of developing atherosclerosis and hypertension, and appears to be a promising novel target for treatment.
Studies performed on animal models provide important knowledge but in a narrow range. Nevertheless, they indicate that salusin β could be a direct target in the treatment of hypertension and heart failure, or an indicator of treatment efficacy. Initial attempts have already been made to use these findings in clinical practice. One promising study on the evaluation of the treatment of hypertension with 110 patients provides interesting and valuable results.30
Recent studies do not directly confirm the connection between elevated levels of salusin β and obesity. However, physical activity in overweight or obese patients brings about a reduction of salusin β and improvement of both lipid profile and inflammatory factors. These data suggest that salusin β could be valuable in the treatment of obesity.
Cohort studies are necessary to confirm the scope and bring us closer to the practical application of salusin β.
Limitations
This review has some limitations. The available literature on salusin β is quite limited and based on a variety of methods, which hinders a systematic review. The division of included papers into sections was based on the main topics of the studies and seemed to be artificial, as each study contained multiple components and had a high degree of overlap. The aim of this classification was to organize the data. Moreover, research conducted in humans is generally based on small groups of patients.
Conclusions
Recent studies confirm that salusin β plays an important role in the development of vascular remodeling, inflammation, hypertension, and atherosclerosis. Additionally, the peptide is related to hyperglycemia and lipid disorders. The widespread activity of salusin β makes it a potential therapeutic target. However, there is a need for additional studies to confirm the potential role of salusin β as a novel target for treatment.
While it could be useful in the prophylaxis or the treatment of the abovementioned disorders, many reports have been performed in animal models, and those conducted in humans are based on small groups of patients and are not always compared with healthy controls. Studies enrolling children are rare. There is a lack of studies conducted in humans confirming the thesis that salusin β is a good predictor of atherosclerosis and metabolic syndrome, hence further investigations are necessary.
