The association between alpha-1 antitrypsin and B-type natriuretic peptide blood levels in healthy African Americans

Abstract

Objective

B-type natriuretic peptide (BNP) is a neurohormone primarily secreted from cardiac ventricles in reaction to increased volume and pressure. The plasma level of BNP is used to measure the mechanical function of the heart and the risk of developing chronic obstructive pulmonary disease (COPD). Alpha-1 antitrypsin (AAT) is a glycoprotein that acts as an inhibitor of serine proteases and plays a crucial role in protecting the lungs against potential harm. AAT deficiency also causes COPD. The data on the prevalence of BNP and AAT in African Americans (AA) are limited and inconsistent. No previous studies have investigated whether any association exists between BNP and AAT. We collected fasting blood samples from 114 AA subjects who provided informed consent. The plasma levels of the biomarkers were measured using ELISA.

Results

Our findings revealed a significant negative correlation between AAT and BNP (r=-0.266, p = 0.01) as well as between AAT and TNF-α (r=-0.242, p = 0.01). Sex hormones (estrogen and testosterone) have a significant relationship with the levels of AAT and BNP. Increasing the AAT and reducing the levels of inflammation can lower the levels of BNP, which in turn could help lower the risk of heart disease in the AA population.

Introduction

B-type natriuretic peptide (BNP) plays a crucial role in the control of intravascular blood volume and vascular tone [1]. Plasma levels of BNP are typically very low; however, in cases of congestive heart failure, the concentration of BNP is increased [2]. The plasma concentrations of BNP have emerged as valuable biomarkers for diagnosing and predicting a range of cardiovascular diseases [3, 4]. The expression of peripheral BNP in patients with heart failure is significantly influenced by obesity, which is a crucial independent factor [5]. Plasma BNP levels are elevated in patients with pulmonary hypertension and chronic lung disease with right ventricular overload [6]. Hypoxia-mediated contraction of small pulmonary arterioles may increase BNP, thus resulting in increased pulmonary arterial pressure and subsequent cardiac stress [7].

African Americans (AA) have higher levels of BNP than the white population [8, 9]. This disparity may be attributed to several factors, including a higher incidence of renal insufficiency, elevated median systolic and diastolic pressures, and a greater incidence of lower median ejection fraction [9]. In contrast, a few studies have shown that AA have lower levels of BNP and NT-proBNP when compared to whites [10,11,12]. Clinical investigations suggest that sex hormones (testosterone and estrogen) may play a role in the regulation of BNP levels [13,14,15,16]. In the general population, plasma BNP levels are greater in women than in men [17,18,19,20]. However, a few studies have reported that men exhibit elevated levels of BNP compared to women [21, 22].

Nishimura et al. showed that plasma BNP levels were slightly but significantly increased during acute exacerbation of chronic obstructive pulmonary disease (COPD) in most subjects [23]. Alpha-1 antitrypsin (AAT) deficiency is also linked to a higher incidence of COPD [24]. AAT plays a crucial role in protecting the lungs against potential harm caused by proteolytic enzymes [25]. AAT, an inhibitor of serine proteases, exhibits anti-inflammatory and immunomodulatory effects [26]. Impairment or reduction in the AAT levels is associated with lung dysfunction and heightened inflammation [27]. Blood levels of AAT have been used to assess lung functions [27, 28].

No previous studies have determined whether any association exists between BNP and AAT levels. This study aimed to address this gap by proposing the hypothesis that there is a negative association between BNP and AAT levels in AA. Further, this study investigated whether the circulating blood levels of estrogen and testosterone correlated with BNP and AAT in AA subjects.

Subject enrollment

Healthy AA individuals aged between 18 and 65 years with no history of any chronic illness were recruited for this study. Documented consent was obtained from all participants. The study protocol was approved by the Louisiana State University Health Sciences Center (LSUHSC) Institutional Review Board (IRB) (Protocol#: H-09-073). Those taking any supplemental vitamins or herbal products were excluded from the study. Women who tested positive for pregnancy or were breastfeeding were not included in the study. The blood pressure of all participants was assessed on the left arm after allowing participants to rest for 10 min and documented. Blood pressure data was categorized as normal (< 120/80 mm Hg), prehypertension (120–139/80–89 mm Hg), and hypertension (> 140/90 mm Hg) [29].

Blood collection

Blood was drawn from the participants after an overnight fast (8 h). Following blood collection, serum tubes for the chemistry profile and EDTA tubes for HbA1c were promptly provided to the LSUHSC clinical laboratories for analyses of HbA1c, total cholesterol, triglycerides, and HDL and LDL cholesterol. Additional tubes of EDTA blood were brought to the research laboratory for plasma isolation and stored at -80 °C.

Biochemical assay

The plasma levels of AAT, BNP, and β-estradiol (Abcam, Cambridge, UK) and TNF-α, MCP-1, CRP, and total testosterone (R&D Systems, MN, USA) were measured using commercially available ELISA kits. All appropriate controls and standards were used as specified by the manufacturer’s kits. In every assay, control samples were analyzed each time to check for any variation on different days of analysis. All the samples were analyzed in duplicate. Unless otherwise mentioned, all chemicals were purchased from Sigma Chemical Co. (St. Louis, MO).

Statistical analyses

Data analyses were performed using SigmaPlot v15 (Inpixon, Palo Alto, CA). Spearman correlation was used to assess the degree of linear association between the variables. The mean values were compared using the unpaired Student’s t-test to determine significant changes in the clinical characteristics. If the data distribution did not follow normality, the correlation was calculated using Mann‒Whitney U or Signed-Ranked tests. A two-sided p-value ≤ 0.05 was considered statistically significant. The percentage changes between the groups were determined using the median values.

Results

A total of 114 AA subjects (25 men and 89 women) participated in this study. The ages, BMIs, and various clinical biomarkers of the subjects were recorded and are presented in Table 1. The biomarker levels were consistent with the values reported in the literature. Among the subjects studied, 24 individuals (21.05%) had normal blood pressure, while 58 (50.87%) were classified as having prehypertension, and 32 (28.07%) were identified with hypertension. Women exhibit a 36.5% lower BNP value than men (Tables 1 and 2). BNP levels are increased in prehypertensive (65%), and hypertensive (112%) men compared to normal BP subjects. There is a notable decrease in BNP levels in prehypertensive women, while hypertensive showed an increase of 8.24% compared to normal BP women (Table 2). Even though the BNP levels between the three blood pressure groups showed a difference they are not statistically significant (p > 0.05) in both men and women subjects. Similarly, no significant correlation (p > 0.05) was seen between normal, prehypertension, and hypertension systolic (r = 0.45, 0.16, 0.24; ns) and diastolic (r = 0.39, 0.15, 0.38; ns) BP respectively with BNP levels.

Table 1 Clinical characteristics of participants

Table 2 Blood pressure ranges and relative BNP values among men and women. Values are presented as median [IQR: 25th − 75th percentile]

Figure 1A-C shows the relationships between BNP and AAT, total testosterone, and β-estradiol. The findings indicate an inverse association between AAT and BNP (r=-0.25, p = 0.01), suggesting that BNP levels tend to decrease as AAT levels increase. BNP was positively correlated with total testosterone (r = 0.214, p = 0.04) and β-estradiol (r = 0.262, p = 0.01), indicating that sex hormones regulate BNP levels.

Fig. 1

The relationship between the plasma levels of BNP and (A) AAT, (B) total testosterone, and (C) β-estradiol in AA subjects. The results indicate a negative impact of BNP and AAT, while total testosterone and β-estradiol exhibited a positive relationship among the AA subjects

Figure 2A and B illustrate the correlations between AAT and total testosterone and β-estradiol in AA subjects. Both total testosterone (r=-0.057, p = 0.542) and β-estradiol (r=-0.232, p = 0.01) exhibited negative relationships with AAT. However, only the association between β-estradiol and AAT was found to be significant. A similar trend was observed in the female subgroup (r=-0.198, p = 0.05) (Fig. 2C).

Fig. 2

The relationship between the plasma levels of AAT and (A) total testosterone and (B-C) β-estradiol in AA subjects. The correlation between plasma levels of AAT and both total testosterone and β-estradiol was found to be negative. This negative effect of β-estradiol was also observed in the female subgroup (C), indicating a similar trend

The associations between AAT and inflammatory markers (TNF-α, CRP, and MCP-1) are depicted in Fig. 3A-C. AAT exhibited a negative association with these inflammatory markers, but only the association with TNF-α was statistically significant (r=-0.233, p = 0.01). These findings suggested that AAT may play a role in regulating the levels of these inflammatory markers.

Fig. 3

The relationship between AAT and the inflammatory markers (A) TNF-α, (B) CRP, and (C) MCP-1 in the AA population. The association between AAT-TNF-α and CRP was negative, suggesting the potential of this combination to reduce inflammation

The results indicate an inverse correlation between BNP and AAT levels, which could potentially be affected by sex hormones.

Discussion

B-type natriuretic peptide is a circulating hormone of cardiac origin that belongs to a group of hormones called natriuretic isopeptides. Initially, it was discovered and identified in the porcine brain, which led to its identification as a brain natriuretic peptide [30]. Subsequent research revealed that BNP is secreted primarily by left ventricular myocytes in heart muscle [31]. These cells release both BNP and its amino-terminal fragments in response to an increase in blood volume and pressure [32]. The precursor of BNP, known as NT-proBNP, is released by the heart muscle and then converted into the active form of BNP [33]. BNP plays a crucial role in protecting the body from the detrimental consequences of increased blood pressure and volume loads [34] and maybe a potential noninvasive marker of pulmonary hypertension [35]. Elevated BNP levels can occur in the early stages of hypertensive disease among AA. Prehypertension is more prevalent among AA [36]. AA have been shown to have a 2- to 3-fold greater incidence of left ventricular hypertrophy (LVH) than white individuals, which subsequently influences cardiac-related risk factors [19]. Despite half of the participants having prehypertension levels, no significant association was seen between BNP levels and hypertension in this study. This might be due to a limited sample size in each subset.

Our study demonstrated a negative relationship between BNP and AAT levels. AAT deficiency is widely recognized as a prominent genetic factor that contributes to the development of COPD [37]. Detecting AAT levels at an early stage would be beneficial for diagnosing diseases, as a delayed diagnosis could lead to the deterioration of clinical conditions, including COPD [26]. AAT is used as a biomarker for detecting lung disorders [27, 28]. This finding suggested that an increase in AAT decreases BNP levels, indicating its potential protective role against increasing BNP levels among AA.

The circulating levels of BNP are heavily influenced by sex, which is regarded as a significant determinant. Studies have provided evidence of contrasting levels of plasma BNP between men and women, with women exhibiting higher BNP levels [17, 18, 38]. Our observations indicated that men had higher plasma concentrations of BNP than women, which was in line with the findings of other investigations [21, 22]. The observed discrepancy could be attributed to the uneven distribution of sample sizes among the male and female subgroups, which is a limitation of the current study.

Clinical investigations have indicated that testosterone may reduce natriuretic peptide levels by enhancing the activity of neprilysin [13]. An increase in free testosterone levels could enhance muscle mass and potentially inhibit the production of natriuretic peptides [14]. Our investigation demonstrated a positive relationship between BNP and total testosterone concentrations. However, no significant correlation was found between free testosterone levels and BNP levels, indicating that testosterone may not play a role in the regulation of BNP. Studies have shown a positive correlation between estrogen and the expression of the genes responsible for cardiac natriuretic peptide expression. Consequently, the higher expression of these genes leads to elevated levels of cardiac natriuretic peptides in women compared to men [15, 16, 39]. Our research findings revealed a positive correlation between estrogen concentration and BNP concentration, suggesting that estrogen has a potential regulatory effect on BNP synthesis. A similar effect was observed in the female subgroup (r = 0.30, p = 0.01) but not in the male subgroup. This finding supports the observation that estrogen influences the plasma levels of BNP to a greater degree in women than in men. However, further investigation is required to fully understand the mechanisms underlying this relationship and its implications for human health.

In contrast, our research revealed a detrimental correlation between sex hormones and AAT levels. Previous studies have demonstrated that various factors, such as BMI, tobacco smoking, and estrogen, can influence the level of AAT. The influence of estradiol on AAT is facilitated through the inhibition of cytochrome P-450 activity [40]. Our results align with these findings, emphasizing the substantial role of estrogen in regulating AAT levels, specifically in women.

AAT is an acute-phase reactant produced mainly by the liver, macrophages, and monocytes, whose concentration increases in plasma during malignancy, inflammation, and infection [41]. The proinflammatory properties of AAT can attract neutrophils, leading to an elevated neutrophil count and increased vulnerability to infection and structural harm [27]. These changes in AAT levels are part of the body’s response to combat the underlying infection or inflammatory process [42]. Impairment or reduction in the AAT levels is associated with lung dysfunction and heightened inflammation [27]. TNF-α is a powerful cytokine responsible for inflammation and the immune response. Our findings demonstrated an inverse relationship between AAT and TNF-α, indicating that elevated AAT levels are associated with a reduction in TNF-α levels.

The current findings showed an inverse association exists between AAT and BNP levels suggesting that AAT may downregulate BNP levels which in turn may reduce the progression of heart disease.

Limitation

This study has limitations. An unequal distribution of subjects between the male and female subgroups affected the findings’ generalizability. In addition, including white subjects would have enhanced the understanding of the comparative differences between white and African American populations in AAT and BNP levels.

Conclusion

Alpha-1 antitrypsin levels in AA are negatively associated with BNP levels, suggesting that higher AAT levels may be linked to lower BNP levels. This study is the first to document the correlation between AAT and BNP in the AA community. Additionally, the study suggested the potential role of β-estradiol in regulating the levels of both BNP and AAT. Furthermore, the AAT is inversely associated with TNF-α, indicating its role in regulating inflammation levels. This study suggests that increasing the levels of AAT may reduce BNP levels, thereby potentially lowering the risk of heart disease.