# Beta3-Adrenergic Receptor Activation Alleviates Cardiac Dysfunction in Cardiac Hypertrophy by Regulating Oxidative Stress

Authors: Mingming Zhang; Yuerong Xu; Jianghong Chen; Chaoshi Qin; Jing Liu; Dong Guo; Rui Wang; Jianqiang Hu; Qing Zou; Jingxiao Yang; Zikuan Wang; Xiaolin Niu
Journal: Oxidative Medicine and Cellular Longevity
 (2021)

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## Abstract

Background. Excessive myocardial oxidative stress could lead to the congestive heart failure. NADPH oxidase is involved in the pathological process of left ventricular (LV) remodeling and dysfunction. β3-Adrenergic receptor (AR) could regulate cardiac dysfunction proved by recent researches. The molecular mechanism of β3-AR regulating oxidative stress, especially NADPH oxidase, remains to be determined. Methods. Cardiac hypertrophy was constructed by the transverse aortic constriction (TAC) model. ROS and NADPH oxidase subunits expression were assessed after β3-AR agonist (BRL) or inhibitor (SR) administration in cardiac hypertrophy. Moreover, the cardiac function, fibrosis, heart size, oxidative stress, and cardiomyocytes apoptosis were also detected. Results. β3-AR activation significantly alleviated cardiac hypertrophy and remodeling in pressure-overloaded mice. β3-AR stimulation also improved heart function and reduced cardiomyocytes apoptosis, oxidative stress, and fibrosis. Meanwhile, β3-AR stimulation inhibited superoxide anion production and decreased NADPH oxidase activity. Furthermore, BRL treatment increased the neuronal NOS (nNOS) expression in cardiac hypertrophy. Conclusion. β3-AR stimulation alleviated cardiac dysfunction and reduced cardiomyocytes apoptosis, oxidative stress, and fibrosis by inhibiting NADPH oxidases. In addition, the protective effect of β3-AR is largely attributed to nNOS activation in cardiac hypertrophy.


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## Body

## 1. Introduction

Despite the progress of therapeutic approaches, congestive heart failure (CHF) remains to be a high morbidity and mortality [1, 2]. Considerable evidence suggests that excessive oxidative stress leads to the CHF [3–5]. Experiments found that oxidative stress was activated in animal models with cardiac hypertrophy [6, 7]. Besides, increased ROS could result in not only cardiac hypertrophy and cardiomyocytes apoptosis but also many other diseases such as acute kidney injury [8–12]. Moreover, patients with CHF were also found to have elevated markers of oxidative stress, suggesting that oxidative stress was increased in failing heart [13].ROS is mainly originated from the NADPH oxidases in cardiovascular system [14]. The typical NADPH oxidases consist NOX, p22phox, p40phox, p47phox, p67phox, and Rac1. When assembled, electron could be transferred from NADPH to molecular oxygen, resulting in the formation of superoxide [15]. The level of NADPH oxidase was increased in an animal model with cardiac hypertrophy and even in CHF patients [5, 16]. Recent studies found that NOX2 deficiency attenuated angiotensin II-induced cardiac hypertrophy [17]. Furthermore, Rac1, an important subunit for NOX2 activation, regulates the occurrence and development of cardiac hypertrophy [18]. Taken together, these results suggested that NAPDH oxidases play an essential role in cardiac hypertrophy. However, clinical application of antioxidants has yielded disappointing results, indicating that the detailed relationship between oxidative stress and heart failing still needs to be explored [19]. Therefore, our study is aimed at exploring the molecular mechanism and finding a novel therapy target to treat heart failure.Accumulating evidence demonstrated that 3 subtypes ofβ-ARs regulate the cardiac function when exposed to stress [20, 21]. The biological function of β1- and β2-AR in mammals is thoroughly studied in the past years [22]. Previous study suggested that β3-AR plays a negative inotropic effect, which is the opposite effect of β1/2-ARs [23]. Furthermore, β3–/– mice exacerbated cardiac hypertrophy and heart failure. However, whether the biological function of β3-AR is mediated by oxidative stress regulation during heart failure is still uncertain. Therefore, it is necessary to clarify the underline mechanism of β3-AR in oxidative stress, especially NADPH oxidase in cardiac hypertrophy.


## 2. Methods Animals

The 8–10-week-old male C57BL6/J mice were randomly allocated as follows: [1] sham group (Sham); [2] transverse aortic constriction group (TAC); [3] TAC + BRL37344 group (TAC+BRL); and [4] TAC + SR59230A group (TAC+SR). The experiments were approved by the Fourth Military Medical University Committee on Animal Care.


### 2.1. Construction of Transverse Aortic Constriction Model

Transverse aortic constriction (TAC) was constructed as previously described. Briefly, mice were anesthetized with 2% isoflurane, endotracheally intubated with a 20G catheter, and ventilated. The transverse aortic arch was surgically accessed. Then, a 25G needle was placed on the transverse aorta, which was secured. Finally, the chest was closed after removing the needle, leaving a stenosis. Mice were administrated with BRL37344 (Tocris Bioscience, Ellisville, Missouri) or SR59230A, respectively, at 0.1 mg/kg/hour via osmotic minipumps.


### 2.2. Primary Cardiomyocytes Culture and Treatments

Primary cardiomyocytes were isolated from the neonatal mouse (1 to 3 days old) hearts as previously described. The cardiomyocytes were treated with hypertrophic agonists, phenylephrine (PE). Then, after 24 hours, the cardiomyocytes were treated with BRL37344 or SR59230A. The groups are as follows: [1] saline; [2] PE; [3] PE+BRL; and [4] PE+ SR.


### 2.3. Cell Size

Primary cardiomyocytes were subjected to immunostaining. Antibody sources were as follows:α-actinin (1 : 200, Abcam); Alexa-594 secondary antibodies (1 : 500, Invitrogen).


### 2.4. Histological Analysis and Immunostaining

Three weeks after TAC operation, mice were sacrificed. Tissue sections of hearts were stained with Masson’s trichrome (Sigma) for detection of fibrosis. The fibrosis-related genes Col1a1, Col3a1, and fibrillin 1 (FBN1) were detected by q-PCR. Meanwhile, wheat germ agglutinin (WGA, Sigma) was used to outline cardiomyocytes. Mean cardiomyocyte cross-sectional area was determined from digitized images and analyzed using the ImageJ program. ANP and BNP were detected by q-PCR. For analyzing the expressions of myocardial NAD(P)H oxidase subunit, immunohistochemical stainings of NOX2, NOX4, and p22phox were performed as previously described. The primary antibodies are NOX2 (1 : 200, Abcam), NOX4 (1 : 200, Abcam), and p22phox (1 : 200, Abcam).


### 2.5. Echocardiography

Echocardiography studies were conducted weekly to monitor heart function as previously described [19]. We followed the methods of Xiaolin Liu et al. (2014).


### 2.6. Cell Apoptosis Assay

Apoptosis in heart tissue was determined based on TUNEL and caspase-3 activity assay as previously described [19].


### 2.7. Measurement of Oxidative Stress Level

The production of O2•- in the LV was measured as previously described. Meanwhile, in situ formation of tissue ROS was detected by staining with DHE and DCF as described recently. Briefly, fresh frozen left ventricular sections were incubated with DHE (2 μM; Molecular Probes) or DCF (4 μM; Molecular Probes), respectively. The activities of SOD and GSH were detected according to the Beyotime Kits.


### 2.8. Protein Preparation and Immunoblotting

Membrane proteins or total cell proteins were extracted from homogenized LV tissue. The primary antibodies included NOX2, NOX4, p22phox, p47phox, p67phox, Rac-1 (1 : 500, Abcam); eNOS, p-eNOSSer1177, p-eNOSThr495, p-eNOSSer114, iNOS, nNOS (1 : 1000, Cell Signaling Technology), β3-adrenergic receptor (1 : 500, Abcam), and β-actin (1 : 5000, Abcam).


### 2.9. Statistical Analysis

The results are presented asmean±SEM. All experiment data were analyzed by the GraphPad Prism8 software. Statistical comparisons for different group were performed using one-way ANOVA followed by Student’s paired, two-tailed t test for two groups’ comparison. P values < 0.05 were considered statistically significant.


## 2.1. Construction of Transverse Aortic Constriction Model

Transverse aortic constriction (TAC) was constructed as previously described. Briefly, mice were anesthetized with 2% isoflurane, endotracheally intubated with a 20G catheter, and ventilated. The transverse aortic arch was surgically accessed. Then, a 25G needle was placed on the transverse aorta, which was secured. Finally, the chest was closed after removing the needle, leaving a stenosis. Mice were administrated with BRL37344 (Tocris Bioscience, Ellisville, Missouri) or SR59230A, respectively, at 0.1 mg/kg/hour via osmotic minipumps.


## 2.2. Primary Cardiomyocytes Culture and Treatments

Primary cardiomyocytes were isolated from the neonatal mouse (1 to 3 days old) hearts as previously described. The cardiomyocytes were treated with hypertrophic agonists, phenylephrine (PE). Then, after 24 hours, the cardiomyocytes were treated with BRL37344 or SR59230A. The groups are as follows: [1] saline; [2] PE; [3] PE+BRL; and [4] PE+ SR.


## 2.3. Cell Size

Primary cardiomyocytes were subjected to immunostaining. Antibody sources were as follows:α-actinin (1 : 200, Abcam); Alexa-594 secondary antibodies (1 : 500, Invitrogen).


## 2.4. Histological Analysis and Immunostaining

Three weeks after TAC operation, mice were sacrificed. Tissue sections of hearts were stained with Masson’s trichrome (Sigma) for detection of fibrosis. The fibrosis-related genes Col1a1, Col3a1, and fibrillin 1 (FBN1) were detected by q-PCR. Meanwhile, wheat germ agglutinin (WGA, Sigma) was used to outline cardiomyocytes. Mean cardiomyocyte cross-sectional area was determined from digitized images and analyzed using the ImageJ program. ANP and BNP were detected by q-PCR. For analyzing the expressions of myocardial NAD(P)H oxidase subunit, immunohistochemical stainings of NOX2, NOX4, and p22phox were performed as previously described. The primary antibodies are NOX2 (1 : 200, Abcam), NOX4 (1 : 200, Abcam), and p22phox (1 : 200, Abcam).


## 2.5. Echocardiography

Echocardiography studies were conducted weekly to monitor heart function as previously described [19]. We followed the methods of Xiaolin Liu et al. (2014).


## 2.6. Cell Apoptosis Assay

Apoptosis in heart tissue was determined based on TUNEL and caspase-3 activity assay as previously described [19].


## 2.7. Measurement of Oxidative Stress Level

The production of O2•- in the LV was measured as previously described. Meanwhile, in situ formation of tissue ROS was detected by staining with DHE and DCF as described recently. Briefly, fresh frozen left ventricular sections were incubated with DHE (2 μM; Molecular Probes) or DCF (4 μM; Molecular Probes), respectively. The activities of SOD and GSH were detected according to the Beyotime Kits.


## 2.8. Protein Preparation and Immunoblotting

Membrane proteins or total cell proteins were extracted from homogenized LV tissue. The primary antibodies included NOX2, NOX4, p22phox, p47phox, p67phox, Rac-1 (1 : 500, Abcam); eNOS, p-eNOSSer1177, p-eNOSThr495, p-eNOSSer114, iNOS, nNOS (1 : 1000, Cell Signaling Technology), β3-adrenergic receptor (1 : 500, Abcam), and β-actin (1 : 5000, Abcam).


## 2.9. Statistical Analysis

The results are presented asmean±SEM. All experiment data were analyzed by the GraphPad Prism8 software. Statistical comparisons for different group were performed using one-way ANOVA followed by Student’s paired, two-tailed t test for two groups’ comparison. P values < 0.05 were considered statistically significant.


## 3. Results

### 3.1.β3-AR Stimulation Alleviated Cardiac Hypertrophy and Remodeling in Pressure-Overloaded Mice

The representative figures of hearts demonstrated that pressure overload caused ventricular dilatation (Figure1(a)). Meanwhile, the heart weight/tibia length ratio (HW/TL ratio) in TAC was significantly increased compared to sham ones (168.9±8.9mg/cm vs. 117.4±9.8mg/cm, P<0.05), indicating that pressure overload successfully induced cardiac pathological remodeling. Moreover, cross-sectional area, LV mass, and wall thickness all increased after TAC operation (Figures 1(b) and 1(d)–1(f)). These results suggested that mice developed evident cardiac hypertrophy induced by TAC. However, 3 weeks of BRL application significantly alleviated LV dilation, cardiac hypertrophy, and cardiac pathological remodeling. ANP and BNP levels significantly decreased after BRL treatment in TAC mice (Figures 1(g) and 1(h)). The size of cardiomyocytes in PE+BRL group was remarkably smaller compared to the PE group (Figures 1(i) and 1(j)). Thus, β3-AR stimulation reduced myocardial hypertrophy and remodeling induced by pressure overload.Figure 1
β3-AR stimulation alleviated cardiac hypertrophy and remodeling in pressure-overloaded mice. (a) Representative images of heart size. (b) Cardiomyocytes size as stained by WGA. (c) HW/TL. (d) Quantitative analysis of cardiomyocytes size. (e, f) LV mass and wall thickness measured by echocardiography. (g, h) The expression level of ANP and BNP. (i, j) Primary cardiomyocytes size as stained by α-actinin. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


### 3.2.β3-AR Stimulation Reduced Fibrosis after TAC

BRL treatment significantly reduced fibrosis compared with the TAC group (Figures2(a) and 2(b)). Moreover, the increasing tendency of interstitial fibrosis was detected in the TAC+SR group, which is a β3-AR-specific inhibitor, although without significance. Moreover, the levels of Col1a1, Col3a1, and FBN1 were all reduced in the TAC mice after BRL treatment (Figures 2(c)–2(e)).Figure 2
β3-AR stimulation reduced fibrosis after TAC. (a, b) The level of cardiac fibrosis. (c–e) The expression of fibrotic remodeling marker genes (Col1a1, Col3a1, and FBN1). The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


### 3.3.β3-AR Stimulation Improved Cardiac Dysfunction in TAC Mice

As shown in Figure3(a), systolic dysfunction was revealed in the TAC and TAC+SR groups. Conversely, BRL treatment enhanced LV anterior wall motion after TAC operation, indicating that β3-AR stimulation could improve cardiac dysfunction induced by pressure overload. Moreover, the LVEDd and LVESd were increased in TAC mice. In addition, the LVEDd and LVESd were significantly decreased in TAC mice after BRL treatment (Figures 3(b) and 3(c)). Similarly, the enhanced LVEF and FS were observed in the TAC+BRL group, indicating that β3-AR stimulation improved cardiac dysfunction after TAC (Figures 3(d) and 3(e)).Figure 3
β3-AR stimulation improved pressure overload-induced cardiac dysfunction. (a) Representative images of echocardiography. (b–e) LVEDd, LVESd, LVEF, and LVFS measured by echocardiography. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


### 3.4. Cardiomyocyte Apoptosis Was Inhibited in TAC Mice after BRL Treatment

Figure4(a) reveals that, compared to the TAC group and TAC+SR group, BRL treatment significantly decreased the cardiomyocyte apoptosis index. The apoptosis index in the TAC+BRL group was 21.67±1.76%, less than that in the TAC group (35.3±3.6%, P<0.05) and TAC+SR group (34.7±4.7%, P<0.05) (Figure 4(b)). Meanwhile, caspase-3 enzymatic activity decreased in the TAC+BRL group compared to the TAC group (73.2±3.2 vs. 131.5±3.7, P<0.05) and TAC+SR group (73.2±3.2 vs. 140.2±6.7, P<0.05) (Figure 4(c)). These data suggested that β3-AR stimulation decreased apoptosis in the pressure-overloaded heart.Figure 4
Cardiomyocyte apoptosis was inhibited in TAC mice after BRL treatment. (a, b) Representative images of TUNEL staining and the apoptosis index. (c) Caspase-3 activity. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)


### 3.5.β3-AR Stimulation Decreased the Oxidative Stress Level in Pressure-Overloaded Mice

The production of ROS, measured by DHE and DCF staining, was significantly increased in the TAC group and TAC+SR group. However, BRL treatment significantly decreased ROS generation in pressure-overloaded mice (Figure5(a)). Meanwhile, the NADPH oxidase activity increased in the TAC group (301.5±19.1 units/min/mg vs. 127.1±13.1 units/min/mg; P<0.05) and TAC+SR group (316.7±24.5 units/min/mg vs. 127.1±13.1 units/min/mg; P<0.05). NADPH oxidase activity was lower in the TAC+BRL group compared to the TAC group (173.5±17.4 units/min/mg vs. 301.5±19.1 units/min/mg; P<0.05, Figure 5(b)). Similarly, the O2•- production was lower in the TAC+BRL group compared with that in the TAC group (52.2±8.4nmol/mg vs. 104.5±13.2nmol/mg; P<0.05; Figure 5(c)). The level of SOD and GSH was increased in the TAC+BRL group compared to the TAC group (Figures 5(d) and 5(e)). These findings indicated that β3-AR stimulation decreased oxidative stress induced by pressure overload.Figure 5
β3-AR stimulation decreased the oxidative stress level in pressure-overloaded mice. (a) Representative images of DHE and DCF staining. (b) NADPH oxidase activity. (c) The production of O2•- in the heart tissues. (d, e) SOD and GSH activity. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


### 3.6. BRL Treatment Regulated the Expression and Activation of NADPH Oxidase Subunits

We assessed the intracellular expressions of NADPH oxidase subunits by Western bolt assay. The expression of membrane-bound subunits, p22phox and NOX2, significantly increased in the TAC group, which was abolished by BRL treatment (Figures6(a)–6(g)). However, the expression of NOX4 was unchanged in all groups. Moreover, the membrane expressions of p47phox, p67phox, and Rac1, which are cytosolic subunits, were significantly upregulated in the TAC group and TAC+SR group. Moreover, BRL treatment resulted in decreased expression of p47phox, p67phox, and Rac1.Figure 6
BRL treatment regulated the expression and activation of NADPH oxidase subunits. (a–g) Western blot analysis of NOX2, NOX4, p22phox, p47phox, p67phox, and Rac1 expression. ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)


### 3.7.β3-AR Stimulation Regulates Activation and Expression of NOS Isoforms

Western blotting assays were performed to evaluateβ3-AR expression in all groups. The decreased β3-AR was observed in the TAC and TAC+SR groups (Figures 7(a) and 7(b)). Conversely, the expression of β3-AR increased after BRL treatment. We further evaluated the expression of NOS isoforms in all groups. First, the expression of total eNOS and phosphorylated eNOSSer114 was unchanged. However, TAC operation significantly decreased the phosphorylation of eNOSThr495 but increased expression of phospho-eNOSSer1177, which were abolished by the BRL treatment. Furthermore, we examined the expression of iNOS and nNOS. There was no difference in iNOS expression in all groups (Figures 7(a) and 7(d)). In contrary, pressure overload decreased the cardiac expression of nNOS, which was increased by the BRL treatment (Figures 7(a) and 7(h)).Figure 7
β3-AR stimulation regulates activation and expression of NOS isoforms. (a–h) Western blot analysis of β3-AR, nNOS, p-eNOSThr495, p-eNOSSer114, p-eNOSSer1177, eNOS, and iNOS. ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)(h)


## 3.1.β3-AR Stimulation Alleviated Cardiac Hypertrophy and Remodeling in Pressure-Overloaded Mice

The representative figures of hearts demonstrated that pressure overload caused ventricular dilatation (Figure1(a)). Meanwhile, the heart weight/tibia length ratio (HW/TL ratio) in TAC was significantly increased compared to sham ones (168.9±8.9mg/cm vs. 117.4±9.8mg/cm, P<0.05), indicating that pressure overload successfully induced cardiac pathological remodeling. Moreover, cross-sectional area, LV mass, and wall thickness all increased after TAC operation (Figures 1(b) and 1(d)–1(f)). These results suggested that mice developed evident cardiac hypertrophy induced by TAC. However, 3 weeks of BRL application significantly alleviated LV dilation, cardiac hypertrophy, and cardiac pathological remodeling. ANP and BNP levels significantly decreased after BRL treatment in TAC mice (Figures 1(g) and 1(h)). The size of cardiomyocytes in PE+BRL group was remarkably smaller compared to the PE group (Figures 1(i) and 1(j)). Thus, β3-AR stimulation reduced myocardial hypertrophy and remodeling induced by pressure overload.Figure 1
β3-AR stimulation alleviated cardiac hypertrophy and remodeling in pressure-overloaded mice. (a) Representative images of heart size. (b) Cardiomyocytes size as stained by WGA. (c) HW/TL. (d) Quantitative analysis of cardiomyocytes size. (e, f) LV mass and wall thickness measured by echocardiography. (g, h) The expression level of ANP and BNP. (i, j) Primary cardiomyocytes size as stained by α-actinin. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)


## 3.2.β3-AR Stimulation Reduced Fibrosis after TAC

BRL treatment significantly reduced fibrosis compared with the TAC group (Figures2(a) and 2(b)). Moreover, the increasing tendency of interstitial fibrosis was detected in the TAC+SR group, which is a β3-AR-specific inhibitor, although without significance. Moreover, the levels of Col1a1, Col3a1, and FBN1 were all reduced in the TAC mice after BRL treatment (Figures 2(c)–2(e)).Figure 2
β3-AR stimulation reduced fibrosis after TAC. (a, b) The level of cardiac fibrosis. (c–e) The expression of fibrotic remodeling marker genes (Col1a1, Col3a1, and FBN1). The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


## 3.3.β3-AR Stimulation Improved Cardiac Dysfunction in TAC Mice

As shown in Figure3(a), systolic dysfunction was revealed in the TAC and TAC+SR groups. Conversely, BRL treatment enhanced LV anterior wall motion after TAC operation, indicating that β3-AR stimulation could improve cardiac dysfunction induced by pressure overload. Moreover, the LVEDd and LVESd were increased in TAC mice. In addition, the LVEDd and LVESd were significantly decreased in TAC mice after BRL treatment (Figures 3(b) and 3(c)). Similarly, the enhanced LVEF and FS were observed in the TAC+BRL group, indicating that β3-AR stimulation improved cardiac dysfunction after TAC (Figures 3(d) and 3(e)).Figure 3
β3-AR stimulation improved pressure overload-induced cardiac dysfunction. (a) Representative images of echocardiography. (b–e) LVEDd, LVESd, LVEF, and LVFS measured by echocardiography. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


## 3.4. Cardiomyocyte Apoptosis Was Inhibited in TAC Mice after BRL Treatment

Figure4(a) reveals that, compared to the TAC group and TAC+SR group, BRL treatment significantly decreased the cardiomyocyte apoptosis index. The apoptosis index in the TAC+BRL group was 21.67±1.76%, less than that in the TAC group (35.3±3.6%, P<0.05) and TAC+SR group (34.7±4.7%, P<0.05) (Figure 4(b)). Meanwhile, caspase-3 enzymatic activity decreased in the TAC+BRL group compared to the TAC group (73.2±3.2 vs. 131.5±3.7, P<0.05) and TAC+SR group (73.2±3.2 vs. 140.2±6.7, P<0.05) (Figure 4(c)). These data suggested that β3-AR stimulation decreased apoptosis in the pressure-overloaded heart.Figure 4
Cardiomyocyte apoptosis was inhibited in TAC mice after BRL treatment. (a, b) Representative images of TUNEL staining and the apoptosis index. (c) Caspase-3 activity. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)


## 3.5.β3-AR Stimulation Decreased the Oxidative Stress Level in Pressure-Overloaded Mice

The production of ROS, measured by DHE and DCF staining, was significantly increased in the TAC group and TAC+SR group. However, BRL treatment significantly decreased ROS generation in pressure-overloaded mice (Figure5(a)). Meanwhile, the NADPH oxidase activity increased in the TAC group (301.5±19.1 units/min/mg vs. 127.1±13.1 units/min/mg; P<0.05) and TAC+SR group (316.7±24.5 units/min/mg vs. 127.1±13.1 units/min/mg; P<0.05). NADPH oxidase activity was lower in the TAC+BRL group compared to the TAC group (173.5±17.4 units/min/mg vs. 301.5±19.1 units/min/mg; P<0.05, Figure 5(b)). Similarly, the O2•- production was lower in the TAC+BRL group compared with that in the TAC group (52.2±8.4nmol/mg vs. 104.5±13.2nmol/mg; P<0.05; Figure 5(c)). The level of SOD and GSH was increased in the TAC+BRL group compared to the TAC group (Figures 5(d) and 5(e)). These findings indicated that β3-AR stimulation decreased oxidative stress induced by pressure overload.Figure 5
β3-AR stimulation decreased the oxidative stress level in pressure-overloaded mice. (a) Representative images of DHE and DCF staining. (b) NADPH oxidase activity. (c) The production of O2•- in the heart tissues. (d, e) SOD and GSH activity. The number of mice (n=6). ∗P<0.05.
(a)(b)(c)(d)(e)


## 3.6. BRL Treatment Regulated the Expression and Activation of NADPH Oxidase Subunits

We assessed the intracellular expressions of NADPH oxidase subunits by Western bolt assay. The expression of membrane-bound subunits, p22phox and NOX2, significantly increased in the TAC group, which was abolished by BRL treatment (Figures6(a)–6(g)). However, the expression of NOX4 was unchanged in all groups. Moreover, the membrane expressions of p47phox, p67phox, and Rac1, which are cytosolic subunits, were significantly upregulated in the TAC group and TAC+SR group. Moreover, BRL treatment resulted in decreased expression of p47phox, p67phox, and Rac1.Figure 6
BRL treatment regulated the expression and activation of NADPH oxidase subunits. (a–g) Western blot analysis of NOX2, NOX4, p22phox, p47phox, p67phox, and Rac1 expression. ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)


## 3.7.β3-AR Stimulation Regulates Activation and Expression of NOS Isoforms

Western blotting assays were performed to evaluateβ3-AR expression in all groups. The decreased β3-AR was observed in the TAC and TAC+SR groups (Figures 7(a) and 7(b)). Conversely, the expression of β3-AR increased after BRL treatment. We further evaluated the expression of NOS isoforms in all groups. First, the expression of total eNOS and phosphorylated eNOSSer114 was unchanged. However, TAC operation significantly decreased the phosphorylation of eNOSThr495 but increased expression of phospho-eNOSSer1177, which were abolished by the BRL treatment. Furthermore, we examined the expression of iNOS and nNOS. There was no difference in iNOS expression in all groups (Figures 7(a) and 7(d)). In contrary, pressure overload decreased the cardiac expression of nNOS, which was increased by the BRL treatment (Figures 7(a) and 7(h)).Figure 7
β3-AR stimulation regulates activation and expression of NOS isoforms. (a–h) Western blot analysis of β3-AR, nNOS, p-eNOSThr495, p-eNOSSer114, p-eNOSSer1177, eNOS, and iNOS. ∗P<0.05.
(a)(b)(c)(d)(e)(f)(g)(h)


## 4. Discussion

Cardiac remodeling, a major determinant of CHF, is associated with pathological cardiac hypertrophy [20]. Initially, cardiac hypertrophy occurs as an adaptive response to maintain normal cardiac function and output by ameliorating ventricular wall stress. However, sustained pressure load induces pathological ventricular hypertrophy, resulting in heart failure and malignant arrhythmias [20]. Our study suggested that sustained pressure overload for 3 weeks induced cardiac hypertrophy and increased the oxidative stress and cardiomyocytes apoptosis, which were associated with impaired cardiac function. But these effects of pressure overload could be abolished by β3-AR stimulation, which was in line with our previous study [24].Oxidative stress is involved in cardiomyocyte apoptosis, cardiac remodeling, cardiac dysfunction, and heart failure [25–28]. Oxidative stress and related mitochondrial damage are strongly associated with the progression of many diseases [29, 30]. Previous study suggested that increased NADPH oxidase activity was observed in end-stage failing myocardium [16]. And increasing evidence suggested the important role of myocardial NADPH oxidase in cardiovascular diseases [31–33]. NADPH oxidase activity is involved in the pathophysiology of congestive heart failure. In our study, β3-AR stimulation alleviated the oxidative stress by inhibiting NADPH oxidases. Moreover, the cardiac protective effect of BRL is mediated through NO generating via the nNOS pathway.The NOX subunit forms a stable heterodimer with the p22phox subunit. Among all NOX submits, NOX2 and NOX4 are mainly enriched in cardiomyocytes. Previous studies revealed that NOX2 deficiency inhibited cardiac hypertrophy [17]. Interestingly, a recent study demonstrated that cardiac dysfunction was exaggerated in NOX4-null mice. Moreover, NOX4 knockout exaggerated cardiac dysfunction when exposed to chronic overload, indicating NOX4 mediates protection against stress [34]. In current study, TAC treatment increased the NOX2 expression, whereas the expression of NOX4 was unchanged in all groups. Therefore, our results suggested that NOX2 is detrimental during pressure overload-induced remodeling. Furthermore, the expression of cytosolic subunits, including p47phox, p67phox, and Rac1, was significantly upregulated in TAC mice. Taken together, we found that the combination of NOX2 with a series of subunits directly activates the NADPH oxidases and generates superoxide, which subsequently results in the pressure overload-induced cardiac injury.It have been demonstrated that 3 subtypes ofβ-ARs may modulate cardiac function. Among them, β1-AR and β2-ARs mediate positive chronotropic and inotropic effects [20, 21]. β3-AR is reported to mediate lipolysis and thermogenesis in adipocytes [35]. However, β3-AR is also involved in cardiovascular system regulation, which may antagonize the effects of β1/2-ARs. Moreover, β3-AR is increased in failing hearts [24]. Meanwhile, β3-AR overexpression attenuated cardiac hypertrophy [36]. Furthermore, β3 knockout exacerbated cardiac hypertrophy induced by pressure overload [24]. In current study, β3-AR stimulation reduced hypertrophy, prevented fibrosis, and preserved cardiac function induced by pressure overload. Furthermore, β3-AR stimulation reduced the superoxide generation in TAC mice. Meanwhile, NADPH oxidase activity was also decreased after β3-AR stimulation, indicating that the protective effect of β3-AR is mediated, at least by part, through inhibiting NADPH oxidases, which contributes to the cardiac oxidative stress induced by pressure overload (Figure 8).Figure 8
Schematic diagram depicting possible mechanisms involved in protective effects ofβ3-AR agonist in cardiac hypertrophy.
(a)(b)β3-AR stimulation could increase NO release by NOS activity [36].Moreover, our previous study has demonstrated that β3-AR knockout exacerbates NOS uncoupling, suggesting that β3-AR regulates cardiac oxidase stress by regulating NO generation through NOS activity [24]. Three NOS isoforms are all associated with NO release. However, which NOS isoform regulates cardiac function by β3-AR signaling still remains unclear. The activity of eNOS is mainly affected by eNOS-PSer1177, eNOS-PThr495, and eNOS-PSer114. Phosphorylation at Ser1177 activates eNOS, whereas phosphorylations at Ser114 and Thr497 inhibit eNOS activity [37]. In the current study, the decreased p-eNOS Thr495 and increased p-eNOSSer1177 were observed in chronic pressure overload after BRL treatment. Therefore, β3-AR stimulation leads to eNOS deactivation rather than activation. In contrary, β3-AR stimulation increased the expression of nNOS. Moreover, both nNOS expression and nNOS-derived NO production regulate cardiac function [30]. The cardioprotective effects of β3-AR were actually abolished in nNOS–/– mice [24]. Based on these results, nNOS may be the primary downstream of β3-AR (Figure 8).In conclusion, we provide evidence thatβ3-AR stimulation regulates the oxidative stress by inhibiting NADPH oxidases, which impaired cardiac function. In addition, these cardioprotective effects of β3-AR are largely attributed to nNOS activation. These inspiring observations provide novel insight into β3-AR as a new target for treating cardiac hypertrophy.



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