# Astragaloside IV Reduces OxLDL-Induced BNP Overexpression by Regulating HDAC

Authors: Wenting Zhang; Xin Wang; Jing Li; Mingyuan Xu; Xiaolu Ren; Huiying Liu; Lu Xu; Jun Shao
Journal: Journal of Healthcare Engineering
 (2021)

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## Abstract

Effective drug intervention is the most important method to improve the prognosis, improve the quality of life, and prolong the life of patients with heart failure. This study aimed to explore the protective effect of astragaloside IV on myocardial cell injury induced by oxidized low-density lipoprotein (OxLDL) and its regulatory mechanism on the increase of brain natriuretic peptide (BNP) caused by myocardial cell injury. The model of myocardial cell injury, protection, and histone deacetylase (HDAC) inhibition in HL-1 mice was established by OxLDL treatment, astragaloside IV intervention, and UF010 coincubation. The effects of OxLDL and astragaloside IV on apoptosis were detected by flow cytometry. The expression level of BNP mRNA and protein in cells was investigated by real-time fluorescence quantification, western blot, and enzyme-linked immunosorbent assay. HDAC activity in nucleus was calibrated by fluorescence absorption intensity. Enzyme-linked immunosorbent assay (ELISA) was applied to test eNOS level in myocardial cells. OxLDL significantly promoted apoptosis, upregulated BNP mRNA, increased BNP protein level inside and outside cells, and decreased eNOS level. Compared with OxLDL treatment group, apoptosis decreased, BNP mRNA expression level decreased, BNP protein concentration decreased, and eNOS level increased significantly combined with low and high concentration astragaloside IV treatment group. HDAC activity significantly increased in OxLDL treatment group and significantly decreased after combined incubation with low and high concentrations of astragaloside IV. Inhibition of HDAC significantly increased eNOS level and decreased BNP protein level. In conclusion, astragaloside IV can reverse the low level of eNOS caused by OxLDL by regulating HDAC activity to protect myocardial cells from oxide damage, which is manifested by the decrease of BNP concentration.


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## Body

## 1. Introduction

European Society of Cardiology [1, 2] has confirmed that heart failure has become one of the most common diseases in the world in the twenty-first century. It is characterized by complicated diagnosis, difficulty in prevention, lack of treatment, and many complications. Most patients with heart failure have serious complications, such as diabetes and hypertension, which makes the treatment of heart failure extremely difficult [3, 4, 5]. There are many researches on drugs for treating heart failure, but the results are not good enough after clinical trials [6, 7]. In recent years, new perspectives have emerged to explore the pathogenesis and intervention methods of heart failure, such as intestinal flora regulation [8], macrophages and inflammatory factors [9, 10], and enzymology [11, 12]. It has become a focus and hot spot to explore the pathogenesis from multiple angles and find effective drug intervention.The heart secretes natriuretic peptide, which is composed of 21–35 amino acid residues. It can promote sodium excretion and urination, reduce the reabsorption of sodium by proximal convoluted tubules, and inhibit the release of aldosterone and plasma antidiuretic hormone. It plays an important role in normal physiology and acts as an antiregulatory hormone in heart failure. Clinical evaluation of its level (for the middle region of B-type natriuretic peptide (BNP), N-terminal proBNP, and N-terminal atrial natriuretic peptide) has become a valuable tool for diagnosing patients with heart failure and risk stratification [13, 14].Astragalus membranaceus is a traditional Chinese medicine commonly used in clinic, which has wide efficacy. Astragaloside IV is the main active ingredient of Astragalus membranaceus [15]. The latest research shows that Astragaloside IV plays an important role in the construction of immune system homeostasis and the treatment of diabetes [16]. The Chinese Pharmacopoeia has pointed out the outstanding contribution of astragaloside IV to cardiovascular treatment. Based on this mechanism, studies have emerged. For example, from the perspective of energy metabolism, astragaloside IV activates peroxisome proliferator-activated receptor a (PPARα), thus stimulating fatty acid ß-oxidation, finally improving mitochondrial function and increasing the efficiency of sarcoplasmic reticulum Ca2+α ATPase (SERCA) in heart failure [17]. Zhen-Hu Liu et al. [18] reported that astragaloside IV can prevent myocardial hypertrophy by inhibiting TBK1/PI3K/AKT activity. Hongliang Dai et al. [19] reported that astragaloside IV inhibited myocardial fibrosis by inhibiting ROS-mediated MAPK activation. Many studies have shown that astragaloside IV inhibits myocardial hypertrophy and fibrosis through energy metabolism related pathways, suggesting that astragaloside IV may have protective effects on myocardial injury and heart failure.Histone deacetylase (HDAC) is very important for tissue-specific gene regulation in development and pathology and has been widely explored in the field of tumor regulation [20]. It regulates the dynamic balance of acetylated histone in vivo together with histone acetylase. HDAC5 promotes the differentiation of monocytes into macrophages, while HDAC3 negatively regulates the development of bone marrow cells. In addition, HDAC inhibitors are also widely applied in antitumor treatment. Recent studies [21] have shown that HDAC can regulate heart Ncx1 and Bnp genes by regulating transcription regulatory complex, which suggest that HDAC has a potential role in regulating BNP to reduce myocardial injury.The role of astragaloside IV on cardiomyocytes remains to be further verified, and the molecular mechanism of this process has not been systematically studied. This study aimed to investigate the protective effect of astragaloside IV on oxidative-low-density-lipoprotein- (OxLDL-) induced myocardial cell injury and its HDAC regulation mechanism on the increase of brain natriuretic peptide (BNP) caused by myocardial cell injury.


## 2. Method

### 2.1. Cell Culture, Grouping, and Intervention

#### 2.1.1. Cell Culture

HL-1 mouse cardiomyocytes (L-0605, Procell Life Science&Technology Co., Ltd.) were cultured in DMED medium (11995073, Gibco, ThermoFisher) mixed with 10% fetal bovine serum (FBS, 16140071, Gibco, ThermoFisher) and 1% penicillin streptomycin double antibody solution (15140–15140, Gibco, ThermoFisher). In carbon dioxide culture environment (37 °C and 5%), HL-1 mouse cardiomyocytes grew healthily. In order to ensure good cell growth, fresh DMED medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was replaced every day. For the purpose of passage and experiment, cells were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip off the cells. After digestion was terminated with 5 ml of DMEM medium, the cells were centrifuged at 2000 rpm at room temperature for 5 min to precipitate. Finally, 5 ml DMEM medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was applied.


#### 2.1.2. Cell Grouping and Intervention

(1)
Blank Ccontrol Group. HL-1 cells were cultured in normal medium.(2)
OxLDL Treatment Group. The myocardial cell injury model was established by OxLDL [22]. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL, Ay-1501, Angyubio) was added. They were incubated for 24 h at 37 °C in 5% carbon dioxide environment.(3)
OxLDL + Low and High-Dose Astragaloside IV Treatment Group. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL) and astragaloside IV (12.5 µM or 50 µM, A111275, Aladdin) were added at 37 °C in 5% carbon dioxide environment. The concentration of astragaloside IV referred to the study of Lei Shang [23] et al.(4)
OxLDL + UF010 Treatment Group. When the cells naturally grew to 1∗106 cells/L, the original medium was discarded, and 5 mL of medium containing OxLDL (25 µg protein/ml) and 2 µM UF010 (HDAC inhibitor, HY-18976, MCE) were added and incubated at 37 °C with 5% CO2 for 24h. The concentration referred to the research of Yunfei Wang et al. [24].Treatment of Cells after Modeling. After the cells were exposed to stimuli (OxLDL, astragaloside IV, UF010) for 24h, they were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip the cells. After the digestion was terminated in 5 ml DMEM medium, the cells were centrifuged at 2000 rpm for 5 min at room temperature for preparation.


### 2.2. Flow Cytometry Was Used to Detect Apoptosis

After digestion, four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. A 500 µL Binding Buffer (KGF005, Jiangsu KeyGEN BioTECH Corp., Ltd) was applied to suspend cells. A 5 µL annexin V-FITC was added, and then, 5 µL propidium iodide (KGA105, Khaki) was added, fully mixed, and incubated at room temperature (20–25°C) for 10 min in dark. At last, the cells were tested using flow cytometry (FACSCantoII, Becton Dickinson, Oxford, UK) and analyzed by FlowJo 9.4 software (Tree Star, Ashland, OR). The excitation wavelength (Ex) = 488 nm and emission wavelength (Em) = 530. The green fluorescence of annexin V-FITC was detected through FITC channel (FL1), and the red fluorescence of PI was detected through FL3 channel.


### 2.3. Western Blot

Four groups of digested cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. RIPA buffer solution (KGP704, Jiangsu KeyGEN BioTECH Corp., Ltd) containing 1% protease inhibitor (87785, ThermoFisher Scientific) was added to extract total protein. The BCA protein assay kit (P0006, Beyotime) was used to determine the protein concentration, which was diluted to 4 µg/µL with RIPA buffer lysate. SDS-PAGE protein loading buffer (P0015, Beyotime) was added, and protein samples were obtained after denaturation at 100°C for 5 min. After SDS-PAGE electrophoresis of 40 µg protein samples in each tunnel, the protein was transferred to PVDF membrane (T2234, ThermoFisher Scientific), and the membrane was sealed with blocking buffer (P0252, Beyotime) at room temperature for 1h. Anti-BNP (1 : 2000, ab243440, Abcam) and Anti-GAPD (1 : 5000, ab181602, Abcam) were added and incubated at 4 °C for a night. TBST (KGP109-T, Jiangsu KeyGEN BioTECH Corp., Ltd) was washed three times, and then, HRP-labeled goat anti-rabbit (Abcam, ab205718, 1 : 10000) secondary antibody was added and incubated at room temperature for two hours. TBST was washed three times and then developed in chemiluminescence system (Millipore Corporation, Billerica) with ECL luminescent solution (P10300, New Cell and Molecular Biotech Co., Ltd). Image J (NIH, Bethesda) was used for gray analysis.


### 2.4. Real-Time Quantitative Polymerase Chain Reaction (qRT-PCR)

The digested four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were incubated for 24h at 37 °C in 5% carbon dioxide environment. They were digested and washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. 1 mL Trizol RNA separation reagent (R0016, Beyotime) was added to break the homogenate thoroughly on ice. The mixture was placed for 5 min and centrifuged at 12000 rpm for 15 min to get supernatant. RNA was precipitated using isopropanol and washed with 75% ethanol solution. Reverse transcription reagent (RR037Q, Takara) was used to reverse transcribe the extracted RNA into cDNA template. TB Green qPCR reagent (RR82LR, Takara) was used for real-time fluorescence quantitative polymerase chain reaction (Applied Biosystems ABI 7500). According to the obtained Ct value, the amplification multiple of the gene relative to the internal reference gene was calculated using 2-∆∆Ct. The amplification conditions were as follows: in the first stage, 95 °C for 30 s; in the second stage, 95 °C for 5s and 60 °C for 30s, and 40 cycles were carried out in this stage; in the third stage, 95 °C for 15s, 60 °C for 60s, and 95 °C for 15s. Sequence of primers: GAPDH: F 5′-AGG TCG GTG TGA ACG GAT TTG-3′, R 5′-TGT AGA CCA TGT AGT TGA GGT CA-3’; BNP: F 5′-CTGCTGGAGCTGATAAGAGA-3′, R 5′-TGCCCAAAGCAGCTTGAGAT-3’.


### 2.5. Elisa Was Used to Detect BNP Level

The four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were incubated at 37°C with 5% carbon dioxide for 24 h. Then, they were centrifuged at 2000g at 4°C for 20 min, the supernatant culture medium was collected, and the content of BNP in the medium was determined using mouse BNP Elisa kit (YS07029 B, Yaji Bio). A diluted protein standard solution was obtained according to the product specification, and blank well, standard well, and sample well were set. 50µL sample was added to each well, and the membrane was sealed with a membrane sealing plate and incubated at 37°C for 30 min. The sealing plate was carefully removed; the liquid was discarded and dried. Each well was added with washing liquid and placed for 30s, and the liquid was then discarded. The steps were repeated five times, and the wells were dried. 50 µL enzyme-labeled reagent was added to each well, and the membrane was sealed with sealing plate and incubated at 37°C for 30 min. The sealing plate was carefully removed, and the liquid was discarded and the wells were dried. Each well was filled with washing liquid, placed for 30 s, and then discard. The steps were repeated five times, and the wells were dried. A 50 L of chromogenic reagent A and 50 L of chromogenic reagent B were added to each well, shaken gently, and incubated at 37°C for 10 min in the dark. A 50 µL of stopping solution was added to each well to stop the reaction. At last, the corresponding absorption value was detected at 450 nm using microplate reader (Absorption 96, Byonoy, Germany), and the BNP protein concentration of each sample was calculated according to the standard curve.


### 2.6. Detection of eNOS Level in Myocardial Cells

The four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) in were used to detect eNOS level in myocardial cells by ELISA kit (ab253219, Abcam). The specific operation steps of Elisa were the same as method 5.


### 2.7. Extraction of Nuclei

To detect the HDAC activity in the nuclei of each cell group, four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were used to extract and purify the nuclei with kits. The whole operation process was carried out on ice (4°C). After constructing the cell model according to the previously mentioned method, the cells were digested and washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 100µL of about 1∗107 cells were collected. 1 mL ice precooled lysis buffer was added and shaken to resuspend cells. A 50 µL reagent A was added, evenly shaken, and mixed. The cell suspension was placed in ice-water bath for 20 min and shaken for 30 s every 5 min. Under a microscope, a large number of nuclei were free and a few cells were not lysed. The suspension was transferred to a centrifuge tube and centrifuged at 4°C. 1000 g of suspension was centrifuged for 3 min, and the supernatant was discarded to obtain a nuclear mixture. The nucleus was resuspended with 500 µL lysis buffer. 1000g of suspension was centrifuged at 4°C for 3 min, and the supernatant was discarded. 500 µL medium buffer A was added to resuspend the precipitate. The resuspension was placed on 1 ml medium buffer B, and 1000g suspension was centrifuged at 4°C for 10 min, and a relatively pure nuclear precipitate was obtained for subsequent experiments.


### 2.8. Detection of Nuclear HDAC Activity

The four groups of nuclear samples prepared successfully in method 7 were used to detect the activity of the enzyme in the nucleus by using HDAC activity detection kit (P-4001, Epigentek). Blank well, standard well, and sample well were set in the sample plate, and 50 µL HDAC substrate, 28µL HDAC detection reagent, and 2 µL nuclear sample were added into the sample well. The membrane was covered and incubated at 37°C for 60 min. Each well was washed with 150 µL washing buffer three times and dried. A 50 µL protein capture antibody was added to each well, shaken slightly, and incubated at 37°C for 60 min. Each well was washed with 150 µL washing buffer three times and dried. 50 µL protein detection antibody was added to each well, shaken slightly, and incubated at 37°C for 35 min. Each well was washed with 150 µL washing buffer three times and dried. 50 µL protein detection antibody was added to each well, shaken slightly, and incubated at 37°C for 35 min. 50 µL developer was added to each well, shaken slightly, and incubated at 37°C for 3 min. At last, the fluorescence value was obtained at 530Ex/590Em nm and the enzyme activity (RFU/h/g) = [(RFU control -RFU blank) -(RFU sample -RFU blank)]/(reaction time∗ protein amount), in which RFU represents relative fluorescence intensity.


### 2.9. Inhibition of Nuclear HDAC Activity

In this study, UF010 (HDAC inhibitor, HY-18976, MCE) was selected to effectively inhibit the activity of HDAC in cells and explore the effect of HDAC on BNP. When the cells naturally grew to 1∗106 cells/L, the original medium was discarded, and 5 mL of medium containing 2 µM UF010 was added and incubated at 37°C with 5% CO2 (the concentration was used according to the study by Yunfei Wang et al. [23]). To verify the inhibitory efficiency of UF010 and provide a stable and reliable cell model for the subsequent BNP detection, the nuclear HDAC activity of four groups of cells (NC group, OxLDL group, UF010 group, and OxLDL + UF010 group) in this part was detected by nuclear extraction and activity detection in Methods 1.7 and 1.8.


### 2.10. Statistical Methods

SPSS 19.0 and GraphPad Prism 8.0 were used for statistical analysis. The counting data were analyzed by one-way ANOVA, and the data were expressed byx ± s. P < 0.05 indicates the difference was statistically significant (∗indicates P < 0.05; ∗∗indicates P < 0.01; ∗∗∗indicates P < 0.001; NS: no significance).


## 2.1. Cell Culture, Grouping, and Intervention

### 2.1.1. Cell Culture

HL-1 mouse cardiomyocytes (L-0605, Procell Life Science&Technology Co., Ltd.) were cultured in DMED medium (11995073, Gibco, ThermoFisher) mixed with 10% fetal bovine serum (FBS, 16140071, Gibco, ThermoFisher) and 1% penicillin streptomycin double antibody solution (15140–15140, Gibco, ThermoFisher). In carbon dioxide culture environment (37 °C and 5%), HL-1 mouse cardiomyocytes grew healthily. In order to ensure good cell growth, fresh DMED medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was replaced every day. For the purpose of passage and experiment, cells were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip off the cells. After digestion was terminated with 5 ml of DMEM medium, the cells were centrifuged at 2000 rpm at room temperature for 5 min to precipitate. Finally, 5 ml DMEM medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was applied.


### 2.1.2. Cell Grouping and Intervention

(1)
Blank Ccontrol Group. HL-1 cells were cultured in normal medium.(2)
OxLDL Treatment Group. The myocardial cell injury model was established by OxLDL [22]. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL, Ay-1501, Angyubio) was added. They were incubated for 24 h at 37 °C in 5% carbon dioxide environment.(3)
OxLDL + Low and High-Dose Astragaloside IV Treatment Group. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL) and astragaloside IV (12.5 µM or 50 µM, A111275, Aladdin) were added at 37 °C in 5% carbon dioxide environment. The concentration of astragaloside IV referred to the study of Lei Shang [23] et al.(4)
OxLDL + UF010 Treatment Group. When the cells naturally grew to 1∗106 cells/L, the original medium was discarded, and 5 mL of medium containing OxLDL (25 µg protein/ml) and 2 µM UF010 (HDAC inhibitor, HY-18976, MCE) were added and incubated at 37 °C with 5% CO2 for 24h. The concentration referred to the research of Yunfei Wang et al. [24].Treatment of Cells after Modeling. After the cells were exposed to stimuli (OxLDL, astragaloside IV, UF010) for 24h, they were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip the cells. After the digestion was terminated in 5 ml DMEM medium, the cells were centrifuged at 2000 rpm for 5 min at room temperature for preparation.


## 2.1.1. Cell Culture

HL-1 mouse cardiomyocytes (L-0605, Procell Life Science&Technology Co., Ltd.) were cultured in DMED medium (11995073, Gibco, ThermoFisher) mixed with 10% fetal bovine serum (FBS, 16140071, Gibco, ThermoFisher) and 1% penicillin streptomycin double antibody solution (15140–15140, Gibco, ThermoFisher). In carbon dioxide culture environment (37 °C and 5%), HL-1 mouse cardiomyocytes grew healthily. In order to ensure good cell growth, fresh DMED medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was replaced every day. For the purpose of passage and experiment, cells were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip off the cells. After digestion was terminated with 5 ml of DMEM medium, the cells were centrifuged at 2000 rpm at room temperature for 5 min to precipitate. Finally, 5 ml DMEM medium (containing 10% FBS and 1% penicillin streptomycin double antibody solution) was applied.


## 2.1.2. Cell Grouping and Intervention

(1)
Blank Ccontrol Group. HL-1 cells were cultured in normal medium.(2)
OxLDL Treatment Group. The myocardial cell injury model was established by OxLDL [22]. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL, Ay-1501, Angyubio) was added. They were incubated for 24 h at 37 °C in 5% carbon dioxide environment.(3)
OxLDL + Low and High-Dose Astragaloside IV Treatment Group. When the cells naturally grew to a density of 1∗106 cells/L, the original culture medium was discarded, and 5 mL of culture medium containing OxLDL (25 µg protein/mL) and astragaloside IV (12.5 µM or 50 µM, A111275, Aladdin) were added at 37 °C in 5% carbon dioxide environment. The concentration of astragaloside IV referred to the study of Lei Shang [23] et al.(4)
OxLDL + UF010 Treatment Group. When the cells naturally grew to 1∗106 cells/L, the original medium was discarded, and 5 mL of medium containing OxLDL (25 µg protein/ml) and 2 µM UF010 (HDAC inhibitor, HY-18976, MCE) were added and incubated at 37 °C with 5% CO2 for 24h. The concentration referred to the research of Yunfei Wang et al. [24].Treatment of Cells after Modeling. After the cells were exposed to stimuli (OxLDL, astragaloside IV, UF010) for 24h, they were incubated with 0.05%- trypsin -EDTA (R001100, Gibco, ThermoFisher) at 37 °C for 5 min to strip the cells. After the digestion was terminated in 5 ml DMEM medium, the cells were centrifuged at 2000 rpm for 5 min at room temperature for preparation.


## 2.2. Flow Cytometry Was Used to Detect Apoptosis

After digestion, four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. A 500 µL Binding Buffer (KGF005, Jiangsu KeyGEN BioTECH Corp., Ltd) was applied to suspend cells. A 5 µL annexin V-FITC was added, and then, 5 µL propidium iodide (KGA105, Khaki) was added, fully mixed, and incubated at room temperature (20–25°C) for 10 min in dark. At last, the cells were tested using flow cytometry (FACSCantoII, Becton Dickinson, Oxford, UK) and analyzed by FlowJo 9.4 software (Tree Star, Ashland, OR). The excitation wavelength (Ex) = 488 nm and emission wavelength (Em) = 530. The green fluorescence of annexin V-FITC was detected through FITC channel (FL1), and the red fluorescence of PI was detected through FL3 channel.


## 2.3. Western Blot

Four groups of digested cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. RIPA buffer solution (KGP704, Jiangsu KeyGEN BioTECH Corp., Ltd) containing 1% protease inhibitor (87785, ThermoFisher Scientific) was added to extract total protein. The BCA protein assay kit (P0006, Beyotime) was used to determine the protein concentration, which was diluted to 4 µg/µL with RIPA buffer lysate. SDS-PAGE protein loading buffer (P0015, Beyotime) was added, and protein samples were obtained after denaturation at 100°C for 5 min. After SDS-PAGE electrophoresis of 40 µg protein samples in each tunnel, the protein was transferred to PVDF membrane (T2234, ThermoFisher Scientific), and the membrane was sealed with blocking buffer (P0252, Beyotime) at room temperature for 1h. Anti-BNP (1 : 2000, ab243440, Abcam) and Anti-GAPD (1 : 5000, ab181602, Abcam) were added and incubated at 4 °C for a night. TBST (KGP109-T, Jiangsu KeyGEN BioTECH Corp., Ltd) was washed three times, and then, HRP-labeled goat anti-rabbit (Abcam, ab205718, 1 : 10000) secondary antibody was added and incubated at room temperature for two hours. TBST was washed three times and then developed in chemiluminescence system (Millipore Corporation, Billerica) with ECL luminescent solution (P10300, New Cell and Molecular Biotech Co., Ltd). Image J (NIH, Bethesda) was used for gray analysis.


## 2.4. Real-Time Quantitative Polymerase Chain Reaction (qRT-PCR)

The digested four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were incubated for 24h at 37 °C in 5% carbon dioxide environment. They were digested and washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 5∗105 cells were collected. 1 mL Trizol RNA separation reagent (R0016, Beyotime) was added to break the homogenate thoroughly on ice. The mixture was placed for 5 min and centrifuged at 12000 rpm for 15 min to get supernatant. RNA was precipitated using isopropanol and washed with 75% ethanol solution. Reverse transcription reagent (RR037Q, Takara) was used to reverse transcribe the extracted RNA into cDNA template. TB Green qPCR reagent (RR82LR, Takara) was used for real-time fluorescence quantitative polymerase chain reaction (Applied Biosystems ABI 7500). According to the obtained Ct value, the amplification multiple of the gene relative to the internal reference gene was calculated using 2-∆∆Ct. The amplification conditions were as follows: in the first stage, 95 °C for 30 s; in the second stage, 95 °C for 5s and 60 °C for 30s, and 40 cycles were carried out in this stage; in the third stage, 95 °C for 15s, 60 °C for 60s, and 95 °C for 15s. Sequence of primers: GAPDH: F 5′-AGG TCG GTG TGA ACG GAT TTG-3′, R 5′-TGT AGA CCA TGT AGT TGA GGT CA-3’; BNP: F 5′-CTGCTGGAGCTGATAAGAGA-3′, R 5′-TGCCCAAAGCAGCTTGAGAT-3’.


## 2.5. Elisa Was Used to Detect BNP Level

The four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were incubated at 37°C with 5% carbon dioxide for 24 h. Then, they were centrifuged at 2000g at 4°C for 20 min, the supernatant culture medium was collected, and the content of BNP in the medium was determined using mouse BNP Elisa kit (YS07029 B, Yaji Bio). A diluted protein standard solution was obtained according to the product specification, and blank well, standard well, and sample well were set. 50µL sample was added to each well, and the membrane was sealed with a membrane sealing plate and incubated at 37°C for 30 min. The sealing plate was carefully removed; the liquid was discarded and dried. Each well was added with washing liquid and placed for 30s, and the liquid was then discarded. The steps were repeated five times, and the wells were dried. 50 µL enzyme-labeled reagent was added to each well, and the membrane was sealed with sealing plate and incubated at 37°C for 30 min. The sealing plate was carefully removed, and the liquid was discarded and the wells were dried. Each well was filled with washing liquid, placed for 30 s, and then discard. The steps were repeated five times, and the wells were dried. A 50 L of chromogenic reagent A and 50 L of chromogenic reagent B were added to each well, shaken gently, and incubated at 37°C for 10 min in the dark. A 50 µL of stopping solution was added to each well to stop the reaction. At last, the corresponding absorption value was detected at 450 nm using microplate reader (Absorption 96, Byonoy, Germany), and the BNP protein concentration of each sample was calculated according to the standard curve.


## 2.6. Detection of eNOS Level in Myocardial Cells

The four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) in were used to detect eNOS level in myocardial cells by ELISA kit (ab253219, Abcam). The specific operation steps of Elisa were the same as method 5.


## 2.7. Extraction of Nuclei

To detect the HDAC activity in the nuclei of each cell group, four groups of cells (blank control group, OxLDL treatment group, OxLDL + low dose astragaloside IV treatment group, and OxLDL + high-dose astragaloside IV treatment group) were used to extract and purify the nuclei with kits. The whole operation process was carried out on ice (4°C). After constructing the cell model according to the previously mentioned method, the cells were digested and washed twice with PBS (centrifuged at 2000 rpm for 5 min), and 100µL of about 1∗107 cells were collected. 1 mL ice precooled lysis buffer was added and shaken to resuspend cells. A 50 µL reagent A was added, evenly shaken, and mixed. The cell suspension was placed in ice-water bath for 20 min and shaken for 30 s every 5 min. Under a microscope, a large number of nuclei were free and a few cells were not lysed. The suspension was transferred to a centrifuge tube and centrifuged at 4°C. 1000 g of suspension was centrifuged for 3 min, and the supernatant was discarded to obtain a nuclear mixture. The nucleus was resuspended with 500 µL lysis buffer. 1000g of suspension was centrifuged at 4°C for 3 min, and the supernatant was discarded. 500 µL medium buffer A was added to resuspend the precipitate. The resuspension was placed on 1 ml medium buffer B, and 1000g suspension was centrifuged at 4°C for 10 min, and a relatively pure nuclear precipitate was obtained for subsequent experiments.


## 2.8. Detection of Nuclear HDAC Activity

The four groups of nuclear samples prepared successfully in method 7 were used to detect the activity of the enzyme in the nucleus by using HDAC activity detection kit (P-4001, Epigentek). Blank well, standard well, and sample well were set in the sample plate, and 50 µL HDAC substrate, 28µL HDAC detection reagent, and 2 µL nuclear sample were added into the sample well. The membrane was covered and incubated at 37°C for 60 min. Each well was washed with 150 µL washing buffer three times and dried. A 50 µL protein capture antibody was added to each well, shaken slightly, and incubated at 37°C for 60 min. Each well was washed with 150 µL washing buffer three times and dried. 50 µL protein detection antibody was added to each well, shaken slightly, and incubated at 37°C for 35 min. Each well was washed with 150 µL washing buffer three times and dried. 50 µL protein detection antibody was added to each well, shaken slightly, and incubated at 37°C for 35 min. 50 µL developer was added to each well, shaken slightly, and incubated at 37°C for 3 min. At last, the fluorescence value was obtained at 530Ex/590Em nm and the enzyme activity (RFU/h/g) = [(RFU control -RFU blank) -(RFU sample -RFU blank)]/(reaction time∗ protein amount), in which RFU represents relative fluorescence intensity.


## 2.9. Inhibition of Nuclear HDAC Activity

In this study, UF010 (HDAC inhibitor, HY-18976, MCE) was selected to effectively inhibit the activity of HDAC in cells and explore the effect of HDAC on BNP. When the cells naturally grew to 1∗106 cells/L, the original medium was discarded, and 5 mL of medium containing 2 µM UF010 was added and incubated at 37°C with 5% CO2 (the concentration was used according to the study by Yunfei Wang et al. [23]). To verify the inhibitory efficiency of UF010 and provide a stable and reliable cell model for the subsequent BNP detection, the nuclear HDAC activity of four groups of cells (NC group, OxLDL group, UF010 group, and OxLDL + UF010 group) in this part was detected by nuclear extraction and activity detection in Methods 1.7 and 1.8.


## 2.10. Statistical Methods

SPSS 19.0 and GraphPad Prism 8.0 were used for statistical analysis. The counting data were analyzed by one-way ANOVA, and the data were expressed byx ± s. P < 0.05 indicates the difference was statistically significant (∗indicates P < 0.05; ∗∗indicates P < 0.01; ∗∗∗indicates P < 0.001; NS: no significance).


## 3. Results

### 3.1. Apoptosis of Myocardial Cells in HL-1 Mice in Each Group

To verify the damage of OxLDL to myocardial cells and further explore the protective effect of astragaloside IV on injured myocardial cells, OxLDL intervention was applied to construct a myocardial injury model. Low (12.5µM) and high (50 µM) concentration astragaloside IV combined intervention were simultaneously used. Flow cytometry showed that the proportion of apoptotic HL-1 cells in OxLDL treatment group was significantly higher than that in blank group (P < 0.001), indicating that the method of inducing myocardial cell injury was stable and reliable. Compared with OxLDL treatment group, the apoptosis in the combined use of astragaloside IV group was significantly declined (all P < 0.001), and the role of high concentration astragaloside IV group was more obvious than that of low-concentration group (P < 0.001). There was no significant difference between the high-concentration group and blank control group (P > 0.05). The results of flow cytometric analysis of HL-1 cardiomyocyte apoptosis indicated that OxLDL can stably induce myocardial cell injury, and astragaloside IV can reduce OxLDL injury in a dose-dependent manner within a certain concentration range (Figure 1).Figure 1
The apoptosis in each group was detected by flow cytometry.


### 3.2. Expression Level of BNP mRNA and Protein in Myocardial Cells of HL-1 Mice in Each Group

To further verify the effects of OxLDL and astragaloside IV on myocardial cell injury, the transcription and protein expression of BNP in HL-1 mouse myocardial cells were detected by real-time fluorescence quantitative PCR and western blot. PCR results (Figure2(a)) showed that BNP mRNA in HL-1 cells was significantly increased after OxLDL intervention (q = 19.01, P < 0.001). BNP mRNA level in cells treated with low and high concentration astragaloside IV was significantly decreased compared with OxLDL intervention group (q1 = 12.33, q2 = 18.26, all P < 0.001). The effect of high-concentration astragaloside group was more obvious than that of low-concentration group (q = 5.93, P = 0.002). There was no significant difference between the high-concentration group and blank control group (q = 0.74, P = 0.951). The western blot results (Figure 2(b)) were consistent with mRNA. The BNP protein level in OxLDL intervention group was significantly higher than that in blank control group (P < 0.001), while astragaloside IV significantly decreased the BNP protein level in cells in a dose-dependent manner (all P < 0.001). The results of mRNA and protein detection suggested that OxLDL significantly upregulated BNP level in HL-1 cardiomyocytes, and astragaloside IV can reverse OxLDL-induced cardiomyocyte injury by reducing the mRNA and protein levels of BNP.Figure 2
Comparison of BNP mRNA and protein expression levels in each group. (a) The expression level of BNP mRNA in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group. (b) Western blot results of BNP protein in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5 µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).
(a)(b)


### 3.3. BNP Protein Concentration in Culture Medium of HL-1 Mouse Cardiomyocytes in Each Group

The secretion and transport of BNP protein by HL-1 cells play important roles in myocardial injury. In this research, the concentration of BNP in cell culture medium was detected by ELISA. ELISA results showed that BNP concentration in cell culture medium intervened by OxLDL was significantly higher than that in blank control group (q = 17.87, P < 0.001), while BNP concentration in cell culture intervened by astragaloside IV was significantly lower than that in OxLDL intervention group (q1 = 9.59, q2 = 15.64, all P < 0.001). BNP concentration in high-concentration astragaloside IV intervention group was not significantly higher than that in blank control group. The results suggested that OxLDL significantly increased the secretion of BNP protein in HL-1 cells, while astragaloside IV decreased the secretion of BNP protein in a dose-dependent manner within a certain concentration range (Figure 3).Figure 3
Comparison of BNP levels in cell culture media of each group. The level of BNP in HL-1 cell culture medium of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5 µM) treatment group, and OxLDL + high-dose astragaloside IV (50µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


### 3.4. HDAC Activity in Myocardial Nuclei of HL-1 Mice in Each Group

In order to explore the mechanism of astragaloside IV protecting myocardial cells from OxLDL injury, we quantitatively explored the protein activity of HDAC by using enzyme activity detection kit. The fluorescence results showed that OxLDL significantly increased HDAC activity in HL-1 cells (q = 16.72, P < 0.001), while both low and high concentrations of astragaloside IV significantly decreased HDAC activity induced by OxLDL (q1 = 17.23, q2 = 30.31, all P < 0.001) in a dose-dependent manner. There was no significant difference in HDAC activity between high-concentration astragaloside group and blank control group (q = 3.64, P = 0.085) (Figure 4).Figure 4
Comparison of HDAC fluorescence activity in nuclei of each group. The fluorescence intensity of HDAC in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


### 3.5. iNOS Level in Cells of Each Group

iNOS function not only targets pathogen killing but also has immune-regulatory effects, such as inhibiting T cell activity. In order to explore the mechanism of HDAC in OxLDL-induced myocardial cell injury, the levels of iNOS in each group of cells were detected by ELISA. OxLDL stimulated myocardial cells, and the iNOS concentration significantly increased (q = 18.03, P < 0.001). Compared with OxLDL group, the iNOS concentration was significantly decreased in a dose-dependent manner (q1 = 6.605, p2< 0.01, q2 = 9.678, p2 < 0.001), as shown in Figure 5.Figure 5
iNOS levels in cells of each group. The levels of iNOS in HL-1 cells in blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


### 3.6. Inhibition of HDAC on BNP Protein Level

Based on the role of astragaloside IV on the inhibition of HDAC activity in HL-1 cells, this study further explored the effect of HDAC on BNP level by western blot. After the application of the inhibitor, the activity of HDAC in nucleus decreased obviously (q = 39.64, P < 0.001, Figure 6(a)). The results of western blot showed that OxLDL significantly upregulated the expression of BNP protein in HL-1 cells, and the expression of BNP protein in HL-1 cells decreased significantly after applying HDAC inhibitor (P < 0.001). These results suggested that HDAC plays a role of a bridge protein for astragaloside IV to protect cardiomyocytes from OxLDL injury.Figure 6
Expression level of BNP protein in each group after inhibiting HDAC activity. (a) The fluorescence intensity level of HDAC in nuclei of blank control group, OxLDL intervention group, UF010, and OxLDL + UF010 groups. (b) Western blot results of BNP protein in blank control group, OxLDL intervention group, UF010, and OxLDL + UF010 groups (∗∗∗indicates P < 0.001; ∗indicates P < 0.05; NS: no significance; n = 5).
(a)(b)


## 3.1. Apoptosis of Myocardial Cells in HL-1 Mice in Each Group

To verify the damage of OxLDL to myocardial cells and further explore the protective effect of astragaloside IV on injured myocardial cells, OxLDL intervention was applied to construct a myocardial injury model. Low (12.5µM) and high (50 µM) concentration astragaloside IV combined intervention were simultaneously used. Flow cytometry showed that the proportion of apoptotic HL-1 cells in OxLDL treatment group was significantly higher than that in blank group (P < 0.001), indicating that the method of inducing myocardial cell injury was stable and reliable. Compared with OxLDL treatment group, the apoptosis in the combined use of astragaloside IV group was significantly declined (all P < 0.001), and the role of high concentration astragaloside IV group was more obvious than that of low-concentration group (P < 0.001). There was no significant difference between the high-concentration group and blank control group (P > 0.05). The results of flow cytometric analysis of HL-1 cardiomyocyte apoptosis indicated that OxLDL can stably induce myocardial cell injury, and astragaloside IV can reduce OxLDL injury in a dose-dependent manner within a certain concentration range (Figure 1).Figure 1
The apoptosis in each group was detected by flow cytometry.


## 3.2. Expression Level of BNP mRNA and Protein in Myocardial Cells of HL-1 Mice in Each Group

To further verify the effects of OxLDL and astragaloside IV on myocardial cell injury, the transcription and protein expression of BNP in HL-1 mouse myocardial cells were detected by real-time fluorescence quantitative PCR and western blot. PCR results (Figure2(a)) showed that BNP mRNA in HL-1 cells was significantly increased after OxLDL intervention (q = 19.01, P < 0.001). BNP mRNA level in cells treated with low and high concentration astragaloside IV was significantly decreased compared with OxLDL intervention group (q1 = 12.33, q2 = 18.26, all P < 0.001). The effect of high-concentration astragaloside group was more obvious than that of low-concentration group (q = 5.93, P = 0.002). There was no significant difference between the high-concentration group and blank control group (q = 0.74, P = 0.951). The western blot results (Figure 2(b)) were consistent with mRNA. The BNP protein level in OxLDL intervention group was significantly higher than that in blank control group (P < 0.001), while astragaloside IV significantly decreased the BNP protein level in cells in a dose-dependent manner (all P < 0.001). The results of mRNA and protein detection suggested that OxLDL significantly upregulated BNP level in HL-1 cardiomyocytes, and astragaloside IV can reverse OxLDL-induced cardiomyocyte injury by reducing the mRNA and protein levels of BNP.Figure 2
Comparison of BNP mRNA and protein expression levels in each group. (a) The expression level of BNP mRNA in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group. (b) Western blot results of BNP protein in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5 µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).
(a)(b)


## 3.3. BNP Protein Concentration in Culture Medium of HL-1 Mouse Cardiomyocytes in Each Group

The secretion and transport of BNP protein by HL-1 cells play important roles in myocardial injury. In this research, the concentration of BNP in cell culture medium was detected by ELISA. ELISA results showed that BNP concentration in cell culture medium intervened by OxLDL was significantly higher than that in blank control group (q = 17.87, P < 0.001), while BNP concentration in cell culture intervened by astragaloside IV was significantly lower than that in OxLDL intervention group (q1 = 9.59, q2 = 15.64, all P < 0.001). BNP concentration in high-concentration astragaloside IV intervention group was not significantly higher than that in blank control group. The results suggested that OxLDL significantly increased the secretion of BNP protein in HL-1 cells, while astragaloside IV decreased the secretion of BNP protein in a dose-dependent manner within a certain concentration range (Figure 3).Figure 3
Comparison of BNP levels in cell culture media of each group. The level of BNP in HL-1 cell culture medium of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5 µM) treatment group, and OxLDL + high-dose astragaloside IV (50µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


## 3.4. HDAC Activity in Myocardial Nuclei of HL-1 Mice in Each Group

In order to explore the mechanism of astragaloside IV protecting myocardial cells from OxLDL injury, we quantitatively explored the protein activity of HDAC by using enzyme activity detection kit. The fluorescence results showed that OxLDL significantly increased HDAC activity in HL-1 cells (q = 16.72, P < 0.001), while both low and high concentrations of astragaloside IV significantly decreased HDAC activity induced by OxLDL (q1 = 17.23, q2 = 30.31, all P < 0.001) in a dose-dependent manner. There was no significant difference in HDAC activity between high-concentration astragaloside group and blank control group (q = 3.64, P = 0.085) (Figure 4).Figure 4
Comparison of HDAC fluorescence activity in nuclei of each group. The fluorescence intensity of HDAC in HL-1 cells of blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


## 3.5. iNOS Level in Cells of Each Group

iNOS function not only targets pathogen killing but also has immune-regulatory effects, such as inhibiting T cell activity. In order to explore the mechanism of HDAC in OxLDL-induced myocardial cell injury, the levels of iNOS in each group of cells were detected by ELISA. OxLDL stimulated myocardial cells, and the iNOS concentration significantly increased (q = 18.03, P < 0.001). Compared with OxLDL group, the iNOS concentration was significantly decreased in a dose-dependent manner (q1 = 6.605, p2< 0.01, q2 = 9.678, p2 < 0.001), as shown in Figure 5.Figure 5
iNOS levels in cells of each group. The levels of iNOS in HL-1 cells in blank control group, OxLDL intervention group, OxLDL + low dose astragaloside IV (12.5µM) treatment group, and OxLDL + high-dose astragaloside IV (50 µM) treatment group (∗∗∗indicates P < 0.001; NS: no significance; n = 5).


## 3.6. Inhibition of HDAC on BNP Protein Level

Based on the role of astragaloside IV on the inhibition of HDAC activity in HL-1 cells, this study further explored the effect of HDAC on BNP level by western blot. After the application of the inhibitor, the activity of HDAC in nucleus decreased obviously (q = 39.64, P < 0.001, Figure 6(a)). The results of western blot showed that OxLDL significantly upregulated the expression of BNP protein in HL-1 cells, and the expression of BNP protein in HL-1 cells decreased significantly after applying HDAC inhibitor (P < 0.001). These results suggested that HDAC plays a role of a bridge protein for astragaloside IV to protect cardiomyocytes from OxLDL injury.Figure 6
Expression level of BNP protein in each group after inhibiting HDAC activity. (a) The fluorescence intensity level of HDAC in nuclei of blank control group, OxLDL intervention group, UF010, and OxLDL + UF010 groups. (b) Western blot results of BNP protein in blank control group, OxLDL intervention group, UF010, and OxLDL + UF010 groups (∗∗∗indicates P < 0.001; ∗indicates P < 0.05; NS: no significance; n = 5).
(a)(b)


## 4. Discussion

Based on the inhibitory effects of astragaloside IV and its derivatives on myocardial hypertrophy and myocardial fibrosis [25], the effects of low and high concentrations on OxLDL-induced myocardial cell injury and its mechanism were explored in this research. The results showed that astragaloside IV could regulate the decrease of HDAC, thereby inhibiting OxLDL-induced BNP production, thus protecting myocardial cells from OxLDL injury, which could induce heart failure. This research highlights the contribution of Chinese medicine in anticardiovascular diseases and makes up for the defect that Chinese medicine has no mechanism to support its pharmacological action against cardiovascular diseases.A series of pathological and physiological overload stimuli (such as excessive exercise and pregnancy) can lead to the growth (hypertrophy) of myocardial cells, the increase of extracellular matrix protein deposition (fibrosis), and necrosis. Excessive apoptosis or phagocytosis can lead to the loss of myocardial cells [26–29], suggesting that myocardial cell injury is an important part in heart failure. Therefore, the myocardial cell injury model was constructed by using OxLDL, and the apoptotic cell markers were counted using flow cytometry. It was found that OxLDL can cause a large number of myocardial cell apoptosis, while astragaloside IV can significantly improve the apoptosis, and high-dose astragaloside IV can even reverse the apoptosis. In other words, astragaloside IV may prevent heart failure from cell deformation and apoptosis.To further verify the effect of astragaloside IV on injured myocardial cells, this study started with molecular markers of myocardial injury. BNP has been commonly used in heart failure. Natriuretic peptide, especially B-type diuretic peptide (BNP), N-terminal pro-B-type diuretic peptide (NT-BNP), central regional atrial natriuretic peptide (MR-proANP), and cardiac troponin are established blood biomarkers in the diagnosis of heart failure and prognosis of HF-related outcomes [16, 30, 31]. In this study, the transcription level and protein content inside and outside cells of BNP intervened by astragaloside IV was investigated. mRNA detection showed that OxLDL could significantly increase BNP expression in cells, suggesting that the transcription level of cells was already in abnormal state. Combined with astragaloside IV intervention, BNP mRNA level was significantly downregulated, and high-dose astragaloside IV mRNA transcription returned to normal levels. The changes of protein level are consistent with mRNA, and astragaloside IV can downregulate the high level of BNP protein induced by OxLDL. As for the level of protein secretion and transport, the content of BNP protein in cell culture medium was detected. The results showed that OxLDL could significantly increase the secretion of BNP protein, while astragaloside IV could reverse this state. To sum up, astragaloside IV reduced BNP level in mRNA, protein expression, and protein secretion, which confirmed that astragaloside IV has certain protective effects on cell damage and may delay the process of heart failure.To explore the mechanism of astragaloside IV in improving the high level of BNP in cardiomyocytes induced by OxLDL, this study focused on the upstream of BNP and explored the possible influencing factors of BNP. According to relevant literature reports, HDAC not only plays a role in the occurrence and development of cancer [32] but also acts in the basic immune process (production and infiltration of inflammatory factors) and the occurrence and development of cardiovascular diseases [33–36]. HDAC inhibitors can effectively inhibit the proliferation of vascular smooth muscle cells and prevent atherosclerosis to some extent. HDAC can cause arrhythmia, and when HDAC is overexpressed, sodium and potassium ions will be maladjusted in varying degrees. HDAC inhibitors can reduce I/R injury, improve cardiac dysfunction, and minimize infarct size by restoring PGC-1 in I/R injury and improving fatty acid oxidation. To sum up, the role of HDAC in cardiovascular system is increasingly prominent, so the role of HDAC in alleviating the high BNP level induced by OxLDL in cardiac myocytes was explored in this study.


## 5. Conclusion

In conclusion, this study found that HDAC activity in nucleus was obviously enhanced after OxLDL treatment, while HDAC activity in nucleus was obviously downregulated after astragaloside IV intervention. The results suggested that astragaloside IV has a significant regulatory effect on HDAC. Then, we simulated the inhibitory effect of astragaloside IV on HDAC and blocked HDAC in cells. The results showed that the high level of BNP induced by OxLDL significantly decreased, suggesting that the inhibitory effect of astragaloside IV on BNP was partly correlated to the inhibition of HDAC. Based on research, we have made progress in exploring the changes of iNOS in protecting myocardial cells from injury by astragaloside IV. The level of iNOS in cells stimulated by OxLDL significantly increased, which further suggested that OxLDL could stably induce myocardial cell injury model, and astragaloside IV could reverse the high level of iNOS. In this study, from the perspective of HDAC enzyme, the inhibitory effect of astragaloside IV on HDAC and BNP induced by OxLDL was revealed, providing hard evidence for the application of astragaloside IV in cardiovascular system.This study also has the following shortcomings, which need to be further improved. First of all, the research on the effect of astragaloside IV on heart failure only involved myocardial cell injury, and the pathogenesis of heart failure is complex. Moreover, there are many incentives and complicated complications, so this study cannot comprehensively discuss the influence of astragaloside IV on all factors. Therefore, the conclusions need to be further expanded and verified. Second, the most basic step is to explore the function of astragaloside IV at the cellular level, and the focus of follow-up research is to explore the pharmacological and toxicological effects of astragaloside IV at the animal level. Finally, the support of clinical data is essential for astragaloside IV to expand to a wider application prospect. This study focused on basic research, and the subsequent inclusion of clinical data is also important. [36].



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