Exploring Regulatory Mechanisms of Atrial Myocyte Hypertrophy of Mitral Regurgitation through Gene Expression Profiling Analysis: Role of NFAT in Cardiac Hypertrophy

Abstract

Background

Left atrial enlargement in mitral regurgitation (MR) predicts a poor prognosis. The regulatory mechanisms of atrial myocyte hypertrophy of MR patients remain unknown.

Methods and Results

This study comprised 14 patients with MR, 7 patients with aortic valve disease (AVD), and 6 purchased samples from normal subjects (NC). We used microarrays, enrichment analysis and quantitative RT-PCR to study the gene expression profiles in the left atria. Microarray results showed that 112 genes were differentially up-regulated and 132 genes were differentially down-regulated in the left atria between MR patients and NC. Enrichment analysis of differentially expressed genes demonstrated that “NFAT in cardiac hypertrophy” pathway was not only one of the significant associated canonical pathways, but also the only one predicted with a non-zero score of 1.34 (i.e. activated) through Ingenuity Pathway Analysis molecule activity predictor. Ingenuity Pathway Analysis Global Molecular Network analysis exhibited that the highest score network also showed high association with cardiac related pathways and functions. Therefore, 5 NFAT associated genes (PPP3R1, PPP3CB, CAMK1, MEF2C, PLCE1) were studies for validation. The mRNA expressions of PPP3CB and MEF2C were significantly up-regulated, and CAMK1 and PPP3R1 were significantly down-regulated in MR patients compared to NC. Moreover, MR patients had significantly increased mRNA levels of PPP3CB, MEF2C and PLCE1 compared to AVD patients. The atrial myocyte size of MR patients significantly exceeded that of the AVD patients and NC.

Conclusions

Differentially expressed genes in the “NFAT in cardiac hypertrophy” pathway may play a critical role in the atrial myocyte hypertrophy of MR patients.

About the Authors:

Tzu-Hao Chang

Affiliation: Graduate Institute of Biomedical Informatics, Taipei Medical University, Taipei, Taiwan

Mien-Cheng Chen

* E-mail: [email protected]

Affiliation: Division of Cardiology and Department of Internal Medicine, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan

Jen-Ping Chang

Affiliation: Division of Cardiovascular Surgery, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan

Hsien-Da Huang

Affiliation: Institute of Bioinformatics and Systems Biology, National Chiao Tung University, Hsinchu, Taiwan

Wan-Chun Ho

Affiliation: Division of Cardiology and Department of Internal Medicine, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan

Yu-Sheng Lin

Affiliation: Division of Cardiology, Chang Gung Memorial Hospital, Chiayi, Taiwan

Kuo-Li Pan

Affiliation: Division of Cardiology, Chang Gung Memorial Hospital, Chiayi, Taiwan

Yao-Kuang Huang

Affiliation: Department of Thoracic and Cardiovascular Surgery, Chang Gung Memorial Hospital, Chiayi, Taiwan

Wen-Hao Liu

Affiliation: Division of Cardiology and Department of Internal Medicine, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan

Chia-Chen Wu

Affiliation: Division of Cardiovascular Surgery, Kaohsiung Chang Gung Memorial Hospital, Chang Gung University College of Medicine, Kaohsiung, Taiwan

Introduction

Mitral regurgitation (MR) is an important cause of heart failure related to valvular heart disease [1]. Left atrial enlargement has prognostic significance in MR patients undergoing mitral valve surgery [2]. Structural remodeling associated with atrial enlargement, especially pathological hypertrophy of myocytes, developed in the left atrial myocardium of patients with MR [3,4]. However, the molecular regulatory mechanisms and functional biological pathways related to the left atrial myocyte hypertrophy of MR patients remain unclear.

In this study, we aimed to systemically explore the crucial differences in the RNA expression pattern between the left atrial myocardium of MR patients and normal subjects, and the molecular regulatory mechanisms and functional biological pathways related to the atrial myocyte hypertrophy using high-density oligonucleotide microarrays and enrichment analysis. The left atrial myocardium of patients with severe aortic valve disease was also used as a reference for gene analysis of the significant pathways as the left atrial size was smaller in patients with aortic valve disease compared to MR patients. The results of this study may recognize some of the differentially expressed genes and related pathways that contribute to the left atrial myocyte hypertrophy in patients with MR.

Methods

Patient Population

This study enrolled 14 patients with symptomatic severe non-ischemic MR in sinus rhythm (age: 58±9 years), and 7 age-matched patients with symptomatic severe aortic valve disease in sinus rhythm (age: 63±7 years; aortic stenosis in 1, aortic regurgitation in 4, combined aortic stenoregurgitation in 2). Exclusion factors include previous myocardial infarction, febrile disorder, infectious or inflammatory disease, autoimmune disease, malignancy, acute or chronic viral hepatitis or use of immunosuppressive drugs. Written informed consent was obtained from each study patient, and the study protocol conforms to the ethical guidelines of the 1975 Declaration of Helsinki as reflected in a priori approval by the Institutional Review Board of Chang Gung Memorial Hospital (100-0067C).

Six normal adult left atrial tissue samples (24-year-old Caucasian male, 27-year-old Caucasian male, 30-year-old Asian male, 60-year-old Caucasian female, 76-year-old Caucasian female and 77-year-old Caucasian male) were purchased from BioChain, Newark, CA, USA, and these 6 normal atrial tissues were used as the normal controls for gene analysis.

Specimen Storage

Atrial tissues of non-ischemic MR patients and aortic valve disease patients with heart failure were sampled from the left atrial free wall during surgery. After excision, some atrial tissues were immediately frozen in liquid nitrogen and stored at –80 Celsius, and some were immediately fixed in 3.7% buffered formalin, then embedded in paraffin, and stored until later study for hematoxylin/eosin staining.

Microarray Analysis and Data Processing

RNAs were extracted from the myocardial samples by using a RiboPureTM kit (Ambion, Grand Island, NY, USA) according to the manufacturer's protocol. RNA quality was assessed using an Agilent 2100 Bioanalyzer (Agilent Technologies Inc, Santa Clara, CA, USA). Samples with optical density ratio 260/280 > 1.8 and RNA integrity number > 7.0 were selected and sent for microarray processing. Two hundred fifty ng of total RNA per sample was used for cRNA production by the RiboPure^TMRNA extraction kit (Ambion, Grand Island, NY, USA). The quality of cRNA was evaluated using the RNA 6000 pico kit (Agilent Technologies, Santa clara,CA,USA) and the Experion automated electrophoresis station (Bio-Rad Laboratories, Inc., Hercules, CA, USA). A total of 750 ng cRNA was used for hybridization to a human HT12-v4 Illumina Beadchip gene expression array (Illumina, San Diego, CA, USA), including 47231 probes and 28688 annotated genes, according to the manufacturer’s protocol. The arrays were scanned and fluorescence signals obtained using the Illumina Bead Array Reader (Illumina, San Diego, CA, USA). Microarray quality control and normalization was performed using Illumina GenomeStudio data analysis software. The expression level of a gene was represented by the average probe intensity. Functional classes were assigned to all known genes using information from the Gene Ontology database available at the website (http://amigo.geneontology.org/cgi-bin/amigo/go.cgi). Additionally, we applied the activation z-score analysis method [5] to measure activation states (increased or decreased) of the pathways affected by differentially expressed genes. The sign of the calculated z-score will reflect the overall predicted activation state of the biological function (<0: decreased, >0: increased).

Quantitative Determination of RNAs by Real-Time RT-PCR

The RNA samples were quantified using a spectrophotometer. First-strand cDNAs were synthesized with reverse transcriptase and oligo (dT) primers. Real-time quantitative PCR was performed on the ABI Prism 7500 FAST sequence detection system (Applied Biosystems, CA, USA), using SYBR Green PCR Master Mix (Applied Biosystems, CA, USA). The results of RNAs were normalized against 18S gene expression (the endogenous control). The selected genes and primer sequences are presented in Table 1. The microRNAs (miRs) were extracted from the tissues by using a RNA MiniPrep kit (Zymo Research, CA, USA) according to the manufacturer’s protocol. Reverse transcription of miRs was performed using the TaqMan™ microRNA reverse transcription kit (Applied Biosystems, CA, USA) according to manufacturer's recommendations. Briefly, 5 ng of miR was combined with deoxyribonucleoside triphosphates, MultiScribe™ reverse transcriptase, and the primer specific for the target miR (Applied Biosystems, CA, USA). The cDNA was combined with the TaqMan™ assay specific for the target miR. The results of miRs were normalized against U6 snRNA (Applied Biosystems, CA, USA). Quantitative RT-PCR values were presented in △Cq units.

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Table 1. Primer Sequences for Real-Time PCR.

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Western Blotting

The protein extracts of human atrial tissues were examined by Western blot analysis. 20μg protein extracts were electrophoresed on 10% acrylamide SDS-PAGE gels and immunoblotted onto polyvinylidene difluoride membranes. The membranes were preblocked for 1 h in TBST (10 mM Tris–HCl pH 7.6, 150 mM NaCl, 0.1% Tween-20) containing 5% w/v nonfat dry milk and then incubated at 4°C overnight with anti-α-sarcomeric actin (Sigma Aldrich, Louis, MO, USA). The result of protein was normalized against GAPDH.

Histological Analysis

Atrial tissue sections were deparaffinized in xylene and rehydrated in decreasing concentrations of alcohol. Slides were then stained with hematoxylin/eosin. Tissue sections were observed under an Olympus BX51 microscope with the analysis including at least 100 randomly selected cells under 400 X magnification. All images of each specimen were captured using an Olympus DP70 camera. Atrial cardiomyocytes were analyzed (UTHSCSA, Image tool, Version 3.0).

Cell Culture and Mechanical Stretching of HL-1 Atrial Myocytes

HL-1 atrial myocytes were plated on silicone rubber culture dishes. HL-1 atrial myocytes were cultured for 24 hours in claycomb medium containing 10% FBS, penicillin and streptomycin. Thereafter, culture medium was changed to serum free claycomb medium and HL-1 atrial myocytes were stretched for an additional 8 hours in the same medium. HL-1 atrial myocytes received 15% uniaxial cyclic stretch at 1 Hz for 8 hours by NST-140 cell stretching system (NEPA GENE, Japan) in the cell stretched study groups. Stretched and control (non-stretched) experiments were carried out simultaneously with the same pool of cells in each experiment to match temperature, CO₂ content, and pH of the medium for the stretched and control HL-1 atrial myocytes.

Immunofluorescence Staining

HL-1 atrial myocytes were fixed for 10 min with 4% paraformaldehyde, then exposed to 0.1% triton-X100, and stained with CytoPainter Phalloidin-iFluor 488 reagent (Abcam, Cambridge, USA), according to the manufacturer's protocol. Nuclei were stained with Hoechst 33258 (1:1000 dilution; Sigma, MO, USA). Four randomly chosen fields per section corresponding to at least fifty cells were examined at high magnification (400X). All images of each specimen were captured using a Leica DMI3000 microscope. Atrial cardiomyocytes were analyzed (UTHSCSA, Image tool, Version 3.0).

Statistical Analysis

Data are presented as mean ± SD (baseline characteristics) or SEM (gene and protein expressions). Categorical variables were compared using chi-square test or Fisher exact test as appropriate. Continuous variables among 3 groups were analyzed by the Kruskal-Wallis Test, and continuous variables between 2 groups were analyzed by the Mann-Whitney Test. Statistical analysis was performed using commercial statistical software (IBM SPSS Statistics 22). All P values were two-sided, and the level of statistical significance was set at 0.05.

Results

Baseline Characteristics of Patients Studied

Table 2 lists the clinical characteristics of the study patients with MR and patients with aortic valve disease. The two groups did not significantly differ in age, or heart failure status. The two groups also did not significantly differ in the preoperative left atrial ejection fraction, left ventricular size and left ventricular ejection fraction. However, the left atrial size was significantly larger in the MR patients than patients with aortic valve disease. Seventy-eight percent of MR patients and forty-two percent of patients with aortic valve disease received renin-angiotensin system blockers (P = 0.102).

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Table 2. Baseline Clinical Characteristics of the Study Patients.

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Identification and Enrichment Analysis of Differential Expression Genes between MR Patients and Normal Subjects

To determine the effect of MR on gene expression, we compared the expression profile in the left atrial free walls of the 7 MR patients to 3 normal subjects (76-year-old Caucasian female, 24-year-old Caucasian male and 27-year-old Caucasian male). A total of 244 differentially expressed genes were discovered by using genefilter R package [6] with the P value < 0.01 (t-test) and a fold-change cut-offs of > 1.5. A total of 112 genes were identified to be differentially up-regulated between MR patients and normal subjects (Table 3), and a total of 132 genes were identified to be differentially down-regulated between MR patients and normal subjects (Table 4), with the heat map graph being depicted in Fig 1. As with the unsupervised hierarchical clustering, the samples and genes were sorted corresponding to their respective groups.

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Fig 1. Unsupervised hierarchical clustering of RNA microarray expression values.

A total of 112 genes were identified to be differentially up-regulated and 132 genes were identified to be differentially down-regulated in the left atria between mitral regurgitation (MR) patients (n = 7) and normal subjects (NC) (n = 3) by using genefilter R package with the P value < 0.01 (t-test) and a fold-change cut-offs of > 1.5. Bar color indicates mRNA expression level. Red indicates up-regulation; black, no change; green, down-regulation.

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Table 3. Selected Signature Upregulated Gene Expression in the Left Atria of Mitral Regurgitation vs. Normal Control.

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Table 4. Selected Signature Downregulated Gene Expressions in the Left Atria of Mitral Regurgitation vs. Normal Control

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To elucidate the molecular mechanisms of MR on left atrial gene expression, we used Ingenuity Pathway Analysis to search for enrichment in predicted function. A network with highest score (P-score = 50, i.e. P value < 10⁻⁵⁰) was generated from 244 differentially expressed genes using Ingenuity Pathway Analysis Global Molecular Network algorithm as depicted in Fig 2, and 26 focused genes were identified to be involved in the network, including PLCE1, PPP3R1, PPP3CB, MEF2C and etc. Top involved canonical pathways in this network included role of nuclear factor of activated T cells (NFAT) in cardiac hypertrophy, cardiac hypertrophy signaling, and calcium signaling (Table 5). Top diseases and functions in this network included cardiovascular system development and function, organ morphology, and organismal development (Table 5). These results demonstrated that the network was significantly associated with cardiac related pathways and functions, such as role of NFAT in cardiac hypertrophy and cardiovascular system development and function.

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Fig 2. The network with highest score (P-score = 50, i.e. P-value < 10⁻⁵⁰) was derived from 244 differentially expressed genes using Ingenuity Pathway Analysis Global Molecular Network algorithm.

The edge in this network represents a relationship between two genes based on Ingenuity Pathways Knowledge Base. The genes with violet border color represent its functions related to cardiovascular system development, such as MEF2C, PLCE1, PPP3CB, and PPP3R1.

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Table 5. Top Involved Canonical Pathways and Top Diseases and Functions in the Network Derived from 244 Differentially Expressed Genes between Mitral Regurgitation Patients and Normal Subjects Using Ingenuity Pathway Analysis Global Molecular Network Algorithm

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Furthermore, we applied the activation z-score analysis method, which was proposed by Andreas Krämer et al [5] in 2014, to measure activation states (increased or decreased) of the pathways affected by differentially expressed genes. We take a statistical approach by defining a quantity (z‐score) that determines whether a biological function has significantly more “increased” predictions than “decreased” predictions (z>0) or vice versa (z<0). In practice, z‐scores greater than 2 or smaller than ‐2 can be considered significant. Only “Role of NFAT in cardiac hypertrophy” pathway (Fig 3) had a z-score of 1.34 and P < 0.02. Thus, according to log2 fold-change values and the predictive activities of the differentially expressed genes significantly involved in NFAT pathway (Table 6), we derived an activation z-score equal to 1.34 which suggests that these differentially expressed genes moderately activate the Role of NFAT in cardiac hypertrophy pathway (Fig 3). The detailed information of differentially expressed genes involved in NFAT pathway was shown in Table 6, implicating that the expression patterns of PPP3CB (Calcineurin A beta), PPP3R1 (Calcineurin B), PLCE1, MEF2C, and CAMK1 played a role in the activation of hypertrophy of atrial myocytes in MR patients compared to normal subjects. The results of the real-time quantitative RT-PCR of these 5 genes were consistent with the RNA microarray data (Table 7).

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Fig 3. The “Role of NFAT in cardiac hypertrophy” pathway.

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Table 6. Log2 Fold Change Values and Predictive Activity of the Differentially Expressed Genes Significantly Involved in Role of NFAT in Cardiac Hypertrophy Pathway

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Table 7. Analysis of mRNA Levels via Quantitative RT-PCR and RNA Microarray

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Additionally, Significant Analysis of Microarrays method [7] was also applied for significant gene analysis. One thousand and eighty-three significant genes were identified with false discovery rate of 0.05, and four out of five focused genes (PLCE1, CAMK1, PPP3R1, and PPP3CB) were also identified using Significant Analysis of Microarrays method. The canonical pathway of Role of NFAT in Cardiac Hypertrophy was still significantly identified using Ingenuity Pathway Analysis with P-value of 0.039 and z-score of 0.83. Therefore, we focused on deciphering and discussing their regulatory roles in cardiac hypertrophy in the following sections and 5 focused NFAT associated genes (PLCE1, PPP3R1, PPP3CB, CAMK1, MEF2C) were studies for experimental validation.

Hypertrophy of Atrial Myocytes in MR Patients Compared to Normal Subjects and Patients with Aortic Valve Disease

The average cell surface area of myocytes in the left atrial tissue of the MR patients (n = 14) significantly exceeded the average cell surface area of myocytes in the left atrial tissue of the patients with aortic valve disease (n = 5) (1145.3±97.1 vs. 637.7±95.7 μm2, P = 0.012) and normal control subjects (n = 3; 20-year-old Asian male and 48-year-old Asian male, purchased from Abcam, Cambridge, UK and 24-year-old Asian male, purchased from BioChain, Newark, CA, USA) (1145.3±97.1 vs. 491.7±60.7 μm2, P = 0.008) (Fig 4). The average nuclear size of myocytes in the left atrial tissues of the MR patients significantly exceeded the average nuclear size of myocytes in the left atrial tissue of the patients with aortic valve disease (198.8±12.0 vs. 135.7±19.8μm2, P = 0.026) and normal control subjects (198.8±12.0 vs. 129.9±15.1 μm2, P = 0.023) (Fig 4). However, the average cell surface area and nucleus size of myocytes in the left atrial tissue did not significantly differ between patients with aortic valve disease and normal subjects (P = 0.456 and P = 0.881, respectively).

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Fig 4.

Histochemical study of hematoxylin, eosin (400 X) stained left atrial tissue sections of (A) mitral regurgitation (MR) patients (n = 14), (B) patients with aortic valve disease (AVD) (n = 5), and (C) normal control (NC) subjects (n = 3). The average cell surface area (D) and average nucleus size (E) per myocyte in the left atrial tissues of MR patients, patients with AVD, and normal control. *P < 0.05. Bar = 50 μm.

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α-Sarcomeric Actin Expression in the Left Atria between MR Patients and Normal Subjects

Four normal adult left atrial tissue samples (66-year-old Caucasian female, 49-year-old Africa American male, 62-year-old Asian female and 78-year-old Caucasian female,) were purchased from BioChain, Newark, CA, USA, and these 4 normal atrial tissues were used as the normal controls for protein analysis.

The expression of α-sarcomeric actin protein (normalized against GAPDH) in the left atrial free wall was significantly up-regulated in the MR patients (n = 10) compared to normal subjects (n = 4) (1.30± 0.07 vs. 0.67± 0.13, P = 0.007).

Comparison of the Gene Expression in the “Role of NFAT in Cardiac Hypertrophy” Pathway in the Left Atrium among MR Patients, Patients with Aortic Valve disease and Normal Subjects

The expressions of mRNAs of PPP3CB (MR, n = 13, normal subjects, n = 6; 14.45±0.35 vs. 16.02±0.19, P = 0.007) and MEF2C (MR, n = 14, normal subjects, n = 5; 15.67±0.56 vs. 17.82±0.36, P = 0.021) in the left atrial free wall were significantly up-regulated in the MR patients compared to normal subjects (Fig 5). However, the expressions of mRNAs of CAMK1 (MR, n = 14, normal subjects, n = 6; 18.57±0.24 vs. 17.43±0.43, P = 0.043) and PPP3R1 (MR, n = 14, normal subjects, n = 6; 14.69±0.42 vs. 13.06±0.27, P = 0.017) in the left atrial free wall were significantly down-regulated in the MR patients compared to normal subjects.

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Fig 5.

Quantitative determination of mRNAs of (A) protein phosphatase 3, catalytic subunit, beta isozyme (PPP3CB), (B) phospholipase C, epsilon 1 (PLCE1), (C) calcium/calmodulin-dependent protein kinase I (CAMK1), (D) protein phosphatase 3, regulatory subunit B, alpha (PPP3R1), and (E) myocyte enhancer factor 2 (MEF2C) by real-time RT-PCR in the left atrial tissues of mitral regurgitation (MR) patients, patients with aortic valve disease (AVD), and normal control (NC) subjects. *P < 0.05.

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The expression of mRNAs of PPP3CB (MR, n = 13, aortic valve disease, n = 6; 14.45±0.35 vs. 17.13±0.49, P = 0.001), MEF2C (MR, n = 14, aortic valve disease, n = 7; 15.67±0.56 vs. 17.37±0.34, P = 0.037) and PLCE1 (MR, n = 13, aortic valve disease, n = 7; 15.68±0.42 vs. 17.10±0.34, P = 0.043) in the left atrial free wall was significantly up-regulated in the MR patients compared to patients with aortic valve disease (Fig 5). However, the expression of mRNAs of CAMK1 (MR, n = 14, aortic valve disease, n = 7; 18.57±0.24 vs. 18.89±0.23, P = 0.502) and PPP3R1 (MR, n = 14, aortic valve disease, n = 7; 14.69±0.42 vs. 13.95±0.21, P = 0.502) in the left atrial free wall did not significantly differ between MR patients and patients with aortic valve disease. These findings implicated that PPP3CB, MEF2C and PLCE1 were associated with the hypertrophy of atrial myocytes in MR patients compared to patients with aortic valve disease.

The expressions of mRNAs of CAMK1 (aortic valve disease, n = 7, normal subjects, n = 6; 18.89±0.23 vs. 17.43±0.43, P = 0.032) and PPP3R1 (aortic valve disease, n = 7, normal subjects, n = 6; 13.95±0.21 vs. 13.06±0.27, P = 0.032) in the left atrial free wall were significantly down-regulated in patients with aortic valve disease compared to normal subjects (Fig 5). However, there was no significant difference in the expressions of mRNAs of PPP3CB (aortic valve disease, n = 6, normal subjects, n = 6; 17.13±0.49 vs. 16.02±0.19, P = 0.150), PLCE1 (aortic valve disease, n = 7, normal subjects, n = 6; 17.10±0.34 vs. 17.16±0.42, P = 0.886) and MEF2C (aortic valve disease, n = 7, normal subjects, n = 5; 17.37±0.34 vs. 17.82±0.36, P = 0.685) in the left atrial free wall between patients with aortic valve disease and normal subjects.

Comparison of the Expressions of miR-1, miR-133 and miR-208 in the Left Atrium between MR Patients and Normal Subjects

The expression of miR-1 in the left atrial tissue was significantly down-regulated in the MR patients (n = 5) compared to normal controls (n = 5) (-2.794±0.561 vs. -5.252±0.807, P = 0.047). The expression of miR-133 in the left atrial tissue was significantly down-regulated in the MR patients (n = 5) compared to normal controls (n = 5) (-0.065±0.334 vs. -2.083±0.691, P = 0.028). The expression of miR-208 in the left atrial tissue was significantly up-regulated in the MR patients (n = 5) compared to normal controls (n = 5) (-0.995±0.415 vs. 1.460±0.918, P = 0.028).

Hypertrophy of HL-1 Atrial Myocytes by Mechanical Stretching

The causal relationship between atrial hypertrophy and atrial dilatation due to volume overload of MR was partly mimicked by mechanical stretching of HL-1 atrial myocytes. The average cell surface area of stretched HL-1 atrial myocytes (experiment number = 6) significantly exceeded that of non-stretched control group (experiment number = 6) (1203.6±89.0 vs. 819.4±43.1 μm2, P = 0.016). The average nucleus size of stretched HL-1 atrial myocytes significantly exceeded that of non-stretched control group (190.2±18.9 vs. 108.3±8.6 μm2, P = 0.009) (Fig 6).

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Fig 6. Immunofluorescence study of average cell surface area and average nucleus size of HL-1 atrial myocytes between the stretched group and non-stretched control group.

Myocyte identification was performed with Phalloidin F-actin (green color). Nucleus identification was performed with Hoechst 33258 (blue color). *P < 0.05. Bar = 80 μm.

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bgThe gene expressions of CAMK1 (4.22±0.07 vs. 4.01±0.06, P = 0.037) and PPP3R1 (3.89±0.08 vs. 3.09±0.06, P = 0.004) (normalized against GAPDH) in the HL-1 atrial myocytes were significantly down-regulated in the stretched HL-1 atrial myocytes compared to the non-stretched control group.

Discussion

This study identifies and reports the alteration of the RNA expression pattern, molecular mechanisms and biological processes involving in atrial myocyte hypertrophy between the left atrial myocardium of MR patients and normal subjects using high-density oligonucleotide microarrays and enrichment analysis. A total of 112 genes were identified to be differentially up-regulated and 132 genes were identified to differentially down-regulated in the left atria between MR patients and normal subjects. Notably, the expression patterns of PPP3CB, PPP3R1, PLCE1, MEF2C and CAMK1 in the “NFAT in cardiac hypertrophy” pathway played a significant role in the activation of hypertrophy of atrial myocytes in MR patients compared to normal subjects.

Activation of “Role of NFAT in Cardiac Hypertrophy” Pathway

Calcineurin/NFAT coupling has been reported to participate in pathological, but not physiological, cardiac hypertrophy [8]. Cardiac hypertrophy is a compensatory response to pathological states and hemodynamic overload. However, pathological hypertrophy leads to atrial myocardial disarrangement and consequently, atrial enlargement that is correlated with poor prognosis in MR patients [2].

The initial phase in the development of myocardial hypertrophy involves factors, such as endothelin-1, angiotensin-II, and adrenergic agonists at the cell membrane, binding to the G-protein coupled receptors (Fig 3), and several interdependent signaling cascades that include G-proteins, GTPases such as Ras, RhoA and Rac, and kinases such as ERK/MAPK and PKC [9]. Prior study showed enhanced expression of Rho-associated kinase in the left atrial myocytes of MR patients [10]. In all of the hypertrophic pathways, NFAT plays a critical role in the development of cardiac hypertrophy. Several studies have shown the importance of Ca²⁺ sensitive signaling molecules, including calcineurins (i.e. PPP3CB, PPP3R1) and CAMK, a calcium/calmodulin-dependent protein kinase, in hypertrophic pathways [11]. Activation of protein kinase C leads to increased Ca²⁺ levels that activate calcineurins. Calcineurin activation leads to the dephosphorylation of NFATc4, allowing its nuclear localization where it cooperates with other transcription factors to participate in the cardiac hypertrophy. In this study, the expression of PPP3CB, the catalytic subunit, was significantly up-regulated in the MR patients compared to normal subjects. However, the expression of PPP3R1, the regulatory subunit B, was significantly down-regulated in the MR patients compared to normal subjects. Overexpression of PPP3R1, also known as modulatory calcineurin-interacting protein-1, has been reported to attenuate left ventricular hypertrophy after myocardial infarction [12]. As shown in Table 6, the predictive activities of the differential trend between PPP3CB and PPP3R1 to the Role of NFAT in cardiac hypertrophy pathway implicated activation of this pathway. The CAMK1 was also found to be significantly down-regulated in MR patients compared to normal subjects in this study. However, the molecular function of CAMK1 in human heart is rarely reported. Further studies are warranted to investigate the role of CAMK1 in the development of atrial hypertrophy in patients with MR.

MEF2 (myocyte enhancer factor 2), especially MEF2C, is an important transcription factor regulating the cardiac gene program during myocardial cell hypertrophy [13]. The activation of MEF2 by CAMK is mediated mainly through the phosphorylation of transcriptional repressors, the class II histone deacetylases, resulting in disrupting the association between MEF2 and histone deacetylases in the nucleus and transcriptional activation of cardiac hypertrophy [14,15]. In this study, MEF2C was found to be significantly up-regulated in MR patients compared to normal subjects.

PLCE (phospholipase C epsilon), an effector of Ras, has been shown to involve in stress-induced hypertrophy and PLCE, scaffolded to muscle-specific A kinase-anchoring protein in cardiac myocytes, responds to hypertrophic stimuli to generate diacylglycerol from phosphatidylinositol 4-phosphate in the Golgi apparatus, in close proximity to the nuclear envelope, to regulate activation of nuclear protein kinase D and hypertrophic signaling pathways [16]. In this study, PLCE1 was found to be up-regulated in MR patients compared to patients with aortic valve disease and normal subjects.

Hypertrophy-Related MicroRNAs

The expressions of antihypertrophic miRs, miR-1 and miR-133, in the left atrial tissue were significantly down-regulated in the MR patients compared to normal controls, while the expression of agonist of the hypertrophic response, miR-208, in the left atrial tissue were significantly up-regulated in the MR patients compared to normal controls [17].

Hypertrophy-Related Decorin and Calponin

Decorin has been reported to promote myoblast proliferation mediated by an endoplasmic reticulum stress-related pathway [18]. Decorin was identified to be differentially down-regulated in MR patients compared to normal subjects in this study. Therefore, decorin might not be involved in the hypertrophy of atrial myocytes in MR patients, which was mainly related to the Role of NFAT in cardiac hypertrophy pathway.

Calponin has been reported to be involved in the hypertrophy of smooth muscle cells [19,20]. Calponin 2 was identified to be differentially down-regulated in MR patients compared to normal subjects in this study. Therefore, calponin 2 might not be involved in the hypertrophy of atrial myocytes in MR patients.

Pathological Hypertrophy of Atrial Myocytes in MR

Atria, like the ventricles, can undergo hypertrophy in response to increased volume and pressure overload. In MR, the volume and pressure in the left atrium are greatly increased. The left atrium of MR patients responds by undergoing chronic dilation, which enables it to accommodate the increased volume without a large increase in pressure because of its increased compliance. However, extreme hypertrophy and dilatation is deleterious because it increases the oxygen demand of the heart and decreases mechanical efficiency. Furthermore, atrial fibrillation, an important risk factor of stroke and systemic embolization, may develop as a consequence of atrial enlargement, and vice versa [21].

Study Limitations

There are several limitations of this study. Firstly, most of the patients with MR received renin-angiotensin system blockers. Therefore, the expression of some genes might have been modified by renin-angiotensin system blockers [22]. However, there was no significant difference in the expressions of PPP3CB (14.66±0.38 vs. 13.28±0.02, P = 0.167), PPP3R1 (14.29±0.41 vs. 16.16±0.91, P = 0.052), PLCE1 (15.95±0.46 vs. 14.22±0.02, P = 0.167), MEF2C (15.99±0.58 vs. 14.46±1.47, P = 0.392) and CAMK1 (18.49±0.29 vs. 18.86±0.38, P = 0.484) between MR patients with renin-angiotensin system blockers (n = 11) vs. MR patients without renin-angiotensin system blockers (n = 3). Secondly, the sample size was relatively small. However, the gene expression pattern by microarray analysis was quite consistent in the same group (Fig 1). Thirdly, the age of the normal subjects (n = 6) was younger than that of MR patients (n = 14) (49±25 vs. 58±9 years, P = 0.620), however the difference did not reach statistical significance. Finally, the microarrays were conducted on frozen, unsorted tissue samples. It is hence impossible to ascertain the sub-tissue or cellular origin of generated data. However, histological analysis did show atrial myocyte hypertrophy of MR patients compared to patients with aortic valve disease and normal subjects, implicating at least some involvement of cellular origin.

Conclusions

Significant hypertrophy developed in the left atrial myocytes of MR patients compared to normal subjects and patients with aortic valve disease. The differentially expressed genes in the “Role of NFAT in cardiac hypertrophy” pathway may play a critical role in the atrial myocyte hypertrophy of MR patients and these differentially expressed genes may serve as potential targets for human MR to prevent the progression of left atrial enlargement and its related complications, such as atrial fibrillation, and heart failure.

Accession Codes

The data discussed in this manuscript have been deposited in NCBI's Gene Expression Omnibus (GEO) and are accessible through GEO Series accession number GSE63045 (http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE63045).

Acknowledgments

The authors would like to thank Chang Gung Medical Foundation Kaohsiung Chang Gung Memorial Hospital Tissue Bank Core Lab (CLRPG8B0033 and CLRPG8E0161) for excellent technical support.

Author Contributions

1. Conceptualization: THC MCC JPC HDH.

2. Data curation: THC MCC JPC WCH.

3. Formal analysis: THC MCC WCH.

4. Funding acquisition: MCC.

5. Investigation: THC MCC JPC WCH WHL.

6. Methodology: THC MCC JPC WCH.

7. Project administration: THC MCC JPC WCH.

8. Resources: JPC YSL KLP YKH WHL CCW.

9. Software: THC.

10. Supervision: THC MCC HDH.

11. Validation: THC MCC WCH.

12. Visualization: THC MCC.

13. Writing – original draft: THC MCC.

14. Writing – review & editing: THC MCC.

Citation: Chang T-H, Chen M-C, Chang J-P, Huang H-D, Ho W-C, Lin Y-S, et al. (2016) Exploring Regulatory Mechanisms of Atrial Myocyte Hypertrophy of Mitral Regurgitation through Gene Expression Profiling Analysis: Role of NFAT in Cardiac Hypertrophy. PLoS ONE 11(12): e0166791. https://doi.org/10.1371/journal.pone.0166791

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