Age-Dependent Protein Expression of Serine/Threonine Phosphatases and Their Inhibitors in the Human Cardiac Atrium.

Ulrich Gergs,Andreas Simm,Theresa Trapp,Joachim Neumann,Rolf-Edgar Silber,Hasan Bushnaq

doi:10.1155/2019/2675972

Abstract

Heart failure and aging of the heart show many similarities regarding hemodynamic and biochemical parameters. There is evidence that heart failure in experimental animals and humans is accompanied and possibly exacerbated by increased activity of protein phosphatase (PP) 1 and/or 2A. Here, we wanted to study the age-dependent protein expression of major members of the protein phosphatase family in human hearts. Right atrial samples were obtained during bypass surgery. Patients (n=60) were suffering from chronic coronary artery disease (CCS 2-3; New York Heart Association (NYHA) stage 1–3). Age ranged from 48 to 84 years (median 69). All patients included in the study were given β-adrenoceptor blockers. Other medications included angiotensin-converting enzyme (ACE) or angiotensin-receptor-1 (AT1) inhibitors, statins, nitrates, and acetylsalicylic acid (ASS). 100 µg of right atrial homogenates was used for western blotting. Antibodies against catalytic subunits (and their major regulatory proteins) of all presently known cardiac serine/threonine phosphatases were used for antigen detection. In detail, we studied the expression of the catalytic subunit of PP1 (PP1c); I1PP1 and I2PP1, proteins that can inhibit the activity of PP1c; the catalytic subunit of PP2A (PP2Ac); regulatory A-subunit of PP2A (PP2AA); regulatory B56α-subunit of PP2A (PP2AB); I1PP2A and I2PP2A, inhibitory subunits of PP2A; catalytic and regulatory subunits of calcineurin: PP2BA and PP2BB; PP2C; PP5; and PP6. All data were obtained within the linear range of the assay. There was a significant decline in PP2Ac and I2PP2A expression in older patients, whereas all other parameters remained unchanged with age. It remains to be elucidated whether the decrease in the protein expression of I2PP2A might elevate cardiac PP2A activity in a detrimental way or is overcome by a reduced protein expression and thus a reduced activity of PP2Ac.

Highlights

In the myocardium, Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors is the main mechanism of cardiac excitationcontraction coupling [1]. e ensuing increase in intracellular Ca2+ concentration is responsible for muscle contraction [1]
Similar experiments were done with all other proteins of interest, namely, the catalytic subunit of PP1, the catalytic subunit PP2Ac, the regulatory subunits A and B56α of PP2A, the inhibitory subunits I1PP2A and I2PP2A of PP2A, the inhibitory subunits I1PP1 and I2PP1 of PP1, and the catalytic and regulatory subunits of calcineurin (PP2B), PP5, and PP6
There was a decreased expression of PP2Ac on protein level in aging (see lanes in the second row from the top in Figure 1(a)) and a decrease of I2PP2A upon aging (see lanes in the second row from bottom in Figure 1(a)). is initial observation was corroborated by studying more samples and performing a statistical analysis

Summary

Introduction

Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors is the main mechanism of cardiac excitationcontraction coupling [1]. e ensuing increase in intracellular Ca2+ concentration is responsible for muscle contraction [1]. Ca2+ is mainly removed from the cytosol by the action of SR Ca2+-ATPase (SERCA) into the SR. Phospholamban itself can be dephosphorylated by two serine/threonine phosphatases, namely, PP1 and PP2A in animal hearts and the human heart [2,3,4,5]. Long-term cardiac specific overexpression of I1 leads upon aging in mouse hearts to decreased systolic contractility, suggesting that low PP1 activity is detrimental for cardiac function in the long run [7]. On the other hand, increased PP1 overexpression in the mouse heart can lead to cardiac hypertrophy and death [8]. In contrast to PP1, another related phosphatase, namely, PP2A, probably mainly

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Conclusion