antioxidants Review Regulation of Cardiac PKA Signaling by cAMP and Oxidants Friederike Cuello 1,2,* , Friedrich W. Herberg 3,* , Konstantina Stathopoulou 1,2 , Philipp Henning 3 and Simon Diering 1,2 1 Institute of Experimental Pharmacology and Toxicology, Cardiovascular Research Center, University Medical Center Hamburg-Eppendorf, Martinistrasse 52, 20246 Hamburg, Germany; [email protected] (K.S.); [email protected] (S.D.) 2 DZHK (German Center for Cardiovascular Research), Partner site Hamburg/Kiel/Lübeck, University Medical Center Hamburg-Eppendorf, Martinistr. 52, 20246 Hamburg, Germany 3 Department of Biochemistry, University Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany; [email protected] * Correspondence: [email protected] (F.C.); [email protected] (F.W.H.); Tel.: +49-(0)-40/7410-57204 (F.C.); +49-(0)-561/804-4511 (F.W.H.); Fax: +49-(0)-40/7410-54876 (F.C.); +49-(0)-561/804-4466 (F.W.H.) Abstract: Pathologies, such as cancer, inflammatory and cardiac diseases are commonly associated with long-term increased production and release of reactive oxygen species referred to as oxidative stress. Thereby, protein oxidation conveys protein dysfunction and contributes to disease progression. Importantly, trials to scavenge oxidants by systemic antioxidant therapy failed. This observation supports the notion that oxidants are indispensable physiological signaling molecules that induce oxidative post-translational modifications in target proteins. In cardiac myocytes, the main driver of cardiac contractility is the activation of the β-adrenoceptor-signaling cascade leading to increased cel- lular cAMP production and activation of its main effector, the cAMP-dependent protein kinase (PKA). PKA-mediated phosphorylation of substrate proteins that are involved in excitation-contraction Citation: Cuello, F.; Herberg, F.W.; coupling are responsible for the observed positive inotropic and lusitropic effects. PKA-actions are Stathopoulou, K.; Henning, P.; counteracted by cellular protein phosphatases (PP) that dephosphorylate substrate proteins and Diering, S. Regulation of Cardiac thus allow the termination of PKA-signaling. Both, kinase and phosphatase are redox-sensitive PKA Signaling by cAMP and and susceptible to oxidation on critical cysteine residues. Thereby, oxidation of the regulatory PKA Oxidants. Antioxidants 2021, 10, 663. and PP subunits is considered to regulate subcellular kinase and phosphatase localization, while https://doi.org/10.3390/ intradisulfide formation of the catalytic subunits negatively impacts on catalytic activity with direct antiox10050663 consequences on substrate (de)phosphorylation and cardiac contractile function. This review article attempts to incorporate the current perception of the functionally relevant regulation of cardiac Academic Editor: Ashley J. Smuder contractility by classical cAMP-dependent signaling with the contribution of oxidant modification. Received: 26 February 2021 Keywords: kinase; phosphatase; cardiac myocyte; contractile function; oxidation Accepted: 20 April 2021 Published: 24 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- The sympathetic nervous system represents the most powerful regulator of cardiac iations. contractile function. Thereby, the monoamine neurotransmitters epinephrine and nore- pinephrine innervate postsynaptic β-adrenoceptors on the surface of cardiac myocytes allowing canonical intracellular signal transduction. This involves the generation of the intracellular second messenger 30-50-cyclic adenosine monophosphate (cAMP) and subse- Copyright: © 2021 by the authors. quent activation of its principle cellular effector enzyme cAMP-dependent protein kinase Licensee MDPI, Basel, Switzerland. (PKA). Incessant investigations are focusing on understanding the isoform-specific, spa- This article is an open access article tiotemporal and structural contributions as well as disease-specific alterations in PKA distributed under the terms and signaling. PKA is “the kinase” in cardiac myocytes that links neurohumoral extracellu- conditions of the Creative Commons lar signals to changes in contractile function. This process termed excitation-contraction Attribution (CC BY) license (https:// coupling occurs via phosphorylation of key substrate proteins [1,2] (Figure1). creativecommons.org/licenses/by/ 4.0/). Antioxidants 2021, 10, 663. https://doi.org/10.3390/antiox10050663 https://www.mdpi.com/journal/antioxidants Antioxidants 2021, 10, x FOR PEER REVIEW 2 of 27 Antioxidants 2021, 10, 663 2 of 26 signals to changes in contractile function. This process termed excitation-contraction cou- pling occurs via phosphorylation of key substrate proteins [1,2] (Figure 1) FigureFigure 1. 1.Scheme Scheme summarizing summarizing the the signaling signaling network network between between PKA, PKA, PP1 PP1 and and PP2A, PP2A, their their inter-action inter- uponactionβAR upon stimulation βAR stimulation and cAMP and production cAMP production or direct or oxidative direct oxidative modification. modification. PKA phosphorylates PKA phos- andphorylates activates and inhibitor activates 1 (I-1),inhibitor which 1 (I-1), inhibits which PP1. inhibits Several PP1. B’-PP2A Several regulatory B‘-PP2A regulatory subunits havesubunits been have been shown to be subject to phosphorylation, B56δ is phosphorylated by PKA, enhancing shown to be subject to phosphorylation, B56δ is phosphorylated by PKA, enhancing PP2A activity PP2A activity by dissociation from the A-subunit. PP2A mediates I-1 dephosphorylation. PKA by dissociation from the A-subunit. PP2A mediates I-1 dephosphorylation. PKA phosphorylates phosphorylates substrate proteins in cardiac myocytes that regulate cardiac contraction, which is substratecounteracted proteins by PP2A in cardiac and PP1 myocytes action. In that addition regulate to the cardiac regulation contraction, by phosphorylation, which is counteracted signaling bysubunits PP2A andlabeled PP1 with action. a star In are addition susceptible to the to oxid regulationation. Green: by phosphorylation, enhanced activity, signaling red: inhibitory subunits labeledeffect. withThe scheme a star are synergizes susceptible aspects to oxidation. of the regula Green:tion enhancedof PKA, PP1 activity, and PP2A red: inhibitoryactivity by effect. phos- The schemephorylation synergizes and simultaneous aspects of the oxidation regulation to oforches PKA,trate PP1 contractility. and PP2A activity Numbers by phosphorylation1–9 referring to the and simultaneousrespective section oxidation in the to manuscript. orchestrate contractility. Numbers 1–9 referring to the respective section in the manuscript. 2. PKA Isoforms 2. PKAPKA Isoforms is a heterotetrameric enzyme that is composed of a dimeric regulatory (R) and twoPKA catalytic is a heterotetramericsubunits. Several enzyme PKA-C thatisoforms is composed are expressed of a dimeric in human regulatory tissue: C (R)α, andCβ twoand catalytic PrKX. C subunits.β isoform Severalexpression PKA-C appears isoforms to be arepredominantly expressed in in human the nervous tissue: and Cα,C theβ andimmune PrKX. system Cβ isoform and PKA-C expressionα is ubiquitously appears to beexpressed predominantly [3,4]. In humans, in the nervous PRKACG and (C theγ) immuneand PrKY system have been and PKA-Cdescribedα is as ubiquitously retrotransposons expressed [5,6]. Association [3,4]. In humans, of PKA-CPRKACG with either(Cγ) andof the PrKY two have existing been types described of regulatory as retrotransposons subunits, RI[ or5,6 RII,]. Association defines the of holoenzyme PKA-C with as either type ofI or the type two II existing PKA. While types the of expression regulatory of subunits, RI and RII RI differs or RII, substantially defines the between holoenzyme tissues, as typetype I I or PKA type activity II PKA. predominates While the expression in the heart of [7]. RI Importantly, and RII differs attempts substantially to quantify between PKA tissues,subunit type abundance I PKA activity in the heart, predominates revealed in high the heartabundance [7]. Importantly, of RI subunits attempts with an to quantifyapprox- PKAimately subunit 10-fold abundance [8]or even in 17-fold the heart, [9] excess revealed over high the catalytic abundance subunits. of RI subunitsThis observation with an approximatelywas rationally 10-foldexplained [8 ]by or RI even functioning 17-fold [as9] a excess cAMP overstorage the allowing catalytic the subunits. formation This of observationbiomolecular was condensates rationally enriched explained in bycAMP RI functioning and PKA activity, as a cAMP critical storage for effective allowing cAMP the formationcompartmentation of biomolecular [10]. Also, condensates the concept enriched of “buffered in cAMP cAMP and diffusion” PKA activity, supports critical the bio- for effectivelogical significance cAMP compartmentation of RI abundance: [10 cAMP]. Also, at the physiological concept of “bufferedconcentrations cAMP is diffusion”predomi- supportsnantly bound the biological to cAMP significance binding sites of and RI abundance: thus immobile cAMP [11]. at Those physiological sites could concentrations potentially isbe predominantly RI subunits. bound to cAMP binding sites and thus immobile [11]. Those sites could potentially be RI subunits. For both, RI and RII regulatory subunits, α and β isoforms have been described [12]. Generally, α isoforms are expressed ubiquitously and β isoforms display enhanced tis- sue specificity.