Cyclic Nucleotide-Directed Protein Kinases in Cardiovascular Inflammation and Growth

Abstract

Cardiovascular disease (CVD), including myocardial infarction (MI) and peripheral or coronary artery disease (PAD, CAD), remains the number one killer of individuals in the United States and worldwide, accounting for nearly 18 million (>30%) global deaths annually. Despite considerable basic science and clinical investigation aimed at identifying key etiologic components of and potential therapeutic targets for CVD, the number of individuals afflicted with these dreaded diseases continues to rise. Of the many biochemical, molecular, and cellular elements and processes characterized to date that have potential to control foundational facets of CVD, the multifaceted cyclic nucleotide pathways continue to be of primary basic science and clinical interest. Cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP) and their plethora of downstream protein kinase effectors serve ubiquitous roles not only in cardiovascular homeostasis but also in the pathogenesis of CVD. Already a major target for clinical pharmacotherapy for CVD as well as other pathologies, novel and potentially clinically appealing actions of cyclic nucleotides and their downstream targets are still being discovered. With this in mind, this review article focuses on our current state of knowledge of the cyclic nucleotide-driven serine (Ser)/threonine (Thr) protein kinases in CVD with particular emphasis on cyclic AMP-dependent protein kinase (PKA) and cyclic GMP-dependent protein kinase (PKG). Attention is given to the regulatory interactions of these kinases with inflammatory components including interleukin 6 signals, with G protein-coupled receptor and growth factor signals, and with growth and synthetic transcriptional platforms underlying CVD pathogenesis. This article concludes with a brief discussion of potential future directions and highlights the importance for continued basic science and clinical study of cyclic nucleotide-directed protein kinases as emerging and crucial controllers of cardiac and vascular disease pathologies.

Keywords: G protein-coupled receptor; Smad3; Stat3; cyclic nucleotide; inflammation; interleukin 6; myocardial infarction; protease-activated receptor; protein kinase; vascular smooth muscle.

Figure 1

Schematic of cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP) signaling. Following activation of adenylate cyclase (AC) by upstream processes including direct ligand agonism, B-adrenergic stimulation, or by stimulatory G protein-coupled receptors (GPCRs), adenosine triphosphate (ATP) is dephosphorylated to yield cyclic AMP and pyrophosphate (PPi). In similar fashion, stimulation of membrane-bound or soluble guanylate cyclase (GC) by natriuretic peptides or gaseous ligands nitric oxide (NO) and/or carbon monoxide (CO), GTP is dephosphorylated to yield cyclic GMP and PPi. Persistence of cyclic nucleotide signaling can be governed by the presence of scaffolding proteins including A-kinase anchoring protein for cyclic AMP or IP₃ receptor-associated cGMP kinase substrate (IRAG) and Huntingtin-associated protein 1 (HAP1) for cyclic GMP, and by degradation into inactive 5′-monophosphates by a family of phosphodiesterases (PDEs). Cyclic AMP can operate through kinase-independent pathways, through binding to cyclic nucleotide-gated (CNG) ion channels or Popeye domain-containing proteins (POPDC), via exchange proteins directly activated by cyclic AMP (EPAC), through non-canonical protein kinases or by activation of PKA. In like manner, cyclic GMP can signal through kinase-independent pathways, by binding to CNG ion channels, through non-canonical protein kinases or via PKG. The predominant protein kinases for cyclic AMP and cyclic GMP, PKA, and PKG, can then stimulate Ser/Thr residues on many diverse downstream effector targets to help control normal physiology and homeostasis as well as wide-ranging pathophysiological processes in cardiac and vascular tissues.

Figure 2

Layers of a blood vessel. Cartoon cross-sectional image of a blood vessel with major layers and cell types depicted. The outermost perivascular fat (A) lends support and anchoring for the vessel as well as mediates adipocyte production and influences cellular metabolism. The outermost layer of the vessel proper, the tunica adventitia (B); is largely a structural layer of the vessel wall and is comprised of extracellular matrix (ECM) containing resident immune cells, an internal vascular supply (vasa vasorum), sparse nerve endings and fibroblasts. The majority of the arterial vessel wall is comprised of the tunica media (C); mostly vascular smooth muscle cells (VSMCs) and ECM. Medial VSMCs are responsible for vasoconstriction and relaxation (i.e., vessel tone) that controls luminal blood flow. The innermost layer of the blood vessel is the tunica intima (D) and is comprised of a single layer of vascular endothelial cells (VECs) that surround the lumen of the vessel, that form a critical interface between flowing blood and the vessel wall, and that communicate with the underlying VSMCs to help regulate tone and direct inflammatory responses. Arrow indicates direction of luminal blood flow.

Figure 3

Overview of mechanisms contributing to cell death following ischemia. Ischemia results in ion channel dysfunction following decreased production of ATP and increased hydrogen ion (H+) production leading to cellular acidosis. Ultimately excessive intracellular calcium (Ca²⁺) triggers activation of phospholipases and the opening of the mitochondrial permeability transition pore (mPTP) in turn inducing cell death.

Figure 4

Overview of mechanisms contributing to reperfusion injury. Reperfusion injury results following revascularization and restoration of blood flow to the ischemic myocardium. As with ischemic injury intracellular calcium (Ca²⁺) overload leads to increased myocardial injury through multiple pathways. Reactive oxygen species (ROS) also contribute to myocardial injury in turn promoting inflammation and altering mitochondrial function.

Figure 5

Classical versus interleukin-6/(IL-6) trans-signaling. A simplified schematic depicts classical IL-6 signaling whereby IL-6 binds to membrane-bound IL-6 receptor (IL-6R) initiating signal transduction via glycoprotein 130 (GP130) to increase intracellular STAT3 via JAK (A); Alternatively, in (B) IL-6 trans-signaling occurs whereby IL-6 binds to soluble IL-6 receptor (sIL-6R) which then complexes to membrane-bound GP130/JAK to initiate STAT3 signal transduction. Inhibition of IL-6 trans-signaling can occur by the decoy soluble GP130 (sGP130) (C).

Figure 6

Generalized G protein-coupled receptors: pH-sensing and protease-activated receptors. A schematic depicting a typical pH-sensing GPCR and a protease-activated receptor (PAR) GPCR under non-activated conditions. Both GPCRs are 7 trans-membrane receptors with an extracellular amino terminus and an intracellular carboxyl end associated with G protein subunits. Activation of the pH-sensing GPCR involves extracellular amino terminal histidine sensing of acidic protons (H+) while PAR activation involves cleavage of the extracellular amino terminus by serine proteases, thrombin, trypsin and other agonists and creation of an activating tethered ligand. These GPCRs then stimulate a cascade of G protein-mediated intracellular signals that have the capacity to govern a wide array of inflammation- and growth-regulatory processes in cardiac and vascular tissues.

Figure 7

Schematic of transforming growth factor-β (TGF-β) signaling. Binding of an active TGF-β ligand initiates the co-localization of the type II (TβR-II) and type I (TβR-I) TGF-β receptors. The TβR-II remains constitutively phosphorylated and when co-localized phosphorylates the Ser/Thr kinase domain (KD) of the TβR-I. The intracellular signal is subsequently transmitted by phosphorylation of Smad proteins, primarily Smad2/3 in cardiovascular tissues. The phosphorylated Smad proteins combine with Smad4, a common Smad, to form a heterotrimeric complex that can then be shuttled through the nuclear membrane ultimately acting as a transcription factor for inflammatory, synthetic, and growth-promoting genes. The cyclic GMP/PKG system is known to inhibit this pathway by sequestering p-Smad in the cytosol, in turn not allowing it to be shuttled into the nucleus and affect gene transcription.