Integrating Phospholipid and Cyclic Nucleotide Signaling: Roles of Phosphodiesterases as Enzymes and Tethers

Abstract

Cells of the cardiovascular system translate incoming extracellular signals from hormones and drugs through binding of cell surface receptors, and activation of intracellular signaling cascades allowing modulation of specific cellular function. cAMP and cGMP are ubiquitous second messengers that activate specific signaling machinery used to promote or inhibit cellular functions such as cell migration, cell adhesion and proliferation. Increases in intracellular cAMP or cGMP levels occurs through activation of adenylyl cyclase (cAMP) or guanylyl cyclase (cGMP) or by inhibition of the cAMP and cGMP hydrolyzing enzymes, cyclic nucleotide phosphodiesterases (PDEs). Cyclic nucleotides achieve signaling specificity through compartmentation, a mechanism allowing effective regulation of cAMP or cGMP signaling in discrete parts of the cell in a spatial and temporal manner. Cells of the cardiovascular system such as platelets, vascular endothelial cells (VECs), vascular smooth muscle cells (VSMCs) maintain cyclic nucleotide compartmentation through coordinating signaling complexes containing a cAMP or cGMP effector protein and PDEs. Studies reported in this thesis demonstrate that human platelets, VECs and VSMCs each contain distinct cyclic nucleotide signaling complexes, and that based on their composition and selective subcellular localization, regulate specific cellular functions. In platelets, subcellular localization of PDE5 results in differential regulation of PDE5 and selective regulation of Ca2+ release from endoplasmic reticulum stores, an initial step in platelet aggregation and provides a potential therapeutic target in preventing thrombosis. VECs utilize multiple signaling systems to regulate cellular function including cAMP signaling pathways and modification of phosphatidylinositols. These studies demonstrate that a PDE3B-based signaling complex allows integration of both cAMP and phosphatidylinositol-3-kinase-γ (PI3Kγ) signals resulting in increased cell adhesion and cell spreading. Finally, studies in VSMCs demonstrate that PDE5 localization in cells allows cAMP/cGMP cross talk through PDE5 and PDE3A. These results are discussed in the context of further understanding the role of PDEs in mediating cAMP and cGMP signaling and modulation of cell function in cells of the cardiovascular system.