PDE3 is a phosphodiesterase. The PDEs belong to at least eleven related gene families, which are different in their primary structure, substrate affinity, responses to effectors, and regulation mechanism. Most of the PDE families are composed of more than one gene. PDE3 is clinically significant because of its role in regulating heart muscle, vascular smooth muscle and platelet aggregation. PDE3 inhibitors have been developed as pharmaceuticals, but their use is limited by arrhythmic effects and they can increase mortality in some applications.
Function
PDE3 enzymes are involved in regulation of cardiac and vascular smooth muscle contractility. Molecules that inhibit PDE3 were originally investigated for the treatment of heart failure, but, because of unwanted arrhythmic side-effects, they are not studied for that indication any longer. Nonetheless, the PDE3 inhibitor milrinone is approved for use in heart failure in intravenous form.
Both PDE3A and PDE3B are expressed in vascular smooth muscle cells and are likely to modulate contraction. Their expression in vascular smooth muscle is altered under specific conditions such as elevated cAMP and hypoxia.
Isoforms and genes
The PDE3 family in mammals consists of two members, PDE3A and PDE3B. The PDE3 isoforms are structurally similar, containing an N-terminal domain important for the localization and a C-terminus end. The 44-amino acid insertion in the catalytic domain differs in the PDE3 isoforms, and the N-terminal portions of the isoforms are quite divergent. PDE3A and PDE3B have strikingly similar pharmacological and kinetic properties, but the distinction is in expression profiles and affinity for cGMP.
The PDE3 family is composed of two genes, PDE3A and PDE3B. In cells expressing both genes, PDE3A is usually dominant. Three different variants of PDE3A (PDE3A1-3) are products of alternate startcodon usage of the PDE3A gene. The PDE3B encodes a single isoform only.
In their full-length both PDE3A and PDE3B contain two N-terminal hydrophobic membrane association regions, NHR1 and NHR2 (figure 2). The difference of the PDE3A1-3 variants lies in whether they include: both NHR1 and NHR2 only NHR2 neither NHR1 nor the NHR2. The last can be predicted to be exclusively on soluble/cytosolic form.
Tissue distribution
PDE3A is mainly implicated in cardiovascular function and fertility but PDE3B is mainly implicated in lipolysis. Table 1 is an overview of localization of the PDE3 isoforms.
In general, PDE3 can be either cytosolic or membrane-bound and has been associated to plasma membrane, sarcoplasmic reticulum, golgi, and nucleus envelope.
PDE3B is predominantly membrane-associated, and is localized to endoplasmic reticulum and microsomal fractions.
PDE3A can be either membrane-associated or cytosolic, depending on the variant and the cell type it is expressed in.
Regulation
PDE3A and PDE3B activity is regulated by several phosphorylation pathways. Protein kinase A and protein kinase B both activate PDE3A and PDE3B via phosphorylation at two different phosphorylation sites (P1 and P2) between NHR1 and NHR2 (figure 2). Hydrolysis of cAMP by PDE3 isoforms is also directly inhibited by cGMP, although PDE3B is only ≈10% as sensitive to cGMP inhibition as PDE3A. The PDE3B has been extensively studied for its importance in mediating the antilipolytic and antiglycogenlytic effect of insulin in adipose and liver tissues. The activation of PDE3B in adipocytes is associated with phosphorylation of serine residue by an insulin-stimulated protein serine kinase (PDE3IK). By blocking insulin activation of PDE3IK, and in turn phosphorylation/activation of PDE3B, the antilipolytic effect of insulin can be antagonized. Activation of PDE3B decreases concentrations of cAMP, which in turn reduces protein kinase A activity. Protein kinase A is responsible for activation of lipase, which induces lipolysis as well as other physiological pathways.
Whether phosphorylation pathways, which regulate activity of PDE3A or PDE3B, could serve as potential drug targets rather than the catalytic domain of the PDE3 enzyme itself is unclear and beyond the scope of this text.