Phosphodiesterase-4 isoforms: intracellular targeting, regulation and potential therapeutic targets

Chronic Obstructive Pulmonary Disease is the fourth leading cause of death in the world. It causes lung damage through chronic inflammation. Raising the levels of a substance called cAMP inside lung cells can stop this. Potential new medicines are drugs that bind tightly to PDE4 enzymes to stop them breaking down cAMP. Unfortunately these drugs cause nausea and vomiting, preventing their use. We’ve found that there are many PDE4 subtypes but that only certain of these need to be inhibited to stop inflammation. We aim to devise novel ways to inhibit these important PDE4 subtypes, rather than all PDE4 enzymes, in order to produce effective medicines with minimal or no side-effects. To do this we will exploit our discovery that individual PDE4 subtypes have unique ‘postcodes’ built into them that allow them to be targeted to exactly the right place in cells for them to do their job properly. Our aim is to find molecules that block this correct targeting so as to stop them working properly, providing new COPD medicines. As our drugs focus on just the PDE4 subtype involved in COPD they should not have the side effects caused by current drugs that inhibit all PDE4 subtypes.

cAMP is a pivotal second messenger able to exert a panoply of cell type specific effects. A multigene family of phosphodiesterases (PDEs) provide the sole means for degrading cAMP and are thus poised to play a key regulatory role in cells. Highly specialised regulatory properties, coupled with cell specific patterns of expression, provide each cell type with a sophisticated and unique means of regulating intracellular cAMP through PDE action. This diversity offers potential for developing highly specific therapeutic agents. There is great interest in developing PDE4 family selective inhibitors for treating Chronic Obstructive Pulmonary Disease (COPD), asthma and other diseases. However, the therapeutic deployment of PDE4 inhibitors has been limited by side effects, such as nausea. It is now appreciated that PDE4 activity is attributable to a large family of isoforms. This has prompted the notion that selective inhibition of particular PDE4 isoforms may maximise therapeutic benefit whilst minimising side effects. Four PDE4 genes produce around 20 isoforms that each have a unique N terminal region and either both UCR1/2 or just UCR2 regulatory domains. Individual PDE4 isoforms have unique functional roles that are attributable to defined intracellular targeting / association with protein complexes. This provides a major underpinning of compartmentalised cAMP signalling. We aim to determine the functional role and basis by which the N terminal and UCR1/2 tailor intracellular targeting, interaction with signal scaffold proteins and regulation by phosphorylation of specific PDE4 isoforms. Our studies have implications for the identification of novel means of generating inhibitors selective for specific PDE4 isoforms by preventing specific PDE4 isoforms from being targeted to their functionally relevant sites in cells. This provides a route to overcome problems in making effective isoform-selective inhibitors by traditional means, namely directed at the active site, as the PDE4 catalytic unit is identical within PDE4 sub-families & very similar between them. Identification of PDE4 interacting proteins has potential for identifying novel biomarkers for COPD. Analysis of post-translational modification of isoforms has potential for appreciating aberrant control of specific PDE4 isoforms in disease. In particular we will focus on (i) PDE4A4, which is selectively up-regulated in COPD macrophages; (ii) PDE4B2, which has been implicated as of importance in regulating macrophage functioning; (iii) PDE4D5, which is up-regulated in macrophages of smokers, regulates beta2AR functioning through beta-arrestin sequestration and is induced by hypoxia in smooth muscle cells and (iv) PDE4D3, which is the major PDE4 in monocytes.