E-type prostanoid receptor 4 (EP4) in disease and therapy

Abstract

The large variety of biological functions governed by prostaglandin (PG) E2 is mediated by signaling through four distinct E-type prostanoid (EP) receptors. The availability of mouse strains with genetic ablation of each EP receptor subtype and the development of selective EP agonists and antagonists have tremendously advanced our understanding of PGE2 as a physiologically and clinically relevant mediator. Moreover, studies using disease models revealed numerous conditions in which distinct EP receptors might be exploited therapeutically. In this context, the EP4 receptor is currently emerging as most versatile and promising among PGE2 receptors. Anti-inflammatory, anti-thrombotic and vasoprotective effects have been proposed for the EP4 receptor, along with its recently described unfavorable tumor-promoting and pro-angiogenic roles. A possible explanation for the diverse biological functions of EP4 might be the multiple signaling pathways switched on upon EP4 activation. The present review attempts to summarize the EP4 receptor-triggered signaling modules and the possible therapeutic applications of EP4-selective agonists and antagonists.

Fig. 1

PGE₂ biosynthesis and receptors. Arachidonic acid is liberated from membrane phospholipids by phospholipase A₂ enzyme activity. Arachidonic acid is converted to the endoperoxide PGG₂ and further reduced to PGH₂ by the action of cyclooxygenase 1 and 2 enzymes. PGE₂ is formed from PGH₂ by PGE synthases and binds to and activates four EP receptor subtypes, designated EP1 to EP4 receptors. These receptors are coupled to different G proteins leading to subsequent activation of specific signal transduction pathways. Besides PGE₂, other prostanoids are also formed from PGH₂, i.e. PGI₂, TXA₂, PGD₂ and PGF_2α. Additionally, PGE₂ can also be converted to PGA₂.

Fig. 2

The structure of the EP4 receptor and its activation of signaling modules. The 488 amino acid sequence of the human EP4 receptor is displayed with color code indicating the single residues conserved in all EPs, the binding sites for the native ligand PGE₂, regions responsible for desensitization and for interaction with EPRAP. Binding of an agonist induces G_αs-dependent activation of adenylate cyclase (AC), formation of cAMP and either activation of Epac (exchange factor activated by cAMP), activation of the PKA-independent AMP-activated protein kinase (AMPK) or activation of PKA and eNOS, or alternatively the transcription factor CREB (cAMP-responsive element-binding protein). Additionally, cAMP can act through inducible cAMP early repressor (ICER). EP4 receptor activation has been found to reduce retinoic acid secretion by ICER-mediated blockade of the expression of retinoic acid dehydrogenase These signals finally mediate vascular relaxation, angiogenesis and protection against cerebral ischemic injury. EP4 receptor becomes rapidly desensitized upon binding of G protein-coupled receptor kinases (GRK) to serine residues on the C-terminus. These residues are phosphorylated and attract β-arrestin initiating receptor internalization, and c-Src which leads to transactivation of EGFR and further activation of PI3K/ERK/Akt kinases. Alternatively, Pertussis toxin-sensitive G_αi can be activated which also may induce the activation of PI3K/ERK pathway. These latter signaling pathways enable migration and metastasis of colorectal carcinoma and inhibit the activation of eosinophils in allergic inflammation. The extended C-terminus of EP4 receptor allows interaction with EPRAP which in turn stabilizes the p105 subunit that prevents the activation of NF-κB and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase kinase 1/2 (MEK/ERK1/2), and inhibits transcription of pro-inflammatory cytokines in activated macrophages.

Fig. 3

The EP4 receptor promotes endothelial barrier function. EP4 receptor activation induces rapid cortical actin polymerization and VE-cadherin expression in the endothelial junctions, which results in markedly increased electrical resistance of the cell monolayer. Furthermore, E-selectin upregulation, and leukocyte adhesion and transendothelial migration as induced by TNF-α treatment is prevented by EP4 receptor activation. EP4 receptor-mediated E-selectin down-regulation is PI3K/PKC-dependent; however, none of the generally used EP4 receptor signaling modules (cAMP, eNOS, Rac1, PI3K, p38, ERK1/2) are involved in the barrier-enhancing effect of EP4 receptor. In contrast, the cAMP/PKA/p38 MAP kinase pathway mediates the EP4 receptor-stimulated release of IL-8 in endothelial cells.

Fig. 4

The effect of EP4 receptor activation in different diseases and cellular processes. Promotion of a respective response is shown by while indicates inhibitory modulation.