Endothelial prostacyclin receptor signaling : importance of nitric oxide and inhibition by apolipoprotein E

Abstract

Prostacyclin is an unstable substance generated by endothelial cells, which plays an important role in inhibiting platelet aggregation and adhesion, and inducing blood vessel dilatation. Prostacyclin receptors (IP) are the endogenous G-protein coupled (GPCR) receptor for prostacyclin. Once activated, they stimulate adenylyl cyclase producing cyclic adenosine monophosphate (cAMP) which in turn activates cAMP-activated protein kinase A (PKA) inhibiting platelet aggregation and inducing relaxation of vascular smooth muscle cells. IP receptor agonists have been used for years to treat pulmonary hypertension. Apolipoprotein E is a plasma glycoprotein synthesized majorly in the liver, spleen, brain and macrophages. As apolipoprotein E promotes the uptake of circulating cholesterol, when it is dysfunctional or absent, hyperlipidemia and atherosclerosis occur. It also exerts protective effects on vascular function by inducing nitric oxide (NO) or prostacyclin production. In isolated blood vessels, prostacyclin-induced relaxations are mediated through endothelial NO generation. NO-mediated relaxations of isolated arteries are blunted by ageing and high fat diet, as well as by apolipoprotein E deletion. The present thesis aims to examine the interaction between prostacyclin (IP) receptor activation and the NO signaling pathway, as well as to determine the influence of apolipoprotein E on IP-mediated vascular responses. The key findings are as follows: 1. In aortae of young healthy mice, IP receptor activation induce endothelium-, NO-, and soluble guanylyl cyclase (sGC)-dependent relaxations which involve both the cyclic guanosine monophosphate (cGMP) and cAMP pathways. Apolipoprotein E deficiency potentiates the relaxations to IP receptor activation in the aorta of the apolipoprotein E-deficient mouse, irrespective of age and diet. 2. IP form heterodimers with thromboxane receptor (TP), leading to reduced vasodilator responses to the IP agonist iloprost and the release of cAMP by the TP agonist U46619. Apolipoprotein E appears to favor the heterodimerization of IP and TP receptors, which may account for the reduced relaxations to iloprost in the wildtype mouse aorta. 3. The NO-sGC-cAMP-PKA pathway plays a critical role in IP receptor agonist-induced relaxations in mouse aorta. The generation of cAMP subsequent to sGC activation requires the activation of smooth muscle IP receptors. 4. Deletion of IP receptors in young ApoE-/- mice does not affect NO-mediated relaxations; however, functional IP receptors play a critical role in preserving vascular function in the presence of cardiovascular risk factors such as ageing, hyperlipidemia and inflammation. NO-sGC-generated cAMP activating PKA is critical for IP receptor stimulation leading to vasodilatation in the mouse aorta, and this pathway is impaired by ageing, hyperlipidemia and apolipoprotein E protein, likely by promoting the heterodimerization between IP and TP receptors. The presence of IP receptors protects against the detrimental effects on vascular function of ageing, high fat diet and inflammation. These findings provide additional information on the IP receptor signaling pathway in mouse blood vessels, in particular as regards the interactions with NO-sGC and with apolipoprotein E, and thus may yield new perspectives for the clinical use of IP receptor agonists, especially in apolipoprotein E-related diseases.