Prostacyclin Synthase Deficiency Leads to Exacerbation or Occurrence of Endothelium-Dependent Contraction and Causes Cardiovascular Disorders Mainly via the Non-TxA2 Prostanoids/TP Axis.

Background: Prostaglandin I₂ synthesized by endothelial COX (cyclooxygenase) evokes potent vasodilation in some blood vessels but is paradoxically responsible for endothelium-dependent constriction (EDC) in others. Prostaglandin I₂ production and EDC may be enhanced in diseases such as hypertension. However, how PGIS (prostaglandin I₂ synthase) deficiency affects EDC and how this is implicated in the consequent cardiovascular pathologies remain largely unknown.

Methods: Experiments were performed with wild-type, Pgis knockout (Pgis^-/-) and Pgis/thromboxane-prostanoid receptor gene (Tp) double knockout (Pgis^-/-Tp^-/-) mice and Pgis^-/- mice transplanted with unfractionated wild-type or Cox-1^-/- bone marrow cells, as well as human umbilical arteries. COX-derived prostanoids were measured by high-performance liquid chromatography-mass spectrometry. Vasomotor responses of distinct types of arteries were assessed by isometric force measurement. Parameters of hypertension, vascular remodeling, and cardiac hypertrophy in mice at different ages were monitored.

Results: PGF_2α, PGE₂, and a trace amount of PGD₂, but not thromboxane A₂ (TxA₂), were produced in response to acetylcholine in Pgis^-/- or PGIS-inhibited arteries. PGIS deficiency resulted in exacerbation or occurrence of EDC ex vivo and in vivo. Endothelium-dependent hyperpolarization was unchanged, but phosphorylation levels of eNOS (endothelial nitric oxide synthase) at Ser1177 and Thr495 were altered and NO production and the NO-dependent relaxation evoked by acetylcholine were remarkably reduced in Pgis^-/- aortas. Pgis^-/- mice developed high blood pressure and vascular remodeling at 16 to 17 weeks and subsequently cardiac hypertrophy at 24 to 26 weeks. Meanwhile, blood pressure and cardiac parameters remained normal at 8 to 10 weeks. Additional ablation of TP (TxA₂ receptor) not only restrained EDC and the downregulation of NO signaling in Pgis^-/- mice but also ameliorated the cardiovascular abnormalities. Stimulation of Pgis^-/- vessels with acetylcholine in the presence of platelets led to increased TxA₂ generation. COX-1 disruption in bone marrow-derived cells failed to affect the development of high blood pressure and vascular remodeling in Pgis^-/- mice though it largely suppressed the increase of plasma TxB₂ (TxA₂ metabolite) level.

Conclusions: Our study demonstrates that the non-TxA₂ prostanoids/TP axis plays an essential role in mediating the augmentation of EDC and cardiovascular disorders when PGIS is deficient, suggesting TP as a promising therapeutic target in diseases associated with PGIS insufficiency.