Abstract 14205: A Role of Inositol 1,4,5-trisphosphate Receptors to Inhibit the Progression of Pulmonary Arterial Hypertension

Abstract

Backgroud: Inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ release channel in response to extracellular stimulation. There are three subtypes of IP3R (IP3R1, 2 and 3) and IP3R2 may be predominantly expressed in the pulmonary arterial smooth muscle cells (PASMCs). Objectives: To elucidate the role of IP3R2 in the progression of pulmonary arterial hypertension (PAH). Methods: The detailed expression pattern of IP3R2 in pulmonary vasculature during development and adulthood was investigated in lung sections of transgenic mice in which a LacZ reporter gene was knocked into the IP3R2 locus. In order to elucidate whether IP3R2 play a role in progression of PAH, we utilized chronic hypoxia (10% O2 for 6 weeks) -induced PAH mouse models. The severity of PAH was evaluated by transthoracic echocardiography, the weight of right ventricle, and the thickness of tunica media of pulmonary arteries, and compared between wild type (WT) and IP3R2 knock out (KO) mice. Proliferation and apoptosis in PASMCs were also assessed in both mice by immuno-staining with anti-phospho-histone H3 (PHH3) antibody and TUNEL assay, respectively. Further, in vitro, capacitative Ca2+ entry was assessed by Ca2+ imaging in primarily cultured PASMCs from WT and IP3R2 KO lungs by loading with fluorescent Ca2+ probe (Fluo-4AM). Results: Strong expression of the IP3R2 was detected in pulmonary arteries, predominantly in medial wall, from embryonic periods to the adulthood. IP3R2 KO mice showed higher RV pressure than those in WT mice by echocardiography. RV hypertrophy and medial wall thickness of PASMCs were more severely progressed in KO. There was no significant increase of PHH3 positive cells, while there was significant decrease of TUNEL positive cells in KO PASMCs compared with WT, suggesting that apoptosis was inhibited in KO PASMCs. Ca2+ imaging revealed that store-operated Ca2+ entry (SOCE) was enhanced in KO compared with WT. Taken together, loss of IP3R2 function leads to the acceleration of the progression of chronic hypoxia-induced PAH, with depressed apoptosis and enhanced SOCE in PASMCs. Conclusions: Although IP3R2 functions in increase of cytosolic Ca2+, it may play an inhibitory role in pathophysiology of PAH by influencing to the apoptosis and SOCE in PASMCs.