Ox-LDL induces PiT-1 expression in human aortic valve interstitial cells

Abstract

BackgroundThe aortic valve interstitial cell (AVIC) has been implicated in the pathogenesis of calcific aortic stenosis. When appropriately stimulated, AVICs undergo a phenotypic change from that of a myofibroblast to that of a bone-forming–like cell. An elevated blood level of low-density lipoprotein (LDL) cholesterol is a clinical risk factor for aortic stenosis, and oxidized LDL (ox-LDL) cholesterol has been consistently found in calcified aortic valve leaflets. However, whether it plays a role in the pathogenesis of aortic stenosis is unknown. The process of aortic valve leaflet calcification has been associated with the deposition of calcium phosphate, mediated in part by the phosphate inorganic transporter 1 (PiT-1), a sodium-phosphate ion cotransporter. Therefore, we hypothesized that ox-LDL induces an osteogenic change in human AVICs marked by the induction of PiT-1. Using isolated human AVICs, the purpose of the present study was to examine the effect of ox-LDL on the expression of PiT-1 and the osteogenic factor bone morphogenetic protein 2 (BMP-2), which is a protein necessary for bone formation. MethodsHuman AVICs were isolated from nonstenotic aortic valves obtained from the explanted hearts of patients undergoing cardiac transplantation (n = 4) and grown in culture. The cells were treated with serum-free media, serum-free media with dimethyl sulfoxide (vehicle control), 40 μg/mL of ox-LDL, or 40 μg/mL of ox-LDL plus 2.5 mM phosphonoformate hexahydrate acid. Phosphonoformate hexahydrate acid is a competitive inhibitor of PiT-1 by mimicking inorganic phosphate. Cell lysis was performed at 24 h after treatment. Cell lysates were analyzed using immunoblot and densitometry for PiT-1 and BMP-2. Statistical analysis was performed using analysis of variance. P < 0.05 was significant. Resultsox-LDL stimulation of AVICs induced an increase in PiT-1 and BMP-2. ox-LDL induced increased production of the phosphate transporter, PiT-1, and the osteogenic factor, BMP-2. Inhibition of PiT-1 with phosphonoformate hexahydrate acid prevented ox-LDL–induced BMP-2 expression. ConclusionsThese data offer mechanistic insight into the pathogenesis of calcific aortic stenosis.