Progression of aortic valve stenosis: TGF-β1 is present in calcified aortic valve cusps and promotes aortic valve interstitial cell calcification via apoptosis

Abstract

Background Aortic valve stenosis characteristically progresses due to cuspal calcification, often necessitating valve replacement surgery. The present study investigated the hypothesis that TGF-β1, a cytokine that causes calcification of vascular smooth muscle cells in culture, initiates apoptosis of valvular interstitial cells as a mechanistic event in cuspal calcification. Methods Noncalcified and calcified human aortic valve cusps were obtained at autopsy or at the time of cardiac surgery. The distributions within cusps of TGF-β1, latent-TGF-β1-associated peptide, and TGF-β receptors were studied using immunohistochemistry. The effects of TGF-β1 on mechanistic events contributing to aortic valve calcification were also investigated using sheep aortic valve interstitial cell (SAVIC) cultures. Results Immunohistochemistry studies revealed that calcific aortic stenosis cusps characteristically contained within the extracellular matrix qualitatively higher levels of TGF-β1 than noncalcified cusps. Noncalcified normal valves demonstrated only focal intracellular TGF-β1. Addition of TGF-β1 to SAVIC cultures led to a cascade of events, including: cellular migration, aggregation, formation of apoptotic-alkaline phosphatase enriched nodules, and calcification of these nodules. The time course of these events in the SAVIC culture system was rapid with nodule formation with apoptosis by 72 hours, and calcification after 7 days. Furthermore, ZVAD-FMK, an antiapoptosis agent (caspase inhibitor), significantly inhibited calcification and apoptosis induced by TGF-β1, but had no effect on nodule formation. However, cytochalasin D, an actin-depolymerizing agent, inhibited nodule formation, but not calcification. Conclusions TGF-β1 is characteristically present within calcific aortic stenosis cusps, and mediates the calcification of aortic valve interstitial cells in culture through mechanisms involving apoptosis.