Role of Pathologic Shear Stress Alterations in Aortic Valve Endothelial Activation

Calcific aortic stenosis is the most common aortic valve (AV) disease and is triggered by an active inflammatory process involving endothelial activation and cytokine expression. Interfacing between the leaflet and the surrounding blood flow, shear stress is presumed to play an important role in endothelial injury. This study investigated the hypothesis that pathologic alterations in shear stress magnitude contribute to valvular endothelial activation via BMP-4and TGF-b1-dependent mechanisms. The fibrosa of porcine AV leaflets was subjected to physiologic, sub-physiologic and supra-physiologic magnitudes of native oscillatory shear stress for 48 h. Endothelial activation was assessed via immunohistochemistry in terms of ICAM-1 and VCAM-1 expressions. Cytokine expression was investigated in terms of BMP-4 and TGF-b1. Proand anti-osteogenic media were used to characterize the role of those cytokines in the shear stress-induced pathological response. Supra-physiologic shear stress increased the expression of all biomarkers in a shear stress magnitude-dependent manner. In contrast, neither physiologic nor sub-physiologic shear stress elicited a pro-inflammatory response. While BMP-4 inhibition and supplementation had limited effects on endothelial activation, TGF-b1 supplementation increased the overall leaflet pro-inflammatory state and TGF-b1 inhibition reduced endothelial activation in response to elevated shear stress. Combined TGF-b1 and BMP-4 inhibition completely suppressed shear stress-induced endothelial activation. The results demonstrate that elevated shear stress activates the valvular endothelium on the fibrosa via a BMP-4and TGFb1-dependent pathway. The suggested synergy between those cytokines also provides new insights into the transduction of valvular hemodynamic alterations into a pathological response. Keywords—Aortic valve, Endothelial activation, Shear stress, Cytokines, Adhesion molecules. INTRODUCTION Calcific aortic stenosis is the most prevalent aortic valve (AV) disease and is present in 8% of the population above 65 years of age. The side-specific formation of calcific nodules on the aortic surface of the leaflets contributes to the obstruction of the left ventricular outflow and can lead ultimately to heart failure. The previously accepted theory that linked valvular calcification to a passive wear-and-tear mechanism has lost support due to new developments that have associated the disease with an active process involving inflammation and ossification. The characterization of calcific lesions suggests that the early stage of the disease is marked by cell proliferation and increased expressions of adhesion molecules, bone morphogenic proteins (BMP) and transforming growth factors-beta (TGF-b).Although the inflammatory stage is thought to be associated with the dysfunction of the leaflet endothelium, the mechanisms contributing to endothelial activation are not well understood. Clinical observations, animal and ex vivo studies have suggested that the hemodynamic stress environment experienced by the leaflets may regulate valvular physiology and pathology. Resulting from the relative motion between the leaflet surface and the surrounding pulsatile blood flow, shear stress is an important component of the valve hemodynamic environment. The particular valve anatomy and leaflet dynamics give rise to a side-specific shear stress defined by a high unidirectional pulsatile shear stress along the ventricular leaflet surface (ventricularis) and a low bidirectional oscillatory shear stress along the aortic surface (fibrosa). This complexity has hampered our understanding of the biological processes regulated by the native valvular fluid shear stresses. Although studies have demonstrated the characteristic Address correspondence to Philippe Sucosky, Department of Aerospace and Mechanical Engineering, University of Notre Dame, 143 Multidisciplinary Research Building, Notre Dame, IN 466575637, USA. Electronic mail: [email protected] Cardiovascular Engineering and Technology, Vol. 1, No. 2, June 2010 ( 2010) pp. 165–178 DOI: 10.1007/s13239-010-0015-5 1869-408X/10/0600-0165/