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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2009;29:936.)
© 2009 American Heart Association, Inc.

Cell Biology/Signaling

Calcification by Valve Interstitial Cells Is Regulated by the Stiffness of the Extracellular Matrix

Cindy Ying Yin Yip; Jan-Hung Chen; Ruogang Zhao; Craig A. Simmons

From the Institute of Biomaterials and Biomedical Engineering (C.Y.Y.Y., J.-H.C., R.Z., C.A.S.), Cardiovascular Sciences Collaborative Program (C.Y.Y.Y.), the Department of Mechanical and Industrial Engineering (J.-H.C., C.A.S.), and the Faculty of Dentistry (C.A.S.), University of Toronto, ON, Canada.

Correspondence to Craig A. Simmons, Department of Mechanical & Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, ON, Canada M5S 3G8. E-mail c.simmons{at}utoronto.ca

Objective— Extensive remodeling of the valve ECM in calcific aortic valve sclerosis alters its mechanical properties, but little is known about the impact of matrix mechanics on the cells within the valve interstitium. In this study, the influence of matrix stiffness in modulating calcification by valve interstitial cells (VICs), and their differentiation to pathological phenotypes was assessed.

Methods and Results— Primary porcine aortic VICs were cultured in standard media or calcifying media on constrained type I fibrillar collagen gels. Matrix stiffness was altered by changing only the thickness of the gels. Calcification did not occur in standard media, regardless of matrix stiffness. However, when VICs were grown in calcifying media on relatively compliant matrices with stiffness similar to that of normal tissue, they readily formed calcified aggregates of viable cells that expressed osteoblast-related transcripts and proteins. In contrast, VICs cultured in calcifying media on stiffer matrices (similar to stenotic tissue) differentiated to myofibroblasts and formed calcified aggregates that contained apoptotic cells. Actin depolymerization reduced aggregation on stiff, but not compliant, matrices. TGF-β1 potentiated aggregate formation on stiff matrices by enhancing -smooth muscle actin expression and cellular contractility, but not on compliant matrices attributable to downregulation of TGF-β receptor I. Cell contraction by VICs inhibited Akt activation and enhanced apoptosis-dependent calcification on stiff matrices.

Conclusions— Differentiation of VICs to pathological phenotypes in response to biochemical cues is modulated by matrix stiffness. Although osteogenic or myofibrogenic differentiation of VICs can result in calcification, the processes are distinct.

Calcification by valve interstitial cells and their differentiation to pathological phenotypes are regulated by the stiffness of the extracellular matrix in vitro. More compliant substrates promote osteogenic differentiation and bone formation, whereas stiffer substrates promote myofibroblast differentiation and calcification associated with apoptotic cells.

Key Words: aortic valve • matrix mechanics • mechanobiology • collagen • sclerosis