Objective: Bone morphogenic protein 2 is found in calcified areas of stenotic aortic valves, and prolonged stimulation of aortic valve interstitial cells with bone morphogenic protein 2 results in increased expression of alkaline phosphatase, indicating a mechanistic role for bone morphogenic protein 2 in aortic valve calcification. The purposes of this study were to assess the effect of bone morphogenic protein 2 on the expression of the osteogenic factors Runx2 and osteopontin in human aortic valve interstitial cells and to determine the signaling mechanisms that mediate the expression of these early osteogenic factors.
Methods: Interstitial cells were isolated from normal and stenotic human aortic valve leaflets, and cellular bone morphogenic protein 2 levels were analyzed by means of immunoblotting. Cultured interstitial cells from normal aortic valves were stimulated with bone morphogenic protein 2 to determine its effect on cellular Runx2 and osteopontin levels.
Results: Interstitial cells from stenotic aortic valves express greater levels of bone morphogenic protein 2 than cells from normal valves. Stimulation of human aortic valve interstitial cells with bone morphogenic protein 2 induced marked increases in Runx2 and osteopontin levels at 48 hours. The changes in Runx2 and osteopontin levels were preceded by phosphorylation of Smad1 and extracellular signal-regulated kinase 1/2 but not p38 mitogen-activated protein kinase. Silencing Smad1 reduced Runx2 and osteopontin levels, whereas inhibition of extracellular signal-regulated kinase 1/2 reduced osteopontin expression without an influence on Runx2 expression.
Conclusions: Interstitial cells of stenotic human aortic valves are characterized by increased bone morphogenic protein 2 levels. A short period of exposure of human aortic valve interstitial cells to bone morphogenic protein 2 induces the expression of Runx2 and osteopontin. The extracellular signal-regulated kinase 1/2 pathway modulates bone morphogenic protein 2-induced osteopontin expression, and the Smad1 pathway plays a role in regulating the expression of both Runx2 and osteopontin induced by bone morphogenic protein 2.