Role of inflammatory response in aortic valve lesions

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Summary Surgical valve replacement is the only available therapy for calcific aortic valve stenosis. Although chronic Inflammation is believed to play a major role in the pathogenesis and progression of aortic valve lesions, the signaling mechanisms that initiate and sustain the inflammatory process in valvular cells remain unclear. Understanding of the inflammatory and osteogenic responses in valvular cells is important for prevention aortic valve lesions and their progression. We demonstrate that stimulating Toll-like receptor 4 (TLR4) with endotoxin up-regulates the expression of bone morphogenetic protein 2 (BMP-2) in human aortic valve myofibroblasts (HAVMFs) and that BMP-2 plays a major role in TLR4-induced osteogenic responses, characterized by expression of Runx2, alkaline phosphatase (ALP) and osteopontin. Further, we found that chronic stimulation of TLR4 promotes aortic valve calcification and thickening in old mice. Preliminary studies show that TLR4 signaling up-regulates the levels of Notch1 intracellular domain (NICD1) in HAVMFs. Neutralizing INF¿, a TRIF-regulated type 1 interferon, abrogates the increase in cellular NICD1 and reduces BMP-2 levels following stimulation of TLR4. Further, inhibition of Notch1 also reduces BMP-2 expression following TLR4 stimulation. Together, these novel findings indicate that INF¿ activates Notch1 and that NICD1 enhances TLR4-induced BMP-2 expression. Interestingly, elevated levels of lNICD1 and BMP-2 coexist in human AVMFs at 24 h following TLR4 stimulation. It is likely that NICD1 plays a role not only in regulating BMP-2 expression in the early phase but also in modulating BMP-2 signaling. Indeed, preliminary studies show that activation of Notch1 enhances BMP-2-induced expression of Runx2. Thus, TLR4 signaling induces the osteogenic responses in human AVMFs, and mechanisms involve the interplay between the BMP-2 and Notch1 pathways. We formulated three interrelated aims to determine the mechanisms of TLR4-induced valvular osteogenic responses: 1) to test the hypothesis that TLR4 signaling induces aortic valve osteogenic responses through BMP-2-mediated up-regulation of Runx2, 2) to test the hypothesis that NICD1 enhances BMP-2 expression through modulation of NF-¿B activity and 3) to test the hypothesis that NICD1 enhances BMP-2 signaling via interaction with Smad1/5/8. These studies will provide important insights into the molecular mechanisms by which pro-inflammatory signaling mediates AVMF osteogenic responses and aortic valve lesions.