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Figure 1 Pathogenesis of Calcific AVS
(A) Current understanding of factors leading to aortic valve stenosis (AVS). All of the genes reported to be associated with AVS in at least 1 published study are shown at the base of the triangle. The next layers show: 1) environmental and clinical factors; 2) metabolic and signaling pathways; and 3) cellular processes associated with AVS. Each layer affects subsequent layers and leads to the final manifestation of the disease. An extra level of complexity arises from the fact that all factors, both genetic and nongenetic, exist in a dynamic network that operates throughout the lifetime of an individual. (B) Two possible disease histories. The first individual (individual 1) carries a high genetic predisposition and develops AVS by the age of 65 years. In contrast, the second individual (individual 2) has a more favorable genetic makeup and develops AVS 2 decades later. (C) Drawing of an aortic valve illustrating the metabolic, signaling, and cellular processes involved in AVS. The potential interplay between cells, molecular mediators, and pathways is depicted in the blood compartment and in the subendothelial region. First, inflammatory cells and atherogenic lipoproteins infiltrate the endothelial layer. The extracellular lipids are subsequently taken up by macrophages to become foam cells. Activated T lymphocytes within the subendothelial region release cytokines, such as tumor necrosis factor- (TNF-), transforming growth factor-β1 (TGF-β1), and interleukin-1β (IL-1β). Interleukin-1β increases local production of matrix metalloproteins (MMPs), which contribute to extracellular matrix remodeling. Macrophages also express osteopontin (OPN), a bone-associated protein. The angiotensin-converting enzyme (ACE) that is colocalized with apolipoprotein B generates angiotensin II (AngII) from angiotensin I (AngI). Angiotensin II stimulates fibroblast expression of lipoprotein-retaining proteoglycan that trap lipids within the subendothelial compartment. The atherogenic milieu up-regulates the expression of bone morphogenic protein 2 (BMP2) and promotes osteogenic signaling pathways such as Runx2/Cbfa1 and Wnt/Lrp5/β-catenin pathways, which are involved in the differentiation of fibroblast/myofibroblast to an osteoblast phenotype. These modified cells then favor the development of calcium nodules and bone formation. Recently, it was shown that NOTCH1 inhibits osteoblast differentiation by repressing the Runx2/Cbfa1 pathway (67). BMI = body mass index.
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