Lysophosphatidic Acid and Cardiovascular Disease Risk

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Veterans have a higher incidence of cardiovascular disease than the general population. Valvular diseases including Calcific Aortic Valve Disease (CAVD) are a particular concern for the aging Veteran Population. At present, there is no medical therapy to delay or reverse CAVD, and the only treatment is valve replacement for severe aortic valve stenosis. CAVD involves remodeling of the heart valve tissue as a consequence of endothelial injury, immune cell infiltration and myofibroblastic / osteogenic differentiation of cells that can ultimately result in valve leaflet thickening and profound calcification. The fibrosis and calcification stiffen the leaflets and can result in leaflet fusion that reduces valve opening and causes valve stenosis. Understanding the molecular mechanisms that drive these changes might lead to the development of much needed therapies for CAVD. In the past funding period we made mouse models to study the roles of a bioactive lipid, lysophosphatidic acid (LPA) in cardiovascular and metabolic disease processes. In the course of these studies we found that mice deficient in the enzyme autotaxin (ATX) that generates LPA were protected from valve calcification and thickening in a commonly used experimental model. We also observed that mice lacking the enzyme lipid phosphate phosphatase 3 (LPP3) that can inactivate LPA exhibited greater valve calcification in this model. These findings are likely translatable to humans because LPP3 levels are decreased during development of human CAVD while ATX accumulates in the valve tissue and ATX binds to lipoprotein (a) particles which are themselves associated with CAVD risk. Valvular Interstitial Cells (VICs) are resident cells of the heart valve tissue that are normally responsible for maintaining the integrity of the heart valves. Pathological differentiation of these cells to myofibroblastic and osteogenic phenotypes is central to the development of CAVD. Consistent with literature reports, our preliminary data shows that mouse and human VICs express LPA selective cell surface receptors. Differentiation of these cells to an osteogenic phenotype and subsequent calcification can be readily observed in culture medium containing serum which is a rich source of LPA. Pharmacological antagonism of LPA receptors blocks osteogenic differentiation and calcification of these cells in culture. In the past funding period we characterized transcriptional circuits that regulate LPP3 expression to understand why expression is increased in inflammation and decreased by heritable variants that associate with increased coronary artery disease risk. These studies provide reagents and a framework for understanding why LPP3 expression is decreased in CAVD. Here we propose to test the broad hypothesis that LPA signaling promotes CAVD. We will test this hypothesis by using mouse models with cell and tissue type selective inactivation of LPA receptors, LPP3 and ATX to identify the cell and tissue types involved the permissive effect of LPA on CAVD with a particular interest in the possibility that secreted ATX and cell surface LPP3 could have non cell autonomous effects on this process. As an orthogonal approach, we will use well characterized experimental therapeutics (ATX inhibitors and LPA receptor antagonists) to validate results from these gene knockout models and evaluate their potential for pharmacological intervention in CAVD. Studies in mouse models will be augmented by experiments using cultured mouse and human VICs where again cells with genetic deficiencies or treatment with small molecule therapeutics can be used to define the role of LPA signaling in osteogenic differentiation and calcification. We will also examine how LPP3 expression is regulated during these processes and test specific hypotheses about why LPP3 expression is decreased during development of CAVD. This research will provide important new information about a pharmacologically tractable lipid signaling pathway that appears to be central to the development of CAVD. This information could lead to new approaches for non- surgical management of CAVD in Veterans.