Atherosclerosis is a chronic inflammatory disease characterized by the accumulation and subsequent modification of low-density lipoprotein (LDL)-cholesterol particles within the arterial wall. Establishing which of the many biological effects of modified LDL demonstrable in vitro are relevant to atherogenesis in vivo, and identifying the mechanisms of action and production of the principal molecular components responsible, remain important goals in the search for new targets for therapy in atherosclerosis. Lysophosphatidylcholine (LPC) is a major product of LDL oxidation and phospholipid hydrolysis by pro-inflammatory phospholipase A2 (PLA2) enzymes. It has been proposed that a principal effect of LPC is to promote sub-endothelial macrophage recruitment into the arterial wall at atherosclerotic foci by directly attracting monocytes and activating endothelial cells. The recently identified role of the G protein-coupled receptor, G2A, as a mediator of chemotaxis to LPC has led to the proposal that this key event is promoted by G2A. Loss of G2A function in atherosclerosis-susceptible low-density lipoprotein receptor knockout (LDLR-/-) mice led to robust suppression of aortic atherosclerosis. Intimal monocyte infiltration at lesion-prone sites of the aorta was significantly suppressed in G2A-deficient LDLR-/- (G2A-/-LDLR-/-) mice. However, elevated HDL-cholesterol levels in G2A-/-LDLR-/- mice revealed possible G2A-mediated effects on lipoprotein metabolism. Thus, G2A provides a pro-atherogenic stimulus in vivo consistent with penetrance of its chemotactic action but to which alterations in lipoprotein metabolism may also contribute. We hypothesize that stimulation of sub-endothelial monocyte infiltration in response to locally generated LPC is the principal mechanism by which G2A promotes atherosclerosis in LDLR-/- mice and that this is mediated independently of G2A-expressing endothelium. We propose that modulation of HDL-cholesterol levels by G2A is due to direct effects of this receptor on macrophage cholesterol efflux and this partially contributes to its pro-atherogenic action. To address these hypotheses, we will generate chimeric LDLR-/- mice in which G2A deficiency or G2A expression is restricted to bone marrow derived cells. HDL composition and macrophage cholesterol efflux will be examined in G2A??/- and G2A-/-LDLR-/- mice to determine the cellular mechanism by which G2A modulates HDL-cholesterol levels in hypercholesterolemic LDLR-/- mice. Finally, the LPC-dependence of G2A-mediated pro-atherogenic action will be investigated by macrophage-restricted overexpression of secretory PLA2 group X in LDLR-/- mice using a macrophage-specific retrovirus. By determining the cell specificity and LPC-dependence of G2A action, these studies will provide a molecular framework for the development of therapeutic approaches to target G2A and thereby attenuate atherosclerosis.
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