Lipid-laden macrophages or "foam cells" play crucial roles in all the stages of atherogenesis, from lesion initiation to plaque rupture. However, our current therapeutic arsenal lacks of drugs primarily targeting the foam cell to prevent atherosclerosis development. Macrophages become foam cells by the uptake of modified low- density lipoproteins (mLDL) and accumulation of their lipids, mostly cholesterol (CHOL) esters (CE), in cytoplasmic lipid droplets (LDs). LDs consist of a core of neutral lipid stabilized and circumscribed by phospholipids, free CHOL (FC) and proteins. The main structural LD-associated proteins are the members of the PAT-family, and in macrophages the most abundant PAT-protein is adipose differentiation-related protein (ADFP). We have shown that ADFP plays a key role in foam cell formation, and its absence severely restricts the ability of macrophages to become foam cells in vitro and in vivo. ADFP is upregulated in human and mouse atherosclerotic lesions, and ADFP ablation in apolipoprotein E-null (apoE-/-) mice is atheroprotective. Enhancing CHOL efflux from macrophages, e.g. by increasing the concentration in plasma of CHOL acceptors or by upregulating the expression of CHOL efflux transporters, protects against atherosclerosis. This also seems to be the atheroprotective mechanism associated to the lack of ADFP, since ADFP-deficient macrophages do not accumulate as much CHOL in the form of CE in LDs, and efflux more CHOL to extracellular acceptors. The broad goal of this proposal is to gain insight on the role of ADFP in LD biology, foam cell formation and atherosclerosis development. In the first aim, we will test in vitro the response, in terms of inflammation and apoptosis, of ADFP-deficient macrophages cultured under high lipid stress conditions. We will also check if the atheroprotection observed in ADFP-/- mice fed regular chow is also observed when mice are challenged with a high fat/high CHOL diet. In the second aim, we will increase the plasma concentration of the CHOL acceptor apolipoprotein A1 (apoA1) with a helper-dependent adenovirus (HDAd) vector system, and assess whether two different approaches to increase CHOL efflux (i.e. increasing extracellular acceptors and limiting the cell's ability to store CHOL) will have additive atheroprotective effects. In the third aim, we will perform proteomic analyses to find the main LD-associated proteins and ADFP interactors in macrophages cultured under lipid-storing and lipid-efflux conditions. We will also perform a global gene expression analysis in foam cells directly isolated from lesions of mice that do or do not express ADFP to determine how the foam cell adjusts to the lack of ADFP. From this broad-based analysis of the role of ADFP in foam cell formation and atherosclerosis we expect to obtain a breadth of understanding that may help to set- up the bases for novel strategies to target the foam cell to prevent atherosclerosis development.
By elucidating the response of ADFP-deficient macrophages to high lipid stress conditions, whether preventing cholesterol accumulation in lipid droplets while increasing the ability of the extracellular milieu to accept cholesterol can work synergistically to prevent atherosclerosis, the molecular mechanisms that govern lipid droplet genesis and degradation in macrophages, how ADFP is involved in the processes, and how the foam cell adjust to the lack of ADFP, these studies will provide a breadth of understanding on foam cell biology that may be very useful if drugs to target the lipid droplet to prevent foam cell formation and atheroclerosis are to be developed.
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