Individuals with apolipoprotein (Apo) E deficiency develop hypercholesterolemia and atherosclerosis. Similarly, ApoE-deficient (ApoE-/-) mice elevate plasma ApoB48-carrying lipoproteins and develop atherosclerosis in a manner that resembles the human disease. The primary cause of atherosclerosis in ApoE-deficient patients and mouse models is the deposition of ApoE-deficient, ApoB48-carrying (E-/B48) lipoproteins in the arterial wall. The deposited lipoproteins recruit monocytes into the arterial intima and transform them into macrophages, which participate in the pathogenesis of atherosclerosis mainly through two mechanisms: 1) releasing inflammatory mediators, and 2) forming foam cells. An uncontrolled macrophage uptake of E-/B48 lipoproteins could be a mechanism underlying foam cell formation in ApoE-/- mice. Our laboratory recently demonstrated that E-/B48 lipoproteins reduce cellular cholesterol efflux from macrophages and down-regulates lysosomal hydrolase expression. In addition, the degradation of E-/B48 lipoproteins by macrophages declined over time. These novel findings indicate that both reduced cholesterol efflux and decreased degradation of E-/B48 lipoproteins could contribute to foam cell formation. Our preliminary studies also revealed that incubation of macrophages with E-/B48 lipoproteins enhanced eukaryotic translation initiationfactor 21 (eIF-2a) phosphorylation, which is linked to one of the unfolded protein response (UPR) signaling pathways. Thus, interaction of E-/B48 lipoproteins with macrophages may activate UPR signaling pathways, which in turn regulate gene expression and induce atherogenic events, such as triggering foam cell formation. In the proposed studies, we will test the hypothesis that activation of UPR signaling pathways is a mechanism by which E-/B48 lipoproteins regulate gene expression, induce foam cell formation and promote atherosclerosis. This project includes four specific aims: 1) to determine whether E-/B48 lipoprotein-induced changes in gene expression result from altered transcription or translation or both in mouse macrophages;2) to determine whether activation of UPR signaling pathways is a mechanism underlying E-/B48 lipoprotein-induced gene expression changes;3) to determine whether activation of UPR pathways is a mechanism underlying E-/B48 lipoprotein-induced foam cell formation;and 4) to determine the effect of inhibiting eIF-2a phosphorylation on atherosclerosis in ApoE-/- mice. If our hypothesis is correct, inactivation of UPR signaling pathways will attenuate E-/B48 lipoprotein-induced gene expression, and suppress foam cell formation and atherosclerosis development.
Myocardial infarction and stroke are the leading caused of death in the United Sates. Atherosclerosis is the primary cause of myocardial infarction and stroke. Formation of lipid-laden foam cells in the vessel wall is the early stage of atherosclerosis. The goal of this proposal is to determine the involvement of endoplasmic reticulum stress in foam cell formation. Endoplasmic reticulum stress is a cellular event that reduces the level of some proteins but increases the level of other proteins in cells. Our preliminary studies indicate that lipoproteins obtained from mice deficient in apolipoprotein E can cause endoplasmic reticulum stress and induce foam cell formation. The experiments designed in this proposal will study whether endoplasmic reticulum stress is the cause of foam cell formation. Finding from this work should contribute to understanding of the molecular mechanism of foam cell formation, and provide strategies for treatment or prevention of atherosclerosis by inhibition of endoplasmic reticulum stress.
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