A lipid-induced RNA-binding protein in atherosclerosis

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PROJECT ABSTRACT Atherosclerosis is a chronic inflammatory vascular disease resulting from maladaptive inflammatory response to an imbalanced lipid metabolism. The cholesterol-laden, foamy macrophages found in plaques play a pivotal role in perpetuating the sterile inflammation that is characteristic of atherosclerosis. Transcriptional control plays a critical role in setting into motion this sterile inflammation. Post-transcriptional mechanisms that operate in atherosclerosis can contribute to resolution of inflammation and promote plaque regression, presenting a therapeutic opportunity. Ribonucleic acid RNA-binding proteins (RBP) alter cytokine and chemokine messenger RNA (mRNA) stability or translation to fine-tune or turn-off the inflammatory response. RBPs also post-transcriptionally regulate key proteins for cholesterol homeostasis and lipid metabolism in macrophages and liver. Despite regulating inflammation, lipid metabolism and cholesterol homeostasis, thereby representing a novel therapeutic opportunity in cardiovascular disease, only a few RBPs and their RNA targets have been directly investigated in atherosclerosis. We made the striking discovery that Fragile X Mental Retardation Protein (FMRP), a widely studied RBP in autism spectrum disorder, is induced by lipids in macrophages and in mouse and human atherosclerotic plaques. We found FMRP associates with and is phosphorylated by the Inositol-Requiring Enzyme-1 (IRE1), a conserved endoplasmic reticulum (ER) stress-sensing kinase/endoribonuclease. ER stress and subsequent IRE1 activation in plaques is causally associated with atherosclerosis. Enhanced IRE1 to FMRP signaling in macrophages may thus promote atherogenesis and represent a novel therapeutic opportunity in atherosclerosis. Our preliminary work shows FMRP inhibition leads to post-transcriptional induction of cholesterol exporters and reduces foam cell formation. Lower cholesterol levels were reported in both FMRP-deficient mice and Fragile X patients, suggesting cholesterol homeostasis is an important target for FMRP. Building on the insight gained through our robust preliminary studies and incorporating additional evidence from literature, we hypothesize that post-transcriptional suppression of cholesterol exporters by the IRE1-phosphorylated FMRP promotes macrophage foam cell formation and atherosclerosis progression. We propose to demonstrate FMRP's role in reverse cholesterol transport, foam cell formation and atherosclerosis in vivo. We will also investigate the consequences of inhibiting IRE1 kinase-mediated FMRP phosphorylation on reverse cholesterol transport, foam cell formation and atherosclerosis in mice. The completion of the proposed studies will illuminate the mechanism of action for this novel IRE1 kinase substrate. The new knowledge gained through these studies could pave the way for the development of effective strategies to prevent atherosclerosis by fine-tuning the homeostatic ER stress response that is pathologically activated by hyerlipidemia.