Ischemic cardiovascular disease (CVD) is caused by atherosclerosis, a lipid-driven inflammatory disease affecting the arteries, which can progress into vulnerable plaques and thrombotic occlusion. The precise molecular mechanisms linking nutrient excess and hyperlipidemia to immune activation remains elusive and the discovery of these mechanisms could lead to novel CVD therapeutics. An important primer for inflammation in dyslipidemia is the chronic metabolic overloading and impairment of anabolic and catabolic organelles. Reduction of organelle stress alleviates insulin resistance and atherosclerosis. Recently, we showed that small molecule inhibitors of Inositol-requiring enzyme -1 (IRE1), a proximal ER stress sensor, counteract atherosclerosis progression. The ER membranes also serve as a nucleation site for RNA-induced silencing complex (RISC), and we made the striking discovery that IRE1 kinase phosphorylates the double stranded RNA-binding protein, the protein activator of the protein kinase R (PACT), that associates with RISC. We found lipid stress induces IRE1 to phosphorylate PACT, which suppresses mitochondrial biogenesis (mito-biogenesis), in part by controlling a miRNA (miR)-181c. Homeostatic mechanisms such as mitophagy (to remove) or mito-biogenesis (to replenish) the malfunctioning mitochondria can counteract inflammation and also operate in atherosclerotic plaque cells. Aberrant activation of IRE1-PACT signaling by lipids block mito-biogenesis and propagate mitochondrial oxidative (MOX) stress and inflammation, indicating inhibition of this pathological signaling could counteract atherosclerosis. PACT is proximal to a locus on human chromosome 2 that is linked to premature coronary artery disease and plasma HDL-C levels. PACT expression is induced during atherosclerosis progression and reduced during regression in mice. We hypothesize that suppressing IRE1-PACT signaling will promote mito-biogenesis and counteract inflammation and atherosclerosis. We will elucidate how PACT regulates mito-biogenesis by discovering PACT?s miR target(s) and their RNA targets that are relevant to mito-biogenesis regulation. We discovered miR-181c is one of these PACT targets that blocks mito-biogenesis. We will directly investigate the impact of PACT and miR-181c on hyperlipidemia-induced mito-biogenesis, inflammation and atherosclerosis in vivo. Based on the discovered targets (for miR-181c and others) we will develop a more specific therapeutic targeting approach (using Locked Nucleic Acid-Target-Site Blockers) to ablate miR and target interaction in atherosclerotic mice. The successful completion of these studies will help define an unprecedented mechanism of immune-metabolic crosstalk between ER and mitochondria by which hyperlipidemia can promote MOX stress, inflammation and atherosclerosis. Understanding the intrinsic operation of this RNA-mediated inter-organelle communication during atherogenesis could pave the way for novel therapeutic approaches targeting this specific immune-metabolic cross talk in CVD.

Public Health Relevance

Low-grade, chronic inflammation is incited by nutrient-excess and hyperlipidemia and underlies the development of cardiovascular diseases (CVD). We will investigate an unprecedented RNA-mediated inter- organelle communication that is promoted by hyperlipidemia and drives inflammation and atherosclerosis. This work will provide insight into how an RNA-binding protein found in the RNA-induced silencing complex and its microRNA target(s) mediate this inter-organelle communication between the endoplasmic reticulum and mitochondria in macrophages during atherogenesis, and thus, may unravel new therapeutic strategies or targets in CVD.

National Institute of Health (NIH)
National Heart, Lung, and Blood Institute (NHLBI)
Research Project (R01)
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Atherosclerosis and Inflammation of the Cardiovascular System Study Section (AICS)
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Chen, Jue
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Cedars-Sinai Medical Center
Los Angeles
United States
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