Cell drinking: a closer look at how macropinocytosis drives cholesterol uptake in atherosclerotic vessels

Abstract

Atherosclerotic vascular disease is the underlying cause of myocardial infarction, stable and unstable angina, stroke, peripheral artery disease and sudden cardiac death. Collectively, these cardiovascular diseases are responsible for the majority of deaths worldwide. Internalization of modified apolipoprotein B–containing lipoproteins by macrophages through scavenger receptor (SR)-mediated pathways is generally viewed as an essential step for the initiation and progression of atherosclerosis. Our studies were designed to investigate the contribution of receptor-independent LDL macropinocytosis to arterial lipid accumulation and atherosclerosis. We developed novel genetic and pharmacological approaches, utilized high resolution imaging techniques and employed unique in vivo lipid quantification assays to investigate the role of macrophage macropinocytosis in the pathogenesis of atherosclerosis. My results demonstrate that the macropinocytosis inhibitor EIPA and selective deletion of a key pathway regulating macropinocytosis in myeloid cells substantially decreased lesion size in both hypercholesterolemic wild type (WT) and SR knockout (CD36-/-/SR-A-/-) mice. Stimulation of macropinocytosis using genetic and physiologically relevant approaches promotes lipoprotein internalization by WT and CD36-/-/SR-A-/- macrophages, leading to foam cell formation. Serial section high-resolution imaging of murine and human atherosclerotic arteries identified for the first time subendothelial macrophages for the first time that demonstrate plasma membrane ruffling, cupping and macropinosome internalization. Immunoelectron microscopy, 3D reconstruction of macrophage foam cells and in vivo LDL tracking demonstrate macrophage internalization of LDL in human and murine atherosclerotic arteries via macropinocytosis. We next performed a large, unbiased-screen of an FDA-approved drug library to identify clinically relevant therapeutic agents that can be repurposed as pharmacological inhibitors of macropinocytosis. Our studies identified a low MW compound (imipramine) that inhibits macrophage macropinocytosis in vitro and in vivo. Imaging, toxicity and selectivity studies demonstrated that imipramine is a potent (IC50 = 130.9 nM), non-toxic (selectivity index CC50/IC50 > 300) and selective inhibitor of macropinocytosis. Repurposing of imipramine to inhibit macropinocytosis in hypercholesterolemic mice substantially decreased plaque development compared with control treatment. Taken together, our findings challenge the SR paradigm of atherosclerosis and identify inhibition of receptor-independent macrophage macropinocytosis as a new therapeutic strategy that may be beneficial in the treatment of atherosclerosis and its cardiovascular consequences.