Global Knockout of LDLRAP1 Regulates Atherosclerosis, Insulin Resistance, and VSMC Foam Cell Formation

Abstract

IntroductionAtherosclerotic vascular syndromes account for 50% of all mortality in the United States, and are a considerable medical and socioeconomic problem contributing to mortality of many conditions including myocardial infarction, stroke, renal failure, and peripheral vascular disease. Recent studies have implicated that as many as 70% of all cells in atherosclerotic lesions are vascular smooth muscle cell (VSMC)‐derived. VSMC express lipid receptors and can uptake oxidized‐LDL to form foam cells, affecting the size and lipid content of atherosclerotic plaques. Low‐Density Lipoprotein Receptor Adaptor Protein 1 (LDLRAP1) is a cytosolic adaptor protein which interacts with the cytoplasmic tail of the LDL receptor, internalizing the receptor when it engages with LDL. Mutations in this gene lead to LDL receptor malfunction and cause Autosomal Recessive Hypercholesterolemia (ARH) in humans. The hypercholesterolemia is assumed to be a result of impaired LDL internalization by hepatocytes. Surprisingly, no investigation into a function for LDLRAP1 in lipid internalization in VSMC, or ramifications on atherogenesis in mice lacking LDLRAP1 has been reported. The purpose of this study was to test the hypothesis that while hepatic expression of LDLRAP1 is beneficial, LDLRAP1 expression in VSMC leads to VSMC foam cell formation and increased atherosclerosis.ResultssiRNA knockdown of LDLRAP1 in human primary VSMC leads to significantly reduced lipid uptake quantified by flow cytometry (p<0.01). Foam cell formation was quantitated in LDLRAP1−/− VSMC – similar to human VSMC, VSMC excised and cultured from aortas of LDLRAP1−/− mice had reduced lipid uptake compared with wild‐type controls (p<0.01). VSMC proliferation is also a key step in atherosclerotic plaque formation, and LDLRAP1−/− VSMC proliferated more slowly than wild‐type controls over a 7‐day period (p<0.05). LDLRAP1−/− mice fed a high‐fat western diet (HFD) for 15 weeks had increased plaque burden compared with wild‐type controls (p<0.01). Unexpectedly, LDLRAP1−/− mice gained significantly more weight on HFD compared with wild‐type control mice (25.72 ± 1.07 vs. 10.95 ± 0.7 grams). Obesity is associated with insulin resistance, and insulin tolerance tests indicated that LDLRAP1−/− mice are insulin resistant.Summary and conclusionsThese data are the first to suggest that lack of LDLRAP1 directly leads to atherosclerosis in mice. LDLRAP1 may play an important role in VSMC foam cell formation, and an unexpected role in metabolism. These data are a first step and imply that knocking out LDLRAP1 specifically in vascular smooth muscle cells may provide a novel approach to reduce vascular smooth muscle foam cell formation and subsequently atherogenesis without inducing the hypercholesterolemia associated with a global knockout.Support or Funding InformationNIH‐NHLBIThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.