MicroRNA 223 Enhances ABCA1 Protein Stability and Supports Efflux in Cholesterol-Burdened Macrophages

Macrophages are present in all vertebrates as part of the innate immune system, which protects from pathogens and scavenges sterol rich, cellular debris and modified lipoproteins. Thus, resident macrophages are prone to excessive levels of intracellular cholesterol esters. Intramacrophage cholesterol esters can efflux via cell surface transporters, ABCA1 and ABCG1, to lipoprotein carriers such as apo-AI and HDL. Systemically, Apo-AI and HDL facilitate trafficking of cholesterol back to the liver, in a process called reverse cholesterol transport. Impaired macrophage cholesterol efflux is a primary factor in the etiology of atherosclerosis. We hypothesized that microRNA 223 (miR-223) regulated macrophage LDL metabolism, due to predicted binding to Sp1 and Sp3 mRNA, transcriptional regulators of ABCA1 expression. Primary mouse (WT, miR-223 KO) macrophages were loaded with acetylated LDL and stimulated with LPS to form an inflammatory foam cell phenotype. miR-223 KO foam cells demonstrated impaired efflux to both apo-AI and HDL. While transcriptional regulation was intact in miR-223 KO foam cells, ABCA1 protein degradation was greatly accelerated. Blockade of both proteasomal and lysosomal degradation pathways rescued miR-223 deficiency-mediated ABCA1 degradation to the WT levels. Our findings demonstrate that miR-223 expression in macrophages is required for maintenance of ABCA1 and ABCG1 proteins.