Objective-ATP-binding cassette transporter A1 (ABCA1) exerts an atheroprotective action through the biogenesis of highdensity lipoprotein in hepatocytes and prevents the formation of foam cells from macrophages. Controlling ABCA1 is a rational approach to improving atherosclerotic cardiovascular disease. Although much is known about the regulatory mechanism of ABCA1 synthesis, the molecular mechanism underpinning its degradation remains to be clearly described. Approach and Results-ABCA1 possesses potential sites of phosphorylation by serine/threonine-protein kinase Pim-1 (Pim-1). Pim-1 depletion decreased the expression of cell surface-resident ABCA1 (csABCA1) and apolipoprotein A-I-mediated [ 3 H]cholesterol efflux in the human hepatoma cell line HepG2, but not in peritoneal macrophages from mice. In vitro kinase assay, immunoprecipitation, and immunocytochemistry suggested phosphorylation of csABCA1 by the long form of Pim-1 (Pim-1L). Cell surface biotinylation indicated that Pim-1L inhibited lysosomal degradation of csABCA1 involving the liver X receptor β, which interacts with csABCA1 and thereby protects it from ubiquitination and subsequent lysosomal degradation. Cell surface coimmunoprecipitation with COS-1 cells expressing extracellularly hemagglutinin-tagged ABCA1 showed that Pim-1L-mediated phosphorylation of csABCA1 facilitated the interaction between csABCA1 and liver X receptor β and thereby stabilized the csABCA1-Pim-1L complex. Mice deficient in Pim-1 kinase activity showed lower expression of ABCA1 in liver plasma membranes and lower plasma high-density lipoprotein levels than control mice. Conclusions-Pim-1L protects hepatic csABCA1 from lysosomal degradation by facilitating the physical interaction between csABCA1 and liver X receptor β and subsequent stabilization of the csABCA1-Pim-1L complex and thereby regulates the circulating level of high-density lipoprotein. Our findings may aid the development of high-density lipoproteintargeted therapy. Katsube et al Protection of Hepatic csABCA1 by Pim-1L 2305mechanism underlying the degradation of ABCA1 remains elusive. Although in vitro and animal studies using inhibitors of the protease calpain and proteasome suggest that they cleave and degrade ABCA1, [16][17][18][19] the molecular entity of calpain and the molecular mechanism of the degradation of ABCA1 involving proteasome have not yet been clearly described. On the basis of the report by Hozoji et al 20 that LXRβ interacts with and post-translationally modulates ABCA1, we recently found that LXRβ protects the cell surface-resident ABCA1 (csABCA1) from ubiquitination through its physical interaction with csABCA1 and not through its transcriptional activity, which prevents lysosomal degradation of csABCA1 through the endosomal sorting complex required for transport (ESCRT) system. 21,22 This mechanism mediates ABCA1 degradation independently of the pathway involving calpain and proteasome. To gain further insight into this finding, the current study focuses on phosphorylation because phosphory...