Liver X Receptor β (LXRβ) Interacts Directly with ATP-binding Cassette A1 (ABCA1) to Promote High Density Lipoprotein Formation during Acute Cholesterol Accumulation

Abstract

Cells have evolved multiple mechanisms for maintaining cholesterol homeostasis, and, among these, ATP-binding cassette protein A1 (ABCA1)-mediated cholesterol efflux is highly regulated at the transcriptional level through the activity of the nuclear receptor liver X receptor (LXR). Here, we show that in addition to its well defined role in transcription, LXR␤ directly binds to the C-terminal region ( Disruption of cellular cholesterol homeostasis can lead to a variety of pathological conditions, including cardiovascular disease (1). ATP-binding cassette protein A1 (ABCA1), 2 an important regulator of cholesterol homeostasis, mediates the release of cellular excess free cholesterol and phospholipids to apolipoprotein A-I (apoA-I), an extracellular acceptor circulating in plasma, to form high density lipoprotein (HDL) (2-5). HDL formation is the only pathway through which excess cholesterol can be eliminated from nonhepatic cells. Defects in ABCA1 cause Tangier disease (6 -8), a condition in which patients have a near absence of circulating HDL, prominent cholesterol-ester accumulation in tissue macrophages, and premature atherosclerotic vascular disease (1, 9).The ABCA1-mediated release of cholesterol is highly regulated at the transcriptional level. When excess cholesterol accumulates in cells, intracellular concentrations of oxysterols increase and activate liver X receptor (LXR), which, in turn, stimulates ABCA1 gene transcription and increased expression of ABCA1 with associated elimination of excess cholesterol (10 -12). However, cholesterol is required for cell function and proliferation, and the intracellular cholesterol concentration must be maintained within a narrow range. Consequently, ABCA1-mediated cholesterol release is also regulated at the post-translational level. Several proteins, including syntrophins (13, 14), JAK2 (15), and LXR␤ (16), have been reported to interact with ABCA1 and modulate its degradation and function. However, the precise mechanism(s) by which ABCA1 activity is regulated post-translationally remains unclear.We previously reported (16) that a fraction of cytosolically localized LXR␤ may interact with ABCA1 on the plasma membrane and modulate the function of ABCA1. In WI-38 and THP-1 cells, endogenous LXR␤ interacts with ABCA1 under conditions in which LXR ligands do not accumulate, i.e. when cholesterol is not in excess. LXR␤ suppresses ABCA1-mediated cholesterol efflux. However, the mechanism by which LXR␤ suppresses ABCA1 functions was not clear. In this study, we identified two leucine residues in the C-terminal region of ABCA1 responsible for the interaction with LXR␤ and showed that LXR␤ interaction suppresses ATP binding to ABCA1 and thereby keeps ABCA1 standby on the plasma membrane for acute cholesterol accumulation. EXPERIMENTAL PROCEDURESMaterials-The LXR ligand TO901317, 22(R)-hydroxycholesterol, 25-hydroxycholesterol, and Alexa Fluor 546 succinimidyl esters were purchased from Cayman Co., Sigma, Stelaroids, and Molecular Probes, respectively. A cholesterol oxidas...

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