OSBP Negatively Regulates ABCA1 Protein Stability

Abstract

LXR is activated by oxysterol derivatives of cholesterol in vivo and is thus positioned to sense the cellular levels of cholesterol and regulate genes required for its elimination (7). The ABCA1-dependent efflux of cholesterol and phospholipids to extracellular apolipoprotein A1 (apoAI) to form nascent high density lipoprotein is the initial and limiting step in the reverse cholesterol transport pathway (reviewed in Ref. 8). Although transcriptional induction of ABCA1 by LXR is a key event in this process, it is now recognized that regulation of ABCA1 protein stability in the plasma membrane (PM) and endosomes is also involved. ApoAI binds and stabilizes ABCA1, resulting in increased PM localization and decreased degradation by thiol proteases (9, 10). Phosphorylation of a PEST sequence in the large cytoplasmic loop of ABCA1 mediates internalization and subsequent degradation by calpain (11-13). Phosphorylation of the PEST sequence is inhibited by apoAI, resulting in increased surface localization and stabilization. ABCA1 also interacts with ␤1-and ␣1-syntrophin, scaffolding proteins that regulate transport through linkage to the cytoskeleton, resulting in stabilization at intracellular sites and the PM (14, 15). Thus, distribution of ABCA1 between the PM and endosomes affects protein stability and efficiency of cholesterol efflux to apoA1. Other sterol-binding proteins could affect cholesterol homeostasis by sequestration or delivery of oxysterol ligands to LXR or by influencing the supply of cholesterol or phospholipids to ABCA1 for efflux to apoA1. A class of binding proteins that could fulfill these functions is the OSBP (oxysterol-binding protein) and ORP (OSBP-related) gene family, which are expressed across eukaryotic phyla and consist of 12 mammalian and seven yeast members (16,17). OSBP and ORPs are characterized by a C-terminal OSBP homology domain that binds oxysterols and cholesterol (18 -20). The majority of family members also contain pleckstrin homology (PH) and FFAT (two phenylalanines in an acidic tract) domains that mediate interaction with phosphatidylinositol phosphates (21, 22) and the resident ER protein vesicle-associated membrane protein-associated protein (VAP) (23, 24), respectively. The PH and FFAT domain of OSBP facilitate partitioning between the ER and Golgi apparatus in response to oxysterol and cholesterol binding (23,25,26). Sterol-dependent partitioning of OSBP between the Golgi and ER does not appear to be directly involved in cholesterol regulation, since depletion by RNA interference did not affect the * This work was support in part by Canadian Institutes of Health ResearchOperating Grant MOP 15284. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. □ S The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1

TNF-α decreases ABCA1 expression and attenuates HDL cholesterol efflux in the human intestinal cell line Caco-2

“…At the end of the incubation, fresh medium containing original treatments was added on apical and basal sides. To inhibit new protein synthesis, 100 µM cycloheximide was added to the apical medium ( 33,34 ). The cells were harvested at various time intervals indicated in Fig.…”

Regulation of High-Density Lipoprotein Metabolism

“…57,58 Overexpression of activator protein 2α also suppresses ABCA1 gene transcription, whereas blockade of this protein with the α(1)-adrenoceptor blocker, doxazosin, increases HDL cholesterol levels in mice. 59 ABCA1 gene transcription as well as cholesterol efflux to apoA-I and the generation of discoidal HDLs are also impaired in people with lysosomal acid lipase deficiency.…”

Functions of OSBP/ORP Family Proteins and Their Relation to Dyslipidemia

“…OSBP knockdown in J774 macrophages also increased ABCA1 expression in the presence and absence of LXR agonists. Their results demonstrate that OSBP opposes the activity of LXR by negatively regulating ABCA1 activity in the cytoplasm by sterol-binding domaindependent protein destabilization (Bowden and Ridgway 2008). Cephalostatin 1, OSW-1, ritterazine B and schweinfurthin A are natural products that potently, and in some cases selectively, inhibit the growth of cultured human cancer cell lines.…”

“…OSBP negatively regulates ATP-binding cassette transporter A1 (ABCA1) protein stability (Bowden and Ridgway 2008). OSBP induces upregulation of SREBP-1c and enhances hepatic lipogenesis (Yan, Lehto et al 2007).…”