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J. Biol. Chem., Vol. 283, Issue 26, 18210-18217, June 27, 2008

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OSBP Negatively Regulates ABCA1 Protein Stability^*

From the Departments of Pediatrics and Biochemistry and Molecular Biology, Atlantic Research Centre, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada

Oxysterol binding to liver X receptors (LXR) increases the transcription of genes involved in cholesterol efflux and disposal, such as ABCA1 (ATP-binding cassette transporter A1). Other cytoplasmic sterol-binding proteins could interact with this pathway by sequestering or delivering substrates and ligands. One potential regulator is OSBP (oxysterol-binding protein), which is implicated in the integration of sterol sensing/transport with sphingomyelin synthesis and cell signaling. Since these activities could impact the cholesterol efflux pathway, we examined whether OSBP was involved in LXR regulation and in expression and activity of ABCA1. Suppression of OSBP in Chinese hamster ovary cells by RNA interference resulted in increased ABCA1 protein expression and cholesterol efflux activity following induction with oxysterols or the synthetic LXR agonist TO901317. OSBP knockdown in J774 macrophages also increased ABCA1 expression in the presence and absence of LXR agonists. OSBP depletion did not affect ABCA1 mRNA levels or LXR activity. Rather, OSBP silencing increased the half-life of ABCA1 protein by 3-fold. Sphingomyelin synthesis was suppressed in OSBP-depleted cells treated with 25-hydroxycholesterol but not TO901317 or 22-hydroxycholesterol and did not correlate with ABCA1 stabilization. Moreover, co-transfection experiments revealed that reduction of ABCA1 protein by OSBP was prevented by a mutation in the sterol-binding domain but not by mutations that abrogated interaction with the Golgi apparatus or endoplasmic reticulum. Thus, OSBP opposes the activity of LXR by negatively regulating ABCA1 activity in the cytoplasm by sterol-binding domain-dependent protein destabilization.

Received for publication, February 4, 2008 , and in revised form, May 1, 2008.

^* This work was support in part by Canadian Institutes of Health Research Operating 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ Recipient of a Nova Scotia Health Research Foundation studentship.

² To whom correspondence should be addressed: Atlantic Research Centre, 5849 University Ave., Dalhousie University, Halifax, Nova Scotia B34 4H7, Canada. Fax: 902-494-1394; E-mail: nridgway{at}dal.ca.

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