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J. Biol. Chem., Vol. 281, Issue 44, 33635-33649, November 3, 2006

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Sitosterol-containing Lipoproteins Trigger Free Sterol-induced Caspase-independent Death in ACAT-competent Macrophages^*

Liping Bao, Yankun Li, Shi-Xian Deng, Donald Landry, and Ira Tabas^¶1

From the Departments of Medicine, Pathology & Cell Biology, and ^¶Physiology & Cellular Biophysics, Columbia University, New York, New York 10032

Sitosterolemia is a disease characterized by very high levels of sitosterol and other plant sterols and premature atherothrombotic vascular disease. One theory holds that plant sterols can directly promote atherosclerosis, but the mechanism is not known. Unesterified, or "free," cholesterol (FC) is a potent inducer of macrophage death, which causes plaque necrosis, a precursor to atherothrombosis. FC-induced macrophage death, however, requires dysfunction of the sterol esterifying enzyme acyl-coenzyme A-cholesterol acyltransferase (ACAT), which likely occurs slowly during lesion progression. In contrast, plant sterols are relatively poorly esterified by ACAT, and so they may cause macrophage death and plaque necrosis in an accelerated manner. In support of this hypothesis, we show here that macrophages incubated with sitosterol-containing lipoproteins accumulate free sterols and undergo death in the absence of an ACAT inhibitor. As with FC loading, sitosterol-induced macrophage death requires sterol trafficking to the endoplasmic reticulum, and sitosterol-enriched endoplasmic reticulum membranes show evidence of membrane protein dysfunction. However, whereas FC induces caspase-dependent apoptosis through activation of the unfolded protein response and JNK, sitosterol-induced death is caspase-independent and involves neither the unfolded protein response nor JNK. Rather, cell death shows signs of necroptosis and autophagy and is suppressed by inhibitors of both processes. These data establish two new concepts. First, a relatively subtle change in sterol structure fundamentally alters the type of death program triggered in macrophages. Understanding the basis of this alteration should provide new insights into the molecular basis of death pathway signaling. Second, sitosterol-induced macrophage death does not require ACAT dysfunction and so may occur in an accelerated fashion. Pending future in vivo studies, this concept may provide at least one mechanism for accelerated plaque necrosis and atherothrombotic disease in patients with sitosterolemia.

Received for publication, July 3, 2006 , and in revised form, August 16, 2006.

^* This work was supported by National Institutes of Health Grants HL75662 and HL081181 and a grant from the Merck/Schering-Plough Investigator-Initiated Studies Program (to I. T.). 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.

¹ To whom correspondence should be addressed: Dept. of Medicine, Columbia University, 630 West 168th St., New York, NY 10032. Tel.: 212-305-9430; Fax: 212-305-4834; E-mail: iat1{at}columbia.edu.

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