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J. Biol. Chem., Vol. 281, Issue 10, 6707-6717, March 10, 2006

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Cholesterol-induced Apoptotic Macrophages Elicit an Inflammatory Response in Phagocytes, Which Is Partially Attenuated by the Mer Receptor^*

Yankun Li, Marie-Christine Gerbod-Giannone, Heather Seitz, Dongying Cui, Edward Thorp, Alan R. Tall, Glenn K. Matsushima, and Ira Tabas^¶1

From the Departments of Medicine and ^¶Anatomy and Cell Biology, and Physiology and Cellular Biophysics, Columbia University, New York, New York 10032 and the Department of Microbiology and Immunology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599

Macrophage apoptosis and the ability of phagocytes to clear these apoptotic cells are important processes in advanced atherosclerosis. Phagocytic clearance not only disposes of dead cells but usually elicits an anti-inflammatory response. To study this process in a model of advanced lesional macrophage death, macrophages rendered apoptotic by free cholesterol loading (FC-AMs) were incubated briefly with fresh macrophages ("phagocytes"). FC-AMs were promptly ingested by the phagocytes, which was dependent upon actin polymerization and the phagocyte Mer receptor. Surprisingly, this brief exposure to FC-AMs triggered a modest proinflammatory response in the phagocytes: tumor necrosis factor- (TNF-) and interleukin (IL)-1 were induced, whereas the levels of transforming growth factor- and IL-10 were not increased. This response required cell contact between the FC-AMs and phagocytes but not FC-AM ingestion. TNF- and IL-1 induction required one or more proteins on the FC-AM surface and was dependent on signaling through extracellular signal-regulated kinase-1/2 mitogen-activated protein kinase and nuclear factor-B in the phagocytes. TNF- production was markedly greater when Mer-defective phagocytes were used, indicating that Mer attenuated the inflammatory response. Interestingly, a more typical anti-inflammatory response was elicited when phagocytes were exposed to macrophages rendered apoptotic by oxidized low density lipoprotein or UV radiation. Thus, the proinflammatory milieu of advanced atherosclerotic lesions may be promoted, or at least not dampened, by contact between FC-induced apoptotic macrophages and neighboring phagocytes prior to apoptotic cell ingestion.

Received for publication, September 27, 2005 , and in revised form, December 23, 2005.

^* This work was supported by National Institutes of Health Grants HL54591 and HL75662 (to I. T.) and American Heart Association Scientist Development Grant 0435364T (to Y. L.). 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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