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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:954-962.)
© 1997 American Heart Association, Inc.

Articles

Expression of the PAF Receptor in Human Monocyte–Derived Macrophages Is Downregulated by Oxidized LDL

Relevance to the Inflammatory Phase of Atherogenesis

Dominique Stengel; Micheline Antonucci; Muriel Arborati; Delphine Hourton; Sabine Griglio; M. John Chapman; ; Ewa Ninio

From INSERM Unité 321, Unité de Recherche sur les Lipoprotéines et l'Athérogénèse, Pavillon Benjamin Delessert, Hôpital de la Pitié, Paris, France.

Correspondence to Dr Dominique Stengel, INSERM Unité 321, Pavillon Benjamin Delessert, Hôpital de la Pitié, 83 Boulevard de l'Hôpital, 75651 Paris Cedex 13, France. E-mail stengel{at}infobiogen.fr

Abstract Human monocyte–derived macrophages play a major role in the initiation and progression of atherosclerotic lesions as a result of the production of a wide spectrum of proinflammatory and prothrombotic factors. Among such factors is a potent inflammatory phospholipid, platelet-activating factor (PAF), which is produced after macrophage activation. Because the cells involved in PAF biosynthesis are typically targets for the bioactions of PAF via specific cell surface receptors, we evaluated the expression of the PAF receptor in human monocyte–derived macrophages. Oxidized LDL (oxLDL) exerts multiple cellular effects that enhance lesion progression; we therefore investigated the potential modulation of expression of the macrophage PAF receptor by oxLDL. [³H]PAF bound to adherent human macrophages with a K_d of 2.1 nmol/L and a B_max of 19 fmol/10⁶ cells; 5300 binding sites per cell were detected. OxLDL (100 µg protein per milliliter) induced a twofold decrease in cellular PAF binding after 3 hours at 37°C. Analysis of macrophage mRNA by reverse transcription–polymerase chain reaction (RT-PCR) revealed two forms corresponding to the PAF receptor, of which the leukocyte type (type 1 promoter) predominated. Expression of PAF receptor mRNA, evaluated by quantitative RT-PCR using an actin or a GAPDH mimic, was progressively reduced (up to 70%) by oxLDL up to 6 hours and remained low for at least 24 hours. Such downregulation was reversible after incubation of the cells for 24 hours in oxLDL-free medium. Addition of forskolin (3 µmol/L) or dibutyryl cAMP (1 mmol/L) to macrophage cultures reproduced the oxLDL-mediated inhibition of PAF receptor expression; carbamyl PAF reduced PAF binding and PAF mRNA to a similar degree (50%). These data demonstrate that atherogenic oxLDL downregulates the expression of both cellular PAF receptors and PAF receptor mRNA in macrophages, consistent with both a diminished bioresponse to PAF and decreased cell motility. Such diminished bioresponse to a powerful antacoid reflects the suppression of an acute inflammatory reaction, thereby leading to chronic, low-level inflammation, such as that characteristic of fatty streaks and more advanced atherosclerotic plaques.

Key Words: platelet-activating factor • scavenger receptor • lipid peroxidation • adenylyl cyclase • cAMP • forskolin

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