Abstract 5068: Interaction Between CD36 and Oxidized LDL Induces Loss of Cell Polarity and Inhibits Macrophage Migration: A Mechanism of Macrophage Trapping
Trapping of lipid-laden macrophages in the arterial intima is a critical but reversible step in atherosclerosis. Previous studies from other laboratories showed that regressed atherosclerotic lesions were characterized by emigration of lipid-laden macrophages. CD36, a class B scavenger receptor expressed on macrophages is a pathogenic factor for foam cell formation and atherosclerosis. We recently showed that oxidized LDL (oxLDL) modulates macrophage cytoskeletal signaling via CD36, inhibiting migration and thus potentially contributing to intimal trapping. We now hypothesize that oxLDL-CD36 inhibition of migration is the result of specific intracellular signals that regulate cell polarity.
Methods/Results: Live cell imaging of murine peritoneal macrophages showed that oxLDL, but not native LDL, induced loss of cell polarity. Macrophages from cd36 null mice did not show this effect. Similarly, macrophages from mice null for Vav, a guanine nucleotide exchange factor recently shown to be a downstream effector of CD36 signaling, did not show oxLDL-induced loss of polarity. Macrophage migration velocity and dynamic movement of macrophage membrane were decreased by oxLDL in wild type macrophages by 85% and 86% respectively, but not in cd36 null or vav null macrophages. These findings appear to be related to activity of myosin II, a recently suggested cell polarity determinant. Western blots for phosphorylated myosin regulatory light chain (MRLC) showed that oxLDL, but not native LDL, induced de-phosphorylation of MRLC by 80%. Enzyme linked immunosorbent assays for GTP-bound Rac, a regulator of MRLC phosphorylation, showed that macrophage Rac-GTPase activity was increased 2.6 fold by oxLDL. These findings were not observed in macrophages from cd36 null or vav null mice. 6-thio-GTP, a Rac-GTPase inhibitor that inhibits binding of Vav to Rac1 abrogated the effect of oxLDL on MRLC dephosphorylation.
Conclusion: OxLDL interaction with CD36 inhibited macrophage migration and caused loss of cell polarity. The mechanism by which this occurs is via Vav-dependent activation of Rac-GTPase and dephosphorylation of MRLC. Activation of this pathway in the vessel wall may induce trapping of macrophages in the arterial intima and promote atherosclerosis.