Anti-Atherogenic Mechanisms of Drebrin

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We discovered that the actin-binding protein, Drebrin, is abundantly expressed in smooth muscle cells (SMCs) and is up-regulated in response to arterial injury in mice and to atherosclerosis in humans. Comparing WT with Dbn-/+ mice, we found that Drebrin inhibits SMC migration and proliferation, both in vitro and in vivo, through stabilization of actin filaments. In studies with SMC-specific Dbn-/- (SMC-Dbn-/-) mice, we found that SMC Drebrin limits angiotensin II-induced remodeling of the ascending aorta, in a manner that correlates with down-regulation of NADPH oxidase 1 (NOX1), decreased SMC reactive oxygen species (ROS) production and reduced vascular inflammation. Thus, Drebrin constrains not only the migratory/proliferative SMC phenotype but also the pro-inflammatory SMC phenotype evoked by vascular injury and angiotensin II. Congruently, we found that atherosclerosis is greater in SMC-Dbn-/-/Ldlr-/- than in congenic SMC-Dbn+/+/Ldlr-/- mice. Because SMC Drebrin negatively regulates atherosclerosis, our goals are to determine the mechanisms by which Drebrin regulates SMC pro-inflammatory signaling and whether enhanced SMC expression of Drebrin can inhibit atherogenesis. To this end, we performed SILAC/mass spectrometry studies on Dbn-/- and congenic WT SMCs. We found that whereas in Dbn-/- SMCs the ROS-defensive enzyme Glutathione-S-transferase µ1 (GSTM1) is down-regulated, the pro-inflammatory cytokine CX3CL1 (fractalkine) is up-regulated: thus we will investigate the role of these proteins in mediating the phenotype of Dbn-/- SMCs. Because endocytosis of NOX1 regulates ROS generation and activation of pro-inflammatory NFκB signaling and Drebrin has been shown to inhibit endocytosis, we will investigate whether Drebrin inhibits NOX1-mediated ROS generation by inhibiting endocytosis. A major focus of our studies will be pro-atherogenic SMC-derived foam cells, which SMC lineage tracing studies have shown, comprise ~40% of foam cells in atherosclerotic lesions. Our Preliminary Studies show that, compared with cognate Dbnflox/flox SMCs, Dbn-/- SMCs induced to transdifferentiate with cholesterol loading exhibited increased expression of the macrophage marker, CD68, and Kruppel-like factor 4 (KLF4), a transcription factor required for SMC-to-foam cell transdifferentiation. By grafting common carotid arteries from Dbnflox/flox and SMC-Dbn-/- mice into carotid arteries of congenic Apoe-/- mice, we also show that Drebrin deficiency augments transdifferentiation of SMCs to CD68+ cells in vivo. We will test the hypothesis that Drebrin inhibits atherogenesis by limiting SMC transdifferentiation into foam cells. To do so, we will establish whether Drebrin inhibits SMC-to-foam cell transdifferentiation and associated pro- inflammatory signaling, both in vitro and in vivo; determine if Drebrin inhibits SMC transdifferentiation through ROS-dependent mechanisms; and define the roles of GSTM1 and CX3CL1 in mediating Drebrin’s inhibitory effects on SMC transdifferentiation and pro-inflammatory signaling.