Abstract 983: Reciprocal Regulation of Caveolin-1 Expression and Bone Morphogenetic Protein Signaling in Mouse Aortic Smooth Muscle Cells
Vascular injury stimulates vascular smooth muscle cell growth by disrupting the physiological balance between growth inhibition and stimulation, promoting the formation of a neointima. Both caveolin-1 (cav-1) and bone morphogenetic protein-2 (BMP2) have been implicated in the negative regulation of vascular smooth muscle cell proliferation. Furthermore, recent studies in pulmonary endothelial cells demonstrate the co-localization and co-sedimentation in a sucrose gradient of the type II BMP receptor (BMPRII) and cav-1. Our goal was to test the hypothesis that cav-1 interacts with and regulates BMPRII-dependent signaling in vascular smooth muscle cells. We demonstrate that BMPRII co-sediments in a sucrose gradient and co-immunoprecipitates with cav-1 in mouse aortic smooth muscle cells (MASMC). This interaction is dynamic, with cav-1 and BMPRII interacting strongly under serum-starved conditions and dissociating upon stimulation of the receptor with its agonist, BMP2. Pretreatment of cells with a cell-permeable peptide containing the scaffolding domain of cav-1 (ap-cav) results in disruption of this interaction. As a consequence of this disruption receptor-mediated phosphorylation of SMAD 1/5/8, which is required for the antiproliferative action of BMP2, was abolished. BMP2-mediated p38 MAPK phosphorylation, conversely, was significantly increased. Interestingly, treatment of MASMC with ap-cav was permissive for BMP-dependent AKT (PKB) phosphorylation, which was not observed in control cells. Similar results were observed in MASMC when lipid rafts were disrupted by the cholesterol-depleting agent methyl-β-cyclodextrin. Unexpectedly, BMP2 was able to regulate cav-1 expression. Under serum-starved conditions cav-1 expression is maximal and decreases upon PDGF or serum treatment. However, pretreatment of MASMC for 30 min with BMP2 blocked PDGF-mediated downregu-lation of cav-1 mRNA and protein as demonstrated by qRT-PCR and Western blot analysis. Taken together, these results suggest that cav-1 and BMP2 play complementary roles in regulating vascular smooth muscle cell proliferation.