Fig. 4. MT1-MMP promotes interactions between PDGFRβ, β3 integrin and LRP1. (A) VSMCs were treated with PDGF-BB for 30 minutes and lysed. The lysates were subjected to immunoprecipitation with polyclonal rabbit anti-LRP1 antibodies. Tyrosine-phosphorylated proteins in the immunoprecipitates were detected by anti-phosphotyrosine antibodies (pY) and LRP1 by mouse monoclonal anti-LRPβ and anti-LRP antibodies. Arrow indicates the phosphorylated protein that co-precipitates with LRP1 and most probably represents PDGFRβ. (B) LRPβ protein levels, as detected by immunoblotting, in the lysates of VSMCs that were treated with PDGF-BB for 3 hours. β-tubulin served as a loading control. Arrow indicates the cleaved fragment of LRP1 β-chain. (C) Wild-type (MT1^+/+) and MT1-MMP-null (MT1^–/–) VSMCs were cultured on polymerized collagen I and were serum starved for 24 hours. The cells were then treated with PDGF-BB (10 ng/ml) for 30 minutes and lysed. The levels of β1- and β3-integrins in the cell lysates were detected by immunoblotting. β-tubulin served as a loading control. (D) The cell lysates were prepared as above and subjected to immunoprecipitation with polyclonal rabbit anti-β1- and β3-integrin antibodies. PDGFRβ, LRPβ, MT1-MMP, and β1- and β3-integrin in the immunoprecipitates were detected by immunoblotting.