Bone morphogenetic protein (BMP-2) is involved in mononuclear cell function and their interaction with endothelial cells

Background: Mononuclear cells play a crucial role in vascular remodelling and cardiovascular disease. Monocytes are recruited to sites of collateral growth where they contribute to arteriogenesis. However, prolonged presence or increased accumulation of these cells can compromise tissue integrity and lead to atherosclerosis. Bone morphogenetic proteins (BMPs) are members of the Transforming growth factor (TGF)-β family of cytokines and play important roles in vascular remodelling and function. It was shown that BMP-2 is up-regulated in calcified atherosclerotic plaques. BMP-2 contributes to calcification and inflammation of the vascular wall. It is unknown whether these effects of BMP-2 are also due to its effects on monocyte function or/and on the monocyte-endothelial cell interactions. Methods: Human monocytes were isolated from peripheral blood through gradient centrifugation and subsequent negative immunological magnet isolation. Monocyte motility was analysed by the modified Boyden chamber migration assay. Protein phosphorylation was assessed by immunoblotting. For endothelial cell adhesion assays, mouse endothelial cells were stimulated with different ligands and mononuclear cell adhesion was analysed. The involvement of different signalling cascades was analysed using specific kinase inhibitors. Results: Here we show that stimulation of primary monocytes with BMP-2 results in monocyte polarization. BMP-2 stimulated monocyte chemotaxis in a dose-dependent manner, while addition of Noggin, a natural BMP-2/4/7 antagonist, hindered monocyte chemotaxis towards BMP-2. Additionally BMP2 induces mononuclear cell adhesion on fibronectin. BMPs signal into the cells by activating both Smad and non-Smad signalling pathways such as PI3K and p38 MAPK pathway. Our results suggest that both PI3K and p38 pathways are involved in BMP-2-induced monocyte migration. Moreover, BMP-2 stimulated monocyte adhesion on endothelial cells by inducing inflammatory responses in endothelial cells in a PI3K and p38 dependent manner. Conclusions: Our results reveal that BMP-2 is a novel stimulator of human monocyte function and suggest that BMP-2 may exert its pro-inflammatory effects and pro-calcification action in atherosclerosis by activating the endothelium and in addition by recruitment of monocytes.