Endothelial microparticles mediate inflammation-induced vascular calcification

Stimulation of endothelial cells (ECs) with TNF-alpha causes an increase in the expression of bone morphogenetic protein-2 (BMP-2) and the production of endothelial microparticles (EMPs). BMP-2 is known to produce osteogenic differentiation of vascular smooth muscle cells (VSMCs). It was found that EMPs from TNF-alpha- stimulated endothelial cells (HUVECs) contained a significant amount of BMP-2 and were able to enhance VSMC osteogenesis and calcification. Calcium content was greater in VSMCs exposed to EMPs from TNF-alpha-treated HUVECs than EMPs from nontreated HUVECs (3.56 +/- 0.57 vs. 1.48 +/- 0.56 mu g/mg protein; P < 0.05). The increase in calcification was accompanied by up-regulation of Cbfa1 (osteogenic transcription factor) and down-regulation of SM22 alpha (VSMC lineage marker). Inhibition of BMP-2 by small interfering RNA reduced the VSMC calcification induced by EMPs from TNF-alpha- treated HUVECs. Similar osteogenic capability was observed in EMPs from both patients with chronic kidney disease and senescent cells, which also presented a high level of BMP-2 expression. Labeling of EMPs with CellTracker shows that EMPs are phagocytized by VSMCs under all conditions (with or without high phosphate, control, and EMPs from TNF-alpha-treated HUVECs). Our data suggest that EC damage results in the release of EMPs with a high content of calcium and BMP-2 that are able to induce calcification and osteogenic differentiation of VSMCs.