Exosomes, also known as extracellular vesicles, are naturally occurring, biocompatible, and bioacive nanoparticles ranging from 40 to 150 nm in diameter. Bone-secreted exosomes play important roles in bone homeostasis, the interruption of which can lead to diseases such as osteoporosis, rheumatoid arthritis, and osteopetrosis. Though the relationship between vascular and bone homeostasis has been recognized recently, the role of vascular endothelial cell (EC)-secreted exosomes (EC-Exos) in bone homeostasis is not well understood. Herein, we found that EC-Exos show more efficient bone targeting than osteoblast-derived exosomes or bone marrow mesenchymal stem cell-derived exosomes. We also found that EC-Exos can be internalized by bone marrow-derived macrophages (BMMs) to alter their morphology. EC-Exos can inhibit osteoclast activity in vitro and inhibit osteoporosis in an ovariectomized mouse model. Sequencing of exosome miRNA revealed that miR-155 was highly expressed in EC-Exos-treated BMMs. The miR-155 level in EC-Exos was much higher than that in BMMs and ECs, indicating that miR-155 was endogenous cargo of EC-derived vesicles. Blockage of BMMs miR-155 levels reversed the suppression by EC-Exos of osteoclast induction, confirming that exosomal miR-155 may have therapeutic potential against osteoporosis. Taken together, our findings suggest that EC-Exos may be utilized as a bone targeting and nontoxic nanomedicine for the treatment of bone resorption disorders.

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Background/Aims: Annexin A2 receptor (AX2R) can mediate annexin A2 signalling and induce apoptosis in a variety of cells, but its role in neovascularization (NV) remains unclear. Krüppel-like transcription factor 2 (KLF2) is known to be expressed in a range of cell types and to participate in a number of processes during development and disease, such as endothelial homeostasis, vasoregulation and vascular growth/remodelling. The aim of our study was to investigate the role of AX2R in NV and the plausible molecular mechanism. Methods: We constructed a eukaryotic overexpression plasmid for AX2R (Lenti-AX2R) by using polymerase chain reaction (PCR). The full-length human AX2R gene was transfected into human retinal endothelial cells (HRECs) and human umbilical vein endothelial cells (HUVECs) using lentivirus vectors to overexpress AX2R. All experiments were divided into three groups: control, negative control (Lenti-EGFP), and Lenti-AX2R.Cell proliferation, cell migration, tube formation, mouse aortic ring assays and mouse matrigel plug assay were applied to analyse the effect of AX2R in NV. Furthermore, we conducted flow cytometry to evaluate whether AX2R could influence the cell cycle. A series of cell cycle-related proteins including cyclin A1, cyclin B1, cyclin D1, cyclin E1, CDK1, and p-CDC2 were detected by WB. The mRNA and protein levels of KLF2, vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor 2 (VEGFR2) were further quantified by RT-PCR and WB to reveal the possible mechanism. Results: Overexpression of AX2R significantly inhibited cell proliferation, migration and tube formation in both types of endothelial cells (ECs), HRECs and HUVECs. It also suppressed vessel sprouting in the mouse aortic ring assay and NV in mouse matrigel plug assay. Furthermore, infection with Lenti-AX2R lentivirus arrested the cell cycle in S/G2 and influenced the expression of a series of cell cycle-related proteins. We also found that the overexpression of AX2R increased the expression of KLF2, mediating VEGF and VEGFR2. Conclusions: Overexpression of AX2R contributes to the inhibition of NV via suppressing KLF2 ubiquitin-dependent protein degradation, which might therefore be a therapeutic option for NV. It could be considered more broadly as an anti-angiogenic agent in the treatment of neovascular-related diseases in the future.