Interaction of hydroxyapatite nanoparticles with endothelial cells: internalization and inhibition of angiogenesis in vitro through the PI3K/Akt pathway

Nano-hydroxyapatite (nano-HAP) has been proposed as a better candidate for bone tissue engineering; however, the interactions of nano-HAP with endothelial cells are currently unclear. In this study, HAP nanoparticles (HANPs; 20 nm np20 and 80 nm np80) and micro-sized HAP particles (m-HAP; 12 μm) were employed to explore and characterize cellular internalization, subcellular distribution, effects of HANPs on endothelial cell function and underlying mechanisms using human umbilical vein endothelial cells (HUVECs) as an in vitro model. It was found that HANPs were able to accumulate in the cytoplasm, and both adhesion and uptake of the HANPs followed a function of time; compared to np80, more np20 had been uptaken at the end of the observation period. HANPs were mainly uptaken via clathrin- and caveolin-mediated endocytosis, while macropinocytosis was the main pathway for m-HAP uptake. Unexpectedly, exposure to HANPs suppressed the angiogenic ability of HUVECs in terms of cell viability, cell cycle, apoptosis response, migration and capillary-like tube formation. Strikingly, HANPs reduced the synthesis of nitric oxide (NO) in HUVECs, which was associated with the inhibition of phosphatidylinositol 3-kinase (PI3K) and phosphorylation of eNOS. These findings provide additional insights into specific biological responses as HANPs interface with endothelial cells.