Calcium enrichment activity initiates extracellular calcium influx-dependent inflammatory response of biologically-derived hydroxyapatite.

Biologically-derived hydroxyapatite is a widely used biomaterial in various clinical applications including bone augmentation. However, the osteogenic application of biological hydroxyapatite is limited by inflammatory responses, and the underlying mechanism remains unknown. The current study aimed to elucidate the molecular mechanisms underlying the inflammatory response to biological hydroxyapatite. Porcine-derived hydroxyapatite (PHA) with two sintering temperatures (800 and 1600 °C), PHA800 and PHA1600, respectively, were prepared. A PHA/macrophage co-culture model was established. Transcriptome, polymerase chain reaction (PCR), and enzyme-linked immunosorbent assay (ELISA) analyses were used to determine the inflammatory effects and the main pathways activated by PHA800 and PHA1600. Intracellular calcium level, PHA-induced calcium enrichment, and related biological effects were used to determine the molecular mechanism at the PHA-cell interface. PHA800 significantly upregulated a TLR4 mediated inflammatory pathway in a calcium influx-dependent manner, and the calcium enrichment activity on the surface of PHA800 promoted calcium influx. In contrast, the calcium enrichment activity on the surfaces of the PHA1600 and PHA800 pretreated groups was attenuated, resulting in decreased calcium influx and mild inflammatory effects. Our results provide a fundamental basis for the development of novel bone substitutes that elicit low levels of inflammation response.