Saos-2 cells cultured under hypoxia rapidly differentiate to an osteocyte-like stage and support intracellular infection by Staphylococcus aureus

The intracellular infection of osteocytes represents a clinically important aspect of osteomyelitis. However, few human osteocyte in vitro models exist and the differentiation of immature osteoblasts to an osteocyte stage typically takes at least 4-weeks of culture, making the study of this process challenging and time consuming. The osteosarcoma cell line Saos-2 has proved to be a useful model of human osteoblast to mature osteocyte differentiation. Culture under osteogenic conditions in a standard normoxic (21% O-2) atmosphere results in reproducible mineralization and acquisition of mature osteocyte markers over the expected 28-35 day culture period. In order to expedite experimental assays, we tested whether reducing available oxygen to mimic concentrations experienced by osteocytes in vivo would increase the rate of differentiation. Cells cultured under 1% O-2 exhibited maximal mineral deposition by 14 days. Early (COLA1, MEPE) and mature (PHEX, DMP1, GJA1, SOST) osteocyte markers were upregulated earlier under hypoxia compared to normoxia. Cells differentiated under 1% O-2 for 14 days displayed a similar ability to internalize Staphylococcus aureus as day 28 cells grown under normoxic conditions. Thus, low oxygen accelerates Saos-2 osteocyte differentiation, resulting in a useful human osteocyte-like cell model within 14 days.