Impact of Sr2+ and hypoxia on 3D triple cultures of primary human osteoblasts,osteocytes and osteoclasts

An in vitro bone triple culture involving human primary osteoblasts, osteocytes and osteoclasts enables the investigation of bone healing factors, drugs or biomaterials in a model system for native bone tissue. The present study analyses the impact of Sr²⁺ as well as hypoxic cultivation (5% O₂ content or chemically induced by Co²⁺) on bone cells. The three cell types were cultivated together in the presence of 100 µM Sr²⁺, hypoxic conditions or in the presence of 75 µM Co²⁺. After cultivation the cell types were separated and analysed on mRNA and protein level individually. In response to Sr²⁺ osteoblasts showed a downregulation of IBSP expression and a stimulation of ALP activity. Osteocyte gene marker expression of PDPN, MEPE, RANKL, OPG, osteocalcin and likewise the amount of secreted osteocalcin was reduced in the presence of Sr²⁺. Activity of osteoclast-specific enzymes TRAP and CAII was enhanced compared to the Sr²⁺ free control. Hypoxic conditions induced by both 5% O₂ or a Co²⁺ treatment led to decreased DNA content of all bone cells and downregulated expression of osteoblast markers ALPL and IBSP as well as osteocyte markers PDPN, RANKL and OPG. In addition, Co²⁺ induced hypoxia decreased gene and protein expression of osteocalcin in osteocytes. In response to the Co²⁺ treatment, the TRAP gene expression and activity was increased. This study is the first to analyse the effects of Sr²⁺ or hypoxia on triple cultures with primary human bone cells. The investigated in vitro bone model might be suitable to reduce animal experiments in early stages of biomaterial and drug development.