Bone cell responsiveness to growth and differentiation factors under hypoxia in vitro

Osteogenic cells contribute to the process of osseointegration and graft consolidation. However, whether the cells survive low oxygen tension and maintain their responsiveness to natural and therapeutic growth and differentiation factors remains unknown.

To determine the effects of low oxygen tension on osteogenic cell viability and responsiveness in vitro, human bone cells were placed into plastic pouches intended to create anaerobic conditions and were either simultaneously or subsequently exposed to supernatants from activated platelets or recombinant bone morphogenetic protein (BMP)-6.

Bone cells cultured for up to 72 hours under hypoxia moderately decreased their metabolic activity, which was paralleled by morphologic changes but not by cleavage of the apoptosis markers caspase-3 and poly(ADP)ribose polymerase. Hypoxia suppressed the mitogenic response of bone cells to platelet-released supernatant and the expression of osteogenic differentiation markers alkaline phosphatase and osteocalcin upon incubation with BMP-6. Stimulation of bone cells with platelet-released supernatant and BMP-6 immediately after re-establishment of normoxia caused a moderate cellular response. However, when bone cells were allowed to recover for 7 days under normoxia, their responsiveness was equal to that of cells not previously exposed to low oxygen tension.

These findings suggest that osteogenic cells can survive transient hypoxia and retain their potential to respond to growth and differentiation factors once normoxia is re-established. The data also implicate that reoxygenation, and thus blood vessel formation, may be an important determinant for the process of osseointegration and graft consolidation.