Sequential analysis of gene expression after an osteogenic stimulus: c-fos expression is induced in osteocytes

We have recently developed an experimental model whereby mechanical stimulation induces osteogenesis in the caudal vertebrae of rats. We used this model to assess expression of genes induced by mechanical loading. Bulk preparations of mRNA extracted after loading did not show > 2-fold increases in expression of mRNA for matrix proteins or growth factors in Northern blotting analysis. c-jun was undectable. However, c-fos showed a 4-fold increase in expression within 60 mins of loading, before returning to control levels by 4 hrs. This increase was associated with intense signals in in situ hybridization, not seen in any nonloaded vertebrae, for c-fos over cortical osteocytes: thus osteocytes respond to mechanical loading with c-fos expression so strongly as to be visible even in the bulk RNA preparations. The results represent persuasive evidence for a role for osteocytes, and for c-fos, in the osteogenic response of bone to mechanical stimulation.