Fluid flow induces enhancement of the Egr-1 mRNA level in osteoblast-like cells: involvement of tyrosine kinase and serum.

It is widely accepted that mechanical loading is necessary to construct the architecture of bone and to maintain bone mass. However, the mechanism of how bone cells respond to mechanical stimuli is not known. To clarify this, we stimulated osteoblast-like MC3T3E1 cells by mechanical shaking of the culture dishes and found that the level of the egr-1 gene, which is an early response gene induced by growth factors or serum and encodes a transcription factor, increased 15-45 min after the shaking, with a peak at 30 min. The egr-1 gene product increased 1 h after the shaking. The egr-1 gene elevation was not blocked by prior exposure to indomethacin, saralasin, Rp-cAMP, A23187, and colchicine, and it was blocked partially by cytochalasin D, H-7, and prolonged exposure to TPA. On the other hand, a prior incubation with cycloheximide, DRB, genistein, herbimycin A, and BAPTA/AM completely blocked the egr-1 gene level enhanced by shaking the culture dishes. Moreover, we found that in serum-deprived cells the egr-1 gene response to shaking was not induced. These results suggested that the egr-1 gene response is regulated at the transcriptional level and that it involves tyrosine kinase as well as labile or de novo protein and requires a particular level of intracellular calcium and serum.