MECHANICAL STIMULATION BY INTERMITTENT HYDROSTATIC COMPRESSION PROMOTES BONE-SPECIFIC GENE-EXPRESSION IN-VITRO

In a previous study of the cellular mechanism underlaying Wolffs law w e showed that mechanical stimulation by intermittent hydrostatic compr ession (IHC) increases bone formation in cultured fetal mouse calvaria e compared to non-stimulated cultures. To test whether mechanical stim uli may modulate bone-specific gene expression, we studied the effect of IHC on alkaline phosphatase (AP) expression and enzyme activity as well as collagen and actin mRNA levels in neonatal mouse calvariae and calvarial bone cells. Two cell populations, one resembling osteoproge nitor (OPR) cells and another resembling osteoblasts (OB) were obtaine d from calvariae by sequential digestion. IHC was applied by intermitt ently (0.3 Hz) compressing the gas- phase of a closed culture chamber (peak stress 13kPa, peak stress rate 32.5 kPas(-1)). In control cultur es of calvariae as well as OB and OPR cells, AP activity and AP-, coll agen-, and actin-mRNA levels all decreased after one or more days, wit h the exception of OPR cell collagen expression which increased during culture. IHC treatment upregulated AP, collagen and actin expression and AP activity in calvariae and OB cells, but decreased collagen expr ession in OPR cells. These results suggest that treatment with IHC pro motes the osteoblastic phenotype in bone organ cultures and in osteobl asts. Osteoprogenitor cells seem to react somewhat differently to mech anical stress than osteoblasts. The loss of bone-specific gene express ion under control culture conditions, in the absence of mechanical sti muli, suggests that the mechanical environment is important in maintai ning the differentiated phenotype of bone cells, and that IHC treatmen t partially restores this environment in bone cell- and organ cultures .