White, C.R.*, Jiang G.L.*, Frangos, J.A.
Temporal gradients in shear stimulate the proliferation of osteoblasts-like through the phosphorylation of erk1/2 and retinoblastoma protein.
Am J Physiol Endrocrinol Metab 283:E383-E389Bone cells are subject to interstitial fluid flow driven by venous pressure and mechanical loading. At rest, interstitial fluid flow is relatively steady. During physical activity such as exercise, rapid dynamic changes in mechanical loading cause transient changes in interstitial fluid flow. Rapid changes in interstitial fluid flow subject bone cells to large temporal gradients in interstitial fluid shear stress. The present study investigates the ability of steady fluid shear stress and temporal gradients in fluid shear to stimulate rat osteoblast-like UMR106 cells proliferation. The role of extracellular signal-regulated kinase (Erk1/2) and retinoblastoma protein (Rb) regulation of cell cycle was also monitored. Pulsatile flow induced a 91% ±28 increase in S phase cells compared to sham static controls. In contrast, steady ramped flow stimulated only an 8% ±10 increase in S phase cells. Consistently, pulsatile flow significantly increased relative cell number by 28%±16 and 67%±16 at 1.5 hr and 24 hrs (respectively). Pulsatile flow significantly increased Erk1/2 phosphorylation relative to control by 388% ±3 and 255% ±56 at 1.5 hr and 24 hrs (respectively). Steady ramped flow reduced Erk1/2 activation relative to control by 43% ±10 after 1.5 hrs, and no change after 24 hrs. Correspondingly, Rb was significantly hyperphosphorylated by pulsatile fluid flow. Treatment of cells with U0126 significantly reduced Erk1/2 phosphorylation, and inhibited progression of the cell cycle via Rb hyperphosphorylation. This suggests that mechanical loading regimes that produce sharp transients in fluid shear stress, such as high impact or high-frequency loading, are potent stimuli of bone cell proliferation.
* Drs White and Jiang are joint primary authors of this work.