Temporal gradients in shear stimulate osteoblastic proliferation via ERK1/2 and retinoblastoma protein

Abstract

The effects of pulsatile flow (temporal gradients in fluid shear) on rat UMR106 cells and rat primary osteoblastic cells were studied. Pulsatile flow induced a 95% increase in S-phase UMR106 cells compared with static controls. In contrast, ramped steady flow stimulated only a 3% increase. Similar patterns of S-phase induction were also observed in rat primary osteoblastic cells. Pulsatile flow significantly increased relative UMR106 cell number by 37 and 62% at 1.5 and 24 h, respectively. Pulsatile flow also significantly increased extracellular signal-regulated kinase (ERK1/2) phosphorylation by 418%, whereas ramped steady flow reduced ERK1/2 activation to 17% of control. Correspondingly, retinoblastoma protein was significantly phosphorylated by pulsatile fluid flow. Inhibition of mitogen-activated protein (MAP)/ERK kinase (MEK)1/2 by U0126 (a specific MEK1/2 inhibitor) reduced shear-induced ERK1/2 phosphorylation and cell proliferation. These findings suggest that temporal gradients in fluid shear stress are potent stimuli of bone cell proliferation. shear stress; pulsatile fluid flow; osteoblasts (RE)MODELING OF BONE in the skeletal system is strongly influenced by mechanical loading and unloading. Exercise has been shown to stimulate increases in bone mass (24), whereas prolonged mechanical unloading results in bone atrophy (36,45). It has been hypothesized that exercise-induced (re)modeling of bone may result from stimulation of osteoblasts by interstitial fluid flow (IFF) through the porous structure of bone (10, 31). Interstitial fluid flows radially outward through the bone cortex and is driven by hydrostatic pressure gradients across the cortex and pressure gradients from mechanical loading (21,23,39,42). Theoretical models of IFF indicate significant fluid shear stresses in the canaliculi ranging between 6 and 30 dyn/cm 2 (44). During physical activity, rapid dynamic changes in mechanical loading cause transient changes in bone marrow pressure and IFF (30, 43). Rapid changes in IFF subject bone cells to large temporal gradients in interstitial fluid shear stress. Temporal shear stress gradients are defined as the rapid change in shear stress over a small period of time (Ͻ0.5 s) at any given location on the cell surface. Steady shear stress is devoid of temporal gradients and can be established if the onset and/or cessation of flow is slowly transitioned. In any given system of flow, if the change in flow is sudden, significant temporal gradients are generated (8,46). Given that similar temporal gradients in shear stress in other cell types have been shown to be promitogenic compared with steady fluid flow (2), repeated temporal gradients in shear stress through the bone matrix may also stimulate osteoblast proliferation (10, 43).A number of investigators have demonstrated that IFF stimulates elaborate mechanochemical signaling cascades in osteoblasts. Fluid shear has been shown to stimulate osteoblast production and release of cAMP, prostaglandin E 2 (PGE 2 ), inositol triphosphate, and nitric oxide ...

Activation of extracellular-signal regulated kinase (ERK1/2) by fluid shear is Ca2+- and ATP-dependent in MC3T3-E1 osteoblasts

“…These kinases have been shown to be important in the proliferation, growth, differentiation in many cell types [19][20][21][22] including osteoblasts [23,24]. Activation of MAPK has been shown to be important in shear-induced increases in OPN [35] and COX-2 mRNA expression [7] and cell proliferation [26]. In the present study, we found that only ERK1/2 was activated by fluid shear, becoming phosphorylated within 5 minutes of the onset of flow with peak activation observed after 30 minutes.…”

“…These members of the MAPK family have been implicated in the regulation of cellular growth, differentiation and apoptosis in numerous cell types [19][20][21][22] including osteoblasts [23,24]. Several studies have shown that ERK1/2 is activated by fluid shear in osteoblasts [7,25,26]. Studies have also shown that Ca 2+ i is important to ERK1/2 activation in osteoblasts [24,27], although it is unclear whether this Ca 2+ i -induced activation results from extracellular Ca 2+ entry or intracellular Ca 2+ release.…”

Tenocyte responses to mechanical loading in vivo: A role for local insulin‐like growth factor 1 signaling in early tendinosis in rats

“…In cultured human osteoblasts, ERK-1/2 blockade prevented shear-induced proliferation and matrix synthesis (43). Tenocytes appear to share elements of a load-sensing mechanism similar to osteoblasts and osteocytes; stretch-activated potassium and calcium channels, internal calcium release, interstitial ATP release, and gap junction signaling all play a role in the proliferative response to membrane deformation, substrate deformation, or fluid shear (43)(44)(45)(46)(47). In elongated tenocytes, proliferation and collagen synthesis in response to ex vivo loading of chicken flexor tendon could be blocked by a gap junction inhibitor (48).…”

The P2Y13 receptor regulates extracellular ATP metabolism and the osteogenic response to mechanical loading

“…This extracellular ATP accumulation may in turn trigger other P2 receptor signaling pathways and cause an increased osteogenic response possibly via ATP-dependent phosphorylation of ERK, (11,17) which then stimulates osteoblastic proliferation and drives the osteogenic response. (18) In conclusion, this study examined the role of P2Y 13 receptor in bone osteogenic response to mechanical loading in vivo and in vitro. Deletion of the P2Y 13 R leads to higher bone formation, mainly in cortical compartment, than WT upon mechanical loading in vivo, possibly because of the lack of P2Y 13 R regulated negative feedback pathway for ATP release.…”