Fluid shear-induced mechanical signaling in MC3T3-E1 osteoblasts requires cytoskeleton-integrin interactions

Abstract

Mechanical stimulation of bone induces new bone formation in vivo and increases the metabolic activity and gene expression of osteoblasts in culture. We investigated the role of the actin cytoskeleton and actin-membrane interactions in the transmission of mechanical signals leading to altered gene expression in cultured MC3T3-E1 osteoblasts. Application of fluid shear to osteoblasts caused reorganization of actin filaments into contractile stress fibers and involved recruitment of β1-integrins and α-actinin to focal adhesions. Fluid shear also increased expression of two proteins linked to mechanotransduction in vivo, cyclooxygenase-2 (COX-2) and the early response gene product c-fos. Inhibition of actin stress fiber development by treatment of cells with cytochalasin D, by expression of a dominant negative form of the small GTPase Rho, or by microinjection into cells of a proteolytic fragment of α-actinin that inhibits α-actinin-mediated anchoring of actin filaments to integrins at the plasma membrane each blocked fluid-shear-induced gene expression in osteoblasts. We conclude that fluid shear-induced mechanical signaling in osteoblasts leads to increased expression of COX-2 and c-Fos through a mechanism that involves reorganization of the actin cytoskeleton. Thus Rho-mediated stress fiber formation and the α-actinin-dependent anchorage of stress fibers to integrins in focal adhesions may promote fluid shear-induced metabolic changes in bone cells.

Release of nitric oxide, but not prostaglandin E2, by bone cells depends on fluid flow frequency

“…Yet, as shown by us and others, MC3T3-E1 cells are sensitive to fluid flow 22,40,[57][58][59] Also, we have shown that, although osteocytes are most sensitive to fluid flow, osteoblasts are sensitive to flow as well. 60 It was shown that MC3T3-E1 cells have the capacity to form calcified bone tissue in vitro and differentiate into osteoblasts and osteocytes 61 and that MC3T3-E1 cells display a time-dependent and sequential expression of osteoblast characteristics analogous to in vivo bone formation.…”

Mechanical force-induced midpalatal suture remodeling in mice

“…For example, application of strain to osteoblasts increases mRNA levels of Cox-2 (cyclooxygenase-2) and the immediate early gene c-fos within an hour [2], as well as the levels of extracellular matrix proteins osteopontin and osteocalcin within 24 hours [3]. In both proliferating and matrix-forming chondrocytes, cyclic mechanical strain can increase parathyroid-hormone-related protein mRNA levels [4].…”

Bone Formation Induced by a Novel Form of Mechanical Loading on Joint Tissue

“…During a routine exercise under earth's gravity, loads transmitted to diaphyseal cortical bone are dissipated at least in part through deformation of viscoelastic joint tissues. The deformation of bone is known to generate strain-induced viscous fluid flow (Piekarski et al, 1977;Tate et al, 2000), and flow-driven shear stress is considered to stimulate proliferation and differentiation of osteoblasts (Pavalko et al, 1998;Reich et al, 1990;Ryder et al, 2001) and osteocytes (Ajubi et al, 1999;KleinNulend et al, 1995). Since some part of bone deformation is generated in joints, we investigated a possibility that direct deformation of joints without any strain in cortical bone would stimulate bone formation in diaphyseal cortical bone.…”