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

Mechanical stimulation of bone induces new bone formation invivo and increases the metabolic activity and gene expression ofosteoblasts in culture. We investigated the role of the actin cytoskeleton and actin-membrane interactions in the transmission ofmechanical signals leading to altered gene expression in cultured MC3T3-E1 osteoblasts. Application of fluid shear to osteoblasts causedreorganization of actin filaments into contractile stress fibers andinvolved recruitment of₁-integrins and -actinin tofocal adhesions. Fluid shear also increased expression of two proteinslinked to mechanotransduction in vivo, cyclooxygenase-2 (COX-2) and theearly response gene product c-fos. Inhibition of actin stress fiberdevelopment by treatment of cells with cytochalasin D, by expression ofa dominant negative form of the small GTPase Rho, or by microinjectioninto cells of a proteolytic fragment of -actinin that inhibits-actinin-mediated anchoring of actin filaments to integrins at theplasma membrane each blocked fluid-shear-induced gene expression inosteoblasts. We conclude that fluid shear-induced mechanical signalingin osteoblasts leads to increased expression of COX-2 and c-Fos througha mechanism that involves reorganization of the actin cytoskeleton.Thus Rho-mediated stress fiber formation and the -actinin-dependentanchorage of stress fibers to integrins in focal adhesions may promotefluid shear-induced metabolic changes in bone cells.