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

Mechanical stimulation of bone induces new bone formation in vivo and incre ases the metabolic activity and gene expression of osteoblasts in culture. We investigated the role of the actin cytoskeleton and actin-membrane inter actions in the transmission of mechanical signals leading to altered gene e xpression in cultured MC3T3-E1 osteoblasts. Application of fluid shear to o steoblasts caused reorganization of actin filaments into contractile stress fibers and involved recruitment of beta(1)-integrins and alpha-actinin to focal adhesions. Fluid shear also increased expression of two proteins link ed to mechanotransduction in vivo, cyclooxygenase-2 (COX-2) and the early r esponse gene product c-fos. Inhibition of actin stress fiber development by treatment of cells with cytochalasin D, by expression of a dominant negati ve form of the small GTPase Rho, or by microinjection into cells of a prote olytic fragment of alpha-actinin that inhibits alpha-actinin-mediated ancho ring of actin filaments to integrins at the plasma membrane each blocked fl uid-shear-induced gene expression in osteoblasts. We conclude that fluid sh ear-induced mechanical signaling in osteoblasts leads to increased expressi on of COX-2 and c-Fos through a mechanism that involves reorganization of t he actin cytoskeleton. Thus Rho-mediated stress fiber formation and the alp ha-actinin-dependent anchorage of stress fibers to integrins in focal adhes ions may promote fluid shear-induced metabolic changes in bone cells.