Prothrombin is converted to thrombin by factor Xa in the cell-associated prothrombinase complex Prothrombin is present in calcified bone matrix and thrombin exerts effects on osteoblasts as well as on bone resorption by osteoclasts We investigated whether (1) osteoclasts display factor Xa-dependent prothrombinase activity and (2) osteoclasts express critical regulatory components upstream of the prothrombinase complex The osteoclast differentiation factor RANKL induced formation of multinucleated TRAP positive cells concomitant with induction of prothrombinase activity in cultures of RAW 264.7 cells and bone marrow osteoclast progenitors Expression analysis of extrinsic coagulation factors revealed that RANKL enhanced protein levels of factor Xa as well as of coagulation factor III (tissue factor) Inhibition assays indicated that factor Xa and tissue factor were involved in the control of prothrombinase activity in RANKL-differentiated osteoclasts, presumably at two stages (1) conversion of prothrombin to thrombin and (2) conversion of factor X to factor Xa, respectively. Activation of the extrinsic coagulation pathway during osteoclast differentiation through induction of tissue factor and factor Xa by a RANKL-dependent pathway indicates a novel role for osteoclasts in converting prothrombin to thrombin. (C) 2010 Elsevier Inc All rights reserved