The role of the plasminogen system in bone resorption in vitro

The plasminogen/plasmin proteolytic cascade plays an important role in extr acellular matrix remodeling. The presence of the two plasminogen activators (PAs), tissue-type plasminogen activator (tPA), and urokinase-type plasmin ogen activator (uPA), and their inhibitor type 1 (PAI-1) in bone cells, sug gests a role in one or more aspects of bone resorption such as osteoclast f ormation, mineral dissolution, and degradation of the organic matrix. These different processes were assayed in vitro using cells derived from mice wi th either tPA (tPA-/-), uPA (uPA-/-), PAI-1 (PAI-1-/-) inactivation or with a combined inactivation (tPA-/-:uPA-/-) and compared with wild-type mice ( WT). First, osteoclast formation, assessed by investigating the number and characteristics of tartrate-resistant acid phosphatase-positive multinuclea ted cells formed in cocultures of primary osteoblasts and bone marrow cells treated with 1 alpha,25-dihydroxyvitamin D-3, was not different between th e different cell types. Second, dentine resorption, an assay for osteoclast activity, was not affected by the combined deficiency of both tPA and uPA. Finally, the ability to degrade nonmineralized bone-like matrix,vas howeve r, significantly reduced in tPA-/-:uPA-/- cells compared with WT cells (28. 1 +/- 0.6%, n = 6 vs. 56.4 +/- 3.1%, n = 6, respectively, p < 0.0001). Surp risingly, collagen proteolysis by bone cells was not dependent on the prese nce of plasmin as suggested by degradation assays performed on type I H-3-c ollagen films. Taken together, these data suggest that the plasminogen acti vator/plasmin system is not required for osteoclast formation, nor for the resorption of the mineral phase, but is involved in the removal of noncolla genous proteins present in the nonmineralized bone matrix.