RELATIONSHIP OF THE PLASMINOGEN ACTIVATOR PLASMIN CASCADE TO OSTEOCLAST INVASION AND MINERAL RESORPTION IN EXPLANTED FETAL METATARSAL BONES/

An attempt was made to establish whether the activation of plasminogen into plasmin is necessary either for the preparatory phases to bone r esorption, involving the recruitment of osteoclast precursors, their m igration toward mineralized surfaces, and their final differentiation, or for the subsequent osteoclastic resorption phase. Ca-45-labeled fe tal (17 day) mouse metatarsals were cultured under conditions in which they pursue their modeling for a few days. In this model, the resorpt ion phase, monitored by the release of Ca-45 into the medium, is entir ely dependent on the preparatory phases affecting osteoclast precursor s. It was, as expected, stimulated by parathyroid hormone (PTH) and 1, 25-dihydroxyvitamin D-3 and inhibited by calcitonin. PTH also enhanced the activity of tissue-type plasminogen activator (PA) in extracts of metatarsals but not that of urokinase (which is, however, the main PA present in the mouse fetal metatarsal culture model). The resorption processes were not dependent on the presence of plasminogen in the med ia, even when the rudiments were precultured with tranexamic acid to r emove their endogenous plasminogen. Moreover, they were not influenced by inhibitors of plasmin, either the plasma inhibitors alpha(2)-antip lasmin, alpha(2)-macroglobulin, and alpha(1)-antitrypsin, or aprotinin , which was tested under a variety of conditions. Aprotinin also did n ot influence the resorption (loss of calcium and hydroxyproline) of 19 day fetal mouse calvariae cultured with PTH in a medium devoid of pla sminogen. It is concluded that the various steps implicated in the bon e resorption processes that occur in the metatarsals and in the calvar iae culture models are not dependent on the activity of plasmin. The f unction of PAs in bone, however, could be exerted through direct prote olysis of extracellular proteins other than plasminogen or be mediated by a molecular structural domain distinct from their catalytic domain .