HGFA mediates HGF conversion by MM cells. (A) MM cells secrete HGFA. To detect the active (34 kDa) form of HGFA, MM-conditioned medium (CM), either pretreated or not with thrombin, as indicated, was immunoblotted with anti-HGFA antibody A-1. (B) HGFA from MM-conditioned medium converts HGF. HGFA(+) or IgG control (-) immunoprecipitates from MM-conditioned medium were analyzed in a HGF conversion assay. (C) HGFA mediates HGF activation by MM cells. The effects of neutralizing antibodies against HGFA (P1-4) and factor XIIa (OT-2), protease inhibitors aprotinin and leupeptin, and C1-inhibitor on HGF activation by recombinant HGFA (left panel) and conditioned medium of MM cell line NCI-H929 (middle panel) and primary MM cells (right panel) were analyzed by the HGF conversion assay. HC indicates immunoglobulin heavy chain. (D) Activation and biologic actions of HGF in the myeloma microenvironment.