GP330 ON TYPE-II PNEUMOCYTES MEDIATES ENDOCYTOSIS LEADING TO DEGRADATION OF PROUROKINASE, PLASMINOGEN-ACTIVATOR INHIBITOR-1 AND UROKINASE-PLASMINOGEN ACTIVATOR INHIBITOR-1 COMPLEX

Glycoprotein 330 (gp330) is a member of a family of receptors related to the low density lipoprotein receptor (LDLR), Although several ligan ds have been shown to bind gp330 in solid-phase assays, the ability of gp330 to mediate ligand endocytosis has not been demonstrated, To dev elop a cellular model for gp330 function we screened a variety of cult ured cell lines and identified several that expressed this protein, in cluding immortalized rat type II pneumocytes and a human and two roden t tumor cell lines, Using type II pneumocytes, endocytosis of a previo usly described gp330 ligand, urokinase (uPA) complexed with plasminoge n activator inhibitor-1 (uPA:PAI-1) and two new ligands, PAI-1 and pro -uPA, was demonstrated, RAP, the 39 kDa receptor-associated protein kn own to antagonize ligand binding to gp330 in solid-phase binding assay s, completely inhibited both internalization and degradation of the ra diolabeled ligands by type II pneumocytes. This suggested that the cle arance of these ligands was dependent on either gp330 or the LDLR-rela ted protein (LRP), which shares several ligand-binding characteristics with gp330, By using polyclonal antibodies to gp330, the cellular int ernalization and degradation of the ligands were inhibited by 30-50%; remaining ligand internalization and degradation activity could be par tially inhibited by polyclonal antibodies against LRP, These findings indicate that gp330, like other LDLR family members, mediates endocyto sis of its ligands, In addition, gp330 acts in concert with LRP in typ e II pneumocytes to mediate clearance of a variety of proteins involve d in plasminogen activation, including uPA:PAI-1 complexes PAI-1 and p ro-uPA, The evidence that gp330 can mediate clearance of these protein s in cultured cells implicates it as having a role in extracellular pr otease regulation in the many specialized epithelia where it is expres sed in vivo.