gp330 on type II pneumocytes mediates endocytosis leading to degradation of pro-urokinase, 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 ligands have been shown to bind gp330 in solid-phase assays, the ability of gp330 to mediate ligand endocytosis has not been demonstrated. To develop a cellular model for gp330 function we screened a variety of cultured cell lines and identified several that expressed this protein, including immortalized rat type II pneumocytes and a human and two rodent tumor cell lines. Using type II pneumocytes, endocytosis of a previously described gp330 ligand, urokinase (uPA) complexed with plasminogen activator inhibitor-1 (uPA:PAI-1) and two new ligands, PAI-1 and pro-uPA, was demonstrated. RAP, the 39 kDa receptor-associated protein known to antagonize ligand binding to gp330 in solid-phase binding assays, completely inhibited both internalization and degradation of the radiolabeled ligands by type II pneumocytes. This suggested that the clearance of these ligands was dependent on either gp330 or the LDLR-related protein (LRP), which shares several ligand-binding characteristics with gp330. By using polyclonal antibodies to gp330, the cellular internalization and degradation of the ligands were inhibited by 30-50%; remaining ligand internalization and degradation activity could be partially inhibited by polyclonal antibodies against LRP. These findings indicate that gp330, like other LDLR family members, mediates endocytosis of its ligands. In addition, gp330 acts in concert with LRP in type II pneumocytes to mediate clearance of a variety of proteins involved in plasminogen activation, including uPA:PAI-1 complexes PAI-1 and pro-uPA.(ABSTRACT TRUNCATED AT 250 WORDS)