Endocytosis and lysosomal delivery of tissue plasminogen activator-inhibitor 1 complexes in Hep G2 cells

Receptor-mediated endocytosis of tissue-type plasminogen activator (t-PA)-plasminogen activator inhibitor type 1 (PAI-1) complexes results in their clearance by Hep G2 cells. After complexes are internalized, the t-PA component is degraded. However, neither the locus of intracellular catabolism nor the fate of PAI-1 has been elucidated. To characterize these aspects of t-PA·PAI-1 catabolism, the subcellular distribution of a prebound cohort of ligand molecules was delineated after internalization at 37°C. ¹²⁵I-t-PA·PAI-1 and t-PA·¹²⁵I-PAI-1 were compared in separate experiments. After ligand uptake, intracellular vesicles were separated on density gradients. Internalized ¹²⁵I-t-PA·PAI-1 concentrated initially in endosomes. After 20 minutes of uptake, the complex began to appear in lysosomes. Subsequently, low molecular weight labeled ligand fragments were detected in culture media. A panel of lysosomotropic agents, including primaquine, chloroquine, ammonium chloride, and a combination of leupeptin and pepstatin A, inhibited degradation. When t-PA·¹²⁵I-PAI-1 rather than ¹²⁵I-t-PA·PAI-1 was internalized, strikingly different results were observed. Although the kinetics of internalization and the intracellular itinerary were indistinguishable for the differently labeled complexes, the ¹²⁵I-PAI-1 component of t-PA·¹²⁵I-PAI-1 resisted rapid degradation. After a rapid loss of t-PA, the ¹²⁵I-PAI-1 moiety persisted in lysosomes for up to 180 minutes. Thus, internalized t-PA·PAI-1 is targeted to lysosomes in which PAI-1 is relatively more stable than t-PA.