Figure 6.

Clathrin-independent uPAR-internalization from the apical domain of polarized MDCK cells is selectively up-regulated by activators of adenylate cyclase or protein kinase C. Filter-grown MDCK cells were preincubated with 100 μM forskolin for 30 min or 250 nM phorbol 12-myristate 13-acetate (PMA) for 15 min before addition of radiolabeled ligands to the apical compartment; 4 nM uPA:PAI-1 without (white columns) or with 100 nm cold RAP (hatched columns), 4 nM DFP-uPA (black columns), or 200 ng/ml ricin (cross-hatched columns). Incubation was continued for a further 8 min at 37°C, and then surface-associated and internal counts were determined as outlined in MATERIALS AND METHODS. The results are expressed as internalization in percentage of uptake in control cells. The means of two experiments with variation <15% are shown.