Inhibition of Transferrin Recycling and Endosome Tubulation by Phospholipase A₂ Antagonists

We report here that a broad spectrum of phospholipase A₂ (PLA₂) antagonists produce a concentration-dependent, differential block in the endocytic recycling pathway of transferrin (Tf) and Tf receptors (TfRs) but have no acute affect on Tf uptake from the cell surface. At low concentrations of antagonists (∼1 μM), Tf and TfR accumulated in centrally located recycling endosomes, whereas at higher concentrations (∼10 μM), Tf-TfR accumulated in peripheral sorting endosomes. Several independent lines of evidence suggest that this inhibition of recycling may result from the inhibition of tubule formation. First, BFA-stimulated endosome tubule formation was similarly inhibited by PLA₂ antagonists. Second, endocytosed tracers were found in larger spherical endosomes in the presence of PLA₂ antagonists. And third, endosome tubule formation in a cell-free, cytosol-dependent reconstitution system was equally sensitive PLA₂ antagonists. These results are consistent with the conclusion that endosome membrane tubules are formed by the action of a cytoplasmic PLA₂ and that PLA₂-dependent tubules are involved in intracellular recycling of Tf and TfR. When taken together with previous studies on the Golgi complex, these results also indicate that an intracellular PLA₂ activity provides a novel molecular mechanism for inducing tubule formation from multiple organelles.