Tumor necrosis factor-induced permeability increase of negatively charged phospholipid vesicles

The effect of human tumor necrosis factor (TNF) on the permeability properties of liposomes containing phosphatidylserine at pH 5-6, as demonstrated by the calcein efflux. However, it did not induce any permeability change in such liposomes at neutral pH. The TNF-induced calcein efflux was also observed when an other acidic lipid was used as a component of the liposomes, i.e., phosphatidic acid or dicetyl phosphate. On the other hand, liposomes composed of neutral phospholipids such as phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin showed little increases in permeability when incubated with TNF above pH 5.0. The TNF-induced permeability change was inhibited by the addition of polyaspartic acid, while it was not affected by the presence of 0.5 mM calcium ions. These data suggest that the negative charges on the liposomal surface trigger the interaction between TNF and liposomes. However, when the pH of the reaction mixture was decreased to 4.5, TNF-induced calcein efflux was observed even from neutral liposomes. When TNF was incubated with 8-anilinonaphthalene-1-sulfonic acid, the fluorescence intensity of this fluorophore increased with a decrease in the pH of the solution from 7 to 5, and a drastic increase in fluorescence was observed at pH 4.5. These data suggest that the hydrophobic region of TNF is also important for liposomal damage. Furthermore, the potencies of TNF and its derivative as to the induction of the permeability change paralleled their cytotoxic effects on mouse L929 cells, suggesting that the effect of TNF on liposomal membranes is related to its biological action.