MECHANISMS BY WHICH THIONIN INDUCES SUSCEPTIBILITY OF S49 CELL-MEMBRANES TO EXTRACELLULAR PHOSPHOLIPASE A(2)

Whereas cells normally resist attack by PLA(2), they become susceptibl e under certain pathological conditions. To ascertain the regulatory m echanisms that induce cellular susceptibility to PLA(2), the effect of thionin on S49 cells was examined in the presence of PLA(2). Thionin alone was unable to evoke hydrolysis of the lipid bilayer. Likewise, t he addition of PLA(2) alone caused production of only a minimal amount of free fatty acid. However, thionin and PLA(2) together resulted in significant hydrolysis of the cell membrane. Thionin caused perturbati on of the bilayer structure as suggested by the changes in the emissio n spectra of laurdan and the permeability of the membrane to propidium iodide. These changes correlated quantitatively with the susceptibili ty of the lipid bilayer to PLA(2). Furthermore, thionin induced a mode st increase in intracellular Ca2+. The source of this Ca2+ was the ext racellular fluid since EDTA in the extracellular medium inhibited the Ca2+ influx. Moreover, cobalt chloride, a universal Ca2+ channel block er, prevented the rise in intracellular Ca2+, the uptake of propidium iodide, and the susceptibility to PLA(2) induced by thionin. In contra st, the changes in the laurdan emission caused by the thionin were not affected by the cobalt. Furthermore, incubation of the cells with the calcium ionophore A23187 also caused the cells to become susceptible to PLA(2). We hypothesize that thionin causes S49 cell membranes to be come susceptible to PLA(2) by a Ca2+-dependent perturbation of the bil ayer structure. (C) 1997 Elsevier Science B.V.