Differential Susceptibility of Normal and Transformed Human Leukocytes to Hydrolytic Attack by Secretory Phospholipase A2

Abstract

Previous experiments with cultured lymphoma cells demonstrated that secretory phospholipase A2 (sPLA2) distinguishes healthy cells from those that are dying by apoptosis or necrosis. This distinction depends on cell membrane properties including the amount of negative charge at the bilayer surface and the strength of interactions among neighboring phospholipids. These results raised two important questions. 1) Does the enzyme's ability to distinguish healthy and dying cells apply to normal human leukocytes? 2) Does sPLA2 differentiate between normal and tumor cells? These questions were addressed by comparing membrane properties and susceptibility to hydrolysis among cultured transformed leukocytes and freshly-isolated human neutrophils and lymphocytes. Membrane properties were assessed by flow cytometry, merocyanine 540 fluorescence spectra, trimethylammonium diphenylhexatriene fluorescence anisotropy, and two-photon scanning microscopy with laurdan. Similar to the behavior of transformed cells, normal human leukocytes resisted hydrolysis by sPLA2. Upon addition of a calcium ionophore, ionomycin, the cells became vulnerable to hydrolysis, again analogous to the results observed with tumor cells. However, several important quantitative distinctions were observed. First, the various types of normal leukocytes responded differently to the enzyme; lymphocytes exhibited significantly greater rates of hydrolysis by sPLA2 compared to granulocytes. Second, hydrolysis was substantially slower in normal cells compared to transformed cells. Third, the time required for ionomycin to induce cells to be attacked by sPLA2 was greater in normal compared to transformed cells. Likewise, changes in membrane physical properties following ionomycin treatment were more subtle in normal cells than they were in transformed cells. These results suggest the possibility that sPLA2 could function as a therapeutic ally during cancer chemotherapy to assist with the demise of tumor cells.