Hypotonic cell swelling induces translocation of the alpha isoform of cytosolic phospholipase A(2) but not the gamma isoform in Ehrlich ascites tumorcells

We demonstrate that two isoforms of the cytosolic phospholipase A(2), cPLA( 2)alpha and cPLA(2)gamma, are present in Ehrlich ascites tumor cells. Both enzymes are almost uniformly distributed throughout the cells under control conditions, as visualized by laser-scanning confocal microscopy. Stimulati on by either hypotonic cell swelling or addition of the Ca2+ ionophore A231 87 results in translocation of cPLA(2)alpha, but not cPLA(2)gamma, to the n ucleus, where it forms hot-spot-like clusters. Our group previously showed that release of radioactively labeled arachidonic acid, incorporated into t he phospholipids of Ehrlich cells, was immediately and transiently increase d on hypotonic cell swelling [Thoroed, S.M., Lauritzen, L., Lambert, I.H., Hansen, H.S. & Hoffmann, E.K. (1997) J. Membr. Biol. 160, 47-58]. We now de monstrate that arachidonic acid is released from the nuclear fraction follo wing hypotonic exposure. Stimulation of Ehrlich cells with A23187 also lead s to an increase in arachidonic acid release from the nucleus. However, as hypotonic cell swelling is not accompanied by any detectable increase in in tracellular concentration of free cytosolic Ca2+ ([Ca2+](i)), stimulus-indu ced translocation of cPLA(2)alpha can also occur without elevation of [Ca2](i). The stimulus-induced translocation of cPLA(2)alpha appears not to be prevented by inhibition of mitogen-activated protein (MAP) kinase activatio n, p38 MAP kinase, tyrosine kinases and protein kinase C, hence, phosphoryl ation is not crucial for the stimulus-induced translocation of cPLA(2)alpha . Disruption of F-actin did not affect the translocation process, thus, an intact F-actin cytoskeleton does not seem to be required for translocation of cPLA(2)alpha.