Hypotonic cell swelling induces translocation of the α isoform of cytosolic phospholipase A2 but not the γ isoform in Ehrlich ascites tumor cells

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. Stimulation by either hypotonic cell swelling or addition of the Ca2+ ionophore A23187 results in translocation of cPLA(2)alpha, but not cPLA(2)gamma, to the nucleus, where it forms hot-spot-like clusters. Our group previously showed that release of radioactively labeled arachidonic acid, incorporated into the phospholipids of Ehrlich cells, was immediately and transiently increased 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 demonstrate that arachidonic acid is released from the nuclear fraction following hypotonic exposure. Stimulation of Ehrlich cells with A23187 also leads to an increase in arachidonic acid release from the nucleus. However, as hypotonic cell swelling is not accompanied by any detectable increase in intracellular concentration of free cytosolic Ca2+ ([Ca2+](i)), stimulus-induced 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 activation, p38 MAP kinase, tyrosine kinases and protein kinase C, hence, phosphorylation 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.