CYTOPLASMIC AND NUCLEAR-LOCALIZATION SITES OF PHOSPHATIDYLINOSITOL 3-KINASE IN HUMAN OSTEOSARCOMA SENSITIVE AND MULTIDRUG-RESISTANT SAOS-2 CELLS

The intracellular localization of phosphatidylinositol 3-kinase (PI 3- kinase) has been analyzed by western blotting, confocal, and electron microscopy immunocytochemistry in human osteosarcoma Saos-2 cells. By western blotting, the enzyme appears to be present in both the cytopla smic and nuclear subfractions. By confocal microscope immunocytochemis try, the cytoplasmic fluorescence is localized in the perinuclear regi on and on a network of filaments, while a diffused signal is present i n the nucleus, except for the nucleolar areas. Ultrastructural analyse s on whole cells and on in situ matrix preparations reveal that nuclea r PI 3-kinase is localized in interchromatin domains, in stable associ ation with inner nuclear matrix components, while the enzyme diffused in the cytosol is partly associated with the cytoskeletal filaments. Q uantitative evaluations indicate that, ina multidrug-resistant variant obtained by continuous exposure of Saos-2 cells to doxorubicin, the a mount of nuclear and cytoplasmic PI 3-kinase is significantly lower th an in the sensitive parental cell line. The nuclear localization of PI 3-kinase and its variation in multidrug-resistant cells, characterize d by a reduced mitotic index, are consistent with the data on the exis tence of a nuclear inositol lipid cycle, which could also utilize 3-ph osphorylated inositides to modulate signal transduction for the contro l of some key functional activities.