Increase of nuclear phosphatidylinositol 4,5-bisphosphate and phospholipase C beta 1 is not associated to variations of protein kinase C in multidrug-resistant Saos-2 cells.

The multidrug resistance (MDR) phenotype that is mediated by an overexpression of P-glycoprotein, has been suggested to be related also to an increased activity of protein kinase C (PKC) and to changes in phospholipid pattern. By electron microscope quantitative immunocytochemistry, we investigated whether PKC and other elements of the polyphosphoinositide signal transduction system are affected in an MDR variant of the human osteosarcoma cell line Saos-2. These cells, which are characterized by an increased expression of P-glycoprotein not only at the plasma membrane but also at the nuclear level, showed increased intranuclear amounts of phosphatidylinositol 4,5-bisphosphate and of phospholipase C beta 1, while both the amount and activity of both nuclear and cellular PKC were not modified with respect to sensitive cells. These results suggest that, in this model, the changes observed in the elements of nuclear signal transduction could be related to previously reported modifications of the MDR phenotype, but that P-glycoprotein phosphorylation is not dependent from increased PKC activity.