Ultrastructural features and P-glycoprotein immunolocalization in Saos-2/DX580 multidrug-resistant human osteosarcoma cells.

The multiple drug type of resistance to anticancer agents (MDR) is mediated by an over-expression of the MDR1 gene product, the P-glycoprotein. This is largely present at the cell surface of MDR cells, mediating the active efflux of cytotoxic molecules, but may be found also intracellularly. In this paper, using Saos-2 human osteosarcoma cells as a model, we provide further evidence of increased presence of P-glycoprotein at the plasma membrane and in the nucleus of MDR cells, where it is closely bound to the nuclear matrix. The structural changes observed in Saos-2 MDR cells, including an increase of the cell surface by the formation of blebs, and a peculiar clustering of chromatin, which are similar to those observed in other MDR cell lines, are likely to be associated with the observed overexpression of the P-glycoprotein at the cell membrane and nuclear level. These findings suggest the existence of more complex, still undetermined, mechanisms underlying the MDR phenomenon.