Inhibition of apoptosis is the cause of resistance to doxorubicin in human breast adenocarcinoma cells.

In our previous paper we have described the isolation and characterization of a doxorubicin (DOX) resistant subline of breast adenocarcinoma SC6 cells. These cells were obtained after the treatment with low, clinically relevant doses of doxorubicin. They became cross-resistant to different wide used cytostatics. The expression of several genes involved in mitotic signal transduction, as well as cathepsins D and L, was similar in both parental and doxorubicin treated cells. The aim of this study was to examine the molecular mechanisms involved in resistance of these cells to doxorubicin. Activity of plasma membrane Pgp was examined in parental and resistant cells due to rhodamine-accumulation assay. The involvement of glutathione (GSH) and glutathione S-transferase (GST) in resistance to doxorubicin was determined in MTT modified assay due to the addition of specific inhibitors: buthionine sulfoximine (for GSH) or ethacrynic acid (for GST). The kinetic of apoptosis was followed after the treatment with DOX in control and SC6 cells by fluorescent microscope. The occurrence of apoptosis was confirmed by analysing DNA fragmentation in agarose gel. Our results indicate that P-glycoprotein, glutathione or glutathione transferases were not involved in resistance of SC6 cells to doxorubicin. However, the apoptosis was inhibited in doxorubicin-resistant cells. Therefore, even low doses of doxorubicin can induce the resistance to this drug due to inhibition of apoptosis.