A P-glycoprotein- and MRP1-independent Doxorubicin-resistant Variant of the MCF-7 Breast Cancer Cell Line with Defects in Caspase-6, -7, -8, -9 and -10 Activation Pathways

Abstract

Background: Several mechanisms are known to cause resistance to chemotherapy in cancer cells, but the mechanisms of drug resistance due to a lack of apoptosis are not well elucidated. Materials and Methods: To understand the mechanisms of resistance to apoptosis induced by doxorubicin (DOX), we developed a DOX-resistant variant of MCF-7 referred to as MCF-7/Adr-20, measured growth inhibition by methylene blue cell survival assay, quantitated apoptosis by annexin V binding assay and detected activation of caspases-6, -7, -8, -9 and -10 in these cells. Results: The resistant cells expressed 20-fold resistance to apoptosis induced by DOX compared to MCF-7 cells. MCF-7/Adr-20 cells did not express MDR1 mRNA or its product P-lycoprotein and they did not overexpress MRP-1. Treating MCF-7 cells with 0.01, 0.1 and 1 μM DOX for 72 h induced 8, 14 and 28% apoptosis, respectively. However, only 1 μM DOX was able to trigger about 8% apoptosis in MCF-7/Adr- 20 cells. Moreover, apoptosis triggered by 0.01 and 0.1 μM DOX in MCF-7 cells was mainly caspase-dependent, but at 1 μM about 70% of apoptosis was caspase-dependent. Western blot analysis revealed that caspase-7 was activated at 0.1 and 1 μM DOX treatment and caspases-6, -8, -9 and 10 were only activated at 1 μM DOX treatment in MCF-7 cells, but none of the caspases checked were activated in MCF-7/Adr-20 cells. Moreover, DOX at 0.01 and 0.1 μM induced p53 and p21^WAF1/CIP-1 to the same extent in both MCF-7 and MCF-7/Adr-20 cells. Therefore, while DOX triggers growth arrest and induces p53 and p21^WAF-1/CIP-1 in these cells, defects in activation of the initiator and executioner caspases play a major role in resistance to apoptosis triggered by DOX.