Acquisition of resistance of pancreatic cancer cells to 2-methoxyestradiol is associated with the upregulation of manganese superoxide dismutase.

Acquired resistance of cancer cells to anticancer drugs or ionizing radiation (IR) is one of the major obstacles in cancer treatment. Pancreatic cancer is an exceptional aggressive cancer, and acquired drug resistance in this cancer is common. Reactive oxygen species (ROS) play an essential role in cell apoptosis, which is a key mechanism by which radio- or chemotherapy induce cell killing. Mitochondria are the major source of ROS in cells. Thus, alterations in the expression of mitochondrial proteins, involved in ROS production or scavenging, may be closely linked to the resistance of cancer cells to radio- or chemotherapy. In the present study, we generated a stable cell line by exposing pancreatic cancer cells to increasing concentrations of ROS-inducing, anticancer compound 2-methoxyestradiol (2-ME) over a 3-month period. The resulting cell line showed strong resistance to 2-ME and contained an elevated level of ROS. We then used a comparative proteomics method to profile the differential expression of mitochondrial proteins between the parental and the resistant cells. One protein identified to be upregulated in the resistant cells was manganese superoxide dismutase (SOD2), a mitochondrial protein that converts superoxide radicals to hydrogen peroxides. Silencing of SOD2 resensitized the resistant cells to 2-ME, and overexpression of SOD2 led the parental cells to 2-ME resistance. In addition, the 2-ME-resistant cells also showed resistance to IR. Our results suggest that upregulation of SOD2 expression is an important mechanism by which pancreatic cancer cells acquire resistance to ROS-inducing, anticancer drugs, and potentially also to IR.