THE ANTICANCER ACTIVITY AND MECHANISMS OF ACTION OF JADOMYCINS IN MULTIDRUG RESISTANT HUMAN BREAST CANCER CELLS

Abstract

Breast cancer is the most common cancer in women, and approximately one third of all breast cancers will ultimately metastasize. Metastatic breast cancer is classified as an incurable disease. The development of multidrug resistance (MDR) is largely responsible for the difficulty associated with its treatment. Jadomycins are natural products biosynthesized by the bacteria Streptomyces venezuelae International Streptomyces Project (ISP) 5230, which have anticancer activity but poorly defined mechanisms of action. Preliminary research in our laboratory showed jadomycins retain their cytotoxic potency in MDR-MCF7 breast cancer cells that overexpress certain ATP-binding cassette (ABC) drug efflux transporters. My research goal was to more completely characterize jadomycin cytotoxicity profiles and pharmacological mechanisms to better understand their potential applications in the treatment of MDR breast cancer. First I show multiple jadomycin analogues retain their cytotoxic potency in ABCB1, ABCC1, or ABCG2-overexpressing MCF7 and ABCB1-overexpressing MDA-MB-231 versus control MCF7 and MDA-MB-231 breast cancer cells. Inhibitors of ABCB1, ABCC1, and ABCG2 minimally affect jadomycin cytotoxicity, suggesting jadomycins are poor ABC transporter substrates. I then show jadomycins have multiple cytotoxicity mechanisms that are influenced by breast cancer cell type. Jadomycins increase intracellular reactive oxygen species (ROS) in MCF7 and MDA-MB-231 cells. In MCF7 cells, inhibition of the antioxidant thioredoxin reductase with auranofin potentiates jadomycin cytotoxicity. Conversely, neutralization of ROS with the antioxidant N-acetylcysteine decreases jadomycin potency, but not efficacy, evidencing a ROS-independent cytotoxicity mechanism. In MDA-MB-231 cells, jadomycins cause DNA double strand breaks and apoptosis. These effects are not blocked by ROS neutralization or enhanced by antioxidant inhibition, further evidencing ROS-independent mechanisms of cytotoxicity. Additional assays show that ROS-independent mechanisms include aurora B kinase inhibition and type II topoisomerase inhibition, which can lead to DNA double strand breaks and apoptosis. These cytotoxicity mechanisms are preserved in the ABC-transporter overexpressing MCF7 and MDA-MB-231 cells, thus helping explain how jadomycins remain effective in these cells. My research is the first to detail the polypharmacology of jadomycins’ anticancer activity. Additionally, I describe the promising jadomycin activity against ABC-transporter overexpressing MDR breast cancer cells, supporting further research into how these natural products may be used as MDR metastatic breast cancer treatments.