Using Phylogenetically Conserved Stress Responses to Discover Natural Products with Anticancer Activity

by Turbyville, Thomas Jefferson.

One unique feature of cancer cells that can be exploited for anticancer drug discovery is their dependence on their own cellular stress responses to survive the stressful acidotic, hypoxic and nutrient-deprived conditions within the tumor. Reasoning that desert organisms surviving under stressful conditions may have evolved to produce small molecule metabolites capable of modulating heat shock protein 90 (Hsp90) function, and/or other cell stress responses, we employed the cellular heat shock response in a moderate-throughput phenotypic assay. This strategy has resulted in the isolation and characterization of a number of small molecule natural products with heat shock induction activity from these organisms. Three such natural products are the subject of this study. In a limited structure-activity relationship (SAR) study, a previously known Hsp90 inhibitor radicicol (RAD), and several structurally related molecules including the fungal metabolite monocillin 1 (MON) were found to interact with Hsp90. In addition, RAD and MON were shown to lead to the degradation of Hsp90 client proteins involved in the cancer cell survival – the estrogen receptor (ER) and the insulin-like growth factor receptor 1 (IGF-1R). We further characterized MON and showed that by targeting the molecular chaperone Hsp90, this compound induces components of the heat shock response at the transcriptional and translational levels, and leads to the acquisition of a thermotolerant phenotype in seedlings of the plant Arabidopsis thaliana. These findings support our 11 hypothesis that there is ecological significance to the elaboration of small molecules that target stress responses. A number of extracts active in our phenotypic assay contained small molecules with no apparent Hsp90 activity. One such extract afforded terrecyclic acid A (TCA) with significant anti-tumor activity against a panel of human cancer cell lines. To characterize the biological activities of TCA we examined three key stress responses— the heat shock, oxidative, and inflammatory responses—and show that TCA destabilizes these pathways associated with cancer cell survival through induction of oxidative stress (ROS), and inhibition of NF-?B transactivation. The isolation of RAD, MON and TCA from Sonoran desert organisms provides proof of principle that we have developed an effective strategy for the discovery of small molecule modulators of cellular stress responses that can serve as leads for the development of new anticancer drugs with novel mechanisms of action. 12