Abstract P5-03-04: Identification of compounds from natural sources with selective activity against triple-negative breast cancer molecular subtypes
Authors: Robles AJ , Du L , Cai S , Risinger AL , Cichewicz RH , Mooberry SL 
Published: May 2016

Triple negative breast cancers (TNBCs) lack expression of the estrogen and progesterone receptors (ER/PR) and do not have amplified HER2. While targeted therapies for ER+/PR+ and HER2-amplified breast cancers have greatly improved patient survival, there are no targeted therapies for TNBCs and no effective therapies to treat metastatic disease. There is a need to identify new therapeutic agents and molecular targets for treating TNBCs, but efforts have been limited by a lack of understanding of the subtypes of these heterogeneous diseases. However, gene expression profiling of TNBC patients recently identified 6 molecular subtypes of TNBC and representative cell lines, providing the first opportunity to identify subtype-specific leads for TNBC.

We performed high-content screening to evaluate novel libraries of extracts from Texas plants and diverse fungal cultures for antiproliferative and/or cytotoxic activity in a panel of cell lines modeling five different TNBC molecular subtypes. The aim was to identify extracts with selective activity in a single cell line. We hypothesized that extracts found to have selective activity in one of these cell lines may target a protein or cellular process critical to the growth of that subtype. We identified 11 extracts with selective activity against cell lines representing four different TNBC molecular subtypes. From a fungal culture we identified a new compound called maximiscin, which was found to have selective cytotoxic efficacy against the MDA-MB-468 cell line of the basal-like 1 subtype. From a plant extract we isolated deguelin, which had selective activity in the MDA-MB-453 cell line, a model of the luminal androgen receptor (LAR) subtype.

The molecular mechanisms of action of each compound were investigated in cell line models. Initial cell cycle studies using flow cytometry showed that maximiscin caused an accumulation of cells in G1 after 18h of treatment. Protein microarray studies indicated that maximiscin increased levels of phospho-p53, which was consistent with the observed G1 accumulation. Based on these findings, we hypothesized that maximiscin induces DNA damage and investigated the effects of maximiscin on the phosphorylation of several DNA damage response proteins. Maximiscin increased phosphorylation of Chk1, Chk2, p53 and H2A.X as soon as 2h after treatment, indicating an accumulation of DNA damage.

Previous studies have shown that LAR TNBC cells are particularly sensitive to PI3K inhibitors in vitro compared to other TNBC subtypes. The effects of deguelin on PI3K-Akt-mTORC1 signaling were evaluated in both MDA-MB-453 and MDA-MB-231 cells. Phosphorylation of both ribosomal protein S6 and 4E-BP1 were dramatically reduced in MDA-MB-453 cells 2h after deguelin treatment. Interestingly this was not observed in MDA-MB-231 cells, suggesting inhibition of mTORC1 signaling may be involved in the selective activity of deguelin in MDA-MB-453 cells. Preliminary studies suggest deguelin may also decrease androgen receptor abundance in MDA-MB-453 cells, indicating multiple molecular mechanisms may be involved in its selective effects. These results demonstrate that compounds with selective activity against TNBC subtypes can be identified from nature.

Citation Format: Robles AJ, Du L, Cai S, Risinger AL, Cichewicz RH, Mooberry SL. Identification of compounds from natural sources with selective activity against triple-negative breast cancer molecular subtypes. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-03-04.