Glaucarubulone glucoside from Castela macrophylla suppresses MCF-7 breast cancer cell growth and attenuates benzo[a]pyrene-mediated CYP1A gene induction

Quassinoids often exhibit antioxidant and antiproliferative activity. Emerging evidence suggests that these natural metabolites also display chemopreventive actions. In this study, we investigated the potential for the quassinoid glaucarubulone glucoside (Gg), isolated from the endemic Jamaican plant Castela macrophylla (Simaroubaceae), to display potent cytotoxicity and inhibit human cytochrome P450s (CYPs), particularly CYP1A enzymes, known to convert polyaromatic hydrocarbons into carcinogenic metabolites. Gg reduced the viability of MCF-7 breast adenocarcinoma cells (IC₅₀ = 121 nm) to a greater extent than standard of care anticancer agents 5-fluorouracil, tamoxifen (IC₅₀ >10 μm) and the tamoxifen metabolite 4-hydroxytamoxifen (IC₅₀ = 2.6 μm), yet was not cytotoxic to non-tumorigenic MCF-10A breast epithelial cells. Additionally, Gg induced MCF-7 breast cancer cell death. Gg blocked increases in reactive oxygen species in MCF-10A cells mediated by the polyaromatic hydrocarbon benzo[a]pyrene (B[a]P) metabolite B[a]P 1,6-quinone, yet downregulated the expression of genes that promote antioxidant activity in MCF-7 cells. This implies that Gg exhibits antioxidant and cytoprotective actions in non-tumorigenic breast epithelial cells and pro-oxidant, cytotoxic actions in breast cancer cells. Furthermore, Gg inhibited the activities of human CYP1A according to non-competitive kinetics and attenuated the ability of B[a]P to induce CYP1A gene expression in MCF-7 cells. These data indicate that Gg selectively suppresses MCF-7 breast cancer cell growth without impacting non-tumorigenic breast epithelial cells and blocks B[a]P-mediated CYP1A induction. Taken together, our data provide a rationale for further investigations of Gg and similar plant isolates as potential agents to treat and prevent breast cancer. Copyright © 2017 John Wiley & Sons, Ltd.

Abstract 2565: Aryl hydrocarbon receptor agonist 5F 203 induces oxidative stress triggering DNA damage and cytoglobin up-regulation in human breast cancer cells

Breakthroughs are needed in breast cancer therapy to improve clinical outcomes. Emerging evidence suggests that tumorigenesis stems, in part, from epigenetically silenced tumor suppressor genes (TSGs) and restoring TSGs may represent a viable strategy to treat breast cancer. We previously found that aryl hydrocarbon receptor (AhR) agonist 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203) exhibits potent cytotoxicity, increases reactive oxygen species (ROS), induces DNA damage and up-regulates the expression of putative tumor suppressor gene cytoglobin in breast cancer cells. In the current study, we seek to delineate the mechanism by which 5F 203 induces DNA damage and cytoglobin expression in susceptible breast cancer cells. We found that 5F 203 activated p38 mitogen activated protein kinase (p38) and c-Jun-N terminal kinase (JNK) signaling in breast cancer cells. Pretreatment with antioxidant N-acetyl-L-cysteine or AhR inhibitor α-naphthoflavone diminished 5F 203-mediated p38 or JNK activation in a cell context-dependent fashion. Pretreatment with pharmacological inhibitors of p38 or JNK suppressed 5F 203-mediated increases in intracellular ROS to suggest the presence of a positive feedback loop. 5F 203 induced oxidative DNA damage in breast cancer cells but not breast epithelial MCF-10A cells unlike AhR agonist benzo[a]pyrene which induced oxidative DNA damage more indiscriminately. Pretreatment with p38 or JNK inhibitors suppressed 5F 203-induced single strand break formation and cytoglobin mRNA expression in breast cancer cells. Our data show 5F 203 confers anticancer activity in breast cancer cells in part by increasing ROS via a positive feedback loop to sustain p38 and JNK activation resulting in DNA damage and cytoglobin restoration.

Citation Format: Leah K. Rowland, Lancelot S. McLean, Petreena Campbell, Cheri N. Watkins, Dain Zylstra, Louisa H. Amis, Maheswari Senthil, Eileen Brantley. Aryl hydrocarbon receptor agonist 5F 203 induces oxidative stress triggering DNA damage and cytoglobin up-regulation in human breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2565. doi:10.1158/1538-7445.AM2015-2565

Aryl Hydrocarbon Receptor Ligand 5F 203 Induces Oxidative Stress That Triggers DNA Damage in Human Breast Cancer Cells

Breast tumors often show profound sensitivity to exogenous oxidative stress. Investigational agent 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F 203) induces aryl hydrocarbon receptor (AhR)-mediated DNA damage in certain breast cancer cells. Since AhR agonists often elevate intracellular oxidative stress, we hypothesize that 5F 203 increases reactive oxygen species (ROS) to induce DNA damage, which thwarts breast cancer cell growth. We found that 5F 203 induced single-strand break formation. 5F 203 enhanced oxidative DNA damage that was specific to breast cancer cells sensitive to its cytotoxic actions, as it did not increase oxidative DNA damage or ROS formation in nontumorigenic MCF-10A breast epithelial cells. In contrast, AhR agonist and procarcinogen benzo[a]pyrene and its metabolite, 1,6-benzo[a]pyrene quinone, induced oxidative DNA damage and ROS formation, respectively, in MCF-10A cells. In sensitive breast cancer cells, 5F 203 activated ROS-responsive kinases: c-Jun-N-terminal kinase (JNK) and p38 mitogen activated protein kinase (p38). AhR antagonists (alpha-naphthoflavone, CH223191) or antioxidants (N-acetyl-l-cysteine, EUK-134) attenuated 5F 203-mediated JNK and p38 activation, depending on the cell type. Pharmacological inhibition of AhR, JNK, or p38 attenuated 5F 203-mediated increases in intracellular ROS, apoptosis, and single-strand break formation. 5F 203 induced the expression of cytoglobin, an oxidative stress-responsive gene and a putative tumor suppressor, which was diminished with AhR, JNK, or p38 inhibition. Additionally, 5F 203-mediated increases in ROS production and cytoglobin were suppressed in AHR100 cells (AhR ligand-unresponsive MCF-7 breast cancer cells). Our data demonstrate 5F 203 induces ROS-mediated DNA damage at least in part via AhR, JNK, or p38 activation and modulates the expression of oxidative stress-responsive genes such as cytoglobin to confer its anticancer action.