Abstract A16: A novel death receptor ligand fabclavine inhibits cancer and cancer stem cell proliferation by extrinsic apoptosis
Authors: Kapur Arvinder , Kajla Mayur , Paskewitz Susan , Cappuccini Allegra , Mehta Pooja , Mehta Geeta , Patankar Manish 
Published: September 2020

Abstract
Many antibiotics produced by bacteria also have antineoplastic activity. Examples of such bacterial compounds include actinomycin D, doxorubicin, mitoxantrone, bleomycin, mitomycin, etoposide, and others. Recently, it was identified that Xenorhabdus budapestnesis (Xbu) produces a secondary metabolite, fabclavine, that has antibiotic property. Here, for the first time we demonstrate that fabclavine exhibits potent cytotoxic activity against ovarian cancer cells and ovarian cancer stem cells and demonstrate its mechanism of action, Fabclavine was isolated and purified from cell-free supernatants of Xbu cultures and analyzed by mass spectrometry. Proliferation assays conducted on several ovarian cancer cell lines Ovcar3, OVCA433, ID8, and breast cancer cell lines MCF7 and 4T1 demonstrate that fabclavine inhibits the cancer cell proliferation at nanomolar (250-400nM) concentration. Fabclavine is ten times more potent than cisplatin in inhibiting cell proliferation of ovarian cancer cell line OVCA33 and breast cancer cell line MCF7. We also tested the effect of fabclavine in 3D culture of ovarian cancer cell lines. The results indicate, even in 3D culture, fabclavine killed 40-50% of the cells at 300nM concentration. In addition, fabclavine was also tested against ovarian cancer stem cells isolated from five ovarian cancer patients and grown in 3D culture. Fabclavine inhibited the growth of stem cells by 70-85%. In cancer cell lines fabclavine induced apoptosis in 20-30% cells after 24-hour treatment. Caspase activity assay for caspase3, caspase8, and caspase9 shows a significant increase in caspase3 and caspse8 activity with no change in caspase9 activity, suggesting an activation of the extrinsic apoptosis pathway in the fabclavine-treated cells. This observation was supported by the increase in phospho-FADD upon exposure to fabclavine. When conjugated to agarose beads, fabclavine efficiently precipitated two death receptors, DR4 and DR5, from ovarian and breast cancer cell extracts. These experiments indicate that fabclavine is a novel DR4/5 ligand and should therefore be considered as a death receptor-targeting agent for the treatment of ovarian and other tumors. We will present data on the comparison of DR4/5 binding to fabclavine versus its natural ligand, TRAIL, and will also present data from PDex and mouse xenograft models on the ex vivo and in vivo effects of fabclavine on the proliferation of ovarian tumors and cancer stem cells.
Citation Format: Arvinder Kapur, Mayur Kajla, Susan Paskewitz, Allegra Cappuccini, Pooja Mehta, Geeta Mehta, Manish Patankar. A novel death receptor ligand fabclavine inhibits cancer and cancer stem cell proliferation by extrinsic apoptosis [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr A16.