Abstract 891: Platinum-induced fatty acids mediate resistance to DNA damaging agents via homologous recombination.
Authors: Daenen Laura GM , Houthuijzen Julia M. , Kruse Fabian , Kersbergen Ariena , Roodhart Jeanine M.L. , Rottenberg Sven , Voest Emile E. 
Published: August 2014

Abstract
Background: We recently identified two fatty acids (PIFAs) that induced chemoresistance in mouse models. At picomolar concentrations, PIFAs 16:4(n-3) and KHT interfered with the antitumor activity of a broad spectrum of chemotherapeutics. Our aim was to unravel the mechanism through which PIFAs prevent chemotherapy-induced tumor cell death. Results: In several animal models, PIFAs were found to interfere with early effects of chemotherapeutics. Four hours after administration of cisplatin significant decreases in proliferation (BrdU) and increases in DNA damage (γH2AX-positive cells) were found in tumors. In contrast, combined therapy with cisplatin and PIFAs normalized proliferation and decreased DNA damage. In cisplatin-treated tumors 31±7% cells were γH2AX-positive as opposed to 20±5% in tumors treated with cisplatin and PIFAs (p&lt;0.01). Similar results were found following exposure of tumor-bearing mice to the topoisomerase inhibitor irinotecan and PIFAs. To study whether the effects of PIFAs were specific to DNA damaging agents, mice were treated with spindle poison paclitaxel or targeted agent sorafenib. PIFAs were not able to interfere with the antitumor activity of these agents, pointing to a DNA damage-dependent mechanism. To analyze whether PIFA-induced decreases in DNA damage following chemotherapy administration were due to prevention of damage or to early repair, the number of γH2AX-positive tumor cells was analyzed at earlier time points after concurrent therapy with cisplatin and PIFAs. In the presence of PIFAs, initial DNA damage was followed by a decrease in the number of γH2AX-positive cells as early as 2 hours after chemotherapy. In the absence of PIFAs DNA damage accumulated beyond this point. We next administered PIFAs 3 hours after chemotherapy and found that also when DNA damage had already been established, PIFAs were able to induce chemoresistance. This corresponded with a decrease of γH2AX-positive cells 4 hours after PIFA administration from 32±3% in tumors treated with cisplatin to 23±2% in the combined therapy group (p&lt;0.01). To prove that PIFAs function via DNA damage repair, we performed experiments in a validated genetically engineered mouse model for BRCA1-deficient breast cancer. Tumors arising in these animals lack DNA repair by homologous recombination (HR). In this model PIFAs were not able to cause chemoresistance, and tumor endpoints were reached after 29 days in the presence or absence of PIFAs. However, partial restoration of HR by concurrent loss of 53BP1 was sufficient to restore PIFA-induced chemoresistance. Tumor endpoints were reached 17 days after therapy in the mice receiving combined treatment as opposed to 22 days in the cisplatin-treated tumors (p&lt;0.001). Conclusion: We show that PIFAs specifically protect tumor cells against DNA damaging chemotherapeutic agents through a mechanism involving functional homologous recombination.
Citation Format: Laura GM Daenen, Julia M. Houthuijzen, Fabian Kruse, Ariena Kersbergen, Jeanine M.L. Roodhart, Sven Rottenberg, Emile E. Voest. Platinum-induced fatty acids mediate resistance to DNA damaging agents via homologous recombination. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 891. doi:10.1158/1538-7445.AM2013-891