Tumor treating fields (TTFields) are an established anti-neoplastic treatment modality in patients with glioblastoma. TTFields are delivered via noninvasive application of low-intensity, intermediate-frequency, alternating electric fields to the region of the tumor. Previous studies have shown that TTFields treatment lead to increased cellular granularity, which is often associated with autophagy. Autophagy has been shown to regulate cell survival and proliferation under stress conditions and to influence cellular response to cytotoxic drugs. This study evaluated the role of autophagy in cancer cells treated with TTFields.

Immunoblot analysis showed significant elevations in levels of lipidated Microtubule Associated Protein Light Chain (LC3-II) in TTFields-treated glioma and lung cancer cells. Increased autophagy following TTFields application in these cell lines was also detected using fluorescence microscopy, where punctate distribution of LC3-II was observed. TEM micrographs demonstrated the presence of autophagy typical, autophagosome-like structures in TTFields-treated U-87 MG cells. Combination of TTFields with autophagy inhibitors, chloroquine and melfoquine, resulted in a significant dose-dependent reduction in cell growth compared with TTFields treatment alone. Inhibition of autophagy with chloroquine triggered apoptosis as indicated by elevated levels of AnnexinV/7AAD double staining. Increased levels of autophagic flux in TTFields-treated cells were not associated with reduced mTOR activity, which was monitored by p70S6K phosphorylation immunoblot analysis.

TTFields are known to exert anti-mitotic effects by disrupting highly dipolar structures that play critical roles in mitosis. Our results demonstrate that TTFields additionally induce cellular autophagy by an mTOR-independent mechanism. TTFields- treated cell lines appear to utilize autophagy as a survival mechanism. Thus, inhibition of autophagy sensitizes tumor cells to TTFields treatment, resulting in elevated apoptotic cell death.

Future studies are warranted to examine the extent to which TTFields-elicited autophagy may affect treatment outcomes and to investigate the therapeutic implications of combining TTFields with autophagy inhibitors in vivo.

Citation Format: Yaara Porat, Anna Shteingauz, Moshe Giladi, Rosa S. Schneiderman, Tali Voloshin, Mijal Munster, Roni Blat, Eilon D. Kirson, Uri Weinberg, Yoram Palti. Cancer cells upregulate autophagy as a survival mechanism in response to tumor treating fields (TTFields) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3315. doi:10.1158/1538-7445.AM2017-3315