Abstract 1431: Autophagic degradation of the endoplasmic reticulum leads to reduced phospholipid biosynthesis in mutant isocitrate dehydrogenase 1 gliomas
Authors: Viswanath Pavithra , Pieper Russell O. , Phillips Joanna J. , Ronen Sabrina M. 
Published: August 2018

Abstract
Introduction: Cancer cells up-regulate phospholipid biosynthesis in order to meet the demands of uncontrolled cell proliferation. Unusually, mutant isocitrate dehydrogenase 1 (IDHmut) gliomas down-regulate levels of the phospholipid precursors phosphocholine and phosphoethanolamine relative to wild-type IDH1 (IDHwt) gliomas. The goal of this study was therefore to investigate whether the phospholipids phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdE) are also down-regulated in IDHmut gliomas.
Methods: Immortalized normal human astrocytes and U87 cells expressing IDHwt or IDHmut enzyme were studied. Steady-state phospholipid levels were measured by 31P-magnetic resonance spectroscopy (MRS) of cell extracts. De novo phospholipid biosynthesis was quantified by 13C-MRS of cells labeled with 56μM [1,2-13C]-choline and [1,2-13C]-ethanolamine. ER area was measured by STORM imaging of calnexin fluorescence.
Results: Steady-state PtdCho and PtdE levels and de novo phospholipid biosynthesis were significantly reduced in IDHmut cells relative to IDHwt. Concomitantly, the activities of CTP:PC cytidylyltransferase (CCT) and CTP:PE cytidylyltransferase (ECT), the rate-limiting enzymes for phospholipid biosynthesis, were significantly reduced in IDHmut cells. CCT and ECT are localized to the endoplasmic reticulum (ER), the site of phospholipid biosynthesis. We therefore measured ER area by super-resolution STORM imaging and found a significant reduction in IDHmut cells relative to IDHwt while cell size remained unchanged. Autophagy of the ER (ER-phagy) can regulate ER area and we found that IDHmut cells showed increased expression of FAM134b, the ER-phagy receptor and higher autophagic flux. These results suggest that IDHmut cells undergo ER-phagy, leading to reduced phospholipid biosynthesis. Silencing FAM134b or the autophagy proteins Atg5 and Atg7 restored ER area and phospholipid biosynthesis and
abrogated clonogenicity of IDHmut cells, linking ER-phagy to reduced phospholipid biosynthesis and pointing to a therapeutic opportunity. In the absence of specific ER-phagy inhibitors, we used the late-stage autophagy inhibitor chloroquine (CQ) to test proof-of-principle and found that CQ significantly inhibited orthotopic IDHmut glioma growth in vivo. Importantly, we observed ER-phagy and down-regulation of phospholipid levels in IDHmut glioma patient biopsies, highlighting the translational validity of our findings.
Conclusions: We find that down-regulation of phospholipid biosynthesis is a unique metabolic outcome of ER-phagy in IDHmut gliomas. While autophagy has been linked to glucose and glutamine metabolism, this study is the first to link autophagy to phospholipid biosynthesis. Importantly, our study identifies a rationale for the development of novel therapies targeting ER-phagy in IDHmut gliomas.
Citation Format: Pavithra Viswanath, Russell O. Pieper, Joanna J. Phillips, Sabrina M. Ronen. Autophagic degradation of the endoplasmic reticulum leads to reduced phospholipid biosynthesis in mutant isocitrate dehydrogenase 1 gliomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1431.