De novo phosphatidylcholine synthesis is required for autophagosome membrane formation and maintenance during autophagy.

ABSTRACT

Macroautophagy/autophagy can enable cancer cells to withstand cellular stress and maintain bioenergetic homeostasis by sequestering cellular components into newly formed double-membrane vesicles destined for lysosomal degradation, potentially affecting the efficacy of anti-cancer treatments. Using 13C-labeled choline and 13C-magnetic resonance spectroscopy and western blotting, we show increased de novo choline phospholipid (ChoPL) production and activation of PCYT1A (phosphate cytidylyltransferase 1, choline, alpha), the rate-limiting enzyme of phosphatidylcholine (PtdCho) synthesis, during autophagy. We also discovered that the loss of PCYT1A activity results in compromised autophagosome formation and maintenance in autophagic cells. Direct tracing of ChoPLs with fluorescence and immunogold labeling imaging revealed the incorporation of newly synthesized ChoPLs into autophagosomal membranes, endoplasmic reticulum (ER) and mitochondria during anticancer drug-induced autophagy. Significant increase in the colocalization of fluorescence signals from the newly synthesized ChoPLs and mCherry-MAP1LC3/LC3 (microtubule-associated protein 1 light chain 3) was also found on autophagosomes accumulating in cells treated with autophagy-modulating compounds. Interestingly, cells undergoing active autophagy had an altered ChoPL profile, with longer and more unsaturated fatty acid/alcohol chains detected. Our data suggest that de novo synthesis may be required to increase autophagosomal ChoPL content and alter its composition, together with replacing phospholipids consumed from other organelles during autophagosome formation and turnover. This addiction to de novo ChoPL synthesis and the critical role of PCYT1A may lead to development of agents targeting autophagy-induced drug resistance. In addition, fluorescence imaging of choline phospholipids could provide a useful way to visualize autophagosomes in cells and tissues. ABBREVIATIONS AKT AKT serine/threonine kinase; BAX BCL2 associated X, apoptosis regulator; BECN1 beclin 1; ChoPL choline phospholipid; CHKA choline kinase alpha; CHPT1 choline phosphotransferase 1; CTCF corrected total cell fluorescence; CTP cytidine-5'-triphosphate; DCA dichloroacetate; DMEM dulbeccos modified Eagles medium; DMSO dimethyl sulfoxide; EDTA ethylenediaminetetraacetic acid; ER endoplasmic reticulum; GDPD5 glycerophosphodiester phosphodiesterase domain containing 5; GFP green fluorescent protein; GPC glycerophosphorylcholine; HBSS hanks balances salt solution; MAP1LC3/LC3 microtubule associated protein 1 light chain 3; LPCAT1 lysophosphatidylcholine acyltransferase 1; LysoPtdCho lysophosphatidylcholine; MRS magnetic resonance spectroscopy; MTORC1 mechanistic target of rapamycin kinase complex 1; PCho phosphocholine; PCYT choline phosphate cytidylyltransferase; PLA2 phospholipase A2; PLB phospholipase B; PLC phospholipase C; PLD phospholipase D; PCYT1A phosphate cytidylyltransferase 1, choline, alpha; PI3K phosphoinositide-3-kinase; pMAFs pancreatic mouse adult fibroblasts; PNPLA6 patatin like phospholipase domain containing 6; Pro-Cho propargylcholine; Pro-ChoPLs propargylcholine phospholipids; PtdCho phosphatidylcholine; PtdEth phosphatidylethanolamine; PtdIns3P phosphatidylinositol-3-phosphate; RPS6 ribosomal protein S6; SCD stearoyl-CoA desaturase; SEM standard error of the mean; SM sphingomyelin; SMPD1/SMase sphingomyelin phosphodiesterase 1, acid lysosomal; SGMS sphingomyelin synthase; WT wild-type.