Laboratory of Molecular Cell Biology, Research Institute for Diseases of Old Age, Juntendo University Graduate School of Medicine, 2-1-1 Hongo, Bunkyo, Tokyo, 113-8421, Japan.
Department of Anatomy and Molecular Cell Biology, Nagoya University Graduate School of Medicine, 65 Tsurumai, Showa, Nagoya, 466-8550, Japan.
Nat Commun. 2020 Sep 8;11(1):4480. doi: 10.1038/s41467-020-18153-w.
Macroautophagy initiates by formation of isolation membranes, but the source of phospholipids for the membrane biogenesis remains elusive. Here, we show that autophagic membranes incorporate newly synthesized phosphatidylcholine, and that CTP:phosphocholine cytidylyltransferase β3 (CCTβ3), an isoform of the rate-limiting enzyme in the Kennedy pathway, plays an essential role. In starved mouse embryo fibroblasts, CCTβ3 is initially recruited to autophagic membranes, but upon prolonged starvation, it concentrates on lipid droplets that are generated from autophagic degradation products. Omegasomes and isolation membranes emanate from around those lipid droplets. Autophagy in prolonged starvation is suppressed by knockdown of CCTβ3 and is enhanced by its overexpression. This CCTβ3-dependent mechanism is also present in U2OS, an osteosarcoma cell line, and autophagy and cell survival in starvation are decreased by CCTβ3 depletion. The results demonstrate that phosphatidylcholine synthesis through CCTβ3 activation on lipid droplets is crucial for sustaining autophagy and long-term cell survival.
自噬体通过形成隔离膜起始，但膜生物发生的磷脂来源仍不清楚。在这里，我们表明自噬体膜包含新合成的磷脂酰胆碱，并且 CTP：磷酸胆碱胞苷转移酶β3（CCTβ3）是肯尼思途径中限速酶的同工型，起着至关重要的作用。在饥饿的小鼠胚胎成纤维细胞中，CCTβ3 最初被募集到自噬体膜，但在长期饥饿后，它集中在由自噬降解产物产生的脂滴上。类小体和隔离膜从这些脂滴周围发出。CCTβ3 的敲低抑制了长期饥饿中的自噬，而过表达则增强了自噬。在成骨肉瘤细胞系 U2OS 中也存在这种依赖 CCTβ3 的机制，并且 CCTβ3 的消耗减少了饥饿中的自噬和细胞存活。结果表明，通过 CCTβ3 在脂滴上的激活合成磷脂酰胆碱对于维持自噬和长期细胞存活至关重要。
