Involvement of ACSL3 in the formation of autophagosomes and lipid droplets during starvation conditions

Acyl-CoA synthetase long-chain (ACSL) catalyzes the conversion of fatty acids into acyl-CoA, which is used for neutral lipid and phospholipid synthesis. Previous studies revealed that yeast Faa1 and mammalian ACSL4 play a crucial role in phagophore expansion by locally synthesizing phospholipids. We found that another member of ACSL protein family, ACSL3, which is involved in lipid droplet biogenesis under energy-rich conditions and is regulated by SYNTAXIN17, also participates in autophagosome formation, but in a different manner. Knockdown of ACSL3 suppressed punctum formation of early autophagosomal marker proteins such as FIP200 and WIPI2 in starved cells, generating nonfunctional multi-membrane autophagosome-like structures. In contrast, ACSL4 suppression blocked autophagosome formation without affecting punctum formation of early autophagosomal marker proteins. Mechanistic analysis revealed that ACSL3 functions independently of its enzymatic activity, while catalytic activity of ACSL4 is required for autophagosome formation as well as LC3 (known as MAP1LC3 proteins) protein lipidation. Furthermore, ACSL3 has been shown to be essential for lipid droplet biogenesis during starvation. These findings establish ACSL3 as a key player in two events in early autophagy: formation of autophagosomes and lipid droplets.