C18ORF32 modulates lipid droplet secretion via secretory autophagy and restrains hepatic steatosis in vivo

Lipid droplets (LDs) are essential for maintaining cellular homeostasis by storing excess lipids, but regulation of their cellular level is crucial to prevent lipotoxicity under normal conditions. In this study, we identified C18ORF32 as a key player in lipid droplet secretion through secretory autophagy pathway. In a high-throughput imaging-based organellar screening, we found that C18ORF32 primarily localizes in autophagosomes and in the endoplasmic reticulum. C18ORF32 interacts with lipid droplets via its N-terminal helix: an amphiphilic region binds to the monolayer membrane leaflet of the LD, while a hydrophobic segment embeds in the droplet’s core, ensuring stable anchoring. Mutations of the clustered aromatic amino acids of the N-terminal amphiphilic region disrupt the proper folding and LD binding of C18ORF32. Additionally, C18ORF32 associates with secretory autophagosomes by binding to C-terminal coiled-coil region of SEC22B through its unstructured C-terminal region. Knockdown of C18ORF32 impairs LD secretion, leading to increased intracellular LD accumulation and reduced extracellular release of triacylglycerols, suggesting C18ORF32’s critical role in secretory LD turnover. In vivo knockdown of C18ORF32 caused NASH-like increased hepatic lipid retention and decreased circulating free fatty acids, indicating impaired lipid droplet export via secretory autophagy. Our data suggest that C18ORF32 promotes the fusion of lipid droplet membranes with secretory autophagosomes, facilitating the release of LDs to extracellular space, thereby mitigating lipotoxicity under physiological conditions. These findings reveal a novel mechanism by which cells regulate lipid droplet homeostasis through secretory autophagy, with C18ORF32 acting as a critical mediator in lipid droplet trafficking and secretion.