The role of ancient ubiquitous protein 1 (Aup1) in regulating hepatic lipid droplet levels, endoplasmic reticulum stress, and inflammation in zebrafish (Danio rerio).

Excessive supplementation of palm oil (PO) in aquafeeds induces hepatic endoplasmic reticulum (ER) stress and inflammatory responses in fish, while lipid droplet (LD) formation has been demonstrated to alleviate stress and inflammation by sequestering lipotoxic lipids. Studies in mammals indicate that ancient ubiquitous protein 1 (Aup1) is a LD-associated protein participating in ER-associated degradation (ERAD) and LD regulation. However, whether Aup1 is involved in the regulation of hepatic LD metabolism, ER stress, and inflammatory responses has not been elucidated in fish. In this study, we cloned zebrafish (Danio rerio) aup1, conducted sequence analysis, and established in vitro hepatocyte models with Aup1 overexpression and knockdown through electroporation of plasmids. Bioinformatics analysis identified zebrafish Aup1 as an unstable, alkaline, hydrophilic transmembrane protein. Subcellular localization demonstrated dual localization of Aup1 on LDs and the ER. Tissue distribution experiments revealed Aup1 was ubiquitously expressed in multiple tissues, with the highest expression in the liver. RT-qPCR and Western blot showed that PO significantly upregulated Aup1 expression in vivo and in vitro, and dual-luciferase reporter assays identified Atf4a as an important transcriptional activator of zebrafish aup1 promoter. Aup1 overexpression markedly improved LD levels as well as triglyceride (TG) content, and reduced ER stress-related genes and pro-inflammatory cytokines expression in zebrafish liver (ZFL) cells. Conversely, Aup1 knockdown exerted opposite effects. These findings indicated that Aup1 could promote LD biogenesis and mitigate ER stress and inflammation. This study may provide novel insights for developing therapeutic strategies against PO-induced hepatic damage in cultured fish species.