Ppar Is Involved in the Transcriptional Regulation of Liver LC-PUFA Biosynthesis by Targeting the 65 Fatty Acyl Desaturase Gene in the Marine Teleost Siganus canaliculatus

As the first marine teleost demonstrated to have the ability of long-chain polyunsaturated fatty acids (LC-PUFA) biosynthesis from C-18 PUFA precursors, the rabbitfish Siganus canaliculatus provides us a unique model for clarifying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts aiming at the replacement of dietary fish oil (rich in LC-PUFA) with vegetable oils (rich in C-18 PUFA precursors but devoid of LC-PUFA). In the study of transcription regulation of gene encoding the 65 fatty acyl desaturase (65 Fads), a rate-limiting enzyme catalyzing the first step of LC-PUFA biosynthesis in rabbitfish, a binding site for the transcription factor (TF), peroxisome proliferator-activated receptor (Ppar), was predicted in 65 fads2 promoter by bioinformatics analysis, and thus the present study focused on the regulatory roles of Ppar on 65 fads2. First, the activity of the 65 fads2 promoter was proved to be downregulated by ppar overexpression and upregulated by treatment of Ppar antagonist (GW9662), respectively, in HEK 293T cells with the dual luciferase reporter assay. Ppar was further confirmed to interact with the promoter by electrophoretic mobility shift assay. Moreover, in S. canaliculatus hepatocyte line (SCHL) cells, GW9662 decreased the expression of ppar together with increase of 65 fads2 mRNA. Besides, 65 fads2 expression was increased by ppar RNAi knockdown and reduced by its mRNA overexpression. Furthermore, knockdown of ppar induced a high conversion of 18:3n-3 to 18:4n-3 and 18:2n-6 to 18:3n-6, while ppar mRNA overexpression led to a lower conversion of that, and finally a significant decrease of 20:4n-6(ARA), 20:5n-3(EPA), and 22:6n-3(DHA) production. The results indicate that Ppar is involved in the transcriptional regulation of liver LC-PUFA biosynthesis by targeting 65 fads2 in rabbitfish, which is the first report of Ppar involvement in the regulation of LC-PUFA biosynthesis in teleosts.