miR-146a is involved in the regulation of vertebrate LC-PUFA biosynthesis by targeting elovl5 as demonstrated in rabbitfish Siganus canaliculatus

Rabbitfish Siganus canaliculatus is the first marine teleost demonstrated to have the ability to synthesize long-chain polyunsaturated fatty acids (LC-PUFA) from C-18 PUFA precursors, and thus provides us a unique model for studying the regulatory mechanisms of LC-PUFA biosynthesis in teleosts. MicroRNAs (miRNAs) were shown to play important roles in the regulation of LC-PUFA biosynthesis in rabbitfish at posttranscriptional level in our previous studies. Here, we focused the roles of miR-146a in such regulation. The expression of miR-146a displayed an inverse pattern with that of elongase 5 (Elovl5), a key enzyme catalyzing the elongation of C-18 (18:4n3 and 18:3n6) and C-20 (20:5n3 and 20:4n6) PUFA in the LC-PUFA biosynthesis, in vivo in liver of rabbitfish reared under different salinities, as well as in vitro in S. canaliculatus hepatocyte line (SCHL) cells incubated with different fatty acids. Bioinformatics analysis predicted that miR-146a may target the 3'UTR of elovl5 directly, which was confirmed by the dual luciferase reporter assays in HEK 293T cells. Overexpression of miR-146a significantly downregulated the expression of elovl5 in SCHL cells, while knockdown of miR-146a showed an opposite effect. Moreover, up-regulation of miR-146a in SCHL cells significantly suppressed the elongation indexes 20:3n6/18:3n6, 20:4n3/18:4n3 and 22:5n3/20:5n3 associated with Elovl5 catalyzing activity, and consequently reduced the contents of LC-PUFA. These results indicate that miR-146a is involved in the regulation of LC-PUFA biosynthesis through inhibiting the mRNA expression and activity of Elovl5 in rabbitfish, which was for the first time to focus on the role of miR-146a in LC-PUFA biosynthesis in vertebrates and will provide a new insight into the regulatory mechanisms of LC-PUFA biosynthesis in teleosts.