Hnf4α is involved in the regulation of vertebrate LC-PUFA biosynthesis: insights into the regulatory role of Hnf4α on expression of liver fatty acyl desaturases in the marine teleost Siganus canaliculatus

Long-chain polyunsaturated fatty acid (LC-PUFA) biosynthesis is an important metabolic pathway in vertebrates, especially fish, considering they are the major source of n-3 LC-PUFA in the human diet. However, most fish have only limited capability for biosynthesis of LC-PUFA. The rabbitfish (Siganus canaliculatus) is able to synthesize LC-PUFA as it has all the key enzyme activities required including Delta 6 Delta 5 Fads2, Delta 4 Fads2, Elovl5, and Elovl4. We previously reported a direct interaction between the transcription factor Hnf4 alpha and the promoter regions of Delta 4 and Delta 6 Delta 5 Fads2, which suggested that Hnf4 alpha was involved in the transcriptional regulation of fads2 in rabbitfish. For functionally investigating it further, a full-length cDNA of 1736-bp-encoding rabbitfish Hnf4 alpha with 454 amino acids was cloned, which was highly expressed in intestine, followed by liver and eyes. Similar to the expression characteristics of its target genes Delta 4 and Delta 6 Delta 5 fads2, levels of hnf4 alpha mRNA in liver and eyes were higher in fish reared at low salinity than those reared in high salinity. After the rabbitfish primary hepatocytes were, respectively, incubated with alverine, benfluorex or BI6015, which were anticipated agonists or antagonist for Hnf4 alpha, the mRNA level of Delta 6 Delta 5 and Delta 4 fads2 displayed a similar change tendency with that of hnf4 alpha mRNA. Furthermore, when the mRNA level of hhf4 alpha was knocked down using siRNA, the expression of Delta 6 Delta 5 and Delta 4 fads2 also decreased. Together, these data suggest that Hnf4 alpha is involved in the transcriptional regulation of LC-PUFA biosynthesis, specifically, by targeting Delta 4 and Delta 6 Delta 5 fads2 in rabbitfish.