Hepatocyte nuclear factor 4α transactivates the mitochondrial alanine aminotransferase gene in the kidney of Sparus aurata

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 Δ6Δ5 Fads2, Δ4 Fads2, Elovl5, and Elovl4. We previously reported a direct interaction between the transcription factor Hnf4α and the promoter regions of Δ4 and Δ6Δ5 Fads2, which suggested that Hnf4α was involved in the transcriptional regulation of fads2 in rabbitfish. For functionally investigating it further, a full-length cDNA of 1736-bp-encoding rabbitfish Hnf4α 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 Δ4 and Δ6Δ5 fads2, levels of hnf4α 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 Hnf4α, the mRNA level of Δ6Δ5 and Δ4 fads2 displayed a similar change tendency with that of hnf4α mRNA. Furthermore, when the mRNA level of hhf4α was knocked down using siRNA, the expression of Δ6Δ5 and Δ4 fads2 also decreased. Together, these data suggest that Hnf4α is involved in the transcriptional regulation of LC-PUFA biosynthesis, specifically, by targeting Δ4 and Δ6Δ5 fads2 in rabbitfish.

Activating transcription factor 4 mediates up-regulation of alanine aminotransferase 2 gene expression under metabolic stress

Alanine aminotransferase (ALT) provides a molecular link between carbohydrate and amino acid metabolism. In humans, two ALT isoforms have been characterized: ALT1, cytosolic, and ALT2, mitochondrial. To gain insight into the transcriptional regulation of the ALT2 gene, we cloned and characterized the human ALT2 promoter. 5'-deletion analysis of ALT2 promoter in transiently transfected HepG2 cells and site-directed mutagenesis allowed us to identify ATF4 as a new factor involved in the transcriptional regulation of ALT2 expression. Quantitative RT-PCR assays showed that the metabolic stressors histidinol and tunicamycin increased ATF4 levels and up-regulated ALT2 in HepG2 and Huh7 cells. Consistently, knock-down of ATF4 decreased ALT2 mRNA levels in HepG2 and Huh-7 cells. Moreover, ATF4 silencing prevented the activating effect of histidinol and tunicamycin on ATF4 and ALT2 expression. Our findings point to ALT2 as an enzyme involved in the metabolic adaptation of the cell to stress.