Synergistic regulation of hepatic Fsp27b expression by HNF4α and CREBH.

The CIDE (cell death-inducing DFF45-like effector) family composed of CIDEA, CIDEB, CIDEC/FSP27 (fat-specific protein 27), has a critical role in growth of lipid droplets. Of these, CIDEB and CIDEC2/FSP27B are abundant in the liver, and the steatotic livers, respectively. Hepatocyte nuclear factor 4α (HNF4α) has an important role in lipid homeostasis because liver-specific HNF4α-null mice (Hnf4aΔHep mice) exhibit hepatosteatosis. We investigated whether HNF4α directly regulates expression of CIDE family genes. Expression of Cideb and Fsp27b was largely decreased in Hnf4aΔHep mice, while expression of Cidea was increased. Similar results were observed only in CIDEC2, the human orthologue of the Fsp27b, in human hepatoma cell lines in which HNF4α expression was knocked down. Conversely, overexpression of HNF4α strongly induced CIDEC2 expression in hepatoma cell lines. Furthermore, HNF4α transactivated Fsp27b by direct binding to an HNF4α response element in the Fsp27b promoter. In addition, Fsp27b is known to be transactivated by CREBH that is regulated by HNF4α, and expression of CREBH was induced by HNF4α in human hepatoma cells. Co-transfection of HNF4α and CREBH resulted in synergistic transactivation and induction of Fsp27b compared to that of HNF4α or CREBH alone. These results suggest that HNF4α, in conjunction with CREBH, plays an important role in regulation of Fsp27b expression.

An HNF4α-microRNA-194/192 signaling axis maintains hepatic cell function.

Hepatocyte nuclear factor 4α (HNF4α) controls the expression of liver-specific protein-coding genes. However, some microRNAs are also modulated by HNF4α, and it is not known whether they are direct targets of HNF4α and whether they influence hepatic function. In this study, we found that HNF4α regulates microRNAs, indicated by marked down-regulation of miR-194 and miR-192 (miR-194/192) in liver-specific Hnf4a-null (Hnf4aΔH) mice. Transactivation of the shared miR-194/192 promoter was dependent on HNF4α expression, indicating that miR-194/192 is a target gene of HNF4α. Screening of potential mRNAs targeted by miR-194/192 revealed that expression of genes involved in glucose metabolism (glycogenin 1 (Gyg1)), cell adhesion and migration (activated leukocyte cell adhesion molecule (Alcam)), tumorigenesis and tumor progression (Rap2b and epiregulin (Ereg)), protein SUMOylation (Sumo2), epigenetic regulation (Setd5 and Cullin 4B (Cln4b)), and the epithelial-mesenchymal transition (moesin (Msn)) was up-regulated in Hnf4aΔH mice. Moreover, we also found that miR-194/192 binds the 3'-UTR of these mRNAs. siRNA knockdown of HNF4α suppressed miR-194/192 expression in human hepatocellular carcinoma (HCC) cells and resulted in up-regulation of their mRNA targets. Inhibition and overexpression experiments with miR-194/192 revealed that Gyg1, Setd5, Sumo2, Cln4b, and Rap2b are miR-194 targets, whereas Ereg, Alcam, and Msn are miR-192 targets. These findings reveal a novel HNF4α network controlled by miR-194/192 that may play a critical role in maintaining the hepatocyte-differentiated state by inhibiting expression of genes involved in dedifferentiation and tumorigenesis. These insights may contribute to the development of diagnostic markers for early HCC detection, and targeting of the miR-194/192 pathway could be useful for managing HCC.