Underexpressed Coactivators PGC1α AND SRC1 Impair Hepatocyte Nuclear Factor 4α Function and Promote Dedifferentiation in Human Hepatoma Cells

Abstract

Hepatocyte nuclear factor 4␣ (HNF4␣) plays critical roles during liver development and in the transcriptional regulation of many hepatic genes in adult liver. Here we have demonstrated that in human hepatoma HepG2 cells, HNF4␣ is expressed at levels as high as in human liver but its activity on target genes is very low or absent. We have discovered that the low expression of key coactivators (PGC1␣, SRC1, SRC2, and PCAF) might account for the lack of function of HNF4␣ in HepG2 cells. Among them, PGC1␣ and SRC1 are the two most important HNF4␣ coactivators as revealed by reporter assays with an Apo-CIII promoter construct. Moreover, the expression of these two coactivators was found to be down-regulated in all human hepatomas investigated. Overexpression of SRC1 and PGC1␣ by recombinant adenoviruses led to a significant up-regulation of well characterized HNF4␣-dependent genes (ApoCIII, ApoAV, PEPCK, AldoB, OTC, and CYP7A1) and forced HepG2 cells toward a more differentiated phenotype as demonstrated by increased ureogenic rate. The positive effect of PGC1␣ was seen to be dependent on HNF4␣. Finally, insulin treatment of human hepatocytes and HepG2 cells caused repression of PGC1␣ and a concomitant down-regulation of ApoCIII, PEPCK, AldoB, and OTC. Altogether, our results suggest that SRC1, and notably PGC1␣, are key coactivators for the proper function of HNF4␣ in human liver and for an integrative control of multiple hepatic genes involved in metabolism and homeostasis. The down-regulation of key HNF4␣ coactivators could be a determinant factor for the dedifferentiation of human hepatomas. Hepatoma cell lines and hepatocellular carcinomas (HCC)3 undergo phenotypic dedifferentiation leading to the loss or low expression of typical hepatic functions such as plasma protein synthesis or xenobiotic detoxification (1-3). Dedifferentiation is a key early event in the pathogenesis of HCC that has been associated with an altered expression of liver-enriched transcription factors (4, 5). Similarly, studies in hepatoma cell lines have revealed that the maintenance of a differentiated hepatic phenotype is dependent on the expression of liver-enriched transcription factor (6, 7).Current research supports the notion that hepatocyte nuclear factor 4␣ (HNF4␣) is one of the most important liverenriched transcription factors for hepatocyte differentiation. HNF4␣ is a highly conserved member of the nuclear receptor superfamily that was initially identified as a factor required for liver-specific gene expression (8). HNF4␣ plays critical roles not only in the specification of the hepatic phenotype during liver development but also in the transcriptional regulation of genes involved in glucose, cholesterol, fatty acids, and xenobiotic metabolism and in the synthesis of blood coagulation factors (9 -12). Disruption of HNF4␣ leads to an early embryonic lethal phenotype associated with a failure of differentiation of visceral endoderm (13). Genome-scale location analysis revealed surprising results for HNF4␣ in hepatocytes. The number...

Direct link between metabolic regulation and the heat-shock response through the transcriptional regulator PGC-1α

“…Because HSF1 is required for several processes that involve normal cell proliferation, such as spleen cell proliferation following immunization (40), muscle regeneration (41), and postnatal growth in C57BL/6 mice (42), it is possible that PGC-1α induction by fasting in liver (4) and muscle (43) represses HSF1 to mediate some of the antiproliferative effects of fasting. These scenarios also may hold true in cancer cells, although the potential effect of PGC-1α on cancer cell growth and survival is controversial, with reports suggesting both positive (44,45) and negative (46)(47)(48)(49)(50) associations between PGC-1α activity and cancer cell survival [including a negative association in HepG2 cells (51)], depending on the cellular context and on PGC-1α expression levels. It thus is possible that PGC-1α effects in cancer depend on a balance between its ability to induce metabolic genes and oxidative stress-response genes, a property that may promote cancer growth, and its ability to repress HSF1 activity.…”

Effects of catechin-rich green tea on gene expression of gluconeogenic enzymes in rat hepatoma H4IIE cells

“…HNF4α gene expression was also down-regulated by GTB, suggesting an association with the down-regulated gene expression of gluconeogenic enzymes. HNF4α is believed to be involved in protein kinase A's regulation of the G6Pase gene (6,19) and has a key role in regulating the PEPCK gene (9,14). Therefore, its down-regulation is expected to cause a decrease in G6Pase and PEPCK expression.…”

CCAAT/Enhancer-binding Protein α (C/EBPα) and Hepatocyte Nuclear Factor 4α (HNF4α) Synergistically Cooperate with Constitutive Androstane Receptor to Transactivate the Human Cytochrome P450 2B6 (CYP2B6) Gene

“…This could result from an altered (mostly repressed) expression of liverenriched transcription factors and co-regulators. We have previously shown that this could be the case for C/EBP␣, HNF4␣, and CAR, three key transcription factors for the hepatic phenotype, which are down-regulated or dysfunctional in human HepG2 cells (16,17,21,27). The functional re-expression of these three factors in HepG2 cells has not only improved the critical pathways associated with a differentiated hepatic phenotype but also activated a number of drug metabolism and disposition enzymes.…”

“…We have previously shown that the re-expression of the coactivator PGC1␣ in HepG2 cells enhances endogenous HNF4␣ activity on particular target genes (17). Consequently, we investigated whether PGC1␣, by coactivating endogenous HNF4␣, is able to improve CYP2B6 expression in cooperation with CAR and C/EBP␣.…”