Underexpressed coactivators PGC1 α and SRC1 impair hepatocyte nuclear factor 4α function and promote dedifferentiation in human hepatoma cells

Hepatocyte nuclear factor 4 alpha ( HNF4 alpha) 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 alpha 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 alpha, SRC1, SRC2, and PCAF) might account for the lack of function of HNF4 alpha in HepG2 cells. Among them, PGC1 alpha and SRC1 are the two most important HNF4 alpha coactivators as revealed by reporter assays with an ApoCIII 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 alpha by recombinant adenoviruses led to a significant up- regulation of well characterized HNF4 alpha - 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 alpha was seen to be dependent on HNF4 alpha. Finally, insulin treatment of human hepatocytes and HepG2 cells caused repression of PGC1 alpha and a concomitant down- regulation of ApoCIII, PEPCK, AldoB, and OTC. Altogether, our results suggest that SRC1, and notably PGC1 alpha, are key coactivators for the proper function of HNF4 alpha in human liver and for an integrative control of multiple hepatic genes involved in metabolism and homeostasis. The down- regulation of key HNF4 alpha coactivators could be a determinant factor for the dedifferentiation of human hepatomas.