Abstract
Hepatocyte nuclear factor 4α (HNF4α) has essential roles in controlling the expression of a variety of genes involved in key metabolic pathways, including gluconeogenesis in the liver. The mechanistic and physiological significance of peroxisome proliferator-activated receptor gamma co-activator-1α (PGC-1α) for HNF4α-mediated transcriptional activation models for gluconeogenic genes is well characterized. However, the transcriptional repression of HNF4α for those genes remains to be examined. In this study, we applied novel proteomic techniques to evaluate the interactions of HNF4α, including those with biochemically labile binding proteins. Based upon our experiments, we identified interferon regulatory factor 2 binding protein 2 (IRF2BP2) as a novel HNF4α co-repressor. This interaction could not be detected by conventional immunoprecipitation. IRF2BP2 repressed the transcriptional activity of HNF4α dependent on its E3 ubiquitin ligase activity. Deficiency of the IRF2BP2 gene in HepG2 cells induced gluconeogenic genes comparable to that of forskolin-treated wild-type HepG2 cells. Together, these results suggest that IRF2BP2 represents a novel class of nuclear receptor co-regulator.