Mitogen-Activated Protein Kinase-Mediated Disruption of Enhancer-Promoter Communication Inhibits Hepatocyte Nuclear Factor 4α Expression

Abstract

Hepatocyte nuclear factor 4 (HNF-4) is a key member of the transcription factor network regulating hepatocyte differentiation and function. Activation of the HNF-4 gene involves physical interaction between a distant enhancer and the proximal promoter region, bound by distinct sets of transcription factors. Here we report that, upon mitogen-activated protein (MAP) kinase activation, HNF-4 expression is downregulated in human hepatoma cells. This effect is mediated by the loss of CEBPα expression. During MAP kinase signaling, the recruitment of HNF-3β and HNF-1α to the HNF-4 enhancer and RNA polymerase II to the proximal HNF-4 promoter was compromised. CBP, Brg1, and TFIIB were also dissociated from the HNF-4 regulatory regions, and the enhancer-promoter complex was disrupted. Interestingly, the extent of nucleosome acetylation did not decrease at either regulatory region, and HNF-6 and HNF-1α, as well as components of the TFIID, remained associated with the proximal promoter during the repressed state. The results point to an absolute requirement of enhancer-promoter communication for maintaining the active state of the HNF-4 gene and provide evidence for a molecular bookmarking mechanism, which may contribute to the prevention of permanent silencing of the locus during the repressed state.

We thank G. Mavrothalassitis for providing the Raf-BXB expression vector and N. Katrakili for technical assistance.

This study was supported by a fellowship from the Onassis Foundation to I.K. and grants from GSRT (PENED-01ED509 and PENED-03ED542) and EU (LSHG-CT-2004-502950 and MTKD-CT2005 029610).