Tumour suppressor p53 down-regulates the expression of the human hepatocyte nuclear factor 4α (HNF4α) gene

The liver is exposed to a wide variety of toxic agents, many of which damage DNA and result in increased levels of the tumour suppressor protein p53. We have previously shown that p53 inhibits the transactivation function of HNF (hepatocyte nuclear factor) 4 alpha 1, a nuclear receptor known to be critical for early development and liver differentiation. In the present study we demonstrate that p53 also down-regulates expression of the human HNF4 alpha gene via the proximal P1 promoter. Overexpression of wild-type p53 down-regulated endogenous levels of both HNF4 alpha protein and mRNA in Hep3B cells. This decrease was also observed when HepG2 cells were exposed to UV irradiation or doxorubicin, both of which increased endogenous p53 protein levels. Ectopically expressed p53, but not a mutant p53 defective in DNA binding (R249S), down-regulated HNF4 alpha P1 promoter activity. Chromatin immunoprecipitation also showed that endogenous p53 bound the HNF4 alpha P1 promoter in vivo after doxorubicin treatment. The mechanism by which p53 down-regulates the P1 promoter appears to be multifaceted. The down-regulation was partially recovered by inhibition of HDAC activity and appears to involve the positive regulator HNF6 alpha. p53 bound HNF6a in vivo and in vitro and prevented HNF6 alpha from binding DNA in vitro. p53 also repressed stimulation of the P1 promoter by HNF6 alpha in vivo. However, since the R249S p53 mutant also bound HNF6 alpha, binding HNF6 alpha is apparently not sufficient for the repression. Implications of the p53-mediated repression of HNF4a expression in response to cellular stress are discussed.