The acetylase activity of p300 is dispensable for MDM2 stabilization

It has been shown that p300 binds to MDM2 and leads to down-regulation of the p53 function. However, it remains unclear whether the acetylase activity of p300 is necessary for regulating MDM2 stability. In this study, we address this issue. First, p300 did not acetylate MDM2 in solution and in cells. Second, overexpression of p300 in cells increased the level of the MDM2 protein but not its mRNA. Similarly, the acetylase-defective p300 AT2 mutant stabilized the MDM2 protein as well. Consistently, the deacetylase inhibitor, trichostatin A, did not significantly affect the half-life of the endogenous MDM2 protein, whereas p300 enhanced the half-life of MDM2. Finally, both wild type and acetylase-defective mutant p300 proteins associated with MDM2 in nuclear body-like structures where MDM2 might be protected from proteasomal degradation. Thus, these results suggest that p300 appears to stabilize MDM2 by retaining this protein in a specific nuclear structure rather than by acetylating it.     

The p300 acetylase is critical for ligand-activated farnesoid X receptor (FXR) induction of SHP

The primary bile acid receptor farnesoid X receptor (FXR) maintains lipid and glucose homeostasis by regulating expression of numerous bile acid-responsive genes, including an orphan nuclear receptor and metabolic regulator SHP. Using SHP as a model gene, we studied how FXR activity is regulated by p300 acetylase. FXR interaction with p300 and their recruitment to the SHP promoter and acetylated histone levels at the promoter were increased by FXR agonists in mouse liver and HepG2 cells. In contrast, p300 recruitment and acetylated histones at the promoter were not detected in FXR-null mice. p300 directly interacted with and acetylated FXR in vitro. Overexpression of p300 wild type increased, whereas a catalytically inactive p300 mutant decreased, acetylated FXR levels and FXR transactivation in cells. While similar results were observed with a related acetylase, CBP, GCN5 did not enhance FXR transactivation, and its recruitment to the promoter was not increased by FXR agonists, suggesting functional specificity of acetylases in FXR signaling. Down-regulation of p300 by siRNA decreased acetylated FXR and acetylated histone levels, and occupancy of FXR at the promoter, resulting in substantial inhibition of SHP expression. These results indicate that p300 acts as a critical coactivator of FXR induction of SHP by acetylating histones at the promoter and FXR itself. Surprisingly, p300 down-regulation altered expression of other metabolic FXR target genes involved in lipoprotein and glucose metabolism, such that beneficial lipid and glucose profiles would be expected. These unexpected findings suggest that inhibition of hepatic p300 activity may be beneficial for treating metabolic diseases.