Modulation of posttranscriptional and posttranslational regulatory processes by histone deacetylase inhibitors
Acetylation is a very critical posttranslational modification in vivo. Targeting lysine residues of various proteins crucially modulates protein functions and interactions. The inhibitors of histone deacetylases are promising anticancer drugs – currently in clinical testing. We could show that the treatment of cells with these histone deacetylase inhibitors is able to modulate cell fate by enhancing conditions that trigger apoptosis by two distinct mechanisms. Firstly, VPA and co-treatment with the chemotherapeutic agent HU induce expression of the pro-apoptotic BH3-only protein BIM. The enhanced BIM expression arises from an increase in transcription of the BIM gene in an AP1-dependent manner. This effect occurs in cultured cells and in primary head and neck cancer cells from patients. We observed that the effect of apoptosis induction following treatment is mainly dependent on the level of BIM protein. This suggests that this therapy might be useful in the clinic. Secondly, incubation of cancer cells with various histone deacetylase inhibitors reduces expression of the class III deacetylase SIRT1. SIRT1 carries out various cellular functions including stress responses and metabolic regulation. During stress conditions, SIRT1 expression is enhanced favouring cell survival by inhibiting apoptosis. We could show for the first time that inhibitors of the classical protein deacetylases (class I, II and IV), that do not target class III enzyme activity, unexpectedly target the class III deacetylase SIRT1 by decreasing its protein levels. The reduced SIRT1 protein amount upon inhibitor treatment is due to changes in SIRT1 mRNA stability. The mRNA binding protein HuR is responsible for this effect. Histone deacetylase inhibitor treatment reduces the cytosolic amount of HuR. Additionally, its binding affinity for SIRT1 mRNA decreases significantly leading to SIRT1 mRNA decay. Furthermore, we identified three novel phosphorylation sites within HuR upon inhibitor treatment. Conceivably, these trigger the changed characteristics of HuR towards SIRT1 mRNA. The loss of SIRT1 upon histone deacetylase inhibitor treatment leads to enhanced sensitivity of cell towards apoptotic stimuli.