Abstract
The proteasome is responsible for the degradation of polyubiquitinated proteins. Inhibition of the proteasome leads to an accumulation of polyubiquitinated proteins and thus to an impairment of the cellular protein homeostasis. To prevent cellular damage on proteasome inhibition there is an up-regulation of several heat shock proteins (Hsps), including Hsp27, Hsp70, and heme oxygenase-1 (HO-1). It was demonstrated that the induction of classical Hsps, such as Hsp27 and Hsp70, is dependent on a HDAC6-dependent mechanism which releases HSF-1 and induces the expression of newly synthesized Hsps. In this study we demonstrate that the up-regulation of HO-1 on proteasome inhibition is mediated by p38MAPK and Nrf-2. Interestingly we found additional evidence, proving the involvement of HDAC6 in the up-regulation of HO-1. By using RNAi technologies against HDAC6 we demonstrate that there is a lack of the expected induction of HO-1, Nrf-2, and phosphorylated p38 (pp38) after proteasome inhibition. Furthermore, we can show that p38 is acetylated in unstressed cells and is a good substrate for HDAC6-mediated deacetylation. Therefore, we propose that on proteasome inhibition HDAC6 deacetylates p38, allowing the subsequent phosphorylation of p38 and resultant activation of NRF-2. NRF-2 enters the nucleus and functions as a transcription factor for HO-1.