SP3 Modulation of stress response by regulation of SIRT1 expression

Project Leader: Thorsten Heinzel

Background and previous work
The NAD+-dependent histone deacetylase SIRT1 is a major regulator of regenerative reactions in stress response and reportedly contributes to life span extension. However, SIRT1 also appears to promote the survival of tumour cells. Since an increase in NAD+ levels (e.g. under fasting conditions) leads to increased expression and activity of SIRT1 it can act as a metabolic sensor. Thus, our analysis of SIRT1 regulation integrates well into CRA1, which aims at the investigation of signalling responses to metabolic stress. SIRT1 enhances cell survival following oxidative and genotoxic stresses, most likely by deacetylating nonhistone proteins including p53, KU70 and FoxO. We identified an unexpected link between class I/II HDAC inhibitors, which do not block the activity of the class III enzyme SIRT1, and reduced SIRT1 expression. We found that HDAC inhibitors in a time- and dose-dependent manner lead to decreased SIRT1 mRNA stability and this appears to depend on posttranslational modifications of the mRNA binding proteins HuR and AUF. SP3 is a project of a qualification fellow of the first recruitment period. Due to maternity leave it will continue into the second funding period of the RTG. For the third recruitment period, SP3 and SP16 will be merged into a single project.

Specific aims and working programme
We will investigate the molecular mechanisms underlying the regulation of SIRT1 mRNA stability including the role of upstream signalling proteins. Our results show that SIRT1 levels are downregulated after genotoxic stress induction and that various HDAC Inhibitors (HDACi) downregulate SIRT1 mRNA and SIRT1 protein levels in a dose-dependent manner. Regulation occurs both, at the posttranslational level via degradation by caspases and at the posttranscriptional level via the mRNA binding protein HuR. HuR changes subcellular localisation and phosphorylation status after HDACi stimulation.
In the third recruitment period our analysis will focus on links between SIRT1 activity and other metabolic regulators. We discovered that SIRT1 is the primary deacetylase regulating acetylation levels of the glucocorticoid receptor (GR) which is involved in the regulation of metabolism, inflammation, development and reproduction. Moreover, AMPK signalling reduces GR acetylation by modulating SIRT1 activity. Taking into account the results of SP3 and SP16, the successor project will focus on the interplay of these key metabolic regulators. This network includes multiple feed-back and feed-forward circuits which lead to non-linear behaviour of the system. We are particularly interested in elucidating how modulation of the activity of these key regulators translates into epigenetic modifications to create a long-term adaptive stress response.