On histone deacetylases in the epigenetic regulation of neural stem and cancer cell fate

Abstract

Development of an organism requires correct spatial and temporal regulation of gene expression. Epigenetic regulation of gene expression has been shown to be an important part of many cellular events. Nucleosomes consist of two subunits of each core histone proteins H2A, H2B, H3, H4, and 146 base pairs of DNA, wrapped around the histones. In this way, DNA can be tightly packaged in the cell. The N-terminal tails, that protrude out from the histone surfaces can be subjected to different covalent modifications, including, acetylation, methylation, phosphorylation, and ubiquitination. These modifications take part in gene regulation by changing the structure of chromatin and by recruiting gene regulatory proteins. Addition of acetyl-groups to the N-terminal of histones is catalyzed by histone acetyltransferases (HATs) whereas removal of the acetyl-groups is carried out by histone deacetylases (HDACs).

Increased acetylation has been correlated with increased gene expression, and decreased acetylation has been correlated with transcriptional repression. HDACs are divided into four families: class I, class II, and class IV histone deacetylases and the class III NAD-dependent enzymes of the sirtuin family. HDACs are expressed early in development and specific HDAC gene deletions show that they are important for many cellular events, such as proliferation, growth, and differentiation. Putative roles for HDACs in neural development come mostly from work done using inhibitors of class I and class II HDACs, and from the association of HDACs to protein complexes that are known to repress neuronal differentiation. HDACs have also been shown to play an important part in cancer development and HDAC inhibitors have been shown to block cell proliferation, promote differentiation, and induce or facilitate apoptosis. The aim of this thesis was to investigate the role of histone deacetylases in neural stem- and cancer cells.

In paper I, we show that HDAC3 is an essential repressor of neuronal differentiation in embryonic neural stem cells, required for repression of BDNF expression and histone H3K9 acetylation. Paper II and III address the use of HDAC inhibitors in cancer treatment and their effect on apoptotic signaling pathways and epigenetic mechanisms. We show that trichostatin A in combination with etoposide, VP16, induce apoptosis via caspase-dependent pathways and the mitochondrial AIFdependent pathway in multi-resistant cancer cells. In addition, we show that cell death promoting effects of valproic acid and trichostatin A depend on the regulation of histone H4K16 acetylation by the histone modifying enzymes hMOF and SIRT1. In paper IV, we show that resveratrol inhibits neuronal differentiation of embryonic neural stem cells in a SIRT1-dependent fashion and that the effects of red wine on embryonic NSCs and cancer cells are toxic and are linked to inhibition of thioredoxin reductase in a resveratrol/SIRT1-independent manner.

In conclusion, this thesis shows that chromatin modifying proteins play essential roles in neural stem cell differentiation and cancer cell characteristics, and contributes to the understanding of epigenetic mechanisms in the regulation of neural stem and cancer cell fate.