The IGF-axis in liver disease : modulation of expression by histone deacetylase inhibitors

Sammanfattning: The Insulin-like growth factor (IGF) axis is composed of a family of ligands, receptors and binding proteins with important functions in the regulation of cell growth, apoptosis, survival, migration and development. The liver is a major provider of several of the IGF-axis members. Alterations to this axis may therefore be of importance in liver tumorigenesis. Enzymes that affect chromatin structure are increasingly being implicated in gene expression. One such group of enzymes are those which affect the acetylation status of the chromatin, histone acetyltransferases and histone deacetylases (HATs and HDACs respectively). The interplay between these enzymes is one of the means by which the cell can control gene transcription. The importance of these enzymes in the control of many cellular processes is only just being elucidated but the activities of these enzymes have been found to be important in transcription, cell cycle progression, gene silencing, lymphocyte and muscle differentiation, DNA replication and the cellular response to DNA damage. The role of histone deactylases in the regulation of gene expression can be examined experimentally using specific inhibitors, one of which is trichostatin A. This thesis is based on studies of the IGF-axis in both primary tumors and experimental manipulation of the axis by histone deacetylase inhibitors in an in vitro cell culture model. An examination of the gene expression profile for IGF-axis members in a series of hepatoblastomas (a pediatric liver tumor), reveals that the expression of many members of this axis are altered in liver tumors. Further investigations reveal that the expression of several important regulatory genes including histone deacetylases are also altered in these tumors. Investigations into the use of the histone deacetylase inhibitor trichostatin A (TSA) in a cell culture system revealed that the effect of this drug is dependent upon cell density. TSA was shown to affect the expression of members of the IGF-axis, in particular the expression of IGF2 and IGFBP-3. Expression of several important cell growth and regulatory genes, including p2l, p27 and TGFß1 were shown to be altered in this cell system upon treatment with TSA. Providing exogenous IGF2 peptide (IGF-II) could modulate the observed effects even further. It was also shown that IGF-II in the presence of IL-2 could affect TSA induced upregulation of HDAC1 mRNA expression.