The regulation of PRH/HHEX by transforming growth factor β

Abstract

The transcription factor PRH/HHEX (Proline-Rich Homeodomain/Haematopoietically Expressed Homeobox) controls cell proliferation, cell differentiation and cell migration/invasion in a diverse range of cell types. Here I show that the gene encoding PRH is deleted in around 4% of sequenced samples from patients with prostate cancer. Additionally, I show that there is increased CpG DNA methylation at this gene in prostate cancer samples compared to samples from normal prostate and that PRH mRNA levels are reduced in prostate cancer. Using immunohistochemistry to stain 96 samples of prostate neoplasia, I show that PRH expression is negatively correlated to tumour differentiation in prostate cancer.
Recent published work has shown that the down-regulation of PRH expression in breast and prostate cancer cells results in increased cell proliferation and increased cancer cell invasion. Moreover, PRH inhibits breast and prostate cancer cell migration and invasion in part at least by activating the transcription of Endoglin, a Transforming Growth Factor β (TGF-β) co-receptor. Here I show that the treatment of immortalised prostate epithelial (PNT2-C2) cells and prostate adenocarcinoma (PC3) cells with TGF-β increases cell migration and induces an epithelial to mesenchymal transition (EMT), with up-regulation of the EMT transcription factor Snail and down-regulation of the epithelial marker E-Cadherin. TGF-β treatment down-regulates the expression of PRH/HHEX in prostate cell lines at both the protein and mRNA level. Moreover, chromatin immunoprecipitation (ChIP) experiments show that the regulation of PRH/HHEX by TGF-β may be via the direct binding of the TGF-β effector pSmad3 at the PRH promoter. I also investigate the effects of PRH over-expression on PC3 cells. PRH expression in these cells re-activates the expression of E-cadherin, reduces the migratory ability of these cells and inhibits cell proliferation.
PRH over-expression also down-regulates the expression of multiple genes involved in the TGF-β signalling pathway including TGFB2 and TGFBR2. ChIP shows that PRH binds to both the TGFB2 and TGFBR2 gene bodies, suggesting a direct effect of PRH on the expression of these genes. Prostate cancer cells are exposed to TGF-β from multiple sources in vivo and blood platelets were tested as a possible source of TGF-β using this model. Platelet treatment was able to increase cell migration, trigger EMT and down-regulate PRH expression in a similar way to TGF-β treatment. Platelets treatment resulted in the up-regulation of pSmad3 and the effect of platelets on cell migration were abolished when TGF-β signalling was blocked.
In summary, the work described in this Thesis shows that TGF-β increases prostate cell migration in part at least by the down-regulation of PRH expression. PRH up-regulates E-cadherin expression and down-regulates cell migration suggesting that TGF-β alters E-cadherin levels and increases cell migration via its effects on PRH. Since PRH also regulates several genes important in TGF-β signalling this creates a feedback loop that enables a more precise control of cell behaviour. Changes in PRH levels or activity are likely to disrupt this control mechanism and to contribute to tumour progression.

Date of Award	19 Mar 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Kevin Gaston (Supervisor)