The long-term goal of this project is to understand the molecular mechanisms of hormone-independent growth and metastasis of prostate cancer (PCA). Our preliminary data demonstrates, for the first time, that tyrosine phosphorylation of the androgen receptor (AR) and its cofactor p300 is associated with hormone refractory growth of PCA cells, suggesting that the hormone refractory tumor cells may utilize autocrine/paracrine factors activating tyrosine kinases in the tumor cells to compensate the loss of androgens under the circumstance of androgen ablation therapy. Several lines of evidence suggest an important role of tyrosine kinase Etk in this process. Etk is up-regulated in over 50% of PCA specimens examined. Etk kinase activity is negatively regulated by tumor suppressors PTEN and p53. The expression level of IL6 and Etk in mouse prostates is elevated in response to castration. The kinase activity of Etk is required for androgen-independent activation of the androgen receptor (AR) by several non-steroid stimuli including IL6, bombesin and neurotensin. The siRNA specific for Etk reduces growth factor-induced tyrosine phosphorylation of AR and inhibits PCA cell proliferation. Overexpression of Etk in PCA cells results in increased expression of the nuclear receptor coactivator p300 which is essential for AR-mediated transcription. Furthermore, targeted expression of Etk in mouse prostate gland results in pathological changes resembling prostatic intraepithelial neoplasia (PIN) and a delayed apoptotic response to castration. We therefore hypothesize that Etk is a key mediator in IL6 signaling in PCA cells and plays an important role in the development of hormone refractory PCA by directly phosphorylating AR and modulating nuclear cofactor p300 activity, leading to activation of AR independent of androgens or sensitization of AR to low levels of androgens.
Our aims are:
Aim 1 : To elucidate the mechanisms underlying the growth factor-induced tyrosine phosphorylation of AR in PCA cells. The effects of tyrosine phosphorylation of AR on its ligand binding activity, its N/C- terminal interaction and the recruitment of its co- factors will be investigated. The contribution of tyrosine phosphorylation of AR to the growth of hormone refractory PCA cells will be assessed by an """"""""AR Replacement"""""""" strategy established in our laboratory. Furthermore, the role of Etk and possibly other tyrosine kinases in growth factor-induced tyrosine phosphorylation of AR will be studied.
Aim 2 : To investigate the role of Etk in modulation of nuclear cofactor p300 activity in PCA cells. The mechanisms by which Etk regulates the promoter activity of p300 will be studied. The tyrosine phosphorylation site of p300 induced by Etk will be identified. Furthermore, the contribution of tyrosine phosphorylation of p300 to its acetyltransferase activity and the AR-mediated transcription activity as well as the hormone-independent growth of PCA cells will be investigated.
Aim 3 : To study the role of Etk in the development of hormone-independence in mouse models. Compound mice which carry multiple genetic modifications will be generated on the Etk transgenic background to study the interaction of Etk with tumor suppressor PTEN in the mouse prostate and their roles in development of hormone-independence in the mouse prostate. Comparative microarray analysis will be performed to reveal the altered gene expression in the Etk transgenic and the compound mice in response to castration, which may lead to discovery of the genes involved in development of hormone-independence.
Showing the most recent 10 out of 15 publications
