Androgen Receptor (AR) modulation through and cAMP signaling in prostate epithelium

点击翻译按钮获取中文摘要

Aim There is currently a great interest in understanding the molecular mechanism that underpin compartmentalisation and cross-talk of intracellular signalling processes in order to appreciate fundamental insights. In this, the cAMP signalling pathway has provided important paradigms. cAMP-phosphodiesterases (PDE's) underpin compartmentalised cAMP signalling in cells. This project focuses on understanding and exploiting the functional role of specific PDE's in male prostate epithelium. In particular defining interactions between the cAMP signalling system and that of the androgen receptor (AR), whose action is required for normal prostate development and function. Androgen Receptor signalling and its modulation by cAMP Prostate development and normal prostate function requires androgen signaling and action (1). The male androgens, in particular the testicular testosterone and its more active metabolite dihydrotestosterone (DHT), exert their action in the prostate through a nuclear hormone receptor, the Androgen Receptor (AR). The development and physiology of the male organism regarding sexual differentiation and maturation is strongly dependent on androgen and AR activity. The 110-kDa AR is a member of the super-family of nuclear receptors, and is activated upon binding of various lipophilic ligands (steroids, retinoids, etc). Ligand binding promotes the association of AR co-regulators, and the protein complex then translocates into the cell nucleus. DNA transcription is induced by binding of AR complexes to AREs (Androgen Responsive Elements) in the promotor region of target genes. The modulation of transcriptional activity of the AR appears to be influenced by a number of co-regulators and phosphorylation events. It has been suggested that AR activity can be induced in a ligand-independent manner by the elevation of cAMP levels. This has been presumed to be mediated via the activation of PKA. However, no analysis has yet been done to determine whether regulation can additionally occur through the recently discovered cAMP signalling system involving the cAMP GTP exchange factor, EPAC. Nevertheless, several potential PKA phosphorylation sites have been identified on the AR (Ser81/94/650) and phosphorylation by PKA has been speculated to provide a mechanism for ligand-independent activation of AR transcription. Thus work is needed to define the parameters that may control PKA phosphorylation at these sites, such as regulation by PDEs and the involvement of AKAPs and PKA isoforms as well as their functional significance. However, it has also been proposed that cAMP cross-talk with the AR may occur (additionally) at the level of CREB (cAMP responsive element-binding protein). This model suggests an indirect effect of PKA on AR stimulation by the phosphorylation of Ser133 of CREB. Such PKA phosphorylation promotes CREB binding to cAMP responsive elements in target candidate genes and the recruitment of additional co-activators, thereby enhancing target gene transcription. The presence of a putative cAMP responsive site (CRE) at the 5'-upstream regulatory region in the PSA gene makes CREB a conceivable candidate for mediating PKA effects. Thus PKA may regulate AR functioning at two localities, the AR itself and also CREB. Although it has been shown consistently over the years that the activation of cAMP-PKA pathway enhances AR transcriptional activity, the exact mechanism for this has not been shown. This effect could be mediated through either direct phosphorylation of the AR by cAMP-PKA, or mediated through indirect PKA phosphorylation effects. Workplan This study aims to identify the modulation of AR signalling by cAMP-PKA. These investigations will provide novel understanding about cAMP compartmentalisation and cross-talk in an important biological system. These multi-disciplinary studies using 'state-of-the-art' technologies will provide a first-rate training for the student involved.