The mechanisms involved in the transition from androgen-dependent to androgen-independent PCa (prostate cancer) remain largely undefined. The AR (androgen receptor) is an androgen-dependent transcription factor and is thought to play an important role in the development of both androgen-dependent and -independent prostatic malignancy. AR-mediated transcription is regulated by the binding of various cofactor proteins to the AR that facilitate transcriptional initiation and elongation. Elucidating the mechanisms by which cofactors regulate AR transcriptional activity may reveal the therapeutic potential of cofactors in PCa. Current models of gene expression indicate that transcription and RNA processing are tightly coupled. In this review, we discuss how the ATP-dependent DEAD box RNA helicase p68, which has established roles in transcription and RNA processing, may function as an ‘adaptor’ or coupling protein to facilitate cross-talk between transcription and RNA processing in AR-regulated genes by controlling the rate of transcriptional initiation/elongation.
- androgen receptor
- p68 DEAD box RNA helicase
- prostate cancer
- RNA polymerase II
- transcription initiation/elongation
Role of AR (androgen receptor) in PCa (prostate cancer)
PCa is currently the second leading cause of cancer deaths in men in the United States . The onset and progression of PCa is driven by the transcriptional function of the AR, a member of the nuclear hormone receptor family of transcriptional factors that activates the expression of numerous androgen-responsive genes. In early stage PCa, changes in the normal cellular homoeostasis of the prostate that affect regulatory genes resulting in growth or survival advantage are regulated by androgens, and androgen ablation therapy by surgical or chemical castration is initially successful in causing tumour regression. However, in later stages of disease progression, conversion into androgen-independent PCa is common, as cancer cells are able to maintain a fully functioning AR in the absence of androgens . Currently, no curative therapeutic agents exist for this metastatic hormone-resistant disease stage .
A suggested mechanism for the maintenance of a functionally active AR in the absence of androgens is by alterations in the expression of AR cofactors leading to increased AR-sensitivity or promiscuous activation of AR by other steroid hormones. During the last decade, a number of proteins (>170) have been proposed to possess AR co-activating or co-repressing functions . AR co-regulators are proteins that ensure competency of the AR, and/or are recruited by the AR to influence AR-mediated transcription by assembling at the promoter and enhancer regions of androgen-responsive genes to facilitate chromatin remodelling and/or enhance the recruitment of the general transcriptional machinery to target promoters.
p68 DEAD box RNA helicase is a novel AR co-activator
p68 is a growth- and developmentally regulated prototypic member of the DEAD box family of proteins, and an established RNA helicase that uses the energy from ATP hydrolysis to rearrange inter- or intra-molecular RNA structures and dissociate RNA–protein complexes. p68 has been found to play a role in a wide range of cellular processes including: unwinding of the U1–5′-SS (splice site) duplex, allowing U6–5′-SS association and formation of the active spliceosome; rRNA processing; ribosome biogenesis and cell proliferation; miRNA (microRNA) processing; and transcription and promotion of mRNA transcript release and transcriptional deactivation in Drosophila (reviewed in [5,6]). There is now considerable evidence indicating that p68 is a transcriptional co-activator of ERα (oestrogen receptor α) and is recruited to ERα-responsive promoters in response to oestrogen: the tumour suppressor p53, being recruited to p53-responsive promoters after DNA damage; MyoD (myogenic differentiation), where p68 has been implicated in directing the assembly of the transcriptome and chromatin remodelling factors (reviewed in [5,6]); and we have shown p68 to be a novel co-activator of the AR and AR-mediated genes (E.L. Clark, F.V. Fuller-Pace and C.N. Robson, unpublished work). Post-translational modifications have also been shown to influence p68 function [7,8] and may explain why the RNA helicase is an important regulator of such distinct functions. Collectively, these findings indicate that p68 plays an important role in transcriptional regulation and this may have implications in tumour development.
An emerging role for p68 in AR co-transcriptional processes?
Current models of gene transcription suggest that transcriptional initiation, elongation and RNA splicing are tightly coupled [9–11]. In light of this, it is highly probable that the AR regulates the transcription of androgen-responsive genes by regulating both transcriptional initiation and elongation events facilitated by cofactor proteins. Recent evidence has suggested that p68 may function as an ‘adaptor’ or ‘coupling’ protein that co-ordinates the tightly integrated processes of transcription and RNA processing [5,12]. Mechanistically, it would be logical that the AR interacts with a cofactor linked to all the steps from transcription to transcript release, facilitating the efficacy of AR-mediated gene transcriptional elongation. Although p68 plays key roles in alternative splicing in some contexts, e.g. SS stability, spliceosome assembly and H-ras splicing, it is unable to directly affect CD44 inclusion . Unpublished evidence indicates that p68 in fact potentiates AR-regulated repression of splicing of a hormone-responsive CD44 variable exon minigene (E.L. Clark, D.J. Elliot and C.N. Robson, unpublished work), underscoring an effect of p68 on AR-dependent splicing via transcriptional co-regulatory mechanisms.
Alternatively, recruitment of p68 to endogenous AR-responsive genes may facilitate spliceosome assembly and increase the rate of RNAPII (RNA polymerase II) elongation, affecting SS recognition and promoting exon-skipping in nascent transcripts. p68 has been shown to interact with the C-terminal domain of RNAPII , and the AR is thought to enhance elongation by interacting with TFIIH (transcription factor IIH) and P-TEFb (positive transcription elongation factor b) , which phosphorylates the C-terminal domain of RNAPII, switching it from a non-processive to a processive form . Future work should examine whether p68 plays an active role in the phosphorylation of RNAPII through facilitating interactions between P-TEFb and RNAPII.
The present review highlights the possibility that p68 may serve as a common link between the transcription initiation/elongation and splicing machinery. It is likely that these RNA-processing activities are strongly connected to efficient transcriptional co-regulation, and p68 could be part of a ‘molecular link’ that involves the coupling of transcriptional initiation and RNA processing, ensuring correct operation of the transcriptional machinery as it tracks along the gene. Further studies will provide the basis for exploring the therapeutic potential of the inhibition of p68 in PCa.
This work was funded by the MRC/Cancer Research UK/Department of Health – Prostate Cancer: Mechanisms of Progression and Treatment (ProMPT) collaboration.
RNA UK 2008: Independent Meeting held at The Burnside Hotel, Bowness on Windermere, Cumbria, U.K., 18–20 January 2008. Organized and Edited by David Elliot (Newcastle, U.K.), Sarah Newbury (Sussex, U.K.) and Alison Tyson-Capper (Newcastle, U.K.).
Abbreviations: AR, androgen receptor; ERα, oestrogen receptor α; PCa, prostate cancer; P-TEFb, positive transcription elongation factor b; RNAPII, RNA polymerase II; SS, splice site
- © The Authors Journal compilation © 2008 Biochemical Society