Mechanisms of androgen receptor signalling via steroid receptor coactivator-1 in prostate.

Abstract

The androgen receptor (AR) is a member of the nuclear receptor superfamily. These ligand-activated transcription factors usually contain two activation functions, a ligand-independent activation function 1(AF1) in the divergent N-terminal domain and a ligand-dependent AF2 in the more conserved Cterminal ligand-binding domain. To promote transcription from target promoters, DNA-bound nuclear receptors recruit coactivator proteins that promote transcription by modifying histones within nucleosomes, resulting in altered topology of chromatin to allow access of the basal transcriptional machinery, or stabilising the pre-initiation complex. It is well known that most coactivators interact with AF2 of many nuclear receptors via conserved, helical LxxLL motifs (where L is leucine and x is any amino acid). The AF2 of the AR is very weak, but we were able to demonstrate that its intrinsic liganddependent activity is potentiated by steroid receptor coactivator-1 (SRC1) and that this region interacts with coactivators via LxxLL motifs. However, a mutant SRC1 coactivator with no functional LxxLL motifs was still able to potentiate AR activity. We found that SRC1 can also be recruited to (and increase activity of) AF1 of the AR via a conserved, glutamine-rich region. Point mutations within this region abolish SRC1 interaction with AF1 and also abolish or severely impair its ability to potentiate AR activity on all promoters tested. Thus the AR interacts with SRC1 via two different regions and the AF1 interaction is functionally the more important, although the contribution of the two interactions varies in a promoter-dependent fashion. SRC1 then potentiates receptor activity via recruitment of CBP/p300, a histone acetyltranferase. This is important in the context of prostate cancer as SRC1 and other coactivators including CBP are coexpressed with AR in the luminal epithelial cells of the prostate, where over 90% of prostate tumours arise. There is a need for effective second-line prostate cancer therapy aimed at blocking the AR pathway when anti-androgen therapy has failed. Since there is growing evidence that nuclear receptor cofactors may be implicated in the progression of hormone-dependent tumours to hormoneindependent states, novel targets could include the interaction of AR with coactivator proteins. We suggest that the N-terminal interaction would be a more specific and effective target in the case of prostate cancer than the LxxLL/AF2 interaction.

Mechanisms of androgen receptor activation in advanced prostate cancer: differential co-activator recruitment and gene expression

“…A previous study of DRG1 regulation in LNCaP cells also showed high specificity for androgens, although in this case SARMs were not studied (Ulrix et al, 1999). This all adds to a growing body of evidence supporting the notion that differential expression of steroid target genes is dependent upon many factors including the promoter, receptor interactions (both intra-receptor and with other proteins) and post-translational modifications (He et al, 2002;Li et al, 2003;Callewaert et al, 2004;Powell et al, 2004). The implications go beyond differential expression of androgen-responsive genes.…”

“…Both SRC-1 and ARA70 are expressed in human prostate cancer cell lines and in normal prostate, in the luminal epithelial cells that also express AR (Gregory et al, 2001;Linja et al, 2001;Hu et al, 2004;Powell et al, 2004;Agoulnik et al, 2005). There is both in vitro and in vivo evidence that SRC-1 is required for optimal androgen signalling and growth in the prostate, since knockdown of SRC-1 attenuated growth and androgendependent gene expression in androgen-sensitive prostate cancer cell lines (Agoulnik et al, 2005), and SRC-1-null mice exhibit attenuation of androgenstimulated prostatic growth (Xu et al, 1998).…”

Molecular Alterations during Progression of Prostate Cancer to Androgen Independence

“…The ligand-binding domain, where androgens bind, results in a conformational change in the AR, where it dissociates from heat shock proteins in the cytoplasm, and localizes to the nucleus (13 ). In the nucleus, the AR binds to specific DNA sequences, called androgen responsive elements, via the DNA-binding domain, promoting further association of factors into a complex, which leads to gene transcription (14 ). Various genes are regulated by the AR, including kallikrein-related peptidase 3 (KLK3), 6 which encodes kallikrein-3 (formerly known as prostate-specific antigen), one of the best cancer biomarkers available.…”

The AR/NCOA1 axis regulates prostate cancer migration by involvement of PRKD1

“…Coactivator proteins are recruited to a ligandbound receptor and thereby enhance its transcriptional activity (Powell et al 2004). So far, there are 300 known coactivators for nuclear receptors, including nuclear receptor coactivator 1 (NCOA1/SRC1), which is a member of the p160 nuclear receptor coactivator family.…”

“…NCOA1 acts as a scaffold protein that houses a central nuclear receptor-interacting domain and two activation domains, which are located at the carboxy-terminus and flank a glutamine-rich region. This glutamine-rich region interacts with the amino-terminal part of the AR (Powell et al 2004). …”