Environmental Factor, September 2010, National Institute of Environmental Health Sciences
MAGE-11 protein in regulation of androgen receptor
By Sophie Bolick
On August 3, the NIEHS Receptor Mechanisms Discussion Group welcomed Elizabeth Wilson, Ph.D., the most recent speaker in its monthly lecture series and an expert on androgen receptor regulation. Hosted by Ken Korach, Ph.D., and John Cidlowski, Ph.D., Wilson discussed her work in a talk on "Regulation of Androgen Receptor Transcription by MAGE-11."
Wilson (http://www.med.unc.edu/biochem/people/faculty/joint/wilson) , who is a professor at the University of North Carolina at Chapel Hill School of Medicine, reported on her research on the primate-specific melanoma antigen gene protein-A11 (MAGE-11). She discussed the discovery of MAGE-11, as well as its binding partners and potential involvement in the development and treatment of prostate cancer.
Androgen receptor binding
The androgen receptor (AR) is a member of the nuclear receptor family of steroid hormone receptors. AR binds with high affinity and specificity the active androgens, testosterone and dihydrotestosterone, which can be disrupted by environmental agents. Similar to other members of the steroid receptor family, AR is comprised of domains important for hormone and DNA binding.
The crystal structure of AR (see text box) shows that the AR ligand binding domain is capable of interacting with the p160 coactivator LXXLL motif as well as its own amino terminal FXXLF motif. According to Wilson, this competition for the ligand binding domain "sets up a dilemma for the androgen receptor because, on the one hand, AR wants to be activated by p160 coactivators. On the other hand, AR has an FXXLF motif within its own structure that binds with higher affinity the same AF2 hydrophobic groove that binds the coactivator LXXLL motif."
Identification of MAGE-11
To address this dilemma, Wilson proposed "that perhaps a protein exists that binds the FXXLF motif and interferes with the AR amino/carboxy interaction and allows the activation of function 2 (AF2) site [in the ligand binding domain] to bind a coactivator." Using a yeast two-hybrid screen to approach this experimentally, she identified MAGE-11.
MAGE-11 stabilizes AR in the absence of ligand because it can bind the FXXLF motif. MAGE-11 is an AR co-regulator that co-localizes with AR in nuclei of human Sertoli cells, endometrium, and granulosa cells. Due to its lack of expression in the mouse and rat, there is a need to establish biological systems to allow study of MAGE-11 in the human.
AR and MAGE-11 in prostate cancer
As Wilson explained, the AR is important at all stages of prostate cancer development and progression. There is an increase in the levels of the AR coactivator TIF2 and MAGE-11 in castration-recurrent prostate cancer. One mechanism for this increase in expression is thought to be due to DNA methylation. The CpG island in the MAGE-11 promoter is extensively methylated in human benign prostatic hyperplasia (BPH) and androgen-dependent prostate cancer, but is hypomethylated in castration-recurrent prostate cancer patient samples.
Tissue microarray studies of patient biopsies stained for MAGE-11 protein expression show evidence for some MAGE-11 staining in BPH samples, even though the MAGE-11 promoter is highly methylated in these samples. While the reason for this may be related to the cell cycle-dependent expression of MAGE-11, it is evident from these studies that MAGE-11 protein expression increases with prostate cancer progression.
MAGE-11 interaction with co-regulators
A series of experiments using AR deletion mutants demonstrated the interaction of MAGE-11 with coactivators TIF2 and CBP/p300. MAGE-11 links the amino terminal region of AR with the p300 histone acetyltransferase to increase AR transcriptional activity. A series of FXXLF-like motif interactions mediate the formation of a transcription complex containing AR, MAGE-11, p300, and TIF2 that is modulated by the AR amino/carboxy interaction.
(Sophie Bolick, Ph.D., is a postdoctoral research fellow in the NIEHS Laboratory of Molecular Carcinogenesis Molecular and Genetic Epidemiology Group.)
Crystal structure of androgen receptor ligand binding domain resolved
The amino terminus of the AR contains an FXXLF motif. The AR ligand binding domain in the carboxyl-terminus contains an AF2 hydrophobic region that interacts with the AR FXXLF motif (the AR amino/carboxy interaction) and with LXXLL motifs of p160 coactivators. In collaboration with Robert Gampe of GlaxoSmithKline, Wilson resolved the crystal structure of the AR ligand-binding domain bound to R1881 and FXXLF or LXXLL peptide. The interaction between the AR FXXLF motif with AF2 suggests an anti-parallel orientation of AR dimers, in which the amino terminus of one AR monomer is oriented across from the carboxy terminus of the opposing monomer.