Date Published: September 24, 2004
Publisher: Public Library of Science
Author(s): Eugene Hur, Samuel J Pfaff, E. Sturgis Payne, Hanne Grøn, Benjamin M Buehrer, Robert J Fletterick
Abstract: Prostate cancer is a leading killer of men in the industrialized world. Underlying this disease is the aberrant action of the androgen receptor (AR). AR is distinguished from other nuclear receptors in that after hormone binding, it preferentially responds to a specialized set of coactivators bearing aromatic-rich motifs, while responding poorly to coactivators bearing the leucine-rich “NR box” motifs favored by other nuclear receptors. Under normal conditions, interactions with these AR-specific coactivators through aromatic-rich motifs underlie targeted gene transcription. However, during prostate cancer, abnormal association with such coactivators, as well as with coactivators containing canonical leucine-rich motifs, promotes disease progression. To understand the paradox of this unusual selectivity, we have derived a complete set of peptide motifs that interact with AR using phage display. Binding affinities were measured for a selected set of these peptides and their interactions with AR determined by X-ray crystallography. Structures of AR in complex with FxxLF, LxxLL, FxxLW, WxxLF, WxxVW, FxxFF, and FxxYF motifs reveal a changing surface of the AR coactivator binding interface that permits accommodation of both AR-specific aromatic-rich motifs and canonical leucine-rich motifs. Induced fit provides perfect mating of the motifs representing the known family of AR coactivators and suggests a framework for the design of AR coactivator antagonists.
Partial Text: The androgen receptor (AR) is the cellular mediator of the actions of the hormone 5-α dihydrotestosterone (DHT). Androgen binding to AR leads to activation of genes involved in the development and maintenance of the male reproductive system and other tissues such as bone and muscle. However, it is the pivotal role of AR in the development and progression of prostate cancer that has led to increasing interest in this nuclear receptor. Presently, hormone-dependent prostate cancer is treated with a combination of strategies that reduce circulating levels of androgens, such as the administration of antiandrogens that compete for the androgen-binding pocket in the core of the C-terminal ligand-binding domain (LBD). The benefits of these treatments are typically transient, with later tumor growth associated with increases in expression levels of AR or its cofactors, or mutations that render AR resistant to antiandrogens (Gregory et al. 2001; Culig et al. 2002; Lee and Chang 2003). Alternative approaches to inhibiting AR transcriptional activity may therefore lie in disrupting critical protein associations the receptor needs for full function.
The crystal structures reported here reveal how AR binds coactivator motifs with bulky aromatic hydrophobic groups and permit construction of a profile of the AR coregulator interface (see Figure 2). In some ways, this interface resembles those of other nuclear receptors: it is an L-shaped hydrophobic cleft comprised of three distinct subsites that bind hydrophobic groups at the +1, +4, and +5 positions in cognate peptides. Moreover, the so-called charge clamp residues (Lys720 and Glu897) bracket the cleft. Nonetheless, the AR coregulator recognition site is unique in that it rearranges upon motif binding to form a long, deep, and narrow groove that accommodates aromatic residues at the +1 and +5 positions (Figure 9). Sequence alignments of AR with other NRs suggest that a unique combination of substitutions at Val730, Met734, and Ile737 combine to permit the formation of a smoother, flatter interaction surface that displays a higher complimentarily to aromatic substituents than to branched aliphatic (see Figure 8). Of these, methionine, the only unbranched hydrophobic amino acid and the most accommodating, at a key position between the +1 and +5 sites, allows the AR AF-2 interface to vary the size and shape of its pockets to associate with a more diverse set of coregulators. GR also contains a methionine residue at this position, raising the possibility that it may also employ induced fit to broaden motif recognition. While naturally occurring mutations in AR have yet to be observed at Met734, it is interesting to note that mutations at Val730 and Ile737 have been reported in patients with prostate cancer and androgen insensitivity, respectively (Newmark et al. 1992; Quigley et al. 1995; Gottlieb et al. 1998).