Functional motifs of the androgen receptor

Abstract

Androgen receptor (AR) transcriptional activity stimulated by high affinity binding of testosterone or dihydrotestosterone involves a dynamic NH2-and carboxyl-terminal (N/C) interaction between an AR NH2-terminal FXXLF motif (23FQNLF27) and a hydrophobic interaction surface in the ligand binding domain known as activation function 2 (AF2). The AR N/C interaction slows the dissociation rate of bound androgen, stabilizes AR and competitively inhibits activity from AF2. Functional significance of the androgen-dependent AR N/C interaction is supported by naturally occurring germline mutations in AF2 that decrease AR transcriptional activity and cause the androgen insensitivity syndrome by interfering with the N/C interaction without altering equilibrium androgen binding affinity. Gain-of-function AR somatic mutations in prostate cancer can enhance the AR N/C interaction in association with increased AR transcriptional activity. Melanoma antigen gene protein-A11 (MAGE-11) is an AR coactivator that competitively inhibits the AR N/C interaction by binding the AR FXXLF motif, but increases AR signaling through mechanisms that appear to contribute to the development and growth of prostate cancer. The androgen receptor (AR) is a ligand-activated nuclear receptor of the steroid receptor subfamily that mediates the developmental and behavioral effects of the biologically active, naturally occurring androgens, testosterone (T) and dihydrotestosterone (DHT). High-affinity androgen binding targets AR to the nucleus where it interacts with androgen response element DNA associated with androgen-regulated genes. The human syndrome of androgen insensitivity, in which 46XY genetic males have a mutation in the AR gene and are phenotypically female, demonstrates the functional requirement for AR to mediate the effects of androgen during normal male sex development. AR transactivation also contributes to prostate cancer development and progression. Structure and function relationships of AR interacting domains are the focus of this review. AR has a multidomain structure characteristic of the steroid hormone family of nuclear receptors (Simental et al. 1991). The carboxyl-terminal ligand binding domain (LBD) binds T and DHT with similar high affinity (Wilson and French 1976). The central DNA binding domain binds androgen response element DNA associated with androgen-regulated genes. Naturally occurring mutations in the AR DNA binding domain result in the androgen insensitivity syndrome (AIS) and demonstrate that AR must bind DNA to mediate the biological effects of T and DHT (Quigley et al. 1995). A hinge region lies between the DNA binding domain and LBD and contains an AR nuclear-targeting signal (Zhou et al. 1994). The AR NH2-terminal region is the least conserved in AR between species and relative to other steroid receptors. The largely unstructured AR NH2-terminal region harbors a strong activation function 1 (AF1) that facilitates tissue-specific gene activation. AF1 is not influenced directly by ligand binding but depends on bound androgen for AR dimerization, nuclear transport, DNA binding, and transcriptional activation. AR transcriptional activity is also mediated by activation function 2 (AF2) in the LBD. AF2 bears no structural similarity to AF1, but both activation regions are involved in recruiting coactivator proteins that link to the transcriptional machinery of chromatin. Considerable research has focused on the structural and functional properties of AF2. Our understanding of AF2 derives in part from crystallographic studies made possible by the highly ordered arrangement of alpha helices in the LBD. AF2 is a hydrophobic surface of the LBD that lies contiguous to the ligand binding pocket and is structurally dependent on bound ligand (He et al. 2004). Functional studies on AR demonstrate an AF2 binding specificity unique among steroid receptors for a set of structurally related amphipathic alpha helical motifs. © 2009 Springer-Verlag New York.