Androgens and androgen receptors (AR) play a pivotal role in expression of the male phenotype. Several diseases, such as androgen insensitivity syndrome (AIS) and prostate cancer, are associated with alterations in AR functions. Indeed, androgen blockade by drugs that prevent the production of androgens and/or block the action of the AR inhibits prostate cancer growth. However, resistance to these drugs often occurs after 2–3 years as the patients develop castration-resistant prostate cancer (CRPC). In CRPC, a functional AR remains a key regulator. Early studies focused on the functional domains of the AR and its crucial role in the pathology. The elucidation of the structures of the AR DNA binding domain (DBD) and ligand binding domain (LBD) provides a new framework for understanding the functions of this receptor and leads to the development of rational drug design for the treatment of prostate cancer. An overview of androgen receptor structure and activity, its actions in prostate cancer, and how structural information and high-throughput screening have been or can be used for drug discovery are provided herein.

Folate receptors (FRα, FRβ and FRγ) are cysteine-rich cell-surface glycoproteins that bind folate with high affinity to mediate cellular uptake of folate. Although expressed at very low levels in most tissues, folate receptors, especially FRα, are expressed at high levels in numerous cancers to meet the folate demand of rapidly dividing cells under low folate conditions1–3. The folate dependency of many tumours has been therapeutically and diagnostically exploited by administration of anti-FRα antibodies, high-affinity antifolates4,5, folate-based imaging agents and folate-conjugated drugs and toxins6–8. To understand how folate binds its receptors, we determined the crystal structure of human FRα in complex with folic acid at 2.8 Å resolution. FRα has a globular structure stabilized by eight disulphide bonds and contains a deep open folate-binding pocket comprised of residues that are conserved in all receptor subtypes. The folate pteroate moiety is buried inside the receptor, whereas its glutamate moiety is solvent-exposed and sticks out of the pocket entrance, allowing it to be conjugated to drugs without adversely affecting FRα binding. The extensive interactions between the receptor and ligand readily explain the high folate-binding affinity of folate receptors and provide a template for designing more specific drugs targeting the folate receptor system.

The androgen receptor (AR), a nuclear transcription factor, mediates male sexual differentiation, and its excessive action is associated with prostate cancer. We have characterized a negative regulatory domain in the AR hinge region, which interacted with filamin A (FLNa), an actin-binding cytoskeletal protein. FLNa interfered with AR interdomain interactions and competed with the coactivator transcriptional intermediary factor 2 to specifically down-regulate AR function. Although full-length FLNa was predominantly cytoplasmic, a C-terminal 100-kDa fragment of FLNa colocalized with AR to the nucleus. This naturally occurring FLNa fragment repressed AR transactivation and disrupted AR interdomain interactions and transcriptional intermediary factor 2-activated AR function in a manner reminiscent of full-length FLNa, raising the possibility that the inhibitory effects of cytoplasmic FLNa may be transduced through this fragment, which can localize to the nucleus and form part of the pre-initiation complex. This unanticipated role of FLNa adds to the growing evidence for the involvement of cytoskeletal proteins in transcription regulation. T he androgen receptor (AR), a member of the steroid͞ nuclear receptor superfamily, mediates male morphogenesis in utero, gametogenesis and prostate growth at puberty, and the development of prostatic cancer in older men. The AR has four principal domains: a large N-terminal transactivation domain (ARTAD), a DNA-binding domain (ARDBD), and a hinge domain, followed by the C-terminal ligand-binding domain (ARLBD). In the absence of androgen, it is generally accepted that AR is cytoplasmic. Androgens bind specifically to a ligandbinding pocket in the lower half of the LBD, causing a conformation change (1), the release of heat-shock proteins, and translocation of the ligand-AR complex to the nucleus, where the DBD interacts with specific response elements on the promoters of target genes. Transactivation functions reside mainly in the ARTAD, and very minimal activity can be demonstrated in the ARLBD (2, 3). Unlike other steroid receptors, transactivity depends on ligand-induced interactions between the ARTAD and ARLBD, and mutations that reduce TAD-LBD interactions affect AR activity, causing androgen insensitivity syndromes (4-6). Like other transcription regulators, the promoter-bound AR serves as a nidus to recruit cofactors that up-regulate (coactivators) or down-regulate (corepressors) AR activity. The most clearly defined class of coactivators is the p160͞steroid receptor coactivator (SRC) family. They include SRC1, transcriptional intermediary factor 2 (SRC2͞TIF2), and SRC3͞TRAM1͞A IB1͞pCIP͞ACTR͞ RAC3, who bind hydrophobic grooves in the LBDs of steroid receptors via LXXLL motifs in their nuclear-receptorinteracting boxes, draw in cAMP-response element-binding protein (CBP͞p300), pCAF, and other cofactors to modulate chromatin and initiate transcription by RNA polymerase II (7). Of the p160͞SRC proteins, TIF2 interacts most strongly with the AR (8) and, in concert with CBP͞p30...