Androgen Receptor (AR)
Androgen receptor (AR) mediates the effects of androgens that are responsible for diverse biological functions, such as development and maintenance of the male reproductive system, as well as involvement in disease states, such as prostate cancer (PCa) (for a review, see Brinkmann (2011)). AR is a ligand-activated transcription factor (TF) that belongs to the steroid hormone receptor (SHR) family within the nuclear receptor (NR) superfamily of TFs (for a review, see Huang et al. (2010)). In addition to AR, the SHR family contains the glucocorticoid receptor (GR), mineralocorticoid receptor (MR), progesterone receptor (PR), and the estrogen receptor (ER). SHRs are structurally and functionally related and mediate the action of steroid hormones that affect nearly all aspects of development and homeostasis; they are also implicated in a number of pathological conditions.
The first evidence for the protein nature of AR from androgen target tissues were published at the end of the 1960s (Fang et al. 1969; Mainwaring 1969; Baulieu and Jung 1970). During the 1970s and 1980s, much effort went into the purification and further characterization of AR from different sources. The human AR cDNA was finally cloned in 1988 and 1989 independently by several groups just a few years after cloning of the GR in 1984 (Chang et al. 1988; Lubahn et al. 1988; Trapman et al. 1988; Tilley et al. 1989). AR is expressed at low to moderate levels in a variety of cell types with high levels present especially in male and female reproductive tissues, adrenal gland, kidney, and skeletal muscle. In addition to androgens, AR can be activated in an androgen-independent manner through other signaling pathways which can significantly contribute to the diversity of AR action.
AR Domains and Function
Based on biochemical studies and similar to other TFs, AR binds stably to its chromatin template in the presence of the ligand. However, development of live cell imaging and advances in green fluorescent protein (GFP) technology have demonstrated dynamic interactions of AR with its specific regulatory sites. Using fluorescence recovery after photobleaching (FRAP) analysis, residence times of these transient interactions were found to be on a time scale of seconds and were influenced by the nature of the ligand (Klokk et al. 2007).
Given the importance of androgens in normal physiology and disease states, several studies have sought to elucidate AR-mediated gene regulatory mechanisms by integrating differential gene expression analysis and identified genome-wide AR target sites using ChIP-chip and ChIP-seq (for a review, see Mills (2014)). These studies started to identify not only the genes that are direct AR targets but also AR-interacting proteins that are important for AR activity, such as transcriptional coregulators and FOXA1 as an AR-associated pioneer factor.
In addition to regulation by its cognate ligand, AR is subject to modification by phosphorylation, acetylation, methylation, sumoylation, and ubiquitination (for a review, see Gioeli and Paschal (2011)). Posttranslational modifications of AR have impact on protein stability, interaction with other proteins, cellular localization, and structure of the receptor itself. With respect to the complexity of AR regulation, the exact consequences of these modifications to AR function in normal physiology and implications for disease states are yet not known.
To better understand the molecular mechanisms of AR action, several AR knockout and knockin mouse models have been generated, both systemic and tissue specific (for a review, see Matsumoto et al. (2013)). The use of these models resulted in the identification of specific AR roles in male fertility, prostate development, muscle function, metabolism and diabetes, immune function, bone metabolism, and several cancers. One such example is the identification of AR functions in Sertoli and granulosa cells in spermatogenesis and folliculogenesis, respectively. In many other target tissues, the AR function and physiology are of complex nature, such as the study of mesenchymal–epithelial interactions in accessory sex glands and relative contribution of AR and estrogen receptor-mediated signaling in the brain and bone.
Non-genomic Actions of AR
For example, following androgen binding, AR interacts with the p85 regulatory subunit of PI3K in the cytosol which then phosphorylates and activates the downstream effector serine–threonine kinase AKT. The AR-mediated phosphorylation of AKT is inhibited by the AR antagonist bicalutamide documenting the direct involvement of AR in this process.
Another example of non-genomic AR action is where androgen-bound AR physically interacts with and activates the tyrosine kinase c-SRC blocking its auto-inhibitory effect. This stimulates the c-SRC/RAF-1/MEK/ERK-2 signaling cascade. One of the targets of c-SRC is the adaptor molecule SH2-containing protein (SHC) which is an upstream regulator of MAPK. c-SRC-activated MAPK pathway modulates AR-mediated transcription by phosphorylating AR and p160 family of steroid receptor coactivators (SRCs) and thereby regulates several cellular processes such as cell proliferation, migration, and differentiation.
Non-genomic AR signaling may also occur in an ERK-independent manner, via activation of mammalian target of rapamycin (mTOR) pathway, or through plasma membrane G protein-coupled receptors (GPCRs) that modulate intracellular Ca2+ concentration.
AR and Disease States
Given the diverse role of androgens in a variety of tissues, perturbation to AR action has been linked to a number of disease conditions. For example, AR dysregulation has been associated with cancer (e.g., prostate, testicular, colorectal), cardiovascular defects (coronary artery disease), immune disease (e.g., type I diabetes), metabolic disorders (i.e., obesity and androgen insensitivity syndrome, osteoporosis), neurological conditions (e.g., Alzheimer’s disease), and other diseases such as Kennedy’s syndrome (for reviews, see Matsumoto et al. (2013) and Shukla et al. (2016)).
AR has been most widely studied in terms of its involvement in PCa (for a review, see Attard et al. (2016)). The key role of androgens in PCa genesis and progression was first observed in 1940s and has formed the basis of androgen ablation/castration therapies. Although initially highly effective, this therapy ultimately fails, and the disease progresses to a castration-resistant state which is associated with a poor prognosis. It has been shown that one of the key proteins in both the androgen-sensitive and castration-resistant PCa is AR. During the progression to castration resistance, AR signaling is maintained despite reduced levels of circulating androgens. Several mechanisms involving AR are activated, such as local PCa tissue androgen production, AR mutations or truncations, AR gene amplification or posttranslational modifications, AR splice variants, increased mitogen signaling, increased expression of AR coactivators, and increased sensitivity of AR to low androgen levels (for a review, see Wang and Tindall (2011)).
Another disease in which dysregulation of AR has a role is spinal and bulbar muscular atrophy (SBMA) or Kennedy’s disease (Kumar et al. 2011). The SBMA is characterized by the degeneration of the motor neurons which are located in the spinal cord and bulbar regions that express AR. It leads to weakness, atrophy, and fasciculation in the limb and bulbar muscles for which no curative treatment is available. This condition is caused by expansion of the CAG repeat encoding a poly-Q stretch in the AR NTD: whereas ~20 repeats are normal in adults, 40–62 repeats result in SBMA. The expanded poly-Q stretch leads to hormone-dependent AR unfolding and toxic gain of function that contributes to the disease phenotype.
A third AR-linked disease, androgen insensitivity syndrome (AIS), is characterized by partial or complete lack of response to androgens in cells which normally would be androgen sensitive (for a review, see Mongan et al. (2015)). This results in the failure of normal masculinization in 46XY male individuals. In AIS patients, the AR gene is frequently mutated in the DBD and/or the LBD making AR lose its function. Mutations can range from complete or partial deletions of the AR gene, which are rare, to point mutations and frame shift mutations. These can disrupt the DNA binding, AR expression levels, ligand-binding specificity, or efficacy which results in at least a partial functional loss in AR. The AIS can present a wide range of phenotypes depending on the degree of residual AR function.
AR is a ligand-activated TF which mediates the biological effects of androgens in diverse tissues. Upon ligand binding, AR translocates into the nucleus, binds AREs on target genes, and regulates transcription. In addition, AR interacts with other TFs or cofactors, as well as initiating rapid non-genomic cellular responses by direct interactions with several signaling molecules at the plasma membrane or in the cytoplasm. Conversely, AR can also be activated by mitogens, cytokines, and growth factors through signaling pathways which can regulate AR/coactivator activity through posttranslational modifications. Dysregulation of AR expression and/or function has been associated with a variety of disease states, including PCa, SBMA, and AIS. Thus, understanding AR function and structure is important for developing therapies in AR-related clinical conditions.