Androgen action and the Androgen Receptor
Introductory Physiology and Pharmacology of Androgens – Endogenous androgens are well known for their lots of functions in promoting sexual differentiation and the induction of the male phenotype. In the male, the two endogenous androgens a lot of active in promoting these effects are testosterone (T) and dihydroxytestosterone (DHT).T is the most quantitatively crucial androgen in systemic circulation while DHT is the most abundant cellular metabolite and a lot of potent androgen in a lot of androgen sensitive tissues (excluding skeletal muscle; Mainwaring 1977).
The physiological effects of androgens have been discussed considering that the 1930’s when several investigators observed that the injection of male urinary extracts into dogs not only promoted androgenic effects on the canine reproductive tract but also caused nitrogen retention or an anabolic effect (Kochakian and Mrulin 1935). considering that then, much information has been gathered about the various anabolic and androgenic effects of exogenous androgens on human physiology (Braunstien 1997). during fetal development androgens are crucial in the proper differentiation of the internal and external male genital systems. Later, during puberty, androgens mediate growth and functional integrity of the scrotum, epididymis, vas deferens, seminal vesicles, prostate, and penis. during this time androgens also stimulate skeletal muscle growth, growth of the larynx, and stimulate the pubertal growth spurt. Both ambisexual hair growth and sexual hair growth as well as sebaceous gland activity are regulated by androgens throughout the life cycle. Finally, androgens also play lots of diverse roles in the adult including: behavioral roles (sexuality, aggression, mood, and cognitive function), guideline of spermatogenesis, guideline of bone metabolism, maintenance of muscle mass and muscle function, various effects on the cardiovascular system, and guideline of prostate cancer (Nieschlag and Behre 1998). This list is far from exhaustive as androgens a lot of likely play roles in nearly every organ and cell of the body. As additionally investigations are conducted, additional physiological effects of endogenous androgens will definitely be uncovered.
Although the prior brief discussion has dealt with the physiological effects of the endogenous androgens T and DHT, it need to be noted that many exogenous steroids have been synthesized in attempts to alter the anabolic to androgenic ratios relative to these two hormones (for a review see Vida 1969). In scientific situations of hypogonadism, T replacement is needed to replace both the anabolic and androgenic effects of the deficient endogenous androgens. In such situations, T therapy alone is warranted. but in other situations of anabolic deficiency such as catabolic wasting syndromes and administration of glucocorticoids, agents that promote anabolism (nitrogen retention) in the absence of androgenic effects are desirable. Although these agents were originally called “anabolic steroids”, no compound has yet been synthesized that completely dissociates anabolic from androgenic effects. as a result these agents are still appropriately termed anabolic androgenic steroids (AAS). Interestingly, subsequent investigations of various anabolic androgenic compounds have demonstrated that lots of (but not all) of the compounds with very low affinity for the androgen receptor have a a lot more complete dissociation of androgenic and anabolic effects (Saartok et al 1984, Dahlberg et al 1981). considering that their relative binding affinities can be as low as 0.01, the mechanism of action of anabolic androgenic steroids might only be directly receptor dependent in a few situations. These situations include substantial intracellular metabolism of the low affinity anabolic androgenic compounds to high affinity compounds or concentration dependent displacement of receptor bound T and DHT by the anabolic androgenic compounds (Gustafsson et al 1984). In addition, even in the absence of viable androgen receptors, these compounds exert androgen certain or anabolic effects in various tissues of the body (Rommerts 1998). These observations may offer indirect evidence for distinct androgen receptor dependent (direct) and androgen receptor independent (indirect) mechanisms of action for the various endogenous and exogenous anabolic androgenic steroids. In fact, Rommerts et al propose that although distinct in some tissues, direct and indirect androgen action may be closely linked in tissues sensitive to both effects (Rommerts 1998). As androgen research becomes a lot more advanced and focuses on examining the androgen receptor, nuclear androgen action elements, and androgen signaling, researchers are getting closer to the desired dissociation of anabolic and androgenic effects.
Androgen action – direct and Indirect Mechanisms
Androgen action on target cells remains only partially characterized and understood. original investigators believed that androgens exerted their effects only through a cytosolic androgen receptor present only in sex-dependent tissues of the body. Today we know the situation to be a lot more complex as both direct or genomic effects as well as indirect or non-genomic effects have been uncovered in nearly every tissue of the body. In addition, androgen receptors have been localized in lots of tissues not thought to be androgen sensitive. using radioligand binding techniques, biochemical exchange assays, and immunohistochemical techniques, it is clear that androgen receptors are Camiseta Selección de fútbol de Polonia present in both cytosolic as well as nuclear cellular compartments (Sar et al. 1990).
Although androgens possess both genomic (direct) and non-genomic (indirect) actions, it has been thought that the majority of their action is through direct activation of DNA transcription by means of high affinity interactions with intracellular Camiseta Leeds United androgen receptors (AR). At least it is though so because these interactions have been studied in the most detail. Although receptor dependent interactions may ultimately turn out to be quantitatively a lot of important, as androgen receptor independent actions continue to be uncovered, the value of these non-genomic interactions may shed new light on androgen’s effects.
It has been demonstrated that some androgen sensitive tissues do not consist of nuclear androgen action elements (ARE). In addition, other androgen sensitive tissues do not consist of viable intracellular androgen receptors due to AR insensitivity, the absence of AR, or AR blockade. As a result, it has been hypothesized that endogenous androgens (T and DHT for example) may act indirectly on cells without the presence of an AR. To this end, it is thought that androgens might can act as mediators of secondary transcription factors; that they might act in the guideline of autocrine and paracrine mediators of gene expression; or that they might influence the secretion of other hormones that mediate androgen effects in distant tissues (Verhoeven and Swinnen 1999). In addition it is thought that some of these effects may be the result of plasma protein bound androgen interaction with extracellular receptors (Rommerts 1998). some of the postulated non-genomic, AR-independent effects of androgens include:
-increases in both liver derived and locally produced IGF-I and IGF-I mRNA (Arnold et al 1996, Mauras et al 1998) -displacement of glucocorticoids from the glucocorticoid receptor and interference of glucocorticoid binding to glucocorticoid action elements (Hickson et al 1990, Danhaive and Rousseau 1986, Danhaive and Rousseau 1988) -the release of several autocrine “andromedins” including androgen induced growth factor, schwannoma-derived growth factor, keratinocyte growth factor, and fibroblast growth factor, to name a few (Tanaka et al 1992, Sonoda et al 1992, Yan et al 1992) -transmembrane influx of extracellular calcium (Koenig et al 1989, Lieberherr and Grosse 1994, Steinsapir et al 1991) -activation of extracellular signal-related kinase cascades by means of binding to a yet unidentified extracellular receptor (Peterziel 1998)
Although the indirect androgen actions discussed above are still subject to speculation, the evidence for androgen receptor independent action is becoming a lot more impressive. direct androgen action, on the other hand, is well characterized.
There is however some ambiguity as to whether androgen binds the AR in the cytosol or in the nuclear membrane. Regardless, the AR is typically bound to heat shock protein 90 that maintains the AR inactive state and the AR hormone binding affinity (Fang et al 1996). Upon binding however, direct androgen action is initiated as inhibitory heat shock proteins are released from the androgen receptor. The AR is then phosphorylated and undergoes a conformational change needed for translocation and dimerization (Grino et al. 1987). Although in the wild-type receptor, this ligand binding is needed for transcriptional activity, one in vivo receptor with a deleted ligand binding domain does posess transcriptional activity. This may indicate that the unliganded binding domain is actually a repressor of receptor action due to conformational constraints in the unbound receptor possessing the ligand binding domain (Jenster et al 1991). once in the nucleus (either by direct binding there or by translocation), the phosphorylated receptor is dimerized and binds to a DNA androgen action element (ARE). The hormone action element, which is also bound by other hormone receptors from this family, is a 15 base pair sequence responsible for transcription initiation. once bound, other transcription regulating proteins or co-activators may also bind the AR-ARE complex to stabilize the promoter of the regulated gene (Shibata et al 1997, Kang 1999). Such co-activators include proteins such as ARA 54, ARA 55, ARA 70, ARA 160 (Yeh et al 1996,Hsiao et al 1999). This binding of such co-factors ultimately results in the guideline of transcription rate. The resultant mRNA from androgen dependent transcription is then processed and transported to ribosomes where it is translated into proteins that can alter cellular function. Although the above mechanism is by far the most predominant, in some tissues there is evidence for a ligand-independent dependent activation of transcriptional activity by means of the AR. As pointed out above, an unliganded receptor with a deletion of the ligand binding domain may possess activity. This indicates activity in the absence of ligand binding. In addition, growth aspects (insulin-like growth factor, keratinocyte growth factor, and epidermial growth factor) as well as protein kinase A activators might be able to induce a transcriptionally active AR in the absence of ligand binding (Culig et al 1995, Nazareth and Weigel 1996). some of these ligand independent transcription activators may act by means of influencing the AR phosphorylation state.
The Androgen Receptor
The androgen receptor is a member of the steroid receptor family of nuclear transcription factors. This family is a group of structurally related nuclear transcription aspects that mediate the action of steroid hormones. The steroid receptor family includes three other receptors including the glucocorticoid receptor, the mineralocorticoid receptor, and the progesterone receptor (Beato 1989). Although there are several regions of each receptor that are heterologous, the ligand-binding and DNA-binding domains are surprisingly highly conserved (Sheffeild-Moore 2000). In addition to their structural homology, these receptors also are related by their ability to activate gene transcription by means of the Camiseta Selección de fútbol de Brasil same DNA hormone action element (Quigley et al 1995).
There are two characterized forms of the androgen receptor. The first, and predominant form, is a 110-114 kDa protein of 910-919 amino acids (Jenster et al 1991, Wilson et al 1992, Liao et al 1989). The second is a smaller 87 kDa protein of about 720-729 amino acids in length that makes up only about 4-26% of the detectible androgen receptors located in varying tissues (Wilson and McPhaul 1996). The relevance of this second form of receptor is unknown, but the full-length receptor has been well-characterized. The isolation and characterization of this form of the human androgen receptor cDNA has allowed for sequencing of its amino acid constituents (Chang et al 1989).
The human androgen receptor is a single polypeptide comprised of four discrete functional domains (Quigley 1998).
The A/B region is the N-terminal domain of the AR and comprises over half of the receptor protein (residues 1-537). Within this domain is a transcription activation region and several regions of homopolymeric amino acid stretches that may be crucial in transcriptional regulation. These amino acid stretches may also be crucial in interactions with other regions of the receptor protein and in figuring out the three-dimensional structure of the receptor. among the four members of the steroid receptor family, this region is poorly conserved both in length and sequence similarity (Evans 1988). The C region of the AR (residues 559-624) is the DNA binding domain. This region is composed of two folded “zinc fingers” which each binding one zinc ion. The first zinc finger is responsible for recognition of the target DNA sequence while the second stabilizes DNA-receptor interaction by contact with the DNA phosphate backbone (Freedman 1992, Berg 1989). between members of the steroid receptor family, this region is the most highly conserved. At the overlap between the between the C and D regions, there is a nuclear targeting sequence (amino acids 617-633) that is responsible for androgen dependent translocation from the cytosol to the nucleus (Jenster 1993). The D region, or hinge region (residues 625-669), seems to be responsible for androgen dependent conformational changes of the AR. In addition, one of the AR phosphorylation sites is located in this region (Zhou et al 1995). Finally, the E r
Introductory Physiology and Pharmacology of Androgens – Endogenous androgens are well known for their lots of functions in promoting sexual differentiation and the induction of the male phenotype. In the male, the two endogenous androgens a lot of active in promoting these effects are testosterone (T) and dihydroxytestosterone (DHT).T is the most quantitatively crucial androgen…