Androgen Signaling

Androgens are recognized as genotropic inducers of a number of physiological functions mainly associated with the development of sexual characteristics. Androgens promote the growth and differentiation of prostate cells through ligand activation of the AR (Androgen Receptor) (Ref.1&2). The AR, upon activation by Androgens, mediates transcription of target genes that modulate growth and differentiation of prostate epithelial cells. AR signaling is crucial for the development and maintenance of male reproductive organs including the prostate gland (Ref.3).

The majority of serum Androgens is complexed with the plasma glycoprotein SHBG (Sex Hormone-Binding Globulin). The plasma protein SHBG binds to a receptor SHBGR (Sex Hormone-Binding Globulin Receptor) on cell membranes to form an SHBG- SHBGR complex. SHBGR transduces its signal through G proteins. Gs-protein coupled SHBG-R modulates AC (Adenylate-Cyclase) with synthesis of cAMP and activation of PKA (Protein Kinase A). PKA activation dephosphorylates AR resulting in its activation (Ref.4 & 5). PKA activation also significantly increases the phosphorylated form of its downstream effector, CREB (Camp Responsive Element-Binding Protein) in the presence of Androgen (Ref.6 & 7). In another signaling, S1P (Sphingosine-1-Phosphate) binds to and signals through several members of GPCRs (G Protein-Coupled Receptors) (Ref.8). The membrane-ARs transmit their signals via G-proteins. After agonist binding to its receptor, the G-alpha subunit releases GDP (Guanosine Diphosphate), binds GTP (Guanosine Triphosphate) and dissociates from the G-beta-gamma protein complex. G beta-gamma subunits cannot be dissociated under non-denaturing conditions, and regulate activity of PLC (Phospholipase C). Activation of PLC causes increased IP3 (Inositol Trisphosphate) and DAG (Diacyl Glycerol) through PIP2 (Phosphatidylinositol-4, 5-Biphosphate) and DAG activates PKC (Protein kinase C) (Ref.4). PKC, on activation, activates PKA which in turn activates Ca2+ channels. The major routes to regulate the intracellular Ca2+ are plasma membrane integrated Ca2+-channels by L-/T-type Ca2+-channels and sarcoplasmic reticulum by IP3-receptors (Ref.4). Androgens stimulate an elevation in intracellular Ca2+ through a G protein-coupled receptor. The elevation of intracellular Ca2+ is detected by specific Ca2+ sensor molecules, including PKC and CaM (Calmodulin), to induce signal transduction cascades and modulation of transcription factor activity. Simultaneously IP3 activates specific receptors in the sarcoplasmic reticulum to release Ca2+ into the cytosol (Ref.5 & 9). Ca2+ functions as a ubiquitous messenger and modulation of intracellular Ca2+ levels regulates a wide range of cellular processes, including proliferation, apoptosis, motility, and gene expression. The Androgen–AR complex interacts with the SH3 domain of Src (Steroid Receptor Coactivator) kinase and leads to a rapid activation of Shc (SHC (Src Homology-2 Domain Containing) Transforming Protein) which rapidly induce the formation of SHC/GRB2 (Growth Factor Receptor-Binding Protein 2)/SOS (Son Of Sevenless) complexes, leading to the stimulation of the MAPK (Mitogen-Activated Protein Kinase) pathway, involving growth factors and growth factor receptors. In cooperation with GRB2, the guanine-nucleotide exchange factor SOS potentiates Ras, by catalyzing the replacement of GDP for GTP. Ras activates Raf-1 which further stimulates MAPK mediating MEK (Mitogen-Activated Protein/Extracellular Signal-Regulated Kinase). In another related signaling, Ras recruits PI3K (Phosphatidylinositol 3-Kinase) phosphorylates and potentiates Akt (v-Akt Murine Thymoma Viral Oncogene Homolog-1) (Ref.4&5)). MAPK directly modifies ligand-dependent activation of AR modulating AREs (Androgen-Receptor Element)-inducible gene transcription downstream of the AR. CREB is a downstream target of active MAPK through the mediation of the RSK (RSK kinase) and p300/CBP (CREB-Binding Protein) bridge the interaction between AR and CREB (Ref.6&10). Activation of the PI3K–Akt pathway promotes AR ubiquitylation and leading to AR degradation via a proteasome-dependent pathway. MDM2 (Mouse Double Minute-2) and Akt form a complex with AR and induce AR ubiquitylation and degradation in a phosphorylation-dependent manner. AR phosphorylation by Akt promotes AR ubiquitylation and subsequent degradation by the 26S proteasome (Ref.11). Phosphorylation of Bcl2 by Raf-1-ERK dependent pathway enhances its anti-apoptotic function. Moreover, Bcl2(B-cell Lymphoma Protein-2) acts dimerizing with and inhibiting pro-apoptotic protein counterparts such as BAD (Bcl2-Antagonist of Cell Death) and the phosphorylation of Bcl2 likely favors the dimerization with BAD. The phosphorylation of BAD is followed by its inactivation (Ref.12). A triple complex between AR, the regulatory subunit p85 of PI3K (PIK3R1) and Src tyrosine kinase is formed in the presence of Androgen, which is negatively regulated by PTEN (Phosphatase and Tensin Homologue) as it function as an AR corepressor. The direct interaction between the AR and PTEN inhibits AR nuclear translocation and promotes AR protein degradation (Ref.7). PTEN also cleaves Caspases 8, 9 and 3 (Ref.13). G12 regulates low molecular weight GTPase RhoA, but when signaling, G-proteins function as dimers because the signal is communicated either by the G-alpha subunit or the G-beta-gamma complex (Ref.4). A signal transduction pathway links the RhoA effector PRK1 (Protein Kinase C-Related Kinase-1) to the transcriptional activation of the AR (Ref.14). Further, in the AR activation process, AR LBD (Ligand Binding Domain) interacts transiently with HSP40 (Heat Shock Proteins-40), HSP70, HSP90. HSP70 and HSP40 also reassociate with the AR in the presence of ligand and facilitate transport of the receptor into the nucleus and this process is accompanied by coregulators of AR, such as ARA55 (Androgen Receptor Coactivator-55) and ARA70 (Ref.7). In the Wnt signaling, Wnt binds to its receptors Frizzled and LRP5/6, activating the downstream component Dishevelled, which in turn inhibits GSK-3â (Glycogen Synthetase Kinase-3Beta), Axin and APC (Adenomatous Polyposis Coli) in the â-Catenin destruction complex. Once stabilized, â-Catenin binds to LEF (Lymphoid Enhancer Factor)/TCF (T-Cell Factor) transcription factors in the nucleus, interacting with the AR and potentiating the AR transcriptional activity in an Androgen-dependent fashion. At the same time, GSK-3â interacts with the AR and suppresses its ability to activate transcription whereas on the other side Akt phosphorylates and inactivates GSK-3â (Ref.7&15)). In the nucleus, DJ-1 interacts directly with the AR whereas DJBP (DJ-1-Binding Protein) binds the DBD of the AR in an Androgen-dependent manner and co-localizes with DJ-1. DJBP represses Androgen-dependent AR transactivation activity by recruiting a HDAC complex. DJ-1 partially restores the activity of the AR by abrogating the DJBP-HDAC complex (Ref.7&16)). Among the other negative regulators of AR, includes Calreticulin, HBO1 (Human Origin Recognition Complex Interacting Protein), Daxx (Death-Domain Associated Protein), Smad3 (Sma and MAD (Mothers Against Decapentaplegic) Related Protein-3)), DAX-1 (DSS-AHC Critical Region on The X Chromosome Protein-1) and cyclin D1 that repress ligand-mediated transcriptional activation of AR. Again, FOXO1 (Forkhead Box O1), as a downstream molecule becomes phosphorylated and inactivated by PI3K/AKT kinase in response to growth factors, and subsequently suppresses ligand mediated AR transactivation. NRIP1 (Nuclear Receptor Interacting Protein 1) also act as a strong AR repressor. NRIP1 reverse TIF-2 (Transcriptional Intermediary Factor-2) dependent overactivation of AR. SUMO-1(Small Ubiquitin-Related Modifier) decreases, whereas SUMO-2 and -3 enhance AR transcriptional activity. AR is covalently modified by SUMO-1 (sumoylated) in an Androgen-enhanced fashion. Whereas, with TR4 (Testicular Orphan Receptor-4), AR interact with it and function as a repressor to down-regulate the TR4 target genes by preventing the TR4 binding to its target DNA (Ref.2, 7, 17&18). The formation of a productive AR transcriptional complex requires the functional and structural interaction of the AR with its coregulators as, CBP, p300, P/CAF (P300/CBP-Associated Factor), SRC-1(Steroid Receptor Co-Activator-1), TIF-2 (Transcriptional Intermediary Factor-2), which contain inherent HAT (Histone Acetylation) activity. Moreover, RanBP9 (Ran-Binding Protein) and HIPK3 (Homeodomain-Interacting Protein Kinase 3) are identified as AR-binding protein. AR interacts with other transcription factors, including NF-kB(Nuclear Factor-kB ), SRF (Serum Response Factor), sp1 (Specificity Protein 1), c-Jun and c-Fos (Cellular Oncogene Fos). An increased activity of another transcription factor Elk-1 also activates c-fos expression and elicits gene transcription independently of AR binding to DNA response elements (Ref. 7, 18&19). Enhanced transcription by the AR depends on the recruitment of RNA polymerase II to promoters of its target genes. This is achieved by the assembly of general transcription factors that make up the PIC (Preinitiation Complex). Formation of the PIC is accomplished by binding of TFIID, which is composed of TBP (TATA-binding protein) and TBP associated factors, in the proximity of the transcriptional start site. TFIIB then binds TBP and recruits RNA polymerase II and TFIIF, which ensures specific interaction of RNA polymerase II at the promoter. TFIIE and TFIIH are recruited to RNA polymerase II to facilitate strand separation, which allows transcription initiation. AR as well binds to HMT (Histone Methyltransferase) in this process (Ref. 7).

AR point mutations changes expression of AR co-regulatory proteins leading to prostate cancers and more frequently tumors. Significant lower levels of Androgen causes CAD (coronary artery disease), thus implying that low circulating Androgen levels are correlated with progression of atherosclerosis. Indeed, numerous risk factors for CAD are associated with Hypotestosteronaemia. Chronic heart failure due to idiopathic dilated cardiomyopathy is also associated with a significant decrease in Androgen level. In neurons, Androgens are also able to rapidly modulate Neuronal synaptic plasticity (Ref.4&20).

References

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Zhu ML, Kyprianou N.

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Chen Y, Sawyers CL, Scher HI.

Curr Opin Pharmacol. 2008 Aug;8(4):440-8.

4.Rapid actions of androgens.

Michels G, Hoppe UC.