C-terminal Ligand-binding Domain Research Articles

Androgen Signaling in Prostate Cancer: When a Friend Turns Foe.

Androgen (AR) signaling is the main signaling for the development of the prostate and its normal functioning. AR is highly specific for testosterone and dihydrotestosterone, significantly contributing to prostate development, physiology, and cancer. All these receptors have emerged as crucial therapeutic targets for PCa. In the year 1966, the Noble prize was awarded to Huggins and Hodge for their groundbreaking discovery of AR. As it is a pioneer transcription factor, it belongs to the steroid hormone receptor family and consists of domains, including DNA binding domain (DBD), hormone response elements (HRE), C-terminal ligand binding domain (LBD), and N-terminal regulatory domains. Structural variations in AR, such as AR gene amplification, LBD mutations, alternative splicing of exons, hypermethylation of AR, and co- regulators, are major contributors to PCa. It's signaling is crucial for the development and functioning of the prostate gland, with the AR being the key player. The specificity of AR for testosterone and dihydrotestosterone is important in prostate physiology. However, when it is dysregulated, AR contributes significantly to PCa. However, the structural variations in AR, such as gene amplification, mutations, alternative splicing, and epigenetic modifications, drive the PCa progression. Therefore, understanding AR function and dysregulation is essential for developing effective therapeutic strategies. Thus, the aim of this review was to examine how AR was initially pivotal for prostate development and how it turned out to show both positive and detrimental implications for the prostate.

The N-terminal activation function AF-1 domain of ERα interacts directly with the C-terminal AF-2-holding ligand-binding domain to recruit the coactivator proteins.

Cryoelectron microscopy (cryo-EM) clarified the quaternary structure of the DNA complex of coactivator-bound estrogen receptor alpha (ERα), revealing the adjacency of the N-terminal domain (NTD) and C-terminal ligand-binding domain (LBD). ERα-NTD and LBD constitute activation function 1 (AF-1) and activation function 2 (AF-2), respectively. These domains are essential for transcription activation. Their spatial proximity was judged to be essential for ERα to recruit the SRC coactivator proteins. In the present study, we first evaluated untethered free ERα-NTD(AF-1) [residues 1-180] and its-truncated desNTD(AF-1)-ERα [residues 181-595] in a luciferase reporter gene assay. ERα-NTD(AF-1) was completely inactive, whereas desNTD(AF-1)-ERα exhibited 66% activity of wild-type ERα. Surprisingly, ERα-NTD(AF-1) was found to inhibit desNTD(AF-1)-ERα markedly. Therefore, assuming that ERα-NTD(AF-1) must also inhibit wild-type full-length ERα, we co-expressed ERα-NTD(AF-1) and full-length ERα. As expected, ERα-NTD(AF-1) inhibited ERα in a dose-dependent manner, but non-competitively for 17β-estradiol. When their intracellular transport was examined immunocytochemically, ERα-NTD(AF-1) showed a distinct translocation from the cytoplasm to the nucleus, despite being expressed solely in the cytoplasm without full-length ERα. This nuclear translocation was attributable to a direct interaction between ERα-NTD(AF-1) and full-length ERα consisting of the nuclear localization signal. The present results demonstrated that, in full-length ERα, the N-terminally tethered NTD(AF-1) domain collaborates with the C-terminal LBD(AF-2) for coactivator recruitment.

Targeted Delivery of AR-V7 siRNA with Bivalent PSMA Aptamers Effectively Suppresses the Growth of Enzalutamide-Resistant Prostate Cancer.

Androgen deprivation therapy has been the primary treatment strategy for advanced prostate cancer (PCa). But most patients develop castration resistance over time. For FDA-approved second-generation androgen receptor (AR) antagonists, including enzalutamide (ENZ) and abiraterone (AA), patients who initially respond to them eventually develop resistance. The key mechanism for resistance to ENZ/AA involves AR splice variants (AR-Vs) and specifically AR-V7. Current AR antagonists cannot target AR-V7 due to its lack of the C-terminal ligand-binding domain (LBD) but keeping the AR N-terminal domain (NTD) which still can activate androgen-responsive genes. Therefore, targeting the AR NTD and AR-V7 is critically important to overcome ENZ resistance. Unfortunately, AR NTD has been considered an "undruggable" target due to the difficulty in defining its three-dimensional (3D) structure. In this context, siRNA is highly suitable to address this undruggable target. However, siRNA cannot freely diffuse into cells, and a carrier is needed. In this regard, nucleic acid-based aptamers are highly suitable for cell type-specific delivery of siRNA in vivo. In this study, we have developed a serum-stable bivalent prostate-specific membrane antigen (PSMA) aptamer-AR-V7 siRNA chimera (PAP). The results show that PAP can knock down both AR-full length and AR-V7 in PSMA-expressing castration-resistant cells. It can resensitize ENZ in cell lines and PCa xenografts. ENZ combined with PAP can significantly inhibit 22Rv1 xenograft growth in mice without experiencing castration. Owing to the low toxicity, PAP has potential to offer a new antiandrogen treatment for current ENZ-resistant PCa.

VDR and deubiquitination control neuronal oxidative stress and microglial inflammation in Parkinson’s disease

Close correlation between vitamin D (VitD) deficiency and Parkinson’s Disease (PD) risk, VitD as an adjuvant treatment promising to improve PD progression. However, VitD excessive intake could induce hypercalcemia and renal damage. Therefore, upregulation of vitD receptor (VDR) is considered a compensatory strategy to overcome VitD insufficiency and alleviate PD symptoms. In this study, we discovered that VDR played antioxidative roles in dopaminergic neurons by decreasing reactive oxygen species (ROS) and maintaining mitochondrial membrane potential. Further, we newly identified VDR downstream events in C. elegans, including glutathione S-transferase (gst) and forkhead box transcription factor class O (daf-16) mediated oxidative stress resistance. VDR upregulation also mitigated microglial activation through inhibition of NLRP3/caspase-1-mediated inflammation and membrane permeabilization. These findings highlight the multifaceted protective effects of VDR in both neurons and microglia against the development of PD. Importantly, we discovered a novel deubiquitinase DUB3, whose N-terminal catalytic domain interacted with the C-terminal ligand-binding domain of VDR to reduce VDR ubiquitination. Identification of DUB3 as an essential player in the deubiquitinating mechanism of VDR provides valuable insights into VDR regulation and its potential as a therapeutic target for PD.

SAT022 A Mechanism Of Resistance To Novel Inhibitors Of The N-Terminal Domain Of Androgen Receptor

Abstract Disclosure: J. Setiawan: None. N.R. Mawji: None. A.H. Tien: None. R.J. Andersen: Stock Owner; Self; ESSA Pharma. M.D. Sadar: Stock Owner; Self; ESSA Pharma. Background: Androgen receptor (AR) is a steroid hormone receptor and a major therapeutic target in prostate cancer. Antiandrogen therapies directed at the AR C-terminal ligand-binding domain (LBD) provide initial benefit to patients with advanced prostate cancer, but they eventually progress to lethal metastatic castration-resistant prostate cancer (CRPC). AR N-terminal domain (NTD) inhibitors are a novel class of antagonists that were developed to overcome resistance mechanisms related to the AR-LBD, including splice variants lacking the LBD (AR-V7). While mutations in the AR-LBD have been well-characterized, the impact of NTD mutations on AR transcriptional activity and drug efficacy remains unclear. The mutation W435L was discovered from clinical samples of prostate cancer from patients treated with antiandrogens. This mutation stabilizes interaction between the AR’s NTD and LBD (N/C interaction) to increase AR transcriptional activity. Tryptophan residues W435 and W397 in AR NTD are essential in the binding site for the AR-NTD inhibitor EPI-7386 that is currently in clinical trials. We hypothesize that tryptophan mutations within the EPI-7386 binding site of AR are a potential mechanism of resistance. Methods: Site-directed mutagenesis was used to create expression vectors with W435 and W397 point mutations in AR. Plasmids were transfected into AR-negative CV-1 cells to compare the transcriptional activity of wild-type (WT) versus mutant ARs. IC50s for each inhibitor were generated for androgen-induced reporter gene constructs. A mammalian two-hybrid system assay was used to assess the impact of inhibitors and AR mutations on N/C interaction. Results: The AR-W435L mutation led to higher IC50s to inhibit androgen-induced MMTV-luciferase activity by NTD inhibitors and antiandrogens. Differences in N/C interaction were observed between WT and mutant ARs treated with EPI-7386. Importantly, the ability of another EPI analog, EPI-7170, to decrease AR-N/C interaction was not significantly impacted by W435L. These data support that loss of efficacy with EPI-7386 is not across the entire class of EPI analogs and may be specific to EPI-7386. Conclusion: Tryptophan mutations in the EPI-7386 binding site are predictive of resistance to this specific antagonist and do not broadly apply to the entire class of EPI analogs. Key differences were revealed between AR-LBD and AR-NTD inhibitors that support the recruitment of different tau regions in the NTD. Presentation: Saturday, June 17, 2023

Abstract 5011: Targeting kinome reprogramming in ESR1 fusion-driven breast cancer

Abstract Background: Transcriptionally active ESR1 gene fusions (ESR1-TAF) are a potent cause of estrogen receptor alpha-positive (ERα+) breast cancer endocrine therapy (ET) resistance. These ESR1-TAF are gain-of-function mutations, exhibiting estrogen-independent cell growth, motility and ET resistance. They are not directly druggable because the ERα C-terminal ligand binding domain (LBD) encoding sequence is replaced with a translocated in-frame partner gene sequence. Herein we utilized proteomic approaches to develop novel targeted therapies against ESR1-TAF driven tumorigenesis. Methods: ESR1 fusion cDNA constructs were expressed in ERα+ breast cancer cell lines (T47D and MCF7). Cell growth was assayed by an Alamar blue assay. A mass spectrometry (MS)-based Kinase Inhibitor Pulldown Assay (KIPA) was employed to identify druggable kinases that are commonly upregulated by diverse ESR1-TAFs. A panel of 22 ERα+ patient-derived xenograft (PDX) models were profiled using proteomics and phosphoproteomics to identify models with sensitivity to RET kinase inhibition. Results: KIPA detected an increased abundance of a receptor tyrosine kinase, RET, in T47D cells expressing ESR1-TAFs in an estrogen-independent manner, compared to stable cell lines expressing transcriptionally inactive fusions as well as wild-type ERα protein. Interestingly, RET was also increased when constitutive activating ERα LBD point mutants, Y537S and D538G, were expressed in breast cancer cells. Inhibition of the RET kinase in vitro by repurposing pralsetinib, an FDA-approved RET inhibitor for advanced thyroid and non-small-cell lung cancers, demonstrated a significant reduction in the growth of cells expressing ESR1-TAFs and ERα LBD mutants. These data nominate RET kinase as a common therapeutic vulnerability for ESR1-TAF expressing breast cancers. Proteomic profiling of 22 biologically heterogenous ERα+ PDX tumors defined targetable pathways and predicted tumor subsets that were responsive to RET inhibition therapy. Organoids and xenografts from the pan-ET resistant WHIM18 PDX (that expresses the ESR1-YAP1 TAF) were inhibited by pralsetinib to a similar extent as the CDK4/6 inhibitor palbociclib. These data provide key preclinical rationale for the consideration of RET inhibition for the treatment of ESR1-TAF-driven ET-resistant breast cancer. Interestingly, the growth of WHIM37 PDX (that expresses ERα D538G) that had low level of RET and high level of GFRα-1, the co-receptor of RET, was also suppressed by pralsetinib. This data suggests that either RET or GFRα-1 is a predictive biomarker for RET inhibitor efficacy. Conclusions: Kinome analysis of ESR1 translocated breast tumors using KIPA followed by drug sensitivity studies nominated RET as a new therapeutic target for ET-resistant ERα+ breast cancer. Citation Format: Xuxu Gou, Beom-Jun Kim, Meenakshi Anurag, Jonathan T. Lei, Meggie N. Young, Matthew V. Holt, Diana Fandino, Craig T. Vollert, Purba Singh, Mohammad A. Alzubi, Anna Malovannaya, Lacey E. Dobrolecki, Michael T. Lewis, Shunqiang Li, Matthew J. Ellis, Charles E. Foulds. Targeting kinome reprogramming in ESR1 fusion-driven breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5011.

Abstract PD10-02: PD10-02 Novel ER fusion detection method to gain insight in fusion prevalence and endocrine resistant mechanisms

Abstract Breast cancer is a leading cause of female mortality and despite advancements in diagnostics and personalized therapeutics, metastatic disease largely remains incurable due to drug resistance. The druggable estrogen receptor (ER, ESR1), overexpressed in two-thirds of all breast cancer, evolves in 30% of tumors exposed to endocrine therapy consequently resulting in treatment resistance. A more recently discovered mechanism of ER mediated endocrine resistance is ER fusion proteins. ER fusions, found predominately in metastatic endocrine resistant disease, harbor ESR1 exons 1-6 fused to an in-frame gene partner due to an ESR1 intron 6 translocation break. Our lab has demonstrated that ER fusion proteins, which lack the C-terminal ligand-binding domain (LBD), recapitulate phenotypes of ER proteins harboring endocrine-resistant point mutations occurring in the LBD. Our research goals aim to 1) determine fusion prevalence and emergence, 2) understand fusion mechanisms of resistance and 3) explore alternative treatment options for our patients. The promiscuous nature of fusion partners hinders fusion detection by traditional sequencing and thus our research team has developed a novel ER fusion detection method, EnRich. The EnRich probe set (4,324 probes in total) is composed of two separate probe pools targeting at 2x tiling intronic and exonic ESR1, the upstream promoter region and select oncogenic genes. To optimize the EnRich pipeline, DNA was extracted from a frozen tumor sample and a PDX model harboring ESR1-DAB2 and ESR1-LPP fusions, respectively. ESR1-DAB2 and ESR1-LPP were accurately detected by quantifying discordant paired-end or split reads mapped to chromosome 6. In addition, liquid biopsies from 15 patients with ER positive advanced breast cancer were also assessed through EnRich to uncover unidentified ESR1 structural variants. Two patient samples were detected to harbor ESR1 fusions (ESR1-CCDC170, ESR1-AKAP12 and ESR1-YAP1) and were further validated in corresponding mRNA. These recurrent and novel fusions were supported with more than 10 reads each, indicating that the EnRich pipeline is an effective and accurate sequencing approach to understanding ER fusion prevalence. Our lab, furthermore, has studied ER fusion proteins mechanistically. We have stably overexpressed ER fusions in transgenic breast cancer cell lines engineered with shRNA targeting the endogenous wildtype ER (ESR1-WT). In this ESR1-WT depleted cellular context, we found that ER fusions (notably ESR1-SOX9 and ESR1-YAP1) demonstrate ER hyperactivation through an estrogen response element (ERE) assay compared to ESR1-WT and a truncated exon 1-6 ESR1 (ESR1∆CTD). Importantly, fusion ERE activity was robust in the absence of the ER ligand, estradiol, as well as in the presence of endocrine therapies, implying ER fusion proteins function in a ligand independent, endocrine resistant mechanism. ER fusion positive cell lines were also enriched in oncogenic phenotypes such as enhanced 3D growth, cell survival via colony formation, and migration in a wound scratch assay. These enhanced metastatic potentials of the ER fusions were observed in both invasive ductal and lobular carcinoma cell lines, albeit at varying magnitudes depending on the 3’ fusion partner and phenotype being assessed. Although the ER fusions harbored unique characteristics due to the C-terminal partner, transcriptomic profiling revealed that enhanced EMT and KRAS signaling signatures were shared among all fusions when compared to the ESR1-WT and ESR1∆CTD cell lines, which may serve as future exploitable drug targets. Comprehensive detection and functional evaluation of ER fusion proteins will provide clinicians and patients with better understanding of tumor endocrine-resistant prevalence and discovery of more effective treatment options. Citation Format: Megan E. Yates, Tiantong Liu, Jagmohan Hooda, Sichun Yang, Riyue Bao, Jennifer M. Atkinson, ADRIAN V. LEE, Steffi Oesterreich. PD10-02 Novel ER fusion detection method to gain insight in fusion prevalence and endocrine resistant mechanisms [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD10-02.

Abstract 881: Functional characterization and clinical prevalence of ESR1 fusions in advanced endocrine resistant breast cancer

Abstract Breast cancer is the most frequently diagnosed women’s cancer with one in eight women being diagnosed during her lifetime. Even within the era of novel therapeutics and improved diagnostic screening, mortality rates remain stable, with an estimated 43,600 deaths occurring in 2021 alone. Malignant transformation of breast cancer to advanced disease is the mainstay driver of mortality and is largely attributed to treatment resistance. Fortunately, identification of therapeutic resistance mechanisms has rapidly transformed our understanding of cancer evasion and is enabling personalized treatment guidance and regimens. Notably, mutations and amplifications of the estrogen receptor (ER, ESR1) remain a prominent source of endocrine therapy resistance. Two-thirds of all breast cancer are positive for ER, making it an attractive druggable target that has revolutionized patient care. Unfortunately, however, 30% of tumors exposed to endocrine therapy subsequently develop resistance. Recently identified ER fusion proteins, discovered in metastatic endocrine resistant disease, result in substitution of the C-terminal ER ligand binding domain with a variety of fused in-frame gene partners due to genomic translocations occurring at ESR1 intron 6. Our lab has demonstrated that ER fusion proteins drive robust ER hyperactivation and metastatic potential independent of ER ligand estradiol and in the presence of endocrine therapies such as Fulvestrant. Overexpression of ER fusion proteins, particularly ESR1-SOX9 and ESR1-YAP1, in invasive ductal and invasive lobular carcinoma models with endogenous ER knockdown, resulted in enhanced 3D growth, increased migration by wound scratch assay, enhanced colony formation as a proxy of survival and ER pathway hyperactivation. Targeted endogenous ER knockdown utilizing short hairpin RNA enhances fusion driven phenotypes that are unique to fusion partner proteins and cellular context. ER fusion prevalence in advanced breast cancer remains a challenge due to the promiscuous fusion partners disrupting traditional sequencing techniques. Our group has developed a novel method of ESR1 fusion detection, EnRich, which exploits the ESR1 fusion breakpoints. Importantly, we have assessed EnRich fusion discovery with the use of circulating tumor DNA (ctDNA) from non-invasive liquid biopsies. Our pilot study of 15 blood ctDNA samples detected three different ESR1 fusion gene products which have been further validated by PCR. EnRich will help us identify and monitor treatment resistance of known and novel ESR1 fusions in patient samples irrespective of treatment regime as well as in longitudinal samples collected from patients undergoing endocrine therapy. The in vitro resistant properties of fusion proteins emphasize the necessity to better detect these mutations in patients with advanced disease and determine more appropriate treatment measures. Citation Format: Megan E. Yates, Tiantong Liu, Jagmohan Hooda, Jennifer M. Atkinson, Adrian V. Lee, Steffi Oesterreich. Functional characterization and clinical prevalence of ESR1 fusions in advanced endocrine resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 881.

Abstract 2650: Targeting the DNA-binding domain of the androgen receptor in castration-resistant prostate cancer

Abstract Background: The androgen receptor (AR), which plays a major role in prostate cancer (PCa), consists of a C-terminal ligand-binding domain (LBD), a hinge domain, a DNA-binding domain (DBD), and an N-terminal domain (NTD). Current AR antagonists and AR signaling inhibitors (ASIs), used in androgen deprivation therapy (ADT) and to prevent disease progression, often fail due to production of alternatively spliced AR variants (AR-Vs) that lack the LBD but retain the DBD, which is essential to AR transcriptional activity. This project examines a series of novel compounds designed to inhibit AR-DNA interaction and study how these compounds affect castration-resistant PCa (CRPC). Methods: Compounds potentially targeting the DBD were used to predict direction and position of compounds when docked to the AR. MTT assays, flow cytometry, immunoblots, qRT-PCR, luciferase assay, and immunofluorescence were used to test compounds on AR and PCa. Drug Affinity Responsive Target Stability (DARTS) tested potential binding of the compounds to the AR. Studies in intact mice were used to determine toxicity of the compounds and the maximum tolerated dose. Results: Of 48 compounds synthesized, C15 had the highest and most consistent efficacy overall. C15 decreased metabolic activity in 22Rv1 and CWR-R1 cells that express AR-Vs and in C4 and C4-2 that do not express AR-Vs. Further, C15 had a reduced effect in AR-null PC3 cells and minimal effect in normal human dermal fibroblast (NHDF) cells. In C4-2 and 22Rv1, C15 significantly reduced PSA mRNA and AR transcriptional activity, and in C4-2, C15 significantly reduced TMPRSS2 and Nrdp1 mRNA levels. C08 and C15 reduced pronase-induced AR proteolysis only when the AR DBD is present. Furthermore, C15 reduces AR binding at the PSA enhancer, but DHT stimulation may weaken this binding inhibition and reduction in AR transcriptional activity. However, combining C15 with an AR inhibitor targeting the LBD (enzalutamide, darolutamide, apalutamide) restores the full inhibitory potential and even reduces metabolic activity and AR transcriptional activity further than either drug alone in both C4-2 and 22Rv1s. No toxicity in intact mice was observed with C15 with the highest dose tested being 50 mg/kg. Conclusion: Our preliminary data demonstrate that C15 directly binds to the DBD of the AR and potently inhibits CRPC cells, including those that express AR-Vs missing the LBD. Although increased DHT levels may weaken C15’s inhibition of the AR DBD, addition of a LBD-targeting AR inhibitor improves the inhibitory potential of both drugs. Based on these results, we conclude that C15 may hold great clinical potential as it successfully inhibits CRPC by targeting the AR DBD with minimal toxicity, and it can improve the effects of standard-of-care LBD-targeting AR inhibitors by reducing AR-V-induced resistance. Citation Format: Thomas M. Steele, Elisabeth A. Messner, Maria Malvina Tsamouri, Salma Siddiqui, Brian Bennion, Ruiwu Liu, Paramita Ghosh. Targeting the DNA-binding domain of the androgen receptor in castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2650.

Identification of SF-1 and FOXL2 and Their Effect on Activating P450 Aromatase Transcription via Specific Binding to the Promoter Motifs in Sex Reversing Cheilinus undulatus.

The giant wrasse Cheilinus undulatus is a protogynous socially hermaphroditic fish. However, the physiological basis of its sex reversal remains largely unknown. cyp19 is a key gender-related gene encoding P450 aromatase, which converts androgens to estrogens. cyp19 transcription regulation is currently unknown in socially sexually reversible fish. We identified NR5A1 by encoding SF-1, and FOXL2 from giant wrasse cDNA and cyp19a1a and cyp19a1b promoter regions were cloned from genomic DNA to determine the function of both genes in cyp19a1 regulation. Structural analysis showed that SF-1 contained a conserved DNA-binding domain (DBD) and a C-terminal ligand-binding domain (LBD). FOXL2 was comprised of an evolutionarily conserved Forkhead domain. In vitro transfection assays showed that SF-1 could upregulate cyp19a1 promoter activities, but FOXL2 could only enhance cyp19a1b promoter transcriptional activity in the HEK293T cell line. Furthermore, HEK293T and COS-7 cell lines showed that co-transfecting the two transcription factors significantly increased cyp19a1 promoter activity. The −120 to −112 bp (5′-CAAGGGCAC-3′) and −890 to −872 bp (5′-AGAGGAGAACAAGGGGAG-3′) regions of the cyp19a1a promoter were the core regulatory elements for SF-1 and FOXL2, respectively, to regulate cyp19a1b promoter transcriptional activity. Collectively, these results suggest that both FOXL2 and SF-1 are involved in giant wrasse sex reversal.

The carbohydrate tail of landomycin A is responsible for its interaction with the repressor protein LanK.

Landomycin A (LaA) is the largest member of the landomycin group of angucyclic polyketides. Its unusual structure and strong anticancer properties have attracted great interest from chemists and biologists alike. This, in particular, has led to a detailed picture of LaA biosynthesis in Streptomyces cyanogenus S136, the only known LaA producer. LanK is a TetR family repressor protein that limits the export of landomycins from S136 cells until significant amounts of the final product, LaA, have accumulated. Landomycins carrying three or more carbohydrate units in their glycosidic chain are effector molecules for LanK. Yet, the exact mechanism that LanK uses to distinguish the final product, LaA, from intermediate landomycins and sense accumulation of LaA was not known. Here, we report crystal structures of LanK, alone and in complex with LaA, and bioassays of LanK's interaction with synthetic carbohydrate chains of LaA (hexasaccharide) and LaE (trisaccharide). Our data collectively suggest that the carbohydrate moieties are the sole determinants of the interaction of the landomycins with LanK, triggering the latter's dissociation from the lanK-lanJ intergenic region via structure conversion of the helices in the C-terminal ligand-binding domain. Analysis of the available literature suggests that LanK represents an unprecedented type of TetR family repressor that recognises the carbohydrate portion of a natural product, and not an aglycon, as it is the case, for example, with the SimR repressor involved in simocyclinone biosynthesis.

Identification and characterization of a serine racemase in the silkworm Bombyx mori.

The pupae of lepidopterans contain high concentrations of endogenous d-serine. In the silkworm Bombyx mori, d-serine is negligible during the larval stage but increases markedly during the pupal stage, reaching 50% of the total free serine. However, the physiological function of d-serine and the enzyme responsible for its production is unknown. Herein, we identified a new type of pyridoxal 5'-phosphate (PLP)-dependent serine racemase (SR) that catalyses the racemization of l-serine to d-serine in B. mori. This silkworm SR (BmSR) has an N-terminal PLP-binding domain that is homologous to mammalian SR and a C-terminal putative ligand-binding regulatory-like domain (ACT-like domain) that is absent in mammalian SR. Similar to mammalian SRs, BmSR catalyses the racemization and dehydration of both serine isomers. However, BmSR is different from mammalian SRs as evidenced by its insensitivity to Mg2+/Ca2+ and Mg-ATP-which are required for activation of mammalian SRs-and high d-serine dehydration activity. At the pupal stage, the SR activity was predominantly detected in the fat body, which was consistent with the timing and localization of BmSR expression. The results are an important first step in elucidating the physiological significance of d-serine in lepidopterans.

AR Structural Variants and Prostate Cancer.

Therapeutic interventions for advanced castration-resistant prostate cancer (CRPC) are focused on inhibiting the androgen receptor (AR) through targeting of its C-terminal ligand binding domain (LBD). However, a significant subset of CRPC patients demonstrate primary resistance to androgen deprivation and anti-androgen therapies, suggesting that other targets, outside of the AR, might be pertinent to the cancer progression. One explanation is the expression of androgen receptor splice variants (AR-Vs). So far, more than 20 AR-Vs have been identified from both prostate cancer cell lines and prostate cancer tissue biopsies. Most of the AR-Vs have a conserved N-terminal domain, but lack the LBD, yet retain the ability to bind DNA and activate downstream signaling. Although it remains unclear whether AR-Vs are principal divers or mere bystanders of CRPC progression, inhibiting AR-Vs, through drugs that target the AR transactivation function outside of the LBD, has been a major emphasis for next generation therapeutics in prostate cancer. This book chapter is dedicated to the role of AR variants and their clinical importance. We will review the initial discovery of AR-Vs, their regulation and prevalence, as well as their biological function in prostate cancer. We will provide an overview of the role of AR-Vs in the development of metastatic CRPC and in promoting clinical treatment failures. Lastly, we will present an introduction to the therapeutic approaches towards developing AR-V-targeted therapies including the continuing progress, the old challenges, and the new prospects.

Drugging the Undruggable: Targeting the N-Terminal Domain of Nuclear Hormone Receptors.

This chapter focuses on the development of drugs targeting the N-terminal domain of nuclear hormone receptors, using progress with the androgen receptor as an example. Historically, development of therapies targeting nuclear hormone receptors has focused on the folded C-terminal ligand-binding domain. Therapies were traditionally not developed to target the intrinsically disordered N-terminal domain as it was considered "undruggable". Recent developments have now shown it is possible to direct therapies to the N-terminal domain. This chapter will provide an introduction of the structure and function of the domains of nuclear hormone receptors, followed by a discussion of the rationale supporting the development of N-terminal domain inhibitors. Chemistry and mechanisms of action of small molecule inhibitors will be described with emphasis on N-terminal domain inhibitors developed to the androgen receptor including those in clinical trials.

Restrained expression of canine glucocorticoid receptor splice variants \u03b1 and P prognosticates fatal disease outcome in SIRS

Glucocorticoids play a central role in the inflammatory response and alleviate the symptoms in critically ill patients. The glucocorticoid action relies on the glucocorticoid receptor (GR) which translocates into the nucleus upon ligand-binding and regulates transcription of a battery of genes. Although the GR is encoded by a single gene, dozens of its splice variants have been described in diverse species. The GRα isoform encodes the full, functionally active protein that is composed of a transactivation, a DNA-binding, and a C-terminal ligand-binding domain. The second most highly expressed receptor variant, the GR-P, is formed by an intron retention that introduces an early stop codon and results in a probably dysfunctional protein with truncated ligand-binding domain. We described the canine ortholog of GR-P and showed that this splice variant is highly abundant in the peripheral blood of dogs. The level of cGRα and cGR-P transcripts are elevated in patients of SIRS and the survival rate is increased with elevated cGRα and cGR-P expression. The ratio of cGRα and cGR-P mRNA did not differ between the survivor and non-survivor patients; thus, the total GR expression is more pertinent than the relative expression of GR isoforms in assessment of the disease outcome.

Development of a Benzothiazole Scaffold-Based Androgen Receptor N-Terminal Inhibitor for Treating Androgen-Responsive Prostate Cancer.

All current clinically approved androgen deprivation therapies for prostate cancer target the C-terminal ligand-binding domain of the androgen receptor (AR). However, the main transactivation function of the receptor is localized at the AR N-terminal domain (NTD). Targeting the AR NTD directly is a challenge because of its intrinsically disordered structure and the lack of pockets for drugs to bind. Here, we have taken an alternative approach using the cochaperone BAG1L, which interacts with the NTD, to develop a novel AR inhibitor. We describe the identification of 2-(4-fluorophenyl)-5-(trifluoromethyl)-1,3-benzothiazole (A4B17), a small molecule that inhibits BAG1L-AR NTD interaction, attenuates BAG1L-mediated AR NTD activity, downregulates AR target gene expression, and inhibits proliferation of AR-positive prostate cancer cells. This compound represents a prototype of AR antagonists that could be key in the development of future prostate cancer therapeutics.

In vitro assessment of pesticides capacity to act as agonists/antagonists of the thyroid hormone nuclear receptors

SummaryChemicals acting as thyroid hormone disruptors (THDs) are of a particular concern for public health, considering the importance of this hormone in neurodevelopment and metabolic processes. They might either alter the circulating level of thyroid hormone (TH) or interfere with the cellular response to the hormonal stimulation. In order to assess this later possibility we selected 39 pesticides and combined several in vitro tests. Reporter assays respectively addressed the transactivation capacity of the full-length TH nuclear receptor TRα1, the transactivation capacity of its C-terminal ligand binding domain, or the ability of the hormone to destabilize the interaction between TRα1 and the transcriptional corepressor NcoR. Although some pesticides elicit a cellular response, which sometimes interferes with TH signaling, RNA-seq analysis provided no evidence that they can act as TRα1 agonists or antagonists. Their neurodevelopmental toxicity in mammals cannot be explained by an alteration of the response to TH.

Structural Insights of Transcriptionally Active, Full-Length Androgen Receptor Coactivator Complexes

Steroid hormone receptors activate gene transcription by binding specific DNA sequences and recruiting coactivators to initiate transcription of their target genes. For most nuclear hormone receptors (NRs), the ligand-dependent activation function domain-2 (AF-2), residing in the C-terminal ligand binding domain (LBD), is a primary contributor to the NR transcriptional activity. In contrast to other steroid receptors such as estrogen receptor-α (ERα), the transcriptional activation function of androgen receptor (AR) is thought to be largely dependent on its ligand-independent activation function domain-1 (AF-1) located in its N-terminal domain (NTD). It remains unclear why AR utilizes a different activation function domain from other steroid receptors despite the fact that NRs share similar domain organizations and have a highly conserved DNA binding domain (DBD) and LBD. Here we present cryo-electron microscopic structures of DNA-bound full-length AR and its functional complex structure with its key coactivators, steroid receptor coactivator-3 (SRC-3) and the histone acetyltransferase p300. The structures reveal that androgen-bound full-length AR dimerization follows a unique head-to-head and tail-to-tail manner with all of the domains involved. The DBD and LBD are located at the center of the dimer interface. The NTDs wrap around the LBDs through their unique intra- and inter-molecular N- and C-terminal interactions and connect to each other. We observe that unlike ERα, AR binds a single SRC-3 molecule along with a single p300 molecule. The AR NTD appears to be the primary site for recruitment of both coactivators. The N-terminal region of SRC-3, rather than its receptor interaction domain, is important for this interaction. The structures presented here highlight the importance of the AR NTD for its transcriptional functions and provide a structural basis for understanding the assembly of the AR:coactivator complex and its domain contributions to transcriptional regulation.

Centrosomal Localization of RXRα Promotes PLK1 Activation and Mitotic Progression and Constitutes a Tumor Vulnerability.

Retinoid X receptor alpha (RXRα), a nuclear receptor of transcription factor, controls various physiological and pathological pathways including cellular growth, proliferation, differentiation, and apoptosis. Here, we report that RXRα is phosphorylated at its N-terminal A/B domain by cyclin-dependent kinase 1 (Cdk1) at the onset of mitosis, triggering its translocation to the centrosome, where phosphorylated-RXRα (p-RXRα) interacts with polo-like kinase 1 (PLK1) through its N-terminal A/B domain by a unique mechanism. The interaction promotes PLK1 activation, centrosome maturation, and mitotic progression. Levels of p-RXRα are abnormally elevated in cancer cell lines, during carcinogenesis in animals, and in clinical tumor tissues. An RXRα ligand XS060, which specifically inhibits p-RXRα/PLK1 interaction but not RXRα heterodimerization, promotes mitotic arrest and catastrophe in a tumor-specific manner. These findings unravel a transcription-independent action of RXRα at the centrosome during mitosis and identify p-RXRα as a tumor-specific vulnerability for developing mitotic drugs with improved therapeutic index.

Combination therapy with androgen receptor N-terminal domain antagonist EPI-7170 and enzalutamide yields synergistic activity in AR-V7-positive prostate cancer.

Resistance of castration-resistant prostate cancer (CRPC) to enzalutamide and abiraterone involves the expression of constitutively active, truncated androgen receptor (AR) splice variants (AR-Vs) that lack a C-terminal ligand-binding domain (LBD). Both full-length AR and truncated AR-Vs require a functional N-terminal domain (NTD) for transcriptional activity thereby providing rationale for the development of ralaniten (EPI-002) as a first-in-class antagonist of the AR-NTD. Here, we evaluated the antitumor effect of a next-generation analog of ralaniten (EPI-7170) as a monotherapy or in combination with enzalutamide in prostate cancer cells that express AR-V7 that were resistant to enzalutamide. EPI-7170 had 8-9 times improved potency compared to ralaniten. Enzalutamide increased levels of AR-V7 and expression of its target genes. Knockdown of AR-V7 restored sensitivity to enzalutamide, indicating a role for AR-V7 in the mechanism of resistance. EPI-7170 inhibited expression of genes transcriptionally regulated by full-length AR and AR-V7. A combination of EPI-7170 and enzalutamide resulted in synergistic inhibition of proliferation of enzalutamide-resistant cells that was consistent with results from cell cycle and clonogenic assays. In addition, this drug enhanced the antitumor effect of enzalutamide in enzalutamide-resistant CRPC preclinical models. Thus, a combination therapy targeting both the NTD and LBD of AR, and thereby blocking both full-length AR and AR-Vs, has potential for the treatment of enzalutamide-resistant CRPC.