Androgen regulation of ribosomal RNA synthesis in LNCaP cells and rat prostate

Effective targeting of RNA polymerase I in treatment-resistant prostate cancer

BACKGROUND

Advanced prostate cancers depend on protein synthesis for continued survival and accelerated rates of metabolism for growth. RNA polymerase I (Pol I) is the enzyme responsible for ribosomal RNA (rRNA) transcription and a rate-limiting step for ribosome biogenesis. We have shown using a specific and sensitive RNA probe for the 45S rRNA precursor that rRNA synthesis is increased in prostate adenocarcinoma compared to nonmalignant epithelium. We have introduced a first-in-class Pol I inhibitor, BMH-21, that targets cancer cells of multiple origins, and holds potential for clinical translation.

METHODS

The effect of BMH-21 was tested in prostate cancer cell lines and in prostate cancer xenograft and mouse genetic models.

RESULTS

We show that BMH-21 inhibits Pol I transcription in metastatic, castration-resistant, and enzalutamide treatment-resistant prostate cancer cell lines. The genetic abrogation of Pol I effectively blocks the growth of prostate cancer cells. Silencing of p53, a pathway activated downstream of Pol I, does not diminish this effect. We find that BMH-21 significantly inhibited tumor growth and reduced the Ki67 proliferation index in an enzalutamide-resistant xenograft tumor model. A decrease in 45S rRNA synthesis demonstrated on-target activity. Furthermore, the Pol I inhibitor significantly inhibited tumor growth and pathology in an aggressive genetically modified Hoxb13-MYC|Hoxb13-Cre|Pten^fl/fl (BMPC) mouse prostate cancer model.

CONCLUSION

Taken together, BMH-21 is a novel promising molecule for the treatment of castration-resistant prostate cancer.

Discovery Proteomics Identifies a Molecular Link between the Coatomer Protein Complex I and Androgen Receptor-dependent Transcription

Aberrant androgen receptor (AR)-dependent transcription is a hallmark of human prostate cancers. At the molecular level, ligand-mediated AR activation is coordinated through spatial and temporal protein-protein interactions involving AR-interacting proteins, which we designate the “AR-interactome.” Despite many years of research, the ligand-sensitive protein complexes involved in ligand-mediated AR activation in prostate tumor cells have not been clearly defined. Here, we describe the development, characterization, and utilization of a novel human LNCaP prostate tumor cell line, N-AR, which stably expresses wild-type AR tagged at its N terminus with the streptavidin-binding peptide epitope (streptavidin-binding peptide-tagged wild-type androgen receptor; SBP-AR). A bioanalytical workflow involving streptavidin chromatography and label-free quantitative mass spectrometry was used to identify SBP-AR and associated ligand-sensitive cytosolic proteins/protein complexes linked to AR activation in prostate tumor cells. Functional studies verified that ligand-sensitive proteins identified in the proteomic screen encoded modulators of AR-mediated transcription, suggesting that these novel proteins were putative SBP-AR-interacting proteins in N-AR cells. This was supported by biochemical associations between recombinant SBP-AR and the ligand-sensitive coatomer protein complex I (COPI) retrograde trafficking complex in vitro. Extensive biochemical and molecular experiments showed that the COPI retrograde complex regulates ligand-mediated AR transcriptional activation, which correlated with the mobilization of the Golgi-localized ARA160 coactivator to the nuclear compartment of prostate tumor cells. Collectively, this study provides a bioanalytical strategy to validate the AR-interactome and define novel AR-interacting proteins involved in ligand-mediated AR activation in prostate tumor cells. Moreover, we describe a cellular system to study how compartment-specific AR-interacting proteins influence AR activation and contribute to aberrant AR-dependent transcription that underlies the majority of human prostate cancers.

Mechanism and Function of Angiogenin in Prostate Cancer

Angiogenin (ANG), the fifth member of the vertebrate-specific ribonuclease (RNase) A superfamily, is a secreted angiogenic ribonuclease strongly up-regulated in human prostate cancers. ANG is translocated to the nucleus in both prostate cancer epithelial cells and endothelial cells to exert its role in prostate cancer progression by mediating tumor angiogenesis, cancer cell survival and proliferation through rRNA biogenesis. ANG-stimulated rRNA is required not only for prostate intraepithelial neoplasia (PIN) formation, but also for androgen-independent growth of prostate cancer cells. Targeting ANG by various antagonists that inhibit its nuclear translocation, function and/or activity has proven to inhibit prostate cancer growth in animal models. Furthermore, the role of ANG in androgen independence has been firmly established, suggesting a strong rationale for therapeutically targeting ANG in the treatment of castration resistant prostate cancer.

Angiogenin mediates androgen-stimulated prostate cancer growth and enables castration resistance

The androgen receptor (AR) is a critical effector of prostate cancer development and progression. Androgen-dependent prostate cancer is reliant on the function of AR for growth and progression. Most castration-resistant prostate cancer (CRPC) remains dependent on AR signaling for survival and growth. Ribosomal RNA (rRNA) is essential for both androgen-dependent and castration-resistant growth of prostate cancer cells. During androgen-dependent growth of prostate cells, androgen-AR signaling leads to the accumulation of rRNA. However, the mechanism by which AR regulates rRNA transcription is unknown. Here, investigation revealed that angiogenin (ANG), a member of the secreted ribonuclease superfamily, is upregulated in prostate cancer and mediates androgen-stimulated rRNA transcription in prostate cancer cells. Upon androgen stimulation, ANG undergoes nuclear translocation in androgen-dependent prostate cancer cells, where it binds to the rDNA promoter and stimulates rRNA transcription. ANG antagonists inhibit androgen-induced rRNA transcription and cell proliferation in androgen-dependent prostate cancer cells. Interestingly, ANG also mediates androgen-independent rRNA transcription through a mechanism that involves its constitutive nuclear translocation in androgen-insensitive prostate cancer cells, resulting in a constant rRNA overproduction and thereby stimulating cell proliferation. Critically, ANG overexpression in androgen-dependent prostate cancer cells enables castration-resistant growth of otherwise androgen-dependent cells. Thus, ANG-stimulated rRNA transcription is not only an essential component for androgen-dependent growth of prostate cancer but also contributes to the transition of prostate cancer from androgen-dependent to castration-resistant growth status.

IMPLICATIONS

The ability of angiogenin to regulate rRNA transcription and prostate cancer growth makes it a viable target for therapy.

Hepatic protein expression networks associated with masculinization in the female fathead minnow (Pimephales promelas)

Endocrine disruptors that act via the androgen receptor (AR) are less well studied than environmental estrogens, and there is evidence that treatment with AR agonists can result in masculinization of female fish. In this study, female fathead minnows (FHM) were exposed to the model nonaromatizable androgen 5-alpha dihydrotestosterone (DHT) (100 μg/L), the ureic-based herbicide linuron (LIN) (100 μg/L), and a mixture of DHT and LIN (100 μg/L each) to better characterize androgen action in females. LIN was used because of reports that this chemical has an antiandrogenic mode of action in fish. After 21d, DHT and LIN treatments resulted in a significant depression of plasma vitellogenin (Vtg) and DHT and DHT+LIN increased the prevalence of nuptial tubercles in female FHMs indicating masculinization. Using iTRAQ and an LTQ Orbitrap Velos, ∼2000 proteins were identified in the FHM liver and the number of proteins quantified after exposures was >1200. Proteins that significantly and consistently changed in abundance across biological replicates included prostaglandin E synthase 3, programmed cell death 4a, glutathione S transferases, canopy, selenoprotein U, and ribosomal proteins. Subnetwork enrichment analysis identified that interferon and epidermal growth factor signaling were regulated by DHT and LIN, suggesting that these signaling pathways are correlated to depressed plasma vitellogenin. These data provide novel insight into hepatic protein networks that are associated with the process of masculinization in teleosts.

Angiogenin as a molecular target for the treatment of prostate cancer

Angiogenin (ANG), a 14 kDa angiogenic ribonuclease, is upregulated in human prostate cancers, especially in hormone refractory diseases, and is the highest upregulated gene in Akt-driven prostate intraepithelial neoplasia (PIN) in mice. ANG has been shown to undergo nuclear translocation in both prostate cancer cells and cancer-associated endothelial cells where it binds to the promoter region of ribosomal DNA (rDNA) and stimulates ribosomal RNA (rRNA) transcription. ANG thus plays an essential role in prostate cancer progression by stimulating both cancer cell proliferation and tumor angiogenesis. A variety of ANG antagonists, including its antisense oligonucleotide, siRNA, soluble binding proteins, monoclonal antibody, enzymatic inhibitors, and nuclear translocation blockers, have all been shown to inhibit prostate cancer in various animal models. Accumulating evidence indicates that ANG is a molecular target for prostate cancer drug development.

Fine temporal analysis of DHT transcriptional modulation of the ATM/Gadd45g signaling pathways in the mouse uterus

In rodents, the uterus of a mature female undergoes changes during the uterine cycle, under the control of steroid hormones. 5alpha-Dihydrotestosterone (DHT) is recognized to play an important role in the regulation of androgen action in normal endometrium. Using microarray technology, a screening analysis of genes responding to DHT in the uterus of ovariectomized mice, has allowed us to highlight multiple genes of the ATM/Gadd45g pathway that are modulated following exposure to DHT. Two phases of regulation were identified. In the early phase, the expression of genes involved in the G2/M arrest is rapidly increased, followed by the repression of genes of the G1/S checkpoint, and by the induction of transcriptional regulators. Later, i.e. from 12 to 24 hr, genes involved in G2/M transition, cytoarchitectural and lipid-related genes are stimulated by DHT while immunity-related genes appear to be differentially regulated by the hormone. These results show that a physiological dose of DHT induces the transcription of genes promoting the cell cycle progression in mice. Profile determination of temporal uterine gene expression at the transcriptional level enables us to suggest that the DHT modulation of genes involved in ATM/Gadd45g signaling in an ATM- or p53-independent manner, could play an important role in the cyclical changes of uterine cells in the mouse uterus.

Dual regulation of upstream binding factor 1 levels by IRS-1 and ERKs in IGF-1-receptor signaling

The Upstream Binding Factor 1 (UBF1) is a nucleolar protein that participates in the regulation of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In 32D myeloid cells expressing the type 1 insulin-like growth factor receptor (IGF-IR), the UBF1 protein (but not its mRNA) is down regulated when the cells are shifted from Interleukin-3 (IL-3) to IGF-1. Ectopic expression of insulin receptor substrate-1 (IRS-1) in these cells inhibits the down-regulation of UBF1. We now show that the stability of UBF1 in 32D-derived cells requires also a signal from the extracellular regulated kinases (ERKs). When ERKs signaling is defective, as in cells over-expressing the insulin receptor (InR) or selected mutants of the IGF-1R, UBF1 is down-regulated, even in the presence of IRS-1. The down-regulation is corrected by the expression of an activated Ha-ras, which stimulates ERKs activity. Mutations at threonines 117 and 201 of UBF1, known to be phosphorylated by ERKs, cause its down-regulation. However, when IRS-2, instead of IRS-1, is ectopically expressed in 32D InR cells, ERKs phosphorylation is increased and UBF is stabilized. Taken together, these results indicate that in 32D-derived myeloid cells expressing either the IGF-IR or the InR, UBF1 levels are regulated by signaling from both IRS proteins and ERKs.

Downregulation of the upstream binding factor1 by glycogen synthase kinase3beta in myeloid cells induced to differentiate

The upstream binding factor 1 (UBF1), one of the proteins that regulate the activity of RNA polymerase I, is downregulated in 32D myeloid cells induced to differentiate into granulocytes, either by the type 1 insulin-like growth factor (IGF-1) or the granulocytic colony stimulating factor (G-CSF). Downregulation of UBF1 is largely due to protein degradation, while mRNA levels are not affected. Inhibition of UBF1 degradation by lithium chloride (LiCl)and lactacystin suggest a role of glycogen synthase kinase beta (GSK3beta) in a proteasome-dependent degradation of UBF. GSK3beta phosphorylates in vitro and in vivo the UBF protein, which has five putative motifs for phosphorylation by GSK3beta. Elimination and/or mutations of these motifs stabilize the UBF1 protein even in cells induced to differentiate. Conversely, a stably transfected, constitutively active GSK3beta accelerates the downregulation of UBF1. We show further that activation of the differentiating protein C/EPBalpha in 32D cells transformed by the oncogenic BCR/ABL protein causes downregulation of UBF1. Finally, inhibition of differentiation of myeloid cells by a dominant negative mutant of Stat3 stabilizes the UBF1 protein, while rapamycin-induced differentiation of myeloid cells downregulates UBF1 levels. Taken together, our results indicate that the induction of granulocytic differentiation in 32D murine myeloid cells causes the degradation of UBF1, via GSK3beta and the proteasome pathway.

A Mechanism for Cell Size Regulation by the Insulin and Insulin-Like Growth Factor-I Receptors

Deletion of the type 1 insulin-like growth factor receptor (IGF-IR) or of the insulin receptor substrate-1 (IRS-1) genes in animals causes a 50% reduction in body size at birth. Decrease in body size is due to both a decreased number of cells and a decreased cell size. Deletion of the insulin receptor (InR) genes results in mice that are normal in size at birth. We have used 32D-derived myeloid cells to study the effect of IGF-IR and InR signaling on cell size. 32D cells expressing the IGF-IR and IRS-1 are almost twice as large as 32D cells expressing the InR and IRS-1. A mechanism for the difference in size is provided by the levels of the upstream binding factor 1 (UBF1), a nucleolar protein that participates in the regulation of RNA polymerase I activity and rRNA synthesis and therefore cell size. When shifted to the respective ligands, UBF1 levels decrease in cells expressing the InR and IRS-1, whereas they remain stable in cells expressing the IGF-IR and IRS-1. The expression of the IGF-IR and IRS-1 is crucial to the stability of UBF1.

Angiogenin is translocated to the nucleus of HeLa cells and is involved in ribosomal RNA transcription and cell proliferation

Angiogenin is an angiogenic protein known to play a role in rRNA transcription in endothelial cells. Nuclear translocation of angiogenin in endothelial cells decreases as cell density increases and ceases when cells are confluent. Here we report that angiogenin is constantly translocated to the nucleus of HeLa cells in a cell density-independent manner. Down-regulation of angiogenin expression by antisense and RNA interference results in a decrease in rRNA transcription, ribosome biogenesis, proliferation, and tumorigenesis both in vitro and in vivo. Exogenous angiogenin rescues the cells from antisense and RNA interference inhibition. The results showed that angiogenin is constitutively translocated into the nucleus of HeLa cells where it stimulates rRNA transcription. Thus, besides its angiogenic activity, angiogenin also plays a role in cancer cell proliferation.

Ku80 is required but not sufficient for Galpha13-mediated endodermal differentiation in P19 embryonic carcinoma cells

We have shown that a constitutively active Galpha13 (Galpha13Q226L) induces differentiation in P19 embryonic carcinoma cells to an endodermal phenotype. In this report, we demonstrate that Ku, a heterodimer of p80 (Ku80) and p70 (Ku70), is upregulated in P19 cells overexpressing Galpha13Q226L. Ku is the regulatory subunit of the DNA-dependent protein kinase and is primarily involved in DNA repair and recombination. Ku80 also is a somatostatin receptor. We show that while overexpression of Ku80 drastically reduced P19 cell proliferation, it was not sufficient to induce endodermal differentiation. However, coexpression of Galpha13Q226L and an antisense Ku80 abrogated the retarded growth rate and endodermal differentiation observed in cells expressing only Galpha13Q226L. Overexpression of Galpha13Q226L or Ku80 downregulated RNA polymerase I-mediated transcriptional activity and overexpression of antisense Ku80 restored the activity to control level. These results suggest that Ku80 is required for Galpha13-mediated endodermal differentiation in P19 cells.

Putative involvement of the histone acetyltransferase Tip60 in ribosomal gene transcription

Tip60 is a histone acetyltransferase (HAT) implicated in a wide range of cellular functions, including mRNA synthesis and DNA repair. In the present report we propose a model based on which Tip60 is actively involved in ribosomal gene transcription through acetylation of UBF, a ribosomal specific transcription factor, as well as through its direct recruitment to the human ribosomal gene promoter, as shown by chromatin immunoprecipitation experiments. Electron microscopy studies revealed that Tip60 resides in sites of active rDNA transcription within the nucleolus, while it co-localizes with UBF as shown by confocal microscopy. In addition, in vivo transcription assays demonstrated that the nucleolar fraction of Tip60 localizes to sites of newly synthesized rRNA. Finally, functional assays established that Tip60 complexes with, and targets UBF for acetylation. The present study underlines the importance of acetylation in rDNA transcription and directly implicates Tip60 in the process of ribosomal gene transcription.

Phosphatidylinositol 3-kinase and mTOR signaling pathways regulate RNA polymerase I transcription in response to IGF-1 and nutrients

Regulation of ribosomal RNA gene transcription by RNA polymerase I (Pol I) is fundamental to ribosome biogenesis and therefore protein translation capacity and cell growth, yet little is known of the key signaling cascades involved. We show here that insulin-like growth factor-1 (IGF-1)-induced Pol I transcription in HEK293 cells is entirely dependent on phosphatidylinositol 3-kinase (PI3K) activity and, additionally, is modulated by the mammalian target of rapamycin (mTOR), which coordinates Pol I transcription with the availability of amino acids. The mitogen-activated protein kinase (MAPK) pathway is weakly stimulated by IGF-1 in these cells and partly contributes to Pol I transcription regulation. Activation of Pol I transcription by IGF-1 results from enhancement of the activity of the Pol I transcription machinery and increased occupancy by SL1 of the endogenous tandemly repeated ribosomal promoters in vivo. The inputs from PI3K, mTOR, and MAPK pathways converge to direct appropriate rRNA gene expression by Pol I in the nucleolus of mammalian cells in response to environmental cues, such as growth factors and nutrients.

Androgen-mediated resistance to apoptosis

BACKGROUND

Previous studies demonstrate that androgen is capable of exerting a protective effect in the androgen-sensitive human prostate cancer cell line LNCaP. Limited studies, however, have addressed the underlying mechanisms involved, in particular the effects of androgen on both pro- and anti-apoptotic gene expression.

METHODS

We investigated the effects of androgen on apoptotic sensitivity and the expression of the caspases and specific members of the Bcl-2 family in the LNCaP cell line. The effects of androgen on NF-kappaB activation were also investigated by using a gel mobility shift assay.

RESULTS

5alpha-Dihydrotestosterone (5-alphaDHT) conferred resistance to radiation (5 Gy) and etoposide-induced apoptosis in the LNCaP cell line. This finding was associated with a time-dependent decrease in the expression of the caspases and pro-apoptotic Bcl-2 family members. 5-alphaDHT did not confer protection against apoptosis in the LNCaP line transfected with the IkappaB super repressor of NF-kappaB, nor in the androgen insensitive PC-3 and DU-145 cell lines.

CONCLUSION

The ability of 5-alphaDHT to raise the apoptotic threshold in the LNCaP cell line by altering specific pro-apoptotic gene expression suggests that androgen may serve as a general survival signal against diverse pathways that ultimately signal for apoptosis. We hypothesize that NF-kappaB serves as an important mediator in androgen survival signaling.

Androgen regulation of the largest subunit of RNA polymerase II in the rat ventral prostate

One of the dramatic changes in the prostate during androgen manipulation is the alteration in cellular content of total RNA - the amount of total RNA in each cell. The abundance of cellular total RNA correlates with the RNA polymerase (RNAP) activity in the prostate. One possible mechanism of androgen regulation of RNAP activity involves the regulation of RNAP expression. Western blot analysis showed that the largest subunit of the RNAP II, an essential component of the transcriptional machinery for mRNA, is indeed regulated by androgens. Castration down-regulates the protein level of RNAP II, whereas androgen replacement up-regulates the protein. However, androgen manipulation does not have consistent effects on the phosphorylation of the C-terminal domain (CTD) of the RNAP II. Androgen regulation of the RNAP II protein expression was also observed in the seminal vesicles but not in the thymus and liver, indicating that androgen regulation of RNAP II protein expression appears to be limited to the male sex accessory organs. These observations suggest that RNAP II plays an essential role in androgen action in male sex accessory organs.

Regulation of RNA polymerase I transcription in yeast and vertebrates

This article focuses on what is currently known about the regulation of transcription by RNA polymerase I (pol I) in eukaryotic organisms at opposite ends of the evolutionary spectrum--a yeast, Saccharomyces cerevisiae, and vertebrates, including mice, frogs, and man. Contemporary studies that have defined the DNA sequence elements are described, as well as the majority of the basal transcription factors essential for pol I transcription. Situations in which pol I transcription is known to be regulated are reviewed and possible regulatory mechanisms are critically discussed. Some aspects of basal pol I transcription machinery appear to have been conserved from fungi to vertebrates, but other aspects have evolved, perhaps to meet the needs of a metazoan organism. Different parts of the pol I transcription machinery are regulatory targets depending on different physiological stimuli. This suggests that multiple signaling pathways may also be involved. The involvement of ribosomal genes and their transcripts in events such as mitosis, cancer, and aging is discussed.

Quantitation of androgen receptor messenger RNA from genital skin fibroblasts by reverse transcription--competitive polymerase chain reaction

To gain further information concerning the regulation by androgen of AR mRNA expression in cultured genital skin fibroblasts (GSF), we first developed a quantitative reverse transcription-competitive polymerase chain reaction (RT-PCR). This method used an ethidium bromide stain analysis of the PCR products for the accurate quantitation of low levels of human androgen receptor (hAR) mRNA in GSF. To control for variations due to sample preparation, and to minimize the disparity of the reverse transcriptase efficiency between samples after the RT procedure, we produced an initial PCR for the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene, and then adjusted the amount of cDNA to that of this housekeeping gene. Competitive PCR for hAR was then immediately performed on normalized cDNA with a competitor DNA that exhibited a 13 bp deletion as compared to the 163 bp for the target fragment, and the PCR products were easily separated by 3.5% agarose gel electrophoresis. This quantitation procedure involved no additional steps, such as enzymatic cleavage of the PCR products, nor the use of radioactivity. In GSF from individuals, we found that the normal amount of AR mRNA was 5.6 attomoles/microg RNA, (+/-1.0, s.e.m.) with an intra- and an inter-assay of 8.4 and 14.7%, respectively. We observed a biphasic pattern of AR mRNA expression in normal human GSF in the presence of physiological concentration of androgen. Quantitative RT-PCR of AR mRNA may be useful for studying AR mRNA expression in experimental or clinical conditions.