RNase L Suppresses Androgen Receptor Signaling, Cell Migration and Matrix Metalloproteinase Activity in Prostate Cancer Cells.

The interferon antiviral pathways and prostate cancer genetics converge on a regulated endoribonuclease, RNase L. Positional cloning and linkage studies mapped Hereditary Prostate Cancer 1 (HPC1) to RNASEL. To date, there is no correlation of viral infections with prostate cancer, suggesting that RNase L may play additional roles in tumor suppression. Here, we demonstrate a role of RNase L as a suppressor of androgen receptor (AR) signaling, cell migration and matrix metalloproteinase activity. Using RNase L mutants, we show that its nucleolytic activity is dispensable for both AR signaling and migration. The most prevalent HPC1-associated mutations in RNase L, R462Q and E265X, enhance AR signaling and cell migration. RNase L negatively regulates cell migration and attachment on various extracellular matrices. We demonstrate that RNase L knockdown cells promote increased cell surface expression of integrin ß1 which activates Focal Adhesion Kinase-Sarcoma (FAK-Src) pathway and Ras-related C3 botulinum toxin substrate 1-guanosine triphosphatase (Rac1-GTPase) activity to increase cell migration. Activity of matrix metalloproteinase (MMP)-2 and -9 is significantly increased in cells where RNase L levels are ablated. We show that mutations in RNase L found in HPC patients may promote prostate cancer by increasing expression of AR-responsive genes and cell motility and identify novel roles of RNase L as a prostate cancer susceptibility gene.

MDM2 regulates the stability of AR, AR-V7, and TM4SF3 proteins in prostate cancer.

Androgen receptor (AR) and its constitutively active splice variant, AR Variant 7 (AR-V7), regulate genes essential for the development and progression of prostate cancer. Degradation of AR and AR-V7 by the ubiquitination proteasomal pathway is important for the regulation of both their protein stability. Our published results demonstrate that the interaction of TM4SF3 with either AR or AR-V7 leads to mutual stabilization due to a reduction in their ubiquitination and proteasomal degradation. These results led us to search for a common E3 ligase for AR, AR-V7, and TM4SF3. Depletion by siRNA of several E3 ligases identified MDM2 as the common E3 ligase. MDM2 inhibition by siRNA depletion or using a pharmacological inhibitor (MDM2i) of its E3 ligase activity led to elevated levels of endogenous AR, AR-V7, and TM4SF3 in prostate cancer cells. MDM2 knockdown in PC-3 cells, which do not express AR, also increased TM4SF3, demonstrating that MDM2 affects the TM4SF3 protein independent of AR. We further demonstrate that MDM2i treatment reduced the ubiquitination of AR and TM4SF3, suggesting that MDM2 can induce the ubiquitination of these proteins. Increased AR and AR-V7 protein levels induced by MDM2i treatment resulted in the expected increased expression of AR-regulated genes and enhanced proliferation and migration of both LNCaP and Enzalutamide-resistant CWR-22Rv1 prostate cancer cells. Thus, our study expands the known roles of MDM2 in prostate cancer to include its potential involvement in the important mutual stabilization that TM4SF3 exhibits when interacting with either AR or AR-V7.

Peptides disrupting TM4SF3 interaction with AR or AR-V7 block prostate cancer cell proliferation.

Androgen receptor (AR) plays a vital role in the development and progression of prostate cancer from the primary stage to the usually lethal stage known as castration-resistant prostate cancer (CRPC). Constitutively active AR splice variants (AR-Vs) lacking the ligand-binding domain are partially responsible for the abnormal activation of AR and may be involved in resistance to AR-targeting drugs occurring in CRPC. There is increasing consensus on the potential of drugs targeting protein-protein interactions. Our lab has recently identified transmembrane 4 superfamily 3 (TM4SF3) as a critical interacting partner for AR and AR-V7 and mapped the minimal interaction regions. Thus, we hypothesized that these interaction domains can be used to design peptides that can disrupt the AR/TM4SF3 interaction and kill prostate cancer cells. Peptides TA1 and AT1 were designed based on the TM3SF3 or AR interaction domain, respectively. TA1 or AT1 was able to decrease AR/TM4SF3 protein interaction and protein stability. Peptide TA1 reduced the recruitment of AR and TM4SF3 to promoters of androgen-regulated genes and subsequent activation of these AR target genes. Peptides TA1 and AT1 were strongly cytotoxic to prostate cancer cells that express AR and/or AR-V7. Peptide TA1 inhibited the growth and induced apoptosis of both enzalutamide-sensitive and importantly enzalutamide-resistant prostate cancer cells. TA1 also blocked the migration and malignant transformation of prostate cancer cells. Our data clearly demonstrate that using peptides to target the important interaction AR has with TM4SF3 provides a novel method to kill enzalutamide-resistant prostate cancer cells that can potentially lead to new more effective therapy for CRPC.

The Transmembrane Protein TM4SF3 Interacts With AR and AR-V7 and is Recruited to AR Target Genes.

Prostate cancer transitions from an early treatable form to the lethal castration-resistant prostate cancer (CRPC). Androgen receptor (AR) and constitutively active AR splice variants, such as AR-V7, may be major drivers of CRPC. Our laboratory recently identified a novel mechanism of AR regulation via the transmembrane protein transmembrane 4 superfamily 3 (TM4SF3), which exhibits a physical interaction, nuclear colocalization, and mutual stabilization with AR. Here, we have mapped the interaction domains within AR and TM4SF3 and discovered that TM4SF3 also physically interacts with AR-V7, regulating its protein stability and the viability of CRPC cells expressing AR-V7. Ubiquitination of TM4SF3 and AR-V7 was detected for the first time and TM4SF3 interaction with either AR or AR-V7 resulted in mutual deubiquitination of both proteins, showing that mutual stabilization results from deubiquitination. Interestingly, nuclear TM4SF3 was co-recruited to the promoters of AR- and AR-V7-regulated genes and required for their expression, showing that TM4SF3 interaction is critical for their transcriptional functions. The results collectively show the multiple critical regulatory functions of TM4SF3 on AR or AR-V7 in prostate cancer cells.

Dual-Acting Peptides Target EZH2 and AR: A New Paradigm for Effective Treatment of Castration-Resistant Prostate Cancer.

Prostate cancer starts as a treatable hormone-dependent disease, but often ends in a drug-resistant form called castration-resistant prostate cancer (CRPC). Despite the development of the antiandrogens enzalutamide and abiraterone for CRPC, which target the androgen receptor (AR), drug resistance usually develops within 6 months and metastatic CRPC (mCRPC) leads to lethality. EZH2, found with SUZ12, EED, and RbAP48 in Polycomb repressive complex 2 (PRC2), has emerged as an alternative target for the treatment of deadly mCRPC. Unfortunately, drugs targeting EZH2 have shown limited efficacy in mCRPC. To address these failures, we have developed novel, dual-acting peptide inhibitors of PRC2 that uniquely target the SUZ12 protein component, resulting in the inhibition of both PRC2 canonical and noncanonical functions in prostate cancer. These peptides were found to inhibit not only the EZH2 methylation activity, but also block its positive effect on AR gene expression in prostate cancer cells. Since the peptide effect on AR levels is transcriptional, the inhibitory peptides can block the expression of both full-length AR and its splicing variants including AR-V7, which plays a significant role in the development of drug resistance. This dual-mode action provides the peptides with the capability to kill enzalutamide-resistant CRPC cells. These peptides are also more cytotoxic to prostate cancer cells than the combination of enzalutamide and an EZH2 inhibitory drug, which was recently suggested to be an effective treatment of mCRPC disease. Our data show that such a dual-acting therapeutic approach can be more effective than the existing front-line drug therapies for treating deadly mCRPC.

Androgen up-regulation of Twist1 gene expression is mediated by ETV1.

Twist1, a basic helix-loop-helix transcription factor that regulates a number of genes involved in epithelial-to-mesenchymal transition (EMT), is upregulated in prostate cancer. Androgen regulation of Twist1 has been reported in a previous study. However, the mechanism of androgen regulation of the Twist1 gene is not understood because the Twist1 promoter lacks androgen receptor (AR)-responsive elements. Previous studies have shown that the Twist1 promoter has putative binding sites for PEA3 subfamily of ETS transcription factors. Our lab has previously identified Ets Variant 1 (ETV1), a member of the PEA3 subfamily, as a novel androgen-regulated gene that is involved in prostate cancer cell invasion through unknown mechanism. In view of these data, we hypothesized that androgen-activated AR upregulates Twist1 gene expression via ETV1. Our data confirmed the published work that androgen positively regulates Twist1 gene expression and further showed that this positive effect was directed at the Twist1 promoter. The positive effect of androgen on Twist1 gene expression was abrogated upon disruption of AR expression by siRNA or of AR activity by Casodex. More importantly, our data show that disruption of ETV1 leads to significant decrease in both androgen-mediated upregulation as well as basal level of Twist1, which we are able to rescue upon re-expression of ETV1. Indeed, we are able to show that ETV1 mediates the androgen upregulation of Twist1 by acting on the proximal region of Twist1 promoter. Additionally, our data show that Twist1 regulates prostate cancer cell invasion and EMT, providing a possible mechanism by which ETV1 mediates prostate cancer cell invasion. In conclusion, in this study we report Twist1 as an indirect target of AR and androgen regulation through ETV1.

c-Jun has multiple enhancing activities in the novel cross talk between the androgen receptor and Ets variant gene 1 in prostate cancer.

The multiple transcriptional roles of c-Jun are shown in a novel cross-talk between the androgen receptor (AR) and its new target gene, Ets variant gene 1 (ETV1). In this report, we show that c-Jun can mediate AR induction of ETV1 expression independent of c-Jun transactivation function. Interestingly, c-Jun can transactivate the cloned ETV1 promoter also in the absence of ligand-activated AR, suggesting two mechanisms by which c-Jun can induce ETV1 expression. In addition, both wild-type c-Jun and a transactivation-deficient mutant can enhance the transcriptional activity of ETV1, as measured by both reporter gene assay and endogenous expression of matrix metalloproteinase genes, well-known targets of Ets proteins. Overexpression of the c-Jun mutant protein also led to increased prostate cancer cell invasion. Immunoprecipitation and immunocytochemistry experiments showed copurification and colocalization of c-Jun with AR or ETV1, suggesting that c-Jun acts on AR or ETV1 via a physical association. Collectively, these results, together with a parallel overexpression of ETV1, c-Jun, and AR in prostate tumors, imply that c-Jun plays a pivotal role in the pathway that connects ligand-activated AR to elevated ETV1 expression, leading to enhanced expression of matrix metalloproteinases and prostate cancer cell invasion.

ETV1 is a novel androgen receptor-regulated gene that mediates prostate cancer cell invasion.

Androgens and the androgen receptor (AR) act in cells by modulating gene expression. Through gene microarray studies, we have identified Ets Variant Gene 1 (ETV1) as a novel androgen-regulated gene. Our data demonstrate that ETV1 mRNA and protein are up-regulated in response to ligand-activated AR in androgen-dependent LNCaP cells, but there is no detectable ETV1 expression in normal prostate cells. The ETV1 promoter is induced by androgens and recruits the AR in the context of chromatin. ETV1-regulated endogenous matrix metalloproteinase genes can be induced by ligand-activated AR. In contrast to the hormone-induced expression in androgen-dependent LNCaP cells, ETV1 expression in androgen-independent LNCaP cells is high and unresponsive to androgen. This androgen-independent ETV1 expression contrasts with the hormone-dependent expression observed for TMPRSS2 in these androgen-independent prostate cancer cells. ETV1 is overexpressed in prostate cancer independent of the TMPRSS2:ETV1 translocation. Disruption of ETV1 expression in both androgen-dependent and androgen-independent prostate cancer cells significantly compromises the invasion capacity of these cells, suggesting an important role for ETV1 in prostate cancer metastasis. Collectively, these results demonstrate that ETV1 expression transitions from androgen-induced to androgen-independent as prostate cancer cells switch from hormone-dependent to hormone-refractory and suggest that this transition may be in part responsible for the elevated levels of ETV1 observed in prostate tumors. Additionally, our data provide an indirect mechanism of AR regulation of gene expression, via the transactivation of the transcription factor ETV1.

Peptide B targets soluble guanylyl cyclase α1 and kills prostate cancer cells.

Among androgen-regulated genes, soluble guanylyl cyclase α1 (sGCα1) is significant in promoting the survival and growth of prostate cancer cells and does so independent of nitric oxide (NO) signaling. Peptides were designed targeting sGCα1 to block its pro-cancer functions and one peptide is discussed here. Peptide B-8R killed both androgen-dependent and androgen-independent prostate cancer cells that expressed sGCα1, but not cells that do not express this gene. Peptide B-8R induced apoptosis of prostate cancer cells. Importantly, Peptide B-8R does not affect nor its cytotoxicity depend on NO signaling, despite the fact that it associates with sGCα1, which dimerizes with sGCß1 to form the sGC enzyme. Just as with a previously studied Peptide A-8R, Peptide B-8R induced elevated levels of reactive oxygen species (ROS) in prostate cancer cells, but using a ROS-sequestering agent showed that ROS was not responsible the cytotoxic activity of Peptide B-8R. Interestingly, Peptide B-8R induced elevated levels of p53 and phosphorylated p38, but neither of these changes is the cause of the peptide's cytotoxicity. Additional drugs were used to alter levels of iron levels in cells and these studies showed that Peptide B-8R activity does not depend on Ferroptosis. Thus, future work will be directed at defining the mechanism of cytotoxic action of Peptide B-8R against prostate cancer cells.

TM4SF3 and AR: A Nuclear Complex that Stabilizes Both Proteins.

Transmembrane 4 superfamily 3 (TM4SF3) was identified as a novel androgen-regulated gene in prostate cancer (PCa) cells. Our data demonstrate that TM4SF3 exhibits androgen-induced repression of the mRNA but up-regulation of the protein. The androgen positive effect on the TM4SF3 protein is of significant interest in view of the procancer functions of both androgens and tetraspanin proteins. Androgen positively regulates TM4SF3 protein stability by inhibiting its proteasome-dependent degradation. This androgen stabilization of TM4SF3 is involved in promoting PCa cell invasion and migration of both androgen-dependent and androgen-independent PCa cells. Although confirming androgen up-regulation of the TM4SF3 protein, we observed that TM4SF3 is localized not only to the membrane, but also, surprisingly, the nuclei of PCa cells. This novel nuclear localization of TM4SF3 depends on androgen-induced nuclear localization of androgen receptor (AR) in both androgen-dependent and androgen-independent PCa cell lines. TM4SF3 interacts with AR both in PCa cell types and in vitro, strongly suggesting a direct interaction. This direct interaction is required for the stabilization of not only TM4SF3, but also remarkably AR, because down-regulation of TM4SF3 resulted in reduced AR protein levels. As expected of an important AR regulator, TM4SF3 regulates androgen-dependent gene expression in and proliferation of PCa cells. Importantly, a direct correlation between AR and TM4SF3 protein levels and nuclear colocalization were also observed in prostate tumors, strongly suggesting that the mutual stabilization resulting from the AR-TM4SF3 interaction is found in tumors and that this interaction is important in PCa biology.

COP9 subunits 4 and 5 target soluble guanylyl cyclase α1 and p53 in prostate cancer cells.

Our laboratory previously has identified soluble guanylyl cyclase α1 (sGCα1) as a direct target of androgen receptor and essential for prostate cancer cell growth via a pathway independent of nitric oxide (NO) signaling. We identified the COP9 signalosome subunit 4 (CSN4) as a novel interacting partner for sGCα1. Importantly, the CSN4-sGCα1 interaction inhibits sGCα1 proteasomal degradation. Consistent with this, disruption of CSN4 led to a significant decrease in prostate cancer cell proliferation, which was significantly but not completely rescued by sGCα1 overexpression, opening the possibility of an additional target of CSN4. Interestingly, immunoprecipitation experiments showed that p53 is found in the CSN4-sGCα1 cytoplasmic protein complex. However, in contrast to sGCα1, p53 protein stability was compromised by CSN4, leading to prostate cancer cell survival and proliferation. Interestingly, we observed that CSN4 was overexpressed in prostate tumors, and its protein level correlates directly with sGCα1 and inversely with p53 proteins, mimicking what was observed in prostate cancer cells. Our data further showed that CSN4 silencing decreased CSN5 protein levels and suggest that the CSN4 effects on sGCα1 and p53 proteins are mediated by CSN5. Lastly, our study showed that caseine kinase-2 (CK2) was involved in regulating p53 and sGCα1 protein stability as determined by both disruption of CK2 expression and inhibition of its kinase activity. Collectively, our study has identified a novel endogenous CSN4-CSN5-CK2 complex with sGCα1and p53 that oppositely controls the stability of these 2 proteins and provides prostate cancer cells an important mechanism for survival and proliferation.

Zinc Finger 280B regulates sGCα1 and p53 in prostate cancer cells.

The Zinc Finger (ZNF) 280B protein was identified as an unexpected target of an shRNA designed for sGCα1. Further analysis showed that these two proteins are connected in another way, with 280B up-regulation of sGCα1 expression. Knock-down and over-expression experiments showed that 280B serves pro-growth and pro-survival functions in prostate cancer. Surprisingly however, these pro-cancer functions of 280B are not mediated by sGCα1, which itself has similar functions in prostate cancer, but by down-regulated p53. The p53 protein is a second target of 280B in prostate cancer, but unlike sGCα1, p53 is down-regulated by 280B. 280B induces p53 nuclear export, leading to subsequent proteasomal degradation. The protein responsible for p53 regulation by 280B is Mdm2, the E3 ubiquitin ligase that promotes p53 degradation by inducing its nuclear export. We show here that 280B up-regulates expression of Mdm2 in prostate cancer cells, and this regulation is via the Mdm2 promoter. To demonstrate an in vivo relevance to this interaction, expression studies show that 280B protein levels are up-regulated in prostate cancer and these levels correspond to reduced levels of p53. Thus, by enhancing the expression of Mdm2, the uncharacterized 280B protein provides a novel mechanism of p53 suppression in prostate cancer.

A peptide against soluble guanylyl cyclase α1: a new approach to treating prostate cancer.

Among the many identified androgen-regulated genes, sGCα1 (soluble guanylyl cyclase α1) appears to play a pivotal role in mediating the pro-cancer effects of androgens and androgen receptor. The classical role for sGCα1 is to heterodimerize with the sGCß1 subunit, forming sGC, the enzyme that mediates nitric oxide signaling by catalyzing the synthesis of cyclic guanosine monophosphate. Our published data show that sGCα1 can drive prostate cancer cell proliferation independent of hormone and provide cancer cells a pro-survival function, via a novel mechanism for p53 inhibition, both of which are independent of sGCß1, NO, and cGMP. All of these properties make sGCα1 an important novel target for prostate cancer therapy. Thus, peptides were designed targeting sGCα1 with the aim of disrupting this protein's pro-cancer activities. One peptide (A-8R) was determined to be strongly cytotoxic to prostate cancer cells, rapidly inducing apoptosis. Cytotoxicity was observed in both hormone-dependent and, significantly, hormone-refractory prostate cancer cells, opening the possibility that this peptide can be used to treat the usually lethal castration-resistant prostate cancer. In mouse xenograft studies, Peptide A-8R was able to stop tumor growth of not only hormone-dependent cells, but most importantly from hormone-independent cells. In addition, the mechanism of Peptide A cytotoxicity is generation of reactive oxygen species, which recently have been recognized as a major mode of action of important cancer drugs. Thus, this paper provides strong evidence that targeting an important AR-regulated gene is a new paradigm for effective prostate cancer therapy.

Soluble guanylyl cyclase α1 and p53 cytoplasmic sequestration and down-regulation in prostate cancer.

Our laboratory has previously identified soluble guanylyl cyclase α1 (sGCα1) as a novel androgen-regulated gene essential for prostate cancer cell proliferation. sGCα1 expression is highly elevated in prostate tumors, contrasting with the low expression of sGCß1, with which sGCα1 dimerizes to mediate nitric oxide (NO) signaling. In studying its mechanism of action, we have discovered that sGCα1 can inhibit the transcriptional activity of p53 in prostate cancer cells independent of either classical mediators of NO signaling or the guanylyl cyclase activity of sGCα1. Interestingly, sGCα1 inhibition of p53-regulated gene expression was gene specific, targeting genes involved in apoptosis/cell survival. Consistent with this, overexpression of sGCα1 makes prostate cancer cells more resistant to etoposide, a chemotherapeutic and apoptosis-inducing drug. Immunoprecipitation and immunocytochemistry assays show a physical and direct interaction between sGCα1 and p53 in prostate cancer cells. Interestingly, sGCα1 induces p53 cytoplasmic sequestration, representing a new mechanism of p53 inactivation in prostate cancer. Analysis of prostate tumors has shown a direct expression correlation between sGCα1 and p53. Collectively, these data suggest that sGCα1 regulation of p53 activity is important in prostate cancer biology and may represent an important mechanism of p53 down-regulation in those prostate cancers that express significant levels of p53.

Use of reporter genes to study promoters of the androgen receptor.

As a transcriptional regulator, the androgen receptor (AR) regulates the expression of many genes that are essential for male sexual differentiation, including the development of both normal prostate and prostate cancer. The AR acts by binding to regulatory DNA sequences found on the promoters of regulated genes. The study of AR activity on such responsive promoters is greatly facilitated by the use of the reporter gene assay, which provides a quantitative and reproducible method for studying the activity of such promoters. Among the several reporter genes that can be used, the genes encoding luciferase (Luc) and chloramphenicol acetyltransferase (CAT) have been used most widely and successfully by researchers interested in AR-regulated promoters. Such studies have led to the identification and characterization of DNA regulatory elements mediating AR activity on responsive promoters and to an improved understanding of how AR regulates the transcription process. Described in this chapter is a method by which to generate and utilize Luc and CAT reporter gene plasmids driven by the promoter of a novel androgen-regulated gene, ETV1.

Expression of a hyperactive androgen receptor leads to androgen-independent growth of prostate cancer cells.

Cellular changes that affect the androgen receptor (AR) can cause prostate cancer to transition from androgen dependent to androgen independent, which is usually lethal. One common change in prostate tumors is overexpression of the AR, which has been shown to lead to androgen-independent growth of prostate cancer cells. This led us to hypothesize that expression of a hyperactive AR would be sufficient for androgen-independent growth of prostate cancer cells. To test this hypothesis, stable lune cancer prostate (LNCaP) cell lines were generated, which express a virion phosphoprotein (VP)16-AR hybrid protein that contains full-length AR fused to the strong viral transcriptional activation domain VP16. This fusion protein elicited as much as a 20-fold stronger transcriptional activity than the natural AR. Stable expression of VP16-AR in LNCaP cells yielded androgen-independent cell proliferation, while under the same growth conditions the parental LNCaP cells exhibited only androgen-dependent growth. These results show that expression of a hyperactive AR is sufficient for androgen-independent growth of prostate cancer cells. To study the molecular basis of this enhanced growth, we measured the expression of soluble guanylyl cyclase-alpha1 (sGCalpha1), a subunit of the sGC, an androgen-regulated gene that has been shown to be involved in prostate cancer cell growth. Interestingly, the expression of sGCalpha1 is androgen independent in VP16-AR-expressing cells, in contrast to its androgen-induced expression in control LNCaP cells. RNA(I)-dependent inhibition of sGCalpha1 expression resulted in significantly reduced proliferation of VP16-AR cells, implicating an important role for sGCalpha1 in the androgen-independent growth of these cells.

SUMO-3 enhances androgen receptor transcriptional activity through a sumoylation-independent mechanism in prostate cancer cells.

Androgens are important for male sexual development, which depend on the cognate receptor, the androgen receptor. The transcriptional activity of the androgen receptor, like other nuclear receptors, is regulated by accessory proteins that can have either positive or negative effects. Through a yeast functional screen, we have identified SUMO-3 as a regulator of androgen receptor activity in prostate cancer cells. SUMO-3 is one of three eukaryotic proteins that become post-translationally conjugated to their target proteins in a manner analogous to the attachment of ubiquitin. In primary prostate epithelial cells, PrEC, and the prostate cancer cells, PC-3, SUMO-3 has a weak negative effect on androgen receptor transcriptional activity. In contrast, SUMO-3 and it close relative SUMO-2 strongly enhance transactivation by endogenous androgen receptor in LNCaP cells. This positive effect is observed in both androgen-dependent and androgen-independent LNCaP cells. Interestingly, SUMO-1, unlike SUMO-3 and SUMO-2, can inhibit, but not stimulate, androgen receptor activity. Mutational analysis of the androgen receptor and SUMO-3 demonstrates that the SUMO-3-positive activity does not depend on either the sumoylation sites of the androgen receptor or the sumoylation function of SUMO-3. Stable overexpression of SUMO-3 in LNCaP cells significantly enhances the androgen-dependent proliferation of these cells. Additionally, siRNA-mediated repression of SUMO-2 significantly inhibits the growth of both androgen-dependent and -independent LNCaP cells. Collectively, these results suggest (i) a novel mechanism for elevating AR activity through the switch of SUMO-3 from a weak negative regulator in normal prostate cells to a strong positive regulator in prostate cancer cells and (ii) a proliferative role for SUMO-3 and SUMO-2 in the growth of prostate cancer cells that is independent of sumoylation of the androgen receptor.

Makorin RING finger protein 1 (MKRN1) has negative and positive effects on RNA polymerase II-dependent transcription.

Through its transcriptional activities, the proto-oncoprotein c-Jun can regulate cellular proliferation, survival, and differentiation. We have established a novel yeast assay that screens for repressors of c-Jun transcriptional activity. This screen led to the identification of a ubiquitously expressed novel RING zinc finger protein, termed Makorin RING zinc finger protein 1 (MKRN1), recently shown to act as an E3 ubiquitin ligase. Overexpression of MKRN1 in mammalian cells inhibited the transcriptional activities of not only c-Jun, but also the nuclear receptors, the androgen receptor, and the retinoic acid receptors. Truncation analysis indicates that both the amino and carboxy termini are required for this transrepression activity. Surprisingly, when fused to the heterologous DNAbinding domain of GAL4, MKRN1 activates, rather than inhibits, a GAL4-responsive reporter plasmid. In addition, truncation of either the amino- or carboxy-terminal half of MKRN1 disrupts its transactivation activity, the same observation that was made on its transrepression activity. These results demonstrate that MKRN1 has transcriptional activity and suggest that its transrepression and transactivation functions are mediated by the same mechanism. Interestingly, disruption of MKRN1's ubiquitin ligase activity does not affect its inhibitory transcriptional activity. Thus, MKRN1 may represent a nuclear protein with multiple nuclear functions, including regulating RNA polymerase II-catalyzed transcription.

Glycogen synthase kinase-3 beta is involved in the phosphorylation and suppression of androgen receptor activity.

Kinases can phosphorylate and regulate androgen receptor activity during prostate cancer progression. In particular, we showed that glycogen synthase kinase-3 beta phosphorylates the androgen receptor, thereby inhibiting androgen receptor-driven transcription. Conversely, the glycogen synthase kinase-3 beta inhibitor lithium chloride suppressed the glycogen synthase kinase-3 beta-mediated phosphorylation of the androgen receptor, thereby enabling androgen receptor-driven transcription to occur. The androgen receptor hinge and ligand-binding domains were important for both the phosphorylation and the inhibition of transcriptional activity of the receptor by glycogen synthase kinase-3 beta. Furthermore, androgen receptor phosphorylation was augmented by LY294002, an indirect inhibitor of protein kinase B/Akt that inhibits glycogen synthase kinase-3 beta. We also showed that the mutation of various phosphorylation sites on glycogen synthase kinase-3 beta affected the ability of these mutants to co-distribute with the androgen receptor in the cell nucleus, also that both glycogen synthase kinase-3beta and androgen receptor proteins can be found in cell nuclei of prostate cancer tissue samples. Because glycogen synthase kinase-3 beta activity is suppressed after the enzyme is phosphorylated by protein kinase B/Akt and Akt activity frequently increases during the progression of prostate cancer, nullification of the glycogen synthase kinase-3 beta-mediated suppression of androgen receptor activity by Akt likely contributes to prostate cancer progression.