NKX3.1 homeodomain protein binds to topoisomerase I and enhances its activity

NKX3.1 Expression Contributes to Epithelial-Mesenchymal Transition of Prostate Cancer Cells

Studies demonstrate that inflammation synergizes with high-grade aggressive prostate tumor development and ultimately metastatic spread, in which a lot of work has been done in recent years. However, the clear mechanism of inflammation inciting prostate cancer remains largely uncharacterized. Our previous study has shown that the conditioned media (CM)-mediated LNCaP cell migration is partially correlated with the loss of expression of the tumor suppressor NKX3.1. Here, we continue to investigate the inflammation-mediated migration of prostate cancer cells, and the role of NKX3.1 in this process to gain insights into cell migration-related changes comprehensively. Earlier, the model of inflammation in the tumor microenvironment have been optimized by our research group; here, we continue to investigate the time-dependent effect of CM exposure together with NKX3.1 changes, in which we observed that these changes play important roles in gaining heterogeneous epithelial-to-mesenchymal transition (EMT) phenotype. Hence, this is an important parameter of tumor progression; we depleted NKX3.1 expression using the CRISPR/Cas9 system and examined the migrating cell clusters after exposure to inflammatory cytokines. We found that the migrated cells clearly demonstrate reversible loss of E-cadherin expression, which is consistent with subsequent vimentin expression alterations in comparison to control cells. Moreover, the data suggest that the AR-mediated transcriptional program also contributes to mesenchymal-to-epithelial transition (MET) in prostate cancer progression. Furthermore, the quantitative proteomic analysis showed that migrated subpopulations from the same cell line presented different phenotypes in which the proteins overexpressed are involved in cell metabolism and RNA processing. According to KEGG pathway analysis, the ABC transporters were found to be the most significant. Thus, the dynamic process of cellular migration favors diverse genetic compositions under changing tumor microenvironments. The different levels of invasiveness are supported by shifting the cells in between these EMT and MET phenotypes.

Tissue immunostaining of candidate prognostic proteins in metastatic and non-metastatic prostate cancer

PURPOSE

Prostate cancer (PCa) lacks specific markers capable of distinguishing aggressive tumors from those with indolent behavior. Therefore, the aim of this study was to evaluate the immunostaining of candidate proteins (PTEN, AKT, TRPM8, and NKX3.1) through the immunohistochemistry technique (IHC) on patients with metastatic and non-metastatic PCa.

METHODS

Tissues from 60 patients were divided into three groups categorized according to prognostic parameters: better prognosis (n = 20), worse prognosis (n = 23), and metastatic (n = 17). Immunostaining was analyzed by a pathologist and staining classifications were considered according to signal intensity: (0) no staining, (+) weak, and (++ and +++) intermediate to strong.

RESULTS

AKT protein was associated (p = 0.012) and correlated (p = 0.014; Tau = - 0.288) with the prognostic groups. The immunostaining for TRPM8 (p = 0.010) and NKX3.1 (p = 0.003) proteins differed between malignant tumor and non-tumoral adjacent tissue as well as for proteins in cellular locations (nucleus and cytoplasm). TRPM8 was independently associated with the ISUP grade ≥ 4 (p = 0.024; OR = 8.373; 95% CI = 1.319-53.164). The NKX3.1 showed positive and predominantly strong immunostaining in all patients in both tumoral and non-tumoral adjacent tissues. All metastatic samples had positive immunostaining, with strong intensity for NKX3.1 (p = 0.021; Tau = - 0.302). In the non-metastatic group, this strong protein staining was not observed in any patients.

CONCLUSION

This study confirmed that NKX3.1 is highly specific for prostate tissue and indicated that NKX3.1, AKT, and TRPM8 may be candidate markers for prostate cancer prognosis.

Gene of the month: NKX3.1

NKX3.1 is a multifaceted protein with roles in prostate development and protection from oxidative stress. Acting as a pioneer factor, NKX3.1 interacts with chromatin at enhancers to help integrate androgen regulated signalling. In prostate cancer, NKX3.1 activity is frequently reduced through a combination of mutational and post-translational events. Owing to its specificity for prostate tissue, NKX3.1 has found use as an immunohistochemical marker in routine histopathology practice.

The Role of Nkx3.1 in Cancers and Stemness

The well-known androgen-regulated homeobox gene, NKX3.1, is located on the short arm of chromosome 8. It is the first known prostate epithelium-specific marker, and is a transcription factor involved in development of the testes and prostate. In addition to specifying the prostate epithelium and maintaining normal prostate secretory function, Nkx3.1 is an established marker for prostate cancer. Over the years, however, this gene has been implicated in various other cancers, and technological advances have allowed determination of its role in other cellular functions. Nkx3.1 has also been recently identified as a factor capable of replacing Oct4 in cellular reprogramming. This review highlights the role of this tumor suppressor and briefly describes its functions, ranging from prostate development to maintenance of stemness and cellular reprogramming.

CRISPR/Cas9-Mediated Point Mutation in Nkx3.1 Prolongs Protein Half-Life and Reverses Effects Nkx3.1 Allelic Loss

NKX3.1 is the most commonly deleted gene in prostate cancer and is a gatekeeper suppressor. NKX3.1 is haploinsufficient, and pathogenic reduction in protein levels may result from genetic loss, decreased transcription, and increased protein degradation caused by inflammation or PTEN loss. NKX3.1 acts by retarding proliferation, activating antioxidants, and enhancing DNA repair. DYRK1B-mediated phosphorylation at serine 185 of NKX3.1 leads to its polyubiquitination and proteasomal degradation. Because NKX3.1 protein levels are reduced, but never entirely lost, in prostate adenocarcinoma, enhancement of NKX3.1 protein levels represents a potential therapeutic strategy. As a proof of principle, we used CRISPR/Cas9-mediated editing to engineer in vivo a point mutation in murine Nkx3.1 to code for a serine to alanine missense at amino acid 186, the target for Dyrk1b phosphorylation. Nkx3.1S186A/-, Nkx3.1+/- , and Nkx3.1+/+ mice were analyzed over one year to determine the levels of Nkx3.1 expression and effects of the mutant protein on the prostate. Allelic loss of Nkx3.1 caused reduced levels of Nkx3.1 protein, increased proliferation, and prostate hyperplasia and dysplasia, whereas Nkx3.1S186A/- mouse prostates had increased levels of Nkx3.1 protein, reduced prostate size, normal histology, reduced proliferation, and increased DNA end labeling. At 2 months of age, when all mice had normal prostate histology, Nkx3.1+/- mice demonstrated indices of metabolic activation, DNA damage response, and stress response. These data suggest that modulation of Nkx3.1 levels alone can exert long-term control over premalignant changes and susceptibility to DNA damage in the prostate. SIGNIFICANCE: These findings show that prolonging the half-life of Nkx3.1 reduces proliferation, enhances DNA end-labeling, and protects from DNA damage, ultimately blocking the proneoplastic effects of Nkx3.1 allelic loss.

Loss of PTEN Accelerates NKX3.1 Degradation to Promote Prostate Cancer Progression

NKX3.1 is the most commonly deleted gene in prostate cancer and a gatekeeper suppressor. NKX3.1 is a growth suppressor, mediator of apoptosis, inducer of antioxidants, and enhancer of DNA repair. PTEN is a ubiquitous tumor suppressor that is often decreased in prostate cancer during tumor progression. Steady-state turnover of NKX3.1 is mediated by DYRK1B phosphorylation at NKX3.1 serine 185 that leads to polyubiquitination and proteasomal degradation. In this study, we show PTEN is an NKX3.1 phosphatase that protects NKX3.1 from degradation. PTEN specifically opposed phosphorylation at NKX3.1(S185) and prolonged NKX3.1 half-life. PTEN and NKX3.1 interacted primarily in the nucleus as loss of PTEN nuclear localization abrogated its ability to bind to and protect NKX3.1 from degradation. The effect of PTEN on NKX3.1 was mediated via rapid enzyme-substrate interaction. An effect of PTEN on Nkx3.1 gene transcription was seen in vitro, but not in vivo. In gene-targeted mice, Nkx3.1 expression significantly diminished shortly after loss of Pten expression in the prostate. Nkx3.1 loss primarily increased prostate epithelial cell proliferation in vivo. In these mice, Nkx3.1 mRNA was not affected by Pten expression. Thus, the prostate cancer suppressors PTEN and NKX3.1 interact and loss of PTEN is responsible, at least in part, for progressive loss of NKX3.1 that occurs during tumor progression. SIGNIFICANCE: PTEN functions as a phosphatase of NKX3.1, a gatekeeper suppressor of prostate cancer.

Oxidative DNA Damage-Mediated Genomic Heterogeneity Is Regulated by NKX3.1 in Prostate Cancer

The 8-hydroxy-2'-deoxyguanosine (8-OHdG) damages are base damages induced by reactive oxygen species. We aimed to investigate the role of Androgen Receptor and NKX3.1 in 8-OHdG formation and repair activation by quantitating the DNA damage using Aklides.NUK system. The data demonstrated that the loss of NKX3.1 resulted in increased oxidative DNA damage and its overexpression contributes to the removal of menadione-induced 8-OHdG damage even under oxidative stress conditions. Moreover, 8-oxoguanine DNA glycosylase-1 (OGG1) expression level positively correlates to NKX3.1 expression. Also in this study, first time a reliable cell-based quantitation method for 8-OHdG damages is reported and used for data collection.

Homeodomain Proteins Directly Regulate ATM Kinase Activity

Ataxia-telangiectasia mutated (ATM) is a serine/threonine kinase that coordinates the response to DNA double-strand breaks and oxidative stress. NKX3.1, a prostate-specific transcription factor, was recently shown to directly stimulate ATM kinase activity through its highly conserved homeodomain. Here, we show that other members of the homeodomain family can also regulate ATM kinase activity. We found that six representative homeodomain proteins (NKX3.1, NKX2.2, TTF1, NKX2.5, HOXB7, and CDX2) physically and functionally interact with ATM and with the Mre11-Rad50-Nbs1 (MRN) complex that activates ATM in combination with DNA double-strand breaks. The binding between homeodomain proteins and ATM stimulates oxidation-induced ATM activation in vitro but inhibits ATM kinase activity in the presence of MRN and DNA and in human cells. These findings suggest that many tissue-specific homeodomain proteins may regulate ATM activity during development and differentiation and that this is a unique mechanism for the control of the DNA damage response.

NK3 homeobox 1 (NKX3.1) up-regulates forkhead box O1 expression in hepatocellular carcinoma and thereby suppresses tumor proliferation and invasion

Hepatocellular carcinoma (HCC) is the leading cause of cancer-related mortality in China, and the molecular mechanism of uncontrolled HCC progression remains to be explored. NK3 homeobox 1 (NKX3.1), an androgen-regulated prostate-specific transcription factor, suppresses tumors in prostate cancer, but its role in HCC is unknown, especially in hepatocellular carcinoma. In the present study, the differential expression analyses in HCC tissues and matched adjacent noncancerous liver tissues revealed that NKX3.1 is frequently down-regulated in human primary HCC tissues compared with matched adjacent noncancerous liver tissues. We also noted that NKX3.1 significantly inhibits proliferation and mobility of HCC cells both in vitro and in vivo Furthermore, NKX3.1 overexpression resulted in cell cycle arrest at the G₁/S phase via direct binding to the promoter of forkhead box O1 (FOXO1) and up-regulation of expression. Of note, FOXO1 silencing in NKX3.1-overexpressing cells reversed the inhibitory effects of NKX3.1 on HCC cell proliferation and invasion. Consistently, both FOXO1 and NKX3.1 were down-regulated in human HCC tissues, and their expression was significantly and positively correlated with each other. These results suggest that NKX3.1 functions as a tumor suppressor in HCC cells through directly up-regulating FOXO1 expression.

Physiological functions of programmed DNA breaks in signal-induced transcription

The idea that signal-dependent transcription might involve the generation of transient DNA nicks or even breaks in the regulatory regions of genes, accompanied by activation of DNA damage repair pathways, would seem to be counterintuitive, as DNA damage is usually considered harmful to cellular integrity. However, recent studies have generated a substantial body of evidence that now argues that programmed DNA single- or double-strand breaks can, at least in specific cases, have a role in transcription regulation. Here, we discuss the emerging functions of DNA breaks in the relief of DNA torsional stress and in promoter and enhancer activation.

Controlling gene expression by DNA mechanics: emerging insights and challenges

Transcription initiation is a major control point for the precise regulation of gene expression. Our knowledge of this process has been mainly derived from protein-centric studies wherein cis-regulatory DNA sequences play a passive role, mainly in arranging the protein machinery to coalesce at the transcription start sites of genes in a spatial and temporal-specific manner. However, this is a highly dynamic process in which molecular motors such as RNA polymerase II (RNAPII), helicases, and other transcription factors, alter the level of mechanical force in DNA, rather than simply a set of static DNA-protein interactions. The double helix is a fiber that responds to flexural and torsional stress, which if accumulated, can affect promoter output as well as change DNA and chromatin structure. The relationship between DNA mechanics and the control of early transcription initiation events has been under-investigated. Genomic techniques to display topological stress and conformational variation in DNA across the mammalian genome provide an exciting new insight on the role of DNA mechanics in the early stages of the transcription cycle. Without understanding how torsional and flexural stresses are generated, transmitted, and dissipated, no model of transcription will be complete and accurate.

RNA Polymerase II Regulates Topoisomerase 1 Activity to Favor Efficient Transcription

We report a mechanism through which the transcription machinery directly controls topoisomerase 1 (TOP1) activity to adjust DNA topology throughout the transcription cycle. By comparing TOP1 occupancy using chromatin immunoprecipitation sequencing (ChIP-seq) versus TOP1 activity using topoisomerase 1 sequencing (TOP1-seq), a method reported here to map catalytically engaged TOP1, TOP1 bound at promoters was discovered to become fully active only after pause-release. This transition coupled the phosphorylation of the carboxyl-terminal-domain (CTD) of RNA polymerase II (RNAPII) with stimulation of TOP1 above its basal rate, enhancing its processivity. TOP1 stimulation is strongly dependent on the kinase activity of BRD4, a protein that phosphorylates Ser2-CTD and regulates RNAPII pause-release. Thus the coordinated action of BRD4 and TOP1 overcame the torsional stress opposing transcription as RNAPII commenced elongation but preserved negative supercoiling that assists promoter melting at start sites. This nexus between transcription and DNA topology promises to elicit new strategies to intercept pathological gene expression.

Controlling gene expression by DNA mechanics: emerging insights and challenges

Transcription initiation is a major control point for the precise regulation of gene expression. Our knowledge of this process has been mainly derived from protein-centric studies wherein cis-regulatory DNA sequences play a passive role, mainly in arranging the protein machinery to coalesce at the transcription start sites of genes in a spatial and temporal-specific manner. However, this is a highly dynamic process in which molecular motors such as RNA polymerase II (RNAPII), helicases, and other transcription factors, alter the level of mechanical force in DNA, rather than simply a set of static DNA-protein interactions. The double helix is a fiber that responds to flexural and torsional stress, which if accumulated, can affect promoter output as well as change DNA and chromatin structure. The relationship between DNA mechanics and the control of early transcription initiation events has been under-investigated. Genomic techniques to display topological stress and conformational variation in DNA across the mammalian genome provide an exciting new insight on the role of DNA mechanics in the early stages of the transcription cycle. Without understanding how torsional and flexural stresses are generated, transmitted, and dissipated, no model of transcription will be complete and accurate.

Regulating NKX3.1 stability and function: Post-translational modifications and structural determinants

BACKGROUND

The androgen-regulated homeodomain transcription factor NKX3.1 plays roles in early prostate development and functions as a prostate-specific tumor suppressor. Decreased expression of NKX3.1 protein is common in primary prostate cancer. Discordance between NKX3.1 mRNA and protein levels during prostate carcinogenesis suggested a key role for post-transcriptional modifications in regulating NKX3.1 protein levels in prostate epithelial cells. Subsequent studies revealed NKX3.1 to be modified post-translationally at multiple sites.

METHODS

We reviewed published literature to identify and summarize post-translational modifications and structural elements critical in regulating NKX3.1 stability and levels in prostate epithelial cells.

RESULTS

NKX3.1 is modified post-translationally at multiple sites by different protein kinases. These modifications together with several structural determinants were identified to play an important role in NKX3.1 stability and biology.

CONCLUSIONS

In this review, we provide a comprehensive overview of the known post-translational modifications and structural features that impact NKX3.1. Defining factors that regulate NKX3.1 in prostate epithelial cells will extend our understanding of molecular changes that may contribute to prostate cancer initiation and progression.

Nkx3.1 controls the DNA repair response in the mouse prostate

BACKGROUND

The human prostate tumor suppressor NKX3.1 mediates the DNA repair response and interacts with the androgen receptor to assure faithful completion of transcription thereby protecting against TMPRSS2-ERG gene fusion. To determine directly the effect of Nkx3.1 in vivo we studied the DNA repair response in prostates of mice with targeted deletion of Nkx3.1.

METHODS

Using both drug-induced DNA damage and γ-irradiation, we assayed expression of γ-histone 2AX at time points up to 24 hr after induction of DNA damage.

RESULTS

We demonstrated that expression of Nkx3.1 influenced both the timing and magnitude of the DNA damage response in the prostate.

CONCLUSIONS

Nkx3.1 affects the DNA damage response in the murine prostate and is haploinsufficient for this phenotype.

NKX3.1 Suppresses TMPRSS2-ERG Gene Rearrangement and Mediates Repair of Androgen Receptor-Induced DNA Damage

TMPRSS2 gene rearrangements occur at DNA breaks formed during androgen receptor-mediated transcription and activate expression of ETS transcription factors at the early stages of more than half of prostate cancers. NKX3.1, a prostate tumor suppressor that accelerates the DNA repair response, binds to androgen receptor at the ERG gene breakpoint and inhibits both the juxtaposition of the TMPRSS2 and ERG gene loci and also their recombination. NKX3.1 acts by accelerating DNA repair after androgen-induced transcriptional activation. NKX3.1 influences the recruitment of proteins that promote homology-directed DNA repair. Loss of NKX3.1 favors recruitment to the ERG gene breakpoint of proteins that promote error-prone nonhomologous end-joining. Analysis of prostate cancer tissues showed that the presence of a TMPRSS2-ERG rearrangement was highly correlated with lower levels of NKX3.1 expression consistent with the role of NKX3.1 as a suppressor of the pathogenic gene rearrangement.

Inflammation contributes to NKX3.1 loss and augments DNA damage but does not alter the DNA damage response via increased SIRT1 expression

The oxidative stress response is a cellular defense mechanism that protects cells from oxidative damage and cancer development. The exact molecular mechanism by which reactive oxygen species (ROS) contribute to DNA damage and increase genome instability in prostate cancer merits further investigation. Here, we aimed to determine the effects of NKX3.1 loss on antioxidant defense in response to acute and chronic inflammation in an in vitro model. Oxidative stress-induced DNA damage resulted in increased H2AX^(S139) phosphorylation (a hallmark of DNA damage), along with the degradation of the androgen receptor (AR), p53 and NKX3.1, upon treatment with conditioned medium (CM) obtained from activated macrophages or H₂O₂. Furthermore, the expression and stability of SIRT1 were increased by CM treatment but not by H₂O₂ treatment, although the level of ATM^(S1981) phosphorylation was not changed compared with controls. Moreover, the deregulated antioxidant response resulted in upregulation of the pro-oxidant QSCN6 and the antioxidant GPX2 and downregulation of the antioxidant GPX3 after CM treatment. Consistently, the intracellular ROS level increased after chronic treatment, leading to a dose-dependent increase in the ability of LNCaP cells to tolerate oxidative damage. These data suggest that the inflammatory microenvironment is a major factor contributing to DNA damage and the deregulation of the oxidative stress response, which may be the underlying cause of the increased genetic heterogeneity during prostate tumor progression.

Ligand-dependent enhancer activation regulated by topoisomerase-I activity

The discovery that enhancers are regulated transcription units, encoding eRNAs, has raised new questions about the mechanisms of their activation. Here, we report an unexpected molecular mechanism that underlies ligand-dependent enhancer activation, based on DNA nicking to relieve torsional stress from eRNA synthesis. Using dihydrotestosterone (DHT)-induced binding of androgen receptor (AR) to prostate cancer cell enhancers as a model, we show rapid recruitment, within minutes, of DNA topoisomerase I (TOP1) to a large cohort of AR-regulated enhancers. Furthermore, we show that the DNA nicking activity of TOP1 is a prerequisite for robust eRNA synthesis and enhancer activation and is kinetically accompanied by the recruitment of ATR and the MRN complex, followed by additional components of DNA damage repair machinery to the AR-regulated enhancers. Together, our studies reveal a linkage between eRNA synthesis and ligand-dependent TOP1-mediated nicking-a strategy exerting quantitative effects on eRNA expression in regulating AR-bound enhancer-dependent transcriptional programs.

TNFα-mediated loss of β-catenin/E-cadherin association and subsequent increase in cell migration is partially restored by NKX3.1 expression in prostate cells

Inflammation-induced carcinogenesis is associated with increased proliferation and migration/invasion of various types of tumor cells. In this study, altered β-catenin signaling upon TNFα exposure, and relation to loss of function of the tumor suppressor NKX3.1 was examined in prostate cancer cells. We used an in vitro prostate inflammation model to demonstrate altered sub-cellular localization of β-catenin following increased phosphorylation of Akt^(S473) and GSK3β^(S9). Consistently, we observed that subsequent increase in β-catenin transactivation enhanced c-myc, cyclin D1 and MMP2 expressions. Consequently, it was also observed that the β-catenin-E-cadherin association at the plasma membrane was disrupted during acute cytokine exposure. Additionally, it was demonstrated that disrupting cell-cell interactions led to increased migration of LNCaP cells in real-time migration assay. Nevertheless, ectopic expression of NKX3.1, which is degraded upon proinflammatory cytokine exposure in inflammation, was found to induce the degradation of β-catenin by inhibiting Akt^(S473) phosphorylation, therefore, partially rescued the disrupted β-catenin-E-cadherin interaction as well as the cell migration in LNCaP cells upon cytokine exposure. As, the disrupted localization of β-catenin at the cell membrane as well as increased Akt^(S308) priming phosphorylation was observed in human prostate tissues with prostatic inflammatory atrophy (PIA), high-grade prostatic intraepithelial neoplasia (H-PIN) and carcinoma lesions correlated with loss of NKX3.1 expression. Thus, the data indicate that the β-catenin signaling; consequently sub-cellular localization is deregulated in inflammation, associates with prostatic atrophy and PIN pathology.

Systems analysis of the prostate tumor suppressor NKX3.1 supports roles in DNA repair and luminal cell differentiation

NKX3.1 is a homeobox transcription factor whose function as a prostate tumor suppressor remains insufficiently understood because neither the transcriptional program governed by NKX3.1, nor its interacting proteins have been fully revealed. Using affinity purification and mass spectrometry, we have established an extensive NKX3.1 interactome which contains the DNA repair proteins Ku70, Ku80, and PARP, thus providing a molecular underpinning to previous reports implicating NKX3.1 in DNA repair. Transcriptomic profiling of NKX3.1-negative prostate epithelial cells acutely expressing NKX3.1 revealed a rapid and complex response that is a near mirror image of the gene expression signature of human prostatic intraepithelial neoplasia (PIN). Pathway and network analyses suggested that NKX3.1 actuates a cellular reprogramming toward luminal cell differentiation characterized by suppression of pro-oncogenic c-MYC and interferon-STAT signaling and activation of tumor suppressor pathways. Consistently, ectopic expression of NKX3.1 conferred a growth arrest depending on TNFα and JNK signaling. We propose that the tumor suppressor function of NKX3.1 entails a transcriptional program that maintains the differentiation state of secretory luminal cells and that disruption of NKX3.1 contributes to prostate tumorigenesis by permitting luminal cell de-differentiation potentially augmented by defects in DNA repair.

Structural and functional interactions of the prostate cancer suppressor protein NKX3.1 with topoisomerase I

NKX3.1 (NK3 homeobox 1) is a prostate tumour suppressor protein with a number of activities that are critical for its role in tumour suppression. NKX3.1 mediates the cellular response to DNA damage by interacting with ATM (ataxia telangiectasia mutated) and by activation of topoisomerase I. In the present study we characterized the interaction between NKX3.1 and topoisomerase I. The NKX3.1 homeodomain binds to a region of topoisomerase I spanning the junction between the core and linker domains. Loss of the topoisomerase I N-terminal domain, a region for frequent protein interactions, did not affect binding to NKX3.1 as was shown by the activation of Topo70 (N-terminal truncated topoisomerase I) in vitro. In contrast, NKX3.1 interacts with the enzyme reconstituted from peptide fragments of the core and linker active site domains, but inhibits the DNA-resolving activity of the reconstituted enzyme in vitro. The effect of NKX3.1 on both Topo70 and the reconstituted enzyme was seen in the presence and absence of camptothecin. Neither NKX3.1 nor CPT (camptothecin) had an effect on the interaction of the other with topoisomerase I. Therefore the interactions of NKX3.1 and CPT with the linker domain of topoisomerase I are mutually exclusive. However, in cells the effect of NKX3.1 on topoisomerase binding to DNA sensitized the cells to cellular toxicity and the induction of apoptosis by low doses of CPT. Lastly, topoisomerase I is important for the effect of NKX3.1 on cell survival after DNA damage as topoisomerase knockdown blocked the effect of NKX3.1 on clonogenicity after DNA damage. Therefore NKX3.1 and topoisomerase I interact in vitro and in cells to affect the CPT sensitivity and DNA-repair functions of NKX3.1.

Functional activation of ATM by the prostate cancer suppressor NKX3.1

The prostate tumor suppressor NKX3.1 augments response to DNA damage and enhances survival after DNA damage. Within minutes of DNA damage, NKX3.1 undergoes phosphorylation at tyrosine 222, which is required for a functional interaction with ataxia telangiectasia mutated (ATM) kinase. NKX3.1 binds to the N-terminal region of ATM, accelerates ATM activation, and hastens the formation of γhistone2AX. NKX3.1 enhances DNA-dependent ATM kinase activation by both the MRN complex and H2O2 in a DNA-damage-independent manner. ATM, bound to the NKX3.1 homeodomain, phosphorylates NKX3.1, leading to ubiquitination and degradation. Thus, NKX3.1 and ATM have a functional interaction leading to ATM activation and then NKX3.1 degradation in a tightly regulated DNA damage response specific to prostate epithelial cells. These findings demonstrate a mechanism for the tumor-suppressor properties of NKX3.1, demonstrate how NKX3.1 may enhance DNA integrity in prostate stem cells and may help to explain how cells differ in their sensitivity to DNA damage.

4FISH-IF, a four-color dual-gene FISH combined with p63 immunofluorescence to evaluate NKX3.1 and MYC status in prostate cancer

NKX3.1 allelic loss and MYC amplification are common events during prostate cancer progression and have been recognized as potential prognostic factors in prostate cancer after radical prostatectomy or precision radiotherapy. We have developed a 4FISH-IF assay (a dual-gene fluorescence in situ hybridization combined with immunofluorescence) to measure both NKX3.1 and MYC status on the same slide. The 4FISH-IF assay contains four probes complementary to chromosome 8 centromere, 8p telomere, 8p21, and 8q24, as well as an antibody targeting the basal cell marker p63 visualized by immunofluorescence. The major advantages of the 4FISH-IF include the distinction between benign and malignant glands directly on the 4FISH-IF slide and the control of truncation artifact. Importantly, this specialized and innovative combined multiprobe and immunofluorescence technique can be performed on diagnostic biopsy specimens, increasing its clinical relevance. Moreover, the assay can be easily performed in a standard clinical molecular pathology laboratory. Globally, the use of 4FISH-IF decreases analytic time, increases confidence in obtained results, and maintains the tissue morphology of the diagnostic specimen.

Antioxidant treatment promotes prostate epithelial proliferation in Nkx3.1 mutant mice

Discordant results in preclinical and clinical trials have raised questions over the effectiveness of antioxidants in prostate cancer chemoprevention. Results from the large-scale Selenium and Vitamin E Cancer Prevention Trial (SELECT) showed that antioxidants failed to prevent, and in some cases promoted, prostate cancer formation in men without a history of the disease. One possible explanation for these alarming results is the notion that the effects of antioxidant treatment on the prostate are modified by specific, intrinsic genetic risk factors, causing some men to respond negatively to antioxidant treatment. Loss of expression of the homeobox transcription factor NKX3.1 in the prostate is frequently associated with human prostate cancer. Nkx3.1 mutant mice display prostatic hyperplasia and dysplasia and are used as a model of the early stages of prostate cancer initiation. While the mechanisms by which Nkx3.1 loss promotes prostate tumorigenicity are not completely understood, published data have suggested that elevated reactive oxygen species (ROS) associated with Nkx3.1 loss may be a causative factor. Here we have tested this hypothesis by treating Nkx3.1 mutant mice with the antioxidant N-acetylcysteine (NAC) for 13 weeks post-weaning. Surprisingly, while NAC treatment decreased ROS levels in Nkx3.1 mutant mouse prostates, it failed to reduce prostatic epithelial hyperplasia/dysplasia. Rather, NAC treatment increased epithelial cell proliferation and promoted the expression of a pro-proliferative gene signature. These results show that ROS do not promote proliferation in the Nkx3.1-null prostate, but instead inhibit proliferation, suggesting that antioxidant treatment may encourage prostate epithelial cell proliferation early in prostate tumorigenesis. Our findings provide new insight that may help explain the increased prostate cancer risk observed with vitamin E treatment in the SELECT trial and emphasize the need for preclinical studies using accurate models of cancer.

The prostate cancer risk locus at 10q11 is associated with DNA repair capacity

Genome-wide association studies (GWAS) have identified several single nucleotide polymorphisms (SNPs) that mildly predict prostate cancer risk. These SNPs are local tagging markers for causal gene alterations. Consideration of candidate genes in the tagged regions would be facilitated by additional information on the particular pathomechanisms which contribute to the observed risk increase. In this study we test for an association of prostate cancer tagging SNPs with alterations in DNA repair capacity, a phenotype that is frequently involved in cancer predisposition. DNA repair capacity was assessed on blood lymphocytes from 128 healthy probands after ionizing irradiation. We used the micronucleus (MN) assay to determine the cellular DNA double-strand break repair capacity and flow cytometry to measure damage induced mitotic delay (MD). Probands were genotyped for a panel of 14 SNPs, each representing an independent prostate cancer risk locus previously identified by GWAS. Associations between germline variants and DNA repair capacity were found for the SNPs rs1512268 (8p21), rs6983267 (8q24) and rs10993994 (10q11). The most significant finding was an association of homozygous rs10993994 T-allele carriers with a lower MN frequency (p=0.0003) and also a decreased MD index (p=0.0353). Cells with prostate cancer risk alleles at rs10993994 seem to cope more efficiently with DNA double strand breaks (less MN) in a shorter time (decreased MD index). This intriguing finding imposes concern about the accuracy of repair, with respect to the cancer risk that is mediated by T genotypes. To date, MSMB (microseminoprotein β) is favored as the causal gene at the 10q11 risk locus, since it was the first candidate gene known to be expressionally altered by rs10993994. Based on the present observation, candidate genes from the contexts of DNA repair and apoptosis may be more promising targets for expression studies with respect to the rs10993994 genotype.

NKX3.1 haploinsufficiency is prognostic for prostate cancer relapse following surgery or image-guided radiotherapy

BACKGROUND

Despite the use of prostate specific antigen (PSA), Gleason-score, and T-category as prognostic factors, up to 40% of patients with intermediate-risk prostate cancer will fail radical prostatectomy or precision image-guided radiotherapy (IGRT). Additional genetic prognosticators are needed to triage these patients toward intensified combination therapy with novel targeted therapeutics. We tested the role of the NKX3.1 gene as a determinant of treatment outcome given its reported roles in tumor initiating cell (TIC) renewal, the DNA damage response, and cooperation with c-MYC during prostate cancer progression.

METHODS

Using high-resolution array comparative genomic hybridization (aCGH), we profiled the copy number alterations in TIC genes using tumor DNA from frozen needle biopsies derived from 126 intermediate-risk patients who underwent IGRT. These data were correlated to biochemical relapse-free rate (bRFR) by the Kaplan-Meier method and Cox proportional hazards models.

RESULTS

A screen of the aCGH-IGRT data for TIC genes showed frequent copy number alterations for NKX3.1, PSCA, and c-MYC. NKX3.1 haploinsufficiency was associated with increased genomic instability independent of PSA, T-category, and Gleason-score. After adjusting for clinical factors in a multivariate model, NKX3.1 haploinsufficiency was associated with bRFR when tested alone (HR = 3.05, 95% CI: 1.46-6.39, P = 0.0030) or when combined with c-MYC gain (HR = 3.88, 95% CI: 1.78-8.49, P = 0.00067). A similar association was observed for patients following radical prostatectomy with a public aCGH database. NKX3.1 status was associated with positive biopsies post-IGRT and increased clonogen radioresistance in vitro.

CONCLUSIONS

Our results support the use of genomic predictors, such as NKX3.1 status, in needle biopsies for personalized approaches to prostate cancer management.

NKX3.1 activates cellular response to DNA damage

The prostate-specific tumor suppressor homeodomain protein NKX3.1 is inactivated by a variety of mechanisms in the earliest phases of prostate carcinogenesis and in premalignant regions of the prostate gland. The mechanisms by which NKX3.1 exercises tumor suppression have not been well elucidated. Here, we show that NKX3.1 affects DNA damage response and cell survival after DNA damage. NKX3.1 expression in PC-3 prostate cancer cells enhances colony formation after DNA damage but has minimal effect on apoptosis. NKX3.1 also diminishes and regulates total cellular accumulation of gammaH2AX. Endogenous NKX3.1 in LNCaP cells localizes to sites of DNA damage where it affects the recruitment of phosphorylated ATM and the phosphorylation of H2AX. Knockdown of NKX3.1 in LNCaP cells attenuates the acute responses of both ATM and H2AX phosphorylation to DNA damage and their subnuclear localization to DNA damage sites. NKX3.1 expression enhances activation of ATM as assayed by autophosphorylation at serine 1981 and activation of ATR as assayed by phosphorylation of CHK1. An inherited mutation of NKX3.1 that predisposes to early prostate cancer and attenuates in vitro DNA binding was devoid of the ability to activate ATM and to colocalize with gammaH2AX at foci of DNA damage. These data show a novel mechanism by which a homeoprotein can affect DNA damage repair and act as a tumor suppressor.

Intronic DNA elements regulate androgen-dependent expression of the murine Nkx3.1 gene

Nkx3.1 is a well-conserved homeobox gene that is involved in development, differentiation and maintenance of prostate epithelial cells. Nkx3.1 expression is induced by androgen in prostate epithelia and, as such, our interest is to understand the mechanism(s) for this androgen-dependent expression in normal epithelial cells. In this report, we show that the region of DNA sequence 2.7 kilobases in front of the mouse Nkx3.1 gene drives enhanced transcription in prostate epithelia cells; however, this segment was not capable of androgen-directed regulation. Among the multiple, potential androgen response elements (AREs) identified by scanning sequences near and within the gene, two sequences within the intron of the murine Nkx3.1 gene were demonstrated to confer androgen-dependent transcription in reporter gene transfection experiments. Each of the elements, termed ARE A and ARE B, contained a 6-base pair core sequence, TGTTCT, that has been described as an androgen receptor half-site binding sequence, separated by 498 base pairs of DNA. Both of the intronic half-sites bind activated androgen receptor from a variety of sources, albeit with different apparent affinities. This region of the Nkx3.1 gene demonstrates a high degree of conservation among diverse species and mutagenesis experiments demonstrated that both elements are required for androgen stimulation. Taken together, our study shows that androgen-dependent transcription of the mouse Nkx3.1 gene is conferred through a noncanonical element within the intron of the gene.

Inflammatory cytokines induce phosphorylation and ubiquitination of prostate suppressor protein NKX3.1

Inflammation of the prostate is a risk factor for the development of prostate cancer. In the aging prostate, regions of inflammatory atrophy are foci for prostate epithelial cell transformation. Expression of the suppressor protein NKX3.1 is reduced in regions of inflammatory atrophy and in preinvasive prostate cancer. Inflammatory cytokines tumor necrosis factor (TNF)-alpha and interleukin-1beta accelerate NKX3.1 protein loss by inducing rapid ubiquitination and proteasomal degradation. The effect of TNF-alpha is mediated via the COOH-terminal domain of NKX3.1 where phosphorylation of serine 196 is critical for cytokine-induced degradation. Mutation of serine 196 to alanine abrogates phosphorylation at that site and the effect of TNF-alpha on NKX3.1 ubiquitination and protein loss. This is in contrast to control of steady-state NKX3.1 turnover, which is mediated by serine 185. Mutation of serine 185 to alanine increases NKX3.1 protein stability by inhibiting ubiquitination and doubling the protein half-life. A third COOH-terminal serine at position 195 has a modulating effect on both steady-state protein turnover and on ubiquitination induced by TNF-alpha. Thus, cellular levels of the NKX3.1 tumor suppressor are affected by inflammatory cytokines that target COOH-terminal serine residues to activate ubiquitination and protein degradation. Our data suggest that strategies to inhibit inflammation or to inhibit effector kinases may be useful approaches to prostate cancer prevention.

Par-4 binds to topoisomerase 1 and attenuates its DNA relaxation activity

The regulation of DNA relaxation by topoisomerase 1 (TOP1) is essential for DNA replication, transcription, and recombination events. TOP1 activity is elevated in cancer cells, yet the regulatory mechanism restraining its activity is not understood. We present evidence that the tumor suppressor protein prostate apoptosis response-4 (Par-4) directly binds to TOP1 and attenuates its DNA relaxation activity. Unlike camptothecin, which binds at the TOP1-DNA interface to form cleavage complexes, Par-4 interacts with TOP1 via its leucine zipper domain and sequesters TOP1 from the DNA. Par-4 knockdown by RNA interference enhances DNA relaxation and gene transcription activities and promotes cellular transformation in a TOP1-dependent manner. Conversely, attenuation of TOP1 activity either by RNA interference or Par-4 overexpression impedes DNA relaxation, cell cycle progression, and gene transcription activities and inhibits transformation. Collectively, our findings suggest that Par-4 serves as an intracellular repressor of TOP1 catalytic activity and regulates DNA topology to suppress cellular transformation.

Integrating differentiation and cancer: the Nkx3.1 homeobox gene in prostate organogenesis and carcinogenesis

Several tissue-specific regulatory genes have been found to play essential roles in both organogenesis and carcinogenesis. In the prostate, the Nkx3.1 homeobox gene plays an important role in normal differentiation of the prostatic epithelium while its loss of function is an initiating event in prostate carcinogenesis in both mouse models and human patients. Thus, the Nkx3.1 homeobox gene provides a paradigm for understanding the relationship between normal differentiation and cancer, as well as studying the roles of homeobox genes in these processes. Here, we review recent findings concerning the roles of Nkx3.1 in development and discuss how its normal function is disrupted in processes of early prostate carcinogenesis.

Ubiquitination by TOPORS regulates the prostate tumor suppressor NKX3.1

The NKX3.1 gene located at 8p21.2 encodes a homeodomain-containing transcription factor that acts as a haploinsufficient tumor suppressor in prostate cancer. Diminished protein expression of NKX3.1 has been observed in prostate cancer precursors and carcinomas. TOPORS is a ubiquitously expressed E3 ubiquitin ligase that can ubiquitinate tumor suppressor p53. Here we report interaction between NKX3.1 and TOPORS. NKX3.1 can be ubiquitinated by TOPORS in vitro and in vivo, and overexpression of TOPORS leads to NKX3.1 proteasomal degradation in prostate cancer cells. Conversely, small interfering RNA-mediated knockdown of TOPORS leads to an increased steady-state level and prolonged half-life of NKX3.1. These data establish TOPORS as a negative regulator of NKX3.1 and implicate TOPORS in prostate cancer progression.

Cancer statistics, 2007

Each year, the American Cancer Society (ACS) estimates the number of new cancer cases and deaths expected in the United States in the current year and compiles the most recent data on cancer incidence, mortality, and survival based on incidence data from the National Cancer Institute, Centers for Disease Control and Prevention, and the North American Association of Central Cancer Registries and mortality data from the National Center for Health Statistics. This report considers incidence data through 2003 and mortality data through 2004. Incidence and death rates are age-standardized to the 2000 US standard million population. A total of 1,444,920 new cancer cases and 559,650 deaths for cancers are projected to occur in the United States in 2007. Notable trends in cancer incidence and mortality rates include stabilization of the age-standardized, delay-adjusted incidence rates for all cancers combined in men from 1995 through 2003; a continuing increase in the incidence rate by 0.3% per year in women; and a 13.6% total decrease in age-standardized cancer death rates among men and women combined between 1991 and 2004. This report also examines cancer incidence, mortality, and survival by site, sex, race/ethnicity, geographic area, and calendar year, as well as the proportionate contribution of selected sites to the overall trends. While the absolute number of cancer deaths decreased for the second consecutive year in the United States (by more than 3,000 from 2003 to 2004) and much progress has been made in reducing mortality rates and improving survival, cancer still accounts for more deaths than heart disease in persons under age 85 years. Further progress can be accelerated by supporting new discoveries and by applying existing cancer control knowledge across all segments of the population.