Transcriptional regulation of the homeobox geneNKX3.1 by all-trans retinoic acid in prostate cancer cells

Epigenetic Control of Gene Expression in the Normal and Malignant Human Prostate: A Rapid Response Which Promotes Therapeutic Resistance

A successful prostate cancer must be capable of changing its phenotype in response to a variety of microenvironmental influences, such as adaptation to treatment or successful proliferation at a particular metastatic site. New cell phenotypes emerge by selection from the large, genotypically heterogeneous pool of candidate cells present within any tumor mass, including a distinct stem cell-like population. In such a multicellular model of human prostate cancer, flexible responses are primarily governed not only by de novo mutations but appear to be dominated by a combination of epigenetic controls, whose application results in treatment resistance and tumor relapse. Detailed studies of these individual cell populations have resulted in an epigenetic model for epithelial cell differentiation, which is also instructive in explaining the reported high and inevitable relapse rates of human prostate cancers to a multitude of treatment types.

Transcriptional regulation of the Nkx3.1 gene in prostate luminal stem cell specification and cancer initiation via its 3' genomic region

NK3 homeobox 1 (Nkx3.1), a transcription factor expressed in the prostate epithelium, is crucial for maintaining prostate cell fate and suppressing tumor initiation. Nkx3.1 is ubiquitously expressed in luminal cells of hormonally intact prostate but, upon androgen deprivation, exclusively labels a type of luminal stem cells named castration-resistant Nkx3.1-expressing cells (CARNs). During prostate cancer initiation, Nkx3.1 expression is frequently lost in both humans and mouse models. Therefore, investigating how Nkx3.1 expression is regulated in vivo is important for understanding the mechanisms of prostate stem cell specification and cancer initiation. Here, using a transgenic mouse line with destabilized GFP, we identified an 11-kb genomic region 3' of the Nkx3.1 transcription start site to be responsible for alterations in Nkx3.1 expression patterns under various physiological conditions. We found that androgen cell-autonomously activates Nkx3.1 expression through androgen receptor (AR) binding to the 11-kb region in both normal luminal cells and CARNs and discovered new androgen response elements in the Nkx3.1 3' UTR. In contrast, we found that, in Pten^-/- prostate tumors, loss of Nkx3.1 expression is mediated at the transcriptional level through the 11-kb region despite functional AR in the nucleus. Importantly, the GFP reporter specifically labeled CARNs in the regressed prostate only in the presence of cell-autonomous AR, supporting a facultative model for CARN specification.

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.

Transcriptomic analysis of murine embryos lacking endogenous retinoic acid signaling

Retinoic acid (RA), an active derivative of the liposoluble vitamin A (retinol), acts as an important signaling molecule during embryonic development, regulating phenomenons as diverse as anterior-posterior axial patterning, forebrain and optic vesicle development, specification of hindbrain rhombomeres, pharyngeal arches and second heart field, somitogenesis, and differentiation of spinal cord neurons. This small molecule directly triggers gene activation by binding to nuclear receptors (RARs), switching them from potential repressors to transcriptional activators. The repertoire of RA-regulated genes in embryonic tissues is poorly characterized. We performed a comparative analysis of the transcriptomes of murine wild-type and Retinaldehyde Dehydrogenase 2 null-mutant (Raldh2^−/−) embryos — unable to synthesize RA from maternally-derived retinol — using Affymetrix DNA microarrays. Transcriptomic changes were analyzed in two embryonic regions: anterior tissues including forebrain and optic vesicle, and posterior (trunk) tissues, at early stages preceding the appearance of overt phenotypic abnormalities. Several genes expected to be downregulated under RA deficiency appeared in the transcriptome data (e.g. Emx2, Foxg1 anteriorly, Cdx1, Hoxa1, Rarb posteriorly), whereas reverse-transcriptase-PCR and in situ hybridization performed for additional selected genes validated the changes identified through microarray analysis. Altogether, the affected genes belonged to numerous molecular pathways and cellular/organismal functions, demonstrating the pleiotropic nature of RA-dependent events. In both tissue samples, genes upregulated were more numerous than those downregulated, probably due to feedback regulatory loops. Bioinformatic analyses highlighted groups (clusters) of genes displaying similar behaviors in mutant tissues, and biological functions most significantly affected (e.g. mTOR, VEGF, ILK signaling in forebrain tissues; pyrimidine and purine metabolism, calcium signaling, one carbon metabolism in posterior tissues). Overall, these data give an overview of the gene expression changes resulting from embryonic RA deficiency, and provide new candidate genes and pathways that may help understanding retinoid-dependent molecular events.

Retinoic acid and androgen receptors combine to achieve tissue specific control of human prostatic transglutaminase expression: a novel regulatory network with broader significance

In the human prostate, expression of prostate-specific genes is known to be directly regulated by the androgen-induced stimulation of the androgen receptor (AR). However, less is known about the expression control of the prostate-restricted TGM4 (hTGP) gene. In the present study we demonstrate that the regulation of the hTGP gene depends mainly on retinoic acid (RA). We provide evidence that the retinoic acid receptor gamma (RAR-G) plays a major role in the regulation of the hTGP gene and that presence of the AR, but not its transcriptional transactivation activity, is critical for hTGP transcription. RA and androgen responsive elements (RARE and ARE) were mapped to the hTGP promoter by chromatin immunoprecipitation (ChIP), which also indicated that the active ARE and RARE sites were adjacent, suggesting that the antagonistic effect of androgen and RA is related to the relative position of binding sites. Publicly available AR and RAR ChIP-seq data was used to find gene potentially regulated by AR and RAR. Four of these genes (CDCA7L, CDK6, BTG1 and SAMD3) were tested for RAR and AR binding and two of them (CDCA7L and CDK6) proved to be antagonistically regulated by androgens and RA confirming that this regulation is not particular of hTGP.

ETS1 regulates NKX3.1 5' promoter activity and expression in prostate cancer cells

BACKGROUND

NKX3.1 controls the differentiation and proliferation of prostatic epithelial cells both during development and in the adult, while its expression is frequently downregulated in prostate cancers. Transcriptional control of NKX3.1 expression and in particular, factors that function via the NKX3.1 5' proximal promoter are poorly characterized.

METHODS

Deletion reporter analyses, bioinformatics, electromobility shift assays (EMSA), chromatin immunoprecipitation (ChIP) and Western blotting were performed to identify and functionally characterize sites of transcription factor binding within the initial 2,062 bp of the NKX3.1 5' promoter.

RESULTS

Deletion reporter studies of the 2,062 bp NKX3.1 5' promoter sequence localized positive transcriptional activity between -1069 and -993. Bioinformatic analyses identified the presence of two overlapping ETS1 binding sites within this region, designated EBS1 and EBS2, which exhibited 82% and 74% homology, respectively, to the ETS consensus binding sequence. EMSA and supershift assays indicated binding of both endogenous ETS1 and a recombinant GST-ETS1 protein solely to EBS1, a result that was confirmed in vivo by ChIP analysis. ETS1 overexpression transactivated NKX3.1 promoter reporter activity and upregulated endogenous NKX3.1 mRNA and protein levels in the LNCaP prostate cancer cell line, demonstrating a functional role for ETS1 in the regulation of NKX3.1 expression.

CONCLUSIONS

ETS1 upregulation of NKX3.1 expression in LNCaP cells is mediated in part via its interaction with an EBS located in the NKX3.1 5' proximal promoter. ETS1 may regulate NKX3.1 during prostate development, with the aberrant ETS1 expression and cellular localization frequently observed in human prostate tumors potentially contributing to the abnormal expression of NKX3.1.

Androgen regulation of the prostatic tumour suppressor NKX3.1 is mediated by its 3' untranslated region

The homeodomain transcription factor NKX3.1 is a prostate-specific tumour suppressor, expression of which is reduced or undetectable in the majority of metastatic prostate tumours. In the normal prostate and in prostate cancer cells, NKX3.1 expression is under tight androgenic control that we have shown to be mediated by its ~2.5 kb 3'UTR (3' untranslated region). Reporter deletion analysis of the NKX3.1 3'UTR identified three regions that were transactivated by DHT (5alpha-dihydrotestosterone) in the AR (androgen receptor)-expressing prostate cancer cell line LNCaP. Reversal of DHT effects by the anti-androgen bicalutamide supported an AR-mediated mechanism, and bioinformatic analysis of the NKX3.1 3'UTR identified canonical AREs (androgen-response elements) in each of the androgen-responsive regions. EMSAs (electrophoretic mobility-shift assays) indicated binding of the AR DNA-binding domain to two of the AREs, a proximal ARE at +2378-2392 from the transcription start site, and a more distal ARE at +3098-3112. ChIP (chromatin immunoprecipitation) analysis provided further evidence of ligand-dependent recruitment of endogenous AR to sequence encompassing each of the two elements, and site-directed mutagenesis and deletion analysis confirmed the contribution of each of the AREs in reporter assays. The present studies have therefore demonstrated that the NKX3.1 3'UTR functions as an androgen-responsive enhancer, with the proximal ARE contributing the majority and the distal ARE providing a smaller, but significant, proportion of the androgen responsiveness of the NKX3.1 3'UTR. Characterization of androgen-responsive regions of the NKX3.1 gene will assist in the identification of transcriptional regulatory mechanisms that lead to the deregulation of NKX3.1 expression in advanced prostate cancers.

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.