Ka-Wing Fong

LncRNA HOTAIR enhances ER signaling and confers tamoxifen resistance in breast cancer

Tamoxifen, an estrogen receptor (ER) antagonist, is the mainstay treatment of breast cancer and the development of resistance represents a major obstacle for a cure. Although long non-coding RNAs such as HOTAIR have been implicated in breast tumorigenesis, their roles in chemotherapy resistance remain largely unknown. In this study, we report that HOTAIR (HOX antisense intergenic RNA) is upregulated in tamoxifen-resistant breast cancer tissues compared to their primary counterparts. Mechanistically, HOTAIR is a direct target of ER-mediated transcriptional repression and is thus restored upon the blockade of ER signaling, either by hormone deprivation or by tamoxifen treatment. Interestingly, this elevated HOTAIR increases ER protein level and thus enhances ER occupancy on the chromatin and potentiates its downstream gene regulation. HOTAIR overexpression is sufficient to activate the ER transcriptional program even under hormone-deprived conditions. Functionally, we found that HOTAIR overexpression increases breast cancer cell proliferation, whereas its depletion significantly impairs cell survival and abolishes tamoxifen-resistant cell growth. In conclusion, the long non-coding RNA HOTAIR is directly repressed by ER and its upregulation promotes ligand-independent ER activities and contributes to tamoxifen resistance.

LncRNA HOTAIR Enhances the Androgen-Receptor-Mediated Transcriptional Program and Drives Castration-Resistant Prostate Cancer

SUMMARY Understanding the mechanisms of androgen receptor (AR) activation in the milieu of low androgen is critical to effective treatment of castration-resistant prostate cancer (CRPC). Here, we report HOTAIR as an androgen-repressed lncRNA, and, as such, it is markedly upregulated following androgen deprivation therapies and in CRPC. We further demonstrate a distinct mode of lncRNA-mediated gene regulation, wherein HOTAIR binds to the AR protein to block its interaction with the E3 ubiquitin ligase MDM2, thereby preventing AR ubiquitination and protein degradation. Consequently, HOTAIR expression is sufficient to induce androgen-independent AR activation and drive the AR-mediated transcriptional program in the absence of androgen. Functionally, HOTAIR overexpression increases, whereas HOTAIR knockdown decreases, prostate cancer cell growth and invasion. Taken together, our results provide compelling evidence of lncRNAs as drivers of androgen-independent AR activity and CRPC progression, and they support the potential of lncRNAs as therapeutic targets.

FOXA1 acts upstream of GATA2 and AR in hormonal regulation of gene expression

Hormonal regulation of gene expression by androgen receptor (AR) is tightly controlled by many transcriptional cofactors, including pioneer factors FOXA1 and GATA2, which, however, exhibit distinct expression patterns and functional roles in prostate cancer. Here, we examined how FOXA1, GATA2 and AR crosstalk and regulate hormone-dependent gene expression in prostate cancer cells. Chromatin immunoprecipitation sequencing analysis revealed that FOXA1 reprograms both AR and GATA2 cistrome by preferably recruiting them to FKHD-containing genomic sites. By contrast, GATA2 is unable to shift AR or FOXA1 to GATA motifs. Rather, GATA2 co-occupancy enhances AR and FOXA1 binding to nearby ARE and FKHD sites, respectively. Similarly, AR increases, but not reprograms, GATA2 and FOXA1 cistromes. Concordantly, GATA2 and AR strongly enhance the transcriptional program of each other, whereas FOXA1 regulates GATA2- and AR-mediated gene expression in a context-dependent manner due to its reprogramming effects. Taken together, our data delineated for the first time the distinct mechanisms by which GATA2 and FOXA1 regulate AR cistrome and suggest that FOXA1 acts upstream of GATA2 and AR in determining hormone-dependent gene expression in prostate cancer.

Dynamic Recruitment of CDK5RAP2 to Centrosomes Requires Its Association with Dynein

CDK5RAP2 is a centrosomal protein known to be involved in the regulation of the γ-tubulin ring complex and thus the organization of microtubule arrays. However, the mechanism by which CDK5RAP2 is itself recruited to centrosomes is poorly understood. We report here that CDK5RAP2 displays highly dynamic attachment to centrosomes in a microtubule-dependent manner. CDK5RAP2 associates with the retrograde transporter dynein-dynactin and contains a sequence motif that binds to dynein light chain 8. Significantly, disruption of cellular dynein-dynactin function reduces the centrosomal level of CDK5RAP2. These results reveal a key role of the dynein-dynactin complex in the dynamic recruitment of CDK5RAP2 to centrosomes.

FOXA1 potentiates lineage-specific enhancer activation through modulating TET1 expression and function.

Forkhead box A1 (FOXA1) is an FKHD family protein that plays pioneering roles in lineage-specific enhancer activation and gene transcription. Through genome-wide location analyses, here we show that FOXA1 expression and occupancy are, in turn, required for the maintenance of these epigenetic signatures, namely DNA hypomethylation and histone 3 lysine 4 methylation. Mechanistically, this involves TET1, a 5-methylcytosine dioxygenase. We found that FOXA1 induces TET1 expression via direct binding to its cis-regulatory elements. Further, FOXA1 physically interacts with the TET1 protein through its CXXC domain. TET1 thus co-occupies FOXA1-dependent enhancers and mediates local DNA demethylation and concomitant histone 3 lysine 4 methylation, further potentiating FOXA1 recruitment. Consequently, FOXA1 binding events are markedly reduced following TET1 depletion. Together, our results suggest that FOXA1 is not only able to recognize but also remodel the epigenetic signatures at lineage-specific enhancers, which is mediated, at least in part, by a feed-forward regulatory loop between FOXA1 and TET1.

Polycomb-mediated disruption of an androgen receptor feedback loop drives castration-resistant prostate cancer

The lethal phenotype of castration-resistant prostate cancer (CRPC) is generally caused by augmented signaling from the androgen receptor (AR). Here, we report that the AR-repressed gene CCN3/NOV inhibits AR signaling and acts in a negative feedback loop to block AR function. Mechanistically, a cytoplasmic form of CCN3 interacted with the AR N-terminal domain to sequester AR in the cytoplasm of prostate cancer cells, thereby reducing AR transcriptional activity and inhibiting cell growth. However, constitutive repression of CCN3 by the Polycomb group protein EZH2 disrupted this negative feedback loop in both CRPC and enzalutamide-resistant prostate cancer cells. Notably, restoring CCN3 was sufficient to effectively reduce CPRC cell proliferation in vitro and to abolish xenograft tumor growth in vivo Taken together, our findings establish CCN3 as a pivotal regulator of AR signaling and prostate cancer progression and suggest a functional intersection between Polycomb and AR signaling in CRPC. Cancer Res; 77(2); 412-22. ©2016 AACR.

Microtubule plus-end tracking of end-binding protein 1 (EB1) is regulated by CDK5 regulatory subunit-associated protein 2

Microtubules are polar cytoskeleton filaments that extend via growth at their plus ends. Microtubule plus-end-tracking proteins (+TIPs) accumulate at these growing plus ends to control microtubule dynamics and attachment. The +TIP end-binding protein 1 (EB1) and its homologs possess an autonomous plus-end-tracking mechanism and interact with other known +TIPs, which then recruit those +TIPs to the growing plus ends. A major +TIP class contains the SXIP (Ser-X-Ile-Pro, with X denoting any amino acid residue) motif, known to interact with EB1 and its homologs for plus-end tracking, but the role of SXIP in regulating EB1 activities is unclear. We show here that an interaction of EB1 with the SXIP-containing +TIP CDK5 regulatory subunit-associated protein 2 (CDK5RAP2) regulates several EB1 activities, including microtubule plus-end tracking, dynamics at microtubule plus ends, microtubule and α/β-tubulin binding, and microtubule polymerization. The SXIP motif fused with a dimerization domain from CDK5RAP2 significantly enhanced EB1 plus-end-tracking and microtubule-polymerizing and bundling activities, but the SXIP motif alone failed to do so. An SXIP-binding-deficient EB1 mutant displayed significantly lower microtubule plus-end tracking than the wild-type protein in transfected cells. These results suggest that EB1 cooperates with CDK5RAP2 and perhaps other SXIP-containing +TIPs in tracking growing microtubule tips. We also generated plus-end-tracking chimeras of CDK5RAP2 and the adenomatous polyposis coli protein (APC) and found that overexpression of the dimerization domains interfered with microtubule plus-end tracking of their respective SXIP-containing chimeras. Our results suggest that disruption of SXIP dimerization enables detailed investigations of microtubule plus-end-associated functions of individual SXIP-containing +TIPs.

Abstract 2080: Polycomb-mediated disruption of an AR-feedback loop drives castration-resistant prostate cancer

Androgen receptor (AR) is an androgen-stimulated transcription factor that critically promotes prostate cancer development and progression. In addition to its conventional role in transcriptional activation, AR also acts as a transcriptional repressor to directly inhibit target gene expression. Through meta-analysis of androgen-regulated expression microarray data, we nominated CCN3/NOV, a CCN family protein, as one of the top AR-repressed genes in prostate cancer, implicating that CCN3 may play a tumor suppressive role in prostate cancer development. Therefore, we sought to investigate the molecular functions of CCN3 and its roles in prostate cancer progression especially in castration-resistant prostate cancer (CRPC). We first showed that CCN3 protein physically interacts with AR. The CCN3 interaction domain was mapped in the N-terminal domain of AR which is also present in constitutively active AR variants. Using immunofluorescence microscopy as well as subcellular fractionation technique, we demonstrated that CCN3 sequesters AR or AR variants into the cytoplasm to inhibit AR nuclear translocation, which in turn suppresses AR chromatin targeting and transcriptional activation. However, this negative feedback loop between AR and AR-repressed gene CCN3is disrupted in CRPC through CCN3 persistent epigenetic silencing by the Polycomb group protein EZH2, rendering AR transcriptional activation and CRPC progression. Taking advantage of this repressive mechanism, we restored CCN3 in CRPC cell model, either through ectopic overexpression or treatment with EZH2 inhibitor, which results in strong suppression of CRPC cell proliferation and anchorage-independent cell growth. Consistently, application of EZH2 inhibitor remarkably reduced tumor size in a castrated mice model. Taken together, our data highlight CCN3 as a novel regulator of AR signaling and support the promise of EZH2-targeting agents in treating CRPC. Citation Format: Ka-Wing Fong, Jonathan Zhao, Jung Kim, Shangze Li, Angela Yang, Bing Song, Laure Rittie, Ming Hu, Ximing Yang, Bernard Perbal, Jindan Yu. Polycomb-mediated disruption of an AR-feedback loop drives castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2080. doi:10.1158/1538-7445.AM2017-2080