Alastair H. Davies

Y-box binding protein-1 induces the expression of CD44 and CD49f leading to enhanced self-renewal, mammosphere growth, and drug resistance.

Y-box binding protein-1 (YB-1) is an oncogenic transcription/translation factor expressed in >40% of breast cancers, where it is associated with poor prognosis, disease recurrence, and drug resistance. We questioned whether this may be linked to the ability of YB-1 to induce the expression of genes linked to cancer stem cells such as CD44 and CD49f. Herein, we report that YB-1 binds the CD44 and CD49f promoters to transcriptionally upregulate their expressions. The introduction of wild-type (WT) YB-1 or activated P-YB-1(S102) stimulated the production of CD44 and CD49f in MDA-MB-231 and SUM 149 breast cancer cell lines. YB-1-transfected cells also bound to the CD44 ligand hyaluronan more than the control cells. Similarly, YB-1 was induced in immortalized breast epithelial cells and upregulated CD44. Conversely, silencing YB-1 decreased CD44 expression as well as reporter activity in SUM 149 cells. In mice, expression of YB-1 in the mammary gland induces CD44 and CD49f with associated hyperplasia. Further, activated mutant YB-1(S102D) enhances self-renewal, primary and secondary mammosphere growth, and soft-agar colony growth, which were reversible via loss of CD44 or CD49f. We next addressed the consequence of this system on therapeutic responsiveness. Here, we show that paclitaxel induces P-YB-1(S102) expression, nuclear localization of activated YB-1, and CD44 expression. The overexpression of WT YB-1 promotes mammosphere growth in the presence of paclitaxel. Importantly, targeting YB-1 sensitized the CD44(High)/CD24(Low) cells to paclitaxel. In conclusion, YB-1 promotes cancer cell growth and drug resistance through its induction of CD44 and CD49f.

Y-box binding protein-1 serine 102 is a downstream target of p90 ribosomal S6 kinase in basal-like breast cancer cells

IntroductionBasal-like breast cancers (BLBC) frequently overexpress the epidermal growth factor receptor (EGFR) and subsequently have high levels of signaling through the MAP kinase pathway, which is thought to contribute to their aggressive behavior. While we have previously reported the expression of Y-box binding protein-1 (YB-1) in 73% of BLBC, it is unclear whether it can be regulated by a component of the MAP kinase signaling pathway. Phosphorylation of YB-1 at the serine 102 residue is required for transcriptional activation of growth-enhancing genes, such as EGFR. Using Motifscan we identified p90 ribosomal S6 kinase (RSK) as a potential candidate for activating YB-1.MethodsInhibition of RSK1 and RSK2 was achieved using siRNA and the small molecule SL0101. RSK1, RSK2, activated RSK and kinase-dead RSK were expressed in HCC1937 cells. Kinase assays were performed to illustrate direct phosphorylation of YB-1 by RSK. The impact of inhibiting RSK on YB-1 function was measured by luciferase assays and chromatin immunoprecipitation.ResultsUsing an in vitro kinase assay, RSK1 and RSK2 were shown to directly phosphorylate YB-1. Interestingly, they were more effective activators of YB-1 than AKT or another novel YB-1 kinase, PKCα. Phosphorylation of YB-1 (serine 102 residue) is blocked by inhibition of the MAP kinase pathway or by perturbing RSK1/RSK2 with siRNA or SL0101. In immortalized breast epithelial cells where RSK is active yet AKT is not, YB-1 is phosphorylated. Supporting this observation, RSK2-/- mouse embryo fibroblasts lose the ability to phosphorylate YB-1 in response to epidermal growth factor. This subsequently interfered with the ability of YB-1 to regulate the expression of EGFR. The RSK inhibitor SL0101 decreased the ability of YB-1 to bind the promoter, transactivate and ultimately reduce EGFR expression. In concordance with these results the expression of constitutively active RSK1 increased YB-1 phosphorylation, yet the kinase-dead RSK did not.ConclusionsWe therefore conclude that RSK1/RSK2 are novel activators of YB-1, able to phosphorylate the serine 102 residue. This provides a newly described mechanism whereby YB-1 is activated in breast cancer. This implicates the EGFR/RSK/YB-1 pathway as an important component of BLBC, providing an important opportunity for therapeutic intervention.

Profiling YB-1 target genes uncovers a new mechanism for MET receptor regulation in normal and malignant human mammary cells

Basal-like breast cancers (BLBCs) are aggressive tumors with high relapse rates and poor survival. We recently reported that >70% of primary BLBCs express the oncogenic transcription/translation factor Y-box binding protein-1 (YB-1) and silencing it with small interfering RNAs (siRNAs) attenuates the growth of BLBC cell lines. To understand the basis of these earlier findings, we profiled YB-1:DNA complexes by chromatin immunoprecipitation (ChIP)-on-chip. Several tumor growth-promoting genes such as MET, CD44, CD49f, WNT and NOTCH family members were identified. In addition, YB-1 and MET are coordinately expressed in BLBC cell lines, as well as in normal human mammary progenitor cells. MET was confirmed to be a YB-1 target through traditional ChIP and gel-shift assays. More specifically, YB-1 binds to −1018 bp on the MET promoter. Silencing YB-1 with siRNA decreased MET promoter activity, transcripts, as well as protein levels and signaling. Conversely, expressing wild-type YB-1 or a constitutively active mutant YB-1 (D102) increased MET expression. Finally, silencing YB-1 or MET attenuated anchorage-independent growth of BLBC cell lines. Together, these findings implicate MET as a target of YB-1 that work in concert to promote BLBC growth.

The Master Neural Transcription Factor BRN2 is an Androgen Receptor Suppressed Driver of Neuroendocrine Differentiation in Prostate Cancer

Mechanisms controlling the emergence of lethal neuroendocrine prostate cancer (NEPC), especially those that are consequences of treatment-induced suppression of the androgen receptor (AR), remain elusive. Using a unique model of AR pathway inhibitor-resistant prostate cancer, we identified AR-dependent control of the neural transcription factor BRN2 (encoded by POU3F2) as a major driver of NEPC and aggressive tumor growth, both in vitro and in vivo Mechanistic studies showed that AR directly suppresses BRN2 transcription, which is required for NEPC, and BRN2-dependent regulation of the NEPC marker SOX2. Underscoring its inverse correlation with classic AR activity in clinical samples, BRN2 expression was highest in NEPC tumors and was significantly increased in castration-resistant prostate cancer compared with adenocarcinoma, especially in patients with low serum PSA. These data reveal a novel mechanism of AR-dependent control of NEPC and suggest that targeting BRN2 is a strategy to treat or prevent neuroendocrine differentiation in prostate tumors. SIGNIFICANCE Understanding the contribution of the AR to the emergence of highly lethal, drug-resistant NEPC is critical for better implementation of current standard-of-care therapies and novel drug design. Our first-in-field data underscore the consequences of potent AR inhibition in prostate tumors, revealing a novel mechanism of AR-dependent control of neuroendocrine differentiation, and uncover BRN2 as a potential therapeutic target to prevent emergence of NEPC. Cancer Discov; 7(1); 54-71. ©2016 AACR.This article is highlighted in the In This Issue feature, p. 1.

YB-1 evokes susceptibility to cancer through cytokinesis failure, mitotic dysfunction, and HER2 amplification

Y-box binding protein-1 (YB-1) expression in the mammary gland promotes breast carcinoma that demonstrates a high degree of genomic instability. In the present study, we developed a model of pre-malignancy to characterize the role of this gene during breast cancer initiation and early progression. Antibody microarray technology was used to ascertain global changes in signal transduction following the conditional expression of YB-1 in human mammary epithelial cells (HMEC). Cell cycle-associated proteins were frequently altered with the most dramatic being LIM kinase 1/2 (LIMK1/2). Consequently, the misexpression of LIMK1/2 was associated with cytokinesis failure that acted as a precursor to centrosome amplification. Detailed investigation revealed that YB-1 localized to the centrosome in a phosphorylation-dependent manner, where it complexed with pericentrin and γ-tubulin. This was found to be essential in maintaining the structural integrity and microtubule nucleation capacity of the organelle. Prolonged exposure to YB-1 led to rampant acceleration toward tumorigenesis, with the majority of cells acquiring numerical and structural chromosomal abnormalities. Slippage through the G1/S checkpoint due to overexpression of cyclin E promoted continued proliferation of these genomically compromised cells. As malignancy further progressed, we identified a subset of cells harboring HER2 amplification. Our results recognize YB-1 as a cancer susceptibility gene, with the capacity to prime cells for tumorigenesis.

Regulation of tumor cell plasticity by the androgen receptor in prostate cancer.

Prostate cancer (PCa) has become the most common form of cancer in men in the developed world, and it ranks second in cancer-related deaths. Men that succumb to PCa have a disease that is resistant to hormonal therapies that suppress androgen receptor (AR) signaling, which plays a central role in tumor development and progression. Although AR continues to be a clinically relevant therapeutic target in PCa, selection pressures imposed by androgen-deprivation therapies promote the emergence of heterogeneous cell populations within tumors that dictate the severity of disease. This cellular plasticity, which is induced by androgen deprivation, is the focus of this review. More specifically, we address the emergence of cancer stem-like cells, epithelial-mesenchymal or myeloid plasticity, and neuroendocrine transdifferentiation as well as evidence that demonstrates how each is regulated by the AR. Importantly, because all of these cell phenotypes are associated with aggressive PCa, we examine novel therapeutic approaches for targeting therapy-induced cellular plasticity as a way of preventing PCa progression.

YB-1 transforms human mammary epithelial cells through chromatin remodeling leading to the development of basal-like breast cancer

There is growing evidence that cancer‐initiation could result from epigenetic changes. Y‐box binding protein‐1 (YB‐1) is a transcription/translation factor that promotes the formation of tumors in transgenic mice; however, the underlying molecular events are not understood. To explore this in a human model system, YB‐1 was expressed in mammary epithelial cells under the control of a tetracycline‐inducible promoter. The induction of YB‐1 promoted phenotypes associated with malignancy in three‐dimensional breast acini cultures. This was attributed to YB‐1 enhancing the expression and activity of the histone acetyltransferase p300 leading to chromatin remodeling. Specifically, this relaxation of chromatin allowed YB‐1 to bind to the BMI1 promoter. The induction of BMI1 engaged the Polycomb complex resulting in histone H2A ubiquitylation and repression of the CDKN2A locus. These events manifested functionally as enhanced self‐renewal capacity that occurred in a BMI1‐dependent manner. Conversely, p300 inhibition with anacardic acid prevented YB‐1 from binding to the BMI1 promoter and thereby subverted self‐renewal. Despite these early changes, full malignant transformation was not achieved until RSK2 became overexpressed concomitant with elevated human telomerase reverse transcriptase (hTERT) activity. The YB‐1/RSK2/hTERT expressing cells formed tumors in mice that were molecularly subtyped as basal‐like breast cancer. We conclude that YB‐1 cooperates with p300 to allow BMI1 to over‐ride p16INK4a‐mediated cell cycle arrest enabling self‐renewal and the development of aggressive breast tumors. Stem Cells 2014;32:1437–1450

Cellular plasticity and the neuroendocrine phenotype in prostate cancer

The success of next-generation androgen receptor (AR) pathway inhibitors, such as abiraterone acetate and enzalutamide, in treating prostate cancer has been hampered by the emergence of drug resistance. This acquired drug resistance is driven, in part, by the ability of prostate cancer cells to change their phenotype to adopt AR-independent pathways for growth and survival. Around one-quarter of resistant prostate tumours comprise cells that have undergone cellular reprogramming to become AR-independent and to acquire a continuum of neuroendocrine characteristics. These highly aggressive and lethal tumours, termed neuroendocrine prostate cancer (NEPC), exhibit reactivation of developmental programmes that are associated with epithelial–mesenchymal plasticity and acquisition of stem-like cell properties. In the past few years, our understanding of the link between lineage plasticity and an emergent NEPC phenotype has considerably increased. This new knowledge can contribute to novel therapeutic modalities that are likely to improve the treatment and clinical management of aggressive prostate cancer.

The Androgen Receptor Bridges Stem Cell-Associated Signaling Nodes in Prostate Stem Cells

The therapeutic potential of stem cells relies on dissecting the complex signaling networks that are thought to regulate their pluripotency and self-renewal. Until recently, attention has focused almost exclusively on a small set of “core” transcription factors for maintaining the stem cell state. It is now clear that stem cell regulatory networks are far more complex. In this review, we examine the role of the androgen receptor (AR) in coordinating interactions between signaling nodes that govern the balance of cell fate decisions in prostate stem cells.

Targeting Prostate Cancer Subtype 1 by Forkhead Box M1 Pathway Inhibition

Purpose: Prostate cancer was recently classified to three clinically relevant subtypes (PCS) demarcated by unique pathway activation and clinical aggressiveness. In this preclinical study, we investigated molecular targets and therapeutics for PCS1, the most aggressive and lethal subtype, with no treatment options available in the clinic. Experimental Design: We utilized the PCS1 gene set and our model of enzalutamide (ENZR) castration-resistant prostate cancer (CRPC) to identify targetable pathways and inhibitors for PCS1. The findings were evaluated in vitro and in the ENZR CRPC xenograft model in vivo. Results: The results revealed that ENZR CRPC cells are enriched with PCS1 signature and that Forkhead box M1 (FOXM1) pathway is the central driver of this subtype. Notably, we identified Monensin as a novel FOXM1-binding agent that selectively targets FOXM1 to reverse the PCS1 signature and its associated stem-like features and reduces the growth of ENZR CRPC cells and xenograft tumors. Conclusions: Our preclinical data indicate FOXM1 pathway as a master regulator of PCS1 tumors, namely in ENZR CRPC, and targeting FOXM1 reduces cell growth and stemness in ENZR CRPC in vitro and in vivo. These preclinical results may guide clinical evaluation of targeting FOXM1 to eradicate highly aggressive and lethal PCS1 prostate cancer tumors. Clin Cancer Res; 23(22); 6923–33. ©2017 AACR.