PELP1: A novel therapeutic target for hormonal cancers

Targeting PELP1 oncogenic signaling in TNBC with the small molecule inhibitor SMIP34

PURPOSE

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer. Oncogenic PELP1 is frequently overexpressed in TNBC, and it has been demonstrated that PELP1 signaling is essential for TNBC progression. The therapeutic utility of targeting PELP1 in TNBC, however, remains unknown. In this study, we investigated the effectiveness of SMIP34, a recently developed PELP1 inhibitor for the treatment of TNBC.

METHODS

To ascertain the impact of SMIP34 treatment, we used seven different TNBC models for testing cell viability, colony formation, invasion, apoptosis, and cell cycle analysis. Western blotting and RT-qPCR were used to determine the mechanistic insights of SMIP34 action. Using xenograft and PDX tumors, the ability of SMIP34 in suppressing proliferation was examined both ex vivo and in vivo.

RESULTS

TNBC cells' viability, colony formation, and invasiveness were all decreased by SMIP34 in in vitro cell-based assays, while apoptosis was increased. SMIP34 treatment promoted the degradation of PELP1 through the proteasome pathway. RT-qPCR analyses confirmed that SMIP34 treatment downregulated PELP1 target genes. Further, SMIP34 treatment substantially downregulated PELP1 mediated extranuclear signaling including ERK, mTOR, S6 and 4EBP1. Mechanistic studies confirmed downregulation of PELP1 mediated ribosomal biogenesis functions including downregulation of cMyc and Rix complex proteins LAS1L, TEX-10, and SENP3. The proliferation of TNBC tumor tissues was decreased in explant experiments by SMIP34. Additionally, SMIP34 treatment markedly decreased tumor progression in both TNBC xenograft and PDX models.

CONCLUSIONS

Together, these findings from in vitro, ex vivo, and in vivo models show that SMIP34 may be a useful therapeutic agent for inhibiting PELP1 signaling in TNBC.

A First-in-Class Inhibitor of ER Coregulator PELP1 Targets ER+ Breast Cancer

Most patients with estrogen receptor alpha-positive (ER+) breast cancers initially respond to treatment but eventually develop therapy resistance with disease progression. Overexpression of oncogenic ER coregulators, including proline, glutamic acid, and leucine-rich protein 1 (PELP1), are implicated in breast cancer progression. The lack of small molecules that inhibits PELP1 represents a major knowledge gap. Here, using a yeast-two-hybrid screen, we identified novel peptide inhibitors of PELP1 (PIP). Biochemical assays demonstrated that one of these peptides, PIP1, directly interacted with PELP1 to block PELP1 oncogenic functions. Computational modeling of PIP1 revealed key residues contributing to its activity and facilitated the development of a small-molecule inhibitor of PELP1, SMIP34, and further analyses confirmed that SMIP34 directly bound to PELP1. In breast cancer cells, SMIP34 reduced cell growth in a dose-dependent manner. SMIP34 inhibited proliferation of not only wild-type (WT) but also mutant (MT) ER+ and therapy-resistant breast cancer cells, in part by inducing PELP1 degradation via the proteasome pathway. RNA sequencing analyses showed that SMIP34 treatment altered the expression of genes associated with estrogen response, cell cycle, and apoptosis pathways. In cell line-derived and patient-derived xenografts of both WT and MT ER+ breast cancer models, SMIP34 reduced proliferation and significantly suppressed tumor progression. Collectively, these results demonstrate SMIP34 as a first-in-class inhibitor of oncogenic PELP1 signaling in advanced breast cancer.

SIGNIFICANCE

Development of a novel inhibitor of oncogenic PELP1 provides potential therapeutic avenues for treating therapy-resistant, advanced ER+ breast cancer.

SETDB1 interactions with PELP1 contributes to breast cancer endocrine therapy resistance

Background

Methyltransferase SETDB1 is highly expressed in breast cancer (BC), however, the mechanisms by which SETDB1 promotes BC progression to endocrine therapy resistance remains elusive. In this study, we examined the mechanisms by which SETDB1 contribute to BC endocrine therapy resistance.

Methods

We utilized therapy sensitive (MCF7 and ZR75), therapy resistant (MCF7-TamR, MCF7-FR, MCF7-PELP1cyto, MCF7-SETDB1) estrogen receptor alpha positive (ER⁺)BC models and conducted in vitro cell viability, colony formation, 3-dimensional cell growth assays to investigate the role of SETDB1 in endocrine resistance. RNA-seq of parental and SETDB1 knock down ER⁺ BC cells was used to identify unique pathways. SETDB1 interaction with PELP1 was identified by yeast-two hybrid screen and confirmed by immunoprecipitation and GST-pull down assays. Mechanistic studies were conducted using Western blotting, reporter gene assays, RT-qPCR, and in vitro methylation assays. Xenograft assays were used to establish the role of PELP1 in SETDB1 mediated BC progression.

Results

RNA-seq analyses showed that SETDB1 regulates expression of a subset of estrogen receptor (ER) and Akt target genes that contribute to endocrine therapy resistance. Importantly, using yeast-two hybrid screen, we identified ER coregulator PELP1 as a novel interacting protein of SETDB1. Biochemical analyses confirmed SETDB1 and PELP1 interactions in multiple BC cells. Mechanistic studies confirmed that PELP1 is necessary for SETDB1 mediated Akt methylation and phosphorylation. Further, SETDB1 overexpression promotes tamoxifen resistance in BC cells, and PELP1 knockdown abolished these effects. Using xenograft model, we provided genetic evidence that PELP1 is essential for SETDB1 mediated BC progression in vivo. Analyses of TCGA datasets revealed SETDB1 expression is positively correlated with PELP1 expression in ER⁺ BC patients.

Conclusions

This study suggests that the PELP1/SETDB1 axis play an important role in aberrant Akt activation and serves as a novel target for treating endocrine therapy resistance in breast cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-022-01520-4.

Inflammation-induced PELP1 expression promotes tumorigenesis by activating GM-CSF paracrine secretion in the tumor microenvironment

The inflammatory tumor microenvironment has been implicated as a major player fueling tumor progression and an enabling characteristic of cancer, proline, glutamic acid, and leucine-rich protein 1 (PELP1) is a novel nuclear receptor coregulator that signals across diverse signaling networks, and its expression is altered in several cancers. However, investigations to find the role of PELP1 in inflammation-driven oncogenesis are limited. Molecular studies here, utilizing macrophage cell lines and animal models upon stimulation with lipopolysaccharide (LPS) or necrotic cells, showed that PELP1 is an inflammation-inducible gene. Studies on the PELP1 promoter and its mutant identified potential binding of c-Rel, an NF-κB transcription factor subunit, to PELP1 promoter upon LPS stimulation in macrophages. Recruitment of c-Rel onto the PELP1 promoter was validated by chromatin immunoprecipitation, further confirming LPS mediated PELP1 expression through c-Rel-specific transcriptional regulation. Macrophages that overexpress PELP1 induces granulocyte-macrophage colony-stimulating factor secretion, which mediates cancer progression in a paracrine manner. Results from preclinical studies with normal-inflammatory-tumor progression models demonstrated a progressive increase in the PELP1 expression, supporting this link between inflammation and cancer. In addition, animal studies demonstrated the connection of PELP1 in inflammation-directed cancer progression. Taken together, our findings provide the first report on c-Rel-specific transcriptional regulation of PELP1 in inflammation and possible granulocyte-macrophage colony-stimulating factor-mediated transformation potential of activated macrophages on epithelial cells in the inflammatory tumor microenvironment, reiterating the link between PELP1 and inflammation-induced oncogenesis. Understanding the regulatory mechanisms of PELP1 may help in designing better therapeutics to cure various inflammation-associated malignancies.

Cell-cell adhesion regulates Merlin/NF2 interaction with the PAF complex

The PAF complex (PAFC) coordinates transcription elongation and mRNA processing and its CDC73/parafibromin subunit functions as a tumour suppressor. The NF2/Merlin tumour suppressor functions both at the cell cortex and nucleus and is a key mediator of contact inhibition but the molecular mechanisms remain unclear. In this study we have used affinity proteomics to identify novel Merlin interacting proteins and show that Merlin forms a complex with multiple proteins involved in RNA processing including the PAFC and the CHD1 chromatin remodeller. Tumour-derived inactivating mutations in both Merlin and the CDC73 PAFC subunit mutually disrupt their interaction and growth suppression by Merlin requires CDC73. Merlin interacts with the PAFC in a cell density-dependent manner and we identify a role for FAT cadherins in regulating the Merlin-PAFC interaction. Our results suggest that in addition to its function within the Hippo pathway, Merlin is part of a tumour suppressor network regulated by cell-cell adhesion which coordinates post-initiation steps of the transcription cycle of genes mediating contact inhibition.

Interaction of transcription factor AP-2 gamma with proto-oncogene PELP1 promotes tumorigenesis by enhancing RET signaling

A significant proportion of estrogen receptor-positive (ER+) breast cancer (BC) initially responds to endocrine therapy but eventually evolves into therapy-resistant BC. Transcription factor AP-2 gamma (TFAP2C) is a known regulator of ER activity, and high expression of TFAP2C is associated with a decreased response to endocrine therapies. PELP1 is a nuclear receptor coregulator, commonly overexpressed in BC, and its levels are correlated with poorer survival. In this study, we identified PELP1 as a novel interacting protein of TFAP2C. RNA-seq analysis of PELP1 knockdown BC cells followed by transcription factor motif prediction pointed to TFAP2C being enriched in PELP1-regulated genes. Gene set enrichment analysis (GSEA) revealed that the TFAP2C-PELP1 axis induced a subset of common genes. Reporter gene assays confirmed PELP1 functions as a coactivator of TFAP2C. Mechanistic studies showed that PELP1-mediated changes in histone methylation contributed to increased expression of the TFAP2C target gene RET. Furthermore, the TFAP2C-PELP1 axis promoted the activation of the RET signaling pathway, which contributed to downstream activation of AKT and ERK pathways in ER+ BC cells. Concomitantly, knockdown of PELP1 attenuated these effects mediated by TFAP2C. Overexpression of TFAP2C contributed to increased cell proliferation and therapy resistance in ER+ BC models, while knockdown of PELP1 mitigated these effects. Utilizing ZR75-TFAP2C xenografts with or without PELP1 knockdown, we provided genetic evidence that endogenous PELP1 is essential for TFAP2C-driven BC progression in vivo. Collectively, our studies demonstrated that PELP1 plays a critical role in TFAP2C transcriptional and tumorigenic functions in BC and blocking the PELP1-TFAP2C axis could have utility for treating therapy resistance.

PELP-1 regulates adverse responses to endocrine therapy in Estrogen Receptor (ER) positive breast cancer

Introduction: Endocrine therapy has played an important role in the management of ER positive breast cancer over recent decades. Despite this, not all patients respond equally to endocrine intervention, which can lead to resistance, associated disease relapse and progression. Previous reports suggest that endocrine agents themselves may induce an invasive phenotype in ER positive breast cancers with low/aberrant expression of E-cadherin. Here we investigate this phenomenon further and provide data supporting a role for the ER co-receptor, PELP-1, in mediating an adverse response to endocrine agents.

Materials and Methods: The effects of tamoxifen, fulvestrant and estrogen withdrawal (as a model for aromatase inhibitor therapy) on the invasive and migratory capacity of endocrine-sensitive MCF-7 and T47D cells, in the presence or absence of functional E-cadherin and/or PELP-1 (using siRNA knockdown), was assessed via Matrigel invasion and Boyden chamber migration assays. The effects of these endocrine therapies alongside E-cadherin/PELP-1 modulation on cell proliferation were further assessed by MTT assay. Western blotting using phospho-specific antibodies was performed to investigate signalling pathway changes associated with endocrine-induced changes in invasion and migration.

Results: Both tamoxifen and fulvestrant induced a pro-invasive and pro-migratory phenotype in ER positive breast cancer cells displaying a high basal expression of PELP-1, which was augmented in the context of poor cell-cell contact. This process occurred in a Src-dependent manner with Src inhibition reversing endocrine induced invasion/migration. While this adverse response was observed using both tamoxifen and fulvestrant therapy, it was not observed under conditions of estrogen withdrawal.

Conclusions: Our data confirms previous reports that anti-estrogens induce an adverse cell phenotype in ER+ breast cancer, particularly in the absence of homotypic cell contact. These results implicate E-cadherin and PELP-1 as potential biomarkers when deciding upon optimum adjuvant endocrine therapy, whereby tumours with high PELP-1/low E-cadherin expression may benefit from estrogen withdrawal therapy via aromatase inhibition, as opposed to ER modulation/antagonism.

PELP1 promotes glioblastoma progression by enhancing Wnt/β-catenin signaling

Background

Glioblastoma (GBM) is a deadly neoplasm of the central nervous system. The molecular mechanisms and players that contribute to GBM development is incompletely understood.

Methods

The expression of PELP1 in different grades of glioma and normal brain tissues was analyzed using immunohistochemistry on a tumor tissue array. PELP1 expression in established and primary GBM cell lines was analyzed by Western blotting. The effect of PELP1 knockdown was studied using cell proliferation, colony formation, migration, and invasion assays. Mechanistic studies were conducted using RNA-seq, RT-qPCR, immunoprecipitation, reporter gene assays, and signaling analysis. Mouse orthotopic models were used for preclinical evaluation of PELP1 knock down.

Results

Nuclear receptor coregulator PELP1 is highly expressed in gliomas compared to normal brain tissues, with the highest expression in GBM. PELP1 expression was elevated in established and patient-derived GBM cell lines compared to normal astrocytes. Knockdown of PELP1 resulted in a significant decrease in cell viability, survival, migration, and invasion. Global RNA-sequencing studies demonstrated that PELP1 knockdown significantly reduced the expression of genes involved in the Wnt/β-catenin pathway. Mechanistic studies demonstrated that PELP1 interacts with and functions as a coactivator of β-catenin. Knockdown of PELP1 resulted in a significant increase in survival of mice implanted with U87 and GBM PDX models.

Conclusions

PELP1 expression is upregulated in GBM and PELP1 signaling via β-catenin axis contributes to GBM progression. Thus, PELP1 could be a potential target for the development of therapeutic intervention in GBM.

The Clinical Value of PELP1 for Breast Cancer: A Comparison with Multiple Cancers and Analysis in Breast Cancer Subtypes

Purpose

Proline, glutamic acid, and leucine-rich protein 1 (PELP1), a novel nuclear receptor (NR) co-regulator, is highly expressed in breast cancer. We investigated its expression in breast cancer subtypes, in comparison with other breast markers as well as cancers from different sites. Its prognostic relevance with different subtypes and other NR expression was also examined in breast cancers.

Methods

Immunohistochemical analysis was performed on totally 1,944 cancers from six different organs.

Results

PELP1 expression rate was the highest in breast cancers (70.5%) among different cancers. Compared to GATA3, mammaglobin and gross cystic disease fluid protein 15, PELP1 was less sensitive than GATA3 for luminal cancers, but was the most sensitive for non-luminal cancers. PELP1 has low expression rate (<20%) in colorectal cancers, gastric cancers and renal cell carcinomas, but higher in lung cancers (49.1%) and ovarian cancers (42.3%). In breast cancer, PELP1 expression was an independent adverse prognostic factor for non-luminal cancers (disease-free survival [DFS]: hazard ratio [HR], 1.403; p=0.012 and breast cancer specific survival [BCSS]: HR, 1.443; p=0.015). Interestingly, its expression affected the prognostication of androgen receptor (AR). AR^posPELP1^lo luminal cancer showed the best DFS (log-rank=8.563, p=0.036) while AR^negPELP1^hi non-luminal cancers showed the worst DFS (log-rank=9.536, p=0.023).

Conclusion

PELP1 is a sensitive marker for breast cancer, particularly non-luminal cases. However, its considerable expression in lung and ovarian cancers may limit its utility in differential diagnosis in some scenarios. PELP1 expression was associated with poor outcome in non-luminal cancers and modified the prognostic effects of AR, suggesting the potential significance of NR co-regulator in prognostication.

STAT3 Interactors as Potential Therapeutic Targets for Cancer Treatment

Signal transducers and activators of transcription (STATs) mediate essential signaling pathways in different biological processes, including immune responses, hematopoiesis, and neurogenesis. Among the STAT members, STAT3 plays crucial roles in cell proliferation, survival, and differentiation. While STAT3 activation is transient in physiological conditions, STAT3 becomes persistently activated in a high percentage of solid and hematopoietic malignancies (e.g., melanoma, multiple myeloma, breast, prostate, ovarian, and colon cancers), thus contributing to malignant transformation and progression. This makes STAT3 an attractive therapeutic target for cancers. Initial strategies aimed at inhibiting STAT3 functions have focused on blocking the action of its activating kinases or sequestering its DNA binding ability. More recently, the diffusion of proteomic-based techniques, which have allowed for the identification and characterization of novel STAT3-interacting proteins able to modulate STAT3 activity via its subcellular localization, interact with upstream kinases, and recruit transcriptional machinery, has raised the possibility to target such cofactors to specifically restrain STAT3 oncogenic functions. In this article, we summarize the available data about the function of STAT3 interactors in malignant cells and discuss their role as potential therapeutic targets for cancer treatment.

PELP1: a key mediator of oestrogen signalling and actions in the brain

Proline-, glutamic acid- and leucine-rich protein 1 (PELP1) is an oestrogen receptor (ER) coregulator protein identified by our collaborative group. Work from our laboratory and others has shown that PELP1 is a scaffold protein that interacts with ERs and kinase signalling factors, as well as proteins involved in chromatin remodelling and DNA repair. Its role in mediating 17β-oestradiol (E₂ ) signalling and actions has been studied in detail in cancer cells, although only recently has attention turned to its role in the brain. In this review, we discuss the tissue, cellular and subcellular localisation of PELP1 in the brain. We also discuss recent evidence from PELP1 forebrain-specific knockout mice demonstrating a critical role of PELP1 in mediating both extranuclear and nuclear ER signalling in the brain, as well as E₂ -induced neuroprotection, anti-inflammatory effects and regulation of cognitive function. Finally, the PELP1 interactome and unique gene network regulated by PELP1 in the brain is discussed, especially because it provides new insights into PELP1 biology, protein interactions and mechanisms of action in the brain. As a whole, the findings discussed in the present review indicate that PELP1 functions as a critical ER coregulator in the brain to mediate E₂ signalling and actions.

Role of Scaffold Protein Proline-, Glutamic Acid-, and Leucine-Rich Protein 1 (PELP1) in the Modulation of Adrenocortical Cancer Cell Growth

PELP1 acts as an estrogen receptor (ER) coactivator that exerts an essential role in the ER's functions. ER coregulators have a critical role in the progression and response to hormonal treatment of estrogen-dependent tumors. We previously demonstrated that, in adrenocortical carcinoma (ACC), ERα is upregulated and that estradiol activates the IGF-II/IGF1R signaling pathways defining the role of this functional cross-talk in H295R ACC cell proliferation. The aim of this study was to determine if PELP1 is expressed in ACC and may play a role in promoting the interaction between ERα and IGF1R allowing the activation of pathways important for ACC cell growth. The expression of PELP1 was detected by Western blot analysis in ACC tissues and in H295R cells. H295R cell proliferation decrease was assessed by A3-(4,5-Dimethylthiaoly)-2,5-diphenyltetrazolium bromide (MTT) assay and [3H] thymidine incorporation. PELP1 is expressed in ACC tissues and in H295R cells. Moreover, treatment of H295R with E2 or IGF-II induced a multiprotein complex formation consisting of PELP1, IGF1R, ERα, and Src that is involved in ERK1/2 rapid activation. PELP1/ER/IGF1R/c-Src complex identification as part of E2- and IGF-II-dependent signaling in ACC suggests PELP1 is a novel and more efficient potential target to reduce ACC growth.

Src Is a Potential Therapeutic Target in Endocrine-Resistant Breast Cancer Exhibiting Low Estrogen Receptor-Mediated Transactivation

Despite the effectiveness of endocrine therapies in estrogen receptor positive (ER+) breast cancer, approximately 40% of patients relapse. Previously, we identified the Focal-adhesion kinase canonical pathway as a major contributor of resistance to estrogen deprivation and cellular-sarcoma kinase (c-src) as a dominant gene in this pathway. Dasatinib, a pan-src inhibitor, has recently been used in clinical trials to treat ER+ patients but has shown mixed success. In the following study, using isogenic cell line models, we provide a potential explanation for these findings and suggest a sub-group that may benefit. A panel of isogenic cell lines modelling resistance to aromatase inhibitors (LTED) and tamoxifen (TAMR) were assessed for response to dasatinib ± endocrine therapy. Dasatinib caused a dose-dependent decrease in proliferation in MCF7-TAMR cells and resensitized them to tamoxifen and fulvestrant but not in HCC1428-TAMR. In contrast, in estrogen-deprived conditions, dasatinib increased the proliferation rate of parental-MCF7 cells and had no effect on MCF7-LTED or HCC1428-LTED. Treatment with dasatinib caused a decrease in src-phosphorylation and inhibition of downstream pathways, including AKT and ERK1/2 in all cell lines tested, but only the MCF7-TAMR showed a concomitant decrease in markers of cell cycle progression. Inhibition of src also caused a significant decrease in cell migration in both MCF7-LTED and MCF7-TAMR cells. Finally, we showed that, in MCF7-TAMR cells, in contrast to tamoxifen sensitive cell lines, ER is expressed throughout the cell rather than being restricted to the nucleus and that treatment with dasatinib resulted in nuclear shuttling of ER, which was associated with an increase in ER-mediated transcription. These data suggest that src has differential effects in endocrine-resistant cell lines, particularly in tamoxifen resistant models, with low ER genomic activity, providing further evidence of the importance of patient selection for clinical trials testing dasatinib utility in ER+ breast cancer.

Hormone-sensing mammary epithelial progenitors: emerging identity and hormonal regulation

The hormone-sensing mammary epithelial cell (HS-MEC-expressing oestrogen receptor-alpha (ERα) and progesterone receptor (PGR)) is often represented as being terminally differentiated and lacking significant progenitor activity after puberty. Therefore while able to profoundly influence the proliferation and function of other MEC populations, HS-MECs are purported not to respond to sex hormone signals by engaging in significant cell proliferation during adulthood. This is a convenient and practical simplification that overshadows the sublime, and potentially critical, phenotypic plasticity found within the adult HS-MEC population. This concept is exemplified by the large proportion (~80 %) of human breast cancers expressing PGR and/or ERα, demonstrating that HS-MECs clearly proliferate in the context of breast cancer. Understanding how HS-MEC proliferation and differentiation is driven could be key to unraveling the mechanisms behind uncontrolled HS-MEC proliferation associated with ERα- and/or PGR-positive breast cancers. Herein we review evidence for the existence of a HS-MEC progenitor and the emerging plasticity of the HS-MEC population in general. This is followed by an analysis of hormones other than oestrogen and progesterone that are able to influence HS-MEC proliferation and differentiation: androgens, prolactin and transforming growth factor-beta1.

Increased expression of proline-, glutamic acid- and leucine-rich protein PELP1 in non-small cell lung cancer

It has been demonstrated that estrogens are able to enhance lung tumorigenesis by estrogen receptor (ER) pathway. ER signaling is a highly complex process that requires a number of different coactivators, including proline-, glutamic acid- and leucine-rich protein-1 (PELP1). We studied PELP1 transcript and protein levels in cancerous and histopathologically unchanged lung tissues obtained from 73 patients diagnosed with non-small cell lung cancer (NSCLC). We observed increased levels of PELP1 transcript (P=0.00001) and protein (P=0.00001) in tumor tissues compared to adjacent histopathologically unchanged tissues. Significant increase of PELP1 transcript/protein level was found in all patients, regardless of gender (males: P=0.0003/P=0.000003; females: P=0.0005/P=0.02), age (≤ 60 patients: P=0.042/P=0.016; >60 patients: P=0.00001/P=0.00001) or histopathological type of tumor (adenocarcinoma [ADC]: P=0.004/P=0.0006; squamous cell carcinoma [SSC]: P=0.0009/P=0.0008). Increased PELP1 transcript/protein levels were also correlated with some lung cancer stage (1a: P=0.07/P=0.02; 1b: P=0.001/P=0.03; 2a: P=0.012/P=0.001), tumor size (T2a: P=0.0006/P=0.001) and lymph node metastasis (N0: P=0.0003/P=0.0006; N1: P=0.017/P=0.003). Moreover, significant increase in PELP1 transcript level in cancer stage 1a (P=0.02) was observed. PELP1 protein content was higher in tumor tissues of patients with cancer stage 3a (P=0.04) and in T1a tumor size (P=0.03). Our studies demonstrate significantly higher amounts of PELP1 transcript and protein in tumor tissues in patients with NSCLC. Moreover, we also determined the association of PELP1 transcript and protein level with some clinicopathological features of NSCLC.

PELP1 overexpression in the mouse mammary gland results in the development of hyperplasia and carcinoma

Estrogen receptor (ER) coregulator overexpression promotes carcinogenesis and/or progression of endocrine related-cancers in which steroid hormones are powerful mitogenic agents. Recent studies in our laboratory, as well as others, demonstrated that the estrogen receptor coregulator PELP1 is a proto-oncogene. PELP1 interactions with histone demethylase KDM1 play a critical role in its oncogenic functions and PELP1 is a prognostic indicator of decreased survival in patients with breast cancer. However, the in vivo significance of PELP1 deregulation during initiation and progression of breast cancer remains unknown. We generated an inducible, mammary gland-specific PELP1-expressing transgenic (Tg) mouse (MMTVrtTA-TetOPELP1). We found more proliferation, extensive side branching, and precocious differentiation in PELP1-overexpressing mammary glands than in control glands. Aged MMTVrtTA-TetOPELP1 Tg mice had hyperplasia and preneoplastic changes as early as 12 weeks, and ER-positive mammary tumors occurred at a latency of 14 to 16 months. Mechanistic studies revealed that PELP1 deregulation altered expression of a number of known ER target genes involved in cellular proliferation (cyclin D1, CDKs) and morphogenesis (EGFR, MMPs) and such changes facilitated altered mammary gland morphogenesis and tumor progression. Furthermore, PELP1 was hyper-phosphorylated at its CDK phosphorylation site, suggesting an autocrine loop involving the CDK-cyclin D1-PELP1 axis in promoting mammary tumorigenesis. Treatment of PELP1 Tg mice with a KDM1 inhibitor significantly reduced PELP1-driven hyperbranching, reversed alterations in cyclin D1 expression levels, and reduced CDK-driven PELP1 phosphorylation. These results further support the hypothesis that PELP1 deregulation has the potential to promote breast tumorigenesis in vivo and represent a novel model for future investigation into molecular mechanisms of PELP1-mediated tumorigenesis.

The social network of PELP1 and its implications in breast and prostate cancers

Proline, glutamic acid- and leucine-rich protein 1 (PELP1) is a multi-domain scaffold protein that serves as a platform for various protein-protein interactions between steroid receptors (SRs) and signaling factors and cell cycle, transcriptional, cytoskeletal, and epigenetic remodelers. PELP1 is known to be a coregulator of transcription and participates in the nuclear and extranuclear functions of SRs, ribosome biogenesis, and cell cycle progression. The expression and localization of PELP1 are dysregulated in hormonal cancers including breast and prostate cancers. This review focuses on the interactive functions and therapeutic and prognostic significance of PELP1 in breast and prostate cancers.

Inhibition of mTOR signaling reduces PELP1-mediated tumor growth and therapy resistance

Proline, Glutamic acid-, and Leucine-rich Protein 1 (PELP1) is a proto-oncogene that modulates estrogen receptor (ER) signaling. PELP1 expression is upregulated in breast cancer, contributes to therapy resistance, and is a prognostic marker of poor survival. In a subset of breast tumors, PELP1 is predominantly localized in the cytoplasm and PELP1 participates in extranuclear signaling by facilitating ER interactions with Src and phosphoinositide 3-kinase (PI3K). However, the mechanism by which PELP1 extranuclear actions contributes to cancer progression and therapy resistance remains unclear. In this study, we discovered that PELP1 cross-talked with the serine/threonine protein kinase mTOR and modulated mTOR signaling. PELP1 knockdown significantly reduced the activation of mTOR downstream signaling components. Conversely, PELP1 overexpression excessively activated mTOR signaling components. We detected the presence of the mTOR signaling complex proteins in PELP1 immunoprecipitates. mTOR-targeting drugs (rapamycin and AZD8055) significantly reduced proliferation of PELP1-overexpressed breast cancer cells in both in vitro and in vivo xenograft tumor models. MCF7 cells that uniquely retain PELP1 in the cytoplasm showed resistance to hormonal therapy and mTOR inhibitors sensitized PELP1cyto cells to hormonal therapy in xenograft assays. Notably, immunohistochemical studies using xenograft tumors derived from PELP1 overexpression model cells showed increased mTOR signaling and inhibition of mTOR rendered PELP1-driven tumors to be highly sensitive to therapeutic inhibition. Collectively, our data identified the PELP1-mTOR axis as a novel component of PELP1 oncogenic functions and suggest that mTOR inhibitor(s) will be effective chemotherapeutic agents for downregulating PELP1 oncogenic functions.

Progesterone receptor-B enhances estrogen responsiveness of breast cancer cells via scaffolding PELP1- and estrogen receptor-containing transcription complexes

Progesterone and estrogen are important drivers of breast cancer proliferation. Herein, we probed ER-alpha and PR cross-talk in breast cancer models. Stable expression of PR-B in PR-low/ER+ MCF7 cells increased cellular sensitivity to estradiol and IGF1, as measured in growth assays performed in the absence of exogenous progestin; similar results were obtained in PR-null/ER+ T47D cells stably expressing PR-B. Genome-wide microarray analyses revealed that unliganded PR-B induced robust expression of a subset of estradiol-responsive ER-target genes, including CathepsinD (CTSD). Estradiol-treated MCF7 cells stably expressing PR-B exhibited enhanced ER Ser167 phosphorylation and recruitment of ER, PR, and the proline, glutamate and leucine rich protein 1 (PELP1) to an estrogen response element (ERE) in the CTSD distal promoter; this complex co-immunoprecipitated with IGF1R in whole cell lysates. Importantly, ER/PR/PELP1 complexes were also detected in human breast cancer samples. Inhibition of IGF1R or PI3K blocked PR-B-dependent CTSD mRNA upregulation in response to estradiol. Similarly, inhibition of IGF1R or PR significantly reduced ER recruitment to the CTSD promoter. Stable knockdown of endogenous PR or onapristone treatment of multiple unmodified breast cancer cell lines blocked estradiol-mediated CTSD induction, inhibited growth in soft agar, and partially restored tamoxifen-sensitivity of resistant cells. Further, combination treatment of breast cancer cells with both onapristone and IGF1R tyrosine kinase inhibitor AEW541 was more effective than either agent alone. In summary, unliganded PR-B enhanced proliferative responses to estradiol and IGF1 via scaffolding of ERalpha/PELP1/IGF1R-containing complexes. Our data provide a strong rationale for targeting PR in combination with ER and IGF1R in patients with luminal breast cancer.

Peptidomimetic targeting of critical androgen receptor-coregulator interactions in prostate cancer

The growth of advanced prostate cancer depends on androgen receptor signalling, however treatment options are limited. Here we report the disruption of specific protein-protein interactions involving LXXLL motifs in androgen receptor-coregulator proteins such as PELP1 using a novel, small molecule peptidomimetic (D2). D2 is stable, non-toxic and efficiently taken up by prostate cancer cells. Importantly, D2 blocks androgen-induced nuclear uptake and genomic activity of the androgen receptor. Furthermore, D2 abrogates androgen-induced proliferation of prostate cancer cells in vitro with an IC50 of 40 nM, and inhibits tumour growth in a mouse xenograft model. D2 also disrupts androgen receptor-coregulator interactions in ex vivo cultures of primary human prostate tumours. These findings provide evidence that targeting androgen receptor-coregulator interactions using peptidomimetics may be a viable therapeutic approach for patients with advanced prostate cancer.

Significance of PELP1/HDAC2/miR-200 regulatory network in EMT and metastasis of breast cancer

Tumor metastasis is the leading cause of death among breast cancer patients. PELP1 is a nuclear receptor coregulator that is upregulated during breast cancer progression to metastasis and is an independent prognostic predictor of shorter survival of breast cancer patients. Here, we show that PELP1 modulates expression of metastasis-influencing microRNAs (miRs) to promote cancer metastasis. Whole genome miR array analysis using PELP1 over expressing and under expressing model cells revealed that miR-200a and miR-141 levels inversely correlated with PELP1 expression. Consistent with this, PELP1 knockdown resulted in lower expression of miR-200a target genes ZEB1 and ZEB2. PELP1 knockdown significantly reduced tumor growth and metastasis compared with parental cells in an orthotopic xenograft tumor model. Furthermore, re-introduction of miR-200a and miR-141 mimetics into PELP1 overexpressing cells reversed PELP1 target gene expression, decreased PELP1 driven migration/invasion in vitro, and significantly reduced in vivo metastatic potential in a preclinical model of experimental metastasis. Our results demonstrated that PELP1 binds to miR-200a and miR-141 promoters and regulates their expression by recruiting chromatin modifier HDAC2 as revealed by ChIP, siRNA and HDAC inhibitor assays. Taken together, our results suggest that PELP1 regulates tumor metastasis by controlling the expression and functions of the tumor metastasis suppressors miR-200a and miR-141.

The prognostic value of estrogen receptor beta and proline-, glutamic acid- and leucine-rich protein 1 (PELP1) expression in ovarian cancer

Background

Proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), a coregulator of the estrogen receptors (ERs) alpha and beta, is a potential proto-oncogene in hormone dependent gynecological malignancies. To better understand the role of PELP1 in epithelial ovarian cancer (EOC), the protein expression and prognostic significance of PELP1 was evaluated together with ERalpha and ERbeta in EOC tissues.

Methods

The expression of PELP1, ERalpha, and ERbeta was characterized in tumor tissues of 63 EOC patients. The prognostic value was calculated performing log-rank tests and multivariate Cox-Regression analysis. In a second step, validation analysis in an independent set of 86 serous EOC patients was performed.

Results

Nuclear PELP1 expression was present in 76.2% of the samples. Prevalence of PELP1 expression in mucinous tumors was significantly lower (37.5%) compared to serous (85.7%) and endometrioid tumors (86.7%). A significant association between PELP1 expression and nuclear ERbeta staining was found (p=0.01). Positive PELP1 expression was associated with better disease-free survival (DFS) (p=0.004) and overall survival (OS) (p=0.04). The combined expression of ERbeta+/PELP1+ revealed an independent association with better DFS (HR 0.3 [0.1-0.7], p=0.004) and OS (HR 0.3 [0.1-0.7], p=0.005). In the validation set, the combined expression of ERbeta+/PELP1+ was not associated with DFS (HR 0.7 [0.4-1.3], p=0.3) and OS (HR 0.7 [0.3-1.4], p=0.3).

Conclusion

Positive immunohistochemical staining for the ER coregulator PELP1, alone and in combination with ERbeta, might be of prognostic relevance in EOC.

PELP1 oncogenic functions involve CARM1 regulation

Estrogen receptor alpha (ERα) is implicated in the initiation and progression of breast cancer and its transcription depends on the modulation of epigenetic changes at target gene promoters via coregulators. There is a critical need to understand the molecular mechanism(s) by which deregulation of epigenetic changes occurs during breast cancer progression. The ERα coregulator PELP1 plays an important role in ERα signaling and is a proto-oncogene with aberrant expression in breast cancer. PELP1 interacts with histones and may be a reader of chromatin modifications. We profiled PELP1's epigenetic interactome using a histone peptide array. Our results show that PELP1 recognizes histones modified by arginine and lysine dimethylation. PELP1 functionally interacts with the arginine methyltransferase CARM1 and their interaction is enhanced by ERα. PELP1-CARM1 interactions synergistically enhance ERα transactivation. Chromatin immunoprecipitation assays revealed that PELP1 alters histone H3 arginine methylation status at ERα target gene promoters. Pharmacological inhibition or small interfering RNA knockdown of CARM1 substantially reduced PELP1 oncogenic functions. The critical role of PELP1 status in modulating arginine methylation status was also observed through in vivo studies where PELP1 knockdown mediated decreased tumorigenesis correlated with decreased arginine dimethylation. Further, immunohistochemical analysis of human breast tumor tissues revealed co-overexpression of PELP1 and CARM1 in a subset of ERα-positive breast tumors. Our findings show PELP1 is a reader of histone arginine methyl modifications and deregulation promotes tumor proliferation via epigenetic alterations at ERα target promoters. Targeting these epigenetic alterations through inhibition of PELP1 and the arginine methyltransferases could be a promising cancer therapeutic.

Targeting the PELP1-KDM1 axis as a potential therapeutic strategy for breast cancer

INTRODUCTION

The estrogen receptor (ER) co-regulator proline glutamic acid and leucine-rich protein 1 (PELP1) is a proto-oncogene that modulates epigenetic changes on ER target gene promoters via interactions with lysine-specific histone demethylase 1 (KDM1). In this study, we assessed the therapeutic potential of targeting the PELP1-KDM1 axis in vivo using liposomal (1,2-dioleoyl-sn-glycero-3-phosphatidylcholine; DOPC) siRNA to downregulate PELP1 expression and KDM1 inhibitors, pargyline and N-((1S)-3-(3-(trans-2-aminocyclopropyl)phenoxy)-1-(benzylcarbamoyl)propyl)benzamide using preclinical models.

METHODS

Preclinical xenograft models were used to test the efficacy of drugs in vivo. Ki-67 and terminal deoxynucleotidyl transferase dUTP nick end-labeling immunohistochemical analysis of epigenetic markers was performed on tumor tissues. The in vitro effect of PELP1-KDM axis blockers was tested using proliferation, reporter gene, chromatin immunoprecipitation and real-time RT-PCR assays. The efficacy of the KDM1 targeting drugs alone or in combination with letrozole and tamoxifen was tested using therapy-resistant model cells.

RESULTS

Treatment of ER-positive xenograft-based breast tumors with PELP1-siRNA-DOPC or pargyline reduced tumor volume by 58.6% and 62%, respectively. In a postmenopausal model, in which tumor growth is stimulated solely by local estrogen synthesis, daily pargyline treatment reduced tumor volume by 78%. Immunohistochemical analysis of excised tumors revealed a combined decrease in cellular proliferation, induction of apoptosis and upregulation of inhibitory epigenetic modifications. Pharmacological inhibition of KDM1 in vitro increased inhibitory histone mark dimethylation of histone H3 at lysine 9 (H3K9me2) and decreased histone activation mark acetylation of H3K9 (H3K9Ac) on ER target gene promoters. Combining KDM1 targeting drugs with current endocrine therapies substantially impeded growth and restored sensitivity of therapy-resistant breast cancer cells to treatment.

CONCLUSION

Our results suggest inhibition of PELP1-KDM1-mediated histone modifications as a potential therapeutic strategy for blocking breast cancer progression and therapy resistance.

Central role for PELP1 in nonandrogenic activation of the androgen receptor in prostate cancer

The ability of 17β-estradiol (E2) to regulate the proliferation of prostate cancer (PCa) cells in the absence of androgen is poorly understood. Here, we show the predominant estrogen receptor (ER) isoform expressed in PCa specimens and cell lines is ERβ. Our data indicate that E2 induces the formation of a complex between androgen receptor (AR), ERβ, and a proline-, glutamic acid-, and leucine-rich cofactor protein 1 (PELP1) in PCa cells. This protein complex is formed on AR's cognate DNA-responsive elements on the promoter in response to E2. Formation of this complex enables the transcription of AR-responsive genes in response to E2. Knockdown of PELP1, AR, or ERβ blocks the assembly of this complex, blocks E2-induced genomic activation of AR-regulated genes, and blocks E2-stimulated proliferation of PCa cells. Overall, this study shows that PELP1 may enable E2-induced AR signaling by forming a protein complex between AR, ERβ, and PELP1 on the DNA, leading to the proliferation of PCa cells in the absence of androgen. PELP1 may bridge the signal between E2 bound to ERβ and AR and thus allow for cross talk between these steroid receptors. These data suggest a novel mechanism of AR activation in the absence of androgens in PCa cells. Our data indicate that disruption of the complex between AR and PELP1 may be a viable therapeutic strategy in advanced PCa.

Regulation of rDNA transcription by proto-oncogene PELP1

Background

Proline-, glutamic acid-, and leucine-rich protein (PELP1) is a novel nuclear receptor coregulator with a multitude of functions. PELP1 serves as a scaffolding protein that couples various signaling complexes with nuclear receptors and participates as a transcriptional coregulator. Recent data suggest that PELP1 expression is deregulated in hormonal cancers, and that PELP1 functions as a proto-oncogene; however, the mechanism by which PELP1 promotes oncogenesis remains elusive.

Methodology/Principal Findings

Using pharmacological inhibitors, confocal microscopy and biochemical assays, we demonstrated that PELP1 is localized in the nucleolus and that PELP1 is associated with the active ribosomal RNA transcription. Cell synchronization studies showed that PELP1 nucleolar localization varies and the greatest amount of nucleolar localization was observed during S and G2 phases. Using pharmacological compounds and CDK site mutants of PELP1, we found that CDK's activity plays an important role on PELP1 nucleolar localization. Depletion of PELP1 by siRNA decreased the expression of pre-rRNA. Reporter gene assays using ribosomal DNA (pHrD) luc-reporter revealed that PELP1WT but not PELP1MT enhanced the expression of reporter. Deletion of nucleolar domains abolished PELP1-mediated activation of the pHrD reporter. ChIP analysis revealed that PELP1 is recruited to the promoter regions of rDNA and is needed for optimal transcription of ribosomal RNA.

Conclusions/Significance

Collectively, our results suggest that proto-oncogene PELP1 plays a vital role in rDNA transcription. PELP1 modulation of rRNA transcription, a key step in ribosomal biogenesis may have implications in PELP1-mediated oncogenic functions.

Integrin-linked kinase 1: role in hormonal cancer progression

Integrin-linked kinase 1 (ILK1) is a serine/threonine kinase that plays important roles in a variety of cellular functions including cell survival, migration and angiogenesis. ILK1 is normally expressed in numerous tissues and activated by growth factors, cytokines and hormones. Dysregulation of ILK1 expression or function is found in several hormonal tumors including breast, ovary and prostate. Emerging evidence suggests that ILK overexpression promotes cellular transformation, cell survival, epithelial mesenchymal transition (EMT), and metastasis of hormonal cancer cells while inhibition of ILK1 reduces tumor growth and progression. The recent development of ILK1 inhibitors has provided novel mechanisms for blocking ILK1 signaling to curb metastasis and therapy resistance of hormonal tumors. This review will focus on recent advances made towards understanding the role of ILK signaling axis in progression of hormonal cancer.

Extranuclear signaling by estrogen: role in breast cancer progression and metastasis

The estrogen receptor (ERa) is implicated in the progression of breast cancer. Hormonal therapies which block ER functions or local and systemic estrogen production are currently used to treat ERa positive breast cancer. Hormonal therapy shows beneficial effects, however, initial or acquired resistance to endocrine therapies frequently occurs, and tumors recur as metastasis. Emerging evidence suggests in addition to exerting its well-studied nuclear functions, ERa also participates in extranuclear signaling that involve growth factor signaling components, adaptor molecules and the stimulation of cytosolic kinases. ERa extranuclear pathways have the potential to activate gene transcription, modulate cytoskeleton, and promote tumor cell proliferation, survival, and metastasis. Cytoplasmic/membrane ERa is detected in a subset of breast tumors and expression of extranuclear components ERa is deregulated in tumors. The extranuclear actions of ER are emerging as important targets for tumorigenic and metastatic control. Inhibition of ERa extranuclear actions has the potential to prevent breast tumor progression and may be useful in preventing ERa positive metastasis. In this review, we summarize the results of recent research into the role of ERa mediated extranuclear actions in breast tumorigenesis and metastasis.