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.

Abstract P5-03-01: Role of Aromatase and Its Inhibitor in Breast Cancer-Induced Tumorigenesis and Bone Metastasis

Aromatase (Aro) is the rate-limiting enzyme that catalyzes the final step in estrogen (E2) biosynthesis. An important strategy to treat hormone-dependent BCa is suppression of estrogen receptor (ER) action by antiestrogens or aromatase inhibitors (AI). Letrozole is a very specific and potent AI. In postmenopausal women, the ovaries cease to make E2 but concentration of E2 in their BCa tissue are maintained at a certain level for survival and proliferation of BCa cells, which is dependent on local E2 formation catalyzed by Aro. Although BCa cells have been shown to express Aro, the local E2 is largely produced by adipose stromal cells in the breast. This raises the question of how ER positive (ER+) metastatic BCa cells survive after they enter blood circulation, where E2 level is very low.

We cultured human ER+ BCa CAMA-1, Aro-transfected MCF-7 and ZR-75-1 (ZR) cells in suspension to mimic circulating BCa cells. Interestingly, suspension culture increased Aro expression, suggesting circulating ER+ BCa may up-regulate intracrine E2 activity for survival after leaving the E2-rich adipose stroma at primary site. The expression of Aro also enhances cell proliferation and supplementation of testosterone (T), the substrate of Aro, stimulates this proliferation further. Notably, while these cells show an increased rate of apoptosis in suspension than in adherent culture, addition of T in suspension culture significantly suppressed the rate of apoptosis and addition of letrozole blocked the T-induced cell survival in suspension culture. To investigate the importance of intracrine E2 in promoting tumorigenesis and metastasis, we implanted Aro-expressing ZR cells orthotopically and intracardiacally (I.C.) into female athymic mice; vector-transfected ZR cells were used as control. While control ZR cells were incapable of forming tumors without E2 supplementation, Aro-expressing cells generated orthotopic tumors with no E2 supplementation after 3-weeks of inoculation. More interestingly, mice with I.C. inoculated Aro-expressing cells also presented distant bone metastasis in the mandible and tibiae/femora after 2-weeks of inoculation, detected by whole mouse fluorescence and bioluminescence imaging as the cells were stably transfected with a luciferase and GFP expression vector. To determine whether growth of orthotopic tumors can be inhibited by systemic administration of an AI, we treated the mice in one group with letrozole at 10 mg/mouse/day and the other group with the vehicle as control after the average tumor volume reached 150 mm3. After 3-weeks, the tumor burden in the letrozole treatment group reduced significantly while tumor burden in the control group increased continuously. Our studies show that suspension culture increases expression of Aro mRNA in several ER+ BCa cell lines, which likely results in increased intracrine E2 signaling and contributes to the survival of these BCa cellsin suspension. This provides a mechanistic insight into how ER+ BCa cells may survive the low E2 condition in circulation and subsequently induce distant metastasis as observed in the I.C. model. Our study provides an important foundation for future investigation on how hormone-dependent BCa cells up-regulate Aro expression in circulation and induce bone metastasis.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-03-01.

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.

Cyclin-dependent kinase-mediated phosphorylation plays a critical role in the oncogenic functions of PELP1

Estrogen receptor (ER) signaling plays an important role in breast cancer progression, and ER functions are influenced by coregulatory proteins. PELP1 (proline-, glutamic acid-, and leucine-rich protein 1) is a nuclear receptor coregulator that plays an important role in ER signaling. Its expression is deregulated in hormonal cancers. We identified PELP1 as a novel cyclin-dependent kinase (CDK) substrate. Using site-directed mutagenesis and in vitro kinase assays, we identified Ser(477) and Ser(991) of PELP1 as CDK phosphorylation sites. Using the PELP1 Ser(991) phospho-specific antibody, we show that PELP1 is hyperphosphorylated during cell cycle progression. Model cells stably expressing the PELP1 mutant that lack CDK sites had defects in estradiol (E2)-mediated cell cycle progression and significantly affected PELP1-mediated oncogenic functions in vivo. Mechanistic studies showed that PELP1 modulates transcription factor E2F1 transactivation functions, that PELP1 is recruited to pRb/E2F target genes, and that PELP1 facilitates ER signaling cross talk with cell cycle machinery. We conclude that PELP1 is a novel substrate of interphase CDKs and that its phosphorylation is important for the proper function of PELP1 in modulating hormone-driven cell cycle progression and also for optimal E2F transactivation function. Because the expression of both PELP1 and CDKs is deregulated in breast tumors, CDK-PELP1 interactions will have implications in breast cancer progression.

B7-H1-dependent sex-related differences in tumor immunity and immunotherapy responses

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) are immunopathogenic in cancers by impeding tumor-specific immunity. B7-homologue 1 (B7-H1) (CD274) is a cosignaling molecule with pleiotropic effects, including hindering antitumor immunity. In this study, we demonstrate sex-dependent, B7-H1-dependent differences in tumor immunity and response to immunotherapy in a hormone-independent cancer, murine B16 melanoma. Antitumor immunity was better in B7-H1(-/-) females versus males as a result of reduced regulatory T cell function in the B7-H1(-/-) females, and clinical response following B7-H1 blockade as tumor immunotherapy was significantly better in wild-type females than in males, owing to greater B7-H1 blockade-mediated reduction of Treg function in females. Wild-type female Tregs expressed significantly lower B7-H1 versus males but were insensitive to estrogen in vitro. Female B7-H1(-/-) Tregs were exquisitely sensitive to estrogen-mediated functional reduction in vitro, suggesting that B7-H1 effects occur before terminal Treg differentiation. Immune differences were independent of known B7-H1 ligands. Sex-dependent immune differences are seldom considered in designing immune therapy or interpreting immunotherapy treatment results. Our data demonstrate that sex is an important variable in tumor immunopathogenesis and immunotherapy responses through differential Treg function and B7-H1 signaling.

PELP1: A novel therapeutic target for hormonal cancers

Recent studies implicate that the estrogen receptor (ER) coregulator proline-, glutamic acid-, and leucine-rich protein (PELP) 1 as playing critical roles in ER-genomic, ER-nongenomic, and ER-signaling cross talk with growth factor signaling pathways. PELP1 expression is deregulated in hormonal cancers and recent studies further elucidated the molecular mechanisms by which PELP1 regulates hormone therapy response. Although PELP1 is important for normal functions of the ER, the possibility to target ER-PELP1 axis appears to be an effective strategy for preventing hormonal carcinogenesis and therapy resistance. Thus, PELP1 may be useful as prognostic marker for hormonal cancers and PELP1 signaling may be useful to generate targeted therapeutics to overcome hormonal therapy resistance.

Extranuclear functions of ER impact invasive migration and metastasis by breast cancer cells

The molecular basis of breast cancer progression to metastasis and the role of estrogen receptor (ER) signaling in this process remain poorly understood. Emerging evidence suggests that ER participates in extranuclear signaling in addition to genomic functions. Recent studies identified proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) as one of the components of ER signalosome in the cytoplasm. PELP1 expression is deregulated in metastatic breast tumors. We examined the mechanism and significance of ER-PELP1-mediated extranuclear signals in the cytoskeletal remodeling and metastasis. Using estrogen dendrimer conjugate (EDC) that uniquely activate ER extranuclear signaling and by using model cells that stably express PELP1 short hairpin RNA (shRNA), we show that PELP1 is required for optimal activation of ER extranuclear actions. Using a yeast two-hybrid screen, we identified integrin-linked kinase 1 (ILK1) as a novel PELP1-binding protein. Activation of extranuclear signaling by EDC uniquely enhanced E2-mediated ruffles and filopodia-like structures. Using dominant-negative and dominant-active reagents, we found that estrogen-mediated extranuclear signaling promotes cytoskeleton reorganization through the ER-Src-PELP1-phosphoinositide 3-kinase-ILK1 pathway. Using in vitro Boyden chamber assays and in vivo xenograft assays, we found that ER extranuclear actions contribute to cell migration. Collectively, our results suggest that ER extranuclear actions play a role in cell motility/metastasis, establishing for the first time that endogenous PELP1 serves as a critical component of ER extranuclear actions leading to cell motility/invasion and that the ER-Src-PELP1-ILK1 pathway represents a novel therapeutic target for preventing the emergence of ER-positive metastasis.

PELP1 is a reader of histone H3 methylation that facilitates oestrogen receptor-alpha target gene activation by regulating lysine demethylase 1 specificity

Histone methylation has a key role in oestrogen receptor (ERalpha)-mediated transactivation of genes. Proline glutamic acid and leucine-rich protein 1 (PELP1) is a new proto-oncogene that functions as an ERalpha co-regulator. In this study, we identified histone lysine demethylase, KDM1, as a new PELP1-interacting protein. These proteins, PELP1 and KDM1, were both recruited to ERalpha target genes, and PELP1 depletion affected the dimethyl histone modifications at ERalpha target genes. Dimethyl-modified histones H3K4 and H3K9 are recognized by PELP1, and PELP1 alters the substrate specificity of KDM1 from H3K4 to H3K9. Effective demethylation of dimethyl H3K9 by KDM1 requires a KDM1-ERalpha-PELP1 functional complex. These results suggest that PELP1 is a reader of H3 methylation marks and has a crucial role in modulating the histone code at the ERalpha target genes.

Abstract 4604: Roscovitine modulates ERα/ERβ ratio: A novel means to target hormonal therapy resistance

Current endocrine therapy for ER+ve breast cancer involves modulating ERα-pathway using either ERα-antagonists or aromatase inhibitors. Despite the positive effects, de novo and/or acquired resistance to endocrine therapies frequently occur. Although mechanisms for hormonal therapy resistance remains elusive, emerging data implicate ER-growth factor signaling cross talk, and alteration in ER subtypes as the major causes of resistance. Most downstream events in the resistance pathways converge upon modulation of cell cycle regulatory proteins; the most conspicuous of which is the activation of Cyclin Dependent Kinase 2 (CDK2) pathway. Roscovitine is one of the most frequently studied and used CDK2 inhibitor. In this study, we examined whether roscovitine confers tumor suppressive advantage to therapy resistant breast epithelial cells by inhibiting CDK functions using three therapy resistant model cells; (a) MCF-7-Tam (acquired Tamoxifen resistance model); (b) MCF-7-CA-LTLT (acquired Letrozole resistance model); (c) MCF-7-HER2 (ER-growth factor signaling cross talk model). Roscovitine at 10 μM concentration substantially reduced the growth of all three resistant model cells in cell proliferation and foci formation assays. FACS analysis revealed that roscovitine treatment increased proportion of cells in G2-M. To increase the drug efficacy, we have developed PLGA nanoparticles containing roscovitine and this mode of delivery significantly reduced the dosage needed to see the growth inhibitory effect in therapy resistant cells to 500 nM. To determine the mechanism, we have determined the relationship between CDK activity, expression of ERα, β subtypes, ER-coregulators, and expression of ER target genes in the presence or absence of roscovitine in resistant cells. Roscovitine substantially affected CDK2 activity and decreased cyclin A levels in the model cells. These studies also revealed an unexpected discovery that CDK inhibitor roscovitine has potential to alter the ratio of ER isoforms with preferential upregulation of ERβ expression with concomitant down regulation of ERα, its coregulators including AIB1 and PELP1. Since many advanced and therapy resistant tumors exhibit loss or reduced expression of ERβ, dual action of roscovitine will provide a novel drug to inhibit both CDKs and to increase the expression of tumor suppressor ERβ. Since deregulation of cell cycle machinery and estrogen receptor signaling contributes to resistance, roscovitine a drug that attacks both these pathways will serve as a double-edged sword to interfere with the resistance mechanisms.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4604.

Abstract 4613: Targeting ER-KDM1 axis to sensitize therapy resistant breast cancer cells to hormonal therapy

Estrogen-induced breast carcinogenesis is characterized by global changes in histone modifications. KDM1 (also known as LSD1) a histone demethylase enzyme, plays a key role in establishing specific histone methylation marks at ERα target gene promoters. Emerging evidence suggests histone methylation imposes ligand dependency; and deregulation of this epigenetic pathway potentially contributes to hormonal independence and adaptive resistance in breast cancer. Our study examined the therapeutic efficacy of pargyline, a KDM1 inhibiting drug, and evaluated the therapeutic benefit to curb growth of therapy resistant breast cancer cells. We used several model cell lines shown to exhibit resistance to hormonal therapy including MCF7-HER2, MCF7-Tam, MCF7-Ca-LTLT, MCF7-PELP1 and parental MCF7 cells as controls. Reporter gene assays showed KDM1 enhanced ERα- mediated transcription in therapy resistant cells. KDM1 functionally interacts with ERα coregulator PELP1 and is co-recruited with PELP1 to ERα target genes. Pargyline treatment substantially inhibited ERα transactivation functions. ChIP analysis revealed distinct activating histone methylation modifications at growth regulatory ERα target genes in aggressively growing and therapy resistant breast cancer cells. Breast cancer models treated with pargyline facilitated reversal of the observed specific modifications and thereby inhibited the growth of breast cancer cells in vitro and in vivo nude mice models. Pargyline also showed significant effect in blocking local estrogen synthesis via alterations in histone methylation modifications at the aromatase promoter. Combinational therapy using three agents that: (a) block ERα's nuclear actions (tamoxifen or letrozole), (b) ERα's extranuclear actions (dasatinib) and (c) ERα's epigenetic modifications (pargyline) showed most promising therapeutic effect compared to single agent therapy on the growth of therapy resistant cells. Our results suggest that histone methylation modifications play a role in therapy resistance and validate the therapeutic potential of pargyline in combinational therapies. Collectively, our results suggests targeting KDM1 axis with pargyline in combination with current endocrine therapies will have better therapeutic effect and may inhibit or delay development of hormonal resistance. This study is funded by Komen grant KG090447.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4613.

Abstract 3669: PELP1: A novel therapeutic target for ovarian cancer progression and metastasis

Ovarian cancer remains a major threat to women's health, especially because of the difficulty in early diagnosis and due to prevalent metastasis. There is critical need to identify novel targets that can be used to curb the progression and metastasis of ovarian cancer. Recent studies implicated a role for nuclear receptors (NR) and their coregulators in ovarian cancer progression and expression of NR-coregulators is shown to be deregulated in ovarian cancer. In this study, we examined whether NR coregulator PELP1 contributes to progression and metastatic potential of ovarian cancer cells and tested if blocking of PELP1 signaling axis will have any therapeutic effect. We have established OVCAR3, SKOV3 and ES2 model cells that stably express PELP1 specific shRNA and these model cells showed ∼80% decrease in PELP1 expression compared to vector transfected cells. PELP1 knock down significantly reduced proliferation of the model cells and also enhanced chemotherapy sensitivity of carboplatin and paclitaxel in ovarian cancer cells. Interestingly, PELP1 down regulated ovarian cancer cells showed cytoskeletal defects with low F-actin structures and increased accumulation of stress fibers. Boyden Chamber and wound healing assays revealed that PELP1 knockdown significantly affect the migratory potential of ovarian cancer cells. Microarray analysis of the model cells using metastasis pathway array revealed that PELP1 down regulation affects the expression of selective genes involved in metastasis including Myc, MTA1, MMP2 and MMP9. Zymography analysis confirmed that PELP1 knockdown caused a decrease in the activation of MMP2 and MMP9 in ovarian cancer cell lines. To examine the in vivo therapeutic potential, we examined whether systemically administered PELP1siRNA in a neurtal nanoliposomal formulation (DOPC) will reduce tumor growth using ovarian cancer xenograft model. Nude mice (n=10) injected with SKOV3ip1 ovarian cancer cells received therapy with either control siRNA-DOPC or PELP1siRNA-DOPC. Results showed that nude mice injected with PELP1-siRNA-DOPC have 54% less number of tumor nodules and exhibited 51% reduction in tumor growth and 84% reduction in ascites volume compared to control siRNA liposome injected mice. IHC examination of the tumors revealed that PELP1-siRNA liposome injected tumors showed low expression of PELP1, PCNA and MMP2. Collectively, these results suggest PELP1 signaling axis as a potential drugable target and PELP1siRNA liposomes could be used as a novel drug for therapeutic targeting of ovarian metastasis. The combination of targeting PELP1 along with the other chemotherapeutic drugs might be an effective new drug regimen for treatment of ovarian cancer progression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3669.

Regulation of aromatase induction by nuclear receptor coregulator PELP1

Estradiol (E2), estrogen receptor (ER), ER-coregulators have been implicated in the development and progression of breast cancer. In situ E2 synthesis is implicated in tumor cell proliferation through autocrine or paracrine mechanisms, especially in post-menopausal women. Several recent studies demonstrated activity of aromatase P450 (Cyp19), a key enzyme that plays critical role in E2 synthesis in breast tumors. The mechanism by which tumors enhance aromatase expression is not completely understood. Recent studies from our laboratory suggested that PELP1 (Proline, Glutamic acid, Leucine rich Protein 1), a novel ER-coregulator, functions as a potential proto-oncogene and promotes tumor growth in nude mice models without exogenous E2 supplementation. In this study, we found that PELP1 deregulation contributes to increased expression of aromatase, local E2 synthesis and PELP1 cooperates with growth factor signaling components in the activation of aromatase. PELP1 deregulation uniquely up-regulated aromatase expression via activation of aromatase promoter I.3/II. Analysis of PELP1 driven mammary tumors in xenograft as well as in transgenic mouse models revealed increased aromatase expression. PELP1-mediated induction of aromatase requires functional Src and PI3K pathways. Chromatin immuno precipitation (ChIP) assays revealed that PELP1 is recruited to the Aro 1.3/II aromatase promoter. HER2 signaling enhances PELP1 recruitment to the aromatase promoter and PELP1 plays a critical role in HER2-mediated induction of aromatase expression. Mechanistic studies revealed that PELP1 interactions with orphan receptor ERRalpha, and histone demethylases play a role in the activation of aromatase promoter. Accordingly, ChIP analysis showed alterations in histone modifications at the aromatase promoter in the model cells that exhibit local E2 synthesis. Immunohistochemical analysis of breast tumor progression tissue arrays suggested that deregulation of aromatase expression occurs in advanced-stage and node-positive tumors, and that cooverexpression of PELP1 and aromatase occur in a sub set of tumors. Collectively, our results suggest that PELP1 regulation of aromatase represent a novel mechanism for in situ estrogen synthesis leading to tumor proliferation by autocrine loop and open a new avenue for ablating local aromatase activity in breast tumors.

The cell fate determination factor DACH1 is expressed in estrogen receptor-alpha-positive breast cancer and represses estrogen receptor-alpha signaling

The Dachshund (dac) gene, initially cloned as a dominant inhibitor of the Drosophila hyperactive EGFR mutant ellipse, encodes a key component of the cell fate determination pathway involved in Drosophila eye development. Analysis of more than 2,200 breast cancer samples showed improved survival by some 40 months in patients whose tumors expressed DACH1. Herein, DACH1 and estrogen receptor-alpha (ERalpha) expressions were inversely correlated in human breast cancer. DACH1 bound and inhibited ERalpha function. Nuclear DACH1 expression inhibited estradiol (E(2))-induced DNA synthesis and cellular proliferation. DACH1 bound ERalpha in immunoprecipitation-Western blotting, associated with ERalpha in chromatin immunoprecipitation, and inhibited ERalpha transcriptional activity, requiring a conserved DS domain. Proteomic analysis identified proline, glutamic acid, and leucine rich protein 1 (PELP1) as a DACH1-binding protein. The DACH1 COOH terminus was required for binding to PELP1. DACH1 inhibited induction of ERalpha signaling. E(2) recruited ERalpha and disengaged corepressors from DACH1 at an endogenous ER response element, allowing PELP1 to serve as an ERalpha coactivator. DACH1 expression, which is lost in poor prognosis human breast cancer, functions as an endogenous inhibitor of ERalpha function.

Extranuclear coactivator signaling confers insensitivity to tamoxifen

PURPOSE

Tamoxifen is one of many standard therapeutic options currently available for estrogen receptor-alpha-positive breast cancer patients. Emerging data have suggested that levels of estrogen receptor coregulatory proteins play a significant role in acquiring resistance to antiestrogen action. It has been suggested that high levels of estrogen receptor coactivators and its mislocalization may enhance the estrogen agonist activity of tamoxifen and contribute to tamoxifen resistance.

EXPERIMENTAL DESIGN

In an effort to understand the impact of nongenomic signaling and its contribution to hormone resistance in a whole-animal setting, we generated a transgenic mouse expressing a cytoplasmic version of proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) mutant defective in its nuclear translocation (PELP1-cyto) and implanted these mice with tamoxifen pellets to assess its responsiveness.

RESULTS

We show that mammary glands from these mice developed widespread hyperplasia with increased cell proliferation and enhanced activation of mitogen-activated protein kinase and AKT as early as 12 weeks of age. Treatment with tamoxifen did not inhibit this hyperplasia; instead, such treatment exaggerated hyperplasia with an enhanced degree of alteration, indicative of hypersensitivity to tamoxifen. Analysis of molecular markers in the transgenic mammary glands from the tamoxifen-treated transgenic mice showed higher levels of proliferation markers proliferating cell nuclear antigen and activated mitogen-activated protein kinase than in untreated PELP1-cyto cell-derived mice. We also found that nude mice with MCF-7/PELP1-cyto cell-derived tumor xenografts did not respond to tamoxifen. Using immunohistochemical analysis, we found that 43% of human breast tumor samples had high levels of cytoplasmic PELP1, which shows a positive correlation between tumor grade and proliferation. Patients whose tumors had high levels of cytoplasmic PELP1 exhibited a tendency to respond poorly to tamoxifen compared with patients whose tumors had low levels of cytoplasmic PELP1.

CONCLUSIONS

These findings suggest that PELP1 localization could be used as a determinant of hormone sensitivity or vulnerability. The establishment of the PELP1-cyto transgenic mouse model is expected to facilitate the development of preclinical approaches for effective intervention of breast tumors using cytoplasmic coregulators and active nongenomic signaling.

Novel mouse model for studying role of ER-nongenomic actions in breast cancer

Abstract #601

Back ground: Estradiol (E2) and estrogen receptor (ER) signaling play a key role in development and progression of breast cancer. ER signaling is complex, involves coregulatory proteins and the status of ER coregulators in tumor cells plays an important role in hormonal responsiveness and tumor progression. In addition, ER also participates in non-genomic signaling events in the cytoplasm, however the significance of non-genomic signaling in mammary tumorigenesis remain unknown. PELP1/MNAR is novel ER coregulator that participates in ER genomic and non-genomic actions. PELP1 expression is deregulated in breast tumors and in a subset of tumors PELP1 is predominantly localized in the cytoplasm. Since PELP1 cytoplamsic localization promotes excessive activation of Src and AKT pathways, we hypothesized that PELP1 mediated excessive activation of ER-nongenomic functions may play a role tumorigenesis. To test this, we have generated MMTV-PELP1cyto TG model that uniquely express PELP1 in the cytoplasm of mammary glands that mimics the pathological situation of PELP1 localization seen breast cancer.
 Methods: As a means of targeting the expression of the PELP1 transgene to the mammary gland, we placed the PELP1cyto cDNA under the control of the MMTV promoter. PELP1 transgene integration was verified by PCR and expression levels by Western and IHC in each founder line. Whole-mount preparations and IHC analysis was performed using Tg and age controlled wild type littermates from different developmental stages. Total protein extracts of mammary gland were used for western blot analysis of nongenomic signaling components.
 Results: Preliminary analysis of mammary gland from PELP-cyto mice showed hyperplasia, increased proliferation as analyzed by PCNA staining. Mammary tumors were observed as early as 32 weeks. No spontaneous mammary tumors were found in the wild type cohort. Pathological analysis revealed that these tumor masses represent full blown mammary adenocarcinomas. Mammary tumors showed excessive activation of nongenomic signaling including activation of Src and AKT pathways. A clear induction of aromatase expression was found in PELP1 tumors compared with the wild-type that showed no aromatase expression in the mammary gland.
 Discussion: We have established and characterized a transgenic mouse model that mimics deregulated ER-nongenomic signaling. Our results suggest that PELP1 is a proto-oncogene and demonstrates its in vivo tumorigenic potential. PELP1 driven tumors are ER+ve, express aromatase, thus provide an interesting in vivo model for studying ER-mediated tumorigenesis and to study effect of local E2 on ER-mediated tumorigenesis.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 601.

Potential role of estrogen receptor coregulator in mitosis

Abstract #5036

Background: Estrogen stimulates breast tissue to increase cell divisions (mitosis) and is implicated in breast cancer progression. ER action is complex and requires functional interactions with coregulators. Proline-, glutamic acid-, and leucine-rich protein (PELP)-1, also known as modulator of nongenomic actions of estrogen receptor (MNAR), is a novel nuclear receptor (ER) coregulator with multitude of functions. Emerging evidence suggest that PELP1 expression is deregulated in breast cancer and serves as a scaffolding protein that couples various signaling complexes with estrogen receptor. In this study we found that ER coregulator PELP1 plays a novel role in mitosis.
 Material and Methods: To understand the mechanism by which ER coregulator PELP1 contribute to breast cancer progression, we have utilized small RNA interference methodology and established breast cancer model cells that stably express PELP1-shRNA (MCF7-PELP1shRNA). FACS analysis was used to determine the cell cycle status of the model cells. Using confocal microscopy, immunoprecipitation, in vitro kinase assays, site directed mutagenesis and Western analysis using phospho-antibodies we studied the mechanism and significance of PELP1 signaling in mitotic progression. We also developed PELP1 siRNA nanoparticles and used them as well as CDK1 inhibitors in cell proliferation studies.
 Results: Down regulation of PELP1 expression resulted in decreased estrogen mediated cell proliferation, delayed mitotic progression and induced accumulation of mitotic cells. Interestingly, PELP1 depleted cells also exhibited multinucleation. Western analysis of various markers of mitotic progression revealed a delay in the kinetics of G2M initiation and progression. Confocal analysis revealed colocalization of CDK1 and PELP1 in G2M. Immunoprecipitation assays demonstrate that endogenous CDK1 form functional complex with PELP1 and Src kinase during mitosis. Using deletion and mutagenesis approach, we have mapped the putative CDK1 phosphorylation sites on PELP1. Down regulation of PELP1 or overexpression of PELP1 mutants (that cannot be phosphorylated by CDK1), reduces the magnitude of Src activation, which is an essential driving force for timely progression of M phase. PELP1 siRNA nanoparticles alone or in combination with CDK1 inhibitors have shown to significantly reduce the proliferation of breast cancer cells and showed increased response in tamoxifen resistant breast cancer cells.
 Discussion: These results suggest that ER coregulator PELP1 play a novel role in G2M progression. Since PELP1 expression is deregulated in breast cancer, PELP1 ability to regulate mitosis could contribute to the progression of cancer by causing genomic instability through the deregulation of mitosis. Taken together our findings suggest that estrogen can promote neoplasia using coregulators by two distinct mechanisms (1) Coregulator induction of target genes and (2) Coregulator mediated actions in mitosis. ER coregulator PELP1 play multiple roles in Estrogen mediated neoplasia, and thus represent a target for novel therapeutic breast cancer strategies by forming the “next generation” of antimitotic drugs.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5036.

Growth factor regulation of estrogen receptor coregulator PELP1 functions via Protein Kinase A pathway

PELP1 (proline-rich, glutamic acid-rich, and leucine-rich protein-1) is a potential proto-oncogene that functions as a coregulator of estrogen receptor (ER), and its expression is deregulated during breast cancer progression. Emerging evidence suggests growth factor signaling crosstalk with ER as one possible mechanism by which breast tumors acquire resistance to therapy. In this study, we examined mechanisms by which growth factors modulate PELP1 functions, leading to activation of ER. Using in vivo labeling assays, we have found that growth factors promote phosphorylation of PELP1. Utilizing a panel of substrate-specific phosphorylated antibodies, we discovered that growth factor stimulation promotes phosphorylation of PELP1 that is recognized by a protein kinase A (PKA) substrate-specific antibody. Accordingly, growth factor-mediated PELP1 phosphorylation was effectively blocked by PKA-specific inhibitor H89. Utilizing purified PKA enzyme and in vitro kinase assays, we obtained evidence of direct PELP1 phosphorylation by PKA. Using deletion and mutational analysis, we identified PELP1 domains that are phosphorylated by PKA. Interestingly, site-directed mutagenesis of the putative PKA site in PELP1 compromised growth factor-induced activation and subnuclear localization of PELP1 and also affected PELP1-mediated transactivation function. Utilizing MCF-7 cells expressing a PELP1 mutant that cannot be phosphorylated by PKA, we provide mechanistic insights by which growth factor signaling regulates ER transactivation in a PELP1-dependent manner. Collectively, these findings suggest that growth factor signals promote phosphorylation of ER coactivator PELP1 via PKA pathway, and such modification may have functional implications in breast tumors with deregulated growth factor signaling.

Role of PELP1/MNAR signaling in ovarian tumorigenesis

Emerging evidence suggests that nuclear receptor (NR) coregulators have potential to act as master genes and their deregulation can promote oncogenesis. Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1/MNAR) is a novel NR coregulator. Its expression is deregulated in hormone-driven cancers. However, the role of PELP1/MNAR in ovarian cancer progression remains unknown. Analysis of serial analysis of gene expression data suggested deregulation of PELP1/MNAR expression in ovarian tumors. Western analysis of PELP1/MNAR in normal and serous ovarian tumor tissues showed 3- to 4-fold higher PELP1/MNAR expression in serous tumors compared with normal ovarian tissues. To examine the significance of PELP1/MNAR in ovarian cancer progression, we have generated model cells that overexpress PELP1/MNAR and ovarian cancer cells in which PELP1/MNAR expression is down-regulated by stable expression of PELP1/MNAR-specific shRNA. Down-regulation of PELP1/MNAR in cancerous ovarian model cells (OVCAR3) resulted in reduced proliferation, affected the magnitude of c-Src and protein kinase B (AKT) signaling, and reduced tumorigenic potential of ovarian cancer cells in a nude mouse model. PELP1/MNAR overexpression in nontumorigenic immortalized surface epithelial cells (IOSE cells) promoted constitutive activation of c-Src and AKT signaling pathways and promoted anchorage-independent growth. Immunohistochemical studies using human ovarian cancer tissue arrays (n = 123) showed that PELP1/MNAR is 2- to 3-fold overexpressed in 60% of ovarian tumors, and PELP1/MNAR deregulation occurs in all different types of ovarian cancer. Collectively, these results suggest that PELP1/MNAR signaling plays a role in ovarian cancer cell proliferation and survival, and that its expression is deregulated in ovarian carcinomas.

PELP1--a novel estrogen receptor-interacting protein

PELP1 (proline-, glutamic acid-, and leucine-rich protein-1) is a novel estrogen receptor (ER)-interacting protein that has been implicated to be important for mediation of both the genomic and nongenomic signaling of 17beta-estradiol (E2). PELP1 contains ten nuclear receptor-interacting boxes (LXXLL motifs), which allow it to interact with ER and other nuclear hormone receptors, a zinc finger, a glutamic acid-rich domain, and two proline-rich domains. The proline-rich regions contain several consensus PXXP motifs, which allow PELP1 to couple the ER with SH3 domain-containing kinase signaling proteins, such as Src and PI3K P85 regulatory subunit. PELP1 is expressed in many different brain regions, including the hippocampus, hypothalamus, and cerebral cortex. Further work has demonstrated that PELP1 is colocalized with ER-alpha in neurons in various brain regions. PELP1 is primarily expressed in neurons, with some expression also observed in glia. Subcellular localization studies revealed that PELP1 is highly localized in the cell nucleus of neurons, with some cytoplasm localization as well, and PELP1 is also localized at synaptic sites. Work in other tissues has demonstrated that PELP1 is critical for nongenomic and genomic signaling by E2, as PELP1 knockdown studies significantly attenuates E2-induced activation of ERK and Akt signaling pathways, and inhibits E2 genomic transcriptional effects on gene expression in breast cancer cells. Preliminary studies in the brain, suggests that similar roles may exist for PELP1 in the brain, but this remains to be established, and further work to characterize the precise roles and functions of PELP1 in the brain are needed.

Regulation of hormonal therapy resistance by cell cycle machinery

Estrogen Receptor (ER) plays a central role in the development and progression of breast cancer. Hormonal therapy substantially improves disease-free survival of ER+ve breast tumors, however acquired resistance to endocrine therapies frequently occur. Emerging data implicate growth factor signaling pathways and their cross talk with ER as major cause of resistance. Both these pathways have been recently shown to use cell cycle machinery as downstream effectors in mediating therapy resistance. Several studies have demonstrated deregulation of cell cycle regulators and their cross talk with ER in therapy resistant tumors. The objective of this article is to review the underlying mechanisms by which tumor cells use cell cycle machinery to override hormonal therapy and to explore cell cycle machinery components as novel therapy targets for overcoming hormonal therapy resistance.

Modulation of in situ estrogen synthesis by proline-, glutamic acid-, and leucine-rich protein-1: potential estrogen receptor autocrine signaling loop in breast cancer cells

In situ estrogen synthesis is implicated in tumor cell proliferation through autocrine or paracrine mechanisms especially in postmenopausal women. Several recent studies demonstrated activity of aromatase, an enzyme that plays a critical role in estrogen synthesis in breast tumors. Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1/MNAR) is an estrogen receptor (ER) coregulator, and its expression is deregulated in breast tumors. In this study, we examined whether PELP1 promotes tumor growth by promoting local estrogen synthesis using breast cancer cells (MCF7) that stably overexpress PELP1. Immunohistochemistry revealed increased aromatase expression in MCF7-PELP1-induced xenograft tumors. Real-time PCR analysis showed enhanced activation of the aromatase promoter in MCF7-PELP1 clones compared with MCF7 cells. Using a tritiated-water release assay, we demonstrated that MCF7-PELP1 clones exhibit increased aromatase activity compared with control MCF-7 cells. PELP1 deregulation uniquely up-regulated aromatase expression via activation of aromatase promoter I.3/II, and growth factor signaling enhanced PELP1 activation of aromatase. PELP1-mediated induction of aromatase requires functional Src and phosphatidylinositol-3-kinase pathways. Mechanistic studies revealed that PELP1 interactions with ER-related receptor-alpha and proline-rich nuclear receptor coregulatory protein 2 lead to activation of aromatase. Immunohistochemistry analysis of breast tumor array showed increased expression of aromatase in ductal carcinoma in situ and node-positive tumors compared with no or weak expression in normal breast tissue. Fifty-four percent (n = 79) of PELP1-overexpressing tumors also overexpressed aromatase compared with 36% (n = 47) in PELP1 low-expressing tumors. Our results suggest that PELP1 regulation of aromatase represents a novel mechanism for in situ estrogen synthesis leading to tumor proliferation by autocrine loop and open a new avenue for ablating local aromatase activity in breast tumors.

Coregulation of estrogen receptor by ERBB4/HER4 establishes a growth-promoting autocrine signal in breast tumor cells

Although crosstalk between cell-surface and nuclear receptor signaling pathways has been implicated in the development and progression of endocrine-regulated cancers, evidence of direct coupling of these signaling pathways has remained elusive. Here we show that estrogen promotes an association between extranuclear estrogen receptor alpha (ER) and the epidermal growth factor receptor (EGFR) family member ERBB4. Ectopically expressed as well as endogenous ERBB4 interacts with and potentiates ER transactivation, indicating that the ERBB4/ER interaction is functional. Estrogen induces nuclear translocation of the proteolytic processed ERBB4 intracellular domain (4ICD) and nuclear translocation of 4ICD requires functional ligand-bound ER. The nuclear ER/4ICD complex is selectively recruited to estrogen-inducible gene promoters such as progesterone receptor (PgR) and stromal cell-derived factor 1 (SDF-1) but not to trefoil factor 1 precursor (pS2). Consistent with 4ICD-selective promoter binding, suppression of ERBB4 expression by interfering RNA shows that 4ICD coactivates ER transcription at the PgR and SDF-1 but not the pS2 promoter. Significantly, ERBB4 itself is an estrogen-inducible gene and the ERBB4 promoter harbors a consensus estrogen response element (ERE) half-site with overlapping activator protein-1 elements that bind ER and 4ICD in response to estrogen. Using a cell proliferation assay and a small interfering RNA approach, we show that ERBB4 expression is required for the growth-promoting action of estrogen in the T47D breast cancer cell line. Our results indicate that ERBB4 is a unique coregulator of ER, directly coupling extranuclear and nuclear estrogen actions in breast cancer. We propose that the contribution of an autocrine ERBB4/ER signaling pathway to tumor growth and therapeutic response should be considered when managing patients with ER-positive breast cancer.

Identifying the estrogen receptor coactivator PELP1 in autophagosomes

Resveratrol, a well-established phytoestrogen and chemopreventive agent, has gained much attention among oncologists because it can act as both estrogen receptor agonist and antagonist, depending on dosage and cell context. It is increasingly accepted that steroidal receptor coregulators may also function in the cytoplasmic compartment. Deregulation and altered localization of these coregulators could influence target gene expression and participate in the development of hormone-responsive cancers. Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1), a novel estrogen receptor (ER) coactivator, plays an important role in the genomic and nongenomic actions of ER. Furthermore, recent studies have shown that differential compartmentalization of PELP1 could be crucial in modulating sensitivity to tamoxifen. In this study, we investigated the role of PELP1 in resveratrol-induced autophagy in lung cancer and salivary gland adenocarcinoma cell lines. Resveratrol reversibly inhibited the growth of these cancer cell lines and induced autophagy, as evidenced by microtubule-associated protein 1 light chain 3 (LC3) up-regulation in a time-dependent and 3-methyladenine-sensitive manner. Confocal microscopic analysis showed that resveratrol induced PELP1 accumulation in autophagosomes with green fluorescent protein-LC3. The intermediary molecule involved in PELP1 accumulation in resveratrol-induced autophagosomes is hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), a trafficking molecule that binds to PELP1. These results identify PELP1 for the first time in autophagosomes, implying that both PELP1 and HRS reallocate to autophagosomes in response to resveratrol treatment, which might be important in the process of autophagy in the cancer cells.

Oncogenic potential of the nuclear receptor coregulator proline-, glutamic acid-, leucine-rich protein 1/modulator of the nongenomic actions of the estrogen receptor

Proline-, glutamic acid-, leucine-rich protein 1 (PELP1), a novel nuclear receptor coactivator, and its expression is deregulated in hormone-dependent cancers, including those of the breast, endometrium, and ovary. PELP1 interacts with estrogen receptor and modulates its genomic and nongenomic functions. In this study, we examined whether PELP1 functions as an oncogene. The overexpression of PELP1 in fibroblasts and epithelial model cells resulted in cellular transformation. PELP1 also enhanced the transformation potential of c-Src kinase in focus formation assays, and PELP1 overexpression potentiated estradiol-mediated cell migratory potential and anchorage-independent growth. Using PELP1-small interfering RNA, we provided evidence that endogenous PELP1 plays an essential role in E2-mediated anchorage-independent growth, cell migration, and cytoskeletal changes. When compared with control vector transfectants, breast cancer cells stably overexpressing PELP1 showed a rapid tumor growth in xenograft studies. Immunohistochemical analysis of PELP1 expression using a tumor progression array of 252 breast carcinomas and normal breast tissue specimens revealed that PELP1 expression is deregulated to a greater degree in higher grade node-positive invasive tumors than in normal breast tissue or ductal carcinoma in situ. Our data suggest that PELP1 is a potential oncogene, that its expression is deregulated during cancer progression, and that PELP1 may play a role in oncogenesis.

Functional and biological properties of the nuclear receptor coregulator PELP1/MNAR

Proline-, glutamic acid-, and leucine-rich protein (PELP)1, also known as modulator of nongenomic actions of the estrogen receptor (MNAR), is a novel nuclear receptor coregulator with a multitude of functions. PELP1/MNAR serves as a scaffolding protein that couples various signaling complexes with nuclear receptors and participates in genomic and nongenomic functions. Recent data suggest that PELP1/MNAR expression is deregulated in several cancers, including breast, endometrial, prostate, and ovarian cancer, and that PELP1/MNAR interacts with several oncogenes. In this review, we summarize the emerging biological properties and functions of PELP1/MNAR.

Estrogen induces expression of BCAS3, a novel estrogen receptor-alpha coactivator, through proline-, glutamic acid-, and leucine-rich protein-1 (PELP1)

We recently reported that the breast carcinoma amplified sequence-3 (BCAS3) gene is regulated by estrogen receptor (ER) alpha. However, the role of ERalpha coactivators in the regulation of BCAS3 expression remains unknown, and information regarding the function of the BCAS3 protein is lacking. Here, we define the contribution of ERalpha coactivators to BCAS3 regulation and identify BCAS3 itself as an ERalpha coactivator in breast cancer cells. We found that PELP1 (proline-, glutamic acid-, and leucine-rich protein-1), a newly described ERalpha coregulator, is recruited to BCAS3 chromatin and activates its expression. Analysis of the BCAS3 sequence for functional motifs and evidence from biochemical fractionation suggested that BCAS3 acts as a transcriptional coactivator. Results from chromatin immunoprecipitation, reporter assays, and expression studies further validated the coactivator function of BCAS3 for ERalpha. BCAS3 physically associated with histone H3 and histone acetyltransferase complex protein P/CAF (p300/CBP-associated factor) and possessed histone acetyltransferase activity. Unexpectedly, BCAS3 required PELP1 to function as a coactivator in ERalpha transactivation activity. In brief, these results highlight a mechanism whereby ERalpha activation triggers a positive feedback loop leading to signal amplification in the cell.

Emerging significance of ER-coregulator PELP1/MNAR in cancer

The estrogen receptors ERalpha and ERbeta have been implicated in the progression of a wide variety of cancers. The actions of ER are regulated by ER coregulator proteins, including proline-, glutamic acid- and leucine-rich-protein-1 (PELP1/MNAR). PELP1 has been shown to participate in both genomic and nongenomic functions of ER. The expression and localization of PELP1/MNAR are deregulated in a wide variety of tumors and have been implicated in the development of hormonal resistance in cancer cell lines. Emerging data suggest that PELP1/MNAR interacts with many proteins and activates several oncogenes, including Src kinase, phosphotidyl inositol 3 kinase (PI3K), and signal transducers and activators of transcription 3 (STAT3). These new results suggest that PELP1/MNAR may act as an oncogene as well as cooperating with other oncogenes. Thus, PELP1/MNAR may contribute to the tumorigenic potential of cancer cells by serving as a scaffolding protein that couples various signaling complexes with ER.

Proline-, glutamic acid-, and leucine-rich protein-1/modulator of nongenomic activity of estrogen receptor enhances androgen receptor functions through LIM-only coactivator, four-and-a-half LIM-only protein 2

Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) is a coregulator of multiple nuclear receptors. Molecular mechanisms of PELP1 function are not completely understood, but its expression is up-regulated in hormonal-dependent cancers. Using a yeast two-hybrid screen, we found that four-and-a-half LIM-only protein 2 (FHL2) interacted with PELP1. FHL2 is a transcriptional regulator that associates with nuclear cofactors, including androgen receptors (ARs), and contains an intrinsic activation domain. PELP1 and FHL2 interact in vitro and in vivo and colocalize in the nuclear compartment. PELP1 interacts with FHL2 via LIM domains 3 and 4 and synergistically enhances the transcriptional activity of FHL2. Src kinase is required for PELP1-mediated enhancement of FHL2 functions because knockdown of Src kinase expression or function abolished PELP1-mediated FHL2 activation functions. PELP1 interacted with AR and enhanced FHL2-mediated AR transactivation functions. PELP1 knockdown by small interfering RNA or PELP1 mutant, which lacks an activation domain, reduced FHL2-mediated AR transactivation. Biochemical analyses revealed a complex consisting of PELP1, FHL2, and AR in prostate cancer cells. PELP1/MNAR expression was elevated in high-grade prostate tumors. Our results suggest that PELP1 functions as a molecular adaptor, coupling FHL2 with nuclear receptors, and PELP1-FHL2 interactions may have a role in prostate cancer progression.

Comprehensive analysis of recent biochemical and biologic findings regarding a newly discovered protein-PELP1/MNAR

Estradiol (E2) and estrogen receptor (ER) signaling have been implicated in the development and progression of several cancers. Emerging evidence suggests that the status of ER coregulators in tumor cells plays an important role in hormonal responsiveness and tumor progression. Proline, glutamic acid, and leucine-rich protein-1 (PELP1/MNAR)-a novel ER coactivator that plays an essential role in the ER's actions and its expression-is deregulated in several hormonal responsive cancers. The precise function of PELP1/MNAR in cancer progression remains unclear, but PELP1 appears to function as a scaffolding protein, coupling ER with several proteins that are implicated in oncogenesis. Emerging evidence suggests that PELP1/MNAR increases E2-mediated cell proliferation and participates in E2-mediated tumorigenesis and metastasis.

9-cis-retinoic acid up-regulates expression of transcriptional coregulator PELP1, a novel coactivator of the retinoid X receptor alpha pathway

Retinoid X receptor alpha (RXRalpha), functioning as either a homodimer or a heterodimer with peroxisome proliferator receptors, is known to be involved in manifesting antiproliferative effects in cells. Consequently, studies of RXRalpha functions and its coregulators have been in the focus for therapeutic approaches against cancer. Here we have discovered that 9-cis-retinoic acid (9-cis-RA), a RXRalpha-specific ligand, up-regulated the expression of transcriptional coregulatory protein PELP1 (proline-, glutamic acid-, and leucine-rich protein 1). PELP1 functioned as a coactivator of RXRalpha, increasing its transactivation function in response to 9-cis-RA as evident by the retinoid X receptor response element-luciferase assays. PELP1 was found to be a binding partner of RXRalpha, and the binding interactions were confirmed both in vitro and in vivo. An electrophoretic mobility shift assay showed greater formation and stability of RXRalpha homodimers on consensus oligonucleotides in PELP1-overexpressing clones in comparison to the pcDNA clones. The presence of PELP1 in these oligonucleotide-bound RXRalpha homodimers was proved by the supershift of the complex when incubated with PELP1-specific antibody. PELP1-overexpressing stable MCF-7 cells exhibited a significantly higher extent of 9-cis-RA-induced apoptosis than the control pcDNA clones. Silencing of PELP1 expression in parental MCF-7 cells and PELP1-overexpressing clones using PELP1-specific RNA-mediated interference compromised the susceptibility to 9-cis-RA-induced apoptosis. PELP1 could also function as a coactivator of the RXRalpha-peroxisome proliferator-activated receptor (PPARgamma) heterodimer as evident by the peroxisome proliferator-activated receptor response element-luciferase assay in response to both 9-cis-RA and PPARgamma-specific ligands. This was reinforced by the higher propensity of PELP1-overexpressing clones to undergo differentiation in response to PPARgamma-specific ligands. This study has revealed a novel facet of PELP1 functions and identified it to be an important potentiator of the antiproliferative effects of 9-cis-RA and PPARgamma-specific ligands.

Novel mechanisms of resistance to endocrine therapy: genomic and nongenomic considerations

Selective estrogen receptor (ER) modulators have been the most commonly used neoadjuvant therapy for hormone-dependent breast cancer. However, resistance to endocrine therapy, either inherent or acquired during treatment, presents a major challenge in disease management. The causes of resistance to hormone therapy are not well understood and are the subject of active investigation. It is increasingly clear that decreasing sensitivity of ER-positive breast cancer cells to antiestrogens is caused by several factors. Cross talk between ER and growth factor signaling has emerged as a critical factor in endocrine resistance. Here, we present evidence that receptor tyrosine kinase signaling also plays a role in resistance by controlling the subcellular localization of ER signaling components. Localization of ER in either the nuclear or cytoplasmic compartments has functional implications. Recent work suggests that dynein light chain 1, a recently identified substrate of p21-activated kinase 1, modulates ER transactivation functions through a novel ER coactivator function. Likewise, receptor tyrosine kinase signaling can also alter the expression of ER coregulators such as metastasis-associated antigen 1, leading to hormonal independence. Furthermore, proline-, glutamic acid-, leucine-rich protein 1, an ER coactivator involved in both genomic and nongenomic signaling pathways, is activated by epidermal growth factor receptor and plays a prominent role in resistance to tamoxifen. These recent advances suggest new targeted therapeutic approaches that may lead to either reversion or prevention of endocrine resistance in breast tumors.

Altered localization of a coactivator sensitizes breast cancer cells to tumor necrosis factor–induced apoptosis

Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) is a novel coregulator of the estrogen receptor that plays a role in both genomic and nongenomic actions of the estrogen receptor. Emerging studies suggest that in addition to the nuclear localization of PELP1, it is predominantly localized in the cytoplasm in human breast tumors, leading to excessive nongenomic signaling and possibly to tamoxifen resistance. The mechanisms underlying resistance to hormones in preclinical model systems remain under intense investigation. In an effort to develop a model system to treat tumor cells with cytoplasmic PELP1 expression and tamoxifen resistance, here we used the cytokine tumor necrosis factor (TNF)-alpha. We found that clones of MCF-7 human breast cancer cells overexpressing PELP1 in the cytoplasm were distinctly sensitive to TNF-alpha-induced apoptosis than were wild-type nuclear PELP1- and pcDNA vector-expressing clones as revealed by cell growth assay, cell cycle analysis, Annexin V staining, and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay. We also found that the clones with cytoplasmic PELP1 overexpression had significantly less antiapoptotic protein Bcl-2 and nuclear factor-kappaB DNA binding, but increased cyclin E expression, further supporting evidence that these cells are sensitive to apoptosis. The mechanism behind TNF-induced apoptosis in these cells involves caspases, as revealed by poly(ADP-ribose) polymerase cleavage and the broad-spectrum caspase inhibitor Z-VAD-inhibited apoptosis. In conclusion, our results suggest that altered localization of PELP1 promotes heightened sensitivity to TNF-alpha in MCF-7 cells, paving the way for developing new treatment strategies for tumors with cytoplasmic PELP1 expression.

Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) interacts with PELP1 and activates MAPK

PELP1 (proline-, glutamic acid-, and leucine-rich protein-1) (also known as the modulator of nongenomic activity of estrogen receptor) plays a role in genomic functions of the estrogen receptor via histone interactions and in nongenomic functions via its influence on the MAPK-Src pathway. However, recent studies have shown that differential compartmentalization of PELP1 could play a crucial role in modulating the status of nongenomic signaling by using molecular mechanisms that remain poorly understood. Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) is an early endosomal protein that plays a role in regulating the trafficking of growth factor-receptor complexes through early endosomes. By using a yeast two-hybrid screen, we identified HRS as a novel PELP1-binding protein providing evidence of a physiologic interaction between HRS and PELP1. The noted HRS-PELP1 interaction was accompanied by inhibition of the basal coactivator function of PELP1 upon estrogen receptor transactivation. HRS was found to sequester PELP1 in the cytoplasm, leading to the activation of MAPK in a manner that is dependent on the epidermal growth factor receptor but independent of the estrogen receptor, Shc, and Src. In addition, stimulation of MAPK and the subsequent activation of its downstream effector pathway, Elk-1, by HRS or PELP1 were found to depend on the presence of endogenous PELP1 or HRS. Furthermore, HRS was overexpressed and correlated well with the cytoplasmic PELP1, increased MAPK, and EGFR status in breast tumors. These findings highlight a novel role of HRS in up-regulating MAPK, presumably involving interaction with PELP1.

Cloning, expression, and localization of MNAR/PELP1 in rodent brain: colocalization in estrogen receptor-alpha- but not in gonadotropin-releasing hormone-positive neurons

MNAR/PELP1 is a recently identified scaffold protein in the human that modulates the nongenomic activity of estrogen receptors by facilitating linkage/cross talk with the Src/Erk activation cascade. We report herein the cloning of rat MNAR/PELP1 and provide new information concerning its distribution in the female rat brain and its degree of colocalization with estrogen receptor-alpha (ER-alpha) and GnRH. PCR-based cloning of MNAR/PELP1 from rat hypothalamus yielded a transcript of approximately 3.4 kb, which shows 86% homology to the published human MNAR/PELP1 sequence and retained all the key binding motifs (PXXP, LXXLL, and glutamic acid clusters) in its primary structure that are known to be critical for its interaction with Src and steroid receptors. RT-PCR revealed that the MNAR/PELP1 transcript is expressed in many regions of the brain, and immunohistochemistry studies showed intense MNAR/PELP1 immunoreactivity (MNAR/PELP1-ir) in areas such as the hypothalamus, cerebral cortex, hippocampus, amygdala, and cerebellum. MNAR/PELP1-ir principally localized in the nucleus, but some cytoplasmic and plasma membrane-associated staining was also observed. MNAR/PELP1-ir was also primarily neuronal, although some localization in glia cells was observed in select brain regions. Colocalization studies revealed that a majority of ER-alpha-positive cells in the brain colocalized MNAR/PELP1-ir. In contrast, MNAR/PELP1-ir rarely colocalized in GnRH neurons. In conclusion, the current study provides evidence that MNAR/PELP1 is expressed in key neural tissues of the rat brain that are known targets of steroid action, that its expression is primarily neuronal, and that MNAR/PELP1-ir is strongly colocalized in ER-alpha, but not GnRH neurons in the rodent brain.

Functional implications of altered subcellular localization of PELP1 in breast cancer cells

It is increasingly accepted that steroidal receptor co-regulators may also function in the cytoplasmic compartment. Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) is a novel coregulator that plays a role in both the genomic and extranuclear actions of estrogen receptors (ER) in hormonally responsive tissues. In this study using breast tumor arrays, we found that PELP1 was localized only in the cytoplasm in 58% of the PELP1-positive breast tumors. To help explain the significance of the cytoplasmic localization of PELP1 in human breast tumors, we created a mutant protein that was expressed only in the cytoplasm (PELP1-cyto) and then generated a model system wherein MCF-7 breast cancer cells were engineered to specifically express this mutant. We found that PELP1-cyto cells were hypersensitive to estrogen but resistant to tamoxifen. PELP1-cyto cells, but not parental MCF-7 cells, formed xenograft tumors in nude mice. In addition, PELP1-cyto cells exhibited increased association of PELP1 with Src, enhanced mitogen-activated protein kinase (MAPK) activation, and constitutive activation of AKT. The altered localization of PELP1 was sufficient to trigger the interaction of PELP1 with the p85 subunit of phosphatidylinositol-3-kinase (PI3K), leading to PI3K activation. In addition, PELP1 interacted with epidermal growth factor receptors and participated in growth factor-mediated ER transactivation functions. Our results suggest that the altered localization of PELP1 modulates sensitivity to antiestrogens, potentiates tumorigenicity, presumably via the stimulation of extranuclear estrogen responses, such as the activation of MAPK and AKT, and also enhance cross-regulation of ER transactivation activity by growth factors.

Proline-, glutamic acid-, and leucine-rich protein-1 is essential in growth factor regulation of signal transducers and activators of transcription 3 activation

Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) is a novel estrogen receptor coactivator that plays an important role in the genomic and nongenomic actions of estrogen receptor by interacting with histones and src-mitogen-activated protein kinase pathway, respectively. A great deal of information has emerged in recent years about the possible role of PELP1 in estrogen receptor signaling. However, the participation and significance of PELP1 in other cellular signaling pathways remains unknown. Using a yeast two-hybrid screen, we identified PELP1 as a novel interacting protein of signal transducers and activators of transcription 3 (STAT3) and found evidence of physiologic interaction between PELP1 and STAT3. We also found that these interactions played a mechanistic role in the positive regulation of STAT3 transcription from synthetic promoters and endogenous target genes such as cyclin D1, c-myc, and c-fos. Overexpression of PELP1 enhanced phosphorylation of STAT3 at Ser727 in a src-mitogen-activated protein kinase-sensitive manner and, conversely, down-regulation of PELP1 compromised growth factor-mediated induction of STAT3 target genes. We also discovered that PELP1 interacts with STAT3 in the nuclear compartment and down-regulation of PELP1 interfered with the recruitment of STAT3 to its target gene promoters. In summary, our results highlight a novel role for PELP1 in growth factor signaling and indicate that PELP1-mediated genomic and nongenomic functions play a role in the growth factor-mediated STAT3 transactivation functions. Such regulatory interactions of PELP1 may have important functional implications in the cross-talk of estrogen receptor and growth factor signaling.

The clinical relevance of steroid hormone receptor corepressors

Steroid hormone receptors are ligand-dependent transcription factors that control a variety of essential physiologic and developmental processes in humans. The functional activity of a steroid receptor is regulated not only by hormones but also by an array of regulatory proteins such as coactivators, corepressors, and chromatin modifiers. Contrary to an earlier notion that corepressors and coactivators exist in separate complexes, these molecules, which have apparently opposite functions, are increasingly being found in the same complex, which allows for efficient transcriptional control mechanisms. These control mechanisms are in turn regulated by an array of post-translational modifications under the influence of upstream and local signaling networks. Because the outcome of steroidal hormone receptor transcriptional complexes is measured in terms of the expression of target genes, any dysregulation of coregulator complexes perturbs normal homeostasis and could contribute to the development and maintenance of malignant phenotypes. Increasing evidence implicating steroid hormone receptors and their coregulators in various pathophysiologic conditions has elicited interest in their structure and biology. Further advances in this field of study should open up a unique window for novel targeted therapies for diseases such as cancer. Here we briefly review the clinical relevance of corepressors, with a particular focus on their role in the development of cancerous phenotypes.

An essential role of Pak1 phosphorylation of SHARP in Notch signaling

The p21-activated kinases (Paks), an evolutionarily conserved family of serine/threonine kinases, play an important role in cytoskeletal reorganization in mammalian cells. The Notch signaling pathway plays an important role in the determination of cell fate/differentiation in a number of organs. Notch signaling is a complex process, and the mechanism by which Notch regulates multiple cellular processes is intriguing. The expression of both Notch and Pak1 has been shown to be deregulated in several human cancers. Using yeast two-hybrid screening, we identified SHARP, one of the Notch signaling components, as a Pak1-interacting protein. We found that SHARP is a physiologic interacting substrate of Pak1, and that this interaction enhances SHARP-mediated repression of Notch target genes. Pak1 phosphorylation sites in SHARP were mapped to Ser3486 and Thr3568 within the SHARP repression domain. Mutation of Pak1 phosphorylation sites in SHARP, inhibition of Pak1 functions by a Pak1-autoinhibitory fragment (amino acids 83-149), or expression of Pak1-specific siRNA interfered with SHARP-mediated repression of Notch target reporter gene activation. These results demonstrate that Pak1-SHARP interaction plays an essential role in enhancing the corepressor functions of SHARP, thereby modulating Notch signaling in human cancer cells.

p21-activated kinase 1 regulates microtubule dynamics by phosphorylating tubulin cofactor B

p21-activated kinase 1 (Pak1) induces cytoskeleton reorganization in part by regulating microtubule dynamics through an elusive mechanism. Using a yeast two-hybrid screen, we identified tubulin cofactor B (TCoB) (a cofactor in the assembly of the alpha/beta-tubulin heterodimers) as an interacting substrate of Pak1. Pak1 directly phosphorylated TCoB in vitro and in vivo on serines 65 and 128 and colocalized with TCoB on newly polymerized microtubules and on centrosomes. TCoB interacted with the GTPase-binding domain of Pak1 and activated Pak1 in vitro and in vivo. In contrast to wild-type TCoB, an S65A, S128A double mutant and knock-down of the endogenous TCoB or Pak1 reduced microtubule polymerization, suggesting that Pak1 phosphorylation is necessary for normal TCoB function. Overexpression of TCoB dramatically increased the number of gamma-tubulin-containing microtubule-organizing centers, a phenotype reminiscent of cells overexpressing Pak1. TCoB was overexpressed and phosphorylated in breast tumors. These findings reveal a novel role for TCoB and Pak1 in regulating microtubule dynamics.

Pak1 phosphorylation of snail, a master regulator of epithelial-to-mesenchyme transition, modulates snail's subcellular localization and functions

The process of epithelial-mesenchymal transition plays a pivotal role in the conversion of early stage tumors into invasive malignancies, and has been shown to be regulated by the zinc finger phosphoprotein, Snail; however, no upstream signaling kinases have been shown to modulate Snail functions. Since the invasiveness of breast cancer cells is also influenced by p21-activated kinase 1 (Pak1) signaling, we investigated Pak1's potential mechanistic role in the regulation of Snail functions. We found for the first time that Pak1 promotes transcription repression activity of Snail from E-cadherin, occludin, and aromatase promoters. Pak1 regulates the repressor activity of Snail by phosphorylating on Ser(246). Pak1 phosphorylation of Snail supports Snail's accumulation in the nucleus as well as its repressor functions. A Ser(246)Ala substitution in Snail or Pak1 knockdown by short interference RNA blocked Pak1-mediated Snail phosphorylation, leading to increased cytoplasmic accumulation of Snail and attenuation of Snail repressor activity in breast cancer cells. The regulation of phosphorylation and function of Snail by Pak1 represents a novel mechanism by which a signaling kinase might contribute to the process of epithelial-mesenchymal transition.

Novel estrogen receptor coactivator PELP1/MNAR gene and ERβ expression in salivary duct adenocarcinoma: potential therapeutic targets

Salivary duct carcinoma (SDC) is a high-grade neoplasm with marked morphological resemblance to mammary duct carcinoma. The novel estrogen receptor (ER)-interacting protein and the proline-, glutamic acid-, and leucine-rich protein 1 ( PELP1 ), also called the modulator of nongenomic activity of ER ( MNAR ), have been shown to activate steroid hormone receptors in mammary carcinomas by nongenomic and genomic mechanisms. The expression and the relationship of this gene to the ER status in SDCs are unknown. We investigated the differential expression of the PELP1 / MNAR and the ERs alpha and beta proteins in SDCs, using Western blotting and immunohistochemistry. Western blot analysis of 7 paired normal and tumor specimens showed increased expression of PELP1 / MNAR and ER beta in 3 and 4 of the SDCs, respectively. No detectable expression of ER alpha in any normal or SDC specimens was noted. Immunohistochemical staining performed on 70 SDCs revealed strong expression of PELP1 / MNAR in 51 (73%) and ER beta in 52 (74%) tumors. PELP1 / MNAR and ER beta were coexpressed in 35 (50%), individually in 17 (24.2%), and negative in 18 (25.7%) tumors. PELP1 / MNAR staining was predominantly cytoplasmic whereas ER beta staining was nuclear and occasionally cytoplasmic in tumor cells. Our results indicate that PELP1 / MNAR and ER beta are coexpressed in most SDCs and may play a role in the pathobiology of these tumors.

Dynein Light Chain 1 Phosphorylation Controls Macropinocytosis*[boxs]

Recent studies have identified dynein light chain-1 (DLC1), a component of the dynein motor, as a p21-activated kinase 1 (Pak1)-interacting substrate with binding sites mapped to amino acids 61-89 of DLC1 and phosphorylation site at serine 88. Here we investigated the role of DLC1 phosphorylation by Pak1 upon the process of macropinocytosis. We found that Pak1 associates with dynein motor and that Pak1-DLC1 interaction starts at the initiation of pinosome formation and persists in early and late endosomes. Pak1 phosphorylation of DLC1 on Ser-88 controls vesicle formation and trafficking functions, as Ser-88 substitution for alanine prevents macropinocytosis. A peptide spanning the C-terminal 19-amino acid region of DLC1 efficiently blocked Ser-88 phosphorylation and macropinocytosis. These results suggest that the regulation of DLC1 by Pak1 is a novel mechanism by which a signaling kinase might influence macropinocytosis.

Regulation of phosphoglucomutase 1 phosphorylation and activity by a signaling kinase

We have identified a novel mechanism of cross-talk between cell signaling and metabolic pathways, whereby the signaling kinase p21-activated kinase 1 (Pak1) binds to, phosphorylates and enhances the enzymatic activity of phosphoglucomutase 1 (PGM), an important regulatory enzyme in cellular glucose utilization and energy homeostasis. Pak1 and PGM were colocalized in model cell systems and showed functional interactions in a physiological setting. Strong direct interaction of PGM with Pak1 but not Pak2, Pak3, or Pak4 was observed. PGM binding was within 75-149 amino acids (aa) of Pak1, while Pak1 binding to PGM was in the N-terminal 96 aa. Pak1-mediated phosphorylation of PGM selectively on threonine 466 significantly increased PGM enzymatic activity and could be blocked by transfection with a dominant-negative Pak1 expression vector and by Pak1-specific small inhibitory RNA. Stable transfection of PGM into PGM-deficient K562 leukemia cells further demonstrated the role of Pak1 in regulating PGM activity. The results presented here provide new evidence that the cell signaling kinase Pak1 is a novel regulator of glucose metabolism through its phosphorylation and regulation of PGM activity. These findings suggest a new mechanism whereby growth factor signaling may coordinately integrate metabolic regulation with established signaling functions of cell cycle regulation and cell growth.

Potential Role of a Novel Transcriptional Coactivator PELP1 in Histone H1 Displacement in Cancer Cells

The estrogen receptor plays an important role in breast cancer progression. Proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), also called modulator of nongenomic activity of estrogen receptor (MNAR), a novel coactivator of estrogen receptor, modulates estrogen receptor transactivation functions. The mechanisms by which PELP1 modulates estrogen receptor genomic functions is not known. Here, using biochemical and scanning confocal microscopic analysis, we have demonstrated nuclear localization and functional implications of PELP1. Subnuclear fractionation showed PELP1 association with chromatin and nuclear matrix fractions. Ligand stimulation promoted recruitment of PELP1 to 17beta-estradiol responsive promoters, its colocalization with acetylated H3, and increased PELP1-associated histone acetyltransferase enzymatic activity. Far Western analysis revealed that PELP1 interacts with histone 1 and 3, with more preference toward histone 1. Using deletion analysis, we have identified the PELP1 COOH-terminal region as the histone 1 binding site. The PELP1 mutant lacking histone 1-binding domain acts as a dominant-negative and blocks estrogen receptor alpha-mediated transcription. Chromatin immunoprecipitation analysis showed a cyclic association and dissociation of PELP1 with the promoter, with recruitment of histone 1 and PELP1 occurring in opposite phases. PELP1 overexpression increased the micrococcal nuclease sensitivity of estrogen response element-containing nucleosomes. Our results provide novel insights about the transcription regulation of PELP1 and suggest that PELP1 participates in chromatin remodeling activity via displacement of histone 1 in cancer cells.

Metastasis-associated protein 1 interacts with NRIF3, an estrogen-inducible nuclear receptor coregulator

The transcriptional activity of estrogen receptor alpha (ER-alpha) is modified by regulatory action and interactions of coactivators and corepressors. Recent studies have shown that the metastasis-associated protein 1 (MTA1) represses estrogen receptor element (ERE)-driven transcription in breast cancer cells. With a yeast two-hybrid screen to clone MTA1-interacting proteins, we identified a known nuclear receptor coregulator (NRIF3) as an MTA1-binding protein. NRIF3 interacted with MTA1 both in vitro and in vivo. NRIF3 bound to the C-terminal region of MTA1, while MTA1 bound to the N-terminal region of NRIF3, containing one nuclear receptor interaction LXXLL motif. We showed that NRIF3 is an ER coactivator, hyperstimulated ER transactivation functions, and associated with the endogenous ER and its target gene promoter. MTA1 repressed NRIF3-mediated stimulation of ERE-driven transcription and interfered with NRIF3's association with the ER target gene chromatin. In addition, NRIF3 deregulation enhanced the responsiveness of breast cancer cells to estrogen-induced stimulation of growth and anchorage independence. Furthermore, we found that NRIF3 is an estrogen-inducible gene and activated ER associated with the ER response element in the NRIF3 gene promoter. These findings suggest that NRIF3, an MTA1-interacting protein, is an estrogen-inducible gene and that regulatory interactions between MTA1 and NRIF3 might be important in modulating the sensitivity of breast cancer cells to estrogen.

Upstream determinants of estrogen receptor-alpha regulation of metastatic tumor antigen 3 pathway

Although recent studies have shown a role of estrogen receptor-alpha (ER) in the regulation of epithelial-to-mesenchymal transition via MTA3, the role of upstream determinants of ER regulation of MTA3 and the underlying molecular mechanism remains unknown. Here we show that MTA3 gene regulation by ER is influenced by dynamic changes in levels of nuclear coregulators. MTA3 promoter has a functional ER element half-site with which MTA1 and HDACs interact under basal conditions. Upon estrogen stimulation, these corepressors are derecruited with concomitant recruitment of ER, leading to increased MTA3 transcription and expression. Genetic inactivation of MTA1 pathway promotes the ability of ER to up-regulate MTA3 expression, whereas knockdown of ER enhances MTA1 association with MTA3 gene. Modulation of ER functions, by corepressors (i.e. MTA1 and MTA1s) or coactivators (i.e. AIB1 and PELP1/MNAR), alters ER recruitment to MTA3 chromatin, MTA3 transcription, and expression of downstream epithelial-to-mesenchymal transition components. These studies provide novel insights into the transregulation of the MTA3 gene and reveal novel roles of upstream determinants in modifying the outcome of MTA3 axis and cell differentiation.