Significance of PELP1 in ER-negative breast cancer metastasis

Breast cancer metastasis is a major clinical problem. The molecular basis of breast cancer progression to metastasis remains poorly understood. PELP1 is an estrogen receptor (ER) coregulator that has been implicated as a proto-oncogene whose expression is deregulated in metastatic breast tumors and whose expression is retained in ER-negative tumors. We examined the mechanism and significance of PELP1-mediated signaling in ER-negative breast cancer progression using two ER-negative model cells (MDA-MB-231 and 4T1 cells) that stably express PELP1-shRNA. These model cells had reduced PELP1 expression (75% of endogenous levels) and exhibited less propensity to proliferate in growth assays in vitro. PELP1 downregulation substantially affected migration of ER-negative cells in Boyden chamber and invasion assays. Using mechanistic studies, we found that PELP1 modulated expression of several genes involved in the epithelial mesenchymal transition (EMT), including MMPs, SNAIL, TWIST, and ZEB. In addition, PELP1 knockdown reduced the in vivo metastatic potential of ER-negative breast cancer cells and significantly reduced lung metastatic nodules in a xenograft assay. These results implicate PELP1 as having a role in ER-negative breast cancer metastasis, reveal novel mechanism of coregulator regulation of metastasis via promoting cell motility/EMT by modulating expression of genes, and suggest PELP1 may be a potential therapeutic target for metastatic ER-negative breast cancer.     

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.     

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 ERRα, 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.     

Protein Kinase A (PKA) Phosphorylation of Shp2 Protein Inhibits Its Phosphatase Activity and Modulates Ligand Specificity

Pathological cardiac hypertrophy (an increase in cardiac mass resulting from stress-induced cardiac myocyte growth) is a major factor underlying heart failure. Src homology 2 domain-containing phosphatase (Shp2) is critical for cardiac function because mutations resulting in loss of Shp2 catalytic activity are associated with congenital cardiac defects and hypertrophy. We identified a novel mechanism of Shp2 inhibition that may promote cardiac hypertrophy. We demonstrate that Shp2 is a component of the protein kinase A anchoring protein (AKAP)-Lbc complex. AKAP-Lbc facilitates PKA phosphorylation of Shp2, which inhibits Shp2 phosphatase activity. We identified two key amino acids in Shp2 that are phosphorylated by PKA. Thr-73 contributes a helix cap to helix αB within the N-terminal SH2 domain of Shp2, whereas Ser-189 occupies an equivalent position within the C-terminal SH2 domain. Utilizing double mutant PKA phosphodeficient (T73A/S189A) and phosphomimetic (T73D/S189D) constructs, in vitro binding assays, and phosphatase activity assays, we demonstrate that phosphorylation of these residues disrupts Shp2 interaction with tyrosine-phosphorylated ligands and inhibits its protein-tyrosine phosphatase activity. Overall, our data indicate that AKAP-Lbc integrates PKA and Shp2 signaling in the heart and that AKAP-Lbc-associated Shp2 activity is reduced in hypertrophic hearts in response to chronic β-adrenergic stimulation and PKA activation. Therefore, although induction of cardiac hypertrophy is a multifaceted process, inhibition of Shp2 activity through AKAP-Lbc-anchored PKA is a previously unrecognized mechanism that may promote this compensatory response.     

Two chimeric receptors of epidermal growth factor receptor and c-Ros that differ in their transmembrane domains have opposite effects on cell growth.

Two chimeric receptors, ER1 and ER2, were constructed. ER1 contains the extracellular and transmembrane (TM) domains derived from epidermal growth factor receptor and the cytoplasmic domain from c-Ros; ER2 is identical to ER1 except that its TM domain is derived from c-Ros. Both chimeras can be activated by epidermal growth factor and are capable of activating or phosphorylating an array of cellular signaling proteins. Both chimeras promote colony formation in soft agar with about equal efficiency. Surprisingly, ER1 inhibits while ER2 stimulates cell growth on monolayer culture. Cell cycle analysis revealed that all phases, in particular the S and G2/M phases, of the cell cycle in ER1 cells were elongated whereas G1 phase of ER2 cells was shortened threefold. Comparison of signaling pathways mediated by the two chimeras revealed several differences. Several early signaling proteins are activated or phosphorylated to a higher extent in ER1 than in ER2 cells in response to epidermal growth factor. ER1 is less efficiently internalized and remains tyrosine phosphorylated for a longer time than ER2. However, phosphorylation of the 66-kDa She protein, activation of mitogen activated protein kinase, and induction of c-fos and c-jun occur either to a lesser extent or for a shorter time in ER1 cells. Cellular protein phosphorylation patterns are also different in ER1 and ER2 cells. In particular, a 190-kDa Shc-associated protein is tyrosine phosphorylated in ER2 but not in ER1 cells. Our results indicate that the TM domains have a profound effect on the signal transduction and biological activity of those chimeric receptors. The results also imply that sustained stimulation of ER1 due to its retarded internalization apparently triggers an inhibitory response that dominantly counteracts the receptor-mediated mitogenic signals. These two chimeras, expressed at similar levels in the same cell type but having opposite effects on cell growth, provide an ideal system to study the mechanism by which a protein tyrosine kinase inhibits cell growth.     

Estrogen receptor signaling pathways in human non-small cell lung cancer

Lung cancer is the most common cause of cancer mortality in male and female patients in the US. The etiology of non-small cell lung cancer (NSCLC) is not fully defined, but new data suggest that estrogens and growth factors promote tumor progression. In this work, we confirm that estrogen receptors (ER), both ERalpha and ERbeta, occur in significant proportions of archival NSCLC specimens from the clinic, with receptor expression in tumor cell nuclei and in extranuclear sites. Further, ERalpha in tumor nuclei was present in activated forms as assessed by detection of ER phosphorylation at serines-118 and -167, residues commonly modulated by growth factor receptor as well as steroid signaling. In experiments using small interfering RNA (siRNA) constructs, we find that suppressing expression of either ERalpha or ERbeta elicits a significant reduction in NSCLC cell proliferation in vitro. Estrogen signaling in NSCLC cells may also include steroid receptor coactivators (SRC), as SRC-3 and MNAR/PELP1 are both expressed in several lung cell lines, and both EGF and estradiol elicit serine phosphorylation of SRC-3 in vitro. EGFR and ER also cooperate in promoting early activation of p42/p44 MAP kinase in NSCLC cells. To assess new strategies to block NSCLC growth, we used Faslodex alone and with erlotinib, an EGFR kinase inhibitor. The drug tandem elicited enhanced blockade of the growth of NSCLC xenografts in vivo, and antitumor activity exceeded that of either agent given alone. The potential for use of antiestrogens alone and with growth factor receptor antagonists is now being pursued further in clinical trials.     

Estrogens Correlate with PELP1 Expression in ER Positive Breast Cancer

The Proline-, glutamic acid- and leucine-rich protein 1 (PELP1) is an estrogen receptor (ER) coactivator and a proto-oncogene known to be deregulated in endocrine cancers. In breast cancer, PELP1 overexpression has been associated with endocrine therapy resistance. Although PELP1 is known to be regulated by estrogens in vitro, its association with estrogen levels within the tissue of breast cancer patients has not previously been assessed. Here, we determined PELP1 mRNA expression levels in paired samples of normal and malignant breast tissue obtained from 32 postmenopausal and 11 premenopausal women. In the total sample set, PELP1 levels were higher in tumors compared to normal breast tissue (P = 0.041). Among postmenopausal women, PELP1 tumor levels correlated positively with estrone (E₁) and estradiol (E₂) levels in both normal tissue (r = 0.543, P = 0.003 and r = 0.601, P = 0.001, respectively) and plasma (r = 0.392, P = 0.053 and r = 0.403, P = 0.046, respectively). Analyzing all ER+ tumors (n = 26), PELP1 correlated positively with E₁ and E₂ in tumor tissue (r = 0.562, P = 0.003 and r = 0.411, P = 0.037, respectively) and normal tissue (r = 0.461, P = 0.018 and r = 0.427, P = 0.030, respectively) in addition to plasma E₁, E₂ and estrone sulphate (E₁S) concentrations (r = 0.576, P = 0.003, r = 0.456, P = 0.025 and r = 0.406, P = 0.049, respectively). Finally, PELP1 correlated positively with ER mRNA (ESR1) (r = 0.553, P = 0.026) in ER+ tumors, whereas a negative association between PELP1 and ESR1 (r = -0.733, P = 0.010) was observed in ER- breast tumors. Taken together, tumor PELP1 mRNA expression is associated with estrogen levels in breast cancer, suggesting a potentially important role of PELP1 in ER+ breast cancer growth in vivo.     

Direct modulation of the protein kinase A catalytic subunit α by growth factor receptor tyrosine kinases

The cyclic-AMP-dependent protein kinase A (PKA) regulates processes such as cell proliferation and migration following activation of growth factor receptor tyrosine kinases (RTKs), yet the signaling mechanisms that link PKA with growth factor receptors remain largely undefined. Here we report that RTKs can directly modulate the function of the catalytic subunit of PKA (PKA-C) through post-translational modification. In vitro kinase assays revealed that both the epidermal growth factor and platelet derived growth factor receptors (EGFR and PDGFR, respectively) tyrosine phosphorylate PKA-C. Mass spectrometry identified tyrosine 330 (Y330) as a receptor-mediated phosphorylation site and mutation of Y330 to phenylalanine (Y330F) all but abolished the RTK-mediated phosphorylation of PKA-C in vitro. Y330 resides within a conserved region at the C-terminal tail of PKA-C that allosterically regulates enzymatic activity. Therefore, the effect of phosphorylation at Y330 on the activity of PKA-C was investigated. The K(m) for a peptide substrate was markedly decreased when PKA-C subunits were tyrosine phosphorylated by the receptors as compared to un-phosphorylated controls. Importantly, tyrosine-phosphorylated PKA-C subunits were detected in cells stimulated with EGF, PDGF, and Fibroblast growth factor 2 (FGF2) and in fibroblasts undergoing PDGF-mediated chemotaxis. These results demonstrate a direct, functional interaction between RTKs and PKA-C and identify tyrosine phosphorylation as a novel mechanism for regulating PKA activity.     

Cytoplasmic PELP1 and ERRgamma Protect Human Mammary Epithelial Cells from Tam-Induced Cell Death

Tamoxifen (Tam) is the only FDA-approved chemoprevention agent for pre-menopausal women at high risk for developing breast cancer. While Tam reduces a woman''s risk of developing estrogen receptor positive (ER+) breast cancer, the molecular mechanisms associated with risk reduction are poorly understood. Prior studies have shown that cytoplasmic proline, glutamic acid and leucine rich protein 1 (PELP1) promotes Tam resistance in breast cancer cell lines. Herein, we tested for PELP1 localization in breast epithelial cells from women at high risk for developing breast cancer and found that PELP1 was localized to the cytoplasm in 36% of samples. In vitro, immortalized HMECs expressing a nuclear localization signal (NLS) mutant of PELP1 (PELP1-cyto) were resistant to Tam-induced death. Furthermore, PELP1-cyto signaling through estrogen-related receptor gamma (ERRγ) promoted cell survival in the presence of Tam. Overexpression of ERRγ in immortalized HMECs protected cells from Tam-induced death, while knockdown of ERRγ sensitized PELP1-cyto expressing HMECs to Tam. Moreover, Tam-induced HMEC cell death was independent of apoptosis and involved accumulation of the autophagy marker LC3-II. Expression of PELP1-cyto and ERRγ reduced Tam-induced LC3-II accumulation, and knockdown of ERRγ increased LC3-II levels in response to Tam. Additionally, PELP1-cyto expression led to the upregulation of MMP-3 and MAOB, known PELP1 and ERRγ target genes, respectively. Our data indicate that cytoplasmic PELP1 induces signaling pathways that converge on ERRγ to promote cell survival in the presence of Tam. These data suggest that PELP1 localization and/or ERRγ activation could be developed as tissue biomarkers for Tam responsiveness.     

Cyclic AMP promotes neuronal survival by phosphorylation of glycogen synthase kinase 3beta

Agents that elevate intracellular cyclic AMP (cAMP) levels promote neuronal survival in a manner independent of neurotrophic factors. Inhibitors of phosphatidylinositol 3 kinase and dominant-inactive mutants of the protein kinase Akt do not block the survival effects of cAMP, suggesting that another signaling pathway is involved. In this report, we demonstrate that elevation of intracellular cAMP levels in rat cerebellar granule neurons leads to phosphorylation and inhibition of glycogen synthase kinase 3beta (GSK-3beta). The increased phosphorylation of GSK-3beta by protein kinase A (PKA) occurs at serine 9, the same site phosphorylated by Akt. Purified PKA is able to phosphorylate recombinant GSK-3beta in vitro. Inhibitors of GSK-3 block apoptosis in these neurons, and transfection of neurons with a GSK-3beta mutant that cannot be phosphorylated interferes with the prosurvival effects of cAMP. These data suggest that activated PKA directly phosphorylates GSK-3beta and inhibits its apoptotic activity in neurons.