Breast cancer is the disease with the highest impact on global health, being metastasis the main cause of death. To metastasize, carcinoma cells must reactivate a latent program called epithelial-mesenchymal transition (EMT), through which epithelial cancer cells acquire mesenchymal-like traits.Glypican-3 (GPC3), a proteoglycan involved in the regulation of proliferation and survival, has been associated with cancer. In this study we observed that the expression of GPC3 is opposite to the invasive/metastatic ability of Hs578T, MDA-MB231, ZR-75-1 and MCF-7 human breast cancer cell lines. GPC3 silencing activated growth, cell death resistance, migration, and invasive/metastatic capacity of MCF-7 cancer cells, while GPC3 overexpression inhibited these properties in MDA-MB231 tumor cell line. Moreover, silencing of GPC3 deepened the MCF-7 breast cancer cells mesenchymal characteristics, decreasing the expression of the epithelial marker E-Cadherin. On the other side, GPC3 overexpression induced the mesenchymal-epithelial transition (MET) of MDA-MB231 breast cancer cells, which re-expressed E-Cadherin and reduced the expression of vimentin and N-Cadherin. While GPC3 inhibited the canonical Wnt/β-Catenin pathway in the breast cancer cells, this inhibition did not have effect on E-Cadherin expression. We demonstrated that the transcriptional repressor of E-Cadherin - ZEB1 - is upregulated in GPC3 silenced MCF-7 cells, while it is downregulated when GPC3 was overexpressed in MDA-MB231 cells. We presented experimental evidences showing that GPC3 induces the E-Cadherin re-expression in MDA-MB231 cells through the downregulation of ZEB1.Our data indicate that GPC3 is an important regulator of EMT in breast cancer, and a potential target for procedures against breast cancer metastasis.

SUMMARY ZEB1 transcription factor is important in both development and disease, including many TGFβ-induced responses, and the epithelial-to-mesenchymal transition (EMT) by which many tumors undergo metastasis. ZEB1 is differentially phosphorylated in different cell types; however the role of phosphorylation in ZEB1 activity is unknown. Luciferase reporter studies and electrophoresis mobility shift assays (EMSA) show that a decrease in phosphorylation of ZEB1 increases both DNA-binding and transcriptional repression of ZEB1 target genes. Functional analysis of ZEB1 phosphorylation site mutants near the second zinc finger domain (termed ZD2) show that increased phosphorylation (due to either PMA plus ionomycin, or IGF-1) can inhibit transcriptional repression by either a ZEB1-ZD2 domain clone, or full-length ZEB1. This approach identifies phosphosites that have a substantial effect regulating the transcriptional and DNA-binding activity of ZEB1. Immunoprecipitation with anti-ZEB1 antibodies followed by western analysis with a phospho-Threonine-Proline-specific antibody indicates that the ERK consensus site at Thr-867 is phosphorylated in ZEB1. In addition to disrupting in vitro DNA-binding measured by EMSA, IGF-1-induced MEK/ERK phosphorylation is sufficient to disrupt nuclear localization of GFP-ZEB1 fusion clones. These data suggest that phosphorylation of ZEB1 integrates TGFβ signaling with other signaling pathways such as IGF-1. This article is protected by copyright. All rights reserved.

ZEB1 is a master regulator of the Epithelial-to-Mesenchymal Transition (EMT) program. While extensive evidence confirmed the importance of ZEB1 as an EMT transcription factor that promotes tumor invasiveness and metastasis, little is known about its regulation. In this work, we screened for potential regulatory links between ZEB1 and multiple cellular kinases. Exploratory in silico analysis aided by phospho-substrate antibodies and ZEB1 deletion mutants led us to identify several potential phospho-sites for the family of PKC kinases in the N-terminus of ZEB1. The analysis of breast cancer cell lines panels with different degrees of aggressiveness, together with the evaluation of a battery of kinase inhibitors, allowed us to expose a robust correlation between ZEB1 and PKCα both at mRNA and protein levels. Subsequent validation experiments using siRNAs against PKCα revealed that its knockdown leads to a concomitant decrease in ZEB1 levels, while ZEB1 knockdown had no impact on PKCα levels. Remarkably, PKCα-mediated downregulation of ZEB1 recapitulates the inhibition of mesenchymal phenotypes, including inhibition in cell migration and invasiveness. These findings were extended to an in vivo model, by demonstrating that the stable knockdown of PKCα using lentiviral shRNAs markedly impaired the metastatic potential of MDA-MB-231 breast cancer cells. Taken together, our findings unveil an unforeseen regulatory pathway comprising PKCα and ZEB1 that promotes the activation of the EMT in breast cancer cells.

Translesion DNA synthesis (TLS) and homologous recombination (HR) cooperate during S-phase to safeguard replication forks integrity. Thus, the inhibition of TLS becomes a promising point of therapeutic intervention in HR-deficient cancers, where TLS impairment might trigger synthetic lethality (SL). The main limitation to test this hypothesis is the current lack of selective pharmacological inhibitors of TLS. Herein, we developed a miniaturized screening assay to identify inhibitors of PCNA ubiquitylation, a key post-translational modification required for efficient TLS activation. After screening a library of 627 kinase inhibitors, we found that targeting the pro-survival kinase AKT leads to strong impairment of PCNA ubiquitylation. Mechanistically, we found that AKT-mediated modulation of Proliferating Cell Nuclear Antigen (PCNA) ubiquitylation after UV requires the upstream activity of DNA PKcs, without affecting PCNA ubiquitylation levels in unperturbed cells. Moreover, we confirmed that persistent AKT inhibition blocks the recruitment of TLS polymerases to sites of DNA damage and impairs DNA replication forks processivity after UV irradiation, leading to increased DNA replication stress and cell death. Remarkably, when we compared the differential survival of HR-proficient vs HR-deficient cells, we found that the combination of UV irradiation and AKT inhibition leads to robust SL induction in HR-deficient cells. We link this phenotype to AKT ability to inhibit PCNA ubiquitylation, since the targeted knockdown of PCNA E3-ligase (RAD18) and a non-ubiquitylable (PCNA K164R) knock-in model recapitulate the observed SL induction. Collectively, this work identifies AKT as a novel regulator of PCNA ubiquitylation and provides the proof-of-concept of inhibiting TLS as a therapeutic approach to selectively kill HR-deficient cells submitted to replication stress.

Background: ZEB1 (Zn Finger E-box Binding Homeobox) transcription factor is important in both development and disease, including the TGFβ-induced epithelial-mesenchymal transition (EMT) by which many tumors undergo metastasis. ZEB1 contains 2 Zn finger domains (ZD1-ZD2) which bind DNA independently. ZEB1 expresses as two phosphorylated forms important for its biological role. We had characterized the role of phosphorylated C-term ZEB1. Aim: Our goal is to uncover the role of signaling pathway/s in the phosphorylation on N-term ZD1 (N-ZEB1) and characterize the functional role of N-ZEB1 in the process of EMT in mammary epithelial cells NMuMG. Results: A 728 aa fragment of N-ZEB1 was overexpressed in HEK293T cells and Immunoprecipitation with N-ZEB1 antibodies followed by western blot analysis with anti phospho kinases (AKT, MEK/ERK, PKC or PKA) substrate Antibodies. The results revealed that N-ZEB1 is solely phosphorylated by PI3K (AKT), MAPK and PKC pathways. IGF1-mediated phosphorylation can regulate nuclear localization of C-term ZEB1. Two N-ZEB1-GFP clones harboring first 490aa (eGFPZ1) and a 2nd clone of 130 aa, eGFPZ2 (included in the former) were transfected to CHO-K1 cells and treated with inhibitors and activators of kinase pathways for 15-60 minutes in serum free medium. Fluorescence microscopy techniques showed that IGF-1 induced relocation of a nuclear eGFPZ1 into the cytosol of CHO-K1 cells; on the contrary, eGFPZ2 was unresponsive to IGF-1. PMA/IONO (activator of PKC) treatment relocated both clones in the cytosol. Later on, NMuMG cells were used to investigate the role of N-ZEB1 in EMT. These epithelial cells, that can turn into mesenchymal type under TGFβ treatment, were stably transfected with eGFPZ2 or empty vector (control). The epithelial markers E-cadherin and actin assessed by Western blot and confocal microscopy were diminished or absent in eGFPZ2 cells. The activity of Matrix Metallo Proteases 2 was also increased in NMuMG-GFPZ2 cells. Also, proliferation, migration and invasion were significantly increased (P<0.001, P<0.001 and P<0.05, respectively) in NMuMG-GFPZ2 cells vs. controls. Taking together, these results show that the cells were induced to EMT by N-ZEB1. Conclusions: We have identified a small fragment in N-ZEB1 that is able to elicit the EMT program in a TGFβ independent manner. In addition, changes in ZEB1 cellular localization induced by either PKC or IGF-1 could regulate this transcription factor biological activity. Citation Format: M Candelaria Llorens, Ana M. Cabanillas. Role of N- ZEB1 in epithelial-mesenchymal transition (EMT). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1988. doi:10.1158/1538-7445.AM2015-1988