MUC1-C oncoprotein as a target in breast cancer: activation of signaling pathways and therapeutic approaches

MUC1-C oncoprotein activates the ZEB1/miR-200c regulatory loop and epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) is activated in cancer cells by ZEB1, a member of the zinc finger/homeodomain family of transcriptional repressors. The mucin 1 (MUC1) heterodimeric protein is aberrantly overexpressed in human carcinoma cells. The present studies in breast cancer cells demonstrate that the oncogenic MUC1-C subunit induces expression of ZEB1 by a NF-κB (nuclear factor kappa B) p65-dependent mechanism. MUC1-C occupies the ZEB1 promoter with NF-κB p65 and thereby promotes ZEB1 transcription. In turn, ZEB1 associates with MUC1-C and the ZEB1/MUC1-C complex contributes to the transcriptional suppression of miR-200c, an inducer of epithelial differentiation. The co-ordinate upregulation of ZEB1 and suppression of miR-200c has been linked to the induction of EMT. In concert with the effects of MUC1-C on ZEB1 and miR-200c, we show that MUC1-C induces EMT and cellular invasion by a ZEB1-mediated mechanism. These findings indicate that (i) MUC1-C activates ZEB1 and suppresses miR-200c with the induction of EMT and (ii) targeting MUC1-C could be an effective approach for the treatment of breast and possibly other types of cancers that develop EMT properties.

Oncogenic MUC1-C promotes tamoxifen resistance in human breast cancer

Tamoxifen resistance of estrogen receptor-positive (ER+) breast cancer cells has been linked in part to activation of receptor tyrosine kinases, such as HER2, and the PI3K-AKT pathway. Mucin 1 (MUC1) is aberrantly overexpressed in about 90% of human breast cancers, and the oncogenic MUC1-C subunit is associated with ERα. The present studies using HER2 overexpressing BT-474 breast cancer cells, which are constitutively resistant to tamoxifen, demonstrate that silencing MUC1-C is associated with (i) downregulation of p-HER2 and (ii) sensitivity to tamoxifen-induced growth inhibition and loss of clonogenic survival. In contrast, overexpression of MUC1-C in tamoxifen-sensitive MCF-7 breast cancer cells resulted in upregulation of p-AKT and tamoxifen resistance. We show that MUC1-C forms complexes with ERα on the estrogen-responsive promoter of Rab31 and that MUC1-C blocks tamoxifen-induced decreases in ERα occupancy. MUC1-C also attenuated tamoxifen-induced decreases in (i) recruitment of the coactivator CREB binding protein, (ii) Rab31 promoter activation, and (iii) Rab31 mRNA and protein levels. The importance of MUC1-C is further supported by the demonstration that targeting MUC1-C with the cell-penetrating peptide inhibitor, GO-203, sensitized tamoxifen-resistant cells to tamoxifen treatment. Moreover, we show that targeting MUC1-C in combination with tamoxifen is highly synergistic in the treatment of tamoxifen-resistant breast cancer cells. Combined, these findings indicate that MUC1-C contributes to tamoxifen resistance.

MUC1 and metastatic cancer: expression, function and therapeutic targeting

MUC1 is a transmembrane mucin that is often overexpressed in metastatic cancers and often used as a diagnostic marker for metastatic progression. The extracellular domain of MUC1 can serve as a ligand for stromal and endothelial cell adhesion receptors, and the cytoplasmic domain engages in several interactions that can result in increased migration and invasion, as well as survival. In this review, we address the role of MUC1 in metastatic progression by assessing clinical studies reporting MUC1 levels at various disease stages, reviewing mouse models utilized to study the role of MUC1 in metastatic progression, discuss mechanisms of MUC1 upregulation, and detail MUC1 protein interactions and signaling events. We review interactions between MUC1 and the extracellular environment, with proteins colocalized in the plasma membrane and/or cytoplasmic proteins, and summarize the role of MUC1 in the nucleus as a transcriptional cofactor. Finally, we review recent publications describing current therapies targeting MUC1 in patients with advanced disease and the stage of these therapies in preclinical development or clinical trials.

Cellular and molecular biology of airway mucins

Airway mucus constitutes a thin layer of airway surface liquid with component macromolecules that covers the luminal surface of the respiratory tract. The major function of mucus is to protect the lungs through mucociliary clearance of inhaled foreign particles and noxious chemicals. Mucus is comprised of water, ions, mucin glycoproteins, and a variety of other macromolecules, some of which possess anti-microbial, anti-protease, and anti-oxidant activities. Mucins comprise the major protein component of mucus and exist as secreted and cell-associated glycoproteins. Secreted, gel-forming mucins are mainly responsible for the viscoelastic property of mucus, which is crucial for effective mucociliary clearance. Cell-associated mucins shield the epithelial surface from pathogens through their extracellular domains and regulate intracellular signaling through their cytoplasmic regions. However, neither the exact structures of mucin glycoproteins, nor the manner through which their expression is regulated, are completely understood. This chapter reviews what is currently known about the cellular and molecular properties of airway mucins.

Expression of underglycosylated MUC1 antigen in cancerous and adjacent normal breast tissues

INTRODUCTION

Mucin 1 antigen (MUC1) is a high-molecular-weight transmembrane glycoprotein with an aberrant expression profile in various malignancies, including breast cancer. Its increased overexpression and underglycosylation in breast cancer is associated with tumor invasiveness and metastatic potential. In this study, we took the next step toward establishing MUC1 as a potential diagnostic, prognostic, and therapeutic target by investigating its expression and posttranslational modification (glycosylation/sialylation).

PATIENTS AND METHODS

In these studies we used a breast cancer tissue microarray (TMA) and fresh-frozen multistage breast cancer tissues. We analyzed in detail the expression of normal and underglycosylated/sialated MUC1 by immunohistochemical techniques, real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR), and various analytic techniques.

RESULTS

We found that changes in cellular localization as well as in upregulation and/or underglycosylation of MUC1 were associated with higher tumor grade. A key finding in this study was that underglycosylated MUC1 (uMUC1) overexpression and sialation were observed in tissues adjacent to tumor but identified as normal on pathology reports.

CONCLUSIONS

These findings suggest that uMUC1 can indeed be used as an early diagnostic marker and provide additional insights into breast cancer management.

Inhibition of the MUC1-C oncoprotein is synergistic with cytotoxic agents in the treatment of breast cancer cells

Mucin 1 (MUC1) is a heterodimeric glycoprotein that is aberrantly overexpressed in most human breast cancers. The oncogenic MUC1-C subunit promotes survival and blocks the apoptotic response to genotoxic anticancer agents. In the present studies, human MCF-7 and ZR-75-1 breast cancer cells were treated with the MUC1-C inhibitor, GO-203, a cell-penetrating peptide that blocks MUC1-C homodimerization and thereby its oncogenic function. Treatment with GO-203 was found to promote the apoptotic response of MCF-7 and ZR-75-1 cells to the therapeutic drugs taxol and doxorubicin (DOX). This effect was (1) attenuated by a pan-caspase inhibitor, and (2) mediated, at least in part, by activation of the effector caspase-7 and cleavage of the downstream substrate PARP. Further analysis of the interaction between GO-203 and taxol using isobolograms, which evaluate the nature of the interaction of two drugs, demonstrated that the combination is highly synergistic. These results were supported by combination index (CI) analysis with values of less than 1. GO-203 was also highly synergistic with DOX in studies of both MCF-7 and ZR-75-1 breast cancer cells. These findings indicate that blocking MUC1-C function could be effective in combination with taxol and DOX for the treatment of breast cancer.

Cooperative interaction between the MUC1-C oncoprotein and the Rab31 GTPase in estrogen receptor-positive breast cancer cells

Rab31 is a member of the Ras superfamily of small GTPases that has been linked to poor outcomes in patients with breast cancer. The MUC1-C oncoprotein is aberrantly overexpressed in most human breast cancers and also confers a poor prognosis. The present results demonstrate that MUC1-C induces Rab31 expression in estrogen receptor positive (ER+) breast cancer cells. We show that MUC1-C forms a complex with estrogen receptor α (ERα) on the Rab31 promoter and activates Rab31 gene transcription in an estrogen-dependent manner. In turn, Rab31 contributes to the upregulation of MUC1-C abundance in breast cancer cells by attenuating degradation of MUC1-C in lysosomes. Expression of an inactive Rab31(S20N) mutant in nonmalignant breast epithelial cells confirmed that Rab31 regulates MUC1-C expression. The functional significance of the MUC1-C/Rab31 interaction is supported by the demonstration that Rab31 confers the formation of mammospheres by a MUC1-C-dependent mechanism. Analysis of microarray databases further showed that (i) Rab31 is expressed at higher levels in breast cancers as compared to that in normal breast tissues, (ii) MUC1+ and ER+ breast cancers have increased levels of Rab31 expression, and (iii) patients with Rab31-positive breast tumors have a significantly decreased ten-year overall survival as compared to those with Rab31-negative tumors. These findings indicate that MUC1-C and Rab31 function in an autoinductive loop that contributes to overexpression of MUC1-C in breast cancer cells.