For further reading: key research

Chronic myelogenous leukemia (CML) is caused by expression of the Bcr-Abl fusion protein in hematopoietic stem cells. The MUC1-C oncoprotein is expressed in CML blasts and stabilizes Bcr-Abl. The present studies demonstrate that treatment of KU812 and K562 CML cells with GO-201, a cell-penetrating peptide inhibitor of MUC1-C oligomerization, downregulates Bcr-Abl expression and inhibits cell growth. In concert with decreases in Bcr-Abl levels, KU812 and K562 cells responded to GO-201 with induction of a differentiated myeloid phenotype as evidenced by increased expression of CD11b, CD11c and CD14. The results also show that the GO-201-treated cells undergo a late apoptotic/necrotic response, consistent with induction of terminal differentiation. Primary CML blasts expressing MUC1 similarly responded to GO-201 with induction of a more differentiated phenotype and late apoptosis/necrosis. In addition, treatment of KU812 xenografts in nude mice was associated with upregulation of CD11 and tumor regression. These findings indicate that CML blasts respond to targeting of the MUC1-C oncoprotein with induction of terminal differentiation.

The MUC1 C-terminal transmembrane subunit (MUC1-C) oncoprotein is a direct activator of the canonical nuclear factor-kB (NF-kB) RelA/p65 pathway and is aberrantly expressed in human multiple myeloma cells. However, it is not known whether multiple myeloma cells are sensitive to the disruption of MUC1-C function for survival. The present studies demonstrate that peptide inhibitors of MUC1-C oligomerization block growth of human multiple myeloma cells in vitro. Inhibition of MUC1-C function also blocked the interaction between MUC1-C and NF-kB p65 and activation of the NF-kB pathway. In addition, inhibition of MUC1-C in multiple myeloma cells was associated with activation of the intrinsic apoptotic pathway and induction of late apoptosis/necrosis. Primary multiple myeloma cells, but not normal B-cells, were also sensitive to MUC1-C inhibition. Significantly, treatment of established U266 multiple myeloma xenografts growing in nude mice with a lead candidate MUC1-C inhibitor resulted in complete tumor regression and lack of recurrence. These findings indicate that multiple myeloma cells are dependent on intact MUC1-C function for constitutive activation of the canonical NF-kB pathway and for their growth and survival.

Epithelia are protected from adverse conditions by a mucous barrier. The secreted and transmembrane mucins that constitute the mucous barrier are largely unrecognized as effectors of carcinogenesis. However, both types of mucins are intimately involved in inflammation and cancer. Moreover, diverse human malignancies overexpress transmembrane mucins to exploit their role in signalling cell growth and survival. Mucins have thus been identified as markers of adverse prognosis and as attractive therapeutic targets. Notably, the findings that certain transmembrane mucins induce transformation and promote tumour progression have provided the experimental basis for demonstrating that inhibitors of their function are effective as anti-tumour agents in preclinical models.

The mucin 1 (MUC1) oncoprotein is aberrantly overexpressed by 90% of human breast cancers. However, there are no effective agents that directly inhibit MUC1 and induce death of breast cancer cells. We have synthesized a MUC1 inhibitor (called GO-201) that binds to the MUC1 cytoplasmic domain and blocks the formation of MUC1 oligomers in cells. GO-201, and not an altered version, attenuates targeting of MUC1 to the nucleus of human breast cancer cells, disrupts redox balance, and activates the DNA damage response. GO-201 also arrests growth and induces necrotic death. By contrast, the MUC1 inhibitor has no effect on cells null for MUC1 expression or nonmalignant mammary epithelial cells. Administration of GO-201 to nude mice bearing human breast tumor xenografts was associated with loss of tumorigenicity and extensive necrosis, which results in prolonged regression of tumor growth. These findings show that targeting the MUC1 oncoprotein is effective in inducing death of human breast cancer cells in vitro and in tumor models.

The MUC1 oncoprotein is aberrantly expressed at high levels in most human carcinomas and certain hematologic malignancies. Estimates are that, of the 1.4 million tumors diagnosed each year in the US, about 900,000 have overexpression of MUC1. MUC1 has thus become a highly attractive target for the development of new anti-cancer agents, including vaccines, antibodies and small molecules. However, there are presently no approved agents that target MUC1, in part because there has been limited information about how MUC1 contributes to the malignant phenotype. Indeed, MUC1 consists of two subunits, and the more recent focus of research on the MUC1 receptor subunit has provided new insights into (i) how MUC1 induces transformation and is of importance to human cancers, and (ii) how to target MUC1 function as an approach for anti-cancer treatment.

Nuclear factor-kB (NF-kB) is constitutively activated in diversehuman malignancies. The mucin 1 (MUC1) oncoprotein is overexpressedin human carcinomas and, like NF-kB, blocks cell death and inducestransformation. The present studies show that MUC1 constitutivelyassociates with NF-kB p65 in carcinoma cells. The MUC1 COOH-terminalsubunit (MUC1-C) cytoplasmic domain binds directly to NF-kB p65and, importantly, blocks the interaction between NF-kB p65 andits inhibitor IkBx. We show that NF-kB p65 and MUC1-C constitutivelyoccupy the promoter of the Bcl-xL gene in carcinoma cells andthat MUC1-C contributes to NF-kB–mediated transcriptionalactivation. Studies in nonmalignant epithelial cells show thatMUC1-C interacts with NF-kB in the response to tumor necrosisfactor-x stimulation. Moreover, tumor necrosis factor-x inducesthe recruitment of NF-kB p65-MUC1-C complexes to NF-kB targetgenes, including the promoter of the MUC1 gene itself. We alsoshow that an inhibitor of MUC1-C oligomerization blocks theinteraction with NF-kB p65 in vitro and in cells. The MUC1-Cinhibitor decreases MUC1-C and NF-kB p65 promoter occupancy andexpression of NF-kB target genes. These findings indicate thatMUC1-C is a direct activator of NF-kB p65 and that an inhibitorof MUC1 function is effective in blocking activation of the NF-kB pathway.

The Mucin 1 (MUC1) oncoprotein is aberrantly overexpressed in diverse human malignancies including breast and lung cancer. Although MUC1 modulates the activity of several transcription factors, there is no information regarding the effects of MUC1 on global gene expression patterns and the potential role of MUC1-induced genes in predicting outcome for cancer patients. We have developed an experimental model of MUC1-induced transformation that has identified the activation of gene families involved in oncogenesis, angiogenesis, and extracellular matrix remodeling. A set of experimentally derived MUC1-induced genes associated with tumorigenesis was applied to the analysis of breast and lung adenocarcinoma cancer databases. A 35-gene MUC1-induced tumorigenesis signature predicts significant decreases in both disease-free and overall survival in patients with breast (n = 295) and lung (n = 442) cancers. The data show that the MUC1 oncoprotein contributes to the regulation of genes that are highly predictive of clinical outcome in breast and lung cancer patients.

The MUC1 heterodimeric transmembrane glycoprotein is aberrantly overexpressed by diverse human carcinomas. Galectin-3 is a β-galactoside binding protein that has also been associated with the development of human cancers. The present results demonstrate that MUC1 induces galectin-3 expression by a posttranscriptional mechanism. We show that the MUC1 C-terminal subunit is glycosylated on Asn-36 and that this modification is necessary for upregulation of galectin-3. N-glycosylated MUC1-C increases galectin-3 mRNA levels by suppressing expression of the microRNA miR-322 and thereby stabilizing galectin-3 transcripts. The results show that, in turn, galectin-3 binds to MUC1-C at the glycosylated Asn-36 site. The significance of the MUC1-C-galectin-3 interaction is supported by the demonstration that galectin-3 forms a bridge between MUC1 and the epidermal growth factor receptor (EGFR) and that galectin-3 is essential for EGF-mediated interactions between MUC1 and EGFR. These findings indicate that MUC1 and galectin-3 function as part of a miR-322-dependent regulatory loop.

The MUC1 protein is aberrantly overexpressed by most human breast carcinomas. We report that the MUC1 C-terminal subunit associates with estrogen receptor α (ERα) and that this interaction is stimulated by 17β-estradiol (E2). MUC1 binds directly to the ERα DNA binding domain and stabilizes ERα by blocking its ubiquitination and degradation. Chromatin immunoprecipitation assays further demonstrate that MUC1 (1) associates with ERα complexes on estrogen-responsive promoters, (2) enhances ERα promoter occupancy, and (3) increases recruitment of the p160 coactivators SRC-1 and GRIP1. In concert with these results, we show that MUC1 stimulates ERα-mediated transcription and contributes to E2-mediated growth and survival of breast cancer cells. These findings provide evidence that MUC1 stabilizes ERα and that this oncoprotein is of importance to the activation of ERα function.

Dysregulation of ß-catenin is of importance to the development of diverse human malignancies. The MUC1 oncoprotein is aberrantly overexpressed by most human carcinomas and associates with ß-catenin. However, the functional significance of the MUC1-ß-catenin interaction is not known. Here, we show that MUC1 increases ß-catenin levels in the cytoplasm and nucleus of carcinoma cells. Previous studies have shown that glycogen synthase kinase 3ß (GSK3ß) phosphorylates ß-catenin and thereby targets it for proteosomal degradation. Consistent with the up-regulation of ß-catenin levels, our results show that MUC1 blocks GSK3ß-mediated phosphorylation and degradation of ß-catenin. To further define the interaction between MUC1 and ß-catenin, we identified a serine-rich motif (SRM) in the MUC1 cytoplasmic domain that binds directly to ß-catenin Armadillo repeats. Mutation of the SRM attenuated binding of MUC1 to ß-catenin and MUC1-mediated inhibition of ß-catenin degradation. Importantly, disruption of the MUC1-ß-catenin interaction with the SRM mutant also attenuated MUC1-induced anchorage-dependent and -independent growth and delayed MUC1-mediated tumorigenicity. These findings indicate that MUC1 promotes transformation, at least in part, by blocking GSK3ß-mediated phosphorylation and thereby degradation of ß-catenin.

The MUC1 oncoprotein is aberrantly overexpressed by most human carcinomas. The present work demonstrates that MUC1 associates with the p53 tumor suppressor, and that this interaction is increased by genotoxic stress. The MUC1 cytoplasmic domain binds directly to p53 regulatory domain. Chromatin immunoprecipitation assays demonstrate that MUC1 coprecipitates with p53 on the p53-responsive elements of the p21 gene promoter and coactivates p21 gene transcription. Conversely, MUC1 attenuates activation of Bax transcription. In concert with these results, MUC1 promotes selection of the p53-dependent growth arrest response and suppresses the p53-dependent apoptotic response to DNA damage. These findings indicate that MUC1 regulates p53-responsive genes and thereby cell fate in the genotoxic stress response.

The MUC1 transforming protein is overexpressed by most human carcinomas. The present studies demonstrate that the MUC1 C-terminal subunit (MUC1 C-ter) localizes to mitochondria in HCT116/MUC1 colon carcinoma cells and that heregulin stimulates mitochondrial targeting of MUC1 C-ter. We also show that MUC1 attenuates cisplatin-induced (1) release of mitochondrial apoptogenic factors, (2) activation of caspase-3, and (3) induction of apoptosis. Moreover, knockdown of MUC1 expression in A549 lung and ZR-75-1 breast carcinoma cells by MUC1siRNA was associated with increased sensitivity to genotoxic drugs in vitro and in vivo. These findings indicate that MUC1 attenuates the apoptotic response to DNA damage and that this oncoprotein confers resistance to genotoxic anticancer agents.