The MUC1-C oncoprotein as a target in hematologic malignancies

MUC1 is a cell surface protein that was identified in the early 1980s based in part on its marked overexpression by breast cancer cells.¹ During the ensuing years, high levels of MUC1 have been documented in many human carcinomas and, in general, overexpression has been associated with a poor prognosis. Although widely recognized as an epithelial membrane protein that is upregulated in carcinomas, MUC1 has also been detected in hematologic malignancies, including leukemias, lymphomas and multiple myeloma.² Collectively, these findings have generated interest in MUC1 as a potential target for the treatment of both carcinomas and hematologic malignancies. However, progress in targeting MUC1 has been limited by a fundamental lack of understanding as to whether MUC1 contributes to cancer and, if so, how. In the current issue of Cancer Biology & Therapy, Yin and colleagues report on the targeting of the MUC1 cytoplasmic tail as an approach to induce terminal differentiation of chronic myelogenous leukemia cells.³ These studies constitute a logical extension of the elegant studies of the Kufe lab on the role of MUC1 in oncogenesis. The implications of these unique findings with regard to the role of MUC1 as an oncoprotein and a therapeutic target are discussed here.

MUC1 is a Heterodimer with Biologically Distinct Subunits

As background, MUC1 is a member of the mucin family, which includes proteins that are characterized by the presence of variable numbers of highly glycosylated tandem repeats.² Mucins are classified into secreted and membrane-bound forms. The secreted mucins appeared early in evolution to provide a protective physical barrier for epithelial cells that line the surface of ducts, as well as organs, such as the aerodigestive tract, that are exposed to the external environment. MUC1 and other transmembrane mucins evolved as a later event in which the mucin structure was linked to a transmembrane domain that effectively positions these proteins at the membrane of the epithelial cell. MUC1 further developed with an intriguing capability to undergo autocleavage of the translated protein into two subunits in the endoplasmic reticulum. The two subunits then form a stable heterodimer involving the regions adjacent to the autocleavage site. The resulting N-terminal subunit, which has been designated MUC1-N, includes the mucin domain. The C-terminal subunit, designated MUC1-C, has a short 58 amino acid extracellular domain, a transmembrane region and a 72 amino acid cytoplasmic tail.²

The MUC1 heterodimer is expressed at relatively low levels by normal epithelial cells and its distribution is restricted to the apical cell membrane. Autocleavage of MUC1 into N- and C-terminal subunits is now understood as a mechanism to facilitate release of the MUC1-N mucin component into the extracellular mucous gel; for example, in the response to stress and to contribute to the protective physical barrier. In addition, the MUC1-C subunit is believed to play a role in the epithelial stress response; that is, an integrated series of signaling events that allow normal epithelial cells to transiently undergo loss of polarity and initiate a process that promotes growth, survival and repair of damage to the epithelial layer.⁴ In carcinoma cells that lose polarity and in malignant hematopoietic cells, MUC1 expression is upregulated and the heterodimer is distributed at high levels throughout the entire cell membrane. Effectively, overexpression of the MUC1-N subunit can result in a barrier that physically protects malignant cells from host defense mechanisms; for example, immune effector cells. In addition and importantly, overexpression of the MUC1-C subunit allows malignant cells, whether of epithelial or hematopoietic origin, to exploit a physiologic stress response that promotes their own growth and survival.

Targeting of MUC1 as a Potential Therapeutic Strategy

Aberrant expression of MUC1 by diverse carcinoma and malignant hematopoietic cells has established the two MUC1 subunits as attractive targets for the development of anti-cancer vaccines, antibodies and small molecules. Indeed, we and others have developed recombinant vaccinia- and fowlpox-based vaccines that express both MUC1 subunits and have entered Phase I and several Phase II clinical trials.⁵ Liposome-based vaccines incorporating the MUC1-N tandem repeats are also under clinical evaluation for the treatment of non-small cell lung cancer and hormone-sensitive breast cancer.² Monoclonal antibodies generated against the MUC1-N tandem repeats have been evaluated as potential therapeutic agents. However, MUC1-N circulates in the plasma of patients with cancer and thus represents a barrier for targeting of anti-MUC1-N antibodies to the surface of malignant cells. Other considerations for targeting MUC1 have included the development of agents against the MUC1-C subunit. Here, a challenge over time has been insufficient evidence about a potential role for MUC1-C in conferring transformation. Moreover, MUC1-C is not a kinase and thus has not been approachable with small molecule strategies that, for example, target a hydrophobic pocket.

Yin et al. now report on the development of an intriguing alternative approach to target the MUC1-C subunit in leukemia cells.³ This approach was based in part on the demonstration that overexpression of the MUC1-C cytoplasmic tail is sufficient to induce transformation of rodent fibroblasts.⁶ In addition, expression of MUC1-C with mutations in the cytoplasmic tail had been shown to function as dominant-negatives that block anchorage-independent growth and tumorigenicity of human carcinoma cells.⁷ In further support of the potential importance of the MUC1-C cytoplasmic tail, this region interacts with certain signaling molecules, such as p53, NFκB and β-catenin, that could contribute to its transforming function.² Yet another observation of significance was that the MUC1-C subunit forms dimers through a CQC motif in the cytoplasmic tail, and that dimerization is necessary for intracellular trafficking of MUC1-C to the nucleus.⁸ These findings indicated that targeting of the MUC1-C subunit at the dimerization motif in the cytoplasmic tail might represent an approach to block its functional involvement in transformation. Indeed, a cell-penetrating peptide that contains the CQC motif was developed to bind to the endogenous MUC1-C cytoplasmic tail in cancer cells and directly block its dimerization and function.⁹ A distinct approach has been the development of cell-penetrating peptides that act as MUC1-C cytoplasmic tail decoys that bind to β-catenin.¹⁰

Targeting the MUC1-C Subunit Induces Terminal Differentiation of CML Cells

Yin and colleagues have now investigated the effects of blocking MUC1-C in chronic myelogenous leukemia (CML) cells that express the Bcr-Abl fusion protein. This line of experimentation seems at first somewhat paradoxical in that MUC1 is recognized as an epithelial membrane antigen. Nonetheless, previous work had shown that MUC1-C is expressed in CML cells.¹¹ CML usually presents as a chronic phase that progresses to blast crisis due to the acquisition of genetic alterations that block myeloid differentiation and promote growth.¹² MUC1 is expressed in CML cells that have entered blast crisis, but not in chronic phase cells.¹¹ In addition, MUC1-C has been shown to interact with Bcr-Abl and block its degradation.¹¹ The demonstration that silencing MUC1-C in CML blasts induces differentiation provided further evidence for involvement of MUC1-C in the pathogenesis of CML.¹¹ In the present publication by Yin et al. a cell-penetrating peptide that blocks MUC1-C dimerization has been shown to induce terminal differentiation of CML blasts.³

CML is effectively treated with small molecule Bcr-Abl inhibitors, such as imatinib and dasatinib.¹² However, patients inevitably develop resistance to these agents by expression of Bcr-Abl with the Y315I mutation.¹² The studies by Yin et al. demonstrate that inhibition of MUC1-C results in the downregulation of Bcr-Abl expression.³ This finding may be due to the effects of MUC1-C on stabilization of the Bcr-Abl protein. Alternatively, the MUC1-C inhibitor may affect other mechanisms, such as transcription, that are responsible for Bcr-Abl expression. Further studies will be needed to fully address how the MUC1-C inhibitor decreases Bcr-Abl levels. Nonetheless, the downregulation of Bcr-Abl expression could represent an alternative approach to small molecule inhibitors of the Bcr-Abl kinase and might be useful in the setting of resistance to these agents. Indeed, consistent with decreases in Bcr-Abl levels, treatment of CML blasts with the MUC1-C inhibitor in culture and in an animal model was associated with reverse of the block in differentiation and a decrease in self-renewal.³

Future Directions?

These important findings by Yin and colleagues highlight certain issues that will need to be addressed in future studies. One question is whether other hematologic malignancies that express MUC1 will respond to MUC1-C inhibitors. MUC1 is expressed in acute myelogenous leukemia (AML), lymphomas and multiple myeloma,² and MUC1-C may also represent a target in these diseases. An overriding issue, however, is whether a MUC1-C inhibitor can be developed for clinical evaluation. The MUC1-C inhibitor used in the present studies is a cell-penetrating peptide. Peptide drugs are susceptible to proteolytic degradation and can have short half-lives in plasma, requiring frequent delivery. The MUC1-C inhibitor has been administered successfully in other animal models;⁹ nonetheless, there may be challenges in the delivery of a peptide to humans. For example, as found during treatment with certain antibodies, peptide drugs might be associated with adverse immune reactions. Peptide drugs against cancer targets involved in protein-protein interactions will increasingly be evaluated in the clinic; however, it is not yet clear whether they will be effective agents. Therefore, another issue is whether the MUC1-C cytoplasmic tail might be amenable to targeting with a small molecule inhibitor that could circumvent some of the potential limitations of peptide drugs.

Commetary to: Yin L, Ahmad R, Kosugi M, Kawano T, Avigan D, Stone R, et al. Terminal differentiation of chronic myelogenous leukemia cells is induced by targeting of the MUC1-C oncoprotein. Cancer Biol Ther. 2010 This issue.