MUC1, an integral membrane mucin associated with the metastatic phenotype, is overexpressed by most human carcinoma cells. The MUC1 cytoplasmic tail (CT) is postulated to function in morphogenetic signal transduction via interactions with Grb2/Sos, c-Src, and ␤-catenin. We investigated intracellular trafficking of the MUC1 CT, using epitope-tagged constructs that were overexpressed in human pancreatic cancer cell lines S2-013 and Panc-1. The MUC1 CT was detected at the inner cell surface, in the cytosol, and in the nucleus of cells overexpressing MUC1. Fragments of the MUC1 CT were associated with ␤-catenin in both cytoplasm and nuclei. Overexpression of MUC1 increased steady state levels of nuclear ␤-catenin but decreased nuclear levels of plakoglobin (␥-catenin). There was no detectable association between plakoglobin and the MUC1 CT. Coimmunoprecipitation experiments revealed that the cytoplasmic and nuclear association of MUC1 CT and ␤-catenin was not affected by disruption of Ca 2؉ -dependent intercellular cadherin interactions. These results demonstrate nuclear localization of fragments of MUC1 CT in association with ␤-catenin and raise the possibility that overexpression of the MUC1 CT stabilizes ␤-catenin and enhances levels of nuclear ␤-catenin during disruption of cadherin-mediated cell-cell adhesion.Human MUC1 is a large, type I transmembrane protein normally expressed on the apical surface of ductal epithelia (1). Full-length MUC1 is synthesized as a single polypeptide chain, which undergoes an early proteolytic cleavage (probably in the endoplasmic reticulum) creating two subunits that remain associated during its post-translational processing and transport to the cell surface (2). The larger of the two fragments contains most of the extracellular domain, including the signal sequence and a tandem repeat domain (3-5). The smaller subunit contains a short extracellular domain, transmembrane domain, and cytoplasmic tail (CT), 1 which are highly conserved across species (88% identity with murine transmembrane and CT sequence) (6). The function of MUC1 is partially elucidated for normal and transformed cells. The extracellular fragment plays a significant role in configuring the adhesive and antiadhesive properties of cells and is believed to contribute to the establishment of molecular structures that protect the cell surface in the relatively harsh environment encountered by different ductal epithelia (7). However, the function of the intracellular portion of the MUC1 cytoplasmic tail (CT) is not known. Indirect evidence suggests that the MUC1 CT is involved in signal transduction, as it contains potential docking sites for Grb2/Sos and ␤-catenin, and can be phosphorylated by GSK-3␤, c-Src, EGFR, and PKC-␦ (7-16). This, together with the general transmembrane structure of the MUC1 molecule, suggests a potential role in morphogenetic signaling; however, little is known about mechanisms by which the MUC1 CT functions in this capacity, and nothing has been reported with regard to the relationship between...

MUC1 is a heterodimeric transmembrane glycoprotein that is overexpressed and aberrantly glycosylated in ductal adenocarcinomas. Differential phosphorylation of the MUC1 cytoplasmic tail (MUC1CT) has been associated with signaling events that influence the proliferation and metastasis of cancer cells. We identified a novel tyrosine phosphorylation site (HGRYVPP) in the MUC1CT by mass spectrometric analysis of MUC1 from human pancreatic adenocarcinoma cell lines. Analyses in vitro and in vivo showed that plateletderived growth factor receptor B (PDGFRB) catalyzed phosphorylation of this site and of tyrosine in the RDTYHPM site. Stimulation of S2-013.MUC1F cells with PDGF-BB increased nuclear colocalization of MUC1CT and B-catenin. PDGF-BB stimulation had no significant effect on cell proliferation rate; however, it enhanced invasion in vitro through Matrigel and in vivo tumor growth and metastases. Invasive properties of the cells were significantly altered on expression of phosphorylation-abrogating or phosphorylation-mimicking mutations at these sites. We propose that interactions of MUC1 and PDGFRB induce signal transduction events that influence the metastatic properties of pancreatic adenocarcinoma. [Cancer Res 2007;67(11):5201-10]

Purpose To investigate the antitumor effects of targeting Src and tubulin in mucinous ovarian carcinoma. Experimental design The in vitro and in vivo effects and molecular mechanisms of KX-01, which inhibits Src pathway and tubulin polymerization, were examined in mucinous ovarian cancer models. Results In vitro studies using RMUG-S and RMUG-L cell lines showed that KX-01 inhibited cell proliferation, induced apoptosis, arrested the cell cycle at the G2/M phase, and enhanced the cytotoxicity of oxaliplatin in the KX-01-sensitive cell line, RMUG-S. In vivo studies showed that KX-01 significantly decreased tumor burden in RMUG-S and RMUG-L mouse models relative to untreated controls, and the effects were greater when KX-01 was combined with oxaliplatin. KX-01 alone and in combination with oxaliplatin significantly inhibited tumor growth by reducing cell proliferation and inducing apoptosis in vivo. PTEN knock-in experiments in RMUG-L cells showed improved response to KX-01. Reverse phase protein array analysis showed that in addition to blocking downstream molecules of Src family kinases, KX-01 also activated acute stress-inducing molecules. Conclusion Our results showed that targeting both the Src pathway and tubulin with KX-01 significantly inhibited tumor growth in preclinical mucinous ovarian cancer models, suggesting that this may be a promising therapeutic approach for patients with mucinous ovarian carcinoma.

The mechanisms by which MUC1 and p120 catenin contribute to progression of cancers from early transformation to metastasis are poorly understood. Here we show that p120 catenin ARM domains 1, 3–5 and 8 mediate interactions between p120 catenin and MUC1, and that these interactions modulate dynamic properties of cell adhesion, motility and metastasis of pancreatic cancer cells. We also show that different isoforms of p120 catenin when co-expressed with MUC1 create cells that exhibit distinct patterns of motility in culture (motility independent of cell adhesion, motility within a monolayer while exchanging contacts with other cells, and unified motility while maintaining static epithelial contacts) and patterns of metastasis. The results provide new insight into the dynamic interplay between cell adhesion and motility and the relationship of these to the metastatic process.