Nuclear Association of the Cytoplasmic Tail of MUC1 and β-Catenin

Abstract

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...

GLI1 facilitates the migration and invasion of pancreatic cancer cells through MUC5AC-mediated attenuation of E-cadherin

“…For instance, a fragment of the MUC1 cytoplasmic tail is cleaved and transported into the nucleus in association with b-catenin, affecting its function as a co-activator for TCF/LEF1 (Wen et al, 2003). Both MUC1 and MUC4 also associate with ErbB tyrosine kinase receptors on the cell membrane and enhance the downstream mitogen-activated protein Figure 6 Immunohistochemical analysis of GLI1, MUC5AC, E-cadherin and b-catenin in PanIN and PDA.…”

“…For instance, the expressions of MUC1, MUC2 and MUC5AC were statistically associated with a shorter survival time for pancreatic cancer patients in the United States (Takikita et al, 2009), suggesting that mucin functions not only to protect the normal epithelial surface from harsh environments but also to regulate cancer cell properties. Indeed, membrane-associated mucins were recently shown to play a crucial role in signal transduction (Schroeder et al, 2001;Jepson et al, 2002;Wen et al, 2003;Hollingsworth and Swanson, 2004;Bafna et al, 2010). However, the role for gel-forming mucins in cancer cells is not fully understood.…”

Galectin-3 regulates MUC1 and EGFR cellular distribution and EGFR downstream pathways in pancreatic cancer cells

“…As MUC1 CT and galectin-3 in the nucleus promote cyclin-D1 expression through the accumulation and/ or stabilization of nuclear b-catenin, respectively (Wen et al, 2003;Song et al, 2009), we studied the levels of cyclin D1 in total cell lysates before and after a 10-min EGF treatment (Figure 4c). Our results show that Sc cells expressed higher levels of cyclin D1 than Sh1 cells in absence or in presence of EGF.…”

“…For example, epidermal growth factor receptor (EGFR) phosphorylates this peptide motif and promotes the binding of MUC1 CT to c-Src and b catenin (Li et al, 2001). MUC1 CT also co-localized with b-catenin in the nucleus thus suggesting that it could be cleaved/ released from the membrane and traffic from the membrane to the cytoplasm and nucleus (Wen et al, 2003).…”

MUC1 Oncoprotein Functions in Activation of Fibroblast Growth Factor Receptor Signaling

“…3B), indicating that this response is not cell type specific. Other studies have shown that MUC1-C, but not MUC1-N, is trafficked to the nucleus (8,9,11,44,45). Analysis of purified nuclei from ZR-75-1 cells showed that FGF1 increases MUC1-C levels (Fig.…”