SMAD4-independent activation of TGF-β signaling by MUC1 in a human pancreatic cancer cell line

Abstract

Pancreatic Ductal Adenocarcinoma (PDA) has a mortality rate that nearly matches its incidence rate. Transforming Growth Factor Beta (TGF-β) is a cytokine with a dual role in tumor development switching from a tumor suppressor to a tumor promoter. There is limited knowledge of how TGF-β function switches during tumorigenesis. Mucin 1 (MUC1) is an aberrantly glycosylated, membrane-bound, glycoprotein that is overexpressed in >80% of PDA cases and is associated with poor prognosis. In PDA, MUC1 promotes tumor progression and metastasis via signaling through its cytoplasmic tail (MUC1-CT) and interacting with other oncogenic signaling molecules. We hypothesize that high levels of MUC1 in PDA may be partly responsible for the TGF-β functional switch during oncogenesis. We report that overexpression of MUC1 in BxPC3 human PDA cells (BxPC3.MUC1) enhances the induction of epithelial to mesenchymal transition leading to increased invasiveness in response to exogenous TGF-β1. Simultaneously, these cells resist TGF-β induced apoptosis by downregulating levels of cleaved caspases. We show that mutating the tyrosines in MUC1-CT to phenylalanine reverses the TGF-β induced invasiveness. This suggests that the tyrosine residues in MUC1-CT are required for TGF-β induced invasion. Some of these tyrosines are phosphorylated by the tyrosine kinase c-Src. Thus, treatment of BxPC3.MUC1 cells with a c-Src inhibitor (PP2) significantly reduces TGF-β induced invasiveness. Similar observations were confirmed in the Chinese hamster ovarian (CHO) cell line. Data strongly suggests that MUC1 may regulate TGF-β function in PDA cells and thus have potential clinical relevance in the use of TGF-β inhibitors in clinical trials.

Tumor -Associated MUC1 Regulates TGF-β Signaling and Function in Pancreatic Ductal Adenocarcinoma

“…In future experiments, we will delineate which of the 7 tyrosines (or combination) may be critical for the TGF-β1 induced degradation of MUC1. In our previous publication, we found no difference in tMUC1 expression levels after 48 hours of TGF-β1 exposure [34] which may have been too late to detect differences. Therefore, we determined that TGF-β1 affects tMUC1 levels immediately (30 minutes) post treatment.…”

“…Our previous studies established c-Src as the key mediator in the interaction of tMUC1 and TGF-β signaling [34]. Therefore, we examined the phosphorylation of c-Src in HPAF-II, CFPAC, and MiaPaca2 cells in response to 10 and 50ng/ml of TGF-β1 treatment for 30 minutes.…”

“…Interestingly, C-Src has been shown to mediate breast cancer cell proliferation and invasion via regulation of mitogen-activated protein kinase (MAPK) activation [25]. In our previous studies, we have demonstrated that tMUC1 modulates TGF-β signaling in cell lines that were engineered to express high tMUC1 [26]. In that report, we established that TGF-β signaling required tyrosine phosphorylation of the tMUC1-CT via tyrosine kinase, c-Src.…”

“…2), and activation of c-Src (Fig. 4); all of which signify activation of the non-canonical pathway associated with cellular proliferation and invasion [34] [41]. Furthermore, high tMUC1 negatively correlated to TGF-βRI expression, a receptor that activates the canonical Smad pathway, (Fig.…”

Inhibition of MUC1 exerts cell-cycle arrest and telomerase suppression in glioblastoma cells

“…However, in the late stage of aggressive and invasive tumors, TGF-β signaling stimulate tumor progression by its pleiotropic activities on the cancer cells which include induction of EMT, migration, invasion, and tumor metastasis 18 , 21 , 22 . MUC1 is reported as a key inducer of EMT and is partially responsible for the functional switch of TGF-β from a tumor suppressor to a tumor promoter during EMT in multiple cancers 23 . However, the role of MUC1 has not been fully elucidated in GBM, although only sporadic reports mentioned the MUC1 in relation to the maintenance of aggressive of gliomas 24 .…”