Mucin1 shifts Smad3 signaling from the tumor-suppressive pSmad3C/p21WAF1 pathway to the oncogenic pSmad3L/c-Myc pathway by activating JNK in human hepatocellular carcinoma cells
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Qiongshu Li1, Guomu Liu1, Hongyan Yuan1, Juan Wang1, Yingying Guo1, Tanxiu Chen1, Ruiping Zhai1, Dan Shao1, Weihua Ni1, Guixiang Tai1
1Department of Immunology, College of Basic Medical Science, Jilin University, Changchun 130021, China
Guixiang Tai, e-mail: email@example.com
Keywords: MUC1, pSmad3L, pSmad3C, HCC, JNK
Received: November 14, 2014 Accepted: December 16, 2014 Published: January 20, 2015
Mucin1 (MUC1) is a transmembrane glycoprotein that acts as an oncogene in human hepatic tumorigenesis. Hepatocellular carcinoma (HCC) cells often gain advantage by reducing the tumor-suppressive activity of transforming growth factor beta (TGF-β) together with stimulation of its oncogenic activity as in MUC1 expressing HCC cells; however, molecular mechanisms remain largely unknown. Type I TGF-β receptor (TβRI) and c-Jun NH2-terminal kinase (JNK) differentially phosphorylate Smad3 mediator to create 2 phosphorylated forms: COOH-terminally phosphorylated Smad3 (pSmad3C) and linker-phosphorylated Smad3 (pSmad3L). Here, we report that MUC1 overexpression in HCC cell lines suppresses TβRI-mediated pSmad3C signaling which involves growth inhibition by up-regulating p21WAF1. Instead, MUC1 directly activates JNK to stimulate oncogenic pSmad3L signaling, which fosters cell proliferation by up-regulating c-Myc. Conversely, MUC1 gene silencing in MUC1 expressing HCC cells results in preserved tumor-suppressive function via pSmad3C, while eliminating pSmad3L-mediated oncogenic activity both in vitro and in vivo. In addition, high correlation between MUC1 and pSmad3L/c-Myc but not pSmad3C/p21WAF1 expression was observed in HCC tissues from patients. Collectively, these results indicate that MUC1 shifts Smad3 signaling from a tumor-suppressive pSmad3C/p21WAF1 to an oncogenic pSmad3L/c-Myc pathway by directly activating JNK in HCC cells, suggesting that MUC1 is an important target for HCC therapy.
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