Oncogenic activation of glypican-3 by c-Myc in human hepatocellular carcinoma

Abstract

Glypican-3 (GPC3) is a heparan sulfate proteoglycan that has an important role in cell growth and differentiation, and its function in tumorigenesis is tissue-dependent. In hepatocellular carcinoma (HCC), the overexpression of GPC3 has been demonstrated to be a reliable diagnostic indicator. However, the mechanisms that regulate the expression and function of GPC3 remain unclear. The oncoprotein c-Myc is a transcription factor that plays a significant role in more than 50% of human tumors. We report here that GPC3 is a transcriptional target of c-Myc and that the expression of c-Myc is also regulated by GPC3, thus forming a positive feedback signaling loop. We found that the overexpression of c-Myc could induce GPC3 promoter-dependent luciferase activity in luciferase reporter experiments. Furthermore, mutational analysis identified c-Myc-binding sites within the GPC3 promoter. The exogenous overexpression of c-Myc increased the endogenous messenger RNA (mRNA) and protein levels of GPC3. Chromatin immunoprecipitation experiments revealed the binding of c-Myc to the endogenous GPC3 promoter, indicating that c-Myc can directly transcriptionally activate GPC3. Interestingly, GPC3 can also elevate c-Myc expression. Overexpression of GPC3 increased c-Myc protein levels, whereas the knockdown of GPC3 reduced c-Myc expression levels. Lastly, the elevated levels of c-Myc correlate with the overexpression of GPC3 in human HCC samples. Conclusion: These data provide new mechanistic insight into the roles of GPC3 and of c-Myc in the development of HCC. (HEPATOLOGY 2012;56:1380-1390 P rimary hepatocellular carcinoma (HCC) is the world's fifth most common cancer, and the third most common cause of cancer deaths. 1 HCC is a common malignant tumor, and causes over 100,000 HCC deaths per year in China. 2 Only 10%-20% of total HCCs can be treated in the early stage by surgery. Most HCCs are diagnosed at late stages, and as a result they progress rapidly, are difficult to treat, and exhibit poor prognosis. Early diagnosis and treatment are critical factors for improving the survival of HCC patients.Recent research has shown that glypican-3 (GPC3) is a specific and sensitive biomarker for the diagnosis of HCC. 3 GPC3 is a proteoglycan that is localized on the cell surface. The abnormal expression of GPC3 in HCC was first reported by Hsu et al. 4 in 1997. In that study, 75% of HCC patients were found to overexpress GPC3 messenger RNA (mRNA), whereas patients with benign liver disease or normal livers exhibited no expression of GPC3 mRNA or protein. Soluble GPC3 protein can be detected in the serum of 50% of HCC patients, demonstrating its value as a diagnostic marker for HCC. 5 The expression of GPC3, its receptor, and of other growth factors coordinate signal transduction pathways that regulate cellular morphology and a variety of cellular behaviors, such as adhesion,

Angiotensin II Enhances Proliferation and Inflammation through AT1/PKC/NF-κB Signaling Pathway in Hepatocellular Carcinoma Cells

“…C-myc is also considered to be oncogene and frequently overexpressed in HCC [39][40][41]. Our present results revealed that Ang II enhanced C-myc immunofluorescent expression in HCC cell lines and the expression was predominantly cell nucleus.…”

Glypican-3 binds to frizzled and plays a direct role in the stimulation of canonical Wnt signaling

“…Work from our laboratory has also shown that, in addition to being a marker of HCC, GPC3 stimulates the growth of this malignancy by promoting canonical Wnt signaling (Capurro et al, 2005). This finding has been confirmed by others (Li et al, 2012). Studies of GPC3-null mice have also demonstrated that GPC3 regulates Wnt signaling in normal embryonic tissues (Song et al, 2005).…”

Advances in the study of oncofetal antigen glypican-3 expression in HBV-related hepatocellular carcinoma

“…There is increasing evidence indicating that the structural requirements for GPC3 activity are cell type-specific, and its core protein is processed by a furin-like convertase. GPC-3 is expressed like a carcinoembryonic antigen, it stains dark brown deep within HCC tissues, and it is found in the cytoplasm and cell membrane (30). Immunohistochemistry has revealed oncofetal GPC- cells of distinct tumors (2).…”