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


CONCLUSIONS
These data provide new mechanistic insight into the roles of GPC3 and of c-Myc in the development of HCC.
Beijing Institute of Liver Disease, Beijing, China.
Li
L
Li
Ronghua
R
Jin
Xiaoli
X
Zhang
Fudong
F
Lv
Lifeng
L
Liu
Daojie
D
Liu
Kai
K
Liu
Ning
N
Li
Dexi
D
Chen
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D001707:Biopsy, Needle; D006528:Carcinoma, Hepatocellular / Q000235:genetics* / Q000473:pathology; D002453:Cell Cycle / Q000235:genetics / Q000502:physiology; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D053673:Glypicans / Q000235:genetics / Q000378:metabolism*; D022781:Hepatocytes / Q000166:cytology / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry; D008113:Liver Neoplasms / Q000235:genetics* / Q000473:pathology; D008297:Male; D008875:Middle Aged; D011401:Promoter Regions, Genetic / Q000235:genetics / Q000502:physiology; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D012494:Sampling Studies; D012680:Sensitivity and Specificity; D046509:Tissue Culture Techniques; D015533:Transcriptional Activation; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
C105937:GPC3 protein, human; D053673:Glypicans; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger