The oncofetal protein glypican-3 is a novel marker of hepatic progenitor/oval cells.

Glypican-3 (Gpc3), a cell surface-linked heparan sulfate proteoglycan is highly expressed during embryogenesis and is involved in organogenesis. Its exact biological function remains unknown. We have studied the expression of Gpc3 in fetal and adult liver, in liver injury models of activation of liver progenitor cells: D-galactosamine and 2-acetylaminofluorene (2-AAF) administration followed by partial hepatectomy (PH) (2-AAF/PH); and in the Solt-Farber carcinogenic model: by initiation with a single dose of diethylnitrosamine and promotion with 2-AAF followed by PH treatment. Gpc3 expression was studied using complementary DNA microarrays, reverse transcriptase-polymerase chain reaction, in situ hybridization (ISH); ISH combined with immunohistochemistry (IHC) and immunofluorescent microscopy. We found that Gpc3 is highly expressed in fetal hepatoblasts from embryonic days 13 through 16 and its expression gradually decreases towards birth. Dual ISH with Gpc3 and alpha-fetoprotein (AFP) probes confirmed that only hepatoblasts and no other fetal liver cells express Gpc3. At 3 weeks after birth the expression of Gpc3 mRNA and protein was hardly detected in the liver. Gpc3 expression was highly induced in oval cell of D-gal and 2-AAF/PH treated animals. Dual ISH/IHC with Gpc3 riboprobe and cytokeratin-19 (CK-19) antibody revealed that Gpc3 is expressed in activated liver progenitor cells. ISH for Gpc3 and AFP performed on serial liver sections also showed coexpression of the two-oncofetal proteins. FACS isolated oval cells with anti-rat Thy1 revealed expression of Gpc3. Gpc3 expression persists in atypical duct-like structures and liver lesions of animals subjected to the Solt-Farber model of initiation and promotion of liver cancer expressing CK-19. In this work we report for the first time that the oncofetal protein Gpc3 is a marker of hepatic progenitor cells and of early liver lesions. Our findings show further that hepatic progenitor/oval cells are the target for malignant transformation in the Solt-Farber model of hepatic carcinogenesis.