The Effects of Glypican-3 Deficiency on Radiosensitivity in Liver Cancer Cells

Abstract

Glypican-3 (GPC-3), a heparan sulfate proteoglycan involved in cellular proliferation, modulates signaling of FGF/FGFR, IGF/IGFR, HGF/Met, Wnt/Frizzled, among others and correlates with survival. GPC-3 is overexpressed in the majority of hepatocellular carcinoma and hepatoblastoma, but not in normal hepatocytes. Accordingly, it is being investigated as a liver cancer-selective target for radiopharmaceutical imaging and therapy. However, the potential linkage between GPC-3 expression and radiosensitivity has not yet been defined. In this study, we investigated the effects of GPC-3 deficiency on radiosensitivity in liver cancer cell lines. CRISPR/Cas9 system was used to engineer GPC-3 knockout variants of liver cancer cell lines, HepG2 & Hep3B, both of which natively express GPC-3. Confirmation of knockout of GPC-3 was evaluated by RT-PCR, western blotting, flow cytometry, immunocytochemistry, and gDNA sequencing. Cell growth and migration were evaluated by BrdU insertion and wound-healing assays, respectively. In vitro radiosensitivity was examined by radiation-induced apoptosis/necrosis (Annexin V-APC and PI staining), cell cycle modification, γH2AX foci formation, and clonogenic assays (6 Gy). Wildtype and knockout lines were engrafted into athymic mice to assess tumor growth kinetics. RT-PCR, western blotting, flow cytometry, and immunocytochemistry all confirmed GPC-3 knockout in both HepG2 and Hep3B cell lines. Nucleotide deletion at exon 3 of the GPC-3 gene was confirmed by gDNA sequencing in HepG2ΔGPC3 and Hep3BΔGPC3. GPC-3 deficiency reduced liver cancer cell proliferation (HepG2ΔGPC3, p = 0.027, and Hep3BΔGPC3, p = 0.031) and migration (HepG2ΔGPC3: 1.5-fold, p<0.001, and Hep3BΔGPC3: 2.3-fold, p<0.001) significantly when compared with wild type. GPC-3 deficiency reduced cell survival and clonogenicity (HepG2ΔGPC3: DEF = 1.23, Hep3BΔGPC3: DEF = 1.23) in liver cancer cells exposed to irradiation (6 Gy). The delayed repair of double-stranded DNA damage was observed in irradiated GPC-3 deficient liver cancer cells. Tumor growth was dramatically delayed by GPC-3 deficiency. Tumor weight measured at 50 (Hep3B) and 60 (HepG2) days after liver cancer cell inoculation corroborated these effects. Knockout lines of HepG2 and Hep3B exhibited decreased cell proliferation, migration, and in vivo tumor growth compared to wildtype. GPC-3 deficiency was associated with increased sensitivity to radiation therapy. Studies identifying the pathways through which this radiosensitivity is mediated are ongoing.