Histological and culture studies with respect to ABCG2 expression support the existence of a cancer cell hierarchy in human hepatocellular carcinoma

In this study, we examined the possible involvement of progenitor cells in the carcinogenesis of human hepatocellular carcinoma (HCC) using tissue specimens and cell lines. We used ATP-binding cassette transporter ABCG2 as a progenitor cell marker. Immunohistochemically, ABCG2(+) hepatocytes were observed in the periportal areas of the dysplastic nodule, and ABCG2(+) cancer cells were also scattered or focally clustered in HCC. We sorted the cultured HCC cells (HuH7 and PLC5) into ABCG2(+) and ABCG2(-) sub-populations and then subcultured them for 4 weeks. ABCG2(+) cells could generate ABCG2(+) and ABCG2(-) progenies during subculture, whereas ABCG2(-) cells bore only ABCG2(-) cells, suggesting that a cancer cell hierarchy with reference to ABCG2 exists in HCC cells and that ABCG2(+) cells reside at the higher rank in that hierarchy. Interestingly, other progenitor cell markers including cytokeratin 19 and alpha-fetoprotein were mainly expressed in ABCG2(+) subpopulations. Conversely, albumin expression was more intense in ABCG2(-) cells. In addition, the expression patterns of transcription factors (GATA6, CCAAT/enhancer-binding protein a, and CCAAT/enhancer-binding protein beta) in ABCG2(+) and ABCG2(-) cells resembled those during normal liver development. In conclusion, this study suggests that cancer cells with ABCG2 expression might play a central role in hepatocarcinogenesis and the maintenance of the cancer cell hierarchy of human HCC.