24 In HCC, CD44 is also an important marker used in combination with other CSC markers to better define the surface phenotype of liver CSCs. For the aforementioned markers, including CD133 and CD90, cells co-expressing either of these markers and CD44

present a more aggressive phenotype than cells with a positive expression of either CD133 or CD90 alone. Yang et al. showed that CD44+ cells developed tumor nodules in immunodeficient mice faster than CD44- cells, whereas lung metastases were only observed in immunodeficient mice transplanted with CD90+CD44+ cells.23 In another study, CD44 was found to be preferentially expressed in a CD133+ population in four HCC cell lines, including Huh7, SMMC-7721, MHCC-LM3 and MHCC-97L. CD133+CD44+ cells exhibit enhanced phosphatase inhibitor library abilities to form tumors, are chemoresistant and express a higher level of “stemness”-associated genes, as compared with their CD133+CD44-

counterparts.25 EpCAM is present in the embryonic liver, bile duct epithelium and proliferating bile ductules in the cirrhotic liver, but it is absent in normal adult hepatocytes.26 An elevated expression of EpCAM was first identified in premalignant hepatic tissues; therefore, this surface protein was suggested to be an early biomarker for HCC.27 Following a cDNA microarray analysis on a clinical cohort of primary HCC tissue, EpCAM+ HCC was linked with the gene signature and the molecular pathway of hepatic progenitor

cells, whereas genes expressed in EpCAM- HCC cells were associated with mature hepatocyte functions.26 These two subtypes BGJ398 purchase of HCC were further stratified into four groups with features resembling different hepatic lineages, and they showed prognostic differences based on the expression of alpha fetoprotein (AFP).26,28 EpCAM+ HCC cells 17-DMAG (Alvespimycin) HCl have also been shown to be highly invasive and tumorigenic, in comparison with their EpCAM- counterparts.28 Recently, a novel cell surface marker, CD13, was identified for potentially dormant and semi-quiescent CSCs in HCC. Haraguchi et al. found that CD13 was commonly enriched in an SP population sorted from Huh7, PLC/PRF/5 and Hep3B cells by gene expression microarray analysis. CD13 was selected as a putative marker to enrich the semi-quiescent liver CSCs because of its predominant distribution during the G1/G0 phase.29 This result suggested that CD13+ cells represent the dormant or slow-growing population that is believed to account for the chemoresistant capacity in HCC. Researchers then assessed the tumorigenic potential of CD13 with two other liver CSC markers, CD133 and CD90. The results clearly showed that the CD13+CD133+ and CD13+CD90- fractions in Huh7 and PLC/PRF/5 cell lines, respectively, initiated tumor formation effectively in limiting-dilution and serial transplantation assays.