Identification and characterization of CD90⁺ cancer stem cells in hepatocellular carcinoma

Abstract

Hepatocellular carcinoma (HCC) is one of the most devastating malignancies worldwide with increasing incidences in both developed and developing countries. Survival rates have not been significantly improved over the past decades in spite of advances in detection and therapies for this disease, suggesting that current treatments may target the wrong cells, and miss the cancer stem cells (CSCs). The cancer stem cell hypothesis presents that tumor formation, proliferation and propagation are driven by a rare subpopulation of chemoresistant CSCs that are not killed by conventional therapies and go on to cause disease relapse. The objective of this study was to identify and characterize CSCs in HCC cell lines and human liver tumor specimens using CD90 as a potential marker. The number of CD90+ cells present in HCC cell lines was found to positively correlate with tumorigenicity potentials. Injection of as few as 2,000 sorted CD90+ cells from HCC cell lines resulted in the formation of tumor nodules in nude mice, whereas no tumors formed for CD90ˉcells in the same model. The tumor xenograft generated by injection of CD90+ cells sorted from previous xenograft in a serial xenotransplantation assay exhibited recapitulation of tumor heterogeneity to original primary tumor and consistent proportion of CD90+ and CD90ˉ cells which demonstrated self-renewal and differentiation capacities of CD90+CSCs. CD45ˉCD90+ cells were detected (0.03%–6.2%) in human liver tumor specimens, but were only present in minute quantities in normal, cirrhotic and non-tumorous tissues. More importantly, CD45ˉCD90+ cells sorted from primary HCC tumor also displayed tumorigenicity, self-renewal and lineage differentiation capacities. CD90+CSCs were found to be more resistant to therapeutic drugs compared to CD90- cells, as reflected by the results of enrichment of the CD90+ CSCs and longer survival rates after chemotherapeutic treatment. The high expression of genes, such as OCT4, MRP3, ABCG2, AKT1, BirC5, BCL2, HA and CD44, in CD90+CSCs may mediate chemoresistance. The majority of CD90+ cells co-expressed CD44, another stem cell marker. Blocking CD44 activities by anti-CD44 antibody increased apoptosis of CD90+ CSCs, sensitized CD90+CSCs to chemotherapeutic drugs in vitro, and decreased tumorigenic and metastatic potentials of CD90+CSCs in vivo, indicating that a therapeutic potential of targeting CD44. However side effects may be problematic due to the endogenous expression of CD44 in healthy tissues and normal lymphocytes. To identify novel gene targets specific to liver CSCs, a sensitive RNA-sequencing (RNA-Seq) technique was used to compare the gene expression profiles between CD90+CSCs sorted from HCC primary tumors and CD90+cells from adjacent non-tumorous tissue (CD90+NTSCs). The up-regulated genes in CD90+CSCs were associated with lipid metabolism, inflammation, and drug resistance. Among the differentially expressed genes, glypican-3 (GPC3) was specifically elevated in CD90+CSCs but not in CD90+NTSCs. Therefore, GPC3 could be a promising gene candidate for HCC therapy as targeting GPC3 should not induce damage to normal liver stem cells. In summary, CD90 is a liver CSCs marker. Identification of CD90+ CSCs in HCC provides new insight into cellular basis of hepatocarcinogenesis, recurrence and metastasis, which opens new avenues for the design of future CSC-targeted therapies.