The liver CSC concept has been acknowledged to explain the molecular diversity of malignant phenotypes in liver cancer. However, many questions remain, including the role of hepatitis viruses, the origin of liver CSCs, the existence and role of the stem cell niche, similarities and differences between normal hepatic stem/progenitor cells and liver CSCs, the timing of CSC emergence, CSC concept universality in liver cancers, and the relationship between CSC plasticity and clonal evolution accompanied by genetic and epigenetic changes (11
Although chronic HBV and HCV infections are two major risk factors for the development of liver cancer, their roles in liver CSCs are largely unknown. It is also unclear whether HBV and HCV infect and replicate in hepatic stem/progenitor cells, but a recent study suggested that HCV could replicate in human fetal hepatocytes (132
). Clinicopathological analysis of surgically resected HCC specimens suggested that EpCAM+
CSCs were more frequently detected in HBV–
HCCs than in HCV-related HCCs, although a validation study using a large independent cohort is required (18
). A recent study supported the role of HBx in the activation of HepG2 cell EpCAM expression accompanied by enhanced cell migration and sphere formation (133
). Continued expression of HCV using a subgenomic replicon system was shown to induce stem cell–like properties, including the activation of CD133, AFP, CK19, and c-Myc (134
). Moreover, TLR4 was induced in HCV NS5A transgenic mice following alcohol exposure, and the resulting HCC showed activation of stem cell signatures including CD133 and Nanog (106
). As liver CSCs are considered resistant to cellular stresses, it should be clarified whether HBV or HCV infection directly induces stemness through interaction with signaling pathways, or whether the results reflect enrichment of stress-resistant CSCs in certain conditions.
The target cell population of malignant transformation is generally controversial in human cancer, but accumulating evidence suggests that cancer heterogeneity may derive from different cells of origin as well as diverse genetic mutations (135
). Recent studies have indicated the spontaneous conversion of non-stem cells to stem-like cells in normal breast epithelial cells and suggested that the biological state of normal cells of origin before transformation may be a determinant of the behavior of their descendants following transformation (136
). Similarly, a recent study suggested the unexpected plasticity of normal mature hepatocytes to dedifferentiate into progenitor cells in rodent (137
), but the relation between the biological state of cells of origin in liver lineages and the descendant liver cancer phenotypes remain to be clarified (138
). Interestingly, a recent study demonstrated that ICC originates from hepatocytes when signaling of Notch and Akt is activated in mice, suggesting a role for hepatobiliary lineage commitment program deregulation in hepatocarcinogenesis (Figure and ref. 142
). In rodents, significant studies have demonstrated that HCC may originate from oval cells as well as hepatocytes (see also a recent review article summarizing important earlier works of experimental chemical hepatocarcinogenesis models; ref. 141
). However, the cellular origin of human HCC, ICC, and HCC-CCA remains elusive.
Stem cell niches have been identified and characterized in many tissues, including the germline, bone marrow, intestine, muscle, skin, hair follicle, mammary gland, and nervous system (143
). However, the niche cells that control the proliferation and self-renewal of liver CSCs as well as normal hepatic stem/progenitors have not yet been clarified.
Current knowledge of CSCs is influenced largely by the biology of normal stem cells, in terms of activated markers and signaling pathways. Therefore, elimination of liver CSCs using these markers and signaling pathways may reduce normal hepatic stem/progenitor cells in CLDs, which may inhibit hepatic regeneration leading to hepatic failure. Thus far, it is unclear whether liver CSCs can be effectively eliminated without affecting normal hepatic stem/progenitor cells. In leukemia, PI3K and downstream mTOR kinase composed of key complexes mTORC1 and mTORC2 are frequently activated in CSCs. Recent studies demonstrated a nonredundant requirement of mTORC1 for both hematopoiesis and PTEN loss–induced leukemogenesis in mice (144
). Interestingly, mTORC2 signaling was also required for leukemogenesis but had little effect on normal hematopoietic stem cell function in PTEN-null adult mice, suggesting that mTORC2 is a potential target for the eradication of leukemia CSCs without affecting normal adult hematopoiesis (145
According to the conventional CSC model, only CSCs are highly tumorigenic and metastatic and can divide asymmetrically to generate non-CSCs, and the frequency of CSCs is maintained at a low level. Therefore, eradication of CSCs alone is considered sufficient for tumor regression. However, recent evidence suggests that non-CSCs de-differentiate to generate CSCs in breast cancer (136
). Hepatocytes may have similar features to stem cells in terms of self-renewal, biliary differentiation, and unlimited cell proliferation under certain conditions (27
). It is plausible that transformed marker-negative cancer cells de-differentiate to acquire features of liver CSCs (Figure ).
In leukemia, the frequency of CSCs is not always maintained at low levels, and clonal evolution by genetic changes may determine the nature and frequency of CSCs (11
). Similarly, the frequency of CD133+
HCCs increases as tumors advance (18
), and the clonal evolution model in collaboration with the CSC model could explain the emergence of certain liver CSCs at later stages. Should this be the case, it is unclear whether liver CSCs exist in all liver cancers at every stage. Moreover, given the diversity of HCC genetic traits revealed by whole genome sequencing, it is reasonable to speculate that no common liver CSCs expressing certain stem cell markers exist in all liver cancers. Rather, liver CSCs are likely to be distinct and different for each cancer according to genetic traits and activated signaling pathways. This warrants further studies to provide better diagnostic and treatment strategies for liver cancer patients.