Deconvolution of the cellular origin in hepatocellular carcinoma: Hepatocytes take the center stage
Authors: Jens U. Marquardt
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. J.U.M. is supported by grants from the German Research Foundation (MA 4443/2‐2), the German Cancer Aid (DKH 110989), and the Volkswagen Foundation (Lichtenberg program). See Article on Page 1163 Hepatocellular carcinoma (HCC) is one of the deadliest, fastest growing cancers worldwide.1 A substantial morphological heterogeneity reflecting a large and diverse range of underlying etiological risk factors significantly impedes therapeutic progress. The majority of HCCs develop on the basis of chronic inflammatory disease, most commonly liver cirrhosis, making HCC a paradigm for inflammation‐driven cancers.2 Histological composition of HCC is context dependent and attributed to the type of liver injury defining cellular target of malignant transformation (i.e., cell of origin). Different hepatic resident cell types, such as hepatocytes, biliary epithelial cells (BECs), and adult stem/progenitor cells, may be targets of transforming events and initiate tumor growth. However, our understanding of the putative cell of origin in HCC is limited, and the subject remains a matter of ongoing scientific discussion.3 This lack of knowledge has broad clinical implications and represents a major limitation for development of novel, effective therapeutic strategies. A long life span and a remarkable regenerative potential define the mature hepatocyte as a prime target for malignant transformation under selective pressure induced by chronic inflammatory cell death.4 However, many chronic liver diseases as well as human HCCs display activation of the markers frequently associated with stemness traits, raising a possibility of stem/progenitor cell origin of liver cancer.5 It has been clearly shown that after excessive hepatic injury, effective liver regeneration and restoration of organ function can be predominantly driven by the resident stem/progenitor cells, making them highly susceptible for malignant transformation.6 Hepatic stem/progenitor cell origin of liver cancer has been recently demonstrated in the context of preexisting chronic liver damage and compensatory hepatocyte proliferation.7 Notably, progenitor‐cell derived HCCs are typically characterized by adverse clinical features and a particular poor clinical outcome.8 Nevertheless, other studies challenge the concept of progenitor cell origin of HCC and provide experimental evidence that hepatocarcinogenesis is driven by transformation of hepatocytes.9 In this case, the molecular profiles of human HCCs resembling stem/progenitor cells may reflect acquisition of the stem‐like traits in hepatocyte‐derived tumors, rather than their progenitor cell origin, supporting a concept of cellular plasticity as a hallmark feature of liver cancer. Another plausible scenario suggests that the diverse hepatocytic lineage cells can undergo malignant transformation and induce HCC upon activation of distinct oncogenic programs.10 However, regardless of the cellular origin, acquisition of some degree of stemness appears to be a common prerequisite for HCC development defining liver tumor biology. These observations highlight the importance of oncogenic reprogramming for HCC initiation and progression. In this issue of Hepatology, Shin et al. performed elegant lineage tracing experiments to shed light on the cellular origin of HCC in the context of hepatotoxin‐induced liver cancer.11 The investigators provide convincing evidence that mature hepatocytes are the dominant target cells of malignant transformation in chemically induced hepatocarcinogenesis (Fig. 1). Shin et al. achieved a highly efficient labeling of hepatocytes (>96%) in Rosa‐YFP (yellow fluorescent protein) mice using adeno‐associated virus (AAV)‐specific induction of Cre. To model human HCC initiation, they then employed two different mouse models of hepatotoxin‐induced hepatocarcinogenesis. First, the well‐established diethylnitrosamine (DEN) model was combined with multiple injections of the hepatotoxin, CCl4, to mimic the permissive inflammatory and fibrotic microenvironment present in human patients. All resulting HCCs and the majority of adenomas (26 of 27) were YFP positive (i.e., derived from the hepatocytes). Interestingly, these tumors did not show an activation of key oncogenes, such as Myc, Notch, and Wnt, but consistently contained populations of the YFP‐positive tumor cells expressing high levels of liver progenitor cell markers EpCAM (epithelial cell adhesion molecule), prominin‐1, and SOX9 (SRY [sex determining region Y]‐box 9) and also displayed high levels of nuclear YAP1 (yes‐associated protein 1). However, the ductular structures in the proximity to tumors generally remained YFP negative, confirming that the YFP‐positive cells with stemness characteristics were indeed tumor cells. Furthermore, Shin et al. showed that although forkhead box (Fox) l1+ progenitor cells were activated during the DEN/CCl4‐induced hepatocarcinogenesis, they had little impact on the generation of hepatocyte nuclear factor 4 alpha‐positive hepatocytes as well as tumor development. Finally, to confirm the importance of hepatocytes for HCC initiation, the investigators applied their lineage tracing approach to another chemically induced HCC model by combining DEN treatment with Fas‐induced liver injury using the constitutive androstane receptor ligand, TCPOBOP. In this model, HCC is mechanistically promoted by activation of Myc‐FoxM1 as well as modulation of B‐cell lymphoma 2 proteins. Again, the resulting tumors were uniformly derived from YFP‐positive hepatocytes and contained cells expressing progenitor cell markers EpCAM, Sox9, and Yap1. Notably, development of tumors in this model was associated with Myc activation whereas Wnt and Notch signaling remained unchanged. Lineage tracing of the Foxl1+ putative progenitor cells did not reveal any significant involvement in HCC development.Figure 1: Hepatocytes as the cellular origin of HCC. Schematic overview of the putative cells of origin in chemically induced hepatocarcinogenesis. Lineage labeling of either hepatocytes or stem/progenitor cells in Rosa‐YFP was performed by AAV‐TGB‐Cre or Foxl1‐Cre mice, respectively. After confirmation of effective labeling of >96% of the different lineage cells, hepatocarcinogenesis was induced by DEN/CCl4 or DEN/TCPOBOP. The resulting tumors were exclusively driven by transformed hepatocytes in both models. Strikingly, stem/progenitor cells had no significant contribution to tumor development. However, subpopulations of hepatocyte‐derived tumor cells showed enrichment of established stemness markers, suggesting that oncogenic reprogramming is important for HCC development. A, artery; BD, bile duct; PV, portal vein; TGB, thyroxine‐binding globulin.The well‐performed study by Shin et al. is an important contribution to dissecting the cellular origin of liver cancer. However, it does not rule out that progenitor cells can be a target of transformation in a scenario where hepatocyte regenerative capacity is exhausted (e.g., during chronic human liver diseases).12 Besides, the Foxl1+ cells may represent only a subfraction of progenitor cells, and therefore the contribution of hepatic progenitor cells to hepatocarcinogenesis might be underestimated. In addition, given that not all hepatocytes seem to contribute to HCC development, it is crucial to determine the exact location, properties, and molecular stimuli leading to their transformation.13 In line with this, recently described distinct subpopulations of diploid hepatocytes defined by location (periportal or pericentral), and the unique self‐renewing capacity may harbor a distinctive, and high, risk for oncogenic transformation.13 Several lines of evidence further indicate that hepatocytes can undergo hepatocyte‐to‐BEC reprogramming in response to acute and chronic biliary injury and give rise to biliary tract cancers.15 Whereas the relative contribution of hepatocytes to primary liver cancer other than HCC remains unclear and may represent a rare event, these observations demonstrate that the oncogenic potential of hepatocytes is highly context dependent and likely stems from the oncogenic field effect determined by the type of hepatic injury. Undoubtedly, the current study demonstrates that in context of the repeated cycles of hepatotoxic injury and regeneration, hepatocytes are the main drivers of malignant transformation. Expression of progenitor cell markers in a subpopulation of hepatocyte‐derived tumor cells supports the concept of oncogenic reprogramming in HCC. Together, the present work convincingly illustrates the importance of malignant transformation of hepatocytes in the context of hepatotoxin‐induced liver cancer. Understanding the cellular origin of liver cancer will pave the way for molecular dissection of the specific oncogenic programs driving hepatocyte transformation and identification of new therapeutic and/or diagnostic targets. The significance of oncogenic reprogramming for HCC development supports the notion that targeting the stemness traits may provide an effective therapeutic strategy for liver cancer. Author names in bold denote shared co‐first authorship.